Theoretical study of the C3S molecule

Abstract : For the most stable linear isomer of C3S in its X-1 Sigma(+) 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, v(1) and v(1)+v(5)-v(5), 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 (1)Pi- X(1)Sigma(+) 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, C-2 + CS and C-3 + S of low lying singlet and triplet states have been investigated.