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

The inclusion of NH<sub>4</sub><sup>+</sup> 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 [UO<sub>2</sub>F<sub>4</sub>(H<sub>2</sub>O)]<sup>2−</sup> in water, it is shown that the inclusion of two NH<sub>4</sub><sup>+</sup> 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 [UO<sub>2</sub>X<sub>5</sub>][NH<sub>4</sub>]<sub>3</sub> (X = F, OH) in water, the fifth fluoride is bound much stronger than the fifth OH<sup>−</sup>. Implications for a recently proposed model for oxygen exchange in uranyl hydroxide are discussed.