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Adding reactivity to structure—reaction dynamics in a nanometer-size oxide ion in water

Eric M. Villa^*,**, C. André Ohlin^*,**, Edina Balogh^*,**, Travis M. Anderson^***, May D. Nyman^*** and William H. Casey^*,**,

^* Department of Chemistry, University of California, Davis, California 95616
^** Department of Geology, University of California, Davis, California 95616
^*** Geochemistry Division, Sandia National Laboratories, Albuquerque, New Mexico 87185; mdnyman{at}sandia.gov

Corresponding author: whcasey{at}ucdavis.edu

We examine oxygen-isotope exchanges in a nanometer-size oxide molecule in water and, separately, both its rates of dissociation and molecular products. This molecule, the decaniobate ion ([H_xNb₁₀O₂₈]^(6-x)-), is at the same size scale as geochemically interesting features on minerals, such as surface polymers and kink sites on growth steps, although it is structurally quite dissimilar. Unlike mineral surface structures, however, we have complete confidence in the aqueous structure of this molecule and it yields a clear spectroscopic signature as it reacts. We thus can follow proton-enhanced isotope exchanges and base-induced dissociation in unprecedented detail and clarity.

The results are surprising and require new thinking about geochemical reactions at the molecular scale. For example, base-induced dissociation of the molecule, which is unprotonated, causes rates of oxygen-isotope exchanges of all structural oxygens to accelerate dramatically. Similarly, protonation of the molecule causes sets of oxygens to react, although protonation is limited. In general, all reactions are via concerted motions of many atoms and the reactivities vary as though the entire structure was responding to changes in solution composition. The site reactivities could not be inferred from the stable structure of the decaniobate molecule because so much of the structure is involved in each exchange event. Thus, computational models must be structurally faithful to an extraordinary degree, and inherently dynamic, or they will miss the essential chemistry.