Potential energy surface of the (A)over-tilde state of NH2 and the role ofexcited states in the N(D-2)+H-2 reaction

We present a global potential energy surface for the (A) over bar state of NH2 (1(2)A') based on application of the reproducing kernel Hilbert space ( RMS) interpolation method to high-quality ab initio (multireference configu ration-interaction) results. This surface correlates adiabatically to the a (1)Delta 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 th at lie near the bottom of a 94 kcal/mol deep well that is accessible from N (D-2) + H-2 by insertion over a 3.4 kcal/mol barrier. This insertion barrie r is about 1 kcal/mol higher in energy than the corresponding insertion bar rier associated with the ground state of NH2(1(2)A "). As a result, the A s tate contributes measurably to both the thermal rate constant for N(D-2) H-2 and the rate for NH(a(1)Delta) production. Extensive quasiclassical tra jectory calculations are performed on the RKHS surface to study the N(2D) H-2 reaction dynamics, with the nonadiabatic rate constant estimated using a capture model. We find that the cross section for ground-state NH produc tion is comparable to that obtained on the ground-state 1A " surface, excep t for a 1 kcal/mol shift upward in the effective threshold due to the diffe rent barrier height. The cross section for NH(a(1)Delta) production has a h igher threshold energy and is about 15% of the ground-state cross section a t energies well above threshold.