An analysis of the methyl rotation dynamics in the S0 (X̃ 1A1) and T1 (ã 3A2) 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, a 3A‘←X 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 C2v–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 V(θ...