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

The photodissociation of methyl thionitrite (CH 3 SNO) 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 S 0 → S 1 (n, π ∗ ) and S 0 → S 2 (π, π ∗ ) transitions correspond to 1A′ → 1A″(n σ , π ∗ ) and 1A′ → 2A′(n π , π ∗ ) excitations, respectively. With respect to the photochemistry of CH 3 SNO, it is found that the potential energy surfaces associated with the low-lying excited states of this molecule (1 1 A″, 2 1 A′, and 2 1 A″) 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.