Quasiclassical trajectory studies of the chlorine–hydrogen system. IV. The effect of reagent vibrational excitation and of the location of the potential energy barrier on the dynamics

Abstract

Three dimensional quasiclassical trajectory calculations were performed for the reaction of chlorine atoms with hydrogen molecules (H2, D2, and HD). Three LEPS semiempirical potential energy surfaces, all having the same barrier heights but with slightly different saddle point locations, were used for the calculations. Surface I, the surface used in our earlier studies of this reaction for the reactants in their ground vibrational state v=0, was employed here to investigate the effect of vibrational excitation of the reactants on the dynamics of this reaction. This was done by carrying out calculations for H2(v=1–4 and v=7), HD(v=1), and for D2(v=1) and comparing the results to those obtained earlier for v=0. The two other surfaces EN and EX were used to investigate the effect of the position of the barrier on the dynamics of the reaction. These two surfaces are very similar to surface I, but in surface EN the barrier is slightly shifted in the direction of the entry valley, while in surface EX it is slightly shifted in the direction of the exit valley. The interatomic distances at the saddle points of surfaces EN and EX differ by only 0.03 to 0.05 Å from those of surface I. For surfaces EN and EX, calculations were carried out for H2(v=0 and 1), D2(v=0), and HD(v=0) and the results were compared to those obtained for surface I. The results of the calculations on surface I show that the rate of reaction is significantly enhanced by the vibrational excitation of the reagents [kr(v=1)/kr(v=0)=400 for Cl+H2 Cl+H2(v) at 300 K)] that vibrational energy of the reactants (Ev) is very efficiently transferred into vibrational energy of the products (Ev′), and that the average scattering angle of the products decreases with the increase of either translational or vibrational energy of the reactants. The results of the calculations on surfaces EN and EX show that the rate constants are very sensitive to the location of the energy barrier, being significantly higher for surface EX than for surfaces I and EN. The energy partitioning among reaction products, as well as the scattering angle at low collision energies, also depend on the position of the potential energy barrier.