Si2CS: A new sulfur-containing molecule with singlet cyclic ground state

The potential energy surface of the possible interstellar molecule Si2CS is investigated at the B3LYP/6-311G(d) level. Eight singlet isomers are located as energy minima and connected by fifteen transition states. Further calculations are performed using the Quadratic CI calculation including single and double substitutions (QCISD/6-311G(2d)) to get more reliable geometrical parameters, harmonic vibrational frequencies, and infrared intensities of the relevant structures. And at the higher (single point) CCSD(T)/6-311+G(2d)//QCISD/6-311G(2d)+ZPVE level, the global state (at 0.0 kcal/mol) corresponds to a planar four-membered ring form. This is followed by the next most stable chain-like isomer SiCSiS (at 1.7 kcal/mol higher the global state) and its geometries is very sensitive to electronic correlation effects. They represent not only high thermodynamic stabilities, but also high kinetic stabilities. These two isomers may be experimentally or astrophysically observable. Meanwhile, the calculated results are compared with the analogous species C3O, C3S, and SiC2S.