Formation and decomposition of uranium–oxygen clusters in fast atom bombardment of dioxouranium(VI) salts

Abstract

Fast atom bombardment (FAB) of uranyl salts in involatile organic matrices produced a long series of clusters of general formula [(UO₂)_xO_y]⁺ with x reaching nearly 40. For each value of x there is a series of values of y, with some value of y exhibiting the greatest abundance for each x value. Smaller clusters (x≤ 22) with even values of x show a stoichiometry of O:U of 2.5:1, while larger cluster (x≥ 24) show a reduced O:U ratio. All clusters with odd values of x adopt a O:U ratio of less than 2.5:1. Near x= 24, odd–even effects appeared in the relative intensities of the clusters. As x→40 all cluster series tend to a final stoichiometry of O:U = 2.5:1. The collision-induced decomposition (CID) of [(UO₂)_xO_y]⁺ produced long sequences of progressively smaller clusters via the loss of UO₂, UO₃ and UO₄ moieties; the detection of particular clusters depends on the collision-gas (argon) pressure. Odd–even alternation effects are apparent in the CID spectra. Clusters of general formula [(ThO₂)_x]⁺ were produced by FAB of thorium(IV) nitrate, with x reaching 20; odd–even alternation effects are prominent when x≥ 14. The relatively complex behaviour exhibited by U–O clusters is associated with electronic effects, particularly the existence and relative stabilities of the +6, +5 and +4 oxidation states of uranium, as also manifested in U–O systems in the solid state, with which comparison can be made.