title: Structure and hydrolysis of the U(IV), U(V), and U(VI) aqua ions from ab initio molecular simulations.
abstract: 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) (U(4+)) and uranyl(V) (UO(2)(+)) ions in aqueous solution. The solvent dipole and first hydrolysis reaction of aqueous uranyl(VI) (UO(2)(2+)) are also probed. The first shell of U(4+) 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 U(4+) matches well with recent experimental data. The first shell of UO(2)(+) is coordinated by five water ligands in the equatorial plane, with the average U═O(ax) and U-O distances being 1.85 Å and 2.54 Å, respectively. Overall, the hydration shell structure of UO(2)(+) closely matches that of UO(2)(2+), except for small expansions in the average U═O(ax) and U-O distances. Each ion strongly polarizes their respective first-shell water ligands. The computed acidity constants (pK(a)) of U(4+) and UO(2)(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 UO(2)(+) is 8.5.
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA. Raymond.Atta-Fynn@pnnl.gov
authors.forename: Raymond
authors.initials: R
authors.lastname: Atta-Fynn
authors.forename: Donald F
authors.initials: DF
authors.lastname: Johnson
authors.forename: Eric J
authors.initials: EJ
authors.lastname: Bylaska
authors.forename: Eugene S
authors.initials: ES
authors.lastname: Ilton
authors.forename: Gregory K
authors.initials: GK
authors.lastname: Schenter
authors.forename: Wibe A
authors.initials: WA
authors.lastname: de Jong