The Temperature Variation of the Electric Moment of Ethylene Chloride

Observations of the molecular polarization of ethylene chloride vapor have been made over a temperature range from 32° to 270°C and over a considerable pressure range at each temperature. These observations show a definite departure from the Debye theory, since the Debye function plotted against temperature gives a definitely curved line. The data might be explained as approximately consistent with an electric moment of value zero and a value of the constant part of the polarization equal to about twice that of the optical refractivity; but for reasons discussed this interpretation is disregarded. It seems necessary to assume that the electric moment varies with temperature, as suggested by Eucken and Meyer. This was to be expected as a result of the hindrance of free rotation due to the interaction of the two atomic groups rotating around the single bond between the two carbon atoms. The possibility of the existence of disturbing effects such as association and adsorption is discussed, but is shown to be improbable, if not untenable, The values of the electric moment are then calculated by the use of a value for the constant part of the polarization, 23.9 ${\mathrm{cm}}^3$, determined by C. S. Hitchcock from the solid state. In the observed temperature interval the electric moment increases with the temperature from 1.1 to 1.5×${10}^{-18\mathrm{}}$ c.g.s.e.s. u.; whereas the value calculated by Meyer for truly free rotation is 2.4×${10}^{-18\mathrm{}}$. This suggests that the rotation is considerably hindered; and in fact the results are entirely in accord with the predictions of Eucken and Meyer. The interaction energy may, however, be sufficiently great that it may be necessary, in order to obtain an accurate theory of this effect, to take into consideration the quantum theory rather than the classical theory of Meyer. The interpretation of other observations, of Meyer and of Sänger, is discussed with reference to certain inconsistencies which have been observed.