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

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.