Ro-vibronic states of the NCS radical in the X2Π state

Abstract

Renner–Teller calculations were performed for the X²Π state of the NCS radical using modified multi-reference configuration interaction (MRCI) three-dimensional potential energy functions (PEFs). For the refinements of the PEFs, existing experimental data were used. In the variational calculations of the ro-vibronic levels for J_total⩽7/2 the full anharmonicity and all angular momenta couplings were considered. NCS has about 260 ro-vibronic states (J=P, J⩽7/2) for energies up to 4000 cm^-1. Due to a large spin–orbit constant, small bending wavenumber, presence of strong anharmonic resonances, high density of interacting states, and strong variation of the spin–orbit coupling with geometry, the Renner–Teller problem is found to be more complex than in other 15 valence electrons ABC molecules. The standard notation for the linear–linear Renner–Teller case was augmented. It allows a qualitative description of various perturbations existing in the ro-vibronic states of NCS. The close lying A²Π and B²Σ⁺ states cross near the equilibrium geometry of the B-state and form a conical intersection in bent structures.