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

Haobin Wang, William L. Hase

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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.

Original languageEnglish
Pages (from-to)3093-3102
Number of pages10
JournalJournal of the American Chemical Society
Issue number13
StatePublished - Apr 2 1997


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