Computational Exploration of the Intersystem Crossing from the X ̃ 3 A 2 to the a ̃ 1 A 1 State in Boryl Nitrenes upon Photoexcitation.

The boryl nitrene CatBN (Cat = catecholato) turns highly reactive toward small inert molecules upon irradiation of its triplet ground state X ̃ 3 A 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 a ̃ 1 A 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 ̃ 3 B 1 to the a ̃ 1 A 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 ̃ 3 B 2 , followed by intersystem crossing to the open-shell singlet state ( b ̃ 1 A 2 ) and internal conversion to a ̃ 1 A 1 . The relevant minimum energy crossing point is about 7-8 kcal mol -1 higher in energy than the Franck-Condon region.