FORMATION AND DECOMPOSITION OF URANIUM-OXYGEN CLUSTERS IN FAST-ATOM-BOMBARDMENT OD DIOXOURANIUM(VI) SALTS

Fast atom bombardment (FAB) of uranyl salts in involatile organic matrices produced a long series of clusters of general formula [(UO2)(x)O-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 less than or equal to 22) with even values of x show a stoichiometry of O:U of 2.5:1, while larger clusters (x greater than or equal to 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 ail cluster series tend to a final stoichiometry of O:U = 2.5:1. The collision-induced decomposition (CID) of [(UO2)(x)O-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 FA of thorium(lv) nitrate, with x reaching 20; odd-even alternation effects are prominent when x greater than or equal to 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