Conservation of El-Sayed's Rules in the Photolysis of Phenyl Azide: Two Independent Decomposition Doorways for Alternate Direct Formation of Triplet and Singlet Phenylnitrene

In the present work, we demonstrate that the formation of triplet and singlet phenylnitrene, when arises from the photodissociation of phenyl azide, fulfills El-Sayed's rules or the electronic angular momentum conservation law. It is shown that the singlet-triplet inversion of phenylnitrene via the vibronic coupling mechanism is not an effective process. In contrast, it is found that there exist two independent dissociation channels that yield alternatively singlet and triplet phenylnitrene. Therefore, a proposed mechanism that violates the angular momentum low is avoided. The clue of the photoreaction lies in the conjunction (energetic and spatial) of two crossing points in a very narrow region of the multidimensional space: (i) an intersystem crossing (ISC1) and (ii) a conical intersection (CI1). The geometrical separation between these two crossing geometries is only 0.016 angstrom measured in Cartesian coordinates, and the respective energy difference is 2 kcal/mol. This work has been performed by application of the complete active space self-consistent field and multiconfigurational second-order perturbation method.