Does the Cl + CH₄ → H + CH₃Cl Reaction Proceed via Walden Inversion?

We report a chemically accurate global ab initio full-dimensional potential energy surface (PES) for the Cl­(²P_3/2) + CH₄ reaction improving the high-energy region of our previous PES [Czakó, G.; Bowman, J. M. <i>Science</i> <b>2011</b>, 334, 343–346]. Besides the abstraction (HCl + CH₃) and the Walden-inversion substitution (H + CH₃Cl) channels, the new PES accurately describes novel substitution pathways via retention of configuration. Quasiclassical trajectory simulation on this PES reveals that the substitution channel opens around 40 kcal/mol collision energy via Walden inversion and the retention cross sections raise from ∼50 kcal/mol. At collision energy of 80 kcal/mol, the retention pathways provide nearly 40% of the substitution cross section, and retention substitution may become the dominant mechanism of the Cl + CH₄ reaction at superhigh collision energies. The substitution probability can be as high as ∼70% at zero impact parameter (<i>b</i>) and decreases rapidly with increasing <i>b</i>, whereas the abstraction opacity function is broad having 5–10% probability over a larger <i>b</i>-range. The high-energy angular distributions show scattering into forward and backward directions for the abstraction (direct stripping) and face-attack Walden-inversion substitution (direct rebound) channels, respectively. Retention can proceed via edge- and vertex-attack pathways producing dominant sideways scattering because the breaking C–H or Cl–H bond is usually at a side position of the forming Cl–C bond