Potential energy surface stationary points and dynamics of the F + CHI double inversion mechanism.

Direct dynamics simulations were performed to study the S2 double inversion mechanism S2-DI, with retention of configuration, for the F + CHI reaction. Previous simulations identified a transition state (TS) structure, i.e. TS0, for the S2-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 FHCHI S2 pre-reaction complex and the FHCHI proton transfer post-reaction complex. Inclusion of TS0 in the S2-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 S2-DI products and ∼70% follow the proposed S2-DI pathway from TS0 to the products. In addition, the Sudden Vector Projection (SVP) method was used to identify which CHI vibrational mode excitations promote access to TS0 and the S2-DI mechanism. Results of F + CHI simulations, with SVP specified mode excitations, are disappointing. With the CH deformations of CHI excited, the S2 single inversion mechanism is the dominant pathway. If the CH stretch modes are also excited, proton transfer dominates the reaction. S2-DI occurs, but with a very small probability of ∼1%. The reasons behind these results are discussed.