Theoretical simulation of Bronsted correlations for proton transfer and methyl transfer: The significance of the Bronsted coefficient β

Brønsted correlations for proton transfer and methyl transfer between pairs of mimicked 4-substituted pyridines have been simulated by means of AMI molecular orbital calculations. The enthalpies of activation and of reaction are well correlated by the Marcus relation. The Brønsted correlation is markedly curved for proton transfer for which the Marcus intrinsic barrier is very low (-5 kJ mol^-1). Conversely, the Brønsted correlation is almost imperceptibly curved for methyl transfer for which the Marcus intrinsic barrier is very high (-152 kJ mol^-1). The slope of the Brønsted correlation provides an approximate measure of the position of the transition structure along the reaction coordinate between the reactant and product encounter complexes. Primary kinetic isotope effects for proton transfer show a broad maximum, centered on ΔH_rxn = 0, when plotted against reaction exo/endothermicity only if computed with respect to isolated acids and bases, but intracomplex kinetic isotope effects show a maximum displaced significantly towards the endothermic proton transfers.