Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory.
The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Daniel
D
Rios
Maria C
MC
Michelini
Ana F
AF
Lucena
Joaquim
J
Marçalo
Travis H
TH
Bray
John K
JK
Gibson
D005740:Gases / Q000737:chemistry; D009405:Neptunium / Q000737:chemistry*; D010084:Oxidation-Reduction; D011005:Plutonium / Q000737:chemistry*; D011789:Quantum Theory; D014501:Uranium / Q000737:chemistry*; D014867:Water / Q000737:chemistry
D005740:Gases; D014867:Water; D014501:Uranium; D011005:Plutonium; D009405:Neptunium