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Absorption and circular dichroism studies of the ethylene and butadiene chromophores

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Absorption and circular dichroism studies of the ethylene and butadiene chromophores

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Content ABSORPTION AND CIRCULAR DICHROISM STUDIES OF THE ETHYLENE AND BUTADIENE CHROMOPHORES by Kenneth Paul Gross A Dissertation Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree DOCTOR OF PHILOSOPHY (Chemi s try ) ^Chemical Physics^ July 1976 UMI Number: DP21816 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI DP21816 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 4 8 1 0 6 -1 3 4 6 UNIVERSITY OF SOUTHERN CALIFORNIA p L T \ TH E G RADUATE SC HO O L ' * ■ U N IV E R S IT Y PARK ^ LOS A N G ELES, C A L IF O R N IA 9 0 0 0 7 • 7 7 G This dissertation, written by .............................Kenneth Paul Gross under the direction of h.lk... Dissertation Com mittee, and approved by all its members, has been presented to and accepted by The Graduate School, in partial fulfillm ent of requirements of the degree of D O C T O R O F P H I L O S O P H Y Dean D ate...... DISSERTATION COMMITTEE To my wife Sandra, to my parents Frank and Myrtle, and to Kathode and Jake. i i Between the Idea and the Reality . . . Falls the Shadow. - - - T. S. Eliot The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. ----- Albert Einstein i i i ACKNOWLEDGEMENTS I would like to express my appreciation to Professor Otto Schnepp, who, as advisor, colleague, and friend has provided me with the essential ingredients needed in the course of this work. His guidance, support and encourage ment throughout my years of graduate studies have proven to be most f u l f i l l i n g . I wish to express my gratitude to Professor Philip J, Stephens who has been instrumental in the development of my formal graduate training and has been a source of many fr u itfu l and interesting discussions, I also thank the other members of the chemical physics group here at USC for providing a broadening and stimulating environment. I am also indebted to the support personnel throughout the University, and would like to acknowledge Barney Poarch and Stan Watson of the physics machine shop, and Tony Reraeikis of the electronics shop for excellent work, and the contribution of many ideas which have proven invaluable in our instrumental design. I would also like to thank Ms, Michele Dea for her expert and painstaking typing of this dissertation. Her efficiency and unfailing perserverance are commendable. Special thanks are due to my wife Sandra, who was understanding and patient throughout. She endured. The financial support of the department of chemistry and of the National Ins ti tute,s*of Heal th is gratefully acknowl edged. v ABSTRACT The absorption and circular dichroism spectra of the optically active straight chain a 1 kenes S, 3-methyl-1- pentene, S, 4-methyl-1-hexene , S, 5-methyl - 1-heptene, and a bicyclic olefin (-)-a-pinene have been measured in the spectral region 200-140 nm. The acyclic olefins were run in the gas phase whereas the (-)-a-pinene study was performed in the solution phase to approximately 160 nm% An intense CD band in a region of weak absorption to * higher energy than the ethylenic tr -* it transition was ★ observed in all olefins and is assigned as t t -* a (^Ag -* The acyclic olefins do not exhibit any well-defined analogue to this CD band in absorption. The CD and absorption spectra of a-pinene in perfluoro- n-hexane demonstrate partial Rydberg character for this state which is shown to be of the mixed valence-Rydberg type. A second prominent CD band at higher energy is evident in the gas phase spectra and is assigned as * °(CH) ^ 77 ^ transitio n, previously character ized in a-pinene, has also been assigned in the acyclic olefins. Furthermore, the spectra of a-pinene in a number of solvents give?; evidence for a low-lying a ^ n valence state that is accidentally degenerate with the t t 3s Rydberg excitation. v i The absorption and circular dichroism spectra of ( - ) - a- phel1andrene and ( - ) -g- phel1andrene have been measured in the spectral region 300-135 nm. Solution spectra were measured in perf1uoro-n-hexane to 160 nm. The spectra were interpreted in terms of the electronic states of the cis- and trans- butadiene chromophores. The over lapping valence and Rydberg states were unambiguously separated by comparison of the vapor and solution phase spectra. For the trans-isomer, a broad intense valence band in the CD near 170 nm has been characterized as magnetic- dipole allowed and electric-dipole forbidden in the parent molecule. This band is interpreted as containing the ( it j it ) Aa state and most likely a (a, it ) B state, y 9 In the gas phase this broad band is overlapped by a structured vibronic system of opposite sign which is assigned to a magnetic-dipole allowed and electric-dipole forbidden it 3d Rydberg transition. In the CD spectrum an electric-dipole allowed valence band on the high energy wing of the N -> Vi transition is assigned as a( 7 aT) t t*, and in the gas phase is overlapped by a Rydberg system characterized as it ^ 3p, which is also el ectri c-di pol e a 11 owed. For the cis-isomer an intense CD band in the neigh- borhood of 160 nm is assigned as a (a,ir ), A^ or the (.ir,ir ) vi i 2 ^ 2 valence state. Evidence for the electric-dipole allowed cis-peak around 155 nm is presented. Two low-energy CD bands near 210 nm and characterized as valence transitions, may correspond to the optically weak 1 ie ★ 1 A-j~ ( t t , it ) state and the a(7a-j) -> t t , state. Like for the trans-chromophore, Rydberg states corresponding to t t ■+ 3p and t t 3d excitations have been assigned and characteri zed. v i i i TABLE OF CONTENTS Page DEDICATION................................................................................................. ii EPIGRAPH.......................................................... i i i ACKNOWLEDGEMENTS ....................................................................................... iv ABSTRACT.......................................................................................................... vi LIST. OF TABLES.........................................................................................xi i LIST OF FIGURES...............................................................................................xiv Chapter I . INTRODUCTION......................... 1 A. E th y le n e ........................................................................... 1 B . 1 ,3-Butadi ene................................................................. 4 C. Circular Dichroism and the Characteri zation of Excited Electronic States. . . 8 I I . EXPERIMENTAL METHODS ....................................................... 10 A. Instrumentation............................................................ 10 B. Materia 1s and P u r i f i c a t i o n ............................... 23 1 . Mpno-ol ef i n s ...................................................... 23 2, D ie n e s .................................................................... 25 I I I , THEORY OF OPTICAL ACTIVITY ......................................... 29 A. Phenomenological and Semi-Classical Considerations ........................................................... 29 B. Circular Dichroism: Quantum-Mechanical Formulation.................................................................... 37 i x TABLE OF CONTENTS (CONTINUED) Page IV. EXPERIMENTAL RESULTS............................................................ 45 A. Mono-olefins................................................................. 45 B. Dienes................................................................................ 63 V. THEORETICAL CONSIDERATIONS: ELECTRONIC STATES OF ETHYLENE AND BUTADIENE. . . . . . . 87 A. Ethylene................................................................ . . . 87 B. 1,3 Butadi ene . . . . .................................................. 94 VI. ASSIGNMENTS AND DISCUSSION.................................................103 A. Mono-o 1 ef.i ns.............................. 103 1. Acyclic-mono-olefins (Gross & SchneppC27H) . . 103 (a) 180 - 1 70 n m .............................................103 (b) 1 95 - 1 85 n m .............................................104 (c) 1 70 - 1 55 n m .............................................106 (d) 160 - 140 n m .............................................110 (e) Vibrational Structure.....................112 2. Cyclic-mono-olefins: a-pinene. . . . 113 (a) 220 - 1 90 n m .............................................114 (b) 1 90 - 1 60 n m .............................. 1 16 B. Dienes.................................................................................117 1. 0- Phel1andrene (trans-butadiene). . . 117 (a) 225 n m .......................................................118 x TABLE OF CONTENTS (CONTINUED) Page (b) 210 - 150 nm; Rydberg State Assi gnments.....................................................119 (c) 210 - 150 nm; Valence State Assignments.....................................................124 (d) 1 50 - 1 35 n m ................................................129 2. a-Phellandrene (cis-butadiene). • . . 130 {a) 260 n m ..............................................................130 (b) 220 - 180 nm; Rydberg State Ass i gnmen t s ............................................ . 130 (c) 230 - 185 nm; Valence State Assignments.....................................................134 (d) 1 8 0-1 50 nm ............................................... 135 REFERENCES................................................ 136 xi LIST OF TABLES Table Page 1. Photon count vs. A; identical conditions for reflection and transmission optics. . . . 14 2. Absorption and circular dichroism features of S ,3-methyl-1 -pentene ( g a s ) .................................. 47 3. Absorption and circular dichroism features of S ,4-methyl-1-hexene (gas)....................................... 49 4. Absorption and circular dichroism features of S, 5-methyl-1 -heptene (gas) .................................. 51 5. Absorption and circular dichroism features of (.-) a-pinene ( g a s ) ...................................................... 55 6 . Absorption and circular dichroism features of (-) a-pinene (in solution; I. perfluoro-n- h e x a n e )........................................................................................ 57 7. Absorption and circular dichroism features of ('-) a-pinene (in solution; I I . 3-methyl- pentane)......................................................................... 59 8 . Absorption and circular dichroism features of (-) a-pinene (in solution; I I I . cyclo- h e x a n e )........................ 61 9A. Absorption and circular dichroism features of (-) g-phel 1 andrene ( g a s ) ....................................... 65 xi i LIST OF TABLES (CONTINUED) Page 9B. Absorption and circular dichroism features of ( - ) ' B-phellandrene (gas). . ......................... 66 10. Absorption and circular dichroism features of (-) B-phel1andrene (in solution; I. per- fl uoro-n-hexane) . ...................................................... 68 11. Absorption and circular dichroism features of (-) B-phel1andrene (in solution; I I . cy- cl ohexane ) ..................................................................... 70 12A. Absorption■and circular dichroism features of (-)■ a-phell andrene (gas)....................................... 76 1-2B. Absorption and circular dichroism features of (-) a-phell andrene (gas)....................................... 77 13. Absorption and circular dichroism features of (-) a-phel1andrene (in solution; I. per- f 1 uoro-n-hexane) ................................................................ 80 14. Absorption and circular dichroism features of (-) ot-phellandrene (in solution; I I . cy- c loh exane).............................................................................. 82 15. Calculated transition energies for ethylene. 108 16. Calculated transition energies for trans- 1.3-butadien e ......................................................................... 121 17. Calculated transition energies for cis- 1 .3-butadien e ......................................................................... 131 x i i i LIST OF FIGURES Figure Page 1. Vacuum ultra vio le t absorption spectrum of ethylene in the gas phase............................. 2 2. Vacuum ultravio let absorption spectrum of 1,3-butadiene in the gas p h a s e .............................. 6 3. Schematic representation of the optical system of the present circular dichroism in s tru m e n t.............................................................................. 11 4. Schematic representation of the optical system of the previous circular dichroism in s tru m e n t . 12 5. Block diagram of the electronics of the circular dichroism instrument.................................. 17 6 . Structures of the compounds investigated . . 24 7. Absorption and circular dichroism of 3- T methyl- 1-pentene in the gas phase......................... 46 8. Absorption and circular dichroism of 4- methyl-1 -hexene in the gas p h a s e ......................... 48 9. Absorption and circular dichroism of 5- methy 1 -1 - heptene in the gas phase......................... 50 10. Absorption and circular dichroism of a-pinene in the gas p h a s e .................................. 54 xi v LIST OF FIGURES (CONTINUED) Page 11. Absorption and circular dichroism of a-pinene in perf 1 uoro-n-hexane.........................................................56 12. Absorption and circular dichroism of a-pinene in 3-methyl-pentane...............................................................58 13. Absorption and circular dichroism of a-pinene i n cycl ohexane........................ 60 14. Absorption and circular dichroism of B - p h e 1 - landrene in the gas phase.................................. . 64 f 15. Absorption and circular dichroism of e-phel- landrene in perf 1 uoro-n-hexane . ................................67 16. Absorption and circular dichroism of 3 -phel- landrene in cyclohexane...................................................69 17. Photoionization efficiency of 3 -phel1andrene in the. gas p h a s e ..................................................................71 18A. Absorption and circular dichroism of a-phel- landrene in the gas phase. 300 - 195 nm . . . 74 18B. Absorption and circular dichroism of a-phel- landrene in the gas phase. 220 - 135 nm . . . 75 19A. Absorption and circular dichroism of a-phel- landrene in perf1uoro-n-hexane. 300 - 195 nm. 78 19B. Absorption and.circular dichroism of a-phel- landrene in perf1uoro-n-hexane. 230 - 160 nm. 79 xv LIST OF FIGURES (CONTINUED) Page 20. Absorption and circular dichroism of ct-phel- landrene in cyclohexane................................................. 81 21. Photoionization efficiency of a-phellan- drene in the gas p h a s e .......................... 83 22. Molecular orbital and symmetry identification of the upper f il l e d and lower empty orbitals of ethy.lene.............................................................................. 89 23. Molecular orbital and symmetry identification Of the upper f i l l e d and lower empty orbitals of trans-1,3 butadiene (C2 h) and cis-1,3 buta diene (C 2 v ) .............................................................................. 97 xy i CHAPTER ONE INTRODUCTION A, Ethylene The electronic spectrum and excited states of ethylene and its derivatives have been, extensively reviewed by Merer and Mulliken £111. Kaldor and Shavitt £2], Robin £3H, Watson and co-workers £4£], and others. This molecule is absolutely fundamental in both molecular spectroscopy and electronic structure calculations since i t is the simplest hydrocarbon with a ir-electron system. The continuous revision of the assignments of the excited states for this simple system, by workers too numerous to mention, is indicative of its elusive excited state electronic struc ture . The vacuum ultra v io le t (below ^ 180 nm) absorption spectrum of ethylene, shown in Figure 1 £4£, was f ir s t reported by Price and Tutte £5J in 1940. The absorption spectrum of ethylene (and alkyl derivatives) should in principle be a good source of information about the excited state properties of the olefinic double bond. Unfortunately, the bands are not easy to interpret due to overlapping states and the inherent diffuseness of the 1 T (nm) 190 180 170 160 150 140 1 .0 H- 0 .8 0.6 < 0.4 O 0.2 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 cm 1 x I0“4 Fig. 1. Vacuum ultraviolet absorption spectrum of ethylsene in the gas phase [4]. spectra. The most prominent feature in the absorption spectrum is the broad and f a ir l y intense band extending from rv 190. nm to ^ 140 nm, with a maximum at about 162 nm (61,700 cm”^1, The identity of this electronic transition is generally agreed upon and is assigned to the rr t t * (N V) transition. Although this band is assigned as ^ / tt ■+ tt its characteristics as a Rydbelrg-1 i ke state or valence-shell state are s t i l l in dispute. The explanation of the vibronic structure observed in the (tt, tt*] state has also been a challenging subject [3J, but will not be dis cussed here. Nearly all features observed at higher energies in the electronic spectrum can be attributed to Rydberg tra n s i tions converging upon the f i r s t ionization potential. They were f i r s t studied by Price and Tutte and later by Wil kinson C6] . The sharp series of intense doublets super imposed on the N -> V transition and starting abruptly at v 175 nm were f i r s t assigned as ir -* 3s I5J. The second member of the series, it 4s is observed at ^ 139 nm. The assignment of the vibronic (tt , 3s) system superimposed on the N V band has been a confusing issue in the past C1J. At this time, however, the assignment to the 3s Rydberg State seems incontestable. An extensive discussion of the Rydberg 3s doublets is given by Robin O U and by Watson 3 and co-workers E4J. The Rydberg series' other than the ns series haye not been unambiguously assigned, The assignments although not consistent £3,7-1 CfJ, include the np series which is el ectri c-di. pole forbidden and the nd series which is electric-dtpole allowed. In general, electron impact measurements on ethylene have not been much more informative than the optical measurements, although the selection rules for excitation are less res tric tiv e. The power of the electron impact technique, however, is manifested in the a b ilit y to dis tinguish between singlet ■ * singlet and singlet t r ip le t excitations £ 1 TJ, whereas for singlet singlet transitions, the characterization of allowed or forbidden character is less clear. It is interesting, though, that anomalous intensity behavior has been observed in ethylene electron impact spectra in the 7-8 ev region £11,12£, and i t has been. suggested that the presence of .an electric-quadrupole (electric-dipole forbidden) singlet state may account for the anomaly, B . 1,3 Butadiene Spectroscopi c work on trans, 1,3 butadiene has been reviewed by Kuppermann and co-workers L13J, and more re- 4 cently by Robin Q33. This molecule has seen a resurgence of current interest since Kohler and co-workers H14-16D , as well as Swofford and McClain C173, have presented exper imental evidence supporting the existence of an optically forbidden state as the lowest lying singlet in various y polyenes. This is in direct contrast to the customarily accepted assignment of a lowest lying optically allowed singlet state of Bu symmetry for all trans-polyenes^( in cluding i butadi ene) . The electronic absorption spectrum of trans, 1,3 butadiene is shown in Figure 2 C18H. Like the ethylene, the most prominent absorption feature ( 2 1 0 nm) is assigned ★ as a i + ir state, in this case designated the N transition. This band is unanimously assigned as due to 1 1 the Ag 1 Bu transition. Unfortunately, agreement stops here, and the rest of the optical spectrum has received a wide variety of excited state assignments. As is evident from Figure 2 most of the absorption spec trum of butadiene is dominated by discrete structure. In fact, recent experimental studies Cl 9-213 have centered their attention on this sharp structure, with the con clusion that all of the structure observed in the optical spectrum of butadiene, can be accounted for in terms of several Rydberg series and the 1 ^Bu valence transition. 5 X( A) 2400 2000 1800 1600 1200 1400 2 5 - IO I o x CH0= C H - C H = CH 20 £ o 46 56 86 76 Fig. 2. Vacuum ultraviolet absorption spectrum of 1,3 butadiene in the gas phase [18]. This is surprising since several 1ow-lying (g , g } and (g > tt 1 valence states have been theoretically predicted to lie in this wavelength region, Mul 1 i ken [ 22 ,233 and Robin £33 have expressed the 'fc opinion that the (_g > n ) V2 and V3 states should lie some where in the vicinity of 175-160 nm, Mulliken, however originally attributed the absorption to the presence of cis-butadiene in equilibrium with the trans-isomer. The V2 , Vg states of trans-butadiene are electric dipole forbidden, and have not been positively identified as yet. Furthermore, on a theoretical basis i t is now widely ac cepted that one of the forbidden ( t t , g * ) states is expected to lie quite low in energy, contrary to earlier theoreti cal studies. Again, as was the case for ethylene, the electron impact spectra have proven to be very useful for the characterization of t r i p l e t states. Most of the higher energy bands observed (singlet states) in the electron impact spectra of butadiene £133, however, have been previously reported in the optical spectra. Exceptions are bands at 170 nm and at 113 nm. The 170 nm band has been assigned [1 33 1 0 • the (g, g*) ;2 ^ Bu state and the 11 3 nm band remains unassigned, Other workers £24,253 feel that the 2 ^Bu state is represented by the f a ir ly strong ab- 7 sorption at about 130 n jn > , C, Circular Dichroism and the Characterization of Excited Electronic States The foregoing is representative of the available optical and electron-impact studies of the ethylene and butadiene systems. It is evident that only a limited amount of information concerning the excited electronic structure of these simple systems has been extracted. Except for the low-lying intense (t t , it ) valence state, higher energy valence transitions have not been widely characterized. Also, there is not a great deal of experi mental data associated with the excited states which are predicted to be electric-dipole forbidden, and hence d if f ic u lt to detect in an absorption measurement. In this work we have chosen the technique of circular dichroism spectroscopy as an aid to the characterization of the excited states for the ethylene and butadiene chromophores, Circular dichroism, unlike the absorption is dependent on the difference in extinction coefficients for right and l e f t circularly polarized light. The CD (Ae. V?, v ) then can be a positive or a negatiye quantity. This property' is often instrumental in resolving overlap ping electronic transitions, which could be obscured in 8 the absorption (e vs. v) spectrum. Also since absorption and CD phenomena associated with the same electronic transition, depend on different quantum-mechanical proper ties, their spectral contours as well as intensities may be quite different. The CD then may reveal transitions that are not evident in absorption spectra. Furthermore the anisotroy factor (g = A e / e ) can lead to the character ization of excited states, since i t can differentiate between electric-dipole allowed (small g) and magnetic- dipole allowed (large g) transitions in the symmetric chromophore. CD spectroscopy is, therefore, a relatively 1ow-resolution technique which can provide a great deal of information from electronic spectra which do not exhibit fine structure. W e have therefore chosen to study several optically active olefins and dien,es, in the vacuum u ltra v io le t. The olefins investigated were, 3-methyl-l- pentene, 4-methyl- 1 -hexene , 5-methyl-1 -heptene , and ct-pinene. The olefin study represents the extension of previous work [261] and can be found in ref. 27. The dienes investigated were a-phellandrene (cis-diene) and B-phel1andrene (trans-diene ) . CHAPTER TW O EXPERIMENTAL METHODS A. Instrumentation The instrument used for the measurement of circular dichroism in the vacuum ultravio let has been previously described [28-30H. The optical system of the present instrument is schematically represented in Fig. 3. The previous configuration of the optical system is depicted in Fig. 4. The instrument is based on a Model 225 McPher son 1 meter normal incidence vacuum monochormator equipped with a 1200 line/mm grating (0.8 nm/mm dispersion at exit s l i t ) blazed at 1500 A. The modified Hinteregger light source [29], is typically operated at a current of 0.6 amp, with a voltage drop of 2.5 KV. In contrast to the com mercial Hinteregger lamp C31U, the capillary has been lengthened and its diameter decreased so that the current density obtained is higher. In addition, the H £ gas is allowed to flow (> 10 torr) through the capillary and out through the air-cooled cathode. The discharge is s ta b il ized by a thoriated tungsten point placed in the center of the cathode along the capillary axis. To prevent corrosion of the anode and cathode, they have been made of stainless steel to replace the commercially available aluminum parts. 1 0 MONO 45' MOD SL CELL PMT Fig. 3. Schematic representation of the optical system of the present circular dichroism instrument (M ONO, monochromator; M, mirror; W , Wollaston; M O D, modulator; SL, separating slit; PMT, photomultiplier tube). M O NO CHROMATOR — LAMP L L WOLL. MOD. SEP. CELL SLIT L PMT g .. 4. Schematic representation of the optical system of the previous circular dichroi instrument (L, lens; WOLL., Wollaston; M O D.,, modulator; SEP. SLIT, separating slit; PM T, photomultiplier tube). The cathode has been equipped with copper cooling fins for maximum heat dissipation. The vacuum systems of the monochromator and the rest of the dichrograph are separated by magnesium fluoride windows. The collimating lenses in the original dichro graph [28 ,291], (Fig. 4), have been replaced by one spheri cal mirror with a focal length of 23 cm and a diameter of 4 1/4 in. [301] (Fig. 3). This conversion to reflection optics has enabled the circular dichroism measurements to be extended out to 135 nm since the light intensity below about 160 nm has shown a dramatic increase (Table 1), while that above 160 nm increased only slightly. Addi tio n a lly , this modification completely eliminates all chromaticity due to the change in refractive index with wavelength suffered by optical lenses, and has also a f forded a less erratic baseline over the spectral range of interest. Some of the anomalies observed previously in the circular dichroism baseline were most lik e ly due to strains and reflections in the lens system. The light from the exit s l i t is reflected a t ^45° and focused onto the photomultiplier tube after passing through all remaining optics and sample cell. Immediately following the mirror is the linear polarizer, a 40° Wol laston prism of magnesium fluoride [32,333> which produces 1 3 TABLE 1. Photon count vs. A;identical conditions9 for reflection and transmission optics. Photon Counts*3 (10^ sec"^) A(nm) Transmission Reflection 200 2.2 2.4 170 1.9 2.0 150 0.95 1.7 146 1.1 2.4 140 0.28 1.0 135 0.090 0.54 130 0.004° 0.045d a) Arbitrary light level. Slits SQy. Lam p Power: 0.62 AMP, 2,5 KV. Hydrogen pressure: 10 Torr., PM T Voltage 3100 V. Grating e ffi- cency: approx. 60% of that achieved when new (measured at 200 nm ). b) Corrected foristray light and dark count. c) 60% stray light at 130 nm (transmission optics). d) 15% stray light at 130 nm (reflection optics). 14 two perpendicu1 arly polarized beams with an angular sepa ration of 1.4°. The two component prisms are not optically contacted and have an air gap of about 0.1 mm. Circular polarization is achieved by passing the linear polarized beams through a stress plate modulator C34D of calcium fluoride, model PEMr2, manufactured by Morvue Electronics, Tigard, Oregon. The stress plate modulator works on the principle of photoelasticity. It is essen t i a l l y a quarter wave plate produced by applying a voltage modulated at 50 KHz to a piezo^electric Quartz driver crystal which is cemented to the calcium fluoride crystal. Both crystals are cut to the dimensions having a resonant frequency of 50 KHz. The stress produced in the quartz is transmitted to the calcium fluoride resulting in the required optical anisotropy. The applied peak voltage is varied with wavelength to keep the retardation at the required quarter-wave value. The two circularly polarized beams are then separated by a rectangular diaphragm a l lowing only one beam to pass through the sample cell CF1g- 3 ) . The extraordinary ray produced by the Wollas ton is selected for its greater intensity at shorter wayelengths. After passing through the sample, the light beam impinges on the photocathode of the photomultiplier. The phototube is a Gencom solar blind G-26H315 with a 15 16 m m diameter CsTe photocathode deposited on a MgF2 entrance window. The sample cells used for gas phase measurements were either constructed from pyrex and/or stainless steel, with polished 1ithiurn f 1uoride or calcium fluoride windows cemented on with low vapor pressure resin. Cell lengths from 1 cm - 12 cm were used. A side arm sample reservoir was used to admit the gas phase sample into the cell. The liquid phase cell used in the vacuum ultra v io le t was made of stainless steel, with the alkali halide windows sealed to the cell with viton o-ring s. A 1 m m thick stainless steel concentric ring spacer was used to set the path length. All cells were evacuated and leak tested with a Helium mass spectrometer leak detector prior to use, A block diagram of the electronics of the circular dichroism instrument is given in Fig. 5. The signal produced at the photomultiplier has the form C35H I - y ECl0“e* cd + 10"ErCd) + (10“e* cd - 1 0 " ^ cd) Sins] (2.1) Where, I = t r a n s m i t t e d l i g h t i n t e n s i t y r e a c h in g the d e t e c t o r I Q = i n c i d e n t l i g h t i n t e n s i t y e , er - extinction coefficients for l e f t and right circularly polarized light respectively c = concentration of the optically active substance in Moles/Li ter 1 6 I MOD. \ Fig. j I O A - O A-2 DC AC REFERENCE MONO X DRIVE AGC PAR PMT RECORDER MOD. POWER SUPPLY PMT POWER SUPPLY 5. Block diagram of the electronics of the circular dichroism instrument (AGC, automatic gain control; PMT, photomultiplier tube; A, preamplifier;. OA-1, OA-2, operational amplifiers; PAR, P A R lock-in amplifier; MOD., modulator). d = length of sample cell in cm 6 = retardation of the photoelastic modulator The retardation 6, is caused by the pressure wave set up in the modulator and is a sinusoidal function of time with a frequency corresponding to the crystal oscillatio n, i.e . 50 KHz. Since s is a function of time, i t is clear that the f i r s t term in equation (2.1) produces a DC signal and the second term which is the difference term produces the AC component. Equation (2.1) can be written: I = Ipc + JAC SinC'5oSina3tD (2.2) where, 50 = the modulation amplitude d ) = the frequency of the modulation 5 = 6 0 S i n c o t The signal from the photomultipiier then consists of a 50 KHz AC signal and a DC background signal. This signal is fed into a pre-amplifier (A; Fig. 5) which is a cur rent to voltage converter. The signal is then separated into AC and DC channels with the AC signal being detected by a phase-sensitive lock-in amplifier (Princeton Applied T M Research Model 186A Synchro-Het lock-in am plifier). The output of the lock-in amplifier is the RMS voltage of the 18 AC signal and is recorded on a strip-chart recorder. The DC signal is fed through two operational amplifiers (OA-1 and OA-2; Fig. 5) which control the amplification and time constant respectively, of the DC component. For circular dichroism measurements the output voltage of QA-1 is maintained at 1 volt by an automatic gain control (AGC) feed-back loop to the photomultiplier power supply. To record transmission spectra the DC component from OA-2 is fed into the recorder with the AGC disconnected. The 50 KHz reference signal for the lock-in amplifier is generated by the power supply for the pho.toel ast i c modulator. The voltage applied to the piezo-electric quartz plate which drives the CaF2 crystal is varied along with the wayelength scan of the monochromator. The modula tor voltage required for quarter-wave retardation is a linear function of wavelength down to 135 nm. The experimentally measured quantity in a circular dichroism measurement is the Vac/Vdc ra tio , where Vac, Vdc are the AC and DC voltages produced after amplifica tion, and are given by V a r = m o " E * c d - 1 0 ~ e r C S i n 6 2 v (2.3) (2.4) - (-j^-)Sin6.{tanh|I(e£ - er )cd/2H> (2.5) 1 9 where K and K' include the proportionality constants con taining photomultiplier sensitivity and amplification factors. Since (e - er )cd/2 is small, one can arrive at an expressive C35□ for the circular dichroism, (e - e^), i r V c l c J d c = Co ( e ^ - e r )cdaAeCd (2.6) where CQ now contains all proportionality constants in cluding the contributing factor from the f i r s t term of the expansion of s i n ( 6Qs i ncot) into a series of Bessel functions: %Si n( 6 q S i nwt) = J-| (6o)Sinwt + J ^ ( 5 )Sin3cot + J5(so)Sin5wt + ’.j - -------------------------V - 0 O f) (2.7) Since one detects at the frequency <u, only the f i r s t term in the expansion need be considered. In practice the constant CQ is determined by calibrating the instrument with a substance of known Ae at a specified wavelength. In this case d-10 Camphorsulphonic Acid (aqueous) was used as the calibrating standard. Circular'Dichroism measurements in the near-uv were performed on a Cary Model 61 CD spectropolarimeter. Calibration of this instrument was also checked with d-10 Camphorsulphonic Acid (aq). 20 Absorption measurements were taken either on a Cary 17 absorption spectrometer (near-uv) or on a McPherson Model 225 vacuum monochromator equipped with a standard double beam attachment. In some instances it was possible to run the transmission spectra in the vacuum uv and to convert the data to absorbance. Photoionization spectra were run on a photoionization mass spectrometer, consisting of a McPherson Model 225 1 meter normal incidence vacuum monochromator equipped with a commercial Hinteregger light source and an Extra- ! nuclear mass spectrometer. The photoionization spectra were only used to determine the adiabatic ionization po tentials for two molecules in this study, and the specifics of the measurement will not be dwelled upon. The details of the measurement will be found in reference 36 and the references found therein. All gas phase CD and absorption measurements were made at ambient temperature. Gas pressures in the sample cells were controlled by monitoring the absorbance at a convenient wavelength. Absolute pressures were determined by the conventional method of maintaining a side arm at a known constant temperature and also by direct measure ment on a differen tial pressure transducer [37H. The l a t ter method was preferred and was used for all extinetion co- 21 efficients (e ) reported in this work. Absorption and CD spectra in the liquid phase were also run at room temperature. Standard ;spprasil cells Cl cm and 1 m m path lengths) were used in the near-uv, while a vacuum-tight stainless steel c e ll, equipped with lithium fluoride or calcium fluoride windows was used for measurements in the vacuum uv. The resolution of the spectroscopic measurements was limited by the spectrometer s l i t width. The instrument resolution for the CD measurements in the vacuum uv is indicated in the figures and is typically 1.6 nm. The spectral resolution for the gas-phase absorption measure ments was 0.16 - 0.4 nm. All liquid phase vacuum uv spec tra were measured with a resolution of 1.6 nm. The spectroscopic measurements in the near uv were performed under variable s l i t conditions. Smooth curves were drawn for both absorption and CD spectra with error bars indicating the noise level where appropriate. Anisotropy factors (g = A e / e . ) were calcula ted by measuring e. and A e for each transition at the band maxima Y). In cases involving overlapping transitions, III C t A the components were carefully resolved to determine the extinction coefficients, and where there was any uncer tainty as to the values of ema^ or A.emaj<, appropriate 22 limits to the g value were ascribed. For a number of selected CD bands amenable to integration, the rotational strengths (R = 0.23 x 10“33 / ) dA ) have been mea- A sured and tabulated. B. Materials and Purification 1. Mono-o1efi ns (-) a-Pinene (IV, Fig. 6) was obtained from Aldrich Chemical Co. The vapor-phase Chromatogram □38□ on a 6 f t . column packed with 15% g ,g ' -oxydipropionitri1e on chromo- sorb W exhibited only a single peak with no evidence of contamination due to impurities. The specific rotation 21 [39] of the sample was measured as Cct□ D = -41.8° (cyclo- ? 0 hexane, c = 0.0211) and [ a ] D = -42.5° (Perfluoro n-hexane, c = 8.7 x 10"^). This is to be compared with a reported value [40] of C a ] ^ = -48.2° (neat). The acyclic olefins S, 3-methyl-1 -pentene, S ,. 4-methyl-1 -hexene, and S, 5- methy1 -1 - heptene (I - I I I , Fig. 6), were kindly donated by Professor Piero Salvadori. The optical purity of the olefins was determined by Pino et al. [41-43]. The S, 2 5 3-methyl-1 - pentene had a specific rotation [a]p = +34.18° (neat; 91% optical p u rity ), the S, 4-methyl-l-hexene had [ a ] ^ = -2.85° (neat; 94% optical p u rity ), and the S, 5-methyl -1 - hepte,ne had [ a ] ^ =+10.35° (neat; 95% optical 23 3n Fig. 6. Structures of the compounds investigated. I. 3-methyl-1-pentene. II. 4-methyl-l- hexene. I I I . 5-methyl-1-heptene. IV. a-pinene. V* a-phellandrene. VI. e- phellandrene. 24 purity). All of the olefins listed above were vacuum d is tille d and thoroughly degassed before use. The absorption and CD spectra of (-) a - Pinene were run in a variety of high-purity solvents. The near-uv spectra from 240-1 90 nm were run in 2 , 2 , 2 - t r i f 1uoroethanol , 3-methyl pentane, cyclohexane, and perf1uoro-n-hexane. The vacuum-uv transmission and CD spectra were measured from 240-158 nm in perfluoro n-hexane. The absolute u lt r a violet cut-off for perfluoro n-hexane for a 1 m m path length is 155 nm. All Pinene spectra were measured using a path length of 1 mm. The absorption and CD spectra of the open-chain ole fins were run i n the gas phase from 200-140 nm. A 1 cm path length cell was employed. The liquid phase spectra of S, 5-methyl-1 - heptene were run in perfluoro n-hexane from 210-158 nm. 2 . D i e n e s (-) a-Phe.l 1 andrene (V, Fig. 6), practical grade, was purchased from ICN-K & K laboratories. GLC analysis showed i t to be approximately .75% pure. (-) B-Phellan- drene (VI, Fig. 6) of approximately 9 5 % - 9 7 % purity was supplied to us by Dr. F. P. McCandless. Both phel1andrenes were d is tille d under reduced pressure in the presence of 25 nitrogen. The d is tille d liquids were then purified by vapor phase chromatography [383 on either a 6 f t . $ , $ 1- oxydipropionitri1e (15% on chromosorb W) column, or a 3 ft. polyphenyl ether (20% on chromosorb P) column. Purity of the final product was checked on both chromatographic columns. The pure samples were estimated to be on the order of 99.8+% pure. Optical rotations [3 93 were measured in two solvents, cyclohexane and perfluoro n-hexane. The specific rotation of R, (-) a - phel1andrene was found to be [ c x 3 q^ = -194° ( cycl ohexane, C = 1 x 10- 3 ) and [a3p^ = -262°(CgF^, C = 6 x 10-4). jh e rotation in cyclohexane ? rj is to be compared to a lite ra tu re value [443 of [«3p = -2010 (.neat) . The specific rotation of R, (-) 3 -phellan- drene is [«3q2 = -19.6° (cydlohexane,: C = 6.7 x 10~3) and [aJg^ = -47Q (perfluoro n-hexane, C = 5 x 10"3). The lite ra tu re value [45J of [a3p^ = -17.5° (neat) for (-) g-phel1andrene is to be compared with the rotation in cyclohexane. It should also be noted that the d-modi- fications of both phel1andrenes were prepared with full optical a c tiv ity by Kuczynski and Zabza [4631. The speci- 2 0 o fic rotation of d, ct-phel 1 andrene [cQp = +185.8 (neat), 2 0 and the specific rotation of d, B-phe 11 andrene [°Dp +17.8P (neat). The reason for the large phellandrene rotations in perfluoro n-hexane (a solvent of low polariza- 26 b i l i t y , and consequently low dielectric constant), is not well understood, but this behavior is most lik e ly due to a population re-distribution of the two possible conforma - tions in these molecules; £, a-phel1andrene has been shown x to exist as an equilibrium mixture of a quasi-axial and quasi-equatorial conformer [47]. Molecular conformations made from Dreiding models convincingly show that e-phellan- drene may also exist as an equilibrium mixture of the same two types of conformer.s. The phellandrene spectra were run in the gas phase as well as in the liquid phase. The absorption and CD spectra of (-) a-phel1andrene in the vapor were run from 300 to 135 nm. The vapor phase (-) 0-phel1andrene spectra were run from 250 to 135 nm. All gas-phase spectral measurements were made in a 12 cm path length pyrex. cell. Liquid phase spectra of both phel1andrenes in cyclohexane were run from 350 to 190 nm. The vacuum-uv CD and trans mission spectra were run from 210 to 158 nm in perfluoro n-hexane. The near-uv CD and absorption spectra were also run in perfluoro n-hexane. A 1 cm or a 1 m m path length cell was used for the liquid phase spectra in the near uv. All liquid phase vacuum-uv measurements were carried out in the 1 m m stainless steel cell described e a rlie r. Photoionization efficiency measurements in the _______ 27 vacuum-uv were carried out for both phel1andrenes> ' Tbe data points, at energies greater than threshhold were taken at rela tive ly large wavelength intervals. The primary concern was with the ionization threshhold region, for the determination of the adiabatic ionization potentials. No attempt was made to measure the complete spectrum in the region of energy covered (7.5 - 8.8 ev). 28 CHAPTER THREE THEORY OF OPTICAL ACTIVITY A quantum-mechanical treatment of the phenomenon of optical ac tivity was f i r s t formulated by Rosenfeld [481 in 1928. This treatment, however, was confined to non absorbing media only. Around the same time, Kuhn and co workers [49,501 investigated and applied the classical theory of optical a c tiv ity to spectral regions where absorption was assumed to occur. Since then, quantum- mechanical as well as semi-cl assical radiation theory has been applied to the formulation of optical a c tiv ity by Condon [511, Condon, Alter and Eyring [521], Kirkwood [53H, M offitt and Moscowitz [541, and others. A. Phenomenological and Semi-Classical Considerations [551 An optically active medium exhibits circular bire fringence and circular dichroism. Circular birefringence is the phenomenon associated with the rotation of the plane of polarization of 1inear po1arized light after the light wave has passed throughan optically active medium. This effect is easily measured in substances that are transparent to the impinging radiation. Circular d i chroism, on the other hand, is characterized by the change 29 from linear to e l l i p t i c polarization for transmitted rad iation in the region of an optically active absorption band. The circular dichroism ( CD) , although present in transparent regions, is measureable only when produced in the v ic in ity of optical absorption. The rotation of linear polarized lig h t, and the degree of e l l i p t i c i t y , 0 , are defined as follows: < ( > = (n_ ~ n + ) u/2c (3.1) 0 = (k _ - k + ) oj/ 2c ( 3 . 2 ) where, < { , = angle of rotation per unit path length e = e l l i p t i c i t y per unit path length a ) = 2 it v , and is the angular frequency of the propagating radiation c = the speed of light n , n+ are the indices of refraction for l e f t and right circularly polarized light respectively k , k , are the absorption coefficients for le f t and — T right circularly polarized light respectively n and <+ are related by the complex index of refraction, N+ , N+ = (n+ - i.<+ ) (3.3) 30 The mean refractive index, n> and mean absorption coef f ic ie n t, e, are then given by: n = ( n _ + n+)/2 (3.4) log, e 2u) e = ---------IL_ (___)[(< + k + ) / 2 ] (3.5) N V c ' s is the decadic extinction coefficient defined by Beer's 1 aw I = I Q 10“eW£ (3.6) w +i ere, I 0 = incident lig h t intensity I v = transmitted light intensity fj = concentration of absorbing substance £ = path length of absorbing substance The rotation data and e l l i p t i c i t y can be most con veniently expressed in terms of the specific rotation, [all* and c i r c u l a r d i c h r o i s m , Ae , given by 1804. 1 M = -------- -» (3.7) tt N 2 1° 910e/180 0\ A.E = E _ _ E + = - { — — ) (3.8) 31 where, e_, e + are the extinction coefficients for le f t and right circularly polarized light N = concentration in M ole/liter N' = concentration in grams/cm The rotation and e l l i p t i c i t y are often expressed in the more common experimental units of degrees per unit length. It is evident then that the absorption coefficients and hence the circular dichroism are related to the ima ginary part of the complex index of refraction, whereas the specific rotation and refractive index are dependent on the real part of the complex index of refraction. The optical rotation and CD, however, only exist when the le f t circularly polarized indices d iffe r from the right c i r cularly polarized indices. For an isotropic medium, n _ - n+ = 0, and <_ - k + = 0. For the presence of optical a c tiv ity , however, n_ - n+ f 0 and <_ - k+ i 0. The essential feature in the semi cl assical theory of optical a c tiv ity is the modification of the electric dis placement and the magnetic induction to account for the variation of the fie ld of the light wave over the molecular dimensions. In the derivation of the mean refractive index and mean extinction coefficient ( electric-dipole absorp- 32 tion) i t is sufficient to assume that the electromagnetic O fi. e 1 d ( j . ^ IQ3 - 10.4 A] does not vary over typical molecu- o lar dimensions (> 1 - 10 A)., The bulk magnetic and elec t r ic properties are given by 5' = eE - g< (3 /3 tl H C 3. 9) 5 * . yH t g* ( a / a t ) E > (.3.10) where, -4 - D - electric displacement ■+ B = magnetic induction e d ielectric constant :p = magnetic permeability (y ^ 1 for nonmagneti c media) -¥• E = electric fie ld strength magnetic fie ld strength g' proportiona1ity constant The microscopic treatment of Rosenfeld E48H, provides the analogous molecular equations, » av -* -V s pte - (e/c)_Ca/at)H + th (3.11 ) mav * 5 h + (e /c )(a /a t)E + yE (3,12) where p ^ and m^ are the average induced electric-dipole and magnetic-dipole moments respectively per individual 33 molecule, c is the speed of lig h t, and a , g , y and $ are appropriate proportionality constants. It has been assumed that the effective local fields of E and H are the same as the impinging electromagnetic fields. This is a good approximation for gases. Rosenfelds equations then, adequately account for the fact that different parts of the molecule will see different electromagnetic field strengths at the same instant of time, Condon C51H has pointed out that terms involving y may be neglected as far as optical rotation is concerned, and since we are dealing with nonmagnetic media, 6=0. The polarizabi1ity af > and the quantity b, express the response of an individual molecule to the perturbing electromagnetic wave. The expressions for a and g, averaged over all directions of the molecule with respect to the electromagnetic fie ld are given by v i < a | jj |o> -< J |.y I a> a = (2/3h ) - l ~ * -------^ ----- (3.13) j v j 2 - v 2 Im (<a| y|j > •<j|m|a >} 6 = ( c/ 3 ith) I -------- (3,14) J ' v where Im { } means imaginary part of { }. Here < a \v | j > and < a|m| 3 > are the el ectric-dipol e and magnetic-dipole transition moment -matrix elements connecting the ground 34 state |a> with the excited state |j>; v. is the frequency J of the transition a j ; v is the frequency of the im pinging radiation, and h is planck's constant. The sum in eq. (3.13) and (3.14) runs over all excited states, j , and i t is im p licitly assumed that there is.only a single state, |a>, accessible to the molecules in thermal equi1i bri urn. Rosenfeld's equation for a and p are however only valid in regions where there is no absorption. I f one is considering optical a c tiv ity in absorbing media, then the equations for a and p (eq. 3.13, 3.14) must be modi fied: v .D . a = ( 2 / 3 h ) I ? 3...1----------------- (3.15) j - * 2 > + Rj P = (c/3irh) I -------=------------ (3.16) . (v . - v ) + i v r . J j o is equal to the full width at half-maximum ( FW HM -) for the absorption line associated with the j th transition. A D. = <a |y | j > • <j|y|a> and is the fam iliar dipole-strength which expresses a measure of the intensity for the absorp tion band associated with the transition from the ground state |a> to the excited state | j >. The rotational strength, Ri , is equal to Im {< a | y | j > •< j | m | a> } and is a \J measure of the intensity of the circular dichroism band 35 associated with the transition a j . The experimentally measured quantities e , and &e ? may he related to the dipole strength and rotatory strength by an appropriate integra tion oyer the absorption band C e vs, v) or CD. band ( m v s v ). corresponding to the electronic transition in question: where NQ i s Avogadro's number. Kuhn [5.6,571] f i r s t introduced the anisotropy factor, g, defined by, and exploited the physical significance of this parameter. It is apparent that the anisotropy factor is proportional to the ratio;;: of the magnet i c-di pol e transition moment to the electric-ddpole transition moment (m/y ). Compari son of the magnitudes of the g-factors for electronic transitions of chirally perturbed chromophores can lead to the character!zation of excited states in terms of the upper state symmetries of the chromophore in question. For example, a re la tiv e ly large g-factor would indicate (3.17) (3.18) 36 that the electronic state is magneti c-di. pol e allowed and electric-dipole forbidden in the symmetric parent chromo- phore, whereas the converse would be true for an excited state whose g-factor is sma11• I t should be stressed that the significance of the absolute magnitudes of the aniso tropy factor in each case will depend on the calibration for the chromophore under study. B. Circular Dichroism: Quantum-Mechanical PormUlation In this section no effort will be made to exhaustively derive or l i s t all steps in the quantum-mechanical formu lation of optical a c tiv ity . -Instead, the pertinent as sumptions and quantum-mechanical expressions will be sketched out. The phenomenon of circular dichroism can be derived quantum-mechanically by a f a ir ly obvious extension of the o r d i n a r y t h e o r y of absorption transition probabilities. Time-dependent perturbation theory is employed to des cribe the interaction of the electromagnetic radiation with the molecular system. The f i r s t expression that is needed is the Hamiltonian function for the system of charged particles in the electromagnetic f ie ld . In deriving the Hamiltonian i t is more convenient to use the vector potential A? and the scalar potential , rather than the electric and magnetic field strengths (E, H) associated with, the interacting radiation. The r e l a t i o n ships between these quantities are given by the equations H = y x A; E * - : l i _ a - v* (3.20) e a t " For an electromagnetic fie ld due to radiation of frequency a) = 2TTV, A may be written A = 1/2 CA'° exp ( i a)t) + A' exp (-iut)II (3.21) I f one neglects all hyperfine and spin interactions (the spin interaction will be zero for singlet electronic states), the Hamiltonian in an external electromagnetic fie ld characterized by A and < j> is H = I {(2m.)-1 ECpt ) - (ei /c)A i : 2 + e . * . } + V (3.22) The sum runs over all particles (electrons and nuclei), p.. is the linear momentum operator, A^ and are the vector and scalar potentials of the fie ld at the i th p article , e^ and m ^ are its charge and mass, and V is the internal coulombic potential energy of the system. For an electromagnetic fie ld associated with a light wave, we can set v ♦ A - 0, and $ - 0, Furthermore, terms in.A. can be neglected since we are only interested in linear- optical phenomena ( e . g, , no external magnetic f ie l d ) . The Hamiltonian can now be written 38 H (3.231 where «. * I f 2* 1 I ’ ’ P* + » i (;3. 24) and i e t Ctit.ce)'1' A 1- p i (3,25) i H is just the Hamiltonian operator for the system in the absence of the electromagnetic fie ld and is independent of time. H .j represents the time-dependent perturbation due to the interaction of the system with the light wave. The problem then reduces to the familiar one where matrix elements of the type < a| H1 -|j> must be calculated to deter mine the probability that a transition will take place from the ground state |a> to an excited state |j>- I f the ap- prgximation is made that A is regarded as a constant over the molecular dimensions, the probability of a transition between two states will be predicted to be solely pro portional to the matrix element for the electric-dipole transition moment between the combining states. This ap proximation is insufficient for the derivation of optical rotatory power, The value of A at any point in the molecule may be expressed by. a Taylor's series expansion in terms of the value of A and its derivatives at the origin of a mole- 39 cule-fixed coordinate system that is determined by the impinging ligh t wave. The vector potential to f i r s t order is given by th.e multipole expansion □ 58H: ^i« " ^poi + ^ ^ Vx^ o x r i^a + ^ ^ Va^B ^ 0 + ( 26 ) where r^ is the position vector of particle i, and a and 3 may be x, y, or z and repeated indices are summed over. The perturbing Hamiltonian .is then H-1 = - I ei (mi c ) " 1 ’ {(A0 *Pi ) + %C(vxA)0 - ( r i x p^D + c V A J (r. P.Q + r . P . ):> (3.27) a 3 o la iB 1.3 la The f i r s t , second, and third terms of Eq. (3.27) repre sent e le c tr ic -d ip o le , magnetic-dipole , and e lec tric - quadrupole interactions respectively. The perturbed wavefunction for the system, i>a ( t ) , can be expanded in terms of the unperturbed eigenfunctions ° . (t ) : J ^ ( . t ) = *°Ct) + I C . . ( t ) * ° ( t ) (.3.28) m a u j j 3 f a The 6 . Ct) are time dependent coefficients and can be ^ 3 A expressed in terms of H -j by using the time-dependent Schrodinger equation and retaining terms f i r s t order in H r 40 c ,(t) S - i / -<<^(t) |H U°(t)>dt (3.29) a J 0 3 < 9 Now the probability per uni;t time for a transition from state |a> to state |j> is given by Ca j Ca! / t ■» (.flf*2! - 1 1 ! i-. ja./c )A®*. - j |-J I a» + ! v < l° ! 0« j I m | a> + { i . u /?c)(v.tA ° ! * , j | q t; : a , ; 2 .;(,1a-,0 2 Ct-1 ) (3.30) where, w- “ to. - w (.3.30) j a j a ' a — < j |y | a > = -< j | £ e1 - r 1 -|a> (electric-dipole 1 transition moment) <j|m|a> = <3 11 e.j (2m.j c ) ~ 1 (r^xp.j ) | a > (magneti d-d i pole transition moment) A < j|q ;■ |a> = < j | £ ei r i ftr i g l ei> (el ectri c-quadrupole transition moment) gCidjg-u) - 1 - exp [i (o> .a- o ) ) t J / U j a-a)) - the value of the vector potential evaluated at the origin and at t = 0, The vector potential for circularly polarized waves is given by Aq | - A° (i + i j ) (3.31 ) 41 where the propagation of the ligh t wave is in the + z direction and the upper sign (+) represents right, and the lower sign (-) l e f t circularly polarized ligh t. It can be shown [581] that the absorption coefficients for circularly polarized light due to the transition a j are given by: o o 2 2 o / k (a - j)du, = ^ (N - N-) ' { ~ |< a|y + | j>| o n n - - 2 a Im[< a | ji + | j > (< j | m ^ _ | a> - — < j I q+ I a> )3} (3.32) where, N,,N- are the number of molecules per unit volume in ® J states a and j respectively a = (n2 + 2 ) /3 m+ = mx 1 1my * = ux i iyy ’ and q± = qzy 1 iqzx and the absorption intensity, k+ , has been integrated over all frequencies, w. The circular dichroism intensity due to the transition a -* j , is then expressed by 00 [ a k ( a - > j) 3 = / { K ( a - > j ) - < +(a->j)}dcj (3.33) 0 ~ This result is for oriented systems which have their molecule-fixed z-axis oriented along the direction of propagation of the light wave. For oriented systems the quadrupole term is necessary in order to assure origin invariance of the absorption coefficients and should not be neglected. However, for molecular systems which are 42 randomly oriented with respect to the external space- fixed coordinate system, the contributions from the quadrupole terms cancel. Finally, for fluids, after averaging overall orientations of the molecules the CD is given by 16air'2 (N , - N . ) Ax (a - * ■ j ) = -------------------- - 1— Im {< a |y | j > • <j | m | a> } (3.34) which is the desired quantum mechanical expression invol ving the rotational strength. For electronic transitions we may write (Na- Nj) = Na > since only the ground state will be populated for molecules in thermal equi1ibriurn. It is apparent that the rotational strength can, in principle, be calculated from the values of the e le c tric - dipole and magnetic-dipole transition moments. Several different approaches have been formulated C51 - 53 ,55 ,59-61 H for the calculation of rotational strengths. It should be mentioned that calculations of this type are not as estab lished as the more common SCF-type M O calculations which only endeavor to calculate excited state energies and electric-dipole oscillator strengths. This is mainly due to the fact that the sign and magnitude of the CD are highly dependent on the accuracy of the wavefunction, and conformation of the system including even minor changes in the charge distributions such as bond lengths and bond angles. The techniques involving the theoretical calcula 43 tion of the circular dichroism are beyond the scope of this dissertation and will not he discussed. 44 CHAPTER FOUR EXPERIMENTAL RESULTS A. Mono-olefins The experimental measurements for the acyclic mono olefins [27] are presented in Figs. 7-9 and in Tables 2-4. The absorption coefficients are aTl decadic and in units of 1iter-mole~^- cm- ^. All spectra were taken with a slit-w idth that provided ample resolution for these systems, and it can be asserted that no spectral features were resolution limited. The gas phase absorption spectra of all three com pounds exhibit a clearly defined but broad band near 180 nm ( e ^ 10,000), which is assumed to represent mainly * 1 i the ir 7 T , A -* B -I,, transition. Some "fine" structure g I u appears on these bands, however the diffuseness prohibits any quantitative vibrational analysis. For all three cases, a minimum occurs near 160 nm followed by increa sing absorption to shorter wavelength without reaching a maximum before the 140 nm lim it of measurement is reached. The CD spectra all have a prominent positive band in the region 170-160 nm and a f a i r l y intense negative band around 155-145 nm. Both of these features move 45 50 CD—*| j* * “ xIO ABSORPTION CD 140 150 160 170 200 190 180 X (nm ) Fig. 7. Absorption ( e ) and circular dichroism ( A e ) of v 3-methyl-1-pentene in the gas phase [273. TABLE 2. Absorption and circular dichroism features of S,3-methyl-l-pentene (gas). ABSORPTION CIRCULAR DICHROISM A (nm) Com m ents e . (lite r mole'‘ cm"^) A (nm) Com m ents A £ (1 iter mole'l cm'^) g d o -4) R(esu) (10-«) 186.5 shoulder 188 • Am ax - 0.3 _> 3. 3a - 27 182.3 shoulder 177.1 Am ax 10,000 176.6 Am ax 1.4 1.4 130 173.2 shoulder 168.4 shoulder 166.6 shoulder 162.7 Am ax 4.5 23a 670 159.3 shoulder 156.7 shoulder 155.0 shoulder 146.3 ■ A m ax - 3.6 > 4a - 600 js . a) ■ > 4 Corrected for overlapping transitions in absorption. 50 60 65 xIO CD 0.5 ABSORPTION - 1 .0 200 190 180 170 150 140 X (nm) Fig. 8. Absorption ( e ) < p n d circular dichroism ( A e ) of _ 4-methyl-1-hexene in the gas phase [27]. TABLE 3. Absorption and circular dichroism features of S,4-methyl-1-hexene (gas). ABSORPTION CIRCULAR DICHROISM a: (nm) Com m ents e (lite r mole"! cm - ' ) a ; (nm) Com m ents A e g (lite r mole- ! cm - ^) (10"5) R(esu) (10- 42) 189.2 shoulder 900a 189.2 A m ax 0.14 >16a 5 183.2 shoulder 178.5 Am ax 8,200 177.1 Am ax - 0.4 5 - 53 173.1 shoulder 173.5 shoulder 168.1 shoulder 168.1 shoulder 163.1 shoulder 164.7 158.2 A m ax shoulder 0.95 52a 100 151.0 A m ax - 0.88 > 13a - 120 a) Corrected for overlapping transitions in absorption • 50 70 c d - H h - 0.5 xlO ABSORPTION -0.5 CD - 1 .0 200 180 170 160 140 150 A(nm) Fig. 9. Absorption (e ) and circular dichroism ( a, e ) of - 5-methyl-1-heptene in the gas phase [27]. U 1 O TA B LE 4. : Absorption!and circular dichroism features.of S,5-methy1-l-heptene (gas.). ABSORPTION CIRCULAR DICHROISM (nm) Com m ents e A Com m ents (1 iter mole"' cm'1) (nm) 4 e (lite r mole"! cm'^) g (10'6) R(esu) (10-42) 188.5 shoulder 185-9 m ax - 0.4 > 400a - 35 183.1 shoulder 178.5 - A m ax 12,800 178.5 Plateau 0.06 5 7 169-° Am av m ax 0.81 270a 90 167.9 shoulder 161.5 shoulder 154.1 A m ax - 0.83 > 863 - 78 a) Corrected for overlapping transitions in absorption. C J 1 successively to longer wavelength with increasing molecu lar chain length. In all compounds there also appears to be a weak positive band, clearly present as a shoulder, on the high energy wing of the 170-160 nm positive CD system. In the region of the tt -*■ tt transition, the CD spectra of the methyl- pentene and the methy1 - hexene exhi bit a positive and a negative band respectively whose maxima coincide with the intense absorption peak at about 180 nm. Methy1 - heptene, however, has only a r e la tiv e ly weak positive shoulder in the v ic in ity of the ★ t t -* t t absorption maximum. About 10 nm to longer wave length than the t t tt* transition both the methyl-pen tene and methyl-hexene exhibit weak CD bands which are opposite in sign to the (tt, t t ) CD bands. It is probable that in the CD spectrum of the methyl-hexene the two lowest energy bands are somewhat overlapped. In con trast to the other olefins, methyl- heptene exhibits a moderately intense negative band some 7 nm to longer wavelength than the (.it, it ) state. The only vibrational structure observed in the CD spectra is diffuse and is observed on the intense positive CD system at 160 nm in methyl-pentene. I t is apparent that the CD spectra clearly exhibit at least four separate electronic tra n s i tions in the 200-140 nm energy region whereas only one 52 absorption band can be located with certainty. A study of the liquid phase CD spectrum of 5-methyl- 1-heptene in per-fluoro-n-hexane was attempted, but the overall intensity was lower by about an order of magni tude relative to the gas phase. As a result the spec trum could not be recorded at a reasonable signal-to- noise level. The absorption and CD spectra for a-pinene are pre sented in Figs.10-13 and Tables 5-8, The gas phase spectra (Fig. 10,Table 5) are taken from an a rtic le by Mason and SchneppH263. The gas phase CD and absorption spectra of this cyclic mono-olefin, except for a long- wavelength shift of about 20 nm, qu a lita tiv e ly resemble the corresponding spectra of the straight chain olefinic systems just described. Naturally, the f i r s t intense broad absorption band (200 nm) is assumed to represent mainly the t t -> t t (N -> V), transition. A one-to-one correspondence between the CD bands and features of the acyclic and cyclic systems is evident. The spectra of a-pinene in solution (Figs. 11-13) however, are particularly revealing. The spectra are f a i r l y smooth and featureless (fine structure) as compared to the gas-phase spectra. The absorption maximum for the (.t t , t t ) transition is the same for both 53 45 60 70 A € 5 - 2 - 240 230 220 210 200 190 180 170 160 150 140 X(nm) Fig. 10. Absorption ( e ) and circular dichroism (A.e) of ot-pinene in the gas phase, [26].. 'c n . o 4 * ' TABLE 5. Absorption and circular dichroism features of (-) a-pinene (gas). ABSO RPTIO N ^CIRCULAR DICHROISM A (nm) Com m ents e (1 iter mole"' cm ""^) A (nm) Com m ents A e 9 (lite r mole-1 cm -1) (10"^) R(esu) (10-40) 220 shoulder 800 222 216 3.0 2.8 2.02 201 broad 4800 202 broad - 7.2 1.5 - 10.8 186.5 overlapping 1700a 188.5 181.5 bands 2800a 185 overlapping 177.5 1600a 184 182 176.5 vibrational 18.3 6 bands 23.2 169 6.0 2.06 163 - 4.3 160 - 6.2 a) < ji Corrected for overlapping transitions. 45 50 60 H K xIO - ABSORPTION -3 X(nm) Fig. 11. Absorption ( e ) and .circular dichroism ( a e ) of a-pinene in perfluoro-n-hexane. 56 TAB LE 6. Absorption and circular dichroism features of (-) a-pinene (in solution). I. (-) a-pinene in per-fluoro-n-hexane ABSO RPTIO N CIRCULAR DICHROISM A Com m ents e (nm) (liter mole"' cm"') A Com m ents (nm) A £ . . , (1 iter mole" c m ) 9 (10"4) R(esu) (io-% ) 218 A m ax - 0.44 > 3a - 5 200 a 5400 • m ax 199 • A m ax - 0.61 1.1 - 9 170 shoulder 174,5 - A m ax 3.5 ^ 20a 92 a) Corrected for overlapping transitions in absorption. c n 4 2 CD ABSORPTION 190 200 210 220 230 240 X (n m ) Fig. 12. Absorption ( e ) and circular dichroism ( A e ) of a-pinene in 3-methyl-pentane. TABLE 7. Absorption and circular dichroism features of (-) ot-pinene (in solution). LI; (-) a-pinene in 3-me-pentane ABSO RPTIO N CIRCULAR DICHROISM A Com m ents e . (nm) (lite r mole"1 cm " ) A Com m ents (nm) A e (lite r mole"^ cm'^) 9 d o -5) R(esu) (10-41) 220 A - m ax - 0.74 > 54a 9.3b 203.7 Am=u 4100 ■ - m ax 204 shoulder (Plateau] 196 shoulder - 0.24 ^ 6 3b a) Corrected for overlapping transitions in absorption. b) Corrected for overlapping C D bands. C J 1 u o 101cm-’) CD ABSORPtlON 240 230 220 210 200 190 -1 X (nm) Fig. 13. Absorption (e) and circular dichroism (Ae) of ct-pinene in cyclohexane. C T i o TABLE 8. Absorption and circular dichroism features of (-) a-pinene (in solution). TM. (-) a-pinene in cyclohexane ABSO RPTIO N CIROULAR DICHROISM A Com m ents e (nm) (lite r mole"1 cm"1) A Com m ents (nm) A e (1 iter mole"^ cm"^) 9 R(esu); (i0"5) (io-4T) A m ax - 0.87 > 50a 12 204 A 4800 • m ax 202 shoulder 196 shoulder a) Corrected for overlapping transitions in absorption. c n the gas phase and the perfluoro-n-hexane solution spectra (200 nm) , whereas the (it 7t t ) absorption system in hydrocarbon solvents is 1 ong-wavelength shifted by approx, 4 nm from the vapor phase value. In all of the CD spectra in solution there is a negative band present at 220 nm. This band is evidently not resolved tn the gas phase spectrum, most lik e ly due to an over lapping transition of opposite sign. Conversely, the gas phase band (positive sign) also found in the v ic in ity of 220 nm is missing in the liquid phase spectra. The perfluoro-n-hexane solution CD spectrum exhibits a well resolved band correspond ing to the ( t t , it * ) absorption band, but in hydrocarbon solvents i t is apparent that the ( it, ir*) CD band is not well resolved, and suffers an apparent variation in rotatory strength. The pro minent positive CD band in per-fluoro-n-hexane at about 175 nm corresponds to the gas phase CD band at 185 nm. This CD'band undergoes a solvent frequency shift of 3 2 00 cm"'' to higher energy, and is considerably broadened in solution. Needless to say, all a-pinene solution spectra are devoid of any vibrational structure. I t should also be noted that the CD intensities in solution are much lower than those in the gas phase, whereas the absorption 62 intensities in the two phases are comparable. B,. Dienes The experimental measurements for the conjugated diene systems are presented in Figs, 14^-21 and Tables9-14. The spectral resolution was found to be quite ample for all of the CD and absorption features recorded. The vapor phase absorption spectrum of the trans- diene, B-phel1andrene CFig.14) is characterized by a very intense absorption band at 225. nm(e ^ 20,000), followed by decreasing absorption to about 175 nm, and then slow increasing absorption until the lim it of measurement is reached. The broad intense band centered at 225 nm, is * * 1 1 assumed to represent mainly the tt2 ^ ^3 » Ag -> ■ 1 transition. Some discernible vibrational structure, although diffuse, appears between 210 and 165 nm. The ( _ t t2 i f 3 *) band ( 225 nm) is extremely broad and diffuse and although some structure is fa in tly v is ib le , i t is badly blurred. At higher energy the only prominent absorption feature is a moderately intense shoulder at about 156 nm. The vapor phase CD spectrum of 3 -phel- landrene consists of a positive band at 225 nm cor responding to the very intense absorption system, f o l lowed by two vibrational progressions of positive sign. 63 XIO3 (cm"'S cn -p> 45 40 65 50 60 20 ABSORPTION - - CD XIO A€ j A. 240 220 180 160 \{nm ) Fig. 14. Absorption (e) and circular dichroism (Ae) of 3-phellandrene in the gas phase. TAB LE 9A. Absorption and circular dichroism features of (-) B-phellandrene (gas). ABSORPTION CIRCULAR DICHROISM A (nm) Com m ents e (1 iter mole"^ cm"^) A : (nm) A p Com m ents (lite r mole" cm"1) g (TO-4) R(esuL (10-40) 233 vib 236 vib 225 A m ax 19,400 226 ''max 3 ,4 1 .8 6.7 219 vib 220 vib 213 vib 214 vib 204 vib, S 203 overlapped by C D 197 191 vib, S vib, S 196.5 191 bands of opposite sign 186 vib, M vib = most likely represents a vibronic feature. S = strong; M = moderate; W = weak CTi c n TAB LE 9B. Absorption and circular dichroism features of (-) e-phellandrene (gas), x (nm) ABSO RPTIO N Com m ents e (1 iter mole"‘ cnf') a: (nm) 182 vib, W . 182.5 177 vib, W 177 172 vib, W 172 167 vib, M 167.5 164 160 156 shoulder 153 144 CIRCULAR DICHROISM Com m ents overlapped by C D bands of opposite sign shoulder shoulder vib = most likely represents a vibronic feature. S = strong; M = moderate; W = weak C T 1 < T > x 10 ic rrf1 ) ABSORPTION “ 10 ------ CD 250 230 210 AC X ( m r c ) Fig. 15. Absorption (e) and circular dichroism ,(&e) of 0-phellandrene in perfluoro-n-hexane. cn TABLE 10. Absorption and circular dichroism features of (-) 3-phellandrene (in solution) • I . (-) 3-phellandrene in per-fluoro-n-hexane ABSO RPTIO N CIRCULAR DICHROISM a : Com m ents e (nm) (1 iter mole" 1 cm"1) A. Com m ents (nm) A e (lite r mole" 1 cm ”1) 9 (10"4) R(esu) (10-40) 236 vib 227.5 xm ax 2,,600 229 A.... v illaX 5.8 2.7 14 221 vib 220 vib ^ 195 shoulder, broad - 0.85 3.4a - 1.5b %187 shoulder ^ 170 shoulder - 10 > 36a vib = most likely represents a vibronic feature, a) = Corrected for overlapping transitions in absorption, o o b) = Corrected for overlapping transitions in CD. 40 xtd^cnfT1 ) 45 50 ABSORPTION CD 20 XlO 260 250 240 230 220 210 200 190 X{nm) Fi.g. 16. Absorption (e) and circular dichroism (Ae) of 3-phellandrene in cyclohexane. •T A B L E 11. Absorption and circular dichroism features of (-) g-phellandrene (in solution). II. (-) g-phellandrene in cyclohexane ABSO RPTIO N CIRCULAR DICHROISM A Com m ents £ 1 , A ' Com m ents (nm) (lite r mole" cm " ) (nm) A e (lite r mole"^ cm"^) 9 /i R(esu) (10-4) (10-40) 240 vib 233.5 ,Am ax 22,400 232.5 . A m ax 7.2 3.2 18 a , 226 vib ^ 200 shoulder - 1.5b /~a, b ^ 6 vib = most likely represents a vibronic feature. a ).= Corrected for overlapping transitions in absorption. b) = Corrected for overlapping transitions in CD. -'j o 7.75 PHOTON ENERGY (evj 8.00 8,25 8.50 8.75 42 I K c D >» k . o X) Iw < > — <. >- 1.0 U z y u U _ U _ Ld I 0.5 h- < N o o fe x 0. o o o ° vO 15 5A j 6 - o -.q g. o 6 qqq. qoX 3 i 2- 1600 1550 o 1500 X(A) 1450 1400 Fig. 17. Photoionization efficiency of B-phellandrene in the gas phase. The positive CD peaks can be matched up with, corresponding structure in the absorption spectrum. The vibrational CD progression at about 200 nm corresponds to a moderately intense absorption system, whereas the more intense higher energy CD progression centered at ^ 170 nm, corresponds to weaker absorption. I t is evident that superimposed upon each set of structured positive bands, there is at least one underlying band of opposite sign. The under lying band at about 2 0 0 nm is weak, and the broad negative CD system centered at about 170 nm, is: very intense, and is overlapped by several f a i r l y intense vibronic features of opposite sign. This strong CD system corresponds to a region where the absorption strength is at a minimum. The remaining features in the CD are distinct shoulders at 153 nm (negative) and at 144 nm (positive). The CD and absorption spectra of e-phel1andrene in solution (Figs.15,15) are instructive since all of the sharp vibronic structure found between 2 1 0 and 160 nm has been blurred. In cyclohexane solution, measurements Can only be made to about 190 nm. As expected, after inspection of the gas-phase CD spectrum, the underlying bands are both negative and structureless. The large CD intensity or rotational strength, of the higher energy band is confirmed in the perfluoro-n-hexane spectrum. 72 Also, a distinct shoulder is observed at around 170 nm in the CD in perfl uoro-n-hexane solvent. The absorption spectrum (in CgF-^l exhibits a band (strongly overlapped) at about 185 nm which corresponds best to the weak negative CD system. Both liquid phase spectra have CD and absorption bands corresponding to the "*Ag -*■ 1 transition observed in the gas phase. These band maxima however, are shifted to longer wavelengths by 2 and 8 nm, the larger shifts being observed in cyclohexane. The vapor phase absorption spectrum of a-phel1andrene (Figs. 18A , 18B),a c i s-d i ene, is much different from the transoid compound. The lowest energy transition, assumed " A 1 1 I to be the ^ irg , A^ 1 &2 excitation, is shifted by some 30 nm to longer wavelength, and is much less intense (e ^ 3500), when compared to the trans - compound. The absorption, however, continues to rise and unlike the trans-diene, the absorption strength between about 180 nm and 135 nm is much larger than in the long wavelength region, The cis-diene has most of its absorption inten sity at high energy, whereas the converse is true for the trans-diene. Two "doublet-1ik e " absorption features appear at ^ 213 nm and ^ 189 nm. There appears to be a very intense absorption band at about 155 nm, and the absorption continues to rise at higher energy until the 73 > ABSORPTION CD XIO -5 300 280 240 220 200 X (n m ) Fig. 18A, Absorption (e) and circular dichroism (Ae) of a-phelTandrene in the gas phase. 300 - 195 nm . XIO' (cm -1 ) 50 55 6 0 65 70 75 ABSORPTION CD XIO - 1 0 220 200 180 160 140 X(nm) Fig. 18B.' Absorption (e) and circular dichroism (ae) of a-phellandrene in the gas phase. 220 - 135 nm . 75 TA B LE 12A. Absorption and circular dichroism features of (-) a-phellandrene (gas). ABSORPTION CIRCULAR DICHROISM . A Com m ents (nm) e (1 iter mole'^ cm " ^) X (nm) Com m ents A e (lite r mole' cm - ’) g (io-4) R(esu) (10-4°) 258 Jm ax 3,350 256 A m ax _ 5 ,4 16 - 17 215 vib 217 overlapping 210.5 vib 211 bands 205 vib, W 205 2.1 18a 199 vib, U 199 195 overlapping 191.5 vib 189.5 bands a) Corrected for overlapping transitions in absorption • TAB LE 12B. Absorption and circular dichroism features of (-) a-phellandrene (gas). ABSO RPTIO N CIRCULAR DICHROISM X (nm) Com m ents e (1 iter mole'' crrH ) X (nm) Com m ents A e (1 iter mole'^ cm"^) g (10-4) R(esu) (10-40) 187 vib 176.5 shoulder 173 shoulder 163 A m ax - 9.3 > 7a - 23 ^ 155 shoulder, (xm ?) m ax 16,000 152 143 shoulder A max(?) a) Corrected for overlapping transitions in absorption, * - s J 35 * 10 (cm” 1) 45 50 ABSORPTION 4 2 200 220 240 280 260 300 X ( n m ) ^ 4 C O Fig. 19A. Absorption (e) .and circular dichroism (& e) of a-phellandrene in perfluoro- n-hexane. 300 - 195 n m . 45 x 1 0 ( c m - 1 ) 50 55 60 h— ABSORPTION 10 ,3 A € 5 -5 230 210 190 170 X(nm] Fig. 19B. Absorption (e) and circular dichroism (Ae) of a-phellandrene in perfluoro-n-hexane. 230 - 160 nm . 79 TAB LE 13. Absorption and circular dichroism features of (-) a-phellandrene (in solution) • T. (-) a-phellandrene in per-fluoro-n-hexane ABSORPTION CIRCULAR DICHROISM A Com m ents e 1 (nm) (lite r mole-1 c m ) A Com m ents A e (nm) (lite r mole- ' cm - ^) g (10-4) R(esu) (10- 40) 259 A 3,700 m ax ’ 260 • A m ax " 5,1 14 - 17 221.5. A m ax(?) 0.8 ^ 10a 0 .6 % 203 shoulder * 198. A max(?) 1.4 ^ 10a 3.2 V l87 shoulder, broad 162 - 10 • m ax V 0 0 o » a) Corrected for overlapping transitions in absorption., C D O 40 50 45 ABSORPTION — CD A€ xIO' 300 290 280 270 260 250 240 230 220 210 200 190 X(nm) Fig. 20. Absorption (e) and circular dichroism (/\e) of a-phellandrene in cyclohexane. TA B LE 14. Absorption and circular dichroism features of (-) a-phellandrene (in solution) • I I . (-) a-phellandrene in cyclohexane ABSORPTION CIRCULAR DICHROISM A Com m ents £ A Com m ents A e (nm) (lite r mole- ' cm - ') (nm) (lite r mole-1 cm - ') g (TO*4) R(esu) (10-40) 264 . A max 3,300 264 . . W ’ 3 ' 3 10 - 12 224 shoulder, weak 224 A (?) 0.8 m sx a , 8a 0.7b 206. W ?) -J.0 a - 2 a) Corrected for overlapping transitions in absorption. b) Corrected for overlapping transitions in CD. 00 ro PHOTO IONIZATION EFFICIENCY (Arbitrary Units) 1.0 0.5 L 6 ~ 1700 P H O T O N E N E R G Y (ev) 7.5 8iO 1650 8.5 O O 1607 A O J— o— oo 1600 1550 XCA) O O 0 1500 o C h 1400 Fig. 21. Photoionization efficiency of a-phellandrene in the gas phase. C O CO lim it of measurement is reached. The gas phase CD spec trum of a,-phel 1 andrene exhibits a negative band corres ponding to the 712 -> tt3* transition, The CD spectrum between 230 nm and 180 nm is complex and the spectrum indicates that at least three separate electronic tra n si tions may be present in this region. The overlapping bands are apparently of opposite sign and therefore not easily sorted out and resolved. I t appears though that the "doublet-like" features in the absorption spectrum may correspond to the negative CD systems in the appro priate energy region. The positive CD band centered at about 205 nm is evidently due to a separate transition. The absorption spectrum exhibits two weak shoulders (ca. 205, 199 nm) which quite probably correspond to this positive CD band. To higher energy, there exists an intense CD band around 163 nm that does not correspond to any well defined absorption band. The CD shoulder at approx. 152 nm may correspond to the intense absorp tion system peaking at 155 nm. Another electronic trans- tion around 143 nm is clearly present in the CD, and is positive in sign. The liquid phase absorption and CD spectra of a-phel landrene CFigs.l9A,19B,20), do not display any of the features ascribed to the gas phase transitions a't 213' and 84 189 nm. Except for three shoulders at 224 nm ( C g H -j 2 » Fig. 20 ), 203 nm (CgF^, Fig.l9B), and ^ 187 nm (C F ^ , Fig. 19B), the only outstanding absorption feature is the (■n^, ^ 3 ) state in the region 260-265 nm corresponding to the gas phase band at 259 nm. The position of this band in the more polar solvent, cyclohexane, is solvent shifted to longer wavelengths to a greater extent ( aa ^ 5 nm). The CD spectrum in cyclohexane is qualitatively similar to the spectrum in perf 1uoro-n-hexane, except for a slight ’ long wavelength shift. The negative CD band centered at about 260-265 nm corresponds to the low-lying ... absorption banddesi--gnated,,; ^ 3 ) • The next two higher energy positive CD bands (less resolved in cyclo hexane) may in fact correspond to weak shoulders in the liquid phase absorption spectra. The lowest energy band corresponds to a very weak shoulder at 224 nm in the absorption spectrum in cyclohexane solvent. The more intense CD system may correspond to the shoulder at ^ 203 nm observed in the absorption spectrum when the solvent is perf1uoro-n-hexane, Nevertheless i t is not entirely clear whether these CD bands represent different elec tronic transitions, or whether they are the same tra n si tion but overlapped by another transition of opposite sign. The negative CD band at about 162 nm, observed in 85 p e r -f1uoro-n-hexane solvent corresponds to the gas phase CD band found at the same energy. The photoionization spectra of a - and 3 -phellandrene (Figs, 17,21) were recorded only for the purpose of ob taining the adiabatic ionization potentials, corresponding to the removal of an electron from the highest occupied it -bonding molecular o rbita l. The spectra were carefully recorded point by point in the ionization "threshold" region and the f i r s t ionization potentials thus determined should be quite relia b le. The data points for the rest of the recorded . spectrum, however, were only intended to give a very qualitative picture of the photoionization efficiency as a function of energy. The I . P .'s are clearly indicated in the figures: 7.72 ev (160.7 nm) and 8.18 ev (J51.5 nm) for a-phel1andrene and 3 -phel1andrene respec-r t i v e 1 y . 86 CHAPTER FIVE THEORETICAL CONSIDERATIONS: ELECTRONIC STATES OF ETHYLENE AND BUTADIENE A. Ethylene The ethylene molecule, being the simplest hydrocar bon with a u-electron system has been the subject of numerous quantum-mechanical calculations. These calcula tions have been of an empirical as well as an a b-initio nature. The earliest theoretical calculations were made by Huckel [162□, Mulliken £63-66], and Penny £67,68]. Other semi-empirical treatments are due to Mulliken and Roothan £69], the zero-differential overlap treatments of Pariser and Parr £70,71], and the more recent calculations of Yaris, Moscowitz, and Berry £72], F.W. Watson et a l . £4], and a CNDO treatment by Hayashi and Nakajima £73] . Some non-empirical calculations which have attempted to describe the important spectroscopic features of ethylene include the calculations of Kaldor and Shavitt £2], M. Robin et al , £74 ,75], Buenker, Peyerimhoff et al. £ 8 ,1 0 ,76, 77], Dunning and co-workers £7 8 ], Basch and MeKoy £79], Schulman et a l . £80], Moscowitz and Harrison £81], and Hjalmars and Kowalewski £7,82.]. Without having to delve into the details of the more sophisticated theoretical calculations, a simple molecu 87 lar orbital scheme describing the low-lying electronic states of ethylene will now be r e v i e w e d . The specific theoretical results used for the experimental assignments will be discussed elsewhere. Ethylene is planar in the ground state and there fore the molecular orbitals and electronic states will be classified by the irreducible representations of the symmetry group 8 2 ^. The molecular coordinate axes used in the molecular orbital description of ethylene are shown in Fig. 22. This is in accordance with the coordi nate system used by Merer and Mul1iken[ TU and therefore the notation of these workers will be used throughout. The z-axis is taken along the C-C bond with the molecule in the y-z plane. The coordinate axes are assigned to the symmetry species as follows: b, = b , b~ = b , J j v -|u zu 5 2 u yu ^3 u = bxu' T* ie e^ec'tronic configuration of ethylene in its ground state as predicted by all electron calculations is then: O a g) 2 ( lb l u ) 2 ( 2 ag) 2 C2 bl u ) 2 (.lb2 u) 2 ( 3ag) 2 ( lb 3 g) 2 ( l b 3u)2 , ’ Ag. The agreement of the IP's from the Photoelectron Spec- trumC83H with the values of the orbital energies (Koop- mans Theorem) predicted by the a b -in itio calculations is good, and supports the a b -in itio orbital ordering. The f i r s t seven orbitals represent the a core, while the 88 < r* (C C ) c r*(C H ) cr* ( CH) I T cr ( C H ) cr (C H ) Fig. 22. Molecular orbital and symmetry identification of the upper filled and lower empty orbitals of ethylene. (United-Atom designations in Parenthesis.) Solid lines indicate electric-dipole allowed transitions and dashed lines indicate magnetic-dipole allowed transitions. 89 (3pcr) 2b9ll(3pTry) 4ag(3s) lb„ (3d?rx) 2 g eighth molecular orbital O b ^ ) is the ir-bonding orbital. In the notation of Mulliken[l H , the ground state is designated the N state. A simple molecular orbital scheme consisting of linear combinations of atomic orbitals (LCAO) is shown in Fig. 22. The MO' s depicted include atomic 2s and 2p functions on the carbon atoms and Is functions on the hydrogen atoms. This simple model will predict the symmetries of the low-lying valence states of the ethylene system. The pure Rydberg states can be described by LCAO's composed of atomic orbitals with principal quantum number >. 3, or by diffuse molecule-centered large-radius , atom-like functions. At this point i t should be pointed out that calculations which are augmented with a basis containing diffuse atom-like functions of higher prin cipal quantum number generally describe the lowest- lying excited states of ethylene as being Rydberg-1ik e . It should also be stressed that the naive MO scheme de picted in F ig . 22 is a diagrammatic simplification and that the virtual orbital energy ordering varies from calculation to calculation. As mentioned e a r lie r , the relative energies of the occupied molecular orbitals are well established by the comparison of various calcula tions with the photoelectron spectrum. It should also 90 be kept in mind that the orbital energies need not alone determine state energies, since the virtual (or direct Hartree-Fock energies) are subject to configuration interaction and electron correlation effects. The lowest energy -ft ■+ ir excitation is achieved by promoting an electron from the l b ^ occupied MO to the 1 b2 g unoccupied MO and will yield the states ^ l u ’ In this study the emphasis is solely on the spin-allowed excited singlet states and hence we wi11 not be concerned with any states of higher m u ltip lic ity . The "*A - -* ^B.^ ■jlp ( i t , tt ) e l e c t r ic d i p o le allowed state has been designated the V stated 1 I t should be mentioned that although the V state has been calculated to contain an appreciable degree of Rydberg character by various authorsC8 , 1 0 , 76-79H, experimental studies of this state have all resulted in findings which are inconsistent with a description of a diffuse upper orbital in this transition. Recently Mul1ikenD343 has suggested that the valence shell exci tation 3ag which he designates Va , is expected to mix somewhat into the ( t t , tt ) state which he designated; V . He concludes that this V - V mixing may well ac- TT TT CT count for the observed valence nature of the state. k The g -* ir (.lbgg) states, originating from the 1 b^g and the 3ag orbitals have symmetries ^B-j^ and re~ 91 spectively and are electric dipole forbidden. These states have been calculated to be s t r ic t ly valence shell in character [8,75,80] and also have been calculated to lie below or energetically close to the ( tt , - t t * ) state [73,75, 80,85] as well as appreciably above i t [4,72,76,78]. The excitation lb„ 4a„ gives rise to a elec- 3u 9 3 3u trie dipole allowed n -> a (CH ) state. Virtu ally all calculations in an expanded basis set predict this state to be Rydberg in nature and composed mainly of 3s atomic orbitals on the carbon atoms. Likewise the electric dipole forbidden ^ 2 g and ^ B -j g states, achieved by electronic ex- citation from the ir O b ^ ) a ( 3 b ^ u) and the ^ ^ ^ 3 u ) ^ o * ( 2 b2 u) molecular orbitals respectively, are calculated to be Rydberg-like in nature which is consistent with their united-atom orbital designations (Fig. 22). The " i 'jlf B2 g is calculated as the ( t t , a (CC^) 3 P0 Rydberg, and the 1 'k B , is identified with the ( i r , a , r u .) 3p Rydberg state. i g \c H; ^y At this point i t is worth noting that there have been se veral calculations solely aimed at calculating the Rydberg- states of the ethylene system [9,10,82,86,87]. It should be stressed that these Rydberg states calculated to be low-lying are classified Rydberg as far as the theore tical description of the state is concerned. The true test of the Rydberg character of an excited electronic 92 state is ultimately determined by experimental observa tions of the electronic states in high pressure gas- phase systems or in condensed phase media. Moreover, it has recently been suggestedI8 8J that in many polyatomic molecules including ethylene, strong mixing between valence and Rydberg configurations can be realized, and that in these mixed states, consisting of nei ther pure valence nor pure Rydberg character the assignment of the state as "valenceM or "Rydberg" on the basis of experi mental observations is less clear. * Even though the low-lying t t o states in ethylene are generally believed to be Rydberg lik e , Robin and co- workers[75J have expressed the opinion that a r e la t iv e ly . low-lying valence shell it -> a transition cannot be ruled out. This is especially true for highly alkylated.or cyclic olefins. Calculations by SchulmanC8 QU have pre- ★ dieted two valence t t a states to be close in energy to the ( t t , t t *) V state. It should also be mentioned that Watson and coworkersf4 D have calculated a rela tiv e ly ^ • low lying tt -> a valence state in ethylene and t h e i r computations on cyclic mono-olefins indicate that a-strain increases the contribution of it a valence states at lower energies. 93 B . 1,3 Butadi ene Butadiene is an important prototype molecule since i t represents the simplest hydrocarbon with a conjugated 7r-electron system. The theoretical si gni f i cance -of this chromophore is evident when one realizes that quantum theoreticians have been continually interested in the ground and excited electronic structure of this simple conjugated system since the early theoretical considera tions by Mul 1 i ken[22 ,23^]. The interpretations of Mulliken in terms of the four Huckel excitations have been reproduced by the more sophisticated semi-empirical c a l culations of Pariser and Parr.E8.9E], ParrE90j, Sannigrahi[91J, Leroy and JaspersE92J, and Allinger and MillerE93j to name a representative few. Configuration-interaction has more recently been found to be important in the description of excited molecular states, and many of the semi-empirical calculations have included some form of Cl (configuration i nteract i on 1 . Recently a b -in itio computational methods have become feasible for larger molecular systems and the f i r s t ab- in it io SCF-CI calculations for cis- and trans- 1,3 buta diene were performed by Buenker and WhittenE94D. Some other 1 1 ab-i ni t i 0 " cal cu 1 ations for Butadiene were reported by Clark and Csi zmadi aE95U , DumbacherE96I], Shih et a 1 C 9 7 □ , 94 and Hosteny et al£98,993. Schulten and Karpl usD ODD, as well as ptber-sD.OT]* have pointed out that the inclusion/") Of doubly-excited configurations in the Cl treatment 'fc leads to a significant energetic reordering of the ) electronic states in both the semi-empirical and a b-initio calculations. The most important result of these Cl cal culations including double-excitations (and higher) is the prediction of a lowest lying ^ electric dipole forbidden state in contrast to the single-excitation results which calculate a strong electric dipole allowed ^Bu state as the lowest lying singlet. The energetic re-ordering of these states of opposite parity, after inclusion of muli- ply excited configurations was f i r s t observed by Buenker and Whitten£94J in their a b -in itio study. The majority of the theoretical treatments of Butadiene have only been concerned with the tt electron system and have involved the hertofore popular theorem of 0 - t t separability. Up until very recently o «-*■ i t states were thought to l i e much higher in energy than the ★ (ir,7r ) states and also the 7r and a electronic structures were considered to be v irtu a lly independent of one another. This view is however rapidly changing and many WQrkers[73,85,94-96H have chosen to include o orbital excitations in their M O treatment of the Butadiene system. 95 An interesting feature of these calculations is the appearance of several a -> tt states among the tt -* i t 1 evels. 1,3 butadiene is generally thought to exist as a mix ture of two planar conformers, s-trans and s-cis, in mobile equilibrium. There has been much experimental evidence supporting the existence of the major conformer having a t ran sco pi anar con forma ti onD02-106l]. There is however evi- dence[107I] that the less stable conformer is probably skew- cis rather than cis-coplanar. In any event,gas-phase microwaye s tudi esQ04,108I] indicate that the "cis" concen tration is most-likely below 1 % in the equilibrium mixture at room temperature, A simple M O scheme depicting the molecular orbitals that will be involved in low-lying electronic transitions for both the cis and trans coplanar forms of 1,3 butadiene is shown in Fig. 23. The M O schemes for both the cis and trans forms are qu alitatively similar, however the selec tion rules for electronic excitation d iffe r considerably. The trans-planar geometry of butadiene belongs to the point-group C2^, whereas the cis-planar form has C£V symmetry. The ground state electronic configuration for trans- 1 ,3 butadi ene i s of course "*A and the following ab-i ni t i o£9,4, 98D or b i tal ordering is 9 6 TRANS C 2h 7bu CIS C 2V 7b, 2bg 2au Ibg lau 7ag 6bu cr cr ■2 a 2b, la2 lb, 7a 6b 2 I Y Fig. 23. Molecular orbital and symmetry identification of the upper filled and lower empty orbitals of trans-1,3 buta diene (C£h) anc* C1S-1>3 butadiene Solid Tines indicate electric-dipole allowed transitions. Dashed lines indicate magnetic-dipole allowed transitions. Double lines indicate transitions which are both electric-dipole and magnetic-dipole allowed. Oa ) 2 0 b ) 2 C2 a ) 2 C2 b ) 2 (3a ) 2 (3b ) 2 ( 4a ) 2 (4b ) 2 ^ g u g u g u g u x (5a ) 2 ( 5b ) 2 ( 6 a ) 2 ( 6 b ) 2 (7a' ) 2(la ) 2(lb ) 2 . g u g u 9 u g F o r t h e c i s - c o n f o r m e r , t h e o r d e r i n g of the o r b i t a l s is s i m i l a r e x c e p t t h a t the 6a-j a n d 5b 2 M O ' s ( c o r r e s p o n d i n g to 6a a n d 5b u o f t h e t r a n s c o n f o r m e r r e s p e c t i v e l y ) are r e v e r s e d . T h e h i g h e s t o c c u p i e d o r b i t a l s are ir-bonding o r b i t a l s and b e l o n g to t he s y m m e t r y r e p r e s e n t a t i o n s l a u a nd 1 b g . T h e r e m a i n i n g o c c u p i e d m o l e c u l a r o r b i t a l s m a k e up t he g- b o n d i n g n e t w o r k . S e m i - e m p i r i c a l c a l c u l a t i o n s g e n e r a l l y p l a c e t h e g ( 7a a ) M O a b o v e t h e tt (la ) o r b i t a l , w h e r e a s y u a b - i n i t i o m e t h o d s a l w a y s p l a c e t he g (7ag) b e l o w the tt (.la •). R e g a r d l e s s o f m e t h o d , the c a l c u l a t i o n s p l a c e t h e s e M O ' s f a i r l y c l o s e t o g e t h e r in a g r e e m e n t w i t h t he p h o t o e l e c t r o n s t u d i e s [ 8 3 ,1093- T he e n e r g e t i c o r d e r i n g of the 6a a n d the 6b M O ’s a r e a l so s o m e t i m e s r e v e r s e d de- p e n d i n g on the t y p e of c a l c u l a t i o n , b u t t h e s e o r b i t a l s are a l m o s t a l w a y s c a l c u l a t e d to be n e a r l y d e g e n e r a t e and m a y d i f f e r by o n l y a f e w t e n t h s of an ev o r so. It s h o u l d a l s o be n o t e d t h a t t h e M I N D O c a l c u l a t i o n s of D e w a r and W o r l e y E n O G p r e d i c t t h a t two a l e v e l s are e n e r g e t i c a l l y l o c a t e d in b e t w e e n t h e tt. (la ) a nd ir0 (lb ) o r b i t a l s . 1 u 2 g 98 T h e p h o t o e l e c t r o n s p e c t r u m i n d i c a t e s t h a t at l e a s t t h r e e m o l e c u l a r o r b i t a l s a r e f o u n d j u s t b e l o w the h i g h e s t o c c u p i e d t t o r b i t a l a n d t h a t all t h r e e o r b i t a l s a r e f o u n d in a p p r o x i m a t e l y a 2 ev e n e r g y r a n g e . T h e s e f a c t s le ad o ne to b e l i e v e t h a t w h e n i n v e s t i g a t i n g t h e e l e c t r o n i c s p e c t r u m of b u t a d i e n e , l o w - l y i n g ( g , i r * ) e x c i t a t i o n s m a y a l s o be p r e s e n t , e s p e c i a l l y f o r a l k y l a t e d or s u b s t i t u t e d s y s t e m s . Fo r t h i s r e a s o n ,• t h e MO s c h e m e s h o w n in Fig. 23 has b een ★ d r a w n to i n c l u d e l o w - l y i n g a t t e x c i t a t i o n s . / * \ T h e s i m p l e MO s c h e m e p r e d i c t s f o u r l o w - l y i n g (tt,tt } e x c i t a t i o n s . C o n s i d e r i n g t he t r a n s c o n f o r m e r f i r s t , we see t h a t the 1T3 ^ a u^ anc* t *ie t r a n s i t i o n s p r o d u c e ^ B u e x c i t e d s t a t e s w h i c h are e l e c t r i c d i p o l e a l l o w e d a n d m a g n e t i c d i p o l e f o r b i d d e n . C o n v e r s e l y e x c i t a t i o n s f r o m - tt (1 b ) -+ ir4 (2b ) a n d 7r 1 (la ) -»■ tr,(2a ) l ^ ^ y I u j u g i v e r i s e to d e g e n e r a t e (in t h e z e r o - o r d e r H u c k e l t r e a t m e n t ) s t a t e s o f s y m m e t r y w h i c h a r e e l e c t r i c d i p o l e f o r b i d d e n a n d m a g n e t i c d i p o l e a l l o w e d . T h e s e v a l e n c e ( t t f tt ) t r a n s i t i o n s h a v e b e en d e s i g n a t e d N -* V^, 1 ^ , V^, a n d V 4 by M u l 1i k e n [ 2 2J . W h e n t he d e g e n e r a t e or q u a s i d e g e n e r a t e ^A„ c o n f i g u r a t i o n s a re a l l o w e d to m i x and y m u l t i p l e e x c i t a t i o n s a r e i n c l u d e d in t h e c o n f i g u r a t i o n i n t e r a c t i o n (Cl) c a l c u l a t i o n t h e s e s t a t e s s p l i t a p p r e c i a bly a n d o n e ^A^ s t a t e ( d e s i g n a t e d is p r e d i c t e d to 99 l i e c l o s e to or o f t e n b e l o w t h e 1 ( N - * V J e l e c t r i c d i p o l e u 1 al l o w e d s t at e . F o r the cis c o n f o r m e r , t he -tt 2 (1 a ^ tt 3 C2_b 1 1 a n d ir - j (i 1 b . j ) -»■ tf 4 C2a 2 ) t r a n s i t i o n s g i v e r i s e to ^ B 2 s t a t e s w h i c h a r e b o t h e l e c t r i c - d i p o l e a n d m a g n e t i c d i p o l e a l l o w e d f o r t h e C 2 v g e o m e t r y . T h e 7r2 ^ a 2 ^ -rT^ ( 2 a 2 ) a n d tt .j (, 1 b i ) -> tt 3 C 2 b ^ ) e x c i t a t i o n s h a v e 1A s y m m e t r y a nd u n l i k e t h e s t a t e s f o r t h e t r a n s c o n f o r m e r , are n o w 9 e l e c t r i c d i p o l e a l l o w e d and m a g n e t i c d i p o l e f o r b i d d e n . A g a i n t h e s e q u a s i - d e g e n e r a t e s t a t e s are s u b j e c t to c o n f i g u r a t i o n m i x i n g and the r e s u l t s o f m o r e r i g o r o u s c a l c u l a t i o n s will be p r e s e n t e d in a l a t e r s e c t i o n . S i n c e the h i g h e s t f i l l e d <7 o r b i t a l s a r e s e e n to lie c l o s e to t h e l o w e s t tt o r b i t a l , as e v i d e n c e d by t h e p h o t o e l e c t r o n s p e c t r u m and t h e o r e t i c a l c a l c u l a t i o n s , v a r i o u s w o r k e r s E 7 3 ,8 5 ,94-93 h a v e c a l c u l a t e d t h a t l o w - l y i n g ic ic a tt e x c i t a t i o n s lie in t h e v i c i n i t y of t he tt -* it s t a t e s . In t h e t r a n s c o n f o r m e r , i n s p e c t i o n o f t h e c o r - ic r e s p o n d i n g M O d i a g r a m of Fig. 23 s h o w s t h a t a it e x c i t a t i o n s o f ^ A u a n d ^B s y m m e t r y w o u l d be e x p e c t e d . T h e s e r e p r e s e n t p a r i t y a l l o w e d [ e l e c t r i c d i p o l e ) g-u, a nd p a r i t y f o r b i d d e n g-g e l e c t r o n i c t r a n s i t i o n s r e s p e c t i v e l y . T h e ^B s y m m e t r y , h o w e v e r , w o u l d be m a g n e t i c - d i p o l e i c a l l o w e d . In t h e c i s - c o n f o r m e r t h e s e ( < 7 , i t ) s t a t e s w o u l d h a v e ^B^ a n d s y m m e t r y r e s p e c t i v e l y , t h e ^A^ b e i n g 1 00 g o v e r n e d by t h e s ame s e l e c t i o n r u l e s as t h e ^B' . s t a t e s l i k e t h e a f o r e m e n t i o n e d ^ 2 s t a t e s a r e b o t h e l e c t r i c and m a g n e t i c d i p o l e a l l o w e d . W o r k e r s w h o ha ve c h o s e n to i n c l u d e (a,ir ) e x c i t a t i o n s in t h e i r c a l c u l a t i o n s a l w a y s p r e d i c t the c ( 7a g ) 7r3 ( 2 a u ), 1 A'u s t a t e (^ B^ f o r c i s - ) to be v e r y n e a r t h e (irgjir 3) V s t ate. T h e s e c a l c u l a t i o n s a l s o p l a c e a (a,it*) ^B s t a t e ( ^ A 2 f o r c i s - ) b e l o w t h e (tt-j ,tt 4 ) s t ate. T h e m o s t c o m p r e h e n s i v e a n d r e l i a b l e a b - i n i t i o c a l c u l a t i o n s on b u t a d i e n e , p e r f o r m e d by S h i h * B u e n k e r a n d P e y e r - i m h o f f H 9 7□» p r e d i c t s s e v e r a l v e r y l o w - l y i n g ^A^ a n d ^B^ s t a t e s f or t h e t r a n s c o n f o r m e r a n d t h e c o r r e s p o n d i n g lo w- l y i n g ^ 1 and ^ 2 c i s - s t a t e s . T h e g a u s s i a n b a s i s set u s e d by t h e s e w o r k e r s w a s a u g m e n t e d w i t h d i f f u s e 3s a n d 3p f u n c t i o n s , a n d t h e s e s t a t e s w e r e of the rr2 + a t y p e w i t h c o n s i d e r a b l e R y d b e r g c h a r a c t e r . T h i s w o u l d s e e m to i m p l y / * t h a t t h e (ct,tt ) s t a t e s a n d n e a r b y tt2 R s t a t e s o f the s a me s y m m e t r y m i g h t a c t u a l l y be m i x e d to s o m e e x t e n t . T h e sa me c an a l s o be s a id a b o u t R y d b e r g a d m i x t u r e , i n t o t he V s t a t e s . A g a i n , a s . w a s d i s c u s s e d in t h e l a s t s e c t i o n {Ch. 5, Sec. A), it m u s t be r e e m p h a s i z e d t h a t the v a l e n c e or R y d b e r g c h a r a c t e r o f c e r t a i n " m i x e d " s t a t e s is not all t h a t c l e a r and t h i s s u b j e c t is c u r r e n t l y in a s t a t e of f l u x , w i t h no well d e f i n e d b o u n d a r i e s or g u i d e l i n e s e x i s t i n g . 1 01 Although not depicted in the MO scheme of F ig .23 , the pure Rydberg states originating from the highest occupied MO and converging upon the f i r s t ionization potential will be described b riefly. The molecular symmetries of atom-like Rydberg or bitals in the centro-symmetric trans conformer of 1,3 butadiene are; ns = a ; np = a , b : nd = a , b •, nf = Q U U y y a , b,,. Transitions to ns ("*B state) and nd (^A„, u u g y g states) have g-parity and are electric dipole forbidden but magnetic dipole allowed. Conversely, transitions to np or nf states either have ^Ay or symmetry depending on magnetic quantum number and spatial orientation. These u-states are electric dipole allowed and magnetic dipole forbi dden. The molecular symmetries for the Rydberg orbitals of the ci s-conf ormer are as follows: ns = a-| ; np = a^ , b-j , b2 ; nd = a] , a ^ , b] , b?; nf = a1 , b] , b2 - I t is apparent that except for the ns series which is electric dipole forbidden and magnetic dipole allowed, the rest of the Rydberg states have electronic components which will be both electric dipole as well as magnetic dipole allowed. In addition, the nd states have components which span all irreducible representations of C2 y ’ whereas the np and nf states do not have components which are to ta lly symme tric . 102 CHAPTER S I X assignments and DISCUSSION A, Mono^olefins The vacuum u ltrav io le t absorption spectra of the alkylated ethylenes are qu alita tiv ely similar to those of the parent molecule ethylene. Many of the features seen in the electronic absorption spectrum of ethylene (Figure 1) are also exhibited in the spectra of the mono olefins. However the spectra of the olefins are much more broad and diffuse and of course the higher energy region of the spectrum differs due to the presence of ic (.a, a ) transitions in the saturated part of the molecule. The CD spectra, on the other hand, possess features that are not at all evident in the absorption spectra. 1. Acycl i c-mono-ol ef i ns (.Gross & Schnepp [2 7 ]). (a) 180 - 170 nm ★ The T T ^ T T region in absorption is assumed to be dominated by the intense band near 180 nm. The assignment is ^Ag -> ■ ^ ^*32 g? ? 9nc* * 1£ls keen similarly assigned by others who Kaye investigated the absorption spectra of mono-oleftnic systems, This assignment serves as a "baseline" for the assignment of the other ethylenic 103 * trans i ti ons. The G r , u , }. absorption peak, along with the analogous; band in the CD does not shift appreciably with varying molecular chain length. This result may be accounted for by considering the fact that the (jr?Tr*l transition involves only out-of-plane orbitals, and there fore, one would expect l i t t l e interaction with the satura ted part of the molecule. Methyl-pentene and methyl - hexene exhibit small to moderate positive and negative CD bands respectively coninciding with the ( t t , t t *) absorp tion band. The CD of methyl-heptene exhibits, only a very weak positive shoulder corresponding to the t t it transition. The g-valuesassociated with these bands are small as expected for an electric-dipole allowed tra n si tion in the symmetric parent chromophore, ethylene. (b) 195 - 185 nm The CD spectra of the olefins to longer wavelengths are more complex. A weak CD band is observed near 190 nm in methyl-pentene and methyl-hexene, which corresponds to the lpw-energy ta il of the (jr,Tr*)„ absorption band. In methyl-heptene, however, the correspond!' ng low-energy band i;s f a ir l y intense and occurs near 185 nm. In all three olefins these bands are opposite in sign to the (jfr,n 1 CD bands, and are best assigned as the low energy 1 04 ethylene Rydberg transition designated t t 3 s (Jt>3L| -*■ The alternative assignment proposed pre viously [273, tt - a * ( S C I (.1 b 3 u - S b ^ , ] A -► although predicted to be low lying in energy by various a b -in itio ca1 cu1 ations C8 , 1Qj , is now thought to be quite an unlikely assignment. It should be borne in mind that the ir 3s Rydberg absorption in various olefins has been assigned and well characterized [ 1 ,74,75 ,111-1 1 3 j . Calcu lations by Watson and co-workers, however [43, have sug gested that the Tk3u 4a^ transition has appreciable Rydberg character but may in fact not be a "pure" Rydberg (n=3) species. In a CD study Drake and Mason [1143 have observed and characterized the ( tt , 3s) Rydberg transition in a-pinene, and have found i t to have appreciable rotational strength of. opposite sign to that of the C7r»Tr*) transition. The large g-factor they observed is explained on the grounds that the ( t t , 3 s ) transition makes only a small con tribution to the absorption. The term value observed for the low energy band in these acyclic olefins is estimated at 22,000-23,000 cm“^ and \s consistent with what one would expect for the 3s Rydberg state in an alkyl olefin E3.3, ' It is not under stood at this time why the ( tt, 3 s ) CD band in methyl - 1 05 heptene is so intense compared to the (tt,3s). bands observed in methyl-pentene and methyl-hexene, (c) 1 70 - .1 55 nm The positive CD bands peaking in the region 170-160 nm are assigned as tt -* a*(CH) Cl ^ 3 u ^ 2 b2 u’ ^ g ^ ^Blg^ in analogy to the assignment of similar CD bands in a - and g-pinene £26^. In all cases studied here, the absorption spectra do not contain separate bands corres ponding to these CD features; in fact, for methyl-pentene the CD peak very nearly coincides with a minimum in absorption. There is however some evidence for corres ponding absorption in the form of the asymmetric shape of the tt it bands which are broadened to shorter wave lengths. For all three compounds, the anisotropy factors (..g = A.e/e) of the tt -+ ■ a (CH), (_ B -| ) transition are large v compared to those of the tt tt transition (Tables 2-4). They are in fact greater by factors of 16, 10 and 57 for methyl-pentene, methyl- hexene, and methyl- heptene , respectively. The large g-yalue$ lend strong support to the assignment as a transition which is magnetic dipole allowed fn ethylene. Ab-initio calculations C8.,-77J for the excited electronic states of ethylene predict that both the 106 1 b3 u ^b2 u (-.so.metimes referred to as ^ -> tt^) and the tt - * a * ( - c c l O b 3 u -»■ 3 b l u ? -> ^ B 2 g ^ t r a n s i t i o n s 1 1 e * near the ^ transition but to lower energy, (Table 15 contains- a representative col 1 ection of the results of some calculations on the low-lying excited states of ethylene). Calculations for a twisted ethylene C 72 ,75J predict a large rotatory power for the tt + a*(CH) tra n s i tion and on this basis the proposed assignment is pre ferred. It is to be noted that all of the calculations, however, predict the lBi g» and states to be quite diffuse and Rydberg like (composed mainly of n=3 basis functions). In fact, the excited orbitals of these Bng states, in the united atom approximation are represented by UAO 3p functions. The strong positive CD band could then be characterized as a Rydberg-like state. Jhis possibility must be considered in view of the fact that o ' . ' the 1 795 A absorption system in isobutene II115□ has been assigned to the v 3p (a*) transition. It should be y pointed out however that the structure associated with this transition is quite discrete, whereas the intense CD bands in question are broad and diffuse. Rydberg state calculations have been undertaken for ethylene H9., 10J and for 2-butene E87J, The magnetic dipole-allowed 3p Rydberg states are again found to lie quite low in energy 107 TABLE 15. Calculated transition energies for ethylene (in ev). Excited State f Buenker Buenker & et ala Peyerimhoffb Fishbach et alc Robin , et ald Yaris et ale ( t t > o * ) [3s3# 1B3u 6.94 7.00 7.11 7.55 — (ir.o*) C3PTry] 5 ]B1g 7.50 7.63 7.66 8.39 8.31 ( i t * a) [3p0l ■ B2g 7.55 7.72 8.42 ( TT s 7T ) [ 3 d i r x 3 , B ^ 8.32 8.12 — 9.40 7.81 ( i t » o * ) [3dol 1B3u 8.50 8.61 — — {a(b3g)j / ) ''la 9.89 9.05 — 9.60 10.91 a. Reference 8 . b. + See Chpater 5; [ o 00 Reference 77. c. Reference 10. ] = UA O description. d. Reference 75. e. Reference 72. q 9 , 1 0 3 ,. T^e calculations, however, place a 3d state in an energy region which, would correspond to the observed positive CD band in question (.i.e., higher in energy than the ? r ir * C*Ag -*■' ^B-]U ) state) , A 3d assignment, however, is clearly impossible since i t would be e le c tric - dipole allowed and magnetic-dipole forbidden in ethylene. The term values determined from the observed CD maxima and the estimated ionization potentials (v e rtic a l) fa ll short of the expected range of term values for 3p Rydberg states in alkyl olefins [3J . Moreover, Rydberg states, by their very nature, would be expected to have l i t t l e rota tory strength, since the transition moments to these large diffuse states may be expected to be small . The positive CD bands in question are seen to shift appreciably to longer wavelength with increasing chain length (unlike the tt + tt band). According to Robin et al . [75J, an examina tion of the molecular orbitals indirectly suggests that the transition energies between valence shell a -*-*• ir states will be much more susceptible to substitution of the ethylene hydrogens by other groups than will be the Rydberg excitations:. At this, point we are inclined to believe that the very prominent positive CD bands in the acyclic mono-olefins are most probably of the mixed valence-Rydberg type (with an appreciable amount of 2p 1 09 valence character!, and are most lik e ly due to the 'Aa y 1 1 •Bj (or Bg■ ) excitations described e arlie r. The second member of the ns Rydberg series, 4s, is quite prominent in the. absorption of the parent molecule ethylene (F i g. 1, % 139 nml and has been assigned £26] in the CD spectrum of a -pinene, In the acyclic olefins the Cir,4s state) is expected to fa ll in the v ic in ity of 155 nm. A positive shoulder at 155-160 nm is observed in all of these olefins. I t is apparent that this shoulder is over lapped by intense CD bands of opposite sign to higher and lower energy, and the position of the underlying CD band cannot be located with certainty. An assignment of this CD band, would therefore be regarded as highly tenuous (d) 160 - 140 nm All three CD spectra exhibit an intense negative short-wavelength band of g-value, that is at least inter- ★ 1 mediate between the g-yalues for the tt tt , (. B^- } and ★ 1 tt -> g C.C-H), ( B-]gi transitions. It should be pointed out that the g-value determined here is only a minimum value and depending on the contribution to the absorption from higher energy states could be much larger, Analogous bands in tran$-cycl ooctene. and the pinenes have previously been assigned [ 2 6 ] as q ( C H) -*• n* , (1 b3g -* 1 b2g , 1 Ag -» ■ 1 B] g) 110 and the same assignment is proposed here, The original assignment was based on .theoretical calculat ions C 72 , 7 5H of rotatory and absorption strengths. This excitation is predicted to be s t r i c t l y valence shell C8v/75,78G in nature. The most recent ethylene a b -in itio calculations of Buenker and Peyerimhoff L77U predict this a tr* state to fa ll at 9.05 ev (see table 15) in good agreement with these alkyl olefins where corresponding transition energies would be expected to be somewhat shifted to lower energy. Other calculations also support this, valence state assign ment [4,80"]. It should be mentioned though that the a - bonding orbital discussed above (Tbgg in ethylene) is most probably delocalized onto the saturated part of the olefin as wel1 . I t has been pointed out by Robin [3,1161!, that the possibility of a transition originating from the pseudo t t orbitals of the a-alkyl groups in certain olefins, to the "regular" C = C t t orbital should not be excluded. In this regard i t is interesting to note that Zeeck [11711, who performed an a b -in itio closed shell MO calculation on propylene, observed that the methyl group interacts strongly with the t t _ electron system in the double bond, and th.at an appreciable amount of ir-character from the methyl group contributes to the occupied tr-molecular 111 orbitals, In an MO study on cyclic mono-olefins Watson and co-workers £4J predicted several low-lying i n ,- n 1 transitions involving p^-type orbitals on the ring. Both higher energy CD bands, however, shift appreciably to longer wavelengths with increasing chain length, thus indicating appreciable interaction with the alkyl chain, and pointing to the predominance of a character in at least one of the combining states. (e) Vibrational Structure In all three olefins, any discernable structure lacks sufficient resolution for definitive vibrational assign- ments. There is some structure on the (n,ir ) bands in the absorption spectra of all three compounds. The struc ture is progressively more blurred with increasing chain length.' The spacings are very irregular and range from ^ 1200 cm"^ to ^ 1800 cm"l. The ethylene ground state C=C stretching frequency (v^) has a value E1□ of 1623 cm~^. The C-C stretch has been observed and assigned in some excited states £3U of ethylene and alkyl ethylenes with a somewhat lower energy, i . e . , 1300-1500 cm~^, These assignments, however, have been confined to Rydberg states. For the ethylene (N -> V1 band, vibrational structure with somewhat irregular spacing between ^ 700 to -v 950 cm"1 ' is 112 best assigned as some combination pf the C = C stretching / C v 2) and -CH2 twisting .(v4l modes C !□ , Addi ttonal ly , Wars he! and Karplus E1]BU present calculations that indicate that twc to ta lly symmetric stretching mcdes as well as a tors i cnal mode are necessary to explain the vibrational structure of the N -» • V band. In view of these assignments, i t is plausible that the diffuse structure observed in the. ^ -n absorption for the olefins repre sents a superposition of vibronic bands which may include vi broni c ..structure, of the Rydberg n ■+ 3s excited state as well. The CD spectra of the olefins are generally lacking in any assignable structure. The it -* a*(CH) (lt»3u 2 b2 u^ transition in methyl-pentene, however, has three peaks superimposed upon i t , having an average spacing of ^ 1380 cm"^., This vibrational interval could be assigned as the C=C stretch. 2 . Cyclic-mono-olefins : a-pinene. When the substitution of the ethylene chromophore involyes more than merely an attached alkyl group, and results for example in a cyclic system such as cyclo^ heocene, the spectra s t i l l show the same general features, but in some cases exhibit some modifications. 113 Cal 2 2 0 - 190 nm The gas and liquid phase spectra of a-pinene are pre sented in Figs. 10-=-13 and Tables 5-8. The clearly defined gas phase absorption and CD band,s at 200 nm are assumed to represent the 1 Ag -> B u, Gt »it ) state and have been pre viously assigned along with the rest of the gas-phase spectra for this compound £26], A comparison of the gas and liquid phase spectra immediately reveals that the positive gas phase "doubl e t - 1 i ke" band at < y 2 2 0 nm is missing in the liquid phase. I t is also evident that the CD spectra in solution are somewhat complex and indicate that the apparent rotatory strengths of the CD bands are quite solvent dependent. This is interesting since the U>tt 1 band rn absorption is clearly located at about 2 0 0 nm in both gas phase and solution phase spectra, without any significant changes in absorption intensity. The CD of a-pinene was previously studied by Drake and Mason C1T4J. Their spectra, in various solvents and temperatures, were recorded in the 230 * 190 nm region. They concluded that the (..tt , 3s) CD band which they assigned to the 2 2 0 nm gas-phase system, shifted to higher energy with a decrease in temperature and that its positive con tribution could be accounted for in the corresponding spectra- They have proposed that i t is present at 49,000 114 cnT^ i n 3-methyl-pentane solvent at ambient temperature. One then might ask: about the identity of the shoulder at ^ 196 nm CFig. 1 2 , Table 7}« The spectrum in cyclohexane solvent (Jig. 13) is also consistent with their assignment, since a positive shoulder is observed at 'v 202 nm. Ac cording to their analysis then, our spectrum in per- f 1ugro-n-he^ane would seem to indicate that the 3s Rydberg band is present at about 2 1 0 nm. Even though the pinene CD spectra in the region of the s7 r ) absorption maximum are quite different from solvent to solvent, i t is puzzling that such a well- defined CD band should be observed at ^ 220 nm in all cases. W e believe that an alternative assignment should be considered, in which the 2 2 0 nm band is assumed to represent a ? -h- u valence transition. This transition is not yisible in the gas phase spectrum, except possibly as the dip observed in the center of the ( tt, 3s) band. This it assignment is consistent with the ORD results of Yogev et a l . Cl 191 who observed Cotton effects for severe! mono-olefins in cyclohexane solution at x > 2 0 0 nm. Their results indicate the presence of a weak valence state k at lower energy than the (tt , tt 1 state. In several condensed phase experiments, Robin and co-workers C75C, have, characterized the presence of a low- 115 lying g yalence shell transition in polycyclic mono olefins with, considerable, "g -s kel etal strain." In fact they have predicted that in biv cycl'ic olefins the g tt valence and n 3s Rydberg transitions are expected to be accidentally degenerate! This prediction is in excellent agreement with the assignment proposed above. Calculations on ethylene by various authors have * placed a valence shell g -> ■ ir transition [80,81 ,8511 in the v ic in ity of the u -> ^ transition, and specific calcula tions on cyclic mono-olefins by Watson et a l . [4[ have indicated that g strain will inevitably increase the contribution of g -.« -► tt transitions at low energy. All of the predicted low-lying a - * - + ir transitions are of g-g character or are weak el ectric-dipole states. This is in fact consistent with the g-values and rotational strengths observed for the 220 nm band (Tables 6 - 8 ). It is not feasible, however, to make a specific a or ic ■n a valence state assignment. Moreover, all a orbitals Will inyariably be extensively delocalized to include the ethylene chromophore as well as the saturated hydrocarbon portion of the molecule, (_bI 1 90 - 1 60 nm The high energy positive CD band observed at a, 175 nm 116 in a-ptnene in perf1uoro-n-hexane solvent (Fig. 11), is assigned to the. u + o*(CH) (163u -> - 2t>2u, ^ ^B ) 9 I g transition. This band corresponds to the intense positive CD system at 1 70-1 60 nm in tile acyclic olefins previously discussed. As can be seen from the gas-phase spectrum (Figure 101, this band has been solvent shifted by ,about 3000 cm”^ to higher energy and has broadened considerably. Some corresponding structure has also been smoothed out in the absorption spectrum. This is consistent with some Rydberg character being attributed to.this transition, but we feel that the fact that the CD band has not been completely washed out, argues in favor of some valence character in the upper state molecular orbital (See Ch. 6 , sec. A(l).:). The large g-factor, as compared to the ir + tt transition (Table 6 ), ju s tifie s the magnetic-dipole allowed assignment for the excited state. Higher energy transitions in the gas phase absorp tion and CD spectra of a-pinene are discussed jn the a rtic le by Mason and Schnepp [26H and will not be reviewed here. B. Dienes 1, g-Phel 1 andrene (.trans-butadi ene), The absorption spectrum of g-phel1andrene, a cyclic 117 monpterpene containing the trans-butadi.ene chromophpre , CFigs. 14-v 1 6 ) ? q u alitatively resembles the electronic absorption spectrum of the parent molecule 1,3 butadiene (Fig. 2). The gas phase; spectrum is shifted some 3000- 4000, cm'^ to lower energy and is much more diffuse and less structured as one would expect for an alkylated chromop hore. (a) 22 5 nm The very intense absorption band centered at ^ 225 nm (e 'v , 2 0 , 0 0 0 ) is assigned as the tt ? (1 b ) '-*■ iT*3 ( 2 a ), ^ 9 ^ 1Ag 1 valence transition. This assignment is widely accepted for trans-dien.es and is substantiated by molecular 1 * orbital calculations which predict the 1 (7T,T r ) state to be very intense,(f ^ 0.4). The gas phase absorption spectrum to higher energy is not very informative, excep ting some barely, resolved structure. The CD spectra contain a positive band which cor responds to the 1 1 Bu state, and is assigned accordingly, the badly blurred structure observed on the ^Ag 1 ^Bu absorption and CD bands is assigned as vibrational struc ture in analogy to the assignment in the parent molecule, 1,3 butadiene. The vibrational interyals are approximately 1 300-1500, cm'^ , which, is reasonable for the C = C stretching 118 vibration (v^.), Shih et a l , J ~ 9 7 J , suggest that the -CH2 twisting mode Cv.12). 4$ well as the C-C stretch. are probable vihromc features in the 1 Bu state, C ; bl 210 - 150. nm; Rydberg State Assignments The distinct positive vibronic features in the gas phase CD spectrum (Fig. 14, Tables 9A, 9B) can be divided into the 200 nm region and the 170 nm region.. The positive CD peaks correspond to the absorption peaks and these features are not observed in solution thereby justifying Rydberg state assignments. The 3 bands in the 200 nm region are assigned as a 7r2(lb ) -> 3p, electric dipole allowed vibronic progression. The spacings are ^ 160Q cm”^ and ^ 1450 cm“^ respectively, both of which are in the neighborhood of the expected C=C stretching mode. Recently, McDiarmid C201 has assigned the origin and several vibronic features of 3p Rydberg States to several f a ir ly intense discrete bands on the high energy wing, of the N -> (_tt2 „tt ) absorption band in 1,3-butadiene. An analysis by Robin C3J is also con sistent with this assignment, Jn a multiphotpn ionization Study of 1 ?3-butadiene by p,M, Johnson 121.1, i t is sug gested that the 1 Au 3P Rydberg state (.admixed with valence character)' is found in the same : spectral region, Further 119 more, calculations by Shih et al. [97J, place two diffuse 1 1 3p-1 ike Au states very near th.e 1 By state, (Table 16 depicts the energetic results of some pertinent theoretical calculations on trans-1,3 butadiene). The allowed nature of the 3p state is exhibited by the f a ir ly intense vibronic components :in the absorption spectrum. The 170 nm Rydberg system is character!' zed by f a ir l y intense positive CD bands (strongly overlapped) and l i t t l e corresponding absorption. Although the gas-phase bands are too badly overlapped to extract any quantitative ani sotropy factors (g), the 170 nm Rydberg system is assumed to be magnetic-dipole allowed in the parent chromophore, butadiene. This system is assigned to a ir?(lb )• -> 3d ^ . y Rydberg state, which has not heretofore been positively identified in butadiene i t s e l f due to its electric-dipole forbidden nature, Recent (.tt,tt ) a b -in itio calculations on trans-butadiene, by Hosteny et a l . [98H, have predicted two nearly degenerate 3d Rydberg-like 1A states with term values of about 1 0,000 cm"/*. The spacings of the six vibronic-1ike features attributed to an electronic 3d state, are quite irregular and range from 1 350-1 750 cm" 1 , McDiarmid's vibronic Rydberg state analysis of 1,3-buta diene C20J is dominated by the occurrence of the C-C stretch C .V 4 , T630 cm"1) and the C-H in plane bend (vg, 1 20 TABLE 16. Calculatec transition energies for trans 1,3 butadiene (in ev). Excited State+ Buenker & Whittena Shih , et al 13’ 5 Hostenyr et alc’ Pariser ■ ... & Parr* 3 Schulten et ale Dumbacher^ * (it s T T ) , 1 ’bu 10.2 6.60 7.05 6.21 6.00 (Bu+) 8.28 ( t t » 7 T ) s 2 \ 7.69 6.67 6.77 7.87 5.81 (Ag- ) 7.18 (ir s ir ) > 3 . \ 12.9 7.79 7.82 8.51 7.87 (Ag+) 10.4 (irsir*), 2 1BU 15.3 7.98 8.061 1 9.50 ---- 12.6 (a sir ) s ’ a U 10.2 — — ---- 9.82 (a sir*), ]B g 13.2 — — — ---- 12.2 (ir s 3 s ) , V -— 6.24 — — ---- ( it ,3pi) , ' a Mu 6 ,50,' 6.76 --- ---- — ( IT s IT ) s 4'Ag -T- 7.871 1 ---- — a. Reference 94 f. Reference 96 + See Chapter 5 . b. Reference 97. c. Reference . 6 Augmented with diffuse basis. 98. d. Reference 89. e. n Diffuse 3d UAO. Reference 123. ro 1 280 cm"."*:). The two other vibrations that were needed to f i t the vi bron i c structure were the C-C-C deformation vg, 512 cnT^l and the asymmetric -CH,, twist (v-^* 522 cm“^). Due to the appearance of rel ati vel y broad (i nt-ri ns i ca 11 y ^unresolved due to large perturbations-.) vibration-like features., in the spectra of 3-phel 1 andrene, any specific vibrational assignments are unwarranted- The structure Observed for the 3d Rydberg state (and the 3p state as well) are most lik ely due to vibrational structure, as demonstrated 'in McDiarmids' Rydberg analysis. It must be borne in mind, that the intrin sic broadness observed in this case, can easily mask closely spaced vibronic bands- as well as the various possible electronic components (3p , 3p , 3d , 3dz , e t c .) , and higher members of the x ■ y At Rydberg series'. I t is understandable then that only gross electronic state assignments are reasonably ju s t i- f i ed. Further evidence for the 3p and 3d assignments pro posed, is supplied by the experimental term values and quantum defects calculated for these Rydberg states. The photoionization spectrum (Fig- 17) indicates an adiabatic ionization potential of 66,000 cnT^, I f one assumes that the f i r s t band in each region represents the Rydherg electronic origin, the 3p state has a term value 1 22 of 16,700 cm-1 and a quantum defect < s = 0.44. On the other hand, the 3d state has a term value of 1 1 ,300 cm"-' and 5 = -0.12. These are entirely reasonable Rydberg para meters for a molecule of this size [13,12011. A negative quantum defect often occurs for nd Rydbergs, and results from perturbations in the molecular (non-hydrogenic) system. It is also reasonable to expect Rydberg vibronic structure since the ir^ orbital ionization in the photo electron spectrum of butadiene £83,1093, displays vibra tional intervals of 1500 cm- 1 , 1200 cm"1, and 520 cm- 1 . The CD spectrum of 3 -phel1andrene does not display any convincing evidence for the magnetic-dipole allowed tt2 3s state, on the high energy wing of the N -*■ V-j absorption band in butadiene that has been assigned and characterized by various workers £3,19-213). This is reasonable, since the 3s Rydberg has the highest quantum defect and would be expected to penetrate most deeply into the core of the molecular framework. The interaction with the highly substituted chromophore could in fact perturb i t in such a way so that i t is smeared out and broadened beyond recognition. Moreover, i t could easily have been shifted closer into the 1 1 Bu state making detection even more d i f f i c u l t . 123 (cl 210 - 150 nm; Valence State Assignments The liquid phase. CD spectra of g-phell andrene (Jigs. 15,16} exhibi-t the presence of at least two valence bands in the 210-150 nm region, sfnce Rydberg structure is smeared out in the 1 tquid phase., A comparison with the gas-phase CD spectrum (Jig. 14). indicates these bands to be negative in sign and overlapped by the 3p and 3d Ryd berg systems described earlier-. The absorption spectrum in perfluoro-n-hexane solution clearly indicates the presence of a moderate electric-dipole allowed valence band at ^ 185 nm. In this regard i t is interesting to note that the absorption spectrum of polycrysta11ine 1,3- butadiene in a solid film at 25°K [3J also reveals a valence band situated in the same spectral region ( i . e . , 175 nm)- The weak CD band observed as a broad shoulder at % 195 nm, is characterized as an electric dipole a l 1 owed and magnetic dipole forbidden state since its g value (Jabl es 10,11 ) is close tp that observed for the 1 state Cwhich is known to be electric-dipole allowed and magnetic-dipole forbidden). This band could correspond to the absorption at about 190-180 nm, but since the CD band is only present as a weak shoulder i t is d i f f ic u l t to estimate its position and band shape. I t is tempting to assign this state to the a(7aa ) + Tr% ( 2 an), 1A_ -* ^A y j u g u 124 valence transition. Several '-ab-initio calculations on butadiene I9.4-.96J have predicted that this state lies v * | just to higher energy of the 1 ''B -state (see Table 16).. CNDO calculations [73,85j have reproduced the same energy ordering. Furthermore, Robin [3D has expressed the opinion that the g ( 7 a ) -> n -,(2a..) state is present in 1 ,3-buta- s , y j * u diene somewhere in the v ic in ity of 60 , 0 0 0 cm"^ , and ★ I Dumbacher C96J, who calculates a low-lying (g ,-tt ) Au state for butadiene (Table 16), has also suggested that 11 ^ the 'A ^ ,7T ^ transition may be found in the same . 9 spectral region. This result seems ju s t ifie d , since the a(7ag) orbital ionization potential is found to lie quite near the tt-j O ay ) orbital ionization potential C83 ,1 09J. I t should be mentioned, however, that the a MO' s in a sub stituted diene will in fact be a combination of the a MO1s of the diene chromophore i t s e l f , and the saturated portion of the molecule,. That is to say, all bonding a-orbitals will be appreciably delocalized. Since 3p Rydberg states of symmetry ^A are expected £97H to fall in the same energy region, and since we have identified an overlapping Cbut distinct) Rydberg 3p state, we would conclude that i f the symmetry assignments are indeed the same (/*Aul* then configuration mixing could occur but the resultant states may s t i l l be predominantly 125 valence or Rydberg in character. In fact for ethylene, M O calculations for. the lowest a -> tr and the. G r ? ir ) states, using an expanded basis set 18,751] haye predicted that the (a s t t * ) states are s t r ic tly valence shell, whereas the Crr»7r*). states are calculated to be somewhat diffuse. This would seem to indicate that C a ,-ir ) states, calculated to have a somewhat contracted outer MO, would not be expected to mix strongly with any nearby Rydberg state, even i f i t were of the same symmetry. Although a ’ c r (7 a g -n-g assignment for the 1 90-1 95 nm CD band is very probable, another assignment, which cannot be excluded would be the electric dipole allowed 2 r u state. This state, calculated to be somewhat diffuse (Table 16, Shih et al . , Hosteny et a l . ) , has been pre dicted to li e some 1 . 0 - 1 .4 ev higher in energy than the 1 state. Kuppermann and co-workers E13H who have studied the electron impact spectra of 1 , 3 butadiene, have assigned a prominent band at 7.3 ev (170 nm) to the 2 ^Bu el ectri c-di pol e allowed stat.e. The band in the electron impact spectrum (assigned to 2 ^B^.) could in fact correspond to the weak negative CD band in g-phel- 1 andrene at 0-19.5 nm.. I t should be mentioned that the electron impact band at 170 nm, does not exactly cor- respond to the. optical Rydberg features seen by McDiarmid 1 26 Q2CT], thereby suggesting the presence of more than one state in that energy region* If., however, the weak nega- tiye CD band is assigned to the g (7an) ^ tt - transition, y ° i t is possible that the electric-dipole allowed 2 ^Bu state could easily be buried under the strong broad intense negative CD system in the 150-185 nm region. The mo.st intense feature in the CD spectrum of 6 -phel’l andrene is the broad negative system in the 185-150 nm region. In the gas phase this system is centered at ^ 170 nm5 i and is strongly overlapped by the positive 3d Rydberg transition. The liquid phase spectrum in per- fluoro-n-hexane (.Fig. 15), reveals the contour of this strong valence system, (unperturbed in solution) to about 160 nm. Maxima ,are evident at about 170 nm, and at about 160 nm.. The anisotropy factor, measured at 170 nm [Table 10). is at least an order of magnitude greater than that obtained for the 1 transition. This, we believe, u ’ is.evidence for the existence of a magnetic-dipole allowed and electric-dipole forbidden state in the parent chromophore trans-1,3-butadiene, Semi-empirical as well as a b -in itio calculations •k I (Jable 161 place the (..t t, 1 3 'A excited state (designa- y ted by Schulten et a l < [100,1213, in the spectral region which corresponds nicely to the gerade state we 127 ^ 1 * 1 observer around 170 nm. The 2 ' A g ( tt ,n ) state ( A g ~ ) is calculated to lie close to or below the 1 . Further- u more for non-symmetry reasons, this state is predicted to have vanishing (or very small) one electron transition moment matrix elements. Therefore, i t is expected that in an absorption or CD experiment this state could easily remain undetected. A closer inspection of the gas phase and liquid phase spectra leads one to believe that there could be more than one magnetic-dipole allowed state in the 185- 150 nm region. The full width at half-maximum (FWHH) for this broad negative system is estimated to be ^ 1 0 , 0 0 0 - 12,000 cm"^, whereas the 1 ^Bu has a FWHH of ^ 5,000-6,000 cm'^. Additionally, the liquid phase CD continues to increase to shorter wavelengths of the shoulder at 170 nm, and although the lim it of measurement is ^ 158 nm, a com parison with the gas-phase spectrum indicates that the presence of another magnetic-dipole allowed state around 160 nm is possible. Since the ^Ag state is expected to have vanishing or small transition moments, and since i t is predicted to be quite low-lying, i t cannot be assigned to a strong CD band in B-phel1andrene. I f there are indeed two strong magnetic-dipole allowed bands in the 185-150 nm region, the other contributing state could 128 be assigned as a o + ir, state. The MO scheme for 9 r - ; s butadiene (Ch. 5; Fig. 23) predicts several a ' -n type transitions of symmetry which would be expected to fa ll in the neighborhood of the ( tt, tt* ) states. MO calcu lations which have included a -+ tt excitations of ^B 9 symmetry [73,94-96], consistently plate them close to a ( t t , t t ) Ag state and below the N V4 , (tt, t t ) transition. ★ 1 Further support for the presence of a ( tt, tt ) Ag state in the 185-150 region is provided by the fact that calculations of rotational strengths for an optically active diene system [60,61,122,123] predict that the ^A 1 * and 'B ( tt, tt ) states are of opposite sign. Our assign ments are therefore consistent with this prediction. (d) 150 - 135 nm The CD above ^ 150 nm reveals the existence of other excited states. This region, however, occurs beyond the f i r s t ionization potential (-^ (Ibg ), 6 6 , 0 0 0 cm- ^, Fig. 17). Any excited state assignments in this region would be regarded as highly tentative since such states could in fact be coupled to the ionization con tinuum. Furthermore, at this energy, transitions of * the o a type would be expected. In view of the above, the higher energy CD bands will not be discussed. 129 2 . a - Phel 1 andrene (ci. s- but^di ene) (a) 260 nm The absorption and CD spectra of a -p h e l1andrene are given in Figs, 18-20 and Tables 12-14. It is immediately apparent that the spectra of this cis-diene (achieved by ring closure), are quite different 'from the trans-diene system. The ^ ( la 2.) ^ (2b^ 1A-j ^B2 transition (.e ^ 3500) corresponds to the absorption and CD bands centered approximately at 255-260 ntm , As predicted by several calculations on the cis-conformer of butadiene, C9 3 ,9 4 •, 96', 97;, 1 0 Ij the 1 ^B2 state is 1 ong-wa vel ength shifted and reduced considerably in absorption intensity relative to the trans-geometry. Table 17 contains some theoretical results for various calculations on the cis conformer of butadiene. The assignment of this fi rst band as the (tt , tt * ) 1 b2 transition is well founded E3J for cyclic dienes, and the position and intensity agree well with the recent vapor-phase absorption spectrum of 1,3 cyclohexadiene C124H. (b) 220. - 180 nm; Rydberg State Assignments The gas phase absorption spectrum to higher energy reveals two "dgubl et^l i, keu bands at ^ 213 and 'v 189 nm respectively, The gas phase CD spectrum in these regions is however quite complex due to severe overlap of neigh- 1 30 TABLE 17. Calculated transition energies for cis-1,3 butadiene (in ev). Buenker & Shih, Pariser Excited State Whitten et alb»n & Parrc Dumbacher (t t » 7 T ) j i ]b2 9.38 6.35 5.91 7.3 (iT jiT T )'» ro 7.54 6.66 8.29 7.1 * (t t **t t ) » 3 A1 12.9 7.89 8.34 11.'1 (ir,IT*), 2 ]B 15.4 ' 7.11 9.25 13.3 (a 57t*) 5 Tb, 10.2 — — 9.6 * (a » t t ) j l A 2 13.4 — — 12.9 (it >3s ) , — ■6.14 — — (ir >3p) , — 6.71, 7. 26 a. Reference 94. b. Reference 97. c. Reference 89. d. * Reference 96. ^ See Chapter 5. n Augmented with diffuse basis. (j> Estimated from Fig. 4, ref. 96. boring electronic transitions. Comparison of the gas phase spectra to the liquid phase spectra immediately establishes the Rydberg nature of these bands. As for g-phel1andrene , i t appears that both Rydberg states have the same sign (negative in this c a s e) i n the CD. The lowest energy Rydberg system (210-220 nm) is f a ir ly prominent in the absorption spectrum and is assigned as being electric-dipole allowed in the parent chromophore. There is a corresponding CD band but i t is strongly over lapped by another CD band of positive sign. It should be mentioned that the corresponding structured absorption band in 1,3 cyclohexadiene has also been assigned to a Rydberg transition q 3,124]. Since the ionization potential for a-phel1andrene has bedn determined (Fig. 21), a term value of < v , 1 6,000 cm"^ and quantum defect, < 5 ^ 0.4 are computed. This band is therefore assignedto a ^ (1 a2 ) * Rydberg state which is electric-dipole allowed as well as magnetic- dipole allowed in the cis-diene chromophore. This assign- ment is consistent with the calculation of Shih et a l . C97] who place two ^ 2 3p, ^B-j states about 3,000-7 ,000 cm- ^ ★ 1 above the lowest ( tt , tt ) B2 state (see Table 17). The higher energy Rydberg system (195-185 nm) is also f a ir l y strong in the absorption spectrum. The negative Rydberg CD peaks at 195 nm and 190 nm are not easily iden t if i e d since they overlap the broad positive CD system 1 32 ranging from 210-185 nm (compare Figs. 18B, 19B). Further more they do not exactly correspond to the Rydberg peaks in the absorption spectrum. Their Rydberg nature, however is amply verified by their disappearance in solution. Taking the lowest Rydberg band 195 nm, CD) a term value of -v 11,000 cm-1 and quantum defect 6 ^ -0.17 are computed. Again, the negative sign is indicative of valence shell or other excited state perturbations. The above values seem to imply that these bands in the 190 nm region can be best classified as 3d Rydberg. These bands are also in the same region (relative to ionization potential) as the corresponding ^-*-3d excitations in 0 -phellan- drene. In the cis-diene geometry, however, the 3d states (5) can be electriC-dipole as well as magnetic-dipole allowed C B ^ , B 2 ) or el ectri c-di pol e allowed only (A-| ) or magnetic-dipole allowed only (A2 )- The observed non correspondence of the absorption and CD bands assigned to tt2 3d, then could be explained by assuming that the absorption and CD are indeed due to 3d excitations but that the electronic components excited (3dyZ, 3dz , etc.) are different in each case. The assigned 3p and 3d bands exhibit some structure (1,100-1,300 cm- ^) in absorption. It is probable that this structure is vibronic in nature. 1 33 (c) 230 - 185 nm; Valence State Assignments The liquid phase CD spectra (Figs. 19A, 19B, 20) clearly exhibit at least two positive valence bands in the 230-185 nm region. The estimated g-values (Tables 13, 14) are on the order of that observed for the 1 tra n si tion. This would seem to indicate that these bands would be electric as well as magnetic-dipole aliowed. However, * 1 since we are comparing all g-values to the (iT,Tr ) 1 band which is both electric as well as magnetic-dipole allowed, i t should be pointed out that this comparison of g-values for excited state assignments is not revealing since in this case the magnitudes of the electric and magnetic moments could vary widely. The very weak CD band at ^ 222 nm in p e r -f1uoro-n- hexane (not as well resolved in cyclohexane) could be assigned to the optically forbidden A^ [ 1 0 0 , 1211] (tt,tt*) ★ I state predicted to fa ll near the lowest ( tt, tt ) B2 state (see Table 17). In the absorption spectra in cyclohexane solvent, a very weak feature is observed which corresponds to the CD maximum at the corresponding wavelength (^ 2 24 nm) . In analogy to the trans-compound, B-phel1andrene, we have assigned the positive CD band at ^ 200 nm to the a(7ai) -*■ it 3 ( 2 b-|) transition. 134 (d) 180 - 1 50 nm The gas and liquid phase spectra both, exhibit a very intense CD band centered at about 160-165 nm. Since there is no corresponding well-defined absorption band i t is d i f f ic u l t to estimate the contribution of this band to the absorption spectrum.- Since there is good reason to believe that most of the absorption is due to the strong band peaking at ^ 155 nm^ we feel that the 162 nm CD band is magnetic-dipole allowed and electric-dipole forbidden in the cis-butadiene chromophore,. The only exclusively magnetic-dipole allowed states possible are the c ■*-+■ tr valence states of symmetry (Ch. 5; Fig. 23). In fact, like the trans diene, relatively low-lying a it states have been calculated to li e in the v ic in ity of the (Tr, Tr*) states (Table 17). However, since the contribution to the absorption is uncertain, the assignment to the ★ 1 171 (Ib-j) tt 4 ( 2 a2) 2 B2 state cannot be excluded. Finally, the presence of the electric-dipole allowed A-|+ cis peak is verified by the rapid increase in absorp tion between ^ 170-16Q nm. The absorption maximum at about 155.nm is assumed tp represent mainly this strongly allowed A-j+ (Ag+ in transl state, This state is predicted to fa ll in this energy region (Jable 17), and has been assigned in a number of cyclic cis-di.enes €3,1,253. As expected for a magnetic-dipole forbidden transition the corresponding CD is small. ' 1 35 REFERENCES 1. A. J . Merer and R.S. Mulliken, Chem. Rev. £9, 639 ( 1 969) . 2. U. Kaldor and I. Shavitt, J. Chem. Phys. £ 8 , 191 (.1 968) . 3. M.B. Robin, "Higher Excited States of Polyatomic Molecules" (Academic Press, New York, 1975), Vol. I I . 4. F,H. Watson, J r ., A.T. Armstrong, and S.P. McGlynn, Theor. Chim. Acta. J_6 , 75 (1 970). 5. W.C. Price and W.T. 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Creator Gross, Kenneth Paul (author)

Core Title Absorption and circular dichroism studies of the ethylene and butadiene chromophores

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