ON D I P O L E - D I P O L E
OF
COMPOUNDS
S. P .
INTERACTIONS
WITH
Apraksin
CARBONYL
and
G. S.
IN S O L U T I O N S
GROUPS
Denisov
UDC 539.196.3 +543.422
When investigating d i p o l e - d i p o l e interactions the m a j o r i t y of investigations pay most attention to the
influence of the mean field of molecules in solution on the p a r a m e t e r s of a band of a given o s c i l l a t o r [1-3].
The r e s u l t s of studies in which fine s t r u c t u r e detected in the v (C =O) band of the IR s p e c t r u m of c e r t a i n
ketches and which have been explained, in p a r t i c u l a r , by the p r e s e n c e of an equilibrium between monomeric
molecules and molecules a s s o c i a t e d by a d i p o l e - d i p o l e mechanism, a r e apparently unreliable [4]. In the
p r e s e n t c o m m u n i c a t i o n the interaction of molecules containing a c a r b o n y l group with molecules p o s s e s s i n g
a l a r g e dipole moment has been investigated by IR spectroscopy. Solutions of earbonyl compounds in CC14
w e r e studied in the p r e s e n c e of t e t r a a l k y l a m m o n i u m halides and in aeetonitrile in c e r t a i n c a s e s . To reduce
e r r o r s linked with a t m o s p h e r i c a b s o r p t i o n the work was conducted at a point in a diffraction s p e c t r o m e t e r
with a small amount of diffuse light [5], the m o n e c h r o m a t o r of which was dried with phosphoric anhydride.
In Fig. 1, (a) r e p r e s e n t s the IR a b s o r p t i o n s p e c t r u m in the v (C = O) region of acetophenone in a solution
containing (CsHi~)4NI. It is seen that on introducing a salt molecule ( ~ ~ 10 to 12 13) into the solution, the
v (C =O) band at 1692 e m -i a c q u i r e s doublet s t r u c t u r e ; a new band appeared on the low frequency side at
1688 c m - i , t h e intensity of which grew with i n c r e a s i n g salt concentration. In spite of the small size of the
shift (N4 c m -1) the fact that the d i s p l a c e m e n t of the band is distinct does not r a i s e doubt since t h e r e a r e
no a b s o r p t i o n bands due to a t m o s p h e r i c w a t e r in the range 1696-1685 e m -1. As was r e c o r d e d in [4] the
c a u s e of the a p p e a r a n c e of the apparent splitting of the v(C =O) band is included in p r e c i s e l y this. An inc r e a s e in solution t e m p e r a t u r e leads to a weakening of the 1688 e m -1 band and a strengthening of the 1692
c m -1 band; t h e s e e h a n g e s with t e m p e r a t u r e a r e r e v e r s i b l e . This indicates the p r e s e n c e in solution of an
equilibrium between two types of aeetophenone molecule viz., free and bonded with salt molecules. In the
p r e s e n c e of dioxan o r t r i e t h y l a m i n e a d i s c r e t e band shift is not o b s e r v e d which indicates the absence of
interactions of a d o n o r - a c c e p t o r bill.
The distinct displacement of the ~ (C =O) band on interaction with salts was also o b s e r v e d for y butyrophenone, benzophenone (Fig. 1, b), and methyl vinyl ketone. In all c a s e s the shift to lower frequency
amounted to 4-6 cm-i~ A s i m i l a r picture was o b s e r v e d on the band at 1688 c m - i for dimethylforrnamide
(Fig. lb), and the lower frequency on forming a complex with Bu4NBr amounted to 8 cm-1. A distinct shift
of the v(C = O) band of dirnethylformamide also a r o s e on interaction with acetonitrile which indicates the
formation of a complex of an e l e c t r o s t a t i c type. A distinct shift has been o b s e r v e d p r e v i o u s l y on the v(C --N) band of acetonitrile [6] and the v(P= O)band of phosphine oxide [7] on interaction with salts.
In the p r e s e n c e of a m m o n i u m salts the v (C =O) band of aliphatic k-etches (acetone, methyl ethyl ketone,
diethyl ketone, and cyclohexanone) is displaced to lower frequency by approximately 4 c m -i and its halfwidth is i n c r e a s e d . The frequency of the band in e s t e r s (ethyl formate, benzyl acetate, and dioctyl phthalate)
is reduced by 1-2 c m - i ; the i n c r e a s e in half width is also less than in ketches. It is interesting that the
band p a r a m e t e r s of (CH3)3CCOCH(CH~) 2 a r e unchanged in the p r e s e n c e of salt which is seemingly caused by
sterle hindrance t o w a r d s the interaetiom C l e a r l y the s t r u c t u r e e x p r e s s e d for the band in e s t e r s and
ketones was not observed; t h e r e f o r e , there is no b a s i s for explaining the picture obtained as the m a n i f e s t a tion of an equilibrium between molecules which a r e free o r interacting w~th salt.
A n a l y s i s of the data obtained showed that the m a x i m u m size of band shift on interaction with salts,
which r e g i s t e r in the s p e c t r u m as a splitting of this band, a r e o b s e r v e d for compounds inwhichthe frequency
Institute of P h y s i c s , Leningrad State University. T r a n s l a t e d f r o m T e o r e t i e h e s k a y a i /~ksperimental'naya Khimiya, Vol. 10, No. 1, pp. 120-122, J a n u a r y - F e b r u a r y , 1974. Original a r t i c l e submitted November
4, 1972.
9 19 75 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced,
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87
D
I
1710
I
1690
a
1670
7690.
1570
1550
b
1710
/ggO ~,
cm "1
C
Fig. 1. IR s p e c t r a of solutions (0.02 M) in CC14: a) a c e t o phenone 1) without additions, 2) and 3) with added 0.02 and
0.2 M (CsHI~)4NI r e s p e c t i v e l y ; b) benzophenone 1) without
additions, 2) and 3) with added 0.5 M (C4Hg)4NBr at t e m p e r a t u r e s of 47 and 16~ respectively; e) dimethylformamide 1)
without additions, 2) and 3) with added 0.3 and 1.0 M a c e t o nitrile respectively.
of v (C =O) lies r e l a t i v e l y low, i.e., in the range 1660-1700 c m -1. The p r e s e n c e of delocalization of the
e l e c t r o n s of the C =O bond (resonance effect) is a c h a r a c t e r i s t i c of t h e s e compounds and this leads to an
i n c r e a s e in the polarity of the C =O bond in c o m p a r i s o n with aliphatic ketones, for example. F o r molecules
in which v (C =O) lies higher at 1780-1810 c m -1 (acetyl chloride, methyl t r i f l u o r a c e t a t e , significant changes
w e r e not o b s e r v e d in the f r e q u e n c y and shape of the ~ (C =O) band in the p r e s e n c e of salt. The high f r e quency of ~ (C =O) in these compounds r e f l e c t s the lowering of the polarity of the C =O bond due to the inductive effect and the field effect. In e s t e r s the i n c r e a s e in bond polarity due to the r e s o n a n c e effect is
compensated by the inductive effect of the e s t e r oxygen which also apparently c a u s e s the intermediate
c h a r a c t e r of the o b s e r v e d s p e c t r a l picture. We r e c o r d that the shift of ~ (C =O) has a tendency to i n c r e a s e
with the growth of the dipole moment of the compounds being investigated. All compounds showed no sign
of association with dioxane o r triethylamine.
The r e s u l t s of the p r e s e n t study, and also of the studies in [6-8], show that oriented van d e r Waals
interactions of dipolar molecules may lead to the f o r m a t i o n of m o l e c u l a r complexes, the d i s s o c i a t i o n e n e r g y
of which significantly i n c r e a s e s the size of kT even at t e m p e r a t u r e s of the o r d e r of 300~
T h e s e data
supplement the existing concept of van der Waals molecules (see r e v i e w [9]) and d e m o n s t r a t e the convention
of the s p e c t r a l c r i t e r i o n (distinct o r smooth shift of frequency), frequently used for the identification of
interaction type.
LITERATURE
1.
2a
3.
4.
5.
6.
7.
8.
9.
88
CITED
G. S. Denisov, in: Spectroscopy o f Interacting Molecules [in Russian], Izd. Leningr. Gos. Un-ta
(1970), p. 170.
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A. E. Lutskii and S. N. Vragova, Opt. i Spektr., 31, 212 (1971).
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