PNNL

The Science

Actinides are key to nuclear processes and exhibit interesting chemical behavior. Researchers observed reactions of thorium and uranium hydride (ThH⁺/UH⁺) cations with oxygen and carbon dioxide by inductively coupled plasma tandem mass spectrometry. The uranium hydride reactions preferentially form uranium dioxide in both reactions. Thorium hydride reacts to preferentially form thorium monoxide. The reaction energies were consistent with previously reported thermochemistry. Extending these results to additional actinides (Th⁺–Am⁺) indicates that the reaction thermochemistry is correlated with the energy required to have two non-f electrons available for reaction. The availability of non-f electrons likely drives this reaction, as non-f electrons are more efficient in forming the necessary reaction intermediates. 

The Impact

Understanding actinide bonding and reactivity is key to advancing nuclear processes. By using actinide hydrides, which are the simplest molecular reactant ion that can be studied, researchers could investigate the energy dependence in forming actinide molecular bonds. If bonding is driven by non-f electrons, as observed in previous work, then the actinide hydride ties up one of the more accessible non-f electrons. Comparing the thermodynamic trends across the actinide series shows a clear difference between the early actinides, where non-f electrons are more accessible, and the transuranic actinides, where non-f electrons are less accessible. Beyond nuclear processes, these insights provide details on molecular bonding that can be applied across chemistry in general.

Summary

The bonding mechanisms of actinides have been a focus of fundamental research in recent decades, but much remains to be learned. Researchers investigated reactions of the simplest actinide-containing species, ThH⁺ and UH⁺, with O₂ and CO₂ using inductively coupled plasma tandem mass spectrometry. The reactions of ThH⁺ and UH⁺ with O₂ are efficient, but the reactions of ThH⁺ and UH⁺ with CO₂ display reduced reaction efficiencies. In the CO₂ reactions, ThO₂⁺ and UO₂⁺ form. However, there is a clear barrier to ThO₂⁺ formation while UO₂⁺ forms through an exothermic, barrierless process. These experimental observations and available thermodynamic information were combined to predict the outcomes of reactions involving later AnH⁺. The anticipated reaction enthalpies for Pa–Am display a clear correlation with the promotion energy of An⁺ to a 6d² electronic configuration, E_p(6d²). A shift in the slope of the correlation of reaction enthalpies and E_p(6d²) suggests that there is likely a change in bonding mechanism that starts with Np⁺. Beginning with Np⁺, the 6d orbitals become less accessible than for earlier An⁺ as measured by E_p(6d²) and this accessibility of the 6d orbitals may drive actinide bonding.

Contact

Richard Cox, Pacific Northwest National Laboratory, Rich.Cox@pnnl.gov 

Funding

The experimental data was collected with support from the Open Call Initiative under the Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). Interpretation of this experimental data and writing this manuscript was supported by a U.S. Department of Energy, Office of Science, Early Career Research Program award in the Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, Heavy Element Chemistry program under FWP 81364. PNNL is a multi-program national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract No. DE-AC05-76RL01830.