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).

An assessment of the gas-phase energetics of neutral and singly and doubly charged cationic actinide monoxides and dioxides of thorium, protactinium, uranium, neptunium, plutonium, americium, and curium is presented. A consistent set of metal-oxygen bond dissociation enthalpies, ionization energies, and enthalpies of formation, including new or revised values, is proposed, mainly based on recent experimental data and on correlations with the electronic energetics of the atoms or cations and with condensed-phase thermochemistry. IntroductionThe thermodynamic properties of actinide (An) oxides are of paramount importance to nuclear science at both the fundamental and applied levels. In the most recent, comprehensive overview of the thermodynamics of actinides and actinide compounds [1], the history of the field and the most reliable current data can be found. As concerns the actinide oxides, that work appropriately indicates that, while a significant number of studies of the solid compounds have been undertaken, gas-phase data are still incomplete. An earlier compilation devoted to the gas-phase thermochemistry of the actinides [2], as well as others of a more general nature [3], are now more than twenty years old and do not reflect the experimental data gathered since then. For instance, recent high-quality spectroscopic studies of thorium and uranium oxides [4] ], which showed significant differences relative to the older accepted values included in the compilations. In recent years, we have carried out a systematic study of the gas-phase thermochemistry of neutral and cationic actinide oxides from thorium to curium using Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) [5,6]. This experimental work was based on the observation of exothermic reactions of An metal or metal-oxide cations with various inorganic and organic molecules. Reactions of singly or doubly charged An cations with oxidizing reagents with a large range of oxygen dissociation energies provided metal-oxygen bond dissociation enthalpies, D[An

The first gas-phase ion chemistry studies of the transuranium actinides Np and Pu by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR-MS) have been performed. Reactions of An+ and AnO+ (An = Th, U, Np, and Pu) with N2O, C2H4O (ethylene oxide), H2O, O2, CO2, NO, and CH2O have been studied, with a focus on the oxidation of transuranium ions. All the An+ ions formed AnO+ with all the oxidants studied, with the exception of Pu+ with CH2O, in accord with the known bond dissociation energies, BDE(An+−O). The reaction efficiencies appear to correlate with the magnitude of the promotion energies from the ground states to reactive-state configurations of the An+ ions. Only N2O and C2H4O oxidized all AnO+ to AnO2 +; CO2 and NO oxidized only UO+; H2O and O2 oxidized UO+, NpO+, and PuO+; and CH2O was unreactive with all the AnO+ ions. The observed formation of PuO2 + from the oxidants weaker than N2O is in conflict with the literature values for BDE(OPu+−O), which we conclude are significantly too low. Charge-transfer “bracketing” experiments to determine the ionization energy of PuO2 were performed, from which a value of 7.03 ± 0.12 eV was obtained. This IE(PuO2) is 2−3 eV lower than the literature values, but in accord with our observations for the oxidation thermodynamics.

Reactions of atomic and ligated dipositive actinide ions, An2+, AnO2+, AnOH2+, and AnO2(2+) (An = Th, U, Np, Pu, Am) were systematically studied by Fourier transform ion cyclotron resonance mass spectrometry. Kinetics were measured for reactions with the oxidants, N2O, C2H4O (ethylene oxide), H2O, O2, CO2, NO, and CH2O. Each of the five An2+ ions reacted with one or more of these oxidants to produce AnO2+, and reacted with H2O to produce AnOH2+. The measured pseudo-first-order reaction rate constants, k, revealed disparate reaction efficiencies, k/k(COL): Th2+ was generally the most reactive and Am2+ the least. Whereas each oxidant reacted with Th2+ to give ThO2+, only C2H4O oxidized Am2+ to AmO2+. The other An2+ exhibited intermediate reactivities. Based on the oxidation reactions, bond energies and formation enthalpies were derived for the AnO2+, as were second ionization energies for the monoxides, IE[AnO+]. The bare dipositive actinyl ions, UO2(2+), NpO2(2+), and PuO2(2+), were produced from the oxidation of the corresponding AnO2+ by N2O, and by O2 in the cases of UO2+ and NpO2+. Thermodynamic properties were derived for these three actinyls, including enthalpies of formation and electron affinities. It is concluded that bare UO2(2+), NpO2(2+), and PuO2(2+) are thermodynamically stable toward Coulomb dissociation to [AnO+ + O+] or [An+ + O2+]. It is predicted that bare AmO2(2+) is thermodynamically stable. In accord with the expected instability of Th(VI), ThO(2+) was not oxidized to ThO2(2+) by any of the seven oxidants. The gas-phase results are compared with the aqueous thermochemistry. Hydration enthalpies were derived here for uranyl and plutonyl; our deltaH(hyd)[UO2(2+)] is substantially more negative than the previously reported value, but is essentially the same as our deltaH(hyd)[PuO2(2+)].

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