Reaction-path dynamics calculations of the Cl + NH(3) hydrogen abstraction reaction: the role of the intermediate complexes.

Using ab initio information at the CCSD(T)/cc-pVTZ level, the reaction path for the Cl + NH(3) hydrogen abstraction reaction was traced, and the coupling terms between the reaction coordinate and the normal modes were analyzed along it. Two intermediate complexes were located in the entry channel and characterized close to the reactants. One of them presents a typical Cl...H-N bond, while the second presents a two-center/three-electron Cl[symbol:see text]N bond. Both complexes are on the reaction path and contribute to the final rate constants. With this information, the rate constants were calculated over the temperature range 200-2000 K, using variational transition state theory with multidimensional tunneling contributions, and were found to reproduce the experimental evidence in the common temperature range. Finally, analysis of the coupling terms showed qualitatively that vibrational excitation of the N-H stretch and the bending and umbrella modes in the reactant NH(3) enhances the forward thermal rate constants, and that, in the products, the H-Cl stretch mode and the bending mode in NH(2) could appear vibrationally excited, although the randomization of the energy in the well in the exit channel might diminish this excitation.