ChemInform Abstract: Hydration Dynamics of Protons from Photon Initiated Acids

Abstract

Proton transfer from the excited states of 1-naphthol and 2-naphthol to the water solvent is studied in water/alcohol mixtures. In pure water, the deprotonation rate of 1-naphthol is insensitive to temperature (..delta..H* approx. O), while that of 2-naphthol exhibits an activation energy of about 2600 cal mol/sup -1/. The deprotonation rates in both molecules decrease nonlinearly as the alcohol concentration increases, becoming effectively zero in pure alcohol. Variation of solvent concentrations in the mixed solvent thus serves as a probe for studying the local reaction environment. Using a Markov random walk theory, a water cluster, (H/sub 2/O)/sub 4 +/- 1/, is identified as the effective acceptor for both molecules, indicating that the structure H/sub 9/O/sub 4//sup +/ plays a direct role in the proton hydration dynamics. At high water concentrations the proton hydration rate is reaction controlled while at high alcohol concentrations it can become diffusion controlled. Proton transfer dynamics in the naphthol systems are similar to the dynamics measured for electron photoionization into water, with the rates having low or zero activation barrier but being entropically inhibited to various degrees. The approx. 1:3 deuterium effect on the rates is in fact derived almost entirely from entropic considerations. A correspondencemore » between the thermodynamic quantities ..delta..H/sup 0//sub i/ and ..delta..S/sup 0//sub i/ and the rate parameters ..delta..H/sup + +/ and ..delta..S/sup + +/ for acid dissociation processes is suggested.« less