| Abstract
| - The structures of seven gas phase identity SN2 reactions of the form CH3X + X- have been characterizedwith seven distinct theoretical methods: RHF, B3LYP, BLYP, BP86, MP2, CCSD, and CCSD(T), inconjunction with basis sets of double and triple ζ quality. Additionally, the energetics of said reactions havebeen definitively computed using focal point analyses utilizing extrapolation to the one-particle limit for theHartree−Fock and MP2 energies using basis sets of up to aug-cc-pV5Z quality, inclusion of higher ordercorrelation effects [CCSD and CCSD(T)] with basis sets of aug-cc-pVTZ quality, and additional auxiliaryterms 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 kcalmol-1. General trends in the energetics, specifically the performance of the density functionals, and thecomponent energies of the focal point analyses are discussed. The utility of classic Marcus theory as a techniquefor barrier predictions has been carefully analyzed. The standard Marcus theory results show disparities of upto 9 kcal mol-1 with respect to explicitly computed results. However, when alternative approaches to Marcustheory, independent of the well-depths, are considered, excellent performance is achieved, with the largestdeviations being under 3 kcal mol-1.
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