In a prior study [Van Stipdonk; et al. J. Phys. Chem. A 2006, 110, 959-970], electrospray ionization (ESI) was used to generate doubly charged complex ions composed of the uranyl ion and acetonitrile (acn) ligands. The complexes, general formula [UO2(acn)n](2+), n = 0-5, were isolated in an 3-D quadrupole ion-trap mass spectrometer to probe intrinsic reactions with H2O. Two general reaction pathways were observed: (a) the direct addition of one or more H2O ligands to the doubly charged complexes and (b) charge-exchange reactions. For the former, the intrinsic tendency to add H2O was dependent on the number and type of nitrile ligand. For the latter, charge exchange involved primarily the formation of uranyl hydroxide, [UO2OH](+), presumably via a collision with gas-phase H2O and the elimination of a protonated nitrile ligand. Examination of general ion fragmentation patterns by collision-induced dissociation, however, was hindered by the pronounced tendency to generate hydrated species. In an update to this story, we have revisited the fragmentation of uranyl-acetonitrile complexes in a linear ion-trap (LIT) mass spectrometer. Lower partial pressures of adventitious H2O in the LIT (compared to the 3-D ion trap used in our previous study) minimized adduct formation and allowed access to lower uranyl coordination numbers than previously possible. We have now been able to investigate the fragmentation behavior of these complex ions completely, with a focus on tendency to undergo ligand elimination versus charge reduction reactions. CID can be used to drive ligand elimination to completion to furnish the bare uranyl dication, UO2(2+). In addition, fragmentation of [UO2(acn)](2+) generated [UO2(NC)](+), which subsequently fragmented to furnish NUO(+). Formation of the nitrido by transfer of N from cyanide was confirmed using precursors labeled with (15)N. The observed formation of [UO2(NC)](+) and NUO(+) was modeled by density functional theory.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
The formation of adduct ions consisting of uranium oxycations and water was studied using an ion trap-secondary ion mass spectrometer. The U(IV) and U(V) species [UO(OH)]+ and [UO2]+ were produced by bombarding the surface of UO3 using molecular primary ions, and the U(VI) species [UO2(OH)]+ was generated by O2 oxidation of [UO(OH)]+ in the gas phase. All three ions formed H2O adducts by termolecular association reactions: [UO(OH)]+ (a U(IV) species) added three water molecules, for a total of five ligands; [UO2]+ (U(V)) added three or four water molecules, for a total of five or six ligands; and [UO2(OH)]+ (U(VI)) added four water molecules for a total of six ligands. Addition of a seventh ligand was not observed in any of the systems. These analyses showed that the optimum extent of ligation increased with increasing oxidation state of the uranium metal. Hard kinetic models were fit to the time-dependent mass spectral data using adaptive simulated annealing (ASA) to estimate reaction rates and rate constants from kinetic data sets. The values determined were validated using stochastic kinetic modeling and resulted in rate data for all forward and reverse reactions for the ensemble of reactive ions present in the ion trap. A comparison of the forward rate constants of the hydration steps showed that in general, formation of the monohydrates was slow, but that hydration efficiency increased upon addition of the second H2O. Addition of the third H2O was less efficient (except in the case of [UO2]+), and addition of the fourth H2O was even more inefficient and did not occur at all in the [UO2(OH)]+ system. Reverse rate constants also decreased with increasing ligation by H2O, except in the case of [UO(OH)(H2O)4]+, which prefers to quickly revert to the trihydrate. These findings indicate that stability of the hydrate complexes [UOyHz(H2O)n]+ increases with increasing n, until the optimum number of ligands is achieved. The results enable correlation of uranium hydration behavior with oxidation state.
Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U+(N2)n (n = 1–8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U+(N2)n indicate that N2 ligands are intact molecules and that there is no insertion chemistry resulting in UN+ or NUN+. Fixed frequency photodissociation at 532 and 355 nm indicate that the U+–N2 bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U+–N2 dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N–N stretch vibration via elimination of N2 molecules. These resonances are observed to be shifted about 130 cm–1 to the red from the free-N2 frequency for complexes with n = 3–8. Density functional theory indicates that U+ is most stable in the sextet state in these complexes and that N2 molecules bind in end-on configurations. The fully coordinated complex is predicted to be U+(N2)8, which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U+–N2 dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.
A uranyl–di-15-crown-5 complex with a unique slipped sandwich structure was synthesized and characterized by infrared spectroscopy and quantum-chemical methods.
A tandem mass spectrometric study of a series of secondary amides of acetylglycine and hippuric acid utilizing electrospray ionization (ESI) was conducted. Among the fragment ions observed was an unusual one, which we have determined to be a nitrilium ion having the structure CH3-C≡N⊕-Ph or Ph-C≡N⊕-Ph by loss of the full mass of glycine as a neutral fragment. A mechanism that we propose involves an initial protonation of the oxygen atom at the N-terminus, followed by cyclization to a five-membered imidazolium ring, and its subsequent collapse to the nitrilium ion. This mechanism is supported by extensive isotopic labels and considerable variation of substituents. A similar study of the amides of acyl β-alanine and acyl γ-aminobutyric acid revealed that the former furnishes the same nitrilium ion, but not the latter. Thus, a six-membered intermediate is also possible and capable of losing the full mass of β-alanine as a neutral fragment. When the size of the ring is forced to be seven-membered, this pathway is blocked. When this study was expanded to include a variety of N-acylproline amides, the nitrilium ion was observed in 100% abundance only when the acyl group was acetyl. Thus a proline effect (involvement of a strained bicyclic [3.3.0] structure) is being observed.
