High-level ab initio studies of unimolecular dissociation of the ground-state $N3$ radical

A comprehensive study of the unimolecular dissociation of the $N3$ radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the $N3$ radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear $N3.$ Three equivalent $C2v$ minima of cyclic $N3$ are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic $N3$ is metastable with respect to ground state products, $N(4S)+N2,$ and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic $N3$ minima. This strongly suggests that cyclic $N3$ possesses a long collision-free lifetime even if formed with substantial internal excitation.