Theoretical insights into photo-induced isomerization mechanisms of phenylsulfinyl radical PhSO•

Sulfinyl radicals (R-SO) play important roles in lots of reactions, while the isomer oxathiyl radicals (R-OS center dot) and the isomerization between them are rarely observed due to the poor stability of R-OS center dot. In this work, the complete active space self-consistent field (CASSCF) and its multi-state second order perturbation (MS-CASPT2) methods were employed to study the photo-induced reaction mechanisms of phenylsulfinyl radical PhSO center dot 1 and its isomer phenoxathiyl radical PhOS center dot 2. Our results show that 1 and 2 have similar singly occupied molecular orbitals in the ground state but different properties in the excited state, which determine their diverse behaviors after irradiation. Radical 1 can generate 2 by light irradiation, but 2 produces isomerization product 3 (2-hydroxyphenylthiyl radical) and ring-opening product 4 (acyclic thioketoketene radical) in two paths via S atom migration intermediate Int1 (2-carbonylcyclohexadienthiyl radical). The former path involves consequent hydrogen shift reactions with a strongly exothermic process while the latter path involves both ring-expansion and ring-opening processes with a high barrier, resulting in a structural and energetic preference for the former path. Moreover, we revealed several conical intersections that participate in the reactions and facilitate the photochemical processes. Our calculations not only remain consistent with and clarify the experimental observations (X. Zeng, et al., J. Am. Chem. Soc., 2018, 140(31), 9972-9978) but also enrich the knowledge of sulfinyl radicals and isomer oxathiyl radicals.

Automated summary

In this study, the photo-induced reaction mechanisms of phenylsulfinyl radical and its isomer phenoxathiyl radical were investigated using computational methods. The results revealed their different properties in the excited state and diverse reaction pathways after irradiation. The calculations were consistent with experimental observations and provided new insights into sulfinyl radicals and isomer oxathiyl radicals.