Potential energy surface of the à state of NH₂ and the role of excited states in the N(²D) + H₂ reaction

We present a global potential energy surface for the à state of NH₂ (1²A') based on application of the reproducing kernel Hilbert space (RKHS) interpolation method to high-quality ab initio (multireference configuration-interaction) results. This surface correlates adiabatically to the a¹ Δ state of NH, with a reaction endoergicity of about 8 kcal/mol, but it can also lead to formation of ground-state NH (exoergic by 29 kcal/ mol) via nonadiabatic (Renner-Teller) interactions for linear HNH geometries that lie near the bottom of a 94 kcal/mol deep well that is accessible from N(²D) + H₂ by insertion over a 3.4 kcal/mol barrier. This insertion barrier is about 1 kcal/mol higher in energy than the corresponding insertion barrier associated with the ground state of NH₂(1²A"). As a result, the à state contributes measurably to both the thermal rate constant for N(²D) + H₂ and the rate for NH(a¹Δ) production. Extensive quasiclassical trajectory calculations are performed on the RKHS surface to study the N(²D) + H₂ reaction dynamics, with the nonadiabatic rate constant estimated using a capture model. We find that the cross section for ground-state NH production is comparable to that obtained on the ground-state 1A" surface, except for a 1 kcal/mol shift upward in the effective threshold due to the different barrier height. The cross section for NH(a¹Δ) production has a higher threshold energy and is about 15% of the ground-state cross section at energies well above threshold.