Simultaneous Occurrence of Solvent Reorganization and Heavy-Atom Reorganization in a Transmethylation Transition State

Article abstract of DOI:10.1021/ja00345a047

The methylation of methoxide anion by S-methyldibenzothiophenium cation in methanol solution from 10 to 25 deg C (ionic strength 0.100), and in binary mixtures of methanol-h and methanol-d at 25 deg C, exhibits (a) a carbon isotope effect at transferring methyl, k₁₂/k₁₃ = 1.072 +/- 0.006 (10 deg C), 1.072 +/- 0.007 (20 deg C), 1.080 +/- 0.011 (25 deg C). (b) an α-deuterium secondary isotope effect at transferring methyl, k_3H/k_3D = 0.996 +/- 0.006 (10 deg C), 0.994 +/- 0.008 (20 deg C), 0.972 +/- 0.012 (25 deg C), and (c) a solvent isotope effect, k_CH3OD/k_CH3OH = 2.04, with the data in mixtures being consistent with transition-state solvation models involving any number of solvent hydrogen bonds to transition-state sites.The results require a transition state with substantial amounts of both C-S bond fission and C-O bond formation and with reorganisation of the initial-state solvation structure being considerably advanced but still incomplete.Specific salt effects are observable and are treated by a modified Setschenow formulation.