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Recent interest in the use of kinetic isotope effects for determination of the mechanism of proton transfer reactions has led to discussion on the role played by three factors?-lo namely, the bending modes of the activated state, the "symmetric" stretching mode of the activated state and proton tunnelling (or, the asymmetric stretching mode of the activated state). It has been shown by Bell1 that, under certain conditions, the contributions of proton tunnelling and of the "symmetric" stretching mode may cancel, which thus implicates the bending mode as the controlling factor in kinetic isotope effects. The present communication aims to show that this alleged cancellation does not have general validity, and that it occurs only under a special set of circumstances. The basis for this claim lies in the fact that both factors are dependent upon the electrochemical free energy change of the proton transfer step and that, whilst the sign of the dependence is identical, one factor tends to increase the magnitude of the isotope effect whilst the other tends to decrease it. The dependence of the degree of proton tunnelling, and the kinetic isotope effect, on the free energy of reaction has been theoretically and experimentally demonstrated by Bockris and Matthewsl1 for the hydrogen evolution reaction a t a mercury electrode. The same authors have also briefly demonstrated12 the theoretical possibility of the dependence of the "symmetric" stretching mode on the free energy of reaction. They show that for proton tunnelling the correction to the kinetic isotope effect is unity for large positive free energies of reaction, AGO, increases with decrease in AGO, going through a maximum at AGO = 0, and decreases towards

The dependence of kinetic isotope effects on the free energy change for proton transfer reactions