Gas-Phase Energetics of Actinide Oxides: An Assessment of Neutral and Cationic Monoxides and Dioxides from Thorium to Curium

Abstract

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

Remarkably High Stability of Late Actinide Dioxide Cations: Extending Chemistry to Pentavalent Berkelium and Californium

“…[11] Based on systematic trendsa cross the actinides eries, D[O-(CmO + )] has been estimated as4 8AE 14 kcal mol À1 ,avalue that is substantially lower than for the earlier AnO 2 + . [21] The authors of that work did not extrapolate theset hermodynamic trends beyond CmO 2 + ;t he present results indicate that D[O-(AnO + )] does not continue to decreaseb eyondC mO 2 + but insteadi ncreases for both BkO 2 + and CfO 2 + .This result is computationally assessed below.…”

Gas-Phase Coordination Complexes of UVIO22+, NpVIO22+, and PuVIO22+ with Dimethylformamide

“…Overall, the ESI mass spectrometry and CID results confirm the transfer of dipositive, hexavalent actinyl complexes of 238 [22] and are at least 2 eV greater than the ionization energies (IEs) of water (12.6 eV) and DMF (9.12 eV) [20], stabilization of dipositive actinyl ions against electron transfer from these neutrals requires charge donation by coordinating ligands. Coordination of the actinyl by electron-donating ligands reduces the effective electron affinity of the complexed dipositive ion such that charge exchange is not energetically favorable.…”

Activation of CH4 by Th+ as Studied by Guided Ion Beam Mass Spectrometry and Quantum Chemistry

“…37,38 Actinide reactivity has been inversely correlated to the promotion energy from the ground level to the first level with two 6d electrons (5f n−3 6d 2 7s or 5f n−2 6d 2 configurations). 39,41 In these studies, the ground state of Th + was reported to have a 6d 2 7s ( 4 F) configuration, thus requiring no promotion.Similar to the oxidation studies, thorium was also observed to be the most reactive of the actinides with several hydrocarbons studied using It is the only actinide to dehydrogenate methane at thermal energies, albeit with low efficiency, k/ k col = 0.009 ± 0.005 36 and k/k col = 0.02 ± 0.01, 35 where the collision rate limit (k col ) is the rate derived from modified variational transition-state/classical trajectory theory. 40 Both studies interpreted their observations to indicate an exothermic reaction, which suggested that D 0 (Th + −CH 2 ) ≥ D 0 (H 2 C−H 2 ) = 457 ± 1 kJ/mol.…”