Density functional study of aqueous uranyl(VI) fluoride complexes

Mixed uranyl aquo fluoro complexes [UO2(H2O)xFy]2−y (y = 1-4; x + y = 4, 5) have been optimized with BLYP and B3LYP density functionals in vacuo and in a polarizable continuum modeling bulk water, and have been studied at the BLYP level with Car-Parrinello molecular dynamics (MD) simulations. Using constrained MD simulations and thermodynamic integration, the computed free binding energy between aqueous uranyl and fluoride, affording [UO2(H2O)4F]+, is in excellent agreement with experiment. With the same technique, five-coordinate [UO2F4(H2O)]2− is indicated to be unstable against loss of the water ligand, as the free energy for dissociation is computed to be ca. −7 kcal/mol in aqueous solution.

An approach based on Car-Parrinello molecular dynamics simulations can reproduce the free binding energy between aqueous uranyl hydrate and fluoride ion very well, in contrast to static continuum models. It is predicted that the pristine tetrafluoride should not bind an additional water ligand in aqueous solution, suggesting significant population of four-coordinate [UO2F4]2− under experimental (EXAFS) conditions.