Graduate Thesis Or Dissertation
 

Probing Aqueous Ion-Pairing and Speciation of Group V Polyoxometalates

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  • Polyoxometalates are discrete anionic oxo clusters of the Group V and VI transition metals that can be conceptualized as molecular metal oxides. Because they are pairable with any alkali cation, a great deal of fundamental chemistry can be gleaned from their study, including solubility trends and insight into cation binding behavior at metal oxide surfaces. This thesis primarily compares the aqueous solution behavior of the two hexacoltanates (hexaniobate and hexatantalate). Crystalline cesium salts of niobo tungstates exhibited a trend of decreasing Cs+ association in the solid state with decreasing charge and increasing tungsten character of the clusters, which was coupled with a narrowing HOMO-LUMO energy gap. The aqueous behavior of alkali and tetramethylammonium salts of the hexacoltanates was investigated via calorimetry, revealing a heightened concentration-dependence on enthalpy of dissolution for the cesium salts compared to the lighter alkali salts. Quadrupolar relaxation NMR and x-ray total scattering experiments were also performed on the cesium salts of the hexacoltanates, revealing a greater degree of Cs+ ion-pairing with hexatantalate. This was corroborated by computational bond energy decomposition calculations, indicating that a greater contribution by the orbital interaction energy term arising from relativistic effects in hexatanalate was responsible for the difference. Finally, attempt at analogous studies with decaniobate revealed a speciation process into much larger niobium clusters driven primarily by alkali cation association. In light of their observed heightened solubilities and unusual speciation in the presence of alkali cations (especially Cs+), polycoltanates constitute model systems in which alkali countercations do not have purely ionic character and instead induce partially-covalent orbital mixing effects. This thesis illuminates the unusual behavior of alkali cations in aqueous polyoxometalate solutions, further highlighting the necessity to consider counterions in order to arrive at complete characterizations of solution processes including speciation, solubility, ion-association, aggregation, and crystallization. These fundamental studies can then be applied to finding effective, reliable mechanisms for sequestering radioactive cesium that has leached into the environment from nuclear reactor meltdowns such as the Fukushima Daiichi disaster.
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