Dynamics of the OH + Cl₂ → HOCl + Cl Reaction: Ab Initio Investigation and Quasiclassical Trajectory Calculations of Reaction Selectivity

Abstract

The selectivity of reactant rotational and vibrational energy upon reactive cross sections for the reaction OH + Cl2 → HOCl + Cl is investigated in detail by performing quasiclassical trajectory (QCT) calculations on a six-dimensional, analytically constructed potential energy surface (PES). The construction of the PES was based on ab initio molecular orbital calculations of the HOClCl system using the unrestricted second-order Møller−Plesset perturbation theory approach. The quantum mechanical investigation produced three stationary points relevant to the title reaction, a transition-state configuration and two HOCl···Cl-type complexes. Geometries, energies, and vibrational frequencies are reported for these structures. The analytical functional form for the PES has been developed following the Schatz−Elgersma formulation incorporating the Sorbie−Murrell potential energy term for the HOCl molecule. Extended three-dimensional QCT calculations at low and moderate collision energies covered several rotational and vibrational reactant states to produce a detailed study of the selectivity upon the reactivity of the system. The resulting dynamical features are the strong enhancement of reactivity when the vibrational content of the breaking bond is raised and the exactly opposite effect when the vibrational excitation of the spectator OH bond is increased. The initial reactant rotation is shown to have a very minor influence on the dynamics.

Pathways for the OH + Cl2 → HOCl + Cl and HOCl + Cl → HCl + ClO Reactions

“…On the HOClCl potential energy surface, we find a total of three complexes and two transition states (TSA, TSB) predicted by the CCSD­(T) method, roughly similar to the density functional method BH&HLYP/aug-cc-pVDZ optimization results of Bryukov and co-workers . However, the exit complex A predicted here by the CCSD­(T) method is intermediate between the TSA and TSB structures predicted by the BH&HLYP/aug-cc-pVDZ DFT method . No other intermediate product is found in the reactant valley for the reaction HOCl + Cl.…”

“…11 However, the exit complex A predicted here by the CCSD(T) method is intermediate between the TSA and TSB structures predicted by the BH&HLYP/aug-cc-pVDZ DFT method. 10 No other intermediate product is found in the reactant valley for the reaction HOCl + Cl. We note that the loosely bound Cl•••H−O−Cl structure is both the exit complex for reaction A and the entrance complex for reaction B.…”

“…Consequently, theoretical investigations of the reaction have been carried out. Kosmas and Drougas and Melissas and co-workers investigated the dynamics of the OH + Cl 2 → HOCl + Cl reaction via quasiclassical dynamical calculations on different potential energy surfaces, some constructed from unrestricted second-order Møller–Plesset perturbation theory. The temperature dependence of the rate coefficients for the OH + Cl 2 → HOCl + Cl reaction has been derived from experimental data, modeling, and thermochemical information by Dellinger and co-workers .…”