Accurate Study of the Two Lowest Singlet States of HN3: Stationary Structures and Energetics at the MRCI Complete Basis Set Limit

Abstract

The two lowest singlet potential energy surfaces of the hydrazoic acid are explored using high-level quantum chemistry calculations, revealing isomeric forms, transition states, and a new path for the nitrogen ring-closing mechanism in the ground state. The reaction of cyc-N3 with hydrogen is shown to proceed through a barrierless path that dissociates to N2 + NH((1)Δ) without reaching the HN3 global minimum. Several intersections between the two states are localized for both N3 isomers near the N3 + H dissociation. The energies of stationary structures and dissociation asymptotes are obtained with the multireference configuration interaction method at the complete basis set limit. A comparison with recent experimental data regarding the H-N3 bond strength and the dissociation barrier in the excited state shows that the present results are of chemical accuracy (~1 kcal mol(-1)).