A comprehensive spectroscopic investigation of α-(2-naphthyl)-N-methylnitrone: a computational study on photochemical nitrone–oxaziridine conversion and thermal E–Z isomerization processes

This comprehensive spectroscopic analysis of alpha-(2-naphthyl)-N-methylnitrone has proposed its photochemical oxaziridine formation and thermal E-Z isomerization mechanisms. The activation energy for the conversion of the unstable non-planar E isomer to the stable planar Z-isomer is found to be 23.7 kcal mol(-1) at the CASSCF/6-31G* level of calculation. A transition state with a negative frequency of 350 cm(-1) is likely to be responsible for this process. Both CASSCF and ONIOM-based studies have revealed that the nitrone-oxaziridine photochemical conversion involves non-radiative decay channels which include biradicaloid conical intersection (CI) points through Hula-twist and terminal one-bond-flip motions, situated at 35-40 kcal mol(-1) below the first excited singlet state (S-1). Following the directions of their gradient-difference vectors, the optimized oxaziridine geometries are obtained. The nature of the low-lying singlet-singlet transitions of these alpha-naphthyl N-methylnitrones is found to be similar to that of the conjugated non-polar polyenes, and differs appreciably from our previously studied long-chain conjugated nitrone systems. The fluorescent S1 state with a radiative lifetime of nanosecond order is populated by the weak upward S-0-S-1 transition (transition moment: 0.3 Debye) and through the decay of the S-2 state, which eventually gets involved in the S-0/S-1 conical intersections.