THE METHYL TORSION MODES OF $S_{0}$ THIOACETONE FROM BULB AND SUPERSONIC JET LASER EXCITATION SPECTROSCOPY

Author Institution: Department of Chemistry, The University of Kentucky; Instituto de Material Science, c/Serrano; Dept, of Chemistry, Brock University; Department of Chemistry, University of Wisconsin - ParksideLaser excitation spectra of thioacetone $(CH_{3})_{2}CS/(CD_{3})_{2}CS$ have been recorded over the region 17250 - 17750 cm at 300 K and under supersonic jet conditions. The pattern of the vibronic bands in the jet spectrum was observed to be very simple and could be assigned to the sulphur wagging and the methy1 torsion modes. Ab initio M. O. calculations were performed with a $4 - 31G + d$ basis. A two dimensional potential surface $V(\Theta_{1}, \Theta_{2})$ was obtained for the state, where $\Theta_{1}$ and $\Theta_{2}$ are the torsional angles of the two groups. This surface was found to have a an energy maximum at $440 cm^{-1}$ and a saddle point at $220 cm^{-1}$. It was incorporated into the two dimensional Hamiltonian matrix and symmetrized by the $G_{36}$ point group into sixteen blocks and solved by the variational method for the nine symmetry species. The supersonic jet spectrum showed clearly the origin bands at $17327.8/17349.8 cm^{-1}$ in $(CH_{3})_{2}CS/ (CD_{3})_{2}CS$. Frequency intervals of $153.2/114.7 cm^{-1}$ between the origin band and a set of hot bands were assigned to fundamental frequencies of the $S_{0}$ ground state. A comparison of the calculated $A_{1}$ fundamentals, 146.78 / 118.07, showed good agreement with the experimental dat