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Dissociation energy and dynamics of HCl and aromatic-water clusters
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Dissociation energy and dynamics of HCl and aromatic-water clusters
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Copyright 2020 Daniel Thomas Kwasniewski DISSOCIA TION ENERGY AND DYNAMICS OF HCl AND AROMA TIC-WA TER CLUSTERS by Daniel Thomas Kwasniewski A Dissertation Presented to the FACULTY OF THE USC GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (CHEMISTRY: CHEMICAL PHYSICS) December 2020 ii To MomMom, with all of my love, For always being a source of encouragement and love throughout my life. This accomplishment is for you! iii Acknowledgements My academic journey has been a wonderful experience, equally stressful, challenging, and rewarding. Luckily, I have been surrounded by many of the most supportive friends, family, and coworkers. First and foremost, I would have never come to the University of Southern California if it was not for Professor Hanna Reisler. Before my senior year of undergrad, I was fortunate to earn a summer research position through the NSF REU program. Her group was so welcoming that I decided to apply to USC in hopes of joining her group as a Ph.D. student. She reluctantly accepted me into her group, and, over the past seven years, she has been one of the most incredibly patient and heartwarming mentors. Dr. Reisler’s infectious enthusiasm and curiosity made believe that I was capable of doing anything in the program, even during the darkest periods of my graduate school career. She mentored and supported me in my current teaching career. As someone who had very little confidence and many weaknesses coming into graduate school, I was able to elevate my abilities under her guidance. Dr. Reisler, if one day you read this section, I just want you to know that I look forward to carrying your enthusiasm for science and your ability to explain complex problems in creative and fun ways into my future classrooms. I am forever grateful to have been mentored by one of the strongest scientific mentors of our generation. As mentioned earlier, I had the pleasure to work with some amazing individuals. I have to first thank Dr. Amit Samanta and Dr. Chirantha Rodrigo for teaching me about the lab instrumentation when I first joined the research group. Without their help, I would have broken most of the equipment in the lab within my first few months, rather than over the course of seven years. While working on the hydrogen bonding project in SSC 612, I had the privilege to work with several wonderful individuals. I was lucky to start on the HCl water project with another then- graduate student, Dr. Kristen Zuraski, who was my friend and colleague for the first five years of iv graduate school. Together, we collaborated on multiple experiments, , wrote papers and proposals, as well as kept each other sane through our coursework, screenings, and qualifying exams. I was also fortunate to be the graduate student mentor to two extremely talented undergraduate students, Sydney Feldman and Mitch Butler, both of whom went on to pursue their doctorates in graduate school. Mitch is a rare soul who has a pure love and curiosity for science, which was infectious to the entire research group. It was an absolute pleasure having him as a coworker and collaborator on the phenol-water project, which was the highlight of my graduate research. Towards the end of my time in SSC 612, I met Dr. Mixtli Campos-Pineda, with whom I had the pleasure of deciphering the mysteries of pyrazine (still a work in progress). On the other side of the lab, I was surrounded by one of the best cast of characters, which included Dr. Ravin Fernando, Dr. Bibek Samanta, and Dr. Subhasish Sutradhar. All three of them were always willing to lend a hand and be a guide throughout my graduate school career. I was so fortunate to have an endless source of encouragement and support from the SSC 610 crew. Bibek, with whom I shared an office for the last two years, made coming to work an absolute joy. Out of all of us, he could change the world one day. When I first came to USC, I was fortunate to be friends with some of the best physical chemistry graduate students. I have to personally thank Bailey Qin, Dr. Natalie Orms, Dr. Arman Sadybekov, and Dr. Han Wool Yoon. Without our late nights studying and coding for classes in the “Wittig Library,” graduate school would have not been as special of a place. All of you will always have a special place in my heart for all of the game nights and food trips across Los Angeles. I would like to also thank Huy Phan, Sean O’Connell and Laura Estergreen for all of their conversations and coffee trips to keep me sane and caffeinated. v Many thanks go out to the professors who have taught me at USC, in particular Dr. Jessica Parr, Dr. Curt Wittig, Dr. Andrey Vilesov, and Dr. Jahn Dawlaty. Dr. Parr served as my mentor for my Burg Teaching Fellowship. Her enthusiasm, passion, and willingness to break away from standard teaching methods were delightful. Dr. Wittig scared the hell out of me my first semester of graduate school, however, he was one of the most encouraging and challenging professors at USC, and I thank him for always pushing me to ask the “good questions.” Dr. Vilesov taught me in a dark hour of his class how to be a Trojan and “Fight On.” Dr. Dawlaty showed me what it was like to be a caring and dedicated teacher at the graduate level. His spectroscopy class will always be one of my favorite courses and I thank him for his never-ending support during our elevator chats in SSC. I would like to thank Michele Dea and Magnolia Benitez for their administrative support. A special thanks to Michele for being the super glue for the Chemistry department; I would not have been able to contribute to the Chemistry Graduate Student Organization or finish my Ph.D. without your support. I would like to thank my undergraduate family at Alvernia University. First, Dr. Rosemarie Chinni, Dr. Joseph Kremer, Kevin Burns, and Professor Elaine Schalck always served as guides throughout my undergraduate career. All of them inspired and encouraged me to do my best, and for that I am forever grateful. I would like to thank my college and high school friends who stood by me throughout these seven years and were always a message away. Keith Kuriger, Brendon McGirr, Chelsea Borror, Jared Phillip, Brandon Bateman, Terry Harrington, Tuan Vo, Raymond Vant, Patrick Price, Jonathan Cudlipp, and David DePalma, thank you for keeping me mentally stable over the past several years. Without each one of you, I would not be the person that I am today. vi I would like to personally thank my friends in Trojan Archery, who, over the past four years, have been a second family to me. During my time period in Trojan Archery, with support from some of the most dedicated volunteers and coaches, we were able to build one of the biggest and most successful collegiate archery programs in California. I cannot wait to see where the future takes all of you during your academic and archery careers. In the meantime, Shoot On! A special thank you goes out to my friend and coach, Terri Ashley-MacQuarrie. I’d like to thank my parents and family for all their love and support. I know that at times they did not really understand exactly what it is that I do here at USC, but I know they can at least see some value in it. Mom and Dad, thank you for all of your support, trips, and phone calls. Last but far from least, I would like to personally thank my fiancé, Alexandra Aloia (Master Alex), who has been by my side for my entire undergraduate and graduate career. It has been a relief to have someone to share my experiences with and I will forever be in debt and appreciative. She has been my rock, my sanity, my soulmate, my proofreader, my “special friend,” and my everything! A special thank you goes out to her for proofreading my entire dissertation. This journey was not an easy one and I hope that I have made all of you proud. For anyone I may have forgotten to mention here, you are no less important, but I only have so much of an attention span. Thank you for helping me see this journey to the end! vii Table of Contents Acknowledgements ...................................................................................................................... iii List of Figures ................................................................................................................................ x List of Tables ............................................................................................................................ xviii Abstract ........................................................................................................................................ xx Chapter 1: Introduction ................................................................................................................ 1 1.1 Hydrogen Bonding ............................................................................................................. 1 1.2 Vibrational Predissociation .............................................................................................. 5 1.3 Background and Motivation for VMI Studies ................................................................ 7 Chapter 1 References ............................................................................................................ 13 Chapter 2: Theoretical Models ................................................................................................... 18 2.1 Ewing’s Model ................................................................................................................. 19 2.2 Prior Distributions ........................................................................................................... 24 Chapter 2 References ............................................................................................................ 29 Chapter 3: Experimental Details ............................................................................................... 31 3.1 Experimental Arrangement ............................................................................................ 32 3.2 Vibrational Predissociation (VP) studied via IR “Action” Photofragment Yield Spectroscopy and IR-UV Depletion Spectroscopy ............................................................. 38 3.2.1 IR “Action” Photofragment Yield Spectroscopy ................................................. 38 3.2.2 IR-UV Depletion Spectroscopy ............................................................................. 39 3.3 Resonance Enhanced Multiphoton Ionization (REMPI) ............................................. 40 3.4 Velocity Map Imaging (VMI) ......................................................................................... 45 3.5 Technical Challenges in Studying Hydrogen Bonded Clusters ................................... 51 3.5.1 Minimizing Background Water in the Interaction Region ................................. 51 3.5.2 Absorption of IR Radiation by Atmospheric Water ........................................... 53 3.5.3 Optimization of Water Fragment Detection ........................................................ 55 3.5.4 Optimization of Molecular Beam Conditions and Laser Conditions for Dimer and Cluster Formation .................................................................................................... 56 3.5.5 Production and Assignment of the HCl-(H2O)3 tetramer ................................... 60 3.5.6 Estimating the Temperature of the Molecular Beam from the Detection of HCl ........................................................................................................................................... 64 viii Chapter 3 References ............................................................................................................ 67 Chapter 4: HCl-(H2O)3Tetramer ............................................................................................... 72 4.1 Introduction ..................................................................................................................... 73 4.2 Experimental Arrangement ............................................................................................ 75 4.3 Methods Used in Theoretical Calculations .................................................................... 78 4.4. Results and Discussion ................................................................................................... 81 4.4.1 IR “Action” Photofragment Yield Spectroscopy Monitoring Pathway 2 ......... 81 4.4.2 Fragment Speed Distributions ............................................................................... 83 4.4.3 Fragment Rotational Energy Distributions ......................................................... 89 4.4.4 Dissociative Trajectories and Lifetimes ................................................................ 93 4.5 Summary and Conclusions ............................................................................................. 96 Chapter 5: Phenol-Water Dimer .............................................................................................. 102 5.1 Introduction ................................................................................................................... 102 5.2 Experimental Details ..................................................................................................... 105 5.3 Results and Discussion .................................................................................................. 108 5.3.1 IR Depletion Spectrum ......................................................................................... 108 5.3.2 REMPI Spectroscopy of H2O Fragments ........................................................... 109 5.3.3 Velocity Map Imaging of the H2O Fragment ..................................................... 111 5.4. Conclusions ................................................................................................................... 120 Chapter 5 References .......................................................................................................... 121 Chapter 6: Pyrazine-H2O .......................................................................................................... 125 6.1 Introduction ................................................................................................................... 125 6.2 Experimental and Theoretical Details ......................................................................... 128 6.2.1 Experimental Details ............................................................................................ 128 6.2.2 Theoretical Details ................................................................................................ 131 6.3 Results and Discussion of the VP of the Pyrazine-H2O Dimer .................................. 132 6.3.1 REMPI Spectroscopy of the Pyrazine Monomer and Pyrazine-H2O Dimer .. 132 6.3.2 IR Depletion Spectroscopy ................................................................................... 135 6.3.3 REMPI spectroscopy of H2O Fragments ........................................................... 138 6.3.4 IR “Action” Spectroscopy of the Pyrazine-H2O dimer ..................................... 140 ix 6.4 Theoretical Calculations ............................................................................................... 142 6.5 Conclusion ...................................................................................................................... 152 Chapter 6 References .......................................................................................................... 154 Chapter 7: Future Work: “Hats Off to Benzene” .................................................................. 159 7.1 Benzene-H2O and Benzene-D2O ................................................................................... 160 7.1.1 Preliminary Experimental Details ...................................................................... 161 7.1.2 Preliminary Experimental Results ...................................................................... 163 7.1.3 Simulated Experimental Results for Bz-H2O and Bz-D2O ............................... 167 7.2 Benzene-HCl .................................................................................................................. 169 7.2.1 Motivation for an Experimental Study of Benzene-HCl ................................... 169 7.2.2 Proposed Experiments with Benzene-HCl ......................................................... 171 Bibliography ............................................................................................................................... 182 Appendix A: MATLAB Programs ........................................................................................... 196 Appendix A.1 Conversion between Pixel, Speed, and Energy Space ............................. 196 Appendix A.2: Beyer Swinehart Algorithm ...................................................................... 197 Appendix A.3: Prior Distribution Program ...................................................................... 200 Appendix A.4: OPO/OPA MATLAB Functions ............................................................... 205 Appendix A References ....................................................................................................... 206 Appendix B: Photoacoustic Cell ............................................................................................... 207 Appendix B References ....................................................................................................... 209 Appendix C: Theoretical Geometries and Energies for Pyrazine-H2O Clusters ................ 210 x List of Figures Figure 1.1: Clusters discussed throughout this dissertation (from left to right): HCl-(H2O)3 (Chapter 4), phenol-H2O (Chapter 5), pyrazine-H2O (Chapter 6), benzene-H(D)2O and benzene-HCl (Chapter 7). ........................................................................................................ 4 Figure 1.2: Diagram 33 depicting the general VP process. This small HCl-H2O cluster provides a simplified example to visualize the bonding. ........................................................................... 6 Figure 1.3: Simplified scheme illustrating the VP of phenol-H2O ................................................. 9 Figure 1.4: Illustration of pyrazine derivatives in Red Bordeaux wine that produce its characteristic flavor. The pyrazine motif is shown in blue. ................................................... 11 Figure 2.1: Potential energy curve, energy terms, and translational wavefunctions for the VP of a weakly bound polyatomic complex (e.g. the HF dimer). Translational and vibrational wavefunctions are shown; * denotes vibrationally excited and “r” is the van der Waals bond length. Figure was reproduced from reference [2]. ................................................................ 21 Figure 2.2: The number of rovibrational states available for the phenol co-fragment when detecting water in the JKa,Kc = 71,6 (704 cm -1 ) rotational state using the prior distribution and direct count of vibrational states. ........................................................................................... 26 Figure 2.3:“IR ON” – “IR OFF” (black) signals for a state-selected H2O fragments obtained following excitation of the free OH stretch of the water moiety of the phenol-water dimer, fitted with an exponential decaying function (blue) based off the Ewing model. The red line represents the prior distribution. The maximum allowed translational energy in this case correspond to D0 = 1960 cm -1 . 15 This was the value used also in the prior calculation. ........ 27 Figure 3.1: Simplified experimental scheme for vibrational predissociation. .............................. 31 Figure 3.2: (Side View) Experimental schematic of the high vacuum chamber depicting the source, main interaction, and detection regions: (1) piezo-electric nozzle; (2) skimmers; (3) electrostatic lens assembly; (4) cryopumping system; (5) detector; (6) ion gauges. ............. 33 Figure 3.3: Top view of chamber showing the anti-collinear propagating laser beam arrangement. M and P refer to alignment mirrors and prisms, respectively. ......................... 35 Figure 3.4: Schematic diagram of the electrostatic lens assembly, Time-of-Flight field free drift tube. R, E, L, and G correspond to the repeller, extractor, lens, and ground electrostatic xi lenses used to achieve VMI conditions. The MCP detector, phosphor screen, and CCD Camera are also shown. .......................................................................................................... 36 Figure 3.5: (Left) Scheme for IR fragment yield “action” spectroscopy of the vibrational predissociation of the HCl-H2O dimer. Following VP, the H2O fragment is detected through a 2+1 REMPI process. (Right) Scheme for IR-UV depletion spectroscopy used to obtain the IR spectrum in the S0 ground state using a 1+1 REMPI as a method of detection via the S1 ← S0 transition. ...................................................................................................................... 40 Figure 3.6:(Left) 2+1 REMPI process via the C̃ 1 B1 ← X ̃ 1 A1 transition. (Right) 1+1 REMPI process via the S1 ← S0 transition of the phenol-water dimer. .............................................. 42 Figure 3.7: Simplified scheme of VMI. Two ions represented by red dots are formed at different initial locations with the same velocity vector, v. They are then accelerated and mapped to the same spot on the detector by the electrostatic ion optics field, E. .................................... 46 Figure 3.8:Diagram of the photofragment imaging approach to measuring the projections of the Newton sphere: (a) Photodissociation of molecules in the molecular beam generates the Newton sphere; (b) The Newton spheres of molecules are then ionized by the ionization laser; (c) Projection of the ion spheres onto the 2D detector; (d) The BASEX reconstruction method utilizes a mathematical transformation of the 2D image back to the 3D data. The data is then converted into speed distributions or center-of-mass translational energy distributions. ........................................................................................................................... 48 Figure 3.9: Comparison of 2+1 REMPI spectrum of the C̃ 1 B1(000) ← X ̃ 1 A1(000) transition of the water monomer without cryopumping (black), with one cryopump (blue), and with two cryopumps (red). .................................................................................................................... 53 Figure 3.10: Diagram of redesigned acrylic dry air box showing the optical path of the IR radiation as it propagates towards the interaction region of the vacuum chamber prior to focusing with a 20 cm focal length lens. ................................................................................ 54 Figure 3.11: Infrared radiation power over time when the box is being purged with dry air. ..... 55 Figure 3.12: (Left) Nozzle-UV time delay scan obtained by monitoring H2O J”Ka,Kc = 32,1 following vibrational predissociation of HCl-(H2O)3. Maximum enhancement can be observed at ~410 µs (Right) IR-UV time delay scan by monitoring H2O J”Ka,Kc = 41,4 upon vibrational predissociation of (H2O)2. 1 IR laser firing was optimized at 65 ns to ensure that the vibrational energy had enough time to couple across vibrations to induce VP. .............. 57 xii Figure 3.13: (Top) Depletion spectrum of (HCl)n-(H2O)m clusters obtained in Helium nanodroplets. 56 Labels indicate cluster size, n:m. (Bottom) The corresponding IR action spectrum 36 (red) obtained by monitoring H2O photofragments in J”Ka,Kc = 32,1, and the background signal from H2O monomers (black). H2O + enhancement in the “action” spectrum was observed at: (a) 3520-3555 cm -1 ; (b) 3580-3590 cm -1 ; (c) 3600-3620 cm -1 ; (d) 3623-3632 cm -1 ; and (e) 3633-3640 cm -1 . .............................................................................. 62 Figure 3.14: Energetically allowed pathways for dissociation of HCl-(H2O)3 following excitation of the H-bonded OH stretch. ................................................................................. 64 Figure 4.1: Simplified Experimental Scheme for the vibrational predissociation of HCl-(H2O)3 ................................................................................................................................................ 72 Figure 4.2: IR action spectrum (red) obtained by monitoring H2O photofragment in J’’Ka,Kc = 32,1 with “IR ON” conditions. The black line shows the “IR OFF” background from H2O monomers. The raw data are shown in lighter color and bold lines show the data with 3- point smoothing. ..................................................................................................................... 82 Figure 4.3: 2 +1 REMPI spectrum of H2O via the C̃ 1 B1 (000) ← X ̃ 1 A1 (000) transition. (Top) The “IR ON” spectrum (red) was obtained by exciting the H-bonded OH stretch of HCl- (H2O)3 at 3550 cm -1 and the “IR OFF” spectrum (black) by recording the background. The arrows mark the following J’Ka,Kc ← J’’Ka,Kc transitions: (a) 71,7 ← 71,6, (b) 20,2 ← 32,1, (c) 40,4/41,4←50,5/51,5, and (d) 20,2 ← 22,1. Assignments are based on the simulated spectrum (300 K) created in PGOPHER (bottom). 34 ............................................................................. 83 Figure 4.4: Speed distributions obtained by monitoring (a) J” = 4 and (b) J” = 6 of the HCl monomer fragments following the dissociation pathway HWWW à H + WWW; and (c) J”Ka,Kc = 22,1 and (d) J”Ka,Kc = 32,1 of H2O fragments from the HWWW à W + HWW pathway. Red and black plots are obtained using “IR ON” and “IR OFF” conditions, respectively. Shaded regions indicate uncertainty in determining the endpoints of the experimental distributions. Arrows indicate the location of endpoints for the speed distributions expected from the calculated dissociation energies. ......................................... 85 Figure 4.5: Experimental and theoretical (using soft ZPE constraints) speed distributions for HCl monomer fragments in rotational levels (a) J”=4 and (b) J” = 6. The large intensity at the origin of the experimental data is the result of monomers in the molecular beam. ......... 87 xiii Figure 4.6: (2+1) REMPI spectra of H 35 Cl (ν = 0) recorded via the (a) f 3 Δ2−Χ 1 Σ + (0, 0) and (b) F 1 Δ2 − Χ 1 Σ + (0,0) intermediate transitions. The background subtracted signal is shown following IR excitation at 3550 cm -1 . .................................................................................... 90 Figure 4.7: Comparison between experimental (black) and calculated (indicated colors) rotational populations for the HCl monomer following dissociation of HWWW. The discrete rotational energy data points for each constraint in the calculations are connected by a line for visual guidance. Experimental REMPI intensities were converted to relative populations using published correction factors. 37, 38 REMPI data obtained using different intermediate states were used and the populations were normalized to J” = 5 for which transitions to all intermediate states were recorded. The experimental and calculated populations are normalized to J” = 3. .............................................................................................................. 90 Figure 4.8: Water monomer rotational energy distributions for Pathway 2 calculated using the indicated constraints. .............................................................................................................. 91 Figure 4.9: Background subtracted 2+1 REMPI spectra (blue) and simulation of the spectra (black) at (a) T = 150 K and (b) T = 300 K. Labeled transitions originate from J’’KaKc rotations: (1) 71,6, (2) 32,1, (3) 50,5, and (4) 22,1. ...................................................................... 92 Figure 4.10: (left) Trajectory snapshots of Pathway 1 for J” = 4 (~6 ps) and (right) Pathway 2 for J” ka,kc= 22,1 (~7 ps). ......................................................................................................... 93 Figure 5.1: Structure of the PhOH-H2O dimer. The angle β=108.7 o represents the angle between the plane of H2O and the H-bond coordinate. 10 ................................................................... 103 Figure 5.2: Experimental scheme for the VP of PhOH-H2O. IR radiation excites one of the OH- stretch fundamental vibrations of PhOH-H2O. (a) The dimer is detected by 1+1 REMPI via its S1 state. (b) H2O fragments in the ground vibrational state are detected by 2+1 REMPI via the C̃ 1 B1(000) state. ............................................................................................................. 105 Figure 5.3: IR Depletion spectra of (a) the H-bonded OH stretch and (b) the free OH stretch of PhOH-H2O. The dimer is probed using 1+1 REMPI via the S1 ← S0 transition at 35,998 cm - 1 . The IR timing alternates between ON/OFF conditions at each frequency. ...................... 108 Figure 5.4: 2 +1 REMPI spectra of H2O fragments recorded via the C̃ 1B1(000) ← X ̃ 1A1(000) transition. The “IR ON – IR OFF” spectrum (black) was obtained by exciting (a) the H- bonded OH stretch of the PhOH moiety at 3522 cm -1 , and (b) the free OH stretch of the H2O xiv moiety at 3744 cm -1 . The “IR OFF” spectrum, obtained by recording the background when the IR laser was fired 2 μs after the UV laser pulse, was subtracted from the “IR ON” spectrum in which the IR laser was fired 65 ns before the UV laser. The arrows mark the !′#$,#&← !”#$,#& transitions monitored in the VMI studies: (a) 20,2 ← 32,1, 20,2 ← 42,3, and 71,7 ← 71,6 and (b) 20,2 ← 32,1 and 20,2 ← 42,3. Assignments are based on simulated spectra (blue) created by the PGOPHER program. 38 ........................................................... 110 Figure 5.5: Left column: “IR ON” (red) and “IR OFF” (black) c.m. translational energy distributions obtained by monitoring state-selected H2O fragments in !”#$,#& levels: (a) 32,1, (c) 42,3, and (e) 71,6 after excitation of the H-bonded OH stretch of PhOH (Pathway 1). Right column: “IR ON – IR OFF” (red) distributions for the same state-selected H2O fragments compared with prior distributions (blue), (b), (d), (f), respectively. The black arrows indicate the maximum allowed translational energies corresponding to D0 = 1960 cm-1.5 This was the value used in the prior calculations as the maximum available energy. .............................................................................................................................................. 114 Figure 5.6: Calculated rovibrational density of states of the PhOH cofragment as a function of energy. In our experiments the available energies are between 858 and 1350 cm -1 for Pathway 1. ............................................................................................................................ 115 Figure 5.7: Left column: “IR ON” (red) and “IR OFF” (black) c.m. translational energy distributions obtained by monitoring state-selected H2O fragments in !”#$,#& levels: (a) 32,1 and (c) 42,3, after excitation of the free OH stretch of the H2O moiety. Right column: “IR ON – IR OFF” (red) signals for the same state-selected H2O, fragments fitted with an exponential decaying function (blue), (b) and (d), respectively. The black dashed lines represent the corresponding prior distributions. The black arrows indicate the maximum allowed translational energies corresponding to D0 = 1960 cm -1 . 5 This was the value used also in the prior calculations and in the decaying function fits. ........................................... 117 Figure 6.1: Structure of the H-bonded Pyrazine-H2O dimer ...................................................... 126 Figure 6.2: Experimental scheme for the VP of pyrazine-H2O. IR radiation excites the OH or CH stretch fundamental vibrations of pyrazine-H2O. (a) The dimer is detected by 1+ n REMPI via the S2 ← S0 transition (),) ∗). (b) H2O fragments in the ground vibrational stated are detected by 2+1 REMPI via the C̃ 1 B1(000) state. ............................................... 129 xv Figure 6.3: S2 ← S0 transition (),) ∗) spectra of Pyrazine + detected by 1+ n REMPI using supersonic molecular beam (black) compared to room temperature experimental results (red) from Stener et. al (2011). 39 ................................................................................................... 133 Figure 6.4: S2 ← S0 transition (),) ∗) spectra of Pyrazine-H2O + detected by 1+ n REMPI ..... 133 Figure 6.5: (top) (),) ∗) spectrum of Pyrazine + at m/z = 80 and (bottom) (),) ∗) spectrum of Pyrazine-H2O + at m/z = 98 detected by 1+ n REMPI. Pyrazine + peaks are representative of the cold monomer and the products of dissociative ionization of larger clusters including the Pyrazine-H2O dimer. ............................................................................................................ 134 Figure 6.6: IR Depletion spectrum of the CH stretch region of pyrazine-H2O. The dimer is probed via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternates between “IR ON” (red) and “IR OFF” (black) conditions at each frequency. .................................................. 135 Figure 6.7: IR Depletion spectrum of the OH stretch of Pyrazine-H2O. The dimer is probed using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternates between “IR ON” and “IR OFF” conditions at each frequency. .......................................... 137 Figure 6.8: 2 +1 REMPI spectra of H2O fragments recorded via the C̃ 1B1(000) ← X ̃ 1A1(000) transition. The “IR ON” (red) and “IR OFF” (black) spectrum were obtained by exciting the OH stretch vibration of the H2O moiety at 3658 cm -1 . The background was recorded when the IR laser was fired 2 μs after the UV laser pulse for “IR OFF” conditions. The IR laser was fired 65 ns before the UV laser for “IR ON” conditions. ............................................. 139 Figure 6.9: IR “Action” spectra (top) of the “free” OH stretch region detecting H2O + photofragments in J”Ka,Kc = 32,1 following the vibrational predissociation of the pyrazine- H2O dimer . IR Depletion spectra (inset) of the “free” OH stretch region of pyrazine-H2O is shown for comparison. The dimer was probed using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing was alternated between “IR ON” and “IR OFF” conditions at each frequency. Note: Daily experiments can take several hours to complete and liquid nitrogen cooling of the detection chamger will not be efficient after 8 hours of continuous use resulting in a gradual increase of the background water signal (See Chapter 3.5.1). .... 140 Figure 6:10: IR “Action” spectra (top) of the CH stretch region detecting H2O + photofragments in J”Ka,Kc = 32,1 following the vibrational predissociation of the pyrazine-H2O dimer . IR Depletion spectra (bottom) of the CH stretch region of pyrazine-H2O. The dimer was probed xvi using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternated between “IR ON” and “IR OFF” conditions at each frequency. .......................................... 141 Figure 6.11: Schematic diagram of the relative energies of pyrazine-(H2O)2 dimer structure. .. 144 Figure 6.12: Schematic diagram of the relative energies of pyrazine-(H2O)2 trimer configurations. ...................................................................................................................... 144 Figure 6.13: Schematic diagram of the vertical excitation energy, Ev, and ionization potential, IP, in eV of pyrazine and two lowest energy conformers of the pyrazine-H2O dimer at the CCSD(T) and EOM-IP-CCSD levels of theory with aug-cc-pVTZ basis set. The energy values were calculated using the difference between the total energy of the ion and neutral molecule. The light blue line separates the dimer from the monomer, where the dimer structures are displayed energetically relative only to each other. ....................................... 151 Figure 7.1: Structure of the H-bonded Bz-H2O dimer. .............................................................. 159 Figure 7.2: Simplified experimental scheme for the VP of H-bonded aromatic dimers. ........... 160 Figure 7.3: S1 ← S0 spectrum of (red) Benzene-D2O + at m/z = 98 and (black) Benzene-H2O + at m/z = 96 detected by 1 + 1 REMPI. The 601 vibration for Benzene-D2O + and Benzene- H2O + were measured at 38,658 and 38662 cm -1 , respectively. ............................................ 164 Figure 7.4: IR depletion spectra of (left) the fundamental OD stretch of Bz-D2O and (right) the fundamental OH stretch of Bz-H2O. The Bz-D2O and Bz-H2O dimers were probed using 1+1 REMPI via the S1 ← S0 transition at 38,658 and 38,662 cm -1 , respectively. The IR timing was alternated between “IR ON” and “IR OFF” conditions at each frequency. ...... 165 Figure 7.5: The S1 ← S0 transition “IR ON” –“IR OFF” spectrum of Bz-D2O + at m/z = 98 detected by 1+1 REMPI. The “IR ON”–“IR OFF” spectrum (red) was obtained by exciting the H-bonded OD stretch at 2671 cm -1 . The “IR OFF” spectrum, obtained by recording the background when the IR laser was fired 2 μs after the UV laser pulse, was subtracted from the “IR ON” spectrum in which the IR laser was fired 65 ns before the UV laser at each frequency. ............................................................................................................................. 165 Figure 7.6: Simulated ET distribution of the VP of the (top) Bz-H2O dimer and (bottom) Bz-D2O dimer. The detected fragments and states are labeled. The red lines indicate vibrational levels of benzene, 43 blue convolutes the vibrational levels with rotational levels. .............. 168 xvii Figure 7.7: Structure of the H-bonded Bz-HCl dimer in the S0 and S1 states. 2 .......................... 170 Figure 7.8: S1 ← S0 spectrum of benzene (black) at m/z = 78 detected by 1+1 REMPI between 40900-42550 cm -1 using a 20 cm f.l. lens, and the same spectral region was monitored for the benzene fragments (red) at m/z = 37 . Note: this one-photon absorption region overlaps with the two-photon absorption region for HCl shown in Figure 7.9 .................................. 171 Figure 7.9: (top) Simulated 2+1 REMPI spectra of HCl + at m/z = 36 in the f 3 Δ2 (ν = 0) ←X 1 Σ + , F 1 Δ2 (ν= 0 ) ←X 1 Σ + , E 1 Σ + (v=0)←X 1 Σ, and V 1 Σ + (v=11,12, and 13)←X 1 Σ + electronic transitions obtained using PGOPHER. (bottom) The corresponding S1 ← S0 spectrum of fragmented benzene + (red) at m/z = 37 detected by 1+1 REMPI, originally between 40900- 42550 cm -1 . In order to show the correlation to HCl, which requires 2 photons, the x-axis (cm -1 ) from Figure 7.8 fragments has been doubled for comparison with the spectra of the benzene fragments (m/z = 37). ............................................................................................. 172 Figure 7.10: Figure from Gord et. al, 41 which illustrates the schematic energy diagrams for a one-color REMPI experiment: (a) non-H-bonded complexes; (b) the H-bonded Bz-HCl energy level diagram with the HCl internal rotor geometry; (c) H-bonded Bz-HCl energy diagram along with the H-bond stretching coordinate, which illustrates the dissociation of the complex. ......................................................................................................................... 173 Figure 7.11: Simulated ET distribution of the VP of the Bz-HCl dimer. The HCl fragment is detected in the labeled rotational state. The red lines indicate vibrational levels of benzene; 43 blue convolutes the vibrational levels with rotational levels. .............................................. 175 Figure B.1: Diagram of photoacoustic cell electronics including microphone, batteries to power microphone, and connection to oscilloscope for data collection. Resistor was 2200 Ohm, capacitor was 0.1 mF, and the system was grounded at the back of the oscilloscope ......... 208 Figure B.2: (Red) Photoacoustic spectrum of 1-3% NH3 in 400 Torr of Helium gas in a vacuum cell at room temperature. (Black) NIST reference spectrum 3 used for OPO/OPA IR calibration. The calibration constant was found to be +82 in February of 2019. ................ 209 xviii List of Tables Table 3.1:Voltages (in V) applied to the electrostatic lens system, MCP detector, and phosphor screen during different modes of data collection ................................................................... 36 Table 3.2: Electronic state transitions used in this work 36 , for the detection of the HCl fragment following the vibrational predissociation of the HCl-(H2O)3 using 2+1 REMPI. Peak positions and descriptive state behavior have been previously reported. 17, 21, 24, 27, 32, 33 ........ 44 Table 3.3: H-bonded OH stretch fundamentals (in cm -1 ) for (H2O)2, (H2O)3, and HCl-(H2O)3 .. 61 Table 4.1: Experimental and Theoretical Values (soft ZPE) for Approximate Peak Positions, Average Speed of the HCl Fragment and Average Translational Energy, Et ........................ 87 Table 4.2: List of Normal Modes using calculated PES for HCl-(H2O)3 2 ................................... 89 Table 6.1: C-H stretch fundaments (in cm -1 ) for pyrazine obtained from Breda et al. 13 compared to the vibrational positions of pyrazine-H2O dimer from this work. ................................... 136 Table 6.2: Optimized geometries of pyrazine, pyrazine-H2O and pyrazine-(H2O)2 at the RIMP2/aug-cc-pVTZ level of theory (Table split between 3 pages). .................................. 145 Table 6.3: Vibrational frequencies in cm -1 , calculated at the RIMP2/aug-cc-pVTZ level of theory, of the stretches of the water moieties of several pyrazine-H2O clusters. † ............... 149 Table 6.4: Comparison between calculated vibrational frequencies (at the RIMP2/aug-cc-pVTZ level of theory) and experimental measurements. ................................................................ 149 Table 6.5: Correction factor required for best fit of calculations with experimental values. ..... 150 Table 6.6: Vertical energy, Ev, and ionization potential, IP, in eV of Pyrazine and Pyrazine-H2O clusters at the CCSD(T) and EOM-IP-CCSD levels of theory with the aug-cc-pVTZ basis set. ........................................................................................................................................ 152 Table C.1: Optimized geometries at the RIMP2/aug-cc-pVTZ level of theory. ........................ 210 Table C.2: Single-point energy calculations of pyrazine-H2O clusters using CCSD(T)/aug-cc- pVTZ .................................................................................................................................... 214 xix Table C.3: Single-point energy calculations of [Pyrazine-H2O] + cluster ions at the CCSD(T)/aug-cc-pVTZ level of theory ............................................................................... 215 xx Abstract The state-to-state vibrational predissociation (VP) dynamics of water clusters with hydrogen chloride or aromatic molecules were studied following infrared excitation of an intramolecular vibrational mode in each cluster. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated center-of-mass translational energy distributions. Product energy distributions and dissociation energies were determined. The cyclic HCl-(H2O)3 tetramer is the largest observed neutral HCl-(H2O)n cluster. The VP of HCl-(H2O)3 was investigated by both theory and experiment, following infrared (IR) laser excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are: HCl + (H2O)3 (Pathway 1) and H2O + HCl-(H2O)2 (Pathway 2). The HCl and H2O monomer fragments were observed by 2+1 REMPI combined with time-of-flight mass spectrometry (TOF-MS), and their rotational energy distributions are inferred and compared to the theoretical results. VMI of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm -1 and 2400 ± 100 cm -1 for Pathway 1 and Pathway 2, respectively. These values are lower, but in the same order as the corresponding calculated dissociation energies: 2426 ± 23 cm -1 and 2826 ± 19 cm -1 . The differences are attributed to contributions of the high-speed tail of the experimental pair-correlated distributions from vibrational hot bands of the clusters. Satisfactory agreement between theory and experiment was achieved when comparing the monomer fragments’ rotational energies, the shapes of the speed distributions, and the average xxi fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from quasi-classical trajectory (QCT) calculations, which are performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are formed in their lowest energy cyclic forms. The VP dynamics of the phenol–water (PhOH–H2O) dimer were studied by detecting H2O fragments and using VMI to infer the internal energy distributions of PhOH cofragments, pair- correlated with selected rotational levels of the H2O fragments. Following IR laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H2O (Pathway 2), dissociation to H2O + PhOH was observed. H2O fragments were monitored state-selectively by using 2+1 REMPI combined with TOF-MS. VMI of H2O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (E T ) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H2O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited. The first observation of the VP of the pyrazine-H2O dimer following excitation of the “free” OH stretch and CH stretch regions was confirmed by the detection of neutral H2O products using REMPI. Following pulsed supersonic expansion, significant rovibrational cooling in the (),) ∗ ) electronic transition was observed for the pyrazine monomer and pyrazine-H2O dimer. xxii Detection of the pyrazine-H2O dimer by 1+ n REMPI and TOF-MS was achieved for the first time. VMI measurements allowed the distinction between translationally cold pyrazine generated in the molecular beam and pyrazine molecules generated in dissociative ionization of higher clusters, which possess kinetic energy. Theoretical calculations indicated that the OH-stretch vibrational peak observed in the experiments corresponds to the non-bonded or “free” hydrogen of the water moiety. Theoretical calculations to characterize the structure and stability of the pyrazine-H2O dimer and trimer and their cations are ongoing. These preliminary establish diagnostics of the clusters to be used in future VMI experiments on the VP dynamics and formation of the H2O fragment, as well as the detection and characterization of Pyrazine-(H2O)2 . 1 Chapter 1: Introduction 1.1 Hydrogen Bonding Water is one of the most ubiquitous molecules on Earth and other interstellar bodies. Hydrogen bonding (H-bonding) gives water its unique properties and has generated a great deal of interest in the scientific community, which has focused its attention on understanding the nature, strength, and dynamics of H-bonds. Without the H-bond, there would not be life on Earth, which makes intermolecular bonding one of the most sought-after phenomena in the search for extraterrestrial life. H-bonding plays an important role in chemical systems ranging from simple dimers in the gas phase to complex biological molecules such as DNA and proteins. Since the early 20 th century, chemists have been fascinated with the nature and dynamics of H-bonded systems. It was in the book, The Nature of the Chemical Bond, 1 that Linus Pauling credited Moore and Winmill 2 with the first mention of the hydrogen bond in 1912. In 1920, Latimer and Rodebush recognized this interaction for its importance and frequent occurrence. 3 Initially, the intermolecular bond between water molecules was determined to have unique properties that could not be described by the known chemical bonding definitions: covalent, ionic, and van der Waals (vdW) forces. The H-bond was reported to range from two orders of magnitude stronger than vdW forces to two orders of magnitude weaker than covalent and ionic bonds. 1 Decades later, Pauling defined that “under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms, instead of only one, so that it may be considered to be acting as a bond between them. This is called the hydrogen bond. 1 ” Weakly bound H-bonds play a crucial role in geometric structures and stereochemistry, relaxation processes, dissociation dynamics, solvation and solvent effects, as well as intermediates in chemical reactions. 2 A century after the first observation of H-bonding, the H-bond definition can be found in every General Chemistry textbook. After decades of research in order to understand and classify the H-bond, IUPAC 4 revised Pauling’s original definition, specifying that: “The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.” 4 The H-bond is denoted as X–H•••Y–Z, where the ••• denotes the H-bond, X–H is the H- bond donor, and Y–Z denotes the H-bond acceptor. In the H-bond donor, X is a highly electronegative atom, most commonly fluorine, oxygen, nitrogen, and chlorine. The requirements of the H-bond acceptor are still not clearly defined, but the bonding typically involves either lone- pairs of electrons or ) -electron clouds. The H-bond strength covers a large range of energies from 0.5 kcal/mol to 40 kcal/mol. 5, 6 The strongest H-bond approaches the strength of covalent bonds (ex. X–H•••X - or X + –H•••Y ) and the weakest H-bond are similar to vdW forces (C–N•••H, O– H•••) , (O–H•••O–H). The strength of the H-bond relies on the strength of its Lewis acid and base. In order to make the H-bond a more tangible concept, theoretical calculations often dissect the interactions that affect the H-bond into electrostatic attraction and steric repulsions. The general definition of the H-bond is imprecise and requires further experimental and theoretical experiments. Over a century of experimental and theoretical work has been devoted to understanding the components that give rise to the H-bond. 7-12 In a ground-up approach, physical chemists have spent the past few decades studying the smallest H-bonding network, the H-bonded cluster, which is described based on the number of subunits found in the cluster (dimer = 2 subunits, trimer = 3 subunits, tetramer = 4 subunits). Some examples of prototypical dimer systems include (HF)2, 3 (HCl)2, (HCl-H2O), (NH3-H2O), (C2H2-H2O), and (H2O)2. 13-21 With this approach, it is believed that the inherent nature of chemical interactions in the conventional condensed phases may be revealed. A pair of molecules in a cluster are often considered to be the precursor to a reaction, which is initiated by a triggering event such as ionization or electronic excitation, which supplies the required energy. Reactions that occur in small clusters are often similar to processes in the conventional phases. Chemical reactivity in clusters depends on the conformation and/or configuration of the intermolecular bonds, which are closely related to the stereochemical processes. Some reactions may be specific to intracluster processes in which the geometric structure allows the reaction to occur. A molecule surrounded by a small number of other molecules represents a molecular model of a solute-solvent system in the condensed phase. Solvent effects in bulk systems may be modeled based on the structure and dynamics found in clusters. Studies on molecular clusters provide us with basic information about common processes encountered in the conventional phases, as well as processes specific to chemistry that occur in clusters, but that may not occur in the bulk. Small molecular dimers have been modeled and studied time and time again. The only direction to progress beyond this exhaustive literature is to study larger systems that resemble more the chemical systems found in aqueous solutions and that apply to biologically relevant molecules. First, in order to better model solute-solvent interactions it is necessary to build the H-bond network by adding individual H-bonded subunits to the dimer to achieve trimers, tetramers, pentamers, etc. (e.g. (H2O)3, (HCl)3, HCl-(H2O)2, HCl-(H2O)3, (NH3)3). Investing these larger clusters with multiple H-bonds will elucidate the many body interactions that are responsible for strengthening the H-bond network and for contributing to the special properties of water. 22-28 In general, these are referred to as cooperative, non-pairwise, or non-additive intermolecular 4 interactions. In order to understand these interactions, joint collaboration between theoretical and experimental research groups are needed to refine our knowledge. H-bonds play a central role in numerous biochemical structures and processes, and, thus, illuminating their characteristic behaviors would be helpful, for example, in protein and enzyme design efforts. 5, 7, 8, 29-31 However, detailed experimental characterizations of the dynamics of H- bonds are sparse. This is due in large part to difficulties in isolating and studying H-bonded systems that are sufficiently small and amenable to experimental interrogation. Clusters of aromatic acids and bases weakly bound to water provide excellent model systems for studying H-bonds at the most fundamental level. 7, 9, 12, 30 Since 1960, H-bonds between dimers with an organic molecule serving as the proton donor or acceptor molecule have been extensively studied and provide a good model for aqueous solutions relevant to biological implications. 9 Figure 1.1: Clusters discussed throughout this dissertation (from left to right): HCl-(H2O)3 (Chapter 4), phenol-H2O (Chapter 5), pyrazine-H2O (Chapter 6), benzene-H(D)2O and benzene- HCl (Chapter 7). As stated earlier, the H-bond is responsible for the unique properties of water, and its clusters with biologically relevant molecules, therefore it is important to understand the mechanisms of its formation and breakup. As the reader can imagine, an H-bond will break when energy that exceeds the bond strength is imparted to the cluster. However, the question remains: when the H-bond breaks, what happens to the excess energy? According to conservation of energy, the excess energy must be accounted for and needs to be disposed of. In fact, the excess energy is 5 distributed in the vibrational, rotational, and translational motions of the dissociated fragments. The focus of this dissertation is on the mechanism by which the energy is distributed amongst the vibrational modes of the cluster prior to H-bond breaking, and how the excess energy is distributed among the fragments. Reported in this dissertation are measurements of the H-bond strength and energy flow dynamics following the dissociation of the following systems (Figure 1.1): HCl- (H2O)3 (Chapter 4) , phenol-H2O (Chapter 5), and pyrazine-H2O (Chapter 6). Future experiments (Chapter 7) on benzene-H2O, benzene-D2O, and benzene-HCl are also discussed. The motivation for studying these systems is detailed in each of their respective sections. 1.2 Vibrational Predissociation Weakly bound complexes have potential energy surfaces with a shallow minimum, which means that their bonds can be easily broken following excitation (see Figure 1.2). 32 According to conservation of energy, the system must have a dissociation energy lower than the vibrational excitation energy in order for dissociation to occur. 32 This vibrational predissociation (VP) of a weakly bound complex occurs by the following sequence: (1) an intramolecular mode within the molecular complex is vibrationally excited; (2) the energy is coupled across vibrational modes and is redistributed; and (3) energy is finally coupled to the H-bond coordinate through couplings to intermolecular modes, leading to dissociation. This type of vibrationally induced, indirect dissociation through a coupling process is commonly referred to as VP, which is the focus of this dissertation. 6 Figure 1.2: Diagram 33 depicting the general VP process. This small HCl-H2O cluster provides a simplified example to visualize the bonding. The clusters of interest are comprised of weakly bound molecules, where the atoms within each molecule are held together via strong covalent bonds, while the molecules themselves are attracted to neighboring molecules by weak interactions such as H-bonds or vdW forces. The discrepancy in bond strength translates into disparities in their respective vibrational frequencies, which results in differing flows of energy between molecules of the cluster and within individual molecules. As a result, there are energy bottlenecks that inhibit the distribution of energy and give rise to nonstatistical product energy distribution. The product energy distributions are described by the Ewing Model (Chapter 2), which is based on momentum or energy gap laws. 34-36 For the predissociation of dimers containing polyatomic molecules such as aromatic molecules and water, state-specific information on energy disposal is rare because of the large density of states. Large nonlinear molecular species with N atoms have 3N-6 modes of vibrations. A molecule such as 7 phenol with 13 atoms will have 33 normal modes of vibration, which result in high density of rovibrational states at energies near the dissociation energy of their clusters. In addition, the existence of low frequency intramolecular modes in polyatomic molecules will be high, which can facilitate energy transfer to the intermolecular modes and lead to statistical-like predissociation mechanisms. 32 Infrared induced dissociation of clusters can provide a testing ground for theories of intramolecular vibrational energy redistribution (IVR) and unimolecular dissociation. The study of the VP of H-bonded clusters and their dissociation dynamics can provide insight into the disposal of excess energy, which reflects the strengths of the vibrational couplings between the different vibrational modes. Velocity Map Imaging (VMI) can be used to examine pair-correlated product state distributions in the VP of H-bonded clusters. VP processes have challenged both experimentalists and theorists, and are still not completely understood. These inefficient processes can exhibit state-specific effects on rotational and vibrational excitations. In the absence of potential energy surfaces, several theoretical models have been developed to help explain experimental observations, which are discussed in detail in Chapter 2. 1.3 Background and Motivation for VMI Studies Evidence for H-bond interactions can be observed in a variety of experimental techniques. In vibrational spectroscopy, evidence for H-bond interactions can be seen in the form of an enhanced IR signal of an intramolecular vibrational transition implicated in the H-bonding of the cluster, accompanied by a red-shift in its wavelength relative to the monomer. This red shift can be related to the geometrical changes upon the formation of the H-bond. In the structure, X–H•••Y– Z, where Y has lone pair or π electrons that effectively pull the positive hydrogen atom towards the more negative Y atom, the X–H bond, as a consequence, is lengthened, causing a shift in the 8 vibrational wavelength. 37-42 The strength of the H-bond is related to the extent of the red shift, and this observation has been exploited in countless spectroscopic studies to understand trends in H- bonding. 4 Most of the information on H-bonds has been uncovered independently by analyzing distinct structural patterns found in X-ray and neutron diffraction, by gas phase rotational spectroscopy, and by frequency shifts in vibrational spectroscopy. However, a more sophisticated combination of methods is needed to determine the underlying dynamics of VP. This is accomplished by combining vibrational and rotational spectroscopies along with VMI, which is a reliable and sensitive method to determine state-specific translational energy distributions. VMI can provide pair-correlated product state distributions following the VP of H-bonded complexes. The spectroscopy, dissociation energy, and product energy distributions of H-bonded water with HCl and aromatic molecules have been investigated by our group utilizing VMI. 10, 25 The specific details of the experimental methods used in the VP studies are discussed in Chapter 3. Descriptions of the creation, characterization, and isolation of these clusters is also given. In order to investigate the potential of H-bonds to break covalent bonds within a molecule and create ionized species, physical chemists have made water-hydracid complexes the prototype system to study mechanisms surrounding H-bonding and solute-solvent interactions in acid/base chemistry. The dynamics of dissociation of HCl clusters is of particular interest, not just for its relative simplicity, but because the solvation of HCl is thought to play an important role in the generation of Cl2 and HOCl in the atmosphere. 43, 44 These species are subsequently photolyzed to generate Cl atoms, which induce chain reactions that destroy ozone. In addition, it is well known that aqueous water can stabilize ions in solution, however the dynamics that lead to ionization are not well understood in small clusters. Chapter 4 continues the ground-up approach to observe 9 trends in H-bonding strengths in mixed HCl-water clusters by starting with a dimer of HCl H- bonded to one water molecule and adding consecutive water molecules. Through the study of mixed neutral water clusters such as HCl-(H2O)n (n = 1-3), it is possible to observe trends in dissociation energy, and the effects that a cyclic H-bonding network may have on cooperativity. 27, 28 Theoretical calculations suggest that ionization occurs at n = 4 or 5 depending on the experimental conditions. 45-51 Chapter 4 discusses the collaborative study of VP of HCl-(H2O)3, the largest cluster for which ionization of HCl is not expected, and identifies H2O and HCL as VP products. Figure 1.3: Simplified scheme illustrating the VP of phenol-H2O Chapters 5 and 6 focus on H-bonding in biologically relevant molecules, phenol (PhOH) and pyrazine. Aromatic molecules play a central role in numerous biochemical structures and processes. Clusters of aromatic molecules weakly bound to water provide excellent model systems for studying H-bonds at the most fundamental level. PhOH and its derivatives are pervasive motifs in biochemical systems, present in the side chain of the amino acid tyrosine, as well as in electron transport, signaling pathways, and other biological processes. It is therefore not surprising that 10 numerous studies have focused on the PhOH-H2O H-bonded dimer in the gas phase to unveil its structure, spectroscopy, and energetics. 52-70 Direct interrogation of the H2O fragment in the VP of PhOH-H2O is discussed in Chapter 5 of this dissertation. The VP dynamics and dissociation energy of the PhOH-H2O dimer (Figure 1.3) were studied by detecting H2O fragments and using VMI to infer the internal energy distribution of the phenol cofragment, pair-correlated with selected rotational levels of the H2O fragments. 71 The pair-correlated internal energy distributions of the phenol cofragment derived via excitation of the H-bonded OH stretch of PhOH were well described by a statistical prior distribution. This result was expected due to the high density of rovibrational states of the phenol cofragment. On the other hand, the corresponding distributions obtained via excitation of the “free” OH stretch of water showed a propensity to populate energy rovibrational levels of PhOH that were higher than expected for a statistical distribution and better agree with an energy-gap model. A brief discussion of the Ewing Model and prior distributions is included in Chapter 2. 32, 34-36 Chapter 6 details initial experimental and theoretical investigations of the VP of aromatic heterocyclic molecules than include nitrogen H-bonded to water. H-bonded molecules, especially those containing heterocyclic nitrogen atoms, play a fundamental role in the structure and function of many biological systems. 31, 72 In the case of oligonucleotides, heterocyclic nitrogen atoms are essential in base-pairing and determine the tertiary structure and function of biopolymers. 31 Pyrazine and its derivatives are aromatic H-bonded acceptors that are good models for components of proteins and nucleotides. Interestingly, derivatives of pyrazine are also of importance to the wine industry because it produces complex herbaceous flavors and scents that depend on its concentration (illustrated in Figure 1.4). Flavors can range from pleasant qualities, like in Red Bordeaux varieties that taste 11 like “fire roasted red peppers,” to undesirable qualities that remind the drinker of “old asparagus water” or “mushy stewed peppers” in the case that the concentration of the substituted pyrazine is too high.” 73 It has been speculated that H-bonding affects taste and flavor profiles. 73, 74 Figure 1.4: Illustration of pyrazine derivatives in Red Bordeaux wine that produce its characteristic flavor. The pyrazine motif is shown in blue. In our studies, we report the first spectral signature of the pyrazine-H2O dimer by excitation via the S2 ← S0 transition (),) ∗ ). We also report the first successful observation of the VP of the pyrazine-H2O dimer by exciting the “free” OH stretch of the water moiety and the CH stretch region of the pyrazine moiety, followed by detecting the H2O fragment and examining IR depletion spectroscopy of the parent complex. To support our experimental findings, we performed electronic structure calculations to compute the geometries, vibrational frequencies, and ionization potentials of pyrazine, the pyrazine-H2O dimer, and the pyrazine-(H2O)2 trimer. 12 The studies of hydracid clusters and aromatic water clusters covered in this dissertation attempt to enhance our understanding of VP dynamics of H-bonded complexes. The knowledge obtained from these studies can be applied to more complex systems found in atmospheric processes and biology. Chapter 7 details preliminary experimental work and insight into a future direction on dimers of small aromatic molecules H-bonded to water. This chapter focuses on benzene-H2O, benzene-D2O, and benzene-HCl, “T-shaped” H-bonded dimers, in order to understand whether different binding motifs can influence energy flow pathways in VP. In the benzene-H2O and -D2O dimers, for example, H2O/ D2O is H-bonded to the ring in a perpendicular geometry with its H atoms pointing towards the ) -cloud of the ring. 82,85,92,96 The forces exerted on the aromatic ring following excitation of the H-bonded OH stretch in the benzene-H2O and -D2O dimers might result in very different vibrational state distributions and/or changes in Et release, especially in comparison to the work discussed in Chapter 5 involving the PhOH-H2O dimer. Using VMI to determine pair-correlated vibrational distributions in the aromatic cofragment as a function of the monitored water fragment level (i.e. available energy) would enable detailed mapping of the dependence of vibrational energy deposited in the cofragment on dimer geometry and H-bonding motifs. 13 Chapter 1 References 1. Pauling, L., The nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry. Cornell University Press: New York: 1939. 2. Moore, T. S.; Winmill, T. F., J. Chem. Soc., Trans. 1912, 101, 1635. 3. Latimer, W. M.; Rodebush, W. H., J. Am. Chem. Soc. 1920, 42, 1419-1433. 4. Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J., Pure and Appl. Chem. 2011, 83 (8), 1619-1636. 5. Desiraju, G. R.; Steiner, T., The weak hydrogen bond instructural chemistry and biology. Oxford University Press.: Oxford, 1999. 6. Glendening, E. D., J. Phys. Chem. A 2005, 109, 11936-11940. 7. Frey, J. A.; Holzer, C.; Klopper, W.; Leutwyler, S., Chemical Reviews 2016, 116 (9), 5614-5641. 8. Meyer, E. A.; Castellano, R. K.; Diederich, F., Angewandte Chemie International Edition 2003, 42 (11), 1210-1250. 9. Mons, M.; Dimicoli, I.; Piuzzi, F., International Reviews in Physical Chemistry 2002, 21 (1), 101-135. 10. Reisler, H., Annu. Rev. Phys. Chem. 2009, 60 (1), 39-59. 11. Scheiner, S., Annu. Rev. Phys. Chem. 1994, 45 (1), 23-56. 12. Zwier, T. S., Annu. Rev. Phys. Chem. 1996, 47, 205-241. 13. Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2010, 114 (36), 9774-9781. 14 14. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134 (37), 15430-15435. 15. Czakó, G.; Wang, Y.; Bowman, J. M., J. Chem. Phys. 2011, 135 (15), 151102. 16. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2009, 113, 10174-10183. 17. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. 18. Pritchard, M.; Parr, J.; Li, G.; Reisler, H.; McCaffery, A. J., Phys. Chem. Chem. Phys. 2007, 9 (47), 6241-6252. 19. Rocher-Casterline, B. E.; Ch'ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134 (21), 211101. 20. Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134, 211101. 21. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2011, 115, 6903-6909. 22. Keutsch, F. N.; Cruzan, J. D.; Saykally, R. J., Chem. Rev. 2003, 103, 2533. 23. Ch'ng, L. C.; Samanta, A. K.; Wang, Y.; Bowman, J. M.; Reisler, H., J. Phys. Chem.A 2013, 117 (32), 7207-16. 24. Samanta, A. K.; Ch'ng, L. C.; Reisler, H., Chem. Phys. Lett. 2013, 575, 1-11. 25. Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700-2709. 26. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chem. Rev. 2016, 116 (9), 4913-4936. 15 27. Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243-4247. 28. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. 29. Desfrançois, C.; Carles, S.; Schermann, J. P., Chem. Rev. 2000, 100 (11), 3943-3962. 30. Dopfer, O.; Fujii, M., Chemical Reviews 2016, 116 (9), 5432-5463. 31. Jeffrey, G. A.; Saenger, W., Hydrogen bonding in biological structure. Springer-Verlag: Berlin: 1991. 32. Baer, T.; Hase, W. L., Unimolecular Reaction Dynamics: Theory and Experiments. Oxford University Press, Inc.: New York, NY, 1996. 33. Ch'ng, L. C. Dissociation Energy and Dynamics of Water Clusters. University of Southern California, Los Angeles, CA, 2013. 34. Ewing, G. E., J. Chem. Phys. 1980, 72, 2096. 35. Ewing, G. E., J. Phys. Chem. 1979, 71, 3143. 36. Ewing, G. E., J. Phys. Chem. 1987, 91 (18), 4662-4671. 37. Barnes, A. J., J. Mol. Struct. 1980, 60, 343-346. 38. Hobza, P.; Havlas, Z., Chem. Rev. 2000, 100 (11), 4253-4264. 39. Joseph, J.; Jemmis, E. D., Journal of the American Chemical Society 2007, 129 (15), 4620- 4632. 40. Leforestier, C., Philos. Trans. R. Soc. London, Ser. A 2012, 370 (1968), 2675-2690. 41. Re, S.; Osamura, Y.; Suzuki, Y.; Schaefer, H. F., J. Chem. Phys. 1998, 109 (3), 973-977. 42. Skvortsov, D.; Choi, M. Y.; Vilesov, A. F., J. Phys. Chem. A 2007, 111 (49), 12711-6. 16 43. Huneycutt, A. J.; Stickland, R. J.; Hellberg, F.; Saykally, R. J., J. Chem. Phys. 2003, 118 (3), 1221-1229. 44. Amirand, C.; Maillard, D., J. Mol. Struct. 1988, 176, 181-201. 45. Mancini, J. S.; Bowman, J. M., Phys. Chem. Chem. Phys. 2015, 17 (9), 6222-6226. 46. Forbert, H.; Masia, M.; Kaczmarek-Kedziera, A.; Nair, N. N.; Marx, D., J. Am. Chem. Soc. 2011, 133, 4062. 47. Masia, M.; Forbert, H.; Marx, D., J. Phys. Chem.A 2007, 111 (49), 12181-12191. 48. Walewski, Ł.; Forbert, H.; Marx, D., Chem Phys Chem 2013, 14 (4), 817-826. 49. Andot, K.; Hynes, J. T., J. Mol. Liq. 1995, 64 (1–2), 25-37. 50. Sugawara, S.; Yoshikawa, T.; Takayanagi, T.; Tachikawa, M., Chem. Phys. Lett. 2011, 501 (4-6), 238-244. 51. Hassanali, A. A.; Cuny, J.; Ceriotti, M.; Pickard, C. J.; Parrinello, M., J. Am. Chem. Soc. 2012, 134 (20), 8557-8569. 52. Bandyopadhyay, I.; Lee, H. M.; Kim, K. S., J. Phys. Chem. A. 2005, 109, 1720-1728. 53. Braun, J. E.; Mehnert, T.; Neusser, H. J., Int. J. Mass Spectrom. 2000, 203, 1-18. 54. Castleman, A. W.; Stanley, R. J., J. Chem. Phys. 1991, 94 (12), 7744-7756. 55. Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millié, P., J. Phys. Chem. A 1998, 102 (25), 4890-4898. 56. Doi, A.; Naohiko, M., J. Chem. Phys. 2008, 129, 154308. 57. Ebata, T.; Mizuochi, N.; Watanabe, T.; Naohiko, M., J. Chem. Phys. 1996, 100, 546-550. 58. Fuke, K.; Kaya, K., Chem. Phys. Lett. 1983, 94 (1), 97-101. 59. Gerhards, M.; Kleinermanns, K., J. Chem. Phys. 1995, 103 (17), 7392-7400. 17 60. Gerhards, M.; Schmitt, M.; Kleinermanns, K.; Stahl, W., J. Chem. Phys. 1996, 104 (3), 967-971. 61. Lipert, R. J.; Bermudez, G.; Colson, S. D., J. Phys. Chem. 1988, 92 (13), 3801-3805. 62. Mazzoni, F.; Pasquini, M.; Pietraperzia, G.; Becucci, M., J. Mol. Struc. 2015, 1090, 2-6. 63. Mikami, N., Bull. Chem. Soc. Jpn. 1995, 68 (3), 683-694. 64. Miyazaki, Y.; Inokuchi, Y.; Ebata, T.; Petkovic, M., Chem. Phys. 2013, 419, 205-211. 65. Oikawa, A.; Abe, H.; Mikami, N.; Mitsuo, I., J. Phys. Chem. 1983, 87 (25), 5083-5090. 66. Petković, M., J. Phys. Chem. A 2011, 116, 364-371. 67. Shimamori, T.; Fujii, A., J. Phys. Chem. A 2015, 119, 1315-1322. 68. Watanabe, T.; Ebata, T.; Tanabe, S.; Mikami, N., J. Chem. Phys. 1996, 105 (2), 408-419. 69. Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K., J. Chem. Phys. 1996, 104 (3), 972-982. 70. Tanabe, S.; Ebata, T.; Fujii, A.; Mikami, N., Chem. Phys. Lett. 1993, 215 (4), 347-352. 71. Kwasniewski, D.; Butler, M.; Reisler, H., Phys. Chem. Chem. Phys. 2019, 21 (26), 13968- 13976. 72. Scheiner, S., Noncovalent Forces. Springer: 2015. 73. Bruni, F.; Di Mino, C.; Imberti, S.; McLain, S. E.; Rhys, N. H.; Ricci, M. A., The Journal of Physical Chemistry Letters 2018, 9 (13), 3667-3672. 74. Roujou De Boubee, D., UC Davis: Viticulture & Enology 2009, 9, 1-3. 18 Chapter 2: Theoretical Models Vibrational predissociation (VP) of a weakly bound complex occurs following the excitation of an intramolecular vibrational mode of one of the molecular subunits of the complex, which eventually couples to the intermolecular dissociation coordinate. Therefore, VP requires some redistribution of energy, and the product energy distributions are sensitive to the efficiency of the coupling between the intermolecular and intramolecular vibrational modes, which usually have a large disparity between their frequencies. Achieving a better understanding of the energy distribution and lifetimes of vibrationally excited, weakly bound complexes such as H-bonded clusters has been of great interest to the scientific community 1 and is the focus of the work in this dissertation. 2-4 Several theoretical models have been developed to explain product energy distribution and vibrational predissociation dynamics of weakly bonded complexes, such as the hydrogen-bonded (H-bonded) complexes discussed in this dissertation. Three of these models have been applied to our recent studies: (1) the Ewing model, which is based on momentum and energy gap laws; (2) prior distributions; and (3) quasiclassical trajectory calculations. The Ewing model 5-7 concludes that, upon the dissociation of weakly bound complexes and following excitation of the intramolecular vibrational mode, the excess energy has the propensity to populate vibration over rotation over translation. On the other hand, the prior distribution model describes statistical behavior and is appealing, because it is based on an unbiased “democratic” model of state populations that imposes no constraints other than energy conservation. The product energy distributions in the VP of the HCl-(H2O)3 tetramer were obtained by Bowman and coworkers 4 by using quasiclassical trajectory calculations, as discussed in Chapter 4. 3, 4, 8 19 2.1 Ewing’s Model Understanding the mechanisms by which the process of vibrational predissociation occurs has proven to be both a theoretical and experimental challenge. Following dissociation, molecular complexes that are composed of only a few atoms often show non-statistical populations among the rotational and vibrational states of fragments. In larger polyatomic complexes that have a high density of states, the excess energy following the excitation of a specific intramolecular mode rapidly couples to other vibrational modes across the complex. The complex with fewer atoms generally shows more state-specific results, due to disparity between the frequencies of the inter- and intramolecular vibrational modes, which leads to a nonstatistical energy distribution in the photodissociated fragments. In the 1970’s, George Ewing extended the momentum (energy) gap law 9, 10 in the article 7 “Selection rules for Vibrational Energy Transfer: Vibrational Predissociation of van der Waals Molecules,” in which he proposed that a “relaxation channel of a vibrationally excited molecule is efficient only when the total change in effective quantum numbers for the process is small. 6 This means that the relaxation process favors the disposition of the maximum amount of energy with the least change in quantum numbers. Thus, the excess energy has the propensity to populate vibration over rotation over translation. When a weakly bound complex is energetically excited and dissociates into two fragments, relaxation can proceed by at least four channels: 1) The energy from the excited vibrational band is completely transferred to the intermolecular stretch coordinate (dissociation coordinate), and the fragments in the ground vibrational state fly away with maximum translational energy (with all of the excess energy). This is the vibration-translational (V-T) channel. 20 2) The fragments have rotational energy as they fly apart, and the relaxation occurs also through the vibrational-rotational (V-T, R) coupling. 3) When the fragments can contain vibrational excitation following dissociation, the relaxation can occur through the vibration-vibration (V-V) channel. 4) The energy flows throughout the complex without predissociation. This intramolecular vibrational redistribution (IVR) proceeds through isoenergetic internal modes of the complex. This occurs when the excitation energy is lower than the bond dissociation energy. When the energy is higher, complete IVR may take place prior to dissociation. Ewing proposed that the rate of vibrational predissociation through the first three channels can be described by the empirical expression: 0 " # = 10 #$ 234[−)(∆8 % +∆8 & +∆8 ' )] = 10 #$ 234[−)(Δ8 ( )] Eq. 2.1 The change in the total effective quantum number (Δ8 ( ) is defined as the sum of the translational (∆8 % ), rotational (∆8 & ), and vibrational (∆8 ' ) quantum numbers. The preexponential factor (10 13 ) gives the typical collision frequency of the monomers through a van der Waals bond. The exponential term, 234[−)(Δ8 ( )], describes the probability that the initial discrete states of the bound complex and the final state of the monomer fragments will mix during the collision, and is a measure of the reluctance of the complex to change quantum numbers during vibrational predissociation. This general propensity rule shows that the relaxation proceeds most efficiently by the channel that corresponds to the smallest change in the total effective quantum number, (Δ8 ( ). The effective translational quantum number change (Δ8 ( ) can be labeled as the difference between the number of nodes (< % /2) of the translational wavefunction of the predissociation 21 fragment pair and the number of nodes (? % ) in the initial van der Waals stretching vibrational wavefunction of the two monomer fragments: Δ8 % ≈ A ) ! * −? % A Eq. 2.2 Figure 2.1: Potential energy curve, energy terms, and translational wavefunctions for the VP of a weakly bound polyatomic complex (e.g. the HF dimer). Translational and vibrational wavefunctions are shown; * denotes vibrationally excited and “r” is the van der Waals bond length. Figure was reproduced from reference [2]. 22 Figure 2.1 6 shows the potential energy curve for the ground and vibrationally excited states along the van der Waals coordinate, as well as the energy terms and translational wavefunction for the V-T channel of the complex. Figure 2.1 also shows the discrepancy in the number of nodes in the van der Waals stretching vibrational wavefunction of the complex and the translational wavefunction of the predissociation fragments. This can be described as a poor Franck-Condon overlap, expected for wavefunctions with an extensively differing number of nodes, which gives rise to inefficient energy transfer in the V-T channel, and is exhibited in the exponential dependence on the change in effective quantum numbers as seen in Eq. 2.1. As depicted in Figure 2.1, Ewing 7 used the (HF)2 dimer as an example of a weakly bound complex to compare the efficiency of energy transfer through the V-T and the V-T, R channels. Given that the Δ8 % value is large in comparison to the number of nodes in the translational wavefunction and that ? % has no nodes, Δ8 ' is therefore equal to one. The total change in the effective quantum number becomes large for the (HF)2 dimer for the V-T channel and makes energy transfer through this channel inefficient. When the Franck-Condon factors are favorable, the lifetime is in turn shortened. Translational energy is transferred to the rotational and/or vibrational energy of the fragments, which have more energy per state and fewer nodes, resulting in a small change in total effective quantum number. Consequently, after VP, fragments are often produced in high rovibrational states. The influence of rotational degrees of freedom is difficult to model during the VP process. The effective rotational quantum number change can be made synonymous with the effective translational quantum number through the following expression, where < & is essentially defined as the rotational quantum number, J, for diatomic molecule. Δ8 & ≈ A ) " * −? + A Eq. 2.3 23 This expression shows that vibrational predissociation through the combined V-T, R channel is more favorable than in the V-T channel. Both the V-T and V-R channels, however, depend on the reduced mass of the cluster and the moment of inertia of the fragments. The V-R channel also has to obey angular momentum conservation, and constraint that is not taken in account by Ewing. For the V-V channel, since the change in vibrational levels can dispose the largest amount of energy with the smallest change in the number of quanta, this channel is expected the most efficient in VP when they are energetically allowed. Finally, the overlap between the initial and final vibrational wavefunctions can be defined in terms of their effective quantum numbers: Δ8 ' ≈ BC? , −? - C = B|Δ?| Eq. 2.4 Where ? , is the vibrational quantum number of the dissociating fragments and ? - is the vibrational quantum number of the vibrationally excited, weakly bound complex. The factor, B, defines the effectiveness of the coupling of the intermolecular hydrogen bond with the intramolecular vibrational motions of the complex. Without coupling of the intramolecular vibrational mode to the dissociation coordinate, there is effectively no VP, and the strength of this coupling affects the dissociation rate. The Ewing energy gap law has been successfully applied to many experimental VP rates and product state distributions of van der Waals and H-bonded small dimers. However, the model is still not completely understood, especially in regard to the observed state specificity in the V-V channel. Several past experiments have examined the effectiveness of coupling between the vibrations of the H-bonded complex and the dissociation coordinate, and have shown which couplings play an important role in explaining the energy disposal in fragments. In the VP of the phenol-H2O dimer, described in Chapter 5, only phenol fragments generated through the excitation 24 of the free OH stretch showed a propensity to populate very high rovibrational levels, while excitation through the H-bonded stretch of the phenol moiety showed a statistical (prior) distribution of energy among all rovibrational states. 2 It is important to emphasize that the laws explained in this section ignore the conservation of angular momentum, which can limit the lover of fragment rotational excitation. 5-7 2.2 Prior Distributions In the case of a bimolecular reaction or dissociation that proceeds through a long-lived intermediate complex, product rovibrational distributions can often be predicted using statistical models. 11 Prior distributions 11 provide a good first-order picture of statistical behavior, especially for larger molecules for which state-to-state experiments are not feasible. This is because they are based on an unbiased “democratic” model of state populations that imposes no constraints other than energy conservation. Neglecting angular momentum conservation should not significantly alter the internal energy distributions of a large polyatomic cofragment because of its high density of internal states. The prior distribution is determined by the available volume of phase space of the product fragments and is constrained only by energy conservation, not angular momentum. The model has been used successfully in accessing shapes of distributions in chemical reactions proceeding via a bound intermediate, 12 unimolecular reaction, 13 and predissociation of dimers, 14 where detailed phase space calculations are unfeasible. For a single dissociation event with a given excess energy, E, we can write the following: E = E '# +E &# +E '* +E &* +E ( Eq 2.5 Where the terms are defined as vibrational (Ev), rotational (ER), and translational (ET) energy for a two-product dissociation. For example, in the case of an aromatic molecule H-bonded to water that 25 dissociates into a large polyatomic molecule (fragment 1) and water fragments (fragment 2), the probability of finding the two fragments in a specific quantum sate can be given by Eq. 2.6: F(E &# ,E &* ,E '# ;E '* ,E ( )HF = # . (0) I(E &# )I(E &* )I(E '# )I(E '* )I(E ( )HE '# HE '* HE &# HE &* HE ( Eq. 2.6 Rotation and vibration of the aromatic co-fragment are described as ER1 and EV1, respectively. The total density of states, I(E), serves to normalize the prior distribution. In our phenol-H2O experiments in which we select for monitoring a specific rotational transition of the water fragment, we can set the vibrational energy (v = 0) and rotational (J = J”Ka, Kc) of water to the following, which is a constant numerical value: I(E &* ) = ! 2 # 2 $ " Eq 2.7 I(E? * ) = (? = 0) Eq. 2.8 In this work, the polyatomic molecule, an aromatic ring such as phenol, pyrazine, or benzene, can be treated as an oblate symmetric top with a 3D rotational density of states, where rotational constants A=B, and C: I(E &# ) = J * 4√6 KE &# #/* Eq 2.9 Translational energy is given in center of mass framework, and the 3D translational density of states is used, where I(E ( ) ∝ E ( #/* : F(E &# ,E &* ,E '# ;E '* ,E ( )HF = # . (0) I(E '# ) J * 4√6 KE &# #/* (1)(1) E ( #/* HE '# HE &# HE ( Eq 2.10 Integrating over the rotations of the aromatic molecule (ER1), we obtain: F(E &# ,E &* ,E '# ;E '* ,E ( )HF = # . (0) I(E '# ) J * 4√6 KE ( #/* HE '# HE ( (E−E '# −E '* −E &* −E ( ) $/* Eq 2.11 26 To simplify the calculations, we discretize the translational energy into 1 cm -1 bin sizes up to the maximum available energy, where: E ( = E 9'9-: = E−E '* −E &* (E '# = 0) Eq. 2.12 For each translational energy level, we compute the probability of formation of each Ev1 vibrational level, and compute the probability, P(ET)k, by summing over probabilities of all allowed individual Ev1 levels: (F(E ( )) ; ∝ ∑ I(E '# ) E ( #/* (E−E '# −E ( ) $/* 0 #%#&' (0 ( <=) 0 %) <= Eq. 2.13 The total probability is proportional to this value: F(E ( ) ∝ (F(E ( )) ; Eq. 2.14 In the case of VP of the phenol-H2O dimer (Chapter 5): Fℎ28OP−Q * R+ℎ? >& → Fℎ28OP (E &# ,E '# )+Q * R(E &* ,E '* ) Eq 2.15 Figure 2.2: The number of rovibrational states available for the phenol co-fragment when detecting water in the JKa,Kc = 71,6 (704 cm -1 ) rotational state using the prior distribution and direct count of vibrational states. 27 The phenol (ER1, Ev1) prior distribution can be obtained by counting all possible harmonic vibrational levels, using the Beyer-Swinehart Algorithm that provides a lower limit for the density of vibrational states, up to the maximum accessible energy (1562 cm -1 in our case). The rotational levels are counted at discrete energy intervals and folded in before counting the density of vibrational states. The density of rovibrational states calculated using the Beyer-Swinehart Algorithm is depicted in the example shown in Figure 2.2, where the number of rovibrational states per cm -1 are calculated for the excitation of the H-bonded OH stretch of the phenol fragment when detecting H2O in JKa,Kc = 71,6. The procedure is similar to one described in the book, Unimolecular Reaction Dynamics: Theory and Experiment by Baer and Hase. 11 Figure 2.3:“IR ON” – “IR OFF” (black) signals for a state-selected H2O fragments obtained following excitation of the free OH stretch of the water moiety of the phenol-water dimer, fitted with an exponential decaying function (blue) based off the Ewing model. The red line represents the prior distribution. The maximum allowed translational energy in this case correspond to D0 = 1960 cm -1 . 15 This was the value used also in the prior calculation. 28 In Figure 2.3, an example of the prior distribution is compared to the Ewing model (discussed in section 2.1) for a velocity map image detecting water dissociating from the phenol- H2O dimer 2 (see Chapter 5). A sample MATLAB code for the Beyer-Swinehart Algorithm and prior distribution used in this dissertation are included in Appendices A.2 and A.3, respectively. 29 Chapter 2 References 1. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chemical Reviews 2016, 116 (9), 4913-4936. 2. Kwasniewski, D.; Butler, M.; Reisler, H., Physical Chemistry Chemical Physics 2019, 21 (26), 13968-13976. 3. Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243-4247. 4. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. 5. Ewing, G. E., J. Phys. Chem. 1979, 71, 3143. 6. Ewing, G. E., J. Chem. Phys. 1980, 72, 2096. 7. Ewing, G. E., J. Phys. Chem. 1987, 91, 4662. 8. Zuraski, K. Photodissociation Dynamics and Energetics of HCl-(H2O)3. University of Southern California, Los Angeles, CA, 2018. 9. Beswick, J. A.; Jortner, J., Chemical Physics Letters 1977, 49 (1), 13-18. 10. Beswick, J. A.; Jortner, J., The Journal of Chemical Physics 1981, 74 (12), 6725-6733. 11. Baer, T.; Hase, W. L., Unimolecular Reaction Dynamics: Theory and Experiments. Oxford University Press, Inc.: New York, NY, 1996. 12. Park, J.-H.; Lee, H.; Kwon, K.-C.; Kim, H.-K.; Choi, Y.-S.; Choi, J.-H., J. Chem. Phys. 2002, 117, 2017-2028. 13. Noble, M.; Qian, C. X. W.; Reisler, H.; Wittig, C., J. Chem. Phys. 1986, 85, 5763-5773. 14. Yoder, L. M.; Parker, J. R.; Lorenz, K. T.; Chandler, D. W., Chem. Phys. Lett. 1999, 302, 602-608. 30 15. Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millié, P., J. Phys. Chem. A 1998, 102 (25), 4890-4898. 31 Chapter 3: Experimental Details In this dissertation, HCl and aromatic water clusters were studied in the gas phase within the collision-free region of a skimmed supersonic molecular beam in a high vacuum chamber. Clusters of interest were selected using infrared (IR) absorption of the hydrogen-bonded (H- bonded) OH stretch fundamental or the free OH stretch of H2O. When the energy absorbed was sufficient, the cluster dissociated into photofragments. The fragments were ionized using resonance enhanced multiphoton ionization (REMPI) of selected quantum states and were subsequently detected mass-selectively. Figure 3.1 shows a simplified depiction of the experimental scheme described herein. Figure 3.1: Simplified experimental scheme for vibrational predissociation. A position-sensitive detector utilizing velocity map imaging (VMI) was used to record the spatial distribution of quantum state-selected fragments following predissociation. Experimental conditions were carefully optimized to maximize the production of selected water clusters for greater detection efficiency of both the fragments and parent clusters. Most notably, optimized sample and molecular beam conditions were essential for the selective production of dimers and small clusters, while optimized laser conditions were crucial to leveraging the best signal-to-noise ratio. 32 3.1 Experimental Arrangement The experimental arrangement as well as details of the supersonic molecular beam assembly and vacuum systems have been described previously. 1-8 The vacuum system consisted of three differentially pumped regions of the vacuum chamber: the source region, the main interaction region, and the detection region (see Figure 3.2). The source region was pumped by a Leybold TMP1000C (1100 L/s N2) turbomolecular pump backed by an Edwards E2M28 mechanical pump. The interaction region was evacuated by a Leybold TMP 361 (345 L/s N2) turbomolecular pump backed by an Alcatel 2021I direct drive pump. The detection region was pumped by a Leybold TMP 1000C (1100 L/s N2) turbomolecular pump backed by an Edwards E2M18 mechanical pump. The pressure in the source and detection regions of the chamber were monitored by an iridium filament glass ion gauge (3/4” 1-075-N) with a Granville-Phillips vacuum gauge controller (350501-0-T1). The pressures were low to mid 10 -8 Torr when the regions were evacuated. The operating pressures of the source and detection regions of the chamber when the pulsed nozzle was running at 10 Hz were 1-2 x 10 -5 and 2-6 x 10 -7 Torr, respectively. A pneumatic valve and a stainless-steel mesh flex trap were installed after each turbomolecular pump and before the mechanical pumps. A cryopumping system consisting of a liquid nitrogen reservoir and a “cold-finger” was attached to the main interaction region of the chamber to reduce ambient water vapor. The molecular beam was generated in the source region by introducing the sample and inert carrier gas at 2 atm total pressure through a home-built piezoelectric nozzle with a 0.5 mm orifice utilizing Physick Instrumente piezo disk translator (P-286.23). The nozzle operated at 10 Hz and was driven by a negative square pulse of 200 - 450 V for a duration of approximately 200 µs. Sample conditions for the molecular beam differed between each experiment and will be 33 described in detail in each chapter of this dissertation. The molecular beam seeded with the sample in a helium carrier gas travelled through the pulsed nozzle and expanded into the source chamber undergoing supersonic expansion. Through this process, collisions between the sample gas and the carrier gas cooled the molecular beam to enable the formation of clusters, which were concentrated at the center of the beam. The molecular beam was then collimated by two skimmers (1.29 and 0.78 mm in diameter; Beam Dynamics, Inc.), which were separated by 4 cm. Figure 3.2: (Side View) Experimental schematic of the high vacuum chamber depicting the source, main interaction, and detection regions: (1) piezo-electric nozzle; (2) skimmers; (3) electrostatic lens assembly; (4) cryopumping system; (5) detector; (6) ion gauges. The center of the molecular beam was expanded into the main interaction region of the chamber where the excitation and ionization laser beams intersect at right angles approximately 5 cm away from the opening of the second skimmer (see Figure 3.3). Focused IR laser radiation (~2- 5 mJ/pulse and ultraviolet (UV) laser radiation (0.2-0.7 mJ/pulse unfocused) were used to first induce dissociation of a selected vibrational mode of a specific cluster, and then to probe individual rotational states of the predissociation fragment state-selectively (Section 3.2.1) or parent cluster 34 (Section 3.2.1), respectively. IR radiation was generated by a tunable OPO/OPA system (Laser Vision, 0.4 cm -1 line width, up to 10 mJ/pulse) pumped by the fundamental (1064 nm) of a seeded Nd:YAG laser (Continuum Precision II, 500 mJ/pulse, 10 Hz). UV radiation was generated by frequency-doubling (Inrad Autotracker III) the output of a dye laser (Continuum ND 6000) pumped by the third harmonic (355 nm) of a Nd:YAG laser (continuum Surelite III, 150-200 mJ/pulse, 10 Hz). The UV laser wavelength was calibrated daily by comparing scans to PGOPHER 9, 10 simulations of specific molecules or wavelength measurement by a UV-Vis wave meter (Coherent Wave Master). The IR wavelength was calibrated using a photoacoustic cell and known IR depletion spectroscopy of H-bonded parent clusters. Timings between the pulsed nozzle, IR laser, and UV laser were controlled by delay generators (Stanford, DG535) through a GPIB interface (National Instruments). The interaction region of the chamber consisted of a four-electrode lens system (Figure 3.4). The electrostatic lens system designed by Ashfold and coworkers 11 was adapted to our experimental setup in 2002. 2 Following dissociation and ionization, the ions produced in the interaction region were focused and accelerated by electric fields generated by the ion optics toward the Time-of-Flight (TOF) spectrometer (60 cm field free drift tube) in the detection chamber. To reduce stray field interactions from outside magnetic fields, both the main interaction region and the detection region are shielded by a µ-metal alloy tube (nickel-iron-copper- molybdenum AD Vance Magnetic, Inc., AD-MU-80). Ions travelled through a drift tube to a position sensitive detector comprised of a 40mm diameter dual-channel microchannel plate (MCP) coupled to a phosphor screen assembly (Beam Imaging Solutions, Inc. Model BOS-40, PN-40- 008). 35 Figure 3.3: Top view of chamber showing the anti-collinear propagating laser beam arrangement. M and P refer to alignment mirrors and prisms, respectively. Typical voltage settings for the electrostatic lens system, MCP detector, and phosphor screen in different modes of detection are shown in Table 3.1. The optimal voltage ratio of the electrostatic lens system was held constant during experiments and was found to be VRepeller : VExtractor : VLens = 2.49 : 2 : 1. 2 Following dissociation, the ions of interest had a low translational energy (500-3000 cm -1 ). Initially, the Newton spheres formed upon dissociation of the water clusters were too small relative to the size of the detector. To counter this, a lower voltage was applied to the electrostatic lens system to obtain a larger Newton sphere and, thus, better image resolution (Table 3.1). 36 Figure 3.4: Schematic diagram of the electrostatic lens assembly, Time-of-Flight field free drift tube. R, E, L, and G correspond to the repeller, extractor, lens, and ground electrostatic lenses used to achieve VMI conditions. The MCP detector, phosphor screen, and CCD Camera are also shown. Table 3.1:Voltages (in V) applied to the electrostatic lens system, MCP detector, and phosphor screen during different modes of data collection Two modes of detection were used for data acquisition: TOF and Imaging. TOF mode was used for spectroscopic investigations in order to optimize optical alignment and timing of the instruments. The current collected by the MCP was amplified 100x (Stanford Research Systems, Model SR445A, 50 Ω input impedance) and monitored by an oscilloscope (Tektronix, TDS 3054, 500 MHz Bandwidth), which was connected to a PC for data transfer using LabView programs (National Instruments). In imaging mode, a charged-coupled device (CCD) camera (LaVision, Imager 3, 13bit, 1280 x 1024-pixel array) recorded images from the phosphor screen and exported the data to a PC for analysis using DaVis software package (LaVision) that included event counting. The detector was gated by a pulse generator (100-300 ns) for mass selection. The 37 recording window (pixel array) and camera exposure time were adjusted for optimal events per image and minimal total data acquisition time. In TOF mode, five types of data were collected: IR photofragment yield spectra; IR depletion spectra; REMPI spectra; nozzle-UV laser time delay scan; and IR-UV laser time delay scan. The IR-UV laser time delay scan was measured by scanning the IR laser firing time with respect to the UV laser firing. Maximum signal was observed when the IR laser fired ~55-80 ns before the UV laser (Section 3.5.4). The IR photofragment yield spectra, IR depletion spectra, and REMPI spectra were collected by alternating “IR ON” and “IR OFF” conditions at each frequency measured. Under “IR ON” conditions, the IR laser was fired ~65 ns prior to the UV laser. Under “IR OFF” conditions, the IR laser was fired 2 µs after the UV laser. Nozzle-UV timings were optimized by time delay scans, which alternated “IR ON” and “IR OFF” conditions at each time step between the nozzle opening and the UV laser firing. The timing that resulted in maximum enhancement or depletion was used (“IR ON” – “IR OFF”). The IR laser conditions (focusing, power) were further optimized to ensure the absorption of only a single photon. The UV laser conditions were optimized to reduce the signal-to-noise ratio as well as to limit parent molecule fragmentation. Laser alignment and delay times were optimized to excite the clusters concentrated at the center of the beam. An improved dry air box was built between the chamber and OPO/OPA to minimize the absorption of IR radiation by atmospheric water (see section 3.5.2). 38 3.2 Vibrational Predissociation (VP) studied via IR “Action” Photofragment Yield Spectroscopy and IR-UV Depletion Spectroscopy VP of clusters describes the process by which sufficient energy acquired from the excitation of one vibrational mode of the cluster undergoes vibrational redistribution in and induces dissociation by breaking relatively weak intermolecular bonds such as hydrogen bonds. VP of H-bonded clusters that include water involves IR laser excitation of either the H-bonded OH stretch fundamental or the free OH stretch of H2O moiety. In order for this to be achieved, a few requirements must be met: (1) the vibrational band excited must be isolated from the vibrational bands of other clusters in order to select the cluster of interest; (2) the energy in the band must couple to the weaker intermolecular bond(s); and (3) the energy must be sufficient enough to break the intermolecular bond(s). In our experiments, we indirectly observed the VP of H-bonded clusters utilizing IR photofragment yield spectroscopy, also referred to as IR “action” spectroscopy, and IR-UV depletion spectroscopy. While these methods are advantageous, they pose certain challenges which are discussed in detail in section 3.5. 3.2.1 IR “Action” Photofragment Yield Spectroscopy IR “action” spectroscopy is a method used in our experiments to obtain IR spectra of a H- Bonded cluster by dissociating it in the ground electronic state (S0) and ionizing the neutral fragment and detecting the it using TOF-MS . A tunable IR laser frequency, vIR, is scanned and signal is detected when it is resonant with one of the normal vibrational modes of an H-bonded cluster. When the energy is sufficient, VP ensues and generates neutral fragments (Figure 3.5, left). REMPI (Section 3.3) is then utilized to state-selectively ionize one of the neutral fragments which is detected mass-selectively using TOF-MS. If IR absorption occurs, there is an increase, or enhancement, in the REMPI signal due to an increase in the population of the monitored state of 39 the fragment generated by VP of the H-bonded cluster. It is important to note that these are “action” spectra; in order to observe enhancement, there must be absorption of IR photons, and this absorption must lead to the production of fragments in specific rotational states. In our experiments, it was advantageous to monitor high rotational levels of the monomer fragments due to the minimal background signal from H2O or HCl monomers in the molecular beam. When monitoring these levels, the center and shape of the observed IR peaks can be altered relative to the absorption spectrum of the cluster, because they may be preferentially correlated with a specific rotational state of the cluster. We were unable to measure the exact temperature of the clusters in the molecular beam, as is explained in Section 3.5.6, but we were able to model and estimate the temperature of the water monomer to be ~15 K in the expansion. 9, 10, 12 In previous studies, our research group was able to model the IR “action” spectrum of the HCl-H2O dimer and found the temperature to be similar to that of the monomer. 13, 14 However, it is possible that contribution from hot bands in larger clusters may further broaden the corresponding peaks. 15 This is discussed in detail in Section 3.5.6 using the case of the HCl-(H2O)3 tetramer as an example. 3.2.2 IR-UV Depletion Spectroscopy IR-UV depletion spectroscopy is a method used to obtain IR spectra for a specific conformer or, in this case, a specific H-bonded parent cluster in either the ground electronic state (S0) or an excited electronic state. Figure 3.5 (right) depicts a typical scheme in which a pulse from a tunable IR laser of frequency vIR is resonant with the fundamental vibration of the parent cluster. In this case, a subsequent UV pulse probing the parent cluster by REMPI will show a depletion compared to the signal intensity at wavelength where there is no IR absorption. The IR and UV lasers must be spatially overlapped, and the IR laser temporally fired prior to the UV laser (~65 ns in these studies). When IR absorption occurs, there is a decrease, or depletion, in the REMPI 40 intensity due to a decrease in population of the electronic ground state or dissociation of the H- bonded cluster. 16 Figure 3.5: (Left) Scheme for IR fragment yield “action” spectroscopy of the vibrational predissociation of the HCl-H2O dimer. Following VP, the H2O fragment is detected through a 2+1 REMPI process. (Right) Scheme for IR-UV depletion spectroscopy used to obtain the IR spectrum in the S0 ground state using a 1+1 REMPI as a method of detection via the S1 ← S0 transition. 3.3 Resonance Enhanced Multiphoton Ionization (REMPI) REMPI is a highly sensitive and state-selective multi-photon ionization technique applied to the spectroscopy of small molecules, atoms, and even h-bonded aromatic clusters, which allows selected molecules to be ionized while other components of the molecular beam remain unchanged. As shown in figure 3.6, REMPI typically involves the resonance of one or two photon excitation with an intermediate electronically excited state followed by absorption of one or more photons of the same or different wavelength to ionize the molecule. Selection rules of multi-photon excitations differ from those of a one-photon transition. As a result, multi-photon REMPI can 41 provide spectroscopic information about states that cannot be reached by one-photon excitation. This experimental technique is similar to that of obtaining a fluorescence excitation spectrum except for the method of detection. In principle, positive ions or photoelectrons may be detected. The sensitivity of detecting ions and, most importantly, the selectivity of this method are highly advantageous. Applying REMPI to our supersonic expansion studies of h-bonded complexes with mass selection through TOF-MS made possible the observation of desired molecules without interference from other molecules, fragments, or parent clusters of water. In the experiments reported in this dissertation, REMPI was used for state-selective detection of several different molecules. In our studies, 2+1 REMPI combined with TOF-MS was used for spectroscopic investigations of water monomers via the C̃ 1 B1 (000) ← X ̃ 1 A1 (000) transition. The water monomer fragments absorbed two UV photons to reach the C̃ -state, and one additional photon was absorbed to reach the ionization continuum (Figure 3.6, left). The REMPI spectrum for water is exceptionally complicated because this asymmetric top molecule (C2v) is highly predissociative in the C̃ 1 B1 state and exhibits congestion among the transitions caused by a high density of rotational states. Ashfold and coworkers 11 as well as Yang et. al 12 studied the REMPI spectra of water and explained the spectroscopy and predissociation mechanisms for the transition via the C̃ 1 B1 state. Based on their findings, it is not possible to derive exact rotational state populations from REMPI spectra of water; our spectra were only compared to rotational temperature simulations, which make best estimates on rotational linestrengths and predissociation lifetimes. 42 Figure 3.6:(Left) 2+1 REMPI process via the C̃ 1 B1 ← X ̃ 1 A1 transition. (Right) 1+1 REMPI process via the S1 ← S0 transition of the phenol-water dimer. The 2+1 REMPI transitions of HCl have been studied extensively and are considered to be well known. 17-35 Each intermediate state is considered to have its own unique advantages and disadvantages for probing specific rotational levels. The HCl electronic states utilized in our experimental scheme for the HCl-(H2O)3 tetramer study 7, 15, 36 are summarized in Table 3.2. A combination of multiple intermediate states was necessary in order to detect the full range of rotational levels of the HCl fragment generated in the HCl-(H2O)3 tetramer VP. For example, the V 1 Σ + state has strong rotational line strengths and is less predissociative than the f 3 Δ2 state, however, the HCl + ion has a strong propensity to dissociate to form Cl + , making it a less than ideal candidate for VMI studies due to high background in the TOF-MS (separation of 1 mass unit). In contrast, the f 3 Δ2 (ν = 0 ) ←X 1 Σ + and F 1 Δ2 (ν= 0 ) ←X 1 Σ + are best for monitoring lower rotational levels. 17, 18, 37 Simulations of H2O and HCl were accomplished using the PGOPHER program 9 developed by Western. 10 The H2O C̃ 1 B1 (000) ← X ̃ 1 A1 (000) REMPI spectrum has been previously published. The H2O C̃ 1 B1 (000) ← X ̃ 1 A1 (000) REMPI spectrum was simulated using published 43 rotational constants and PGOPHER. 9, 10, 12 The contour fitting method was used to calculate rotational temperatures of the REMPI spectra. 38 In our study of the HCl-(H2O)3 tetramer, the rotational spectrum of the tetramer could not be simulated and compared to the observed IR action spectrum, because the spectroscopic constants have not yet been measured. Instead, the temperature of the monomers in the beam was used to estimate the temperature of the clusters at the interaction region. The approximate temperature was then used to estimate the internal energy of the tetramer, which is important in estimating dissociation energies, which is discussed in detail in Section 3.5.6 and Chapter 4. 7, 36 1+1 REMPI detection of PhOH-(H2O)2-4 has been reported in detail previously. 39, 40 The UV radiation frequency for the detection of PhOH-H2O has been reported at 35,998 cm -1 . The PhOH-H2O spectrum was frequency calibrated using the published PhOH-H2O spectrum at 35,995-36,400 cm -1 . 41 UV radiation ionized the PhOH-H2O dimer by 1+1 REMPI while scanning through the S1 ← S0 band of the dimer (Figure 3.6 (Right)). This spectrum was also utilized to monitor molecular beam conditions for optimum production of the PhOH-H2O dimer. The REMPI spectrum of the pyrazine monomer has been reported previously in room- temperature gas phase. 42-44 In our experiments, pyrazine was initially detected using several different electron transitions to determine the existence of pyrazine in the molecular beam. The REMPI spectra of the S1 ←S0 (8,)), S2 ←S0 (),)), and 3s Rydberg states were measured for pyrazine, while monitoring m/z = 80. Additionally, a different REMPI spectrum was observed when monitoring m/z = 98 (pyrazine-water) around the S2 ←S0 (),)) transition (36350-39600 cm -1 ) using TOF-MS. As discussed in Chapter 6, this REMPI scheme was assigned to be a 1+ n (n=1, 2, 3… photons) based on electronic structure calculations. 44 Table 3.2: Electronic state transitions used in this work 36 , for the detection of the HCl fragment following the vibrational predissociation of the HCl-(H2O)3 using 2+1 REMPI. Peak positions and descriptive state behavior have been previously reported. 17, 21, 24, 27, 32, 33 Electronic State Rotational Peaks for the HCl monomer Electronic State J States range (cm -1 ) Description ─ 0-4 ─ Rotationally cold monomers ionized in the molecular beam create a large background signal f 3 Δ 2 (ν = 0 ) ←X 1 Σ + 3-6 82,023 – 82,032 Excellent for Imaging, very low Cl + population. Predissociative electronic state cannot observe J ≥ 9. V 1 Σ + (v=11)←X 1 Σ + 5-8 83,787 – 84,104 Highly photodissociative state leading to a large population of Cl + ions due to close proximity to dissociation reaction coordinate. The J=9 rotational state is overlapped with a strong Cl + from the J=3 peak of the E 1 Σ + (v=0)state. Large Cl + ion signal has been observed at J=8 due to photodissociation of HCl. V 1 Σ + (v=12)←X 1 Σ + 10-11 83,955-84,055 Highly photodissociative state leading to an increased population of Cl + ions. Has been used previously for imaging with success. J=12 peak strongly overlapped by J=8 of V 1 Σ + (v=11) state. F 1 Δ 2 (ν= 0 ) ←X 1 Σ + 5-8 82,800-82,825 Propensity towards predissociation of the HCl monomer. Low rotational levels have also been observed experimentally. J>9 has never been imaged. Never imaged for rotational- cofragment distribution 45 3.4 Velocity Map Imaging (VMI) VMI is a powerful tool used worldwide for studying energy distributions in chemical reaction dynamics. 45, 46 This method combines the state-selective and highly sensitive REMPI detection (Section 3.3) and two-dimensional (2D) imaging of photodissociation products to study state-resolved photochemical reaction dynamics. The technique was first pioneered by Chandler and Houston in 1987 for the purpose of studying photochemical reactions. 47 The resolution of the imaging method was drastically improved by Eppink and Parker in 1997, who replaced the conventional grid electrodes with an electrostatic ion lens with open electrodes. 48 As a result, under optimized conditions, the electric field arrangements allow ion particles with the same initial velocity vector that originate at different initial distances from the ion lens axis to arrive at the same point in the detector. This effectively nullifies the hindrance of the finite size of the interaction volume of the laser and the molecular beam cross section. Velocity distributions of photofragments measured by VMI can reveal an abundance of information about the dissociation process, including speed and angular distributions of products, product branching ratios, orientation and alignment, pair-correlated energy distributions of co- fragments, and recoil anisotropy parameters (ß). In respect to the conservation of energy and momentum, this method allows for the determination of dissociation energy with spectroscopic accuracy and gives insight into isomerization pathways and fragmentation mechanisms. VMI can also be used for the detection of photoelectrons. Photoelectron VMI and other variations of this technique are useful for the study of cation vibrational structures and for information regarding the nature of intermediate excited states, including resonances of rovibronic states and conical 46 intersections. Isomerization and nonadiabatic transitions can also affect photoelectron images, which reflect underlying dynamics. 45, 46 In VMI mode, briefly discussed in Section 3.1, pulsed laser radiation induced photodissociation of molecules in the molecular beam. The dissociated fragments were then ionized by the same or another laser utilizing REMPI. The generated ion fragment was then extracted and accelerated into the TOF-MS detection region by the ion optics. The detector system was made up of a MCP detector coupled with a phosphor screen. Figure 3.7 shows a simplified scheme of VMI in a coordinate system where x and y are parallel to the detector plane, and z, which represents the path that the molecular beam travels, is perpendicular to it. Figure 3.7: Simplified scheme of VMI. Two ions represented by red dots are formed at different initial locations with the same velocity vector, v. They are then accelerated and mapped to the same spot on the detector by the electrostatic ion optics field, E. 47 Ions at different initial spatial locations, but with the same velocity vector, v, were accelerated and mapped to the same spot on the detector by the electrostatic ion optics lens system. The location where the ion hits the detector was recorded by a CCD camera using an event counting program (DaVis). The angle and distance of each ion spot from the center of the image are θ and r, respectively. The image was collected over many thousands of laser shots by integrating the signal intensity on an external computer. The image is a 2D projection of a three- dimensional (3D) velocity distribution. The 2D projection of the Newton spheres on the detector was formed from ion fragments with different velocities (v) and angles (θ). The distance of the spot from the center of the image is the radius (r) in pixels, which is proportional to velocity, v. Conservation of energy and momentum directly relate the ion velocities of the detected fragments to their co-fragments. As shown in Figure 3.8, the parent molecule was formed in the molecular beam and subsequently dissociated by IR laser radiation. Fragments in selected quantum states were ionized by UV laser radiation, allowing the co-fragments to be in specific quantum states, vi, ji; vj, jj; and vk, jk. The Newton sphere is smaller when the co-fragment is in a quantum state with high internal energy. This is due to the conservation of energy and momentum, which determines that the corresponding excess energy available for translational energy decreases when the internal energy of the cofragment increases. 48 Figure 3.8: Diagram of the photofragment imaging approach to measuring the projections of the Newton sphere: (a) Photodissociation of molecules in the molecular beam generates the Newton sphere; (b) The Newton spheres of molecules are then ionized by the ionization laser; (c) Projection of the ion spheres onto the 2D detector; (d) The BASEX reconstruction method utilizes a mathematical transformation of the 2D image back to the 3D data. The data is then converted into speed distributions or center-of-mass translational energy distributions. The 3D velocity distributions were recovered from the 2D image projection and converted to fragment speed distributions in pixel space using the basis set expansion (BASEX) Abel transform method and centroiding. 49 Speed distributions were converted to center of mass (c.m) translational energy distributions using momentum conservation and the appropriate Jacobian. 50 The speed distribution T(?) is obtained from the velocity distribution by integration over all angles. When the VMI arrangement has fixed voltages, the following relationship is true: ? ∝ U ∝ # √? Eq. 3.1 49 Where the velocity, v, is proportional to the radius of the image, r, which is inversely proportional to the square of the mass of the charged particle. However, the c.m translational energy (E ( @.? ) of the charged particle does not depend on m and is proportional to the square of the radius. E ( @.? = VU * Eq. 3.2 The c.m translational energy is proportional to the maximum kinetic energy release of the detected particle: E ( @.? = VU * = # * W? * Eq. 3.3 Where V is a magnification factor directly related to two factors: (1) the ratio of the voltages applied to the electrostatic ion lens; and (2) the geometry of the experimental setup. These factors imply that V is not dependent on mass, but is system dependent. In order to determine V, the experimental imaging system must be calibrated by measuring the radii of a well-defined translational energy release in molecular photodissociation. The system discussed in this dissertation was calibrated by the photodissociation of the diatomic molecule, O2, where the translational energy distributions can be determined. 48 Under our experimental conditions, this was 5.2U = ? for detection of H2O, 3.66U = ? for HCl detection, and 2.28U = ? for phenol detection. The E ( @.? distribution of the charged particle can be plotted using Eq. 3.2 and the corresponding Jacobian: T(E) = T(U) B+ B0 = , (+) *C+ Eq. 3.4 To find E ( @.? , conservation of momentum must be followed for the dissociation of two fragments moving in opposite directions in the c.m. system: W # ? ⃗ # = −W * ? ⃗ * Eq. 3.5 0 = W # ? ⃗ # +W * ? ⃗ * Eq. 3.6 50 This can be arranged to the following: ? ⃗ * * = W # * ? ⃗ # * W * * This expression can be substituted into Eq. 3.7 for the total c.m translational energy distribution of the two products: E ( @.? = # * W # ? ⃗ # * + # * W * ? ]]]⃗ * * Eq. 3.7 To give the following: E ( @.? = # * W # ? ⃗ # * + # * W * J ? ) * ' D⃗ ) * ? * * K Eq. 3.8 E ( @.? = # * ? ⃗ # * J ? ) (? * F? ) ) ? * K Eq. 3.9 If W * +W # is equal to the mass of the parent cluster, ^, and W * = ^−W # , where m1 is the mass of the monitored product, m, then the total c.m. translational energy distribution of the two products can be potted using: E ( @.? = E G G" ? Eq. 3.10 The speed can be calculated from the experimental radius using the following relationship between r and v: _ *C ? U = ? Eq. 3.11 And using the following Jacobian: B' B+ = _ *C ? Eq. 3.12 The speed distribution T(?)in velocity units can be obtained from the speed distribution in pixels: T(?) = T(U) B+ B' = T(U)_ ? *C Eq. 3.13 51 3.5 Technical Challenges in Studying Hydrogen Bonded Clusters When studying water clusters, several technical challenges can arise from the following: (1) water background in the vacuum chamber; (2) IR absorption by water in the lab atmosphere before the laser beam enters the vacuum chamber; (3) inefficient detection of water fragments; (4) difficulty optimizing sample conditions; (5) assignment of IR “action” spectra; and (6) estimating the temperature of the molecular beam. An inherent problem of working in a vacuum chamber is the presence of background water, which can affect experimental signals. This was minimized by cryopumping the interaction region of the vacuum chamber during data collection and baking the chamber periodically (Section 3.5.1). Atmospheric water absorption of IR radiation was reduced by constructing an improved dry box along the path of the laser beam (Section 3.5.2). Water fragment detection by REMPI is challenging due to the predissociative nature of the excited electronic state. The detection efficiency was improved by optimizing the laser focusing conditions and optic path (Section 3.5.3). Production of aromatic-H2O dimers was maximized and formation of higher order clusters was minimized (section 3.5.4). Conditions were also optimized for the production and assignment of the HCl-(H2O)3 tetramer (section 3.5.5). 3.5.1 Minimizing Background Water in the Interaction Region The interaction region was modified in the past and has been discussed previously in other work. 1 In brief, the interaction region included two cryopumping systems cooled by liquid nitrogen traps in order to reduce background water inside the interaction region of the vacuum chamber. The system consisted of a “cold finger” connected to the outside of the chamber by steel tubing and cooled by liquid nitrogen in a Styrofoam reservoir. Figure 3.9 shows a comparison of a 2+1 REMPI scan of water monomer in the low rotational level region with and without cryopumping. By adding one cryopump, it was found that the background water signal was reduced by over a 52 factor of 5 in a less than 20 minutes. The second cryopump further reduced the signal, but was rarely used in order to conserve liquid nitrogen. The cryosystem was limited to 8-hour operation windows due to the following: (1) solidification of atmospheric water in the cryopumps; (2) saturation of ice on the cold surface inside of the chamber. Dry air was pumped through the cryosystem following each 8-hour window to remove excess water from the system. The chamber was also baked for 2-4 days every 2-4 weeks in order to induce the removal of background water and hydrocarbons that would collect on the walls of the chamber over time. The vacuum chamber was insulated with fiberglass and baked using heating tape controlled by Variac variable transformers. The temperature was kept at a gradient for each region of the chamber, growing in temperature between 90-105 o C in going from the source region to the detector, respectively. The temperature in each region was monitored separately using a thermocouple controller (Auber Instruments, SYL-2342). Before baking, the nozzle was removed and the chamber was evacuated. After baking, the nozzle and source region were passivated by flowing the sample gas mixture. 53 Figure 3.9: Comparison of 2+1 REMPI spectrum of the C̃ 1 B1(000) ← X ̃ 1 A1(000) transition of the water monomer without cryopumping (black), with one cryopump (blue), and with two cryopumps (red). 3.5.2 Absorption of IR Radiation by Atmospheric Water The IR frequency of the H-bonded and free OH stretches of the water clusters we study directly coincide with those of atmospheric water. As a result, the IR power would drop significantly as the laser beam passed through the air before reaching the vacuum chamber. An existing acrylic glass box was redesigned and reinstalled around the laser beam path and was purged with dry air during experiments. Approximately ~60 psi of air was propogated through a membrane air dryer (Parker Balston 64-02) on the way from the fume hood to the dry air box (Figure 3.19). At the IR frequency of atmospheric water, the IR laser intensity at the exit of the box increased from ~2 mJ/pulse to ~9 mJ/pulse in approximately 6-10 minutes when the box was closed and purged with dry air (Figure 3.11). The dry air box was designed with a hinged sealed 54 lid for easy optical alignment of two IR mirrors inside the box. IR radiation was directed through the box and towards the interaction region of the chamber. Figure 3.10: Diagram of redesigned acrylic dry air box showing the optical path of the IR radiation as it propagates towards the interaction region of the vacuum chamber prior to focusing with a 20 cm focal length lens. In the HCl-(H2O)3, phenol-H2O, and pyrazine-H2O experiments described in this dissertation, the OPO/OPA laser frequency was calibrated by scanning the absorption spectrum of NH3 51 using a photoacoustic cell. 6 The previously recorded phenol-H2O depletion spectrum was also used as a calibration of the OPO/OPA after initial confirmation of its peaks position in comparison to those measured in the literature. 52, 53 55 Figure 3.11: Infrared radiation power over time when the box is being purged with dry air. 3.5.3 Optimization of Water Fragment Detection The electronically excited states of water have been established as predissociative and, as a result, the signal-to-noise ratio of the water REMPI spectra is low. 11 In order for ionization to compete effectively with predissociation and achieve a reasonable signal-to-noise ratio, a medium to high UV laser beam fluence was needed, requiring the use of a 20 cm focal length (f.l) lens. Another complication comes from spectral congestion of the rovibronic spectral lines, which is aggravated by a high UV fluence, causing partial saturation and spectral overlap with nearby transitions. Maximum UV laser power was not used in order to avoid power broadening and reduced resolution in our imaging experiments from a buildup of space charge. The UV laser beam shape was improved over time by shortening the overall laser path length, eliminating a half dozen optical prisms between the ND 6000 Dye laser and the vacuum chamber, and tedious and careful optical alignment. 56 3.5.4 Optimization of Molecular Beam Conditions and Laser Conditions for Dimer and Cluster Formation Molecular beam conditions were optimized for each specific cluster in order to maximize its formation and the detection of photofragments from its VP rather than from larger clusters that also exist in the beam. The temperature of the molecular beam controls the size distribution of the clusters; the colder the beam, the higher the concentration of larger clusters or “snowballs,” and the hotter the beam, the higher the propensity to form monomers (background H2O) and dimers. The molecular beam cooling conditions were adjusted by optimizing the sample concentration, backing pressure, and nozzle voltage. It was essential to first optimize the nozzle-UV time delay due to the dependence of cluster formation on this parameter. The optimization of the time delay between nozzle opening and UV laser firing, as well as the delay between the IR and UV laser firings, ensured that the lasers fired at correct times with respect to each other. The UV and IR laser focusing conditions were optimized to minimize multi-photon absorption in larger clusters. The timing of the lasers was controlled by delay generators and optimized for maximum photofragment yield and depletion signals from the clusters of interest. The 2+1 REMPI detection of the monomer of interest was first optimized by careful laser alignment and focusing. Attempts were then made to observe enhancement of the REMPI signal after firing the IR dissociating laser. The IR laser frequency was tuned to coincide with the specific vibrational mode of the cluster of interest in order to induce vibrational predissociation. The IR laser was set to fire ~65 ns before the UV laser to ensure efficient dissociation of the cluster prior to ionization of the fragment. The nozzle-UV time delay scan was performed by alternating “IR ON” and “IR OFF” conditions at each time step. Fragment ions displayed an enhanced signal following the vibrational predissociation of a cluster. The optimal timing was selected based on 57 the largest enhancement of the monomer signal. When no enhancement was observed, other IR frequencies or other rotational levels in the REMPI transitions were utilized. Figure 3.12: (Left) Nozzle-UV time delay scan obtained by monitoring H2O J”Ka,Kc = 32,1 following vibrational predissociation of HCl-(H2O)3. Maximum enhancement can be observed at ~410 µs (Right) IR-UV time delay scan by monitoring H2O J”Ka,Kc = 41,4 upon vibrational predissociation of (H2O)2. 1 IR laser firing was optimized at 65 ns to ensure that the vibrational energy had enough time to couple across vibrations to induce VP. The IR laser firing time with respect to the UV laser firing was optimized to ensure that the vibrational energy had enough time to effectively couple across vibrations to induce VP. Figure 3.12 shows an example of a UV-nozzle time delay scan for VP of the HCl-(H2O)3 tetramer, as well as the IR-UV time delay scan following VP of the (H2O)2 dimer. When enhancement was observed, the IR-UV timing was further optimized, but it proved to be rather insensitive to the types of clusters studied by photofragment yield “action” spectroscopy. This timing became critical, however, when observing IR depletion spectroscopy of the parent clusters found in the studies of aromatic-H-bonded clusters. The cold temperature of the molecular beam promoted the formation of large clusters rather than the dimers that are of interest to our studies. It was 58 determined that monitoring the leading edge of the pulse could increase the number of dimers relative to larger clusters for two reasons: warmer molecular beam temperature and velocity slip, in which lighter species, such as the dimers, travel faster than heavier species toward the interaction region. When enhancement was found in the nozzle-UV time delay scans, the optimum time delay was set for the acquisition of REMPI spectra of fragments. This led to the careful optimization of UV laser conditions discussed previously (Section 3.5.3). Lastly, IR fragment yield “action” spectroscopy was recorded. In these measurements, it was important to reduce the IR fluence in order to avoid saturation effects and multi-photon absorption, which is manifest in frequency independent enhancement throughout the IR spectral region, results in translational energy exceeding the energetically allowed maximum, and causes increased formation of fragments with high velocities. With HCl-(H2O)3, PhOH-H2O, and Pyrazine-H2O clusters, contributions from larger or similar size clusters were observed in the IR spectra. Conditions were carefully optimized to reduce these contributions and to identify an isolated single vibrational mode of the dimer in the IR spectra. PhOH-H2O was generated in the pulsed molecular beam by bubbling He gas (Gilmore, 99.9999%) at 2 atm through 10 mL of liquid water and passing the mixture over 200 mg of solid phenol (Sigma-Aldrich, 99.5%) at room temperature (vapor pressure 0.4 Torr). PhOH was shielded from light to avoid sample degradation. Expansion conditions (H2O and PhOH concentration, and He backing pressure) were optimized to maximize the signal of the PhOH-H2O dimer and to minimize the concentration of higher order phenol-water clusters. PhOH-H2O dimer formation was rather insensitive to the backing pressure of the sample; however, generation of higher order clusters was found to be sensitive to the order of mixture preparation, i.e. whether He gas was passed over solid phenol and then through liquid water, or bubbled through a PhOH-H2O solution. 59 The sample preparation method chosen was 2 atm of He gas passing through 10 mL of liquid water and passing the mixture over 200 mg of solid phenol. Low IR fluence was used for the excitation of both the H-bonded and free OH stretching modes [20-cm f.l. lens; 1.5 mJ/pulse] at 3522 cm -1 and 3744 cm -1 , respectively. The IR frequency was calibrated using the photoacoustic spectrum of gaseous NH3. Low IR fluence was also used to prevent multi-photon absorption when recording the fragment yield IR “action” spectra. To obtain an acceptable signal-to-noise ratio in water REMPI spectroscopy, a reasonably high UV laser fluence was used, but the UV fluence was kept low, at around ~0.2 mJ/pulse to minimize space charge that reduces the VMI image resolution as well as lead to power broadening of the REMPI spectra. In contrast, fewer conditions could be optimized for the Pyrazine-H2O dimer. The overall concentrations of H2O and pyrazine in the sample were limited by the vapor pressure of water and pyrazine at room temperature. Pyrazine-H2O was generated in the pulsed molecular beam by bubbling He gas (Gilmore, 99.999%) at 2 atm through 10 mL of liquid water with ~2 g of Pyrazine dissolved in solution. It was determined that production of the pyrazine-H2O dimer was very sensitive to the backing pressure of the sample, requiring the source region pressure to be kept at ~8 x 10 -6 to generate an adequate signal-to-noise ratio. Low IR fluence was used for the excitation of the OH stretch of the Pyrazine-H2O dimer at 3660 cm -1 . The IR frequency was calibrated using the photoacoustic spectrum of NH3. A lower UV fluence was necessary to reduce fragmentation of neutral pyrazine monomer and pyrazine-H2O dimer in the molecular beam. This was achieved with a 20 cm f.l. lens and with the UV power at about ~0.3 mJ/pulse. 60 3.5.5 Production and Assignment of the HCl-(H2O)3 tetramer As mentioned earlier, the assignment of vibrational modes in H-bonded clusters is complicated due to the red shift in the spectral frequency. Spectra can become exceedingly complicated as degrees of freedom increase with increasing cluster size. This can lead to spectral overlap, congestion, and peak broadening which make accurate peak assignments difficult to achieve. This proved to be a particular issue during the study of the HCl-(H2O)3 tetramer. 7, 15, 36 There are three major spectroscopic IR regions for mixed HCl-water clusters: the HCl stretch region, H-bonded OH stretch region, and free OH stretch region of the water moieties. The HCl stretch vibrations mixed HCl-H2O clusters have been assigned previously, 54 however, according to theory, excitation of the HCl stretch fundamental does not supply enough energy for VP of the HCl-(H2O)3 tetramer. 15 The free OH region for most clusters is considered too congested due to the lack of red-shifting between the water monomer asymmetric stretch vibration at 3755 cm -1 and the free OH vibration of the H-bonded clusters of interest. 55 This left only the H-bonded OH stretch as a viable option to probe the HCl-(H2O)3 tetramer. Though studied in Helium droplets, 56 this region lacked assignment in the gas phase, which are required to assure that the H- bonded OH vibration is isolated from other clusters. Sample conditions were varied by changing the concentration of HCl (water only, 1% HCl, 2% HCl, and 5% HCl with 2 atm Helium carrier gas). Based on the dependence of the relative intensities of spectral peaks on HCl concentration, along with comparisons with the spectroscopic studies in Helium droplets 56 , the3530-3555 cm -1 spectral band [peak (a) in Figure 3.13] was assigned as the to the H-bonded OH stretch of the HCl-(H2O)3 tetramer. Table 3.3 summarizes the results and includes comparisons with prior assignments. 61 Table 3.3: H-bonded OH stretch fundamentals (in cm -1 ) for (H2O)2, (H2O)3, and HCl-(H2O)3 Published values from: (a) Ayers et. al 57 , (b) Schriver et. al 58 , (c) Zischang et. al 56 , (d) Paul et. al 59 , (e) Fárnìk and coworkers 60 (tentative), (f) Rocher-Casterline et. al 14 , (g) Ch’ng et. al 1 , and (h) this work 7, 15, 36 . “Action” spectra were obtained using instrumentation as shown in Figure 3.13. Experimental conditions to produce the HCl-(H2O)3 tetramer were optimized by varying the partial pressures and relative concentrations of HCl and water in the supersonic expansion. When only water (0.6% H2O backed by 2 atm He gas) was introduced into the chamber, the IR “action” spectra of the water dimer (H2O)2 and trimer (H2O)3 showed peaks at 3602 cm -1 and 3538 cm -1 , respectively. When a trace amount of HCl was added to the sample (0.5-1% HCl), peaks [marked (a) – (d) in Figure 3.13] were observed while monitoring the H2O fragments, and these correlated to small mixed HCl water clusters with a relatively low concentration of HCl. These peaks rose and fell as expected with the variations in HCl concentration. With the addition of more HCl (2% HCl), peaks (b) and (e) emerged. Increasing the HCl concentration to as high as 3- 5% led to the appearance of broader peaks, which extended to lower frequencies and correlated to the production of larger (HCl)n(H2O)m clusters. This correlation was also observed in slit-jet absorption experiments by Fárnìk and coworkers 60 . The spectral locations, shapes, and pressure dependencies of the peaks were reproducible. The production of HCl-water mixed clusters was insensitive to the pressure in the source region and the voltage applied to the opening of the nozzle. 62 Figure 3.13: (Top) Depletion spectrum of (HCl)n-(H2O)m clusters obtained in Helium nanodroplets. 56 Labels indicate cluster size, n:m. (Bottom) The corresponding IR action spectrum 36 (red) obtained by monitoring H2O photofragments in J”Ka,Kc = 32,1, and the background signal from H2O monomers (black). H2O + enhancement in the “action” spectrum was observed at: (a) 3520- 3555 cm -1 ; (b) 3580-3590 cm -1 ; (c) 3600-3620 cm -1 ; (d) 3623-3632 cm -1 ; and (e) 3633-3640 cm -1 . Zischang et. al. 56 performed Helium droplet studies on the H-bonded OH stretch region of HCl-water mixed clusters. In comparing our results, peak (a) at 3550-3555 cm -1 best fits the lower frequency peak of the two peaks in this region, which were assigned to the 1:3 cluster in Helium droplets. This peak is red-shifted relative to the peaks in the region assigned to the 0:2, 1:2, 2:1, and 2:2 clusters in Helium droplets, which are all greater than 3590 cm -1 . While monitoring HCl rotational levels, we recorded a band with a similar shape and position, which we preliminarily labeled as a cluster with m>n, correlated with the production of HCl fragments. Schriver et. al 58 63 reported a peak at a similar IR frequency (3554 cm -1 ) in Ar matrix studies and gave the same assignment. After careful consideration, peak (a) was assigned to the HCl-(H2O)3 tetramer. Zischang et. al 56 also reported a 1:3 peak at higher frequencies, however, no enhancement was observed. This may be due to the possibility that energetically higher lying isomers, which are trapped during the fast cooling process in Helium droplets, relax better to the lowest lying isomers at the warmer temperatures of the supersonic expansion and therefore are not observed. Another reason for this discrepancy is that our data shows a higher concentration of HCl fragments in the H-bonded OH region following IR excitation, which begs the question of whether or not peak (a) belongs to the non-ionized HCl-(H2O)4 pentamer. We do not believe this to be the case based on the following studies. Mancini and Bowman published the calculated structure and energetics of both the ionized and non-ionized forms of 1:4 cluster. 61 According to their electronic structure calculations, the ionized 1:4 cluster (i.e. their cluster with HCl ionized) is the most stable form, whereas the non-ionized isomer lies approximately 3-6 kJ/mol higher in energy. Zischang et. al 56 calculated the frequencies of the OH stretching bands of the non-ionized HCl-(H2O)4 pentamer using a harmonic approximation, and concluded that their experimental pressure dependence studies in Helium droplets did not support the presence of this cluster and could not assign it to any of their observed peaks. Based on these results, the H-bonded OH stretch bands of the 1:4 cluster, either fully or partially ionized, is expected to red-shift to around ~3200 cm -1 and become much broader. In conclusion, this cluster has not been observed in Helium droplet spectra in the HCl or the free OH stretch regions. 54, 56 It is also unlikely that the measured H2O fragment signal originates from the dissociation of the (H2O)3 trimer (3538 cm 1 ). The reason is that both HCl and water were introduced into the molecular beam, and enhancement in both fragment signals was observed. 15, 36 64 While monitoring the HCl fragment, peak (a) was more intense and had a distinctive shape. The dissociation energy value as well as the shape of the velocity distributions of the water trimer, studied previously, are characteristically different. Further details on the experimentation, dissociation energy, and product state distributions related to the excitation of the H-bonded OH stretch (Figure 3.14) will be discussed in Chapter 4. Figure 3.14: Energetically allowed pathways for dissociation of HCl-(H2O)3 following excitation of the H-bonded OH stretch. 3.5.6 Estimating the Temperature of the Molecular Beam from the Detection of HCl As discussed in Chapter 4 and our recently published work, 15 the rotational spectrum of the HCl-(H2O)3 tetramer cannot be simulated and compared to the observed IR action spectrum, because the spectroscopic constants have not yet been measured. Instead, the temperature of the monomers in the beam was used to estimate the temperature of the clusters at the interaction region. The approximate temperature was then used to estimate the internal energy of the tetramer, which is important in estimating dissociation energies. Because experimental conditions (concentrations ratios, backing pressure, time delay between the pulsed molecular beam and the 65 ionization beam, etc.) were different for Pathways 1 and 2, in order to optimize the signal of each monomer fragment, independent temperature analyses for the clusters were required. For Pathway 2, the H2O monomer was monitored by 2+1 REMPI via the C̃ 1 B1 ← X ̃ 1 A1 transition. Direct comparison to PGOPHER simulations developed by Western et al. were used to estimate the H2O monomer temperature. 9, 12 A monomer temperature of 15 ± 5 K was reported in our previous work, 17 corresponding to a rotational energy of 10 ± 3 cm -1 in the tetramer. For Pathway 1, direct comparison to PGOPHER simulations could not be employed to determine HCl populations, as a model that incorporates the predissociation of the intermediate states has not yet been developed. Instead, populations were determined by recording 2+1 REMPI spectra, fitting smooth curves to the spectral peaks, integrating the peak areas, and correcting for predissociation of the intermediate states by using the experimentally calibration factors determined previously. 21 The question arose on whether or not the calibration factors would be applicable to the present work by utilizing a different instrument because the factors are dependent on experimental conditions. Conditions were similar between our work 36 and the previous experimental studies, 21 these small deviations were incorporated into the error bars. The sensitivity of these small deviations were tested by converting the area of the REMPI peaks to populations using two different data sets of calibration factors, which gave a difference of 3 K for the temperature of the molecular beam. 21, 22 In comparison to the total error, the observed effect of using the calibration factor was relatively small. Temperatures were then estimated from Boltzmann plots of the populations obtained by using different intermediate states. 7, 15 Because several rotational states had to be monitored, we used multiple intermediate states to confidently estimate populations (displayed in Table 3.2). In total, five branches were observed from two intermediate states: the S- and R- branches for the 66 F 1 Δ2 state, and the Q-, S-, and R- branches of the f 3 Δ2 state. There was consistency in the relative populations derived from all of the branches; however, due to factors such as partial saturation observed in the Q-branch and high error bars in the S-branch correction factors, the R-branches yielded the most reliable results. From these, the rotational temperature was estimated to be 44 ± 10 K. The error incorporates deviations in day to day experimental temperature fluctuations in the molecular beam, estimated error in the published calibration constants, and deviations from differences in the published experimental conditions. The HCl fragment rotational state distributions were needed for comparisons with theoretical calculations. In contrast to observing the HCl monomers in the beam, “IR ON” and “IR OFF” conditions were used to observe the HCl fragments after dissociation, and to isolate this signal from the HCl monomers in the beam. In addition to the five branches used for estimating the beam temperature, the V 1 Σ + (ν’ = 11) intermediate state (where correction factors to account for predissociation are not required) was also used to observe higher rotational levels. 62 Results of the fragment rotational state distribution are reported in our recent publication 15 as well as in Chapter 4 of this dissertation. 67 Chapter 3 References 1. Ch'ng, L. C., Dissociation Energy and Dynamics of Water Clusters. Ph.D. Dissertation, University of Southern California, 2013. 2. Dribinski, V., Photoelectron and Ion Imaging Studies of the Mixed Valence/Rydberg Excited Stated of the Chlormethyl Radical, CH2Cl and Nitric Oxide Dimer (NO2)2. Ph.D. Dissertation, University of Southern California, 2004. 3. Federov, I., Photoelectron and Ion Imaging Investigations of Spectroscopy, Photoionization, and Photodissociation Dynamics of Diazomethane and Diazirine. Ph.D. Dissertation, University of Southern California, 2009. 4. Parr, J. A., Imaging the State-Specific Vibrational Predissociation of Hydrogen Bonded Coplexes. Ph.D. Dissertation, University of Southern California, 2007. 5. Potter, A. B., Ion Imaging Studies of the Spectroscopy and Photodissociation Dynamics of CHloromethyl Radical and Nitric Oxide Dimer. Ph.D. Dissertation, University of Southern California, 2005. 6. Rocher, B. E., Velocity Map Imaging of the State-Specific Vibrational Predissociation of Water-Containing Hydrogen-Bonded Complexes. Ph.D. Dissertation, University of Southern California, 2011. 7. Zuraski, K., Photodissociation Dynamics and Energetics of HCl-(H2O)3. Ph.D. Dissertation, University of Southern California, 2018. 8. Zyrianov, M., Photoinitiated Decomposition of HNCO. Ph.D. Dissertation, University of Southern California, 1998. 9. Western, C. M., PGOPHER, a Program for Simulating Rotational Structure, http://pgopher.chm.bris.ac.uk. 68 10. Western, C. M., J. Quant. Spectrosc. Radiat. Transfer, 2017, 186, 221-242. 11. Ashfold, M. N. R., J. M. Bayley and R. N. Dixon, Chemical Physics, 1984, 84, 35-50. 12. Yang, C.-H., G. Sarma, J. J. ter Muelen, D. H. Parker and C. M. Western, Phys. Chem. Chem. Phys., 2010, 12, 13983-13991. 13. Casterline, B. E., A. K. Mollner, L. C. Ch'ng and H. Reisler, J. Phys. Chem. A, 2010, 114, 9774-9781. 14. Rocher-Casterline, B. E., A. K. Mollner, L. C. Ch'ng and H. Reisler, J. Phys. Chem. A, 2011, 115, 6903-6909. 15. Zuraski, K., Q. Wang, D. Kwasniewski, J. M. Bowman and H. Reisler, J. Chem. Phys., 2018, 146, 204303. 16. Hollas, J. M., High Resolution Spectroscopy, John Wiley & Sons1998. 17. Green, D. S., G. A. Bickel and S. C. Wallace, Journal of Molecular Spectroscopy, 1991, 150, 303-353. 18. Green, D. S., G. A. Bickel and S. C. Wallace, Journal of Molecular Spectroscopy, 1991, 150, 354-387. 19. Green, D. S., G. A. Bickel and S. C. Wallace, Journal of Molecular Spectroscopy, 1991, 150, 388-469. 20. Green, D. S. and S. C. Wallace, The Journal of Chemical Physics, 1992, 96, 5857-5877. 21. Rudić, S., D. Ascenzi and A. J. Orr-Ewing, Chemical Physics Letters, 2000, 332, 487-495. 22. Rudić, S., C. Murray, D. Ascenzi, H. Anderson, J. N. Harvey and A. J. Orr-Ewing, The Journal of Chemical Physics, 2002, 117, 5692-5706. 23. Kvaran, Á., Á. Logadóttir and H. Wang, The Journal of Chemical Physics, 1998, 109, 5856-5867. 69 24. Callaghan, R., S. Arepalli and R. J. Gordon, J. Chem. Phys., 1987, 86, 5273-5280. 25. Chichinin, A. I., C. Maul and K.-H. Gericke, J. Chem. Phys., 2006, 124, 224324. 26. Douglas, A. E. and F. R. Greening, Cana. J. Phys., 1979, 57, 1650-1661. 27. Ginter, D. S. and M. L. Ginter, J. Mol. Spectrosc., 1981, 90. 28. Kauczok, S., C. Maul, A. I. Chichinin and K. H. Gericke, J. Chem. Phys., 2010, 133, 414- 420. 29. Kvaran, A., W. H. and B. G. Waage, Can. J. Phys., 2001, 79, 197-210. 30. Ni, H., J. Serafin and J. Valentini, J. Chem. Phys., 2000, 113, 3055-3066. 31. Spiglanin, T. A., D. W. Chandler and D. H. Parker, Chem. Phys. Letters, 1987, 10, 414- 420. 32. Tilford, S. G., M. L. Ginter and J. T. Vanderslice, J. Mol. Spectrosc, 1970, 33, 505. 33. Tilford, S. G. and M. L. Ginter, J. Mol. Spectrosc., 1971, 40, 568. 34. Maul, C., C. A. I. and K. H. Gericke, J. Atom. Moles. and Opt. Phys., 2011, 410108. 35. Korolik, M., Molecule surface interactions in hydrogen chloride/magnesium oxide. Ph.D. Dissertation, University of Southern California, 1999. 36. Zuraski, K., D. Kwasniewski, A. K. Samanta and H. Reisler, J. Phys. Chem. Lett., 2016, 7, 4243-4247. 37. Green, D. S., G. A. Bickel and S. C. Wallace, J. Mol. Spectrosc., 1991, 150, 303-353. 38. Western, C. M., personal communication. 39. Berden, G., W. L. Meerts, M. Schmitt and K. Kleinermanns, J. Chem. Phys., 1996, 104, 972-982. 40. Fuke, K. and K. Kaya, Chem. Phys. Lett., 1983, 94, 97-101. 41. Zwier, T. S., Annu. Rev. Phys. Chem., 1996, 47, 205-241. 70 42. Stener, M., P. Decleva, D. M. P. Holland and D. A. Shaw, J. Phys. B: At. Mol. Opt. Phys., 2011, 44, 075203. 43. Yamazaki, I., T. Murao, T. Yamanaka and K. Yoshihara, Faraday Discussions of the Chemical Society, 1983, 75, 395-405. 44. Turner, R. E., V. Vaida, C. A. Molini, J. O. Berg and D. H. Parker, Chem. Phys., 1978, 28, 47-54. 45. Suits, A. G. and R. E. Continetti, Imaging in Chemical Dynamics, American Chemical Society, Washington, D.C., 2001. 46. Whitaker, B. J., Imaging in Molecular Dynamics. Technology and Applications, Cambridge University Press2003. 47. Chandler, D. W. and P. L. Houston, The Journal of Chemical Physics, 1987, 87, 1445- 1447. 48. Eppink, A. T. J. B. and D. H. Parker, Review of Scientific Instruments, 1997, 68, 3477- 3484. 49. Dribinski, V., A. Ossadtchi, V. A. Mandelshtam and H. Reisler, Rev. Sci. Instrum., 2002, 73, 2634-2642. 50. Mooney, J. and P. Kambhampati, The Journal of Physical Chemistry Letters, 2013, 4, 3316-3318. 51. Kleiner, I., L. R. Brown, G. Tarrago, Q.-L. Kou, N. Piqu é, G. Guelachvili, V. Dana and J.-Y. Mandin, Journal of Molecular Spectroscopy, 1999, 193, 46-71. 52. Tanabe, S., T. Ebata, A. Fujii and N. Mikami, Chem. Phys. Lett., 1993, 215, 347-352. 53. Watanabe, T., T. Ebata, S. Tanabe and N. Mikami, J. Chem. Phys., 1996, 105, 408-419. 71 54. Flynn, S. D., D. Skvortsov, A. M. Morrison, T. Liang, M. Y. Choi, G. E. Douberly and A. F. Vilesov, The Journal of Physical Chemistry Letters, 2010, 1, 2233-2238. 55. Skvortsov, D., S. J. Lee, M. Y. Choi and A. F. Vilesov, The Journal of Physical Chemistry A, 2009, 113, 7360-7365. 56. Zischang, J., D. Skvortsov, M. Y. Choi, R. A. Mata, M. A. Suhm and A. F. Vilesov, The Journal of Physical Chemistry A, 2015, 119, 2636-2643. 57. Ayers, G. P. and A. D. E. Pullin, Spectrochimica Acta Part A: Molecular Spectroscopy, 1976, 32, 1695-1704. 58. Schriver, A., B. Silvi, D. Maillard and J. P. Perchard, The Journal of Physical Chemistry, 1977, 81, 2095-2102. 59. Paul, J. B., C. P. Collier, R. J. Saykally, J. J. Scherer and A. O'Keefe, The Journal of Physical Chemistry A, 1997, 101, 5211-5214. 60. Fárnı ́ k, M., M. Weimann and M. A. Suhm, The Journal of Chemical Physics, 2003, 118, 10120-10136. 61. Mancini, J. S. and J. M. Bowman, Phys. Chem. Chem. Phys., 2015, 17, 6222-6226. 62. Korolik, M., D. W. Arnold, M. J. Johnson, M. M. Suchan, H. Reisler and C. Wittig, Chemical Physics Letters, 1998, 284, 164-170. 72 Chapter 4: HCl-(H2O)3Tetramer In this chapter, the vibrational predissociation dynamics of the cyclic HCl-(H2O)3 tetramer is investigated by theory, quasiclassical trajectory calculations, and experiment, following the infrared excitation of the hydrogen bonded OH stretch fundament. The following energetically possible pathways: HCl-(H2O)3 à HCl + (H2O)3 (Pathway 1) and HCl-(H2O)3 à H2O + HCl- (H2O)2 (Pathway 2) are reported. The HCl and H2O monomer fragments are observed by 2+1 REMPI combined with TOF-MS, and their rotational energy distributions are inferred and compared to the theoretical results. Utilizing velocity map imaging, monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and theory are broad and structureless, encompassing the entire range of allowed speed for both pathways. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments rotational energies, the shapes of the speed distributions, and the average fragment speeds, and center of mass translational energies. Insights into the dissociation mechanisms and lifetime are gained from theoretical results. The work described in this chapter has been published previously. 1, 2 Figure 4.1: Simplified Experimental Scheme for the vibrational predissociation of HCl-(H2O)3 73 4.1 Introduction A general introduction, experimental setup and data analysis of hydrogen- bonded clusters have been presented in previous chapters. Only information relevant to the HCl-(H2O)3 tetramer will be presented in this chapter. Water-hydracid complexes are prototype systems for the study of hydrogen bond (H-bond) properties and the mechanisms of acid solvation. The unique properties of water arise from its ability to form H-bonds, which often leads to the stabilization of intermediates, the shifting of molecular vibrational energies, and the lowering of activation barriers for reactions. 3, 4 The ability of water to stabilize ions is also well known; 5 however, a detailed understanding of the dynamics leading to acid ionization within mixed clusters is uncertain. A bottom-up approach aims to identify trends in H-bonds dissociation through isolation of small mixed clusters. This starts with dimers containing one water and one hydrogen halide species, and then sequentially adding solvent water molecules to compare properties of larger cyclic networks. The present work focuses on the vibrational predissociation (VP) of the HCl-(H2O)3 tetramer (referred to henceforth as HWWW), the largest neutral HCl-(H2O)n cluster observed without ionization to the acid. Addition of one or two water molecules to this mixed cluster is expected to initiate proton migration to generate a stable solvent ion pair. 6-13 Although the existence and structure of the HWWW tetramer was debated in the past, the most recent calculations confirm that it is stable and its optimized geometry is cyclic, as shown in Fig. 4.1. 6-15 An investigation of this mixed cluster is also interesting because there are two close-lying dissociation channels that can be accessed by excitation of the intramolecular OH-stretch fundamentals, yielding HCl and H2O monomer fragments: HCl-(H2O)3 → HCl + (H2O)3 HCl-(H2O)3 → H2O + HCl-(H2O)2 74 An open question, for example, is whether the final trimer products, (H2O)3 (WWW) and HCl-(H2O)2 (HWW), have cyclic or open-chair configurations. The cyclic forms of the trimers are the most stable energetically, but the transition states required for their formation via VP of the tetramer are tighter and thus disfavored by entropy. The theoretical/experimental examination presented here strives to answer this intriguing question. As discussed in Chapters 1 and 3, the investigation of the VP of the HWWW tetramer poses challenges for both experiment and theory. In experiments, separating the infrared (IR) transition of the H-bonded OH stretch of the desired tetramer from transitions of other (HCl)m-(H2O)n clusters was key. In recent spectroscopic assignments in He droplets, 16 the H-bonded OH-stretch vibrations of several pure water and mixed water-HCl clusters were identified. In spite of the close proximity of transitions of several clusters, HWWW has IR bands that are isolated from other clusters. In supersonic molecular beams, however, these bands are broader and partially overlap. In our experimental work below, we present arguments that the broad band at 3530-3550 cm -1 is associated with one of the H-bonded OH stretch vibrations of HWWW, and we identified H2O monomers as VP products. 1 From the highest translational energy release in the VP of HWWW, we infer that the dissociation energy (D0) for Pathway 2 was 2400 ± 100 cm -1 . The theoretical results presented in this chapter were performed by our long time collaborators at Emory University. 1-3, 11, 17-25 Theoretical studies of the VP dynamics of HWWW are challenging, requiring an accurate potential energy surface (PES) that describes the cluster at the high energies of dissociation to the two product channels of interest here. (The need to run thousands of trajectories for roughly 10 picoseconds rules out an ab initio direct dynamics approach). Here the PES employed is based on high-level ab initio many-body components, as 75 described in detail and tested previously by Mancini and Bowman. 21 Structures were optimized and vibrational frequencies were determined using different approximations, which confirmed the vibrational assignments of the experimentally observed IR spectra of mixed clusters. Previous calculations for dimers and trimers gave H-bond dissociation energies that were in close agreement with experiment . These studies are summarized in recent papers and review articles. 3, 18, 19, 23, 24, 26 In addition, quasiclassical trajectory (QCT) calculations performed on these highly-accurate PESs were used successfully before to determine the VP dynamics and dissociation energies of (H2O)2, HCl-H2O, (H2O)3, and (HCl)3. 18, 19, 22, 24 In this chapter, the results of a joint experimental and theoretical study of the predissociation of HWWW via Pathways 1 and 2 are presented. In addition to being the first tetramer species to be investigated in detail, it is also the first mixed HCl-water VP dynamics study for a system larger than dimer. We report our best experimental and theoretical estimates for the dissociation energies for Pathway 1 and 2, rotational energy distributions for the HCl and H2O monomer fragments, and fragment speed distributions. Moreover, we gain deeper insight into the predissociation dynamics by analyzing dissociative trajectories, obtained by QCT calculations, for the two dissociation pathways. These indicate that cyclic trimer fragments are formed in the two pathways, and the dissociation mechanism follows trends congruent with smaller neutral cluster work. 4.2 Experimental Arrangement The experimental procedures are similar to those used in studies of smaller H-bonded clusters such as NH3-H2O 27 , HCl-H2O 26, 28 , HCl-C2H2 29 , (H2O)3 19 , and phenol-H2O. 30 The HWWW tetramer and other mixed HCl/H2O clusters were formed in a supersonic molecular beam through a 0.5 mm orifice of a pulsed valve (~150 μs opening time) operating at 10 Hz. For Pathway 76 1 and 2, the sample mixture and backing pressure were optimized for the maximum signal of the HWWW at a concentration of 0.6% H2O and 2% HCl (Matheson Trigas, 99.995%) with 2 atm of a He carrier gas (Gilmore, 99.999%). Samples were prepared by transferring H2O by vacuum distillation to an evacuated bulb followed by adding gaseous HCl to the bulb in a fume hood and then filling by the carrier gas. At these concentrations, signals from pure water clusters, and from higher order HCl/H2O clusters were minimized as discussed in Chapter 3.2.1). The mixture was then introduced into a high vacuum chamber maintained at a base pressure of ~2.5 x 10 -8 torr. The skimmed molecular beam was intersected at right angles by two counter propagating laser beams in the interaction region of the chamber. Focused IR laser radiation (~3.5 mJ/pulse, 20 cm f.l. lens) was used to excite the H-bonded OH stretch of the cluster at 3550 cm -1 and focused UV radiation (0.1-0.4 mJ/pulse, 20 cm f.l. lens) was used to ionize state-selected H2O and HCl fragments. IR laser radiation was generated by an OPO/OPA laser system (LaserVision, up to ~8 mJ/pulse, ~0.4 cm -1 linewidth) pumped by the fundamental of a seeded Nd:YAG laser (Continuum Powerlite 8000). “IR ON” signals were collected while exciting the OH stretch of the tetramer, whereas “IR Off” signals corresponded to an off peak 3652 cm -1 background position where signals from other (HCl)n-(H2O)m clusters were at a minimum. For each pathway, the laser conditions (timing, focusing, and power) were optimized to enhance the signal from HWWW while minimizing signals from other clusters. The UV radiation was generated by frequency- doubling (Inrad Autotracker III) the output of a dye laser (Continuum ND 6000, Coumarin 480 for HCl, and Coumarin 500 for H2O) pumped by a Nd:YAG (Continuum Surelite III). The IR frequency was calibrated using ammonia in a photoacoustic cell. 31 The UV radiation was calibrated using a simulated REMPI spectra of H2O and HCl. 32-34 77 Experiments were run using two different data acquisition modes: (1) time-of-flight mass spectrometry (TOF-MS) for recording IR action spectra, 2+1 REMPI spectra, and optimizing the timings and alignment of the instrument; and (2) velocity map imaging (VMI) for deriving pair- correlated rovibrational distributions of the cofragments by monitoring specific rotational levels of the HCl or H2O monomer fragments. Raw images were converted to fragment speed distributions in pixel space using the BASEX method, 35 and the speed distributions were converted to center-of-mass (c.m.) translational energy distributions using the appropriate Jacobian and momentum coservation. 36 Using the TOF-MS mode for Pathway 1, scans were taken across the 2+1 REMPI transitions of HCl to determine the relative populations of individual rotational states and to set an upper limit for the dissociation energy. To encompass the full range of all the energetically possible rotational levels of HCl, transitions through the following intermediate states were used: f 3 Δ2 (υ’=0) state for J” = 3-6, F 1 Δ2 (υ’=0) state for J” = 5-8, and the V 1 Σ + (υ’ = 11 and 12) state for J” = 5-11. 22, 37, 38 As discussed in Chapter 3, the spectra were analyzed by comparison to PGOPHER 32 simulations generated with constants from Green et al. 39-41 While scanning across these transitions, “IR ON” and “IR OFF” signals were collected intermittently at each wavelength position. REMPI peaks were fit with a Gaussian function and integrated. The difference between the “IR ON” and “IR OFF” values were then used to infer relative HCl fragment populations after correction by appropriate calibration constants, which take into account predissociation in the upper electronic states. 37, 38 Population distributions of the HCl monomer fragment obtained via different intermediate states were normalized to each other by using J” = 5, which was accessed by all three transitions. 78 For Pathway 2, the C̃ 1B1 (000) ← X ̃ 1A1 (000) transition was used for H2O photofragment imaging and REMPI spectroscopy. 2+1 REMPI spectra were compared to simulations using PGOPHER, with spectroscopic data from Yang et al. 1, 32, 34 In this case, it was not feasible to infer rotational state populations due to spectral congestion and the complexity of the 2+1 REMPI spectra. Instead, temperatures of the water monomers and fragments were estimated using comparisons to PGOPHER simulations, which include experimentally determined predissociation lifetimes of rotational levels in the C̃ 1 B1(000) state. 1, 34 Background signals from water monomers coming from the molecular beam were estimated from REMPI scans obtained under “IR OFF” conditions. The difference between the “IR ON” and “IR OFF” spectra gave the signals from the monomer fragments. The temperatures of the molecules in the beam were estimated from the temperatures of the monomers under the conditions optimized for the VP studies of each pathway. They were 15 ± 5 K and 44 ± 10 K for H2O and HCl monomers, respectively, determined at the crossing point of the molecular and laser beams at the interaction region. 4.3 Methods Used in Theoretical Calculations As noted above, the PES used was a highly accurate many-body potential constructed by Mancini et al. 21 Accurate dissociation energies for numerous dissociation pathways, including the high energy breakup to H+W+W+W, were calculated using De values from complete basis set (CBS) calculations and zero-point energies (ZPE) using an unbiased diffusion Monte Carlo (DMC) method for the HWWW tetramer and various product fragments. The results of relevance here are as follows: De values are 3008 and 3634 cm -1 for Pathways 1 and 2, respectively, and the corresponding D0 values are 2426 ± 23 and 2826 ± 19 cm -1 . In the next section, we compare these values to the experimental results. The VP dynamics reported here are based on QCT calculations 79 very similar to those reported previously, 1-3, 17, 24 and further detail can be found in our collaborative paper 2 and will be briefly discussed here. The trajectories were initiated at the global minimum configuration of HWWW, depicted in Figure 4.1 and assignment of harmonic ZPE for each mode followed by exciting the mode of interest. From the normal mode analysis (NMA) using the PES, 2 one of the O-H stretch modes, whose frequency is only 20 cm -1 higher than the experimental energy, was the mode excited in this study. 1 This mode has a high oscillator strength as well and corresponds to the mode excited experimentally. After the total vibrational energy was assigned, an ensemble of trajectories was generated by randomly distributing the normal mode displacement and momentum for each mode. Then the normal coordinates and momenta were transformed into Cartesian counterparts. During the transformation, small spurious angular momentum was generated and then removed so that the total angular momentum is zero. The removal of rotational energy resulted in loss of total energy, so the iteration was corrected by a scaling step to increase harmonic displacement and momenta to compensate for the energy loss. Finally, the total energy is the sum of anharmonic ZPE (17683 cm -1 ), calculated using DMC, and excitation energy used in the experiment (3550 cm -1 ), 21233 cm -1 . Then trajectories were propagated using a velocity-Verlet algorithm with a step size of 0.06 fs. Trajectories were terminated when any individual bond length exceeded 16 Å. The total energy of the trajectories is roughly seven times larger than the PES De for either pathway, leading to relatively prompt dissociation (see below); however, as expected, many trajectories result in products with less energy than ZPE, as discussed in more detail below. Final product channels were categorized by the distance between the monomers. The HWWW complex dissociates classically into numerous products, of which only those in Pathways 80 1 and 2 are rigorously open when considering ZPE of the fragments. 2 Thus, distances between all possible fragments were monitored and the products of a dissociated trajectory were identified based on a condition that the shortest intermolecular distance between this monomer and other fragments was greater than 6.5 Å. Overall, 48885 trajectories were run, of which 15 242 dissociated to Pathway 1, and 19 838 to Pathway 2, with no consideration of ZPE of the fragments. We return to this branching ratio in the next section. The average termination time for both channels was approximately 7 ps. Note that the remaining configurations belong to other dissociation products but they are discarded, because these channels have D0 values higher than the excitation energy, 2 so they are rigorously energetically forbidden. As a result, it is safe to say that the HCl fragment comes only from Pathway 1, and the water fragment only from Pathway 2. The analysis of the fragments’ internal energies was done in the standard way, with subsequent comparison to the experiments in mind. Specifically, for each trajectory corresponding to Pathway 1 the magnitude of the classical angular momentum of HCl (in atomic units), j, was obtained. From this, J was obtained from j 2 = J(J+1) and rounded to the nearest integer. The same procedure was applied to obtain J for the H2O product following Pathway 2. Then, the angular momenta were removed so that the remaining energy was the vibrational energy, and the difference with the total internal energy was the rotational energy. In this way, the vibrational and rotational energies of the fragments were obtained. The c.m. translational energy, Et, was calculated directly using Et, = # * `v 2 , where ` is the reduced mass of the two fragments and v is the relative speed. In the experiments, Et, was calculated from the measured fragment speed as described in Section II. Standard histogram binning was done for the HCl and H2O rotations. For the H2O fragments, selected JKa, Kc levels were observed in the experiments, whereas in the QCT calculations, while 81 the corresponding J was obtained similarly as for HCl, JKa, Kc states were determined by binning QCT rotational energies to the corresponding experimental rotational energies. As noted above, most of the fragments are formed with less energy than the ZPE. Thus several standard constraints were applied by comparing the calculated vibrational energy to the DMC-calculated ZPE of the fragments. The “soft ZPE constraint” requires Evib(HCl-(H2O)2) + Evib(H2O) > ZPE(HCl-(H2O)2) + ZPE(H2O) in Pathway 2 and Evib((H2O)3) + Evib(HCl) > ZPE((H2O)3) + ZPE(HCl) in Pathway 1, while the “hard ZPE constraint” requires each fragment to have a vibrational energy greater than the ZPE of the fragment. Lastly, we consider a “hard ZPE on monomer,” which requires only the dissociated water or HCl to satisfy the ZPE condition. Ideally, the hard ZPE constraint should be applied; however, this is problematic in the present case, because it results in the rejection of such a large number of trajectories that final conditions such as rotational distributions are statistically highly uncertain. 4.4. Results and Discussion 4.4.1 IR “Action” Photofragment Yield Spectroscopy Monitoring Pathway 2 As discussed in Chapter 3.5.5, characterization of the fundamental OH stretch region was crucial for our studies as it allowed for tagging HWWW while at the same time inducing dissociation. Zischang et al. 16 have shown that based on experimental observations and ab-initio calculations, several transitions are likely associated with the H-bonded OH stretch fundamentals of HCl-(H2O)3 in Helium nanodroplets: 3560.1 cm -1 , 3546.8 cm -1 , 3476.2 cm -1 , and 3438.5 cm -1 . We have obtained IR action spectra in a supersonic molecular beam (Helium carrier gas) in the range 3520-3655 cm -1 by monitoring state-selected H2O fragments (J’’Ka,Kc = 22,1, 32,1, 50,5/51,5, 71,6) by 2+1 REMPI. Our results show several peaks, whose intensities vary with the HCl:H2O ratio in the expansion. Based on our measured pressure and concentration dependencies of the 82 action spectra, and comparisons with previous spectroscopic work, we assign the intense broad peak observed at 3530-3555 cm -1 to excitation of the H-bonded OH stretch fundamental of HCl- (H2O)3. This is most likely analogous to the 3546.8 cm -1 peak observed in the Helium droplet study [See Chapter 3.5.5] VP was induced by pulsed laser excitation in this band at 3550 cm -1 , which has minimum overlap with other HCl/H2O mixed clusters and pure water clusters. Figure 4.2 shows a representative spectrum obtained by detecting H2O(J’’Ka,Kc = 32,1) in the region of 3520- 3560 cm -1 . Similar action spectra were observed by detecting HCl(J”) states, albeit with higher background from larger clusters and lower signal to noise ratio. Figure 4.2: IR action spectrum (red) obtained by monitoring H2O photofragment in J’’Ka,Kc = 32,1 with “IR ON” conditions. The black line shows the “IR OFF” background from H2O monomers. The raw data are shown in lighter color and bold lines show the data with 3-point smoothing. A photofragment yield spectrum was obtained by exciting HWWW at 3550 cm -1 and scanning the UV laser frequency in the region of the C̃ 1 B1 (000) ← X ̃ 1 A1 (000) H2O transition. Figure 4.3 displays the 2 +1 REMPI spectrum of H2O fragments and the background spectrum of water monomers. Fast predissociation in the C̃-state and spectral congestion limit the state selective detection of H2O. 26 The highest isolated rotational state detected in our study was J”Ka,Kc = 71,6 (704 cm -1 ), which sets an upper limit to D0 at approximately 2850 cm -1 for Pathway 2. A 83 more accurate value is obtained by VMI. By careful selection of the UV wavelength, 32, 34 images were obtained by monitoring H2O fragments in J”Ka,Kc = 22,1, 32,1, and 50,5/51,5. Figure 4.3: 2 +1 REMPI spectrum of H2O via the C̃ 1 B1 (000) ← X ̃ 1 A1 (000) transition. (Top) The “IR ON” spectrum (red) was obtained by exciting the H-bonded OH stretch of HCl-(H2O)3 at 3550 cm -1 and the “IR OFF” spectrum (black) by recording the background. The arrows mark the following J’Ka,Kc ← J’’Ka,Kc transitions: (a) 71,7 ← 71,6, (b) 20,2 ← 32,1, (c) 40,4/41,4←50,5/51,5, and (d) 20,2 ← 22,1. Assignments are based on the simulated spectrum (300 K) created in PGOPHER (bottom). 34 4.4.2 Fragment Speed Distributions Experimental speed distributions of monomer fragments were obtained by reconstruction of velocity mapped raw images obtained by 2+1 REMPI detection of either HCl or H2O in selected rotational levels. Pixels on the detector were converted to speeds using calibration factors obtained by comparison with previously published speed distributions. 1, 26 Due to the high density of internal states in the trimer co-fragments, the speed distributions displayed no clear structural features, and appeared statistical-like. In this case, only endpoints of the distributions could be used to estimate the dissociation energy. This is in contrast to small dimers, such as HCl-H2O, 84 where distinct structures correlated with rotational states of the cofragment were exploited to obtain precise D0 values. 26 Nevertheless, the endpoints of the speed distributions were reproducible, allowing us to estimate the dissociation energies for each pathway. The endpoints were clearer for Pathway 2 due to much higher signal/noise ratios. Figure 4.4 shows the speed distributions of the HCl and H2O monomer fragments monitored in the indicated rotational states. The shaded areas in Figure 4.4 depict our estimates for the ranges of acceptable endpoints in each distribution. From the ranges for the HCl J” = 4 and 6 speed distributions, we estimate D0 for Pathway 1 to be 2100 ± 300 cm -1 . Having a much higher signal/noise ratio, the corresponding ranges for Pathway 2 are smaller, resulting in D0 = 2400 ± 100 cm -1 . 1 As noted above, the calculated dissociation energies are 2426 ± 23 cm -1 and 2826 ± 19 cm -1 for Pathways 1 and 2, respectively. These values correspond to the cyclic form of the trimer products (see below). The fragment speeds corresponding to the calculated D0 values are indicated by arrows in Figure 4.4. Experimentally, there are two problems in determining D0 from the highest speed. First, as described above, assigning the endpoint of the distribution has inherent uncertainty because of the low signal/noise values in the tail region; therefore, for Pathway 1 only ranges of acceptable values can be determined. Second, when the parent cluster has significant internal energy, especially in the form of vibrational energy, the highest translational energy may be correlated with hot bands, in which case the experimentally determined dissociation energy is only a lower limit to D0 of the cold tetramer. 85 Figure 4.4: Speed distributions obtained by monitoring (a) J” = 4 and (b) J” = 6 of the HCl monomer fragments following the dissociation pathway HWWW à H + WWW; and (c) J”Ka,Kc = 22,1 and (d) J”Ka,Kc = 32,1 of H2O fragments from the HWWW à W + HWW pathway. Red and black plots are obtained using “IR ON” and “IR OFF” conditions, respectively. Shaded regions indicate uncertainty in determining the endpoints of the experimental distributions. Arrows indicate the location of endpoints for the speed distributions expected from the calculated dissociation energies. 86 Following conversion of fragment speeds to c.m. translational energy distributions, D0 was obtained by using conservation of energy: Evib,rot (int) + EIR = D0 + Erot(monomer) + Evib,rot (cofragment) + Et Eq 4.1 where Evib,rot (int) is the internal rovibrational energy of the HWWW parent cluster, EIR is the energy imparted by IR excitation, Erot(monomer) is the (known) energy of the monitored HCl or H2O rotational levels, and Evib,rot (cofragment) is the rovibrational energy of the cofragment, which is zero when the c.m. translational energy (Et) release reaches the maximum value allowed by energy conservation. The greatest difficulty is in assessing Evib,rot (int). The average rotational energy of the cluster is estimated from the rotational temperatures of the monomers: 30 ± 15 cm -1 and 10 ± 5 cm -1 for HCl and H2O, respectively. However, it is impossible to assess accurately the vibrational energy of the cluster, because its low-energy intermolecular vibrations may not be cooled efficiently in the supersonic expansion. Additional insight can be gained by comparing the experimental pair-correlated speed distributions to the ones calculated by QCT. The speed distributions calculated for HCl J” = 4 and J” = 6 rotational levels (Pathway 1) are shown in Figure 4.5. The QCT distributions calculated by using soft ZPE constraints are compared to the experimental distributions plotted for comparison. This constraint was chosen as the best compromise between accuracy and the number of trajectories. Clearly the experimental and calculated peaks of the distributions appear similar. In addition to comparing the peaks of the pair-correlated speed distributions, we also compared the average fragment speeds and average c.m. translational energies, and the comparisons are summarized in Table 4.1. As seen in Figure 4.5 and Table 4.1, the theoretical calculations and experimental distributions have similar peak positions, shapes and average speeds, but the 87 experimental distributions include low-intensity tails that extend to higher speeds, leading to the disagreement in the D0 values. The calculated D0 values are higher for both pathways than those observed experimentally, but in both cases Pathway 1 (HCl fission) has the lower energy. The corresponding QCT calculations for specific J’’Ka, Kc levels of the water fragment (Pathway 2) could not be done because of insufficient number of trajectories for specific rotational levels. Figure 4.5: Experimental and theoretical (using soft ZPE constraints) speed distributions for HCl monomer fragments in rotational levels (a) J”=4 and (b) J” = 6. The large intensity at the origin of the experimental data is the result of monomers in the molecular beam. Table 4.1: Experimental and Theoretical Values (soft ZPE) for Approximate Peak Positions, Average Speed of the HCl Fragment and Average Translational Energy, Et HCl Rotation J”= 4 J” = 6 Experimental Theoretical Experimental Theoretical Peak Position 270 m/s 260 m/s 290 m/s 210 m/s Average Speed 329 m/s 303 m/s 324 m/s 297 m/s Average E t 273 cm -1 264 cm -1 264 cm -1 250 cm -1 The similarly of the results for the two investigated modes and the statistical-like appearance of the speed distributions are not surprising, given the large density of states of the reactants and products. The Beyer-Swinehart algorithm was used to calculate a lower limit for the 88 density of states of the excited parent and fragment clusters. The vibrational density of states of the tetramer were obtained using the harmonic vibrational states calculated by Bowman and coworkers in Table 4.2 and was determined to be approximately 1x10 6 /cm -1 . The corresponding values for the water trimer products were obtained using the calculated intermolecular frequencies listed in Table 7 in Reference [42], which range from 5 to 20/cm -1 for 500-1000 cm -1 of excess energy available. 42, 43 Previous comparisons of theory and experiment for D0 values of small H-bonded clusters such (H2O)2, (HCl)3, and H2O-HCl showed excellent agreement. 3, 18, 22, 24, 28 In these cases, the experimental D0 values were obtained by exploiting distinct structural features in each distribution, rather than using the endpoints. As mentioned above, a plausible explanation for the present disagreement is the participation of hot bands captured by IR excitation at 3550 cm -1 , which result in fragments with higher maximum speeds. In this case, the parent cluster would have excess internal energy, Evib,rot (int)which is not taken into account properly in determining D0. The possible participation of hot bands is also supported by the observed large width of the H-bonded OH-stretch transition being excited, 1 which may include contributions from hot bands. This band lies in the proximity of transitions of other clusters (see Chapter 3.5.5), and from a spectroscopic perspective, the 3550 cm -1 excitation energy within the broad band was chosen because of its isolation from other mixed-clusters transitions (Figure 4.2). 16 According to calculations, there are 11 low-energy intermolecular vibrations in the tetramer parent within 400 cm -1 of the ground vibrational state, which makes the contribution of hot bands a likely explanation for the difference between theory and experiment (See Table 4.2 for a complete list of calculated vibrational frequencies). However, because the majority of the fragments’ signals derive from cold clusters, we expect the average speed and peak positions to give a better match to the theoretical results 89 than D0, as is indeed the case. Fragments from hot clusters would be manifest mainly in the low- intensity tail of the speed distributions. Hot band transitions of clusters have been observed before under our experimental expansion conditions, but not in lower temperature studies, such as in He droplets. 27 Table 4.2: List of Normal Modes using calculated PES for HCl-(H2O)3 2 4.4.3 Fragment Rotational Energy Distributions The rotational energy distribution of the HCl fragment following Pathway 1 was determined both by theory and experiment. Figure 4.6 shows examples of 2+1 REMPI spectra (after background subtraction) obtained via the f 3 Δ2−Χ 1 Σ + (0,0) and F 1 Δ2 − Χ 1 Σ + (0,0) intermediate transitions. An upper limit of D0 ≤ 2830 cm -1 is obtained for Pathway 1 from J” = 8, the highest HCl rotational level that could be observed with confidence. Populations in J” = 0-2 could not be obtained because of large background from HCl monomers. Figure 4.7 shows the relative rotational state populations plotted as a function of rotational energy along with calculated rotational energy distributions. The QCT derived distributions show the same general trend as the experiment but decrease more slowly with increasing energy. The hard and soft ZPE constraints both show that the rotational energy cut-offs correspond to J” = 10 in the HCl monomer fragment. There are multiple explanations for the discrepancy between theory and experiment, including the 90 known difficulty in interpreting quantum rotations using classical calculations as well as the experimental inability to detect reliably rotational levels greater than J” = 8 above the background. Overall, we consider the agreement between theory and experiment satisfactory. Figure 4.6: (2+1) REMPI spectra of H 35 Cl (ν = 0) recorded via the (a) f 3 Δ2−Χ 1 Σ + (0, 0) and (b) F 1 Δ2 − Χ 1 Σ + (0,0) intermediate transitions. The background subtracted signal is shown following IR excitation at 3550 cm -1 . Figure 4.7: Comparison between experimental (black) and calculated (indicated colors) rotational populations for the HCl monomer following dissociation of HWWW. The discrete rotational energy data points for each constraint in the calculations are connected by a line for visual guidance. Experimental REMPI intensities were converted to relative populations using published correction factors. 37, 38 REMPI data obtained using different intermediate states were used and the populations were normalized to J” = 5 for which transitions to all intermediate states were recorded. The experimental and calculated populations are normalized to J” = 3. 91 The calculated rotational energy distributions for the water fragment following Pathway 2 were calculated and shown in Figure: 4.8. A total of 1058 trajectories with a hard ZPE constraint and 14223 with a soft ZPE constraint produced rotational energy maxima of 760 cm-1 and 1011 cm-1, respectively. With the theoretically calculated D0, the expected rotational cut-off should be 724 cm-1 by conservation of energy. The cut-off of rotational energy under both soft and hard ZPE obeys this theoretical upper limit and hard ZPE matches the cut-off very well. Figure 4.8: Water monomer rotational energy distributions for Pathway 2 calculated using the indicated constraints. Experimentally, the H2O 2+1 REMPI spectrum recorded via the C̃ 1 B1(000) state is too congested to obtain a detailed H2O rotational state distribution. However, the spectrum reported in Fig. 4.2 1 could be fit fairly well with a temperature of 300 K for transitions originating from higher rotational levels (for example, J’’KaKc = 50,5 and 70,7) by using the PGOPHER simulation [Figure 92 4.9, (a)]. 34 On the other hand, transitions from the lower rotational levels, such as J’’KaKc = 22,1 and 32,1, clearly fit better with a lower temperature of ~ 150 K [Figure 4.9, (b)]. The corresponding average rotational energies are about 200 and 100 cm -1 . It must be born in mind, however, that the predissociation factors that are included in the fit are less accurate for high rotational levels of the C̃ 1 B1(000), which may affect the fit. 44 Another possible reason for the higher temperature corresponding to the higher rotational levels is contributions from hot bands, as discussed above. Boltzmann plots of the calculated distributions were fit well (at low rotational energies) with temperatures of 327, 306, 205 and 160 K for no constraint, a hard constraint on the water monomer, soft constraints on both ZPEs, and hard constraints on both, respectively (see supplemental theoretical information found in Reference 2 for more detail). From these results, it appears that the rotational distribution of the water fragment is statistical-like, and can be described within the temperature range of 150-300 K (rotational energies of 100-200 cm -1 ). Figure 4.9: Background subtracted 2+1 REMPI spectra (blue) and simulation of the spectra (black) at (a) T = 150 K and (b) T = 300 K. Labeled transitions originate from J’’KaKc rotations: (1) 71,6, (2) 32,1, (3) 50,5, and (4) 22,1. 93 4.4.4 Dissociative Trajectories and Lifetimes Figure 410: (left) Trajectory snapshots of Pathway 1 for J” = 4 (~6 ps) and (right) Pathway 2 for J” ka,kc= 22,1 (~7 ps). Examination of dissociative trajectories by the Bowman group 2 can shed light on mechanisms; snapshots from two representative trajectories are shown in Figure 4.10. As stated above, the calculated dissociation energies for Pathways 1 and 2 correspond to cyclic trimer fragments. Indeed, it is evident from Figure 4.10 that just prior to dissociation, the polyatomic fragment forms a cyclic structure, sometimes with the monomer fragment still attached to the trimer ring by a hydrogen bond. In fact, the product water trimer ring breaks and reform many times during the dissociation until the monomer (HCl or H2O) eventually separates, and there are many intermediate structures seen in each trajectory. No low-energy open-chain stable trimer fragments were found in the calculations, and we conclude that the VP products are a monomer and a cyclic fragments. The fragments may evolve via intermediates that have the monomer H- 94 bonded to the cyclic trimer, as seen for example in the HCl trajectory in Figure 4.10 (left). The multiple break-up and formation of hydrogen bonds appear to be a common motif in the VP of small dimers. In our work, we have seen them in trajectories of VP of (H2O)2, (H2O)3, and (HCl)3. 3, 18, 19, 22 The vibrational predissociation lifetimes for the HWWW tetramer are significantly shorter than that of smaller cluster systems. Both pathways show a lifetime of about 7 ps, compared to the water trimer (84% dissociated at 10.5 ps) and the water dimer (84% dissociated at 25 ps). 3, 24 Based solely on cluster size, the trend is that larger clusters have shorter dissociation lifetimes. This trend may be explained partly by the time it takes for the initial OH-stretch excitation to couple to low- energy intermolecular vibrational levels of the cluster. For example, the lifetime of the HCl trimer is much longer than that of the H2O trimer, because of the existence of a bending mode in the water monomer, which facilitates coupling to the intermolecular modes in the latter. 19, 22 The lifetime of > 1 ns for the HCl trimer is indeed correlated with the persistence of its coupled HCl stretch modes. 22 In the present work, the excited H-bonded OH-stretch of the tetramer is likely to couple more efficiently to the intermolecular modes than in the water trimer because of the higher density of states of intermolecular modes in the tetramer. Also, the nominal OH-stretch vibration has contributions from other motions, which may facilitate coupling. Therefore, it is reasonable that the lifetimes of larger clusters become shorter, though at some point the statistical nature of the predissociation process will cause the rates to decrease with cluster size. In large clusters, we expect that energy disposition would be more statistical-like, although because of the relatively large energy separation between the rotational levels of the HCl monomer, some deviations from statistical behavior would not be surprising. Overall, however, the VP of HWWW is typical of that of neutral clusters, exhibiting no sign of impending ionization. In this study, we experimentally 95 probe only the neutral H2O and HCl fragments; therefore, we are unable to see either ionization to H3O + and Cl - or zwitterionic behavior. Fárník et. al raised the possibility of the latter and speculated that it might be evidenced in their IR spectrum of the mixed clusters by the observation of a “broad hump” (~150 cm -1 ) at 2180 cm -1- . 45 The branching ratio for the two pathways can be obtained from the QCT calculations; however, the results are very sensitive to the ZPE constraint used, as expected. With no ZPE constraint, the branching ratio for H+WWW and W+HWW is 0.77 to 1, with the soft ZPE is 0.99 to 1 and with the hard ZPE it is 1.76 to 1. With the hard ZPE and using the theory values of D0 for the two pathways, the excess energy for the H+WWW channel 1124 cm -1 (3550-2426 cm -1 ) and for W+HWW it is 724 cm -1 (3550-2826 cm -1 ) – a difference of roughly 400 cm -1 . However, for the PES the difference is 500 cm -1 , owing to the differences in the PES De and the CBS values. In any case, the difference in energy certainly favors the H+WWW channel over the W+HWW one. However, without the ZPE constraint the W+HWW channel is slightly favored even though the difference in electronic energies is even larger, namely 600 cm -1 from the CBS values and 700 cm -1 from the PES values. Naively, there are three ways to eliminate a water monomer and only one way to eliminate HCl, which would lead to a branching ratio of 0.33 to 1 if the two channels were isoenergetic. This ratio is not observed, even without any ZPE constraint, suggesting that the branching ratio is not a simple statistical one. Unfortunately, because of the different experimental conditions used in optimizing the signals from the two VP pathways, the branching ratio cannot be obtained experimentally. 96 4.5 Summary and Conclusions The VP of cyclic HWWW, the largest HCl-(H2O)n cluster that is not ionized, has been investigated by both theory and experiment. The cluster was excited experimentally at 3550 cm -1 corresponding to an H-bonded OH stretch fundament and HCl and H2O have been identified as the products of Pathways 1 and 2, respectively. QCT Calculations show that these pathways terminate in the corresponding cyclic products, WWW and HWW, and their D0 values are 2426 ± 23 cm -1 and 2826 ± 19 cm -1 , respectively. The corresponding experimental values, estimated from the endpoints of the fragments’ speed distributions, are lower but in the same order: 2100 ± 300 cm -1 and 2400 ± 100 cm -1 . Arguments are presented that the most likely reason for the discrepancy is contributions from hot bands of the clusters, which contribute mainly to the high-speed tails. The shapes of the HCl pair-correlated speed distributions, their peaks and average, and the average c.m. translational energies all show better agreement between theory and experiment, with small deviations attributed mainly to the high-speed tails. Likewise, the rotational energy distributions of the HCl and H2O monomer fragments show satisfactory agreement between theory and experiment. Trajectory calculations show that the dissociation lifetime is considerable and during each trajectory, terminating in either HCl or H2O monomer fragments, H-bonds are broken and reformed many times, until the monomer detaches completely, leaving a cyclic trimer as the cofragment. Such behavior is typical in the VP of other small clusters of HCl and/or H2O, where the rate limiting step is often the initial coupling of the excited OH or HCl stretch vibration to other intramolecular and intermolecular vibrations of the cluster, followed by energy randomization in the excited cluster and finally dissociation. 97 Except for the branching ratios of Pathways 1 and 2, which are calculated to be nonstatistical, energy partitioning in the other degrees of freedom appear statistical-like, as it is typical of clusters with high density of vibrational states. Overall, the VP of the HWWW is typical of that of other neutral clusters of comparable size, and does not show evidence of impending ionization. 98 Chapter 4 References 1. Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243-4247. 2. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. 3. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chem. Rev. 2016, 116 (9), 4913-4936. 4. Scheiner, S., Noncovalent Forces. Springer: 2015. 5. Bondybey, V. E.; Beyer, M.; Achatz, U.; Joos, S.; Niedner-Schatteburg, G., Israel Journal of Chemistry 1999, 39 (3‐4), 213-219. 6. Forbert, H.; Masia, M.; Kaczmarek-Kedziera, A.; Nair, N. N.; Marx, D., Journal of the American Chemical Society 2011, 133 (11), 4062-4072. 7. Masia, M.; Forbert, H.; Marx, D., The Journal of Physical Chemistry A 2007, 111 (49), 12181-12191. 8. Andot, K.; Hynes, J. T., Journal of Molecular Liquids 1995, 64 (1), 25-37. 9. Sugawara, S.; Yoshikawa, T.; Takayanagi, T.; Tachikawa, M., Chemical Physics Letters 2011, 501 (4), 238-244. 10. Chaban, G. M.; Gerber, R. B.; Janda, K. C., The Journal of Physical Chemistry A 2001, 105 (36), 8323-8332. 11. Mancini, J. S.; Bowman, J. M., Phys. Chem. Chem. Phys. 2015, 17 (9), 6222-6226. 12. Vargas-Caamal, A.; Cabellos, J. L.; Ortiz-Chi, F.; Rzepa, H. S.; Restrepo, A.; Merino, G., Chemistry – A European Journal 2016, 22 (8), 2812-2818. 99 13. Guggemos, N.; Slavíček, P.; Kresin, V. V., Physical Review Letters 2015, 114 (4), 043401. 14. Packer, M. J.; Clary, D. C., The Journal of Physical Chemistry 1995, 99 (39), 14323- 14333. 15. Odde, S.; Mhin, B. J.; Lee, S.; Lee, H. M.; Kim, K. S., The Journal of Chemical Physics 2004, 120 (20), 9524-9535. 16. Zischang, J.; Skvortsov, D.; Choi, M. Y.; Mata, R. A.; Suhm, M. A.; Vilesov, A. F., The Journal of Physical Chemistry A 2015, 119 (11), 2636-2643. 17. Zuraski, K. Photodissociation Dynamics and Energetics of HCl-(H2O)3. University of Southern California, Los Angeles, CA, 2018. 18. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134, 15430-15435. 19. Ch'ng, L. C.; Samanta, A. K.; Wang, Y.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2013, 117, 7207-7216. 20. Czakó, G.; Wang, Y.; Bowman, J. M., The Journal of Chemical Physics 2011, 135 (15), 151102. 21. Mancini, J. S.; Bowman, J. M., The Journal of Physical Chemistry Letters 2014, 5 (13), 2247-2253. 22. Mancini, J. S.; Samanta, A. K.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2014, 118 (37), 8402-8410. 23. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700. 100 24. Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700-2709. 25. Wang, Y.; Bowman, J. M., The Journal of Chemical Physics 2011, 135 (13), 131101. 26. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2011, 115, 6903-6909. 27. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2009, 113, 10174-10183. 28. Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2010, 114 (36), 9774-9781. 29. Li, G.; Parr, J.; Federov, I.; Reisler, H., Phys. Chem. Chem. Phys. 2006, 8, 2915-2924. 30. Kwasniewski, D.; Butler, M.; Reisler, H., Physical Chemistry Chemical Physics 2019, 21 (26), 13968-13976. 31. Kleiner, I.; Brown, L. R.; Tarrago, G.; Kou, Q.-L.; Piqu é, N.; Guelachvili, G.; Dana, V.; Mandin, J.-Y., Journal of Molecular Spectroscopy 1999, 193, 46-71. 32. Western, C. M. PGOPHER, a Program for Simulating Rotational Structure. http://pgopher.chm.bris.ac.uk. 33. Western, C. M., J. Quant. Spectrosc. Radiat. Transfer 2017, 186, 221-242. 34. Yang, C.-H.; Sarma, G.; ter Muelen, J. J.; Parker, D. H.; Western, C. M., Phys. Chem. Chem. Phys. 2010, 12, 13983-13991. 35. Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H., Rev. Sci. Instrum. 2002, 73 (7), 2634-2642. 36. Mooney, J.; Kambhampati, P., The Journal of Physical Chemistry Letters 2013, 4 (19), 3316-3318. 101 37. Korolik, M.; Arnold, D. W.; Johnson, M. J.; Suchan, M. M.; Reisler, H.; Wittig, C., Chemical Physics Letters 1998, 284 (3), 164-170. 38. Rudić, S.; Ascenzi, D.; Orr-Ewing, A. J., Chemical Physics Letters 2000, 332 (5), 487- 495. 39. Green, D. S.; Bickel, G. A.; Wallace, S. C., Journal of Molecular Spectroscopy 1991, 150 (2), 303-353. 40. Green, D. S.; Bickel, G. A.; Wallace, S. C., Journal of Molecular Spectroscopy 1991, 150 (2), 354-387. 41. Green, D. S.; Bickel, G. A.; Wallace, S. C., Journal of Molecular Spectroscopy 1991, 150 (2), 388-469. 42. Keutsch, F. N.; Cruzan, J. D.; Saykally, R. J., Chem. Rev. 2003, 103, 2533. 43. Jung, J. O.; Gerber, R. B., The Journal of Chemical Physics 1996, 105 (23), 10332-10348. 44. Western, C. M., (private communication). 45. Fárník, M.; Weimann, M.; Suhm, M. A., J. Chem. Phys. 2003, 118, 10120. 102 Chapter 5: Phenol-Water Dimer In this chapter, the vibrational predissociation (VP) dynamics and dissociation energy of the phenol-H2O dimer were studied by detecting H2O fragments and using velocity map imaging to infer the internal energy distribution of the phenol (PhOH) cofragment, pair-correlated with selected rotational levels of the H2O fragments. Following infrared excitation of the hydrogen bonded OH stretch of PhOH (Pathway 1) and asymmetric OH stretch of water (Pathway 2), dissociation to H2O and PhOH were observed. The pair-correlated internal energy distributions of the PhOH cofragment derived via Pathway 1 were well described well by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. An introduction to the theoretical models were discussed in Chapter 2, while the experimental techniques were discussed in detail in Chapter 3 of this Dissertation. The work discussed in this chamber has been published previously. 1 5.1 Introduction Recent advances in biological research have created an immense drive not only to broaden but also to deepen our understanding of the fundamental interactions that govern the behavior of biologically relevant systems. Hydrogen bonds (H-bonds) play a central role in numerous biochemical structures and processes, and thus elucidating their characteristic behavior should be helpful, for example, in protein and enzyme design efforts. However, detailed experimental characterizations of the dynamics of H-bonds are sparse. This is due in large part to difficulties in isolating and studying H-bonded systems that are sufficiently small and amenable to experimental interrogation. 103 Clusters of small molecules weakly bound to water provide excellent model systems for studying H-bonds at the most fundamental level. Phenol and its derivatives are ubiquitous in biochemical systems, such as the side chain of the amino acid tyrosine, and phenolic compounds play essential roles in electron transport, signaling pathways, and other biological processes. It is therefore not surprising that numerous studies have focused on the phenol-water (PhOH-H2O) H- bonded dimer in the gas-phase and elucidated its structure, spectroscopy, and energetics. 2-20 The geometry of PhOH-H2O in the ground electronic state is shown in Figure 5.1. 10 Bond lengths and angles were determined using microwave spectroscopy. 10 The angle β between the plane of H2O and the H-bond coordinate is 108.7 o . The PhOH and H2O moieties are individually planar, but the planes are mutually nearly perpendicular. Figure 5.1: Structure of the PhOH-H2O dimer. The angle β=108.7 o represents the angle between the plane of H2O and the H-bond coordinate. 10 Experimental studies of the PhOH-H2O dimer have focused on infrared (IR) and ultraviolet (UV) spectroscopy; energy transfer following vibrational excitation; and determination of the H- bond dissociation energy (D0). Courty et al. 5 and Braun et al. 3 independently determined D0 by similar methods. In both studies, thermochemical cycles based on the energies of several ground (S0) and excited (S1) electronic state transitions in bare PhOH and PhOH-H2O were used. D0 values of 1960 ± 40 cm -1 and 1916 ± 30 cm -1 were determined by Courty et al. 5 and Braun et al. 3 , respectively. Miyazaki et al. 14 examined the vibrational dynamics of the PhOH-H2O dimer and its 104 deuterated analog by using time-resolved IR-UV pump-probe spectroscopy; they inferred the mechanisms and timescales of intramolecular vibrational energy redistribution (IVR) in the parent cluster after excitation of the H-bonded phenol OH(D) stretch vibration. The only velocity map imaging (VMI) study that examined PhOH-H2O was reported by Mazzoni et al. 12 Using available data on the electronic spectroscopy of PhOH, measurements of the electronic spectrum of PhOH-H2O, and photoelectron images of the ionized dimer, these investigators obtained a D0 value that was in good agreement with previous determinations. 3, 5 They also attempted to obtain the translational energy distribution of the PhOH fragment ion produced by two-photon two-color ionization, but due to energy and momentum conservation, the ion’s velocity was too low to allow a detailed study. Several studies from our group have employed VMI to investigate the vibrational predissociation (VP) dynamics of H-bonded dimers, as well as to derive the predissociation dynamics and accurate D0 values. 21-35 To date, direct interrogation of the H2O fragment in the VP of PhOH-H2O has not been reported. This is largely due to predissociation in the upper electronic state used for state-selected 2+1 Resonance Enhanced Multi-Photon Ionization (REMPI) detection of the H2O fragment and spectral congestion of the rovibronic transitions. 36 In this chapter, we report the first study of the energetics and VP dynamics of the PhOH-H2O dimer obtained by examining the H2O fragment. VMI was exploited to obtain complementary information on the PhOH cofragment. Our goal was to elucidate and characterize more completely the H-bond predissociation dynamics of PhOH-H2O upon excitation of the two different OH stretch vibrations: the H-bonded OH stretch fundamental of PhOH and the free OH stretch of H2O. The dynamical information derived from the experiments described herein provides fundamental insights into the H-bonding interactions in the PhOH-H2O dimer. 105 5.2 Experimental Details VP of the PhOH-H2O dimer generated in a pulsed supersonic molecular beam was studied following IR laser excitation of either the H-bonded OH stretch fundamental of PhOH or the free OH stretch of H2O. Three experimental methods were utilized in the data collection: (1) Time-of- Flight mass spectrometry (TOF-MS) combined with 2 + 1 REMPI for spectroscopic investigations of H2O fragments; (2) TOF-MS combined with 1+1 REMPI for spectroscopic investigations of the PhOH-H2O dimer; and (3) VMI for deriving internal energy distributions of the PhOH cofragment (undetected fragment) as well as for estimating D0 for PhOH-H2O → PhOH + H2O. Figure 5.2: Experimental scheme for the VP of PhOH-H2O. IR radiation excites one of the OH- stretch fundamental vibrations of PhOH-H2O. (a) The dimer is detected by 1+1 REMPI via its S1 state. (b) H2O fragments in the ground vibrational state are detected by 2+1 REMPI via the C̃ 1 B1(000) state. Figure 5.2 depicts the laser excitation scheme. Upon excitation of the OH-stretch fundamental of the PhOH or H2O moiety, energy couples to the H-bond dissociation coordinate 106 and VP ensues. The excess energy is distributed among the center-of-mass (c.m.) translational energy, the rotational levels of H2O, and the rovibrational levels of PhOH. The experimental procedures are similar to those used in previous H-bonded cluster studies. 21-27, 33 PhOH-H2O was generated in the pulsed molecular beam by bubbling He gas (Gilmore, 99.9999%) at 2 atm through 10 mL of liquid water and passing the mixture over 200 mg of solid phenol (Sigma-Aldrich, 99.5%) at room temperature (vapor pressure 0.4 Torr). PhOH was shielded from light to avoid sample degradation. The cluster sample was then expanded through a 0.5 mm orifice of a pulsed piezoelectric valve (~200 μs opening time) operating at 10 Hz. Expansion conditions (H2O and PhOH concentration, and He backing pressure) were optimized to maximize the signal of the PhOH-H2O dimer and minimize the concentration of higher order phenol-water clusters. The skimmed molecular beam was intersected at right angles by two counter-propagating laser beams in the interaction region. IR radiation [1.5 mJ/pulse, ~0.4 cm -1 linewidth, focused by a 20-cm focal length (f.l.) lens] excited the H-bonded OH stretch or the free OH of PhOH-H2O at 3522 cm -1 and 3744 cm -1 , respectively. IR radiation was generated by an optical parametric oscillator/amplifier (OPO/OPA) system (LaserVision), pumped by radiation from a seeded Nd:YAG laser (Continuum Precision II 8000). The IR frequency was calibrated using the photoacoustic spectrum of gaseous NH3. 37 UV radiation for the detection of H2O at 80,353-80,808 cm -1 was generated by frequency- doubling (Inrad Autotracker III) the output of the dye laser (Continuum ND 6000, Coumarin 500) pumped by a Nd:YAG laser (Continuum Surelite); the spectra were frequency calibrated by the known 2+1 REMPI spectrum of H2O. 36 Tightly focused UV radiation (~0.2 mJ/pulse, lens f.l. = 20 cm; 0.4 cm -1 linewidth) ionized state-selected H2O fragments while scanning through the C̃ 1 B1(000) ← X ̃ 1 A1(000) transition using 2+1 REMPI. The H2O REMPI spectra were simulated 107 using the PGOPHER program 38 with rotational constants from Yang et al. 36 From the rotational temperature of background H2O monomers in the molecular beam, we estimated the dimer temperature at 25 ± 10 K. UV radiation for the detection of PhOH-H2O at 35,998 cm -1 was generated by frequency doubling the output of the dye laser (Coumarin 540). The PhOH-H2O spectrum was frequency calibrated using published phenol-water cluster spectra at 35,995-36,400 cm -1 . 13 Unfocused UV radiation (0.3 mJ/pulse, 0.4 cm -1 linewidth) ionized the PhOH-H2O dimer by 1+1 REMPI while scanning through the S1 ← S0 band of the dimer. Spectra were collected by alternating “IR ON” and “IR OFF” conditions at each frequency. In “IR ON,” the IR laser was fired 65 ns before the UV laser, and in “IR OFF” the IR laser was fired 2 µs after the UV laser. The UV laser conditions for each experiment were varied to optimize the signal-to-noise ratio. Laser timings were adjusted by using delay generators (Stanford, DG535) controlled through a GPIB interface (National Instruments). The VMI and TOF-MS arrangement has been described previously. 21-27, 33 Briefly, the apparatus consists of a four-electrode ion acceleration assembly, a 60-cm field-free drift tube, and a microchannel plate (MCP) detector coupled to a phosphor screen (Beam Imaging Solutions, Inc.) that is monitored by a charge coupled device (CCD) camera (LaVision, Imager). In VMI mode, two-dimensional projections were collected using an event counting method (DaVis) and reconstructed to three-dimensional images using the BASEX method. 39 Speed distributions were obtained by summing over the angular distribution of each radius and were converted to c.m. ET distributions using momentum conservation, the appropriate Jacobian, 40 and calibration constants obtained from previous experiments. 24 108 5.3 Results and Discussion 5.3.1 IR Depletion Spectrum Figure 5.3: IR Depletion spectra of (a) the H-bonded OH stretch and (b) the free OH stretch of PhOH-H2O. The dimer is probed using 1+1 REMPI via the S1 ← S0 transition at 35,998 cm -1 . The IR timing alternates between ON/OFF conditions at each frequency. The H-bonded and free OH stretch fundamental transitions of the PhOH-H2O dimer have previously been assigned and characterized in the gas phase. 6, 7, 13, 17, 18, 20, 41 In this study, IR depletion spectra were obtained by scanning the frequency of the IR laser while monitoring the vibrationless ground state of PhOH-H2O by REMPI via the S1 ← S0 transition. The position and shape of the dimer peaks agree with the previously reported depletion spectra. Figure 5.3 shows the depletion spectra of the H-bonded and free OH stretch transitions of PhOH-H2O. The observed vibrational band for the H-bonded OH stretch, centered at 3522 cm -1 , is well isolated from neighboring OH stretching bands of higher order clusters such as PhOH-(H2O)2 (3505 cm -1 and 3550 cm -1 ) 20 and (H2O)3 (3536 cm -1 ) 23 . The observed vibrational band of the free OH stretch, centered at 3744 cm -1 , is located between the close-lying free OH stretch vibrations of PhOH- 109 (H2O)2 and (H2O)2 at 3725 cm -1 and 3729 cm -1 , respectively, and the v1 symmetric stretch of the H2O monomer at 3755 cm -1 . 42 High backing pressure and H2O concentration can result in the formation of higher clusters; therefore, we optimized the expansion conditions to minimize the formation of larger clusters, as described in Section 2. The temperature of the molecular beam was adjusted to maximize the concentration of dimers. It is essential to ensure that the H2O fragments are produced following one-photon absorption by PhOH-H2O. To achieve this, great care was taken to minimize multi- photon absorption. This was achieved by reducing the IR laser fluence and slightly defocusing the radiation passing through the 20-cm IR lens. Decreasing the IR fluence lowers the signal-to-noise ratio, and the H2O signal is further reduced due to the large number of accessible fragment monomer states as well as predissociation from the upper state in the H2O REMPI scheme. 36 5.3.2 REMPI Spectroscopy of H2O Fragments In the REMPI and VMI measurements of H2O fragments, the H-bonded and free OH stretch fundamentals were excited at 3522 cm -1 (Pathway 1) and 3744 cm -1 (Pathway 2). The excitation energy was (more than) sufficient to induce VP. Figure 5.4 displays the “IR ON – IR OFF” 2+1 REMPI spectra of H2O fragments following VP. Spectra were obtained by scanning the UV laser frequency in the region of the C̃ 1 B1(000) ← X ̃ 1 A1(000) H2O transition. As stated previously, fast predissociation in the C̃ state and spectral congestion limit the state-selective detection of H2O. Nevertheless, the 2+1 REMPI spectra were simulated fairly well by rotational temperatures. The H2O fragment rotational distribution via Pathway 1 was fit best with a temperature of 165 ± 25 K, which corresponds to an average rotational energy of 115 ± 17 cm -1 . The excess energy in VP in this case is 1562 ± 60 cm -1 . In the experiments, rotational levels of the H2O fragments up to !” 2 # ,2 $ = 71,6 (704 cm -1 energy) could be clearly observed. The REMPI 110 spectrum obtained for Pathway 2 was fit with a rotational temperature of 310 ± 25 K, corresponding to an average rotational energy of 216 ± 17 cm -1 . The excess energy in this case is 1784 ± 60 cm -1 , and rotational levels up to !” 2 # ,2 $ = 85,3 and 83,6 were observed, which have energies of 1255 and 1006 cm -1 , respectively. The higher H2O fragment temperature of Pathway 2 is consistent with these observations. This can be clearly seen in Figure 5.4 by comparing the signal intensities in the low rotational energy region (> 80480 cm -1 ) and the high rotational energy region (< 80480 cm -1 ). Figure 5.4: 2 +1 REMPI spectra of H2O fragments recorded via the C̃ 1B1(000) ← X ̃ 1A1(000) transition. The “IR ON – IR OFF” spectrum (black) was obtained by exciting (a) the H-bonded OH stretch of the PhOH moiety at 3522 cm -1 , and (b) the free OH stretch of the H2O moiety at 3744 cm -1 . The “IR OFF” spectrum, obtained by recording the background when the IR laser was fired 2 μs after the UV laser pulse, was subtracted from the “IR ON” spectrum in which the IR laser was fired 65 ns before the UV laser. The arrows mark the !′ 2 # ,2 $ ← !” 2 # ,2 $ transitions monitored in the VMI studies: (a) 20,2 ← 32,1, 20,2 ← 42,3, and 71,7 ← 71,6 and (b) 20,2 ← 32,1 and 20,2 ← 42,3. Assignments are based on simulated spectra (blue) created by the PGOPHER program. 38 There is not enough excess energy following excitation of the H-bonded OH stretch to populate one quantum of bending vibration of H2O at 1595 cm -1 . 43 However, the excess energy is just high enough to populate this level when exciting the free OH stretch. We searched for evidence 111 of this excitation in the 2+1 REMPI spectrum obtained via the C̃ 1B1(000) ← X ̃ 1A1(010) H2O transition, but the signal-to-noise ratio was far too low to obtain evidence for this pathway. 5.3.3 Velocity Map Imaging of the H2O Fragment The isolated rotational transitions of H2O (000) fragments used for imaging were: !′ 2 # ,2 $ ← !” 2 # ,2 $ = 20,2 ← 32,1, 20,2 ← 42,3, and 71,7 ← 71,6. The energies of the ground electronic state rotational levels of these transitions are 212, 300, and 704 cm -1 , respectively. The pair-correlated ET distributions were derived from the images as described in Section 2, and using conservation of energy: hvIR + Eint (PhOH-H2O) = D0 + ET + Erot (H2O) + Erot,vib (PhOH) Eq. 5.1 In Eq. 5.1, hvIR denotes the excitation energy of one of the OH stretch vibrations; Eint (PhOH-H2O) is the internal energy of the dimer, estimated at about 20 cm -1 based on the temperature of H2O monomers in the beam; Erot (H2O) is the energy of the monitored rotational level of H2O; and Erot,vib (PhOH) is the rovibrational energy of the PhOH cofragment. The state-specific c.m. ET distributions encode dynamical information about the VP process and provide indirect measurements of the internal energy distributions of the PhOH cofragments pair-correlated with each monitored H2O rotational level. Below we describe separately results obtained following excitation of the PhOH H-bonded OH stretch (Pathway 1) and the H2O free OH stretch (Pathway 2). 5.3.3.1 Pathway 1 Figure 5.5 displays the c.m. ET distributions obtained following excitation of the H-bonded OH stretch of the PhOH moiety of PhOH-H2O by monitoring several H2O !” 2 # ,2 $ levels. The arrows indicate the maximum allowed translational energies corresponding to D0 = 1960 cm -1 , the 112 value measured by Courty et al. 5 The observed end points of all three images are in good agreement with this value, as well as with those reported by Neusser et al. (1916±30 cm -1 ) 3 and Mazzoni et al. (1975±60 cm -1 ) 12 . The angular distributions of the images were isotropic, reflecting the fact that the lifetime of the dimer is of the order of tens of picoseconds. 14 Based on the findings of Miyazaki et al. 14 and considering the high density of rovibrational states in the PhOH cofragment, we did not expect distinct structures in the images. Indeed, the ET distributions obtained by monitoring H2O fragments with different internal energies show no reproducible structures, and the three images sample the entire range of energetically accessible states. The shapes of the ET distributions as well as the high density of states of the PhOH cofragment suggest that the distributions following VP via Pathway 1 are statistical-like. Therefore, the observed ET distributions were compared to statistical predictions—specifically, the microcanonical prior distribution of product energies. 44 The prior distributions provide a good first-order picture of statistical behavior because they are based on an unbiased “democratic” model of state populations that imposes no constraints other than energy conservation (Chapter 2.2) The model is based on volumes in phase space for each degree of freedom and involves no dynamics. It has been used successfully, for example, in assessing shapes of distributions in chemical reactions proceeding via a bound intermediates, 45 unimolecular reactions, 46 and predissociation of dimers, 47 where detailed phase space calculations are unfeasible. For the case of the PhOH-H2O dimer, the relative probability of observing products with translational energy ET at energy E = Eavail – Erot (H2O) is: 44 F 0 (Ea; E)HEa = I ( ( E ( ) I +I%,'-J (E−E ( ) HEa Eq. 5.2 113 where I ( (E ( ) is the translational density of states and I +I%,'-J (E−E ( ) is the rovibrational density of sates of the phenol fragment at energy E−E ( . This simple model gives pair-correlated microcanonical statistical ET distributions to which the measured distributions are compared. The calculated distributions for the three monitored levels of H2O are shown as the smooth blue lines in the right panels of Fig 5.5 along with the background subtracted experiment ET distributions. Neglecting angular momentum conservation should not significantly alter the internal energy distributions of the PhOH cofragment because of its high density of internal states. The model also assumes complete IVR among levels, disregarding symmetry. On the other hand, as demonstrated before, the rotational angular momentum of the fragments cannot be too large because there is insufficient anisotropy in the potential energy surface of weakly bound dimers to support a large torque. 29, 31, 32, 44 We therefore limited the rotational angular momentum of the PhOH fragment to 115 cm -1 (165 K) based on the temperature of the H2O fragment obtained experimentally from Pathway 1. 114 Figure 5.5: Left column: “IR ON” (red) and “IR OFF” (black) c.m. translational energy distributions obtained by monitoring state-selected H2O fragments in !” 2 # ,2 $ levels: (a) 32,1, (c) 42,3, and (e) 71,6 after excitation of the H-bonded OH stretch of PhOH (Pathway 1). Right column: “IR ON – IR OFF” (red) distributions for the same state-selected H2O fragments compared with prior distributions (blue), (b), (d), (f), respectively. The black arrows indicate the maximum allowed translational energies corresponding to D0 = 1960 cm-1.5 This was the value used in the prior calculations as the maximum available energy. 115 Figure 5.6: Calculated rovibrational density of states of the PhOH cofragment as a function of energy. In our experiments the available energies are between 858 and 1350 cm -1 for Pathway 1. We computed the harmonic vibrational density of states of the PhOH cofragment using the Beyer-Swinehart algorithm with fundamental vibrational levels listed in Roth et al. 48 and Schumm et al. 46 The algorithm counts all possible harmonic vibrational levels up to the maximum accessible energy (1562 cm -1 , in our case), and therefore provides a lower limit for the density of vibrational states. Although PhOH is an asymmetric top, for the purpose of our computations, it was sufficient to approximate it’s geometry as an oblate symmetric top with A = B and C rotational constants of 0.0597 and 0.1885 cm -1 , respectively. 19 In our implementation, the rotational levels are first counted at discrete energy intervals and folded in before counting the density of vibrational states. The procedure is similar to the one described in ref. 47, which also provides sample computer programs. 47 This procedure gave the dependence of the rovibrational density of states of PhOH, ρrot,vib, on the available energy, as shown in Figure 5.6. As expected, the density of rovibrational states is high, and can reach 1 x 10 5 /cm -1 when the fragment translational energy is low. Finally, we obtained the prior ET distributions for the maximum available energies corresponding to each of the monitored H2O !” 2 # ,2 $ rotational levels. The pair-correlated prior ET distributions are in good agreement with the experimental results, and they support our assertion that the VP of PhOH-H2O is statistical-like when the H-bonded OH stretch fundamental is excited. 116 5.3.3.2 Pathway 2 To the best of our knowledge this is the first report of the VP dynamics of a mixed dimer of water (HX-H2O) induced by excitation of the free OH stretch vibration of the H2O moiety. This study was made possible because in contrast to many other dimers of H2O, the fundamental vibrational transition of the free OH stretch of PhOH-H2O was separated from IR transitions of other clusters, as described above. The experiments, however, are more challenging for this pathway because the free OH stretch has a lower oscillator strength than the H-bonded OH transition, and at the same time, the excitation requires using low IR fluences to minimize multiphoton dissociation of larger clusters. As a result, the signals were much smaller than those obtained when exciting the H-bonded OH stretch. Nevertheless, two isolated rotational levels of the H2O (000) fragment could be utilized for imaging. Figure 5.7 shows the c.m. ET distributions derived from velocity map images when monitoring the transitions: 20,2 ← 32,1 and 20,2 ← 42,3. The c.m. ET distributions for Pathway 2 appear qualitatively different from those observed for Pathway 1. Indeed, pair-correlated prior distributions fail to capture accurately the behavior of the lower-ET region, which corresponds to higher cofragment internal energies; it appears that the populations of the higher rovibrational levels of the PhOH cofragment are underestimated by the prior statistical model. We compared our results to a model proposed by Ewing, 49-51 who used the energy gap law to predict trends in the VP rates of van der Waals dimers of small molecules. In addition to predicting higher VP rates when the energy gap is minimized, Ewing proposed that the “relaxation channel of a vibrationally excited molecule is efficient only when the total change in effective quantum numbers for the process is small.” 49 In other words, the energy transfer process favors channels with the smallest change in quantum numbers, resulting in a propensity to populate vibration over rotation over translational energy release. To exhibit this propensity, we used an 117 exponentially decaying function to fit the observed ET distributions. This function included a single fit parameter, C, which was the same for both rotational states shown in Figure 5.7: b = 2 " 60 + Eq. 5.3 As seen in Figure 5.7, the ET distributions obtained by VMI are fit better with this function than with the statistical prior distribution. Figure 5.7: Left column: “IR ON” (red) and “IR OFF” (black) c.m. translational energy distributions obtained by monitoring state-selected H2O fragments in !” 2 # ,2 $ levels: (a) 32,1 and (c) 42,3, after excitation of the free OH stretch of the H2O moiety. Right column: “IR ON – IR OFF” (red) signals for the same state-selected H2O, fragments fitted with an exponential decaying function (blue), (b) and (d), respectively. The black dashed lines represent the corresponding prior distributions. The black arrows indicate the maximum allowed translational energies corresponding to D0 = 1960 cm -1 . 5 This was the value used also in the prior calculations and in the decaying function fits. 118 5.3.3.3 VP mechanism The internal energy distributions in fragments of H-bonded dimers following VP range from clearly nonstatistical to statistical-like. When the two subunits of the dimer have a low density of internal states, the internal state distributions often obey propensity rules suggested by Ewing as described briefly above. 49-51 In these cases IVR is incomplete, and the internal state distributions conform to the momentum (or energy) gap law. 29, 31, 32, 44, 52, 53 Three dimers containing H2O (or an isotope thereof) as a subunit, H2O-HCl 21, 24 , H2O-H2O 22, 25 and D2O-D2O, 22 were examined experimentally in detail. In all three cases the rovibrational energy distribution of the H2O fragment exhibited a clear propensity to populate rovibrational states with high internal energy. Thus, the distributions were found to be nonstatistical obeying Eq. 5.3, similar to those of other dimers with small subunits. 35, 53 Statistical-like distributions were observed only when the H2O was part of a larger cluster, such as (H2O)3 23 or HCl-(H2O)3. 26, 27 In these larger clusters the density of states is large and the couplings between the subunits are more efficient; which in turn leads to complete IVR prior to VP. The VP of PhOH-H2O is intermediate between the cases discussed above. Due to the large density of states in PhOH, the dimer may exhibit efficient IVR following excitation of the OH stretch. Miyazaki et al. 14 determined energy transfer rates in bare PhOH and PhOH-H2O by using real time picosecond IR-UV measurements, and found that following excitation of the H-bonded OH stretch of PhOH in PhOH-H2O, IVR in the dimer was faster than in bare PhOH. 14 By analyzing the rise and fall curves of several internal levels of the PhOH moiety populated by IVR in PhOH- H2O and PhOD-D2O, they estimated that the IVR lifetime in these dimers is of the order of 10-30 ps, which is shorter by about a factor of 4-5 than the corresponding VP lifetime, estimated at 40 and 100 ps, respectively. 14 They used a model based on anharmonic force fields 14, 16 to examine 119 both IVR within the PhOH moiety and the slower energy transfer process involving the intermolecular modes. They also concluded that the experimental VP rates were in reasonable agreement with the ones calculated by RRKM theory. 14 The results reported here following excitation of the H-bonded OH stretch of PhOH-H2O show statistical energy distributions in the PhOH fragments and therefore reinforce the previous conclusion that complete IVR precedes VP. Achieving complete IVR prior to dissociation is the first step to a statistical internal energy product distribution. Moreover, the test afforded by our results is more rigorous because it is based on pair-correlated distributions in the PhOH fragment; this removes the effects of inherent averaging over some degrees of freedom. The H2O fragment rotational distribution, inferred from the REMPI spectrum, is well described by a temperature of 165 K and appears to be statistical as well. We are not aware of any experiments that investigated the IVR in PhOH-H2O following excitation of the free OH stretch of H2O. However, we can use for comparison results of IVR in the PhOH dimer following excitation of its H-bonded and free OH stretch vibrations studied by Ebata et al. 54 The authors of that study observed clear site specificity in the IVR lifetimes, which were 5 and 14 ps for the H-bonded and free OH, respectively. The latter value was similar to the IVR lifetime determined for the PhOH monomer. On the other hand, they found that the ensuing VP rate was faster for free OH excitation than for the H-bonded case, apparently due to incomplete IVR prior to dissociation. Our results, which indicate nonstatistical internal energy distributions in the PhOH cofragments, also suggest incomplete IVR prior to dissociation. Furthermore, the H2O fragment rotational temperature following excitation of the free OH stretch is significantly higher than the corresponding one for the H-bonded OH stretch. Predissociation in the upper electronic state of H2O used for the REMPI detection and spectral 120 congestion prevent us from observing a clear propensity to populate high rotational levels of H2O; however, for free OH excitation we detected rotational levels higher than those observed with H- bonded stretch excitation. Also, the increase in rotational temperature—from 165 to 310 K—seems to be larger than what would be expected by the slight increase in excitation energy for the free OH stretch excitation. We, therefore, suggest that following free OH stretch excitation, VP in PhOH-H2O takes place before complete IVR in the PhOH moiety is achieved, and that the rovibrational energy distributions are hotter than expected by statistical considerations. 5.4. Conclusions The VP dynamics of the phenol-water (PhOH-H2O) dimer were studied by using velocity map imaging of H2O fragments to infer the internal state distributions of PhOH co-fragments, pair- correlated with selected rotational levels of the H2O fragment. The parent cluster was excited at two different frequencies corresponding to the H-bonded OH stretch of the PhOH (Pathway 1) and the free OH stretch of the H2O (Pathway 2) of the PhOH-H2O dimer. We conclude that the predissociation dynamics depends on the OH stretch level initially excited. The results found in this study for Pathway 1, as well as previous results involving PhOH-H2O 14 and the PhOH dimer, 54 suggest that complete IVR occurs prior to VP. On the other hand, our results for Pathway 2, inferred from the c.m. ET distributions of VMI and the H2O 2+1 REMPI spectrum, suggest incomplete IVR prior to VP. Pathway 2 shows a propensity to populate rovibrational levels of PhOH higher in energy than those predicted by a statistical model, and show better agreement with an energy-gap model. 121 Chapter 5 References 1. Kwasniewski, D.; Butler, M.; Reisler, H., Physical Chemistry Chemical Physics 2019, 21 (26), 13968-13976. 2. Bandyopadhyay, I.; Lee, H. M.; Kim, K. S., J. Phys. Chem. A. 2005, 109, 1720-1728. 3. Braun, J. E.; Mehnert, T.; Neusser, H. J., Int. J. Mass Spectrom. 2000, 203, 1-18. 4. Castleman, A. W.; Stanley, R. J., J. Chem. Phys. 1991, 94 (12), 7744-7756. 5. Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millié, P., J. Phys. Chem. A 1998, 102 (25), 4890-4898. 6. Doi, A.; Naohiko, M., J. Chem. Phys. 2008, 129, 154308. 7. Ebata, T.; Mizuochi, N.; Watanabe, T.; Naohiko, M., J. Chem. Phys. 1996, 100, 546-550. 8. Fuke, K.; Kaya, K., Chem. Phys. Lett. 1983, 94 (1), 97-101. 9. Gerhards, M.; Kleinermanns, K., J. Chem. Phys. 1995, 103 (17), 7392-7400. 10. Gerhards, M.; Schmitt, M.; Kleinermanns, K.; Stahl, W., J. Chem. Phys. 1996, 104 (3), 967-971. 11. Lipert, R. J.; Bermudez, G.; Colson, S. D., J. Phys. Chem. 1988, 92 (13), 3801-3805. 12. Mazzoni, F.; Pasquini, M.; Pietraperzia, G.; Becucci, M., J. Mol. Struc. 2015, 1090, 2-6. 13. Mikami, N., Bull. Chem. Soc. Jpn. 1995, 68 (3), 683-694. 14. Miyazaki, Y.; Inokuchi, Y.; Ebata, T.; Petkovic, M., Chem. Phys. 2013, 419, 205-211. 15. Oikawa, A.; Abe, H.; Mikami, N.; Mitsuo, I., J. Phys. Chem. 1983, 87 (25), 5083-5090. 16. Petković, M., J. Phys. Chem. A 2011, 116, 364-371. 17. Shimamori, T.; Fujii, A., J. Phys. Chem. A 2015, 119, 1315-1322. 18. Watanabe, T.; Ebata, T.; Tanabe, S.; Mikami, N., J. Chem. Phys. 1996, 105 (2), 408-419. 122 19. Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K., J. Chem. Phys. 1996, 104 (3), 972-982. 20. Tanabe, S.; Ebata, T.; Fujii, A.; Mikami, N., Chem. Phys. Lett. 1993, 215 (4), 347-352. 21. Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2010, 114 (36), 9774-9781. 22. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134, 15430-15435. 23. Ch'ng, L. C.; Samanta, A. K.; Wang, Y.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2013, 117, 7207-7216. 24. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2011, 115, 6903-6909. 25. Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134, 211101. 26. Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243-4247. 27. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. 28. Mancini, J. S.; Samanta, A. K.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2014, 118 (37), 8402-8410. 29. McCaffery, A. J.; Pritchard, M.; Reisler, H., J. Phys. Chem. 2009, 112, 412-418. 30. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2009, 113, 10174-10183. 123 31. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. 32. Pritchard, M.; Parr, J.; Li, G.; Reisler, H.; McCaffery, A. J., Phys. Chem. Chem. Phys. 2007, 9, 6241-6252. 33. Samanta, A. K.; Ch'ng, L. C.; Reisler, H., Chem. Phys. Lett. 2013, 575, 1-11. 34. Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700-2709. 35. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chem. Rev. 2016, 116 (9), 4913-4936. 36. Yang, C.-H.; Sarma, G.; ter Muelen, J. J.; Parker, D. H.; Western, C. M., Phys. Chem. Chem. Phys. 2010, 12, 13983-13991. 37. Kleiner, I.; Brown, L. R.; Tarrago, G.; Kou, Q.-L.; Piqu é, N.; Guelachvili, G.; Dana, V.; Mandin, J.-Y., Journal of Molecular Spectroscopy 1999, 193, 46-71. 38. Western, C. M., J. Quant. Spectrosc. Radiat. Transfer 2017, 186, 221-242. 39. Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H., Rev. Sci. Instrum. 2002, 73 (7), 2634-2642. 40. Mooney, J.; Kambhampati, P., J. Phys. Chem. Lett. 2013, 4 (19), 3316-3318. 41. Ebata, T.; Watanabe, T.; Mikami, N., J. Phys. Chem. 1995, 99 (16), 5761-5764. 42. Kuma, S.; Slipchenko, M. N.; Kuyanov, K. E.; Momose, T.; Vilesov, A. F., J. Phys. Chem. A. 2006, 110, 10046-10052. 43. Shimanouchi, T., National Bureau of Standards 1972, 1, 1-160. 44. McCaffery, A. J., Phys. Chem. Chem. Phys. 2004, 6, 1637-1657. 124 45. Roth, W.; Imhof, P.; Gerhards, M.; Schumm, S.; Kleinermanns, K., Chem. Phys. 2000, 252, 247-256. 46. Schumm, S.; Gerhards, M.; Roth, W.; Gier, H.; Kleinermanns, K., Chem. Phys. Lett. 1996, 263, 126-132. 47. Baer, T.; Hase, W. L., Unimolecular Reaction Dynamics: Theory and Experiments. Oxford University Press, Inc.: New York, NY, 1996. 48. Roth, W.; Imhof, P.; Gerhards, M.; Schumm, S.; Kleinermanns, K., Chem. Phys. 2000, 252, 247-256. 49. Ewing, G. E., J. Phys. Chem. 1987, 91, 4662. 50. Ewing, G. E., J. Chem. Phys. 1980, 72, 2096. 51. Ewing, G. E., J. Phys. Chem. 1979, 71, 3143. 52. Miller, R. E., Acc. Chem. Res. 1990, 23 (1), 10-16. 53. Miller, R. E.; Oudejans, L., Annu. Rev. Phys. Chem. 2001, 52, 607-637. 54. Ebata, T.; Kayano, M.; Sato, S.; Mikami, N., J. Phys. Chem. A 2001, 105, 8623-8628. 125 Chapter 6: Pyrazine-H2O This chapter details initial experimental and theoretical investigations of the vibrational predissociation (VP) of aromatic heterocyclic nitrogen molecules hydrogen-bonded (H-bonded) to water. We report the first spectral signature of the pyrazine-H2O dimer by excitation via the S2 ← S0 transition (),) ∗ ). We also report the first successful observation of the VP of the pyrazine- H2O dimer by exciting the “free” OH stretch of the water moiety and the CH stretch region of the pyrazine moiety, followed by detecting the H2O fragment as well as examining IR depletion spectroscopy of the parent complex. To support our experimental findings, we performed electronic structure calculations to compute the geometries, vibrational frequencies, and ionization potentials of pyrazine, the pyrazine-H2O dimer, and the pyrazine-(H2O)2 trimer. The results described herein serve as a preliminary diagnostic for future experiments. 6.1 Introduction Biological processes occur in an aqueous environment and, thus, the understanding of H- bonding is of practical interest to both the experimental and theoretical communities. 1 H-bonded molecules, especially those containing heterocyclic nitrogen atoms, play a fundamental role in the structure and function of many biological systems. 2, 3 In the case of oligonucleotides, heterocyclic nitrogen atoms are essential in base-pairing, and determine the tertiary structure and function of biopolymers. 2 Pyrazine, a model molecule for components of proteins and nucleotides, is particularly appealing in this respect. However, detailed experimental characterizations of the dynamics of H-bonds are sparse. This is due in large part to difficulties in isolating and studying H-bonded systems that are sufficiently small and responsive to experimental interrogation. It had been speculated for many years that the ) electron cloud of an aromatic ring could act as a hydrogen-bond acceptor. The position of the water moiety with respect to the ) electron 126 cloud was first confirmed by rotational spectroscopy of the benzene-water dimer obtained in supersonic expansion; the study demonstrates that the water molecule acts as a nearly free rotor with both hydrogens pointed towards the ) electron cloud above the plane of the benzene ring. 4, 5 In recognition that pyrazine, which contains a heterocyclic nitrogen atom, could potentially H- bond via the ) electron cloud or the nitrogen lone pairs, extensive experimental spectroscopic studies were pursued. 6-13 Figure 6.1: Structure of the H-bonded Pyrazine-H2O dimer Evidence of H-bond formation between pyrazine and water has been obtained from electronic spectra recorded in solution 8, 14 and in argon matrix studies, 9 but efforts to observe these complexes in supersonic expansion have been unsucessful. 10 Recently, the structure of the pyrazine-water dimer (Figure 6.1) was determined, demonstrating that the water moiety is H- bonded to the nitrogen atom in the ground electronic state. 15 This is in agreement with the study by Baba et. al in which H-bonds between hydrogen bonding solvent molecules and the pyrazine solute resulted in a large blue shift in the (8,) ∗ ) absorption spectrum, but showed only minimal changes to the fluorescence spectrum. 14 127 To date, direct interrogation of the vibronic spectrum of the pyrazine-H2O dimer has not been successful in the gas phase. Recent theoretical investigations by Cai and Reimers 16 suggest that the pyrazine-H2O dimer, upon electronic excitation to the (8,) ∗ ) state, weakens in H-bond strength by 1.0 kcal/mol. In contrast, upon excitation to the (),) ∗ ) state, the H-bond strength increases by 0.4 kcal/mol. Cai and Reimers’ calculations also predict that the (),) ∗ ) absorption spectrum of the pyrazine-H2O dimer should display a small red shift (~160 cm -1 ), in contrast to the expected blue shift (~500 cm -1 ) upon absorption to the (8,) ∗ ) state. The predicted shifts are consistent with previous experimental observations in solution 14 and theoretical calculations 17 for the (8,) ∗ ) state. This prediction might also explain the unsuccessful results of Wanna et al., 10 who used supersonic molecular jet spectroscopy and two-color Time-of-Flight Mass Spectrometry (TOF-MS) to study the H-bonded pyrazine-H2O dimer through the (8,) ∗ ) state. Cai and Reimers 16 predict that vertical excitation to the (8,) ∗ ) state will cause rapid predissociation of the dimer, whereas the vertical excitation energy to the (),) ∗ ) state leaves the excited dimer with internal energy less than or equal to the its dissociation energy, suggesting that the pyrazine-H2O dimer may be long-lived in the (),) ∗ ) state. Based on Cai and Reimers’ prediction of the long-lived nature of the (),) ∗ ) state, the direct interrogation of the Pyrazine-H2O dimer utilizing 1+ n REMPI is presented here. In this study, we report the first successful detection of the pyrazine-H2O dimer in the vibrational ground state of through excitation of the (),) ∗ ) state. Detection of the H2O fragment following VP of the dimer was observed using TOF-MS and 2+1 REMPI spectroscopy. Previous extensive studies of the pyrazine monomer with room temperature samples, which exhibited broad spectral features, were reproduced to give clearer spectral calibration references. 6, 7, 9-12 Electronic structure calculations were performed to corroborate our findings. This work serves as a foundation for 128 future experiments that will more extensively examine the VP of the pyrazine-H2O dimer by using Velocity Map Imaging, and extend them to new investigations of the pyrazine-(H2O)2 trimer. 6.2 Experimental and Theoretical Details 6.2.1 Experimental Details Vibrational predissociation (VP) of the pyrazine-H2O dimer was generated in a pulsed supersonic molecular beam by IR laser excitation of the OH stretch fundamental of the H2O moiety or the CH stretch of the pyrazine moiety. Two experimental methods were utilized in the data collection: (1) Time-of-Flight mass spectrometry (TOF-MS) combined with 2 + 1 REMPI for spectroscopic investigations of H2O fragments; (2) TOF-MS combined with 1+ n REMPI for spectroscopic investigations of the pyrazine-H2O dimer. Figure 6.2 depicts the laser excitation scheme. Upon excitation of the OH or CH stretch fundamental of the H2O or pyrazine moiety, respectively, energy couples to the H-bond dissociation coordinate and VP ensues. The excess energy is distributed among the center-of-mass (c.m.) translational energy, the rotational levels of H2O, and the rovibrational levels of pyrazine. The experimental procedures are similar to those used in previous H-bonded cluster studies. 18-28 Pyrazine-H2O was generated in the pulsed molecular beam by bubbling He gas (Gilmore, 99.999%) at 2 atm through 10 mL of H2O with ~2 g of solid pyrazine (Sigma-Aldrich, >99%) dissolved in solution at room temperature (vapor pressure 10.8 Torr). Pyrazine was shielded from light to avoid sample degradation. The sample was then expanded through a 0.5 mm orifice of a pulsed piezoelectric valve (~200 μs opening time) operating at 10 Hz. Expansion conditions (H2O and pyrazine concentration, and He-backing pressure) were optimized to maximize the signal of the pyrazine-H2O dimer and to minimize the concentration of higher order pyrazine-water clusters. The skimmed molecular beam was intersected at right angles by two counter-propagating 129 laser beams in the interaction region. IR radiation [1.5 mJ/pulse, ~0.4 cm -1 linewidth, focused by a 20 cm focal length (f.l.) lens] excited the H-bonded OH stretch or the CH stretch of pyrazine- H2O at 3658 cm -1 and 3040 cm -1 , respectively. IR radiation was generated by an optical parametric oscillator/amplifier (OPO/OPA) system (LaserVision), pumped by radiation from a seeded Nd:YAG laser (Continuum Precision II 8000). The IR frequency was calibrated using the photoacoustic spectrum of gaseous NH3. 29 Figure 6.2: Experimental scheme for the VP of pyrazine-H2O. IR radiation excites the OH or CH stretch fundamental vibrations of pyrazine-H2O. (a) The dimer is detected by 1+ n REMPI via the S2 ← S0 transition (),) ∗ ). (b) H2O fragments in the ground vibrational stated are detected by 2+1 REMPI via the C̃ 1 B1(000) state UV radiation for the detection of H2O at 80,353-80,808 cm -1 was generated by frequency- doubling (Inrad Autotracker III) the output of the dye laser (Continuum ND 6000, Coumarin 500) pumped by a Nd:YAG laser (Continuum Surelite); the spectra were frequency calibrated by the known 2+1 REMPI spectrum of H2O. 30 Tightly focused UV radiation (~0.2 mJ/pulse, lens f.l. = 20 cm; 0.4 cm -1 linewidth) ionized state-selected H2O fragments while scanning through the C̃ 130 1 B1(000) ← X ̃ 1 A1(000) transition using 2+1 REMPI. The REMPI spectrum of H2O was simulated using the PGOPHER program with rotational constants from Yang et al. 30 From the rotational temperature of background H2O monomers in the molecular beam, we estimated the dimer temperature at 25 ± 10 K. UV radiation for the detection of pyrazine-H2O at 36,818 cm -1 was generated by frequency doubling the output of the dye laser (Coumarin 540). Unfocused UV radiation (0.3 mJ/pulse, 0.4 cm -1 linewidth) ionized the pyrazine-H2O dimer by 1+ n REMPI while scanning through the S2 ← S0 band of the dimer. Spectra were collected by alternating “IR ON” and “IR OFF” conditions at each frequency. In “IR ON,” the IR laser was fired 65 ns before the UV laser, and in “IR OFF” the IR laser was fired 2 µs after the UV laser. The UV laser conditions for each experiment were varied to optimize the signal-to-noise ratio. Laser timings were adjusted by using delay generators (Stanford, DG535) controlled through a GPIB interface (National Instruments). Pyrazine was ionized and detected through the S2← S0 (),) ∗ ) band and was used to frequency calibrate the position of the pyrazine-H2O dimer using previously published pyrazine spectra. 6, 31 The VMI and TOF-MS arrangement has been described previously. 18-28 Briefly, the apparatus consists of a four-electrode ion acceleration assembly, a 60-cm field-free drift tube, and a microchannel plate (MCP) detector coupled to a phosphor screen (Beam Imaging Solutions, Inc.) that is monitored by a charge coupled device (CCD) camera (LaVision, Imager). In VMI mode, two-dimensional projections were collected using an event counting method (DaVis) and reconstructed to three-dimensional images using the BASEX method. 32 Speed distributions were obtained by summing over the angular distribution of each radius and were converted to c.m. ET distributions using momentum conservation, the appropriate Jacobian, 33 and calibration constants obtained from previous experiments. 21 131 6.2.2 Theoretical Details Electronic structure calculations took place using the QChem 5.2 suite of programs. 34 Geometry optimizations of pyrazine, pyrazine-H2O, and pyrazine-(H2O)2 were performed on the ground state using at the Resolution-of-Identity (RI) MP2 level of theory 35 with the aug-cc-pVTZ basis set. 36 Single-point energy calculations of the optimized geometries were carried out at the CCSD(T) level 37 with the aug-cc-pVTZ basis set. The relative energies of the different conformers of the pyrazine-H2O and pyrazine-(H2O)2 clusters were calculated. Vibrational frequencies were calculated at the RIMP2 level of theory with the aug-cc-pVTZ basis set in order to identify the frequencies of the CH and OH stretch fundamentals. In addition, single-point energy calculations at the CCSD(T)/aug-cc-pVTZ level of theory were carried out for the pyrazine cation and the two conformers of the pyrazine-H2O cluster ion. Vertical ionization energies were calculated as the difference between the ground state single-point energy and the ion single-point energy at the same geometry. Errors arising from using the electron configuration of the ground state as a single electron configuration reference of the ion, as well as the difference between the vertical and adiabatic ionization energies, were assessed by carrying out multireference calculations using the Equation-of-Motion ionization potential (EOM-IP) CCSD level of theory 38 with the aug-cc-pVTZ basis set, and with the doubly and triply excited determinants correction to the EOM-CC method. 132 6.3 Results and Discussion of the VP of the Pyrazine-H2O Dimer 6.3.1 REMPI Spectroscopy of the Pyrazine Monomer and Pyrazine-H2O Dimer After a solution of pyrazine in water was introduced into the He carrier gas, the REMPI spectrum of the pyrazine monomer ion was monitored at m/z=80 (Figure 6.3). Additionally, a different REMPI spectrum was observed when monitoring the pyrazine-water dimer at m/z = 98, which is the mass of the pyrazine-water dimer (Figure 6.4). As mentioned earlier, Pyrazine-H2O dimer has not been previously detected using REMPI as an ionization scheme and, moreover, the ionization potential and energetics of the dimer are still experimentally unknown. In our study, we excite the dimer to the S2 state with one photon via the S2 ← S0 (),) ∗ ) transition at 36350-39600 cm -1 , ionize it with one or more photons (1 + n =1, 2, 3….), and detect it at m/z = 98 using TOF- MS. The jet-cooled spectrum of the pyrazine monomer measured at 15 K shows well-separated lines but increasing congestion towards shorter wavelengths. Previously recorded S2 ← S0 transition (),) ∗ ) spectra for the pyrazine monomer between 36400-43500 cm -1 (230-275 nm) typically reveal several broad band transitions with no defining vibrational structure at room temperature. 11, 34, 39 Both spectra become progressively congested towards shorter wavelengths like those recorded in previous studies involving the pyrazine monomer. 11, 40 Figure 6.3 shows a comparison of the REMPI spectrum of the (),) ∗ ) transition utilizing supersonic molecular beam with gas phase absorption spectrum obtained using synchrotron radiation in a gas cell, both spectra were taken while monitoring pyrazine (m/z = 80). 39 133 Figure 6.3: S2 ← S0 transition (),) ∗ ) spectra of Pyrazine + detected by 1+ n REMPI using supersonic molecular beam (black) compared to room temperature experimental results (red) from Stener et. al (2011). 39 Figure 6.4: S2 ← S0 transition (),) ∗ ) spectra of Pyrazine-H2O + detected by 1+ n REMPI 134 Figure 6.5: (top) (),) ∗ ) spectrum of Pyrazine + at m/z = 80 and (bottom) (),) ∗ ) spectrum of Pyrazine-H2O + at m/z = 98 detected by 1+ n REMPI. Pyrazine + peaks are representative of the cold monomer and the products of dissociative ionization of larger clusters including the Pyrazine- H2O dimer. The pyrazine-H2O dimer was ionized by REMPI through the S2 ← S0 transition (),) ∗ ). The pyrazine-H2O spectrum was recorded by scanning the frequency of the UV laser from 36350- 39600 cm -1 as shown in Figures 6.4 and 6.5. The spectrum shows narrow lines, attesting to the stability of the excited dimer as was predicted by the theoretical investigation by Cai and Reimers. 16 We are able to distinguish between species formed in the expansion and those that are the dissociation products of higher clusters by determining the kinetic energy released (KER) for each REMPI peak using velocity map imaging (VMI). Nascent species created in the supersonic expansion have zero KER, whereas fragments either of neutral or ionic clusters show non-zero KER. Some of the observed pyrazine monomer peaks have the same spectral positions as the pyrazine-water dimer, and these have non-zero KER. In this way, we are able to filter spectral lines 135 of nascent species from those of dissociation fragments. We used spectral lines of the dimer that were cold translationally and showed only small signals from pyrazine products. (Figure 6.5). 6.3.2 IR Depletion Spectroscopy Figure 6.6: IR Depletion spectrum of the CH stretch region of pyrazine-H2O. The dimer is probed via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternates between “IR ON” (red) and “IR OFF” (black) conditions at each frequency. The CH stretch fundamental transitions for the pyrazine monomer have been assigned previously. 13, 41 The vibrational spectrum of the pyrazine-H2O dimer has not been previously assigned or characterized in the gas phase. IR depletion spectrum of pyrazine-H2O was obtained by scanning the CH stretch region between 2960-3120 cm -1 , while monitoring the vibrationless ground state by REMPI via the S2 ← S0 transition and m/z = 98 in TOF-MS at 36,818 cm -1 (Figure 6.6). In this way we have identified three spectral features that we assign as CH stretch vibrations— at 3105-3120, 3060-3080, and 3040-3050 cm -1 . These bands are close in energy to those in the corresponding pyrazine monomer, which is expected because the CH bonds do not participate 136 directly in H-bonding. The observed vibrational bands, summarized in Table 6.1, are compared to the vibrational assignments by Breda et al. 13 as well as Martin and van Alsenoy 41 for the pyrazine monomer detected in Argon matrix and vapor phase, respectively. Table 6.1: C-H stretch fundaments (in cm -1 ) for pyrazine obtained from Breda et al. 13 compared to the vibrational positions of pyrazine-H2O dimer from this work. Mode a Monomer Argon Matrix b Crystal T = 10 K b Vapor c Theory b Pyrazine-H 2O d 1 3062 3102 3105-3120 2 (12) 3085.9 19 3067.7 3064.2 3069 3096.1 3060-3080 15 2961.2 3051.2 3069 3082.6 11 3053 3081.5 2 + 16 3021.1 3034.3 3040-3050 12 + 16 3016.2 3011.2 2 + 20 3002.9 12 + 20 2969.1 2973.1 Published values from: (a) Mode numbering follows what is used by Boese and Martin 42 (b) Calculated vibrational frequencies, and observed infrared spectra for pyrazine monomer isolated in argon matrix and crystal phase. 13 (c) Raman active bands were obtained for the melted compounds from Martin and Van Alsenoy 41 , which includes reassignments from Ref [ 13 ]. (d) Observed IR Depletion spectra for pyrazine-H2O in our current work. The IR depletion spectra of the “free” OH stretch region were obtained for pyrazine-H2O through the same detection scheme described above. Figure 6.7 shows the depletion spectrum of the pyrazine-H2O following excitation of the “free” OH stretch. The observed vibrational band for the OH stretch is centered at 3658 cm -1 (with a small shoulder at 3664 cm -1 ). Originally, when looking for depletion near the OH stretch region, we expected an IR peak for the H-bonded OH stretch of water at 3400-3500 cm -1 , and a peak for the “free” OH at 3600-3700 cm -1 , based on our electronic structure calculations (See Section 6.4) and comparisons with similar dimers involving 137 the N—H H-bond. We did see efficient depletion at 3658 cm -1 , however, we have not yet been able to observe depletion at 3400-3500 cm -1 , which is surprising. We repeated this measurement by monitoring depletions in other vibronic lines of the dimer, but depletion for the H-bonded OH stretch has still not been successfully detected. The IR depletion of pyrazine-H2O is only observed with the addition of pyrazine into the molecular beam and at m/z = 98 in TOF-MS measurements. Figure 6.7: IR Depletion spectrum of the OH stretch of Pyrazine-H2O. The dimer is probed using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternates between “IR ON” and “IR OFF” conditions at each frequency. 138 6.3.3 REMPI spectroscopy of H2O Fragments In this study, we have measured 2+1 REMPI signals of H2O fragments following the excitation of the “free” OH stretch fundamental at 3658 cm -1 and the CH stretch region of the dimer, demonstrating that the excitation energy was more than sufficient to induce VP. Because water fragment signals can also be generated by the predissociation of water clusters and higher order pyrazine-water clusters, the correspondence in frequencies between dimer depletion, and fragment signal is a reassuring observation. Figure 6.8 displays the “IR ON” and “IR OFF” 2+1 REMPI spectra of H2O fragments following excitation in the “free” OH fundamental transition and VP. Spectra were obtained by scanning the UV laser frequency in the region of the C̃ 1 B1(000) ← X ̃ 1 A1(000) H2O transition and fixing the IR laser frequency at the maximum of the depletion peak. As stated previously (see Chapter 3.3), fast predissociation in the C̃ state and spectral congestion limit the state-selective detection of H2O. Rotational levels of the H2O fragments up to ! 2 # ! 2 $ = 71,6 (704 cm -1 energy) were observed in our experiments. It is not known if there is enough excess energy following excitation of the OH stretch to populate one quantum of bending vibration of H2O at 1595 cm -1 . 43 We searched for evidence of this excitation in the 2+1 REMPI spectrum obtained via the C̃ 1B1(000) ← X ̃ 1A1(010) H2O transition, but the signal-to-noise ratio was far too low to obtain evidence for this pathway. The 2+1 REMPI spectra of the water monomer fragments were simulated fairly well by rotational temperatures. Temperatures of the water monomers and fragments were estimated by comparison to a PGOPHER simulation, which includes experimentally determined predissociation lifetimes of rotational levels in the C̃ 1B1(000) state. 27, 28, 30 The H2O fragment rotational distribution through excitation of the OH stretch was fit best with a temperature of 150 ± 25 K, which corresponds to an average rotational energy of 104 ± 17 cm -1 . Rotational level assignments 139 are also based on simulated spectra created by the PGOPHER program. 44 The temperature of the background H2O monomer signal was estimated to be 15 ± 5 K based on REMPI scans under “IR OFF” conditions. Experiments will soon be underway by future group members to determine the KER distribution of the fragments by monitoring selected rotational levels of water using VMI. Figure 6.8: 2 +1 REMPI spectra of H2O fragments recorded via the C̃ 1B1(000) ← X ̃ 1A1(000) transition. The “IR ON” (red) and “IR OFF” (black) spectrum were obtained by exciting the OH stretch vibration of the H2O moiety at 3658 cm -1 . The background was recorded when the IR laser was fired 2 μs after the UV laser pulse for “IR OFF” conditions. The IR laser was fired 65 ns before the UV laser for “IR ON” conditions. 140 6.3.4 IR “Action” Spectroscopy of the Pyrazine-H2O dimer The IR photofragment yield spectroscopy, also referred to as “IR action spectroscopy,” for the detection of water following the VP of the dimer is shown in Figures 6.9 and 6.10 in the region of OH stretch and CH stretch, respectively. High backing pressure and H2O concentration can result in the formation of higher clusters; therefore, we optimized the expansion conditions to minimize the formation of larger clusters, as described previously. 26-28 Figure 6.9: IR “Action” spectra (top) of the “free” OH stretch region detecting H2O + photofragments in J”Ka,Kc = 32,1 following the vibrational predissociation of the pyrazine-H2O dimer . IR Depletion spectra (inset) of the “free” OH stretch region of pyrazine-H2O is shown for comparison. The dimer was probed using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing was alternated between “IR ON” and “IR OFF” conditions at each frequency. Note: Daily experiments can take several hours to complete and liquid nitrogen cooling of the detection chamger will not be efficient after 8 hours of continuous use resulting in a gradual increase of the background water signal (See Chapter 3.5.1). 141 To maximize the signal from the mixed dimer, it was essential to ensure that the H 2O fragments were produced following one-photon absorption. Great care was taken to minimize multi-photon absorption by reducing the IR laser fluence (thereby lowering the signal-to-noise ratio), and by slightly defocusing the radiation passing through the 20 cm IR lens. The H 2O signal was further reduced due to the large number of accessible monomer fragment states as well as predissociation from the upper state in the H 2O REMPI scheme. 30 Production of the H 2O + photofragment was not observed when pyrazine was the only species present in the molecular beam. Figure 6:10: IR “Action” spectra (top) of the CH stretch region detecting H2O + photofragments in J”Ka,Kc = 32,1 following the vibrational predissociation of the pyrazine-H2O dimer . IR Depletion spectra (bottom) of the CH stretch region of pyrazine-H2O. The dimer was probed using 1+ n REMPI via the S2 ← S0 transition at 36,818 cm -1 . The IR timing alternated between “IR ON” and “IR OFF” conditions at each frequency. 142 6.4 Theoretical Calculations Geometry optimizations of pyrazine, pyrazine-H2O, and pyrazine-(H2O)2 complexes were performed for the ground state using QChem 5.2 34 at the Resolution-of-Identity (RI) MP2 35 level of theory with the aug-cc-pVTZ 36 basis set. The choice of basis functions is similar to the work of Cai and Reimers on ground and excited states of pyrazine-water clusters, 16, 45 albeit with a wavefunction method rather than a density functional. Table 6.2 shows the bond lengths and angles obtained from optimized geometries of pyrazine and of different conformers of pyrazine-H2O and pyrazine-(H2O)2 clusters (See Appendix C.1 for Geometry Z-matrix). Single-point energy calculations of the optimized geometries were carried out at the CCSD(T) 37 level of theory with the aug-cc-pVTZ basis set (See Appendix C.2). The optimized geometries of the Pyrazine-H2O agree with previous theoretical studies performed by Cai and Reimers. 16 From the single-point calculations of the total basis set energy, the relative energies of the different conformers of the pyrazine-H2O dimer and pyrazine-(H2O)2 trimer were calculated, as shown in Figure 6.11 and 6.12, respectively. The lowest two geometrical configurations for the Pyrazine-H2O dimer were the “side” conformer, where H2O is hydrogen bonded to the nitrogen atom of the pyrazine ring. The next geometry was the “top” conformer, which has water hydrogen bonded to the pyrazine ) electron cloud. The “top” conformation was found to be 3.73 kcal/mol higher in relative energy to the “side” conformer, which is in agreement with Cai and Reimers 16 and was to be expected based on the electronegativity of the nitrogen atom. This was also predicted with other azobenzenes such as pyridine and pyrimidine. 45 Theoretical calculations for the pyrazine-(H2O)2 trimer, show that the “bridge” structure shown in Table 6.2 and Figure 6.12 is preferred for the H2O-Pyrazine-H2O configuration, because 143 the complex is stabilized by a second (weaker) H-bond between the ring and the other water moiety. Larger cooperativity is expected in the bridge structure than in the linear structure, where each water has an equal sharing of the lone pair electrons located on each of the nitrogen atoms of the diazine ring. Similar bridge configurations have been found in theoretical investigations of pyrimidine-water clusters 46 . Vibrational frequencies were calculated at the RIMP2 level of theory with the aug-cc- pVTZ basis set in order to identify vibrational stretches characteristic of H-bonded cluster in molecular beam experiments. Table 6.3 shows the calculated values of different OH stretches for the pyrazine-H2O clusters. The deviation between experiment and theory was assessed by comparison of calculated frequencies of several water clusters, especially those including H—N H-bond. 144 Figure 6.11: Schematic diagram of the relative energies of pyrazine-(H2O)2 dimer structure. Figure 6.12: Schematic diagram of the relative energies of pyrazine-(H2O)2 trimer configurations. 145 Table 6.2: Optimized geometries of pyrazine, pyrazine-H2O and pyrazine-(H2O)2 at the RIMP2/aug-cc-pVTZ level of theory (Table split between 3 pages). Molecule Level of theory Bond Lengths and Angles Pyrazine RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.082 Å C—C = 1.392 Å C—N = 1.339 Å Pyrazine-H 2O (side) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.082 Å C—C = 1.392 Å C—N = 1.339 Å H2O “free” O—H = 0.961 Å “H-bonded” O—H = 0.974 Å HB length/Angle H•••N = 1.941Å NNH = 176.01 o HOH = 104.56 o Pyrazine-H 2O (top) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.083 Å C—C = 1.394 Å C—N = 1.339 Å H2O “H-bonded” O—H = 0.963 Å O—H = 0.963 Å HB length/Angle H•••N = 2.798 Å H•••N = 2.708 Å NHO = 132.38 o NHO = 150.51 o HOH = 103.74 o 146 H 2O-Pyrazine-H 2O (Z) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.083 Å C—C = 1.392 Å C—N = 1.339 Å H2O “free” O—H = 0.961 Å “H-bonded” O—H = 0.972 Å HB length/Angle H•••N = 1.971Å H•••N = 1.971 Å NHO = 152.18 o HOH = 104.94 o Close/Far to water (C)NCH = 116.53 o (F)NCH = 117.15 o H 2O-Pyrazine-H 2O (E) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.083 Å C—C = 1.392 Å C—N = 1.340 Å H2O “free” O—H = 0.961 Å “H-bonded” O—H = 0.972 Å HB length/Angle H•••N = 1.971Å H•••N = 1.971 Å NHO = 149.89 o HOH = 105.05 o Close/Far to water (C)NCH = 116.60 o (F)NCH = 117.15 o 147 Pyrazine-(H 2O) 2 (Bridge) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.083 Å C—C = 1.394 Å C—N = 1.343 Å H2O Bound to Pyrazine “free” O—H = 0.960 Å “H-bonded” O—H = 0.976 Å Bound to H2O “free” O—H = 0.961 Å “H-bonded” O—H = 0.982 Å HB length/Angle H•••N = 1.862 Å CH•••O = 2.257 Å H•••O = 1.845 Å NHO = 166.85 o HOH = 140.93 o HOH = 105.21 o HOH = 104.90 o NCH = 116.89 o Pyrazine-(H 2O) 2 (Linear) RIMP2/aug-cc-pVTZ Pyrazine C—H = 1.083 Å C—C = 1.394 Å C—N = 1.343 Å H2O “Bound to H2O” O—H = 0.967 Å “Bound to Pyrazine” O—H = 0.960 Å Acceptor Water “free” O—H = 0.962 Å HB length/Angle Bound to Pyrazine H•••N = 2.019 Å H•••O= 2.545 Å Bound to H2O OH•••O = 1.989 Å NHO = 147.85 o CHO = 112.32 o HOH = 105.42 o HOH = 104.42 o NCH = 116.44 o NCH = 117.08 o 148 Frequency calculations such as those shown in Table. 6.3 are known to be larger than the experimental values, and correction factors need to be estimated. Table 6.4 shows the deviations between calculated and experimental vibrational frequencies of different OH stretches in clusters containing hydrogen bonds. Calculations at the RIMP2/aug-cc-pVTZ level of theory are shown in Table 6.5. The difference between calculations and experimental measurements is systematic. Calculations show that, similarly to other clusters involving hydrogen bonding to nitrogen, there is a significant separation between the “free” OH stretch (~3600-3700 cm -1 ) and the bonded OH stretch (~3400-3500 cm -1 ). Furthermore, for the bridge and linear configurations of the pyrazine- (H2O)2 trimer, the separation between the OH stretch bound to oxygen and the OH stretch bound to nitrogen is also significant (~100 cm -1 ). Our results and comparisons to spectra of other azines and diazines confirm the expectation of the H-bonded OH stretch of the dimer in the range of 3400-3500 cm -1 . Additional single-point energy calculations at the CCSD(T)/aug-cc-pVTZ level of theory were carried out for the ions of pyrazine and conformers of the pyrazine-H2O cluster. 149 Table 6.3: Vibrational frequencies in cm -1 , calculated at the RIMP2/aug-cc-pVTZ level of theory, of the stretches of the water moieties of several pyrazine-H2O clusters. † Molecule OH (free) OH---O OH---N Pyrazine-H2O (side) 3904.36 NA 3603.69 Pyrazine-H2O (top) NA NA 3803.29 a 3919.79 b H2O-Pyrazine-H2O (Z) 3906.58 c 3905.96 d NA 3642.18 c 3637.09 d H2O-Pyrazine-H2O (E) 3908.95 c 3908.40 d NA 3642.06 c 3637.16 d Pyrazine-(H2O)2 (bridge) 3893 .05 f 3905.86 e 3597.35 3446.55 Pyrazine-(H2O)2 (linear) 3817.67 a 3936.49 b 3795.08 3679.23 †Data in Table 6.3 is displayed without experimental correction factors found in Table 6.5 a Symmetric stretch. b Asymmetric stretch. c Symmetric stretch between water moieties. d Asymmetric stretch between water moieties. e Free OH stretch of the HOH---O water moiety. Table 6.4: Comparison between calculated vibrational frequencies (at the RIMP2/aug-cc-pVTZ level of theory) and experimental measurements. Molecule OH (free) OH---O OH---N Theory Experiment Theory Experiment Theory Experiment H2O 3822.87 a 3948.59 b 3655.8 47 3755.1 47 - - - - (H2O)2 3814.44 a 3934.69 b 3914.54 c 3600 48 3730 48 3714 48 3716.72 3602 22 - - (H2O)3 3906.69 d 3905.70 d 3904.63 d - - - 3578.99 f 3641.67 g 3651.40 - - 3536 23 - - Ammonia-H2O 3908.38 - - - 3588.20 3485 19 t-Butylamine- H2O 3898 3715 49 - - 3464.97 3420 49 Trimethylamine- H2O 3899.04 3720 49 - - 3419 3375 49 Phenol-H2O 3810.16 a 3931.37 b 3744 26 3640.37 3522 26 - - Pyridine-H2O 3903.79 3701 50 - - 3553.46 50 3400 50 Pyrimidine-H2O 3904.78 3703 50 - - 3615.49 3468 50 a Symmetric stretch. b Asymmetric stretch. c Free OH stretch of the HOH---O water moiety. d Vibrational frequencies calculated for each water molecule. f Symmetric stretch between two water molecules. g Asymmetric stretch between two water molecules. 150 Table 6.5: Correction factor required for best fit of calculations with experimental values. Experiment / Theory (%) Average Std. deviation Free OH 95.05 0.36 OH --- O 96.34 0.81 OH --- N 97.41 1.49 In order to better characterize and understand the REMPI detection scheme for the pyrazine-H2O dimer, additional single-point energy calculations at the CCSD(T)/aug-cc-pVTZ level of theory were carried out for the ions of pyrazine and the two lowest energy two conformers of the pyrazine-H2O dimer (Appendix C.3). Vertical excitation energy, Ev, without Zero-point Energy (ZPE) correction, was calculated as the difference between the ground state single-point energy and the ion single-point energy at the same geometry. Error arising from using the electron configuration of the ground state as a single electron configuration reference of the ion, 38, 51 as well as the relation between the vertical energy and the ionization potential, was assessed by carrying out calculations with multireference using the Equation-of-Motion ionization potential (EOM-IP) CCSD 51 level of theory with the aug-cc-pVTZ basis set (Figure 6.13). 52 Calculation of Ev was also implemented by Cai and Reimers 16 as an initial benchmark because it closely reflects the ionization potential of the molecule with an error of ~0.2 eV. EOM-IP-CCSD was used to calculate the energy of removing the electron in the manner of Koopman’s theorem (but more rigorously), which means that zero-point energy correction is not needed. Calculation of the Ionization Potential by EOM-IP-CCSD is close to Ev for the monomer and dimer, and shows good agreement with the pyrazine monomer experimental data available in the NIST database. EOM-IP CCSD reveals the pyrazine-H2O dimer to have an IP of 10.13 eV; however, with a 1+1 REMPI, we would only be able to excite the dimer and reach 9.0 - 9.8 eV upon absorption of a second photon. This 151 explains why we designate our tentative assignment as a 1 + n REMPI scheme, where n may be any number of photons used to reach or exceed the ionization of the dimer. Figure 6.13: Schematic diagram of the vertical excitation energy, Ev, and ionization potential, IP, in eV of pyrazine and two lowest energy conformers of the pyrazine-H2O dimer at the CCSD(T) and EOM-IP-CCSD levels of theory with aug-cc-pVTZ basis set. The energy values were calculated using the difference between the total energy of the ion and neutral molecule. The light blue line separates the dimer from the monomer, where the dimer structures are displayed energetically relative only to each other. Table 6.6 shows Ev as well as the ionization potential using the EOM-IP-CCSD/aug-cc- pVTZ level of theory for pyrazine and pyrazine-H2O clusters. It can be seen that the calculated Ev is in good agreement with the ionization potential calculated from the EOM-IP-CCSD method. In the case of pyrazine, the vertical energy (9.71 eV) and the ionization potential from EOM-IP- CCSD (9.76 eV) are close to the vertical ionization potential of 9.63 eV. 7 152 Table 6.6: Vertical energy, Ev, and ionization potential, IP, in eV of Pyrazine and Pyrazine-H2O clusters at the CCSD(T) and EOM-IP-CCSD levels of theory with the aug-cc-pVTZ basis set. Molecule IP (EOM-IP-CCSD) Pyrazine 9.71 9.76 Pyrazine-H2O (side) 10.29 10.13 Pyrazine-H2O (top) 10.00 10.08 In order to understand the reason why we could not observe the expected H-bonded OH vibration, we are taking a closer look at the ionization of pyrazine and its dimer. Further work will comprise the calculations of ionization potentials of the conformers of pyrazine-H2O using the EOM-CC-[2,3]/aug-cc-pVTZ level of theory. This information will allow us to assess the nature of the ionization potential with a multireference method that makes use of triple excitations. A lower value of the ionization potential with respect to the one calculated from EOM-IP-CCSD or by CCSD(T) will indicate that the calculations needed for molecular beam experiments are well described by Ev, from CCSD(T) single-point calculations, or by using a multireference method with only single and double excitations like EOM-IP-CCSD. With this information, calculation of the vertical and adiabatic ionization potentials for the conformers of pyrazine-(H2O)2 trimer will be carried out. 6.5 Conclusion In summary, we report the first observation of the VP of the pyrazine-H2O dimer following excitation of the “free” OH stretch and CH stretch region, which was confirmed by the detection of neutral H2O products using REMPI. Following pulsed supersonic expansion, significant rovibrational cooling of the (", " ∗ ) state was observed for the pyrazine monomer and pyrazine- H2O dimer. We also present here the first report of pyrazine-H2O detected through 1+ n REMPI using TOF-MS. VMI measurements allow us to distinguish between translationally cold pyrazine 153 generated in the molecular beam and pyrazine molecules generated in dissociative ionization of higher clusters, which possess kinetic energy. Theoretical calculations confirmed that the OH- stretch vibrational peak observed in our experiments can be assigned to the non-bonded or “free” hydrogen of the water moiety. Theoretical calculations to characterize the structure and stability of the pyrazine-H2O dimer and trimer and their cations are ongoing. Further VMI experiments are underway to observe the VP dynamics of the formation of the H2O fragment as well as the detection and characterization of Pyrazine-(H2O)2. 154 Chapter 6 References 1. Zwier, T. S., Annual Review of Physical Chemistry, 1996, 47, 205-241. 2. Jeffrey, G. A. and W. Saenger, Hydrogen bonding in biological structure, Springer-Verlag: Berlin1991. 3. Scheiner, S., Noncovalent Forces, Springer2015. 4. Gutowsky, H. S., T. Emilsson and E. Arunan, J. Chem. Phys, 1993, 99, 4883. 5. Suzuki, S., P. G. Green, R. E. Bumgarner, S. Dasgupta, W. A. Goddard and G. A. Blake, Science, 1992, 257, 942. 6. Bolovinos, A., P. Tsekeris, J. Philis, E. Pantos and G. Andritsopoulos, Journal of Molecular Spectroscopy, 1984, 103, 240-256. 7. Gleiter, R., E. Heilbronner and V. Hornung, Helvetica Chimica Acta, 1972, 55, 255-274. 8. Marzzacco, C., Journal of the American Chemical Society, 1973, 95, 1774-1777. 9. Rossetti, R. and L. E. Brus, The Journal of Chemical Physics, 1979, 70, 4730-4736. 10. Wanna, J., J. A. Menapace and E. R. Bernstein, The Journal of Chemical Physics, 1986, 85, 777-784. 11. Yamazaki, I., T. Murao, T. Yamanaka and K. Yoshihara, Faraday Discussions of the Chemical Society, 1983, 75, 395-405. 12. Turner, R. E., V. Vaida, C. A. Molini, J. O. Berg and D. H. Parker, Chemical Physics, 1978, 28, 47-54. 13. Breda, S., I. D. Reva, L. Lapinski, M. J. Nowak and R. Fausto, Journal of Molecular Structure, 2006, 786, 193-206. 14. Baba, H., L. Goodman and P. C. Valenti, Journal of the American Chemical Society, 1966, 88, 5410-5415. 155 15. Caminati, W., L. B. Favero, P. G. Favero, A. Maris and S. Melandri, Angewandte Chemie International Edition, 1998, 37, 792-795. 16. Cai, Z. L. and J. R. Reimers, J. Phys. Chem. A, 2007, 111, 954-962. 17. Monte, S. A. d., T. Müller, M. Dallos, H. Lischka, M. Diedenhofen and A. Klamt, Theoretical Chemistry Accounts, 2004, 111, 78-89. 18. Casterline, B. E., A. K. Mollner, L. C. Ch'ng and H. Reisler, J. Phys. Chem. A, 2010, 114, 9774-9781. 19. Mollner, A. K., B. E. Casterline, L. C. Ch'ng and H. Reisler, J. Phys. Chem. A, 2009, 113, 10174-10183. 20. Rocher-Casterline, B. E., L. C. Ch’ng, A. K. Mollner and H. Reisler, J. Chem. Phys., 2011, 134, 211101. 21. Rocher-Casterline, B. E., A. K. Mollner, L. C. Ch'ng and H. Reisler, J. Phys. Chem. A, 2011, 115, 6903-6909. 22. Ch'ng, L. C., A. K. Samanta, G. Czakó, J. M. Bowman and H. Reisler, J. Am. Chem. Soc., 2012, 134, 15430-15435. 23. Ch'ng, L. C., A. K. Samanta, Y. Wang, J. M. Bowman and H. Reisler, J. Phys. Chem. A, 2013, 117, 7207-7216. 24. Parr, J. A., G. Li, I. Federov, A. J. McCaffery and H. Reisler, J. Phys. Chem. A, 2007, 111, 7589-7598. 25. Samanta, A. K., L. C. Ch'ng and H. Reisler, Chem. Phys. Lett., 2013, 575, 1-11. 26. Kwasniewski, D., M. Butler and H. Reisler, Physical Chemistry Chemical Physics, 2019, 21, 13968-13976. 156 27. Zuraski, K., D. Kwasniewski, A. K. Samanta and H. Reisler, J. Phys. Chem. Lett., 2016, 7, 4243-4247. 28. Zuraski, K., Q. Wang, D. Kwasniewski, J. M. Bowman and H. Reisler, J. Chem. Phys., 2018, 146, 204303. 29. Kleiner, I., L. R. Brown, G. Tarrago, Q.-L. Kou, N. Piqu é, G. Guelachvili, V. Dana and J.-Y. Mandin, Journal of Molecular Spectroscopy, 1999, 193, 46-71. 30. Yang, C.-H., G. Sarma, J. J. ter Muelen, D. H. Parker and C. M. Western, Phys. Chem. Chem. Phys., 2010, 12, 13983-13991. 31. Streibel, T., F. Hafner K Fau - Mühlberger, T. Mühlberger F Fau - Adam, R. Adam T Fau - Zimmermann and R. Zimmermann. 32. Dribinski, V., A. Ossadtchi, V. A. Mandelshtam and H. Reisler, Rev. Sci. Instrum., 2002, 73, 2634-2642. 33. Mooney, J. and P. Kambhampati, J. Phys. Chem. Lett., 2013, 4, 3316-3318. 34. Shao, Y., Z. Gan, E. Epifanovsky, A. T. B. Gilbert, M. Wormit, J. Kussmann, A. W. Lange, A. Behn, J. Deng, X. Feng, D. Ghosh, M. Goldey, P. R. Horn, L. D. Jacobson, I. Kaliman, R. Z. Khaliullin, T. Kuś, A. Landau, J. Liu, E. I. Proynov, Y. M. Rhee, R. M. Richard, M. A. Rohrdanz, R. P. Steele, E. J. Sundstrom, H. L. Woodcock, P. M. Zimmerman, D. Zuev, B. Albrecht, E. Alguire, B. Austin, G. J. O. Beran, Y. A. Bernard, E. Berquist, K. Brandhorst, K. B. Bravaya, S. T. Brown, D. Casanova, C.-M. Chang, Y. Chen, S. H. Chien, K. D. Closser, D. L. Crittenden, M. Diedenhofen, R. A. DiStasio, H. Do, A. D. Dutoi, R. G. Edgar, S. Fatehi, L. Fusti-Molnar, A. Ghysels, A. Golubeva-Zadorozhnaya, J. Gomes, M. W. D. Hanson-Heine, P. H. P. Harbach, A. W. Hauser, E. G. Hohenstein, Z. C. Holden, T.-C. Jagau, H. Ji, B. Kaduk, K. Khistyaev, J. Kim, J. Kim, R. A. King, P. Klunzinger, D. 157 Kosenkov, T. Kowalczyk, C. M. Krauter, K. U. Lao, A. D. Laurent, K. V. Lawler, S. V. Levchenko, C. Y. Lin, F. Liu, E. Livshits, R. C. Lochan, A. Luenser, P. Manohar, S. F. Manzer, S.-P. Mao, N. Mardirossian, A. V. Marenich, S. A. Maurer, N. J. Mayhall, E. Neuscamman, C. M. Oana, R. Olivares-Amaya, D. P. O’Neill, J. A. Parkhill, T. M. Perrine, R. Peverati, A. Prociuk, D. R. Rehn, E. Rosta, N. J. Russ, S. M. Sharada, S. Sharma, D. W. Small, A. Sodt, T. Stein, D. Stück, Y.-C. Su, A. J. W. Thom, T. Tsuchimochi, V. Vanovschi, L. Vogt, O. Vydrov, T. Wang, M. A. Watson, J. Wenzel, A. White, C. F. Williams, J. Yang, S. Yeganeh, S. R. Yost, Z.-Q. You, I. Y. Zhang, X. Zhang, Y. Zhao, B. R. Brooks, G. K. L. Chan, D. M. Chipman, C. J. Cramer, W. A. Goddard, M. S. Gordon, W. J. Hehre, A. Klamt, H. F. Schaefer, M. W. Schmidt, C. D. Sherrill, D. G. Truhlar, A. Warshel, X. Xu, A. Aspuru-Guzik, R. Baer, A. T. Bell, N. A. Besley, J.-D. Chai, A. Dreuw, B. D. Dunietz, T. R. Furlani, S. R. Gwaltney, C.-P. Hsu, Y. Jung, J. Kong, D. S. Lambrecht, W. Liang, C. Ochsenfeld, V. A. Rassolov, L. V. Slipchenko, J. E. Subotnik, T. Van Voorhis, J. M. Herbert, A. I. Krylov, P. M. W. Gill and M. Head-Gordon, Molecular Physics, 2015, 113, 184-215. 35. Weigend, F., A. Köhn and C. Hättig, The Journal of Chemical Physics, 2002, 116, 3175- 3183. 36. Kendall, R. A., T. H. Dunning and R. J. Harrison, The Journal of Chemical Physics, 1992, 96, 6796-6806. 37. Raghavachari, K., G. W. Trucks, J. A. Pople and M. Head-Gordon, Chemical Physics Letters, 1989, 157, 479-483. 38. Krylov, A. I., Annual Review of Physical Chemistry, 2008, 59, 433-462. 158 39. Stener, M., P. Decleva, D. M. P. Holland and D. A. Shaw, J. Phys. B: At. Mol. Opt. Phys., 2011, 44, 075203. 40. Streibel, T., H. K., F. Mühlberger, T. Adam and R. Zimmermann, Appl . Spectrosc., 60, 72-79. 41. Martin, J. M. L. and C. Van Alsenoy, The Journal of Physical Chemistry, 1996, 100, 6973- 6983. 42. Boese, A. D. and J. M. L. Martin, The Journal of Physical Chemistry A, 2004, 108, 3085- 3096. 43. Shimanouchi, T., National Bureau of Standards, 1972, 1, 1-160. 44. Western, C. M., J. Quant. Spectrosc. Radiat. Transfer, 2017, 186, 221-242. 45. Reimers, J. R. and Z.-L. Cai, Physical Chemistry Chemical Physics, 2012, 14, 8791-8802. 46. Howard, A. A., N. I. Tschumper Gs Fau - Hammer and N. I. Hammer, J. Phys. Chem. A, 2010, 114, 6803-6810. 47. Kuma, S., M. N. Slipchenko, K. E. Kuyanov, T. Momose and A. F. Vilesov, J. Phys. Chem. A., 2006, 110, 10046-10052. 48. Page, R. H., J. G. Frey, Y. R. Shen and Y. T. Lee, Chemical Physics Letters, 1984, 106, 373-376. 49. Millen, D. J. and G. W. Mines, Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics, 1977, 73, 369-377. 50. Destexhe, A., J. Smets, L. Adamowicz and G. Maes, The Journal of Physical Chemistry, 1994, 98, 1506-1514. 51. Stanton, J. F. and J. Gauss, The Journal of Chemical Physics, 1994, 101, 8938-8944. 52. Hirata, S., M. Nooijen and R. J. Bartlett, Chemical Physics Letters, 2000, 326, 255-262. 159 Chapter 7: Future Work: “Hats Off to Benzene” This chapter details preliminary experimental work and insight into a future direction on dimers of small, biologically relevant aromatic molecules. As mentioned in the previous chapter, it is possible for H-bonds to form with the "-cloud of an aromatic ring. This is known to occur for benzene, a simple molecule ideal for the purpose of this study. In the benzene-water (Bz-H2O) dimer, for example, water is bound to the ring in a perpendicular geometry with its H atoms pointing towards the "-cloud of the ring (Figure 7.1). 1-4 The vibrational predissociation (VP) dynamics of benzene-water, currently unknown, could provide a fundamental benchmark for other aromatic molecules that may exhibit similar behavior in aqueous biological environments. Figure 7.1: Structure of the H-bonded Bz-H2O dimer. A unique capability of our research group is to determine, using VMI, pair-correlated vibrational distributions in the aromatic cofragment as a function of the monitored water fragment level (i.e. as a function of Eavail), which enables detailed mapping of the dependence of vibrational energy in the cofragment on dimer geometry and type of H-bonding. State-specific fragment detection has rarely been attempted in the dissociation of clusters, and data on state-specific energy disposal is scarce. Our studies, which address energy disposal at the pair-correlated level, would open a new window into the VP dynamics of these systems, and, together with future theoretical calculations, would provide a mechanistic description of how H-bonding and geometry influence energy disposal. 160 This chapter proposes a systematic study of energy disposal in the VP of mixed dimers of benzene with selected subunits. Noncovalent interactions of water with aromatic molecules have served as models for hydrophilic or hydrophobic interactions of molecules of chemical and biological relevance. 1, 5-11 In the proposed studies in section 7.1, we will use detection of H2O or D2O fragments in the ground (000) and bend (010) vibrational states, as in our previous work on water clusters. 12-16 In section 7.2, we will discuss the VP of Bz-HCl in order to provide a complete picture of the "-cloud binding motif. The methodology used in these proposed experiments is illustrated in Figure 6.2 and has been discussed in detail in Chapter 3 of this Dissertation and applied in Chapters 4-6. 17-19 Figure 7.2: Simplified experimental scheme for the VP of H-bonded aromatic dimers. 7.1 Benzene-H2O and Benzene-D2O Dimers of water with aromatic molecules exhibit two types of H-bonding: p-bonding, in which the water molecule is above the aromatic plane with its H atom(s) pointing toward the plane, and %-bonding, which is near planar and has a more traditional H-bonding structure, with water serving as either the donor or acceptor. 1, 2, 16, 20, 21 Ring substituents play a crucial role in H- bonding, as demonstrated in many spectroscopic studies that have been summarized in reviews 1, 2, 161 20-24 . Bz-H2O complexes are arguably the most thoroughly studied, both experimentally and theoretically, as models for interactions with the benzene "-cloud. According to theory, these interactions are dominated by dispersion. 2, 21 This is in contrast to the work described in Chapter 5 17 , where electrostatic interaction dominates the %-bonding in PhOH-H2O, which can be viewed as a substituted water dimer in which an H-atom of one water molecule is replaced by the phenyl group, with H2O serving as the acceptor. 1, 2, 21, 25-29 In matrix isolation studies 30 , OH(D) stretch fundamentals and their shifts relative to the water monomer have been identified, as have been several vibrational levels of the benzene partner. 30 The most comprehensive work on Bz-water clusters is that of Zwier and coworkers 2 will serve as a starting point for the preliminary experimental results presented below as a proof of concept. 8 7.1.1 Preliminary Experimental Details The experimental procedures in the current and future work are similar to those used in previous H-bonded cluster studies and those discussed in this chapter. 14, 17-19, 31-37 VP of the Bz- H2O and Bz-D2O dimers was induced in a pulsed supersonic molecular beam by IR laser excitation of the OH or OD stretch fundamental. The experimental method utilized in the data collection was TOF-MS combined with 1+1 REMPI for spectroscopic investigations of both dimers. Upon excitation of the OH or OD stretch fundamental, energy coupled to the H-bond dissociation coordinate and VP ensued. The excess energy was distributed among the center-of-mass (c.m.) translational energy (Et), the rotational levels of H2O or D2O, and the rovibrational levels of benzene. The sample mixture and backing pressure were optimized for maximum signal of the either Bz-H2O or Bz-D2O at a concentration of 0.6% H2O or 0.6% D2O (Millipore-Sigma, 99.9%) respectively, with 0.6% benzene (Millipore-Sigma, >99.9%) and 2 atm of a He carrier gas 162 (Gilmore, 99.999%). Samples were prepared by transferring H2O or D2O to an evacuated bulb, adding benzene by vacuum distillation, and filling the bulb with He carrier gas. At these concentrations, signals from pure water clusters and from higher order clusters were minimized (discussed in Chapter 3.5.4). The mixture was then introduced into a high vacuum chamber maintained at a base pressure of ~2.5 x 10 -8 torr. Benzene was shielded from light to avoid sample degradation. The sample was then expanded through a 0.5 mm orifice of a pulsed piezoelectric valve (~200 μs opening time) operating at 10 Hz. Expansion conditions (H2O, D2O, and benzene concentration, and He-backing pressure) were optimized to maximize the signal of the benzene dimers and to minimize the concentration of water clusters. The skimmed molecular beam was intersected at right angles by two counter-propagating laser beams in the interaction region. IR radiation [1.5 mJ/pulse, ~0.4 cm -1 linewidth, focused by a 20 cm focal length (f.l.) lens] excited the H-bonded OH/OD stretch of the Bz-H2O at 3631 cm -1 and 2671 cm -1 for Bz-D2O, respectively. IR radiation was generated by an optical parametric oscillator/amplifier (OPO/OPA) system (LaserVision), pumped by radiation from a seeded Nd:YAG laser (Continuum Precision II 8000). The IR frequency was calibrated using the photoacoustic spectrum of gaseous NH3. 38 UV radiation for the detection of Bz-D2O + and Bz-H2O + at 38,658 and 38662 cm -1 , respectively, was generated by frequency doubling the output of the dye laser (Coumarin 522B). Unfocused UV radiation (~0.15 mJ/pulse, 0.4 cm -1 linewidth) ionized the Bz-H2O and -D2O dimers by 1+1 REMPI while scanning through the S1 ← S0 band of the dimers. Spectra were collected by alternating “IR ON” and “IR OFF” conditions at each frequency. In “IR ON,” the IR laser was fired 65 ns before the UV laser, and in “IR OFF,” the IR laser was fired 2 µs after the UV laser. The UV laser conditions for each experiment were varied to optimize the signal-to-noise ratio. Laser timings were adjusted by using delay generators (Stanford, DG535) controlled through a 163 GPIB interface (National Instruments). Zwier et. al 2, 8 have previously ionized and detected the Bz-H2O and Bz--D2O dimers through the S1← S0 transition and their spectra was used to frequency calibrate our spectra for both dimers. The VMI and TOF-MS arrangement has been described previously. 14, 17-19, 31-37 Briefly, the apparatus consists of a four-electrode ion acceleration assembly, a 60-cm field-free drift tube, and a microchannel plate (MCP) detector coupled to a phosphor screen (Beam Imaging Solutions, Inc.) that is monitored by a charge coupled device (CCD) camera (LaVision, Imager). In future experiments, VMI mode will be employed to collect two-dimensional projections using an event counting method (DaVis) and to reconstruct three-dimensional images using the BASEX method. 39 Speed distributions will be obtained by summing over the angular distribution of each radius and converting to c.m. ET distributions using momentum conservation, the appropriate Jacobian, 40 and calibration constants obtained from previous experiments. 34 7.1.2 Preliminary Experimental Results As a proof of concept, preliminary one-color 1+1 REMPI spectra of the Bz-H2O and Bz- D2O dimers were monitored at m/z=96 and m/z=98, respectively, using TOF-MS (Figure 7.3). We excited each dimer to the S1 state with one photon via the S1 ← S0 transition at 38640-38800 cm -1 and ionized it with an additional photon. Zwier et. al 3, 41 used the 6 " # vibronic band in their studies of these dimers because of good S/N ratio and separation from the rovibronic congestion surrounding the 0 " " bandhead. In our experiments, the peak positions for both dimers are separated by 4 cm -1 , which is in good agreement with previous experimental studies. 2 The 6 " # vibration was used as a transition to probe the IR depletion of the dimers. One observation based on the UV spectra, is that the Bz-D2O dimer tended to have a larger overall signal in the molecular beam, which is indicative of the stability of the dimer following UV-only excitation. 164 Figure 7.3: S1 ← S0 spectrum of (red) Benzene-D2O + at m/z = 98 and (black) Benzene-H2O + at m/z = 96 detected by 1 + 1 REMPI. The 6 " # vibration for Benzene-D2O + and Benzene-H2O + were measured at 38,658 and 38662 cm -1 , respectively. The IR depletion spectra of the H-bonded OH stretch region were obtained for Bz-H2O using the same detection scheme described above. The observed vibrational band for the OH stretch is centered at 3631 cm -1 , which is in good agreement with previous experimental studies. 2 To the best of our knowledge, gas phase IR depletion spectra for Bz-D2O following the excitation of the H-bonded OD stretch were observed for this first time. The vibrational band is centered around 2671 cm -1 , which is in agreement with matrix isolation studies. 30 The IR depletion of Bz- H2O and Bz-D2O is only observed with the addition of benzene into the molecular beam and while monitoring m/z=96 and m/z=98, respectively, in the TOF-MS measurements. Figure 7.4 shows the depletion spectrum of Bz-D2O (left) following excitation of the H-bonded OD stretch and Bz- H2O (right) following excitation of the H-bonded OH stretch. 165 Figure 7.4: IR depletion spectra of (left) the fundamental OD stretch of Bz-D2O and (right) the fundamental OH stretch of Bz-H2O. The Bz-D2O and Bz-H2O dimers were probed using 1+1 REMPI via the S1 ← S0 transition at 38,658 and 38,662 cm -1 , respectively. The IR timing was alternated between “IR ON” and “IR OFF” conditions at each frequency. Figure 7.5: The S1 ← S0 transition “IR ON” –“IR OFF” spectrum of Bz-D2O + at m/z = 98 detected by 1+1 REMPI. The “IR ON”–“IR OFF” spectrum (red) was obtained by exciting the H-bonded OD stretch at 2671 cm -1 . The “IR OFF” spectrum, obtained by recording the background when the IR laser was fired 2 μs after the UV laser pulse, was subtracted from the “IR ON” spectrum in which the IR laser was fired 65 ns before the UV laser at each frequency. 166 The results described above served as a preliminary diagnostic for subsequent experiments on other H-bonded aromatic dimers of interest, PhOH-H2O (Chapter 5) and pyrazine-H2O (Chapter 6), and will also inform future experiments. COVID-19, unfortunately, interrupted the completion of the full study on benzene dimers. Continued efforts will first focus on the VP of Bz-D2O because the OD-stretch energy requires less energy for excitation, which reduces Eavail by ~ 1000 cm -1 relative to Bz-H2O. Also, the excited electronic state of D2O used in REMPI is less dissociative than that of H2O as observed in previous experiments. 35, 41 In the past, our group was able to identify rotational levels of water fragments up to J = 15. 14 As seen in our experiments and Ar matrix work, 42 the intramolecular vibrations in Bz-D2O show similar shifts to Bz-H2O, and sufficient energy is available to populate the (010) level of D2O (1170 cm -1 ). When monitoring this vibrational level, the maximum Eavail is only ~ 600 cm -1 , corresponding to low vibrational levels in the benzene cofragment. If, however, VP dynamics favor D2O (000), higher vibrational levels in benzene are likely to be excited. After observing the IR depletion spectra for Bz-D2O + , the “IR ON” –“IR OFF” REMPI spectrum via the S1 ← S0 transition of the dimer following IR excitation and VP is shown in Figure 7.5. The H-bonded OD stretch was excited at 2671 cm -1 . The excitation energy was (more than) than sufficient to induce VP, where the dissociation energy, D0, has been previously measured to be 935 cm -1 . 1 The spectrum was obtained by scanning the UV laser frequency in the region near the 6 " # vibration of the S1 ← S0 transition. Figure 7.5 also shows a large depletion signal at the position of the 6 " # vibronic band for Bz-D2O + while monitoring m/z=98. While the observed intensity of the 6 " # vibronic band of Bz-D2O + (in Figure 7.3) is low under UV-only (“IR OFF”) conditions, which is due to UV induced dimer fragmentation, the dimer depletion signal is understandably higher, because IR absorption precedes the UV radiation. TOF-MS mass 167 signatures for the benzene monomer and Bz-D2O dimer fragmentation could be observed at m/z = 13, 25, and 37. Observing these peak positions was important in determining the viability of future studies, because there are no notable overlaps with the mass peaks for H2O and D2O, at m/z = 16 and 18, respectively. As discussed in previous chapters, ionization and detection of H2O + and D2O + are accomplished via 2+1 REMPI spectroscopy, which requires a higher UV fluence. In section 7.2 of this chapter, the focus will be on the fragmentation peak at m/z = 37 in relation to proposed future experiments concerning the VP of the Bz-HCl dimer, for which the HCl + photofragment overlaps at m/z = 36. 7.1.3 Simulated Experimental Results for Bz-H2O and Bz-D2O To demonstrate the expected resolution in these experiments, we simulated pair-correlated Et distributions for dimers of benzene with H2O and D2O. As the internal energy of the H2O /D2O fragment decreases, Eavail increases, which allows for the population of higher benzene vibrational levels in the Et distributions. Figure 7.6 displays simulations using our experimental results from section 7.1.2 and the rovibrational levels of benzene. 12, 43 The expected Et release is based on monitoring selected rotational levels of H2O and D2O in the (000) and (010) states. The simulations were set with a 225 K rotational temperature, which is typical of larger fragments due to angular momentum constraints, 44-46 and similar to observations form studies involving PhOH-H2O (Chapter 5) and Pyrazine-H2O (Chapter 6). If energy transfer follows the Ewing rules, 4, 47, 48 we expect significant energy transfer to the (010) bend excitation of H2O/D2O. The Et ranges in Figure 7.6 are typical for VP experiments with dimers. Figure 7.6 (left) shows expected results for Bz (v) + H2O(000) or D2O(000), while Figure 7.6 (right) shows expected results for the Bz (v) + H2O and D2O (010) pathway. Excitation of the H-bonded H2O/D2O stretch vibration will lead to intramolecular vibrational redistribution 168 (IVR) within the benzene cofragment, which will be reflected in the ET release of the H2O/D2O fragment. For this study, we will be able to access the full ensemble of available rotational states of H2O and D2O to observe a complete picture of the VP dynamics of both dimers. The proposed benzene experiments seek to answer whether energy transfer prior to VP in the Bz-H2O and Bz- D2O dimer will be efficient resulting in complete IVR and a statistical-like pair-correlated Et distribution. Figure 7.6: Simulated ET distribution of the VP of the (top) Bz-H2O dimer and (bottom) Bz-D2O dimer. The detected fragments and states are labeled. The red lines indicate vibrational levels of benzene, 43 blue convolutes the vibrational levels with rotational levels. 169 7.2 Benzene-HCl 7.2.1 Motivation for an Experimental Study of Benzene-HCl Though this dissertation has so far focused on the VP dynamics of dimers and clusters with a more traditional linear H-bond, our research group also has an interest in the state-specific behavior of the VP of T-shaped dimers. 49-52 Pritchard et. al 46 obtained pair-correlated product state distributions in the VP of C2H2-H(D)Cl by using VMI to monitor selected H(D)Cl (J) photofragments, 46, 49, 53 where acetylene can engage in H-bonds via its "-bond. The distributions yielded D0 = 700 and 755 cm -1 for C2H2-HCl and C2H2-DCl, respectively, in excellent agreement with theory. 54, 55 Excitation of the asym-CH stretch 46, 53, 56 resulted in one quantum of CC stretch of the acetylene fragment, with the remaining energy distributed nearly statistically among available acetylene rotational levels and to translational energy, Et. 57, 58 In C2H2-DCl, 46 DCl (v = 1) was not detected, despite sufficient energy, and only acetylene bending levels were populated. The bottom line is that clear propensity rules have yet to be established for fragment vibrational excitation. In comparison, the study by Parr et. al involving C2H2-NH3, 49 where NH3 is H-bonded to the hydrogen of the C2H2 moiety through a traditional linear H-bond, showed that VP (D0 = 900 cm -1 ) proceeded only via pathways that included energy transfer across the H-bond. Namely, the NH3 fragment following asym-CH stretch excitation of the dimer contained one or two quanta in the umbrella mode, while the C2H2 fragment exhibited excited bending levels corresponding to low Et. Interestingly, other pathways with just as low an Et release were not observed. In the VP of NH3-H2O (D0 = 1540 cm -1 ), 50 excitation of the H-bonded OH stretch again yielded NH3 with one or two umbrella quanta, whereas excitation of the near-isoenergetic water bend was not observed. These results attest to the puzzling nature of vibrational state specificity. 170 In future experiments, the Bz-HCl dimer would serve as a good alternative to the Bz-H2O dimer due to its simpler REMPI detection scheme for the HCl fragment, which was discussed in Chapter 3 and demonstrated in Chapter 4. Bz-HCl has a T-shaped H-bond similar to C2H2-HCl, as displayed in Figure 7.7. In previous experiments, our group has obtained images by monitoring HCl up to J = 12, which corresponds to the internal energy of HCl in excess of 1600 cm -1 . 15 In addition, there is still uncertainty surrounding the value of the D0 of this dimer, which can be resolved with VMI. According to Mons et. al, 1 the D0 for Bz-HCl is approximately ~1000 cm -1 , which would leave more than ~1600 cm -1 for Eavail following HCl stretch excitation. Zwier et. al 59 detected the 6 " # vibronic transition of Bz-HCl utilizing 1+1 REMPI through the S1← S0 transition at 38,735 cm -1 . The IR vibrational frequency of the HCl stretch for Bz-HCl has yet to be measured experimentally. The HCl stretch would be red shifted from the HCl monomer, which is indicative of the strength of the H-bond. The HCl stretch for C2H2-HCl 46, 60 was at 2806.9 cm -1 , and the dimer had a D0 value of 700 cm -1 , whereas the HCl stretch of HCl-H2O dimer was located at 2723 cm -1 and D0 was 1334 cm -1 . 31, 33 Based on previous experiments, we estimate the HCl stretch of Bz-HCl to be located around ~2800 cm -1 , which will be detected using IR depletion spectroscopy by probing the 6 " # vibronic transition using 1+1 REMPI at 38,735 cm -1 at m/z = 114.5. Figure 7.7: Structure of the H-bonded Bz-HCl dimer in the S0 and S1 states. 2 171 7.2.2 Proposed Experiments with Benzene-HCl In our future studies, following the VP of Bz-HCl, the HCl fragment would be detected and ionized with 2+1 REMPI, which would require a high UV fluence. This would be accomplished with the use of a 20 cm f.l. lens, such as the one used for the detection of H2O in previous chapters. Fragmentation of the benzene monomer and excited -H2O and -D2O dimers was observed at m/z = 37, which was discussed in 7.1.2. For the S1 ← S0 transition (40900-42550 cm - 1 ), shown in Figure 7.8, the benzene monomer and one of its fragments were detected at m/z = 78 and m/z = 37, respectively. Unfortunately, this spectrum overlaps with several two-photon electronic transitions of HCl (m/z = 36) between (81800 and 85100 cm -1 ). Figure 7.8: S1 ← S0 spectrum of benzene (black) at m/z = 78 detected by 1+1 REMPI between 40900-42550 cm -1 using a 20 cm f.l. lens, and the same spectral region was monitored for the benzene fragments (red) at m/z = 37 . Note: this one-photon absorption region overlaps with the two-photon absorption region for HCl shown in Figure 7.9 A list of two-photon electronic states for HCl is presented in Table 3.2. In comparison to the benzene monomer, the intensity of the fragmentation peaks appears to be relatively small within the HCl region even with focused conditions. In Figure 7.9, the 1+1 REMPI spectrum of 172 the benzene monomer fragments (m/z = 37) taken from Figure 7.8 is overlaid onto PGOPHER simulations of the 2+1 REMPI spectrum of the f 3 Δ2 (ν = 0) ←X 1 Σ + , F 1 Δ2 (ν = 0 ) ←X 1 Σ + , and V 1 Σ + (v = 11,12, and 13)←X 1 Σ + electronic state transitions. 61 The rotational temperature and linewidths of the HCl + REMPI simulation were set to match the same experimental conditions used during the detection of HCl following the VP of HCl-(H2O)3 (Chapter 4). The F 1 Δ2 and V 1 Σ + HCl electronic states do not overlap with the benzene fragment peaks and have been utilized previously for VMI. Following the VP of Bz-HCl, HCl rotational levels of J = 5-12 would be accessible for imaging based on energetics and frequency positions of benzene and the m/z = 37 fragment ions. Figure 7.9: (top) Simulated 2+1 REMPI spectra of HCl + at m/z = 36 in the f 3 Δ2 (ν = 0) ←X 1 Σ + , F 1 Δ2 (ν= 0 ) ←X 1 Σ + , E 1 Σ + (v=0)←X 1 Σ, and V 1 Σ + (v=11,12, and 13)←X 1 Σ + electronic transitions obtained using PGOPHER. (bottom) The corresponding S1 ← S0 spectrum of fragmented benzene + (red) at m/z = 37 detected by 1+1 REMPI, originally between 40900-42550 cm -1 . In order to show the correlation to HCl, which requires 2 photons, the x-axis (cm -1 ) from Figure 7.8 fragments has been doubled for comparison with the spectra of the benzene fragments (m/z = 37). 173 Gord et. al, 41 observed another experimental complication, which is the UV-induced dissociation of Bz-HCl dimer following 1+1 REMPI. Photofragments detected following the dissociation determined the UV-induced process to be 95% efficient. Gord et. al 41 proposed the following dissociation mechanism for the H-bonded Bz-HCl complex following 1+1 REMPI: )*−,-. (/ " )+ℎ2 → )*−,-. (/ # )+ℎ2 → [)*−,-.] $ +6 % → )* $ +,-.+6 % Figure 7.10: Figure from Gord et. al, 41 which illustrates the schematic energy diagrams for a one- color REMPI experiment: (a) non-H-bonded complexes; (b) the H-bonded Bz-HCl energy level diagram with the HCl internal rotor geometry; (c) H-bonded Bz-HCl energy diagram along with the H-bond stretching coordinate, which illustrates the dissociation of the complex. In their study, Gord et. al 41 argued that the efficient dissociation was a consequence of the H-bonded geometry of the neutral complex (Figure 7.7). As the molecule was excited via S1←S0 transition, it retained the structure of the neutral complex. When the cluster was ionized with an additional photon, the lowest energy structure for the neutral complex was decidedly a highly unstable repulsive geometry for the ionic complex as displayed in Figure 7.10. The H-bonded minimum in the neutral complex is the highest energy geometry for the ionic complex. Our experimental methods can compensate for the UV-induced dissociation process, which was 174 demonstrated in our preliminary results observed for the IR-UV depletion of Bz-D2O. The 6 " # vibration for Bz-D2O + and Bz-H2O + in Figure 7.3 suffered from a similar UV-induced dissociation process. In comparison to the 6 " # vibration in Figure 7.5, we detected a large IR depletion signal . The IR excitation laser firing 65 ns before the UV ionization laser vibrationally selected the dimer, which allowed VP to occur prior to UV induced dissociation. In Figure 7.5, a large depletion signal was observed for the 6 " # vibration for the Bz-D2O dimer in comparison to the peak intensity observed in Figure 7.3. To demonstrate the expected resolution and possible results for these experiments, we simulated pair-correlated ET distributions for Bz-HCl. As the internal energy of the HCl fragment decreases, the ET distributions reflect additional benzene vibrational levels. Figure 7.11 displays simulations calculated with the same method as those presented for Bz-H2O and -D2O in Section 7.1.3. The simulations were created based on the previous experimental studies 12, 66 and assumptions that were presented in this section. The expected ET release was based on selected rotational levels in the ground electronic state of HCl. The simulations were set for a 225 K rotational temperature, which is typical of larger fragments due to angular momentum constraints 44-46 In these simulations, the HCl stretch vibration of the Bz-HCl dimer was estimated to be ~2800 cm -1 with an approximate D0 of ~1000 cm -1 . In the simulated ET distributions, the dissociating HCl fragment was assumed to be at two energetically accessible extremes based on the available electronic states, J = 5 and J =12. The proposed VMI experiments would provide information on energy transfer pathways within the dimer subunits and across the H-bonds. These experiments would give insight into the energetics for VP, which are given in the following equation: ℎ2 &' +7 ()* = 9 " +7 +,* (,-.)+7 -(.,+,* ()* :;<=>?@6AB)+7 1 Eq. 7.1 175 Figure 7.11: Simulated ET distribution of the VP of the Bz-HCl dimer. The HCl fragment is detected in the labeled rotational state. The red lines indicate vibrational levels of benzene; 43 blue convolutes the vibrational levels with rotational levels. Several of the values in Eq. 7.1, including the IR vibrational frequency of the HCl stretch and the D0 of the Bz-HCl dimer, have not been confirmed experimentally. The proposed experiments would first obtain the vibrational frequency through IR-UV depletion spectroscopy by probing the 6 " # vibronic transition using 1+1 REMPI at 38,735 cm -1 at m/z = 114.5, as discussed in 7.2.1. As mentioned, the IR vibration is estimated to be around ~2800 cm -1 , which would be enough to induce VP. Once the IR position is confirmed, IR “action” photofragment yield spectroscopy will be employed to find HCl enhancement using the HCl electronic states at m/z = 36 (outlined in 7.2.2).. The IR “action” spectrum does not only confirm VP, but provides knowledge on the rotational populations of the HCl fragments following VP. A combination of HCl electronic states will be used to minimize the production of background HCl from UV induced photodissociation, as well as too minimize photodissociation of the HCl monomer discussed in section 3.3. Images will be collected using VMI on an ensemble of rotational states of the HCl fragment, which can range from J = 5 (312 cm -1 ) to as high as J = 12 (1628 cm -1 ). Utilizing VMI to obtain ET distributions will reveal the strength of the H-bond by allowing us to determine the 176 D0 and the underlying dynamics. The excess energy from detecting the HCl fragments in different rovibrational states enables the examination of the correlation between Eavail and population distributions. The results can be modeled with statistical or nonstatistical models as discussed in Chapter 2 of this dissertation and in our previous work. 13, 15, 16, 31-34, 46, 62-65 7.3 Conclusions In summary, the detailed spectroscopic work published on dimers of aromatic molecules H-bonded to water has influenced our choices to focus on dimers with relevance to biological systems. 5, 7, 20, 66, 67 In previous experiments, our group has examined the VP of dimers with two H-bond binding motifs: (1) perpendicular H-bonding to the "-bond, and (2) % in-plane bonding to one of the atoms in the aromatic molecule, with water serving as donor or acceptor. The main question we would address in these future studies is how different geometries and H-bonding types in the dimers of benzene evolve into fragment vibrational state distributions. It would be enlightening to know whether different vibrational modes (in-plane bend, out-of-plane bend, torsion, stretch, etc.) in the aromatic fragment are preferred for different binding motifs. It is likely that, in keeping with Ewing’s propensity rules, energy disposal would favor levels associated with low Et. The nature of such state specificity, however, remains an open question, as we have seen that larger fragment may favor a statistical energy distribution of internal states. We need experimental results, like those described above, to create the databases necessary for identifying vibrational propensity rules. High level electronic structure and dynamics calculations are still needed in partnership with state-of-the-art experiments in order to obtain a comprehensive picture of VP dynamics of H-bonded clusters. Future experiments involving H2O, D2O, and HCl H- bonded to benzene would help bridge the gap between dimers of small molecules and those involving larger aromatic species. 177 Chapter 7 References 1. Mons, M.; Dimicoli, I.; Piuzzi, F., International Reviews in Physical Chemistry 2002, 21 (1), 101-135. 2. Zwier, T. S., Annu. Rev. Phys. Chem. 1996, 47, 205-241. 3. Fredericks, S. Y.; Jordan, K. D.; Zwier, T. S., The Journal of Physical Chemistry 1996, 100 (19), 7810-7821. 4. Ewing, G. E., J. Chem. Phys. 1980, 72, 2096. 5. Meyer, E. A.; Castellano, R. K.; Diederich, F., Angewandte Chemie International Edition 2003, 42 (11), 1210-1250. 6. Caminati, W.; Favero, L. B.; Favero, P. G.; Maris, A.; Melandri, S., Angew. Chem. Int. Edit. 1998, 37 (6), 792-795. 7. Melandri, S.; Sanz, M. E.; Caminati, W.; Favero, P. G.; Kisiel, Z., Journal of the American Chemical Society 1998, 120 (44), 11504-11509. 8. Zwier, T. S., The Journal of Physical Chemistry A 2006, 110 (12), 4133-4150. 9. Becucci, M.; Melandri, S., Chemical Reviews 2016, 116 (9), 5014-5037. 10. Řezáč, J.; Hobza, P., Chemical Reviews 2016, 116 (9), 5038-5071. 11. Biedermann, F.; Schneider, H.-J., Chemical Reviews 2016, 116 (9), 5216-5300. 12. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem.A 2011, 115 (25), 6903-6909. 13. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134 (37), 15430-15435. 14. Samanta, A. K.; Ch'ng, L. C.; Reisler, H., Chem. Phys. Lett. 2013, 575, 1-11. 178 15. Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700-2709. 16. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chem. Rev. 2016, 116 (9), 4913-4936. 17. Kwasniewski, D.; Butler, M.; Reisler, H., Phys. Chem. Chem. Phys. 2019, 21 (26), 13968-13976. 18. Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243-4247. 19. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. 20. Dopfer, O.; Fujii, M., Chemical Reviews 2016, 116 (9), 5432-5463. 21. Frey, J. A.; Holzer, C.; Klopper, W.; Leutwyler, S., Chemical Reviews 2016, 116 (9), 5614-5641. 22. Braun, J. E.; Mehnert, T.; Neusser, H. J., Int. J. Mass Spectrom. 2000, 203, 1-18. 23. Brutschy, B., Chem. Rev. 2000, 100 (11), 3891-3920. 24. Ebata, T.; Fujii, A.; Mikami, N., International Reviews in Physical Chemistry 1998, 17 (3), 331-361. 25. Kim, K. S.; Tarakeshwar, P.; Lee, J. Y., Chem. Rev. 2000, 100 (11), 4145-4186. 26. Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millié, P., J. Phys. Chem. A 1998, 102 (25), 4890-4898. 27. Parthasarathi, R.; Subramanian, V.; Sathyamurthy, N., The Journal of Physical Chemistry A 2005, 109 (5), 843-850. 179 28. Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K., J. Chem. Phys. 1996, 104 (3), 972-982. 29. Gerhards, M.; Schmitt, M.; Kleinermanns, K.; Stahl, W., J. Chem. Phys. 1996, 104 (3), 967-971. 30. Engdahl, A.; Nelander, B., The Journal of Physical Chemistry 1985, 89 (13), 2860-2864. 31. Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2010, 114 (36), 9774-9781. 32. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2009, 113, 10174-10183. 33. Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134, 211101. 34. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2011, 115, 6903-6909. 35. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134, 15430-15435. 36. Ch'ng, L. C.; Samanta, A. K.; Wang, Y.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2013, 117, 7207-7216. 37. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. 38. Kleiner, I.; Brown, L. R.; Tarrago, G.; Kou, Q.-L.; Piqu é, N.; Guelachvili, G.; Dana, V.; Mandin, J.-Y., Journal of Molecular Spectroscopy 1999, 193, 46-71. 39. Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H., Rev. Sci. Instrum. 2002, 73 (7), 2634-2642. 180 40. Mooney, J.; Kambhampati, P., J. Phys. Chem. Lett. 2013, 4 (19), 3316-3318. 41. Gord, J. R.; Garrett, A. W.; Bandy, R. E.; Zwier, T. S., Chemical Physics Letters 1990, 171 (5), 443-450. 42. Engdahl, A.; Nelander, B., J. Phys. Chem. 1990, 94 (25), 8777-8780. 43. Maslen, P. E.; Handy, N. C.; Amos, R. D.; Jayatilaka, D., The Journal of Chemical Physics 1992, 97 (6), 4233-4254. 44. McCaffery, A. J., Phys. Chem. Chem. Phys. 2004, 6, 1637-1657. 45. McCaffery, A. J.; Marsh, R. J., J. Chem. Phys. 2002, 117 (20), 9275-9285. 46. Pritchard, M.; Parr, J.; Li, G.; Reisler, H.; McCaffery, A. J., Phys. Chem. Chem. Phys. 2007, 9 (47), 6241-6252. 47. Ewing, G. E., J. Phys. Chem. 1987, 91, 4662. 48. Ewing, G. E., J. Phys. Chem. 1979, 71, 3143. 49. Parr, J. A. Imaging the State-Specific Vibrational Predissociation of Hydrogen Bonded Coplexes. University of Southern California, Los Angeles, CA, 2007. 50. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem.A 2009, 113 (38), 10174-83. 51. Hilpert, G.; Fraser, G. T.; Pine, A. S., J. Chem. Phys. 1996, 105 (15), 6183-6191. 52. Liu, Y.; Suhm, M. A.; Botschwina, P., Phys. Chem. Chem. Phys. 2004, 6 (19), 4642- 4651. 53. Li, G.; Parr, J.; Fedorov, I.; Reisler, H., Phys. Chem. Chem. Phys. 2006, 8 (25), 2915- 2924. 54. Carcabal, P.; Brenner, V.; Halberstadt, N.; Millie, P., Chem. Phys. Lett. 2001, 336 (3- 4), 335-342. 181 55. Çarçabal, P.; Broquier, M.; Chevalier, M.; Picard-Bersellini, A.; Brenner, V.; Millié, P., J. Chem. Phys. 2000, 113 (12), 4876-4884. 56. Oudejans, L.; Miller, R. E., J. Phys. Chem.A 1999, 103 (25), 4791-4797. 57. Oudejans, L.; Miller, R. E., Annu. Rev. Phys. Chem. 2001, 52, 607-637. 58. Dayton, D. C.; Block, P. A.; Miller, R. E., J Phys Chem 1991, 95 (7), 2881-2888. 59. Gotch, A. J.; Zwier, T. S., The Journal of Chemical Physics 1990, 93 (10), 6977-6986. 60. McCaffery, A. J.; Pritchard, M.; Reisler, H., J. Phys. Chem. 2009, 112, 412-418. 61. Green, D. S.; Bickel, G. A.; Wallace, S. C., J. Mol. Spectrosc. 1991, 150 (2), 303-353. 62. McCaffery, A. J.; Pritchard, M.; Reisler, H., J. Phys. Chem.A 2010, 114 (9), 2983-2990. 63. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. 64. Rocher-Casterline, B. E.; Ch'ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134 (21), 211101. 65. Samanta, A. K.; Ch'ng, L. C.; Reisler, H., Chem. Phys. Lett. 2013, 575, 1-11. 66. Desfrançois, C.; Carles, S.; Schermann, J. P., Chem. Rev. 2000, 100 (11), 3943-3962. 67. Jeffrey, G. A.; Saenger, W., Hydrogen bonding in biological structure. Springer-Verlag: Berlin: 1991. 182 Bibliography Amirand, C.; Maillard, D., J. Mol. Struct. 1988, 176, 181-201. Andot, K.; Hynes, J. T., J. Mol. Liq. 1995, 64 (1), 25-37. Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J., Pure and Appl. Chem. 2011, 83 (8), 1619-1636. Ashfold, M. N. R.; Bayley, J. M.; Dixon, R. N., Chem. Phys. 1984, 84 (1), 35-50. Ayers, G. P.; Pullin, A. D. E., Spectrochim. Acta. A. 1976, 32 (11), 1695-1704. Baba, H.; Goodman, L.; Valenti, P. C., J. Am. Chem. Soc.1966, 88 (23), 5410-5415. Baer, T.; Hase, W. L., Unimolecular Reaction Dynamics: Theory and Experiments. Oxford University Press, Inc.: New York, NY, 1996. Bandyopadhyay, I.; Lee, H. M.; Kim, K. S., J. Phys. Chem. A. 2005, 109, 1720-1728. Barnes, A. J., J. Mol. Struct. 1980, 60, 343-346. Becucci, M.; Melandri, S., Chemical Reviews 2016, 116 (9), 5014-5037. Berden, G.; Meerts, W. L.; Schmitt, M.; Kleinermanns, K., J. Chem. Phys. 1996, 104 (3), 972- 982. Beswick, J. A.; Jortner, J., Chem. Phys. Lett. 1977, 49 (1), 13-18. Beswick, J. A.; Jortner, J., J. Chem. Phys.1981, 74 (12), 6725-6733. Biedermann, F.; Schneider, H.-J., Chemical Reviews 2016, 116 (9), 5216-5300. Boese, A. D.; Martin, J. M. L., J. Phys. Chem. A.2004, 108 (15), 3085-3096. Bolovinos, A.; Tsekeris, P.; Philis, J.; Pantos, E.; Andritsopoulos, G., J. Mol. Spectrosc. 1984, 103 (2), 240-256. 183 Bondybey, V. E.; Beyer, M.; Achatz, U.; Joos, S.; Niedner-Schatteburg, G., Israel Journal of Chemistry 1999, 39 (3‐4), 213-219. Braun, J. E.; Mehnert, T.; Neusser, H. J., Int. J. Mass Spectrom. 2000, 203, 1-18. Breda, S.; Reva, I. D.; Lapinski, L.; Nowak, M. J.; Fausto, R., J. Molec. Struct. 2006, 786 (2), 193-206. Bruni, F.; Di Mino, C.; Imberti, S.; McLain, S. E.; Rhys, N. H.; Ricci, M. A., J. Phys. Chem.Letters 2018, 9 (13), 3667-3672. Brutschy, B., Chem. Rev. 2000, 100 (11), 3891-3920. Cai, Z. L.; Reimers, J. R., J. Phys. Chem. A 2007, 111, 954-962. Callaghan, R.; Arepalli, S.; Gordon, R. J., J. Chem. Phys. 1987, 86, 5273-5280. Caminati, W.; Favero, L. B.; Favero, P. G.; Maris, A.; Melandri, S., Angew. Chem. Int. Edit. 1998, 37 (6), 792-795. Carcabal, P.; Brenner, V.; Halberstadt, N.; Millie, P., Chem. Phys. Lett. 2001, 336 (3-4), 335- 342. Çarçabal, P.; Broquier, M.; Chevalier, M.; Picard-Bersellini, A.; Brenner, V.; Millié, P., J. Chem. Phys. 2000, 113 (12), 4876-4884. Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2010, 114 (36), 9774-9781. Castleman, A. W.; Stanley, R. J., J. Chem. Phys. 1991, 94 (12), 7744-7756. Ch'ng, L. C. Dissociation Energy and Dynamics of Water Clusters. University of Southern California, Los Angeles, CA, 2013. Ch'ng, L. C.; Samanta, A. K.; Czakó, G.; Bowman, J. M.; Reisler, H., J. Am. Chem. Soc. 2012, 134 (37), 15430-15435. 184 Ch'ng, L. C.; Samanta, A. K.; Wang, Y.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2013, 117, 7207-7216. Chaban, G. M.; Gerber, R. B.; Janda, K. C., J. Phys. Chem. A.2001, 105 (36), 8323-8332. Chandler, D. W.; Houston, P. L., J. Chem. Phys.1987, 87 (2), 1445-1447. Chichinin, A. I.; Maul, C.; Gericke, K.-H., J. Chem. Phys. 2006, 124, 224324. Courty, A.; Mons, M.; Dimicoli, I.; Piuzzi, F.; Brenner, V.; Millié, P., J. Phys. Chem. A 1998, 102 (25), 4890-4898. Czakó, G.; Wang, Y.; Bowman, J. M., J. Chem. Phys.2011, 135 (15), 151102. Dayton, D. C.; Block, P. A.; Miller, R. E., J Phys Chem 1991, 95 (7), 2881-2888. Desfrançois, C.; Carles, S.; Schermann, J. P., Chem. Rev. 2000, 100 (11), 3943-3962. Desiraju, G. R.; Steiner, T., The weak hydrogen bond instructural chemistry and biology. Oxford University Press.: Oxford, 1999. Destexhe, A.; Smets, J.; Adamowicz, L.; Maes, G., J. Phys. Chem.1994, 98 (5), 1506-1514. Doi, A.; Naohiko, M., J. Chem. Phys. 2008, 129, 154308. Dopfer, O.; Fujii, M., Chemical Reviews 2016, 116 (9), 5432-5463. Douglas, A. E.; Greening, F. R., Cana. J. Phys. 1979, 57, 1650-1661. Dribinski, V. Photoelectron and Ion Imaging Studies of the Mixed Valence/Rydberg Excited Stated of the Chlormethyl Radical, CH2Cl and Nitric Oxide Dimer (NO2)2. University of Southern California, Los Angeles, CA, 2004. Dribinski, V.; Ossadtchi, A.; Mandelshtam, V. A.; Reisler, H., Rev. Sci. Instrum. 2002, 73 (7), 2634-2642. Ebata, T.; Fujii, A.; Mikami, N., Int. Rev. Phys. Chem. 1998, 17 (3), 331-361. Ebata, T.; Kayano, M.; Sato, S.; Mikami, N., J. Phys. Chem. A 2001, 105, 8623-8628. 185 Ebata, T.; Mizuochi, N.; Watanabe, T.; Naohiko, M., J. Chem. Phys. 1996, 100, 546-550. Ebata, T.; Watanabe, T.; Mikami, N., J. Phys. Chem. 1995, 99 (16), 5761-5764. Engdahl, A.; Nelander, B., J. Phys. Chem.1985, 89 (13), 2860-2864. Engdahl, A.; Nelander, B., J. Phys. Chem. 1990, 94 (25), 8777-8780. Eppink, A. T. J. B.; Parker, D. H., Rev. Sci. Instrum. 1997, 68 (9), 3477-3484 Ewing, G. E., J. Phys. Chem. 1979, 71, 3143. Ewing, G. E., J. Chem. Phys. 1980, 72, 2096. Ewing, G. E., J. Phys. Chem. 1987, 91, 4662. Fárník, M.; Weimann, M.; Suhm, M. A., J. Chem. Phys. 2003, 118, 10120.. Federov, I. Photoelectron and Ion Imaging Investigations of Spectroscopy, Photoionization, and Photodissociation Dynamics of Diazomethane and Diazirine. University of Southern California, Los Angeles, CA, 2009. Flynn, S. D.; Skvortsov, D.; Morrison, A. M.; Liang, T.; Choi, M. Y.; Douberly, G. E.; Vilesov, A. F., J. Phys. Chem. Lett. 2010, 1 (15), 2233-2238. Forbert, H.; Masia, M.; Kaczmarek-Kedziera, A.; Nair, N. N.; Marx, D., J. Am. Chem. Soc. 2011, 133 (11), 4062-4072. Fredericks, S. Y.; Jordan, K. D.; Zwier, T. S., J. Phys. Chem.1996, 100 (19), 7810-7821. Frey, J. A.; Holzer, C.; Klopper, W.; Leutwyler, S., Chemical Reviews 2016, 116 (9), 5614-5641. Fuke, K.; Kaya, K., Chem. Phys. Lett. 1983, 94 (1), 97-101. Gerhards, M.; Kleinermanns, K., J. Chem. Phys. 1995, 103 (17), 7392-7400. Gerhards, M.; Schmitt, M.; Kleinermanns, K.; Stahl, W., J. Chem. Phys. 1996, 104 (3), 967-971. Ginter, D. S.; Ginter, M. L., J. Mol. Spectrosc. 1981, 90 (117). Gleiter, R.; Heilbronner, E.; Hornung, V., Helvetica Chimica Acta 1972, 55 (1), 255-274. 186 Glendening, E. D., J. Phys. Chem. A 2005, 109, 11936-11940. Gord, J. R.; Garrett, A. W.; Bandy, R. E.; Zwier, T. S., Chem. Phys. Lett. 1990, 171 (5), 443- 450. Gotch, A. J.; Zwier, T. S., J. Chem. Phys.1990, 93 (10), 6977-6986. Green, D. S.; Bickel, G. A.; Wallace, S. C., J. Mol. Spectrosc. 1991, 150 (2), 303-353. Green, D. S.; Bickel, G. A.; Wallace, S. C., J. Mol. Spectrosc. 1991, 150 (2), 303-353. Green, D. S.; Bickel, G. A.; Wallace, S. C., J. Mol. Spectrosc. 1991, 150 (2), 354-387. Green, D. S.; Bickel, G. A.; Wallace, S. C., J. Mol. Spectrosc. 1991, 150 (2), 388-469. Green, D. S.; Wallace, S. C., J. Chem. Phys.1992, 96 (8), 5857-5877. Guggemos, N.; Slavíček, P.; Kresin, V. V., Physical Review Letters 2015, 114 (4), 043401. Gutowsky, H. S.; Emilsson, T.; Arunan, E., J. Chem. Phys.1993, 99, 4883. Hassanali, A. A.; Cuny, J.; Ceriotti, M.; Pickard, C. J.; Parrinello, M., J. Am. Chem. Soc. 2012, 134 (20), 8557-8569. Hilpert, G.; Fraser, G. T.; Pine, A. S., J. Chem. Phys. 1996, 105 (15), 6183-6191. Hirata, S.; Nooijen, M.; Bartlett, R. J., Chem. Phys. Lett. 2000, 326 (3), 255-262. Hobza, P.; Havlas, Z., Chem. Rev. 2000, 100 (11), 4253-4264. Hollas, J. M., High Resolution Spectroscopy. 2nd. ed.; John Wiley & Sons: 1998. Howard, A. A.; Tschumper Gs Fau - Hammer, N. I.; Hammer, N. I., J. Phys. Chem. A 2010, 114, 6803-6810. Huneycutt, A. J.; Stickland, R. J.; Hellberg, F.; Saykally, R. J., J. Chem. Phys. 2003, 118 (3), 1221-1229. Jeffrey, G. A.; Saenger, W., Hydrogen bonding in biological structure. Springer-Verlag: Berlin: 1991. 187 Joseph, J.; Jemmis, E. D., J. Am. Chem. Soc.2007, 129 (15), 4620-4632. Jung, J. O.; Gerber, R. B., J. Chem. Phys.1996, 105 (23), 10332-10348. Kauczok, S.; Maul, C.; Chichinin, A. I.; Gericke, K. H., J. Chem. Phys. 2010, 133, 414-420. Kendall, R. A.; Dunning, T. H.; Harrison, R. J., J. Chem. Phys.1992, 96 (9), 6796-6806. Keutsch, F. N.; Cruzan, J. D.; Saykally, R. J., Chem. Rev. 2003, 103, 2533. Kim, K. S.; Tarakeshwar, P.; Lee, J. Y., Chem. Rev. 2000, 100 (11), 4145-4186. Kleiner, I.; Brown, L. R.; Tarrago, G.; Kou, Q.-L.; Piqu é, N.; Guelachvili, G.; Dana, V.; Mandin, J.-Y., J. Mol. Spectrosc. 1999, 193, 46-71. Korolik, M. Molecule surface interactions in hydrogen chloride/magnesium oxide. University of Southern California, Los Angeles, CA, 1999. Korolik, M.; Arnold, D. W.; Johnson, M. J.; Suchan, M. M.; Reisler, H.; Wittig, C., Chem. Phys. Lett. 1998, 284 (3), 164-170. Krylov, A. I., Annu. Phys. Chem. 2008, 59 (1), 433-462. Kuma, S.; Slipchenko, M. N.; Kuyanov, K. E.; Momose, T.; Vilesov, A. F., J. Phys. Chem. A. 2006, 110, 10046-10052. Kvaran, A.; H., W.; Waage, B. G., Can. J. Phys. 2001, 79, 197-210. Kvaran, Á.; Logadóttir, Á.; Wang, H., J. Chem. Phys.1998, 109 (14), 5856-5867. Kwasniewski, D.; Butler, M.; Reisler, H., Phys. Chem. Chem. Phys. 2019, 21 (26), 13968-13976. Latimer, W. M.; Rodebush, W. H., J. Am. Chem. Soc. 1920, 42, 1419-1433. Leforestier, C., Philos. Trans. R. Soc. London, Ser. A 2012, 370 (1968), 2675-2690. Li, G.; Parr, J.; Fedorov, I.; Reisler, H., Phys. Chem. Chem. Phys. 2006, 8 (25), 2915-2924. Lipert, R. J.; Bermudez, G.; Colson, S. D., J. Phys. Chem. 1988, 92 (13), 3801-3805. Liu, Y.; Suhm, M. A.; Botschwina, P., Phys. Chem. Chem. Phys. 2004, 6 (19), 4642-4651. 188 Martin, J. M. L.; Van Alsenoy, C., J. Phys. Chem.1996, 100 (17), 6973-6983. Mancini, J. S.; Bowman, J. M., Phys. Chem. Lett. 2014, 5 (13), 2247-2253. Mancini, J. S.; Samanta, A. K.; Bowman, J. M.; Reisler, H., J. Phys. Chem. A 2014, 118 (37), 8402-8410. Mancini, J. S.; Bowman, J. M., Phys. Chem. Chem. Phys. 2015, 17 (9), 6222-6226. Marzzacco, C., J. Am. Chem. Soc.1973, 95 (6), 1774-1777. Masia, M.; Forbert, H.; Marx, D., J. Phys. Chem. A.2007, 111 (49), 12181-12191. Maslen, P. E.; Handy, N. C.; Amos, R. D.; Jayatilaka, D., J. Chem. Phys.1992, 97 (6), 4233- 4254. Maul, C.; I., C. A.; Gericke, K. H., J. Atom. Moles. and Opt. Phys. 2011, 410108. Mazzoni, F.; Pasquini, M.; Pietraperzia, G.; Becucci, M., J. Mol. Struc. 2015, 1090, 2-6. McCaffery, A. J., Phys. Chem. Chem. Phys. 2004, 6, 1637-1657. McCaffery, A. J.; Marsh, R. J., J. Chem. Phys. 2002, 117 (20), 9275-9285. McCaffery, A. J.; Pritchard, M.; Reisler, H., J. Phys. Chem. 2009, 112, 412-418. McCaffery, A. J.; Pritchard, M.; Reisler, H., J. Phys. Chem.A 2010, 114 (9), 2983-2990. Melandri, S.; Sanz, M. E.; Caminati, W.; Favero, P. G.; Kisiel, Z., J. Am. Chem. Soc.1998, 120 (44), 11504-11509. Meyer, E. A.; Castellano, R. K.; Diederich, F., Angewandte Chemie International Edition 2003, 42 (11), 1210-1250. Mikami, N., Bull. Chem. Soc. Jpn. 1995, 68 (3), 683-694. Millen, D. J.; Mines, G. W., Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chem. phys. 1977, 73 (3), 369-377. Mikami, N., Bull. Chem. Soc. Jpn. 1995, 68 (3), 683-694. 189 Miller, R. E., Acc. Chem. Res. 1990, 23 (1), 10-16. Miller, R. E.; Oudejans, L., Annu. Rev. Phys. Chem. 2001, 52, 607-637. Miyazaki, Y.; Inokuchi, Y.; Ebata, T.; Petkovic, M., Chem. Phys. 2013, 419, 205-211. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem.A 2009, 113 (38), 10174-83. Mollner, A. K.; Casterline, B. E.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A 2009, 113, 10174- 10183. Mons, M.; Dimicoli, I.; Piuzzi, F., Int. Rev. Phys. Chem. 2002, 21 (1), 101-135. Monte, S. A. d.; Müller, T.; Dallos, M.; Lischka, H.; Diedenhofen, M.; Klamt, A., Theoretical Chemistry Accounts 2004, 111 (2), 78-89. Mooney, J.; Kambhampati, P., J. Phys. Chem. Lett. 2013, 4 (19), 3316-3318. Moore, T. S.; Winmill, T. F., J. Chem. Soc., Trans. 1912, 101, 1635. Ni, H.; Serafin, J.; Valentini, J., J. Chem. Phys. 2000, 113, 3055-3066. Noble, M.; Qian, C. X. W.; Reisler, H.; Wittig, C., J. Chem. Phys. 1986, 85, 5763-5773. Odde, S.; Mhin, B. J.; Lee, S.; Lee, H. M.; Kim, K. S., J. Chem. Phys. 2004, 120 (20), 9524- 9535. Oikawa, A.; Abe, H.; Mikami, N.; Mitsuo, I., J. Phys. Chem. 1983, 87 (25), 5083-5090. Oudejans, L.; Miller, R. E., J. Phys. Chem.A 1999, 103 (25), 4791-4797. Oudejans, L.; Miller, R. E., Annu. Rev. Phys. Chem. 2001, 52, 607-637. Packer, M. J.; Clary, D. C., J. Phys. Chem.1995, 99 (39), 14323-14333. Page, R. H.; Frey, J. G.; Shen, Y. R.; Lee, Y. T., Chem. Phys. Lett. 1984, 106 (5), 373-376. Park, J.-H.; Lee, H.; Kwon, K.-C.; Kim, H.-K.; Choi, Y.-S.; Choi, J.-H., J. Chem. Phys. 2002, 117, 2017-2028. 190 Parr, J. A. Imaging the State-Specific Vibrational Predissociation of Hydrogen Bonded Coplexes. University of Southern California, Los Angeles, CA, 2007. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. Parr, J. A.; Li, G.; Federov, I.; McCaffery, A. J.; Reisler, H., J. Phys. Chem. A 2007, 111 (31), 7589-7598. Parthasarathi, R.; Subramanian, V.; Sathyamurthy, N., J. Phys. Chem. A.2005, 109 (5), 843-850. Paul, J. B.; Collier, C. P.; Saykally, R. J.; Scherer, J. J.; O'Keefe, A., J. Phys. Chem. A.1997, 101 (29), 5211-5214. Pauling, L., The nature of the Chemical Bond and the Structure of Molecules and Crystals: An Introduction to Modern Structural Chemistry. Cornell University Press: New York: 1939. Petković, M., J. Phys. Chem. A 2011, 116, 364-371. Potter, A. B. Ion Imaging Studies of the Spectroscopy and Photodissociation Dynamics of Chloromethyl Radical and Nitric Oxide Dimer. University of Southern California, Los Angeles, CA, 2005. Pritchard, M.; Parr, J.; Li, G.; Reisler, H.; McCaffery, A. J., Phys. Chem. Chem. Phys. 2007, 9 (47), 6241-6252. Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M., Chem. Phys. Lett. 1989, 157 (6), 479-483. Re, S.; Osamura, Y.; Suzuki, Y.; Schaefer, H. F., J. Chem. Phys. 1998, 109 (3), 973-977. Reimers, J. R.; Cai, Z.-L., Phys. Chem. Chem. Phys. 2012, 14 (25), 8791-8802. Reisler, H., Annu. Rev. Phys. Chem. 2009, 60 (1), 39-59. Řezáč, J.; Hobza, P., Chemical Reviews 2016, 116 (9), 5038-5071. 191 Rocher, B. E. Velocity Map Imaging of the State-Specific Vibrational Predissociation of Water- Containing Hydrogen-Bonded Complexes. University of Southern California, 2011. Rocher-Casterline, B. E.; Ch'ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134 (21), 211101. Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H., J. Chem. Phys. 2011, 134, 211101. Rocher-Casterline, B. E.; Mollner, A. K.; Ch'ng, L. C.; Reisler, H., J. Phys. Chem. A. 2011, 115 (25), 6903-6909. Rossetti, R.; Brus, L. E., J. Chem. Phys.1979, 70 (10), 4730-4736. Roth, W.; Imhof, P.; Gerhards, M.; Schumm, S.; Kleinermanns, K., Chem. Phys. 2000, 252, 247-256 Roujou De Boubee, D., UC Davis: Viticulture & Enology 2009, 9, 1-3. Rudić, S.; Ascenzi, D.; Orr-Ewing, A. J., Chem. Phys. Lett. 2000, 332 (5), 487-495. Rudić, S.; Murray, C.; Ascenzi, D.; Anderson, H.; Harvey, J. N.; Orr-Ewing, A. J., J. Chem. Phys.2002, 117 (12), 5692-5706. Samanta, A. K.; Ch'ng, L. C.; Reisler, H., Chem. Phys. Lett. 2013, 575, 1-11. Samanta, A. K.; Czakó, G.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Acc. Chem. Res. 2014, 47, 2700-2709. Samanta, A. K.; Wang, Y.; Mancini, J. S.; Bowman, J. M.; Reisler, H., Chem. Rev. 2016, 116 (9), 4913-4936. Scheiner, S., Annu. Rev. Phys. Chem. 1994, 45 (1), 23-56. Scheiner, S., Noncovalent Forces. Springer: 2015. Shimamori, T.; Fujii, A., J. Phys. Chem. A 2015, 119, 1315-1322. 192 Schriver, A.; Silvi, B.; Maillard, D.; Perchard, J. P., J. Phys. Chem.1977, 81 (22), 2095-2102. Schumm, S.; Gerhards, M.; Roth, W.; Gier, H.; Kleinermanns, K., Chem. Phys. Lett. 1996, 263, 126-132. Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.; Kussmann, J.; Lange, A. W.; Behn, A.; Deng, J.; Feng, X.; Ghosh, D.; Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.; Khaliullin, R. Z.; Kuś, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.; Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.; Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire, E.; Austin, B.; Beran, G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.; Brown, S. T.; Casanova, D.; Chang, C.-M.; Chen, Y.; Chien, S. H.; Closser, K. D.; Crittenden, D. L.; Diedenhofen, M.; DiStasio, R. A.; Do, H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti- Molnar, L.; Ghysels, A.; Golubeva-Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.; Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.; Jagau, T.-C.; Ji, H.; Kaduk, B.; Khistyaev, K.; Kim, J.; Kim, J.; King, R. A.; Klunzinger, P.; Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.; Laurent, A. D.; Lawler, K. V.; Levchenko, S. V.; Lin, C. Y.; Liu, F.; Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.; Manzer, S. F.; Mao, S.-P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.; Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O’Neill, D. P.; Parkhill, J. A.; Perrine, T. M.; Peverati, R.; Prociuk, A.; Rehn, D. R.; Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.; Sodt, A.; Stein, T.; Stück, D.; Su, Y.- C.; Thom, A. J. W.; Tsuchimochi, T.; Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel, J.; White, A.; Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z.-Q.; Zhang, I. Y.; Zhang, X.; Zhao, Y.; Brooks, B. R.; Chan, G. K. L.; Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M. S.; Hehre, W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar, D. G.; Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley, N. A.; Chai, J.-D.; Dreuw, A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S. R.; Hsu, C.-P.; Jung, Y.; Kong, J.; Lambrecht, D. S.; Liang, W.; Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van Voorhis, T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head- Gordon, M., Mol. Phys. 2015, 113 (2), 184-215. 193 Scheiner, S., Noncovalent Forces. Springer: 2015. Shimamori, T.; Fujii, A., J. Phys. Chem. A 2015, 119, 1315-1322. Shimanouchi, T., National Bureau of Standards 1972, 1, 1-160. Skvortsov, D.; Lee, S. J.; Choi, M. Y.; Vilesov, A. F., J. Phys. Chem. A. 2009, 113 (26), 7360- 7365. Spiglanin, T. A.; Chandler, D. W.; Parker, D. H., Chem. Phys. Letters 1987, 10, 414-420. Stanton, J. F.; Gauss, J., J. Chem. Phys.1994, 101 (10), 8938-8944. Stener, M.; Decleva, P.; Holland, D. M. P.; Shaw, D. A., J. Phys. B: At. Mol. Opt. Phys. 2011, 44 (7), 075203. Streibel, T.; K., H.; Mühlberger, F.; Adam, T.; Zimmermann, R., Appl . Spectrosc. 60 (1), 72- 79. Sugawara, S.; Yoshikawa, T.; Takayanagi, T.; Tachikawa, M., Chem. Phys. Lett.2011, 501 (4), 238-244. Suits, A. G.; Continetti, R. E., Imaging in Chemical Dynamics. American Chemical Society: Washington, D.C., 2001. Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A.; Blake, G. A., Science 1992, 257 (5072), 942. Tanabe, S.; Ebata, T.; Fujii, A.; Mikami, N., Chem. Phys. Lett. 1993, 215 (4), 347-352. Tilford, S. G.; Ginter, M. L., J. Mol. Spectrosc. 1971, 40, 568. Tilford, S. G.; Ginter, M. L.; Vanderslice, J. T., J. Mol. Spectrosc 1970, 33, 505. Turner, R. E.; Vaida, V.; Molini, C. A.; Berg, J. O.; Parker, D. H., Chem. phys. 1978, 28 (1), 47-54. 194 Vargas-Caamal, A.; Cabellos, J. L.; Ortiz-Chi, F.; Rzepa, H. S.; Restrepo, A.; Merino, G., Chem. Eur. J. 2016, 22 (8), 2812-2818. Walewski, Ł.; Forbert, H.; Marx, D., Chem Phys Chem 2013, 14 (4), 817-826 Wang, Y.; Bowman, J. M., J. Chem. Phys. 2011, 135 (13), 131101. Wanna, J.; Menapace, J. A.; Bernstein, E. R., J. Chem. Phys.1986, 85 (2), 777-784. Watanabe, T.; Ebata, T.; Tanabe, S.; Mikami, N., J. Chem. Phys. 1996, 105 (2), 408-419. Weigend, F.; Köhn, A.; Hättig, C., J. Chem. Phys.2002, 116 (8), 3175-3183. Western, C. M., (private communication). Western, C. M. PGOPHER, a Program for Simulating Rotational Structure. http://pgopher.chm.bris.ac.uk. Western, C. M., J. Quant. Spectrosc. Radiat. Transfer 2017, 186, 221-242. Whitaker, B. J., Imaging in Molecular Dynamics. Technology and Applications. Cambridge University Press: 2003. Yamazaki, I.; Murao, T.; Yamanaka, T.; Yoshihara, K., Faraday Discussions of the Chemical Society 1983, 75 (0), 395-405. Yang, C.-H.; Sarma, G.; ter Muelen, J. J.; Parker, D. H.; Western, C. M., Phys. Chem. Chem. Phys. 2010, 12, 13983-13991. Yoder, L. M.; Parker, J. R.; Lorenz, K. T.; Chandler, D. W., Chem. Phys. Lett. 1999, 302, 602- 608. Zischang, J.; Skvortsov, D.; Choi, M. Y.; Mata, R. A.; Suhm, M. A.; Vilesov, A. F., J. Phys. Chem. A.2015, 119 (11), 2636-2643. Zuraski, K. Photodissociation Dynamics and Energetics of HCl-(H2O)3. University of Southern California, Los Angeles, CA, 2018. 195 Zuraski, K.; Kwasniewski, D.; Samanta, A. K.; Reisler, H., J. Phys. Chem. Lett. 2016, 7, 4243- 4247. Zuraski, K.; Wang, Q.; Kwasniewski, D.; Bowman, J. M.; Reisler, H., J. Chem. Phys. 2018, 146, 204303. Zwier, T. S., Annu. Rev. Phys. Chem. 1996, 47, 205-241. Zwier, T. S., J. Phys. Chem. A.2006, 110 (12), 4133-4150. Zyrianov, M. Photoinitiated Decomposition of HNCO. University of Southern California, Los Angeles, CA, 1998. 196 Appendix A: MATLAB Programs Appendix A.1 Conversion between Pixel, Speed, and Energy Space %% Import Data in Pixels from Origin data = xlsread('DanPhenolExpt3_J=7'); % Import Data Pixels_x=data(:,1); % Assign x values (pixels) Pixels_y = data(:,2); % Assign y values, P(pixels) %% Conversion to Speed Distribution a = 5.2; % mass dependent factor for H2O % a = 3.669 mass dependent factor for HCl % a = 2.28 mass dependent factor for phenol % Plot speed distribution Speed_x= a.*Pixels_x; Speed_y = (1/a).*Pixels_y; figure(1) ylabel('Counts'); xlabel('Speed (m/s)'); title (Speed H2O J=7) plot (Speed_x,Speed_y,'k','LineWidth',2) %% Conversion to Energy Distribution M = 112; % Mass of Parent Cluster (Phenol-H2O) m = 94; % Mass of Cofragment (Phenol) CC = 0.081; % Calibration constant for experimental system Energy_x = (Pixels_x).^2.*(CC*(1000/4000)*(M/m)); Energy_y = Pixels_y.*(1/(((a.*(M-m)).^2).*Pixels_x)); figure(2) % Plot energy distribution plot (Energy_x,Energy_y,'k','LineWidth',2) title (H2O JkaKc = 7(1,6)) xlabel('Energy, Wavenumbers'); ylabel('Counts'); %% 197 Appendix A.2: Beyer Swinehart Algorithm This function is to be used in correlation with the prior distribution program (Appendix A.3). The Beyer-Swinehart program was recreated from the C program found in reference [1]. 1 This algorithm mimics the one provided in the Baer and Hase textbook in so far as it discretizes the number of bins rather than the energies themselves. The bin size is akin to an energy step; the advantage of the bin counting method is that it counts and folds in state degeneracies and sorts them into bins simultaneously. The indices are adjusted by +1 in order to assign index 1 (i.e. the zeroth bin) to correspond to E = 0. If v1 = 40 cm -1 and the bin size is 10 cm -1 , then this would correspond to index 5 in the density of sates vector, which stores the counts for each bin. The round () function addresses decimal values and directs MATLAB in which bin to place the value. For example, a result of 4.2 cm -1 would be sorted into <n.5, where the positive integer, n, goes into the n th bin. function rho = BeyerSwine(modes_mat,E_max,bin_size) rho = zeros(1,round(E_max/bin_size)+1); rho(1) = 1; for j = 1:numel(modes_mat) for i = modes_mat(j):bin_size:E_max rho(round(i/bin_size)+1) = rho(round(i/bin_size)+1) + rho(round((i-modes_mat(j))/bin_size)+1); end end 198 Program to calculate the rovibrational density of states The MATLAB program developed to calculate the rovibrational density of states using the Beyer- Swinehart algorithm is shown below: IR = 3522; D0 = 1960; %-- Rotations --% % The PhOH constants are taken from Berden et al. (1996), J. Chem. Phys. 103(15) % Constants in MHz units --> B_rot0 = 2619.236; A_rot0 = 5650.515; C_rot0 = 1789.855; % Constants in cm^-1 --> % converted online A_rot1 = 0.1885; B_rot1 = 0.0874; C_rot1 = 0.0597; Vibrational levels -- dimer and fragments modes_mat = [225 300 404 408 504 526 618 686 749 821 881 958 978 999 1026 1071 1145 1167 1174 1261]; % matrix of FUNDAMENTAL vibrational levels of the cofragment (PhOH) Compute the rovibrational density of states E_max = IR-D0; E_max_lab = num2str(E_max); rot_temp = 119 bin_size = 60; % bin_lab = num2str(bin_size); Compute a rotational density of states vector ('!') rho = zeros(1,round(E_max/bin_size)+1); E_v2_mat = bin_size*(0:(length(rho)-1)); rho = 2/(A_rot1*((C_rot1)^(1/2))).*((E_v2_mat).^(1/2)); % rho(1) = 1; 199 Beyer-Swinehart Algorithm implementation on the 'rho' vector for j = 1:numel(modes_mat) for i = modes_mat(j):bin_size:120 rho(round(i/bin_size)+1) = rho(round(i/bin_size)+1) + rho(round((i- modes_mat(j))/bin_size)+1); end end E_rot1_mat = [0 0 212 300 0 0 704]; J = input('J = '); % Manipulate THIS J_Det = E_rot1_mat(J); E_av = E_max - J_Det % rho_v2 = SmoothRho2(rho); E_rovib_mat = E_v2_mat(E_v2_mat<=E_max); rho_v2 = rho(1:numel(E_rovib_mat)); 200 Appendix A.3: Prior Distribution Program The following program was written in collaboration with Mitchell Butler † to model the vibrational predissociation of the phenol-H2O dimer. This program was used to calculate the prior distribution of the phenol cofragment following predissociation, which is discussed in detail in Chapter 2.2 and Chapter 5 of this dissertation. † College of Medicine, University of Illinois at Chicago, Chicago, Il 60607 Experimental parameters IR = 3520; % IR Excitation energy in wavenumbers D0 = 1960;% Dissociation Energy in wavenumbers E_max = IR-D0; E_rot1_mat = [0 0 212 300 0 0 704]; % matrix that stores the rotational levels of the detected fragment % in this case, H2O J = 1; % manipulates the E_rot_1_mat matrix with stored H2O fragment rotations E_rot1 = E_rot1_mat(J); Cofragment (#2) rotational and vibrational levels and density of states %-- Rotations --% % The PhOH constants are taken from Kleinermanns et al. (1996), J. Chem. Phys. 103(15) % Constants in MHz units --> B_rot0 = 2619.236; A_rot0 = 5650.515; C_rot0 = 1789.855; % Constants in cm^-1 --> % converted online A_rot1 = 0.1885; B_rot1 = 0.0874; C_rot1 = 0.0597; % % OBLATE TOP: % % Set A = B % --> the off-axis O-H in the inertial axis used to compute the B constant creates the % observed difference in the A and B constants % NOTE: the rotational constant is INVERSELY proportional to the 201 % moment of inertia (I) along a given axis A_rot = A_rot1; B_rot = A_rot; % % PROLATE TOP: % % Set C = B % C_rot = C_rot1; % B_rot = C_rot; % (Rotational) Density of States % --> write the constant of proportionality between rho_j2 and (E_j2)^(1/2) CC_j2 = (2/(A_rot*((C_rot1)^(1/2)))); %-- Vibrations --% % levels taken from Kleinermanns et al. (2000), Chem. Phys. 252 % --> see Table 1 and Table 2 ? modes from the [12] reference in the IR, % Raman column of Table 1 AND the list in Table 2 of modes observed in the % DF spectrum vib_modes_co = [225 300 404 408 448 504 525 618 686 749 809 821 881 958 978 998 1008 1025 1036 1052 1174 1220 1252 1261 1337 1457 1481]; % matrix of vibrational levels of the cofragment (PhOH) % --> These modes are ONLY from Table 1 (which are the fundamentals) %vib_modes_co = [225 300 404 408 504 526 618 686 749 821 881 958 978 999 1026 1071 1145 1167 1174 1261]; % matrix of FUNDAMENTAL vibrational levels of the cofragment (PhOH) vib_modes_co = vib_modes_co(vib_modes_co <= (E_max-E_rot1)); % (Vibrational) Density of States % --> Use the Beyer-Swinehart algorithm to compute rho_v2 bin_size = 60; % must be a multiple of the E_max rho_v2 = BeyerSwine(vib_modes_co,E_max,bin_size); % "Smooth" The Vibrational Density of States to Make it a Monotonically % Increasing Function --> Try to Eliminate bin(E_T) dependence smoothRho = SmoothRho2(rho_v2); Compute probability of finding PhOH-H2O in a specific ET state C = 2.5*10^-6; CC_Tot = C*(2/3)*CC_j2; % full calibration constant, which incorporates the PhOH rotational constants, as well as the 4/3 prefactor E_v1 = 0; % for our images to date, we're detecting H20 in v = 0, j = 3, 4, or 7 E_j1 = E_rot1; G_j1 = 1; 202 % bin_ET = bin_size; % specify the bin size for the discretization of E_T space bin_ET = 1; E_av1 = E_max - E_v1 - E_j1; E_T_mat = 0:bin_ET:E_av1; E_v2_mat = bin_size*(0:(numel(smoothRho)-1)); % make a matrix that stores all the E_v2 values --> each bin in the density of states vector corresponds to a v2 energy % must incorporate the v2 = 0 state % E_v2_mat = 0:bin_size:E_max; E_v2_ltd = E_v2_mat(E_v2_mat<E_av1); max_ind = numel(E_v2_ltd); % figure; % plot(E_v2_ltd,rho_v2(1:max_ind)) figure; plot(E_v2_ltd,smoothRho(1:max_ind)) total_count = sum(smoothRho) Algorithm without cofragment (PhOH) rotations à PhOH(J=0) P_mat2 = zeros(1,numel(E_v2_ltd)); E_T_flipd = zeros(1,numel(E_v2_ltd)); G_v2_mat = zeros(1,numel(E_v2_ltd)); for p = 1:length(E_v2_ltd) E_v2 = E_v2_ltd(p); E_T = E_av1-E_v2; E_T_flipd(p) = E_T; G_v2_index = find(E_v2_ltd == E_v2); G_v2 = smoothRho(G_v2_index); G_v2_mat(p) = G_v2; P_mat2(p) = G_j1*G_v2*((E_T)^(1/2)); end P_mat1 = P_mat2; E_T_mat1 = E_T_flipd; 203 Plot the prior distributions %--Set Up to Plot a Stick Spectrum of the PhOH Vibrations in E_T Space--% v2_pos_mat = E_av1-vib_modes_co; v2_pos_mat = v2_pos_mat(v2_pos_mat>0); stick_height = (max(P_mat1)/4)*ones(1,length(v2_pos_mat)); % Plot the P(v1,j1;E) as a function of Translational Energy figure; x2_axis = E_T_mat1; hold on plot(x2_axis,P_mat1,'r*-'); xlim([-10,max(x2_axis)+10]); hold on stem(v2_pos_mat,stick_height) Import Experimental Data This portion of the MATLAB program allows the user to import experimental data from Excel. The data is then converted to energy space using the correct Jacobian factor and is plotted with a prior distribution. data1 = importdata('/Users/dan/Desktop/FreeOH J=321.xlsx'); %names_mat = string(fieldnames(master.data)); % extract the names of the fields within the struct 'master' + store them in 'names_mat' % Use the strcat() function and the extracted field name to write the line % of code 'data1 = master.data.(fieldname);' % Use the eval() function to evaluate the written expression --> puts data1 % in the workspace as the desired variable %num_file = 3; % manipulate THIS %data_name = names_mat(num_file); %data_name2 = char(data_name); %data_label = data_name2(2:end-3); %data1_exp = strcat('data1 = master.data.',data_name,';'); %eval(data1_exp); pixels1 = data1(:,3); counts1 = data1(:,7); % IR OFF counts data counts2 = data1(:,7); % IR ON counts data 204 cc = 5.25; % calibration constant for H20 detection [pixels --> velocity] % cc = 2.28; % calibrtion constant for PhOH detection [pixels --> velocity] % Strange thing going on w/ this beta parameter --> find when we divide it % by 2 (from the original 0.081) it makes the images in energy space match % beautifully w/ the expected/allowed maximum energies beta = 0.081/2; % calibration constant based on images of speciese of known Translational Energy Release mDet = 18; % mass of detected fragment in atomic units mCo = 94; % mass of cofragment in atomic units M = mDet + mCo; cM = ((M)/(mCo)); % "center of mass" conversion parameter mass_ratio = (mCo/mDet)^(1/2); % MUST account for the ratio of the fragment masses ==> there IS a dependence on mass % mass_ratio = 1; alpha = beta*(1000/4000)*mass_ratio; % aggregate "magnifying parameter" -- (1000/400) = voltage ratio IR = 3740; D0 = 1960; % Transform to Energy Space -- Jacobian AND Calibration Parameter E_axis = cM*(alpha*(pixels1).^(2)); % transforms pixels to Energy (E) and then to c.m. Translational Energy E_T E1counts = counts2.*((1./pixels1))*(1/(2*alpha)); E2counts = counts1.*((1./pixels1))*(1/(2*alpha)); Net_counts = E1counts-E2counts; E_max = IR-D0-E_rot1; E_axis = E_axis(E_axis<=E_max); Net_counts = Net_counts(1:length(E_axis)); y2_Boltz = log(Net_counts); x2_Boltz = E_axis; % ---- % 205 Appendix A.4: OPO/OPA MATLAB Functions OPO/OPA Difference Frequency Generation (DFG) Equation: 7 ( :@ %# )= 10 2 D # #"34 )5 − # 678 )5 + # 9 E+:>.FG=>BF;A :;AHB>AB Eq. A.1 where I is the wavelength generated by the OPO/OPA prior to DFG, and the calibration constant is determined using the photoacoustic cell discussed in Appendix B. The following function allows the user to determine what wavelength to input into the LaserVision OPO/OPA system at USC SSC 612 in order to generate the desired IR radiation. The user must input the wavelength in units of wavenumbers, as is described in the following sequence: 1. Input Wavelength (cm -1 ) for the Phenol-H2O Dimer 2. Enter command: OPO (3522) 3. ans = 778.9074 nm [wavelength for difference frequency generation (DFG)] function[y]=OPO(x) y= (((x-82)/10^7)+1/532-1/1064)^-1; end The following function allows the user to determine the wavelength of IR radiation that the LaserVision OPO/OPA system at USC SSC 612 is generating based on DFG using MATLAB: 1. Input Wavelength (cm -1 ) used in DFG by OPO/OPA 2. Enter command: OPA (778.9074) 3. ans = 3522 (Energy in wavenumbers generated by OPO/OPA) function[y]=OPA(x) y= (10^7)*((1/1064)-(1/532)+(1/x))+82 end 206 Appendix A References 1. Baer, T. and Hase, W.L, Unimolecular Reaction Dynamics: Theory and Experiments, Oxford University Press, Inc., New York, NY, 1996. 207 Appendix B: Photoacoustic Cell The photoacoustic effect is over a century old and its application as a detection method has been used since the late 1960’s in absorption spectroscopy, kinetic studies, calorimetry, trace gas detection, and photoacoustic imaging. 2 In short, a molecule which has absorbed radiation may lose it through emission or a collision. In the gas phase, at only a few Torr, loss of vibrational energy is mainly through collisions. As a result of collisions between molecules, vibrational energy is transferred to the collisional partner, which receives it in the form of translational energy. If the radiation source is modulated, the temperature of the collision partner oscillates and a pressure variation in the form of sound wave travels through the gaseous medium. The sound wave can be detected by a microphone attached to a gas cell containing the sample. Photoacoustic spectroscopy is accomplished when a pulsed laser source changes frequency as it is fired through the gaseous region. When the sample absorbs a photon from the pulsed light, the constituent molecules become thermally excited, and periodic heat flows from the sample to the surrounding gaseous medium creating a pressure wave which can be detected. The photoacoustic cell (shown in Figure B.1) was used to calibrate the wavelength of the OPO/OPA infrared laser system during the work described in this dissertation. The diagram illustrates the electronic connections inside and outside of the vacuum cell. The 9V battery that powered the microphone (PC-mount omnidirectional microphone element with 50-10,000 Hz frequency response) was connected through vacuum sealed leads via a BNC connection. The output of the microphone was connected to another BNC connector that was directed towards the oscilloscope. The path length of the laser beam through the cell was approximately 0.15 meters. 208 Figure B.1: Diagram of photoacoustic cell electronics including microphone, batteries to power microphone, and connection to oscilloscope for data collection. Resistor was 2200 Ohm, capacitor was 0.1 mF, and the system was grounded at the back of the oscilloscope The vacuum cell was filled with 1-3% NH3 in 300-500 torr of Helium. For data collection, the oscilloscope window was set to 4 ms to collect the positive area of the signal from the microphone. Signal was recorded using Labview (National Instruments), while scanning the frequency of the IR laser. Results are shown in Figure B.2, which were compared to FTIR data from the NIST database to determine the offset (calibration constant) in the IR frequency. The offset is variable dependent on the crystal alignment of the OPO/OPA system and is dependent on the frequency region being scanned. It is recommended that the OPO/OPA be recalibrated every year. The calibration constant was found to be + 82 cm -1 in 2019. 209 Figure B.2: (Red) Photoacoustic spectrum of 1-3% NH3 in 400 Torr of Helium gas in a vacuum cell at room temperature. (Black) NIST reference spectrum 3 used for OPO/OPA IR calibration. The calibration constant was found to be +82 in February of 2019. Appendix B References 1. Hollas, J.M. High Resolution Spectroscopy, 2nd. edn., 1998. 2. Kleiner, I. Brown, L. R. Tarrago, G. Kou, Q.-L. Piqué, N. Guelachvili, G. Dana, V. and Mandin, J.-Y., Journal of Molecular Spectroscopy, 1999, 193, 46-71. 210 Appendix C: Theoretical Geometries and Energies for Pyrazine-H2O Clusters Table C.1: Optimized geometries at the RIMP2/aug-cc-pVTZ level of theory. Molecule Level of theory Optimized Z matrix Pyrazine RIMP2/aug-cc-pVTZ N 0 1 C 1 1.339192 H 2 1.083030 1 116.932160 C 1 1.339192 2 115.407184 3 180.000000 0 H 4 1.083030 1 116.932168 2 -180.000000 0 C 4 1.393221 1 122.296404 2 0.001827 0 H 6 1.083030 4 120.771454 1 180.000000 0 N 6 1.339192 4 122.296413 1 -0.002068 0 C 8 1.339191 6 115.407180 4 0.002058 0 H 9 1.083030 8 116.932195 6 179.999069 0 Pyrazine-H2O (side) RIMP2/aug-cc-pVTZ N 0 1 C 1 1.339472 H 2 1.082679 1 116.940076 C 1 1.339502 2 116.115842 3 -179.715773 0 H 4 1.082704 1 116.918342 2 179.711732 0 C 2 1.392538 1 121.806715 3 179.876427 0 H 6 1.082739 2 120.726651 1 179.978453 0 N 6 1.339123 2 122.317766 1 -0.029973 0 C 8 1.339119 6 115.636146 2 -0.105661 0 H 9 1.082734 8 116.955794 6 -179.882512 0 H 1 1.941056 2 122.643618 3 -4.498081 0 O 11 0.974109 1 175.042664 2 -95.549060 0 H 12 0.960583 11 104.567364 1 -173.511066 0 211 Pyrazine-H2O (top) RIMP2/aug-cc-pVTZ 0 1 N C 1 1.340079 H 2 1.082907 1 116.952364 C 1 1.340310 2 115.592238 3 179.798417 0 H 4 1.082951 1 116.958046 2 -179.789363 0 C 4 1.393350 1 122.189812 2 0.846436 0 H 6 1.082982 4 120.848050 1 179.461002 0 N 6 1.340383 4 122.209074 1 -0.007281 0 C 8 1.339986 6 115.589766 4 -0.823949 0 H 9 1.082869 8 116.952669 6 -179.739074 0 H 1 2.708225 2 86.635417 3 -95.532883 0 O 11 0.962778 1 132.380919 2 -58.155244 0 H 12 0.962653 11 103.737627 1 -5.352839 0 H2O-Pyrazine-H2O (Z) RIMP2/aug-cc-pVTZ N 0 1 C 1 1.339084 H 2 1.082452 1 117.151432 C 1 1.340635 2 116.291793 3 179.834497 0 H 4 1.082855 1 116.533902 2 -179.796332 0 C 2 1.391573 1 121.884356 3 -179.970678 0 H 6 1.082452 2 120.963794 1 179.968400 0 N 6 1.339085 2 121.885102 1 -0.000000 0 C 8 1.340644 6 116.291473 2 0.136110 0 H 9 1.082853 8 116.533573 6 179.800827 0 H 1 1.970785 2 143.119282 3 5.534759 0 O 11 0.972416 1 152.179198 2 167.402993 0 H 12 0.960649 11 104.941394 1 154.334899 0 H 8 1.970910 6 143.173971 2 174.580329 0 O 14 0.972416 8 152.102079 6 -167.653094 0 H 15 0.960647 14 104.944757 8 -154.541558 0 212 H2O-Pyrazine-H2O (E) RIMP2/aug-cc-pVTZ N 0 1 C 1 1.339167 H 2 1.082361 1 117.130051 C 1 1.340658 2 116.265449 3 179.996489 0 H 4 1.082953 1 116.598729 2 -179.988851 0 C 2 1.391874 1 121.958631 3 180.000000 0 H 6 1.082951 2 121.626462 1 -179.982232 0 N 6 1.340686 2 121.775879 1 0.006353 0 C 8 1.339145 6 116.265217 2 -0.004489 0 H 9 1.082363 8 117.131446 6 180.000000 0 H 1 1.972702 2 144.760616 3 0.289638 0 O 11 0.972450 1 149.890689 2 179.478882 0 H 12 0.960555 11 105.051336 1 -179.986944 0 H 8 1.972760 6 98.957453 2 -179.836596 0 O 14 0.972445 8 149.892530 6 0.251056 0 H 15 0.960555 14 105.050651 8 179.963555 0 Pyrazine-(H2O)2 (Bridge) RIMP2/aug-cc-pVTZ N 0 1 C 1 1.339950 H 2 1.082723 1 117.137130 C 1 1.342869 2 116.455972 3 -179.828801 0 H 4 1.083935 1 116.894761 2 179.520027 0 C 2 1.391651 1 121.759855 3 179.943265 0 H 6 1.082843 2 120.685073 1 -179.969043 0 N 6 1.339168 2 122.297626 1 0.007996 0 C 8 1.339232 6 115.601765 2 -0.095914 0 H 9 1.082691 8 116.888233 6 -179.939703 0 H 1 1.862361 2 130.884733 3 -0.026071 0 O 11 0.982221 1 166.846794 2 -173.348559 0 H 12 0.961174 11 105.210043 1 -126.390744 0 H 12 1.845354 11 97.094464 1 -5.518247 0 O 14 0.975524 12 163.701929 11 -3.651218 0 H 15 0.960471 14 104.899383 12 146.957186 0 213 Pyrazine-(H2O)2 (Linear) RIMP2/aug-cc-pVTZ 0 1 N C 1 1.338931 H 2 1.082741 1 117.082061 C 1 1.340659 2 116.044282 3 -179.996562 0 H 4 1.083067 1 116.444469 2 -179.994357 0 C 2 1.392533 1 121.924454 3 -179.998148 0 H 6 1.082943 2 120.712115 1 179.991567 0 N 6 1.339177 2 122.324480 1 -0.005589 0 C 8 1.339433 6 115.528369 2 0.000000 0 H 9 1.082819 8 116.902220 6 -179.993609 0 H 1 2.019899 2 146.544900 3 -0.262909 0 O 11 0.969598 1 147.854108 2 -179.449136 0 H 12 0.966591 11 105.417456 1 -178.290721 0 O 13 1.988506 12 168.690490 11 -171.124437 0 H 14 0.962170 13 109.317466 12 -64.632328 0 H 14 0.962280 13 105.871631 12 47.327593 0 214 Table C.2: Single-point energy calculations of pyrazine-H2O clusters using CCSD(T)/aug-cc- pVTZ Molecule Level of Theory Energy in basis set (Hartree) Pyrazine CCSD(T)/aug-cc-pVTZ SCF energy = -262.77061626 MP2 energy = -263.80695039 CCSD correlation energy = -1.04430192 CCSD total energy = -263.81491817 CCSD(T) correlation energy = -0.05839560 CCSD(T) total energy = -263.87331377 Pyrazine-H 2O (side) CCSD(T)/aug-cc-pVTZ SCF energy = -338.83580115 MP2 energy = -340.14640180 CCSD correlation energy = -1.32199984 CCSD total energy = -340.15780099 CCSD(T) correlation energy = -0.06790170 CCSD(T) total energy = -340.22570269 Pyrazine-H 2O (top) CCSD(T)/aug-cc-pVTZ SCF energy = -338.82979032 MP2 energy = -340.14042809 CCSD correlation energy = -1.32202851 CCSD total energy = -340.15181883 CCSD(T) correlation energy = -0.06793664 CCSD(T) total energy = -340.21975547 H 2O-Pyrazine-H 2O (Z) CCSD(T)/aug-cc-pVTZ SCF energy = -414.90018977 MP2 energy = -416.48603070 CCSD correlation energy = -1.60046756 CCSD total energy = -416.50065733 CCSD(T) correlation energy = -0.07751307 CCSD(T) total energy = -416.57817041 H 2O-Pyrazine-H 2O (E) CCSD(T)/aug-cc-pVTZ SCF energy = -414.90023926 MP2 energy = -416.48634369 CCSD correlation energy = -1.60065650 CCSD total energy = -416.50089577 CCSD(T) correlation energy = -0.07752897 CCSD(T) total energy = -416.57842473 Pyrazine-(H 2O) 2 (bridge) CCSD(T)/aug-cc-pVTZ SCF energy = -414.90427917 MP2 energy = -416.49092581 CCSD correlation energy = -1.60137880 CCSD total energy = -416.50565797 CCSD(T) correlation energy = -0.07774596 CCSD(T) total energy = -416.58340392 Pyrazine-(H 2O) 2 (linear) CCSD(T)/aug-cc-pVTZ SCF energy = -414.90020983 MP2 energy = -416.48262176 CCSD correlation energy = -1.59809838 CCSD total energy = -416.49830822 CCSD(T) correlation energy = -0.07707484 CCSD(T) total energy = -416.57538306 215 Table C.3: Single-point energy calculations of [Pyrazine-H2O] + cluster ions at the CCSD(T)/aug- cc-pVTZ level of theory Molecule Level of Theory Energy in basis set (Hartree) [Pyrazine] + CCSD(T)/aug-cc-pVTZ SCF energy = -262.40195121 MP2 energy = -263.46780722 CCSD correlation energy = -1.04682517 CCSD total energy = -263.44877638 CCSD(T) correlation energy = -0.06764490 CCSD(T) total energy = -263.51642128 [Pyrazine-H 2O] + (side) CCSD(T)/aug-cc-pVTZ SCF energy = -338.47664666 MP2 energy = -339.73470509 CCSD correlation energy = -1.29628043 CCSD total energy = -339.77292709 CCSD(T) correlation energy = -0.07474131 CCSD(T) total energy = -339.84766841 [Pyrazine-H 2O] + (top) CCSD(T)/aug-cc-pVTZ SCF energy = -338.44941601 MP2 energy = -339.79055864 CCSD correlation energy = -1.32531433 CCSD total energy = -339.77473034 CCSD(T) correlation energy = -0.07736118 CCSD(T) total energy = -339.85209152
Abstract (if available)
Abstract
The state-to-state vibrational predissociation (VP) dynamics of water clusters with hydrogen chloride or aromatic molecules were studied following infrared excitation of an intramolecular vibrational mode in each cluster. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated center-of-mass translational energy distributions. Product energy distributions and dissociation energies were determined. ❧ The cyclic HCl-(H₂O)₃ tetramer is the largest observed neutral HCl-(H₂O)ₙ cluster. The VP of HCl-(H₂O)₃ was investigated by both theory and experiment, following infrared (IR) laser excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are: HCl + (H₂O)₃ (Pathway 1) and H₂O + HCl-(H₂O)₂ (Pathway 2). The HCl and H₂O monomer fragments were observed by 2+1 REMPI combined with time-of-flight mass spectrometry (TOF-MS), and their rotational energy distributions are inferred and compared to the theoretical results. VMI of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm⁻¹ and 2400 ± 100 cm⁻¹ for Pathway 1 and Pathway 2, respectively. These values are lower, but in the same order as the corresponding calculated dissociation energies: 2426 ± 23 cm⁻¹ and 2826 ± 19 cm⁻¹. The differences are attributed to contributions of the high-speed tail of the experimental pair-correlated distributions from vibrational hot bands of the clusters. Satisfactory agreement between theory and experiment was achieved when comparing the monomer fragments’ rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from quasi-classical trajectory (QCT) calculations, which are performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are formed in their lowest energy cyclic forms. ❧ The VP dynamics of the phenol–water (PhOH–H₂O) dimer were studied by detecting H₂O fragments and using VMI to infer the internal energy distributions of PhOH cofragments, pair-correlated with selected rotational levels of the H₂O fragments. Following IR laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H₂O (Pathway 2), dissociation to H₂O + PhOH was observed. H₂O fragments were monitored state-selectively by using 2+1 REMPI combined with TOF-MS. VMI of H₂O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (ET) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H₂O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited. ❧ The first observation of the VP of the pyrazine-H₂O dimer following excitation of the “free” OH stretch and CH stretch regions was confirmed by the detection of neutral H₂O products using REMPI. Following pulsed supersonic expansion, significant rovibrational cooling in the (π,π^*) electronic transition was observed for the pyrazine monomer and pyrazine-H₂O dimer. Detection of the pyrazine-H₂O dimer by 1+ n REMPI and TOF-MS was achieved for the first time. VMI measurements allowed the distinction between translationally cold pyrazine generated in the molecular beam and pyrazine molecules generated in dissociative ionization of higher clusters, which possess kinetic energy. Theoretical calculations indicated that the OH-stretch vibrational peak observed in the experiments corresponds to the non-bonded or “free” hydrogen of the water moiety. Theoretical calculations to characterize the structure and stability of the pyrazine-H₂O dimer and trimer and their cations are ongoing. These preliminary establish diagnostics of the clusters to be used in future VMI experiments on the VP dynamics and formation of the H₂O fragment, as well as the detection and characterization of Pyrazine-(H₂O)₂
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University of Southern California Dissertations and Theses
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Asset Metadata
Creator
Kwasniewski, Daniel Thomas
(author)
Core Title
Dissociation energy and dynamics of HCl and aromatic-water clusters
School
College of Letters, Arts and Sciences
Degree
Doctor of Philosophy
Degree Program
Chemistry
Publication Date
10/19/2020
Defense Date
08/03/2020
Publisher
University of Southern California
(original),
University of Southern California. Libraries
(digital)
Tag
action spectroscopy,clusters,dimer,hydrogen bond,OAI-PMH Harvest,phenol,phenol-water,pyrazine,pyrazine-water,REMPI,tetramer,velocity map imaging,vibrational predissociation,VMI,water
Language
English
Contributor
Electronically uploaded by the author
(provenance)
Advisor
Reisler, Hanna (
committee chair
), Dawlaty, Jahan Mansoor (
committee member
), Kresin, Vitaly (
committee member
)
Creator Email
Daniel.t.kwasniewski@gmail.com,kwasniew@usc.edu
Permanent Link (DOI)
https://doi.org/10.25549/usctheses-c89-382982
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UC11666331
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etd-Kwasniewsk-9052.pdf (filename),usctheses-c89-382982 (legacy record id)
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etd-Kwasniewsk-9052.pdf
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382982
Document Type
Dissertation
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Kwasniewski, Daniel Thomas
Type
texts
Source
University of Southern California
(contributing entity),
University of Southern California Dissertations and Theses
(collection)
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The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...
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Tags
action spectroscopy
clusters
dimer
hydrogen bond
phenol
phenol-water
pyrazine
pyrazine-water
REMPI
tetramer
velocity map imaging
vibrational predissociation
VMI
water