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Vibrational sum frequency generation studies of electrode interfaces

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Vibrational sum frequency generation studies of electrode interfaces

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Content Vibrational Sum Frequency Generation studies of Electrode Interfaces by Joel Gabriel Patrow A Dissertation Presented to the Faculty of The USC Graduate School University of Southern California In Partial Fulllment of the Requirements for the Degree Doctor of Philosophy (Chemistry) May 2019 Copyright 2018 Joel Gabriel Patrow ii Table of Contents List of Figures v List of Tables ix Acknowledgements xi 1 Introduction 1 1.1 Electric Fields at Electrochemical Interfaces . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Lewis Acid-Base Adducts at the Interface. . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Theory and Practice of Vibrational Sum Frequency Generation Spectroscopy 5 2.1 Modeling vSFG processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Direct Spectroscopic Measurement of Interfacial Electric Fields 13 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Experiment and Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.3 Result and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.3.1 Stark shift in the absence of electrochemical current: . . . . . . . . . . . . . . 19 3.3.2 Stark shift in the presence of electrochemical current: . . . . . . . . . . . . . 27 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4 Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spec- troscopy 31 A Typical IR and Visible Upconversion Spectra 47 B Tensors and the Second Order Hyperpolarizability 49 B.1 Hooke's Law as a Motivating Example of Tensors . . . . . . . . . . . . . . . . . . . . 49 B.2 Tensor Notaion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 B.3 Using Tensor Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 B.4 Tensors and Symmetry Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 C Supporting Information for Direct Spectroscopic Measurement of Interfacial Electric Fields 57 C.1 Modeling the Electric Field at the Electrode-Electrolyte Interface . . . . . . . . . . . 57 C.2 Representative vSFG Data of the Potential and Ionic Concentration Dependent Data 63 iii TABLE OF CONTENTS C.2.1 Representative vSFG Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 C.2.2 Representative Fit Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 66 D Supporting Information to Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy 75 D.1 Spectral Fitting Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 D.2 Electric Field Calculations of MBN and MBN/BCF . . . . . . . . . . . . . . . . . . 80 References 87 iv List of Figures 1.1 Frustrated Lewis acid-base pair B(C 6 F 5 ) 3 (acid) and P(C 4 H 12 ) 3 (base). As can be seen the central a toms boron and phosphorous are very crowded by their bulky substituents. Therefore, they do not react. . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 Schematic showing vSFG spectroscopy in the non-colinear propogating geometry. The vSFG signal comes o the surface at an angle separate from the IR and Vis pulses. 6 2.2 Energy level diagram depicting a vSFG process. = 1 is the rst vibrational state of a given vibrational mode and V is a virtual state. . . . . . . . . . . . . . . . . . . . . 7 2.3 Double sided Feynman diagram depicting a SFG process. . . . . . . . . . . . . . . . 9 3.1 (a) A picture of the experimental cell used to perform the experiment. (b) A diagram showing a cross section of the cell and the accessories. (c) A representative spectrum (blue) and its t (red) of the positive scan for V = 0.0 V with a 1000mM KCl concentration. The dip around 2230 cm 1 is the nitrile stretch Representative data including the t. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.2 Correlation between (a) the current and (b) the nitrile stretch frequency measured as a function of applied bias. A clear correspondence is observed between regions of non-zero current and the change in vibrational frequency of the nitrile stretch which reports the local electric eld. When current is nearly zero, a large potential drop across the interface can be maintained and the interface behaves more like a capacitor. The increase in potential leads to a linear variation of the eld and consequently a linear variation in the vibrational frequency. When the interface passes current, it is akin to a capacitor shorted via a low resistance with diminished ability to maintain a potential drop. Therefore the dependence on potential vanishes. . . . . . . . . . . . 20 3.3 (a) A plot of the frequency change as a function of applied potential 0 and ionic concentration n. (b) A slice through this plot at 0.3 V. Relating these frequency shifts to eld variations is the central goal of this section. . . . . . . . . . . . . . . . 21 3.4 A picture of the model. Region 1 is composed of the 4-MBN SAM with thickness d. Region 2 is the diuse ionic layer. The potential drop across the SAM is linear, while across the ionic layer it follows the Gouy-Chapman theory. Note that the potentials displayed here ( ) are not referenced to the reference electrode potential. See chapter C.1 for further details. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 v LIST OF FIGURES 3.5 (a) A slice of ( 0 ;n) for 0 = 0:3V (blue) for the positive scan with a slice of the t at 0 = 0:3V (red). (b) Slice of ( 0 ;n) for n = 10mM (blue) for the positive scan with a slice of the t at n = 10mM (red). The table contains the extracted tting parameters of both the positive and negative scans along with the average values of the two scans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 3.6 A plot of the electric eld F = F( 0 ;n) F(0;n) experienced by the SAM layer based on the model and the experimentally retrieved parameters. The plot gives the general behavior of how the interfacial eld varies with ionic concentration and potential. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.7 A circuit diagram that can qualitatively explain the observed dierence in vibrational frequency shift of the nitrile probe molecule in the current-carrying and polarizing conditions. When no electrochemical current ows (corresponding to the reverse- biased diode), the eld near the interface builds up linearly with potential, analogous to charging the capacitors. At potentials for which the redox reaction becomes possible, the leaked current prevents further build up of eld across the SAM layer. Increase in potential under this condition yields more current but not more polarization of the probe molecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 4.1 (A) vSFG spectra of MBN monolyaer on gold before and after adduction formation with BCF. (B) FTIR spectra of benzonitrile before and after adduct formation with BCF. Cartoons are representations of LP interaction on the surface and in the bulk. 37 4.2 (A-C) SFG spectra of MBN and MBBN (before and after adduct formation) at full and diluted surface coverages. The table shows the nitrile frequencies and the frequency dierences between adducted and un-adducted molecules. . . . . . . . . . 39 4.3 Calculated nitrile frequency shifts with respect to N-B tilt angle. Deviation from 0 results into poor overlap of the donor and acceptor orbitals in the Lewis adduct and consequently red shifts the nitrile frequency. . . . . . . . . . . . . . . . . . . . . . . . 40 4.4 (A) Calculated nitrile frequencies in the presence of applied electric elds for benzoni- trile and benzonitrile/BF 3 adduct using !B97X-D. While the benzonitrile frequency conforms to a linear Stark shift model (see gure D.3 in the appendix), the frequency shift of the adduct shows a discontinuity around 6.5 MV/cm. This discontinuity changes with DFT functional, however, is always found at relatively small electric elds (see gure D.4 in the appendix). (B) The Lewis donor-acceptor (nitrogen and boron) interatomic distance as a function of applied electric eld. The discontinuity in (A) corresponds to a large shift in the donor acceptor-distance, 0.4 A, indicating the partial dissociation of the adduct at that eld value. . . . . . . . . . . . . . . . 43 A.1 (a) A spectrum of the upconverted IR pulse re ected o of bare Au. The large dip between 2300 - 2400 cm 1 comes from ambient CO 2 adsorption. We enclose the experimental setup and evacuate the enclosure with dried and CO 2 free air. (b) The visible upconversion pulse. The pulse is made by passing the output of a regeneratively amplied femtosecond Ti:Sapph laser through a 4f lter to narrow the pulse to a FWHM of 0.05 nm (8 cm 1 ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 B.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 B.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 B.3 Methane molecule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 vi LIST OF FIGURES C.1 A picture of the model that describes potential variation near the interface. The interface is divided into a dielectric layer composed of the 4-MBN SAM and a diuse Gouy-Chapman layer. The potential at the electrode and ionic concentration in the bulk are the experimentally controllable variables. The potential prole across the interface is obtained by applying Gauss's law at the boundaries as described in the text. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 C.2 Positive scan zoomed in spectra for V = 0, 0.5 and -0.8 V. The dip arises from the CN stretch. At small ionic concentrations, the dierence in frequency \peak" position for 0.0 V and 0.5 V is small and increases with larger ionic concentrations. It is also apparent that the change in frequency at -0.8 V and 0.0 V is small at low ionic concentrations as discussed in the main body of the publication. . . . . . . . . . . . 64 C.3 Same as the above gure except this is for the negative scan. . . . . . . . . . . . . . 65 C.4 Plots of the center wavelength ! CN extracted from the ts as a function of applied potential and ionic concentration. Panels (a) and (b) correspond to positive and negative scan as explained in the experimental section. As discussed in the main text, for potentials where negligible current is drawn (0.0 - 0. 5 V), is linear with respect to 0 . However, as can be seen for the 100 mM and 1000 mM negative scans, behaves linearly only after reaching 0.3 V for the rst time along the scan. Furthermore, the last applied potential for all the negative scans was 0.1 V, not 0.0 V. Because of this, linear extrapolations were used to obtain the missing values of , including the 100 mM and 1000 mM reference frequencies 0 . . . . . . . . . . . . . . 66 C.5 The plot of the frequency change for the negative scan. As can be seen, it matches the frequency changes for the positive scan (shown in the main text) quite well. . . . 72 C.6 A representative current-time plot at -0.2 V. Notice that at rst, there is a transient signal from the voltage jump. However, after about 10 seconds, the current begins to level o and stabilize. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 D.1 A cartoon of computational setup we have used. . . . . . . . . . . . . . . . . . . . . 81 D.2 The contour plots on top are taken at dierent slices between the two meshes. The colors on the contour indicate electric eld magnitudes in units of MV/cm. The electric eld magnitude changes by at most 0.08 MV/cm across the 64 A 2 represented in the plots. The lower two plots display the invariance of the electric eld along the z direction. The left plot shows the point (x,y) the z direction traces are taken at in the right plot. The largest variance between the traces is on the order of 0.02 MV/cm while total change along a specic trace is about 0.01 MV/cm. . . . . . . . 82 D.3 Plot of the computational frequencies for benzonitrile as a function of applied electric eld. The tting equation is displayed in the inset. The main parameter we use to benchmark our method of calculating frequency changes in response to electric elds is . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 D.4 Plot of the computational nitrile frequencies for the Benzonitrile/BF 3 adduct calcu- lated using both !B97X-V and !B97X-V. As can be seen, the electric eld at which the adduct partially dissociates is dependent on how we account for the Van der Waals interactions. However, we note that both the!B97X-V and!B97X-V partially dissociate the adduct at relatively weak electric elds, 6.5 MV=cm and 9 MV=cm 84 D.5 FTIR spectra of MBN and Benzonitrile in dimethylformamide. . . . . . . . . . . . . 85 vii LIST OF FIGURES viii List of Tables C.1 Fit Parameters for the Electric Field Study . . . . . . . . . . . . . . . . . . . . . . . 66 D.1 Fit parameters for Lewis Acid-Base Study (MBN) . . . . . . . . . . . . . . . . . . . 75 D.2 Fit parameters for Lewis Acid-Base Study (MBBN) . . . . . . . . . . . . . . . . . . 78 ix LIST OF TABLES x Acknowledgements I would like to start by thanking my advisor, Professor Jahan Dawlaty. He has consistently inspired me to think for myself and to understand things on a fundamental level. Additionally, his patience and humility as a teacher never ceases to amaze me. I am grateful to have gotten the opportunity to work with him for the past four and a half years. I would also like to thank the other professors who have oered me guidance both scientically and professionally. Specifcally, I would like to address Professor Stephen Bradforth and Professor Alexander Benerskii. Both have helped me understand my research better and have provided valuable feedback every Wednesday evening during group meetings. My ability to communicate scientic material has undoubtedly increased because of their help. There are also many previous and current graduate students I would like to thank. Dr. Shayne Sorenson and I have spent many hours gathering SFG data together in the dark laser room. Our conversations about science, life and parenting made hours of data gathering seem much shorter. I would also like to thank Dr. Yi Wang, Dr. Shima Hagighat, Dr. Eric Driscoll, Ryan Hunt, Dr. Sohini Sarkar, Anuj Pennathur and Matthew Voegtle. They were wonderful lab-mates and were always willing to oer ideas or lend an ear. I would also like to the thank my good friend Dianna Zeegers. Over many years now she has been a continual source of inspiration and I am forever grateful for our discussions about life. Now, I would like to thank my family because without their support I would not be here. I would like to start with my father, Miles Patrow. He has always believed in me and has supported me in so many ways. From helping me nancially to giving me an outlet to vent to talking Milwaukee xi CHAPTER 0. ACKNOWLEDGEMENTS baseball with me, he has always been there for me. He helped me believe that I can do whatever I want to. I would like to thank my mother, Jeanette Robbins for encouraging me to always be creative and listening to me complain about whatever is going on. Whenever I have been frustrated she has answered my phone calls and is always willing to talk. I would like to thank my brother, David, and sister, Chessa, for making my 3 hour daily commutes much more bearable. Finally, I would like to thank my wife, Michelle Ambelang. So much of what I have been able to accomplish has only been possible because she has been at my side. She moved across the country with me so I could chase my dream. She has comforted me when I was struggling, she has laughed with me when things went well. She has been so supportive and I am forever grateful and honored to be her husband. xii Chapter 1 Introduction Heterogeneous catalysis and interfaces are ubiquitously found together. The interfaces between the catalytic material and the reactants can be very small, like that found in Raney Nickel, 1 or very large, like that found in electrochemistry. However, the important thing to note is that whether on the microscopic or macroscopic scale, the catalytic material and the reactants make an interface and it is at this interface that a large amount of the chemistry of heterogeneous catalysis occurs. This work is largely concerned with electrochemical interfaces. Understanding these interfaces is important for renewable energy applications. The electrochemical conversion of renewable energy to chemical energy that can then be stored and used accordingly is essential to the widespread implementation of renewable energy technologies. 2{4 Traditionally, electrochemistry and electrochemical interfaces have been studied using a variety of voltammetric techniques where the electrode potential and current are the key experimental parameters and observables. Though these techniques are highly useful and have yielded fundamental insight regarding interfacial processes and electrochemical reactions, they rely on accurately decomposing the current response into contributions from the interface and contributions from everything else (response of the bulk), which includes the motion of ions, reactants and products to and from the interface. It is better to directly study the electrochemical interface using techniques that probe only the interface. Studying any interface, however, can be particularly dicult. One reason (among many) for this 1 CHAPTER 1. INTRODUCTION is because interfaces, which are typically only nanometers wide, have very small responses and are therefore dicult to measure. This issue is only compounded by the fact that interfaces are almost always buried. This is especially true for the electrochemical interface, where one side is composed of the electrode and the other is composed of the electrolyte. One way to study interfacial processes is to use a spectroscopic technique called vibrational sum frequency generation spectroscopy (vSFG), which is an interface specic non linear spectroscopy (i.e. signal is obtained only from the interface). Because it is interface specic it is ideally suited for directly studying interfaces. The specics of vSFG and the associated theory can be found in chapter 2 and in chapter B. Brie y, in this work, vSFG has been used to monitor vibrational chromophores covalently attached to surfaces of interest. Because the chromophores are attached at specic locations within the electrochemical interface their vibrational response is a direct reporter of the interface. Specically, nitrile vibrational frequencies have been used to probe both the electrostatic environment and the extent of interaction between lewis acid base pairs at the electrode interface. 1.1 Electric Fields at Electrochemical Interfaces An important "quantity" to electrochemical processes is the interfacial electric eld. Near the electrode surface, the electric eld is expected to be quite large and, therefore, play an important role in electrochemical catalysis. 5,6 For example, during an electrochemical reaction, unstable intermediates can be generated. If these intermediates have a dipole, then they can be stabilized in the presence of these electric elds. Additionally, molecules in the presence of strong electric elds are polarized and this can aect their reactivity. The importance of electric elds to catalysis is not a new discovery, however. Work by Warshel 7,8 and Boxer 5,9 have shown that strong electric elds play a large role in enzyme catalysis. Despite the clear importance of interfacial electric elds to electrochemistry, very few studies have directly measured them. 6,10{15 This is likely due to the diculty associated with their measurement, as discussed above. One way to accomplish this is by monitoring the vibrational Stark shift of well calibrated vibrational chromophores attached to the electrode surface. In chapter 3, the 2 1.2. LEWIS ACID-BASE ADDUCTS AT THE INTERFACE. details of a study examining the electric elds at the electrode interface using this method is outlined. Additionally, a simple electrostatic model is developed and used to better understand the experimental results. 1.2 Lewis Acid-Base Adducts at the Interface. One interesting facet of homogeneous catalysis that has been developing recently is the use of frustrated Lewis acid base pairs (FLPs) to drive chemical reactions. In FLPs, the lewis acids and lewis bases want to react, however, they are unable to because of sterich hinderance (see gure 1.1). For example, if tris(penta uorophenyl) borane (B(C 6 F 5 ) 3 ) and tri-tert-butylphosphine (P(C 4 H 12 ) 3 ) are dissolved in toluene, they do not react to form a lewis acid-base adduct. However, upon the addition of hydrogen gas (H 2 ), precipitation of the ionic compounds [HB(C 6 F 5 ) 3 ] [HP(C 4 H 12 ) 3 ] + occurs. Where the H 2 molecule has been heterolytically cleaved into a hydride and a proton. 16 Because water and hydrogen splitting are central components of many renewable energy storage devices, integrating FLPs into electrochemical architecture holds promise. The main idea being if FLPs could be tethered to the surface of an electrode, then utilization of their inherent ability to cleave H 2 molecules could drastically improve the eciency of converting chemical energy to electrical energy. In chapter 4, the formation of interfacial lewis acid-base adducts at the surface of gold thin lms is reported utilizing B(C 6 F 5 ) 3 as the frustrated lewis acid. Several aspects of the adduct formation is discussed including the dierences between adduct formation in the bulk compared to that formed at the surface. 3 CHAPTER 1. INTRODUCTION B(C 6 F 5 ) 3 P(C 4 H 12 )3 Figure 1.1: Frustrated Lewis acid-base pair B(C 6 F 5 ) 3 (acid) and P(C 4 H 12 ) 3 (base). As can be seen the central a toms boron and phosphorous are very crowded by their bulky substituents. Therefore, they do not react. 4 Chapter 2 Theory and Practice of Vibrational Sum Frequency Generation Spectroscopy Vibrational Sum Frequency Generation (vSFG) spectroscopy is a second order non linear spectro- scopic method that is often used to study interfaces due to the fact that it is a interface specic spectroscopy. The surface specicity of vSFG is a manifestation of the symmetry of the interface. A detailed mathematical discussion of this can be found in the chapter B. Using non linear opitical formalism, the vSFG polarization is described as: ~ P (2) = (2) : ~ E IR ~ E Vis (2.1) Where (2) is the second order susceptibility of the interface being studied and ~ E IR and ~ E Vis are infrared (IR) and visible light sources, respectively. Typically, the IR and visible light sources are pulsed lasers because (2) is very small and in order to create a polarization that is measurable very large electric elds, like those found in femtosecond laser pulses, are needed. 17 5 CHAPTER 2. THEORY AND PRACTICE OF VIBRATIONAL SUM FREQUENCY GENERATION SPECTROSCOPY Figure 2.1: Schematic showing vSFG spectroscopy in the non-colinear propogating geometry. The vSFG signal comes o the surface at an angle separate from the IR and Vis pulses. The name sum frequency comes from the fact that when the two laser beams (IR and visible laser beams) are overlapped in space and time at an interface, a new beam of light is generated at frequency ! vSFG = ! IR +! Vis . The mechanism of the vSFG signal can be understood in a couple of dierent manners. Figure 2.2 qualitatively describes a vSFG process. If we assume that the IR light is resonant with the surface, then a coherence between the ground vibrational state and the rst vibrational state is created upon interaction with the IR light. Then, the visible light excites the vibrational coherence to a virtual state (Raman transition) which then radiates the vSFG signal putting the system back in the ground state. The direction of the the emitted light is found by conservation of momentum: ~ k vSFG = ~ k IR + ~ k Vis . Thus, if the IR and visible beams are non co-linear, then the vSFG signal will be emitted in a separate direction (see gure 2.1). A quantitatively simple way to show where the "sum" in sum frequency generation comes from can be found by a quick analysis of equation 2.1. We start by expressing the electric elds as temporally oscillating plane waves. We can then recast equation 2.1 as: ~ P (2) = (2) :E IR e i! IR t E Vis e i! Vis t ^ e IR ^ e vis (2.2) =E IR E Vis e i(! IR +! Vis )t ^ e IR ^ e vis (2.3) As can be seen, via simple algebra, we have shown that the second order polarization will oscillate at a frequency equal to the sum of the two incident light sources. It is important to note that we 6 2.1. MODELING VSFG PROCESSES = Figure 2.2: Energy level diagram depicting a vSFG process. = 1 is the rst vibrational state of a given vibrational mode and V is a virtual state. have used the dipole approximation here. 2.1 Modeling vSFG processes Modeling vSFG processes is done using the non-linear optics mathematical formalism that is outlined in books by Mukamel, Shen and Boyd. 18{20 The description found here can be found elsewhere. 21{23 A useful way to understand the mathematics involved in modeling non linear optical processes is with double sided Feynman diagrams (see gure 2.3). We will use it a map to construct the response function of the system. We note that for a system composed of a metal surface with a vibrational chromophore attached to it, we have two dierent response functions; one for the metal and one for the vibrational chromophore. The vibrational chromophore starts in state [ aa ] chrom and the metal starts in state [ aa ] met : [ aa ] chrom + aa e i met Note that the brackets with the subscript merely denote which terms correspond to the chromophore and which to metal. This is because the transition dipole operators and density matrix terms etc. are dierent between the chromophore and metal. The e i term simply accounts for the fact the the 7 CHAPTER 2. THEORY AND PRACTICE OF VIBRATIONAL SUM FREQUENCY GENERATION SPECTROSCOPY metal and chromophore signal will interfere and can have dierent phases. Then both the chromophore and the metal interact with the IR light source which puts them in coherences: [ aa ba ] chrom + aa e i ba met After the interaction, the system evolves under the time evolution operator. It is important to note that this operator is dierent for vibrational chromophore than the metal. The interaction between the IR light and the vibrational chromophore is a resonant one, while the interaction between the metal and the IR light is non resonant. Therefore, we have: h aa ba e (i! ba ba )1 i chrom + aa e i ba ( 1 ) met where the exponent term (e (i! ba ba )1 ) is the time evolution operator (for the vibrational chro- mophore) with an added dephasing term, ba to simulate relaxation of the coherence. It is important to note that this is not the only way to include relaxation. Rather, it is just a simple way to do so. For the metal, because the interaction is non resonant, the dephasing time is so short that we approximate the response as a delta function. Additionally, it is important to note that 1 is a dierence between times, not an absolute point in time (see gure 2.3). After time 1 , the system interacts with the visible light source: h aa ba e (i! ba ba )1 Vb i chrom + aa e i ba ( 1 ) Vb met Because the interaction with the visible light is a non resonant interaction for both the chromophore and metal, the time evolution operators are approximated as delta functions. Thus, we have: h aa ba e (i! ba ba )1 Vb ( 2 ) i chrom + aa e i ba ( 1 ) Vb ( 2 ) met 8 2.1. MODELING VSFG PROCESSES ⟩⟨ ⟩⟨ ⟩⟨ ' ( Figure 2.3: Double sided Feynman diagram depicting a SFG process. Finally, the system interacts with the vacuum emitting light and ending up back in the ground state: h aa ba e (i! ba ba )1 Vb ( 2 ) Va i chrom + aa e i ba ( 1 ) Vb ( 2 ) Va met Finally, to enforce causality, we multiply both terms by heaviside functions of 1 and 2 . This ensures that prior to the time of interaction, the response function is zero. Now, we can write our response function as: R (2) ( 1 ; 2 ) = h ( 1 )( 2 ) aa ba e (i! ba ba )1 Vb ( 2 ) Va i chrom + ( 1 )( 2 ) aa e i ba ( 1 ) Vb ( 2 ) Va met (2.4) Now, the second order polarization, P (2) can be expressed as follows: P (2) (t) = Z 1 0 Z 1 0 R (2) ( 1 ; 2 )E Vis (t 2 )E IR (t 1 2 )d 1 d 2 (2.5) 9 CHAPTER 2. THEORY AND PRACTICE OF VIBRATIONAL SUM FREQUENCY GENERATION SPECTROSCOPY Inputing our response function from equation 2.1, we have: P (2) (t) = aa ba Vb Va R 1 0 R 1 0 ( 1 )( 2 )e (i! ba ba )1 ( 2 )E Vis (t 2 )E IR (t 1 2 )d 1 d 2 chrom + aa ba Vb Va e i R 1 0 R 1 0 ( 1 )( 2 )( 1 )( 2 )E Vis (t 2 )E IR (t 1 2 )d 1 d 2 met (2.6) If we assume that the electric elds are plane waves, then the integral over 2 dies in the chromophore term and both the integrals in the metal term die as well because of the delta functions. Thus, we are left with: P (2) (t) = aa ba Vb Va E Vis (t) Z 1 0 ( 1 )e (i! ba ba )1 E IR (t 1 )d 1 chrom + aa ba Vb Va e i E Vis (t)E IR (t) met Now, let us do the following manipulations. We will set A = [ aa ba Vb Va ] chrom and we will set B = [ aa ba Vb Va ] met . Additionally, we will make the following substitution, f( 1 ) = ( 1 )e (i! ba ba )1 . Then we have: P (2) (t) =BE Vis (t) Z 1 0 f( 1 )E IR (t 1 )d 1 +Ae i E Vis (t)E IR (t) (2.7) To obtain the frequency domain polarization, we simply take the Fourier transform of equation 2.7: P (2) (!) =BF E Vis (t) Z 1 0 f( 1 )E IR (t 1 )d 1 +Ae i F [E Vis (t)E IR (t)] whereF denotes a Fourier transform. Now, recall that the Fourier transform of two functions multiplied together in the time domain is equal to the convolution of the two functions in the frequency domain. Therefore, we have: P (2) (!) =BE Vis (!)F Z 1 0 f( 1 )E IR (t 1 )d 1 +Ae i E Vis (!)E IR (!) 10 2.1. MODELING VSFG PROCESSES where the symbols denotes a convolution. Now, we note that theF R 1 0 f( 1 )E IR (t 1 )d 1 is simply the Fourier transform of a convolution. Remember that the Fourier transform of a convolution in the time domain equals multiplication in the frequency domain. Therefore, we have: P (2) (!) =BE Vis (!) [E IR (!)f(!)] +Ae i E Vis (!)E IR (!) We note that the experimentally, to keep IR resonant features from blurring out because of the convolutions, we make the visible pulse as narrow as possible in the frequency domain, such that we can approximate it as a delta function: E Vis (!) =E Vis (!). Then, we note that for a given function, g(!), the convolution with a delta function, (!) yields g(!)(!) =g(!).Therefore, we have the following: P (2) (!) =BE Vis E IR (!)f(!) +Ae i E Vis E IR (!) Where we now have a function dependent on the strength of the visible electric eld. Finally, we remember that f(!) =F h ( 1 )e (i! ba ba )1 i = Z 1 0 e (i! ba ba )1 e i!1 = 1 i(! ba +!) + ba Now, we will approximate the the IR light pulse as a gaussian (E IR (!) = E IR e !!g !c 2 ) and absorb the constants together. Thus, we nally have: P (2) (!) = B i(! ba +!) + ba +Ae i e !!g !c 2 (2.8) As will be seen in later chapters, we will use this expression to t our vSFG data and back out valuable information about interfaces. 11 CHAPTER 2. THEORY AND PRACTICE OF VIBRATIONAL SUM FREQUENCY GENERATION SPECTROSCOPY 12 Chapter 3 Direct Spectroscopic Measurement of Interfacial Electric Fields 3.1 Introduction Interfacial electric eld and the related molecular polarization are the central quantities that govern interfacial charge transfer. 6,24{28 This is especially true for electrochemistry where the majority of chemical reactions of interest occur at the interface between the electrode and electrolyte. The externally applied potential on the working electrode is related to the thermodynamic drive of electrochemical reactions. This potential is rapidly screened by the adjacent electrolyte, giving rise to large electric elds near the surface. This eld polarizes the molecules near the electrode and at large enough values ionizes them. Several models of the potential in an electrolyte near an electrode exist, including the classic Gouy-Chapman model, the Gouy-Chapman-Stern model 29 and the Bockris-Devanathan-M uller model (BDM). 30 Electrochemical methods such as electrochemical impedance spectroscopy and dierential capacitance are used in conjunction with equivalent circuit models to examine interfacial processes. 31{33 While such methods are useful, they do not directly probe the interface and lack spatial resolution. Therefore, to fully understand interfacial processes, 13 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS it is desirable to directly probe the interface and measure such elds by spectroscopic means. Previous spectroscopic measurements of the interfacial elds have largely been accomplished via two methods. The rst is using second harmonic generation (SHG) where the net magnitude of the SHG signal is shown to be proportional to the potential drop near an interface. 34{40 However, this method lacks spatial resolution since it relies on integrated non-resonant SHG signal across the entire interface. The second method is using a vibrational Stark shift reporter tethered near a surface. The frequency shift of the vibrational mode of the reporter molecule is measured and related to the local eld via the rst order Stark eect equation. This allows for control over the location at which the potential is measured. Stark shift reporters have been used to estimate electric eld strengths in proteins and en- zymes 41{44 and to evaluate potentials in bulk heterojunction solar cells. 45 In a previous work, we have measured and modeled the solvation eld that a Stark reporter feels at the boundary of a conductor and a dielectric. 46 Several previous studies have reported vibrational Stark shifts of tethered chromophores at the conductor/electrolyte interface. 12,13,15 However, to our knowledge, a systematic study of the dependence of the eld measured by Stark shift spectroscopy on a wide range of ionic concentration, and correlation of these elds with electrochemical currents has not been reported before. The value of local eld changes signicantly with ionic concentration and electrochemical current and understanding this behavior is important. In this work, rst we examine the ionic concentration dependence of local eld based on a model by Smith and White and further developed by Hildebrandt. 6,26,47,48 We show that our Stark shift spectroscopy in combination with this model can be used to obtain relative changes in interfacial eld values with applied potential. Second, and importantly, we show that the interfacial eld tracks the externally applied potential only for a polarized electrode when no current is passing. Under the conditions that a current is traversing the interface, the local eld and hence the polarization of the probe molecule, is xed to a constant value and does not vary with changing applied potential. This eect is most manifest at low ionic concentrations, in which hysteresis due to mass transport is minimal. We provide a simple interpretation of this observation that to our knowledge is not obvious by other methods. This manuscript is arranged as follows. First, we present the experimental techniques and data 14 3.2. EXPERIMENT AND DATA ANALYSIS analysis. Second we present a brief synopsis of the local eld model by Smith and White and show the experimentally veriable prediction of the model for the dependence of eld on ionic strength. Then we show that the experimental results are consistent with the functional form of this prediction over three orders of magnitude in ionic concentration. Next, we show that the local measurement of the eld yields vastly dierent results when current is drawn across the interface. We provide an interpretation of this observation and discuss its consequences for electrochemical charge transfer theories. 3.2 Experiment and Data Analysis Sample Preparation. Self-assembled monolayers (SAMs) of 4-Mercaptobenzonitrile (4-MBN) were prepared on silicon wafers with a 10 nm Ti adhesion layer and 100 nm of Au purchased from LGA Thin Films, Inc. Wafers were cleaned by sonication in acetone twice, then in ethanol twice for 8 minutes each time, then immersed in a 0.03 M solution of 4-MBN in ethanol for at least 24 hours, which ensures full surface coverage for good signal quality. 49 After soaking in the 4-MBN solution, the wafers were removed and again sonicated twice in ethanol for 8 minutes each. Electrochemical Cell Modied for VSFG. Electrochemical Vibrational Sum Frequency Generation (VSFG) spectra of the SAM modied Au samples were taken in a demountable liquid FTIR cell (International Crystal Laboratories) modied for this purpose (see gure 3.1). The back window of the cell was removed and replaced with the SAM containing wafer and a 15 m Te on spacer was placed directly on the sample surface. The front window of the demountable cell is a 4 mm thick CaF 2 window with small holes drilled to allow access to the cavity formed by the te on spacer between the wafer and the window. The entire assembly is then held rmly together using stainless steel plates and screws. The electrolyte was introduced by a large syringe through one of the lling ports of the cell and the reservoir was subsequently attached to the other port. By design, the large syringe contained an excess of electrolyte that was used to replace the electrolyte in the cell throughout the duration of the experiment. The working electrode was the SAM modied Au samples with a wire contacting the Au, the reference electrode was Ag/AgCl (purchased from 15 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS 5 mm thick CaF 2 window 25 µm Teflon spacer 100 nm gold on Si Wafer covered with a monolayer of 4- MBN Metal Support Electrolyte outlet Counter Electrode Reference Electrode Contact to Working Electrode (a) (b) Filling Syringe 2200 2210 2220 2230 2240 2250 2260 15 20 25 30 35 40 wavenumber (cm −1 ) SFG signal (a.u.)) (c) Figure 3.1: (a) A picture of the experimental cell used to perform the experiment. (b) A diagram showing a cross section of the cell and the accessories. (c) A representative spectrum (blue) and its t (red) of the positive scan for V = 0.0 V with a 1000mM KCl concentration. The dip around 2230 cm 1 is the nitrile stretch Representative data including the t. Gamry) and the counter electrode was a Au disk electrode. Potentials were applied using a Gamry Reference 3000 potentiostat. Electrochemical VSFG measurements. A 1 kHz regeneratively amplied Ti:Sapph laser (Coherent) was used to generate ultrafast near IR pulses. A portion (1 W) of this was directed to an optical delay stage followed by a 4f lter to signicantly narrow the spectrum, while another portion (2 W) was directed to an OPA (Coherent OPerA Solo) equipped with a AgGaS 2 crystal for dierence frequency generation of mid IR pulses. The 4f lter used two transmissive volume phase gratings (BaySpec, Inc), two cylindrical lenses and a variable width slit to lter the near IR pulse to a spectral width of 8.0 cm 1 , centered at 787.62 nm. Typical spectra of both the near IR upconversion pulses as well as the broadband mid IR pulses can be found in chapter A of the appendix. Pulse energies were measured at the sample position to be 8.43 J for the near IR and 7.56 J for the mid IR. VSFG spectra were acquired by focusing these two pulses together on the sample using a common parabolic mirror and overlapping them in time. The angles of incidence relative to normal on the front face of the CaF2 window were measured to be about 45° and 59° for the mid-IR and near-IR pulses, respectively. Using Snell's law and tabulated data for refractive 16 3.2. EXPERIMENT AND DATA ANALYSIS indices, 50,51 the angles of incidence on the sample for the near-IR pulses are estimated to be 32° and 40° for the mid-IR and near-IR pulses respectively. The resulting VSFG signal was collected with a second parabolic mirror and passed through a notch lter and a short pass lter to reject the majority of the scattered near-IR photons. The SFG was then sent to a spectrometer (Horiba iHR320) with a CCD camera (Princeton Instruments Pixis 300) for spectral analysis. With the input slit of the spectrometer set to 0.05 m, and using an 1800 gr/mm grating, the theoretically achievable spectral resolution was 0.05 nm (about 1 cm 1 in the spectral range of interest), which is well below the width of the near IR upconversion pulse. Spectral resolution of the SFG spectra are, thus, limited by the 8 cm 1 width of the near IR upconversion pulses. We point out that peaks shifts smaller than this value can be measured as has been discussed in a previous publication 46 and which will be shown here. The electrochemical VSFG studies were carried out as a function of both electrolyte concentration and applied potential. The electrolyte used was KCl in water with concentrations of 1 mM, 10 mM, 50 mM, 100 mM and 1000 mM. For each concentration, a cyclic voltage scan was performed where the applied potential was scanned from 0.0 V to 0.5 V to -0.8 V to -0.1 V with a step size of 0.1 V. For this paper, this voltage scan is referred to as the positive scan. The reverse scan was also performed from 0.0 V to -0.8 V to 0.5 V to 0.1 V. This is referred to as the negative scan. After application of each potential step a transient current was observed. VSFG spectra were obtained only after this transient current had decayed and a steady electrochemical response was observed. At each potential, three VSFG spectra were obtained each with 60 second integration times. Additionally, between each voltage step, the electrolyte in the cell was replaced by fresh electrolyte with the syringe. To obtain the center frequencies of the nitrile stretch, we t our data with the following expression: 21 I SFG (!) = A NR e i + B !! CN +i CN 2 exp (!! g ) 2 2 g ! + 10 (3.1) The above model assumes that the total VSFG signal is composed of a non-resonant background 17 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS signal from the gold, which interferes with a resonant signal from the adsorbed monolayer (4-MBN). The non-resonant background is modeled with amplitude, A NR , and phase, . The resonant signal assumes a lorentzian lineshape with apparent amplitude, B, center frequency, ! CN and width, CN . Both of these signals are multiplied by the IR pulse which is assumed to be Gaussian with center frequency, ! g and width 2 g . A background spectrum was obtained by blocking the IR pulses and collecting a spectrum under otherwise identical conditions. This background was subtracted from the raw VSFG spectra, however an additional constant term C BG was necessary to account for the scatter of the upconversion beam. 3.3 Result and Discussion According to the linear Stark eect, when the electric eld experienced by a molecule changes by F , the vibrational frequency changes by: h =~ ~ F: (3.2) where is the dierence in dipole moments of the ground and excited vibrational states. In our case, we will always refer ~ F relative to the eld corresponding to zero applied potential 0 = 0. The value of ~ F experienced by the SAM layer depends on both the applied potential 0 and ionic concentration n and is the focus of our investigation. To reveal this relationship, we measure the vibrational frequency change as a function of the experimentally controlled parameters 0 andn. Note that for positive potentials ~ F and ~ point in opposite directions resulting into a positive frequency shift. Also, for convenience, we will consider equation 3.2 in scalar form, in which is the projection of ~ along the electric eld which is normal to the surface. Furthermore, our previous work 46 suggests that the molecules are oriented close to the surface normal. The experimental relationship between and 0 for a xed ionic concentration of 10 mM KCl is plotted in gure 3.2b. As can be seen in the gure, the observed frequency shift is qualitatively dierent for positive versus negative potentials. In the potential range of 0.0 V - 0.5 V, the frequency 18 3.3. RESULT AND DISCUSSION shift appears linear with respect to potential. Negligible hysteresis is observed between forward and reverse scans. This linear dependence on potential is observed for all ionic concentrations used and is shown in the gure C.4 in chapter C.2 section C.2.1. In contrast, for negative biases, the frequency shift with respect to potential displays dierent behavior. For low ionic concentrations a negligible shift with respect to potential was observed. While for large ionic concentrations, frequency shifts show large hysteresis with respect to forward and reverse scans. As will be discussed later, these dierences in behavior between the negative and positive potentials is related to the electrochemical current that traverses the interface. In particular, at negative potentials, a signicant amount of electrochemical current is observed, while for positive potentials the current is negligible. We will discuss these two regimes separately. 3.3.1 Stark shift in the absence of electrochemical current: As seen in gure 3.2, for positive potentials, negligible current is drawn. Under such conditions the applied potential polarizes the interface. We observe a linear increase in as a function of increasing potential for all ionic concentrations used. We emphasize, however, that these changes in vibrational frequency are a function of both applied potential and ionic concentration, ( 0 ;n), as seen in gure 3.3. For KCl concentrations of 1mM to 100mM, increases rapidly. However, for the 1000 mM KCl solution, little increase is observed. As stated earlier, changes in are due to changes in the local electric eld. Therefore, to better understand our data, we invoke a model that relates the local eld in the SAM layer to both the applied potential and the ionic concentration. The model is based on the work by Smith and White 47 and the details are given in the chapter C.1 of the appendix. In brief, the model assumes a dielectric SAM in contact with a conducting electrode and an electrolyte described by the Gouy-Chapman theory (see gure C.1). The electric eld in the SAM layer is the relevant quantity of the model and is expressed as: F ( 0 ;n) = 2 ( 0 PZC ) 2 d + 1 (3.3) 19 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 −6 −4 −2 0 2 Applied Potential (V) Current (µA) −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 −0.5 0 0.5 1 1.5 2 2.5 Applied Potential ∆ν CN (cm −1 ) (a) (b) ≈ ∆ ≈ (V) Figure 3.2: Correlation between (a) the current and (b) the nitrile stretch frequency measured as a function of applied bias. A clear correspondence is observed between regions of non-zero current and the change in vibrational frequency of the nitrile stretch which reports the local electric eld. When current is nearly zero, a large potential drop across the interface can be maintained and the interface behaves more like a capacitor. The increase in potential leads to a linear variation of the eld and consequently a linear variation in the vibrational frequency. When the interface passes current, it is akin to a capacitor shorted via a low resistance with diminished ability to maintain a potential drop. Therefore the dependence on potential vanishes. 20 3.3. RESULT AND DISCUSSION 10 0 10 1 10 2 10 3 0 0.1 0.2 0.3 0.4 0 1 2 3 10 0 10 1 10 2 10 3 0 0.1 0.2 0.3 0.4 0 1 2 3 ∆ ( − ) (a) 10 0 10 1 10 2 10 3 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 KCl Concentration (mM) (b) ∆ ( − ) Figure 3.3: (a) A plot of the frequency change as a function of applied potential 0 and ionic concentration n. (b) A slice through this plot at 0.3 V. Relating these frequency shifts to eld variations is the central goal of this section. where is the inverse Debye length, 1 and 2 are the dielectric constants of the SAM and the diuse layer respectively, d is the eective thickness of the SAM, 0 is the applied potential, and PZC is the potential of the point of zero charge. The inverse Debye length depends on ionic concentration n as = p n, where = ze p 220kT . Inserting this into equation 3.3 yields: F ( 0 ;n) = 2 p n( 0 PZC ) 2 d p n + 1 (3.4) The above is the electric eld in the SAM. Since we are concerned with changes in electric eld with respect to zero applied potential, ~ F (0;n) = 2 PZC 2d p n+1 , the relevant expression is: F ( 0 ;n) = ~ F ( 0 ;n) ~ F (0;n) = 2 p n 0 2 d p n + 1 (3.5) 21 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Au + + + + + + + + + - + + - - - - - - - - - - - - - - - - - + + + + + + + + + - - - - + - + + Potential () d + + + - - - - - - + + + 1 2 Figure 3.4: A picture of the model. Region 1 is composed of the 4-MBN SAM with thickness d. Region 2 is the diuse ionic layer. The potential drop across the SAM is linear, while across the ionic layer it follows the Gouy-Chapman theory. Note that the potentials displayed here ( ) are not referenced to the reference electrode potential. See chapter C.1 for further details. 22 3.3. RESULT AND DISCUSSION which according to equation 3.2 is related to the change in vibrational frequency as: h = 2 p n 0 2 d p n + 1 (3.6) For convenience, we rearrange the above to: = p n A p n +B 0 (3.7) where, A = d h (3.8) B = 1 h 2 : (3.9) Thus, according to the model, the Stark frequency shift, , should be proportional to the applied potential, 0 . As stated earlier, a linear relationship is observed for positive biases. The above expression is the experimentally veriable outcome of the model and we use it to t our ( 0 ;n) data for both the positive and negative scans (gure 3.5). As can be seen, this model captures the dependence of on 0 and n very well. Two limiting cases are pointed out here. First, by setting the eective SAM thickness to zero (d = 0) and 1 = 2 in equation 3.7, we reclaim the Gouy-Chapman model. In this limit, has a square root dependence on n which does not t the data. This underlines the fundamental change SAMs can have on interfacial electric elds. Furthermore, it shows the importance of explicitly including SAM layers when constructing models for surface modied electrodes. The second limiting case is seen for large n. In this regime, 0 A . This limiting value of , which is observed experimentally, occurs because the potential drop across the SAM and the potential drop across the electrolyte in the diuse layer are intimately related. When n is very large, the electrolyte behaves like a conductor and the entire system resembles a capacitor. Therefore, any further increase in ionic concentration no longer aects the potential drop across the SAM. This explains the asymptotic 23 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Fit Parameters A (V/cm -1 ) B(V /cm -1 ) Positive Scan 0.178 +/- 0.009 0.34 +/- 0.06 NegativeScan 0.22 +/- 0.02 0.4 +/- 0.1 Average 0.198 +/- 0.009 0.39 +/- 0.06 0 0.1 0.2 0.3 0.4 0.5 −0.5 0 0.5 1 1.5 2 Applied Potential (V) ∆ν CN (cm −1 ) (a) (b) 10 0 10 1 10 2 10 3 0 0.5 1 1.5 2 KCl Concentration (mM) ∆ν CN (cm −1 ) Figure 3.5: (a) A slice of ( 0 ;n) for 0 = 0:3V (blue) for the positive scan with a slice of the t at 0 = 0:3V (red). (b) Slice of ( 0 ;n) for n = 10mM (blue) for the positive scan with a slice of the t at n = 10mM (red). The table contains the extracted tting parameters of both the positive and negative scans along with the average values of the two scans. value of in the experiment. From the ts of both the positive and negative scan data, the average values for the parameters A and B are shown in gure 3.5. From the average t parameters, values for and the SAM dielectric constant, 1 , can be estimated. Using equation 3.8, if the SAM thickness is assumed to be 7.15 A (the nitrogen to sulfur distance calculated using DFT at the B97XD as implemented in Q-Chem), then the Stark tuning rate is, = 0:022 0:001D. This value is smaller than our previous estimation of the 4-MBN Stark tuning rate ( = 0:030D). 46 In our previous work, we developed a model for the interfacial electric eld present at an unbiased SAM covered electrode in contact with a dielectric. It was found that depended on the third power of the SAM thickness 24 3.3. RESULT AND DISCUSSION d 3 . Therefore, small deviations in d can lead to large uncertainty in according to that model. In this work, however, linearly depends on d and is less sensitive to our chosen value of the SAM thickness. We also point out that this value for the Stark tuning rate is closer to previous estimates from the literature 42,52 for similar molecules like benzonitrile ( = 0:012 0:018D). The parameter B is related to the dielectric constants of the SAM layer and the electrolyte (see equation 3.9). Using our value for and a value of 78 30,47 for 2 , we nd 1 = 6 1. This value is of the same order as other related SAMs, such as biphenyl monolayers, from previous theoretical estimations. 53 This is also consistent with intuition. The dielectric constant of, 4-MBN in the bulk is expected to have a similar dielectric constant to that of benzonitrile ( = 26). However, at the surface of an electrode, where 4-MBN is no longer able to rotate to the extent it could in the bulk, 1 is expected to be smaller. 30 It is possible that the dielectric constant 2 used above will have spatial variation near the electrode. Therefore an uncertainty in the value of 2 used above should be expected. However, the uncertainty in 1 is fairly insensitive to uncertainty in 2 . For example, when using an arbitrary error of 2 = 50 % the uncertainty in 1 only increase to a value of2. Using the model and our experimental values for , A and B, we plot the change in electric eld as a function of potential and concentration (see gure 3.6). Of course, for any observed frequency change, it is possible to estimate F using only . The model, however, quantitatively establishes the relationship between applied potential, ionic concentration and electric eld. The utility of this plot lies in the fact that the value of F , which was determined using experimentally derived parameters, can now be obtained using only the bulk quantities 0 and n. We emphasize that this model does not take into account the known hydrogen bonding interaction between water and the nitrile probe. 41,54{56 To do so would require going beyond an electrostatic model. However, we are able to t the data quite well. This suggest that either the changes in the hydrogen bonding interactions have similar dependance on 0 and n as F does or that the hydrogen bonding interaction is insensitive to our changes in 0 and n. Further experimental and theoretical studies examining how interfacial hydrogen bonds are aected by these quantities are necessary to better understand the role that hydrogen bond plays in interfacial electric eld. 25 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS 10 0 10 1 10 2 10 3 0 0.2 0.4 0.6 0.8 1 0 5 10 15 ∆ (MV/cm) Figure 3.6: A plot of the electric eld F = F( 0 ;n) F(0;n) experienced by the SAM layer based on the model and the experimentally retrieved parameters. The plot gives the general behavior of how the interfacial eld varies with ionic concentration and potential. 26 3.3. RESULT AND DISCUSSION 3.3.2 Stark shift in the presence of electrochemical current: At negative potentials less than -0.1 V, an electrochemical current is drawn. As shown in gure 3.2, the frequency of the nitrile stretch does not vary with potential at low ionic concentrations. We will interpret this observation in this section. We emphasize that the reported current at each potential does not arise from capacitive transients. As explained in the experimental section, after application of each potential step, we waited for the transient current to subside. The values reported in gure 3.2 are from steady current that was sustained for the duration of SFG spectral integration time. The possibility of the current coming from the reduction of the SAM layer can be ruled out for two reasons. First, the potentials that thiol SAMs desorb at are usually below -1.0 V vs Ag/Ag/Cl. 57,58 Where, the most negative potential we applied was -0.8 V. Secondly, the current that we observed for each applied potential was steady for three minutes (the amount of time it took to get the three spectra for each potential). See the gure C.6 in chapter C.2 section C.2.1 for further details. If the SAM was desorbing, a decrease in the current magnitude would be expected as more and more molecules were reduced. Therefore, the current must arise from reduction of an electron acceptor in the electrolyte. Since we are using a simple electrolyte, the most likely candidate for the current is the reduction of protons i.e. hydrogen evolution. A small current due to reduction of protons even at such low potentials can not be ruled out. As stated, we are interested in explaining why the vibrational frequency, which is a proxy for local electric eld, does not vary with changing potential in this region. This behavior can be qualitatively understood with an equivalent circuit model as shown in gure 3.7. An electrochemical current turns on after an onset potential and has asymmetric behavior with respect to potential just like a diode. The diode in the circuit model accounts for this. Its asymmetric behavior is responsible for the current-carrying and polarizing regimes manifested in gure 3.2. The capacitor in the model represents the polarization of the interface, as discussed in the previous section. The resistors R 1 and R 2 represent the charge transfer and mass transfer resistances respectively. At positive potentials there is no electrochemical current and corresponds to the reverse-bias 27 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS R 1 R 2 D C 2 A A - A A - C 1 e - Figure 3.7: A circuit diagram that can qualitatively explain the observed dierence in vibrational frequency shift of the nitrile probe molecule in the current-carrying and polarizing conditions. When no electrochemical current ows (corresponding to the reverse-biased diode), the eld near the interface builds up linearly with potential, analogous to charging the capacitors. At potentials for which the redox reaction becomes possible, the leaked current prevents further build up of eld across the SAM layer. Increase in potential under this condition yields more current but not more polarization of the probe molecule. 28 3.4. CONCLUSION of the diode in the model. In this situation, with increasing potential a eld builds up inside the capacitor. As discussed in the previous section, this is measured by the vibrational frequency of the Stark reporter at the interface. At negative potentials, where current is drawn, the current passes through two resistors in series R 1 and R 2 . However, this time, with increasingly negative potentials, more current passes through the resistors and, if R 1 <<R 2 , negligible increase in eld build up inside the capacitor is expected. This \leaky capacitor" model for the interface polarization can explain why the electric eld, and correspondingly the vibrational frequency of the nitrile probe, does not change when a current is passing. This does not mean that the eld within the capacitor is absent. Rather, it means that the eld does not change with increasing potential. The data conrms that R 1 is much smaller than R 2 because negligible shifts in frequency are observed as potential is changed. We emphasize that this observation is distinct form the cases where the redox reactants or products are being monitored, as reported recently for adsorbed CO on silver electrode. 14 In our experiment, we believe that the SAM layer is a bridge for electron transfer and is not part of irreversible redox chemistry, as explained previously. 3.4 Conclusion This work has examined the behavior of interfacial electric elds as a function of potential and ionic concentration using a vibrational Stark shift reporter. We have found two distinct regimes of dependence of eld on external potential. When no current is drawn and the electrode is polarized, the eld seems to linearly increase with applied potential. The resulting slope depends on ionic concentration and follows an interfacial model that includes a capacitive SAM layer and ionic diuse layer. This model allows for estimating electric eld variations near the surface. In the regime when current is drawn, no change in eld with respect to potential is observed. This can be explained by a \leaky capacitor" model, in which passage of current across the interface prevents build up of eld. To our knowledge, observation of this eect by a local measurement of a eld probe has not been reported before. This work underlines the complexities of local electric eld and the importance of 29 CHAPTER 3. DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS their direct measurement in functionalized electrodes. While several important ndings are reported, this work also introduces some important questions. First, we do not know how the structure of the SAM on the electrode changes with applied potential and concentrations. While the changes are expected to be small, they may aect how the interfacial eld depends on external parameters and on how VSFG Stark shift measurements are interpreted. Such structural changes may be the cause for variation of the relative phase of the resonant VSFG signal with respect to the non-resonant background. Second, we are aware of the in uence of hydrogen bonding on the Stark shift probe, as reported by us previously. It is desirable to address this eect in a direct way in our model. This requires microscopic modeling of the electrolyte and more experiments that are beyond the scope of this work. We hope that further theoretical research can address this. If this issue is resolved, the frequency of the nitrile probe in pure water can serve as a reasonable reference for measurement of electric eld. 30 Chapter 4 Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy Lewis acid and base interactions are a cornerstone of chemistry describing a wide range of chemical phenomena. Nearly 100 year ago, G.N. Lewis generalized the concept of acidity and basicity to encompass all electron pair acceptors as Lewis acids (LA) and electron pair donors as Lewis bases (LB). 59 The bond resulting from the interaction of the LA and LB Lewis pair (LP) is known as a dative bond and the complex is referred to as a Lewis adduct. Frustrated Lewis Pairs (FLPs) are special cases in which the LA and LB interaction is energetically favorable, but the orbital overlap is sterically hindered. While FLPs were rst reported several decades ago, 60{62 their reincarnation has spawned a new eld in contemporary chemistry, mainly due to their potential for catalysis. 63{66 It is shown that the environment within unquenched FLPs is highly polarizing and therefore can heterolytically cleave non-polar bonds. The eld of FLP research is rapidly growing and new instances of their potential for catalysis are being demonstrated. 67{70 Heterogeneous reactions, including electrochemical reactions, have also been described in terms 31 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY of LP interactions on the surface. Surface sites are often described as Lewis acidic (e.g. oxygen in a metal oxide) or basic (e.g. metal ions in metal oxides). 71{74 In some cases, surface catalytic reactions have been described as cooperative interaction between FLPs. 71,75 Designing tethered FLPs near surface requires fundamental understanding of how the interfacial environment aects LP interactions. Compared to bulk chemistry, both measurements and theoretical models of interfacial interactions are more complicated. Chemical and physical properties are known to be modied at an interface, particularly when an external bias is applied. From a molecular orbital theory perspective, LP interactions are expected to be sensitive to orbital overlap and energetics, which are in turn in uenced by steric hindrance, 64,66 orbital energetics, 76 and solvation and local electrostatics. 77{80 The asymmetric environment of an interface can aect all of these factors and therefore can in uence interfacial LP interactions. For the above reasons, it is necessary to bridge the understanding of bulk and surface LP interactions. Such a connection will be useful in tailoring surface properties, for example for designing catalytic surface sites or incorporating molecular catalysts on electrode surfaces. In particular, extending the concept of frustration to heterogeneous environments requires physical and chemical knowledge of interfacial LPs. A promising prospect is to synergistically augment the polarizing eects of a biased interface with the polarizing functionality of interfacial FLPs. These ideals are the motivation for our work. In this work, we report the preparation of surface LPs, their measurement, and comparison to their corresponding bulk counterparts. We use the classic tris(penta uorophenyl) borane as our Lewis acid. Following convention, 81 we shall refer to it as BCF, after the elements involved in the compound. The central boron of BCF has an empty Lewis acidic p-type orbital that can accept electrons from Lewis bases. It has been used as LA in many FLP studies. 64,66 The LB in our work is the lone pair of the nitrile group in self-assembled monolayers of 4-mercaptobenzonitrile on gold. This LB is chosen due to the sensitivity of the nitrile stretch frequency to the LP interaction. 76{78 While nitriles are not very strong LBs, 82 the sensitivity of their vibrational frequency to LP interactions makes them ideal choices for this study. We measure the signature of LP interaction at an interface using vibrational Sum Frequency Generation (vSFG), which is a sensitive and surface-specic 32 spectroscopic technique. Methods for measurement of the strength of LP interactions in the bulk exist that are based on nuclear magnetic resonance (NMR) of either the donor or acceptor atoms. 83,84 Employing such methods for understanding surface interactions, especially in the presence of a potential, is challenging and not readily possible. In contrast, vibrational spectroscopy of the nitrile stretch is relatively easy to implement both in the bulk and on the surface. Therefore, the nitrile stretch is a suitable probe of the LP interactions in both environments. We have measured and compared the vibrational frequency of the nitrile stretch both for LP adducts in the bulk and on the surface. In this work we will describe the observed dierences between them and explore mechanisms that in uence the strength of interfacial LP interactions. We describe the role of interfacial frustration, surface electric elds, and electronic energy level alignments. We hope this work will form a basis for a systematic understanding of interfacial LPs and, consequently, their use for catalyzing chemical reactions. Six dierent self-assembled monolayers (SAMs) were prepared on gold thin lm wafers (silicon with a 10 nm Ti adhesion layer and 100 nm of Au) purchased from LGA Thin Films, Inc. Prior to surface modication, the wafers were cleaned by sonication in ethanol twice. Each sonication process was performed in fresh solution for 8 minutes. Then, the wafers were immersed overnight in the thiol solutions in ethanol that form the SAM. To make the full coverage SAMs 0.03 M solutions of either 4-Mercaptobenzonitrile (MBN) or 4-mercaptobiphenylcarbonitrile (MBBN) were used. To make diluted SAMs, we used solutions of MBN mixed with benzenethiol in ratios of 1:4 and 1:8 respectively, with the total thiol concentration still maintained at 0.03 M. The same procedure was used for MBBN diluted monolayers. After soaking in the solutions, the wafers were removed and cleaned by sonication in ethanol twice. Each sonication process was performed in fresh solution for 8 minutes. The wafers were soaked in closed containers lled with saturated solutions of BCF in cyclohexane for approximately one hour. Each type of wafer was soaked in a separate BCF solution to avoid any contamination. The BCF solutions were prepared under a constant ow of nitrogen gas in the hood to prevent the reaction of BCF with water in the atmosphere. The wafers were then removed from their containers under an atmosphere free of CO 2 and H 2 O and allowed to dry. Once the 33 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY cyclohexane had evaporated, spectra were obtained in the same atmosphere. A 1 kHz regeneratively amplied Ti:Sapph laser (Coherent) was used to generate ultrafast near IR pulses. A portion (1 W) of this was directed to an optical delay stage followed by a 4f lter to signicantly narrow the spectrum, while another portion (2 W) was directed to an OPA (Coherent OPerA Solo) equipped with a AgGaS 2 crystal for dierence frequency generation of mid IR pulses. The 4-f lter used two transmissive volume phase gratings (BaySpec, Inc), two cylindrical lenses and a variable width slit to lter the near IR pulse to a spectral width of 8.0 cm 1 , centered at 787.10 nm. Pulse energies were measured at the sample position to be 8 J for the near IR and 7 J for the mid IR. vSFG spectra were acquired by focusing these two pulses together on the sample using a common parabolic mirror and overlapping them in time. The resulting vSFG signal was collected with a second parabolic mirror and passed through a 750 nm short pass lter to reject the majority of the scattered near-IR photons. The vSFG signal was then sent to a spectrometer (Horiba iHR320) with a CCD camera (Syncerity) for spectral analysis. Spectra of the six dierent samples were obtained both before and after soaking in the BCF solutions. All spectra were obtained under a CO 2 and H 2 O free atmosphere. For each sample, a total of six spectra were obtained: three before soaking and three after. Each spectrum was obtained by integrating for 120s. To obtain the center frequencies of the nitrile stretch, we t our data with the following expression: 21 I SFG () = A NR e i + X k B k CN;k +i CN;k ! 2 exp ( g ) 2 2 g ! (4.1) The above model assumes that the total vSFG signal is composed of a non-resonant background signal from the gold, which interferes with the resonant signals from the adsorbed monolayer. The non-resonant background is modeled with amplitude, A NR , and phase, . The resonant signal assumes a Lorentzian lineshape with amplitude, B, center frequency, CN and width, CN . Both of these signals are multiplied by the IR pulse which is assumed to be Gaussian with center frequency, g and width 2 g . A background spectrum was obtained by walking o the timing of IR-near IR 34 overlap under otherwise identical conditions. This background was subtracted from the raw vSFG spectra. For a full list of the tting parameters, see section D.1 in the appendix. FTIR spectra of benzonitrile and the benzonitrile/BCF adduct were obtained using a Bruker Vertex 80 FTIR spectrometer. For the benzonitrile spectrum, 0.2 mmol of benzonitrile was mixed into 2 mL of mineral oil. A few drops of the solution were sandwiched between two calcium ouride plates and held together using a demountable liquid FTIR cell (International Crystal Laboratories). For the adduct, the same procedure was followed except 0.03 mmol of BCF was mixed into the mineral oil as well as the benzonitrile. All computations were performed using Q-Chem. 85 Geometry optimizations and frequency calculations for the angle dependent calculations were performed using DFT with the B3LYP functional and the 6-31G(d,p) basis set. For the electric eld calculations, geometry optimizations and frequency calculations were done performed using !B97X-D and !B97X-V. For computational ease, BF 3 was used as a model instead of BCF because they have similar Lewis acidities. 81 Furthermore, for investigation of the trends in frequency shift as a function of angle and electric eld, the simplicity of this model is appealing. The angle dependent calculations were performed by optimizing the benzonitrile/BF 3 complex and then performing a series of frequency calculations where the angle displayed in gure 4.3 was swept from =30 to = 30 where 0 was assigned to the optimized linear geometry. The angles that were swept all lie in the plane which contains the benzene ring (see gure 4.3). Other degrees of freedom were held constrained while scanning the angle. The electric eld calculations were performed by placing the molecule between two meshes composed of point charges of opposite charge. The number of point charges for each mesh and the distance between the meshes were chosen such that the spatial variation of the electric eld between the two meshes was negligible. The direction and strength of the electric eld was controlled by the amount of charge on each point charge. For various electric elds spanning from -17 to 17 MV/cm, geometry optimizations were performed followed by frequency calculations. These calculations were performed for both benzonitrile and the benzonitrile/BF 3 adduct. The method produced the correct Stark tuning for benzonitrile which serves as a benchmark. For more information regarding how 35 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY these calculations were performed, see section D.2 in the appendix The main experimental parameter that we use to investigate adduct formation is = a ua (4.2) where, a and ua are the nitrile frequencies of the adducted and un-adducted compounds respectively. As can be seen in gure D.5, for both the surface and the bulk, the nitrile frequency blue shifts drastically upon adduct formation. The reason for this blue shift is well understood and discussed in detail elsewhere. 86,87 Brie y, the nitrogen lone pair has anti-bonding character across the CN triple bond. When the CN/BCF adduct is formed, some electron density from the nitrogen lone pair is donated to the empty p-type orbital on boron thereby strengthening the CN bond. We will use this frequency change as a parameter to measure the strength of adduct formation. Interestingly, in our work, we observe that for the surface (+84cm 1 ) is smaller than for the bulk (+90cm 1 ). This implies that the adduct formed at the surface is weaker than the adduct formed in the bulk. Here, we explore three factors that may contribute to this dierence between the surface and bulk values. These factors are frustration, surface electric elds, and electronic energy level alignment. We will start by examining the in uence of surface frustration or hinderance. We hypothesize that the relative geometry of the nitrile and BCF molecules at the surface will result into steric hinderance leading to poor donor-acceptor orbital overlap. This is analogous to hindrance in molecular FLPs in the literature. 63{66 As mentioned in the experimental section, our gold surface is fully saturated with SAM of MBN. It is expected that the nitrile donor orbitals will not be as accessible as the bulk because of the crowding of the molecules in the SAM. Additionally, the large penta uorophenyl substituents around the Lewis acidic boron make the acceptor orbital relatively inaccessible. Therefore, the extent of lone pair donation will be limited, resulting into a weaker adduct with a smaller . To test this hypothesis, we performed two experiments. First, we varied the surface concentration of MBN by diluting it with benzenethiol. Second, we adsorbed MBBN, a longer molecule than MBN, 36 2100 2150 2200 2250 2300 2350 2400 wavenumber (cm -1 ) Transmission (a.u.) 2100 2150 2200 2250 2300 2350 2400 wavenumber (cm -1 ) SFG Signal (a.u.) SFG Spectra Fits ∆ = − A B Before Adduct After Adduct ∆ = − Before Adduct After Adduct On the Surface In the Bulk N Figure 4.1: (A) vSFG spectra of MBN monolyaer on gold before and after adduction formation with BCF. (B) FTIR spectra of benzonitrile before and after adduct formation with BCF. Cartoons are representations of LP interaction on the surface and in the bulk. 37 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY onto gold in varying surface concentrations as well. Both experiments were attempts at relieving the postulated surface frustration. By diluting the surface concentrations of the nitrile with a smaller molecule, benezenethiol, we anticipated an increase in the ability of the nitrile to interact with the BCF molecules. This would be especially true in the MBBN case as it is roughly twice the length of benzenethiol. Therefore the donor nitrile orbitals should stand above the neighboring benzenethiol molecules and be more accessible for adduct formation. For these experiments, we would expect to see an increase in as we dilute the surface concentrations of MBN and MBBN. However, as can be seen in the table in gure 4.2, no signicant dierence in is observed across dierent surface concentrations for MBN and MBBN. The above suggests at least two possibilities. Either the observed associated with adduct formation is not sensitive to the environment surrounding the nitrile or the MBN and MBBN form phase separated domains or islands. If the latter were the case, we would not expect to see any real deviations in because, regardless of surface concentration, most MBN and MBBN molecules will be surrounded by like molecules rather than standing above the benzenethiol background. Additionally, islanding would be consistent with the actual frequencies ua and a values. As can be seen in the table in gure 4.2, for MBN and MBBN, these values are relatively invariant across the dierent surface concentrations. If uniform mixing occurred in the SAMs, then dierent nitrile stretching frequencies would be expected for the full coverage cases compared to the mixed cases, especially for MBBN. Many examples in the literature also suggest islanding occurs in mixed monolayer systems. 88{93 However, most of the previous studies examined mixed monolayers where the two component molecules are quite dierent. There, the mixed monolayers are often composed of aliphatic and alcoholic molecules or alkyl molecules with drastically dierent chain lengths. Because of the possibility of islanding, it is unclear if our experiment relieved the hypothesized surface frustration. Since frustration is critically related to the functionality of FLPs, studying this eect in more detail in the future is benecial. Relative orientation of the donor and acceptor orbitals also aects overlap and therefore is a contributor to frustration of the Lewis interaction. We examine the molecular orientations of the MBN and MBBN molecules at the surface as a possible cause of the dierence in values for the 38 2100 2150 2200 2250 2300 2350 Full Coverage wavenumber (cm −1 ) SFG Signal (a.u.) MBN MBN w/ BCF MBBN MBBN w/ BCF 2100 2150 2200 2250 2300 2350 2400 1:4 Coverage wavenumber (cm −1 ) SFG Signal (a.u.) MBN MBBN MBBN w/ BCF MBN w/ BCF 2100 2150 2200 2250 2300 2350 2400 1:8 Coverage wavenumber (cm −1 ) SFG Signal (a.u.) MBN w/ BCF MBBN w/ BCF MBN MBN Full 1:4 1:8 MBN =.±. =.±. ∆=.±. =.±. =.±. ∆=.±. =.±. =.±. ∆=.±. MBBN =.±. =.±. ∆=.±. =.±. =.±. ∆=.±. =.±. =.±. ∆=.±. A B C Figure 4.2: (A-C) SFG spectra of MBN and MBBN (before and after adduct formation) at full and diluted surface coverages. The table shows the nitrile frequencies and the frequency dierences between adducted and un-adducted molecules. surface and bulk. We computationally investigated how the tilt angle between the nitrogen and boron (gure 4.3) can change the adducted nitrile frequency. As explained in the computational methods section, it is reasonable to use BF3 as a model for BCF. The results of the frequency calculations as a function of the tilt angle between the donor and acceptor orbitals are shown in gure 4.3. For =30 (or 30 ) we see a change in CN of about 10cm 1 compared to = 0 . Intuitively, at = 0 o the overlap between the nitrogen lone pair and the boron p-type orbital is maximized. Any angular deviation should decrease this overlap, thereby, softening the CN bond. Because BCF is a bulky molecule, if the surface nitrile molecules are tilted, the MBN/BCF adduct would be unable to adopt the linear geometry ( = 0 ) because this would push the penta uorophenyl rings into the surrounding surface molecules. Instead the adduct would form at an angle and this would be, at least in part, responsible for the observed dierence between the bulk and surface. This is consistent with the large body of literature on SAMs on gold indicating that the molecules assume a tilted geometry with respect to the surface. 94{97 If this proposal is true, then it is an example of how surface tilt angle in uences Lewis acid-base chemistry. Estimation of the molecular tilt angles 39 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY -30 -20 -10 0 10 20 30 2394 2396 2398 2400 2402 2404 2406 Nitrogen-Boron Tilt Angle (degrees)  CN (cm -1 ) Figure 4.3: Calculated nitrile frequency shifts with respect to N-B tilt angle. Deviation from 0 results into poor overlap of the donor and acceptor orbitals in the Lewis adduct and consequently red shifts the nitrile frequency. found at the surface is possible using polarized vSFG, which is the subject of ongoing study by us. Dierences in solvation electric eld between the bulk and the surfaces may also manifest as variations. A dipolar molecule immersed in a polarizable medium polarizes its surrounding. This induced polarization, in its own turn, acts upon the molecule via an electric eld known as the solvation eld. The canonical model of this eect is the Onsager solvation theory. As an extension of the Onsager theory to the interfaces, we have previously shown that the image dipole eld created when a molecule is close to a conductor can be thought of as a solvation eld. Here, we explore the possible dierences these solvation elds may have on adduct formation. As we have shown previously, the strength of the solvation electric eld in the bulk compared to that of the surface of a gold electrode can be dierent. 46 Furthermore, bulk nitrile adducts have been shown to be sensitive their dielectric environment, as well. 77{79 To examine if this contributes to our dierence in values, we performed frequency calculations of benzonitrile and a benzonitrile/BF 3 adduct 40 in dierent electric elds. The results are displayed in gure 4.4. Experimentally, the behavior of benzonitrile in electric elds is well reproduced by the linear vibrational Stark eect and serves as a benchmark for our computational studies. The linear Stark eect states that changes in vibrational frequency follow the relation: = 0 F, where is the vibrational frequency in the presence of the electric eld, F. The unperturbed vibrational frequency is 0 and is the dierence in dipole moment between the ground and excited vibrational states. From previous reports, 42,98 = 0:3 cm 1 MV=cm for benzonitrile, with an error of at least0:03; cm 1 MV=cm , in part due to uncertainty in local eld correction. By linearly tting the vibrational frequency of benzonitrile in our computation (see gure D.3 in the appendix) we nd that = 0:38 cm 1 MV=cm . Given the level of theory employed, it is a reasonable match with the experimental value. Therefore, the anomalous behaviour of the adduct that we will describe next can not be attributed to the deciency in computation. Interestingly, the behavior of adducted benzonitrile is dramatically dierent from the linear Stark variation of frequency. For negative electric elds, the adducted nitrile frequency is relatively unaected. When a relatively weak electric eld of under 10 MV=cm is applied to the adduct, a large discontinuity is observed whereby CN jumps down by about 30 cm 1 . The exact value of the eld necessary to cause this is likely subject to Van der Waals interactions since values ranging from 6-9 MV=cm are obtained when using various levels of theory (see gure D.4 in the appendix). After the discontinuity, the CN becomes invariant again. This behavior is quite remarkable for two reasons. First, at no point within the range of the electric elds used here is the conventional Stark eect (i.e linear dependence) observed. Second, upon comparing gures 4.4 A and B, we note that the discontinuity observed in adducted nitrile frequency is mirrored by a discontinuity in the distance between the nitrogen and boron nuclei, R NB . At the discontinuity, R NB jumps from about 1.9 A to about 2.4 A. This suggest that the adduct partially dissociates under the application of a relatively weak electric eld (6-9 MV cm ). The existence of two stable bond lengths for nitrile/boron adducts, as suggested by these calculations, is previously observed in experiments. In the condensed phase R NB 1:6 A and in the gas phase R NB 2:4 A have been reported using crystallographic and rotational spectroscopic methods respectively. 99{101 Additionally, Phillips 41 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY and coworkers have shown that the adducts are stable in solid noble gas matrices 76,79 and that for media with dielectric constants > 3 the adduct forms with a shorter bond length, while for < 3 it assumes the longer bond length. The dielectric constant of a medium in uences the solvation electric eld 46,102 felt by the adduct. Solvation elds in this range of dielectric constants are a few MV/cm. 46 This supports the results presented in gure 4.4 indicating bond dissociation under the in uence of a few MV/cm eld. A consequence of this would be that formation of the adduct in the presence of high dielectric constant solvents should be dicult. In our work, we have observed that the adduct dissociates in ambient air, which is likely due to water vapor interacting with BCF. It is reasonable to propose that if an organic solvent is a stronger Lewis base than the surface nitrile, it will dissociate the adduct and dissolve the BCF away from the surface. We have not been able to create the adduct in solution, even in solvents that are not particularly Lewis basic such as cyclohexane. All of these experimental observations support the plausibility of the relatively weak eld causing partial dissociation of the adduct suggested in our computation. We propose a reasonable electronic origin for the breaking of the bond under the in uence of the eld. A positive electric eld will polarize the lone pair away from boron and more towards nitrogen. Therefore the Lewis basicity of the lone pair becomes a function of the applied electric eld. At large enough elds (> 10 MV/cm), the lone pair is no longer basic enough to hold on to the Lewis acid as strongly and the adduct partially dissociates. This observation links the Lewis basicity of a chemical entity with electric eld, and is anticipated to have wider implications for all Lewis interactions. In general, all Lewis bases will be expected to weaken upon application of an external eld that polarizes the lone pair away from the Lewis acid and make it less available for bonding. Our observation hints at a connection between eld strength and Lewis basicity, which we believe is useful both for homogenous and heterogeneous chemistry. To begin comparing the electric eld eects between the surface and the bulk we note that, based on previous models of solvation elds, the electric eld the molecules experience due to solvation will be negative. Therefore, to address the possible eects of solvation elds on , we examine the left side of gure 4.4. If we knew the local electric elds at the surface and in the bulk of both the adducted and un-adducted molecules, all we would need is to nd the corresponding frequencies for 42 −15 −10 −5 0 5 10 15 2380 2400 2420 2440 2460 Electric Field (MV/cm) ν CN (cm −1 ) −15 −10 −5 0 5 10 15 1.6 1.8 2 2.2 2.4 2.6 Electric Field (MV/cm) R N−B (Å) A Benzonitrile/BF 3 Benzonitrile ∆= ) ∆= ) ∆= ) . Å B Figure 4.4: (A) Calculated nitrile frequencies in the presence of applied electric elds for benzonitrile and benzonitrile/BF 3 adduct using !B97X-D. While the benzonitrile frequency conforms to a linear Stark shift model (see gure D.3 in the appendix), the frequency shift of the adduct shows a discontinuity around 6.5 MV/cm. This discontinuity changes with DFT functional, however, is always found at relatively small electric elds (see gure D.4 in the appendix). (B) The Lewis donor-acceptor (nitrogen and boron) interatomic distance as a function of applied electric eld. The discontinuity in (A) corresponds to a large shift in the donor acceptor-distance, 0.4 A, indicating the partial dissociation of the adduct at that eld value. 43 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY each from the gure and calculate their . However, the models for the bulk and surface solvent electric elds depend sensitively on the ground state dipole moment, relative size, and refractive index of the molecule, all of which are not known for the adduct. Experimental determination of the local elds is desirable, for example through measuring and modeling its solvatochromic shift in a variety of solvents and the conventional Stark spectroscopy. 42,98 However, such experiments for the adduct are quite dicult due to its instability and reactivity. In spite of all this, from gure 4.4 it is clear that dierent electric elds experienced by the adducted and un-adducted nitrile result into dierent values. Unfortunately, for the reasons described above, we do not have a reliable way of estimating the electric eld that the adduct experiences. Future work may resolve this problem. Another possible factor in uencing the observed values is the energy level alignment of the interacting orbitals in nitrile and BCF. It is noted that we are using dierent LBs when comparing the bulk and the surface adducts. In the bulk we use benzonitrile and at the surface we use either MBN or MBBN. The reason for this choice is because we need the attachment capability of the thiol to tether the nitrile to the surface. We suspect that this may in uence the Lewis basicity of the nitrile group. The dierence between these molecules may be signicant enough to modify the donor orbital energies, and therefore their Lewis basicity. The extent to which these LBs donate electron density to BCF may vary and dierences in values may be expected accordingly. Furthermore, previous work examining nitrile/BCF adducts report dierent values for dierent LBs. 103 For example, p-nitrobenzonitrile and acetornitrile adducts formed with BCF were found to have = 99cm 1 and = 114cm 1 , respectively. The root cause of these dierent values was not addressed in that work, however, it is reasonable to suggest it may arise because of the orbital energy dierences between the LBs. An analogous line of thought has been used previously to address the dierence between the observed values of acetonitrile complexes with BF 3 and BCl 3 . 86 There, the authors noted that the calculated orbital energy of the empty boron p-type orbital of BCl 3 was closer to the acetonitrile donor orbital and argue this was responsible for the dierence in values for the acetonitrile/(BF 3 or BCl 3 ) complexes. While the dierence between benzonitrile and MBN is likely not as dramatic as that between p-nitrobenzonitrile and acetonitrile, it may still be important. We have found that the unsubstituted benzonitrile (dissolved in dimethylformamide) has its nitrile 44 frequency at 2228cm 1 , while that of MBN in the same solvent is at 2226cm 1 (see gure D.5 in the appendix). This red shift would imply more electron donation to the nitrile end of the molecule. However, the known Hammett constant for thiol is slightly positive ( p = 0.15) indicative of slight electron withdrawing. Although these two observations are inconsistent with each other, it does show that the thiol group could have at least a slight electronic contribution to the Lewis basicity of the nitrile. In any event, this contribution does not seem to entirely account for the larger 6cm 1 shift observed in this work. The reason that bulk experiments were performed with benzonitrile instead of MBN is that the thiol group can attack the Lewis acid and the desired nitrile adduct would not form. In analyzing the energetics of Lewis adducts, it is often customary to use Energy Decomposition Analysis (EDA) to isolate the electrostatic contributions from covalent contributions. 80,104 In particular, it is noted that a signicant contribution to the bond energy arises from electrostatic interactions. It is important to analyze the consequence of this for the surface adduct. The electrostatic contribution, E(es), is dened as the electrostatic interaction between the LA and LB that are distorted to the nal adduct geometry and brought together to the nal adduct distance, but the orbital interactions and Pauli exchange have not yet occurred. 105 In our case, the LB is on a metal surface. Therefore, the LA will not only interact with the surface-tethered LB, but also with its own image charge in the metal. The last interaction is favorable (attractive) and should result into a more stable bond. We have discussed this eect in the context of the dipole of the adduct feeling a favorable solvation eld in the previous paragraphs. However, mentioning it in the context of EDA provides an alternative avenue of thinking about it, and can potentially generate interest in explicitly calculating the EDA of surface bound adducts. Finding absolute orbital energies can be done via photoelectron spectroscopy, however, we point out that these experiments, which are already quite dicult, become even more problematic when examining molecules at the surface of a conductor. Furthermore, an experiment where we tune the orbital energies and leave other properties that in uence the adduct formation alone would be nearly impossible. Our surface adduct data exemplies this latter point. In our experiments, we co-adsorbed both MBN and MBBN with benzenethiol to the gold surface. It is reasonable to assume 45 CHAPTER 4. INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY that, MBN and MBBN have dierent orbital energies. However, our data does not re ect this. Rather, we observe no dierence in between the two LBs. This does not imply orbital energy alignment is not contributing to our observed data. It could be that orbital energy dierence are not major contributors or it could be that other eects, including the ones mentioned earlier such as surface frustration, are compensating for any changes caused by the orbital energy dierences. In conclusion, we have reported the formation and study of Lewis acid base adducts at the surface of gold. The nitrile vibrational frequency change upon adduct formation at the surface is smaller than that of the bulk. We postulate that this dierence may arise from numerous mechanisms including surface frustration, solvation electric eld dierences between the surface and the bulk, and energy level alignment between the donor-acceptor orbitals of the LPs. It is not clear if any one of these candidates is solely responsible for the observed dierences between the surface and bulk or if all the mechanisms contribute comparably and what we observe is the total net eect. Future studies are needed to address this issue. We hope this work serves as an impetus to studies further examining LPs at interfaces. Some recent work has begun explore the possible eects that FLPs have on electrochemical processes. 106 It may be possible to tether known homogenous catalytically active FLPs on a surface to augment the polarizing eect of an electrochemical interface for driving specic reactions. 46 Appendix A Typical IR and Visible Upconversion Spectra 47 APPENDIX A. TYPICAL IR AND VISIBLE UPCONVERSION SPECTRA 1900 2000 2100 2200 2300 2400 2500 0 0.2 0.4 0.6 0.8 1 Wavenumber (cm −1 ) Normalized SFG Intensity 785 786 787 788 789 790 0 2000 4000 6000 8000 10000 12000 Wavelength (nm) Intensity (CCD Counts) (a) (b) Figure A.1: (a) A spectrum of the upconverted IR pulse re ected o of bare Au. The large dip between 2300 - 2400 cm 1 comes from ambient CO 2 adsorption. We enclose the experimental setup and evacuate the enclosure with dried and CO 2 free air. (b) The visible upconversion pulse. The pulse is made by passing the output of a regeneratively amplied femtosecond Ti:Sapph laser through a 4f lter to narrow the pulse to a FWHM of 0.05 nm (8 cm 1 ) 48 Appendix B Tensors and the Second Order Hyperpolarizability B.1 Hooke's Law as a Motivating Example of Tensors Tensors are a mathematical construction originally derived to couple things that would otherwise be orthogonal. An easy way to understand their utility is to work through a motivating example. We will use the ball and spring example (Hook's law). From gure B.2, we can write the force that the ball feels after it has been stretched by x as follows: ~ f =(kx)^ x + 0^ y + 0^ z where ~ f is the restoring force the ball feels, k is the spring constant and x is the displacement of the ball from its equilibrium position. The thing to note here is that spring was pulled in the ^ x direction and the restoring force it feels is in the ^ x direction as well. However, we can imagine a more complicated ball and spring example. As can be seen in gure B.2, we have two springs and that when we move the by the same x as before, we develop two forces. The important point being that one of the those forces has a component in the ^ y direction. 49 APPENDIX B. TENSORS AND THE SECOND ORDER HYPERPOLARIZABILITY ∆ Figure B.1: ∆ Figure B.2: We can express the the forces the ball feels as follows: ~ f 1 =(k 1 x)^ x + 0^ y + 0^ z ~ f 2 =(k 2;x x)^ x (k 2;y x)^ y + 0^ z where, ~ f 1 and ~ f 2 are the forces from each spring, k 1 is the spring constant of spring 1 while k 2;x and k 2;y are the spring constants of spring 2 in the ^ x and ^ y directions, respectively. Combining ~ f 1 and ~ f 2 we can write that the total force of the spring is: ~ f = [(k 1 +k 2;x )x] ^ x (k 2;y x)^ y + 0^ z We can see that by displacing the ball in only the ^ x direction produces forces in both the ^ x and the ^ y directions. We can extend this then to the more complicated case of the molecule methane where we approximate the chemical bonds that hold atoms together as springs. If we move the central carbon atom (see gure B.3), we actually will feel forces in three dierent directions (^ x, ^ y and ^ z). We can 50 B.2. TENSOR NOTAION describe the total force in this case as: ~ f =(k xx x +k xy y +k xz z)^ x (k yx x +k yy y +k yz z)^ y (k zx x +k zy y +k zz z)^ z We can also choose to write this expression in matrix form: ~ f = 2 6 6 6 6 4 k xx k xy k xz k yx k yy k yz k zx k zy k zz 3 7 7 7 7 5 2 6 6 6 6 4 x y z 3 7 7 7 7 5 We note that the matrix that contains the spring constants is a tensor. It couples motions in one direction to forces in another direction. This is exactly what a tensor is supposed to do. That is, the spring constant is actually a tensor! B.2 Tensor Notaion Tensors are noted by their rank. A 0th order tensor is just simply a scalar, a 1st order tensor is a vector, a 2nd order tensor is a matrix, a 3rd order tensor is a cube, a 4th order tensor is some weird 4 dimensional object and so on. Typical notation for a few of these tensors can be found below: Scaler: 6:022 10 23 Vector: ~ A =a x ^ x +a y ^ y +a z ^ z Matrix: 2 6 6 6 6 4 k xx k xy k xz k yx k yy k yz k zx k zy k zz 3 7 7 7 7 5 51 APPENDIX B. TENSORS AND THE SECOND ORDER HYPERPOLARIZABILITY However, when we get above a matrix, we see that we do not necessarily have a very good way to write down a 3rd rank or above tensor. However, we can rewrite 1st rank and above tensors in the following manner: Vector: ~ A =a x ^ x +a y ^ y +a z ^ z = X i=x;y;z a i ^ i Matrix: 2 6 6 6 6 4 k xx k xy k xz k yx k yy k yz k zx k zy k zz 3 7 7 7 7 5 = X i;j=x;y;z k ij ^ i ^ j . Where the summations are tensor notation. Note that the ^ i and ^ j are actually just vectors. Essentially, tensor notation takes any tensor higher than a 0th rank tensor and decomposes it into vectors. We can transform from the tensor notation back to the typical notation fairly simply. For a 1st rank tensor, nothing really needs to be done mathematically other than just expanding out the sum. For the 2nd rank tensor, we expand out the sum, write out the vectors for each term and the multiply the two vectors together using the inner product method. As an example, lets do the rst three terms in the sum (the xx, xy and xz terms). k xx 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 +k xy 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 0 1 0 3 7 7 7 7 5 +k xz 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 0 0 1 3 7 7 7 7 5 =k xx 2 6 6 6 6 4 1 0 0 0 0 0 0 0 0 3 7 7 7 7 5 +k xy 2 6 6 6 6 4 0 1 0 0 0 0 0 0 0 3 7 7 7 7 5 k xz 2 6 6 6 6 4 0 0 1 0 0 0 0 0 0 3 7 7 7 7 5 = 2 6 6 6 6 4 k xx k xy k xz 0 0 0 0 0 0 3 7 7 7 7 5 52 B.3. USING TENSOR NOTATION Note that this type of multiplication is accomplished by taking the rst element of the rst vector and multiply the entire second vector by this value; this then becomes the rst row of the matrix. Then take the second value of rst vector and multiply the entire second vector by this value; this then becomes the second row of the matrix. Then take the third value of the rst vector and multiply the entire second vector by this value; this then becomes the third and nal row of the matrix. If we do this for every term in the summation, we would end up with 9 dierent matrices. Note that each individual matrix in the sum has only one non zero term in it. When all of the matrices are summed up we obtain the entire spring constant matrix (tensor). This type of method to go back and forth between tensor notation and the typical notation can be expanded to higher rank tensors. For example we can discuss a 3rd rank tensor, which would be a cube. In this case, two of the vectors would make a matrix like we just discussed. Then the rst value of the leftover vector would multiply the entire matrix and this would be the rst slice of the cube. Then the second value of the leftover vector would multiply the entire matrix and this would be second slice of the cube. Then the third value of the leftover vector would multiply the entire matrix and this would be the third and nal slice of the cube. The summation for the 3rd rank tensor contains 27 dierent cubes each with only one non zero element. The same thing could be done with a 4th rank or higher tensor. However, at some point, this ceases to be useful. What is useful is to use the tensor notation to do tensor mathematics as will be seen in the next section. B.3 Using Tensor Notation We will now use this new tensor notation that we have developed to examine the taylor series expansion of the polarization of a material in terms of electric elds: ~ P = (1) ~ E + (2) : ~ E 1 ~ E 2 + (3) . . . ~ E 1 ~ E 2 ~ E 3 +::: (B.1) in this expansion, (1) , (2) and (3) are 2nd, 3rd and 4th rank tensors, respectively. The rst question that people have upon seeing the equation B.1 is what are the little dots? Well in the rst 53 APPENDIX B. TENSORS AND THE SECOND ORDER HYPERPOLARIZABILITY Figure B.3: Methane molecule term of the equation is simply a dot product between the rst order susceptibility (1) (which is 2nd rank tensor, i.e. a matrix) and an electric eld (a 1st rank tensor, i.e. a vector). If we were to try to understand this in the typical notation and not tensor notation, this would be dicult to process and seem not very intuitive. However, in tensor notation, this dot product is much easier to perform. Let us work through it then: (1) ~ E = x;y;z X i;j (1) ij ^ i ^ j X k E k ^ k (B.2) Then taking the dot product between the ^ j and ^ k direction we end up killing the summation over k and we end up with: (1) ~ E = x;y;z X i;j (1) ij E j ^ i (B.3) The second term in equation B.1 contains the double dot product between the second order susceptibility (2) and two electric elds. Again we can understand this term using our tensor notation: (2) : ~ E 1 ~ E 2 = x;y;z X i;j;k (2) ijk ^ i ^ j ^ k : x;y;z X m E m ^ m x;y;z X n E n ^ n (B.4) 54 B.4. TENSORS AND SYMMETRY OPERATORS Au ! Now, we simply take the dot product between ^ k and ^ m and then take the dot product between ^ j and ^ n. The summations over m and n are killed and we end up with the following expression: (2) : ~ E 1 ~ E 2 = x;y;z X i;j;k (2) ijk E k E j ^ i (B.5) This, of course can be then extended to higher order terms of the polarization expansion. B.4 Tensors and Symmetry Operators Because this document is concerned with vSFG, we will now discuss how the symmetry of the interface leads to a non zero (2) . We will be examining the system seen in gure B.4. We simply have a nitrile molecule attached to gold. In this case, the molecule is perfectly vertical. The surface system is has some response that is encapsulated by (2) = P x;y;z i;j;k (2) ijk ^ i ^ j ^ k. We start by noting that if we were to rotate the gold/molecule system by 90 , we would not change the system in any distinguishable way. The same is true for a 180 rotation as well. However, one question that is raised is how can we examine these rotations mathematically? We need to perform symmetry operations on the tensor and we need a way to do this. We can accomplish this by using the rotational matrices and operating on each vector in each tensor element. Let's start by 55 APPENDIX B. TENSORS AND THE SECOND ORDER HYPERPOLARIZABILITY operating on the (2) xxx ^ x^ x^ x term. We will rotate it 180 about the z-axis: (2) xxx ^ x^ x^ x = (2) xxx 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 we will rotate each vector180 (2) xxx 2 6 6 6 6 4 1 0 0 0 1 0 0 0 1 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 0 1 0 0 0 1 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 0 1 0 0 0 1 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 = (2) xxx 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 = (2) xxx 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 2 6 6 6 6 4 1 0 0 3 7 7 7 7 5 However, we note that the a 180 rotation about the z-axis should not change the response of the system. Therefore, (2) xxx = (2) xxx , and thus, (2) xxx = 0. However, if we were to do this for the (2) zzz , where we rotate 180 about the y-axis, we would do almost the exact same mathematics (except the roational matrix would be dierent along with the vectors that we are operating on), however, in the end, we are unable to say that we did not change the system. Therefore, via symmetry arguments, (2) zzz 6= (2) zzz 6= 0. Using these arguments and dierent symmetry operators we can show that for a centrosymmetric system (i.e. rotations about the x-axis, y-axis and z-axis do not chnage the system in a distinguishable way), (2) = 0. The only dierence for the gold/molecule system just discussed is that for rotations about the y or x-axis, the system is distinguishable, and, therefore, (2) 6= 0. 56 Appendix C Supporting Information for Direct Spectroscopic Measurement of Interfacial Electric Fields C.1 ModelingtheElectricFieldattheElectrode-Electrolyte Interface A variant of the Smith and White model for potential drop at an electrochemical interface in the presence of dielectric monolayer 47 is used here to explain the ionic concentration dependence of the electric eld observed in our experiments. We split the interface into two regions. The rst layer is a dielectric layer that we assign to the self-assembled monolayer (SAM). The second layer is the diuse Gouy-Chapman layer. Figure C.1 shows a diagram of this model. The potential drop across the SAM is assumed to be linear while in the diuse layer it is assumed to follow the Gouy-Chapman 57 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Au + + + + + + + + + - + + - - - - - - - - - - - - - - - - - + + + + + + + + + - - - - + - + + Potential () d + + + - - - - - - + + + 1 2 Box 1 Box 2 Box 3 Gaussian Boxes Figure C.1: A picture of the model that describes potential variation near the interface. The interface is divided into a dielectric layer composed of the 4-MBN SAM and a diuse Gouy-Chapman layer. The potential at the electrode and ionic concentration in the bulk are the experimentally controllable variables. The potential prole across the interface is obtained by applying Gauss's law at the boundaries as described in the text. 58 C.1. MODELING THE ELECTRIC FIELD AT THE ELECTRODE-ELECTROLYTE INTERFACE model. Therefore, the electric elds in each region are: ~ F = 8 > > > < > > > : 0 s d for Region 1 q 8kTn 2 0 sinh( ze 2kT ( (x) B )) for Region 2 (C.1) where kT is the thermal energy, n is the ionic concentration, 2 is the dielectric constant of the diuse layer, 0 is the permittivity of free space, e is the elementary charge, and z is the charge of the ions. We seek an expression for the electric eld the SAM molecules feel as a function of the applied potential and the ionic concentration. Of course, the above expression for Region 1 is such an expression, however, the value of S depends on potential and concentration and needs to be determined. We will apply Gauss' law for the three boxes seen in gure C.1. Gauss' law relates the charge enclosed by a Gaussian box to the electric eld created by the enclosed charge as shown below: Z ~ Fd ~ A = Q enclosed 0 (C.2) In our system, we use rectangular Gaussian boxes and assume that the electric eld lines are normal to the electrode. Then Gauss's law can be written as: = ( R 0 j ~ F R j L 0 j ~ F L j) (C.3) where, the subscripts \R" and \L" refer to the right and left sides of the gaussian box, and charge density is dened as Q enclosed A . Analysis of Box 1 We will begin with equation C.3. The electric eld on the right,j ~ F R j, is simply equation C.1 for Region I. The electric eld,j ~ F L j, is zero because inside of a conductor, an electric eld can not be 59 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS sustained. Therefore, we have: 0 = 1 0 0 s d where 0 is the charge density on the electrode surface and 1 is the dielectric constant of the SAM. Analysis of Box 3 Because the results of box 3 will make it easier to examine box 2, we will examine it rst. Both the right and left sides of box 3 are inside of Region 2, therefore, we will use the electric eld expression in equation C.1 for Region 2 for both sides of the box. The right side of the box is positioned such that it is in the \bulk" solution. Therefore, for the right side, (x) = B and j ~ F R j = 0 because we have referenced the potential in the diuse layer to the potential in the bulk. On the left side, we are innitesimally close to the junction between Region 1 and Region 2. Therefore, (x) = S and we have the following: B = 2 0 q 8kTn 2 0 sinh( ze 2kT ( S B )) Analysis of Box 2 Here, we have no charge enclosed by the box. The drawing does make it appear as if we have enclosed some charge from the diuse layer, however, similar to our analysis of box 3, the width of box 2 is innitesimally small and we actually enclose no free charges. Therefore we insert the electric elds on the two sides of the boundary between regions 1 and 2 into equation C.3, while keeping = 0. 0 = 2 0 q 8kTn 2 0 sinh( ze 2kT ( S B )) 1 0 0 s d In the above expression, it is worth noting that the rst and second terms are really just B and 0 , respectively, and therefore consistent with the neutrality requirement: 0 + B = 0 60 C.1. MODELING THE ELECTRIC FIELD AT THE ELECTRODE-ELECTROLYTE INTERFACE If we collect all of our charge density expressions, then we have the following set of equations: 0 = 1 0 0 S d I (C.4) B = 2 0 q 8kTn 2 0 sinh( ze 2kT ( S B )) (C.5) 0 = 0 + B (C.6) At this point, it is important to note that the applied potential, 0 , is the potential of the electrode with respect to a reference electrode, 0 = 0 ref , where, for our case, ref is the Ag/AgCl electrode potential. Because of this, we will make use of the following equations: 0 = 0 ref (C.7) S = S B (C.8) PZC = B ref (C.9) where PZC is the potential of the point of zero charge. Using the above expressions, we want an expression for S instead of S . The next step is to use equations C.4-C.9 to nd an expression for S . We start by linearizing equation C.5 and substituting in S . Therefore, B = 2 0 S (C.10) where, = ze q 2n 2 0 kT , which is the inverse Debye length. From here, we substitute this expression along with equation C.4 into equation C.6 which yields, 1 0 0 S d 2 0 S = 0: (C.11) 61 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Next, we can use equations C.7, C.8 and C.9 to recast 0 S . 1 0 0 PZC S d 2 0 S = 0 (C.12) solving for S yields S = 1 ( 0 PZC ) 2 d+ 1 (C.13) From here, we obtain an expression for the electric eld, ~ F , that the SAM experiences by plugging equation C.13 into the rst equation found below. ~ F = 0 1 0 = B 1 0 = 2 0 S 1 0 (C.14) Thus, we have, after some rearrangement, the following expression ~ F = 2 ( 0 PZC ) 2 d+ 1 (C.15) The above is the expression for the electric eld that the SAM layer experiences as a function of ionic concentration n and applied potential 0 . As explained in the main text, we observe that our experimental measurement of electric eld as a function of ionic concentration follows the above form for conditions that the electrode is polarized and negligible current ows. We point out that the origianl model by Smith and White includes the Stern layer. However, its inclusion does not alter the dependence of ~ F on either applied potential, 0 , or ionic concentration n. Because of this, our our experiment is insensitive to the inclusion of the Stern layer and we have chosen to exclude it from the model. 62 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA C.2 Representative vSFG Data of the Potential and Ionic Concentration Dependent Data C.2.1 Representative vSFG Spectra 63 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS 2200 2220 2240 2260 15 20 25 30 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V 0.5 V −0.8 V [KCl] = 1mM 2200 2220 2240 2260 15 20 25 30 35 40 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V 0.5 V −0.8 V [KCl] = 10mM 2200 2220 2240 2260 10 15 20 25 30 35 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V 0.5 V −0.8 V [KCl] = 50mM 2200 2220 2240 2260 15 20 25 30 35 40 45 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V 0.5 V −0.8 V [KCl] = 100mM 2200 2220 2240 2260 10 20 30 40 50 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V 0.5 V −0.8 V [KCl] = 1000mM Figure C.2: Positive scan zoomed in spectra for V = 0, 0.5 and -0.8 V. The dip arises from the CN stretch. At small ionic concentrations, the dierence in frequency \peak" position for 0.0 V and 0.5 V is small and increases with larger ionic concentrations. It is also apparent that the change in frequency at -0.8 V and 0.0 V is small at low ionic concentrations as discussed in the main body of the publication. 64 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA [KCl] = 1mM [KCl] = 10mM [KCl] = 50mM [KCl] = 100mM [KCl] = 1000mM 2200 2210 2220 2230 2240 2250 2260 15 20 25 30 35 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V −0.8 V 0.5 V 2200 2220 2240 2260 20 30 40 50 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V −0.8 V 0.5 V 2200 2220 2240 2260 15 20 25 30 35 40 45 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V −0.8 V 0.5 V 2200 2220 2240 2260 20 30 40 50 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V −0.8 V 0.5 V 2200 2220 2240 2260 20 40 60 80 100 wavenumber (cm −1 ) SFG signal (a.u.) 0.0 V −0.8 V 0.5 V Figure C.3: Same as the above gure except this is for the negative scan. 65 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS −0.500.5 2228 2229 2230 2231 2232 2233 2234 Applied Potential (V) CN Stretch Frequency (cm −1 ) 1 mM 10 mM 50 mM 100 mM 1000 mM −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 −1 0 1 2 3 4 Applied Potential (V) ∆ν CN (cm −1 ) −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 −2 −1 0 1 2 3 4 Applied Potential ∆ν CN (cm −1 ) Figure C.4: Plots of the center wavelength ! CN extracted from the ts as a function of applied potential and ionic concentration. Panels (a) and (b) correspond to positive and negative scan as explained in the experimental section. As discussed in the main text, for potentials where negligible current is drawn (0.0 - 0. 5 V), is linear with respect to 0 . However, as can be seen for the 100 mM and 1000 mM negative scans, behaves linearly only after reaching 0.3 V for the rst time along the scan. Furthermore, the last applied potential for all the negative scans was 0.1 V, not 0.0 V. Because of this, linear extrapolations were used to obtain the missing values of , including the 100 mM and 1000 mM reference frequencies 0 . C.2.2 Representative Fit Parameters The table seen below shows the t parameters of representative data. This is done because the total number of spectra obtained exceeds 280. We report the average values and their standard deviations for both the positive and negative scans. From the standard deviations, it is seen that the values obtained from the ts were highly repeatable. Table C.1: Fit Parameters using the model equation: I SFG (!) = A NR e i + B !!CN+iCN 2 exp (!!g) 2 2 g + 10. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. 1mM, 0.0 V A NR 4.24 0.03 5.0 0.2 B 17.1 0.4 19 1 CN 6.0 0.4 6.0 0.2 66 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA Fit Paramaters Cont. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. g 118 6 155 8 1.40 0.02 1.52 0.03 ! CN 2230.5 0.2 2229.5 0.2 ! g 2270 4 2293 4 1mM, 0.5 V A NR 4.1 0.1 4.2 0.1 B 18.3 0.8 18.5 0.8 CN 6.27 0.08 6.0 0.2 g 124 2 150 12 1.39 0.01 1.42 0.04 ! CN 2231.30 0.01 2230.80 0.09 ! g 2276 2 2301 14 1mM, -0.8 V A NR 3.93 0.02 4.5 0.3 B 16 1 16 3 CN 5.9 0.2 5.6 0.2 g 127 10 157 6 1.31 0.02 1.45 0.04 ! CN 2230.1 0.2 2229.5 0.2 ! g 2276 8 2304 9 10mM, 0.0 V A NR 5.2 0.2 6.0 0.1 B 18 1 19 1 CN 5.4 0.1 5.4 0.2 g 125 6 142 8 1.38 0.05 1.39 0.06 67 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Fit Paramaters Cont. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. ! CN 2231.1 0.3 2230.5 0.2 ! g 2265 4 2286 6 10mM, 0.5 V A NR 4.83 0.2 4.8 0.2 B 26.9 0.8 27 1 CN 6.12 0.06 5.9 0.1 g 119 10 143 7 1.43 0.04 1.42 0.03 ! CN 2232.8 0.1 2232.7 0.3 ! g 2262 7 2284 10 10mM, -0.8 V A NR 4.4 0.2 4.8 0.2 B 15 1 10.5 0.6 CN 5.02 0.01 4.6 0.2 g 119 3 146 11 1.04 0.06 1.1 0.1 ! CN 2231.0 0.3 2230.1 0.4 ! g 2259 5 2282 14 50mM, 0.0 V A NR 4.42 0.01 5.64 0.03 B 28 2 26.0 0.5 CN 6.53 0.08 6.29 0.03 g 107.0 0.5 119 1 1.39 0.03 1.454 0.005 ! CN 2230.0 0.2 2229.50 0.02 ! g 2247 1 2264 1 68 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA Fit Paramaters Cont. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. 50mM, 0.5 V A NR 4.84 0.06 4.8 0.2 B 31.7 0.8 30 1 CN 6.77 0.05 6.6 0.1 g 111 3 131 6 1.428 0.004 1.43 0.02 ! CN 2232.0 0.1 2231.8 0.1 ! g 2257 4 2270 5 50mM, -0.8 V A NR 4.3 0.2 4.8 0.1 B 28.1 0.3 20.9 0.8 CN 6.2 0.1 5.86 0.05 g 122 13 133 4 1.21 0.01 1.33 0.02 ! CN 2230.2 0.1 2228.9 0.1 ! g 2259 12 2273 5 100mM, 0.0 V A NR 5.42 0.06 5.6 0.1 B 28.5 0.6 33 2 CN 7.68 0.08 6.65 0.05 g 120 4 152 8 1.283 0.008 1.191 0.006 ! CN 2229.2 0.2 2230.5 0.1 ! g 2254 3 2265 8 100mM, 0.5 V A NR 5.57 0.09 5.43 0.03 69 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS Fit Paramaters Cont. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. B 25.4 0.8 24.7 0.4 CN 7.5 0.1 7.0 0.1 g 126 9 153 7 1.32 0.02 1.35 0.01 ! CN 2231.6 0.2 2231.2 0.1 ! g 2255 3 2275 4 100mM, -0.8 V A NR 5.0 0.1 5.0 0.1 B 32 1 36 1 CN 7.2 0.1 6.44 0.08 g 127 2 143 1 1.07 0.02 1.12 0.04 ! CN 2230.6 0.1 2229.5 0.1 ! g 2255 5 2268 5 1000mM, 0.0 V A NR 4.98 0.07 6.9 0.2 B 29.9 0.4 37 1 CN 7.1 0.2 5.85 0.03 g 118 2 136 6 1.268 0.002 0.92 0.01 ! CN 2229.0 0.1 2232.0 0.08 ! g 2256.6 0.7 2262 9 1000mM, 0.5 V A NR 6.34 0.04 6.04 0.08 B 29.3 0.8 32.8 0.4 CN 6.9 0.2 6.50 0.06 70 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA Fit Paramaters Cont. Conc./Potential Parameter Avg(pos. scan) Std. Avg(neg. scan) Std. g 115 4 125.0 0.6 1.30 0.02 1.38 0.01 ! CN 2231.7 0.2 2231.4 0.1 ! g 2252 3 2254 2 1000mM, -0.8 V A NR 3.9 0.1 4.4 0.3 B 39 2 49 4 CN 6.95 0.05 6.5 0.1 g 178 23 178 30 0.12 0.02 -0.02 0.04 ! CN 2229.9 0.1 2232.1 0.3 ! g 2313 24 2316 34 71 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS 10 0 10 1 10 2 10 3 0 0.1 0.2 0.3 0.4 0 0.5 1 1.5 2 2.5 ∆ (cm -1 ) Figure C.5: The plot of the frequency change for the negative scan. As can be seen, it matches the frequency changes for the positive scan (shown in the main text) quite well. 72 C.2. REPRESENTATIVE VSFG DATA OF THE POTENTIAL AND IONIC CONCENTRATION DEPENDENT DATA 0 50 100 150 200 −10 −8 −6 −4 −2 0 time (s) current (µA) Figure C.6: A representative current-time plot at -0.2 V. Notice that at rst, there is a transient signal from the voltage jump. However, after about 10 seconds, the current begins to level o and stabilize. 73 APPENDIX C. SUPPORTING INFORMATION FOR DIRECT SPECTROSCOPIC MEASUREMENT OF INTERFACIAL ELECTRIC FIELDS 74 Appendix D Supporting Information to Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy D.1 Spectral Fitting Parameters A full list of the tting parameters for the un-adducted molecules can be found below: Table D.1: Fit Parameters using the model equation: I SFG (!) = A NR e i + B !!CN+iCN 2 exp (!!g) 2 2 g . Nitrile/Surf. Coverage Parameter Avg Value 95% MBN, 100% A NR 10.72 0.04 75 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY Fit Paramaters Cont. Nitrile/Surf. Coverage Parameter Avg Value 95% B 69 3 CN 7.2 0.8 g 88.4 0.5 1.20 0.02 ! CN 2229.2 0.2 ! g 2274.1 0.4 MBN, 25% A NR 11.00 0.03 B 36 1 CN 7.0 0.3 g 92.0 0.5 1.14 0.04 ! CN 2227.8 0.3 ! g 2273.0 0.4 MBN, 1.25% A NR 10.27 0.03 B 28 1 CN 6.1 0.3 g 88.3 0.5 1.20 0.04 ! CN 2228.0 0.3 ! g 2273.5 0.4 MBBN, 100% A NR 11.35 0.04 B 53 2 CN 6.2 0.2 76 D.1. SPECTRAL FITTING PARAMETERS Fit Paramaters Cont. Nitrile/Surf. Coverage Parameter Avg Value 95% g 86.0 0.5 1.50 0.02 ! CN 2231.1 0.2 ! g 2271.6 0.3 MBBN, 25% A NR 12.92 0.05 B 50 2 CN 6.5 0.3 g 91.0 0.6 1.36 0.03 ! CN 2230.4 0.3 ! g 2270.2 0.4 MBBN, 1.25% A NR 12.00 0.04 B 45 1 CN 6.2 0.2 g 87.6 0.4 1.42 0.02 ! CN 2230.8 0.2 ! g 2271.1 0.3 A full list of the tting parameters for the adducted molecules can be found below: 77 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY Table D.2: Fit parameters using the model equation: I SFG (!) = A NR e i + B1 !!CN;1+iCN;1 + B2 !!CN;2+iCN;2 2 exp (!!g ) 2 2 g Nitrile/Surf. Coverage Parameter Avg Value 95% MBN-BCF, 100% A NR 6.39 0.06 B 1 49 2 B 2 50 3 CN;1 12.5 0.5 CN;2 17 1 g 102.0 0.8 1.47 0.03 ! CN;1 2229.6 0.5 ! CN;2 2314.0 0.6 ! g 2262.5 0.8 MBN-BCF, 25% A NR 6.50 0.09 B 1 15 2 B 2 63 3 CN;1 13 1 CN;2 14.7 0.8 g 84.3 0.8 1.43 0.05 ! CN;1 2229.2 0.7 ! CN;2 2310.5 0.8 ! g 2255.3 0.7 MBN-BCF, 1.25% A NR 6.75 0.09 78 D.1. SPECTRAL FITTING PARAMETERS Fit Paramaters Cont. Nitrile/Surf. Coverage Parameter Avg Value 95% B 1 15 2 B 2 79 4 CN;1 14 2 CN;2 14.6 0.7 g 91 1 1.40 0.04 ! CN;1 2232.6 0.8 ! CN;2 2312.0 0.8 ! g 2248.0 0.8 MBBN-BCF, 100% A NR 6.84 0.08 B 1 38 2 B 2 63 3 CN;1 11.4 0.6 CN;2 14.6 0.7 g 82.4 0.8 1.87 0.04 ! CN;1 2233.5 0.5 ! CN;2 2313.8 0.6 ! g 2274.4 0.8 MBBN-BCF, 25% A NR 7.7 0.1 B 1 36 3 B 2 63 3 CN;1 14 1 CN;2 16.2 0.8 79 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY Fit Paramaters Cont. Nitrile/Surf. Coverage Parameter Avg Value 95% g 85.4 0.8 1.79 0.05 ! CN;1 2233.2 0.6 ! CN;2 2314.5 0.8 ! g 2269.8 0.8 MBBN-BCF, 1.25% A NR 7.28 0.08 B 1 40 3 B 2 61 3 CN;1 13.0 8 CN;2 14.7 0.7 g 82.7 0.7 1.77 0.04 ! CN;1 2235.6 0.5 ! CN;2 2313.8 0.6 ! g 2270.5 0.8 D.2 Electric Field Calculations of MBN and MBN/BCF To examine how electric elds aect the nitrile frequencies of we placed benzonitrile and adducted benzonitrile between two meshes of point charges. As discussed in the main text, the number of point charges and their separation distance was chosen such that the electric eld between them was approximately spatially invariant see gure D.2. We used a separation distance between the meshes of 200 A where one mesh was placed at z = 100 A and the other was placed at z = -100 80 D.2. ELECTRIC FIELD CALCULATIONS OF MBN AND MBN/BCF z y x 45 Å 200 Å Mesh with 1 Å - separation between point charges Figure D.1: A cartoon of computational setup we have used. A. Each mesh was a square centered around the point (x=0,y=0) with a total of 2025 constituent point charges (45X45). The point charges are separated by 1 A. The molecules examined were approximately centered about the origin (x=0,y=0,z=0) with the nitrile pointing in the positive z direction. A graphic from IQmol is found in gure D.1. The electric eld between the two meshes follows the following expression: ~ F = 4050 X i=1 a i q 4 0 xx i ((xx i ) 2 + (yy i ) 2 + (zz i ) 2 ) 3=2 ^ x + yy i ((xx i ) 2 + (yy i ) 2 + (zz i ) 2 ) 3=2 ^ y + zz i ((xx i ) 2 + (yy i ) 2 + (zz i ) 2 ) 3=2 ^ z ! (D.1) where, a i is the charge of the ith point charge; x i , y i and z i is the location of the ith point charge. The upper limit of the sum, 4050, is the total number of point charges used in each calculation. Note 81 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY 5.6 MV/cm 5.5 MV/cm Z = −6.5 Å x position (Å) y position (Å) −5 0 5 −5 0 5 5.53 5.55 5.57 5.59 5.61 z = 0 Å x position (Å) y position (Å) −5 0 5 −5 0 5 5.47 5.49 5.51 5.53 5.55 z = 6.5 Å x position (Å) y position (Å) −5 0 5 −5 0 5 5.53 5.55 5.57 5.59 5.61 z = 0 Å x position (Å) y position (Å) −5 0 5 −5 0 5 5.47 5.49 5.51 5.53 5.55 x=2,y=0 x=0,y=0 x=−1.5,y=3 x=−2.5,y=−3.5 −5 0 5 5.3 5.4 5.5 5.6 5.7 5.8 z position (Å) Electric Field Magnitude (MV/cm) x=0,y=0 x=2,y=0 x=−1.5,y=3 x=−2.5,y=−3.5 Figure D.2: The contour plots on top are taken at dierent slices between the two meshes. The colors on the contour indicate electric eld magnitudes in units of MV/cm. The electric eld magnitude changes by at most 0.08 MV/cm across the 64 A 2 represented in the plots. The lower two plots display the invariance of the electric eld along the z direction. The left plot shows the point (x,y) the z direction traces are taken at in the right plot. The largest variance between the traces is on the order of 0.02 MV/cm while total change along a specic trace is about 0.01 MV/cm. that there are 2025 on each mesh. The strength of the electric eld and its direction were varied by varying the magnitude and sign of a i . For each mesh, the sign of the charge is constant. For each electric eld computation,ja i j is constant and does not vary from charge to charge or mesh to mesh. Displayed in gure D.2 is the magnitude of the above vector expression for the electric eld, ~ F . The benzonitrile and benzonitrile/BCF adduct are approximately placed at the midway point between the two meshes and geometry optimized while between the meshes. After optimization, frequency calculations are performed while the molecule is still between the meshes. To benchmark our method of calculating vibrational frequencies in the presence of an electric eld, we examined the behavior of benzonitrile. As stated in the main text, the nitrile frequency of benzonitrile changes according to the linear Stark eect. As can be seen from the gure below, our model we reproduce the linear behavior quite well. Upon tting our computational data for 82 D.2. ELECTRIC FIELD CALCULATIONS OF MBN AND MBN/BCF −20 −10 0 10 20 2385 2390 2395 2400 2405 Electric Field (MV/cm) ν CN (cm −1 ) = −∆ ∆=. / 3 Figure D.3: Plot of the computational frequencies for benzonitrile as a function of applied electric eld. The tting equation is displayed in the inset. The main parameter we use to benchmark our method of calculating frequency changes in response to electric elds is . benzonitrile, we nd = 0:35 0:05 cm 1 MV=cm . 83 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY −15 −10 −5 0 5 10 15 2380 2400 2420 2440 2460 2480 Electric Field (MV/cm) ν CN (cm −1 ) wB97X−V wB97X−D Figure D.4: Plot of the computational nitrile frequencies for the Benzonitrile/BF 3 adduct calculated using both !B97X-V and !B97X-V. As can be seen, the electric eld at which the adduct partially dissociates is dependent on how we account for the Van der Waals interactions. However, we note that both the !B97X-V and !B97X-V partially dissociate the adduct at relatively weak electric elds, 6.5 MV=cm and 9 MV=cm 84 D.2. ELECTRIC FIELD CALCULATIONS OF MBN AND MBN/BCF 2180 2200 2220 2240 2260 2280 0.4 0.45 0.5 0.55 0.6 Transmission (a.u.) Frequency (cm -1 ) MBN Benzonitrile Figure D.5: FTIR spectra of MBN and Benzonitrile in dimethylformamide. 85 APPENDIX D. SUPPORTING INFORMATION TO INTERFACIAL LEWIS ACID-BASE ADDUCT FORMATION PROBED BY VIBRATIONAL SPECTROSCOPY 86 References [1] Nishimura, S., et al. Handbook of heterogeneous catalytic hydrogenation for organic synthesis; Wiley New York etc, 2001. [2] Chen, H.; Cong, T. N.; Yang, W.; Tan, C.; Li, Y.; Ding, Y. Progress in natural science 2009, 19, 291{312. [3] Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. Applied energy 2011, 88, 981{1007. [4] Stambouli, A. B.; Traversa, E. Renewable and sustainable energy reviews 2002, 6, 433{455. [5] Gorin, C. F.; Beh, E. S.; Kanan, M. W. Journal of the American Chemical Society 2011, 134, 186{189. [6] Murgida, D. H.; Hildebrandt, P. Accounts of chemical research 2004, 37, 854{861. [7] Warshel, A.; Levitt, M. Journal of molecular biology 1976, 103, 227{249. [8] Warshel, A. Proceedings of the National Academy of Sciences 1978, 75, 5250{5254. [9] Fried, S. D.; Boxer, S. G. Annual review of biochemistry 2017, 86, 387{415. [10] Baldelli, S. Accounts of chemical research 2008, 41, 421{431. [11] Ge, A.; Videla, P. E.; Lee, G. L.; Rudshteyn, B.; Song, J.; Kubiak, C. P.; Batista, V. S.; Lian, T. The Journal of Physical Chemistry C 2017, 121, 18674{18682. 87 REFERENCES [12] Oklejas, V.; Sjostrom, C.; Harris, J. M. JACS 2002, 124, 2408{2409. [13] Oklejas, V.; Sjostrom, C.; Harris, J. M. J. Phys. Chem. B 2003, 107, 7788{7794. [14] Rey, N. G.; Dlott, D. Journal of Electroanalytical Chemistry (2016) http://dx.doi.org/10.1016/j.jelechem.2016.12.023, [15] Schkolnik, G.; Salewski, J.; Millo, D.; Zebger, I.; Franzen, S.; Hildebrandt, P. Int. J. Mol. Sci. 2012, 13, 7466{7482. [16] Welch, G. C.; Stephan, D. W. Journal of the American Chemical Society 2007, 129, 1880{1881. [17] Lambert, A. G.; Davies, P. B.; Neivandt, D. J. Applied Spectroscopy Reviews 2005, 40, 103{145. [18] Mukamel, S. Principles of nonlinear optical spectroscopy; Oxford university press New York, 1995; Vol. 29. [19] Shen, Y.-R. New York, Wiley-Interscience, 1984, 575 p. 1984, [20] Boyd, R. W. Nonlinear optics; Elsevier, 2003. [21] Shalhout, F. Y.; Malyk, S.; Benderskii, A. V. J. Phys. Chem. Lett. 2012, 3, 3493. [22] Laaser, J. E.; Xiong, W.; Zanni, M. T. The Journal of Physical Chemistry B 2011, 115, 2536{2546. [23] The online notes by Andrei Tokmako are really good resources for anyone trying to understand the mathematical formalism of nonlinear optics. [24] Ardo, S.; Sun, Y.; Staniszewski, A.; Castellano, F. N.; Meyer, G. J. J. Am. Chem. Soc. 2010, 132, 6696{6709. [25] Chung, M. S.; Mayer, A.; Miskovsky, N. M.; Weiss, B. L.; Cutler, P. H. Ultramicroscopy 2013, 132, 41{47. [26] Murgida, D. H.; Hildebrandt, P. J. Phys. Chem. B 2001, 105, 1578{1586. 88 REFERENCES [27] Pensack, R. D.; Asbury, J. B. J. Am. Chem. Soc. 2009, 131, 15986{15987. [28] Vanbel, M. K.; Su, C. Y.; Locquet, J. P.; Verbiest, T. J. Phys. Chem. C 2014, 118, 1919{1924. [29] Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons, Inc., 1980. [30] Bockris, J. O.; Devanathan, M. A. V.; M uller, K. Proc. Roy. Soc. A 1963, 274, 55{79. [31] Janek, R. P.; Fawcett, R. W.; A, U. J. Phys. Chem. B 1997, 101. [32] Lockett, V.; Horne, M.; Sedev, R.; Rodopoulos, T.; Ralston, J. Phys. Chem. Chem. Phys. 2010, 12, 12499{12512. [33] Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222{4228. [34] Richmond, G. L.; Rojhantalab, H. M.; Robinson, J. M.; Shannon, V. L. J. Opt. Soc. Am. B 1987, 4, 228{236. [35] Lantz, J. M.; Baba, R.; Corn, R. M. J. Phys. Chem. 1993, 97, 7392{7395. [36] Lantz, J. M.; Corn, R. M. J. Phys. Chem. 1994, 98, 4899{4905. [37] Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. B 1998, 102, 6331{6336. [38] Achtyl, J. L.; Vlassiouk, I. V.; Surwade, S. P.; Fulvio, P. F.; Dai, S.; Geiger, F. M. The Journal of Physical Chemistry B 2014, 118, 7739{7749, PMID: 24517192. [39] Geiger, F. M. Annu. Rev. Phys. Chem. 2009, 60, 61. [40] Jena, K. C.; Hore, D. K. The Journal of Physical Chemistry C 2009, 113, 15364{15372. [41] Fried, S. D.; Bagchi, S.; Boxer, S. G. Science 2014, 346, 1510{1514. [42] Fried, S. D.; Boxer, S. G. Acc. Chem. Res. 2015, 48, 998{1006. [43] Fafarman, A. T.; Boxer, S. G. J. Phys. Chem. B 2010, 114, 13536{13544. [44] Bagchi, S.; Boxer, S. G.; D, F. M. J. Phys. Chem. B 2012, 116, 4034{4042. 89 REFERENCES [45] Siebert-Henze, E.; Lyssenko, V. G.; Fischer, J.; Tietze, M.; Brueckner, R.; Schwarze, M.; Vandewal, K.; Ray, D.; Riede, M.; Leo, K. AIP Adv. 2014, 4, 047134{047144. [46] Sorenson, S. A.; Patrow, J. G.; Dawlaty, J. M. Journal of the American Chemical Society 2017, 139, 2369{2378. [47] Smith, C. P.; White, H. S. Analytical chemistry 1992, 64, 2398{2405. [48] Lecomte, S.; Hildebrandt, P.; Soulimane, T. J. Phys. Chem. B 1999, 103, 10053{10064. [49] Humbert, C.; Busson, B.; Six, C.; Gayral, A.; Gruselle, M.; Villain, F.; Tadjeddine, A. J. Electroanal. Chem. 2008, 621, 314{321. [50] Li, H. H. J. Phys. Chem. Ref. Data 1980, 9, 161{289. [51] Hale, G. M.; Querry, M. R. Appl. Opt. 1973, 12, 555{563. [52] Bagchi, S.; Fried, S. D.; Boxer, S. G. JACS 2012, 134, 10373{10376. [53] Natan, A.; Kuritz, N.; Kronik, L. Adv. Funct. Mater. 2010, 20, 2077{2084. [54] Aschaenburg, D. J.; Moog, R. S. J. Phys. Chem. B 2009, 113, 12736{12743. [55] Deb, P.; Haldar, T.; Kashid, S. M.; Banerjee, S.; Chakrabarty, S.; Bagchi, S. J. Phys. Chem. B 2016, 120, 4034{4046. [56] Choi, J.-H.; Oh, K.-I.; Lee, H.; Cho, M. J. Chem. Phys 2008, 128, 134506. [57] Muglali, M. I.; Erbe, A.; Chen, Y.; Barth, C.; Koelsch, P.; Rohwerder, M. Electrochimica Acta 2013, 90, 17{26. [58] Deodato, J. D. C.; Lee, T. R.; Baldelli, S. J. Phys. Chem. C 2014, 118, 29126{29134. [59] Lewis, G. N. Valence and the Structure of Atoms and Molecules; Chemical Catalog Com.; New York, 1923. [60] Brown, H. C.; Schlesinger, H.; Cardon, S. Z. Journal of the American Chemical Society 1942, 64, 325{329. 90 REFERENCES [61] Wittig, G.; Benz, E. European Journal of Inorganic Chemistry 1959, 92, 1999{2013. [62] Tochtermann, W. Angewandte Chemie International Edition in English 1966, 5, 351{371. [63] Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124{1126. [64] Stephan, D. W.; Erker, G. Angewandte Chemie International Edition 2010, 49, 46{76. [65] Staubitz, A.; Robertson, A. P.; Sloan, M. E.; Manners, I. Chemical reviews 2010, 110, 4023{4078. [66] Stephan, D. W. Journal of the American Chemical Society 2015, 137, 10018{10032. [67] M omming, C. M.; Otten, E.; Kehr, G.; Fr ohlich, R.; Grimme, S.; Stephan, D. W.; Erker, G. Angewandte Chemie International Edition 2009, 48, 6643{6646. [68] Chen, D.; Wang, Y.; Klankermayer, J. Angewandte Chemie 2010, 122, 9665{9668. [69] Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angewandte Chemie 2007, 119, 8196{8199. [70] Pereira, J. C. M.; Sajid, M.; Kehr, G.; Wright, A. M.; Schirmer, B.; Qu, Z.-W.; Grimme, S.; Erker, G.; Ford, P. C. Journal of the American Chemical Society 2013, 136, 513{519. [71] Ghuman, K. K.; Hoch, L. B.; Szymanski, P.; Loh, J. Y.; Kherani, N. P.; El-Sayed, M. A.; Ozin, G. A.; Singh, C. V. Journal of the American Chemical Society 2016, 138, 1206{1214. [72] Stair, P. Journal of the American Chemical Society 1982, 104, 4044{4052. [73] Metiu, H.; Chr etien, S.; Hu, Z.; Li, B.; Sun, X. The Journal of Physical Chemistry C 2012, 116, 10439{10450. [74] Sun, J.; Baylon, R. A.; Liu, C.; Mei, D.; Martin, K. J.; Venkitasubramanian, P.; Wang, Y. Journal of the American Chemical Society 2015, 138, 507{517. [75] Lu, G.; Zhang, P.; Sun, D.; Wang, L.; Zhou, K.; Wang, Z.-X.; Guo, G.-C. Chemical Science 2014, 5, 1082{1090. 91 REFERENCES [76] Phillips, J. A. Theoretical Chemistry Accounts 2017, 136, 16. [77] Hattori, R.; Suzuki, E.; Shimizu, K. Journal of molecular structure 2005, 750, 123{134. [78] Eigner, A. A.; Rohde, J. A.; Knutson, C. C.; Phillips, J. A. The Journal of Physical Chemistry B 2007, 111, 1402{1407. [79] Buchberger, A. R.; Danforth, S. J.; Bloomgren, K. M.; Rohde, J. A.; Smith, E. L.; Gar- dener, C. C.; Phillips, J. A. The Journal of Physical Chemistry B 2013, 117, 11687{11696. [80] Wrass, J. P.; Sadowsky, D.; Bloomgren, K. M.; Cramer, C. J.; Phillips, J. A. Physical Chemistry Chemical Physics 2014, 16, 16480{16491. [81] Lawson, J. R.; Melen, R. L. Inorganic chemistry 2017, 56, 8627{8643. [82] Oliveri, I. P.; Maccarrone, G.; Di Bella, S. The Journal of organic chemistry 2011, 76, 8879{8884. [83] Beckett, M. A.; Strickland, G. C.; Holland, J. R.; Varma, K. S. Polymer 1996, 37, 4629{4631. [84] Sivaev, I. B.; Bregadze, V. I. Coordination Chemistry Reviews 2014, 270, 75{88. [85] Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T.; Wormit, M.; Kussmann, J.; Lange, A. W.; Behn, A.; Deng, J.; Feng, X., et al. Molecular Physics 2015, 113, 184{215. [86] Cho, H.-G.; Cheong, B.-S. Journal of Molecular Structure: THEOCHEM 2000, 496, 185{198. [87] Cho, H.-G.; Cheong, B.-S.; Kim, K.-W. Bull. Korean Chem. Soc. 1998, 19, 909{910. [88] Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330{1341. [89] Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. The Journal of Physical Chemistry 1994, 98, 563{571. [90] Stranick, S.; Parikh, A.; Tao, Y.-T.; Allara, D.; Weiss, P. The Journal of Physical Chemistry 1994, 98, 7636{7646. 92 REFERENCES [91] Schreiber, F. Progress in surface science 2000, 65, 151{257. [92] Brewer, N. J.; Leggett, G. J. Langmuir 2004, 20, 4109{4115. [93] Hobara, D.; Ota, M.; Imabayashi, S.-i.; Niki, K.; Kakiuchi, T. Journal of electroanalytical chemistry 1998, 444, 113{119. [94] Fenter, P.; Eisenberger, P.; Liang, K. Physical review letters 1993, 70, 2447. [95] Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216{1218. [96] Dannenberger, O.; Weiss, K.; Himmel, H.-J.; J ager, B.; Buck, M.; W oll, C. Thin Solid Films 1997, 307, 183{191. [97] Vericat, C.; Vela, M.; Benitez, G.; Gago, J. M.; Torrelles, X.; Salvarezza, R. Journal of Physics: Condensed Matter 2006, 18, R867. [98] Bagchi, S.; Fried, S. D.; Boxer, S. G. Journal of the American Chemical Society 2012, 134, 10373{10376. [99] Reeve, S.; Burns, W.; Lovas, F. J.; Suenram, R.; Leopold, K. The Journal of Physical Chemistry 1993, 97, 10630{10637. [100] Burns, W.; Leopold, K. Journal of the American Chemical Society 1993, 115, 11622{11623. [101] Phillips, J.; Giesen, D.; Wells, N.; Halfen, J.; Knutson, C.; Wrass, J. The Journal of Physical Chemistry A 2005, 109, 8199{8208. [102] Onsager, L. Journal of the American Chemical Society 1936, 58, 1486{1493. [103] Jacobsen, H.; Berke, H.; D oring, S.; Kehr, G.; Erker, G.; Fr ohlich, R.; Meyer, O. Organometallics 1999, 18, 1724{1735. [104] Frenking, G.; Wichmann, K.; Fr ohlich, N.; Loschen, C.; Lein, M.; Frunzke, J.; Ray on, V. M. Coordination chemistry reviews 2003, 238, 55{82. 93 REFERENCES [105] Hopgarten, M. v.; Frenking, G. Wiley Interdisciplinary Reviews: Computational Molecular Science 2012, 2, 43{62. [106] Lawrence, E. J.; Oganesyan, V. S.; Hughes, D. L.; Ashley, A. E.; Wildgoose, G. G. Journal of the American Chemical Society 2014, 136, 6031{6036. 94

Abstract (if available)

Abstract This work is largely concerned with electrochemical interfaces. Studying any interface, however, can be particularly difficult. One reason (among many) for this is because interfaces, which are typically only nanometers wide, have very small responses and are therefore difficult to measure. This issue is only compounded by the fact that interfaces are almost always buried. This is especially true for the electrochemical interface, where one side is composed of the electrode and the other is composed of the electrolyte. One way to study interfacial processes is to use a spectroscopic technique called vibrational sum frequency generation spectroscopy (vSFG), which is an interface specific non linear spectroscopy. Because it is interface specific it is ideally suited for directly studying interfaces. In this work, vSFG has been used to monitor vibrational chromophores covalently attached to surfaces of interest. Because the chromophores are attached at specific locations within the electrochemical interface their vibrational response is a direct reporter of the interface.

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Asset Metadata

Creator Patrow, Joel Gabriel (author)

Core Title Vibrational sum frequency generation studies of electrode interfaces

School College of Letters, Arts and Sciences

Degree Doctor of Philosophy

Degree Program Chemistry

Publication Date 03/15/2019

Defense Date 03/15/2019

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag electric fields,electrodes,interfaces,OAI-PMH Harvest,SFG,spectroscopy,sum frequency generation spectroscopy,ultrafast spectroscopy

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Dawlaty, Jahan (committee chair), Benderskii, Alex (committee member), Ravichandran, Jayakanth (committee member)

Creator Email hbar34@outlook.com,patrow@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-134374

Unique identifier UC11675750

Identifier etd-PatrowJoel-7168.pdf (filename),usctheses-c89-134374 (legacy record id)

Legacy Identifier etd-PatrowJoel-7168.pdf

Dmrecord 134374

Document Type Dissertation

Format application/pdf (imt)

Rights Patrow, Joel Gabriel

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions 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...

Repository Name University of Southern California Digital Library

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