An analytical potential energy surface and variational transition-state theory kinetics of the NH3 + Br → NH2 + HBr hydrogen-abstraction reaction

Abstract

Abstract

We report the first full-dimensional analytical potential energy surface (PES) and a variational transitionstate theory (VTST) study of the endothermic gas-phase reaction NH3 + Br −−→ NH2 + HBr, completing the halogen series (NH3 + F, Cl, Br) of the group’s analytical surfaces. Stationary points and energetics were benchmarked at the CCSD(T)/CBS level with the bromine 3d shell frozen; a −3.51 kcal/mol spinorbit correction was added to the NH3 + Br asymptote for the Br(2P) splitting. The benchmark locates a genuine, product-like (Hammond) saddle 17.6 kcal/mol above reactants but submerged 2.1 kcal/mol below the NH2+HBr asymptote (zero-point-and spin–orbit-corrected). A valence-bond/molecular-mechanics PES was calibrated to reproduce these stationary points to better than 0.25 kcal/mol; it retains an exact analytical gradient and dedicated van der Waals wells for the entrance (NH3···Br) and exit (NH2···HBr) complexes. Canonical variational theory with multidimensional tunneling (µOMT) yields forward rate constants over 200 K to 2000 K. Because the adiabatic barrier lies below the product asymptote, the tunneling coefficient is unity and the reverse reaction NH2 +HBr −−→ NH3 +Br exhibits a negative apparent activation energy, as measured for the carbon analogue CH3 +HBr. Quasi-classical trajectories on the same surface, once product zero-point leakage is accounted for, reproduce the variational forward rate as an independent check of the barrier. In the absence of experimental data, the forward rate constants are presented as predictions anchored to the high-level energetics.