William Casey
Bill Casey - website
email: whcasey@ucdavis.edu
Reaction dynamics in large nanometer-size oxide ions in water (CASEY)
The Casey group examines dynamics in nanometer-size oxide ions in water, such as the actinyl peroxyl clusters, and employs primarily NMR, ESI-MS and basic solution-chemistry methods. His group will initially spend their time devising methods of synthesizing pure model compounds in gram quantities, such as the U32.
With the model clusters, they want to examine reaction dynamics in actinyl-peroxyl sites, including pathways for polymerization, dissociation and oxygen-isotope-exchanges. It is essential for Casey's work that monospecific solutions of the target molecules be made for study in order to suppress unwanted side reactions. In most cases, the molecules, such as the U24 cluster or the U(VI)-peroxyl monomers, are made isotopically enriched in 17O or 18O and crystallized. The crystals are then dissolved in isotopically normal water and the reactions followed by NMR or mass spectroscopically. The Casey group typically employs NMR spectroscopy, X-ray structural analysis and Electrospray Ionization Mass Spectrometry (ESI-MS) methods to isolate the molecules, characterize them and to understand the reaction pathways. Purity is essential and synthesis and isolation is usually the most difficult part of the research. An example is shown in Figure 1 where a combination of isotope-exchange and ESI-MS methods detailed the dissociation pathways for decaniobate ions and in Figure 2 where progressive substitution of peroxyl for oxyl sites are followed in detail, much as we propose to accomplish for the actinyl peroxyl clusters.
Figure 1: By following rates of steady isotopic exchange in all structure sites using NMR and ESI-MS method, (Villa, Ohlin et al. 2008) showed how the decaniobate ion [Nb10O286-(aq)] reacts with solution over a wide range in pH, including the pathways for dissociation and isotopic equilibration. At pH<9, the molecule dissociates to release a tetrameric fragment at a highly coordinated μ3-oxo while leaving other structural oxygens in the remaining hexamer intact. The rate of dissociation is much slower than most steady rates of isotopic exchange at the structural oxygens, indicating with unprecedented clarity how this nanometer-size ion interacts with water.
Figure WHC-2 - The stability of an aqueous multimeric cluster in water is best shown using a combination of spectroscopies. Here are the ESI-MS spectra (top) of a solution of decaniobate ion in water (Figure 1 center and right) undergoing steady replacement of terminal oxo sites peroxyls, converting the molecule into metastable monoperoxo- and diperoxo-substituted clusters. All species molecules are followed separately using 17O-NMR (bottom) and the lifetime of the various peroxyl forms can be established. In this case, the decaniobate molecule decomposes into a stable hexaniobate molecule with peroxyl substitutions at the terminal oxygens, much like the U24 cluster (Ohlin, Villa et al. 2008).
References
Balogh, E., T. A. Anderson, et al. (2007). "Rates of oxygen-isotope exchange between sites in the [HxTa6O19](8-x)-(aq) Lindqvist ion and aqueous solutions-comparisons to [HxNb6O19](8-x)-(aq)." Inorganic Chemistry 46(17): 7032-7039.
Balogh, E., A. M. Todea, et al. (2007). "Rates of ligand exchange between >FeIII-OH2 functional groups on a nanometer-size aqueous cluster and bulk solution." Inorganic Chemistry 46(17): 7087-7092.
Black, J. R., M. Nyman, et al. (2006). "Rates of oxygen exchange between the [HxNb6O19](8-x)-(aq) Lindquist ion and aqueous solutions." Journal of the American Chemical Society 128(45): 14712-14720.
Casey, W. H. (2006). "Large aqueous aluminum-hydroxide molecules." Chemical Reviews 106(1): 1-16.
Casey, W. H. and J. R. Rustad (2007). "Reaction dynamics, molecular clusters and aqueous geochemistry." Annual Reviews of Earth Science 35: 21-46.
Evans, R. J., J. R. Rustad, et al. (2008). "Calculating geochemical reaction pathways - Exploration of the inner-sphere water exchange mechanism in Al(H2O)63+(aq) + nH2O with ab initio calculations and molecular dynamics." J. Phys. Chem. A 112: 4125-4140.
Houston, J. R., M. O. Olmstead, et al. (2006). "Substituent effects in five oxo-centered Rh(III) trimers." Inorg. Chem. 45: 7799-7805.
Ohlin, C. A., E. M. Villa, et al. (2008). "Remarkable differences in the reactivity of two similar polyoxoniobates to hydrogen peroxide." Angew Chem Int Ed in press.
Ohlin, C. A., E. M. Villa, et al. (2008). "The [Ti12Nb6O44]10- ion - a new type of polyoxometalate structure." Angewandte Chemie, International Edition 47(30): 5634-5636.
Rustad, J. R., J. S. Loring, et al. (2004). "Oxygen-exchange pathways in aluminum polyoxocations." Geochimica et Cosmochimica Acta 68(14): 3011-3017.
Stack, A. G., J. R. Rustad, et al. (2005). "Modeling water exchange on an aluminum polyoxocation." J. Phys. Chem. 109: 23771-23775.
Swaddle, T. W., J. Rosenqvist, et al. (2005). "Kinetic evidence for five-coordination in AlOH(aq)2+ ion." Science 308(1450-1453.).
Villa, E. M., C. A. Ohlin, et al. (2008). "Reaction dynamics of the decaniobate ([HxNb10O28](6-x)-) ion in water." Angew Chem Int Ed 47: 1-4.
Wang, J., J. R. Rustad, et al. (2007). "Calculation of water-exchange rates on aqueous polynuclear clusters and at oxide-water interfaces." Inorganic Chemistry 46(8): 2962 - 2964.
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