Gas-Phase Reactivity of Actinides Monocations with NH 3 : ICP-MS Experiments Combined with a DFT Study

The reactivity of actinide monocations (Th , U , Np , Pu , Am , and Cm ) with NH 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 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 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 adduct followed by N-H bond insertion with the formation of HAnNH and H AnNH intermediate species and subsequent H loss. The trend in reactivity across the actinides is largely due to the first energy barrier and formation of the HAnNH 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.