PNNL

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 kinetic energy dependences of ThH+ and UH+ reactions 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, Ep(6d2), although a shift in the slope 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 Ep(6d2), and the accessibility of the 6d orbitals may drive actinide bonding.