Solvation Ultrafast Dynamics of Reactions. 8. Acid-Base Reactions in Finite-Sized Clusters of Naphthol in Ammonia, Water, and Piperidine

Abstract

In this contribution, studies of the dynamics of proton-transfer reactions 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. Phys. Lett. 1994, 228, 3691. 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 for proton transfer have been found to be critically dependent on the number and type of solvent molecules: n = 2 for piperidine and n = 3 for ammonia; no proton transfer was observed for water up to n = 21. With subpicosecond time resolution, we observe a biexponential transient for the n = 3 cluster with ammonia.From these observations and the high accuracy of the fits, we provide the rate of the proton transfer at short times and the solvent reorganization at longer times. From studies of the effect of the total energy, the isotope substitution, and the number and type of solvent molecules, we discuss the nature of the transfer and the interplay between the local structure of the base solvent and the dynamics. The effective shape of the potential energy surface is discussed by considering the anharmonicity of the reactant states and the Coulombic interaction of ion-pair product states. Tunneling is related to the nature of the potential and to measurements specific to the isotope effect and energy dependence. Finally, we discuss a simple model for the reaction in finite-sized clusters, which takes into account the proton affinity and the dielectric shielding of the solvent introduced by the local structure.

Femtosecond real-time probing of reactions. XXI. Direct observation of transition-state dynamics and structure in charge-transfer reactions

“…The parameters for V 2 are C 12 ϭ5.875ϫ10 5 eV Å 12 and C 6 ϭ540 eV Å. 6 These were obtained by assuming a vdW well depth of 1000 cm Ϫ1 and an equilibrium D-I separation of 3.6 Å. The available energies are assumed to be localized in the I-I bond and are adjusted by initiating the trajectories at three different I-I separations to match the available energies of three dissociation asymptotes.…”

“…The advantage of using a vdW complex as the reaction precursor in the fs real-time studies is twofold: 2,3 ͑1͒ the zero-of-time of the reaction is precisely defined and ͑2͒ the impact geometry is defined by the structure of the vdW complex. These bimolecular CT reactions are unique among the different classes of CT reactions studied on the fs and ps time scales, which include unimolecular harpooning reactions, 4 intramolecular CT reactions, 5 proton transfer in acid-base reactions, 6 harpooning reactions of XeϩI 2 ͑Ref. 7͒ and, more recently, XeϩCl 2 .…”

Dynamics of clusters: From elementary to biological structures

“…The first system we studied was that of 1-naphthol (1-NpOH) as the acid and ammonia, piperidine, or water as the base solvent, with the number (n) of solvent molecules varying (20)(21)(22). Time-resolved fluorescence and ps and fs ionization mass spectrometry were carried out to investigate the dynamics of each cluster.…”

“…The scope of phenomena studied varied, from the influence of one-atom solvation and caging (13)(14)(15)(16)(17)(18)(19), to stepwise solvation of acid-base (20)(21)(22) and isomerization reactions (23,24) in clusters of up to 30 solvent molecules. Both electron (25)(26)(27)(28) and proton transfer (20)(21)(22)29) processes were also studied, elucidating for the former one of the classics for Mulliken's dative bonded complexes, that of benzene-iodine, and for the latter the dynamics of single and double proton transfer.…”

“…Both electron (25)(26)(27)(28) and proton transfer (20)(21)(22)29) processes were also studied, elucidating for the former one of the classics for Mulliken's dative bonded complexes, that of benzene-iodine, and for the latter the dynamics of single and double proton transfer. With mass selection, we investigated ionic clusters, the premier being solvated electrons (30) and dioxygen anions (31)(32)(33); both are important in biological activities.…”