Gas-Phase Reactivity of Actinides Monocations with NH3: ICP-MS Experiments Combined with a DFT Study.

Abstract

The reactivity of actinide monocations (Th+, U+, Np+, Pu+, Am+, and Cm+) with NH3 gas was studied in the reaction cell of an inductively coupled plasma-mass spectrometer (ICP-MS). Only Th+, U+, Np+, and Cm+ react completely with NH3 to form AnNH+, contrary to Pu+ and Am+. Differences in reactivity are found between U+/Pu+, Pu+/Cm+, and Am+/Cm+, which could resolve isobaric interferences in ICP-MS. DFT calculations were performed across the first half of the actinide series. The calculated reaction energy between An+ and NH3 reproduces the experimental trends in reactivity with Th+ > Pa+ > U+ > Np+ > Ac+ > Cm+ > Pu+ > Am+. The reaction path involves the initial formation of an AnNH3+ adduct followed by N-H bond insertion with the formation of HAnNH2+ and H2AnNH+ intermediate species and subsequent H2 loss. The trend in reactivity across the actinides is largely due to the first energy barrier and formation of the HAnNH2+ intermediate species. This limiting step is energetically unfavorable for the Pu+ and Am+ cations. For these cations, the excitation energy required to achieve a reactive configuration with two non-f electrons available for bonding is too high.