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The computational resolution and characterization of lanthanide-ligand complex structures in varying solution pH conditions
AdvisorCantu, David C
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Lanthanide (Ln), or rare earth, elements are used in a wide variety of technologies, for which they need to be in a purified form. Lanthanides can be obtained from mining and are separated from other non-Ln elements. After the mining process, solvent extraction is used to separate specific Ln elements from solutions containing many different Ln elements. Ligand selectivity to specific Ln ions is key to separating rare earth elements from each other. Ligand selectivity can be quantified with relative stability constants, which can be predicted by calculating binding energies of Ln-ligand complexes. The first part of this work details the prediction of relative stability constants from Ln-ligand complex binding energies of ethylenediaminetetraacetic acid complexed with La3+, Eu3+, Gd3+, and Lu3+ ions. The binding energy calculations are based on the solution structures of Ln-ligand complexes resolved with density functional theory ab initio molecular dynamics simulations. The second part of this work characterizes the changes in solution structures of Gd3+ complexed with diethylenetriamine pentaacetate over a pH range ~0 to ~11 by utilizing a combination of ab initio and classical molecular dynamics simulations. These structures reveal that as pH decreases, the ligand uncoordinates from the Gd3+ ion, allowing for more anion and water molecules to bind with the Gd3+ ion which is in qualitative agreement with experimental stability constants that show stronger binding of diethylenetriamine pentaacetate with Gd3+ in basic conditions.