HR: 16:45h
AN: V14B-04    [Abstracts]
TI: Rates of Ligand Exchange at Fe(III) Sites in a Nanometer-Size Aqueous Ion from 17O- NMR
AU: * Balogh, E
EM: ebalogh@ucdavis.edu
AF: Dept of Chemistry, Dept. of Geology, University of California, Davis, CA 95616, United States
AU: Casey, W H
EM: whcasey@ucdavis.edu
AF: Dept of Chemistry, Dept. of Geology, University of California, Davis, CA 95616, United States
AB: Some of the pressing questions in geochemistry concern the dynamics and reactivities of individual functional groups at mineral surfaces, meaning their Broensted acid-base properties, the rates of ligand substitution and the rates of electron exchange. The field relies heavily on computer simulation because so few experimental models exist with well-constrained aqueous surface structures. We take a different approach. We use nanometer-size aqueous oxide clusters with well-known structures to isolate the key functional groups for spectroscopic study. Here we discuss a particularly relevant example and report rates of exchange between waters bound to isolated Fe(III) atoms at the surface of a 2.5-nm size nanoacid and bulk solution. The molecule [Mo72Fe30] is part of a class of nanometer-size ions called Keplerates. We apply the 17O-NMR line-broadening method and show that the rates are not dissimilar to Fe(III) monomer ions. A correlation suggests itself between Fe-OH2 bond lengths and rates over several orders of magnitude. We can estimate rates of ligand substitution at mineral surfaces if the correlation can be expanded and if bond lengths can be estimated for surface functional groups. Such a result was recently established for Al(III)-hydroxide minerals (Wang et al. 2007, Inorg. Chem. 46, 2962-2964).
DE: 0317 Chemical kinetic and photochemical properties
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8412 Reactions and phase equilibria (1012, 3612)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting