Electronic states, conical intersections, and spin-rovibronic spectroscopy of the nitrogen oxide sulfide radical.

Highly correlated ab initio methods are used to investigate the lowest electronic states of doublet and quartet spin multiplicities for SNO. One-dimensional cuts of the three-dimensional potential energy surfaces (3D-PESs) of these electronic states along the stretch and bend coordinate are calculated. Several avoided crossings and conical intersections are located for bent and linear configurations. The dynamics on the excited electronic states of SNO are very complex, and suggest that multi-step mechanisms should occur to populate the ground state via radiationless processes or lead to predissociation. In addition, our calculations show that the ground (X̃(2)A(')) and the first excited (Ã(2)A(")(Π)) states of this radical form a linear-bent Renner-Teller system. They correlate to the SNO(1(2)Π) state at linearity. Systematic studies of both components are performed using standard coupled cluster approaches, explicitly correlated coupled cluster technique, and multi-configurational methods in connection with large basis sets. Core-valence and scalar relativistic effects are examined. For both electronic states, the 3D-PESs are mapped in internal coordinates at the RCCSD(T)-F12b∕cc-pVTZ-F12 level. The analytical representations of these potential energy surfaces are incorporated later into perturbative and variational treatments of the nuclear motions. A set of spectroscopic parameters and spin-rovibronic levels calculated variationally are presented. Strong anharmonic resonances are found. These new results allow for the reassignment of earlier experimental IR bands of SNO trapped in cooled argon matrices.