Role of vibrational and translational energy in the OH + NH3 reaction: a quasi-classical trajectory study.

Issues such as mode selectivity and Polanyi rules are connected to the effects of vibrational and translational energy in dynamics studies. Using the heavy-light-heavy OH(ν) + NH3(ν) gas-phase reaction, these effects were analyzed by performing quasi-classical trajectory calculations, at low and high collision energies (3.0 and 10.0 kcal mol(-1)), based on an analytical potential energy surface developed by our group. While the independent vibrational excitation of the NH3(ν) modes increases the reactivity by a factor of ∼1.1-2.8 with respect to the vibrational ground-state at both collision energies, OH(ν) stretching acts as a spectator mode. With respect to mode selectivity, we find a different behavior for both reactants. Thus, while the OH(ν) vibrational excitation is maintained in the products, indicating a certain degree of mode selectivity, the vibrational excitation of the NH3(ν) modes is not retained in the products; furthermore, the independent excitation of the N-H asymmetric and symmetric stretch modes leads to similar reaction probabilities, indicating negligible mode selectivity. For this early transition state reaction, translational energy is more effective in driving the reaction than an equivalent amount of energy in vibration, thus extending the validity of Polanyi rules to this polyatomic system. Finally, these results were interpreted on the basis of the existence of little or negligible intramolecular vibrational redistribution in the reactants before collision, while the nonconservation of the zero-point energy has a strong influence.