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 NH 2 (1 2 A‘) 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 a 1 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( 2 D) + H 2 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 NH 2 (1 2 A‘ ‘). As a result, the à state contributes measurably to both the thermal rate constant for N( 2 D) + H 2 and the rate for NH(a 1 Δ) production. Extensive quasiclassical trajectory calculations are performed on the RKHS surface to study the N( 2 D) + H 2 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(a 1 Δ) production has a higher threshold energy and is about 15% of the ground-state cross section at energies well above threshold.