VIBRATIONAL SPECTRA OF SINGLE CRYSTALS AND POLYCRYSTALLINE FILMS OF CH2CL2 AND CH2BR2

Polarized infrared spectra of single crystals formed in a miniaturized high-pressure cell and the infrared and Raman spectra of polycrystalline films of CH2C12 and CH2Br2 have been measured. From their vibrational spectra it is suggested that the CH 2C12 molecules occupy C2 sites in a hexagonal D3h, crystal system and that the CH2Br2 molecules are on C, sites in a hexagonal C6v or D6h crystal system. Comparison of the vibrational spectra of the two methylene halides indicates that the intermolecular forces present in the solid phases of the two compounds have different origins. In CH2C12 there appears to be a considerable dipole-dipole contribution to the intermolecular forces. Conversely, in CH2Br2 atom-atom interactions (hydrogen-hydrogen repulsions) seem to account for the intermolecular forces. The Raman spectrum of a mixed crystalline sample of CH2C12 in an CH2Br2 lattice has been recorded in order to separately assign the components of bands due to the chlorine isotopes from those due to the factor-group splittings.