Potential energy surface stationary points and dynamics of the F^- + CH₃I double inversion mechanism

Direct dynamics simulations were performed to study the S_N2 double inversion mechanism S_N2-DI, with retention of configuration, for the F^- + CH₃I reaction. Previous simulations identified a transition state (TS) structure, i.e. TS0, for the S_N2-DI mechanism, including a reaction path. However, intrinsic reaction coordinate (IRC) calculations from TS0 show it is a proton transfer (PT) TS connected to the F^-⋯HCH₂I S_N2 pre-reaction complex and the FH⋯CH₂I^- proton transfer post-reaction complex. Inclusion of TS0 in the S_N2-DI mechanism would thus involve non-IRC atomistic dynamics. Indeed, trajectories initiated at TS0, with random ensembles of energies as assumed by RRKM theory, preferentially form the S_N2-DI products and ∼70% follow the proposed S_N2-DI pathway from TS0 to the products. In addition, the Sudden Vector Projection (SVP) method was used to identify which CH₃I vibrational mode excitations promote access to TS0 and the S_N2-DI mechanism. Results of F^- + CH₃I simulations, with SVP specified mode excitations, are disappointing. With the CH₃ deformations of CH₃I excited, the S_N2 single inversion mechanism is the dominant pathway. If the CH stretch modes are also excited, proton transfer dominates the reaction. S_N2-DI occurs, but with a very small probability of ∼1%. The reasons behind these results are discussed.