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

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.