Molecular Dynamics and Free Energy Calculations of Dicyclohexano-18-crown-6 Diastereoisomers with Sm²⁺, Eu²⁺, Dy²⁺, Yb²⁺, Cf²⁺, and Three Halide Salts in Tetrahydrofuran and Acetonitrile Using the AMOEBA Force Field

Abstract

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 (Sm2+, Eu2+, Dy2+, and Yb2+) and one divalent actinide halide salt (Cf2+) bound to three diastereoisomers of DCH18C6. Dy2+, Yb2+, Cf2+, DCH18C6, and tetrahydrofuran (THF) solvent were parameterized for the AMOEBA polarizable force field for the first time, whereas existing parameters for Sm2+ and Eu2+ were utilized from our previous efforts. A coordination number (CN) of six for Ln2+/An2+–O solvated in THF indicated that the cations interacted almost entirely with the oxygens of the polyether ring. A CN of one for Ln2+/An2+-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 Dy2+ and Yb2+ 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 Sm2+ in all DCH18C6 diastereoisomers solvated in THF were calculated. The binding free energy of the cis-syn-cis diastereoisomer was the most favorable, followed by cis-anti-cis, and then trans-anti-trans. Finally, two major types of conformation were observed for each diastereoisomer that were related to the electrostatic interactions and charge density of the cations.

Free Energy Calculations and Conformational Analysis of Dibenzo-30-crown-10 with Sm2+, Eu2+, and Three Halide Salts in THF Using the AMOEBA Force Field

“…Thus, the system may take much longer for the oxygen atoms in DB30C10 to outcompete the halides if at all. These findings are consistent with our previous study of the DCH18C6 trans-anti-trans isomer complexed to the same cations, where the mentioned isomer is capable of folding around the cation (depending on the ionic radius of the cation). The normalized RDF plots of SmCl 2 /Br 2 -DB30C10 and EuCl 2 /Br 2 -DB30C10 are provided in Figure , highlighting the Sm 2+ –O and Sm 2+ –Cl – /Br – (magnified for clarity) interactions.…”

“…The RDF plot (Figure right, orange curves) of this stage has the highest peak, suggesting a significant interaction for Sm 2+ –I – and a CN of two since the crown ether has not folded yet. This coordination environment, indeed, is very similar to that of DCH18C6, which was previously studied by the authors . As such, it might suggest why this smaller crown ether is a promising complexant in lanthanide separation.…”

“…Comparison of the binding free energy of SmBr 2 -DCH18C6 with SmBr 2 -DB30C10 (−18.62 and −18.45 kcal/mol, respectively) finds the value to be close for both crown ethers where DCH18C6 slightly has a better complexation. Although in the systems where Br – is employed as the counteranion DB30C10 does not fold, it is of interest to investigate the binding free energy of SmI 2 -DB30C10 when folded and observe if the binding free energy is larger than that of the SmI 2 -DCH18C6 system.…”

“…This coordination environment, indeed, is very similar to that of DCH18C6, which was previously studied by the authors. 15 As such, it might suggest why this smaller crown ether is a promising complexant in lanthanide separation. In addition, the comparison of the absolute binding free energies between Sm 2+ and DB30C10 and cis-syn-cis isomer of DCH18C6 15 yielded similar values of −18.45 and −18.62 kcal/mol, respectively (Table 3).…”

“…2 Given this limitation, molecular dynamics (MD) simulations provide a good alternative for studying conformational change in these systems. 13,14 In our previous work, 15 we studied the dicyclohexano-18crown-10 (DCH18C6) system (Figure 1, right) with several divalent lanthanides, which was expanded in the current work toward the study of a larger crown ether, dibenzo-30-crown-10 (Figure 1, left). Considering the highly charged species present in these systems, the use of a polarizable force field was essential for obtaining accurate results.…”