Solvation Effects on Association Reactions in Microclusters: Classical Trajectory Study of H+Cl(Ar)n

Abstract

The role of solvent effects in association reactions is studied. Classical trajectory studies of the system H + Cl(Ar)n, (n=1,12) are used to investigate the influence of size, structure and internal energy of the "microsolvation" on the H + Cl association reaction. The following effects of solvating the chlorine in an Arn cluster are found. In the H + ClAr system there is a large "third body" effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H + ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long lifetimes. These resonance states decay into HCl + Ar. At low collision energies(E=10 kJ/mol) for H + Cl(Ar)12, the H + Cl association shows a sharp threshold effect with cluster temperature. For temperatures of about T=45 K the cluster is liquidlike, and the reaction probability is high. For T<40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. At high collision energies (E=100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)12 cluster. At this energy, the solid-liquid phase change is found not to increase the reaction probability.