Effect of Pressure on the Proton-Transfer Rate from a Photoacid to a Solvent. 2. 2-Naphthol-6-sulfonate in Water

The reversible proton dissociation and geminate recombination of the photoacid 2-naphthol-6-sulfonate is studied as a function of pressure in liquid water. Our time-resolved experimental data are analyzed by the reversible diffusion-influenced chemical reaction model. The proton-transfer rate increases significantly with pressure. At 10 kbar the rate increases by about a factor of 8. The main pressure effect is the decrease of the distance between the proton donor and acceptor. The pressure dependence is explained using an approximate stepwise two-coordinate proton-transfer model. The increase in rate as a function of pressure manifests the strong dependence of proton tunneling on the distance between the two heavy atoms which decreases with an increase of pressure.