Gas-Phase Reactivity of Actinides Monocations with\nNH<sub>3</sub>: ICP-MS Experiments Combined with a DFT Study

The reactivity of\nactinide monocations (Th<sup>+</sup>, U<sup>+</sup>, Np<sup>+</sup>, Pu<sup>+</sup>, Am<sup>+</sup>, and Cm<sup>+</sup>) with NH<sub>3</sub> gas was studied in the reaction cell of an\ninductively coupled plasma-mass spectrometer (ICP-MS). Only Th<sup>+</sup>, U<sup>+</sup>, Np<sup>+</sup>, and Cm<sup>+</sup> react\ncompletely with NH<sub>3</sub> to form AnNH<sup>+</sup>, contrary\nto Pu<sup>+</sup> and Am<sup>+</sup>. Differences in reactivity are\nfound between U<sup>+</sup>/Pu<sup>+</sup>, Pu<sup>+</sup>/Cm<sup>+</sup>, and Am<sup>+</sup>/Cm<sup>+</sup>, which could resolve isobaric\ninterferences in ICP-MS. DFT calculations were performed across the\nfirst half of the actinide series. The calculated reaction energy\nbetween An<sup>+</sup> and NH<sub>3</sub> reproduces the experimental\ntrends in reactivity with Th<sup>+</sup> > Pa<sup>+</sup> >\nU<sup>+</sup> > Np<sup>+</sup> > Ac<sup>+</sup> > Cm<sup>+</sup> > Pu<sup>+</sup> > Am<sup>+</sup>. The reaction path\ninvolves the initial\nformation of an AnNH<sub>3</sub><sup>+</sup> adduct followed by N–H\nbond insertion with the formation of HAnNH<sub>2</sub><sup>+</sup> and H<sub>2</sub>AnNH<sup>+</sup> intermediate species and subsequent\nH<sub>2</sub> loss. The trend in reactivity across the actinides is\nlargely due to the first energy barrier and formation of the HAnNH<sub>2</sub><sup>+</sup> intermediate species. This limiting step is energetically\nunfavorable for the Pu<sup>+</sup> and Am<sup>+</sup> cations. For\nthese cations, the excitation energy required to achieve a reactive\nconfiguration with two non-f electrons available for bonding is too\nhigh.