Molecular Dynamics and Free Energy Calculations of Dicyclohexano-18-crown-6 Diastereoisomers with Sm 2+ , Eu 2+ , Dy 2+ , Yb 2+ , Cf 2+ , and Three Halide Salts in Tetrahydrofuran and Acetonitrile Using the AMOEBA Force Field.

With the continual development of lanthanides (Ln) in current technological devices, an efficient separation process is needed that can recover greater amounts of these rare elements. Dicyclohexano-18-crown-6 (DCH18C6) is a crown ether that may be a promising candidate for Ln separation, but additional research is required. As such, molecular dynamics (MD) simulations have been performed on four divalent lanthanide halide salts (Sm, Eu, Dy, and Yb) and one divalent actinide halide salt (Cf) bound to three diastereoisomers of DCH18C6. Dy, Yb, Cf, DCH18C6, and tetrahydrofuran (THF) solvent were parameterized for the AMOEBA polarizable force field for the first time, whereas existing parameters for Sm and Eu were utilized from our previous efforts. A coordination number (CN) of six for Ln/An-O solvated in THF indicated that the cations interacted almost entirely with the oxygens of the polyether ring. A CN of one for Ln/An-N solvated in acetonitrile for systems containing iodide suggested that the N atom of acetonitrile was competitive with I for cation interactions. Fluctuation between five and six CNs for Dy and Yb suggested that although the cations remained in the polyether ring, the size of the ring may not be an ideal fit as these cations possess comparatively smaller ionic radii. Gibbs binding free energies of Sm in all DCH18C6 diastereoisomers solvated in THF were calculated. The binding free energy of the diastereoisomer was the most favorable, followed by , and then . Finally, two major types of conformation were observed for each diastereoisomer that were related to the electrostatic interactions and charge density of the cations.