Reports of the Academy of Sciences of the USSR

1. Vol. 131, No. 4

Physical Chemistry

Yu. G. Borodko and Corresponding Member of the Academy of Sciences of the USSR Ya. K. Syrkin

Temperature and Concentration Dependence of the Frequencies and Intensities of Raman Scattering Lines of Mixtures of Ketones with Chloroform

The study of IR absorption spectra and Raman scattering spectra of the acetone—chloroform system showed that the frequencies of the stretching vibrations of the carbonyl group and of the C—H group of chloroform do not change in comparison with their values in the individual substances. Meanwhile, the intensity of the corresponding bands increases significantly ((^{1-4})). The increase in intensity in this case is the result of the formation of a molecular compound. This example shows that the formation of a hydrogen bond may be reflected to a considerable extent in the intensities and to a lesser extent in the vibration frequencies.

To clarify the nature of the interaction, we investigated the temperature dependence of the frequencies and intensities of Raman scattering lines in the nonpolar solvent hexane, and also in ketone—chloroform mixtures. Upon lowering the temperature, the interaction manifests itself more distinctly, since the thermal disorientation that hinders the formation of molecular compounds decreases. Below we present experimental data on the concentration and temperature dependence of the Raman scattering lines corresponding to the stretching vibrations of the C=O bonds of ketones and of the C—H, C—Cl bonds of chloroform in the systems, as well as in the corresponding individual substances.

In Tables 1 and 2 the integral intensities (I = \int I_\nu d\nu) are given, expressed on an arbitrary scale, where the intensity of the Raman scattering line of an individual substance at (t = +30^\circ)C, referred to one mole, is taken as 100.

The measurements were performed on an ISP-51 spectrograph with photoelectric recording of the spectrum. The optical part of the temperature apparatus consisted of a thermostated cuvette. The required temperature was attained by evaporation of liquid nitrogen. When measuring intensities at different temperatures it is necessary to take into account the change in the ratio (n^2/d), where (n) is the refractive index and (d) the density. In our experiments the greatest change in this quantity over the temperature interval from (+30^\circ) to (-90^\circ) is about 4%. The error in measuring intensities is 10%, and in measuring frequencies (\pm 2\ \text{cm}^{-1}). The temperature from (+30) to (-30^\circ) was measured with an accuracy of up to (1^\circ). The errors in measuring lower temperatures did not exceed (3^\circ).

The experiments show that the intensity of the C=O and C—H lines increases in the ketone + chloroform system and decreases in solutions of these substances in hexane. The intensity of the C—Cl line in both cases changes practically not at all (Fig. 1). The constancy of the frequency and intensity of the C—Cl line indicates that the intermolecular interaction affects mainly the C=O bonds of the ketone and the C—H bond of chloroform. The picture of hydrogen-bond formation is complicated by the fact that ketones at room temperature are strongly associated and, apparently, partially dimerized. This con—

Table 1

Concentration dependence of frequencies and intensities

Table 2

Temperature dependence of frequencies and intensities

is confirmed by the increase in the vibration frequency of the carbonyl group of ketones in hexane when ketone dimers, associated probably in the form of quadrupoles, dissociate:

[
\begin{array}{c}
\mathrm{H_3C} \
\quad \backslash \
\mathrm{C}=\mathrm{O} \
/ \quad \cdot \quad \cdot \
\mathrm{H_3C} \quad \cdot \quad \cdot \quad \mathrm{CH_3} \
\quad \cdot \quad \cdot \quad / \
\mathrm{O}=\mathrm{C} \
\quad \backslash \
\mathrm{CH_3}
\end{array}
]

Such an assumption agrees with experiments on the dielectric polarization of acetone solutions, which changes from 160 cm³ in extremely dilute solutions in nonpolar solvents to (\simeq 40) cm³ in pure liquid acetone ((^5)). In the presence of (\mathrm{CHCl_3}), the vibration frequency of the carbonyl group does not change, despite the fact that chloroform causes dissociation of the dimers. This is explained, possibly, by the fact that simultaneously with the dissociation of the dimers there occurs the formation of a molecular compound with chloroform, and moreover

Fig. 1. Concentration dependence of the intensities of chloroform lines:
1 — intensity of the C—H line (3019 cm(^{-1})) in acetone solution;
2 — intensity of the C—Cl line (667 cm(^{-1})) in acetone and hexane solutions;
3 — intensity of the C—H line (3019 cm(^{-1})) in hexane solution

Fig. 2. Temperature dependence of the intensity of the acetone line (1708 cm(^{-1})):
1 — in chloroform solution (6.5 mol/l);
2 — pure acetone;
3 — in hexane solution (7.5 mol/l)

the vibration frequencies of the carbonyl group in dimers and in the molecular compound are very close. Another reason for the constancy of the frequencies is the relatively low concentration of complexes at room temperature. Thus, when the temperature is lowered to (-90^\circ), the vibration frequency of the C—H bond of chloroform, (\Delta\nu = 3019\ \mathrm{cm}^{-1}), in the acetone—chloroform system decreases to 3010 cm(^{-1}), whereas in pure chloroform it remains unchanged down to its melting temperature.

The change in the intensity of the carbonyl-group line in liquid acetone, in a solution of acetone in hexane, and in a mixture of acetone with chloroform as a function of temperature is shown in Fig. 2.

The intensity of the lines of the carbonyl bond and of the C—H bond of chloroform is composed of the intensity of the lines of isolated molecules ((I_m)) and of molecules associated in the complex ((I_k)), where (I_k > I_m). The enthalpy of formation ((\Delta H)) of the acetone + chloroform molecular compound can be estimated from the temperature course of the change in the integral intensity of the line with frequency 3019 cm(^{-1}) of chloroform. If it is assumed that the intensity of the 3019 cm(^{-1}) line of chloroform at a given temperature is composed of the intensities of the lines of monomeric molecules and of molecules associated into a complex, which are not resolved, then

[
I_T =
\left[
I_m + (I_k - I_m)\frac{C_k}{C_0^{\mathrm{chl}}}
\right]
\left(
\frac{1}{1 - e^{-h\nu/kT}}
\right),
]

where (I_{\tau}) is the intensity of the line at the given temperature; (I_{m}) is the intensity of the 3019 cm(^{-1}) line in hexane, extrapolated to infinite dilution; (I_{k}) is the intensity of the 3019 cm(^{-1}) line in acetone, extrapolated to infinite dilution; (C_{k}) is the concentration of the molecular compound; (C_{0}^{\mathrm{chl}}) is the initial concentration of chloroform.

The factor (\dfrac{1}{1-e^{-h\nu/kT}}) for (\Delta \nu = 3019) cm(^{-1}) remains practically constant in the temperature interval studied. Therefore one may take

[
I_{\tau}=I_{m}+(I_{k}-I_{m})\frac{C_{k}}{C_{0}^{\mathrm{chl}}}.
]

From the experimental data we calculated the equilibrium constants for the reaction

[
\begin{array}{ccc}
\mathrm{H}{3}\mathrm{C} & & \mathrm{H} \}\mathrm{C
\diagdown & & \diagdown \
\mathrm{CO}+\mathrm{HCCl}{3} & \rightleftarrows & \mathrm{C}=\mathrm{O}\ldots \mathrm{H}-\mathrm{CCl} \
\diagup & & \diagup \
\mathrm{H}{3}\mathrm{C} & & \mathrm{H}}\mathrm{C
\end{array}
]

at temperatures of 301, 263, and 243°K, and obtained the values 0.2, 0.44, and 0.75 l/mole. From these quantities we found the enthalpy and entropy of the reaction: (\Delta H \simeq -3350) cal/mole, and (\Delta S=-14.3) entropy units. According to the data of Melvin-Hughes from vapor elasticities ((^{6})), (\Delta H=-4070) cal/mole, while according to Pimentel’s data, obtained from proton magnetic resonance, (\Delta H=-2500) cal/mole ((^{7})). The values we found lie in between. The standard free energy of formation of the molecular compound is 910 cal/mole. This explains its instability at elevated temperature.

Moscow Institute of Fine Chemical Technology
named after M. V. Lomonosov

Received
30 XII 1959

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