Revealing the active role of the terminal CNO moiety in the photochemical oxaziridine conversion process of some chemopreventive retinylnitrones through hybrid QM:QM and QM:MM ONIOM calculations

•ONIOM calculations on retinylnitrones have produced photo-conversion route to oxaziridines.•Choice of CNO moiety as the active site in QM:QM and QM:MM has produced nitrone-oxaziridine path.•An optimal interaction from the low-level part is required to produce the studied photochemical route.•A non-radiative decay path is followed through a low-lying conical intersection in this process.•The E–Z isomerizations of these nitrones are found to be non-photochemical in nature.