Computation of Accurate Activation Barriers for Methyl-Transfer Reactions of Sulfonium and Ammonium Salts in Aqueous Solution

摘要: The energetics of methyl-transfer reactions from dimethylammonium, tetramethylammonium, and trimethylsulfonium to dimethylamine were computed with density functional theory, MP2, CBS-QB3, quantum mechanics/molecular mechanics (QM/MM) Monte Carlo methods. At the CBS-QB3 level, gas-phase activation enthalpies are be 9.9, 15.3, 7.9 kcal/mol, respectively. MP2/6-31+G(d,p) in best agreement results. effects aqueous solvation on these studied polarizable continuum model, generalized Born/surface area (GB/SA), QM/MM simulations utilizing free-energy perturbation theory which PDDG/PM3 semiempirical Hamiltonian for QM explicit TIP4P water molecules MM region used. In phase, all proceed more slowly when compared gas since charged reactants stabilized than transition structure geometries delocalized positive charges. order obtain aqueous-phase free energies, energies corrected obtained single-point conductor-like model GB/SA calculations stationary points along reaction coordinate.