Kinetics of F^- + CH₃Cl S(N)2 nucleophilic substitution

Statistical theory is used to calculate the F^- + CH₃Cl → FCH₃+ Cl^- rate constant versus relative translational energy E(re1) and CH₃Cl temperature. The calculations are performed on a potential energy surface derived from MP2 and QCISD(T) ab initio calculations with the 6-311++G(2df,2pd) basis set. At best, statistical theory only qualitatively reproduces the dependence of the experimental rate constant on translational energy and temperature. Using the height of the central barrier with respect to the pre-reaction complex as an adjustable parameter, the experimental rate constant at E(re1) = 0.9 kcat/mol and T = 296 K may be fit by statistical theory with a central barrier 3 kcal/mol higher than that determined from the QCISD(T) calculation. The calculation of the S(N)2 rate constant is insensitive to whether the unified statistical model or standard RRKM branching ratio expression is used. Also, including an harmonicity for the F^- - - -CH₃Cl complex does not affect the calculated rate constant. A comparison of statistical rate constants with parametrized trajectory capture rate constants suggests that, during the entrance channel capture dynamics, the F^- + CH₃Cl relative translation and CH₃Cl rotation motions are strongly and weakly coupled at low and high E(re1), respectively.