THE ANHARMONIC-FORCE FIELD OF THIOFORMALDEHYDE, H2CS, BY AB-INITIO METHODS

The quartic force field of thioformaldehyde has been calculated ab ini tio using large basis sets and augmented coupled cluster methods. Calc ulated fundamentals are in excellent agreement with experiment, as is the most important Coriolis coupling constant. Computed values for the anharmonicity, rovibrational coupling, and centrifugal distortion con stants of the four isotopomers (H2CS)-S-32, (H2CS)-S-34, (HDCS)-S-32, and (D2CS)-S-32 have been reported. Predictions have been made for all vibrational transitions from the ground state to excited states with at most two quanta for these isotopomers, both using second-order pert urbation theory corrected for Darling-Dennison resonance and using vib rational SCF-CI calculations. For (D2CS)-S-32, perturbation theory per forms quite well; for the other isotopomers, performance is poorer for states involving excitation of the out-of-plane bend and, for the (H2 CS)-S-32 and (H2CS)-S-34 isotopomers, also for the antisymmetric bend that is in severe Coriolis resonance with it. A possible explanation h as been suggested. (C) 1994 Academic Press, Inc.