Theoretical rovibrational characterization of the cis/trans-HCSH and H2SC isomers of the known interstellar molecule thioformaldehyde

Abstract

The missing sulfur of the interstellar medium may be hiding in molecules for which there are no spectral data yet available for any astrophysical observation, molecules like isomers of H2CS a known interstellar compound. Every known interstellar molecule is a pointer to the presence and detectability of related molecules in the ISM. This quantum chemical study provides the necessary spectral data for the observation of cis-, trans-, and 3A HCSH as well as for 1A1 and 3A″ H2SC. Benchmarks of the CCSD(T)-F12/cc-pVTZ-F12 quartic force field for the known thioformaldehyde provide anharmonic vibrational frequencies of with an average of 1.9 cm−1 of gas-phase experiment demonstrating reliability for the computed fundamental frequencies. The 1A1 H2SC form has bright spectral features in the infrared and millimeter-wave regimes, but is the highest-energy species of this set. The trans-HCSH isomer is the lowest-energy (43.69 kcal/mol or 1.90 eV) isomer next to thioformaldehyde, but the slightly higher cis-HCSH isomer has greater intensities for its brightest fundamental frequencies and a larger dipole moment making it potentially more likely to be observed. These data determined here may also assist in laboratory characterization of these molecules and how their chemistry will likely progress.