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 Delta pK(a) units and yielded k(r)(0) = 3 X 10(11) s(-1) and G(a)(0) = 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 Delta pK(a) units, yielded k(r)(0) = 2 X 10(11) s(-1) and G(a)(0) = 2.5kcal/mol. (C) 1997 Elsevier Science B.V.