Structure and growth of oxides on polycrystalline nickel surfaces

The oxidation of polycrystalline nickel (Ni) metal surfaces after exposure to oxygen gas (O₂) at 25 and 300 °C and pressures near 130 Pa, was studied using X‐ray photoelectron spectroscopy (XPS). Oxide structures involving both divalent (Ni²⁺) and trivalent (Ni³⁺) species could be distinguished using Ni 2p spectra, while surface adsorbed O₂ and atomic oxygen (O) species could be differentiated from bulk oxide (O^2?) using O 1s spectra. Oxide thicknesses and distributions were determined using QUASES^?, and the average oxide thickness was verified using the Strohmeier formula. The reaction kinetics for oxide films grown at 300 °C followed a parabolic mechanism, with an oxide thickness of greater than 4 nm having formed after 60 min. Exposure at 25 °C followed a direct logarithmic mechanism with an oxide growth rate about four to five times slower than at 300 °C. Reaction of a Ni (100) single crystal under comparable conditions showed much slower reaction rates compared to polycrystalline specimens. The higher reaction rate of the polycrystalline materials is attributed to grain boundary transport of Ni cations. Oxide thickness was measured on a microscopic scale for polycrystalline Ni exposed to large doses of O₂ at 25 and 300 °C. The thickness of oxide was not strongly localized on this scale. However, the QUASES^? analysis suggests that there is localized growth on a nanometric scale—the result of island formation. Copyright © 2007 John Wiley & Sons, Ltd.     

Environmental applications of stable xenon and radioxenon monitoring

Characterization of transuranic waste is needed for decisions about waste site remediation. Soil-gas sampling for xenon isotopes can be used to define the locations of spent fuel and transuranic waste. Radioxenon in the subsurface is characteristic of transuranic waste and can be measured with extreme sensitivity using large-volume soil-gas samples. Measurements at the Hanford Site showed ¹³³Xe and ¹³⁵Xe levels indicative of ²⁴⁰Pu spontaneous fission. Stable xenon isotopic ratios from fission are distinct from atmospheric xenon background. Neutron capture by ¹³⁵Xe produces an excess of ¹³⁶Xe in reactor-produced xenon, providing a means of distinguishing spent fuel from separated transuranic material.     

Neutron diffraction studies on a 4-coordinate hydrogen atom in an yttrium cluster

A four-coordinate hydrogen atom has been unambiguously located, by single-crystal neutron diffraction for the first time, in the center of the tetrahedral metal complex Y4H8(Cp')4(THF) [Cp'=C5Me4(SiMe3)]. The core of the molecule consists of a tetranuclear cluster with one interstitial, one face-bridging, and six edge-bridging hydride ligands. The compound was prepared via the reaction of YCp'(CH2SiMe3)2(THF) with gaseous H2. Neutron data were collected on a 4 mm3 crystal at the Quasi-Laue diffractometer VIVALDI at ILL (Grenoble)1a and on an 8 mm3 crystal at the SXD diffractometer at ISIS (Didcot). The final agreement factor is R = 8.9% for 4171 reflections. The existence of 4-coordinate hydrogen now completes the series of high-connectivity hydride ligands located in the interstitial cavities of molecular cluster complexes. We had previously reported the existence of 6-coordinate hydrogen in the octahedral cavity of [HCo6(CO)15]- in 1979, and 5-coordinate hydrogen in the square pyramidal cavities of [H2Rh13(CO)24]3- in 1997, also via single-crystal neutron analyses.     

Uranium and thorium hydride complexes as multielectron reductants: a combined neutron diffraction and quantum chemical study

The unusual uranium reaction system in which uranium(4+) and uranium(3+) hydrides interconvert by formal bimetallic reductive elimination and oxidative addition reactions, [(C(5)Me(5))(2)UH(2)](2) (1) ? [(C(5)Me(5))(2)UH](2) (2) + H(2), was studied by employing multiconfigurational quantum chemical and density functional theory methods. 1 can act as a formal four-electron reductant, releasing H(2) gas as the byproduct of four H(2)/H(-) redox couples. The calculated structures for both reactants and products are in good agreement with the X-ray diffraction data on 2 and 1 and the neutron diffraction data on 1 obtained under H(2) pressure as part of this study. The interconversion of the uranium(4+) and uranium(3+) hydride species was calculated to be near thermoneutral (~-2 kcal/mol). Comparison with the unknown thorium analogue, [(C(5)Me(5))(2)ThH](2), shows that the thorium(4+) to thorium(3+) hydride interconversion reaction is endothermic by 26 kcal/mol.     

Polyatomic ions in inductively coupled plasma–mass spectrometry: Part II: Origins of N2H and HxCO ions using experimental measurements combined with calculated energies and structures

Several polyatomic ions in inductively coupled plasma–mass spectrometry are studied experimentally and by computational methods. Novel calculations based on spin-restricted open shell second order perturbation theory (ZAPT2) and coupled cluster (CCSD(T)) theory are performed to determine the energies, structures and partition functions of the ions. These values are combined with experimental data to evaluate a dissociation constant and gas kinetic temperature (T_gas) value. In our opinion, the resulting T_gas value can sometimes be interpreted to deduce the location where the polyatomic ion of interest is generated. The dissociation of N₂H⁺ to N₂⁺ leads to a calculated T_gas of 4550 to 4900 K, depending on the computational data used. The COH⁺ to CO⁺ system yields a similar temperature, which is not surprising considering the similar energies and structures of COH⁺ and N₂H⁺. The dissociation of H₂CO⁺ to HCO⁺ leads to a much lower T_gas (< 1000 to 2000 K). Finally, the dissociation of H₂COH⁺ to HCOH⁺ generates a T_gas value between those from the other H_xCO⁺ ions studied here. All of these measured T_gas values correspond to formation of extra polyatomic ion in the interface or extraction region. The computations reveal the existence of isomers such as HCO⁺ and COH⁺, and H₂CO⁺ and HCOH⁺, which have virtually the same m/z values and need to be considered in the interpretation of results.     

Toluene dioxygenase-catalyzed cis-dihydroxylation of benzo[b]thiophenes and benzo[b]furans: synthesis of benzo[b]thiophene 2,3-oxide

Enzymatic cis-dihydroxylation of benzo[b]thiophene, benzo[b]furan and several methyl substituted derivatives was found to occur in both the carbocyclic and heterocyclic rings. Relative and absolute configurations and enantiopurities of the resulting dihydrodiols were determined. Hydrogenation of the alkene bond in carbocyclic cis-dihydrodiols and ring-opening epimerization/reduction reactions of heterocyclic cis/trans-dihydrodiols were also studied. The relatively stable heterocyclic dihydrodiols of benzo[b]thiophene and benzo[b]furan showed a strong preference for the trans configuration in aqueous solutions. The 2,3-dihydrodiol metabolite of benzo[b]thiophene was utilized as a precursor in the chemoenzymatic synthesis of the unstable arene oxide, benzo[b]thiophene 2,3-oxide.     

The aging of silicate glass surfaces in humid air

Slicate glass surfaces were observed over several weeks in normally humid laboratory air by atomic force microscopy. Auger spectroscopy and lateral force microscopy measurements were also made. Samples in the study include; vitreous quartz, bulk insulation glass, bulk and fibrous textile e-glass, and bulk container glass. Various features developed on these surfaces are consistent with patterns seen in crystal growth from supersaturated solution. Auger spectroscopy reveals the presence of chemically bound carbon and supports the hypothesis that these crystals are alkali and alkali earth carbonate species. The presence of these crystals is expected to affect the bonding of a coating to silicate surfaces.