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

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 k_D⁺/k_H⁺=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.