Simulation studies of the Cl^- + CH₃I S_N2 nucleophilic substitution reaction: Comparison with ion imaging experiments

In the previous work of Mikosch Science 319, 183 (2008)10.1126/science. 1150238, ion imaging experiments were used to study the Cl^- CH ₃I → 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)ECPd 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 CH ₃Cl 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 BhandHECPd 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 andor 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.