Scientific Publications 2005
A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
W
2005. "Reoxidation of Bioreduced Uranium Under Reducing Conditions." Environmental Science and Technology 39(16):6162-6169. doi:10.1021/es048236g Abstract Uranium mining and processing for nuclear weapons and fuel have left thousands of sites with toxic levels of this actinide in soil and ground water1. An emerging strategy for remediating such environments involves using organic carbon to promote microbially-mediated reduction and precipitation of insoluble U(IV) minerals2-4. Although previous U bioreduction studies have shown promising results, they were of short duration (up to a few months). Our longer-term (20 months) laboratory study using historically contaminated sediment has alarmingly shown that microbial reduction of U was transient even under reducing (methanogenic) conditions. Uranium was reductively immobilized during the first 100 days, but later (150 to 600 days) reoxidized and mobilized, although a microbial community capable of reducing U(VI) remained through the end of the experiment. The formation of Ca2UO2(CO3)3 complexes5 (caused by the elevated carbonate concentration from microbial respiration and presence of calcium) drove the U(IV)/U(VI) reduction potential to much more reducing conditions. Fe(III) and Mn(IV) were found to be likely terminal electron acceptors (TEAs) for U reoxidation. Thus, U remediation by organic carbon based reductive precipitation is not sustainable in calcareous, neutral to alkaline soils and ground waters
2005. "Void Formation during Early Stages of Passivation: Initial Oxidation of Iron Nanoparticles at Room Temperature." Journal of Applied Physics 98(9):094308(7). doi:10.1063/1.2130890 Abstract The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some uncertainties. Exposure of most clean metals to air or oxygen results in the nearly instant formation of an oxide layer. Because this initial layer normally forms in a relatively uncontrollable manner, the atomic level understanding of the initial oxidation is limited in comparison to the abundant experimental observation and theoretical derivation on thickening of oxidation layer on metal surface at high temperature. We report in this letter mMicrostructurale characterization of iron nanoparticles oxide passivated with iron oxide shell nanoparticleswere studied using high resolution transmission electron microscopy (HRTEM) and high angle annular dark-filed (HAADF) imaging in aberration corrected scanning transmission electron microcopy (STEM). Voids were readily observed on both small single crystal -Fe nanoparticles formed in a sputtering process and the more complex particles created by reduction of an oxide by hydrogen. Although the formation of hollow spheres of nanoparticle has been engineered for Co at higher temperatures1, they occur for iron at room temperature and provide insight into the initial oxidation processes of iron. The examination of nanoparticles allows study of some processes and mechanisms that are not as easily observed for films or other types of studies where sample preparation artifacts have been cause of some controversy. For example, void formation has been noticed in the single crystal -Fe nanoparticles as a consequence of iron outward diffusion during the initial oxidation at room temperature. There exists a critical size of ~ 8 nm for which the iron has been fully oxidized, leading to a hollow iron oxide nanoparticle. For particles larger than the critical size, an iron/iron oxide core-shell structure was formed and voids reside at the interface between the oxide shell and the iron core. The present observation provides new insight for tailoring of metal/metal-oxide core-shell structured nanoparticles for applications related to optics, magnetism, and nanoelectronics.
2005. "Direct Imaging of Quantum Antidots in MgO Dispersed with Au Nanoclusters." Applied Physics Letters 87(15):153104 (3). Abstract Quantum antidots (a vacancy cluster) have been proposed to account for the non-linear optical behavior of magnesium oxide (MgO) dispersed with Au nanoclusters [1,2]. In this paper, we provide convincing evidence, based on direct observation using high-angle annular dark-filed imaging in aberration corrected scanning transmission electron microscope, that vacancies in excess of Au atoms are clustering together to form antidots at the immediate neighborhood of the Au clusters, leading to a spatially associated Au nanocluter and antidot. The antidots show a terraced layer structure and are typically facetted along the MgO{100} planes. Furthermore, we also directly observed that Au atom substitutes for Mg atom in the MgO lattice, which is consistently supported by image calculations.
2005. "Microstructure of Co-doped TiO₂ (110) Rutile by Ion Implantation." Journal of Applied Physics 97(7):99-104. Abstract Co-doped rutile TiO₂ was synthesized by injecting Co ions into single crystal rutile TiO₂ using high energy ion implantation. Microstructures of the implanted specimens were studied in detail using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), electron diffraction, and HRTEM image simulations. The spatial distribution and conglomeration behavior of the implanted Co ions, as well as the point defect distributions induced by ion implantation, show strong dependences on implantation conditions. Uniform distribution of Co ions in the rutile TiO₂ lattice was obtained by implanting at 1075 K with a Co ion fluence of 1.25x10¹⁶ Co/cm². Implanting at 875 K leads to the formation of Co metal clusters. The precipitated Co metal clusters and surrounding TiO₂ matrix exhibit the orientation relationships Co<110>//TiO₂[001] and Co{111}//TiO₂(110). A structural model representing the interface between Co metal clusters and TiO₂ is developed based on HRTEM imaging and image simulations.
2005. "Atomic Resolution Imaging of Au Nanocluster Dispersed in TiO₂, SrTiO₃, and MgO." Journal of the American Ceramic Society 88(11):3184-3191. Abstract Gold nanoclusters dispersed in single crystal TiO₂, MgO, and SrTiO₃ have been prepared by ion implantation at 300 K - 975 K and subsequent annealing at 1275 K for 10 hours. High resolution transmission electron microscopy and high-angle-annular-dark-field (HAADF) imaging in aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the gold nanoclusters dispersed materials. STEM-HAADF imaging with atomic resolution has directly revealed for all three materials that Au atoms occupy cations lattice positions. Cavities of up to several tens nanometers were observed in the TiO₂ and SrTiO₃. The cavities and gold clusters appear to be spatially associated in SrTiO₃. The nanometer-sized cavities and the Au cluster are faceted along the same lattice plane of the matrix, indicating that the interfacial energy defined by the Au cluster and the matrix follows the same order of the surface energy for different lattice plane.
2005. "Self-assembling of nanocavities in TiO2 dispersed with Au nanoclusters." Physical Review. B, Condensed Matter 72(24):245421, 1-5. doi:10.1103/PhysRevB.72.245421 Abstract There has been considerable research effort on tailoring the non-linear optical properties of dielectric materials by dispersing nanometer-sized metallic clusters in them. It has been proposed that the optical response of this type of material is related to the quantum antidots (a vacancy cluster), which is spatially located at the interface between the metal cluster and the dielectric matrix. In order to clarify the vacancy clustering behavior as well as its correlation with Au clustering, single crystal TiO2 has been implanted with Au ions at 975 K and subsequently annealed at 1275 K for 10 hours. A characteristic self-assembling of nano-cavities along the boundary between the region of Au clusters and the free surface has been observed in the present system. These cavities are faceted along TiO2(110) and have a size of ~10 nm. High angle annular dark-field (HAADF) imaging in an aberration corrected scanning transmission electron microscope (STEM) revealed that vacancy clusters of ~ 2 nm in size also exist in the Au populated regions. Formation of cavities in Au-irradiated TiO2 strongly indicates that vacancy clustering processes prevail over Frenkel-pair recombination. Furthermore, the Au atoms substitution for Ti in TiO2 is also directly observed by STEM-HAADF imaging and by channeling Rutherford backscattering spectrometry (RBS).
2005. "Sticky Ice Grains Aid Planet Formation: Unusual Properties of Cryogenic Water Ice." The Astrophysical Journal 620(2):1027-1032, Pt. 1. Abstract For planets to form in the nebula around a new-born star, they must hurry: Ina few million years or less the star's stellar winds will disperse much of the remaining dust (Briceno, et. al 2001). How do the dust grains stick together fast enough to form gravitationally bound planetsimals? The distinct properties of cryogenic (5-100 K) amorphous water ice, which composes or coats the grains, could be the key. Measurements are presented that show this ice readily acquires persistant macroscopic electric dipoles, strongly enhancing grain-grain adhesion. It is also highly mechanically inelastic (about 10% rebound). Together these explain this efficient sticking.
2005. "Iron Oxide-Gold Core-Shell Nanoparticles and Thin-Film Assembly." Journal of Materials Chemistry 15(18):1821-1832. Abstract This paper reports findings of an investigation of the synthesis of monolayer-capped iron oxide and core (iron oxide)–shell (gold) nanocomposite and their assembly towards thin film materials. Pre-synthesized and size-defined iron oxide nanoparticles were used as seeding materials for the reduction of gold precursors, which was shown to be effective for coating the iron oxide cores with gold shells (Fe oxide@Au). The unique aspect of our synthesis is the formation of Fe oxide@Au core–shell nanoparticles with controllable surface properties. The novelty of our assembly strategy is the exploitation of the ligand-exchange reactivity at the gold shells for the thin film assembly of the core–shell nanoparticles. The core–shell nanocomposites and assemblies have been characterized using TEM, XRD, XPS, FTIR, TGA, and DCP-AES techniques. In addition to evidence from TEM detection of the change in particle size, UV-Vis observation of the change in the surface plasmon resonance band, and XRD detection of disappearance of the magnetite diffraction peaks after coating the gold shell, the formation of the core–shell morphology was further confirmed by DCP-AES composition analysis of Au and Fe in the molecularly-mediated thin film assembly of Fe oxide@Au particles. The interparticle ligand exchange–precipitation chemistry at the gold shell is to our knowledge the first example demonstrating the inter-shell reactivity for constructing thin films of Fe oxide@Au particles. The results have provided important insights into the design of interfacial reactivities via core–shell nanocomposites for magnetic, catalytic and biosensing applications.
2005. "Adsorption and Reaction of Methanol on Stoichiometric and Defective SrTiO3(100) Surfaces ." Journal of Physical Chemistry B 109(10):4507-4513. doi:10.1021/jp048338t Abstract The adsorption and reaction of methanol (CH3OH) on stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces have been investigated using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and first-principles density-functional calculations. Methanol adsorbs mostly nondissociatively on the stoichiometric SrTiO3(100) surface that contains predominately Ti4+ cations. Molecular desorption of methanol from the stoichiometric surface is observed at ~250 K whereas the multilayer is found to desorb at ~160 K. Theoretical calculations predict weak adsorption of methanol on TiO2–terminated SrTiO3(100) surfaces, in agreement with the experimental results. However, the reduced SrTiO3(100) surface containing substantial amounts of Ti3+ cations exhibits higher reactivity towards adsorbed methanol and H2, C2H4, and CO are the major decomposition products observed on the reduced surface. The surface defects on the reduced SrTiO3(100) surface are partially reoxidized upon saturation exposure of CH3OH onto this surface at 300 K.
2005. "Nuclear Magnetic Resonance Studies of Aluminosilicate Gels Prepared in High-Alkaline and Salt-Concentrated Solutions." Journal of Non-crystalline Solids 351(43-45):3435-3442. Abstract Solid-state 29Si, 27Al, and 23Na MAS (magic angle spinning) NMR techniques in combination with x-ray powder diffraction (XRD) are used to characterize aluminosilicate gels as a function of composition, pH, and reaction times. These gels were prepared at 80 oC using initial solutions with low Si/Al ratios, high alkaline and salt concentrations that are characteristic of nuclear tank wastes. XRD data show that cancrinite and sodalite are the main crystalline phases in the aluminosilicate gels produced. It is found that the pH and the salt content have significant effects on the nature of the aluminosilicate gels. Higher pH appears to increase the rate of crystallization, the degree of overall crystallinity and the percentage of cancrinite phases in aluminosilicate gels, whereas the high salt concentration promotes the formation of cancrinite and sodalite and prohibits the formation of other zeolites. Complementary to XRD, NMR is extremely useful for providing the information on the structure of amorphous intermediate gels with no long-range order.
2005. "Monodispersed core-shell Fe3O4@Au nanoparticles ." Journal of Physical Chemistry B 109(46):21593-21601. Abstract The ability to synthesize and assemble monodispersed core-shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe3O4)-shell (An) nanoparticles with high monodispersity. Fe3O4 nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe3O4 nanoparticles (Fe3O4@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core-shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core-shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core-shell nanoparticles. The SQUID data reveal a decrease in magnetization and blocking temperature and an increase in coercivity for Fe3O4@Au, reflecting the decreased coupling of the magnetic moments as a result of the increased interparticle spacing by both gold and capping shells. Implications of the findings to the design of interfacial reactivities via core-shell nanocomposites for magnetic, catalytic, and biological applications are also briefly discussed.
2005. "The Ground and Two Lowest-lying Singlet Excited Electronic Statesof Copper Hydroxide (CuOH)." Journal of Chemical Physics 123(1):014313 1-13. doi:10.1063/1.1944726 Abstract The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1∑+ and 1∏ stationary points, and for the bent ground-state X˜ 1A', and excited-states 2 1A" and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas–Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X˜ 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol−1 for the 2 1A' state and 52.6 kcal mol−1 for the 1 1A" state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1 1A" state (53.1 kcal mol−1) than those for the corresponding 2 1A' state (57.6 kcal mol−1), suggesting that the 1 1A" state might be the first singlet excited state while the 2 1A' state might be the second singlet excited state.
2005. "Intramolecular Rotation through Proton Transfer: [Fe(eta(5)-C5H4CO2-)(2)] Versus [(eta(5)-C5H4CO2-)Fe(eta(5)-C5H4CO2H)]." Angewandte Chemie International Edition 44(37):6022-6024. Abstract We report a photoelectron spectroscopic study of doubly charged (η5C5H4CO2-)Fe(η5-C5H4CO2-) (1) and singly charged (η5C5H4CO2-)Fe(η5C5H4CO2H) (2). It is shown that strong intramolecular coulomb repulsion keeps 1 in the trans-form, in which the two –CO2- groups on the cyclopentadieny ligands are oriented opposite to each other, whereas 2 assumes the cis-form owing to a strong intramolecular H-bond. We estimate a rotational barrier of 1.4 eV for 1 and 0.6 eV for 2. A proton transfer to 1 would result in a 112° intramolecular rotation, whereas deportation of 2 would result in a similar intramolecular rotation. Thus 1 and 2 form a model molecular rotor system, controlled by a proton transfer.
2005. "Vibrational Cooling in A Cold Ion Trap: Vibrationally Resolved Photoelectron Spectroscopy of Cold C60- Anions." Journal of Chemical Physics 123(5):051106-1-4. Abstract We demonstrate vibrational cooling of anions via collisions with a background gas in an ion trap attached to a cryogenically controlled cold head (10 – 400 K). Photoelectron spectra of vibrationally cold C60- anions, produced by electrospray ionization and cooled in the cold ion trap, have been obtained. Relative to spectra taken at room temperature, vibrational hot bands are completely eliminated, yielding well resolved vibrational structures and a more accurate electron affinity for neutral C60. The electron affinity of C60 is measured to be 2.683 ± 0.008 eV. The cold spectra reveal complicated vibrational structures for the transition to the C60 ground state due to the Jahn-Teller effect in the ground state of C60-. Vibrational excitations in the two Ag modes and eight Hg modes are observed, providing ideal data to assess the vibronic couplings in C60-.
2005. "Photoelectron Spectroscopy and Electronic Structures of Fullerene Oxides: C60Ox- (x=1-3)." Journal of Physical Chemistry A 109(49):11089-11092. Abstract We report a photoelectron spectroscopy (PES) study on a series of fullerene oxides, C600x- (x = 1-3). The PES spectra reveal one isomer for C600x-, two isomers for C6002-, and multiple isomers for C6003- . Compared to C60, the electronic structures of C600x are only slightly perturbed, resulting in similar anion photoelectron spectra. The electron affinity of C600x was observed to increase only marginally with the number of oxygen atoms, x, from 2.683 eV for C60, to 2.745 eV for C600, and 2.785 eV/2.820 eV for C6002 (two isomers). We also carried out theoretical calculations, which confirmed the observed isomers and showed that all the fullerene oxides are in the form of epoxide. The PES and theoretical calculations, as well as molecular orbital analysis, indicate that addition of oxygen atoms to the C60 cage only modifies the local carbon network and leave the rest of the fullerene cage largely intact geometrically and electronically.
2005. "Observation of Weak C-H...O Hydrogen Bonding by Unactivated Alkanes." Angewandte Chemie International Edition 44(31):4968-4972. doi:10.1002/anie.200501349 Abstract Weak C-H…O hydrogen bonding has been recognized to play a major role in biological molecular structures and functions. A newly developed low-temperature photoelectron spectroscopy apparatus is used here to study the C-H…O hydrogen bonding between unactivated alkanes and the carboxylate functional group. We observed that gaseous linear carboxylates, CH3(CH2)nCO2-, assume folded structures at low temperatures due to weak C-H…O hydrogen bonding between the terminal CH3 and CO2- groups for n≥5. Temperature-dependent studies showed that the folding transition depends on both the temperature and the aliphatic chain length. Theoretical calculations revealed that for n = 3-8, the folded conformations are more stable than the linear structures, but C-H…O hydrogen bonding only forms for species with n≥5 due to steric constraint in the smaller species. One C-H…O hydrogen-bond is formed in the n = 5 and 6 species, whereas two C-H…O hydrogen-bonds are formed for n = 7 and 8. Comparison of the photoelectron spectral shifts for the folded relative to the linear conformations yielded lower limits for the strength of the C-H…O hydrogen-bonds in CH3(CH2)nCO2-, ranging from 1.2 kcal/mol for n = 5 to 4.4 kcal/mol for n = 8.
2005. "Oxidation of Polymer-derived SiAICN Ceramics." Journal of the American Ceramic Society 88(11):3075-3080. doi:10.1111/j.1551-2916.2005.00542.x Abstract The oxidation behavior of polymer-derived amorphous SiAlCNs was studied in the temperature range of 900°–1200°C. The results revealed that while at 900°C the oxidation of the SiAlCNs follows typical parabolic kinetics, at higher temperatures the oxidation rates of the materials decrease with annealing time. Long-term oxidation rate of the SiAlCNs is much lower than the lowest values reported for chemical vapor deposition of SiC and Si3N4. Structures of the oxide scales were studied using solid-state nuclear magnetic resonance. We proposed that oxide scales formed for the SiAlCNs possess a unique network structure of the oxide scale in which aluminum atoms block the path of oxygen diffusion, thus lowering the oxidation rates. Such a unique structure was likely formed gradually with annealing time, leading to a continuous decrease in oxidation rate.
2005. "Complexation of Cm(III)/Eu(III) with Silicate in Basic Solutions." Radiochimica Acta 93(12):741-748. Abstract The complexation of Cm(III) and Eu(III) with dissolved silica was studied by time resolved laser fluorescence spectroscopy (TRLFS) in basic solutions over a range of total silica concentrations and ionic strengths (NaNO3). In highly basic solutions, both the fluorescence spectra and lifetime data indicate the formation of Eu(III)/Cm(III) complexes with oligomeric silicates as well as hydroxide groups and/or nitrate in the presence of concentrated NaNO3. At high silica concentration the inner-sphere complexation caused the shift of the fluorescence spectral maximum for Cm(III)(aq) from 594 nm to up to 607 nm and a significant increase of the hypersensitive 5D0 → 7F2 band around 615 nm relative to the non-hypersensitive 5D0 → 7F1 band at 592 nm for Eu(III). At the same time, the fluorescence lifetime increased from 68 s to up to 202 s for Cm(III) in 0.1 M NaNO3 and from 115 s to 1.8 ms for Eu(III) in 3.0 M and 5.0 M NaNO3, consistent with the removal of 6 or more water molecules upon silicate complexation. Linear correlations between the spectral intensity of Cm(III) complexes and the concentrations of the dissolved silicates suggest that Cm(III) complexation with the silicate dimer, Si2O2(OH)22-, may play a role.
2005. "Fluorescence Spectroscopy of U(VI)-Silicates and U(VI)-Contaminated Hanford Sediment." Geochimica et Cosmochimica Acta 69(6):1391-1403. doi:10.1016/j.gca.2004.08.028 Abstract Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ~ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O=U=O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm-1 for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60 % of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although, an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.
2005. "Electronic Structure of the Hydroxo and Methoxo Oxometalate Anions MO3(OH)- and MO3(OCH3)- (M=Cr, Mo, and W)." Journal of Physical Chemistry A 109(51):11771-11780. Abstract The electronic structure of the mononuclear hydroxo MO3(OH)- and methoxo MO3(OCH3)- Group 6 oxometalate anions (M ) Cr, Mo, and W) were examined by photodetachment photoelectron spectroscopy and electronic structure calculations at the density functional and CCSD(T) levels of theory. All of the anions exhibited high electron binding energies (>4.9 eV), with the lowest-energy detachment features arising from oxygen 2p-based orbitals. The combined experimental and theoretical results allowed the change in molecular orbital energy levels to be investigated as a function of metal (Cr, Mo, or W) and ligand (-OH, -OCH3). A number of fundamental thermodynamic properties of the anions and corresponding neutrals were predicted on the basis of the theoretical calculations. The calculations indicate high O-H bond dissociation energies for MO2(OR)(O-H) (R ) H, CH3) and MO3(O-H), consistent with their high Brønsted acidities (just below that of H2SO4 in the gas phase) and the high ionization energies of their conjugate base anions. This suggests that the corresponding radicals should readily abstract H atoms from organic molecules.
2005. "A Study of Spectral Integration and Normalization in NMR-based Metabonomic Analyses ." Journal of Pharmaceutical and Biomedical Analysis 39(3-4):830-836. Abstract Metabonomics involves the quantitation of the dynamic multivariate metabolic response of an organism to a pathological event or genetic modification (Nicholson, Lindon and Holmes, 1999). The analysis of these data involves the use of appropriate multivariate statistical methods. Exploratory Data Analysis (EDA) linear projection methods, primarily Principal Component Analysis (PCA), have been documented as a valuable pattern recognition technique for 1H NMR spectral data (Brindle et al., 2002, Potts et al., 2001, Robertson et al., 2000, Robosky et al., 2002). Prior to PCA the raw data is typically processed through four steps; (1) baseline correction, (2) endogenous peak removal, (3) integration over spectral regions to reduce the number of variables, and (4) normalization. The effect of the size of spectral integration regions and normalization has not been well studied. We assess the variability structure and classification accuracy on two distinctly different datasets via PCA and a leave-one-out cross-validation approach under two normalization approaches and an array of spectral integration regions. This study indicates that independent of the normalization method the classification accuracy achieved from metabonomic studies is not highly sensitive to the size of the spectral integration region. Additionally, both datasets scaled to mean zero and unity variance (auto-scaled) has higher variability within classification accuracy over spectral integration window widths than data scaled to the total intensity of the spectrum.
2005. "Defects and Ion-Solid Interactions in Silicon Carbide." Materials Science Forum 475-479(1-5):1345-1350. Abstract Atomic-level simulations are used to determine defect production, cascade-overlap effects, and defect migration energies in SiC. Energetic C and Si collision cascades primarily produce single interstitials, mono-vacancies, antisite defects, and small defect clusters, while amorphous clusters are produced within 25% of Au cascades. Cascade overlap results in defect stimulated cluster growth that produces amorphization. The good agreement of disordering behavior and changes in volume and elastic modulus obtained computationally and experimentally provides atomic-level interpretation of experimentally observed features. Simulations indicate that close-pair recombination activation energies range from 0.24 to 0.38 eV, and long-range migration energies for interstitials and vacancies have been determined.
2005. "Bond-valence Sums for Tc—O Systems from EXAFS Data." Inorganica Chimica Acta 358(4):865-874. Abstract Literature data for structures containing exclusively Tc—O bonds were used to calculate unit-valence parameters R0 for Tc(VII), Tc(VI), Tc(V) (six- and five-coordinate), Tc(IV), and Tc(III). A second method of estimating R0 was developed to validate the calculated values for these oxidation states because crystallographic data are limited. The method was first tested and shown to be valid using literature data for Cr, Mn, Fe, and Co complexes. The validated R0 values for Tc were used to calculate bond-valence sums (BVS) for Tc solids and aqueous solutions using EXAFS data for the bond distances and coordination numbers. The calculated BVS showed good agreement with the expected values for the assumed Tc oxidation states.
2005. "Trimethyl Acetate on TiO₂(110): Preparation and Anaerobic Photolysis." Journal of Physical Chemistry B 109(25):12417-12430. Abstract The preparation and anaerobic ultraviolet photolysis of trimethyl acetate (TMA) on rutile TiO₂(110) have been examined with an emphasis on reaction paths. Substrates for photolysis were prepared by dosing trimethyl acetic acid at 100, 300 and 550 K. The chemistry was characterized by mass spectrometry during dosing, and by H₂O adsorption and temperature programmed desorption after dosing. Using temperature programmed desorption after photolysis and mass spectrometry during photolysis, the products ejected and retained during photolysis were sought. The photolysis results are interpreted using the following mechanistic model. Photons with energies exceeding 3 eV create electronhole pairs in the substrate. With probabilities of 10-₅ or lower, the holes initiate TMA chemistry by extracting an electron from the π orbital of the carboxylate moiety. The accompanying electrons are trapped at the surface and inhibit this chemistry. The electron deficient intermediate, TMA*, decarboxylates to form CO₂ and either chemisorbed t-butyl, (-C(CH₃)₃), or physisorbed i-butene. For photolysis at 100 or 200 K, the (-C(CH₃)₃) accumulates and there is a slow photon-driven secondary reaction that, with a source of H, hydrogenates adsorbed t-butyl to physisorbed i-butane. For photolysis at 300 K, (-C(CH₃)₃) thermally reacts to form and desorb i-butene and ibutane during photolysis.
2005. "An Investigation of Lanthanum Coordination Compounds by Using Solid-State 139La NMR Spectroscopy and Relativistic Density Functional Theory." Chemistry - a European Journal 12(1):159-168. Abstract Lanthanum-139 NMR spectra of stationary samples of several solid LaIII coordination compounds have been obtained at applied magnetic fields of 11.75 and 17.60 T. The breadth and shape of the 139La NMR spectra of the central transition are dominated by the interaction between the 139La nuclear quadrupole moment and the electric field gradient (EFG) at that nucleus; however, the influence of chemical-shift anisotropy on the NMR spectra is non-negligible for the majority of the compounds investigated. Analysis of the experimental NMR spectra reveals that the 139La quadrupolar coupling constants (CQ) range from 10.0 to 35.6 MHz, the spans of the chemical-shift tensor (W) range from 50 to 260 ppm, and the isotropic chemical shifts (diso) range from -80 to 178 ppm. In general, there is a correlation between the magnitudes of CQ and W, and diso is shown to depend on the La coordination number. Magneticshielding tensors, calculated by using relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) and incorporating scalar only or scalar plus spin–orbit relativistic effects, qualitatively reproduce the experimental chemical-shift tensors. In general, the inclusion of spin–orbit coupling yields results that are in better agreement with those from the experiment. The magnetic-shielding calculations and experimentally determined Euler angles can be used to predict the orientation of the chemical-shift and EFG tensors in the molecular frame. This study demonstrates that solidstate 139La NMR spectroscopy is a useful characterization method and can provide insight into the molecular structure of lanthanum coordination compounds.
2005. "Heteroepitaxial Growth and Structural Analysis of Epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001)." Journal of Materials Research 20(5):1250-1256. Abstract We have grown epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001) rutile by oxygen plasma assisted molecular beam epitaxy. High resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction (RHEED) and x-ray diffraction pole figures confirm that the film is composed of four different in-plane orientations rotated by 90º relative to one another. For a given Fe₂O₃ unit cell, the lattice mismatch along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions is nominally +67%. However, due to a three-fold repetition of the slightly distorted square symmetry of anion positions within the Fe₂O₃ unit cell, there is a coincidental anion alignment along the [0001]Fe₂O₃ and [100]TiO₂ directions which results in an effective lattice mismatch of only -0.02% along this direction. The lattice mismatch is nearly 10% in the orthogonal [1120]Fe₂O₃ and [100]TiO₂ directions. The film is highly ordered and well registered to the substrate despite a large lattice mismatch in one direction. The film grows in registry with the substrate along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions and nucleates dislocations along the orthogonal [1120]Fe₂O₃ [100]TiO₂ directions.
2005. "Geophysical Imaging of Stimulated Microbial Biomineralization." Environmental Science and Technology 39(19):7592-7600. doi:10.1021/es0504035 Abstract Understanding how microorganisms influence the physical and chemical properties of the subsurface is hindered by our inability to observe microbial dynamics in real time and with high spatial resolution. Here, we investigate the use of noninvasive geophysical methods to monitor biomineralization at the laboratory scale during stimulated sulfate reduction under dynamic flow conditions. Alterations in sediment characteristics resulting from microbe-mediated sulfide mineral precipitation were concomitant with changes in complex resistivity and acoustic wave propagation signatures. The sequestration of zinc and iron in insoluble sulfides led to alterations in the ability of the pore fluid to conduct electrical charge and of the saturated sediments to dissipate acoustic energy. These changes resulted directly from the nucleation, growth, and development of nanoparticulate precipitates along grain surfaces and within the pore space. Scanning and transmission electron microscopy (SEM and TEM) confirmed the sulfides to be associated with cell surfaces, with precipitates ranging from aggregates of individual 3-5 nm nanocrystals to larger assemblages of up to 10-20 ím in diameter. Anomalies in the geophysical data reflected the distribution of mineral precipitates and biomass over space and time, with temporal variations in the signals corresponding to changes in the aggregation state of the nanocrystalline sulfides. These results suggest the potential for using geophysical techniques to image certain subsurface biogeochemical processes, such as those accompanying the bioremediation of metalcontaminated aquifers.
2005. "Mechanism of Hierarchical Porosity Formation in Silica Thin Films using Cellulose Nitrate." Microporous and Mesoporous Materials 84(1-3):201-210. Abstract Ordered mesoporous silica thin films have potential applications as, e.g., sorbents, catalysts, and analytical instruments. In addition to high internal surface areas, these applications also require high permeability, which can be obtained by engineering hierarchical porosity into the film. A novel approach uses cellulose nitrate to generate a system of larger pores (15-30 nm dia.) connected by the smaller mesopores (2-10 nm dia.) generated using traditional organic surfactants. The unimolecular reaction for cellulose nitrate deflagration avoids the usual diffusion limitations of traditional combustion synthesis. A family of hierarchically porous films was produced using a range of surfactant mixtures, and exhibited a range of surface areas along with relatively impermeable ‘skins’ on the film free boundaries. These skins were formed by an extrusion mechanism due to pore expansion coupled with the lateral (in-plane) symmetry constraints unique to thin films. The skin thickness depends primarily on the ratio of the large pore spacing (l0) over its diameter (d). The specific surface area (SA, m2/g) of the film increases monotonically as l0/d decreases, reaching a maximum at about l0/d~0.67. Precipitous reductions in SA for smaller values of l0/d are caused by pore intersections and the associated excluded surface areas. At high pore intersection (low l0/d), the film structure evolves from ‘a solid with pores’ to ‘pores surrounded by a cage-like silica matrix.’ The cage-like film structure is less susceptable to thin film cracking behavior, but is probably in a more metastable state susceptable to thermal or hydrothermal exposure. This metastability might be mitigated by controlling the structure of the skin through several avenues, including processing conditions (e.g., drying rates or thermal schedules) and the sol-gel chemistry.
2005. "Pore Shape Evolution in Mesoporous Silica Thin Films: from Circular to Elliptical to Rectangular ." Journal of Non-crystalline Solids 351(27-29):2217-2223. Abstract The synthetically tailored morphologies of mesoporous silica thin films are important for applications to sensing, separations, and catalysis. Sol-gel methods using surfactant templates can produce a variety of mesoporous phases, including one composed of long cylindrical micelles aligned parallel to the substrate. This hexagonal phase is particularly useful for analysis of the thin film stresses that produce the circular-elliptical-rectangular micelle shape evolution as the film dries. A simple linear elastic model employing an effective medium concept is proposed to describe this shape evolution using a minimal set of parameters: mesopore aspect ratios are predicted from the film’s known Poisson ratio and its measured thickness strain. The model may be of general utility for guiding the structural design of thin film mesoporous materials. Additional analysis reveals that the material fails when the internal stress system departs significantly from the plane stress condition. This occurs for severe gel shrinkage when the micelle shape exceeds its maximum aspect ratio, defined by conservation of volume and the geometric constraints of adjacent micelles. Localized stresses associated with micelle pressurization cause failure of the small silica ligaments between micelle ends, thus limiting the range of mesopore aspect ratios attainable for the structural tailoring of these materials.
2005. "Mechanical Stability of Templated Mesoporous Silica Thin Films." Microporous and Mesoporous Materials 85(3):260-266. Abstract Mesoporous silica thin films about 1 micron thick were prepared by spin casting using several organic templates that provide a range of pore structures from disordered (sponge-like) to more ordered (honeycomb-like) 2D hexagonal arrays. Nanoindentation measurements indicate that the elastic modulus (E), and thus the density, of the pore wall material are substantially lower than for fused silica. The corresponding lower dielectric constant for pore walls was used to calculate film porosities (P) with recent correlations from the literature. Curve fits to the standard modulus vs. porosity correlation, E/Eporewall=(1-P)n, for the films gave lower n=2.2 for the honeycomb-like film with higher E, but higher n=2.5 for the sponge-like film with lower E, in contrast to theoretical expectations (n=2 for sponge-like and n=3 for honeycomb-like). Although the dielectric constant depends primarily on first-order structural information (P), the elastic modulus of these structurally imperfect films required second-order (pore wall thickness/diameter ratio) and third-order (pore connectivity) parameters to resolve the data. The power law exponent n can vary continuously, depending on the details of the mesostructure, and should not be assumed a' priori unless justified by detailed structural information
2005. "Slow-MAS NMR: A New Technology for In Vivo Metabolomic Studies." Drug Discovery Today. Technologies 2(3):291-294. doi:10.1016/j.ddtec.2005.08.009 Abstract Improvements in the ability to obtain detailed in vivo metabolic information have been identified as key elements of better understanding the efficacy and toxicity of new therapies. A new NMR technology called LOCMAT is discussed that yields substantially increased spectral resolution of spatially localized in vivo 1H NMR metabolite spectra, as illustrated by measurements in the liver and heart of a live mouse. Thus, LOCMAT promises to significantly enhance the utility of NMR spectroscopy for biomedical research.
2005. "Temperature-Dependent Photoelectron Spectroscopy of Methyl-Benzoate Anions: Observation of Steric Effect in Ortho-Methyl-Benzoate." Journal of Physical Chemistry A 109(50):11395-11400. Abstract Temperature-dependent photoelectron spectra of benzoate anion (C6H5CO2-) and its three methyl-substituted isomers (o-, m-, p-CH3C6H4CO2-) have been obtained using a newly developed low-temperature photoelectron spectroscopy apparatus that features an electrospray source and a cryogenically controlled ion trap. Detachment channels due to removing electrons from the carboxylate group and benzene ring electrons were distinctly observed. Well-resolved vibrational structures were obtained in the lower binding energy region due to the OCO bending modes, except for o-CH3C6H4CO2-, which yielded broad spectra even at the lowest ion trap temperature (18 K). Theoretical calculations revealed a large geometry change in the OCO angles between the anion and neutral ground states, consistent with the broad ground state bands observed for all species. A strong steric effect was observed between the carboxylate and the methyl group in o-CH3C6H4CO2-, such that the -CO2- group is pushed out of the plane of the benzene ring by ~25 degrees and its internal rotational barrier is significantly reduced. The low rotational barrier in o-CH3C6H4CO2-, which makes it very difficult to be cooled vibrationally, and the strong coupling between the OCO bending and CO2 torsional modes yielded the broad PES spectra for this isomer. It is shown that there is no C-H…O hydrogen bond in o-CH3C6H4CO2- and the interaction between the carboxylate and methyl groups in this anion is found to be repulsive in nature.
2005. "Probing the Low-Barrier Hydrogen Bond in Hydrogen Maleate in the Gas Phase: A Photoelectron Spectroscopy and ab Initio Study." Journal of Physical Chemistry A 109(47):10633-10637. Abstract The strength of the low-barrier hydrogen bond in hydrogen maleate in the gas phase was investigated by low-temperature photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of maleic and fumaric acid monoanions (cis-/trans-HO2CCHdCHCO2 -) were obtained at low temperatures and at 193 nm photon energy. Vibrational structure was observed for trans-HO2CCHdCHCO2 - due to the OCO bending modes; however, cis-HO2CCHdCHCO2 - yielded a broad and featureless spectrum. The electron binding energy of cis-HO2CCHdCHCO2 - is about 1 eV blue-shifted relative to trans-HO2CCHdCHCO2 - due to the formation of intramolecular hydrogen bond in the cis-isomer. Theoretical calculations (CCSD(T)/ aug-cc-pVTZ and B3LYP/aug-cc-pVTZ) were carried out to estimate the strength of the intramolecular hydrogen bond in cis-HO2CCHdCHCO2 -. Combining experimental and theoretical calculations yields an estimate of 21.5 ( 2.0 kcal/mol for the intramolecular hydrogen bond strength in hydrogen maleate.
2005. "Increased Protein Identification Capabilities Through Novel Tandem MS Calibration Strategies ." Journal of Proteome Research 4(4):1434-1441. Abstract High mass measurement accuracy is critical for confident protein identification and characterization in proteomics research. Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is a unique technique which can provide unparalleled mass accuracy and resolving power. However, the mass measurement accuracy of FTICR-MS can be affected by space charge effects. Here we present a novel internal calibrant-free calibration method that corrects for space charge-induced frequency shifts in FTICR fragment spectra called Calibration Optimization on Fragment Ions (COFI). This new strategy utilizes the information from fixed mass differences between two neighboring peptide fragment ions (such as y1 and y2) to correct the frequency shift after data ollection. COFI has been successfully applied to LC-FTICR fragmentation data. Mascot MS/MS ion search data demonstrate that most of the fragments from BSA tryptic digested peptides can be identified using a much lower mass tolerance window after applying COFI to LC-FTICR-MS/MS of BSA tryptic digest. Furthermore, COFI has been used for multiplexed LC-CID-FTICR-MS which is an attractive technique because of its increased duty cycle and dynamic range. After the application of COFI to a multiplexed LC-CID-FTICR-MS of BSA tryptic digest, we achieved an average measured mass accuracy of 2.49 ppm for all the identified BSA fragments.
2005. "Effects of Varied pH, Growth Rate and Temperature using Controlled fermentation and Batch culture on Matrix Assisted Laser Desorption/Ionization Whole Cell Protein Fingerprints. ." Journal of Microbiological Methods 62(3):259-271. Abstract Abstract Rapid identification of microorganisms using matrix assisted laser desorption/ionization (MALDI) is a rapidly growing area of research due to the minimal sample preparation, speed of analysis and broad applicability of the technique. This approach relies on protein markers to identify microorganisms. Therefore, variations in culture conditions that affect protein expression may limit the ability of MALDI-MS to correctly identify an organism. We have expanded our efforts to investigate the effects of culture conditions on MALDI-MS protein signatures to examine the effects of pH, growth rate and temperature. Continuous cultures maintained in bioreactors were used to maintain specific growth rates and pH for E. coli HB 101. Despite measurable morphological differences between growth conditions, the MALDI-MS data associated each culture with the appropriate library entry (E. coli HB 101 generated using batch culture on a LB media), independent of pH or growth rate. The lone exception was for a biofilm sample collected from one of the reactors which had no appreciable degree of association with the correct library entry. Within the data set for planktonic organisms, variations in growth rate created the largest variation between fingerprints. The effect of varying growth temperature on Y. enterocolitica was also examined. While the anticipated effects on phenotype were observed, the MALDI-MS technique provided the proper identification.
