Conformer equilibria in dichloroacetic acid solutions

Spectrochimica Acta, Vol. 32A, pp. 1379 to 1382. Pergamon Press 1976. Printed in Northern Ireland

Conformer equilibria in dichloroacetic acid solutions L. M. BABKOW,** V. V.

VASHCHINSKAYA,*

M. A. KOVNER,**

G. A. PUCHKOVSKAYA,Yu. YA. FIALKOV* Institute of Physics. Ukrainian SSR Academy of Sciences, Kiev, USSR;* Kiev Polytechnical Institute, Kiev, USSR; **Saratov State University, Saratov, USSR (Received 1 October 1975) Abstract--The vibratiofml spectra of dichloroacetic acid in the solvents with different dielectric constants (e = 2-35) have been studied. Observed concentration and temperature dependences of i.r. band intensities in the stretching and deformation vibrations regions for monomer molecules showed the presence of two spectrally different conformers due to the internal rotation around the C---C bond. The forms and frequencies of normal vibrations of molecules were computed for several types of the conformers with different angles of rotation q~. The comparison of experimental and calculated frequencies and dipole moments allowed to identify two forms of dichloroacetic acid conformers existing in the solutions studied. The influence of dielectrical permeability upon the dichloroacetic acid conformational equilibrium constants is examined.

INTRODUCTION

W h e n a halogen atom is introduced in the a position of a carboxylic acid, two rotational isomers coexist. They differ by the orientation of the carboxylic group relative to other atoms [1, 2]. The presence of two rotational isomers leads to a doublet structure of some absorption bands in vibrational spectra of diluted solutions of m o n o h a l o g e n substituted carboxylic acids. The absorption band, corresponding to the vibration of the carbonyl group C~------O,first of all belongs to such bands. This band is the most intense one in the i.r. spectrum and its doublet structure becomes most distinct. By analogy with monohalogensubsitituted carboxylic acids, in particular, with monochloroacetic acid [2], one may suppose that rotational isomers also exist in dihalogensubstituted carboxylic acids. In this case it is interesting to interpret the vibrational spectra of these acids which in solution are a mixture of at least two spectrally discernible rotational isomers. O n e must elucidate also the influence of a solvent upon the c o n f o r m e r equilibrium constant. This work deals with dichloroacetic acid. EXPERIMENTAL

In order to confirm the supposition of rotational isomers existence for dichloroacetic acid and to make clear the influence of the solvent on the conformer equilibrium constant we have investigated the i.r. spectra of dichloroacetic acid, dissolved in heptane, cyclobexane, carbon tetrachloride, benzene, chiorobenzene and nitrobenzene over the temperature range from 16 to 80°C. Infrared 1

spectra of solutions of acetic, monochloroacetic and trichloroacetic acids in carbon tetrachloride are also measured. The solution concentration was 10 - t 10 -3 mol/l. The spectra were obtained on the spectrophotometers UR-10 in the 4000-400cm -1 region and FIS-3 in the 400-50 cm -1 region. MEASUREMENT, RESULTS A N D DISCUSSION

The measured i.r. absorption spectrum of dichloroacetic acid contains about 30 bands, which can be assigned to m o n o m e r i c molecules vibrations. O n the Fig. 1 a scheme of measured frequencies of stretching vibrations of the carbonyl group in m o n o m e r s of acetic, mono-, di- and trichloroacetic acids, dissolved in carbon tetrachloride, is presented. O n e can see from Fig. 1, that the introduction of a chlorine atom in the acetic acid molecule leads to appearance of two carbonyl bands, which are due to existence of two optically different isomers [2]. O n e of these bands is characterized by a more high frequency (1792 cm -1) as c o m p a r e d with the corresponding frequency in acetic acid ( 1 7 6 2 c m - t ) , whereas the position of the other band is altered insignificantly. The introduction of the second chlorine atom leads to a slight displacement of the considered bands (1768, 1788 cm-t). In the i.r. spectrum of the trichloroacetic acid only one band with the increased frequency (1790 cm -1) becomes apparent. The doublet structure of the absorption band, corresponding to stretching vibration of the ~ O group, in the i.r. spectrum of the dichloroacetic acid indicates the presence of its two

1379

1380

L.M. BABKOW,V. V. V.ASHCHINSKAYA,M. A. KOVNER,G. A. PUCHKOVSKAYAand Yu. YA. FIALKOV

I I

l

CCt3COOH CHCt2CO0 H

l

I CH2OCOOH

J I 1760

cm-I

J CH3COOH 1790

Fig. 1. Positions of v ( ~ ) frequencies in absorption spectra of monomeric acetic, mono-, di- and trichloroacctic acids dissolved in CCl4. rotational isomers in the solution. The doublet structure of the band is still conserved in going to other solvents. The doublet frequencies v~= 1768 cm -~ and v2 = 1788 cm-1 are independent on the acid concentration and on the temperature and nature of the solvent. The only exception from our selected solvents is nitrobenzene, in which the considered frequencies of dichloroacetic acid are v~ = 1766 cm -~ and v2 = 1778 cm -]. Temperature changing differently influences the intensities of the doublet bands. The integrated intensity of the higher frequency band v2 increases with the temperature increasing more than the intensity of the lower frequency one. Thus, the frequency v2 = 1788 cm -~ corresponds to the vibration of the C = O - b o n d of rotational isomer, which is stable at high temperatures, whereas the frequency Vl = 1768 cm -1 corresponds to the C = O stretching vibration of the isomer stable at low temperatures. In Fig. 2 the temperature dependences of the integrated intensities relation I(v2)/l(vl) of the dichloroacetic acid in carbon tetrachloride and chlorobenzene are shown. This de-

pendence is linear. Detailed investigation of the intensity temperature dependence is pertormed also for some bands in the 400-650cm -1 and l l 0 0 - 1 3 0 0 c m -1 regions. On the basis of these investigations the band with frequencies 520, 1163cm -1 are assigned to the high temperature isomer, whereas the bands with frequencies 542, 608, l l 3 0 c m -~ are assigned to the low temperature one. In Fig. 3 two possible limit forms of dichloroacetic acid isomers are indicated. These two isomers differ by their symmetry: one of them belongs to Cs, the other being asymmetric. We suppose the asymmetrical conformer to be a high temperature and assign to it the frequencies v 2 ( ~ O ) = 1788cm -~ and 520, 1163 cm-L The symmetrical

H(t)I't!0")

H(2)

CL(z)

q:~.~2H

/q

III

0(2) q4 ~ H ( z } Qo

/ Oc Pcc

Cl(:t}

"~

/

Io(?jH(2)

2.0--

1.5

H(,)I

qs

7,c~/_Yo] 7bc~ -

°°\

O{n

Fig. 3. Newman projections and internal vibrational coordinates of dichloroacetic acid rotational isomers.

I.O

0.5

I

50

I 40 t °C

I 50

Fig. 2. Temperature dependence of integrated intensities relation (Iv2/Ivl) of v(C----O) absorption bands for CHCIECOOH in carbon tetrachloride (1) and chlorobcnzene (2).

conformer exists at low temperature and the frequencies v1(C=-O)=1768cm -1 and 542, 608, l l 3 0 c m -] correspond to it. With the help of dipole moments of separate bonds [3] we have appreciated the dipole moments of the symmetrical and asymmetrical rotational isomers to be p.~= 1.05 D and tx2=2.58 D. The experimental value of the dipole moment of monomer of dichloracetic acid at room temperature is 1.09 D [4]. If two conformers possess different polarity, the change of the dielectric permeability of the solvent

Conformer equilibria in dichloroacetic acid solutions

0.8

0.6

I

0.4

o

I o.~

I 0.2

I o.3

I 0.4

I/E

Fig. 4. 1/e dependence of integrated intensities relations (lv2/Ivt) for di- (1) and monochloroacetic (2) acids. C~------O. 1M. m u s t b e a c c o m p a n i e d by a c h a n g e of the c o n f o r m a tional e q u i l i b r i u m constant. In this work a n investigation of c o n f o r m a t i o n a l e q u i l i b r i u m of d i c h l o r o a c e t i c acid was carried o u t in solvents with different values of the dielectric p e r m e a b i l i t y E. In Fig. 4 t h e d e p e n d e n c e of t h e

1381

i n t e g r a t e d intensities r e l a t i o n I(v2)/I(vl) f r o m 1/E is shown, w h e r e e is c h a n g i n g in the 2 - 3 4 . 8 interval. It is clear t h a t increasing of the dielectric p e r m e a b i l i t y of the solvent is a c c o m p a n i e d by increasing of the intensity of t h e b a n d with vz = 1788 cm -~, w h e r e a s the intensity of t h e b a n d with Vl = 1768 cm ~ decreases. T h e polarity a n d dielectric p e r m e a b i l i t y c h a n g e in the same way in going from o n e solvent to a n o t h e r . C o n s e q u e n t l y o n e c a n affirm t h a t in going f r o m a less p o l a r solvent to a m o r e p o l a r o n e t h e intensity of b a n d with f r e q u e n c y u2 = 1788 cm -~ increases. T h e r e f o r e this f r e q u e n c y m u s t be assigned to s t r e t c h i n g v i b r a t i o n of the C----O-bond of the a s y m m e t r i c a l r o t a t i o n a l isomer. T h e a b s o r p t i o n b a n d with f r e q u e n c y u I = 1768 cm -1 c o r r e s p o n d s to the s y m m e t r i c a l isomer. This is true for the m o n o c h l o r o a c e t i c acid too. In its s p e c t r u m a n a n a l o g o u s d e p e n d e n c e of the i n t e g r a t e d i n t e n sities r e l a t i o n I(u2)/I(uO f r o m the 1/e value ( v l = 1765, v2 = 1792 cm -~) is o b s e r v e d (see Fig. 4). T o confirm the p r o p o s e d a s s i g n m e n t of the d o u b let b a n d s a n d to i n t e r p r e t the whole dichloroacetic acid i.r. s p e c t r u m we h a v e calculated the f r e q u e n cies a n d the n o r m a l m o d e s of its b o t h isomers.

Table 1. Vibrational frequencies of the dichloroacetic acid monomer and their interpretation Experiment i.r.-spectrum u(cm -I ) 178 205 233 272 304 325 520 542 608

Asymmetrical isomer u(cm -I) normal mode 179

Calculation Symmetrical isomer u(cm- 1) symm. normal mode

T,c,/3a4,/32,/33

209 280 321

/33 P~, 3'.o/3.4,/32 /3a4, 3',c

527 534

p K

648

645

Tab, /3a4, Tac, q3

667 765 840 915 1130 1163 1199 1235 1275 1310 1330 1768 1788 3003 3520

687 776

O,/33 0, q2,/32

930

Oc,/3.4

1149 1192

/3a4 o/13, o/12

1267 1315

/31

1788 2974 3498

198 199

A" A'

~1, 0

323 342

A' A'

/3,4, T,c a

536 537 600

A" A' A"

K /3,4T, b p

685

A'

Tab, q

836 906 1151 1177(?)

A" A' A' A"

p,/3, q /3.4, Q~ /3,4 0/

1238

A'

/3z, ot

1335 1768

A' A'

/3, T.~, Qa Qb

2974 3502

A' A'

ql q4

fl, T.~

/3.4, 3'.b, Qa Qb ql q4

1382

L.M. BABKOW, V. V. VASHCHINSKAYA,M. A. KOVNER, G. A. OUCHKOVSKAYAand Yu. YA. FIALKOV

CALCULATION OF FREQUENCIES AND NORMAL MODES AND SHORT DISCUSSION

The calculation is carried out by the El'yashewich-Stepanov method. The geometrical parameters are taken from [5]. In a preliminary calculation version the force fields of both isomers were taken as the same, whereas the inverse kinetic energy matrix were taken different. The force constants were transfered from acetic acid [6] and dichloroacetonitrile [7]. The twisting vibration was not taken into account. The secular equations have been solved on the computer "M-22." The experimental and calculated frequencies are in good agreement. But, as the calculation has shown, the difference in the inverse kinetic energy matrix of both isomers has not influenced on all calculated frequencies. In particular, the difference between stretching C-----O-vibrations frequencies of the two isomers was only 2 cm -1. It follows that the observed doublet must be explained by different force fields of both isomers. Such a possibility was indicated by BELLAMY [1], who explained the frequency increase of the stretching C------O-vibration in halogensuhstituted carboxylic acids by a "field effect." The latter consists accordingly to [1] of an electrostatic interaction between a carbonyl oxygen atom and nearly situated polar halogen atom in the plane of atoms, forming the carbonyl group. This interaction deforms the electronic shell, altering the force constant of the C ~ O - b o n d and its frequency.

We have evaluated the difference between the C==O-force constants of the two isomers. To do this we have performed a variation of force constants characterizing the interaction between the C = O coordinates using the experimental frequencies and the normal modes. The difference between the force constants is 0.5 106 cm-2(Kc~ = 21.23 • 106 for asymmetrical isomer and Kc, = 20.71 • 10 6 c m -2 for the symmetrical one). The frequencies and normal modes calculations results are presented in Table 1, where they are compared with experimental frequencies of the monomeric dichloroacetic acid, dissolved in carbon tetrachloride.

REFERENCES

[1] L. BELLAMY, Novye Dannye po IK Spectram Slozhnykh Molecul. Mir, 1971. [2] G. I. GIrqZBtJRN, B. N. TARASOV,JOKh, 42, 2470 (1972). [3] J. W. SMITH,Electric Dipole Moments. London, 1955. [4] O. A. OsIPOV, V. I. MINKIN, A. D. GARNOVSKI. Spravochnik po Dipolnym Momentam. Vysshaya shkola, Moscow, 1971. [5] G. H. KWEI,R. F. CURL.Jr. J. Chem. Phys., 32, 1592 (1960). [6] L. M. SVERDLOW,Isvestiya A N SSSR, ser. phys., 17, 567 (1953). [7] M. V. VOLKENSTEIN, L. A. GRIBOV, M. A. EL'YASHEVICH, B. I. STEPANOV,

Nauka, Moscow, 1972.

Kolebaniya molecul.