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Ultraviolet photoelectron spectroscopy : a study of unstable molecules Colbourne, David 1979

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ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY: A STUDY OF UNSTABLE MOLECULES by DAVID COLBOURNE B.Sc. L a n c h e s t e r P o l y t e c h n i c , C o v e n t r y , E n g l a A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES (Department o f C h e m i s t r y ) We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA June 1979 0 D a v i d C o l b o u r n e , 1979 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department n f OHBM/^T/^y  The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 :-6 BP 75-5 1 ! E ABSTRACT The technique of ultraviolet photoelectron spectroscopy (UV PES) has been used to detect unstable molecules in the gas phase. The production of the species under: investigation can be continuously monitored, allowing experimental conditions to be altered to optimize the yield of the desired product. It has therefore proven possible to develop methods of producing high purity gas phase samples of four different groups of unstable molecules; hypochlorites (HOCJi, CH^OCil, C 2H 5OC£, (CH3)3COCJO i • halamines • (SH CH, NHC& , NCJ>3, CH3NHC£, (CH3)2NCA, CH3NC2,2, NH Br, CH^HBr, (CH3)2NBr, CH3NBr2, NHF2, NDF2) , substituted ketenes (C£HC=C=0, C£ 2C=C= 0 , BrHC=C=0, B'r2C=C=0, CH3HC=C=0, (CH 3) 2C=C=0) and formic anhydride, (HCO ) 2 0 . For the latter compound the stable anhydrides (CH 3CO ) 2 0 , (CF^CO'^O, (CCilF CO) 0 , (CC^CO^O were also investigated for comparison. The Hel photoelectron spectra thus obtained are free from impuritie in most cases, this permitting the valence electronic energy levels of these molecules below 21.22eV to be fully studied. The effect of haloge or alkyl substitution upon the functional group has been determined providing information on electronic interactions within the molecule e.g. inductive, resonance and through bond. Several of the molecules studied here have never been observed before in the gas phase eg. CH3NHBr, CH3NBr2, CJL2C=C=0, Br2C=C=0 and many are sufficiently small that a f u l l assignment of the Hel photo-- i i i -e l e c t r o n s p e c t r u m can be g i v e n on t h e b a s i s o f t h e o b s e r v e d v i b r a t i o n a l f i n e s t r u c t u r e ( p r o v i d i n g i n f o r m a t i o n on t h e i o n i c v i b r a t i o n a l f r e q u e n c i e s ) , c o m p a r i s o n w i t h known i s o e l e c t r o n i c m o l e c u l e s and t h e s u b s t i t u e n t e f f e c t s o b s e r v e d w i t h i n t h e group. I n c a s e s where a d d i t i o n a l s u p p o r t f o r an a s s i g n m e n t was n e c e s s a r y t h e e x p e r i m e n t a l i o n i z a t i o n p o t e n t i a l s have been compared t o s e m i - e m p i r i c a l m o l e c u l a r o r b i t a l c a l c u l a t i o n s (CNDO/2, HAM 3) w i t h i n t h e l i m i t s o f Koopmans' theorem. - iv. — TABLE OF CONTENTS Page CHAPTER I : INTRODUCTION 1 CHAPTER I I : FUNDAMENTAL CONCEPTS IN PHOTOELECTRON SPECTROSCOPY 6 2.1 I n t r o d u c t i o n . . 6 2.2 D i r e c t P h o t o i o n i z a t i o n 7 2.3 Franck-Condon P r i n c i p l e . . . . 10 2.4 S e l e c t i o n R u l e s 13 2.5 C o n f i g u r a t i o n I n t e r a c t i o n 16 2.6 A u t o i o n i z a t i o n 17 2.7 V a r i a t i o n o f C r o s s S e c t i o n w i t h P h o t o n Energy 19 2.8 S p i n - o r b i t s p l i t t i n g , J a h n - T e l l e r e f f e c t ; Renner e f f e c t 21 2.9 Through Space and Through Bond I n t e r a c t i o n s 23 2.10 Assi g n m e n t o f PE s p e c t r a and M o l e c u l a r O r b i t a l C a l c u l a t i o n s . . 26 CHAPTER I I I : THE PE SPECTROMETER 31 3.1 The vacuum system... 31 3.2 The a n a l y z e r 34 3.3 The l i g h t s o u r c e 37 CHAPTER IV: HYP0CHL0R0US ACID AND ALKYL HYPOCHLORITES 39 4.1 I n t r o d u c t i o n 39 4.2 E x p e r i m e n t a l 41 4.3 R e s u l t s 43 - v = 4.4 Discussion 53 4.4.1 Hypochlorous acid 53 4.4.2 Alkyl hypochlorites 57 4.5 Conclusion 61 CHAPTER V: THE HALAMINES AND THE METHYL HALAMINES 64 5.1 Introduction 64 5.2 Experimental 68 5.3 Results 74 5.4 Discussion 93 5.4.1 Chloramines 93 5.4.2 Methyl substituted chloramines 101 5.4.3 Methyl substituted bromamines 107 5.4.4 Bromamines 110 5.4.5 Difluoramine 114 5.4.6 Ionization from N lone pair orbital 118 5.5 Conclusion 125 CHAPTER VI: SUBSTITUTED KETENES 127 6.1 Introduction 127 6.2 Experimental 129 6.3 Results 133 6.4 Discussion 144 6.4.1 Monosubstituted ketenes 148 6.4.2 Disubstituted ketenes 156 6.5 Conclusion 164 - v i - -CHAPTER VII: ALIPHATIC ANHYDRIDES... 166 7.1 Introduction 166 7.2 Experimental 168 7.3 Results 170 7.4 Discussion 174 7.5 Conclusion 192 CHAPTER VIII: SUMMARY AND FURTHER WORK 193 REFERENCES 199 - v i i — LIST OF TABLES Page T a b l e 4.1 E x p e r i m e n t a l and t h e o r e t i c a l r e s u l t s f o r HOC£ 47 4.2 E x p e r i m e n t a l v e r t i c a l I P ' s f o r t h e a l k y l h y p o c h l o r i t e s 51 5.1 E x p e r i m e n t a l v e r t i c a l I P ' s and a s s i g n m e n t s f o r t h e u n s u b s t i t u t e d c h l o r a m i n e s 79 5.2 E x p e r i m e n t a l v e r t i c a l I P ' s and a s s i g n m e n t s f o r t h e m e t h y l s u b s t i t u t e d c h l o r a m i n e s 80 5.3 E x p e r i m e n t a l v e r t i c a l I P ' s and a s s i g n m e n t s f o r t h e m e t h y l s u b s t i t u t e d bromamines 83 5.4 E x p e r i m e n t a l v e r t i c a l I P ' s and a s s i g n m e n t s f o r NH^Br and p r e d i c t e d v e r t i c a l I P ' s of NHBr 2 84 5.5 E x p e r i m e n t a l and c a l c u l a t e d I P ' s f o r NHF 2 90 5.6 FWHM v a l u e s o f t h e f i r s t PE bands o f s u b s t i t u t e d ammonias 119 6.1 E x p e r i m e n t a l v e r t i c a l I P ' s f o r t h e m o n o s u b s t i t u t e d k e t e n e s 136 6.2 E x p e r i m e n t a l v e r t i c a l I P ' s f o r t h e d i s u b s t i t u t e d k e t e n e s 137 7.1 E x p e r i m e n t a l I P ' s f o r some a l i p h a t i c a n h y d r i d e s 173 7.2 E x p e r i m e n t a l and c a l c u l a t e d I P ' s f o r a c e t i c and t r i f l u o r o a c e t i c a n h y d r i d e s 179 7.3 E x p e r i m e n t a l and c a l c u l a t e d I P ' s f o r f o r m i c a c i d and f o r m i c a n h y d r i d e 186 - v i i i — LIST OF FIGURES Page Figure 2.1 The Franck-Condon Principle in electronic transitions 11 2.2 Plots of transition moments vs. photoelectron energy (E n) for various atomic orbitals 20 3.1 Vertical Cross Section of PE spectrometer 33 3.2 Hemispherical Analyzer unit 36 4.1 Hel photoelectron spectra of:-a) Ci^O b) vapour above a concentrated aqueous solution of C^O c) vapour above a dilute aqueous solution of CZ^O d) vapour above an extremely dilute aqueous solution of CJ^O 44 4.2 a) Expansions of the f i r s t and second bands in the Hel photoelectron spectrum of H0C£ b) The third and fourth bands in the Hel photoelectron spectrum of H0C£ - partially masked by the A^-^  state of H20+ 48 4.3 Hel photoelectron spectra of the alkyl hypochlorites (Me0C£, Et0C£, t-Bu0C£) 50 4.4 Expansion of the f i r s t bands in the Hel photoelectron spectra of MeOC£ and EtOC£ 52 4.5 Correlation diagram for the valence isoelectronic series Ctt^CZ, H0C£, C&2 and FC£ 56 4.6 Correlation diagram for H0C£ and i t s alkyl derivatives 62 -" i x — F i g u r e Page 5.1 H e l p h o t o e l e c t r o n s p e c t r a o f a) NH 2C£ b) NHC£ 2 c) NC£3 75 5.2 N e l p h o t o e l e c t r o n s p e c t r a o f a) NH 2C£ b) NHC£ 2 c) N C £ 3 76 5.3 E x p a n s i o n of a) f i r s t b) second c) t h i r d bands o f N e l p h o t o e l e c t r o n s p e c t r u m o f NH 2C£ 77 5.4 H e l p h o t o e l e c t r o n s p e c t r a o f a) CH 3NHC£ b) CH3NC£ 2 c) (CH 3) 2NC£ 81 5.5 E x p a n s i o n o f t h e second bands i n t h e H e l p h o t o e l e c t r o n s p e c t r a o f a) (,CH3)2NC£ b) CH 3NHC£ 82 5.6 H e l p h o t o e l e c t r o n s p e c t r a o f a) CH„NHBr b) CH„NBr 0 c) ( C H 3 ) 2 N B r 7 85 5.7 E x p a n s i o n o f a) se c o n d band o f ( C H 3 ) 2 N B r b) second band o f CH 3NHBr c) t h i r d band o f C H 3 N B r 2 86 5.8 H e l p h o t o e l e c t r o n s p e c t r a o f t h e NH3 + B r 2 gas phase r e a c t i o n a) e x c e s s B r 2 b) e x c e s s NH 3 87 5.9 E x p a n s i o n o f t h e 10-12eV r e g i o n i n t h e gas phase r e a c t i o n o f NH 3 and B r 2 a) e x c e s s B r 2 b) e x c e s s NH 3 88 5.10 D e t a i l o f t h e f i r s t band i n t h e PE s p e c t r u m o f NH 2Br 89 5.11 H e l p h o t o e l e c t r o n s p e c t r u m o f NHF 2 91 5.12 E x p a n s i o n o f t h e f i r s t band i n t h e PE s p e c t r u m o f NHF 2 92 5.13 C o r r e l a t i o n d i a g r a m f o r t h e i s o e l e c t r o n i c s e r i e s CH 3C£, NH 2C£, H0C£ and FC£ 97 5.14 E f f e c t o f Me and C£ s u b s t i t u t i o n upon t h e f i r s t I P ' s o f s u b s t i t u t e d ammonias 106 5.15 C o r r e l a t i o n d i a g r a m f o r t h e m e t h y l c h l o r a m i n e s and m e t h y l bromamines 108 5.16 E f f e c t o f Me and B r s u b s t i t u t i o n upon t h e f i r s t I P ' s o f s u b s t i t u t e d ammonias 113 C o r r e l a t i o n d i a g r a m f o r t h e i s o e l e c t r o n i c s e r i e s C H 2 F 2 , NHF 2 and F 20 P o t e n t i a l energy c u r v e s f o r p y r a m i d a l and p l a n a r s p e c i e s H e l p h o t o e l e c t r o n s p e c t r a o f p r o p a n o y l c h l o r i d e , b r o m o a c e t y l bromide and c h l o r o a c e t y l c h l o r i d e H e l p h o t o e l e c t r o n s p e c t r a o f 2 m e t h y l p r o p a n o y l c h l o r i d e , t r i b r o m o a c e t y l b r omide and t r i c h l o r o a c e t y l c h l o r i d e H e l p h o t o e l e c t r o n s p e c t r a o f t h e m o n o s u b s t i t u t e d k e t e n e s H e l p h o t o e l e c t r o n s p e c t r a o f t h e d i s u b s t i t u t e d k e t e n e s E x p a n s i o n s o f t h e f i r s t band i n t h e H e l p h o t o e l e c t r o n s p e c t r a o f t h e m o n o s u b s t i t u t e d k e t e n e s E x p a n s i o n s o f t h e f i r s t band i n t h e H e l p h o t o e l e c t r o n s p e c t r a o f t h e d i s u b s t i t u t e d k e t e n e s E x p a n s i o n o f t h e second and f o u r t h PE bands o f m o n o c h l o r o k e t e n e and t h e s i x t h band of d i c h l o r o k e t e n e H e l p h o t o e l e c t r o n s p e c t r u m o f k e t e n e The i n - p l a n e and o u t - o f - p l a n e IT o r b i t a l s i n c a r b o n d i o x i d e , k e t e n e and m o n o c h l o r o k e t e n e C o r r e l a t i o n d i a g r a m f o r t h e k e t e n e s H e l p h o t o e l e c t r o n s p e c t r a o f f o r m i c a c i d and f o r m i c a n h y d r i d e H e l p h o t o e l e c t r o n s p e c t r a o f a c e t i c a n h y d r i d e and i t s h a l o g e n a t e d d e r i v a t i v e s The t h r o u g h bond i n t e r a c t i o n i n 3 - d i c a r b o n y l s - x i -F i g u r e Page 7.4 C o r r e l a t i o n d i a g r a m f o r e x p e r i m e n t a l and c a l c u l a t e d I P ' s o f a c e t i c and t r i f l u o r o a c e t i c a n h y d r i d e s 180 7.5 H e l p h o t o e l e c t r o n s p e c t r a o f t r i c h l o r o a c e t i c a c i d and c h l o r o d i f l u o r o a c e t i c a c i d 184 7.6 C o r r e l a t i o n d i a g r a m f o r e x p e r i m e n t a l and c a l c u l a t e d I P ' s o f f o r m i c a n h y d r i d e 188 7.7 P l o t o f t h e f i r s t I P ' s v s . FWHM o f t h e f i r s t PE band f o r t h e a n h y d r i d e s 191 ACKNOWLEDGEMENTS I would l i k e t o t a k e t h i s o p p o r t u n i t y t o e x p r e s s my a p p r e c i a t i o n t o Dr. D.C. F r o s t and Dr. C A . McDowell f o r t h e i r s u p p o r t and i n t e r e s t t h r o u g h o u t t h i s work. My s i n c e r e t h a n k s go t o Dr. N.P.C. Westwood f o r h i s g u i d a n c e and c o n t a g i o u s e n t h u s i a s m and f o r h i s c a r e f u l r e a d i n g o f t h i s m a n u s c r i p t . I a l s o w i s h t o thank Dr. C. K i r b y f o r a s s i s t a n c e i n p r o o f r e a d i n g t h e t y p e d t h e s i s . I w o u l d a l s o l i k e t o thank Dr. M.C.L. G e r r y and Dr. A . J . Merer f o r u s e f u l d i s c u s s i o n s and t o Dr. F. Aubke f o r t h e use of h i s l a b o r a t o r y and f a c i l i t i e s . F i n a l l y I would l i k e t o acknowledge t h e a s s i s t a n c e o f t h e m e c h a n i c a l , e l e c t r i c a l , g l a s s b l o w i n g and i l l u s t r a t i o n s t a f f o f t h e C h e m i s t r y department of U.B.C. - 1 -CHAPTER ONE INTRODUCTION P h o t o e l e c t r o n s p e c t r o s c o p y i s a t e c h n i q u e based on the p h o t o e l e c t r i c e f f e c t a n d measures the b i n d i n g e n e r g i e s of e l e c t r o n s i n m o l e c u l e s . A sample i s i r r a d i a t e d w i t h monochromatic r a d i a t i o n of s u f f i c i e n t energy to i o n i z e the m o l e c u l e and the e l e c t r o n s thus e j e c t e d a r e energy a n a l y z e d . The d i f f e r e n c e between the energy of r a d i a t i o n (hv) and t h a t o f t h e e j e c t e d e l e c t r o n (KE^) i s e q u a l t o t h e b i n d i n g energy or i o n i z a t i o n p o t e n t i a l ( I P ) , assuming the r e c o i l energy o f the i o n can be n e g l e c t e d . Hv = KE + IP (1.1) e A p h o t o e l e c t r o n (PE) spectrum i s a p l o t o f e l e c t r o n c o u n t s v e r s u s e l e c t r o n energy and the IP thus o b t a i n e d can be equated t o the n e g a t i v e of the e i g e n v a l u e f o r the m o l e c u l a r o r b i t a l i n q u e s t i o n by u s i n g the (2) a p p r o x i m a t i o n known as Koopmans' theorem Two ty p e s o f e x c i t i n g r a d i a t i o n a r e commonly used; a d i s c h a r g e i n a s u i t a b l e r a r e gas to produce e s s e n t i a l l y monochromatic r a d i a t i o n i n the vacuum u l t r a v i o l e t ( u v ) ( c a . 5-50eV) and s o f t X-ray s o u r c e s (100 to 1500eV, eg. MgKa or A £ K a ) . The two ty p e s of r a d i a t i o n o b v i o u s l y d i f f e r c o n s i d e r a b l y i n energy and t h e r e f o r e the range of b i n d i n g e n e r g i e s t h a t can be s t u d i e d a l s o d i f f e r . R a d i a t i o n i n the vacuum uv w i l l i o n i z e o n l y t h e v a l e n c e s h e l l e l e c t r o n s and when t h i s t y p e of sou r c e i s used the t e c h n i q u e i s r e f e r r e d to as UV PES, UPS o r si m p l y PES. I t i s g e n e r a l l y a p p l i e d t o samples - 2 -i n t h e gas phase, b u t i s a l s o used t o s t u d y s o l i d s u r f a c e s ( h e r e i n g e n e r a l l y r e f e r r e d t o as p h o t o e m i s s i o n ) and more r e c e n t l y , l i q u i d s . T h i s t h e s i s p r e s e n t s r e s u l t s o b t a i n e d f o r gas phase samples u s i n g s u c h vacuum uv l i g h t s o u r c e s . A d i s c h a r g e i n h e l i u m i s most commonly used because i t produces one main l i n e i n t h e vacuum uv and i s t h e most e n e r g e t i c o f t h e s i m p l e r a r e gas r e s o n a n c e l i n e s . T h i s i s t h e so c a l l e d He I a l i n e a t 584A, e q u i v a l e n t t o an energy of 21.22eV. The n e x t most o i n t e n s e l i n e p r o duced i n such a d i s c h a r g e i s t h e He IB l i n e a t 537A h a v i n g a p p r o x i m a t e l y 2% of t h e i n t e n s i t y o f t h e He I a l i n e . The n a t u r a l l i n e w i d t h i s a l s o v e r y n a r r o w (3.3 x 10 A) and t h e r e f o r e i t i s m a i n l y i n s t r u m e n t a l f a c t o r s t h a t l i m i t " t h e r e s o l u t i o n t h a t can be a t t a i n e d . T h i s i s however u s u a l l y s u f f i c i e n t t o a l l o w t h e v a r i o u s v i b r a t i o n a l l e v e l s i n t h e i o n t o be d i s t i n g u i s h e d f o r s i m p l e m o l e c u l e s . (3-7) I n some s p e c i f i c c a s e s r o t a t i o n a l s t r u c t u r e may be p a r t i a l l y r e s o l v e d . O ther r a r e gases can be used i n t h e s e l i g h t s o u r c e s , b u t more t h a n one l i n e o f s i g n i f i c a n t i n t e n s i t y i n the vacuum uv i s pr o d u c e d . Neon i s w i d e l y u s e d , g i v i n g l i n e s a t 744 and 736A (16.67 and 16.85eV r e s p e c t i v e l y ) w i t h an i n t e n s i t y r a t i o o f a p p r o x i m a t e l y 1:7 depending upon t h e d i s -c h a r g e c o n d i t i o n s . I n a l l t h e s e d i s c h a r g e s p o s s i b l e i m p u r i t y gases may g i v e l i n e s o f s u f f i c i e n t energy t o produce i o n i z a t i o n . The most commonly o c c u r i n g a r e t h e ^.nitrogen N I l i n e a t 10.92eV and t h e hydrogen Lyman a l i n e a t 10.20eV. These of c o u r s e can be e l i m i n a t e d u s i n g h i g h p u r i t y gases but i n some ca s e s t h e i r p r e s e n c e must be c o n s i d e r e d . - 3 -The He l a l i n e i t s e l f has i n s u f f i c i e n t energy to i o n i z e a l l valence e l e c t r o n s (eg. e l e c t r o n s i n o r b i t a l s that are uniquely molecular i n n a t u r e ) . The resonance fluorescence of s i n g l y i o n i z e d He atoms gives the He I I l i n e at 304A (40.81eV) thus extending the range of study to cover the more t i g h t l y bound valence e l e c t r o n s . Under the normal operating c o n d i t i o n s of a discharge lamp the i n t e n s i t y of t h i s l i n e i s l e s s than 1% of the Hela l i n e . However when a much higher current d e n s i t y i s used and the pressure i s reduced,the r e l a t i v e i n t e n s i t y of the He I I l i n e can be increased to usable l e v e l s . The i n c r e a s i n g a v a i l a b i l i t y of such a l i g h t source i s g i v i n g a d d i t i o n a l i n f o r m a t i o n on p h o t o i o n i z a t i o n cross s e c t i o n s ( p r o b a b i l i t i e s ) as w e l l as a l l o w i n g study of the deeper o r b i t a l s of the v a r i o u s molecular species. The other major source of photons i s s o f t X-rays and here the technique i s commonly r e f e r r e d to as ESCA ( E l e c t r o n Spectroscopy f o r Chemical A n a l y s i s ) , X-ray PES or XPS. O r i g i n a l l y t h i s was a p p l i e d to s o l i d samples, but i s now r o u t i n e l y a p p l i e d to gaseous samples and l i q u i d s . The MgKa and A£Ka l i n e s have energies of 1253.6 and 1486.6eV r e s p e c t i v e l y and so can penetrate a l l but the most t i g h t l y bound core l e v e l s . The major disadvantage of these sources i s t h e i r n a t u r a l l i n e -width which approaches leV. This not only means that the v a r i o u s v i b r a t i o n a l l e v e l s of the ion which are produced when a valence e l e c t r o n i s e j e c t e d cannot be resolved,but i n d i v i d u a l c l o s e l y spaced valence l e v e l s themselves are not d i s t i n g u i s h e d . ESCA i s t h e r e f o r e mainly used f o r the study of core l e v e l s which are b a s i c a l l y atomic i n nature, but somewhat perturbed by t h e i r environment (chemical s h i f t s ) . As - 4 -each element has atomic l e v e l s of known energy X-ray PES can be used in q u a l i t a t i v e chemical a n a l y s i s . Other X-ray sources that have been used are the M£ l i n e s of zirconium and yttrium^' 9'^"^ (151.4 and 132.3eV r e s p e c t i v e l y ) . These "intermediate" energy sources are important in the study of the cross section dependence of a p a r t i c u l a r i o n i z a t i o n as a function of i o n i z i n g r a d i a t i o n energy. The major problem with these sources i s that they have extensive s a t e l l i t e l i n e s and are d i f f i c u l t to maintain in operational condition eg. oxidation of the anode target occurs r e a d i l y . Another source that has emerged over the l a s t few years i s that of synchrotron r a d i a t i o n . This provides a continuous, high i n t e n s i t y source of polarized photons from lOeV to lOkeV. Unfortunately there are a l i m i t e d number of synchrotrons a v a i l a b l e . Their use has l a r g e l y been r e s t r i c t e d to investigations of cross-section and angular dependence where the high i n t e n s i t y polarized beam i s most valuable. The i n i t i a l development of UV PES was by T u r n e r w h o used the Helot resonance l i n e as a photon source. Measurement of photoelectron (12) energies in photoionization studies had- been c a r r i e d out e a r l i e r ; , but were r e s t r i c t e d to energies of le s s than l l e V because a f l u o r i t e window was used to separate the photon source from the ion chamber. (13) The h i s t o r i c a l development of X-ray PES has been reviewed from i t s inception at the beginning of the century to i t s r e v i t a l i z a t i o n in the 1950's by K. Siegbahn and c o - w o r k e r s . Several books have been published dealing with the general f i e l d of electron spectros-(16—22} C20—22} copy , some dealing s p e c i f i c a l l y with UV PES . There i s - 5 -also a journal dedicated to a l l aspects of electron spectroscopy (Journal of Electron Spectroscopy and Related Phenomena) and there (23) have been several i n t e r n a t i o n a l conferences on the subject Most stable, v o l a t i l e compounds have already been studied by UV PES in order to gain information about t h e i r e l e c t r o n i c structure. With the improvements i n spectrometer design the technique has been applied to l e s s e a s i l y handled substances such as transient molecules and free r a d i c a l s . This thesis i s p a r t i c u l a r l y r e l a t e d to the study of reactive and unstable compounds using a spectrometer s p e c i f i c a l l y designed f o r such purposes. Those compounds are often reaction intermediates and are of both t h e o r e t i c a l and chemical i n t e r e s t . Their solution chemistry may well be extensive while l i t t l e or nothing i s known about t h e i r gas phase existence. However the possible presence of these compounds i n the i n t e r s t e l l a r or ci r c u m s t e l l a r regions and in the stratosphere makes t h e i r gas phase study important. PES o f f e r s a convenient method of monitoring production of compounds in the gas phase allowing conditions to be al t e r e d " i n s i t u " to obtain optimum y i e l d s of the desired species. This approach has been adopted i n t h i s work, often r e s u l t i n g i n a reactive or unstable compound being obtained pure or as the major species i n the gas phase. This work i s the f i r s t PE study for most of the molecules investigated and in some instances represents the f i r s t gas phase i n v e s t i g a t i o n of any kind. These compounds include several small molecules whose PE spectra can be f u l l y interpreted, an important feature when t e s t i n g the v i a b i l i t y of a molecular o r b i t a l c a l c u l a t i o n and the v a l i d i t y of Koopmans1, theorem. - 6 -CHAPTER TWO FUNDAMENTAL CONCEPTS IN PHOTOELECTRON SPECTROSCOPY 2.1 I n t r o d u c t i o n The theory relevant to PES has been discussed i n d e t a i l i n s e v e r a l b o o k s ^ ^ and r e v i e w s ^ 4 ^ and w i l l not be de a l t w i t h i n depth here, although some of the c h a r a c t e r i s t i c features that can be observed i n a PE spectrum w i l l be discussed more f u l l y . Such features are oft e n the main guide i n i n t e r p r e t i n g a spectrum, but a d d i t i o n a l a s s i s t a n c e can be s u p p l i e d from c a l c u l a t i o n s of va r i o u s s o p h i s t i c a t i o n and some mention of these w i l l be made. The i o n i z a t i o n energy as obtained from equation 1.1 can be w r i t t e n as:-IP = E. + E + + E * (2.1) A v i b r o t where E i s the s o - c a l l e d a d i a b a t i c i o n i z a t i o n energy ( t r a n s i t i o n from the lowest v i b r a t i o n a l l e v e l of the n e u t r a l molecule to the lowest v i b r a t i o n a l l e v e l of the i o n ) . E .,+ and E + are the v i b r a t i o n a l and vi b r o t r o t a t i o n a l energies of the p o s i t i v e i o n . Equation 1.1 can t h e r e f o r e be r e w r i t t e n to g i v e : -KE = hv - E - E * - E * (2.2) e A v i b r o t The value of E + i s r e l a t i v e l y small and r o t a t i o n a l f i n e s t r u c t u r e r o t J has been seen i n only a few cases 7 1 1 using high r e s o l u t i o n (< lOmeV). - 7 -+ E ., v e r y o f t e n has a v a l u e t h a t a l l o w s v a r i o u s v i b r a t i o n a l l e v e l s t o be r e s o l v e d u s i n g UV PES. eg. w i t h a t y p i c a l r e s o l u t i o n of 50meV s t r u c t u r e of 500cm ^ can e a s i l y be r e s o l v e d . Such s t r u c t u r e g i v e s v a l u a b l e i n f o r m a t i o n about the m o l e c u l a r i o n and the e x t e n t of geometry r e o r g a n i z a t i o n upon i o n i z a t i o n . 2.2 D i r e c t P h o t o i o n i z a t i o n T h i s i s the b a s i c p r o c e s s i n v o l v e d i n PES. An e l e c t r o n i s e j e c t e d from a m o l e c u l e Y(X;u") i n i t s ground e l e c t r o n i c s t a t e X and v i b r a t i o n a l l e v e l f " ( = 0 u n l e s s v i b r a t i o n a l l y e x c i t e d ) t o form the m o l e c u l a r i o n Y + ( x , a, b, c....; v' = 0, 1....)' where x, a, b... r e p r e s e n t t h e e l e c t r o n i c s t a t e s and v' a r e t h e v i b r a t i o n a l l e v e l s . The m o l e c u l a r ground s t a t e i s c h a r a c t e r i z e d by the e i g e n f u n c t i o n V w h i l e the f i n a l s t a t e ( i o n + p h o t o e l e c t r o n ) i s c h a r a c t e r i z e d by V', b o t h b e i n g f u n c t i o n s of t h e e l e c t r o n r and n u c l e a r R c o o r d i n a t e s . The p r o b a b i l i t y o f t h e above t r a n s i t i o n i s determined by the square of the t r a n s i t i o n moment i n t e g r a l . Y ( X ; y " ) + h v ' + Y + ( x , a , b , c . ; V' = 0, 1 ) + e (2.3) M = <V" Ep ¥ '> (2.4) where p i s the d i p o l e moment o p e r a t o r , t h e sum e x t e n d i n g over a l l e l e c t r o n s i and n u c l e i j . - 8 -(12) U s i n g t h e Born-Oppenheimer a p p r o x i m a t i o n t h e e i g e n f u n c t i o n s can be s e p a r a t e d i n t o t h e p r o d u c t of e l e c t r o n i c and n u c l e a r f u n c t i o n s . ¥(r;R) = V ( r ; R ) V (R) (2.5) e n ^ ( r j R ) has a p a r a m e t r i c dependence upon t h e i n s t a n t a n e o u s n u c l e a r c o n f i g u r a t i o n and ^ (R) x s dependent upon a p a r t i c u l a r e l e c t r o n i c c o n f i g u r a t i o n . The n u c l e a r f u n c t i o n can be e x p r e s s e d as a p r o d u c t of v i b r a t i o n a l ^ and r o t a t i o n a l f w a v e f u n c t i o n s assuming i n t e r a c t i o n between t h e two t y p e s o f m o t i o n can be n e g l e c t e d . * (R) = (hv (R)VT(R) (2.6) n R v T S i m i l a r l y t h e d i p o l e o p e r a t o r can be s e p a r a t e d i n t o an e l e c t r o n i c and n u c l e a r dependent p a r t . £ . . p = £ . p -. + E . p • (2.7) Thus e q u a t i o n (2.4) can be expanded t o g i v e M = f/V * " ( r ; R ) ( i ) > f * "(R) V * "(R) | £p' + Xp n i 3 Ve '(r;R) (h^ ' ( R U '(R)drdR (2.8) R v T S e p a r a t i n g a c c o r d i n g t o e l e c t r o n i c and n u c l e a r d i p o l e o p e r a t o r s - ? -M = f(hv *"(R)V ,*"(R) ( J - ) * '(R)* .'(R)dryi' * " ( r ; R ) | Z p I V ' ( r ; R ) d r R v x R v T e . e e x + f(k)V *"(R)¥ *"(R)|Sp/ Uhv '(R)* '(R)dR.Tl' * " ( r ; R ) f ' ( r ; R ) d r (2.9) R v T ' . n R v T e e J As e l e c t r o n i c e i g e n f u n c t i o n s b e l o n g i n g to d i f f e r e n t e l e c t r o n i c s t a t e s a r e o r t h o g o n o l the second term i n e q u a t i o n (2.9) v a n i s h e s f o r e l e c t r o n i c o t r a n s i t i o n s . As r o t a t i o n a l s t r u c t u r e i s u s u a l l y u n r e s o l v e d i n PES (13) the r o t a t i o n a l p a r t of e q u a t i o n (2.9) •>? ( P O can be n e g l e c t e d and a s i m p l e r e q u a t i o n r e s u l t s . M = H *"(R)V '(R)dR-.H' * " ( r ; R ) |Zp .1* ' ( r ; R ) d r (2.10) v v e . e e l The second i n t e g r a l i s the m a t r i x element of the e l e c t r i c d i p o l e moment (M^ ,) f o r a g i v e n n u c l e a r c o n f i g u r a t i o n (R) and i n most c a s e s v a r i e s ' o n l y s l i g h t l y w i t h R. The f i r s t term g i v e s the v i b r a t i o n a l o v e r l a p between the n e u t r a l and i o n i c s t a t e s . U s i n g e q u a t i o n (2.10) t h e . p h o t o i o n i z a t i o n t r a n s i t i o n p r o b a b i l i t y i s a p p r o x i m a t e l y g i v e n by:-Pa.iM..(r;R)| 2l<! " ( R ) | T '(R)>| 2 (2.11) e v v The v i b r a t i o n a l o v e r l a p i n t e g r a l i s c a l l e d the Franck-Condon f a c t o r and i s l a r g e l y r e s p o n s i b l e f o r t h e r e l a t i v e i n t e n s i t i e s of the v i b r a t i o n a l bands i n p h o t o i o n i z a t i o n t r a n s i t i o n s . T h i s i n t e g r a l does n o t v a n i s h by o r t h o g o n a l i t y because ' and a r e v i b r a t i o n a l f u n c t i o n s b e l o n g i n g t o d i f f e r e n t e l e c t r o n i c s t a t e s . - 10 -2.3 Franck-Condon p r i n c i p l e . E q u a t i o n 2.11 c o n t a i n s t h e wave m e c h a n i c a l s t a t e m e n t o f t h e Franck-Condon p r i n c i p l e w h i c h s t a t e s t h a t t h e i n t e n s i t y o f a v i b r a t i o n a l band i n an e l e c t r o n i c a l l y a l l o w e d 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 t h e a b s o l u t e s quare o f t h e o v e r l a p i n t e g r a l o f t h e v i b r a t i o n a l w a v e f u n c t i o n s of t h e i n i t i a l and f i n a l s t a t e s . The d e r i v a t i o n assumes t h a t t h e Born-Oppenheimer a p p r o x i m a t i o n h o l d s and an e l e c t r o n i c a l l y a l l o w e d t r a n s i t i o n i s c o n s i d e r e d ^ ^ . T h i s p r i n c i p l e i n d i c a t e s t h a t t r a n s i t i o n s a r e f a v o u r e d when t h e r e i s a l a r g e o v e r l a p between t h e two v i b r a t i o n a l w a v e f u n c t i o n s b u t does n o t s p e c i f y a " v e r t i c a l t r a n s i t i o n " n o r i s t h e r e m e n t i o n made of t h e t i m e d u r a t i o n o f an e l e c t r o n i c t r a n s i t i o n r e l a t i v e t o a v i b r a t i o n a l f r e q u e n c y . Maximum o v e r l a p f a v o u r s , b u t does n o t r e q u i r e , t r a n s i t i o n s i n w h i c h t h e r e l a t i v e p o s i t i o n s o f t h e n u c l e i a r e the same i n t h e two s t a t e s . P o t e n t i a l e n e r g y c u r v e s f o r a h y p o t h e t i c a l d i a t o m i c m o l e c u l e , AB, i n i t s ground s t a t e and v a r i o u s i o n i c s t a t e s a r e shown i n f i g . 2.1. T h i s d e m o n s t r a t e s i n a q u a l i t a t i v e manner how t h e b o n d i n g c h a r a c t e r i s t i c s o f an e l e c t r o n can be r e l a t e d t o t h e band shape i n a PE sp e c t r u m . The ground i o n i c s t a t e (x) i s a t t a i n e d by l o s s o f a non b o n d i n g e l e c t r o n , r e s u l t i n g i n l i t t l e o r no change i n a t o m i c c o o r d i n a t e s . Assuming t h e i n i t i a l m o l e c u l a r s t a t e t o be i n i t s ground v i b r a t i o n a l l e v e l ( y " = 0 ) , maximum o v e r l a p o f t h e v i b r a t i o n a l e i g e n f u n c t i o n s i s o b t a i n e d when t h e f i n a l s t a t e i s a l s o i n i t s ground v i b r a t i o n a l l e v e l (u ' = 0 ) . O v e r l a p between v" = 0 and.-other v i b r a t i o n a l l e v e l s of t h e f i n a l s t a t e i s m i n i m a l . T h i s r e s u l t s i n a v e r y s h a r p and i n t e n s e PE band r e p r e s e n t i n g - 11 -THE FRANCK CONDON PRINCIPLE Predissociation 6. Curve Crowing 5. Dissociation < < *. Strong Bondi *9 2. A n t i b o n d / n g i. Nonbonding BOND LENGTH F i g . 2.1. - 12 -the 0 0 or a d i a b a t i c t r a n s i t i o n , w i t h o n l y minor v i b r a t i o n a l components r e p r e s e n t i n g t r a n s i t i o n s t o h i g h e r i o n i c v i b r a t i o n a l l e v e l s . The second i o n i c s t a t e (a) ( f i g . 2.1) i s o b t a i n e d when an a n t i b o n d i n g e l e c t r o n i s removed from the m o l e c u l e . T h i s r e s u l t s i n a d e c r e a s e i n the i n t e r n u c l e a r d i s t a n c e and i n c u r s s i g n i f i c a n t o v e r l a p between the i n i t i a l s t a t e v i b r a t i o n a l e i g e n f u n c t i o n and the v i b r a t i o n a l e i g e n -f u n c t i o n s of s e v e r a l d i f f e r e n t l e v e l s i n the i o n . The PE band thus c o n s i s t s of an extended v i b r a t i o n a l p r o g r e s s i o n where the t r a n s i t i o n w i t h maximum i n t e n s i t y ( v e r t i c a l IP) i s now d i f f e r e n t from the a d i a b a t i c t r a n s i t i o n . T h i s p r o g r e s s i o n r e p r e s e n t s the v i b r a t i o n a l l e v e l s i n the i o n and t h e i n t e r v a l s w i l l be d i f f e r e n t from the ground s t a t e m o l e c u l e . In t h i s case an a n t i b o n d i n g e l e c t r o n has been e j e c t e d , g i v i n g a s t r o n g e r bond i n the i o n than i n the m o l e c u l e , and thus the v i b r a t i o n a l f r e q u e n c y w i l l be l a r g e r i n the i o n . I o n i c s t a t e (b) g i v e s a s i m i l a r Franck-Condon env e l o p e , but i n t h i s case the v i b r a t i o n a l i n t e r v a l i n the i o n w i l l be s m a l l e r than i n the ground s t a t e m o l e c u l e as a bonding e l e c t r o n has been e j e c t e d . The next i o n i c s t a t e (c) ( f i g . 2.1) a g a i n r e s u l t s from the l o s s of a bonding e l e c t r o n , but i n t h i s c ase the Franck-Condon r e g i o n extends p a s t the d i s s o c i a t i o n l i m i t of t h e i o n . There i s some o v e r l a p w i t h the bound s t a t e s , g i v i n g v i b r a t i o n a l s t r u c t u r e i n the PE band, which smoothly converge u n t i l the continuum i s r e a c h e d . The more extreme case i s f o r the i o n i c s t a t e (d) where the Franck-Condon r e g i o n i s c o n f i n e d to the continuum i . e . i o n i z a t i o n to a r e p u l s i v e p o t e n t i a l s u r f a c e and a r e s u l t i n g u n s t r u c t u r e d band. - 13 -F i n a l l y f o r case (e) t h e r e i s a c r o s s i n g o r c l o s e approach of a r e p u l s i v e p o t e n t i a l s u r f a c e and a bound p o t e n t i a l . The w a v e f u n c t i o n s d e s c r i b i n g the two s t a t e s become mixed and the r e p u l s i v e s t a t e puts l i m i t s on the l i f e t i m e of the i o n . T h i s i s p r e d i s s o c i a t i o n and r e s u l t s i n a PE band w i t h d i s c r e t e v i b r a t i o n a l s t r u c t u r e below the c r o s s i n g , which broadens i n the r e g i o n of the c r o s s i n g , due to the u n c e r t a i n t y p r i n c i p l e , to f i n a l l y g i v e a continuum. In a p o l y a t o m i c m o l e c u l e i t i s o f t e n t h e case t h a t more than one v i b r a t i o n a l mode i s e x c i t e d i n a p a r t i c u l a r i o n i z a t i o n event. In such s i t u a t i o n s the v i b r a t i o n a l p r o g r e s s i o n s may o v e r l a p r e s u l t i n g i n a band showing no apparent s t r u c t u r e . In l a r g e r m o l e c u l e s i t i s t h i s type of band t h a t i s common. However i t i s s t i l l p o s s i b l e to draw some c o n c l u s i o n s about the change i n geometry upon i o n i z a t i o n from the band shape. Another r e a s o n f o r • s t r u c t u r e l e s s bands i n a PE spectrum i s i n s u f f i c i e n t r e s o l u t i o n - when low f r e q u e n c y v i b r a t i o n s a r e e x c i t e d . Under the c o n d i t i o n s of a t y p i c a l experiment i s i s o f t e n i m p o s s i b l e to r e s o l v e v i b r a t i o n a l p r o g r e s s i o n s of l e s s than 300cm 1 r e s u l t i n g i n f e a t u r e l e s s bands. 2.4 S e l e c t i o n r u l e s i ) E l e c t r o n i c P h o t o e l e c t r o n t r a n s i t i o n s a r e a l l o w e d whenever the i n t e g r a l s of e q u a t i o n 2.10 a r e n o n - z e r o . The most important t y p e of p h o t o e l e c t r o n t r a n s i t i o n i n v o l v e s e j e c t i o n of a s i n g l e e l e c t r o n and p . i n e q u a t i o n 2.10 - 14 -can be approximated as a o n e - e l e c t r o n d i p o l e o p e r a t o r . A t r a n s i t i o n i s a l l o w e d i f the p r o d u c t o f the i r r e d u c i b l e r e p r e s e n t a t i o n s V of the s p e c i e s i n the m a t r i x element M (r;R) ( i . e . rC^  "") x T(p ) x r(¥ ')) e e e e i s t o t a l l y symmetric w i t h r e s p e c t t o a l l symmetry elements of t h e m o l e c u l a r p o i n t group f o r a t l e a s t one component of p^ ( t h e s e t r a n s f o r m i n the m o l e c u l a r p o i n t group as the c a r t e s i a n v e c t o r s x, y and z ) . A T h i s means the p r o d u c t r(1' ") x T(V '') has to b e l o n g t o the same e e symmetry s p e c i e s as one of the components of F ( p ^ ) . In a p h o t o e l e c t r o n t r a n s i t i o n the f i n a l s t a t e w a v e f u n c t i o n ¥ ' c o n s i s t s o f t h e w a v e f u n c t i o n s e of the p o s i t i v e i o n ¥ + and of t h e unbound e l e c t r o n A . The f u n c t i o n e e <f>^  i s a o n e - e l e c t r o n continuum wave and can assume a n g u l a r momentum v a l u e s such t h a t t h e i r r e d u c i b l e r e p r e s e n t a t i o n rCP ') i s of t h e c o r r e c t A , symmetry f o r t h e pro d u c t T(V "') x T(V^') to f u l f i l l the above c o n d i t i o n . T h i s means t h a t a l l o n e - e l e c t r o n p h o t o i o n i z a t i o n t r a n s i t i o n s a r e a l l o w e d . ( i i ) S p i n The e l e c t r o n i c e i g e n f u n c t i o n s i n c l u d i n g s p i n ¥ can be t r e a t e d as a product o f an o r b i t a l and a s p i n f u n c t i o n (V = ¥ x Y ) es e s i f s p i n - o r b i t c o u p l i n g i s s m a l l . The d i p o l e o p e r a t o r does not o p e r a t e on s p i n c o o r d i n a t e s and so the t r a n s i t i o n moment can be f a c t o r i z e d . <y *"|p. k '><¥ "lY '> (2.12) e e' e s s As s p i n f u n c t i o n s c o r r e s p o n d i n g to d i f f e r e n t s p i n v a l u e s a r e o r t h o n o r m a l , the second i n t e g r a l i n 2.12 i s z e r o f o r s t a t e s o f - 15 -d i f f e r e n t s p i n . The s p i n s e l e c t i o n r u l e i s thus AS = 0. F o r c l o s e d -s h e l l ground s t a t e s t h e s p i n f o r t h e i n i t i a l s t a t e i s 0 and t h e r e f o r e the s p i n of t h e f i n a l s t a t e (combined s p i n of t h e p o s i t i v e i o n and the p h o t o e l e c t r o n ) must a l s o be 0. As the s p i n of an e l e c t r o n i s 1/2 the i o n i c s t a t e must be a d o u b l e t . ( i i i ) V i b r a t i o n a l As M-(r;R) i n eq. 2.11 i s always f i n i t e f o r a p h o t o e l e c t r o n e t r a n s i t i o n , i t i s t h e Franck-Condon o v e r l a p i n t e g r a l t h a t d etermines whether a t r a n s i t i o n o c c u r s from a c e r t a i n v i b r a t i o n a l l e v e l of t h e i n i t i a l s t a t e (v") 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 of t h e f i n a l s t a t e ( y ' ) . F o r t h i s i n t e g r a l to be non-zero t h e i n t e g r a n d i n eq. 2.11 must be symmetric w i t h r e s p e c t to a l l symmetry o p e r a t i o n s of t h e p o i n t group. T h i s means o n l y v i b r a t i o n a l l e v e l s of the same v i b r a t i o n a l s p e c i e s i n t h e i n i t i a l and f i n a l s t a t e s can combine t o g i v e a non-ze r o i n t e g r a l . Assuming t h e m o l e c u l e t o be i n the t o t a l l y symmetric z e r o p o i n t v i b r a t i o n a l l e v e l of t h e ground s t a t e , p h o t o e l e c t r o n t r a n s i t i o n s can o n l y o c c u r t o t o t a l l y symmetric l e v e l s of the i o n i c s t a t e . T h e r e f o r e o n l y p r o g r e s s i o n s i n s i n g l e quanta of t o t a l l y symmetric modes of t h e i o n a r e n o r m a l l y o b s e r v e d . The v i b r a t i o n a l l e v e l s o f a n t i s y m m e t r i c v i b r a t i o n s a r e symmetric f o r even V and so a c c o r d i n g t o the s e l e c t i o n r u l e , t r a n s i t i o n s can occur from the z e r o p o i n t v i b r a t i o n a l l e v e l o f t h e ground s t a t e t o a n t i -symmetric v i b r a t i o n a l l e v e l s o f t h e i o n i c s t a t e when V i s even, i . e . p r o g r e s s i o n s i n do u b l e quanta can be seen. The i n t e n s i t i e s of such - 16 -t r a n s i t i o n s - a r e d e t e r m i n e d from t h e Franck-Condon p r i n c i p l e and f o r a p r o g r e s s i o n i n an a n t i s y m m e t r i c mode i t can be shown t h a t the (13) a d i a b a t i c t r a n s i t i o n i s the most i n t e n s e even when t h e r e i s a change i n geometry between the m o l e c u l e and i o n . F o r a p p r e c i a b l e i n t e n s i t y i n t r a n s i t i o n s where Ay =^ 0 t h e r e has t o be a v e r y l a r g e d i f f e r e n c e i n the a n t i s y m m e t r i c v i b r a t i o n i n the two s t a t e s and such p r o g r e s s i o n s i n double quanta a r e u s u a l l y e x t r e m e l y weak. 2.5 C o n f i g u r a t i o n i n t e r a c t i o n The above s e l e c t i o n r u l e s were d i s c u s s e d on the assumption t h a t p h o t o i o n i z a t i o n i s a s i n g l e s t e p o n e - e l e c t r o n p r o c e s s . The w a v e f u n c t i o n s of the i n i t i a l and f i n a l s t a t e s d i f f e r by one m o l e c u l a r o r b i t a l i n t h i s c a s e and t h i s i s the s i t u a t i o n t h a t a l l o w s the measured IP's to be d i r e c t l y equated to t h e d i f f e r e n t o r b i t a l e n e r g i e s of t h e n e u t r a l m o l e c u l e . However t h e r e a r e cases where a PE spectrum cannot be a s s i g n e d on the b a s i s of one band per o r b i t a l because the e l e c t r o n i c c o n f i g u r a t i o n of the m o l e c u l a r i o n d i f f e r s from t h a t o f the ground s t a t e by more than one m o l e c u l a r o r b i t a l . E j e c t i o n of an e l e c t r o n i s accompanied by a s i m u l t a n e o u s e x c i t a t i o n of a n o t h e r e l e c t r o n i n t o (18—20) an u n o c c u p i e d o r b i t a l to g i v e a "doubly e x c i t e d " s t a t e , and to d e s c r i b e such t w o - e l e c t r o n p r o c e s s e s a more c o r r e c t c o n f i g u r a t i o n i n t e r a c t i o n e x pansion of w a v e f u n c t i o n s has to be used i n p r e f e r e n c e to t h e s i m p l e m o l e c u l a r o r b i t a l d e s c r i p t i o n . The t r a n s i t i o n moment i n t e g r a l i n eq. 2.10 would then t a k e t h e form o f : -- 17 -CO . . M : = Z c..<f" p $,> (2.13) e i = i U where §^ i s a p a r t i c u l a r e l e c t r o n i c c o n f i g u r a t i o n b e l o n g i n g to a c e r t a i n s p i n and symmetry s p e c i e s . T r a n s i t i o n s to "a-:final s t a t e ¥ a r e a l l o w e d i f any components of the summation a r e n o n - z e r o . Such t r a n s i t i o n s have not been found t o be common i n UV PES when He l l r a d i a t i o n i s used. However when He I I i s used the i n c r e a s e d photon energy p e r m i t s t r a n s i t i o n s to the more h i g h l y e x c i t e d s t a t e s of m o l e c u l a r (19-21) i o n s f o r which c o n f i g u r a t i o n i n t e r a c t i o n assumes a s i g n i f i c a n t r o l e The i n t e r p r e t a t i o n of s p e c t r a i n v o l v i n g such t r a n s i t i o n s becomes d i f f i c u l t because t w o - e l e c t r o n p r o c e s s e s t h a t i n v o l v e e j e c t i o n of one e l e c t r o n and e x c i t a t i o n of a n o t h e r may encompass a l a r g e number of i o n i c s t a t e s I n f o r m a t i o n on e x c i t e d s t a t e s of i o n s i s s c a r c e and o p t i c a l d a t a on the n e u t r a l s p e c i e s i s u s u a l l y used as a guide i n i n t e r p r e t i n g such (22) s p e c t r a . T h i s assumes t h a t the IP of a n e u t r a l e x c i t e d s t a t e m o l e c u l e i s the same as f o r the ground s t a t e of t h e n e u t r a l m o l e c u l e . C o n c l u s i o n s based on t h i s assumption which n e g l e c t s changes i n n u c l e a r s h i e l d i n g i n the m o l e c u l e and i o n , s h o u l d be t r e a t e d w i t h c a u t i o n and i d e n t i f i c a t i o n of the s t a t e s cannot be c o n s i d e r e d c e r t a i n . 2.6 A u t o i o n i z a t i o n Another phenomenon t h a t can sometimes be e n c o u n t e r e d i n UV PES i s a u t o i o n i z a t i o n . The r a d i a t i o n used i s s u f f i c i e n t l y e n e r g e t i c to e x c i t e e l e c t r o n s o t h e r than the most l o o s e l y bound to d i s c r e t e n e u t r a l s t a t e s above the i o n i z a t i o n t h r e s h o l d . In such c i r c u m s t a n c e s a r a d i a t i o n -- 18 -l e s s t r a n s i t i o n can o c c u r from the n e u t r a l e x c i t e d s t a t e to t h e i o n -i z a t i o n continuum. T h i s a u t o i o n i z a t i o n p r o c e s s i s o b served i n a PE spectrum as i r r e g u l a r i t i e s i n the i n t e n s i t i e s of the l e v e l s i n a v i b r a t i o n a l p r o g r e s s i o n and a l s o a b r o a d e n i n g of v i b r a t i o n a l bands -14 due to the s h o r t l i f e t i m e (#10 sec) of the a u t o i o n i z i n g s t a t e s . The p r o c e s s can be r e p r e s e n t e d by:-* + M + h v - ^ M -> M + e where M i s the e x c i t e d n e u t r a l s t a t e , which has to be a Rydberg s t a t e t h a t converges on the second o r h i g h e r I P ' s . The energy of the e x c i t i n g photons has to c o i n c i d e w i t h . t h a t of the a u t o i o n i z i n g s t a t e i n which case t h e r e i s a h i g h p r o b a b i l i t y of e x c i t i n g to the n e u t r a l s t a t e w i t h a w e l l d e f i n e d v i b r a t i o n a l quantum number. I f the l i f e t i m e of t h e a u t o i o n i z i n g s t a t e i s g r e a t e r than one v i b r a t i o n a l p e r i o d , t h e PE spectrum w i l l be dependent on the Franck-Condon f a c t o r s f o r t h e r a d i a t i o n -l e s s t r a n s i t i o n from the a u t o i o n i z i n g s t a t e to t h e f i n a l s t a t e of the i o n . In t h i s case the v a l u e of v" w i l l not g e n e r a l l y be z e r o and t h e v i b r a t i o n a l s t r u c t u r e i n the spectrum may be extended c o n s i d e r a b l y . An example of t h i s i s the Ne I e x c i t e d PE spectrum of o x y g e n ^ 1 7 ^ , where the f i r s t band i s q u i t e d i f f e r e n t from t h a t i n the He I spectrum where a u t o i o n i z a t i o n does n o t o c c u r . In f a c t most e f f e c t s of t h i s k i n d have been observed u s i n g e i t h e r Ne o r Ar resonance l i n e s . I t i s i n t h i s 12-17eV r e g i o n where the d e n s i t y of e x c i t e d and i o n i c s t a t e s g i v e s a h i g h p r o b a b i l i t y f o r a u t o i o n i z a t i o n . In the He I s p e c t r a the p r o b a b i l i t y of such p r o c e s s e s i s low and, problems from a u t o i o n i z a t i o n a r e not u s u a l l y e n c o u n t e r e d . - 19 -2.7 V a r i a t i o n of Cross S e c t i o n w i t h Photon energy Band i n t e n s i t i e s i n PE s p e c t r a a r e dependent on many v a r i a b l e s i n c l u d i n g the photon f l u x , s a m p l e p r e s s u r e , s o l i d a n g l e of e l e c t r o n s a n a l y z e d , d i r e c t i o n of s o l i d a n g l e , type of a n a l y z e r , r e t a r d i n g p o t e n t i a l s , (23-27) s e n s i t i v i t y o f d e t e c t o r , as w e l l as the energy of the photon s o u r c e As t h e r e a r e so many v a r i a b l e s , a b s o l u t e v a l u e s of i o n i z a t i o n c r o s s s e c t i o n s a r e not e a s i l y o b t a i n e d , but g e n e r a l t r e n d s i n i n t e n s i t i e s on changing the l i g h t s o u r c e can be u s e f u l i n i n t e r p r e t i n g a PE spectrum. T h i s i s p a r t i c u l a r l y so f o r p h o t o i o n i z a t i o n of l o n e p a i r s where, t o a r e a s o n a b l e a p p r o x i m a t i o n , o n l y atomic c r o s s s e c t i o n s need be c o n s i d e r e d w i t h o u t i n t r o d u c i n g two-centre terms as f o r a bonding o r b i t a l (24) Schweig and T h i e l • have c a l c u l a t e d t h e v a r i a t i o n of atomic c r o s s s e c t i o n s w i t h p h o t o e l e c t r o n energy f o r s e v e r a l s p e c i e s and t h e r e s u l t s a r e shown i n f i g . 1,2. "' There a r e ob v i o u s s i m i l a r i t i e s i n the r e s u l t s f o r the v a r i o u s o r b i t a l s , i n t h a t a l l c r o s s s e c t i o n s i n c r e a s e r a p i d l y a t low p h o t o e l e c t r o n e n e r g i e s t o r e a c h a maximum which then s l o w l y decays. However one d i f f e r e n c e which i s s i g n i f i c a n t enough t o be n o t i c e d by experiment i s t h a t the more d i f f u s e n = 3 o r b i t a l s have maxima a t lower p h o t o e l e c t r o n e n e r g i e s than those w i t h n = 2. U s i n g a lower energy photon s o u r c e w i l l thus g i v e a more i n t e n s e band f o r an i o n i z a t i o n from a p r e d o m i n a n t l y 29 3p o r b i t a l r e l a t i v e t o one w i t h s i g n i f i c a n t 2p c h a r a c t e r , and t h i s can be observed when u s i n g Ne I i n s t e a d of He I (see Ch. 5 ) . There a r e o t h e r t r e n d s i n f i g . 2.2 which can r e s u l t i n more s u b t l e changes i n i n t e n s i t i e s of PE bands u s i n g d i f f e r e n t l i g h t s o u r c e s because of d i f f e r e n c e s - 20 -Er, ( e V ) Plots of transition moment vs. photoelectron energy(E ) f o r various atomic orbital?, (from ref. 24 ) F i g . 2.2 - 21 -i n s:p c h a r a c t e r . F o r i n s t a n c e the graphs i n f i g . 2.2 f o r s type o r b i t a l s show a s h a r p e r i n c r e a s e a f t e r t h r e s h o l d than do p- t y p e o r b i t a l s . E x p e r i m e n t a l c r o s s s e c t i o n s i n t h i s r e g i o n can be o b t a i n e d u s i n g Ne I , He I and He I I r a d i a t i o n and t h e s e p r e d i c t e d trends' have been (27) v e r i f i e d f o r p and s based o r b i t a l s i n N^ and CO 2.8 S p i n - o r b i t s p l i t t i n g ; J a h n - T e l l e r e f f e c t ; Renner e f f e c t A l l of t h e s e e f f e c t s can l i f t t he degeneracy of an i o n i c s t a t e and have been observed i n t h e PE s p e c t r a o f h i g h symmetry m o l e c u l e s 30 31 32 (eg. s p i n - o r b i t s p l i t t i n g of HX and X^ , J a h n - T e l l e r i n NH^ and 33 34 CH^ and the Renner e f f e c t i n H^O ) . As the m o l e c u l e s s t u d i e d i n t h i s work a r e of low symmetry such e f f e c t s a r e n o t seen ( a l t h o u g h s p i n - o r b i t 35 c o u p l i n g i s observed i n some low symmetry m o l e c u l e s ) and w i l l t h e r e -f o r e n ot be d i s c u s s e d i n d e t a i l . E j e c t i o n of an e l e c t r o n from a c l o s e d s h e l l atom or m o l e c u l e r e s u l t s i n an i o n w i t h s p i n = 1/2. I f t h e o r b i t a l concerned i s degenerate the r e s u l t i n g i o n i c s t a t e i s c h a r a c t e r i z e d by an o r b i t a l a n g u l a r momentum g r e a t e r than z e r o . C o u p l i n g o f t h i s o r b i t a l a n g u l a r momentum and the s p i n a n g u l a r momentum l i f t s the degeneracy t o g i v e s t a t e s w i t h d i f f e r e n t t o t a l a n g u l a r momentum (38) (39) The J a h n - T e l l e r and Renner e f f e c t s o c c u r through i n t e r a c t i o n of the e l e c t r o n i c and n u c l e a r m o t i o n . T h i s i n t e r a c t i o n can be d e s c r i b e d by a p e r t u r b a t i o n approach i n which t h e H a m i l t o n i a n i s expanded i n terms of a T a y l o r s e r i e s i n the normal c o o r d i n a t e s of v i b r a t i o n Q - 22 -# - ( q ; Q ) l 0 + ^ ~~r ^T i 0 Q r + ••• (2.14) q o r r * 3Q q o The f i r s t term i n t h i s e q u a t i o n i s the z e r o - o r d e r H a m i l t o n i a n ("pj ). The i n t e r a c t i o n between two degenerate f u n c t i o n s and ^^ i n the degenerate s t a t e Y can be e x p r e s s e d by the v a l u e of the m a t r i x element <f |"^-f |¥ > which when expanded u s i n g the o p e r a t o r i n eq. 2.14 g i v e s k 1 o 1 p r k 1 30 1 p r 2 2 2. r k 1 . 2 1 £ 2: r s k'30:9Q £ r 3Q. r , s 'r s The p e r t u r b a t i o n terms of e q u a t i o n 2.15 cause a s p l i t t i n g o f t h e e l e c t r o n i c degeneracy i f the normal c o o r d i n a t e s t h a t a r e a c t i v e i n the d i s p l a c e m e n t a r e non . t o t a l l y " s y m m e t r i c . S p l i t t i n g due t o the l i n e a r p e r t u r b a t i o n term i s the J a h n - T e l l e r e f f e c t and s p l i t t i n g due to (39) the q u a d r a t i c term i s the Renner e f f e c t . Symmetry c o n s i d e r a t i o n s l e a d to t h e l i n e a r term i n e q u a t i o n 2.15 to be always 0 f o r an asymmetric v i b r a t i o n i n a l i n e a r m o l e c u l e . The J a h n - T e l l e r e f f e c t cannot o c c u r t h e r e f o r e i n l i n e a r m o l e c u l e s and i s o n l y seen i n n o n - l i n e a r m o l e c u l e s . However the q u a d r a t i c term i s always p e r m i s s i b l e i n a l i n e a r m o l e c u l e and so the Renner e f f e c t w i l l o c c u r i f t h e m a t r i x elements of t h e q u a d r a t i c term a r e l a r g e enough to cause s i g n i f i c a n t s p l i t t i n g . The t h e o r y of b o t h the R e n n e r a n d the J a h n - T e l l e r e f f e c t s h a v e been d e a l t w i t h i n d e t a i l e l s e where. - 23 -2.9 Through Space and Through Bond I n t e r a c t i o n s T h i s i s a concept t h a t i s u s e f u l i n the e x p l a n a t i o n o f i n t e r a c t i o n s (42-44) between l o c a l i z e d o r b i t a l s or f u n c t i o n a l groups . To a f i r s t a p p r o x i m a t i o n a chromophore (eg- C = 0) i s c o n s i d e r e d t o have l o c a l i z e d o r b i t a l s which can i n t e r a c t w i t h o t h e r s o f the same symmetry. Hoffmann has e x p l a i n e d such i n t e r a c t i o n s i n terms o f p e r t u r b a t i o n t h e o r y which (45) l e a d s t o some g e n e r a l c o n c l u s i o n s h e l p f u l to the e x p e r i m e n t a l i s t I f t h e s o l u t i o n s of the S c h r o d i n g e r e q u a t i o n f o r some H a m i l t o n i a n H Q a r e known, t h e energy of t h e i t h l e v e l a f t e r i n t e r a c t i o n H' w i t h o t h e r l e v e l s i s t o second o r d e r g i v e n b y : -| H . . f E. = E. + £ o 1^ (2.16) l i . i. &.—&. z'h- i J and i s d e s c r i b e d by the w a v e f u n c t i o n :-V . = H<° + E o 1 ^ V? (2 .17 ) where H..' = HY^ d T - and V. i s the wavef u n c t i o n d e s c r i b i n g the i t h l e v e l b e f o r e i n t e r a c t i o n . These e q u a t i o n s a p p l y t o non-degenerate c a s e s , and n e g l e c t o v e r l a p between i n t e r a c t i n g w a v e f u n c t i o n s . These e q u a t i o n s l e a d t o t h e c o n c l u s i o n t h a t changes i n energy l e v e l s and w a v e f u n c t i o n s a r e p a i r w i s e a d d i t i v e i . e . i n d i v i d u a l p e r t u r b a t i o n s can be added l i n e a r l y . I f the i n t e r a c t i o n o f two l e v e l s ( l a b e l l e d a and b) i s c o n s i d e r e d the p e r t u r b a t i o n e q u a t i o n s g i v e : -- 24 -= T J O . ab F = F 4-a a E°-E° a b where E° < E° (2.18) a b I H ' J 2 E, = E° + a b Jb b E°-E° b a i 12 [ H ^ [ i s always p o s i t i v e so the d i r e c t i o n of the p e r t u r b a t i o n i s dependent upon the denominator. As E° < E° i t f o l l o w s t h a t E < E° a b a a and E n > E ° . In g e n e r a l terms t h i s l e a d s to t h e c o n c l u s i o n t h a t when b b two l e v e l s i n t e r a c t t h e lower l e v e l i s s t a b i l i z e d and the upper l e v e l i s d e s t a b i l i z e d i . e . t h e energy l e v e l s r e p e l each o t h e r . I f the w a v e f u n c t i o n s a r e c o n s i d e r e d , the e q u a t i o n s a r e : -H ' a a E -E, b a b (2.19) H ' b b E^-E a b a The s i g n of i s important and has g e n e r a l l y been found t o be o p p o s i t e to t h e o v e r l a p a l t h o u g h t h i s i s n o t proven. However i t i m p l i e s t h a t p o s i t i v e o v e r l a p produces s t a b i l i z a t i o n . U s i n g t h i s assumption, i f S , i s p o s i t i v e , H / w i l l be n e g a t i v e and E - E, i s ab ab a b a l s o n e g a t i v e so V? w i l l mix i n t o V° w i t h a p o s i t i v e s i g n i . e . b a - 25 -¥ = ¥° + c l Y " , ; ¥ = r 3 - cU' 0 (2.20) a a b b b a Thus when two o r b i t a l s i n t e r a c t t h e lower energy one mixes i n t o i t s e l f t h e h i g h e r energy one i n a bonding way w h i l e t h e h i g h e r energy o r b i t a l mixes i n t o i t s e l f the lower one i n an a n t i b o n d i n g way, o r t h e upper c o m b i n a t i o n takes the node. I m p l i e d i n the above e q u a t i o n s i s t h a t the e x t e n t of i n t e r a c t i o n i s i n v e r s e l y p r o p o r t i o n a l to t h e d i f f e r e n c e i n e n e r g i e s of t h e un-p e r t u r b e d l e v e l s . For t h e extreme case where the i n t e r a c t i n g o r b i t a l s a r e d egenerate t h e m i x i n g i s f i r s t o r d e r and d i f f e r e n t p e r t u r b a t i o n p r o c e d u r e s from the above a r e a p p l i c a b l e . However the c o n c l u s i o n s a r r i v e d a t i n the more g e n e r a l c a s e a r e s t i l l t r u e . T h e r e f o r e two degenerate o r b i t a l s ^ and % w i t h an u n p e r t u r b e d energy E q form two new o r b i t a l s d e s c r i b e d by x ^ + x-2 a n c ^ - X*2 w i t h e n e r g i e s of E q + H ^ and E - H / and the upper o r b i t a l i s noded. o ab D i r e c t s p a t i a l o v e r l a p of two chromophores i s termed through space (TS) i n t e r a c t i o n , and can g e n e r a l l y be n e g l e c t e d i f t h e two c e n t r e s ° (46) a r e more than 2.5A a p a r t . I f i n t e r a c t i o n i n v o l v e s o v e r l a p of the n or TT o r b i t a l s on the chromqphore w i t h s e m i - l o c a l i z e d a o r b i t a l s of t h e c o r r e c t symmetry i n the r e s t o f t h e m o l e c u l e i t i s s a i d t o be of the through bond (TB) t y p e . T h i s i n t e r a c t i o n can extend over much (47) l a r g e r d i s t a n c e s than the TS i n t e r a c t i o n . In cases where b o t h types of i n t e r a c t i o n a r e p o s s i b l e they may r e i n f o r c e or oppose each o t h e r , but the s i t u a t i o n i s s i m p l i f i e d because the TS e f f e c t i s g e n e r a l l y much l a r g e r than the TB one. -• 26 -Examples of PE s p e c t r a where these c o n c e p t s a r e i n v o k e d a r e numer-(42-47) ous and a r e p a r t i c u l a r l y u s e f u l i n t h e d i s c u s s i o n of t h e PE s p e c t r a of o r g a n i c m o l e c u l e s . Some d i k e t o n e s p e c i e s have been s t u d i e d w i t h p a r t i c u l a r r e f e r e n c e to th e i n t e r a c t i o n between the CO e n t i t i e s and c h a p t e r 7 p r e s e n t s the PE s p e c t r a of some a c i d a n h y d r i d e s where such i n t e r a c t i o n s a r e d i s c u s s e d . 2.10 Assignment of PE s p e c t r a and M o l e c u l a r O r b i t a l (MO) C a l c u l a t i o n s I n t e r p r e t a t i o n o f a PE spectrum to o b t a i n a d e f i n i t i v e assignment of the bands i n the spectrum to m o l e c u l a r o r b i t a l s i n t h e p a r e n t mole-c u l e can be a d i f f i c u l t p r o c e d u r e . The IP's of some o r b i t a l s can be e x p e c t e d to be i n a p a r t i c u l a r range (eg. c h l o r i n e l o n e p a i r i o n -i z a t i o n s u s u a l l y l i e between l l - 1 3 e V ) but a f i n a l assignment w i l l o f t e n be a i d e d by f a c t o r s a l r e a d y d i s c u s s e d i n t h i s c h a p t e r i . e . F r a n c k -Condon e n v e l o p e s , v i b r a t i o n a l s t r u c t u r e , i n t e n s i t y changes w i t h d i f f e r e n t e x c i t i n g r a d i a t i o n and t hrough space and t hrough bond i n t e r a c t i o n s , e t c . I t i s a l s o o f t e n i m p ortant to study a s e r i e s of compounds where s u b s t i t u e n t e f f e c t s can be o b s e r v e d or where the m o l e c u l e s i n v o l v e d a r e v a l e n c e i s o e l e c t r o n i c . T h i s a l l o w s a c o r r e l a t i o n of IP's and subsequent t r e n d s i n the s e r i e s w i l l h o p e f u l l y s u p p o r t a p a r t i c u l a r assignment. In some cases t h e r e w i l l be s u f f i c i e n t i n f o r m a t i o n a v a i l a b l e i n such a s e r i e s to a l l o w a f u l l assignment. However i t i s sometimes n e c e s s a r y to seek a d d i t i o n a l e v i d e n c e from MO c a l c u l a t i o n s which a r e a v a i l a b l e i n v a r i o u s forms. - 27 -(49) The s e l f - c o n s i s t e n t f i e l d (SCF) MO c a l c u l a t i o n i s most w i d e l y used where the Fock m a t r i x i s c a l c u l a t e d i n an i t e r a t i v e p r o c e s s t o f i n a l l y g i v e e i g e n v a l u e s which a r e then u s u a l l y c o r r e l a t e d w i t h IP's v i a Koopmans' theorem. The so c a l l e d ab i n i t i o c a l c u l a t i o n i n v o l v e s e v a l u a t i o n of a l l i n t e g r a l s and m i n i m i z a t i o n of t h e t o t a l energy of the system. T h i s i s ex p e n s i v e i n computer time but i s expec t e d t o g i v e r e l i a b l e e i g e n v a l u e s . The l e a s t s o p h i s t i c a t e d SCF c a l c u l a t i o n s a r e t h o s e of the s e m i - e m p i r i c a l t y p e eg. CNDO (complete n e g l e c t o f d i f f e r e n t i a l o v e r l a p ) where a l l i n t e g r a l s i n v o l v i n g d i f f e r e n t i a l o v e r l a p a r e neglected^"*" . Such programmes r e q u i r e l e s s computer time but g i v e l e s s r e l i a b l e e i g e n v a l u e s . Between t h e two extremes a r e many proc e d u r e s t h a t n e g l e c t o v e r l a p to v a r y i n g degrees eg. INDO^"^ , M I N D 0 ^ 5 6 \ S P I N D 0 ^ 5 7 \ Other c a l c u l a t i o n s t h a t g i v e e s t i m a t e s of e i g e n v a l u e s a r e the H i i c k e l m o l e c u l a r o r b i t a l (HMO) and the e m p i r i c a l l y (49 58) p a r a m e t e r i z e d extended H i i c k e l m o l e c u l a r o r b i t a l (EHMO) ' methods. The f i r s t of these e s t i m a t e s the e i g e n v a l u e s o f TT o r b i t a l s and has t h e r e f o r e found l i t t l e use i n i n t e r p r e t i n g PE s p e c t r a . The EHMO method however i n c l u d e s a l l v a l e n c e e l e c t r o n s , u s i n g the l i n e a r com-b i n a t i o n o f atomic o r b i t a l s (LCAO) form f o r MOs, and the energy i s mi n i m i z e d w i t h r e s p e c t to the AO c o e f f i c i e n t s . The many a p p r o x i m a t i o n s made i n t h i s method u s u a l l y l e a d t o c a l c u l a t e d o r b i t a l e n e r g i e s b e i n g h i g h by l-3eV. In a l l the methods mentioned Koopmans' theorem i s invok e d t o o b t a i n IP's from t h e c a l c u l a t e d e i g e n v a l u e s . As was mentioned p r e v i o u s l y t h i s i s an a p p r o x i m a t i o n ^ ^ which can l i m i t t h e u s e f u l n e s s of such - 28 -c a l c u l a t i o n s . Not o n l y can the a b s o l u t e v a l u e s be d i f f e r e n t from e x p e r i m e n t a l v a l u e s but the o r d e r i n g of the l e v e l s can be changed. One assumption i n Koopmans' theorem i s t h a t the c o r r e l a t i o n energy i s the same i n the m o l e c u l e and th e i o n . C o r r e l a t i o n e f f e c t s a r i s e l a r g e l y from p a i r i n t e r a c t i o n s between the e l e c t r o n s , a n d s i n c e the i o n has one l e s s e l e c t r o n than the m o l e c u l e , the c o r r e l a t i o n energy w i l l be d i f f e r e n t and u s u a l l y l e s s i n the i o n than the m o l e c u l e . Koopmans• theorem a l s o assumes t h e r e i s no r e o r i e n t a t i o n of o r b i t a l s upon i o n i z a t i o n so t h a t a l l i n t e g r a l s i n the H a r t r e e - F o c k e q u a t i o n a r e i d e n t i c a l i n the i o n and m o l e c u l e . I t i s known t h a t the use of Koopmans' theorem does i n f a c t g i v e the i n c o r r e c t o r d e r i n g of IP's 59 14 14 6 i n some a p p a r e n t l y s i m p l e cases (N , CO , CS , F^ ) even when e x t e n s i v e a b • i n i t i o c a l c u l a t i o n s a r e used. The breakdown of Koopmans' theorem may w e l l be w i d e s pread and e v i d e n c e i s o c c u r i n g t h a t t h i s i s so. T h e r e f o r e i n t e r p r e t a t i o n based s o l e l y on any c a l c u l a t i o n s h o u l d a t b e s t be c o n s i d e r e d t e n t a t i v e and t h e a p p r o x i m a t i o n s under-s t o o d . A b e t t e r approach would be t o p e r f o r m ab i n i t i o c a l c u l a t i o n s s e p a r a t e l y on the m o l e c u l e and i o n (ASCF method) } but t h i s o b v i o u s l y i n c r e a s e s computer time c o n s i d e r a b l y and has n o t been w i d e l y used. There a r e o t h e r t y p e s of c a l c u l a t i o n a v a i l a b l e t h a t do n o t r e l y ( 61) on Koopmans' theorem (eg. M u l t i p l e s c a t t e r i n g X a ( s c a t t e r e d wave), (62) Many Body Greens f u n c t i o n s ) which a r e becoming i n c r e a s i n g l y i m p ortant a l t h o u g h they a r e m a i n l y l i m i t e d to t h e r e a l m of the t h e o r e t i c i a n . C a l c u l a t i o n s t h a t i n v o l v e p e r t u r b a t i o n c o r r e c t i o n s to Koopmans' theorem - 29 -("63") have a l s o been used f o r sim p l e m o l e c u l e s . The r e s u l t s o b t a i n e d to t h i r d o r d e r a r e g e n e r a l l y w i t h i n 0.5eV of t h e e x p e r i m e n t a l I P ' s , and t h i s type of c a l c u l a t i o n has been used as an a i d to i n t e r p r e t a t i o n f o r a s p e c i f i c m o l e c u l e i n t h i s t h e s i s and i s d i s c u s s e d f u r t h e r i n c h a p t e r 4. o (64 65) A r e c e n t s e m i e m p i r i c a l p r o c e d u r e d e v e l o p e d by A s b r i n k e t a l . ' c a l c u l a t e s IP's independent o f Koopmans' Theorem by u s i n g a t r a n s i t i o n s t a t e p r o c e d u r e t h a t i n v o l v e s h a l f an e l e c t r o n b e i n g d i f f u s e l y removed from t h e m o l e c u l e . The HAM'3 (Hydrogenic Atoms i n M o l e c u l e s ) programme i s based on an e x t e n s i o n of S l a t e r ' s atomic s h i e l d i n g c o n s t a n t s to f i t an MO model. I t can o n l y be used f o r m o l e c u l e s c o n t a i n i n g C, H, N, 0 and F and has been h e a v i l y p a r a m e t e r i z e d by matching c a l c u l a t e d r e s u l t s w i t h e x p e r i m e n t a l IP's f o r around 80 m o l e c u l e s c o n t a i n i n g t h e s e atoms. ( 6 6 ) I n i t i a l r e s u l t s u s i n g t h i s method have been e n c o u r a g i n g ' but i t i s l i m i t e d by the p a r a m e t e r i z a t i o n ( i n f a c t i t i s p a r a m e t e r i z e d i n -c o r r e c t l y f o r t h e CN g r o u p ) . In t h i s t h e s i s many of the m o l e c u l e s s t u d i e d c o n t a i n second ( o r h i g h e r ) row atoms so HAM 3 c o u l d not be used. In c h a p t e r s 5 and 7 PE r e s u l t s a r e r e p o r t e d f o r m o l e c u l e s c o n t a i n i n g o n l y f i r s t row atoms and HAM..3 c a l c u l a t i o n s a r e compared w i t h a CNDO typ e c a l c u l a t i o n and the r e s u l t s c i t e d w i t h 1 the e x p e r i m e n t a l v a l u e s . At the p r e s e n t time c a l c u l a t i o n s can be an a i d to i n t e r p r e t i n g a PE spectrum but s h o u l d be used w i t h o t h e r c o r r o b o r a t i n g e v i d e n c e (eg. s u b s t i t u e n t e f f e c t s , v i b r a t i o n a l s t r u c t u r e , e t c . ) . The r e v e r s e - 30 -of the problem i s t h a t a PE spectrum i n t e r p r e t e d i n d e p e n d e n t l y of a c a l c u l a t i o n can be used to t e s t the r e l i a b i l i t y of a p a r t i c u l a r t y p e of c a l c u l a t i o n i . e . to check the v e r a c i t y of quantum m e c h a n i c a l p r o c e d u r e s . A form of s y m b i o s i s t h e r e f o r e e x i s t s between the PE s p e c t r o s c o p i s t and t h e t h e o r e t i c i a n , as an IP i s an e x p e r i m e n t a l o b s e r v a b l e which i s t h e o r e t i c a l l y m e a n i n g f u l . - 31 -CHAPTER THREE  THE PE SPECTROMETER A PE s p e c t r o m e t e r c o n s i s t s of a vacuum chamber c o n t a i n i n g an i o n i z i n g s o u r c e , an i o n i z a t i o n chamber t o g e t h e r w i t h an i n l e t system., and an e l e c t r o n a n a l y z e r . A system of e l e c t r o n i c s i s then r e q u i r e d to s u p p l y and c o n t r o l the v o l t a g e s n e c e s s a r y to o p e r a t e the e l e c t r o n a n a l y z e r . A l l the work r e p o r t e d h e r e i n v o l v e d u s i n g an u l t r a v i o l e t (1 2) PE s p e c t r o m e t e r whose d e s i g n has been d e s c r i b e d f u l l y e lsewhere ' The i n s t r u m e n t w i l l o n l y be d i s c u s s e d b r i e f l y t h e r e f o r e , w i t h p a r t i c u l a r emphasis on the f e a t u r e s r e l e v a n t to the study of u n s t a b l e and r e a c t i v e s p e c i e s . 3.1 The Vacuum System The d e s i g n of the pumping system i s of s p e c i a l importance i n the study of s h o r t l i v e d s p e c i e s because i t i s n e c e s s a r y t o have a s h o r t t r a n s p o r t time f o r th e s p e c i e s under i n v e s t i g a t i o n from the g e n e r a t i o n p o i n t to the i o n i z a t i o n chamber. I t i s a l s o important to r e s t r i c t the amount of compound e n t e r i n g t h e e l e c t r o n a n a l y z e r and t o remove the compound from the s p e c t r o m e t e r b e f o r e d e c o m p o s i t i o n and d e p o s i t i o n of i h v o l a t i l e m a t e r i a l . T h i s e n s u r e s the optimum performance of the a n a l y z e r f o r the g r e a t e s t p o s s i b l e time as any d e p o s i t i o n of m a t e r i a l on the e l e c t r o n o p t i c s r e s u l t s i n ihhomogeneous p o t e n t i a l s , c a u s i n g a l o s s i n e f f i c i e n c y and r e s o l u t i o n . - 32 -A schematic c r o s s s e c t i o n of t h e s p e c t r o m e t e r i s shown i n f i g . 3.1. The t o t a l volume of the i o n i z a t i o n chamber, the a n a l y z e r and e l e c t r o n m u l t i p l i e r h o u s i n g i s around 200cc w i t h the a n a l y z e r and m u l t i p l i e r h o u s i n g b e i n g pumped by a 2" d i f f u s i o n pump. The i o n i z a t i o n chamber i s pumped by a 4" d i f f u s i o n pump v i a a t h r o t t l i n g v a l v e and t h i s can p r o v i d e the r a p i d removal of compound from the s p e c t r o m e t e r (pumping _3 speed of 1000£/sec f o r a i r at p r e s s u r e s below 3 x 10 t o r r ) . The s m a l l volume of the s p e c t r o m e t e r and t h i s f a s t pumping f a c i l i t y means the i n s t r u m e n t can be pumped down q u i c k l y a f t e r b e i n g l e t up to a i r . T h i s i s an important c o n s i d e r a t i o n i n s t u d i e s where the i n l e t system has to be changed or c l e a n e d f r e q u e n t l y . Three a p e r t u r e s t h a t s e r v e as p a r t of the e l e c t r o n o p t i c s s e p a r a t e the i o n i z a t i o n r e g i o n from the a n a l y z e r . These a r e a t the bottom of the i o n i z a t i o n chamber (0.030"), the l e n s (0.100") and the e n t r a n c e to the a n a l y z e r (0.020") and i t i s t h i s arrangement t h a t m i n i m i z e s the amount of sample e n t e r i n g the a n a l y z e r and p r o v i d e s the i n i t i a l f o c u s s i n g and s c a n n i n g of the e j e c t e d e l e c t r o n s . An i o n i z a t i o n gauge m o n i t o r s t h e p r e s s u r e immediately below the a n a l y z e r . The base p r e s s u r e i n the s p e c t r o m e t e r i s around 1 x 10 ^ t o r r and when i n o p e r a t i o n t h e p r e s s u r e i n the a n a l y z e r i s u s u a l l y i n the range 5 x 10 ^ t o r r t o 1 x 10 ~ * t o r r . The p r e s s u r e i n t h e i o n i z a t i o n chamber i s , o f c o u r s e , h i g h e r and i s of the o r d e r of O . O l t o r r f o r an —6 a n a l y z e r p r e s s u r e of 5 x 10 t o r r . - 33 -PE SPECTROMETER VERTICAL CROSS SECTION « * brass ° O-rings sapphire balls — teflon scale 2 *diffusion pump F i g . 3.1 - 34 -3.2 The A n a l y z e r The 180° h e m i s p h e r i c a l a n a l y z e r i s an i n t e g r a l p a r t of t h e s p e c t r o -meter and l i k e the r e s t o f the s p e c t r o m e t e r i s c o n s t r u c t e d of b r a s s . The i n n e r hemisphere ( r a d i u s 1") i s screwed to the s u p p o r t i n g p l a t e , b e i n g i n s u l a t e d from ground by a V i t o n 0 - r i n g and a t e f l o n s h e a t h , and i s s e a t e d on t h r e e s a p p h i r e b a l l s . The o u t e r hemisphere ( r a d i u s 1.5") i s s e a t e d on the top of t h e 2" d i f f u s i o n pump b e i n g i n s u l a t e d a g a i n by a t e f l o n s p a c e r and V i t o n 0 - r i n g . I t i s a l s o connected t o the s u p p o r t i n g p l a t e by f o u r n y l o n screws, i n s u l a t i o n from t h i s b e i n g a c h i e v e d by a mylar s p a c e r and V i t o n 0 - r i n g . The h e m i s p h e r i c a l s u r f a c e s a r e g o l d p l a t e d and l i k e the r e s t o f t h e e l e c t r o n o p t i c s a r e c o a t e d w i t h c o l l o i d a l g r a p h i t e which reduces background due to s c a t t e r e d e l e c t r o n s . I t a l s o s e r v e s as a p r o t e c t i v e l a y e r f o r the hemispheres and can be s i m p l y removed a f t e r " sample p o i s o n i n g and r e p l a c e d by a new c o a t i n g . to be The t h e o r e t i c a l r e s o l u t i o n f o r such a n . a n a l y z e r can be shown ( 2 ) . A E l / 2 W 2R o where A E ^ ^ i s t n e h a l f w i d t h of the PE l i n e ( t h e s t a n d a r d b e i n g the 2 ^3/2 b a n < ^ °^ ' E •*~s t' r i e e l e c t r o n energy which i s the pass energy of the a n a l y z e r and W i s t h e di a m e t e r of t h e e n t r a n c e and e x i t a p e r t u r e s , which i s 0.020" i n t h i s c a s e . R i s the p a t h r a d i u s of the e l e c t r o n o - 35 -which i s 1.25" f o r t h i s a n a l y z e r . S u b s t i t u t i n g t h e s e f i g u r e s i n t o the above e q u a t i o n g i v e s a r e s o l u t i o n o f 16meV f o r a pass energy of 2eV. When the a n a l y z e r s u r f a c e s of t h e s p e c t r o m e t e r a r e c l e a n an e x p e r i m e n t a l r e s o l u t i o n of around 30meV can be o b t a i n e d under such c o n d i t i o n s but i s somewhat l e s s (^50meV) under normal o p e r a t i n g c o n d i t i o n s . In f i g . 3.2 the v o l t a g e s a p p l i e d to the e l e c t r o n o p t i c s a r e shown. V o l t a g e V ^ i s s e t u s u a l l y between 1 and 2V, the b a l a n c e ( t h e r a t i o of v o l t a g e s a p p l i e d t o the hemispheres) h a v i n g been s e t f o r optimum (1 2) performance ' . V i s a p o t e n t i a l used t o r e t a r d the p h o t o e l e c t r o n s R to the pass energy of th e a n a l y z e r , and i s a p p l i e d to the cup which s e r v e s as the i o n i z a t i o n chamber. To r e c o r d a spectrum i t i s t h i s v o l t a g e t h a t i s scanned, a l l o t h e r s b e i n g c o n s t a n t . T h i s has the advantage of m a i n t a i n i n g the same pass energy f o r a l l p h o t o e l e c t r o n s and thus the r e s o l u t i o n and t h e s e n s i t i v i t y of the s p e c t r o m e t e r a r e c o n s t a n t over the f u l l range of the spectrum. The ramp v o l t a g e r e q u i r e d to d r i v e the scan i s s u p p l i e d by a F a b r i t e k model 1072, through a ramp a m p l i f i e r a c r o s s a maximum of 1024 c h a n n e l s f o r a f u l l s c a n . The F a b r i t e k a l s o s t o r e s the s i g n a l o f each c h a n n e l a l l o w i n g the b u i l d up of counts by m u l t i p l e s c a n s . The l e n s v o l t a g e V can be a d j u s t e d to maximize the s i g n a l on a p a r t i c u l a r band of a spectrum and f o c u s e s the e l e c t r o n s b e f o r e e n t e r i n g the a n a l y z e r . To s h i e l d the e j e c t e d p h o t o e l e c t r o n s from extran e o u s magnetic f i e l d s (eg. e a r t h ' s magnetic f i e l d , power s u p p l i e s , f u r n a c e s , e t c . ) the whole s p e c t r o m e t e r i s e n c l o s e d i n a s e t of t h r e e F i g . 3.2 - 37 -m u t u a l l y p e r p e n d i c u l a r H e l m o l t z c o i l s . The v o l t a g e s i n these c o i l s a r e s e t to compensate f o r e x t e r n a l f i e l d s by m aximizing the s i g n a l . The e l e c t r o n s a r e d e t e c t e d w i t h a C h a n n e l t r o n e l e c t r o n m u l t i p l i e r (Type B 318 AL/01)." O p t i m a l r e s p o n s e i s o b t a i n e d by a p p l y i n g 300V t o the horn of the d e t e c t o r and a m p l i f y i n g t h e s i g n a l a c r o s s 3.5kV. The s i g n a l from the m u l t i p l i e r i s passed through a p r e - a m p l i f i e r ( M o d i f i e d V a r i a n type) to an a m p l i f i e r / d i s c r i m i n a t o r (Harshaw NA-15) and r a t e m e t e r (Harshaw NR-10). The s i g n a l p u l s e s a r e s t o r e d i n the F a b r i t e k 1072. 3.3 The L i g h t Source The l i g h t s o u r c e used on t h e s p e c t r o m e t e r i s an a i r c o o l e d gas d i s c h a r g e powered by a M i c r o t r o n 200 microwave power s o u r c e . D i s c h a r g e of h e l i u m (or neon) i n a q u a r t z tube (6mm OD) c o n s t r i c t e d at the s p e c t r o m e t e r end by a 1.5mm ID boron n i t r i d e tube p r o v i d e s i n t e n s e He I ( o r Ne I) r a d i a t i o n . An 87.5mm l e n g t h of c a p i l l a r y ( 0 . 7 5 m m ID) c o l l i m a t e s the beam i n t o t h e i o n i z a t i o n chamber. A gap of around 5mm s e p a r a t e s t h i s c o l l i m a t i n g c a p i l l a r y , from t h e d i s c h a r g e tube p r o v i d i n g a removal p o i n t f o r unwanted He ( o r Ne) which i s pumped away b e f o r e p o s s i b l e e n t r y i n t o the s p e c t r o m e t e r . Under i d e a l c o n d i t i o n s , w i t h t h i s s p e c t r o m e t e r d e s i g n , t h i s lamp p r o v i d e s s u f f i c i e n t i n t e n s i t y o f He I to o b t a i n ^50,000cps 2 f o r the Ar ^2>/2 Pea^- a t a n a n a l y z e r pass energy of 2eV and a r e s o l u t i o n of 30meV. When u s i n g Neon the c o u n t i n g r a t e i s s l i g h t l y - 38 -b e t t e r (50,000cps a t 1.6eV pass energy and a r e s o l u t i o n of 25meV). T h i s means t h a t the time r e q u i r e d to r e c o r d a spectrum of an u n s t a b l e or r e a c t i v e s p e c i e s i s such t h a t the e x p e r i m e n t a l c o n d i t i o n s can be kept r e a s o n a b l y s t a b l e , t h e r e b y m i n i m i z i n g t h e e f f e c t due to p r e s s u r e f l u c t u a t i o n s , f i e l d v a r i a t i o n s and i n s t a b i l i t i e s i n the e l e c t r o n i c s . - 39 -CHAPTER FOUR HYPOCHLOROUS ACID AND ALKYL HYPOCHLORITES 4.1 I n t r o d u c t i o n A c o n c e n t r a t e d aqueous s o l u t i o n : o f c h l o r i n e i s known to c o n t a i n an a p p r e c i a b l e c o n c e n t r a t i o n o f h y p o c h l o r o u s a c i d ^ (HOC£) . The a c i d cannot be o b t a i n e d i n the pure s t a t e however as i t e x i s t s i n e q u i l i b r i u m w i t h o t h e r s p e c i e s , e.g. H 20 + C ^ O ' v N 2H0C£ ( R e f s . 2-5) C £ 2 ( a q ) ^ s H + + C£ + HOC£ ( R e f s . 1 and 6) The a c i d can be c o n s i d e r e d an u n s t a b l e i n t e r m e d i a t e i n the s o l u t i o n c h e m i s t r y o f c h l o r i n e and c h l o r i n e monoxide. R e a c t i o n o f h y p o c h l o r o u s a c i d w i t h an a l c o h o l r e s u l t s i n the f o r m a t i o n o f t h e a l k y l h y p o c h l o r i t e s . R - OH + HOCJ> s S R - OCi + H„0 Many such a l k y l e s t e r s have been o b t a i n e d i n s o l u t i o n s of o r g a n i c s o l v e n t s which decompose s l o w l y i n the d a r k ^ 7 ^ . RCH 2 - OCI > RCHO + HC£ RCH 2 - OC£H-HC£ > CJ>2 + R - CI^OH R 3 COC£ > R - CO - R + RC£ - 40 -T e r t i a r y b u t y l h y p o c h l o r i t e has been r e p o r t e d to be q u i t e s t a b l e i n such s o l u t i o n s under normal c o n d i t i o n s . In the gas phase the s t a b i l i t y of these compounds i s l e s s c e r t a i n and methyl h y p o c h l o r i t e vapours a r e known t o be e x p l o s i v e ^ 7 ^ . A l t h o u g h t h e r e has been some gas phase s p e c t r o s c o p i c i n v e s t i g a t i o n of the pa r e n t m o l e c u l e , HOCXJ i t was sug g e s t e d as r e c e n t l y as 1976 t h a t ( 8") i t had not d e f i n i t e l y been shown to e x i s t i n the gas phase . Vapour p r e s s u r e d a t a does i n f a c t i n d i c a t e t h a t the f r a c t i o n o f H0C£ i n the (3 4 9) vapour phase above an aqueous s o l u t i o n of C ^ O i s e x t r e m e l y s m a l l ' ' . However microwave measurements have b e e n - c a r r i e d o u t ^ ^ and the r e s u l t s show t h a t the 0-H and 0-C£ bond l e n g t h s a r e 0.959 and 1.6895A': r e s p e c t i v e l y w i t h a bond a n g l e o f 102° 29'. The microwave s p e c t r a of methyl h y p o c h l o r i t e ^ " ^ ' " ^ and e t h y l h y p o c h l o r i t e ^ " ^ have a l s o been r e c o r d e d . The methyl d e r i v a t i v e was p r e p a r e d by r e a c t i o n o f c h l o r i n e w i t h an a l k a l i n e methanol s o l u t i o n a t 0°C, w i t h the vapours of t h e r e s u l t a n t s o l u t i o n b e i n g a l l o w e d t o e n t e r the a b s o r p t i o n c e l l , which was c o o l e d t o -78°C. E t h y l h y p o c h l o r i t e was p r e p a r e d d i r e c t l y i n the a b s o r p t i o n c e l l c o o l e d t o -78°C by r e a c t i o n of c h l o r i n e n i t r a t e (CMO^) w i t h e t h a n o l . In n e i t h e r case was a pure gas phase sample o f the h y p o c h l o r i t e o b t a i n e d . V i b r a t i o n a l s p e c t r a o f H0C£ i n t h e gas phase have a l s o been o b t a i n e d ^ 7 a l t h o u g h the o r i g i n a l e s t i m a t e o f the bond a n g l e (113°) (19) has been shown to be i n c o r r e c t . S o l u t i o n s o f t h e a l k y l h y p o c h l o r i t e s (20) have been used t o o b t a i n v i b r a t i o n a l i n f o r m a t i o n . F o r c e c o n s t a n t s (21 22 23) and thermodynamic p r o p e r t i e s have been r e p o r t e d f o r the a c i d ' ' - 4 1 -( 2 4 - 2 7 ) The u l t r a v i o l e t spectrum of HOC£ has been s t u d i e d b o t h i n s o l u t i o n ' ( 2 5 2 7 ) and i n t h e gas phase ' . . Recent i n t e r e s t i n HOC£ has been d i r e c t e d ( 2 8 - 3 0 ) towards i t s p o s t u l a t e d r o l e i n t h e s t r a t o s p h e r e a r e c e n t ( 3 1 ) t h e o r e t i c a l i n v e s t i g a t i o n - s u g g e s t i n g t h a t i t i s an i m p o r t a n t r e s e r v o i r ( 2 7 ) . . ... f o r i n e r t c h l o r i n e . The l a t e s t u l t r a v i o l e t r e s u l t s however ' i n d i c a t e t h a t t h e p h o t o d i s s o c i a t i o n o f H0C£ i n t h e s t r a t o s p h e r e would be s i g n i f i c a n t , thus making i t unimportant i n such a r o l e . A l t h o u g h t h e s o l u t i o n c h e m i s t r y o f H0C£ and i t s a l k y l d e r i v a t i v e s have been e x t e n s i v e l y i n v e s t i g a t e d t h e e l e c t r o n i c s t r u c t u r e o f t h e s e mo l e c u l e s i s l e s s w e l l u n d e r s t o o d . There a r e a few c a l c u l a t i o n s on ( 3 2 - 3 5 ) H0C£ and one (Ref. 3 5 ) g i v e s a comprehensive e v a l u a t i o n i f I P ' s . through p e r t u r b a t i o n c o r r e c t i o n s t o Koopmans' theorem. However t h i s work i s t h e f i r s t t o p r e s e n t e x p e r i m e n t a l IP's f o r H0C£ and i t s a l k y l d e r i v a t i v e s . The s p e c i e s FC£ and CR^CH which a r e i s o e l e c t r o n i c w i t h H0C£, have ( 3 6 - 4 2 ) been e x t e n s i v e l y s t u d i e d by PES 5 - and w i l l p r o v i d e a comparison w i t h t h e PE spectrum o f H0C£ p r e s e n t e d h e r e . 4.2 E x p e r i m e n t a l Hypochlorous a c i d was o b t a i n e d from an aqueous s o l u t i o n o f c h l o r i n e monoxide ( C £ 2 0 ) w i t h which i t i s i n e q u i l i b r i u m . C £ 2 0 + H 20 *s=* 2H0C£ The C £ 2 0 was f i r s t p r e p a r e d by o x i d a t i o n of C £ 2 i n carbon t e t r a c h l o r i d e •.-*••? (43 44) at 0°C u s i n g «mercury-IIoxide ' . The r e s u l t a n t s o l u t i o n was t h e n shaken - 42 -w i t h d e i o n i z e d water which e x t r a c t e d t h e CZ^O w h i l e excess CZ^ remained i n t h e o r g a n i c phase. The aqueous s o l u t i o n o f CZ^O was kept:"in t h e dark a t -30°C to m i n i m i z e d e c o m p o s i t i o n . To r e c o r d t h e PE spectrum o f H0C£ t h e vapour above t h e s o l u t i o n ( m a i n t a i n e d a t 0°C) was l e t d i r e c t l y i n t o t h e i o n i z a t i o n chamber of the s p e c t r o m e t e r . The a c t u a l c o n d i t i o n s of o p e r a t i o n had t o be c a r e f u l l y o p t i m i z e d because t h e c o n c e n t r a t i o n o f H0C£ i n th e gas phase above an aqueous s o l u t i o n o f CH^O w a s found t o be e x t r e m e l y low i n agreement w i t h (3 4 9) t h e vapour p r e s s u r e measurements ' ' . U s i n g a c o n c e n t r a t e d s o l u t i o n o f CZ^O i t was t h i s s p e c i e s ( a l o n g w i t h R^O) t h a t dominated t h e PE spectrum r e c o r d e d . As t h e C ^ O w a s removed and t h e s o l u t i o n became more d i l u t e an i n c r e a s i n g p r o p o r t i o n o f H0C£ was o b s e r v e d i n t h e spectrum. F i n a l l y a l l t h e CZyO was removed from t h e vapour phase and t h e PE spectrum of HOC£ i n the p r e s e n c e of a l a r g e excess of water was o b t a i n e d . A l l attempts t o remove t h e R^O by t h e use of v a r i o u s t r a p p i n g p r o c e d u r e s and d e s s i c a n t s were u n s u c c e s s f u l due t o t h e c o n v e r s i o n o f HOC£ to CZ^O, presumably from removal o f a water m o l e c u l e from two HOC£ m o l e c u l e s . The sequence o f s p e c t r a o b t a i n e d d u r i n g t h i s s ampling p r o c e d u r e a r e shown i n F i g . 4:1. The a l k y l h y p o c h l o r i t e s were a l l p r e p a r e d by r e a c t i o n o f a sodium h y p o c h l o r i t e s o l u t i o n w i t h the a p p r o p r i a t e a l c o h o l . The sodium hypo-c h l o r i t e s o l u t i o n was made by p a s s i n g c h l o r i n e t h r o u g h a 20% sodium h y d r o x i d e s o l u t i o n at 0°C u n t i l t h e r e was excess h a l o g e n p r e s e n t . T h i s s o l u t i o n . c o u l d be kept a t 5°C w i t h o u t s e r i o u s d e c o m p o s i t i o n . - 43 -The alcohol (methanol, ethanol or t-b u t y l alcohol) was added to t h i s hypochlorite s o l u t i o n at 0°C i n a r a t i o of approximately 1:25. The mixture was immediately pumped into the spectrometer through a cold trap (-95°C for methyl and ethyl hypochlorites, -48°C f or t- b u t y l hypochlorite). This served to remove water and the s t a r t i n g alcohol from the vapour phase allowing the PE spectrum of the appropriate a l k y l hypochlorite to be obtained. The only other species seen i n the gas phase was chlorine which could be pumped away p r e f e r e n t i a l l y . Unlike the PE spectrum of HOC£, these sampling techniques have enabled clean spectra of the esters to be obtained. F i g . 4; 3 show the PE spectra of methyl, e t h y l and t-b u t y l hypochlorites (CH 3OC£, C ^ O C J i , ft-BuOCJi) . ) A CNDO/BW c a l c u l a t i o n has been performed f o r CH 3OC£ (Table 4.2). 4.3 Results i ) HOC& ^ The PE spectra obtained during the sampling of the vapours above an aqueous Ci^O s o l u t i o n are shown i n F i g . 4:1, and i t can be seen that the features assigned to i o n i z a t i o n events from HOC£ are indeed very weak. When a CaZl^ trap was introduced between the sample so l u t i o n and the spectrometer i n order to remove the H^ O from the gas phase, a PE spectrum of C^O was obtained and t h i s i s shown i n F i g . 4.1a. (45) This compares favourably with the spectrum previously published F i g . 4.1b shows the PE spectrum obtained f o r a concentrated Cl^O s o l u t i o n when no attempt was made to remove the H^O, i t s presence i n the gas - 44 F i g . 4.1 He l p h o t o e l e c t r o n s p e c t r a of a) b) c * 2 o vapour above a c o n c e n t r a t e d aqueous s o l u t i o n of CSL^O c) vapour above" a d i l u t e aqueous s o l u t i o n of CJ^O d) vapour above an extremely d i l u t e aqueous s o l u t i o n of C£-2° ( v e r t i c a l l i n e s c o r r e s p o n d to IP's of HOC£ computed by p e r t u r -b a t i o n c o r r e c t i o n s t o Koopmans' theorem - r e f 35) IONIZATION POTENTIAL (eV) - 45 -phase being evidenced by the sharp peak at 12.62eV and broad bands in the regions 14-16 and 17.5-20eV. In this case the Cl^O peaks are s t i l l quite intense and a weak band around 11.6eV is assigned to chlorine by (46 47) comparison with i t s known PE spectrum ' . There is also an additional band at 11.2eV appearing between the f i r s t IP's of CH^O and Ci^. Upon careful examination the familiar Franck-Condon envelope of the second band of H^ O (14-16eV) is seen to be somewhat perturbed at - 14.5 and 15.5eV, Continued ; monitoring of the PE spectrum over several hours revealed gradual changes in the gas phase composition. The excess CH^ disappeared and the CH^O diminished appreciably to!',leave structured bands at 11.2 and 12.3eV (the latter had previously been hidden by the second band of CSL^O) . This situation is represented by the PE spectrum in Fig. 4.1c. which confirms the presence of two additional bands in the region of the second band of H^ O. Eventually a l l the Cl^O disappeared and the PE spectrum shown in Fig. 4.Id was obtained. This consists of the PE spectrum of H^ O together with the aforementioned peaks at 11.2 and 12.3eV which together with the two bands hidden under the second band of water are assigned to the previously undetected species HOC£. No other species that would be formed in this reaction and showing the observed features in the PE spectrum can be envisaged. The results obtained are in (35) excellent agreement with some good calculations on HOC£ , showing less than 0.37eV discrepancy between the experimental and calculated values for the f i r s t four IP's. Fig. 4.Id shows the calculated values as vertical lines above the spectrum. Identical calculations for Ci^O (35) also gave extremely good agreement with the experimental PE spectrum - 46 -(45) of t h a t m o l e c u l e , t h e r e b y g i v i n g some su p p o r t to t h e i r v e r a c i t y . The f i r s t and second IP's of HOC£ a r e p r e d i c t e d t o l i e between the f i r s t and second IP's of CZ^O and t h i s i s observed e x p e r i m e n t a l l y . That t h e o b s e r v e d f e a t u r e s i n F i g . 4.1 a r e i n d e e d due t o HOC£ i s f u r t h e r e s t a b l i s h e d by comparison w i t h t h e PE s p e c t r a o f t h e a l k y l h y p o c h l o r i t e s (see b e l o w ) . The f i r s t two IP v a l u e s of t h e h y p o c h l o r i t e s c o r r e l a t e v e r y w e l l w i t h t h e r e s u l t s quoted f o r HOC£ and th e c o r r e s p o n d i n g band shapes and v i b r a t i o n a l s t r u c t u r e a r e a l s o c o n s i s t e n t w i t h t h e HOC£ spectrum. The f i r s t two bands i n t h e H e l PE spectrum of HOC£ a r e shown i n g r e a t e r d e t a i l i n F i g . 4.2a. The t h i r d and f o u r t h bands which a r e 2 + p a r t l y h i d d e n by the s t a t e o f ^ 0 a r e shown i n F i g . 4.2b. C a r e f u l a n a l y s i s o f many s p e c t r a c o n t a i n i n g d i f f e r e n t r a t i o s o f CZ^O, H^O and H0C£ • - , (49 50) • ' ' and- Comparison, w i t h h i g h r e s o l u t i o n HO. ' s p e c t r a i n d i c a t e s t-the p r e s e n c e of two bands w i t h maxima a t 14.6 ± 0.1 and 15.6 ± O.leV. There i s a l s o a s m a l l c o n t r i b u t i o n from t h e weaker, h i g h e r IP's of C£^0 i n t h i s r e g i o n but t h e observed f e a t u r e s can be o b t a i n e d i n t h e absence o f CZ^O and t h e i r i n t e n s i t y changes show t h a t they a r e a s s o c i a t e d w i t h the f i r s t two IP's of H0C£. E x p e r i m e n t a l v a l u e s f o r t h e f i r s t f o u r IP's t o g e t h e r w i t h any a s s o c i a t e d v i b r a t i o n a l s t r u c t u r e a r e g i v e n i n T a b l e 4.1 w i t h t h e proposed (35) assignments. The c a l c u l a t e d v a l u e s a r e a l s o i n c l u d e d t o show the e x c e l l e n t agreement. The a d d i t i o n a l f i f t h IP ( c a l c u l a t e d v a l u e 17.73eV) 2 + i s e x p e c t e d t o be broad and weak, l y i n g under the band o f ^ 0 (17-20eV) and so was not d e t e c t e d i n t h i s work. - 47 -Table 4.1. Experimental and Theoretical Results for HOC?. B a n d Experimental I P ( e V ) a , b T h e o r e t i c a l 3 5 Assignment 1 (11.12) 830cm"1 ( 4 E ) G A 11-22 11.21 3a." 2 (12.09) 12.27 700cm - 1 12.06 10a' 1250cm"1 14.6 14.23 2a" 15.6 15.29 9a' 17.73 8a' a) A l l IP's ± O.OleV, except bands 3 and 4 (+ O.leV) Adiabatic IP's in brackets. .b) Vi b r a t i o n a l structure ± 50cm _ 1, except 1250cm"1 ± 80cm _ 1. 1st Band a 2nd Band i i n i n I I I I — 1 3rd Band 4th Band b H , 0 + 2 A , ! I I I 1 I T I I I I I I I I I t I I I | I I I I [ I I I I | ' I | I I I 1 | I I I ! [ 1 I I I | 11.0 12.0 14 15 16 17 IONIZATION P O T E N T I A L (eV) F i g . 4.2. a) Expansions of the f i r s t and second bands i n the Hel p h o t o e l e c t r o n spectrum of HOC£ b) The t h i r d and f o u r t h bands i n the H e l p h o t o e l e c t r o n spectrum of HOC''- - p a r t i a l l y masked by the 2A^ s t a t e o f + 00 - 49 -Alkyl hypochlorites The three alkyl hypochlorites studied here were a l l obtained in good yields in the gas phase thus permitting clean PE spectra to be obtained. The reaction mixtures contained l i t t l e excess alcohol and the sampling procedures previously described, were successful in removing this .'from the gas phase. The PE spectra of the alcohols are known ( 5 1' 5 2 ). H20 was almost totally removed by the cold traps only appearing as-a minor contaminant in the spectrum of t-BuOC£. in the PE spectrum of CH^ OCX, weak bands from C&2 were usually observed, presumably from decomposition, although there was no evidence for formaldehyde^"^ or HCtf/"'^ the other expected decomposition products^ 7^ . (53) In the case of t-BuOC£, methyl chloride was seen in the PE spectrum i f the solution was l e f t standing overnight at room temperature. However (54) the other possible decomposition product, acetone , was again not observed. The spectra of CR^CZ, C^OCJl and t-BuOC£ are shown in Fig. 4.3. The low IP region contains two distinctive bands (compare HOC£) with similar Franck Condon envelopes in a l l cases. The f i r s t band in CH3OC£ and again in C^ H^ OCA shows resolvable, vibrational structure, and this is illustrated in more detail in Fig. 4.4. The IP values determined, together with the vibrational frequencies, are given in Table 4.2. For C2H,.OC£ and particularly t-BuOC£ the high IP region consists of broad overlapping bands and thus only band maxima are listed which may not precisely correspond to individual IP's. The results for the CNDO/BW calculation for CH3OC£ are included in Table 4.2 although - 50 -J 1 1 11 u 1 MeOCI i i i i i i i i 1 1 1 1 EtOCI • * i i i i i i 1 ' 1 H 2 C llA t-BuOCI 1 1 1 1 1 1 1 1 1 10 12 14 16 18 20 IONIZATION P O T E N T I A L (eV) F i g . 4.3. H e l p h o t o e l e c t r o n s p e c t r a o f the a l k y l h y p o c h l o r i t e s (MeOC£, EtOC£, t-Bu0C£) - 51 -Table 4.2 Experimental v e r t i c a l IP's for the a l k y l hypochlorites CH 3 0CJ> C^ O C J l t-BuOCJ. EXP. ( a ) (CALC.) ( b ) EXP. ( C ) EXP. ( C ) (10.39) (10.13) 10.49(4a") 12.56(12a') 10.23 9.90 (700cm - 1) (700cm - 1) 11.75(12a') 12.63(lla') 11.53 11.07 13.08(lla') 12. 72 (4a") 12.35 11.82 13.53(3a") 13.47(3a") 12.96 12.22 (13.53) (10a') 14.58(10a') 13.90 12.98 16! 05 (2a") 15.14(9a') 14.33 14.48 17.10(9a') 15.95(3a") 15.28 15.80 16.50 16.58 17.02 18.68 a) F i r s t two exp. IP's ± O.OleV; other exp. IP's ± 0.05eV. Vi b r a t i o n a l frequency ± 50cm--!-. b) Results of CNDO/BW cal c u l a t i o n using experimental geometry 1^. c) Results below double l i n e are peak maxima but may not correspond to i n d i v i d u a l IP's. V i b r a t i o n a l frequency ± 50cm--'-. - 52 -F i g . 4.4. Expansion of t h e f i r s t bands i n the H e l p h o t o e l e c t r o n s p e c t r a of MeOC£ and EtOC£ - 53 -the correspondence with the experiemental IP's is poor. The calculation predicts a l l the IP's in the Hel region to be between 12.5 and 16.0eV and the ordering of the levels cannot be relied upon when they are predicted to be so close. It is also l i k e l y that use of Koopmans' theorem w i l l give an incorrect ordering, as is the case for WCl, and thus the calcu-lation w i l l not be used further. 4.4 Discussion The PE spectrum of HOC£ w i l l be discussed separately and then the alkyl hypochlorites w i l l be considered in relationship to the parent acid. 4.4.1 HOC& Comparison of the PE spectrum of HOC£ and the valence isoelectronic (46 47) species Cl^ ' provides a basis for the assignment of the HOCJ, spectrum. In electronegativity terms C£ and OH are very close and thus inductive effects are unimportant in this comparison. The degeneracy + 2 of the f i r s t ionic state of C£„ ( rr ) with a vertical IP of 11.60eV ^ 8 is therefore s p l i t in EOCi by the presence of the off-axis H atom giving rise to antibonding in-plane (a') and out-of-plane (a") orbitals. The a level i s destabilized by 0.38eV to become the outermost orbital of HOC£ (vertical IP = 11.22eV) and the a' level i s stabilized by 0.67eV to become the second orbital of HOC£ (vertical IP = 12.27eV). It i s to be expected that the out-of-plane orbital is less affected than the i n -plane orbital which i s stablized by a His combination. The f i r s t band of HOC£ (Fig. 4.2a) shows a vibrational progression of 830 50cm extending over.at least five members. Comparing this value to the neutral ground state frequencies for symmetrical (a ') - 54 -v i b r a t i o n a l modes [ v ^ = 3609cm ^ (0-H s t r e t c h ) ; =1239cm (bend); = 725cm ^ (0-C£ s t r e t c h ) ] t h e o b s e r v e d s t r u c t u r e i s a s s i g n e d t o an e x c i t a t i o n o f t h e 0-C£ s t r e t c h i n g f r e q u e n c y . The i n c r e a s e from t h e n e u t r a l ground s t a t e f r e q u e n c y i s i n a c c o r d w i t h t h e assignment o f . t h i s band to an a n t i b o n d i n g c o m b i n a t i o n of piT o r b i t a l s on t h e 0 and C£ atoms ( 3 a " ) . The PE spectrum o f h y p o f l u o r o u s a c i d (HOF) has p r e v i o u s l y been r e p o r t e d ^ " * ^ and t h e f i r s t band (13.0eV) i s . l i k e w i s e a s s i g n e d t o an i o n i z a t i o n from an o u t - o f - p l a n e o r b i t a l and shows s i m i l a r v i b r a t i o n a l s t r u c t u r e . The second band of H0C£ 10a' i s a l s o r e s o l v e d i n t o v i b r a t i o n a l components ( F i g . 4.2a), t h e Franck-Condon en v e l o p e b e i n g somewhat s i m i l a r t o t h e f i r s t band. The o b s e r v e d f r e q u e n c y of 700 ± 50cm p r o b a b l y c o r r e s p o n d s to an e x c i t a t i o n of b e i n g almost unchanged from t h e n e u t r a l m o l e c u l e . There i s e v i d e n c e f o r an a d d i t i o n a l p r o g r e s s i o n of 1250 ± 80cm which would c o r r e s p o n d t o t h e b e n d i n g f r e q u e n c y v^, a l s o e x c i t e d by i o n i z a t i o n from t h e i n - p l a n e o r b i t a l . T h e r e was no v i b r a t i o n a l s t r u c t u r e r e s o l v e d on t h e second band o f HOF.^"^ The low symmetry of H0C£ ( C g ) and t h e p r o x i m i t y i n energy of t h e 0 (53) and C£ l o n e p a i r s (eg. H^O 12.62eV; HC£ 12.78eV) v ' means t h a t a l l t h e v a l e n c e o r b i t a l s i n v o l v e e x t e n s i v e m i x i n g between t h e 0 and C£ atomic o r b i t a l s . I t i s t h e r e f o r e d i f f i c u l t t o s p e c i f i c a l l y a s s i g n e i t h e r o f t h e (33.). f i r s t two bands to an 0 o r C£ nonbonding ( l o n e p a i r ) l e v e l . C a l c u l a t i o n s do i n f a c t p l a c e most e l e c t r o n d e n s i t y on t h e C£ atom f o r b o t h o r b i t a l s i n v o l v e d i n t h e f i r s t two i o n i z a t i o n p r o c e s s e s . - 55 -The second band o f Cl* a t 1 4 . 4 3 e V ( 4 6 ) ( 1 4 . 3 3 e V ( 4 7 ) ) ( 2n u) and t h e 2 + t h i r d band a t 16eV ( E ) can be c o r r e l a t e d w i t h t h e h i g h e r IP's of g HOC£"! w i t h the degenerate s t a t e b e i n g s p l i t i n the non l i n e a r m o l e c u l e . The t h i r d band o f H0C£ a t 14.6 + O.leV ( F i g . 4.2b) i s a s s i g n e d t o the 2a" o r b i t a l , the bonding o u t - o f - p l a n e c o m b i n a t i o n o f 0 and Cl 2p o r b i t a l s , and has almost the same IP v a l u e as t h e second band o f Cl^. The f o u r t h band o f HOC£ a t 15.6 t O.leV ( F i g . 4b) i s a s s i g n e d to a p r e d o m i n a t e l y 2 + + O-Cl a bonding o r b i t a l (9a') c o r r e s p o n d i n g t h e t h e E s t a t e o f C£„ , which i s e x p e c t e d t o be d e s t a b i l i z e d upon f o r m i n g t h e bent m o l e c u l e , due t o a g r e a t e r m i x i n g of a l l the l e v e l s . The c o r r e l a t i o n o f IP's f o r H0C£ and C £ 2 i s shown i n F i g . 4.5. I n c l u d e d i n t h e diagram a r e the r e s u l t s f o r the i s o e l e c t r o n i c s p e c i e s C H 3 C £ and FC£. T h i s demonstrates t h e t r e n d of t h e a X-C£ bond energy which r e f l e c t s t h e p o l a r i z a t i o n o f t h e X-C£ bond s u p p o r t i n g the assignment of t h i s l e v e l i n HOC£. The i n - p l a n e TT bonding l e v e l (8a') t h e r e f o r e g i v e s r i s e t o the f i f t h band o f HOC£ which has a c a l c u l a t e d IP o f 17.73eV but was not o b s e r v e d 2 i n t h e e x p e r i m e n t a l spectrum, presumably b e i n g o v e r l a p p e d by the s t a t e of R^o"1". The 8a ' l e v e l w i l l have c o n s i d e r a b l e 0-H a bonding 2 c h a r a c t e r , c a u s i n g the l a r g e s t a b i l i z a t i o n r e l a t i v e t o the n s t a t e o f u G £ 2 + . The 0-C£ a o r b i t a l coming between the ir bonding l e v e l s i s i n a c c o r d w i t h the c a l c u l a t i o n s ' ^ . The assignment o f the H e l PE spectrum of H0C£ i s i n agreement w i t h (35) the c a l c u l a t i o n s i n v o l v i n g p e r t u r b a t i o n c o r r e c t i o n s t o Koopmans' theorem^ (33 35) I t i s however worth n o t i n g t h a t Koopmans' theorem c a l c u l a t i o n s ' f a i l - 56 -10T 11 > I2h < 13 U J 1 4 O Q_ o r -< M Z O 15-16-17 18j 19 a \ IL a ^ ^  - -y \ e \ 7 \ \ / \ \ \ C H 3 C I HOCI CI FCI F i g . 4.5. C o r r e l a t i o n diagram f o r the v a l e n c e i s o e l e c t r o n i c s e r i e s C H 3 C £ , H0C£, C& 2 and FCS, - 57 -to p r e d i c t t h i s o r d e r i n g ( p r e d i c t i n g a", a', a ' , a", a' i n s t e a d o f a", a', a", a', a')« T h i s i s y e t .another example o f low l y i n g v i r t u a l a o r b i t a l s (35) c a u s i n g a l a r g e c o r r e c t i o n t o Koopmans' theorem f o r TT o r b i t a l s A s i m i l a r e f f e c t was p r e d i c t e d f o r t h e l a ' l e v e l o f HOF and has (41) been observed i n C£F ( i s o e l e c t r o n i c w i t h H O C £ ) . 4.4.2 A l k y l H y p o c h l o r i t e s - • The PE s p e c t r a o f the a l k y l h y p o c h l o r i t e s a r e c o m p l i c a t e d by t h e i n t r o d u c t i o n o f CH^ and CH^ a and TT l e v e l s . However t h e f i r s t two IP's a r e s e p a r a t e d from t h e s e l e v e l s i n a l l cases and can be d i r e c t l y c o r r e l a t e d w i t h t h e f i r s t two IP's o f HOC£ ( i . e . o u t - o f - p l a n e and i n - p l a n e a n t i b o n d i n g combinations of 0 and C£ p o r b i t a l s ) . The expe c t e d d e s t a b i l i z a t i o n of t h e s e l e v e l s due t o t h e i n d u c t i v e e f f e c t o f t h e a l k y l groups i s ob s e r v e d . ( F u r t h e r d e s t a b i l i z a t i o n by h y p e r c o n j u g a t i o n may a l s o o c c u r -see l a t e r . ) The f i r s t band i n t h e PE spectrum o f C H 3 O C £ ( v e r t i c a l IP = 10.49eV) e x h i b i t s a v i b r a t i o n a l p r o g r e s s i o n o f 700 ± 50cm \ e x t e n d i n g o v e r a t l e a s t f o u r members. T h i s i s a s s i g n e d t o an e x c i t a t i o n o f t h e 0-C£ s t r e t c h i n g f r e q u e n c y which i s o n l y s l i g h t l y i n c r e a s e d from t h e n e u t r a l ground s t a t e f r e q u e n c y o f 688cm . T h i s i n d i c a t e s t h a t t h e 4 a " o r b i t a l i n t h e methyl d e r i v a t i v e i s l e s s 0-C£ a n t i b o n d i n g than t h e c o r r e s p o n d i n g 3a" o r b i t a l i n H0C£ f o r which a s u b s t a n t i a l i n c r e a s e i n t h e 0-C£ s t r e t c h i n g f r e q u e n c y was observed upon i o n i z a t i o n . The i n t r o d u c t i o n of t h e me t h y l group d e s t a b i l i z e s t h i s o r b i t a l by 0.73eV. - 58 -The second band i n CR^OCH (11.75eV) has a s i m i l a r band w i d t h and i n t e n s i t y t o t h e f i r s t band a l t h o u g h no v i b r a t i o n a l s t r u c t u r e c o u l d d e f i n i t e l y be r e s o l v e d . There i s some e v i d e n c e f o r a p r o g r e s s i o n o f ~ 600cm but t h i s c o u l d be due to i n t e r f e r e n c e from t h e f i r s t band o f ( a d i a b a t i c IP = 11.49eV, v ' = 645cm" "*") , a ' d e c o m p o s i t i o n p r o d u c t o f CH^OCtf, and d i f f i c u l t t o remove from t h e gas phase. The d e s t a b i l i z a t i o n of t h i s o r b i t a l (12a') r e l a t i v e t o t h e c o r r e s p o n d i n g 10a' o r b i t a l i n H0C£, i s 0.52eV. The o u t - o f - p l a n e o r b i t a l i s t h e r e f o r e d e s t a b i l i z e d t o a g r e a t e r e x t e n t t h a n t h e i n - p l a n e o r b i t a l by i n t r o d u c t i o n of a met h y l group. T h i s i s p o s s i b l y due to a resonance e f f e c t such as h y p e r c o n j u g a -( 5 7 ) t i o n (see b e l o w ) . The remainder o f t h e H e l PE spectrum o f CR^OCZ c o n s i s t s o f 3 d i s t i n c t bands, w i t h a s h o u l d e r on one a t ~ .13:5eV. -There a r e however expeeted:.to be 5 IP's i n t h i s r e g i o n ( t h e i n - p l a n e and o u t - o f - p l a n e bonding c o m b i n a t i o n s of t h e 0 and C£ p o r b i t a l s , a a 0-C£ bond and two CH^ i o n i z a t i o n events)". The assignment o f bands i n t h i s r e g i o n i s based on a c o r r e l a t i o n (52) w i t h t h e PE s p e c t r a o f H0C£ and met h y l a l c o h o l • I n t h e a l c o h o l t h e i n - p l a n e CH^ i o n i z a t i o n o c c u r s a t 12.6eV and t h e IP a t 13.08eV i n m e t h y l h y p o c h l o r i t e i s l i k e w i s e a s s i g n e d . S i m i l a r l y t h e IP at 16.05eV i n t h e met h y l h y p o c h l o r i t e i s c o r r e l a t e d w i t h t h e o u t - o f - p l a n e CH^ i o n i z a t i o n a t 15.6eV i n met h y l a l c o h o l . The f i n a l IP i n met h y l h y p o c h l o r i t e a t 17.10eV i s a s s i g n e d t o t h e 9 a ' i o n i z a t i o n , e q u i v a l e n t t o the 8 a ' i n H0C£ (not o b s e r v e d but c a l c u l a t e d t o be 17.73eV) and i s e s s e n t i a l l y t h e i n - p l a n e bonding c o m b i n a t i o n o f 0 and C£ p o r b i t a l s . - 59 -The c o r r e s p o n d i n g o u t - o f - p l a n e bonding c o m b i n a t i o n i s a s s i g n e d t o t h e peak a t 13.53eV (14.6eV i n H 0 C £ ) . The 10a' o r b i t a l r e p r e s e n t s a p r e -dominantly 0-C£ a bonding o r b i t a l and would be expected t o be c o n s i d e r a b l y d e s t a b i l i z e d r e l a t i v e t o t h e c o r r e s p o n d i n g 9 a ' o r b i t a l i n H0G£ (15.6eV). I o n i z a t i o n from t h i s o r b i t a l i s thus a l s o a s s i g n e d t o the 13-14eV band. The e x a c t o r d e r i n g o f t h e IP's i n t h i s band i s o f c o u r s e n ot c o m p l e t e l y c e r t a i n . The PE spectrum o f C2H^0C£ ( F i g . 4.3) i s f u r t h e r c o m p l i c a t e d by t h e a d d i t i o n a l l e v e l s o f t h e l a r g e r a l k y l group. However t h e f i r s t two IP's ar e s e p a r a t e d from the r e s t o f t h e spectrum and can be d i r e c t l y c o r r e l a t e d t o t h e f i r s t two bands i n CH^OCA. The Franck-Condon envelopes f o r t h e s e two i o n i z a t i o n s a r e v e r y s i m i l a r f o r t h e two h y p o c h l o r i t e s , w i t h t h e f i r s t band i n bo t h cases showing 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 obs e r v e d f r e q u e n c y i n C^H^OCi!, o f 700 ± 50cm ^ i s i n f a c t w i t h i n e x p e r i m e n t a l e r r o r o f t h e v a l u e f o r CH^OCA, and i s a g a i n a s s i g n e d t o an e x c i t a t i o n o f the 0-C£ s t r e t c h i n g f r e q u e n c y (688cm i n - t h e n e u t r a l ground s t a t e o f . , , (20), bo t h m o l e c u l e s ) . The i n d u c t i v e s h i f t observed i n s u b s t i t u t i n g a C^B.^ f o r a CH^ group i s as expected f o r t h e f i r s t two bands. The f i r s t IP i n C^H^OCJl ( v e r t i c a l = 10.23eV) i s 0.26eV lower than t h e c o r r e s p o n d i n g v a l u e i n CR^OCZ w h i l e t h e second IP shows a s i m i l a r s h i f t o f 0.22eV. The h i g h e r IP r e g i o n i n t h e PE spectrum of C2H,.0C£ c o n s i s t s o f o v e r -l a p p i n g bands w i t h no c h a r a c t e r i s t i c f e a t u r e s . Nine band maxima o r s h o u l d e r s can be d i s t i n g u i s h e d and a r e l i s t e d i n T a b l e 4.2, but because of :the c o m p l e x i t y o f t h i s r e g i o n no d e f i n i t i v e assignment i s made. - 60 -However t h e o u t - o f - p l a n e bonding c o m b i n a t i o n o f 0 and C£ p o r b i t a l s and t h e a bonding 0-C£ o r b i t a l a r e expe c t e d i n t h e 12-14.5eV r e g i o n by comparison w i t h t h e CH^OCA r e s u l t s . S i m i l a r l y t h e i n - p l a n e bonding c o m b i n a t i o n would be expe c t e d i n t h e 16-17eV r e g i o n and thus t h e band w i t h the maximum a t 15.28eV i s p r o b a b l y due to an i o n i z a t i o n from an o r b i t a l l o c a l i z e d m a i n l y on t h e e t h y l group. I n t h e PE spectrum o f t-BuOC£ ( F i g . 4.3) t h e f i r s t two IP's a r e a g a i n s e p a r a t e d from t h e r e s t o f t h e spectrum. I n t t h i s c a s e no v i b r a t i o n a l s t r u c t u r e was r e s o l v e d on e i t h e r band b u t t h e Franck-Condon envelopes a r e s i m i l a r t o t h o s e o b s e r v e d i n t h e o t h e r h y p o c h l o r i t e s . The IP v a l u e s of:..the f i r s t and second bands of t-BuOG£ a r e 9.90 and 11.07eV r e s p e c t i v e l y , to g i v e a s p l i t t i n g between t h e two l e v e l s o f 1.17eV. T h i s compares t o v a l u e s o f 1.05, 1.26, 1.30eV f o r H0C£, C H 3 0 C £ and C 2 H 5 O C £ . The i n c r e a s e d s p l i t t i n g i n t h e a l k y l h y p o c h l o r i t e s i s due to a g r e a t e r d e s t a b i l i z a t i o n o f t h e o u t - o f - p l a n e l e v e l r e l a t i v e t o t h e i n - p l a n e l e v e l on s u b s t i t u t i n g an a l k y l group f o r a hydrogen. T h i s s p l i t t i n g i s t h e r e f o r e a measure o f t h e r e l a t i v e d e s t a b i l i z a t i o n o f t h e two l e v e l s and has an o r d e r i n g C^H^ ~ CH^ > t-Bu > H which i s not t h e expected one f o r p u r e l y i n d u c t i v e e f f e c t s . I t would t h e r e f o r e appear t h e r e a r e a l s o r e sonance e f f e c t s p r e s e n t . Except f o r t h e s i m i l a r v a l u e s f o r t h e . e t h y l and m e t h y l group t h e o r d e r i n g i s t h a t g i v e n by Baker and Nathan f o r h y p e r c o n j u g a t i o n ( 58) e f f e c t s ( i . e . CH^ > C 2H^ > t-Bu > H) as o b t a i n e d from k i n e t i c s t u d i e s . The e f f e c t o f h y p e r c o n j u g a t i o n has p r e v i o u s l y been used i n i n t e r p r e t i n g ( 5 9 ) . PE s p e c t r a v ' . The h i g h e r IP r e g i o n i n t h e PE spectrum of ^ t-BuOC£ i s v e r y complex due t o t h e number o f i o n i z a t i o n e v e n t s . Thus o n l y peak maxima a r e g i v e n - 61 -i n T a b l e 4.2 and t h e s e may not c o r r e s p o n d to i n d i v i d u a l I P ' s . F i g . 4.6 shows the c o r r e l a t i o n between t h e IP's f o r t h e a l k y l h y p o c h l o r i t e s and t h e p a r e n t a c i d . T h i s demonstrates the d e s t a b i l i z a t i o n o f t h e o r b i t a l s due t o t h e i n d u c t i v e (and resonance) e f f e c t s o f t h e a l k y l groups. The t r e n d e s t a b l i s h e d i s f i n a l e v i d e n c e t h a t t h e i d e n t i f i c a t i o n o f t h e u n s t a b l e s p e c i e s HOC£ i s i n d e e d v a l i d . 4.5 C o n c l u s i o n Hypochlorous a c i d has been o b t a i n e d i n t h e gas phase by sampling t h e vapours above a v e r y d i l u t e aqueous s o l u t i o n o f c h l o r i n e d i o x i d e C £ 2 0 + H 20 5=s 2H0C£ The PE spectrum of HOC£..could o n l y be o b t a i n e d i n t h e p r e s e n c e o f a l a r g e excess of water, which c o u l d not be removed from t h e vapour phase w i t h o u t t h e concomitant c o n v e r s i o n of HOC£ t o CH^O. The f i r s t two PE bands of HOC£ were s e p a r a t e d from t h e bands due to water and c o u l d t h e r e f o r e be s t u d i e d i n d e t a i l . The t h i r d and f o u r t h IP's were seen as s h o u l d e r s 2 + on t h e band of H^O w i t h t h e f i f t h IP b e i n g t o t a l l y o b s c u r e d _„ by t h e 2 + band o f R^O . The spectrum a t t r i b u t e d t o H0C£ was a s s i g n e d on t h e b a s i s o f a comparison w i t h t h e v a l e n c e i s o e l e c t r o n i c s p e c i e s , C&2 a n d HOF. R e s u l t s f r o m . c a l c u l a t i o n s on H0C£ i n v o l v i n g p e r t u r b a t i o n (35) c o r r e c t i o n s t o Koopmans' theorem a r e a l s o . i n good agreement w i t h t h e e x p e r i m e n t a l r e s u l t s g i v e n i n t h i s work. - 62 -F i g . 4.6 C o r r e l a t i o n diagram f o r HOC£ and i t s a l k y l d e r i v a t i v e s - 63 -The PE s p e c t r a o f t h r e e a l k y l h y p o c h l o r i t e s ( m e t h y l , e t h y l and t -b u t y l ) have a l s o been o b t a i n e d , w i t h t h e pure compounds b e i n g o b t a i n e d i n t h e gas phase. The f i r s t two IP's ( o u t - o f - p l a n e and i n - p l a n e combinations o f t h e 0 and C£ l o n e p a i r s ) o f t h e s e a l k y l d e r i v a t i v e s c o r r e l a t e w e l l w i t h t h e c o r r e s p o n d i n g IP's a s s i g n e d t o t h e p a r e n t a c i d and g i v e s f u r t h e r c redance t o t h e i d e n t i f i c a t i o n of HOC£. The spectrum o f CE^OCZ can be i n t e r p r e t e d by comparison w i t h t h e s p e c t r a o f HOC£ and CH^OH. However t h e s p e c t r a of t h e e t h y l and t - b u t y l d e r i v a t i v e s a r e c o m p l i c a t e d by o v e r l a p p i n g bands due to C-H typ e i o n i z a t i o n s i n t h e r e g i o n above 12eV and a f u l l assignment i s not g i v e n f o r t h e s e compounds. - 64 -CHAPTER FIVE  THE HALAMINES AND THE METHYLHALAMINES T h i s c h a p t e r c o n s i d e r s the p r e p a r a t i o n and PE s p e c t r a o f gaseous c h l o r a m i n e s , m e t h y l c h l o r a m i n e s , bromamines, methylbromamines and d i f l u o r a m i n e . 5.1. I n t r o d u c t i o n The simple u n s u b s t i t u t e d c h l o r a m i n e s , NH„ Ca , a r e known to be J-n n (1 2) r a t h e r u n s t a b l e m o l e c u l e s ' and c o n s e q u e n t l y have n o t been e x t e n s i v e l y i n v e s t i g a t e d i n the f r e e s t a t e . Monochloramine ( N ^ C i i ) and p a r t i c u l a r l y d i c h l o r a m i n e (NHCJ^) r e a d i l y decompose to g i v e N^ and NH^C£ as major p r o d u c t s . In f a c t N H C ^ has never been s t r i c t l y c h a r a c t e r i z e d i n the gas phase. There a r e however numerous s o l u t i o n r e a c t i o n s i n v o l v i n g (1 2) th e s e m o l e c u l e s ' , one of the most im p o r t a n t i n v o l v i n g NH^CA as (3) an i n t e r m e d i a t e i n the R a s c h i g p r o c e s s f o r the s y n t h e s i s o f h y d r a z i n e . In aqueous s o l u t i o n the r e a c t i o n o f NH^ and Ci^ to g i v e the c h l o r a m i n e s (4 5) has been shown to be pH dependent ' w i t h f o r m a t i o n o f o n l y N H 2 C £ above pH 8.5, some NHC^ between pH 8.5 and 4.4 and some n i t r o g e n t r i c h l o r i d e ( N C ^ ) below pH 4.4. These r e s u l t s agree w i t h e a r l i e r work w i t h minor d i f f e r e n c e s i n pH l i m i t s . . The e q u i l i b r i u m o f t h e s e t h r e e s p e c i e s i n aqueous s o l u t i o n i n v o l v e s h y p o c h l o r o u s a c i d as an i n t e r m e d i a t e ^ ' ^ , (a s p e c i e s t h a t has been c o n s i d e r e d i n Chapter 4). T h i s pH dependence i n the c h l o r i n a t i o n of ammonia has p e r m i t t e d i n v e s t i g a t i o n of the a b s o r p t i o n s p e c t r a o f the c h l o r a m i n e s i n s o l u t i o n ^ ' ^ ' ^ and some v i b r a t i o n a l s p e c t r a i n the gas phase . There has a l s o been an e v a l u a t i o n o f the thermodynamic p r o p e r t i e s o f t h e s e t h r e e m o l e c u l e s . - 65 -The microwave spectrum of NH^ CiL has been obtained^"*"2 "*""^ by mixing Cl^O and NH^  in the gas phase. It is known that this species i s also formed by a direct gas phase mixing of C&2 and N H ^ ' ^ \ The gas n o 1 9 ) phase structure of the powerful explosive NC&3 has been studied ' ( O f\ 0 / \ and despite serious experimental d i f f i c u l t i e s , the vibrational (Of o c \ and gas phase ultraviolet spectra ' of the f u l l y chlorinated species have been obtained. A proposed PE spectrum of NC&3 has been previously presented ^ ^a) ^ t transpires that an impure sample^^^was prepared resulting in discrepancies in the spectrum. The methyl substituted chloramines are somewhat more stable than the unsubstituted species. The absorption spectra of the three methyl derivatives (CH3NHC£, CH3N.C&2, ( C H ^ N C J O have been studied in sol-(8 27 28) ution ' ' with the reaction between CH^ NrL^  and C&2 also being found to be pH dependent. The vibrational spectra of the three molecules (7Pi—31 ^  in solution have also been reported In the gas phase the molecular structures have been investigated u u i . A-ce ' (32,33) , . . . (34,35) by both electron dxffraction and microwave spectroscopy The PE spectrum of the most stable of the methylchloramines, (CH 3) 2NC£, has previously been reported in a study of the corresponding P and As A • (36) derivatives (1 2 27) The bromamines are less stable than the analogous chloramines ' ' and have therefore not been as extensively studied. Monobromamine (NH2Br) has been prepared in ether solution by reaction of (35) (39) ammonia and bromine at 0°C or -60°C with dibromamine (NHBr2) (40-41) being formed under slightly modified conditions . The - 66 -a b s o r p t i o n s p e c t r a of such s o l u t i o n s i n d i c a t e t h a t an e q u i l i b r i u m (42) e x i s t s between NH^, NR^Br and NHBr^ • The p r e p a r a t i o n o f b o t h (43. 44) bromamines-in a q u e o u s ~ s o l u t i o n - h a s : b e e n • c l a i m e d ' w i t h i d e n t i f i c a t i o n (45) b e i n g based on the a b s o r p t i o n s p e c t r a o b t a i n e d . Other work i s i n agreement w i t h these r e s u l t s and a l s o p o s t u l a t e s the f o r m a t i o n of n i t r o g e n t r i b r o m i d e (NBr^). Only NH^Br has p r e v i o u s l y been p r e p a r e d i n the gas p h a s e ^ 4 ^ , a l b e i t i n an impure s t a t e , by r e a c t i n g a l a r g e e x cess of NH^ w i t h Br^ d i l u t e d w i t h . As w i t h the c h l o r a m i n e s , '-\ the m e t h y l d e r i v a t i v e s a r e more s t a b l e than the u n s u b s t i t u t e d s p e c i e s . A l t h o u g h n e v e r i s o l a t e d , the m e t h y l (28) bromamines have been p r e p a r e d i n aqueous s o l u t i o n and t h e i r (28) i n f r a r e d and u l t r a v i o l e t a b s o r p t i o n s p e c t r a so o b t a i n e d . I n e t h e r s o l u t i o n the a b s o r p t i o n s p e c t r a i n d i c a t e t h a t CH^NHBr, CH^NB^ and CH^NrL^ (42) e x i s t i n e q u i l i b r i u m . The most s t a b l e bromamine, (CH^^NBr, has been o b t a i n e d i n the gas phase but o n l y i n the p r e s e n c e of an e x c e s s o f one of the r e a c t a n t s . Of the s i m p l e f l u o r a m i n e s , o n l y d i f l u o r a m i n e (NRT^) and n i t r o g e n (47) t r i f l u o r i d e (NF^) have been c h a r a c t e r i z e d . The f u l l y f l u o r i n a t e d d e r i v a t i v e has been p r e v i o u s l y s t u d i e d by P E S ^ 4 ^ and so has not been i n v e s t i g a t e d i n t h i s work. There has a l s o been c o n s i d e r a b l e i n t e r e s t (49) i n NHF2 s i n c e i t s f i r s t p r e p a r a t i o n i n 1959 when i t was i n i t i a l l y (49) thought u s e f u l as a r o c k e t p r o p e l l a n t . The p h y s i c a l p r o p e r t i e s , i n c l u d i n g the e x p l o s i v e n a t u r e of the compound were e s t a b l i s h e d i n t h e e a r l y w o r k ^ " ^ . In s o l u t i o n i t i s p o s s i b l e t o o b t a i n h i g h y i e l d s of - 67 -NHF2 by s i m p l y f l u o r i n a t i n g u r e a and s u b s e q u e n t l y h y d r o l y s i n g . S e v e r a l o t h e r p r e p a r a t i v e methods have been p r o p o s e d . In the gas phase r e a c t i o n o f and NH^, d i f l u o r a m i n e i s one of s e v e r a l p r o d u c t s formed (57) but t h e r e i s no e v i d e n c e f o r f o r m a t i o n of monofluoramine(NH^F) , as would be expected by a n a l o g y w i t h the NR^/Cl^ system. t h e r e f o r e (58) remains unknown a l t h o u g h c l a i m s f o r i t s e x i s t a n c e have been made The microwave and v i b r a t i o n a l s p e c t r a o f NHF^ have been o b t a i n e d i n the gas phase. T h i s c h a p t e r c o n s t i t u t e s the f i r s t comprehensive i n v e s t i g a t i o n of the e l e c t r o n i c s t r u c t u r e o f the N-halamines by PES. T h i s a l l o w s the e f f e c t s o f h a l o g e n and m e t h y l s u b s t i t u t i o n on the v a l e n c e o r b i t a l s (64 65) and on the b a r r i e r t o i n v e r s i o n f o r t h e ground i o n i c s t a t e ' to be s t u d i e d . One of the more im p o r t a n t f e a t u r e s o f t h i s work i s t h a t i t demonstrates how r e l a t i v e l y pure samples of these s p e c i e s can be o b t a i n e d f o r gas phase i n v e s t i g a t i o n . N e l as w e l l as H e l PE s p e c t r a a r e p r e s e n t e d f o r the s i m p l e c h l o r a m i n e s to p r o v i d e i n f o r m a t i o n on t h e v a r i a t i o n o f c r o s s - s e c t i o n w i t h i o n i z i n g r a d i a t i o n , thus g i v i n g a d d i t i o n a l e v i d e n c e to support the proposed a s s i g n m e n t s . At the c o m p l e t i o n and p u b l i c a t i o n o f most of t h i s work, H e l and H e l l PE s p e c t r a o f m^l, N H C ^ 6 1 ) , N H 2 B r ( 6 2 ) , N H B r ^ 6 2 ^ , CH^NHBr and CH^NBr,^ were p u b l i s h e d by workers i n A u s t r a l i a . I n t h i s c ase a l l the compounds were p r e p a r e d by t h e gas phase m i x i n g of NH^ o r CH 3NH 2 w i t h the a p p r o p r i a t e h a l o g e n . The s p e c t r a o b t a i n e d thus i n v o l v e d - 68 -a m i x t u r e of compounds and a d i g i t a l s u b t r a c t i o n p r o c e s s was used to remove the f e a t u r e s from s t a r t i n g m a t e r i a l s and d e c o m p o s i t i o n p r o d u c t s . (The s o - c a l l e d " s p e c t r u m - s t r i p p i n g " p r o c e d u r e ) . I n s e v e r a l c a s e s t h i s r e s u l t e d i n a l o s s o f d e t a i l i n the spectrum o f the "pur e " halamine-so o b t a i n e d . I n the work d e s c r i b e d h e r e , wherever p o s s i b l e methods were d e v i s e d t o obta i n . i t h e pure compounds i n t h e gas phase a l l o w i n g " c l e a n " s p e c t r a t o be o b t a i n e d d i r e c t l y . I n a l l o t h e r r e s p e c t s the s p e c t r a a r e i n g e n e r a l agreement w i t h the r e s u l t s p r e s e n t e d i n t h i s t h e s i s . 5.2. E x p e r i m e n t a l 1. Chloramines The method used t o p r e p a r e the c h l o r a m i n e s (except f o r NC£^) was e s s e n t i a l l y the same as t h a t of C h a p i n ^ 4 ' " ^ and M e t c a l f e t a l . ^ ' ^ , s l i g h t l y m o d i f i e d f o r the purpose o f t h e s e e x p e r i m e n t s . The a p p r o p r i a t e amine was mixed w i t h an aqueous s o l u t i o n o f sodium h y p o c h l o r i t e (NaOC£) under d i f f e r e n t pH c o n d i t i o n s and the r e s u l t a n t c h l o r a m i n e was l e d i n t o the PE s p e c t r o m e t e r where the p u r i t y c o u l d be m o n i t o r e d . i ) P r e p a r a t i o n o f N-aOC& s o l u t i o n C£^ was bubbled i n t o a 20% NaOH s o l u t i o n a t 0°C u n t i l t h e r e was an excess of the h a l o g e n . To mi n i m i z e d e c o m p o s i t i o n t h i s s o l u t i o n was s t o r e d i n the dark a t -10°C u n t i l needed. - 69 -i i ) m2cz_ Aqueous NH^ (30%) was added dropwise t o the s o l u t i o n o f NaOC£ a t 0°C u n t i l i t was a l k a l i n e (pH 9-11), w i t h no e v i d e n c e f o r excess CZ^. NaOC£(aq) + NH (aq) > NH C£(aq) + Na0H(aq) i i i ) NHCS^ A s o l u t i o n o f NH^CJi, as p r e p a r e d above, was made n e u t r a l o r s l i g h t l y a c i d i c (pH 5-7) by a d d i t i o n o f d i l u t e HNO.^ . + H 2 ° + 2NH„C£ + H — ±—->- N H C £ 0 + NH, 1 0C£ 1 * i v ) CH NHC& A s l i g h t e x c e ss o f CH^N^ s o l u t i o n (40%) was q u i c k l y added to the h y p o c h l o r i t e s o l u t i o n . I f i n s u f f i c i e n t amine was added o r i f i t was added s l o w l y , the d i c h l o r o d e r i v a t i v e , CH^NCJ^* w a s formed as w e l l . However a l a r g e e x c ess of CH^N^ r e s u l t e d i n the amine b e i n g dominant i n the observed PE spectrum. The exa c t r a t i o of amine h y p o c h l o r i t e used t o o b t a i n optimum y i e l d s of CH^NHCA was t h e r e f o r e c r i t i c a l and had t o be found by t r i a l as the e x a c t c o n c e n t r a t i o n o f t h e NaOC£ s o l u t i o n was not known. - 70 -v) C H 3 N C £ 2 T h i s was p r e p a r e d by the dropwise a d d i t i o n of CH^NR^ to the h y p o c h l o r i t e s o l u t i o n . I t became apparent that.CH^NC.^ i s o n l y s p a r i n g l y s o l u b l e i n water and under these r e a c t i o n c o n d i t i o n s , t h e m e t h y l e h l o r a m i n e was formed as an i m m i s c i b l e l i q u i d . I t was thus p o s s i b l e t o o b t a i n samples of CH^NCJ^ f r e e from CH^NHCA and w i t h o u t the n e c e s s i t y o f a d j u s t i n g the pH. v i ) ( C H 3 ) 2 N C £ T h i s was p r e p a r e d by the a d d i t i o n o f ( CH 3) 2NH s o l u t i o n (40%) to the h y p o c h l o r i t e s o l u t i o n u n t i l i t was s l i g h t l y a l k a l i n e . The sampling t e c h n i q u e to o b t a i n the gas phase PE s p e c t r a of the above compounds was common to a l l the s o l u t i o n s d e s c r i b e d above. The sample f l a s k was a t t a c h e d on l i n e t o the s p e c t r o m e t e r , e v a c u a t e d , and the v o l a t i l e components s l o w l y pumped i n t o the i o n i z a t i o n chamber v i a a U - t r a p packed w i t h CaC& 2 and m a i n t a i n e d a t -37°C. Under s u i t a b l e pumping c o n d i t i o n s t h i s removed the H 20 from the gas phase a l l o w i n g a c l e a n PE spectrum of the r e s p e c t i v e c h l o r a m i n e to be o b t a i n e d . v i i ) N C £ 3 An a d a p t a t i o n of a known p r o c e d u r e was used to p r e p a r e N C £ 3 . A few mm p r e s s u r e of C& 2 was a l l o w e d t o s t a n d over m o i s t ammonium s u l p h a t e c o n t a i n e d i n a U - t r a p a t t a c h e d to the s p e c t r o m e t e r . A f t e r about 1 hour the r e a c t i o n was complete, w i t h t h e NC& 3 e v i d e n t as a y e l l o w o i l on the s i d e s o f the a p p a r a t u s . - 71 -3C£„ + 4(NH.) oS0. -> •- N C £ 0 + 4NH.HS0. + 3NH.C£ 2 4 2 4 3 4 4 4 The sample was then a d m i t t e d t o the s p e c t r o m e t e r . D u r i n g t h i s p r o c e d u r e e x p l o s i o n s sometimes o c c u r e d w i t h the sudden change i n p r e s s u r e . Only s m a l l q u a n t i t i e s o f NCtf^ were thus p r e p a r e d and a l l g l a s s w a r e was e n c l o s e d w i t h the n e c e s s a r y s h i e l d i n g . I t s h o u l d a l s o be mentioned t h a t NH^C^ was p r e p a r e d by the d i r e c t (16) gas phase m i x i n g o f NH^ and Cl^ but was never o b t a i n e d pure by t h i s means as e v i d e n c e d by the PE spectrum. By i n c r e a s i n g the r a t i o of CSi^'.Ml^, the f o r m a t i o n of NHCJl^ was a l s o a c h i e v e d but a g a i n i n the p r e s e n c e o f s t a r t i n g m a t e r i a l s and Nr^Cft. S m a l l q u a n t i t i e s o f NC& 3 were a l s o produced when a l a r g e e x cess o f CSi^ was used i n the r e a c t i o n . As the PE s p e c t r a o b t a i n e d d u r i n g t h e s e r e a c t i o n s always r e p r e s e n t e d s e v e r a l s p e c i e s they a r e not p r e s e n t e d h e r e , w i t h p r e f e r e n c e b e i n g g i v e n t o the c l e a n s p e c t r a o b t a i n e d as d e s c r i b e d above. T h i s however was the t e c h n i q u e used by the A u s t r a l i a n group t o produce s p e c t r a o f m i x t u r e s , s u b s e q u e n t l y o b t a i n i n g ' c l e a n ' s p e c t r a by s u b t r a c t i o n p r o c e d u r e s . 2. Bromamines The methylbromamines were p r e p a r e d i n an analogous manner to the m e t h y l c h l o r a m i n e s , u s i n g sodium hypobromite s o l u t i o n i n s t e a d o f sodium h y p o c h l o r i t e . An i d e n t i c a l sampling p r o c e d u r e was found t o be a p p r o p r i a t e and a g a i n c l e a n s p e c t r a o f the r e s p e c t i v e bromamines were o b t a i n e d . U n f o r t u n a t e l y a l l attempts to p r e p a r e the si m p l e u n s u b s t i t u t e d - 72 -bromamines by t h i s s o l u t i o n r e a c t i o n were u n s u c c e s s f u l . A d d i t i o n o f ammonia s o l u t i o n t o the sodium hypobromite s o l u t i o n r e s u l t e d i n a v i g o r o u s r e a c t i o n w i t h the e v o l u t i o n o f N 2 , which was the o n l y s p e c i e s observed i n the PE spectrum when the p r e v i o u s l y d e s c r i b e d sampling p r o c e d u r e was employed. A d i r e c t gas phase s y n t h e s i s of monobromamine (by r e a c t i n g NH^ and Br^) was t h e r e f o r e d e v i s e d , based on the g e n e r a t o r used i n the (16) monochloramine p r e p a r a t i o n . There i s a l s o e v i d e n c e t h a t dibromamine was formed i n t h i s r e a c t i o n and c o n s e q u e n t l y t h e NH^rBr^ r a t i o , and f l o w r a t e were e x t e n s i v e l y v a r i e d to t r y and improve the y i e l d o f t h i s dibromo s p e c i e s . As w i t h t h e case i n v o l v i n g the monochloramine gas phase s y n t h e s i s , e x cess r e a c t a n t s were always p r e s e n t and the observed PE spectrum of monobromamine i s somewhat c o m p l i c a t e d by the c o r r e s p o n d i n g s p e c t r a o f the s t a r t i n g r e a c t a n t s , NH^ and B r 2 -3. D i f l u o r a m i n e passed through an 8% aqueous u r e a s o l u t i o n a t 0°C u n t i l the h a l o g e n was no .longer b e i n g absorbed (as e v i d e n c e d by the r e l e a s e o f from a KI s o l u t i o n used to t e s t the gas m i x t u r e a f t e r p a s s i n g t h r o u g h the urea, s o l u t i o n ) . The method used to p r e p a r e d i f l u o r a m i n e i s e s s e n t i a l l y t h a t o f P a r k e r and F r e e m a n ^ 5 2 ^ . . F l u o r i n e gas, d i l u t e d w i t h n i t r o g e n , was C=0 + 2 F 2 C=0 + 2HF - 73 -At t h i s p o i n t i t was assumed t h a t s u f f i c i e n t " i , - l d i f l u o r o u r e a ' had been formed f o r the purpose , o f t h i s experiment. The s o l u t i o n c o u l d be s t o r e d a t 5°C w i t h o u t s e r i o u s d e c o m p o s i t i o n but a t ambient temperatures d i f l u o r a m i n e and CC^ were formed i n s u f f i c i e n t q u a n t i t i e s to be d e t e c t e d by PES under s u i t a b l e sampling p r o c e d u r e s . The sample s o l u t i o n was a t t a c h e d d i r e c t l y t o t h e PE s p e c t r o m e t e r and the v o l a t i l e components l e d i n t o t h e i o n i z a t i o n chamber v i a a -78°C and a -126°C t r a p . The former t r a p removed t h e H^O from the gas phase and under c o n t r o l l e d pumping c o n d i t i o n s most o f the d i f l u o r a m i n e was c o l l e c t e d i n the -126°C t r a p i n p r e f e r e n c e t o the CC^) which was pumped away through the s p e c t r o m e t e r . I s o l a t i n g the -126°C t r a p from the sample s o l u t i o n , and a l l o w i n g i t s c o n t e n t s t o e n t e r the s p e c t r o m e t e r as the t r a p warmed enabled a c l e a n spectrum of Nltt^ t o be o b t a i n e d . The o n l y o t h e r s p e c i e s o c c a s i o n a l l y d e t e c t e d i n low y i e l d i n the PE spectrum was n i t r o u s o x i d e , which presumably i s a p r o d u c t o f the h y d r o l y s i s o f NHE 9 • 2 C=0 + H 20 > 2NHF- + + CO. + (NH„)„CO 2NIIF- 2 + H 20 > N„0 + 4HF However no HF was observed i n any of the s p e c t r a and i t i s assumed i t r e a c t e d w i t h the g l a s s w a r e to g i v e S i F ^ which was s u b s e q u e n t l y t r a p p e d o u t . - 74 -D e u t e r a t e d d i f l u o r a m i n e was p r e p a r e d i n t h e same manner, but u s i n g D^O as the s o l v e n t i n s t e a d o f H^O. In t h i s c a s e i t was assumed a l a r g e excess o f D^O would ensure p r o d u c t i o n of c o m p l e t e l y d e u t e r a t e d d i f l u o r o u d e s p i t e t h e f a c t t h a t non d e u t e r a t e d u r e a was used. In a l l cases the PE s p e c t r a o b t a i n e d f o r t h e halamines were c a l i b r a t e d w i t h t h e known IP's o f CH^I, H^O, and A r . R e s o l u t i o n of t h e s p e c t r o m e t e r d e t e r i o r a t e d d u r i n g t h e c o u r s e of r e c o r d i n g a spectrum and c o u l d not be m a i n t a i n e d at b e t t e r than 40meV.-5.3. R e s u l t s The H e l and N e l PE s p e c t r a o f NH C£,'NHC£ 2 and N C £ 3 a r e shown i n F i g s . 5.1 and 5.2 r e s p e c t i v e l y . The H e l s p e c t r a o f N H C £ 2 and NC£^ show the p r e s e n c e of N 2 a r i s i n g from d e c o m p o s i t i o n o f t h e s e m o l e c u l e s . A s u p e r i o r c o u n t i n g r a t e a l l o w e d t h e c o r r e s p o n d i n g N e l s p e c t r a t o be o b t a i n e d b e f o r e d e c o m p o s i t i o n became ap p a r e n t , and thus t h e s e s p e c t r a are r e l a t i v e l y c l e a n . The H e l PE s p e c t r a o f t h e m e t h y l c h i o r a m i n e s ar e shown i n F i g . 5.4 and r e p r e s e n t s p e c t r a o f t h e v i r t u a l l y pure compounds. The f i r s t t h r e e bands o f N H 2 C £ show v i b r a t i o n a l f i n e s t r u c t u r e and t h i s i s shown i n d e t a i l f o r t h e N e l spectrum i n F i g . 5.3. • Of a l l t h e o t h e r c h l o r a m i n e s o n l y C H 3 N H C £ and CH^NC^ show v i b r a t i o n a l s t r u c t u r e (on t h e sharp second band) and t h i s i s shown i n d e t a i l i n F i g . 5.5. There a r e i n d i c a t i o n s t h a t i f s u f f i c i e n t r e s o l u t i o n c o u l d be m a i n t a i n e d ( < 25meV)the f i r s t b r o a d bands i n N H C £ 2 and NC£^ would a l s o show a s s o c i a t e d v i b r a t i o n a l s t r u c t u r e . - 75 -a i i i NH2CI i i i i i i i b A I < i i NHCI2 I I I I I I I c I J l 1 1 i NCI3 l l l l l l ! 10 12 14 16 18 IONIZATION POTENTIAL (eV) F i g . 5.1. H e l p h o t o e l e c t r o n s p e c t r a of a ) N H 2 C £ b) NHC£ c) NCJ?3 - 76 -a i i i i NH2CI i - i b A) I I 1 1 1 f i i i i JHCI2 1 1 1 c fi • i i I 1 NCI3 1 1 1 1 1 1 1 1 — 1 t I I    L_ 10 12 14 16 , 18 IONIZATION POTENTIAL (eV) F i g . 5.2 N e l p h o t o e l e c t r o n s p e c t r a o f a) NH C£ b) NHC£ c) N C £ 3 1 st Band a T—I—I—I—I 1—I—T"f 2nd Band Ne I 744A 3rd Band 111111 r *—i l I | i i I l 1 i I i i | ' 1 1 1 1 1 r—i—i—i—i—i | i i r 10.0 11.0 120 12.3 13.0 14.0 IONIZATION POTENTIAL (eV) F i g . 5.3 Expansions o f a) f i r s t b) second c) t h i r d bands of the Ne l p h o t o e l e c t r o n spectrum of N H 2 C £ - 78 -The IP^s f o r t h e c h l o r a m i n e s a r e g i v e n i n T a b l e s 5.1 and 5.2 t o g e t h e r w i t h t h e proposed assignments and any a s s o c i a t e d v i b r a t i o n a l s t r u c t u r e . The H e l PE s p e c t r a o f t h e methylbromamines* a r e shown i n F i g . 5.6 and a r e a g a i n r e l a t i v e l y c l e a n , t h e o n l y contaminant b e i n g a t r a c e o f water e v i d e n c e d by t h e sharp peak a t 12.62eV. The second bands of CH^NHBr and (CH^^NBr show v i b r a t i o n a l s t r u c t u r e as does t h e t h i r d band of CH^NB^ and t h e s e bands a r e shown i n g r e a t e r d e t a i l i n F i g . 5.7. The IP's t o g e t h e r w i t h t h e assignments and t h e measured v i b r a t i o n a l s t r u c t u r e a r e c o l l e c t e d i n T a b l e 5.3. The s p e c t r a o f NH^Br, b o t h i n t h e p r e s e n c e of excess B r ^ ^ ^ ^ , and excess N H ^ * ^ a r e shown i n F i g . 5.8a and 5.8b. An e x p a n s i o n o f t h e low IP r e g i o n under t h e same c o n d i t i o n s i s shown i n F i g . 5.9 and t h e r e s o l v e d v i b r a t i o n a l s t r u c t u r e on t h e f i r s t band of N ^ B r i s shown i n F i g . 5.10. The measured IP's f o r monobromamine (and dibromamine) and th e a s s o c i a t e d v i b r a t i o n a l s t r u c t u r e ar e g i v e n i n T a b l e 5.4. I t s h o u l d be n o t e d t h a t the H e l PE s p e c t r a o f a l l t h e c h l o r a m i n e s and bromamines have a shadow l i n e (Helg) a r i s i n g from t h e i n t e n s e second IP which sometimes i n t e r f e r e s w i t h t h e lower i n t e n s i t y f i r s t IP, p a r t i c u l a r l y a t t h e onset (eg. NH^CJl F i g . 5.1a). T h i s causes some d i f f i c u l t y i n o b t a i n i n g an accurate':, v a l u e f o r t h e a d i a b a t i c IP o f t h e f i r s t band. F o r t h e N e l s p e c t r a t h i s o f c o u r s e i s not a problem but even so, t h e weak Franck-Condon f a c t o r s f o r t h e a d i a b a t i c t r a n s i t i o n may mean t h a t t h e quoted a d i a b a t i c IP's .are i n e r r o r by one v i b r a t i o n a l quantum. - 79 -Table 5.1. Experimental V e r t i c a l IP's and assignments for NH CI, NHC12 and NC1 3 NH CI (9.85) 10.52 11.92 13.50 15.72 17.50 V ,cm 760 580 970 3040 450 -1 5a' 2a" 4a' 3a' l a " NHC1 (9.98) 10.56 11.88 12.39 12.50 14.57 16.11 17.39 •2-6a' 4a" 5a' 3a" 4a' 2a" 3a' NCI (10.12) 10.69 11.66 12.08 13.02 15.41 16.70 •3-4a x l a2 4e 3e 3 a l 2e a) Adiabatic IP's in brackets. Valence l e v e l numbering. b) NH2CI. See also Ref. 20. A l l values ± O.OleV, except bands 4,5 and 6 (± 0.05eV). A l l v i b r a t i o n a l structure ± 40cm~l. c) NHC12. A l l values ± 0.02eV, except 9.98 and 12.50 (± 0.05eV). d) NCI . A l l values ± 0.02eV, except 10.12 (± O.leV) and 15.41 and 16.70 (+ 0.05eV) - 80 -Table 5.2. Experimental V e r t i c a l I P ' s 3 and assignments for CH3NHC1, ( C H ^ N C l and CH 3NC1 2. CH 3NHCl b -1 v ,cm (CH 3) 2NC1 C -1 v ,cm CH 0NCl„ d (9.19) (8.67) (9 35) 9.80 10a 9.31 8a' 10.01 8a' 11.52 750 9a 11.22 960 5a" 11.45 5a" 12.42 8a 11.93 7a' 11 96 7a' 13.68 7a 13.15 4a" 12 14 4a" 15.05 6a 13.69 6a' 13 20 6a' 15.75 5a 14.31 3a" 14 11 3a" 16.99 4a 15.85 16.34 } 5a' 2a" 4a' 15 16 54 53 5a' 2a" 4a' a) Adiabatic IP's in brackets. Valence l e v e l numbering. Vi b r a t i o n a l structure, ± 50cm~l. b) CH NHC1. A l l values + 0.02-eV, except 15.05 (± 0.05eV) and 15.75 (± O.leV). c) (CH 3) 2NC1. F i r s t 3 IP's ± 0.02eV. The re s t , ± 0.05eV. d) CH 3NC1 2. A l l values + 0.02eV, except 15.54 and 16.53(± 0.05eV), - 81 -a C H 3 N H C 1 1 1 1 1 1 1 I I b 1 1 _/vl I l l l CH3NCU 1 1 1 1 1 1 1 1 C 1 AJ (CH3)2NCI 1 1 1 1 i 1 1 _L li 1 1 I 1 I 1 J t I L_ 8 10 12 14 16 18 IONIZATION POTENTIAL (eV) F i g . 5.4. H e l p h o t o e l e c t r o n s p e c t r a of a) CH NHC£ b) CH^C.C,, c ) ( C H 3 ) 2 N C £ - 82 -1 I . . . • (1 t I i i • i 1 i i I 11.0 11.5 11.5 12.0 IONIZAT ION P O T E N T I A L (eV) F i g . 5.5. E x p a n s i o n of the second bands i n the H e l p h o t o e l e c t r o n s p e c t r a o f a) (CH,) 9NC£ b) CH^NHC£. - 83 -Table 5.3. Experimental . IP's and assignments for CH^NHBr, (CH^NBr and CH NBr CH 3NHBr b (CH 3) 2NBr b CH 3NBr 2 C IP(eV) v (cm ) IP( eV) v (cm ) IP(eV) tf v (cm ) (9.12) (8. 61) (9.15) 9.60 10a 9. 14 8a' 9.62 8a' 10.86 830 9a 10. 55 320 5a" 1040 10.56 5a" 11.68 8a 11.18 7a' 11.02 300cm"1 7a' 13.22 7a 12. 84 4a" 11.33 4a" 14.64 6a 13. 33 6a' 12.28 6a' 15.63 5a 14. 20 3a" 13.19 3a" 16.66 4a 15. 55 5a' 14.88 5a' 16. 08 2a" 15.78 2a" 16. 36 4a' 16.30 4a' a) Adiabatic IP's in brackets. Valence l e v e l numbering. V i b r a t i o n a l structure ± 50cm~l. b) CH3NHBr and (CH^NBr. F i r s t three IP's ± 0.02eV, the rest ± 0.05eV c) CH NBr . F i r s t f i v e IP's + 0.02eV, the rest ±0.05eV - 84 -T a b l e 5.4. E x p e r i m e n t a l v e r t i c a l IP's and assignments f o r NH^Br to{ pettier w i t h p r e d i c t e d v e r t i c a l IP's f o r NHBr 2 NH^Br C NHBr 2 IP(eV) v (cm ) P r e d i c t e d I P b Observed IP (9.72) 10.18 650 5 a ' 10.15 6a' 11.19 840 2 a " 10.96 4 a " 12.69 4a' 11.42 11.43 5a' 15.0 3a' 11.67 11.57 3a" 13.35 4a' a) A d i a b a t i c IP i n b r a c k e t s . V a l e n c e l e v e l numbering. V i b r a t i o n a l s t r u c t u r e - 50cm - 1. b) O b t a i n e d by IP(NHC1 2) x I P ( C H 3 N B r 2 ) / I P ( C H 3 N C 1 2 ) . c) F i r s t two IP's ± 0.02eV. Rest ± 0.05eV - 85 -a A I I i CH3NHBr H 20 i i i i i i i i i b | CH3NBr2 i i i i i i i i i c A J (CH3)2NBr 1 1 t i l l —1 1 1 — l l l l l l I 1 — — J ——J I 9 II 13 15 17 19 IONIZATION P O T E N T I A L (eV) F i g . 5.6. H e l p h o t o e l e c t r o n s p e c t r a o f a) CH-NHBr b) CH NBr c) ( C H 3 ) 2 N B r - 87 -10 12 14 16 18 IONIZATION P O T E N T I A L (eV) F i g . 5.8. H e l p h o t o e l e c t r o n s p e c t r a of the NH^ + B r 2 gas phase r e a c t i o n a) excess Br b) excess NH - 88 -- 89 -NH3 I . . i i I • • ' J I ' i i I I 1 1 1 J 9 7 5 e V 1025 F i g . 5.10 D e t a i l o f t h e f i r s t band i n the PE spectrum of NH Br - 90 -T a b l e 5.5. E x p e r i m e n t a l v e r t i c a l IP's and c a l c u l a t e d IP's f o r NHF„ fa) EXP. I P V ' (eV) v (cm ) C a l c . IP(eV) HAM 3 (11.65) 12.44 585 ± 30 6a' 12.65 (6a') (540 ± 30) 15.55 5a', 4 a " 15.20 (4a ' 0 15.24 (5a') 15.96 3a" 15.70 (3a") 17.90 4a' 17.78 (4a') 19.75 3a', 2 a " 18.2 7 (2a"). 19.87 (3a') a) A d i a b a t i c IP i n b r a c k e t s . V a l e n c e l e v e l .numbering F i r s t f o u r v e r t i c a l IP's ± 0.02eV, r e s t ± 0.05eV. b) R e s u l t s f o r NDF„ g i v e n i n b r a c k e t s . i l l l I I I I I I L 12 14 16 18 2 0 IONIZATION P O T E N T I A L (eV) F i g . 5.11. H e l p h o t o e l e c t r o n spectrum of NHF 2 ( v e r t i c a l l i n e s r e p r e s e n t r e s u l t s o f HAM 3 c a l c u l a t i o n ) F i g . 5.12. Expansion of the f i r s t band i n the PE spectrum of NHF - 93 -The H e l PE spectrum o f d i f l u o r a m i n e i s shown i n F i g . 5.11 w i t h t h e measured IP's c o l l e c t e d i n T a b l e 5.5. The r e s o l v e d v i b r a t i o n a l s t r u c t u r e on t h e f i r s t band i s shown i n d e t a i l i n F i g . 5.12.'. R e s u l t s o f the HAM 3 c a l c u l a t i o n f o r HNF 2"are a l s o g i v e n i n - T a b l e 5.5. These were .performed to a s s i s t , ' w i t h t h e i d e n t i f i c a t i o n , o f groups of; IP' s to i n d i v i d -u a l m o l e c u l a r o r b i t a l s . 5.4. D i s c u s s i o n The assignments g i v e n f o r t h e PE s p e c t r a of t h e s i m p l e c h l o r a m i n e s w i l l be d i s c u s s e d i n d e t a i l from which, assignment of t h e PE s p e c t r a o f t h e bromamines and methyl s u b s t i t u t e d halamines f o l l o w s d i r e c t l y . To f a c i l i t a t e comparison t h e r e f o r e o n l y t h e v a l e n c e o r b i t a l s a r e l a b e l l e d . The PE spectrum of d i f l u o r a m i n e w i l l be d i s c u s s e d s e p a r a t e l y as t h e r e a r e few apparent s i m i l a r i t i e s w i t h t h e PE s p e c t r a of t h e o t h e r h a l a m i n e s . The complete s e r i e s o f compounds s t u d i e d i n t h i s c h a p t e r w i l l t hen be d i s c u s s e d w i t h r e g a r d to t h e b a r r i e r t o i n v e r s i o n i n t h e ground i o n i c s t a t e . 5.4.1. The c h l o r a m i n e s Monochloramine has \Cs ;symmetry and thus t h e c h l o r i n e l o n e p a i r s i n t h i s m o l e c u l e t r a n s f o r m as a ' and a". One of t h e s e ( a f ) has t h e c o r r e c t symmetry t o i n t e r a c t w i t h t h e N l o n e p a i r g i v i n g a n t i b o n d i n g and bonding c o m b i n a t i o n s , w h i l e t h e o u t - o f - p l a n e C£ o r b i t a l (a") remains r e l a t i v e l y non-bonding. T h i s can be r e p r e s e n t e d by t h e f o l l o w i n g diagram. - 94 -eV cd •rH 4-1 c CU 4-1 o PM C o •rH 4J ni N •H PS O 11-12-h £ 13-+-nN a ' V a " \ v. ,> TiCZ > a ' / The f i r s t t h r e e bands i n t h e PE spectrum o f NH^CJi a r e a s s i g n e d on t h i s b a s i s . The Franck-Condon e n v e l o p e o f t h e f i r s t band ( a d i a b a t i c and v e r t i c a l I P ' s , 9.85 and 10.52eV r e s p e c t i v e l y ) i s s i m i l a r t o t h a t i n (68") th e PE spectrum o f NH-j ( a d i a b a t i c and v e r t i c a l I P ' s , 10.15 and 10.88eV r e s p e c t i v e l y ) and c o n f i r m s i t s i d e n t i t y as t h e N l o n e p a i r ( 1 0 a ' ) , b e i n g d e s t a b i l i s e d i n NH^ CJ!, by t h e i n c o r p o r a t i o n o f some a n t i b o n d i n g c h l o r i n e 3p o r b i t a l c h a r a c t e r . I n t h i s c a s e t h e r e s o n a n c e . i n t e r a c t i o n of t h e c h l o r i n e i s s t r o n g e r than t h e i n d u c t i v e e f f e c t r e s u l t i n g i n a s h i f t t o lower IP. The r e s o l v e d v i b r a t i o n a l p r o g r e s s i o n f o r t h i s band ( F i g . 5.3a) has an average f r e q u e n c y o f 760cm \ e x t e n d i n g o v e r some n i n e members, and becoming d i f f u s e beyond t h e maximum. The n a t u r e o f t h i s p r o g r e s s i o n i s not c o m p l e t e l y c e r t a i n as t h e ob s e r v e d f r e q u e n c y c o u l d be due t o e i t h e r an e x c i t a t i o n o f t h e H^N wag" (v^'") o r t h e N-C& s t r e t c h (v^") ( t h e n e u t r a l ground s t a t e v a l u e s a r e 1032cm "*" and 686cm "*" r e s p e c t i v e l y ) -, x(10) r e s p e c t i v e l y ) - 95 -An i n c r e a s e i n t h e N-C£ s t r e t c h i n g f r e q u e n c y upon i o n i z a t i o n would be e x p e c t e d due t o t h e a n t i b o n d i n g n a t u r e o f t h i s m o l e c u l a r o r b i t a l , w h i l e a d e c r e a s e i n t h e NH^ wagging f r e q u e n c y might be e x p e c t e d due t o an i n c r e a s e i n t h e i n t e r b o n d a n g l e r e s u l t i n g from i o n i z a t i o n from t h e N l o n e p a i r . I t i s i n f a c t l i k e l y : t h a t b o t h modes a r e e x c i t e d , as t h e c o r - .' r e s p o n d i n g band i n N^CN. (10.65eV) e x h i b i t s e x c i t a t i o n o f t h r e e d i f f e r e n t m'odes^^. However i t i s important t o a s s i g n t h e - o b s e r v e d v i b r a t i o n a l s t r u c -t u r e i n N^Ctf,-as. i t c o u l d p r o v i d e I n f o r m a t i o n on the b a r r i e r t o i n v e r s i o n f o r t h e f i r s t i o n i c s t a t e . T h i s problem i s d i s c u s s e d f u r t h e r i n 5.4.6. The second band i n t h e PE spectrum o f NH^CA i s v e r y sharp and i n t e n s e , i n d i c a t i v e o f an i o n i z a t i o n from a non-bonding o r b i t a l ( F i g . 5.3b). T h i s i s i n a c c o r d w i t h t h e s i m p l e m o l e c u l a r o r b i t a l d iagram g i v e n above, which p r e d i c t s t h e second IP t o i n v o l v e t h e c o m p l e t e l y non-bonding CZ l o n e p a i r o r b i t a l ( 3 a " ) . The weak v i b r a t i o n a l s t r u c t u r e a s s o c i a t e d w i t h t h i s band (580, 970 and 3040cm "S p r o b a b l y c o r r e s p o n d s to v^, and (N-H s t r e t c h ) , t h e s e f r e q u e n c i e s b e i n g r e l a t i v e l y unchanged from t h e n e u t r a l ground s t a t e f r e q u e n c i e s . The t h i r d band ( F i g . 5.3c) w i t h a d i a b a t i c and v e r t i c a l IP's of 13.15 and 13.50eV r e s p e c t i v e l y i s a s s i g n e d t o t h e i n - p l a n e bonding c o m b i n a t i o n of N and CSL p o r b i t a l s ( 9 a ' ) . The o b s e r v e d v i b r a t i o n a l p r o g r e s s i o n of 450cm i s a s s i g n e d t o a reduced v a l u e of v^, t h e N-C£ s t r e t c h i n g f r e q u e n c y , i n a c c o r d w i t h t h e bonding c h a r a c t e r o f t h i s o r b i t a l . The l o s s o f s t r u c t u r e on t h i s band above t h e maximum c o u l d be due t o e i t h e r d i s s o c i a t i o n , o v e r l a p p i n g o f s e v e r a l f r e q u e n c i e s , o r to a l a c k of r e s o l u t i o n . (61 ^ I n f a c t P e e l e t a l h a s - s u g g e s t e d a - d i s s o c i a t i o n p r o c e s s - 96 -There a r e two re m a i n i n g bands i n t h e H e l PE spectrum o f N H 2 C £ which a r e br o a d and weak, o c c u r i n g a t 15.72 and 17.50eV. These a r e a s s i g n e d t o s i n g l e i o n i z a t i o n p r o c e s s e s g i v i n g a ' t o t a l o f 5 IP's i n t h e H e l r e g i o n , i n agreement w i t h r e s u l t s from t h e v a l e n c e i s o e l e c t r o n i c s p e c i e s (Ctt^Cl, C ^ e t c . ) . The f o u r t h band (15.72eV) i s a s s i g n e d t o t h e N-C£ a bonding o r b i t a l ( 8 a ' ) , b e i n g i n t h e a n t i c i p a t e d p o s i t i o n (see F i g . 5.13), l e a v i n g t h e f i f t h band (17.50eV) t o be a s s i g n e d t o t h e TT NH^ o r b i t a l (2a") somewhat, s t a b i l i z e d by i n c o r p o r a t i o n o f H Is f u n c t i o n s . As a f i n a l comment i t s h o u l d be n o t e d t h a t NH^CJi i s i s o e l e c t r o n i c w i t h HOCJc- (see c h a p t e r 4 ) ; However t h e e l e c t r o n i c s t r u c t u r e s o f t h e two m o l e c u l e s i n v o l v e e x t e n s i v e m i x i n g and a d e f i n i t e c o r r e l a t i o n o f m o l e c u l a r o r b i t a l s i s d i f f i c u l t . I t i s however i n t e r e s t i n g t o compare the PE s p e c t r a o f NH2CJL and HOC£ w i t h t h a t o f t h e v a l e n c e i s o e l e c t r o n i c s p e c i e s h y d r o x y l a n i i n e ; NH 2OH^ 7^ . T h i s m o l e c u l e s h a r e s f e a t u r e s o f bo t h N H 2 C £ and HOC£ and t h e o r b i t a l e n e r g i e s o f NH 2OH l i e n i c e l y between t h o s e o f t h e two m o l e c u l e s s t u d i e d i n t h i s work, as shown i n t h e c o r r e l a t i o n diagram ( F i g . 5.13). R e s u l t s f o r CE^Cl, C J ^ a n d F C ^ a r e i n c l u d e d i n t h e diagram f o r completeness. The t r e n d i n t h e a X-CSL bond energy i n t h e i s o e l e c t r o n i c s e r i e s CR^CSL through t o FC£ r e f l e c t s t h e p o l a r i z a t i o n o f the X-Cl bond, and as su g g e s t e d p r e v i o u s l y s u p p o r t s t h e assignment o f the f o u r t h band i n N H 2 C £ . The assignments a r e e s s e n t i a l l y i n agreement w i t h t h o s e o f t h e (61 ^ o t h e r workers . There a r e however some minor d i f f e r e n c e s i n p r e c i s e IP v a l u e s , a l t h o u g h t h e ones r e p o r t e d h e r e a r e b e l i e v e d t o be more - 97 -CH3CI CINH2 NH2OH HOCI C l 2 CIF F i g . 5.13. C o r r e l a t i o n diagram f o r t h e i s o e l e c t r o n i c s e r i e s CH3C£, NH2C&, HOC£ and FCI . NH 2OH and C&2 a r e a l s o i n c l u d e d f o r completeness. - 98 -a c c u r a t e h a v i n g been o b t a i n e d from pure samples, i n s t e a d o f r e s u l t i n g from s t r i p p e d s p e c t r a . F u r t h e r c h l o r i n a t i o n of NH^C^ to g i v e NHC£ p r o v i d e s t h e s p e c t r a shown i n F i g . 5.1b (Hel) and 5.2b ( N e l ) . The f i r s t band ( v e r t i c a l IP = 10.56eV) corr e s p o n d s t o t h e f i r s t band i n NH^CA ( v e r t i c a l IP = 10.52eV), and a l t h o u g h i t i s a s s i g n e d t o t h e N l o n e p a i r (6a') t h e r e i s a g a i n c o n s i d e r a b l e admixture o f c h l o r i n e 3p c h a r a c t e r . F o r NHCfl.^ t h e resonance e f f e c t i s overcome by t h e i n d u c t i v e e f f e c t o f c h l o r i n e i n c r e a s i n g t h e IP s l i g h t l y r e l a t i v e t o T^^C-^-. I n t h i s work 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 was r e s o l v e d on t h i s band but a weak v i b r a t i o n a l p r o g r e s s i o n o f 560cm ^ i s r e p o r t e d i n t h e work by L i v e t t et a l . ^  ^ from a h i g h r e s o l u t i o n . , s i n g l e sweep. T h i s was a s s i g n e d t o an e x c i t a t i o n o f t h e NC&2 symmetric bending v i b r a t i o n ( < 400cm ^ i n t h e ground s t a t e m o l e cule) . T h i s band w i l l be:'further d i s c u s s e d i n 5.4.6. w i t h r e g a r d t o t r e n d s i n FWHM f o r t h e halamines and t h e p o s s i b l e geometry changes upon:".ionization. The r e g i o n between 11 and 13eV i n t h e PE spectrum o f NHC&2 shows the p r e s e n c e o f two bands w i t h maxima a t 11.88 and 12.39eV. The second band has t h e g r e a t e r i n t e n s i t y and a s l i g h t s h o u l d e r at 12.50eV and i s t h e r e f o r e a s s i g n e d t o two i o n i z a t i o n p r o c e s s e s . J u s t i f i c a t i o n f o r t h i s i s o b t a i n e d from the spectrum o f CH^NCJ!^ (see below) where the near a c c i d e n t a l degeneracy i s s p l i t f u r t h e r t o g i v e two d i s t i n c t I P ' s . These t h r e e bands i n MLCl^ a r e a s s i g n e d t o e s s e n t i a l l y c h l o r i n e l o n e p a i r s , and as the f i r s t ( a t 11.88eV) i n c r e a s e s i n r e l a t i v e i n t e n s i t y - 99 -i n t h e N e l spectrum ( F i g . 5.2b) i t i s a s s i g n e d t o t h e 4 a " l e v e l ( a l m ost pure CH 3p). The second band i s t h e r e f o r e a s s i g n e d t o t h e 5a' ( p r o b a b l y at 12.39eV) and t h e 3a" o r b i t a l s ( p r o b a b l y a t 12.50eV). Above 14eV t h e r e a r e t h r e e r e l a t i v e l y b r o a d and weak bands o b s e r v e d i n t h e H e l spectrum and t h e s e a r e a s s i g n e d t o i n d i v i d u a l i o n i z a t i o n e v e n t s . The band at 14.57eV i s a s s i g n e d to'ithe b onding c o u n t e r p a r t of t h e f i r s t IP, b e i n g t h e bonding c o m b i n a t i o n of n i t r o g e n and c h l o r i n e p o r b i t a l s ; . " the" e q u i v a l e n t band i n NH^C^, o c c u r s a t 13.50eV. The f i n a l weak band at 17.39eV i s a s s i g n e d to an o r b i t a l i n v o l v i n g c o n s i d e r a b l e N-H bonding c h a r a c t e r i s t i c (3a') and can be compared to t h e band at 17.50eV i n NH^Cil. T h i s l e a v e s the band a t 16.11eV which i s a s s i g n e d t o an N C £ 2 a bonding l e v e l ( 2 a " ) . The N e l spectrum ( F i g . 5.2b) i n d i c a t e s t h a t t h e band at 14.57eV d e c r e a s e s i n i n t e n s i t y r e l a t i v e t o t h e band at 16.11eV but t h i s i s d i f f i c u l t t o q u a n t i f y because o f the s t e e p r i s e i n t h e c u t - o f f p o i n t of t h e spectrum. However t h e band a t 16.11eV does seem to show a s h i f t i n t h e maxima and the p o s s i b l i t y o f a u t o -i o n i z a t i o n cannot be e x c l u d e d . (61) A g a i n t h e assignments agree w i t h t h o s e o f t h e l a t e r work and t h e IP's a r e a l l w i t h i n O.leV. The H e l PE spectrum o f NC£^ ( F i g . 5.1c) p r e s e n t e d h e r e shows some s i g n i f i c a n t d i f f e r e n c e s from t h a t p r e v i o u s l y r e p o r t e d ^ 2 ^ 3 ^ . W h i l e t h e v a l u e of 10.69eV f o r t h e v e r t i c a l IP of t h e f i r s t band i s i n good agreement w i t h t h e p r e v i o u s work,the p o s i t i o n o f t h e sharp second band - 100 -shows a l a r g e d i s c r e p a n c y . The v a l u e o f 11.66eV quoted h e r e i s s i g n i f i c a n t l y h i g h e r than t h e r e p o r t e d v a l u e o f 11.2eV. However i t has been!.learnt t h a t t h e e a r l i e r w ork^ 2^ 3^ i n v o l v e d an impure sample of NCX.^, c o n t a i n i n g and QZ^^°^ . V i s u a l s u b t r a c t i o n of t h e C£^ a p p a r e n t l y removed the r e l a t i v e l y weak and c o i n c i d e n t 11.66eV band. T h i s l e f t t h e 11.2eV band as an unknown i m p u r i t y which a l s o i n t e r f e r e d w i t h t h e band a t 12.08eV. The f o u r t h IP i s a g a i n i n good agreement w i t h t h e r e s u l t g i v e n h e r e (13.02eV). H i g h e r IP's a t 15.41 and 16.70eV a r e a l s o seen i n t h i s work, which were not p r e v i o u s l y r e p o r t e d . To e s t a b l i s h beyond any doubt t h e : . i d e n t i f i c a t i o n o f t h i s m o l e c u l e , NC£ 3 was produced by t h r e e d i f f e r e n t methods: r e a c t i o n of CZ^ w i t h s o l i d ( N H ^ ^ S O ^ ^ ^ ; gas phase r e a c t i o n o f NH^ w i t h excess CZ^; and r e a c t i o n i n aqueous s o l u t i o n o f NaOC£ and NH^. I n a l l cases t h e same s p e c i e s was o b s e r v e d . I t s h o u l d a l s o be n o t e d t h a t v i o l e n t e x p l o s i o n s , a w e l l e s t a b l i s h e d c h a r a c t e r i s t i c o f NCJl^, o c c u r r e d d u r i n g t h e s e e x p e r i m e n t s , u s u a l l y upon changing t h e p r e s s u r e . T h i s compound i s t h e r e f o r e not to be t r e a t e d l i g h t l y . The f i r s t band i n NC£^ i - s a s s i g n e d to t h e N l o n e p a i r o r b i t a l (4a') somewhat s t a b i l i z e d compared to t h e c o r r e s p o n d i n g bands i n M ^CJl and NHCJ^ • T h i s i s i n p a r t due t o t h e i n d u c t i v e e f f e c t o f c h l o r i n e and t h e s m a l l e r resonance i n t e r a c t i o n ( t h e next a^ o r b i t a l i s a t 1 5 . 4 l e V ) . Under symmetry the c h l o r i n e l o n e p a i r s t r a n s f o r m as a^ + a^ +2e., The unique a^ o r b i t a l i s 100% l o c a l i z e d on t h e c h l o r i n e atoms and -so - 101 -( t h e r e s h o u l d be l i t t l e o r no change o f t h i s l e v e l i n t h e s e r i e s N C£^ to SbC& 3. ( v i z . , N C £ 3 , 11.66eV; P C £ 3 , 11.70eV; AsCJ^, 11.60eV; S b C £ 3 , (26s. 71 72) 11.57eV ' ' ) . The same i s t r u e f o r t h e analogous bromides and i o d i d e s ^ 7 ^ ' 7 2 ^ , and so l e n d s f u r t h e r e v i d e n c e f o r t h e p o s i t i o n o f t h e la^ l e v e l a t 11.66eV. I n t h e Ne I spectrum ( F i g . 5.2) t h i s band shows a r e l a t i v e i n c r e a s e i n i n t e n s i t y i n a c c o r d w i t h i t s assignment t o a pure c h l o r i n e 3p l e v e l . The i n t e n s e bands a t 12.08 and 13.02eV a r e a s s i g n e d t o t h e 4e and 3e l e v e l s , b e i n g e s s e n t i a l l y c h l o r i n e l o n e p a i r s . I n t h e N e l spectrum the band a t 13.02eV i s o b s e r v e d t o d e c r e a s e somewhat r e l a t i v e t o t h e o t h e r , i n d i c a t i v e o f some i n c r e a s e d n i t r o g e n c h a r a c t e r i n t h i s o r b i t a l , and i t s s t a b i l i z a t i o n r e f l e c t s some bonding c h a r a c t e r . By comparison t o P C £ 3 and AsCi^^'^^ t h e bands a t 15.41 and 16.70eV a r e a s s i g n e d t o t h e 3a^ and 2e N-C£ bonding l e v e l s r e s p e c t i v e l y . The o v e r a l l t r e n d s o f a l l o r b i t a l s from N C £ 3 t o S b C £ 3 a r e t h e r e f o r e as (73) e x p e c t e d . A r e c e n t a b - i n i t i o c a l c u l a t i o n l e n d s f u r t h e r e v i d e n c e f o r t h i s assignment. 5.4.2 Methylchloramines The Hel spectra of the methylchloramines (Fig. 5.4) show considerable correspondence with the non-methylated derivatives and therefore the assignments are f a i r l y straightforward. Thus the PE spectrum of CH3NHC£ (Fig. 5.4a) shows three distinct IP's at 9.80, 11.52 and 12.42eV which may be directly correlated with the f i r s t three IP's of NH 2C£ after incurring the usual shift due to CH3 substitution. - 102 -The Franck-Condon envelopes a r e s i m i l a r a l t h o u g h no v i b r a t i o n a l s t r u c t u r e was r e s o l v e d on t h e 1 s t and 3rd bands of CH^NHCJl. The second band of t h e m e t h y l d e r i v a t i v e i s a g a i n sharp i n d i c a t i n g s u b s t a n t i a l non-bonding c h a r a c t e r , ( F i g . 5.5a), w i t h t h e o b s e r v e d v i b r a t i o n a l p r o g r e s s i o n of 750cm b e i n g a s s i g n e d t o an e x c i t a t i o n of t h e N-C s t r e t c h i n g f r e q u e n c y (895, 875cm ^ i n t h e m o l e c u l a r ground s t a t e ) ( ^ l ) i n < } i c a t i n g not a l l the e l e c t r o n d e n s i t y i s on the C£ atom. T h i s i s based o n t h e r e s u l t f o r CH^NHBr • (see below) i n the absence of.which, one might be tempted t o p r e d i c t an e x c i t -a t i o n of an N^C£ -frequency."'"As l a i g h t b e " ~ e x p e c t e d t h i s band shows the l e a s t s h i f t upon m e t h y l a t i o n (-0.4eV), w h i l e s h i f t s o f -0.72 and -1.08eV a r e o b s e r v e d f o r t h e f i r s t and t h i r d bands r e s p e c t i v e l y . The l a r g e r s h i f t o f t h e t h i r d band i s i n a c c o r d w i t h t h e l a r g e r i n c o r p o r a t i o n o f t h e m e t h y l c h a r a c t e r i n t h i s o r b i t a l . T h i s m o l e c u l e has t h e l o w e s t symmetry (C^) o f a l l m o l e c u l e s s t u d i e d h e r e and t h e r e f o r e a l l o r b i t a l s have " a " symmetry l e a d i n g t o e x t e n s i v e m i x i n g . T h i s r e s u l t s i n t h e h i g h e r IP's b e i n g s p r e a d out w i t h f o u r d i s t i n c t maxima b e i n g o b s e r v e d (13.68, 15.05, 15.75 and 16.99eV). These r o u g h l y c o r r e s p o n d t o two CH^ i o n i z a t i o n s , a C-N-C£ and a NrlCH^ IT bonding o r b i t a l . There i s a r e a s o n a b l e c o r r e s p o n d e n c e between t h e s e f o u r IP's and t h e f o u r h i g h e r (74) .IP's"-of CH^NH^ . An a l t e r n a t i v e approach i s t o view the PE spectrum of CH^NHCJt. as i n t e r m e d i a t e i n t h e replacement of t h e CH^ groups i n (74) ( C H ^ N H by C£ atoms t o u l t i m a t e l y g i v e NHCJ!^ (see a b o v e ) . Thus t h e b r o a d bands due t o CH^ i o n i z a t i o n s i n t h e 13-17eV r e g i o n o f t h e spectrum of ( C H ^ N H t h i n out i n C H 3 N H C £ and N H C £ 2 , b e i n g r e p l a c e d by s h a r p e r bands i n t h e l l - 1 3 e V r e g i o n c o r r e s p o n d i n g t o t h e c h l o r i n e l o n e p a i r s . - 103 -F u r t h e r s u b s t i t u t i o n by a CH^ group g i v e s (CH 3); 2NC£ ( F i g . 5.4c), t h e PE spectrum o f which has p r e v i o u s l y been r e p o r t e d i n a s t u d y o f some Y3fi-v s u b s t i t u t e d phosphorus compounds . The s p e c t r a a r e e s s e n t i a l l y t h e same except t h a t t h e v a l u e s f o r t h e f i r s t t h r e e IP's determined i n t h i s work (9.31, 11.22 and 11.93eV) show a c o n s i s t e n t d i s c r e p a n c y o f ~ 0.12eV from t h e e a r l i e r work. T h i s i s somewhat s u r p r i s i n g as t h e bands have f a i r l y w e l l - d e f i n e d maxima but i t i s f e l t t h e r e s u l t s g i v e n h e r e a r e c o r r e c t as c o n s i s t e n t v a l u e s were o b t a i n e d u s i n g s e v e r a l c a l i b r a n t s ( C H 3 I , H 20, A r ) . A d i r e c t c o r r e l a t i o n can a g a i n be made between t h e f i r s t t h r e e IP's o f ( C H 3 ) 2 N C £ and C H 3 N H C £ w i t h t h e e x p e c t e d s h i f t due t o CH 3 s u b s t i t u t i o n b e i n g o b s e r v e d and so t h e assignments a r e made a c c o r d i n g l y . The second sharp band (11.22eV) shows the l e a s t s h i f t (-0.3eV) r e l a t i v e t o CH NHC£, s t i l l e x h i b i t i n g c o n s i d e r a b l e non bonding c h a r a c t e r . The o b s e r v e d v i b r a t i o n a l p r o g r e s s i o n of 960 ± 50cm ^ ( F i g . 5.5b) i s p r o b a b l y e i t h e r a reduced CH 3 d e f o r m a t i o n o r an NC 2 s t r e t c h i n g v i b r a t i o n ( m o l e c u l a r ground s t a t e v a l u e s o f ~ 1200cm ^ and 899cm ^ r e s p e c t i v e l y ) . The p r e s e n c e of two methyl groups i n t h i s m o l e c u l e r e s u l t s i n a complex PE spectrum above 12.5eV, b u t t h e IP's s e p a r a t e i n t o two d i s t i n c t r e g i o n s . I n t h e band c e n t r e d around 13.7eV t h e r e a r e maxima a t 13.15 and 13.69eV w i t h a s h o u l d e r a t 14.31eV. These a r e a s s i g n e d t o t h r e e I P ' s , two CH 3 l e v e l s ( a " and a') and a a C-N l e v e l (a") ( T a b l e 5.2). The f i n a l band (~ 16eV) shows two maxima a t 15.85 and 16.34eV but i s a s s i g n e d t o t h r e e i o n i z a t i o n events t o g i v e a t o t a l o f n i n e IP's i n t h e - 104 -(74) Hel region as observed for the isoelectric species (CH^^N . The three IP's are assigned to two CH^ levels (a'' and a') and another a C-N level ( a " ) . A comparison with the phosphorus analogue (CH^^PC^"^ shows a good correspondnece with the f i r s t three IP's, although the chlorine lone pairs appear more delocalized in the phosphorus compound. The P-C o orbitals are also at much lower energy than the N-C a orbitals and are therefore not masked by the CH^ ionization. The spectrum of (CH^^NCJ!. can be correlated with the isoelectronic (74) species (CH^^N . A reduction in the intensity of the 12-14eV band in (CH.j)2NC£ relative to (CH^^N i s accompanied by the introduction of two bands in the ll-12eV region (C£ lone pairs) as the number of CH^ ionizations is reduced. The perfluoro alkylated amines (CF^^NH and (CF^) ^blCZ have been studied and in the chloro derivative three distinct IP's are observed at 11.45, 13.0 and 14.30eV corresponding to the f i r s t three IP's of (CH^^NCi!,, inductively shifted by the fluorine atoms. The non-bonding level (13.0eV) i s again least affected by the substituents. The f i n a l methylchloramine i s CH^ NCJ!^  the PE spectrum of which is shown in Fig. 5.4b. The f i r s t five IP's at 10.01, 11.45, 11.96, 12.14 and 13.20eV correlate directly with the f i r s t five IP's in NHCS,2, although as mentioned earlier the third and fourth IP's are easily distinguished in CH^C^. The nitrogen lone pair at 10.01eV(8a') is shifted by -0.55eV upon introducing the CH^ group while the chlorine non-bonding levels show a shift of -0.4eV. In the more stable phosphorus compound CH^PCJI^J the chlorine lone pairs merge into a broad band at - 105 -~ 11.90eV, but a r e e a s i l y d i s t i n g u i s h a b l e i n CH^NCi!^ and a r e a s s i g n e d t o 5a"(11.45eV), 7a'(11.96eV) and 4a"(12.14eV) by comparison w i t h N H C £ 2 < The f i f t h IP at 13.20eV(6a') i s th e bonding c o u n t e r p a r t o f the f i r s t band and shows a s h i f t of-1.37eV r e l a t i v e t o t h e e q u i v a l e n t IP i n NHCji^' T h i s l a r g e s h i f t i s i n a c c o r d w i t h t h e i n c o r p o r a t i o n o f s i g n i f i c a n t m e t h y l c h a r a c t e r i n t o t h i s o r b i t a l . Above 13.5eV t h e r e a r e t h r e e maxima (14.11, 15.54 and 16.53eV which a r e a s s i g n e d t o f o u r o r b i t a l s t o g i v e a t o t a l o f n i n e IP's i n t h e H e l r e g i o n ( c f . t h e i s o e l e c t r o n i c ( C H ^ ^ N C i ) . These f o u r o r b i t a l s i n v o l v e t h e CH 3 group ( a " and a') a C-N(a') and o N C £ 2 ( a " ) w i t h two IP's b e i n g p l a c e d i n t h e l a s t band. The PE spectrum of CH^PCJ^ shows two b r o a d bands above 13eV^''~^ which may be c o r r e l a t e d t o t h e f o u r h i g h e r TP Is of C H 3 N C £ 2 . The f u l l s e r i e s o f u n s u b s t i t u t e d and m e t h y l s u b s t i t u t e d c h l o r a m i n e s a l l o w s a comprehensive s t u d y o f t h e e f f e c t o f C£ and CH 3 s u b s t i t u t i o n upon t h e a b s o l u t e p o s i t i o n o f t h e f i r s t band o f ammonia, and t h i s i s summarized i n F i g . 5.14. The h o r i z o n t a l and v e r t i c a l arrows r e f e r t o t h e replacement o f H by CH 3 and C£ r e s p e c t i v e l y . The d i a g o n a l arrows c o r r e s p o n d t o replacement of C£ by C H 3 < The l e f t hand column i l l u s t r a t e s t h a t t h e i n d u c t i v e e f f e c t o f s u c c e s s i v e l y r e p l a c i n g H i n NH 3 by c h l o r i n e i s c o u n t e r b a l a n c e d by t h e resonance e f f e c t . Thus t h e f i r s t IP i n t h e s e r i e s NH 3, N H 2 C £ , N H C £ 2 a n d N C £ 3 a t f i r s t d e c r e a s e s and t h e n s l o w l y i n c r e a s e s b u t n e v e r a t t a i n s t h e v a l u e i n NH 3. S u c c e s s i v e m e t h y l sub-s t i t u t i o n (rows) i s n o t a d d i t i v e s i n c e t h e d e c r e a s e o f t h e f i r s t IP i s reduced by t h e p r e s e n c e o f C£ atoms and may be a t t r i b u t e d t o competing 10.88 NhU - 1 . 2 4 9.64 - MeNH - 0 6 7 - 0 . 36 8.97 Me2NH + 0 . 3 4 0.53 8 .44 -Me 3 N .1 10.52 ' n y p 9 . 8 0 9.31 + 0 . 0 4 10.56 NHCL + 0.13 T 10.69 NCU + 0 2 1 10.01 - MeNCL i9< F i g . 5.14. 2>, E f f e c t o f Me and CI s u b s t i t u t i o n upon the f i r s t IP's o f s u b s t i t u t e d ammonias. H o r i z o n t a l arrows, replacement of H by Me; V e r t i c a l arrows, replacement o f H by CI; D i a g o n a l arrows, replacement o f C£ by Me. o - 107 -e l e c t r o n i c e f f e c t s and d e r e a l i z a t i o n . The d i a g o n a l r e l a t i o n s h i p s a r e t h e most c o n s i s t e n t w i t h t h e replacement o f any Cl atom by a CH^ group d e c r e a s i n g t h e f i r s t IP by a r e a s o n a b l y c o n s t a n t f a c t o r o f 0.78 ± O.leV. T h i s t r e n d i s i n agreement w i t h t h e b a s i c i t i e s of t h e s e m o l e c u l e s w i t h n o t f o r m i n g a d d u c t s ^ 7 ^ ' 7 ^ , u n l i k e t h e m e t h y l c h l o r a m i n e s and HCHp-gN^-'** 0^ . T h i s r e f l e c t s the s u b s t l t u e n t ' s e l e c t r o n e g a t i v i t y . which, as I t increases,"removes charge from, t h e n i t r o g e n making i t u n a v a i l a b l e f o r (27) d o n a t i o n . NrL^Cj!, i s a l s o known to be a poor e l e c t r o n donor 5.4.3 Methylbromamines Comparison o f t h e H e l s p e c t r a o f t h e methylbromamines ( F i g . 5.6) and m e t h y l c h l o r a m i n e s ( F i g . 5.4) i l l u s t r a t e s a d i r e c t c o r r e s p o n d e n c e o f I P ' s . The assignments f o r t h e bromine d e r i v a t i v e s which f o l l o w d i r e c t l y from t h e d i s c u s s i o n i n s e c t i o n 5.4.2 w i l l not be r e p e a t e d , a l t h o u g h c e r t a i n f e a t u r e s o f t h e s p e c t r a a r e worth comment. The n u m e r i c a l v a l u e s a r e summarized i n t h e c o r r e l a t i o n diagram ( F i g . 5.15). As e x p e c t e d a l l IP's of t h e methylbromamines a r e lower than t h o s e o f t h e c o r r e s p o n d i n g c h l o r o - d e r i v a t i v e s . The e f f e c t i s more pronounced f o r t h o s e bands i n v o l v i n g c o n s i d e r a b l e Br 4p c h a r a c t e r , i . e . h a l o g e n l o n e p a i r s and nN + nX c o m b i n a t i o n s . The bands i n t h e methylbromamines a r e more sp r e a d out than t h e c o r r e s p o n d i n g bands i n t h e m e t h y l c h l o r a m i n e s and t h e i n d i v i d u a l IP's become more apparent, e s p e c i a l l y i n t h e CH^ r e g i o n ( ~ 12.5eV). I t i s a l s o i n t e r e s t i n g t h a t t h e t h i r d and f o u r t h bands o f CH^NB^ a r e c o m p l e t e l y s e p a r a t e d whereas i n NHCJ^ they a r e e f f e c i t v e l y degenerate and o n l y s l i g h t l y s e p a r a t e d i n CH.JNC&2' P -00 IONIZATION POTENTIAL (eV) l 10 I U l U l 3 n fD O rt i-l =r P. ^ fD M P1a4 cu H rt O P-3 O (u 3 3 P- CU p p. (D OJ cn OQ l-i 03 3 o p. rt Cr fD 3 fD rt D* <^ r-1 O ^ M O P SU 3 P-3 fD CO § D Q cn Q a o o a CO to a Q ro cn O D D \ s OJ CD -J> cn Q Q D O Q O J CD D Q O I. I II -N" cn a a O a CO a a> o CD X CD a> -z. Q ro CD -z. CD —^  ro IV o fD ro Z CD CO D - 801 -- 109 -V i b r a t i o n a l f i n e s t r u c t u r e was o b s e r v e d o n l y on t h e bands a s s i g n e d to h a l o g e n non-bonding o r b i t a l s ( F i g . 5.7), (compare the m e t h y l c h l o r a m i n e s ) . The second band o f CH^NHBr shows a v i b r a t i o n a l p r o g r e s s i o n o f 830 ± 50cm \ somewhat h i g h e r than t h a t seen f o r C H 3 N H C £ + ( 7 5 0 ± 50cm 1 ) . The assignment t o an N-C£' s t r e t c h i n g f r e q u e n c y r a t h e r than an N-X s t r e t c h i n g f r e q u e n c y i s thus p r e f e r r e d . On t h e second band o f ( C H 3 ) 2 N B r two p r o g r e s s i o n s a r e observed, one of 320 ± 50cm ^ not p r e v i o u s l y seen - f o r ( C H 3 ) 2 N C £ + 'and t h e o t h e r of 1040 ± 50cm c o r r e s p o n d i n g to t h e e x c i t a t i o n o f 960 ± 50cm 1 i n (CH^X^NCii, + ( e x c i t a t i o n o f an NC^ s t r e t c h i n g f r e q u e n c y o r a reduced CH^ deformation)". The lower f r e q u e n c y c o u l d be a reduced N-Br s t r e t c h i n g f r e q u e n c y (525cm 1 f o r m o l e c u l a r (CH^) 2NBr) a l t h o u g h t h i s seems u n l i k e l y due t o the non-bonding n a t u r e of t h i s t r a n s i t i o n . .ikel? (30) The more l i y a l t e r n a t i v e i s an NC 2 d e f o r m a t i o n (390cm f o r m o l e c u l a r ( C H 3 ) 2 N B r ) The t h i r d band o f CH 3NBr 2 shows some v i b r a t i o n a l s t r u c t u r e not seen i n C H 3 N C £ 2 presumably because of o v e r l a p p i n g bands. The p r o g r e s s i o n of 300 ± 50cm ^ i s p r o b a b l y s i m i l a r t o t h a t o b s e r v e d i n ( C H 3 ) 2 N B r . The assignments g i v e n h e r e a r e i n agreement w i t h t h o s e o f C a r n o v a l e .. (63^ et a l f o r CR^NHBr and CH 3NBr 2, which were p u b l i s h e d a f t e r c o m p l e t i o n of t h i s work. There a r e however s l i g h t d i f f e r e n c e s i n IP v a l u e s which a r e a t t r i b u t e d t o t h e spectrum s t r i p p i n g t e c h n i q u e used by C a r n o v a l e et a l . The r e s u l t s g i v e n h e r e a r e o b t a i n e d from t h e PR s p e c t r a o f the. pure compounds• and a r e t h e r e f o r e o f h i g h e r a c c u r a c y . The spectrum s t r i p p i n g t e c h n i q u e a l s o has t h e e f f e c t o f r e d u c i n g r e s o l u t i o n and t h e r e f o r e 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 was o b s e r v e d . - 110 -5.4.4. Monobromamine and dibromamine These a r e d i s c u s s e d a f t e r t h e m e t h y l d e r i v a t i v e s , a s pure samples of the s i m p l e bromamines c o u l d not be p r e p a r e d and t h e r a t i o n a l e f o r p r o p o s i n g t h e d e t e c t i o n of NHB^ i s p a r t l y based on r e s u l t s o b t a i n e d f o r CH 3NBr 2. The H e l PE spectrum of NH^Br s h o u l d show f i v e bands, analogous to t h o s e of NH^CJljbut the s p e c t r a o b t a i n e d from the gas phase r e a c t i o n o f B r^ and NH^ ( F i g . 5.8) always i n v o l v e the p r e s e n c e o f a t l e a s t one of t h e i n i t i a l r e a c t a n t s and v a l u e s c o u l d t h e r e f o r e o n l y be o b t a i n e d f o r t h e f i r s t f o u r I P ' s . The r e m a i n i n g band i s expected to be of low 2 i n t e n s i t y ( c f . NH^CJl) and i s presumably obscured' by t h e broad E s t a t e o f N H 3 + ( 1 5 - 1 8 e V ) ( 6 8 ) . The f i r s t - h a n d of „NH B r - w i t h a d i a b a t i c and v e r t i c a l IP's of 9.72; and 10.18eV ( F i g . 5.10) c o r r e s p o n d s to the n i t r o g e n l o n e p a i r somewhat d e s t a b i l i z e d by t h e i n c o r p o r a t i o n of some a n t i b o n d i n g bromine 4p c h a r a c t e r . T h i s i s s h i f t e d by -0.34 and -0.70eV from t h e c o r r e s p o n d i n g bands i n N ^ C i l and NH 3 r e s p e c t i v e l y . The observed v i b r a t i o n a l p r o g r e s s i o n has a mean v a l u e o f 650 ± 50cm compared t o 760 ± 40cm f o r NH C& +. The o n l y v i b r a t i o n a l d a t a a v a i l a b l e f o r NH Br i s t h a t i n v o l v i n g vN-Br -1 (2) (540cm' ) and t h i s i s a p o s s i b l e e x c i t a t i o n as t h e o r b i t a l i s N-Br a n t i b o n d i n g . The o t h e r a l t e r n a t i v e t h a t was c o n s i d e r e d f o r NH^CJi ( s e c t i o n 5.4.1 and 5.4.6) i s a reduced v a l u e of the H 2N-Br bending f r e q u e n c y ( e s t i m a t e d t o be ~ 800cm ^ i n th e m o l e c u l a r ground s t a t e ) . - I l l -The second band of NlL^Br ( F i g . 5.9) i s sharp and i n t e n s e , analogous to t h e c o r r e s p o n d i n g band i n NH^Cfl,, but s h i f t e d by -0.73eV. From c a r e f u l s t u d y o f many s p e c t r a w i t h v a r y i n g Br" 2 .'NH^ . r a t i o s i t i s apparent t h a t t h e r e a r e t h r e e o t h e r , much weaker peaks i n t h i s r e g i o n which a r e not a s s o c i a t e d w i t h e i t h e r B r 2 or NH^. The one a t 11.29eV i s d e f i n i t e l y a s s o c i a t e d w i t h the i n t e n s e second band of Nrl^Br and i s a s s i g n e d t o a v i b r a t i o n a l component a t 840 ± 50cm \ p r o b a b l y c o r r e s p o n d i n g to t h e R^N-Br bending f r e q u e n c y . However, t h e o t h e r two peaks a t 11.43 and 11.57eV a r e not a s s o c i a t e d w i t h t h i s i o n i z a t i o n event as e v i d e n c e d by i n t e n s i t y d i f f e r e n c e s a p p a r e n t i n s p e c t r a o b t a i n e d w i t h v a r y i n g NH^iBr^ r a t i o s . These a r e t h e r e f o r e t e n t a t i v e l y a s s i g n e d t o t h e t h i r d and f o u r t h IP's of dibromamine NHB^. The r a t i o n a l e f o r t h i s i s t w o f o l d ; f i r s t l y t h e r e l a t i v e i n t e n s i t i e s of t h e s e bands i n c r e a s e w i t h excess B r ^ , c o n d i t i o n s most l i k e l y t o produce h i g h e r brominated s p e c i e s ( c f . gaseous r e a c t i o n of C& 2 a n ( i » a n ^ s e c o n d l y t h e o b s e r v e d v a l u e s a g r e e w e l l w i t h those p r e d i c t e d from a s i m p l e s c a l i n g f a c t o r , ( T a b l e 5.4) g i v e n by, IP(NHBr 2) = I P ( N H C £ 2 ) x I P ( C H 3 N B r 2 ) IP(CH 3NC£ 2) I t s h o u l d be noted however t h a t t h e p r e s e n c e o f N B r 3 cannot be r u l e d out and may c o n t r i b u t e to t h e s e f e a t u r e s . The t h i r d band of NH 2Br, the N-Br bonding e q u i v a l e n t of t h e f i r s t band has t h e same band envelope as t h e c o r r e s p o n d i n g band of NH 2C£, - 112 -b e i n g s h i f t e d by -0.81eV to a v e r t i c a l v a l u e of 12.69eV. The magnitude of t h i s s h i f t i n d i c a t e s a much l a r g e r c o n t r i b u t i o n from a bromine 4p o r b i t a l t h a n was t h e c a s e f o r t h e f i r s t IP. The spectrum shown i n F i g . 5.8b shows a s l i g h t t r a c e o f r e s i d u a l Br (^ILL ) on the h i g h 2. -£u energy s i d e o f t h i s band. The f o u r t h IP i s l o c a t e d a t t h e onset of t h e band due to t h e 2 E s t a t e of N H » + and p a r t i a l l y o v e r l a p s t h e 2 E + s t a t e of B r „ + . 3 g 2 The e s t i m a t e d v e r t i c a l v a l u e f o r t h i s IP i s 15.0 ± O.leV compared to 15.72eV f o r the c o r r e s p o n d i n g band of NH^C^ w i t h t h e Franck-Condon envelope b e i n g broad and weak i n b o t h c a s e s . The even weaker f i f t h band, a l t h o u g h observed f o r N r ^ C ^ i s not seen i n the s p e c t r a o f NH^Br p r e s e n t e d h e r e as i t l i e s under t h e broad second band of NH^ (15-18eV). The PE spectrum f o r NH^Br p r e s e n t e d h e r e i s v e r y s i m i l a r t o t h a t (62) r e p o r t e d by N a g y - F e l s o b u k i et a l . , where a spectrum s t r i p p i n g t e c h n i q u e was a g a i n used to remove f e a t u r e s due to r e a c t a n t g a s e s . T h i s l o c a t e d the f i f t h IP a t 16.93eV. There a r e some minor d i f f e r e n c e s between the two r e s u l t s , v i z . . t h e a b s o l u t e p o s i t i o n o f t h e sharp second band and t h e magnitude of t h e a s s o c i a t e d v i b r a t i o n a l s t r u c t u r e . The l a t t e r i s p a r t i c u l a r l y c o n f u s e d by t h e p r e s e n c e o f bands due to NHBr^. The s t r i p p e d spectrum a l s o r e v e a l s an " i m p u r i t y " band a t 10.9eV which i s i n c l o s e agreement "with t h e p r e d i c t e d v a l u e of 10.98eV f o r the second band, of NHBr^ ( T a b l e 5.4). A f u r t h e r " s t r i p p i n g " o f the N r ^ B r spectrum (62) was used to o b t a i n a spectrum of NHBr^ , but t h e r e s u l t can o n l y be c o n s i d e r e d as an a p p r o x i m a t i o n as t h e p r e s e n t e d spectrum appears to be h i g h l y smoothed and i n d i v i d u a l IP's above 12eV cannot be d i s t i n g u i s h e d . 10.88 N H 3 0.70 -124 9.64 MeNB -0.67 -0.04 8.97 Me2NH + 0.17 -0.53 8.44 Me 5N 10.18 ' n R p 9.60 ' _ n 4 R 9.14 NH 2Br : ^ - M e N H B r - ^ - Me 2NBr -0.08 1010 NBr, +0.02 • 9.62 MeNBr .0 F i g . 5.16. E f f e c t o f Me and Br s u b s t i t u t i o n upon the f i r s t IP's of s u b s t i t u t e d ammonias. H o r i z o n t a l arrows, replacement of H by Me; V e r t i c a l arrows, replacement of H by Br; D i a g o n a l arrows, replacement of Br by Me. ( E s t i m a t e d v a l u e s f o r NHBr and NHBr^.) - 114 -Analogous t o the c h l o r a m i n e s , t h e e f f e c t of Br and CH^ s u b s t i t u t i o n upon the a b s o l u t e p o s i t i o n o f the f i r s t band o f NH^ can be s t u d i e d and t h i s i s summarized i n F i g . 5.16. In t h i s c a s e t h e h o r i z o n t a l and v e r t i c a l arrows r e f e r t o replacement o f H by CH^ and Br r e s p e c t i v e l y w i t h the d i a g o n a l arrows c o r r e s p o n d i n g t o replacement o f Br by CH^. As w i t h t h e c h l o r a m i n e s t h e f i r s t IP of NH^ i s s u b s t a n t i a l l y d e s t a b i l i z e d by t h e a n t i b o n d i n g h a l o g e n i n t e r a c t i o n and from t h e observed s h i f t s f o r the compounds s t u d i e d , t h e v e r t i c a l IP's f o r the f i r s t bands of NHBr^ and NBr^ have been e s t i m a t e d . The former r e s u l t i s i n good agreement w i t h t h a t e s t i m a t e d by the s c a l i n g p r o c e d u r e ( T a b l e 5.4). The d i a g o n a l r e l a t i o n s h i p s a r e a g a i n t h e most c o n s i s t e n t w i t h replacement o f a Br atom by a CH^ group d e c r e a s i n g t h e f i r s t IP by a f a c t o r of 0.56 ± 0.14eV. 5.4.5. D i f l u o r a m i n e The H e l PE spectrum o f d i f l u o r a m i n e , NHF^, ( F i g . 5.11) i s somewhat u n l i k e t h e s p e c t r a f o r the p r e v i o u s l y d i s c u s s e d h a l a m i n e s . The f i r s t band o f NHF 2 i s r e l a t i v e l y more i n t e n s e than f o r t h e c o r r e s p o n d i n g bands of t h e c h l o r a m i n e s or bromamines, and t h e i o n i z a t i o n s from t h e ha l o g e n non-bonding l o n e p a i r s a r e not as i n t e n s e o r sh a r p . F i v e d i s t i n c t maxima a r e observed i n t h e H e l PE spectrum o f NHF^ w h i l e seven IP's a r e expec t e d from comparison w i t h NHC^' C a l -(81) c u l a t i o n s on the i s o e l e c t r o n i c s p e c i e s , methylene f l u o r i d e ( C H 2 F 2 ) , and oxygen d i f l u o r i d e ( F 2 0 ) a l s o p r e d i c t seven IP;s i n t h e H e l r e g i o n and assignments were made on t h i s b a s i s a l t h o u g h seven maxima a r e not observed i n e i t h e r c a s e . E a r l i e r assignments df t h e F 2 O s p e c t r a had - 115 -(83 84) been u n c e r t a i n ' . The PE spectrum of NHF^ p r e s e n t e d h e r e c o r r e l a t e s q u i t e w e l l w i t h t hose o f and ( F i g . 5.17). A HAM 3 c a l c u l a t i o n on NHF^ a l s o g i v e s good agreement w i t h the e x p e r i m e n t a l IP's ( T a b l e 5.5) which i s not unexpected s i n c e b o t h CR^F^ and F 2 0 were used i n t h e i n i t i a l p a r a m e t e r i z a t i o n -of HAM 3. The f i r s t band i n t h e PE spectrum of NHF 2 ( a d i a b a t i c and v e r t i c a l IP's of 11.65 and 12.44eV r e s p e c t i v e l y ) i s a s s i g n e d to a n i t r o g e n l o n e p a i r w i t h c o n s i d e r a b l e f l u o r i n e a n t i b o n d i n g c h a r a c t e r (6a')> T h i s r e p r e s e n t s a s h i f t o f +1.56eV and 1.88eV- r e l a t i v e to t h e f i r s t bands of ammonia and NHCJc^, i n d i c a t i n g t h a t t h e i n d u c t i v e e f f e c t o f F f a r outweighs t h e d e s t a b i l i z a t i o n due t o the resonance i n t e r a c t i o n . The r e s o l v e d v i b r a t i o n a l f i n e s t r u c t u r e on t h i s band, which i s l o s t j u s t a f t e r the maximum, has an average s p a c i n g of 585 ± 30cm ^. There a r e two p o s s i b l e assignments f o r t h i s v i b r a t i o n a l f r e q u e n c y . The NF 2 d e f o r m a t i o n mode ( v ^ = 500cm i n t h e n e u t r a l m o l e c u l e ) c o u l d be e x c i t e d , b e i n g s l i g h t l y i n c r e a s e d i n the i o n , w h i l e the o t h e r p o s s i b i l i t y i n v o l v e s e x c i t a t i o n of t h e NH bending v i b r a t i o n (v = _1 < 6 0> 1307cm i n the n e u t r a l m o l e c u l e ) ; reduced i n t h e i o n due t o p e n e t r a t i o n of t h e i n v e r s i o n b a r r i e r and a c o r r e s p o n d i n g f r e q u e n c y h a l v i n g . The same band i n the d e u t e r a t e d s p e c i e s , N D F 2 +, shows a v i b r a t i o n a l p r o g r e s s i o n o f 540 ± 30cm "*" c o n s i s t e n t l y lower than i n NHF 2 +, but a l t h o u g h n o t - o u t s i d e the e r r o r l i m i t s . T h i s problem w i l l be c o n s i d e r e d f u r t h e r " i n t h e d i s c u s s i o n o f t h e f i r s t IP's f o r the whole s e r i e s o f halamines ( s e c t i o n 5.4.6). - 116 -The next band i n the PE spectrum of NHF^ shows;.two maxima (15.55 and 15.96eV) but has been a s s i g n e d t o t h r e e i o n i z a t i o n e v e n t s by com-p a r i s o n w i t h CH. 2F 2 and F 2 0 . The HAM 3 c a l c u l a t i o n p l a c e s 2IP's i n the f i r s t peak (5a'and 4a") and a s i n g l e IP i n t h e second peak (3a") i n agreement w i t h the assignment f o r t h e i s o e l e c t r o n i c m o l e c u l e s . These t h r e e i o n i z a t i o n events can be c o n s i d e r e d to a f i r s t a p p r o x i m a t i o n as a r i s i n g from the f l u o r i n e non bonding " l o n e p a i r s " , a l t h o u g h due to t h e low symmetry of the molecule, c o n s i d e r a b l e m i x i n g i s i n v o l v e d . Above 17eV t h e r e a r e two f u r t h e r bands which a r e a s s i g n e d t o t h r e e IP's i n a c c o r d a n c e w i t h t h e r e q u i r e m e n t f o r seven IP's i n t h e H e l r e g i o n . By comparison w i t h ^ 2 ^ 2 a n < ^ F 2 ° 5.17) one IP i s a s s i g n e d t o t h e band w i t h a maximum a t 17 .90eV(4a'). T h i s can be c o n s i d e r e d as the bonding c o u n t e r p a r t o S • the f i r s t IP and r e p r e s e n t s a s h i f t o f 3.33eV r e l a t i v e t o t h e c o r r e s p o n d i n g band i n NHCJl^, i n agreement w i t h the l a r g e F c o n t r i b u t i o n to t h i s o r b i t a l . The f i n a l band (maximum 19.75eV) i s t h e r e f o r e a s s i g n e d to two IP's ( 3 a ' and 2a") which a r e b a s i c a l l y a a NHF 2 l e v e l and a a NF 2 bonding c o m b i n a t i o n . I t s h o u l d be noted however t h a t t h e HAM 3 c a l c u l a t i o n p r e d i c t s 2IP's i n t h e band c e n t r e d a t 17.90eV ( 4 a ' and 2a") ( T a b l e 5.5) and t h e f i n a l assignment must be c o n s i d e r e d t e n t a t i v e . The g e n e r a l f e a t u r e s of t h e spectrum of NHF 2 c o r r e s p o n d w e l l w i t h those i n t h e PE spectrum of d i f l u o r o p h o s p h i n e ^ ^ a l t h o u g h o n l y f i v e IP's were a s s i g n e d t o t h e H e l r e g i o n . L i t t l e d i s c u s s i o n of t h e r e s u l t s was g i v e n however, and the assignment g i v e n h e r e f o r NHF (where the bands a r e more spread out) i s f e l t t o be a p p r o p r i a t e - 117 -12 1 3 1 2 0 -21 -6 a ' V s 2 b, S X 2 b, ~ 1 4 | QJ g 1 6 f - 10. ~ - - - 3 ^ > . 6 c L 4 b , O I 1a, Q -17 2 1 8 -z 1b x ^ O X 3 a ' 2 a " s \ \ 3 b 2 C H 2 F 2 NHF 2 F 2 0 F i g . 5.17. C o r r e l a t i o n diagram f o r t h e i s o e l e c t r o n i c s e r i e s C H ^ ,NHF 2and F 2 0 - 118 -f o r PHF^. P a r t i c u l a r l y c h a r a c t e r i s t i c ^ f o r b o t h m o l e c u l e s i s t h e band be-tween 15 and 16eV, a s s i g n e d t o t h e f l u o r i n e noh bonding 2 p , o r b i t a l s i n NHIV,. As a f i n a l comment on t h e spectrum o f NHF^ i t s h o u l d be n o t e d t h a t a c o r r e l a t i o n can be made w i t h t h e PE spectrum o f NHC&2 o n t h e b a s i s o f t h e expe c t e d s h i f t s . A l l IP's a r e i n c r e a s e d i n NHI?^, t ^ i e l e a s t a f f e c t e d b e i n g t h e 6a' o r b i t a l which i s e s s e n t i a l l y t h e N l o n e , p a i r and i s s h i f t e d by +1.88eV. T h i s compares t o t h e s h i f t o f t h e h a l o g e n l o n e p a i r s o f around +3.4eV. 5.4.6. Ionization of the N lone pair Ionization from the N lone pair orbital of ammonia and i t s derivatives produces an increase in the interbond angle with a resultant decrease in the barrier to inversion at the N centre. The f i r s t bands in the PE spectra of the halamines are thus of interest with regard to information on geometry changes upon ionization and this does appear to be significant in a l l cases, as evidenced by the broad Franck-Condon envelopes (FWHM values are given in table 5.6). For the parent molecule, NH^ , ionization from the N lone pair orbital is known to produce a planar i o n ^ ^ , corresponding to a change in interbond angle of 13°. The barrier to inversion in the ion is thus either zero or very close to zero compared to a value of ~ 0.25eV in the ground state molecule^^ . The f i r s t PE band of NH^  has a FWHM of l.OeV with a very distinctive vibrational progression of 950cm ^ spread (68^ over .18-19 members . This corresponds to t n e bending mode which produces the inversion, being hardly unchanged from the molecular ground state value. This is somewhat surprising as a "frequency halving" - 119 -T a b l e 5.6. FWHM v a l u e s o f t h e f i r s t PE band i n s u b s t i t u t e d ammonias. Compound FWHM ( NH 3 1.0 N H 2 a 0.70 N H C £ 2 0.63 N C £ 3 0.57 CH 3 N H C £ 0.71 C H 3 N C £ 2 0.61 ( C H 3 ) 2 N C £ 0.64 ( C H 3 ) 2 N B r 0.61 CH 3NBr 2 0.53 CH 3NHBr 0.63 NHF 2 0.80 a) V a l u e s ± 0.02eV i n a l l cases - 120 -might be e x p e c t e d i n t h e i o n compared t o t h e ground s t a t e m o l e c u l e (91) because o f t h e d i f f e r e n t p o t e n t i a l f u n c t i o n s (see F i g . 6.18). Such a f r e q u e n c y h a l v i n g i s o b s e r v e d i n t h e PE s p e c t r a o f t h e o t h e r Group V h y d r i d e s which a l l have f i r s t i o n i c s t a t e s w i t h low b a r r i e r s t o i n v e r s i o n . The v i b r a t i o n a l s t r u c t u r e on t h e f i r s t -PE bands-of. t h e s e compounds has a f r e q u e n c y c o r r e s p o n d i n g t o v^'/2 once t h e b a r r i e r i s overcome and so t h e case o f NH^ seems t o be an anomaly/which i s not f u l l y u n d e r s t o o d . On t h e f i r s t PE bands o f t h e halamines i t i s t h e r e f o r e p o s s i b l e t h a t f r e q u e n c y l . h a l v i n g o f t h e be n d i n g modes s e n s i t i v e t o i n v e r s i o n may be o b s e r v e d . Of t h e PE s p e c t r a r e p o r t e d i n t h i s work v i b r a t i o n a l s t r u c t u r e i s o b s e r v e d on t h e f i r s t bands o f o n l y t h r e e m o l e c u l e s - NH^CJi, NH^Br and NHF2 — w i t h i n f o r m a t i o n on t h e f i n e s t r u c t u r e o f NHCJi^ a l s o b e i n g ( g j \ a v a i l a b l e . However the p o s s i b i l i t y o f more than one v i b r a t i o n a l mode b e i n g e x c i t e d upon i o n i z a t i o n o f t h e N l o n e p a i r causes d i f f i c u l t y i n a s s i g n i n g t h e ob s e r v e d s t r u c t u r e . F o r the m o n o s u b s t i t u t e d s p e c i e s t h e f r e q u e n c i e s i n t h e i o n (760 and -650CHT 1 f o r N H 2 C £ * and NH^Br* r e s p e c t i v e l y ) c o u l d be a s s i g n e d t o e i t h e r ', t h e N-X s t r e t c h i n g f r e q u e n c y (686 and 540cm ^ i n t h e m o l e c u l a r ground s t a t e s ) o r ' t h e NH^ wag t h a t can r e s u l t i n i n v e r s i o n a t t h e N c e n t r e (ground s t a t e m o l e c u l a r v a l u e ' f o r N H ^ J ^ 1 0 ^ of 1032 cm which i s presumably reduced t o ~ 850cm ^ f o r NE^Br) . F o r the l a t t e r v i b r a t i o n a l m o d e v i b r a t i o n a l l e v e l s above t h e b a r r i e r t o i n v e r s i o n would be expe c t e d t o show a fr e q u e n c y h a l v i n g . The b a r r i e r t o i n v e r s i o n f o r t h e ground s t a t e m o l e c u l e - 121 -5.18 P o t e n t i a l energy c u r v e s f o r p l a n a r and p y r a m i d a l s p e c i e s . - 122 -Ml^Ci, is known to have an upper limit of 4000cm ^ ( 0 . 5 e V ) w h i c h w i l l be substantially reduced in the ground ionic state. The FWHM of the f i r s t PE band of N^Ci is 0.7eV and assuming a barrier to inversion for the ion of < 0.5eV, barrier penetration should occur upon ionization with the resultant reduction in the v^' frequency. The observed frequency however is not obviously v^'/2 and'there is no definite evidence for frequency halving in the progression although the onset is i l l defined and could be informative under higher resolution. For this reason the vibrational structure is assigned to ' in preference to ' which should however not be totally dismissed as an exact frequency halving of the inversion mode is not always seen, as evidenced by the PE spectrum of NH^  . A much higher resolution (lOmeV) spectrum is necessary to give a definitive assignment. It is likely that both ' and ' are excited (also possibly v^' > the s c i s s o r s ) a n d the convergence of two or more vibrational modes is probably responsible for the loss of structure 0.77eV past the onset of this band (Fig. 5.3a) A high resolution optical spectrum would be useful in this regard. The proposed assignment of the vibrational structure to ' can be rationalized on the basis of an increased frequency in the ion relative to the molecule in agreement with ionization from an orbital which is N-C£ antibonding. The equivalent assignment is proposed for the vibrational structure observed on the f i r s t PE band of NH^Br with again being larger in the ion than in the ground state molecule. For the dihalogenated species, NHF2, the possible assignments -the observed vibrational progesssion (5,85'cm "*") are v^'> the wag - 123 -(NH bending) which s h o u l d be s e n s i t i v e t o i n v e r s i o n (1307cm i n t h e m o l e c u l a r ground s t a t e ) o r ' t h e NHF 2 b e n d i n g mode (500cm ^ i n t h e m o l e c u l a r ground s t a t e ) . In t h e ground m o l e c u l a r s t a t e t h e b a r r i e r t o i n v e r s i o n has been (86 87) c a l c u l a t e d t o be between 1.49 and 1.81eV ' but i s expe c t e d t o be s i g n i f i c a n t l y M o w e r i n t h e ground i o n i c s t a t e . The FWHM of t h e f i r s t PE band i s 0.80eV ( F i g . 5.12) and so i t i s c o n c e i v a b l e t h a t b a r r i e r p e n e t r a t i o n o c c u r s upon i o n i z a t i o n o f t h e N l o n e p a i r . ::The a c t u a l p r o g r e s s i o n o b s erved on t h e band extends from c l o s e t o t h e onset t o p a s t t h e band maximum and c o u l d be v 2'/2,_ c l o s e t o h a l f the m o l e c u l a r gound s t a t e f r e q u e n c y . I f t h i s i s s o , t h e i o n i c s t a t e must have a low b a r r i e r t o i n v e r s i o n (~ 0.25eV) above which t h e r e s o l v e d p r o g r e s s i o n shows no o b v i o u s changes i n f r e q u e n c y . The onset o f t h e band i s i l l - d e f i n e d which i s a p o s s i b l e i n d i c a t i o n of p e r t u r b a t i o n of v i b r a t i o n a l l e v e l s c l o s e t o t h e b a r r i e r maximum but c o u l d a l s o be due t o a poor s i g n a l r n o i s e r a t i o . i n t h i s r e g i o n . The a l t e r n a t i v e assignment i s t o ' ( N F 2 s c i s s o r s ) b e i n g s l i g h t l y i n c r e a s e d from the ground m o l e c u l a r s t a t e f r e q u e n c y . The ob s e r v e d p r o g r e s s i o n o f 540 ± 30cm on t h e e q u i v a l e n t PE band o f NDF 2 ( c f . 585 ± 30cm on NHF 2) s u p p o r t s t h i s assignment as t h e i s o t o p e e f f e c t i s s m a l l , i n a c c o r d w i t h a v i b r a t i o n a l mode w i t h l i t t l e H in v o l v e m e n t (ground m o l e c u l a r s t a t e f r e q u e n c i e s a r e 500cm f o r b o t h NHF 2 and D N F 2 ^ ^ ) . A much l a r g e r i s o t o p e e f f e c t would be expe c t e d f o r t h e NF 2 wag o r N-H bend, v^'' (The m o l e c u l a r ground s t a t e f r e q u e n c i e s - 124 -a r e 1307 and 1008cm f o r NHF^ and NDi^ r e s p e c t i v e l y ) . The v i b r a t i o n a l p r o g r e s s i o n on'"the f i r s t PE bands o f NHF^ and NDF^ i s thus a s s i g n e d t o ' i n p r e f e r e n c e t o However, as w i t h t h e monohalamines, i t i s l i k e l y t h a t more than one v i b r a t i o n a l mode i s e x c i t e d upon i o n i z a t i o n o f t h e N l o n e p a i r and h i g h e r r e s o l u t i o n s p e c t r a c o u l d w e l l r e s o l v e p r o g r e s s i o n s i n b o t h modes. The l o s s o f s t r u c t u r e p a s t t h e maximum of t h e f i r s t PE band i s a t t r i b u t e d t o t h e convergence o f s e v e r a l v i b r a t i o n a l modes ( v^ ' , ' and p o s s i b l y v ^ ' , t h e N-F s t r e t c h i n g f r e q u e n c y which might a l s o be expected t o be excited).. R e c e n t l y p u b l i s h e d work on NHCJ^ has c l a i m e d t o r e s o l v e a p r o g r e s s i o n of 560cm on t h e f i r s t PE b a n d ^ 6 1 ^ . T h i s was a l s o a s s i g n e d t o v.. , t h e s c i s s o r s . T h i s r e p r e s e n t s an i n c r e a s e o f around 40% from t h e m o l e c u l a r ground s t a t e v a l u e . ( < 400cm ^) which appears t o be e x c e s s i v e when compared t o t h e r e s u l t s f o r NH^. In t h e work p r e s e n t e d h e r e no s t r u c t u r e was r e s o l v e d on t h e f i r s t band of NHCJt^ a n < * i s i n f a c t p o o r l y ( 6 1 ) d e f i n e d i n t h e spectrum p r e s e n t e d by L i v e t t e t a l . The l a c k o f d a t a f o r NDCJc^ and t h e q u a l i t y of d a t a f o r NHCJ^ thus makes i t d i f f i c u l t t o compare r e s u l t s f o r t h e c h l o r o and f l u o r o d e r i v a t i v e s . I t i s o b v i o u s from a l l t h e r e s u l t s d i s c u s s e d above t h a t t h e r e a r e a m b i g u i t i e s i n assignment o f t h e v i b r a t i o n a l f r e q u e n c i e s o b s e r v e d f o r t h e f i r s t i o n i c s t a t e o f t h e h a l a m i n e s . U n t i l h i g h e r r e s o l u t i o n s p e c t r a a r e a v a i l a b l e , e i t h e r o p t i c a l o r p h o t o e l e c t r o n , a d e f i n i t i v e assignment cannot be made and thus any c o n c l u s i o n s c o n c e r n i n g t h e geometry changes upon i o n i z a t i o n and t h e r e s u l t a n t i n v e r s i o n b a r r i e r - 125 -i n t h e i o n have t o be c o n s i d e r e d t e n t a t i v e . However q u a l i t a t i v e c o n s i d e r a t i o n o f t h e Franck-Condon envelope of t h e f i r s t PE band of t h e s e m o l e c u l e s suggest s i g n i f i c a n t geometry change upon i o n i z i n g t h e N l o n e p a i r and i t i s b e l i e v e d h i g h r e s o l u t i o n gas phase o p t i c a l s p e c t r a would i n d e e d show d e f i n i t e e v i d e n c e f o r b a r r i e r p e n e t r a t i o n i n t h e i o n and an a c c u r a t e v a l u e f o r t h e i n v e r s i o n b a r r i e r c o u l d t h e n be o b t a i n e d . 5.5. Conclusion,' The H e l PE s p e c t r a o f t h e simple? halamines-"and the m e t h y l s u b s t i t u t e d d e r i v a t i v e s have been presented.". By: sampling t h e vapours above aqueous s o l u t i o n s o f t h e s e u n s t a b l e s p e c i e s , and w i t h t h e use of j u d i c i o u s t r a p p i n g procedures, pure samples of many o f t h e s e compounds were o b t a i n e d i n t h e gas phase. NH^^r and NHB^ c o u l d not be d e t e c t e d u s i n g such e x p e r i m e n t a l p r o c e d u r e s due t o r a p i d d e c o m p o s i t i o n i n s o l u t i o n , and had to be p r e p a r e d by a d i r e c t gas phase m i x i n g o f NH^ and B r ^ r e s u l t i n g i n "impure" s p e c t r a . T h i s p r o c e d u r e . a l l o w e d a"comprehensive study of the h a l o g e n and m e t h y l s u b s t i t u e n t e f f e c t s on the v a l e n c e l e v e l IP's i n t h e H e l r e g i o n a n d - f o r some g e n e r a l t r e n d s t o be e s t a b l i s h e d -(see: F i g s . 5.14 and 5.16). The f i r s t band i n th e PE spectrum of t h e s e compounds i s p a r t i c u l a r l y i n t e r e s t i n g as i t r e p r e s e n t s i o n i z a t i o n from t h e N l o n e p a i r which i s accompanied by a s i g n i f i c a n t change i n geometry as e v i d e n c e d by t h e Franck-Condon envelope o f t h e band. Where v i b r a t i o n a l s t r u c t u r e - 126 -i s o b s erved on the band t h e r e i s d i f f i c u l t y i n i t s assignment and i t i s b e l i e v e d t h i s w a r r a n t s f u r t h e r work when h i g h e r r e s o l u t i o n i s p o s s i b l e . A q u a l i t a t i v e e s t i m a t i o n o f t h e r e l a t i v e s t a b i l i t i e s o f t h e s e compounds can be made on the b a s i s o f t h e i r r a t e s o f d e c o m p o s i t i o n as e x p e r i e n c e d d u r i n g t h e s e e x p e r i m e n t s . T h i s i s somewhat s u b j e c t i v e but i s an i n d i c a t i o n o f t h e ease o f h a n d l i n g o f t h e s e compounds ( C H 3 ) 2 N C £ > C H 3 N C £ 2 ~ ( C H ^ N B r > CH^HCJl > CH 3NBr 2 > CH 3NHBr > NHF 2 > NH^CJc. > N C £ 3 > NHC2_2 » NH^Br > NHBr 2 - 127 -CHAPTER SIX SUBSTITUTED KETENES 6.1. I n t r o d u c t i o n Ketene (CH2=C=0) i s an important a c e t y l a t i n g r e a g e n t i n o r g a n i c s y n t h e s i s . A l t h o u g h i t p o l y m e r i z e s r e a d i l y above 0°C the common p r e p a r a t i v e methods based on p y r o l y s i s of a c e t i c a c i d , a c e t o n e o r a c e t i c a n h y d r i d e a t reduced p r e s s u r e g i v e good y i e l d s . The s i m p l e ketene m o l e c u l e has been w i d e l y i n v e s t i g a t e d s p e c t r o s c o p i c a l l y s i n c e (2-7) i t p o s s e s s e s a l i n e a r C=C=0 c h a i n , w i t h the v i b r a t i o n a l s p e c t r a and geometry^ 8 " ^ b e i n g w e l l e s t a b l i s h e d . The H e l PE spectrum i s a l s o w e l l e s t a b l i s h e d ^ " * " "*"^ w i t h a f u l l assignment b e i n g g i v e n i n one (12) c a s e which i s i n good agreement w i t h c a l c u l a t i o n s u s i n g t h e (12) p s e u d o n a t u r a l o r b i t a l c o u p l e d e l e c t r o n p a i r approach and (14) p e r t u r b a t i o n c o r r e c t i o n s to Koopmans' theorem . Thus t h e p a r e n t m o l e c u l e i s w e l l u n d e r s t o o d and t h i s p r o v i d e s a s t a r t i n g p o i n t f o r t h e n o v e l d e r i v a t i v e s d e s c r i b e d i n t h i s c h a p t e r . The m e t h y l s u b s t i t u t e d ketene has not been as e x t e n s i v e l y s t u d i e d as i t r a p i d l y combines w i t h water, o r g a n i c a c i d s o r i t s e l f . However the microwave spectrum o f m e t h y l k e t e n e has been o b t a i n e d > - ^ X f r o m a gas phase p y r o l y s i s of p r o p i o n i c a n h y d r i d e , w h i l e a s i m i l a r p y r o l y s i s (18) of m a l o n i c a n h y d r i d e has been used to o b t a i n b o t h microwave and (19) i n f r a r e d s p e c t r a o f d i m e t h y l k e t e n e . T h i s d i s u b s t i t u t e d ketene (19) a l s o p o l y m e r i z e s r e a d i l y under normal c o n d i t i o n s . The f i r s t IP o f b o t h the m e t h y l d e r i v a t i v e s has been p r e v i o u s l y r e p o r t e d from a (13) PE s t u d y i n v o l v i n g p y r o l y s i s and subsequent e l i m i n a t i o n of HC£ - 128 -from a s u b s t i t u t e d a c e t y l c h l o r i d e . However no o t h e r d a t a were g i v e n and the s p e c t r a were not shown. In the same work the f i r s t IP o f monochloroketene was a l s o quoted, t h i s b e i n g t h e f i r s t r e p o r t e d i n v e s t i g a t i o n of any h a l o g e n a t e d ketene i n the gas phase. Attempts to produce d i c h l o r o k e t e n e by a s i m i l a r p y r o l y s i s were a p p a r e n t l y f ,(13) u n s u c c e s s f u l D i c h l o r o k e t e n e ^ 2 ^ and d i b r o m o k e t e n e ^ 2 " ^ have been p r e p a r e d i n s o l u t i o n by the d e h a l o g e n a t i o n of the a p p r o p r i a t e a c i d bromide w i t h z i n c , but c o u l d not be i s o l a t e d due to p o l y m e r i z a t i o n . I d e n t i f i c a t i o n was based m a i n l y on " i n s i t u " r e a c t i o n s of t h e ketene s o l u t i o n s a l t h o u g h l i m i t e d i r d a t a was a l s o "obtained on what were b e l i e v e d to be monomeric s p e c i e s . D i c h l o r o k e t e n e has a l s o been p r e p a r e d i n s o l u t i o n by t h e (22) d e h y d r o c h l o r i n a t i o n of d i c h l o r o a c e t y l c h l o r i d e D i f l u o r o k e t e n e i s r e p o r t e d to be u n s t a b l e above -5°C but has been proposed as a s h o r t l i v e d i n t e r m e d i a t e i n the r e a c t i o n of bromodi-(23) f l u o r o a c e t y l bromide s o l u t i o n s w i t h z i n c . T h i s i s i n disagreement (24) w i t h e a r l i e r work which c l a i m e d t h a t t h e d i f l u o r o d e r i v a t i v e behaved i n a s i m i l a r f a s h i o n to ketene i t s e l f and c o u l d be d i s t i l l e d w i t h e t h e r . D i c y c a n o k e t e n e has a l s o been r e c e n t l y proposed as a (25) r e a c t i v e i n t e r m e d i a t e T h i s c h a p t e r p r e s e n t s t h e H e l PE s p e c t r a of b o t h mono- and d i s u b s t i t u t e d k e t e n e s which have been p r e p a r e d i n h i g h y i e l d s i n gas phase p y r o l y s i s e x p e r i m e n t s . T h i s r e p r e s e n t s t h e f i r s t e v e r e v i d e n c e f o r t h e gas phase e x i s t e n c e of t h r e e o f t h e m o l e c u l e s and a l l o w s a d e t a i l e d i n v e s t i g a t i o n o f s u b s t i t u e n t e f f e c t s upon the k e t e n e m o l e c u l a r o r b i t a l s . - 129 -6.2. E x p e r i m e n t a l 1. M o n o s u b s t i t u t e d k e t e n e s These were a l l p r e p a r e d by t h e gas phase p y r o l y s i s of the a p p r o p r i a t e l y s u b s t i t u t e d a c e t y l h a l i d e s i n a d e h y d r o h a l o g e n a t i o n (13) r e a c t i o n s i m i l a r t o t h e one p r e v i o u s l y r e p o r t e d RCH 2COX — > RCH=C=0 + HX Vapours from a l i q u i d sample were c o n t i n u o u s l y l e d i n t o a q u a r t z tube (250mm l o n g ; 7mm ID) a t t a c h e d d i r e c t l y t o the s p e c t r o m e t e r . The c e n t r a l 120mm s e c t i o n o f the tube was hea t e d by a s i m p l e , non-i n d u c t i v e l y wound f u r n a c e which was m a i n t a i n e d a t around 700°C a t the v e r y c e n t r e . A f t e r t h e p y r o l y s i s r e g i o n ammonia gas was added t o t h e gas f l o w as r e q u i r e d , i n o r d e r t o t i t r a t e out t h e hydrogen h a l i d e , w i t h the r e m a i n i n g gas phase s p e c i e s e n t e r i n g the PE s p e c t r o m e t e r . By c a r e f u l adjustment o f f l o w r a t e , pumping and p y r o l y s i s temperature, optimum y i e l d s o f t h e ketene c o u l d be o b t a i n e d . The a c i d h a l i d e s s u c c e s s f u l l y p y r o l y s e d i n t h i s " f r e e f l o w " system were c h l b r o a c e t y l c h l o r i d e ( C H 2 C £ C 0 C £ ) , b r o m o a c e t y l bromide ( C H ^ r C O B r ) , and p r o p a n o y l c h l o r i d e ( C H 3 C H 2 C O C £ ) ( a l l o b t a i n e d from Eastman C h e m i c a l Co.) g i v i n g c h l o r o k e t e n e , bromoketene and meth y l k e t e n e r e s p e c t i v e l y . In each case a p y r o l y s i s temperature o f over 700°C was found t o be n e c e s s a r y i n o r d e r t o ensure t h a t t h e m a j o r i t y o f t h e s t a r t i n g m a t e r i a l was decomposed. However temperatures i n excess o f 800°C r e s u l t e d i n e x c e s s i v e d e c o m p o s i t i o n of t h e h a l o k e t e n e s , as e v i d e n c e d by t h e p r e s e n c e of l a r g e amounts o f CO i n the f i n a l PE spectrum. - 130 -2. D i s u b s t i t u t e d k e t e n e s D i m e t h y l k e t e n e -? was p r e p a r e d i n an analogous manner t o t h e m o n o s u b s t i t u t e d k e t e n e s . Thus p y r o l y s i s o f 2 m e t h y l p r o p a h o y l c h l o r i d e ( C H 3 ) 2 C H C O C £ ( o b t a i n e d from BDH) a t 750°C r e s u l t e d i n the f o r m a t i o n of d i m e t h y l k e t e n e and HC£ w i t h no d e t e c t a b l e s i d e p r o d u c t s . The HC£ c o u l d a g a i n be t i t r a t e d out w i t h NH^ a l l o w i n g a c l e a n PE spectrum o f the k e t e n e t o be o b t a i n e d . Attempts t o o b t a i n d i c h l o r o k e t e n e by p y r o l y s i s o f d i c h l o r o a c e t y l c h l o r i d e (CHC£ 2C0C£) were however u n s u c c e s s f u l . A l t h o u g h HC£ was e l i m i n a t e d a t temperatures i n excess of 650°C t h e o n l y s p e c i e s o b s e r v e d i n t h e PE spectrum were t e t r a c h l o r o e t h y l e n e and CO. Presumably the ketene was formed but decomposed'.immediately a t t h e s e e l e v a t e d temperatures i C H C £ 2 C 0 C £ -> [C£ C:C0] + HC£ : C C £ 2 + CO C£„C = CC£„ The same r e s u l t has p r e v i o u s l y been o b s e r v e d f o r t h e p y r o l y s i s o f (13) t r i c h l o r o a c e t a l d e h y d e (CC£ CHO) ' . An a l t e r n a t i v e p r o c e d u r e was t h e r e f o r e adopted f o r t h e p r e p a r a t i o n o f the d i h a l o k e t e n e s i n v o l v i n g a d e h a l o g e n a t i o n of a c i d h a l i d e s i n the gas phase u s i n g m e t a l l i c z i n c . T h i s r e a c t i o n was found t o o c c u r at lower temperatures ( ~500°C) than t h e s i m p l e d i r e c t p y r o l y s i s . - 131 -The g e n e r a l arrangement was t h e same as f o r t h e p y r o l y s i s experiments except t h a t "mossy" Zn ( M a l l i n c k r o d t r e a g e n t g r a d e ) , p r e v i o u s l y washed i n d i l u t e n i t r i c a c i d , f o l l o w e d by ac e t o n e , was p l a c e d i n t h e hea t e d s e c t i o n o f t h e q u a r t z t u b e . Vapours from the l i q u i d a c i d h a l i d e were then p a s s e d over t h e h e a t e d z i n c which was m o l t e n (m.p. 41 9 . 4 ° C ) , t o produce t h e ketene which was immediately pumped i n t o t h e s p e c t r o m e t e r . CX 3COX(g) + ZN(A) > X 2C=C=0(g) + ZnX (s) D i c h l o r o k e t e n e and dibromoketene were p r e p a r e d i n t h i s manner s t a r t i n g from t r i c h l o r o a c e t y l c h l o r i d e (Eastman C h e m i c a l Co.) and t r i -b r omoacetylbromide ( p r e p a r e d as d e s c r i b e d b e l o w ) . Temperatures o f 500°C o r more ensured almost complete r e a c t i o n o f t h e a c i d c h l o r i d e w h i l e s u f f i c i e n t " r e a c t i v e " Z n remained i n t h e he a t e d r e g i o n . By o p t i m i z i n g t h e f l o w r a t e and pumping speed good y i e l d s o f t h e h a l o -ketenes were o b t a i n e d . At t h e s e temperatures Zn d i f f u s e d o n l y s l o w l y out o f t h e hot zone t o condense on t h e c o o l e r r e g i o n s o f t h e q u a r t z tube. The l i m i t i n g f a c t o r t o t h e a c t i v e l i f e t i m e o f t h e Zn was i n f a c t t h e r a t e o f s u r f a c e c o n t a m i n a t i o n w i t h C from a f u r t h e r d e h a l o g e n a t i o n r e a c t i o n (see s e c t i o n 6.3). To t e s t t h e g e n e r a l a p p l i c a b i l i t y o f t h i s p r e p a r a t i v e method ketene and monochloroketene were a l s o produced by the r e a c t i o n o f Zn w i t h m o n o c h l o r o a c e t y l c h l o r i d e and d i c h l o r o a c e t y l c h l o r i d e r e s p e c t i v e l y . - 132 -CH 2C£COC£(g) + Zn(£) — - — > H 2C=C=0(g) + Z n C £ 2 ( £ ) CHG£ 2COC£(g) + Zn(£) — > HC£C=C=0(g) + ZnC£fe-,) Y i e l d s o f monochloroketene comparable w i t h t h e s i m p l e d i r e c t p y r o l y s i s of C H 2 C £ C O C £ c o u l d be o b t a i n e d ( s e e s e c t i o n 6^4). 3. P r e p a r a t i o n of t r i b r o m o a c e t y l bromide 2.5 gms of t r i b r o m o a c e t i c a c i d were d i s s o l v e d i n 25ml o f t o l u e n e and r e f l u x e d w i t h 0.3ml of PBr^ f o r two h o u r s . 3CBr 3COOH + P B r 3 > CB^COBr + H 3 P 0 3 The t o l u e n e was then d i s t i l l e d o f f (b.p. 110.6°C) u n t i l o n l y 3ml o f >; solutibn'.was l e f t i n t h e f l a s k . Most o f t h e r e m a i n i n g t o l u e n e was t h e n vacuum d i s t i l l e d o f f , tp^.leave a brown l i q u i d which was t r a n s f e r r e d onto t h e PE s p e c t r o m e t e r . The r e m a i n i n g t o l u e n e was c a r e f u l l y pumped o f f through the s p e c t r o m e t e r a l l o w i n g m o n i t o r i n g o f t h e r e s u l t a n t vapour above t h e l i q u i d . The PE spectrum e v e n t u a l l y o b t a i n e d i n d i c a t e d a h i g h p e r c e n t a g e of one compound which c o u l d be c o r r e l a t e d w i t h t h e PE spectrum o f t r i c h l o r o a c e t y l c h l o r i d e . The l i q u i d o b t a i n e d was t h e r e f o r e assumed t o be m a i n l y t r i b r o m o a c e t y l bromide a l t h o u g h t h e e x a c t p u r i t y was not determined. - 133 -6.3 R e s u l t s The H e l PE s p e c t r a o f t h e a c i d h a l i d e s used as s t a r t i n g m a t e r i a l s i n t h e s e experiments a r e shown i n F i g s . 6.1 and 6.2. These s p e c t r a a r e p r e s e n t e d f o r r e f e r e n c e o n l y and no attempt has been made t o a s s i g n them h e r e , a l t h o u g h some a c e t y l h a l i d e s have p r e v i o u s l y been s t u d i e d by PES. I n a l l cases t h e low IP r e g i o n c o n s i s t s o f a c a r b o n y l oxygen l o n e p a i r and t h e h a l o g e n l o n e p a i r s . The H e l PE s p e c t r a o f t h e m o n o s u b s t i t u t e d and d i s u b s t i t u t e d k e t e n e s a r e shown i n F i g s . 6.3 and 6.4 r e s p e c t i v e l y . The measured IP's and any a s s o c i a t e d v i b r a t i o n a l f i n e s t r u c t u r e a r e c o l l e c t e d i n T a b l e s 6.1 and 6.2. F o r t h e m e t h y l k e t e n e s t h e IP r e g i o n above 12eV c o n s i s t s o f b r o a d o v e r l a p p i n g bands and a l t h o u g h band maxima a r e g i v e n i n t h e t a b l e s , t h e s e may not be i n d i v i d u a l IP v a l u e s . The IP's f o r ketene a r e a l s o l i s t e d i n t h e t a b l e s f o r comparison. F o r t h e h a l o k e t e n e s t h e r e a r e c e r t a i n i m p u r i t i e s o b s e r v e d i n t h e s p e c t r a and t h e s e a r e marked i n the f i g s . D e c o m p o s i t i o n of mono-bromoketene i s q u i t e s e v e r e as e v i d e n c e d by t h e r e l a t i v e l y i n t e n s e (27) CO peak a t 14.01eV and peaks t h a t can be a s s i g n e d t o c i s and t r a n s /oo\ (9Q^ d i b r o m o e t h y l e n e , and bromine , ( F i g . 6.3) Br A C=C=0 : CHBr + CO ClH Br„(cis and t r a n s ) D e c o m p o s i t i o n of monochloroketene a l s o appears t o o c c u r t o some e x t e n t as CO i s a g a i n p r e s e n t i n t h e PE spectrum, a l t h o u g h t h e d i c h l o r o -- 134 -CH3CH2COCI IONIZATION POTENTIAL (eV) 6.1 H e l p h o t o e l e c t r o n s p e c t r a o f p r o p a n o y l c h l o b r o moacetyl bromide and c h l o r o a c e t y l c h l o r - 135 -HCI (CH3)2CHCOCI _l I 1 1 1-CBr3C0Br / \ I I . I I J X— _ j i _ i CCI3COCI < ' 1 1 1 J 1 1 ii 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) F i g . 6.2 H e l p h o t o e l e c t r o n s p e c t r a o f 2 m e t h y l p r o p a n o y l c h l o r i d e , t r i b r o m o a c e t y l bromide and t r i c h l o r o -a c e t y l c h l o r i d e Table 6.1. Experimental V e r t i c a l IP's (eV) for the monosubstituted ketenes "> H = C = o ( a ) C " 3 > - c - „ < » H > - C - o ( c ) H c = o ( c> (d) (d) (d) (d) 9.63 1080 (2b x) 8.91 1150 9.16 390±80 9.25 1080 (3a") 2220 2200 1080 2220 2220±100 U.2 (2b 2) 13.32 11.37 12.15 950 (8a') 2150 15.0 950 ( l b ^ 14.14 12.25 13.10 (2 a") 16.3 1020 ( l b 2 ) 15.23 13.85 14.54 950 (7a') 16.8 (4a^) 15.5 14.80 15.06 (la") 18.2 1210 ( 3 3 l ) 16.64 15.95 16.39 (6a') 17.65 17.01 17.20 (5 a') - 18.0 18.30 (4 a') a) Results quoted from r e f . 12. O r b i t a l assignments in parentheses - valence l e v e l numbering. b) F i r s t IP + 0.02eV, second IP ± 0.05eV. Results below the l i n e are band maxima and not necessarily individual IP's. c) F i r s t 4 IP's ± 0.02eV, rest ± 0.05eV. Or b i t a l assignments in patentheses - valence l e v e l -1 numbering d) Associated v i b r a t i o n a l frequencies(in cm ) of the ion (± 50cm~l unless otherwise stated). Table 6.2. Experimental V e r t i c a l IP's (eV) for the disubstituted ketenes H > H = C = o < a ) C H 3 > = CH 3 c - o ( b ) Br (c) = C = 0 Br C l C l o ( c ) (d) (d) (d) (d) 9.63 1080 (2b 1) 8.38 1270 8.90 370±60 9.08 320+60 (3b x) 2220 2220 1120 1100 2100±100 2200 1A.2 (2b 2) 12.6A 11.19 12.18 (3b 2) 15.0 950 ( l b x ) 13.56 11.58 12.53 ( l a 2 ) 10.3 1020 ( l b 2 ) 13.91 11.87 12.85 (7a x) 16.8 ( 4 a i ) 15.5 12.98 13.90 (2b x) 18.2 1210 16.15 13.99 14.92 950 (2b 2) 17.02 14.26 15.64 (Ib^) 16.04 16.76 ( e a p 16.36 17.18 ( l b 2 ) 17.68 18.13 ( 5 a i ) a) Results quoted from r e f . 12. Or b i t a l assignments in parentheses - valence l e v e l numbering. b) F i r s t IP ± 0.02eV, second IP ± 0.05eV. Results below the l i n e are band maxima and not necessarily individual IP's. c) F i r s t 6 IP's ± 0.02eV, rest 0.05eV. Or b i t a l assignments in parentheses - valence l e v e l numbering. d) Associated v i b r a t i o n a l frequencies (in cm - 1) of the ion (± 50 cm _ 1unless otherwise stat - 138 -Br H c = c = o CI H c = c = o 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) F i g . 6.3 H e l p h o t o e l e c t r o n s p e c t r a of the m o n o s u b s t i t u t e d k e t e n e s - 139 -IONIZATION POTENTIAL (eV) F i g . 6.4 H e l p h o t o e l e c t r o n s p e c t r a o f t h e d i s u b s t i t u t e d k etenes - 140 -e t h y l e n e s a r e not obse r v e d . There i s however some u n r e a c t e d c h l o r o -a c e t y l c h l o r i d e p r e s e n t i n t h e PE spectrum as e v i d e n c e d by t h e peaks at ~ 11.8 and ~ 12.5eV (s h a d e d ) . CO i s t h e o n l y major i m p u r i t y o b s e r v e d i n t h e s p e c t r a o f d i c h l o r o -k etene and i s b e l i e v e d t o be formed from a f u r t h e r d e h a l o g e n a t i o n r e a c t i o n o f t h e ket e n e . A C £ 2 C = C = 0 + Zn > Z n C £ 2 + C + CO (28) No t e t r a c h l o r o e t h y l e n e was o b s e r v e d i n t h e PE spectrum which s u g g e s t s t h a t a s i m p l e t h e r m a l d e c o m p o s i t i o n o f t h e s u b s t i t u t e d ketene (as o b s e r v e d f o r monobromoketene) i s not t a k i n g p l a c e . The o n l y major i m p u r i t y observed i n t h e PE spectrum o f dibromoketene i s i n f a c t monobromoketene. The r e a s o n f o r t h i s i s presumably an i m p u r i t y i n t h e s t a r t i n g m a t e r i a l . One p o s s i b i l i t y t o be c o n s i d e r e d i s t h e p r e s e n c e of d i b r o m o a c e t y l bromide which would be expe c t e d t o g i v e bromoketene upon r e a c t i o n w i t h Zn. A s i m p l e p y r o l y s i s o f t h e s t a r t i n g m a t e r i a l r e s u l t e d i n complete d e c o m p o s i t i o n above 700°C w i t h t h e subsequent f o r m a t i o n o f HBr as w e l l as CO and B r 2 . T h i s can i n f a c t be e x p l a i n e d i n terms o f d e c o m p o s i t i o n o f a m i x t u r e o f dibromo-and t r i b r o m o a c e t y l bromide. 7 0 0 °C CHBr 2C0Br > HBr + B r 2 + CO + C 7 o o ° r C B r 3 C 0 B r > 2 B r 2 + CO + C The o t h e r p o s s i b l e i m p u r i t y i n t h e s t a r t i n g m a t e r i a l i s t r i b r o m o -a c e t i c a c i d , e i t h e r from h y d r o l y s i s of t h e a c i d bromide o r u n r e a c t e d ° ^ C = C = 0 Br ^ ^C=C=0 H CI ^ .c=c=o | 1 1 1 1 1 1 I k • i 1 i i i i i i i * • J • i 1 i I trans C ? H ? B r ? j 1 • cis C2H2Br2 fl J ' ' ' VI 1 Y\ HBr V H i 1 537A1 . ! I 1 1 1 . . 1 , . | 1 I ' 1 1 I K —i—1.1 i i i i i i i i i 9 . 0 9.5 9.5 10.0 9.5 10.0 IONIZATION P O T E N T I A L (eV) F i g . 6.5 Expan s i o n s o f t h e f i r s t band i n the H e l p h o t o e l e c t r o n s p e c t r a o f t h e m o n o s u b s t i t u t e d ketenes E x p a n s i o n s of t h e f i r s t band i n th e H e l o f t h e d i s u b s t i t u t e d ketenes p h o t o e l e c t r o n s p e c t r a 12.5 15.0 16.0 15.0 16.0 IONIZATION POTENTIAL (eV) U J F i g . 6.7 E x p a n s i o n s o f t h e second and f o u r t h PE bands of monochloroketene and the s i x t h PE band of d i c h l o r o k e t e n e - 144 -a c i d t h a t was n e v e r c o n v e r t e d t o t h e a c i d bromide w i t h t h e PBr^. I t i s , however, d i f f i c u l t t o e n v i s a g e t h i s f o r m i n g monobromoketene upon r e a c t i o n w i t h Zn and t h e r e i s no PE e v i d e n c e f o r any o t h e r r e a c t i o n o r f o r any r e m a i n i n g , u n r e a c t e d s t a r t i n g m a t e r i a l . I t i s t h e r e f o r e f e l t t h a t t h e r e i s some d i b r o m o a c e t y l bromide i n t h e s t a r t i n g m a t e r i a l , t h e PE spectrum of which would o v e r l a p t h a t o f t r i b r o m o a c e t y l bromide thus making i t s i d e n t i f i c a t i o n r a t h e r d i f f i c u l t . The expanded s p e c t r a o f t h e f i r s t bands of the m o n s u b s t i t u t e d and d i s u b s t i t u t e d k etenes a r e shown i n F i g s . 6.5 and 6.6 r e s p e c t i v e l y . The i n t e r f e r i n g s p e c i e s a r e a g a i n i d e n t i f i e d f o r t h e bromoketenes. Monochloroketene e x h i b i t s v i b r a t i o n a l s t r u c t u r e on t h e second and fourth 1-bands and t h i s i s shown i n d e t a i l i n F i g . .6.7. D i c h l o r o k e t e n e has v i b r a t i o n a l s t r u c t u r e on t h e 6 t h band and t h i s i s a l s o shown i n F i g . 6.7. The PE spectrum o f ketene produced by d e h a l o g e n a t i o n of c h l o r o -a c e t y l c h l o r i d e i s shown i n F i g . 6.8. 6.4 D i s c u s s i o n The PE spectrum o f k e t e n e ( F i g . 6.8) has been i n t e r p r e t e d on t h e b a s i s of r e l i a b l e c a l c u l a t i o n s ^ " ' " 2 ' " ' " ^ and t h e o b s e r v e d v i b r a t i o n a l (12) f i n e s t r u c t u r e . The f i r s t and second IP's ( a d i a b a t i c t r a n s i t i o n s 9.63 and 13.84eV) aire a s s i g n e d , t o th e non-bonding (nb) ( w i t h r e s p e c t to t h e C-C-0 framework) o u t - o f - p l a n e and i n - p l a n e ^ t y p e o r b i t a l s r e s p e c t i v e l y . ( E q u i v a l e n t t o t h e 2TT ([n')' o r b i t a l i n t h e i s o e l e c t r o n i c > l i n e a r CO ) . The t h i r d and f o u r t h IP's ( a d i a b a t i c t r a n s i t i o n s 14.60 >c=o F i g . 6.8 H e l p h o t o e l e c t r o n spectrum of ketene - 146 -and 16.08eV) a r e a s s i g n e d t o t h e bonding (b) ( w i t h r e s p e c t t o t h e C-C-0 f work) o u t - o f - p l a n e and i n - p l a h e TT t y p e o r b i t a l s r e s p e c t i v e l y ( e q u i v a l e n t ) t o t h e . l ' " ' (11^). o r b i t a l i n t h e i s o e l e c t r o n i c CG^) . In t h e f o l l o w i n g d i s c u s s i o n t h e s e f i r s t f o u r ketene IP's w i l l t h e r e f o r e be r e f e r r e d to as the o u t - o f - p l a n e o r i n - p l a n e -n^ o r T o r b i t a l s f o r which t h e approximate atomic c o e f f i c i e n t s a r e shown i n F i g . 6.9. Two f u r t h e r IP's o f ketene observed i n t h e H e l r e g i o n (16.8 and 18.2eV) a r e a s s i g n e d t o a t y p e o r b i t a l s . T h i s assignment p r o v i d e s a s t a r t i n g p o i n t f o r t h e i n t e r p r e t a t i o n o f t h e PE s p e c t r a o f t h e s u b s t i t u t e d k e t e n e s f o r which a g e n e r a l comparison can be made w i t h t h a t o f t h e p a r e n t m o l e c u l e . The f i r s t band i n the PE s p e c t r a o f a l l t h e k e t e n e s i s v e r y c h a r a c t e r i s t i c i n t h a t w e l l r e s o l v e d 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 n v o l v i n g e x c i t a t i o n of two main p r o g r e s s i o n s (C=C and C=0 s t r e t c h e s ) a r e o b s e r v e d i n a l l cases:.: In ketene i t s e l f t h i s o u t - o f - p l a n e non-bonding TT o r b i t a l i s s e p a r a t e d from t h e i n - p l a n e non-bonding'TT o r b i t a l by 4.4eV l e a v i n g the 10-14eV r e g i o n c o m p l e t e l y f r e e of I P ' s . In t h e h a l o g e n a t e d k e t e n e s i t i s i n t h i s r e g i o n t h a t i o n i z a t i o n e v e n t s o r i g i n a t i n g from e s s e n t i a l l y h a l o g e n l o n e p a i r s a r e observed and so t h e r e i s no o v e r l a p p i n g of t h e h a l o g e n - t y p e IP's w i t h t h o s e ^ a s s o c i a t e d m a i n l y w i t h f t h e ketene m o i e t y . Thus the i n t e r a c t i o n s o f t h e h a l o g e n Xp l e v e l s w i t h t h e a and IT k e t e n e framework can be i n v e s t i g a t e d . F o r t h e m e t h y l k e t e n e s t h i s i s not t h e case as IP's w i t h m a i n l y CH^ p c h a r a c t e r o v e r l a p w i t h the ketene based IP's i n t h e r e g i o n above 12eV. T h i s makes i t d i f f i c u l t t o c o r r e l a t e band maxima w i t h i n d i v i d u a l IP's and to a s c e r t a i n t h e e x t e n t of p e r t u r b a t i o n of t h e k e t e n e based IP' s. o=c=o c=c=o H -C=C=0 > O) < r— U J 9-10-11 12-13-o 14-O N 1! 16-17-18-&c5 2TT G%«P G I / o u t - o f - p l a n e i n - p l a n e i n - p l a n e lTT . 6 6 0 F i g . 6.9 The i n - p l a n e and o u t - o f - p l a n e TT o r b i t a l s o f carbon d i o x i d e , k e t e n e and monochloroketene a) F o r assignments see t e x t . - 148 -These g e n e r a l f e a t u r e s i n t h e PE s p e c t r a o f t h e s u b s t i t u t e d ketenes p r o v i d e t h e b a s i s f o r a more d e t a i l e d d i s c u s s i o n o f t h e i n d i v i d u a l s p e c t r a . 6.4.1. M o n o s u b s t i t u t e d ketenes Ketene has symmetry and s i x IP's have been a s s i g n e d i n t h e H e l r e g i o n ^ " ^ ' ( T a b l e 6.1). S u b s t i t u t i o n o f a s i n g l e h a l o g e n o r me t h y l w i l l reduce the symmetry t o C g and s h o u l d i n t r o d u c e two e x t r a IP's i n t h e H e l r e g i o n . I n the C ^ v p o i n t g r o u p , o r b i t a l s t h a t a r e i n t h e p l a n e of t h e m o l e c u l e can be o f e i t h e r a^ o r b^ symmetry w h i l e o r b i t a l s t h a t a r e out of t h e p l a n e o f t h e m o l e c u l e a r e r e p r e s e n t e d by e i t h e r a^ o r b^ symmetry. I n symmetry i n - p l a n e and o u t - o f - p l a n e o r b i t a l s t r a n s f o r m as a ' and a " r e s p e c t i v e l y and t h e r e i s thus c o n s i d e r a b l e i n t e r a c t i o n of o r b i t a l s o f t h e same symmetry w i t h a r e s u l t a n t m i x i n g of c h a r a c t e r . However f o r t h e purposes o f t h i s d i s c u s s i o n t h e o r b i t a l s can be d e s c r i b e d t o a f i r s t a p p r o x i m a t i o n by t h e i r major l o c a l i z a t i o n . a ) ' C h l o r o k e t e n e ' • The H e l PE spectrum o f c h l o r o k e t e n e i s shown i n F i g . 6.3, w i t h t h e p r e s e n c e o f a l i t t l e CO b e i n g e v i d e n c e d by t h e sharp peak at 14.01eV. Some s t a r t i n g m a t e r i a l a l s o remains, t h e most i n t e n s e peaks o f which a r e due t o c h l o r i n e l o n e p a i r i o n i z a t i o n s i n the l l - 1 3 e V r e g i o n and i n t h e spectrum t h e s e f e a t u r e s a r e shaded i n . The f i r s t IP (3a") o f c h l o r o k e t e n e has c o i n c i d e n t a d i a b a t i c and v e r t i c a l t r a n s i t i o n s a t 9.25eV ( F i g . 6.5). T h i s compares w i t h a v a l u e of 9.63eV f o r t h e f i r s t IP o f ketene and i s l i k e w i s e a s s i g n e d - 149 -t o t h e o u t - o f - p l a n e Tr7, o r b i t a l . The s u b s t i t u t i o n of a c h l o r i n e f o r nb a hydrogen thus s l i g h t l y d e s t a b i l i z e s 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 o f ketene i n d i c a t i n g t h a t t h e a n t i b o n d i n g resonance i n t e r a c t i o n w i t h t h e C£ 3p o r b i t a l i s g r e a t e r than t h e i n d u c t i v e e f f e c t ( s e e F i g . 6.9). (13) The p r e v i o u s l y r e p o r t e d v e r t i c a l IP f o r c h l o r o k e t e n e i s 9.35eV which i s c l o s e t o the p o s i t i o n of t h e second v i b r a t i o n a l component (9.38eV) o f t h e band as measured i n t h i s work. The r e l a t i v e i n t e n s i t i e s o f t h e f i r s t t h r e e v i b r a t i o n a l components a r e i n f a c t v e r y s i m i l a r , and o n l y a f t e r t h e s t u d y of many s p e c t r a was i t c l e a r t h a t t h e a d i a b a t i c t r a n s i t i o n i s a l s o t h e s t r o n g e s t and t h e r e f o r e l a b e l l e d v e r t i c a l . As w i t h ketene i t s e l f , t h e o b s e r v e d s t r u c t u r e has been a s s i g n e d t o two v i b r a t i o n a l p r o g r e s s i o n s , a l t h o u g h w i t h o u t c a r e f u l e x a m i n a t i o n i t might appear o n l y one s e r i e s was p r e s e n t . The f i r s t p r o g r e s s i o n o f 1080 ± 50cm ^ i s a s s i g n e d t o an e x c i t a t i o n of t h e C=C s t r e t c h i n g f r e q u e n c y w i t h t h e second p r o g r e s s i o n of 2220 ± 50cm b e i n g a s s i g n e d t o t h e C=0 s t r e t c h i n g f r e q u e n c y . There a r e no m o l e c u l a r ground s t a t e f r e q u e n c i e s f o r c h l o r o k e t e n e but they a r e s i m i l a r t o t h o s e (12) o b s e r v e d f o r t h e f i r s t i o n i c s t a t e o f ketene and a r e t h e r e f o r e assumed t o a r i s e from s i m i l a r v i b r a t i o n a l e x c i t a t i o n s . The c o r r e s p o n d i n g band i n t h e i s o e l e c t r o n i c s p e c i e s c h l o r o a l l e n e ^ " ^ ( v e r t i c a l IP = 9.57eV) e x h i b i t s p r o g r e s s i o n s i n b o t h t h e C-C£ (720cm ^) and C=C=C (1130cm "*") s t r e t c h i n g modes and i t i s p o s s i b l e t h a t a s i m i l a r C-C£ v i b r a t i o n c o u l d be e x c i t e d i n c h l o r o k e t e n e but i s too weak t o be observed a t t h e r e s o l u t i o n of t h i s experiment (~ 40meV). The second IP o f c h l o r o k e t e n e (12.15eV) appears as a s h a r p , i n t e n s e peak i n the PE spectrum, i n d i c a t i v e o f a non-bonding o r b i t a l . - 150 -T h i s i s i n a c c o r d w i t h t h e assignment of t h i s IP t o t h e i n - p l a n e Cl l o n e p a i r ( 8 a ' ) . T h ere a r e two weak v i b r a t i o n a l p r o g r e s s i o n s o f 950 ± 50 and 2150 ± 50cm 1 a s s o c i a t e d ( F i g . 6.7) w i t h t h i s i o n i z a t i o n , but a d e f i n i t e assignment o f t h e s e f r e q u e n c i e s i s d i f f i c u l t w i t h o u t d a t a f o r t h e ground s t a t e m o l e c u l e . They a r e however, c l o s e t o t h e e x p e c t e d v a l u e s f o r t h e C=C and C=0 s t r e t c h i n g f r e q u e n c i e s which would i n d i c a t e some ( a l b e i t s m a l l ) i n t e r a c t i o n of t h i s l o n e p a i r w i t h t h e i n - p l a n e k e t e n e TT system. The t h i r d IP ( v e r t i c a l = -13.10eV) i s a s s i g n e d to t h e o u t - o f -p l a n e Cl l o n e p a i r ( 2 a " ) , and i s t h e C-C£ bonding c o u n t e r p a r t o f t h e f i r s t IP: The c o r r e s p o n d i n g band i n the PE spectrum i s s i g n i f i c a n t l y b r o a d e r than f o r t h e i n - p l a n e Cl l o n e p a i r , i n a c c o r d w i t h t h e bonding c h a r a c t e r i s t i c s . In t h e PE spectrum o f c h l o r o a l l e n e t h e i n - p l a n e Cl l o n e p a i r (11.54eV) i s a l s o much s h a r p e r than t h e c o r r e s p o n d i n g o u t -o f - p l a n e l o n e p a i r (12.87eV). The f i n a l f i v e IP's i n t h e H e l PE spectrum of c h l o r o k e t e n e can be c o r r e l a t e d w i t h IP's i n t h e spectrum of k e t e n e . The f o u r t h and f i f t h bands of c h l o r o k e t e n e ( v e r t i c a l IP's = 14.54 and 15.06eV) p a r t i a l l y o v e r l a p as do t h e second and t h i r d bands o f k e t e n e ( v e r t i c a l IP's 14.2 and 15.0eV). C a l c u l a t i o n s ^ 1 2 ' 1 ^ p r e d i c t t h e s e two IP's i n ketene t o c o r r e s -pond to t h e i n - p l a n e (2b^) non-bonding co m b i n a t i o n .of p o r b i t a l s ( a d i a b a t i c ) IP = 13.84eV) and t h e o u t - o f - p l a n e ( l b ^ ) bonding c o m b i n a t i o n of p o r b i t a l s ( a d i a t a t i c IP = 14.60eV). These o r b i t a l c h a r a c t e r i s t i c s a r e shown i n approximate form i n F i g . 6.9. The assignment g i v e n f o r c h l o r o k e t e n e i s made on t h i s b a s i s , w i t h t h e i n - p l a n e o r b i t a l ( 7 a r ) a t lower IP than - 151 -the o u t - o f - p l a n e o r b i t a l ( l a ' " ) . T h i s g i v e s a s p l i t t i n g o f 5.29eV f o r t h e o u t - o f - p l a n e and i n - p l a n e k e t e n e based TT ^  o r b i t a l s o f c h l o r o -ketene compared t o a v a l u e o f 4.55eV f o r k e t e n e . However t h e assignment of t h e 7a' and l a " o r b i t a l s i s open t o doubt as t h e v i b r a t i o n a l s t r u c t u r e observed on t h e f o u r t h band o f c h l o r o k e t e n e ( F i g . 6.7) i s v e r y s i m i l a r to t h a t observed f o r t h e o u t - o f - p l a n e bonding l e v e l o f ketene (both f r e q u e n c i e s a r e 950 ~t 50cm ^) s u g g e s t i n g t h a t t h e o r b i t a l s c o u l d be sw i t c h e d i n c h l o r o k e t e n e and t h a t t h e f o u r t h IP i s a s s o c i a t e d w i t h t h e l a " o r b i t a l . T h i s would mean t h a t the o u t - o f - p l a n e TT^ o r b i t a l was d e s t a b i l i z e d by the a d d i t i o n of c h l o r i n e , a s i t u a t i o n t h a t i s d i f f i c u l t to envisage= as t h i s i s the lowest l y i n g o u t - o f - p l a n e o r b i t a l . An assignment d i r e c t l y comparable t o ketene i s t h e r e f o r e p r e f e r r e d w i t h t h e v i b r a t i o n a l s t r u c t u r e f o r b o t h k e t e n e and c h l o r o k e t e n e i n v o l v i n g an e x c i t a t i o n o f t h e C=C s t r e t c h i n g f r e q u e n c y ( w i t h a p o s s i b i l i t y o f an u n d e r l y i n g C=0 s t r e t c h of ~ 1900cm "*") a l t h o u g h t h e y a r e a s s o c i a t e d w i t h d i f f e r e n t o r b i t a l s . In t h e i s o e l e c t r o n i c s p e c i e s , c h l o r o a l l e n e , t h e c o r r e s p o n d i n g i n -p l a n e (a') -Tr k o r b i t a l and t h e o u t - o f - p l a n e (a") TT^ o r b i t a l s a r e w e l l s e p a r a t e d h a v i n g IP's of 10.55 and - 15.1eV r e s p e c t i v e l y . The IP f o r t h e o u t - o f - p l a n e ( l a " ) TT^ o r b i t a l i n c h l o r o k e t e n e i s t h e r e f o r e v e r y s i m i l a r ( v e r t i c a l = 15.0eV) w h i l e the i n - p l a n e (7a') TT ^  o r b i t a l i s g r e a t l y s t a b i l i z e d ( v e r t i c a l IP = 14.54eV), presumably from i n c o r p o r a t i o n of s i g n i f i c a n t C=0 bonding c h a r a c t e r . By comparison w i t h ketene t h e r e m a i n i n g bands i n t h e PE spectrum of c h l o r o k e t e n e a r e a s s i g n e d t o the i n - p l a n e TT^ o r b i t a l and to two a l e v e l s . On the b a s i s o f t h e s h i f t s o b s e r v e d on r e p l a c i n g a - 152 -c h l o r i n e w i t h a bromine ( s e e l a t e r ) , t h e IP a t 16.39eV i s a s s i g n e d t o a a i o r b i t a l (6a') which has s i g n i f i c a n t C-C£ c h a r a c t e r . Thus t h e peak at 17.20eV i s a s s i g n e d . t o t h e i o n i z a t i o n from the i n - p l a n e TT^  (5a') o r b i t a l (compared t o 16.3eV i n k e t e n e ) . T h i s g i v e s a s p l i t t i n g o f t h e ketene based TT, l e v e l s i n c h l o r o k e t e n e of 2.14eV compared t o 1.3eV b i n t h e p a r e n t m o l e c u l e ( s e e F i g . 6.9). T h i s l e a v e s t h e f i n a l weak f e a t u r e a t 18.30eV t o be a s s i g n e d t o a a o r b i t a l (4a') located'.mainly on t h e C-C-0 framework. The e q u i v a l e n t band of ke t e n e (18.2eV) i s a l s o o f low i n t e n s i t y i n t h e H e l spectrum s u g g e s t i n g c o n s i d e r a b l e s c h a r a c t e r . I n c h l o r o k e t e n e t h e s e l a s t t h r e e IP's ( 6 a ' , 5a', 4a') w i l l i n v o l v e c o n s i d e r a b l e m i x i n g as they a r e a l l o f t h e same symmetry and t h i s i s e v i d e n c e d by t h e near e q u a l s p a c i n g s o f the c o r r e s p o n d i n g bands i n t h e PE spectrum. T h i s i s a l s o o b served i n c h l o r o a l l e n e where t h e 3 IP's o c c u r a t 15.1, 16.1 and ~ 17.2eV w i t h t h e assignment b e i n g d i r e c t l y , n (30) comparable b) Bromoketene The H e l PE spectrum o f bromoketene i s shown i n F i g . 6.3 but i n c l u d e s f e a t u r e s from s e v e r a l . ' . i m p u r i t i e s . CO i s p r e s e n t t o a g r e a t e r e x t e n t than i n t h e case f o r c h l o r o k e t e n e , w i t h t h e peak a t 14.01eV b e i n g r e l a t i v e l y i n t e n s e and t h e s t r u c t u r e d TT^ band w i t h a v e r t i c a l IP a t 16.91eV a l s o q u i t e o b v i o u s . F o r bromoketene t h e f i r s t band - 153 -( a d i a b a t i c IP = 9.16eV) i s not f u l l y r e s o l v e d due to o v e r l a p w i t h t h e f i r s t bands of b o t h c i s d i b r o m o e t h y l e n e ( a d i a b a t i c and v e r t i c a l IP's (28) of 9'.50 and 9.63eV r e s p e c t i v e l y ) and t r a n s d i b r o m o e t h y l e n e ( a d i a b a t i c (28) and v e r t i c a l IP's of 9.44 and 9.55eV r e s p e c t i v e l y ) . T h i s i s shown i n d e t a i l i n F i g . 6.5 where t h e v i b r a t i o n a l p r o g r e s s i o n s ,of t h e v a r i o u s s p e c i e s a r e i n d i c a t e d . Peaks due t o e x c i t a t i o n o f t h e f i r s t i o n i c 2 2 s t a t e o f HBr ( II, and ITqy/) by t h e H e l 3 l i n e a r e a l s o p r e s e n t i n t h i s spectrum, a r e s u l t o f not removing t h e h a l o g e n a c i d when i n v e s t i g a t i n g t h i s r e g i o n o f t h e spectrum. The bromoethylenes have t h e i r most i n t e n s e peaks i n t h e h a l o g e n l o n e p a i r r e g i o n w i t h t h e c i s isomer h a v i n g IP's a t 10.74, 11.23, 11.53 and 12.86eV and the t r a n s isomer at 11.04 ( d e g e n e r a t e ) , 11.57 and 12.90eV. A f u r t h e r c o m p l i c a t i o n i n t h i s r e g i o n i s produced from IP's 2 2 + a s s o c i a t e d w i t h t h e . Ilq. and IL s t a t e s o f B r . a t 10.51 and 10.86eV 72 h 2 r e s p e c t i v e l y . A l l t h e f e a t u r e s due t o i m p u r i t i e s i n t h e spectrum o f bromoketene . a r e shaded i n F i g . 6.3b w i t h the;:weak broad peak around 13eV h a v i n g c o n t r i b u t i o n s from b o t h t h e bromoethylenes and bromine The a c t u a l assignment f o r t h e PE spectrum of bromoketene f o l l o w s d i r e c t l y from t h a t g i v e n f o r c h l o r o k e t e n e . The f i r s t IP (3a") has c o i n c i d e n t a d i b a t i c and v e r t i c a l t r a n s i t i o n s a t 9.16eV, o n l y s l i g h t l y lower than f o r c h l o r o k e t e n e (9.25eV). T h i s i s t o be e x p e c t e d from comparison w i t h t h e i s o e l e c t r o n i c b r o m o a l l e n e w h i c h has a f i r s t IP of 9.46eV,only O.leV l e s s than c h l o r o a l l e n l ^ ^ T h e comparisons made f o r c h l o r o a l l e n e and c h l o r o k e t e n e a r e i n f a c t g e n e r a l l y a p p l i c a b l e t o thebromo:analogues and w i l l thus not be d i s c u s s e d a g a i n . - 154 -As e x p e c t e d t h e f i r s t IP o f bromoketene i s thus lower than f o r k e t e n e and a g a i n i n d i c a t e s t h e a n t i b o n d i n g i n t e r a c t i o n o f t h e h a l o g e n 4p o r b i t a l w i t h t h e ketene o u t - o f - p l a n e TT system. A l t h o u g h t h e p r e s e n c e of t h e bromoethylenes does not a l l o w complete r e s o l u t i o n o f t h i s band ( F i g . 6.5) i t can be:.seen t h a t two main v i b r a t i o n a l p r o g r e s s i o n s a r e e x c i t e d i n t h e k e t e n e . The observed f r e q u e n c i e s o f 1080 ± 50 and 2220 ± 100cm - 1 can a g a i n be a s s i g n e d t o the C=C and C=0 s t r e t c h i n g modes. There i s a l s o e v i d e n c e f o r a n o t h e r much weaker p r o g r e s s i o n of 390 ± 80cm 1 which i s p r o b a b l y due t o a Br-C-H bending mode. The second and t h i r d IP's o f bromoketene (11.37 and 12.25eV) a r e the i n - p l a n e (8a') and o u t - o f - p l a n e (2a") bromine l o n e p a i r s showing a s h i f t o f ~ 0.8eV from the c o r r e s p o n d i n g IP's o f c h l o r o k e t e n e . The non-bonding n a t u r e o f t h e i n - p l a n e l o n e p a i r i s a g a i n i n d i c a t e d by t h e sharp PE band w h i l e t h e o u t - o f - p l a n e l e v e l g i v e s a much b r o a d e r band i n d i c a t i v e o f i t s bonding c h a r a c t e r i s t i c s . The i n - p l a n e IT ^ o r b i t a l (7a') g i v e s r i s e t o t h e IP a t 13.85eV b u t u n l i k e 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 o f c h l o r o k e t e n e (14.54eV) no v i b r a t i o n a l s t r u c t u r e i s observed.' T h i s may be p a r t l y due t o t h e o v e r l y i n g p r e s e n c e of t h e CO peak at 14.01eV. The s p l i t t i n g o f t h e k e t e n e b a s e d l 7 7 . ^ o r b i t a l s f o r bromoketene i s thus 4.69eV, which i s l e s s than i n c h l o r o k e t e n e but g r e a t e r than i n k e t e n e . The o u t - o f - p l a n e o r b i t a l ( l a " ) appears as a b r o a d band c e n t r e d a t 14.80eV, s h i f t e d by -0.26eV from t h e c o r r e s p o n d i n g IP i n c h l o r o k e t e n e . The i n - p l a n e TT.; o r b i t a l (5a') i s 2 a s s i g n e d t o t h e band a t 17.01eV which i s o v e r l a p p e d by t h e s t a t e o f CO-. T h i s r e p r e s e n t s a s h i f t o f -0.19eV from the c o r r e s p o n d i n g band - 155 -i n c h l o r o k e t e n e and g i v e s a v a l u e of 2.21eV f o r the s p l i t t i n g o f t h e k e t e n e based TC, o r b i t a l s i n bromoketene ( c f . t o 2.14eV i n c h l o r o -b k e t e n e ) . The peak at 15.95eV i s a s s i g n e d t o t h e a o r b i t a l w i t h c o n s i d e r a b l e C-Br c h a r a c t e r (6a')> a s h i f t o f -0.46eV r e l a t i v e t o e c h l o r o k e t e n e . T h i s l e v e l thus shows a g r e a t e r s h i f t upon changing t h e h a l o g e n than does the 7a' l e v e l and i s t h e r e a s o n f o r a s s i g n i n g t h e 6a' and la' l e v e l s as g i v e n . There i s some e v i d e n c e f o r t h e 5a' i o n i z a t i o n a t around 18.0eV a l t h o u g h t h i s f e a t u r e i s even weaker than i n t h e spectrum of c h l o r o k e t e n e . c) M e t h y l k e t e n e M e t h y l k e t e n e i s t h e most s t a b l e o f t h e m o n o s u b s t i t u t e d k e t e n e s s t u d i e d h e r e w i t h a p p a r e n t l y no d e c o m p o s i t i o n a t t h e r e a c t i o n temperatures as e v i d e n c e d by t h e absence of CO i n t h e PE spectrum ( F i g . 6.3). However, as mentioned p r e v i o u s l y t h e spectrum i s c o m p l i c a t e d by o v e r -l a p p i n g o f bands i n the IP r e g i o n above 12eV. Only t h e f i r s t band w i t h c o i n c i d e n t a d i a b a t i c and v e r t i c a l t r a n s i t i o n s a t 8.91eV i s o b v i o u s l y due t o a s i n g l e i o n i z a t i o n event ( F i g . 6.5). As w i t h t h e o t h e r k e t e n e s t h i s i s a s s i g n e d t o t h e o u t - o f - p l a n e j r . ^ l e v e l (3a") which r e p r e s e n t s a s h i f t of -0.77eV r e l a t i v e t o t h e c o r r e s p o n d i n g IP o f k e t e n e , i n a c c o r d w i t h t h e i n d u c t i v e e f f e c t of a methyl group. Two v i b r a t i o n a l p r o g r e s s i o n s h a v i n g f r e q u e n c i e s o f 1150 ± 50 and 2200 ± 50cm ^ r e s p e c t i v e l y a r e r e s o l v e d . By analogy w i t h o t h e r ketenes t h e s e v a l u e s can be a s s i g n e d t o C=C and C=0 s t r e t c h i n g f r e q u e n c i e s a l t h o u g h a g a i n no v i b r a t i o n a l d a t a i s a v a i l a b l e f o r the ground s t a t e m o l e c u l e . The f i r s t IP - 156 -of methylketene had previously been reported as 8.95eVVXJ>' in good agreement with the value obtained here. Assignment of the rest of the spectrum is d i f f i c u l t although i t seems reasonable that the band with a maximum at around 13.3eV could be the in-plane T T ^ ionization. This would give a splitting of ~ 4.4eV for the ketene based .IT ^  orbitals in methylketene which compares to 4.55eV for ketene i t s e l f . As a total of 8 IP's is expected in the Hel region this leaves a total of 6 IP's (four a' and two a" levels) to be assigned to the bands observed between 13.5 and 18.0eV. It i s possible the weak band at 17.65eV could be associated with the ionization from the a orbital with mainly C-C-0 character (5a f) as this has been observed to have a low cross-section in the Hel spectra of the other ketenes discussed so far. No assignment i s offered for the rest of the spectrum because of the severe overlap of bands. However by comparison with the other isoelectronic ketenes a total of. 8 IP's is expected under 19eV. 6.4.2 Disubstituted ketenes Addition of another halogen or methyl group to the monosubstituted ketenes introduces a further two IP's into the Hel region giving a total of ten IP's. The assignment of the spectra i s made by comparison with the corresponding monsubstituted species and the parent ketene. The symmetry of the disubstituted ketenes studied here is the same as for ketene (C^y) a n t^ thus the orbital symmetries are directly comparable. a) Dichloroketene The PE spectrum of the dichloro derivative i s shown in Fig. 6.4 with the only observable impurity being due to CO (vertical IP's of - 157 -14.01 and 16.91eV). As mentioned i n s e c t i o n 6.3 t h i s i s b e l i e v e d t o be due to the Zn r e a c t i n g f u r t h e r w i t h the d i c h l o r o k e t e n e . The f i r s t IP o f d i c h l o r o k e t e n e (3b^) has c o i n c i d e n t a d i a b a t i c and v e r t i c a l t r a n s i t i o n s a t 9.08eV ( F i g . 6.6) which r e p r e s e n t s a s h i f t of -0.17eV from t h e f i r s t IP o f c h l o r o k e t e n e . T h i s i s i n agreement w i t h the assignment t o t h e o u t - o f - p l a n e TT ^ o r b i t a l somewhat d e s t a b i l i z e d by t h e i n t r o d u c t i o n o f a n t i b o n d i n g C£ 3p c h a r a c t e r ( c f . k e t e n e 2b^ i o n i z a t i o n a t 9.63eV). The Franck-Condon envelopes o f t h e f i r s t bands of c h l o r o k e t e n e and d i c h l o r o k e t e n e a r e a l s o v e r y s i m i l a r , i n t h a t i t might appear t h a t o n l y one main v i b r a t i o n a l p r o g r e s s i o n was e x c i t e d . However by comparison w i t h t h e o t h e r ketenes t h e 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 a s s i g n e d t o t h e C=C and C=0 s t r e t c h i n g f r e q u e n c i e s (1100 ± 50 and 2200 ± 50cm 1 r e s p e c t i v e l y ) . The o n l y m o l e c u l a r ground s t a t e f r e q u e n c y r e p o r t e d ^ ^ i s t h e C=0 s t r e t c h a t 1940cm which i s s i g n i f i c a n t l y lower t h a n the observed ground s t a t e i o n i c v a l u e . T h i s s u g g e s t s t h i s o r b i t a l i s C=0 a n t i b o n d i n g (and presumably, t h e r e f o r e C=C bonding) as i s t h e case i n ketene ( F i g . 6.9). A weaker p r o g r e s s i o n w i t h a f r e q u e n c y o f 320 ± 60cm 1 a l s o appears t o be e x c i t e d h e r e and t h i s most l i k e l y c o r r e s p o n d s t o a C C ^ bending mode ( c f . C C ^ bend of 285cm" 1 i n C £ 2 C O ( 3 1 ) ) • The next f o u r IP's o f d i c h l o r o k e t e n e ( v e r t i c a l v a l u e s of 12.18, 12.53, 12.85 and 13.90eV) can be a s s i g n e d t o e s s e n t i a l l y C£ l o n e p a i r s . I n C^ v symmetry t h e s e t r a n s f o r m as a^, a^, b ^ and b^. The a^ l e v e l i s expected to be t h e most non-bonding as t h i s symmetry r e p r e s e n t a t i o n - 158 -i s u n i q ue. In t h e PE spectrum t h e peak a t 12.53eV i s t h e s h a r p e s t and i s thus a s s i g n e d t o t h e l a ^ i o n i z a t i o n . S i m i l a r l y t h e b r o a d e s t o f t h e f o u r peaks a s s i g n e d t o t h e C£ l o n e p a i r s i s the one a t 13.90eV and i s thus a s s i g n e d t o t h e 2b^ i o n i z a t i o n , w hich i s the C-CSL bonding c o u n t e r p a r t o f t h e f i r s t IP. T h i s i s o v e r l a p p e d by t h e sharp peak a t 14.01eV due to CO. I n o r d e r t o maximize '. t h e s e p a r a t i o n o f o r b i t a l s of t h e same symmetry t h e bands at 12.18 and 12.85 a r e a s s i g n e d t o t h e 3 b 2 and 7a^ i o n i z a t i o n s r e s p e c t i v e l y . The next band o f d i c h l o r o k e t e n e ( a d i a b a t i c and v e r t i c a l IP's o f 14.53 and 14.92eV r e s p e c t i v e l y ) has a v e r y s i m i l a r Franck-Condon envelope t o t h e f o u r t h band o f c h l o r o k e t e n e a t 14.54eV ( F i g . 6.7) and i s l i k e w i s e a s s i g n e d t o t h e i o n i z a t i o n from t h e i n - p l a n e : TT o r b i t a l (2b^) . The v i b r a t i o n a l p r o g r e s s i o n o f 950 ± 50cm 1 i s t h e r e f o r e a s s i g n e d t o an e x c i t a t i o n of t h e C=C s t r e t c h . T h i s g i v e s a s p l i t t i n g o f 5.84eV f o r t h e ketene based TT l e v e l s i n d i c h l o r o k e t e n e compared nb t o 5.29eV i n c h l o r o k e t e n e and 4.55eV. i n th e u n s u b s t i t u t e d k e t e n e . The peak a t 15.64eV by ana l o g y t o k e t e n e and c h l o r o k e t e n e i s thus a s s i g n e d t t h e o u t - o f - p l a n e TT^ i o n i z a t i o n (lb-^) w i t h the c o r r e s p o n d i n g i n - p l a n e i o n i z a t i o n (lh^) a s s i g n e d t o t h e peak at 17.18eV. T h i s g i v e s a s p l i t t i n o f t h e k e t e n e based o r b i t a l s o f 1.54eV which i s i n t e r m e d i a t e between t h e v a l u e s f o r c h l o r o k e t e n e (2.14eV) and k e t e n e (1.3eV). T h i s l e a v e s t h e peak a t 16.76eV to be a s s i g n e d t o t h e a o r b i t a l w i t h s i g n i f i c a n t C-CZ bonding c h a r a c t e r (6a^) which compares t o a v a l u e o f 16.39eV i n c h l o r o k e t e n e . The f i n a l band at 18.13eV i s t h e r e f o r e t h e i o n i z a t i o n - 159 -from the C-C-0 bonding o r b i t a l (5a') h a r d l y 'changed from the v a l u e i n k etene (18.2eV). I t i s however s i g n i f i c a n t l y more i n t e n s e i n c h l o r o k e t e n e than i n e i t h e r ketene o r c h l o r o k e t e n e which may be a r e s u l t o f i n c o r p o r a t i o n o f C£ 3s and 3p c h a r a c t e r i n s t e a d of H I s . b) Dibromoketene The H e l PE spectrum o f dibromoketene i s shown i n F i g . 6.4. CO^ i s p r e s e n t as an i m p u r i t y as e v i d e n c e d by t h e sharp peak a t 18.08eV (£ ^ + ) w i t h the lower IP's a l l b e i n g o v e r l a p p e d by bands . o f dibromoketene. The major i m p u r i t y however i s bromoketene, the PE spectrum of which has been d i s c u s s e d above. The most o b v i o u s c o n t r i b u t i o n s from t h i s mono-s u b s t i t u t e d ketene a r e t h e sharp peak a t 11.37eV ( 8 a 7 ) and t h e b r o a d e r band a t 12.25eV (2a") which were b o t h a s s i g n e d t o bromine l o n e p a i r s . At lower IP t h e f i r s t band of bromoketene (9.16eV) o v e r l a p s the f i r s t band of dibromoketene ( a d i a b a t i c IP = 8.90eV) which i s thus p o o r l y r e s o l v e d ( F i g . 6.6). Other f e a t u r e s t h a t can be a t t r i b u t e d t o bromo-ket e n e a r e t h e broad band a t l 4 . 8 0 e V ( l a " ) and t h e i l l - d e f i n e d band a t 17.01eV(5a'). IP's of bromoketene a t 13.85 and 15.95eV o v e r l a p ones a s s i g n e d t o dibromoketene r e s u l t i n g i n i n c r e a s e d i n t e n s i t i e s f o r t h e c o r r e s p o n d i n g PE bands. The i m p u r i t y bands b e i n g i d e n t i f i e d , t h e PE spectrum of dibromo-ketene can be a s s i g n e d by c o r r e l a t i o n w i t h t h a t o f d i c h l o r o k e t e n e . The f i r s t band, as mentioned, i s c o m p l i c a t e d by the p r e s e n c e of bromoketene but i f t h i s i s taken i n t o c o n s i d e r a t i o n t h e a d i a b a t i c - 160 -t r a n s i t i o n (8.90eV) a l s o appears to be the v e r t i c a l t r a n s i t i o n . T h i s i s a d e c r e a s e of 0.26eV r e l a t i v e t o bromoketene, a g a i n i n d i c a t i n g t h e C-X a n t i b o n d i n g n a t u r e o f t h i s o r b i t a l ( 3 b ^ ) . The v i b r a t i o n a l s t r u c t u r e i s a t t r i b u t e d t o the e x c i t a t i o n o f t h e C=C and C=0 s t r e t c h i n g modes (1120 1 50 and 2100 ± 100cm" 1 r e s p e c t i v e l y ) and a C B r 2 bending mode (370 1 60cm 1.) a l t h o u g h above 9.16eV the p r o g r e s s i o n s o v e r l a p t h e f i r s t band o f bromoketene. There i s a s h o u l d e r on t h e low IP s i d e o f t h e a d i a b a t i c t r a n s i t i o n o f dibromoketene e q u i v a l e n t t o an energy d i f f e r e n c e o f ~ 350cm 1 and i s p o s s i b l y a hot band of the b e n d i n g mode. The weaker p r o g r e s s i o n i n t h e b e n d i n g mode of the i o n i s q u i t e d i s t i n c t i n t h i s case and tends t o g i v e c r e d e n c e t o t h e s u g g e s t i o n t h a t s i m i l a r b e n d i n g modes a r e e x c i t e d i n the c o r r e s p o n d i n g i o n i z a t i o n s o f d i c h l o r o -ketene and bromoketene. The o n l y m o l e c u l a r ground s t a t e f r e q u e n c y r e p o r t e d i s a g a i n the C=0 s t r e t c h (1970cm "*") and t h e v a l u e o b s e r v e d h e r e i n d i c a t e s the C=0 a n t i b o n d i n g n a t u r e of t h i s o r b i t a l . The f o u r IP's a t t r i b u t a b l e t o dibromoketene i n t h e r e g i o n 11-13.5eV a r e a s s i g n e d t o t h e bromine l o n e p a i r s , w i t h the o r d e r i n g b e i n g d i r e c t l y analogous to the c h l o r i n e l o n e p a i r s i n d i c h l o r o k e t e n e . Thus the bands a t 11.19, 11.58, 11.87 and 12.98eV a r e a s s i g n e d t o the 3 b 2 > l a 2 , 7a^ and 2b^ o r b i t a l s r e s p e c t i v e l y and a l l show a d e c r e a s e o f j u s t under l.OeV from t h e c o r r e s p o n d i n g bands i n d i c h l o r o k e t e n e (12.18, 12.53, 12.85 and 13.90eV). The non-bonding and bonding c h a r a c t e r i s t i c s o f t h e l a 2 and 2b^ l e v e l s r e s p e c t i v e l y a r e a g a i n i n d i c a t e d by the Franck-Condon envelopes of t h e c o r r e s p o n d i n g PE bands. - 161 -The next band o f dibromoketene has a maximum a t 13.99eV w i t h a s h o u l d e r a t 14.26eV and i s a s s i g n e d t o two i o n i z a t i o n e v e n t s (2b^ and l b ^ r e s p e c t i v e l y , a l t h o u g h the p r o x i m i t y of t h e IP's c o u l d mean t h a t t h e s e a r e s w i t c h e d ) . The i n - p l a n e •. n' i o n i z a t i o n (2b„, 13.99eV) nb z g i v e s a s p l i t t i n g o f t h e ketene based 'TC IP's i n dibromoketene of 5.09eV, somewhat i n c r e a s e d from the v a l u e observed f o r bromoketene (4.69eV). An i n c r e a s e i n t h e s p l i t t i n g was a l s o observed i n g o i n g from c h l o r o k e t e n e t o d i c h l o r o k e t e n e . The l b ^ i s t h e o u t - o f - p l a n e IT i o n i z a t i o n w i t h the c o r r e s p o n d i n g i n - p l a n e i o n i z a t i o n ( I t ^ ) b e i n g one of two i o n i z a t i o n s a s s i g n e d t o the next band i n the PE spectrum. Under c a r e f u l e x a m i n a t i o n t h e r e a r e two maxima i n t h i s band (16.04 and 16.36eV), but a g a i n any assignment t o l e v e l s so c l o s e must be open t o doubt. To be c o n s i s t e n t w i t h d i c h l o r o k e t e n e t h e IP a t 16.04eV i s a s s i g n e d t o t h e 6a^ l e v e l (a,with C-Br c h a r a c t e r ) and t h e one at 16.36eV to t h e l e v e l . T h i s g i v e s a s p l i t t i n g o f 2.10eV f o r t h e ketene based IP's o f dibromoketene s l i g h t l y reduced from the v a l u e o f 2.21eV a r r i v e d a t f o r bromoketene. T h i s s p l i t t i n g was a l s o l e s s i n d i c h l o r o k e t e n e compared t o c h l o r o k e t e n e . F i n a l l y ".the peak at 17.68eV i s a s s i g n e d t o t h e o t h e r a l e v e l (5a') which" i s C-C-0 b o n d i n g . c) D i m e t h y l k e t e n e The PE spectrum of d i m e t h y l k e t e n e ( F i g . 6.4) was o b t a i n e d f r e e from i m p u r i t i e s i n d i c a t i n g r e l a t i v e s t a b i l i t y compared t o the h a l o -d e r i v a t i v e s , b u t as w i t h m ethylketene t h e r e i s c o n s i d e r a b l e o v e r l a p of bands above 12eV r e s u l t i n g i n o n l y a l i m i t e d assignment. - 162 -The f i r s t band ( v e r t i c a l and a d i a b a t i c IP's = 8.38eV) "shows the c h a r a c t e r i s t i c v i b r a t i o n a l p r o g r e s s i o n of i o n i z a t i o n from t h e o u t -o f - p l a n e if k o r b i t a l (3b^) . The o b s e r v e d f r e q u e n c i e s f o r t h e C=C and C=0 s t r e t c h e s i n t h e i o n a r e 1270 ± 50 and 2200 ± 50cm 1 r e s p e c t i v e l y ( F i g . 6.6). F o r t h i s m o l e c u l e t h e r e a r e n e u t r a l ground s t a t e f r e q u e n c i e s a v a i l a b l e f o r comparison: The corresponding" C=C and C=0 s t r e t c h i n g f r e q u e n c i e s a r e 1392 and 2134cm - 1 r e s p e c t i v e l y f 1 - \ • I t t h e r e f o r e .appears t h a t t h e 3b^ o r b i t a l i n d i m e t h y l k e t e n e i s C=C bonding and C=0 a n t i b o n d i n g , w i t h t h e C=C s t r e t c h reduced i n the i o n and t h e C=0 s t r e t c h i n c r e a s e d . (12) T h i s i s as e x p e c t e d by comparison w i t h the 2b^ o r b i t a l of k e t e n e The IP i s s h i f t e d by -0.53eV compared t o th e c o r r e s p o n d i n g IP of m e t h y l -ketene i n a c c o r d w i t h t h e i n d u c t i v e ; e f f e c t o f an e x t r a m e t h y l group. The v a l u e o f t h e f i r s t IP g i v e n h e r e i s i n agreement w i t h t h a t p r e v i o u s l y r e p o r t e d The n e x t band i n t h e spectrum (12.64eV) i s t e n t a t i v e l y a s s i g n e d t o the i n - p l a n e ir- , i o n i z a t i o n (3b„) t o g i v e a s p l i t t i n g o f t h e ketene nb z based TT , o r b i t a l s i n d i m e t h y l k e t e n e of 4.26eV, s l i g h t l y reduced from nb th e p roposed v a l u e o f 4.4eV i n m e t h y l k e t e n e . By comparison w i t h m e t h y l k e t e n e t h e peak a t 17.02eV c o u l d be a s s i g n e d to t h e i o n i z a t i o n from the a o r b i t a l which i s C-C-0 bonding. The remainder o f t h e spectrum c o n s i s t s o f o v e r l a p p i n g bands and i s not a s s i g n e d , but i n t o t a l s h o u l d comprise 7 I P ' s . The proposed assignment f o r t h e s u b s t i t u t e d k etenes i s summarized i n t h e c o r r e l a t i o n diagram shown i n F i g . 6.10 where the g e n e r a l c o r r e s p o n d e n c e of t h e s p e c t r a i s demonstrated. The r e s u l t s f o r ketene a r e t h o s e of t h e p r e v i o u s l y p u b l i s h e d w o r k ^ 1 2 ^ . IONIZATION POTENTIAL (eV) C D a* £ o 0 0 n 1 i i i i i i i i r - G9I -- 164 -6.5. Conclusion^ Mono- and d i s u b s t i t u t e d k e t e n e s have been o b t a i n e d i n t h e gas phase i n good y i e l d s a l l o w i n g t h e i r PE s p e c t r a t o be r e c o r d e d . A s i m p l e t h e r m a l d e h y d r o h a l o g e n a t i o n of s u i t a b l y s u b s t i t u t e d a c i d h a l i d e s was used t o o b t a i n monochloro-,. monobromo-, monomethyl- and m i m e t n y l -d e r i v a t i v e s o f k e t e n e but t h e temperatures i n v o l v e d i n such p y r o l y s i s (~ 700°C)resulted i n some d e c o m p o s i t i o n o f t h e h a l o g e n a t e d k e t e n e s . The PE spectrum o f d i c h l o r o k e t e n e c o u l d not i n f a c t be o b t a i n e d i n t h i s manner,with o n l y i t s d e c o m p o s i t i o n p r o d u c t s b e i n g o b s e r v e d . A more v e r s a t i l e p r e p a r a t i v e method f o r t h e s u b s t i t u t e d k e t e n e s appears t o be t h e d e h a l o g e n a t i o n of a c i d c h l o r i d e s u s i n g z i n c . The r e a c t i o n temperature (~ 500°C) i s s i g n i f i c a n t l y lower t h a n t h e p y r o l y s i s experiments a l l o w i n g b o t h d i c h l o r o - and dibromoketene t o be o b t a i n e d i n good y i e l d s by t h e d e h a l o g e n a t i o n p r o c e s s . To demonstrate t h e o v e r a l l a p p l i c a b i l i t y of t h e r e a c t i o n , m o n o c h l o r o k e t e n e and k e t e n e were a l s o p r e p a r e d i n t h i s manner. For "gas phase s t u d i e s t h e d e h a l o g e n a t i o n r e a c t i o n a l s o has t h e advantage t h a t t h e ketene i s t h e o n l y v o l a t i l e p r i m a r y p r o d u c t w i t h no e v i d e n c e f o r t h e z i n c h a l i d e s b e i n g seen i n the PE s p e c t r a . The PE s p e c t r a o f t h e k e t e n e s a r e c h a r a c t e r i z e d by t h e f i r s t band which e x h i b i t s two main v i b r a t i o n a l p r o g r e s s i o n s i n a l l c a s e s (C=C and C=0 s t r e t c h e s ) and c o r r e s p o n d s t o i o n i z a t i o n from t h e ketene o u t -o f - p l a n e T o r b i t a l . T h i s i s d e s t a b i l i z e d i n t h e h a l o g e n a t e d ketenes due t o i n c o r p o r a t i o n o f a n t i b o n d i n g h a l o g e n p c h a r a c t e r . - 165 -The assignment g i v e n f o r t h e PE s p e c t r a o f t h e s u b s t i t u t e d k e t e n e s i s based on t h e i n t e r p r e t a t i o n o f t h e spectrum of t h e p a r e n t m o l e c u l e and on t h e o b s e r v e d s h i f t s i n IP's f o r t h e s p e c i e s s t u d i e d . F o r the h a l o g e n a t e d k e t e n e s a f u l l c o r r e l a t i o n of IP's i n t h e H e l r e g i o n i s p o s s i b l e as i o n i z a t i o n events from o r b i t a l s l o c a t e d m a i n l y on t h e k e t e n e moiety and t h o s e l o c a t e d m a i n l y on t h e s u b s t i t u e n t s a r e w e l l s e p a r a t e d . T h i s i s not t h e c a s e f o r t h e m e t h y l d e r i v a t i v e s where o v e r l a p o f IP's i s a s e v e r e problem. - 166 -CHAPTER SEVEN  FORMIC ANHYDRIDE AND OTHER ALIPHATIC ANHYDRIDES 7.1 I n t r o d u c t i o n Compounds c o n t a i n i n g c a r b o n y l f u n c t i o n a l groups have been w i d e l y i n v e s t i g a t e d by PES t o g i v e i n f o r m a t i o n c o n c e r n i n g e l e c t r o n i c i n t e r a c t i o n s . The s p l i t t i n g o f t h e oxygen l o n e p a i r IP's i n d i c a r b o n y l compounds p r o v i d e s a p a r t i c u l a r l y c o n v e n i e n t measure of t h e t h r o u g h bond i n t e r a c t i o n s The a - d i c a r b o n y l s , where the j u x t a p o s i t i o n of t h e two c a r b o n y l groups might be expected t o g i v e l a r g e i n t e r a c t i o n s , have been e x t e n s i v e l y (2-5) s t u d i e d by PES . F o r t h e s i m p l e s t c a s e , g l y o x a l , much t h e o r e t i c a l work has a l s o been c a r r i e d out i n an attempt t o r a t i o n a l i z e t h e r e l a t i v e c o n t r i b u t i o n s o f t h e t h r o u g h space and t h r o u g h bond m e c h a n i s m s ^ . Data on 3 - d i c a r b o n y l s i s more s p a r s e , a l t h o u g h as might be e x p e c t e d , t h e i n t e r a c t i o n s have g e n e r a l l y been found t o be l e s s than t h o s e i n (9) o t - d l c a r b o n y l s . S i m i l a r l y the ^ - d i c a r b o n y l s i n v e s t i g a t e d show (9) l i t t l e i n t e r a c t i o n between the oxygen l o n e p a i r s C a r b o n y l compounds t h a t c o n t a i n o t h e r f u n c t i o n a l oxygen atoms g i v e a more c o m p l i c a t e d PE; spectrum i n t h e r e g i o n of oxygen l o n e p a i r s ( t y p i c a l l y 10-13eV). For i n s t a n c e , t h e s i m p l e s t c a r b o x y l i c a c i d , HCOOH and i t s a l k y l a t e d d e r i v a t i v e s have been found t o have a non-bonding ( n Q ) o r b i t a l ( i n the p l a n e of the m o l e c u l e ) from the c a r b o n y l oxygen, and an a n t i b o n d i n g (TT^) o r b i t a l ( o u t - o f - p l a n e ) from the h y d r o x y l oxygen , . , , . (10111) which a r e c l o s e i n energy The PE spectrum of o r g a n i c a n h y d r i d e s would be e x p e c t e d t o p r e s e n t t h e s e d i f f i c u l t i e s because o f t h e p r e s e n c e of a non c a r b o n y l oxygen. - 167 -Only two such s p e c i e s have p r e v i o u s l y been s t u d i e d by PES, t h e c y c l i c , - t , . . , . . (12,13,14) , ,. a n h y d r i d e s m a l e i c and s u c c i n i c and problems m assignments were enco u n t e r e d . F o r m i c a n h y d r i d e i s the s i m p l e s t a c y c l i c a n h y d r i d e but decomposes r e a d i l y a t room temperature t o g i v e f o r m i c a c i d and carbon monoxide. There has been r e c e n t i n t e r e s t i n t h i s a n h y d r i d e r e g a r d i n g i t s p o s s i b l e i n t e r m e d i a c y i n the r e a c t i o n of f o r m i c a c i d on s u r f aces . The PE i n v e s t i g a t i o n and c h a r a c t e r i z a t i o n of t h i s u n s t a b l e s p e c i e s i n the gas phase,which i s d i s c u s s e d i n t h i s c h a p t e r , w i l l a l l o w p o s s i b l e f u t u r e i d e n t i f i c a t i o n o f t h e same s p e c i e s on s u r f a c e s . The s p e c t r a of o t h e r a c y c l i c a n h y d r i d e s have a l s o been o b t a i n e d f o r c o m p a r a t i v e i n f o r m a t i o n . Of p a r t i c u l a r i n t e r e s t i s t h e s p e c t r a of a c e t i c a n h y d r i d e which i s i s o e l e c t r o n i c w i t h a c e t y l a c e t o n e , a compound s t u d i e d i n some d e t a i l by P E S ^ 1^ ^ \ Two of t h e s e a c c o u n t s ^ 1 ^ ' d e a l w i t h the k e t o form w h i l e the o t h e r s c o n s i d e r o n l y the e n o l tautomer. A c o m p a r a t i v e s t u d y o f a l i p h a t i c a n h y d r i d e s i s c o m p l i c a t e d by t h e v a r i o u s s t r u c t u r e s t h a t such s p e c i e s may assume. There a r e t h r e e p o s s i b l e p l a n a r geometries f o r an a n h y d r i d e (XCO^O. °\/0\^° X \ / ° \ ^ 0 X \ / 0 \ / X c c c c c c I I I! I II II X X O X 0 0 "W" form "SICKLE" form "U" form and an i n f i n i t e number of n o n - p l a n a r s t r u c t u r e s dependent upon the magnitude of the d i h e d r a l a n g l e s . - 168 -F o r f o r m i c a n h y d r i d e an e l e c t r o n . d i f f r a c t i o n study i n d i c a t e d a " s i c k l e " t y p e s t r u c t u r e w i t h d i h e d r a l a n g l e s of 17 ± 6° and 26 ± 9 ° ^ ^ . R e s u l t s i n d i c a t i n g the t w i s t i n g o f t h e f o r m y l groups out of t h e p l a n e (21) were a l s o o b t a i n e d by NMR . However more r e c e n t microwave s t u d i e s , w h i l e c o n f i r m i n g t h e " s i c k l e " n a t u r e of t h e s t r u c t u r e , show t h e .molecule (22 23) t o be q u i t e p l a n a r ' . An ab i n i t i o m o l e c u l a r o r b i t a l study a g r e e s (24) w i t h the microwave d a t a , t h e c o n c l u s i o n b e i n g t h a t f o r m i c a n h y d r i d e has symmetry, w i t h c o p l a n a r c a r b o n y l groups. E l e c t r o n d i f f r a c t i o n d a t a f o r a c e t i c a n h y d r i d e show i t t o have a "U"-type s t r u c t u r e w i t h a d i h e d r a l a n g l e of 78.8° between two p l a n a r (25) a c e t y l fragments . A s i m i l a r s t r u c t u r e has been shown f o r s o l i d • Y.26) m o n o c h l o r o a c e t i c a n h y d r i d e by X-ray d i f f r a c t i o n , a d i h e d r a l a n g l e o f 43° b e i n g i n d i c a t e d . In t h i s work i t has t h e r e f o r e been assumed t h a t a l l o t h e r h a l o - d e r i v a t i v e s of a c e t i c a n h y d r i d e p o s s e s s s i m i l a r s t r u c t u r e s w i t h v a r y i n g d i h e d r a l a n g l e s . Thus f o r m i c a n h y d r i d e has a d i f f e r e n t symmetry from t h e o t h e r a n h y d r i d e s and a d i r e c t comparison of PE s p e c t r a becomes d i f f i c u l t . However the g e n e r a l t r e n d s i n e l e c t r o n i c i n t e r a c t i o n s i n t h i s c l a s s o f compounds can be o b t a i n e d . 7.2 E x p e r i m e n t a l Formic a n h y d r i d e , (HCO^O, was p r e p a r e d by t h e d e h y d r a t i o n of f o r m i c a c i d u s i n g N , N ' - d i c y c l o h e x y l c a r b o d i i m i d e (DCC). T h i s d e h y d r a t i n g agent has been used i n an i n e r t s o l v e n t a t low temperature to o b t a i n (27) s o l u t i o n s o f the a n h y d r i d e , but i n o r d e r to o b t a i n pure gas phase - 169 -samples f o r these' experiments ari» i n - s i t u r e a c t i o n i n t h e absence o f s o l v e n t s was developed. Formic a c i d a t low p r e s s u r e was passe d through a U - t r a p c o n t a i n i n g a m i x t u r e o f g l a s s wool and DCC. To m i n i m i z e d e c o m p o s i t i o n o f t h e r e s u l t a n t a n h y d r i d e i n t o f o r m i c a c i d and carbon monoxide the t r a p was m a i n t a i n e d a t -20°C and t h e sample was pumped i n t o t h e s p e c t r o m e t e r u s i n g the f a s t pumping f a c i l i t y . 2HC00H + C-H-.r-N^N^C-H-- > (HC0) o0 6 11 6 11 2 + C-H-.-NHC0NH-C,H.1 6 11 6 11 (HCO) 0 > HCOOH + CO The o t h e r a n h y d r i d e s ( a c e t i c , (CH^CO^O; t r i f l u o r o a c e t i c , ( C F 3 C O ) 2 0 ; t r i c h l o r o a c e t i c , ( C C £ 3 C O > 2 0 ; c h l o r o d i f l u o r o a c e t i c , ( C C £ F 2 C O ) 2 0 ) were commercial samples (Eastman C h e m i c a l s ) . I n a l l cases h y d r o l y s i s p r o d u c t s were i n i t i a l l y o b s e r v e d when r e c o r d i n g t h e PE s p e c t r a , b u t once t h e i n l e t system had been c o n d i t i o n e d c l e a n s p e c t r a o f t h e a n h y d r i d e s were o b t a i n e d . For t h i s r e a s o n s h o r t i n l e t systems were used. A l l s p e c t r a were c a l i b r a t e d u s i n g t h e IP's o f m e t h y l i o d i d e and argon. F o r f o r m i c a n h y d r i d e t h e p r e s e n c e o f carbon monoxide s e r v e d as an a d d i t i o n a l c a l i b r a n t . CNDO/2 c a l c u l a t i o n s were c a r r i e d out f o r f o r m i c , a c e t i c and t r i f l u o r o a c e t i c a n h y d r i d e s t o a s s i s t i n the i n t e r -(28") p r e t a t i o n o f the s p e c t r a . HAM 3 c a l c u l a t i o n s were performed f o r f o r m i c a n h y d r i d e f o r comparison p u r p o s e s . - 170 -7.3 R e s u l t s F i g u r e 7.1a shows the PE spectrum of f o r m i c a c i d and F i g . 7.1b shows the spectrum o f t h e a c i d a f t e r passage over DCC. In t h e l a t t e r c a s e t h e f i r s t two bands of f o r m i c a c i d l o s e t h e i r s t r u c t u r e as a n o t h e r s p e c i e s appears i n the spectrum. The p r e s e n c e of an a d d i t i o n a l component i s c o n f i r m e d by t h e e x t r a peak at 16.08eV i n F i g . 7.1b. The spectrum shown i n F i g . 7.1c r e p r e s e n t s complete c o n v e r s i o n of t h e a c i d t o t h e a n h y d r i d e and a l t h o u g h some CO i s p r e s e n t as a d e c o m p o s i t i o n p r o d u c t any r e s u l t a n t f o r m i c a c i d i s o n l y a v e r y minor c o n s i t i u e n t , as i t s c h a r a c t e r i s t i c v i b r a t i o n a l s t r u c t u r e i s c o m p l e t e l y a b s e n t . The PE spectrum of DCC has a l s o been r e c o r d e d t o ensure t h a t i t does not c o n t r i b u t e t o t h e o b s e r v e d e f f e c t s . Comparison of F i g . 7.1a and 7.1c r e v e a l o t h e r , more s u b t l e d i f f e r e n c e s and t h e s e w i l l be d i s c u s s e d l a t e r . The PE s p e c t r a o f a c e t i c a n h y d r i d e and i t s h a l o - d e r i v a t i v e s a r e shown i n F i g . 7.2. As w i t h t h e f o r m i c a n h y d r i d e , no v i b r a t i o n a l s t r u c t u r e was o b s e r v e d f o r any band, t h i s p r o b a b l y b e i n g p a r t due t o t h e d e l e t e r i o u s ' e f f e c t t h e compounds had on t h e r e s o l u t i o n o f t h e s p e c t r o m e t e r . The e x p e r i m e n t a l IP's f o r a l l t h e a n h y d r i d e s s t u d i e d a r e g i v e n i n T a b l e 7.1. The f i r s t t h r e e IP's r e p r e s e n t i n g i o n i z a t i o n s from t h e oxygen l o n e p a i r s a r e the main i n t e r e s t i n t h e s p e c t r a and s e p a r a t e i o n i z a t i o n e v e n t s i n t h i s r e g i o n a r e r e s o l v e d i n most c a s e s . The r e l a t i v e p o s i t i o n of t h e s e f i r s t IP's p r o v i d e i n f o r m a t i o n on the e l e c t r o n i n t e r a c t i o n s . The h i g h e r IP's c o n s i s t m a i n l y o f b r o a d , superimposed bands. - 171 -II 13 15 17 19 IONIZATION POTENTIAL (eV) 7.1 H e l p h o t o e l e c t r o n s p e c t r a of a) f o r m i c a c i d b) f o r m i c a c i d over c) f o r m i c a n h y d r i d e - 172 -10 12 14 16 18 IONIZATION POTENTIAL (eV) 7.2 H e l p h o t o e l e c t r o n s p e c t r a of a c e t i c a n h y d r i d e and i t s h a l o g e n a t e d d e r i v a t i v e s - 173 -T a b l e 7.1. I o n i z a t i o n p o t e n t i a l s (eV) of the a n h y d r i d e s (HCO) 20 (CH 3CO) 20 ( C C 1 3 C 0 ) 2 0 ( C C 1 F 2 C 0 ) 2 0 ( C F ^ O ^ O 11.34 12.49 12.64 10.69 11.24 11.66 12.02 12.95 11.24 11.81 11.64 12.49 12.00 13.09 13.38 14.57 13.47 12.51 14.5 15.06 14.97 14.10 12.69 14.8 15.80 16.12 14.7 12.99 15.93 16.59 16.8 15.41 13.16 16.5 17.73 17.3 16.69 15.3 17.3 18.74 17.75 15.82 16.78 a) V e r t i c a l I P ' s . b) A l l IP's above the l i n e a r e i n d i v i d u a l IP's (± 0.02eV) and a r e d i s c u s s e d i n the t e x t . Below the l i n e the numbers r e f e r t o o b s e r v e d maxima o n l y , (± 0.05eV) and a r e not n e c e s s a r i l y i n d i v i d u a l I P ' s . - 174 -7.4 D i s c u s s i o n As mentioned e a r l i e r , a n h y d r i d e s can be c l a s s i f i e d as 3 - d i c a r b o n y l s and an i n t e r a c t i o n between the two c a r b o n y l groups i s t o be e x p e c t e d , e i t h e r t h r o u g h space o r t h r o u g h bond^"^ . The magnitude of th e t h r o u g h space i n t e r a c t i o n i s dependent upon the s p a t i a l s e p a r a t i o n of t h e two groups,which i f s u f f i c i e n t l y s m a l l w i l l l e a d to an o r b i t a l o v e r l a p r e s u l t i n g i n non-degenerate o r b i t a l s . F o r 3 - d i c a r b o n y l s w i t h o r C symmetry t h i s g i v e s a bonding c o m b i n a t i o n which i s symmetric t o r o t a t i o n t h r o u g h 180° (C^ a x i s ) (and w i t h r e s p e c t t o a p l a n e of symmetry ( c O f ° r C type m o l e c u l e s ) . The a n t i b o n d i n g c o m b i n a t i o n (A) i s a n t i s y m m e t r i c t o t h e s e o p e r a t i o n s and l i e s above t h e symmetric(S) l e v e l . The s e p a r a t i o n of S and A l e v e l s can be f u r t h e r i n c r e a s e d by t h e t h r o u g h bond i n t e r a c t i o n which i n v o l v e s t h e lower l y i n g a l e v e l s o f t h e a p p r o p r i a t e symmetry. In g - d i c a r b o n y l s t h e d i r e c t o v e r l a p between the k e t o groups i s s m a l l because of the l a r g e s p a t i a l s e p a r a t i o n and thus the t h r o u g h bond i n t e r a c t i o n i s dominant. The m i x i n g of the n and a o r b i t a l s i s r e p r e s e n t e d d i a g r a m a t i c a l l y i n F i g . 7.3. F o r a c e t y l a c e t o n e ( i s o e l e c t r o n i c w i t h a c e t i c a n h y d r i d e ) and 1,3 propanedione ( i s o e l e c t r o n i c w i t h f o r m i c a n h y d r i d e ) c a l c u l a t i o n s have shown t h e S l e v e l t o l i e below the A l e v e l w i t h the s p l i t t i n g b e i n g dependent upon t h e g e o m e t r y ^ ' . The s p l i t t i n g was found to d e c r e a s e i n t h e o r d e r "U" > "W" > " 9 0 ° " where t h e " 9 0 ° " form i s t h e "U" form w i t h a d i h e d r a l a n g l e o f 180°. The above d i s c u s s i o n and t e r m i n o l o g y i s v a l i d f o r t h e a n h y d r i d e s s t u d i e d , b u t cannot be s t r i c t l y a p p l i e d t o f o r m i c a n h y d r i d e which has - 175 -7.3 The through bond (TB) i n t e r a c t i o n i n g. d i c a r b o n y l s - 176 -the " s i c k l e " s t r u c t u r e . In t h e C g p o i n t group t h e r e i s no symmetric and a n t i s y m m e t r i c r e p r e s e n t a t i o n s f o r t h e i n - p l a n e o r b i t a l s . However some c o r r e l a t i o n between the C g and s p e c i e s f o r the c a r b o n y l oxygen l o n e p a i r s can be o b t a i n e d by c o n s i d e r i n g the o r b i t a l diagrams: H H H X) ( i ) H (2) A 180° r o t a t i o n about the C^O band c o n v e r t s the " s i c k l e " form to the "U" form w i t h 1 b e i n g the a n t i s y m m e t r i c c o m b i n a t i o n and 2 t h e symmetric one. F o r comparison purposes t h e s e p s e u d o - e q u i v a l e n c e s w i l l be used i n f u r t h e r d i s c u s s i o n . The c e n t r a l oxygen atom i n the a n h y d r i d e s w i l l produce an a d d i t i o n a l oxygen l o n e p a i r (b i n symmetry). The r e s u l t a n t PE band w i l l o c c u r i n t h e same r e g i o n as t h e c a r b o n y l oxygen l o n e p a i r s m o d i f y i n g t h e i r p o s i t i o n by the i n d u c t i v e e f f e c t . T h i s o r b i t a l i s a l s o expected to have a resonance i n t e r a c t i o n w i t h the c a r b o n y l TT system which has the same symmetry. The low IP r e g i o n i n t h e s p e c t r a o f t h e s e compounds w i l l thus c o n t a i n 3 bands from oxygen l o n e p a i r s and from the above d i s c u s s i o n - 177 -t h e f i r s t IP i n a n h y d r i d e s w i l l be t h e A c o m b i n a t i o n of c a r b o n y l l o n e p a i r s . The c o r r e s p o n d i n g a c i d s have o n l y two IP's from oxygen l o n e p a i r s , t h e n^ and TT l e v e l s . These ex p e c t e d t h r e e bands a r e o b s e r v e d i n t h e s i m p l e s t a n h y d r i d e h a v i n g symmetry. A c e t i c a n h y d r i d e ; (CH 3CO> 20: The f i r s t IP f o r t h i s m o l e c u l e i s a t 10.69eV compared t o 9.60eV f o r t h e i s o e l e c t r o n i c a c e t y l a c e t o n e i n i t s k e t o f o r m ^ ^ . The s h i f t of 1.09eV can be e x p l a i n e d by t h e i n d u c t i v e e f f e c t o f an a oxygen. The second IP i n a c e t y l a c e t o n e o c c u r s at 1 0 . 1 5 e V ^ ^ .to g i v e a s p l i t t i n g o f t h e c a r b o n y l oxygen l o n e p a i r s o f 0.55eV. I n a c e t i c a n h y d r i d e t h e second IP o c c u r s a t 11.24eV which i s 0.55eV h i g h e r than i t s f i r s t IP. I t i s assumed t h e r e f o r e t h a t t h i s c o r r e s p o n d s t o t h e S c o m b i n a t i o n of c a r b o n y l oxygen l o n e p a i r s as t h e s p l i t t i n g i n t h e two i s o e l e c t r o n i c compounds i s e x p e c t e d t o be o f t h e same o r d e r ( t h e y happen t o be i d e n t i c a l ) . The s p l i t t i n g i s a l s o i n t h e known range of v a l u e s f o r (9) many g-diketones . Both t h e a n h y d r i d e and d i k e t o n e a r e n o n - p l a n a r (25) (29) w i t h d i h e d r a l a n g l e s of 78.8° and 48.6° r e s p e c t i v e l y and t h e c e n t r a l oxygen atom i n t h e a n h y d r i d e s s i m p l y s h i f t s b o t h oxygen l o n e p a i r c o m b i n a t i o n s t o h i g h e r IP by 1.09eV. The t h i r d IP i n a c e t i c a n h y d r i d e (11.81eV) i s t h e r e f o r e a s s i g n e d t o t h e c e n t r a l oxygen TT o r b i t a l . (14) In comparing the c y c l i c s p e c i e s , s u c c i n i c a n h y d r i d e and 1 , 3 - c y c l o p e n t a n e d i o n e ^ ^ ,: t h e : i n c l u s i o n o f a c e n t r a l oxygen atom - 178 -r e s u l t s i n a s h i f t o f the c a r b o n y l oxygen l o n e p a i r s o f 1.27 and 1.20eV. T h i s i n d u c t i v e s h i f t i s o f comparable magnitude t o t h a t o b s e r v e d i n t h e a c y c l i c compounds and i l l u s t r a t e s t h a t t h e s e p a r a t i o n o f A and S l e v e l s i s v e r y s i m i l a r f o r t h e two types o f compounds(0.8eV f o r t h e c y c l i c a n h y d r i d e and 0.87eV f o r t h e c y c l i c d i k e t o n e ) . S u c c i n i c a n h y d r i d e has a "W" typ e c o n f i g u r a t i o n and f o r a c e t i c a n h y d r i d e w i t h a "U" t y p e s t r u c t u r e a g r e a t e r s e p a r a t i o n o f A and S l e v e l s might be ex p e c t e d a c c o r d i n g t o t h e c a l c u l a t i o n s o f Houk et a l . ^ 1 ^ and Hug e t a l . ^ ^ . T h i s i s not t h e case however, because a c e t i c a n h y d r i d e i s non p l a n a r w i t h an a c t u a l geometry a p p r o a c h i n g t h e " 9 0 ° " t y p e o f Hug e t a l . , where t h e s p l i t t i n g i s reduced. F o r a c e t y l a c e t o n e a CNDO/2 c a l c u l a t i o n p r e d i c t s a s e p a r a t i o n of 0.17eV^ 1 6^ w h i l e a MINDO/2 c a l c u l a t i o n p r e d i c t s a v a l u e o f 0.73eV^ 1 7^. A CNDO/2 c a l c u l a t i o n performed i n t h i s work ( T a b l e 7.2) p r e d i c t s a s e p a r a t i o n of 2.30eV between t h e A and S l e v e l s f o r a p l a n a r a c e t i c a n h y d r i d e and a v a l u e o f 1.59eV when t h e e x p e r i m e n t a l geometry i s used ( s e e F i g . 7.4). The A l e v e l i s p l a c e d a t lower IP i n bo t h c a s e s . A HAM 3 c a l c u l a t i o n ( T a b l e 7.2) g i v e s t h e c o r r e c t o r d e r i n g but a s e p a r a t i o n o f o n l y 0.09eV between t h e A and S l e v e l s . In a c e t i c a n h y d r i d e t h e s e p a r a t i o n between t h e c a r b o n y l oxygen l o n e ° (25) p a i r s i s 3.:11A p r o d u c i n g n e g l i g a b l g t h r o u g h space i n t e r a c t i o n . The O s i t u a t i o n i n a c e t y l a c e t o n e i s the same w i t h a s e p a r a t i o n o f 2.11k (29) between t h e oxygens . F o r a l l t h e C^ a n h y d r i d e s s t u d i e d i t i s t h e r e -f o r e assumed t h a t t h r o u g h bond i n t e r a c t i o n i s predominant, as w i t h (6) o t h e r 3 - d i c a r b o n y l s - 179 -T a b l e 7.2. E x p e r i m e n t a l and C a l c u l a t e d IP's (eV) f o r A c e t i c and T r i f l u o r o a c e t i c A n h y d r i d e A c e t i c A n h y d r i d e T r i f l u o r o a c e t i c A n h y d r i d e C a l c u l a t e d C a l c u l a t e d E x p e r i m e n t a l (±0.02) fa") CND0/2 K J CND0/2^ b ) HAM 3 ( b ) E x p e r i m e n t a l (±0.02) CND0/2 3 10.69(A) 12.69(A) 13.01(A) 10.54(A) 12.02(A) 14.48(A) 11.24(S) 14.60(TT) 14.56(TT) 10.63(S) 12.95(S) 16. 60(IT) 11.81(TT) 14.99(S) 14.60(S) 11.77(TT) 13.38(ir) 16.9KS) 16.30 15.77 12.14 18.09 16.66 12.21 19.32 17.77 13.12 18.52 13.74 19.45 13.93 19.89 14.42 14.76 a) P l a n a r geometry. b) E x p e r i m e n t a l geometry. - 180 -EXPERIMENTAL 9 -l O -ll -12-13-14 15 H (CHjCO^O CALCULATED HI (cci 3coi 2o (CF2ClCO)2C (CFjCO^O § {CHjCO^O0 0 ) (ii) ( C H ^ C O ^ O 5 (cr3co)2o; S F i g . 7.4 C o r r e l a t i o n diagram of e x p e r i m e n t a l and c a l c u l a t e d I P's f o r a c e t i c and t r i f l u o r o a c e t i c a n h y d r i d e s a) C a l c u l a t i o n performed u s i n g e x p e r i m e n t a l geometry ( r e f . 25) ( i ) HAM 3 ( i i ) CNDO/2 - r e s u l t s s h i f t e d u n i f o r m l y by 2.eV b) C a l c u l a t i o n (CNDO/2) performed u s i n g p l a n a r geometry - r e s u l t s s h i f t e d u n i f o r m l y by 2 eV - 181 -The h i g h e r IP r e g i o n i n a c e t i c a n h y d r i d e i s complex w i t h o v e r l a p p i n g bands due t o a and > C-C and C-H o r b i t a l s . D i s t i n g u i s h i n g i n d i v i d u a l IP's i s d i f f i c u l t and t h e band maxima g i v e n i n T a b l e 7.1 may not be good IP v a l u e s . The TrCO l e v e l s , which would be e x p e c t e d i n the 14-16eV r e g i o n , a r e thus not a s s i g n e d . T r i f l u o r o a c e t i c a n h y d r i d e ; (CF^CO^O The IP v a l u e s i n t h e f u l l y f l u o r i n a t e d a n h y d r i d e a r e a l l i n c r e a s e d w i t h r e s p e c t to a c e t i c a n h y d r i d e due t o t h e i n d u c t i v e e f f e c t o f t h e f l u o r i n e . I n t r i f l u o r o a c e t y l a c e t o n e b o t h t h e f i r s t two IP's a r e s h i f t e d (18") by cji, 1.6eV compared to a c e t y l a c e t o n e and a s i m i l a r s h i f t i s o b s e r v e d h e r e f o r t h e f i r s t t h r e e IP's of t h e r e s p e c t i v e a n h y d r i d e s . A l t h o u g h the r e s u l t s f o r t h e a c e t y l a c e t o n e s a r e f o r t h e e n o l form the t r e n d i s i l i m p o r t a n t and s u g g e s t s t h e r e i s no d r a m a t i c change i n t h e o r d e r i n g o f t h e f i r s t 3 IP's i n (CI^CO^O. These have v a l u e s o f 12.02, 12.95 and 13.38eV f o r t h e v e r t i c a l t r a n s i t i o n s . The o v e r l a p of t h e second and t h i r d IP's makes a d e f i n i t i v e assignment d i f f i c u l t , b u t i t seems r e a s o n a b l e t o assume the o r d e r i n g i s t h e same as f o r a c e t i c a n h y d r i d e (A, S, Tr:). The CND0/2 c a l c u l a t i o n ( T a b l e 7.2) does i n f a c t p r e d i c t t h e TT o r b i t a l t o l i e between the A and S o r b i t a l s but from t h e r e s u l t s f o r a c e t i c a n h y d r i d e t h i s c a l c u l a t i o n is'known t o e x aggerate t h e A and S s p l i t t i n g ( T a b l e 7.2) and to g i v e t h e i n c o r r e c t o r d e r i n g . The e x p e r i m e n t a l s p l i t t i n g f o r t h e c a r b o n y l oxygen l o n e p a i r s i s thus 0.93eV, somewhat l a r g e r t h a n f o r a c e t i c a n h y d r i d e (0.55eV). Comparison o f peak w i d t h s i n th e PE s p e c t r a o f t h e two a n h y d r i d e s suggest t h a t t h e - 182 -oxygen l o n e p a i r s a r e more d e l o c a l i z e d i n t h e p e r f l u o r o a n h y d r i d e (FWHM o f 0.42 and 0.69eV f o r (CH 3CO) 20 and ( C F 3 C 0 ) 2 0 r e s p e c t i v e l y ) - w h i c h would be expe c t e d t o r e s u l t i n a reduced s p l i t t i n g i f i t was due t o a thro u g h space i n t e r a c t i o n . The i n c r e a s e d s p l i t t i n g f o r t r i f l u o r o a c e t i c a n h y d r i d e i s a f u r t h e r i n d i c a t i o n t h a t i t i s t h e through bond i n t e r a c t i o n t h a t i s important i n t h e s e m o l e c u l e s . The i n c r e a s e d i n t e r a c t i o n may be a s u b s t i t u e n t e f f e c t o r may be caused by a more f a v o u r a b l e d i h e d r a l a n g l e which i s a t p r e s e n t unknown f o r ( C F 3 C O ) 2 0 . CND0/2 c a l c u l a t i o n s on ( C H 3 C 0 ) 2 0 . c e r t a i n l y p r e d i c t an i n c r e a s e d s e p a r a t i o n o f the A and S l e v e l s w i t h a d e c r e a s e d d i h e d r a l a n g l e ( F i g . 7.4). The IP r e g i o n above 14eV i n t h e PE spectrum o f ( C F 3 C O ) 2 0 i s a g a i n complex and i n d i v i d u a l I P 's have not been a s s i g n e d . F u r t h e r c o m p l i c a t i o n a r i s e s fo»r . t h i s m o l e c u l e because o f t h e p r e s e n c e of i o n i z a t i o n p r o c e s s e s a r i s i n g from t h e f l u o r i n e atoms i n the 15-19eV range. T r i c h l o r o a c e t i c a n h y d r i d e ; ( C C £ 3 C O ) 2 0 I o n i z a t i o n events from b o t h oxygen and c h l o r i n e l o n e p a i r s o c c u r i n t h e l l - 1 4 e V range o f t h e PE spectrum f o r t h i s a n h y d r i d e . O v e r l a p of bands i s so s e v e r e t h a t o n l y the f i r s t band a t 11.24eV can r e a s o n a b l y be a t t r i b u t e d t o a s i n g l e i o n i z a t i o n event and i s a s s i g n e d t o t h e a n t i s y m m e t r i c c o m b i n a t i o n o f c a r b o n y l oxygen l o n e p a i r s , a s h i f t o f 0.55eV r e l a t i v e t o t h e f i r s t IP o f a c e t i c a n h y d r i d e . A s i m i l a r i n d u c t i v e s h i f t o f 0.67eV i s ob s e r v e d f o r t h e m o l e c u l e s CH CHO (10.21eV) ^ 3 0 ^ and (31) CCJ^CHO (10.88eV) . Other maxima ob s e r v e d i n t h e PE spectrum of ( C C £ 3 C O ) 2 0 a r e g i v e n i n T a b l e 7.1 and t h e PE spectrum o f t h e a c i d CC£~C00H i s shown i n F i g . 7.5 f o r completeness. - 183 -C h l o r o d i f l u o r o a c e t i c a n h y d r i d e ; ( C C £ F 2 C O ) 2 0 The c h l o r i n e l o n e p a i r l e v e l s a g a i n c o m p l i c a t e t h e low IP r e g i o n o f t h e PE spectrum o f t h i s m o l e c u l e , but as t h e r e a r e o n l y two c h l o r i n e atoms, o v e r l a p o f bands i s l e s s s e v e r e t h a n i n ( C C i ^ C O ) , ^ . Two peaks can be d i s t i n g u i s h e d from the i n t e n s e band at 13.09eV due t o t h e c h l o r i n e non bonding l e v e l s . The f i r s t o f t h e s e a t 11.66eV i s a s s i g n e d t o t h e a n t i s y m m e t r i c c o m b i n a t i o n of c a r b o n y l oxygen l o n e p a i r s , a s h i f t of 0.42eV r e l a t i v e t o t h e f i r s t band of ( C C ^ C O ^ O , i n a c c o r d w i t h t h e i n d u c t i v e e f f e c t o f f l u o r i n e . ' The second IP a t 12.49eV i s a s s i g n e d t o t h e c o r r e s p o n d i n g symmetric c o m b i n a t i o n by c o r r e l a t i o n w i t h t h e r e s u l t s f o r ( C H 3CO) 20 and ( C F 3 C O ) 2 0 . T h i s g i v e s a s p l i t t i n g o f 0.83SV f o r t h e A and S l e v e l s , i n t e r m e d i a t e between t h a t o f t h e perhydro and p e r f l u o r o a n h y d r i d e s . The oxygen TH o r b i t a l cannot be d i s t i n g u i s h e d s i n c e i t i s most p r o b a b l y h i d d e n under t h e i n t e n s e peak at 13.09eV No attempt has been made t o a s s i g n t h e r e s t of t h e spectrum, t h e o b s e r v e d maxima b e i n g g i v e n i n T a b l e 7.1. A g a i n t h e PE spectrum o f t h e c o r r e s p o n d i n g a c i d i s g i v e n i n F i g . 7.5 f o r completeness. Formic a n h y d r i d e ; (HCO) 20 The PE spectrum o f f o r m i c a n h y d r i d e ( F i g . 7.1c) i s v e r y s i m i l a r t o t h a t o f f o r m i c a c i d and c o u l d i n f a c t be m i s t a k e n f o r a p o o r l y r e s o l v e d spectrum o f t h e a c i d u n t i l a c a r e f u l comparison o f th e s p e c t r a i s made. - 184 -C C U C O O H 12 14 16 18 IONIZATION POTENTIAL (eV) F i g . 7.5 H e l p h o t o e l e c t r o n s p e c t r a o f t r i c h l o r o a c e t i c a c i d and c h l o r o d i f l u o r a c e t i c a c i d - 185 -The f i r s t two bands i n t h e PE spectrum o f f o r m i c a c i d have FWHM v a l u e s o f 0.42 and 0.35eV r e s p e c t i v e l y w h i l e t h e c o r r e s p o n d i n g v a l u e s f o r t h e f i r s t two bands of f o r m i c a n h y d r i d e a r e 0.28 and,.0.48eV. The maximum, c o r r e s p o n d i n g t o t h e v e r t i c a l IP, o f t h e f i r s t band i n (HCO)^© has a v a l u e o f 11.34eV compared t o a v e r t i c a l IP o f 11.52eV f o r t h e f i r s t band i n t h e a c i d ^ 1 ^ . The second band o f (HCO) 0 can be r e s o l v e d i n t o two components upon c a r e f u l s t u d y and i s a s c r i b e d t o two i o n i z a t i o n p r o c e s s e s (12.49 and 12.64eV). The r e l a t i v e band w i d t h s o f t h e a c i d and a n h y d r i d e support such an assignment and t h e next band a t 14.57eV i s too f a r removed i n energy t o c o n s t i t u t e t h e t h i r d IP o f t h e a n h y d r i d e . I t i s worth n o t i n g a t t h i s p o i n t t h a t CNDO/2 and HAM 3 c a l c u l a t i o n s p r e d i c t t h e PE spectrum o f t h e s e two compounds to.be v e r y s i m i l a r i n the low IP r e g i o n (Table. 7.3). I n f o r m i c a n h y d r i d e t h e f i r s t IP i s a s s i g n e d t o t h e c a r b o n y l l o n e p a i r c o m b i n a t i o n w i t h atomic c o e f f i c i e n t s o f t h e type p r e v i o u s l y d e s i g n a t e d 1. T h i s was d i r e c t l y compared to t h e a n t i s y m m e t r i c c o m b i n a t i o n (A) i n symmetry. The o t h e r c o m b i n a t i o n ( d e s i g n a t e d 2) must be a s s i g n e d t o one o f t h e two bands a t 12.49 and 12.64eV, t o g i v e a s p l i t t i n g between the c a r b o n y l oxygen l o n e p a i r c o m b i n a t i o n s o f e i t h e r 1.15 o r 1.30eV. There a r e no f e a t u r e s i n t h e e x p e r i m e n t a l spectrum t o f a v o u r one p a r t i c u l a r assignment so the f i r s t v a l u e (12.49eV) has been taken t o c o r r e s p o n d t o the l o n e p a i r c o m b i n a t i o n t o g i v e an o r d e r i n g comparable t o t h e a n h y d r i d e s . By d e f a u l t t h e peak a t 12.64eV i s t h e r e f o r e a s s i g n e d t o the oxygen if" o r b i t a l . - 186 -Table 7.3. Experimental and Calculated IP's (eV) for Formic acid and Formic Anhydride. Formic acid Formic Anhydride EXP. 3 HAM 3 b CND0/2b EXP. HAM 3° CND0/2 c CNDO/S0 CNDO/Kv 11.51 11.37 14.77 11.34 11 21 14.50 12.42 12 69 12.51(a") 12.82(a') 15.34(a ) 12 49 11.68 15.26 a" 13 08 12 70 14.74 14.81 17.61 12 64(a") 12 89(a") 15.50 13 23 a" 13 81 a" 15.72(a") 16.08(a") 22.06(a ) 14 57 14 51(a") 18.75 14 18 13 91 17.13 16.82 22.23 14 97 14 68 18.85 a" 14 32 14 31 -17.7 17.70 23.65 16 .12 15 86 .19.66 14 37(a") 14 92 a" 16 .8 16 97 22.91 16 39 15 76 a" 17 .3 17 15 (a ) 24.02 a" 16 61(a") 16 13 18 .3 18 42 24.94 17 .50 16 .58 a) Results from r e f . 10. b) Geometry from r e f . 34. c) Geometry from r e f . 23. - 187 -The CNDO/2 c a l c u l a t i o n ( T a b l e 7.3) c a r r i e d out u s i n g t h e e x p e r i m e n t a l geometry ( s i c k l e ) p r e d i c t s a s p l i t t i n g o f l.OeV f o r t h e two l o n e p a i r c o m binations w h i l e p u t t i n g t h e Tr/ l e v e l between them. The HAM 3 c a l c u l a t i o n g i v e s an o r d e r i n g i n agreement w i t h t h e proposed assignment but p r e d i c t s a s p l i t t i n g o f o n l y 0.54eV. F o r a c e t i c a n h y d r i d e t h i s c a l c u l a t i o n a l s o p r e d i c t e d a s m a l l e r s p l i t t i n g than t h e e x p e r i m e n t a l l y determined one. A comparison o f the e x p e r i m e n t a l r e s u l t s w i t h t h e s e c a l c u l a t i o n s i s summarized i n F i g . 7.6. The s p l i t t i n g o f c a r b o n y l l e v e l s i n f o r m i c a n h y d r i d e i s s i g n i f i c a n t l y l a r g e r than i n any of t h e a n h y d r i d e s . T h i s cannot be due t o a c o n t r i b u t i o n of through space i n t e r a c t i o n as t h e c a r b o n y l oxygen o s e p a r a t i o n i n t h e s i c k l e form i s g r e a t e r than 3A. The e x t e n t o f through bond i n t e r a c t i o n c o u l d be g r e a t e r f o r t h i s case because o f t h e c o p l a n a r geometry f a v o u r i n g i n t e r a c t i o n w i t h t h e a l e v e l s . However t h e c a r b o n y l oxygen l o n e p a i r s a r e n o n - e q u i v a l e n t i n C symmetry as one cannot be s c o n v e r t e d t o t h e o t h e r by r o t a t i o n o r r e f l e c t i o n . The two c a r b o n y l . oxygen l o n e p a i r s c o u l d t h e r e f o r e be expec t e d t o be non-degenerate ( 9 ) i n a s i m p l e z e r o o r d e r p e r t u r b a t i o n model . The e f f e c t o f t h e thro u g h bond i n t e r a c t i o n w i l l be t o i n c r e a s e t h i s z e r o - o r d e r s p l i t t i n g . The h i g h IP r e g i o n i n t h e PE spectrum o f (HCO^O i s c o n s i d e r a b l y l e s s c o m p l i c a t e d than t h e o t h e r a n h y d r i d e s d i s c u s s e d because t h e r e a r e fewer v a l e n c e e l e c t r o n s . U n f o r t u n a t e l y t h e r e i s no v i b r a t i o n a l s t r u c t u r e to a s s i s t t h e i n t e r p r e t a t i o n and o n l y a t e n t a t i v e assignment can be made i n t h i s r e g i o n . B e s i d e s the CNDO/2 and HAM 3 c a l c u l a t i o n s , C N D O / S - 188 -EXPERIMENTAL CALCULATED ( H C O ) P O ( H C O ) 2 O U ( i ) d i ) (HCO)2CV IT F i g . 7.6 C o r r e l a t i o n diagram f o r e x p e r i m e n t a l and c a l c u l a t e d IP's of f o r m i c a n h y d r i d e . a) C a l c u l a t i o n u s i n g e x p e r i m e n t a l geometry ( i ) HAM 3 ( i i ) CNDO/2 - r e s u l t s s h i f t e d u n i f o r m l y by 3eV b) C a l c u l a t i o n (CNDO/2) u s i n g p l a n a r "U" c o n f i g u r a t i o n - r e s u l t s s h i f t e d u n i f o r m l y by 3eV - 189 -and CNDO/BW c a l c u l a t i o n s have been performed f o r ' ; t h i s m o l e c u l e ( T a b l e 7.3). The CNDO/2 and CNDO/S c a l c u l a t i o n s p r e d i c t t h e f i r s t two bands t o c o n s i s t of one and two IP's r e s p e c t i v e l y which i s as ob s e r v e d . The CNDO/BW c a l c u l a t i o n however p r e d i c t s t h e f i r s t and second bands t o c o n s i s t o f two IP's each and g e n e r a l l y p r e d i c t s IP's t o be much c l o s e r i n energy than i s found..* In. t h e ."higher IP r e g i o n two groups o f t h r e e IP's a r e p r e d i c t e d by t h e CNDO/2 and CNDO/S c a l c u l a t i o n s w h i l e t h e HAM 3 c a l c u l a t i o n p r e d i c t s seven IP's between 14 and 20eV, w i t h two v e r y c l o s e around 14.5eV ( T a b l e 7.3). To account f o r t h e number of expected i o n -i z a t i o n s t h e t h i r d band i n t h e e x p e r i m e n t a l spectrum (14.57eV w i t h a s h o u l d e r a t 14.97eV) has been a s s i g n e d t o t h r e e I P ' s , two a i o n i z a t i o n s and a TT CO i o n i z a t i o n . The o t h e r c a r b o n y l IP i s t h e r e f o r e e x p e c t e d to l i e i n the band between 16 and 17.5eV a s i t h i s w i l l g i v e a r e a s o n a b l e TT-./TT s p l i t t i n g based on o t h e r d i c a r b o n y l compounds ' . F o r g l y o x a l (32 33) th e c a r b o n y l TT IIP v a l u e s a r e 14.0 and 15.4eV ' and s u b s t i t u t i o n of an i n t e r m e d i a t e oxygen atom i n d u c t i v e l y s h i f t s t h e s e i o n i z a t i o n s t o h i g h e r IP. The l e v e l s w i l l be f u r t h e r s t a b i l i z e d from a resonance i n t e r a c t i o n w i t h t h e oxygen T r(a") l o n e p a i r . The TT: l e v e l i n f o r m i c a c i d o c c u r s at 15.72eV. The band between 16 and 17.5eV shows two d i s t i n c t maxima a t 16.12 2 and 17.3eV. A l t h o u g h t h i s r e g i o n i s o v e r l a p p e d by t h e II s t a t e o f C 0 + (16.8 - 17.5eV) t h e r e does appear t o be a t h i r d maximum around 16.8eV. T h i s f e a t u r e , a l t h o u g h weak, i s always p r e s e n t and i s a s s i g n e d t o one of t h r e e IP's i n t h i s r e g i o n , t h e o t h e r TT. l e v e l and two more a l e v e l s . - 190 -There i s a n o t h e r v e r y weak band at 18.3eV° which a g a i n appears c o n s i s t e n t l y i n a l l s p e c t r a . T h i s c o u l d w e l l be an a d d i t i o n a l a l e v e l o f (HCO^O which would g i v e a t o t a l o f t e n IP's i n t h e H e l PE spectrum i n agreement w i t h t h e HAM 3 c a l c u l a t i o n ( T a b l e 7.3). The o t h e r p o s s i b l i t y i s a' band due t o t h e d e h y d r a t i n g agent DCC whose PE spectrum g i v e s i t s most i n t e n s e band i n t h i s r e g i o n . The former e x p l a n a t i o n i s p r e f e r r e d as DCC has a r e l a t i v e l y low vapour p r e s s u r e and t h e e x p e r i m e n t a l c o n d i t i o n s ( f a s t pumping, low r e a c t i o n temperature) make i t u n l i k e l y DCC would c o n t r i b u t e t o t h e PE s i g n a l . F i g u r e s 7.4 and 7.6 show a c o r r e l a t i o n ( e x p e r i m e n t a l and t h e o r e t i c a l ) of t h e f i r s t t h r e e IP's o f a l l t h e a n h y d r i d e s s t u d i e d . F o r t h e C^ an h y d r i d e s t h e c a r b o n y l o x y g e n l l o n e p a i r s e p a r a t i o n i n c r e a s e s as t h e e l e c t r o n e g a t i v i t y o f t h e s u b s t i t u e n t s i n c r e a s e s which s u g g e s t s t h a t t h e through bond i n t e r a c t i o n i n t h e s e m o l e c u l e s i s dependent upon t h e i n d u c t i v e e f f e c t o f t h e s u b s t i t u e n t s . The m o l e c u l e (HCO^O cannot be i n c l u d e d i n t h i s g e n e r a l i z a t i o n f o r reasons a l r e a d y d i s c u s s e d . The FWHM o f t h e A l e v e l i n t h e a n h y d r i d e s i s a l s o dependent upon t h e i n d u c t i v e e f f e c t o f t h e s u b s t i t u e n t s . The v a l u e s a r e (CH^CO^O, 0.42eV < ( C C £ 3 C O ) 2 0 , 0.52eV, < ( C C £ F 2 C 0 ) 2 0 , 0.61eV < ( C F 3 C O ) 2 0 , 0.69eV and a p l o t o f t h e r e s p e c t i v e f i r s t IP's a g a i n s t t h e s e v a l u e s ( F i g . 7.7) g r a p h i c a l l y i l l u s t r a t e s t h e t r e n d o f d e r e a l i z a t i o n o f t h e MO w i t h i n d u c t i v e e f f e c t . As expected (HC0) 20 does not f i t i n t o t h i s t r e n d , t h e c a r b o n y l oxygen l o n e p a i r s b e i n g s u b s t a n t i a l l y more non-bonding i n t h i s c ase (FWHM = 0.28eV) than f o r any of the C 2 a n h y d r i d e s . The b a r y c e n t r e o f t h e c a r b o n y l oxygen l o n e p a i r s i s a l s o p a r t i c u l a r l y - 191 -FWHM of first IP (eV) F i g . 7.7 P l o t of the f i r s t IP's v s . FWHM of the f i r s t PE band f o r th e a n h y d r i d e s - 192 -s t a b i l i z e d i n (HCO)^O because o f a c o n j u g a t i v e i n t e r a c t i o n o f t h e c a r b o n y l groups w i t h t h e c e n t r a l oxygen l o n e p a i r f a c i l i t a t e d by t h e p l a n a r geometry. 7.5 C o n c l u s i o n Formic a n h y d r i d e ((HCO^O) t h e s i m p l e s t a l i p h a t i c a n h y d r i d e has been o b t a i n e d pure i n t h e gas phase and i t s H e l PE spectrum o b t a i n e d . T h i s may be of some r e l e v a n c e t o t h e d e c o m p o s i t i o n o f f o r m i c a c i d (HCOOH) on s o l i d s u r f a c e s . 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 o f t h e CNDO type (CNDO/2, CNDO/S) i n d i c a t e some co r r e s p o n d e n c e w i t h t h e e x p e r i m e n t a l I P ' s , p a r t i c u l a r l y i n the low IP r e g i o n . O v e r a l l however, t h e HAM 3 c a l c u l a t i o n seems t o p e r f o r m more c o n s i s t e n t l y f o r t h i s l i m i t e d d a t a s e t . The PE s p e c t r a o f o t h e r a l i p h a t i c a n h y d r i d e s have been o b t a i n e d f o r comparison and r e s u l t s used t o study t h e i n t e r a c t i o n of t h e c a r b o n y l oxygen l o n e p a i r s . The ob s e r v e d s p l i t t i n g between t h e A and S l e v e l s was found t o be comparable t o p r e v i o u s s t u d i e s on 3 - d i k e t o n e s and has been d i s c u s s e d i n terms o f a th r o u g h bond mechanism. F o r t h e a n h y d r i d e s w i t h C^ symmetry a l a r g e r i n t e r a c t i o n was ob s e r v e d f o r t h e f l u o r o d e r i v a t i v e s than f o r a c e t i c a n h y d r i d e . T h i s c o u l d be a sub-s t i t u e n t e f f e c t w i t h t h e e l e c t r o n e g a t i v e f l u o r i n e s c a u s i n g an i n c r e a s e d i n t e r a c t i o n o f t h e l o n e p a i r s w i t h t h e lower l y i n g a l e v e l s , b u t may a l s o be due t o d i f f e r e n c e s i n d i h e d r a l a n g l e f o r t h e a n h y d r i d e s under i n v e s t i g a t i o n . - 193 -CHAPTER EIGHT  SUMMARY AND FURTHER WORK In the work d e s c r i b e d i n t h i s t h e s i s t h e t e c h n i q u e of UV PES has been used to d e t e c t and c o n t i n u o u s l y m o n i t o r t h e p r o d u c t i o n of u n s t a b l e s p e c i e s i n t h e gas phase. T h i s has p e r m i t t e d t h e e x p e r i m e n t a l c o n d i t i o n s t o be changed i n s i t u i n o r d e r t o maximize t h e y i e l d s o f t h e s p e c i e s under i n v e s t i g a t i o n . Thus, u n s t a b l e compounds have been o b t a i n e d i n t h e gas phase, e i t h e r pure o r i n h i g h y i e l d s , a l l o w i n g a d e t a i l e d s t u d y d f t h e i r v a l e n c e e l e c t r o n i c s t r u c t u r e i n t h e H e l r e g i o n (21.2eV). Even when o n l y low y i e l d s o f an u n s t a b l e compound c o u l d be o b t a i n e d eg. HOC£ ( c h a p t e r 4 ) , i t has been found p o s s i b l e t o m i n i m i z e the number o f " i m p u r i t y " s p e c i e s and so o b t a i n ^ P E s p e c t r a of s u f f i c i e n t q u a l i t y t o a l l o w i d e n t i f i c a t i o n o f t h e u n s t a b l e s p e c i e s . In t h e p a r t i c u l a r c a s e of HOC£ t h i s was important i n o r d e r t o compare e x p e r i m e n t a l IP's t o t h o s e p r e d i c t e d by u s i n g p e r t u r b a t i o n c o r r e c t i o n s t o Koopmans'! t h e o r e m ^ ^ and thus f u r t h e r e s t a b l i s h t h e v e r a c i t y o f t h i s t y p e of c a l c u l a t i o n . The e x p e r i m e n t a l p r o c e d u r e s employed t o obt a i n t h e u n s t a b l e compounds o b s e r v e d i n t h i s t h e s i s can be s e p a r a t e d i n t o t h r e e c a t e g o r i e s : -a) E x t r a c t i o n from s o l u t i o n ; t h i s was found t o be most s u c c e s s f u l f o r t h e h y p o c h l o r i t e s ( c h a p t e r 4) and t h e h a i a m i n e s ( c h a p t e r 5 ) . The p a r t i c u l a r v o l a t i l e u n s t a b l e s p e c i e s was p r e p a r e d i n aqueous s o l u t i o n and t h e vapours above the s o l u t i o n l e d i n t o t h e s p e c t r o m e t e r v i a s u i t a b l e c o l d t r a p s to remove water and o t h e r s i d e p r o d u c t s . b) H i g h temperature p y r o l y s i s of a vapour; a method used t o produce s u b s t i t u t e d ketenes from a c i d h a l i d e s ( c h a p t e r 6 ) . T h i s has t h e - 194 -advantage o f b e i n g d i r e c t and s i m p l e to.;control, b u t has t h e i n h e r e n t d i s a d v a n t a g e t h a t t h e h i g h temperatures i n v o l v e d (~ 750°C) can p o s s i b l y r e s u l t i n complete t h e r m a l d e c o m p o s i t i o n of t h e p r e c u r s o r and t h e u n s t a b l e m o l e c u l e . c) In s i t u homogeneous r e a c t i o n s ; t h i s method i n v o l v e s p a s s i n g a gas o v e r a s o l i d o r l i q u i d t o produce an u n s t a b l e m o l e c u l e t h a t can be immediately t r a n s f e r r e d i n t o t h e s p e c t r o m e t e r i n t h e gas phase w i t h o u t f u r t h e r d e c o m p o s i t i o n . T h i s was t h e method used t o produce f o r m i c a n h y d r i d e , (HCO^O, from f o r m i c a c i d vapour and N,N d i c y c l o h e x y l -c a r b o d i i m i d e s o l i d ( c h a p t e r 7 ) , n i t r o g e n t r i c h l o r i d e , NCii.^, from c h l o r i n e gas and ammonium s u l p h a t e ( c h a p t e r 5), and h a l o g e n a t e d ketenes from a c i d h a l i d e vapours and m o l t e n z i n c ( c h a p t e r 6 ) . The l a t t e r method i s shown t o be more v e r s a t i l e than t h e s i m p l e p y r o l y s i s o f an a c i d h a l i d e , s i n c e t h e temperatures r e q u i r e d a r e f a r lower (~ 500°C) Four groups o f compounds ( h y p o c h l o r i t e s , h a l a m i n e s , s u b s t i t u t e d k e t e n e s and a c i d a n h y d r i d e s ) have thus been i n v e s t i g a t e d by UV PES, and i n most c a s e s r e p r e s e n t s t h e f i r s t PE study of t h e s e m o l e c u l e s . In c a s es where t h e PE s p e c t r a had p r e v i o u s l y been o b t a i n e d , more d e t a i l was observed i n t h i s work. (eg. NC& 3 - p r e v i o u s work i n v o l v e d an impure (2) sample , and monochloroketene and t h e methylketenes - o n l y t h e f i r s t (3) IP's p r e v i o u s l y r e p o r t e d ) . F o r t h r e e of t h e ketenes (monobromoketene, dibromoketene and d i c h l o r o k e t e n e ) t h i s i s i n f a c t t h e f i r s t gas phase study o f any k i n d and l e a d s t h e way to f u r t h e r gas phase i n v e s t i g a t i o n . I n a l l f o u r groups of compounds t h e s u b s t i t u e n t e f f e c t s on t h e v a l e n c e - 195 -e l e c t r o n i c energy l e v e l s have been f o l l o w e d w i t h p a r t i c u l a r emphasis b e i n g g i v e n t o h a l o g e n and m e t h y l s u b s t i t u t i o n . T h i s t h e s i s has t h e r e f o r e p r e s e n t e d g e n e r a l p r e p a r a t i v e methods, t h a t i n c o n j u n c t i o n w i t h UV PES can be used t o s t u d y u n s t a b l e m o l e c u l e s i n t h e gas phase. A l t h o u g h t h e number of m o l e c u l e s s t u d i e d i n each of t h e groups o f compounds has been r e l a t i v e l y h i g h , i n o r d e r to e s t a b l i s h t r e n d s i n t h e IP v a l u e s , t h e r e a r e e x t e n s i o n s o f t h i s work which s h o u l d prove f e a s i b l e and u s e f u l . In c h a p t e r 4 r e s u l t s f o r t h e h y p o c h l o r i t e s were p r e s e n t e d and i t i s b e l i e v e d t h i s c o u l d be extended to t h e hypobromites. Low y i e l d s o f h y p o c h l o r o u s a c i d were o b t a i n e d i n t h i s work and i t t h e r e f o r e seems u n l i k e l y t h a t t h e PE spectrum o f hypobromous a c i d i t s e l f w i l l be p o s s i b l e i n t h e n e a r f u t u r e , as t h e bromo-acid i s l e s s s t a b l e than t h e (4) c h l o r o - a n a l o g u e . However a d d i t i o n a l s t a b i l i t y o f t h e a l k y l hypo-c h l o r i t e s s u g g e s t s t h a t good y i e l d s o f t h e a l k y l h ypobromites may w e l l be o b t a i n e d f o r UV PES s t u d i e s . In c h a p t e r 5 t h e a m b i g u i t y i n a s s i g n i n g t h e v i b r a t i o n a l s t r u c t u r e observed on t h e PE band 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 t h e N l o n e p a i r o f t h e halamines was d i s c u s s e d i n some d e t a i l . I t was mentioned t h a t b e t t e r r e s o l u t i o n would p o s s i b l y , have h e l p e d s o l v e t h i s problem and t h e r e f o r e i t i s f e l t t h a t i f s u f f i c i e n t l y ! h i g h r e s o l u t i o n s p e c t r o -meters (< lOmeV) become a v a i l a b l e t h a t t h e m o l e c u l e s i n c h a p t e r 5 s h o u l d be s t u d i e d a g a i n , a l o n g w i t h t h e i r d e u t e r o a n a l o g u e s . - 196 -The m e t h y l h a l a m i n e s o f f e r t h e p o s s i b l i t y o f p r o d u c i n g t h e u n s t a b l e halo-imines.' by" a s i m p l e p y r o l y s i s and e l i m i n a t i o n "of ..HX eg. C H 3 N C £ - — - — > H 2C = NC£ + HC£ C H 3NBr 2 — - — > H 2C = NBr + HBr The e x p e r i m e n t a l p r o c e d u r e s o u t l i n e d i n c h a p t e r 5 r e s u l t e d i n pure samples o f t h e d i h a l a m i n e s and w i t h a f a s t pumping f a c i l i t y on a PE s p e c t r o m e t e r t h e s u b s t i t u t e d imines c o u l d be d e t e c t e d b e f o r e f u r t h e r d e c o m p o s i t i o n , o r t r i m e r i z a t i o n o c c u r r e d . H 2C = NC£ — - — > HCN + HC£ y c y c l i c t r i m e r " N-chloroimine' !'is a l s o e x p e c t e d t o be formed i n the r e a c t i o n of N H 2 C £ and f o r m a l d e h y d e ^ . The gas phase p r e p a r a t i o n of the s u b s t i t u t e d k e t e n e s d e s c r i b e d i n c h a p t e r 6 o f f e r s t h e p o s s i b i l i t y o f t h e s e m o l e c u l e s b e i n g i n v e s t i g a t e d by o t h e r s p e c t r o s c o p i c t e c h n i q u e s . L i t t l e i n f o r m a t i o n i s a v a i l a b l e on t h e s e compounds and t h e r e i s o b v i o u s l y c o n s i d e r a b l e scope f o r f u r t h e r work,both i n terms.of c h e m i c a l r e a c t i v i t y and p h y s i c a l p r o p e r t i e s . W i t h r e g a r d to f u r t h e r PE s t u d i e s i n v o l v i n g s u b s t i t u t e d k e t e n e s i t would be d e s i r a b l e t o extend t h e i n v e s t i g a t i o n t o t h e f l u o r o - d e r i v a t i v e s , - 197 -i n p a r t i c u l a r d i f l u o r o k e t e n e where t h e p e r f l u o r o e f f e c t s h o u l d be f (L\ o b s e r v a b l e . The d i f f i c u l t y w i t h t h i s may be i n o b t a i n i n g t h e s u i t a b l e p r e c u r s o r , b r o m o d i f l u o r o a c e t y l bromide, which i s thought t o r e a c t w i t h Z n ^ . A CBrF 2COBr + Zn > F 2C=C=0 + Z n B r 2 Work i n s o l u t i o n i n d i c a t e s t h a t d i f l u o r o k e t e n e i s l e s s s t a b l e than e i t h e r t h e c h l o r o - o r bromo-analogues ^ ^and e x t r e m e l y s h o r t t r a n s p o r t time from p r o d u c t i o n t o s p e c t r o m e t e r w i l l be n e c e s s a r y . In t h e work on t h e ketenes i t was sugg e s t e d t h a t d e c o m p o s i t i o n o c c u r r e d v i a t h e s u b s t i t u t e d methylene r a d i c a l s . X 2C=C=0 —-—> :CX 2 + CO C 2 X 4 Thus by p y r o l y z i n g t h e k e t e n e s , t o t a l d e c o m p o s i t i o n c o u l d be ensured and i f t h i s was ac c o m p l i s h e d c l o s e t o a s p e c t r o m e t e r w i t h a f a s t pumping f a c i l i t y the d i r a d i c a l s p e c i e s c o u l d p o s s i b l y be d e t e c t e d . The same r e s u l t may w e l l be seen by p y r o l y z i n g t h e a c i d h a l i d e a t temperatures above t h a t r e q u i r e d t o form the ketene A CHX 2COX > :CX 2 + CO + HX C 2 X 4 - 198 -A g a i n t r a n s p o r t time from the p y r o l y s i s r e g i o n t o the s p e c t r o m e t e r would be the c r i t i c a l f e a t u r e . The PE s p e c t r a o f the a c i d a n h y d r i d e s , a l t h o u g h c o m p l i c a t e d by t h e number of I P ' s , gave i n f o r m a t i o n on the through bond i n t e r a c t i o n o f t h e c a r b o n y l oxygen l o n e p a i r s . F u r t h e r i n v e s t i g a t i o n o f the a n h y d r i d e s would p r o b a b l y o n l y g i v e l i m i t e d a d d i t i o n a l i n f o r m a t i o n but t h e work c o u l d be extended t o 8 9 i n c l u d e the i s o e l e c t r o n i c i m i d e s (eg. (HCO) 2NH,/(CH 3CO) 2NH e t c . ) ' . I o n -i z a t i o n s a s s o c i a t e d w i t h the 0 l o n e p a i r s s h o u l d be e a s i l y d i s t i n g u i s h e d from the r e s t o f the spectrum a l l o w i n g " t h e e x t e n t o f the thr o u g h bond i n t e r -a c t i o n t o be i n v e s t i g a t e d . The work d e s c r i b e d i n t h i s t h e s i s has thus shown how UVPES can be used to m o n i t o r t h e p r e p a r a t i o n o f n o v e l c h e m i c a l s p e c i e s , u n s t a b l e m o l e c u l e s , r e a c t i o n i n t e r m e d i a t e s e t c . . The r e s u l t i n g PE s p e c t r a can be i n t e r p r e t e d i n o r d e r t o o b t a i n i n f o r m a t i o n on the e l e c t r o n i c s t r u c t u r e o f such m o l e c u l e s . 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