Frankfurt 2006 – wissenschaftliches Programm
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MO: Molekülphysik
MO 41: Hauptvortr
äge
MO 41.2: Hauptvortrag
Mittwoch, 15. März 2006, 11:10–11:40, H12
Ultrafast bimolecular proton transfer in water: from direct exchange to sequential hopping — •Erik T. J. Nibbering1, Omar F. Mohammed1, Jens Dreyer1, Dina Pines2, and Ehud Pines2 — 1Max Born Institut für Nichtlineare Optik und Kurzzeitspektroskopie, D-12489 Berlin, Germany — 2Department of Chemistry, Ben Gurion University of the Negev, Beersheva 84125, Israel
Bimolecular acid-base neutralization reactions in aqueous solution involve, according to Eigen and Weller, diffusive motion, and upon formation of the reactive encounter complex, an exchange of a proton. Until now the nature of this encounter pair had not been elucidated. Proton transfer may occur in a tight complex between acid and base directly bound by a hydrogen bond, forming the reaction coordinate. An encounter complex may also involve an acid and base connected via a water bridge [1]. We now have been able to determine the ultrafast proton transfer rates of these tight and loose acid-base complexes with femtosecond infrared spectroscopy. We use the dye stain pyranine as optically triggered acid (photoacid ROH) as a means to follow in real time its reaction with carboxylic bases B-. By monitoring infrared-active marker modes we are able, in the case of the loose complexes, to follow the sequential, von Grotthuss type, hopping of the proton from the photoacid, through an intermediate water bridge, to the base [2]. The transient spectra of the hydrated proton band indicate a hydronium cation H3O+ stabilized in an Eigen configuration in an ionic complex RO- .. H3O+ .. B- . [1]. M. Rini et al., Science 301, 349 (2003); J. Chem. Phys. 121, 9593 (2004). [2]. O. F. Mohammed et al., Science 310, 83 (2005).