Gas-Phase Uranyl, Neptunyl, and Plutonyl: Hydration and Oxidation Studied by Experiment and Theory

Abstract

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).

Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase

“…Anion mass spectra were acquired using the following parameters: solution flow rate, 60 L.h Torr and reproducibility of hydration rates of UO 2 (OH) + established that the water pressure was constant to within <10%. 20 The helium buffer gas pressure in the trap is constant at ~10 -4 Torr.…”

Synthesis and Hydrolysis of Uranyl, Neptunyl, and Plutonyl Gas-Phase Complexes Exhibiting Discrete Actinide–Carbon Bonds

“…140 kJ mol −1 exothermic association of a water molecule with the metal center of cationic AnO 2 + to create a much stronger An x+ −OH 2 electrostatic bond. 33 The reaction proceeds via the formation of a transition state (TS) in which the water molecule is inserted into the inner sphere of the reaction complex, with one of the H atoms retaining a hydrogen bond with an acetate and the other interacting with the CH 3 ligand ( Figure 7); in this TS the An−O distance is essentially the same, ca. 62 the energies of the transition state, intermediate II, and products are all shifted to higher energy for neptunyl relative to uranyl, with the result that although the ratedetermining barrier is the TS, the relative rates correlate with the net reaction exothermicities.…”

Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

“…Several computational studies were performed on neutral and ionic neptunium mono-and dioxides at SO-CASPT2 and various DFT levels resulting in theoretical ionization energies, 12,38 vibrational frequencies, 13 and dissociation energies. 38,39 The hydration and oxidation reactions of NpO 2 + in the gaseous phase were studied by DFT assisting electrospray experiments, 40 while NpO 2 2+ served as model compound (focusing on the bond distance and vibrational frequencies) in a benchmark study of two-component relativistic DFT methods. 41 The goal of the present study is the systematic evaluation of the gas-phase electronic spectra of the neutral and ionic NpO and NpO 2 species by means of multireference relativistic ab initio calculations on the basis of our previous studies on the electronic ground states of these species.…”