Analytical Potential Energy Surface and Variational Transition-State Theory Kinetics of the NH3 + Br → NH2 + HBr Hydrogen-Abstraction Reaction

The hydrogen-abstraction reaction NH3 + Br → NH2 + HBr is strongly endothermic and has no measured rate coefficient. We report a full-dimensional analytical potential energy surface (PES) for it and variational transition-state theory (VTST) rate coefficients in both directions over 200 to 2000 K. CCSD(T)/CBS benchmarks, with a −3.51 kcal/mol spin–orbit correction on the reactant asymptote, place a very late saddle point 17.6 kcal/mol above the reactants but 2.1 kcal/mol below the NH2 + HBr asymptote once zero-point and spin–orbit corrections are included. A valence-bond/molecular-mechanics PES calibrated against that benchmark reproduces the stationary-point energies to 0.5 kcal/mol and the adiabatic barrier to 0.25 kcal/mol. The submerged barrier has two consequences. In the forward direction, it empties the ground-state tunneling window and makes the variational rate an upper bound, because flux is counted at energies from which products are inaccessible; quasi-classical trajectories on the same surface, once their zero-point leakage is removed, agree with the corrected variational rate within a factor of 3 over 1000 to 2000 K. In the reverse direction, a two-step treatment with long-range capture agrees with conventional transition-state theory above 400 K but falls by a factor of 3.4 below it at 200 K, and the activation energy is negative below 1000 K (−2.8 kcal/mol at 298 K), as measured for CH3 + HBr. The rate coefficients are predictions.