Carbon-13 isotope effects on proton transfers from carbon

Abstract

Carbon-13 isotope effects at C-2 have been determined on the rates of elimination from 2-phenylethyl-dimethylsulphonium and -trimethylammonium salts with hydroxide ion in mixtures of dimethyl sulphoxide and water. The effects are consistently larger than those predicted from model calculations for a semi-classical (without tunnelling) isotope effect. The most reasonable interpretation is that the form of the dependence of the isotope effect on solvent composition is controlled by the semiclassical effect with a substantial tunnel effect superimposed.