WATER-STRUCTURE AFFECTED BY A WEAK BASE

The behavior of the photon-initiated weak base 6-methoxyquinoline was compared in the neat solvents heptane, cyclohexane, acetonitrile, alcohols, and water and also in aqueous two-component-solvent mixtures containing alcohol, dioxane, and acetonitrile. In water, the strong intramolecular nonradiative decay rate knr competes with the intermolecular proton abstraction rate kab. Lifetime and quantum yield measurements at various temperatures show that kab in aqueous solutions is not measurably sensitive to temperature, while knr has an activation energy ΔE of about 1.25 kcal/mol. From the mixed-solvent studies, a water cluster (H2O)4±1 is found to be the effective proton donor. The significance of this specific water cluster as a proton donor and acceptor is discussed in terms of the microscopic structure of liquid water and the hydration of OH-(H2O)3 associated with the weak base. Surprising, perhaps, is the fact that proton transfer from a weak acid to water exactly parallels the mechanism of proton transfer to a weak base from water. © 1990, American Chemical Society. All rights reserved.