A classical trajectory study of the effect of reaction barrier height on the vibrational energy transfer efficiency in the Cl + HCl system

Abstract

The effects of reaction barrier height and initial rotational excitation of the reactants on the overall rate of H atom exchange between atomic chlorine and HCl ( v = 0) and on the 0 -+ 1 vibrational excitation of HCI via reactive and nonreactive collisions have been investigated using quasiclassical trajectory techniques. Two empirical LEPS potential energy surfaces were employed in the calculations having reaction barrier heights of 9.84 and 7 05 kcal mol-'. Trajectory studies of planar collisions were carried out on each surfacc over a range of relative translational energies with the ground-state HCl collision partner given initial rotational excitation corresponding J = 0 , 3 , and 7. Initial molecular rotation was found to be relatively inefficient in promoting the H atom exchange; the computed rate coefficient for H atom exchange between C1 + HCl (ZI = 0, J = 7) was only 4 times larger than that for C1 + HCl (ZI = 0, J = 0). The vibrational excitation rate coefficient exhibited a stronger dependence on initial molecular rotational excitation. The observed increase in the vibrational excitation rate coefficient with increasing initial molecular rotational excitation was due primarily to nonreactive intermolecular R + V energy transfer. The vibrational excitation rate coefficients increase with decreasing reaction barrier height.

Symmetric atom exchange reaction Cl' + HCl. An arduous task for theory and experiment

“…In contrast, calculations employing surfaces with a small potential energy well are in better accord with the experiments. 15 The best agreement with the experimentally observed rates was obtained with a barrier height around 4 kJ mol"1.16 More direct experimental information is seriously needed.…”

Whence the Energy Term of the Rate Constant?

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The analytical representation of electronic potential-energy surfaces

“…On the other hand, several high-quality theoretical dynamics studies have been carried out on several of the surfaces, and the dependence of dynamical results on certain features of the surfaces is known to be especially strong. (Smith and Wood, 1973;Thommarson and Berend, 1973;Smith, 1975;%'ilkins, 1975;Thompson, 1982), but most of these surfaces were not specifically optimized for Cl+HC1. Instead they were derived for the well-studied H+ Clz~HCl+ Cl rearrangement that occurs on the same surface.…”