Linking Solution Structures and Energetics: Thorium Nitrate Complexes

Seeking predictive insights into how metal-ion speciation impacts solution chemistry as well as the composition and structure of solid-precipitates, thorium correlations, with both solvent and other solute ions, were quantitatively probed in a series of acidic, nitrate/perchlorate solutions held at constant ionic strength. Difference pair-distribution functions (dPDF), obtained from high-energy X-ray scattering (HEXS) data, provide unprecedented structural information on the number of Th ligating ions in solution and how they change with increasing nitrate concentration. A fit of the end member solution, Th (4 m perchloric acid and no nitrate), reveals a homoleptic Th aqua ion with 10 waters in its first coordination shell. Analyses of the acidic solutions containing nitrate reveal exclusively bidentate NO₃^- complexation with Th, consistent with published solid-state M^IV nitrate structures, where M^IV = Ce, Th, U, Np, Pu. Metrical fits of Th coordination as a function of nitrate concentration are used to calculate Th-NO₃ stability constants, information important to a molecular-scale description of reaction energetics. The coordination environments of Th in solution were compared with single-crystal structures obtained from their precipitates, Th(NO₃)₄(H₂O)₄ and Th(NO₃)₄(H₂O)₃·(H₂O)₂. Relative stabilities of the solid-state compounds, assessed based on the results of molecular quantum chemical calculations, reveal the importance of including an accurate description of complexed waters when predicting relative energetics of dissolved ions in aqueous solution.