Multiconfigurational second-order perturbation study of the photochemical decomposition of methyl thionitrite

The photodissociation of methyl thionitrite (CH3SNO) has been computationally studied by means of CAS-SCF and MS-CASPT2 methods. The analysis of the multiconfigurational wavefunction corroborates the assignments of the absorption spectra, the visible and UV bands assigned to S0 → S1(n, π∗) and S0 → S2(π, π∗) transitions correspond to 1A′ → 1A″(nσ, π∗) and 1A′ → 2A′(nπ, π∗) excitations, respectively. With respect to the photochemistry of CH3SNO, it is found that the potential energy surfaces associated with the low-lying excited states of this molecule (11A″, 21A′, and 21A″) are repulsive along the NO elimination coordinates. For this reason, population of such excited states leads to NO extrusion as the primary process, in agreement with experiments.