Scientific Publications 2007
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2007. "Reduction of Pertechnetate [Tc(VII)] by Aqueous Fe(II) and the Nature of Solid Phase Redox Products." Geochimica et Cosmochimica Acta 71(9):2137-2157. doi:doi:10.1016/j.gca.2006.10.025 Abstract The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is strongly dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 10-9atm O2) in the absence of atmospheric or aqueous carbonate. The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Tc(IV)/Fe form was observed when 11 µmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. The heterogeneous reaction was more rapid, but not quantified. The kinetics of homogeneous reduction were slow at pH 7, but increased dramatically at pH 8, and correlated with the concentration of Fe(OH)+ and Fe(OH)2o(aq). Wet chemical and Fe-x-ray absorption near edge spectroscopy measurements (XANES) indicated that both Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox reaction products. 57Fe- Mössbauer, extended x-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements on the solid phase redox products that contained 13-15% Tc indicated that they were poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with a Tc2O10 octahedral dimer bound in bidentate edge-sharing mode to an octahedral FeO6 surface or vacancy site in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2•nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2•nH2O(s) as a result of its intraparticle/intragrain residence. It is suggested that precipitates of this nature may form in anoxic sediments or groundwaters, and that the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.
2007. "On the Effect of Particle Alignment in the DMA." Aerosol Science and Technology 41(2):112-124. Abstract The Differential Mobility Analyzer (DMA) is designed to Measure particle mobility diameter, which for spherical particles is equal to particle volume equivalent diameter. In contrast, the mobility diameter of aspherical particles is a function of the particle shape and orientation. The magnitude of the DMA electric fields is such that it can cause aspherical particles to align preferentially in a specific orientation. The same electric field and the sheath flow rate (qsh) define the particle mobility diameter. But, the fact that particle orientation depends on the electric field makes the dynamic shape factor and hence the mobility diameter depend on qsh. Here, we describe an operating procedure that relies on a tandem DMA system, in which the second DMA is operated at a number of qsh, to obtain information about particle shape by measuring the effect of particle alignment on the particle mobility diameter. We show how the relationship between the mobility diameter and qsh can even be used to physically separate particles according to their shapes. In addition, we explore the use of simultaneous measurements of particle alignment and particle vacuum aerodynamic diameters to gain further information on particle shape and account for particle alignment in the calculations of dynamic shape factor. We first test this approach on doublets and compact triplets of PSL spheres, for which the orientation dependent dynamic shape factors are known. We then investigate applications on a number of polydisperse particle systems of various shapes.
2007. "Measurements and Interpretation of the Effect of a Soluble Organic Surfactant on the Density, Shape and Water Uptake of Hygroscopic Particles." Journal of Aerosol Science 38(9):903-923. Abstract A large fraction of atmospheric particles are composed of hygroscopic salts that are mixed with variety of organic molecules, of which surfactants represent an interesting and potentially important class. Because of the tendency of surfactant molecules to coat the particles' surface, a monolayer has the potential to completely alter the particles' interactions with the rest of the atmosphere. Given the important role that is played by the interaction of particles with the ambient relative humidity it is critical to develop an understanding of the impact surfactants may exert on particle hygroscopic properties. We present an experimental study of the relationship between the concentrations of a soluble surfactant that is internally mixed with two different hygroscopic salts and particle density, shape and water uptake. We show that the observed particle density provides evidence that the density of the surfactant fraction changes with concentration and that once this is properly taken into account the water uptake data can quantitatively be understood.
2007. "Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy." Journal of the American Chemical Society 129(10):3022-3026. doi:10.1021/ja068601z Abstract TiO2 is a wide-band gap semiconductor and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n– clusters for n = 1–10 have been obtained at 193 (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n– clusters for n > 1 appears to be localized in a tricoodinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.
2007. "Delta-Aromaticity in Ta303-." Angewandte Chemie International Edition 46(31):4277-4280. doi:10.1002/anie.200700442 Abstract We report experimental and theoretical evidence of -aromaticity, which is discovered in the Ta3O3 – cluster via a combined photoelectron spectroscopy and ab initio study. Well-resolved low-lying electronic transitions are observed in the photoelectron spectra of Ta3O3 – and are compared with ab initio calculations, which show that the Ta3O3 – cluster possesses a planar D3h triangular structure. Chemical bonding analyses reveal that among the five valence molecular orbitals responsible for the multi-center metal-metal bonding there is a completely bonding delta and orbital from the 5d atomic orbitals of Ta. The totally delocalized multi-center bond renders -aromaticity for Ta3O3 – and represents a new mode of chemical bonding. Ta3O3 – is the first -aromatic molecule confirmed experimentally and theoretically, suggesting that -aromaticity may exist in many multi-nuclear, low oxidation state transition-metal compounds.
2007. "Vibrationally Resolved Photoelectron Spectroscopy of BO- and BO2-: A Joint Experimental and Theoretical Study." Journal of Physical Chemistry A 111(6):1030-1035. doi:10.1021/jp0666939 Abstract We report a photoelectron spectroscopy and computational study of two simple boron oxide species: BO– and BO2–. Vibrationally-resolved photoelectron spectra are obtained at several photon energies (355, 266, 193, and 157 nm) for the 10B isotopomers, 10BO– and 10BO2–. In the spectra of 10BO–, we observe transitions to the 2 sigma + ground state and the 2 pi excited state of 10BO at an excitation energy of 2.96 eV. The electron affinity of 10BO is measured to be 2.510 plus/minus 0.015 eV. The vibrational frequencies of the ground states of 10BO– and 10BO, and the 2 pi excited state are measured to be 1,725 plus/minus 40, 1,935 30, and 1,320 plus/minus 40 cm-1, respectively. For 10BO2–, we observe transitions to the 2 pi g ground state and two excited states of 10BO2, 2 pi u and 2 signa u+, at excitation energies of 2.26 and 3.04 eV, respectively. The electron affinity of 10BO2 is measured to be 4.46 plus/minus 0.03 eV and the symmetrical stretching vibrational frequency of the 2 pi u excited state of 10BO2 is measured to be 980 plus/minus 30 cm-1. Both density functional and ab initio calculations are performed to elucidate the electronic structure and chemical bonding of the two boron oxide molecules. Comparisons with the isoelectronic AlO– and AlO2– species and the closely related molecules CO, N2, CN–, and CO2 are also discussed.
2007. "Probing the Electronic Structure of Early Transition Metal Oxide Clusters: Polyhedral Cages of (V2O5)n- (n=2-4) and (M2O5)2- (M=Nb, Ta)." Journal of the American Chemical Society 129(43):13270-13276. doi:10.1021/ja0750874 S0002-7863(07)05087-1 Abstract Vanadium oxide clusters, (V2O5)n, have been predicted to possess interesting polyhedral cage structures, which may serve as ideal molecular models for oxide surfaces and catalysts. Here we examine the electronic properties of these oxide clusters via anion photoelectron spectroscopy for (V2O5)n − (n = 2-4), as well as for the 4d/5d species, Nb4O10 − and Ta4O10 −. Wellresolved photoelectron spectra have been obtained at 193 and 157 nm and used to compare with density functional calculations. Very high electron affinities and large HOMO-LUMO gaps are observed for all the (V2O5)n clusters. The HOMO-LUMO gaps of (V2O5)n, all exceeding that of the band gap of the bulk oxide, are found to increase with cluster size from n = 2-4. For the M4O10 clusters, we find that the Nb/Ta species yield similar spectra, both possessing lower electron affinities and larger HOMO-LUMO gaps relative to V4O10. The structures of the anionic and neutral clusters are optimized; the calculated electron binding energies and excitation spectra for the global minimum cage structures are in good agreement with the experiment. Evidence is also observed for the predicted trend of electron delocalization versus localization in the (V2O5)n − clusters. Further insights are provided pertaining to the potential chemical reactivities of the oxide clusters and properties of the bulk oxides.
2007. "Boronyls as Key Structural Units in Boron Oxide Clusters: B(BO)2- and B(BO)3-." Journal of the American Chemical Society 129(30):9254-9255. doi:10.1021/ja072611y Abstract BO- is isoelectronic with CN-.1 However, in comparison to CN-, which is an important ligand in inorganic and biomolecules, the chemistry of BO- is relatively unknown. The electron affinity (EA) of BO (2.51 eV)2,3 is much smaller than that of CN (3.86 eV),4 which may explain the fact that CN- is a stable anion in solution, but BO- is not. However, the electronic structure and bond strength of BO- are similar to those of CN-, suggesting that it may be a robust chemical unit and can retain its structural integrity in chemical compounds. In a recent study, we indeed found that BO behaves like a monovalent structural unit in its bonding to Au in AunBO- (n ) 1-3) clusters.5 Theoretical calculations also suggested that carbon boronyls (CBO)n (n ) 3-7) are stable species on the potential-energy surfaces.6 Here we report a photoelectron spectroscopy (PES) and theoretical study on two boron oxide clusters, B3O2 - and B4O3 -, which are shown to possess a D¥h (3ªg) linear and D3h (2A2¢¢) triangular structure, respectively, and can be viewed as two and three boronyl groups bonded to a single B atom.
2007. "Interaction between Water Molecules and Zinc Sulfide Nanoparticles Studied byTemperature-Programmed Desorption and Molecular Dynamics Simulations." Journal of Physical Chemistry A 111(23):5008-5014. doi:10.1021/jp0688916 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. We have investigated the bonding of water molecules to the surfaces of ZnS nanoparticles (2-3 nm sphalerite) using temperature-programmed desorption (TPD). The activation energy for water desorption was derived as a function of the surface coverage through kinetic modeling of the experimental TPD curves. The binding energy of water equals the activation energy of desorption if it is assumed that the activation energy for adsorption is nearly zero. Molecular dynamics (MD) simulations of water adsorption on 3 and 5 nm sphalerite nanoparticles provided insights into the adsorption process and water binding at the atomic level. Water binds with the ZnS nanoparticle surface mainly via formation of Zn-O bonds. As compared with bulk ZnS crystals, ZnS nanoparticles can adsorb more water molecules per unit surface area due to the greatly increased curvature, which increases the distance between adjacent adsorbed molecules. Results from both TPD and MD show that the water binding energy increases with decreasing the water surface coverage. We attribute the increase in binding energy with decreasing surface water coverage to the increasing degree of surface under-coordination as removal of water molecules proceeds. MD also suggests that the water binding energy increases with decreasing particle size due to the further distance and hence lower interaction between adsorbed water molecules on highly curved smaller particle surfaces. Results also show that the binding energy, and thus the strength of interaction of water, is highest in isolated nanoparticles, lower in nanoparticle aggregates, and lowest in bulk crystals. Given that water binding is driven by surface energy reduction, we attribute the decreased binding energy for aggregated as compared to isolated particles to the decrease in surface energy that occurs as the result of inter-particle interactions.
2007. "Application of Electron Transfer Dissociation Mass Spectrometry in Analyses of Non-enzymatically Glycated Peptides." Rapid Communications in Mass Spectrometry 21(5):661-666. doi:10.1002/rcm.2884 Abstract Non-enzymatic glycation of peptides and proteins by D-glucose has important implications in diabetes mellitus research, particularly in the context of development of diabetic complications. The fragmentation behavior of glycated peptides produced from reaction of D-glucose with lysine residues was investigated by electron transfer dissociation (ETD) and collision induced dissociation (CID) tandem mass spectrometry. It was found that high abundance ions corresponding to various degrees of neutral water losses, as well as furylium ion production, dominate the CID spectra, and that the sequence informative b and y ions were rarely observed when Amadori-modified peptides were fragmented. Contrary to what was observed under CID conditions, ions corresponding to neutral losses of water or furylium ion production were not observed in the ETD spectra. Instead, abundant and almost complete series of c and z type ions were observed regardless of whether the modification site was located in the middle of the sequence or close to the N-terminus, greatly facilitating the peptide sequencing. This study strongly suggests that ETD is a better technique for proteomics studies of non-enzymatically glycated peptides and proteins.
2007. "Transparent thin-film transistor with self-assembled nanocrystals." Nanotechnology 18(19):Art. No. 195204. doi:10.1088/0957-4484/18/19/195204 Abstract The paper presents the fabrication of transparent devices using nanofabrication and nanomaterials. Clear and functional structures can be constructed over a large area with highly transparent nanocrystals via layer-by-layer self-assembly. The 12 nm SnO2 and SiO2 nanocrystals can form ultra-thin and uniform multilayers, serving as the active and insulating layers, respectively. Transistors fabricated by this method have a high optical transparency and reliable electrical performance. The averaged optical transmittance is higher than 85%. The usage of nanofabrication and nanomaterials solves some of the current fabricating problems for transparent devices. This technique is applicable to other transistor devices and circuits.
2007. "Enrichment and Analysis of Nonenzymatically Glycated Peptides: Boronate Affinity Chromatography Coupled with Electron-Transfer Dissociation Mass Spectrometry." Journal of Proteome Research 6(6):2323-2330. doi:doi:10.1021/pr070112q Abstract Non-enzymatic glycation of peptides and proteins by D-glucose has important implications in the pathogenesis of diabetes mellitus, particularly in the development of diabetic complications. However, no effective high-throughput methods exist for identifying proteins containing this low abundance post-translational modification in bottom-up proteomic studies. In this report, phenylboronate affinity chromatography was used in a two-step enrichment scheme to selectively isolate first glycated proteins and then glycated, tryptic peptides from human serum glycated in vitro. Enriched peptides were subsequently analyzed by alternating electron transfer dissociation (ETD) and collision induced dissociation (CID) tandem mass spectrometry. It was observed that ETD fragmentation mode resulted in a significantly higher number of glycated peptide identifications (87.6% of all identified peptides) versus CID mode (17.0% of all identified peptides), when utilizing dual glycation enrichment on both the protein and peptide level. This study illustrates that phenylboronate affinity chromatography coupled with LC-MS/MS with ETD as the fragmentation mode is an efficient approach for analyses of glycated proteins and can have broad applications in studies of diabetes mellitus.
2007. "A Method for Selective Enrichment and Analysis of Nitrotyrosine-Containing Peptides in Complex Proteome Samples." Journal of Proteome Research 6(6):2257-2268. doi:10.1021/pr0606934 Abstract Elevated levels of protein tyrosine nitration have been found in various neurodegenerative diseases and aging related pathologies; however, the lack of an efficient enrichment method has prevented the analysis of this important low level protein modification. We have developed an efficient method for specific enrichment of nitrotyrosine containing peptides that permits nitrotyrosine peptides and specific nitration sites to be unambiguously identified with LC-MS/MS. The method is based on the derivatization of nitrotyrosine into free sulfhydryl groups followed by high efficiency enrichment of sulfhydryl-containing peptides with thiopropyl sepharose beads. The derivatization process starts with acetylation with acetic anhydride to block all primary amines, followed by reduction of nitrotyrosine to aminotyrosine, then derivatization of aminotyrosine with N-Succinimidyl S-Acetylthioacetate (SATA), and finally deprotecting of S-acetyl on SATA to form free sulfhydryl groups. This method was evaluated using nitrotyrosine containing peptides, in-vitro nitrated human histone 1.2, and bovine serum albumin (BSA). 91% and 62% of the identified peptides from enriched histone and BSA samples were nitrotyrosine derivatized peptides, respectively, suggesting relative high specificity of the enrichment method. The application of this method to in-vitro nitrated mouse brain homogenate resulted in 35% of identified peptides containing nitrotyrosine (compared to only 5.9% observed from the global analysis of unenriched sample), and a total of 150 unique nitrated peptides covering 102 proteins were identified with a false discovery rate estimated at 3.3% from duplicate LC-MS/MS analyses of a single enriched sample.
2007. "Radiation detector resolution over a continuous energy range." Applied Physics Letters 91(09):094105, 1-3. doi:10.1063/1.2776978 Abstract Urgent need for new radiation detector materials with excellent energy resolution at room temperature has prompted research efforts on both accelerated materials discovery and efficient techniques that can investigate candidate materials to identify material properties relevant to detector performance. In this work, an ion-beam approach is demonstrated to obtain energy resolution in both semiconductor detectors and scintillators over a continuous energy range. For semiconductors, the energy resolution of a silicon detector was measured as a function of He+ ion energy, and the values from an extrapolation to high energies are in good agreement with the literature data from alpha measurements. For the scintillators, benchmark crystals of BGO, YAP(Ce) and CsI(Tl) subject to He+ irradiation were investigated, and the corresponding energy resolution is compared with gamma-ray tests on the same crystals. The agreement of energy resolution between the ion and gamma measurements indicates that the ion approach can be used to predict energy resolution of candidate materials in thin film form (a few tens m thick) or a small crystals (a few mm3), when large crystals necessary for gamma-ray testing are unavailable.
2007. "Stopping power measurements of He ions in Si and SiC by time-of-flight spectrometry." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 261(1-2):1180-1183. doi:10.1016/j.nimb.2007.04.276 Abstract Electronic energy loss is the fundamental mechanism accountable for the response of materials to ions. Helium particles are a product from alpha decay in nuclear waste materials, and helium ions are widely used as projectiles in ion beam analysis. Current work introduces a straightforward approach to determine electronic stopping powers of He ions in Si and SiC over a continuous range of energies. In transmission geometry, the energy loss of He ions in self-supporting stopping foils of Si and SiC is measured using a Time-of-Flight (TOF) set-up. The energy of individual heavy ions prior to impingement on the foil is determined from its TOF data; the exit energy after the stopping foil is essentially measured using the Si detector, for which every channel has been calibrated using TOF data without the stopping foil present. The measured stopping powers demonstrate excellent repeatability of this experimental approach and good reliability as compared with pervious data from the literature and theoretical predictions.
2007. "Direct Visualization of 2-Butanol Adsorption and Dissociation on TiO2(110)." Journal of Physical Chemistry C 111(7):3021-3027. doi:10.1021/jp067461c Abstract Atomically resolved scanning tunneling microscopy (STM) images of identical regions of a TiO2(110) surface were gathered before and after controlled doses of 2-butanol (CH3CH2CH(OH)CH3) at ambient temperature (~ 300 K). When dosing is initiated, 2-butanol preferentially adsorbs at bridge-bonded oxygen vacancy (BBOv) sites and dissociates via O—H, not C—O, bond scission to form paired 2-butoxy and hydroxyl species evidenced by two local maxima in STM line profiles. The measured separation is 0.4 nm, slightly larger than the measured separation (0.3 nm) between neighboring bridge-bonded oxygen anions in the surface unit cell of TiO2(110). As the dose increases, but before all the BBOv are occupied, there is direct STM evidence of hydroxyl proton hopping to an adjacent oxygen anion row. This process is facilitated by species bound to 5-coordinate Ti4+ rows, presumably undissociated 2-butanol, that hop slowly compared the STM imaging time scale. The backbones of these mobile species are centered over the Ti4+ rows with preference for lying parallel to these rows. On the other hand, the carbon backbones of the 2-butoxy species that fill BBOv's are centered over the O2- rows and prefer an orientation perpendicular to these rows. As the oxygen vacancy concentration increases from 0.4 to 11 % and 2-butanol is dosed the ratio of mobile species to 2-butoxy species decreases for doses that do not fill all the BBOv.
2007. "Imaging Intrinsic Diffusion of Bridge-Bonded Oxygen Vacancies on TiO2(110)." Physical Review Letters 99(12):paper #126105. doi:10.1103/PhysRevLett.99.126105 Abstract Since oxygen atom vacancies play a central role in the behavior of oxide materials, determining their properties is widely pursued both experimentally and theoretically. As a function of temperature between 340 and 420 K, we report here the first measurements and calculations of the intrinsic mobility of bridge-bonded oxygen atom vacancies on a TiO2(110) rutile surface. Under conditions where interference by adsorbates was negligible, isothermal atomically-resolved scanning tunneling microscope images that track individual vacancies in real time show that, vacancies migrate exclusively along bridging oxygen atom rows. The hopping rate increases exponentially with increasing temperature with experimental activation energy of 1.15 eV. Density functional theory calculations are in very good agreement giving an energy barrier for hopping of 1.034 eV. Both theory and experiment, indicate a short-range repulsive interaction between vacancies on a given bridge-bonded oxygen atom row.
2007. "Metal Chlorides in Ionic Liquid Solvents Convert Sugars to 5-Hydroxymethylfurfural." Science 316(5831):1597-1600. doi:10.1126/science.1141199 Abstract Sugars were converted to hydroxymethylfurfural (HMF) at high yield in ionic liquids without the addition of Bronsted acids. Very small amount of certain metal halides significantly reduced the fructose dehydration barrier in ionic liquids producing HMF at high yields. Most remarkably, glucose, a common sugar molecule, was selectively converted to HMF in good yield in ionic liquids containing a small amount of CrCl2. Thus CrCl2 is unique among metal chlorides tested for its effectiveness in both isomerizing glucose as well as dehydrating fructose. Only negligble amount of levulinic acid was formed in the reactions. The catalytic activity of metal chlorides for sugar conversion in ionic liquids is perhaps related to hydroxyl group of the sugar forming metal complexes with the unsaturated metal center.
2007. "A new route to improved glucose yields in cellulose hydrolysis." Journal of Biobased Materials and Bioenergy 1(2):210-214. Abstract An unusual inverse temperature-dependent pathway was discovered for cellulose decrystallization in trifluoroacetic acid (TFA). Cellulose was completely decrystallized by TFA at 0 °C in less than 2 hours, a result not achieved in 48 hours at 25°C in the same medium. The majority of TFA used in cellulose decrystallization was recycled via a vacuum process. The small remaining amount of TFA was diluted with water to make a 0.5% TFA solution and used as a catalyst in dilute acid hydrolysis. After one minute, under batch conditions at 185 °C, the glucose yield reached 63.5% without production of levulinic acid. In comparison, only 15.0% glucose yield was achieved in the hydrolysis of untreated cellulose by 0.5% H2SO4 under the same condition. Further improvement of glucose yield is possible by optimizing reaction conditions. Alternatively, the remaining TFA can be completely removed by water while keeping the regenerated cellulose in a highly amorphous state. This regenerated cellulose is much more reactive than untreated cellulose in hydrolysis reactions, but still less reactive than corn starch. The lower temperatures and shorter reaction times with this activated cellulose makes it possible to reduce operating costs and decrease byproduct yields such as HMF and levulinic acid.
2007. "Inverse Temperature-Dependent Pathway of Cellulose Decrystallization in Trifluoroacetic Acid." Journal of Physical Chemistry B 111(19):5295-5300. doi:10.1021/jp070253f Abstract Abstract An unusual inverse temperature-dependent pathway was observed during cellulose decrystallization in trifluoroacetic acid (TFA). Decreasing the TFA treatment temperature accelerated the cellulose decrystallization process. It took only 100 minutes to completely decrystallize cellulose at 0 °C in TFA, a result not achieved in 48 hours at 25°C in the same medium. There was neither cellulose esterification nor a change of cellulose macrofibril morphology by TFA treatment at 0 °C. Our IR data suggest that TFA molecules are present as cyclic dimers when they penetrate into crystalline cellulose regions, transforming crystalline cellulose to amorphous cellulose. The TFA cyclic dimer does not form strong hydrogen bonds with cellulose since the IR vibration frequency of the carbonyl group of the dimer molecule remained unchanged after the dimer diffused into the cellulose matrix. On the other hand, the rate of TFA penetration into the cellulose matrix was greatly retarded at higher temperatures where monomeric TFA esterification took place on the external surface of crystalline cellulose. At elevated temperatures esterification of TFA monomers with cellulose, as well as water released from the esterification reaction, inhibit the diffusion rate of TFA into the cellulose crystalline region and decreases the TFA swelling capability. Based on experimental observations, our study indicates that cellulose decrystallization does not require that solvent molecules form strong hydrogen bonds with cellulose.
2007. "Interactions between cellulose and N-Methylmorpholine-N-oxide." Carbohydrate Polymers 67(1):97-103. doi:10.1016/j.carbpol.2006.04.019 Abstract X-ray diffraction (XRD), 13C Solid State Nuclear Magnetic Resonance (NMR) and Fourier Transform Infrared (FTIR) Spectroscopy were used to investigate interactions between N-methylmorpholine-N-oxide (NMMO) and Iα cellulose. Cellulose II precipitated from NMMO/H2O/Iα cellulose solution by adding excess water. Comparing hydrolysis activities, reprecipitated cellulose was three times more reactive than untreated Iα cellulose. Solid mixtures comprising equal masses of powdered I cellulose and NMMO were heated to various temperatures and cooled to room temperature. Analysis after cooling showed that during heating, the NMMO crystal structure collapsed between 70 and 100 C (80 to 110 oC below pure NMMO). and I cellulose became amorphous.. When the protocol involved heating to 150 C, FTIR data suggest that H2O, hydrogen-bonded to NMMO, can be displaced by cellulose, leaving H2O either hydrogen bonded to cellulose or unbound and physically confined within the cellulose matrix.
2007. "Studying Cellulose Fiber Structure by SEM, XRD, NMR and Acid Hydrolysis." Carbohydrate Polymers 68(2):235-241. doi:10.1016/j.carbpol.2006.12.013 Abstract Cotton linters were partially hydrolyzed in dilute acid and the morphology of remaining macrofibrils studied with Scanning Electron Microscopy (SEM) under various magnifications. The crystal region (microfibril bundles) in the macrofibrils was not altered by hydrolysis, and only amorphous cellulose was hydrolyzed and leached out from the macrofibrils. The diameter of microfibril bundles was 20-30 nm after the amorphous cellulose was removed by hydrolysis. XRD experiments confirm the unaltered diameter of the microfibrils after hydrolysis. The strong stability of these microfibril bundles in hydrolysis limits both the total sugar monomer yield and the size of nano particles or rods produced in hydrolysis. The large surface potential on the remaining microfibril bundles drives the agglomeration of macrofibrils.
2007. "Attractive Noncovalent Interactions inthe Mechanism of GrubbsSecond-Generation Ru Catalysts forOlefin Metathesis." Organic Letters 9(10):1967-1970. doi:10.1021/ol0705548 Abstract Second-generation ruthenium carbenoid catalysts for olefin metathesis are a hundred to a thousand times more active than first-generation catalysts, despite a slower initiation step. A new density functional capable of treating medium-range correlation energy shows that the relative rates of generation of the catalyst are determined by attractive noncovalent interactions.
2007. "Size-Selective Supramolecular Chemistry in a Hydrocarbon Nanoring." Journal of the American Chemical Society 129(27):8440-8442. doi:10.1021/ja071884q 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. Supramolecular chemistry has been a very active area since the seminal work of Lehn.1-4 Macrocycles are versatile building blocks for supramolecular design. Conjugated macrocycles with a beltlike shape in which the conjugated orbitals are hybridized orbitals directed radially5-7 (as compared to pure p-orbitals normal to a plane in polyacenes and polyaphenes) provide especially intriguing opportunities for such design; carbon nanotubes8 and hydrocarbon nanorings6,9-15 are exemplary cases.
2007. "Thermochemical Kinetics of Hydrogen-Atom Transfers between Methyl, Methane, Ethynyl, Ethyne, and Hydrogen." Journal of Physical Chemistry A 111(21):4632-4642. doi:10.1021/jp070252n 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. Saddle point properties of three symmetric and one asymmetric hydrogen transfer and the energy of reaction of the asymmetric reactions are investigated in the present work. These reactions were calculated by various density functionals, many of which were developed in recent years, by coupled cluster theory, and by multicoefficient correlation methods based on wave function theory. Instead of comparing calculated results to “semi-experimental” values, we compared them to very accurate theoretical values (e.g., to values obtained by the Weizmann-1 method). Coupled cluster theory and the multicoefficient correlation methods MC-QCISD/3 and MCQCISD-MPW are very accurate for these reactions with mean unsigned errors below 0.94 kcal/mol. Diagnostics for multireference character add additional reliability to these results. The newly developed hybrid density functional M06-2X shows very good performance for these reactions with a mean unsigned error of only 0.77 kcal/mol; the BHandHLYP, MPW1K, and BB1K density functionals, can also predict these reactions well with mean unsigned errors less than 1.42 kcal/mol.
2007. "A Mechanism For Carbon Nanosheet Formation." Carbon 45(11):2229-2234. doi:10.1016/j.carbon.2007.06.017 Abstract The growth, structure and properties of a two-dimensional carbon nanostructure-carbon nanosheet produced by radio frequency plasma enhanced chemical vapor deposition have been investigated. The effects of deposition parameters on the structure and properties of carbon nanosheets were also investigated. A growth model has been described proposing that atomically thin graphene sheets result from a balance between deposition through surface diffusion and etching by atomic hydrogen, and that the observed vertical orientation of these sheets results from the interaction of the plasma electric field with their anisotropic polarizability.
2007. "On The Chemical Bonding of Gold In Auro-Boron Oxide Clusters AunBO-(n=1-3)." Journal of Physical Chemistry A 111(9):1648-1658. doi:10.1021/jp0657437 Abstract During experiment on Au-B alloy clusters, an auro-boron oxide cluster Au2BO- was observed to be an intense peak dominating the Au-B mass spectra, along with weaker signals for AuBO- and Au3BO-. Well-resolved photoelectron spectra have been obtained for the three new oxide clusters, which exhibit an odd-even effect in their electron binding energies. Au2BO- is shown to be a closed shell molecule with a very high electron binding energy, whereas AuBO and Au3BO neutrals are shown to be closed shell species with large HOMO-LUMO gaps, resulting in relatively low electron affinities. Density functional calculations were performed for both AunBO- (n = 1-3) and the corresponding HnBO- species in order to evaluate the analogy in bonding between gold and hydrogen in B-Au clusters. The combination of experiment and theory allowed us to establish the structures and chemical bonding of these tertiary clusters. We find that the first gold atom does mimic hydrogen and interacts with the BO unit to produce a linear AuBO- structure. This unit preserves its identity when interacting with additional gold atoms: a linear Au-[AuBO] complex is formed when adding one extra Au atom and two isomeric Au2-[AuBO] complexes are formed when adding two extra Au atoms. Since BO- is isoelectronic to CO, the AunBO- species can be alternatively viewed as Aun interacting with a BO- unit. The structures and chemical bonding in AunBO- are compared to those in the corresponding AunCO complexes.
2007. "Adsorption, Desorption, and Diffusion of Nitrogen in a Model Nanoporous Material: I. Surface Limited Desorption Kinetics in Amorphous Solid Water." Journal of Chemical Physics 127(18):Art. No. 184707. doi:10.1063/1.2790432 Abstract The adsorption and desorption kinetics of N2 on porous amorphous solid water (ASW) films were studied using molecular beam techniques, temperature programmed desorption (TPD), and reflection-absorption infrared spectroscopy (RAIRS). The ASW films were grown on Pt(111) at 23 K by ballistic deposition from a collimated H2O beam at various incident angles to control the film porosity. The experimental results show that the N2 condensation coefficient is essentially unity until near saturation, independent of the ASW film thickness. This means that N2 transport within the porous films is rapid. The TPD results show that the desorption of a fixed dose of N2 shifts to higher temperature with ASW film thickness. Kinetic analysis of the TPD spectra shows that a film thickness rescaling of the coverage dependent activation energy curve results in a single master curve. Simulation of the TPD spectra using this master curve results in a quantitative fit to the experiments over a wide range of ASW thicknesses (up to 1000 layers, ~0.5 mm). The success of the rescaling model indicates that N2 transport within the porous film is rapid enough to maintain a uniform distribution throughout the film on a time scale faster than desorption.
2007. "Adsorption, Desorption, and Diffusion of Nitrogen in a Model Nanoporous Material: II. Diffusion Limited Kinetics in Amorphous Solid Water." Journal of Chemical Physics 127(14):Art. No. 184708. doi:10.1063/1.2790433 Abstract Tykhon Zubkov, R. Scott Smith, Todd R. Engstrom, and Bruce D. Kay The adsorption, desorption, and diffusion kinetics of N2 on thick (up to ~9 mm) porous films of amorphous solid water (ASW) films were studied using molecular beam techniques and temperature programmed desorption (TPD). Porous ASW films were grown on Pt(111) at low temperature (<30 K) from a collimated H2O beam at glancing incident angles. In thin films (<1 mm), the desorption kinetics are well described by a model that assumes rapid and uniform N2 distribution throughout the film. In thicker films, (>1 mm), N2 adsorption at 27 K results in a non-uniform distribution where most of N2 is trapped in the outer region of the film. Redistribution of N2 can be induced by thermal annealing. The apparent activation energy for this process is ~7 kJ/mol, which is approximately half of the desorption activation energy at the corresponding coverage. Blocking adsorption sites near the film surface facilitates transport into the film. Despite the onset of limited diffusion, the adsorption kinetics are efficient, precursor-mediated and independent of film thickness. An adsorption mechanism is proposed, in which a high-coverage N2 front propagates into a pore by the rapid transport of physisorbed 2nd layer N2 species on top of the 1st layer chemisorbed layer.
