Effect of Counterions on the Structure and Stability of Aqueous Uranyl(VI) Complexes. A First-Principles Molecular Dynamics Study

The inclusion of NH₄⁺ as counterions in Car−Parrinello molecular dynamics (CPMD) simulations of anionic uranyl(VI) complexes is proposed as a viable approach to modeling “real” aqueous solutions. For [UO₂F₄(H₂O)]²⁻ in water, it is shown that the inclusion of two NH₄⁺ ions strengthens the bond between uranyl and the water ligand by ca. 2 kcal/mol, improving the accordance with experiment. According to CPMD simulations for [UO₂X₅][NH₄]₃ (X = F, OH) in water, the fifth fluoride is bound much stronger than the fifth OH⁻. Implications for a recently proposed model for oxygen exchange in uranyl hydroxide are discussed.