Molecular Rotation in Crystalline Disubstituted Ethanes

Abstract

The dielectric constants and conductivities of ethylene chloride, ethylene bromide, ethylene cyanide, ethylene diamine and ethylene thiocyanate have been measured over a wide temperature range in the solid and liquid states and in the frequency band 1–100 kc. Thermal measurements have revealed hitherto unreported transitions in ethylene cyanide at —38° to —46° and in ethylene bromide at —23° to —25°. The ethylene cyanide molecule rotates about all three axes in the crystal between the transition and melting temperatures. The internal rotational motion of atomic groups about the central C–C bond, which is believed to characterize this molecule in the liquid state, is not observably restricted by solidification. This internal motion is believed to facilitate molecular rotation in the crystal by increasing the time‐average symmetry of the molecule. An atomic polarization of about 17 cc below the transition is ascribed to the persistence of some degree of torsional vibration within the molecule even in the low temperature phase of the crystal. In none of the other crystals does the polar form of the molecule rotate below the freezing point nor is there evidence of unusual intramolecular motion except possibly in ethylene thiocyanate. It is probable, however, that the trans form of the ethylene bromide molecule rotates about its long axis between the transition and melting points.