Vibrational relaxation of HCl(v=1,2,3,6) by H and Cl atoms

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Abstract
Rate coefficients are calculated for the collisional de‐excitation of vibrationally excited HCl molecules by H and Cl atoms. Three‐dimensional classical trajectories of the collisional dynamics of these reactions are calculated by using London–Eyring–Polanyi–Sato (LEPS) potential‐energy surfaces. The results indicate that (1) the rapid deactivation of vibrationally excited HCl by H and Cl atoms can be explained if bound complexes are postulated for both HClH and ClHCl; (2) H‐ and Cl‐atom exchange are efficient mechanisms for relaxing vibrationally excited HCl by Cl and H atoms, respectively; (3) multiple quantum transitions are important in these de‐excitation processes; and (4) the major energy transfer processes are vibration to translation in the deactivation of HCl by H atoms and vibration to rotation in the deactivation of HCl by Cl atoms. The vibrational relaxation of HCl by H atoms is faster than vibrational relaxation of HCl by Cl atoms. Room‐temperature over‐all relaxation rates kv from level v for HCl(v) by H atoms are k2/k1=1.7, k3/k1=2.2, and k6/k1=2.6, where k1=3.0×1013 cm3 mole−1⋅sec−1; for deactivation of HCl(v) by Cl atoms, they are k2/k1=1.7, k3/k1=2.2, and k6/k1 =8.4, where k1=4.5×1012 cm3 mole−1⋅sec−1. For v=1,2, and 3, these over‐all rates show a linear dependence with v. The potential‐energy wells of 3.8 kcal/mole predicted for the HClH surface and 0.77 kcal/mole predicted for the ClHCl surface have an effect on the collision dynamics corresponding to these surfaces as vibrational energy in the reagent is increased. The collision dynamics on the surface with the deeper potential‐energy well are such that the deactivation rate for reaction H+HCl(v=6) shows a much smaller enhancement than that for reaction Cl+HCl(v=6), where the potential‐energy well is not as deep. Increased vibrational energy in the reagents does not necessarily enhance the over‐all reaction rates in reactions that occur mostly by formation of collision complexes rather than by direct interaction. The results of the calculations are compared with the experimental rates for the de‐excitation of HCL (v=1,2,3) by Cl atoms and HCl(v=1) by H atoms. Rate coefficients are provided for many reactions that were not measured experimentally.

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