Solvent Modulation of the Dynamics of Hydride Transfer

Article abstract of DOI:10.1021/j100178a021

A two-coordinate model is used to discuss hydride transfer between substituted acridans and substituted quinolinium ions.Both the donor and the acceptor can be regarded as analogues of the enzymatic cofactor, NAD⁺.One coordinate is used for solvent and heavy atom motions; the other is used for the hydrogenic motion which produces the covalency change.In hydroxylic solvents the primary kinetic isotope effect is maximized and constant, and the Broensted α indicates a critical configuration equally resembling reactants and products.However, in certain polar, aprotic solvents, most notably dimethyl sulfoxide (DMSO), the isotope effect is sharply reduced and the Broensted α indicates a productlike critical configuration.These changes are attributed to a barrier in the solvent coordinate.Solvents like DMSO, which are known to be relatively good Lewis bases, are thought to respond relatively slowly to the relocation of positive charge in the reacting molecules.This shifts the rate-limiting step to the solvent coordinate.Hydroxylic solvents, on the other hand, have a fast component in their dielectric relaxation spectrum, which permits them to respond quickly to the relocation of charge.Thus, the overall bottleneck is in the hydrogenic coordinate.