The QM: QM and QM:MM ONIOM calculations on the retinylnitrones( 13-trans and 13-cis) have revealed the presence of low-lying CNO-kinked conical intersections (65-70 kcal/mol above their ground states) on the nitrone-oxaziridine photo-conversion path. Following the directions of their gradient difference vectors, optimized oxaziridine structures were obtained at the CASSCF/6-31 G*:HF/STO-6G and CASSCF/631G*:UFF (with electronic embedding) level taking the CNO moiety as the high-level part. Inclusion of dynamic correlation in the high-level shifts the oxaziridine energies to higher values by 12-23 kcal/mol. A transition state (Delta E-activation = 30 kcal/mol) with imaginary frequency of 361i cm(-1) is found to be responsible for thermal E-Z isomerization of 13-trans compound. (C) 2015 Elsevier B.V. All rights reserved.