Abstract. The speciation and reactivity of uranium are topics of sustained interest because of their importance to the development of nuclear fuel processing methods, and a more complete understanding of the factors that govern the mobility and fate of the element in the environment. Tandem mass spectrometry can be used to examine the intrinsic reactivity (i.e., free from influence of solvent and other condensed phase effects) of a wide range of metal ion complexes in a species-specific fashion. Here, electrospray ionization, collision-induced dissociation, and gas-phase ion-molecule reactions were used to create and characterize ions derived from precursors composed of uranyl cation (U

Past studies of fragmentation reactions of doubly-charged uranyl (UO2 2+ ) complexes have been impeded by very rapid water addition reactions that cause H2O adducts to dominate product ion spectra.The fragmentation of uranyl-acetone (aco) complexes ([UO2(aco)n] 2+ , n=1-5), generated by electrospray ionization, is revisited here using: (a) collisional activation in a linear ion trap (LIT) mass spectrometer in which the level of background H2O is significantly lower, and (b) infrared photodissociation (IRMPD, 10.6 µm) in the LIT and a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. Lower levels of adventitious H2O in the LIT provided access to fragmentation of [UO2(aco)n] 2+ , n=1-5. For n=3-5, direct elimination of aco ligands is the favored fragmentation pathway. For n=1 and 2, charge reduction reactions are dominant. For [UO2(aco)2] 2+ , the most abundant product ion is [UO2(aco)] + , while UO2 + is observed following collision-induced dissociation (CID) of [UO2(aco)] 2+ . Minor peaks corresponding to ligated [UO2OH] + are also observed. The IRMPD experiments in the FT-ICR yielded highly accurate mass measurements that confirm composition assignments, and shed light on dissociation reactions in a gasphase environment that is entirely free of adventitious H2O. For [UO2(aco)n] 2+ , n=3-5, the primary photodissociation channel is direct aco elimination, along with charge-reduction pathways that involve intra-complex proton transfer and formation of species that contain enolate ligands. Similar pathways are observed for IRMPD measurements in the LIT.

UO2 H(+) is generated from [UO2 OCH3 ](+) by transfer of H from the methyl group. Variable-energy and variable-time CID experiments suggest that the apparent threshold for production of UO2 H(+) is lower than for UO2 (+) , but the pathway is kinetically less favored for the former than for the latter. CID experiments reveal that [UO2 OCH2 CH3 ](+) dissociates to generate [UO2 CH3 ](+) , a relatively rare species with a U-C bond, and [UO2 (O = CH2 )](+) .