Analytical potential energy surface and kinetics of the NH(3) + H --> NH(2) + H(2) hydrogen abstraction and the ammonia inversion reactions.

Based on accurate electronic structure calculations, a new analytical potential energy surface (PES) was fitted to simultaneously describe the hydrogen abstraction reaction from ammonia by a hydrogen atom, and the ammonia inversion. Using a wide spectrum of properties of the reactive system (equilibrium geometries, vibrational frequencies, and relative energies of the stationary points, topology of the reaction paths, and points on the reaction swaths) as reference, the resulting analytical PES reproduces reasonably well the input ab initio information obtained at the CCSD(T)/cc-pVTZ level, which represents a severe test for the new surface. As a first application, on this analytical PES we perform an extensive kinetics study using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 200-2000 K. For the hydrogen abstraction reaction, the forward rate constants reproduce the experimental measurements, while the reverse ones are slightly underestimated. Another severe test of the new surface is the analysis of the kinetic isotope effects (KIEs). The KIEs between unsubstituted and all deuterated reactions agree with experiment in the common temperature range. For the ammonia inversion reaction, the splitting of the degenerate vibrational levels of the double well due to the tunneling contribution, which is very important in this reaction representing 93% of the reactivity at 200 K, was calculated for the NH(3) and ND(3) species. The values found were 3.6 and 0.37 cm(-1), respectively, which although higher than experimental values, reproduce the experimental behavior on isotopic substitution.