Potential energy surface and quasiclassical trajectory studies of the N(²D) + H₂ reaction

We present a global potential energy surface for the 1 A″ state of NH₂ based on application of the reproducing kernel Hilbert space interpolation method to high quality ab initio (multireference configuration interaction) results. Extensive quasiclassical trajectory calculations are performed on this surface to study the N(²D) + H₂/D₂ reaction dynamics. Comparison is made with calculations on the lower level [first order configuration interaction (FOCI)] surface of Kobayashi, Takayanagi, Yokoyama, Sato, and Tsunashima (KTYST). We find a saddle point energy of 2.3 (1.9) kcal/mol for the perpendicular approach for the second order configuration interaction (SOCI) (SOCI with Davidson correction) surfaces, and a collinear stationary point energy of 5.5 (4.6) kcal/mol. The ordering of these stationary points is reversed compared to the corresponding FOCI results, and the only true reaction path on our surface is perpendicular. The primary reaction mechanism is determined to be C_2v insertion to produce short lived (100-300 fs) NH₂ intermediates. Angular distributions are found to be primarily forward-backward symmetric, with a slight bias towards backward scattering at low energies. Decay of the NH₂'s occurs before energy is fully randomized, so the product vibrational distributions are a little hotter than statistical - with vibrational population ratios NH(v″=1)/NH(v″=0) = 0.8 and ND(v″=1)/ND(v″ = 0) = 0.9 (near threshold). These ratios, and other aspects of the vibrational product distributions are in excellent agreement with recent laser induced fluorescence studies.