The geometry and force field of thioformaldehyde

A six-dimensional variational code, recently presented for the determination of the rovibrational energy levels of formaldehyde, has been used to refine a quartic potential surface of thioformaldehyde and simultaneously to optimize its geometry. The parameters have been adjusted to obtain as good agreement as possible with the known J = 0 vibrational energy levels and the corresponding observed J = 1 rotational lines of both H2CS and D2CS, where known. This is a highly nonlinear problem, and it is not possible to refine separately the geometry and force field. We have simultaneously optimized the HCS bond angle, the quadratic force field, and a partial cubic and quartic force field. Fermi and Coriolis resonances have been reproduced. The predicted geometry is CH = 1.0856 Angstrom, CS = 1.6110 Angstrom, HCS = 121.88 degrees. Predictions are made for overtones and combination bands of H2CS and D2CS. Furthermore, a prediction is made for the unobserved fundamental nu(5) of D2CS, which lies very close to 3 nu(6). (C) 1998 Academic Press.