Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]+[F(HF)2.3]- Ionic Liquid.

A new methodology for gas-phase uranium ion formation is described in which UO2 is dissolved in neat N-ethyl,N'-methylimidazolium fluorohydrogenate ionic liquid [EMIm+][F(HF)2.3-], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF6- (a U(V) species) was observed at m/z = 352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF6- was the base peak, and it was accompanied by a less intense UF5- that most likely was formed by elimination of a fluorine radical from UF6-. Formation of UF6- required dissolution of UO2 followed by or concurrent with oxidation of uranium from the + 4 to the + 5 state and finally formation of the fluorouranate. Dissolution of UO3 produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF6- provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations. Graphical abstract ᅟ.

Gamma-radiolytic stability of new methylated TODGA derivatives for minor actinide recycling.

The stability against gamma radiation of MeTODGA (methyl tetraoctyldiglycolamide) and Me2TODGA (dimethyl tetraoctyldiglycolamide), derivatives from the well-known extractant TODGA (N,N,N',N'-tetraoctyldiglycolamide), were studied and compared. Solutions of MeTODGA and Me2TODGA in alkane diluents were subjected to (60)Co γ-irradiation in the presence and absence of nitric acid and analyzed using LC-MS to determine their rates of radiolytic concentration decrease, as well as to identify radiolysis products. The results of product identification from three different laboratories are compared and found to be in good agreement. The diglycolamide (DGA) concentrations decreased exponentially with increasing absorbed dose. The MeTODGA degradation rate constants (dose constants) were uninfluenced by the presence of nitric acid, but the acid increased the rate of degradation for Me2TODGA. The degradation products formed by irradiation are also initially produced in greater amounts in acid-contacted solution, but products may also be degraded by continued radiolysis. The identified radiolysis products suggest that the weakest bonds are those in the diglycolamide center of these molecules.

Iron Fluoroanions and Their Clusters by Electrospray Ionization of a Fluorinating Ionic Liquid.

Metal fluoroanions are of significant interest for fundamental structure and reactivity studies and for making isotope ratio measurements that are free from isobaric overlap. Iron fluoroanions [FeF(4)](-) and [FeF(3)](-) were generated by electrospray ionization of solutions of Fe(III) and Fe(II) with the fluorinating ionic liquid 1-ethyl-3-methylimidazolium fluorohydrogenate [EMIm](+)[F(HF)(2.3)](-). Solutions containing Fe(III) salts produce predominately uncomplexed [FeF(4)](-) in the negative ion spectrum, as do solutions containing salts of Fe(II). This behavior contrasts with that of solutions of FeCl(3) and FeCl(2) (without [EMIm](+)[F(HF)(2.3)](-)) that preserve the solution-phase oxidation state by producing the gas-phase halide complexes [FeCl(4)](-) and [FeCl(3)](-), respectively. Thus, the electrospray-[EMIm](+)[F(HF)(2.3)](-) process is oxidative with respect to Fe(II). The positive ion spectra of Fe with [EMIm](+)[F(HF)(2.3)](-) displays cluster ions having the general formula [EMIm](+) (n+1)[FeF(4)](-) n, and DFT calculations predict stable complexes, both of which substantiate the conclusion that [FeF(4)](-) is present in solution stabilized by the imidazolium cation. The negative ion ESI mass spectrum of the Fe-ionic liquid solution has a very low background in the region of the [FeF(4)](-) complex, and isotope ratios measured for both [FeF(4)](-) and adventitious [SiF(5)](-) produced values in close agreement with theoretical values; this suggests that very wide isotope ratio measurements should be attainable with good accuracy and precision when the ion formation scheme is implemented on a dedicated isotope ratio mass spectrometer.

Generation of gas-phase zirconium fluoroanions by electrospray of an ionic liquid.

RATIONALE: New approaches for forming anions are sought that have strong abundance and no isobaric overlap, attributes that are compatible with the measurement of isotope ratios. Fluoroanions are particularly attractive because fluorine is monoisotopic, and thus will not have overlapping isobars with the isotope of interest. Since many elements do not have positive electron affinity values, they do not form stable negative atomic ions, and hence are not compatible with isotope ratio measurement using high sensitivity isotope ratio mass spectrometers such as accelerator mass spectrometers. METHODS: Zirconium fluoroanions were prepared using the fluorinating ionic liquid (IL) 1-ethyl-3-methylimidazolium fluorohydrogenate, which was used to generate abundant [ZrF5](-) ions using electrospray ionization. The IL was dissolved in acetonitrile, combined with a dilute solution of either Zr(4+) or ZrO(2+), and then electrosprayed. Mass analysis and collision-induced dissociation experiments were conducted using a time-of-flight mass spectrometer. Cluster structures were predicted using density functional theory calculations. RESULTS: The fluorohydrogenate IL solutions generated abundant [ZrF5](-) ions starting from solutions of both Zr(4+) and ZrO(2+). The mass spectra also contained IL-bearing cluster ions, whose compositions indicated the presence of [ZrF6](2-) in solution, a conclusion supported by the structural calculations. Rinsing out the zirconium-IL solution with acetonitrile decreased the IL clusters, but enhanced [ZrF5](-), which was sorbed by the polymeric electrospray supply capillary, and then released upon rinsing. This reduced the ion background in the mass spectrum. CONCLUSIONS: The fluorohydrogenate-IL solutions are a facile way to form zirconium fluoroanions in the gas phase using electrospray. The approach has potential as a source of fluoroanions for isotope ratio measurements, which would enable high-sensitivity measurement of minor zirconium isotopes without overlapping isobars caused by the charge carrier (i.e., the monoisotopic fluorine atoms).

Fluorohydrogenate cluster ions in the gas phase: electrospray ionization mass spectrometry of the [1-ethyl-3-methylimidazolium(+)][F(HF)2.3(-)] ionic liquid.

Electrospray ionization of the fluorohydrogenate ionic liquid [1-ethyl-3-methylimidazolium][F(HF)2.3] ionic liquid was conducted to understand the nature of the anionic species as they exist in the gas phase. Abundant fluorohydrogenate clusters were produced; however, the dominant anion in the clusters was [FHF(-)], and not the fluoride-bound HF dimers or trimers that are seen in solution. Density functional theory (DFT) calculations suggest that HF molecules are bound to the clusters by about 30 kcal/mol. The DFT-calculated structures of the [FHF(-)]-bearing clusters show that the favored interactions of the anions are with the methynic and acetylenic hydrogen atoms on the imidazolium cation, forming planar structures similar to those observed in the solid state. A second series of abundant negative ions was also formed that contained [SiF5(-)] together with the imidazolium cation and the fluorohydrogenate anions that originate from reaction of the spray solution with silicate surfaces.

Infrared multiple photon dissociation spectroscopy of group I and group II metal complexes with Boc-hydroxylamine.

RATIONALE: Hydroxamates are essential growth factors for some microbes, acting primarily as siderophores that solubilize iron for transport into a cell. Here we determined the intrinsic structure of 1:1 complexes between Boc-protected hydroxylamine and group I ([M(L)](+)) and group II ([M(L-H)](+)) cations, where M and L are the cation and ligand, respectively, which are convenient models for the functional unit of hydroxamate siderphores. METHODS: The relevant complex ions were generated by electrospray ionization (ESI) and isolated and stored in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Infrared spectra of the isolated complexes were collected by monitoring (infrared) photodissociation yield as a function of photon energy. Experimental spectra were then compared to those predicted by density functional theory (DFT) calculations. RESULTS: The infrared multiple photon dissociation (IRMPD) spectra collected are in good agreement with those predicted to be lowest-energy by DFT. The spectra for the group I complexes contain six resolved absorptions that can be attributed to amide I and II type and hydroxylamine N-OH vibrations. Similar absorptions are observed for the group II cation complexes, with shifts of the amide I and amide II vibrations due to the change in structure with deprotonation of the hydroxylamine group. CONCLUSIONS: IRMPD spectroscopy unequivocally shows that the intrinsic binding mode for the group I cations involves the O atoms of the amide carbonyl and hydroxylamine groups of Boc-hydroxylamine. A similar binding mode is preferred for the group II cations, except that in this case the metal ion is coordinated by the O atom of the deprotonated hydroxylamine group.

Rapid analysis of single droplets of lanthanide-ligand solutions by electrospray ionization mass spectrometry using an induction-based fluidics source.

Electrospray ionization mass spectra of lanthanide coordination complexes were measured by launching nanoliter-sized droplets directly into the aperture of an electrospray ionization mass spectrometer. Droplets ranged in size from 102 nL to 17 nL, while metal concentrations were 293 µM. The sample solution was delivered to a source capillary by a nanoliter dispenser at a rate of 21 nL/s, and droplets were ejected from the capillary by pulsing a potential onto the capillary. The end of the capillary was situated in front of the mass spectrometer and aimed directly at the aperture. The period and power of the electrical pulse was controlled by a digital energy source. The intensity of the extracted ion time profiles from the experiment showed reproducible production of lanthanide nitrato-anion complexes (Ce, Tb, and Lu). The integrated ion intensities of the complexes were reproducible, having relative standard deviations on the order 10% for anions, and 10-30% for cations. The integrated ion intensities were proportional to the droplet size, and the response was linear from about 100 to 650 pmol. However, the intercept is not zero, indicating a nonlinear response at lower analyte quantities or droplet sizes. Cation complexes were generated in separate experiments that corresponded to lanthanide nitrate ion pairs coordinated with the separations ligand octyl,phenyl,(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO). Experiments showed a preference for formation of CMPO complexes with Ln(3+) having larger ionic radii. The relative standard deviation values of the cation abundance measurements were somewhat higher for the more highly coordinated complexes, which are also less stable. The mass spectral quality was high enough to measure the ratios of the minor isotopic ions to a high degree of accuracy. The approach suggests that the methodology has utility for analysis of solutions where the sample quantity is limited, or where the sampling efficiency of a normal ESI source is limiting on account of hazards derived from the sample solution.

Oxidative degradation of bis(2,4,4-trimethylpentyl)dithiophosphinic acid in nitric acid studied by electrospray ionization mass spectrometry.

RATIONALE: The selective separation of the minor actinides (Am, Cm) from the lanthanides is a topic of ongoing nuclear fuel cycle research, and dithiophosphinic acids are candidate ligands in these processes. Ligand instability has been noted under radiolytic and harsh acid conditions but explicit degradation pathways for ligands such as bis(2,4,4-trimethylpentyl)-dithiophosphinic acid (CyxH), the major compound in the commercial product Cyanex 301, have been elusive. METHODS: Organic solutions of CyxH were contacted with aqueous solutions of HNO(3), and their degradation was studied by analyzing samples from these experiments by direct infusion electrospray ionization mass spectrometry. Ions were identified using accurate mass measurement and collision-induced dissociation. RESULTS: The positive ion spectra contained cationized CyxH cluster ions, and oxidatively coupled species (designated Cyx(2)) cationized by either H or Na. The Cyx(2)-derived ions increased with acid contact time. The negative ion spectra consisted almost entirely of the CyxH conjugate base. The negative ion spectra of the HNO(3)-contacted samples also contained conjugate bases corresponding to the dioxo and perthio derivatives of CyxH. CONCLUSIONS: CyxH is oxidized by acid contact to form the coupled species Cyx(2), and the dioxo species arise from subsequent oxidation of Cyx(2). Oxidative coupling increases with contact time, and with higher HNO(3) concentrations. The direct infusion measurements provided a simple approach for assessing degradation pathways and kinetics.

Determination of CMPO using HPLC-UV.

Octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) is an extractant proposed for selective separation of radionuclide metals from used nuclear fuel solutions using solvent extraction. Radiolysis reactions can degrade CMPO and reduce separation performance and hence methods for measuring the concentration of CMPO are needed. A novel high performance liquid chromatography (HPLC) method was developed for measuring CMPO in dodecane that featured a low pH buffer, octanol as a co-solvent with 2-propanol, and ultraviolet (UV) detection. Validation data indicated that the HPLC-UV method for CMPO determination provided good linearity, sensitivity, accuracy and precision. Method performance was evaluated using CMPO samples that had undergone radiolysis, and the results showed a decrease in CMPO concentration and the appearance of degradation products. The degradation products were identified using electrospray ionization mass spectrometry, which also showed formation of CMPO-nitric acid complexes that account for the apparent loss of CMPO in an acidic environment, independent of irradiation.

Infrared multiple-photon dissociation spectroscopy of group II metal complexes with salicylate.

Ion trap tandem mass spectrometry with collision-induced dissociation, and the combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations, were used to characterize singly charged, 1:1 complexes of Ca(2+), Sr(2+) and Ba(2+) with salicylate. For each metal-salicylate complex, the CID pathways are: (a) elimination of CO(2) and (b) formation of [MOH](+) where M = Ca(2+), Sr(2+) or Ba(2+). DFT calculations predict three minima for the cation-salicylate complexes which differ in the mode of metal binding. In the first, the metal ion is coordinated by O atoms of the (neutral) phenol and carboxylate groups of salicylate. In the second, the cation is coordinated by phenoxide and (neutral) carboxylic acid groups. The third mode involves coordination by the carboxylate group alone. The infrared spectrum for the metal-salicylate complexes contains a number of absorptions between 1000 and 1650 cm(-1), and the best correlation between theoretical and experimental spectra is found for the structure that features coordination of the metal ion by phenoxide and the carbonyl O of the carboxylic acid group, consistent with the calculated energies for the respective species.

Recovery of phosphonate surface contaminants from glass using a simple vacuum extractor with a solid-phase microextraction fiber.

Recovery of chemical contaminants from fixed surfaces for analysis can be challenging, particularly if it is not possible to acquire a solid sample to be taken to the laboratory. A simple device is described that collects semi-volatile organic compounds from fixed surfaces by creating an enclosed volume over the surface, then generating a modest vacuum. A solid-phase microextraction (SPME) fiber is then inserted into the evacuated volume where it functions to sorb volatilized organic contaminants. The device is based on a syringe modified with a seal that is used to create the vacuum, with a perforable plunger through which the SPME fiber is inserted. The reduced pressure speeds partitioning of the semi-volatile compounds into the gas phase and reduces the boundary layer around the SPME fiber, which enables a fraction of the volatilized organics to partition into the SPME fiber. After sample collection, the SPME fiber is analyzed using conventional gas chromatography/mass spectrometry. The methodology has been used to collect organophosphorus compounds from glass surfaces, to provide a simple test for the functionality of the devices. Thirty minute sampling times (ΔT(vac)) resulted in fractional recovery efficiencies that ranged from 10(-3) to >10(-2), and in absolute terms, collection of low nanograms was demonstrated. Fractional recovery values were positively correlated to the vapor pressure of the compounds being sampled. Fractional recovery also increased with increasing ΔT(vac) and displayed a roughly logarithmic profile, indicating that an operational equilibrium is being approached. Fractional recovery decreased with increasing time between exposure and sampling; however, recordable quantities of the phosphonates could be collected three weeks after exposure.

Investigation of uranyl nitrate ion pairs complexed with amide ligands using electrospray ionization ion trap mass spectrometry and density functional theory.

Ion populations formed from electrospray of uranyl nitrate solutions containing different amides vary depending on ligand nucleophilicity and steric crowding at the metal center. The most abundant species were ion pair complexes having the general formula [UO(2)(NO(3))(amide)(n=2,3)](+); however, singly charged complexes containing the amide conjugate base and reduced uranyl UO(2)(+) were also formed as were several doubly charged species. The formamide experiment produced the greatest diversity of species resulting from weaker amide binding, leading to dissociation and subsequent solvent coordination or metal reduction. Experiments using methyl formamide, dimethyl formamide, acetamide, and methyl acetamide produced ion pair and doubly charged complexes that were more abundant and less abundant complexes containing solvent or reduced uranyl. This pattern is reversed in the dimethylacetamide experiment, which displayed lower abundance doubly charged complexes, but augmented reduced uranyl complexes. DFT investigations of the tris-amide ion pair complexes showed that interligand repulsion distorts the amide ligands out of the uranyl equatorial plane and that complex stabilities do not increase with increasing amide nucleophilicity. Elimination of an amide ligand largely relieves the interligand repulsion, and the remaining amide ligands become closely aligned with the equatorial plane in the structures of the bis-amide ligands. The studies show that the phenomenological distribution of coordination complexes in a metal-ligand electrospray experiment is a function of both ligand nucleophilicity and interligand repulsion and that the latter factor begins exerting influence even in the case of relatively small ligands like the substituted methyl-formamide and methyl-acetamide ligands.

Structure of [M + H - H(2)O](+) from protonated tetraglycine revealed by tandem mass spectrometry and IRMPD spectroscopy.

Multiple-stage tandem mass spectrometry and collision-induced dissociation were used to investigate loss of H(2)O or CH(3)OH from protonated versions of GGGX (where X = G, A, and V), GGGGG, and the methyl esters of these peptides. In addition, wavelength-selective infrared multiple photon dissociation was used to characterize the [M + H - H(2)O](+) product derived from protonated GGGG and the major MS(3) fragment, [M + H - H(2)O - 29](+) of this peak. Consistent with the earlier work [ Ballard , K. D. ; Gaskell , S. J. J. Am. Soc. Mass Spectrom. 1993 , 4 , 477 - 481 ; Reid , G. E. ; Simpson , R. J. ; O'Hair , R. A. J. Int. J. Mass Spectrom. 1999 , 190/191 , 209 -230 ], CID experiments show that [M + H - H(2)O](+) is the dominant peak generated from both protonated GGGG and protonated GGGG-OMe. This strongly suggests that the loss of the H(2)O molecule occurs from a position other than the C-terminal free acid and that the product does not correspond to formation of the b(4) ion. Subsequent CID of [M + H - H(2)O](+) supports this proposal by resulting in a major product that is 29 mass units less than the precursor ion. This is consistent with loss of HN horizontal lineCH(2) rather than loss of carbon monoxide (28 mass units), which is characteristic of oxazolone-type b(n) ions. Comparison between experimental and theoretical infrared spectra for a group of possible structures confirms that the [M + H - H(2)O](+) peak is not a substituted oxazolone but instead suggests formation of an ion that features a five-membered ring along the peptide backbone, close to the amino terminus. Additionally, transition structure calculations and comparison of theoretical and experimental spectra of the [M + H - H(2)O - 29](+) peak also support this proposal.

Variable denticity in carboxylate binding to the uranyl coordination complexes.

Tris-carboxylate complexes of uranyl [UO(2)](2+) with acetate and benzoate were generated using electrospray ionization mass spectrometry, and then isolated in a Fourier transform ion cyclotron resonance mass spectrometer. Wavelength-selective infrared multiple photon dissociation (IRMPD) of the tris-acetato uranyl anion resulted in a redox elimination of an acetate radical, which was used to generate an IR spectrum that consisted of six prominent absorption bands. These were interpreted with the aid of density functional theory calculations in terms of symmetric and antisymmetric -CO(2) stretches of the monodentate and bidentate acetate, CH(3) bending and umbrella vibrations, and a uranyl O-U-O asymmetric stretch. The comparison of the calculated and measured IR spectra indicated that the predominant conformer of the tris-acetate complex contained two acetate ligands bound in a bidentate fashion, while the third acetate was monodentate. In similar fashion, the tris-benzoate uranyl anion was formed and photodissociated by loss of a benzoate radical, enabling measurement of the infrared spectrum that was in close agreement with that calculated for a structure containing one monodentate and two bidentate benzoate ligands.

Infrared multiple photon dissociation spectroscopy of sodium and potassium chlorate anions.

The structures of gas-phase, metal chlorate anions with the formula [M(ClO(3))(2)](-), M = Na and K, were determined using tandem mass spectrometry and infrared multiple photon dissociation (IRMPD) spectroscopy. Structural assignments for both anions are based on comparisons of the experimental vibrational spectra for the two species with those predicted by density functional theory (DFT) and involve conformations that feature either bidentate or tridentate coordination of the cation by chlorate. Our results strongly suggest that a structure in which both chlorate anions are bidentate ligands is preferred for [Na(ClO(3))(2)](-). However, for [K(ClO(3))(2)](-) the best agreement between experimental and theoretical spectra is obtained from a composite of predicted spectra for which the chlorate anions are either both bidentate or both tridentate ligands. In general, we find that the overall accuracy of DFT calculations for prediction of IR spectra is dependent on both functional and basis set, with best agreement achieved using frequencies generated at the B3LYP/6-311+g(3df) level of theory.

Spectroscopic evidence for mobilization of amide position protons during CID of model peptide ions.

Infrared multiple photon dissociation (IRMPD) spectroscopy was used to study formation of b2+ from nicotinyl-glycine-glycine-methyl ester (NicGGOMe). IRMPD shows that NicGGOMe is protonated at the pyridine ring of the nicotinyl group, and more importantly, that b2+ from NicGGOMe is not protonated at the oxazolone ring, as would be expected if the species were generated on the conventional bn+/yn+ oxazolone pathway, but at the pyridine ring instead. IRMPD data support a hypothesis that formation of b2+ from NicGGOMe involves mobilization and transfer of an amide position proton during the fragmentation reaction.

Infrared spectrum of potassium-cationized triethylphosphate generated using tandem mass spectrometry and infrared multiple photon dissociation.

Tandem mass spectrometry and wavelength-selective infrared photodissociation were used to generate an infrared spectrum of gas-phase triethylphosphate cationized by attachment of K(+). Prominent absorptions were observed in the region of 900 to 1300 cm(-1) that are characteristic of phosphate P=O and P-O-R stretches. The relative positions and intensities of the IR absorptions were reproduced well by density functional theory (DFT) calculations performed using the B3LYP functional and the 6-31+G(d), 6-311+G(d,p) and 6-311++G(3df,2pd) basis sets. Because of good correspondence between experiment and theory for the cation, DFT was then used to generate a theoretical spectrum for neutral triethylphosphate, which in turn accurately reproduces the IR spectrum of the neat liquid when solvent effects are included in the calculations.

Cerium oxyhydroxide clusters: formation, structure, and reactivity.

Cerium oxyhydroxide cluster anions were produced by irradiating ceric oxide particles by using 355 nm laser pulses that were synchronized with pulses of nitrogen gas admitted to the irradiation chamber. The gas pulse stabilized the nascent clusters that are largely anhydrous [Ce(x)O(y)] ions and neutrals. These initially formed species react with water, principally forming oxohydroxy species that are described by the general formula [Ce(x)O(y)(OH)(z)](-) for which all the Ce atoms are in the IV oxidation state. In general, the extent of hydroxylation varies from a value of three OH per Ce atom when x = 1 to a value slightly greater than 1 for x >or= 8. The Ce(3) and Ce(6) species deviate significantly from this trend: the x = 3 cluster accommodates more hydroxyl moieties compared to neighboring congeners at x = 2 and 4. Conversely, the x = 6 cluster is significantly less hydroxylated than its x = 5 and 7 neighbors. Density functional theory (DFT) modeling of the cluster structures shows that the hydrated clusters are hydrolyzed, and contain one-to-multiple hydroxide moieties, but not datively bound water. DFT also predicts an energetic preference for formation of highly symmetric structures as the size of the clusters increases. The calculated structures indicate that the ability of the Ce(3) oxyhydroxide to accommodate more extensive hydroxylation is due to a more open, hexagonal structure in which the Ce atoms can participate in multiple hydrolysis reactions. Conversely the Ce(6) oxyhydroxide has an octahedral structure that is not conducive to hydrolysis. In addition to the fully oxidized (Ce(IV)) oxyhydroxides, reduced oxyhydroxides (containing a Ce(III) center) are also formed. These become more prominent as the size of the clusters increases, suggesting that the larger ceria clusters have an increased ability to accommodate a reduced Ce(III) moiety. In addition, the spectra offer evidence for the formation of superoxide derivatives that may arise from reaction of the reduced oxyhydroxides with dioxygen. The overall intensity of the clusters tends to monotonically decrease as the cluster size increases; however, this trend is interrupted at Ce(13), which is significantly more stable compared to neighboring congeners, suggesting formation of a dehydrated Keggin-type structure.

IRMPD spectroscopy of anionic group II metal nitrate cluster ions.

Anionic group II metal nitrate clusters of the formula [M(2)(NO(3))(5)](-), where M(2) = Mg(2), MgCa, Ca(2), and Sr(2), are investigated by infrared multiple photon dissociation (IRMPD) spectroscopy to obtain vibrational spectra in the mid-IR region. The IR spectra are dominated by the symmetric and the antisymmetric nitrate stretches, with the latter split into high and low-frequency components due to the distortion of nitrate anion symmetry by interactions with the cation. Density functional theory (DFT) is used to predict geometries and vibrational spectra for comparison to the experimental spectra. Calculations yield two stable isomers: the first one contains two terminal nitrate anions on each cation and a single bridging nitrate ("mono-bridging"), while the second structure features a single terminal nitrate on each cation with three bridging nitrate ligands ("tri-bridging"). The tri-bridging isomer is calculated to be lower in energy than the mono-bridging one for all species. Theoretical spectra of the tri-bridging structure provide a better qualitative match to the experimental infrared spectra of [Mg(2)(NO(3))(5)](-) and [MgCa(NO(3))(5)](-). However, the profile of the low-frequency nu(3) band for the Mg(2) complex suggests a third possible isomer not predicted by theory. The IRMPD spectra of the Ca(2) and Sr(2) complexes are better reconciled by a weighted summation of the spectra of both isomers suggesting that a mixture of structures is present.

Addition of H2O and O2 to acetone and dimethylsulfoxide ligated uranyl(V) dioxocations.

Gas-phase complexes of the formula [UO2(lig)]+ (lig = acetone (aco) or dimethylsulfoxide (dmso)) were generated by electrospray ionization (ESI) and studied by tandem ion-trap mass spectrometry to determine the general effect of ligand charge donation on the reactivity of UO2(+) with respect to water and dioxygen. The original hypothesis that addition of O2 is enhanced by strong sigma-donor ligands bound to UO2(+) is supported by results from competitive collision-induced dissociation (CID) experiments, which show near exclusive loss of H2O from [UO2(dmso)(H2O)(O2)]+, whereas both H2O and O2 are eliminated from the corresponding [UO2(aco)(H2O)(O2)]+ species. Ligand-addition reaction rates were investigated by monitoring precursor and product ion intensities as a function of ion storage time in the ion-trap mass spectrometer: these experiments suggest that the association of dioxygen to the UO2(+) complex is enhanced when the more basic dmso ligand was coordinated to the metal complex. Conversely, addition of H2O is favored for the analogous complex ion that contains an aco ligand. Experimental rate measurements are supported by density function theory calculations of relative energies, which show stronger bonds between UO2(+) and O2 when dmso is the coordinating ligand, whereas bonds to H2O are stronger for the aco complex.