SOLVATION ULTRAFAST DYNAMICS OF REACTIONS .8. ACID-BASE REACTIONS IN FINITE-SIZED CLUSTERS OF NAPHTHOL IN AMMONIA, WATER, AND PIPERIDINE

In this contribution, studies of the dynamics of proton-transfer react ions in solvent cages are presented, building on earlier work [Breen, J. J.; et al. J. Chem. Phys. 1990, 92, 805. Kim, S. K.; et al. Chem. P hys. Lett. 1994, 228, 369]. The acid-base system studied in a molecula r beam is l-naphthol as a solute and ammonia, piperidine, or water as the solvent, with the number of solvent molecules (n) varying. The rat es and threshold for proton transfer have been found to be critically dependent on the number and type of solvent molecules: n = 2 for piper idine and n = 3 for ammonia; no proton transfer was observed for water up to n = 21. With subpicosecond time resolution, we observe a biexpo nential transient for the n = 3 cluster with ammonia. From these obser vations and the high accuracy of the fits, we provide the rate of the proton transfer at short times and the solvent reorganization at longe r times. From studies of the effect of the total energy, the isotope s ubstitution, and the number and type of solvent molecules, we discuss the nature of the transfer and the interplay between the local structu re of the base solvent and the dynamics. The effective shape of the po tential energy surface is discussed by considering the anharmonicity o f the reactant states and the Coulombic interaction of ion-pair produc t states. Tunneling is related to the nature of the potential and to m easurements specific to the isotope effect and energy dependence. Fina lly, we discuss a simple model for the reaction in finite-sized cluste rs, which takes into account the proton affinity and the dielectric sh ielding of the solvent introduced by the local structure.