Far Infrared Spectra of Solid Methyl Halides

The infrared spectra of solid CH3Cl, CH3Br, CH3I, CD3Cl, CD3Br, and CD3I have been measured in the spectral region from 130 to 30 cm—1 at 78°K. Two sharp bands were found in the cases of the chloride and bromide, and three in the case of the iodide. These absorptions are interpreted as arising from lattice vibrational modes. The isotopic shifts exclude libration about the symmetry axis as a possible assignment for any of the bands, however, definite assignments to B-axis librations or translational modes cannot be made with confidence.