Structure and Hydrolysis of the U(IV), U(V), and U(VI) Aqua Ions from Ab Initio Molecular Simulations

Ab initio molecular dynamics simulations at 300 K based on density functional theory, are performed to study the hydration shell geometries, solvent dipole, and first hydrolysis reaction of the uranium (IV) (U4+) and uranyl(V) (UO2+) ions in aqueous solution. The solvent dipole and first hydrolysis reaction of aqueous uranyl(VI) (UO22+) are also probed. The first shell of U4+ is coordinated by 8-9 water ligands, with an average U-O distance of 2.42 angstrom. The average first shell coordination number and distance are in agreement with experimental estimates of 8-11 and 2.40-2.44 angstrom, respectively. The simulated EXAFS of U4+ matches well with recent experimental data. The first shell of UO2+ is coordinated by five water ligands in the equatorial plane, with the average U=O-ax and U-O distances being 1.85 angstrom and 2.54 angstrom, respectively. Overall the hydration shell structure of UO2+ closely matches that of UO22+, except for small expansions in the average U=O-ax U-O distances. Each strongly polarizes their respective first-shell water ligands. The computed acidity constants (pK(a)) of U4+ and UO22+ are 0.93 and 4.95, in good agreement with the experimental values of 0.54 and 5.24 respectively. The predicted pK(a) value of UO2+ is 8.5.