Effect of solvation on the dynamics of H + CH3 association

Abstract

The gas phase association of CH 3 with the HAt2 cluster to form a vibrationally/rotationally excited CH* 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 + C H 3 ab initio potential and 12-6 Lennard-Jones A r -Ar, A r -C, and A r -H potentials. Classical trajectory calculations performed with the chemical dynamics computer program VENUS are used to investigate the CH3 + HAra-* CH4*+ At2 reaction dynamics. Reaction is dominated by a mechanism in which the CH 3 "strips" the H-atom from HAr2 during large impact parameter collisions. For a large initial relative translational energy the CH 3 + HAr2-+CH~'+ Ar2 cross section is the same as that for H + CH3 association, so that HAt2 acts like a "heavy" H-atom. However, at a low initial relative translational energy, the long-range A r 2 ---C H 3 attractive potential apparently makes the CH 3 + HAr2 association cross section larger than that for H + CH3. Partitioning of energy to the CH* and Ar 2 products is consistent with a stripping mechanism. The initial and final relative translational energies are nearly identical and the CH* rotational energy is controlled by the initial CH3 rotational energy. The velocity and orbital tilt scattering angles, O(v~, vs) and O(l, ll), respectively, are consistent with the stripping mechanism. On average only a small amount of the product energy is partitioned to Ar 2 vibration/rotation and C H * + Ar2 relative translation.