Low Frequency Vibrations of Molecular Crystals. XIII. CH 3 AsCl 2,CH 3 AsBr 2,CH 3 Asl 2,and CD 3 Asl 2

The ir spectra from 4000 to 33 cm⁻¹ have been recorded of CH₃AsCl₂, CH₃AsBr₂, CH₃Asl₂, and CD₃Asl₂. The Raman spectra of each of these compounds have been recorded and depolarization values measured for CH₃AsCl₂ and CH₃AsBr₂. The observed spectra have been successfully interpreted on the basis of a pyramidal skeletal structure of C_s symmetry. The observed values for the Teller–Redlich product rule for the assignments of the normal modes of the CH₃Asl₂ and CD₃Asl₂ molecules are in reasonable agreement with the theoretical values for both symmetry species. The As–CH₃ torsional mode was tentatively assigned in the ir spectrum of solid CH₃AsBr₂ at 134 cm⁻¹ and the torsional barrier calculated to be 461×30 cm⁻¹ (1.32×0.08 kcal/mole). The low frequency bending modes of solid CH₃AsI₂ and CD₃AsI₂ have been found to show correlation field splitting and have been successfully explained in terms of the four molecules per unit cell with a space group of C_2h⁶.