The Development of a Classical Force Field To Determine the Selectivity of an Aqueous Fe³⁺–EDA Complex for TcO₄^– and SO₄^2–

A classical force field has been developed in order to investigate the selective exchange of oxyanions (TcO₄^– vs SO₄^2–) with other ligands (H₂O, Cl^–) to an aqueous Fe³⁺–ethylenediamine (EDA) complex. Potentials of mean force for a range of exchange reactions were generated using umbrella sampling and classical molecular dynamics simulations in order to calculate the affinity of each oxyanion for the Fe³⁺–EDA complex in aqueous solution. In order to accurately introduce a degree of specificity for the interaction of Fe³⁺ with each ligand type, force field parameters were tuned to match the results of density functional theory calculations. Preferential exchange of H₂O, Cl^–, and SO₄^2– for TcO₄^– via an interchange mechanism is observed, in agreement with experimental observations. Both the relative solvation entropies and enthalpies of the anions were found to be critically important factors governing the magnitude of the observed selectivities. These results have important implications for the design and modeling of functionalized materials for the remediation of land contaminated with radioactive ⁹⁹Tc.