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

Bronsted correlations for proton transfer and methyl transfer between pairs of mimicked 4-substituted pyridines have been simulated by means of AM1 mo lecular orbital calculations. The enthalpies of activation and of reaction are well correlated by the Marcus relation. The Bronsted correlation is mar kedly curved for proton transfer for which the Marcus intrinsic barrier is very low (similar to 5 kJ mol(-1)). Conversely, the Bronsted correlation is almost imperceptibly curved for methyl transfer for which the Marcus intri nsic barrier is very high (similar to 152 kJ mol(-1)). The slope of the Bro nsted correlation provides an approximate measure of the position of the tr ansition structure along the reaction coordinate between the reactant and p roduct encounter complexes. Primary kinetic isotope effects for proton tran sfer show a broad maximum, centered on Delta H-rxn = 0, when plotted agains t reaction exo/endothermicity only if computed with respect to isolated aci ds and bases, but intracomplex kinetic isotope effects show a maximum displ aced significantly towards the endothermic proton transfers.