Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2 CS have been recorded over the region 16 800-18 500 cm-1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T 1 ←-S0, ã 3 A ″ ← X̃ 1A1, results from an n → π* 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 C2υ-Cs 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° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise-counterclockwise directions. Restricted Hartree-Fock (RHF) calculations were used to generate the F(θ1,θ2) 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 ν17 (b1) mode of S0 (CH3)2CS/(CD3)2 CS were found to agree with the experimental values, 153.2/114.7 cm-1. © 1991 American Institute of Physics.

The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work. D. J. C. acknowledges the support of the National Science Foundation through Grant No. CHE-8914403.

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