Mechanism of Proton Transfer in Acid—Base Reactions

It is proposed that proton transfer in acid—base reactions goes by a quantum‐mechanical tunnel effect. The predominance of the tunnel effect in these reactions is attributed to the recognition that the proton transfer can proceed alonga hydrogen bond between the two reactants and the distance which the proton has to travel is only 0.4 to 0.6 Å. On this basis some calculations have been carried out with a one‐dimensional Eckart potential barrier, the (apparent) activation energy of the proton transfer being identified with the endothermicity (Q) of the proton‐transfer reaction: $$\mathrm{AH}+\mathrm{B}\to \mathrm{A}+\mathrm{HB}\mathrm{-Q.}$$ For the primary isotope effect for proton and deuteron transfer, it can be shown that increase of the (apparent) activation energy for the deuteron transfer (due to the difference in the zero‐point energies) necessarily also affects the magnitude of the transmission coefficient and that this effect can compensate, to a greater or lesser extent, for the increased mass of the deuteron. The Bro/nsted relation in acid—base catalysis can also be derived, as a first approximation, from such a model and without any further a prioriassumptions between activation energies and free energies.