AB-INITIO AND SPECTROSCOPIC STUDY OF DIMETHYL SULFIDE - AN ANALYSIS OF THE TORSIONAL SPECTRA OF (CH3)(2)S AND (CD3)(2)S

The far-infrared spectra of dimethyl sulfide (CH3)(2)S and (CD3)(2)S h ave been determined from ab initio calculations and a number of the we aker bands reassigned to cross sequence transitions between the higher torsional levels. For this purpose, the potential energy surface for the double methyl rotation was calculated with the MP4/6-31G(df,p)/MP2 /6-31G(df,p) basis set. The energy values, as well as the kinetic para meters, were fitted to double Fourier expansions as functions of the r otational angles. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G(36) nonrigid group. The torsiona l bands of the infrared active transitions were found to be clustered into quartets. For most of the bands the torsional splittings were cal culated to be too small to be resolved. The intensities of the transit ions to the torsional components were found to be roughly proportional to their nuclear spin weights with the G --> G component being the mo st intense.