Infrared Spectrum in Polarized Light of CH2Br2 and CD2Br2. Determination of the Crystal Structure

The infrared spectra of crystalline CH₂Br₂ and CD₂Br₂ were measured at liquid‐nitrogen temperature, between 4000 and 200 cm⁻¹. Splitting of the normal modes was observed for several fundamentals. These data, together with polarization measurements on polycrystalline oriented films, were used to study the crystal structure of CH₂Br₂. The technique of mixed crystals was used to prove the nature of the observed crystal splittings. The data accumulated on this molecule are consistent with a monoclinic $C_{\text{2h}}^5{\text{(P2}}_1\mathrm{\hspace{0.17em}/\hspace{0.17em}c)}$ unit cell or with a triclinic $C_g^1\mathrm{(p̄)}$ cell with two sets of sites. The differences are discussed in terms of close packing.