Kinetics of F- + CH3Cl SN2 Nucleophilic Substitution

Abstract

Statistical theory is used to calculate the F- + CH3Cl → FCH3 + Cl- rate constant versus relative translational energy Erel and CH3Cl temperature T. 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 Erel = 0.9 kcal/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 SN2 rate constant is insensitive to whether the unified statistical model or standard RRKM branching ratio expression is used. Also, including anharmonicity for the F-- - -CH3Cl complex does not affect the calculated rate constant. A comparison ...