EXCITED-STATE PROTON-TRANSFER REACTIONS IN SUBCRITICAL AND SUPERCRITICAL WATER

The isobaric rates of excited-state deprotonations of 2-naphthol by ac etate and berate anions exhibit only modest deviations from Arrhenius- like behavior from ambient temperature to nearly the critical temperat ure of water (T-c = 374 degrees C). In contrast, the rates of deproton ation by ammonia and water exhibit marked deviations from Arrhenius-li ke behavior and go through a maximum at high temperatures. These obser vations establish a fundamental difference in how the rates of charge- generating reactions, such as proton transfers to neutral molecules li ke ammonia and water, and those in which ionicity is unchanged, such a s proton transfers to acetate and berate 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-ge nerating reactions. These results are interpreted using transition sta te theory and compared with several molecular dynamics-free energy per turbation simulations. At temperatures above 250 degrees C, contact io n pair formation further inhibits deprotonation. The formation of cont act ion pairs is evident in both the time-resolved fluorescence and st eady-state fluorescence spectra. Near the critical point, where solven t properties vary widely with pressure, the bimolecular rate constant for 2-naphthol deprotonation by ammonia increases by nearly an order o f magnitude over the pressure range from 3000 to 5000 psia. This effec t is caused by the large changes in solvent density induced by pressur e changes and leads to electrostriction about the polar transition sta te.