Solvent dependence of kinetic hydrogen isotope effects for simple proton transfer

Kinetic hydrogen isotope effects for some simple proton transfer reactions, involving 2-nitropropane, nitroethane, and ethyl 2-methylacetoacetate as subtrates, have been measured in a variety of solvent mixtures (dimethyl sulphoxide-water, acetonitrile-water, and 2,2,2-trifluoroethanol-water). Both rates and isotope effects vary considerably as the solvent composition is altered, but they show no correlation with the effect of the solvent on the overall free energy changes for the reactions. The results are discussed in relation to current theories of isotope effects. (author)