Effect of solvation on the dynamics of H + CH₃ association

The gas phase association of CH₃ with the HAr₂ cluster to form a vibrationally/rotationally excited CH₄^* 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 → CH₄^* + 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 → CH₄^* + 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 CH₄^* and Ar₂ products is consistent with a stripping mechanism. The initial and final relative translational energies are nearly identical and the CH₄^* 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 CH₄^* + Ar₂ relative translation.