The Structure of Para-di-chloro-benzene

Abstract

Two low frequency spectra of the crystal of para-di-chloro-benzene are reproduced, one showing the frequencies 27, 54, and 93 and the other 45, 57, and 84 cm−1. The former represent the spectra of a single crystal very slowly cooled from melt, and the latter of a crystal rapidly cooled. The three frequencies in the spectra of the single crystal have been shown to be rotational oscillations, the smallest frequency representing an oscillation about the axis of greatest moment of inertia which is the axis perpendicular to the plane of the molecule. From the x-ray data it is noticed that the molecules in the crystals of para-di-chloro-benzene and para-di-bromo-benzene are differently orientated, but they can be brought into juxtaposition by a rotation about this axis. The different orientations of the molecules in these crystals are however incompatible with the fact that these crystals are isomorphous and form mixed crystals. It has therefore been postulated that the crystal of para-di-chloro-benzene has a tendency to become disordered by the excitation of free rotation about the axis perpendicular to the plane of the molecule which results in the vanishing of the frequency 27 as noticed in the second spectrum. The apparently discordant results of Sircar and Venkateswaran on the temperature independence of Laue spots and Raman spectra and also those of Sircar and Vuks on the spectra of mixed crystals have been explained on this basis, and the frequencies of mixed single crystals of para-di-chloro- and para-di-bromo-benzenes have been calculated in good agreement with the observational data.