Rotating Polar Groups in ``Organic'' Molecules

The temperature dependence of the electric moment, observed in the case of molecules with rotating polar groups, permits a quantitative explanation in terms of intra-molecular forces opposing free rotation of the groups. Specifically, the following questions have been raised and treated: I. The statistical weight to be associated with a configuration. II. Approximation of the periodic potential function by its first and second Fourier component. III. An application to 1,2-dichloroethane is given and a plausible potential of interaction between the two CH2Cl groups is obtained. The result suggests the presence of repulsive forces between two bound chlorine atoms at distances as large as 4.0 Angstrom. IV. The related question of the thermal equilibrium ratio of the cis and trans isomers of dichloroethylene is discussed. A new explanation of the observed value is proposed, which attributes about equal heat of formation to both isomeric forms.