Direct measurement of intrinsic proton transfer rates in diffusion-controlled reactions

Acid–base reactions are usually treated as diffusion-controlled. In this study we directly measure the intrinsic proton transfer rates at the reaction contact of several naphthols photoacids–carboxylic base pairs. Deviations from the diffusion-controlled limit result from the finite intrinsic proton transfer rates at contact and these rates correlate well with the total free-energy change following the reactions. The correction was made over 8 ΔpKa units and yielded kr0=3×1011 s−1 and Ga0=2.9 kcal/mol for the activationless proton transfer rate and the intrinsic free-energy barrier, respectively. A similar behavior is found when the proton dissociation rates of all previously studied naphthol-like photoacids are included in the correlation. The combined correlation, over 11 ΔpKa units, yielded kr0=2×1011 s−1 and Ga0=2.5 kcal/mol.