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Unformatted text preview: Infrared Spectroscopy of Discrete Uranyl Anion Complexes Gary S. Groenewold,* Anita K. Gianotto, and Michael E. McIlwain Idaho National Laboratory, Idaho Falls, Idaho Michael J. Van Stipdonk* and Michael Kullman Wichita State Uni V ersity, Wichita, Kansas David T. Moore, Nick Polfer, and Jos Oomens FOM Instituut V oor Plasmafysica Rijnhuizen, Nieuwegein, The Netherlands Ivan Infante and Lucas Visscher Vrije Uni V ersiteit Amsterdam, The Netherlands Bertrand Siboulet DEN/DRCP/SCPS, CEA Marcoule, 30207, Bagnols-sur-Ce ` ze cedex, France Wibe A. de Jong Pacific Northwest National Laboratory, Richland, Washington Recei V ed: September 11, 2007; In Final Form: October 25, 2007 The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO 2 2 + ) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO 2 A(S) n ] + , where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n )- 3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO 2 A] + ) were as low or lower than measurements for [UO 2 ] 2 + ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30- 40 cm- 1 higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO 2 A] + produced [UO 2 AS] + , which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO 2 AS] + were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO 2 AS] + species having different solvent neutrals, values decreased with increasing neutral nucleophilicity. Introduction The chemical behavior of uranium in general, and the linear uranyl dication [UO 2 ] 2 + in particular, are diverse on account of the relative ease of redox processes, 1 and the availability of f and d orbitals 2- 4 for complex formation. The latter has a profound affect on the solubility of the element....
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