Dynamical and spectroscopic studies of nonrigid molecules. Application to the visible spectrum of thioacetaldehyde

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Abstract
The methyl torsion and aldehydic hydrogen wagging modes are studied theoretically in thioacetaldehyde, in both ground and first triplet excited states. For this purpose, the potential energy surfaces were determined by ab initio restricted Hartree Fock and unrestricted Hartree Fock calculations with 4–31 basis set +d orbitals on the sulphur atom. The two electronic states were found to have different preferred conformations. The singlet state exhibits a planar eclipsed conformation, whereas the structure of the triplet state is anti-eclipsed and pyramidal. It was found that a potential function, which was constructed from a symmetry adapted double Fourier expansion of the wagging and torsional coordinates, gave a reasonable fit to the energy points. The two-dimensional Schrödinger equations for the torsion and wagging motions were solved for both singlet and triplet states, taking into account the internal symmetry and the appropriate basis. The relative band locations and the intensities (Franck–Condon factors) were evaluated from the eigenvalues and the eigenvectors. Spectra were calculated for the thioacetaldehyde CH3CHS/CH3CDS/CD3CHS/CD3CDS isotopomers. New assignments for the higher bands in the CH3CHS spectrum are proposed.