Direct Measurement of Intrinsic Proton Transfer Rates in Diffusion Controlled Reactions

AUTHORS

Pines E. , Magnes B-Z. , Lang M.J. , Fleming G.R. . Chemical Physics Letters. 1997 ; 281(4-6). 413-420

ABSTRACT

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 ΔpK_a units and yielded k_r⁰=3×10¹¹ s⁻¹ and G_a⁰=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 ΔpK_a units, yielded k_r⁰=2×10¹¹ s⁻¹ and G_a⁰=2.5 kcal/mol.