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

Crown ether complexes have been tailored for use in industrialseparations of lanthanides (Ln) as a part of rare earth mining andrefining. Dibenzo-30-crown-10 (DB30C10) is one of the most efficientcomplexants for the separation of rare earth mixtures based on thecation size. To understand the origin of this complexation, moleculardynamics (MD) simulations of DB30C10 have been performed using differentcombinations of divalent Sm and Eu and three halide salts Cl-, Br-, and I- in tetrahydrofuran(THF) solvent. DB30C10 was parameterized here for the polarizableatomic multipole optimized energetics for biomolecular simulation(AMOEBA) force field, and the existing parameters of THF, Sm2+, and Eu2+ were employed from our previous efforts. Thelarge conformational fluctuations present in the DB30C10 systems werefound to be dependent both on the identity of the lanthanide and halidecomplexes. For Cl- and Br- systems,there were no observed conformational changes at 200 ns, while inI(-) systems, there were two conformational changeswith Sm2+ and one with Eu2+ within that sametimeframe. In SmI2-DB30C10, there were three stages ofconformational changes. In the first stage, the molecule is unfolded,in the second stage, the molecule is partly folded, and finally, inthe third stage, the molecule is completely folded. Lastly, the Gibbsbinding free energies of DB30C10 with SmBr2 and EuBr2 have been computed, which resulted in nearly identical & UDelta;G (comp) values for each lanthanide with Sm2+ being slightly more favorable. Considering the folding mechanismof the SmI2 system with DB30C10, the Gibbs binding freeenergies of DB30C10 and dicyclohexano-18-crown-6 (DCH18C6) with SmI2 were calculated separately and compared to probe their complexationaffinities, in which the former was found to be more favorable.