Effects of vibrational and rotational energies on the lifetime of the pre-reaction complex for the F⁻ + CH₃I S_N2 reaction

Direct dynamics simulations were performed to investigate unimolecular dynamics of the F⁻⋯HCH₂I pre-reaction complex for the F⁻ + CH₃I S_N2 reaction. The simulations were performed for a total energy commensurate with the 0.32 eV collision energy considered in previous experiments and simulations of this reaction. Two excitation patterns of the complex were considered for the simulations: one with the excitation energy primarily in vibration and the other with the energy primarily in rotation. For the latter equal amounts of energy were added about the three external rotation axes of the complex, giving rise to J and K rotation quantum numbers of 357 and 66 for the initial excitation. For the vibrational excitation the unimolecular dynamics agrees with RRKM theory and dissociation of the complex to the F⁻ + CH₃I reactants is negligible, since the barrier for this reaction is 20.2 kcal/mol in contrast to the 2.4 kcal/mol barrier for accessing the S_N2 transition state (TS). The unimolecular dynamics are much different for the rotational excitation simulation. They are non-RRKM and the instantaneous unimolecular rate constant for F⁻⋯HCH₂I decomposition increases with time. Apparently, this arises from K-mixing and vibration/rotation energy redistribution. At the end of the 10 ps simulation, the instantaneous simulation rate constant for accessing the S_N2 transition state is an order of magnitude smaller than the harmonic RRKM rate constant with the K quantum number treated as an active degree of freedom, indicating that K-mixing and vibration/rotation energy redistribution are not complete on this time scale. For rotational excitation, dissociation to the F⁻ + CH₃I reactants is the dominant unimolecular pathway, instead of forming the S_N2 products. This is a result of a much higher rotational barrier at the S_N2 TS than for the TS leading to F⁻ + CH₃I.