Nitrenes as intermediates in the thermal decomposition of aliphatic azides

N2 extrusion from hydrazoic acid, methyl azide, and ethyl azide to yield the corresponding nitrene has been studied with high‐level ab initio calculations. Geometry optimizations of stationary points and surfaces crossing seams were carried out with the complete active space self‐consistent field (CASSCF) method, and their energies were reevaluated with the second‐order multireference perturbation (CASPT2) theory and corrected by the zero‐point energy (ZPE). The analytic harmonic frequencies calculated at the CASSCF level have been used in the ZPE corrections. The decomposition reaction is a competitive mechanism between a spin‐allowed and a spin‐forbidden channel, giving the nitrene either in the singlet or triplet states. The energy barrier height for XNN2 bond fission is approximately the same in both channels for each azide, respectively. The spin‐orbit (HSO) interactions were determined at the minimum energy point on the seam of crossing between the singlet and triplet surfaces, the value ranges from 43.9 cm−1 in hydrazoic acid to 43.3 cm−1 in ethyl azide.