AN ANALYSIS OF THE METHYL ROTATION DYNAMICS IN THE S0(X1A1) AND T1(A3A2) STATES OF THIOACETONE, (CH3)2CS AND (CD3)2CS FROM PYROLYSIS JET SPECTRA

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800-18 500 cm-1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T1 <-- S0, approximately a 3A" <-- approximately X 1A1, results from an n <-- pi-* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out-of-plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2-upsilon-C(s) molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree-Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36-degrees from the molecular plane and the methyl groups were rotated by 10.93-degrees in clockwise-counterclockwise directions. Restricted Hartree-Fock (RHF) calculations were used to generate the V(theta-1,theta-2) potential surface governing methyl rotation for the S0 state. This was incorporated into a two-dimensional Hamiltonian, symmetrized for the G36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the nu-17 (b1) mode of S0 (CH3)2CS/(CD3)2CS were found to agree with the experimental values, 153.2/114.7 cm-1.