Free Energy Calculations and Conformational Analysis of Dibenzo-30-crown-10 with Sm²⁺, Eu²⁺, and Three Halide Salts...

Crown ether complexes have been tailored for use in industrial separations of lanthanides (Ln) as a part of rare earth mining and refining. Dibenzo-30-crown-10 (DB30C10) is one of the most efficient complexants for the separation of rare earth mixtures based on the cation size. To understand the origin of this complexation, molecular dynamics (MD) simulations of DB30C10 have been performed using different combinations of divalent Sm and Eu and three halide salts Cl^-, Br^-, and I^- in tetrahydrofuran (THF) solvent. DB30C10 was parameterized here for the polarizable atomic multipole optimized energetics for biomolecular simulation (AMOEBA) force field, and the existing parameters of THF, Sm²⁺, and Eu²⁺ were employed from our previous efforts. The large conformational fluctuations present in the DB30C10 systems were found to be dependent both on the identity of the lanthanide and halide complexes. For Cl^- and Br^- systems, there were no observed conformational changes at 200 ns, while in I^- systems, there were two conformational changes with Sm²⁺ and one with Eu²⁺ within that same timeframe. In SmI₂-DB30C10, there were three stages of conformational changes. In the first stage, the molecule is unfolded, in the second stage, the molecule is partly folded, and finally, in the third stage, the molecule is completely folded. Lastly, the Gibbs binding free energies of DB30C10 with SmBr₂ and EuBr₂ have been computed, which resulted in nearly identical ΔG_comp values for each lanthanide with Sm²⁺ being slightly more favorable. Considering the folding mechanism of the SmI₂ system with DB30C10, the Gibbs binding free energies of DB30C10 and dicyclohexano-18-crown-6 (DCH18C6) with SmI₂ were calculated separately and compared to probe their complexation affinities, in which the former was found to be more favorable.