Uncovering the role of the stationary points in the dynamics of the F− + CH3I reaction

Abstract

We describe an analysis method which assigns geometries to stationary points along (quasi)classical trajectories. The method is applied to the F⁻ + CH₃I reaction, thereby uncovering the role of the minima and transition states in the dynamics of the S_N2 inversion, S_N2 retention via front-side attack and double inversion, induced inversion, and proton-transfer channels. Stationary-point probability distributions, stationary-point-specific trajectory orthogonal projections, root-mean-square distance distributions, transition probability matrices, and time evolutions of the stationary points reveal long-lived front-side (F⁻⋯ICH₃) and hydrogen-bonded (F⁻⋯HCH₂I) complexes in the entrance channel and significant post-reaction ion-dipole complex (FCH₃⋯I⁻) formation in the S_N2 exit channel. Most of the proton-transfer stationary points (FH⋯CH₂I⁻) participate in all the reaction channels with larger distance deviations than the double-inversion transition state. Significant forward–backward transitions are observed between the minima and transition states indicating complex, indirect dynamics. The utility of distance and energy constraints is also investigated, thereby restricting the assignment into uniform configuration or energy ranges around the stationary points.