Effect of solvation on the dynamics of H + CH3 association

Abstract

The gas phase association of CH3 with the HAr2 cluster to form a vibrationally/rotationally excited CH 4 * molecule is used as a model to study microscopic solvation dynamics. A potential energy surface for the reactive system is constructed from a previously fitted H + CH3 ab initio potential and 12-6 Lennard-Jones Ar-Ar, Ar-C, and Ar-H potentials. Classical trajectory calculations performed with the chemical dynamics computer program VENUS are used to investigate the CH3 + HAr2 → CH 4 * + Ar2 reaction dynamics. Reaction is dominated by a mechanism in which the CH3 “strips” the H-atom from HAr2 during large impact parameter collisions. For a large initial relative translational energy the CH3 + HAr2 → CH 4 * + Ar2 cross section is the same as that for H + CH3 association, so that HAr2 acts like a “heavy” H-atom. However, at a low initial relative translational energy, the long-range Ar2—CH3 attractive potential apparently makes the CH3 + HAr2 association cross section larger than that for H + CH3. Partitioning of energy to the CH 4 * and Ar2 products is consistent with a stripping mechanism. The initial and final relative translational energies are nearly identical and the CH 4 * rotational energy is controlled by the initial CH3 rotational energy. The velocity and orbital tilt scattering angles, θ(v i ,v f ) and θ(l i ,l f ), respectively, are consistent with the stripping mechanism. On average only a small amount of the product energy is partitioned to Ar2 vibration/rotation and CH 4 * + Ar2 relative translation.