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UBC Theses and Dissertations

Photoelectron spectroscopy of unstable molecules : multiply bonded nitrogen compounds MacDonald, Charles Bruce 1978

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PHOTOELECTRON SPECTROSCOPY OF UNSTABLE MOLECULES: MULTIPLY BONDED NITROGEN COMPOUNDS By CHARLES BRUCE HacDONALD B.Sc. (Hon.), Saint Mary's U n i v e r s i t y , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard. THE UNIVERSITY OF BRITISH COLOMBIA September, 197 8 (c) C h a r l e s Bruce MacDonald, 1978 In presenting th i s thes i s in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary shal l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th i s thesis for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my written permission. Department of CHEMISTRY The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date S e p t . 14 , 1978 i i ABSTRACT As p a r t o f a c o n t i n u i n g 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 u n s t a b l e and t r a n s i e n t m o l e c u l e s , the vacuum u l t r a — v i o l e t p h o t o e l e c t r o n s p e c t r a of t h i r t e e n compounds ( CH 2 = NCH 3, CH3CH=NH, H 2C (CN ) z , Cl^C (CN}^ , Br--, C (CN) 2 , ClNCO, BrNCO, INCO, C1NNN, BrNNN, C1SCN, BrSCN, and (SCN>2 ) have been o b t a i n e d . The s p e c t r a have been a s s i g n e d u s i n g v i b r a t i o n a l a n a l y s i s o f t h e band shapes, m o l e c u l a r o r b i t a l t h e o r y , comparisons t o s i m i l a r and i s o e l e c t r o n i c m o l e c u l e s , and v a r i a t i o n s o f r e l a t i v e band c r o s s - s e c t i o n s w i t h photon energy. A l l the m o l e c u l e s s t u d i e d c o n t a i n m u l t i p l y bonded n i t r o g e n , and most i n v o l v e pseudohalogen f u n c t i o n a l groups. T h i s work p r o v i d e s the f i r s t gas phase d e t e c t i o n and s p e c t r o s c o p i c r e s u l t s f o r s e v e r a l halogan p s e u d o h a l i d e m o l e c u l e s . The a z i d e , i s o c y a n a t e , and t h i o c y a n a t e f u n c t i o n a l groups a r e i n v e s t i g a t e d by f o l l o w i n g the p e r t u r b a t i o n o f an o f f - a x i s halogen atom. The e f f e c t o f r e p l a c i n g a halogen wi t h a cyano group i s a l s o demonstrated i n s e v e r a l i n s t a n c e s . C o r r e l a t i o n of p h o t o e l e c t r o n s p e c t r a of cyano m o l e c u l e s , w i t h t h e i r h alogen analogues i s shown t o be u s e f u l i n i n t e r p r e t i n g t h e r e s u l t s . i i i S mall, s u b s t i t u t e d imines belong t o a group of molecules of cu r r e n t a s t r o p h y s i c a l i n t e r e s t . High y i e l d p r e p a r a t i v e routes f o r CH^=NCH^ and CH^CH=NH have been developed, and the molecules s t u d i e d by p h o t o e l e c t r o n spectroscopy. i v TABLE OF CONTENTS Page CHAPTER I : INTRODUCTION 1 CHAPTER I I : BACKGROUND CONSIDERATIONS TO PES .,,„ 5 2.1 I n t r o d u c t i o n ................. 5 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 The Franck—Condon Factor ..... 9 2.4 S e l e c t i o n Rules 12 2.5 M u l t i - e l e c t r o n Processes ..,,..„, 13 2.6 A u t o i o n i z a t i o n ............... 15 2.7 Use of C a l c u l a t i o n i n PES ..... 17 2.8 C r o s s - S e c t i o n Dependance on Photon Energy .... 19 2.9 Through Space and Through Bond I n t e r a c t i o n s and the Sum Rule 23 2. 10 General C r i t e r i a f o r A s s i g n i n g PE Sp&c tIrs. 25 CHAPTER I I I : THE PHOTOELECTRON SPECTROMETER AND GENERAL EXPERIMENTAL CONSIDERATIONS 3.1 I n t r o d u c t i o n ................. 28 3.2 The Spectrometer Vacuum System 29 3.3 The Analyz er . ,...,......... .,, „ 32 3. if The L i g h t Source ............. 34 3.5 Spectrometer Performance 35 V CHAPTER IV: UNSTABLE IHINES; PE SPECTRA OF N-METHYL METHYLENIMINE (CH^NCH^) AND C—M ET HY L METHYLENIMINE <CH3CH=NH) U.I I n t r o d u c t i o n ................. 37 4.2 Experimental .................. 38 4.3 Results 40 4.4 Assignment ................... 46 4.5 D i s c u s s i o n 52 CHAPTER V: M ALO N 0 N IT RILE, %C{CN) a , DICHLOROMALONONITRILE, CL 2C(CN)_2 , AND DIBS OMO M AL ON O N ITS IL E , BR^CCCN)^ 5.1 I n t r o d u c t i o n 55 5.2 Experimental ................. 56 5.3 R e s u l t s ...................... 57 5.4 Assignments ................... 64 CHAPTER VI: HALOGEN ISOCY AN ATES, XN CO {X=Cl, Br, I) AND HALOGEN AZIDES, XN 3(X=Cl, Br) 6.1 I n t r o d u c t i o n ................. 73 6.2 Experimental ................. , 75 6.3 Re s u l t s . . . . . . . . . . . . . . . . . . . . . . 7 7 6.4 Assignments .................. 84 6.5 D i s c u s s i o n ................... 88 6.6 S t r u c t u r e s ................... 101 CHAPTER VII: THIOCYANOGEN HALIDES, XSCN (X=Cl, B r ) , AND THIOCYANOGEN (SCN)^ 7.1 I n t r o d u c t i o n ................. 106 7.2 Experimental 107 7» 3 Res u I t s •••••••••*«••••••••«•* -j 10 9 Assignments ••••••,*•*•••*•••••*•• 115 v i CHAPTER VII: SUMMARY, CONCLUSIONS, AND FURTHER WORK 8.1 Summary 129 8.2 Conclus i o n s 131 8.3 F u r t h e r S t u d i e s .............. 132 REFERENCES: ................................../ 135 v i i LIST OF TABLES T a b l e 4.1 E x p e r i m e n t a l I P ' s f o r (CH 2=NCH 3) 3 and CH 2=NCH 3 TABLE 4.2 E x p e r i m e n t a l I P ' s f o r (CH3CH=NH)3 and CH3CH=NH TABLE 5.1 E x p e r i m e n t a l I P ' s f o r H 2C(CN) 2 , C]£{CN) 2 , and Br^C {CN)2 . 42 44 62 TABLE 6.1 E x p e r i m e n t a l I P ' s and Assignments f o r t h e Halogen A z i d e s 80 TABLE 6.2 E x p e r i m e n t a l I P ' s and Assignments f o r t h e Halogen Isocyan ates 83 TABLE 6.3 Bond Angles and Band S p l i t t i n g s f o r the A z i d e s and I s o c y a n a t e s 89 TABLE 6.4 S t r u c t u r a l Data f o r Some A z i d e s and I s o c yana t e s 104 TABLE 7.1 E x p e r i m e n t a l I P ' s and Assignments f o r C1SCN, BrSCN, and (SCN) a 112 v i i i LIST OF FIGURES FIGURE 2.1 The Franck-Condon P r i n c i p l e i n Photoelec t r o n Spectroscopy 10 FIGURE 2. 2 Au t o i o n i z a t i o n i n the Ne I PE Spectrum of Oxygen. 16 FIGURE 2.3 P l o t s o f T r a n s i t i o n Moment Vs. Ph o t o e l e c t r o n Energy (E n) fo r Various Atomic O r b i t a l s . 21 FIGURE 3.1 V e r t i c a l C r o s s - S e c t i o n Spectrometer. of PE 31 FIGURE 3.2 Hemispherical Analyzer Unit. , 33 FIGURE 4.1 He I Ph o t o e l e c t r o n Spectra of N - t r i m e t h y l h e x a h y d r o - s - t r i a z i n e , and N-methyl Methylenimine, CH 2=NCH 3. 41 FIGURE 4. 2 He I Spectra o f C - t r i m e t h y l h e x a d y d r o - s - t r i a z i n e , and C-methyl methylenimine, CH3CH=NH. 43 FIGURE 4.3 He I PE Spectrum o f 2-methyl P y r i d i n e (cx-picoline) . 45 i x F i g u r e 4. 4 FIGURE 4.5 FIGURE 4.6 FIGURE 5. 1 FIGURE 5. 2 FIGURE 5.3 FIGURE 5.4 FIGURE 5.5 FIGURE 5.6 FIGURE 5.7 FIGURE 6.1 C o r r e l a t i o n Diagram f o r the Molecules Trans-methyl D i a z i n e , N— methyl Methyl enimine, Propylene, C-Methyl methylene and acetaldehyde 50 C o r r e l a t i o n of PES and HAM3 R e s u l t s f o r CH 2=NH, C H 2 = N C H 3 , and C H o C H=NH. 51 C o r r e l a t i o n o f Experimental and HAM3 IP's f o r CHn=N©fl. 54 He I PE Spec t r a of C l 2 C ( C N ) 2 and Br 2C (CN)o . .................................. 58 He I PE Spec t r a o f E1C(CE)Z , C 1 2 C ( C N ) 2 , and BvzC(CU)2 i n the 11—15 eV Region. ............................... ... 59 Ne I PE Spec t r a o f H^CfCN)^, C 1 2 C ( C N ) 2 , and Br^C(CN) a i n the 11—15 eV Region. , 60 Ne I PE Spec t r a o f H 2C(CN) 3, C l a C ( C N ) 5 , and Br 2C(CN) 2 C o r r e c t e d f o r the Ne 16.65 eV L i n e . The High IP's of H 2C(CN) 2 . , The Through Space and Through Bond I n t e r a c t i o n s i n H^CtCN)^. C o r r e l a t i o n of Experimental IP's of H 2CBr a, Br AC(CN) 2 , H 2C(CN) A , C1 2C(CN) 5 , and H^  Cd.z « He I PE Sp e c t r a o f C l N 3 and BrN^ 61 63 65 71 78 F i g u r e 6.2 The F i r s t PE Bands of ClN^ and BrN^. 79 FIGURE 6.3 PE Spectra of ClNCO, BrNCO, and INCO. 81 FIGURE 6.4 The F i r s t and Second PE Bands of ClNCO, BrNCO, and INCO. 82 FIGURE 6.5 The E f f e c t of O f f - A x i s Angle i n X N 3 . 86 FIGURE 6.6 PES Re s u l t s f o r HN3 and HNCO by Various Authors. 87 FIGURE 6.7 C a l c u l a t i o n s f o r HNg I n c l u d i n g HAM3 Resu l t s . 95 FIGURE 6.8 C a l c u l a t i o n s f o r HNCO I n c l u d i n g HAM3 Re s u l t s . 96 FIGURE 6.9 The F i f t h IP of C l N 3 and the F i f t h to Seventh IP's of BrN 3 . 97 FIGURE 6.10 The F i f t h t o Seventh IP's of ClNCO. 99 FIGURE 6.11 The Experimental IP's of BrNCO, and ClNCO as a Function o f t h e Valence State IP's of T h e i r Respective Halogens. INCO, 102 FIGURE 7.1 He I PE Spectra o f ClSCN, BrSCN, and (SCN) a . 110 FIGURE 7.2 Ne I PE Spectra of ClSCN, BrSCN, and (SCN)^. . 111 FIGURE 7.3 The F i r s t PE Bands of BrSCN and ClSCN. ,.-113 FIGURE 7.4 The Second t o Six t h PE Bands of ClSCN and BrSCN. 114 x i F i g u r e 7.5 C o r r e l a t i o n of PES R e s u l t s f o r S C l ^ , ClSCN, and S (CN) 2 . FIGORE 7.6 The F i r s t Two PE Bands of (SCN ) a . , FIGURE 7.7 C o r r e l a t i o n o f PES Results f o r HSSH, FSSF, (SCN)^ , C l S S C l , and BrSSBr. FIGURE 7.8 He I and Ne I PE Spectra of (SCN), , i n t h e 13.2-14.6 eV Region. 122 124 126 127 x i i ACKNOWLEDGEMENTS I would l i k e to take t h i s o p portunity t o thank P r o f e s s o r D.C. F r o s t f o r h i s support.. Also my s i n c e r e thanks go to Dr. N. P. C. Westwood f o r h i s encouragement and guidance during my s t a y a t U.B.C, and to Dave Colburne f o r h e l p f u l d i s c u s s i o n s . I would l i k e to thank the N a t i o n a l Research C o u n c i l f o r generous f i n a n c i a l a i d . I wish t o acknowledge the a s s i s t a n c e of the mechanical, 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 . Most of a l l I would l i k e to thank my f a m i l y . 1 CHAPTER ONE I n t r o d u c t i o n 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 (PES) i s a n e x p e r i m e n t a l t e c h n i q u e g i v i n g a d i r e c t measurement o f t h e b i n d i n g e n e r g i e s o f e l e c t r o n s i n atoms and m o l e c u l e s . I r r a d i a t i o n o f g a s e o u s s a m p l e m o l e c u l e s w i t h s u f f i c i e n t l y h i g h f r e q u e n c y (vacuum UV ) m o n o c h r o m a t i c l i g h t w i l l p r o d u c e some p h o t o e l e c t r o n s h a v i n g k i n e t i c e n e r g y E^. The b i n d i n g e n e r g y o r i o n i z a t i o n p o t e n t i a l ( IP) o f t h e e l e c t r o n , and t h e f r e g u e n c y o f t h e l i g h t s o u r c e (v) d e t e r m i n e E/, by t h e c o n s e r v a t i o n o f e n e r g y r u l e . lw = I P • Eft (1-1) K i n e t i c e n e r g y o f t h e m o l e c u l a r i o n i s n e g l e c t e d a s c o n s e r v a t i o n o f momentum shows i t t o be i n s i g n i f i c a n t . I n PE s p e c t r a t h e y i e l d (on a r e l a t i v e s c a l e ) o f p h o t o e l e c t r o n s h a v i n g e n e r g y E^ i s p l o t t e d as a f u n c t i o n o f I P ., 2 The I P ' s m e asured i n PES a r e c l o s e l y r e l a t e d t o t h e e l e c t r o n i c e n e r g y l e v e l s o f t h e m o l e c u l e . The s i m p l e s t a p p r o x i m a t i o n i s Koopmans' Theorem 1 w h i c h a s sumes _ I P t o be t h e e i g e n v a l u e o f t h e m o l e c u l a r o r b i t a l v a c a t e d . As e l e c t r o n s i n any o r b i t a l h a v i n g I P < i w may be p h o t o e j e c t e d one may s t u d y a l l {or most) o f t h e v a l e n c e o r b i t a l s . A PE s p e c t r u m c o n s i s t s o f a number o f bands e a c h , by Koopmans' t h e o r e m , r e p r e s e n t i n g i o n i z a t i o n f r o m a p a r t i c u l a r m o l e c u l a r o r b i t a l . The g e n e r a l f i e l d o f e l e c t r o n s p e c t r o s c o p y e n c o m p a s s e s many t e c h n i q u e s r e l a t e d t o PES. T h e s e may a n a l y z e g a s e s o r s o l i d s u s i n g i o n i z i n g s o u r c e s s u c h a s p h o t o n s , e l e c t r o n s , o r e x c i t e d a t o m s , and may i n v e s t i g a t e t h e i n t e n s i t y d e p e n d a n c e on a n g l e o r i o n i z i n g e n e r g y . X - r a y 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 (XPS, o r ESCA - E l e c t r o n S p e c t r o s c o p y f o r C h e m i c a l A n a l y s i s ) i s v e r y s i m i l a r t o PES. By v i r t u e o f i t s h i g h e r e n e r g y l i g h t s o u r c e s ( Ex. Mg l i n e a t 1245eV) b o t h v a l e n c e and c o r e o r b i t a l s may be s t u d i e d . However t h e n a t u r a l w i d t h o f s o u r c e l i n e s l i m i t s r e s o l u t i o n t o a b o u t 0.5 e V , o b s c u r i n g s p e c t r a l d e t a i l i n t h e v a l e n c e r e g i o n . E x a m i n a t i o n o f t h i s d e t a i l r e g u i r e s n e a r m o n o c h r o m a t i c vacuum u l t r a v i o l e t s o u r c e s l i k e t h o s e u s e d i n PES. T h e s e a l l o w a c c e s s t o o n l y p a r t o f t h e v a l e n c e r e g i o n o f most m o l e c u l e s , however n a t u r a l l i n e w i d t h s a r e s o n a r r o w t h a t r e s o l u t i o n i s l i m i t e d by v a r i o u s i n s t r u m e n t a l c o n s i d e r a t i o n s r a t h e r t h a n by t h e s o u r c e l i n e w i d t h . The i d e a l l i g h t s o u r c e w o u l d be e n e r g e t i c e n o u g h t o show t h e w h o l e v a l e n c e r e g i o n , w o u l d be m o n o c h r o m a t i c , and v e r y i n t e n s e . E x c e p t i n g 3 p e r h a p s s y n c h r o t r o n r a d i a t i o n , s u c h a s o u r c e i s n o t a v a i l a b l e . Gas d i s c h a r g e s o u r c e s a r e u s e d , t h e He I l i n e (.21.22 eV o r 584 A) b e i n g f a r t h e most common. The i n c r e a s i n g u s e o f He I I ( 40.81 eV ) l i g h t s o u r c e s r e p r e s e n t s a s i g n i f i c a n t a d v a n c e i n PES t e c h n o l o g y . The p h o t o e l e c t r i c e f f e c t was e x p l a i n e d i n 1905 by E i n s t e i n 2 , h o w e v e r t h e f i r s t PE s p e c t r o m e t e r s were n o t b u i l t u n t i l t h e e a r l y 1960 » s . 3 S i n c e t h e n s p e c t r o m e t e r s h a v e b e e n i m p r o v e d and h a v e become c o m m e r c i a l l y a v a i l a b l e , a n d t h e t e c h n i q u e h a s become w e l l known. I n f a c t s p e c t r a o f most s m a l l , e a s i l y h a n d l e d a nd v o l a t i l i z e d compounds have been c a t a l o g u e d . PES i s t h e s u b j e c t o f s e v e r a l e x t e n s i v e r e v i e w s . T h e v o l u m e o f p u b l i s h e d l i t e r a t u r e , e s p e c i a l l y i n t h e " J o u r n a l o f E l e c t r o n S p e c t r o s c o p y a nd B e l a t e d Phenomena," t e s t i f i e s t o t h e u t i l i t y and i m p o r t a n c e o f PES. B o r e r e c e n t l y PES has been a p p l i e d t o t h e s t u d y o f r e a c t i v e and t r a n s i e n t c o m p o u n d s 1 * , w h i c h i s t h e s u b j e c t o f t h i s t h e s i s . S u c h m o l e c u l e s a r e a p p e a l i n g a s t h e y may be s p e c i a l l y a c t i v e r e a g e n t s , o r may be i n v o l v e d as r e a c t i o n i n t e r m e d i a t e s , a r e o f p a r t i c u l a r t h e o r e t i c a l i n t e r e s t , o r a r e p e r h a p s among t h e g r o u p o f known i n t e r s t e l l a r m o l e c u l e s . The e x p e r i m e n t a l h a n d l i n g o f t h e s e compounds r e q u i r e s s p e c i a l p r o c e d u r e s w h i c h u s u a l l y c a n n o t be u n d e r t a k e n w i t h c o n v e n t i o n a l PE s p e c t r o m e t e r s . They o f t e n must be p r o d u c e d i n an • o n - l i n e * f l o w - s y s t e m c o n n e c t e d t o t h e s p e c t r o m e t e r ' s s a m p l e i n l e t . , W h i l e t h e r e a r e e x p e r i m e n t a l d i f f i c u l t i e s , t h e r e s u l t s o f t h e work g i v e d e t a i l e d i n f o r m a t i o n on t h e e l e c t r o n i c 4 structure of some of the simplest molecules containing various functional groupings. In particular, multiply bonded nitrogen species containing the ^C=N-, -C=N, -N-C-0, — NNN, and -SCH groups have been studied by the author. The PE results are presented and analyzed, hopefully leading to an understanding of the chemistry of the molecules themselves, and also of the electronic structure of the functional group (s) involved. 5 CHAPTER TWO B a c k g r o u n d C o n s i d e r a t i o n s t o PES The t h e o r e t i c a l a s p e c t s o f PES h a v e been c o n s i d e r e d by v a r i o u s a u t h o r s . 6 , 8 , 9 , 1 0 , 1 2 I n t h i s c h a p t e r t h e f u n d a m e n t a l c o n c e p t s n e c e s s a r y f o r d i s c u s s i o n o f PES w i l l be s u m m a r i z e d w i t h no a t t e m p t a t t h e o r e t i c a l r i g o r . T h e s e i d e a s a r e u s e d i n s u b s e q u e n t c h a p t e r s where t h e y a p p l y t o p a r t i c u l a r c a s e s . 2.1 I n t r o d u c t i o n To b e g i n d i s c u s s i o n o f p h o t i o n i z a t i o n , e g . 1:1 i s shown i n e x p a n d e d f o r m . E k = ho - I P - ^ E v - A E T (2.1) The a d i a b a t i c i o n i z a t i o n p o t e n t i a l ( I P ) i s t h e e n e r g y d i f f e r e n c e b e t w e e n t h e i o n i c s t a t e i n v o l v e d a n d t h e g r o u n d 6 s t a t e m o l e c u l e , b o t h i n t h e i r g r o u n d v i b r a t i o n a l and r o t a t i o n a l l e v e l s , A E ^ and ^~Et a r e t h e c h a n g e s i n v i b r a t i o n a l a n d r o t a t i o n a l e n e r g i e s r e s p e c t i v e l y . ^ E t i s s m a l l a n d c a n o n l y be r e s o l v e d i n s p e c i a l c a s e s . 1 6 , 1 7 a t o r d i n a r y t e m p e r a t u r e s most s a m p l e m o l e c u l e s a r e i n t h e g r o u n d v i b r a t i o n a l s t a t e . So ^ E v = E*,, t h e v i b r a t i o n a l e n e r g y o f t h e i o n . S h o u l d t h e m o l e c u l e be i n an e x c i t e d v i b r a t i o n a l l e v e l o ne w i l l s e e one o r more " h o t b a n d s " i n t h e s p e c t r u m . E x a m p l e s o f t h e s e a r e s e e n i n C h a p t e r 6 i n w h i c h h o t band s c o r r e s p o n d i n g t o v '=0*-v , ,=1 a r e n o t e d . , v' r e f e r s t o t h e v i b r a t i o n a l l e v e l o f t h e i o n a n d v " t o t h a t o f t h e m o l e c u l e . The p r e s e n c e o f h o t b a n d s i s e a s i l y v e r i f i e d by c h a n g e s i n s a m p l e t e m p e r a t u r e . T h i s c h a n g e s t h e p o p u l a t i o n o f t h e e x c i t e d v i b r a t i o n a l l e v e l s i n t h e m o l e c u l e and t h u s t h e i n t e n s i t y o f t h e h o t b a n d s . M o d i f y i n g e g . 2.1 a s d e s c r i b e d above g i v e s I P = t w - E k - E»> (2.2) ft PE s p e c t r u m t h e r e f o r e c o n s i s t s o f a number o f b a n d s , some p e r h a p s h a v i n g r e s o l v e d v i b r a t i o n a l c o m p o n e n t s . The v i b r a t i o n a l s e p a r a t i o n s a r e t h o s e o f t h e i o n , and a r e u s u a l l y c h a n g e d somewhat f r o m 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 m o l e c u l e . E a c h b a n d r e p r e s e n t s an e l e c t r o n i c s t a t e o f t h e m o l e c u l a r i o n w h i c h i s e n e r g e t i c a l l y s e p a r a t e d f r o m t h e g r o u n d s t a t e m o l e c u l e by I P . Band i n t e n s i t i e s depend upon p h o t o i o n i z a t i o n c r o s s - s e c t i o n s and i n s t r u m e n t a l b e h a v i o u r . 7 while the d e t a i l s of band shape may depend upon s e v e r a l f a c t o r s to be mentioned l a t e r . 2.2 D i r e c t P h o t o i o n i z a t i o n In d i r e c t p h o t o i o n i z a t i o n a molecule i n the ground e l e c t r o n i c s t a t e i n t e r a c t s with a photon, producing a ph o t o e l e c t r o n and i o n . The p r o b a b i l i t y of p h o t o i o n i z a t i o n i s determined by the sguare of the t r a n s i t i o n moment i n t e g r a l M, which i s given by • H = *\ f n\t P| f) (2.3) y/" i s the e i g e n f u n c t i o n of the molecule, y/1 t h a t of the i o n and p h o t o e l e c t r o n , and p the d i p o l e moment operator. The sum extends over i e l e c t r o n s and j n u c l e i . Expanding y/n i n t o e l e c t r i c a l , v i b r a t i o n a l , and r o t a t i o n a l wavefunctions by the Born-Oppenheimer a p p r o x i m a t i o n 1 8 , g i v e s ^ « { r , B ) = ^ e » ( r ;H ) ±y»(B) y» ( B) (2.4) R y£», \fjj*t and i/g a r e the e l e c t r o n i c , v i b r a t i o n a l , and r o t a t i o n a l wavefunctions. ^" has only parametric dependence on the n u c l e a r c o o r d i n a t e s R. p may a l s o be separated i n t o an e l e c t r o n i c and nu c l e a r part. 8 (2. 5) Equation 2.3 may now be expanded. M = ^1R) i^*[R) * ; ( R ) * ; ( R ) d r . ^*Cr;R) dr The second term i s zero as e l e c t r o n i c e i q e n f u n c t i o n s belonqing 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 are orthoqonal. The r o t a t i o n a l wavefunctions may be neqlected i n the case where r o t a t i o n a l s t r u c t u r e i s not r e s o l v e d . 1 9 Now M can be w r i t t e n as B « (#MH)| ft ( 8 ) ) ( y ^ ( r : B ) J £ P | %* (r;F)) (2.7) The second i n t e q r a l of eq. 2.7 q i v e s t h e c r o s s - s e c t i o n f o r p h o t o i o n i z a t i o n to a p a r t i c u l a r i o n i c s t a t e . I t i s approximately constant over the width of a s p e c t r a l band and determines the band i n t e n s i t y . The a t t e n u a t i n g f i r s t i n t e q r a l i s the Franck-Condon i n t e q r a l which determines band shape. 9 2.3 The F r a n c k - C o n d o n F a c t o r I E c a s e s where v i b r a t i o n a l s t r u c t u r e i s r e s o l v e d f o r a PE band i t i s t h e F r a n c k - C o n d o n f a c t o r 2 0 - 2 2 w h i c h d e t e r m i n e s t h e i n t e n s i t y o f e a c h v i b r a t i o n a l peak. B o t h t h e v i b r a t i o n a l l e v e l s p a c i n g and t h e s h a p e o f t h e F r a n c k -Condon (FC) e n v e l o p e a r e c r i t i c a l i n s p e c t r a l a s s i g n m e n t . The 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 s p r o p o r t i o n a l t o t h e FC i n t e g r a l , w h i c h i s j u s t t h e o v e r l a p o f i n i t i a l and f i n a l v i b r a t i o n a l w a v e f u n e t i o n s . , The FC f a c t o r may be c o n s i d e r e d w i t h r e f e r e n c e t o t h e p o t e n t i a l c u r v e f o r a d i a t o m i c m o l e c u l e and i t s i o n ( F i g 2 . 1 ) . The v i b r a t i o n a l w a v e f u n c t i o n w i l l h a v e a h i g h a m p l i t u d e i n o n l y a s m a l l r e g i o n ( t h e FC r e g i o n ) a r o u n d t h e e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e . L a r g e o v e r l a p o f and i s f a v o u r e d by u n c h a n g e d o r s l i g h t l y c h a n g e d n u c l e a r p o s i t i o n s . T h i s d e f i n e s t h e *FC r e g i o n ' . F i g . 2.1 shows t h e p o t e n t i a l e n e r g y c u r v e f o r t h e g r o u n d s t a t e m o l e c u l e and s e v e r a l s t a t e s o f t h e m o l e c u l a r i o n , t o g e t h e r w i t h t h e PE s p e c t r u m o f a f i c t i o n a l d i a t o m i c m o l e c u l e . The FC r e g i o n i s i n d i c a t e d by t h e s h a d e d a r e a . S h o u l d t h e w a v e f u n c t i o n o f t h e n t h v i b r a t i o n a l l e v e l o f an i o n i c s t a t e be l a r g e i n t h i s r e g i o n t h e n l a r g e o v e r l a p w i l l r e s u l t . U s i n g t h i s a s a g u i d e t h e c o n c e p t o f FC e n v e l o p e s c a n be c o n s i d e r e d i n some d e t a i l . I n F i g . 2.1, t/J* h a s a p o t e n t i a l e n e r g y c u r v e n e a r l y d i r e c t l y a b o v e y". T h e e q u i l i b r i u m i n t e r n u c l e a r d i s t a n c e i s 10 T H E F R A N C K - C O N D O N P R I N C I P L E I N 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 Predisiociation By Curve Crossing Dissociation Bonding Antibonding Nonbonding B O N D L E N G T H FIGURE 2:1 11 u n c h a n g e d i n d i c a t i n g a n o n b o n d i n g e l e c t r o n h a s been e j e c t e d , and t h e FC f a c t o r i s by f a r l a r g e s t f o r v ' = 0 . The r e s u l t a n t PE b a n d i s d i s t i n c t i v e i n t h a t t h e a d i a b a t i c I P i s a l s o t h e v e r t i c a l (most i n t e n s e v i b r a t i o n a l component) and o n l y a few o t h e r weak v i b r a t i o n a l b a n d s a p p e a r . Such a s h a r p b a n d i s g e n e r a l l y f o u n d on i o n i z a t i o n f r o m a " n o n b o n d i n g " o r b i t a l . S t a t e y/* h a s s e v e r a l v i b r a t i o n a l l e v e l s w i t h i n t h e FC r e g i o n , r e s u l t i n g i n a b r o a d FC e n v e l o p e w i t h v'=2 t h e v e r t i c a l I P . T h i s t y p e o f band i n d i c a t e s a s h i f t i n i n t e r n u c l e a r d i s t a n c e and i n t h i s c a s e an a n t i b o n d i n q o r b i t a l i s i n v o l v e d . , S t r e n g t h e n i n g o f t h e bond r e s u l t s i n a v i b r a t i o n a l f r e q u e n c y w h i c h i s i n c r e a s e d f r o m t h e v a l u e i n t h e m o l e c u l e i t s e l f . R emoval o f a b o n d i n g e l e c t r o n w o u l d g i v e a s i m i l a r FC e n v e l o p e b u t w i t h a d e c r e a s e d v i b r a t i o n a l f r e q u e n c y , a s f o r . S t a t e has s o c h a n q e d t h e i n t e r n u c l e a r d i s t a n c e t h a t p a r t o f t h e FC r e g i o n o v e r l a p s t h e c o n t i n u u m . The v i b r a t i o n a l s p a c i n g s i n t h e PE band s m o o t h l y c o n v e r g e . The more e x t r e m e c a s e o f i/* shows no v i b r a t i o n a l s t r u c t u r e a t a l l . T h e s e b a n d s i n v o l v e s t r o n g l y b o n d i n g o r b i t a l s . , S t a t e o v e r l a p s w i t h a d i s s o c i a t i n g s t a t e . Lower v* v a l u e s b e h a v e n o r m a l l y b u t i n c r e a s i n g v i b r a t i o n a l e x c i t a t i o n r a i s e s t h e p r o b a b i l i t y o f i n t e r s y s t e m c r o s s i n g , and e v e n t u a l l y v i b r a t i o n a l s t r u c t u r e i n t h e PE b a n d c o m p l e t e l y d i s a p p e a r s . U n l i k e ip*, t h i s b a n d does n o t show a smooth c o n v e r g e n c e o f v i b r a t i o n a l b a n d s , b u t r a t h e r a b r o a d e n i n g and l o s s o f s t r u c t u r e w i t h o u t a g r a d u a l d e c r e a s e o f v i b r a t i o n a l f r e q u e n c y . 12 T h i s q u a l i t a t i v e d i s c u s s i o n o f FC e n v e l o p e s i n t e r m s o f F i g . 2.1 d e m o n s t r a t e s t h e u t i l i t y o f v i b r a t i o n a l s t r u c t u r e i n a s s i g n i n g s p e c t r a . I n p o l y a t o m i c m o l e c u l e s s e v e r a l v i b r a t i o n a l modes may be e x c i t e d a t o n c e , g i v i n g two o r more s e r i e s on a s i n g l e b and. The mode e x c i t e d may g i v e some h i n t t o l o c a l i z a t i o n o f t h e o r b i t a l i n v o l v e d . F o r e x a m p l e i o n i z a t i o n f r o n t h e n i t r o g e n l o n e p a i r 13a,) o r b i t a l o f NH3 e x c i t e s t h e NH o u t o f p l a n e b e n d i n g v i b r a t i o n r e s u l t i n q i n a b r o a d band w i t h 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 a b o u t 9 0 0 c m - 1 . 1 3 O v e r l a p p i n g v i b r a t i o n a l s e r i e s a n d c l o s e n e s s o f t h e v i b r a t i o n a l c o m p o n e n t s may t e n d t o o b s c u r e s t r u c t u r e f o r l a r g e r m o l e c u l e s . W h i l e t h e p r i n c i p l e s a r e t h e same i n p c l y a t o m i c s , t h e s i t u a t i o n i s more c o m p l i c a t e d and v e r y o f t e n no s t r u c t u r e i s s e e n , e s p e c i a l l y f o r l a r g e r and h e a v i e r m o l e c u l e s . V i b r a t i o n a l s e l e c t i o n r u l e s a r e o f some i m p o r t a n c e a nd t h e s e a r e m e n t i o n e d i n t h e n e x t s e c t i o n . 2.4 S e l e c t i o n B u l e s B e f o r e c o n s i d e r i n g t h e more u s u a l m e a n i n g o f t h e t e r m s e l e c t i o n r u l e s i t s h o u l d be p o i n t e d o u t t h a t Koopmans* Theorem i t s e l f may be c o n s i d e r e d a p s e u d o s e l e c t i o n r u l e . 2 7 T h a t i s , t h e " s h a k e — u p " t y p e m u l t i - e l e c t r o n p r o c e s s i s uncommon, and one g e n e r a l l y s e e s o n l y s i n q l e e l e c t r o n p r o c e s s e s . I n f a c t m u l t i — e l e c t r o n p r o c e s s e s do o c c a s i o n a l l y o c c u r and w i l l be e x a m i n e d i n a s e p a r a t e s e c t i o n . 1 3 The s e l e c t i o n r u l e s f o r one e l e c t r o n t r a n s i t i o n s are generated by c o n s i d e r i n g t h a t eguation 2.7 must be t o t a l l y symmetric. The e l e c t r o n i c i n t e g r a l can be made t o t a l l y symmetric f o r any i o n i c s t a t e as the p h o t o e l e c t r o n can c a r r y whatever angular momentum i s r e g u i r e d . I o n i z a t i o n from a c l o s e d s h e l l , s i n g l e t molecule g i v e s the doublet s t a t e of the p o s i t i v e i o n . A l l one 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 are allowed. The v i b r a t i o n a l i n t e g r a l of eguation 2.7 must a l s o be t o t a l l y symmetric. Assuming that i s the t o t a l l y symmetric ground v i b r a t i o n a l l e v e l r e q u i r e s t h a t p/J be t o t a l l y symmetric., T h i s i s so f o r any l e v e l of a t o t a l l y symmetric v i b r a t i o n a l mode. N o n t o t a l l y symmetric modes are symmetric f o r v*=0,2,4... Thus e x c i t a t i o n of a n o n t o t a l l y symmetric v i b r a t i o n i n v o l v e s *freguency d o u b l i n g * . 2.5 H u l t i - e l e c t r o n Processes Supernumerary bands i n the PE s p e c t r a of some molecules can r e s u l t from t w o - e l e c t r o n processes, Hany e l e c t r o n i c s t a t e s of a molecular i o n can not be d e s c r i b e d i n terms of removal of a s i n g l e e l e c t r o n from the ground s t a t e molecule, as was assumed i n the previous d i s c u s s i o n . These correspond to PE processes i n v o l v i n g i o n i z a t i o n of one e l e c t r o n , and simultaneous e x c i t a t i o n of a s e c o n d . 3 0 Such a t w o - e l e c t r o n t r a n s i t i o n i s f o r b i d d e n by the p s e u d o s e l e c t i o n r u l e f a c e t of Keopmans' th e o r e m . 2 7 T h i s "shake-up" phenomena i s u s u a l l y a s s o c i a t e d with more e n e r g e t i c photons to which more h i g h l y e x c i t e d i o n i c s t a t e s with l a r g e r c o n f i g u r a t i o n i n t e r a c t i o n (CIJ r o l e s are a v a i l a b l e . 2 9 Thus shake—up i s more common i n He I I s p e c t r a 3 0 - 3 3 than i n He I s p e c t r a , and i s e s p e c i a l l y important i n X P S . 3 , 7 - 9 Nevertheless such f o r b i d d e n t r a n s i t i o n s are p o s s i b l e i n He I s p e c t r a . One notable example i s the t r a n s i e n t molecule C S . 3 3 - 3 5 By u s i n g a CI d e s c r i p t i o n of the i o n i c s t a t e , such f o r b i d d e n t r a n s i t i o n s may be e x p l a i n e d . The p r o b a b i l i t y o f t r a n s i t i o n t o a »doubly e x c i t e d 1 , CI i o n i c s t a t e i s p r o p o r t i o n a l to the s t a t e ' s o v e r l a p with an i o n i c s t a t e which may be formed by a simple o n e - e l e c t r o n t r a n s i t i o n f r o n the ground s t a t e m o l e c u l e . 3 0 , 3 4 The f o r b i d d e n t r a n s i t i o n may be c o n s i d e r e d t o borrow i n t e n s i t y from the allowed t r a n s i t i o n by C I . 3 0 In cases where a CI band appears Koopmans' theorem i s c l e a r l y not a good a p p r o x i m a t i o n , 2 7 however the s c a r c i t y of data on e x c i t e d i o n i c s t a t e s makes Siegbahn's approach a p p e a l i n g . 3 * That i s , one assumes the sum of the a p p r o p r i a t e IP and the corresponding e x c i t a t i o n energy i n the n e u t r a l molecule, equals th e i o n i z a t i o n energy g i v i n g the CI band. The assumption has been c r i t i c a l l y d i s c u s s e d , 3 2 yet appears to be of some u s e . 3 2 - 3 * A l t e r n a t i v e l y one may c a l c u l a t e e x c i t a t i o n e n e r g i e s f o r the i o n . 3 2 , 3 3 C a l c u l a t e d r e l a t i v e i n t e n s i t i e s have a l s o been obtained with some s u c c e s s . 3 2 , 3 3 E i t h e r of these techniques may h e l p i d e n t i f y the r e s u l t a n t i o n i c s t a t e . 15 G e n e r a l l y i n He I PES t h e r e i s a one t o one c o r r e s p o n d e n c e o f I P ' s and m o l e c u l a r o r b i t a l s . T h i s o b s e r v a t i o n s u p p o r t s a o n e — e l e c t r o n t r e a t m e n t and has a c t u a l l y e n h a n c e d t h e g e n e r a l u s e o f t h e m o l e c u l a r o r b i t a l a p p r o a c h , 2 7 C o n v e r s e l y , m u l t i - e l e c t r o n p r o c e s s e s o c c a s i o n a l l y o c c u r and s o d e m o n s t r a t e t h e i m p o r t a n c e o f C I . 2.6 A u t o i o n i z a t i o n I n some c a s e s t h e i n c i d e n t p h o t o n e n e r g y c o i n c i d e s w i t h a s u p e r - e x c i t e d s t a t e o f t h e m o l e c u l e . P h o t o e x c i t a t i o n t o a s u p e r — e x c i t e d m o l e c u l a r s t a t e , f o l l o w e d by a r a d i a t i o n l e s s t r a n s i t i o n t o an i o n i c s t a t e i s c a l l e d a u t o i o n i z a t i o n . A u t o i o n i z a t i o n r e s u l t s i n an i o n i c s t a t e w h i c h may a l s o be r e a c h e d by d i r e c t p h o t o i o n i z a t i o n . The FC e n v e l o p e f o r a u t o i o n i z a t i o n i s c h a n g e d f r o m t h a t f o r d i r e c t p h o t o i o n i z a t i o n a s t h e o v e r l a p o f v i b r a t i o n a l w a v e f u n c t i o n s o f a l l t h r e e s t a t e s must be c o n s i d e r e d . T h i s i s d e m o n s t r a t e d by t h e Ne I s p e c t r u m o f NO 4 3 and o f 0 2 .*° The Ne I s p e c t r u m o f 0 2 i s shown i n F i g . 2.2a. The Ne I l i n e a t 16.85eV g i v e s t h e i n t e n s e b a n d s a b o v e 13 eV by an a u t o i o n i z i n g p r o c e s s w h i l e t h e l i n e a t 16.67eV g i v e s t h e w e a k e r s e r i e s o f b a n d s by d i r e c t p h o t o i o n i z a t i o n . F i g . 2.2 d e m o n s t r a t e s t h e i n v o l v e m e n t of an e x c i t e d m o l e c u l a r s t a t e . As a r e s o n a n c e a b s o r p t i o n i s i n v o l v e d a c h a n g e o f p h o t o n e n e r g y w i l l e l i m i n a t e a u t o i o n i z a t i o n . The 16 FIGURE 2:2 Autoionization in the Ne I PE spectrum of oxygen, (a) The main series shows autoionization by the Nel (16.85 eV) l i n e . The Nel (16.67 eV) l i ne gives the weaker series showing the normal Frank-Condon envelope, (from ref. IO) (b) A schematic diagram showing the direct and autoionizing trans i t ions of oxygen, (from ref. 6) 17 p r o b a b i l i t y o f s e e i n g a u t o i o n i z a t i o n d e c r e a s e s w i t h t h e h i g h e r p h o t o n e n e r g i e s o f He I and He I I s o u r c e s , a s t h e r e a r e g e n e r a l l y f e w e r bound s t a t e s a t h i g h e r e n e r g y . 2.7 Use o f C a l c u l a t i o n s i n PES Koopmans 1 Theorem has a l r e a d y b e e n m e n t i o n e d and i t s i m p o r t a n c e i n PES d e s e r v e s a d d i t i o n a l comment. T h i s a p p r o x i m a t i o n n e g l e c t s r e l a x a t i o n e f f e c t s , and d i f f e r e n c e s i n e l e c t r o n c o r r e l a t i o n a n d r e l a t i v i s t i c e n e r g y b e t w e e n t h e m o l e c u l e and i o n , 3 6 W h i l e i t i s h o p e d t h e s e a p p r o x i m a t i o n s w i l l c a n c e l , Koopmans* t h e o r e m w i l l u s u a l l y o v e r e s t i m a t e I P ' s . 2 7 , 3 7 D e s p i t e t h e a c k n o w l e d g e d l i m i t a t i o n s i t h a s b e e n w i d e l y u s e d i n P E S , l a r g e l y b e c a u s e o f t h e a v a i l a b i l i t y o f v a r i o u s MO c a l c u l a t i o n s . I t h a s been t h e u s u a l i n t e r f a c e b e t w e e n PES a n d t h e o r y . 2 7 B r e a k d o w n s o f Koopmans* t h e o r e m a r e known i n many c a s e s ( i e . N 2 3 7 , CO *, CS *, F ^ 2 7 ) and i n f a c t may be q u i t e common, b u t t h e h i g h g u a l i t y H a r t r e e - F o c k c a l c u l a t i o n s r e q u i r e d t o d e t e c t b r e a k d o w n a r e o f t e n n o t a v a i l a b l e , 2 7 N e v e r t h e l e s s Koopmans* t h e o r e m c o n t i n u e s t o be u s e d w i t h c o n s i d e r a b l e s u c c e s s . 2 5 , 2 7 I n a p p l y i n g Koopman*s t h e o r e m one c o m p a r e s t h e 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 w i t h e x p e r i m e n t a l I P ' s . Seme d e g r e e o f c o n f i d e n c e must be e s t a b l i s h e d b y t e s t i n g t h e c a l c u l a t i o n . T h i s i s b e s t done by c o m p a r i n g c o m p u t e d and e x p e r i m e n t a l t r e n d s f o r a s e r i e s o f r e l a t e d m o l e c u l e s . 18 Commonly used c a l c u l a t i o n s (CNDO 3 8, CND0/2 3*, INDO*°, MINDO**, SPINDO* 2, etc.) o f t e n p r e d i c t the t r e n d s i n IP's r e l i a b l y , while g i v i n g i o n i z a t i o n e n e r g i e s which are not e s p e c i a l l y good., D i f f e r e n t c a l c u l a t i o n s may give c o n f l i c t i n g r e s u l t s perhaps changing the o r d e r i n g of o r b i t a l e n e r g i e s , so i t i s u s e f u l to have s e v e r a l methods a v a i l a b l e . T h i s a l l o w s some c h o i c e of the best c a l c u l a t i o n f o r the f a m i l y of molecules under examination. More advanced computing programs c a l c u l a t e IP's without r e l i a n c e on Koopmans* theorem. These allow c a l c u l a t i o n of IP's which may toe b e t t e r than Hartree-Fock e i g e n v a l u e s . A r ecent review of M u l t i p l e S c a t t e r i n g X * 3 IMS-Xa), and other methods, i s given by C o n n e l l y , * * and S l a t e r * 5 . A technique 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 to Koopmans* theorem has been developed by D. P Chong e t a l . * 6 - * 8 A l s o a many body. Green's F u n c t i o n approach has been developed by Cederbaum e t a l . * * The r e c e n t l y completed HAM3 5 5 - 5 3 (Hydrogenic Atoms i n Molecules) program of E. Lindholm e t a l has been used where a p p l i c a b l e i n t h i s t h e s i s . The program i n v o l v e s a s e m i e m p i r i c a l MO model based on an expanded and s o p h i s t i c a t e d v e r 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 . The s h i e l d i n g values are h e a v i l y parameterized ( f o r the atoms H,C,N,0, and F) by matching c a l c u l a t e d r e s u l t s with the PE and other data f o r about 120 molecules. IP's are c a l c u l a t e d independently of Koopmans' theorem by c a l c u l a t i n g the e n e r g i e s of the f i n a l and i n i t i a l s t a t e s , 5 2 o r by a S l a t e r t r a n s i t i o n s t a t e . 5 0 - 5 2 T h i s program i s new 19 and r e l a t i v e l y untested. I t i s l i m i t e d i n a p p l i c a b i l i t y , and has been c r i t i c i z e d f o r "bad p h y s i c s " 5 4 and i t s very heavy p a r a m e t e r i z a t i o n . 5 * t s s N evertheless where t e s t e d i t has been demonstrated to g i v e very accurate I P ' s . 5 0 - 5 3 Hhere p o s s i b l e the program has been used i n t h i s t h e s i s , p a r t l y as a c o n t i n u i n g demonstration of the program's c a p a b i l i t y and p a r t l y as a s u p p o r t i v e assignment c r i t e r i o n . 2.8 C r o s s - s e c t i o n Dependance on Photon Energy Observed PE band i n t e n s i t i e s depend on p h o t o i o n i z a t i o n c r o s s — s e c t i o n , photon f l u x , sample pressure, the s o l i d angle of e l e c t r o n s a n a l y z e d , the d i r e c t i o n of the s o l i d angle, the 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 , s e n s i t i v i t y of the e l e c t r o n d e t e c t o r , e t c . Because of the number of v a r i a b l e s , a b s o l u t e c r o s s — s e c t i o n s cannot be obtained e a s i l y . Instrumental energy d i s c r i m i n a t i o n may a c t u a l l y cause s e n s i t i v i t y to be a f u n c t i o n of e l e c t r o n energy. D i r e c t comparison of s p e c t r a measured on d i f f e r e n t machines i s d i f f i c u l t . Despite these problems the v a r i a t i o n of c r o s s - s e c t i o n with photon energy can sometimes be used as an a i d i n a s s i g n i n g s p e c t r a . D e t a i l e d c r o s s — s e c t i o n c a l c u l a t i o n s p r e d i c t v a r i a t i o n s i n r e l a t i v e band i n t e n s i t i e s with photon energy, and t h i s has been v e r i f i e d e x p e r i m e n t a l l y . 5 * - * 0 The c r o s s - s e c t i o n f o r p h o t o i o n i z a t i o n f o r an LCAO-MO 20 has been separated i n t o two t e r m s . S 4 , 5 6 The f i r s t term i s a sum of c r o s s — s e c t i o n s of the atomic o r b i t a l s i n v o l v e d , weighted by t h e i r c o n t r i b u t i o n to the molecular o r b i t a l . The second i s a two centered term c o r r e c t i n g f o r d i f f r a c t i o n and geometric f a c t o r s . 5 6 i n the case of lone p a i r o r b i t a l s the f i r s t term predominates and the c r o s s — s e c t i o n depends l a r g e l y on the one atomic o r b i t a l i n v o l v e d . 6 4 On t h i s b a s i s , c r o s s - s e c t i o n trends f o r "lone p a i r " o r b i t a l s are expected t o depend p r i m a r i l y on the atom and l e s s on the molecule as a whole. T h i s g e n e r a l i z a t i o n i s a u s e f u l assignment c r i t e r i o n . The c r o s s - s e c t i o n behaviour of the one c e n t e r terms has been c a l c u l a t e d f o r a v a r i e t y of molecules by Schweig and T h i e l , and t h e i r r e s u l t s are shown i n F i g . 2.3. In a l l cases the c r o s s — s e c t i o n r i s e s r a t h e r r a p i d l y at t h r e s h o l d , reaches a maximum a t some value of E n , and then s l o w l y decays. S e v e r a l important t r e n d s a r e co n t a i n e d i n the f i g u r e and these correspond to gen e r a l p a t t e r n s which v a r i o u s experimenters have noted. The c r o s s — s e c t i o n a l behaviour of o r b i t a l s has been di s c u s s e d i n terms of s:p c h a r a c t e r , and a l s o i n terms of o r b i t a l s i z e . 5 8 Using a plane wave d e s c r i p t i o n of the p h o t o e l e c t r o n , the t r a n s i t i o n moment f o r a molecular o r b i t a l i s p r o p o r t i o n a l to i t s ov e r l a p with the p h o t o e l e c t r o n . For maximum ove r l a p the wavelength of the phot o e l e c t r o n i s comparable t o the o r b i t a l s i z e . 6 4 , 8 5 Thus a d i f f u s e MO has i t s maximum c r o s s - s e c t i o n at a somewhat lower p h o t o e l e c t r o n e n e r g y . 6 4 T h i s i s shown i n F i g . 2.3 by comparing the p o s i t i o n s of the maxima o f the n=2 and n=3 o r b i t a l s . The more d i f f u s e n=3 o r b i t a l s g e n e r a l l y 21 FIGURE 2:3 Plots of t rans i t ion moment vs. photoelectron energy(E ) for various atomic o r b i t a l s . (from ref. 50) 22 have maxima at l o s e r p h o t o e l e c t r o n energies. The t r e n d has been seen i n the i n c r e a s e d c r o s s — s e c t i o n s of 3s and 3p r e l a t i v e t o 2s and 2p d e r i v e d MO's on changing to lower energy p h o t o n s . 6 5 T h i s i s p a r t i c u l a r l y e v i d e n t i n comparing PES and X P S 7 , 6 5 but i s a l s o t rue f o r He I I , He I , and Ne I sources. In F i g . 2.3, the much l a r g e r s l o p e s j u s t above t h r e s h o l d f o r the s r e l a t i v e t o the p o r b i t a l s i n d i c a t e how s:p r a t i o s a f f e c t the c r o s s - s e c t i o n dependance on photon energy. T h i s behaviour i s demonstrated by comparing lone p a i r and s based o r b i t a l s 6 0 , as i n the case of the sulphur lone p a i r s of the t h i o c y a n a t e molecules (Chapter 7). F i g . 2.3 a l s o shows t h a t i t i s i n the r e g i o n of the Ne I, He I and He I I source e n e r g i e s that the e f f e c t s of changing l i g h t source are t h e l a r g e s t . 5 9 The v a r i a t i o n s may be used t o c l a r i f y a case of a c c i d e n t a l degeneracy, as i n HCN. 5 a, 6 1 In t h i s case a ptr and Tr o r b i t a l show d i f f e r e n t behaviour i n changing sources. One cannot e a s i l y p r e d i c t the t r e n d i n a case as s u b t l e as t h i s without c a r e f u l l y examining c l o s e l y r e l a t e d molecules, as the o- and tr o r b i t a l s have s i g n i f i c a n t two c e n t e r terras i n t h e i r c r o s s - s e c t i o n s . In Chapter 7 a p a r t i c u l a r case i s examined i n some d e t a i l . The g e n e r a l trends of F i g . 2.3 are co n s i d e r e d and the d i s t i n c t i o n between i n and out of plane rr o r b i t a l s i s made on the b a s i s of the observed i n t e n s i t y changes noted i n c l o s e l y r e l a t e d molecules. 2.9 T h r o u g h S p a c e and T h r o u g h Bond I n t e r a c t i o n s and t h e Sum E u l e The t h r o u g h s p a c e (TS) and t h r o u g h bond (TB) a p p r o a c h t o m o l e c u l a r o r b i t a l s h a s b e e n d e v e l o p e d by £. H o f f mann and o t h e r s t o e x p l a i n i n t e r a c t i o n s o f l o c a l i z e d s e t s o f o r b i t a l s o r f u n c t i o n a l g r o u p s . 6 5 - 6 7 The v a l e n c e o r b i t a l s a r e d i v i d e d i n t o an u n d e r l y i n g f r a m e w o r k o f c r o r b i t a l s a nd a s u p e r i m p o s e d s y s t e m o f TT and n o r b i t a l s . , C h r o m o p h o r e s a r e c o n s i d e r e d t o h a v e , t o a f i r s t a p p r o x i m a t i o n , l o c a l i z e d o r b i t a l s w h i c h may i n t e r a c t w i t h o t h e r s o f t h e same symmetry. As t h e o r b i t a l e n e r g i e s a r e c o n s i d e r e d p e r t u r b e d by t h e i n t e r a c t i o n s , t h e c o n c e p t o f o r b i t a l e n e r g i e s • b e f o r e h a n d ' a f t e r * i n t e r a c t i o n p e r m e a t e s t h e l i t e r a t u r e . C o n s i d e r a t i o n o f t h e b a s i c p e r t u r b a t i o n e q u a t i o n s a l l o w s e v e r a l c o n c l u s i o n s t o be d r a w n . 6 2 I n t h e a b s e n c e o f t h e i n t e r a c t i o n t h e i t h o r b i t a l h a v i n g wavef u n c t i o n if/io h a s e n e r g y E i 0. , The e n e r g y and w a v e f u n c t i o n f-L a f t e r i n t e r a c t i o n H* i s g i v e n by e q u a t i o n 2.8. The e q u a t i o n shows i n t e r a c t i o n t o be p a i r w i s e a d d i t i v e a n d shows t h a t t h e l e v e l s w i l l r e p e l . A l s o t h e m a q n i t u d e i s l+l i — (2.8) 24 i n v e r s e l y p r o p o r t i o n a l t o the unperturbed s e p a r a t i o n . Within the proposed model t h e r e are two types of i n t e r a c t i o n s p o s s i b l e : (1) a d i r e c t TS or s p a t i a l o v e r l a p of the tr or n o r b i t a l s g i v i n g symmetric and antisymmetric combinations which are e g u a l l y s t a b i l i z e d and d e s t a b i l i z e d r e s p e c t i v e l y ; (2) a TB i n t e r a c t i o n i n v o l v i n g o v e r l a p of the tr or n o r b i t a l s with the cr o r b i t a l s of the same symmetry. The c l a s s i c example o f TS i n t e r a c t i o n s i s the s p l i t t i n g o f the rr l e v e l s of norbornadiene** , 6 3 , 6 . s # 6 « , and the TB e f f e c t i s demonstrated by 1 , 4 - d i a z a - b i c y c l o — b i c y c l o f2.2.21 octane ( D A B C O ) . 6 3 , 6 4 , 6 7 In t h i s case the symmetric combination of the n i t r o g e n lone p a i r s i s d e s t a b i l i z e d r e l a t i v e t o the antisymmetric combination v i a c o u p l i n g with a symmetric cr l e v e l , The antisymmetric combination of n i t r o g e n lone p a i r s i s s t a b i l i z e d through c o u p l i n g with the antisymmetric cr* o r b i t a l . Dependant upon the p a r t i c u l a r case the TB and TS e f f e c t s may r e i n f o r c e or oppose each o t h e r . 6 3 IS i n t e r a c t i o n depends upon geometry and i s g e n e r a l l y n e g l e c t e d i f the i n t e r a c t i n g s i t e s are more than 2.5A a p a r t . 6 3 The TB e f f e c t however, may be a p p r e c i a b l e ever a c o n s i d e r a b l e d i s t a n c e . 6 4 In c o n s i d e r i n g i n t e r a c t i o n s a s e r i e s of eguations l i k e e guation 2.9 may be w r i t t e n . , E i = El0 • A E T S • * E T B (2.9) These eguations are used i n a p a r t i c u l a r case d i s c u s s e d i n Chapter 4. The p a i r w i s e nature of the e f f e c t i s such that 25 on summing over a l l o r b i t a l s o f the same symmetry one f i n d s , LEL = £Ei0 C2.10) Eg. 2.10 i s a statement of the "Sum Rule* which s t a t e s t h a t the sum of e i g e n v a l u e s of a s e c u l a r equation e q u a l s the d i a g o n a l s u m . 6 8 - 7 2 That i s , the t o t a l energy i s u n a f f e c t e d by the o r b i t a l i n t e r a c t i o n s . 6 8 T h i s r u l e may be invoked with no assumptions about the nature of the i n t e r a c t i o n s themselves. As an assignment c r i t e r i o n the sum r u l e may be used to p r e d i c t the number of PE bands. To do t h i s E;0 values f o r each type of molecular o r b i t a l a r e r e g u i r e d and these are u s u a l l y chosen from the IP's of simple m o l e c u l e s . 6 9 , 7 1 Experience has shown the sum r u l e to be q u i t e g e n e r a l . 6 8 — 7 2 2.10 General C r i t e r i a f o r A s s i g n i n g PE Spectra Host of the concepts used i n s p e c t r a l assignment have alre a d y been d i s c u s s e d . T h i s s e c t i o n presents an overview, g i v i n g some p e r s p e c t i v e t o the process., (1) A s t a r t i n g p o i n t i s a molecular o r b i t a l p i c t u r e of the molecule., T h i s may r e v e a l how many and which molecular o r b i t a l s are i n v o l v e d or may even c o n s i s t of a more d e t a i l e d through space and through bond approach. (2) The band shapes which are c o n t r o l l e d by Franck-Condon i n t e g r a l s may allow 26 i d e n t i f i c a t i o n o f nonbonding as opposed t o bonding or a r t i b o n d i n g o r b i t a l s . Anomalies i n the band envelope may i n d i c a t e more than one i o n i z a t i o n process. (3) Comparison of observed v i b r a t i o n a l s t r u c t u r e with IB data i s most i n f o r m a t i v e . I d e n t i f i c a t i o n of the v i b r a t i o n a l mode may suggest l o c a l i z a t i o n o f the o r b i t a l , while a change i n freguency from the molecular value r e f l e c t s a d i f f e r e n c e i n geometry and hence the nature of the o r b i t a l i n v o l v e d . (4) The a c t u a l p o s i t i o n of the band i s important as one always expects i o n i z a t i o n from a p a r t i c u l a r type of o r b i t a l — say a c h l o r i n e l o n e p a i r - t o be i n a c e r t a i n r e g i o n of the spectrum — about 11 to 13 eV. (5) The i n t e n s i t y of the band i s of l i m i t e d importance. However, i n cases of degeneracy anomalously int e n s e bands can r e s u l t . where t h e r e i s some symmetry l e a d i n g t o degeneracy, s t r u c t u r e due to s p i n - o r b i t s p l i t t i n g may r e s u l t . Heavier atoms are u s u a l l y i n v o l v e d here and the t r e n d i n a s e r i e s i n c l u d i n g l i g h t e r s u b s t i t u e n t s <ex. a f a m i l y of h a l i d e s ) , i s h e l p f u l i n i d e n t i f i c a t i o n . , S i m i l a r l y J a h n - T e l l e r and Renner d i s t o r t i o n can l i f t o r b i t a l degeneracy. As the molecules i n t h i s t h e s i s are of low symmetry n e i t h e r of these e f f e c t s have been observed, A comprehensive treatment of both e f f e c t s i s given i n r e f e r e n c e 6, (6) The v a r i a t i o n of r e l a t i v e band i n t e n s i t y with photon energy can be o f c o n s i d e r a b l e importance i n assignment. (7) One assignment a i d not yet d i s c u s s e d i n d e t a i l i s the 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 . R eplacing a subgroup i n the molecule has a marked, o f t e n p r e d i c t a b l e 27 e f f e c t . Molecules are g e n e r a l l y s t u d i e d i n r e l a t e d groups to take advantage of such trends i n making assignments. The chemical s i g n i f i c a n c e of PE r e s u l t s i s c l e a r l y enhanced by s t u d i e s o f f a m i l i e s o f r e l a t e d or i s o e l e c t r o n i c molecules. T h i s approach i s used throughout the t h e s i s , h o p e f u l l y demonstrating i t s m e r i t s . In cases of low symmetry and when r e l i a b l e c a l c u l a t i o n s are u n a v a i l a b l e t h i s approach becomes p a r t i c u l a r l y important. (8) C o r r e l a t i o n o f PE s p e c t r a with c a l c u l a t i o n s i s a common technigue. The u t i l i t y of the c o r r e l a t i o n must be e s t a b l i s h e d by demonstrating the c a l c u l a t i o n ' s a b i l i t y t o reproduce the PE r e s u l t s f o r s i m i l a r molecules. T h i s check i s important whether Koopmans* theorem i s i n v o l v e d or not. (9) The Sum Bule has been mentioned e a r l i e r and i t may be used t o p r e d i c t the number of PE bands. (10) A u t o i o n i z a t i o n may be a c o m p l i c a t i o n , i n some cases d r a s t i c a l l y a l t e r i n g band shape. I t i s i d e n t i f i a b l e by changing the photon source. (11) Franck-Condon envelopes may a l s o be a f f e c t e d by hot bands. T h i s i s a p a r t i c u l a r concern i n the production of t r a n s i e n t s by p y r c l y s i s . , A change of sample temperature w i l l i d e n t i f y the hot bands, (12) F i n a l l y the p o s s i b i l i t y of m u l t i - e l e c t r o n processes must not be o v e r l o o k e d . This phenomenon w i l l g e n e r a l l y be spotted by the appearance of an unexpected a d d i t i o n a l PE band. 28 CHAPTER THREE The Phot o e l e c t r o n Spectrometer and General Experimental C o n s i d e r a t i o n s 3 . 1 I n t r o d u c t i o n A p h o t o e l e c t r o n spectrometer r e q u i r e s an i o n i z i n g source, a sample i n l e t l e a d i n g t o an i o n i z a t i o n r e g i o n , an e l e c t r o n energy a n a l y z e r with an e l e c t r o n d e t e c t o r and s u i t a b l e r e c o r d i n g equipment, and an a p p r o p r i a t e vacuum system. The design of the spectrometer must be compatible with the type of PES s t u d i e s planned. The two spectrometers used f o r t h i s work were s p e c i f i c a l l y designed f o r the study o f s h o r t - l i v e d molecules. To t h i s end sample i n l e t s must be e a s i l y adaptable as the means of producing v a r i o u s t r a n s i e n t s may d i f f e r c o n s i d e r a b l y , , G e n e r a l l y the i s l e t 29 must a l l o w r a p i d t r a n s i t o f t h e s a m p l e f r o m i t s p r o d u c t i o n p o i n t , t o t h e i o n i z a t i o n c h a m b e r , c o n t a c t i n g a s l i t t l e and a s i n e r t a s u r f a c e a s p o s s i b l e . E q u a l l y r a p i d r e m o v a l o f s a m p l e f r o m t h e i o n i z a t i o n r e g i o n i s n e c e s s a r y , i m p l y i n g t h a t i n c o n t r a s t t o c o n v e n t i o n a l PE s p e c t r o m e t e r s t h e s e m a c h i n e s w i l l e x p e r i e n c e a r a p i d f l o w o f l a r q e v o l u m e s o f q a s . The s a m p l e s t h e m s e l v e s may be c o r r o s i v e o r may t e n d t o decompose i n t h e s p e c t r o m e t e r . The o r i q i n a l s p e c t r o m e t e r h a s been d e s c r i b e d i n d e t a i l . 7 3 , 7 * The newer m a c h i n e i s l a r g e l y a c o p y b u t w i t h s e v e r a l p e r f o r m a n c e a l t e r i n g c h a n g e s . The g e n e r a l d e s i g n w i l l be d e s c r i b e d b r i e f l y a n d p e r f o r m a n c e w i l l be d i s c u s s e d . 3.2 The S p e c t r o m e t e r Vacuum S y s t e m The vacuum s y s t e m i s p r o b a b l y t h e most c r i t i c a l a r e a o f t h e o r i g i n a l s p e c t r o m e t e r ' s d e s i g n . R a p i d t r a n s i t o f t h e u n s t a b l e s p e c i e s i n t o , t h e n away f r o m t h e i o n i z a t i o n r e g i o n i s t h e p r i m a r y g o a l . H o p e f u l l y t h i s i s a c c o m p l i s h e d p r i o r t o any d e c o m p o s i t i o n . D e c o m p o s i t i o n i n t r o d u c e s i m p u r i t i e s i n t o t h e s p e c t r u m , and may a l s o l e a d t o d e p o s i t i o n o f p o l y m e r s e t c . , i n t h e i o n i z a t i o n chamber. T h i s c o a t s t h e e l e c t r o n o p t i c s w i t h a n o n c o n d u c t i n g m a t e r i a l , r e s u l t i n g i n a b u i l d u p o f s t a t i c c h a r g e s c a u s i n g a l o s s o f e f f i c i e n c y and r e s o l u t i o n o f t h e o p t i c s , a nd p o s s i b l y a ' d r i f t i n g ' p r o b l e m 30 due t o slowly changing s u r f a c e p o t e n t i a l s . ft schematic c r o s s - s e c t i o n of the spectrometer 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, 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 housing i s about 200 cc. The ana l y z e r and e l e c t r o n m u l t i p l i e r housing are pumped by a 2" d i f f u s i o n pump. (The newer spectrometer has f a r l e s s e f f i c i e n t pumping i n t h i s r e g i o n . I n f a c t the e l e c t r o n m u l t i p l i e r i s not pumped d i r e c t l y but v i a the a n a l y z e r , which i s i t s e l f l e s s e f f i c i e n t l y pumped. The e f f e c t s of t h i s on performance are d i s c u s s e d l a t e r . ) 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 via a t h r o t t l i n g v a l v e . The anal y z e r and i o n i z a t i o n r e g i o n s a r e separated by three c i r c u l a r a p e r t u r e s : the aperture at 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 entrance to the a n a l y z e r (0.020"). T h i s setup minimizes sample e n t e r i n g the a n a l y z e r , as the bulk of sample i s pumped away from the top by the 4" d i f f u s i o n pump. Pressure i s monitored by an i o n i z a t i o n gauge immediately below the ana l y z e r . Sample pressure i s adjusted by changing the flow r a t e i n t o the spectrometer, or by a d j u s t i n g the t h r o t t l i n g v a l v e on the 'top pumping*. 31 VERTICAL CROSS-SECTION OF PE SPECTROMETER materials E=D brass . l • sapphire balls • O-rings »— tefbn 4" Diffusion pump 4 electron multiplier-lens system ... ' a 2"Diffusion pump FIGURE 3:1 32 3.3 The Analyzer A 180° h e m i s p h e r i c a l a n a l y z e r i s used. The r e s o l u t i o n f o r such an an a l y z e r i s a p p r o x i m a t e l y 7 3 where AE-^ i s the h a l f width of the PE l i n e , E the e l e c t r o n ' s energy, which i s the pass energy of the a n a l y z e r , W the diameter of the entrance and e x i t a p e r a t u r e s , and R0 the path r a d i u s of the e l e c t r o n . In t h i s case R0 = 1.25" g i v i n g a r e s o l u t i o n o f 0.8 percent. For a 2eV e l e c t r o n t h i s r e p r e s e n t s a t h e o r e t i c a l r e s o l u t i o n of 16meV. R e s o l u t i o n i s measured using the 2P 3 / i band of Ar and a r e s o l u t i o n of 25 meV i s e a s i l y o b t a i n e d when the system i s c l e a n . The v o l t a g e s a p p l i e d to the a n a l y z e r and l e n s system are shown i n F i g . 3.2.. An e l e c t r o n passes through the ana l y z e r i f the c e n t r i f u g a l and e l e c t r i c f o r c e s are balanced. The spectrometer i s run i n the f o l l o w i n g way. The value of V a i s adjusted t o a predetermined value, u s u a l l y about 2V. i s used to r e t a r d the p h o t o e l e c t r o n s t o the pass energy of the a n a l y z e r . Using Ar or the sample under study, VR i s tuned t o a band maximum. VL i s then a d j u s t e d t o maximize the co u n t i n g r a t e . Because magnetic f i e l d s from m e t a l l i c o b j e c t s and e l e c t r i c a l equipment i n the FIGURE 3:2 v i c i n i t y , and the e a r t h ' s magnetic f i e l d a f f e c t the e l e c t r o n ' s path, the e n t i r e spectrometer i s enclosed i n a s e t of Helmholtz c o i l s . The c u r r e n t s through the c o i l s are a d j u s t e d to optimize s i g n a l . VR , VL , and the Helmholtz c o i l c u r r e n t s are c y c l i c a l l y a d j u s t e d to maximize the counting r a t e on the PE band. To r e c o r d a spectrum VR i s scanned h o l d i n g a l l e l s e constant. By m a i n t a i n i n g a c o n s t a n t V u, and hence c o n s t a n t pass energy f o r the a n a l y z e r , the r e s o l u t i o n and s e n s i t i v i t y of the spectrometer are almost constant over the f u l l range of the spectrum. The system i s c o n s t r u c t e d of brass, using V i t o n 0—rings,, i n s u l a t i n g t e f l o n s p a c e r s , nylon screws, and s a p h i r e b a l l s as r e q u i r e d . The h e m i s p h e r i c a l s u r f a c e s are g o l d p l a t e d . A l l e l e c t r o n o p t i c s are coated with g r a p h i t e , r e d u c i n g the background noise l e v e l by absorbing s c a t t e r e d e l e c t r o n s . E l e c t r o n s are detected by a Channeltron e l e c t i o n m u l t i p l i e r . The s i g n a l i s f e d v i a a pre-amp t o an a m p l i f i e r , and then to a multichannel a n a l y z e r which s t o r e s the data from m u l t i p l e scans. 3.4 The L i g h t Source The l i g h t s o u r c e 7 * i s an a i r cooled gas discharge lamp powered by a Microtron—200 microwave power source. Helium and neon are the gases used g i v i n g the He I <21.22eV) or Me I (16.85eV) l i n e s . Discharge i s i n a guartz tube 35 c o n s t r i c t e d a t the spectrometer end by a 1.5 mm ID boron n i t r i d e tube. A gap of about 1 cm. separates t h i s from the 1 mm ID c o l l i m a t i n g c a p i l l i a r y tube l e a d i n g to the i o n i z a t i o n chamber. The pumping po r t f o r removal of the lamp gas i s i n the r e g i o n o f t h i s gap, and l e a d s to a mechanical pump. 3.5 Spectrometer Performance The spectrometer*s performance has been d i s c u s s e d 7 3 , 7 4 and demonstrated by a long l i s t of p u b l i c a t i o n s . R e s o l u t i o n i s improved by d e c r e a s i n g Via and hence the pass energy, however t h i s i s at the expense of counting r a t e . Ose of a Ne discharge r e q u i r e s s m a l l e r r e t a r d i n g p o t e n t i a l s f o r the same V,2 and t h i s i n c r e a s e s r e s o l u t i o n without decrease of counting r a t e . In f a c t c o u n t i n g r a t e s are h i g h e r as the lamp seems t o d i s c h a r g e Ne more e f f i c i e n t l y . ' When st u d y i n g t r a n s i e n t s , r e s o l u t i o n i s u s u a l l y no b e t t e r than about 40meV. T h i s i s due t o the problems of decomposition causing a b u i l d u p o f s u r f a c e d e p o s i t s , or simply t o the c o r r o s i v e nature of some molecules. The problem of d r i f t i n g due to s u r f a c e charging i s g e n e r a l l y the r e s o l u t i o n l i m i t i n g f a c t o r . The newer spectrometer, while l a r g e l y a copy of the f i r s t , has l e s s e f f i c i e n t pumping i n the a n a l y z i n g and 36 d e t e c t i o n r e g i o n . The o r i g i n a l machine has s i x pumping por t s to the a n a l y z e r (one i s shown i n F i g . 3.1) and one d i r e c t l y to the e l e c t r o n m u l t i p l i e r . However the newer instrument has only a s i n g l e h a l f i n c h diameter port d r i l l e d d i r e c t l y through the s i d e of the outer hemisphere, and c o n s t r i c t e d by a wire mesh t o maintain a homogeneous f i e l d . The e l e c t r o n m u l t i p l i e r housing i s not d i r e c t l y pumped. T h i s seems to be the only major d i f f e r e n c e between the machines, ap p a r e n t l y causing the d i f f e r e n c e s i n performance. Using Ar the o r i g i n a l machine under i d e a l c o n d i t i o n s can have about 50,000 cps at 25meV while the newer instrument has about 10,000 cps at 30meV. When studying t r a n s i e n t s the e f f i c i e n c y and r e s o l u t i o n worsening nature of the samples a f f e c t s the newer machine more than i t a f f e c t s the o r i g i n a l spectrometer. While performing the experiments of Chapter 6 i t was noted t h a t these compounds a t t a c k e d the e l e c t r o n m u l t i p l i e r g i v i n g s e v e r a l hundred cps background counting r a t e s , when u s i n g the newer machine. The o r i g i n a l machine d i d not have t h i s problem as sample apparently d i d not reach the m u l t i p l i e r . T h i s suggests t h a t the l e s s e f f i c i e n t l y pumped newer machine has a f a r h i g h e r pressure i n the a n a l y z e r and d e t e c t o r r e g i o n , reducing c o u n t i n g r a t e s d r a s t i c a l l y . For s t r u c t u r a l reasons the necessary m o d i f i c a t i o n s e n t a i l major changes and t h e r e f o r e have cot yet been made. 3 7 CHAPTER EOUJR Unstable Imines: PE S p e c t r a o f M-Methyl Methylenimine (CHi=NCH3) and C-Methyl Methylenimine (CH3CH=NH) 4.1 I n t r o d u c t i o n Imines are important as r e a c t i o n i n t e r m e d i a t e s and while many are w e l l known, the s m a l l e r imines are q u i t e u n s t a b l e , tending t o hydrolyze and p o l y m e r i z e . 1 So while the ^C=M- group i s i s o e l e c t r o n i c with )c=0 and /C=c' i t has cot been as thoroughly 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 . The PE s p e c t r a of SCH=NR» (R #R» = Me 2, 3, E t * , Ph«, and l a r g e r a l k y l s u b s t i t u e n t s ) and Me^G^NPh6 have been recorded., The si m p l e s t imine, methylenimine has a l s o been examined by P E S . 7 , 8 Methylenimine i s a known i n t e r s t e l l a r molecule, and by analogy with aldehydes the i n t e r s t e l l a r presence of the s m a l l methylated imines s t u d i e d here i s e x p e c t e d . 9 , 1 0 CH2N=CH;l i n p a r t i c u l a r i s a strong p o s s i b i l i t y as i t s 38 h y d r o g e n a t i o n p r o d u c t ( C H 3 ) 2 NH i s p o s t u l a t e d a s an i n t e r s t e l l a r m o l e c u l e a l t h o u g h i t h a s n o t y e t t e e n o b s e r v e d . * 1 N - m e t h y l m e t h y l e n i m i n e h a s been s t u d i e d by m i c r o w a v e t z , 1 3 and i n f r a — r e d s p e c t r o s c o p y , 1 * , 1 5 and w h i l e l e s s w e l l c h a r a c t e r i z e d , C - m e t h y l m e t h y l e n i m i n e h a s a l s o been e x a m i n e d by m i c r o w a v e 1 6 a n d i n f r a — r e d s p e c t r o s c o p y . * 5 CH 2=NCHj and CH 3CH=NH a r e p r o d u c e d by low p r e s s u r e p y r o l y s i s o f t h e i r r i n g t r i m e r s , N - t r i m e t h y l h e x a h y d r o - s - t r i a z i n e and C — t r i m e t h y l h e x a h y d r o - s — t r i a z i n e r e s p e c t i v e l y . T h i s r o u t e t o CH 2 = NCH 3 i s w e l l d o c u m e n t e d 1 7 , however i n t h e l a t e r c a s e t h e p r o c e s s i s r a t h e r more c o m p l i c a t e d . 1 8 , 1 9 I n t h i s c h a p t e r t h e p r o d u c t i o n and PE s p e c t r a o f CH 2=NCH 3 and CH 3CH=NH a r e d i s c u s s e d . The p r e s e n t r e s u l t s a r e c o n s i d e r e d i n l i g h t o f t h e v e r y r e c e n t l y p u b l i s h e d PE s p e c t r u m o f f o r m a l d o x i m e <CH2 = N 0 H ) . 2 0 4.2 E x p e r i m e n t a l (1) N — t r i m e t h y l h e x a h y d r o — s — t r i a z i n e was p r e p a r e d by s l o w l y m i x i n g e q u a l v o l u m e s o f 3 3 % a g u e o u s m e t h y l a m i n e and 40$ a g u e o u s f o r m a l d e h y d e . 1 2 , 2 1 The r e a c t i o n was q u i t e e x o t h e r m i c a n d t h e m i x t u r e was h e l d a t 0°C i n an i c e b a t h . Upon a d d i t i o n o f KOH t h e t r i a z i n e s e p a r a t e d . I t was f u r t h e r d r i e d o v e r KOH and was t h e n vacuum d i s t i l l e d . H - m e t h y l m e t h y l e n i m i n e was p r e p a r e d b y l o w p r e s s u r e 39 {<0.1 t o r r ) p y r o l y s i s o f N - t r i m e t h y l h e x a h y d r o - s - t r i a z i n e i n a f l o w s y s t e m l e a d i n g d i r e c t l y i n t o t h e s p e c t r o m e t e r . The t r i a z i n e was p a s s e d t h r o u g h an 8mm. I D g u a r t z p y r o l y s i s t u b e p a c k e d f o r a b o u t 15 cm. w i t h A l ^ O ^ and S±02 c h i p s . , The p y r o l y s i s t u b e was h e a t e d by a P t wound f u r n a c e . The s p e c t r u m was r e c o r d e d o v e r a r a n g e o f t e m p e r a t u r e s and o n l y t h e t r i m e r was d e t e c t e d below 80°C. Above t h i s t e m p e r a t u r e p y r o l y s i s t o o k p l a c e , D u r i n g u s e , t h e s u r f a c e o f t h e A l 2 0 3 a n d S i O ^ c h i p s became c h a r r e d r e d u c i n g t h e i r c a t a l y t i c e f f i c i e n c y , and r e q u i r i n g h i g h e r t e m p e r a t u r e s f o r c o m p l e t e p y r o l y s i s . C o m p l e t e p y r o l y s i s o c c u r r e d a s l o w a s 270°C f o r a f r e s h s y s t e m however t e m p e r a t u r e s up t o 370°C were s o m e t i m e s r e q u i r e d . O n l y one p y r o l y s i s p r o d u c t was s e e n i n t h i s t e m p e r a t u r e r a n g e . Some r e s i d u a l H^Q and H 2CO f r o m t h e t r i a z i n e p r e p a r a t i o n a p p e a r i n t h e s p e c t r a . (2) C — t r i m e t h y l h e x a h y d r o — s — t r i a z i n e was p r e p a r e d by c o n d e n s a t i o n o f 30% a q u e o u s ammonia ( i n s l i q h t e x c e s s ) w i t h a c e t a l d e h y d e a t 0 ° C . 2 2 , 2 3 A s o l i d m a t e r i a l i n s o l u b l e i n C H C l ^ was f o r m e d on s t a n d i n q . Upon d e h y d r a t i o n i n vacuum l a r q e c u b i c c r y s t a l s s o l u b l e i n C H C I 3 were o b t a i n e d . T h e s e o b s e r v a t i o n s a r e c o n s i s t a n t w i t h p r e v i o u s work i n w h i c h an a l d e h y d e - a m m o n i a was f o r m e d , t h e n d e h y d r a t e d t o g i v e t h e t r i a z i n e . 2 2 , 2 3 The d e h y d r a t e d m a t e r i a l was vacuum s u b l i m e d s e v e r a l t i m e s e l i m i n a t i n q a v i s c o u s , y e l l o w i m p u r i t y . C - m e t h y l m e t h y l e n i m i n e was p r o d u c e d by p y r o l y s i s a s d e s c r i b e d a b o v e . P u r e CH 3CH=NH was n e v e r o b t a i n e d a s two a d d i t i o n a l p r o d u c t s a p p e a r e d . Ammonia and 2 - m e t h y l p y r i d i n e ( a — p i c o l i n e ) were a l w a y s p r e s e n t and t h e i r f o r m a t i o n w i l l be 40 d i s c u s s e d l a t e r . U n p y r o l y z e d t r i m e r was t r a p p e d a t —78°C. B e s t r e s u l t s were o b t a i n e d a t a f u r n a c e t e m p e r a t u r e o f a b o u t 170°C w i t h s l o w f l o w r a t e s . S p e c t r a o f t h e i m i n e s were c a l i b r a t e d w i t h t h e known I P ' s o f A r , H A , H 20, HCN, and B^CO. 4.3 R e s u l t s (1) The PE s p e c t r a o f N — t r i m e t h y l h e x a h y d r o — s — t r i a z i n e a n d t h e monomer CH^=NCH 3 a r e shown i n F i g . 4. 1, The I P ' s a r e l i s t e d i n T a b l e 4.1. 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 s e c o n d b and o f t h e i m i n e i s 1320±60cm _ l. The PE s p e c t r u m o f C - t r i m e t h y l h e x a h y d r o - s - t r i a z i n e shown i n F i g . , 4 . 2 a shows e v i d e n c e o f an i m p u r i t y a s t h e peak a t a b o u t 9.8eV i s o f v a r i a b l e i n t e n s i t y . I n f a c t t h e t r i m e r i s n o t f u l l y c h a r a c t e r i z e d and one may e x p e c t t h e p r e s e n c e o f o t h e r p o l y m e r i c s p e c i e s . 2 2 , 2 3 P y r o l y s i s o f t h e t r i m e r g i v e s t h e c o m p l i c a t e d s p e c t r a o f F i g . 4.2b,c. The i m p u r i t y i s c * - p i c o l i n e • a n d i t s i n t e n s i t y i s e n h a n c e d i n F i g . 4.2c f o r c o m p a r i s o n t o t h e s p e c t r u m o f c x - p i c o l i n e i n F i g . 4.3. D e s p i t e t h e i m p u r i t i e s , t h e i n d i v i d u a l I P ' s h a v e been i d e n t i f i e d a n d n o t e d i n T a b l e 4.2. The s e c o n d band o f CH aCH=NH h a s a v i b r a t i o n a l s e r i e s o f 1260±60cm - 1. 41 -I 1 ! I I I 8 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) FIGURE 4:1 a. He I photoelectron spectrum of N-trimethylhexahydro-s-tri azine. b. He I photoelectron spectrum of N-methyl methylenimine, CH ?=NCHV 42 TABLE 4.1 Experimental IP s for (CH2=NCH3)3 and CH2=NCF-3 (CH2=NCH3)3 CH2=NCH3 8.33 n 9.90b 8.71 7T 11.38b (v', 1320160 cm"1) 11.74 13.35b 13.0 15.1 14.A 15.8 15.55 19.38 16.3 16.7 18.5 20.5 b. V e r t i c a l IP's (in eV); higher bands may comprise more than one single ionization process. (See tex t ) . ±0.02 eV. A l l other IP's ±0.05 eV. 43 i i I 1 I I 1 1 1 1 ' 1 '— 8 10 12 14 16 18 20 IONIZATION POTENTIAL (eV) FIGURE 4:2 a. He I PE spectrum of C-trimethylhexahydro-s - t r i a z i ne . b. He I PE spectrum of C-methyl methylenimine, CH3CH=NH. c. He I PE spectrum of CH3CH=NH plus other pyrolys is products (see tex t ) . TABLE 4.2 Experimental IP's 3 for (CH CH=NH) and CH-CH=NH (CH3CH=NH)3 CH3CH=NH 8.45 n 10.18 b 8.79 7T 11.44 b (v', 1260±60 cm - 1) 11.6 13.62 12.5 14.3 13.9 15.3 14.3 16.93 16.2 19.09 V e r t i c a l IP's (in eV); higher bands may comprise more than one single i o n i z a t i o n process. (See t e x t ) . ±0.02 eV. A l l other IP's ±0.05 eV. 45 a J i i i i \ i i i i i — i — i — 8 10 12 14 16 18 2 0 IONIZATION POTENTIAL (eV) FIGURE 4:3 He I PE spectrum of 2-methyl pyridine (a -p ico l ine) . 46 4.4 A s s i g n m e n t (1) The N - m e t h y l t r i m e r g i v e s p e a k s a t 8.33 and 8.71 e V , w e l l s e p a r a t e d f r o m t h e r e s t o f t h e s p e c t r u m . By c o m p a r i s o n w i t h t h e r e l a t e d t r i o x a n e 2 4 , t h e s e a r e a s s i g n e d t o t h e t h r e e n i t r o g e n l o n e p a i r s w h i c h t r a n s f o r m a s a, and e u n d e r C 3 v s y m m e t r y (assumed f o r t h e C-N r i n g ) . From t h e r e l a t i v e i n t e n s i t i e s i t a p p e a r s t h a t t h e e l e v e l i s s t a b i l i z e d r e l a t i v e t o t h e a, l e v e l . The h i g h e r I P r e g i o n (>11 eV) c o n s i s t s o f m a i n l y C-N and CH rr and cr o r b i t a l s . P y r o l y s i s o f t h e t r i m e r g i v e s C H I N C H 3 w i t h f i r s t and s e c o n d v e r t i c a l I P * s a t 9.90 and 11.38 eV. The f i r s t band i s f e a t u r e l e s s and f a i r l y s y m m e t r i c and t h e s e c o n d band 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 1320±60 cm-*. T h e s e two b a n d s u n d o u b t e d l y c o r r e s p o n d t o t h e n ( a * ) and >r(a") o r b i t a l s r e s p e c t i v e l y . , 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 rr band c o r r e s p o n d s t o t h e C=N s t r e t c h i n g f r e q u e n c y (16 60 cm -* i n t h e g r o u n d s t a t e m o l e c u l e * 4 , 1 5 ) , somewhat r e d u c e d i n t h e i o n . The c o r r e s p o n d i n g b a n d s i n m e t h y l e n i m i n e (CH 2 = NH) a r e a t 10.55(n) and 12.45 e V ( f r ) 7 , 8 , t h e s e c o n d b a n d h a v i n g 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 1370 cm—*. The d i m e t h y l d e r i v a t i v e , CH 3CH=NCH shows bands a t 9.5 (n) and 10.67 ev {rr) w i t h no o b s e r v e d f i n e s t r u c t u r e . 3 Thus s u b s t i t u t i n g a s i n g l e C f l 3 g r o u p a t t h e n i t r o g e n o f CH 2 = NH i n t r o d u c e s an n l e v e l s h i f t o f -0.65 eV a n d a tr l e v e l s h i f t o f -1.07 eV. A d d i t i o n o f a f u r t h e r C H 3 g r o u p a t t h e c a r b o n r e d u c e s t h e n and rr l e v e l s by 0.40 and 0.71 eV 47 r e s p e c t i v e l y . F o r t h e c o r r e s p o n d i n g i s o e l e c t r c n i c d i a z i n e s 2 S , t h e n and TT s h i f t s f o r s i n g l e m e t h y l a t i o n a r e —0.45 and -1.5 eV r e s p e c t i v e l y , and i n t r o d u c i n g a s e c o n d CH3 g r o u p p r o d u c e s f u r t h e r s h i f t s o f - 0.6 and -1.2 eV. The t r e n d s f o r t h e i m i n e s a r e t h e r e f o r e a s e x p e c t 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 on t h e s e c o n d band c o n f i r m i n g i t s o r i g i n t o be t h e rr o r b i t a l . From MO a r g u m e n t s , and by c o m p a r i s o n w i t h t r a n s - m e t h l y d i a z i n e 2 5 and p r o p y l e n e 2 6 , 2 7 , s i x o r s e v e n v a l e n c e o r b i t a l s a r e e x p e c t e d i n t h e He I e n e r g y r e g i o n , and s e v e n a r e o b s e r v e d . The l a s t , a t 19.38 eV i n v o l v e s m a i n l y c a r b o n a n d n i t r o g e n 2 s o r b i t a l s . The r e m a i n i n g f o u r bands a r e l o c a t e d a t 1 3 . 3 5 , 15,1 and two a t 15.8 e v . One b a nd i n t h e r e g i o n 15-16 eV i s most l i k e l y t h e b o n d i n g (a") l e v e l , l e a v i n g t h r e e cr o r b i t a l s . Due t o t h e low ( C S ) s y m m e t r y o f t h i s m o l e c u l e e x t e n s i v e m i x i n g c a n o c c u r a mongst t h e s e i n - p l a n e cr o r b i t a l s . The t h i r d b a n d a t 13, 35 eV, w h i c h i s a s s i g n e d t o a s i n g l e i o n i z a t i o n p r o c e s s p r o b a b l y o r i g i n a t e s f r o m what may be t e r m e d a C-N tr MO c o r r e s p o n d i n g t o t h e band a t 15.2 ev i n C H ^ N H . 7 , 8 The t r e n d s i n F i g . 4.4 show a c l e a r c o r r e l a t i o n f o r t h e t h i r d I P * s o f t h e s e r i e s o f i s o e l e c t r o n i c m o l e c u l e s . The r e m a i n i n g b a n d s a r e r a t h e r more d i f f i c u l t t o a s s i g n . However t h e b o n d i n g rr o r b i t a l b e i n g w e l l s e p a r a t e d f r o m t h e o t h e r a " o r b i t a l , may be e x p e c t e d t o show a more r e g u l a r t r e n d t h a n t h e s t r o n g l y i n t e r a c t i n g a» o r b i t a l s i n t h e same r e g i o n ( F i g . 4 . 4 ) . F o r t h i s r e a s o n i t i s a s s i g n e d t o t h e band a t 15.55 eV. The HAM3 and e x p e r i m e n t a l r e s u l t s 48 f o r CH 2=NH, C H ^ N C H j , and CH 3CH=NH a r e shown i n F i g . . 4 . 5 . The a g r e e m e n t i s q u i t e g o o d , p a r t i c u l a r l y f o r CH A=NH, s o some c o n f i d e n c e i n t h e c a l c u l a t e d I P * s i s w a r r a n t e d . The HAM3 r e s u l t s s u p p o r t t h e p r o p o s e d a s s i g n m e n t o f t h e a" l e v e l . (2) The C - m e t h y l t r i m e r d i s p l a y s a b r o a d l o w e n e r g y b a n d w i t h maximum a t 8.45 eV and a s h o u l d e r a t 8.79 eV. I t i s p o s s i b l e t h a t t h e a, and e o r b i t a l s a r e r e v e r s e d c o m p a r e d t o t h o s e i n t h e N - m e t h y l t r i m e r . As m e n t i o n e d a b o v e , t h e peak a t 9.8 eV i s n o t a s s o c i a t e d w i t h t h e mai n s p e c t r u m . The h i g h e r I P r e g i o n c o n s i s t s m a i n l y o f C - C , C-N, and CH rr a n d cr o r b i t a l s . F i g . 4.2b shows t h e s p e c t r u m o f CH 3CH=NH o b t a i n e d by p y r o l y s i s o f t h e t r i m e r . A l t h o u g h t h e n a t u r e o f t h e t r i m e r h a s n o t been f u l l y c h a r a c t e r i s e d , c a r e f u l p y r o l y s i s a t l o w p r e s s u r e (<0. 1 t o r r ) , and l o w t e m p e r a t u r e (170°C) p r o d u c e s CH 5CH=NH as t h e mai n p r o d u c t . The s p e c t r u m i s c o m p l e t e l y i n a c c o r d w i t h t h a t e x p e c t e d . Ammonia a nd < x - p i c o l i n e a r e a l s o p r e s e n t i n l o w y i e l d s . By c o m p a r i s o n w i t h t h e N - m e t h y l s p e c i e s , we e x p e c t t h e rr o r b i t a l i n a p p r o x i m a t e l y t h e same p o s i t i o n , a n d t h e n i t r o g e n l o n e p a i r t o be s l i g h t l y s t a b i l i z e d , a s t h e C H 3 g r o u p i s now l o c a t e d on t h e C atom. The f i r s t I P ( a ' ) i s a t 10.18 eV (9.90 eV i n C H ; ? = N C H 3 ) and t h e s e c o n d I P <a") a t 11.44 eV (11.38 eV i n C H 2 = N C H 3 ) . The o r i g i n o f t h e s e c o n d b a n d i s c o n f i r m e d by i t s v i b r a t i o n a l s t r u c t u r e . I t 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 1260±60 c m - 1 (1320±60 cm-* i n C H T = N C H ? ) r e d u c e d f r o m t h e g r o u n d s t a t e m o l e c u l a r C=N 49 s t r e t c h i n g f r e q u e n c y (1659 c m - 1 t r a n s ; f o r t h e c i s i s o m e r , 1652 c m - * ) 1 7 . A g a i n f o u r I P ' s a r e e x p e c t e d i n t h e r e g i o n 13-18 eV. The f u r t h e r I P a t 19.09 eV i n v o l v e s m a i n l y c a r b o n 2 s o r b i t a l s . I n d i v i d u a l I P ' s a p p e a r a t 13.62, 14.3, 15.3, and 16.93 eV, more e v e n l y s p a c e d t h a n i n CH a=NCH 5. As b e f o r e a s s i g n m e n t i s d i f f i c u l t i n t h i s r e g i o n . However one may e x p e c t t h e )r b o n d i n g o r b i t a l t o be s t a b i l i z e d r e l a t i v e t o t h e v a l u e o f 14.8 eV i n CH 3CH=CH F i g . 4.4 now shows a r e g u l a r t r e n d t o w a r d s s t a b i l i z a t i o n o f t h e >r band a s more e l e c t r o n e g a t i v e a t o m s a r e i n v o l v e d . The a* l e v e l s w h i c h a r e e x p e c t e d t o i n t e r a c t a n d m i x s t r o n g l y show l e s s r e g u l a r t r e n d s . F i g . ,4.5 c o m p a r e s t h e HAM3 a n d e x p e r i m e n t a l r e s u l t s f o r CH^CH=NH, and t h e c a l c u l a t i o n a q r e e s w i t h t h e p r o p o s e d a s s i q n m e n t . The I P ' s f o r t h e c i s c o n f i g u r a t i o n were a l s o c o m p u t e d s h o w i n q a l a r g e d e s t a b i l i z a t i o n o f t h e t h i r d I P i n c o n f l i c t w i t h an e a r l i e r c a l c u l a t i o n . 1 s The g e o m e t r i e s c h o s e n f o r t h e s e c a l c u l a t i o n s were b a s e d on i n c o m p l e t e m i c r o w a v e s p e c t r a r e s u l t s 1 6 and a r e p r o b a b l y t h e r e a s o n f o r t h e p o o r e r g u a l i t y o f t h e c a l c u l a t i o n s . The s p e c t r u m o f CH 3CH=NH i s d e f i n i t e l y o b s c u r e d b y ammonia a n d c t - p i c o l i n e . , T h i s r e s u l t s i n some u n c e r t a i n t y a s t o e x a c t band p o s i t i o n s and d e f i n i t e l y q i v e s t h e h i g h e r I P r e q i o n a c l u t t e r e d a p p e a r a n c e . I t i s a l s o p o s s i b l e t h a t some c i s i s o m e r i s p r e s e n t due t o i n v e r s i o n a t t h e n i t r o g e n atom. The t r a n s i s o m e r h a s b e e n c a l c u l a t e d t o be a b o u t 1,7 K c a l / m o l e more s t a b l e t h a n t h e c i s . 1 5 I t i s c o n c e i v a b l e t h a t C H 3 N = N H C H 3 C H - CH2 C H 3 N = C H 2 C H 3 C H = 0 C H 3 C H = NH 10-k)a'' > CD _J < (n)0 2H LJ fe 14-\{TT)Q" a' o N o I6H a'-a" a' a / 2 0 J a/ FIGURE 4:4 Correlation diagram for the molecules trans-methyl diazine, N-methyl methylenimine, propylene, C-methyl methylenimine, and acetaldehyde. C H 0 - - N H . , P E S HAM3 10 1 1 12 13 14 15 16 1 / B 19 20 21 C H I N C H 3 P E S a ' K A M I C H 3 C H = N H P E S HAM3 HAM3 ( t r a n s ) ( c i s ) FIGURE 4:5 Correlat ion of PES and HAM3 results for CH2=NH , CH2=NCH3 , and CH3CH=NH. 52 t h e t r a n s t o c i s c o n v e r s i o n c o u l d t a k e p l a c e i n t h e p y r o l y s i s e x p e r i m e n t . 4.5 D i s c u s s i o n The c h e m i s t r y i n v o l v e d i n p y r o l y s i s o f t r i a z i n e s i s o f some g e n e r a l i n t e r e s t . 1 8 (See C h a p t e r 6 f o r t h e p r e p a r a t i o n o f ClNCO) T h i s r o u t e t o t h e monomer i s p a r t i c u l a r l y n e a t a s t h e r e a r e no c o - p r o d u c t s . However i n t h e CH 5CH=NH e x p e r i m e n t s ammonia a n d c t - p i c o l i n e were p r o d u c e d , oc—pi co l i n e was i d e n t i f i e d by i t s known PE s p e c t r u m 2 8 , 2 9 w h i c h i s shown i n F i g . 4.3. The v i b r a t i o n a l s e r i e s on t h e f i r s t band c o n f i r m s t h e i d e n t i f i c a t i o n . I t h a s b e e n known f o r some t i m e t h a t p y r i d i n e d e r i v a t i v e s a r e f o r m e d by h e a t i n g a l d e h y d e - a m m o n i a s . 1 8 , 2 3 I n t h e p r e s e n t i n s t a n c e o c - p i c o l i n e f o r m a t i o n c o u l d n o t be p r e v e n t e d n o r c o u l d i t be removed by t r a p p i n g . The l a t e r o b s e r v a t i o n i s u n e x p e c t e d a s one w o u l d e x p e c t «.-picoline t o be r e a d i l y t r a p p e d . T hus i t i s p r o b a b l e t h a t t h e p r o d u c t r e s u l t s f r o m r e c o m b i n a t i o n o f t h e i m i n e f o l l o w e d by r i n g c l o s u r e . The r e a c t i o n i s t h e T s c h i t s c h i b a b i n r e a c t i o n : 3 CH, CH-NH 3 I 53 T h i s r e a c t i o n a l s o e x p l a i n s t h e o r i g i n o f NH 3. A l t e r n a t i v e s o u r c e s o f t h e N — m e t h y l a n d C — m e t h y l i m i n e s , i n c l u d i n g p y r o l y s i s o f a z i r i d i n e ( C B a - C H 2 - N H ) 3 2 , and e t h y l a m i n e , CH^CH^NH 2 3 1 , 3 2 were t r i e d . . I n t h e f o r m e r c a s e i t «as p o s t u l a t e d t h a t clT^CH^NH p y r o l y z e d a t 525°C may g i v e CH 3CH=NH {<20%) and CH A=NCH 3 (80%) . 3 ° CH-^NCHj was t h e m a j o r p r o d u c t o f t h i s r e a c t i o n b u t no C%CH=NH was d e t e c t e d . The a d d i t i o n a l f o r m a t i o n o f Cz E2, HCN, MeNC and NHj i n d i c a t e s t h a t t h i s p y r o l y s i s c a n go f u r t h e r t h a n s i m p l e i n l i n e f o r m a t i o n . The p y r o l y s i s o f CH 3CH 2NH 2 i s c o m p l i c a t e d . 3 1 , 3 2 CHjCN and HCN a r e e x p e c t e d a t h i g h t e m p e r a t u r e s . , , A l s o e x p e c t e d a r e v a r i o u s i m i n e s and v i n y l a m i n e , CH^=CHNH X, t h e l a t t e r h a v i n g b een d e t e c t e d by m i c r o w a v e s p e c t r o s c o p y . 3 2 The d e s i r e d p r o d u c t CH 3CH=NH i s p o s t u l a t e d a s an i n t e r m e d i a t e . The PE e x p e r i m e n t s p r o d u c e d m a i n l y HCN, MeCN, C^H 2 a n d NHj upon p y r o l y s i s b e t w e e n 750 and 1000°C, and an a d d i t i o n a l u n i d e n t i f i e d m a t e r i a l . T h e r e was no o b v i o u s e v i d e n c e f o r CHjCH^NH o r i t s e n amine t a u t o m e r , CH 2=CHNH 2. S u b s e q u e n t t o t h i s work t h e PE s p e c t r u m o f f c r o a l d o x i m e , CH 2=N0fl, a p p e a r e d i n t h e l i t e r a t u r e . 2 0 The f i r s t t h r e e I P ' s c o r r e s p o n d t o t h e tr , n , and tr b o n d i n g o r b i t a l s r e s p e c t i v e l y . T h i s o r d e r i n g i s c o n f i r m e d by a r e d u c e d C=N s t r e t c h i n g f r e q u e n c y on t h e f i r s t b a n d . 2 0 T h i s a g r e e s w i t h t h e HAM3 r e s u l t s u s i n g t h e g e o m e t r y o b t a i n e d f r o m m i c r o w a v e d a t a . 3 3 The HAM3 and e x p e r i m e n t a l r e s u l t s a r e shown i n F i g . 4.6. CH2= NOH FIGURE 4:6 Correlat ion of experimental and HAM3 IP's for CH 2 = NOH. 55 CHAPTER F I V E M a l o n o n i t r i l e , H 2 C ( C N ) A , D i c h l o r c m a l o n o n i t r i l e , C l ^ C ( C N ) ^ , a n d D i b r c m o n a l c n o n i t r i l e , B r ^ C l C N ) ^ . 5.1 I n t r o d u c t i o n The PE s p e c t r a o f many CN c o n t a i n i n g m o l e c u l e s a r e known. Of p a r t i c u l a r i n t e r e s t a r e t h o s e o f H C N 1 - * , N C N 3 S , 6 , C l C H ^ C N 7 , * , S(CN) a», and ( S C N ) 2 ( C h a p t e r 7) a l l o f w h i c h h a v e been s t u d i e d w i t h more t h a n one i o n i z i n g s o u r c e . I n a d d i t i o n (CH)^,***, <CH J} 2 C (CN)^ * *, CH3CN 7, CH^CH^CN?, CI3CCN 7, and C{CN) 4 1 2 a r e s i m i l a r s m a l l m o l e c u l e s whose PE s p e c t r a h a v e been r e c o r d e d . , U n f o r t u n a t e l y e v e n ab i n i t i o SCF c a l c u l a t i o n s do n o t r e p r o d u c e o r b i t a l e n e r g i e s f o r c y a n o compounds, a n d o f t e n e m p i r i c a l a s s i g n m e n t s must be made f o r t h e s e s y s t e m s . H 2C{CN}^ has p r e v i o u s l y been i n v e s t i g a t e d u s i n g PES and a s i m p l e MO m o d e l , I n t h e p r e s e n t s t u d y He I and Ne I s p e c t r a were r e c o r d e d s u p p l y i n g a d d i t i o n a l i n f o r m a t i o n i n 56 s u p p o r t o f t h e e a r l i e r a s s i g n m e n t . The i n v e s t i g a t i o n was t h e n a l s o e x t e n d e d t o i n c l u d e t h e d i h a l o s p e c i e s , u s i n g a s i m i l a r MO a p p r o a c h . The I R and Raman s p e c t r a o f H 2C(CN)^ 1 3 ^ * , l s , C l ^ C ( C N ) ^ 1 6 , and B r ^ C ( C N ) 2 **; and t h e m i c r o w a v e s p e c t r u m o f H 2 C ( C M ) ^ 1 7 h a v e b e e n r e p o r t e d . T h e r e i s a l s o a c o m p r e h e n s i v e r e v i e w a r t i c l e on t h e r e a c t i o n s o f t h e s e c o m p o u n d s . 1 8 I t i s e x p e c t e d f r o m t h e PE s p e c t r a o f s i m i l a r compounds, t h a t t h e rrCN and n ^ i o n i z a t i o n e v e n t s w i l l be c o n c e n t r a t e d i n a s m a l l r e g i o n o f t h e s p e c t r u m ( a b o u t 12.5-14.5 eV) . A l s o , i n b o t h C l ^ C ( C » ) 2 and B r 2 C ( C M } 2 t h e h a l o g e n l o n e p a i r i o n i z a t i o n s w i l l be c l o s e i n e n e r g y , and may e v e n o v e r l a p t h e yp and n^ I P ' s . Thus b e c a u s e t h e s e t e n e x p e c t e d v a l e n c e I P ' s may be g u i t e c l o s e t o g e t h e r , i t was hoped t h a t i n t e n s i t y v a r i a t i o n s w i t h d i f f e r e n t p h o t o n e n e r g i e s ( C h a p t e r 2) w o u l d be an a i d t o a s s i g n i n g t h e c o m p l i c a t e d s p e c t r a . 5.2 E x p e r i m e n t a l H 2 C ( C N } 2 (Eastman) was p u r i f i e d by r e c r y s t a l i z a t i o n f r o m e t h e r , and was d r i e d i n vacuum b e f o r e u s e . C l 2 C ( C U ) ^ ( A l d r i c h ) was u s e d w i t h o u t p u r i f i c a t i o n . B r ^ C (CN)^ was p r e p a r e d by a d d i t i o n o f B r ^ t o a g u e o u s H 2 C ( C N ) 2 as d e s c r i b e d i n t h e l i t e r a t u r e . 1 9 57 S p e c t r a were c a l i b r a t e d w i t h A r , N^, a n d M e l . B o t h He I (21.22 eV) and He I (16.85 eV) l i g h t s c u r c s were u s e d . The 16.67 eV l i n e i n t h e Ne d i s c h a r g e h a s a n i n t e n s i t y o f a b o u t 15-208 t h a t o f t h e main l i n e . The 'shadow 1 s p e c t r u m c a u s e d by t h i s l i n e was removed by a d i g i t a l s u b t r a c t i o n p r o c e d u r e u s i n g t h e m u l t i c h a n n e l a n a l y z e r . A f t e r r e c o r d i n g a Ne I s p e c t r u m , t h e r e t a r d i n g p o t e n t i a l ( V R ) was s h i f t e d by -0.177 eV. T h i s s u p e r i m p o s e d t h e 16.85 eV l i n e s i g n a l w i t h t h e p r e r e c o r d e d 16.67 eV s p e c t r u m . The s u b s e g u e n t d a t a was t h e n a c c u m u l a t e d i n a s u b t r a c t i n g mode. R e m o v a l o f t h e •shadow' s p e c t r u m was m o n i t o r e d u s i n g t h e L* band o f 8^+. T h i s t e c h n i q u e removed t h e a p p r o x i m a t e l y 15% 'shadow' s p e c t r u m d i s p l a c e d by 0.177 eV, and c r e a t e d a n a p p r o x i m a t e l y 21 n e g a t i v e shadow s p e c t r u m d i s p l a c e d by 0.354 e v . F i g . 5.4 was o b t a i n e d i n t h i s way. When c o n s i d e r i n g i n t e n s i t y d i f f e r e n c e s between bands i n t h e He I a n d Ne I s p e c t r a . F i g . 5,4 was u s e d . B e c a u s e f i n e s p e c t r a l d e t a i l s a r e l o s t t h r o u g h i n a c c u r a c i e s i n t h e s u b t r a c t i o n p r o c e s s , t h e He I s p e c t r a were u s e d f o r a l l m e a s u r e m e n t s . 5.3 R e s u l t s The He I PE s p e c t r a o f H 2 C ( C N ) 2 , C l ^ C ( C N ) 2 , and B r a C ( C N ) ^ a r e shown i n f i g u r e s 5.1 and 5.2. The Ne I s p e c t r a a r e shown i n F i g , 5.3, and t h e Ne I s p e c t r a c o r r e c t e d f o r t h e 16.67 eV l i n e a r e shown i n F i g . 5.4, I P ' s 58 1 1 1 hh CI2C(CN)2 1 1 1 1 1 1 ft k 1 Br2C(CN)2 i i i i i i IONIZATION POTENTIAL (eV) FIGURE 5:1 He I PE spectra of C1 2C(CN) 2 and Br2C(CN) 59 11 12 13 14 IONIZATION POTENTIAL (eV) FIGURE 5:2 He I PE spectra of H 2C(CN) 2, C1 2C(CN) 2, and Br 7C(CN), in the 11-15 eV region. I i • I I . I . L 11 12 13 14 15 IONIZATION POTENTIAL (eV) FIGURE 5:3 Ne I PE spectra of H 2 C(C;0 2 > C1 2C(CN) 2 > and Br„C(CN) 9 in the 11-15 eV region. IONIZATION POTENTIAL (eV) FIGURE 5:4 Ne I PE spectra of H 2C(CN) 2, C1 2 C(CN) 2 > and Br-C(CN), corrected for the Ne 16.65eV l i n e . TABLE 5:1 Experimental IP's 3 for H 2C(CN) 2, C1 2C(CN) 2, and Br2C(CN) H2C(CN)2 C12C(CN)2 Br 2C(CN) 2 IP i V vib. mode MO IP IP 12.72 970 *35cm"1 2438^50 2850^80 C-H C N C-C str. str. str. 2 b l 12.58 12.78 11.48 11.87 13.14 4b 2 12.92b 12.01b 13.42 5a^ 13.26 12.405 13.59 3b2 13.59 12.94b 13.91 l a 2 13.84b 13.22 14.06b 517±40 CCC bend 4a 2 14.20 13.45 17.51b 18.15b 19.34b 669±35 1350±50 CH„ C-C def. str. 3a .j l b l 2b 2 14.31 14.58b 17.26 17.79 13.70 13.87 14.11 16.42b 17.03b a IP in eV tn.O? eV b IP i0.05 eV 18 19 IONIZATION POTENTIAL (eV) FIGURE 5:5 The high IP's of H 2C(CN) 2. 64 an d v i b r a t i o n a l f r e q u e n c e s a r e l i s t e d i n T a b l e 5.1. The h i g h e r I P r e g i o n o f H 2 C ( C N ) ^ i s shown i n d e t a i l i n F i g . 5.5. The He I r e s u l t s f o r H 2 C ( C N ) 2 a r e e s s e n t i a l l y i n a g r e e m e n t w i t h t h o s e r e p o r t e d e a r l i e r . 1 1 5.4 A s s i g n m e n t s (1) H 2 C ( C N ) 2 The He I PE s p e c t r u m h a s p r e v i o u s l y b een r e c o r d e d and a s s i g n e d , 1 1 The 12 . 5-14 eV r e g i o n c o n t a i n s t h e b a n d s f r o m t h e f o u r rrCN ,ht , a ^ # b 5 ) a nd t h e two n N (a, /b 2) o r b i t a l s . T h e r e a r e a l s o some v i b r a t i o n a l c o m p o n e n t s w h i c h a r e n o t e a s i l y d i s t i n g u i s h e d f r o m t h e i n d i v i d u a l i o n i z a t i o n p r o c e s s e s . T h i s g i v e s a t o t a l o f 10 d i s t i n c t p e a k s i n t h e 12 ,5-14 eV r e g i o n , w h i c h must be a s s i g n e d t o s i x I P ' s . The t h r o u g h s p a c e (TS) a n d t h r o u g h bond {TB) a p p r o a c h was u s e d i n t h e p r e v i o u s a s s i g n m e n t 1 1 ( F i g . 5 , 6 ) . CNDO/2 and EHMO c a l c u l a t i o n s p r e d i c t t h e o r d e r o f TB i n t e r a c t i o n s t o be t b (b, ) > t b ( a , ) > t b ( b - j ) , 1 1 As t h e r e i s o n l y one o r b i t a l h a v i n g a 2 s y m m e t r y , i t e x p e r i e n c e s no TB i n t e r a c t i o n and r e m a i n s a t a p p r o x i m a t e l y t h e same e n e r g y . The nN o r b i t a l s a r e n e a r l y o r t h o g o n a l t o t h e rT^'s and t h e r e f o r e t h e s e two t y p e s o f o r b i t a l s a r e assumed t o u n d e r g o no TS i n t e r a c t i o n , 1 1 The r a t i o o f A V ^ / £ ^ V = 0.47 (See F i g . 5,6) was c h o s e n f r o m t h e c a l c u l a t e d S l a t e r t y p e o v e r l a p i n t e g r a l s , f o r w h i c h e r r o r s a r e assumed t o c a n c e l i n t a k i n g 65 FIGURE 5:6 The Through Space and Through Bond interact ions in H 2C(CN) 2. ( f romre f 11) aC N=14.1eV, ATT i i=0.5eV, Air a=1.0eV, tb(b 1 )=1.7eV, tb (a ] )=0.6eV, tb(b 2)=0.5eV. t h e r a t i o . By v a r y i n g AtT^ , t h e d a t a was f i t t e d t o t h e i n t e r a c t i o n scheme o f F i g . 5.6. T h i s was p o s s i b l e f o r o n l y one o f t w e n t y — f o u r f e a s i b l e a s s i g n m e n t s w h i c h were c o n s i d e r e d . The f i n a l a s s i g n m e n t i s g i v e n i n T a b l e 5.1. The p r e s e n t e x p e r i m e n t s e m p l o y e d two l i g h t s o u r c e s . F o r SCCN)^ t h e i n t e n s i t y d e p e n d a n c e o n p h o t o n e n e r g y i s q u i t e p r o n o u n c e d 9 s i n c e t h e r e a r e l a r g e d i f f e r e n c e s i n c r o s s - s e c t i o n f o r S 3p and C a n d N 2p o r b i t a l s . However f o r H^C(CN)^ ( F i g . 5.2, 5.3, 5.4) t h e r e l a t i v e i n t e n s i t y c h a n g e s f o r most bands a r e n o t s o d r a m a t i c a s o n l y f i r s t row a t o m s a r e i n v o l v e d . The 13.59 eV ( n N ) b a n d shows a c o n s i d e r a b l y l o w e r i n t e n s i t y i n t h e Ne I s p e c t r u m , r e l a t i v e t o t h e s u r r o u n d i n g rrCN b a n d s . T h i s i s t y p i c a l b e h a v i o u r f o r a CN g r o u p i n g , a n d r e p r o d u c e s o b s e r v a t i o n s f o r HCN 2 , N C N j s , * , S (CN)^ 9 , ClCH-jCN 7 , 8 , C l S C N , BrSCN, and (SCN) 2 (Ch. 7) . The p r e s e n t r e s u l t s a l s o i n d i c a t e g ood a g r e e m e n t w i t h t h e e a r l i e r v i b r a t i o n a l f r e g u e n c e s . I n p a r t i c u l a r t h 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 on t h e 17.5 eV b a nd a r e c o n f i r m e d . T h a t o f 1350 c m - 1 i s a r e d u c e d CH^ d e f o r m a t i o n f r e g u e n c y i m p l y i n g H-H b o n d i n g c h a r a c t e r , t h u s c o n f i r m i n g t h a t t h i s b a n d r e p r e s e n t s t h e fj-(a, ) o r b i t a l . (2) B r ^ C ( C N ) ^ T h i s i s c o n s i d e r e d f i r s t s i n c e t h e PE s p e c t r u m ( F i g . 5.1) c o n s i s t s o f t h r e e d i s t i n c t r e g i o n s . The 11.5-12.5 eV r e g i o n i s a s s i g n e d t o t h e n g r o r b i t a l s , t h e 12.9-14,5 e¥ r e g i o n t o t h e 7TCN and nN o r b i t a l s , and t h e h i g h e r b a n d s r e p r e s e n t cr o r b i t a l s . The yr c / V a n d nN r e g i o n i s a b o u t 1.2 eV w i d e , o n l y s l i g h t l y r e d u c e d f r o m i t s w i d t h i n 67 H^C(CN) 2 . , I n a l l t h e r e a r e t e n o r b i t a l s e x p e c t e d i n t h e r e g i o n f r o m a b o u t 11-14.5 eV, and t e n d i s t i n c t b a n d s a r e o b s e r v e d { F i g . 5 . 2 ) . S i n c e t h e b a n d s a r e s o c l o s e a d e f i n i t i v e a s s i g n m e n t c a n n o t be made, ho w e v e r t h e most p l a u s i b l e i n t e r p r e t a t i o n i s p r e s e n t e d i n t h e f o l l o w i n g d i s c u s s i o n . As i n H^C(CN) 2 , t h e n N o r b i t a l s a r e assumed n o t t o i n t e r a c t t h r o u g h s p a c e w i t h t h e 'WcN o r b i t a l s . ,. S i m i l a r l y t h e y w i l l n o t i n t e r a c t t h r o u g h s p a c e w i t h t h e n g r o r b i t a l s , a s t h e Br...N=C— s e p a r a t i o n i s a b o u t 3.7A. I n t e r a c t i o n o f t h e TrCN and n ^  o r b i t a l s w i l l be s m a l l due t o t h e e n e r g y d i f f e r e n c e . A l s o t h e I S i n t e r a c t i o n and i n d u c t i v e e f f e c t s o f t h e h a l o g e n a t o m s may be assumed t o a p p r o x i m a t e l y c a n c e l e a c h o t h e r . T h i s was f o u n d f o r t h e m o l e c u l e s F ^ C C l ^ and B r ^ C C l ^ . z o The TS i n t e r a c t i o n s among t h e n g r o r b i t a l s w i l l be s i m i l a r t o t h o s e i n H^CBr^ s i n c e t h e g e o m e t r y i s e x p e c t e d t o be a p p r o x i m a t e l y t h e same. A l s o t h e TS i n t e r a c t i o n s among t h e "rrCN o r b i t a l s w i l l be s i m i l a r t o t h o s e i n H ^ C ( C N ) 2 . 1 1 As t h e r e i s no t b ( a ^ ) , t h e a^ o r b i t a l s a c t a s a k i n d o f i n t e r n a l s t a n d a r d . The b a n d a t 12.40 eV ( F i g s . 5.2, 5.4) shows a d e c r e a s e d i n t e n s i t y i n t h e Ne I s p e c t r u m , r e l a t i v e t o t h e r e s t o f t h e ng^ r e g i o n . T h i s s u g g e s t s an o r b i t a l i n t e r a c t i n g w i t h o r b i t a l s o f h i g h e r I P , and t h u s i t c a n n o t be t h e n g ^ (a^) o r b i t a l , a s s u m i n g no s i g n i f i c a n t i n t e r a c t i o n w i t h t h e / r c / V ( a a ) o r b i t a l . The ( b a ) o r b i t a l must be a t h i g h e r I P t h a n t h e n ^ f b , ) o r b i t a l a s t h e i n i t i a l l y d e g e n e r a t e n ^ { a a ) 68 and b| o r b i t a l s a r e s p l i t by TS i n t e r a c t i o n , t h e b, o r b i t a l b e i n g 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 o r b i t a l . The b ( o r b i t a l may be f u r t h e r d e s t a b i l i z e d by TB i n t e r a c t i o n , w h i l e t h e r e i s no TB e f f e c t a c t i n g on t h e a ^ o r b i t a l . (A s i m i l a r s i t u a t i o n a r i s e s f o r t h e b^ and a 2 irCN o r b i t a l s shown i n F i g . 5.6.) T h e r e f o r e t h e a^ o r b i t a l must l i e i n t h e s e c o n d m a j o r band s e q u e n c e . I t i s t h e r e f o r e a s s i g n e d t o t h e I P a t 12.01 eV, a l t h o u g h t h e band a t 11.87 eV i s a l s o a p o s s i b i l i t y . The n ^ ( b , ) o r b i t a l must now be a s s i g n e d t o e i t h e r t h e f i r s t o r t h e s e c o n d I P . I t i s a s s i g n e d t o t h e f i r s t (11.48 eV) band s i n c e t h e c o r r e s p o n d i n g o r b i t a l i n H^CBr^ and B r ^ C C l ^ g i v e s t h e f i r s t PE band. A l s o i n H^CBr^ i t i s t h e b r o a d e s t ng^ b a n d , w h i c h i s t h e c a s e f o r t h i s a s s i g n m e n t o f B r ^ C ( C N ) ^ . T h i s l e a v e s t h e n g r (a ,) o r b i t a l , w h i c h i s a s s i g n e d t o t h e 12.40 eV b a n d . The l o w e r i n t e n s i t y i n t h e Ne I s p e c t r u m , r e l a t i v e t o t h e o t h e r n ^ o r b i t a l s s u g g e s t s t h e r e i s some m i x i n g o f c h a r a c t e r f r o m more s t a b l e a j o r b i t a l s . T h i s a s s i g n m e n t o f t h e ng^ r e g i o n g i v e s t h e same o r d e r i n g a s i n H^CBr^ ( F i g . 5.8) and B r 2 C C l 2 . 2 0 The tT C N and nN r e g i o n a s a w h o l e shows a s l i g h t l y l o w e r i n t e n s t y i n t h e Ne I s p e c t r u m , r e l a t i v e t o t h e ng r r e g i o n ( F i g . 5,2, 5 . 4 ) . T h i s t r e n d i s a l s o a p p a r e n t i n C l ^ C l C N ) ^ a n d h a s b e e n n o t e d f o r CICH^CN u s i n g He I a n d He I I r a d i a t i o n . 7 The TCN , n N s y s t e m i s s l i g h t l y c o m p a c t e d i n o v e r a l l w i d t h c o m p a r e d t o H^C(CN)^ b u t i s s h i f t e d o n l y a b o u t 0.2 eV t o h i g h e r I P . , The 12.94 eV band h a s a s h a r p F r a n c k - C o n d o n e n v e l o p e s i m i l a r t o t h e f i r s t {bf) band o f H^C(CN) 2 , a l t h o u g h i n B r ^ C ( C N ) ^ no v i b r a t i o n a l c o m p o n e n t s a r e r e s o l v e d . T h i s band t h e n , i s a s s i g n e d t o t h e ff ( b ( ) o r b i t a l t h u s m a k i n g i t t h e f i r s t irCN i o n i z a t i o n i n b o t h m o l e c u l e s . The i n t e n s i t y d e p e n d a n c e on p h o t o n e n e r g y f o r t h e n w ( b 2 ) o r b i t a l i s c e r t a i n l y n o t a s s t r o n g i n B r ^ C ( C N ) ^ as i n H^C(CU) 2 , and t h e o r b i t a l c a n n o t be i d e n t i f i e d on t h i s b a s i s . The o r d e r i n g i n t h e )rCN, n w r e g i o n i s t e n t a t i v e l y a s s i g n e d t o be t h e same a s i n E2C(C^)211 { F i g . 5 . 8 ) . B e c a u s e o f t h e c l o s e n e s s o f t h e PE b a n d s , l a c k o f v i b r a t i o n a l s t r u c t u r e , and t h e s m a l l i n t e n s i t y d e p e n d a n c e on p h o t o n e n e r g y , no f i r m e r a s s i g n m e n t c a n be made. However t h i s a s s i g n m e n t d o e s a g r e e w i t h r e s u l t s f o r t h e v a l e n c e i s o e l e c t r o n i c (CH^ )^ C (CN)^ . i * I t p l a c e s t h e n / V { a , ) band a t 13.45 eV and t h e n w { b 2 ) band a t 13.70 eV. W i t h t h i s a s s i g n m e n t t h e r a t i o o f t h e c r o s s - s e c t i o n s f o r t h e s e n^, o r b i t a l s i s r o u g h l y t h e same a s i n H^CfCN)^ a n d , ( C H j ) ^ C ( C N ) 2 f o r t h e He I s p e c t r a . The 16.42 and 17.03 eV b a n d s a r e a s s i g n e d t o a , and b, r o r b i t a l s r e s p e c t i v e l y . T h e s e c o r r e l a t e w i t h H^CBr^ a l t h o u g h t h e r e l a t i v e o r d e r i n g i s a g a i n n o t c o m p l e t e l y d e f i n i t e . From F i g . . 5 . 8 , a bz o r b i t a l i s a l s o e x p e c t e d i n t h e h i g h I P r e g i o n . T h i s may h a v e been m i s s e d due t o a s m a l l c r o s s - s e c t i o n (as i n H ^ C B r 2 ) , o r may l i e j u s t o u t s i d e t h e He I r a n g e . (3) C l ^ C (CN)^ The s p e c t r a o f C l ^ G ( C N ) 2 ( F i g . 5 . 1 , 5 .2 , 5.3) show t h a t 70 t h e n C | and Vr t A , , n^ r e g i o n s a r e much c l o s e r i n e n e r g y t h a n t h e c o r r e s p o n d i n g r e g i o n s o f B r ^ C j C N ) ^ . T h i s i s e x p e c t e d a s t h e n c | o r b i t a l s a r e i n h e r e n t l y a t h i g h e r I P t h a n t h e n g)_ o r b i t a l s . W i t h t e n i o n i z a t i o n p r o c e s s e s o v e r a 2.5 eV r e g i o n i t i s i m p o s s i b l e t o make s p e c i f i c a s s i g n m e n t s . I n ClCH^CN t t h e n a h a l o g e n o r b i t a l s show i n c r e a s e d i n t e n s i t y r e l a t i v e t o t h e 'rrCN o r b i t a l s , on c h a n g i n g f r o m He I I t o He I . I n Br^CCCN)^ t h i s t r e n d was o b s e r v e d w i t h H e l and N e l r a d i a t i o n . C o m p a r i n g f i g u r e s 5.2 and 5.4 i t i s c l e a r t h a t t h e b r o a d band f r o m a b o u t 12.3-13 eV shows e n h a n c e d i n t e n s i t y i n t h e Ne I s p e c t r u m . T h i s band i s a s s i g n e d t o t h e f o u r n c ^ o r b i t a l s . I n f a c t t h e b a nd shows t h r e e p e a k s a t 12. 58, 12.78, and 12.92 eV. The TTCH , n N r e g i o n i s c o m p l i c a t e d a n d d e f i n i t e i d e n t i f i c a t i o n o f t h e i n d i v i d u a l I P ' s i s n o t p o s s i b l e . I t a p p e a r s t h a t t h e r e a r e s e p a r a t e b a n d s a t a b o u t 13.26, 13.59, 13.84, 14.20, 14.31, and 14.58 eV { F i g . 5 . 2 ) . The c o m p o s i t e band i n c l u d i n g t h e 14.20, 1 4 . 3 1 , a n d 14.58 eV i o n i z a t i o n e v e n t s shows a s l i g h t l y h i g h e r i n t e n s i t y i n t h e Ne I s p e c t r u m . T h i s p r o b a b l y i n c l u d e s t h e v r C A /(a,) and y r ^ O ^ ) o r b i t a l s . The l o w i n t e n s i t y band a t 13.84 may i n v o l v e t h e n ^ i a , ) o r b i t a l . The d e e p e r l y i n g b a n d s a t 17.26 and 17,97 eV a r e a s s i g n e d t o a , a n d bt cr o r b i t a l s r e s p e c t i v e l y , f o l l o w i n g t h e a s s i g n m e n t o f H^C(CN)^. B e c a u s e t h e n c ) and Tc^ , nN s y s t e m s a r e so c l o s e e n e r g e t i c a l l y , one c a n n o t d i s c o u n t i n t e r a c t i o n s b e t w e e n t h e two g r o u p s . T h i s c o m p l i c a t e s t h e s i t u a t i o n e v e n f u r t h e r . The d a s h e d c o n n e c t i o n l i n e s i n F i g . 5.8 a r e s p e c u l a t i v e and 71 FIGURE 5:7 Correlat ion of experimental IP's of H ^ B ^ , Br 2 C(CN) 2 , H 2C(CN) 2, C1 2C(CN) 2 > and H 2CC1 2. 72 no a t t e m p t a t a comp le te a s s i gnment i s made. N e v e r t h e l e s s t h e g r o s s f e a t u r e s o f the s p e c t r a p e r m i t i d e n t i f i c a t i o n o f t h e n . and rr. n., r e g i o n s . 73 CHAPTER- 6 H a l o g e n I s o c y a n a t e s , XNCO (X=C1, B r , I) and H a l o g e n A z i d e s , XN^ (X=C1, B r ) 6.1 I n t r o d u c t i o n A t t a c h i n g an o f f — a x i s atom t o t h e n e a r l i n e a r a z i d e o r i s o c y a n a t e f u n c t i o n a l g r o u p s w i l l h a v e i m p o r t a n t s p e c t r o s c o p i c c o n s e q u e n c e s . The d e g e n e r a t e b o n d i n g and a n t i b o n d i n g tr o r b i t a l s o f t h e l i n e a r m o l e c u l e w i l l be s p l i t i n t o two p a i r s o f i n - p l a n e and o u t — o f — p l a n e b o n d i n g and a n t i b o n d i n g o r b i t a l s i n t h e b e n t <C 5) m o l e c u l e . The m a g n i t u d e o f t h e s p l i t t i n g f o r e a c h s e t o f o r b i t a l s w i l l d e pend upon t h e o f f - a x i s a n g l e and upon i n t e r a c t i o n o f t h e o f f - a x i s atom w i t h t h e l i n e a r g r o u p i n g , t h r o u g h m i x i n g o f t h e a p p r o p r i a t e symmetry o r b i t a l s . The h a l o g e n a z i d e s (XN 3) an d h a l o g e n i s o c y a n a t e s (XNCO) p r o v i d e good e x a m p l e s o f t h i s e f f e c t . I n t h e s e m o l e c u l e s t h e h a l o g e n p o r b i t a l s w i l l be shown t o i n t e r a c t w i t h t h e rr a n d cr s y s t e m s o f t h e n e a r 74 l i n e a r g r o u p . , As t h e h a l o g e n i s o c y a n a t e s t e n d t o p o l y m e r i z e and t h e a z i d e s a r e e x p l o s i v e , t h i s i s t h e f i r s t e x t e n s i v e s p e c t r o s c o p i c s t u d y o f t h e s e m o l e c u l e s . * T h i s i s a l s o t h e f i r s t r e p o r t o f BrN 3 and INCO i n t h e g a s p h a s e , a l t h o u g h b o t h a r e known i n s o l u t i o n . 2 ClNCO has been shown by e l e c t r o n d i f f r a c t i o n 3 and m i c r o w a v e * s p e c t r o s c o p y t o h a v e a bend a n g l e o f a b o u t 119°, c o m p a r e d t o a r o u n d 140° f o r CH 3NCO a n d NCNCO, and a b o u t 128° f o r t h e a c i d , HNCO. s The m i c r o w a v e s p e c t r u m o f C l N ^ 6 g i v e s a bond a n g l e o f 108,7°, compared t o a n g l e s i n t h e r a n g e o f 114-120°for C H 3 N 3 , NCN 3 and HN 3. 5 I n a l l o f t h e s e compounds t h e NCO o r NNN g r o u p s a r e w i t h i n 10° o f b e i n g l i n e a r . 3 - 6 The a n g l e s f o r t h e c h l o r i n e p s e u d o h a l i d e s a r e s i g n i f i c a n t l y l e s s t h a n f o r a n a l o g o u s compounds. O t h e r i n t e r e s t i n g f e a t u r e s i n c l u d e t h e o b s e r v a t i o n t h a t t h e C l - N bond i s l o n g e r i n t h e a z i d e {1.745 A ) 6 t h a n i n t h e i s o c y a n a t e ( 1 . 7 0 0 3 , 1,703*) a l t h o u g h N > N - C1 i s g r e a t e r f o r t h e a z i d e {724cm~* » 7 , 1 8 - , 2 8 ) t h a n t h e i s o c y a n a t e (710cm-* 1 9 , 2 9 ) » T h e s e v i b r a t i o n a l f r e q u e n c e s a r e a c t u a l l y g u i t e h i g h c o m p a r e d t o t h e v a l u e o f 8 1 8 c m - 1 f o r N C I , 1 7 The v a l u e s o f ^ N - B r f o r B r N 3 and NBr a r e 687 and 6 9 1 c m - 1 r e s p e c t i v e l y , * 7 The h i g h v a l u e f o r t h e h a l o g e n a z i d e s u g g e s t s t h e p r e s e n c e o f some XN d o u b l e b o n d i n g c h a r a c t e r . The c h e m i c a l b e h a v i o u r o f t h e s e compounds s u g g e s t s a p o s i t i v e c h a r g e on c h l o r i n e , a n o t h e r u n u s u a l p r o p e r t y o f t h e s e compounds. The PE s p e c t r a o f t h e p a r e n t a c i d s , H N 3 7 _ * * and HNCO 7 , 8 , * 2 h a v e b e e n e x t e n s i v e l y i n v e s t i g a t e d by s e v e r a l 75 authors, and PE s p e c t r a are known f o r the methyl, s i l y l , and germyl s u b s t i t u t e d a n a l o g u e s * 8 There i s c o n s i d e r a b l e disagreement over assignments, and t h i s w i l l be c o n s i d e r e d l a t e r . Apart from aromatics, other a z i d e s and i s o c y a n a t e s t h a t have been s t u d i e d by PES i n c l u d e cyanogen a z i d e N C N 3 1 3 , 1 4 cyanogen i s o c y a n a t e NCNCO, 1 3 and v i n y l isocyanate»s CH^CHNCO. Thi s chapter d i s c u s s e s the pre p a r a t i o n and PE s p e c t r a of XNj (X=C1, Br) and XMCO (X=C1, Br, I) . The e l e c t r o n i c s t r u c t u r e of the Mj and NCO groups are d i s c u s s e d with r e f e r e n c e to the e f f e c t of halogen s u b s t i t u t i o n on the parent a c i d s . A l s o the s t r u c t u r e of the compounds are dis c u s s e d i n l i g h t of the PE s p e c t r a . 6.2 Experimental (1) BrB^ a n a " C L N 3 W € r e prepared by r e a c t i o n of NaS^ with B r s and C l 2 . i 6 , 1 7 , 1 8 Beactions were c a r r i e d out i n a flow system d i r e c t l y e n t e r i n g the spectrometer. The halogen, a t pressures o f about 1—3 t o r r , was passed through a column packed with moist iiaNj supported on g l a s s wool. I n i t i a l r e s u l t s showed onl y N^, i n d i c a t i n g decomposition. C o o l i n g the column to 0°C while maintaining slow flow r a t e s o f halogen gave s p e c t r a of the halogen a z i d e s and water. Excess water was removed by a f o l l o w i n g t r a p o f C a C l ^ held at 0°C. C o n t r o l o f flow r a t e was c r u c i a l . At low flow 76 r a t e s some halogen a z i d e decomposed g i v i n g N 2. Too f a s t a flow gave some unreacted halogen and untrapped water. C a r e f u l , even packing of the column was c r i t i c a l f o r t h i s balance. Because t h e s e a z i d e s are v i o l e n t l y e x p l o s i v e and e a s i l y d etonated, 1 care was taken to prevent condensation i n any p a r t of the apparatus. They were found to be g u i t e safe as gases at low pressure. No attempt was made to prepare F N g which i s e x p l o s i v e even as a gas, or I U 3 which i s expected t o condense very e a s i l y . (2) ClNCO was prepared by p y r o l y t i c decomposition of t r i c h l o r o i s o c y a n u r i c a c i d 1 , - 2 2 (Eastman Chemical Company) i n a flow system. T h i s compound i s an s — t r i a z i n e , as were the t r i m e r s p y r o l i z e d to produce t h e imines d i s c u s s e d i n Chapter 4. The t r i m e r was vapourized at about 200°C, then p y r o l i z e d i n a column packed with g l a s s wool and held at about 30 0°C. Harming of t r i m e r was accompanied by an i n i t i a l r e l e a s e of some C l a which disappeared i n a few hours, a f t e r which pure ClNCO was observed. There was no evidence of c a r b c e y l i s o c y a n a t e , CO(NCO)^, even at higher temperatures and p r e s s u r e s . 1 9 , 2 1 (3) BrNCO was prepared by r e a c t i o n of B r A (1—3 t o r r ) with AgNCO at 150°C i n a flow system. AgNCO was prepared by mixing agueous s o l u t i o n s of KNCO and AgNO . I t was washed and d r i e d , packed i n a column supported on g l a s s wool, and degassed f o r s e v e r a l hours under vacuum a t 150°C. Complete d r y i n g i s e s s e n t i a l as BrNCO was e a s i l y h ydrolyzed. Sy n t h e s i s of t r i b r o m o i s o c y a n u r i c a c i d f o r p y r o l y s i s was 77 a l s o t r i e d . The r e a c t i o n of B r ^ with t r i c h l o r o i s o c y a n u r i c a c i d was c a r r i e d o u t 2 * , 2 5 g i v i n g high y i e l d s of B r C l . P y r o l y s i s was d i f f i c u l t and gave a mixture of products. INCO was produced hy the same method as BrNCO,with the AgNCQ at 190°C. INCO i s even more s u s c e p t i b l e to h y d r o l y s i s than BrNCO. I t a p p a r e n t l y decomposed g i v i n g 1^ and a s o l i d , n e c e s s i t a t i n g r a p i d pumping i n t o the spectrcmeter. The r a p i d flow allowed some 1^ to pass over the AgNCO unreacted, and 1^ was never completely e l i m i n a t e d . A l l p r e p a r a t i o n s were i n flow systems l e a d i n g d i r e c t l y i n t o the spectrometer. S p e c t r a were c a l i b r a t e d with the known IP's of HNCO, H^O, CO^, N^ and Ar. 6.3 R e s u l t s The He I PE s p e c t r a of C I N 3 and BrNj are shown i n F i g . 6.1 the BrN^ spectrum shows t r a c e s of Br^ and N^ «ith the f i f t h IP d i r e c t l y under the JL* s t a t e of N+. The f i r s t IP of each molecule i s shown on an expanded s c a l e i n F i g , .6.2. F i g . 6.9 shows the l a s t three bands of ClN^ and BrN^. The s p e c t r a of C1NC0, BrNCO and INCO are shown i n F i g . 6.3. R e s i d u a l 1^ i s seen i n the INCO spectrum. The f i r s t two bands f o r C1NC0, BrNCO and INCO are expanded i n F i g . 6.4. Hot bands are e v i d e n t i n a l l of these. , F i g . 6.10 shows the l a s t three bands of ClNCO on an expanded s c a l e . 7B J I I I I I I I I I U 10 12 14 16 18 2 0 IONIZATION POTENTIAL (eV) FIGURE 6:1 He I PE spectra of C1N~ and BrN CIN3 — i — i — i — r j 1 1 ' ' ' -I 10.0 10.5 IONIZATION P O T E N T I A L ( e V ) FIGURE 6 :2 The f i r s t PE bands of C1N-, and 80 TABLE 6:1 EXPERIMENTAL IP's AND ASSIGNMENTS FOR THE HALOGEN AZIDES C£N-Band IP(eV) 1. 2. 3. 4. 5. 6. 7. (10.20)' 10.38 12.00 13.39 13.39 15.96 17.39 18.53 v*(cm"^) 730±40 2100±60 1280±60 6rN. IP(eV) (10.00)' 10.08 11.38 12.44 12.75 15.62 17.04 18.01 v-tcrn"1) 710±40 1970±80 1940±80 Assignment 3a" 8a-2a" 7a-6a' l a " 5a-a. Vertical IP's (adiabatic IP's in brackets). All bands ±0.02 eV except the first (±0.01 eV) and bands 6 and 7 (±0.05 eV). b. The true adiabatic IP may l ie one quantum lower. c. See text. 81 i J i I J J 1 1 1 ' J -10 12 14 16 18 2 0 IONIZATION POTENTIAL (eV) FIGURE 6:3 He I PE spectra of ClNCO, BrNCO, and INCO. FIGURE 6:4 The f i r s t and second PE C1NC0, BrNCO, and INCO. n GO TABLE 6:2 EXPERIMENTAL IP'S AND ASSIGNMENTS FOR THE HALOGEN ISOCYANATES*1 C£NCO BrNCO INCO Band IP(eV) v'(cnf 1) IP(eV) v 'tcnf 1 ) IP(eV) v'(cm _ 1) Assignment^ 1 (10.72) 10.86 1110 and 600±40 (10.46) 10.46 1100 and 520±40 (9.89) 420 and 9.89 2070±40 3 a " 2 11.64 11.20 10.58 8a' 3 13.87 13.04 12.16 2 a " 4 13.87 13.21 12.53 7a' 5 16.32 910±80 15.98 15.60 l a " 6 17.59 950 or 1900±50 17.12 700 to 850 16.39 6a' 7 18.61 1100±80 18.15 750±100 17.55 5a' a Vertical IP's (adiabatic IP's in brackets). All bands ±0.02 eV except the f irst (tO.Ol eV) b See text. 84 IP's and v i b r a t i o n a l f r e q u e n c i e s f o r the halogen a z i d e s and halogen i s o c y a n t e s are l i s t e d i n Tables 6.1 and 6.2 r e s p e c t i v e l y . 6.4 Assignments The nonbonding and bonding TT o r b i t a l s of the l i n e a r molecule w i l l each be s p l i t i n a bent c o n f i g u r a t i o n g i v i n g f o u r IP's. These i n - p l a n e and out—of—plane tr o r b i t a l s are r e f e r r e d t o as the a" (nb) , a*(nb), a " ( b ) , and a» (b) o r b i t a l s . 7 The halogen p o r b i t a l s w i l l give r i s e t o two a d d i t i o n a l IP's. C o n s i d e r i n g the r e s u l t s f o r the a c i d s 7 - 1 2 a t l e a s t one cr o r b i t a l i s expected i n the He I spectrum, g i v i n g a t o t a l o f seven or e i g h t IP's. The f i r s t two IP's of a l l f i v e molecules are assigned t o the nonbonding o u t — o f — p l a n e (3a") and in—p l a n e (8a') o r b i t a l s r e s u l t i n g from the l i f t i n g of the degeneracy of the nonbonding 3 )r l e v e l i n a l i n e a r molecule. T h i s agrees with e a r l i e r r e s u l t s f o r H N 3 7 - 1 1 , HNCO7,*,*2, NC N ^ 1 3 , 1 4 and NCNCO. 1 3 I t w i l l be shown l a t e r t h a t there i s e x t e n s i v e halogen p c h a r a c t e r ( i n p a r t i c u l a r , N-X antibonding character) i n these two o r b i t a l s . The t h i r d (7a") and f o u r t h (7a') bands of a l l f i v e molecules a re assigned to o r b i t a l s i n v o l v i n g some degree o f halogen p o r b i t a l c h a r a c t e r . However they cannot s t r i c t l y be termed " l o n e p a i r s " s i n c e the c o n s i d e r a b l e mixing with the pseudohalide 85 grouping must be co n s i d e r e d (they are N-X bonding i n c h a r a c t e r ) . In C1N5 and ClNCO these two o r b i t a l s are superimposed a t about 13.4 and 13.9 eV r e s p e c t i v e l y , but the high r e l a t i v e i n t e n s i t i e s suggest the presence o f two I P ' s . I n B r N 5 , BrNCO, and INCO the degeneracy i s removed by str o n g e r i n t e r a c t i o n with the rr<nb) o r b i t a l s which are e n e r g e t i c a l l y c l o s e r i n the bromo and iodo molecules. The f i r s t f o u r IP's of a l l f i v e molecules are t h e r e f o r e e a s i l y assigned by comparison to known molecules. The s p l i t t i n g of o r b i t a l e n e r g i e s i n the bent (C s) molecule i s c o n s i d e r e d i n F i g , 6,5 i n comparison to the h y p o t h e t i c a l l i n e a r molecule. The f i g u r e i s intended only as a g u a l i t a t i v e i n d i c a t i o n o f the e f f e c t o f the o f f - a x i s angle, and w i l l not be co n s i d e r e d i n depth u n t i l much l a t e r . The f i f t h IP of the halogen a z i d e s i s r e l a t i v e l y sharp, having a Franck—Condon envelope s i m i l a r to the t h i r d IP of HNj, which i s the per o r b i t a l , 7 - 1 1 Therefore i t i s assigned t o the 6a», per o r b i t a l which has some l o c a l i z a t i o n on the t e r m i n a l n i t r o g e n . The f i f t h IP of the halogen i s o c y a n a t e s i s much broader and may not i n v o l v e a s i m i l a r o r b i t a l . The l a s t two IP's of the halogen a z i d e s , and the l a s t three IP's of the halogen i s o c y a n a t e s are not e a s i l y a s s i g n e d . T h i s i s p a r t l y due to the c o n f l i c t i n g assignments f o r the PE s p e c t r a of HN-g and HNCO. The c o n f l i c t s mainly i n v o l v e the o r d e r i n g and s e p a r a t i o n of the a" and a* tr bonding o r b i t a l s . The va r i o u s assignments f o r HN 3 and HNCO are summarized i n F i g . 6.6. T h i s problem w i l l be considered l a t e r , i n l i g h t of the present r e s u l t s . FIGURE 6:5 The e f fect of o f f -ax i s angle in XN 3. 87 10 CP < E 141 LU \— O CL o M z o 16 18 H N , H N , H N Ref. 7,8 Ref. 9 a" nb a" nb a',a"b a' a"b 3 Ref. 10,11 a" nb 12- a1 nb a1 nb o1 nb a'b H N C O H N C O Ref. 7,8 Ref. 12 a" nb a" nb a1 nb a' nb a", a'b a a; a ' b 20 a ' b a"b 22 [ FIGURE 6:6 PES results for HN3 and HNCO by various authors. 88 The r e m a i n i n g I P ' s of t h e XNg and XNCO m o l e c u l e s s i l l be a s s i g n e d a f t e r a more d e t a i l e d e x a m i n a t i o n of t h e f i r s t f o u r I P ' s . 6.5 D i s c u s s i o n (1) The 3a" and 8a* non-bonding IT o r b i t a l s . The Franck-Condon e n v e l o p e s o f t h e f i r s t two bands o f H»3 and HNCO i n d i c a t e t h e o u t — c f - p l a n e and i n - p l a n e n a t u r e o f t h e non-bonding rr o r b i t a l s . The f i r s t (a") band i s a s i n g l e i n t e n s e t r a n s i t i o n w i t h weak v i b r a t i o n a l s t r u c t u r e , and t h e second (a«) band i s broad due t o an o f f — a x i s a n g l e change upon i o n i z a t i o n . The s e p a r a t i o n between t h e f i r s t two I P ' s i s ex p e c t e d t o i n c r e a s e w i t h i n c r e a s i n g o f f - a x i s a n g l e . Thus the energy d i f f e r e n c e and bond a n g l e s a re -1.45 eV and 114° f o r M3; and 0.79 eV and 128° f o r HNCO.5 The a c t u a l I P ' s a r e 10.72, 12.24 eV f o r HNj; and 11.60, 12.39 eV f o r HNCO. The bond a n g l e (Z.X-N-C o r /.X-N-N) i s s m a l l e r i n the hal o g e n a t e d compounds th a n i n the parent a c i d s (Table 6.3). The s u b s t i t u t i o n o f a halogen f o r the hydrogen atom d e s t a b l i l i z e s t he f i r s t two o r b i t a l s (assuming Koopmans' theorem), and t h e s m a l l e r a n g l e t e n d s t o i n c r e a s e t h e i r s e p a r a t i o n , A'B, I n CIN3, AE = 1.62 eV, i n c r e a s e d from 1.45 ev i n HN 3. F o r C1NCO, AE = 0 . 7 8 e¥, about t h e same as HKCO. A ! i s l a r g e r f o r t h e a z i d e which i s more be n t , a c t u a l l y the 85 TABLE 6:3 Bond Angles and Band Sp l i t t ings for the Azides and Isocyanates Bond angle (Ref.) AE (eV) P(7a') •IP(2a") AE' (eV) Column 2 + 3 HN3 114° (5) 1.45 1.45 C1N3 ; 108.7° (6) 1.62 0.0 1.62 BrN 3 109° a 1.30 0.31 1.61 HNCO ClNCO BrNCO INCO 128° (5) 119° (3,4) 119° a <119° 3 0.79 0.78 (0.91) b 0.74 0.68 0.0 0.17 0.37 0.79 0.78 (0.91) b 0.91 1.05 a Angle estimated on the basis of AE' value. b Measured using the v'=0 t rans i t i on for the 3a" band. This corresponds to the ve r t i ca l t rans i t ion in BrNCO and INCO. A l l unmarked values involve ve r t i ca l t rans i t i ons . 90 AlE values are not completely accounted f o r by changes i n the o f f — a x i s angle. I n t e r a c t i o n s between the 8a* and 7a» o r b i t a l s tend to decrease AE0 simultaneously l i f t i n g the degeneracy of the 7a* and 2a" o r b i t a l s . , T h i s trend i s g u i t e apparent i n the s e r i e s ClNCO, BrNCO, INCO (see F i g . 6,3, Tab l e 6.3) where the 7a ,-2a" s p l i t c l e a r l y i n c r e a s e s with decreasing s e p a r a t i o n of the 8a § non-bonding IT o r b i t a l and the 7a» halogen p based o r b i t a l . For BrN 3, BrNCO, and INCO the values o f AE are a c t u a l l y lower than f o r the a c i d s . However i f the 7a*—2a" s p l i t t i n g i s added to AE g i v i n g ^E*, a l l v a l u e s are above the AE s e p a r a t i o n i n the a c i d s . (Table 6,3). I t should be noted t h a t the values quoted are f o r the s e p a r a t i o n s between the maxima, and so some d i f f e r e n c e s between molecules must a l s o i n c o r p o r a t e the o b s e r v a t i o n that the v e r t i c a l IP's occur at v' = 0 f o r BrNCO and INCO, at v' = 1 f o r ClNCO and B r N 3 and a t v»=2 f o r C1N 3. Thus i n g e n e r a l , t h e maxima f o r the c h l o r o s u b s t i t u t e d a z i d e s and i s o c y a n a t e s occur at v*+1, compared to the corresponding bromo and iodo compounds, ( F i g . 6,3, 6.4). S t r u c t u r a l data are not known f o r the bromo and iodo molecules, and so the values of AE* obtained above provide an i n d i c a t i o n of the expected s t r u c t u r e s . AE* f o r BrN 3 i s i d e n t i c a l to t h a t f o r C1N 3 s u g g e s t i n g t h a t they have s i m i l a r angles (ca. 109°), T h i s i s not unexpected by comparison with the s t r u c t u r a l l y r e l a t e d n i t r o s y l h a l i d e s , CINQ and BrNO which a l s o have s i m i l a r bond angles (~114°). 2 7 For the i s o c y a n a t e s , using the ^E' value f o r ClNCO from v'=0 on the f i r s t band, the val u e s f o r ClNCO and BrNCO become v i r t u a l l y 91 i d e n t i c a l ( c a . 0 .91 e V ) , a l t h o u g h t h e v a l u e f o r INCO i s somewhat l a r g e r ( A E * = 1.05 eV) . I n t h e a b s e n c e o f o t h e r f a c t o r s t h i s i m p l i e s t h a t t h e bond a n g l e f o r t h i s m o l e c u l e i s somewhat s m a l l e r t h a n t h a t f o r t h e c h l o r o and bromo i s o c y a n a t e s , w h i c h p r o b a b l y h a v e s i m i l a r bond a n g l e s o f a b o u t 119°. The f i r s t band o f a l l f i v e m o l e c u l e s h a s v i b r a t i o n a l s t r u c t u r e , a n d c o m p a r e d t o t h e a c i d s i t i s b r o a d e r , a n d t h e s e c o n d b and i s more i n t e n s e ( F i g . 6.3, 6 . 4 ) . The f i r s t band o f C l N ^ h a s f i v e v i b r a t i o n a l i n t e r v a l s a v e r a g i n g 730±40cm-», an d t h e f i r s t b a n d o f B r N 3 h a s s i x i n t e r v a l s o f 710+40cm- 1. F o r CIN3 t h e v e r t i c a l t r a n s i t i o n i s p l a c e d a t v*=2 however t h e t r u e maximum i s p r o b a b l y b e t w e e n v'=1 and v'=2. T h e r e i s no d e f i n i t i v e v i b r a t i o n a l a n a l y s i s o f t h e s e c ompounds, h o w e v e r a s s u m i n g o n l y ft1 modes a r e i n v o l v e d , t h e p o s s i b i l i t i e s a r e t h e N-X s t r e t c h i n g v i b r a t i o n (N;+) a n d t h e i n - p l a n e NNN b e n d ( . * 7 , * 8 , 2 8 The m o l e c u l a r g r o u n d s t a t e v> v a l u e s v N - C l and ^ N - B r a r e 723 and 687cn>-» r e s p e c t i v e l y . * 7 T h e s e numbers a r e s u p r i s i n g l y c l o s e t o e a c h o t h e r , a n d t o t h e o b s e r v e d i o n i c v a l u e s . The v 3 f r e q u e n c e s a r e 1144 and 1 1 6 0 c m - 1 f o r t h e g r o u n d m o l e c u l a r s t a t e s o f C l N j and B r N j r e s p e c t i v e l y . * 7 By a n a l o g y w i t h t h e i s o c y a n a t e s ( b e l o w ) and HN 3, t h e NNN b e n d i n g v i b r a t i o n i s t h e more l i k e l y . T h i s w o u l d i n v o l v e a r e d u c t i o n i n s p a c i n g a b o u t t h e same a s i n t h e f i r s t b a n d o f HN^ (1265 t o 850 1 0 o r 980 9 c n r i ) . I t i s g u i t e p o s s i b l e t h a t b o t h f r e q u e n c i e s a r e e x c i t e d . From t h e known v i b r a t i o n a l s p e c t r a o f ClNCO 1 9 , 2 9 and B r N C O 2 3 , 2 5 i t a p p e a r s t h a t t h e e x c i t a t i o n o f a b o u t 1 1 0 0 c m - 1 c o r r e s p o n d s t o t h e NCO s y m m e t r i c s t r e t c h i n g f r e g u e n c y |\^) o f 1309 a n d 1296cm-* i n t h e m o l e c u l a r g r o u n d s t a t e s o f ClNCO an d BrNCO r e s p e c t i v e l y . T h i s r e d u c t i o n i n f r e g u e n c y c o r r e s p o n d s t o t h a t o f t h e f i r s t band o f HNCO where v ' ^ I O S O c m - 1 a n d v " = 1 3 2 7 c m - 1 , and a g r e e s w i t h t h e above a s s i g n m e n t f o r t h e a z i d e s . The weaker p r o g r e s s i o n c o u l d r e p r e s e n t t h e NCO i n - p l a n e b e n d (N>J«=708 and 6 8 9 c m - 1 f o r ClNCO and BrNCO r e s p e c t i v e l y ) . However, i n v i e w o f t h e downward t r e n d o f i o n i c f r e g u e n c i e s (s;* =600, 5 2 0 , 4 2 0 c m - 1 f o r C l N C O , BrNCO, and INCO) a n d m o l e c u l a r g r o u n d s t a t e v a l u e s {<J*'=603, 4 7 3 c m - 1 f o r ClNCO, BrNCO) i t i s a s s i g n e d t o t h e N—X s t r e t c h i n g mode O"* T h i s mode becomes r e l a t i v e l y more i m p o r t a n t g o i n g f r o m ClNCO t o INCO, a s d o e s t h e h a l o g e n l o n e p a i r c h a r a c t e r i n t h e n o n - b o n d i n g Tr o r b i t a l s . The i o n i c v a l u e s a r e t h e same o r l a r g e r t h a n t h e m o l e c u l a r g r o u n d s t a t e v a l u e s a s e x p e c t e d f o r t h i s o r b i t a l w h i c h has some N-X a n t i b o n d i n g c h a r a c t e r . T h e r e i s no a v a i l a b l e I R d a t a f o r INCO h o w e v e r t h e o b s e r v e d v a l u e o f 4 2 0 c m - 1 i s a b o u t t h a t e x p e c t e d f o r an i n c r e a s e d N - l s t r e t c h i n g f r e g u e n c y . The f r e g u e n c y o f 2070cm- 1 p r o b a b l y r e s u l t s f r o m t h e NCO a s y m m e t r i c s t r e t c h i n g v i b r a t i o n w h i c h i s a b o u t 2 2 00cm- 1 f o r t h e m o l e c u l a r g r o u n d s t a t e o f ClNCO and BrNCO. The f i r s t a n d s e c o n d b a n d s o f t h e i s o c y a n a t e s become p r o g r e s s i v e l y n a r r o w e r and more i n t e n s e c h a n g i n g f r o m ClNCO t o BrNCO t o INCO. The same t r e n d i s s e e n f o r t h e a z i d e s ( F i g . 6.1, 6 . 3 ) . I n b o t h t h e a z i d e s and i s o c y a n a t e s t h e v e r t i c a l t r a n s i t i o n f o r t h e f i r s t band i s one guantum l o w e r i n t h e brcmc t h a n i n t h e c h l o r c m o l e c u l e ( F i g . 6.2, 6 . 4 ) . T h e s e o b s e r v a t i o n s s u g g e s t i n c r e a s i n g h a l o g e n p c h a r a c t e r i n t h e 3 a " and 8 a f o r b i t a l s on s u b s t i t u t i o n o f a h e a v i e r h a l o g e n atom. As m e n t i o n e d e a r l i e r , t h e i n c r e a s e d m i x i n g f o r a h e a v i e r h a l o g e n i s l i n k e d t o t h e l o w e r v a l e n c e s t a t e I P o f t h e h a l o g e n o r b i t a l s , w h i c h a p p r o a c h e s t h e 3 a " and 8a* o r b i t a l e n e r g i e s . I n INCO t h e f i r s t t wo b a n d s a r e c l e a r l y s h a r p e r a nd more i n t e n s e t h a n t h e t h i r d a n d f o u r t h b a n d s . CNDO/BW c a l c u l a t i o n s , u s i n g e s t i m a t e d g e o m e t r i e s f o r BrNCO and INCO, c o n f i r m t h a t h a l o g e n c h a r a c t e r i n t h e t h e 3 a " and 8a» o r b i t a l s i n c r e a s e s f r o m C1NC0 t o INCO, INCO i s a c t u a l l y c a l c u l a t e d t o have g r e a t e r I 5p c h a r a c t e r i n t h e f i r s t two b a n d s t h a n i n t h e t h i r d and f o u r t h b a n d s . C l e a r l y t h e PES r e s u l t s s u g g e s t s t r o n g m i x i n g o f h a l o g e n p and /r(nb) c h a r a c t e r i n t h e f i r s t f o u r I P ' s . The i m p l i e d rr c h a r a c t e r i n t h e NX bond has been s u g g e s t e d e a r l i e r 1 , 2 b a s e d o n t h e h i g h NX s t r e t c h i n g f r e g u e n c i e s f o r t h e g r o u n d s t a t e m o l e c u l e s . I t i s n o t e w o r t h y t h a t I B d a t a shows t h e NX s t r e t c h i n g f r e g u e n c y f o r b o t h t h e a z i d e s and i s o c y a n a t e s i s n e a r e r t h e v a l u e f o r t h e NX m o l e c u l e f o r t h e bromo t h a n t h e c h l o r o compounds. <2) The 2 a " a n d 7a* O r b i t a l s . I t i s c l e a r t h a t t h e s e a r e n o t h a l o g e n l c n e p a i r o r b i t a l s , b u t a r e mi x e d w i t h t h e 3 a " and 8a» o r b i t a l s . However, t h e y do show t h e c h a r a c t e r i s t i c s h i f t o f h a l o g e n b a s e d o r b i t a l s , t h e a v e r a g e p o s i t i o n s b e i n g 13.4 and 12.6 eV 94 f o r c h l o r i n e and b r o m i n e a z i d e , and 13.9, 13.1 and 12.35 eV f o r c h l o r i n e , b r o m i n e and i o d i n e i s o c y a n a t e . The d i f f e r e n c e s i n p o s i t i o n o f t h e s e bonds f o r t h e a z i d e s and t h e i s o c y a n a t e s a r e r e l a t e d t o t h e i n c r e a s e d n u c l e a r c h a r g e o f o x y g e n c o m p a r e d t o n i t r o g e n . The s e p a r a t i o n o f t h e s e o r b i t a l s i n B r N 3 , BrNCO, and INCO s u p p o r t s t h e a s s i g n m e n t o f t s o o r b i t a l s t o t h e t h i r d b a n d o f C l N j and ClNCO. pcr(a») a nd d e e p e r l e v e l s . B e y ond 15 eV t h e r e a r e t h r e e b a n d s i n a l l t h e m o l e c u l e s . I n t h i s r e g i o n t h e a z i d e s and i s o c y a n a t e s may d i f f e r i n o r b i t a l e n e r g y o r d e r i n g , a n d i t i s i n t h i s r e g i o n t h a t t h e a s s i g n m e n t s o f t h e p a r e n t a c i d s a r e c o n f u s e d . F i r s t t h e a s s i g n m e n t s o f t h e a c i d s ( F i g . 6.6) w i l l be c o n s i d e r e d w i t h r e f e r e n c e t o t h e e x t e n s i v e c a l c u l a t i o n s on H N 3 « , 10,35-38 ( F i g . 6.7) and H N C O 1 2 , 3 3 , 3 S , 3 a — •<» ( F i g . 6 . 8 ) . The HAM3 r e s u l t s a r e i n c l u d e d d e s p i t e t h e f a c t t h a t HN^ was u s e d i n t h e p a r a m e t e r i z a t i o n o f t h e p r o g r a m . The c a l c u l a t i o n s p r e d i c t v e r y s m a l l s p l i t t i n g o f t h e rr b o n d i n g o r b i t a l s i n b o t h c a s e s . T h i s i m p l i e s t h a t t h e s e o r b i t a l s a r e t h e f o u r t h a n d f i f t h I P ' s i n H N 3 and t h e t h i r d and f o u r t h i n HNCO. T h i s c o n c u r s w i t h t h e a s s i g n m e n t i n t h e o r i g i n a l p u b l i c a t i o n ( F i g . 6.6, R e f . 7) and a g r e e s w i t h t h e o r d e r i n g o f HNCS. 7 The f i f t h I P o f t h e a z i d e s ( F i g . 6.9) c o r r e s p o n d s t o t h e t h i r d I P o f HNj and i s a s s i g n e d t o t h e 6 a * (per) o r b i t a l . W i t h r e s p e c t t o t h e a c i d i t i s s t a b i l i z e d a t 0.49 eV i n C 1 N 3 a n d 0.15 eV i n B r N 3 . T h i s i s p r o b a b l y due t o some i n c l u s i o n 95 FtGURE 6:7 Calculations for HN^including HAM3 resu l t s . FIGURE 6:8 Calculations for HNCO.including HAM3 resu l t s . 98 of X s c h a r a c t e r . T h i s band has a s t r o n g 00 t r a n s i t i o n i n HN 3, CIN3, BrN^ and N CN j . 1 3 The v i b r a t i o n a l s t r u c t u r e of 1 940cm—-1 f o r BrN^ i s the asymmetric NNN s t r e t c h i n g freguency reduced from 2070cm - 1 f o r the ground s t a t e molecule. The f i f t h IP of C1N 5 shows s t r u c t u r e o f 2100 and 1280cm - 1 assigned to the asymmetric and symmetric NNN s t r e t c h i n g f r e q u e n c i e s which are 207 2 and 1144cm - 1 f o r the molecule. The v i b r a t i o n a l s t r u c t u r e i s s t r o n g evidence f o r l o c a l i z a t i o n i n the NNN grouping. F i g . 6.9 shows the 15-19 eV r e g i o n of BrN 3 f r e e of any t r a c e s o f N^. The s i x t h and seventh bands at about 17 and 18 eV i n BrN^, and 17.4 and 18.5 eV i n ClN^ are broad with s n a i l c r o s s — s e c t i o n s . C o n s i d e r i n g the r e s u l t s f o r UUj 7 and NCN^ 1 3, i t appears these bands are the 1a" and 5a* o r b i t a l s r e s p e c t i v e l y . These r e s u l t from l i f t i n g the degeneracy of the ir bonding o r b i t a l i n the l i n e a r molecule, and so the higher IP band i s assigned to the 5a* o r b i t a l . T h i s d i s a g r e e s with the CNDO/BW and with the previous ab i n i t i o r e s u l t . * 1 However the 5a* band i s expected t o be s t a b i l i z e d by i n c l u s i o n of some C l 3s c h a r a c t e r . The i s o c y a n a t e molecules a l s o have three IP's i n t h i s r e g i o n but with very d i f f e r e n t Franck-Condon envelopes compared to the a z i d e s ( F i g . 6.10). T h i s i s expected as i n t h i s r e g i o n the d i f f e r e n c e s i n the NNN and NCO groups due to d i f f e r e n t n uclear charges w i l l be pronounced. The f i f t h band shows the s m a l l e s t s h i f t upon changing halogens. In the a z i d e s t h i s was the case f o r the a' (per) o r b i t a l . Ab i n i t i o , * 1 and CNDO/B8 c a l c u l a t i o n s concur with t h i s FIGURE 6:10 The f i f t h to seventh IP 's of ClNCO. 100 a s s i g n m e n t . However t h i s d i s a g r e e s w i t h t h e o r d e r i n g assumed f o r HNCO. A more l i k e l y a s s i g n m e n t i s b a s e d on t h e f a c t t h a t t h e per o r b i t a l f o r t h e a z i d e s was s l i g h t l y s t a b i l i z e d w i t h r e s p e c t t o t h e a c i d . As i n HNCO t h e .per o r b i t a l a p p e a r s t o o c c u r a t 17.4 eV,* t h e s i x t h b a n d o f t h e h a l o g e n i s o c y a n a t e s i s a s s i g n e d t o t h e 6a*(pcrj o r b i t a l . I t o c c u r s a t 17.59, 17.12 a n d 16.39 e ? f o r C l N C O , BrNCO, and INCO r e s p e c t i v e l y . The d e s t a b i l i z i n g t r e n d w i t h h e a v i e r h a l o g e n o c c u r s f o r t h e a z i d e s a l s o . T h i s band c l e a r l y h a s a s h a r p e r F r a n c x - C o n d o n e n v e l o p e , e s p e c i a l l y i n ClNCO a nd BrNCO. T h e r e i s seme p a r t i a l l y 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 i s band f o r ClNCO. The measurement i s u n c e r t a i n and i s e i t h e r a b o u t 950 o r 1900cm- 1. BrNCO a l s o a p p e a r s t o show s t r u c t u r e o f a b o u t 7 0 0 - 8 5 0 c n r - 1 . These f r e g u e n c i e s may r e f e r t o t h e s y m m e t r i c ( a n d a s y m m e t r i c ) NCO s t r e t c h i n g v i b r a t i o n s w h i c h i s c o n s i s t e n t w i t h t h e o b s e r v e d s t r u c t u r e on t h e per band o f t h e a z i d e s . T h i s i n t e r p r e t a t i o n t h e n a s s i g n s t h e 1 a M and 5 a ' b e n d i n g rf o r b i t a l s t o t h e f i f t h and s e v e n t h I P * s f o r t h e i s o c y a n a t e s r e s p e c t i v e l y . The f i f t h I P o f ClNCO ( s e e F i g . 6.10) h a s a v i b r a t i o n a l s e r i e s o f a b o u t 910±80cm- 1 w h i c h p r o b a b l y i s t h e NNN s y m m e t r i c s t r e t c h i n g f r e g u e n c y {v^'= 1309cm- 1) , somewhat r e d u c e d upon i o n i z a t i o n f r o m t h e b o n d i n g o r b i t a l . The l a s t I P c o r r e s p o n d s t o t h e 5a* b o n d i n g r r o r b i t a l . , I n BrNCO t h e r e a p p e a r s t o be weak s t r u c t u r e o f a b o u t 7 5 0 1 8 0 c m - 1 on t h i s b a nd. I n ClNCO ( s e e F i g . 6.10) t h e r e i s s t r u c t u r e o f a b o u t 1 100180cm- 1. 101 Assignment of the l a s t t hree bands i s not c e r t a i n , however the proposed assignment agrees w e l l with the a z i d e s . The Tr s p l i t ranges from 2.29 eV f o r ClNCO to 1.95 eV f o r INCO. The trends of the halogen a z i d e s ( i n c r e a s i n g d e s t a b i l i z a t i o n o f the per o r b i t a l and decreasing s e p a r a t i o n of the rr bonding o r b i t a l s with heavier halogen s u b s t i t u e n t s ) are reproduced by the above assignment f o r the i s o c y a n a t e s . The assignment a l s o c o r r e l a t e s w e l l with the PE r e s u l t s f o r the halogen t h i o c y a n a t e s (see Chapter 7 ) . I t i s i n t e r e s t i n g t o note that as f o r the l i n e a r m c nchalogencacetylenes, 3 1 the IP's of the halogen i s o c y a n a t e s are a l i n e a r f u n c t i o n of the IP's of the f r e e halogen atoms. T h i s i s i l l u s t r a t e d i n F i g . 6.11 which i s a p l o t of the measured IP's as a f u n c t i o n of the valence s t a t e IP's f o r the halogens ( I ^ ) . 3 0 The c o r r e l a t i o n appears q u i t e l i n e a r f o r the known i s o c y a n a t e s , and the data have been e x t r a p o l a t e d to p r e d i c t the PE spectrum of the unknown molecule FNCO. The HAH3 c a l c u l a t i o n has a l s o been c a r r i e d out f o r FNCO 3 2 and those r e s u l t s are i n c l u d e d i n F i g . 6.11. 6.6 S t r u c t u r e s The PE s p e c t r a of the halogen a z i d e s and halogen i s o c y a n a t e s have been anal y z e d , and the i n t e r a c t i o n of halogen p with the >r and cr o r b i t a l s of the near l i n e a r pseudohalide group has been noted. The s i z e of the o f f - a x i s 1D2 o Q . S 17 h 2 18 r-11 FIGURE 6:11 13 14 UX) (eV) 15 16 17 18 The experimental IP ' s -of INCO,BrNCO, and C1NC0 as a function of the valence state IP 's of the respective halogens. The HAM3 results for FNCO are included on the far r ight s ide. 103 bond a n g l e h a s been r e l a t e d t o t h e s p l i t t i n g o f t h e 3 a " and 8 a * , a n d t h e 2 a " and 7a* o r b i t a l s . P r e v i o u s d i s c u s s i o n s o f t h e bond a n g l e s h a v e e m p l o y e d r e s o n a n c e s t r u c t u r e s . * - 6 The main r e s o n a n c e f o r m s f o r HN^ a r e , H H . \ + \ /V = w= A/ N — N = N - +-I JL w h i l e HNCO i s p r e d o m i n a t e l y o f t h e f o r m H \ N - C = O The e f f e c t o f s t r u c t u r e I I i n HNj t e n d s t o s h o r t e n t h e t e r m i n a l NN bond l e n g t h a n d g i v e a more b e n t c o n f i g u r a t i o n . As t h e c o r r e s p o n d i n g f o r m i n HNCO i s u n f a v o u r e d , HN 3 i s more b e n t t h a n HNCO ( T a b l e 6 . 4 ) . I n t h e h a l o g e n a t e d m o l e c u l e s s i m i l a r s t r u c t u r e s h a v e b e e n p r o p o s e d * , * a n d i n g e n e r a l a z i d e s a r e more b e n t t h a n t h e i s o c y a n a t e s ( T a b l e 6 . 4 ) . F o r C l N ^ and C1NCO t h e c h l o r i n e n u c l e a r q u a d r u p l e c o u p l i n g c o n s t a n t s s u g g e s t m i n o r c o n t r i b u t i o n s f r o m t h e r e s o n a n c e f o r m s * , 6 + +• cl • c i s o " w h i c h a g r e e w i t h t h e t r a n s bend f o u n d i n b o t h c a s e s * , 6 ( T a b l e 6 . 4 ) . The PES r e s u l t s s u g g e s t a n g l e s o f a b o u t 119° f o r ClNCO an d BrNCO, and a s m a l l e r a n g l e f o r INCO. FNCO h a s been c a l c u l a t e d t o ha v e an a n g l e o f 135°. 3 3 , 3 * T h i s t r e n d o f a 104 TABLE 6:4 STRUCTURAL DATA FOR SOME AZIDES AND ISOCYANATES3 Isocyanates Ref. b ^XNC rX-N rN-C rC-0 ^NCO PE ref. HNCO MW. 5 128.1 0.987 1.207 1.171 180C 7,8,12 CH3NC0 E.D. 5 MW. 5 140.3 140 1.450 1.437 1.168 1.207 1.202 1.171 180C 180 c 8 SiH3NC0 E.D. 5 MW. 5 151.7 180 1.703 1.699 1.216 1.150 1.164 1.179 8 NCNCO MW. 5 140 1.283 1.218C 1.165c 180C 13 ClNCO E.D. 3 MW. 4 118.2 119.4 1.700 1.703 1.227 1.218 1.156 1.165 171.0 171.4 d trans-CH2=CHNC0 cis-CH2=CHNC0 MW. 15 MW. 15 138.4 142.4 1.382 1.382 1.207c 1.207C 1.171 c 1.171 c 15 15 Azides Ref. ' X N a N b rX-N d r V N b rN.-N b c zN N. N a b c PE ref. HN3 MW. 5 114.1 0.975 1.237 1.133 7-11 CH 3N 3 E.D. 5 MW. 5 116.8 117 1.468 1.46 1.216 1.24 1.130 1.13 8 S iH 3 N 3 E.D. 5 123.8 1.719 1.304 1.125 8 NCN3 MW. 5 120.2 1.312 1.252 1.133 180 c 13,14 C1N3 MW. 6 108.7 1.745 1.252 1.133 172 d ! I a. Angles in degrees, bond lengths in Angstroms. b. E.D. = Electron d i f f r a c t i o n , MW. = Microwave c. Assumed d. This work 105 l e s s b e n t c o n f i g u r a t i o n f o r a more e l e c t r o n e g a t i v e h a l o g e n a c c o m p a n i e s t h e d e c r e a s i n g i m p o r t a n c e o f t h e X + o s t r u c t u r e . A more c o m p l e t e t r e a t m e n t o f t h i s p r o b l e m s h o u l d i n c l u d e a B a l s h d i a g r a m , * 7 , * 8 I n t h e a b s e n c e o f s u i t a b l e c a l c u l a t i o n s . F i g . 6.5 h a s b e e n c o n s t r u c t e d t o p r o v i d e a g u a l i t a t i v e p i c t u r e o f t h e e f f e c t o f t h e o f f — a x i s a n g l e . The a " o r b i t a l s a r e e x p e c t e d t o be a f f e c t e d m i n i m a l l y by b e n d i n g . The 8a* o r b i t a l i s s t a b i l i z e d i n a g r e e m e n t w i t h t h e PES r e s u l t s , and t h e 2 a " and 7a* o r b i t a l s a r e s p l i t by i n t e r a c t i o n s w i t h t h e 3 a " and 8 a * o r b i t a l s . The 6 a * , pcr o r b i t a l w i l l be q u i t e i m p o r t a n t i n d e t e r m i n i n g t h e bond a n g l e . I n t h e b e n t XNCO o r XNj m o l e c u l e t h i s o r b i t a l i s s t a b i l i z e d by t h e a d d i t i o n o f XN rr o v e r l a p . The b e h a v i o u r o f t h e 4 a ' p r o r b i t a l i n t h e a c i d s i s j u s t t h e o p p o s i t e , a s t h e o v e r l a p w i t h t h e H 1s o r b i t a l d e c r e a s e s o n b e n d i n g , w i t h r e s u l t a n t d e s t a b i l i z a t i o n . I t a p p e a r s l i k e l y t h a t c o n s i d e r a t i o n s s u c h a s t h i s w i l l b e s t e x p l a i n t h e s t r u c t u r e s o f t h e s u b s t i t u t e d a z i d e and i s o c y a n a t e m o l e c u l e s . E x p e r i m e n t a l g e o m e t r i e s f o r INCO, BrNCO, a n d BrN^ w o u l d be most i n f o r m a t i v e . The use o f r e l i a b l e c a l c u l a t i o n s w i t h G r e e n ' s f u n c t i o n s * * o r p e r t u r b a t i o n s t o Koopmans' t h e o r e m * 5 w o u l d h e l p t o c h a r a c t e r i z e t h e o r b i t a l s more e x a c t l y and w o u l d y i e l d a more d e f i n i t i v e a s s i g n m e n t f o r t h e h i g h I P r e g i o n o f t h e i s o c y a n a t e s . 106 CHAPTER SEVEN T h i o c y a n o g e n H a l i d e s , XSCN ( X = C l , B r ) , and T h i o c y a n o g e n {SCN)^ 7.1 I n t r o d u c t i o n T h i o c y a n a t e s have r e c e i v e d l i t t l e a t t e n t i o n i n PES, howe v e r t h e i r g e n e r a l c h e m i s t r y i s w e l l known, and t h e m o l e c u l e s s t u d i e d i n t h i s c h a p t e r a r e t h e s u b j e c t s o f two c o m p r e h e n s i v e r e v i e w s . 1 , 2 The SCN g r o u p c a n bond a t e i t h e r s u l p h u r o r n i t r o g e n , f o r m i n g RSCN o r RNCS compounds. U s u a l l y o ne f o r m i s g r e a t l y f a v o u r e d 1 a s i n t h e a c i d w h i c h i s a t l e a s t 9 5 % HNCS i n t h e gas p h a s e . 3 The PE s p e c t r u m o f HNCS i n known.? The PE s p e c t r u m o f t h e c y a n o d e r i v a t i v e S (CN| a has a l s o b een r e p o r t e d . 6 The XSCN m o l e c u l e s s t u d i e d h e r e a r e v a l e n c e i s o e l e c t r o n i c w i t h t h e XNCO and XNj m o l e c u l e s d e s c r i b e d i n C h a p t e r 6. The known g e o m e t r i e s o f C H 3 S C N 3 o , S C l - ^ 3 1 , S { C N ) 2 * s u g g e s t t h a t C l S C N and BrSCN have n e a r l i n e a r SCN g r o u p s w i t h an XSC a n g l e o f a b o u t 100°. 107 T h i s s t r u c t u r a l r e s e m b l a n c e t o t h e XNCO and XN3 m o l e c u l e s s u g g e s t s t h a t some o f t h e c o n c e p t s d e v e l o p e d i n C h a p t e r 6 may a l s o a p p l y t o t h e XSCN m o l e c u l e s . The XSCN m o l e c u l e s a r e p o l a r i z e d w i t h t h e h a l o g e n n e g a t i v e 1 , u n l i k e t h e h a l o g e n a z i d e s a n d h a l o g e n i s o c y a n a t e s . , P r e v i o u s s p e c t r o s c o p i c s t u d i e s h a v e p r o v i d e d I B 3 2 and OV 8 s p e c t r a f o r C l S C N , B r S C N , and (SCN)^ . T h i s c h a p t e r o u t l i n e s p r e p a r a t i v e p r o c e d u r e s f o r t h e gas p h a s e p r o d u c t i o n o f C l S C N , B r SCN, and ( S C N ) A . T h i s i s t h e f i r s t r e p o r t o f t h e s e m o l e c u l e s as g a s e s , and i s t h e f i r s t t i m e t h e y h a v e r e c e i v e d d e t a i l e d s p e c t r o s c o p i c s t u d y . 7.2 E x p e r i m e n t a l <D (SCN)^ AgSCN was p r e p a r e d b y m i x i n g a g u e o u s s o l u t i o n s o f KSCN an d AgNO . T h e p r e c i p i t a t e was washed, d r i e d , s t o r e d i n vacuum a n d d a r k . A 10mm OD p y r e x t u b e was p a c k e d , f o r a l e n g t h o f a b o u t 15 cm, w i t h AgSCN powder s u p p o r t e d on g l a s s w o o l . The c o l u m n was d e g a s s e d a t 160°C f o r up t o two d a y s . T h i s d r y i n g p r o c e d u r e s o m e t i m e s l e f t some r e s i d u a l w a t e r w h i c h a p p e a r e d i n t h e s p e c t r a , b u t d i d n o t seem t o i n t e r f e r e w i t h p r o d u c t i o n o f ( S C N ) ^ , o r C l S C N , o r BrSCN. C h l o r i n e was p a s s e d t h r o u g h t h e c o l u m n g i v i n g {SCN)^, w h i c h p a s s e d d i r e c t l y i n t o t h e s p e c t r o m e t e r . As (SCN)^ had p r e v i o u s l y b e e n i s o l a t e d a s an e x p l o s i v e s o l i d 1 2 , no a t t e m p t s were made 108 t o t r a p r e a c t i o n p r o d u c t s o r t o remove w a t e r by t r a p p i n g . No t r a c e o f C l ^ was e v e r n o t e d i n t h e PE s p e c t r a o f ( S C N ) ^ . O n l y t h e one p r o d u c t , ( S C N ) 2 , was s e e n f o r t h e f i r s t s e v e r a l h o u r s o f r e a c t i o n . (SCN)^ was a l s o p r e p a r e d i n i d e n t i c a l f a s h i o n u s i n g B r ^ r a t h e r t h a n C l ^ , , (2) C1SCN D u r i n g p r o d u c t i o n o f (SCN)^ u s i n g C l ^ a s e c o n d r e a c t i o n p r o d u c t a p p e a r e d a f t e r s e v e r a l h o u r s . E v e n t u a l l y (SCN)^ d i s a p p e a r e d a n d a p u r e s p e c t r u m o f t h e new p r o d u c t , C l S C N , was o b t a i n e d . T h i s c h a n g e o f p r o d u c t upon • p a r t i a l s a t u r a t i o n * o f t h e c o l u m n w i t h C l ^ i s e x p l a i n e d by t h e g e n e r a l r e a c t i o n scheme p r o p o s e d i n s e c t i o n 7.4. E v e n t u a l l y u n r e a c t e d C l 2 emerged f r o m t h e c o l u m n . O n l y (SCN)^ , C l S C N , a n d C l ^ (and H^O) were d e t e c t e d i n t h e PE s p e c t r a , s u g g e s t i n g t h a t C l ^ S C N 1 3 c a n n o t be p r e p a r e d f o r g a s p h a s e s t u d y by t h i s method. (3) BrSCN D u r i n g p r o d u c t i o n o f (SCN)^ w i t h B r ^ a c h a n g e o f p r o d u c t was n o t e d , a f t e r s e v e r a l h o u r s . T h i s p r o d u c t , B r S C N , gave a PE s p e c t r u m q u i t e s i m i l a r t o C l S C N . No e v i d e n c e o f Br^SCN 1 3 was n o t e d i n t h e PE s p e c t r a . An a l t e r n a t e s o u r c e o f BrSCN was a l s o u s e d , C l ^ was p a s s e d t h r o u g h t h e AgSCN c o l u m n g i v i n g ( S C N ) ^ . B r ^ was t i t r a t e d i n t o t h e (SCN)^ f l o w , d o w n s t r e a m f r o m t h e AgSCN. T h i s p r o d u c e d BrSCN i n h i g h y i e l d s . I n b o t h p r e p a r a t i o n s o f B r S CN, B r ^ and (SCN)^ a p p e a r e d a s i m p u r i t i e s , however t h e y p r o v e d o n l y a m i n o r i n t e r f e r e n c e i n t h e s p e c t r a . D u r i n g a l l t h e t h i o c y a n a t e s t u d i e s a y e l l o w - r e d 109 m a t e r i a l was d e p o s i t e d t h r o u g h o u t t h e i n l e t and r e a c t i o n t u b e s . T h i s m a t e r i a l was p r o b a b l y p o l y m e r i c . 1 , 1 3 , 1 * The He I a n d Ne I s p e c t r a were r e c o r d e d and c a l i b r a t e d w i t h t h e known I P ' s o f C H ^ I , A r , N^, and H^O. 7 . 3 B e s u i t s The He I PE s p e c t r a o f C l S C N , BrSCN, a n d (SCN)^ a r e shown i n P i g . 7.1. The He .1 s p e c t r a i n F i g . 7,2 r e v e a l s i g n i f i c a n t i n t e n s i t y v a r i a t i o n s f o r a c h a n g e o f p h o t o n e n e r g y . The v a l u e s o f t h e I P ' s , and f i n e s t r u c t u r e n o t e d f o r some b a n d s , a r e l i s t e d i n T a b l e 7.1, The f i r s t PE bands o f BrSCN and ClSCN a r e e x p a n d e d i n F i g , 7,3. I n b o t h c a s e s t h e v e r t i c a l t r a n s i t i o n a p p e a r s t o be a t v*=1 a l t h o u g h BrSCN may h a v e o n e a d d i t i o n a l v i b r a t i o n a l b a n d a t l o w e r I P , The s e c o n d t o s i x t h b a n d s o f ClSCN and BrSCN a r e e x p a n d e d i n F i g . 7.4. I n C l S C N t h e s i x t h b a n d shows weak v i b r a t i o n a l s t r u c t u r e . F i g . 7 . 6 shows d e t a i l s o f t h e f i r s t two PE bands o f ( S C N ) 5 . I n t h i s s p e c t r u m t h e s e c o n d b a n d i s w a t e r f r e e , h o wever w a t e r was o f t e n p r e s e n t , and t h e r e i s some w a t e r i n t h e s p e c t r u m o f (SCN)^ i n F i g , 7.2. F i g . 7.8 i s an e x p a n s i o n o f t h e t h i r d a n d f o u r t h b a n d s o f ( S C N ) ^ . B o t h o f t h e s e b a n d s w i l l be shown t o i n v o l v e more t h a n one i o n i z a t i o n p r o c e s s . 1 I 1 1 1 1 ClSCN I i i i i i J 11 •a, 'Pili I 1 BrSCN 1 1 1 1 1 1 M i i i 1 (SCN)2 i i i i i i 10 12 14 16 18 IONIZATION POTENTIAL (eV) FIGURE 7:1 He I PE spectra of ClSCN,BrSCN, and (SCN) 2. I ClSCN I l I I l I l u 10 12 14 16 IONIZATION P O T E N T I A L (eV) Ne I PE spectra of ClSCN,BrSCN, and(SCN) 2 < TABLE 7:1 EXPERIMENTAL IP'S AND ASSIGNMENTS FOR ClSCN, BrSCN, AND (SCN)2 ClSCN BrSCN IP(eV) (10.47) 10.52 12.67 13.04 13.28 13.65 14.47 15.44 17.55 v'(cm~') 570130 650*60 IP(eV) (10.24) 10.30 11.88 12.38 12.63 13.43 14.00 14.9 17.2 v ' U n T 1 ) 450±30 Assignment 3a" 2a" 8a' 7a' 6a' la" 5a' 4a' ISCNJL IP(eV) (10.63) 11.00 11.32 12.40 13.41 13.49 13.67 13.96 14.23 15.18 16.43 18.27 v'(cm"^) 900t80 Assignment n~(a) r»s(b) (a) aSCN aSCN a s s (a ) (b) (a) (b) (b) (a) (a).(b) a Vertical IP's (adiabatic IP's in brackets). Al„l bands i0.02eV except where indicated, b See text. ro 113 1 0 . 2 10.4 10.4 10.6 I O N I Z A T I O N P O T E N T I A L ( e V ) FIGURE 7:3 The f i r s t PE bands of BrSCN and ClSCN. 114 ClSCN J I L i i ' I I I I I L 13 14 15 I I ' ' I I I 1 1 L. 12 13 14 IONIZAT ION P O T E N T I A L ( e V ) FIGURE 7:4 The second to s ixth PE bands of ClSCN and BrSCN. 115 The s p e c t r a i n F i g . 7.2, and t h e BrSCN s p e c t r u m i n F i g . 7.3 were r e c o r d e d u s i n g Ne I r a d i a t i o n . , A l l o t h e r s p e c t r a w e re r e c o r d e d u s i n g t h e He I l i g h t s o u r c e . 7.4 A s s i g n m e n t s (1) As ( S C N ) ^ , B rSCN, and C l S C N h a v e n o t p r e v i o u s l y been p r e p a r e d by t h e p r e s e n t m e t h o d s , t h e i r i d e n t i f i c a t i o n must f i r s t be e s t a b l i s h e d . The i d e n t i f i c a t i o n i s b a s e d on t h e a n a l o g o u s n a t u r e o f t h e p r e s e n t e x p e r i m e n t s t o t h e e a r l i e r s o l u t i o n p r e p a r a t i o n s . S e v e r a l w o r k s d e s c r i b e t h e l a t t e r t o p i c . 1 , 2 , 1 2 # 1 3 , 1 S , 1 6 The s o l u t i o n s a r e p r e p a r e d by r e a c t i n g C l ^ o r B r ^ w i t h a m e t a l t h i o c y a n a t e , i n a s u i t a b l e s o l v e n t . V a r i o u s m e t a l t h i o c y a n a t e s h a v e b e e n u s e d , n o t a b l y P b i S C N ) ^ and A g S C N . * , 1 2 The s o l v e n t s w h i c h h a v e been used i n c l u d e C C l ^ , C H C l ^ , b e n z e n e , e t h e r , and CS^.* A c r u c i a l c o n s i d e r a t i o n i s d r y n e s s o f t h e s o l v e n t . * , 2 , 8 , 1 5 , 1 7 The r e a c t i o n m i x t u r e i s o f t e n c o o l e d , h owever t h i s i s p r i m a r i l y t o p r e s e r v e t h e u n s t a b l e p r o d u c t s , 1 , 8 a n d t h e r e a c t i o n c a n be done a t room t e m p e r a t u r e . 8 , 1 6 The r a t i o o f h a l o g e n t o m e t a l t h i o c y a n a t e u s e d , d e t e r m i n e s w h e t h e r ( S C N ) 2 o r XSCN i s p r o d u c e d . * , 1 2 , 1 * The o v e r a l l r e a c t i o n scheme f o r t h e s o l i d — s o l u t i o n s y s t e m may be s u m m a r i z e d a s : * # 2 , 8 , 1 2 , * 3 , 1 s , 1 8 MSCN XSCN , (SCN) The p r e s e n t r e s u l t s i n d i c a t e t h i s scheme a l s o a p p l i e s 116 t o t h e s o l i d - g a s s y s t e m . H i t h a f r e s h r e a c t i o n c o l u m n , AgSCN i s i n v a s t e x c e s s and C l ^ o r B r ^ r e a c t t o p r o d u c e (SCN)^ . As t h e q u a n t i t y o f AgSCN i s d i m i n i s h e d , XSCN i s s t i l l f o r m e d , b u t emerges f r o m t h e c o l u m n u n r e a c t e d . The a l t e r n a t e s o u r c e o f BrSCN i n v o l v e d t i t r a t i o n o f B r ^ i n t o a f l o w o f (SCN)^ , e s t a b l i s h i n g t h e g a s p h a s e r e a c t i o n o f t h e s e compounds. The y e l l o w — r e d d e p o s i t i o n i n t h e i n l e t t u b e s , h a s been n o t e d i n t h e s o l u t i o n r e a c t i o n s . 1 , 3 2 A l l t h e e v i d e n c e o f t h e p r e s e n t e x p e r i m e n t s s u g g e s t s t h e s o l i d - g a s s y s t e m b e h a v e s j u s t a s t h e s o l i d — s o l u t i o n s y s t e m . A d d i t i o n a l l y i t w i l l be shown t h a t t h e PE s p e c t r a a r e c o n s i s t e n t w i t h t h e f o r m u l a e C l S C N , BrSCN, and (SCN)^. (2) C l S C N and BrSCN The He I s p e c t r a o f C l S C N a n d BrSCN ( F i g . 7.1) show o b v i o u s s i m i l a r i t i e s . The f i r s t band (3a") h a s a 0.22 eV l o w e r I P i n BrSCN t h a n C l S C N , and i n b o t h c a s e s i t i s w e l l s e p a r a t e d f r o m t h e r e s t o f t h e s p e c t r u m . F i g . 7,3 shows t h e f i r s t b a n d s t o i n v o l v e v i b r a t i o n a l e x c i t a t i o n o f 570±30 c m - 1 f o r C l S C N and 450±30 cm- 1 f o r BrSCN. The c h a n g e i n f r e q u e n c y s u g g e s t s t h e v i b r a t i o n may be t h e X-S s t r e t c h i n g mode. The I B v a l u e s f o r t h e g r o u n d m o l e c u l a r s t a t e s o f C l S C N and BrSCN a r e 520 and 451 cm- 1 r e s p e c t i v e l y . 3 2 The i o n i c f r e q u e n c y f o r C l S C N i s i n c r e a s e d c o m p a r e d t o t h e m o l e c u l a r v a l u e , i n d i c a t i n g some C I S a n t i b o n d i n g c h a r a c t e r i n t h e 3 a " o r b i t a l . A s i m i l a r o b s e r v a t i o n was made c o n c e r n i n g t h e 3 a H o r b i t a l s o f t h e X N ^ and XNCO m o l e c u l e s 117 (Ch. 6) . B o t h t h e l o w I P and t h e e n h a n c e d i n t e n s i t i e s i n t h e Ne I s p e c t r a ( F i g . 7.2) c o m p a r e d t o t h e He I s p e c t r a , i n d i c a t e t h a t t h e 3 a " band i n v o l v e s a s u l p h u r l o n e p a i r o r b i t a l ( s e e Ch. 2 ) . The same i n t e n s i t y b e h a v i o u r u s i n g He I I , He I and Ne I , i s s e e n i n t h e f i r s t band o f S ( C N ) ^ w h i c h i s a s s i g n e d t o a s u l p h u r l o n e p a i r o r b i t a l . 6 B e c a u s e o f t h e s i m i l a r i t i e s o f t h e XSCN m o l e c u l e s (Ch. 6) t h e same t e r m i n o l o g y f o r t h e o r b i t a l s w i l l be use d . T h i s makes t h e 3 a " o r b i t a l o f C l S C N and BrSCN a W(nb) o r b i t a l , h owever t h e l o w I P and i n t e n s i t y v a r i a t i o n w i t h p h o t o n e n e r g y v e r i f y t h a t i t h a s a s u b s t a n t i a l s u l p h u r 3p o r b i t a l c o n t r i b u t i o n . E x c i t a t i o n o f t h e SX s t r e t c h i n g v i b r a t i o n i n d i c a t e s t h e i n v o l v e m e n t o f h a l o g e n p a t o m i c o r b i t a l s . , The 3 a " b a n d o f BrSCN i s somewhat n a r r o w e r t h a n t h e 3a" b a n d o f C l S C N , r e p r o d u c i n g a t r e n d o f t h e XNCO a n d X N j m o l e c u l e s (Ch. 6 ) . T h i s t r e n d f o r XNCO and XN^ a c c o m p a n i e d i n c r e a s e d h a l o g e n p i n v o l v e m e n t i n t h e 3 a " o r b i t a l , f o r h e a v i e r h a l o g e n s u b s t u e n t s . T h i s may a l s o o c c c u r f o r t h e XSCN m o l e c u l e s . C o r r e l a t i o n o f t h e PE d a t a o f C l S C N a n d BrSCN i s g r e a t l y a i d e d b y t h e i n t e n s i t y c h a n g e s upon c h a n g i n g p h o t o n e n e r g y ( F i g . 7 . 1 , 7 . 2 ) . (The Ne d i s c h a r g e p r o d u c e s two l i n e s . The 16.67 eV l i n e i s a b o u t 18% o f t h e main l i n e a t 16.85 eV, and must be a c c o u n t e d f o r i n e x a m i n i n g Ne I s p e c t r a . ) C o r r e l a t i o n o f t h e PE b a n d s o f C l S C N and BrSCN i s i n d i c a t e d i n T a b l e 7.1. The 7 a * o r b i t a l o f C l S C N and t h e 8a* o r b i t a l o f BrSCN show a d e c r e a s e i n i n t e n s i t y on c h a n g i n g t o l o w e r p h o t o n e n e r g y , r e l a t i v e t o t h e 3a" 118 o r b i t a l s . These o r b i t a l s c l e a r l y i n v o l v e f a r l e s s sulphur 3p c h a r a c t e r than the 3a" o r b i t a l s . Using the terminology of Chapter 6, these are the a*, )T(nb) o r b i t a l s . The second (2a") and t h i r d (8a*) bands of ClSCN, and the second (2a") and f o u r t h (7a*) bands of BrSCN show an i n c r e a s e d i n t e n s i t y f o r lower photon energy, r e l a t i v e to a l l bands e x c e p t i n g the 3a" bands. T h i s i n t e n s i t y i n c r e a s e i s i n d i c a t i v e of halogen p o r b i t a l c h a r a c t e r c o n t r i b u t i n g t o these o r b i t a l s (see Ch. 2). In f a c t the f i r s t f o u r IP*s r e p r e s e n t o r b i t a l s which i n v o l v e c o n s i d e r a b l e mixing of the symmetry e g u i v a l e n t n^ and tr(nb) o r b i t a l s . T h i s i s i n d i c a t e d by the s t r o n g i n t e r a c t i o n s between the 8a* and 7a* o r b i t a l s f o r both molecules and i s expected by analogy to the molecules i n Chapter 6. T h i s a c t u a l l y r e s u l t s i n an i n t e r c h a n g i n g of the main atomic o r b t a l c o n t r i b u t i o n s i n the 8a* and 7a* bands on changing the halogen from C l to Br. The 77(nb) o r b i t a l i s the 8a* o r b i t a l i n ClSCN and the 7a' o r b i t a l i n BrSCN, while the 7a« o r b i t a l i n ClSCN and the 8a' o r b i t a l i n BrSCN i n v o l v e halogen p atomic o r b i t a l s . Mixing of the out-of—plane s u l p h u r lone p a i r and halogen p o r b i t a l s has been e s t a b l i s h e d e a r l i e r with r e f e r e n c e to the v i b r a t i o n a l s t r u c t u r e and Franck-Condon envelopes of the 3a" bands of ClSCN and BrSCN. T h i s i n d i c a t e s some su l p h u r 3p c h a r a c t e r w i l l be present i n the 2a" o r b i t a l s . T h i s w i l l tend t o i n c r e a s e the c r o s s - s e c t i o n of the 2a" o r b i t a l i n the Ne I spectrum, even more than i f i t were purely a halogen lone pair.„ On t h e other hand the 8a* o r b i t a l of ClSCN and 7a* o r b i t a l of BrSCN mix with the a*, Tf(nb) o r b i t a l s which 119 have been shown to have l i t t l e sulphur 3p c o n t r i b u t i o n . T h i s suggests t h a t the 2a" o r b i t a l s should experience a g r e a t e r i n c r e a s e i n c r o s s — s e c t i o n on changing to lower photon energy, than the 8a" o r b i t a l o f CISCN or the 7a" o r b i t a l of BrSCN. In f a c t t h i s i s observed ( F i g . 7.1,7.2) c o n f i r m i n g the assignment of the second PE band, f o r both molecules, t o be o f a" symmetry. In c o n s i d e r i n g the p o s i t i o n s of the f i r s t f our IP's of ClSCN and BrSCN, the i n t e r a c t i o n of the 8a« and 7a« o r b i t a l s i s g u i t e important. Having i d e n t i f i e d the 2a" and 8a* o r b i t a l s of ClSCN, and t h e 2a" and 7a» o r b i t a l s of BrSCN as having l a r g e halogen p c h a r a c t e r , a s p l i t t i n g of 0.37 eV i n ClSCN and 0.75 eV i n BrSCN i s obtained f o r these halogen p c o n t a i n i n g o r b i t a l s . The i n c r e a s e i s , of course, due to the i n t e r a c t i o n with the a* o r b i t a l which i s i n t e r p o s e d between the 2a" and 7a' o r b i t a l s of BrSCN. The c e n t e r of g r a v i t y of the 2a" and 8a' o r b i t a l s of ClSCN i s at 0.60 eV higher IP than the c e n t r o i d of the 2a" and 7a' o r b i t a l s of BrSCN, r e p r e s e n t i n g a normal s h i f t f o r the IP of halogen lone p a i r o r b i t a l s on changing from C l to Br. The f i f t h band of ClSCN and BrSCN i s assigned to the 6a» (per) o r b i t a l . I t d i s p l a y s a sharp Franck-Condon envelope s i m i l a r t o the per band i n the halogen a z i d e s (Ch. 6), and i n both cases has some l o c a l i z a t i o n on the t e r m i n a l n i t r o g e n atom. Thus the p o s i t i o n of the band i s s h i f t e d only 0.22 eV to lower IP from ClSCN to BrSCN. A s i m i l a r s m a l l s h i f t of 0.34 eV was noted f o r the halogen a z i d e s (Ch. 6). The p o s i t i o n of t h e per band i s about 15.8 120 eV fox the halogen a z i d e s (Ch. 6) and 13.5 eV f o r the halogen t h i o c y a n a t e s . T h i s s h i f t r e f l e c t s the presence of the sulphur 3p atomic o r b i t a l . Beyond the f i f t h IP, there are t h r e e more PE bands f o r each molecule. T h i s i s one more band than f o r the molecules of Chapter 6. The h i g h e s t IP a t 17.55 eV f o r ClSCN and 17.2 eV f o r BrSCN r e p r e s e n t s the 4a*, cr o r b i t a l which has c o n s i d e r a b l e s c h a r a c t e r and i s t h e r e f o r e weak i n the He I spectrum ( F i g . 7.1). The s i x t h (1a") and seventh (5a') bands are the i n - , and o u t — o f - p l a n e vr bonding o r b i t a l s . These have broad Franck-Condon envelopes, as found f o r the halogen a z i d e s and halogen i s o c y a n a t e s , and i s i n keeping with the bonding nature of the o r b i t a l s . There i s about a 0.5 eV s h i f t t o lower IP from ClSCN to BrSCN, and the s p l i t t i n g between them changes s l i g h t l y from 0.97 to 0.90 eV, As i n Chapter 6, one expects the e f f e c t of the o f f - a x i s atom to s t a b i l i z e the 5a* o r b i t a l r e l a t i v e to the 1a" o r b i t a l . In a s s i g n i n g these two bands the r e l a t i v e i n t e n s i t y change f o r the Ne I and He I s p e c t r a ( F i g , 7,1, 7.2) must be e x p l a i n e d . F i r s t the t r e n d s f o r i n - , and out—of-plane tr bonding o r b i t a l s i n other s m a l l molecules must be e s t a b l i s h e d . In SjjCN)^ the o u t - o f - p l a n e Tr bands l o s e i n t e n s i t y i n the Ne I (compared to He I and He II) spectrum, r e l a t i v e to the i n plane IT bands. 6 In NCNj the o u t — o f - p l a n e Tr band l o s e s i n t e n s i t y i n the Ne I (compared to He I) spectrum, r e l a t i v e to the i n - p l a n e rr band. The i n t e r p r e t a t i o n of the PE spectrum of HNj given i n Chapter 6, i n d i c a t e s t h a t the i n - , and c u t — o f — p l a n e >r 121 b e n d i n g o r b i t a l s f o l i o s t h e s a n e t r e n d i n t h e He I a n d He I I s p e c t r a . F o r t h e s e s i m i l a r , s m a l l m o l e c u l e s t h e o u t — o f - p l a n e tr b o n d i n g b a n d l o s e s i n t e n s i t y f o r l o w e r p h o t o n e n e r g y , r e l a t i v e t o t h e i n - p l a n e Yr b o n d i n g band. W h i l e t h i s w i l l d e p e n d t o some e x t e n t on t h e n a t u r e o f t h e p a r t i c u l a r l i n e a r g r o u p i n g , i t may a l s o a p p l y h e r e . The s i x t h band f o r b o t h C l S C N a nd BrSCN, l o s e s i n t e n s i t y on c h a n g i n g t o l o w e r p h o t o n e n e r g y , r e l a t i v e t o t h e s e v e n t h band. The s i x t h and s e v e n t h bands a r e a s s i g n e d t o t h e 1 a M and 5 a * o r b i t a l s r e s p e c t i v e l y f o l l o w i n g ! t h e a b o v e t r e n d f o r i n t e n s i t y c h a n g e s , and a g r e e i n g w i t h t h e a s s i g n m e n t s f o r t h e XNCO and X N 3 m o l e c u l e s (Ch. 6 ) . The s i x t h band o f C l S C N ( F i g . 7.4) h a s 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 650 cm- 1. T h i s i s p r o b a b l y a r e d u c e d CS s t r e t c h i n g f r e q u e n c y , w h i c h i s 678 c m - 1 f o r t h e g r o u n d s t a t e m o l e c u l e , 3 2 c o n f i r m i n g t h e b o n d i n g n a t u r e o f t h e o r b i t a l . F i g . 7.5 c o r r e l a t e s t h e PE r e s u l t s f o r S C 1 2 , C l S C N , and S (CN)^ . The a" o r b i t a l s show p a r t i c u l a r l y l i n e a r s h i f t s upon r e p l a c e m e n t o f C l b y CN. The 3 a " band i s s h i f t e d 0.85 eV f r o m S C l ^ t o C l S C N , a nd 0.80 eV f r o m C l S C N t o S ( C N ) ^ . The 4a» o r b i t a l o f C l S C N i s SCN b o n d i n g , a n d i n c l u d e s c o n s i d e r a b l e s u l p h u r 3 s and c a r b o n 2 s c o n t r i b u t i o n s . , T h i s c o r r e s p o n d s t o t h e 19.7 eV (b^) o r b i t a l o f S ( C N ) ^ w h i c h i s CS b o n d i n g , 6 and t h e o r b i t a l o f S C l ^ w h i c h i s w e a k l y S C I b o n d i n g . 1 8 The l a r g e s h i f t ( F i g . 7.5) i n v o l v e s r e p l a c e m e n t o f C l o r b i t a l s w i t h C o r b i t a l s w h i c h have an i n h e r e n t l y h i g h e r I P . The o r i g i n a l PE s t u d y o f S C 1 2 s u g g e s t e d t h a t t h e 13 . 9 1 , 1 4 .67, and 15.70 eV ba n d s may c o r r e s p o n d t o t h e 3b,, 12:2 S C I 2 C l S C N S ( C N ) 2 FIGURE 7;5 Correlat ion of PES results for SCI ClSCN , and S(CN) 7. 123 10a,, and 7b^ o r b i t a l s r e s p e c t i v e l y , on the b a s i s of ab i n i t i o SCF c a l c u l a t i o n s . 1 8 while t h i s d i s a g r e e s with an assignment based on Halsh d i a g r a m s , 1 8 i t concurs with the present assignment f o r ClSCN., T h i s suggests that Koopmans* theorem i s obeyed i n S C l ^ , d e s p i t e e a r l i e r doubts concerning i t s a p p l i c a b i l i t y . 1 8 (3) (SCN) 2 IB and Baman data have shown the s t r u c t u r e of {SCN)^ i s N=C-S-S-C=N.*,2 One expects s i m i l a r i t i e s to the d i s u l p h u r d i h a l i d e m o l e c u l e s 2 6 which have the s t r u c t u r e XSSX. The f i r s t band of (SCN)^ ( F i g . 7.6) resembles the f i r s t bands of HSSH and XSSX ( X = F , C l , B r ) 2 6 and i n a l l cases the f i r s t band i s assigned to sulphur lone p a i r o r b i t a l s . For HSSH, 2 6 XSSX, 2 6 and CH^SSCH^ 2 8 the tWWlphur lone p a i r o r b i t a l s are s p l i t by a through space i n t e r a c t i o n g i v i n g n +(a) and n~ (b) combinations. The magnitude of the s p l i t t i n g i s p r i m a r i l y a f u n c t i o n of the d i h e d r a l angle i n the C^ molecules, as o v e r l a p o f the lone p a i r o r b i t a l s i s a f u n c t i o n of the d i h e d r a l angle. Maximum o v e r l a p and s p l i t t i n g would occur i n the planar C s geometry, while the minimum o v e r l a p and s p l i t t i n g " i s expected f o r a d i h e d r a l angle of about 9 0 ° . 2 9 , 3 3 Through bond i n t e r a c t i o n s have a l s o been c o n s i d e r e d 2 6 but are less'"important. For HSSH, FSSF, C1SSC1 and BrSSBr the d i h e d r a l "Wangles are 90.6°, 87.9°, 81.8°, and 8 3 . 5 ° r e s p e c t i v e l y ; and the s p l i t t i n g s of the sulphur lone p a i r o r b i t a l s are 0.27, 0.41, 0.2, and 0.25 eV r e s p e c t i v e l y . <^'~~^ shows a s p l i t t i n g of 0.95 eV and has a 124 FIGURE 7:6 The f i r s t two PE bands of (SCN) 125 d i h e d r a l a n g l e o f a b o u t 6 0 ° . 2 8 The 0.32 eV s p l i t t i n g i n t h e f i r s t band o f ( S C N ) ^ ( F i g . 7.6) s u g g e s t s a d i h e d r a l a n g l e a p p r o a c h i n g 90°. I n f a c t t h i s i s t h o u g h t t o be so f o r a l l u n c o n s t r a i n e d d i s u l p h i d e s . 3 3 The b and a t 12.40 eV ( F i g . 7.6) shows a s t r o n g v i b r a t i o n a l component o f 1520 c m - 1 . T h i s i s a s s i g n e d t o t h e CN s t r e t c h i n g mode w h i c h h a s a f r e q u e n c y o f 2171 c m - 1 i n t h e g r o u n d s t a t e m o l e c u l e . 3 2 T h i s band i s t h e r e f o r e a s s i g n e d t o a o r b i t a l . F i g . 7.7 c o r r e l a t e s t h e PE d a t a f o r HSSH, F S S F , (SCN)^ , C1SSC1, and Br S S B r . » J * , 2 8 The a s s i g n m e n t o f t h e {SCN)^ s p e c t r u m i n t e r m s o f c h a r a c t e r i s t i c 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 due t o s t r o n g m i x i n g i n s y m m e t r y , h o w e v e r f o l l o w i n g t h e t r e n d s i n F i g . 7.7 g i v e s a f e a s i b l e i n t e r p r e t a t i o n . The band a t a b o u t 13.5 eV ( F i g . 7.8) i n v o l v e s more t h a n one i o n i z a t i o n p r o c e s s . I n t h e Ne I s p e c t r u m t h e 13.67 eV s h o u l d e r h a s e n h a n c e d i n t e n s i t y , r e l a t i v e t o t h e m a i n peak. The band a l s o i n v o l v e s a s h o u l d e r a t a b o u t 13.41 eV w h i c h i s a s s i g n e d t o a t h i r d I P . The two PO^CAJ o r b i t a l s a r e a s s i g n e d t o t h e I P ' s a t 13.41 and 13.49 eV. I n t h e C 2 m o l e c u l e , * i t h a d i h e d r a l a n g l e n e a r 90° t h e po~scN o r b i t a l s w i l l be n e a r l y o r t h o g o n a l , and c o n s e q u e n t l y one may e x p e c t a s m a l l s p l i t t i n g . A l s o , t h e per band i s s h i f t e d o n l y a b o u t o.22 eV f r o m C l S C N t o B r S C N , i n d i c a t i n g i t t o be r e l a t i v e l y i n s e n s i t i v e t o t h e s u b s t i t u e n t g r o u p a t s u l p h u r . The p c r g ^ b a n d s o f (SCN)^ may be e x p e c t e d t o o c c u r a t a b o u t t h e same I E , w h i c h i s a b o u t 13.5 eV. F i n a l l y t h e b a n d s a t 13.41 and 13.49 eV a p p e a r t o ha v e f a i r l y s h a r p F r a n c k - C o n d o n e n v e l o p e s FIGURE 7:7 Correlat ion of PES results for HSSH, FSSF, (SCN) 2, C1SSC1, and BrSSBr. 128 resembling the per bands o f ClSCN and BrSCN ( F i g . 7.8). The IP a t 13.67 eV has an i n c r e a s e d i n t e n s i t y a t lower photon energy, r e l a t i v e to the 13.5 eV bands (Fig. 7.8). T h i s probably r e f l e c t s a sulphur p c o n t r i b u t i o n , and t h i s band i s assigned t o the og $ o r b i t a l . The c o r r e l a t i o n of the 0~5S o r b i t a l s i n F i g . 7.7 supports the assignment. F o l l o w i n g the t r e n d i n the d i s u l p h u r d i h a l i d e s , there i s probably c o n s i d e r a b l e i n t e r a c t i o n between the a (crS s ) and the a (ir) o r b i t a l s i n (SCN)^ . The f o u r t h PE band i s a l s o assigned to two IP's, a t 13.96 and 14.23 eV. R e l a t i v e t o the 13.96 eV i o n i z a t i o n , the 14.23 eV i o n i z a t i o n shows a small i n c r e a s e i n i n t e n s i t y i n the Ne I spectrum ( F i g . 7.8). There i s some v i b r a t i o n a l s t r u c t u r e i n v o l v e d i n the band, probably o v e r l a p p i n g s e r i e s from the two IP's. The p r o g r e s s i o n s i n v o l v e d are i n the range 600-700 cm - 1 and correspond t o the CS s t r e t c h i n g v i b r a t i o n which i s about 670cm _ i f o r the ground s t a t e m o l e c u l e , 8 The band i s assigned t o two Yr o r b i t a l s . The f o u r t h )r o r b i t a l i s assigned t o the 15.18 eV band. F i g , 7,7 i n c l u d e s t e n t a t i v e c o n n e c t i o n l i n e s f o r these bands. Above 15,18 eV are two more PE bands. These b r i n g the t o t a l number of IP's to e l e v e n which i s the number expected i f one adds t o the nine IP's of FSSF, two a d d i t i o n a l I P's from the n i t r o g e n lone p a i r s of the CN groups. There i s a p o s s i b i l i t y t h a t a tw e l t h IP i n v o l v i n g s u lphur 3s c h a r a c t e r would have low c r o s s - s e c t i o n , as i n C1SSC1, 2 6 and would e a s i l y be missed. These l a s t two bands r e p r e s e n t cr bonding o r b i t a l s having some s o r b i t a l c h a r a c t e r . 129 CHAPTER EIGHT Summary, C o n c l u s i o n s , and F u r t h e r S t u d i e s 8.1 Summary The PES r e s u l t s d e s c r i b e d i n t h i s t h e s i s r e p r e s e n t p a r t o f a n o n g o i n g p r o j e c t t o p r o d u c e u n s t a b l e and t r a n s i e n t m o l e c u l e s i n t h e g a s p h a s e . The g e n e r a l theme h e r e r e v o l v e s a r o u n d compounds c o n t a i n i n g m u l t i p l y bonded n i t r o g e n , s e v e r a l o f w h i c h h a v e n o t p r e v i o u s l y b een p r o d u c e d i n t h e gas p h a s e . Thus i n C h a p t e r 4 t h e u n s t a b l e i m i n e s N - m e t h y l m e t h y l e n i m i n e (CH^N^H^) and C - m e t h y l m e t h y l e n i m i n e (CHjCH=NH) were p r o d u c e d by i n s i t u p y r o l y s i s t e c h n i q u e s . T h e i r PE s p e c t r a a r e a s s i g n e d by c o m p a r i s o n w i t h i s o e l e c t r c n i c m o l e c u l e s , and t h e p a r e n t i m i n e m e t h y l e n i m i n e (CH X=NH). The h i g h y i e l d p r e p a r a t i v e r o u t e s w h i c h were 130 developed, provide e x c e l l e n t sources of these model a s t r o p h y s i c a l molecules, whereas the previous routes used to ob t a i n microwave s p e c t r a were l e s s s a t i s f a c t o r y , and d i d not provide pure samples. S e v e r a l new pseudohalogen compounds have been prepared as gases f o r the f i r s t time, Thus t h e halogen a z i d e s XN^ (X=C1, B r ) , and halogen i s o c y a n a t e s XNCO (X=C1, Br, I) were produced i n the pure s t a t e , and detected by PES, The d i s c u s s i o n of these molecules (Chapter 6) i n v o l v e s the i n t e r a c t i o n s of the o r b i t a l s of the l i n e a r t r i a t c m i c grouping, with the o f f - a x i s halogen s u b s t i t u e n t . I n t e r e s t i n g l y the valence i s o e l e c t r o n i c halogen t h i o c y a n a t e s d i s c u s s e d i n Chapter 7, are bonded at s u l p h u r , g i v i n g ClSCN and BrSCN, The present PES study i s the f i r s t r e p o r t of these t h i o c y a n a t e molecules i n the gas phase. The He I and Ne I s p e c t r a were recorded f o r the halogen t h i o c y a n a t e s , and i n d i c a t e a l a r g e r e l a t i v e c r o s s - s e c t i o n dependance on photon energy, and suggest an assignment s i m i l a r to those of the halogen azide and halogen i s o c y a n a t e molecules. The d i p s e u d o h a l i d e thiocyanogen, (SCN)^, was detected during p r e p a r a t i o n of the halogen t h i o c y a n a t e s . I t s complicated PE spectrum i s assigned i n Chapter 7, by comparison t o the known d i s u l p h u r d i h a l i d e molecules. Here a l s o , the Hel and Nel s p e c t r a were recorded, and the r e l a t i v e i n t e n s i t y changes were used i n the assignment. In summary, the m u l t i p l y bonded n i t r o g e n compounds s t u d i e d by the author may be c l a s s i f i e d as belonging to three g e n e r a l areas o f i n v e s t i g a t i o n : (1) Imines ( CH,N=CH^, 131 and CHjCH=NH ) ; (2) Replacement of a hydrogen atom with a halogen , or the pseudohalogen cyano group ( H aC(CN) 2, X^C(CN)^, (SCN)^ ); (3) The i n t e r a c t i o n of an o f f - a x i s halogen with a l i n e a r t r i a t o m i c pseudohalogen f u n c t i o n a l group ( X N 3 , XNCO, XSCN). 8.2 C o n c l u s i o n s Most of the molecules i n the t h e s i s i n v o l v e pseudohalogen f u n c t i o n a l groups. A pseudohalogen i s l o o s e l y d e f i n e d as a u n i v a l e n t f u n c t i o n a l group which forms compounds with a s i m i l a r range of elements as the halogens. The a z i d e , i s o c y a n a t e , t h i o c y a n a t e , and cyano groups are the common pseudohalogens. These form compounds with the halogens (ex, ClNCO, CIN3, ClSCN, ClCN) and may r e p l a c e a hydrogen atom i n simple molecules (ex. H^C(CN)^, CHjSCN, CH^CH^Nj, CH^CH^NCO). They may a l s o form compounds with each other (ex. NCNCO, NCN 3, S(CN)^), and the dimers (SCN)^ and (CN)^ are known. Only the cyano pseudohalogen group, which has two valence e l e c t r o n s more than a halogen, has been thoroughly s t u d i e d by PES, In terms of symmetry the yrCN o r b i t a l s of the cyano group correspond to the lone p a i r s o f a halogen, w h i l e a halogen has no c o u n t e r p a r t f o r the n^ o r b i t a l . Thus replacement of a halogen by a cyano group i n t r o d u c e s one a d d i t i o n a l o r b i t a l . The I P 1 of the rrraad nN bands are u s u a l l y c l o s e i n energy, and i n comparison to 132 halogen lone p a i r s are g e n e r a l l y i n t e r m e d i a t e between those o f c h l o r i n e and f l u o r i n e . The Ug, NCO, and SCN pseudohalogen groups are more complex. These near l i n e a r groups have two )r<nb), two jrib), and one per o r b i t a l i n the He I energy range. Each p a i r of o r b i t a l s i s s p l i t and s h i f t e d by the presence of an o f f - a x i s s u b s t i t u e n t , the extent of the s e p a r a t i o n of the o r b i t a l s r e f l e c t i n g the magnitude o f the o f f — a x i s angle. Thus the PE spectrum i s h i g h l y dependant on the bond angle, and upon i n t e r a c t i o n s between the s u b s t i t u e n t atom or group and the l i n e a r grouping. , The p(r o r b i t a l has some l o c a l i z a t i o n on the t e r m i n a l n i t r o g e n or oxygen atom, and t h e r e f o r e i s l e s s i n f l u e n c e d by the nature of the s u b s t i t u e n t atom or group than are the o r b i t a l s . I t s p o s i t i o n i s l a r g e l y determined by the t e r m i n a l atom nuclear charge. Thus replacement of a halogen atom by a cyano group w i l l r e s u l t i n r e l a t i v e l y e a s i l y i n t e r p r e t a b l e changes i n the PE spectrum, while a molecule c o n t a i n i n g an a z i d e , i s o c y a n a t e , or t h i o c y a n a t e group w i l l have a more complicated spectrum. 8.3 F u r t h e r S t u d i e s The m a t e r i a l i n t h i s t h e s i s should lead to f u t u r e s t u d i e s of r e l a t e d systems. The l i t e r a t u r e r e p o r t s t h a t s e v e r a l unstable molecules d i r e c t l y r e l a t e d t o those s t u d i e d 133 h e r e have p r e v i o u s l y been d e t e c t e d by v a r i o u s methods. One v o u l d hope t h a t t h e s e c o u l d be p r o d u c e d f o r s t u d y by FES, S e v e r a l p o s s i b l e p r o j e c t s a r e s u g g e s t e d h e r e . To e x t e n d t h e s t u d y o f t h e u n s t a b l e i m i n e s i t i s d e s i r a b l e t o i n t r o d u c e new s u b s t i t u e n t s o n t o t h e p a r e n t i m i n e CH^=KH. One s u c h m o l e c u l e i s t h e N - c h l o r o d e r i v a t i v e CH A=NC1, and h e r e one may e x p e c t t o f i n d some i n t e r a c t i o n b e t w e e n t h e c h l o r i n e l o n e p a i r s and t h e TT a n d n^ o r b i t a l s . The p r e p a r a t i o n o f CH^=NCl i n v o l v e s N H^Cl ( i t s e l f an u n s t a b l e m o l e c u l e ) and CH3CHO. T h i s y i e l d s CH^=NCl w h i c h may t h e n t r i m e r i z e t o g i v e 1,3,5 t r i c h l o r o h e x a h y d r o - s - t r i a z i n e . 1 , 2 , 3 Thus t h i s s y s t e m i s a n a l o g o u s t o t h e p r e p a r a t i o n s o f t h e i m i n e s i n C h a p t e r 4. F l u o r i n e d e r i v a t i v e s o f t h e p a r e n t i m i n e , CH^=Nfl, a r e a l s o known. A m o l e c u l e c l o s e l y r e l a t e d t o CHjCH=NH i s i t s e n a m i n e t a u t o m e r v i n y l a m i n e , CH^=CHNH i, w h i c h i s a l s o o f a s t r o p h y s i c a l i n t e r e s t , One w o u l d e x p e c t a c o m p a r i s o n o f PE d a t a f o r CH^CH^NH^and C f l ^ C H N H w o u l d p r o v i d e some i n s i g h t i n t o t h e p r o b l e m o f e n a m i n e , i m i n e t a u t o m e r i s m , V i n y l a m i n e h a s b e e n p r e p a r e d by p y r o l y s i s o f e t h y l a m i n e , a n d d e t e c t e d by m i c r o w a v e s p e c t r o s c o p y . 5 I n a c o n t i n u a t i o n o f t h e s t u d i e s o f p s e u d o h a l o g e n s , t h e m o l e c u l e s ONCN a n d ONSCN may a l s o be i n v e s t i g a t e d . I n b o t h m o l e c u l e s one may e x p e c t t o d e t e c t i n t e r a c t i o n s b e t w e e n t h e tr s y s t e m s o f t h e NO and t h e p s e u d o h a l o g e n g r o u p . These m o l e c u l e s a r e f o r m e d by r e a c t i o n o f N 0 C 1 w i t h AgCN o r AgSCN g i v i n g ONCN 7 and ONSCN 8 r e s p e c t i v e l y . 134 A more d i r e c t e x t e n s i o n o f C h a p t e r s 6 and 7 i n v o l v e s r e a c t i o n o f w i t h AgSCN. O n l i k e t h e r e a c t i o n w i t h C l ^ and B r ^ { g i v i n g C l S C N and B r S C N ) , t h i s f o r m s t h e bond a t n i t r o g e n g i v i n g I NCS. Thus t h i s m o l e c u l e i s e x p e c t e d t o be d i r e c t l y a n a l o g o u s t o INCO, and i n p a r t i c u l a r one w o u l d e x p e c t a c o m p a r i s o n o f t h e s e m o l e c u l e s t o p r o v i d e a more d e f i n i t i v e a s s i g n m e n t o f t h e l a s t t h r e e h a l o g e n i s o c y a n a t e b a n d s . I n p a r t i c u l a r t h e s h i f t o f t h e ptr o r b i t a l s h o u l d be t o l o w e r I P , and a n e n h a n c e d i n t e n s i t y i n t h e N e l s p e c t r u m i s e x p e c t e d , due t o t h e d i f f e r e n t c r o s s — s e c t i o n o f t h e s u l p h u r 3p o r b i t a l . More g e n e r a l l y i t h a s b e e n n o t e d t h a t p y r o l y s i s o f t r i a z i n e s h a s p r o d u c e d C=N compounds on t h r e e o c c a s i o n s , C1NCO, CH^=NCHj, and CHjCH=NH. T h i s r e a c t i o n may p r o v e u s e f u l w i t h o t h e r t r i a z i n e s . F i n a l l y , t h e p s e u d o h a l o g e n (X . i ) s t u d i e s i n v o l v e d s e v e r a l r e a c t i o n s o f t h e t y p e «< Xpseud) + R X M X + * I*pseud) And t h i s may p r o v e t o work f o r o t h e r R g r o u p s . 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