Abstract
The reversible proton dissociation and geminate recombination of three photoacids, 2-naphthol, 2-naphthol-7-sulfonate, and 2-naphthol-8-sulfonate, are studied at various pressures in water. The results are compared with the results of the pressure dependence study we recently conducted for 2-naphthol-6-sulfonate in water and 5,8-dicyanonaphthol in ethanol and propanol. Our time-resolved experimental data are analyzed by the reversible diffusion-influenced chemical reaction model. The proton transfer rate increases significantly with pressure: at ∼11 kbar, the rate increases by a factor in the range of 7.5−10. The pressure dependence is explained using an approximate stepwise two coordinate proton-transfer model. The model is compared with the Landau−Zener curve-crossing proton tunneling formulation. The increase in rate, as a function of pressure, manifests, on one hand, the strong dependence of proton tunneling on the distance between the two oxygen atoms involved in the process, which decreases with an increase of pressure, and, on the other hand, the small change in the water relaxation rate as a function of pressure.
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