Gas-Phase Reactions of Bare and Ligated Uranium Ions with Sulfur Hexafluoride

Reactions of bare and ligated uranium ions with sulfur hexafluoride were studied in a quadrupole ion trap mass spectrometer. Bare U ⁺ was found to react rather efficiently with SF ₆ (k/k _ADO ∼ 0.4) to produce both UF _n ⁺ (n = 1, 2, 3, 4) and SF _n ⁺ (n = 1, 2, 3). Whereas the UF ⁺/SF ₆ reaction rate was essentially the same as that for U ⁺/SF ₆, both UF ₂ ⁺ and UF ₃ ⁺ were inert; this is attributed to a repulsive interaction between UF _n ⁺ and SF ₆ when n exceeds 1. Reactions of UF ⁺ (k/k _ADO ∼ 0.2) and UF ₂ ⁺ (k/k _ADO ∼ 0.05) with H ₂O resulted in both F → OH exchange and oxidation. In contrast, UF ₃ ⁺ reacted very efficiently with H ₂O (k/k _ADO ∼ 1), exhibiting only F → OH exchange. The primary ion products of the UO ⁺/SF ₆ reaction (k/k _ADO ∼ 0.2) were SF ₃ ⁺ and UOF ₂ ⁺; those of the UOH+/SF ₆ reaction (k/k _ADO ∼ 0.3) were SF ₃ ⁺ and UOF ⁺. The reaction results are discussed in the context of a previously proposed reaction model, the distinctive chemistry of uranium, and thermodynamic considerations. The results illuminate the nature of uranium as well as general aspects of the interaction of bare and ligated transition-metal ions with SF ₆. Results for collision-induced dissociation (CID) of selected uranium molecular ions support the concept of CID being a quasithermal process under these experimental conditions, with rearrangements prior to fragmentation possible for certain ions.