A novel probe of photochemical reaction mechanisms of sulfine from vertical and adiabatic excitations: Nonadiabatic molecular dynamics simulation

By using SA3-CASSCF(14e,10o)/def2-TZVP level of theory, we have performed on-the-fly trajectory surface hoping molecular dynamic simulation to probe photochemical reaction mechanisms of the sulfine CH2SO from both vertical and adiabatic excitations to the bright S2 state. We found that there are two conical intersections (S2/S1-CI and S1/S0-CI) governing S–O dissociation, C–S rotation, and C–S–O cyclization motions. When vertically excited to the S2 state in the Franck–Condon region, the photo-reaction channel of CH2SO is all S–O dissociation that has almost completely taken place on the S2 state within about 50 fs. For adiabatic excitation (2 eV lower in energy than vertical excitation) starting from S2 state minimum, 91% sampling trajectories go back to planar CH2SO (S0-minimum) and the rest of 9% isomerize to cyclized oxathiirane CH2S(O) in excited-state lifetime about 800 fs. The results simulated using SA3-CASSCF(14e,10o)/def2-TZVP are confirmed by simulation with SA5-CASSCF(14e,10o)/def2-TZVP, including two triplet states. The present studies not only help to comprehensively understand photo-reaction mechanisms of sulfines but also are very helpful for analyzing photochemical mechanisms of substituted sulfines where E/Z isomerization can be achieved.