Dynamics of the O( ) + HCl reaction on the electronic state: A new ab initio potential energy surface, quasi-classical trajectory study, and comparison to experiment

A new potential energy surface for the lowest electronic state of the O( ) + HCl system is presented. This surface is based on electronic energies calculated at the multireference configuration interaction level of theory with the Davidson correction (MR-CI+Q) using the Dunning cc-pVTZ one-electron basis sets. The ab initio energies thus obtained are scaled using the Scaled External Correlation (SEC) method of Brown and Truhlar. The SEC-scaled energies are fitted to a simple analytical expression to yield a potential energy surface which correlates the reactants O( ) + HCl( ) to the products OH( ) + Cl( ). The reaction barrier on this surface lies at an O-H-Cl angle of 131.4 at an energy of 9.78 kcal/mol above the asymptotic O + HCl minimum. This barrier is 1.3 kcal/mol higher than that on the potential energy surface obtained by Koizumi, Schatz, and Gordon (KSG) [J. Chem. Phys. 95, 6421 (1991)] and 1.1 kcal/mol lower than the S2 surface of Ramachandran, Senekowitsch, and Wyatt (RSW) [J. Mol. Struct. (Theochem) 454, 307 (1998)]. The dynamics of the reaction O( ) + HCl ( ) OH( ) + Cl on this potential surface is studied using quasi-classical trajectory (QCT) propagation and the results are compared to the experimental observations of Zhang et al. [R. Zhang, W. J. van der Zande, M. J. Bronikowski, and R. N. Zare, J. Chem. Phys. 94, 2704 (1991)]. The broad distribution of collision energies in the experiment is modeled by computing weighted averages of the quantities of interest with the weighting factor at each collision energy determined by the collision energy distribution.