Effect of pressure on proton transfer rate in aqueous solutions: A picosecond study

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Abstract
Time resolved picosecond fluorescence studies of excited aqueous 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS) have been carried out at pressures up to the ice transition point of H2O and D2O. The proton transfer rates derived from these studies exhibit a large linear increase with pressure from 8×109 s−1 at 1 atm and 294 K to 2.5×1010 s−1 at the liquid→ice VI transition point at 9 kbar and 294 K. In D2O, the deuteron transfer rate also increases linearly with pressure from 2.7×109 s−1 at 1 atm to 1010 s−1 at 8 kbar. The rates between H2O and D2O were found to differ by a factor of 3, which is consistent with the isotope effect kD+/kH+=1/3 which is maintained across the pressure range studied. From these results, an activation volume ΔV‡0 of −6 cc/mol is obtained. These results which show different trends than previous NMR and proton conductance studies are discussed in terms of a microscopic model.