Excited-State Proton Transfer Reactions in Subcritical and Supercritical Water

The isobaric rates of excited-state deprotonations of 2-naphthol by acetate and borate anions exhibit only modest deviations from Arrhenius-like behavior from ambient temperature to nearly the critical temperature of water ( T c = 374 °C). In contrast, the rates of deprotonation by ammonia and water exhibit marked deviations from Arrhenius-like behavior and go through a maximum at high temperatures. These observations establish a fundamental difference in how the rates of charge-generating reactions, such as proton transfers to neutral molecules like ammonia and water, and those in which ionicity is unchanged, such as proton transfers to acetate and borate anions, depend on temperature. The loss of local water structure and changes in dielectric constant with temperature have a much more profound influence on the charge-generating reactions. These results are interpreted using transition state theory and compared with several molecular dynamics−free energy perturbation simulations. At temperatures above 250 °C, contact ion pair formation further inhibits deprotonation. The formation of contact ion pairs is evident in both the time-resolved fluorescence and steady-state fluorescence spectra. Near the critical point, where solvent properties vary widely with pressure, the bimolecular rate constant for 2-naphthol deprotonation by ammonia increases by nearly an order of magnitude over the pressure range from 3000 to 5000 psia. This effect is caused by the large changes in solvent density induced by pressure changes and leads to electrostriction about the polar transition state.