Crystallographic and Spectroscopic Studies of Solid Methylene Bromide, Methylene Iodide and 1,1-DICHLOROETHENE

The crystal structures of the three solid phases of methylene iodide and the stable phase of deuterated methylene bromide have been determined using the neutron powder profile structure refinement technique. For CH _2I_2, both phase I at 50 K and phase III at 16 K, and for CD _2Br_2, the stable phase at 15 K, have eight molecules per unit cell on general sites of the space group C2/c. The structure of CH _2I_2, phase II, has been determined at 16 K, 30 K, 50 K and 255 K. This phase is face centred orthorhombic, space group Fmm2, with four molecules per unit cell on sites with C_ {rm 2V} symmetry. Molecular bond lengths and angles, corrected for thermal motions, are given for all structures. The lattice energy of each structure has been calculated using an empirical potential. The most stable phase of CH_2I _2 is phase II, but the energy differences between all three phases are small. Neutron powder diffraction profiles of 1,1-dichloroethene -d_2 (CD_2CC l_2) at 12 K and 120 K have been recorded. The diffraction peaks are indexed on the basis of a non-primitive hexagonal unit cell containing 32 molecules. The molecular positions are not refined. The far infrared spectra of crystalline CH _2CCl_2 and CD_2CCl_2 have been recorded from 30 cm^{ -1} to 400 cm^{-1} at 20 K and 80 K. The corresponding Raman spectra have been recorded from 20 cm^{-1} to 3200 cm^{-1} at the same temperatures. The lattice spectra contain 29 Raman and 15 infrared active modes. A centrosymmetric crystal structure is indicated by these spectra. The lattice modes are assigned on the basis of their isotopic frequency shifts. Crystal field and isotopic splittings are observed in the internal mode spectra at 20 K.