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 reactions in solvent cages are presented, building earlier work [Breen, J. J.; et al. J. Chem. Phys. 1990, 92, 805. Kim, S. K.; et al. Chem. Phys. Lett. 1994, 228, 369]. The acid-base system studied in a molecular beam is 1-naphthol as a solute and ammonia, piperidine, or water as the solvent, with the number of solvent molecules (n) varying. The rates and threshold proton transfer have been found to be critically dependent on the number and type of solvent molecules: = 2 for piperidine and n = 3 for ammonia; no proton transfer was observed for water up ton = 21. With time resolution, we observe a biexponential transient for the n = 3 cluster with ammonia. these observations and the high accuracy of the fits, we provide the rate of the proton transfer at short and the solvent reorganization at longer times. From studies of the effect of the total energy, the substitution, and the number and type of solvent molecules, we discuss the nature of the transfer and interplay between the local structure of the base solvent and the dynamics. The effective shape of the energy surface is discussed by considering the anharmonicity of the reactant states and the Coulombic of ion-pair product states. Tunneling is related to the nature of the potential and to measurements to the isotope effect and energy dependence. Finally, we discuss a simple model for the reaction in -sized clusters, which takes into account the proton affinity and the dielectric shielding of the solvent by the local structure.