Insights into the Excited-State Deactivation and Photoproduct Formation of Pyridine N-Oxides

The photochemical behavior of substituted pyridine Noxides is characterized by complex rearrangements culminating in the formation of valuable photoproducts. In this study, we employ ab initio methods to investigate the mechanistic pathways governing the transformation of pyridine N-oxides into oxaziridine-like derivatives upon photoactivation. Using the CAS(10,8)/cc-pVDZ approach with NEVPT2 corrections, we elucidate the electronic and geometric distortions associated with the S1 excited state, conical intersections, and the subsequent ground-state product formation. Our results reveal that the deactivation of the S1 excited state is driven by an out-of-plane rotation of the N-O oxygen atom, resulting in the formation of a lone pair over the nitrogen atom. Along this excited-state reaction pathway, the N-O bond undergoes significant weakening, while a C=C double bond emerges mainly on the excited state. The deactivation at the minimum-energy conical intersection leading to the ground state reveals the formation of an oxaziridine-like intermediate, which subsequently converts into a 1,2-oxazepine derivative.