2-Naphthol: A simple example of proton transfer effected by water structure

Abstract

Using picosecond spectroscopic techniques, the proton transfer process of 2-naphthol in its first excited singlet state is investigated in water/methanol mixtures at different temperatures. The proton transfer rate is found to increase as the temperature increases, and to decrease as the methanol concentration increases. A Markov random walk theory previously used to electron transfer kinetics is employed to analyze the data. By this method, a water cluster containing 4±1 members is shown to be the proton acceptor. Suggestively, a ‘‘four-cluster’’ structure (H9O4)+ has been hypothesized as the most probable hydrating unit for the proton in acid–base equilibria, charge transfer, and other chemical systems. The observed activation energy of 3.45 kcal/mol in pure water is attributed to the energy required for rearrangement of the hydrogen bonding in the normal water structure to form the proton accepting cluster. Proton transfer kinetics thus exactly parallels electron transfer kinetics in aqueous media. The critical size of 4±1 molecules and a specific structure of the water cluster apparently determine the controlling rates for both electron and proton charge transfer processes in aqueous media.