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) (UO2 2+) are also probed. The first shell of U4+ is coordinated by 8–9 water ligands, with an average U–O distance of 2.42 Å. The average first shell coordination number and distance are in agreement with experimental estimates of 8–11 and 2.40–2.44 Å, 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Oax and U–O distances being 1.85 Å and 2.54 Å, respectively. Overall, the hydration shell structure of UO2 + closely matches that of UO2 2+, except for small expansions in the average UOax and U–O distances. Each ion strongly polarizes their respective first-shell water ligands. The computed acidity constants (pK a) of U4+ and UO2 2+ 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.