The mechanism of photoconversion of cyclic dinitrone to oxaziridine and dioxaziridine: A computational investigation of an experimentally reported photochemical reaction

The unexplored photochemistry behind the conversion of cyclic dinitrone to oxaziridine and dioxaziridine has been investigated computationally. The mechanism behind the experimentally reported photo‐conversion reaction of 2,2,4,5‐tetramethyl‐2H‐imidazole‐1,3‐dioxide has been analyzed. The allowed S0‐S1 transition in this cyclic dinitrone system is characterized by a transition moment value of 3 Debye. The photo‐excitation to the vertically excited S1 state is subsequently followed by a passage through low‐lying conical intersection(S0/S1). This photoproduct oxaziridine can undergo reverse transformation to the parent dinitrone through a transition state (barrier height 30–40 kcal/mol) with imaginary frequency value close to 1000i cm−1. Under photo‐irradiation, oxaziridine of this dioxide system can undergo strongly allowed singlet‐singlet transition, characterized by HOMO LUMO excitation with transition moment value close to 2.70 Debye. Subsequently, formations of the trans‐ and cis‐dioxaziridines take place through S0/S1 conical intersections. Computational studies on 2,2,4,5‐tetramethyl‐2H‐imidazole‐1,3‐dioxide have revealed that its photo‐conversion to oxaziridine and subsequent dioxaziridine formation pathways include low‐lying S0/S1 conical intersections. The reverse conversion of oxaziridine to the parent dinitrone happens through a transition state on the ground state surface.