Ab Initio Potential Energy Surfaces and Vibrational Spectra of Thioformaldehyde

The potential energy surfaces (PESs) and vibrational spectra of thioformaldehyde (H 2 CS) in the high energy region have been calculated using ab initio calculations. The harmonic frequencies of H 2 CS were first calculated at the CCSD(T)-F12a/cc-pVTZ-F12 level and the potential was represented by a multimode expansion to obtain one-dimensional (1D) and two-dimensional (2D) PESs expanded along the normal coordinates. Anharmonic vibration frequencies of H 2 CS in the high energy region were calculated using the vibrational multi-reference configuration interaction (VMRCI) method which is applicable to systems with strong anharmonic resonances and high lying vibrational states (overtones or combination bands), the results obtained not only correctly reproduce the mixing of the vibrational bands between the overtones and the combination bands of H 2 CS due to the coupling of ν 4 and ν 6 , but also verify the reliability of the PESs. The infrared (IR) spectra and Raman spectra of the molecular vibrations are plotted, and intensity borrowing due to resonance is observed in the IR spectra, while the stretching of the C–H bond has a relatively large effect on the Raman intensity. These data provide a reference for laboratory characterization and interstellar observations.