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UBC Theses and Dissertations

Pes studies of some five-membered ring compounds Bain, Alex D. 1972

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PES STUDIES OF SOME FIVE-MEMBERED RING COMPOUNDS BY ALEX D. BAIN B.Sc. U n i v e r s i t y of Toronto, 1970 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA May, 1972 In present ing t h i s thes is in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y sha l l make i t f r e e l y a v a i l a b l e for reference and study. I fu r ther agree that permission for extensive copying o f th is t h e s i s for s c h o l a r l y purposes may be granted by the Head of my Department or by h is representa t ives . It is understood that copying or p u b l i c a t i o n o f th is t h e s i s fo r f i n a n c i a l gain sha l l not be allowed without my wr i t ten permiss ion . Department of Chemistry .  The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada Date - i i -ABSTRACT Recently there has been considerable inte r e s t i n studying the interactions of o r b i t a l s which were formerly thought of as l o c a l i z e d and i s o l a t e d . The technique of Photoelectron Spectroscopy (PES) i s well suited for t h i s and PE spectra are presented here for the series of f i v e membered r i n g molecules consisting of tetrahydrofuran, Y-butyrolactone, 2,5-dihydrofuran, y-crotonolactone, 1,3-dioxolane, ethylene carbonate, vinylene carbonate, JJ-cyclo-pentene 1,3-dione, maleimide and maleic anhydride. In these spectra the IP's of the carbonyl oxygen lone pairs and the higher IT l e v e l s have been assigned and trends i n these IP's established i n the s e r i e s . For the carbonyl oxygen lone p a i r , the s h i f t i n IP caused by various groups (oxygen atoms a to the carbonyl, double bonds i n the r i n g and addit-io n a l carbonyl groups 8 to the one i n question) are found to be approximately additive, which suggests that inductive ef f e c t s are important. However, the pattern of TT IP's must be r a t i o n a l i z e d i n terms of both inductive and resonance e f f e c t s . F i n a l l y some correlations with UV spectra for the l a s t three molecules are presented. - i i i -TABLE OF CONTENTS Page CHAPTER ONE:. INTRODUCTION 1 CHAPTER TV/O: THEORETICAL CONSIDERATIONS 4 2:1 I n t r o d u c t i o n 4 2:2 Shapes of. the Bands 6 2:3 P o s i t i o n s of the Bands 12 2:4 I n t e r a c t i o n s of O r b i t a l s 17 CHAPTER THREE: EXPERIMENTAL 21 3:1 I n t r o d u c t i o n 21 3:2 I n d i v i d u a l Molecules 25 3:2:1 T e t r a h y d r o f u r a n 25 3 : 2 : 2 y - B u t y r o l a c t o n e 25 3 : 2 : 3 2 , 5-Dihydrofuran 28 3:2:4 y - C r o t o n o l a c t o n e 30 3:2:5 1 , 3-Dioxolane 32 3:2:6 E t h y l e n e Carbonate 32 3:2:7 V i n y l e n e Carbonate 37 3:2:8 4-Cyclopentene 1 , 3-dione 39 3:2:9 Maleimide 43 3:2:10 M a l e i c Anhydride 45 3:3 D i s c u s s i o n 48 3:3:1 I n t r o d u c t i o n 48 3:3:2 Behaviour of the Carbonyl Lone P a i r s 50 - i v -Page 3 : 3 : 3 TT L e v e l s 53 3 : 3 : 4 The D i c a r b o n y l Compounds 66 3 : 4 C o n c l u s i o n 73 REFERENCES 75 - V -LIST OF TABLES Table Page 3 : 1 IP's of Y-Butyrolactone 2 7 3 : 2 E l e c t r o n D e n s i t i e s of Y - B u t y r ° l a c t o n e ••• 2 7 3 : 3 IP's of 2 , 5 - D i h y d r o f u r a n 2 9 3 : 4 • E l e c t r o n D e n s i t i e s of 2 , 5 - D i h y d r o f u r a n .. 2 9 3 : 5 IP's of y-Crotonolactone 3 1 3 : 6 E l e c t r o n D e n s i t i e s of y-Crotonolactone .. 3 1 3 : 7 IP's of 1 ,3-Dnoxolane 3 4 3 : 8 E l e c t r o n D e n s i t i e s of 1 , 3 - D i o x o l a n e 3 4 3 : 9 IP's of Ethylene Carbonate 3 6 3 : 1 0 E l e c t r o n D e n s i t i e s of Ethylene Carbonate 3 6 3 : 1 1 IP's of Vinylene Carbonate 3 8 3 . 1 2 E l e c t r o n D e n s i t i e s of Vinylene Carbonate 3 8 3 : 1 3 IP's of 4-Cyclopentene 1 , 3-Dione 4 1 3 : 1 4 E l e c t r o n D e n s i t i e s of 4-Cyclopentene 1 ,3-Dione 42 3 : 1 5 IP's of Maleimide 4 4 3 : 1 6 E l e c t r o n D e n s i t i e s of Maleimide 4 4 3 : 1 7 IP's of Maleic Anhydride 46 3 : 1 8 E l e c t r o n D e n s i t i e s of Maleic Anhydride .. 4 7 3 : 1 9 Carbonyl Lone P a i r IP's 5 5 3 : 2 0 TT IP's 6 5 3 : 2 1 The Dicarb o n y l Compounds 7 0 - v i -LIST OF FIGURES Figure Page 1 The Franck Condon P r i n c i p l e 8 2 Schematic Diagram of the Photoelectron Spectrometer 22 3 PE Spectra of Tetrahydrofuran, y-Butyrolactone, 2,5-Dihydrofuran, y-Crotonolactone 26 4 PE Spectra of 1,3-Dioxolane, Ethylene Carbonate, Vinylene Carbonate 33 5 PE Spectra of 4-Cyclopentene 1,3-Dione, Maleimide, Maleic Anhydride 40 6 Carbonyl Oxygen Lone Pair IP's 54 7 Observed vs. Calculated Carbonyl Oxygen Lone Pair IP's 56 8 Correlation of PE and UV Spectra of the Dicarbonyl Compounds 71 - v i i -ACKNOWLEDGEMENTS F i r s t , I would l i k e to express my deep g r a t i t u d e to my s u p e r v i s o r , Dr. D. C. F r o s t , f o r h i s help and understand-i n g , which were much appreciated and to Dr. C. A. McDowell f o r h i s i n t e r e s t throughout my work. Dr. L. Weiler and Dr. F. G. Her r i n g a l s o deserve thanks f o r p o i n t i n g me i n the r i g h t d i r e c t i o n . I am g r a t e f u l to my colleagues i n the l a b ; Dr. D. Chadwick, Dr. R. A. N. McLean, Dr. J.-C. B t i n z l i , Dr. A. K a t r i b , Mr. S. T. Lee, Mr. A. J . Burak and e s p e c i a l l y Dr. A. B. Cornford f o r t h e i r help and f o r the many d i s c u s s i o n s I had with them. Al s o I wish to thank Mrs. Margaret Wight f o r p a t i e n t l y t y p i n g t h i s t h e s i s and the t e c h n i c a l s t a f f of the Chemistry Department f o r t h e i r able a s s i s t a n c e . F i n a l l y , I would l i k e t o thank the N a t i o n a l Research C o u n c i l of Canada and the U n i v e r s i t y of B r i t i s h Columbia f o r t h e i r f i n a n c i a l support. CHAPTER 1 INTRODUCTION " Now, i f we are ready, w i l l you watch w h i l s t I f l a s h one quantum of X-rays on to the atom? I may not h i t the atom the f i r s t time; i n that case, of course, you w i l l not see i t . Try again; t h i s time the quantum has h i t the e l e c t r o n . Look sharp, and n o t i c e where i t i s . I s n ' t i t there? Bother! I must have blown the e l e c t r o n out of the atom. " A. S. Ed d i n g t o n 1 (1927) Eddington was annoyed when h i s X-ray knocked the e l e c t r o n out of the atom, but had he measured the energy of t h i s e j e c t e d e l e c t r o n , he would have performed an experiment i n Photoel e c t r o n Spectroscopy. 2—^6 The p r i n c i p l e of Photo e l e c t r o n Spectroscopy (PES) i s as f o l l o w s . Work must be done to separate the e l e c t r o n from the atom or molecule, but e s s e n t i a l l y a l l the excess energy o r i g i n a l l y contained i n the quantum of r a d i a t i o n must be c a r r i e d away by the pho t o e l e c t r o n i n the form of k i n e t i c energy. I f the i o n i z a t i o n p o t e n t i a l (IP) i s defined as t h i s work done, i . e . the energy necessary to remove the e l e c t r o n from the molecule, then equation (1:1) holds hv = IP + T (1:1) where T i s the k i n e t i c energy of the e l e c t r o n . According - 2 -to t h i s equation, t h e r e f o r e , i f monochromatic r a d i a t i o n i s used, IP's can be obtained simply by measuring the k i n e t i c energy of the photoelectrons produced. W. C. P r i c e J has drawn an analogy between t h i s procedure and back t i t r a t i o n s i n c o n v entional chemistry. The usefulness of measuring IP's l i e s i n the f a c t that they r e f l e c t the energies of the o r b i t a l s that the e l e c t r o n s occupy i n the molecule. This i s because, from a r a t h e r naive but i n s t r u c t i v e viewpoint, the process of i o n i z a t i o n i s j u s t the removal of an e l e c t r o n from one of the occupied o r b i t a l s , l e a v i n g an i o n which i s regarded as unchanged from the n e u t r a l molecule, except that there i s an e l e c t r o n missing from one of the o r b i t a l s . Therefore, the IP, which i s the energy r e q u i r e d f o r t h i s i o n i z a t i o n , i s approximately the negative of the energy of the e l e c t r o n i n that p a r t i c u l a r o r b i t a l . Furthermore, i n f o r m a t i o n about the composition and bonding p r o p e r t i e s of the o r b i t a l s can a l s o be obtained. The term PES u s u a l l y r e f e r s to the branch of the f i e l d i n which vacuum UV photons are used as the i o n i z i n g r a d i a t i o n . Siegbahn ' and co-workers have developed the use of s o f t X-rays, w i t h energies of s e v e r a l hundred eV, but t h i s branch ( u s u a l l y c a l l e d ESCA) commonly st u d i e s only the i n n e r s h e l l o r b i t a l s , whereas the work des c r i b e d i n t h i s t h e s i s i s concerned only with valence s h e l l molecular o r b i t a l s . For organic molecules, t h i s i n c l u d e s the o r b i t a l s that comprise TT bonds - 3 -and the so c a l l e d lone p a i r s of heteroatoms. U n t i l r e c e n t l y , unless there i s conjugation, these groups have u s u a l l y been considered as i s o l a t e d and independent p a r t s of the molecule, but, i n f a c t , appreciable i n t e r a c t i o n s occur. I t . i s the purpose of t h i s t h e s i s to study some of these i n t e r a c t i o n s , using a s e r i e s of f i v e membered r i n g compounds as a b a s i s . CHAPTER 2 THEORETICAL CONSIDERATIONS 2:1 I n t r o d u c t i o n In the i n t r o d u c t i o n , i t was pointed out that i f a photon of s u f f i c e n t energy f a l l s on a molecule, then a ph o t o e l e c t r o n w i l l be e j e c t e d w i t h a k i n e t i c energy T, which i s r e l a t e d to the photon energy by the i o n i z a t i o n p o t e n t i a l ( I P ) . hv = IP + T (2:1) The molecular i o n formed can be i n i t s ground s t a t e or i n one of i t s e x c i t e d s t a t e s , depending on the o r b i t a l that the e l e c t r o n came from, but i n any case an IP j u s t represents the d i f f e r e n c e i n energy between the n e u t r a l molecule and the p a r t i c u l a r i o n i c s t a t e . However, sin c e the d e f i n i t i o n of a molecular s t a t e i n c l u d e s not only the e l e c t r o n i c c o n f i g u r a t i o n but a l s o the amount of v i b r a t i o n and r o t a t i o n , then the IP, as def i n e d by equation (2:1), i s r e a l l y made up of three p a r t s IP = IP + AE + AE (2:2) 0 v i b r o t where the three terms are, r e s p e c t i v e l y , the d i f f e r e n c e i n e l e c t r o n i c , v i b r a t i o n a l and r o t a t i o n a l energy of the molecule - 5 -before and a f t e r i o n i z a t i o n . The f i r s t term i n equation (2:2), IP , i s known as the 0 a d i a b a t i c IP and represents the energy d i f f e r e n c e between the ground s t a t e of the n e u t r a l molecule and the lowest v i b r a t i o n a l and r o t a t i o n a l l e v e l of the i o n i c s t a t e . In other words, i t i s the minimum amount of energy necessary to remove an e l e c t r o n from a given o r b i t a l , or e q u i v a l e n t l y , to create an i o n i n a given s t a t e . Since removing an e l e c t r o n from a d i f f e r e n t o r b i t a l creates a d i f f e r e n t i o n i c s t a t e , then f o r each of these s t a t e s there i s an a d i a b a t i c IP and the value of t h i s IP gives the energy of the s t a t e r e l a t i v e to the ground s t a t e . •37 The a d i a b a t i c IP i s at l e a s t f i v e or s i x eV J I f o r most molecules and so i t i s f a r l a r g e r than the other terms i n e xpression (2:2) which account f o r changes i n v i b r a t i o n a l and r o t a t i o n a l energy. In f a c t , r o t a t i o n a l spacings are almost always too s m all to be r e s o l v e d (but see references 3 8 , 3 9 , 40) and since i o n i z a t i o n does not g r e a t l y e x c i t e r o t a t i o n , t h i s c o n t r i b u t i o n to the observed f i n e s t r u c t u r e i s u s u a l l y ignored. Therefore, a PE spectrum, which i s j u s t a p l o t of the number of photoelectrons produced per u n i t of time as a f u n c t i o n of t h e i r k i n e t i c energy, w i l l show a s e r i e s of bands due to i o n i z a t i o n s from d i f f e r e n t o r b i t a l s , and w i t h i n these bands there may be v i b r a t i o n a l f i n e s t r u c t u r e . The a d i a b a t i c IP i s u s u a l l y taken to be the energy - 6 -corresponding to the onset of the band, or the f i r s t v i b r a t i o n a l peak, i f these are r e s o l v e d . The p o s i t i o n of the band maximum, which represents the energy of the most probable t r a n s i t i o n to the i o n , i s a l s o measured and i s c a l l e d the v e r t i c a l IP. As w i l l be seen, t h i s v e r t i c a l IP represents the energy of the o r b i t a l i n v o l v e d b e t t e r than the a d i a b a t i c IP, and so i f j u s t one value i s quoted, i t i s u s u a l l y the v e r t i c a l IP. However, before any experimental r e s u l t s are quoted, some theory i s necessary to e x p l a i n the shapes and p o s i t i o n s of the bands i n a PE spectrum. 2:2 Shapes of the Bands: A p p l i c a t i o n of the Franck-Condon P r i n c i p l e t o PES Since most molecules are i n t h e i r ground v i b r a t i o n a l s t a t e s at room temperature, f i n e s t r u c t u r e i n a PE spectrum i s u s u a l l y due to t r a n s i t i o n s t o e x c i t e d v i b r a t i o n a l s t a t e s of the i o n (although 'hot bands', t r a n s i t i o n s from other v i b r a t i o n a l s t a t e s of the n e u t r a l molecule, have been observed**"*"). Therefore, the shape of a band i s determined by the extent of t h i s v i b r a t i o n a l e x c i t a t i o n , which, i n t u r n , i s governed by the Franck-Condon p r i n c i p l e . In c l a s s i c a l terms, t h i s p r i n c i p l e s t a t e s that since the time r e q u i r e d f o r an e l e c t r o n i c t r a n s i t i o n ( i n t h i s case, — 1 R i o n i z a t i o n ) i s about 10 seconds, which i s much s h o r t e r -lb than the c h a r a c t e r i s t i c time f o r nuclear motions 10 sec.) then the n u c l e i w i l l r e t a i n t h e i r p o s i t i o n s a f t e r i o n i z a t i o n . In terms of a p o t e n t i a l energy diagram (see Figure 1), t h i s means that the l i n e r e p r e s e n t i n g the t r a n s i t i o n i s v e r t i c a l , i . e . there i s no change i n the coordinates of the n u c l e i during i o n i z a t i o n . However, i f the n u c l e i have d i f f e r e n t e q u i l i b r i u m • p o s i t i o n s i n the i o n , then immediately a f t e r i o n i z a t i o n they w i l l be i n an unstable s t a t e , so they w i l l move towards the new e q u i l i b r i u m p o s i t i o n , and s t a r t to v i b r a t e . Therefore, v i b r a t i o n a l e x c i t a t i o n i n a PE band i m p l i e s that there has been a change i n geometry on i o n i z a t i o n and so the e l e c t r o n , or r a t h e r the o r b i t a l from which i t was removed, must have been important i n the bonding. For more q u a n t i t a t i v e work, a wave mechanical p i c t u r e , hp such as that given by Herzberg i s necessary. From time dependent p e r t u r b a t i o n theory, the p r o b a b i l i t y of l i g h t i n d u c i n g a t r a n s i t i o n from s t a t e \p" to s t a t e ip* i s p r o p o r t i o n a l 2 to R , where R = <T|»' |M|TJ>"> ( 2 : 3 ) In t h i s e x p r e s s i o n , the wave f u n c t i o n s depend on both the nuclear and e l e c t r o n i c coordinates and M = V'q.r. i s the d i p o l e moment operator. The q^ and r ^ are the charges and p o s i t i o n s of a l l the p a r t i c l e s ( n u c l e i and e l e c t r o n s ) and so the d i p o l e moment M can be s p l i t i n t o M + M , where M i s n e n the nuclear part and M depends only on the e l e c t r o n i c c o o r d i n a t e s . - 8 -T h e F r a n c k - C o n d o n P r i n c i p l e observed PE spectrum r *• PE Spectrum of H 2 0 (from ref.15) Figure 1 - 9 -Also the Born-Oppenheimer approximation i s invoked, i n order that the t o t a l wave f u n c t i o n can be fa c t o r e d i n t o a v i b r a t i o n a l wave f u n c t i o n and an e l e c t r o n i c part ^ . The express i o n v e f o r R now becomes R = <^'iJ>f|M + M U ' V > (2:4) e v e n e v This can be s p l i t i n t o two terms by breaking up the d i p o l e moment operator, then each term can be fa c t o r e d i n t o an i n t e g r a l over nuclear coordinates and one over the e l e c t r o n s ' p o s i t i o n s . However, < i f ; ' | ^ " > = 0, since the e l e c t r o n i c wave f u n c t i o n s are e e orthogonal, so the p r o b a b i l i t y of t r a n s i t i o n i s p r o p o r t i o n a l t o (2:6) The f a c t o r R i n (2:6) i s known as the e l e c t r o n i c e t r a n s i t i o n moment and i s d i f f e r e n t f o r each o r b i t a l , but i s approximately constant f o r a l l the v i b r a t i o n a l t r a n s i t i o n s w i t h i n a given band. The p r o b a b i l i t y of t r a n s i t i o n to a c e r t a i n v i b r a t i o n a l l e v e l then, i s p r o p o r t i o n a l to <^ '|^ "> which i s known as the Franck-Condon f a c t o r f o r that v v p a r t i c u l a r t r a n s i t i o n . Therefore, the i n t e n s i t i e s of the v i b r a t i o n a l peaks i n a band of a PE spectrum w i l l r e f l e c t the s i z e s of the Franck-Condon f a c t o r s , so that the t r a n s i t i o n - 10 -w i t h the l a r g e s t Franck-Condon f a c t o r w i l l be the v e r t i c a l t r a n s i t i o n , by ana l o g y w i t h the c l a s s i c a l model. F o r example, i n a d i a t o m i c m o l e c u l e , t h e Franck-Condon f a c t o r i s a n - i n t e g r a l o f the form where Ar i s the change i n e q u i l i b r i u m bond l e n g t h . I f Ar i s z e r o o r v e r y s m a l l , t h e n the v i b r a t i o n a l wave f u n c t i o n s w i l l r e t a i n t h e i r o r t h o g o n a l i t y and so o n l y the 0-0 band w i l l have a p p r e c i a b l e i n t e n s i t y , f o l l o w e d by one o r two much weaker components. I f Ar i s ' l a r g e , however, t h e n t h i s o r t h o g o n a l i t y w i l l be d e s t r o y e d and t h e Franck-Condon f a c t o r s w i l l be non-zero f o r many t r a n s i t i o n s , w h i c h means t h a t a l o n g v i b r a t i o n a l p r o g r e s s i o n w i l l be o b s e r v e d . As i n the c l a s s i c a l argument, t h e p r e s e n c e of a l o n g v i b r a t i o n a l p r o g r e s s i o n means t h a t t h e r e has been a change i n t h e geometry on i o n i z a t i o n . I n f a c t , e x p e r i m e n t a l Franck-Condon f a c t o r s have been used t o c a l c u l a t e t h i s change i n geometry and hence deduce the s t r u c t u r e o f the i o n ' The a p p l i c a t i o n o f t h e s e i d e a s can be seen i n t h e PE 11 46 49-52 spectrum o f w a t e r ' ' > which i s r e p r o d u c e d from r e f e r e n c e 15 i n F i g u r e 1. The f i r s t band c o r r e s p o n d s t o i o n i z a t i o n from the l b ^ o r b i t a l , w h i c h i s t h e oxygen l o n e p a i r t h a t p r o j e c t s up out o f t h e p l a n e o f the m o l e c u l e and i s e s s e n t i a l l y non-bonding. That v e r y l i t t l e change i n (2:7) - 11 -geometry and bonding occurs on i o n i z a t i o n i s shown by the intense peak i n the PE spectrum at the a d i a b a t i c IP, and a l s o from the observation that the v i b r a t i o n a l frequencies i n the i o n are about the same as f o r the n e u t r a l molecule. In f a c t the changes i n bond length and angle are estimated to be only 0.06 A and 5° r e s p e c t i v e l y . The H 0 + i o n formed i n the i o n i z a t i o n corresponding to 2 2 the second band i s i n a s t a t e , the same e l e c t r o n i c c o n f i g u r a t i o n as the f i r s t e x c i t e d s t a t e of NH 2, which i s ho 54 known to be l i n e a r or only s l i g h t l y bent » J- > y J . Therefore a l a r g e change of bond angle (from 105° to almost 180°) i s expected on i o n i z a t i o n . This i s confirmed i n the PE spectrum by the long p r o g r e s s i o n i n the bending v i b r a t i o n and the s e p a r a t i o n of the a d i a b a t i c and v e r t i c a l IP's by about 1.0 eV. F i n a l l y , the t h i r d band i n the water spectrum shows that not e v e r y t h i n g i n PES i s as simple as the f i r s t two bands might i n d i c a t e . The v i b r a t i o n a l p a t t e r n i s i r r e g u l a r and the peaks are broad and not w e l l r e s o l v e d . This complexity i s probably caused by more than one v i b r a t i o n a l mode being 49 e x c i t e d and Turner a t t r i b u t e s the broadening to the f a c t that the i o n i c s t a t e may have a short l i f e t i m e . This i m p l i e s that by the u n c e r t a i n t y p r i n c i p l e , the energy of the s t a t e i s not w e l l defined and the l i n e s w i l l be broadened. For example, -14 / i f the l i f e t i m e of the s t a t e i s about 10 seconds (about - 12 -one v i b r a t i o n a l p e r i o d ) , then t h i s u n c e r t a i n t y broadening i s about 100 meV. In g e n e r a l , i n a polyatomic molecule i t i s only the t o t a l l y symmetric v i b r a t i o n s that are u s u a l l y e x c i t e d . This i s because the ground s t a t e i s t o t a l l y symmetric and so, i n order that the Franck-Condon f a c t o r be non-zero, the v i b r a t i o n a l wave f u n c t i o n of the i o n must co n t a i n a t o t a l l y symmetric r e p r e s e n t a t i o n a l s o . Moreover, the v i b r a t i o n s that are e x c i t e d are those that " c a r r y " the molecule from the geometry of the n e u t r a l to that of the i o n . For i n s t a n c e , i n the second band of water, the change was from bent to l i n e a r and i t was the bending v i b r a t i o n that was e x c i t e d . Therefore, an a n a l y s i s of the v i b r a t i o n a l s t r u c t u r e of a band i n a PE spectrum i s very u s e f u l i n determining the type of o r b i t a l i n v o l v e d and i t s bonding p r o p e r t i e s . 2:3 P o s i t i o n s of the Bands: T h e o r e t i c a l p r e d i c t i o n s of IP's. Since an IP i s j u s t the d i f f e r e n c e i n energy between two e l e c t r o n i c s t a t e s then the values of the I P ' s , and hence the p o s i t i o n s of the bands i n a PE spectrum, can be p r e d i c t e d by c a l c u l a t i n g the energies of these two s t a t e s and s u b t r a c t i n g . However, i n p r a c t i c e , t h i s i s not as simple as i t sounds. Most c a l c u l a t i o n s of molecular energies are done usi n g the 55 Hartree-Fock method , which neglects any energy due t o - 13 -c o r r e l a t i o n of the e l e c t r o n s ' motion, so t h a t the H a r t r e e -Fock energy i s always l a r g e r than the t r u e energy. Even though most of the c o r r e l a t i o n e r r o r c a n c e l s out i n the s u b t r a c t i o n of the e n e r g i e s , the c o r r e l a t i o n energy, which i s the d i f f e r e n c e between the Hartree-Fock and the t r u e energy, i s s m a l l e r i n the i o n because i t has fewer e l e c t r o n s . T h e r e f o r e the IP's c a l c u l a t e d by t h i s method tend to be . ,, 60 too s m a l l T h i s argument depends on the f a c t that the c a l c u l a t i o n s should be "good" ones, near the Hartree-Fock l i m i t . However, most molecules have a c l o s e d s h e l l c o n f i g u r a t i o n , so the i o n formed w i l l have an u npaired e l e c t r o n . Moreover, except f o r the f i r s t IP, the i o n w i l l be i n an e x c i t e d s t a t e , and c a l c u l a t i o n s on such systems are n o t o r i o u s l y d i f f i c u l t t o perform. In s p i t e of a l l t h i s , energy d i f f e r e n c e c a l c u l a t i o n s 56 57 have been used^ » J , but t h e r e i s a much e a s i e r way. 58 59 60 T h i s method, commonly known as Koopmans' theoreirr ' J ' i s , i n p r i n c i p l e , l e s s a c c u r a t e but i t i s much s i m p l e r because only a c a l c u l a t i o n on the n e u t r a l molecule i s needed. The essence of the theorem i s a statement i n mathematical terms o f the simple p i c t u r e of i o n i z a t i o n g i v e n i n the i n t r o d u c t i o n , i n which the i o n i s c o n s i d e r e d to have the same m o l e c u l a r o r b i t a l s as the o r i g i n a l molecule, except t h a t t h e r e i s an e l e c t r o n m i s s i n g from one of them. T h e r e f o r e , Koopmans' theorem s t a t e s t h a t the IP i s j u s t the n e g a t i v e of the o r b i t a l energy o f the o r b i t a l i n v o l v e d . - in -In order to de r i v e the theorem, l e t <t> . . . . d> be 1 n the molecule o r b i t a l s of the n e u t r a l molecule, so that the t o t a l wave f u n c t i o n i s the determinant ¥ = U 2 (j> 2 | ( 2 : 8 ) ' 1 n These o r b i t a l s are obtained, i n the Hartree-Pock method, as s o l u t i o n s of the Fock equations F<j). = e.<J> ( 2 : 9 ) l l i where F = H +T](2J - K ), H i s the one e l e c t r o n part of the j J j Hamiltonian, and J . and K. are the coulomb and exchange J J operators. The energy of such a system i s given by E° = 2 £ h . + £ ( 2 J . . - K . J where h = <<j> I H U > ( 2 : 1 0 ) i i i J i j " ^ I ' V V K u - < w * i " The h can be thought of as the k i n e t i c energy plus the p o t e n t i a l energy of nuclear a t t r a c t i o n of a s i n g l e e l e c t r o n moving i n the o r b i t a l <f>. , and the J . . and K.. account f o r the e i i j i j r e p u l s i v e and exchange i n t e r a c t i o n s between e l e c t r o n s . In terms of these q u a n t i t i e s , the o r b i t a l e n e rgies, the e 's, which, i f o r a c l o s e d s h e l l system, are j u s t the e x p e c t a t i o n values of the Fock operator, are given by e. = h. + £(2J. . - K. J (2:11) i i j i j i j i . e . they are the sum of the k i n e t i c energy plus the e l e c t r o n i c r e p u l s i o n s . For t h i s c l o s e d s h e l l system the t o t a l energy may be re-expressed i n terms of the o r b i t a l energies t o give (2:12) E° = 2^e. - E ( 2 J ± j - K ± J ) (2:12) i . e . the t o t a l energy i s j u s t the sum of the o r b i t a l energies', but the r e p u l s i o n s have been counted t w i c e , so they must be subt r a c t e d . 58 Koopmans then showed that i f an e l e c t r o n i s removed from one of the o r b i t a l s , say <J> , and i f the i o n i s des c r i b e d n i n terms of a l i n e a r combination of a l l the o r b i t a l s <bs i ( i n c l u d i n g <!>n)> then the best l i n e a r combination (the one that minimizes the energy d i f f e r e n c e between the n e u t r a l and the ion) i s the determinantal f u n c t i o n . * i o n ' 1*1 V •••• * n - l O (2:13) This i s j u s t the o r i g i n a l determinant w i t h one s p i n o r b i t a l m i s s i n g . I f the energy of t h i s wave f u n c t i o n i s c a l c u l a t e d , i t i s found to be E. = 2 £ h , + h + Y(2JA . - K. .) i o n ^ i n *y i j i j _ (2:14) + >-(2J. - K. ) - 16 -Therefore the d i f f e r e n c e i n energy between the i o n and the n e u t r a l , the IP, i s given by E. - E° = h + V*(2J. - K. ) ( 2 : 1 5 ) i o n n j n j n But t h i s i s j u s t equal to the o r b i t a l energy e ! Thus the usual form of Koopmans1 theorem i s obtained - the IP i s equal to the negative of the o r b i t a l energy. In r e a l i t y , however, the geometry of the molecule changes on i o n i z a t i o n , as do the o r b i t a l s of the other e l e c t r o n s , and Koopmans' theorem neglects t h i s r e o r g a n i z a t i o n energy. Part of t h i s e r r o r i s c a n c e l l e d by the d i f f e r e n c e i n c o r r e l a t i o n energy, but i t s t i l l i s found that IP's c a l c u l a t e d by Koopmans' theorem are u s u a l l y too large^" 1". In order t o compare them w i t h experimental numbers, the t h e o r e t i c a l values are u s u a l l y t y i ,64 62 reduced by an e m p i r i c a l amount , which depends on the type of c a l c u l a t i o n . In t h i s work the se m i - e m p i r i c a l CNDO/2 6 S 63 and INDO 5 methods of Pople et a l . have been used, and fo r the IP's that have been assigned, i t i s found t h a t , on the average, INDO p r e d i c t s s m a l l e r IP's than CNDO/2^, but.the unsealed o r b i t a l energies are s t i l l too la r g e by about 2 eV fo r INDO, whereas CNDO/2 overestimates by about 3 eV. The a p p l i c a t i o n of Koopmans' theorem i m p l i e s that a c a l c u l a t i o n has been done on the i o n i c s t a t e , a l b e i t w i t h a very r e s t r i c t e d b a s i s set (the o r i g i n a l o r b i t a l s ) . However, t h i s p s e u d o - c a l c u l a t i o n i s n e c e s s a r i l y done wi t h the same - 17 -geometry as the n e u t r a l molecule, and so the o r b i t a l energies should be compared wi t h v e r t i c a l I P ' s , r a t h e r than a d i a b a t i c values, because the v e r t i c a l IP a l s o i m p l i e s a neglect of the geometry change. I t has been found that care must be taken i n usi n g Koopmans' theorem, p a r t i c u l a r l y w i t h 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 , because the or d e r i n g of the l e v e l s i s not always r e l i a b l e '. However, i f the theorem i s used i n t e l l i g e n t l y , i t can be very u s e f u l i n a s s i g n i n g PE s p e c t r a . 2:4 I n t e r a c t i o n s of O r b i t a l s . Although the d e l o c a l i z e d molecular o r b i t a l s obtained from an SCF c a l c u l a t i o n present a f a i r l y accurate p i c t u r e of the e l e c t r o n i c s t r u c t u r e of a molecule, i t i s u s u a l l y e a s i e r to understand and i n t e r p r e t a PE spectrum i n terms of more f a m i l i a r l o c a l i z e d o r b i t a l s . For example, when a band i s assigned t o , say, an oxygen lone p a i r , t h i s means tha t most of the e l e c t r o n d e n s i t y i s on the oxygen atom, but there are al s o c o n t r i b u t i o n s to that o r b i t a l from other parts of the molecule. Recently, Hoffmann 6 8 , 6 9 » 7 0 and o t h e r s 7 1 , 7 2 » 3 4 have shown that although the concept of i s o l a t e d f u n c t i o n a l groups and l o c a l i z e d o r b i t a l s i s h e u r i s t i c - a l l y u s e f u l , some account must be taken of the i n t e r a c t i o n s between these o r b i t a l s . For i n s t a n c e , i t might be thought t h a t the two IT bonds i n 1, 4 - 18 -cyclohexadiene ( 2 ) are degenerate, because they are not conjugated and are u s u a l l y regarded as i s o l a t e d and c h e m i c a l l y e q u i v a l e n t . However, e x p e r i m e n t a l l y ^ ^ i t i s found that they mix to form a symmetric and an antisymmetric combination and the energy d i f f e r e n c e between these two o r b i t a l s i s 1 eV, which i s c e r t a i n l y s i g n i f i c a n t i n chemical terms. More f o r m a l l y , consider two o r b i t a l s \\i and ty^ which are completely i s o l a t e d from each other, and l e t have lower energy. Then i n a h y p o t h e t i c a l process, l e t these two be brought together u n t i l they take up t h e i r molecular p o s i t i o n s As they are brought together, they s t a r t to " f e e l " one another and so t h e i r energies and wave f u n c t i o n s are perturbed. I f overlap i s neglected, these p e r t u r b a t i o n s are given by ( 2 : 1 6 ) 7 4 |H' | 2 E = E + 1 2 1 10 E = E + E 1 0 |H}2 " E 2 0 I2 E - E 20 10 H' 1 ? E 10 H' 12 - E .20 ( 2 : 1 6 ) ij, = \p + — ij, 2 20 E - E 10 20 10 - 19 -so that the energy l e v e l s move apart, or " r e p e l " one another, by an amount 2|H I /(E__ - E n_) and both o r b i t a l s are mixtures 12 20 10 of the o r i g i n a l o r b i t a l s . Note a l s o that i t i s only o r b i t a l s that are close i n energy that mix s i g n i f i c a n t l y , because of the f a c t o r ( E 2 Q - E 1 Q ) i n the denominator. Hoffmann t h e r e f o r e suggests e s t i m a t i n g what the energies of the unperturbed l e v e l s would have been, from measurements on model compounds, and using the energy d i f f e r e n c e before and a f t e r p e r t u r b a t i o n as a measure of the i n t e r a c t i o n . He has i d e n t i f i e d two mechanisms by which these i s o l a t e d groups may i n t e r a c t . F i r s t , there may be d i r e c t "through space" i n t e r a c t i o n , i n which the o r b i t a l s overlap enough to cause a s p l i t t i n g o f the energies, f o r i n s t a n c e , the two TT bonds of norbornadiene (1) i n t e r a c t through space to give 1 2 a s p l i t t i n g of that even when 0.85 eV. However, i n some cases i t i s found d i r e c t overlap i s almost zero, there i s s t i l l - 20 -s u b s t a n t i a l c oupling of two groups. For example, i f "through space" i n t e r a c t i o n were the only mechanism, i t would be expected that the s p l i t t i n g i n cyclohexadiene (2) would be s m a l l e r than norbornadiene ( 1 ) , since the overlap of the two TT bonds i s s m a l l e r , but i n f a c t i t i s l a r g e r . This i s because the TT bonds. have mixed w i t h o r b i t a l s of a p p r o p r i a t e symmetry i n the a framework, causing a "through bond" i n t e r a c t i o n . These ideas are admittedly only approximate, but they are u s e f u l i n r a t i o n a l i z i n g PE s p e c t r a , e s p e c i a l l y of complicated organic molecules. CHAPTER 3 EXPERIMENTAL 3 : 1 I n t r o d u c t i o n A l l of the spectr a i n t h i s t h e s i s were taken on a 75 spectrometer described by A. B. Cornford , which i s described i n Figure 2 . The 2 1 . 2 2 eV photons were produced i n a 2450 MHz micro-wave discharge i n helium, and passed through a glass c a p i l l a r y tube i n t o a c o l l i s i o n chamber. The sample vapour was leaked i n t o the c o l l i s i o n chamber through a G r a n v i l l e P h i l i p s leak valve u n t i l the pressure i n the whole system was about 2 x 10 J t o r r . The photoelectrons produced escaped through a hole i n the c o l l i s i o n chamber and entered a 2 1/2 i n c h mean rad i u s h e m i s p h e r i c a l e l e c t r o s t a t i c a n a l y s e r . I f they had the r i g h t energy, they were t r a n s m i t t e d by t h i s e l e c t r o n mono-chromator and were detected by a Channeltron e l e c t r o n m u l t i -p l i e r s i t u a t e d beyond the e x i t s l i t . Each e l e c t r o n produced a p u l s e , which was f i r s t p r e a m p l i f i e d and then passed t o a Harshaw NA 15 a m p l i f i e r - d i s c r i m i n a t o r and f i n a l l y to a Harshaw NR 10 ratemeter, which produced an output p r o p o r t i o n a l t o the number of counts per second. This s i g n a l could be p l o t t e d out d i r e c t l y on a chart r e c o r d e r , or accumulated i n a Fabri-Tek model 1 0 6 2 m u l t i c h a n n e l a n a l y s e r . - 22 -SCHEMATIC DIAGRAM OF THE PHOTOELECTRON SPECTROMETER To Pulse-Counting System To Sample Inlet Collision Chamber To Diffusion Pump To Differential Pumping Discharge Tube To Microwave Generator To Helium Source Figure 2 In the a c t u a l operation of the spectrometer, the analyser was set to transmit e l e c t r o n s of a c e r t a i n f i x e d energy (about one t o two eV) and the spectrum was scanned by v a r y i n g the r e t a r d i n g v o l t a g e between the c o l l i s i o n chamber and the entrance s l i t . The energy s c a l e was c a l i b r a t e d by s i m u l t a n -eously f l o w i n g i n a small amount of a c a l i b r a n t gas ( u s u a l l y n i t r o g e n , or one of the rare gases) and measuring the IP's of the compound r e l a t i v e to the a c c u r a t e l y known values f o r the c a l i b r a n t 7 ^ . Because a moving charge i s d e f l e c t e d by a magnetic f i e l d , e f f o r t s must be made t o cancel out such f i e l d s i n the r e g i o n of the ana l y s e r . This was achieved by the use of Helmholtz c o i l s to balance these magnetic f i e l d s and a l s o by the use of magnetic s h i e l d i n g m a t e r i a l (Mu met a l ) ; but i t was found that the r e s i d u a l f i e l d was s t i l l about the order of t h i r t y m i l l i g a u s s . The ba l a n c i n g f i e l d s from the Helmholtz c o i l s must a l s o be adjusted p e r i o d i c a l l y to give the best i n t e n s i t y and r e s o l u t i o n . For t h i s work the r e s o l u t i o n was about twenty to tw e n t y - f i v e m i l l i v o l t s (FWHM of one of the peaks of the argon spectrum) and the i n t e n s i t y was about f i v e thousand counts f o r argon, but f o r the l a r g e r molecules count r a t e s of one hundred per second or fewer were t y p i c a l . When the spectrum had been accumulated long enough (about ten to twenty scans, or about two hours i f the f u l l memory of the Fabri-Tek was used) t o give a good s i g n a l t o - 24 -no i s e r a t i o , i t was p l o t t e d out on an X-Y r e c o r d e r . A d i a b a t i c IP's were taken as e i t h e r the f i r s t v i b r a t i o n a l peak, or the onset of an u n r e s o l v e d band, s i n c e i t was assumed t h a t the lowest v i b r a t i o n a l l e v e l o f the i o n was populated by the 45' i o n i z a t i o n . V e r t i c a l IP's were measured from the peak of the band, but i f bands o v e r l a p , these may be i n a c c u r a t e . For i n s t a n c e , the true v e r t i c a l IP of the band at lower e n e r g i e s w i l l always be l a r g e r than the measured v a l u e . The s p e c t r a were measured from the r e c o r d e r p l o t s and i t was assumed t h a t the v o l t a g e s c a l e was l i n e a r . Values of the IP's are a c c u r a t e to about ± 0.02 eV and v i b r a t i o n a l f r e q u e n c i e s t o about ±100 cm - 1. A l s o t a b u l a t e d f o r each molecule are the r e s u l t s of MO c a l c u l a t i o n s which were performed. O r b i t a l e n e r g i e s are prese n t e d d i r e c t l y , without any s c a l i n g , and f o r the o r b i t a l s of i n t e r e s t , approximate e l e c t r o n d e n s i t i e s are a l s o t a b u l a t e d . For any atom, these are the sums of the squares of the c o e f f i c i e n t s o f the atomic o r b i t a l s on that c e n t r e , and g i v e an i d e a where t h a t p a r t i c u l a r o r b i t a l i s l o c a l i z e d . F i n a l l y , a l l o f the molecules are assumed t o have a p l a n a r s k e l e t o n , and so the o r b i t a l s are named a c c o r d i n g to the C,, or C p o i n t group. I f the molecule i s c: , the 2v s 2v plane o f the molecule i s taken as the YZ plane, with the a x i s a l o n g the Z d i r e c t i o n , which i m p l i e s t h a t the TT type o r b i t a l s t r a n s f o r m as a„ or b,. 3 : 2 I n d i v i d u a l M o l e c u l e s 3:2:1 T e t r a h y d r o f u r a n No c a l c u l a t i o n s were c a r r i e d out f o r t h i s m o l e c u l e because o n l y one low l y i n g IP i s e x p e c t e d , t h e l o n e p a i r o f the oxygen atom,- which would t r a n s f o r m as b^ i f the m o l e c u l e were p l a n a r . The a d i a b a t i c IP i s 9-^ 3 eV and the v e r t i c a l IP o f t h i s o r b i t a l i s 9.75 eV ( c f . 9-417 eV from Rydberg 77 11 s e r i e s , 9.42 from p r e v i o u s PE s p e c t r a and 9.54 from p h o t o i o n i z a t i o n 7 8 ) . 3:2:2 y - B u t y r o l a c t o n e T h i s c h e m i c a l was o b t a i n e d from the Eastman Co. and b e f o r e use i t s p u r i t y was checked by NMR. T a b l e s 3:1 and 3:2 summarize the o b s e r v e d spectrum and t h e r e s u l t s o f the c a l c u l a t i o n s . The f i r s t band was a s s i g n e d t o the c a r b o n y l l o n e p a i r because the v i b r a t i o n a l f r e q u e n c y o f 1520 cm" 1 p r o b a b l y r e p r e s e n t s t h e s t r e t c h i n g v i b r a t i o n o f t h e C=0 bond ( f o r i n s t a n c e , t h e analogous band i n the PE s pectrum o f f o r m i c a c i d 7 9 has a v i b r a t i o n o f 1460 cm-"'" which T u r n e r ^ has a s s i g n e d t o the c a r b o n y l s t r e t c h ) . T h i s w i l l be seen i n t h e o t h e r m o l e c u l e s i n t h e s e r i e s , t h a t t h e f o r m a l l o n e p a i r on the c a r b o n y l oxygen atom i s r e a l l y q u i t e i m p o r t a n t i n the b o n d i n g . - 26 -Tetrahydro-furan Vertical IP(eV) b 9.75 • i - -i r C f . 7-Butyro-lactone a' 10.2 5 a" 10.94 -i 1 r ~i r 2,5-Dihydro-furan b 1 9.14 b 1 10.59 a ,a Y-Crotono-lactone a" 10.57 a' 10.70 a" 11.43 — i i _ 1 1 1 1 — 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) Figure 3 - 2 7 -y-Eutyrolactone Table 3:1 Assignment Adiabatic Vertical v' IP (eV) IP (eV) (cm - 1) (eV) CNDO INDO (eV) 10. 06 10. 25 10.91* 1 5 2 0 12.83 14.11 1 2 . 0 3 1 3 . 3 8 Table 3:2 Electron Densities Assignment Orbital Energy Atom 1 Atom 2 Atom 3 CNDO INDO CNDO INDO 12.83 12. 03 14.11 13-38 .48 . 5 2 .34 . 2 9 . 1 6 • 1 9 .43 . 5 1 . 0 8 . 0 6 . 0 3 . 0 2 - 28 -The second band was assigned to the highest occupied TT l e v e l which i s mainly l o c a l i z e d on the oxygen atom i n the r i n g . As i n the case of t e t r a h y d r o f u r a n , the band shows l i t t l e v i b r a t i o n a l s t r u c t u r e . The r e s u l t s of the c a l c u l a t i o n s confirm the o r d e r i n g of these two l e v e l s . 3:2:3 2 j5-Dihydrofuran The sample of t h i s compound, obtained from the Columbia Chemical Co., contained water and so before use i t was d r i e d over magnesium sulphate and molecular sieves and i t s p u r i t y checked by NMR. The r e s u l t s of the experiments and the c a l c u l a t i o n s are given i n Tables 3:3 and 3:4. The s t r u c t u r e used i n the c a l c u l a t i o n s i s the one that was used f o r the i n t e r p r e t a t i o n of the f a r IR spectrum Both of the f i r s t two bands are i o n i z a t i o n s from TT type l e v e l s d e r i v e d from the combination of the C=C TT bond and the oxygen lone p a i r . Because the f i r s t IP of cyclopentene i s lower than that of t e t r a h y d r o f u r a n , the f i r s t band of dihydro-furan i s expected to be mainly C=C bonding, w i t h s m a l l e r c o n t r i b u t i o n s from the oxygen lone p a i r . This i s confirmed i n the spectrum by the e x c i t a t i o n of a v i b r a t i o n w i t h a frequency of 1400 cm-"1", which probably corresponds to the C=C s t r e t c h . The second band was assigned to the o r b i t a l mainly d e r i v e d from the oxygen lone p a i r , because the band la c k s any - 29 -2,5-Dihydrofuran Table 3:3 Assignment Adiabatic Vertical IP (eV) IP (eV) (cm x) CNDO (eV) INDO (eV) 9.14 10. 30 9.14 10.59 950 1400 11.55 16.14 11. 24 14.95 Table 3:4 Electron Densities Assignment Orbital Energy Atom 1 Atom 2 Atom 3 CNDO INDO 11.55 11. 24 10 15 04 04 . 21 • 34 CNDO INDO 16.14 14.95 71 68 01 00 14 16 - 30 -d i s t i n c t v i b r a t i o n a l s t r u c t u r e , as was observed i n the previous two molecules. The c a l c u l a t i o n s p r e d i c t that there should be one or two a o r b i t a l s between these two TT l e v e l s , but t h i s may be due to the f a c t that these 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 sometimes overestimate the se p a r a t i o n between IP's of the 81 same symmetry . A l s o , as w i l l be seen, assignment of the second band as a a i o n i z a t i o n would lead to an unreasonably l a r g e s e p a r a t i o n of the two TT IP's. 3 : 2 : 4 y-Crotonolactone This compound was prepared i n t h i s l a b o r a t o r y according O p to the method of P r i c e and Judge and a n a l y s i s by NMR and IR showed no i m p u r i t i e s . The f i r s t band i n the spectrum c o n s i s t s of two overlapped i o n i z a t i o n s , as shown by the f a c t that the f i r s t two peaks of the band are r e l a t i v e l y narrow, but the t h i r d i s broadened and asymmetric, due to the second i o n i z a t i o n . The prominent v i b r a t i o n a l p r o g r e s s i o n i s d i f f i c u l t to a s s i g n d e f i n i t i v e l y , but i t i s probably the C=C s t r e t c h i n g mode, which means that the i o n i z a t i o n i s from the highest occupied ir l e v e l . This means that the un d e r l y i n g i o n i z a t i o n i s the carbonyl lone p a i r , since the p o s i t i o n of t h i s IP should be about the same as i n b u t y r o l a c t o n e , but the value of the IP cannot be measured a c c u r a t e l y , because of problems w i t h the bands - 31 -Y-Crotonolactone H H Table 3:5 Assignment Adiabatic Vertical IP (eV) IP (eV) V ' l (cm"1) CNDO (eV) INDO (eV) a" 10.21 10.57 1420 13 . 12 12.-a' 10.70 13 .74 12.. a" 11.43 Table 3:6 1 6 .70 15. Electron Densities iignment Orbital Energy Atom 1 Atom 2 Atom 4 Atom 5 Atom CNDO 13-12 .19 .35 .11 .14 .03 INDO 12.65 .26 .36 .09 . 11 .02 CNDO 13-74 . 1 6 . 6 1 .01 .12 .05 INDO 12.72 .17 .63 .01 . 11 .03 CNDO 1 6 . 70 . 5 1 .00 .24 .25 .00 INDO 15.85 .47 .00 .26 .27 .00 - 32 -ove r l a p p i n g . The c a l c u l a t i o n s support t h i s assignment of the TT l e v e l being the f i r s t o r b i t a l and followed by the lone p a i r . F i n a l l y , the t h i r d band represents the other TT l e v e l d e r i v e d from the o r b i t a l s of d i h y d r o f u r a n , and again, no v i b r a t i o n a l f i n e s t r u c t u r e i s evident. 3:2:5 1,3-Dioxolane K&K L a b o r a t o r i e s s u p p l i e d the sample of t h i s compound and before the spectrum was run, i t s p u r i t y was checked by O o Q h NMR. The molecule i s thought t o be non-planar 2 3 , but the assignments are made assuming the C poi n t group, s i n c e 2v d e v i a t i o n s from p l a n a r i t y would not change the arguments much. The f i r s t two bands correspond to i o n i z a t i o n from the symmetric and antisymmetric combinations of the lone p a i r s of the two oxygen atoms. I t i s not obvious which of these w i l l have the lower IP, but the second band appears sharper and hence l e s s bonding and was t h e r e f o r e assigned to the a o r b i t a l , which has a node down the centre of the molecule. 2 The c a l c u l a t i o n s a l s o give t h i s assignment, but t h i s order must not be regarded as c o n c l u s i v e . 3:2:6 Ethylene Carbonate P r a c t i c a l grade ethylene carbonate was obtained from 33 T 1 1 1 r O —| , : 1 , 1 , 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) Figure 4 - 3 4 -1 , 3 ~ D i o x o l a n e H H Assignment A d i a b a t i c V e r t i c a l v' IP (eV) IP (eV) ( c m - 1 ) CNDO (eV) b l 9 . 7 9 1 0 . 1 2 1 3 a 2 1 0 . 6 2 14 T a b l e 3 : 8 E l e c t r o n D e n s i t i e s Assignment O r b i t a l Atom 1 Atom Energy CNDO 1 3 . 6 5 . 0 8 • 2 9 b l INDO 1 2 . 8 7 . 0 5 . 3 1 CNDO 14. 42 . 0 0 . 2 2 a 2 INDO 1 3 . 7 9 . 0 0 . 2 7 INDO (eV) 1 2 . 8 7 1 3 . 7 9 - 35 -the Eastman company, p u r i f i e d by s u b l i m a t i o n and i t s p u r i t y v e r i f i e d by NMR. Traces of water appeared i n some of the sp e c t r a , but the p o s i t i o n s of the peaks i n a PE spectrum of water do not i n t e r f e r e with the bands of i n t e r e s t here. Again, there i s evidence that the s k e l e t o n of the molecule i s not s t r i c t l y p l a n a r 8 ^ 5 8 6 , but the point group was used f o r the reasons given e a r l i e r . The f i r s t band c o n s i s t s of at l e a s t two I P ' s , but the overlap of the bands i s so great that the values of the v e r t i c a l IP's are only approximate. These two IP's are the b^ o r b i t a l , which i s mainly the carbonyl oxygen lone p a i r , and the b o r b i t a l which r e s u l t s from the i n t e r a c t i o n of the carbonyl TT bond and the b^ combination of the oxygen lone p a i r s . I t i s d i f f i c u l t to say which o r b i t a l has the lower IP, but the c a l c u l a t i o n s and the trends observed i n the other molecules i n the s e r i e s i n d i c a t e that the b^ i s at higher energy (lower IP) and the spectrum has been assigned a c c o r d i n g l y . There appears to be some v i b r a t i o n a l s t r u c t u r e at the f r o n t of the band c o n s i s t i n g of a pr o g r e s s i o n i n one of the r i n g v i b r a t i o n s at 1000 cm - 1 and (probably) the C=0 s t r e t c h at 1500 cm - 1 ( c f . 1 8 6 8 cm - 1 i n the n e u t r a l , , 87,88\ molecule 5 ). The second band of the spectrum may c o n t a i n the a^ combination of the oxygen lone p a i r s , but no assignment has been made because t h i s band probably contains another IP and - 36 -E t h y l e n e Carbonate 1 0 Assignment A d i a b a t i c IP (eV) V e r t i c a l IP (eV) v ' l (cm" 1) CNDO (eV) INDO (eV) b 2 10.89 11.5 1000 1500 14.53 13.69 b l 11.8 Table 3: 10 14.77 14. 06 E l e c t r o n D e n s i t i e s Assignment O r b i t a l Energy Atom 1 Atom 2 Atom 4 Atom 5 CNDO 14.53 • 58 .18 . 02 . 00 2 * INDO 13.69 .59 .18 . 01 . 00 CNDO 14.77 .43 .23 .03 . 01 1 INDO 14 . 0 6 .42 .24 . 02 . 01 - 37 -no h e l p f u l v i b r a t i o n a l f i n e s t r u c t u r e could be r e s o l v e d . 3:2:7 Vinylene Carbonate Vinylene carbonate was obtained from the A l d r i c h company and an NMR spectrum was taken of the sample to measure i t s p u r i t y . The geometry of the molecule has been worked out from 8Q the microwave spectrum * and i t was t h i s that was used i n the c a l c u l a t i o n s . The f i r s t IP i s too low t o be deriv e d from any of the o r b i t a l s of ethylene carbonate, and t h e r e f o r e i t must be assigned to a IT l e v e l , which comes from the a d d i t i o n of a double bond. The v i b r a t i o n a l f i n e s t r u c t u r e supports t h i s , because the frequency of 1380 cm-"'" i s i n the r i g h t r e g i o n f o r a double bond s t r e t c h i n an i o n ( c f . 1320 cm - 1 i n the 96 analogous band i n cyclopentene^ ). The second band represents i o n i z a t i o n from the carbonyl lone p a i r o r b i t a l , but again i t i s seen that i t has extensive bonding c h a r a c t e r . The prominent v i b r a t i o n e x c i t e d has an i n t e r v a l of 15^0 cm-"1" which i n d i c a t e s that i t i s probably the carbonyl s t r e t c h i n g mode, as might be expected. F i n a l l y , o v erlapping the onset of the a i o n i z a t i o n s , there i s a r e p r o d u c i b l e s e r i e s of peaks which form a v i b r a t -i o n a l p r o g r e s s i o n w i t h an i n t e r v a l of 1500 cm - 1. This would i n d i c a t e that the o r b i t a l i n v o l v e d i s a s s o c i a t e d with the carbonyl group, and t h e r e f o r e t h i s band has been assigned t o - 38 -Vinylene Carbonate 1 0 Assignment A d i a b a t i c V e r t i c a l IP (eV) IP (eV) (cm - 1) CNDO (eV) b l 9 .67 9. 84 1380 12. 43 800 b 2 11.34 11. 91 1540 14 . 24 980 b l 13. 2 17. 53 Table 3:12 E l e c t r o n D e n s i t i e s Assignment O r b i t a l Atom 1 Atom 2 Atom 4 Atom .Energy CNDO 12.43 .26 .18 . 00 .19 b l INDO 11 . 65 .26 • 19 . 00 .18 CNDO 14.24 .59 .18 . 02 . 00 b 2 .62 INDO 13-35 .17 . 01 .01 CNDO 17.53 . 42 .00 .16 . 20 b l INDO 16.99 . 42 .01 .16 . 21 INDO (eV) 11.65 13.35 16.99 - 3 9 -the second highest TT l e v e l , which correlates with the highest 77 o r b i t a l of ethylene carbonate. 3 : 2 : 8 4-Cyclopentene 1 ,3-Dione on Dr. G. H. DePuy^ kindly provided a sample of t h i s compound, which was r e p u r i f i e d by r e c r y s t a l l i z a t i o n from ether. Unfortunately, no s t r u c t u r a l data are av a i l a b l e , but the molecule i s most probably planar, belonging to the point group C 2 v. The f i r s t and second bands represent ionizations from the antisymmetric (b 2) and symmetric (a^) combinations of the carbonyl oxygen lone pairs, respectively, but the vibrations excited are impossible to i d e n t i f y at the moment. The prominent v i b r a t i o n i n the t h i r d band has an i n t e r v a l of about 1400 cm-"'", which would suggest a carbon-carbon double bond v i b r a t i o n . Therefore t h i s band i s assigned to a delocalized TT l e v e l whose main bonding character i s between the doubly bonded carbon atoms. The band at 1 6 . 5 8 eV, showing a v i b r a t i o n a l progression with i n t e r v a l 1470 cm-"'" was unexpected, but gives some additi o n a l insight into the bonding. From the v i b r a t i o n a l structure, i t follows that t h i s represents a TT l e v e l and so i t was assigned to the symmetry determined a^ o r b i t a l . This has a node along the C^ axis of the molecule and i s antibonding between the doubly bonded carbons, hence at higher energy IONIZATION P O T E N T I A L (eV) F i g u r e 5 - 41 -4-Cyclopentene 1,3-Dlone H H H H Table 3:13 Assignment Adiabatic Vertical CNDO INDO IP (eV) IP (eV) (cm-1) - (eV) (eV) 9.76 9.86 350 13.25 12.24 10.63 10.63 300 14.44 13.42 11.16 11.16 1400 13.47 13.25 16.58 16.58 1470 17.71 17.47 _ h2 -Table 3:14 Electron Densities Assignment Orbital Atom 1 Energy Atom 2 Atom 3 Atom 6 CNDO INDO CNDO INDO CNDO INDO CNDO INDO 1 3 . 2 5 1 2 . 24 14 . 44 1 3 . 42 1 3 . 4 7 1 3 . 2 5 1 7 . 7 1 1 7 . 4 7 . 1 7 . 1 7 . 1 2 . 1 0 . 0 1 . 0 2 . 0 0 . 0 0 06 05 04 03 05 04 21 21 .05 .04 .08 .07 .14 .15 .01 .01 . 2 7 . 2 9 • 3 1 . 3 3 . 2 7 . 2 7 . 2 8 . 2 8 - 43 -(lower IP) than the third TT level which has b^ symmetry. 3:2:9 Maleimide This compound was purchased from the Aldrich Company and i t was checked for impurities by NMR. Because of i t s low vapour pressure a special inlet system was built, which bypassed the leak valve and admitted the sample vapour to the c o l l i s i o n chamber directly through a half-inch stopcock. The sample container was heated to 40°C with a water bath, and this gave sufficient pressure in the co l l i s i o n chamber to obtain a spectrum. However, the intensity was low and as a result the signal-to-noise ratio in the spectra was small. The f i r s t band was assigned to a TT orbital consisting mainly of the lone pair of the nitrogen atom, with contributions from the double bond and the carbonyl groups. The vibrational interval of 1150 cm-"'" seems to be too low for a C=0 or a C=C stretching mode but i t may correspond to the C-N-C stretch which has been observed at 1350 cm - 1 in the 91 neutral molecule^ . After the f i r s t band there is a series of three distinct peaks. In this region of the spectrum there should be ionizations from the symmetric and antisymmetric combinations Q2 of the carbonyl oxygen lone pairs, but the UV spectrum shows^ two low TT -*• T T * transitions and so there may be another TT ionization as well. Also, i f the last two peaks of the series - 4 4 -Table 3 : 1 5 Assignment Adiabatic IP (eV) Vertical v' IP (eV) (cm - 1) CNDO (eV) INDO (eV) b l 1 0 . 3 1 1 0 . 3 1 1 1 2 5 5 0 0 1 3 . 0 3 12.46 b2 1 0 . 8 6 1 3 . 0 5 1 2 . 0 6 a i 1 1 . Table 40 3 : 1 6 14. 24 1 3 - 2 7 Electron Densities Assignment Orbital Energy Atom 1 At cm 2 Atom 3 Atom 6 CNDO h -. 1 3 . 0 3 . 3 4 . 0 2 . 0 2 . 2 8 D l INDO 12.46 . 3 9 . 0 1 . 0 2 . 2 8 CNDO 13.05 . 1 0 . 0 6 . 0 8 . 2 7 ^ INDO 1 2 . 0 6 . 1 1 .04 . 0 7 . 3 0 CNDO 14. 24 . 0 9 . 0 3 . 1 1 . 3 0 a l INDO 1 3 . 2 7 • 0 7 . 0 1 . 0 9 • 3 2 - 45 -are considered as one i o n i z a t i o n , then the i n t e n s i t i e s of the two carbonyl lone p a i r bands are q u i t e d i f f e r e n t , i n contrast to the two other molecules i n t h i s s e r i e s . Therefore, the f i r s t and t h i r d peaks of the s e r i e s have been assigned as the carbonyl lone p a i r IP's and the peak i n the middle i s very t e n t a t i v e l y assigned as another TT i o n i z a t i o n . 3 : 2 : 1 0 M a l e i c Anhydride Reagent grade maleic anhydride was used without f u r t h e r p u r i f i c a t i o n . A c r y s t a l s t r u c t u r e 9 3 showed that the molecule i s s l i g h t l y non-planar i n the s o l i d , but i t i s assumed that i t i s planar i n the gas phase, because of the extensive conjugation. The f i r s t IP represents i o n i z a t i o n from the antisymmetric (b^) combination of the carbonyl lone p a i r s , as i n cyclopentene dione. The v i b r a t i o n at 9 5 0 cm - 1 i s some u n s p e c i f i e d s k e l e t a l mode, but that at 1 5 3 0 cm - 1 i s most probably the C=0 s t r e t c h i n g mode which has been assigned as 1 8 8 0 cm - 1 i n the n e u t r a l O i l molecule^ . Because of i t s s i m i l a r i t y to the TT IP's of the other molecules i n t h i s s e r i e s , the second band i s assigned to the highest occupied TT l e v e l . C o n t r i b u t i o n s to t h i s o r b i t a l from both the oxygen lone p a i r and the carbon-carbon double bond are expected to be important but i t i s d i f f i c u l t to assi g n the v i b r a t i o n that i s e x c i t e d , and so the bonding p r o p e r t i e s of - 46 -Maleic Anhydride Table 3:17 Assignment A d i a b a t i c V e r t i c a l IP (eV) IP (eV) a l 10. 98 11.83 14.20 11.06 11.83 12.44 14. 56 v - l (cm L ) 950 1530 930 1500 1630 CNDO (eV) 13.96 14. 25 14. 54 17.14 INDO (eV) 13.02 13.78 13.62 16.34 - 47 -Table 3:18 Electron Densities Assignment Orbital Atom 1 Atom 2 Atom 4 Atom Energy CNDO 13.96 .13 .25 .05 .08 b 2 .14 .04 INDO 13.02 .29 .07 CNDO 14.25 .19 .28 .03 .10 b l INDO 13.78 .27 .27 . 02 .06 CNDO 14. 54 .27 .25 .03 .09 l INDO 13.62 .29 .24 . 02 .09 CNDO 17.14 .49 .00 . 00 .25 b l INDO 16.34 .43 . 00 .00 .28 - 48 -t h i s o r b i t a l cannot be deduced. The t h i r d band represents the a combination of the 1 carbonyl lone p a i r s but no v i b r a t i o n a l f i n e s t r u c t u r e i s d i s c e r n a b l e . J u s t before the onset of the a i o n i z a t i o n s , there i s another band which d i s p l a y s a v i b r a t i o n a l p r o g r e s s i o n w i t h an i n t e r v a l of I63O cm--'-. This probably corresponds to the carbonyl s t r e t c h i n g mode, and since another low l y i n g TT i o n i z a t i o n i s expected, then t h i s band was assigned as b,. 3 : 3 D i s c u s s i o n 3 : 3 : 1 I n t r o d u c t i o n A l l of these molecules, or at l e a s t t h e i r s k e l e t o n s , are assumed to be plan a r , and from t h i s assumption there f o l l o w s a ri g o r o u s s e p a r a t i o n o f a and TT type l e v e l s . The former are defined to be those o r b i t a l s which are symmetric w i t h respect to r e f l e c t i o n i n the place of the molecule, whereas the l a t t e r are antisymmetric, which i s i n keeping w i t h the usual p i c t u r e of s i n g l e and double bonds i n a pl a n a r molecule. However, t h i s d e f i n i t i o n of a TT o r b i t a l i s more general. The heteroatom lone p a i r that occupies the p o r b i t a l perpend-i c u l a r to the molecular plane i s i n a TT o r b i t a l , a ccording to t h i s c r i t e r i o n , and a TT o r b i t a l can be cons t r u c t e d out of combinations of the two C-H s i n g l e bonds i n a methylene - 49 -group. The r e s u l t i s that the o r b i t a l corresponding to a fo r m a l l y l o c a l i z e d TT bond can have c o n t r i b u t i o n s from a l l such groupings. In terms of group theory, i n the C po i n t s group any o r b i t a l that transforms as the a" r e p r e s e n t a t i o n i s a TT o r b i t a l whereas a' represents a type o r b i t a l s . S i m i l a r l y , i n C • the TT o r b i t a l s transform as b, or a^, 2v 1 2 and the a o r b i t a l s have b^ or a^ symmetry. The importance of t h i s c l a s s i f i c a t i o n according to symmetry i s that two o r b i t a l s of d i f f e r e n t symmetries can have no net ov e r l a p , and furthermore any Hamiltonian matrix elements between them are n e c e s s a r i l y zero. Therefore, conjugation or other resonance i n t e r a c t i o n s depending on the overlap of e l e c t r o n clouds can only occur between o r b i t a l s of l i k e symmetry. The two carbon-carbon TT bonds i n butadiene can conjugate because the molecule i s pl a n a r and both o r b i t a l s have the same symmetry, but i n a l l e n e , the two TT bonds are perpendicular and there i s no d i r e c t i n t e r a c t i o n . However, i n d i r e c t i n t e r a c t i o n s do take place between o r b i t a l s of d i f f e r e n t symmetries, and these can be understood i n terms of the i n d u c t i v e e f f e c t . For i n s t a n c e , a s t r o n g l y e l e c t r o n e g a t i v e atom near a carbon-carbon TT bond w i l l withdraw e l e c t r o n d e n s i t y and hence increase the e f f e c t i v e n u c l e a r charge on carbon. This means that the energies of the p o r b i t a l s that make up the TT bond w i l l be lowered, and so the IP of t h i s o r b i t a l w i l l be g r e a t e r . Therefore, the i n t e r a c t i o n - 50 -of the e l e c t r o n e g a t i v e atom and the TT bond through the i n d u c t i v e e f f e c t has l e d to a s t a b i l i z i n g o f the TT bond. Of course, i n d u c t i v e e f f e c t s w i l l a l t e r a l l the o r b i t a l s , r e g a r d l e s s o f symmetry, but with o r b i t a l s of d i f f e r e n t symmetries the e f f e c t stands out because i t i s the major mode of i n t e r a c t i o n . T h e r e f o r e , i n a s s e s s i n g the change caused by adding a s u b s t i t u e n t t o a molecule account must be taken f i r s t o f how the i n d u c t i v e e f f e c t s have a l t e r e d a l l the o r b i t a l s and then the resonance i n t e r a c t i o n s between o r b i t a l s of l i k e symmetries must be es t i m a t e d . 3:3:2 Behaviour of the Carbonyl Lone P a i r s . In compounds c o n t a i n i n g a c a r b o n y l group, a r e l a t i v e l y low IP i s u s u a l l y observed c o r r e s p o n d i n g t o i o n i z a t i o n from the lone p a i r on the c a r b o n y l oxygen. The exact composition of t h i s o r b i t a l depends on the p a r t i c u l a r molecule, but the main component i s the oxygen p o r b i t a l t h a t l i e s i n the plane, i . e . i t i s p e r p e n d i c u l a r t o the TT bond. Thus, f o r pl a n a r molecules, t h i s i s a a type o r b i t a l and so i t can i n t e r a c t w i t h the a framework and a c q u i r e some bonding c h a r a c t e r . T h i s i s evidenced by the f a c t t h a t many o f the bands a s s i g n e d t o t h i s o r b i t a l show v i b r a t i o n a l s t r u c t u r e , f o r example, t h i s band i n v i n y l e n e carbonate has a long v i b r a t i o n a l p r o g r e s s i o n (at l e a s t seven members) and the v e r t i c a l t r a n s i t i o n occurs at v'=4. Furthermore, s i n c e i t i s a a o r b i t a l , the s h i f t i n the IP i n d u c e d by a change i n the TT s t r u c t u r e , such as t h e a d d i t i o n o f a doub l e bond, w i l l be m o s t l y due t o the i n d u c t i v e e f f e c t , because t h e o v e r l a p o f the l o n e p a i r w i t h t h e a framework i s not e x p e c t e d t o change a p p r e c i a b l y . These changes a c t as p e r t u r b a t i o n s o f the c a r b o n y l l o n e p a i r and t h e r e f o r e , s i n c e f i r s t o r d e r p e r t u r b a t i o n s a r e c u m u l a t i v e , the e f f e c t s o f s u b s t i t u t i n g v a r i o u s groups on the IP o f t h i s o r b i t a l s h o u l d be appr o x -i m a t e l y a d d i t i v e . F o r the e i g h t compounds c o n t a i n i n g c a r b o n y l groups the s h i f t o f t h e c a r b o n y l l o n e p a i r IP from c y c l o p e n t a n o n e was f i t t e d t o an e x p r e s s i o n o f t h e form AIP = n 6 + n <5 + n 6 (3:1) o o c=c c=c c=o c=o where n . n and n are t h e numbers o f oxygen atoms a o c=c c=o t o t h e c a r b o n y l , t h e number o f c a r b o n - c a r b o n d o u b l e bonds i n the r i n g , and t h e number o f o t h e r c a r b o n y l groups 8 t o the c a r b o n y l i n q u e s t i o n ( f o r example, m a l e i c a n h y d r i d e has n = 1, n =1). The S's a r e the s h i f t s o f the IP i n d u c e d o c=c by t h e s e groups. I n the case o f t h e d i c a r b o n y l compounds, t h e r e a r e , o f c o u r s e , two c a r b o n y l l o n e p a i r I P ' s and so t h e mean o f the two was used. The l e a s t squares v a l u e s o f t h e parameters were found t o be 6 = 1.18 eV, 5 = 0.24, 6 = 0.92 o c=c ' c=o and a p l o t o f the o b s e r v e d IP v e r s u s the v a l u e s c a l c u l a t e d - 52 -from e q u a t i o n 1 i s g i v e n i n the diagram ( F i g u r e 7). The v a l u e g i v e n above f o r the e f f e c t o f a d d i n g d i f f e r e n t groups i s not t r u l y an i n d u c t i v e parameter, because the i n t e r a c t i o n s . w i t h the a o r b i t a l s have been n e g l e c t e d and as w i l l be seen i n the d i c a r b o n y l compounds, t h e s e can be s u b s t a n t i a l . However, the good f i t o f o b s e r v e d t o c a l c u l a t e d I P ' s means t h a t the model f i t s t h i s v e r y r e s t r i c t e d s e r i e s o f m o l e c u l e s q u i t e w e l l . These parameters a l s o f i t t h e few o t h e r c a r b o n y l compounds t h a t have been s t u d i e d by PES F o r i n s t a n c e , from p r o p i o n a l d e h y d e (CH CH CHO) t o a c r o l e i n 3 2 (CH = CHCHO) the c a r b o n y l l o n e p a i r IP s h i f t s by 0.14 eV 2 from 9-97 eVyj t o 10.11 eV , which i s c o n s i s t e n t w i t h the change i n d u c e d by a c a r b o n - c a r b o n double bond i n t h i s s e r i e s , 0.24 eV. The e f f e c t o f an oxygen atom a t o t h e c a r b o n y l can be seen i n the two m o l e c u l e s a c e t a l d e h y d e and f o r m i c a c i d . 95 I n the f o r m e r , t h e c a r b o n y l l o n e p a i r IP i s 10.22 eV , whereas t h e c o r r e s p o n d i n g i o n i z a t i o n i n f o r m i c a c i d comes at 11.51 e V 7 9 , a s h i f t o f 1.3 eV, a g a i n i n agreement w i t h the v a l u e o b t a i n e d here o f 1.2 eV. The parameter g i v i n g the e f f e c t o f a c a r b o n y l group 8 t o a n o t h e r c a r b o n y l i s p r o b a b l y t h e l e a s t r e l i a b l e because o n l y two such compounds were i n c l u d e d i n t h e c a l c u l a t i o n s and b e s i d e s , t h e mean o f the c a r b o n y l l o n e p a i r I P ' s may not be t h e b e s t measure o f the "average" IP o f t h e two groups. However, f o r t h e s i x compounds - 53 -with j u s t one c a r b o n y l group a f i t to an e x p r e s s i o n of the form AIP = n 6 + n 6 (3:2) G = C C = C O O was performed, i n order to see i f the d i c a r b o n y l compounds a f f e c t e d the other parameters, but the values o b t a i n e d from t h i s f i t were not s i g n i f i c a n t l y d i f f e r e n t from the o t h e r f i t . 3 : 3 : 3 TT L e v e l s . The TT o r b i t a l s i n a l l these molecules can be thought of as a r i s i n g from combinations of three d i f f e r e n t f u n c t i o n a l groups, or r a t h e r the TT o r b i t a l s of these groups, - a carbon-carbon double bond, a c a r b o n y l group and a heteroatom ( u s u a l l y oxygen) which makes up p a r t of the r i n g . U s u a l l y one of these i s the dominant c o n t r i b u t o r , so t h a t a TT o r b i t a l of a complicated molecule may be d e s c r i b e d , f o r i n s t a n c e , as the oxygen lone p a i r p e r t u r b e d by a nearby c a r b o n y l group. However, i n o r d e r t o use t h i s p i c t u r e of a d e l o c a l i z e d TT o r b i t a l as b e i n g the r e s u l t o f i n t e r a c t i o n s between s i m p l e r c o n s t i t u e n t s , a model must be chosen to show what the s i t u a t i o n would have been i f t h e r e had been no i n t e r a c t i o n . For example, i f butadiene i s c o n s i d e r e d t o be the r e s u l t of p u t t i n g t o g e t h e r two e t h y l e n e molecules, then i n order to estimate the i n t e r -a c t i o n , the c h a r a c t e r i s t i c s of the unperturbed e t h y l e n e molecule must be known. - 54 -CARBONYL OXYGEN LONE PAIR IP'S 9.01 10.0L p 11.0L z UJ o 0. Z o I-N 12.0L o o o o o o ft II II II II II a a a a a a a tf'tftr Figure 6 Compound Cyclopentanone Cyclopentene 2-one Cyclopentene 3-one y-Butyrolactone Y-Crotonolactone Ethylene carbonate Vi n y l e n e carbonate Cyclopentene dione M a l e i c anhydride Table 3:19 Carbonyl Lone P a i r IP's Obs. IP A IP ( r e l . to A IP ( c a l c . (eV) cyclopentanone) 9.25 0 . 0 0 . 0 9.34 .1 .24 9 .14 .2 .24 10 .26 1.0 1 .18 10.70 1.5 1 .42 11 .47 2 .2 2 .36 11 .91 2.7 2 . 6 0 10.25 1.0 1.16 11.75 2 .5 2 .34 - 56 -P L O T O F O B S E R V E D IP S H I F T S vs. T H O S E 3.0 2.0 -o CD 5 1.0 O CO o a. < 0 C A L C U L A T E D B Y E Q U A T I O N 3:1 • / / / s 1.0 2.0 3.0 A IP (observed) Figure 7 - 57 -The c h o s e n m o d e l compounds e x h i b i t i n g a c a r b o n - c a r b o n d o u b l e b o n d , a c a r b o n y l g r o u p a n d an u n p e r t u r b e d o x y g e n a t o m i n t h e r i n g a r e , r e s p e c t i v e l y , c y c l o p e n t e n e ( 3 ) , c y c l o p e n t -anone ( 4 ) , and t e t r a h y d r o f u r a n ( 5 ) . a a 3 4 5 The I P o f t h e TT b o n d i n c y c l o p e n t e n e i s r e p o r t e d t o be 96 9.18 eV a n d t h i s p r e s e n t w o r k h a s f o u n d t h e v e r t i c a l I P o f t e t r a h y d r o f u r a n t o be 9 .75 eV. U n f o r t u n a t e l y , t h e I P o f t h e c a r b o n y l TT b o n d i n c y c l o p e n t a n o n e h a s n o t be a s s i g n e d - ^ , b u t t h e a n a l o g o u s I P i n f o r m a l d e h y d e i s 14 . 0 9 e Y ^ , s o t h a t t h i s TT I P i n a f i v e membered r i n g i s c e r t a i n l y g r e a t e r t h a n t h a t o f t h e o t h e r two g r o u p s . S i n c e t h e s e m o l e c u l e s a r e p l a n a r , t h e TT o r b i t a l s a r e p a r a l l e l t o e a c h o t h e r a n d a r e o f t e n on o p p o s i t e s i d e s o f t h e r i n g , s o t h e d i r e c t t h r o u g h s p a c e i n t e r a c t i o n i s e x p e c t e d 70 t o be s m a l l 1 . T h e r e f o r e , i f i n t e r a c t i o n s b e t w e e n o r b i t a l s a r e o b s e r v e d , t h r o u g h b o n d c o u p l i n g must be t h e m a i n c a u s e , b u t s i n c e t h e s e o r b i t a l s a r e a l l o f t h e same s y m m e t r y , t h i s c a n t a k e p l a c e i n two ways. As e x p l a i n e d b e f o r e , t h e s e a r e by t h e r e s o n a n c e a n d t h e i n d u c t i v e e f f e c t s . The f i r s t molecule to be considered i s 2,5 dihydrofuran ( 6 ) , i n which an oxygen atom i s j o i n e d to a carbon-carbon double bond by two methylene groups. There are two TT IP'S and i n Hoffmann's p i c t u r e the s h i f t s of these IP's from the values i n -the model compounds are a measure of the i n t e r a c t i o n of the two o r b i t a l s . The observed IP's are 9.14 eV and 10.59 eV, whereas the unperturbed IP's are 9.18 eV i n cyclopentene and 9-75 eV i n t e t r a h y d r o f u r a n . Part of the observed increase i n the s e p a r a t i o n of the two IP's can be a s c r i b e d to a resonance e f f e c t i n which the two methylene groups couple the lone p a i r to the double bond, so t h a t they " f e e l " and r e p e l each other. A s i m i l a r s i t u a t i o n i s observed i n 1,4 cyclohexadiene ( 2 ) , i n which two double bonds are coupled by two methylene groups. The two o r b i t a l s are degenerate i n the absence of i n t e r a c t i o n s , but c o u p l i n g them together leads to a s p l i t t i n g of the TT IP's of 1.0 e V 7 0 . However, i n the case of dihydrofuran i n d u c t i v e e f f e c t s must a l s o be i n c l u d e d . Oxygen i s s t r o n g l y e l e c t r o n e g a t i v e and the replacement of s p J h y b r i d i z e d carbons by the sp - 59 -type a l s o causes an i n d u c t i v e s t a b i l i z a t i o n of the o r b i t a l s , so that these two i n d u c t i v e e f f e c t s r a i s e the IP of both o r b i t a l s . This i n d u c t i v e s t a b i l i z a t i o n may not, however, be the same f o r the two o r b i t a l s and so part of the observed s p l i t t i n g of the l e v e l s may be caused by i n d u c t i v e e f f e c t s . Therefore, f o r the highest occupied o r b i t a l i n dihydrofuran the resonance e f f e c t works to decrease the IP, r e l a t i v e to cyclopentene, but the i n d u c t i v e e f f e c t i n c r e a s e s i t , w i t h the r e s u l t that the f i r s t IP of t h i s compound i s 9.14 eV, almost the same as i n cyclopentene, 9.18 eV. However, f o r the other T T o r b i t a l , the resonance and i n d u c t i v e e f f e c t s r e i n f o r c e each other, r a i s i n g the second IP from 9.75 eV i n t e t r a h y d r o f u r a n to 10.59 eV i n dihydrofuran. The a d d i t i o n of a carbonyl group to dihydrofuran forming crotonolactone (7) adds another TT o r b i t a l at much lower energy C = C 0 o 7 (higher I P ) , so that the o r b i t a l s of dihydrofuran can serve - 60 -as models f o r the upper two TT l e v e l s of crotonolactone. Prom t h i s viewpoint, the IP's of both these o r b i t a l s i n c r e a s e , but the s e p a r a t i o n between them i s decreased to 0.9 eV from l.H eV i n d i h y drofuran. The increase i n both IP's i s c l e a r l y due to the i n d u c t i v e e f f e c t of the carbonyl group, which withdraws e l e c t r o n d e n s i t y from the a l e v e l s , as was seen i n the trends of the carbonyl lone p a i r IP's, but the decrease i n s e p a r a t i o n i s mainly due to resonance e f f e c t s . One of the r e s u l t s of p e r t u r b a t i o n theory i s that the amount that one l e v e l mixes w i t h and perturbs another i s i n v e r s e l y p r o p o r t i o n a l to the energy d i f f e r e n c e between the tvjo l e v e l s . Thus, l e v e l s that are close i n energy are s t r o n g l y a f f e c t e d , but d i s t a n t ones are hardly changed. This i s the s i t u a t i o n i n crotonolactone. The T T o r b i t a l of the carbonyl group i s w e l l below both the l e v e l s of d i h y d r o f u r a n , which leads t o both of them being increased i n energy by the resonance r e p u l s i o n . However, the lower of the two i s c l o s e r ( i n energy) to the carbonyl group and thus f e e l s the e f f e c t more s t r o n g l y . I t i s increased i n energy more than the upper l e v e l and the s e p a r a t i o n of the l e v e l s i s decreased. a -\ \ s c=o - 6 1 -At f i r s t s i g h t , i t looks as i f the a d d i t i o n of another carbonyl group to crotonolactone to form maleic anhydride (8) 8 should decrease the s e p a r a t i o n of the f i r s t two TT l e v e l s s t i l l f u r t h e r . E x p e r i m e n t a l l y , i t i s found that the s e p a r a t i o n increases from 0.92 eV" i n the lactone to 2.4 eV i n maleic anhydride. This occurs because symmetry now plays a p a r t . M a l e i c anhydride belongs to the C po i n t group, which means that the 2v carbonyl o r b i t a l s must be considered i n . terms of symmetry adapted o r b i t a l s . In t h i s case, the carbonyl TT bonds form a symmetric and an antisymmetric combination, with the a n t i -symmetric combination at higher energy (s i n c e i t has a node down the middle of the molecule). In terms of group theory, the symmetric combination transforms as b_^  and the a n t i -symmetric one as a . However, the two highest TT l e v e l s , the 2 ones that are of i n t e r e s t here, and both of b_^  symmetry and cannot i n t e r a c t with the a combination of the carbonyl groups. 2 The only other b o r b i t a l i s the b^ combination of the ca r b o n y l s , which has been s t a b i l i z e d , r e l a t i v e to crotono-l a c t o n e . Therefore, i f the s e p a r a t i o n of the two highest - 62 -occupied TT levels of both crotonolactone and maleic anhydride i s r a t i o n a l i z e d i n terms of a carbonyl function at lower energy which forces the two o r b i t a l s together, then maleic anhydride has a larger separation because the carbonyl group i s further away i n energy. C=0 The PE spectra of the series 1,3-dioxolane, ethylene carbonate and vinylene carbonate, a l l with two oxygen atoms i n the ri n g , can be explained i n s i m i l a r terms. In 1,3-dioxolane (9), the oxygen lone pairs form symmetric (b^) and antisymmetric o o o o I I I I 9 10 (a^) combinations, although the nomenclature may not be rigorously applicable because-the molecule i s probably non-planar. The IP's of both these o r b i t a l s are s h i f t e d upwards - 63 -from t e t r a h y d r o f u r a n by i n d u c t i v e e f f e c t s and then s p l i t by about 0.5 eV, mainly through the i n f l u e n c e of the methylene group at the "2" p o s i t i o n , between the two oxygens. An a^ o r b i t a l has a node through t h i s p o s i t i o n and so i t w i l l be u n a f f e c t e d , but there i s a b combination of the C-H a bonds 1 which w i l l push the b oxygen lone p a i r s to higher energy 1 and hence to lower IP. This c o u p l i n g by the methylene group i s a form of hyperconjugation and i s the same e f f e c t that was seen i n dihydrofuran. There i s an a combination of the 2 C-H a bonds at the base of the r i n g , which can i n t e r a c t w i t h the oxygen lone p a i r s , but t h i s i s expected to be a l e s s e r e f f e c t , because the overlap i s s m a l l e r . Also the n o n - p l a n a r i t y of the r i n g probably decreases a l l the c o u p l i n g somewhat, because the overlap of the o r b i t a l s that i s necessary f o r the i n t e r a c t i o n i s l e s s . The a d d i t i o n of a carbonyl group t o give ethylene carbon-ate (10) s h i f t s a l l the o r b i t a l s to higher energy, as expected from i n d u c t i v e e f f e c t s , so that the f i r s t IP of dioxolane i s 10.12 eV, but the f i r s t TT i o n i z a t i o n of ethylene carbonate i s 11.82 eV. I f the molecule i s assumed to be p l a n a r , then the C=0 TT o r b i t a l transforms as b and can i n t e r a c t w i t h the b 1 1 combination of the oxygen lone p a i r s . The a o r b i t a l w i l l not 2 be a f f e c t e d , but the b combination w i l l be d e s t a b i l i z e d by 1 t h i s i n t e r a c t i o n and thus the f i r s t TT IP of ethylene carbonate can be c o n f i d e n t l y assigned as b . U n f o r t u n a t e l y , i t was not - 64 -p o s s i b l e to as s i g n the i o n i z a t i o n from the a o r b i t a l i n the 2 spectrum, but i t seems probable that i t i s contained i n the second band i n the spectrum at 13-75 eV. O II I—I 11 F i n a l l y , v i n y l e n e carbonate (11) i s regarded as formed by the i n c o r p o r a t i o n of a C=C double bond i n ethylene carbonate. The f i r s t IP i s the r e s u l t of the p e r t u r b a t i o n of t h i s double bond, but the resonance and i n d u c t i v e e f f e c t s work against each other. The l a t t e r e f f e c t i s dominant, so that the f i r s t T T IP i s s h i f t e d up by 0.66 eV from that i n cyclopentene to 9.84 eV. However, f o r the second TT IP, the two e f f e c t s add together, s h i f t i n g t h i s IP from 11.8 eV i n ethylene carbonate to about 13.2 eV i n the present case. O II o II _ 65 _ Table 3:20 TT IP's Compound O r b i t a l IP Cyclopentanone b^ >11.3 T e t r a h y d r o f u r a n b 9.75 Cyclopentene b_^  9 . 1 8 y - b u t y r o l a c t o n e a" 10.94 2,5-dihydrofuran b, 9.14 b£ 10.59 y - c r o t o n o l a c t o n e a" 10.57 a" 11.43 1,3-dioxolane b 10.12 a 1 10.62 Ethylene carbonate b^ 1 1 . 8 2 V i n y l e n e carbonate b 9.84 bj- 13.2 Cyclopentene dione b 11 . 1 6 a^ 16.58 Maleimide b 10.31 1 M a l e i c anhydride b 1 1 . 8 3 b 1 14.20 - 66 -3:3:4 The Dicarbonyl Compounds. The three molecules, 4-cyclopentene 1,3-dione (12), maleimide (13) and maleic anhydride (8) are i s o e l e c t r o n i c , a l l having 36 valence e l e c t r o n s , and so the PE sp e c t r a of the three show s i m i l a r i t i e s . Each spectrum e x h i b i t s three bands at low IP (maleimide may have four) which correspond to a d e l o c a l i z e d TT l e v e l and the symmetric and antisymmetric (a and b ) combinations of the carbonyl oxygen lone p a i r s , 1 2 but the r e l a t i v e and absolute p o s i t i o n s of these IP's c l e a r l y change. 99 To use Hoffmann's p i c t u r e , the carbonyl lone p a i r s are at f i r s t degenerate and then are s p l i t by through-space or i n d i r e c t through-bond i n t e r a c t i o n . Therefore, even though they are probably not s p l i t s y m m e t r i c a l l y , the mean of the two IP's can be used as a measure of the carbonyl lone p a i r IP before the i n t e r a c t i o n s have l i f t e d the degeneracy. I t i s found that t h i s mean IP increas e s from 10.15 eV i n cyclopentene dione to 11.15 eV i n maleimide and 11.75 eV i n maleic anhydride, f o l l o w i n g the increase i n e l e c t r o n e g a t i v i t y from carbon through n i t r o g e n to oxygen. As was seen w i t h the other 1 2 13 14 - 67 -c a r b o n y l compounds s t u d i e d , i n a r e l a t e d s e r i e s o f m o l e c u l e s t h e p o s i t i o n o f t h e c a r b o n y l l o n e p a i r I P seems t o be d e t e r -m i n e d m a i n l y be i n d u c t i v e e f f e c t s . S i n c e t h e d i s t a n c e between t h e o x y g e n atoms o f t h e two o c a r b o n y l g r o u p s i n t h e s e m o l e c u l e s i s a b o u t 5 A, d i r e c t o v e r l a p w i l l be e s s e n t i a l l y z e r o , so t h a t t h e o b s e r v e d s p l i t t i n g s qq must be due t o t h r o u g h bond i n t e r a c t i o n s . B o t h H o f f m a n n ' s ^ c a l c u l a t i o n s on some m o d e l s y s t e m s and t h o s e done f o r t h i s work p r e d i c t t h a t t h e a n t i s y m m e t r i c (h^) c o m b i n a t i o n w i l l be a t h i g h e r e n e r g y ( l o w e r I P ) w h i c h i s t h e way t h a t t h e s p e c t r a have been a s s i g n e d . T h i s c a n be r a t i o n a l i z e d by s a y i n g t h a t t h e s p l i t t i n g o c c u r s b e c a u s e o f i n t e r a c t i o n s w i t h t h e a o r b i t a l s o f t h e framework, and s i n c e t h e b ^ o r b i t a l o f t h e r i n g s t r u c t u r e has a node, i t may be e x p e c t e d t o be a t h i g h e r e n e r g y t h a n a c o r r e s p o n d i n g a ^ t y p e . T h e r e f o r e , s i n c e t h e change i n e n e r g y due t o t h e i n t e r a c t i o n v a r i e s i n v e r s e l y as t h e d i f f e r e n c e i n e n e r g y , t h e n t h e b^ c a r b o n y l l o n e p a i r w i l l be d e s t a b i l i z e d more t h a n t h e a n a l o g o u s a_^ o r b i t a l . . . b2 C=0 a1fb ^ 2 { = V \ \ / - a 2 a l e v e l s - 68 -For cyclopentene dione the observed s p l i t t i n g of 0 . 8 eV f i t s n i c e l y w i t h that observed i n 2 , 2 , 4 , 4 t e t r a m e t h y l c y c l o -butanedione (14) of 0 . 7 eV and a s p l i t t i n g of 0 .9 eV i n para benzoquinone ( 1 5 ) . On going to maleimide, the s p l i t t i n g decreases to 0 .6 eV, probably because the s e p a r a t i o n of the a o r b i t a l s and the lone p a i r s increases (the s e p a r a t i o n of the mean of the carbonyl IP's and the a onset i n cyclopentene dione i s 2 . 7 eV, i n maleimide i t i s 3 -3 eV). However, i n maleic anhydride the s p l i t t i n g i ncreases to 1.4 eV, which i s more d i f f i c u l t to e x p l a i n . The s e p a r a t i o n of the carbonyl IP's and the a onset i s 3 .2 eV, about the same as i n maleimide, but without a more complete knowledge of the o r b i t a l s i n v o l v e d i t i s impos s i b l e to draw any f i r m c o n c l u s i o n s . For the TT l e v e l s , simple Httckel c a l c u l a t i o n s 1 ^ p r e d i c t that the highest occupied TT l e v e l i n each molecule i s l o c a l i z e d completely on the carbonyl oxygens and the carbob-carbon double bond. I f t h i s were t r u e , the TT IP would be expected to increase i n the order CH <NH<0, as d i d the lone p a i r I P ' s , 15 14 - 69 -whereas the observed order i s NH<CH <0. This i s a l s o the order 2 of the IP's of p y r o l l i d i n e 1 0 1 (16), c y c l o p e n t e n e 9 6 (3) and tet r a h y d r o f u r a n ( 5 ) , ( 8 . 4 l eV, 9 -18 eV, 9 -75 eV, r e s p e c t i v e l y ) which i m p l i e s that there must be some p a r t i c i p a t i o n of the heteroatom lone p a i r . However, the o r b i t a l i s c e r t a i n l y d e l o c a l i z e d over the whole molecule and so there i s not much value i n a s s i g n i n g the IP t o one p a r t i c u l a r f u n c t i o n a l group. H A d d 16 Recently, the UV s p e c t r a of these molecules have been 92 102 10? i n v e s t i g a t e d ^ ' 3 J and the r e s u l t s are given i n Table 3:21, In a l l cases, the T T * o r b i t a l i s of a^ symmetry, and so i t has no de n s i t y on the heteroatom. Therefore i t should have approximately the same energy i n each molecule, and the trends i n the UV s p e c t r a should f o l l o w the trends i n the PE 104 s p e c t r a . However, care must be taken because the UV t r a n s i t i o n energy i s not j u s t the d i f f e r e n c e i n o r b i t a l energies. For t r a n s i t i o n s to the s i n g l e t s t a t e the energy i s given by AE.. = e. - e. - J . . + 2K.. (3:1) i j J i i j i j where J and K are the coulomb and exchange i n t e g r a l s i j i j between the two o r b i t a l s . These i n t e g r a l s may lead to the Table 3:21 The D i c a r b o n y l Compounds Compound UV T r a n s i t i o n Energy (eV) PES O r b i t a l IP (eV) Cyclopentene dione n-»-Tr* n - * T r * Tr->-Tr* 2.6 3.3 5.4 a. (n) (n) ( T T ) 9. 8 1 1 0 . 6 3 1 1 . 1 6 Maleimide n+Tr* T r + T r * T T - * - 7 T * 3.3 3.9 5.6 a: (n) (n) ( T T ) 10.76 11. 40 10. 31 Maleic anhydride n->Tr* n+ T r * T T - > - T T * 3.7 4.3 5.8 (n) (n) (rr) 11. 06 12. 44 11.83 - 7 1 -C O R R E L A T I O N O F P E A N D UV S P E C T R A O F T H E D I C A R B O N Y L C O M P O U N D S 13 1 2 11 10 b] (77*) a , ( n ) bg(n) a i ( n ) b 2 ( n ) bi(7r) Q i ( n )  b,(7r) b 2 ( n ) IONIZATION P O T E N T I A L S > > O CC UJ z UJ 7T—»-7r* TT—^TT* \7T—*.7T*' n—-7r*,.—-~~ 7T—+JT* n-~.Tr* n ——TT U V T R A N S I T I O N E N E R G I E S n—»-77~ F i g u r e 8 - 72 -s i t u a t i o n where the or d e r i n g of the UV t r a n s i t i o n s may not be the same as the order of the o r b i t a l s i n the PE spectrum. For example, i t i s found that the T r ^ - r r * t r a n s i t i o n s i n azo-methane are at s h o r t e r wavelength than would be expected from o r b i t a l energy c o n s i d e r a t i o n s and comparison w i t h the n->-Trs t r a n s i t i o n s , because of the l a r g e exchange i n t e g r a l . In the present s e r i e s of molecules, the f i r s t n-»-rr* t r a n s i t i o n i s one i n which the n o r b i t a l i s assigned as the b 2 combination of the carbonyl oxygen lone p a i r s and from the data i t i s seen that the energy of t h i s t r a n s i t i o n f o l l o w s the same trend as the IP of t h i s o r b i t a l CH <NH<0. 2 The t r a n s i t i o n from the a^ combination of the carbonyl lone p a i r s i s e l e c t r i c d i p o l e f o r b i d d e n , but such t r a n s i t i o n s are observed 1 1"^ and so the second n-»-rr* t r a n s i t i o n i n c y c l o -pentene dione and maleic anhydride may be from t h i s o r b i t a l . For the f i r s t T T ^ T T * t r a n s i t i o n s i n these molecules, the energies r e f l e c t the trends i n the b o r b i t a l s of the three 1 molecules: the order i s NH<CH <0. However, as i n azomethane, 2 these t r a n s i t i o n s are at sh o r t e r wavelength than expected, so that even though the f i r s t IP of maleimide i s of TT type, the f i r s t UV t r a n s i t i o n i s an n-*Tr* one. This may a l s o e x p l a i n why only one n-*-Tr* t r a n s i t i o n was observed i n maleimide, because the 7 T - > T T * t r a n s i t i o n may not have been blue s h i f t e d enough to uncover the other n+ir* t r a n s i t i o n . The UV data are th e r e f o r e c o n s i s t e n t w i t h the PE spe c t r a and show that u s e f u l - 73 -c o r r e l a t i o n s can be made w i t h i n a r e l a t e d s e r i e s of molecules 3 : 4 Conclusion. In summary, i t has been found that the trends i n the IP's of a s e r i e s of c l o s e l y r e l a t e d s e r i e s of f i v e membered r i n g compounds can be understood i n r e l a t i v e l y simple terms. The IP of the oxygen lone p a i r of a carbonyl group i n a complicated molecule can be estimated by simply summing the e f f e c t s of the various f u n c t i o n a l groups that make up the molecule, and as long as the r e l a t i o n s between the molecules are c l o s e , the r e s u l t s can be accurate to a few tenths of an eV. For these f i v e membered r i n g s , i t has been found that an oxygen atom a to the carbonyl group s h i f t s the IP of the lone p a i r by about 1 . 2 eV, a double bond i n the r i n g i n c r e a s e s i t by about 0 . 2 eV and the e f f e c t of another carbonyl group i s somewhat u n c e r t a i n , but i t seems t o inc r e a s e the lone p a i r IP by about 0 . 9 eV. S i m i l a r l y , the TT IP'S can be r a t i o n a l i z e d by combining these i n d u c t i v e e f f e c t s w i t h the phenomenon of o r b i t a l s mixing and r e p e l l i n g each other. P r e d i c t i o n s are more d i f f i c u l t i n t h i s case because both e f f e c t s must be estimated, and only the net e f f e c t i s measurable, but the r e s u l t s are c o n s i s t e n t w i t h the simple p i c t u r e s of o r b i t a l i n t e r a c t i o n . Therefore, as long as s u i t a b l e care i s e x e r c i s e d i n t h e i r a p p l i c a t i o n , these simple h e u r i s t i c ideas can be very u s e f u l - 74 -i n u n r a v e l l i n g the PE s p e c t r a of r a t h e r complicated organic molecules. - 75 -BIBLIOGRAPHY 1. A. S. Eddington, The Mature of the P h y s i c a l World, Cambridge U n i v e r s i t y P r e s s , 1929. 2. M. I. A l Joboury and D. W. Turner, J . Chem. Phys., 3 7 , 3 0 0 7 , ( 1 9 6 2 ) . 3 . M. I. A l Joboury and D. W. Turner, J . 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