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Electron paramagnetic resonance studies of adsorbed species Pelman, Alan Irwin 1971

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ELECTRON PARAMAGNETIC RESONANCE STUDIES OF ADSORBED SPECIES  by  ALAN IRWIN PELMAN B . S c , U n i v e r s i t y o f B r i t i s h . C o l u m b i a , 1966  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY In t h e Department or Chemistry  We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d  THE UNIVERSITY OF BRITISH COLUMBIA April,  19 71  In p r e s e n t i n g t h i s t h e s i s  in p a r t i a l  fulfiIment  o f the requirements  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, the L i b r a r y  s h a l l make i t f r e e l y  available for  I agree  r e f e r e n c e and  I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s  It  i s understood that copying or  thesis  permission.  Depa rtment The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada  Date  or  publication  o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my written  that  study.  f o r s c h o l a r l y purposes may be g r a n t e d by the Head of my Department by h i s r e p r e s e n t a t i v e s .  for  Supervisor:  .C.A. McDowell  ABSTRACT  E l e c t r o n p a r a m a g n e t i c resonance t e c h n i q u e s have been used t o i n v e s t i g a t e t h e n a t u r e and p o s s i b l e e f f e c t s o f a d s o r p t i o n of gaseous s p e c i e s on s e v e r a l a d s o r b e n t s , i n p a r t i c u l a r  several  s y n t h e t i c z e o l i t e s , a t t e m p e r a t u r e s from 77°K upwards.  Analysis of  the  s p e c t r a o b t a i n e d has been a i d e d t h r o u g h computer s i m u l a t i o n o f t h e  v a r i o u s s p e c t r a and comparison o f t h e s e t o t h e a c t u a l o b s e r v e d spectra. The m o l e c u l e c h l o r i n e d i o x i d e ( C l O ^ ) has been s t u d i e d i n v a r i o u s low t e m p e r a t u r e m a t r i c e s b u t l i t t l e has been p u b l i s h e d f o r "'CIO2 i n t h e adsorbed s t a t e .  An attempt was made t o f i n d an a d s o r b e n t  such t h a t an i n e r t m a t r i x might be a p p r o x i m a t e d , t o g i v e a base from which t o make c o m p a r i s o n s . gel,  To t h i s end, a d s o r b e n t s i n c l u d i n g  silica  s y n t h e t i c z e o l i t e s 13X, 10X, 4A, 5A, Na-mordenite and  H-mordenite were i n v e s t i g a t e d .  The r e s u l t s v a r y between t h o s e from  s i l i c a g e l , where s p e c t r a y i e l d i n g EPR parameters s i m i l a r t o o t h e r >•  .  .  .  m a t r i c e s were o b t a i n e d , t o t h o s e from 13X where i t was e v i d e n t t h a t two d i s t i n c t a d s o r p t i o n s i t e s o f t h e C l O ^ were p r e s e n t .  I n t h e 13X  as i n t h e o t h e r s y n t h e t i c z e o l i t e s , EPR parameters m a r k e d l y  different  from o t h e r s t u d i e s were found and were a t t r i b u t e d t o t h e i n t e n s e e l e c t r o s t a t i c f i e l d s present i n these z e o l i t e s . at  Results obtained  room t e m p e r a t u r e f o r t h e s e a d s o r b e n t s ranged from C l O ^ m o l e c u l e s  f r e e l y r o t a t i n g i n t h e cages o f t h e z e o l i t e s t o o t h e r m o l e c u l e s having hindered r o t a t i o n s .  Nitrogen  dioxide  ( N0_  ) was  view t o f i n d i n g s i m i l a r i n t e r a c t i o n s . as for  Cl-2  also i n v e s t i g a t e d with  A l t h o u g h changes as marked  compared t o o t h e r s t u d i e s were not  observed,  s y n t h e t i c z e o l i t e H-mordenite y i e l d e d s p e c t r a c l o s e l y those  i n s o l i d ^2^4  obtained  a  matrices.  the  approximating  I t i s proposed t h e NC^  molecules  a r e caged i n t h e numerous s i d e p o c k e t s emanating from the main channels in this The  z e o l i t e and  i s o l a t e d from o t h e r NO^  r e s u l t i n g s p e c t r a are s t r i k i n g l y more r e s o l v e d than  obtained  u s i n g o t h e r a d s o r b e n t s and  simulations  enabled  accurate  adsorption  found w i t h  of n i t r i c  oxide  the other molecules.  a r e a c t i o n o f the NO  those  computer  A new  w i t h H-mordenite and  ( NO  ) produced an  s p e c i e s was  s p e c i e s does not from a d s o r p t i o n  c o u l d not be removed at  c o n t a i n n i t r o g e n as i d e n t i c a l s p e c t r a were o f ^NO  and  were u n s u c c e s s f u l .  radical The  The  new  obtained  "^NO.  Attempts t o observe s p e c t r a which c o u l d be the d i f l u o r o a m i n o  effect  formed from  room t e m p e r a t u r e , i n d i c a t i n g a s t r o n g bond t o the s u r f a c e .  having  molecules.  to be made.  The not  are e f f e c t i v e l y  from a d s o r p t i o n  assigned  of t e t r a f l u o r o h y d r a z i n e  s p e c t r a observed were a s s i g n e d  no h y p e r f i n e s t r u c t u r e and  to  an a n i s o t r o p i c g  to a species  tensor.  iii  TABLE OF CONTENTS Abstract  Page i  L i s t o f Tables  vi  L i s t o f Figures  v i i  Acknowledgments  X  CHAPTER ONE:  INTRODUCTION  CHAPTER TWO:.  ADSORPTION  .  1  1 5  2.1  Surfaces  6  2.2  C l a s s i f i c a t i o n o f Isotherms  7  2.3  Volume F i l l i n g o f Pores  H  2.4  Adsorption Forces  12  2.4.1 2.4.2  P o l a r i z a t i o n Energy F i e l d - d i p o l e Energy.  14 14  2.4.3  F i e l d G r a d i e n t - q u a d r u p o l e Energy  15  CHAPTER THREE:  ZEOLITES  16  3.1  Adsorption i n Zeolites  21  3.2  Structures of Zeolites 3.2.1 X,Y Type 3.2.1.1 C a t i o n P o s i t i o n s 3.2.2 A Type 3.2.2.1 C a t i o n P o s i t i o n s 3.2.3 M o r d e n i t e 3.2.3.1 C a t i o n P o s i t i o n s  25 25 28  CHAPTER FOUR: 4.1  Theory 4.1.1 4.1.2 .4.1.3 •4.1.4  CHAPTER FIVE: 5.1  3 0  32 3 4  36  ELECTRON PARAMAGNETIC RESONANCE ,  3 8  38  E l e c t r o n i c Zeeman I n t e r a c t i o n The H y p e r f i n e I n t e r a c t i o n . Other I n t e r a c t i o n s EPR S p e c t r a ADSORPTION STUDIES  EPR S t u d i e s o f R a d i c a l s  on S u r f a c e s  '  45 46 48 49 5 2  54  XV  TABLE OF CONTENTS ( c o n t . )  •5.2  5.1.1  N o n - Z e o l i t i c Adsorbents  5.1.2  Zeolitic  Adsorbents  Special Adsorption  CHAPTER SIX:  EXPERIMENTAL  6.1  Vacuum System  6.2  Sample Tubes  6.3  Adsorbents  6.4  Sample P r e p a r a t i o n  6.5  6.6  Gases 6.5.1 6.5.2 ,. 6.5.3 6.5'. 4  Effects  Chlorine Dioxide Nitrogen Dioxide N i t r i c Oxide Tetraf lurohydrazine  N  C10 N0„ NO 2^4  2  Spectrometers  CHAPTER SEVEN:  ANALYSIS OF ELECTRON PARAMAGNETIC RESONANCE SPECTRA  CHAPTER EIGHT:  CHLORINE DIXOIDE, C 1 0  2  8.1  S i l i c a Gel  8.2  Na and H-mordenite  8.3  4A and 5A S y n t h e t i c Z e o l i t e s  8.4  13X S y n t h e t i c Z e o l i t e  8.5  10X S y n t h e t i c Z e o l i t e  8.6  L i t h i u m Exchanged 13X S y n t h e t i c Z e o l i t e  8.7  Discussion  CHAPTER NINE:  NITROGEN DIOXIDE, N 0  9.1  S i l i c a Gel  9.2  13X S y n t h e t i c Z e o l i t e  2  TABLE OF CONTENTS  (cont.)  Page  9.3  H-mordenite  116  9.4  Discussion  121  CHAPTER TEN:  NITRIC OXIDE, NO  130  10.1  S i l i c a Gel  131  10.2  13X S y n t h e t i c Z e o l i t e  132  10.3  H-mordenite  132  10.4  Discussion  140  CHAPTER ELEVEN:  DIFLUORAMINO RADICAL, Yl?  144  11.1  H-mordenite  145  11.2  Discussion  147  CHAPTER TWELVE: REFERENCES APPENDIX  SUMMARY  151 . 156 166  vi  LIST OF TABLES  Page  1.  The p r i n c i p a l v a l u e s o f t h e h y p e r f i n e and g t e n s o r s f o r C l O ^ i n v a r i o u s media.  85  2.  The p r i n c i p a l v a l u e s o f t h e h y p e r f i n e and g t e n s o r s f o r N 0 i n v a r i o u s media.  113  , The p r i n c i p a l v a l u e s o f " t h e h y p e r f i n e and g t e n s o r s f o r NO i n v a r i o u s media.  133  2  3.  4.  g - y a l u e s o f t h e spectrum o b s e r v e d when N^F^ was adsorbed on H-mordenite a t 77°K.  148  vii  LIST OF FIGURES Page 1.  Five d i f f e r e n t types o f adsorption isotherms, as c l a s s i f i e d by Brunauer, Deming, Deming and T e l l e r .  2.  The fundamental b u i l d i n g b l o c k s o f z e o l i t e s : (a) S i O ^ t e t r a h e d r o n (b) AIO4 t e t r a h e d r o n .  3.  The t r u n c a t e d o c t a h e d r o n , o r s o d a l i t e cage.  20  4.  Cation positions f o r zeolites of varying Si/Al rations: (a) 1 / 2 (b) 1 / 1 .  22  5i  The s t r u c t u r a l framework o f t h e X t y p e synthetic zeolite. '  26  6.  The t y p e I I 2 6 - h e d r b n cage, o r f f a u j a s i t e cage. .  27  7.  C a t i o n s i t e s i n Na 1 3 X s y n t h e t i c z e o l i t e .  8.  The s t r u c t u r a l framework o f t h e A t y p e synthetic zeolite.  31  9.  The.type I 2 6 - h e d r o n cage.  33  10.  The s t r u c t u r a l framework o f s y n t h e t i c mordenite: (a) c h a r a c t e r i s t i c c h a i n s t r u c t u r e (b) c r o s s - s e c t i o n a l a r e a o f a c h a i n .  35  11.  Cation p o s i t i o n s i n s y n t h e t i c mordenite. Aluminium and S i l i c o n a t t h e c e n t e r s o f each t e t r a h e d r o n a r e n o t shown.  37  12.  L o r e n t z i a n and G a u s s i a n f i r s t d e r i v a t i v e c u r v e s .  44  13.  a)  A s c h e m a t i c diagram o f t h e vacuum system used i n t h e s e e x p e r i m e n t s , The.sample tubes used i n t h e s e e x p e r i m e n t s .  64  Q u a r t z dewar used f o r v a r i a b l e t e m p e r a t u r e EPR .experiments. A V a r i a n V - 4 5 4 6 l i q u i d n i t r o g e n dewar.  66  b) 14'..  a) b)  8  . 1 8  29  15.  B l o c k diagram o f a V a r i a n E - 3 X-band S p e c t r o m e t e r system.  70  16.  The m o l e c u l a r and m a g n e t i c f i e l d c o o r d i n a t e system. '••  75  y m  LIST OF FIGURES ( c o n t . )  G e n e r a l i z e d l i n e s h a p e s o f powder EPR s p e c t r a f o r a s p e c i e s w i t h no h y p e r f i n e structure: (a) a x i a l l y symmetric g t e n s o r (b) f u l l y a n i s o t r o p i c g tensor. The m o l e c u l a r a x i s system f o r c h l o r i n e d i o x i d EPR spectrum o f c h j o r i n e d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d at 77°K. EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d a t room t e m p e r a t u r e . Computer s i m u l a t e d EPR. spectrum o f c h l o r i n e d i o x i d e adsorbed on s i l i c a gel> r e c o r d e d . a t room t e m p e r a t u r e . EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d at 77°K. • ' • :•• EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d a t room t e m p e r a t u r e Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d a t room t e m p e r a t u r e . EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on 4A s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. C o m p u t e r s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on 4A s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. :  EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K.  ix  LIST OF FIGURES ( c o n t . ) Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d at 77°K. EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d at 77°K. Computer s i m u l a t e d EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum (assuming a x i a l l y symmetric g and h y p e r f i n e t e n s e r s ) c f n i t r o g e n d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on H-mordenite, r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on H-mordenite, r e c o r d e d at 77°K. EPR spectrum o f n i t r o g e n d i o x i d e adsorbe on H-irsordenite, r e c o r d e d a t 77°K. Computer s i m u l a t e d EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on H-mordenite, r e c o r d e d . a t 77°K. EPR spectrum o f n i t r i c o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. Computer;simulated EPR spectrum o f n i t r i c o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K. :  X  LIST OF FIGURES ( c o n t . ) 42.  EPR spectrum o f  14 N n i t r i c o x i d e adsorbed  £M£  pn H-mordenite, r e c o r d e d a t 77°K. 43. 44.  45.  46.  15 EPR spectrum o f ' N n i t r i c o x i d e adsorbed on H-mordenite, r e c o r d e d a t 77°K. 14 Computer s i m u l a t e d EPR spectrum o f N n i t r i c o x i d e adsorbed on H-mordenite, r e c o r d e d a t 77°K. 15 Computer s i m u l a t e d EPR spectrum o f N n i t r i c o x i d e adsprbed on H-mordenite, r e c o r d e d a t 77°K. EPR spectrum o b s e r v e d a f t e r a d s o r p t i o n o f n i t r i c o x i d e cm. H^mordenit;e, f o l l o w e d by e v a c u a t i o n , r e c o r d e d a t 77°K.  137 138  139  141  47.  EPR spectrum o b s e r v e d a f t e r a d s o r p t i o n o f N2F4 on- H-mordenite, r e c o r d e d a t 77°K.  146  48.  Computer s i m u l a t e d EPR s p e c t r a o f s p e c i e s formed on a d s o r p t i o n o f N2F4 on H-mordenite, r e c o r d e d a t 77°K: (a) i s o t r o p i c g and h y p e r f i n e t e n s p r (b) a n i s o t r o p i c g t e n s o r , no h y p e r f i n e s p l i t t i n g .  149  ACKNOWLEDGEMENT  •• . .  I am g r a t e f u l t o Dr. C A . McDowell f o r h i s i n t e r e s t and. s u p p o r t t h r o u g h o u t t h e c o u r s e o f my g r a d u a t e studies. I would a l s o lj.ke t o e x p r e s s my a p p r e c i a t i o n t o Drs, P. Raghunathan, C.L. Gardner and J.B. Farmer f o r t h e i r h e l p f u l comments pn t h i s t h e s i s ; t o Mr. John T a i t f o r h i s i-nvaluable a s s i s t a n c e d u r i n g t h e p r e p a r a t i o n o f t h e t h e s i s , t o t h e o t h e r members o f t h i s l a b o r a t o r y f o r t h e i r . h e l p f u l d i s c u s s i o n s ; t o Mr, Tom Markus f o r t h e c a r e o f t h e EPR s p e c t r o m e t e r s ;  and l a s t > b u t c e r t a i n l y h o t l e a s t , t o my  wife S y l v i a f o r her patienqe  and h e l p d u r i n g t h e f i n a l  stages  of p r e p a r a t i o n o f t h e m a n u s c r i p t . The  awards from t h e N a t i o n a l Research C o u n c i l o f  Canada r e c e i v e d d u r i n g my g r a d u a t e s t u d i e s a r e a l s o g r a t e f u l l y acknowledged.  -1-  CHAPTER ONE  INTRODUCTION  S u r f a c e s t u d i e s have grown i n i n t e r e s t o v e r r e c e n t years due i n p a r t t o b o t h i n c r e a s e d knowledge o f t h e s t r u c t u r e o f s u r f a c e s and a l s o t o t h e a p p l i c a t i o n o f d i f f e r e n t to the study o f t h i s area.  techniques  Adsorption studies, surface structure  s t u d i e s and s t u d i e s o f r e a c t i o n s on s u r f a c e s a l l have c o n t r i b u t e d g r e a t l y t o our knowledge o f s u r f a c e phenomena.  A great deal o f  i n t e r e s t and a c t i v i t y i n t h e a p p l i c a t i o n o f s p e c t r o s c o p i c t o problems i n b o t h c a t a l y s i s and s u r f a c e c h e m i s t r y  techniques  i s evident.  f a p t t h a t i n f o r m a t i o n can be o b t a i n e d a t t h e m o l e c u l a r  The  level rather  than t h e system l e v e l has been a d r i v i n g f o r c e f o r c o n t i n u i n g r a p i d growth i n t h i s a r e a o f i n t e r e s t . No one s p e c t r o s c o p i c t e c h n i q u e  can hope t o p r o v i d e  -2-  a l l t h e i n f o r m a t i o n a v a i l a b l e from s t u d i e s o f adsorbed s p e c i e s . Perhaps t h e most s u c c e s s f u l and w i d e l y used a p p l i c a t i o n has been that o f i n f r a r e d spectroscopy.  Many r e v i e w s have been w r i t t e n on  i n f r a r e d s p e c t r o s c o p y as a p p l i e d t o t h e s t u d y o f s u r f a c e s ( f o r example [1-6]).  Gamma-ray resonance  spectroscopy, although  relatively  u n e x p l o r e d a t p r e s e n t i n t h i s r e g a r d , has a l s o been used t o some extent [ 7 ] . Magnetic Resonance t e c h n i q u e s , b o t h e l e c t r o n paramagnetic ; resonance, h e r e i n a f t e r c a l l e d EPR, and n u c l e a r m a g n e t i c  resonance,  h e r e i n a f t e r c a l l e d NMR, h o l d promise o f p r o v i d i n g answers t o some o f t h e c o m p l i c a t e d s i t u a t i o n s t h a t a r i s e on s u r f a c e s i n systems i n v o l v i n g the g a s - s o l i d i n t e r f a c e . techniques i s s t i l l been v a r i e d . adsorbed [8-fll]).  The a p p l i c a t i o n o f t h e s e  at a r e l a t i v e l y e a r l y stage.  NMR s t u d i e s have  The d e t e c t i o n o f r e l a x a t i o n phenomena o f m o l e c u l e s  on s u r f a c e s has been a prime a r e a o f i n t e r e s t  ( f o r example  Other NMR s t u d i e s have i n c l u d e d such e f f e c t s as t h e study  o f c h e m i c a l s h i f t s on v a r i o u s s u r f a c e s [12] and s t u d i e s o f adsorbed water  [13-15]. The a p p l i c a t i o n o f EPR t o t h e s t u d y o f adsorbed  m o l e c u l e s has been v e r y p r o d u c t i v e . to  The a b i l i t y o f t h i s  d e t e c t s m a l l c o n c e n t r a t i o n s o f paramagnetic  technique  s p e c i e s and t o  r e l a t e t h e u n p a i r e d e l e c t r o n charge d i s t r i b u t i o n t o t h e m o l e c u l a r s t r u c t u r e has made t h i s an e x t r e m e l y u s e f u l method o f i n v e s t i g a t i n g systems i n v o l v i n g s u r f a c e s . Many r e a c t i o n s which o c c u r a t s u r f a c e s a l s o i n v o l v e paramagnetic  species.  I t i s possible to stabilize highly reactive  -3-  m o l e c u l e s e i t h e r by a d s o r p t i o n i n t o porous media such as z e o l i t e s or m o l e c u l a r s i e v e s ( f o r example [ 1 6 - 1 8 ] ) .  Controlled reaction  o f t h e s e ' t r a p p e d ' m o l e c u l e s would then be p o s s i b l e t o produce s p e c i f i c p r o d u c t s , whereas c o r r e s p o n d i n g r e a c t i o n s i n t h e gas phase may n o t be as s e l e c t i v e .  Under s u i t a b l e c o n d i t i o n s , i n f o r m a t i o n  about t h e adsorbed s p e c i e s such as i t s i d e n t i t y , s t a b i l i t y , m o t i o n a l s t a t e , c h e m i c a l s t r u c t u r e , and i n t e r a c t i o n w i t h v a r i o u s s u r f a c e f i e l d s can thus be o b t a i n e d . The  i n t e r a c t i o n s between paramagnetic m o l e c u l e s and i t s .  s u r r o u n d i n g s can g r e a t l y a f f e c t t h e EPR spectrum.  When t h e m o l e c u l e s  under s t u d y a r e adsorbed on a s u r f a c e o r i n some way t r a p p e d , one would n a t u r a l l y expect some d i f f e r e n c e s i n t h e EPR parameters from those observed f o r ' f r e e ' m o l e c u l e s . be r e f e r r e d t o as m a t r i x i n t e r a c t i o n s .  These i n t e r a c t i o n s  Matrix interactions  will  determine  the a b i l i t y o f a paramagnetic m o l e c u l e t o r o t a t e o r r e o r i e n t about v a r i o u s m o l e c u l a r axes.  These i n t e r a c t i o n s can a l s o p e r t u r b t h e  wave f u n c t i o n s o f t h e m o l e c u l e and thus produce changes i n t h e components o f b o t h t h e g and h y p e r f i n e t e n s o r s o f t h e m o l e c u l e s . T h i s s t u d y i s concerned w i t h t h e EPR s p e c t r a o f paramagnetic m o l e c u l e s adsorbed on s y n t h e t i c z e o l i t e s and on s i l i c a gel,  and t h e e f f e c t s o f such a d s o r p t i o n on t h e EPR p a r a m e t e r s .  B r i e f i n t r o d u c t i o n s on a d s o r p t i o n and t h e s t r u c t u r e o f z e o l i t e s are  g i v e n f o r completeness.  I n f a c t , a knowledge o f t h e s u r f a c e  s t r u c t u r e o f the adsorbents i s extremely b e n e f i c i a l t o the i n t e r p r e t a t i o n o f t h e observed phenomena.  -4-  A c h a p t e r on EPR i s i n c l u d e d b u t t h e r e a d e r i s r e f e r r e d t o o t h e r sources f o r a more d e t a i l e d coverage  of the theory.  Chapter F i v e g i v e s some i n s i g h t i n t o t h e a p p l i c a t i o n o f v a r i o u s s p e c t r o s c o p i c t e c h n i q u e s t o t h i s r e s e a r c h a r e a and p r e s e n t s background on t h e a p p l i c a t i o n o f t h e EPR t e c h n i q u e t o some s u r f a c e phenomena. The remainder  of this thesis includes a discussion  o f t h e e x p e r i m e n t a l t e c h n i q u e s and t h e r e s u l t s o b t a i n e d from t h e v a r i o u s systems i n v e s t i g a t e d .  -5-  CHAPTER TWO ADSORPTION The  components o f a s o l i d  ( i o n s , atoms, o r m o l e c u l e s )  are s u b j e c t t o f o r c e s which a r e i n e q u i l i b r i u m deep w i t h i n t h e l a t t i c e b u t a r e unbalanced n e a r t h e s u r f a c e . a t t r a c t i v e force f i e l d only extending  T h i s r e s u l t s i n an  a few angstroms, b u t enough  to a t t r a c t m o l e c u l e s o f a l i q u i d o r gas i n t h e immediate p r o x i m i t y . These f o r c e s cause m o l e c u l e s t o become a t t a c h e d t o t h e s u r f a c e , the phenomenon b e i n g known as a d s o r p t i o n .  The term was i n t r o d u c e d  by Kayser [19] i n 1881 t o denote t h e c o n d e n s a t i o n surfaces.  Desorption  o f gases on f r e e  i s t h e complementary p r o c e s s , t h e removal  o f gases from t h e s u r f a c e , w h i l e t h e s u r f a c e i s termed t h e adsorbent.  The p h y s i c a l a d s o r p t i o n bond d e r i v e s from s i m i l a r  c o h e s i o n a l f o r c e s as t h o s e r e s p o n s i b l e f o r c o n d e n s a t i o n chemical  adsorption or chemisorption  whereas  a l t e r s the nature of the  -6-  adsorbed s p e c i e s .  A d s o r p t i o n i s commonly measured i n terms  of  the mass adsorbed as a f u n c t i o n o f p r e s s u r e , t h e measurements undertaken at c o n s t a n t t e m p e r a t u r e .  The r e s u l t i n g p l o t s a r e  termed a d s o r p t i o n isotherms„  2.1  Surfaces. I t i s c o n v e n i e n t t o d i s t i n g u i s h between e x t e r n a l  and  i n t e r n a l s u r f a c e s when c o n s i d e r i n g the l a r g e a v a i l a b l e s u r f a c e areas o f the adsorbents n o r m a l l y used.  The e x t e r n a l s u r f a c e  o f a s o l i d f r e q u e n t l y r e p r e s e n t s no more than one p e r c e n t o f t h e t o t a l s u r f a c e a c c e s s i b l e t o gas m o l e c u l e s , t h e a d d i t i o n a l i n t e r n a l s u r f a c e a r i s i n g from the w a l l s o f t h e p o r e s , c r a c k s o r i n t e r s t i c e s w i t h i n t h e solid.  I t i s obvious a l s o t h a t t h e s m a l l e r the p a r t i c l e s , the  l a r g e r w i l l be t h e e x t e r n a l s u r f a c e .  The d e m a r c a t i o n l i n e between  t h e s e two k i n d s o f s u r f a c e s i s a r b i t r a r y , but t h e term  'internal  s u r f a c e ' , t h e n , would comprise the w a l l s o f a l l c r a c k s , pores c a v i t i e s which are deeper than t h e y are wide.  and  This i n t e r n a l surface  must o f c o u r s e be open t o the e x t e r i o r o f t h e s o l i d and i n porous s o l i d s i s g e n e r a l l y s e v e r a l o r d e r s o f magnitude g r e a t e r t h a n the external surface.  The concern o f t h i s s t u d y i s w i t h porous  having large i n t e r n a l  systems  surfaces.  A c o n v e n i e n t c l a s s i f i c a t i o n o f pores has been g i v e n by D u b i n i n [20].  Pores o f w i d t h below %2oR  are termed m i c r o p o r e s , t h o s e  o f w i d t h above 200$ are termed macropores,  w h i l e those i n between  are c o n s i d e r e d t r a n s i t i o n a l o r i n t e r m e d i a t e p o r e s .  -7-  2.2  C l a s s i f i c a t i o n of  Isotherms.  Many a d s o r p t i o n i s o t h e r m s have now  been d e t e r m i n e d  have been found t o be o f f i v e d i f f e r e n t t y p e s , each t y p e of  a d i f f e r e n t s u r f a c e makeup.  Deming, Deming and T e l l e r  [21]  and  characteristic  These have been c l a s s i f i e d by and are shown i n f i g u r e  Brunauer,  1.  A d s o r p t i o n i s o t h e r m s a r e g e n e r a l l y a n a l y z e d by r e f e r e n c e to  an e q u a t i o n i n which the c a p a c i t y o f a complete  appears as a parameter.  monolayer  Knowing the c r o s s - s e c t i o n a l a r e a o f the  a d s o r b a t e m o l e c u l e s , t h e s p e c i f i c s u r f a c e a r e a o f the adsorbent  can  be c a l c u l a t e d from the monolayer c a p a c i t y . Type I i s o f main i n t e r e s t i n t h i s s t u d y and i s o u t l i n e d below.  Langmuir [22]  was  t h e f i r s t t o attempt  an  i n t e r p r e t a t i o n o f a d s o r p t i o n phenomena and type I i s o t h e r m s  are  commonly c a l l e d Langmuir i s o t h e r m s . The  i s o t h e r m i s c h a r a c t e r i z e d by t h e e q u a t i o n  1+aP  where V i s the volume o f vapour adsorbed V , the volume o f vapour adsorbed a constant.  a t an e q u i l i b r i u m p r e s s u r e  a t f u l l monolayer coverage;  and  P;  a,  I t i s obvious from o b s e r v a t i o n o f t h e i s o t h e r m t h a t  a s a t u r a t i o n o f t h e s u r f a c e appears t o o c c u r a t h i g h e r gas p r e s s u r e s , not always the case as seen f o r the o t h e r t y p e s . Langmuir assumed t h a t i n i t i a l l y , a s u r f a c e would condense on i t .  a l l gas m o l e c u l e s  Once c o m p l e t e l y covered by  striking adsorbate  ORDINAlES: Adsorption, ABSCISSAE": Relative (scaled  ( m g /g)  vapour 0 to  pressure 1.0)  FIGURE 1. F i v e d i f f e r e n t types o f a d s o r p t i o n i s o t h e r m s , as c l a s s i f i e d by Rrunauer, Deming, Deming and T e l l e r .  P/P  0  -9-  m o l e c u l e s , f u r t h e r c o n d e n s a t i o n would cease s i n c e the s u r f a c e f o r c e s would be n e u t r a l i z e d .  S a t u r a t i o n i n t h i s i n s t a n c e i s i n the  o f a s i n g l e monolayer over the s u r f a c e .  form  Before t h i s l i m i t i s  r e a c h e d , p a r t o f the s u r f a c e must be vacant and Langmuir assumed a dynamic e q u i l i b r i u m between the c o n d e n s a t i o n o f gas m o l e c u l e s the f r e e s u r f a c e and the e v a p o r a t i o n o f condensed from the o c c u p i e d s u r f a c e .  hitting  molecules  The r a t e o f c o n d e n s a t i o n s h o u l d be  p r o p o r t i o n a l t o the s p e c i f i c s u r f a c e S; the p r e s s u r e o f the adsorbate P; and the f r a c t i o n o f the s u r f a c e not y e t c o v e r e d ,  (1 - 0 ) ,  so t h a t :  r a t e of condensation = uSP(l - 6 ) where u i s a c o n s t a n t .  (2-2)  The r a t e o f e v a p o r a t i o n i s a l s o p r o p o r t i o n a l  t o the s p e c i f i c s u r f a c e S; the f r a c t i o n o f s u r f a c e a l r e a d y c o v e r e d , 6; and the r a t e e v a p o r a t i o n would o c c u r i f the s u r f a c e were c o m p l e t e l y covered, V ,  such t h a t  r a t e o f e v a p o r a t i o n = Sv9.  (2-3)  At s o r p t i o n e q u i l i b r i u m , SuP(l-G) = Sv6 By d e f i n i t i o n , 8 = V/V^  (2-4) and r e p l a c i n g  by the c o n s t a n t a,  e q u a t i o n (2-4) becomes the Langmuir e q u a t i o n g i v e n by e q u a t i o n The use o f the Langmuir e q u a t i o n i s l i m i t e d a t the p r e s e n t time e n t i r e l y t o c h e m i s o r p t i o n s t u d i e s , assuming here t h a t s u r f a c e does not exceed a monolayer.  (2-1). almost coverage  -10-  Type I i s o t h e r m s a r e f r e q u e n t l y encountered i n a d s o r p t i o n s t u d i e s o f microporous s o l i d s , case b e i n g a complete f i l l i n g  the s a t u r a t i o n i n t h i s  o f t h e pores w i t h a d s o r b a t e m o l e c u l e s .  Any s l i g h t r i s e i n t h e i s o t h e r m would then come from m u l t i l a y e r a d s o r p t i o n on t h e r e l a t i v e l y microporous  small external surface of these  solids.  E x t e n s i o n s o f t h e Langmuir t h e o r y o f a d s o r p t i o n have been made, i n c l u d i n g t h e p o s s i b i l i t y o f m u l t i l a y e r a d s o r p t i o n . In 1938, Bruhauer, Emmett and T e l l e r  [23] proposed a t h e o r y which  r e t a i n e d t h e Langmuir concept o f dynamic e q u i l i b r i u m b u t extended the  process t o include m u l t i l a y e r adsorption.  the  condensation-evaporation characteristics  I t was assumed t h a t o f t h e second and  subsequent l a y e r s a r e t h e same as t h o s e o f the s u r f a c e o f t h e bulk adsorbate.  The assumptions f o r t h e i n i t i a l  same as f o r Langmuir.  P V(P -P)  P  1 V c m  monolayer a r e t h e  The e q u a t i o n i s c h a r a c t e r i z e d by  +  P P  o  c-l V c m  (2-5)  i s t h e s a t u r a t e d vapour p r e s s u r e o f t h e a d s o r b a t e and c i s a  c o n s t a n t r e l a t e d t o t h e d i f f e r e n t i a l h e a t o f a d s o r p t i o n by the e q u a t i o n c = expCCHL-H )/RT)  (2-6)  -11-  where H  and H  a r e the h e a t s o f a d s o r p t i o n i n t h e f i r s t  and the heat o f l i q u i f i c a t i o n , r e s p e c t i v e l y . reduces  t o the Langmuir e q u a t i o n when P/P  i s very  large. The  BET  Q  Equation  layer (2-5)  i s v e r y low and c  t h e o r y , as i t i s c a l l e d , i s s t i l l  the b e s t known  and most w i d e l y used today f o r b o t h porous and non-porous  adsorbents.  Whichever t h e o r y i s used, however, t o r e p r e s e n t a p h y s i c a l a d s o r p t i o n i s o t h e r m , agreement i s r a r e l y complete between t h e f o r m u l a experimental r e s u l t s .  T h i s i s due t o the the assumptions o f e n e r g e t i c  homogeneity o f the a d s o r p t i o n s i t e s and a l s o o f a g r a d u a l of a polymolecular adsorption l a y e r . v a l i d f o r the porous adsorbents 2.3  and  formation  These assumptions are not  i n use  today.  Volume F i l l i n g o f P o r e s . Numerous e x p e r i m e n t a l and t h e o r e t i c a l s t u d i e s i n r e c e n t  years  ( f o r example [24]) l e a d t o t h e c o n c l u s i o n t h a t a d s o r p t i o n  i n micropores  d i f f e r s q u a l i t a t i v e l y from a d s o r p t i o n on wide pore  and non-porous a d s o r b e n t s . used i n t h i s s t u d y .  Microporous  The concepts  adsorbents  ' s u r f a c e ' and  o n l y have been  'adsorption i n layers'  l o s e t h e i r p h y s i c a l meaning i n these systems and i t i s n a t u r a l t o expect t h a t a d s o r p t i o n i n m i c r o p o r e s micropore  a d s o r p t i o n space, W .  a d s o r b e n t s , the v a l u e o f V  q  m  leads t o a f i l l i n g of a l i m i t e d  When working w i t h  o f t h e BET  microporous  and Langmuir e q u a t i o n s  may  not be c o n s i d e r e d as equal t o t h e volume o f the monomolecular l a y e r c o v e r i n g the s u r f a c e o f the a d s o r b e n t . o f the volume o f t h e m i c r o p o r e s ,  I t s v a l u e i s near t o t h a t  and t h e r e f o r e a l s o t o W , the  -12-  c o n s t a n t o f the D u b i n i n - Radushkevich  equation.  This  equation  i s c h a r a c t e r i s t i c o f a d s o r p t i o n i s o t h e r m s o b t a i n e d from on microporous W  a = __o  a d s o r b e n t s , and i s g i v e n by /BT -expj  V  A P j l o g _s  2  N  Vi - V ' 2  experiments  [25]  2  (2-7)  P  V i s the volume o f t h e amount adsorbed, a c o n s t a n t independent o f temperature  a; T, the temperature;  B,  and r e p r e s e n t i n g t h e b a s i c  c h a r a c t e r i s t i c o f the porous s t r u c t u r e o f t h e adsorbent;  3,  the  a f f i n i t y c o e f f i c i e n t g i v e n by the r a t i o o f the d i f f e r e n t i a l m o l a r work o f a d s o r p t i o n o f a g i v e n vapour t o t h a t o f a vapour chosen as a s t a n d a r d ; P , the s a t u r a t e d vapour p r e s s u r e o f t h e s o r b a t e ; and P, t h e p r e s s u r e o f the a d s o r b a t e .  The  constants W  q  then c h a r a c t e r i z e the a d s o r p t i v e p r o p e r t i e s o f t h e g i v e n whereas P- , 3,  and B adsorbent  and V d e s c r i b e t h e a d s o r p t i v e p r o p e r t i e s o f the  adsorbate. At the p r e s e n t s t a t e o f the t h e o r y o f a d s o r p t i o n i n t e r a c t i o n s , s u f f i c i e n t l y complete i n f o r m a t i o n on the a d s o r p t i o n f i e l d i n micropores  can be o b t a i n e d o n l y from a d s o r p t i o n  The t h e o r y t h e r e f o r e , has a somewhat phenomenological  experiments,  c h a r a c t e r and  i s being c o n s t a n t l y r e - i n v e s t i g a t e d . 2.4  Adsorption  Forces.  London [26] i n 1 9 3 0 , showed t h a t t h e r e was  a very general  f o r c e between atoms such t h a t A  . * ~xV D=  where <J>  i s the: p o t e n t i a l  "  (2  8)  -13-  energy o f t h e two i s o l a t e d atoms s e p a r a t e d by a d i s t a n c e X; A, a constant  r e l a t e d t o t h e p o l a r i z a b i l i t i e s o f t h e atoms and n an  i n t e g e r , u s u a l l y g i v e n as 6.  The n e g a t i v e  s i g n denotes a t t r a c t i o n .  T h i s f o r c e i s termed a d i s p e r s i o n f o r c e and a r i s e s as a s m a l l p e r t u r b a t i o n o f t h e motions o f o r b i t a l e l e c t r o n s on each l e a d i n g t o a t t r a c t i o n o f t h e atoms.  other  Dispersion forces are additive  such t h a t an a d s o r b a t e m o l e c u l e near t h e s u r f a c e o f an adsorbent e x p e r i e n c e s a t o t a l a t t r a c t i o n which i s t h e sum o f a l l p a i r s o f interactions. In a d d i t i o n , s h o r t range r e p u l s i v e f o r c e s a r e a l s o u n i v e r s a l l y a s s o c i a t e d w i t h p h y s i c a l a d s o r p t i o n , g i v e n by  *  R  1  V  where B i s a c o n s t a n t  v  (2-9)  and m an i n t e g e r , g e n e r a l l y much l a r g e r than, h.  Consequently, the r e p u l s i o n i s important only at very short of  distances  separation. I t i s assumed t h e n , t h a t b o t h r e p u l s i o n and a t t r a c t i o n  energies  o f t h i s t y p e have t h e same form and t h e t o t a l p o t e n t i a l i s  g e n e r a l l y g i v e n by  *  =  ; * D  +  * R  =  - f n  where m > n. .This e q u a t i o n physiochemical introduced  systems.  +  f m  ^  has been a p p l i e d t o a v a r i e t y o f  A r e l a t i o n o f t h i s form was f i r s t  i n t o the theory  o f gases by Lennard-Jones [27] where  n = 6 and m =•12 and e q u a t i o n  (2-10) i s g e n e r a l l y r e f e r r e d t o as t h e  Lennard-Jones (6-12) p o t e n t i a l .  1  -14-  Other a t t r a c t i o n f o r c e s a r e a l s o p r e s e n t i f t h e adsorbent i s i o n i c i n n a t u r e and t h e a d s o r b a t e p o l a r .  Strong  f i e l d s F a r e known t p be p r e s e n t on i o n i c s u r f a c e s .  electrostatic B a r r e r [28] has  d e f i n e d v a r i o u s energy terms t h a t c o n t r i b u t e t o t h e p h y s i c a l bond i n t h e s e systems.  These a r e :  P o l a r i z a t i o n energy  fy^,  F i e l d - d i p o l e energy $p > F i e l d g r a d i e n t - q u a d r u p o l e energy Q ) ^ , D i p o l e - d i p o l e energy $^> Quadrupole-quadrupole  2.4.1  Polarization  D i p o l e - q u a d r u p o l e energy  a n  d  energy ^QQ*  Energy.  P o l a r i z a t i o n a r i s e s when t h e adsorbent i s h e t e r o p o l a r , c r e a t i n g l o c a l e l e c t r o s t a t i c f i e l d s which may p o l a r i z e a d s o r b a t e m o l e c u l e s h a v i n g some p o l a r i z a b i l i t y .  ?p = - | F  Then,  (2-11),  2  The s t r e n g t h o f t h i s i n t e r a c t i o n i s o b v i o u s l y d i r e c t l y dependent on b o t h a and F. 2.4.2  Field-dipole  Energy.  M o l e c u l e s p o s s e s s i n g permanent d i p o l e moments a l s o i n t e r a c t w i t h F, t h e energy o f i n t e r a c t i o n g i v e n by  *  F|J  = - FycosG  (2-12)  where y i s t h e d i p o l e moment o f t h e adsorbed m o l e c u l e and 8 t h e angle t h e a x i s , o f t h e d i p o l e makes w i t h t h e f i e l d . (l>P  I t i s expected  w i l l assume an a p p r e c i a b l e v a l u e o n l y i f t h e a d s o r b a t e  can approach w i t h i n a s h o r t d i s t a n c e o f t h e s u r f a c e [ 2 9 ] .  molecule  -15-  2.4.3  F i e l d G r a d i e n t - quadrupole Energy. Recently,  t h e importance o f t h e p r e s e n c e o f a  permanent quadrupole i n c e r t a i n a d s o r b a t e m o l e c u l e s has been recognized  [30] .  A q u a d r u p o l e i s p i c t u r e d as a r i s i n g  separation  o f e q u a l and o p p o s i t e  being proportionate separation  d i p o l e s , t h e magnitude o f t h e moment  t o t h e p r o d u c t o f t h e d i p o l e moment and t h e  of the dipoles.  The"local  a s s o c i a t e d w i t h them a f i e l d g r a d i e n t with molecules possessing The  from  f i e l d s F w i l l n o r m a l l y have F which can i n t e r a c t  powerfully  permanent quadrupole moments.  1  i n t e r a c t i o n s o f the poles also contribute t o the  bond energy though t h e i r c o n t r i b u t i o n s a r e n o r m a l l y much s m a l l e r than those p r e v i o u s l y mentioned. Dispersion  f o r c e s , t h e n , a r e always p r e s e n t when  considering p h y s i c a l adsorption has  and, u n l e s s  the adsorbate molecule  a permanent d i p o l e moment, w i l l r e p r e s e n t t h e major c o n t r i b u t i o n  to the t o t a l adsorption  energy,  E l e c t o s t a t i c forces are present i f  the s o l i d i s i o n i c and become s i g n i f i c a n t and perhaps predominant i f the adsorbed m o l e c u l e has a l a r g e d i p o l e moment. I t i s evident  t h a t t h e e x a c t f o r c e s i n v o l v e d depend upon  the p h y s i c a l and c h e m i c a l p r o p e r t i e s adsorbent.  The f a v o u r e d a d s o r p t i o n  by t h e s e p r o p e r t i e s , ,  o f b o t h t h e a d s o r b a t e and s i t e s a r e a l s o determined  -16-  CHAPTER THREE  ZEOLITES Over 200 y e a r s ago, a Swedish m i n e r a l o g i s t and c h e m i s t , Baron C r o n s t e d t , observed t h a t c e r t a i n m i n e r a l s appeared b o i l a t t h e same time when h e a t e d .  t o m e l t and  He named t h e s e m i n e r a l s z e o l i t e s  from t h e Greek words "zeo" meaning t o b o i l and " l i t h o s " meaning s t o n e . L i t t l e a t t e n t i o n was g i v e n these z e o l i t e s u n t i l t h e 1920's when t h e i r s e l e c t i v e a d s o r p t i o n p r o p e r t y was n o t i c e d . McBain [ 3 1 ] , i n d i s c u s s i n g the s i g n i f i c a n c e  o f t h e s e r e s u l t s , c o i n e d t h e term  "molecular s i e v e s " f o r these z e o l i t e s . began a thorough i n v e s t i g a t i o n  I n t h e l a t e 1930*s, B a r r e r [32]  o f the adsorptive p r o p e r t i e s o f these  m a t e r i a l s which l e d t o c o n s i d e r a b l e i n t e r e s t among t h e s c i e n t i f i c community. About 40 z e o l i t e s o c c u r i n n a t u r e b u t much i n t e r e s t has  -17-  a l s o been g i v e n t o s y n t h e t i c v a r i e t i e s .  Barrer synthesized the z e o l i t e  m o r d e n i t e and s e v e r a l o t h e r s y n t h e t i c v a r i e t i e s  [33-35].  e a r l y 1950's many d i f f e r e n t s y n t h e t i c z e o l i t e s had been  By t h e prepared  i n t h e L i n d e r e s e a r c h l a b o r a t o r y [36, 3 7 ] . Some a r e analogs o f z e o l i t e m a t e r i a l s ; o t h e r s , new v a r i e t i e s n o t found i n n a t u r e .  Many  p r e s e n t - d a y commercial o p e r a t i o n s s i m p l y were n o t p o s s i b l e o r p r a c t i c a l p r i o r t o t h e advent o f these m a t e r i a l s .  They have p e r m i t t e d t h e  development o f s e l e c t i v e a d s o r p t i o n as a p r a c t i c a l a l t e r n a t i v e t o t h e long e s t a b l i s h e d s e p a r a t i o n methods o f d i s t i l l a t i o n , e x t r a c t i o n and f r a c t i o n a l  absorption,  crystallization.  Molecular sieves ( z e o l i t e s ) are c r y s t a l l i n e metal . aluminosilicates with a three-dimensional s t r u c t u r e o f SiO. and A10. t e t r a h e d r a .  4  '  i n t e r c o n n e c t i n g network  The fundamental b u i l d i n g  4  b l o c k o f any z e o l i t e c r y s t a l i s a t e t r a h e d r o n o f f o u r oxygen i o n s surrounding  a s i l i c o n o r aluminium i o n ( f i g u r e 2 ) . The t r i v a l e n c y  o f aluminium causes t h e AIO^ t e t r a h e d r o n t o be n e g a t i v e l y charged r e q u i r i n g an a d d i t i o n a l c a t i o n t o e l e c t r i c a l l y n e u t r a l i z e t h e system. balance  The oxygens a r e s h a r e d between n e i g h b o u r i n g t h e charge o f t h e s i l i c o n i o n .  t e t r a h e d r a and  The charge b a l a n c i n g  cations  are t h e exchangeable i o n s o f the z e o l i t e s t r u c t u r e . The  remainder o f t h e b u i l d i n g b l o c k s o f t h e z e o l i t e s , i n  order o f i n c r e a s i n g complexity and  are:  (a) r i n g s ; (b) p r i m a r y  cages;  (c) secondary cages and c h a n n e l s . Rings;are  by oxygen b r i d g e s .  formed o f t h e s i l i c o n and aluminium t e t r a h e d r a The cages a r e composed o f v a r i o u s s i z e d r i n g s  SILICON ALUMINIUM  OXYGEN CATION  FIGURE 2. . The fundamental b u i l d i n g b l o c k s o f z e o l i t e a) S l O . t e t r a h e d r o n b) A l p . t e t r a h e d r o n .  -19-  so t h a t a c c e s s t o them i s governed by t h e r i n g d i m e n s i o n s . pore opening o f t h e 4-membered r i n g s  The  (tetrahedra) i s n e g l i g i b l e .  The 6-membered r i n g s have an opening o f 2.2.% d i a m e t e r .  8-membered  r i n g s have a pore d i a m e t e r o f 4.3 A* w h i l e 12-membered r i n g s have a pore d i a m e t e r o f 8.9 X. The s t r u c t u r e s o f many z e o l i t e s c o n s i s t o f s i m p l e arrangements  o f p o l y h e d r a formed from t h e r i n g s .  The t r u n c a t e d  o c t a h e d r o n , a l s o known as t h e s o d a l i t e cage, i s a w e l l known example o f such a p r i m a r y cage ( f i g u r e 3 ) . T h i s cage c o n t a i n s 24 s i l i c o n (aluminium) t e t r a h e d r a and i s composed o f s i x 4-membered r i n g s and e i g h t 6-membered r i n g s .  The f r e e d i a m e t e r o f t h e i n t e r n a l c a v i t y i s  o 6.6 A, and access i s through t h e 6-membered r i n g s . Secondary  cages appear on t h e p a c k i n g o f t h e s i m p l e r p r i m a r y  cages t o form t h e t o t a l z e o l i t e s t r u c t u r e .  Cages o f i n t e r e s t a r e  d i s c u s s e d when t h e s t r u c t u r e s o f s p e c i f i c z e o l i t e s a r e r e v i e w e d . A s t r u c t u r a l formula o f the type Me  , [(A10 ) ( S i O J ] -M HJD x/n 2^x ^ 2 y 2 L v  o  J  i s o f t e n used t o i l l u s t r a t e t h e r e l a t i o n between c h e m i c a l c o m p o s i t i o n and structure of zeolites.  Me s t a n d s f o r t h e m e t a l i o n s ; x,y and n a r e  i n t e g e r s ; and M i s t h e number o f H^O m o l e c u l e s i n t h i s u n i t c e l l f o r m u l a . The p o r t i o n i n b r a c k e t s r e p r e s e n t s t h e framework s t r u c t u r e . The r a t i o y/x v a r i e s between 1 and 5. r u l e o f Loewenstein  [ 3 8 ] , A10  A c c o r d i n g t o an e m p i r i c a l  t e t r a h e d r a can be j o i n e d o n l y t o  -20-  © Silicon  or  Aluminium  A r5 •4 •3  O 0 xygen  •2 •1 L  FIGURE  3.  The  truncated octahedron, or s o d a l i t e  0  cage.  SiO^ t e t r a h e d r a and n e v e r t o a n o t h e r AIO^ the l i m i t t o the r a t i o o f 1:1.  The  tetrahedron,  thus g i v i n g  f a c t t h a t o n l y a l i m i t e d number  o f s i l i c o n / a l u m i n i u m r a t i o s are observed would i n d e e d i n d i c a t e t h a t t h e r e i s an o r d e r i n g i n the r i n g s o f the A l and S i .  The  metal  i o n s needed f o r charge compensation occupy s i t e s a d j a c e n t t o c a v i t i e s i n the z e o l i t e s and with other ions. common, t r i - ,  the  are g e n e r a l l y a v a i l a b l e f o r exchange  A l t h o u g h mono- and d i - v a l e n t i o n s are the most  t e t r a - and  even p e n t a - v a l e n t  S y n t h e t i c v a r i e t i e s c o n t a i n i n g Ge^  and Ga^  +  i o n s have been found. +  substituted for S i ^  +  and  A l " * have a l s o been p r e p a r e d [39,. 40] . 5  3.1  Adsorption  in Zeolites.  As a.consequence o f t h e i r porous s t r u c t u r e , z e o l i t e s are i n many cases a b l e t o c o n t a i n a d s o r b a t e m o l e c u l e s i n g r e a t v a r i e t y and y e t i n a h i g h l y s e l e c t i v e manner. i s composed o f c o n t i n u o u s ,  Since t h e i r s t r u c t u r e  o f t e n i n t e r p e n e t r a t i n g channel systems,  e n t r y i s governed by r i n g s o f v a r i o u s dimensions periodically.throughout The  located  the s t r u c t u r e .  v a r i e t y and dimensions o f the v a r i o u s  adsorbates  c a p a b l e o f e n t e r i n g the z e o l i t e s i s t h e r e f o r e c o n t r o l l e d not by  the  dimensions o f the c a v i t i e s , but by the dimensions o f the r i n g s or "windows" p e r m i t t i n g a c c e s s t o them.  Owing t o r i n g  puckering,  not a l l r i n g s c o n t a i n i n g the same number o f t e t r a h e d r a equivalent  i n s i z e [41].  are  S t r u c t u r e s w i t h , f o r example, 8-membered  r i n g s can t h e r e f o r e e x e r t a wide range o f m o l e c u l a r s i e v i n g b e h a v i o u r based on r i n g d i s t o r t i o n  alone.  -22-  The number, s i z e , v a l e n c y o f z e o l i t i c c a t i o n s have i m p o r t a n t  and  l o c a t i o n i n the  e f f e c t s on the s i z e and  o f the e n t r y pores t o the l a r g e r c a v i t i e s . p r o f o u n d e f f e c t on a d s o r p t i o n e n e r g i e s .  The  c a t i o n s are  present  They are  i n t o 6 - r i n g windows which do not n o r m a l l y  main a c c e s s t o the channel system.  shape  They a l s o have a  i n the same channels as the a d s o r b a t e m o l e c u l e s . recessed  lattice  often  f u n c t i o n as  the  Sometimes the c a t i o n s are a l s o  located i n polyhedra  which are not themselves a b l e t o h o l d  adsorbate molecules.  These c a t i o n s , o f c o u r s e , would not  the m i g r a t i o n o f a d s o r b a t e m o l e c u l e s .  hinder  Other c a t i o n s , however,  may  remain n e a r windows c o n t r o l l i n g access t o the pore system o f the z e o l i t e .  T h i s i n f l u e n c e may  be moderated i n t h r e e ways  [42,43]: 1.  Changing the s i z e o f the c a t i o n s t h r o u g h exchange (K  2.  +  ^Na , +  Changing the number o f c a t i o n s through exchange -tr- Ca  (2Na 3.  f o r example)  , f p r example)  Changing the number o f c a t i o n s t h r o u g h s y n t h e s i s (NaAl £ S i , f o r example)  The  e f f e c t o f the t h i r d c o n s i d e r a t i o n u s i n g s y n t h e s i s i s t w o - f o l d .  Besides  removing t h e i n f l u e n c i n g c a t i o n , a g i v e n r i n g s i z e  decrease s l i g h t l y w i t h h i g h e r s i l i c o n content are s l i g h t l y s h o r t e r than A l - 0 bonds. a f f e c t the p o s i t i o n s o f the c a t i o n s than a 1:1  The  may  [44] s i n c e S i - 0 bonds  S i / A l r a t i o may  (figure 4).  Anything  also other  r a t i o o f S i / A l w i l l g r e a t l y a f f e c t the arrangement  o f the c a t i o n . w i t h r e s p e c t t o the t e t r a h e d r a charge i t i s b a l a n c i n g .  -23-  FIGURE 4.ratios';  Cation position for zeolites (a) (b) 1/1  1/2  of v a r y i n g  Si/Al  -24-  T h i s i s e s p e c i a l l y t r u e when mono-valent c a t i o n s are  replaced  by d i - o r t r i - v a l e n t ones. In a d d i t i o n t o the pore geometry o f the z e o l i t e s , various adsorption  forces discussed p r e v i o u s l y a l s o determine  s e l e c t i v i t y i n adsorption. becomes v e r y  the  The  p o l a r i t y o f the a d s o r b a t e m o l e c u l e s  i m p o r t a n t s i n c e s t r o n g i n t e r a c t i o n s may  occur  between the z e o l i t e and p o l a r a d s o r b a t e m o l e c u l e s . C l u s t e r s o f m o l e c u l e s are p r e s e n t i n the c a v i t i e s when they are s a t u r a t e d .  These c l y s t e r s may  o t h e r c l u s t e r s t h r o u g h the windows.  be j o i n e d by c o n t a c t  The  number of m o l e c u l e s i n  c l u s t e r i s not n e c e s s a r i l y an i n t e g e r s i n c e a m o l e c u l e may between two  with  be  any  shared  c a v i t i e s or cages i f i t happens t o be l o c a t e d i n the  window between the two.  When the c a v i t i e s are not s a t u r a t e d  and  the number o f a d s o r b a t e m o l e c u l e s i s s m a l l , t h e y are d i s t r i b u t e d , not n e c e s s a r i l y u n i f o r m l y ,  throughout the e n t i r e a c c e s s i b l e pore  volume. A l t h o u g h the e n t i r e pore volume i s a v a i l a b l e f o r adsorption, and w i l l  c e r t a i n adsorption  s i t e s are more f a v o u r e d t h a n  n e c e s s a r i l y be f i l l e d f i r s t .  These are due  the c a t i o n s which are exposed i n the c r y s t a l l a t t i c e .  others  p r i m a r i l y to These  c a t i o n s a c t as s i t e s o f s t r o n g p o s i t i v e charge which e l e c t r o s t a t i c a l l y a t t r a c t the n e g a t i v e  ends o f p o l a r m o l e c u l e s .  M o l e c u l e s can  have d i p o l e s induced i n them under the i n f l u e n c e o f t h e s e charges.  localized  These i n d u c e d d i p o l e s a r e , however, f a r weaker and  strongly attracted.  also  less  -25-  5.2 5.2.1  Structures of Zeolites. X,Y Type. The c r y s t a l s t r u c t u r e o f t h e s y n t h e t i c  zeolites  types X and Y i s s i m i l a r t o t h a t o f t h e n a t u r a l l y o c c u r r i n g faujasite  [45].  arrangement  The framework  consists of a tetrahedral  o f s o d a l i t e cages, i n a diamond t y p e l a t t i c e ,  by hexagonal f a c e s w i t h s i x b r i d g e oxygen i o n s [46,47]  linked  (figure 5).  The u n i t c e l l f o r m u l a f o r t h e t y p e 13X s y n t h e t i c z e o l i t e i s  N a  96«  A 1 0  2^96^  S i 0  2^96' '  2 6 4 H  2°  T h i s i s a sodium X s i e v e and has t h e same c h a r a c t e r i s t i c s t r u c t u r e as t h e sodium Y s i e v e except f o r a lower S i / A l and c o n s e q u e n t l y more sodium i o n s p e r u n i t c e l l .  ratio  The r a t i o i s  u s u a l l y 1:1 f o r t h e X s t r u c t u r e and 17:7 f o r t h e Y. The volume e n c l o s e d by t h i s a r r a y o f cages i s t h e supercage, i n t h i s case termed a t y p e I I 26-hedron cage, o r f a u j a s i t e ( f i g u r e 6 ) . I t i s composed o f 48 atoms o f s i l i c o n 96 oxygen atoms.  cage  (aluminium) and  The cage has 18 square f a c e s , f o u r 6-membered  r i n g s , and f o u r 12-membered r i n g s .  The l a t t e r a r e the most i m p o r t a n t  p o r t s o f e n t r y i n t o t h e supercage.  The openings o f t h e s e r i n g s i s  a p p r o x i m a t e l y 8-9 A* and t h e i n t e r n a l d i a m e t e r o f t h e cage i s 12.5 A. The volume o f t h e supercage i s about 850 A* whereas t h e volume o f 0  3  the s o d a l i t e cages i s about 160 A . Thus t h e r e a r e t h r e e cage t y p e s p r e s e n t i n t y p e X z e o l i t e s : t h e f a u j a s i t e cages, t h e s o d a l i t e cages and t h e hexagonal p r i s m s  -26-  FIGURK 5. The . s t r u c t u r a l thetic zeolite.  framework o f the X type s y n -  -27-  FIGURE 6.  The' t y p e I I 2 6 - h e d r o n  cage,  or faujasite  cage.  -28-  formed by the b r i d g i n g oxygens j o i n i n g the s o d a l i t e cages. hexagonal p r i s m  c a v i t i e s can u s u a l l y o n l y be e n t e r e d  The  from  the s o d a l i t e cages t h r o u g h the h e x a g o n a l f a c e s where the opening i s about 2 X i n d i a m e t e r . two  The  type X s t r u c t u r e t h e r e f o r e  independent, three dimensional  networks o f c a v i t i e s - one  s o d a l i t e cages l i n k e d t h r o u g h h e x a g o n a l p r i s m s and  one  o f the  cages l i n k e d by s h a r i n g r i n g s o f 12 t e t r a h e d r a - the two interconnected  3.2.1.1  super-  by r i n g s o f 6 t e t r a h e d r a .  Cation Positions.  B r o u s s a r d and  13X,  Shoemaker [46] were a b l e t o l o c a t e p r e c i s e l y o n l y  48 out o f the 80 N a  +  cations r e q u i r e d per u n i t c e l l of  their  X-ray s t u d i e s o f a calcium-exchanged n a t u r a l f a u j a s i t e  by P i c k e r t , Rabo and  associates  [48,49] y i e l d e d a more e x p l i c i t  p i c t u r e o f the c a t i o n d i s t r i b u t i o n . described  of  systems  From a c r y s t a l l o g r a p h i c s t u d y o f s y n t h e t i c Na  sampleo  contains  (figure 7).  sites  Three c a t i o n s i t e s were  (16 p e r u n i t c e l l ) are l o c a t e d i n  the i n t e r i o r o f ; t h e hexagonal p r i s m s ,  p o s i t i o n e d between  two  puckered 6-membered r i n g s i n s i x - f o l d c o o r d i n a t i o n t o oxygen. S j i s e f f e c t i v e l y h i d d e n from the z e o l i t e s u r f a c e as a consequence o f i t s i n t i m a t e c o o r d i n a t i o n t o the framework i o n s . (32 p e r u n i t c e l l ) are found i n the hexagonal f a c e s r i n g s ) at the mouths o f the s o d a l i t e cages. t h r e e - f o l d oxygen i o n c o o r d i n a t i o n ,  The  sites  The  sites  (6-membered  c a t i o n s h e r e have are l o c a t e d next  t o the 4-membered r i n g s on the s u r f a c e o f the s u p e r c a g e .  The  of preference  .  o f c a t i o n s seems t o be S  over S  T T  over S  T T T  order  -29-  FIGURE  7.  C a t i o n s i t e s i n Na  13X  synthetic  zeolite.  -30-  S i n c e S j and S ^ j s i t e s a r e more t h a n s u f f i c i e n t t o accommodate t h e b i v a l e n t c a t i o n s , t h e S^^^ s i t e s a r e p r o b a b l y  only populated  i n the  u n i v a l e n t forms o f t h e zeolites„ The sodium i o n can be r e p l a c e d by a m u l t i t u d e o f o t h e r s , depending on i o n s i z e and charge.  Among those more commonly  •+ ,+ + ++ ^ exchanged a r e L i , K , Rb , Cs , Ca , S r +  + +  ++ '-, ++ , Ba , and Cu .  The  sodium can a l s o be r e p l a c e d by ammonium i o n s and t h e s e i n t u r n decomposed t o y i e l d a d e c a t i o n a t e d  zeolite.  Replacement o f sodium i o n s f o r c a l c i u m i o n s d e c r e a s e s the p e r m e a b i l i t y o f t h e z e o l i t e from a p p r o x i m a t e l y  13 X i n 13X  t o 10 X i n 10X, a c a l c i u m exchanged form o f t h e sodium Thus, d e c r e a s i n g  t h e number o f c a t i o n s (2Na  +  £ Ca ) + +  decreases the a d s o r p t i v e a b i l i t y o f the X z e o l i t e .  zeolite.  actually T h i s i s due t o  the f a c t t h a t t h e c a t i o n s i n i t i a l l y r e p l a c e d a r e those i n t h e hexagonal p r i s m s , which have no e f f e c t on t h e pore openings i n t h e zeolite.  Replacement o f t h e c a t i o n s a t t h e open r i n g s o f t h e  supercage a c t u a l l y i n c r e a s e s t h e r e t a r d i n g e f f e c t o f t h e c a t i o n i c p o t e n t i a l due t o t h e i n c r e a s e d s i z e and charge o f t h e c a t i o n s .  3.2.2  A Type. : In t h e A type s t r u c t u r e , t h e p r i m a r y  o f s o d a l i t e cages. square f a c e s  cage a l s o c o n s i s t s  I n t h i s i n s t a n c e they a r e j o i n e d t h r o u g h t h e  (4-membered r i n g s ) by f o u r b r i d g i n g oxygen i o n s i n a  c u b i c a r r a y [50] (see f i g u r e 8 ) .  I n t h e sodium form, commonly c a l l e d  4A, t h e s t r u c t u r e i s r e p r e s e n t e d by t h e f o r m u l a Na ((A10 ) (Si0 ) ).27H 0. 1 2  2  1 2  2  1 2  2  -31-  FIGURE 8. The s t r u c t u r a l framework of the A type synthetic zeolite.  -32-  As seen from t h e f o r m u l a , t h e S i / A l r a t i o i n t h i s case s h o u l d be 1:1.  T h i s type o f s t a c k i n g o f t h e s o d a l i t e cages g i v e s a r o u g h l y  s p h e r i c a l s u p e r c a g e , termed a t y p e I 26-hedron I t c o n s i s t s o f t h e same number o f s i l i c o n  cage ( f i g u r e 9 ) .  (aluminium) atoms, 48,  and oxygen atoms, 96, as t h e supercage o f t h e X t y p e s t r u c t u r e . In terms o f r i n g s i z e s composing rings  t h e cage, i t has e i g h t e e n 4-membered  (square f a c e s ) , e i g h t 6-membered r i n g s  8-membered r i n g s  (octagons).  (hexagons), and s i x  The d i a m e t e r o f t h i s 26-hedron  cage  o o3 xs 11.4 A and t h e volume i s 775 A . As i n the X type s t r u c t u r e , t h e r e a r e t h r e e cage t y p e s : the supercage, t h e s o d a l i t e cage and t h e square p r i s m s , formed by the oxygen atoms l i n k i n g t h e s o d a l i t e cages.  The s u p e r c a g e s , sometimes  c a l l e d t r u n c a t e d c u b o o c t a h e d r a , a r e found i n a c u b i c w i t h r e s p e c t t o each o t h e r .  arrangement  Access i s through t h e 8-membered  r i n g s w i t h a pore d i a m e t e r o f 4.2 X,. and a r e t h e l a r g e s t 8-membered r i n g s t o be found i n z e o l i t e s s i n c e t h e r i n g i s p l a n a r [ 5 1 ] .  The  A type s t r u c t u r e t h e r e f o r e c o n s i s t s o f one t h r e e - d i m e n s i o n a l network o o f c a v i t i e s h a v i n g a maximum d i a m e t e r o f 11.4 A and a minimum o f 4.2 %.  Access t o t h e s o d a l i t e cages i s through t h e d i s t o r t e d  6-membered r i n g s o f d i a m e t e r 2.2 A* b u t access i s o n l y through t h e central cavity 3.2.2.1  system.  Cation Positions. The a v a i l a b l e p o s i t i o n s f o r c a t i o n s i n t h e A t y p e s t r u c t u r e  are a t t h e c e n t e r o f t h e e i g h t 6-membered r i n g s o f t h e s o d a l i t e  cages  at the c o r n e r s o f t h e supercage, s i t e A, and i n 12 a v a i l a b l e p o s i t i o n s  -33-  FIGURF,  9.  The type I 26-hedron cage.  -34-  a d j a c e n t t o t h e 8-membered r i n g s d e f i n i n g t h e s u p e r c a g e , s i t e B. For 4A, t h e sodium form, e i g h t o f t h e t w e l v e c a t i o n s o f a u n i t  cell  are found i n s i t e A, w h i l e t h e o t h e r f o u r a r e s t a t i s t i c a l l y d i s t r i b u t e d i n t o t h e twelve s i t e B l o c a t i o n s [50,52]. is therefore  f i l l e d p r e f e r e n t i a l l y t o s i t e B.  Site A  Replacement o f t h e  sodium i o n s by c a l c i u m  i o n s t o form t h e 5A s y n t h e t i c  actually increases-the  e f f e c t i v e opening t o t h e c e n t r a l p o r e  system t o a p p r o x i m a t e l y 5 $ i n d i a m e t e r . i o n s a r e r e p l a c e d by s i x c a l c i u m  zeolite,  S i n c e t h e t w e l v e sodium  i o n s , t h e s e w i l l be l o c a t e d i n  s i t e A, l e a v i n g t h e 8-membered r i n g s c l e a r e r and y i e l d i n g a l a r g e r access t o t h e supercage.  The sodium i o n s may a l s o be r e p l a c e d  by i o n s such as L i , K , R b , C s , T l , A g , N H , M g , +  +  +  +  +  +  +  2 +  4  Ba , Hg , Cd , Zn 2 +  2 +  2 +  2 +  , Co  2 +  , and N i  2 +  Sr  2 +  ,  .  3.2.3 M o r d e n i t e . The  z e o l i t e mordenite belongs t o that  classification  c h a r a c t e r i z e d b y t h e predominance o f 5-membered r i n g s o f tetrahedra.  The g e o m e t r i c a l  pattern of the a l u m i n o s i l i c a t e  framework i s d i f f e r e n t from t h e A and X t y p e s t r u c t u r e s i n t h a t t h e buildup  i s o f c h a i n s r a t h e r than o f p o l y h e d r a ( f i g u r e 10a).  There  are s i x p o s s i b l e s i m p l e s t r u c t u r e s formed by d i f f e r e n t l a t e r a l bondings o f t h e c h a i n s t o one a n o t h e r and a c r o s s - s e c t i o n o f t h a t found i n m o r d e n i t e i s shown i n f i g u r e 10b. The r e s u l t i s a t w o - d i m e n s i o n a l , t u b u l a r pore system, u n l i k e t h e t h r e e - d i m e n s i o n a l pore systems o f A and X s t r u c t u r e s  [53].  The u n i t c e l l o f an i d e a l  sodium m o r d e n i t e i s g i v e n by t h e f o r m u l a Na  g  • (A10 ) 2  • (Si0 ) 2  4 Q  • 24H 0 2  -35-  (a)  (b)  FIGURE itc (a) (b)  10. The s t r u c t u r a l framework o f s y n t h e t i c morden • • c h a r a c t e r i s t i c chain structure c r o s s - s e c t i o n a l area o f a'chain  -36-  Thus, m o r d e n i t e c o n t a i n s  a h i g h e r S i / A l r a t i o , 5:1.  i n f i g u r e 10b, t h e channels a r e c i r c u m s c r i b e d of tetrahedra;  As shown  by 12-membered r i n g s  l a r g e d e v i a t i o n s from p l a n a r i t y , however make a  p l a n a r p r o j e c t i o n much s m a l l e r t h a n a 12-membered r i n g o f t h e X t y p e structure.  The major and minor d i a m e t e r s a r e thus 7.0 X and 5.8 A*,  respectively.  These l a r g e channels a r e i n t e r s e c t e d p e r p e n d i c u l a r l y  by s m a l l e r c h a n n e l s c i r c u m s c r i b e d minimum f r e e d i a m e t e r o f 3.9 X  by 8-membered r i n g s h a v i n g a  and l e a d i n g t o t h e n e x t main c h a n n e l .  However, h a l f w a y t o t h e n e i g h b o u r i n g main c h a n n e l , t h e s i d e channels branch through two d i s t o r t e d 8-membered r i n g s o f 2.8 A* f r e e d i a m e t e r which open i n t o t h e main c h a n n e l .  3.2.3.1  Cation P o s i t i o n s . In Na-mordenite, a sodium i o n r e s t s a t t h e c e n t e r o f each  d i s t o r t e d 8-membered r i n g , e f f e c t i v e l y i s o l a t i n g t h e main channels from one another, and l e a v i n g each main channel l i n e d w i t h two rows o f s i d e p o c k e t s [ 5 4 ] . These p o c k e t s have a low r a t i o o f volume t o c r o s s - s e c t i o n a l a r e a o f t h e i r e n t r a n c e s ( f i g u r e 1 1 ) . The o t h e r c a t i o n s a r e l o c a t e d i n t h e main channels and occupy a t random some o f the 8- and 1 2 - f o i d p o s i t i o n s a v a i l a b l e [ 5 3 ] .  -37-  O o  xygen  HI O x y g e n O  in p l a n e  of  paper  Cations  11. C a t i o n p o s i t i o n s i n s y n t h e t i c lnordeni te Aluminium arid S i l i c o n a t the c e n t e r s o f each t e t r a hedron a r e n o t shown.  FIGURE  -38-  CHAPTER FOUR ELECTRON PARAMAGNETIC RESONANCE 4.1  Theory. * The b a s i s o f e l e c t r o n p a r a m a g n e t i c resonance (EPR) i s  concerned w i t h t h e i n t r i n s i c s p i n o f an e l e c t r o n and i t s a s s o c i a t e d magnetic moment.  An a p p l i e d magnetic f i e l d H a l l o w s o n l y  certain  d i s c r e t e o r i e n t a t i o n s o f the precessing dipoles with respect to the magnetic f i e l d , t h e o r i e n t a t i o n s c o r r e s p o n d i n g t o d i f f e r e n t energy levels. the  I r r a d i a t i o n o f t h e system w i t h e l e c t r o m a g n e t i c energy o f  a p p r o p r i a t e f r e q u e n c y i n d u c e s t r a n s i t i o n s between t h e s e magnetic  energy  levels.  *  The term e l e c t r o n s p i n r e s o n a n c e , ESR, i s l e s s g e n e r a l than EPR, s i n c e t h e former does n o t t a k e i n t o account o r b i t a l magnetism.  -39-  Th e energy f o r t h e s e t r a n s i t i o n s i s g i v e n by the  hv  =  equation  gm  (4-1)  where h i s P l a n c k ' s c o n s t a n t ; v the f r e q u e n c y o f t h e r a d i a t i o n ; a n u m e r i c a l f a c t o r o f t e n c l o s e t o 2; (3, t h e Bohr magneton; and the magnetic f i e l d .  H,  A magnetic f i e l d o f 3000 gauss r e q u i r e s a  frequency o f about 9 G i g a h e r t z t o induce t h e t r a n s i t i o n s . corresponds  g,  This  t o a w a v e l e n g t h o f a p p r o x i m a t e l y 3 c e n t i m e t e r s , which i s i n  the microwave r e g i o n o f t h e e l e c t r o m a g n e t i c spectrum. R e l a x a t i o n p r o c e s s e s must n e c e s s a r i l y be p r e s e n t such t h a t t h e energy absorbed by s p i n s i n the h i g h e r energy l e v e l can  be  d i s s i p a t e d i n such a manner as t o p e r m i t t h e i r r e t u r n t o the ground energy l e v e l .  Otherwise  p o p u l a t i o n between t h e energy l e v e l s would  e q u a l i z e and a b s o r p t i o n would cease.  This e s s e n t i a l l y i s achieved  through the phenomenom o f ' s p i n - l a t t i c e r e l a x a t i o n ' , where the ' s p i n system' i n t e r a c t s w i t h i t s s u r r o u n d i n g s  i n such a way  as t o  p r o v i d e paths f o r t h i s p r o c e s s , and a l s o through s p i n - s p i n r e l a x a t i o n . The p o p u l a t i o n o f these two at  l e v e l s , when i n t h e r m a l e q u i l i b r i u m  a g i v e n f i e l d and t e m p e r a t u r e ,  may  be r e p r e s e n t e d by  the  Boltzmann e q u a t i o n .  Thus, i f t h e p o p u l a t i o n s o f the upper and  l e v e l s are  respectively,  and  lower  -40-  where 3, g, and H a r e d e f i n e d constant;  i n e q u a t i o n ( 4 - 1 ) ; k, Boltzmann's  and T , t h e s p i n t e m p e r a t u r e d e f i n e d by e q u a t i o n (4-2) i n  terms o f the i n s t a n t a n e o u s r e l a t i v e p o p u l a t i o n s levels.  I f the population  o f t h e two s p i n  d i f f e r e n c e a t a g i v e n t i m e t i s AN, e q u i l i b -  r i u m w i l l be reached a t a r a t e g i v e n by  dAN/dt = da /dt. - d N / d t  (4-3)  2  Given that  and  a r e the p r o b a b i l i t i e s o f t r a n s i t i o n s from t h e  upper and lower l e v e l s r e s p e c t i v e l y , we can w r i t e dAN/dt =  2W  9  V  (4-4) kT  where T now i s the l a t t i c e t e m p e r a t u r e .  From e q u a t i o n (4-4)  i t i s e a s i l y shown t h a t dAN dt  _  2W 1  (AN -AN) °  (4-5)  which has t h e s o l u t i o n  AN - A N [ 1 - expC-t/T ) ]  (4-6)  q  where  = 1/2W.  T^, t h e s p i n - l a t t i c e r e l a x a t i o n t i m e i s seen as  the i n v e r s e o f a l a t t i c e - i n d u c e d t r a n s i t i o n p r o b a b i l i t y .  The s p i n -  l a t t i c e r e l a x a t i o n i s thus c h a r a c t e r i z e d by a r e l a x a t i o n time T^ and  t h e s p i n system t r a n s f e r s energy t o t h e l a t t i c e a t t h e r a t e  S i m i l a r l y , t h e r e l a x a t i o n t i m e T^ c h a r a c t e r i z e s  1/T^.  a spin-spin relaxation  p r o c e s s , a p r o c e s s which depends on t h e e f f e c t o f l o c a l m a g n e t i c f i e l d s g e n e r a t e d by n e i g h b o r i n g  spins.  -41-  A consequence o f the e x i s t e n c e o f t h e s e r e l a x a t i o n p r o c e s s e s i s t h a t t h e s p e c t r a l l i n e s observed f o r t h e t r a n s i t i o n s between the s p i n l e v e l s have a f i n i t e w i d t h and are o f t e n d i s c u s s e d i n terms o f a  'lineshape . 1  V a r i o u s mechanisms may o f these s p e c t r a l l i n e s .  be r e s p o n s i b l e f o r b r o a d e n i n g  P o r t i s [55] has c l a r i f i e d  the d i s t i n c t i o n  between the two main c l a s s e s o f b r o a d e n i n g , homogeneous b r o a d e n i n g and inhomogeneous b r o a d e n i n g .  Homogeneous b r o a d e n i n g i s t h a t  a s s o c i a t e d w i t h t r a n s i t i o n s between s p i n l e v e l s which are not themselves s h a r p l y d e f i n e d but are somewhat broadened.  Thermal  e q u i l i b r i u m o f the s p i n system i s m a i n t a i n e d throughout resonance  as  the energy absorbed from the microwave f i e l d i s d i s t r i b u t e d t o a l l the s p i n s .  Sources o f homogeneous b r o a d e n i n g i n c l u d e [ 5 5 ] :  (a) s p i n - l a t t i c e r e l a x a t i o n ; (b) d i p o l a r i n t e r a c t i o n between s p i n s ; (c) i n t e r a c t i o n w i t h the r a d i a t i o n f i e l d ; o f e x c i t a t i o n through the sample.  and  An inhomogeneously  (d) d i f f u s i o n broadened  c o n s i s t s of a s p e c t r a l d i s t r i b u t i o n of i n d i v i d u a l resonant merged t o form an o v e r a l l l i n e s h a p e .  like  line  lines  The d i s t i n c t i o n between  homogeneous and inhomogeneous b r o a d e n i n g i s t h a t the inhomogeneous b r o a d e n i n g comes from i n t e r a c t i o n s e x t e r n a l t o t h e s p i n system and/must be s l o w l y v a r y i n g o v e r t h e time r e q u i r e d f o r a spin t r a n s i t i o n .  Inhomogeneities i n the m a g n e t i c  field  energy t o be t r a n s f e r r e d o n l y t o t h o s e s p i n s whose l o c a l s a t i s f y the resonance c o n d i t i o n . broadened  cause fields  The resonance i s thus a r t i f i c i a l l y  i n an inhomogeneous manner.  Other s o u r c e s o f inhomogeneous  -42-  broadening are [55]:  (a) h y p e r f i n e i n t e r a c t i o n ; and (b) a n i s o t r o p y  broadening. The shape o f t h e a b s o r p t i o n spectrum i s thus determined by t h e types o f i n t e r a c t i o n s between t h e environment system.  and t h e s p i n  The w i d t h s o f t h e s e l i n e s , however, depends on t h e s t r e n g t h  of t h e i n t e r a c t i o n s and t h e r e l a x a t i o n t i m e .  A system where  r e l a x a t i o n i s c o n t r o l l e d by s p i n - l a t t i c e i n t e r a c t i o n s and t h e r m a l e q u i l i b r i u m o f t h e s p i n system i s m a i n t a i n e d throughout  resonance  has a l i n e s h a p e approximated by a L o r e n t z i a n f u n c t i o n [ 5 6 ] , c h a r a c t e r i z e d by t h e e q u a t i o n  f (H-H  )  =  2 A H i  3  (4-7)  u[(H-H ) Q  2  AH ] 2  +  AHj^ h e r e r e p r e s e n t s t h e w i d t h o f t h e a b s o r p t i o n l i n e a t h a l f the maximum i n t e n s i t y , and 'H , t h e resonance f i e l d .  I t i s customary  however, i n EPR, t o d i s p l a y t h e f i r s t d e r i v a t i v e o f t h e spectrum. A l t h o u g h many i n t e r a c t i o n s i n f l u e n c e t h e l i n e w i d t h , t h e H e i s e n b e r g u n c e r t a i n t y p r i n c i p l e s e t s t h e u l t i m a t e minimum w i d t h which may be s t a t e d as  A H  %  =itf  C") 4  where T now c o r r e s p o n d s t o t h e r e l a x a t i o n t i m e .  8  E i t h e r T^ o r T^  can be t h e c o n t r o l l i n g r e l a x a t i o n t i m e , o r b o t h may be i n f l u e n t i a l . I t i s thus p o s s i b l e , i n c e r t a i n c a s e s , t o d e t e r m i n e r e l a x a t i o n times from t h e observed  spectra.  Another commonly encountered  lineshape i s a Gaussian f u n c t i o n  -43-  [56],  characterized by  f  ( -V H  - is^  « P  f ^ # - ]  This generally occurs i n an inhomogeneous spin system described above.  Gaussian and Lorentzian lineshapes are compared i n f i g u r e 12.  Although these two lineshape functions are the most common, combinations and v a r i a t i o n s of these have also been observed and are described i n reference [57]. When the nucleus also possesses a magnetic moment, i t can i n t e r a c t with the magnetic f i e l d and the e l e c t r o n i c magnetic moment. This may r e s u l t , not i n l i n e broadening, but i n the appearance of resolved hyperfine s t r u c t u r e . This hyperfine mechanism accounts f o r the m u l t i p l e t character of the spectrum. EPR i s w e l l covered i n many a r t i c l e s and reviews  The theory of  ( f o r example  [58-61]) and only pertinent theory w i l l be f u r t h e r discussed. The problem of expressing i n t e r a c t i o n s a f f e c t i n g e l e c t r o n i c energy l e v e l s i s u s u a l l y approached through the a p p l i c a t i o n of the Hamiltonian operator.  When applied t o the time-dependent  Schrodinger equation, t h i s approach y i e l d s the eigenvalues and eigenfunctions of the permitted energy l e v e l s .  Abragam and Pryce [62]  have shown that the behaviour of a spin system can be described by a 1  spin-Hamiltonian', a p a r t i c u l a r part of the o v e r a l l  Hamiltonian.  Perturbation theory i s generally used i n the s o l u t i o n of the energy levels.  This representation has the same e f f e c t as r e p l a c i n g the  i n t e r a c t i o n of the f i e l d with the o r b i t a l angular momentum by an a n i s o t r o p i c coupling between the e l e c t r o n spin and the e x t e r n a l  -44-  FIGURE .12. curves.  L o r e n t z i a n and G a u s s i a n f i r s t  derivative  -45-  magnetic f i e l d , t h e s p i n h e r e now b e i n g termed t h e f i c t i c i o u s s p i n . A p p l i c a t i o n s o f t h i s s p i n - H a m i l t o n i a n approach t o EPR a r e c o n s i d e r e d i n r e v i e w s by Bleaney and Stevens [ 6 3 ] , Bowers and Owens [64] and C a r r i n g t o n and Longuet-Higgins  [65].  The s p i n - H a m i l t o n i a n f o r a system c o n s i s t i n g o f one e l e c t r o n w i t h s p i n S=%  and a n u c l e u s  o f s p i n I may be w r i t t e n as  £%f= -BH-g.S + ftS.J_.I_ - -frYl/H + I/Q-I.  (4-10)  where t h e terms r e p r e s e n t e l e c t r o n i c Zeeman, h y p e r f i n e , n u c l e a r Zeeman and n u c l e a r quadrupole i n t e r a c t i o n , r e s p e c t i v e l y . 4.1.1  E l e c t r o n i c Zeeman I n t e r a c t i o n . The most g e n e r a l e x p r e s s i o n r e p r e s e n t i n g t h e Zeeman  i n t e r a c t i o n between a magnetic f i e l d H and t h e e l e c t r o n s p i n S i s g i v e n by J^f  = 3 H-g-S  H and S_ a r e e x p r e s s e d  (4-11) as v e c t o r s , and g, t h e s p e c t r o s c o p i c  s p l i t t i n g f a c t o r , o r g v a l u e , i s u s u a l l y expressed r a t h e r than t h e : f r e e e l e c t r o n v a l u e g the c o n s t a n t g  g  g  = 2.0023.  when (a) t h e e l e c t r o n possesses  o n l y , and (b) t h e g t e n s o r i s i s o t r o p i c .  as a t e n s o r The g f a c t o r equals  s p i n a n g u l a r momentum  D e v i a t i o n s from g  e  a r e due  to o r b i t a l magnetic moment c o n t r i b u t i o n s , due t o s p i n o r b i t c o u p l i n g , which a l t e r t h e e f f e c t i v e m a g n e t i c moment and g i s o f t e n found t o be anisotropic.  The a n i s o t r o p y may be d e s c r i b e d by t h e t e n s o r g, which  -46-  has t h e form  XX 'yx 'zx  g  g  g  xy  g  xz (4-12)  yy 'zy  The S_, t h e n , does n o t g e n e r a l l y  g  zz  r e p r e s e n t t h e pure s p i n , and i s o f t e n  termed t h e e f f e c t i v e o r f i c t i c i o u s  spin.  EPR may be d e s c r i b e d as t h e measurement o f t h e Zeeman energywhich  i s generally  o f the order of 0 - 1 cm . -1  In  essence, EPR i s concerned w i t h t h e manner i n which t h e o t h e r H a m i l t o n i a n terms p e r t u r b o r a r e p e r t u r b e d by t h i s Zeeman energy. 4.1.2  The H y p e r f i n e I n t e r a c t i o n . The h y p e r f i n e , o r e l e c t r o n  spin-nuclear  spin  i n t e r a c t i o n r e s u l t s from t h e i n t e r a c t i o n o f t h e magnetic moment of t h e u n p a i r e d e l e c t r o n  and t h e magnetic moment o f any n u c l e i  within  i t s orbital.  ways.  The f i r s t i s e s s e n t i a l l y t h e c l a s s i c a l i n t e r a c t i o n o f t h e two  dipoles  T h i s i n t e r a c t i o n a r i s e s " i n two q u i t e  s e p a r a t e d by a d i s t a n c e r .  different  I t would t h e n be e x p e c t e d  t h a t t h i s i n t e r a c t i o n s h o u l d depend upon t h e i r mutual C o n s e q u e n t l y , we r e f e r t o i t as t h e a n i s o t r o p i c  orientation.  or dipolar  hyperfine  interaction. The second form o f i n t e r a c t i o n i s n o n - c l a s s i c a l known as t h e Fermi o r c o n t a c t i n t e r a c t i o n . the u n p a i r e d e l e c t r o n  density  I t i s d e t e r m i n e d by  a t t h e n u c l e u s , and i s i s o t r o p i c .  o v e r a l l h y p e r f i n e s p l i t t i n g o b s e r v e d , t h e n , would c o n s i s t anisotropic  component  and i s  o f an  superimposed upon an i s o t r o p i c term.  The  -47-  Rapid r e o r i e n t a t i o n o f the paramagnetic s p e c i e s , f o r example i n s o l u t i o n , can average the a n i s o t r o p i c s t o z e r o . The  e x p r e s s i o n f o r the h y p e r f i n e i n t e r a c t i o n i n the  spin Hamiltonian  i s g i v e n by  J^ff  = *S.T.I  (4-13)  where T i s a t e n s o r r e p r e s e n t i n g the c o u p l i n g between the e l e c t r o n and n u c l e a r s p i n a n g u l a r momentum v e c t o r s , S and combination may  o f b o t h d i p o l a r and c o n t a c t terms.  I , and i s a The  d i p o l a r term  be w r i t t e n as  ^ d i p :  SeSlBeBl  =  I-S r  ~  3(1.r) ( S T ) r*>  (4-14)  and the c o n t a c t o r Fermi term w r i t t e n as ^font.  =  A  oi-I  (4-15) ,  where  A  o  =  -  (if)  8 gie ei e  e  I ^(o)|  2  (4-i6)  2 and  |^(0)|  nucleus.  i s the s p i n ' d e n s i t y ' o f the u n p a i r e d e l e c t r o n a t the Here g j and 3 j are the n u c l e a r g f a c t o r and magneton  d e f i n e d c o r r e s p o n d i n g l y t o those f o r the e l e c t r o n ; r_, the r a d i u s v e c t o r between the e l e c t r o n and n u c l e a r moments; r , i s the d i s t a n c e between them; and A ,the i s o t r o p i c h y p e r f i n e c o u p l i n g The  constant.  t e n s o r form o f T i s i d e n t i c a l t o t h a t p r e v i o u s l y  d e s c r i b e d f o r g, a l t h o u g h  c o n t r i b u t i o n s from b o t h d i p o l a r and  -48-  c o n t a c t i n t e r a c t i o n s may be s e p a r a t e d i n each term.  T T  XX  yx  •T zx  T T T  xy yy zy  T T T  Thus  xz yz  (4-17)  zz  and  T . .' = A  6.. + BAA  (4-18)  In g e n e r a l , t h e h y p e r f i n e i n t e r a c t i o n ( o f t h e o r d e r -2-1 0-10  cm  ) i s found t o be s m a l l e r than t h e Zeeman l e v e l s , each  l e v e l b e i n g s p l i t i n t o 21+1 s u b l e v e l s . 4.1.3  Other I n t e r a c t i o n s . N u c l e a r q u a d r u p o l e i n t e r a c t i o n s a r e even s m a l l e r i n  magnitude b u t on o c c a s i o n may have t o be i n c l u d e d t o e x p l a i n EPR r e s u l t s s a t i s f a c t o r i l y .  F o r n u c l e i w i t h s p i n I > h,  the  n u c l e a r quadrupole i n t e r a c t i o n may be e x p r e s s e d by = I-Q-I  (4-19)  where Q i s a g a i n r e p r e s e n t e d by a t e n s o r , o f t h e same form as f o r g and T. The e f f e c t i s a s m a l l b u t c a l c u l a b l e s h i f t o f t h e hyperfine  lines. The  l a s t term t o be mentioned i s t h e i n t e r a c t i o n o f  the n u c l e a r moments w i t h t h e magnetic f i e l d , t h e n u c l e a r Zeeman i n t e r a c t i o n , expressed as giBiI-H  (4-20)  -49-  A l l symbols are as d e f i n e d p r e v i o u s l y . g e n e r a l l y s m a l l and may  4.1.4  EPR  T h i s term i s a l s o  be i g n o r e d i n a f i r s t o r d e r  treatment.  Spectra. The  a n i s o t r o p i e s p r e s e n t i n b o t h the g f a c t o r and  h y p e r f i n e s p l i t t i n g s cause the EPR  spectrum t o depend on  the  the  o r i e n t a t i o n o f the s p e c i e s under c o n s i d e r a t i o n w i t h r e s p e c t t o the e x t e r n a l magnetic f i e l d .  Studies of oriented species i n s i n g l e  c r y s t a l s a r e n e c e s s a r i l y l e n g t h y and o f t e n r e q u i r e c o n s i d e r a b l e refinement  o f e x p e r i m e n t a l p r o c e d u r e and m a t h e m a t i c a l  analysis to  a c h i e v e a h i g h degree o f p r e c i s i o n i n t h e e v a l u a t i o n o f the g :  and h y p e r f i n e t e n s o r s . I f the p a r a m a g n e t i c m o l e c u l e s polycrystalline  are c o n t a i n e d i n a  or amorphous h o s t , as i s u s u a l l y the case i n  s t u d i e s on s u r f a c e s , the observed  EPR  spectrum w i l l be a complex  s u p e r p o s i t i o n o f l i n e s due t o a l l o r i e n t a t i o n s o f the randomly oriented molecules.  T h i s i s not n e c e s s a r i l y t o say t h a t the  molecul  are themselves randomly o r i e n t e d w i t h r e s p e c t t o the s u r f a c e , but r a t h e r the a d s o r p t i o n s i t e s are randomly o r i e n t e d .  Information  be o b t a i n e d from such o b s e r v a t i o n s and i s g e n e r a l l y a c h i e v e d  can  by  computing s p e c t r a l l i n e shapes f o r a number o f commonly o c c u r r i n g c o n d i t i o n s f o r known or guessed p r i n c i p a l v a l u e s o f the g hyperfine tensors.  and  The p r i n c i p a l v a l u e s are t h o s e o b t a i n e d  on  d i a g o n a l i z a t i o n o f the r e s p e c t i v e t e n s o r s . Sands [66] o b t a i n e d a resonance l i n e s h a p e by assuming a random d i s t r i b u t i o n o f s p i n o r i e n t a t i o n s , and then a v e r a g i n g  the  -50-  r e s o n a n t magnetic f i e l d s o v e r a l l o r i e n t a t i o n s .  S i m i l a r methods  of c a l c u l a t i n g t h e s e s o - c a l l e d powder o r p o l y c r y s t a l l i n e s p e c t r a have been d e v e l o p e d by Bloembergen and Rowland  [ 6 7 ] , K o b i n and P o o l e [68]  and Kneubuhl [ 6 9 ] . In d e r i v i n g t h e o r e t i c a l l i n e s h a p e s f o r t h e s e media, c o m p l e t e l y random o r i e n t a t i o n w i t h r e s p e c t t o t h e e x t e r n a l magnetic f i e l d on a m a c r o s c o p i c s c a l e i s assumed.  Thus t h e m i c r o s c o p i c  environment may be o r d e r e d o r d i s o r d e r e d w i t h o u t a f f e c t i n g t h e v a l i d i t y of the c a l c u l a t i o n s . F o l l o w i n g t h e t r e a t m e n t o f Sands [ 6 6 ] , an example o f a c a l c u l a t i o n o f a powder . l i n e s h a p e f o r an a x i a l l y symmetric case i s outlined.  For the resonance c o n d i t i o n g i v e n i n e q u a t i o n ( 4 - 1 ) , we  have •g = ( g / / c o s 9 + g j 2  sin e)^ 2  (4-21)  where 0 r e l a t e s t h e p o s i t i o n o f t h e g t e n s o r t o t h e a p p l i e d magnetic f i e l d  (see f i g u r e 16, page 7 5 ) . Here g  p r i n c i p a l g v a l u e s and g  x x  = g  y y  = g|  and g^  gy » %  = gjj .  o r i e n t a t i o n s a r e e q u a l l y p r o b a b l e , one must sum absorptions.  xx>  V  a r e  zz  the  Since a l l  o v e r a l l the  With t h e f r e q u e n c y v c o n s t a n t and sweeping t h e magnetic  f i e l d H, t h e n a b s o r p t i o n o f energy w i l l o c c u r a t f i e l d s g i v e n by H = j- {gj/ c o s 6 + 2  for  each 8.  g^sin e)" 2  %  (4-22)  The number o f s p i n s N h a v i n g an o r i e n t a t i o n w i t h  r e s p e c t t o the a p p l i e d magnetic f i e l d between  8 and 8 + d8 i s  - 5 1 -  g i v e n by  dN = (No/2)sined6  where N  (4T23)  i s t h e t o t a l number o f s p i n s .  o  T h i s becomes, from  r  equation  (4-^22) ^ / u ^ r r 2._ 2 , , 2 _ 2 ,h dN = ( N / 2 ) ( 4 H S / H - ) [ ( ; - g | ) [ 2 ( H / H ) - g | ] dH M  i  0  where H  = h /2g* v  q  spectrum. Chapter  g  /  m  m  2  A  u  (4-24)  0  ^ p l o t o f d N / ^ vs H y i e l d s t h e e x p e c t e d powder  P l o t s o f t h i s and a l s o o f a n i s o t r o p i c cases a r e g i v e n i n  Seven.  CHAPTER FIVE  ADSORPTION STUDIES As i n t r o d u c e d e a r l i e r , many s p e c t r o s c o p i c t e c h n i q u e s found wide use i n a d s o r p t i o n s t u d i e s . more common t e c h n i q u e s  have  Reviews i n v o l v i n g some o f t h e  have been g i v e n i n t h e i n t r o d u c t i o n .  This  c h a p t e r w i l l be concerned w i t h s t u d i e s u s i n g t h e EPR t e c h n i q u e and t h e i r a p p l i c a t i o n t o t h e s u r f a c e s under c o n s i d e r a t i o n .  The l i s t o f  s t u d i e s g i v e n i s by no means complete, b u t y i e l d s enough i n s i g h t i n t o t h e scope o f EPR i n t h i s  area.  I d e a l l y , one may e x p e c t t o o b t a i n t h r e e t y p e s o f i n f o r m a t i o n from s p e c t r o s c o p i c  experiments:  1,  The i d e n t i t y o f t h e a c t i v e s i t e s on t h e s u r f a c e s  2,  The i d e n t i t y o f p h y s i c a l l y adsorbed o r chemisorbed s p e c i e s  3,  The n a t u r e adsorbed  o f t h e i n t e r a c t i o n between an a c t i v e s i t e and an molecule.  -53-  The  terra ' a c t i v e s i t e '  s i t e on a s u r f a c e c a p a b l e  i s broadly defined.  o f a d s o r b i n g m o l e c u l e s from t h e gas phase,  o r can r e f e r t o s p e c i f i c s i t e s which induce c h e m i c a l is often d i f f i c u l t  although EPR  i s universally applicable  o b t a i n i n g a l l t h r e e types o f i n f o r m a t i o n .  techniques  reactions. I t  t o d i s t i n g u i s h between them.  No s p e c t r o s c o p i c t e c h n i q u e for  I t can be a  M a g n e t i c resonance  a r e most u s e f u l i n o b t a i n i n g i n f o r m a t i o n o f t y p e 1  i t i s p o s s i b l e i n some cases t o o b t a i n types 2 and 3 a l s o .  techniques  complicated  h o l d c o n s i d e r a b l e p r o m i s e o f e l u c i d a t i n g some o f t h e  and c o n f u s i n g s i t u a t i o n s t h a t a r i s e on s u r f a c e s i n systems  i n v o l v i n g the g a s - s o l i d i n t e r f a c e , S t u d i e s o f t h e n a t u r e o f s u r f a c e s t h e m s e l v e s , p r i o r t o any a d s o r p t i o n , comprised much o f t h e e a r l i e r work o f t h e EPR i n t h i s area.  technique  Low t e m p e r a t u r e s t u d i e s o f v a r i o u s carbon samples gave  narrow EPR s i g n a l s due t o v a r i o u s f r e e r a d i c a l s p r e s e n t on t h e s u r f a c e [70,71].  F u r t h e r s t u d i e s o f t h e e f f e c t s o f added gaseous oxygen t o  these samples ( f o r example [72^74]) produceid v a r i o u s RC - 0 - 0 radicals.  Not a l l r a d i c a l c e n t e r s a r e n e c e s s a r i l y on t h e s u r f a c e ;  however, t h e s e r e a c t i o n s w i t h oxygen presumably i n v o l v e r a d i c a l s at t h e i n t e r f a c e . Most s t u d i e s o f t h i s type have as t h e i r main i n t e r e s t i n t e r n a l r a t h e r than s u r f a c e e f f e c t s and c o n s e q u e n t l y d i s c u s s e d here..  w i l l n o t be  More r e c e n t s t u d i e s o f t h i s type have i n v o l v e d t h e  use o f r a d i a t i o n and t h e study o f t h e d e f e c t s formed i n v a r i o u s substances [75T78].  EPR t e c h n i q u e s  here provide very u s e f u l i n f o r m a t i o n  about t h e type o f d e f e c t , i t s environment and c r y s t a l f i e l d symmetry.  -54-  5.1 5.1.1  EPR  S t u d i e s o f R a d i c a l s on  Surfaces.  N o n - Z e o l i t i c Adsorbents. Many o f t h e e a r l i e r s t u d i e s o f p a r a m a g n e t i c s p e c i e s  s u r f a c e s were done on g l a s s e s , s i l i c a g e l s , a l u m i n a s , and v a r i o u s c a t a l y t i c s u r f a c e s .  on  semiconductors,  An e a r l y study by Faber  and  Rogers [79] i n v o l v e d a d s o r p t i o n o f manganese ( I I ) , copper ( I I ) , and  oxy  vanadium (IV) on v a r i o u s c a t i o n and a n i o n exchange r e s i n s , a c t i v a t e d c h a r c o a l , z e o l i t e and s i l i c a g e l .  T h e i r purpose was  an attempt t o  f u r t h e r u n d e r s t a n d the bonding and environment o f t r a n s i t i o n i o n s i n unknown s u r r o u n d i n g s  on the b a s i s o f t h e i r EPR  spectra.  R u s s i a n workers c a r r i e d out s t u d i e s o f f r e e r a d i c a l s produced on s i l i c a g e l s u r f a c e s i n t h e e a r l y 1960's.  Hydrogen  atoms were produced by low t e m p e r a t u r e y - i r r a d i a t i o n o f t h e gel  silica  s u r f a c e , the atoms b e i n g produced from the adsorbed w a t e r m o l e c u l e s  o r from t h e s u r f a c e h y d r o x y l groups [80-82].  The  hydrogen  atoms formed were s t a b i l i z e d on the s u r f a c e and the i n f l u e n c e o f the s u r f a c e on the EPR  parameters s t u d i e d .  f i n e s p l i t t i n g was  found t o agree c l o s e l y w i t h v a l u e s f o r a f r e e  hydrogen atom a l t h o u g h  The magnitude o f the hyper-  the w i d t h o f the components v a r i e d w i t h  n a t u r e o f the s u r f a c e under s t u d y .  T h i s s u g g e s t s t h a t the  o f the atoms t o t h e s u r f a c e must o c c u r w i t h o u t i n the s p i n d e n s i t y o f the u n p a i r e d  the  'binding'  s i g n i f i c a n t change  e l e c t r o n i n the atom.  A  d e f i n i t e i n t e r a c t i o n , however, between the s u r f a c e and t h e atom i s i n d i c a t e d by l i n e w i d t h v a r i a t i o n s depending on the type o f s u r f a c e . Accurate  q u a n t i t a t i v e a n a l y s i s o f t h e s e e f f e c t s was  not thought p o s s i b l  -55-  owing t o t h e l a c k o f r e l i a b l e d a t a c o n c e r n i n g o f such s o l i d s .  the surface s t r u c t u r e  Wide v a r i a t i o n s i n s u r f a c e p r o p e r t i e s e x i s t among  v a r i o u s s i l i c a g e l s [ 8 3 ] , t h e d i f f e r e n c e s b e i n g caused by c o n c e n t r a t i o n o f s u r f a c e h y d r o x y l groups and d i f f e r i n g degrees o f s u r f a c e or c r y s t a l l i n i t y .  regularity  O t h e r f r e e r a d i c a l s have been s t a b i l i z e d on  s i l i c a g e l s u r f a c e s , n o t a b l y e t h y l and m e t h y l [84-88],  i n each case  the r a d i c a l b e i n g formed on i r r a d i a t i o n o f adsorbed m o l e c u l e s on t h e surface. The  s t a b i l i z a t i o n o f f r e e r a d i c a l s at the surface o f s o l i d s  i s o f c o n s i d e r a b l e i n t e r e s t i n r e g a r d t o heterogeneous c a t a l y s i s and s u r f a c e s t r u c t u r e ; EPR t e c h n i q u e s to b o t h areas„  have p r o v i d e d v a l u a b l e  information  Benzene adsorbed on s i l i c a g e l , when i r r a d i a t e d w i t h  u l t r a v i o l e t l i g h t , produced p h e n y l and benzene c a t i o n dimer r a d i c a l s  r a d i c a l s , benzene c a t i o n r a d i c a l s [89].  Radiolys'is o f monocarboxylic  a c i d s adsorbed on s i l i c a g e l [90] has been s t u d i e d u s i n g t h e EPR technique  t o o b t a i n information concerning  the adsorbed s t a t e and.also  t h e r a d i c a l s produced i n  the nature o f t h e i r thermal  motion.  Monomeric and d i m e r i c c a t i o n r a d i c a l s have a l s o been observed i n Y - i r r a d i a t e d b e n z e n e - s i l i c a g e l systems [ 9 1 ] .  Other r a d i c a l ions >  have a l s o been formed by d i r e c t i n t e r a c t i o n o f adsorbates solid  [92-93.];.  with the  These a r e g e n e r a l l y produced as a r e s u l t o f e l e c t r o n  t r a n s f e r from t h e a d s o r b e n t t o t h e a d s o r b a t e h a v i n g  a high electron  affinity. Porous V y c p r g l a s s has a l s o p r o v i d e d a  convenient  s t a b i l i z i n g medium f o r f r e e r a d i c a l s , T u r k e v i t c h and F u j i t a [94] r e p o r t e d t h e s t a b i l i z a t i o n o f t h e methyl r a d i c a l a t room temperature  .  -56-  and s t u d i e d i t s r e a c t i v i t y w i t h v a r i o u s added g a s e s .  Further  studies  [95-97] o f the methyl r a d i c a l on porous g l a s s have been c a r r i e d out and r e s u l t s have i n d i c a t e d b o t h p h y s i c a l l y t r a p p e d r a d i c a l s and t h o s e which have i n t e r a c t e d w i t h s u r f a c e s i t e s . e x p l o r e the g e n e r a l u s e f u l n e s s  The  aim was  o f porous g l a s s as a f r e e r a d i c a l  h o s t and/or the r e l a t i o n s h i p between a p o s s i b l e c a t a l y s t and r a d i c a l host.  A novel type of methyl r a d i c a l trapped  small hyperfine coupling constant  the adsorbed r a d i c a l  free  i n porous  Vycor g l a s s at.77°K has r e c e n t l y been r e p o r t e d , h a y i n g  methyl r a d i c a l , probably  to  an e x t r e m e l y  compared t o t h a t o f the  planar  i n d i c a t i n g a non-planar s t r u c t u r e f o r  [98].  As mentioned p r e v i o u s l y , workers i n the a r e a o f geneous c a t a l y s i s have e x t e n s i v e l y employed the EPR  hetero-  technique.  Knowledge o f the mechanisms o f heterogeneous c a t a l y s i s may  be  obtained  from i n v e s t i g a t i o n s o f the e l e m e n t a r y a c t s i n v o l v e d , and o f t h e s t r u c t u r e s and p r o p e r t i e s o f i n t e r m e d i a t e s The  taking part i n c a t a l y t i c reactions.  resonance s i g n a l can p r o v i d e e v i d e n c e as t o t h e n a t u r e  paramagnetic s p e c i e s on o r i n the s u r f a c e : and a l s o as t o s t r u c t u r e and c h e m i c a l  composition  o f the c a t a l y s t .  the  V a r i a t i o n s i n the  s i g n a l produced by d i f f e r e n t methods o f p r e p a r a t i o n and may  o f the  processing  a l s o p r o v i d e i n f o r m a t i o n on the c a t a l y s t s t r u c t u r e and the  o f the chemical  bonds formed on  adsorption.  A n o t h e r p o s s i b i l i t y o f a p p l y i n g EPR c a t a l y s i s problems i s a l s o a v a i l a b l e . o f chemical  nature  t o heterogeneous  T h i s would i n v o l v e the  study  r e a c t i o n s and o f the a d s o r p t i o n p r o c e s s w i t h a view  -57-  t o o b t a i n i n g s i g n a l s from l a b i l e i n t e r m e d i a t e p r o d u c t s on c a t a l y t i c surface.  Petcherskaya  et a l [99] have shown t h e  method, t o be a p p l i c a b l e i n i n v e s t i g a t i o n s o f p r o p e r t i e s , chemical  composition  various oxide c a t a l y s t s . been p u b l i s h e d  5.1.2  the EPR  crystalline  and e l e c t r o n i c p r o p e r t i e s o f  S i m i l a r s t u d i e s and r e v i e w s t h e r e o n have  [100,101],  Z e o l i t e Adsorbents. Various  mainly  due  z e o l i t e s have found w i d e s p r e a d use as a d s o r b e n t s  t o the c r y s t a l l o g r a p h i c a l l y w e l l - d e f i n e d s t r u c t u r e and  a l s o t o some knowledge o f the e l e c t r o n i c p r o p e r t i e s o f the As mentioned i n an e a r l i e r c h a p t e r , , a  very important  surface.  characteristic  o f z e o l i t e s i s t h a t i t i s p o s s i b l e t o v a r y the e l e c t r o n i c s t r u c t u r e o f the s u r f a c e by a s i m p l e s u b s t i t u t i o n p f v a r i o u s c a t i o n s d i f f e r e n t s i z e s and The  of  charge w h i l e the l a t t i c e remains unchanged.  l o c a t i o n o f the c a t i o n s can be assumed t o be the same, p r o v i d e d  t h e r e i s not a l a r g e s i z e d i f f e r e n c e .  S t a m i r e s and T u r k e v i t c h  s t u d i e d y - i r r a d i a t e d s y n t h e t i c z e o l i t e s v a r y i n g the S i / A l  ratio.  Most o f the d e f e c t s produced are p a r a m a g n e t i c c e n t e r s and  EPR  [102]  has proved u s e f u l i n p r o v i d i n g i n f o r m a t i o n about the t y p e o f d e f e c t and i t s environment.  The  same a u t h o r s [103]  also studied  a d s o r p t i o n o f a number o f m o l e c u l e s on these z e o l i t e s . charge-transfer  Electron  complexes were found when m o l e c u l e s w i t h low  p o t e n t i a l s were adsorbed. the z e o l i t e s as a c c e p t o r s  The  purpose of. the s t u d y was  the  ionization  t o examine  i n r e a c t i o n s o f t h i s t y p e , and because o f  t h e i r c r y s t a l l i n e s t r u c t u r e , show the e x i s t e n c e o f w e l l - d e f i n e d  -58-  electron accepting s i t e s i n the l a t t i c e . S t u d i e s o f r a d i c a l c a t i o n s formed on t h e a d s o r p t i o n o f a r o m a t i c hydrocarbons on z e o l i t e s ' a r e a l s o p r e s e n t ( f o r example..[104]).  R a d i c a l s produced by i r r a d i a t i o n p f adsorbed  s p e c i e s have a l s o been i n v e s t i g a t e d . mesitylene  Electron irradiation of  adsorbed on 13X produced s e v e r a l r a d i c a l s  Adsorbed 0^.  [105].  s p e c i e s on v a r i o u s Y type z e o l i t e s have a l s o been  s t u d i e d ( f o r example [106,107]). has been t r a p p e d  1  The h i g h l y r e a c t i v e m e t h y l r a d i c a l  i n a z e o l i t e m a t r i x and s t a b i l i z e d f o r l o n g p e r i o d s  at temperatures below 90°K [ 1 0 8 ] .  The r a d i c a l was generated  Y - i r r a d i a t i o n o f methane on type A z e o l i t e . was  i n the l i t e r a t u r e  by  The f r e e r a d i c a l NO^  produced by t h e d i r e c t r e a c t i o n o f NO^ and atomic oxygen  and t r a p p e d w i t h i n t h e s i e v e c a v i t i e s o f 13X [ 1 0 9 ] . :  The c a t a l y t i c p r o p e r t i e s p f m o l e c u l a r - s i e v e  been r e c o g n i z e d  z e o l i t e s have  f o r many y e a r s , b u t i n t e n s i v e i n v e s t i g a t i o n has g o t t e n  under way o n l y w i t h i n t h e l a s t two decades.  Z e o l i t e s are suggested  as c a t a l y s t s i n such r e a c t i o n s as c r a c k i n g i s o m e r i z a t i o n and a l k y l a t i o n [110].  EPR can be used f o r s t r u c t u r a l d e t e r m i n a t i o n s o f  t h e c a t a l y s t s , which h e l p s t o i d e n t i f y t h e c a t a l y t i c 5.2  Special Adsorption  centers.  Effects.  P h y s i c a l adsorption i s a r e v e r s i b l e process  and m o l e c u l e s  so adsorbed may be e a s i l y removed from t h e s u r f a c e b y pumping. Chemisbrption  u s u a l l y i n v o l v e s s t r o n g e r f o r c e s and i s o f t e n .  i r r e v e r s i b l e a t moderate t e m p e r a t u r e s .  Weak c h e m i s p r p t i o n  i n d i s t i n g u i s h a b l e from p h y s i c a l a d s o r p t i o n .  i s often  Perturbation of the  -59T  adsorbed  m o l e c u l e s , d i s t i n c t from a c h e m i c a l r e a c t i o n between t h e  s u r f a c e and a d s o r b a t e , i s g e n e r a l l y c o n s i d e r e d t o be a p h y s i c a l adsorption process. The r o t a t i o n a l freedom o f p h y s i c a l l y adsorbed an i m p o r t a n t f a c t o r t o be c o n s i d e r e d . temperature,  t h e adsorbed  molecules i s  Depending on t h e a d s o r b i n g  m o l e c u l e may have no a x i s o f f r e e r o t a t i o n ,  p o s s i b l y f r e e r o t a t i o n about an a x i s p e r p e n d i c u l a r t o t h e s u r f a c e o r , even f r e e r o t a t i o n , about t h r e e : m u t u a l l y p e r p e n d i c u l a r axes.  The  p o s s i b i l i t y o f h i n d e r e d r o t a t i o n about any o r a l l t h e s e axes i s a l s o to.be c o n s i d e r e d and i n many cases appears t o be i m p o r t a n t . o r i e n t a t i o n p f t h e adsorbed  molecules  i s . a l s o o f importance.  depends on t h e v a r i o u s a d s o r p t i o n f o r c e s , p r e s e n t i n a g i v e n adsorbent  system.  The This adsorbate-  I f t h e s u r f a c e o r c a v i t i e s o f t h e adsorbent  are c o n s i d e r e d as t h e h o s t m a t r i x t o t h e adsorbed  molecules  then i t i s  C l e a r t h e m a t r i x can have a pronounced e f f e c t on t h e m o l e c u l e .  This  n e c e s s a r i l y a f f e c t s t h e EPR spectrum and i t may p o s s i b l y a f f e c t t h e spectrum r e c o r d e d by any o t h e r s p e c t r o s c o p i c t e c h n i q u e .  These w i l l be  termed m a t r i x i n t e r a c t i o n s and w i l l be d i s c u s s e d i n more d e t a i l when t h e e x p e r i m e n t a l r e s u l t s a r e i n t e r p r e t e d . Other a d s o r p t i o n e f f e c t s need a l s o t o be c o n s i d e r e d . E l e c t r o s t a t i c f o r c e s p l a y an i m p p r t a n t r o l e , i n p h y s i c a l a d s o r p t i o n . e q u a t i o n s o f e l e c t r o s t a t i c s , g i v e n by e q u a t i o n s  (2-11) and (2-12) i n  Chapter Two, may be a p p l i e d t o t h e a d s o r p t i o n o f gases on z e o l i t i c molecular sieves.  The s e p a r a t i o n o f m o l e c u l e s by these s i e v e s i s  due i n l a r g e p a r t , n o t t o t h e s i z e o f t h e m o l e c u l e s , b u t by e l e c t r o s t a t i c f o r c e s between t h e a d s o r b a t e and t h e s t r o n g e l e c t r i c  The  -60-  f i e l d s present  i n the s i e v e s .  be d i s c u s s e d at  I t i s these e l e c t r i c f i e l d s which w i l l  present.  K i n g and Benson [ 1 1 1 ] , i n e x p l a i n i n g the low  temperature  a d s o r p t i o n o f the hydrogen i s o t o p e s on a l u m i n a , have shown t h a t the adsorbent has v e r y s t r o n g s u r f a c e e l e c t r i c f i e l d s , over v a r i o u s s i t e s on the s u r f a c e . equations  (2-11) and  They have s u c c e s s f u l l y used  (2-12) f o r t h e i r r e s u l t s .  found t o a r i s e from normal s t r u c t u r a l v a c a n c i e s l a t t i c e s , v a c a n c i e s wh^ch were p r e s e n t neutrality.  These same authors  distributed  The  f i e l d s were,  i n the  to maintain  crystal  electrical  a l s o found e v i d e n c e [112]  that  e l e c t r o s t a t i c i n t e r a c t i o n s a l s o p l a y a dominant r o l e i n the p h y s i c a l a d s o r p t i o n o f gases on z e o l i t e s . hydrogen c o u l d be s e p a r a t e d  I t was  found t h a t o- and  on s y n t h e t i c z e o l i t e s 5A and  13X.  pr  In t h i s  case, s e p a r a t i o n must be r e l a t e d t o some type o f h i n d e r e d r o t a t i o n s i n c e these s p e c i e s d i f f e r o n l y i n r o t a t i o n a l energy.  Strong  e l e c t r q s t a t i c f o r c e s can produce such l a r g e b a r r i e r s t o r o t a t i o n . The  o r i g i n o f these e l e c t r i c f i e l d s was  then i n v e s t i g a t e d .  I t was  found t h a t the c a t i o n s , because o f t h e i r l o c a l uncompensated charge, produce v e r y s t r o n g e l e c t r i c f i e l d s and t h e s e c a t i o n s s e r v e as adsorption s i t e s . s i e v e s ' a r e due present  i n the The  Thus the s i e v i n g p r o p e r t i e s o f these  o n l y i n p a r t t o the s i z e o f the cages and  the  'molecular channels  lattice. c a t i o n d e n s i t y o f the u n i t c e l l o f z e o l i t e s can  v a r i e d s y s t e m a t i c a l l y by v a r y i n g the AIO^ SiO^ content between w e l l - d e f i n e d l i m i t s .  be  c o n t e n t w i t h r e s p e c t t o the Removal o r s u b s t i t u t i o n  o f the c a t i o n s can a l s o c r e a t e changes i n the s i t e s a v a i l a b l e f o r  -61-  adsorption.  Rabo e t a l [113] s t u d i e d t h e e f f e c t o f t h e c a t i o n on t h e  c a t a l y t i c a c t i v i t y o f v a r i o u s s y n t h e t i c z e o l i t e s by comparing t h e sodium form, t h e c a l c i u m form and t h e d e c a t i o n i z e d form o f t h e zeolites.  They found a p o s i t i v e r e l a t i o n s h i p between t h e number o f  c a t i o n s i t e s and t h e c a t a l y t i c a c t i v i t y .  F u r t h e r r e p o r t s by these  and o t h e r workers have s u b s t a n t i a t e d t h e importance o f t h e s e l a r g e e l e c t r o s t a t i c f i e l d s o f t h e c a t i o n s as a d s o r p t i o n and c a t a l y t i c [114,115],  centers  The p o l a r i z a t i o n o f t h e adsorbed m o l e c u l e s by t h e  e l e c t r o s t a t i c f i e l d s has a l s o been suggested as b e i n g w i t h the c a t a l y t i c a c t i v i t y o f the z e o l i t e s .  associated  C a l c u l a t i o n s by  H o i j t i n k [116,117] on.the p o l a r i z a t i o n o f a r o m a t i c m o l e c u l e s i n a l i n e a r e l e c t r i c f i e l d give support t o t h i s hypothesis. and B a r r e r  Gibbons  [118,119] have c a l c u l a t e d t h e e l e c t r o s t a t i c energy  c o n t r i b u t i o n s t o adsorption energies  f o r m o l e c u l e s w i t h b o t h d i p o l e and  quadrupole moments f o r v a r i o u s c a t i o n exchanged z e o l i t e s .  I t was  t h u s ^ p o s s i b l e t o see t h e e f f e c t o f s i z e and charge o f t h e c a t i o n s on these  energies. Adsorption  according  t o t h e s e e l e c t r o s t a t i c models o f  i n t e r a c t i o n between t h e a d s o r b a t e and t h e s t r o n g e l e c t r i c present  fields  i n t h e z e o l i t e s can a l s o p r e d i c t the p r e f e r r e d o r i e n t a t i o n •  o f t h e m o l e c u l e s on t h e s u r f a c e .  The e l e c t r i c f i e l d normal t o t h e  s u r f a c e w i l l a c t i n t h e d i r e c t i o n o f g r e a t e s t p o l a r i z a b i l i t y and cause t h i s t o be. t h e p r e f e r r e d o r i e n t a t i o n .  P o l a r molecules should  then  be r e a d i l y o r i e n t e d by t h e i n t e r n a l f i e l d s o f t h e s o l i d and one s h o u l d be a b l e t o p r e d i c t t h i s o r i e n t a t i o n . The m o l e c u l e s would a l s o be assumed t o e x e c u t e s m a l l o s c i l l a t i o n s about an e q u i l i b r i u m p o s i t i o n with respect to the surface.  -62-  CHAPTER SIX  :  EXPERIMENTAL 6.1  Vacuum System. The m a t e r i a l s used i n t h i s study were h a n d l e d i n a  p y r e x g l a s s vacuum system c o n s t r u c t e d i n t h e C h e m i s t r y department g l a s s b l p w i n g shop a t U.B.C.  T e f l o n stopcocks  with  'viton'  C u r i n g s were used i n t h e gas h a n d l i n g p a r t so as n o t t o i n t r o d u c e i m p u r i t i e s v i a r e a c t i o n w i t h any stopcock  grease.  The s t o p c o c k s  were manufactured by Ace G l a s s I n c o r p o r a t e d , V i n e l a n d , New J e r s e y . Where g r e a s e was n e c e s s a r y ,  a Haloflurocarbon  KEL-F #90 g r e a s e ,  o f 3M Company, was u s e d .  a product  lubricant,  u n r e a c t i v e t o most c o r r o s i v e o r r e a c t i v e c h e m i c a l s .  KEL-F i s q u i t e Pumping was v i a  a 'Veecb' o i l d i f f u s i o n pump backed by a Welsch Duo S e a l r o t a r y pump. The u l t i m a t e vacuum was o f t h e o r d e r o f 10 ^ t o r r .  Both an NRC  Thermocouple vacuum gauge and an NRC I o n i z a t i o n gauge were used as  -63-  p r e s s u r e measuring  devices.  c o n j u n c t i o n w i t h t h e s e gauges. given i n f i g u r e  6.2  An NRC  Model 831 d e t e c t o r was used i n  A diagram o f t h e vacuum system i s  13a.  Sample Tubes. F i g u r e }3b shows t h e sample tubes used i n the  Quartz t u b i n g o f 4 mm.  experiments.  o u t e r d i a m e t e r was used f o r the p a r t o f the  tube t o be p l a c e d i n the EPR s p e c t r o m e t e r .  The d i a m e t e r  was  d e t e r m i n e d by t h e s i z e o f the l i q u i d n i t r o g e n dewar t o be used f o r low temperature  experiments.  A t e f l o n s t o p c o c k was used h e r e  t o p r e v e n t any. p o s s i b l e r e a c t i o n s o f the sample w i t h g r e a s e .  also Glass  wool p l u g s were p l a c e d above t h e sample and i n t h e c o n s t r i c t i o n t o p r e v e n t p o s s i b l e s c a t t e r i n g o f t h e sample d u r i n g e v a c u a t i o n . c y l i n d r i c a l f u r n a c e was f u r n a c e was  6.3  A small  used which f i t around the sample t u b e .  c a p a b l e o f temperatures  The  i n e x c e s s p f 673°K.  Adsorbents. The L i n d e D i v i s i o n o f t h e Union C a r b i d e C o r p o r a t i o n k i n d l y  s u p p l i e d samples o f the s y n t h e t i c z e o l i t e s 4A, 5A, 10X and  13X.  The samples were w h i t e powders,and d i d not have any added b i n d e r s . The u s u a l commercial  form o f t h e s e z e o l i t e s i s p e l l e t s o f v a r i o u s s i z  and a c l a y b i n d e r i s added t p f a c i l i t a t e the m o l d i n g .  As  an  a d s o r p t i o n m a t e r i a l , the b i n d e r i s r e l a t i v e l y i n e r t but may unknown i m p u r i t i e s [ 1 2 0 ] .  G e n e r a l l y , the p e l l e t s are a p p r o x i m a t e l y  15 p e r cent b i n d e r , so these s p e c i a l samples were r e q u e s t e d . number f o r the 4A i s 470017; t h e 5A, M580031; 10X, 1370014.  :  .  introduce  The l o t  1080001; and  13X,  -64-  a £ a  a  in c a o  Q. >>  3 O +->  £  ^ "O  o  L.  teflon stopcock  cn (TJ  o "5  o  a) or <3; Z3  rJ  CT)  C  o  o  0)  to  7s L.  a.  -o-  L.  £ o  c m  q r a d e d seal  / CL) C  £  glass- wool  u O u a  "glass wool 4  (a)  m m o.d. (b)  FIGURE .1.3. a) A s c h e m a t i c diagram o f the vacuum system used' i n t h e s e e x p e r i m e n t s . b) The sample tubes used i n t h e s e e x p e r i m e n t s .  -65-  Th e s y n t h e t i c m o r d e n i t e s ( Z e o l o n s ) were s u p p l i e d by the  N o r t o n Company o f W o r c e s t e r , Mass., i n t h e same form as were t h e  Linde products.  The l o t number o f t h e sodium m o r d e n i t e i s  HB 79-80E and t h a t f o r t h e hydrogen m o r d e n i t e , HB 91-92E. The s i l i c a g e l samples used were o f a t h i n l a y e r s o r b e n t marketed by t h e M a l l i n c k r o d t Chemical Works. is  chromagraphic  The brand name  S i l l C A R TLC-7GF.  The i o n s i n t h e L i n d e s y n t h e t i c z e o l i t e s were exchanged f o l l o w i n g s t a n d a r d p r o c e d u r e s .  S h e r r y [121] g i v e s an  account o f t h e exchange p r o p e r t i e s o f v a r i o u s z e o l i t e s and a l s o d e s c r i b e s t h e c o n d i t i o n s r e q u i r e d f o r a number o f s p e c i f i c 6.4  exchanges.  Sample P r e p a r a t i o n . Sample tubes c o n t a i n i n g t h e a d s o r b e n t s were degassed  for  a p e r i o d o f g e n e r a l l y 4-6 hours a t a t e m p e r a t u r e o f a p p r o x i m a t e l y  523°K a t a p r e s s u r e o f l e s s than 5x10*"^ t o r r .  T h i s p e r i o d was  s u f f i c i e n t t o remove any water from t h e a d s o r b e n t s .  The gases t o be  s t u d i e d were then adsorbed onto t h e s u r f a c e s a t room t e m p e r a t u r e for  several minutes.  The p r e s s u r e o f gas adsorbed v a r i e d f o r t h e  d i f f e r e n t systems and w i l l be g i v e n i n each a p p r o p r i a t e s e c t i o n . EPR s p e c t r a were r e c o r d e d a t 77°K on t h e s p e c t r o m e t e r s to be d e s c r i b e d l a t e r i n t h i s c h a p t e r .  A V a r i a n V-4546  liquid  n i t r o g e n dewar, shown i n f i g u r e 14b, was used f o r t h e low t e m p e r a t u r e studies.  The dewar was f a b r i c a t e d e n t i r e l y from s e l e c t e d q u a r t z t o  pass u l t r a v i o l e t l i g h t w i t h a minimum o f background  signals.  -66-  7  ^  5 m m i.d;  11mm o.d.  im  •11 m m  (a) •FIGURE 14. a) Q u a r t z dev/ar used f o r v a r i a b l e EPR e x p e r i m e n t s .  (b) temperature  b) A V a r i a n V-4546 l i q u i d n i t r o g e n dev/ar.  i.d.  o.d.  -67-  V a r i a b l e temperature experiments were performed u s i n g a s p e c i a l l y d e s i g n e d q u a r t z dewar, shown i n f i g u r e 14a.  Dry n i t r o g e n  gas was p a s s e d t h r o u g h a heat exchanger p l a c e d i n a l a r g e dewar f i l l e d w i t h a c o o l a n t such as l i q u i d n i t r o g e n o r a d r y i c e - a c e t o n e mixture.  The gas was c o o l e d t o t h e approximate t e m p e r a t u r e o f t h e  c o o l a n t and passed through t h e q u a r t z dewar, c o o l i n g t h e sample. The temperature a t t h e sample was c o n t r o l l e d by t h e r a t e o f f l o w o f the gas and was measured u s i n g a c o p p e r - c o n s t a n t a n thermocouple. The wave g u i d e n e a r t h e c a v i t y was kept f r e e from condensed m o i s t u r e by p a s s i n g d r y n i t r o g e n gas t h r o u g h  6.5  it.  Gases.  6.5.1.  C h l o r i n e D i o x i d e ClO^. The c h l o r i n e d i o x i d e used i n t h i s s t u d y was k i n d l y  s u p p l i e d by P r o f e s s o r F. Aubke o f t h i s U n i v e r s i t y .  In h i s method  o f p r e p a r a t i o n [ 1 2 2 ] , a m i x t u r e o f 12.2 gm. o f p o t a s s i u m c h l o r a t e , 10 gm. o x a l i c a c i d and a c h i l l e d s o l u t i o n o f 10.8 gm. o f c o n c e n t r a t e d s u l f u r i c a c i d i n 40 m l . o f water was s l o w l y h e a t e d oh a steam b a t h ( t h e mole r a t i o KCIO^ : H ^ C ^ - 2 H 0 : H S 0 2  2  4  was  1:0-8:1.1)  The r e a c t i o n i s c h a r a c t e r i z e d by t h e f o l l o w i n g e q u a t i o n :  2KC10  3  + 2H S0  The C 1 0  2  2  + H C 0 - 2 H 0 -> 2C1C> + 2CC> + 4H 0 + 2KHS04.  4  and C 0  2  4  2  2  2  2  (6-1)  produced were passed t h r o u g h a P ^ d r y i n g tube and U  2  c o o l e d t o 195°K. the CO,,.  2  2  Pumping on t h e sample a t t h i s temperature removed  F u r t h e r p u r i f i c a t i o n was a c h i e v e d t h r o u g h a t r a p t o t r a p  d i s t i l l a t i o n from 195°K t o 77°K.  The C 1 0  2  was s t o r e d i n a d r y i c e -  -68-  t r i c h l o r o e t h y l e n e b a t h a t 195°K. „  6.5.2.  N i t r o g e n . D i o x i d e NC^. The n i t r o g e n d i o x i d e used i n t h i s s t u d y was p u r c h a s e d  from Matheson o f Canada, L i m i t e d . c o n t a i n e r s was > 99.5 p e r c e n t .  The p u r i t y o f t h e gases i n t h e  F u r t h e r p u r i f i c a t i o n was a c h i e v e d  by pumping on t h e s o l i d i f i e d gas i n a c o n t a i n e r immersed i n a d r y ice-acetone bath.  6.5.3  The p u r i f i e d gas was s t o r e d i n a g l a s s sample b u l b .  N i t r i c Oxide NO. 14 The n i t r i c o x i d e ,  NO, was p u r c h a s e d from Matheson o f  Canada, L i m i t e d , and t h e r e p o r t e d p u r i t y i s > 98.5 p e r c e n t . was passed through  The gas  a t r a p immersed i n an i s o p e n t a n e - p e n t a n e - d r y  bath at approximately  ice  133°K and s t o r e d i n a g l a s s sample b u l b .  F u r t h e r p u r i f i c a t i o n was a c h i e v e d by pumping on t h e sample a t l i q u i d nitrogen  temperature. The  ^-N s u b s t i t u t e d sample o f n i t r i c o x i d e was p u r c h a s e d  from t h e Isomet C o r p o r a t i o n , New J e r s e y and had a r e p o r t e d p u r i t y o f > 99.3 p e r c e n t 6.5.4  1 5  N0.  Tetrafluorohydrazine ^ F ^ . The  t e t r a f l u o r o h y d r a z i n e used i n t h i s s t u d y was purchased  from A i r P r o d u c t s  and Chemicals I n c o r p o r a t e d , P e n n s y l v a n i a .  The  r e s e a r c h grade gas had a r e p o r t e d p u r i t y o f > 99 p e r c e n t and was used d i r e c t l y from t h e c o n t a i n e r w i t h o u t any f u r t h e r p u r i f i c a t i o n .  -69-  6.6  Spectrometers. The m a j o r i t y o f t h e measurements were c a r r i e d o u t  on a V a r i a n E-3 X-band EPR s p e c t r o m e t e r system.  The o p e r a t i n g  f r e q u e n c y o f t h e k l y s t r o n i s 8.8 t o 9.6 GH^, t r a n s m i t t e d t o t h e c a v i t y t h r o u g h a 4-port c i r c u l a t o r .  The magnetic  f i e l d of this  system was s u p p l i e d by a f o u r i n c h e l e c t r o m a g n e t h a v i n g a u s a b l e a i r gap o f 1.2 i n c h e s , c a p a b l e o f homogeneous magnetic excess o f 6 k i l o g a u s s . 70 mG l i n e s .  fields i n  Homogeneity was such as t o a l s o r e s o l v e  The m a g n e t i c  f i e l d was modulated  t h r o u g h a 100 kH Li  f i e l d m o d u l a t i o n u n i t i n t h i s system.  A V a r i a n E-4531 c a v i t y o f  r e c t a n g u l a r mode TEJQ2 was used f o r t h e e x p e r i m e n t s .  This  s p e c t r o m e t e r system has a c a l i b r a t e d f i e l d c o n t r o l and a l s o c a l i b r a t e d f r e q u e n c y and power m e t e r s .  The magnetic  was measured as a f u n c t i o n o f t h e p r o t o n resonance w h i l e b e i n g f r e q u e n c y modulated  field  i n an NMR  probe,  by a magnetometer c o n s t r u c t e d by t h e  C h e m i s t r y Department E l e c t r o n i c s Shop.  The magnetometer o u t p u t was  d i s p l a y e d on an H e w l e t t - P a c k a r d 5245L f r e q u e n c y c o u n t e r . was  intensity  This counter  a l s o used t o measure t h e microwave f r e q u e n c y o f t h e k l y s t r o n by  means o f a 5255A f r e q u e n c y c o n v e r t e r .  A b l o c k diagram o f t h i s EPR  system i s shown i n f i g u r e 15. A p o r t i o n o f t h e e x p e r i m e n t a l s p e c t r a was r e c o r d e d u s i n g a V a r i a n V-4500 100 kH^ EPR s p e c t r o m e t e r m o d i f i e d by t h e C h e m i s t r y Department E l e c t r o n i c s Shop.  The o p e r a t i n g f r e q u e n c y i s about 9 GH .  A s t a n d a r d V a r i a n r e c t a n g u l a r c a v i t y , model V-4531 was used w i t h t h i s spectrometer.  A maximum f i e l d o f about 9 k i l o g a u s s was a t t a i n a b l e  from t h e V a r i a n V-4012A 12 i n c h magnet h a v i n g a 2.5 i n c h gap between  -70-  MICROWAVE BRIDGE CONSOLE POWER SUPPLY E-007  DKTECTOR CRYSTAL  1  KLYSTRON AND  |  CIRCULATOR  j  EX-100  100  AFC  kHz  M O D E-201  RECEIVER  [ TRANS.  I p  R E  EPR SIG.  . A M P  I  T  (THRU FUNCTION SWITCH)  TO CONSOLE UNITS  100  WAVEGUIDE  kHz  MODULATION  9 . 5 GHz P O W E R F O R EPR SAMPLE EXCITATION EPR SIGNAL (PHASE D E T . ) REFLECTION FROM EPR S A M P L E IN C A V I T Y  MAG.  POLE  FUNCTION MOD.  SWITCH  E-205  COIL  4—  (2)  EXT.REC.  «cj 30  Hz  + 100  kHz  SUMMATION  U  1  FIELD SIGNAL  A 3 0 Hz S A W T O O T H  SIGNAL  TO M A G N E T COILS  EPR SIG.  A  MAGNET POWER SUPPLY . E-005  MAGNETIC FIELD CONTROL E-202  OSCILLOSCOPE CHECKOUT E-200  FIELD SCAN  FIGURE.15. B l o c k d i a g r a m o f a V a r i a n E-3 X-band Spectrometer, system.  RECORDER E-80  EXT. REC.  -71-  the p o l e s . performed  Frequency  and f i e l d i n t e n s i t y measurements were  as p r e v i o u s l y d e s c r i b e d . F u r t h e r measurements were c a r r i e d out on a s p e c t r o m e t e r  s i m i l a r t o t h a t j u s t d e s c r i b e d , a l t h o u g h a V a r i a n V-3900 magnet capable o f 15 k i l o g a u s s s u p p l i e d t h e magnetic  field.  A  V a r i a n V-2501 F i e l d a i l Mark I I M a g n e t i c F i e l d R e g u l a t o r the magnetic essentially  field.  controlled  The remainder o f t h e s p e c t r o m e t e r was  the same.  -72-  ' CHAPTER-SEVEN •  ANALYSIS OF ELECTRON PARAMAGNETIC RESONANCE SPECTRA The  t a s k o f a s s i g n i n g n u m e r i c a l v a l u e s t o t h e parameters  i n t h e s p i n H a m i l t o n i a n g i v e n by e q u a t i o n c a s e s , be q u i t e f o r m i d a b l e .  (4-10) c a n , i n some  The c o n s i s t e n c y o f t h e a s s i g n e d  must be checked t h r o u g h , g e n e r a l l y by means o f a t h e o r e t i c a l  values calculation.  V a r i o u s methods used f o r the c a l c u l a t i o n o f resonance f i e l d s have been r e v i e w e d by Swalen and Gladney [ 1 2 3 ] , and some computer programs a v a i l a b l e t o t h i s end a r e d i s c u s s e d .  Gladney [123,124]  has w r i t t e n a program w h i c h , though r e s t r i c t e d , i s g e n e r a l l y a p p l i c a b l e t o many EPR problems. p u b l i s h e d on t h e s u b j e c t  Many papers have s i n c e been  [125-128].  A c o m p l e t e l y g e n e r a l and e x t r e m e l y v e r s a t i l e method o f resonance f i e l d c a l c u l a t i o n has r e c e n t l y been p u b l i s h e d  [129].  A method i s proposed f o r c a l c u l a t i n g EPR t r a n s i t i o n f i e l d s f o r a g e n e r a l s p i n H a m i l t o n i a n w i t h no r e s t r i c t i o n s .  The method has a l s o  -73-  been supplemented w i t h the The  c a l c u l a t i o n of t r a n s i t i o n p r o b a b i l i t i e s .  i n c l u s i o n o f a l i n e s h a p e t o the  positions  enables one The  involves  to s i m u l a t e EPR  spectra.  spectrum o f a p o l y c r y s t a l l i n e or powder sample  a s p a t i a l average over d i f f e r e n t o r i e n t a t i o n s  with respect fields  EPR  c a l c u l a t e d resonance f i e l d  to the d i r e c t i o n o f the magnetic f i e l d , the  f o r each o r i e n t a t i o n b e i n g c a l c u l a t e d u s u a l l y by  aforementioned methods. s o l v i n g the  s p i n problem d e s c r i b e d  by  be  [130]  g i v e n f o r completeness.  simple method to s o l v e  McClung  i n the  n e g l e c t e d and  the g and  lattice.  hyperfine  g i v e n f o r a paramagnetic s p e c i e s I.  The  t e n s o r s are  i n the  of  the  for the  spectra  will  recently published  a  orthorhombic  Hamiltonian i s  sense t h a t n u c l e a r Zeeman and  simultaneously diagonalized  nuclear spin  has  o f EPR  a s p i n H a m i l t o n i a n f o r an  paramagnetic' c e n t e r i n a r i g i d restricted  one  a s p i n H a m i l t o n i a n and  simulation  spins  resonance  A b r i e f summary o f a method g i v e n  a p p l i c a t i o n o f these r e s u l t s to the  the  o f the  quadrupole terms  assumed to  same a x i s system.  be  The  w i t h e l e c t r o n s p i n S=h  are  solution is  and  one  Although somewhat l e s s g e n e r a l i n a p p l i c a t i o n ,  technique i s i n s t r u c t i v e . The  spin Hamiltonian i s then  | T H  where the are not  +  h S-T-I_  symbols have been d e f i n e d  necessarily  whereas (7-1)  (7-1)  previously.  Experimental  data  c o l l e c t e d i n a m o l e c u l a r c o o r d i n a t e frame,  operates i n a m o l e c u l a r frame.  We  must r e f e r t o some  ^74-  experimental  o r l a b o r a t o r y frame o f r e f e r e n c e .  A ' t e n s o r frame'  i s d e f i n e d as t h e a x i s system i n which t h a t t e n s o r i s d i a g o n a l . For our s p i n H a m i l t o n i a n , t h e g t e n s o r frame i s t a k e n as t h e molecular  frame.  F o r t h i s example, t h e T t e n s o r i s a l s o  d i a g o n a l i n t h e m o l e c u l a r frame.  Generally, Euler  coordinate  t r a n s f o r m a t i o n s a r e used t o reduce t h e frames t o t h e form wanted for  stydy.  These t r a n s f o r m a t i o n s c o r r e s p o n d  v e c t o r networks by t h e a p p r o p r i a t e a n g l e s . case when the m o l e c u l e  t o r o t a t i o n of the T h i s w i l l o f t e n be t h e  i s i n a h o s t m a t r i x such as a c r y s t a l and  i t s molecular coordinates are i m p l i c i t l y defined w i t h respect to  t h e c r y s t a l axes.  Appropriate transformations are also required  when t h e g and h y p e r f i n e t e n s o r frames a r e n o t c o i n c i d e n t .  In t h i s  case, g e n e r a l l y t h e g t e n s o r frame i s taken as t h e m o l e c u l a r and t h e h y p e r f i n e t e n s o r frame t r a n s f o r m e d  accordingly.  McLung uses t h e f o l l o w i n g t e c h n i q u e Hamiltonian.  frame  to solve the  The e l e c t r o n s p i n o p e r a t o r , S_, i s q u a n t i z e d  the d i r e c t i o n o f g-H t o a l l o w e x a c t treatment  along  o f t h e Zeeman term.  Q u a n t i z a t i o n o f t h e n u c l e a r s p i n o p e r a t o r I_, a l o n g t h e d i r e c t i o n o f T-S^j then l e a d s t o an e x p r e s s i o n f o r t h e h y p e r f i n e i n t e r a c t i o n which i s most s u i t a b l e f o r a p e r t u r b a t i o n t r e a t m e n t . r e l a t i o n s h i p s o f t h e frames used.  F i g u r e 16 shows t h e  G e n e r a l l y H_ i s a l a r g e s t a t i c  magnetic f i e l d a p p l i e d a l o n g t h e l a b o r a t o r y Z a x i s . l a b o r a t o r y frame and ( x , y , z ) , t h e m o l e c u l a r  frame.  (X,Y,Z) i s t h e The a n g l e s 8 and  <}> r e l a t e t h e p o s i t i o n o f t h e a p p l i e d magnetic f i e l d t o t h e g and T t e n s o r frames (the m o l e c u l a r  frame).  -75-  y  X  FIGURE 16. system.  The m o l e c u l a r and magnetic f i e l d c o o r d i n a t e  -76-  The  hyperfine  t e r m may g e n e r a l l y b e t r e a t e d a s a  p e r t u r b a t i o n which s p l i t s of t h e s p i n system.  Using  = .gBoHoMs  M M! S  +  I,  s  second-rprder p e r t u r b a t i o n  M >  levels functions  ,  x  theory  ™SMl  h  Zeeman  the zero-order basis  M >|  |S,  E  the electronic  [131] y i e l d s  - h V ^ T ^  the eigenvalues  [S ( S + l )  - Mg ]Mj ?  2g6 HoT 0  2 /2„2 2 „2 , - 2 I (g T - g T )sm 'zz^ a a zz &  &  J  cos  £ £ T  2 2 Jl ' 2 , 2 . 2 xx yy^ xx " V ' „ 2 2 £ g T (  +  g  g  T  S  i  n  .2 sin  2  <j) c o s (j>  h  2 2 MjMg  2  T  2 2 2 g T T a zz 2 2 g T-  +  +  6  b  ^o o H  2 2 2 g ^ T a x x yy 2 2 a &  2 2 2 2 2 2 2 g g g T (T -T ) xx yy zz zz^ xx yy^ 2 4 2 2 g g T T cr a  to  2  b  cos  2  2 2 0 s i n cj) c o s (f)  &  h [I(I 2  x  + 1) - M ] M 2  S  (7-2)  4gMo  -77-  g = Cg^sin  where  9  2  +  g^cos  2 2 g = (g cos <j> + "a xx •  T = (g T sin 2  2  2  9  T = (g T cos a xx xx 2  E,, ,, M ,M S  ,3 , Q  2  2  2  a  g yy  +  <J> +  (7-3)  2 ^ s i n cj>)  (7-4)  2  g^T  .  0)^  2  g  2 z  cos  2  T sin yy yy 2  2  .  0 ) ^  2  (JoVg ^ J  b  (7-5)  (7-6) • J  are the e i g e n v a l u e s o f t h e b a s i s f u n c t i o n s named above:  T  the Bohr magneton; H , the magnitude o f the a p p l i e d magnetic  f i e l d ; h, P l a n c k ' s c o n s t a n t , w i t h a l l o t h e r terms b e i n g p r e v i o u s l y defined.  In the l i m i t o f a x i a l symmetry, t h i s r e s u l t reduces t o t h a t  of Bleaney (132). . Once the magnetic f i e l d H i s computed f o r v a r i o u s v a l u e s o f 0 and cj) f o r t h e a p p r o p r i a t e Mg and M^ v a l u e s , i t i s p o s s i b l e t o s i m u l a t e t h e EPR spectrum.  An a p p r o p r i a t e l i n e shape must be  added t o each t r a n s i t i o n , u s u a l l y i n the form o f a G a u s s i a n o r a L o r e n t z i a n l i n e shape. such a program.  Mr. J.C. T a i t o f t h i s l a b o r a t o r y has w r i t t e n  D i f f e r e n t programs may be used t o g e n e r a t e the  resonance f i e l d s f o r use i n t h i s program, the c h o i c e b e i n g d e t e r m i n e d by the c o m p l e x i t y o f the problem. C o n s i d e r a b l e i n f o r m a t i o n i s a v a i l a b l e from powder s p e c t r a , even though the observed EPR spectrum i s a complex s u p e r p o s i t i o n o f l i n e s due t o a l l o r i e n t a t i o n s of. t h e randomly o r i e n t e d m o l e c u l e s . The major l o s s o f i n f o r m a t i o n i s d e r i v e d from the f a c t t h a t t h e o r i e n t a t i o n of t h e m o l e c u l e i n t h e h o s t cannot be determined from the  spectrum a l o n e .  L i n e - s h a p e s o f powder s p e c t r a have been d i s c u s s e d  -78-  by a number o f authors i n Chapter Four.  and r e f e r e n c e has been made t o some o f these  Assuming t h a t t h e g and h y p e r f i n e t e n s o r s a r e  d i a g o n a l i z e d i n t h e same frame and c o n s i d e r i n g , f o r t h e p r e s e n t , no magnetic n u c l e u s line-shapes.  i n the molecule,  f i g u r e 17 shows some g e n e r a l i z e d  F i g u r e 17a d e p i c t s t h e spectrum e x p e c t e d f o r a s p e c i e s  w i t h an a x i a l l y symmetric g t e n s o r , i . e . g The  = g  '= g l and g  = g//.  s o l i d l i n e denotes the i d e a l i z e d a b s o r p t i o n and t h e b r o k e n  a possible real absorption.  line  The r e a l a b s o r p t i o n , r e p r e s e n t e d by  some smoothly v a r y i n g l i n e - s h a p e , c o n s i s t s o f many sources o f broadening. for  F i g u r e 17b d e p i c t s t h e spectrum, b o t h i d e a l i z e d and r e a l ,  a species with a f u l l y a n i s o t r o p i c g tensor.  I t i s r e a d i l y seen  t h a t t h e s e complex l i n e shapes c o n t a i n a number o f s h a r p , observable  peaks.  These c o r r e s p o n d  readily  t o m o l e c u l e s which a r e o r i e n t e d  so t h a t t h e magnetic f i e l d l i e s a l o n g one o f t h e p r i n c i p a l axes o f the m o l e c u l e ,  t h e r e s u l t . b e i n g t h a t t h e components o f t h e g and  T t e n s o r s a r e r e a d i l y determined from t h e p o s i t i o n s o f t h e s e The  lines.  e x p l a n a t i o n l i e s i n t h e f a c t t h a t when t h e magnetic f i e l d  along one o f t h e m o l e c u l a r  lies  a x e s , t h e resonance f i e l d f o r t h e p a r t i c u l a r  t r a n s i t i o n under s t u d y i s a maximum o r a minimum w i t h r e s p e c t t o v a r i a t i o n s i n 9. and cj>.  T h i s r e s u l t s i n a p i l i n g - u p o f t h e number o f  randomly o r i e n t e d m o l e c u l e s whose resonance f i e l d s a r e i n t h e v i c i n i t y o f one o f t h e p r i n c i p a l axes, and causes an abrupt and r e a d i l y observable  change i n t h e i n t e n s i t y o f t h e EPR a b s o r p t i o n a t t h e s e p o i n t s . When:a m a g n e t i c n u c l e u s  i s present  i n the molecule, the  s p e c t r a are more c o m p l i c a t e d , b u t can g e n e r a l l y be a n a l y z e d o f t h e a b s o r p t i o n curves  just discussed.  Complications  i n terms .  may a r i s e  -79-  FIGURE 17. G e n e r a l i z e d li'neshapes o f powder EPR s p e c t r a f o r a s p e c i e s w i t h no h y p e r f i n e s t r u c t u r e . a) a x i a l l y symmetric g t e n s o r b) f u l l y a n i s o t r o p i c g t e n s o r  -80-  through  t h e appearance o f e x t r a l i n e s owing t o t h e o c c u r r e n c e o f  u s u a l l y f o r b i d d e n t r a n s i t i o n s , o r due t o s t a t i o n a r i t i e s caused when the g and h y p e r f i n e t e n s o r s t r y t o s h i f t t h e l i n e s i n o p p o s i t e directions. One can t h e n , under f a v o u r a b l e c o n d i t i o n s , d e t e r m i n e t h e components o f t h e g and h y p e r f i n e t e n s o r s from a powder EPR spectrum. In c o m p l i c a t e d i n s t a n c e s , approximate s t a r t i n g v a l u e s f o r t h e s e t e n s o r s . c o u l d p r o b a b l y be o b t a i n e d from t h e s p e c t r a , and more p r e c i s e d e t e r m i n a t i o n done by f i t t i n g a computer s i m u l a t e d spectrum t o t h e observed  spectrum.  Powder EPR s p e c t r a cannot p r o v i d e i n f o r m a t i o n  about t h e o r i e n t a t i o n  o f t h e p r i n c i p a l axes o f t h e g and h y p e r f i n e  t e n s o r s w i t h r e s p e c t t o t h e m o l e c u l a r axes, n o r can i n f o r m a t i o n be o b t a i n e d about which component i s a s s o c i a t e d w i t h a s p e c i f i c axis.  T h i s must be determined by comparison w i t h t h e o r e t i c a l  o f these q u a n t i t i e s a l o n g t h e v a r i o u s m o l e c u l a r  axes.  molecular estimates  CHAPTER EIGHT  CHLORINE DIOXIDE,  ClCy  C h l o r i n e d i o x i d e i s one o f t h e few s t a b l e gases t h a t i s paramagnetic i n i t s normal c h e m i c a l c h l o r i n e atom i s I=2~«  u  u  e  i n  state.  The n u c l e a r s p i n o f t h e  p a r t perhaps t o i t s e x t r e m e l y h i g h  r e a c t i v i t y , i n v e s t i g a t i o n s u s i n g EPR t e c h n i q u e s extensive.  have ° t been n  S e v e r a l y e a r s ago, Bennett and Ingram [133] r e p o r t e d  the spectrum o f C 1 0  2  i n a dilute f l u i d solution of ethyl alcohol.  The spectrum c o n s i s t e d o f a b r o a d l i n e a t room t e m p e r a t u r e , s e p a r a t i n g i n t o f o u r components on c o o l i n g .  More r e c e n t s t u d i e s o f C l O ^ i n  v a r i o u s s o l v e n t s a t low temperatures have produced b e t t e r r e s o l v e d s p e c t r a , t h e r e s o l u t i o n i n some cases b e i n g good enough t o d i s t i n g u i s h 37 the h y p e r f i n e s p l i t t i n g s due t o t h e  C l isotope  [134].  The spectrum r e p o r t e d i n r i g i d s u l f u r i c a c i d a t 77°K i s somewhat b e t t e r r e s o l v e d , a l t h o u g h  complex [135].  Here a g a i n ,  features  -82-  due t o t h e  CI i s o t o p e may be d i s t i n g u i s h e d from t h o s e due t o t h e 35  predominant  C I . I r r a d i a t e d p o t a s s i u m p e r c h l o r a t e has p r o v i d e d  a s o u r c e o f t r a p p e d C l O ^ m o l e c u l e s i n t h e c r y s t a l environment. independent s t u d i e s [136,137] o f t h e CIC^ m o l e c u l e i n such have l e d t o s i m i l a r a n a l y s i s o f t h e s p e c t r a .  Two  crystals  The p r i n c i p a l v a l u e s  of b o t h t h e g and h y p e r f i n e t e n s o r s were o b t a i n e d from t h e s e s p e c t r a whereas t h e r i g i d s o l u t i o n s p e c t r a c o u l d y i e l d w i t h c e r t a i n t y o n l y one p r i n c i p a l v a l u e o f each o f t h e s e t e n s o r s . 8.1  S i l i c a Gel. S i l i c a g e l was i n i t i a l l y chosen as an adsorbent due i n p a r t  to  i t s h i g h s u r f a c e a r e a and t h e f a c t t h a t t h e s i l i c o n n u c l e u s 29 " •'• (except f o r . S i o f n a t u r a l abundance l e s s t h a n 5%) does n o t have  a nuclear spin. environment  T h i s would p r o v i d e a m a g n e t i c a l l y i n e r t  e l i m i n a t i n g a p o s s i b l e source o f l i n e broadening.  A d s o r p t i o n o f CIC^ a t room temperature a t p r e s s u r e s h i g h e r than -3 8 x 10  t o r r produced  when c o o l e d t o 77°K.  a bright yellow colouring of the s i l i c a g e l S p e c t r a r e c o r d e d f o r t h e s e p r e s s u r e s were  composed o f e x t r e m e l y broad l i n e s i n d i c a t i n g f a r t o o h i g h a c o n c e n t r a t i o n o f ClO^. l a y e r must be adsorbed.  To a v o i d d i p o l a r b r o a d e n i n g , l e s s than a monoP r e s s u r e s o f l e s s than 1 x 10  produced m u c h , c l e a r e r s p e c t r a , w i t h r e a d i l y r e s o l v a b l e  t o r r of ClO^ features.  There was no c o l o u r a t i o n o f t h e s i l i c a g e l a t t h e s e p r e s s u r e s . An e q u i l i b r a t i o n time o f up t o 20 minutes was needed f o r maximum s i g n a l s t r e n g t h , i n d i c a t i n g slow s o r p t i o n o f t h e CIC^ m o l e c u l e s throughout t h e s u r f a c e o f t h e s i l i c a g e l .  The samples c o u l d be  -83-  s t o r e d a t room temperature and r e c o o l e d t o 77 K w i t h no l o s s of s i g n a l . CIC^  Pumping a t room t e m p e r a t u r e removes v e r y  little  i n d i c a t e d by v e r y l i t t l e change i n t h e spectrum, b u t t h e  ClO^ may be removed from t h e s i l i c a g e l by pumping a t h i g h e r temperatures. . The s u r f a c e o f t h e s i l i c a g e l i s not u n i f o r m ,  and a  large v a r i e t y of adsorption centers are p o s s i b l e with the p o s s i b i l i t y o f 'densely' p o p u l a t e d  areas on the s u r f a c e .  This  would account f o r t h e i n a b i l i t y t o observe s p e c t r a w i t h a s m a l l enough l i n e w i d t h t o d i s t i n g u i s h a l l t h e f e a t u r e s  X  o  F i g u r e 18  clearly.  r-84-  In c o n f o r m i t y w i t h e a r l i e r works, t h e a x i s system f o r t h e magnetic parameters o f t h e C 1 0  2  has been chosen so t h a t the z - a x i s  l i e s along t h e t w o - f o l d symmetry a x i s , y i s p e r p e n d i c u l a r t o z i n the p l a n e o f the m o l e c u l e ,  and x, t h e t h i r d o r t h o g o n a l  axis  (see f i g u r e 1 8 ) . F i g u r e 19 shows a t y p i c a l spectrum o f CIO,, adsorbed on s i l i c a g e l , recorded 35 isotopes  a t 77°K.  C h l o r i n e has two n a t u r a l l y o c c u r r i n g  • 37 C l and  C l i n the r a t i o o f approximately  3:1.  Both  i s o t o p e s have a n u c l e a r s p i n o f I = 3/2, and t h e r a t i o o f t h e i r magnetic moments i s 0.82089:0.68329.  The observed spectrum i s then 35 37  e s s e n t i a l l y a s u p e r p o s i t i o n o f two s p e c t r a , w i t h t h e r a t i o o f the c o r r e s p o n d i n g  C10  2  and  C10 , 2  h y p e r f i n e components g i v e n by t h e  r a t i o o f the r e s p e c t i v e magnetic moments. The v a l u e s f o r the components o f t h e h y p e r f i n e and g t e n s o r s were o b t a i n e d by comparison o f t h e r e c o r d e d  spectra to simulated  s p e c t r a u s i n g the v a r i o u s programs p r e v i o u s l y mentioned. s i m u l a t e d spectrum i s shown i n f i g u r e 20. be a n i s o t r o p i c .  EPR  The  Both T and g were found t o  The r e s u l t s a r e t a b u l a t e d i n T a b l e 1.  The main d i s t i n g u i s h a b l e f e a t u r e s o f t h e spectrum a r e those a s s o c i a t e d w i t h the x component o f t h e h y p e r f i n e t e n s o r , p a r t i c u l a r l y those a s s o c i a t e d w i t h nij=±3/2. spectrum.  These a r e the o u t e r l i n e s o f the  Both t h e y and z components a r e c o n c e n t r a t e d  i n the c e n t r a l  p o r t i o n o f the spectrum. F i g u r e 21 shows a spectrum o f C 1 0 recorded  a t room t e m p e r a t u r e .  d i s c e r n a b l e here.  Features  2  adsorbed on s i l i c a g e l ,  due t o t h e i s o t o p e s a r e not  From the spectrum, t h e C10_ appears t o be f r e e l y  TABLE 1 Reference  Linewidth used f o r simulations ('gauss)  H y p e r f i n e components (gauss) For •T XX  T  3 5  C10  yy  only  2  T  zz  ,A o  g xx  g yy  Medium  g . zz  137  79.9  -13.4  -12.5  18.0  2.0018  2.0167  2.0111  KCIO^  137  72.7  - 9.6  -10.0  18.0  2.0025  2.017  2.011  H S0  136  74.7  -10.8  -11.5  17.5  2.0016  2.01667  2.01214  KC10, 4  135  70.5  2  4  '"H SO  2.0015  ?  77°K  @  77°K "  @  106°K  @  77°K  ADSORBED ON .... .  ± 0.0005  ± 0 . 2 GAUSS  .@  t h i s work  76.1  -17.0.  - 7.9 ,  17.1  2.0023  2.0123  " • 2.0115  t h i s work  74.9  -16.7  - 7.8  16.8  2.0023  2.0123  2.0115  3.5  silica gel  @  77°K  H-mordenite  @  77°K i  t h i s work  77.0  -17.2  - 8.0  17.3  2.0023  2.0123  2.0115  8.0  Na-mordenite @  t h i s work  82.2  -18.4  - 8.5  18.4  2.0023  2.0123  2.0115  2.5  4A  @ • ' 77°K  t h i s work  81,6  - 1 8 . 3  - 8.5  18.3  2.0023  2.0123  2.0115  5A  @  t h i s work  84.5  -18.9  - 8.8  18.9  2.0023  2.0123  2.0115  2.0  13X s i t e I I I  §  77°K  t h i s work  77.5  -17.4  - 8.0  17.3  2.0023  2.0123  2.0115  2.0-  13X s i t e I I  @  77°K  t h i s work  79.0  .-17.7  - 8.2  17.7  2.0023  2.0123  2.0115  t h i s work  77.2  -17.3  - 8.0  17.3  2.0023  2.0123  2.0115  .  77°K  77°K  10X  @  77°K  LiX  @  77°K  j  -86-  FIGURE .19. EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d a t 77° K.  FIGURE 20. Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on s i l i c a g e l , r e c o r d e d a t 77° K.  -38-  -89-  r o t a t i n g on t h e s i l i c a g e l s u r f a c e o r i n t h e p o r e s , and t h e i s o t r o p i c parameters a r e i n c l u d e d i n T a b l e 1.  A s i m u l a t e d spectrum i s shown  i n f i g u r e 22.  8.2  Na and H-Mbrdenite. ClO^ adsorbed  on H-mordenite and r e c o r d e d a t 77°K y i e l d s  e s s e n t i a l l y an i d e n t i c a l spectrum t o t h a t o b t a i n e d on s i l i c a g e l .  The  components o f t h e h y p e r f i n e t e n s o r a r e s l i g h t l y d i f f e r e n t and a r e l i s t e d i n T a b l e 1.  These, a l o n g w i t h t h o s e a s s o c i a t e d w i t h t h e o t h e r  a d s o r b e n t s , w i l l be d i s c u s s e d  later.  Na-mordenite produces some i n t e r e s t i n g r e s u l t s .  Under  s i m i l a r c o n d i t i o n s o f sample p r e p a r a t i o n , t h e spectrum a t 77°K appears much b r o a d e r and cannot be improved by pumping. the sample shows no c o l o u r .  Unlike s i l i c a g e l ,  A t y p i c a l spectrum i s shown i n f i g u r e 23.  The x components o f t h e h y p e r f i n e t e n s o r a r e s t i l l  readily  d i s c e r n a b l e and t h e v a l u e s a r e l i s t e d i n T a b l e 1.  The spectrum  r e c o r d e d a t room temperature,  u n l i k e that of s i l i c a g e l , indicates  t h a t some f e a t u r e s may have been p a r t i a l l y averaged due t o some m o t i o n a l p r o c e s s o f t h e CIC^. The most l i k e l y would be a r o t a t i o n about t h e z a x i s , a v e r a g i n g t h e h y p e r f i n e and g t e n s o r components o f t h e x and y axes.  A spectrum s i m u l a t e d under t h e s e c o n d i t i o n s however,  does n o t match t h e observed suggests  spectrum (see f i g u r e s  that the r o t a t i o n i s hindered.  temperatures  show  o n l y a decrease  25 and 26).  Spectra recorded at higher  i n s i g n a l h e i g h t and i t i s l i k e l y  d i f f u s i o n o f t h e CIC^ molecules w i l l o c c u r a t t h e s e e l e v a t e d At 373°K, t h e CIO  This  temperatures.  i s c o m p l e t e l y removed from.the Na-mordenite.  FIGURE 22'.' C o m p u t e r s i m u l a t e d EPR s p e c t r u m o f c h l o r i n e d i o x i d e ' a d s o r b e d on s i l i c a g e l , r e c o r d e d a t room temperature.  FIGURE 23. EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d a t 77° K. . .  -92-  F i g u r e 24 shows a s i m u l a t e d spectrum f o r CIC^ adsorbed on Na-mordenite, r e c o r d e d at 77°K.  F i g u r e s 25 and 26 show ClO^  adsorbed on Na-mordenite, r e c o r d e d at room t e m p e r a t u r e , o b s e r v e d and simulated, respectively..  8.5  4A and 5A S y n t h e t i c  Zeolites.  F i g u r e 27 shows a t y p i c a l spectrum o f ClO^ adsorbed on 4A s y n t h e t i c z e o l i t e , r e c o r d e d a t 77°K.  The l i n e w i d t h i s g r e a t l y  decreased from t h a t observed on e i t h e r s i l i c a g e l o r the s y n t h e t i c mordenites. the  C o n s e q u e n t l y , the f e a t u r e s i n t h e c e n t r a l p o r t i o n of.  spectrum ( c o r r e s p o n d i n g t o t h e y and z components o f the hyper-  f i n e s p l i t t i n g ) are b e t t e r d e f i n e d .  There i s a s u b s t a n t i a l  increase  i n the magnitude o f t h e components o f the h y p e r f i n e t e n s o r , and t h e r e s u l t s a r e g i v e n i n T a b l e 1. c o r r e s p o n d i n g t o C10  F i g u r e 28 shows a s i m u l a t e d spectrum  adsorbed on 4A.  A d s o r p t i o n on 5A s y n t h e t i c z e o l i t e a g a i n y i e l d s a s i m i l a r spectrum and has not been shown h e r e .  The outermost f e a t u r e s o f t h e  spectrum are somewhat broadened, which p o s s i b l y s u g g e s t s the e x i s t e n c e of  two a d s o r p t i o n s i t e s .  T h i s w i l l be d i s c u s s e d l a t e r .  Spectra  r e c o r d e d a t room temperature a r e markedly changed, a l t h o u g h the ClO^ does not appear t o be f r e e l y r o t a t i n g .  A p a r t i a l r o t a t i o n or  some o t h e r form o f h i n d e r e d r o t a t i o n appears e v i d e n t .  The  h y p e r f i n e and g t e n s o r s are g i v e n i n T a b l e 1. 8.4.  13X S y n t h e t i c  Zeolite.  A t y p i c a l spectrum o f C10„  adsorbed on 13X  synthetic  -94-  .FT.GURE 25. EPR spectrum of. c h l o r i n e d i o x i d e , adsorbed on Na-mordenite, r e c o r d e d a t room t e m p e r a t u r e .  ICURE 26. Computer s i m u l a t e d EPR spectrum o f c h l o r i n e d i o x i d e adsorbed on Na-mordenite, r e c o r d e d a t room temperature.  -96-  27. EPR spectrum o f c h l o r i n e d i o x i d e y n t h e t i c z e o l i t e , r e c o r d e d a t 77° K.  adsorbed  -98-  z e o l i t e i s shown i n f i g u r e 29.  The spectrum was r e c o r d e d a t 77°K.  A s i m u l a t e d spectrum i s shown i n f i g u r e 30. the  I t i s e v i d e n t from  spectrum t h a t two a d s o r p t i o n s i t e s a r e p r e s e n t i n t h e z e o l i t e .  The outermost components (itij = ± 3/2, x a x i s ) show t h i s q u i t e  clearly.  A l i n e w i d t h d i f f e r e n c e between the two s i t e s e n a b l e s them t o be more readily  d i s t i n g u i s h e d , p a r t i c u l a r l y i n the c e n t r a l p o r t i o n o f the  spectrum.  T a b l e 1 l i s t s t h e components o f t h e h y p e r f i n e and g t e n s o r s  f o r the two s i t e s . F u r t h e r pumping i n c r e a s e s t h e r e s o l u t i o n o f t h e l i n e s a l t h o u g h t h e ClO^ can be removed from n e i t h e r s i t e by pumping a t room temperature.  V a r i a b l e t e m p e r a t u r e ( a n n e a l i n g t y p e ) experiments were  performed i n t h e hope t h a t t h e C l O ^ would be removed from one o f t h e s i t e s .  preferentially  U n f o r t u n a t e l y , l i n e broadening at temperatures  h i g h e r than 77°K made i t i m p o s s i b l e f o r a c c u r a t e o b s e r v a t i o n s t o be made.  I t i s apparent t h a t t h e ClO^ does n o t remain r i g i d l y t r a p p e d  i n the z e o l i t e as t h e t e m p e r a t u r e i s r a i s e d , b u t t h e e x a c t t y p e o f motion c o u l d not be d e t e r m i n e d . 8.5  IPX S y n t h e t i c  Zeolite.  The c a l c i u m exchanged form o f t h e 13X z e o l i t e , 10X, was a l s o used as an a d s o r b e n t .  The s p e c t r a r e c o r d e d a t 77°K were  similar  t o t h o s e on 13X a l t h o u g h t h e p r e s e n c e o f two s i t e s was not as o b v i o u s . The reasons f o r t h i s w i l l be d i s c u s s e d l a t e r . than t h a t observed on 13X.  The l i n e w i d t h i s b r o a d e r  The spectrum a t room temperature was  s i m i l a r t o t h a t observed on s i l i c a g e l , i n d i c a t i n g C10  ?  i n 10X a t t h i s t e m p e r a t u r e .  free rotation of  T h i s i s u n l i k e t h e 13X sample,  -99-  -100-  -101-  where the m o t i o n o f the ClO^ was  s t i l l h i n d e r e d a t room  temperature.  T a b l e 1 g i v e s the components o f the h y p e r f i n e and g t e n s o r s . 8.6  L i t h i u m Exchanged 13X S y n t h e t i c Z e o l i t e . . L i t h i u m was  exchanged f o r the sodium i n a sample o f  and CIO2 then adsorbed as b e f o r e .  The  spectrum r e c o r d e d a t 77°K  d i d not show two d i s t i n c t s i t e s as d i d the 13X, somewhat b r o a d e r .  13X  and t h e l i n e s were  T a b l e 1 g i v e s the components o f the h y p e r f i n e  and  g tensors.  8.7  Discussion. CIO2 i s a bent m o l e c u l e  o f the C  p o i n t group.  2 v  and has the symmetry p r o p e r t i e s  Q u a l i t a t i v e d i s c u s s i o n s o f the  s t r u c t u r e o f t h i s type o f m o l e c u l e [138]  and Walsh [139].  electronic  (AB ). have been g i v e n by M u l l i k e n 2  F o l l o w i n g the m o l e c u l a r - c o r r e l a t i o n diagrams  g i v e n by these a u t h o r s , the ground s t a t e has the c o n f i g u r a t i o n  ... The b  1  (lbp  2  (3b ) 2  2  (la ) 2  2  (4  o r b i t a l ' c o n s i s t s o f the p  & 1  )  2  (2bp  o r b i t a l s o f the c h l o r i n e and  w i t h p o s s i b l e a d m i x t u r e from the c h l o r i n e d r  ,-\  xz  oxygens  o r b i t a l , and i s a n t i '  bonding. Although expected  an i s o t r o p i c h y p e r f i n e s p l i t t i n g would not  be.  from an e l e c t r o n i n a b^ o r b i t a l , the odd e l e c t r o n i s expected  t o cause a p o l a r i z a t i o n o f the i n n e r s - o r b i t a l s on the c h l o r i n e and oxygen atoms. component.  T h i s would i n t r o d u c e a s m a l l i s o t r o p i c h y p e r f i n e  An a n i s o t r o p i c h y p e r f i n e t e n s o r w i t h the maximum p r i n c i p a l  v a l u e observed  when the f i e l d i s p e r p e n d i c u l a r t o the m o l e c u l a r  plane  -102-  (alorig t h e x - a x i s ) i s a l s o e x p e c t e d , o f o p p o s i t e s i g n t o t h e s m a l l e r i n - p l a n e p r i n c i p a l v a l u e s , s i n c e the unpaired e l e c t r o n i s i n a b j orbital. The d e v i a t i o n s from the f r e e e l e c t r o n v a l u e g , g e n e r a l l y termed g - s h i f t s , may  be r e p r e s e n t e d by the g e n e r a l  formula  (excluding d o r b i t a l s )  A  gii  =  ( i i > CV . .E E f  c  c  X  r  where f ( c ^ ,  A , C 1  A) Q  A  p)  (8-1) •  2  i s a f u n c t i o n o f the s p i n - o r b i t c o u p l i n g c o n s t a n t s  on the atbms c h l o r i n e and oxygen (A  and A ) , and the p r o d u c t s o f t h e  ;  c o e f f i c i e n t s o f the o r b i t a l s on t h e atoms.  The denominator i s the  energy d i f f e r e n c e o f the two s t a t e s which are m i x i n g .  The  various  s t a t e s which a r e . a l l o w e d t o mix and c o n t r i b u t e t o the g - t e n s o r be determined  u s i n g group t h e o r y [182] .  g-shift affects-g  For C 1 0 , 2  t h e dominant  and i s expected t o be l a r g e and p o s i t i v e .  s h i f t i n the x d i r e c t i o n , Ag  s h o u l d be c l o s e t o z e r o , and  The  negligible  XX  i f d o r b i t a l s are n e g l e c t e d . and l e s s than  may  '. A  S  Z Z  ^  s  expected  t o be  positive,  Ag yy &  The  s t r u c t u r a l parameters f o r c h l o r i n e d i o x i d e have been  o b t a i n e d by C u r l e t a l [140,141] as a r e s u l t o f a microwave s t u d y , and Ward [142] who resolution  IR  combined UV  data.  :  spectroscopic data with high  The r e s u l t s are summarized below. r e f e r e n c e 140,  141  reference  r Cl-0  (8)  1.471  1.472  ^OCIO  (°)  117.6  117.4  142  -103-  Using the preceding  information, the r e s u l t s o f c h l o r i n e  d i o x i d e adsorbed on v a r i o u s s u r f a c e s may be a n a l y z e d .  The r e s u l t s  o b t a i n e d from t h e p r e s e n t work and those o f . p r e v i o u s workers a r e summarized i n T a b l e 1. A general discussion o f the adsprptioh of c h l o r i n e dioxide on these s u r f a c e s i s u s e f u l p r i o r t p t h e d i s c u s s i o n p f t h e i n d i v i d u a l cases.  C h l o r i n e d i o x i d e has been found t o p o s s e s s a s u b s t a n t i a l  d i p o l e moment, 1.784 D. [ 1 4 3 ] .  T h i s d i p o l e moment, t o g e t h e r  the quadrupole moment due t o t h e c h l o r i n e n u c l e u s important The  with  w i t h 1=3/2, p l a y  r o l e s i n t h e a d s o r p t i o n as d e t a i l e d p r e v i o u s l y i n Chapter Two.  s t r o n g a t t r a c t i v e f o r c e due t p t h e i o n i c charges o f t h e adsorbent  i n t e r a c t s w i t h t h e s e m u l t i p o l e moments and i s c h a r a c t e r i z e d by the changes observed i n t h e components o f t h e g and h y p e r f i n e  tensors  as compared t o t h e s e components observed f o r c h l o r i n e d i o x i d e i s o l a t e d i n o t h e r media. I t i s important  i n ^his discussion  t o analyze  these  observed parameter changes i n terms o f a d s o r p t i o n - i . e . t h e s i t e o f adsorption; the p o s i t i o n of the c h l o r i n e dioxide i n r e l a t i o n t o the t r a p p i n g s i t e ; any movement o f t h e c h l o r i n e d i o x i d e on t h e s u r f a c e or a t . t h e  site. Buckingham £144] has c o n s i d e r e d t h e i n t e r a c t i o n p o t e n t i a l  energy u ^ o f two.charge d i s t r i b u t i o n s 1 and 2 p o s s e s s i n g q and m u l t i p o l e moments \i, 0, .... <f>„ F' , i  3  ^  charge  are the p o t e n t i a l  and i t s d e r i v a t i v e s a t t h e c e n t e r o f mass o f 2 due t o t h e charges o f 1.  -104-  Hence  u  12  =  ¥ 2  " ^2 2z " ^ 2 2 z z " F  e  F  ( 8 _ 2 )  Assuming t h e a d s o r p t i o n c e n t e r t o be a p o s i t i v e charge as p r e v i o u s l y d i s c u s s e d f o r t h e z e o l i t e s , then t h e f a v o r a b l e r e l a t i v e orientations f o r a charge-dipole  i n t e r a c t i o n and a charge-  quadrupole i n t e r a c t i o n a r e as f o l l o w s :  +  charge-dipole where + r e p r e s e n t s represents for  charge-quadrupole  a p o s i t i v e charge; — ^ - r e p r e s e n t s  a quadrupole.  a d i p o l e ; and |  The a d s o r p t i o n i n the z e o l i t e s expected  ClO^ a c c o r d i n g t o t h i s model i s d i s c u s s e d i n t h e next  paragraph. These same a d s o r p t i o n s i t e s  ( p o s i t i v e charges) can produce  strong e l e c t r i c f i e l d s which, i n a d d i t i o n t o p r o v i d i n g a d d i t i o n a l a t t r a c t i v e f o r c e s f o r a d s o r p t i o n , can a l s o determine t h e r e l a t i v e p o s i t i o n o f t h e adsorbed m o l e c u l e .  In t h e p r e s e n c e o f an e l e c t r i c  f i e l d the d i p o l e s ( o r induced d i p o l e s i f t h e m o l e c u l e does not possess a permanent moment) a r e o r i e n t e d i n t h e same d i r e c t i o n as t h e field.  I t has been shown [145] t h a t even f o r p r o n o u n c e d l y  aniso-  t r o p i c d i p o l a r m o l e c u l e s , t h e mean p o l a r i z a b i l i t y i n a homogeneous e x t e r n a l e l e c t r i c f i e l d i s p r a c t i c a l l y equal t o (a +a~+a )/., where  -105-  a  j» 2' a  a n c  * 3 a  a r e  t n  e molecular p o l a r i z a b i l i t i e s i n the three  axes.  We thus expect c h l o r i n e d i o x i d e t o be adsorbed on t h e s u r f a c e s s t u d i e d as  0 cr  The c h l o r i n e atom i s assumed t o be t h e p o s i t i v e end o f t h e m o l e c u l e . Assuming s u b s t a n t i a l l y s t r o n g a d s o r p t i o n ,  t h e o n l y p r o b a b l e movement  a s i d e from p o s s i b l e s l i g h t wagging as i n d i c a t e d by t h e arrows i n the f i g u r e , would.be a r o t a t i o n about t h e z a x i s o f t h e m o l e c u l e ( b i s e c t i n g t h e O-Cl-0 bond a n g l e ) . C a l c u l a t i o n s o f t h e s e e l e c t r i c f i e l d s have been performed by P i c k e r t e t a l [48] and more r e c e n t l y by Dempsey [146] and a p p l i e d t o v a r i o u s  zeolites.  The c a l c u l a t i o n s were performed  by 'growing' t h e c r y s t a l on a computer.  The b a s i c q u a n t i t y o f  i n t e r e s t i s t h e e l e c t r o s t a t i c p o t e n t i a l a t e i t h e r an i o n s i t e j o r at a p o i n t i n f r e e space.  Thus  ' •r'hsiyv.j  "  C8  3)  where q. , i s t h e charge a t i o n i i n t h e b a s i s a t l a t t i c e p o i n t k, d i s t a n c e r . , . from t h e p o t e n t i a l p o i n t j . The e l e c t r o s t a t i c J > 1 1  K  energy o f t h i s b a s i s i s  X qj<f>j i  C8-4)  -106-  summed over t h e i o n s o f t h e b a s i s o r some p r o p o r t i o n o f t h e s e , depending on the symmetry.  Another q u a n t i t y o f i n t e r e s t i s t h e  e l e c t r o s t a t i c f i e l d F g i v e n by  F = - grad cf>  (8-5)  E v a l u a t i o n o f <f> at any p o i n t i n t h e c r y s t a l was done u s i n g t h e t r a n s f o r m a t i o n method o f Ewald [147].  Values o f t h e f i e l d F and t h e  components o f t h e f i e l d g r a d i e n t t e n s o r were a l s o d e r i v e d by Dempsey u s i n g t h e Ewald method.  T h e i r r e s u l t s w i l l be a p p l i e d t o  the EPR s p e c t r a o f c h l o r i n e d i o x i d e adsorbed on t h e s y n t h e t i c z e o l i t e 13X and these i n t u r n r e l a t e d t o t h e o t h e r  adsorbents.  Before d i s c u s s i n g t h e parameters o b t a i n e d from t h e spectrum o f C l O ^ adsorbed on 13X, i t s h o u l d be p o i n t e d out t h a t t h e s e parameters were o b t a i n e d w i t h o u t  i n c l u d i n g t h e quadrupole i n t e r a c t i o n term  in the spin Hamiltonian.  Byberg e t a l [136] have shown t h i s  to be o f importance i n t h e C l O ^ m o l e c u l e t r a p p e d  interaction  i n irradiated  KCIO^, i n p a r t i c u l a r g i v i n g r i s e t o f o r b i d d e n * t r a n s i t i o n s o f h i g h 1  i n t e n s i t y i n c e r t a i n molecular magnetic, f i e l d .  o r i e n t a t i o n s with respect to the  When t h e o f f - d i a g o n a l t e n s o r elements o f ^ ^ ^  become comparable t o t h e d i a g o n a l p a r t s q f ^ ^ ^ . mixing o f nuclear s p i n states occurs.  a  n  c  ^^^  considerabl  The s e l e c t i o n r u l e  AMj. = 0  (8-6)  breaks down and t r a n s i t i o n s w i t h  AMj = + 1 , - A M  = ± 2-  (8-7)  -107-  become o b s e r v a b l e  as w e l l .  S e v e r a l s p e c t r a were s i m u l a t e d i n c l u d i n g  the quadrupole i n t e r a c t i o n but the observed l i n e w i d t h masked these f e a t u r e s . experimental due  The  d e v i a t i o n s o f the s i m u l a t e d s p e c t r a from the  s p e c t r a , i n p a r t i c u l a r l i n e i n t e n s i t i e s , are presumed  to t h i s  interaction.  Figure  29 c l e a r l y shows two d i s t i n c t a d s o r p t i o n  on the 1 3 X , the h y p e r f i n e s p l i t t i n g c o n s t a n t s d i f f e r i n g between the two  sites.  The  constants  sites  considerably  are i n f a c t much l a r g e r than  those p r e v i o u s l y observed i n o t h e r media (see T a b l e 1 ) .  The  two  s i t e s are v e r y l i k e l y a s s o c i a t e d w i t h the s u r f a c e c a t i o n s at s i t e s  SJJ  and  SJJJ, The  as d e s c r i b e d i n Chapter Three. l a r g e change i n the parameters i n d i c a t e s t h a t  the  i n t e n s e e l e c t r i c f i e l d s a s s o c i a t e d w i t h the c a t i o n s d i s t o r t the e l e c t r o n i c s t r u c t u r e o f the CIC^.  Both s i t e s show an  increased  h y p e r f i n e s p l i t t i n g c o n s t a n t i n d i c a t i n g an i n c r e a s e i n u n p a i r e d e l e c t r o n d e n s i t y at the c h l o r i n e n u c l e u s .  Our model o f  ClO^  adsorbed i s such t h a t the d i p o l e moment i s o r i e n t e d a l o n g the  electric  f i e l d d i r e c t i o n , w i t h the oxygen end o f the m o l e c u l e c l o s e s t t o the c a t i o n . One  would then expect a net s h i f t i n e l e c t r o n d e n s i t y towards the  oxygen end o f the m o l e c u l e and a d e c r e a s e i n the h y p e r f i n e constants. occupies  The  o p p o s i t e i n f a c t i s the case.  The  unpaired  the a n t i b o n d i n g b^ o r b i t a l and the u n p a i r e d  d e n s i t y s h i f t s towards the c h l o r i n e .  splitting electron  electron  T h i s e f f e c t a r i s e s because i t  i s e n e r g e t i c a l l y .more f a v o r a b l e f o r the two e l e c t r o n s i n the bonding o r b i t a l t o be c l o s e r t o the c a t i o n than t o have the s i n g l y  occupied  -108-  antibonding o r b i t a l close t o the c a t i o n .  Orthogonality conditions  ensure t h a t i f t h e bonding o r b i t a l s h i f t s towards t h e c a t i o n , the a n t i b o n d i n g o r b i t a l must s h i f t away from i t .  The  experimental  r e s u l t s agree w i t h t h e proposed model. The  p r e v i o u s l y mentioned c a l c u l a t i o n s o f Dempsey o f t h e e l e c t r i c  f i e l d s i n 13X show t h e f i e l d a t S^^. t o be much l a r g e r than t h a t a t 'SJJ.  I t i s reasonable  SJJJ.  s h i f t to  then, t o assign the s i t e w i t h the l a r g e s t  The t h i r d s i t e , S j , as o u t l i n e d i n Chapter Three, i s  i n a c c e s s i b l e t o adsorbed m o l e c u l e s and t h e r e f o r e i s n o t o b s e r v e d . Two o t h e r z e o l i t e s h a v i n g t h e same b a s i c X s t r u c t u r e as 13X were a l s o used as a d s o r b e n t s . 13X  i s o f course  sodium.  10X i s a c a l c i u m z e o l i t e whereas t h e  I t i s expected t h a t o n l y one s i t e  be observed f o r a 100 p e r c e n t  should  exchanged form s i n c e two sodium  c a t i o n s a r e r e p l a c e d by a s i n g l e d o u b l y - c h a r g e d c a l c i u m , thus l e a v i n g site  SJJJ  zeolite  unoccupied.  In f a c t , t h e m a n u f a c t u r e r s s t a t e t h a t t h e  i s o n l y 75 p e r c e n t  sodium c a t i o n s i n s i t e s  exchanged, l e a v i n g t h e p o s s i b i l i t y o f and  a v a i l a b l e f o r a d s o r p t i o n , as  the c a t i o n s i n these s i t e s a r e exchanged a f t e r those i n S^. spectrum o f C 1 0  2  on 10X i s c o n s e q u e n t l y  The  l e s s r e s o l v e d than t h e 13X  but o n l y one site.-appears t o be p r e s e n t . A l i t h i u m exchanged X s t r u c t u r e z e o l i t e i s expected t o show two d i s t i n c t s i t e s s i m i l a r t o 13X, w i t h i n c r e a s e d s h i f t s i n t h e h y p e r f i n e s p l i t t i n g c o n s t a n t s due t o t h e s m a l l e r s i z e o f t h e l i t h i u m cation.  Only one d i s t i n c t s i t e was observed a l t h o u g h  second s i t e , much.less p o p u l a t e d , were e v i d e n t . assume t h a t t h e S  indications of a  I t i s reasonable t o  s i t e c o n t a i n s adsorbed water r e t a i n e d from t h e  -109-  exchange p r o c e s s and so i s u n a v a i l a b l e f o r a d s o r p t i o n o f  CIC^.  The observed  s i t e i s then a s s o c i a t e d w i t h S J J , and approximates  SJJ  An i n c r e a s e d e l e c t r i c f i e l d i n t e n s i t y i s p o s t u l a t e d f o r  i n 13X.  the l i t h i u m z e o l i t e due t o the i n c r e a s e d c h a r g e / s i z e r a t i o . decreased  s i z e o f the c a t i o n , however, decreases  The  the e x t e n t o f exposure  of the c a t i o n to t h e c a v i t i e s p r o v i d i n g l e s s c o n t a c t w i t h t h e molecules  that of  ClO^  even though t h e s p e c i f i c e l e c t r i c f i e l d i n t e n s i t y at t h e  cation surface i s stronger. The  spectrum observed  on s i l i c a g e l , a l t h o u g h  less  r e s o l v e d than on the z e o l i t e s , i s h e l p f u l i n a n a l y z i n g the parameters obtained.  The  temperature  i s o t r o p i c s p l i t t i n g c o n s t a n t o b t a i n e d from t h e room  spectrum i s 1 7 . 1 gauss.  T h i s i s i n good agreement w i t h  those o b t a i n e d from t h e o t h e r s t u d i e s mentioned and i s t o be e x p e c t e d , s i n c e t h i s s h o u l d v a r y l i t t l e from medium t o medium.  The  i s o t r o p i c s p l i t t i n g c o n s t a n t o b t a i n e d from t h e a n i s o t r o p i c spectrum (i.e. T  xx  = A  o  both i s o t r o p i c observed. o f the  + B  xx  ; the observed  splitting.T ^ " xx  [A ] and a n i s o t r o p i c [B  £  i s composed o f r  ] p a r t s ) agrees w i t h t h a t  T h i s o f f e r s a d d i t i o n a l s u p p o r t t o t h e a s s i g n e d values^  parameters. Some mention s h o u l d be made o f t h e r a t h e r l a r g e v a l u e  a s s i g n e d t o the h y p e r f i n e component a l o n g the y - a x i s o f the molecule  ( a c r o s s the oxygens i n the p l a n e o f t h e m o l e c u l e ) .  I t seems  r e a s o n a b l e t o expect some change i n t h i s component from the ' f r e e s t a t e ' v a l u e due t o t h e manner i n which t h e CIC^ i s adsorbed.  A  decrease  i n the O-Cl-0 bond a n g l e i s p r o b a b l e , p o s s i b l y a c c o u n t i n g f o r t h i s observed  change.. Agreement o f t h e c a l c u l a t e d i s o t r o p i c v a l u e t o  -110-  t h a t observed l e n d s s u p p o r t t o t h i s v i e w . The parameters o b t a i n e d from t h e s p e c t r a observed on the z e o l i t e s 4A and 5A a r e i n agreement w i t h t h e arguments proposed for  the o t h e r z e o l i t e s .  The e l e c t r i c f i e l d s produced by t h e c a t i o n s  are  l e s s i n t e n s e than t h o s e i n 13X as i n d i c a t e d by t h e parameters.  The  c a t i o n s i t e s are l e s s w e l l d e f i n e d as compared t o 13X and o n l y one s i t e appears e v i d e n t . C h l o r i n e d i o x i d e adsorbed on t h e m o r d e n i t e samples i n d i c a t e a l s o a much l e s s i n t e n s e e l e c t r i c f i e l d sites.  at the a d s o r p t i o n  The spectrum r e c o r d e d a t room temperature i n d i c a t e s some  motion o f t h e CIC^, a l t h o u g h somewhat more r e s t r i c t e d than t h a t observed on s i l i c a g e l .  R o t a t i o n about t h e z - a x i s o f t h e ClO^ m o l e c u l e seems  most p r o b a b l e , but a spectrum s i m u l a t e d f o r t h i s case does n o t agree w i t h t h e observed spectrum and a r e s t r i c t e d r o t a t i o n i s assumed. A c c u r a t e measurements o f t h e h y p e r f i n e and g t e n s o r components was not p o s s i b l e due t o t h e l a r g e observed  linewidth.  The measured components o f t h e g t e n s o r were i n agreement w i t h t h o s e p r e d i c t e d f o r a m o l e c u l e such as CIO • Z.  the  g  i s close to XX  f r e e - s p i n v a l u e , as i s g e n e r a l l y found f o r an e l e c t r o n i n a b^  o r b i t a l composed o f p^-atomic o r b i t a l s .  Comparing t h i s t o t h e Se02  r a d i c a l [148] where a n e g a t i v e Ag i s a s s o c i a t e d w i t h admixture of the s e l e n i u m d • l e v e l i n t o t h e b, o r b i t a l , i t i s r e a s o n a b l e t o xz . 1 ' assume l i t t l e p a r t i c i p a t i o n o f t h e 3d c h l o r i n e o r b i t a l i n C102v a l u e s f o r Ag  and A g  z z  The  a r e a l s o i n agreement w i t h t h e o r y , A g ^ h a v i n g  a l a r &g e rp o s i t i v e .-v a l u e w i t h Ag&  xx  < Ags  z z  <  Ag& y y  -111-  - CHAPTER NINE  NITROGEN DIOXIDE, NQ  2  N i t r o g e n d i o x i d e , l i k e CIO,,, i s a s t a b l e p a r a m a g n e t i c m o l e c u l e whose normal c h e m i c a l technique  s t a t e i s a gas.  has f r e q u e n t l y been used t o s t u d y t h i s m o l e c u l e i n t h e  gaseous and l i q u i d phases [149,150]. present  The EPR  One o f t h e purposes o f t h e  study was t o compare t h e spectrum o f t h e adsorbed m o l e c u l e  t o t h e w e l l - e s t a b l i s h e d s p e c t r a o f NO^ i n a v a r i e t y o f environments. P a r t i c u l a r a t t e n t i o n w i l l be g i v e n t o comparison o f s p e c t r a on o t h e r a d s o r b e n t s and i n v a r i o u s m a t r i c e s . The r e p o r t e d s p e c t r a o f NO^ i n v a r i o u s  polycrystalline  media g e n e r a l l y show l i n e w i d t h s o f t h e o r d e r o f 10-20 gauss, l i m i t i n g t h e amount o f d e t a i l which can be r e s o l v e d [151,152]. r e c e n t s t u d i e s o f NO  i n N„0  More  have produced s p e c t r a w i t h much s m a l l e r  -112-  linewidths, revealing greater d e t a i l  [150, 1 5 3 ] . EPR s p e c t r a o f  NO^ adsorbed on z i n c o x i d e [154, 155] and on magnesium o x i d e [156] are of  9.1  also not w e l l r e s o l v e d .  Comparison o f t h e s e s p e c t r a t o t h o s e  t h i s study w i l l be made i n t h e d i s c u s s i o n .  S i l i c a Gel. The spectrum r e c o r d e d a t 77°K f o r NO^ adsorbed on s i l i c a  gel  i s shown i n f i g u r e 31. The e x p e c t e d p a t t e r n o f t h r e e groups 14  of  l i n e s due t o t h e i n t e r a c t i o n o f t h e odd e l e c t r o n w i t h t h e  n u c l e u s , which has a n u c l e a r s p i n 1=1, was o b s e r v e d . of  N  The c o m p l e x i t y  t h e spectrum a r i s e s from t h e f a c t t h a t b o t h t h e g and h y p e r f i n e  tensors are a n i s o t r o p i c .  The spectrum i s c o m p l i c a t e d f u r t h e r by  broadening o f those l i n e s a s s o c i a t e d with t r a n s i t i o n s  involving  nij = ± 1 compared t o t h o s e w i t h m^. = 0, t o g e t h e r w i t h an o v e r l a p p i n g o f some l i n e s . some f e a t u r e s .  The observed l i n e w i d t h a l s o  T a b l e 2 g i v e s t h e g and h y p e r f i n e t e n s o r components  d e r i v e d from t h e computer s i m u l a t i o n o f t h e . s p e c t r u m . the  computer  overshadows  fitted  spectrum.  F i g u r e 32 shows  The spectrum observed on s i l i c a g e l  i s comparable i n r e s o l u t i o n t o t h o s e o b t a i n e d on o t h e r a d s o r b e n t s as y e t r e p o r t e d i n t h e l i t e r a t u r e . When t h e t e m p e r a t u r e was r a i s e d from 77°K a b r o a d e n i n g of the  t h e spectrum o c c u r r e d .  S p e c i f i c changes  i n t h e spectrum o c c u r i f  adsorbed NO^ b e g i n s t o r o t a t e about a g i v e n a x i s on warming.  The  l i n e w i d t h o f t h e spectrum even a t 77°K makes i t d i f f i c u l t t o d i s t i n g u i s h a x i a l l y symmetric t e n s o r s from c o m p l e t e l y a n i s o t r o p i c ones.  TABLE 2 H y p e r f i n e components (gauss)  g-value •  Reference  T  T XX  T yy  A zz  g  0  xx  g  g zz  yy.  s  1  {  medium  154  52  47  65  54.6  2.007  1.994  2.003  a d s o r b e d on ZnO @ 77°K  156  53.0  49.0  66.4  56.5  2.005  1.9915  2.002  adsorbed.on MgO  156  50.0  47.9  66.4  54.8  2.0058  1.9920  2.00.18  solid NO  @  77°K  153  50.3  48.2  67.3  55.25  2.0061  1.9922  2.0022  s o l i d N^O  @  77°K  g 77°K  •  150  50.2  49.6  68.3  56.0  2.0065  (+0.2) gauss  t h i s work  52.3  48.7  67.8  56.3  1.9960  2.0029  . solid N 0 2  4  @  77°K  (±0.0005)  2.0051  1.9926  2.0019  adsorbed on s i l i c a gel @ 77°K  t h i s work  53.1  51.0  65.5  56.5  2.0066  1.9956  2.0029  adsorbed on 13X § 77°K  t h i s work  50.1  48.1  66.7  55.1  2.0062  1.9926  2.0025  adsorbed on H-mordenite ' (a 77°K  t h i s work  71.1  67.7  93.8  77.5  2.0062  1.9926  adsorbed on H-morderiite  2.0025 i •  @ 77°K  -114-  -115-  -116-  Th e i n c r e a s e d b r o a d e n i n g due t o t h e i n c r e a s e i n t e m p e r a t u r e makes i t impossible i n t h i s case.  At a p p r o x i m a t e l y 200°K, t h e NO^  appears t o be f r e e l y r o t a t i n g on t h e s i l i c a g e l , s i n c e t h e s t r u c t u r e on t h e t h r e e groups o f l i n e s i s c o m p l e t e l y broadened.  Above t h i s  t e m p e r a t u r e , t h e spectrum c o u l d n o t be d i s c e r n e d from t h e background.  When t h e sample was r e c o o l e d , t h e s i g n a l was r e c o v e r e d  unchanged.  9.2  -15X S y n t h e t i c  Zeolite.  The spectrum a t 77°K o f NO^ adsorbed on 13X s y n t h e t i c z e o l i t e i s s i m i l a r t o t h a t observed on s i l i c a g e l . The parameters d e r i v e d from the. spectrum are g i v e n i n T a b l e 2. i s somewhat more e v i d e n t i n t h i s case.  Line broadening  D i f f e r e n t i a t i o n between  a x i a l l y symmetric and f u l l y a n i s o t r o p i c t e n s o r s i s v e r y d i f f i c u l t . F i g u r e 33 shows a t y p i c a l spectrum w h i l e f i g u r e s 34 and 35 show computer s i m u l a t e d s p e c t r a f o r f u l l y a n i s o t r o p i c and a x i a l l y symmetric t e n s o r s respectively. A comparison o f f i g u r e s 34 and 35 shows t h e s i m i l a r i t y o f t h e two s p e c t r a and t h e d i f f i c u l t y t h a t might be encountered i n a n a l y z i n g s p e c t r a w i t h t h e l i n e w i d t h s g e n e r a l l y found.  The  spectrum c o r r e s p o n d i n g t o a x i a l l y symmetric t e n s o r s c o u l d be caused by r o t a t i o n o f t h e NO^ about t h e z - a x i s .  No spectrum was observed  at room t e m p e r a t u r e .  9.3  H-Mordenite. F i g u r e . 36 shows a t y p i c a l spectrum o f NO  adsorbed  - 1 1 7 -  -118-  FIGURE 34. Computer s i m u l a t e d EPR spectrum o f n i t r o g e n d i o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77° K.  -121-  on H-mordenite,  r e c o r d e d a t 77°K.  I t s h o u l d be n o t e d t h a t  much h i g h e r p r e s s u r e s o f NG^ were needed t o observe s p e c t r a than . those r e q u i r e d f o r ClO^.  P r o b a b l e reasons f o r t h i s phenomenon  w i l l be g i v e n i n t h e d i s c u s s i o n .  An obvious f e a t u r e o f the  spectrum i s the reduced l i n e w i d t h as compared t o NCh, adsorbed on o t h e r s u r f a c e s y i e l d i n g a w e l l r e s o l v e d a n i s o t r o p i c s e t o f triplets.  The r e s o l u t i o n compares t o t h a t observed f o r NO^  t r a p p e d i n a N^O^  m a t r i x a t 77°K as r e p o r t e d by Schaafsma e t a l [153]  and James e t a l [ 1 5 0 ] .  A computer s i m u l a t e d spectrum i s shown  i n f i g u r e 37 and the r e s u l t s a r e g i v e n i n T a b l e 2. of the narrow  linevyidth w i l l  The  consequences  a l s o be d i s c u s s e d l a t e r .  The spectrum f o r ^NG^  adsorbed on H-mordenite  i n f i g u r e 38, and a s i m u l a t e d spectrum i n f i g u r e 39.  The  i s shown parameters  o b t a i n e d a r e g i v e n i n T a b l e 2 and agree w i t h t h o s e expected f o r with I =  h. An attempt was made t o r e c o r d s p e c t r a a t h i g h e r temperatures  w i t h a view t o o b t a i n i n f o r m a t i o n on p o s s i b l e m o t i o n a l p r o c e s s e s o f the  NO^  s i n c e the narrow l i n e w i d t h a t 77°K s h o u l d enable any  new  f e a t u r e s t o be e a s i l y seen.  U n f o r t u n a t e l y , t h i s was not the  case and l i n e b r o a d e n i n g at h i g h e r t e m p e r a t u r e s o b s c u r e d a l l details. 9.4  Discussion..' NO^,  •  l i k e ClO^ i s a l s o a bent m o l e c u l e w i t h t h e symmetry  p r o p e r t i e s o f the C„  p o i n t group.  F o l l o w i n g t h e approach  of  -123-  $  FIGURE 33. EPR s p e c t r u m o f * N n i t r o g e n d i . o x i d e s o r b e d on H - m o r d e n i t e , r e c o r d e d a t 77° K.  ad-  -124-  -125-  Walsh [ 1 3 9 ] , t h e ground s t a t e has t h e c o n f i g u r a t i o n  ...  (3a ) (lb ) (3b ) (la ) (4a. ), 2  1  2  1  2  2  2  2  1  ^  The u n p a i r e d e l e c t r o n o c c u p i e s an a^ o r b i t a l , d e l o c a l i z e d and c o n s t r u c t ed from b o t h s and p o r b i t a l s on t h e c e n t r a l n i t r o g e n atom.  The  h y p e r f i n e s p e c t r a s h o u l d thus d i s p l a y a c o n s i d e r a b l e i s o t r o p i c s p l i t t i n g and t h e a n i s o t r o p y s h o u l d be such t h a t i t s maximum v a l u e occurs when t h e magnetic f i e l d .  axis o f the molecule  i s a l i g n e d along the  The assignment o f t h e m o l e c u l a r axes i s shown below ,  N—-*y  where t h e x - a x i s i s p e r p e n d i c u l a r t o t h e p l a n e o f t h e m o l e c u l e . The dominant g - s h i f t , as w i t h C 1 0 ,  1 S  2  expected t o  be along t h e y - a x i s a l t h o u g h i n t h i s case i t w i l l be n e g a t i v e . These g - s h i f t s a r e determined equation  (8-1).  The g-value  from t h e g e n e r a l f o r m u l a g i v e n by a l o n g t h e d i r e c t i o n o f t h e maximum  v a l u e o f t h e h y p e r f i n e t e n s o r s h o u l d be c l o s e t o t h e f r e e s p i n value or s l i g h t l y greater. be s m a l l and p o s i t i v e .  The s h i f t a l o n g z, ^ %  z z  w i l l also  -126-  T a b l e 2 i n c l u d e s t h e r e s u l t s o f NO^ observed i n v a r i o u s m a t r i c e s and adsorbed on s u r f a c e s o t h e r t h a n t h o s e s t u d i e d h e r e . The r e s u l t s o f t h i s . w o r k a r e a l s o i n c l u d e d .  Comparison  o f the  spectrum o f adsorbed NO^ w i t h t h e w e l l - e s t a b l i s h e d s p e c t r a i n a v a r i e t y o f environments  i s the subject of the following  discussion.  The d i p o l e moment o f NQ^ i s c o n s i d e r a b l y l e s s than t h a t o f ClO^, b e i n g .29 D [157,158], and.on t h i s b a s i s a l o n e , one would expect somewhat weaker a d s o r p t i o n i n comparable  situations.  The  n i t r o g e n n u c l e u s has a n u c l e a r s p i n 1 = 1  and so a quadrupole moment  can a l s o a f f e c t t h e a b s o r p t i o n spectrum.  The weaker a d s o r p t i o n  f o r c e s a r e s u b s t a n t i a t e d by the. l o s s o f spectrum on warming t h e sample t o room t e m p e r a t u r e .  This i s i n contrast to the r e s u l t s of  C o l b u r n e t a l [159] who observed t h e spectrum o f r a p i d l y t u m b l i n g NO^ m o l e c u l e s a t room temperature i n 13X z e o l i t e s .  The p r e s s u r e s  o f NO;, i n e q u i l i b r i u m w i t h t h e z e o l i t e s were,however, s e v e r a l o r d e r s o f magnitude l a r g e r t h a n those i n t h e p r e s e n t e x p e r i m e n t s .  The  p r e s s u r e s needed t o observe t h e NO^ s p e c t r a were however,, much h i g h e r than those needed f o r C l O ^ i n d i c a t i n g a much reduced a d s o r p t i o n attraction. The expected o r i e n t a t i o n o f NO^ on a d s o r p t i o n d i f f e r s  from  t h a t o f CIQ2 s i n c e t h e d i p o l e moment i s a l i g n e d i n t h e o p p o s i t e direction.  One would e x p e c t , t h e n , t h a t t h e n i t r o g e n n u c l e u s w i l l  be c l o s e s t t o t h e a d s o r p t i o n s i t e s .  R o t a t i o n about t h e m o l e c u l a r  z - a x i s would seem more p r o b a b l e , t h e n , i n t h i s case than w i t h CIO .  -127-  The  r e s u l t s observed  from the a d s o r p t i o n on s i l i c a g e l  are q u i t e s i m i l a r t o those r e p o r t e d f o r the a d s o r p t i o n o f on MgO  [156].  The  i s o t r o p i c h y p e r f i n e s p l i t t i n g o f 56.3  i n good agreement w i t h , and v a r i e s l i t t l e other surfaces.  NO^ gauss i s  from, t h a t observed  on  In f a c t , the i s o t r o p i c p o r t i o n o f the h y p e r f i n e  t e n s o r changes l i t t l e between the s u r f a c e s s t u d i e d and NO^ t r a p p e d i n o t h e r media.  The  f i e l d s inherent i n these  molecules  different  environments v a r y from v e r y weak i n the i n e r t gas m a t r i c e s t o v e r y s t r o n g i n the s y n t h e t i c z e o l i t e s .  T h i s i m p l i e s t h a t the  s-character  o f the m o l e c u l a r o r b i t a l o f the n i t r o g e n i s not a p p r e c i a b l y a f f e c t e d by the s u r r o u n d i n g s are observed  o f the m o l e c u l e .  S m a l l changes,however  f o r the a n i s o t r o p i c components, but are s m a l l e r than  the l i n e w i d t h used f o r the s i m u l a t i o n s , agrees w i t h t h a t expected  The  assignment o f g v a l u e s  f o r t h i s molecule:.: g  i s greater  than  XX  the f r e e s p i n v a l u e g ; g i s l e s s than g ; and g very n e a r l y e yy e z z . equals g .  T h i s i s i n accordance w i t h the work on the  e  molecule  C0~  [160].  The 13X  isoelectfonic  observed  spectrum o f NO^  adsorbed on the  zeolite  i s not as s t r i k i n g as t h a t f o r ClO^ f o r s e v e r a l r e a s o n s .  D i r e c t evidence obvious.  The  f o r two d i s t i n c t a d s o r p t i o n s i t e s i s not  l i n e s h a p e i s somewhat d i f f e r e n t from t h a t  immediately observed  on the o t h e r s u r f a c e s and the spectrum i s b e s t s i m u l a t e d u s i n g a L o r e n t z i a n r a t h e r t h a n a Gaussian  lineshape function.  v a l u e s are s i m i l a r . t o those observed  The  on o t h e r s u r f a c e s and  g the  d e v i a t i o n s i n the a n i s o t r o p i c components o f the h y p e r f i n e t e n s o r  are  -128-  l e s s than t h e l i n e w i d t h .  The o n l y n o t i c e a b l e e f f e c t o f a d s o r p t i o n  i s the d i f f e r e n t lineshape.  The s t r o n g e l e c t r i c f i e l d s i n t h e  c a v i t i e s o f t h e z e o l i t e do n o t have t h e pronounced e f f e c t s o b s e r v e d i n t h e case o f ClO^.  T h i s i s s u r p r i s i n g i n t h a t t h e approach o f  the n i t r o g e n n u c l e u s i s much c l o s e r t o t h e o r i g i n o f t h e f i e l d s than was t h e c h l o r i n e n u c l e u s , due o f c o u r s e t o t h e proposed mode of adsorption.  '  NO^ adsorbed on H-mordenite y i e l d e d a spectrum which enabled a more p r e c i s e assignment o f p a r a m e t e r s .  The h y p e r f i n e  components a r e c l e a r l y seen and t h e g t e n s o r r e a d i l y measured. The assignment i s v e r y c l o s e t o t h a t o f Schaafsma  et a l [153],  o f t h e components o f b o t h t h e g and h y p e r f i n e t e n s o r s . a d s o r p t i o n s i t e s i n H-mordenite  The  a r e thought t o be i n t h e s i d e  p o c k e t s l i n i n g t h e main c y l i n d r i c a l tubes o f t h e s t r u c t u r e (see F i g u r e 1 1 ) , Each pocket has space s u f f i c i e n t ' f o r o n l y one m o l e c u l e r e d u c i n g b r o a d e n i n g due t o r e c o m b i n a t i o n o f t h e r a d i c a l t o form N^O^ and a l s o d i p o l a r b r o a d e n i n g caused by o t h e r NC^ m o l e c u l e s . In t h e experiments o f Schaafsma  e t a l , s o l i d N^O^ was  chqsen as t h e h o s t m a t r i x due t o i t s i n e r t n e s s towards NO^ and t h e absence o f any i n t e r n a l e l e c t r i c f i e l d s , b e i n g a m o l e c u l a r r a t h e r than an i o n i c m a t r i x .  D i s t o r t i o n o f t h e NO^ due t o s p a t i a l  effects  s h o u l d a l s o be m i n i m i z e d s i n c e t h e s t r u c t u r e o f t h e guest and h o s t m o l e c u l e s are- t h e same.  In l i g h t o f the s i m i l a r i t y o f the  parameters o b t a i n e d from t h e spectrum o f NO^ adsorbed on H-mordenite  t o t h o s e i n s o l i d N„0., i t appears a d s o r p t i o n , i n t h i s c a s e ,  -129-  has  l i t t l e o r no e f f e c t on t h e NO^.  I t i s l i k e l y , then, that  the ' t r a p p i n g p o c k e t s ' o f HT-mordenite s e r v e o n l y as i s o l a t i o n cages f o r t h e NO^  molecules  electronic structure.  and have l i t t l e e f f e c t on i t s  This i s i n accord w i t h the  observed f o r ClO^ adsorbed  parameters  on t h i s same z e o l i t e , the e f f e c t b e i n g  the s m a l l e s t f o r a l l t h e z e o l i t e s s t u d i e d .  -130-  CHAPTER TEN NITRIC OXIDE, NO N i t r i c o x i d e i s another s t a b l e p a r a m a g n e t i c m o l e c u l e which n o r m a l l y e x i s t s i n t h e gas phase.  B e r i n g e r arid C a s t l e [161]  have a n a l y z e d i n d e t a i l t h e spectrum o f NO i n t h e gas phase, where c o m p l e x i t i e s due t o o r b i t a l , s p i n and r o t a t i o n a l are p r e s e n t .  interactions  E a r l y attempts t o d e t e c t NO t r a p p e d i n r a r e gas  m a t r i c e s were u n s u c c e s s f u l [ 1 6 2 ] .  That i t does n o t g i v e r i s e t o a  d e t e c t a b l e spectrum i n t h e s e m a t r i c e s i s n o t s u r p r i s i n g as t h e i n t e r a c t i o n w i t h t h e environment i s p r o b a b l y n o t s u f f i c i e n t t o . quench t h e o r b i t a l motion o f t h e paramagnetic e l e c t r o n s u f f i c i e n t l y . T h i s s h a l l be d i s c u s s e d below. The NO m o l e c u l e i n i t s ground s t a t e i s n o t p a r a m a g n e t i c . NO i s a  2 ' TT m o l e c u l e .  • 2 The ground s t a t e o f t h e m o l e c u l e ( TTJ ) i s  nonmagnetic s i n c e s p i n and o r b i t a l a n g u l a r moments a r e a n t i p a r a l l e l , and t h e o r b i t a l magnetism  j u s t cancels the spin  magnetism.  J  -131-  The  paramagnetic c h a r a c t e r o f NO r e s u l t s from t h e ^3/2  st^tej  a consequence o f t h e s p i n and o r b i t a l momenta b e i n g a l i g n e d . 2  7i\^2 s t a t e i s s e p a r a t e d by 121 cm  states are appreciably populated  -1  from t h e ground s t a t e .  The Both  except a t t e m p e r a t u r e s below.  about 50°K. S o r p t i o n and magnetic s u s c e p t i b i l i t y  s t u d i e s on n i t r i c  o x i d e - s i l i c a g e l systems [163, 164] have i n d i c a t e d a p a r t i a l quenching o f t h e o r b i t a l a n g u l a r momentum.  I t thus appears t h a t  c e r t a i n environments may quench t h e o r b i t a l a n g u l a r momentum and enable t h e EPR. s p e c t r a t o be r e c o r d e d .  Recently, Lunsford  [165-167],  Gardner and Weinberger [168], and Hoffman and N e l s o n [169] have 2 r e p o r t e d s p e c t r a o f NO i n a zeolites,  TT s t a t e adsorbed on MgO, ZnO and v a r i o u s  i n which t h e o r b i t a l momentum seems s u b s t a n t i a l l y  quenched by t h e s u r f a c e f i e l d s o f t h e a d s o r b e n t s .  These s u r f a c e  f i e l d s were s t u d i e d as w e l l as t h e e f f e c t o f t h e a d s o r p t i o n on t h e NO, The  r e s u l t s t o be p r e s e n t e d  p r e v i o u s l y reported, although  here are i n accord w i t h  i n t h e p r e s e n t work a r e a c t i o n o f  the.NO w i t h c e r t a i n s u r f a c e s was observed i n a d d i t i o n .  Preparation  o f t h e samples i s t h e same as f o r t h e a d s o r p t i o n o f NO^. 10.1  Silica  those  Gel.  Attempts were made t o observe t h e spectrum o f NO adsorbed on s i l i c a g e l a t 77°K b u t were u n s u c c e s s f u l .  Although  Solbakken e t a l [163, 164] r e p o r t e d t h a t t h e f i r s t m o l e c u l e s  -132-  adsprbed a t t h i s temperature were i n t h e e x c i t e d s t a t e  '"'3/2  a n  ^  c o n t i n u e d up t o n e a r l y monolayer c o v e r a g e , no spectrum was observed. 10.2  13X S y n t h e t i c Z e o l i t e . The speptrum observed f o r NO adsorbed on 13X s y n t h e t i c  z e o l i t e a t 77°K< i s shown i n f i g u r e 40.  No h y p e r f i n e s t r u c t u r e was  e v i d e n t and t h e spectrum appears s i m i l a r t o t h a t r e p o r t e d by Gardner e t a l [168].  The parameters were a s s i g n e d by comparison  t o t h e s i m u l a t e d spectrum, f i g u r e 41, and a r e g i v e n i n T a b l e 3. S i n c e any h y p e r f i n e s t r u c t u r e i s a p p a r e n t l y l e s s than t h e l i n e w i d t h , the e f f e c t s o f t h i s and any o t h e r b r o a d e n i n g i n t e r a c t i o n s were i n c l u d e d i n t h e l i n e w i d t h and t h e spectrum was s i m u l a t e d u s i n g an a x i a l l y symmetric g t e n s o r .  10.3  H-Mordenite. The spectrum observed a t 77°K f o r NO adsorbed on  s y n t h e t i c H-mordenite i t a l s o s i m i l a r t o t h a t r e p o r t e d by Gardner e t a l [168] and t o t h a t o f L u n s f o r d  [165] r e p o r t e d on MgO.  Some s t r u c t u r e i s e v i d e n t and as an a i d t o a n a l y s i s , t h e spectrum o f "^NO was a l s o .recorded. 14  Figures 42 and:43 show t h e s p e c t r a f o r  15 NO and  NO r e s p e c t i v e l y . S i m u l a t i o n was attempted i n a s i m i l a r  manner t p t h a t o f . t h e 13X sample u s i n g an; a x i a l l y symmetric g t e n s o r , b u t now i n c l u d i n g h y p e r f i n e s p l i t t i n g s . T a b l e 3.  The r e s u l t s a r e shown i n  I t i s o b v i o u s from t h e r e s u l t i n g s p e c t r a , shown i n 14 15 f i g u r e s 44 and 45 r e s p e c t i v e l y f o r NO and NO, t h a t t h e s i t u a t i o n  ' TABLE 3 g values 3ferenc-e  £  (gauss) H y p e r f i n e components g "zz  R yy  xx  b  g l  T  T  T. yy  xx  •-  "  :  Medium  ZZ '  g//  165  1.996  1.996  1.89  35  169  1.994  1.994  1.873  28  166  1.997  1.997  1.91  31  168  1.970  1.970  1.7S6  168  1.967  1.967  1.773  168  1.990  < 10  14 NO. adsorbed @ • 77°-K 14 NO a d s o r b e d 3 77°K -14 . NO a d s o r b e d @ 77°K 14 NO..adsorbed • @ 77°K 14 NO a d s o r b e d § 77°K 14 NO a d s o r b e d . § 77°K  on MgO  on 4A  on ZnS  :  -  . •  .. 1.990  1.859  ±0.001  +0.001  ±0.01  t h i s work  1.967  1.967  1.78  t h i s work  1.994  1.994  1.S7  23  this-work v  1.994 .  1.994 •  1.87 '  38  L  V.-  '  '  •  •  ±1  on 13X  on  H mordenite _  on 5A  L i n e w i d t h used f o r s i m u l a t i o n s (gauss) 90.0  35.0  ,v  •  :  35.0  14 NO adsorbed on 13X @ 77°K 14 NO adsorbed on H-mordenite • .. Q 77°K ^NO  adsorbed on 8 77°K '  H-mordenite  -134-  FIGURE 40. EPR spectrum o f n i t r i c o x i d e adsorbed on 13X s y n t h e t i c z e o l i t e , r e c o r d e d a t 77° K.  -135-  FIGURR 4 1 . C o m p u t e r s i m u l a t e d EPR s p e c t r u m o f n i t r i c oxi.de a d s o r b e d on 13X s y n t h e t i c ' z e o l i t e , r e c o r d e d a t 77° K. •'  -136-  H  100  GAUSS  <———1>  H  FIGURE 42. ERR spectrum>of N n i t r i c oxide on Ilr-mordenite, r e c o r d e d a t 77° K.  adsorbed  -137-  He  100  GAUSS  15 FIGURE 43. EPR spectrum o f • • N. n i t r i c o x i d e adsorbed on H-mordenite, r e c o r d e d a t 77° K.  rl38-  361.45S  -139-  -140-  i s more complex than t h i s .  Both samples were evacuated t o remove  as irjuch adsorbed s p e c i e s as p o s s i b l e w i t h the r e s u l t i n g shown i n f i g u r e 46,  spectrum  The spectrum t h e n observed a t 77°K was i d e n t i c a l  i n b o t h cases and showed c o n s i d e r a b l e s t r u c t u r e . 10.4  Discussion. I n t e r a c t i o n o f t h e s u r f a c e f i e l d s w i t h AB  type  T r - r a d i c a l s quenches t h e o r b i t a l momentum o f t h e s e r a d i c a l s .  An  *  unsymmetrieal environment (TT x  l i f t s the degeneracy  o f the 2pir  qrbitals  and TT o r b i t a l s , d e f i n i n g the N-0 bond as t h e z - a x i s ) . y  F o r t h e NO  *  m o l e c u l e , t h e u n p a i r e d e l e c t r o n w i l l be i n the 2piT  x  level, i n t h e ,  absence o f any, s p i n - o r b i t i n t e r a c t i o n . 2 . E x p l i c i t formulae f o r t h e g t e n s o r o f an e l e c t r o n s t a t e were g i v e n by K a n z i g e t a l [ 1 7 0 ] :  i n a TT  A i s the c r y s t a l f i e l d  s p l i t t i n g ; A the s p i n - o r b i t c o u p l i n g c o n s t a n t ; E, t h e e f f e c t i v e 2 2 energy d i f f e r e n c e between the ;Tr l e v e l s and the £ l e v e l s ; and k, the e f f e c t i v e g f a c t o r f o r t h e o r b i t a l c o n t r i b u t i o n (k=l f o r t h e f r e e m o l e c u l e ) . The e q u a t i o n s are g i v e n by  (10-3)  -141-  -142-  The  apparent a x i a l symmetry o f the observed  s p e c t r a (due  t o the l i n e w i d t h ) i n d i c a t e s A/E must be s m a l l .  mainly  The c a l c u l a t i o n s  o f Gardner et a l on the 13X and H-mordenite [168] w i l l not r e p e a t e d h e r e s i n c e the same adsorbents The  were used.  s p e c i e s formed on a d s o r p t i o n o f NO  on H-mprdenite  and subsequent pumping p f the.sample, i s indeed c u r i o u s . a b l e t h a t the NO  [171]  0  I t i s reaspn  r  i s e a s i l y removed by pumping s i n c e i t s d i p o l e moment  i s much l e s s than t h a t o f NO,,. Stogryn  be  The  c o n t a i n i n g molecule a d s o r p t i o n o f ^NO  A v a l u e o f 0.158  0 was  r e p o r t e d by  s p e c i e s i s . o b v i o u s l y not due t o a n i t r p g e n s i n c e no change i n s t r u c t u r e was and ^NO,  observed  the. n u c l e a r s p i n o f ^ N  on  I=h  being  14 i n c o n t r a s t t o 1=1  for  N.  Numerous attempts, a t a n a l y s i s u s i n g  Computer s i m u l a t e d spep^ra were made w i t h no s u c c e s s . o f more than one s p e c i e s i s p o s s i b l e but no evidence  The  presence  for this  was  g i v e n by t e s t s o f i n c r e a s i n g the microwave power l e v e l . The  r e a c t i o n o f NO w i t h the s u r f a c e o f H-mordenite  has produced a s p e c i e s s t r o n g l y a t t a c h e d t o the s u r f a c e . t o remove the s p e c i e s by pumping i s e v i d e n c e  of t h i s .  Failure  It is likely  the NO has r e a c t e d w i t h some p a r t o f the s u r f a c e t o form a s p e c i e s w h i c h , i f not chemisorbed, i s v e r y s t r o n g l y a t t a c h e d . [ 1 7 2 ] , when s t u d y i n g the a b s o r p t i o n o f NO i n f r a r e d spectroscopy,  found t h a t NO was  T e r e n i n and  cp-workers  oh v a r i o u s z e o l i t e s u s i n g adsorbed as N 0 2  f r e e d by the r e a c t i o n p r o b a b l y b e i n g adsorbed.  ?  the oxygen  T h e i r assignment  was  shown t o be c o r r e c t by a d s o r b i n g N^O  directly.  The  N^O  would account f o r i d e n t i c a l s p e c t r a b e i n g observed  formation f o r ^NO  14 NO  s i n c e N„0  i s not p a r a m a g n e t i c .  The  r e a c t i o n o f the oxygen  pf  and  -143-  atpm w i t h the H-rjnordenite would t h e n be r e s p p n s i b l e f o r the observed spectrum.  -144-  CHAPTER ELEVEN DIFUJORAMINO RADICAL, N p  2  The NF^ r a d i c a l e x i s t s i n e q u i l i b r i u m w i t h i t s dimer . t e t r a f l u o r p h y d r a z i n e , N^F^, a t normal t e m p e r a t u r e s .  The  d i s s o c i a t i o n o f N^F^ i n t o NF^ r a d i c a l s has been s t u d i e d p r e v i o u s l y ( f o r example [173, 1 7 4 ] ) , and i t has been shown t h a t t h e d i f l u o r a m i n o r a d i c a l i s q u i t e s t a b l e and i s c a p a b l e o f e x i s t i n g i n the f r e e s t a t e .  indefinitely  At rpom t e m p e r a t u r e and-atmospheric  pressure,  the r a d i c a l i s p r e s e n t t o the e x t e n t o f o n l y 0.05 p e r c e n t .  The  r a d i c a l c o n c e n t r a t i o n reaches 90 p e r cent o n l y a t 573°K and one atmosphere,  a t 423°K and 1 mm, o r a t 298°K and 10 ^  atmospheres.  The EPR spectrum observed i n t h e gas phase c o n s i s t e d o f a s i n g l e b r o a d l i n e showing no h y p e r f i n e s t r u c t u r e due t o e i t h e r the n i t r o g e n o r t h e f l u o r i n e s [173].  I s o t r o p i c s p e c t r a showing  -145-  r e s o l v e d h y p e r f i n e s t r u c t u r e have been o b s e r v e d f o r NF„ d i s s o l v e d i n p p r f l u o r p d i m e t h y l h e x a n e [175] and i n l i q u i d  [174].  Adrian  e t a l [176] s t u d i e d t h e KV^ r a d i c a l i n an argon m a t r i x , b u t were u n a b l e t o o f f e r any f i r m i d e n t i f i c a t i o n o f t h e a n i s o t r o p i c components. Farmer e t a l [177] r e p o r t e d r e s u l t s f o r NF- i n b o t h argon and k r y p t o n m a t r i c e s a t 4.2°K.  Unfprtunately, n e i t h e r o f these studies  y i e l d e d the a n i s o t r o p i c h y p e r f i n e parameters. K a s a i and Whipple  More r e c e n t l y ,  [178] s t u d i e d t h e r a d i c a l i n a neon m a t r i x a t  4°K and were a b l e t o a s s i g n t;he o b s e r v e d p r i n c i p a l t e n s o r components t o the m o l e c u l a r axes.  A r e c e n t paper by McDpweTl e t a l [179] r e p o r t e d .  a d e t a i l e d s t u d y o f how an i n e r t gas m a t r i x and a p p r o p r i a t e p h y s i c a l . c o n d i t i o n s t o g e t h e r may i n f l u e n c e t h e n a t u r e and e x t e n t o f t h e o r i e n t a t i o n p f a p a r a m a g n e t i c s p e c i e s , u s i n g NF^ as an example, The w o r k . a c c o m p l i s h e d a complete a n a l y s i s o f t h e s p e c t r a a d a l s o n  a temperature, s t u d y . R e s u l t s o f t h e a d s o r p t i o n o f t h e NF^ r a d i c a l on H-mordenite 11.1  are reported here,  H-Mprdenite. The spectrum o b s e r v e d f o r ^ F ^ adsorbed on  at 77°K i s shown i n f i g u r e 47.  H-mordehite  S e v e r a l experiments were attempted  w i t h v a r y i n g c o n c e n t r a t i o n s b u t t h i s was t h e o n l y r e p r o d u c i b l e spectrum o b s e r v e d .  The o b s e r v e d s p l i t t i n g s a r e n o t s i m i l a r t o t h o s e  p r e v i o u s l y observed f o r t h e NF^ r a d i c a l i n o t h e r media.  H-mordenite  was used as t h e a d s o r b e n t f o r t h e s e e x p e r i m e n t s s i n c e i t has  -146-  -147-  produced consistent r e s u l t s with the other r a d i c a l s .  Computer  simulations of the pbserved spectrum are shown i n f i g u r e 48.  The  parameters are given i n Table 4. ,11.2  Discussion. The NF^ r a d i c a l i s valence i s p e l e c t r o n i c with CIC^,  the unpaired e l e c t r o n i n a b^ antibonding TT o r b i t a l .  haying  The ground,  2  e l e c t r o n i c s t a t e of tjie molecule i s  Bj, The  expected EPR  spectrum  of the r a d i c a l should show hyperfine s p l i t t i n g due to both the f l u o r i n e s and the nitrogen.  The lack of hyperfine s t r u c t u r e could  be a t t r i b u t e d to r a p i d recombination recombination  of the r a d i c a l s , i n f a c t , r a p i d  could even o b l i t e r a t e the e n t i r e spectrum.,  The  spectrum observed i n 5A molecular sieve by Colburn et a l [180] did, indeed show w e l l resolved hyperfine s t r u c t u r e with measured 14  19 N and  F couplings of 16 and 56 gauss r e s p e c t i v e l y .  reported g value was 2.009.  The  I t was assumed the I^F^ was screened  out  of the z e o l i t e e l i m i n a t i n g much l i n e broadening and the spectrum was due to f r e e l y r o t a t i n g NF  2  radicals.  The spectrum observed i n t h i s study has been a t t r i b u t e d to a species having an a n i s o t r o p i c g tensor with no  observable  hyperfine s t r u c t u r e . Table 4 gives the assigned values.  Figure 48a  i s an attempt to simulate the spectrum as due to an i s o t r o p i c g value with observed s p l i t t i n g s due to a nitrogen nucleus.  A  good f i t could not be obtained with regard t° e i t h e r i n t e n s i t i e s or l i n e p o s i t i o n s . As was the case vvith NO adsorbed on H-mordenite, the NF  or N F. ?  has probably reacted with the surface to fprm a  -148-  TABLE 4  t h i s work f i g u r e 48b  Isotropic Hyperfine component (gauss)  g-values (± 0.0005)  Reference  8  xx  2.0151  g  A, o .  hz  yy  2.0025  2.0084  t h i s work ' f i g u r e 48a  TI—?—n—|—  :  ^— •  ,  . ,  —p.  1  •" i ri  1  —  ,  —  '7.8  2.0100 J—.  ,  ii  • i'j.—,,.  ini,  1  -149-  (a)  •3219.593  234:599  '3249.599  3254.529  .593  FIELD (GRUSS)  (b)  r  i  3215.559-  '3228.2£9  3230.529  — * T  • -3253.659  FIELD (GfiUSS)  32S6.3S9  -.3279.059  .FIGURE 48. Computer s i m u l a t e d EPR s p e c t r a o f s p e c i e s formed on a d s o r p t i o n o f N F on H-mordenite, r e c o r d e d ,0 -2 4 a t 77^ K a) i s o t r o p i c g and h y p e r f i n e t e n s o r M  a n i s o t r o o i c a t e n s o r , no h v o c r f i n c  -150-  non-paramagnetic s p e c i e s and a p a r a m a g n e t i c s p e c i e s h a v i n g observable hyperfine s t r u c t u r e .  no  -151:-  GHAPTER. TWELVE SUMMARY T h i s chapter- i s i n t e n d e d  as a summary o f t h e work completed;  i n t h i s t h e s i s w i t h a view t o p o s s i b l e f u r t h e r a p p l i c a t i o n s o f s t u d i e s i n t h i s area.,  G r e a t e r amounts o f i n f o r m a t i o n are. s t e a d i l y -  becoming a v a i l a b l e on t h e t o p o l o g y  o f t h e v a r i o u s surfaces, s t u d i e d ,  the a r e a where l a c k o f knowledge has been t h e most  outstanding.  More d e t a i l e d c o n c l u s i o n s c o u l d t h e n be r e a c h e d c o n c e r n i n g t h e i n t e r a c t i o n s a t these g a s - s o l i d i n t e r f a c e s . The main s p e c i e s which has been s t u d i e d h e r e , c h l o r i n e d i o x i d e , has shown w i d e l y d i f f e r e n t i n t e r a c t i o n s w i t h t h e v a r i o u s a d s o r b e n t s used.  The H-mordenite samples y i e l d e d EPR s p e c t r a  h a v i n g measured parameters t h e l e a s t changed from those i n media o t h e r t h a n a d s o r b e n t s .  T h i s i n d i c a t e s t h e C1C*  obtained molecules  -152-  are p h y s i c a l l y t r a p p e d  i n the i n t e r i o r o f t h i s z e o l i t e ,  having  l i t t l e i n t e r a c t i o n w i t h the i n t e r n a l e l e c t r o s t a t i c f i e l d s . i n t e r a c t i o n w i t h s i l i c a g e l i s somewhat s i m i l a r , a l t h o u g h amorphous s t r u c t u r e o f this,, adsorbent makes i t d i f f i c u l t  The the  to  q u a n t i t a t i v e l y p l a c e the CIC^ m o l e c u l e s i n any p a r t i c u l a r a r e a i t s internal surface.  13X,  s t r u c t u r e which enables one, experiments,  on the o t h e r hand, has  an  of  ordered  from d a t a o b t a i n e d from the  t o v i s u a l i z e the a c t u a l a d s o r p t i o n s i t e s i n v o l v e d .  These have been d i s c u s s e d i n C h a p t e r E i g h t . o t h e r z e o l i t e s may  The  results for  s i m i l a r l y be a n a l y z e d w i t h r e g a r d  a d s o r p t i o n s i t e s and i n t e r a c t i o n s w i t h the i n t e r n a l  the  to surface  fields. A p u b l i c a t i o n concerning synthetic zeolites The  a study o f ClO^ adsorbed  [181] has r e c e n t l y appeared i n the  s p e c t r a o b t a i n e d on the z e o l i t e s 13X  and  literature.  10X were not  i n terms o f a c t u a l a d s o r p t i o n s i t e s , p r o b a b l y due  on  t o the  analyzed fact  t h a t s u c c e s s f u l computer s i m u l a t i o n o f the s p e c t r a c o u l d not obtained.  The  two d i s t i n g u i s h a b l e s i t e s o b s e r v e d i n the  study were not n o t i c e d .  The  be  present  l i n e w i d t h s f o r the s p e c t r a r e p o r t e d i n  t h e i r p u b l i c a t i o n would have o b l i t e r a t e d t h e s e  features.  T h i s same paper by P i e t r z a k and Wood a l s o c o n t a i n e d study o f NG^  adsorbed on t h e s e same z e o l i t e s .  comparable s p e c t r a t o these o b t a i n e d synthetic zeolite.  The paper  contained  i n t h i s s t u d y f o r the  R e s u l t s o b t a i n e d h e r e f o r N0  9  a  were not  13X  -153-  s i g n i f i c a n t l y d i f f e r e n t from those o f NC^ s t u d i e d i n o t h e r media. In f a c t , t h e s p e c t r a observed on t h e s y n t h e t i c z e o l i t e H-mordenite i s i n e x c e l l e n t agreement t o t h a t f o r N0„ i n an N O . m a t r i x , b o t h media s t u d i e d a t 77°K. N0  2  The proposed a d s o r p t i o n s i t e s f o r t h e  m o l e c u l e s i n t h i s z e o l i t e a r e t h e s m a l l p o c k e t s l i n i n g t h e main  passage-ways.  I n t h e case o f ClO^,  t h e observed s p e c t r a c o r r e l a t e d  w e l l w i t h t h e s t r u c t u r e s o f t h e 13X z e o l i t e s , whereas f o r NO^ t h i s was n o t s o . S p e c i f i c a d s o r p t i o n s i t e s may o n l y be a s s i g n e d t o t h e z e o l i t e , H-mordenite. has  This i s a t t r i b u t e d t o the f a c t that C10  a l a r g e r d i p o l e moment t h a n N 0  2  2  which enables i t t o i n t e r a c t  more s t r o n g l y w i t h s p e c i f i c c a t i o n s i t e s i n 13X.  I n H-mordenite,  these s i t e s a r e n o t as w e l l - d e f i n e d , and c o u p l e d w i t h t h e s m a l l d i p p l moment o f t h e N 0 , t h e observed s p e c t r a suggest t h e m o l e c u l e s t o be 2  confined i n these s i d e The  pockets.  s p e c t r a observed f o r n i t r i c o x i d e adsorbed on  v a r i o u s z e o l i t e s show y e t a n o t h e r p o s s i b l e e f f e c t o f a d s o r p t i o n . While t h e s p e c t r a which a r e f i r s t apparent on a d s o r p t i o n a r e s i m i l a r t o those observed b y o t h e r s on a v a r i e t y o f s u r f a c e s , pumping o f t h e sample t o d e c r e a s e . t h e s u r f a c e y i e l d s a new spectrum.  c o n c e n t r a t i o n o f t h e NO.on t h e This i s assigned t o a species  formed by a r e a c t i o n o f t h e NO m o l e c u l e s w i t h t h e s u r f a c e , s p e c i e s has been shown n o t t o c o n t a i n n i t r o g e n .  T h i s new  This i s  c o n f i r m e d b y t h e f a c t t h a t i d e n t i c a l EPR s p e c t r a a r e observed 15 f o r both  14 NO and  NO.  The observance o f c h e m i c a l  reactions  on s u r f a c e s e i t h e r t h r o u g h t h e f o r m a t i o n o f a new s p e c i e s o r a  -154-  change i n the s p e c t r a f o r the o r i g i n a l s p e c i e s i s then  another  a r e a w i t h wide p o s s i b i l i t i e s . The use t h e n , o f the EPR  t e c h n i q u e , i n the s t u d y o f the  g a s - s o l i d i n t e r f a c e can g e n e r a l l y be c a t e g o r i z e d i n t h r e e  areas:  I n f o r m a t i o n about the n a t u r e o f the s u r f a c e s i s p o s s i b l e i n many cases and the o p p o r t u n i t y f o r study h e r e i s limited  o n l y t o the number o f s u r f a c e s which y i e l d EPR The  a d d i t i o n o f gaseous m o l e c u l e s  widens the scope c o n s i d e r a b l y . ClO^, t h i s t e c h n i q u e may enables  to these  signals.  surfaces  In t h e s e c a s e s , as was  found f o r  p r o v i d e an " i n e r t matrix!' which perhaps  the s p e c i e s i n q u e s t i o n t o be s t u d i e d w i t h g r e a t e r  f a c i l i t y than o t h e r EPR  t e c h n i q u e s , o r may  even p r o v i d e a means  o f study where o t h e r s have not as y e t been found.  Included i n t h i s  area a l s o a r e the p o s s i b i l i t i e s o f r e a c t i o n o f the gaseous molecules  w i t h t h e s u r f a c e s t o y i e l d new  adsorbed s p e c i e s , o r even s p e c t r a now b e f o r e the a d s o r p t i o n , none was case w i t h n i t r i c o x i d e .  The  evident.  new  t o the s u r f a c e , whereas The  l a t t e r was  the  o p p o r t u n i t i e s are e x t r e m e l y  large  l a s t g e n e r a l a r e a where the use o f EPR has  found  in this particular  The  due  species; either a  area.  v a l u e i n these s t u d i e s i s i n the a r e a o f the dynamical  behaviour  o f the adsorbed m o l e c u l e s .  The motion o f the adsorbed  molecules,  e i t h e r h i n d e r e d o r f r e e may  be s t u d i e d at a v a r i e t y o f temperatures  by t h i s t e c h n i q u e .  The p u b l i c a t i o n o f P i e t r z a k and Wood  mentioned e a r l i e r was  such a s t u d y , a l t h o u g h  i t was  not  [181] completely  -155-  s u c c e s s f u l i n t h e ease o f CIO2. The  o p p o r t u n i t i e s f o r f u t u r e work i n t h i s  area,  complemented by o t h e r s p e c t r o s c o p i c , t e c h n i q u e s , l o o k p r o m i s i n g . The EPR t e c h n i q u e has c e r t a i n l y n o t been e x p l o r e d t o i t s f u l l e s t i n any o f t h e t h r e e areas mentiqned.  S i n c e knowledge o f t h e  s u r f a c e s i s v i t a l t o an. u n d e r s t a n d i n g  o f t h e r e s u l t s , t h e more  accurate the information a v a i l a b l e i n t h i s area, the b e t t e r the conclusions.  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Symons, The S t r u c t u r e o f InorganjLc R a d i c a l s , E l s e v i e r P u b l i s h i n g Co., London, 1967, p. 242,  -166APPENDIX  I M P L I C I T  R G A L * 3 { A - H , 0 - Z )  pI  H ( 9  ME N S I  I AA2(  UN  L , 9 1 )  9 1, 9 1 ) , A A L R H 2 (  2 C 6 ( 9 1  , 9 1 )  3 S T 2 ( 9 1 )  , PHI  , T P ( 9 L,91)  , G ( 9 1 , 9 1 ) ,  9 1 , 9 i ) > C  91  ( 9 1 . ) ,. S P ( 9 j  , 9 1 )  } , C P ( 9 1  ,C5R2rH]  , C T 2 ( 9 l >  U  G 2 ( 9  , C 2 ( 91  ) , S T (  , G A L ^ H 2 l 9 i )  I,9  , 9 1)  9 1 ) , C T ( t  91  P H I i l { 9 1 )  i ) , A ( 9  L,9  I),  , C 4 ( 9 \ , 9 1)  , C i? ( 9 1  ) , S P 2 < 9 1 ) , C P 2 (  , 91  9 j )  ) ,  ,  »riTLE(20)  R t A 0 ( 5 , 3 3 3 ) T I T L E 8 9 4 )  R E AO K O U N T = 0 R  EA  a9  D ( 5 ,  R E A D  NTI  0)  ME S » N T i l F . T A ,  G X , G Y , G Z , F R E  ( '5, 3 9 1 ) A X , A Y , A Z  W R I T E ( 6 , B 3 3 W R I T E (  )T  1  NPHI  Q, $ P I N '  !  '  •  .'  '  " ~  ~  ITL.E  6 , 3 9 9 ) F R E Q  W K I T E ( 6 , 8 3 Q ) G X , G Y , G Z W R I T E ( 6 , 3 9 6 ) A X , A Y , A Z W R I T E  ( 6 ,  3 9 7 ) S P I N , , M T H E T A , M ? H I  3 B 3  F O R M A T ( 2 0  3 9 0  F O R M A T  ( 3 T 1 0 .5  )  891  F O R M A T  ( "3 F-1 0 . 3  )  3 9 4  .  :  A 4 )  F O R M A T ( 3 1 3 )  8 9 9  FORMAT  3 3 0  F O R M A K  ( I X ,  8 9 8  FORMA T ( IX,  8 9 7  F O R M A T !  • T H E  j X ,  ' AX=  ' , F 1 0 .  L X , ••' 5 P I N -  S T A R T = S C L O C K ( 0 . 0 F R E Q = F  F R E Q U E N C Y .  RE4}  IS  = V,F10.!>,J  ' G.X  3 , »  ' * F L O . 1,  •  .  S F I O . 5 , 1  G Y f  It  A Y ?  J , r I  F l O . 5 , 0,3>  N THE T A =  ' , F 1 0 . 5  • '  A Z ~  ' , f  • ,I3,«  )  3 9 9 6 2 6 4 0 1 - 0 6  4  A Y = A'Y * G Y * L . 3 9 9 6 2 6 D + 0 6 AZ = AZ « G Z * 1 . 3 9 9 6 2 6 4 0 + 0 6 ;  G X 2 = G X * G X : G Y 2 - G Y * G Y G Z 2 = G Z * G Z  • ••  AX 2 = AX * AX  : .  A Y 2 - A Y * A Y  i  '  : •  A Z 2 = A Z * A Z AX Y 2~ A X 2 * A Y G A X 2 =  2  GX2*-VX2  GAY2s=GY  2 * A Y  2  G A Z 2 = G Z 2 * A Z 2  B O O . HH=6  9 2 7 3 2 0 - 2 0 . 6 2 5  l7l"J-2  7'  R A D = 1 . 7 4 5 3 3 9 2 5 1 9 9 4 3 0 - 0 2  D P H I = N P H I , : IF  ( N P H l  . G T .  1) Q P H I = 9 0 .  D O / F L O A T I N P H I r-i )  A I = ( A X 2 * A X 2 - 2 . 0 0 * A X 2 * A Y 2 + A Y 2 -? A Y 2 ) * G X 2 * G Y A 2 - A 1 * G A Z 2 A 3 - H H / 6 0  ' •  A 3 = A 3 * A 5 A 4=5 P I N  *J  SJM N + 1 . 0 0 )  ' )  G Z -  * l . 0 0 + 06.  AX = A X * G X * U  MHZ•  1  2  :  ),  1 0 . 3 , » ' .N1PHU  IN  G A U S S '  * , I 3 )  -167r A 6 = A5*=FRE'J  11  SMI=SPIN Bl=SMi*SMI B 2 = A 3 * 8 1 / 2 . 0 D 0 ; B 3 - A 3 * ( A 4 - ii I ) / 4 . O Q 0 B 4 = A 5 » S M I  '  I F ( S M I . L T . S P I N ) G Q  TO  56  PHI.CU =0.0 00 P H I D ( 1 ) = P H T  ( 1)  A A P = D P H I * K 4 0 D I V = 1 . 0 / F L 0 4 T ( N T H E T A * - 1 )  1) = i  C T ( C T 2 (  1)  .0  0  = 1 . 0 0  S T ( l ) = 0 . J O S T 2 ( 1 ) = 0 . 3 0 DO  17  I = 2 , N T H F T A  C T ( 1 ) - = C T  P  I - 1 ) - 0 I V  F(  I )  ) = <)S J••<T ( u o o - c r 2 ( s r 2.{ n = s r ( i ) * s r (11 ST{  17  (  > =C T ( I ) f C  T2 ( I I  I  ))  C I F { N P H i  . L t . . l)Gi3  OfJ  9 3  PHI  { I ) - P H I.(  90  Tp  ' l ? 2 » N P H I  6 0  ••  I ~ 1 ) + A A P,  P H I O t I ) = P H I D ( [ r l J t D P H I  6 0  DO  8 9  NP =  S P I N P ) •  L , N P H l  = D S I N ( P H I  ( N P )  C P ( N P > = O C O S ( P H I S P 2 1 N P  ) r S >:( M P ) * S P (  C P 2 ( N P ) - C * M N '  )  I N P ) ) NP )  ) * C P < N P )  ,  C S P 2 ( N P ) = C P 2 { N P ) - S P 2 GAL 89  (  NP)  ) •- G X 2 * C P 2. ( N ? ) * G Y 2 * S P 2 ( N P  PH 2 { NP  )  CONTINUE:  •  DO  4 3  00  4 3  rtP^l-.NPHl NT = I jJiTJlE T A  ,  G ( N T , N 9 ) = u S ):< T { i i A L P H 2 ( N P ) * S T 2 ( N T ) +GZ.2 * C T 2 ( N T ) ) G 2 { N T f fNiP ) - G (•'•) T , N P ) 4? r { N T , N f j ) 3  A A L P H 2 ( A ( N T ,  1i  ii P  T ,  h ? )  > «= J :S Q  = ( G A X 2 * C ? 2 ( N P ) + G A Y 2 * S P 2 ( N P ) >/ 0 2 ( N T , N P ) T { G 2 ( N T ,:^ P ) * A A L P H 2 ( N T , N P ) * S T 2 ( N T ) + G A Z  7  2*  C T 2 (, N T ) ) / G ( N T  IMP). A A 2 ( N T  t  N P )•= A ( N T  N  T  P ) * A ( N T t N P )  C 1 ( N T , N P ) ='M ( N T tiPi) C 2 (  NT r NP » - G A L P H 2  ) ^ G 2 ( NT » N P ) * G 2 ( N T , NP  CS  j = C 4 (  NP  C 6 ( N T , t M P ) TP( N T , N P 4B  1122  )'*C  2 ( NT , NP  )  N T , N>> ) T A A L P H 2 ( N T , N P )  = - v 2 ( N T , N ? ) * p 4 ) = " G 2 2 f G A L P H 2  ( NT  ( NP)  , N P ) / G 2 ( N T , N P )  C O N T I N U E  r  56  ( NP ) «?AA2 ( N T , HP )  C4 ( NT , NP ( N T ,  •  :  * A \L P H 2 ( N T , N P )  r  DO  5B  DO  5  B  3  N P = l.,NPf|I  3  NT = I,NT  ~ ( B 4 r A ( N T  .VJ2 =  r  A  A6  ) / G ( N T  A  ( ( G 7.2 v ( C M I -SO  2 ( NT)  N T ,NP)  * C SP2  2 ) / C 2 ( N T , M P ) <N G A L P H 2 ( 3 T , N P )  tN  P)  ..W><=-J:i  C C = tt 2 1 P ) +• (  f  MET N I?) -  )  X = B 3 2 - C e * " 4 .  00  - G A L P H2(  ( NP)  ) / C 4  NP  ) * A X Y 2  NP  ) * A Z 2 )  * * 2^S  T 2 ( N T ) #C T 2 ( N T ) ) /'C 6 { N T ,  ( N T , N P ) ) •+ ( B 3 / G 2 ( N T , N P ) ) * ( ( C L ( N T , N P ) * A 7 . ) /C  I ( NT , NP i+ iA 2 - C  T2 ( NT)  *Q^!>2 ( N P  ) J-/C5 !  -168-  H{NT,NP)=(-83+DSQ*T(X))/2.pQ C 53 C  CONTINUE I FJSM_I_.LT ._SP (N._OR,KOONT GT.O )G0 TO XI 11 r  iiiiRI T f i ( I , 6 0 3 ) ( C J ( I )  , 1= L , N THET  A )  W R I T F ( 1 , P O O ) ( P H I 0 ( I ) , 1 = 1 , N P H I > 1111  W R I T E ( 6 , 1 0 0 0 ) WRI DO  T E ( o , 1 0 Q 2 ) P H I D I 1 ) 7 0  N P = I , N P H f '  I F ( N P . G T . DO  8 0  1 ) W R I T E ( 6 ,  .MT= I,  1 0 03 ) P H I D ( N P )  NTH E T A i 4  K=NT+1 L =NT  f  2  M=NT«-3 W^I T E { 6 , 1 0 0 1 ) H ( N T , N P ) .  1 H (  80  C  70  M,  N P ) , T P ( M , MP )  O  N  T  I  U  E  '' '  1  ~  F O R M A T  1001  F 0 K M A T < 5 X ,  10Q2  ~  (K,NPnH{L,NP)  t T P (L » NP )  • '  I L H 1 ,3 ( / > , 3X , ^ ( • F I E L 3 ( G A U $ S  F O R M A T ! 3  t+(F3.  *P H 1~ ' >F 5  F O R M A T ! 3 ( / ) oo  3o  2 , 5 X , F 3 .  ~  ^  ^  n  t  1»' '  i =i t f i P - H i  '  1 , 6 0 1 ) [ H ( J , I ) , J * l , N T H E T A )  WRI T t ( 1 , 6 0 2 ) ( T P ( J , I ) , J = l , N T H E T A )  600  2 X ) / / )  ,  31  3 1  TY» ,  0 c GR LE S ' )  , ' P H I - ' , - F 5. 1 / )  W R I T F ( PQ  ) • , 3 X , • I \l T E N , S I  5 , 5 X ) )  30  5Q1  ,Tf»  '  CONTINUE:  LOOO  100  N  , T P i N T ,N P ) , H ( K , N P )  i = 1 t N P H l  '  F O R M A T ( 1 J F 6 . 2 ) •  F O R M A T  ( 10Fci . 2 )  60 2  F O R M A T ! 1 0 F 3.5)  603  FORM A T ( 1 J F O  .4 )  C  5  SMI -O - 1 . 0 0 I F ( S M I .LT . - S P I N ) G O T O 5 GO TO 11 ' T I M E = 5 C L g C - ^ ( START) WR I T£ ( 6 , 35 p ) T IMF NI'IMC-5-iNTIKFS-l  KOUNT^KO'JNTt-l .35 0  IF(NT IMES.GT.Q) FORMA TJ J H 0 _ _ T j STOP ' END f  GO T O 4 Mb R E jUIi<FI)<  » F 3 . 3 ,  '  S E C O N 0 S J / )_  '  T  

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