Infrared Spectra and Structures of Solid CH4 and CD4 in Phases I and II

Abstract

This paper reports the results of extensive infrared absorption measurements of CH4, CD4, and mixed isotopic crystals containing CH4, CD4, CHD3, and CH3D at low concentration in matrices of light and heavy methane in crystalline phases I and II. The interpretation of the spectra, based upon the Halford–Hornig approach, provides information on the crystal structures. The spectra indicate that the two solid phases of CH4 are similar to the two higher temperature phases of CD4. In phase I, there is apparently considerable motion: either the molecules undergo large angular oscillations or even rotational diffusion. In phase II, there exist two types of sites for the molecules: some molecules undergo quantized rotation similar to that found in noble gas matrices while others order with a local symmetry that is probably slightly distorted from Td[Formula: see text] to D2d[Formula: see text]. Acceptable factor groups are then limited to Td and Oh, but the former is unlikely for it could yield the f.c.c. lattice found by diffraction experiments only by a particular choice of the parameters, whereas the latter leads to the f.c.c. space group [Formula: see text] (Fm 3c).