We report here that electrospray ionization (ESI) of uranyl nitrate dissolved in a mixture of H2O and acetone causes the formation of doubly charged, gas-phase complexes containing UO2 2+ “solvated” by neutral ligands. Using mild conditions, the dominant species observed in the ESI mass spectrum contained the uranyl ion coordinated by five acetone ligands, consistent with proposed most-stable structures in the solution phase. However, chemical mass shift data, ion peak shapes, and a plot of fractional ion abundance versus ion desolvation temperature suggest that in the gas phase, and under the ion-trapping and ejection conditions imposed, complexes with five equatorial acetone ligands are less stable than those with four. Multiple-stage tandem mass spectrometry showed that uranyl-acetone complexes dissociate via the elimination of acetone ligands and through pathways that involve reactive collisions with adventitious H2O in the ion trap. At no point was complete removal of ligands to generate the UO2 2+ ion achieved. ESI was also used to generate complex ions of similar composition and ligand number but different charge state for an investigation of the influence of complex charge on the tendency to add ligands by gas-phase association reactions. We found that the addition of a fifth acetone molecule to complexes initially containing four equatorial ligands is more facile for the doubly charged species. The singly charged complex shows a significant back-reaction to eliminate the fifth ligand, suggesting an intrinsic difference in the preferred coordination number for the U(VI) and U(V) complexes in the gas phase.

Electrospray ionization was used to generate doubly charged complex ions composed of the uranyl ion and nitrile ligands. The complexes, with general formula [UO2(RCN)n]2+, n = 0-5 (where R=CH3-, CH3CH2-, or C6H5-), were isolated in an ion-trap mass spectrometer to probe intrinsic reactions with H2O. For these complexes, two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-reduction reactions. For the latter, the reactions produced uranyl hydroxide, [UO2OH], complexes via collisions with gas-phase H2O molecules and the elimination of protonated nitrile ligands.

Multiple-stage tandem mass spectrometry was used to characterize the dissociation pathways for complexes composed of (1) the uranyl ion, (2) nitrate or hydroxide, and (3) water or alcohol. The complex ions were derived from electrospray ionization (ESI) ϩ . The abundance of the species was greater than expected based on previous experimental measurements of the (slow) hydration rate for UO 2 ϩ when stored in the ion trap. To account for the production of the hydrated product, a reductive elimination reaction involving reactive collisions with water in the ion trap is proposed. T he speciation and reactivity of uranium is a topic of sustained interest because species-dependent chemistry [1] controls processes ranging from nuclear fuel processing [2] to mobility and fate in the geologic subsurface [3,4]. The solution chemistry of uranium is dominated by the uranyl dication, UO 2 2ϩ , which is known to form complexes with a range of ligands [1]. Specific interaction with solvent will significantly influence the physico-chemical behavior of the uranyl ion and its complexes, and this has motivated investigations of complex composition and stability using infrared spectroscopy and extended X-ray absorption fine structure [5][6][7][8][9][10][11]. Unfortunately, explicit control over the interactions of solvent and nonsolvent ligands with the uranyl ion is difficult, which makes the study of species-dependent uranium behavior complicated. To gain a better understanding of the intrinsic interactions between different uranyl species and solvent, we have begun an investigation of uranyl-anion complexes in the gas phase using ion-trap mass spectrometry (IT-MS). Several recent reports have demonstrated that intrinsic metal and metal complex chemistry can be investigated by the (controlled) addition of reagent gas to an ion trap [12][13][14][15][16][17][18][19][20][21][22], or by way of the presence of H 2 O and other small molecule contaminants within the He bath gas used to collisionally cool ions and improve trapping efficiency [23][24][25]. The reactions of uranium ions in the gas phase have been the subject of several earlier investigations. Studies by , and by Schwarz and coworkers [30] have probed the reactions between U ϩ and UO ϩ and organic compounds such as alkanes and alcohols. Armentrout and Beauchamp [31] investigated the oxidation of U ϩ using small molecules such as O 2 , CO, CO 2 , COS, and

Theoretical calculations suggest a novel two-electron three-atom bonding scheme for complexation of O 2 with U(V) compounds, leading to the stabilization of superoxo complexes in the side-on (eta (2)) configuration. This binding motif is likely to play an important role in the oxidative processes involving trans-uranium compounds having valence 5f phi electrons.