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Model Identity SN2 Reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2):  Marcus Theory Analyzed

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journal contribution
posted on 24.11.2005, 00:00 by Jason M. Gonzales, Wesley D. Allen, Henry F. Schaefer
The structures of seven gas phase identity SN2 reactions of the form CH3X + X- have been characterized with seven distinct theoretical methods:  RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), in conjunction with basis sets of double and triple ζ quality. Additionally, the energetics of said reactions have been definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for the Hartree−Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher order correlation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliary terms for core correlation and scalar relativistic effects. Final net activation barriers for the reactions are = −0.8, = 1.6, = 28.7, = 14.3, = 13.8, = 28.6, and = 25.7 kcal mol-1. General trends in the energetics, specifically the performance of the density functionals, and the component energies of the focal point analyses are discussed. The utility of classic Marcus theory as a technique for barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of up to 9 kcal mol-1 with respect to explicitly computed results. However, when alternative approaches to Marcus theory, independent of the well-depths, are considered, excellent performance is achieved, with the largest deviations being under 3 kcal mol-1.