A determination of site symmetries in crystalline methane at low temperatures from the infrared spectra of mixed isotopic crystals

The infrared spectra of uncoupled CH4, CHD3 and CD4 molecules are shown to be consistent with T or T-d site symmetry in phase II of crystalline CH4 and CD4. In phase III of CD4 the site symmetry is low; it does not contain any threefold or higher axis of symmetry. Acceptable sites groups are D-2, C-2v, C-2, C-S and C-1.