Theoretical study of the C3S molecule

For the most stable linear isomer of C3S in its X1Σ+ state a six-dimensional potential energy surface (PES) has been calculated ab initio by coupled cluster – connected triples (CCSD(T)) method. The analytic form of the PES has been transformed in a quartic force field in dimensionless normal coordinates and employed in calculations of spectroscopic constants using second-order perturbation theory. The PES and the full kinetic energy operator in internal coordinates have been used to calculate variationally the anharmonic ro-vibrational energies for J=0 and J=1. The two experimental band origins of C3S observed in the gas phase, ν1 and ν1+ν5−ν5, agree very well with the theoretical values. The anharmonic ro-vibrational levels, including the bending modes up to 2200 cm−1, are reported. The singlet ground state PES has a saddle point at about 1.25 eV above the linear minimum and two other higher lying cyclic local minima. The only dipole- and spin-allowed electronic transition between 0 and 5 eV is calculated to be the 1Π−X1Σ+ transition with a vertical transition energy of 353.2 nm in good agreement with the matrix value of 378 nm. The dissociative paths C + C2S, C2 + CS and C3 + S of low lying singlet and triplet states have been investigated.