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

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The reactivity of actinide monocations (Th +, U +, Np +, Pu +, Am +, and Cm + ) with NH 3 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 NH 3 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 NH 3 reproduces the experimental trends in reactivity with Th + > Pa + > U + > Np + > Ac + > Cm + > Pu + > Am + . The reaction path involves the initial formation of an AnNH 3 + adduct followed by N–H bond insertion with the formation of HAnNH 2 + and H 2 AnNH + intermediate species and subsequent H 2 loss. The trend in reactivity across the actinides is largely due to the first energy barrier and formation of the HAnNH 2 + 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.