Trajectory studies of S_N2 nucleophilic substitution. 7. F^- + CH₃Cl → FCH₃ + Cl^-

The PES(F,Cl) analytic potential energy surface, developed previously, is used in a trajectory study of the F^- + CH₃Cl S_N2 reaction. The trajectory S_N2 rate constants, as a function of reactant relative translational energy E_rel and CH₃Cl temperature, are in good overall agreement with the experimental rate constants and those calculated using an ion-molecule capture/RRKM statistical model applied to PES(F,Cl). The latter agreement exists even though the reaction dynamics is decidedly nonstatistical. For high E_rel the reaction is direct. At lower E_rel there is evidence for formation of an ion-molecule complex; however, its lifetime is too short for complete energy randomization to occur. The velocity scattering angle is isotropic at low E_rel but becomes anisotropic with forward scattering as E_rel is increased. The reaction exothermicity is primarily partitioned to product vibration, in disagreement with a previous experimental study. Energy transfer from the reactants to products is very selective. Excess reactant relative translational energy almost exclusively goes to product relative translation. Similarly, C-Cl stretch excitation goes to product vibration. For a 300 K CH₃Cl rotational temperature, the total angular momentum is dominated by the reactant orbital angular momentum, which is strongly correlated with the product orbital angular momentum.