A Computational Study Revealing the Unexplored Mechanisms of the Photo‐isomerization of 4H‐imidazole N‐oxides and the Reverse Thermal Isomerization of their Photo‐Products

The current computational study explores the photo-isomerization mechanisms of 4H-imidazole N-oxide systems leading to the formations of their experimentally reported photoproducts. Additionally, the reverse thermal isomerization reactions are also investigated. Quantum mechanical studies on 5-phenyl-4,4-dimethyl-4H-imidazole 3-oxides (IMO) and 1,3-dioxides(IMDO) with substitutions at 2-position reveal their excited state decay pathways forming the oxaziridine and trans,cis-dioxaziridine photoproducts, respectively. The vertically excited S₂ state of 2-methyl-substituted IMO undergoes S₂/S₁ and S₀/S₁ conical intersections(CIs) through an upward CNO twist to form the respective oxaziridine. On the other hand, in the 2-phenyl-substituted IMO, a pathway through three consecutive CIs (S₃/S₂,S₂/S₁,S₀/S₁) leads to a downward twisted oxaziridine. The methyl-substituted IMDO is found to form both upward and downward twisted oxaziridines through low-lying S₀/S₁ CIs. The S₀-S₂ photo-excitation of these oxaziridines gives rise to their respective trans-dioxaziridines. The energy barriers of the oxaziridine→parent nitrone conversion processes are much lower in the studied IMO systems(27-33 kcal/mol) than their 2H-imidazole N-oxide analogues(~38 kcal/mol) which supports the experimentally reported faster thermal isomerizations of the former. These barrier heights in 4H-imidazole 3-oxides decrease from 2-methyl to 2-phenyl-substituted systems, and in the latter, it decreases with the increasing electron donating ability of the groups on phenyl (order of barrier height:-NO₂>-OMe>-NMe₂).