Computational Exploration of the Intersystem Crossing from the X̃³A₂ to the ã¹A₁ State in Boryl Nitrenes upon Photoexcitation

Abstract

The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X ̃3A 2 with light of wavelength λ > 550 nm. A computational study of a model boryl nitrene using complete active space self-consistent field (CASSCF) theory provides evidence for the population of the highly reactive electronic state ã1A 1 upon irradiation. Potential energy scans connecting different critical points (minima, minimum energy crossing points, and conical intersections) reveal two possible pathways that could relax photoexcited boryl nitrene from the Franck−Condon region of A ̃3B 1 to the ã1A 1 state minimum. Considering the energy barriers to relaxation from one electronic state to another and the magnitude of spin−orbit couplings, the energetically most favorable pathway involves photoexcitation to A ̃3B 2 , followed by intersystem crossing to the open-shell singlet state (b 1A 2 ) and internal conversion to ã1A 1 . The relevant minimum energy crossing point is about 7−8 kcal mol −1 higher in energy than the Franck−Condon region.