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Electron spin resonance studies of small free radicals trapped in inert matrices at 4.2 degrees K Gerry, Michael Charles Lewis 1962

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ELECTRON SPIN RESONANCE STUDIES OP SMALL FREE RADICALS TRAPPED IN INERT MATRICES AT 1}..20K. by MICHAEL CHARLES LEWIS GERRY B.A., UNIVERSITY OF BRITISH COLUMBIA, i 9 6 0 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o f CHEMISTRY We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA August, . 1 9 6 2 . In presenting t h i s t h e s i s i n p a r t i a l f u l f i l 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 t h a t the L i b r a r y s h a l l make i t freely-a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department o r by h i s r e p r e s e n t a t i v e s . I t i s understood t h a t 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 g a i n s h a l l not be allowed without my w r i t t e n permission. Department of CHEH>S>Tft-Y  The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date AUGUST Sfejqka  (1)' ABSTRACT Small f r e e r a d i c a l s trapped i n s o l i d argon, krypton and carbon t e t r a c h l o r i d e a t i|..20K have been s t u d i e d u s i n g e l e c t r o n s p i n resonance (ESR). An attempt was made to determine whether the methylene r a d i c a l , produced by the p h o t o l y s i s o f diazomethane and ketene trapped i n the s o l i d m atrix, has a t r i p l e t ground s t a t e . No s i g n a l d e f i n i t e l y a t t r i b u t a b l e to the methylene r a d i c a l was observed. I t i s p o s t u l a t e d that the zero f i e l d s p l i t t i n g due to the s p i n - s p i n c o u p l i n g of the un p a i r e d e l e c t r o n s broadened any ESR s i g n a l beyond detectability„ The ESR s i g n a l of trapped methyl r a d i c a l s was observed i n some experiments, and i t i s suggested that they were formed by a b s t r a c t i o n of hydrogen atoms from another d e p o s i t e d m a t e r i a l by methylene r a d i c a l s . An experiment i n which diazomethane was p h o t o l y s e d i n the presence of D2O i n an argon m a t r i x at I)..20K y i e l d e d an ESR s i g n a l which may p o s s i b l y have been due to the CH2D r a d i c a l . An i n v e s t i g a t i o n has been c a r r i e d out of the po p u l a t i o n s of the r o t a t i o n a l l e v e l s of methyl r a d i c a l s produced by the p h o t o l y s i s of trapped methyl i o d i d e and dimethyl mercury at if.2°K. For thermal e q u i l i b r i u m f r e e l y r o t a t i n g r a d i c a l s should populate o n l y the ground s t a t e a t t h i s temperature, but i t was found that the lowest two l e v e l s were both populated. I t i s suggested that e i t h e r there was ( i i ) n o t t h e r m a l e q u i l i b r i u m , o r , more l i k e l y , t h e m e t h y l r a d i c a l s were u n d e r g o i n g h i n d e r e d r o t a t i o n . Room t e m p e r a t u r e e q u i l i b r i u m m i x t u r e s o f N£F^ - N F 2 were t r a p p e d i n t h e t h r e e m a t r i c e s a t Ij..2°K, a n d ESR a b s o r p t i o n due t o t h e t r a p p e d NF£ r a d i c a l s was o b s e r v e d . T h r e e l i n e s were o b s e r v e d a t t h i s t e m p e r a t u r e , w i t h t h e c e n t r e one o f g r e a t e r a m p l i t u d e a n d s m a l l e r l i n e w i d t h t h a n t h e c' o u t e r two. D u r i n g warmup t h e a m p l i t u d e s and w i d t h s o f t h e s e l i n e s became a p p r o x i m a t e l y e q u a l a n d two f u r t h e r t r i p l e t s a p p e a r e d , s y m m e t r i c a l l y d i s t r i b u t e d a b o u t t h e c e n t r e l i n e . F r o m t h e warmup s p e c t r a t h e 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 c o n s t a n t s f o r f l u o r i n e a n d n i t r o g e n h a v e b e e n d e d u c e d t o be l68 a n d L|_8 m c . / s e c . r e s p e c t i v e l y . I t i s s u g g e s t e d t h a t t h e r a d i c a l s u n d e r w e n t s l o w i s o t r o p i c r o t a t i o n a t lx..2.uK. The d e g r e e o f s - c h a r a c t e r o f t h e m o l e c u l a r o r b i t a l c o n t a i n i n g t h e u n p a i r e d e l e c t r o n i s d i s c u s s e d i n t h e l i g h t o f t h e 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 c o n s t a n t s . An u n s u c c e s s f u l a t t e m p t t o f i n d h y p e r f i n e a n d r o t a t i o n a l s t r u c t u r e i n t h e ESR s i g n a l o f t h e K F 2 r a d i c a l i n t h e gas p h a s e was c a r r i e d o u t . The - p h o t o l y s i s o f C F 3 I i n k r y p t o n a n d c a r b o n t e t r a c h l o r i d e m a t r i c e s a t i ^ . 2 ° K y i e l d e d a v e r y c o m p l i c a t e d ESR s p e c t r u m . A p h a s e r e v e r s a l o f some o f t h e l i n e s was o b s e r v e d . A b r o a d s i n g l e l i n e was o b s e r v e d when C F 3 I i n c a r b o n t e t r a c h l o r i d e was i r r a d i a t e d a t 77°K. A t t h e t i m e o f w r i t i n g no d e f i n i t e i n t e r p r e t a t i o n o f t h e s p e c t r a c a n be s u g g e s t e d . ( v i i i ) ACKNOWLEDGMENTS Many thanks to P r o f e s s o r C.A. McDowell f o r h i s gre a t i n t e r e s t and help i n t h i s work. To Dr. J.B. Parmer I am most g r a t e f u l f o r h i s much needed guidance throughout the experiments. I a l s o thank Mr. R. Muelchen and Mr. J. S a l l o s , who were always most h e l p f u l , e s p e c i a l l y a t the times when things went wrong. With many members of the Department of Chemistry I have had d i s c u s s i o n s on v a r i o u s matters concerning t h i s r e s e a r c h ; I thank them very much. Also, many members of the department p r o v i d e d me with compounds; I am a l s o g r a t e f u l to them. Many thanks to Dr. J.A. B e l l and Dr. C.B. Colburn f o r t h e i r communicstions. I must thank a l s o the members of the t e c h n i c a l s t a f f o f the department f o r t h e i r h e l p . I am g r a t e f u l to the N a t i o n a l Research C o n c i l o f Canada f o r a Bursary (196O-61) and a Studentship ( 1 9 6 1 - 6 2 ) . ( i i i ) TABLE OF CONTENTS Page A b s t r a c t i L i s t of Tables v L i s t of F i g u r e s v i Acknowledgments v i i i Chapter One GENERAL INTRODUCTION 1 1-1 Free R a d i c a l s at Low Temperatures 1 1- 2 B a s i c P r i n c i p l e s of E l e c t r o n S p i n Resonance 3 Chapter Two GENERAL EXPERIMENTAL METHODS - 9 2- 1 D e s c r i p t i o n of the E l e c t r o n S p i n Resonance Spectrometer 9 2-2 The L i q u i d H eli u m Dewar System 10 2- 3 P r o d u c t i o n of Trapped R a d i c a l s 12 Chapter Three THE METHYLENE RADICAL IIL 3 - 1 I n t r o d u c t i o n 1I4. 3 - 2 E l e c t r o n S p i n Resonance of T r i p l e t S t a t e s , with p a r t i c u l a r c o n s i d e r a t i o n to 3 ^ methylene i n a s t a t e 17 3-3 Experimental Methods 2lx. A. Ketene 21L B. Diazomethane 2$ 3—i|- R e s u l t s and D i s c u s s i o n 28 Chapter Four POPULATIONS OF ROTATIONAL STATES OF THE METHYL RADICAL 3k-I 4.-I I n t r o d u c t i o n 3k-( i v ) rage I4.-2 Experimental Methods 37 If.-3 R e s u l t s and D i s c u s s i o n 38 Chapter F i v e THE NF 2 RADICAL I4JL 5 -1 I n t r o d u c t i o n 1|1 5 - 2 Experimental Methods f o r Trapping the N F 2 R a d i c a l I4.2 5 - 3 R e s u l t s l[3 (a) S p e c t r a a t i t . 2 0 K ij.3 (b) Warmup S p e c t r a I4.6 (c) NO2 Trapped i n Krypton I4.7 5 - l j . D i s c u s s i o n lj.8 5 - 5 A Gas Phase Study of the NF2 R a d i c a l 51+ Chapter S i x THE TRIFLUOROMETHYL RADICAL 58 6 - 1 I n t r o d u c t i o n 58 6 - 2 Experimental Methods 58 6 - 3 R e s u l t s and D i s c u s s i o n 59 Chapter Seven CONCLUSIONS 6 l B i b l i o g r a p h y 62 (v) LIST OF TABLES Page Table I E i g e n v a l u e s and E i g e n f u n c t i o n s of a T r i p l e t S t a t e of a L i n e a r Molecule, when a Magnetic F i e l d i s O r i e n t e d P e r p e n d i c u l a r to the Symmetry Axis Table I I The R e s u l t s of the Experiments I n v o l v i n g the Search f o r Methylene Table I I I Term Values of the Lowest States of F r e e l y R o t a t i n g Methyl R a d i c a l s Table IV Numerical Data from the ESR Spectra of NF 2 R a d i c a l s Trapped i n Various M a t r i c e s at Table V R e s u l t s of High Temperature Experiments I n v o l v i n g the NF 2 R a d i c a l i n the Gas Phase 22 29 35 56 ( v i ) LIST OF FIGURES To f o l l o w pa F i g u r e 1 S p l i t t i n g of E l e c t r o n i c Energy L e v e l s by a Magnetic F i e l d and an I s o t r o p i c Proton H y p e r f i n e I n t e r a c t i o n 7 F i g u r e 2 ESR Spectrometer D e t e c t i o n System 1 0 F i g u r e 3 ESR Spectrometer Automatic Frequency C o n t r o l System 1 0 F i g u r e ij. L i q u i d Helium Dewar 1 0 F i g u r e 5 Energy L e v e l Diagram f o r the Sp i n H a m i l t o n i a n of E q u a t i o n ( 2 8 ) f o r a T r i p l e t State with a Zero F i e l d S p l i t t i n g of 3 3 Kmc./sec.. 2 1 F i g u r e 6 Vacuum System f o r CH2N2 P r e p a r a t i o n (Method of de Boer and Backer) 2 7 F i g u r e 7(a) Vacuum System f o r CH2N2 P r e p a r a t i o n (Method of B e l l ) (b) Y-shaped tube 2 7 F i g u r e 8 A T y p i c a l ESR Spectrum from the P h o t o l y s i s of CH 2N 2 i n Argon at LL..2°K 3 0 F i g u r e 9 ESR Spectrum from the P h o t o l y s i s of CH2N2 i n the Presence of D2O 3 0 F i g u r e 1 0 I n t e n s i t i e s of the H y p e r f i n e L i n e s of the ESR Spectrum of the Methyl R a d i c a l i n i t s Lowest R o t a t i o n a l Energy States 3 8 ( v i i ) F i g u r e 11 F i g u r e 12 F i g u r e 13 F i g u r e ill. F i g u r e 15 F i g u r e l 6 F i g u r e 17 F i g u r e 18 F i g u r e 19 To f o l l o w page The Two F i g h F i e l d . L i n e s of the ESR Spectrum of the Methyl R a d i c a l 38 ESR Spectrum o f NF2 i n Argon (M/R=300) at L|_.2°K I4J4. ESR Spectrum of N F 2 i n Argon (M/R-1200) at J.|_. 2°K I4J4. ESR Spectrum of NF 2 i n Krypton (M/R=300) a t lu. 2°K i|J+ ESR S p e c t r a of N F 2 during Warmup l±l\. ESR S p e c t r a of N F 2 i n GCT.^ (M/R=1200) i|4 ESR S p e c t r a of N0 2 i n Krypton 1|4 ESR Spectrum Obtained on I r r a d i a t i o n of CF3I i n Krypton (M/R=100) at J+.2°K 59 ESR Spectra Obtained on I r r a d i a t i o n of CF3I i n CCIlj. (M/R=6o-75) 59 1. CHAPTER ONE. GENERAL INTRODUCTION  1-1 Free R a d i c a l s a t Low Temperatures. Study o f any chemical e n t i t y r e q u i r e s that there be p r e s e n t a s u f f i c i e n t q u a n t i t y of that substance to give an observable s i g n a l on i n t e r a c t i o n w i t h an a p p r o p r i a t e measuring apparatus. For the case of a r e a c t i v e s p e c i e s , of which most f r e e r a d i c a l s are e x c e l l e n t examples, o b t a i n i n g such a q u a n t i t y i s o f t e n a major problem. I t i s u s u a l l y s o l v e d e i t h e r by produc i n g the species i n such l a r g e numbers t h a t they may be s t u d i e d before r e a c t i n g (as i n f l a s h p h o t o l y s i s ) , or by p r e v e n t i n g them from c o n t a c t i n g anything w i t h which they may r e a c t (as i n m a t r i x i s o l a t i o n ) . In the experiments d i s c u s s e d i n t h i s t h e s i s the l a t t e r method, ma t r i x i s o l a t i o n , i s used f o r study o f small f r e e r a d i c a l s . The r a d i c a l s are prevented from r e a c t i n g by t r a p p i n g them i n a ma t r i x o f s o l i d i n e r t gas or another i n e r t m a t e r i a l . R i g i d i t y of the mat r i x i s necessary f o r e f f e c t i v e t r a p p i n g of the r a d i c a l s , s i n c e a n o n - r i g i d m a t r i x allows them to d i f f u s e and hence recombine. R i g i d i t y i s achieved by r e d u c i n g thermal motion o f the atoms or molecules of the m a t r i x m a t e r i a l almost to zero with some c o o l a n t such as l i q u i d n i t r o g e n (b.p.77°K) or l i q u i d h elium (b.p.l4-.2°K); the r e f r i g e r a n t used w i l l depend on the nature o f the m a t r i x . S t u d i e s by Pimentel and others (1) have shown t h a t f o r e f f e c t i v e t r a p p i n g the temperature must be below about f o r t y p e r c e n t of the m e l t i n g p o i n t of the matrix, and as l i q u i d h e lium achieves t h i s purpose adequately f o r the matrices used predominantly here ( s o l i d argon and krypton) i t has been 2 used as the r e f r i g e r a n t . Of the v a r i o u s methods used f o r s t u d y i n g trapped f r e e r a d i c a l s , e l e c t r o n s p i n resonance (ESR) i s one of the most u s e f u l . In the f i r s t p l a c e , s i n c e the presence of an u n p a i r e d e l e c t r o n i s necessary f o r ESR d e t e c t i o n a trapped substance can be i d e n t i f i e d as a r a d i c a l r a t h e r than a molecule with a c l o s e d s h e l l . The nature of any c o u p l i n g of the u n p a i r e d s p i n w i t h other angular momenta ( o r b i t a l momentum, n u c l e a r s p i n s , r o t a t i o n a l motion) r e v e a l s much about the nature of the r a d i c a l 12 and i t s environment. A l s o , ESR can d e t e c t as few as 10 r a d i c a l s , an advantage of immense importance i n studying s p e c i e s d i f f i c u l t to produce. The major disadvantage of ESR i s the complexity and c o s t of the apparatus, but i f t h i s can be overcome the .technique i s e x c e l l e n t . The work d e s c r i b e d i n t h i s t h e s i s i s r a t h e r s i m i l a r to that c a r r i e d out by the ESR group at the Johns Hopkins U n i v e r s i t y • ( 2 ) - ( 1 2 ) . They o b t a i n f r e e r a d i c a l s by d e p o s i t i n g a m i x t u r e o f m a t r i x gas and decomposition products of a microwave discharge , o or u l t r a v i o l e t i r r a d i a t i o n on a sapphire needle at ix.,2. K, or by i r r a d i a t i n g a s o l i d d e p o s i t of reagent and matrix with u l t r a v i o l e t l i g h t . These v/orkers have s t u d i e d a number of s p e c i e s , I n c l u d i n g hydrogen (2) (6) (7) (8) and n i t r o g e n (If.) (5) atoms, amino (3) and a l k y l (9) r a d i c a l s as w e l l as HGO ( 1 0 ) , CN (12) and N 0 2 ( 1 1 ) , and have done much work i n t e r p r e t i n g t h e i r p r o p e r t i e s when trapped. The work d e s c r i b e d i n t h i s t h e s i s extends the s t u d i e s to v a r i o u s other r a d i c a l s , whose ESR s p e c t r a are i n t e r p r e t e d J.n a s i m i l a r f a s h i o n . 1 - 2 B a s i c P r i n c i p l e s of E l e c t r o n S p i n Resonance E l e c t r o n s p i n resonance s t u d i e s c o n s i s t e s s e n t i a l l y of p l a c i n g f r e e r a d i c a l s i n a magnetic f i e l d and i n v e s t i g a t i n g t r a n s i t i o n s between Zeeman l e v e l s of any non compensated e l e c t r o n angular momenta. The Ha m i l t o n i a n f o r such l e v e l s i s : (1) where ^ = e l e c t r o n i c g - f a c t o r ; |2> = Bohr magneton; H = magnetic f i e Id; J~-= [_•+• S - t o t a l angular momentum operator of the e l e c t r o n s ; = o r b i t a l angular momentum operator; $> - s p i n angular momentum op e r a t o r . In most f r e e r a d i c a l s L = O so t h a t ( i s w r i t t e n : In t h i s case ^ 2: 2 • In a st r o n g magnetic f i e l d , which i s u s u a l l y the case i n an ESR experiment, S i s qua n t i z e d along H , so tha t i f i s t r e a t e d as a f i r s t order p e r t u r b a t i o n of the e l e c t r o n i c p o t e n t i a l and k i n e t i c e n e r g i e s i t s energy l e v e l s a re: W, * Ms ^fHo (3) where \\s- p r o j e c t i o n o f the e l e c t r o n s p i n i n the d i r e c t i o n o f H J Ho - t n e magnitude of the magnetic f i e l d . If S , the quantum number of the t o t a l s p i n , i s equation (3) becomes: If. The magnetic dipole selection rule AJ^s+| allows transitions between the leve l s at energies of: A -v ~- 3pH 0 (5) where/t\ = Planck 1 s constant; T) = frequency. In actual fact these equations are applicable only when the radicals are i n f a i r l y rapid i s o t r o p i c motion, for the g-factor has d i r e c t i o n a l properties, and should be written as a tensor. Thus, equation (2) should read: ft, =/3H-G-S (6) where G i s the g-factor tensor. S i s nevertheless quantized i n the f i e l d d i r e c t i o n , so that equation (5) should be: An?= | 3 H 0 ( ? H - G - t H ) ( 7 ) where ^ H i s a unit vector i n the f i e l d d i r e c t i o n . However, when the rad i c a l s are i n i s o t r o p i c motion the g-factor term i s constant for a l l sample orientations and equation (5) i s applicable. The following additional Hamiltonian, due to magnetic inter-actions of the nuclei i n the r a d i c a l , i s superimposed on $&t: ftr-DsVj (8) j where Ij i s the nuclear spin operator of nucleus j . The f i r s t term, due to the spin-spin i n t e r a c t i o n of the unpaired electron and nucleus j , i s cal l e d the hyperfine (hf) int e r a c t i o n , and ftj i s c a l l e d the hf int e r a c t i o n tensor. The second term i s the 5 . nuclear Zeeman energy which (as i n this case) i s often small enough to be neglected. Y>j Ij i s the nuclear moment of nucleus j , The hyperfine i n t e r a c t i o n i s written as a tensor because, l i k e the g-factor, i t has d i r e c t i o n a l properties. Nevertheless CLj may be subdivided into two parts: Qj = AjU + I j (9) where LI i s the unit dyadic. Equation (8) may be rewritten: (AjS-UL - I j + S-Ij-Ij) Aj and Bj are given by the following: ( 1 0 ) 3^ : i l l ..s ( 1 1 ) ( 1 2 ) J J where >Tj i s the length of Jr^ , the vector between nucleus j . and the electron; 5 (-**j)-Dirac delta function. I t w i l l be noted that Ajhas no d i r e c t i o n a l properties and i s thus c a l l e d the i s o t r o p i c hf constant. Also requires that Aj=Ounless there i s a f i n i t e p r o b a b i l i t y of finding the electron at nucleus j , i . e . unless the o r b i t a l containing the unpaired electron has some s-character at nucleus j . > which i s traceless, does have d i r e c t i o n a l character and i s c a l l e d the anisotropic hf in t e r a c t i o n tensor. Let us assume that the g-factor i s i s o t r o p i c and work i n a molecule - fi x e d coordinate system whose axes xyz are coincident with the p r i n c i p a l axes of &j . The hyperfine interaction super-imposes an extra s p l i t t i n g on the electron Zeeman s p l i t t i n g , making the energy l e v e l s ( 1 3 ) : w - w , + * B -[3 ^ C ^ f ^ M ^ X - 5in3lj2 where ( i ^ i l j i s the projection of Ij on the magnetic f i e l d ; are spherical polar coordinates of the z-axis with respect to the external f i e l d ; X represents the orientation of the molecule ab out the z—axis; i f Bjx? Bjy * ^ j^* the three p r i n c i p a l values of Qj , then: Bp* Bjy --i(B> + Bja) Bj i s ca l l e d the anisotropic hf s p l i t t i n g constant. Note that for a x i a l symmetry Bj* " Bjy a n <i a n d equation (13) becomes: 2 (15) In the frequently found case of lAj|^|Bjl , t h i s becomes: W= Ms3pH. *I n,H)j{Aj+Dj(3«-*e-0j at) For spherical symmetry (including the case when the rad i c a l s are •D. r o t a t i n g at a frequency Q^zl ) (1^ 4-) > tb-G anisotropic part 7 averages to zero, making the energy l e v e l s : w = M s 3 p H 0 ^ A j M s ( r g . ( 1 7 ) Transitions occur according to the selection rules A =+1, ^ ( H l ^ O a t energies of: (18) J These are shown schematically i n Figure 1 for the case of 5= (the quantum number of the t o t a l spin of nucleus j) = -|. The equations show that the s h i f t due to B jvaries with the orientation of the r a d i c a l i n the magnetic f i e l d . I f one i s studying a sample containing randomly oriented radicals the main effect of Bj i s often to broaden the l i n e s due to the i s o t r o p i c hf i n t e r a c t i o n , sometimes changing th e i r shape (l6)(10), and other times making them undetectable. The ESR transitions observed i n these experiments involve the absorption of microwave power. The power absorbed i s proportional to the population difference between the lower (|V/\s = --^and upper (j^ ^^-j l e v e l s . Such a difference i s usually found, since very often the spin system i s i n thermal equilibrium with i t s surroundings, and the populations of the l e v e l s follow a -JBoltzmann d i s t r i b u t i o n . This d i s t r i b u t i o n i s maintained, even when microwave power i s absorbed, by a mechanism (called spin-l a t t i c e relaxation) which depopulates the excited l e v e l f a s t enough to r e t a i n thermal equilibrium. Sometimes, however, the s p i n - l a t t i c e relaxation rate cannot keep up with the power absorption, so that the population difference, and hence the To F o l l o w Page 7 M =i , ^7K s 2 "A" -( Ms=5 :M=4 7fT gHH + A o 2 gHH A o 2 1 1 ^ " 2 ; M r 2 H=0 H=H H=H o o no hyper f ine hyper f ine splitting splitt ing produced by one proton F i g u r e ! SPLITTING OF ELECTRONIC ENERGY LEVELS BY A MAGNETIC FIELD A N D A N ISOTROPIC PROTON HYPERFINE INTERACTION. 8. power - absorbed, decreases. A watch has to be kept for this phenomenon, c a l l e d power saturation, especially at low temperatures, for i t may wreak havoc with ESR measurements, broadening the absorption l i n e s out of proportion. 9-CHAPTER TWO  GENERAL EXPERIMENTAL METHODS •2-1 D e s c r i p t i o n of the E l e c t r o n Spin Resonance Spectrometer The X-band spectrometer, which was designed i n t h i s l a b o r a t o r y , uses a V a r i a n V -153 k l y s t r o n as a microwave source. Energy from the k l y s t r o n e n t e r s a magic tee br i d g e connected on one s i d e to a microwave c a v i t y r e s o n a t i n g i n the ^ EQ.^ mode and on the other to a r e s i s t i v e l o a d ; a s i l i c o n diode d e t e c t o r i s connected to the t h i r d arm. With the d e t e c t o r as an i n d i c a t o r the k l y s t r o n c a v i t y s i z e and r ' e f l e c t o r v o l t a g e are a d j u s t e d u n t i l power absorbed i n the c a v i t y i s balanced by that absorbed i n the r e s i s t i v e l o a d . F i n a l l y , by s e t t i n g up a s l i g h t inbalance with an a t t e n u a t o r i n the r e s i s t i v e l o a d arm a b i a s c u r r e n t of ca.lOO microamperes i s passed through the d e t e c t o r to i n c r e a s e the s i g n a l - t o - n o i s e r a t i o . The k l y s t r o n frequency i s l o c k e d to that of the c a v i t y by an automatic frequency c o n t r o l (AFC) with a c a r r i e r channel of 10 k c . / s e c . F u r t h e r a t t e n u a t o r s i n the waveguide system, one between the k l y s t r o n and the magic tee, and the other between the magic tee and the c a v i t y , a l l o w the microwave power to the c a v i t y t o be reduced to one m i l l i w a t t and below. In the microwatt range the AFC works only with d i f f i c u l t y , so the r e s i s t i v e l o a d can be r e p l a c e d by another c a v i t y towhich the AFC can be connected at a h i g h power l e v e l w hile the c a v i t y c o n t a i n i n g the sample i s operated at a low power l e v e l . The" microwave frequency i s measured with a Hewlett Packard $2\\/^>2$/S\0 frequency counter, and the microwave power i s monitored with a t h e r m i s t o r and a Hewlett Packard I4.3O power meter. 10 The magnetic f i e l d i s produced by a Varian six-inch electro-magnet with a 2 . 5 inch pole gap, and f i t t e d with r i n g shim pole caps. I t i s measured with a proton resonance magnetometer. The l a t t e r consists of a 3 mm. probe c o i l containing gl y c e r o l , which i s inserted into the magnet gap beside the cavity, and i s connected to a marginal o s c i l l a t o r which i s frequency modulated at 20 cycles/sec. The proton resonance signal i s displayed on an oscilloscope and a signal generator loosely coupled to the o s c i l l a t o r i s tuned to zero beat; the frequency of the generator i s then measured with the counter. The magnetic f i e l d i s modulated at 1}.00 cycles/sec. or 20 cycles/sec. with a pair of a u x i l i a r y c o i l s mounted round the cavity and signals are recorded as f i r s t derivatives of magnetic s u s c e p t i b i l i t y vs. magnetic f i e l d . The very high signal-to-noise r a t i o i n the spectrometer i s due to the use of narrow band amplification techniques. Block diagrams of the detection and AFC systems are i n Figures 2 and 3 . 2 - 2 The Liquid Helium Dewar System The l i q u i d helium dewar i s based on the design of Duerig and Mador (l6), and i s shown i n cross section i n Figure II.. The centre container holds the l i q u i d helium and i s shielded by - 7 - 8 l i q u i d nitrogen; the vacuum envelope i s pumped to 10 - 1 0 mm.Hg with an o i l d i f f u s i o n pump and a rotary o i l pump, and the pressure i n the system i s measured with an NRC type 507 i o n i z a t i o n gauge. The microwave cavity i s attached d i r e c t l y to the l i q u i d helium container, and i n i t s centre Is an 0 . 1 5 inch sapphire rod on which gaseous samples are condensed. In order to minimize .Co F o l l o w Pacre 10 Klystron Br idge S y s t e m Diode Cav i ty s Audio Preampl i f ier S igna l Ampl i f ier B a n d P a s s F i l t e r Synch rove r t e r ^ Low P a s s Fi l ter R e c o r d e r Modulat ion Coils 7K 400 cycles/sec. Genera to r F igure 2. E S R S p e c t r o m e t e r DETECTION S Y S T E M . F o l l o v "ape 10 Klystron - j -Br idge System Diode Cavity Audio Preamplif ier i 10 kc./sec. Ampl i f ier Reflector Klystron Power Supply Phase Detector 10 kc./sec. Generator F igure 3. E S R Spectrometer AUTOMATIC FREQUENCY CONTROL SYSTEM. To F o l l o w Page 10 Liqu Liquid H Stainless Steel Waveguide Target Radiation Shield -Modulation Coils Mounted Inside Capillary Figure 4 Liquid Helium Dewar. 1 1 . e v a p o r a t i o n of l i q u i d helium due to e n t r y of heat from e x t e r n a l sources the c a v i t y i s connected to the e x t e r n a l c i r c u i t r y by a l e n g t h of Type 30L\ s t a i n l e s s s t e e l waveguide having a w a l l t h i c k n e s s of 0 . 0 1 i n c h . In the end w a l l of the c a v i t y have been m i l l e d h o r i z o n t a l s l o t s to a l l o w i r r a d i a t i o n of the sample i n s i t u ; the s l o t s are 0 . 3 mm. wide and allow %Q% of the i n c i d e n t r a d i a t i o n to enter the c a v i t y . A h o l e i n the bottom of the c a v i t y -permits i n s e r t i o n of a l / l 6 i n . o u t s i d e diameter, 0 . 0 0 5 i n . w a l l t h i c k n e s s s t a i n l e s s s t e e l c a p i l l a r y through which the gaseous samples e n t e r . The f o l l o w i n g procedure was used to f i l l the dewar with l i q u i d helium. At l e a s t one, and p r e f e r a b l y three or more hours before the h e l i u m t r a n s f e r the r a d i a t i o n s h i e l d was f i l l e d w i th l i q u i d n i t r o g e n . Then the l i q u i d h e l i u m was siphoned from a storage dewar through a 3 / l 6 i n . o u t s i d e diameter, 0 . 0 1 0 i n . w a l l t h i c k n e s s s t a i n l e s s s t e e l t r a n s f e r tube en c l o s e d i n a vacuum j a c k e t . The p r e s s u r e on the l i q u i d helium i n the storage dewar was about \ pound/square i n c h above atmospheric d u r i n g a t r a n s f e r o p e r a t i o n . Helium was l i q u e f i e d i n an adjacent C o l l i n s c r y o s t a t and gas evolved from the dewar was r e c o v e r e d f o r f u r t h e r use. Only by t r i a l and e r r o r , and then from experience, c o u l d the o p e r a t o r decide when the dewar was f u l l ; i n d i c a t i o n s of the dewar's being empty were such t h i n g s as c e s s a t i o n of motion of the gas h o l d e r i n the h e l i u m r e c o v e r y system and an i n c r e a s e i n the p r e s s u r e i n the vacuum system. I t i s hoped t h a t a depth i n d i c a t o r u s i n g a superconducting tantalum wire, which i s now 12. i n the process o f being c o n s t r u c t e d , w i l l e l i m i n a t e the guesswork i n h a n d l i n g the l q u i d helium. 2-3 P r o d u c t i o n of Trapped R a d i c a l s Vacuum techniques were used i n h a n d l i n g samples. The f o l l o w i n g procedure was used i n p r e p a r i n g gas mixtures. An evacuated, blackened three l i t r e bulb was f i l l e d to an a p p r o p r i a t e p r e s s u r e ( u s u a l l y 1-5 mm. Hg) wit h a reagent gas (R) from which a l l a i r had been p r e v i o u s l y removed. Enough m a t r i x gas (M) was then added to make up the d e s i r e d mole r a t i o (Ivl/R). I f p o s s i b l e the sample was l e f t to stand f o r a few days to allow mixing of the reagent and m a t r i x . In many cases, however, t h i s c o u l d not be done, and mixing was o f t e n promoted by warming the bottom of the b u l b . Two lamps were used t o i r r a d i a t e the samples. These were a General E l e c t r i c A-H6 1000 watt mercury a r c lamp and a General E l e c t r i c H85A3/UV 85 watt mercury arc lamp; both were water c o o l e d . R a d i a t i o n from the lamp was f o c u s s e d on the sapphire ro d by a p a i r of quartz l e n s e s s e t i n a brass h o l d e r . The h o l d e r was c o n s t r u c t e d i n such a way t h a t the space between the lenses c o u l d be f i l l e d w i t h water as a f i l t e r f o r any i n f r a r e d r a d i a t i o n from the lamp. P i l l i n g w i t h water was necessary when u s i n g the A-H6 lamp as otherwise the heat caused the l i q u i d helium to b o i l o f f r a t h e r r a p i d l y . Use of the water f i l t e r was not necessary f o r the H 85A3/UV lamp. Ph o t o l y s e s were c a r r i e d out on the s o l i d d e p o s i t s u s i n g the f u l l arc of the lamp; attempts to i n c r e a s e the e f f i c i e n c y of r a d i c a l p r o d u c t i o n by i r r a d i a t i n g d u r i n g d e p o s i t i o n had l i t t l e e f f e c t . 1 3 . The course of a c t i o n i n an ESR experiment was as f o l l o w s . A f t e r the a p p r o p r i a t e lamp had been fo c u s s e d on the sapphire r o d and the dewar had been f i l l e d w i t h l i q u i d helium, the sample was passed from the bulb, through a f l o w r e g u l a t o r at a s e t t i n g found by t r i a l - a n d - e r r o r to g i v e the most e f f i c i e n t r a t e of d e p o s i t i o n , and thence onto the coo l e d sapphire r od v i a the s t a i n l e s s s t e e l c a p i l l a r y . The sample was added e i t h e r u n t i l the d e p o s i t c o u l d be seen (by l o o k i n g through the waveguide) to be l-2mm. t h i c k (as i n the case of experiments r e q u i r i n g p h o t o l y s i s ) or u n t i l a f a i r l y l a r g e s i g n a l was found (as i n the experiments i n v o l v i n g NO^ and NF^). When d e p o s i t i o n was complete the lamp and microwave system were turned on, the frequency was measured, the microwave power was s e t a t an a p p r o p r i a t e l e v e l and the d e s i r e d magnetic f i e l d r e g i o n was scanned i n search of an ESR s i g n a l . The above was the ge n e r a l technique used i n st u d y i n g o s o l i d samples at ij..2 K. S p e c i a l treatments were o f t e n necessary, depending on the sample and experiment, and are d e s c r i b e d i n the s e c t i o n s d i s c u s s i n g the i n d i v i d u a l r a d i c a l s . CHAPTER THREE  THE METHYLENE RADICAL 3-1 I n t r o d u c t i o n Throughout the p a s t decade much i n t e r e s t has been ce n t r e d about the methylene r a d i c a l (CH^). In the main the cause of such i n t e r e s t has been i t s two non-bonded e l e c t r o n s , and much work has been done to t r y to determine what happens to them i n v a r i o u s c o n d i t i o n s . C h i e f l y , however, most c o n s i d e r a t i o n has been gi v e n to the ground s t a t e and the d e c i s i o n as to whether i t i s a s i n g l e t or a t r i p l e t . T h e o r e t i c a l p r e d i c t i o n s o f the nature of v a r i o u s s t a t e s of methylene have been as d i v e r s e as the number of papers i n v o l v e d , and have depended on the assumptions and treatments used and on the d e t a i l of the c a l c u l a t i o n . For example, P o s t e r and B oys (17) used a v a r i a t i o n method employing c o n f i g u r a t i o n 3 i n t e r a c t i o n and p r e d i c t e d the ground s t a t e to be B^, w i t h an o H-C-H bond angle o f 129 . On the oth e r hand Lennard-Jones ( 1 8 ) , c o n t i n u i n g some work of M u l l i k e n (19)* drew some c o r r e l a t i o n diagrams between i s o e l e c t r o n i c s p e c i e s u s i n g molecular o r b i t a l theory and decided t h a t the ground s t a t e of methylene should be "^A^ . Walsh (20) went f u r t h e r , drawing c o r r e l a t i o n diagrams f o r v a r i o u s species at d i f f e r e n t c o n f i g u r a t i o n s , and although he 1 d e c i d e d that the ground s t a t e o f methylene should be A^ he p o i n t e d out t h a t i f the molecule were l i n e a r i t would be . Jordan and Longuet-Higgins ( 2 1 ) , u s i n g a m o d i f i e d valence bond approach, and Pedley ( 2 2 ) , from heat of formation c o n s i d e r a t i o n s , have r e c e n t l y concluded t h a t the ground s t a t e should be the 1 5 . l i n e a r t r i p l e t . Prom these examples the d i v e r s i t y of the p r e d i c t i o n s i s apparent. Experimental f i n d i n g s seem more and more to favour the t r i p l e t as the ground s t a t e , although the evidence i s i n d i r e c t . In the p h o t o l y s i s of diazomethane i n the presence of c i s - 2 - b u t e n e Woodworth e t a l . (23) found s t e r e o s p e c i f i c a d d i t i o n of methylene to the o l e f i n to form c i s - 1 , 2 - d i m e t h y l c y c l o p r o p a n e . Such s t e r e o s p e c i f i c i t y would be expected, they p o i n t out, i f the methylene were i n a s i n g l e t s t a t e because c o n s e r v a t i o n of s p i n would f a c i l i t a t e d i r e c t a d d i t i o n to the double bond. A d d i t i o n of t r i p l e t methylene would have a d i r a d i c a l as an intermediate which would have a s u f f i c i e n t l i f e t i m e to permit l o s s of the s t e r e o s p e c i f i c i t y as w e l l as f o r m a t i o n of 3-methyl-l-butene. Anet e t a l . (2l\.) and Prey (25) d i d i n f a c t f i n d the l a t t e r phenomenon when the experiment was done i n h i g h p r e s s u r e s of i n e r t gas and concluded t h a t methylene, o r i g i n a l l y formed i n the s i n g l e t s t a t e , had been degraded to a t r i p l e t ground s t a t e . Attempts to t r a p methylene and perform u l t r a v i o l e t and i n f r a r e d s t u d i e s on i t have always y i e l d e d complicated s p e c t r a . Pimentel e t a l . ( 2 6 ) ( 2 7 ) ( 2 8 ) have made numerous i n f r a r e d s t u d i e s on the trapped p h o t o l y s i s products of diazomethane, and have based arguments concerning trapped methylene on the presence of a b s o r p t i o n s whose disappearance on warmup c o i n c i d e d with the f o r m a t i o n of e t h y l e n e . The most r e c e n t papers on such i n v e s t i g a t -i o n s , by G o l d f a r b and Pimentel (28) and by Robinson and McCarty ( 2 9 ) , ' p r e s e n t complex and o f t e n c o n f l i c t i n g r e s u l t s In i n f r a r e d and u l t r a v i o l e t s t u d i e s of the phenomenon.. The presence of 1 6 . tautomers of diazomethane and e x t r a p h o t o l y s i s products such as CH was p r o b a b l y the cause of the c o m p l i c a t i o n . Robinson and o McCarty found some abs o r p t i o n s near 3200A which they thought might have had as a lower s t a t e L$ methylene, but i n view of the evidence of Herzberg (30) t h i s assignment appears d o u b t f u l . In a l l , p r e v i o u s work on trapped methylene has been h i g h l y i n c o n c l u s i v e , f o r s l i g h t changes i n experimental c o n d i t i o n s seem to have f a r r e a c h i n g e f f e c t s . In gas phase s p e c t r o s c o p i c s t u d i e s of the f l a s h p h o t o l y s i s of diazomethane Herzberg e t a l . ( 3 0 ) ( 3 D have found the most sugg e s t i v e evidence f o r t r i p l e t methylene. The experiments, done i n a h i g h p r e s s u r e of i n e r t gas, r e v e a l e d f e a t u r e s i n the vacuum u l t r a v i o l e t which c o u l d be due to l i n e a r methylene . o (C-H bond d i s t a n c e 1.03A). He a l s o found f e a t u r e s i n the v i s i b l e whose lower s t a t e was bent ("'"A^ ), but because a g r e a t e r p ressure of i n e r t gas was r e q u i r e d to produce the f e a t u r e s i n the u l t r a v i o l e t he suggested t h a t the lower s t a t e of t h i s t r a n s i t i o n ( Ij ) was the ground s t a t e . In the experiments d e s c r i b e d here an attempt was made to i n v e s t i g a t e the p h o t o l y s i s products of ketene (CH^CO) and diazomethane (CH^N ) trapped at lx..2°K,in s o l i d argon and krypton. Ketene was t r i e d f i r s t because of the dangerous p r o p e r t i e s of diazomethane even though e a r l i e r s t u d i e s by Pimentel (32) of the p h o t o l y s i s i n s i t u had r e v e a l e d no decomposition, presumably because CH^ and CO had recombined i n the m a t r i x cage. Diazomethane has the advantage t h a t CH^ does not r e a c t with the other photo-l y s i s product, N p , and thus the cage has l i t t l e e f f e c t , once 1 7 . fragmentation has oc c u r r e d . I t was hoped that a t r i p l e t s t a t e f o r trapped methylene c o u l d be found by ESR. In view of the low temperature to be used and the number of c o l l i s i o n s t h a t methylene co u l d undergo i n the s o l i d i t i s l i k e l y t h a t any t r i p l e t s t a t e found would be the ground s t a t e . The experiments are d e s c r i b e d i n the f o l l o w i n g s e c t i o n s . An experiment i n v o l v i n g the p h o t o l y s i s of CH^l^ i n the presence of D^O i s a l s o d i s c u s s e d . 3 - 2 E l e c t r o n S p i n Resonance of T r i p l e t S t a t e s , with p a r t i c u l a r c o n s i d e r a t i o n to methylene i n a £~ s t a t e . J A t r i p l e t s t a t e possesses two coupled u n p a i r e d e l e c t r o n s ( i . e . S - l ), and consequently should be d e t e c t a b l e by ESR. The i n t e r a c t i o n s d e s c r i b e d i n equations (6) and (8) s t i l l e x i s t , but there i s a f u r t h e r one r e s u l t i n g from the s p i n - s p i n i n t e r -a c t i o n of the e l e c t r o n s , which i s o f t e n of very g r e a t magnitude and complicates matters c o n s i d e r a b l y . Data on ESR of t r i p l e t s t a t e s are r a t h e r sparse, although some do e x i s t (33) - ( 3 7 ) . F o r two u n p a i r e d e l e c t r o n s i n a t r i p l e t s t a t e c o n t r i b u t i o n from s p i n - s p i n i n t e r a c t i o n can be adequately d e s c r i b e d by the f o l l o w i n g H a miltonian: _ — \ Hss --a? {-^zi — s — } d 9 ) where S, , S a , are s p i n o p e r a t o r s of the i n d i v i d u a l e l e c t r o n s and -f i s the i n t e r e l e c t r o n i c v e c t o r of l e n g t h >T . (Because the s p i n p a r t of the t r i p l e t wave f u n c t i o n i s symmetric the s p a t i a l p a r t must be antisymmetric, so that always "f^Oand the contact term i n ^ C s s v a n i s h e s ) . I f now e q u a t i o n (19) i s expanded i n t o 1 8 . components i n C a r t e s i a n c o o r d i n a t e s ^ g J b e c o m e s , a f t e r some a l g e b r a i c m a n i p u l a t i o n s : 3 3 2 K $ +s c V 3 - <; -S S V 2 o ) + + 2 (3 S,^  S2y S,'S2)] L e t us now co n s i d e r CH i n the a x i a l l y symmetric l i n e a r T s t a t e . . 2 3 In t h i s s t a t e the s p i n wave f u n c t i o n i s a t r i p l e t , the o r b i t a l angular momentum about the mol e c u l a r (z) a x i s i s zero, and the s p a t i a l wave f u n c t i o n i s symmetric to i n v e r s i o n through the centre of symmetry but antisymmetric to r e f l e c t i o n i n any plane r u n n i n g through the z - a x i s . In t h i s case any i n t e g r a t i o n s of over the s p a t i a l p a r t o f the wave f u n c t i o n cause the f i r s t f o u r terms to v a n i s h l e a v i n g only One can take as zero order s p i n f u n c t i o n s of the t r i p l e t s t a t e : |M, = -I> = ft(3, where o( andp mean spins p a r a l l e l and a n t i p a r a l l e l to the molecular a x i s and s u b s c r i p t s 1 and 2 mean e l e c t r o n s 1 and 2. The s t a t e s j e q . (22)] are e i g e n f u n c t i o n s of $ Z 5 5 being the t o t a l s p i n o p e r a t o r . Now S " Sj+ S^and i t s z-component S^=S,^,+ 1 9 . whence: S S = S 2 • S * < +2S.-S. A l s o , so t h a t : s,JK> • fK> s*lM«> =!ins) S,V|Ms> = i I ns > 3 ? a - OS'-S*) I ( 2 6 ) (23) (21+) (25) One can r e p r e s e n t the s p a t i a l p a r t of the zero-order wave f u n c t i o n by 4 (^^ *1 )^, where >T( are the s p a t i a l c o o r d i n a t e s of the two e l e c t r o n s , so that the o v e r a l l zero-order wave f u n c t i o n i s : f - T ( ^ ) l M s ) (27) The o v e r a l l Hamiltonian, i g n o r i n g h y p e r f i n e and n u c l e a r Zeeman terms, which are small compared to the e l e c t r o n Zeeman term and ^ t ^ g , becomes: 4 r 5 ( 3 S j - S ! ) (28) B ecause of the a x i a l symmetry one can consid e r components of M p e r p e n d i c u l a r to the mole c u l a r a x i s as being along the x - a x i s . 2 0 . In t h i s case the mat r i x f o r the Zeeman term i s o 0 H IB F o r ^CSjit i s where T i s the integ: ;ral (^(T,,^ 4 ^ (29) (30) ( 3 D The o v e r a l l H a m i l t o n i a n i s then: ft-I 3 P H-/JT -2T At zero f i e l d , i . e . when H^H^O, equation (31) becomes: % -Ao -2T o) (32) \ 0 O T / The s t a t e s 1') ) are thus e i g e n s t a t e s of l b under these c o n d i t i o n s , the f i r s t h a v i n g energy"2Tand the l a t t e r two being degenerate a t an e n e r g y T . The s e p a r a t i o n , 3 T , c a l l e d the z e r o - f i e l d s p l i t t i n g , i s the cause of much of the d i f f i c u l t y i n d e t e c t i n g t r i p l e t s t a t e s by ESR. I t depends s o l e l y on the s p a t i a l p a r t of the wave f u n c t i o n and can be very l a r g e . F or 21. oxygen molecules (35)(38) I t has been found to be 92,200 mc./sec, and Coope (39) has estimated t h a t f o r l i n e a r methylene i t i s at l e a s t 32,890 mc./sec. These energi e s are very much g r e a t e r than those of 9500 mc./sec. used i n most X-band ESR spectrometers, and may n e c e s s i t a t e use of s p e c i a l techniques. Indeed, ESR s t u d i e s of 0^  l n c l a t h r a t e s r e q u i r e d use of a s p e c i a l c a v i t y and p u l s e d magnetic f i e l d s (35). I f a magnetic f i e l d i s o r i e n t e d along the z - a x i s the Hamiltonian m a t r i x becomes IT * 3 p H t o \ 0 -2T O 0 (33) C> T-qpH 0/ l i > ^ lo> ^ are again e i g e n s t a t e s of$L, t h i s time at energies of T+-<}|SHo1-2T jT-*-gpH6. These are shown diag r a m m a t i c a l l y i n F i g u r e 5 f o r a zero f i e l d s p l i t t i n g of 33*000 mc./sec. When a r a d i o frequency magnetic f i e l d i s o r i e n t e d p e r p e n d i c u l a r to M (as i n the p r e s e n t c a s e ) , magnetic d i p o l e t r a n s i t i o n s occur a c c o r d i n g to the s e l e c t i o n r u l e AM s-±lat energies of | - g^Hol. The A M s = t Z t r a n s i t i o n i s f o r b i d d e n . I t must be remembered t h a t i n most ESR experiments the frequency of the microwave c a v i t y ( i . e . the energy t r a n s i t i o n studied) i s kept constant and the magnitude of the zero f i e l d s p l i t t i n g may be such that some t r a n s i t i o n s , though allowed, may not be observable i n a g i v e n experimental s i t u a t i o n . I f now the magnetic f i e l d i s a p p l i e d p e r p e n d i c u l a r to the molecular a x i s ( i . e . along the x - a x i s ) the Hamiltonian m a t r i x becomes: T o F o l l o w Fap-e 2 1 H( ki logauss) Figure 5. Energy Level D iagrams for the Spin Hamiltonian of Equation (28) for a Triplet S ta te w i t h a Zero Field Split t ing of 33 k m c / s e c . (3if) 22. The e i g e n v a l u e s and t h e i r c o r r e s p o n d i n g e i g e n f u n c t i o n s are g i v e n i n Ta b l e I below: TABLE I E i g e n v a l u e s and E i g e n f u n c t i o n s o f a T r i p l e t S t a t e of a L i n e a r M o l e c u l e when a Mag n e t i c F i e l d i s O r i e n t e d P e r p e n d i c u l a r t o the Symmetry A x i s . E i g e n v a l u e E i g e n f u n c t i o n T -T t J 9T* + ^q a p *H e \-Tt£ 2 Z Ym C H> - 1 0 ) The magnetic d i p o l e t r a n s i t i o n s a re now a l l o w e d between a l l t h r e e e i g n e n s t a t e s , and o c c u r a t e n e r g i e s o f -> '—' and / 9T2+ T ^ j ^ H * • O b s e r v a t i o n o f t r a n s i t i o n s i s s u b j e c t to the same e x p e r i m e n t a l r e s t r i c t i o n s as f o r the case o f H H y . A diagram of the energy l e v e l s i s g i v e n i n F i g u r e 5. I n the case o f the magnetic f i e l d b e i n g n e i t h e r p a r a l l e l n or p e r p e n d i c u l a r t o the m o l e c u l a r a x i s the e i g e n s t a t e s become f u r t h e r c o m b i n a t i o n s o f the b a s i s s t a t e s . The energy l e v e l s change w i t h the f i e l d , as w e l l as the angl e between the f i e l d and the m o l e c u l a r a x i s , i n a c o m p l i c a t e d f a s h i o n . As a r e s u l t 23. the f i e l d s a t which the t r a n s i t i o n s may occur a l s o have a complex dependence on the angle. The experiments to be d e s c r i b e d here concern attempts to trap methylene i n a p o l y c r y s t a l l i n e m a t r i x of s o l i d i n e r t gas. A number of p o s s i b l e s i t u a t i o n s may a r i s e ; the f i r s t i s t h a t the r a d i c a l s are h e l d r i g i d l y i n the m a t r i x . In t h i s case one o b t a i n s a l l o r i e n t a t i o n s of the r a d i c a l s with r e s p e c t to the f i e l d . T r a n s i t i o n s occur over a continuous range of f i e l d s about ~JjT> ^ e magnitude of the range being dependent on the zero f i e l d s p l i t t i n g . I t i s v e r y l i k e l y that t h i s broadening would prevent d e t e c t i o n of any t r a n s i t i o n at f i e l d s c l o s e to t h a t corresponding to g = 2. In the f i e l d r e g i o n of g ° 1)., d e t e c t i o n i s more l i k e l y , but c o u l d occur only i f 3T< ^ § /fto, (-0,= c a v i t y frequency) (3^). In view of Coope's p r e d i c t e d value f o r 3 T ^ 32,890 mc./sec. and the frequency of 95>00 mc./sec. used i n the experiments i t i s u n l i k e l y t h a t such a c o n d i t i o n would be met. The r e s u l t i s that i f the r a d i c a l s are h e l d r i g i d l y i t i s most improbable t h a t they would be d e t e c t a b l e . I f the r a d i c a l s are i s o t r o p i c a l l y r o t a t i n g at f r e q u e n c i e s very much gr e a t e r than then a s i m i l a r s i t u a t i o n to that found f o r the a n i s o t r o p i c p a r t of the h f i n t e r a c t i o n would occur, namely t h a t the zero f i e l d s p l i t t i n g would average to zero. In t h i s case t r a n s i t i o n s o c c u r r i n g i n the f i e l d r e g i o n of g = 2 would be d e t e c t a b l e . For h i n d e r e d r o t a t i o n the s i t u a t i o n becomes much more complicated. For the case of 0 2 trapped i n q u i n o l c l a t h r a t e s , a s i t u a t i o n r a t h e r s i m i l a r to t h a t found i n the p r e s e n t 2k. experiments, the zero f i e l d s p l i t t i n g i s merely reduced (38). I t has been suggested t h a t there i s a hindrance p o t e n t i a l to r o t a t i o n o f the form V0 ( l-cosftf), where f i s the angle between the m o l e c u l a r (z) a x i s and the d i r e c t i o n o f minimum p o t e n t i a l i n the cage (z'"). This reduces the zero f i e l d s p l i t t i n g to 3T'= 3T£|r^cos1§)(W - and transforms the Hamiltoniani& to ft-gpH-S + 3T'[Sj.*- •HlS+0] ( 3 5 ) Q u a n t i z a t i o n of S i s now i n the d i r e c t i o n o f z'. In t h i s case the r a d i c a l s would not be d e t e c t a b l e a t g = 2. 3-3 Experimental Methods  A, Ketene The ketene used was k i n d l y s u p p l i e d by Dr. G. B. P o r t e r , who had prepared i t by p y r o l y s i n g a c e t i c anhydride. As ketene b o i l s at - 56°G and r e a c t s with water to gi v e poisonous a c e t i c a c i d vapour i t was necessary to keep i t i n l i q u i d n i t r o g e n . I t s p u r i t y was checked i n a mass spectrometer, and the sm a l l amounts of ethylene and carbon monoxide found i n t h i s way were removed - o by f r e e z i n g and warming the sample between 77 and 195 K. Gas mixtures of ketene and Matheson Research Grade argon were prepared at m a t r i x r a t i o s (M/R) of 100 and 300. The g e n e r a l d e p o s i t i o n technique d e s c r i b e d e a r l i e r was m o d i f i e d by bypassing the b r a s s flow r e g u l a t o r to reduce any metal c a t a l y s e d p o l y m e r i z a t i o n of the ketene (although the sample had n e v e r t h e l e s s to pass through the s t a i n l e s s s t e e l c a p i l l a r y i n t o the c a v i t y ) . The General E l e c t r i c A-H6 lamp was used to i r r a d i a t e the s o l i d sample. Magnetic f i e l d s i n the r e g i o n of g = 2 and g = k were scanned i n se a r c h i n g f o r methylene resonance; the microwave power v a r i e d 2 5 . between f i v e and 100 m i l l i w a t t s . B. Diazome thane Diazomethane was prepared i n the laboratory before each attempt to produce methylene. Since diazomethane i s both highly toxic and explosive i t was necessary to carry out the preparation i n a fume cupboard and to wear protective clothing while handling the compound. Two methods were used for the sample preparations. The one used for a l l the experiments involving the search for methylene used the synthesis method of de Boer and Backer (1±0) f o r gaseous diazomethane. The apparatus i s shown schematically i n Figure 6 . The procedure was as follows. A solution of 3 gm. KOH i n 3 ml. water and 25 ml. 2-(2-ethoxyethoxy) ethanol was o warmed with a heating pad to 50-50 C i n the s p e c i a l l y modified 125 ml. long stemmed f l a s k . To this was added dropwise a solution of 6 . 5 gm. N-methyl N-nitroso p-toluenesulfonamide ("Diaz-ald" from Al d r l c h Chemical Company) i n 10 ml. anisole, and the diazomethane produced i n the ensuing reaction was extracted by bubbling i n e r t gas (argon or helium) gently through the mixture. The gases were cooled i n a condenser, passed through a trap containing KOH p e l l e t s to remove any water, cooled o further i n a trap at 19<5 K (C0 2- acetone), and the diazomethane was f i n a l l y condensed i n a trap at 77°K ( l i q u i d N^). The system of traps was closed to the atmosphere and any non-condensable gas was pumped o f f . The trap containing diazomethane was warmed , o o to 1 9 5 K, and the compound d i s t i l l e d to a t h i r d trap at 77 K. o F i n a l l y the t h i r d trap was warmed to 1 9 5 K and a middle f r a c t i o n 26. o f the gas e v a p o r a t i n g from t h i s was c o l l e c t e d i n a bulb and t r a n s p o r t e d to the vacuum system connected to the ESR spectrometer. The p u r i t y of the diazomethane was checked i n a Perkin-Elmer 21 I n f r a r e d Spectrometer and although peaks were observed a t 2800-2900 c m j 1 (the a l i p h a t i c C-H s t r e t c h r e g i o n ; the peaks may have been due to an i m p u r i t y or p o s s i b l y a c y c l i c GE^2 dimer) a l l others were those r e p o r t e d f o r diazomethane and the sample was used. (A f u r t h e r middle f r a c t i o n of the sample o b t a i n e d as above f a i l e d to reduce the r e l a t i v e i n t e n s i t y of the peak a t 2800-2900 cm. - 1.) Samples f o r ESR study were prepared i n the u s u a l way a t m a t r i x r a t i o s , from 100 to 300 u s i n g Mathe.son Research Grade argon and krypton as m a t r i c e s . Because diazomethane decomposes on l o n g s tanding the samples had to be used a f t e r only one or two hours mixing. The samples were d e p o s i t e d i n the u s u a l way, and the A-H6 lamp was used f o r the i r r a d i a t i o n . The search f o r an ESR s i g n a l f o l l o w e d the same procedure as f o r ketene. In the experiment i n v o l v i n g D^O a s l i g h t m o d i f i c a t i o n of the method of Dr. J . A. B e l l (1x2) was used f o r the diazomethane s y n t h e s i s . I t was adopted i n an attempt to reduce the number of p o s s i b l e i m p u r i t i e s i n the sample, f o r i t e l i m i n a t e d the n e c e s s i t y of h e a t i n g and u s i n g a c a r r i e r gas and p e r m i t t e d the use o f a mercury d i f f u s i o n pump, which the p r e v i o u s apparatus d i d not do. The apparatus i s shown i n F i g u r e 7 and the method used was as f o l l o w s . Using the Y-shaped tube shown i n F i g u r e 7 dry N-methyl N - n i t r o s o p-toluene sulfonamide was added i n vacuo to a s o l u t i o n of KOH i n ethylene g l y c o l which had been p r e v i o u s l y degassed and c o o l e d to 0°G to prevent decomposition of any 27. diazomethane formed. ( I t was necessary to prepare the KOH s o l u t i o n by d i s s o l v i n g the hydroxide slowly i n ethylene g l y c o l at room temperature; h e a t i n g caused the s o l u t i o n to t u r n y e l l o w with a simultaneous i n c r e a s e i n v i s c o s i t y , p o s s i b l y due to the f o r m a t i o n of a polymer.) The vapours produced were e x t r a c t e d from the g e n e r a t i n g system by s l i g h t pumping, passed through a tube c o n t a i n i n g g l a s s wool to c a t c h any d r o p l e t s which might o have bumped out of the s o l u t i o n , and condensed i n a t r a p at 77 K. Any ethylene condensing i n the trap was removed by a f l a s h d i s t i l l a t i o n and the diazomethane remaining was condensed i n a bulb and t r a n s p o r t e d to the other bulb as b e f o r e . The whole procedure was c a r r i e d out i n the dark i n order to minimize p h o t o l y s i s of diazomethane. In the p r e p a r a t i o n of the gas mixture the three l i t r e bulb was f i l l e d with the diazomethane sample to ij. mm.Hg and the pressure was i n c r e a s e d to 15 mm. Hg by a d d i t i o n o f DgO. ( A l l a i r had been p r e v i o u s l y removed from the D^O by f r e e z i n g to 19^ 5 K, pumping and then warming with the pump o f f . The procedure was repeated s i x times.) F i n a l l y argon, the m a t r i x gas, was added to b r i n g the t o t a l p r e s s u r e to j u s t over 30 cm. Hg. A f t e r the gases had been mixing a t room temperature f o r about an hour the sample was i 0 d e p o s i t e d on the sapphire needle at lx.,2 K i n the u s u a l way and the s o l i d was i r r a d i a t e d with the f u l l a r c of the A-H6 lamp. The f i e l d r e g i o n of g = 2 was searched f o r an ESR s i g n a l u s i n g a microwave power l e v e l of 2-5 mw. to Manometer to Pump Figure 6. Vacuum System for C h y s ^ Preparation (Method of de Boer and Backer). To F o l l o w Page ^1 Glass Wool Y- shaped Tube raps Figure 7(a). Vacuum System for CH2N2 Preparation (Method of Bell). B - K socket Side View End View Figure 7(b). Y- shaped Tube. 28. 3 - l l R e s u l t s and D i s c u s s i o n The r e s u l t s of the experiments i n v o l v i n g the search f o r methylene are shown i n Table I I . The most obvious f a c t s i n the t a b l e are t h a t no d e f i n i t e , r e p r o d u c i b l e s i g n a l a t t r i b u t a b l e to methylene was found, and on numerous occasions methyl r a d i c a l s were obtained. Furthermore, when the wide s i g n a l was obtained i t was almost always i n c o n j u n c t i o n with that of the methyl r a d i c a l (except i n r u n 6 , and i n t h i s run d i f f i c u l t i e s were encountered w i t h the spectrometer). A l s o a suggestion of a s i g n a l a t g =l\. was found only once (run 8) and t h i s was d e t e c t e d o n l y a t extreme s e t t i n g s of f i e l d modulation, amplitude and s i g n a l l e v e l . I t should be n o t i c e d a l s o that the methyl s i g n a l and the wide one were produced from both ketene and diazomethane when the m a t r i x r a t i o was 1 0 0 , but not when i t was 3 0 0 . I t seems most improbable that trapping of methylene r a d i c a l s would have been l e s s l i k e l y at M/R = 300 than a t M/R = 100 and i t must appear that the methyl R a d i c a l s and the r a d i c a l s c a u s i n g the wide s i g n a l were produced i n the same p r o c e s s . I f t h i s were the case then the ESR a b s o r p t i o n of trapped methylene r a d i c a l s was p r o b a b l y not detected. Methyl r a d i c a l s c o u l d have been prepared e i t h e r by the p h o t o l y s i s of an i m p u r i t y or by the a b s t r a c t i o n of hydrogen atoms from something by methylene. P h o t o l y s i s of methyl i o d i d e i s a w e l l known example of the f i r s t type of r e a c t i o n , while the r e a c t i o n of methylene w i t h propane (lj-3) i s an example of the second. I t i s by no means imp o s s i b l e that a b s t r a c t i o n to produce the methyl r a d i c a l and a l a r g e r i g i d l y trapped r a d i c a l a c t u a l l y occurred, and the f a c t t h a t the r a d i c a l s were observed only at the low m a t r i x r a t i o , when something other than i n e r t gas was more 2 9 . TABLE I I The R e s u l t s of the Experiments I n v o l v i n g the Search f o r Methylene Experiment Sample M a t r i x R a t i o R e s u l t f 1 CH CO/Ar 2 300 n i l 2 CH CO/Ar 2 300 s l i g h t i n d i c a t i o n of s i g n a l 3 CH 2C0/Ar 300 n i l If- CH 2C0/Ar 300 n i l CH N /Ar 2 2 ioo""" at g r 2 wide s i g n a l ; CH^ pr e s e n t 6 CH N /Ar 2 2 100 a t g = 2 p o s s i b l e wide s i g n a l 7 CH 2CO/Ar 100 CH^ prese n t 8 CH N /Ar 2 2 7 100 at g = 2 s t r o n g wide s i g n a l ; CH p r e s e n t . 3 a t g = Ix. p o s s i b l e wide s i g n a l 9 • CH ¥ /Ar 2 2' 100 at g = 2 weak wide s i g n a l ; CH^ present 10 CH N /Ar 2 2 - at g = 2 weak wide signal;. CH^ presen t n CH 2N 2/Kr 1 0 0 - 3 0 0 n i l # 12 CH 2N 2/Kr 2 0 0 - 3 0 0 n o t h i n g d e f i n i t e D i e t h y l e ther was p r e s e n t i n the sample. # A pyrex f i l t e r was used. 3 0 . a c c e s s i b l e to methylene, adds weight to the argument. To t e s t the a b s t r a c t i o n h y p o t h e s i s the experiment i n v o l v i n g GH^S^ i n the presence of D^O was c a r r i e d out. Although the ease of a b s t r a c t i o n o f hydrogen atoms from water would be l e s s than t h a t from hydrocarbons because of the gr e a t e r bond energy i n water, i t was hoped that the r a d i c a l CELjD c o u l d be prepared. D^O was chosen, however, because i t i s e a s i l y o b t a i n e d and because any 0D generated i n an a b s t r a c t i o n would probably not be de t e c t e d because i t s g - f a c t o r i s not 2 . (However, i n i c e , a p o l a r matrix, the o r b i t a l angular momentum may be quenched and the r a d i c a l d e t e c t e d (i|l).) i n t h i s f i e l d r e g i o n . The s e p a r a t i o n o f the l i n e s i n 0D i s about 6 gauss.) P o s s i b l e p h o t o l y s i s o f D^O was dis c o u n t e d because the energy of the r a d i a t i o n was too low to break the 0D bond. A spectrum was obtained i n the experiment and i s shown i n Fi g u r e 9 . The f o u r l i n e s i g n a l of methyl was very prominent, but there were some other a b s o r p t i o n s i n t e r s p e r s e d , the most n o t i c e a b l e b e i n g a t r i p l e t between the two most i n t e n s e methyl l i n e s . Between the outer methyl l i n e s there were f u r t h e r weak a b s o r p t i o n s . A l l the l i n e s were very narrow,,suggesting t h a t the r a d i c a l s p roducing them showed some degree of r o t a t i o n a l freedom i n the matrix. Now the ESR spectrum o f CH^D would probably show three sets of t r i p l e t s s y m m e t r i c a l l y arranged about the f r e e s p i n g-value with the ce n t r e s of each t r i p l e t about 2% gauss a p a r t and each l i n e i n the t r i p l e t 3 . 7 gauss a p a r t . The expected i n t e n s i t y r a t i o of the l i n e s would be 1 : 1 : 1 : 2 : 2 : 2 : 1 : 1 : 1 . The l i n e s found i n the centre t r i p l e t surrounded a g - f a c t o r o f 2 . 0 0 1 5 - . 0 0 0 2 and were IL.2 ± 0 . 3 gauss apart, very n e a r l y the s e p a r a t i o n expected (but very much l e s s Do Folio-:: Fa~e 3 0 r30G^ H—> Me Me Figure 8. A Typical ESR Spectrum from the Photolysis of C H 2 N 2 in Argon at 4.2°K. The lines marked by Me are due to the CH3-radical. G denotes gauss. Po F o l l o w Pa.qe 30 Figure 9. ESR Spectrum from the Photolysis of C H 2 N 2 in the Presence of D 2 0 . Lines marked by Me are due to the CH3 radical. G denotes gauss. H e is the magnetic field where g=2.0023. 31. than 6 gauss found f o r OD). The strange shape of the t r i p l e t may w e l l have been caused by i t s being superimposed on the broad resonance found e a r l i e r i n c o n j u n c t i o n w i t h the p r o d u c t i o n of CH^ r a d i c a l s from diazomethane. Although i t was d i f f i c u l t to i d e n t i f y any t r i p l e t s amongst the outer peaks the low s i g n a l -t o - n o i s e l e v e l may have been o b s c u r i n g them. Thus the prod-u c t i o n of CH^Dcwas suggested; time c o n s i d e r a t i o n s prevented f u r t h e r p u r s u i t of the problem. The appearance of the spectrum of o r d i n a r y methyl i n the water experiment was h a r d l y s u r p r i s i n g . The m a t r i x r a t i o Al? + D 0/011^2 was only 8 0 , i d e a l c o n d i t i o n s a c c o r d i n g to the p r e v i o u s experiments f o r p r o d u c t i o n of methyl r a d i c a l s . The gr e a t i n t e n s i t y of the methyl spectrum compared with that o f the other l i n e s c o u l d be a t t r i b u t e d to g r e a t e r ease of a b s t r a c t i o n of H from CH than of D from OD. In a l l the experiment added impetus to the suggestion t h a t the methyl r a d i c a l s found e a r l i e r arose through an a b s t r a c t i o n mechanism. In c o n s i d e r i n g the problem of the non appearance of a s i g n a l due to methylene a v e r y obvious q u e s t i o n comes to mind, viz.,were any methylene r a d i c a l s trapped? The a b s t r a c t i o n mechanism j u s t d i s c u s s e d , i f c o r r e c t , suggests that they were produced, although i t does not say f o r how l o n g . I t i s p o s s i b l e t h a t methylene r a d i c a l s may have decomposed or t h a t other products may have been obtained, a s i t u a t i o n suggested by the data of Robinson and McCarty ( 2 9 ) . Pimentel (28) however, d i d manage to trap a p r e c u r s o r f o r ethylene, which was almost c e r t a i n l y methylene, u s i n g r a d i a t i o n of energies s i m i l a r to 3 2 . those used here. I t i s i n c o n c e i v a b l e t h a t methylene could be produced a t a m a t r i x r a t i o of 100 but not one of 3 0 0 , and one thus suggests t h a t i t was indeed trapped and the reason f o r the l a c k of an ESR s i g n a l must be found elsewhere. Of course, i f the trapped r a d i c a l s were not i n a t r i p l e t s t a t e no s i g n a l c o u l d havelbeen obtained. In view of the r e s u l t s of Herzberg (30) t h i s i s an u n l i k e l y s i t u a t i o n , f o r as was p o i n t e d out e a r l i e r any trapped r a d i c a l s were probably i n the ground s t a t e and the ground s t a t e i s almost c e r t a i n l y the t r i p l e t ( ). Furthermore the o b s e r v a t i o n of a b s t r a c t i o n may i n d i c a t e the presence of a t r i p l e t , f o r Richardson e t a l . (l[$) have suggested that s i n g l e t methylene may show random i n s e r t i o n i n t o carbon hydrogen bonds whereas the t r i p l e t may show a b s t r a c t i o n . Trapped t r i p l e t methylene r a d i c a l s were thus very probable. I f any trapped methylene r a d i c a l s were h e l d r i g i d l y i n the matrix, then they would have been undetectable i n these experiments because of the h i g h z e r o - f i e l d s p l i t t i n g . Again we have an u n l i k e l y s i t u a t i o n , f o r the ESR spectrum of methyl r a d i c a l s , which are of comparable s i z e , i n d i c a t e s r o t a t i o n of t h i s r a d i c a l i n the m a t r i x and methylene should do l i k e w i s e (although r o t a t i o n v i a the tunnel e f f e c t i s much more l i k e l y f o r methyl than f o r methylene)> Now a t 2°K f r e e l y r o t a t i n g methylene s h o u l d be i n the ground (J=0 ) a n d f i r s t e x c i t e d (J=l ) r o t a t i o n a l s t a t e s , and the f r e q u e n c i e s of r o t a t i o n s hould be zero and ij..8 x 1 0 + 1 1 s e c . " 1 r e s p e c t i v e l y . Since the z e r o - f i e l d s p l i t t i n g i s between 3«3 x 10"^ and about 7 x l 0 1 0 s e c . " 1 then f o r the r a d i c a l s i n the J = l s t a t e i t i s very p o s s i b l e t h a t the e f f e c t of the e l e c t r o n s p i n - s p i n 3 3 . i n t e r a c t i o n c o u l d be averaged to zero and ESR t r a n s i t i o n s c o u l d be observable a t g = 2. The l a c k of an observed s i g n a l suggests t h a t t h i s was not the case. Most l i k e l y the s i t x i a t i o n was s i m i l a r to th a t of oxygen i n a q u i n o l c l a t h r a t e ( 3 5 ) , namely t h a t the r a d i c a l s were undergoing h i n d e r e d r o t a t i o n and th a t i n the cage there was a d i r e c t i o n of minimum p o t e n t i a l energy. As w i l l be d i s c u s s e d l a t e r i n the t h e s i s methyl r a d i c a l s i n an argon m a t r i x probably undergo h i n d e r e d r o t a t i o n , and the s i t u a t i o n was probably s i m i l a r f o r methylene. The zero f i e l d s p l i t t i n g would have been merely reduced, and as the cages were randomly o r i e n t e d i n the p o l y -c r y s t a l l i n e m a t r i x the e f f e c t i n the experiments would have been the same as f o r r i g i d l y trapped r a d i c a l s . 3k. CHAPTER FOUR POPULATIONS OF ROTATIONAL STATES OF THE METHYL RADICAL  Ii-—X I n t r o d u c t i o n . In the ESR spectrum of the NH^ r a d i c a l one would expect at f i r s t glance that the i n t e n s i t i e s of the h f peaks due to the hydrogen n u c l e i should be i n the r a t i o 1:2:1, with three due to n i t r o g e n of i n t e n s i t y r a t i o 1:1:1 superimposed on each. I n s t e a d Foner et a l . (3) found a l l nine l i n e s to be of approx-i m a t e l y equal i n t e n s i t y , and McConnell (lj.7) proposed an e x p l a n a t i o n based on the premises that the r a d i c a l s were r o t a t i n g f r e e l y and that the s t a t i s t i c a l d i s t r i b u t i o n of the r a d i c a l s i n r o t a t i o n a l s t a t e s corresponded to thermal e q u i l i b r i u m . Using data f o r ND^ Jen (Ij_8) c a r r i e d the argument f u r t h e r and decided from the peak i n t e n s i t i e s t h a t both ND^ and NH^ were undergoing h i n d e r e d r o t a t i o n i n the m a t r i x . In the ESR spectrum of the CH CKCOOH)^ r a d i c a l H e l l e r (J4.9) found f o u r peaks due to h f i n t e r a c t i o n of the e l e c t r o n and the methyl protons, whose i n t e n s i t y r a t i o was 1:3:3:1 at room temperature but decreased to 1:1.1|5:1.1|5:1 at ij.,2 K . From c o n s i d e r a t i o n s s i m i l a r to those of McConnell he decided t h a t the methyl group underwent n e a r l y f r e e r o t a t i o n about the C-C bond a t I)..2 0 K. The ESR spectrum of methyl r a d i c a l s i s w e l l known- (£). I t contains f o u r sharp l i n e s , c e n t r e d about the f r e e s p i n g-value, about 2k gauss a p a r t . In t h i s chapter a treatment s i m i l a r to those of H e l l e r and McConnell i s made to t r y to determine the p o p u l a t i o n s of the r o t a t i o n a l l e v e l s of methyl r a d i c a l s trapped 35. i n s o l i d argon at I4..2 K. L e t us assume the methyl r a d i c a l s 2 ' t to be i n t h e i r p l a n a r A ground s t a t e with the C-H d i s t a n c e 2 1.08 A (30) . They are thus symmetric tops, of p o i n t group D , 3h and t h e i r r o t a t i o n a l energy term values f o r f r e e r o t a t i o n are give n by: F(J,K)-- BJtH)+(A-B)K* (36) where 6 = g TT 11 ^ ^ A ~ g Tlae IA > C i s t t i e v e l o c i t y of l i g h t ; I A , I B a r e moments of i n e r t i a about the t h r e e f o l d and twofold symmetry axes r e s p e c t i v e l y ; J * 0,1 , 2 ... j K " 0t - | , - Z ... - J . The lowest l e v e l s are gi v e n i n Table I I I . TABLE I I I Term Values of the Lowest S t a t e s of F r e e l y R o t a t i n g Methyl R a d i c a l s . J K Term value (cm. "*") 0 0 0 1 • 1 l i f . 5 1 0 19.3 2 2 38.6 I f f r e e l y r o t a t i n g r a d i c a l s f o l l o w a Boltzmann d i s t r i b u t i o n at [|_.2°K, then g r e a t e r than 99 percent of them w i l l be i n the ground r o t a t i o n a l s t a t e . Using the Born- Oppenheimer approximation the o v e r a l l wave f u n c t i o n can be w r i t t e n as the product of the e l e c t r o n i c , v i b r a t i o n a l , r o t a t i o n a l and n u c l e a r wave f u n c t i o n s ( 5 0 ) ( 5 l ) : (37) 3 6 . Since methyl r a d i c a l s c o n t a i n protons, which are fermions of s p i n the P a u l i P r i n c i p l e says t h a t 4* must be antisymmetric to exchange of two of these protons. R o t a t i o n of l 8 0 ° about one of the twofold axes performs such an exchange, whereas r o t a t i o n o o f 120 about the t h r e e f o l d a x i s i s e q u i v a l e n t to exchange of two p a i r s of p r o t o n s , i s thus antisymmetric and symmetric r e s p e c t i v e l y to these two o p e r a t i o n s and i s t h e r e f o r e of s p e c i e s &2 °f the r o t a t i o n a l subgroup (5l)• The r a d i c a l s trapped at q..2 K may be assumed to be i n t h e i r ground e l e c t r o n i c and v i b r a t i o n a l s t a t e s (symmetry species and r e s p e c t i v e l y ) . I f one now c o n s i d e r s the n u c l e i one f i n d s e i g h t p o s s i b l e d i f f e r e n t c o n f i g u r a t i o n s of the three s p i n s , which can be combined i n t o e i g h t orthonormal e i g e n s t a t e s as f o l l o w s : (38) N 5 3 7 . The f i r s t f o u r are t o t a l l y symmetric ( s p e c i e s A-^), and correspond to n u c l e a r s p i n s of - V 2 , ^^2' ~ " V 2 > - " V 2 r e s p e c t i v e l y . The. l a t t e r f o u r make up two doubly degenerate s t a t e s (species E ) , and correspond to n u c l e a r s p i n s of " V ^ , Z* ~ ~^/Z> ~ V 2 r e s p e c t i v e l y . In order f o r the o v e r a l l s t a t e to be of species A 2, % must be of s p e c i e s A.^  when % i i s A^ ( s i n c e A,2 x A^ = A 2 ) , or i t must be of s p e c i e s E when 4^ i s E ( s i n c e E x E - A-j+A,-,+ E ) . Of the two most important low l y i n g r o t a t i o n a l s t a t e s being c o n s i d e r e d here, the s t a t e J - 0 , K-0 has symmetry A , whereas the J-\ 5 K= - I l e v e l has symmetry E. The J = l , K=0 s t a t e , being of r o t a t i o n a l s p e c i e s A 2, i s completely absent. Since there are f o u r A^ n u c l e a r s p i n f u n c t i o n s , and two E s p i n f u n c t i o n s the A^ r o t a t i o n a l l e v e l s have s t a t i s t i c a l weight [(_, whereas the E r o t a t i o n a l l e v e l s have weight 2 . The i n t e n s i t y of a h y p e r f i n e l i n e i s p r o p o r t i o n a l to the p o p u l a t i o n of the n u c l e a r l e v e l i n q u e s t i o n . Thus, i f the methyl r a d i c a l s are i n the ground i r o t a t i o n a l s t a t e the f o u r h y p e r f i n e l i n e s w i l l be i n an i n t e n s i t y r a t i o 1 : 1 : 1 : 1 , corresponding to 3 X X 3 n u c l e a r spins / 2 , /2' ~ /2' ~ ' 2 r e s p e c t i v e l y . However, i f the r o t a t i o n a l s t a t e i s a l s o populated the i n t e n s i t i e s of the 1 / 2 and - " V 2 l i n e s i n c r e a s e and i n the l i m i t of b o t h r o t a t i o n a l l e v e l s being e q u a l l y p o p u l a t e d the i n t e n s i t y r a t i o reaches 1 : 3 : 3 : 1 . This i s shown s c h e m a t i c a l l y i n F i g u r e 1 0 . [)_-2 Experimental Methods Two sources of methyl r a d i c a l s were employed i n the experiments. B and A Reagent Grade methyl i o d i d e was k i n d l y s u p p l i e d by Dr. G. G. S. Dutton, .and although small q u a n t i t i e s 3 8 . of i m p u r i t i e s were found by gas chromatography i t was used with-out f u r t h e r p u r i f i c a t i o n because of the w e l l known nature of the ESR spectrum of the methyl r a d i c a l . Dimethyl mercury, the other source, was used without f u r t h e r p u r i f i c a t i o n because of i t s h i g h t o x i c i t y . Exhausts from a l l pumps used i n the experiments i n v o l v i n g dimethyl mercury were d i r e c t e d e i t h e r i n t o a fume cupboard or out-s i d e . Matheson Research Grade argon was the matrix, and gas mixtures (M/R = 100) were prepared, mixed and d e p o s i t e d i n the u s u a l way. The A-R"6 lamp was found to be the most e f f e c t i v e i n p r e p a r i n g the r a d i c a l s . The s p e c t r a were obtained by scanning very s l o w l y over the peaks, with the i d e a of doing a g r a p h i c a l i n t e g r a t i o n , s i n c e the i n t e n s i t y of a t r a n s i t i o n i s p r o p o r t i o n a l to the area under the a b s o r p t i o n peak. Microwave power of 5 - 1 0 mw. was used, although i t was d i s c o v e r e d a f t e r the experiments were complete t h a t there was a p o s s i b i l i t y of s l i g h t s a t u r a t i o n . lj . -3 R e s u l t s and D i s c u s s i o n The d e r i v a t i v e t r a c i n g s of two of the ESR a b s o r p t i o n l i n e s used i n the measurements are shown i n F i g u r e 1 1 ; they are the two h i g h e s t f i e l d peaks of the spectrum. Although a g r a p h i c a l i n t e g r a t i o n was attempted the value o b t a i n e d f o r the peak i n t e n s i t y r a t i o was h i g h l y i n a c c u r a t e because of the l a r g e amount of guesswork i n v o l v e d i n f i n d i n g the exact p o s i t i o n t h at a c e r t a i n slope should be p l a c e d . Furthermore, the l a r g e t a i l on e i t h e r s i d e of the more i n t e n s e l i n e , which was p a r t l y r e s p o n s i b l e f o r the l a r g e d iscrepancy, d i d not f i t i n t o the L o r e n t z i a n shape of the sharper p a r t of the l i n e and may have been due to a superimposed broad a b s o r p t i o n . To F o l l o w Page 38 Total M = - 3 A A< J=1 K=fT 14.5 cm: 1 J=0 K=0 A i A ^ Symmetry of Total Eigenf unction Symmetry of Nuclear Spin E igenf unction S y m m e t r y of Non Spin E i g e n f u n c t ion FigurelO- Intensit ies of the Hyper f i ne Lines of the ESR S p e c t r u m of the Methy l R a d i c a l in its Lowest Rotat iona l Ene rgy S ta tes . To Follow Page 38 Figure 11. The Two High Field Lines of the ESR S p e c t r u m of the Methyl Radical* 3 9 . As a r e s u l t the f o l l o w i n g procedure was used to determine the a b s o r p t i o n i n t e n s i t i e s . S ince both peaks had e s s e n t i a l l y a L o r e n t z i a n l i n e shape, and s i n c e both had the same width be-tween the p o i n t s of maximum and minimum slope, the area under an a b s o r p t i o n peak was p r o p o r t i o n a l to i t s amplitude i n the d e r i v a t i v e t r a c i n g . Thus these h e i g h t s were measured and found to be i n the r a t i o 2.8:1 f o r the l a r g e r peak to the s m a l l e r peak. T h i s r a t i o may w e l l have been a b i t h i g h i n favour of the l a r g e r peak because of the p o s s i b i l i t y of a c o n t r i b u t i o n from the superimposed broad a b s o r p t i o n . N e v e r t h e l e s s i t i s qu i t e apparent t h a t the i n t e n s i t y r a t i o of 1:1, expected I f only the ground r o t a t i o n a l s t a t e of the r a d i c a l s were populated, was not found,, and that many of the r a d i c a l s were e x c i t e d above the ground s t a t e . One of the p o s s i b l e reasons why the i n t e n s i t y r a t i o was not 1:1 i s t h a t there was not thermal e q u i l i b r i u m and that the upper r o t a t i o n a l l e v e l was po p u l a t e d . T h i s was not an i m p o s s i b i l i t y , f o r i f one were to condense a room temperature e q u i l i b r i u m mixture of methyl r a d i c a l s at lx..2°K there would be o r i g i n a l l y a d i s t r i b u t i o n r a t i o of 1:1 between the two lowest r o t a t i o n a l l e v e l s (although thermal e q u i l i b r i u m would e v e n t u a l l y be a c h i e v e d ) . The reason i s t h a t combinations between l e v e l s of d i f f e r e n t s p e c i e s (apart from n u c l e a r spin) are f o r b i d d e n . Now the r a d i c a l s s t u d i e d here were prepared a t Ij..2°K, and although they c o u l d a l l have been produced i n the ground s t a t e the upper s t a t e might e a s i l y have been populated as w e l l . In t h e i r s t u d i e s of NH^ r a d i c a l s trapped from the gas phase at if.2 K Robinson and McCarty (li6) found, however, t h a t the room temperature d i s t r i b u t i o n was not preser v e d , a f a c t which they suggested was due to a breakdown of the symmetry s e l e c t i o n r u l e caused by i n t e r a c t i o n of the n u c l e a r spins with the s p i n of the unp a i r e d e l e c t r o n . I f such a p o s s i b l e s i t u a t i o n were achieved i n the present experiments, then thermal e q u i l i b r i u m c o u l d probably e a s i l y have been e s t a b l i s h e d . A v e r y l i k e l y cause of the upper r o t a t i o n a l l e v e l being populated c o u l d have been the presence of a p o t e n t i a l b a r r i e r to f r e e ' r o t a t i o n . E f f e c t s of such b a r r i e r s have been co n s i d e r e d by Koehler and Dennison (52) f o r the case of i n t e r n a l r o t a t i o n of a methyl group i n methanol, and by Meyer et a l . (38) f o r the r o t a t i o n of 0^ i n a q u i n o l c l a t h r a t e . In these cases the two lowest l e v e l s came c l o s e r together as the p o t e n t i a l b a r r i e r was i n c r e a s e d , and e v e n t u a l l y c o a l e s c e d i n t o p a r t of a v i b r a t i o n a l mode. Thus, when the p o t e n t i a l b a r r i e r was h i g h the upper r o t a t i o n a l l e v e l c o u l d be populated with thermal e q u i l i b r i u m . Now the van der Waals r a d i u s of the methyl r a d i c a l along a CH o bond i s 2 . 2 8 A, whereas the r a d i u s of a s u b s t i t u t i o n a l h o l e i n o an argon l a t t i c e i s 1 . 8 7 A . Although the van der Waals r a d i u s does not n e c e s s a r i l y give the e f f e c t i v e r a d i u s of the r a d i c a l ? , and although the t r a p p i n g s i t e s of the p o l y c r y s t a l l i n e m a t r i x need not be the s u b s t i t u t i o n a l h o l e s , y e t n e v e r t h e l e s s these f i g u r e s i n d i c a t e that i f some s o r t o f r o t a t i o n were o c c u r r i n g , then i t was probably g r e a t l y h i n d e r e d . To t e s t t h i s h y p o t h e s i s i t w i l l probably be necessary to reduce the sample temperature c o n s i d e r a b l y , and t h i s w i l l r e q u i r e an apparatus f o r pumping on the l i q u i d helium to reach temperatures w e l l below the o normal b o i l i n g p o i n t of if.2 K. I f l . CHAPTER FIVE THE N F £ RADICAL 5-1 I n t r o d u c t i o n I t i s now w e l l known t h a t t e t r a f l u o r o h y d r a z i n e , ^2^1$.' d i s s o c i a t e s i n t o NF r a d i c a l s a t room temperature (53) a c c o r d i n g 2 to the f o l l o w i n g equation: N F, 2IF - 20 k c a l . 2 If 2 L i k e other polyatomic r a d i c a l s , N F 2 i s expected to e x h i b i t ESR a b s o r p t i o n , and such has been observed by P i e t t e et a l . (51+) f o r NF^ i n the gas phase. The s i g n a l observed by them was a s i n g l e l i n e , of width lOlf gauss, c e n t r e d a t g = 2.010, and although h f i n t e r a c t i o n s and c o u p l i n g of the e l e c t r o n s p i n w i t h the r o t a t i o n a l angular momentum of the r a d i c a l would be expected i n such circumstances, the l a c k of s p l i t t i n g was e x p l a i n e d as being due to a smearing out of the s i g n a l by c o l l i s i o n a l e f f e c t s . The N F, - NF_ e q u i l i b r i u m has a cl o s e analogy i n the w e l l 2 if 2 known d i s s o c i a t i o n of N 0, to N0 o. Farmer e t a l . have found t h a t 2 I f 2 although s i m i l a r c o l l i s i o n a l broadening of an N0 2 s i g n a l i s pr e s e n t a t p r e s s u r e s above 5 ram. Hg, N0 2 may n e v e r t h e l e s s be trapped i n i n e r t m a t r i c e s and show h y p e r f i n e i n t e r a c t i o n s due to the n i t r o g e n n u c l e a r s p i n ( 5 5 ) . A s i m i l a r study of NF^ was t h e r e f o r e thought to be f e a s i b l e . In t h i s case one would expect h y p e r f i n e i n t e r -a c t i o n s with both the n i t r o g e n ( l N = I ) and f l u o r i n e ( I F 3 t ) n u c l e i , as opposed to only the n i t r o g e n i n N0 2, with consequent c o m p l i c a t i o n of the. spectrum. The study i s made more i n t e r e s t i n g by the l a c k of data i n the l i t e r a t u r e on f l u o r i n e h y p e r f i n e i n t e r a c t i o n s , and i|2. i t v/as hoped that a comparison of the h f s p l i t t i n g s i n NH2 (3) and WPg c o u l d be made. An attempt to d e t e c t h f and r o t a t i o n a l s p l i t t i n g s i n the gas phase was a l s o made and i s r e p o r t e d i n s e c t i o n 5 of t h i s chapter. 5 -2 Experimental Method f o r Trapping the NF? R a d i c a l T e t r a f l u o r o h y d r a z i n e , 99*1% pure, was o b t a i n e d from A i r Products and Chemicals Inc., Allentown, Pa., and, as r e p o r t e d i m p u r i t i e s should not i n t e r f e r e i n the experiments i t was used without f u r t h e r p u r i f i c a t i o n . The compound i s t o x i c and under c e r t a i n c o n d i t i o n s may be e x p l o s i v e (56) (57) •• I t was t h e r e f o r e handled i n a s i m i l a r manner to diazomethane, namely u s i n g a fume cupboard, w i t h the operator wearing p r o t e c t i v e c l o t h i n g . P r e c a u t i o n s were taken to keep t e t r a f l u o r o h y d r a z i n e away from o r g a n i c m a t e r i a l s , e s p e c i a l l y hydrocarbon pump o i l , and K e l - F # 90 h a l o c a r b o n grease was used on the stopcocks.. (For the s m a l l q u a n t i t i e s of t e t r a f l u o r o h y d r a z i n e used Apiezon grease was found s a t i s f a c t o r y i n the vacuum system where the f i n a l gas mixtures f o r ESR study were prepared.) The m a t r i c e s used were Matheson Research Grade argon and krypton, as w e l l as F i s h e r S p e c troanalysed carbon t e t r a c h l o r i d e . Experiments were c a r r i e d out at mole r a t i o s (M/R) of 300 and 1200 i n both argon and krypton, and 1200 i n carbon t e t r a c h l o r i d e . Trapping was achieved by d e p o s i t i n g the room temperature e q u i l i b r i u m mixture of N2F[|_, N F 2 and m a t r i x gas on the sapphire r o d which had been cooled to l i q u i d h e l i u m temperature. Such a procedure prevented combination of N F 2 r a d i c a l s which would otherwise have o c c u r r e d on c o o l i n g . D e p o s i t i o n was continued u n t i l a v e r y h i g h s i g n a l - t o - n o i s e r a t i o was obtained. o S p e c t r a were obtained at l\..2 K and throughout warmup, u n t i l the ESR s i g n a l disappeared. No a p p r e c i a b l e s a t u r a t i o n was observed at a microwave power l e v e l of one m i l l i w a t t and t h i s was used In a l l the experiments. R e s u l t s of an experiment i n which NO^ was trapped a t a mole r a t i o (M/R) of approximately 300 are a l s o d e s c r i b e d h e r e . The m a t r i x was Matheson Research Grade krypton, and the sample was the room temperature e q u i l i b r i u m mixture of Matheson NO^ - ^ ^ i j . * The microwave power was one m i l l i w a t t . 5-3 R e s u l t s o (a) S p e c t r a a t I4..2 K. The ESR s p e c t r a found f o r the NF^ r a d i c a l trapped i n v a r i o u s m a t r i c e s at lj . . 2°K are shown i n F i g u r e s 1 2 , 1 3,ll|. and l6. In g e n e r a l a l l the s p e c t r a showed three major l i n e s , with the centre one of g r e a t e r amplitude but lower peak-to-peak width than the other two. The low f i e l d peak had g r e a t e r amplitude than t h a t at h i g h f i e l d , but the two were of comparable width. Numerical data concerning l i n e widths and s e p a r a t i o n s , as w e l l as the g - f a c t o r at the p o i n t of zero slope of the centre peak,are g i v e n i n Table IV. In argon (F i g u r e s 12 and 13) some f a i r l y r e s o l v a b l e s t r u c t u r e was e v i d e n t . On the centre l i n e a bump was found at both c o n c e n t r a t i o n s approximately f i v e gauss to h i g h f i e l d of the p o i n t of zero s l o p e , and a t M/R = 1200 there was a l s o s t r u c t u r e on the low f i e l d side of the l i n e three gauss from the same p o i n t . (A c l o s e l o o k at the spectrum found a t M/R = 300 r e v e a l s v e s t i g e s of t h i s s t r u c t u r e ) . The other two l i n e s were very much smoother, TABLE 17 Numerical Data from the ESR Spectra of NF Radicals Trapped in Various Matrices at 1*.2°K. This table gives the separations,as well as the line widths between points of maximum and minimum slope, of the three centre lines of the spectra. The g-factor given i s that of the point of zero slope of the centre peak. Matrix Mole Ratio (M/R) Low Fiel d Line Width (gauss) Separation (gauss) Centre Line Width (gauss) Separation (gauss) High F i e l d Line Width (gauss) g-Factor Ar 300 5.9*1.0 19.0*1.0 3.3*1.0 17.1**1.0 5.7*1.0 2.00i*8*.000i* Ar 1200(A)* 3.3*0.5 16.7*0.5 1.1**0.5 16.3*0.5 2.8*0.5 2.0050*.00OU Ar 1200(B)* 3.3*0.5 17.1**0.5 1.7*0.5 16.9*0.5 U.9*0.5 2.005l* ± .OOOl* Kr 300 8.5*0.5 19.1**0.5 3.3*1.0 16.3*0.5 9.0*0.5 2.0OU7*.0O0l* CC1 1* 1200 J 11.7*0.5 7.7*0.5 17.2*0.5 .# 2.00l*9*.CO0U Two experiments were carried out i n argon at M/R • 1200. These l i n e widths are unspecified because of the nebulous nature of the peaks ( see Fig. 16) . To Follow Page \ H > k 1 1 Figure 12. ESR Spectrum of N F 2 in Argon (M/R=300) at 42°K. G denotes gauss. H e is the magnetic field where g=2.0023. i-3 O Figure 13. ESR Spectrum of N F 2 in Argon (M/R=1200) at 4.2°K. G denotes gauss. H is the magnetic field where g=2.0023. To F o l l o w Page Ijif \ ^20G^ 1 H—' T Figure 14. ESR Spectrum of N F 2 in Krypton (M/R = 300)at 4.2°K. G denotes gauss. H e is the magnetic field where g= 2.0023. To F o l l o v Farre I4J4. . 15. ESR Spectra of N F 2 during Warmup-To Follow Page lih. Figure 16. ESR Spectra of N F 2 in CCl^ (M/R=1200). G denotes gauss. H e is the magnetic f ield where g=2.0023. ^ o l l o w Page I4J4-h 5 0 G H H-at 4.2 K Figure 17. ESR Spectra of N 0 2 in Krypton. G = gauss. H is the field where g=2.0023. after 2 hours warmup k5. although there was some s t r u c t u r e on the low f i e l d peak a t M/R = 3 0 0 . I t i s i n t e r e s t i n g to note from the above t a b l e t h a t at M/R = 1 2 0 0 the peak-to-peak widths were i n general about h a l f those a t M/R = 3 0 0 . However data changed c o n s i d e r a b l y from run to run on the same sample (see the two h i g h f i e l d peak widths i n argon at M/R = 1 2 0 0 ) , and so pro b a b l y l i t t l e c o n c l u s i o n can be drawn from t h i s o b s e r v a t i o n . The divergence i n the two peak widths j u s t citedw.as probably due to s l i g h t d i f f e r e n c e s i n d e p o s i t i o n c o n d i t i o n s , although they were supposedly the same. In k r y p t o n (Figure ll}.) much of the s t r u c t u r e was absent. On the centre l i n e the bump to h i g h f i e l d was reduced to a shoulder, while the ones on the low f i e l d s i d e of the centre peak and on the low f i e l d peak were m i s s i n g . I t i s p o s s i b l e i n t h i s case that averaging of the a n i s o t r o p i c p a r t s of the h f i n t e r -a c t i o n s and g - f a c t o r may have been appearing because of the probable l a r g e r s i z e of the krypton t r a p p i n g s i t e s than the argon s i t e s . At M/R = 3 0 0 i n the i n e r t gas m a t r i c e s two f u r t h e r a b s o r p t i o n s c o u l d be seen, one on e i t h e r s i d e of the main t r i p l e t . There were two very n o t i c e a b l e f e a t u r e s of these l i n e s : (a) The one at h i g h f i e l d was f u r t h e r from the centre than the one a t low f i e l d . (b) N e i t h e r peak c r o s s e d the b a s e l i n e , suggesting that they were due to a broadening of the main a b s o r p t i o n r a t h e r than i n d i v i d u a l t r a n s i t i o n s . The spectrum o b t a i n e d when NF^ was trapped at l i q u i d h elium temperature i n carbon t e t r a c h l o r i d e was very much d i s t o r t e d i n comparison to those found i n the i n e r t gases (see IL6. F i g u r e l 6 ) . The two outer peaks of the t r i p l e t , compared to the centre one, were v e r y weak indeed, and the centre peak was very d i s t o r t e d , b e i n g of a p e c u l i a r shape and very much wider than that found i n argon and krypton. A l s o , the c e n t r e -t o - c e n t r e s e p a r a t i o n of the low f i e l d and centre l i n e s was 12 gauss, as opposed to approximately 18 i n the i n e r t gases. However the g - f a c t o r at the p o i n t where the centre peak c r o s s e d the b a s e l i n e was 2.00i|-9 £ .OOOij., the same as i n the i n e r t gases. (b) Warmup Spe c t r a E s s e n t i a l l y the same changes were observed d u r i n g warmup i n both the argon and krypton m a t r i c e s . The peak-to-peak widths of the two outer l i n e s decreased, t h e i r amplitutes i n c r e a s e d and the bumps on the c e n t r e peak faded. Apparently any a n i s o t r o p i e s of the g - f a c t o r and the h f i n t e r a c t i o n s were being averaged out, and i t i s probable that with l e s s r i g i d i t y i n the l a t t i c e the r a d i c a l s were begi n n i n g to r o t a t e . I t became apparent that i n the l i m i t o f i s o t r o p i c r o t a t i o n the three l i n e s , nowl7±l gauss apart and centred about S = 2 . 0 0 5 3 ± . 0 0 0 6 , would be of equal i n t e n s i t y and they were t h e r e f o r e a s s i g n e d to h f i n t e r a c t i o n with a nucleus of s p i n one > namely n i t r o g e n - ill.. Warmup s p e c t r a i n both argon and krypton are shown i n F i g u r e ll j . . I t w i l l be n o t i c e d that b e s i d e s the centre t r i p l e t there are two f u r t h e r t r i p l e t s , one on each si d e of the centre one, n e i t h e r of which was observed at It.2°K. The i n d i v i d u a l l i n e s of each t r i p l e t are 1 7 ± 2 gauss apa r t and the centre l i n e 4 7 . of each t r i p l e t i s 60 i 2 gauss from the centre l i n e of the spectrum. An assignment of the I n t e r a c t i o n of 60 - 2 gauss to the i s o t r o p i c h f i n t e r a c t i o n constant f o r the f l u o r i n e atom was suggested. Now, i f t h i s were the c o r r e c t assignment the amplitude r a t i o of the outer t r i p l e t s to the main one would be expected to be 1 : 2 : 1 . That found was somewhat g r e a t e r , but the t r a n s i e n t nature of the spectrum, the f a c t that the l i n e s of the outer t r i p l e t s were somewhat wider than those of the main t r i p l e t , and the constancy of the s p l i t t i n g s a l l tend to o v e r r u l e t h i s o b j e c t i o n , and the assignment i s probably v a l i d . In carbon t e t r a c h l o r i d e the s i g n a l i n t e n s i t y decreased s t e a d i l y d u r i n g warmup, but there was no s i g n of the s t r u c t u r e found i n the i n e r t gases. F i g u r e l6 shows the s i g n a l o b tained about two hours a f t e r the s t a r t of warmup. 'The i n t e n s i t i e s of the two outer peaks r e l a t i v e to the centre one were very , o much g r e a t e r ; a l s o there was a peak, not observed at if.2 K, about s i x t y gauss to h i g h f i e l d of the t r i p l e t . The o r i g i n of t h i s peak i s unknown, but i t was probably caused by an i m p u r i t y . I t was s t i l l p r e s e n t many hours a f t e r warmup had s t a r t e d , when o the c a v i t y was a t 77 K, and a f t e r the t r i p l e t had disappeared, (c) NO 2 Trapped i n Krypton A f t e r probable averaging of a n i s o t r o p i e s i n the h f i n t e r -a c t i o n and the g - f a c t o r had been observed f o r the NF r a d i c a l 2 d u r i n g warmup, an experiment was c a r r i e d out to see i f the same co u l d be observed f o r NO^ i n krypton. The spectrum o b t a i n e d at if . 2°K ( F igure 17) was very s i m i l a r to that found i n other m a t r i c e s ( 1 1 ) ( 5 5 ) . Throughout warmup, however, the s t r u c t u r e , u n l i k e t hat of the NF^ r a d i c a l , d i d not disappear, and i t seems t h a t the a n i s o t r o p i c s remained. 5-ij- D i s c u s s i o n L e t us consi d e r the s i g n a l s o b t a i n e d at lx..2°K i n the l i g h t o f the warmup s p e c t r a found i n the i n e r t gases. I f i t i s assumed t h a t a l l h f and g - f a c t o r a n i s o t r o p i c s were averaged out i n the warmup s p e c t r a equation (18) i s a p p l i c a b l e i n d i s c u s s i n g t r a n s i t i o n s , and should be w r i t t e n f o r the case of NF^ as f o l l o w s : * - > = 3 P H . * E MrOF +/\„(iu (39> where and ( M i ) N are components of the n u c l e a r spins of f l u o r i n e and n i t r o g e n n u c l e i ; AF and AN are the i s o t r o p i c h f s p l i t t i n g constants of f l u o r i n e and n i t r o g e n r e s p e c t i v e l y . Now i n N P g there are two f l u o r i n e - 19 atoms of n u c l e a r s p i n |r and one n i t r o g e n - l l f atom of n u c l e a r s p i n 1. Thus both and N °an have v a l u e s 1 , 0 , - 1 and equation (39) p r e d i c t s n i n e t r a n s i t i o n s . I t i n d i c a t e s a l s o t h a t the three n i t r o g e n peaks should be superimposed on each of the three due to f l u o r i n e or v i c e v e r s a . The h i g h i n t e n s i t y o f the centre t r i p l e t compared w i t h the other two i n d i c a t e s t h a t the former i s the case f o r the NPg r a d i c a l . Now i f the a n i s o t r o p i c p a r t of the h f s p l i t t i n g were to have an i n f l u e n c e the outer t r i p l e t s should show e f f e c t s of both the f l u o r i n e and n i t r o g e n a n i s o t r o p y , whereas the main one should show onl y those due to n i t r o g e n a n i s o t r o p y . F u r t h e r -more f o r the centre l i n e of the main t r i p l e t b o t h and (^Ai should be zero, so that the h f i n t e r a c t i o n should have no e f f e c t on i t whatsoever. 49 These i n f e r e n c e s appear to be j u s t i f i e d i n the s p e c t r a . o o b t a i n e d a t lx.,2 K. The two outer peaks of the t r i p l e t have been observed to be of g r e a t e r width and s m a l l e r amplitude than the centre l i n e , c o n s i s t e n t with the s u g g e s t i o n that the outer peaks can be i n f l u e n c e d by a n i s o t r o p y of the n i t r o g e n h f i n t e r a c t i o n whereas the centre one cannot. Furthermore the narrowness of the centre l i n e tends to d i s c o u n t other forms of broadening. The f a c t t h a t on warmup the l i n e widths of the outer peaks decreased whereas t h e i r amplitudes i n c r e a s e d adds f u r t h e r impetus to the s u g g e s t i o n . I f one c a r r i e s the i n f e r e n c e f u r t h e r i t appears t h a t the a n i s o t r o p i c p a r t of the f l u o r i n e h f i n t e r a c t i o n i s very l a r g e , so l a r g e i n f a c t t h a t i t can . o broaden the i s o t r o p i c p a r t beyond d e t e c t a b i l i t y at.4.2 K. On warmup, however, I t i s averaged out. I t may be p o i n t e d out i n p a s s i n g that Gordy e t a l . ( 5 8 ) ( 5 9 )obtained s t r u c t u r e i n a s i g n a l a t room temperature from X - i r r a d i a t e d t e f l o n which they assi g n e d to f l u o r i n e h f i n t e r a c t i o n s ; t h i s s t r u c t u r e was broadened c o n s i d e r a b l y at 90°K, a phenomenon r a t h e r s i m i l a r that observed here f o r NF . 2 The s t r u c t u r e of the c e n t r e peak found f o r NF trapped i n argon may be r e l a t e d to a n i s o t r o p y i n the g - f a c t o r tensor. Kneubuhl (15) has shown how the l i n e shape of an ESR s i g n a l of randomly o r i e n t e d r a d i c a l s i n which there i s . n o h f s p l i t t i n g can be r e l a t e d to the p r i n c i p a l v a l u e s of the g - f a c t o r tensor. A d r i a n et a l . have used h i s c a l c u l a t i o n s to determine these v a l u e s f o r N 0 2 (11) and DCO (10 ) . Now the l i n e shape found f o r NF_ was somewhat d i f f e r e n t from those shown by Kneubuhl and by 5 0 . A d r i a n , but the s t r u c t u r e may be s i m i l a r l y r e l a t e d to the g -f a c t o r . Some i n t e r e s t i n g comparisons can be made between the s p e c t r a of NO^ and NF^. The ESR spectrum of trapped N0^ has very d e f i n i t e s t r u c t u r e which can be r e l a t e d to the g - f a c t o r and h f a n i s o t r o p i e s ( 1 1 ) . In argon the spectrum shows N0 2 to be a x i a l l y symmetric, although i t heed not n e c e s s a r i l y be so, and A d r i a n p o s t u l a t e d s l i g h t h i n d e r e d r o t a t i o n of the NO^ , to account f o r t h i s . An attempt to do a s i m i l a r treatment f o r NF^ ended i n f a i l u r e because of the great width and smoothness of the outer peaks of the t r i p l e t . Indeed, the smoothness of the l i n e s suggests slow, r a t h e r i s o t r o p i c r o t a t i o n of the r a d i c a l s , and the ease w i t h which the peaks sharpened on warmup tends to corr o b o r a t e t h i s i d e a . I t i s a l s o I n t e r e s t i n g ' t h a t on warmup the h f a n i s o t r o p y f a i l e d to be averaged out f o r NO^ but was appa r e n t l y removed f o r NF . Now removal of the h f a n i s o t r o p y r e q u i r e s a tumbling 2 Q frequency « // , D b e i n g the a n i s o t r o p i c h f s p l i t t i n g constant, F o r n i t r o g e n In NO^ ^/fo i s 211 3 m c . / s e c , so that d u r i n g warm-up NO^ never underwent tumbling motion of frequency g r e a t e r than t h i s . On the other hand f o r f l u o r i n e i n NF^ must be much g r e a t e r than t h i s , i n order to broaden the i s o t r o p i c p a r t beyond d e t e c t a b i l i t y , y e t d u r i n g warmup i t was averaged out q u i c k l y i n both argon and kr y p t o n . A clue to the causes of these i n t e r e s t i n g phenomena may be i n the shapes and s i z e s of the two r a d i c a l s . NO^ i s somewhat more elongated and more bulky than N F ^ j f o r i t s bond angle i s 51. 13k° with N - O l e n g t h of 1.197 2, whereas the bond angle i n NF^ i s 10i)..2O i f the N - P d i s t a n c e i s 1.37 A (6o). Using the van der Waals r a d i i f o r N, P and 0 of 1 .5, 1.35 and l.lj . 0 X r e s p e c t i v e l y (6l) one obtains approximate i s o t r o p i c van der o Waals r a d i i of 2.7 and 2.1}. A f o r NOg and NPg r e s p e c t i v e l y . These f i g u r e s may have some b e a r i n g on the apparent ease of NP to undergo i s o t r o p i c motion i n comparison with the 2 d i f f i c u l t y of NO . The two outer peaks i n the M/R = 300 s p e c t r a at if. 2 K are noteworthy i n t h a t they a p p a r e n t l y d i d not appear at h i g h e r matrix r a t i o s and t h a t they d i d not c r o s s the b a s e l i n e . I t i s very p o s s i b l e t h a t they were due to broadening of the main t r i p l e t and may have been a s i g n of peaks found d u r i n g warmup. A r a t h e r c u r i o u s s i m i l a r i t y i s found i n comparing the centre t r i p l e t of NP with the spectrum of trapped CN r a d i c a l s 2 (12) . Both showed a t r i p l e t , with the centre l i n e of very much g r e a t e r amplitude than the other two, and w i t h the outer l i n e s broad but smooth. Furthermore, i n both the outer l i n e s narrowed and i n c r e a s e d i n amplitude during warmup. The l i n e s e p a r a t i o n s were d i f f e r e n t , of course, but the s i m i l a r i t y of the s p e c t r a i s remarkable. The s p e c t r a obtained i n carbon t e t r a c h l o r i d e were very strange. This was one of the f i r s t times that t h i s m a t r i x had been used at lf . 2°K. Because of i t s h i g h m e l t i n g p o i n t a lengthy study of the warmup s p e c t r a found f o r the i n e r t gases was hoped f o r , but t h i s was not to be. The small i n t e n s i t y o f the outer peaks of the main t r i p l e t , along with the absence of the outer t r i p l e t s d u r i n g warmup suggests l e s s r o t a t i o n a l freedom 5 2 . f o r the r a d i c a l s here than i n other m a t r i c e s . The f r e e z i n g p o i n t of carbon t e t r a c h l o r i d e i s much h i g h e r than those of argon and krypton, and i t i s consequently more r i g i d than these at 2°K. This was p r o b a b l y the main cause of the observed broadening, although i t should be mentioned t h a t enough motion of the m a t r i x was found to allow d i f f u s i o n and hence recombination of the r a d i c a l s . L e t us now compare the i s o t r o p i c spectrum of NF^ with the spectrum of NH ( 3 ) . For NP the observed s p l i t t i n g s of 2 2 " , 1 7 - 1 gauss and 60 ± 2 gauss f o r the n i t r o g e n and f l u o r i n e h f i n t e r a c t i o n s correspond to i s o t r o p i c h f onstants of I4.8 i 3 and l 6 8 * 6 mc./sec. r e s p e c t i v e l y . In NH^ these are 2 8 . 9 and 67.O mc./sec. f o r n i t r o g e n and hydrogen r e s p e c t i v e l y . Wow the i s o t r o p i c h f s p l i t t i n g constant f o r nucleus j , a f t e r i n t e g r a t i o n over the s p a t i a l p a r t of the wave f u n c t i o n , i s given by where <Jj and ^ x a r e the n u c l e a r g - f a c t o r of nucleus j and the n u c l e a r magneton r e s p e c t i v e l y , and i s the value of the unpaired e l e c t r o n wave f u n c t i o n a t nucleus j . The n u c l e a r g - f a c t o r s f o r f l u o r i n e - 1 9 and hydrogen are approximately equal, b e i n g 5 . 2 5 and 5 . 6 r e s p e c t i v e l y , and with the other f a c t o r s i n Aj being numerical constants any d i f f e r e n c e s i n the i s o t r o p i c h f constants must a r i s e mainly from the I 4^ (°)| term. Now the u n p a i r e d e l e c t r o n s i n both NH^ and NP^ are both expected to be i n o r b i t a l s having the molecular plane as a nodal plane ( 2 0 ) . Since A; would be zero i f the e l e c t r o n were p u r e l y i n such an 5 3 . o r b i t a l , the true o r b i t a l must have some s-c h a r a c t e r to g i v e a non - zero constant. This i s u s u a l l y e x p l a i n e d by admixing the ground s t a t e by c o n f i g u r a t i o n i n t e r a c t i o n w i t h an e x c i t e d s t a t e i n which the unpaired e l e c t r o n i s i n an o r b i t a l with some s - c h a r a c t e r . I s o t r o p i c h f i n t e r a c t i o n due to the hydrogen n u c l e i i n NHg r e s u l t s from the u n p a i r e d e l e c t r o n being i n an o r b i t a l w i t h some hydrogen I s - c h a r a c t e r . I f t h i s o r b i t a l were p u r e l y the Is of hydrogen the h f constant would be ll±20 mc./sec. I t i s a c t u a l l y 67 mc./sec. Thus, i n the molecular wave f u n c t i o n the degree of l s - c h a r a c t e r from each hydrogen atom may be con s i d e r e d to be if.7 p e r c e n t . On the other hand the f l u o r i n e atoms i n NF^ have a 2s o r b i t a l a v a i l a b l e and s i n c e e x c i t e d s t a t e s w i t h the u n p a i r e d e l e c t r o n i n the Is s t a t e would be of much h i g h e r energy than those w i t h i t i n the 2s o r b i t a l c o n f i g u r a t i o n i n t e r a c t i o n s w i l l be assumed to i n v o l v e e x c i t e d s t a t e s with the unpaired e l e c t r o n i n the l a t t e r . Using the s e l f - c o n s i s t e n t f i e l d wave f u n c t i o n s of Brown (62) f o r the f l u o r i n e atom one f i n d s t h a t 68 x 10 2^" cm."^ f o r the f l u o r i n e 2s s t a t e , so t h a t i f the e l e c t r o n were p u r e l y here the s p l i t t i n g constant would be l f 2 , 5 0 0 mc./sec. As i t i s a c t u a l l y l 6 8 mc./sec. the molecular wave f u n c t i o n may be s a i d to have f l u o r i n e 2 s - c h a r a c t e r to the extent of O.if p e r c e n t from each f l u o r i n e atom. F o r n i t r o g e n , which a l s o has a 2s o r b i t a l a v a i l a b l e , the c o n f i g u r a t i o n i n t e r a c t i o n i s again most l i k e l y to i n v o l v e e x c i t e d s t a t e s with the unp a i r e d e l e c t r o n here. From the s e l f - c o n s i s t e n t f i e l d c a l c u l a t i o n o f Hartree and Ha r t r e e (63) the value of f o r the 2s o r b i t a l of atomic n i t r o g e n may be estimated as 33 x 10 2^" cm."^ Since the g - f a c t o r of n i t r o g e n - l i f i s O.ifO 54-t h i s g i v e s a s p l i t t i n g constant of l$lx.O mc./sec. For NH^ and NF^ the constants are 2 8 . 9 and I4.8 mc./sec. g i v i n g the molecular wave f u n c t i o n s 1 . 9 and 3.1 p e r c e n t n i t r o g e n 2 s - c h a r a c t e r r e s p e c t i v e l y . 5-5 A Gas Phase Study of the NF 2 R a d i c a l Although P i e t t e et a l . (54) found a gas phase spectrum of NF^ the a b s o r p t i o n was very broad, and showed no s i g n of - s t r u c t u r e , i n s p i t e of the f a c t t h a t the u n p a i r e d e l e c t r o n 'spin should couple w i t h the spins of both the n i t r o g e n and f l u o r i n e n u c l e i and a l s o w i t h the r o t a t i o n a l angular momentum of the r a d i c a l . The l a c k was due to c o l l i s i o n a l e f f e c t s at the pressure used, and i t was f e l t t h a t the s t r u c t u r e should be observable at a lower p r e s s u r e . P r e v i o u s work of Farmer i n t h i s l a b o r a t o r y had shown t h i s to be so f o r the case of NO^, and a c c o r d i n g l y an attempt was made to f i n d such s t r u c t u r e i n NF^. A i r f r e e samples had to be prepared very c a r e f u l l y , as the s l i g h t e s t t r a c e s of paramagnetic oxygen molecules can broaden the s t r u c t u r e of the ESR spectrum of the' NF^, p r e s e n t i n very low c o n c e n t r a t i o n s , beyond d e t e c t a b i l i t y . The s i l i c a sample tubes, and e s p e c i a l l y the c o n s t r i c t i o n where the tubes were sea l e d , were heated to 1+00 C and c o n t i n u o u s l y pumped f o r at l e a s t f o u r and u s u a l l y e i g h t hours to remove a l l t r a c e s of oxygen. A f t e r t h i s o p e r a t i o n the sample tube was f i l l e d with the N F - NF e q u i l i b r i u m mixture to the p r e s s u r e d e s i r e d 2 1}. 2 and the tube was then s e a l e d with a blowtorch. During these o p e r a t i o n s , a l l done i n a fume cupboard, the operator wore a face mask and heavy p r o t e c t i v e c l o t h i n g to guard a g a i n s t 5 5 . p o s s i b l e e x p l o s i o n s . A multipurpose ESR spectrometer with f i e l d modulation at 1 0 0 kc./sec. was used i n these s t u d i e s . The f i r s t sample i n v e s t i g a t e d was prepared i n 10 mm. t u b i n g with the N^ - NF^ e q u i l i b r i u m mixture a t 3 - 10 mm. Hg p r e s s u r e , and was run a t room temperature. There was no s i g n of the broad s i g n a l r e p o r t e d by P i e t t e e t a l . , and s i n c e he found h i s f i r s t s i g n a l w e l l above room temperature a l l f u r t h e r s t u d i e s were c a r r i e d out at temperatures above ambient. The V a r i a n V - V a r i a b l e Temperature A c c e s s o r y was used to r a i s e the temperature. N i t r o g e n from a c y l i n d e r was passed through a c a l c i u m c h l o r i d e d r y i n g tube, heated by a c o i l and passed through a dewar i n t o the c a v i t y . The temperature was monitored with a copper -constantan thermocouple at the entrance to the c a v i t y . The sample tubes used were if mm. s i l i c a . The r e s u l t s of the h i g h temperature experiments a t v a r i o u s p r e s s u r e s are shown i n Table V. I t i s apparent t h a t no s t r u c t u r e was found a t the p r e s s u r e s used and t h a t the s i g n a l would be l o s t i n the n o i s e at any p r e s s u r e s lower than 3 mm. Hg. Probably there were simply not enough r a d i c a l s p r e s e n t i n the tube. Now an e q u i l i b r i u m c a l c u l a t i o n a t 3 0 0°C i n d i c a t e s t h a t t e t r a f l u o r o h y d r a z i n e i s almost completely d i s s o c i a t e d a t that temperature, so an e x p l a n a t i o n of the weak s i g n a l must be found elsewhere. I t i s l i k e l y t h a t at the h i g h temperature used the t e t r a f l u o r o h y d r a z i n e was a t t a c k i n g the s i l i c a tube, e s p e c i a l l y i f there were any very s l i g h t t r a c e s of water or p o s s i b l y 56. TABLE V R e s u l t s o f h i g h temperature e x p e r i m e n t s i n v o l v i n g the NF r a d i c a l i n the gas phase. P r e s s u r e a t Room Temperature R e s u l t 15 cm. Hg n i l 10 mm. Hg Weak s i g n a l , 34-68 gauss wide superimposed on the g e n e r a l d r i f t , about g = 2.017±-.-001. I n t e n s i t y , i n c r e a s e d f rom l67° to 207°G. No s t r u c t u r e . 3mm. Hg Same s i g n a l , much weaker - n e a r l y l o s t i n n o i s e , even a t temperatures up to 0 300 C. No s t r u c t u r e . 5 7 . hydrogen f l u o r i d e p r e s e n t . Water c o u l d e a s i l y have been there, s i n c e the t e t r a f l u o r o h y d r a z i n e was s t o r e d i n a g l a s s trap i n vacuo a t room temperature p r i o r to use. At the h i g h temperatures i n the sample tube the t e t r a f l u o r o h y d r a z i n e was p r o b a b l y d i s a p p e a r i n g and being r e p l a c e d by s i P||. . I f t h i s were the case then the s m a l l s i g n a l was probably due to l a c k of NP r a d i c a l s i n the sample tube. 0 \ 5 8 . CHAPTER SIX THE TRIFLUOROMETHYL RADICAL 6-1 I n t r o d u c t i o n Attempts were made to t r a p the t r i f l u o r o m e t h y l r a d i c a l o (CF^) i n an i n e r t m a t r i x at 1+.2 K. The method used was to i r r a d i a t e CF^I trapped i n the s o l i d with the hope t h a t the C-I bond would break i n s i m i l a r f a s h i o n to the same bond i n methyl i o d i d e . That t h i s bond should break on i r r a d i a t i o n was suggested by p h o t o l y s i s (6J4.) and p y r o l y s i s (65) experiments w i t h gaseous CF I. 3 Previous attempts to trap the CF^ r a d i c a l have been made. F l o r i n e t a l . (66) c a r r i e d out ESR s t u d i e s of gamma i r r a d i a t e d o s o l i d CF. . At 77 K they found a f o u r l i n e s i g n a l very s i m i l a r k • 0 to t h a t of the methyl r a d i c a l , while at 24..2 K t h e i r spectrum was so complicated t h a t i t d e f i e d i n t e r p r e t a t i o n . Mastrangelo ( 6 7 )-trapped the products of a microwave discharge i n C^F^ a t 77°K and used chemical data to i d e n t i f y one of the d e p o s i t i o n products as the CF^ r a d i c a l . 3 6-2 Experimental Method Dr. W. R. C u l l e n k i n d l y s u p p l i e d the CF^I used i n these experiments. The very s m a l l q u a n t i t i e s of i m p u r i t i e s were removed i n a Perkin-Elmer Vapor Fractometer, and a i r was removed by d i s t i l l a t i o n ±a vacuo. Matheson Research Grade argon and krypton, as w e l l as F i s h e r S p e c t r o a n a l y s e d carbon t e t r a c h l o r i d e , were* the m a t r i c e s , and the mole r a t i o s (M/R) were 1 0 0 , 1 0 0 , and 60 - 75 r e s p e c t i v e l y . M i x i n g and sample d e p o s i t i o n were c a r r i e d out i n the u s u a l way. The H85A3/UV lamp was found 59-to be the most s u c c e s s f u l In p r e p a r i n g the r a d i c a l s . The f i e l d r e g i o n of g = 2 was scanned, and microwave power l e v e l s of about 1 - 1 0 mw. were used. 6 - 3 R e s u l t s and D i s c u s s i o n Only weak signs of a s i g n a l were ob t a i n e d when CP I 3 trapped i n argon was i r r a d i a t e d . However i n krypton and carbon t e t r a c h l o r i d e s p e c t r a were o b t a i n e d e a s i l y and are shown i n F i g u r e s 18 and 19. T h e i r most s t r i k i n g f e a t u r e s were the g r e a t o v e r a l l width and complexity, and a l s o the presence of l i n e s i n which there was a p p a r e n t l y a phase r e v e r s a l ( i . e . they appeared as though they were due to emission r a t h e r than a b s o r p t i o n ) . The cause of the r e v e r s e d peaks i s unknown. P o s s i b l y they were r e l a t e d i n some way to the s p i n - l a t t i c e r e l a x a t i o n time T^ (the time i n which an i n i t i a l excess of energy g i v e n to the spins w i l l f a l l to 1 / e of i t s v a l u e ) , perhaps i n a s i m i l a r f a s h i o n to the peaks of atomic hydrogen found by Jen et a l . ( 5 ) . They found the a b s o r p t i o n s i g n a l to have a component l a g g i n g o the modulation frequency by 90 » which they f e l t was due to the - modulation frequency b e i n g of the order of magnitude of 1 / 1 How much i n f l u e n c e t h i s had i n the p r e s e n t case i s u n c e r t a i n at the time of w r i t i n g . However one f u r t h e r p o i n t should be made. When carbon t e t r a c h l o r i d e was used as the m a t r i x there was a s m a l l p i e c e of d i p h e n y l p i c r y l h y d r a z y l (D.P.P.H.) on the ta r g e t which gave a superimposed s i g n a l . During warmup the phase of the. e n t i r e s i g n a l , except f o r the D.P.P.H. l i n e , was r e v e r s e d i n d i c a t i n g t h at e i t h e r the motion of the m a t r i x or the temperature had a very profound i n f l u e n c e on the spectrum. Figure 18. ESR Spect rum Obtained on Irradiation of CF^I in Krypton (M/R=100) at 4.2°K. G denotes gauss. Fig.19. ESR Spectra Obtained on Irradiation of CF 3I in CCl 4 (M/R^60-75) The line marked " D " is due to D.PRH. G= gauss. 6o. In l a t e r attempts to reproduce the r e s u l t s d i f f i c u l t y was encountered i n o b t a i n i n g a s i g n a l . Complete assignments of the l i n e s must await an e x p l a n a t i o n of the phase r e v e r s a l . I n s p e c t i o n of the spectrum r e v e a l e d l i t t l e apparent system (expected were f o u r l i n e s of r a t h e r the same p a t t e r n as that f o r the methyl r a d i c a l ) . I t may p o s s i b l y be that more than one r a d i c a l was i n v o l v e d , and i f they had d i f f e r e n t r e l a x a t i o n times then one may be able to account f o r the phase s h i f t . S u f f i c e i t to say, however, that the great o v e r a l l spread of 288 gauss was c o n s i s t e n t w i t h the l a r g e spreads found f o r other f l u o r i n e compounds ( 5 8 ) ( 5 9 ) (see a l s o chapter 5*) I t may a l s o be mentioned i n p a s s i n g that a sample of 15 percent CP I i n carbon t e t r a c h l o r i d e was i r r a d i a t e d at 77°K 3 with the A-H6 lamp. The sample was h e l d i n a p i e c e of i+mm. s i l i c a t u b i n g and was c o o l e d i n a V a r i a n V-lx5l\b L i q u i d N i t r o g e n Accessory. A s i n g l e ESR l i n e was observed, of peak-to-peak width 2 3 . 0 ± 0 . 5 gauss at g = 2 . 0 0 9 ± . 0 0 1 . The s i g n a l disappeared on warmup, and the p u r p l e c o l o u r of molecular i o d i n e was observed i n the s o l u t i o n a t room temperature. This r e s u l t was q u i t e i ° d i f f e r e n t from t h a t o b tained a t 4 . 2 K, and reemphasized the need f o r f u r t h e r work on the system. 6 1 . CHAPTER SEVEN  CONCLUSIONS The scope of the experiments j u s t d e s c r i b e d seems to be r a t h e r l i m i t e d . The reason f o r t h i s statement i s t h a t the technique appears to be r e s t r i c t e d to the study of sma l l f r e e r a d i c a l s , s i n c e i n l a r g e r ones a n i s o t r o p i c h f broadening i s o f t e n so g r e a t as to make i n t e r p r e t a t i o n s of the s p e c t r a very d i f f i c u l t , i f not i m p o s s i b l e . Furthermore the l a r g e cage e f f e c t of the m a t r i x makes quantum y i e l d s f o r p h o t o l y s e s i n s i t u v e r y s m a l l . Since the supply of l i q u i d helium i s l i m i t e d i n each experiment s t u d i e s of most r a d i c a l s have to be made at s i g n a l - t o - n o i s e r a t i o s r a t h e r lower than d e s i r a b l e . T h i s statement does not i n c l u d e two important types of experiments, f o r which f r u i t f u l s t u d i e s can be made, namely those i n v o l v i n g ready made r a d i c a l s l i k e NF^ which can be d e p o s i t e d from the gas phase and those i n v o l v i n g the p h o t o l y s i s of compounds l i k e diazomethane, i n which one of the fragments i s a s t a b l e molecule. A p o s s i b l e method of overcoming the quantum y i e l d d i f f i c u l t y may be to use h i g h energy r a d i a t i o n . The lamps used i n the experiments were medium and h i g h p r e s s u r e mercury a r c s , with the r e s u l t that h i g h energy resonance r a d i a t i o n was s e l f -absorbed. Use of a low p r e s s u r e resonance lamp would probably i n c r e a s e the quantum y i e l d . D e p o s i t i o n of r a d i c a l s prepared i n the gas phase (with the use of h i g h i n t e n s i t y u l t r a v i o l e t r a d i a t i o n or a microwave discharge) would a l s o overcome the quantum y i e l d d i f f i c u l t y f o r here the cage e f f e c t would be absent. However, f o r t h i s type of experiment the apparatus must be completely redesigned. 62. BIBLIOGRAPHY 1. G. C. Pimentel, Formation and Trapp i n g of Free R a d i c a l s , ed. A. M. B ass and H.P. Broida.New York, Academic Press, i 9 6 0 , chapter If.. 2. O.K. Jen, S.N. Foner, E.L. Cochran and V.A. Bowers, Phys. Rev., lpjj., dlih (1956). 3 . S.N. Foner, E.L. Cochran, V.A. Bowers and O.K. Jen, Phys. Rev. L e t t e r s , 1, 91 (1958). I 4 . . S.N. Foner, C.K. Jen, E.L. Cochran and V.A. Bowers, J.Chem. Phys., 28, 351 (1958). 5. C.K. Jen, S.N. Foner, E.L. Cochran and V.A. Bowers, Phys. 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