This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 7497–7508 7497 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 7497–7508 Quasi-classical trajectory study of the role of vibrational and translational energy in the Cl( 2 P) + NH 3 reaction M. Monge-Palacios, J. C. Corchado and J. Espinosa-Garcia* Received 13th March 2012, Accepted 4th April 2012 DOI: 10.1039/c2cp40786h A detailed state-to-state dynamics study was performed to analyze the effects of vibrational excitation and translational energy on the dynamics of the Cl( 2 P) + NH 3 (v) gas-phase reaction, effects which are connected to such issues as mode selectivity and Polanyi’s rules. This reaction evolves along two deep wells in the entry and exit channels. At low and high collision energies quasi-classical trajectory calculations were performed on an analytical potential energy surface previously developed by our group, together with a simplified model surface in which the reactant well is removed to analyze the influence of this well. While at high energy the independent vibrational excitation of all NH 3 (v) modes increases the reactivity by a factor E1.1–2.9 with respect to the vibrational ground-state, at low energy the opposite behaviour is found (factor E 0.4–0.9). However, when the simplified model surface is used at low energy the independent vibrational excitation of all NH 3 (v) modes increases the reactivity, showing that the behaviour at low energies is a direct consequence of the existence of the reactant well. Moreover, we find that this reaction exhibits negligible mode selectivity, first because the independent excitation of the N–H symmetric and asymmetric stretch modes, which lie within 200 cm 1 of each other, leads to reactions with similar reaction probabilities, and second because the vibrational excitation of the reactive N–H stretch mode is only partially retained in the products. For this ‘‘late transition-state’’ reaction, we also find that vibrational energy is more effective in driving the reaction than an equivalent amount of energy in translation, consistent with an extension of Polanyi’s rules. Finally, we find that the non-reactive events, Cl( 2 P)+NH 3 ( v) - Cl( 2 P) + NH 3 (v 0 ), lead to a great number of populated vibrational states in the NH 3 ( v 0 ) product, even starting from the NH 3 (v = 0) vibrational ground state at low energies, which is unphysical in a quantum world. This result is interpreted on the basis of non-conservation of the ZPE per mode. 1. Introduction The Cl( 2 P) + NH 3 (v) - HCl + NH 2 hydrogen abstraction reaction has been little studied, either experimentally or theoretically, possibly because the chemistry of the reaction of ammonia with chlorine atoms is very complex with many intermediate fast reactions being involved. 1 Experimentally, to the best of our knowledge, only two kinetics measurements have been reported, 2,3 and theoretically, we know of only four studies. 3–6 Using different theoretical levels, these studies showed that the reaction evolves through a reactant complex in the entry channel, a saddle point, and a product complex in the exit channel. Based exclusively on high-level ab initio calculations, we recently developed the first analytical potential energy surface, PES-2010, for this polyatomic system. 7 We found that the ab initio information used in the fit is well reproduced by the new PES-2010 surface, especially the barrier height and the depth of the wells, and that this surface reproduces the experimental forward rate constants in the common temperature range. However to the best of our knowledge, no dynamics information is available about this reactive system, either experimental or theoretical, and only recently our group has begun a series of dynamics studies using quasi-classical (QCT) and quantum- mechanical (QM) calculations to analyze the role that these complexes in the entry and exit channels play in the dynamics and the atomic-level mechanisms. 8 Due to the presence of the reactant well, we found different mechanisms of reaction depending on the collision energy: indirect at low (o3 kcal mol 1 ) and direct at high (>5 kcal mol 1 ) collision energies. 8 To shed more light on this reaction, in the present paper we focus attention on the dynamics connected with vibrationally excited ammonia molecules, which is related to issues such as bond and mode selectivity or the problem of which motion (vibration or translation) is more effective in driving the reaction. In the case of polyatomic systems, Zare and co-workers 9–12 and Crim and co-workers 13–15 analyzed the bond and mode selectivity, focusing their attention on the reaction of H and Cl atoms with Departamento de Quı´mica Fı´sica, Universidad de Extremadura, 06071 Badajoz, Spain. E-mail: joaquin@unex.es PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by UNIVERSIDAD DE EXTREMADURA on 09 May 2012 Published on 04 April 2012 on http://pubs.rsc.org | doi:10.1039/C2CP40786H View Online / Journal Homepage / Table of Contents for this issue