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 CH₂Cl₂ and CH₂Br₂ have been measured. From their vibrational spectra it is suggested that the CH₂Cl₂ molecules occupy $C_2$ sites in a hexagonal $D_{\text{3h}}$ crystal system and that the CH₂Br₂ molecules are on $C_s$ sites in a hexagonal $C_{\text{6υ}}$ or $D_{\text{6h}}$ 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 CH₂Cl₂ 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 CH₂Cl₂ 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.