Application of picosecond time-correlated single photon counting technique to acid-base problems in chemistry

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 non-radiative decay rate k competes with the intermolecular proton abstraction rate k , . Lifetime and quantum yield measurements at various temperatures show that k , in aqueous solutions is not measurably sensitive to temperature, while k has an activation energy AE of about 1.25 kcal/mol. From the mixed solvent studies, a water cluster (H^O). - 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 (H20)^ associated with the weak base. Surprisingly, 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.