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Nuclear magnetic resonance investigations of organosilicon compounds Hunter, Brian K. 1966

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NUCLEAR MAGNETIC RESONANCE INVESTIGATIONS OF ORGANOSILICON COMPOUNDS by BRIAN K. HUNTER B . S c . ( h o n s . ) , U n i v e r s i t y o f B r i t i s h C o lumbia, 196^ A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n t h e Department o f C h e m i s t r y We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September, 1966 In p r e s e n t i n g 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 of the requirements for an advanced degree a t 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 f r e e l y a v a i l a b l e f o r reference and study, I f u r t h e r agree t h a t p e r m i s s i o n - f o r 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 or by h i s r e p r e s e n t a t i v e s . Tt i s understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a in s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of v. \iLiAAJL*i'f\r\j The U n i v e r s i t y o f " B r i t i s h Columbia Vancouver 8, Canada i ABSTRACT The n u c l e a r magnetic resonance s p e c t r a o f a number of o r g a n o s i l i e o n compounds a r e examined. The 7.95 Mcs. s i l i c o n -29 s p e c t r a a r e o b t a i n e d f o r t h e s e compounds and t h e c h e m i c a l s h i f t s a r e c o n s i d e r e d i n d e t a i l . I t i s proposed t h a t t h e t r e n d s o b s e r v e d a r e the r e s u l t o f a c o m p e t i t i o n between two o p p o s i n g e f f e c t s ; a p a r a m a g n e t i c s h i f t i n f l u e n c e d by changes i n t h e e f f e c t i v e n u c l e a r charge on s i l i c o n and a d i a m a g n e t i c s h i f t i n f l u e n c e d by (p—»d)TT bon d i n g . The concept o f t h e c o m p e t i t i v e s h i f t mechanism r a t i o n a l i z e s t h e c h e m i c a l s h i f t s o b s e r v e d f o r s e r i e s o f t h e t y p e M e x s i ^ L r _ x (where: X = OMe, OEt, OAc, C l , P and C^H^) but does n o t e x p l a i n t h e s h i f t s of s i l i c o n h y d r i d e s . T h i s concept a l s o r a t i o n a l i z e s t h e t r e n d s 119 o b s e r v e d i n 7 S n c h e m i c a l s h i f t s b u t , l o g i c a l l y , does n o t 1 a p p l y t o J C c h e m i c a l s h i f t s where the d - o r b i t a l c o n t r i b u t i o n must v a n i s h . None of t h e o b s e r v e d c o u p l i n g c o n s t a n t s c o r r e l a t e w i t h t h e ^ S i c h e m i c a l s h i f t s . A v a i l a b l e t h e o r i e s a r e found t o be i n a d e q u a t e t o d e s c r i b e t h e c o u p l i n g c o n s t a n t s i n t h e s e m o l e c u l e s . A new l o n g range c o u p l i n g i s o b s e r v e d between th e m e t h y l groups on s i l i c o n a t t a c h e d t o oxygen and c h l o r i n e w i t h J r .„„ o f about 0.35 c.p.s. i i TABLE OF CONTENTS Page INTRODUCTION 1 G e n e r a l Theory o f N.M.R. 5 C h e m i c a l S h i f t Theory 9 S p i n - S p i n C o u p l i n g Theory 12 (p —*d)TT Bonding i n O r g a n o s i l i e o n Compounds 1*+ N.M.R. o f O r g a n o s i l i e o n Systems 16 EXPERIMENTAL 20 N u c l e a r M a g n e t i c Resonance Measurements 20 Sample P r e p a r a t i o n 23 RESULTS 25 DISCUSSION 30 R e l a x a t i o n Times 30 A c c u r a c y o f S i l i c o n - 2 9 C h e m i c a l S h i f t Data 32 S i l i c o n - 2 9 C h e m i c a l S h i f t s 3*+ C o u p l i n g C o n s t a n t s *+5 BIBLIOGRAPHY hQ APPENDICES 52 S i l i c o n - 2 9 C h e m i c a l S h i f t s 52 Tln-119 C h e m i c a l S h i f t s 5^ Lead-207 C h e m i c a l S h i f t s 55 i i i LIST OF TABLES T a b l e Page 1. C a l c u l a t e d and Observed S i - X and Sn-X Bond Lengths 15 2. N.M.R. and Bond Length Data f o r S i l L ^ F ^ Compounds 17 3. S i g n s o f C o u p l i n g C o n s t a n t s i n O r g a n o s i l i c o n Systems 18 h. P r e p a r a t i o n o f F l u o r o s i l a n e s 2h 5. 1-*C and 2 9 S i R e l a x a t i o n Times 25 6. N.M.R. Data f o r M e x S i ( 0 M e ) ^ x 25 7. N.M.R. Data f o r M e ^ K O E t ) ^ 26 8. N.M.R. Data f o r M e x S l ( 0 A c ) i + _ x 26 9. N.M.R. Data f o r Me S i C l ^ . 27 10. N.M.R. Data f o r Me S i F l f _ x 28 11. N.M.R. Data f o r M e x S i H ^ _ x 28 12. N.M.R. Data f o r M E X S I ( C 6 H 5 ) h , _ x 2 7 13. M i s c e l l a n e o u s 2 ^ S i C h e m i c a l S h i f t s . 29 lk. P a r a m a g n e t i c S h i f t s and P e r c e n t a g e Observed 35 i v LIST OF FIGURES Figure Page 1 . The Laboratory Coordinate System f o r the N.M.R. Experiment 6 2. (p-*d)Tf Bonding Orbitals i n ( S i H ^ N 16 3 . 1 9 F Chemical Shifts f o r R xSiF l f_ < x Compounds 1 8 h, 1 3 C and 2 9 S i Chemical Shift s i n MeJM(OR)^ Compounds 1 9 5. (a) 2 9 S i Spectrum of Me^SiH with (MeO)^Si as Internal Standard 22 (b) 2 9 S i Spectrum of Me^SiH with (MeO)^Si as External Standard 22 6. Recorder Traces For Adiabatic Fast Passage T 1 Measurement Of (Me^Si) 20 23 7 . Plot Of In ( 1 + I t / I Q ) vs. t for 1 3 C and 2 9 S i Compounds 3 1 8. 2 9 S i Spectrum of C ^ S i h ^ 33 9 . 2 9 S i Chemical Sh i f t Of M e ^ K O M e ) ^ Series 3 ^ F i g u r e Page 10. 2 9 S i C h e m i c a l S h i f t o f Me S i X v s . D i f f e r e n c e i n E l e c t r o n e g a t i v i t y o f S i and X 36 11. 2 9 S i C h e m i c a l S h i f t o f M e x S i ( 0 M e ) ^ _ x i n Terms of ^<Tp and A <f y 38 12. C h e m i c a l S h i f t o f M e x S i C l l f - x 39 13. 1 1 9 S n C h e m i c a l S h i f t s f o r ( n - B u ) x S n C l l + _ _ x h2 J[k. 1 1 9 S n C h e m i c a l S h i f t s o f M e ^ n V i ^ h2 15. 1 3 C C h e m i c a l S h i f t s o f CH^X v s . ( x x - x c ) hh 16. H i g h F i e l d 1 3 C S a t e l l i t e s o f (a) M e 2 S i ( 0 M e ) 2 and (b) Me^SKOMe) h7 v i ACKNOWLEDGEMENTS I w i s h t o thank Dr. L. W. Reeves f o r h i s h e l p and encouragement d u r i n g t h e co u r s e o f t h i s i n v e s t i g a t i o n . A l s o , I would l i k e t o thank A. Lee Smith o f t h e Dow-Corning C o r p o r a t i o n who k i n d l y donated a number o f compounds used i n t h i s work. 1 INTRODUCTION Nuclear Magnetic Resonance (N.M.R.) has, i n the past two decades, become one of the most commonly applied spectroscopic techniques used by chemists. However, at present, chemists use only four n u c l e i on a routine basis; Hydrogen-1, Fluorine-19, Phosphorus-31, and Boron-11. Of the 130 "non-routine" n u c l e i , only one, Carbon-13? has received any large amount of attention. Studies of non-routine n u c l e i are' hampered by the low inherent nuclear s e n s i t i v i t i e s of most of these n u c l e i . From 1 2 a b r i e f consideration of the Bloch equations J one may calculate the r e l a t i v e N.M.R. signal strength f o r a given nucleus. This s e n s i t i v i t y i s usually expressed r e l a t i v e to protons. We s h a l l consider the slow passage absorption mode experiment usually 1 1 9 used f o r routine H and 7 F N.M.R. spectra. From the o r i g i n a l Bloch treatment of Magnetic Resonance, the voltage induced i n the receiver c o i l may be written, V= Kwtf, X" o induced voltage which i s out of phase with the applied radio frequency f i e l d a constant which depends upon the geometry of the apparatus angular frequency, i n radians sec."* 1, of the applied radio frequency f i e l d magnitude, i n gauss, of the radio frequency f i e l d Bloch magnetic s u s c e p t i b i l i t y where: V -K = CO = H, = X-2 a - 1 W . X z T . 7? ( M , -(2) where: = T , -T , = 3T T h e r e f o r e : V = Larmor p r e c e s s i o n f r e q u e n c y i n r a d i a n s secT e q u i l i b r i u m p a ramagnetic s u c e p t i b i l i t y . l o n g i t u d i n a l o r s p i n - l a t t i c e r e l a x a t i o n t i m e i n seconds. t r a n s v e r s e o r s p i n - s p i n r e l a x a t i o n time i n seconds. -1 -1 m a g n e t o g y r i c r a t i o i n r a d i a n s gauss s e c . 1 To (3) T h i s v o l t a g e r e a c h e s a maximum f o r two c o n d i t i o n s w h i c h may be r e a c h e d s i m u l t a n e o u s l y ; 1. A t r e s o n a n c e , (4) where: f-/^ = magnitude o f t h e a p p l i e d magnetic f i e l d i n gauss. 2. At b e g i n n i n g o f s a t u r a t i o n , 3 Thus, f o r maximum s i g n a l ; K y W, HO X> W'O T * 4 2. 4 F o r n o n - v i s c o u s l i q u i d s ; so t h a t , From t h e C u r i e Law: - A/ 2. 2. o where: 3kT ft1 ) r (fa) (7) A/ x k = number o f n u c l e i p e r c.c. = n u c l e a r s p i n = n u c l e a r magnetic moment i n n u c l e a r magnetons = Boltzmann's c o n s t a n t = 1 .38 X 1 0 7 gm.cm. 2sec."* 2deg." 1 "J*" = a b s o l u t e t e m p e r a t u r e i n degrees K e l v i n Thus; V = 3 k T I +1 d o ) F o r c o n s t a n t K, H D, and T, F o r c o n s t a n t K,oJ 0> and T, ( I X ) One may c a l c u l a t e t h e r e l a t i v e s i g n a l s t r e n g t h o f a g i v e n n u c l e u s u s i n g t h e above e q u a t i o n s and known magnetic 29 moments and i s o t o p i c abundances. F o r y S i i n T e t r a m e t h y l s i l a n e t h e s e n s i t i v i t y a t c o n s t a n t magnetic f i e l d i s -#. 15* 10"-^  and f o r 1 3 C i n Benzene i t i s 1.71+ V 1 0 _ 1 + . I n s p i t e o f t h e s e l o w s e n s i t i v i t i e s , t h e n u c l e a r magnetic r e s o n a n c e s o f many n o n - r o u t i n e n u c l e i have been o b s e r v e d . *+ ") 6 Three r e v i e w s o f Carbon -13 N.M.R. have appeared. Most o f the work on 1^C has i n v o l v e d t h e s t u d y o f c h e m i c a l s h i f t s i n p a r t i c u l a r s e r i e s o f o r g a n i c molecules. 1 F o r example, s u b s t i t u t e d 1 7 - 1 0 a r o m a t i c systems have r e c e i v e d c o n s i d e r a b l e a t t e n t i o n . ' C a r b o n y l compounds have a l s o • b e e n examine i n d e t a i l . 1 1 1 ^ The 1 3 c a r b o n y l c a r b o n i s i d e a l y s u i t e d t o J C N.M.R. i n v e s t i g a t i o n s i n t h a t i t u s u a l l y g i v e s s i n g l e t r e s o n a n c e s w h i c h a r e c h e m i c a l l y 1 3 s h i f t e d f r o m most o t h e r J C r e s o n a n c e s . 1 3 Most o f t h e J C work t o da t e has examined e l e c t r o n i c 13 e f f e c t s on s h i e l d i n g s i n r a t h e r c o m p l i c a t e d systems. However, 17 S p i e s e c k e and S c h n e i d e r ' have examined a number o f CH^X and CH-jCH^X compounds f o r e f f e c t s o f e l e c t r o n e g a t i v i t y and magnetic 1 1 " } a n i s o t r o p y o f s u b s t i t u e n t s upon th e H and J C c h e m i c a l s h i f t s . 18 1 ~\ I n a d d i t i o n , G r a n t and P a u l have r e p o r t e d t h e ->C c h e m i c a l s h i f t s f o r some 50 carbons i n a l k a n e systems u s i n g an improved 19 13 t e c h n i q u e . ' No at t e m p t s have been made t o c o r r e l a t e J C s h i f t s w i t h t h e remainder o f group IV because o f the l i m i t e d d a t a a v a i l a b l e . O t her n u c l e i i n group IV have been examined and some c h e m i c a l s h i f t s r e p o r t e d . Silicon-29 resonances have been examined i cr on 29 by L a u t e r b u r and co - w o r k e r s ^ ' and a l i s t o f 7 S i c h e m i c a l 21 s h i f t s has been c o m p i l e d . (See Appendix A.) Baker has observe d 2 9 S i s p e c t r a u s i n g t h e INDOR ( i n t e r n u c l e a r d o u b l e resonance) method but has n o t r e p o r t e d any c h e m i c a l s h i f t s . Germanium -73 i s n o t l i k e l y t o be t o o u s e f u l f o r N.M.R. c h e m i c a l s h i f t s t u d i e s because i t s l a r g e s p i n (9/2) and lo w m a g n e t o g y r i c r a t i o (2^ r a d i a n s gauss"" s e c . " f o r '-'Ge v s . 655 r a d i a n s gauss" -1 1 73 s e c . f o r H) make o b s e r v a t i o n o f ' JGe resona n c e s v e r y d i f f i c u l t . ' 2 2 Tin -119 has been o b s e r v e d by L a u t e r b u r and Burke and by Reeves <y\ oh. 119 and c o - w o r k e r s . J ' A t a b l e o f 7 S n c h e m i c a l s h i f t s appears i n ap p e n d i x B. Lead-207 has been obse r v e d by P i e t t e and Weaver y 26 207 and by Rocard and co-workers and a few 'Pb c h e m i c a l s h i f t s have been c o m p i l e d . (See Appendix C.) The n u c l e a r magnetic resonances o f many o t h e r n u c l e i 27 have been o b s e r v e d , i n c l u d i n g : ' 2 H 3 7 Ci 9 3Hb « C o « C u 127x 1 9 9 H g 2 7 A l 205 T 1 3*<a 8 1 B r G e n e r a l Theory Of N.M.R. The n u c l e a r magnetic resonance experiment i s b e s t d e s c r i b e d 2 ^ ' 2 9 by c o n s i d e r i n g a c o o r d i n a t e system as shown i n f i g u r e 1. The s t a t i c magnetic f i e l d , H Q, i s a p p l i e d i n t h e z d i r e c t i o n and the o s c i l l a t i n g r a d i o f r e q u e n c y f i e l d i s a p p l i e d a l o n g t h e x d i r e c t i o n . The r a d i o f r e q u e n c y ( r . f . ) f i e l d i s g i v e n by: 2H,C/0 = 2 Hi (to* co-t) (is) H o X Figure 1. The laboratory coordinate system f o r the N.M.R. experiment. The r . f . f i e l d may also be written as the sum of two vectors of magnitude, , rotating i n the xy plane with angular frequencies ± . We now consider a rotating reference frame, S', which rotates about the laboratory z axis with an angular frequency + <JL> . One r . f . component i s now fi x e d along the x 1 axis and the other rotates with an angular frequency of -2<*> and may be ignored. The ef f e c t i v e magnetic f i e l d , H f f., may be written: (14) where; 1 i f we write, = - n then, where: ( l b ) - s L The angle,©, between H Q and H e f f goes from 0 to TT depending on whether one i s sweeping the magnetic f i e l d from above or below resonance. The angle 0 may be calculated from: S U A © =• c o s e = In the rotating reference frame the magnetization precesses about H"ef£ with an angular v e l o c i t y , li*0 above resonance, 0 ^ 90° at resonance, e = 90 o below resonance, 0 > 90 I f H Q i s f a r above resonance, H-eff i s p r a c t i c a l l y p a r a l l e l to H . If we now sweep down through resonance, the magnetization 8 w h i c h was p a r a l l e l t o H Q, remains p a r a l l e l t o and f i n a l l y ends up a n t i p a r a l l e l t o H Q. D u r i n g the passage of resonance a m a g n e t i z a t i o n i s i n d u c e d i n t h e x ' y ! p l a n e w h i c h i n t u r n i n d u c e s a v o l t a g e i n t h e r e c e i v e r c o i l w h i c h i s a l i g n e d a l o n g t h e y a x i s . t i where: i B l o c h has shown t h a t t h i s v o l t a g e i s p r o p o r t i o n a l t o : $ = H - w / r H The s i g n o f the v o l t a g e depends upon whether H Q i s i n i t i a l l y g r e a t e r t h a n or l e s s t h a n . 29 S l i c h t e r ' has shown t h a t i f , t h e r e w i l l be a complete i n v e r s i o n o f m a g n e t i z a t i o n . T h i s r e p r e s e n t s an a d i a b a t i c passage. P h y s i c a l l y , the a d i a b a t i c passage o c c u r s i n such a way t h a t t h e a n g l e , & , between th e m a g n e t i z a t i o n and KQff remains c o n s t a n t . B e g i n n i n g t h e passage above r e s o n a n c e , 0 e q u a l s 0 ° and b e g i n n i n g t h e passage below resonance t h e a n g l e & e q u a l s 1 8 0 ° . F a s t passage r e q u i r e s t h a t the t i m e r e q u i r e d t o sweep t h r o u g h resonance be v e r y s h o r t w i t h r e s p e c t t o T^, t h e s p i n -l a t t i c e r e l a x a t i o n t i m e . T h i s c o n d i t i o n means t h a t v e r y few n u c l e i change energy l e v e l d u r i n g the passage of r e s o n a n c e . When T 1 i s l o n g , one can e a s i l y s a t i s f y t h e c o n d i t i o n s f o r a d i a b a t i c f a s t passage because one can sweep s l o w enough t o s a t i s f y the c o n d i t i o n , ^LMs j 2f f o r r e a s o n a b l e v a l u e s o f and s t i l l be sweeping f a s t w i t h r e s p e c t t o T^. I n t h e above d i s c u s s i o n i t has been assumed t h a t t h e system i s i n t h e r m a l e q u i l i b r i u m a t t h e b e g i n n i n g o f t h e sweep. 2. F o r many n u c l e i , i n c l u d i n g S i l i c o n - 2 9 , the s p i n - l a t t i c e r e l a x a t i o n t i m e , T.j, i s l o n g ; o f the o r d e r o f a m i n u t e . T h i s means t h a t one must w a i t s e v e r a l minutes between sweeps i n o r d e r t o a l l o w t h e system t o r e a c h t h e r m a l e q u i l i b r i u m . Now, c o n s i d e r what happens i f , i n s t e a d o f a l l o w i n g the system t o e q u i l i b r a t e a f t e r our i n i t i a l sweep, we sweep back t h r o u g h resonance s h o r t l y a f t e r our i i i n i t i a l passage of r e s o n a n c e . The i n i t i a l s i g n a l i s p r o p o r t i o n a l t o the e q u i l i b r i u m m a g n e t i z a t i o n , M . A f t e r t h e a d i a b a t i c p a s s a g e , t h e m a g n e t i z a t i o n i s i n v e r t e d t o -M . I f we now a l l o w t h e system t o r e l a x , t h e z component o f t h e m a g n e t i z a t i o n , M , i s g i v e n by: • _ _ \ M l - M , ( l - 2 e (22) The i n t e n s i t y o f the second s i g n a l i s p r o p o r t i o n a l t o -M . Thus, t Ar \ I i » _ H z = - (\-i<r h') so t h a t , / T J \ i A p l o t o f v s . t s h o u l d g i v e a s t r a i g h t l i n e w i t h s l o p e 1/T^ and i n t e r c e p t o f I n 2 . C h e m i c a l S h i f t Theory C h e m i c a l s h i f t s were f i r s t o b s e r v e d by K n i g h t 3 0 i n 19^9. S i n c e t h e n , t h e r e have been many at t e m p t s t o c a l c u l a t e c h e m i c a l s h i f t s . Most t h e o r i e s o f c h e m i c a l s h i f t a r e based upon m o d i f i c a t i o n s o f Ramsey's t h e o r y w h i c h was f i r s t p r oposed i n 1950-^ u s i n g p e r t u r b a t i o n t h e o r y . The c h e m i c a l s h i f t i s u s u a l l y c a l c u l a t e d as a s c r e e n i n g c o n s t a n t ; where: U c _ U 0 ( l - o r ) <*r> 10 where: |4t = r e s o n a n t f i e l d o f i ^ * 1 s p e c i e s U - a p p l i e d f i e l d (f- - s c r e e n i n g c o n s t a n t o f i s p e c i e s 32 ' P o p l e ^ has e x p r e s s e d t h i s s c r e e n i n g u s i n g t h e breakdown proposed by S a i k a and S l i c h t e r . ^ 3 where: (f^ - s c r e e n i n g c o n s t a n t f o r n u c l e u s A CJL* - d i a m a g n e t i c c o n t r i b u t i o n (Tp** = paramagnetic c o n t r i b u t i o n = c o n t r i b u t i o n f r o m c i r c u l a t i o n s on A olUoc o t h e r atoms C = c o n t r i b u t i o n f r o m i n t e r a t o m i c c i r c u l a t i o n s w h i c h cannot be l o c a l i z e d ^ ^ i s a l s o known as t h e Lamb^^ term and f o r randomly t u m b l i n g m o l e c u l e s i t may be c a l c u l a t e d as f o l l o w s : 3mc l J I where: f r = d i s t a n c e f r o m n u c l e u s ( r ) = e l e c t r o n d e n s i t y a t d i s t a n c e The d i a m a g n e t i c term depends on t h e e l e c t r o n d e n s i t y near t h e n u c l e u s and c o n s e q u e n t l y upon t h e e l e c t r o n e g a t i v i t y o f s u b s t i t u e n t s a t t a c h e d t o t h e n u c l e u s under c o n s i d e r a t i o n . The p a r a m a g n e t i c term,U p , r e s u l t s f r o m the m i x i n g o f the ground and e x c i t e d e l e c t r o n i c s t a t e s i n t h e a p p l i e d f i e l d . 1 1 The paramagnetic term i s l a r g e o n l y i f the e x c i t a t i o n i n v o l v e s t r a n s f e r of an e l e c t r o n between p or d - o r b i t a l s . ^ F o r hydrogen, t h e p c h a r a c t e r o f the hydrogen o r b i t a l s i s s m a l l 1 so t h a t t h e paramagnetic c o n t r i b u t i o n t o H c h e m i c a l s h i f t s i s n e g l i g i b l e . However, f o r most n u c l e i e x c e p t t h e l i g h t ones 7 9 such as ' L i and 'Be the paramagnetic term i s dominant. The i n t e r n u c l e a r c o n t r i b u t i o n , ( 7 " * B , depends upon t h e n a t u r e of B and a l s o t h e i n t e r n u c l e a r s e p a r a t i o n , r ^ , but s i n c e i t i n v o l v e s a 1 / r 3 term t h i s c o n t r i b u t i o n w i l l be s m a l l e x c e p t f o r 1H. The d e l o c a l i z e d c o n t r i b u t i o n , cr*'**6'06', ±s p r o b a b l y s i g n i f i c a n t o n l y f o r r i n g c u r r e n t s i n a r o m a t i c systems. The o n l y term we s h a l l c o n s i d e r w i l l be t h e paramagnetic p a r t s i n c e i t w i l l be most s e n s i t i v e t o changes i n c h e m i c a l environment.' S c h n e i d e r and Buckingham c a l c u l a t e d p a r a m a g n e t i c s h i e l d i n g s f o r mercury, t h a l l i u m , and l e a d u s i n g t h e f o l l o w i n g f o r m u l a : k L ( L - M ) CT P = ^ — — <v I r 12 T T * m 2 c 2 A E where: ^ 0 = ground s t a t e wave f u n c t i o n 7. = t h e sum o f the squares o f t h e t o t a l o r b i t a l a n g u l a r momenta o f t h e e l e c t r o n s w i t h o( and {3 s p i n s . A El = average e x c i t a t i o n energy, i n ergs S i n c e , -3 i s t he v a l u e of r -3 _ -r3 2_M a . , - 3 I™) ^1 LA^U+'A) f o r an e l e c t r o n which c o n t r i b u t e s t o t h e 12 o r b i t a l a n g u l a r momentum, where: Z r ~ = e f f e c t i v e n u c l e a r charge % - Bohr r a d i u s = 0 . 5 2 9 Angstroms n , l - = quantum numbers o f t h e c o n t r i b u t i n g o r b i t a l s S u b s t i t u t i n g , one o b t a i n s : ~ They compare t h e c a l c u l a t e d s h i f t s w i t h e x p e r i m e n t a l v a l u e s by c o n s i d e r i n g t h e pa r a m a g n e t i c s h i f t t o be z e r o f o r t h e 2+ 3+ 2+ s p h e r i c a l i o n s Hg , T 1 J , and Pb . T h e i r agreement i s r e m a r k a b l y good when one c o n s i d e r s t h e s i m p l i c i t y o f t h e i r t r e a t m e n t . A more r e c e n t t r e a t m e n t by Jameson and Gutowsky^ 7 based on v a l e n c e bond and L.C.A.O. c a l c u l a t i o n s has been a p p l i e d t o Xenon f l u o r i d e s . - 3 T h i s approach i s r a t h e r c o m p l i c a t e d and i s f a r t o o cumbersome t o be a p p l i e d i n most c a s e s . W i t h p r e s e n t t h e o r i e s one c a n , a t b e s t , q u a l i t a t i v e l y e x p l a i n o b s e r v e d c h e m i c a l s h i f t s f o r most n o n - r o u t i n e n u c l e i . S p i n - S p i n C o u p l i n g Theory Much of the f i n e s t r u c t u r e i n N.M.R. s p e c t r a i s a r e s u l t o f i n t e r a c t i o n s between t h e n u c l e a r s p i n s . S p i n c o u p l i n g a r i s e s f r o m t h e s m a l l magnetic f i e l d i n d u c e d a t a g i v e n n u c l e u s by o t h e r 1 n u c l e a r moments w i t h i n t h e m o l e c u l e . T h i s e f f e c t i s t r a n s m i t t e d v i a t h e bonding e l e c t r o n s and g e n e r a l l y a t t e n u a t e s r a p i d l y as t h e number o f i n t e r v e n i n g bonds i n c r e a s e s . The magnitude o f t h i s i n t e r a c t i o n , w h i c h i s indepen-. d e nt o f t h e a p p l i e d f i e l d , i s g i v e n by t h e c o u p l i n g c o n s t a n t , J , i n c y c l e s p e r second. 13 Most e a r l y t h e o r e t i c a l t r e a t m e n t s • o f c o u p l i n g con-s t a n t s i n v o l v e d p r o t o n / p r o t o n - c o u p l i n g s and cannot be a p p l i e d t o c o u p l i n g s between o t h e r n u c l e i . However, Juan and G u t o w s k y ^ 9 * ^ have c o n s i d e r e d J 13r-rr and J 29o,-u f r o m a Lti b i n ^ 2 v a l e n c e bond t r e a t m e n t . These, and o t h e r w o r kers ' , have hi assumed t h a t , i n Ramsey's J t r e a t m e n t o f s p i n c o u p l i n g , t h e F e r m i c o n t a c t term i s dominant. T h i s term may be w r i t t e n a s : ( S I ) where: $y = m a g n e t o g y r i c r a t i o o f X, H Afc = average e l e c t r o n i c e x c i t a t i o n energy \f\ = a n o r m a l i z a t i o n f a c t o r o/u. = s c h a r a c t e r o f o r b i t a l s u sed f o r bonding |v\S (^0)) [ = v a l u e o f s e l e c t r o n wave f u n c t i o n a t ' n u c l e u s hh Smith has assumed t h a t t h e J u a n and Gutowsky f o r m u l a t i o n a l s o a p p l i e s t o JY_CH* J = c o * s l o £ a.K (m) i n f o ) t ^ where: c/ y = s c h a r a c t e r o f X o r b i t a l C L X ( A S ) = h y p e r f i n e c o u p l i n g c o n s t a n t o f ns e l e c t r o n on X h y p e r f i n e c o u p l i n g c o n s t a n t o f 1 s e l e c t r o n on H A ^ = average e x c i t a t i o n energy o f e l e c t r o n on X OL 1^ We can see from the above two equations that changes i n coupling constants r e f l e c t either changes i n e x c i t a t i o n energy or h y b r i d i z a t i o n of the coupled n u c l e i . (p ->d) TT Bonding i n Organosilieon Compounds The p o s s i b i l i t y of the d - o r b i t a l s of s i l i c o n being used i n the formation of (p-*d)TT bonds has been, discussed at considerable length i n a number of a r t i c l e s . J The need to modify the simple cr bond picture becomes apparent when one compares calculated and experimental values of Si-X bond lengths. Schomaker and Stevenson^ have shown that one may lf7 calculate bond lengths from consideration of covalent r a d i i 1 ' 1+8 and Pauling e l e c t r o n e g a t i v i t i e s , where: dA 8 = A - B bond length fA , f$ = covalent r a d i i of A and B % p(g = Pauling e l e c t r o n e g a t i v i t i e s p = a constant = 0.09 Table 1 gives calculated and observed values of bond lengths f o r a number of Si-X and Sn-X bonds. It can be seen that while a correction f o r e l e c t r o -n e g a t i v i t y and hence i o n i c character gives some idea of bond length, there are s t i l l problems. For example, there i s a noticeable shortening of the Si-0 and Si-F bonds and the Si-H bond i s longer than the calculated value. The apparent lengthening of the Si-H bond i s probably not s i g n i f i c a n t because of the f a c t that both the covalent radius of hydrogen and i t s e l e c t r o n e g a t i v i t y are rather i n d e f i n i t e l y defined. For example i f one uses one-half the bond length i n the hydrogen molecule as the covalent radius one obtains a Si-H bond length of 1 . 5 0 Angstroms which i s too long. However, the 15 T a b l e - J C a l c u l a t e d And Observed S i - X And Sn-X Bond Lengths Bond . c a l c j • obs. (»*9) Compound S i - H 1 A 2 A 1 A 8 A S i - C 1 .88 1 .89 S i - 0 1 .76 1.6W S i - P 1 .69 1 .56 S l - C l 2 . 0 5 2.01 S i - B r 2 . 2 2 2 . 1 5 S i - I 2.kk 2 A 6 Sn-H 1 .66 1 .70 Sn-C 2.11 2 . 1 8 Sn-F 1 .92 i o n i c S n - C l 2 . 2 8 2.31 Sn-Br 2M 2.kh S n l l 2 . 6 7 2.6h S i ( C H 3 ) ^ Si(OMe) SiF S i C l ; S i B r h CH^SnH^ S n C C H ^ SnF^. S n C l ^ SnBr^ s h o r t e n i n g o f the S i - 0 and S i - F bonds must be a c c o u n t e d f o r i n some way. h e r cv\ _ cro A number o f workers y j have d e s c r i b e d t h e r o l e and e f f e c t s o f t h e d - o r b i t a l s upon t h e bonding and c h e m i s t r y o f s i l i c o n . The most o b v i o u s case r e q u i r i n g t h e use of some h i g h e r h y b r i d i z a t i o n beyond 3 s p ^ on s i l i c o n i s t h e h e x a f l u o r o -2-s i l i c a t e i o n S1F7 . T h i s has been shown^ t o be o c t a h e d r a l : 3 2 2-p r o b a b l y u s i n g sp d h y b r i d i z e d s i l i c o n . W h i l e S i F ^ and t h e t r i m e t h y l a m i n e a dducts o f f l u o r o s i l a n e s - ^ ' - ^ do n o t r e q u i r e the use of (p-»d)TT b o n d i n g , t h e y do show t h a t t h e e n e r g i e s o f t h e d - o r b i t a l s a r e low enough, a t l e a s t i n t h e pr e s e n c e o f e l e c t r o n e g a t i v e g r o u p s , t o e n t e r i n t o 3 s p d t y p e h y b r i d s . T r i s - s i l y l a m i n e and t r i m e t h y l a m i n e show a number o f i n t e r e s t i n g c o n t r a s t s w h i c h p o i n t toward a (p-»d)"7T bonding arrangement. ( S i E % K N i s a much weaker e l e c t r o n donor t h a n 16 (CH^)^N and (SiH^)-^N i s p l a n a r ^ ' whereas (CH^)^N i s p y r a m i d a l . These r e s u l t s can b o t h be e x p l a i n e d on th e b a s i s o f the f o l l o w i n g b o n d i n g scheme: F i g u r e 2 (p —»d)TT Bonding O r b i t a l s I n ( S i H - ^ N p The n i t r o g e n i s sp h y b r i d i z e d w i t h t h e l o n e p a i r d e l o c a l i z e d f r o m t h e r e m a i n i n g p - o r b i t a l i n t o a d t y p e o r b i t a l on t h e s i l i c o n s . T h i s same t y p e o f d e r e a l i z a t i o n c o u l d a l s o be r e s p o n s i b l e f o r t h e l a r g e S i - O - S i bond a n g l e (130 ) 7 i n h e x a m e t h y l d i s i l o x a n e and d i s i l o x a n e . T h i s e f f e c t c o u l d a l s o be a r e s u l t o f s t e r i c r e p u l s i o n b u t i t i s n o t l i k e l y t h a t t h e e f f e c t would be so l a r g e i n t h e l i g h t o f t h e c o n s i d e r a b l e l o n e p a i r / l o n e p a i r r e p u l s i o n w h i c h would be i n t r o d u c e d . The bon d i n g may i n v o l v e sp -<f bonds on oxygen t o s i l i c o n w i t h t h e l o n e p a i r s i n t h e r e m a i n i n g p - o r b i t a l s b a c k - d o n a t i n g i n t o t h e s i l i c o n d - o r b i t a l s . N.M.R. Of O r g a n o s i l i e o n Systems The most s t r i k i n g o b s e r v a t i o n one can make about t h e N.M.R. s t u d i e s o f o r g a n o s i l i e o n systems i s t h a t i t i s v i r t u a l l y i m p o s s i b l e t o c o r r e l a t e t h e c h e m i c a l s h i f t s and c o u p l i n g JLZ c o n s t a n t s w i t h any parameter. F o r example, c o n s i d e r t h e d a t a i n t a b l e 2 on t h e s e r i e s , S i H F Y . w h i c h E b s w o r t h has ^ ^8^0 ' x H~x 7 c o m p i l e d . T h e p r o t o n s h i f t o f S i H ^ and t h e f l u o r i n e c h e m i c a l s h i f t o f S i F ^ a r e b o t h d i s t i n c t l y d i f f e r e n t f r o m t h e r e s t o f the s e r i e s . The S i - H c o u p l i n g c o n s t a n t s a r e f a i r l y r e g u l a r b u t t h e S i - F c o u p l i n g c o n s t a n t s a r e i r r e g u l a r i n t h a t a maximum o c c u r s f o r S i H 2 F 2 . The S i - H and S i - F bond l e n g t h s d e c r e a s e as one goes f r o m S i H ^ t o S i F ^ . T a b l e 2 N.M.R. And Bond Length Data F o r SiH^F), Compounds £ ( a ) £(b) T ( c ) T ( c ) T ( c ) ,(c) ,(d) H F °SiE d S i F HF a S i H a S i F S i H ^ 6.80 — 202.5 -- — l.h-77 A — S i H 3 F 5.2^ - 217 229.0 281 5^.8 1.1+73 1.59^  A S i H 2 F 2 5.29 151 282 297.8 60.5 1.^7 1.577 S i H F 3 5A9 109.5 381.7 27^ .8 96.3 1.^ 6 1.565 S i F ^ — 163.3 — 178(e) — — 1.5^  (a) i n T" u n i t s , r e f . 58 (b) i n p.p.m. fr o m C F C l ^ r e f . 58 ( c ) r e f . 59 (d) r e f . 53 (e) r e f . 60 S c h n e l l and Rochow have obse r v e d s i m i l a r e f f e c t s i n t h e 1 9 F s h i f t s i n t h e s e r i e s , R ^ - S i F ^ , w i t h R = Me o r E t and x = 0, 1, 2, or 3. (See F i g u r e 3.) These r e s u l t s have been p a r t i a l l y e x p l a i n e d on t h e b a s i s of changes i n ( p - * d ) T T b o n d i n g b u t no f i r m c o n c l u s i o n s have been r e a c h e d . Bruhe has examined t h e c h e m i c a l s h i f t s and c o u p l i n g c o n s t a n t s i n t h e s e r i e s , C l n ( C H 3 ) 3 _ n S i O R . He o b s e r v e s changes i n t h e e f f e c t i v e e l e c t r o n e g a t i v i t y o f t h e oxygen i n t h e s e m o l e c u l e s and a t t r i b u t e s t h e s e changes t o (p-*d)TT bonding f r o m c h l o r i n e and oxygen t o s i l i c o n . Changes i n c o u p l i n g c o n s t a n t a r e a t t r i b u t e d t o v a r i a t i o n s i n h y b r i d i z a t i o n t h r o u g h (p-*d)TT b o n d i n g . Schmidbaur has a l s o examined t h e c h e m i c a l s h i f t s and c o u p l i n g c o n s t a n t s and p o i n t s out how (p-»d)"TT bonding can p l a y an i m p o r t a n t p a r t i n changing b o t h 1 8 p.p. 1.00-150 -100 T? 3<^F -R^Fj F i g u r e 3 . 1 9 F C h e m i c a l S h i f t s F o r R v S i F k v Compounds. c h e m i c a l s h i f t s and c o u p l i n g c o n s t a n t s . Danyluk >^ h a s , by d o u b l e r e s o n a n c e , d e t e r m i n e d t h e s i g n s o f s e v e r a l c o u p l i n g c o n s t a n t s i n o r g a n o s i l i c o n systems. C o u p l i n g s and s i g n s a r e g i v e n i n t a b l e 3« The s i g n o f J 2 9 Q . ? U i s o p p o s i t e t o t h a t o f J 1 3 C H but s i n c e t h e m a g n e t o g y r i c r a t i o s o f ' S i and J C a r e o f o p p o s i t e s i g n , t h i s i s t o be e x p e c t e d . T a b l e 3 S i g n s Of C o u p l i n g C o n s t a n t s I n O r g a n o s i l i c o n Systems C o u p l i n g S i g n M o l e c u l e J 2 9 S i F J H C S i F J 2 9 S i H J H C S i H J 2 9 S i C C H ( c i s )  J 2 9 S i C C H ( t r a n s )  JHCH + (assumed) + (assumed) Me^SiF Me^SiF M e 2 S i H C l M e 2 S i H C l M e . S i 3 \ C = C H CI \ H 12 5 20 29 L a u t e r b u r ^ ' has measured the 7 S i c h e m i c a l s h i f t s o f a number of o r g a n o s i l i e o n compounds. He p o i n t s out a major -lo 29 d i f f e r e n c e between J C and 7 S i . s h i f t s f o r t h e s e r i e s Me^jMCOR)^ ; where M i s C or S i . (He does n o t s t a t e what R i s . ) (See F i g u r e h.) L a u t e r b u r e x p l a i n s t h i s d i f f e r e n c e on t h e b a s i s o f d - o r b i t a l p a r t i c i p a t i o n i n b a c k - c o o r d i n a t i o n . I n a d d i t i o n , p h e n y l and v i n y l groups s h i e l d s i l i c o n and d e s h i e l d c a r b o n , c o n s i s t e n t w i t h d o n a t i o n f r o m th e TT-bonds i n t o s i l i c o n d - o r b i t a l s . To d a t e , no a t t e m p t s have been made t o e x p l a i n the 29 d e t a i l s o f ' S i c h e m i c a l s h i f t s . F i g u r e h. 1 3 C and 2 9 S i C h e m i c a l S h i f t s I n M e ^ l ( O R ) ^ Compounds 20 EXPERIMENTAL N u c l e a r M a g n e t i c Resonance Measurements P r o t o n N.M.R. s p e c t r a were r u n a t 100 Mcs. on a V a r i a n A s s o c i a t e s HA-100 s p e c t r o m e t e r . Benzene ( d r y , r e d i s t i l l e d r e a g e n t grade) was used as an i n t e r n a l s t a n d a r d u n l e s s o t h e r w i s e n o t e d . However, a l l c h e m i c a l s h i f t s a r e r e p o r t e d i n p.p.m. fr o m t e t r a m e t h y l s i l a n e (T.M.S.); a n e g a t i v e v a l u e denotes a s h i f t t o low f i e l d . S p e c t r a were r u n on f i e l d sweep mode u s i n g t h e benzene s i g n a l t o p r o v i d e an i n t e r n a l f i e l d f r e q u e n c y l o c k . L i n e p o s i t i o n s were measured by r u n n i n g t h e s p e c t r a on 50 c.p.s. — 2 sweep w i d t h and 1000 second sweep time ( 1 .2 X 10 m i l l i g a u s s second ) and p l a c i n g f r e q u e n c y c a l i b r a t i o n markers on e i t h e r s i d e o f t h e peak. I n t h i s way, c o u p l i n g c o n s t a n t s may be measured t o ± 0 . 0 5 c.p.s. and c h e m i c a l s h i f t s t o b e t t e r t h a n ± 0.01 p.p.m. F l u o r i n e N.M.R. s p e c t r a were r u n on t h e V a r i a n A s s o c i a t e s HA-100 o p e r a t i n g on HR mode a t 9 .^1 Mcs. M o n o f l u o r o t r i c h l o r o -methane ( F r e o n - 1 1 ; Matheson Canada L i m i t e d ) was used as an i n t e r n a l s t a n d a r d . C o u p l i n g c o n s t a n t s and c h e m i c a l s h i f t s were measured u s i n g the a u d i o s i d e b a n d method. By t h i s method, c o u p l i n g c o n s t a n t s may be measured t o — 0 . 1 c.p.s. and c h e m i c a l s h i f t s t o ± 0 . 0 1 p.p.m;/ S i l i c o n - 2 9 N.M.R. s p e c t r a were r u n on a s p e c t r o m e t e r system assembled i n t h e department; l a r g e l y f r o m V a r i a n A s s o c i a t e s p a r t s . The system o p e r a t e s a t 7«95 Mcs. and 9«l+1 K i l o g a u s s ; t h i s f i e l d c o r r e s p o n d s t o ^ 0 . 0 Mcs. resonance f o r p r o t o n s . F i e l d sweep i s p r o v i d e d by a V a r i a n Model V3507 s l o w sweep u n i t and s p e c t r a a r e r e c o r d e d on a V a r i a n G-10 r e c o r d e r . Good b a s e l i n e s t a b i l i t y i s a c h i e v e d by u s i n g r e g u l a t e d power s u p p l i e s f o r t h e V^-311 r a d i o - f r e q u e n c y u n i t . B+ v o l t a g e (+300 v o l t s d.c.) i s 21 p r o v i d e d by a Lambda E l e c t r o n i c s Corp., Model 3 2 ( 1 $ r e g u l a t i o n ) power s u p p l y and t h e + 1 2 . 0 v o l t s d.c. f i l a m e n t v o l t a g e by a E l e c t r o n i c R e s e a r c h A s s o c i a t e s , I n c . , Model T.R. 32-*+ ( 0 . 0 1 $ r e g u l a t i o n ) power s u p p l y . A l l s p e c t r a a r e r u n a t h i g h power •i ( > 1 0 0 m i l l i g a u s s ) and r a p i d passage (A/1 gauss minute ) i n d i s p e r s i o n mode. The s i g n a l s p r o b a b l y c o n t a i n some a b s o r p t i o n mode b u t t h e r e i s v e r y l i t t l e a p p a r e n t d i s t o r t i o n o f t h e p e a k s , (See F i g u r e 5 » ) I t i s n e c e s s a r y t o w a i t a p p r o x i m a t e l y 5 minutes between sweeps because o f t h e l o n g r e l a x a t i o n t i m e s o f t h e S i l i c o n - 2 9 n u c l e i . Samples were r u n as n e a t l i q u i d s i n t h i n w a l l 10 mm. o.d. p y r e x tubes w i t h i n a c o n c e n t r i c 1 5 mm. o.d. p y r e x t h i n w a l l t e s t tube c o n t a i n i n g t h e e x t e r n a l s t a n d a r d ( u s u a l l y (MeCO^Si). A sample o f t r i m e t h y l s i l a n e , Me^SiH, was used as a s t a n d a r d f o r sweep c a l i b r a t i o n . The s i l i c o n - 2 9 spectrum g i v e s two l i n e s w i t h a s p a c i n g o f 18*+ c.p.s. (measured f r o m t h e p r o t o n spectrum) w h i c h c o r r e s p o n d s t o 2 3 p.p.m. a t 7 . 9 5 Mcs. F o r r o u t i n e measurements th e f o l l o w i n g p r o c e d u r e was u s e d . The s t a n d a r d sample was r u n on i n c r e a s i n g and d e c r e a s i n g sweep and t h e sample t o be measured was t h e n r u n a t t h e same sweep r a t e s e t t i n g a t l e a s t f o u r t i m e s . The s t a n d a r d sample was t h e n r u n a g a i n . By comparing t h e peak s e p a r a t i o n s i n t h e s t a n d a r d and t h e sample, t h e s i l i c o n - 2 9 s h i f t s may be measured t o ± 1 . 0 p.p.m. 29 The r e l a x a t i o n t i m e s o f t h e ^,'Si i n h e x a m e t h y l d i -s i l o x a n e , t e t r a m e t h o x y s i l a n e , and t e t r a m e t h y l s i l a n e were measured by t h e f o l l o w i n g p r o c e d u r e . The power l e v e l was i n c r e a s e d as h i g h as p o s s i b l e w i t h o u t changing probe b a l a n c e and t h e sweep r a t e i n c r e a s e d t o about 10 t i m e s t h e v a l u e used f o r r o u t i n e measurement. The f i e l d was t h e n swept t h r o u g h r e s o n a n c e . A f t e r a t i m e , t , t h e f i e l d was swept t h r o u g h r e s o n a n c e i n t h e o p p o s i t e d i r e c t i o n . These s i g n a l s were r e c o r d e d on a Sanborn Model 151 r e c o r d e r and one i s a b l e t o measure t h e change i n i n t e n s i t y f r o m I t o 1^. a t t i m e t . By w a i t i n g f o r s e v e r a l minutes a f t e r t h e second sweep, one can r e p e a t t h e p r o c e s s a g a i n f o r a d i f f e r e n t v a l u e o f t . Two 22 £ - — 7 9.5~ f>f>** F i g u r e 5b. ? 9 S i spectrum o f Me^Si w i t h (MeCO^Si as e x t e r n a l s t a n d a r d s u c h p a i r s o f s i g n a l s a r e shown i n f i g u r e 6. A p l o t o f I n (1 - I^/IQ) v s . t i s a s t r a i g h t l i n e w i t h s l o p e and i n t e r c e p t I n 2. Because o f t h e c o m p l i c a t e d r e l a x a t i o n p r o c e s s e s w h i c h may go on, t h i s method p r o b a b l y g i v e s o n l y an e s t i m a t e o f and s h o u l d n o t be used e x c e p t as an o r d e r o f magnitude v a l u e . F i g u r e 6 Recorder T r a c e s F o r A d i a b a t i c F a s t Passage T 1 Measurement Of (Me^SDpO Carbon - 1 3 s p e c t r a were r u n a t 7»95 Mcs. u s i n g t h e 29 same p r o c e d u r e as f o r 7 S i e x c e p t t h a t t h e magnetic f i e l d was red u c e d t o 7.^5 K i l o g a u s s . The samples were r u n as 60 volume p e r c e n t s o l u t i o n s i n CSg and a c o n v e n i e n t v a l u e o f J 13QJJ i n the sample was used f o r sweep c a l i b r a t i o n . I n t h e compounds o f i n t e r e s t , m e t h y l c h l o r o s i l a n e s , t h e J C re s o n a n c e s a r e q u a r t e t s 1 3 and a r e r a t h e r d i s t o r t e d . The a c c u r a c y o f t h e J C s h i f t s i s re d u c e d t o ±. 5 p.p.m. by t h i s d i s t o r t i o n . r e l a x a t i o n t i m e s were measured f o r Benzene, CS 9,.and t h e c a r b o n y l c a r b o n 29 i n acetone u s i n g t h e same p r o c e d u r e used f o r y S i r e l a x a t i o n t i m e s . Sample P r e p a r a t i o n i Many o r g a n o s i l i c o n compounds were p u r c h a s e d from 2it s e v e r a l s o u r c e s . (N.M.R. S p e c i a l t i e s Co.; P e n i n s u l a r Chem-R e s e a r c h Corp; A l d r i c h C h e m i c a l Co.) S e v e r a l samples were k i n d l y donated by the Dow C o r n i n g C o r p o r a t i o n . The purc h a s e d and donated samples were used w i t h o u t f u r t h e r p u r i f i c a t i o n . S e v e r a l samples were p r e p a r e d by s t a n d a r d t e c h n i q u e s ^ 7 as l i s t e d below. Standard vacuum t e c h n i q u e s were used where n e c e s s a r y . Samples were u s u a l l y p u r i f i e d by d i s t i l l a t i o n and p u r i t i e s checked by N.M.R. s p e c t r a r u n on a V a r i a n A-60. The s i l a n e s , ( C g H ^ S i H g , ( C g H ^ S i H ^ , S i H ^ , Me^SiH, M e 2 S i H 2 , and MeSiH^ were p r e p a r e d by r e d u c t i o n o f t h e c o r r e s p o n d i n g c h l o r i d e s w i t h L i A l H ^ under a d r y n i t r o g e n environment. The m e t h y l and p h e n y l f l u o r i d e s were p r e p a r e d from t h e c h l o r i d e s u s i n g v a r i o u s f l u o r i n a t i n g agents as shown i n t a b l e h. S i l i c o n T e t r a f l u o r i d e (SiF>,) was p r e p a r e d from t h e fiP> r e a c t i o n o f N a 2 S i F ^ and HgSO^ on s i l i c a - g e l . The p h e n y l m e t h y l s i l a n e s were p r e p a r e d f r o m P h e n y l l i t h i u m and t h e c o r r e s p o n d i n g m e t h y l c h l o r o s i l a n e s . T r i p h e n y l s i l a n e was p r e p a r e d from Phenyl'magnesium Bromide and T r i c h l o r o s i l a n e . T a b l e h P r e p a r a t i o n Of F l u o r o s i l a n e s Compound F l u o r i n a t i n g Agent M e 3 S i F A g F / E t 2 0 M e 2 S i F 2 P b F 2 / r e f l u x M e S i F 3 HF/EtpO ( l o w y i e l d ) 4> 2 S i F 2 H F / E t 2 0 21 RESULTS Table 5 1 3 C and 2 9 S i R e l a x a t i o n Times Compound T^  Benzene 1*+ sec, Acetone (carbonyl carbon) 19 CS 2 35 (MeO^Si 73 ( M e 3 S i ) 2 0 hO Me^Si 16 Table 6 N.M.R. Data For Me^Si(OMe)), ^ ™ " ^ ™ ~ ~ ^ ~ — " ' '" »I1.PI1I«III _X_ — ' ( " <f(a) r (b) r (b) -r O Q T oo T 1^ Cc) 6 S i 6 Me 6 OMe d ^ S i C H J ^SiOCH J 1 JCH p.p.m. p.p.m. p. p. r.. c.p.s. c.p.s. c.p.s. -79 .5 0.0 6.62 118.1 -96 .5 -0.21 - 3 A 9 6 .65 H - . 13 118.0 -77 -0.19 -3.55 7.2 3.90 118.5 -38 -0.12 -3.58 8.35 3.82 119.1 0 -3.60 3.55 Me^Si Me^SiOMe Me 2Si(0Me) 2 MeSKOMe)^ Si(OMe)^ (a) S i l i c o n - 2 9 chemical s h i f t i n p.p.m. from (MeOVSi (b) H chemical s h i f t i n p.p.m. from T.M.S. (c) J 1 3 C H i s f o r methyl group. 26 T a b l e 7 N.M.R. fData For, M e ^ S i C O E t ) l f - x { (a) 0 S i C (b) d Me J 2 9 S i C H J 1 3 C H p.p.m. p.p.m. c.p.s. c.p.s. - 7 9 . 5 -93 -72 -3h +k 0 . 0 - 0 . 2 3 - 0 . 2 0 - 0 . 1 6 6.62 6.67 7 .08 8 .35 118.1 118.0 118.2 119.2 Me^Si Me^SiOEt M e 2 S i ( 0 E t ) 2 M e S i ( O E t ) 3 S K O E t ) ^ (a) S i l i c o n - 2 9 c h e m i c a l s h i f t i n p.p.m. from (MeO vVSi (b) H c h e m i c a l s h i f t i n p.p.m. from T.M.S. T a b l e 8 N.M.R. Data F o r Me S i ( 0 A c ) ^ C (a) O S i C (b) 0 Me J 2 9 S i C H J 1 3 C H p.p.m. p.p.m. c.p.s. c.p.s. Me^Si - 7 9 . 5 0 6.62 118.1 Me^SiOAc M e 2 S i ( 0 A c ) 2 -103 - 0 . 3 6 7.10 119.5 - 8 5 - 0 . 5 3 7.77 121.3 M e S i ( O A C ) 3 -35 - 0 . 6 7 9 . 5 123.^ S K O A c ) ^ - 5 — — — (a) S i l i c o n - 2 9 c h e m i c a l s h i f t i n p.p.m. fr o m ( M e O ^ S i (b) 1 H c h e m i c a l s h i f t i n p.p.m. from T.M.S. 22 T a b l e 9 N.M.R. Data F o r Me S l C l ) f _ r (a) c (b) r (c) T O Q T 1 , 0 S i O C O Me J ^ V S i C H J 15CE p.p.m. p.p.m. p.p.m. c.p.s. c.p.s. Me^Si -79.5 190 0 6.62 118.1 M e 3 S i C l -115 176 -0.39 7.00 120.77 M e 2 S i C l 2 -120 190 -0A5 7.61 123.6 M e S i C l 3 -96 210 -1.05 9.07 125.5 S i C l ^ -63 — — — — (a) " S i c h e m i c a l s h i f t i n p.p.m. from (MeO)^Si (b) 1 3 C c h e m i c a l s h i f t i n p.p.m. f r o m C S 2 ( c ) 1 H s h i f t i n p.p.m. fr o m T.M.S. N.M.R. Data Fo r M e x S i ( C 6 H 5 V x _ C (a) d S i C (b) ° M e J 2 9 S i C H J 1 3 C H p .p.m. p.p.m. c.p.s. c.p.s. -79.5 -75 -72 -67 (c) 0 -0.26 -0.52 -0.7^ 6.62 6.60 6.60 6.6*t 118.1 119.0 120.5 (c) M e ^Si Me^Si 4> Me 2Si4> 2 MeSi4>3 (a) 2 9 S i c h e m i c a l s h i f t r e l a t i v e t o ( M e O V S i -i ^ (b) H c h e m i c a l s h i f t r e l a t i v e t o Me^Si (c) t o o i n s o l u b l e t o be measured T a b l e 10 N.M.R. Data F o r M e v S i F ^ _ v  S l ) <f Me^ ^ F C ) J 2 9 S i F J 2 9 S i C H J 1 3 C H J] p.p.m. p.p.m. p.p.m. c.p.s. c.p.s. c.p.s. c.p.s Me^Si - 7 9 . 5 0 6 . 6 2 118.1 M e 3 S i F - 1 1 0 - 0 . 1 9 158 276 7.*+0 1 2 0 . 0 7 . 2 3 M e 2 S i F 2 - 6 % - 2 8 130 7 . 6 0 - 6 . 2 5 M e S i F ^ (d) + 0 . 2 133 278 8 . 8 S i F ^ (d) 161 .8 178 — (a) 2 9 S i c h e m i c a l s h i f t r e l a t i v e t o (MeO)^Si (b) 1 H c h e m i c a l s h i f t f r o m T.M.S. ( O 1 9 F c h e m i c a l s h i f t from CFC1-, (d) t o o v o l a t i l e t o be measured T a b l e 11 N.M.R. Data For Me^SiH),, 6 S i p.p.m. X ^ 0 Me p.p.m. X ^ 6 H p.p.m. J S i H c.p.s. J 2 9 S i C H c.p.s. J 1 3 C H c.p.s. JHCS3H c.p.s. Me^Si Me^SiH M e 2 S i H 2 M e S i H 3 S i H ^ - 7 9 . 5 -61 - 3 8 (c) ( c ) 0 - 0 . 2 0 -0.26 - 0 . 2 7 20 -h.03 - 3 . 7 1 (c) 1 8 3 . 8 1 8 8 . 5 19^.6 2 0 2 . 5 6.62 7 . 2 0 7 . 5 7 . 9 118.1 1 1 9 . 3 5 1 2 0 . 8 1 2 3 . 6 3 . 7 6 >+.i5 h.67 (a) 2 9 S i c h e m i c a l s h i f t from ( M e O ^ S i (b) 1 H c h e m i c a l s h i f t f r o m T.M.S. (c) t o o v o l a t i l e t o be measured 22 Table 13 Miscellaneous 2 9 S i Chemical S h i f t s . Chemical S h i f t ^ Compound (p.p.m.) V i S i ( O E t ) 3 -15.5 C6H^SiR"3 -18 C 6H^Si ( 0 E t ) 3 -19 (MeO) 6Si 2 -27 ( C6 H 5 ) 2 S i H 2 ( C 6 H 5 ) 2 S i ( 0 E t ) 2 -»+5 ( C 6 H 5 ) 2 S i F 2 -^9 V i ^ S i -57 M e 6 S i 2 -59 Me(C 6H^) 2SiH -60 [(MeOjgMeSi] 2 -72 E t 2 S i F 2 -80 ClCH 2SiMe 3 -81.5 (Me 3Si ) 2 0 -83.5 Me 3Si — O —(^--1-8*. -92 (a) r e l a t i v e to (Mefc^Si 30 DISCUSSION R e l a x a t i o n t i m e s 13 29 The r e l a x a t i o n t i m e s measured f o r J C and 7 S i a r e c a l c u l a t e d f r om t h e s l o p e s o f the p l o t s o f I n (1 + v s . t 69 shown i n f i g u r e 7 and a r e l i s t e d i n t a b l e 5. M c C o n n e l l and Holm 7 1 3 have d i s c u s s e d J C | r e l a x a t i o n i n terms of a n i s o t r o p i c c h e m i c a l s h i e l d i n g . They e s t i m a t e t h e T^ o f C S 2 t o be about 60 seconds and " a p p a r e n t l y l o n g " f o r C C l ^ . I n most cases o b s e r v e d h e r e , t h e dominant f a c t o r c o n t r o l l i n g t h e magnitude of the r e l a x a t i o n t ime i s the d i p o l a r s p i n - s p i n c o u p l i n g . T h i s a r i s e s f r o m t h e magnetic f i e l d i n d u c e d a t one n u c l e u s by a n o t h e r . As the m o l e c u l e tumbles i n the a p p l i e d magnetic f i e l d , t h i s i n d u c e d magnetic f i e l d r e s u l t s i n a m o d u l a t i o n o f the magnetic f i e l d a t t h e n u c l e u s w h i c h may i n d u c e t r a n s i t i o n s i n t h e n u c l e a r energy l e v e l s . The e f f e c t upon th e r e l a x a t i o n t ime i s p r o p o r t i o n a l t o (jU,y*z) and r ^ 2 ; where r 1 2 i s t h e i n t e r n u c l e a r s e p a r a t i o n . I n C S 0 , t h e r e a r e no o t h e r m a g n e t i c n u c l e i i n the m o l e c u l e t o c o n t r i b u t e t o the J C r e l a x a t i o n so T 1 i s l o n g . I n benzene and f o r t h e c a r b o n y l c a r b o n i n a c e t o n e , t h e J C i s c o u p l e d t o one or more p r o t o n s w h i c h l e a d s t o a r e d u c t i o n i n T 1. I n ( M e O ^ S i , t h e i n t e r n u c l e a r d i s t a n c e i s l a r g e so t h a t t h e d i p o l a r c o u p l i n g i s l e s s e f f e c t i v e i n p r o v i d i n g a means f o r t h e n u c l e i t o r e l a x . I n ( M e ^ S i ^ O and i n M e ^ S i , t h e i n t e r n u c l e a r d i s t a n c e i s r e d u c e d , r e d u c i n g T^. The s h o r t e r T^ i n Me^Si t h a n i n (Me^Si^O. i s a r e s u l t o f a ; g r e a t e r number of p r o t o n s c o u p l e d t o t h e s i l i c o n . The 1 3 C T ^ s a r e s h o r t e r t h a n t h o s e f o r 2 9 S i i n t h e compounds measured. T h i s d i f f e r e n c e i s p r o b a b l y a r e s u l t o f th e g r e a t e r n u c l e a r magnetic moment of Carbon-13 and a l s o o f 1 3 t h e l o w e r symmetry of the C m o l e c u l e s measured. S i n c e t h e c h e m i c a l s h i f t i s a d i r e c t i o n a l p r o p e r t y i t may be a n i s o t r o p i c . I n most N.M.R. work, t h i s a n i s o t r o p y i s n o t i m p o r t a n t because random m o t i o n i n t h e l i q u i d causes t h e o b s e r v e d s h i e l d i n g t o be an average v a l u e . However, t h i s a n i s o t r o p y may have a marked 31 F i g u r e 7 P l o t Of I n (1 + I t / I Q ) v s » * F o r c And ^ S i Compounds 32 69 e f f e c t on ' As t h e m o l e c u l e t u m b l e s , t h e magnetic n u c l e i , i n e f f e c t "see" a v a r y i n g m agnetic f i e l d w h i c h may i n d u c e 1 3 t r a n s i t i o n s between the n u c l e a r energy l e v e l s . A l l o f the J C compounds, f o r w h i c h T^'s were measured, would be ex p e c t e d t o e x h i b i t a n i s o t r o p i c s h i e l d i n g s ; CS^ and Me 2C0 on t h e b a s i s o f m o l e c u l a r symmetry and benzene because o f the r i n g c u r r e n t . I t would be u s e f u l t o measure f o r analogous c a r b o n and s i l i c o n 1 29 A-\ compounds t o c l a r i f y w h i c h shows l o n g e r T ^ s ; S i or ^C. A c c u r a c y o f S i l i c o n - 2 9 C h e m i c a l S h i f t Data 29 The e r r o r i n t h e S i s h i f t measurements i s e s t i m a t e d t o be ±1 p.p.m. T h i s e s t i m a t e i s based on two f a c t s . F i r s t , t he s t a n d a r d d e v i a t i o n on a s e r i e s o f seven measurements of the c h e m i c a l s h i f t i n a (MeOVSi/T,M.S. sample was found t o be 29 0 . M - p.p.m. Second, many samples g i v e 7 S i s p e c t r a which a r e s p l i t i n t o m u l t i p l e t s by d i r e c t 2 9 S i - H c o u p l i n g . (See F i g u r e 8.) The 29 c o u p l i n g c o n s t a n t s may be measured from t h e 7 S i spectrum and •i compared t o the v a l u e o b t a i n e d f r o m t h e H spectrum. The v a l u e s 29 f r o m 7 S i s p e c t r a c o n s i s t e n t l y agree t o ± 5 c.p.s. w i t h t h o s e o b t a i n e d f r o m 1 H s p e c t r a . F o r example, J 2 9 S i H i n (C^H^^SiMeH was f o u n d , f r o m 2 9 S i spectrum, t o be 190-200 c.p.s. and, fr o m 1 H spect r u m , 196 c.p.s. I n some c a s e s , f o r example the m e t h y l -29 c h l o r o s i l a n e s , t h e 7 S i spectrum cannot be ob s e r v e d e x c e p t under v e r y f a s t sweep and h i g h e r t h a n normal power c o n d i t i o n s . The e r r o r s a r e c o n s e q u e n t l y somewhat l a r g e r ; about ±5 p.p.m. A l s o , some s i g n a l s , p a r t i c u l a r l y t e t r a v i n y l s i l a n e , a r e c o n s i d e r a b l y broadened by s p i n c o u p l i n g and t h e e r r o r i s i n c r e a s e d t o ± 3 p.p.m. 29 W h i l e t h e random e r r o r i n t h e 7 S i measurements i s i 1 p.p.m., t h e r e i s a s y s t e m a t i c e r r o r i n t h e measurements w h i c h has n o t been t a k e n i n t o a c c o u n t . S i n c e an e x t e r n a l s t a n d a r d i s b e i n g u s e d , a c o r r e c t i o n s h o u l d be made f o r d i f f e r -ences i n b u l k s u s c e p t i b i l i t y o f t h e v a r i o u s samples.-' T h i s 21 F i g u r e 8 2 9 S i Spectrum Of C ^ S i R ^ c o r r e c t i o n may be a p p l i e d u s i n g t h e f o l l o w i n g e q u a t i o n , where: & sphere = s p h e r i c a l o r t r u e c h e m i c a l s h i f t & obs = o b s e r v e d c h e m i c a l s h i f t X v r e f = v o l u m e s u s c e p t i b i l i t y o f r e f e r e n c e 9( v sample = v ° l u m e s u s c e p t i b i l i t y o f sample The b u l k s u s c e p t i b i l i t y c o r r e c t i o n w i l l be l e s s t h a n 1 p.p.m. and can p r o b a b l y be i g n o r e d . As a check, t h e c h e m i c a l s h i f t between Me^SiH and ( M e O ^ S i was measured f o r i n t e r n a l and e x t e r n a l s t a n d a r d s . F o r i n t e r n a l s t a n d a r d t h e s h i f t i s 62.6 p.p.m. and f o r e x t e r n a l s t a n d a r d i t i s 61 . 5 p.p.m. I t i s q u e s t i o n a b l e , whether o r n o t t h i s d i f f e r e n c e i s s i g n i f i c a n t , I n any c a s e , t h e s m a l l e r r o r i n t r o d u c e d by f a i l u r e t o a p p l y a b u l k s u s c e p t i b i l i t y c o r r e c t i o n i s bound t o be p r e f e r a b l e t o t h e p o s s i b i l i t y o f l a r g e r e r r o r s i n t r o d u c e d by s o l v e n t e f f e c t s , S i l i c o n - 2 9 C h e m i c a l S h i f t s 29 Any attempt t o e x p l a i n t h e obser v e d y S i c h e m i c a l s h i f t s must be v e r y q u a l i t a t i v e and somewhat s p e c u l a t i v e . We 29 s h a l l a t t e m p t t o e x p l a i n g e n e r a l t r e n d s i n S i c h e m i c a l s h i f t 119 13 1 and w i l l b r i e f l y examine t h e 7 S n and J C c h e m i c a l s h i f t s f o r s i m i l a r i t i e s and d i f f e r e n c e s . The 2 9 S i ' c h e m i c a l s h i f t s f o r t h e s e r i e s Me S i ( O M e ) ^ a r e l i s t e d i n t a b l e 6 and t h e t r e n d i s shown g r a p h i c a l l y i n f i g u r e 9. T h i s t r e n d i s a l s o f o l l o w e d f o r Me S i C O E t ) ^ ^ and f o r Me S i ( 0 A c ) L . (See t a b l e s 7 and 8.) The e x i s t e n c e o f a minimum i n t h e c h e m i c a l s h i f t i s i n d i c a t i v e o f two competing e f f e c t s b e i n g p r e s e n t . X F i g u r e 9 2 9 S i C h e m i c a l S h i f t Of Me SKOMe)^ S e r i e s I t has been shown t h a t t h e para m a g n e t i c c o n t r i b u t i o n t o the c h e m i c a l s h i f t w i l l i n most cases be t h e dominant c o n t r i b u t i o n . U s i n g t h e Schneider-Buckingham t r e a t m e n t , 3 ^ t h e paramagnetic s h i f t s maybe c a l c u l a t e d f o r most n u c l e i . A number o f such s h i f t s a r e l i s t e d i n t a b l e 1*f, w i t h t h e p e r -centage t h a t t h e obser v e d c h e m i c a l s h i f t range forms o f t h e 12 T a b l e 1H- P a r a m a g n e t i c S h i f t s And P e r c e n t a g e Observed -1175 -2300 -1750 -Moo -¥fOO 27$ 27% 8% h5% 300% c a l c u l a t e d p a r a m a g n e t i c term. The l a r g e Pb c h e m i c a l s h i f t r ange i n c l u d e s a number o f s o l i d s and l e a d m e t a l so t h a t i t i s 29 n o t s u r p r i s i n g t h a t t h e s i m p l e t h e o r y f a i l s . The v e r y s m a l l S i c h e m i c a l s h i f t range i n d i c a t e s t h a t w h i l e t h e t o t a l p a r a m a g n e t i c s h i e l d i n g i s l a r g e t h e o b s e r v e d changes i n s h i e l d i n g a r e r e l a t i v e l y s m a l l . I t i s i m p o r t a n t t o r e a l i z e t h a t t h e s e l e c t i o n o f a s t a n d a r d f r o m w h i c h c h e m i c a l s h i f t s a r e measured i s a r b i -t r a r y . The t r u e z e r o o f c h e m i c a l s h i f t i s an u n s h i e l d e d n u c l e u s . 29 I n t h e case o f ' S i t h e r e i s no s p e c i e s f o r w h i c h t h e paramag-n e t i c s h i e l d i n g would be s m a l l . S c h n e i d e r and Buckingham assume 2+ t h a t Pb i n s o l u t i o n would have a s m a l l p a r a m a g n e t i c s h i e l d i n g so t h a t t h e 3000 p.p.m. s h i f t between PbNO^ s o l u t i o n and PbCMe)^ may be used as a measure o f t h e p a r a m a g n e t i c s h i e l d i n g . A l l 29 of t h e s i l i c o n compounds f o r w h i c h t h e ' S i s h i f t s have been measured c o n t a i n s i l i c o n i n a t e t r a c o o r d i n a t e d environment o f c o v a l e n t bonds. Hence, t h e paramagnetic c o n t r i b u t i o n w i l l be l a r g e i n a l l c a s e s . s m a l l changes i n p a r a m a g n e t i c s h i e l d i n g i t i s r e a s o n a b l e t o assume t h a t o t h e r f a c t o r s may a l s o s i g n i f i c a n t l y a f f e c t t h e t o t a l s h i e l d i n g . I n an attempt t o m i n i m i z e t h e number o f v a r i a b l e s we have examined compounds of t h e t y p e Me SiXu and as a f u n c t i o n o f e l e c t r o n e g a t i v i t y w i t h o u t w o r r y i n g about major changes i n m o l e c u l a r geometry. I n f i g u r e 10 we have I f we t a k e t h e p o i n t o f v i e w t h a t we a r e o b s e r v i n g 36 p l o t t e d ( M e ^ S i 2 - Me^SiX) v s . t h e e l e c t r o n e g a t i v i t y d i f f e r e n c e between S i and X. The p o i n t s f o r S i , I , B r , and C l l i e w i t h i n 1 3 p.p.m. of a s t r a i g h t l i n e , i m p l y i n g a l i n e a r dependence o f t h e c h e m i c a l s h i f t on e l e c t r o n e g a t i v i t y . N, 0, and F l i e on a d i f f e r e n t s t r a i g h t l i n e w h i c h i s n o t p a r a l l e l t o t h e f i r s t l i n e and does n o t pass t h r o u g h t h e o r i g i n . T h i s d e v i a t i o n c o i n c i d e s w i t h t h e known a b i l i t y of n i t r o g e n , oxygen and f l u o r i n e t o back donate t o s i l i c o n u s i n g (p-» d)7T bonds. We s h a l l now c o n s i d e r the observed s h i e l d i n g t o be 21 d i v i d e d i n t o two p a r t s as f o l l o w s : 0" - cr p + cr* where: CT = o b s e r v e d s h i e l d i n g Op" = p a r a m a g n e t i c s h i e l d i n g = c o n t r i b u t i o n t o s h i e l d i n g f r o m s u b s t i t u e n t s I n our c o n t e x t , <7" w i l l be c o n s i d e r e d as t h e d i a m a g n e t i c s h i e l d i n g w h i c h r e s u l t s f r om t h e d o n a t i o n o f e l e c t r o n s i n t o t h e 3 d -orbitals o f t h e s i l i c o n . <T^ i s n e g a t i v e so t h a t i n c r e a s e s i n Zaf.~ caused by s u b s t i t u t i o n o f e l e c t r o n e g a t i v e groups on t o e n pg s i l i c o n w i l l l e a d t o l o w f i e l d s h i f t s of t h e ' S i r e s o n a n c e s . < T X i s p o s i t i v e and i n c r e a s e s i n occupancy of t h e d - o r b i t a l s 29 w i l l l e a d t o h i g h f i e l d s h i f t s of t h e 7 S i r e s o n a n c e s . Thus, a c o m b i n a t i o n of s u b s t i t u t i o n of e l e c t r o n e g a t i v e groups and back-d o n a t i o n on t o s i l i c o n l e a d s t o a c o m p e t i t i o n of two e f f e c t s . The o bserved c h e m i c a l s h i f t w i l l be t h e sum o f t h e changes i n s c r e e n i n g , A o~p and A d " * . We now assume t h a t Op i s l i n e a r l y dependent upon the d i f f e n e n c e i n e l e c t r o n e g a t i v i t y between X and S i and w i l l change l i n e a r l y w i t h s u b s t i t u t i o n . I f we assume t h a t , i n f i g u r e 1 0 , t h e l i n e t h r o u g h S i , B r , I , and C I d e t e r m i n e s the dependence of O p upon e l e c t r o n e g a t i v i t y , we can e x t r a p o l a t e i t and d e t e r m i n e what the c h e m i c a l s h i f t would be f o r M e ^ SiF, Me^Si-O-R and M e 3 S i - N R 2 compounds i f t h e r e were no c o n t r i b u t i o n f r om b a c k - d o n a t i o n . We can t h e n e x t r a p o l a t e t h e change i n s h i f t f r o m Me^Si t o Me^SiX t o S i X ^ . See f i g u r e 1 1 . The d i f f e r e n c e between A<Tp and t h e o bserved c h e m i c a l s h i f t i s t h e n th e c o n t r i b u t i o n f r om Ao*"*. From f i g u r e 11 , i t i s a p p a r e n t t h a t t h e change i n C T X becomes more e f f e c t i v e as one adds more s u b s t i t u e n t s t o t h e s i l i c o n . As C r a i g ^ 0 and E b s w o r t h ^ 3 have p o i n t e d out t h e f o r m a t i o n of a p a r t i a l p o s i t i v e charge a t t h e s i l i c o n w i l l cause an i n c r e a s e i n the e f f e c t i v e n e s s of t h e ( p — * d ) T T b o n d i n g . The s u b s t i t u t i o n o f e l e c t r o n e g a t i v e groups 2d w i l l t e n d t o i n c r e a s e t h e p o s i t i v e charge on t h e s i l i c o n . Thus, t h e change i n (j~x g o i n g f r o m Me^Si t o Me^SiX would be expected t o be l e s s t h a n t h e change g o i n g from MeSiX^ t o S i X ^ . T h i s i s what i s e x p e r i m e n t a l l y o b s e r v e d . J a f f e ^ 2 has c a l c u l a t e d o v e r l a p i n t e g r a l s f o r S i - 0 , S i - F and S i - C l bonds. F o r S i - 0 and S i - F t h e o v e r l a p i n t e g r a l i s 0 . 2 0 and f o r S i - C l i t i s 0 . 1 5 . T h e r e f o r e , w h i l e ( p ^ d ) T T bonding i s p o s s i b l e f o r S i C l bonds i t i s l i k e l y t o be l e s s e f f e c t i v e t h a n i n S i - 0 or S i - F bonds. The d a t a f o r t h e Me S i C l ^ . s e r i e s i s g i v e n i n t a b l e 9 and f i g u r e 1 2 shows the t r e n d graph-i c a l l y . The A cTp c o n t r i b u t i o n i s s m a l l e r t h a n f o r t h e Me Si(OMe)), v s e r i e s because of t h e s m a l l e r e l e c t r o n e g a t i v i t y o f C l . A l s o , t h e A <f c o n t r i b u t i o n i s much s m a l l e r because of t h e l e s s e r a b i l i t y o f c h l o r i n e t o back d o n a t e . Thus, th e minimum i n t h e c h e m i c a l s h i f t o c c u r s a t M e ~ S i C l 0 . ko The d a t a f o r t h e s e r i e s Me S i F ^ i s i n c o m p l e t e (See PQ t - x t a b l e 10.) b u t from the t h r e e 7 S i s h i f t s a v a i l a b l e one can a t l e a s t comment on t h e t r e n d . I n s p i t e o f t h e g r e a t e r e l e c t r o -n e g a t i v i t y o f f l u o r i n e o v er c h l o r i n e , t h e c h e m i c a l s h i f t o f Me^SiF i s v i r t u a l l y t h e same as t h a t o f M e ^ S i C l . A l s o , t h e c h e m i c a l s h i f t o f M e 2 S i F 2 i s o n l y k.5 p.p.m. t o low f i e l d of T.M.S. B o t h o f t h e s e f a c t s a r e c o n s i s t e n t w i t h a l a r g e CT* t e r m . S i n c e f l u o r i n e i s known t o back-donate v e r y r e a d i l y , a h i g h occupancy o f t h e s i l i c o n d - o r b i t a l s i s n o t u n e x p e c t e d . The c h e m i c a l s h i f t s f o r t h e M e x S i ( C 6 H 5 ' ) L r _ x a r e g i v e n i n t a b l e 12. The s i l i c o n - 2 9 resonance moves r e g u l a r l y t o h i g h f i e l d b u t t h e changes a r e q u i t e s m a l l ; 13 p.p.m. fr o m Me^Si t o Me S i t C ^ H ^ ) ^ . S i n c e , t h e s i l i c o n i s bound t o f o u r carbons i n a l l members o f t h e s e r i e s i t i s n o t l i k e l y t h a t t h e paramagnetic term w i l l e x h i b i t much change. P h e n y l groups have e l e c t r o n s i n t h e i r TT-bonding o r b i t a l s w h i c h may back-donate i n t o t h e d-o r b i t a l s o f s i l i c o n . However, any b a c k - d o n a t i o n w i l l be much s m a l l e r t h a n f o r t h e h a l o g e n s and w i l l n o t become more e f f e c t i v e as t h e s u b s t i t u t i o n i s i n c r e a s e d . A s i m i l a r e f f e c t would be ex p e c t e d f o r t h e v i n y l s i l a n e s , and i t s h o u l d be o f t h e same g e n e r a l o r d e r o f magnitude. The m e t h y l s i l a n e s , M e x s i H L f _ x > appear t o show a g e n e r a l i n c r e a s e i n c h e m i c a l s h i f t toward S i H ^ . However, MeSiH^ and S i H ^ a r e b o t h v e r y v o l a t i l e and t h e 2 9 S i s h i f t s a r e n o t a v a i l a b l e . The r e a s o n f o r t h e s e h i g h f i e l d s h i f t s i s n o t known but appears t o be g e n e r a l f o r S i - H compounds. F o r example, ( C ^ H ^ S i H ^ i s t o h i g h f i e l d o f ( C 6 H ^ ) 2 S i H 2 w h i c h i s t o h i g h f i e l d o f M e ( C 6 H ^ ) 2 S i H . (See t a b l e 13.) T a b l e 13 and a p p e n d i x A l i s t a number o f m i s c e l l a n e o u s 2 9 S i c h e m i c a l s h i f t s and a number o f comments can be made. The s h i f t s i n appendix A may be c o n v e r t e d t o p.p.m. from (MeOj^Si by s u b t r a c t i n g 58 p.p.m. G e n e r a l l y , s p e a k i n g p h e n y l and v i n y l s u b s t i t u t i o n l e a d s t o a h i g h f i e l d s h i f t . F o r example, V i S K O E t ) ^ and C 6 H ^ S i ( 0 E t ) ^ a r e about 15 p.p.m. t o h i g h f i e l d o f M e S i ( 0 E t ) 3 and V i S i ( 0 E t ) 3 i s t o h i g h f i e l d o f C 6 H ^ S i ( 0 E t ) 3 . A l s o , ( C 6 R y 2 S i ( 0 E t ) 2 i s 25 p.p.m. t o h i g h f i e l d o f M e 2 S i ( 0 E t ) 2 . However, Me 2(C£H^)SiH and M e ( C ^ H ^ ) 2 S i H a r e b o t h a t v i r t u a l l y t h e same resonance p o s i t i o n as Me^SiH. The s u b s t i t u t i o n of m e t h o x y l groups f o r m e t h y l groups u s u a l l y l e a d s t o a h i g h f i e l d s h i f t o f 29 t h e S i resonance b u t t h i s i s not s t r a i g h t f o r w a r d f o r the s e r i e s ( M e O ) ^ S i 2 , (MeOpMeSiJg and Me^Si,^. The o r d e r o b s e r v e d i s ( M e O ) 6 S i 2 t o h i g h f i e l d o f M e 6 S i 2 w i t h (MeO)^IeSi a t low f i e l d . The c h e m i c a l s h i f t o f S i B r ^ (+ 10 p.p.m. fr o m (MeO)^Si) i s somewhat d i f f i c u l t t o r a t i o n a l i z e u n l e s s t h e h i g h e r p o l a r i z -a b i l i t y o f bromine t h a n c h l o r i n e makes ( p - ? d ) T T b o n d i n g more f a v o u r a b l e . W h i l e t h e concept o f a c o m p e t i t i o n between t h e pa r a m a g n e t i c s h i e l d i n g and a d i a m a g n e t i c s h i e l d i n g r e s u l t i n g f r o m ( p - * d)TT b o n d i n g , e x p l a i n s t h e g r o s s f e a t u r e s o f 2 < 7 S i c h e m i c a l s h i f t s , t h e r e a r e a few minor p o i n t s w h i c h cannot be e x p l a i n e d . T h i s concept o f c h e m i c a l s h i f t a l s o appears t o a p p l y 119 t o t h e 7 S n c h e m i c a l s h i f t s o f the s e r i e s (n-Bu) SnClu x whi c h 22 have been measured by L a u t e r b u r . (See Appendix B.) I n f i g u r e 119 13 we have p l o t t e d t h e 7 S n c h e m i c a l s h i f t o f (n-Bu) S n C l ^ „ a g a i n s t x. The two v a l u e s f o r n - B u 2 S n C l 2 a r e f o r acetone (- 77 p.p.m.) and C S 2 ( - 11*+ p.p.m.) s o l u t i o n s . S i n c e acetone i s a p o l a r s o l v e n t i t ' i s l i k e l y t o e x h i b i t a l a r g e r s o l v e n t e f f e c t 119 t h a n C S 2 . I n any c a s e , t h e 7 S n s h i f t s o f t h i s s e r i e s f o l l o w t h e same t r e n d as t h e Me Si(OMe)^ s e r i e s . S i n c e t i n has v a c a n t ^ - d - o r b i t a l s , ( p - * d)TT bonding i s p o s s i b l e and s h o u l d be more e f f e c t i v e f o r c h l o r i n e t o t i n b a c k - d o n a t i o n t h a n f o r 119 c h l o r i n e t o s i l i c o n . T h e r e f o r e , t h e 7 S n s h i f t s f o r t h e t i n c h l o r i d e s behave more l i k e t h e s i l i c o n a l k o x i d e s t h a n t h e s i l i c o n c h l o r i d e s . The c h e m i c a l s h i f t o f M e 2 S n C l 2 has been measured o n l y i n acetone s o l u t i o n and MeSnCl^ o n l y i n acetone and i n water s o l u t i o n . The g e n e r a l t r e n d i s t h e same as f o r th e n - b u t y l c h l o r i d e s i f one c o n s i d e r s t h a t s o l v a t i o n e f f e c t s l e a d to. h i g h f i e l d s h i f t s . The s u b s t i t u t i o n o f p h e n y l and v i n y l groups f o r m e t h y l groups l e a d t o h i g h f i e l d s h i f t s o f 119 the 7 S n resonance and v i n y l , a g a i n , appears t o be more e f f e c t i v e t h a n p h e n y l . F o r example, M e 2 S n V i 2 and M e 2 S n ( C g H ^ ) 2 a r e s h i f t e d 79.^ p.p.m. and 59.8 p.p.m. r e s p e c t i v e l y t o h i g h k2 i cm • so-o --STO . 4 0 F i g u r e 13 1 1 9 S n C h e m i c a l S h i f t s F o r ( n - B u ) x S n C l l f _ x f i e l d of Me^Sn. F o r t h e m e t h y l v i n y l s t a n n a r i e s t h e t r e n d i s a l m o s t l i n e a r . (See f i g u r e 1*+.) Thus, t h e c o m p e t i t i o n o f two s h i e l d i n g s g i v e s a r a t i o n a l , but q u a l i t a t i v e , d e s c r i p t i o n o f the g e n e r a l f e a t u r e s o f 119 Sn c h e m i c a l s h i f t s . i hi I f the p r e c e e d i n g e x p l a n a t i o n o f c h e m i c a l s h i f t i s c o r r e c t , i t s h o u l d n o t a p p l y t o Carbon -13 c h e m i c a l s h i f t s . I n o t h e r words, analogous c a r b o n compounds s h o u l d n o t show t h e same t r e n d s as s i l i c o n o r t i n . The r e a s o n f o r t h i s i s t h a t t h e d - o r b i t a l c o n t r i b u t i o n , (T*, must be z e r o f o r c a r b o n . F i g u r e h (page 19) shows the t r e n d s i n 1 3 C and 2 9 S i c h e m i c a l s h i f t s f o r t h e two s e r i e s o f compounds ( C H ^ ^ C O R ) ^ ^ . ^ W h i l e t h e R group i s n o t s t a t e d , i t makes no d i f f e r e n c e t o our 13 d i s c u s s i o n . The J C s h i f t s e x h i b i t a g e n e r a l d e c r e a s e f r o m Me^C t o C ( 0 R ) i + c o n s i s t e n t w i t h an i n c r e a s e i n p a r a m a g n e t i c s h i e l d i n g as e l e c t r o n e g a t i v e groups a r e added. S p i e s e c k e and 17 13 S c h n e i d e r ' have measured the' J C c h e m i c a l s h i f t s of a number 1 3 o f s u b s t i t u t e d methanes. I n f i g u r e 15 we have p l o t t e d t h e J C c h e m i c a l s h i f t o f CH^X r e l a t i v e t o t h e t e r m i n a l carbons i n neopentane a g a i n s t t h e d i f f e r e n c e i n e l e c t r o n e g a t i v i t i e s , 1 3 X - X . W h i l e t h e J C s h i f t s do n o t e x h i b i t a s i m p l e depend-ence upon e l e c t r o n e g a t i v i t y , t h e d e v i a t i o n s w h i c h do appear a r e c e r t a i n l y d i f f e r e n t f r om t h e s i m i l a r p l o t f o r 2 9 S i s h i f t s i n 1 3 Me.SiX compounds. (See F i g u r e 10.) The d e v i a t i o n s i n  0 C 17 s h i f t have been d e s c r i b e d by S p i e s e c k e and S c h n e i d e r ' i n terms o f changes i n magnetic a n i s o t r o p y and by S c h a e f e r 7 ^ and co-workers i n terms o f i n t r a m o l e c u l a r d i s p e r s i o n e f f e c t s . 1 3 F o r t h e s e s i m p l e examples t h e t r e n d s i n • J C c h e m i c a l s h i f t s a r e 29 d i f f e r e n t f r o m t h o s e i n 7 S i and a r e e x p l a i n e d on a d i f f e r e n t b a s i s . The 1 3 C c h e m i c a l s h i f t s o f the M e x S i C l l f _ x s e r i e s e x h i b i t a t r e n d i n c h e m i c a l s h i f t w h i c h appears t o be s i m i l a r t o t h e 2 9 S i s h i f t s . However, i t i s u n l i k e l y t o be a r e s u l t o f t h e same e f f e c t . More l i k e l y , t h e 1 3 C s h i f t s r e f l e c t a change i n t h e e f f e c t i v e e l e c t r o n e g a t i v i t y o f t h e s i l i c o n bound t o i t . The e l e c t r o n e g a t i v i t y o f the s i l i c o n w i l l be governed by t h e number of c h l o r i n e s bound t o i t and a l s o t h e e x t e n t o f back-d o n a t i o n f r o m c h l o r i n e . More 1 3 C s h i f t s w i l l have t o be measured f o r c a r b o n bound t o s i l i c o n and o t h e r group f o u r elements b e f o r e t h i s b e h a v i o r can be e x p l a i n e d . hi Coupling Constants 3 9 Gutowsky and Juan-" have considered J 13QJJ and J 2 9 S ^ H from a valence bond treatment and have reached the f o l l o w i n g c o n c l u s i o n s . The magnitude of the coupling i s dependent almost e n t i r e l y upon the s-character of the s i l i c o n or carbon. The p o l a r i t y of the bond does not a f f e c t the coupling constant. The a d d i t i o n of an e l e c t r o n e g a t i v e sub-s t i t u e n t to methane inpreases the s-character by, i n e f f e c t , i n c r e a s i n g the p-character of the C-X bonding o r b i t a l s . S i l i c o n , however, has a l e s s e r a f f i n i t y f o r the s e l e c t r o n s on carbon than does hydrogen. Therefore, J 13 C £ i s reduced i n Me^Si (118 .1 c.p.s.) from methane (125 c.p.s.). I f the e f f e c t i v e nuclear charge of s i l i c o n , which amounts to i t s e l e c t r o n e g a t i v i t y , increases one would expect the value of J 13QJJ to i n c r e a s e . From examining the values of J 1 3 C H i n the s e r i e s Me Si(OMe)^ , Me Si(OAc)^ and Me S i C l u , t a b l e s 6 , 8 and 9> one can see t h a t indeed there i s a r e g u l a r increase i n J 13 C JJ. Furthermore, the values of J 1 3 C H i n d i c a t e t h a t the order of i n c r e a s i n g e f f e c t i v e nuclear charge on s i l i c o n i s Cl^OAc^OMe. This i s 2 9 the same order p r e d i c t e d from the ' S i chemical s h i f t s i n the Me^SiX compounds. This simple approach breaks down f o r M e x s i H L , . _ x and M e x s i ( c 6 H 5 , ) l j . _ x > t a b l e s 11 and 1 2 , where, i n s p i t e of the f a c t t h a t l i t t l e or no change i n % e££ i s expected on s i l i c o n , J 1 3QJJ shows a r e g u l a r increase toward MeSiX^. At present, t h i s r e s u l t i s unexplained. I f we use equation 3 2 , page 1 3 , as a guide to the f a c t o r s governing J w e should be able to make some comment on the changes i n J 2 9 s ^ Q J J. I t i s u n l i k e l y t h a t , f o r the compounds measured, a g i ( 3 s ) , a^ ( 1 s ) , o r A E x w i l l change. We must t h e r e f o r e consider changes i n s-character alone to be r e s p o n s i b l e f o r changes i n J 2 9 S i C j j . I f the e l e c t r o n e g a t i v i t y of s u b s t i t u e n t s on s i l i c o n a f f e c t s J 2 9 g i C g i n the same way as e l e c t r o n e g a t i v e s u b s t i t u e n t s a f f e c t J 1 3 Q H i n s u b s t i t u t e d methanes, J 2 9 S i C H s h o u l d increase as e l e c t r o n e g a t i v e groups are added. The expected behavior i s observed f o r Me S i X ^ ^ ; h6 where: S = OMe, OEt, OAc and C I , t a b l e s 6, 7, 8, and 9. A l s o , t h e s e r i e s M e x S i ( C ^ H ^ ) i + _ x e x h i b i t s v i r t u a l l y no change i n J 2 9 S i C j j w h i c h i s c o n s i s t e n t w i t h no change i n e f f e c t i v e n u c l e a r charge on s i l i c o n . The s e r i e s ' Me SiRY Y , a g a i n , shows a g e n e r a l i n c r e a s e i n J 2 9 g i Q H . I t must be assumed t h a t s i m p l e t h e o r i e s of s p i n - s p i n c o u p l i n g cannot be extended t o systems i n v o l v i n g heavy n u c l e i . T h i s i s p r o b a b l y because of t h e g r o s s a s s umptions w h i c h must be made w i t h r e s p e c t t o t h e e l e c t r o n wave f u n c t i o n s . A c a r e f u l e x a m i n a t i o n o f t h e 1 3 C s a t e l l i t e s of s e v e r a l m e t h y l s i l i c o n compounds r e v e a l s f i n e s t r u c t u r e due t o a l o n g range HCSiCH c o u p l i n g . (See F i g u r e 16.) The compounds, ( M e ^ S i ^ O and Me^SiX (where: X = OMe, OEt, OAc and CI) have 1 3 C s a t e l -l i t e s w h i c h a r e s p l i t i n t o seven l i n e s . The compounds M e 0 S i X 0 1 3 (where: X = OMe, OEt, OAc and CI) have J C s a t e l l i t e s w h i c h are q u a r t e t s . I n a l l c a s e s , J^CSiCH = ° » 3 5 0 . 1 c.p.s. T h i s c o u p l i n g i s o b s e r v a b l e because p r o t o n s a t t a c h e d t o 1 3 C a r e not e q u i v a l e n t 1 2 t o p r o t o n s a t t a c h e d t o C. The magnitude of t h i s c o u p l i n g i s p r o b a b l y r e l a t e d t o t h e (p d)TTbonding i n t h e s e compounds. I n g e n e r a l , t h e c o u p l i n g c o n s t a n t d a t a does n o t r e f l e c t t h e environment of t h e s i l i c o n i n t h e s e compounds. The c o u p l i n g between n u c l e i i s dominated by i n t e r a c t i o n s between the S - o r b i t a l s and i n t h e case of s i l i c o n , a t l e a s t , t h e s - e l e c t r o n s do n o t appear t o be a f f e c t e d i n t h e same way as t h e p and d e l e c t r o n s w h i c h dominate th e S i l i c o n - 2 9 c h e m i c a l s h i f t s . The p r o t o n c h e m i c a l s h i f t d a t a shows, i n most c a s e s , o n l y s m a l l changes w h i c h cannot be a t t r i b u t e d t o any s p e c i f i c e f f e c t . 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Chem. k±, 2969 (1963). 5 2 APPENDIX A S i 2 9 Chemical S h i f t s ^> 2 Chemical S h i f t Compound (p.p.m.) [ ( C H 3 ) 3 S i O ] l f S i * 81.5 S i B r ^ 68 Sodium s i l i c a t e s o l u t i o n 6k (C-E^\S± 59.5 [ ( C H 3 ) 3 S i O ] 3 S i C H 3 *f 2.5 C 6 H 5 S i H 3 39 ( C 2 H 5 0 ) 3 S i C H 3 2115 C 6H^(CH 3)SiH 2 1*f.8 (CH 3SiHO) 5 13.1 ( C ^ S i H O ) ^ 1 2 ( C 6 H 5 ) 2 S i H 2 11.-8 [ ( C H 3 ) 2 S i 0 } x 0 [ ( C H ^ S i O ] ^ -2.2 [C g H y C H ^ S i O ] ^ -2.3 C 6H^(CH 3) 2SiH -k.5 S i C l ^ - 6 [ ( C H 3 ) 2 S i O ] 3 ( C 6 H 6 s o l u t i o n ) -12.1 (CH 3) 3SiCH==CH 2 J 1 5 > 1 [ ( C H 3 ) 2 S i H ] 2 0 -17.2 ( C H 3 ) 3 S i C 6 H 5 -1 ? . 8 ( C 6 H 5 ) 2 C H 3 S i O C 2 H 5 -18.2 [ ( C H 3 ) 2 S i N H ] 3 -18.7 [ ( C H 3 ) 3 S i ] 2 N H -19 . 8 (CH 3) 3SiCH 2OH -21 . 8 ( C H ^ S i _22.0 [ ( C H 3 ) 3 S i ] 2 C H 2 -22.5 (CH 3) 3SiCH 2NH 2 -23.2 ( C H 3 ) 3 S i C H 2 C 6 H 5 -23.k 21 APPENDIX A (Cont'd) S i 2 9 Chemical Shifts ^' 2 0 (CH 3) 3Si(CH 2) 3CH 3 -23.8 (CH 3) 3SiCH 2Cl -25.5 (CH3)3SiCH2NCS -28.2 t(CH 3) 3Si ] 2 0 -28.7 (CH 3) 3SiOC 2H^ -29.h [(CH 3 ) 3 Si6] 3SiCH"3 -29.+ CH 3SiCl 3 - 3 0 . [(CH^^Siql^Si - 3 0 . h (CH 3) 3SiI -30.6 (CH3) 2Si(CH 2C1)CHC1 2 -31.2 [(CH 3) 3Sid] 3PO -35 (CH 3) 3SiBr -WA (CH3)3SiCl -51 (CH3)3,SiP -53.1 ( C H 3 ) 2 S i C l 2 -5+ (CH.KSiOSO.H -57.6 5k APPENDIX B 1 1 9 S n Chemical S h i f t s 2 2 ' 2 ^ Me^Sn = 0 Chemical Sh i f t Compound (p.p.m.) Snl^ (CS 2 solution)' 1701 SnSO^ (aqueous) 909 SnBr^ 638 Na 2[Sn(0H) 6] (aqueous) 592 K 2QSn(0H) 6l (aqueous) 590 SnCl 2.2H 2 0 (aqueous, I+.85M) 521 CH3SnCl^. (aqueous) *+80 S n C l 2 (tetrahydrofuran) 236 (n-C l fH 9) 23n(0Ac) 2 195 Vi^Sn 165 SnCl^ 150 (C 6H^) 3(cyclohexyl)Sn 11*+ (CH 3) 6Sn 2 109 V i 2 S n B u 2 86. *f (nrBu) 6Sn 2 79.5 (CH 3) 2Sn -Vi 2 79.*+ (CH 3) 2Sn(C 6Hy 2 59.8 (C 6H ?) 3SnCl , 1+6.0 V i 2 S n C l 2 1+0.9 (CH 3) 3SnVi 3 5 A (CH ) 3SnC 6H 5- 30.3 (n-Bu^Sn 12 n-BuSnCl 3 3 ( C H 3 ) 2 S n C l 2 (acetone solution) -.36 n-Bu 2SnCl 2 (acetone solution) -71 (CH 3) 2SnBr 2 -7I+.3 n-Bu 2SnCl 2 (CS 2 solution) -11^ (CH 3) 3SnBr -130.7 n-Bu 3SnCl (CH.KSnCl -158.6 51 APPENDIX C 2 ° 7 P b C h e m i c a l S h i f t s 2 ^ ' 2 6 ' 3 6 C h e m i c a l S h i f t Compound (p.p.m.) Pb m e t a l 0 P b 0 2 powder +6,900 PbO y e l l o w +7,+00 PbSe +8,700 PbS +10,100 PbTe +10,800 P b ( C 2 H 3 0 2 ) 2 s i n g l e c r y s t a l +10,900 PbO r e d +11,200 P M C H ^ +11,1+00 P b ( C 2 R " 3 0 2 ) 2 s o l u t i o n +12,300 P b ( C 2 0 l f ) 2 s o l i d +12,300 P b ( Z i r c o n a t e ) s o l i d +12,500 P b ( C l O ^ s o l u t i o n +1^,100 PbCO^ s i n g l e c r y s t a l +1l+,lf00 P b ( N 0 3 ) 2 s o l u t i o n + 1l+,lK)0 Pb(N0 3).'H 20 +15,200 PbNO. +15,200 

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