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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.Sc, U n i v e r s i t y of 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 the Department or Chemistry We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 19 71 In p resent ing t h i s t h e s i s in p a r t i a l f u l f i I m e n t o f the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e fo r reference and study. I f u r t h e r agree t h a t permiss ion fo r e x t e n s i v e copying o f t h i s t h e s i s fo r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s r e p r e s e n t a t i v e s . It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l not be a l lowed without my w r i t t e n p e r m i s s i o n . 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 Supervisor: .C.A. McDowell ABSTRACT E l e c t r o n paramagnetic resonance techniques have been used t o i n v e s t i g a t e the nature and p o s s i b l e e f f e c t s of adsorption of gaseous species on s e v e r a l adsorbents, i n p a r t i c u l a r s e v e r a l s y n t h e t i c z e o l i t e s , at temperatures from 77°K upwards. A n a l y s i s of the s p e c t r a obtained has been aided through computer s i m u l a t i o n of the various s p e c t r a and comparison of these to the a c t u a l observed s p e c t r a . The molecule c h l o r i n e d i o x i d e ( ClO^ ) has been s t u d i e d i n v a r i o u s low temperature matrices but l i t t l e has been p u b l i s h e d f o r "'CIO2 i n the adsorbed s t a t e . An attempt was made to f i n d an adsorbent such t h a t an i n e r t matrix might be approximated, to give a base from which to make comparisons. To t h i s end, adsorbents i n c l u d i n g s i l i c a g e l , 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 vary between those 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 to other >• . . . matrices were obtained, to those from 13X where i t was evident 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 of the ClO^ were present. In the 13X as i n the other s y n t h e t i c z e o l i t e s , EPR parameters markedly d i f f e r e n t from other s t u d i e s were found and were a t t r i b u t e d to the int 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 . R e s ults obtained at room temperature f o r these adsorbents ranged from ClO^ molecules f r e e l y r o t a t i n g i n the cages o f the z e o l i t e s to other molecules having hindered r o t a t i o n s . Nitrogen dioxide ( N0_ ) was also investigated with a view to 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 . Although changes as marked as for Cl-2 compared to other studies were not observed, the 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 spectra c l o s e l y approximating those obtained i n s o l i d ^2^4 matrices. It i s proposed the NC^ molecules are caged i n the numerous side pockets emanating from the main channels i n t h i s z e o l i t e and are e f f e c t i v e l y i s o l a t e d from other NO^ molecules. The r e s u l t i n g spectra are s t r i k i n g l y more resolved than those obtained using other adsorbents and enabled accurate computer simulations to be made. The adsorption of n i t r i c oxide ( NO ) produced an e f f e c t not found with the other molecules. A new species was formed from a r e a c t i o n of the NO with H-mordenite and could not be removed at room temperature, i n d i c a t i n g a strong bond to the surface. The new species does not contain nitrogen as i d e n t i c a l spectra were obtained from adsorption of ^NO and "^NO. Attempts to observe spectra which could be assigned to the difluoroamino r a d i c a l from adsorption of tetrafluorohydrazine were unsuccessful. The spectra observed were assigned to a species having no hyperfine structure and an a n i s o t r o p i c g tensor. i i i TABLE OF CONTENTS Page Abstract i L i s t of Tables v i L i s t of Figures v i i Acknowledgments X 1 CHAPTER ONE: INTRODUCTION . 1 CHAPTER TWO:. ADSORPTION 5 2.1 Surfaces 6 2.2 C l a s s i f i c a t i o n of Isotherms 7 2.3 Volume F i l l i n g of Pores H 2.4 Adsorption Forces 12 2.4.1 P o l a r i z a t i o n Energy 14 2.4.2 F i e l d - d i p o l e Energy. 14 2.4.3 F i e l d Gradient-quadrupole Energy 15 CHAPTER THREE: ZEOLITES 16 3.1 Adsorption i n Z e o l i t e s 21 3.2 St r u c t u r e s of Z e o l i t e s 25 3.2.1 X,Y Type 25 3.2.1.1 Cation P o s i t i o n s 28 3.2.2 A Type 3 0 3.2.2.1 Cation P o s i t i o n s 32 3.2.3 Mordenite 3 4 3.2.3.1 Cation P o s i t i o n s 36 CHAPTER FOUR: ELECTRON PARAMAGNETIC RESONANCE 3 8 4.1 Theory , 38 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 45 4.1.2 The Hyperfine I n t e r a c t i o n . ' 46 .4.1.3 Other I n t e r a c t i o n s 48 •4.1.4 EPR Spectra 49 CHAPTER FIVE: ADSORPTION STUDIES 5 2 5.1 EPR Studies of Radicals on Surfaces 54 X V TABLE OF CONTENTS (cont.) 5.1.1 N o n - Z e o l i t i c Adsorbents 5.1.2 Z e o l i t i c Adsorbents •5.2 S p e c i a l Adsorption E f f e c t s CHAPTER SIX: EXPERIMENTAL 6.1 Vacuum System 6.2 Sample Tubes 6.3 Adsorbents 6.4 Sample Pre p a r a t i o n 6.5 Gases 6.5.1 C h l o r i n e Dioxide C10 2 6.5.2 Nitrogen Dioxide N0„ ,. 6.5.3 N i t r i c Oxide NO 6.5'. 4 T e t r a f lurohydrazine N2^4 6.6 Spectrometers CHAPTER SEVEN: ANALYSIS OF ELECTRON PARAMAGNETIC RESONANCE SPECTRA CHAPTER EIGHT: CHLORINE DIXOIDE, C10 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 Sy n t h e t i c Z e o l i t e 8.5 10X Sy n t h e t i c Z e o l i t e 8.6 Lithium Exchanged 13X S y n t h e t i c Z e o l i t e 8.7 D i s c u s s i o n CHAPTER NINE: NITROGEN DIOXIDE, N0 2 9.1 S i l i c a Gel 9.2 13X Sy n t h e t i c Z e o l i t e TABLE OF CONTENTS (cont.) Page 9.3 H-mordenite 116 9.4 Di s c u s s i o n 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 Di s c u s s i o n 140 CHAPTER ELEVEN: DIFLUORAMINO RADICAL, Yl? 144 11.1 H-mordenite 145 11.2 D i s c u s s i o n 147 CHAPTER TWELVE: SUMMARY 151 REFERENCES . 156 APPENDIX 166 v i LIST OF TABLES Page 1. The p r i n c i p a l values of the hyperfine and g tensors 85 f o r ClO^ i n various media. 2. The p r i n c i p a l values of the hy p e r f i n e and g tensors 113 f o r N0 2 i n va r i o u s media. 3. , The p r i n c i p a l values of"the h y p e r f i n e and g tensors 133 f o r NO i n various media. 4. g-yalues of the spectrum observed when N^F^ was 148 adsorbed on H-mordenite at 77°K. v i i LIST OF FIGURES Page 1 . Five d i f f e r e n t types of adsorption isotherms, 8 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 of z e o l i t e s : . 1 8 (a) SiO^ tetrahedron (b) AIO4 tetrahedron. 3 . The truncated octahedron, or s o d a l i t e cage. 2 0 4 . Cation p o s i t i o n s f o r z e o l i t e s of var y i n g 2 2 S i / A l r a t i o n s : (a) 1 / 2 (b) 1 / 1 . 5 i The s t r u c t u r a l framework o f the X type 2 6 s y n t h e t i c z e o l i t e . ' 6 . The type I I 26-hedrbn cage, or ffau j a s i t e 2 7 cage. . 7 . Cation 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 . 2 9 8 . The s t r u c t u r a l framework of the A type 3 1 s y n t h e t i c z e o l i t e . 9 . The.type I 26-hedron cage. 3 3 1 0 . The s t r u c t u r a l framework of s y n t h e t i c 3 5 mordenite: (a) c h a r a c t e r i s t i c chain s t r u c t u r e (b) c r o s s - s e c t i o n a l area of a chain. 1 1 . Cation p o s i t i o n s i n s y n t h e t i c mordenite. 3 7 Aluminium and S i l i c o n at the centers of each tetrahedron are not shown. 1 2 . L o r e n t z i a n and Gaussian f i r s t d e r i v a t i v e curves. 4 4 1 3 . a) A schematic diagram of the vacuum system 6 4 used i n these experiments, b) The.sample tubes used i n these experiments. 14'.. a) Quartz dewar used f o r v a r i a b l e temperature 6 6 EPR .experiments. b) A Varian V - 4 5 4 6 l i q u i d n i t r o g e n dewar. 1 5 . Block diagram of a Var i a n E - 3 X-band 7 0 Spectrometer system. 1 6 . The molecular and magnetic f i e l d coordinate 7 5 system. '• y m LIST OF FIGURES (cont.) Ge n e r a l i z e d lineshapes of powder EPR spe c t r a f o r a species w i t h no hy 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 tensor (b) f u l l y a n i s o t r o p i c g tensor. The molecular a x i s system f o r c h l o r i n e d i o x i d EPR spectrum of 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 , recorded at 77°K. Computer simulated EPR spectrum of 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 , recorded at 77°K. EPR spectrum of 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 , recorded at room temperature. Computer simulated EPR. spectrum of c h l o r i n e d i o x i d e adsorbed on s i l i c a gel> recorded.at room temperature. EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at 77°K. Computer simulated EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at 77°K. • ' • :•• EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at room temperature Computer simulated EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at room temperature. EPR spectrum of 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 , recorded at 77°K. Computer :simulated EPR spectrum of 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 , recorded at 77°K. EPR spectrum of 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 , recorded at 77°K. i x LIST OF FIGURES (cont.) Computer simulated EPR spectrum of 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 , recorded at 77°K. EPR spectrum of 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 , recorded at 77°K. Computer simulated EPR spectrum of 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 , recorded at 77°K. EPR spectrum of 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 , recorded at 77°K. Computer simulated EPR spectrum of nit 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 , recorded at 77°K. Computer simulated EPR spectrum (assuming a x i a l l y symmetric g and hy p e r f i n e tensers) 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 , recorded at 77°K. EPR spectrum of n i t r o g e n d i o x i d e adsorbed on H-mordenite, recorded at 77°K. Computer simulated EPR spectrum of n i t r o g e n d i o x i d e adsorbed on H-mordenite, recorded at 77°K. EPR spectrum of n i t r o g e n d i o x i d e adsorbe on H-irsordenite, recorded at 77°K. Computer simulated EPR spectrum of nit r o g e n d i o x i d e adsorbed on H-mordenite, recorded.at 77°K. EPR spectrum of n i t r i c oxide adsorbed on 13X s y n t h e t i c z e o l i t e , recorded at 77°K. Computer;simulated EPR spectrum o f n i t r i c oxide adsorbed on 13X s y n t h e t i c z e o l i t e , :recorded at 77°K. X LIST OF FIGURES (cont.) 14 £ M £ 42. EPR spectrum of N n i t r i c oxide adsorbed pn H-mordenite, recorded at 77°K. 15 43. EPR spectrum of ' N n i t r i c oxide adsorbed 137 on H-mordenite, recorded at 77°K. 14 44. Computer simulated EPR spectrum of N 138 n i t r i c oxide adsorbed on H-mordenite, recorded at 77°K. 15 45. Computer simulated EPR spectrum of N 139 n i t r i c oxide adsprbed on H-mordenite, recorded at 77°K. 46. EPR spectrum observed a f t e r a d s o r p t i o n 141 of n i t r i c oxide cm. H^mordenit;e, followed by evacuation, recorded at 77°K. 47. EPR spectrum observed a f t e r adsorption o f 146 N2F4 on- H-mordenite, recorded at 77°K. 48. Computer simulated EPR sp e c t r a of species 149 formed on adsorption of N2F4 on H-mordenite, recorded at 77°K: (a) i s o t r o p i c g and hyperf i n e tenspr (b) a n i s o t r o p i c g tensor, no h y p e r f i n e s p l i t t i n g . ACKNOWLEDGEMENT • . . I am g r a t e f u l to Dr. C A . McDowell f o r h i s i n t e r e s t and. support throughout the course of my graduate s t u d i e s . I would a l s o lj.ke to express my a p p r e c i a t i o n to 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 ; to Mr. John T a i t f o r h i s i-nvaluable a s s i s t a n c e during the pr e p a r a t i o n of the t h e s i s , to the other members of 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 ; to Mr, Tom Markus f o r the care of the EPR spectrometers; and last> but c e r t a i n l y hot l e a s t , to my wife S y l v i a f o r her patienqe and help during the f i n a l stages of p r e p a r a t i o n of the manuscript. The awards from the N a t i o n a l Research C o u n c i l of Canada re c e i v e d during my graduate s t u d i e s are als o g r a t e f u l l y acknowledged. -1-CHAPTER ONE  INTRODUCTION Surface s t u d i e s have grown i n i n t e r e s t over recent years due i n par t to both increased knowledge of the s t r u c t u r e of surfaces and a l s o to the a p p l i c a t i o n of d i f f e r e n t techniques to the study of t h i s area. Adsorption s t u d i e s , surface s t r u c t u r e s t u d i e s and stu d i e s of r e a c t i o n s on surfaces a l l have c o n t r i b u t e d g r e a t l y to our knowledge of surface phenomena. A great deal of i n t e r e s t and a c t i v i t y i n the a p p l i c a t i o n of spe c t r o s c o p i c techniques to problems i n both c a t a l y s i s and surface chemistry i s e v i d e n t . The fapt that i n f o r m a t i o n can be obtained at the molecular l e v e l r a t h e r than the system l e v e l has been a d r i v i n g f o r c e f o r co n t i n u i n g r a p i d growth i n t h i s area of i n t e r e s t . No one sp e c t r o s c o p i c technique can hope to provide -2-a l l the in 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 of adsorbed s p e c i e s . Perhaps the most s u c c e s s f u l and widely used a p p l i c a t i o n has been th a t of i n f r a r e d spectroscopy. Many reviews have been w r i t t e n on i n f r a r e d spectroscopy as a p p l i e d to the study o f surfaces ( f o r example [1-6]). Gamma-ray resonance spectroscopy, although r e l a t i v e l y unexplored at present i n t h i s regard, has a l s o been used to some extent [ 7 ] . Magnetic Resonance techniques, both 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 nuclear magnetic resonance, h e r e i n a f t e r c a l l e d NMR, ho l d promise of p r o v i d i n g answers to some of the complicated s i t u a t i o n s that a r i s e on surfaces 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 . The a p p l i c a t i o n of these techniques i s s t i l l 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 been v a r i e d . The d e t e c t i o n of r e l a x a t i o n phenomena of molecules adsorbed on surfaces has been a prime area of i n t e r e s t ( f o r example [ 8 - f l l ] ) . Other NMR s t u d i e s have included such e f f e c t s as the study of chemical s h i f t s on various surfaces [12] and stu d i e s of adsorbed water [13-15]. The a p p l i c a t i o n of EPR to the study of adsorbed molecules has been very p r o d u c t i v e . The a b i l i t y of t h i s technique to detect small concentrations of paramagnetic species and to r e l a t e the unpaired e l e c t r o n charge d i s t r i b u t i o n to the molecular s t r u c t u r e has made t h i s an extremely u s e f u l method of 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 occur at surfaces a l s o i n v o l v e paramagnetic s p e c i e s . I t i s p o s s i b l e to s t a b i l i z e h i g h l y r e a c t i v e -3-molecules e i t h e r by adsorption i n t o porous media such as z e o l i t e s or molecular sieves ( f o r example [16-18]). C o n t r o l l e d r e a c t i o n of these 'trapped' molecules would then be p o s s i b l e to produce s p e c i f i c products, whereas corresponding r e a c t i o n s i n the gas phase may not 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 the adsorbed species such as i t s i d e n t i t y , s t a b i l i t y , motional s t a t e , chemical 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 various surface f i e l d s can thus be obtained. The i n t e r a c t i o n s between paramagnetic molecules and i t s . surroundings can g r e a t l y a f f e c t the EPR spectrum. When the molecules under study are adsorbed on a surface or i n some way trapped, 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 the EPR parameters from those observed f o r 'f r e e ' molecules. These i n t e r a c t i o n s w i l l be r e f e r r e d to as matrix i n t e r a c t i o n s . M a t r i x i n t e r a c t i o n s determine the a b i l i t y of a paramagnetic molecule to r o t a t e or r e o r i e n t about various molecular axes. These i n t e r a c t i o n s can a l s o perturb the wave funct i o n s of the molecule and thus produce changes i n the components of both the g and h y p e r f i n e tensors of the molecules. This study i s concerned with the EPR s p e c t r a of paramagnetic molecules 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 g e l , and the e f f e c t s of such adsorption on the EPR parameters. B r i e f i n t r o d u c t i o n s on adsorption and the s t r u c t u r e of z e o l i t e s are given f o r completeness. In f a c t , a knowledge of the surface s t r u c t u r e of the adsorbents i s extremely b e n e f i c i a l to the i n t e r p r e t a t i o n of the observed phenomena. -4-A chapter on EPR i s in c l u d e d but the reader i s r e f e r r e d to other sources f o r a more d e t a i l e d coverage of the theory. Chapter Five gives some i n s i g h t i n t o the a p p l i c a t i o n of various s p e c t r o s c o p i c techniques to t h i s research area and presents background on the a p p l i c a t i o n of the EPR technique to some surface phenomena. The remainder of t h i s t h e s i s i n c l u d e s a d i s c u s s i o n of the experimental techniques and the r e s u l t s obtained from the various systems i n v e s t i g a t e d . -5-CHAPTER TWO  ADSORPTION The components of a s o l i d ( i o n s , atoms, or molecules) are subject to forces which are i n e q u i l i b r i u m deep w i t h i n the l a t t i c e but are unbalanced near the su r f a c e . This r e s u l t s i n an a t t r a c t i v e f o r c e f i e l d only extending a few angstroms, but enough to a t t r a c t molecules of a l i q u i d or gas i n the immediate p r o x i m i t y . These forces cause molecules to become attached to the s u r f a c e , the phenomenon being known as adsorption. The term was introduced by Kayser [19] i n 1881 to denote the condensation of gases on f r e e s u r f a c e s . Desorption i s the complementary process, the removal of gases from the s u r f a c e , w h i l e the surface i s termed the adsorbent. The p h y s i c a l adsorption bond deriv e s from s i m i l a r cohesional forces as those r e s p o n s i b l e f o r condensation whereas chemical adsorption or chemisorption a l t e r s the nature of the -6-adsorbed s p e c i e s . Adsorption i s commonly measured i n terms of the mass adsorbed as a f u n c t i o n of pressure, the measurements undertaken at constant temperature. The r e s u l t i n g p l o t s are termed adsorption isotherms„ 2.1 Surfaces. I t i s convenient to 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 surfaces when co n s i d e r i n g the large a v a i l a b l e surface areas of the adsorbents normally used. The e x t e r n a l surface of a s o l i d f r e q u e n t l y represents no more than one percent of the t o t a l surface a c c e s s i b l e to gas molecules, the a d d i t i o n a l i n t e r n a l surface a r i s i n g from the w a l l s of the pores, cracks or i n t e r s t i c e s w i t h i n the s o l i d . I t i s obvious a l s o that the 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 the e x t e r n a l s u r f a c e . The demarcation l i n e between these two kinds of surfaces i s a r b i t r a r y , but the term ' i n t e r n a l s u r f a c e ' , then, would comprise the w a l l s of a l l c r a c k s , pores and c a v i t i e s which are deeper than they are wide. This i n t e r n a l surface must of course be open to the e x t e r i o r of the 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 orders of magnitude gr e a t e r than the e x t e r n a l s u r f a c e . The concern of t h i s study i s with porous systems having large i n t e r n a l s u r f a c e s . A convenient c l a s s i f i c a t i o n of pores has been given by Dubinin [20]. Pores of width below %2oR are termed micropores, those of width above 200$ are termed macropores, w h i l e those i n between are considered t r a n s i t i o n a l or intermediate pores. -7-2.2 C l a s s i f i c a t i o n of Isotherms. Many adsorption isotherms have now been determined and have been found to be of f i v e d i f f e r e n t types, each type c h a r a c t e r i s t i c of a d i f f e r e n t surface makeup. These have been c l a s s i f i e d by Brunauer, Deming, Deming and T e l l e r [21] and are shown i n f i g u r e 1 . Adsorption isotherms are g e n e r a l l y analyzed by reference to an equation i n which the c a p a c i t y of a complete monolayer appears as a parameter. Knowing the c r o s s - s e c t i o n a l area of the adsorbate molecules, the s p e c i f i c surface area of 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 of main i n t e r e s t i n t h i s study and i s o u t l i n e d below. Langmuir [22] was the f i r s t t o attempt an i n t e r p r e t a t i o n of adsorption phenomena and type I isotherms are commonly c a l l e d Langmuir isotherms. The isotherm i s c h a r a c t e r i z e d by the equation 1+aP where V i s the volume of vapour adsorbed at an e q u i l i b r i u m pressure P; V , the volume of vapour adsorbed at f u l l monolayer coverage; and a, a constant. I t i s obvious from observation of the isotherm that a s a t u r a t i o n of the surface appears to occur at higher gas pressures, not always the case as seen f o r the other types. Langmuir assumed that i n i t i a l l y , a l l gas molecules s t r i k i n g a surface would condense on i t . Once completely covered by adsorbate O R D I N A l E S : A d s o r p t i o n , ( m g / g ) A B S C I S S A E " : R e l a t i v e vapour p ressure P/P 0 ( s c a l e d 0 to 1.0) FIGURE 1. Fi v e d i f f e r e n t types of adsorption isotherms, as c l a s s i f i e d by Rrunauer, Deming, Deming and T e l l e r . -9-molecules, f u r t h e r condensation would cease s i n c e the surface forces 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 instance i s i n the form of a s i n g l e monolayer over the surface. Before t h i s l i m i t i s reached, part of the surface must be vacant and Langmuir assumed a dynamic e q u i l i b r i u m between the condensation of gas molecules h i t t i n g the f r e e surface and the evaporation of condensed molecules from the occupied s u r f a c e . The r a t e of condensation should be p r o p o r t i o n a l to the s p e c i f i c surface S; the pressure of the adsorbate P; and the f r a c t i o n of the surface not yet covered, (1 - 0 ) , so t h a t : r a t e of condensation = uSP(l - 6 ) (2-2) where u i s a constant. The r a t e of evaporation i s a l s o p r o p o r t i o n a l to the s p e c i f i c surface S; the f r a c t i o n of surface already covered, 6; and the r a t e evaporation would occur i f the surface were completely covered, V , such that r a t e of evaporation = 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 (2-4) By d e f i n i t i o n , 8 = V/V^ and r e p l a c i n g by the constant a, equation (2-4) becomes the Langmuir equation given by equation (2-1). The use of the Langmuir equation i s l i m i t e d at the present time almost e n t i r e l y to chemisorption s t u d i e s , assuming here t h a t surface coverage does not exceed a monolayer. -10-Type I isotherms are f r e q u e n t l y encountered i n adsorption s t u d i e s o f microporous s o l i d s , the s a t u r a t i o n i n t h i s case being a complete f i l l i n g of the pores with adsorbate molecules. Any s l i g h t r i s e i n the isotherm would then come from m u l t i l a y e r adsorption on the r e l a t i v e l y small e x t e r n a l surface of these microporous s o l i d s . made, i n c l u d i n g the 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 theory which r e t a i n e d the Langmuir concept of dynamic e q u i l i b r i u m but extended the process to i n c l u d e m u l t i l a y e r a d s o r p t i o n . I t was assumed that the condensation-evaporation c h a r a c t e r i s t i c s of the second and subsequent l a y e r s are the same as those of the surface of the bulk adsorbate. The assumptions f o r the i n i t i a l monolayer are the same as f o r Langmuir. The equation i s c h a r a c t e r i z e d by Extensions of the Langmuir theory of adsorption have been P 1 P P c - l (2-5) V(P -P) + V c m o V c m P i s the saturated vapour pressure of the adsorbate and c i s a constant r e l a t e d to the d i f f e r e n t i a l heat of adsorption by the equation c = expCCHL-H )/RT) (2-6) -11-where H and H are the heats of adsorption i n the f i r s t l a y e r and the heat of 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 . Equation (2-5) reduces to the Langmuir equation when P/P Q i s very low and c i s very l a r g e . The BET theory, as i t i s c a l l e d , i s s t i l l the best known and most widely used today f o r both porous and non-porous adsorbents. Whichever theory i s used, however, to represent a p h y s i c a l adsorption isotherm, agreement i s r a r e l y complete between the formula and experimental r e s u l t s . This i s due to the the assumptions of e n e r g e t i c homogeneity of the adsorption s i t e s and a l s o o f a gradual formation of a polymolecular adsorption l a y e r . These assumptions are not v a l i d f o r the porous adsorbents i n use today. 2.3 Volume F i l l i n g of Pores. Numerous experimental and t h e o r e t i c a l s t u d i e s i n recent years ( f o r example [24]) lead to the c o n c l u s i o n that adsorption i n micropores d i f f e r s q u a l i t a t i v e l y from adsorption on wide pore and non-porous adsorbents. Microporous adsorbents only have been used i n t h i s study. The concepts 'surface' and 'adsorption i n l a y e r s ' lose 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 to expect that adsorption i n micropores leads to a f i l l i n g of a l i m i t e d micropore adsorption space, Wq. When working w i t h microporous adsorbents, the value of V m of the BET and Langmuir equations may not be considered as equal to the volume of the monomolecular l a y e r covering the surface of the adsorbent. I t s value i s near to that of the volume of the micropores, and t h e r e f o r e a l s o to W , the -12-constant of the Dubinin - Radushkevich equation. This equation i s c h a r a c t e r i s t i c of adsorption isotherms obtained from experiments on microporous adsorbents, and i s given by [25] W a = __o V /BT 2 A P N 2 -expj j l o g _s Vi 2 - V ' (2-7) P V i s the volume of the amount adsorbed, a; T, the temperature; B, a constant independent of temperature and r e p r e s e n t i n g the b a s i c c h a r a c t e r i s t i c of the porous s t r u c t u r e of the adsorbent; 3, the a f f i n i t y c o e f f i c i e n t given by the r a t i o o f the d i f f e r e n t i a l molar work of adsorption of a given vapour to that of a vapour chosen as a standard; P , the s a t u r a t e d vapour pressure of the sorbate; and P, the pressure of the adsorbate. The constants Wq and B then c h a r a c t e r i z e the adsorptive p r o p e r t i e s of the given adsorbent whereas P- , 3, and V describe the adsorptive p r o p e r t i e s of the adsorbate. At the present s t a t e of the theory of adsorption 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 adsorption f i e l d i n micropores can be obtained only from adsorption experiments, The theory t h e r e f o r e , has a somewhat phenomenological character 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 that there was a very general for c e between atoms such that A . *D = ~xV (2"8) where <J> i s the: p o t e n t i a l -13-energy of the two i s o l a t e d atoms separated by a distance X; A, a constant r e l a t e d to the p o l a r i z a b i l i t i e s of the atoms and n an i n t e g e r , u s u a l l y given as 6. The negative s i g n denotes a t t r a c t i o n . This 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 small p e r t u r b a t i o n of the motions of o r b i t a l e l e c t r o n s on each other leading to a t t r a c t i o n of the atoms. D i s p e r s i o n forces are a d d i t i v e such that an adsorbate molecule near the surface of an adsorbent experiences a t o t a l a t t r a c t i o n which i s the sum of a l l p a i r s of i n t e r a c t i o n s . In a d d i t i o n , short range r e p u l s i v e forces are 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 , given by * R V 1 v (2-9) where B i s a constant 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. 1 Consequently, the r e p u l s i o n i s important only at very short distances of s e p a r a t i o n . I t i s assumed then, that both r e p u l s i o n and a t t r a c t i o n energies of t h i s type have the same form and the 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 given by * = ; * D + * R = - f n + f m ^ where m > n. .This equation has been a p p l i e d to a v a r i e t y of physiochemical systems. A r e l a t i o n of t h i s form was f i r s t introduced i n t o the theory of gases by Lennard-Jones [27] where n = 6 and m =•12 and equation (2-10) i s g e n e r a l l y r e f e r r e d to as the Lennard-Jones (6-12) p o t e n t i a l . -14-Other a t t r a c t i o n forces are a l s o present i f the adsorbent i s i o n i c i n nature and the adsorbate p o l a r . Strong e l e c t r o s t a t i c f i e l d s F are known tp be present on i o n i c s u r f a c e s . Barrer [28] has defined v a r ious energy terms that c o n t r i b u t e to the p h y s i c a l bond i n these systems. These are: 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 gradient-quadrupole energy Q ) ^ , D i p o l e - d i p o l e energy $^> Dipole-quadrupole energy a n d Quadrupole-quadrupole energy ^QQ* 2.4.1 P o l a r i z a t i o n Energy. P o l a r i z a t i o n a r i s e s when the adsorbent i s het 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 adsorbate molecules having some p o l a r i z a b i l i t y . Then, ?p = - | F 2 (2-11), The s t r e n g t h of t h i s i n t e r a c t i o n i s obviously d i r e c t l y dependent on both a and F. 2.4.2 F i e l d - d i p o l e Energy. Molecules possessing 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, the energy of i n t e r a c t i o n given by * F | J = - FycosG (2-12) where y i s the d i p o l e moment of the adsorbed molecule and 8 the angle the axis, of the d i p o l e makes with the f i e l d . I t i s expected (l>P w i l l assume an appreciable value only i f the adsorbate molecule can approach w i t h i n a short d i s t a n c e of the surface [29]. -15-2.4.3 F i e l d Gradient - quadrupole Energy. Recently, the importance of the presence of a permanent quadrupole i n c e r t a i n adsorbate molecules has been recognized [30] . A quadrupole i s p i c t u r e d as a r i s i n g from separation of equal and opposite d i p o l e s , the magnitude of the moment being p r o p o r t i o n a t e to the product of the d i p o l e moment and the separation of the d i p o l e s . T h e " l o c a l f i e l d s F w i l l normally have ass o c i a t e d w i t h them a f i e l d gradient F which can i n t e r a c t p o w e r f u l l y w i t h molecules possessing permanent quadrupole moments. 1 The i n t e r a c t i o n s of the poles a l s o c o n t r i b u t e to the bond energy though t h e i r c o n t r i b u t i o n s are normally much s m a l l e r than those p r e v i o u s l y mentioned. D i s p e r s i o n f o r c e s , then, are always present when con s i d e r i n g p h y s i c a l adsorption and, unless the adsorbate molecule has a permanent d i p o l e moment, w i l l represent the 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 molecule has a large d i p o l e moment. I t i s evident that the exact forces i n v o l v e d depend upon the p h y s i c a l and chemical p r o p e r t i e s of both the adsorbate and adsorbent. The favoured adsorption s i t e s are a l s o determined by these properties,, -16-CHAPTER THREE ZEOLITES Over 200 years ago, a Swedish m i n e r a l o g i s t and chemist, Baron Cronstedt, observed that c e r t a i n minerals appeared to melt and b o i l at the same time when heated. He named these minerals z e o l i t e s from the Greek words "zeo" meaning to b o i l and " l i t h o s " meaning stone. L i t t l e a t t e n t i o n was given these z e o l i t e s u n t i l the 1920's when t h e i r s e l e c t i v e adsorption property was n o t i c e d . McBain [31], 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 of these r e s u l t s , coined the term "molecular s i e v e s " f o r these z e o l i t e s . In the l a t e 1930*s, Barrer [32] began a thorough i n v e s t i g a t i o n of the adsorptive p r o p e r t i e s of these m a t e r i a l s which l e d t o considerable i n t e r e s t among the s c i e n t i f i c community. About 40 z e o l i t e s occur i n nature but much i n t e r e s t has -17-a l s o been given to 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 mordenite and s e v e r a l other s y n t h e t i c v a r i e t i e s [33-35]. By the 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 prepared i n the Linde research l a b o r a t o r y [36, 37]. Some are analogs of z e o l i t e m a t e r i a l s ; others, new v a r i e t i e s not found i n nature. Many present-day commercial operations simply were not p o s s i b l e or p r a c t i c a l p r i o r to the advent of these m a t e r i a l s . They have permitted the development of s e l e c t i v e adsorption as a p r a c t i c a l a l t e r n a t i v e to the long e s t a b l i s h e d s e p a r a t i o n methods of d i s t i l l a t i o n , a b s o r p 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 c r y s t a l l i z a t i o n . Molecular sieves ( z e o l i t e s ) are c r y s t a l l i n e metal . a l u m i n o s i l i c a t e s with a three-dimensional i n t e r c o n n e c t i n g network s t r u c t u r e of SiO. and A10. t e t r a h e d r a . The fundamental b u i l d i n g 4 ' 4 block of any z e o l i t e c r y s t a l i s a tetrahedron of four oxygen ions surrounding a s i l i c o n or aluminium i o n ( f i g u r e 2). The t r i v a l e n c y of aluminium causes the AIO^ tetrahedron to 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 to e l e c t r i c a l l y n e u t r a l i z e the system. The oxygens are shared between neighbouring t e t r a h e d r a and balance the charge of the s i l i c o n i o n . The charge bal a n c i n g c a t i o n s are the exchangeable ions of the z e o l i t e s t r u c t u r e . The remainder of the b u i l d i n g blocks of the z e o l i t e s , i n order of i n c r e a s i n g complexity are: (a) r i n g s ; (b) primary cages; and (c) secondary cages and channels. Rings;are formed of the s i l i c o n and aluminium t e t r a h e d r a by oxygen b r i d g e s . The cages are composed of va r i o u s s i z e d r i n g s SILICON A L U M I N I U M O X Y G E N CATION FIGURE 2. . The fundamental b u i l d i n g b l o c k s of z e o l i t e a) SlO. tetrahedron b) A l p . tetrahedron. -19-so that access to them i s governed by the r i n g dimensions. The pore opening of the 4-membered r i n g s (tetrahedra) i s n e g l i g i b l e . The 6-membered r i n g s have an opening of 2.2.% diameter. 8-membered r i n g s have a pore diameter of 4.3 A* w h i l e 12-membered r i n g s have a pore diameter of 8.9 X. The s t r u c t u r e s of many z e o l i t e s c o n s i s t o f simple arrangements of polyhedra formed from the r i n g s . The truncated octahedron, a l s o known as the s o d a l i t e cage, i s a w e l l known example of such a primary cage ( f i g u r e 3 ). This cage contains 24 s i l i c o n (aluminium) t e t r a h e d r a and i s composed of s i x 4-membered r i n g s and eight 6-membered r i n g s . The f r e e diameter of the 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 the 6-membered r i n g s . Secondary cages appear on the packing of the simpler primary cages to form the t o t a l z e o l i t e s t r u c t u r e . Cages of i n t e r e s t are discussed when the s t r u c t u r e s of s p e c i f i c z e o l i t e s are reviewed. A s t r u c t u r a l formula of the type Me , [(A10 o) ( S i O J ] -M HJD x/n L v 2^x ^ 2 Jy 2 i s o f t e n used to i l l u s t r a t e the r e l a t i o n between chemical composition and s t r u c t u r e of z e o l i t e s . Me stands f o r the metal i o n s ; x,y and n are i n t e g e r s ; and M i s the number of H^ O molecules i n t h i s u n i t c e l l formula. The p o r t i o n i n brackets represents the 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. According t o an e m p i r i c a l r u l e of Loewenstein [38], A10 t e t r a h e d r a can be j o i n e d only to -20-© S i l i c o n or A l u m i n i u m O 0 xygen A r 5 •4 •3 •2 •1 L 0 F I G U R E 3. The truncated octahedron, or s o d a l i t e cage. SiO^ t e t r a h e d r a and never to another AIO^ tetrahedron, thus g i v i n g the l i m i t to the r a t i o of 1:1. The f a c t t h a t only a l i m i t e d number of 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 indeed i n d i c a t e that there i s an ordering i n the r i n g s of the A l and S i . The metal ions needed f o r charge compensation occupy s i t e s adjacent to the c a v i t i e s i n the z e o l i t e s and are g e n e r a l l y a v a i l a b l e f o r exchange with other i o n s . Although mono- and d i - v a l e n t ions are the most common, t r i - , t e t r a - and even penta-valent ions have been found. 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^ + s u b s t i t u t e d f o r S i ^ + and A l " 5 * have a l s o been prepared [39,. 40] . 3.1 Adsorption i n Z e o l i t e s . As a.consequence of 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 able to contain adsorbate molecules i n great v a r i e t y and yet i n a h i g h l y s e l e c t i v e manner. Since t h e i r s t r u c t u r e i s composed of continuous, o f t e n i n t e r p e n e t r a t i n g channel systems, entry i s governed by r i n g s of various dimensions located p e r i o d i c a l l y . t h r o u g h o u t the s t r u c t u r e . The v a r i e t y and dimensions of the various adsorbates capable of 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 of the c a v i t i e s , but by the dimensions of the r i n g s or "windows" p e r m i t t i n g access to them. Owing to 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 of t e t r a h e d r a are equivalent i n s i z e [41]. S t r u c t u r e s w i t h , f o r example, 8-membered ri n g s can t h e r e f o r e exert a wide range of molecular s i e v i n g behaviour based on r i n g d i s t o r t i o n alone. -22-The number, s i z e , valency and l o c a t i o n i n the l a t t i c e of z e o l i t i c c a t i o n s have important e f f e c t s on the s i z e and shape of the entry pores to the l a r g e r c a v i t i e s . They a l s o have a profound e f f e c t on adsorption energies. The c a t i o n s are present i n the same channels as the adsorbate molecules. They are o f t e n recessed i n t o 6-ring windows which do not normally f u n c t i o n as the main access to the channel system. Sometimes the cations are a l s o located i n polyhedra which are not themselves able to h o l d adsorbate molecules. These c a t i o n s , of course, would not hinder the m i g r a t i o n of adsorbate molecules. Other c a t i o n s , however, may remain near windows c o n t r o l l i n g access to the pore system of the z e o l i t e . This i n f l u e n c e may be moderated i n three ways [42,43]: 1. Changing the s i z e of the c a t i o n s through exchange (K + ^ N a + , f o r example) 2. Changing the number of c a t i o n s through exchange (2Na -tr- Ca , f p r example) 3. Changing the number of c a t i o n s through syn t h e s i s (NaAl £ S i , f o r example) The e f f e c t of the t h i r d c o n s i d e r a t i o n using s y n t h e s i s i s t w o - f o l d . Besides removing the i n f l u e n c i n g c a t i o n , a given r i n g s i z e may decrease s l i g h t l y w i t h higher s i l i c o n content [44] s i n c e S i - 0 bonds are s l i g h t l y s h o r t e r than Al-0 bonds. The S i / A l r a t i o may a l s o a f f e c t the p o s i t i o n s of the c a t i o n s ( f i g u r e 4). Anything other than a 1:1 r a t i o of S i / A l w i l l g r e a t l y a f f e c t the arrangement of the c a t i o n . w i t h respect to 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 p o s i t i o n f o r z e o l i t e s of v a r y i n g S i / A l (a) 1/2 (b) 1/1 -24-This i s e s p e c i a l l y t r u e when mono-valent cat i o n s are replaced by d i - or t r i - v a l e n t ones. In a d d i t i o n to the pore geometry of the z e o l i t e s , the 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 a d s o r p t i o n . The p o l a r i t y of the adsorbate molecules becomes very important s i n c e strong 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 adsorbate molecules. C l u s t e r s of molecules are present 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 be j o i n e d by contact w i t h other c l u s t e r s through the windows. The number of molecules i n any 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 molecule may be shared between two c a v i t i e s or cages i f i t happens to 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 of adsorbate molecules i s s m a l l , they 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. Although the e n t i r e pore volume i s a v a i l a b l e f o r adsorption, c e r t a i n adsorption s i t e s are more favoured than others and w i l l 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 p r i m a r i l y to the cations which are exposed i n the c r y s t a l l a t t i c e . These cati o n s act as s i t e s of strong 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 negative ends of p o l a r molecules. Molecules can a l s o have d i p o l e s induced i n them under the i n f l u e n c e of these l o c a l i z e d charges. These induced d i p o l e s are, however, f a r weaker and l e s s s t r o n g l y a t t r a c t e d . -25-5.2 Str u c t u r e s of Z e o l i t e s . 5.2.1 X,Y Type. The c r y s t a l s t r u c t u r e of the s y n t h e t i c z e o l i t e s types X and Y i s s i m i l a r to t h a t of the n a t u r a l l y o c c u r r i n g f a u j a s i t e [45]. The framework c o n s i s t s of a t e t r a h e d r a l arrangement of s o d a l i t e cages, i n a diamond type l a t t i c e , l i n k e d by hexagonal faces with s i x bridge oxygen ions [46,47] ( f i g u r e 5). The u n i t c e l l formula f o r the type 13X s y n t h e t i c z e o l i t e i s N a96« A 1 02^96^ S i 02^96' ' 2 6 4 H 2 ° This i s a sodium X s i e v e and has the 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 the sodium Y sieve except f o r a lower S i / A l r a t i o and consequently more sodium ions per u n i t c e l l . The r a t i o i s u s u a l l y 1:1 f o r the X s t r u c t u r e and 17:7 f o r the Y. The volume enclosed by t h i s array of cages i s the supercage, i n t h i s case termed a type I I 26-hedron cage, or f a u j a s i t e cage ( f i g u r e 6 ). I t i s composed of 48 atoms of s i l i c o n (aluminium) and 96 oxygen atoms. The cage has 18 square faces, four 6-membered r i n g s , and four 12-membered r i n g s . The l a t t e r are the most important ports of entry i n t o the supercage. The openings of these r i n g s i s approximately 8-9 A* and the i n t e r n a l diameter of the cage i s 12.5 A. The volume of the supercage i s about 850 A* whereas the volume of 0 3 the s o d a l i t e cages i s about 160 A . Thus there are three cage types present i n type X z e o l i t e s : the f a u j a s i t e cages, the s o d a l i t e cages and the hexagonal prisms -26-FIGURK 5. The . s t r u c t u r a l framework of the X type syn-t h e t i c z e o l i t e . -27-FIGURE 6. The' t y p e I I 26-hedron cage, o r f a u j a s i t e 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. The hexagonal prism c a v i t i e s can u s u a l l y only be entered from the s o d a l i t e cages through the hexagonal faces where the opening i s about 2 X i n diameter. The type X s t r u c t u r e t h e r e f o r e contains two independent, three dimensional networks of c a v i t i e s - one of s o d a l i t e cages l i n k e d through hexagonal prisms and one of the super-cages l i n k e d by sharing r i n g s of 12 t e t r a h e d r a - the two systems interconnected by r i n g s of 6 t e t r a h e d r a . 3.2.1.1 Cation P o s i t i o n s . From a c r y s t a l l o g r a p h i c study of s y n t h e t i c Na 13X, Broussard and Shoemaker [46] were able to l o c a t e p r e c i s e l y only 48 out of the 80 Na + cations r e q u i r e d per u n i t c e l l of t h e i r sampleo X-ray st u d i e s of 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 a s s o c i a t e s [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 of the c a t i o n d i s t r i b u t i o n . Three c a t i o n s i t e s were described ( f i g u r e 7). s i t e s (16 per 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 of;the hexagonal prisms, 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 to oxygen. Sj i s e f f e c t i v e l y hidden from the z e o l i t e surface as a consequence of i t s i n t i m a t e c o o r d i n a t i o n to the framework i o n s . The s i t e s (32 per u n i t c e l l ) are found i n the hexagonal faces (6-membered r i n g s ) at the mouths of the s o d a l i t e cages. The c a t i o n s here have 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 , sites are l o c a t e d next to the 4-membered r i n g s on the surface of the supercage. The order of preference of c a t i o n s seems to be S over S T T over S T T T . - 2 9 -F I G U R E 7. Cation s i t e s i n Na 13X s y n t h e t i c z e o l i t e . -30-Since Sj and S^j s i t e s are more than s u f f i c i e n t to accommodate the b i v a l e n t c a t i o n s , the S^^^ s i t e s are probably only populated i n the u n i v a l e n t forms of the zeolites„ The sodium i o n can be replaced by a mu l t i t u d e of others, depending on ion s i z e and charge. Among those more commonly • + + , + + ++ ^ + + ++ '-, ++ exchanged are L i , K , Rb , Cs , Ca , Sr , Ba , and Cu . The sodium can a l s o be replaced by ammonium ions and these i n t u r n decomposed to y i e l d a decationated z e o l i t e . Replacement of sodium ions f o r calcium ions decreases the p e r m e a b i l i t y of the z e o l i t e from approximately 13 X i n 13X to 10 X i n 10X, a calcium exchanged form of the sodium z e o l i t e . Thus, decreasing the number of cat i o n s (2Na + £ C a + + ) a c t u a l l y decreases the adsorptive a b i l i t y of the X z e o l i t e . This i s due to the f a c t that the cations i n i t i a l l y r eplaced are those i n the hexa-gonal prisms, which have no e f f e c t on the pore openings i n the z e o l i t e . Replacement of the cat i o n s at the open r i n g s of the supercage a c t u a l l y increases the r e t a r d i n g e f f e c t of the c a t i o n i c p o t e n t i a l due to the increased s i z e and charge of the c a t i o n s . 3.2.2 A Type. : In the A type s t r u c t u r e , the primary cage a l s o c o n s i s t s of s o d a l i t e cages. In t h i s i nstance they are j o i n e d through the square faces (4-membered r i n g s ) by four b r i d g i n g oxygen ions i n a cubic array [50] (see f i g u r e 8). In the sodium form, commonly c a l l e d 4A, the s t r u c t u r e i s represented by the formula N a 1 2 ( ( A 1 0 2 ) 1 2 ( S i 0 2 ) 1 2 ) . 2 7 H 2 0 . -31-FIGURE 8. The s t r u c t u r a l framework of the A type s y n t h e t i c z e o l i t e . -32-As seen from the formula, the S i / A l r a t i o i n t h i s case should be 1:1. This type of s t a c k i n g o f the s o d a l i t e cages g i v e s a roughly s p h e r i c a l supercage, termed a type I 26-hedron cage ( f i g u r e 9 ) . I t c o n s i s t s of the same number of s i l i c o n (aluminium) atoms, 48, and oxygen atoms, 96, as the supercage of the X type s t r u c t u r e . In terms of r i n g s i z e s composing the cage, i t has eighteen 4-membered rin g s (square f a c e s ) , e i g h t 6-membered r i n g s (hexagons), and s i x 8-membered r i n g s (octagons). The diameter of t h i s 26-hedron cage o o3 xs 11.4 A and the volume i s 775 A . As i n the X type s t r u c t u r e , there are three cage types: the supercage, the s o d a l i t e cage and the square prisms, formed by the oxygen atoms l i n k i n g the s o d a l i t e cages. The supercages, sometimes c a l l e d truncated cubooctahedra, are found i n a cubic arrangement with respect to each other. Access i s through the 8-membered rin g s w i t h a pore diameter of 4.2 X,. and are the l a r g e s t 8-membered ri n g s to be found i n z e o l i t e s s i n c e the r i n g i s planar [51]. 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 of one three-dimensional network o of c a v i t i e s having a maximum diameter of 11.4 A and a minimum of 4.2 %. Access to the s o d a l i t e cages i s through the d i s t o r t e d 6-membered r i n g s of diameter 2.2 A* but access i s only through the c e n t r a l c a v i t y system. 3.2.2.1 Cation P o s i t i o n s . The a v a i l a b l e p o s i t i o n s f o r cat i o n s i n the A type s t r u c t u r e are at the center of the eight 6-membered r i n g s o f the s o d a l i t e cages at the corners of the 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-F I G U R F , 9. The type I 26-hedron cage. -34-adjacent to the 8-membered r i n g s d e f i n i n g the supercage, s i t e B. For 4A, the sodium form, eight of the twelve c a t i o n s of a u n i t c e l l are found i n s i t e A, while the other four are 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 the twelve s i t e B l o c a t i o n s [50,52]. S i t e A i s t h e r e f o r e f i l l e d p r e f e r e n t i a l l y to s i t e B. Replacement of the sodium ions by calcium ions to form the 5A s y n t h e t i c z e o l i t e , a c t u a l l y increases-the e f f e c t i v e opening to the c e n t r a l pore system to approximately 5 $ i n diameter. Since the twelve sodium ions are replaced by s i x calcium i o n s , these w i l l be loc a t e d i n s i t e A, le a v i n g the 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 to the supercage. The sodium ions may a l s o be replaced by ions such as L i + , K +, Rb +, C s + , T l + , Ag +, NH 4 +, Mg 2 +, S r 2 + , B a 2 + , H g 2 + , C d 2 + , Z n 2 + , C o 2 + , and N i 2 + . 3.2.3 Mordenite. The z e o l i t e mordenite belongs to that c l a s s i f i c a t i o n c h a r a c t e r i z e d by the predominance of 5-membered r i n g s of tet r a h e d r a . The geometrical p a t t e r n 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 the A and X type s t r u c t u r e s i n that the buildup i s of chains r a t h e r than of polyhedra ( f i g u r e 10a). There are s i x p o s s i b l e simple 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 of the chains to one another and a c r o s s - s e c t i o n of that found i n mordenite i s shown i n f i g u r e 10b. The r e s u l t i s a two-dimensional, t u b u l a r pore system, u n l i k e the three-dimensional pore systems of A and X s t r u c t u r e s [53]. The u n i t c e l l of an i d e a l sodium mordenite i s given by the formula Na g • (A10 2) • ( S i 0 2 ) 4 Q • 24H 20 -35-(a) (b) FIGURE 10. The s t r u c t u r a l framework of s y n t h e t i c morden i t c • • (a) c h a r a c t e r i s t i c chain s t r u c t u r e (b) c r o s s - s e c t i o n a l area o f a'chain -36-Thus, mordenite contains a hi g h e r S i / A l r a t i o , 5:1. As shown i n f i g u r e 10b, the channels are circumscribed by 12-membered r i n g s of t e t r a h e d r a ; 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 planar p r o j e c t i o n much sm a l l e r than a 12-membered r i n g of the X type s t r u c t u r e . The major and minor diameters are thus 7.0 X and 5.8 A*, r e s p e c t i v e l y . These large channels are 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 channels circumscribed by 8-membered r i n g s having a minimum f r e e diameter of 3.9 X and leading to the next main channel. However, halfway to the neighbouring main channel, the 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 diameter which open i n t o the main channel. 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 at the center of 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 the main channels from one another, and l e a v i n g each main channel l i n e d with two rows of si d e pockets [54]. These pockets have a low r a t i o of volume to c r o s s - s e c t i o n a l area of t h e i r entrances ( f i g u r e 11). The other cat i o n s are located i n the main channels and occupy at random some of the 8- and 12-foid p o s i t i o n s a v a i l a b l e [53]. -37-O o xygen HI O x y g e n in p l a n e of paper O C a t i o n s F I G U R E 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 at the centers of each t e t r a -hedron are not shown. -38-CHAPTER FOUR  ELECTRON PARAMAGNETIC RESONANCE 4.1 Theory. * The b a s i s of e l e c t r o n paramagnetic resonance (EPR) i s concerned with the i n t r i n s i c s p i n of 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 allows only c e r t a i n d i s c r e t e o r i e n t a t i o n s of the precessing d i p o l e s with respect to the magnetic f i e l d , the o r i e n t a t i o n s corresponding to d i f f e r e n t energy l e v e l s . I r r a d i a t i o n of the system with electromagnetic energy of the appropriate frequency induces t r a n s i t i o n s between these magnetic energy l e v e l s . * The term e l e c t r o n s p i n resonance, ESR, i s le s s general than EPR, since the former does not take i n t o account o r b i t a l magnetism. -39-Th e energy f o r these t r a n s i t i o n s i s given by the equation hv = gm (4-1) where h i s Planck's constant; v the frequency of the r a d i a t i o n ; g, a numerical f a c t o r o f t e n c l o s e to 2; (3, the Bohr magneton; and H, the magnetic f i e l d . A magnetic f i e l d of 3000 gauss r e q u i r e s a frequency of about 9 Gigahertz to induce the t r a n s i t i o n s . This corresponds to a wavelength of approximately 3 centimeters, which i s i n the microwave region of the electromagnetic spectrum. R e l a x a t i o n processes must n e c e s s a r i l y be present such that the energy absorbed by spins i n the higher energy l e v e l can be d i s s i p a t e d i n such a manner as to permit t h e i r r e t u r n to the ground energy l e v e l . Otherwise p o p u l a t i o n between the energy l e v e l s would eq u a l i z e and absorption would cease. This e s s e n t i a l l y i s achieved through the phenomenom of ' s p i n - l a t t i c e r e l a x a t i o n ' , where the 'spin system' i n t e r a c t s w i t h i t s surroundings i n such a way as to provide paths f o r t h i s process, 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 of these two l e v e l s , when i n thermal e q u i l i b r i u m at a given f i e l d and temperature, may be represented by the Boltzmann equation. Thus, i f the populations of the upper and lower l e v e l s are and r e s p e c t i v e l y , -40-where 3, g, and H are defined i n equation (4-1); k, Boltzmann's constant; and T , the s p i n temperature de f i n e d by equation (4-2) i n terms of the instantaneous r e l a t i v e populations of the two s p i n l e v e l s . I f the po p u l a t i o n d i f f e r e n c e at a given time t i s AN, e q u i l i b -rium w i l l be reached at a r a t e given by dAN/dt = da /dt. - dN 2/dt (4-3) Given that and are the p r o b a b i l i t i e s of t r a n s i t i o n s from the 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 = 2W9 where T now i s the l a t t i c e temperature. From equation (4-4) i t i s e a s i l y shown that dAN _ 2W (AN -AN) (4-5) dt 1 ° which has the s o l u t i o n AN - AN q[1 - expC-t/T )] (4-6) where = 1/2W. T^, the s p i n - l a t t i c e r e l a x a t i o n time i s seen as the i n v e r s e of 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 the s p i n system t r a n s f e r s energy to the l a t t i c e at the r a t e 1/T^. S i m i l a r l y , the r e l a x a t i o n time T^ c h a r a c t e r i z e s a s p i n - s p i n r e l a x a t i o n process, a process which depends on the e f f e c t of l o c a l magnetic f i e l d s generated by neighboring s p i n s . V kT (4-4) - 4 1 -A consequence of the existence of these r e l a x a t i o n processes i s that the s p e c t r a l l i n e s observed f o r the t r a n s i t i o n s between the sp i n l e v e l s have a f i n i t e width and are o f t e n discussed i n terms of a ' l i n e s h a p e 1 . Various mechanisms may be r e s p o n s i b l e f o r broadening of these s p e c t r a l l i n e s . 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 of broadening, homogeneous broadening and inhomogeneous broadening. Homogeneous broadening i s that associated with 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 sharply defined but are somewhat broadened. Thermal e q u i l i b r i u m of the s p i n system i s maintained 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 to a l l the s p i n s . Sources of homogeneous broadening i n c l u d e [55]: (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 l i k e 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 ; and (d) d i f f u s i o n of e x c i t a t i o n through the sample. An inhomogeneously broadened l i n e 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 l i n e s merged to form an o v e r a l l lineshape. The d i s t i n c t i o n between homogeneous and inhomogeneous broadening i s t h a t the inhomogeneous broadening comes from i n t e r a c t i o n s e x t e r n a l to the s p i n system and/must be sl o w l y v a r y i n g over the time r e q u i r e d f o r a s p i n t r a n s i t i o n . Inhomogeneities i n the magnetic f i e l d cause energy to be t r a n s f e r r e d only to those spins whose l o c a l f i e l d s s a t i s f y the resonance c o n d i t i o n . The resonance i s thus a r t i f i c i a l l y broadened i n an inhomogeneous manner. Other sources of 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) anisotropy broadening. The shape o f the absorption spectrum i s thus determined by the types of i n t e r a c t i o n s between the environment and the s p i n system. The widths of these l i n e s , however, depends on the st r e n g t h of the i n t e r a c t i o n s and the r e l a x a t i o n time. 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 thermal e q u i l i b r i u m of the s p i n system i s maintained throughout resonance has a lineshape approximated by a L o r e n t z i a n f u n c t i o n [56], c h a r a c t e r i z e d by the equation f (H-H ) = 2 A H i 3 (4-7) u [ ( H - H Q ) 2 + AH 2] AHj^ here represents the width of the absorption l i n e at h a l f the maximum i n t e n s i t y , and 'H , the resonance f i e l d . I t i s customary however, i n EPR, to d i s p l a y the f i r s t d e r i v a t i v e of the spectrum. Although many i n t e r a c t i o n s i n f l u e n c e the l i n e w i d t h , the Heisenberg u n c e r t a i n t y p r i n c i p l e sets the u l t i m a t e minimum width which may be s t a t e d as A H % = i t f C4"8) where T now corresponds to the r e l a x a t i o n time. E i t h e r T^ or T^ can be the c o n t r o l l i n g r e l a x a t i o n time, or both 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 cases, to determine r e l a x a t i o n times from the observed s p e c t r a . Another commonly encountered lineshape i s a Gaussian f u n c t i o n -43-[56], characterized by f ( H - V - i s ^ « P f ^ # - ] This generally occurs i n an inhomogeneous spin system described above. Gaussian and Lorentzian lineshapes are compared i n figure 12. Although these two lineshape functions are the most common, combinations and variations of these have also been observed and are described i n reference [57]. When the nucleus also possesses a magnetic moment, i t can interact with the magnetic f i e l d and the electronic 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 structure. This hyperfine mechanism accounts for the multiplet character of the spectrum. The theory of EPR i s well covered i n many a r t i c l e s and reviews (for example [58-61]) and only pertinent theory w i l l be further discussed. The problem of expressing interactions affecting electronic energy levels i s usually approached through the application of the Hamiltonian operator. When applied to the time-dependent Schrodinger equation, t h i s approach yields the eigenvalues and eigenfunctions of the permitted energy levels. Abragam and Pryce [62] have shown that the behaviour of a spin system can be described by a 1spin-Hamiltonian', a p a r t i c u l a r part of the overall Hamiltonian. Perturbation theory i s generally used i n the solution of the energy levels. This representation has the same effect as replacing the interaction of the f i e l d with the o r b i t a l angular momentum by an anisotropic coupling between the electron spin and the external -44-FIGURE .12. curves. L o r e n t z i a n and Gaussian f i r s t d e r i v a t i v e -45-magnetic f i e l d , the s p i n here now being termed the 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 of t h i s spin-Hamiltonian approach to EPR are considered i n reviews by Bleaney and Stevens [63], Bowers and Owens [64] and C a r r i n g t o n and Longuet-Higgins [65]. The spin-Hamiltonian f o r a system c o n s i s t i n g of one e l e c t r o n w i t h s p i n S=% and a nucleus of 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 the terms represent 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 nuc 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 general expression r e p r e s e n t i n g the Zeeman i n t e r a c t i o n between a magnetic f i e l d H and the e l e c t r o n s p i n S i s given by J^f = 3 H-g-S (4-11) H and S_ are expressed as v e c t o r s , and g, the spe 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 , or g value, i s u s u a l l y expressed as a tensor r a t h e r than t h e : f r e e e l e c t r o n value g g = 2.0023. The g f a c t o r equals the constant g g when (a) the e l e c t r o n possesses s p i n angular momentum only, and (b) the g tensor i s i s o t r o p i c . Deviations from g e are 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 to s p i n o r b i t c o u p l i n g , which a l t e r the e f f e c t i v e magnetic moment and g i s o f t e n found to be a n i s o t r o p i c . The anisotropy may be described by the tensor g, which -46-has the form XX g x y g x z 'yx gyy 'zx g'zy g z z (4-12) The S_, then, does not g e n e r a l l y represent the pure s p i n , and i s o f t e n termed the e f f e c t i v e or f i c t i c i o u s s p i n . EPR may be described as the measurement of the Zeeman e n e r g y w h i c h i s g e n e r a l l y of the order of 0 - 1 cm - 1. In essence, EPR i s concerned w i t h the manner i n which the other Hamiltonian terms perturb or are perturbed by t h i s Zeeman energy. 4.1.2 The Hyperfine I n t e r a c t i o n . The h y p e r f i n e , or e l e c t r o n s p i n - n u c l e a r s p i n i n t e r a c t i o n r e s u l t s from the i n t e r a c t i o n of the magnetic moment of the unpaired e l e c t r o n and the magnetic moment of any n u c l e i w i t h i n i t s o r b i t a l . This 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 d i f f e r e n t ways. The f i r s t i s e s s e n t i a l l y the c l a s s i c a l i n t e r a c t i o n of the two dip o l e s separated by a dis t a n c e r . I t would then be expected that t h i s i n t e r a c t i o n should depend upon t h e i r mutual o r i e n t a t i o n . Consequently, we r e f e r to i t as the a n i s o t r o p i c or d i p o l a r h y p e r f i n e i n t e r a c t i o n . The second form of 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 and i s known as the Fermi or contact i n t e r a c t i o n . I t i s determined by the unpaired e l e c t r o n d e n s i t y at the nucleus, and i s i s o t r o p i c . The 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 observed, then, would c o n s i s t of an a n i s o t r o p i c component superimposed upon an i s o t r o p i c term. -47-Rapid r e o r i e n t a t i o n of 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 to zero. The expression 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 s p i n Hamiltonian i s given by J^f f = *S.T.I (4-13) where T i s a tensor r e p r e s e n t i n g the coupling between the e l e c t r o n and nuclear s p i n angular momentum v e c t o r s , S and I , and i s a combination of both d i p o l a r and contact terms. The d i p o l a r term may be w r i t t e n as I-S 3(1.r) ( S T ) ^ d i p : = SeSlBeBl ~ (4-14) r r*> and the contact or Fermi term w r i t t e n as ^ f o n t . = Aoi-I (4-15) , where A o = - (if) 8egieeei I ^ (o)| 2 (4-i6) 2 and | ^ ( 0 ) | i s the s p i n 'density' of the unpaired e l e c t r o n at the nucleus. Here gj and 3j are the nuclear g f a c t o r and magneton defined correspondingly to those f o r the e l e c t r o n ; r_, the radius 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 distance between them; and A ,the i s o t r o p i c h y p e r f i n e coupling constant. The tensor form of T i s i d e n t i c a l to that p r e v i o u s l y described f o r g, although c o n t r i b u t i o n s from both d i p o l a r and -48-contact i n t e r a c t i o n s may be separated i n each term. Thus T T T XX xy xz T T T yx yy yz •T T T zx zy zz (4-17) and T . .' = A 6.. + BAA (4-18) In g e n e r a l , the hy p e r f i n e i n t e r a c t i o n (of the order - 2 - 1 0 - 1 0 cm ) i s found to be s m a l l e r than the Zeeman l e v e l s , each l e v e l being 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 . Nuclear quadrupole i n t e r a c t i o n s are even s m a l l e r i n magnitude but on occasion may have to be in c l u d e d to 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 . For n u c l e i with s p i n I > h, the nuclear quadrupole i n t e r a c t i o n may be expressed by = I - Q - I (4-19) where Q i s again represented by a tensor, of the same form as f o r g and T. The e f f e c t i s a small but c a l c u l a b l e s h i f t of the hyperfine l i n e s . The l a s t term to be mentioned i s the i n t e r a c t i o n of the n u c l e a r moments with the magnetic f i e l d , the nuc l e a r Zeeman i n t e r a c t i o n , expressed as g i B i I - H (4-20) -49-A l l symbols are as defined p r e v i o u s l y . This term i s a l s o g e n e r a l l y small and may be ignored i n a f i r s t order treatment. 4.1.4 EPR Spectra. The a n i s o t r o p i e s present i n both the g f a c t o r and the hyperfine s p l i t t i n g s cause the EPR spectrum to depend on the o r i e n t a t i o n of the species under c o n s i d e r a t i o n w i t h respect to the e x t e r n a l magnetic f i e l d . Studies of o r i e n t e d species i n s i n g l e c r y s t a l s are n e c e s s a r i l y lengthy and o f t e n r e q u i r e considerable refinement of experimental procedure and mathematical a n a l y s i s to achieve a high :degree of p r e c i s i o n i n the e v a l u a t i o n of the g and h y p e r f i n e t e n s o r s . I f the paramagnetic molecules are contained i n a p o l y c r y s t a l l i n e or amorphous host, as i s u s u a l l y the case i n studies 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 of l i n e s due to a l l o r i e n t a t i o n s of the randomly o r i e n t e d molecules. This i s not n e c e s s a r i l y to say that the molecul are themselves randomly o r i e n t e d w i t h respect to the s u r f a c e , but r a t h e r the adsorption s i t e s are randomly o r i e n t e d . Information can be obtained from such observations and i s g e n e r a l l y achieved by computing s p e c t r a l l i n e shapes f o r a number of 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 values of the g and hyperfine t e n s o r s . The p r i n c i p a l values are those obtained on d i a g o n a l i z a t i o n of the r e s p e c t i v e t e n s o r s . Sands [66] obtained a resonance lineshape by assuming a random d i s t r i b u t i o n of s p i n o r i e n t a t i o n s , and then averaging the -50-resonant magnetic f i e l d s over 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 these s o - c a l l e d powder or p o l y c r y s t a l l i n e s p e c t r a have been developed by Bloembergen and Rowland [67], Kobin and Poole [68] and Kneubuhl [69]. In d e r i v i n g t h e o r e t i c a l lineshapes f o r these media, completely random o r i e n t a t i o n w i t h respect to the e x t e r n a l magnetic f i e l d on a macroscopic s c a l e i s assumed. Thus the microscopic environment may be ordered or disordered without a f f e c t i n g the v a l i d i t y of the c a l c u l a t i o n s . F o llowing the treatment of Sands [66], an example of a c a l c u l a t i o n of a powder .lineshape f o r an a x i a l l y symmetric case i s o u t l i n e d . For the resonance c o n d i t i o n given i n equation (4-1), we have •g = ( g / / cos 29 + g j s i n 2 e ) ^ (4-21) where 0 r e l a t e s the p o s i t i o n of the g tensor to the a p p l i e d magnetic f i e l d (see f i g u r e 16, page 75). Here gxx> gyV» %zz a r e the p r i n c i p a l g values and g x x = g y y = g | and g^ = gjj . Since a l l o r i e n t a t i o n s are e q u a l l y probable, one must sum over a l l the absorptions. With the frequency v constant and sweeping the magnetic f i e l d H, then absorption of energy w i l l occur at f i e l d s given by H = j- {gj/ cos 26 + g ^ s i n 2 e ) " % (4-22) f o r each 8. The number of spins N having an o r i e n t a t i o n w i t h respect to the a p p l i e d magnetic f i e l d between 8 and 8 + d8 i s - 5 1 -given by dN = (No/2)sined6 (4T23) where N i s the t o t a l number of s p i n s . This becomes, from o r equation (4-^22) ^ / u ^ r r 2._ 2 , M m , m 2 _ 2 ,h A u (4-24) dN = ( N 0 / 2 ) ( 4 H S / H - i ) [ ( g ; / - g | ) [ 2 ( H 0 / H ) - g | ] 2 dH where H q = h v/2g* ^ p l o t of d N / ^ vs H y i e l d s the expected powder spectrum. P l o t s of t h i s and a l s o of a n i s o t r o p i c cases are given i n Chapter Seven. CHAPTER FIVE ADSORPTION STUDIES As introduced e a r l i e r , many sp e c t r o s c o p i c techniques have found wide use i n adsorption s t u d i e s . Reviews i n v o l v i n g some of the more common techniques have been given i n the i n t r o d u c t i o n . This chapter w i l l be concerned w i t h s t u d i e s using the EPR technique and t h e i r a p p l i c a t i o n to the surfaces under c o n s i d e r a t i o n . The l i s t of stud i e s given i s by no means complete, but y i e l d s enough i n s i g h t i n t o the scope of EPR i n t h i s area. I d e a l l y , one may expect to o b t a i n three types o f information 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 of the a c t i v e s i t e s on the surfaces 2, The i d e n t i t y of p h y s i c a l l y adsorbed or chemisorbed species 3, The nature of the 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 adsorbed molecule. -53-The terra ' a c t i v e s i t e ' i s broadly d e f i n e d . I t can be a s i t e on a surface capable of adsorbing molecules from the gas phase, or can r e f e r to s p e c i f i c s i t e s which induce chemical r e a c t i o n s . I t i s o f t e n d i f f i c u l t to d i s t i n g u i s h between them. No sp e c t r o s c o p i c technique i s u n i v e r s a l l y a p p l i c a b l e f o r o b t a i n i n g a l l three types o f i n f o r m a t i o n . Magnetic resonance techniques are 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 type 1 although i t i s p o s s i b l e i n some cases to o b t a i n types 2 and 3 a l s o . EPR techniques h o l d considerable promise of e l u c i d a t i n g some of the complicated and confusing s i t u a t i o n s that a r i s e on surfaces 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 , Studies of the nature o f surfaces themselves, p r i o r to any adsorption, comprised much of the e a r l i e r work of the EPR technique i n t h i s area. Low temperature s t u d i e s of various carbon samples gave narrow EPR s i g n a l s due to various f r e e r a d i c a l s present on the surface [70,71]. Further s t u d i e s o f the e f f e c t s of added gaseous oxygen to these samples ( f o r example [72^74]) produceid v a r i o u s RC - 0 - 0 r a d i c a l s . Not a l l r a d i c a l centers are n e c e s s a r i l y on the su r f a c e ; however, these 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 the i n t e r f a c e . Most s t u d i e s of 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 surface e f f e c t s and consequently w i l l not be discussed here.. More recent s t u d i e s of t h i s type have i n v o l v e d the use of r a d i a t i o n and the study of the defects formed i n va r i o u s substances [75T78]. EPR techniques here provide very u s e f u l i n f o r m a t i o n about the type of 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 EPR Studies of Radicals on Surfaces. 5.1.1 N o n - Z e o l i t i c Adsorbents. Many of the e a r l i e r s t u d i e s of paramagnetic species on surfaces were done on g l a s s e s , s i l i c a g e l s , aluminas, semiconductors, and various c a t a l y t i c s u r f a c e s . An e a r l y study by Faber and Rogers [79] i n v o l v e d adsorption of manganese ( I I ) , copper ( I I ) , and oxy vanadium (IV) on various c a t i o n and anion 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 to f u r t h e r understand the bonding and environment of t r a n s i t i o n ions i n unknown surroundings on the b a s i s of t h e i r EPR s p e c t r a . Russian workers c a r r i e d out s t u d i e s of f r e e r a d i c a l s produced on s i l i c a g e l surfaces i n the 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 of the s i l i c a gel s u r f a c e , the atoms being produced from the adsorbed water molecules or from the surface hydroxyl groups [80-82]. The hydrogen atoms formed were s t a b i l i z e d on the surface and the i n f l u e n c e of the surface on the EPR parameters s t u d i e d . The magnitude of the hyper-f i n e s p l i t t i n g was found to agree c l o s e l y with values f o r a f r e e hydrogen atom although the width of the components v a r i e d w i t h the nature of the surface under study. This suggests that the 'binding' of the atoms to the surface must occur without s i g n i f i c a n t change i n the s p i n d e n s i t y of the unpaired 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 surface and the 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 of 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 of these e f f e c t s was not thought p o s s i b l -55-owing to the lack of r e l i a b l e data concerning the surface s t r u c t u r e of such s o l i d s . Wide v a r i a t i o n s i n surface p r o p e r t i e s e x i s t among various s i l i c a gels [83], the d i f f e r e n c e s being caused by conc e n t r a t i o n of surface hydroxyl groups and d i f f e r i n g degrees of surface r e g u l a r i t y or c r y s t a l l i n i t y . Other 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 , notably e t h y l and methyl [84-88], i n each case the r a d i c a l being formed on i r r a d i a t i o n of adsorbed molecules on the surface. The s t a b i l i z a t i o n of f r e e r a d i c a l s at the surface of s o l i d s i s of considerable i n t e r e s t i n regard to heterogeneous c a t a l y s i s and surface s t r u c t u r e ; EPR techniques have provided v a l u a b l e i n f o r m a t i o n to both areas„ 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 phenyl r a d i c a l s , benzene c a t i o n r a d i c a l s and benzene c a t i o n dimer r a d i c a l s [89]. Radiolys'is of monocarboxylic acids adsorbed on s i l i c a g e l [90] has been s t u d i e d using the EPR technique to o b t a i n i n f o r m a t i o n concerning the r a d i c a l s produced i n the adsorbed s t a t e and.also the nature of t h e i r thermal motion. Monomeric and dimeri 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 [91]. 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 of adsorbates w i t h the s o l i d [92-93.];. These are 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 the adsorbent to the adsorbate having a high e l e c t r o n a f f i n i t y . Porous Vycpr glass has a l s o provided a convenient s t a b i l i z i n g medium f o r free 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] reported the s t a b i l i z a t i o n of the methyl r a d i c a l at 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 various added gases. Further s t u d i e s [95-97] of the methyl r a d i c a l on porous glass 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 both p h y s i c a l l y trapped r a d i c a l s and those which have i n t e r a c t e d w i t h surface s i t e s . The aim was to explore the general usefulness of porous glass as a f r e e r a d i c a l host 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 f r e e r a d i c a l host. A novel type of methyl r a d i c a l trapped i n porous Vycor glass at.77°K has r e c e n t l y been reported, haying an extremely small h y p e r f i n e coupling constant compared to that of the planar methyl r a d i c a l , probably 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 the adsorbed r a d i c a l [98]. As mentioned p r e v i o u s l y , workers i n the area of hetero-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 technique. Knowledge of the mechanisms of 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 of the elementary acts i n v o l v e d , and of the s t r u c t u r e s and p r o p e r t i e s of intermediates t a k i n g part i n c a t a l y t i c r e a c t i o n s . The resonance s i g n a l can provide evidence as to the nature of the paramagnetic species on or i n the surface: and a l s o as to the s t r u c t u r e and chemical composition of the c a t a l y s t . 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 of p r e p a r a t i o n and processing may a l s o provide 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 nature of the chemical bonds formed on adsorption. Another p o s s i b i l i t y of applying EPR to heterogeneous 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 . This would i n v o l v e the study of chemical r e a c t i o n s and of the adsorption process with a view -57-to o b t a i n i n g s i g n a l s from l a b i l e intermediate products on the c a t a l y t i c s u r f a c e . Petcherskaya et a l [99] have shown the EPR method, to 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 of c r y s t a l l i n e p r o p e r t i e s , chemical composition and e l e c t r o n i c p r o p e r t i e s of various oxide c a t a l y s t s . S i m i l a r s t u d i e s and reviews thereon have been published [100,101], 5.1.2 Z e o l i t e Adsorbents. Various z e o l i t e s have found widespread use as adsorbents mainly due to 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 to some knowledge of the e l e c t r o n i c p r o p e r t i e s of the s u r f a c e . As mentioned i n an e a r l i e r chapter,,a very important c h a r a c t e r i s t i c of z e o l i t e s i s that i t i s p o s s i b l e to vary the e l e c t r o n i c s t r u c t u r e of the surface by a simple s u b s t i t u t i o n pf v a r i o u s c a t i o n s of d i f f e r e n t s i z e s and charge w h i l e the l a t t i c e remains unchanged. The l o c a t i o n of the c a t i o n s can be assumed t o be the same, provided there i s not a l a r g e s i z e d i f f e r e n c e . Stamires and T u r k e v i t c h [102] 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 r a t i o . Most of the defects produced are paramagnetic centers and EPR 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 type of defect and i t s environment. The same authors [103] a l s o s t u d i e d the adsorption of a number of molecules on these z e o l i t e s . E l e c t r o n charge-transfer complexes were found when molecules with low i o n i z a t i o n p o t e n t i a l s were adsorbed. The purpose of. the study was to examine the z e o l i t e s as acceptors i n r e a c t i o n s of t h i s type, and because of 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 existence of w e l l - d e f i n e d -58-e l e c t r o n accepting s i t e s i n the l a t t i c e . Studies of r a d i c a l c a t i o n s formed on the adsorption of aromatic hydrocarbons on zeo l i t e s ' are a l s o present i n the l i t e r a t u r e ( 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 pf adsorbed species have a l s o been i n v e s t i g a t e d . E l e c t r o n i r r a d i a t i o n of mesitylene adsorbed on 13X produced s e v e r a l r a d i c a l s [105]. Adsorbed 0^. species on various Y type z e o l i t e s have a l s o been 1 s t u d i e d ( f o r example [106,107]). The h i g h l y r e a c t i v e methyl r a d i c a l has been trapped 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 long periods at temperatures below 90°K [108]. The r a d i c a l was generated by Y - i r r a d i a t i o n of methane on type A z e o l i t e . The f r e e r a d i c a l NO^ was produced by the d i r e c t r e a c t i o n of NO^ and atomic oxygen and trapped w i t h i n the siev e c a v i t i e s of 13X [109]. : The c a t a l y t i c p r o p e r t i e s pf molecular-sieve z e o l i t e s have been recognized f o r many years, but i n t e n s i v e i n v e s t i g a t i o n has gotten under way only w i t h i n the 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 cra 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 determinations of the c a t a l y s t s , which helps to i d e n t i f y the c a t a l y t i c centers. 5.2 S p e c i a l Adsorption E f f e c t s . P h y s i c a l adsorption i s a r e v e r s i b l e process and molecules so adsorbed may be e a s i l y removed from the surface by pumping. Chemisbrption u s u a l l y i n v o l v e s stronger forces and i s o f t e n . i r r e v e r s i b l e at moderate temperatures. Weak chemisprption i s oft e 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 adsorption. P e r t u r b a t i o n of the - 5 9 T adsorbed molecules, d i s t i n c t from a chemical r e a c t i o n between the surface and adsorbate, i s g e n e r a l l y considered to be a p h y s i c a l adsorption process. The r o t a t i o n a l freedom of p h y s i c a l l y adsorbed molecules i s an important f a c t o r to be considered. Depending on the adsorbing temperature, the adsorbed molecule may have no a x i s of 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 to the surface o r, even f r e e r o t a t i o n , about three:mutually 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 of hindered r o t a t i o n about any or a l l these axes i s a l s o to.be considered and i n many cases appears to be important. The o r i e n t a t i o n pf the adsorbed molecules i s . a l s o of importance. This depends on the various adsorption forces, present i n a given adsorbate-adsorbent system. I f the surface or c a v i t i e s of the adsorbent are considered as the host matrix to the adsorbed molecules then i t i s Clear the matrix can have a pronounced e f f e c t on the molecule. This n e c e s s a r i l y a f f e c t s the EPR spectrum and i t may p o s s i b l y a f f e c t the spectrum recorded by any other s p e c t r o s c o p i c technique. These w i l l be termed matrix i n t e r a c t i o n s and w i l l be discussed i n more d e t a i l when the experimental r e s u l t s are i n t e r p r e t e d . Other adsorption e f f e c t s need a l s o to be considered. E l e c t r o s t a t i c forces p l a y an impprtant role, i n p h y s i c a l a d s o r p t i o n . The equations of e l e c t r o s t a t i c s , given by equations (2-11) and (2-12) i n Chapter Two, may be a p p l i e d to the adsorption of gases on z e o l i t i c molecular s i e v e s . The sep a r a t i o n of molecules by these sieves i s due i n large p a r t , not to the s i z e of the molecules, but by e l e c t r o s t a t i c forces between the adsorbate and the strong e l e c t r i c -60-f i e l d s present i n the s i e v e s . I t i s these e l e c t r i c f i e l d s which w i l l be discussed at present. King and Benson [111], i n e x p l a i n i n g the low temperature adsorption of the hydrogen isotopes on alumina, have shown that the adsorbent has very strong surface e l e c t r i c f i e l d s , d i s t r i b u t e d over various s i t e s on the s u r f a c e . They have s u c c e s s f u l l y used equations (2-11) and (2-12) f o r t h e i r r e s u l t s . The f i e l d s were, found to a r i s e from normal s t r u c t u r a l vacancies i n the c r y s t a l l a t t i c e s , vacancies wh^ch were present to maintain e l e c t r i c a l n e u t r a l i t y . These same authors a l s o found evidence [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 adsorption of gases on z e o l i t e s . I t was found that o- and pr hydrogen could be separated on s y n t h e t i c z e o l i t e s 5A and 13X. In t h i s case, separation must be r e l a t e d to some type of hindered r o t a t i o n s i n c e these species d i f f e r only 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 large b a r r i e r s to r o t a t i o n . The o r i g i n of 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 that the c a t i o n s , because of t h e i r l o c a l uncompensated charge, produce very s t r o n g e l e c t r i c f i e l d s and these c a t i o n s serve as the adsorption s i t e s . Thus the s i e v i n g p r o p e r t i e s of these 'molecular s i e v e s ' are due only i n p a r t to the s i z e o f the cages and channels present i n the l a t t i c e . The c a t i o n d e n s i t y of the u n i t c e l l of z e o l i t e s can be 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^ content w i t h respect to the SiO^ content between w e l l - d e f i n e d l i m i t s . Removal or s u b s t i t u t i o n of the cations can a l s o create changes i n the s i t e s a v a i l a b l e f o r -61-adsorption. Rabo et a l [113] s t u d i e d the e f f e c t of the c a t i o n on the c a t a l y t i c a c t i v i t y of various s y n t h e t i c z e o l i t e s by comparing the sodium form, the calcium form and the d e c a t i o n i z e d form of the z e o l i t e s . They found a p o s i t i v e r e l a t i o n s h i p between the number of c a t i o n s i t e s and the c a t a l y t i c a c t i v i t y . Further r e p o r t s by these and other workers have s u b s t a n t i a t e d the importance of these large e l e c t r o s t a t i c f i e l d s of the cat i o n s as adsorption and c a t a l y t i c centers [114,115], The p o l a r i z a t i o n of the adsorbed molecules by the 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 being a s s o c i a t e d w i t h the c a t a l y t i c a c t i v i t y of the z e o l i t e s . 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 aromatic molecules i n a l i n e a r e l e c t r i c f i e l d g i ve support to t h i s hypothesis. Gibbons and Barrer [118,119] have c a l c u l a t e d the 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 to adsorption energies f o r molecules with both d i p o l e and quadrupole moments f o r various c a t i o n exchanged z e o l i t e s . I t was th u s ^ p o s s i b l e t o see the e f f e c t of s i z e and charge of the cations on these energies. Adsorption according to these e l e c t r o s t a t i c models of i n t e r a c t i o n between the adsorbate and the strong e l e c t r i c f i e l d s present i n the 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 • of the molecules on the su r f a c e . The e l e c t r i c f i e l d normal to the surface w i l l act i n the d i r e c t i o n o f gr 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 to be. the 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 the i n t e r n a l f i e l d s of the s o l i d and one should be able to p r e d i c t t h i s o r i e n t a t i o n . The molecules would a l s o be assumed to execute small 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 s u r f a c e . -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 handled i n a pyrex glass vacuum system constructed i n the Chemistry department glassblpwing shop at U.B.C. Te f l o n stopcocks with ' v i t o n ' Curings were used i n the gas handling p a r t so as not to introduce 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 stopcocks were manufactured by Ace Glass Incorporated, Vineland, New Jerse y . Where grease was necessary, a Haloflurocarbon l u b r i c a n t , KEL-F #90 grease, a product o f 3M Company, was used. KEL-F i s q u i t e unreactive to most c o r r o s i v e or r e a c t i v e chemicals. 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 Seal r o t a r y pump. The u l t i m a t e vacuum was of the order of 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-pressure measuring devices. An NRC Model 831 detector was used i n conjunction w i t h these gauges. A diagram of the vacuum system i s given i n f i g u r e 13a. 6.2 Sample Tubes. Figure }3b shows the sample tubes used i n the experiments. Quartz tubing of 4 mm. outer diameter was used f o r the part of the tube to be placed i n the EPR spectrometer. The diameter was determined by the s i z e of the l i q u i d n i t r o g e n dewar to be used f o r low temperature experiments. A t e f l o n stopcock was used here a l s o to prevent any. p o s s i b l e r e a c t i o n s of the sample w i t h grease. Glass wool plugs were placed above the sample and i n the c o n s t r i c t i o n to prevent p o s s i b l e s c a t t e r i n g of the sample during evacuation. A small c y l i n d r i c a l furnace was used which f i t around the sample tube. The furnace was capable of temperatures i n excess pf 673°K. 6.3 Adsorbents. The Linde D i v i s i o n of the Union Carbide Corporation k i n d l y s u p p l i e d samples of 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 white powders,and d i d not have any added b i n d e r s . The usual commercial form of these z e o l i t e s i s p e l l e t s of various s i z and a c l a y binder i s added tp f a c i l i t a t e the molding. As an adsorption 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 introduce unknown i m p u r i t i e s [120]. G e n e r a l l y , the p e l l e t s are approximately 15 per cent b i n d e r , so these s p e c i a l samples were requested. The l o t number f o r the 4A i s 470017; the 5A, M580031; 10X, 1080001; and 13X, 1370014. : . -64-<3; Z3 rJ CT) 7s L. a. in a cn (TJ o "5 a) or o to L. £ o c m £ a £ a c o 3 O ^ +-> "O a £ Q. >> o L. - o -u O u a o C o 0) CL) C teflon stopcock glass- wool ( a ) q r a d e d seal / "glass wool 4 m m o.d. ( b ) FIGURE .1.3. a) A schematic diagram of the vacuum system used' i n these experiments. b) The sample tubes used i n these experiments. -65-Th e s y n t h e t i c mordenites (Zeolons) were s u p p l i e d by the Norton Company of Worcester, Mass., i n the same form as were the Linde products. The l o t number of the sodium mordenite i s HB 79-80E and that f o r the hydrogen mordenite, HB 91-92E. The s i l i c a g e l samples used were of a t h i n l a y e r chromagraphic sorbent marketed by the M a l l i n c k r o d t Chemical Works. The brand name i s S i l l C A R TLC-7GF. The ions i n the Linde 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 standard procedures. Sherry [121] gives an account of the exchange p r o p e r t i e s of various z e o l i t e s and a l s o describes the c o n d i t i o n s r e q u i r e d f o r a number of s p e c i f i c exchanges. 6.4 Sample P r e p a r a t i o n . Sample tubes c o n t a i n i n g the adsorbents were degassed f o r a p e r i o d of g e n e r a l l y 4-6 hours at a temperature of approximately 523°K at a pressure of l e s s than 5x10*"^ t o r r . This p e r i o d was s u f f i c i e n t to remove any water from the adsorbents. The gases to be studied were then adsorbed onto the surfaces at room temperature f o r s e v e r a l minutes. The pressure of gas adsorbed v a r i e d f o r the d i f f e r e n t systems and w i l l be given i n each appropriate s e c t i o n . EPR s p e c t r a were recorded at 77°K on the spectrometers to be described l a t e r i n t h i s chapter. A Varian V-4546 l i q u i d n i t r o g e n dewar, shown i n f i g u r e 14b, was used f o r the low temperature s t u d i e s . 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 quartz to pass u l t r a v i o l e t l i g h t with a minimum of background s i g n a l s . -66-5mm i.d; 11mm o.d. (a) 7 ^ im i.d. •11 mm o.d. ( b ) •FIGURE 14. a) Quartz dev/ar used f o r v a r i a b l e temperature EPR experiments. b) A Va r i a n V-4546 l i q u i d n i t r o g e n dev/ar. -67-V a r i a b l e temperature experiments were performed using a s p e c i a l l y designed quartz dewar, shown i n f i g u r e 14a. Dry n i t r o g e n gas was passed through a heat exchanger placed i n a l a r g e dewar f i l l e d w i t h a coolant such as l i q u i d n i t r o g e n or a dry ice-acetone mixture. The gas was cooled to the approximate temperature of the coolant and passed through the quartz dewar, c o o l i n g the sample. The temperature at the sample was c o n t r o l l e d by the r a t e of flow of the gas and was measured using a copper-constantan thermocouple. The wave guide near the c a v i t y was kept f r e e from condensed moisture by passing dry n i t r o g e n gas through i t . 6.5 Gases. 6.5.1. C h l o r i n e Dioxide ClO^. The c h l o r i n e d i o x i d e used i n t h i s study was k i n d l y s u p p l i e d by Pr o f e s s o r F. Aubke of t h i s U n i v e r s i t y . In h i s method of p r e p a r a t i o n [122], a mixture of 12.2 gm. of potassium 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. of concentrated s u l f u r i c a c i d i n 40 ml. of water was slo w l y heated oh a steam bath (the mole r a t i o KCIO^ : H^C^-2H 20 : H 2S0 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 the f o l l o w i n g equation: 2KC10 3 + 2H 2S0 4 + H 2C 20 4-2H 20 -> 2C1C>2 + 2CC>2 + 4H 20 + 2KHS04. (6-1) The C10 2 and C0 2 produced were passed through a P 2 U ^ drying tube and cooled to 195°K. Pumping on the sample at t h i s temperature removed the CO,,. Further p u r i f i c a t i o n was achieved through a trap to trap d i s t i l l a t i o n from 195°K to 77°K. The C10 2 was stored i n a dry i c e --68-t r i c h l o r o e t h y l e n e bath at 195°K. „ 6.5.2. Nitrogen.Dioxide NC^. The n i t r o g e n d i o x i d e used i n t h i s study was purchased from Matheson of Canada, L i m i t e d . The p u r i t y of the gases i n the containers was > 99.5 percent. Further p u r i f i c a t i o n was achieved by pumping on the s o l i d i f i e d gas i n a container immersed i n a dry ice-acetone bath. The p u r i f i e d gas was stored i n a glass sample bulb. 6.5.3 N i t r i c Oxide NO. 14 The n i t r i c oxide, NO, was purchased from Matheson of Canada, L i m i t e d , and the reported p u r i t y i s > 98.5 percent. The gas was passed through a t r a p immersed i n an isopentane-pentane-dry i c e bath at approximately 133°K and stored i n a glass sample bulb. Further p u r i f i c a t i o n was achieved by pumping on the sample at l i q u i d n i t r o g e n temperature. The ^-N s u b s t i t u t e d sample of n i t r i c oxide was purchased from the Isomet Corporation, New Jersey and had a reported p u r i t y of > 99.3 percent 1 5N0. 6.5.4 T e t r a f l u o r o h y d r a z i n e ^ 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 study was purchased from A i r Products and Chemicals Incorporated, Pennsylvania. The research grade gas had a reported p u r i t y o f > 99 percent and was used d i r e c t l y from the container without 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 of the measurements were c a r r i e d out on a Va r i a n E-3 X-band EPR spectrometer system. The operating frequency of the k l y s t r o n i s 8.8 to 9.6 GH^ , t r a n s m i t t e d to the c a v i t y through a 4-port c i r c u l a t o r . The magnetic f i e l d of t h i s system was su p p l i e d by a four in c h electromagnet having a usable a i r gap of 1.2 inches, capable of homogeneous magnetic f i e l d s i n excess of 6 k i l o g a u s s . Homogeneity was such as to a l s o r e s o l v e 70 mG l i n e s . The magnetic f i e l d was modulated through a 100 kH Li f i e l d modulation u n i t i n t h i s system. A Varian E-4531 c a v i t y of rec t a n g u l a r mode TEJQ2 was used f o r the experiments. This spectrometer 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 frequency and power meters. The magnetic f i e l d i n t e n s i t y was measured as a f u n c t i o n of the proton resonance i n an NMR probe, while being frequency modulated by a magnetometer constructed by the Chemistry Department E l e c t r o n i c s Shop. The magnetometer output was di s p l a y e d on an Hewlett-Packard 5245L frequency counter. This counter was a l s o used to measure the microwave frequency of the k l y s t r o n by means of a 5255A frequency converter. A block diagram of 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 the experimental s p e c t r a was recorded using a Varian V-4500 100 kH^ EPR spectrometer m o d i f i e d by the Chemistry Department E l e c t r o n i c s Shop. The operating frequency i s about 9 GH . A standard 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 of about 9 k i l o g a u s s was a t t a i n a b l e from the Varian V-4012A 12 inch magnet having a 2.5 inch gap between -70-M I C R O W A V E B R I D G E E X - 1 0 0 C O N S O L E P O W E R S U P P L Y E - 0 0 7 T D K T E C T O R A F C C R Y S T A L 1 I K L Y S T R O N A N D | C I R C U L A T O R j p R E . A M P I T O C O N S O L E U N I T S W A V E G U I D E M A G . P O L E E P R S I G . 100 kHz M O D E-201 R E C E I V E R [ T R A N S . ( T H R U F U N C T I O N S W I T C H ) 9 .5 GHz P O W E R F O R E P R S A M P L E E X C I T A T I O N E P R S I G N A L ( P H A S E D E T . ) R E F L E C T I O N F R O M E P R S A M P L E I N C A V I T Y M O D . C O I L (2) F I E L D S I G N A L F U N C T I O N S W I T C H E - 2 0 5 30 Hz + 100 kHz «cj S U M M A T I O N 100 kHz M O D U L A T I O N U 1 T O M A G N E T C O I L S A A 30 Hz S A W T O O T H S I G N A L M A G N E T P O W E R S U P P L Y . E - 0 0 5 E P R S I G . 4 — E X T . R E C . E X T . R E C . M A G N E T I C F I E L D C O N T R O L E - 2 0 2 O S C I L L O S C O P E C H E C K O U T E - 2 0 0 R E C O R D E R E - 8 0 F I E L D S C A N FIGURE.15. Block diagram of a Var i a n E-3 X-band Spectrometer, system. -71-the p o l e s . Frequency and f i e l d i n t e n s i t y measurements were performed as p r e v i o u s l y described. Further measurements were c a r r i e d out on a spectrometer s i m i l a r to that j u s t d e scribed, although a V a r i a n V-3900 magnet capable of 15 k i l o g a u s s s u p p l i e d the magnetic f i e l d . A Varian V-2501 F i e l d a i l Mark I I Magnetic F i e l d Regulator c o n t r o l l e d the magnetic f i e l d . The remainder of the spectrometer was e s s e n t i a l l y the same. -72-' CHAPTER-SEVEN • ANALYSIS OF ELECTRON PARAMAGNETIC RESONANCE SPECTRA The task of a s s i g n i n g numerical values to the parameters i n the s p i n Hamiltonian given by equation (4-10) can, i n some cases, be q u i t e formidable. The consistency of the assigned values must be checked through, g e n e r a l l y by means of a t h e o r e t i c a l c a l c u l a t i o n . Various methods used f o r the c a l c u l a t i o n of resonance f i e l d s have been reviewed by Swalen and Gladney [123], and some computer programs a v a i l a b l e to t h i s end are discussed. Gladney [123,124] has w r i t t e n a program which, 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 to many EPR problems. Many papers have si n c e been published on the subject [125-128]. A completely general and extremely v e r s a t i l e method of 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 published [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 general s p i n Hamiltonian 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 with 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 . The i n c l u s i o n of a lineshape to the calculated resonance f i e l d positions enables one to simulate EPR spectra. The EPR spectrum of a p o l y c r y s t a l l i n e or powder sample involves a s p a t i a l average over d i f f e r e n t orientations of the spins with respect to the d i r e c t i o n of the magnetic f i e l d , the resonance f i e l d s f or each o r i e n t a t i o n being calculated u s u a l l y by one of the aforementioned methods. A b r i e f summary of a method given for solving the spin problem described by a spin Hamiltonian and the a p p l i c a t i o n of these r e s u l t s to the simulation of EPR spectra w i l l be given f o r completeness. McClung [130] has recently published a simple method to solve a spin Hamiltonian f o r an orthorhombic paramagnetic' center i n a r i g i d l a t t i c e . The Hamiltonian i s r e s t r i c t e d i n the sense that nuclear Zeeman and quadrupole terms are neglected and the g and hyperfine tensors are assumed to be simultaneously diagonalized i n the same axis system. The s o l u t i o n i s given for a paramagnetic species with electron spin S=h and one nuclear spin I. Although somewhat less general i n a p p l i c a t i o n , the technique i s i n s t r u c t i v e . The spin Hamiltonian i s then | T H + h S-T-I_ (7-1) where the symbols have been defined previously. Experimental data are not n e c e s s a r i l y c o l l e c t e d i n a molecular coordinate frame, whereas (7-1) operates i n a molecular frame. We must r e f e r to some ^74-experimental or l a b o r a t o r y frame of reference. A 'tensor frame' i s defined as the a x i s system i n which that tensor i s d i a g o n a l . For our s p i n Hamiltonian, the g tensor frame i s taken as the molecular frame. For t h i s example, the T tensor i s a l s o diagonal i n the molecular frame. G e n e r a l l y , E u l e r coordinate transformations are used to reduce the frames to the form wanted f o r stydy. These transformations correspond to r o t a t i o n of the ve c t o r networks by the appropriate angles. This w i l l o f t e n be the case when the molecule i s i n a host matrix 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 the c r y s t a l axes. Appropriate transformations are a l s o r e q u i r e d when the g and h y p e r f i n e tensor frames are not 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 the g tensor frame i s taken as the molecular frame and the hyper f i n e tensor frame transformed a c c o r d i n g l y . McLung uses the f o l l o w i n g technique to solve the Hamiltonian. The e l e c t r o n s p i n operator, S_, i s quantized along the d i r e c t i o n of g-H to allow exact treatment of the Zeeman term. Q u a n t i z a t i o n of the nuclear s p i n operator I_, along the d i r e c t i o n of T-S j^ then leads to an expression f o r the 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 treatment. Figure 16 shows the r e l a t i o n s h i p s of the frames used. G e n e r a l l y H_ i s a large s t a t i c magnetic f i e l d a p p l i e d along the l a b o r a t o r y Z a x i s . (X,Y,Z) i s the lab o r a t o r y frame and ( x , y , z ) , the molecular frame. The angles 8 and <}> r e l a t e the p o s i t i o n of the a p p l i e d magnetic f i e l d to the g and T tensor frames (the molecular frame). -75-y X FIGURE 16. system. The molecular and magnetic f i e l d coordinate -76-The h y p e r f i n e term may g e n e r a l l y be t r e a t e d as a p e r t u r b a t i o n w h i c h s p l i t s t h e e l e c t r o n i c Zeeman l e v e l s o f t h e s p i n s y s tem. U s i n g t h e z e r o - o r d e r b a s i s f u n c t i o n s | S , M s > | I, M x > , second-rprder p e r t u r b a t i o n t h e o r y [131] y i e l d s t h e e i g e n v a l u e s EM SM! = .gBoHoMs + h ™ S M l - h V ^ T ^ [S (S+ l ) - Mg ?]Mj 2g60HoT 2 / 2 „ 2 2 „2 , - 2 I (g T - g T ) s m ' z z ^ & a & a zzJ cos £ £ T 2 2 ( J l ' 2 , 2 . 2 + g x x g y y ^ T x x " V S i n ' „ 2 2 T2 £ g T . 2 2 s i n <j) cos (j> 2 2 h MjMg 2 ^ o H o 2 2 2 2 2 2 g T T g ^ T a zz + 6 a xx yy 2 2 g T-2 2 & a 2 2 2 2 2 2 2 + g g g T (T -T ) toxxbyybzz z z ^ xx yy^ 2 4 2 2 g g T T & c r a 2 2 2 cos 0 s i n cj) cos (f) x h 2 [ I ( I + 1) - M 2]M S 4 g M o (7-2) -77-where g = C g^sin 2 9 + g ^ c o s 2 0 ) ^ (7-3) 2 2 2 2 ^ g = (g cos <j> + g s i n cj>) 2 (7-4) "a xx • ayy . T = ( g 2 T 2 s i n 2 9 + g ^ T 2 z c o s 2 0 ) ^ (7-5) T = ( g 2 T 2 c o s 2 <J> + g 2 T 2 s i n 2 (JoVg (7-6) a xx xx . yy yy ^ J b • J E,, ,, are the eigenvalues of the b a s i s f u n c t i o n s named above: MS,MT , 3 Q , the Bohr magneton; H , the magnitude of the a p p l i e d magnetic f i e l d ; h, Planck's constant, with a l l other terms being p r e v i o u s l y d e f i n e d . In the l i m i t of a x i a l symmetry, t h i s r e s u l t reduces to that of Bleaney (132). . Once the magnetic f i e l d H i s computed f o r various values of 0 and cj) f o r the appropriate Mg and M^ v a l u e s , i t i s p o s s i b l e to simulate the EPR spectrum. An appropriate l i n e shape must be added to each t r a n s i t i o n , u s u a l l y i n the form of a Gaussian or a Lorent z i a n l i n e shape. Mr. J.C. T a i t of t h i s l a b o r a t o r y has w r i t t e n such a program. D i f f e r e n t programs may be used to generate the resonance f i e l d s f o r use i n t h i s program, the choice being determined by the complexity of the problem. Considerable infor 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 of l i n e s due to a l l o r i e n t a t i o n s of. the randomly o r i e n t e d molecules. The major loss of inform a t i o n i s d e r i v e d from the f a c t that the o r i e n t a t i o n of the molecule i n the host cannot be determined from the spectrum alone. Line-shapes of powder s p e c t r a have been discussed -78-by a number of authors and reference has been made to some of these i n Chapter Four. Assuming that the g and h y p e r f i n e tensors are d i a g o n a l i z e d i n the same frame and c o n s i d e r i n g , f o r the present, no magnetic nucleus i n the molecule, f i g u r e 17 shows some g e n e r a l i z e d line-shapes. Figure 17a d e p i c t s the spectrum expected f o r a species w i t h an a x i a l l y symmetric g tensor, i . e . g = g '= g l and g = g//. The s o l i d l i n e denotes the i d e a l i z e d absorption and the broken l i n e a p o s s i b l e r e a l a b s o r p t i o n . The r e a l a b s o r ption, represented by some smoothly v a r y i n g line-shape, c o n s i s t s of many sources of broadening. Figure 17b de p i c t s the spectrum, both i d e a l i z e d and r e a l , f o r a species w i t h 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 that these complex l i n e shapes cont a i n a number of sharp, r e a d i l y observable peaks. These correspond to molecules which are o r i e n t e d so that the magnetic f i e l d l i e s along one of the p r i n c i p a l axes of the molecule, the r e s u l t . b e i n g that the components of the g and T tensors are r e a d i l y determined from the p o s i t i o n s of these l i n e s . The explanation l i e s i n the f a c t that when the magnetic f i e l d l i e s along one of the molecular axes, the resonance f i e l d f o r the p a r t i c u l a r t r a n s i t i o n under study i s a maximum or a minimum w i t h respect to v a r i a t i o n s i n 9. and cj>. This r e s u l t s i n a p i l i n g - u p of the number of randomly o r i e n t e d molecules whose resonance f i e l d s are i n the v i c i n i t y of one of the 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 the i n t e n s i t y of the EPR absorption at these p o i n t s . When:a magnetic nucleus i s present i n the molecule, the spe c t r a are more complicated, but can g e n e r a l l y be analyzed i n terms . of the absorption curves j u s t discussed. Complications may a r i s e -79-FIGURE 17. Generalized li'neshapes of powder EPR s p e c t r a f o r a species 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 tensor b) f u l l y a n i s o t r o p i c g tensor -80-through the appearance of e x t r a l i n e s owing to the occurrence of u s u a l l y forbidden t r a n s i t i o n s , or due to 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 tensors t r y to s h i f t the l i n e s i n opposite d i r e c t i o n s . One can then, under favourable c o n d i t i o n s , determine the components of the g and hy p e r f i n e tensors from a powder EPR spectrum. In complicated i n s t a n c e s , approximate s t a r t i n g values f o r these tensors.could probably be obtained from the s p e c t r a , and more p r e c i s e determination done by f i t t i n g a computer simulated spectrum to the observed spectrum. Powder EPR s p e c t r a cannot provide i n f o r m a t i o n about the o r i e n t a t i o n of the p r i n c i p a l axes of the g and hy p e r f i n e tensors w i t h respect to the molecular axes, nor can informa t i o n be obtained about which component i s as s o c i a t e d w i t h a s p e c i f i c molecular a x i s . This must be determined by comparison with t h e o r e t i c a l estimates of these q u a n t i t i e s along the various molecular axes. CHAPTER EIGHT CHLORINE DIOXIDE, ClCy C h l o r i n e d i o x i d e i s one of the few s t a b l e gases t h a t i s paramagnetic i n i t s normal chemical s t a t e . The nuc l e a r s p i n of the c h l o r i n e atom i s I=2~« u u e i n p a r t perhaps to i t s extremely high r e a c t i v i t y , i n v e s t i g a t i o n s using EPR techniques have n°t been extensive. Several years ago, Bennett and Ingram [133] reported the spectrum of C10 2 i n a d i l u t e f l u i d s o l u t i o n of e t h y l a l c o h o l . The spectrum c o n s i s t e d of a broad l i n e at room temperature, separating i n t o four components on c o o l i n g . More recent s t u d i e s of ClO^ i n various solvents at 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 , the r e s o l u t i o n i n some cases being good enough to 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 to the C l isotope [134]. The spectrum reported i n r i g i d s u l f u r i c a c i d at 77°K i s somewhat b e t t e r r e s o l v e d , although complex [135]. Here again, features -82-due to the CI isotope may be d i s t i n g u i s h e d from those due to the 35 predominant CI. I r r a d i a t e d potassium p e r c h l o r a t e has provided a source of trapped ClO^ molecules i n the c r y s t a l environment. Two independent s t u d i e s [136,137] of the CIC^ molecule i n such c r y s t a l s have led to s i m i l a r a n a l y s i s of the s p e c t r a . The p r i n c i p a l values of both the g and h y p e r f i n e tensors were obtained from these s p e c t r a whereas the r i g i d s o l u t i o n s p e c t r a could y i e l d w i t h c e r t a i n t y only one p r i n c i p a l value of each of these tensors. 8.1 S i l i c a G e l . 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 pa r t to i t s high surface area and the f a c t that the s i l i c o n nucleus 29 " •'• (except f o r . S i of n a t u r a l abundance l e s s than 5%) does not have a nuclear s p i n . This would provide a m a g n e t i c a l l y i n e r t environment e l i m i n a t i n g a p o s s i b l e source of l i n e broadening. Adsorption of CIC^ at room temperature at pressures higher than -3 8 x 10 t o r r produced a b r i g h t yellow c o l o u r i n g of the s i l i c a g e l when cooled to 77°K. Spectra recorded f o r these pressures were composed of extremely broad l i n e s i n d i c a t i n g f a r too high a concentration of ClO^. To avoid d i p o l a r broadening, l e s s than a mono-l a y e r must be adsorbed. Pressures of le s s than 1 x 10 t o r r of ClO^ produced much,clearer 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 f e a t u r e s . There was no c o l o u r a t i o n of the s i l i c a g e l at these pressures. An e q u i l i b r a t i o n time of up to 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 of the CIC^ molecules throughout the surface of the s i l i c a g e l . The samples could be -83-stored at room temperature and recooled to 77 K with no l o s s of s i g n a l . Pumping at room temperature removes very l i t t l e CIC^ i n d i c a t e d by very l i t t l e change i n the spectrum, but the ClO^ may be removed from the s i l i c a g e l by pumping at higher temperatures. . The surface of the s i l i c a g e l i s not uniform, and a large v a r i e t y of adsorption centers are p o s s i b l e w i t h the p o s s i b i l i t y of 'densely' populated areas on the surface. This would account f o r the i n a b i l i t y to observe s p e c t r a w i t h a small enough l i n e w i d t h to d i s t i n g u i s h a l l the features c l e a r l y . X o Figure 18 r - 8 4 -In conformity w i t h e a r l i e r works, the a x i s system f o r the magnetic parameters o f the C10 2 has been chosen so that the z-axis l i e s along the two-fold symmetry a x i s , y i s perpe n d i c u l a r to z i n the plane of the molecule, and x, the t h i r d orthogonal a x i s (see f i g u r e 18). Figure 19 shows a t y p i c a l spectrum of CIO,, adsorbed on s i l i c a g e l , recorded at 77°K. Ch 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 35 • 37 isotopes C l and C l i n the r a t i o of approximately 3:1. Both isotopes have a nuc l e a r s p i n of I = 3/2, and the r a t i o of 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 , C10 2 and C10 2, with the r a t i o of the corresponding h y p e r f i n e components given by the r a t i o of the r e s p e c t i v e magnetic moments. The values f o r the components o f the hype r f i n e and g tensors were obtained by comparison of the recorded s p e c t r a to simulated EPR spec t r a using the various programs p r e v i o u s l y mentioned. The simulated spectrum i s shown i n f i g u r e 20. Both T and g were found to be a n i s o t r o p i c . The r e s u l t s are t a b u l a t e d i n Table 1. The main d i s t i n g u i s h a b l e features of the spectrum are those a s s o c i a t e d w i t h the x component of the hy p e r f i n e tensor, 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. These are the outer l i n e s of the spectrum. Both the y and z components are concentrated i n the c e n t r a l p o r t i o n of the spectrum. Figure 21 shows a spectrum of C10 2 adsorbed on s i l i c a g e l , recorded at room temperature. Features due to the isotopes are not di s c e r n a b l e here. From the spectrum, the C10_ appears to be f r e e l y TABLE 1 Reference 137 Hyperfine components (gauss) For 3 5 C 1 0 2 only Linewidth used f o r si m u l a t i o n s ('gauss) Medium • T XX T T , A g g g yy z z o xx yy . z z KCIO^ . @ 77°K 79.9 -13.4 -12.5 18.0 2.0018 2.0167 2.0111 137 72.7 - 9.6 -10.0 18.0 2.0025 2.017 2.011 H 2S0 4 @ 77°K " 136 74.7 -10.8 -11.5 17.5 2.0016 2.01667 2.01214 KC10, @ 106°K 4 135 70.5 2.0015 '"H?SO @ 77°K ±0.2 GAUSS ± 0.0005 ADSORBED ON .... . t h i s work 76.1 -17.0. - 7.9 , 17.1 2.0023 2.0123 " • 2.0115 3.5 s i l i c a g e l @ 77°K t h i s work 74.9 77.0 -16.7 - 7.8 16.8 2.0023 2.0023 2.0023 2.0123 2.0123 2.0123 2.0115 2.0115 2.0115 H-mordenite @ 77°K i t h i s work -17.2 - 8.0 17.3 8.0 Na-mordenite @ 77°K t h i s work 82.2 -18.4 - 8.5 18.4 2.5 4A @ • ' 77°K t h i s work t h i s work 81,6 84.5 - 1 8 . 3 - 8.5 18.3 2.0023 2.0123 2.0115 2.0115 5A @ 77°K -18.9 - 8.8 18.9 2.0023 2.0123 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 2.0115 10X @ 77°K j t h i s work 77.2 -17.3 - 8.0 17.3 2.0023 2.0123 LiX @ 77°K - 8 6 -FIGURE .19. EPR spectrum of 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 , recorded at 77° K. FIGURE 20. Computer simulated EPR spectrum of 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 , recorded at 77° K. -38--89-r o t a t i n g on the s i l i c a g e l surface or i n the pores, and the i s o t r o p i c parameters are in c l u d e d i n Table 1. A simulated 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 recorded at 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 to th a t obtained on s i l i c a g e l . The components of the h y p e r f i n e tensor are s l i g h t l y d i f f e r e n t and are l i s t e d i n Table 1. These, along w i t h those a s s o c i a t e d with the other adsorbents, w i l l be discussed l a t e r . 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 of sample p r e p a r a t i o n , the spectrum at 77°K appears much broader and cannot be improved by pumping. U n l i k e s i l i c a g e l , the sample shows no co l o u r . A t y p i c a l spectrum i s shown i n f i g u r e 23. The x components of the hyper f i n e tensor are s t i l l r e a d i l y d i s c e r n a b l e and the values are l i s t e d i n Table 1. The spectrum recorded at room temperature, u n l i k e that of s i l i c a g e l , i n d i c a t e s that some features may have been p a r t i a l l y averaged due to some motional process of the CIC^. The most l i k e l y would be a r o t a t i o n about the z a x i s , averaging the hy p e r f i n e and g tensor components of the x and y axes. A spectrum simulated under these c o n d i t i o n s however, does not match the observed spectrum (see f i g u r e s 25 and 26). This suggests that the r o t a t i o n i s hindered. Spectra recorded at higher temperatures show only a decrease i n s i g n a l height and i t i s l i k e l y d i f f u s i o n of the CIC^ molecules w i l l occur at these elevated temperatures. At 373°K, the CIO i s completely removed from.the Na-mordenite. FIGURE 22'.' Computer 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 'adsorbed on s i l i c a g e l , r e c o r d e d a t room temp-e r a t u r e . FIGURE 23. EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at 77° K. . . -92-Figure 24 shows a simulated spectrum f o r CIC^ adsorbed on Na-mordenite, recorded at 77°K. Figures 25 and 26 show ClO^ adsorbed on Na-mordenite, recorded at room temperature, observed and simulated, r e s p e c t i v e l y . . 8.5 4A and 5A S y n t h e t i c Z e o l i t e s . Figure 27 shows a t y p i c a l spectrum of ClO^ adsorbed on 4A s y n t h e t i c z e o l i t e , recorded at 77°K. The l i n e w i d t h i s g r e a t l y decreased from that observed on e i t h e r s i l i c a g e l or the s y n t h e t i c mordenites. Consequently, the features i n the c e n t r a l p o r t i o n of. the spectrum (corresponding to the y and z components of the hyper-f i n e s p l i t t i n g ) are b e t t e r defined. There i s a s u b s t a n t i a l i n c r e a s e i n the magnitude of the components of the h y p e r f i n e tensor, and the r e s u l t s are given i n Table 1. Figure 28 shows a simulated spectrum corresponding to C10 adsorbed on 4A. Adsorption on 5A s y n t h e t i c z e o l i t e again y i e l d s a s i m i l a r spectrum and has not been shown here. The outermost features of the spectrum are somewhat broadened, which p o s s i b l y suggests the existence of two adsorption s i t e s . This w i l l be discussed l a t e r . Spectra recorded at room temperature are markedly changed, although the ClO^ does not appear to 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 other form of hindered r o t a t i o n appears evident. The h y p e r f i n e and g tensors are given i n Table 1. 8.4. 13X S y n t h e t i c Z e o l i t e . A t y p i c a l spectrum of C10„ adsorbed on 13X s y n t h e t i c -94-.FT.GURE 25. EPR spectrum of. c h l o r i n e dioxide, adsorbed on Na-mordenite, recorded at room temperature. ICURE 26. Computer simulated EPR spectrum of c h l o r i n e d i o x i d e adsorbed on Na-mordenite, recorded at room temperature. -96-27. EPR spectrum of c h l o r i n e d i o x i d e adsorbed y n t h e t i c z e o l i t e , recorded at 77° K. -98-z e o l i t e i s shown i n f i g u r e 29. The spectrum was recorded at 77°K. A simulated spectrum i s shown i n f i g u r e 30. I t i s evident from the spectrum that two adsorption s i t e s are present i n the 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 c l e a r l y . 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 enables them to be more r e a d i l y 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 of the spectrum. Table 1 l i s t s the components of the h y p e r f i n e and g tensors f o r the two s i t e s . Further pumping increases the r e s o l u t i o n o f the l i n e s although the ClO^ can be removed from n e i t h e r s i t e by pumping at room temperature. V a r i a b l e temperature (annealing type) experiments were performed i n the hope that the ClO^ would be removed p r e f e r e n t i a l l y from one of the s i t e s . U n f o r t u n a t e l y , l i n e broadening at temperatures higher than 77°K made i t impossible f o r accurate observations to be made. I t i s apparent that the ClO^ does not remain r i g i d l y trapped i n the z e o l i t e as the temperature i s r a i s e d , but the exact type of motion could not be determined. 8.5 IPX S y n t h e t i c Z e o l i t e . The calcium exchanged form of the 13X z e o l i t e , 10X, was a l s o used as an adsorbent. The s p e c t r a recorded at 77°K were s i m i l a r to those on 13X although the presence of two s i t e s was not as obvious. The reasons f o r t h i s w i l l be discussed l a t e r . The l i n e w i d t h i s broader than that observed on 13X. The spectrum at room temperature was s i m i l a r to that observed on s i l i c a g e l , i n d i c a t i n g f r e e r o t a t i o n of C10 ? i n 10X at t h i s temperature. This i s u n l i k e the 13X sample, - 9 9 --100--101-where the motion of the ClO^ was s t i l l hindered at room temperature. Table 1 gives the components of the h y p e r f i n e and g t e n s o r s . 8.6 Lithium Exchanged 13X S y n t h e t i c Z e o l i t e . . Lithium was exchanged f o r the sodium i n a sample of 13X and CIO2 then adsorbed as before. The spectrum recorded at 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, and the l i n e s were somewhat broader. Table 1 gives the components of the h y p e r f i n e and g tensors. 8.7 D i s c u s s i o n . CIO2 i s a bent molecule and has the symmetry p r o p e r t i e s of the C 2 v p o i n t group. Q u a l i t a t i v e d i s c u s s i o n s of the e l e c t r o n i c s t r u c t u r e of t h i s type of molecule (AB 2). have been given by M u l l i k e n [138] and Walsh [139]. Following the m o l e c u l a r - c o r r e l a t i o n diagrams given by these authors, the ground s t a t e has the c o n f i g u r a t i o n ... ( l b p 2 ( 3 b 2 ) 2 ( l a 2 ) 2 ( 4 & 1 ) 2 (2bp , - \ The b 1 o r b i t a l ' c o n s i s t s of the p o r b i t a l s of the c h l o r i n e and oxygens with p o s s i b l e admixture from the c h l o r i n e d o r b i t a l , and i s a n t i -r xz ' bonding. Although 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. expected 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 to cause a p o l a r i z a t i o n of the inner s - o r b i t a l s on the c h l o r i n e and oxygen atoms. This would introduce a small i s o t r o p i c h y p e r f i n e component. An a n i s o t r o p i c h y p e r f i n e tensor w i t h the maximum p r i n c i p a l value observed when the f i e l d i s p e r p e n d i c u l a r to the molecular plane -102-(alorig the x - a x i s ) i s a l s o expected, of opposite s i g n to the s m a l l e r in-plane 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 o r b i t a l . 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 value g , g e n e r a l l y termed g - s h i f t s , may be represented by the general formula (excluding d o r b i t a l s ) A g i i = f ( c i c i > XCV Ap) (8-1) . . E r E 2 • where f ( c ^ , A C 1 , A Q) i s a f u n c t i o n of the s p i n - o r b i t c oupling constants on the atbms; c h l o r i n e and oxygen (A and A ), and the products of the c o e f f i c i e n t s of the o r b i t a l s on the atoms. The denominator i s the energy d i f f e r e n c e of the two s t a t e s which are mixing. The various s t a t e s which are.allowed to mix and c o n t r i b u t e to the g-tensor may be determined using group theory [182] . For C10 2, the dominant g - s h i f t a f f e c t s - g and i s expected to be large and p o s i t i v e . The s h i f t i n the x d i r e c t i o n , Ag should be c l o s e to zero, and n e g l i g i b l e XX '. i f d o r b i t a l s are neglected. A S Z Z ^ s expected to be p o s i t i v e , and l e s s than 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 obtained by C u r l et a l [140,141] as a r e s u l t of a microwave study, and Ward [142] who combined : UV s p e c t r o s c o p i c data w i t h high r e s o l u t i o n IR data. The r e s u l t s are summarized below. r Cl-0 (8) ^OCIO (°) reference 140, 141 1.471 117.6 reference 142 1.472 117.4 -103-Using the preceding i n f o r m a t i o n , the r e s u l t s of c h l o r i n e d i o x i d e adsorbed on various surfaces may be analyzed. The r e s u l t s obtained from the present work and those of.previous workers are summarized i n Table 1. A general d i s c u s s i o n o f the adsprptioh of c h l o r i n e d i o x i d e on these surfaces i s u s e f u l p r i o r tp the d i s c u s s i o n pf the 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 to possess a s u b s t a n t i a l d i p o l e moment, 1.784 D. [143]. This d i p o l e moment, together w i t h the quadrupole moment due to the c h l o r i n e nucleus with 1=3/2, play important r o l e s i n the adsorption as d e t a i l e d p r e v i o u s l y i n Chapter Two. The strong a t t r a c t i v e f o r c e due tp the i o n i c charges of the adsorbent i n t e r a c t s with these 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 the components of the g and hy p e r f i n e tensors as compared to these 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 other media. I t i s important i n ^ h i s d i s c u s s i o n to analyze these observed parameter changes i n terms of adsorption - i . e . the s i t e of adsorption; the p o s i t i o n of the c h l o r i n e d i o x i d e i n r e l a t i o n to the trapping s i t e ; any movement of the c h l o r i n e d i o x i d e on the surface or at.the s i t e . Buckingham £144] has considered the i n t e r a c t i o n p o t e n t i a l energy u ^ of two.charge d i s t r i b u t i o n s 1 and 2 possessing charge q and m u l t i p o l e moments \i, 0, .... <f>„ F' , i 3 ^ 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 at the center of mass of 2 due to the charges of 1. -104-Hence u12 = ¥ 2 " ^2 F2z " ^ e 2 F 2 z z " ( 8 _ 2 ) Assuming the adsorption center t o be a p o s i t i v e charge as p r e v i o u s l y discussed f o r the z e o l i t e s , then the favorable r e l a t i v e o r i e n t a t i o n s 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 are as f o l l o w s : + charge-dipole charge-quadrupole where + represents a p o s i t i v e charge; —^-represents a d i p o l e ; and | represents a quadrupole. The adsorption i n the z e o l i t e s expected f o r ClO^ according t o t h i s model i s discussed i n the next paragraph. These same adsorption 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 to 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 forces f o r adsorption, can a l s o determine the r e l a t i v e p o s i t i o n of the adsorbed molecule. In the presence of an e l e c t r i c f i e l d the d i p o l e s (or induced d i p o l e s i f the molecule does not possess a permanent moment) are o r i e n t e d i n the same d i r e c t i o n as the f i e l d . I t has been shown [145] that even f o r pronouncedly aniso-t r o p i c d i p o l a r molecules, the 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 to (a +a~+a )/., where -105-aj» a2' a n c* a 3 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 to be adsorbed on the surfaces studied as 0 cr The c h l o r i n e atom i s assumed to be the p o s i t i v e end of the molecule. Assuming s u b s t a n t i a l l y strong a d s o r p t i o n , the only probable movement aside 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 the arrows i n the f i g u r e , would.be a r o t a t i o n about the z a x i s of the molecule ( b i s e c t i n g the O-Cl-0 bond angle). C a l c u l a t i o n s of these 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 et a l [48] and more r e c e n t l y by Dempsey [146] and a p p l i e d to various z e o l i t e s . The c a l c u l a t i o n s were performed by 'growing' the c r y s t a l on a computer. The b a s i c q u a n t i t y of i n t e r e s t i s the e l e c t r o s t a t i c p o t e n t i a l at e i t h e r an ion s i t e j or at a point i n f r e e space. Thus ' •r'hsiyv.j C8"3) where q. , i s the charge at i o n i i n the b a s i s at l a t t i c e p o i n t k, distance r. , . from the 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 1 J K > 1 energy of t h i s b a s i s i s X qj<f>j C8-4) i -106-summed over the ions of the b a s i s or some p r o p o r t i o n of these, depending on the symmetry. Another q u a n t i t y of i n t e r e s t i s the e l e c t r o s t a t i c f i e l d F given by F = - grad cf> (8-5) Eva l u a t i o n of <f> at any point i n the c r y s t a l was done usi n g the transformation method of Ewald [147]. Values of the f i e l d F and the components of the f i e l d gradient tensor were a l s o derived by Dempsey using the Ewald method. Th e i r r e s u l t s w i l l be a p p l i e d to the EPR spec t r a of c h l o r i n e d i o x i d e adsorbed on the s y n t h e t i c z e o l i t e 13X and these i n turn r e l a t e d to the other adsorbents. Before d i s c u s s i n g the parameters obtained from the spectrum of ClO^ adsorbed on 13X, i t should be pointed out that these parameters were obtained without i n c l u d i n g the quadrupole i n t e r a c t i o n term i n the s p i n Hamiltonian. Byberg et a l [136] have shown t h i s i n t e r a c t i o n to be of importance i n the ClO^ molecule trapped i n i r r a d i a t e d KCIO^, i n p a r t i c u l a r g i v i n g r i s e to 1 f o r b i d d e n * t r a n s i t i o n s of high i n t e n s i t y i n c e r t a i n molecular o r i e n t a t i o n s w i t h respect to the magnetic, f i e l d . When the o f f - d i a g o n a l tensor elements o f ^ ^ ^ become comparable to the diagonal p a r t s q f ^ ^ ^ . a n c ^ ^ ^ co n s i d e r a b l mixing of nuclear s p i n s t a t e s occurs. 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,-AM = ± 2- (8-7) - 1 0 7 -become observable as w e l l . Several s p e c t r a were simulated 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 . The d e v i a t i o n s of the simulated s p e c t r a from the experimental 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 due to t h i s i n t e r a c t i o n . Figure 29 c l e a r l y shows two d i s t i n c t adsorption s i t e s on the 13X, the hyperfine s p l i t t i n g constants d i f f e r i n g c o n s i d e r a b l y between the two s i t e s . The constants 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 other media (see Table 1 ) . The two s i t e s are very l i k e l y a s s o c i a t e d w i t h the surface c a t i o n s at s i t e s SJJ and SJJJ, as described i n Chapter Three. The large change i n the parameters i n d i c a t e s that the intense e l e c t r i c f i e l d s a s s o ciated 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 of 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 constant i n d i c a t i n g an increase i n unpaired 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 nucleus. Our model of ClO^ adsorbed i s such that the d i p o l e moment i s o r i e n t e d along the e l e c t r i c f i e l d d i r e c t i o n , w i t h the oxygen end of the molecule c l o s e s t to 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 of the molecule and a decrease i n the h y p e r f i n e s p l i t t i n g constants. The opposite i n f a c t i s the case. The unpaired e l e c t r o n occupies the antibonding b^ o r b i t a l and the unpaired e l e c t r o n d e n s i t y s h i f t s towards the c h l o r i n e . This 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 fav 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 to be c l o s e r to the c a t i o n than to have the s i n g l y occupied -108-antibonding o r b i t a l c l o s e to the c a t i o n . Orthogonality c o n d i t i o n s ensure that i f the bonding o r b i t a l s h i f t s towards the c a t i o n , the antibonding 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 the 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 of Dempsey of the e l e c t r i c f i e l d s i n 13X show the f i e l d at S^^. to be much l a r g e r than that at 'SJJ. I t i s reasonable then, to assign the s i t e w i t h the l a r g e s t s h i f t to SJJJ. 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 to adsorbed molecules and t h e r e f o r e i s not observed. Two other z e o l i t e s having the 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 adsorbents. 10X i s a calcium z e o l i t e whereas the 13X i s of course sodium. I t i s expected that only one s i t e should be observed f o r a 100 percent exchanged form s i n c e two sodium cations are replaced by a s i n g l e doubly-charged calcium, thus l e a v i n g s i t e SJJJ unoccupied. In f a c t , the manufacturers s t a t e that the z e o l i t e i s only 75 percent exchanged, l e a v i n g the p o s s i b i l i t y of sodium cations i n s i t e s and a v a i l a b l e f o r ad 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 are exchanged a f t e r those i n S^. The spectrum of C10 2 on 10X i s consequently l e s s r e s o l v e d than the 13X but only one site.-appears to be present. 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 to show two d i s t i n c t s i t e s s i m i l a r to 13X, with increased s h i f t s i n the hyperf i n e s p l i t t i n g constants due to the smaller s i z e of the l i t h i u m c a t i o n . Only one d i s t i n c t s i t e was observed although i n d i c a t i o n s of a second s i t e , much.less populated, were evident. I t i s reasonable to assume that the S s i t e contains adsorbed water r e t a i n e d from the -109-exchange process and so i s u n a v a i l a b l e f o r adsorption of 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 that of S J J i n 13X. An increased 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 the l i t h i u m z e o l i t e due to the increased charge/size r a t i o . The decreased s i z e of the c a t i o n , however, decreases the extent of exposure of the c a t i o n to the c a v i t i e s p r o v i d i n g l e s s contact w i t h the ClO^ molecules even though the 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 the c a t i o n surface i s stronger. The spectrum observed on s i l i c a g e l , although l e s s resolved 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 i s o t r o p i c s p l i t t i n g constant obtained from the room temperature spectrum i s 17.1 gauss. This i s i n good agreement with those obtained from the other s t u d i e s mentioned and i s to be expected, s i n c e t h i s should vary l i t t l e from medium to medium. The i s o t r o p i c s p l i t t i n g constant obtained from the a n i s o t r o p i c spectrum ( i . e . T = A + B ; the observed s p l i t t i n g . T i s composed of xx o xx ^ £" xx r both i s o t r o p i c [A ] and a n i s o t r o p i c [B ] p a r t s ) agrees w i t h that observed. This o f f e r s a d d i t i o n a l support to the assigned values^ of the parameters. Some mention should be made of the r a t h e r l a r g e value assigned to the h y p e r f i n e component along the y-axis of the molecule (across the oxygens i n the plane of the molecule). I t seems reasonable to expect some change i n t h i s component from the 'free s t a t e ' value due to the manner i n which the CIC^ i s adsorbed. A decrease i n the O-Cl-0 bond angle i s probable, p o s s i b l y accounting f o r t h i s observed change.. Agreement of the c a l c u l a t e d i s o t r o p i c value to -110-that observed lends support to t h i s view. The parameters obtained from the spe c t r a observed on the z e o l i t e s 4A and 5A are i n agreement with the arguments proposed f o r the other 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 the cat i o n s are l e s s intense than those i n 13X as i n d i c a t e d by the parameters. The c a t i o n s i t e s are l e s s w e l l defined as compared to 13X and only one s i t e appears evident. C h l o r i n e d i o x i d e adsorbed on the mordenite samples i n d i c a t e a l s o a much le s s intense e l e c t r i c f i e l d at the adsorption s i t e s . The spectrum recorded at room temperature i n d i c a t e s some motion of the CIC^, although somewhat more r e s t r i c t e d than that observed on s i l i c a g e l . Rot a t i o n about the z-axis of the ClO^ molecule seems most probable, but a spectrum simulated f o r t h i s case does not agree with the 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. Accurate measurements of the hyp e r f i n e and g tensor components was not p o s s i b l e due to the large observed l i n e w i d t h . The measured components of the g tensor were i n agreement with those p r e d i c t e d f o r a molecule such as CIO • g i s c l o s e to Z. XX the f r e e - s p i n v a lue, 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 of p^-atomic o r b i t a l s . Comparing t h i s to the Se02 r a d i c a l [148] where a negative Ag i s a s s o c i a t e d w i t h admixture of the selenium d • l e v e l i n t o the b, o r b i t a l , i t i s reasonable to xz . 1 ' assume l i t t l e p a r t i c i p a t i o n of the 3d c h l o r i n e o r b i t a l i n C102- The values f o r Ag and A g z z are a l s o i n agreement w i t h theory, A g ^ having a large p o s i t i v e . v a l u e with Ag < Ag < Ag & r - & x x s z z & y y -111-- CHAPTER NINE NITROGEN DIOXIDE, NQ2 Nitrogen d i o x i d e , l i k e CIO,,, i s a s t a b l e paramagnetic molecule whose normal chemical s t a t e i s a gas. The EPR technique has f r e q u e n t l y been used to study t h i s molecule i n the gaseous and l i q u i d phases [149,150]. One of the purposes of the present study was to compare the spectrum of the adsorbed molecule to the w e l l - e s t a b l i s h e d s p e c t r a of NO^ i n a v a r i e t y of 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 given to comparison of spec t r a on other adsorbents and i n various m a t r i c e s . The reported s p e c t r a of NO^ i n various p o l y c r y s t a l l i n e media g e n e r a l l y show l i n e w i d t h s of the order of 10-20 gauss, l i m i t i n g the amount of d e t a i l which can be re s o l v e d [151,152]. More recent s t u d i e s of NO i n N„0 have produced s p e c t r a w i t h much sm a l l e r -112-l i n e w i d t h s , r e v e a l i n g g r e a t e r d e t a i l [150, 153]. EPR s p e c t r a of NO^ adsorbed on z i n c oxide [154, 155] and on magnesium oxide [156] are a l s o not w e l l r e s o l v e d . Comparison of these sp e c t r a to those of t h i s study w i l l be made i n the d i s c u s s i o n . 9.1 S i l i c a G e l . The spectrum recorded at 77°K f o r NO^ adsorbed on s i l i c a g e l i s shown i n f i g u r e 31. The expected p a t t e r n of three groups 14 of l i n e s due to the i n t e r a c t i o n of the odd e l e c t r o n w i t h the N nucleus, which has a nuclear s p i n 1=1, was observed. The complexity of the spectrum a r i s e s from the f a c t that both the g and hy p e r f i n e tensors are a n i s o t r o p i c . The spectrum i s complicated f u r t h e r by broadening of those l i n e s a s s ociated with t r a n s i t i o n s i n v o l v i n g nij = ± 1 compared to those with m^. = 0, together w i t h an over-lapping of some l i n e s . The observed l i n e width a l s o overshadows some f e a t u r e s . Table 2 gives the g and hy p e r f i n e tensor components derived from the computer s i m u l a t i o n of the.spectrum. Figure 32 shows the computer f i t t e d spectrum. 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 those obtained on other adsorbents as yet reported i n the l i t e r a t u r e . When the temperature was r a i s e d from 77°K a broadening of the spectrum occurred. S p e c i f i c changes i n the spectrum occur i f the adsorbed NO^ begins to r o t a t e about a given a x i s on warming. The l i n e w i d t h of the spectrum even at 77°K makes i t d i f f i c u l t to d i s t i n g u i s h a x i a l l y symmetric tensors from completely a n i s o t r o p i c ones. TABLE 2 Reference Hyperfine components (gauss) g-value -• T XX T yy T zz A 0 g x x 2.007 gyy. 1 g { medium s z z 154 52 47 65 54.6 1.994 2.003 adsorbed on ZnO @ 77°K 156 53.0 49.0 66.4 56.5 2.005 1.9915 2.002 adsorbed.on MgO g 77°K 156 50.0 47.9 66.4 54.8 2.0058 1.9920 1.9922 2.00.18 s o l i d N O @ 77°K 153 50.3 48.2 67.3 55.25 2.0061 2.0022 s o l i d N^O @ 77°K • 150 50.2 49.6 68.3 56.0 2.0065 1.9960 2.0029 . s o l i d N 2 0 4 @ 77°K (+0.2) gauss (±0.0005) t h i s work 52.3 48.7 67.8 56.3 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 2.0025 adsorbed on H-morderiite i • @ 77°K -114--115--116-Th e increased broadening due to the increase i n temperature makes i t impossible i n t h i s case. At approximately 200°K, the NO^ appears to be f r e e l y r o t a t i n g on the s i l i c a g e l , s i n c e the s t r u c t u r e on the three groups of l i n e s i s completely broadened. Above t h i s temperature, the spectrum could not be discerned from the background. When the sample was recooled, the s i g n a l was recovered unchanged. 9.2 -15X S y n t h e t i c Z e o l i t e . The spectrum at 77°K of 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 to that observed on s i l i c a g e l . The parameters derived from the. spectrum are given i n Table 2. Line broadening i s somewhat more evident i n t h i s case. 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 tensors i s very d i f f i c u l t . Figure 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 simulated 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 tensors r e s p e c t i v e l y . A comparison of f i g u r e s 34 and 35 shows the s i m i l a r i t y of the two s p e c t r a and the d i f f i c u l t y that 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 the l i n e w i d t h s g e n e r a l l y found. The spectrum corresponding to a x i a l l y symmetric tensors could be caused by r o t a t i o n of the NO^ about the z - a x i s . No spectrum was observed at room temperature. 9.3 H-Mordenite. Figure. 36 shows a t y p i c a l spectrum of NO adsorbed - 1 1 7 -- 1 1 8 -FIGURE 34. Computer simulated EPR spectrum of 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 , recorded at 77° K. -121-on H-mordenite, recorded at 77°K. I t should be noted that much higher pressures of NG^ were needed to observe s p e c t r a than . those r e q u i r e d f o r ClO^. Probable reasons f o r t h i s phenomenon w i l l be given i n the d i s c u s s i o n . An obvious fe a t u r e of the spectrum i s the reduced l i n e w i d t h as compared to NCh, adsorbed on other surfaces 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 set of t r i p l e t s . The r e s o l u t i o n compares to that observed f o r NO^ trapped i n a N^O^ matrix at 77°K as reported by Schaafsma et a l [153] and James et a l [150]. A computer simulated spectrum i s shown i n f i g u r e 37 and the r e s u l t s are given i n Table 2. The consequences of the narrow linevyidth w i l l a l s o be discussed l a t e r . The spectrum f o r ^NG^ adsorbed on H-mordenite i s shown i n f i g u r e 38, and a simulated spectrum i n f i g u r e 39. The parameters obtained are given i n Table 2 and agree with those expected f o r with I = h. An attempt was made to record s p e c t r a at higher temperatures with a view to ob t a i n i n f o r m a t i o n on p o s s i b l e motional processes of the NO^ s i n c e the narrow l i n e w i d t h at 77°K should enable any new features to 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 broadening at higher temperatures obscured a l l d e t a i l s . 9.4 Discussion..' • NO^, l i k e ClO^ i s a l s o a bent molecule with the symmetry p r o p e r t i e s o f the C„ po i n t group. Following the approach of - 1 2 3 -$ FIGURE 33. EPR s p e c t r u m o f * N n i t r o g e n di.oxide ad-s o r b e d on H-mordenite, r e c o r d e d a t 77° K. -124--125-Walsh [139], the ground s t a t e has the c o n f i g u r a t i o n ... ( 3 a 1 ) 2 ( l b 1 ) 2 ( 3 b 2 ) 2 ( l a 2 ) 2 ( 4 a . 1 ) , ^ The unpaired e l e c t r o n occupies an a^ o r b i t a l , d e l o c a l i z e d and construct-ed from both s and p o r b i t a l s on the c e n t r a l n i t r o g e n atom. The hyperf i n e s p e c t r a should thus d i s p l a y a considerable i s o t r o p i c s p l i t t i n g and the anisotropy should be such that i t s maximum value occurs when the axi s of the molecule i s a l i g n e d along the magnetic f i e l d . The assignment of the molecular axes i s shown below , N—-*y where the x-axis i s pe r p e n d i c u l a r to the plane of the molecule. The dominant g - s h i f t , as with C10 2, 1 S expected to be along the y-axis although i n t h i s case i t w i l l be negative. These g - s h i f t s are determined from the general formula given by equation (8-1). The g-value along the d i r e c t i o n of the maximum value of the hyper f i n e tensor should be c l o s e t o the free s p i n value or s l i g h t l y g r e a t e r . The s h i f t along z, ^ % z z w i l l a l s o be small and p o s i t i v e . -126-Table 2 incl u d e s the r e s u l t s of NO^ observed i n various matrices and adsorbed on surfaces other than those s t u d i e d here. The r e s u l t s of this.work are a l s o i n c l u d e d . Comparison of the spectrum of adsorbed NO^ w i t h the 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 of environments i s the subject of the f o l l o w i n g d i s c u s s i o n . The d i p o l e moment of NQ^ i s conside r a b l y l e s s than that of ClO^, being .29 D [157,158], and.on t h i s b a s i s alone, one would expect somewhat weaker adsorption i n comparable s i t u a t i o n s . The ni t r o g e n nucleus has a nuclear s p i n 1 = 1 and so a quadrupole moment can a l s o a f f e c t the absorption spectrum. The weaker adsorption forces are s u b s t a n t i a t e d by the. l o s s of spectrum on warming the sample to room temperature. This i s i n cont r a s t to the r e s u l t s of Colburn et a l [159] who observed the spectrum of r a p i d l y tumbling NO^ molecules at room temperature i n 13X z e o l i t e s . The pressures of NO;, i n e q u i l i b r i u m w i t h the z e o l i t e s were,however, s e v e r a l orders of magnitude l a r g e r than those i n the present experiments. The pressures needed to observe the NO^ sp e c t r a were however,, much higher than those needed f o r ClO^ i n d i c a t i n g a much reduced adsorption a t t r a c t i o n . The expected o r i e n t a t i o n of NO^ on adsorption d i f f e r s from that of CIQ2 s i n c e the d i p o l e moment i s a l i g n e d i n the opposite d i r e c t i o n . One would expect, then, that the n i t r o g e n nucleus w i l l be c l o s e s t to the adsorption s i t e s . R o t a t i o n about the molecular z-axis would seem more probable, then, 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 adsorption on s i l i c a g e l are q u i t e s i m i l a r to those reported f o r the adsorption of NO^ 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 of 56.3 gauss i s i n good agreement w i t h , and v a r i e s l i t t l e from, that observed on other s u r f a c e s . In f a c t , the i s o t r o p i c p o r t i o n of the h y p e r f i n e tensor changes l i t t l e between the surfaces s t u d i e d and NO^ molecules trapped i n other media. The f i e l d s inherent i n these d i f f e r e n t environments vary from very weak i n the i n e r t gas matrices to very strong i n the s y n t h e t i c z e o l i t e s . This i m p l i e s that the s-character of the molecular o r b i t a l of 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 surroundings of the molecule. Small changes,however are observed 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 , The assignment of g values agrees with that expected f o r t h i s molecule:.: g i s g r e a t e r than XX the f r e e s p i n value 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 e . This i s i n accordance with the work on the i s o e l e c t f o n i c molecule C0~ [160]. The observed spectrum of NO^ adsorbed on the z e o l i t e 13X i s not as s t r i k i n g as that f o r ClO^ f o r s e v e r a l reasons. D i r e c t evidence f o r two d i s t i n c t adsorption s i t e s i s not immediately obvious. The lineshape i s somewhat d i f f e r e n t from that observed on the other surfaces and the spectrum i s best simulated using a Lorentzian r a t h e r than a Gaussian lineshape f u n c t i o n . The g values are s i m i l a r . t o those observed on other surfaces and 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 of the h y p e r f i n e tensor are -128-l e s s than the l i n e w i d t h . The only n o t i c e a b l e e f f e c t of adsorption i s the d i f f e r e n t lineshape. The strong e l e c t r i c f i e l d s i n the c a v i t i e s of the z e o l i t e do not have the pronounced e f f e c t s observed i n the case of ClO^. This i s s u r p r i s i n g i n that the approach of the n i t r o g e n nucleus i s much c l o s e r to the o r i g i n of the f i e l d s than was the c h l o r i n e nucleus, due of course to the proposed mode of adso r p t i o n . ' 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 of parameters. The h y p e r f i n e components are c l e a r l y seen and the g tensor r e a d i l y measured. The assignment i s very cl o s e to that of Schaafsma et a l [153], of the components of both the g and hy p e r f i n e t e n s o r s . The adsorption s i t e s i n H-mordenite are thought to be i n the s i d e pockets l i n i n g the main c y l i n d r i c a l tubes of the s t r u c t u r e (see Figure 11), Each pocket has space s u f f i c i e n t ' f o r only one molecule reducing broadening due to recombination of the r a d i c a l to form N^O^ and a l s o d i p o l a r broadening caused by other NC^ molecules. In the experiments of Schaafsma et a l , s o l i d N^O^ was chqsen as the host matrix due to i t s i n e r t n e s s towards NO^ and the absence of any i n t e r n a l e l e c t r i c f i e l d s , being a molecular r a t h e r than an i o n i c matrix. D i s t o r t i o n of the NO^ due to s p a t i a l e f f e c t s should a l s o be minimized s i n c e the s t r u c t u r e of the guest and host molecules are- the same. In l i g h t of the s i m i l a r i t y of the parameters obtained from the spectrum of NO^ adsorbed on H-mordenite to those i n s o l i d N„0., i t appears adsorption, i n t h i s case, -129-has l i t t l e or no e f f e c t on the NO^. I t i s l i k e l y , then, that the 'trapping pockets' of HT-mordenite serve only as i s o l a t i o n cages f o r the NO^ molecules and have l i t t l e e f f e c t on i t s e l e c t r o n i c s t r u c t u r e . This i s i n accord w i t h the parameters observed f o r ClO^ adsorbed on t h i s same z e o l i t e , the e f f e c t being the smallest f o r a l l the 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 oxide i s another s t a b l e paramagnetic molecule which normally e x i s t s i n the gas phase. Beringer arid C a s t l e [161] have analyzed i n d e t a i l the spectrum of NO i n the gas phase, where complexities due to o r b i t a l , s p i n and r o t a t i o n a l i n t e r a c t i o n s are present. E a r l y attempts to detect NO trapped i n ra r e gas matrices were unsuccessful [162]. That i t does not give r i s e to a detec t a b l e spectrum i n these matrices i s not s u r p r i s i n g as the i n t e r a c t i o n with the environment i s probably not s u f f i c i e n t t o . quench the o r b i t a l motion of the paramagnetic e l e c t r o n s u f f i c i e n t l y . This s h a l l be discussed below. The NO molecule i n i t s ground s t a t e i s not paramagnetic. 2 ' • 2 NO i s a TT molecule. The ground s t a t e of the molecule ( TTJ ) i s nonmagnetic s i n c e s p i n and o r b i t a l angular moments are a n t i p a r a l l e l , and the o r b i t a l magnetism j u s t cancels the s p i n magnetism. J -131-The paramagnetic character of NO r e s u l t s from the ^3/2 s t ^ t e j a consequence of the s p i n and o r b i t a l momenta being a l i g n e d . The 2 -1 7i\^2 s t a t e i s separated by 121 cm from the ground s t a t e . Both st a t e s are appreciably populated except at temperatures below. about 50°K. Sorp 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 the o r b i t a l angular momentum. I t thus appears that c e r t a i n environments may quench the o r b i t a l angular momentum and enable the EPR. s p e c t r a to be recorded. Recently, Lunsford [165-167], Gardner and Weinberger [168], and Hoffman and Nelson [169] have 2 reported s p e c t r a of NO i n a TT s t a t e adsorbed on MgO, ZnO and various z e o l i t e s , i n which the o r b i t a l momentum seems s u b s t a n t i a l l y quenched by the surface f i e l d s of the adsorbents. These surface f i e l d s were s t u d i e d as w e l l as the e f f e c t of the adsorption on the NO, The r e s u l t s to be presented here are i n accord w i t h those p r e v i o u s l y reported, although i n the present work a r e a c t i o n o f the.NO with c e r t a i n surfaces was observed i n a d d i t i o n . P r e p a r a t i o n of the samples i s the same as f o r the adsorption of NO^. 10.1 S i l i c a G e l . Attempts were made to observe the spectrum of NO adsorbed on s i l i c a g e l at 77°K but were un s u c c e s s f u l . Although Solbakken et a l [163, 164] reported that the f i r s t molecules -132-adsprbed at t h i s temperature were i n the e x c i t e d s t a t e '"'3/2 a n ^ continued up to n e a r l y monolayer coverage, 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 at 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 evident and the spectrum appears s i m i l a r to t h a t reported by Gardner et a l [168]. The parameters were assigned by comparison to the simulated spectrum, f i g u r e 41, and are given i n Table 3. Since any hy p e r f i n e s t r u c t u r e i s apparently l e s s than the l i n e w i d t h , the e f f e c t s of t h i s and any other broadening i n t e r a c t i o n s were included i n the l i n e w i d t h and the spectrum was simulated using an a x i a l l y symmetric g tensor. 10.3 H-Mordenite. The spectrum observed at 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 to that reported by Gardner et a l [168] and to that of Lunsford [165] reported on MgO. Some s t r u c t u r e i s evident and as an a i d to a n a l y s i s , the spectrum of "^ NO was al s o .recorded. Figures 42 and:43 show the s p e c t r a f o r 14 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 tp that of.the 13X sample using an; a x i a l l y symmetric g tensor, but 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 . The r e s u l t s are shown i n Table 3. I t i s obvious from the 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, that the s i t u a t i o n ' TABLE 3 3ferenc-e g values (gauss) Hyperfine components £ R g xx byy "zz T xx T yy T. ZZ ' •- " : Medium 165 g l g// 1.996 1.996 1.89 35 < 10 14 NO. adsorbed on MgO @ • 77°-K 169 1.994 1.994 1.873 28 14 NO adsorbed on 4A 3 77°K 166 1.997 1.997 1.91 31 -14 . NO adsorbed on ZnS @ 77°K 168 1.970 1.970 1.7S6 14 : NO..adsorbed on 13X • @ 77°K 168 1.967 1.967 1.773 14 NO adsorbed on H _mordenite § 77°K 168 1.990 . • .. 1.990 1.859 - 14 NO adsorbed on 5A . § 77°K ±0.001 +0.001 ±0.01 ±1 Line width used f o r si m u l a t i o n s (gauss) t h i s work 1.967 1.967 1.78 90.0 14 NO adsorbed on 13X @ 77°K t h i s work 1.994 1.994 1.S7 23 38 35.0 14 NO adsorbed on H-mordenite • .. Q 77°K this-work v 1.994 . V.- 1.994 • 1.87 ' L ' ' • • ,v • : 35.0 ^NO adsorbed on H-mordenite 8 77°K ' -134-FIGURE 40. EPR spectrum of n i t r i c oxide adsorbed on 13X s y n t h e t i c z e o l i t e , recorded at 77° K. -135-FIGURR 41. Computer 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 G A U S S < — — — 1 > H FIGURE 42. ERR spectrum>of N n i t r i c oxide adsorbed on Ilr-mordenite, recorded at 77° K. -137-H e 100 G A U S S 15 FIGURE 43. EPR spectrum of • • N. n i t r i c oxide adsorbed on H-mordenite, recorded at 77° K. r l 3 8 -361.45S -139 --140-i s more complex than t h i s . Both samples were evacuated to remove as irjuch adsorbed species as p o s s i b l e w i t h the r e s u l t i n g spectrum shown i n f i g u r e 46, The spectrum then observed at 77°K was i d e n t i c a l i n both cases and showed considerable s t r u c t u r e . 10.4 D i s c u s s i o n . I n t e r a c t i o n of the surface f i e l d s w i t h AB type Tr-radicals quenches the o r b i t a l momentum of these r a d i c a l s . An * unsymmetrieal environment l i f t s the degeneracy of the 2pir q r b i t a l s (TT and TT o r b i t a l s , d e f i n i n g the N-0 bond as the z - a x i s ) . For the NO x y * molecule, the unpaired e l e c t r o n w i l l be i n the 2piTx level, i n t h e , absence of 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 the g tensor of an e l e c t r o n i n a TT s t a t e were given by Kanzig et a l [170]: 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 coupling constant; E, the 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 the o r b i t a l c o n t r i b u t i o n (k=l f o r the f r e e molecule). The equations are given by (10-3) -141--142-The apparent a x i a l symmetry of the observed s p e c t r a (due mainly to 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 . The c a l c u l a t i o n s of Gardner et a l on the 13X and H-mordenite [168] w i l l not be repeated here s i n c e the same adsorbents were used. The species formed on adsorption of NO on H-mprdenite and subsequent pumping pf the.sample, i s indeed c u r i o u s . I t i s reaspn r able that the NO i s e a s i l y removed by pumping si n c e i t s d i p o l e moment i s much l e s s than that of NO,,. A value of 0.158 0 was reported by Stogryn [171] 0 The species i s . o b v i o u s l y not due to a n i t r p g e n c o n t a i n i n g molecule s i n c e no change i n s t r u c t u r e was observed on adsorption of ^NO and ^NO, the. nuclear s p i n of ^ N being I=h 14 i n c o n t r a s t to 1=1 f o r N. Numerous attempts, at a n a l y s i s using Computer simulated spep^ra were made with no success. The presence of more than one species i s p o s s i b l e but no evidence f o r t h i s was given by t e s t s of 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 of NO w i t h the surface of H-mordenite has produced a species s t r o n g l y attached to the s u r f a c e . F a i l u r e to remove the species by pumping i s evidence of t h i s . I t i s l i k e l y the NO has reacted w i t h some part of the surface to form a species which, i f not chemisorbed, i s very s t r o n g l y attached. Terenin and cp-workers [172], when studying the absorption of NO oh various z e o l i t e s using i n f r a r e d spectroscopy, found that NO was adsorbed as N 20 ? the oxygen freed by the r e a c t i o n probably being adsorbed. T h e i r assignment was shown to be c o r r e c t by adsorbing N^O d i r e c t l y . The formation pf N^ O would account f o r i d e n t i c a l s p e c t r a being observed f o r ^NO and 14 NO s i n c e N„0 i s not paramagnetic. The r e a c t i o n of the oxygen - 1 4 3 -atpm with the H-rjnordenite would then be r e s p p n s i b l e f o r the observed spectrum. -144-CHAPTER ELEVEN DIFUJORAMINO RADICAL, Np 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^, at normal temperatures. The d i s s o c i a t i o n of 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, 174]), and i t has been shown that the di 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 capable of e x i s t i n g i n d e f i n i t e l y i n the f r e e s t a t e . At rpom temperature and-atmospheric pressure, the r a d i c a l i s present to the extent of only 0.05 per cent. The r a d i c a l c oncentration reaches 90 per cent only at 573°K and one atmosphere, at 423°K and 1 mm, or at 298°K and 10 ^ atmospheres. The EPR spectrum observed i n the gas phase c o n s i s t e d of a s i n g l e broad l i n e showing no hy p e r f i n e s t r u c t u r e due to e i t h e r the n i t r o g e n or the 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-resolve d h y p e r f i n e s t r u c t u r e have been observed f o r NF„ d i s s o l v e d i n pprfluorpdimethylhexane [175] and i n l i q u i d [174]. Adrian et a l [176] st u d i e d the KV^ r a d i c a l i n an argon m a t r i x , but were unable 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 of the a n i s o t r o p i c components. Farmer et a l [177] reported r e s u l t s f o r NF- i n both argon and krypton matrices at 4.2°K. Unfp r t u n a t e l y , n e i t h e r of these s t u d i e s 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. More r e c e n t l y , Kasai and Whipple [178] st u d i e d the r a d i c a l i n a neon matrix at 4°K and were able to assign t;he observed p r i n c i p a l tensor components to the molecular axes. A recent paper by McDpweTl et a l [179] reported . a d e t a i l e d study of how an i n e r t gas matrix and appropriate p h y s i c a l . con d i t i o n s together may i n f l u e n c e the nature and extent of the o r i e n t a t i o n pf a paramagnetic s p e c i e s , u s i n g NF^ as an example, The work.accomplished a complete a n a l y s i s o f the sp e c t r a a nd a l s o a temperature, study. Results of the adsorption of the NF^ r a d i c a l on H-mordenite are reported here, 11.1 H-Mprdenite. The spectrum observed f o r ^ F ^ adsorbed on H-mordehite at 77°K i s shown i n f i g u r e 47. Several experiments were attempted with varying concentrations but t h i s was the only r e p r o d u c i b l e spectrum observed. The observed s p l i t t i n g s are not s i m i l a r to those p r e v i o u s l y observed f o r the NF^ r a d i c a l i n other media. H-mordenite was used as the adsorbent f o r these experiments s i n c e i t has -146-- 1 4 7 -produced consistent results with the other r a d i c a l s . Computer simulations of the pbserved spectrum are shown i n figure 48. The parameters are given i n Table 4. ,11.2 Discussion. The NF^ ra d i c a l i s valence ispelectronic with CIC^, haying the unpaired electron i n a b^ antibonding TT o r b i t a l . The ground, 2 electronic state 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 fluorines and the nitrogen. The lack of hyperfine structure could be attributed to rapid recombination of the r a d i c a l s , i n f a c t , rapid recombination could even obliterate the entire spectrum., The spectrum observed i n 5A molecular sieve by Colburn et a l [180] did, indeed show well resolved hyperfine structure with measured 14 19 N and F couplings of 16 and 56 gauss respectively. The reported g value was 2.009. It was assumed the I^F^ was screened out of the zeol i t e eliminating much l i n e broadening and the spectrum was due to freely rotating NF 2 r a d i c a l s . The spectrum observed i n this study has been attributed to a species having an anisotropic g tensor with no observable hyperfine structure. Table 4 gives the assigned values. Figure 48a i s an attempt to simulate the spectrum as due to an isotropic 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° either i n t e n s i t i e s or l i n e positions. 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 Reference g-values (± 0.0005) I s o t r o p i c Hyperfine component (gauss) t h i s work f i g u r e 48b 8 x x g y y hz A, o . 2.0151 2.0084 2.0025 t h i s work ' f i g u r e 48a 1 •" i ri 1 — TI— ? — n — | — : ^— • 2.0100 , . , —p. , — '7.8 J—. , ii • i ' j . — , , . in i , 1 -149-(a) •3219.593 234:599 '3249.599 3254.529 FIELD (GRUSS) .593 (b) i r — * T 3215.559- '3228.2£9 3230.529 • -3253.659 32S6.3S9 -.3279.059 FIELD (GfiUSS) .FIGURE 48. Computer simulated EPR s p e c t r a of species formed on ad s o r p t i o n of N F on H-mordenite, recorded ,0 -- 2 4 at 77^ K a) i s o t r o p i c g and h y p e r f i n e tensor 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 - 1 5 0 -non-paramagnetic species and a paramagnetic species having no observable h y p e r f i n e s t r u c t u r e . -151:-GHAPTER. TWELVE SUMMARY This chapter- i s intended as a summary of the work completed; i n t h i s t h e s i s w i t h a view to 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 of stud i e s i n t h i s area., Greater amounts o f inf o r m a t i o n are. steadily-becoming a v a i l a b l e on the topology of the various surfaces, s t u d i e d , the area where lack of knowledge has been the most outstanding. More d e t a i l e d conclusions could then be reached concerning the i n t e r a c t i o n s at these g a s - s o l i d i n t e r f a c e s . The main species which has been s t u d i e d here, c h l o r i n e d i o x i d e , has shown widely 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 the various adsorbents used. The H-mordenite samples y i e l d e d EPR s p e c t r a having measured parameters the l e a s t changed from those obtained i n media other than adsorbents. This i n d i c a t e s the C1C* molecules -152-are p h y s i c a l l y trapped i n the i n t e r i o r of 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 . The i n t e r a c t i o n with s i l i c a g e l i s somewhat s i m i l a r , although the amorphous s t r u c t u r e of this,, adsorbent makes i t d i f f i c u l t to q u a n t i t a t i v e l y place the CIC^ molecules i n any p a r t i c u l a r area of i t s i n t e r n a l s u r f a c e . 13X, on the other hand, has an ordered s t r u c t u r e which enables one, from data obtained from the experiments, to v i s u a l i z e the a c t u a l adsorption s i t e s i n v o l v e d . These have been discussed i n Chapter E i g h t . The r e s u l t s f o r the other z e o l i t e s may s i m i l a r l y be analyzed with regard to adsorption 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 surface f i e l d s . A p u b l i c a t i o n concerning a study of ClO^ adsorbed on s y n t h e t i c z e o l i t e s [181] has r e c e n t l y appeared i n the l i t e r a t u r e . The s p e c t r a obtained on the z e o l i t e s 13X and 10X were not analyzed i n terms of a c t u a l adsorption s i t e s , probably due to the f a c t that s u c c e s s f u l computer s i m u l a t i o n of the s p e c t r a could not be obtained. The two d i s t i n g u i s h a b l e s i t e s observed i n the present study were not n o t i c e d . The l i n e w i d t h s f o r the s p e c t r a reported 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 these f e a t u r e s . This same paper by P i e t r z a k and Wood a l s o contained a study of NG^ adsorbed on these same z e o l i t e s . The paper contained comparable s p e c t r a to these obtained i n t h i s study f o r the 13X s y n t h e t i c z e o l i t e . Results obtained here f o r N0 9 were not -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 of NC^ s t u d i e d i n other media. In f a c t , the s p e c t r a observed on the 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 to that f o r N0„ i n an NO. ma t r i x , both media s t u d i e d at 77°K. The proposed adsorption s i t e s f o r the N0 2 molecules i n t h i s z e o l i t e are the small pockets l i n i n g the main passage-ways. In the case of ClO^, the 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 the s t r u c t u r e s of the 13X z e o l i t e s , whereas f o r NO^ t h i s was not so. S p e c i f i c adsorption s i t e s may only be assigned to the z e o l i t e , H-mordenite. This i s a t t r i b u t e d to the f a c t that C10 2 has a l a r g e r d i p o l e moment than N0 2 which enables i t to i n t e r a c t more s t r o n g l y with s p e c i f i c c a t i o n s i t e s i n 13X. In H-mordenite, these s i t e s are not as w e l l - d e f i n e d , and coupled w i t h the small d i p p l moment of the N0 2, the observed s p e c t r a suggest the molecules to be confined i n these s i d e pockets. The s p e c t r a observed f o r n i t r i c oxide adsorbed on various z e o l i t e s show yet another p o s s i b l e e f f e c t of adsor p t i o n . While the spec t r a which are f i r s t apparent on adsorption are s i m i l a r to those observed by others on a v a r i e t y of s u r f a c e s , pumping of the sample to decrease.the c o n c e n t r a t i o n of the NO.on the surface y i e l d s a new spectrum. This i s assigned to a species formed by a r e a c t i o n of the NO molecules with the su r f a c e , This new species has been shown not to contain n i t r o g e n . This i s confirmed by the f a c t that i d e n t i c a l EPR sp e c t r a are observed 15 14 f o r both NO and NO. The observance of chemical r e a c t i o n s on surfaces e i t h e r through the formation of a new species or 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 species i s then another area w i t h wide p o s s i b i l i t i e s . The use then, of the EPR technique, i n the study of 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 categorized i n three areas: Information about the nature of the surfaces i s p o s s i b l e i n many cases and the opportunity f o r study here i s l i m i t e d only to the number of surfaces which y i e l d EPR s i g n a l s . The a d d i t i o n of gaseous molecules to these surfaces widens the scope c o n s i d e r a b l y . In these cases, as was found f o r ClO^, t h i s technique may provide an " i n e r t matrix!' which perhaps enables the species i n question to 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 other EPR techniques, or may even provide a means of study where others have not as yet been found. Included i n t h i s area a l s o are the p o s s i b i l i t i e s of r e a c t i o n of the gaseous molecules w i t h the surfaces to y i e l d new s p e c i e s ; e i t h e r a new adsorbed s p e c i e s , or even s p e c t r a now due to the s u r f a c e , whereas before the adsorption, none was evident. The l a t t e r was the case w i t h n i t r i c oxide. The o p p o r t u n i t i e s are extremely large i n t h i s p a r t i c u l a r area. The l a s t general area where the use of EPR has found value i n these s t u d i e s i s i n the area of the dynamical behaviour of the adsorbed molecules. The motion of the adsorbed molecules, e i t h e r hindered or f r e e may be s t u d i e d at a v a r i e t y of temperatures by t h i s technique. The p u b l i c a t i o n of P i e t r z a k and Wood [181] mentioned e a r l i e r was such a study, although i t was not completely -155-s u c c e s s f u l i n the ease of 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 other spectroscopic, techniques, look promising. The EPR technique has c e r t a i n l y not been explored t o i t s f u l l e s t i n any of the three areas mentiqned. Since knowledge of the surfaces i s v i t a l to an. understanding of the r e s u l t s , the more accurate the i n f o r m a t i o n 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. To t h i s end, perhaps a combination of XRD and EPR techniques would prove very v a l u a b l e . M o d i f i c a t i o n s to the surfaces could then be s t u d i e d i n regard to t h e i r e f f e c t on the observed s p e c t r a . A s i n g l e species could then be s t u d i e d i n , g r e a t e r d e t a i l by v a r y i n g the surface c o n d i t i o n s s y s t e m a t i c a l l y . In any Case, the fu t u r e leaves much to be discovered i n t h i s area. -156-REFERENCES •1. R.P. Eischens and W.A. 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McDowell, H. Nakajima, and P. Raghunathan, Can, J . Chem 48, 805 (1970). 180. C.B. Colburn, R. E t t i n g e r , and F.A. Johnson, Ihorg, Chem., 3_, 455 (1964), 1 181. T.M„ P i e t r z a k and D.E. Wood, J . Chem. Phys., 5_3, 2454 (1970) 182.. P.W. Atkins and M.C.R. Symons, The S t r u c t u r e of 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, -166-APPENDIX I M P L I C I T R G A L * 3 { A - H , 0 - Z ) p I M E N S I U N H ( 9 L , 9 1 ) , T P ( 9 L , 9 1 ) , G ( 9 1 , 9 1 ) , G 2 ( 9 I , 9 i ) , A ( 9 L,9 I), I A A 2 ( 9 1 , 9 1 ) , A A L R H 2 ( 9 1 , 9 i ) > C U 9 1 , 9 1 ) , C 2 ( 9 1 , 9 1 ) , C 4 ( 9 \ , 9 1 ) , C i? ( 9 1 , 9 1 ) , 2 C 6 ( 9 1 , 9 1 ) , P H I ( 9 1 . ) ,. S P ( 9 j } , C P ( 9 1 ) , S T ( 9 1 ) , C T ( 9 1 ) , S P 2 < 9 1 ) , C P 2 ( 9 j ) , 3 S T 2 ( 9 1 ) , C T 2 ( 9 l > ,C5R2rH] , G A L ^ H 2 l 9 i ) t P H I i l { 9 1 ) » r i T L E ( 2 0 ) R t A 0 ( 5 , 3 3 3 ) T I T L E RE AO 8 9 4 ) N T I M E S » N T i l F . T A , N P H I K O U N T = 0 R E A D ( 5 , a 9 0 ) G X , G Y , G Z , F R E Q, $ P I N R E A D ( ' 5 , 3 9 1 ) A X , A Y , A Z 1 ! ' ' • .' ' " ~ ~ W R I T E ( 6 , B 3 3 ) T I T L . E W R I T 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 A 4 ) 3 9 0 F O R M A T ( 3 T 1 0 . 5 ) 8 9 1 F O R M A T ( "3 F-1 0 . 3 ) 3 9 4 F O R M A T ( 3 1 3 ) 8 9 9 F O R M A T ( I X , • T H E F R E Q U E N C Y . I S . S F I O . 5 , 1 M H Z • ' ) 3 3 0 F O R M A K j X , ' G.X = V,F10.!>,J G Y f It F l O . 5 , 1 G Z - ' , F 1 0 . 5 ), 8 9 8 F O R M A T ( I X , ' A X = ' , F 1 0 . 3 , » A Y ? J , r I 0,3> • ' A Z ~ ' , f : 1 0 . 3 , » ' I N G A U S S ' 8 9 7 F O R M A T ! L X , ••' 5 P I N - ' * F L O . 1 , • N T H E T A = • , I 3 , « . N 1 P H U * , I 3 ) S T A R T = S C L O C K ( 0 . 0 ) F R E Q = F RE4} * l . 0 0 + 0 6 . A X = A X * G X * U 3 9 9 6 2 6 4 0 1 - 0 6 A Y = A 'Y * G Y * L . 3 9 9 6 2 6 4 D + 0 6 A Z = A Z « ; 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 • •• A X 2 = A X * A X : . ' A Y 2 - A Y * A Y i : • A Z 2 = A Z * A Z A X Y 2 ~ A X 2 * A Y 2 G A X 2 = G X 2 * - V X 2 G A Y 2 s = G Y 2 * A Y 2 G A Z 2 = G Z 2 * A Z 2 B O O . 9 2 7 3 2 0 - 2 0 H H = 6 . 6 2 5 l 7 l " 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 , : I F ( 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 2 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 S J M N + 1 . 0 0 )  -167r A 6 = A 5 * = F R E ' J SMI=SPIN 11 Bl=SMi*SMI B 2 = A 3 * 8 1 / 2 . 0 D 0 ; B 3 - A 3 * ( A 4 - i i I ) / 4 . O Q 0 B 4 = A 5 » S M I ' C I F ( S M I . L T . S P I N ) G Q T O 5 6 P H I . C U = 0 . 0 0 0 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 ) C T ( 1) = i . 0 0 C T 2 ( 1 ) = 1 . 0 0 S T ( l ) = 0 . J O S T 2 ( 1 ) = 0 . 3 0 D O 1 7 I = 2 , N T H F T A C T ( 1 ) - = C T ( I - 1 ) - 0 I V P T 2 ( I > = C T ( I ) f C F( I ) S T { I ) = <)S J••<T ( u o o - c r 2 ( I ) ) 17 s r 2.{ n = s r ( i ) * s r (11 I F { N P H i . L t . . l ) G i 3 T p 6 0 O f J 9 3 ' l ? 2 » N P H I ••  P H I { I ) - P H I.( I ~ 1 ) + A A P , 9 0 P H I O t I ) = P H I D ( [ r l J t D P H I 6 0 D O 8 9 N P = L , N P H l S P I N P ) = D S I N ( P H I ( N P ) ) • C P ( N P > = O C O S ( P H I I N P ) ) S P 2 1 N P ) r S >:( M P ) * S P ( N P ) C P 2 ( N P ) - C * M N ' , ) * C P < N P ) C S P 2 ( N P ) = C P 2 { N P ) - S P 2 ( N P ) G A L P H 2 { N P ) •- G X 2 * C P 2. ( N ? ) * G Y 2 * S P 2 ( N P ) 8 9 C O N T I N U E : D O 4 3 rtP^l-.NPHl • 0 0 4 3 N T = I j J i T J l E 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? r3 { N T , N f j ) A A L P H 2 ( 1 i T , h ? ) = ( G A X 2 * C ? 2 ( N P ) + G A Y 2 * S P 2 ( N P ) > / 0 2 ( N T , N P ) A ( N T , ii P > «= J :S Q 7 T { G 2 ( N T , : ^ P ) * A A L P H 2 ( N T , N P ) * S T 2 ( N T ) + G A Z 2 * C T 2 (, N T ) ) / G ( N T I M P ) . A A 2 ( N T t N P )•= A ( N T T N P ) * A ( N T t N P ) : •  C 1 ( N T , N P ) ='M ( N T rtiPi) * A \L P H 2 ( N T , N P ) C 2 ( N T r N P » - G A L P H 2 ( N P ) « ? A A 2 ( N T , H P ) C 4 ( N T , N P ) ^ G 2 ( N T » N P ) * G 2 ( N T , N P ) ' * C 2 ( N T , N P ) C S ( N T , N P j = C 4 ( N T , N>> ) T A A L P H 2 ( N T , N P ) C 6 ( N T , t M P ) = - v 2 ( N T , N ? ) * p 4 ( N T , N P ) T P ( N T , N P ) = " G 2 2 f G A L P H 2 ( N P ) / G 2 ( N T , N P ) 4 B C O N T I N U E r 5 6 D O 5 B N P = l . , N P f | I D O 5 3 N T = I , N T M E T A 1 1 2 2 B 3 ~ ( B 4 r A ( N T f N I?) - A 6 ) / G ( N T t N P ) r.VJ2 = ..W><=-J:i C C = tt 2 ( ( G 7.2 v ( C M N T , N P ) - G A L P H 2 ( N P ) * A Z 2 ) * * 2 ^ S T 2 ( N T ) # C T 2 ( N T ) ) / ' C 6 { N T , 1 P ) +• ( A I - S O 2 ( N T ) * C S P 2 ( N P ) ) / C 4 ( N T , N P ) ) •+ ( B 3 / G 2 ( N T , N P ) ) * ( ( C L ( N T , N P ) * A 7 . 2 ) / C 2 ( N T , M P ) <N G A L P H 2 ( N P ) * A X Y 2 ) / C I ( N T , N P i + i A 2 - C T 2 ( N T ) * Q ^ ! > 2 ( N P ) J - / C 5 ! 3 T , N P ) ) X = B 3 2 - C e * " 4 . 0 0 -168-H{NT,NP)=(-83+DSQ*T(X))/2.pQ C 5 3 CONTINUE C I FJSM_I_.LT ._SP (N._OR,KOONTrGT.O )G0 TO XI 11 iiiiRI T f i ( I , 6 0 3 ) ( C J ( I ) , 1 = L , N T H E T A ) W R I T F ( 1 , P O O ) ( P H I 0 ( I ) , 1 = 1 , N P H I > 1 1 1 1 W R I T E ( 6 , 1 0 0 0 ) W R I T E ( o , 1 0 Q 2 ) P H I D I 1 ) D O 7 0 N P = I , N P H f ' I F ( N P . G T . 1 ) W R I T E ( 6 , 1 0 0 3 ) P H I D ( N P ) D O 8 0 .MT= I , N T H E T A i 4 K = N T + 1 L = N T f 2 M = N T « - 3 W ^ I T E { 6 , 1 0 0 1 ) H ( N T , N P ) , T P i N T , N P ) , H ( K , N P ) , T f » ( K , N P n H { L , N P ) t T P ( L » N P ) . 1 H ( M , N P ) , T P ( M , M P ) '' ' • 8 0 C O N T I N U E 1 ' ~ ~ ' ~ ^ n ^ 7 0 C O N T I N U E : L O O O F O R M A T I L H 1 , 3 ( / > , 3 X , ^ ( • F I E L 3 ( G A U $ S ) • , 3 X , • I \ l T E N , S I T Y » , 2 X ) / / ) 1 0 0 1 F 0 K M A T < 5 X , t+(F3. 2 , 5 X , F 3 . 5 , 5 X ) ) 1 0 Q 2 F O R M A T ! * P H 1 ~ ' > F 5 t 1» ' ' 0 c G R L E S ' ) 1 0 0 3 F O R M A T ! 3 ( / ) , ' P H I - ' , - F 5 . 1 / ) , oo 3o i =i t f i P - H i ' 3 0 W R I T F ( 1 , 6 0 1 ) [ H ( J , I ) , J * l , N T H E T A ) PQ 3 1 i = 1 t N P H l ' 3 1 W R I T t ( 1 , 6 0 2 ) ( T P ( J , I ) , J = l , N T H E T A ) 6 0 0 F O R M A T ( 1 J F 6 . 2 ) 5 Q 1 • F O R M A T ( 1 0 F c i . 2 ) 6 0 2 F O R M A T ! 1 0 F 3.5) 6 0 3 F O R M A T ( 1 J F O .4 ) C S M I -O - 1 . 0 0 I F ( S M I .LT . - S P I N ) G O T O 5 GO TO 11 5 ' T I M E = 5 C L g C - ^ ( S T A R T ) W R I T £ ( 6 , 3 5 p ) T I M F N I ' I M C - 5 - i N T I K F S - l KOUNT^KO'JNTt-l I F ( N T I M E S . G T . Q ) GO T O 4 . 3 5 0 FORMA TJ J H 0 _ f _ T j Mb R E jUIi<FI)< » F 3 . 3 , ' S E C O N 0 S J / )_ S T O P ' ' END T 

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