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 CH 4 , CD 4 , and mixed isotopic crystals containing CH 4 , CD 4 , CHD 3 , and CH 3 D 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 CH 4 are similar to the two higher temperature phases of CD 4 . 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 T d [Formula: see text] to D 2d [Formula: see text]. Acceptable factor groups are then limited to T d and O h , 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).