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 X̃3A2 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 ã1A1 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 Ã3B1 to the ã1A1 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 Ã3B2, followed by intersystem crossing to the open-shell singlet state (b̃1A2) and internal conversion to ã1A1. The relevant minimum energy crossing point is about 7-8 kcal mol-1 higher in energy than the Franck-Condon region.