Thermodynamic insight into AnO2+ bonding from ThH+/UH+ reactions studied by inductively coupled plasma tandem mass spectrometry

Abstract

The bonding mechanisms of actinides have been a focus of fundamental research over the past few decades. In the present study, reactions of the simplest actinide-containing species, ThH⁺ and UH⁺, with O₂ and CO₂ are investigated by inductively coupled plasma tandem mass spectrometry. The reactions of ThH⁺ and UH⁺ with O₂ are efficient, and the reactions of ThH⁺ and UH⁺ with CO₂ display reduced reaction efficiencies. For both reactions involving CO₂, ThO₂⁺ and UO₂⁺ are observed; however, there is a clear barrier to ThO₂⁺ formation whereas UO₂⁺ forms through an exothermic, barrierless process. The experimental observations and available thermodynamic information are used to predict the outcomes of reactions involving the 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), although 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⁺. Similar shifts have also been noted in previous studies. Beginning with Np⁺, the 6d orbitals become less accessible than they are for the earlier An⁺ as measured by E_p(6d²), and this accessibility of the 6d orbitals may drive actinide bonding.