Simulation studies of the Cl- + CH3I SN2 nucleophilic substitution reaction: comparison with ion imaging experiments.

In the previous work of Mikosch et al. [Science 319, 183 (2008)], ion imaging experiments were used to study the Cl(-) + CH3I → ClCH3 + I(-) reaction at collision energies E(rel) of 0.39, 0.76, 1.07, and 1.9 eV. For the work reported here MP2(fc)/ECP/d direct dynamics simulations were performed to obtain an atomistic understanding of the experiments. There is good agreement with the experimental product energy and scattering angle distributions for the highest three E(rel), and at these energies 80% or more of the reaction is direct, primarily occurring by a rebound mechanism with backward scattering. At 0.76 eV there is a small indirect component, with isotropic scattering, involving formation of the pre- and post-reaction complexes. All of the reaction is direct at 1.07 eV. Increasing E(rel) to 1.9 eV opens up a new indirect pathway, the roundabout mechanism. The product energy is primarily partitioned into relative translation for the direct reactions, but to CH3Cl internal energy for the indirect reactions. The roundabout mechanism transfers substantial energy to CH3Cl rotation. At E(rel) = 0.39 eV both the experimental product energy partitioning and scattering are statistical, suggesting the reaction is primarily indirect with formation of the pre- and post-reaction complexes. However, neither MP2 nor BhandH/ECP/d simulations agree with experiment and, instead, give reaction dominated by direct processes as found for the higher collision energies. Decreasing the simulation E(rel) to 0.20 eV results in product energy partitioning and scattering which agree with the 0.39 eV experiment. The sharp transition from a dominant direct to indirect reaction as E(rel) is lowered from 0.39 to 0.20 eV is striking. The lack of agreement between the simulations and experiment for E(rel) = 0.39 eV may result from a distribution of collision energies in the experiment and/or a shortcoming in both the MP2 and BhandH simulations. Increasing the reactant rotational temperature from 75 to 300 K for the 1.9 eV collisions, results in more rotational energy in the CH3Cl product and a larger fraction of roundabout trajectories. Even though a ClCH3-I(-) post-reaction complex is not formed and the mechanistic dynamics are not statistical, the roundabout mechanism gives product energy partitioning in approximate agreement with phase space theory.