SPECTRA
AND
CONTAINING P. O~
P. M.
CONJUGATION HEAVY
ATOMS
Shorygin, Nefedov,
B. and
IN
V. S.
MOLECULES
Lopatin, P. Kolesnikov
UDC 543.424:541.57:547.258.11
The investigation of the R a m a n s p e c t r a of diatomic and c e r t a i n other s i m p l e m o l e c u l e s h a s m a d e it p o s s i b l e to e s t a b l i s h the b a s i c p r i n c i p l e s and m a k e a n u m b e r of g e n e r a l i z a t i o n s on the i n t e r r e l a t i o n s h i p between the intensity of the R a m a n lines and the nature of the c h e m i c a l bonds [1]. C o m p a r i s o n s of the physical and c h e m i c a l constants not with the intensities of the R a m a n lines t h e m s e l v e s , but with the d e r i v a t i v e s of the p o l a r i z a b i l i t y with r e s p e c t to the n o r m a l nuclear coordinate, 0 o r / 0 Q , or with the values of the d e r i v a t i v e of the p o l a r i z a b i l i t y of the m o l e c u l e s with r e s p e c t to the change in the length of a definite c h e m i cal bend q (i.e., with r e s p e c t to the n a t u r a l coordinate) 0 o r / a q , a r e e x t r e m e l y indicative. A c c o r d i n g to the widely used s e m i e m p i r i c a l f o r m u l a s of Woodward [2] and Lippincott [3] (see also [1]), the value of 0 / 0q is l a r g e r , the g r e a t e r the multiplicity of the bond and the higher the d e g r e e of covalency. F o r c o m pounds in which the d e r i v a t i v e s 0 a ~ ~q a r e s m a l l and v a r y within c o m p a r a t i v e l y s m a l l l i m i t s , a f a i r l y good c o r r e s p o n d e n c e was obtained between the e x p e r i m e n t a l v a l u e s of 0 a / 0 q and those calculated a c c o r d ing to these f o r m u l a s . H o w e v e r , the question of the l i m i t s of applicability of the a b o v e - m e n t i o n e d f o r m u l a s is net e n t i r e l y c l e a r . In view of this, the c i r c l e of objects c o n s i d e r e d was expanded, and s u p p l e m e n t a r y data w e r e obtained on compounds p o s s e s s i n g e l e c t r o n i c a b s o r p t i o n bands in a m o r e long-wave region of the s p e c t r u m . In the s e r i e s of t e t r a h a l i d e s of e l e m e n t s of g r o u p I V B , YHal 4, the UV a b s o r p t i o n bands a r e shifted into the m o r e long-wave region as the t r a n s i t i o n to h e a v i e r a t o m s Y and Hal p r o c e e d s (Figs. 1 and 2). The data of the UV s p e c t r a , on the one hand, and the Woodward f o r m u l a , on the o t h e r hand, gave a b a s i s for expecting that the intensity of the R a m a n line of the v a l e n c e fully s y m m e t r i c a l v i b r a t i o n for SnI 4, and all the m o r e for P b I 4, should be e x t r e m e l y l a r g e . We w e r e unable to obtain the R a m a n s p e c t r u m of SnI4 a f t e r excitation with the blue line of the m e r c u r y s p e c t r u m (u 22,938 e r a - l ) , but in the c a s e of excitation with the line 15,803 cm -1 of a H e - N e l a s e r , f o r solutions of SnI4 in benzene, a v e r y intense line 149 em -1 , c o r r e s p o n d i n g to the valence s y m m e t r i c a l v i b r a t i o n of S n - I (I~ = 6000 units) w a s o b s e r v e d . The d e r i v a t i v e 0 a / O q , calculated in a f i r s t a p p r o x i m a tion of an additive s c h e m e , p r o v e d equal to 19 A* 2 , while the value of the d e r i v a t i v e obtained by e x t r a p o l a tion to 22,938 e m -1 r e a c h e d -~40 A 2 (all the data for a solution of SnI 4 in benzene). The a f o r e m e n t i o n e d actually c o r r e s p o n d s to a tendency for an i n c r e a s e in the intensity and a a / O q in the t r a n s i t i o n to bonds with heavy a t o m s , which is evidently a s s o c i a t e d with a d e c r e a s e in the energsr of the e l e c t r o n i c t r a n s m i s s i o n o ~ o* under conditions of c o m p a r a t i v e l y s m a l l o v e r l a p p i n g of the atomic e r b i t a l s of the heavy a t o m . Calculation a c c o r d i n g to the Lippincott f o r m u l a gives a value of 5.86 ~2 for 0 a / O q of Srff4, which is f a r s m a l l e r than follows f r o m the e x p e r i m e n t . Somewhat b e t t e r c o r r e s p o n d e n c e between the o b s e r v e d and calculated values can be achieved if we a s s u m e an i n c r e a s e in the multiplicity of the Y - H a l bonds in compounds containing h e a v y a t o m s . This a s s u m p t i o n a g r e e s with the data of the M S s s b a u e r s p e c t r a f o r SnI4, which indicates that the S n - I bond has a pronounced (~20%) 7 r - c h a r a c t e r [4]. But in this v a r i a t i o n a l s o , the d i s c r e p a n c i e s f r o m the e x p e r i m e n t still r e m a i n e x t r e m e l y substantial. It is m o r e i m p o r t a n t to take into c o n s i d e r a t i o n the substantial deviation f r o m additivity in the optical p r o p e r t i e s of the t e t r a i o d i d e m o l e c u l e s . The UV a b s o r p t i o n bands of the t e t r a i o d i d e s lie f a r c l o s e r to the visible portion of the s p e c t r u m than those of the monoiodides (see Fig. 2), and the individual Y - - I bonds cannot be c o n s i d e r e d N. D. Z e l i n s k i i Institute of Organic C h e m i s t r y , A c a d e m y of Sciences of the USSR, Moscow. T r a n s l a t e d f r o m I z v e s t i y a A k a d e m i i N a u k SSSR, Scriya K h i m i c h e s k a y a , No. 3, pp. 562-566, M a r c h , 1975. Original a r t i c l e submitted M a y 31, 1974. 9 1975 Plenum Publishing Corporan'on, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.
488
~, cm -~ 9mole-1, liter 8, cm -t .mole -~ liter
4000
9
2000
;i
l
\ 11 IgOO \ \ ._ ./ ,
150
:000
\
t,\',\
,1\
./ ',
\~ \
,\ ~
175
Z00 ~., n m
" 200
"
300
~ qOQ
500
\rim
Fig. 2 Fig. 1 Fig. 1. UV s p e c t r a of c e r t a i n c h l o r o - d e r i v a t i v e s of methane: 1) CHsC1; 2) CHC13; 3) CC14. Fig. 2.
UV s p e c t r a : 1) CHsI; 2) CI4; 3) (CH3)sSnI; 4) SnI4.
as zones of local e l e c t r o n i c excitation. .two-center fragment.
And yet, the Lippincott formula [3] p e r t a i n s to one independent
Calculation a c c o r d i n g to the Woodward formula [2] p r e d i c t s a substantial i n c r e a s e in the intensity and a ~ / 0 q with i n c r e a s i n g atomic n u m b e r of Y and Hal in the entire s e r i e s of t e t r a h a l i d e s ; but the degree of i n c r e a s e is not so substantial as it follows from the e x p e r i m e n t for SnBr 4 and Sr~4 (for the l a t t e r the o2 f o r m u l a gives 13.9 A ). In [5-7] (in c o n t r a s t to [3, 8]), we used an extrapolation of the values of 0 ~ / 0 q , d e t e r m i n e d e x p e r i mentally, to a z e r o frequency of the radiation exciting the R a m a n s p e c t r u m (u - - 0). The extrapolation was i n t e r p r e t e d as an exclusion of the influence of " r e s o n a n c e amplification of the R a m a n line" on the c a l culated p a r a m e t e r s of the electronic polarizability. A c c o r d i n g to [5], the experimental values c o r r e c t e d in this way a r e in better a g r e e m e n t with the Lippincott formula. This is due to the specifics of the s i m plified model of the chemical bond with a potential of the type of the 5-function, on which it is based. Such a model cannot r e n d e r the a r r a n g e m e n t of the e l e c t r o n i c levels of excitation and has some substantiation only in the region of frequencies u v e r y far from the electronic absorption bands. Let us note that when p ~ 0, the change not only in the e l e c t r o n i c , but also in the nuclear polarizability should have a significant r o l e ; t h e r e f o r e , the values of a G / 8 q , extrapolated to ~ -- 0, a r e only indirectly related to the Raman s p e c t r a . Extrapolation to a z e r o frequency by no m e a n s eliminates the r e q u i r e m e n t s that follow from the additive s c h e m e , and the influence of nonlocality of the e l e c t r o n i c excitations. As is shown by Fig. 2, the deviations f r o m additivity in the UV s p e c t r a m a y be v e r y great. The Lippincott formula for the polarizability gives better a g r e e m e n t with the e x p e r i m e n t than for a G / a q . This is p a r t l y due to the fact that the contributions of different levels of excitation to c~ (in cont r a s t to the contributions to 8 G / 0 q ) have the same signs. A significant role is played by the fact that a c o n s i d e r a t i o n of the levels of excitation of all the e l e c t r o n s is important for the polarizability (many of them lie in a v e r y s h o r t - w a v e region of the s p e c t r u m ) , while for 8 G / a q a consideration of the levels of excitation only of the valence electrons is sufficient. Consequently, the "effective frequency" of a b s o r p tion for G is situated in a substantially m o r e s h o r t - w a v e region than the "effective frequency" for 0 ~ / 8 q . The higher degree of the r e s o n a n c e denominator for 8 ~ / a q than for ~, which p r o m o t e s an i n c r e a s e in the role of the close levels of excitation, is also significant. L e t us note that in the derivation of the calculation f o r m u l a in [3] a c e r t a i n t r a n s i t i o n from the s - a t o m to the a - m o l e c u l e with r e p l a c e m e n t of the m e a n square value of the coordinate of the e l e c t r o n by the c o ordinates of the nuclei, was p e r m i t t e d ; it can be stated that the a p p e a r a n c e of the factor r '~ is essentially a consequence of the a r b i t r a r y p r o c e d u r e . This i n a c c u r a c y was m a n i f e s t e d to the g r e a t e r degree in the f o r m u l a for 8 a / 0 q ; the values calculated with its aid show a substantially p o o r e r c o r r e s p o n d a n c e to the e x p e r i m e n t than those calculated a c c o r d i n g to the e m p i r i c a l formula of Woodward. In c e r t a i n c a s e s , in a s e r i e s of s i m i l a r compounds, the Lippincott f o r m u l a cannot r e n d e r even the general tendency of the v a r i a t i o n of 8o~/0q as a fumction of the change in the p a r a m e t e r s of the chemical bond. Thus, the values of 8 ~ / 8 q calculated a c c o r d i n g to this f o r m u l a for the PbI 4 molecule (5.2 ~2) a r e substantially s m a l l e r o2 than for SnI4. The Woodward f o r m u l a for 8 ~ / 8 q of the P b - I bond of the PbI 4 m o l e c u l e s gives 19.1 A , which is substantially g r e a t e r than for SaT4. These r e s u l t s a r e in better a g r e e m e n t with the UV s p e c t r a which indicate a pronounced tendency for an i n c r e a s e in a ~ / 8 q for YHal 4 in the sequence to m o l e c u l e s
489
c, cm- 1. mole- 1. liter \
30000
2 i \
20000
with i n c r e a s i n g l y heavy Y a t o m s . An analogous situation also e x i s t s for the m o l e c u l e s Y(CH3)4, f o r which a monotonic i n c r e a s e in ~ ~ / ~ q is o b s e r v e d experimemtally in the sequence f r o m Si to P b [9]. Calculation a c c o r d i n g to the Lippincott f o r m u l a gave a substantially s m a l l e r value of a oz/~q for Pb(CH3) 4 than f o r the Sn(CH3) 4 m o l e c u l e . And y e t , the Lippincott f o r m u l a is widely u s e d by a n u m b e r of authors for judgments of changes in the m u l t i plicity of c h e m i c a l bonds [10] and even for the calculation of f o r c e coefficients [11]. The conclusions drawn in these studies have no s e r i o u s substantiation.
Although in the m o l e c u l e s YHal 4 the Y - H a l bond is p a r t i a l l y ionic, and the d i f f e r e n c e s in the integral intensity and ~ oz/~q a r e p a r t i a l l y a s s o c i a t e d with d i f f e r e n c e s in the p o l a r i t y of the bond, in m o l e c u l e s of the type of AlknY L7 I I, i ! 75 20g 225 - Y A l k n , the Y - Y bond is nonpolar and covalent. T h e r e f o r e , it can be a s ]t ~ H i l l s u m e d that the intensity of the line of the valence v i b r a t i o n of Y - Y in the Fig. 3. UV s p e c t r a of R a m a n s p e c t r a will be e x t r e m e l y substantial, e s p e c i a l l y if Y is a heavy atom. g e r m a n i u m compounds: E x p e r i m e n t s showed that in the R a m a n s p e c t r u m of hexamethyldistannane 1) (CH3).~GeGe(CH~),~; 2) (CH3)3Sn-Sn(CH3)~, the line 190 c m -1, c o r r e s p o n d i n g to the v a l e n c e v i b r a t i o n CI~GeCH2CH2GeC13. of S n - S a a c c o r d i n g to [12], actually is v e r y intense (Too= 200 units). This line is m o r e than 10 t i m e s as s t r o n g as the c o r r e s p o n d i n g line in the R a m a n s p e c t r u m of h e x a m e t h y l e t h a n e . H o w e v e r , its intensity is substantially lower than follows f r o m a c a l c u l a tion a c c o r d i n g to the Lippineott o r Woodward f o r m u l a (500 and 2750 units, r e s p e c t i v e l y . lO001
02
T e n t a t i v e e s t i m a t e s show that the d e r i v a t i v e a ( ~ / ~ q for the S n - S n bond, which is equal to 4 1 A , c o n t r a r y to the IAppincott f o r m u l a , i s s e v e r a l t i m e s s m a l l e r than f o r the Sn--I bonds in SaT4 (although the l a t t e r is ionic to a substantial d e g r e e ) . In a c c o r d with this, the UV bands of distannane lie in a s h o r t e r wave r e gion than the bands of S~I4 [13]. Taking the a b s o r p t i o n s p e c t r u m of 12 into c o n s i d e r a t i o n , we can a s s u m e that the iodine a t o m s (and the t r a n s f e r of c h a r g e to the iodine atom) a r e of e s p e c i a l l y g r e a t i m p o r t a n c e for the o b s e r v e d e l e c t r o n i c t r a n s i t i o n in the SnI4 m o l e c u l e . The m o d e r a t e l y high intensity of the line 190 c m -1 in the R a m a n s p e c t r u m of hexamethyldistannane is c o m b i n e d with a v e r y high intensity of another s t r o n g l y p o l a r i z e d line, 518 c m "1 (I > 3000 units). The Sn - C bonds should m o s t p a r t i c i p a t e in the v i b r a t i o n with frequency 518 cm -l, and it can be called the "valence s y m m e t r i c a l v i b r a t i o n of the S n - C bonds." A c o m p a r i s o n with the s p e c t r u m of t e t r a m e t h y l s t a n n a n e [in Sn(CH3)4 the intensity of the line of the S n - C v i b r a t i o n is equal to 350 units] shows that the intensity of the line u n d e r c o n s i d e r a t i o n i n c r e a s e s in distannane d i s p r o p o r t i o n a t e l y to the i n c r e a s e in the n u m b e r of S n - C bonds, and to a substantially g r e a t e r d e g r e e (despite the fact that the S a - C bonds and the SB-Sn bond in the h e x a m e t h y l d i s t a n a a n e m o l e c u l e v i b r a t e in opposite p h a s e s ) . T h i s p e r m i t s us to a s s u m e that in the s y s t e m of C - S n - S n - C bonds t h e r e is an i n t e r a c t i o n between the e l e c t r o n shells of the S n - C bonds, which can be c o n s i d e r e d a s a , a - c o n j u g a t i o n . In the c a s e of such conjugation, we m i g h t expect an a l t e r n a t i o n of the signs of ~xqn along the chain (variations of the equilibrium v a l u e s of the bond lengths) in the c a s e of e l e c t r o n i c excitation with the t r a n s f e r of an e l e c t r o n to the l o w e r vacant m o l e c u l a r o r b i t a l . The a l t e r n a tion of signs should c o r r e s p o n d to a change in the a r r a n g e m e n t of the nodes of the wave function o r , in other w o r d s , a change in the signs of the contributions of the a t o m i c o r b i t a l s of the neighboring a t o m s in the chain. The e x p e r i m e n t a l data of [13, 14] indicate a b a t h o e h r o m i c shift of the UV band of distannane by ~200 /~ in c o m p a r i s o n with Sn(CH3) 4. The l o n g - w a v e band of distannane lies in the region of 2100 A and, a c c o r d L~g to [13], can be a s s i g n e d to the S n - S n c h r o m o p h o r e ; h o w e v e r , the g r e a t e r intensity of the line 518 cm -t c a n be evaluated a s an indication of an e x t r e m e l y s u b s t a n t i a l contribution of the S n - C bonds and n o n l o c a l i zation of the e l e c t r o n i c excitation Ca shift of the wave functions of the S n - C and S n - S n c h r o m o p h o r e s ) . If we c o m p a r e the b a t h o c h r o m i c shifts of the bands in the sequence f r o m Sn(CH3) 4 to (CH3).~SnSa(CH~)3 and f r o m Sr,(CH3)4 to SnI4, the l a t t e r p r o v e s to be m u c h g r e a t e r . T h i s is in qualitative a g r e e m e n t with the data d i s c u s s e d above on the intensity of the lines in the R a m a n s p e c t r u m . It is m o r e difficult to s p e a k of quantitative c o r r e s p o n d e n c e since this r e q u i r e s m o r e c o m p l e t e i n f o r m a t i o n on the origin of the UV a b s o r p t i o n bands. Under the conditions of a b s e n c e of a v i b r a t i o n a l s t r u c t u r e of the UV hand of distannane, the i n f o r m a t i o n is difficult to obtain, and data on the intensity of the lines in the R a m a n s p e c t r u m a r e a useful supplement.
490
It is also e x t r e m e l y interesting to c o m p a r e the deviations f r o m additivity in the optical p r o p e r t i e s of the s y s t e m s C - S n - S n - C and S n - C - C - S n . Since we do not have the n e c e s s a r y p r e p a r a t i o n s a v a i l able, we shall l i m i t o u r s e l v e s to a c o m p a r i s o n of two s i m i l a r s y s t e m s in germanium compounds - (CH~).~GeGe(CH3)3 and C13GeC}I2CH2GeCI3, for which r a t h e r s i m i l a r UV absorption c u r v e s have been obtained (Fig. 3). F o r both germanium compounds the intensity of the absorption is substantially higher than might have been expected judging by the s p e c t r a of GeX4. A m o r e pronounced bathochromic shift is o b s e r v e d for the s y s t e m C - G e - G e - C . This c o r r e s p o n d s to the assumption of m o r e favorable conditions of o v e r lapping of the wave functions, describing the upper occupied and lower vacant o r b i t a l s of the C - G e and G e - C f r a g m e n t s (with predominant contributions of the atomic orbitals of Ge). EXPERIMENTAL
METHOD
T h e Raman s p e c t r a w e r e m e a s u r e d on a Coderg PHO-1 s p e c t r o m e t e r (OKG LG-36 radiation s o u r c e , H e - N e , ~ 15,803 em-1; s c a t t e r i n g at an angle of ~ 9 ( r ) . The integral intensities of the lines (calculated p e r mole) w e r e m e a s u r e d by the internal standard method; the intensity of the line 313 cm "l in the s p e c t r u m of CC14 was taken as 100, and the derivative ~ a / a q for the C -C1 bond of the CC14 molecule in the o2 liquid phase was a s s u m e d equal to 3.35 A . The UV s p e c t r a w e r e obtained on a Specord UV-VIS s p e c t r o p h o t o m e t e r (Z > 200 am) and a vacuum s p e c t r o m e t e r , based on a VMR-2 m o n o c h r o m a t o r (Z < 200 nm). Hexamethyldistannane was produced by the r e a c t i o n of (CH3)3SnC1 with sodium in liquid NH 3, a c c o r d ing to [15], mp 23 ~ bp 182 ~ (756 ram). Its t r e a t m e n t with iodine powder led to (CH3)3SnI, mp 190 ~ (with dec.). H e x a m e t h y l d i g e r m a n e was p r e p a r e d by the interaction of (CH3)3GeC1 with potassium in the absence of a solvent [15], bp 86 ~ (160 ram), n ~ 1.4614. The compound C13GeCH2CH2GeCI~ was produced by the r e a c t i o n of t r i e h l o r o g e r m a n e with ethylene according to [16], mp 50 ~ bp 82-84 ~ (0.4 mm). The authors would like to e x p r e s s t h e i r gratitude to V. T. Aleksanyan and A. V. Iogansen for p r o viding the facilities for m e a s u r i n g the Raman s p e c t r a , to V. A. Petukhov for aid in m e a s u r i n g the UV s p e c t r a , and to V. I. Shiryaev for aid in synthesizing c e r t a i n compounds. CONCLUSIONS 1. The possibility and limits of applicability of the s e m i e m p i r i c a l f o r m u l a s of Woodward and Lippineott, r e l a t i n g the intensities of the Raman lines and the p a r a m e t e r s of the chemical bonds, w e r e investigated. The limitation of such functions was d e m o n s t r a t e d for compounds containing atoms from the lower p e r i o d s of the periodic s y s t e m . 2. The e x t r e m e l y substantial intensity of the Raman line of the valence s y m m e t r i c a l vibration of S n - C in the s p e c t r u m of hexamethyldistannane and the data of the UV s p e c t r a p e r m i t t e d us to establish that in the s y s t e m of C - S n - S n - C bonds t h e r e is an interaction between the e l e c t r o n clouds of the C - S n bonds, which can be c o n s i d e r e d as a ,a-conjugation. LITERATURE 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
CITED
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