Excited-State Proton Transfer Reactions in Subcritical and Supercritical Water

Abstract

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.

Noncatalytic Organic Synthesis Using Supercritical Water: The Peculiarity Near the Critical Point

“…The activity profiles obtained during the purification of the sAP [10,11] and its thermo-deactivation at 69 8C show that the activity toward the hydrolysis of the phosphodiesterase substrate bis-p-nitrophenylphosphate (BNPP) [12] is always parallel to the activities toward the aminopeptidase substrates Leu-p-nitroanilide (Leu-pNA) and Lys-pNA (see data in ref. [13] and Table 1).…”

“…The diastereoselectivity of the CÀC bond forming steps [9] was higher than with [Cp 2 TiCl 2 ] as catalyst [3b,c] (8: 86:14 vs. 97.5:2.5; 9: 60:40 vs. 81:19; 10: 70:30 vs. 87:13) or with stoichiometric amounts of reagent. [10] In summary we have demonstrated for the first time that the catalytic opening of meso-epoxides by electron transfer can be performed with high enantioselectivity and high diastereoselectivity with 1 as rationally designed catalyst.…”

“…The organization of 4 ± 10 water molecules is required to solvate the transferred protons, [10] and hence the water structure near the critical pressure becomes less able to accommodate the ejected protons. That is, the ªlocalº proton concentration would be extremely high when the transferring proton cannot escape.…”

Spectroscopic Investigations of Intermolecular Interactions in Supercritical Fluids

“…Most of the dearth of spectroscopic data arises from the fact that it is relatively difficult to work with and generate supercritical water. Johnston and co-workers have developed special optical cells and used absorbance spectroscopy [21] in tandem with steady-state and time-resolved fluorescence [22,23] to characterize the acidbase behavior of a model solute in supercritical water. This work has focused on the behavior of t3-naphthol, a fluorescent compound in which the lowest excited singlet state is significantly more acidic (pK; = 2.5) than its ground state (pKa = 9.5) in liquid water near ambient conditions.…”

“…This deviation was attributed to the unique properties of supercritical water that affected deprotonation at high temperatures in ways not well-described by extrapolation of the Arrhenius model. Most recently, Johnston's group extended this work to study excited-state proton transfer reactions from t3-naphthol dissolved in supercritical water [23]. They determined that deprotonation of t3-naphthol by acetate and borate anions in supercritical water leads to very small deviations from Arrhenius behavior.…”