(531f) Calculating Stability Constants of Lanthanide-Ligand Complexes in Solution

The process feasibility of solvent extraction to separate rare earth elements, or lanthanides, from each other depends on extractants, or ligands, that selectively bind particular rare earth elements. The extraction separation factor between two rare earth elements can be predicted from relative lanthanide-ligand binding energies. To calculate lanthanide-ligand binding energies, the solution structure of lanthanide-ligand complexes must be identified, which includes lanthanide ion, ligand, solvent and anion molecules. To properly model lanthanide ion interactions with other molecules, it is necessary to consider the electronic structure due to binding reactions, and dynamics to sample molecular conformations of the ligand, solvent, and anion molecules. To resolve the structures of lanthanide-ligand complexes in solution, we perform ab initio molecular dynamics (AIMD) simulations, predict extended X-ray absorption fine structure (EXAFS) spectra, and measure EXAFS spectra to directly compare with simulation. Predicted structures show lanthanide-oxygen, and lanthanide-nitrogen, distances that are within ~0.05 Å of experimental values. With the structures determined from simulation, relative lanthanide-ligand binding energies are quantified with electronic structures calculations, and compared with stability constants from experiment.