An experimental and theoretical study of ring closing dynamics in HN3

Abstract

We report on an H(D)-atom Rydberg tagging experiment for H(D)N₃ photolysis providing detailed dynamical information on the wavelength dependence of the H(D) + N₃ channel. We observe subtle yet striking changes in the photochemical dynamics as the photolysis energy passes through ∼5.6 eV. In addition to producing linear azide with an average of ∼40% of available energy appearing as translation, a second H(D)-atom producing channel grows in above this energy releasing only about 15%. An observed (inverse) isotope effect suggests that statistical decomposition on S₀ is unimportant. High level ab initio quantum chemical calculations reveal a transition state to cyclization of the N₃ moiety in H(D)N₃ on the first excited singlet (S₁) surface that is close in energy to the experimentally observed threshold energy for this “slow channel”. Furthermore, the translational energy release of the “slow channel” is energetically consistent with cyclic-N₃ formation. This work provides the clearest presently available insights into how ring closure can occur in azide photochemistry.