Effect of counterions on the structure and stability of aqueous uranyl(VI) complexes. A first-principles molecular dynamics study.

The inclusion of NH(4)(+) 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(2)F(4)(H(2)O)](2-) in water, it is shown that the inclusion of two NH(4)(+) 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(2)X(5)][NH(4)](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.