Solvation Effects on Association Reactions in Microclusters: Classical Trajectory Study of H+Cl(Ar)n
Burkhard Schmidt and R. Benny Gerber
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.
Third Body ("Chaperon") Effect
The third-body effect is illustrated for the case of associative H+Cl collisions where one of the reactands is complexed to an Ar atom. Although the binding energy of the van-der-Waals complex is extremely weak (approx. 1 kJ/mol), the "evaporation" of the single rare gas atom is sufficient to release the excess energy and to bring the H-Cl complex under the dissociation threshold thus yielding a stable diatomic product. Depending on the initial geometry of the reactants, the HCl product molecule is scattered forward or backward. In special cases, long-lived orbiting resonances are found. Note also the high vibrational and rotational excitation of the nascent HCl molecule.
The cage effect on reactive collisions is demonstrated here for collision of a hydrogen atom with a Cl atom wrapped in a complete (icosahedral) solvation shell containing 12 rare gas (Ar) atoms. For extremely low scattering energies, the H atom cannot penetrate the cage and may be trapped in the van der Waals potential on the surface of the cluster. For high enough energies and for suitable orientation of the collision partners, H and Cl can recombine, where the extremely hot HCl molecule evaporates the solvation completely on a timescale of a few ps.
The effect of reactant masses becomes more important when both collision partners are inside a microsolvation cluster. While two heavy partners easily recombine each other by pushing the solvent particles aside, the same is not necessarily true for a light reactant.
For suitable collision energies, one can also find associative collisions of pure rare gas clusters.
The animations were produced by the visualization center of the Hebrew University in Jerusalem, Israel .