Carbon isotope effects on proton transfers from carbon, and the question of hydrogen tunneling

A study of β-carbon-13 isotope effects on reactions of 2- phenylethyldimethylsulfonium and -trimethylammonium ions with hydroxide ion in mixtures of water and dimethyl sulfoxide was made. Both reactions show maxima in k/sub H//k/sub D/ as the percentage of dimethyl sulfoxide is varied. The resulting isotope effects, expressed as the percentage by which k12/k13 exceeds unity, are compared with previously reported k/sub H//k/sub D/ values. Evidence is reported that the tunnel effect makes an important contribution to hydrogen isotope effects in proton transfers but does not control the form of the dependence of the isotope effect on the strength of the attacking base