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1

Aqueous tape casting of reaction bonded aluminium oxide (RBAO)  

Microsoft Academic Search

Reaction bonding of alumina (RBAO) has been proved to be a successful route to make high-performance alumina. However, the aluminium metal in the starting mixtures prevents using aqueous processing techniques in the initial phase. Here a route is presented for aqueous tape casting of RBAO by hydrophobization of the starting powder and preparing a suitable tape cast suspension. Thermal analysis

M. D. Snel; FMM Snijkers; JJ Luyten; AA Kodentsov

2007-01-01

2

Insertion reactions of difluorocarbene generated by pyrolysis of hexafluoropropene oxide to O H bond  

Microsoft Academic Search

ROCHF2-type fluorinated ethers were synthesized by the reaction of hexafluoropropene oxide (HFPO) with alcohol or phenol. In this reaction, although the insertion reaction of difluorocarbene to OH bond and the nucleophilic attack of alcohol or phenol to HFPO were competition, the insertion reaction proceeded predominantly to give fluorinated ether in the case of low nucleophilic alcohol or phenol. In addition,

Junji Mizukado; Yasuhisa Matsukawa; Heng-dao Quan; Masanori Tamura; Akira Sekiya

2005-01-01

3

Effects of aluminum and zirconia contents on the reaction bonded aluminum oxide process  

Microsoft Academic Search

The effects of aluminum and ZrO2 contents on the reaction and sintering of reaction bonded aluminum oxide (RBAO) were investigated. It was apparent that ZrO2-containing RBAO powders with higher initial aluminum contents (>45 vol%) were increasingly more difficult to react and sinter. During oxidation in air, samples often underwent a self-propagating high-temperature synthesis (SHS) reaction which led to catastrophic failure.

Paul Martin Sheedy

2003-01-01

4

Oxidation behavior in reaction-bonded aluminum-silicon alloy/alumina powder compacts  

SciTech Connect

Goal of this research is to determine the feasibility of producing low-shrinkage mullite/alumina composites by applying the reaction-bonded alumina (RBAO) process to an aluminum-silicon alloy/alumina system. Mirostructural and compositional changes during heat treatment were studied by removing samples from the furnace at different steps in the heating schedule and then using optical and scanning electron microscopy, EDS and XRD to characterize the powder compacts. Results suggest that the oxidation behavior of the alloy compact is different from the model proposed for the pure Al/alumina system.

Yokota, S.H.

1992-12-01

5

Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si  

SciTech Connect

Although strain is used in semiconductor technology for manipulating optical, electronic, and chemical properties of semiconductors, the understanding of the microscopic phenomena that are affected or influenced by strain is still incomplete. Second-harmonic generation data obtained during the air oxidation of H-terminated (111) Si reveal the effect of compressive strain on this chemical reaction. Even small amounts of strain manipulate the reaction kinetics of surface bonds significantly, with tensile strain enhancing oxidation and compressive strain retarding it. This dramatic change suggests a strain-driven charge transfer mechanism between Si-H up bonds and Si-Si back bonds in the outer layer of Si atoms.

Gokce, Bilal; Aspnes, David E.; Gundogdu, Kenan [Department of Physics, North Carolina State University, Raleigh North Carolina 27695 (United States)

2011-03-21

6

Effects of aluminum and zirconia contents on the reaction bonded aluminum oxide process  

NASA Astrophysics Data System (ADS)

The effects of aluminum and ZrO2 contents on the reaction and sintering of reaction bonded aluminum oxide (RBAO) were investigated. It was apparent that ZrO2-containing RBAO powders with higher initial aluminum contents (>45 vol%) were increasingly more difficult to react and sinter. During oxidation in air, samples often underwent a self-propagating high-temperature synthesis (SHS) reaction which led to catastrophic failure. This reaction and cracking behavior was more pronounced with increasing aluminum and ZrO2 contents of the powders. Subsequently, it was shown that the SHS reaction was actually two combustion phenomena: a thermal explosion reaction on the surface of the sample between aluminum and oxygen, which (in ZrO2-containing samples) triggered a self propagating aluminothermic reduction of ZrO2, forming Al2O3 and Al 3Zr. Therefore, methods for controlling the rate of the initial oxidation reaction were effective since both SHS reactions were prevented. Despite the use of controlled firing, initial samples with increasing aluminum contents proved difficult to densify. It was found that in all RBAO samples (regardless of ZrO2 content), the reactively formed Al 2O3 underwent the gamma to alpha-Al2O 3 transformation, which resulted in the development of a vermicular microstructure. In ZrO2-containing RBAO samples, this transformation was inhibited and occurred concurrently with the start of densification. In addition, the start of bulk shrinkage in these samples was delayed and the densification rates were decreased in comparison to samples without ZrO 2. This ultimately resulted in a decrease in the limiting density to which ZrO2-containing RBAO samples could be sintered. Surprisingly, in samples without ZrO2, increasing the aluminum content did not appear to have any effects upon the densification behavior of RBAO. In examining RBAO samples with similar aluminum contents but increasing ZrO2 contents, it became apparent that the grain growth inhibiting action of the ZrO2 prevented the complete removal of the vermicular microstructure. Thus, in ZrO2 containing RBAO samples with higher aluminum contents, where there was more reactively formed Al2O 3, the vermicular structure was even more difficult to eliminate.

Sheedy, Paul Martin

7

Consequences of Metal-Oxide Interconversion for C-H Bond Activation during CH4 Reactions on Pd Catalysts.  

PubMed

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C-H bonds in CH4 react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C-H activation routes change from oxidative addition to H-abstraction and then to ?-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*-*) to oxygen atom (O*-O*) pairs and ultimately to Pd cation-lattice oxygen pairs (Pd(2+)-O(2-)) in PdO. The charges in the CH3 and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved. Homolytic C-H dissociation prevails on bare (*-*) and O*-covered surfaces (O*-O*), while C-H bonds cleave heterolytically on Pd(2+)-O(2-) pairs at PdO surfaces. On bare surfaces, C-H bonds cleave via oxidative addition, involving Pd atom insertion into the C-H bond with electron backdonation from Pd to C-H antibonding states and the formation of tight three-center (H3CPdH)(?) transition states. On O*-saturated Pd surfaces, C-H bonds cleave homolytically on O*-O* pairs to form radical-like CH3 species and nearly formed O-H bonds at a transition state (O*CH3()*OH)(?) that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd(2+) and vicinal O(2-), Pdox-Oox , cleave C-H bonds heterolytically via ?-bond metathesis, with Pd(2+) adding to the C-H bond, while O(2-) abstracts the H-atom to form a four-center (H3C(?-)PdoxH(?+)Oox)(?) transition state without detectable Pdox reduction. The latter is much more stable than transition states on *-* and O*-O* pairs and give rise to a large increase in CH4 oxidation turnover rates at oxygen chemical potentials leading to Pd to PdO transitions. These distinct mechanistic pathways for C-H bond activation, inferred from theory and experiment, resemble those prevalent on organometallic complexes. Metal centers present on surfaces as well as in homogeneous complexes act as both nucleophile and electrophile in oxidative additions, ligands (e.g., O* on surfaces) abstract H-atoms via reductive deprotonation of C-H bonds, and metal-ligand pairs, with the pair as electrophile and the metal as nucleophile, mediate ?-bond metathesis pathways. PMID:24083571

Chin, Ya-Huei Cathy; Buda, Corneliu; Neurock, Matthew; Iglesia, Enrique

2013-10-01

8

Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping  

SciTech Connect

It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds.

Gokce, B.; Aspnes, D. E.; Lucovsky, G.; Gundogdu, K. [Physics Department, North Carolina State University, Raleigh, North Carolina 27695 (United States)

2011-01-10

9

Reactions of Hf +, Ta +, and W + with O 2 and CO: Metal carbide and metal oxide cation bond energies  

Microsoft Academic Search

The reactions of Hf+, Ta+, and W+ with O2 and CO are studied as a function of translational energy in a guided ion beam tandem mass spectrometer. All three reactions with O2 form diatomic metal oxide cations in exothermic reactions that occur at the collision rate. In the CO systems, formation of both diatomic metal oxide and metal carbide cations

Christopher S. Hinton; Fengxia Li; P. B. Armentrout

2009-01-01

10

Oxidation Effects on the Mechanical Properties of SiC Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites.  

National Technical Information Service (NTIS)

The room temperature mechanical properties of SiC fiber reinforced reaction bonded silicon nitride composites were measured after 100 hrs exposure at temperatures to 1400 C in nitrogen and oxygen environments. The composites consisted of approx. 30 vol pe...

R. T. Bhatt

1989-01-01

11

Facile Access to Tuneable Schwartz's Reagents: Oxidative Addition Products from the Reaction of Amide N?H Bonds with Reduced Zirconocene Complexes.  

PubMed

On the tracks of Schwartz's reagent: Two zirconocene hydrido amidate complexes are synthesized by formal oxidative addition of amide N?H bonds to reduced zirconocene fragments. Insertion reactions with alkenes show a different behavior than Schwartz's reagent by forming branched insertion products. The insertion product and the hydrido complex are characterized by X-ray analysis. PMID:24038845

Haehnel, Martin; Yim, Jacky C-H; Schafer, Laurel L; Rosenthal, Uwe

2013-09-03

12

Sintered-reaction Bonded Silicon Nitride Densified by a Gas Pressure Sintering Process Effects of Rare Earth Oxide Sintering Additives  

SciTech Connect

Reaction-bonded silicon nitrides containing rare-earth oxide sintering additives were densified by gas pressure sintering. The sintering behavior, microstructure and mechanical properties of the resultant specimens were analyzed. For that purpose, Lu2O3-SiO2 (US), La2O3-MgO (AM) and Y2O3-Al2O3 (YA) additive systems were selected. Among the tested compositions, densification of silicon nitride occurred at the lowest temperature when using the La2O3-MgO system. Since the Lu2O3-SiO2 system has the highest melting temperature, full densification could not be achieved after sintering at 1950oC. However, the system had a reasonably high bending strength of 527 MPa at 1200oC in air and a high fracture toughness of 9.2 MPa m1/2. The Y2O3-Al2O3 system had the highest room temperature bending strength of 1.2 GPa

Lee, S. H. [Korea Institute of Materials Science; Ko, J. W. [Korea Institute of Materials Science; Park, Y. J. [Korea Institute of Materials Science; Kim, H. D. [Korea Institute of Materials Science; Lin, Hua-Tay [ORNL; Becher, Paul F [ORNL

2012-01-01

13

Transition-metal-free oxidative coupling reactions for the formation of C-C and C-N bonds mediated by TEMPO and its derivatives.  

PubMed

The application of nitroxides for the development of new synthetic methods and their implementation in polymer chemistry, material science and beyond is one of the major research topics in our laboratory in the institute of organic chemistry at the WWU Mnster. This short review focuses on our recent progress towards nitroxide-based transition-metal-free oxidative coupling reactions. The demand for organic surrogates for transition metals in such transformations is in our eyes unquestionable, since environmental and economic issues have become progressively more important in recent years. For this purpose, the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) is shown to be a highly efficient oxidant for the homo- and cross-coupling of Grignard reagents. This powerful C-C bond forming strategy allows the generation of conjugated polymers from bifunctional Grignard reagents. Moreover, cross-coupling of alkynyl Grignard compounds and nitrones can be accomplished under aerobic atmosphere with catalytic amounts of TEMPO. It is also shown that TEMPO-derived N-oxoammonium salts can act as suitable oxidants for formation of C-N bonds between non-preactivated benzoxazoles and secondary amines under metal-free conditions. PMID:22871285

Murarka, Sandip; Wertz, Sebastian; Studer, Armido

2012-01-01

14

Oxidation of natural targets by dimethyl dioxirane: Regio and stereospecific reactions on enol double bond of bioactive nor quinone methide triterpenes  

Microsoft Academic Search

Nor quinone methide triterpenes, like pristimerin and tingenone, possess one enol double bond (C3C4). They also have an extended conjugation with an additional double bond (C7C8). When these compounds are treated with dimethyl dioxirane, regio and stereoespecific oxidation occurs only on the enol double bond.

Antonio G. Gonzlez; Nelson L. Alvarenga; Ana Estvez-Braun; Angel G. Ravelo; Rafael Estvez-Reyes

1996-01-01

15

Oxidatively induced abstraction reactions. A synthetic approach to low-coordinate and reactive early transition metal complexes containing metal-ligand multiple bonds.  

PubMed

A library of low-coordinate titanium and vanadium complexes containing terminal metal-ligand multiply bonded functionalities such as alkylidenes, alkylidynes, and imides have been prepared by one-electron oxidatively induced alpha-hydrogen abstraction reactions. In the case of the alkylidene motif, the nucleophilic nature of the M-C multiple bond permits subsequent reactions such as alpha-hydrogen migration to generate other rare functionalities such as phosphinidene-alkyl and imide-alkyls. Identifying and fine-tuning of the supporting ancillary ligand on the metal has allowed the isolation of kinetically stable titanium alkylidene and phosphinidene systems. The former is a key functionality to generate transient titanium alkylidynes, which readily engage in intermolecular C-H activation reactions of arenes and alkanes, and the ring-opening metathesis of aromatic substrates such as pyridines. In this Account, we describe several synthetic strategies to achieve reactive functionalities, functionalities that were previously portrayed as "incompatible" or "too kinetically reactive" with 3d early transition metals. PMID:17115721

Mindiola, Daniel J

2006-11-01

16

Reaction-bonded mullite/zirconia composites  

SciTech Connect

The feasibility of fabricating dense, low-shrinkage, mullite/ZrO{sub 2} composites based on the reaction bonding of alumina (RBAO) process and the reaction sintering of zircon is examined. Compacts pressed from an attrition-milled powder mixture of Al, Al{sub 2}O{sub 3} and zircon were heated in air according to a two-step heating cycle. The phase evolution and microstructural development during reaction bonding were traced by X-ray diffraction, nuclear magnetic resonance, and scanning electron microscopy on samples extracted from various points along the heating cycle. It is seen that, as in conventional RBAO, Al oxidizes to {gamma}-Al{sub 2}O{sub 3} which then transforms to {alpha}-Al{sub 2}O{sub 3} between 1,100 and 1,200 C. The zircon dissociation commences at {approximately} 1,400 C and is practically complete by 1,500 C. Mullite enriched in Al{sub 2}O{sub 3} forms initially, but 3:2 stoichiometry is attained in the final product which consists of mullite, t- and m-ZrO{sub 2}, and residual {alpha}-Al{sub 2}O{sub 3}. The flexure strength of the composite is superior to that of pure mullite, and {approximately} 80% of the strength is retained up to 1,200 C. Although there was no toughness enhancement relative to mullite, this should be achievable by optimizing the fabrication procedure.

Lathabai, S.; Hay, D.G. [CSIRO, Clayton, Victoria (Australia). Div. of Materials Science and Technology; Wagner, F.; Claussen, N. [Technische Univ. Hamburg-Harburg, Hamburg (Germany). Advanced Ceramics Group

1996-01-01

17

Csp(2)-N Bond Formation via Ligand-Free Pd-Catalyzed Oxidative Coupling Reaction of N-Tosylhydrazones and Indole Derivatives.  

PubMed

In a fresh approach to the synthesis of N-vinylazoles, a ligand-free palladium catalytic system was found to promote the Csp(2)-N bond-forming reaction utilizing N-tosylhydrazones and N-H azoles. This process shows functional group tolerance; di-, tri-, and tetrasubstituted N-vinylazoles were obtained in high yields. Under the optimized conditions, the reaction proceeds with high stereoselectivity depending on the nature of the coupling partners. PMID:23899168

Roche, Maxime; Frison, Gilles; Brion, Jean-Daniel; Provot, Olivier; Hamze, Abdallah; Alami, Mouad

2013-08-14

18

First insertion of NO into a transition-metal cluster-carbon bond: regioselective formation, structure, and reactions of the first alkanenitrile oxide complexes  

SciTech Connect

The chemistry of NO in the presence of transition metals is receiving considerable current attention because of its role in air pollution, its potential in organic synthesis by carbon-nitrogen bond formation, and an increasing interest in its basic features. The nitrosyl cation has been reacted with many mono and polynuclear metal systems, leading mainly to substitution and reduction. Insertion into alkyl and aryl metal bonds in mono-metallic complexes is documented. The unprecedented title reaction and some preliminary chemistry of the products are reported here. 27 references, 1 figure.

Goldhaber, A.; Vollhardt, K.P.C.; Walborsky, E.C.; Wolfgruber, M.

1986-02-05

19

New LTCC-hexaferrites by using reaction bonded glass ceramics  

Microsoft Academic Search

Hexaferrites are usually prepared according to the standard mixed oxide method with high sintering temperatures of up to 1350C, which are not suitable for low temperature cofired ceramics (LTCC) technology. In this work, the sintering temperature of BaFe12O19 was reduced to 900C by the development of reaction-bonded glass ceramics systems for LTCC-hexaferrites. Low amounts of reactive glasses (<7vol.%) based on

R. Karmazin; O. Dernovsek; N. Ilkov; W. Wersing; A. Roosen; M. Hagymasi

2005-01-01

20

Sensor/ROIC Integration using Oxide Bonding  

SciTech Connect

We explore the Ziptronix Direct Bond Interconnect (DBI) technology [2] for the integration of sensors and readout integrated circuits (ROICs) for high energy physics. The technology utilizes an oxide bond to form a robust mechanical connection between layers which serves to assist with the formation of metallic interlayer connections. We report on testing results of sample sensors bonded to ROICs and thinned to 100 {micro}m.

Ye, Zhenyu; /Fermilab

2009-02-01

21

Large kinetic isotope effects in methane oxidation catalyzed by methane monooxygenase: evidence for C-H bond cleavage in a reaction cycle intermediate.  

PubMed

The reduced hydroxylase component (MMOH) of soluble methane monooxygenase (MMO) from Methylosinus trichosporium OB3b reacts with O2 and CH4 to produce CH3OH and H2O in a single-turnover reaction. Transient kinetic analysis of this reaction has revealed at least five and probably six intermediates during the turnover [Lee, S.-K., Nesheim, J. C., & Lipscomb, J. D. (1993) J. Biol. Chem. 268, 21569-21577; Liu, Y., Nesheim, J. C., Lee, S.-K., & Lipscomb, J. D. (1995) J. Biol. Chem. 270, 24662-24665]. One intermediate, termed compound Q, reacts with CH4 to yield enzyme-bound product. It is shown here that the deuterium kinetic isotope effect (KIE) for the reaction of compound Q with CH4 is 50-100, which is one of the largest effects observed to date. The rate constants for the reactions of the deuterated homologs of methane decrease monotonically as the deuterium content increases, suggesting that a large primary isotope effect dominates. The KIEs determined by analyzing the products after a single turnover have the following values: 1:1 CH4:CD4 (19); CD3H (12); CD2H2 (9); and CH3D (4). The KIE values determined by directly observing the reactive intermediate and by monitoring product ratios are all large, consistent with complete C-H bond breaking in the oxygenation step of the reaction. However, the differences in the KIE values determined by these two methods suggest that the reaction is more complex than currently proposed. A modified mechanism introducing the possibility of hydrogen-atom reabstraction by an intermediate methyl radical is proposed. PMID:8756490

Nesheim, J C; Lipscomb, J D

1996-08-01

22

Stretching the bonds of chemical reactions  

NASA Astrophysics Data System (ADS)

The effects of reagent stretch-excitation on the gas-phase Cl+CH 4 ? HCl+CH3 reaction are examined experimentally using a photoinitiated reaction technique in which the reagents are prepared by direct infrared absorption or stimulated Raman pumping and the products are state-selectively ionized and detected in a time-of-flight mass spectrometer. Our measurements indicate that the rovibrational distributions and state-selected differential cross sections of the HCl and CH3 products from the symmetric and antisymmetric stretch excited reactions are indistinguishable. This result suggests that the mechanisms of these nearly isoenergetic vibrationally excited reactions are similar, despite theoretical and experimental measurements that suggest the symmetric stretch enhances the reaction rate more than the antisymmetric stretch. Furthermore, we have demonstrated mode- and bond-selectivity in the Cl+CH2D2 reaction. Excitation of the first C-H overtone of CH2D2 leads to a preference for hydrogen abstraction over deuterium abstraction by at least a factor of 20, whereas excitation of the first C-D overtone of CH2D2 reverses this preference by at least a factor of 10. Reactions with CH2D 2 prepared in a local mode containing two quanta in one C-H oscillator |2000>- or in a local mode containing one quantum each in two C-H oscillators |1100> lead to products with significantly different rotational, vibrational, and angular distributions, although the vibrational energy for each mode is nearly identical. These measurements represent the first example of mode selectivity observed in a differential cross section, and they demonstrate that vibrational excitation can be used to direct the reaction pathway of the Cl+CH2D2 reaction. Our results indicate that reagent stretch-excitation enhances the reaction rate by localizing energy along the reaction coordinate, thereby widening the cone of acceptance and allowing reactive collisions at high impact parameter. The chlorine atom appears to react with a single C-H oscillator, while the methyl radical is largely a spectator throughout the course of the reaction. Moreover, our results are consistent with a model in which the impact parameter governs the angular distributions of the product.

Bechtel, Hans A.

23

Oxidant sensing by reversible disulfide bond formation.  

PubMed

Maintenance of the cellular redox balance is crucial for cell survival. An increase in reactive oxygen, nitrogen, or chlorine species can lead to oxidative stress conditions, potentially damaging DNA, lipids, and proteins. Proteins are very sensitive to oxidative modifications, particularly methionine and cysteine residues. The reversibility of some of these oxidative protein modifications makes them ideally suited to take on regulatory roles in protein function. This is especially true for disulfide bond formation, which has the potential to mediate extensive yet fully reversible structural and functional changes, rapidly adjusting the protein's activity to the prevailing oxidant levels. PMID:23861395

Cremers, Claudia M; Jakob, Ursula

2013-07-16

24

Resin Bonding to Oxide Ceramics for Dental Restorations  

Microsoft Academic Search

Bonding to silica-based dental ceramics using etching techniques and silanization is well established, but for bonding of dental oxide ceramics with little or no silica no particular method is generally accepted. Therefore, the purpose of this review is to summarize the current knowledge on bonding to dental oxide ceramics. Approved bonding methods to dental oxide ceramics require first cleaning, roughening

Matthias Kern

2009-01-01

25

Bond ionicities of high-Tc oxides  

NASA Astrophysics Data System (ADS)

A semiempirical method for the calculation of the bond ionicity, which is applicable even to extremely anisotropic systems such as copper-oxide superconductors, is proposed as a generalization of the Phillips-Van Vechten-Levine scheme. The value of the ionicity calculated for oxides generally tends to increase as the crystal strain becomes more tensile. It is characteristic of cuprate superconductors that the values of the ionicity are high compared with other 3d transition-metal oxides. Also significant are the extremely high ionicities in the direction normal to the CuO2 planes and the relatively high covalencies of the intraplanar bonds. These crystal-chemical characteristics may be intimately related to the remarkable insulator-to-metal transition and the associated high-Tc superconductivity in the layered copper-oxide systems.

Tanaka, Shigenori

1994-02-01

26

Cyclic oligomer of oxide clusters through a siloxane bond. Synthesis and structure of reaction products of alpha2-mono-lacunary Dawson polyoxometalate with tetrachlorosilane and tetraethoxysilane.  

PubMed

The title compound with the formula (Me2NH2)13H5[{alpha2-P2W17O61(Si2O)}3(micro-O)3].39H2O Me2NH(2)-1 was obtained in 12.8% (0.258 g scale) yield by a 1:2 molar ratio reaction of alpha2-mono-lacunary Dawson polyoxometalate (POM) [alpha2-P2W17O61]10- with SiCl4 in water and unequivocally characterized by complete elemental analysis, TG/DTA, FTIR, (solution and solid-state) 31P NMR, solid-state 29Si NMR and X-ray crystallography. [Note: the moieties of the polyoxoanions in Me2NH(2)-1 and Me2NH(2)-2 are abbreviated simply as 1 and 2, respectively.] X-Ray crystallography revealed that the molecular structure of polyoxoanion 1 was a cyclic trimer consisting of three alpha2-mono-lacunary Dawson POM subunits constituted through intra- and intermolecular siloxane bonds. The compound resulted in an approximate Cs symmetry, but not the idealized C3v symmetry, because one Dawson unit was unequivalently incorporated into the cyclic framework. This compound is the first example of a siloxane-bonding cyclic trimer of POMs and its formation suggests that the oxygen atoms in the lacunary site of POM are reactive just like the surface oxygen atoms of silica. The formation of the cyclic trimer was also clarified by two control experiments: (1) the reaction of [alpha2-P2W17O61]10- with Si(OEt)4 under acidic conditions (pH 1), and (2) oligomerization under 6 M aqueous HCl acidic conditions (pH approximately 0) of the reactive monomer units (RMUs: tentatively assigned as "[alpha2-P2W17O61{O(SiOH)2}]6-"), derived from the cleavage of Ph-Si bonds by the thermal degradation of [alpha2-P2W17O61{O(SiPh)2}]6-. On the other hand, the reactions of alpha2-mono-lacunary Dawson POMs with GeX4 (X=OEt, Cl) gave the monomeric species of mono-germanium-substituted Dawson POM (Me2NH2)7[alpha2-P2W17O61(GeOH)].5H2O Me2NH(2)-2, which was characterized by CHN analysis, TG/DTA, FTIR, X-ray crystallography, and (solid-state and solution) 31P NMR spectroscopy. X-ray crystallography revealed that compound Me2NH(2)-2 was a monomeric species, but the Ge site per formula unit was not determined due to disorder of alpha-Dawson structure. PMID:19587998

Kurashina, Takayuki; Aoki, Shotaro; Hirasawa, Ryota; Hasegawa, Takeshi; Kasahara, Yuhki; Yoshida, Shoko; Yoza, Kenji; Nomiya, Kenji

2009-06-05

27

Ceramic oxide bonds using calcium aluminosilicate glasses  

Microsoft Academic Search

Glasses with compositions falling in the Ca-Al-Si-O system are used to bond oxide ceramics. Alumina, zirconia and an alumina-zirconia\\u000a composite are used as ceramic components. Joints are obtained at temperatures slightly higher than the melting temperature\\u000a of glass which, for the selected compositions, change from 1210 to 1410C. The degree of interaction between the ceramic components\\u000a and the glass interlayer

L. Esposito; A. Bellosi

2005-01-01

28

Hydrodenitrogenation chemistry. I. Cleavage of alkylcarbon-nitrogen bonds, methane and ammonia formation in the HDN reaction of 1,2,3,4-tetrahydroquinoline with a nickel oxide catalyst supported on silica/alumina  

SciTech Connect

The hydrodenitrogenation reaction (HDN) is one of the most important industrial processes used in the refining of petroleum feedstocks and involves the removal of the nitrogen atom, as ammonia, from polynuclear heteroaromatic nitrogen compounds at high temperatures and high pressures of hydrogen gas (350-500/sup 0/C and 2000 psi). It is interesting to note that most of the reported heterogeneous catalysts require the complete hydrogenation of both the nitrogen heterocyclic ring and the aromatic ring before carbon-nitrogen bond cleavage can occur. A major breakthrough in the technical and economic aspects of the HDN reaction would take place if, in fact a catalyst could be found that would selectively cleave the C-N bond din the saturated nitrogen ring and subsequently produce ammonia, without substantial reduction of the aromatic rings, at lower temperatures as well as lower pressures of hydrogen gas. In this note, the authors report on a highly loaded nickel oxide catalyst (50% by weight Ni), supported on silica/alumina, that will effectively provide some of the criteria for an ideal HDN catalyst. 8 references.

Fish, R.H.; Thormodsen, A.D.; Moore, R.S.; Perry, D.L.; Heinemann, H.

1986-11-01

29

Interface-confined oxide nanostructures for catalytic oxidation reactions.  

PubMed

Heterogeneous catalysts, often consisting of metal nanoparticles supported on high-surface-area oxide solids, are common in industrial chemical reactions. Researchers have increasingly recognized the importance of oxides in heterogeneous catalysts: that they are not just a support to help the dispersion of supported metal nanoparticles, but rather interact with supported metal nanoparticles and affect the catalysis. The critical role of oxides in catalytic reactions can become very prominent when oxides cover metal surfaces forming the inverse catalysts. The source of the catalytic activity in homogeneous catalysts and metalloenzymes is often coordinatively unsaturated (CUS) transition metal (TM) cations, which can undergo facile electron transfer and promote catalytic reactions. Organic ligands and proteins confine these CUS cations, making them highly active and stable. In heterogeneous catalysis, however, confining these highly active CUS centers on an inorganic solid so that they are robust enough to endure the reaction environment while staying flexible enough to perform their catalysis remains a challenge. In this Account, we describe a strategy to confine the active CUS centers on the solid surface at the interface between a TM oxide (TMO) and a noble metal (NM). Among metals, NMs have high electron negativity and low oxygen affinity. This means that TM cations of the oxide bind strongly to NM atoms at the interface, forming oxygen-terminated-bilayer TMO nanostructures. The resulting CUS sites at the edges of the TMO nanostructure are highly active for catalytic oxidation reactions. Meanwhile, the strong interactions between TMOs and NMs prevent further oxidation of the bilayer TMO phases, which would otherwise result in the saturation of oxygen coordination and the deactivation of the CUS cations. We report that we can also tune the oxide-metal interactions to modulate the bonding of reactants with CUS centers, optimizing their catalytic performance. We review our recent progress on oxide-on-metal inverse catalysts, mainly the TMO-on-Pt (TM = Fe, Co, and Ni) systems and discuss the interface-confinement effect, an important factor in the behavior of these catalytic systems. We have studied both model catalyst systems and real supported nanocatalysts. Surface science studies and density functional theory calculations in model systems illustrate the importance of the oxide-metal interfaces in the creation and stabilization of surface active centers, and reveal the reaction mechanism at these active sites. In real catalysts, we describe facile preparation processes for fabricating the oxide-on-metal nanostructures. We have demonstrated excellent performance of the inverse catalysts in oxidation reactions such as CO oxidation. We believe that the interface confinement effect can be employed to design highly efficient novel catalysts and that the inverse oxide-on-metal catalysts may find wide applications in heterogeneous catalysis. PMID:23458033

Fu, Qiang; Yang, Fan; Bao, Xinhe

2013-03-04

30

Tannin oxidation: intra- versus intermolecular reactions.  

PubMed

Grape and apple condensed tannin fractions were autoxidized at high concentrations (5 g/L) in aqueous solutions and analyzed by thiolysis (depolymerization followed by HPLC analysis) and small angle X-ray scattering (SAXS). Structural parameters of native (unoxidized) tannin polymers were derived from SAXS according to the wormlike chain model: the length per monomer is 15 A, the length of the statistical segment 17 A, and the cross section of the macromolecule has a radius within the range 3-4.5 A. The rather short length of the statistical segment is an effect of the different location of interflavanol linkages, which cause a loss of orientational correlation between successive monomers. Oxidation created new bonds that were resistant to thiolysis, and, according to thiolysis, some of these new bonds were intramolecular. However, according to SAXS, oxidation at high tannin concentration caused the weight average degree of polymerization to increase, indicating that intermolecular reactions took place as well, creating larger macromolecules. In the case of the smaller grape seed tannins, these intermolecular reactions took place "end to end" leading to the formation of longer linear macromolecules, at least in the earlier stages of oxidation. In the case of the larger apple tannins, the SAXS patterns were characteristic of larger branched macromolecules. Accordingly, the intermolecular reactions were mainly "end to middle". This is in agreement with the higher probabilities of "end to middle" reactions arising from a higher ratio extension unit/terminal unit in the latter case. PMID:20831276

Poncet-Legrand, Cline; Cabane, Bernard; Bautista-Ortn, Ana-Beln; Carrillo, Stphanie; Fulcrand, Hlne; Prez, Javier; Vernhet, Aude

2010-09-13

31

Cu-catalyzed cross-dehydrogenative coupling: A versatile strategy for C-C bond formations via the oxidative activation of sp3 C-H bonds  

PubMed Central

Cu-catalyzed cross-dehydrogenative coupling (CDC) methodologies were developed based on the oxidative activation of sp3 CH bonds adjacent to a nitrogen atom. Various sp, sp2, and sp3 CH bonds of pronucleophiles were used in the Cu-catalyzed CDC reactions. Based on these results, the mechanisms of the CDC reactions also are discussed.

Li, Zhiping; Bohle, D. Scott; Li, Chao-Jun

2006-01-01

32

Cross-Dehydrogenative Coupling Reactions of sp 3 -Hybridized CH Bonds  

Microsoft Academic Search

\\u000a New methodologies in cross-coupling reaction using CH bonds as substrates is of great interest due to the challenges associated\\u000a with CH bond activation and the potential to streamline synthesis by the elimination of pre-activation of coupling reagents.\\u000a In this chapter, recent developments in oxidative cross-coupling reactions will be presented with the focus on the functionalization\\u000a of sp3 CH bonds with

Woo-Jin Yoo; Chao-Jun Li

2010-01-01

33

Low temperature silicon direct bonding for application in micromechanics: bonding energies for different combinations of oxides  

Microsoft Academic Search

Plain or structured hydrophillic silicon wafers covered with native oxide or with thermally grown oxide layers have been directly bonded at room temperature; afterwards, the samples were annealed at 100C to 400C. There is a significant difference in the observed bonding energy depending on the wafer pairing chosen. If one or both wafers are covered with a native oxide layer,

Gertrud Kruter; Andreas Schumacher; Ulrich Gsele

1998-01-01

34

Manganese(III)-promoted reactions for formation of carbonheteroatom bonds  

Microsoft Academic Search

Manganese(III) reagent is an important one- electron oxidant for initiation of free-radical reactions and formation of carboncarbon\\u000a bonds. The reactions are usually carried out under mild conditions, have high regio- and stereo-selectivities, and good functional\\u000a group tolerance. Summarized in this article are the developments of manganese (III)-based reactions for the formation of carbonheteroatom\\u000a bonds including carbonoxygen, carbonphosphorus, carbonsulfur, and carbonnitrogen

Xiang-Qiang Pan; Jian-Ping Zou; Wei Zhang

2009-01-01

35

?-Bond maximization of graphene in hydrogen addition reactions  

NASA Astrophysics Data System (ADS)

Thermodynamic stability of graphene hydrides increases in an approximately linear way with the numbers of ?-bonds they contain. Thus, ?-bond maximization is the primary driving force for hydrogen addition reactions of graphene. The previously reported thermal preference of sp2/sp3-phase separation of graphene hydrides is a straightforward effect of ?-bond maximization. Although not well applicable to hydroxylation and epoxidation, the ?-bond maximization principle also holds approximately for the fluorination reactions of graphene. The findings can be used to help locate the lowest-energy structures for graphene hydrides and to estimate the hydrogenation energy without first-principles calculations.Thermodynamic stability of graphene hydrides increases in an approximately linear way with the numbers of ?-bonds they contain. Thus, ?-bond maximization is the primary driving force for hydrogen addition reactions of graphene. The previously reported thermal preference of sp2/sp3-phase separation of graphene hydrides is a straightforward effect of ?-bond maximization. Although not well applicable to hydroxylation and epoxidation, the ?-bond maximization principle also holds approximately for the fluorination reactions of graphene. The findings can be used to help locate the lowest-energy structures for graphene hydrides and to estimate the hydrogenation energy without first-principles calculations. Electronic supplementary information (ESI) available: Computational details, Figures, energy and Cartesian coordinates of graphene hydrides and epoxides. See DOI: 10.1039/c1nr11048a

Gao, Xingfa; Zhao, Yuliang; Liu, Bo; Xiang, Hongjun; Zhang, Shengbai B.

2012-02-01

36

The Quantum Mechanics of Chemical Reactions Involving Conjugate Double Bonds  

Microsoft Academic Search

The various theories of conjugate double bonds are discussed on the basis of quantum mechanics. The potential energy surfaces for the addition of diatomic molecules to such bonds are calculated by the generalized Heitler-London method. It is shown that various mechanisms are possible for such reactions and that such effects as the steric repulsions of the various inactive groups in

Henry Eyring; Albert Sherman; George E. Kimball

1933-01-01

37

Photochemistry of Intermolecular C-H Bond Activation Reactions.  

National Technical Information Service (NTIS)

The major goal of this project has been to gain photophysical and photochemical insight as to why transition-metal organometallic systems facilitate intermolecular C-H bond activation reactions with various hydrocarbon substrates. The emphasis of the rese...

A. J. Lees

2000-01-01

38

Reactions of actinide ions with ethylene oxide.  

PubMed

Naked and oxo-ligated actinide (An) monopositive ions were reacted with ethylene oxide, cyclo-C(2)H(4)O (EtO). Along with An = U, Np, Pu and Am, ions of two lanthanide (Ln) elements, Ln = Tb and Tm, were studied for comparison. Metal and metal oxide ions, M(+), MO(+) and MO(2)(+), were generated by laser ablation and immediately reacted with EtO. Unreacted and product ions were detected by time-of-flight mass spectrometry. It was apparent that the overall reaction cross-sections decreased in the order U(+) > or = Np(+) > Pu(+) > Am(+). A primary reaction channel for each studied metal was the formation of MO(+) from M(+), in accord with the expected exothermicity of oxygen abstraction from EtO. For U, Np and Pu, the dioxides were also major products, indicating OAn(+)--O dissociation energies of at least 350 kJ mol(-1), the energy required for O-atom abstraction from EtO. For Am, Tb and Tm, the dioxides were only very minor products, reflecting the stabilities of the trivalent states and resistance to oxidation to higher valence states; the structures/bonding in these MO(2)(+) are intriguing given that the formal pentavalent bonding state is effectively unattainable. It was demonstrated that EtO, unlike more thermochemically favorable but kinetically restricted O-donors, is effective at achieving facile oxidation of actinide metal ions to the monoxide, and to the dioxide if the second O-abstraction reaction is exothermic. Several intriguing minor products were also identified, most of which incorporate metal--oxygen bonding and are attributed to the oxophilicity of the f-block elements; the contrast to the behavior of first-row d-block transition elements is striking in this regard. Particularly noteworthy was the formation of MH(4)(+) (and MOH(4)(+)), evidently via abstraction of all four H atoms from a single C(2)H(4)O molecule; the structures/bonding in these novel 'hydride' species are indeterminate and warrant further attention. PMID:11312520

Gibson, J K

2001-03-01

39

Oxidative 1,2-difunctionalization of activated alkenes with benzylic C(sp(3))-H bonds and aryl C(sp(2))-H bonds.  

PubMed

DTBP (di-tert-butyl peroxide) is utilized to mediate oxidative 1,2-difunctionalization of activated alkenes with an aryl C(sp(2))-H bond and a benzylic C(sp(3))-H bond for the synthesis of functionalized oxindoles. This reaction is a new organomediated strategy for alkene difunctionalization facilitated by Lewis acids. PMID:24113153

Zhou, Ming-Bo; Wang, Cheng-Yong; Song, Ren-Jie; Liu, Yu; Wei, Wen-Ting; Li, Jin-Heng

2013-10-24

40

Bonding of chlorophenols on iron and aluminum oxides  

SciTech Connect

The adsorption of 10 chlorophenols on synthetic, naturally occurring iron and aluminum oxides was studied to elucidate the mechanism of binding and relative bond strength of the chlorine-substituted phenols was identified by spectroscopic methods. Chlorophenolates were found to be chemisorbed on oxide surfaces via an inner-sphere coordination. Chlorophenols also bonded on oxides by weak physical forces (H bonding and condensation), but these types of weak bonding were identified only when adsorption occurred from the vapor phase onto dry surfaces. Physisorbed chlorophenols, unlike chemisorbed molecules, were readily removed from oxide surfaces by washing with water. Poorly crystallized iron and aluminum oxides showed similar mechanisms of chlorophenol binding, although the bond for chlorophenolate chemisorbed on iron oxide was stronger than that on aluminum oxide. Only physically adsorbed chlorophenols were detected on crystalline gibbsite, suggesting that the dominant (001) crystal face, with surface hydroxyl groups doubly coordinated to Al, was not specifically reactive with the chlorophenols. Chemisorption, however, was identified on the crystalline iron, geothite. From the extent of perturbation of aromatic ring electrons, the surface bond strength for chlorophenolates on aluminum oxide was found to correlate with the Lewis basicity of the phenolate anions (the higher the pK{sub {alpha}} of the chlorophenols, the stronger the surface bond). Nevertheless, the amount of chlorophenol adsorbed on noncrystalline iron oxide at controlled pH of 5.4 was limited by the extent of deprotonation (the lower the pK{sub a}, the more adsorption).

Kung, K.S.; McBride, M.B. (Cornell Univ., Ithaca, NY (United States))

1991-04-01

41

A Nanoleakage Perspective on Bonding to Oxidized Dentin  

Microsoft Academic Search

The mechanism responsible for sodium-hypochlorite-induced reduction in dentin bond strength and its reversal with reducing agents is unknown. This study examined the relationship between nanoleakage and reversal of compromised bonding to oxidized dentin. Acid-etched dentin was completely depleted of demineralized collagen matrix when sodium hypochlorite was used. Specimens were bonded with two single-bottle dentin adhesives. They were immersed in ammoniacal

C. K. Y. Yiu; F. Garca-Godoy; F. R. Tay; D. H. Pashley; S. Imazato; N. M. King; S. C. N. Lai

2002-01-01

42

Oxidation and Reduction Reactions in Organic Chemistry  

ERIC Educational Resources Information Center

|A variety of approaches to the concept of oxidation and reduction appear in organic textbooks. The method proposed here is different than most published approaches. The oxidation state is calculated by totaling the number of heterogeneous atoms, [pi]-bonds, and rings. A comparison of the oxidation states of reactant and product determine what

Shibley, Ivan A., Jr.; Amaral, Katie E.; Aurentz, David J.; McCaully, Ronald J.

2010-01-01

43

Phase Evolution in Boride-Based Cermets and Reaction Bonding onto Plain Low Carbon Steel Substrate  

NASA Astrophysics Data System (ADS)

Reaction sinter bonding is a process that aims to bond two materials for improvement in properties through reactive sintering technique. The process has been effectively used to sinter hard materials like borides in situ which not only possess excellent oxidation resistance, good corrosion resistance but also resistant to abrasive wear. Sinter bonding is a unique surface modification process achieved through powder metallurgy and is competent with other techniques like boronizing sintering and sinter-brazing since it eliminates the additional operations of heat treatment and assembly and removes the inherent setbacks with these processes. This study focuses on identifying the phase evolution mechanism using characterization tools like x-ray diffractometry and energy dispersive spectroscopy and study of sinter bonding of the boron containing precursors (Mo-Cr-Fe-Ni-FeB-MoB) onto plain carbon steel. A microstructure containing Fe-based matrix dispersed with complex borides develops with temperature in the tape cast sheets. A fivefold increase in hardness between plain carbon steel in wrought condition and sinter bonded steel was observed. The multilayer consisted of a reaction zone adjacent to the interface and was investigated with the composition profile and hardness measurements. A model of sinter bonding between the cermet and the steel has also been proposed.

Palanisamy, B.; Upadhyaya, A.

2012-04-01

44

Theoretical investigation of the reaction of Mn+ with ethylene oxide.  

PubMed

The potential energy surfaces of Mn(+) reaction with ethylene oxide in both the septet and quintet states are investigated at the B3LYP/DZVP level of theory. The reaction paths leading to the products of MnO(+), MnO, MnCH(2)(+), MnCH(3), and MnH(+) are described in detail. Two types of encounter complexes of Mn(+) with ethylene oxide are formed because of attachments of the metal at different sites of ethylene oxide, i.e., the O atom and the CC bond. Mn(+) would insert into a C-O bond or the C-C bond of ethylene oxide to form two different intermediates prior to forming various products. MnO(+)/MnO and MnH(+) are formed in the C-O activation mechanism, while both C-O and C-C activations account for the MnCH(2)(+)/MnCH(3) formation. Products MnO(+), MnCH(2)(+), and MnH(+) could be formed adiabatically on the quintet surface, while formation of MnO and MnCH(3) is endothermic on the PESs with both spins. In agreement with the experimental observations, the excited state a(5)D is calculated to be more reactive than the ground state a(7)S. This theoretical work sheds new light on the experimental observations and provides fundamental understanding of the reaction mechanism of ethylene oxide with transition metal cations. PMID:22148680

Li, Yuanyuan; Guo, Wenyue; Zhao, Lianming; Liu, Zhaochun; Lu, Xiaoqing; Shan, Honghong

2011-12-22

45

Transfer of semiconductor and oxide films by wafer bonding and layer cutting  

NASA Astrophysics Data System (ADS)

Material integration by wafer bonding and layer transfer is one of the main approaches to increase functionality of semiconductor devices and to enhance integrated circuits (IC) performance. Even though most mismatches such as different lattice constants betweeen bonding materials present no obstacle for wafer direct bonding, thermal stresses caused by thermal mismatches must be minimized by low temperature bonding to avoid debonding, sliding or cracking. In order to achieve a strong bond at low temperatures, two approaches may be adopted: 1) Bonding at room temperature by hydrogen bonding of OH, NH, or FH terminated surfaces followed by polymerization to form covalent bonds. Within this approach the key is to remove the by-products of the reaction at the bonding interface. 2) Direct formation of a covalent bond between clean surfaces without adsorbents in ultra high vacuum conditions. Low temperature bonding allows bonding processed wafers for technology integration. Layer transfer requires uniform thinning of one wafer of a bonded pair. The most promising technology involves a buried embrittled region by hydrogen implantation. A layer with a thickness corresponding to the hydrogen implantation depth is then transferred onto a bonded desired substrate by either splitting due to internal gas pressure or by forced peeling as long as the bonding energy is higher than the fracture energy in the embrittled region at the layer transfer temperature. This approach is quite generic in nature and may be applied to almost all materials. We have found that B+H co-implantation and/or H implantation at high temperatures can significantly lower the splitting temperature. However, the wafer temperature during H implantation has to be within a temperature window that is specific for each material. The experimentally determined temperature windows for some semiconductors and single crystalline oxides will be given.

Tong, Qin-Yi; Huang, Li-Juan; Gsele, Ulich M.

2000-07-01

46

CH bond functionalizations with palladium(II): intramolecular oxidative annulations of arenes  

Microsoft Academic Search

Oxidative annulations for the synthesis of carbocycles were developed using a catalytic palladium(II) system. Indoles with pendant olefin tethers were oxidatively cyclized to form annulated products. Electron-rich aromatic systems were also investigated, culminating in the synthesis of benzofurans and dihydrobenzofurans by a similar protocol. These reactions were demonstrated to proceed by an initial CH bond functionalization event, followed by olefin

Eric M. Ferreira; Haiming Zhang; Brian M. Stoltz

2008-01-01

47

Rhodium catalyzed chelation-assisted C-H bond functionalization reactions.  

PubMed

Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon-carbon and carbon-heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required prefunctionalization to carry out these reactions adds cost and reduces the availability of the starting reagents. The use of C-H bond activation in lieu of prefunctionalization has presented a tantalizing alternative to classical cross-coupling reactions. Researchers have met the challenges of selectivity and reactivity associated with the development of C-H bond functionalization reactions with an explosion of creative advances in substrate and catalyst design. Literature reports on selectivity based on steric effects, acidity, and electronic and directing group effects are now numerous. Our group has developed an array of C-H bond functionalization reactions that take advantage of a chelating directing group, and this Account surveys our progress in this area. The use of chelation control in C-H bond functionalization offers several advantages with respect to substrate scope and application to total synthesis. The predictability and decreased dependence on the inherent stereoelectronics of the substrate generally result in selective and high yielding transformations with broad applicability. The nature of the chelating moiety can be chosen to serve as a functional handle in subsequent elaborations. Our work began with the use of Rh(I) catalysts in intramolecular aromatic C-H annulations, which we further developed to include enantioselective transformations. The application of this chemistry to the simple olefinic C-H bonds found in ?,?-unsaturated imines allowed access to highly substituted olefins, pyridines, and piperidines. We observed complementary reactivity with Rh(III) catalysts and developed an oxidative coupling with unactivated alkenes. Further studies on the Rh(III) catalysts led us to develop methods for the coupling of C-H bonds to polarized ? bonds such as those in imines and isocyanates. In several cases the methods that we have developed for chelation-controlled C-H bond functionalization have been applied to the total synthesis of complex molecules such as natural products, highlighting the utility of these methods in organic synthesis. PMID:22148885

Colby, Denise A; Tsai, Andy S; Bergman, Robert G; Ellman, Jonathan A

2011-12-08

48

Rhodium catalyzed chelation-assisted C-H bond functionalization reactions  

PubMed Central

Conspectus Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon-carbon and carbon-heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required pre-functionalization to carry out these reactions adds cost and reduces the availability of the starting reagents. The use of C-H bond activation in lieu of pre-functionalization has presented a tantalizing alternative to classical cross-coupling reactions. Researchers have met the challenges of selectivity and reactivity associated with the development of C-H bond functionalization reactions with an explosion of creative advances in substrate and catalyst design. Literature reports on selectivity based on steric effects, acidity, and electronic and directing group effects are now numerous. Our group has developed an array of C-H bond functionalization reactions that take advantage of a chelating directing group, and this Account surveys our progress in this area. The use of chelation control in C-H bond functionalization offers several advantages with respect to substrate scope and application to total synthesis. The predictability and decreased dependence on the inherent stereoelectronics of the substrate generally result in selective and high yielding transformations with broad applicability. The nature of the chelating moiety can be chosen to serve as a functional handle in subsequent elaborations. Our work began with the use of Rh(I) catalysts in intramolecular aromatic C-H annulations, which we further developed to include enantioselective transformations. The application of this chemistry to the simple olefinic C-H bonds found in ?,?-unsaturated imines allowed access to highly substituted olefins, pyridines, and piperidines. We observed complementary reactivity with Rh(III) catalysts and developed an oxidative coupling with unactivated alkenes. Further studies on the Rh(III) catalysts led us to develop methods for the coupling of C-H bonds to polarized ? bonds such as those in imines and isocyanates. In several cases the methods that we have developed for chelation-controlled C-H bond functionalization have been applied to the total synthesis of complex molecules such as natural products, highlighting the utility of these methods in organic synthesis.

Colby, Denise A.; Tsai, Andy S.; Bergman, Robert G.; Ellman, Jonathan A.

2011-01-01

49

Uranium Oxidation: Characterization of Oxides Formed by Reaction with Water.  

National Technical Information Service (NTIS)

Three different uranium oxide samples have been characterized with respect to the different preparation techniques. Results show that the water reaction with uranium metal occurs cyclically forming laminar layers of oxide which spall off due to the strain...

E. L. Fuller J. B. Condon M. H. Eager N. R. Smyrl

1983-01-01

50

Transient liquid phase bonding of ferritic oxide dispersion strengthened alloys  

NASA Astrophysics Data System (ADS)

Oxide dispersion strengthened (ODS) alloys possess excellent properties including resistance to oxidation, corrosion, creep and thermal fatigue. In addition, ferritic ODS alloys exhibit resistance to void swelling and are of particular interest to the nuclear industry. The present study involves the joining of fuel cans to end caps that will be utilized in the nuclear industry. Mechanically alloyed (MA) ODS alloys possess coarse columnar grain structure strengthened with nanosize yttria dispersoids. In that past, fusion welding techniques resulted in microstructural disruption leading to poor joints. This work investigated joining of two ferritic MA ODS alloys, MA956 and PM2000, using; (a) Transient liquid phase (TLP) bonding and (b) Solid-state diffusion bonding. TLP bonds were prepared with MA956 and PM2000 in the unrecrystallized and recrystallized conditions using electron beam physical vapor deposited (EBPVD) boron thin films as interlayers. The use of thin interlayers reduced the amount of substrate dissolution and minimized the bondline microstructural disruption. Different bond orientations were also investigated. Successful bonds with better microstructural continuity were obtained when substrates were joined in the unrecrystallized condition followed by post bond recrystallization heat treatment with the substrate faying surface aligned along the working (extrusion or rolling) direction than when substrates were aligned perpendicular to the working direction. This was attributed to the number of yttria stringers cut by the bondline, which is less when the substrate faying surface is lying parallel to the working direction than when the substrate faying surface is lying perpendicular to the working direction. Solid-state diffusion bonding was conducted using MA956 and PM2000 in the unrecrystallized and recrystallized conditions. Bonding occurred only when an unrecrystallized substrate was involved. Bonding occurred at unusually low stresses. This may be attributed to the grain boundary diffusion, owing to submicron grain size of the unrecrystallized substrates. Post bond heat treatment was conducted in order to induce recrystallization in the bonds. Room temperature mechanical testing was conducted on the bonds and the bulk. Bond shear strengths and tensile strengths of up to 80% and 110% of bulk, respectively, were obtained. Defects in the bulk material such as porosity and unwanted fine grain formation were observed. Pore formation at the bondline during post bond heat treatment seems to decrease the bond strength. These defects were attributed to prior thermomechanical history of the materials.

Krishnardula, Venu Gopal

51

Isolation, Characterization of an Intermediate in an Oxygen Atom-Transfer Reaction, and the Determination of the Bond Dissociation Energy  

SciTech Connect

Redox reactions coupled with the formal loss or gain of an oxygen atom are ubiquitous in chemical processes. Such reactions proceed through the reduction of the donor center (XO) and the oxidation of the acceptor (Y) molecule. Among many examples of the metal centered oxygen atom transfer (OAT) reactivity, those involving molybdenum complexes have been widely investigated due to their involvement in mononuclear molybdenum enzymes. The heat of reaction of the overall atom transfer process can be expressed as a difference between the bond dissociation energies (BDEs) of the oxygen-donor(X) and oxygen-acceptor(Y) bond, i.e., H=DX=o-DY=O.

Nemykin, Victor N.; Laskin, Julia; Basu, Partha

2004-07-19

52

Imparting catalyst control upon classical palladium-catalyzed alkenyl C-H bond functionalization reactions.  

PubMed

The functional group transformations carried out by the palladium-catalyzed Wacker and Heck reactions are radically different, but they are both alkenyl C-H bond functionalization reactions that have found extensive use in organic synthesis. The synthetic community depends heavily on these important reactions, but selectivity issues arising from control by the substrate, rather than control by the catalyst, have prevented the realization of their full potential. Because of important similarities in the respective selectivity-determining nucleopalladation and ?-hydride elimination steps of these processes, we posit that the mechanistic insight garnered through the development of one of these catalytic reactions may be applied to the other. In this Account, we detail our efforts to develop catalyst-controlled variants of both the Wacker oxidation and the Heck reaction to address synthetic limitations and provide mechanistic insight into the underlying organometallic processes of these reactions. In contrast to previous reports, we discovered that electrophilic palladium catalysts with noncoordinating counterions allowed for the use of a Lewis basic ligand to efficiently promote tert-butylhydroperoxide (TBHP)-mediated Wacker oxidation reactions of styrenes. This discovery led to the mechanistically guided development of a Wacker reaction catalyzed by a palladium complex with a bidentate ligand. This ligation may prohibit coordination of allylic heteroatoms, thereby allowing for the application of the Wacker oxidation to substrates that were poorly behaved under classical conditions. Likewise, we unexpectedly discovered that electrophilic Pd-?-alkyl intermediates are capable of distinguishing between electronically inequivalent C-H bonds during ?-hydride elimination. As a result, we have developed E-styrenyl selective oxidative Heck reactions of previously unsuccessful electronically nonbiased alkene substrates using arylboronic acid derivatives. The mechanistic insight gained from the development of this chemistry allowed for the rational design of a similarly E-styrenyl selective classical Heck reaction using aryldiazonium salts and a broad range of alkene substrates. The key mechanistic findings from the development of these reactions provide new insight into how to predictably impart catalyst control in organometallic processes that would otherwise afford complex product mixtures. Given our new understanding, we are optimistic that reactions that introduce increased complexity relative to simple classical processes may now be developed based on our ability to predict the selectivity-determining nucleopalladation and ?-hydride elimination steps through catalyst design. PMID:22111756

Sigman, Matthew S; Werner, Erik W

2011-11-23

53

Imparting Catalyst-Control upon Classical Palladium-Catalyzed Alkenyl C-H Bond Functionalization Reactions  

PubMed Central

Conspectus The functional group transformations carried out by the palladium-catalyzed Wacker and Heck reactions are radically different, but they are both alkenyl C-H bond functionalization reactions that have found extensive use in organic synthesis. The synthetic community depends heavily on these important reactions, but selectivity issues arising from control by the substrate, rather than control by the catalyst, have prevented the realization of their full potential. Because of important similarities in the respective selectivity-determining nucleopalladation and ?-hydride elimination steps of these processes, we posit that the mechanistic insight garnered through the development of one of these catalytic reactions may be applied to the other. In this Account, we detail our efforts to develop catalyst-controlled variants of both the Wacker oxidation and the Heck reaction to address synthetic limitations and provide mechanistic insight into the underlying organometallic processes of these reactions. In contrast to previous reports, we discovered that electrophilic palladium catalysts with non-coordinating counterions allowed for the use of a Lewis basic ligand to efficiently promote TBHP-mediated Wacker oxidation reactions of styrenes. This discovery led to the mechanistically guided development of a Wacker reaction catalyzed by a palladium complex with a bidentate ligand. This ligation may prohibit coordination of allylic heteroatoms, thereby allowing for the application of the Wacker oxidation to substrates that were poorly behaved under classical conditions. Likewise, we unexpectedly discovered that electrophilic Pd-?-alkyl intermediates are capable of distinguishing between electronically inequivalent CH bonds during ?-hydride elimination. As a result, we have developed E-styrenyl selective oxidative Heck reactions of previously unsuccessful electronically non-biased alkene substrates using arylboronic acid derivatives. The mechanistic insight gained from the development of this chemistry allowed for the rational design of a similarly E-styrenyl selective classical Heck reaction using aryldiazonium salts and a broad range of alkene substrates. The key mechanistic findings from the development of these reactions provide new insight into how to predictably impart catalyst control in organometallic processes that would otherwise afford complex product mixtures. Given our new understanding, we are optimistic that reactions that introduce increased complexity relative to simple classical processes may now be developed based on our ability to predict the selectivity-determining nucleopalladation and ?-hydride elimination steps through catalyst design.

Sigman, Matthew S.; Werner, Erik W.

2011-01-01

54

Laccase-catalyzed carboncarbon bond formation: oxidative dimerization of salicylic esters by air in aqueous solution  

Microsoft Academic Search

The laccase catalyzed oxidative dimerization of salicylic esters, a rare example of a laccase-catalyzed carboncarbon bond formation, was studied. This reaction allows the use of air as stoichiometric oxidant and proceeds in aqueous solution. The preparative scope and the mechanism of the method, which provides a new and convenient access to functionalized biaryls under mild conditions, were investigated.

Sabine Ciecholewski; Elke Hammer; Katrin Manda; Gopal Bose; Van T. H. Nguyen; Peter Langer; Frieder Schauer

2005-01-01

55

Identification of products containing {single_bond}COOH, {single_bond}OH, and {single_bond}C{double_bond}O in atmospheric oxidation of hydrocarbons  

SciTech Connect

Atmospheric oxidation of hydrocarbons by hydroxyl radicals and ozone leads to products containing {single_bond}COOH, {single_bond}OH, and {single_bond}C{double_bond}O functional groups. The high polarity of such compounds precludes direct GC-MS analysis. In addition, many such compounds often exist in a single sample at trace levels. An analytical method has been developed to identify compounds containing one or more functional groups of carbonyl, carboxy, and hydroxy in atmospheric samples. In the method, {single_bond}C{double_bond}O groups are derivatized using O-(2,3,4,5,6-pentafluorobenzyl) hydroxy amine(PFBHA), and {single_bond}COOH and {single_bond}OH groups are derivatized using a silylation reagent N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA). The derivatives are easily resolved by a GC column. The chemical ionization mass spectra of these derivatives exhibit several pseudomolecular ions, allowing unambiguous determination of molecular weights. Functional group identification is accomplished by monitoring the ions in the electron ionization mass spectra that are characteristic of each functional group derivative: m/z 181 for carbonyl and m/z 73 and 75 for carboxyl and hydroxy groups. The method is used to identify products in laboratory studies of ozone oxidation of {alpha}-pinene and {Delta}{sup 3}-carene.

Yu, J.; Flagan, R.C.; Seinfeld, J.H. [California Inst. of Tech., Pasadena, CA (United States)

1998-08-15

56

Reversal of Compromised Bonding to Oxidized Etched Dentin  

Microsoft Academic Search

The mechanism responsible for hydrogenperoxide- or sodium-hypochlorite-induced reductions in dentin bond strength is unknown. This in vitro study tested the hypothesis that these oxidizing agents were responsible by attempting to reverse the effect with sodium ascorbate, a reducing agent. Human dentin was treated with these oxidants before or after being acid-etched and with or without post-treatment with sodium ascorbate. They

S. C. N. Lai; Y. F. Mak; G. S. P. Cheung; R. Osorio; M. Toledano; R. M. Carvalho; F. R. Tay; D. H. Pashley

2001-01-01

57

Oxidation-reduction reactions of metal ions.  

PubMed Central

Several metal or metalloid ions exist in multiple oxidation states and can undergo electron transfer reactions that are important in biological and environmental systems. There are endogenous metal ions such as iron, copper, and cobalt that participate in oxidation-reduction reactions with species of oxygen like molecular dioxygen, superoxide, and hydrogen peroxide. These reactions may be modulated by endogenous reducing agents such as glutathione, ascorbate, and tocopherol. The reactions can be described in terms of thermodynamics through the use of standard electrode potentials. A favorable reaction will depend on the concentrations of the reactants and may depend on the pH and/or on the presence of organic ligands that form complexes with the metal or metalloid. Arsenate (As(V)) can react with glutathione in buffered aqueous solutions to produce arsenite (As(III)) and oxidized glutathione. This reaction may be important in the methylation reactions of arsenic. Arsenic species can decrease the red blood cell levels of reduced glutathione, but the products of oxidation and the mechanism of oxidation are more complex than those found in water alone. Chromium (VI) is thought to interact with DNA after first reacting with a reducing agent such as glutathione to form lower oxidation states of chromium. These examples illustrate the importance of oxidation-reduction reactions for toxic metals and metalloids.

Carter, D E

1995-01-01

58

Tailoring oxidation degrees of graphene oxide by simple chemical reactions  

SciTech Connect

High quality graphene oxide (GO) with controllable degrees of oxidation was synthesized by simple chemical reactions inspired by approaches to unzip single wall carbon nanotubes using strong oxidizing agents. As compared to the conventional Hummers method, these reactions are less exo-therm involved without emission of toxic gases. The structural characteristics of the synthesized GO with various oxidation degrees were evaluated by x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy, thermal gravimetric analysis, and UV-vis-IR spectroscopy. GO with tailored degrees of oxidation displays tunable optoelectronic properties and may have a significant impact on developing graphene- or GO-based platforms for various technological applications.

Wang Gongkai [Key Laboratory for Anisotropy and Texture of Materials of Ministry of Education, Northeastern University, Shenyang, Liaoning 110004 (China); Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 (United States); Sun Xiang; Lian Jie [Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 (United States); Liu Changsheng [Key Laboratory for Anisotropy and Texture of Materials of Ministry of Education, Northeastern University, Shenyang, Liaoning 110004 (China)

2011-08-01

59

Cu-Catalyzed Esterification Reaction via Aerobic Oxygenation and C-C Bond Cleavage: An Approach to ?-Ketoesters.  

PubMed

The Cu-catalyzed novel aerobic oxidative esterification reaction of 1,3-diones for the synthesis of ?-ketoesters has been developed. This method combines C-C ?-bond cleavage, dioxygen activation and oxidative C-H bond functionalization, as well as provides a practical, neutral, and mild synthetic approach to ?-ketoesters which are important units in many biologically active compounds and useful precursors in a variety of functional group transformations. A plausible radical process is proposed on the basis of mechanistic studies. PMID:24032593

Zhang, Chun; Feng, Peng; Jiao, Ning

2013-09-26

60

Graphite Oxidation Thermodynamics/Reactions  

SciTech Connect

The vulnerability of graphite-matrix spent nuclear fuel to oxidation by the ambient atmosphere if the fuel canister is breached was evaluated. Thermochemical and kinetic data over the anticipated range of storage temperatures (200 to 400 C) were used to calculate the times required for a total carbon mass loss of 1 mgcm-2 from a fuel specimen. At 200 C, the time required to produce even this small loss is large, 900,000 yr. However, at 400 C the time required is only 1.9 yr. The rate of oxidation at 200 C is negligible, and the rate even at 400 C is so small as to be of no practical consequence. Therefore, oxidation of the spent nuclear fuel upon a loss of canister integrity is not anticipated to be a concern based upon the results of this study.

Propp, W.A.

1998-09-01

61

Oxidative Reactions with Nonaqueous Enzymes  

SciTech Connect

The objective of this work is to demonstrate a proof-of-concept of enzymatic oxidative processing in nonaqueous media using alkene epoxidation and phenolic polymerization as relevant targets. This project will provide both the fundamental and applied investigations necessary to initiate the implementation of oxidative biocatalysts as commercially relevant alternatives to chemical processing in general, and to phenolic polymerizations and alkene epoxidation specifically. Thus, this work will address the Bioprocessing Solicitation Area to: (1) makes major improvements to phenolic polymerization and alkene epoxidation technologies; (2) is expected to be cost competitive with competing conventional processes; and (3) produces higher yields with less waste.

Jonathan S. Dordick; Douglas Clark; Brian H Davison; Alexander Klibanov

2001-12-30

62

Oxidative folding of leech-derived tryptase inhibitor via native disulfide-bonded intermediates.  

PubMed

Leech-derived tryptase inhibitor (LDTI), comprising 46 residues and a fold stabilized by three disulfide bonds, is the only protein known to inhibit human beta-tryptase with high affinity. The present work examines its oxidative folding and reductive unfolding with chromatographic and disulfide analysis of the trapped intermediates. LDTI folds and unfolds through a sequential oxidation of its cysteine residues that give rise to the accumulation of a few one- and two-disulfide intermediates. Three species containing two native disulfide bonds (IIa, IIb, and IIc) are detected in LDTI folding, but only one (IIb) seems to be productive and oxidizes into the native structure. Stop/go experiments indicate that the intermediates IIa and IIc must reduce or rearrange their disulfide bonds to reach the productive route. The acquisition of the native structure is extremely fast and efficient, probably influenced by the low levels of non-native three-disulfide (scrambled) isomers occurring along the reaction. Finally, the Cys14-Cys40 disulfide bond, buried in native LDTI and formed in IIa and IIb intermediates, appears to be a key factor for both the initiation of folding and the stability of this molecule. Together, the derived data provide a molecular basis for development of new LDTI variants with altered properties. PMID:18004973

Arolas, Joan L; Bronsoms, Slvia; Aviles, Francesc X; Ventura, Salvador; Sommerhoff, Christian P

2008-01-01

63

The Proximal Hydrogen Bond Network Modulates Bacillus subtilis Nitric-oxide Synthase Electronic and Structural Properties  

PubMed Central

Bacterial nitric-oxide synthase (NOS)-like proteins are believed to be genuine NOSs. As for cytochromes P450 (CYPs), NOS-proximal ligand is a thiolate that exerts a push effect crucial for the process of dioxygen activation. Unlike CYPs, this catalytic electron donation seems controlled by a hydrogen bond (H-bond) interaction between the thiolate ligand and a vicinal tryptophan. Variations of the strength of this H-bond could provide a direct way to tune the stability along with the electronic and structural properties of NOS. We generated five different mutations of bsNOS Trp66, which can modulate this proximal H-bond. We investigated the effects of these mutations on different NOS complexes (FeIII, FeIICO, and FeIINO), using a combination of UV-visible absorption, EPR, FTIR, and resonance Raman spectroscopies. Our results indicate that (i) the proximal H-bond modulation can selectively decrease or increase the electron donating properties of the proximal thiolate, (ii) this modulation controls the ?-competition between distal and proximal ligands, (iii) this H-bond controls the stability of various NOS intermediates, and (iv) a fine tuning of the electron donation by the proximal ligand is required to allow at the same time oxygen activation and to prevent uncoupling reactions.

Brunel, Albane; Wilson, Adjele; Henry, Laura; Dorlet, Pierre; Santolini, Jerome

2011-01-01

64

Insertion reactions into Pd[bond]O and Pd[bond]N bonds: preparation of alkoxycarbonyl, carbonato, carbamato, thiocarbamate, and thioureide complexes of palladium(II).  

PubMed

Mononuclear palladium hydroxo complexes of the type [Pd(N[bond]N)(C(6)F(5))(OH)] [(N[bond]N = 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (Me(2)bipy), 1,10-phenanthroline (phen), or N,N,N',N'-tetramethylethylenediamine (tmeda)] have been prepared by reaction of [Pd(N[bond]N)(C(6)F(5))(acetone)]ClO(4) with KOH in methanol. These hydroxo complexes react, in methanol, with CO (1 atm, room temperature) to yield the corresponding methoxycarbonyl complexes [Pd(N[bond]N)(C(6)F(5))(CO(2)Me)]. Similar alkoxycarbonyl complexes [Pd(N[bond]N)(C(6)F(5))(CO(2)R)] (N[bond]N = bis(3,5-dimethylpyrazol-1-yl)methane); R = Me, Et, or (i)Pr) are obtained when [Pd(N[bond]N)(C(6)F(5))Cl] is treated with KOH in the corresponding alcohol ROH and CO is bubbled through the solution. The reactions of [Pd(N[bond]N)(C(6)F(5))(OH)] (N[bond]N = bipy or Me(2)bipy) with CO(2), in tetrahydrofuran, lead to the formation of the binuclear carbonate complexes [(N[bond]N)(C(6)F(5))Pd(mu-eta(2)-CO(3))Pd(C(6)F(5))(N[bond]N)]. Complexes [Pd(N[bond]N)(C(6)F(5))(OH)] react in alcohol with PhNCS to yield the corresponding N-phenyl-O-alkylthiocarbamate complexes [Pd(N[bond]N)(C(6)F(5))[SC(OR)NPh

Ruiz, Jos; Martnez, M Teresa; Florenciano, Flix; Rodrguez, Venancio; Lpez, Gregorio; Prez, Jos; Chaloner, Penny A; Hitchcock, Peter B

2003-06-01

65

C-H bond functionalization: An aminated reaction  

NASA Astrophysics Data System (ADS)

Selective conversion of C-H bonds into C-N bonds to form N-heterocycles would streamline the synthesis of these important structural motifs. Now, an iron(II) catalyst has been developed that can transform alkyl azides into cyclic secondary amines by controlling the iron imido intermediate to react only with the nearby aliphatic C-H bond.

Driver, Tom G.

2013-09-01

66

Chemisorption and the electronic structure of transition metal oxides and transition metals bonded to oxide surfaces  

Microsoft Academic Search

Some simple transition metal oxides such as NiO have incomplete atomic subshells and yet are highly insulating. This implies a high degree of electronic localization and also indicates that the chemical bonds are saturated. In this respect these oxides are similar to alkaline earth oxides such as MgO. The analogy extends to their surface properties. MgO and NiO, for example,

A. Barry Kunz

1985-01-01

67

Surface Femtochemistry: Investigation and Optimization of Bond-Forming Chemical Reactions  

NASA Astrophysics Data System (ADS)

We investigate femtosecond laser-induced surface reactions by varying the properties of the surface, the reactant gases, and the laser. In optimal control experiments, we selectively manipulate the bond-forming catalytic reactions.

Nuernberger, Patrick; Wolpert, Daniel; Weiss, Horst; Gerber, Gustav

68

Designing catalysts for functionalization of unactivated C-H bonds based on the CH activation reaction.  

PubMed

In an effort to augment or displace petroleum as a source of liquid fuels and chemicals, researchers are seeking lower cost technologies that convert natural gas (largely methane) to products such as methanol. Current methane to methanol technologies based on highly optimized, indirect, high-temperature chemistry (>800 C) are prohibitively expensive. A new generation of catalysts is needed to rapidly convert methane and O(2) (ideally as air) directly to methanol (or other liquid hydrocarbons) at lower temperatures (~250 C) and with high selectivity. Our approach is based on the reaction between CH bonds of hydrocarbons (RH) and transition metal complexes, L(n)M-X, to generate activated L(n)M-R intermediates while avoiding the formation of free radicals or carbocations. We have focused on the incorporation of this reaction into catalytic cycles by integrating the activation of the CH bond with the functionalization of L(n)M-R to generate the desired product and regenerate the L(n)M-X complex. To avoid free-radical reactions possible with the direct use of O(2), our approach is based on the use of air-recyclable oxidants. In addition, the solvent serves several roles including protection of the product, generation of highly active catalysts, and in some cases, as the air-regenerable oxidant. We postulate that there could be three distinct classes of catalyst/oxidant/solvent systems. The established electrophilic class combines electron-poor catalysts in acidic solvents that conceptually react by net removal of electrons from the bonding orbitals of the CH bond. The solvent protects the CH(3)OH by conversion to more electron-poor [CH(3)OH(2)](+) or the ester and also increases the electrophilicity of the catalyst by ligand protonation. The nucleophilic class matches electron-rich catalysts with basic solvents and conceptually reacts by net donation of electrons to the antibonding orbitals of the CH bond. In this case, the solvent could protect the CH(3)OH by deprotonation to the more electron-rich [CH(3)O](-) and increases the nucleophilicity of the catalysts by ligand deprotonation. The third grouping involves ambiphilic catalysts that can conceptually react with both the HOMO and LUMO of the CH bond and would typically involve neutral reaction solvents. We call this continuum base- or acid-modulated (BAM) catalysis. In this Account, we describe our efforts to design catalysts following these general principles. We have had the most success with designing electrophilic systems, but unfortunately, the essential role of the acidic solvent also led to catalyst inhibition by CH(3)OH above ~1 M. The ambiphilic catalysts reduced this product inhibition but were too slow and inefficient. To date, we have designed new base-assisted CH activation and L(n)M-R fuctionalization reactions and are working to integrate these into a complete, working catalytic cycle. Although we have yet to design a system that could supplant commercial processes, continued exploration of the BAM catalysis continuum may lead to new systems that will succeed in addressing this valuable goal. PMID:22482496

Hashiguchi, Brian G; Bischof, Steven M; Konnick, Michael M; Periana, Roy A

2012-04-06

69

Polyorganosilazane preceramic binder development for reaction bonded silicon nitride composites  

SciTech Connect

This study has examined the use of two commercially available polyorganosilazanes for application as preceramic binders in a composite composed of silicon carbide fibers in a reaction bonded silicon nitride (RBSN) matrix. Ceramic monolithic and composite samples were produced. Density of monolithic and whisker reinforced RBSN samples containing the polysilazane binder was increased. Mercury intrusion porosimetry revealed a significant decrease in the pore sizes of samples containing a polyorganosilazane binder. Electron micrographs of samples containing the preceramic binder looked similar to control samples containing no precursor. Overall, incorporation of the polysilazane into monolithic and whisker reinforced samples resulted in significantly increased density and decreased porosity. Nitriding of the RBSN was slightly retarded by addition of the polysilazane binder. Samples with the preceramic binders contained increased contents of {alpha} versus {beta}-silicon nitride which may be due to interaction of hydrogen evolved from polysilazane pyrolysis with the nitriding process. Initial efforts to produce continuous fiber reinforced composites via this method have not realized the same improvements in density and porosity which have been observed for monolithic and whisker reinforced samples. Further, the addition of perceramic binder resulted in a more brittle fracture morphology as compared to similar composites made without the binder.

Mohr, D.L.; Starr, T.L. [Georgia Tech Research Inst., Atlanta, GA (United States)

1992-11-01

70

Polyorganosilazane preceramic binder development for reaction bonded silicon nitride composites  

SciTech Connect

This study has examined the use of two commercially available polyorganosilazanes for application as preceramic binders in a composite composed of silicon carbide fibers in a reaction bonded silicon nitride (RBSN) matrix. Ceramic monolithic and composite samples were produced. Density of monolithic and whisker reinforced RBSN samples containing the polysilazane binder was increased. Mercury intrusion porosimetry revealed a significant decrease in the pore sizes of samples containing a polyorganosilazane binder. Electron micrographs of samples containing the preceramic binder looked similar to control samples containing no precursor. Overall, incorporation of the polysilazane into monolithic and whisker reinforced samples resulted in significantly increased density and decreased porosity. Nitriding of the RBSN was slightly retarded by addition of the polysilazane binder. Samples with the preceramic binders contained increased contents of [alpha] versus [beta]-silicon nitride which may be due to interaction of hydrogen evolved from polysilazane pyrolysis with the nitriding process. Initial efforts to produce continuous fiber reinforced composites via this method have not realized the same improvements in density and porosity which have been observed for monolithic and whisker reinforced samples. Further, the addition of perceramic binder resulted in a more brittle fracture morphology as compared to similar composites made without the binder.

Mohr, D.L.; Starr, T.L. (Georgia Tech Research Inst., Atlanta, GA (United States))

1992-11-01

71

Novel Ti-O-Ti bonding species constructed in a metal-oxide cluster: reaction products of bis(oxalato)oxotitanate(IV) with the dimeric, 1,2-dititanium(IV)-substituted Keggin polyoxotungstate.  

PubMed

The preparation and structural characterization of a novel Ti-O-Ti bonding complex constructed in a dilacunary alpha-Keggin polyoxometalate (POM), [[{Ti(ox)(H2O)}4(mu-O)3](alpha-PW10O37)](7-) (H2ox = oxalic acid) (1a), are described. The water-soluble, crystalline complex with a formula of K6H[1a].0.5KCl.10H2O (1p) was prepared as the bulk sample in 28.0% (0.51 g scale) yield in a 1:4 molar-ratio reaction of the dititanium(IV)-substituted, dimeric form of an alpha-Keggin POM, K10[(alpha-1,2-PW10Ti2O39)2].18H2O, with the titanium(IV) source K2[TiO(ox)2].2H2O in HCl-acidic solution (pH 0.08). Prior to formation of 1p, the KCl-free crystalline compound (1c) obtained was characterized with X-ray crystallography. The compound 1p was unequivocally characterized with complete elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, and solution (31P, 183W, and 13C) NMR spectroscopy. The molecular structure of 1a was determined. The POM 1a in the solid state was composed of the four octahedral Ti groups (four guests), i.e., the two Ti-O-Ti groups linked with the mu-O atom, incorporated to the two adjacent, octahedral vacant sites (two hosts) in the dilacunary Keggin POM. The formation of 1a, as well as the recently found POM [{Ti(ox)(H2O)}2(mu-O)](alpha-PW11O39)](5-) (2a), was strongly dependent on the reaction with [TiO(ox)2](2-), i.e., the anionic titanium(IV) complex as the titanium(IV) source. The POM 1a is contrasted to most titanium(IV)-substituted POMs consisting of a combination of a monolacunary site (one host) and an octahedral Ti group (one guest) and also contrasted to 2a as a combination of a monolacunary site (one host) and two octahedral Ti groups or a Ti-O-Ti group (two guests). PMID:16999405

Hayashi, Kunihiko; Murakami, Hideyuki; Nomiya, Kenji

2006-10-01

72

Reaction of tantalum pentoxide with erbium oxide  

Microsoft Academic Search

An investigation of the reaction of erbium oxide with tantalum pentoxide in a wide range of concentrations and temperatures has revealed the existence of regions of solid solutions based on ErTaO, erbium oxide, and tantalum pentoxide; the lattice constants of ErTaO, E rTaO, and ErTaO have been calculated, the densities and melting points of erbium tantalates have been determined, and

E. N. Isupova; E. P. Savchenko; E. K. Keler

1975-01-01

73

Mechanistic insights from reactions between copper(II)-phenoxyl complexes and substrates with activated C-H bonds.  

PubMed

The reactivities of two copper(II)-phenoxyl analogues of the oxidized, active form of the metalloenzyme galactose oxidase, [1tBu2]+ and [2tBu2]+, have been studied using the substrates benzyl alcohol and 9,10-dihydroanthracene, for a total of four reactions. The reaction stoichiometries in all cases show a 2:1 ratio of oxidant to benzaldehyde or anthracene product, indicating that [1tBu2]+ and [2tBu2]+ behave ultimately as only one-electron oxidants, but the reaction kinetics each indicate that only a single copper(II)-phenoxyl complex is involved in the rate-determining step. For each substrate, rate laws indicate that [1tBu2]+ and [2tBu2]+ react by different mechanisms: one proceeds by a simple bimolecular reaction, while the other first enters into a substrate-binding equilibrium before subsequently reacting by an intramolecular reaction. The reactions proceeding by the latter mechanism have faster overall rates, which correlates to a lower entropic barrier for the substrate-binding mechanism. Correlation of the reaction rates with the C-H bond dissociation energies of substrates as well as significant deuterium kinetic isotope effects indicates that the rate-determining steps involve hydrogen atom abstraction from the activated C-H bonds. A variable-temperature study (268-308 K) of the nonclassical KIE of the [1tBu2]+/benzyl alcohol reaction (kH/kD = 15 at 298 K) failed to show evidence for quantum tunneling. The rapid sequence by which a second 1 equiv of copper(II)-phenoxyl oxidant completes the reaction after the rate- and product-determining hydrogen atom abstraction step cannot be probed kinetically. Comparisons are made to the reactivities of other copper(II)-phenoxyl complexes reported in the literature and to galactose oxidase itself. PMID:15792472

Pratt, Russell C; Stack, T Daniel P

2005-04-01

74

Galvanic replacement reactions in metal oxide nanocrystals.  

PubMed

Galvanic replacement reactions provide a simple and versatile route for producing hollow nanostructures with controllable pore structures and compositions. However, these reactions have previously been limited to the chemical transformation of metallic nanostructures. We demonstrated galvanic replacement reactions in metal oxide nanocrystals as well. When manganese oxide (Mn3O4) nanocrystals were reacted with iron(II) perchlorate, hollow box-shaped nanocrystals of Mn3O4/?-Fe2O3 ("nanoboxes") were produced. These nanoboxes ultimately transformed into hollow cagelike nanocrystals of ?-Fe2O3 ("nanocages"). Because of their nonequilibrium compositions and hollow structures, these nanoboxes and nanocages exhibited good performance as anode materials for lithium ion batteries. The generality of this approach was demonstrated with other metal pairs, including Co3O4/SnO2 and Mn3O4/SnO2. PMID:23704569

Oh, Myoung Hwan; Yu, Taekyung; Yu, Seung-Ho; Lim, Byungkwon; Ko, Kyung-Tae; Willinger, Marc-Georg; Seo, Dong-Hwa; Kim, Byung Hyo; Cho, Min Gee; Park, Jae-Hoon; Kang, Kisuk; Sung, Yung-Eun; Pinna, Nicola; Hyeon, Taeghwan

2013-05-24

75

CO bond cleavage on supported nano-gold during low temperature oxidation.  

PubMed

The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO. PMID:21152570

Carley, Albert F; Morgan, David J; Song, Nianxue; Roberts, M Wyn; Taylor, Stuart H; Bartley, Jonathan K; Willock, David J; Howard, Kara L; Hutchings, Graham J

2010-12-10

76

Influences of interface oxidation on transmission laser bonding of wafers for microsystem packaging  

Microsoft Academic Search

In the fabrication of micro-devices and systems, wafer bonding offers a unique opportunity for constructing complicated three-dimensional structures. In this paper, a wafer bonding technique, called transmission laser bonding (TLB), is studied with focus on the effects of interface oxidation and contact pressure on the bonding strength. The TLB is implemented for bonding Pyrex glass-to-silicon wafers, with and without interface

Ampere A. Tseng; Jong-Seung Park; George P. Vakanas; Hongtao Wu; Miroslav Raudensky; T. P. Chen

2007-01-01

77

Copper-catalyzed B-h bond insertion reaction: a highly efficient and enantioselective C-B bond-forming reaction with amine-borane and phosphine-borane adducts.  

PubMed

A copper-catalyzed B-H bond insertion reaction with amine- and phosphine-borane adducts was realized with high yield and enantioselectivity under mild reaction conditions. The B-H bond insertion reaction provides a new C-B bond-forming methodology and an efficient approach to chiral organoboron compounds. PMID:24025045

Cheng, Qing-Qing; Zhu, Shou-Fei; Zhang, Yong-Zhen; Xie, Xiu-Lan; Zhou, Qi-Lin

2013-09-13

78

Selective oxidation of unactivated C-H bonds by supramolecular control.  

PubMed

Efficient methods for dioxirane-based selective C-H bond oxidation by supramolecular control in H(2)O have been developed. With ?-cyclodextrin as the supramolecular host, site-selective oxidation of the terminal over the internal tertiary C-H bond of 3,7-dimethyloctyl esters 3a-c was achieved. In addition, ?-cyclodextrin selectively enhanced the C-H bond oxidation of cumene in a mixture of cumene and ethyl benzene in H(2)O. Through (1)H NMR studies, the selectivity in C-H bond oxidation could be attributed to the inclusion complex formation between ?-cyclodextrin and the substrates. PMID:22411003

Fung, Yat-Sing; Yan, Siu-Cheong; Wong, Man-Kin

2012-03-13

79

Why (1 0 0) terraces break and make bonds: oxidation of dimethyl ether on platinum single-crystal electrodes.  

PubMed

A surface structural preference for (1 0 0) terraces of fcc metals is displayed by many bond-breaking or bond-making reactions in electrocatalysis. Here, this phenomenon is explored in the electrochemical oxidation of dimethyl ether (DME) on platinum. The elementary C-O bond-breaking step is identified and clarified by combining information obtained from single-crystal experiments and density functional theory (DFT) calculations. Experiments on Pt(1 0 0), Pt(5 1 0), and Pt(10 1 0) surfaces show that the surface structure sensitivity is due to the bond-breaking step, which is unfavorable on step sites. DFT calculations suggest that the precursor for the bond-breaking step is a CHOC adsorbate that preferentially adsorbs on a square ensemble of four neighboring atoms on Pt(1 0 0) terraces, named as "the active site". Step sites fail to strongly adsorb CHOC and are, therefore, ineffective in breaking C-O bonds, resulting in a decrease in activity on surfaces with increasing step density. Our combined experimental and computational results allow the formulation of a new mechanism for the electro-oxidation of DME as well as a simple general formula for the activity of different surfaces toward electrocatalytic reactions that prefer (1 0 0) terrace active sites. PMID:23980577

Li, Hongjiao; Calle-Vallejo, Federico; Kolb, Manuel J; Kwon, Youngkook; Li, Yongdan; Koper, Marc T M

2013-09-13

80

Carboncarbon bond construction on solid support: triethylborane-induced radical reactions of oxime ethers  

Microsoft Academic Search

The triethylborane-induced solid-phase radical reaction was studied. The solid-phase radical reaction of oxime ether anchored to Wang resin proceeded smoothly to give the ?-amino acid derivatives. The carboncarbon bond-forming radical reaction of TentaGel OH resin-bound glyoxylic oxime ether proceeded even in aqueous media.

Hideto Miyabe; Azusa Nishimura; Yumi Fujishima; Takeaki Naito

2003-01-01

81

Iron Oxide/Aluminum Fast Thermite Reaction  

NASA Astrophysics Data System (ADS)

The self-sustained thermite reaction between iron oxide (Fe2O3) and aluminum (Al) is commonly slow. This paper presents an experimental study to evaluate the reaction regression rates of these thermite mixtures with or without continuous electrical discharge. Two thermite compositions are tested: stoichiometric (Al2O3 and Fe as the only products) and over aluminized. To generate a fast and stable front propagation in thermite material, a long channel configuration is used and the reaction is assisted with a continuous electrical discharge between two copper plates (external confinement). The reaction velocity is evaluated by the regression rate, which is measured by optical methods. The assisted reaction is compared to the original self-sustained reaction, as a function of the composition. The electrical discharge level is also measured with a current monitor and oscilloscope. The thermodynamic properties of reaction products are predicted by a thermochemical code, named THOR, assuming an isobar adiabatic combustion and using HL equation of state.

Morgado, J.; Dura~Es, L.; Campos, J.; Portugal, A.

2004-07-01

82

Graphene-hemin hybrid material as effective catalyst for selective oxidation of primary C-H bond in toluene  

NASA Astrophysics Data System (ADS)

An effective hemin catalyst on graphene support for selective oxidation of primary C-H bond in toluene is reported with an over 50% conversion rate achieved at mild conditions. Significantly this hybrid material shows catalytic efficiency in toluene oxidation with selectivity towards benzoic acid. The role of graphene support is discussed here as providing large contact area between the catalyst and the substrate, maintaining hemin in catalytically active monomer form, attracting electron to promote site isolation, as well as protecting hemin from oxidative degradation during the reaction. Moreover, graphene is suggested to largely alter the final product selectivity, due to the different ?-? interaction strength between the graphene support and the substrate/oxidized products. With longer reaction time, overall conversion rate tends to maintain relatively unchanged while toluene undergoes a series of oxidation to convert mostly to benzoic acid.

Li, Yongjia; Huang, Xiaoqing; Li, Yujing; Xu, Yuxi; Wang, Yang; Zhu, Enbo; Duan, Xiangfeng; Huang, Yu

2013-05-01

83

Fabrication and simultaneous bonding of metal matrix composite by combustion synthesis reaction  

Microsoft Academic Search

The combustion synthesis and bonding of TiB2\\/Cu composite were simultaneously performed in a single process. The heat of reaction of TiB2 formation was high enough to form the bonded interface between the composite and an aluminum rod. Intermetallic compounds consisting of copper and aluminum were observed at the interface.

Y. J. Kwon; M. Kobashi; T. Choh; N. Kanetake

2004-01-01

84

Formation of Porous Surface Layers in Reaction Bonded Silicon Nitride During Processing.  

National Technical Information Service (NTIS)

An effort was undertaken to determine if the formation of the generally observed layer of large porosity adjacent to the as-nitride surfaces of reaction bonded silicon nitrides could be prevented during processing. Isostatically pressed test bars were pre...

N. J. Shaw T. K. Glasgow

1979-01-01

85

New Reduced Ternary Molybdenum Oxide with Infinite Mo-Mo Bonded Chains.  

National Technical Information Service (NTIS)

A novel reduced ternary oxide of molybdenum is reported. The phase contains infinite octahedral chains of bonded molybdenum atoms with an average oxidation state of 2.375. The pure material can be obtained using stoichiometric quantities of manganese moly...

C. D. Carlson

1986-01-01

86

High temperature heterogeneous reaction kinetics and mechanisms of tungsten oxidation  

Microsoft Academic Search

Tungsten, which is a material used in many high temperature applications, is limited by its susceptibility to oxidation at elevated temperatures. Although tungsten has the highest melting temperature of any metal, at much lower temperatures volatile oxides are formed during oxidation with oxygen containing species. This differs from many heterogeneous oxidation reactions involving metals since most reactions form very stable

Justin L. Sabourin

2010-01-01

87

Oxygen radical-mediated oxidation reactions of an alanine peptide motif - density functional theory and transition state theory study  

PubMed Central

Background Oxygen-base (O-base) oxidation in protein backbone is important in the protein backbone fragmentation due to the attack from reactive oxygen species (ROS). In this study, an alanine peptide was used model system to investigate this O-base oxidation by employing density functional theory (DFT) calculations combining with continuum solvent model. Detailed reaction steps were analyzed along with their reaction rate constants. Results Most of the O-base oxidation reactions for this alanine peptide are exothermic except for the bond-breakage of the C?-N bond to form hydroperoxy alanine radical. Among the reactions investigated in this study, the activated energy of OH ?-H abstraction is the lowest one, while the generation of alkylperoxy peptide radical must overcome the highest energy barrier. The aqueous situation facilitates the oxidation reactions to generate hydroxyl alanine peptide derivatives except for the fragmentations of alkoxyl alanine peptide radical. The C?-C? bond of the alkoxyl alanine peptide radical is more labile than the peptide bond. Conclusion the rate-determining step of oxidation in protein backbone is the generation of hydroperoxy peptide radical via the reaction of alkylperoxy peptide radical with HO2. The stabilities of alkylperoxy peptide radical and complex of alkylperoxy peptide radical with HO2 are crucial in this O-base oxidation reaction.

2012-01-01

88

Selectivity in Selenoxide and Telluroxide Eliminations as Double Bond Forming Reactions  

Microsoft Academic Search

Some aspects of the oxidation of organic selenides and tellurides leading to selenoxide and telluroxide eliminations to form a double bond have been reviewed as follows : (i) the rate of oxidation of selenides to selenoxides in comparison with that of sulfides to sulfoxides, (ii) the stereochemistry of eliminations, (iii) the reactivity order of sulfoxide, selenoxide, and telluroxide eliminations, (iv)

Sakae UEMURA; Akio TOSHIMITSU

89

Effects of thermal cycling on thermal expansion and mechanical properties of SiC fiber-reinforced reaction-bonded Si3N4 composites  

SciTech Connect

Thermal expansion curves for SiC fiber-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 C in nitrogen and in oxygen. The effects of fiber/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fiber direction for both bonding cases were similar to that of the weakly bonded composites, but those normal to the fiber direction for both bonding cases resulted in no net dimensional changes at room temperature, and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly bonded SiC/RBSN composites the most, resulting in loss of strain capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fiber and RBSN matrix due to oxidation of the carbon-rich fiber surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the main reasons for the poor mechanical performance of these composites.

Bhatt, R.T.; Palczer, A.R.

1994-10-01

90

Effects of thermal cycling on thermal expansion and mechanical properties of SiC fiber-reinforced reaction-bonded Si3N4 composites. Technical memo  

SciTech Connect

Thermal expansion curves for SiC fiber-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 deg C in nitrogen and in oxygen. The effects of fiber/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fiber direction were between the measured curves for the strongly and weakly bonded composites, but those normal to the fiber direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature, and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly bonded SiC/RBSN composites the most, resulting in loss of straln capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fiber and RBSN matrix due to oxidation of the carbon-rich fiber surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the maln reasons for the poor mechanical performance of these composites.

Bhatt, R.T.; Palczer, A.R.

1994-10-01

91

Adsorption and abstraction reactions of HCl on a single Si(100) dangling bond  

NASA Astrophysics Data System (ADS)

On a Si(100)-(21) surface with abundant dangling bonds, reaction of HCl molecules at room temperature is dominated by exothermic dissociative adsorption of H and Cl on two adjacent dangling bonds. This coadsorption reaction is blocked for an isolated dangling bond, yet surprisingly endothermic H or Cl abstractive adsorption occurs, as observed by in situ scanning tunneling microscopy. On an isolated dimer dangling bond (DB) pair, coadsorption of H and Cl is common as expected, but adsorption of a pair of abstracted Cl or H from two HCl molecules also occurs. These results, complemented by theoretical calculations, indicate that dissociative adsorption and abstractive reaction of a multiatom gas molecule can be initiated at a single DB by forming an intermediate adsorption state.

Li, Hong-Dao; Chang, Chan-Yuen; Chien, Ling-Ying; Chang, Shih-Hsin; Chiang, T.-C.; Lin, Deng-Sung

2011-02-01

92

The fluoroform?ethylene oxide complex exhibits a CH?O anti-hydrogen bond  

Microsoft Academic Search

A new, recently discovered type of intermolecular bond, called an anti-hydrogen bond, is found in the fluoroform?ethylene oxide complex. The complex structure and properties were predicted on the basis of counterpoise-corrected gradient optimization performed at a correlated ab initio level. Formation of the CH?O anti-hydrogen bond is manifested by an 0.1 pm contraction of the CH bond and a significant

Pavel Hobza; Zden?k Havlas

1999-01-01

93

Cross-coupling reactions involving metal carbene: from c?c/c-C bond formation to C-h bond functionalization.  

PubMed

Cross-coupling reactions involving metal-carbene are emerging as a new type of carbon-carbon bond-forming reaction. The aim of this JOCSynopsis is to provide an overview of the most recent development of these reactions, focusing on the use of diazo compounds (or in situ formed diazo compounds from N-tosylhydrazones) as carbene precursors. In addition, the integration of this type of reaction with C-H bond functionalization is also surveyed. PMID:24134640

Liu, Zhenxing; Wang, Jianbo

2013-09-19

94

Thermochemistry and reaction paths in the oxidation reaction of benzoyl radical: C6H5C(?O).  

PubMed

Alkyl substituted aromatics are present in fuels and in the environment because they are major intermediates in the oxidation or combustion of gasoline, jet, and other engine fuels. The major reaction pathways for oxidation of this class of molecules is through loss of a benzyl hydrogen atom on the alkyl group via abstraction reactions. One of the major intermediates in the combustion and atmospheric oxidation of the benzyl radicals is benzaldehyde, which rapidly loses the weakly bound aldehydic hydrogen to form a resonance stabilized benzoyl radical (C6H5C()?O). A detailed study of the thermochemistry of intermediates and the oxidation reaction paths of the benzoyl radical with dioxygen is presented in this study. Structures and enthalpies of formation for important stable species, intermediate radicals, and transition state structures resulting from the benzoyl radical +O2 association reaction are reported along with reaction paths and barriers. Enthalpies, ?fH298(0), are calculated using ab initio (G3MP2B3) and density functional (DFT at B3LYP/6-311G(d,p)) calculations, group additivity (GA), and literature data. Bond energies on the benzoyl and benzoyl-peroxy systems are also reported and compared to hydrocarbon systems. The reaction of benzoyl with O2 has a number of low energy reaction channels that are not currently considered in either atmospheric chemistry or combustion models. The reaction paths include exothermic, chain branching reactions to a number of unsaturated oxygenated hydrocarbon intermediates along with formation of CO2. The initial reaction of the C6H5C()?O radical with O2 forms a chemically activated benzoyl peroxy radical with 37 kcal mol(-1) internal energy; this is significantly more energy than the 21 kcal mol(-1) involved in the benzyl or allyl + O2 systems. This deeper well results in a number of chemical activation reaction paths, leading to highly exothermic reactions to phenoxy radical + CO2 products. PMID:21942384

Sebbar, Nadia; Bozzelli, Joseph W; Bockhorn, Henning

2011-09-23

95

Catalytic hydrocarbon reactions over supported metal oxides. Final report, August 1, 1986--July 31, 1995  

SciTech Connect

Oxide catalysis plays a central role in hydrocarbon processing and improvements in catalytic activity or selectivity are of great technological importance because these improvements will translate directly into more efficient utilization of hydrocarbon supplies and lower energy consumption in separation processes. An understanding of the relationships between surface structure and catalytic properties is needed to describe and improve oxide catalysts. The approach has been to prepare supported oxides that have a specific structure and oxidation state and then employ these structures in reaction studies. The current research program is focused on studying the fundamental relationships between structure and reactivity for two important reactions that are present in many oxide-catalyzed processes, partial oxidation and carbon-carbon bond formation. During the course of these studies the author has: (1) developed methods to form and stabilize various Mo and W oxide structures on silica; (2) studied C-H abstraction reactions over the fully oxidized cations; (3) studied C-C bond coupling by metathesis and reductive coupling of aldehydes and ketones over reduced cation structures; and (4) initiated a study of hydrogenation and hydrogenolysis over reduced cation structures.

Ekerdt, J.G.

1995-10-20

96

Theory of chemical bonds in metalloenzymes - Manganese oxides clusters in the oxygen evolution center -  

NASA Astrophysics Data System (ADS)

In early 1980 we have initiated broken-symmetry (BS) MO theoretical calculations of transition-metal oxo species M = O (M = Ti,V,Cr,Mn,Fe,Ni,Cu) to elucidate the nature of d?-p? and d?-p? bonds. It has been concluded that high-valent M = O species such as [Mn(IV) = O]2+ and [Fe(IV) = O]2+ exhibit electrophilic property in a sharp contrast with nucleophilic character of low-valent M = O bonds: [M(II)O2-]0, and closed-shell d?-p? bonds of high-valent M = O species often suffer the triplet-instability, giving rise to open-shell (BS) configurations with significant metal-diradical (MDR) character: M-O: note that these bonds are therefore regarded as typical examples of strongly correlated electron systems. Because of the MDR character, 1,4-metal diradical mechanism was indeed preferable to four-centered mechanism in the case of addition reaction of naked Mn(IV) = O to ethylene. Recently the manganese-oxo species have been receiving renewed interest in relation to catalytic cycle of oxygen evolution from water molecules in the photosynthesis II (PSII) system. Accumulated experimental results indicate that this process is catalyzed with four manganese oxide clusters coordinated with calcium ion (CaMn4O4). Past decade we have performed BS MO theoretical investigations of manganese oxide clusters related to CaMn4O4. These calculations have elucidated that high-valent Mn(X) = O (X = IV,V) bonds exhibit intermediate MDR character (y=40-60%) in the case of total low-spin (LS) configuration but the MDR character decreases with coordination of Ca2+ and water molecules. While the MDR character of the Mn-oxo bonds becomes very high at the high-spin (HS) configuration. Our computational results enabled us to propose two possible mechanisms on the theoretical ground: (A) electrophilic (EP) mechanism and (B) radical coupling (RC) mechanism. The theoretical results indicate that the EP mechanism is preferable for the low-spin (LS) state in polar media like in the protein environments (native OEC), whereas the RC mechanism is feasible at the state without such environmental stabilization: local singlet and local triplet diradical mechanisms are proposed for the OO coupling process. Possibilities of EP and RC mechanisms are examined in comparison with a lot of experimental results accumulated and theoretical results with several groups.

Yamaguchi, K.; Shoji, M.; Saito, T.; Isobe, H.; Yamada, S.; Nishihara, S.; Kawakami, T.; Kitagawa, Y.; Yamanaka, S.; Okumura, M.

2012-12-01

97

Bond-valence-sum study on possible candidates for high-Tc oxide superconductors  

NASA Astrophysics Data System (ADS)

Relationships between crystal structures and electronic states of layered transition-metal oxides have been analyzed in the light of bond valence sums. Parameters are introduced representing excess charge and internal strain in the central MO2 planes (M = 3d transition metal) and characterizing the electronic states of those oxides. Correlations between the superconducting transition temperature and those bond-valence-sum parameters are investigated for the high-Tc cuprate compounds and the possibility of making nonsuperconducting oxides superconducting is discussed.

Tanaka, Shigenori; Fukushima, Noburu; Niu, Hiromi; Ando, Ken

1990-11-01

98

Prediction of Reliable Metal-PH? Bond Energies for Ni, Pd, and Pt in the 0 and +2 Oxidation States  

SciTech Connect

Phosphine-based catalysts play an important role in many metal-catalyzed carbon-carbon bond formation reactions yet reliable values of their bond energies are not available. We have been studying homogeneous catalysts consisting of a phosphine bonded to a Pt, Pd, or Ni. High level electronic structure calculations at the CCSD(T)/complete basis set level were used to predict the M-PH? bond energy (BE) for the 0 and +2 oxidation states for M=Ni, Pd, and Pt. The calculated bond energies can then be used, for example, in the design of new catalyst systems. A wide range of exchange-correlation functionals were also evaluated to assess the performance of density functional theory (DFT) for these important bond energies. None of the DFT functionals were able to predict all of the M-PH3 bond energies to within 5 kcal/mol, and the best functionals were generalized gradient approximation functionals in contrast to the usual hybrid functionals often employed for main group thermochemistry.

Craciun, Raluca; Vincent, Andrew J.; Shaughnessy, Kevin H.; Dixon, David A.

2010-06-21

99

Methods for forming complex oxidation reaction products including superconducting articles  

SciTech Connect

This patent describes a method for producing a superconducting complex oxidation reaction product of two or more metals in an oxidized state. It comprises positioning at least one parent metal source comprising one of the metals adjacent to a permeable mass comprising at least one metal-containing compound capable of reaction to form the complex oxidation reaction product in step below, the metal component of the at least one metal-containing compound comprising at least a second of the two or more metals, and orienting the parent metal source and the permeable mass relative to each other so that formation of the complex oxidation reaction product will occur in a direction towards and into the permeable mass; and heating the parent metal source in the presence of an oxidant to a temperature region above its melting point to form a body of molten parent metal to permit infiltration and reaction of the molten parent metal into the permeable mass and with the oxidant and the at least one metal-containing compound to form the complex oxidation reaction product, and progressively drawing the molten parent metal source through the complex oxidation reaction product towards the oxidant and towards and into the adjacent permeable mass so that fresh complex oxidation reaction product continues to form within the permeable mass; and recovering the resulting complex oxidation reaction product.

Rapp, R.A.; Urquhart, A.W.; Nagelberg, A.S.; Newkirk, M.S.

1992-04-07

100

Chemical bonding and reactions at the Pd\\/Si interface  

Microsoft Academic Search

The electronic structure of the clean Pd\\/Si(111) and Pd\\/Si(100) interfaces has been investigated using angle-integrated and angle-resolved ultraviolet photoemission (UPS) and Auger electron spectroscopies (AES), in conjunction with transmission electron microscopy (TEM), work-function, and low-energy electron diffraction measurements. Since the interface is highly reactive, studies were made by processing the reaction in two ways: (i) sequential annealing steps of thick

G. W. Rubloff; P. S. Ho; J. F. Freeouf; J. E. Lewis

1981-01-01

101

Sulfur radical cation-peptide bond complex in the one-electron oxidation of S-methylglutathione.  

PubMed

Neighboring group participation was investigated in the *OH-induced oxidation of S-methylglutathione in aqueous solutions. Nanosecond pulse radiolysis was used to obtain the spectra of the reaction intermediates and their kinetics. Depending on the pH, and the concentration of S-methylglutathione, pulse irradiation leads to different transients. The transients observed were an intramolecularly bonded [>S thereforeNH2]+ intermediate, intermolecularly S thereforeS-bonded radical cation, alpha-(alkylthio)alkyl radicals, alpha-amino-alkyl-type radical, and an intramolecularly (S thereforeO)+-bonded intermediate. The latter radical is of particular note in that it supports recent observations of sulfur radical cations complexed with the oxygen atoms of peptide bonds and thus has biological and medical implications. This (S thereforeO)+-bonded intermediate had an absorption maximum at 390 nm, and we estimated its formation rate to be >or=6x10(7) s(-1). It is in equilibrium with the intermolecularly S thereforeS-bonded radical cation, and they decay together on the time scale of a few hundred microseconds. The S thereforeS-bonded radical cation is formed from the monomeric sulfur radical cation (>S*+) and an unoxidized S-methylglutathione molecule with the rate constant of 1.0x10(9) M(-1) s(-1). The short-lived [>S thereforeNH2]+ intermediate is a precursor of decarboxylation, absorbs at approximately 390 nm, and decays on the time scale of hundreds of nanoseconds. Additional insight into the details of the association of sulfur radical cations with the oxygen atoms of the peptide bonds was gained by comparing the behavior of the S-methylglutathione (S thereforeO+-bonded five-membered ring) with the peptide gamma-Glu-Met-Gly (S thereforeO+-bonded six-membered ring). Conclusions from experimental observations were supported by molecular modeling calculations. PMID:17602483

Bobrowski, Krzysztof; Hug, Gordon L; Pogocki, Dariusz; Marciniak, Bronislaw; Schneich, Christian

2007-06-29

102

On the Electronic Nature of Low-Barrier Hydrogen Bonds in Enzymatic Reactions  

Microsoft Academic Search

The electronic nature of low-barrier hydrogen bonds (LBHBs) in enzymatic reactions is discussed based on combined low temperature neutron and x-ray diffraction experiments and on high level ab initio calculations by using the model substrate benzoylacetone. This molecule has a LBHB, as the intramolecular hydrogen bond is described by a double-well potential with a small barrier for hydrogen transfer. From

Birgit Schiott; Bo Brummerstedt Iversen; Georg Kent Hellerup Madsen; Finn Krebs Larsen; Thomas C. Bruice

1998-01-01

103

Formation of chemical bonds with visible light: The sensitized oxidation of iodide and water  

NASA Astrophysics Data System (ADS)

This thesis reports on visible light sensitized oxidation chemistry that drives the formation of I-I and O-O bonds in solution and at surfaces. Chapter 1 introduces the reader to a general overview of the fundamental iodine redox chemistry related to the making and breaking of I-I bonds. The relevance of I-I bonds to the functionality to dye-sensitized solar cells (DSSCs) is discussed. In Chapter 2, the importance of strong photo-oxidants for solar water splitting and the transient generation of free iodine atoms are examined. The synthesis of the photo-oxidants was completed by Jovan Giaimuccio. Chapter 3 further expands on the formation of iodine atoms from potent photo-oxidants and the cleaving of I-I bonds from the direct photochemistry of I3-. Maria Abrahamsson is gratefully acknowledged for assistance with low temperature measurements. Chapter 4 addresses the mechanistic details of iodide oxidation and concerted I-I bond formation as compared with sequential oxidation and bond forming steps. The synthesis of the photo-catalysts was completed by Andras Marton. A limitation for producing functional water oxidation photocatalysts is the generation of significant concentrations of oxidizing equivalents to drive the multi-electron water oxidation chemistry necessary to form O-O bonds. The design of visible light absorbing, high surface area semiconductors with minimized distances for hole diffusion to the solution interface could improve water oxidation efficiency and are discussed in Chapter 5. Su Kim is acknowledged for assistance in assembling high surface area semiconductor electrodes for water splitting. Within Appendix 1 we discuss the synthesis and characterization of the rutile, anatase, and brookite phase nanoparticles of TiO2, which are known to be photo-oxidants capable of oxidizing water to O2. David Reyes-Coronado is gratefully acknowledged for the synthesis of TiO2 nanoparticles during an extended research collaboration with our laboratories.

Gardner, James M.

104

Direct imaging of covalent bond structure in single-molecule chemical reactions.  

PubMed

Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways. PMID:23722428

de Oteyza, Dimas G; Gorman, Patrick; Chen, Yen-Chia; Wickenburg, Sebastian; Riss, Alexander; Mowbray, Duncan J; Etkin, Grisha; Pedramrazi, Zahra; Tsai, Hsin-Zon; Rubio, Angel; Crommie, Michael F; Fischer, Felix R

2013-05-30

105

Dependence of cryogenic strength of hydroxide catalysis bonded silicon on type of surface oxide  

NASA Astrophysics Data System (ADS)

Hydroxide catalysis bonding is a joining technique used in the construction of highly stable opto-mechanical systems including quasi-monolithic silica suspensions for first and second generation gravitational wave detectors. Future generations of detector are likely to operate at cryogenic temperatures necessitating a change in testmass/suspension material. A promising candidate material is silicon, which requires an oxide surface layer for hydroxide catalysis bonding to be reliable. Here, we present first results showing the influence of the type of oxide layer applied on bond strength, measured at room temperature and 77 K, and identify preferred oxide deposition methods.

Beveridge, N. L.; van Veggel, A. A.; Cunningham, L.; Hough, J.; Martin, I. W.; Nawrodt, R.; Reid, S.; Rowan, S.

2013-01-01

106

Dependence of oxide surface structure on surface topology and local chemical bonding  

SciTech Connect

The atomic geometries of the charge neutral surfaces of several oxides exhibiting different crystal structures and varying participation of O(2{ital p}) electrons in the chemical bonding have been calculated using tight-binding total energy models. Surface structures have been computed for exemplary cubic (MgO), wurtzite (ZnO), {beta}-tridymite, and ideal {beta}-cristobalite (SiO{sub 2}) oxides. The cubic oxide exhibits a minimum energy structure involving small outward relaxations of the oxygens and inward relaxations of the cations. For the cleavage faces of wurtzite ZnO, large bond-length-conserving relaxations occur because the surface atoms can relax without appreciable distortion of the local bond lengths. The charge neutral faces of {beta}-tridymite and ideal {beta}-cristobalite SiO{sub 2} also undergo bond-length-conserving relaxations. Thus the mechanism for the surface relaxation of tetrahedrally coordinated oxides is significantly different from that of the cubic oxides as is the role of the oxygen {ital p} electrons in the surface chemical bonding. Most importantly, the surface structural chemistry of charge neutral oxide surfaces reflects the interplay of both surface topology (via the ability to undergo bond-length-conserving relaxations) and local coordination chemistry (via the participation of {ital p} electrons in bonding versus nonbonding surface state bands).

LaFemina, J.P. (Molecular Science Research Center, Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352 (USA)); Duke, C.B. (Xerox Webster Research Center, 800 Phillips Road 0114-38D, Webster, New York 14580 (USA))

1991-05-01

107

Scale-up of microwave nitridation of sintered reaction bonded silicon nitride parts. Final report  

SciTech Connect

Scale-up were performed in which microwave heating was used to fabricate reaction-bonded silicon nitride and sintered reaction-bonded silicon nitride (SRBSN). Tests were performed in both a 2.45 GHz, 500 liter and a 2.45 GHz, 4000 liter multimode cavities. The silicon preforms processed in the studies were clevis pins for diesel engines. Up to 230 samples were processed in a single microwave furnace run. Data were collected which included weight gains for nitridation and sintering studies were performed using a conventional resistance-heated furnace.

Tiegs, T.N.; Kiggans, J.O.; Garvey, G.A.

1997-10-01

108

SiC continuous fiber-reinforced Si3N4 by infiltration and reaction bonding  

SciTech Connect

Difficulties encountered in the production of long-fiber reinforced ceramic-matrix composites via hot pressing and CVI can be circumvented through the use of the infiltration/consolidation of the long-fiber preforms with submicron particle slurries and reaction-bonding the resulting matrix, as is presently demonstrated for the case of SiC fibers and Si/Si3N4 particle slurries. High temperature fiber degradation during reaction bonding is avoided through the use of additives that lower the nitridation temperature and shorten process time. Attention is given to the microstructure and mechanical properties thus obtained; nearly complete impregnation is achieved. 12 refs.

Brandt, J.; Rundgren, K.; Pompe, R.; Swan, H.; O'meara, C.; Lundberg, R.; Pejryd, L.

1992-10-01

109

Transition-metal-catalyzed laboratory-scale carbon-carbon bond-forming reactions of ethylene.  

PubMed

Ethylene, the simplest alkene, is the most abundantly synthesized organic molecule by volume. It is readily incorporated into transition-metal-catalyzed carbon-carbon bond-forming reactions through migratory insertions into alkylmetal intermediates. Because of its D2h symmetry, only one insertion outcome is possible. This limits byproduct formation and greatly simplifies analysis. As described within this Minireview, many carbon-carbon bond-forming reactions incorporate a molecule (or more) of ethylene at ambient pressure and temperature. In many cases, a useful substituted alkene is incorporated into the product. PMID:24105881

Saini, Vaneet; Stokes, Benjamin J; Sigman, Matthew S

2013-09-17

110

C-H bond activation by aluminum oxide cluster anions, an experimental and theoretical study.  

PubMed

Aluminum oxide cluster anions are produced by laser ablation and reacted with n-butane in a fast flow reactor. A reflectron time-of-flight mass spectrometer is used to detect the cluster distribution before and after the reactions. Aluminum oxide clusters Al?O4,6? and Al?O?? can react with n-C?H?? to produce Al?O4,6H? and Al?O??, respectively, while cluster Al?O?? reacts with n-C?H?? to produce both the Al?O?H? and Al?O?H??. The theoretical calculations are performed to study the structures and bonding properties of clusters Al?O4,6? and Al?O6,7? as well as the reaction mechanism of Al?O?? + n-C?H??. The calculated results show that the mononuclear oxygen-centred radicals (O??) on Al?O4,6? and Al?O??, and oxygen-centred biradical on Al?O?? are the active sites responsible for the observed hydrogen atom abstraction reactivity. Furthermore, mechanism investigation of the O?? generation in Al?O?? upon O? molecule adsorption on un-reactive Al?O?? is performed by theoretical calculations. PMID:23807463

Tian, Li-Hua; Ma, Tong-Mei; Li, Xiao-Na; He, Sheng-Gui

2013-08-21

111

Reactions of Aromatic Phosphate Esters with Metals and Their Oxides  

Microsoft Academic Search

Previous results obtained in this laboratory have shown that tricresyl phosphate vapors preferentially react with iron and its oxides in the order of the oxidation state (reaction rate of Fe3+Fe2+>Fe+>Fe). The reaction kinetics is attributed to a Lewis acid, FenOm, reacting with the basic phosphate ester, (ArO)3PO. The current research further investigates the role of the oxide in the reaction

C. S. Saba; N. H. Forster

2002-01-01

112

Computational study of peptide bond formation in the gas phase through ion-molecule reactions.  

PubMed

A computational study of peptide bond formation from gas-phase ion-molecule reactions has been carried out. We have considered the reaction between protonated glycine and neutral glycine, as well as the reaction between two neutral glycine molecules for comparison purposes. Two different mechanisms, concerted and stepwise, were studied. Both mechanisms show significant energy barriers for the neutral reaction. The energy requirements for peptide bond formation are considerably reduced upon protonation of one of the glycine molecules. For the reaction between neutral glycine and N-protonated glycine the lowest energy barrier is observed for the concerted mechanism. For the reaction between neutral glycine and protonated glycine at carbonyl oxygen, the preferred mechanism is the stepwise one, with a relatively small energy barrier (23 kJ mol(-1) at 0 K) and leading to the lowest-lying protonated glycylglycine isomer. In the case that the reaction could be initiated by protonated glycine at hydroxyl oxygen the process would be barrier-free and clearly exothermic. In that case peptide bond formation could take place even under interstellar conditions if glycine is present in space. PMID:23817675

Redondo, Pilar; Martnez, Henar; Cimas, Alvaro; Barrientos, Carmen; Largo, Antonio

2013-08-21

113

Why do cycloaddition reactions involving C60 prefer [6,6] over [5,6] bonds?  

PubMed

The origin of the experimentally known preference for [6,6] over [5,6] bonds in cycloaddition reactions involving C60 has been computationally explored. To this end, the Diels-Alder reaction between cyclopentadiene and C60 has been analysed by means of the recently introduced activation strain model of reactivity in combination with the energy decomposition analysis method. Other issues, such as the aromaticity of the corresponding transition states, have also been considered. These results indicate that the major factor controlling the observed regioselectivity is the more stabilising interaction between the deformed reactants in the [6,6] reaction pathway along the entire reaction coordinate. PMID:23576307

Fernndez, Israel; Sol, Miquel; Bickelhaupt, F Matthias

2013-04-10

114

Oxidative addition of methane and benzene CH bonds to rhodium center: A DFT study  

Microsoft Academic Search

A density functional theory study on mechanisms of the oxidative addition of methane and benzene CH bonds to the rhodium center containing Cp and PMe3 ligands has been performed. Our calculated results confirm that the CH bond cleavage from a sigma complex to a hydride alkyl complex is the rate-determining step. Compared with the case of methane CH bond, the

Siwei Bi; Zhenwei Zhang; Shufen Zhu

2006-01-01

115

Potential energy surfaces for CH bond cleavage reactions  

SciTech Connect

Ab initio, multi-reference, configuration interaction calculations are reported for CH{sub 4}{leftrightarrow}CH{sub 3}+H, CH{sub 3}F{leftrightarrow}CH{sub 2}F+H, CH{sub 2}F{sub 2}{leftrightarrow}CHF{sub 2}+H, and CHF{sub 3}{leftrightarrow}CF{sub 3}+H. Two equivalent, barrier-less paths are found for the CH{sub 3}+H recombination, two inequivalent, barrier-less paths are found for the CH{sub 2}F+H and CHF{sub 2}+H recombinations (depending on which side of the radical the H atom approaches), and only one barrier-less path is found for the CF{sub 3}+H recombination. Minimum energy path for H atom approaching CF{sub 3} from the concave side is predicted to have a barrier of 27 kcal/mole. Both minimum energy path energies and transitional frequencies as function of R{sub CH} for all 4 reactions are predicted to be similar.

Harding, L.B.

1996-12-31

116

Relationship of bond strengths to selectivity in heterogeneous surface reactions: Mercaptoethanol and ethanedithiol on Ni(110)  

SciTech Connect

The reaction selectivities and bonding configurations in mercaptoethanol (HSCH{sub 2}CH{sub 2}OH) and ethanedithiol (HSCH{sub 2}CH{sub 2}SH) on Ni(110) were determined and found to be correlated to the relative strengths of the C-S and C-O bonds. Mechanistic details of the mercaptoethanol reaction have been elucidated. Mercaptoethanol reacts with Ni(110) to form ethanol, acetaldehyde, methane, CO, and hydrogen. Mercaptoethanol reactivity is dominated by thiolate formation as suggested by the S 2p core level binding energy. Near 200-250K, the C-S bond cleaves with the evolution of ethanol and formation of surface ethoxide which is easily discernible from vibrational spectroscopy. The ethoxide dehydrogenates to form acetaldehyde, which either desorbs or reacts to form methane and CO. In dramatic contrast, the structurally similar ethanedithiol apparently adsorbs in a bidentate fashion, resulting in a dithiolate which selectively splits out thylene and produces surface sulfur. The difference in the reactivity of ethanedithiol and mercaptoethanol is understood in terms of the inactivity of the Ni(110) surface toward C-O bond scission, which is primarily a reflection of the strength of C-O bonds compared with C-S bonds. 35 refs., 8 figs., 2 tabs.

Huntley, D.R. [Oak Ridge National Lab., TN (United States)

1995-08-24

117

Methods for forming complex oxidation reaction products including superconducting articles  

SciTech Connect

This patent describes a method for producing a complex oxidation reaction product of two or more metals in an oxidized state. Positioning a parent metal source comprising at least one first metal of the two or more metals adjacent to a permeable mass comprising at least one metal-containing compound capable of reaction to form the complex oxidation reaction product, the metal component of the at least one metal-containing compound comprising at least a second of the two or more metals, and orienting the parent metal source and the permeable mass relative to each other so that formation of the complex oxidation reaction product will occur in a direction towards and into the permeable mass; heating the parent metal source in the presence of a vapor-phase oxidant; and recovering the resulting complex oxidation reaction product.

Rapp, R.A.; Urquhart, A.W.; Nagelberg, A.S.; New Kirk, M.S.

1990-09-11

118

Vibrationally controlled chemistry: mode- and bond-selected reaction of CH3D with Cl.  

PubMed

Selective vibrational excitation controls the competition between C-H and C-D bond cleavage in the reaction of CH(3)D with Cl, which forms either HCl + CH(2)D or DCl + CH(3). The reaction of CH(3)D molecules with the first overtone of the C-D stretch (2nu(2)) excited selectively breaks the C-D bond, producing CH(3) exclusively. In contrast, excitation of either the symmetric C-H stretch (nu(1)), the antisymmetric C-H stretch (nu(4)), or a combination of antisymmetric stretch and CH(3) umbrella bend (nu(4) + nu(3)) causes the reaction to cleave only a C-H bond to produce solely CH(2)D. Initial preparation of C-H stretching vibrations with different couplings to the reaction coordinate changes the rate of the H-atom abstraction reaction. Excitation of the symmetric C-H stretch (nu(1)) of CH(3)D accelerates the H-atom abstraction reaction 7 times more than excitation of the antisymmetric C-H stretch (nu(4)) even though the two lie within 80 cm(-1) of the same energy. Ab initio calculations and a simple theoretical model help identify the dynamics behind the observed mode selectivity. PMID:16851984

Yoon, Sangwoon; Holiday, Robert J; Crim, F Fleming

2005-05-01

119

Reaction Bonded Silicon Nitride Prepared from Wet Attrition-Milled Silicon.  

National Technical Information Service (NTIS)

Silicon powder wet milled in heptane was dried, compacted into test bar shape, helium-sintered, and then reaction bonded in nitrogen-4 volume percent hydrogen. As-nitrided bend strengths averaged approximately 290 MPa at both room temperature and 1400 C. ...

T. P. Herball T. K. Glasgow N. J. Shaw

1980-01-01

120

Enantioselective aldol reactions catalyzed by chiral phosphine oxides.  

PubMed

The development of enantioselective aldol reactions catalyzed by chiral phosphine oxides is described. The aldol reactions presented herein do not require the prior preparation of the masked enol ethers from carbonyl compounds as aldol donors. The reactions proceed through a trichlorosilyl enol ether intermediate, formed in situ from carbonyl compounds, which then acts as the aldol donor. Phosphine oxides activate the trichlorosilyl enol ethers to afford the aldol adducts with high stereoselectivities. This procedure was used to realize a directed cross-aldol reaction between ketones and two types of double aldol reactions (a reaction at one/two ? position(s) of a carbonyl group) with high diastereo- and enantioselectivities. PMID:23828817

Kotani, Shunsuke; Sugiura, Masaharu; Nakajima, Makoto

2013-07-04

121

Density-functional theory studies of adsorbate bonding and structure on metal, metal oxide, and semiconductor surfaces  

NASA Astrophysics Data System (ADS)

In this thesis density functional theory is used to investigate the bonding and structure of adsorbates on a variety of metal, metal oxide, and semiconductor surfaces. Aspects of important catalytic processes such as partial oxidation and NO reduction are studied. Oxygen on Rh(111) was compared to oxygen on Pt(111), using cluster models of the surface, in order to determine the differences in binding that lead to differences in partial oxidation activity of the two surfaces. Oxygen is more strongly bound to Rh than Pt due to stronger covalent bonding, which affects subsequent reactivity. Rh selectively activates one C-H bond of methyl bound to the surface, explaining experimental vibrational measurements of the same system. Another catalyst used for partial oxidation, MoO3, is investigated, in particular to determine the chemical character of the different oxygens present in the material. Coverage effects are important, especially in methyl binding. At 1 ML coverages of methyl, formaldehyde and formyl are formed when methyl is placed over bridging oxygens, while at 0.5 ML coverages the methyl remains intact. Surface defects are also important in the activity of MoO3 for partial oxidation. In particular, the presence of a terminal oxygen vacancy enhances the dissociation of methane to methyl and hydrogen. Dinitrosyl is investigated as an intermediate of NO reduction on a surface defect of MoO3(010). The bonding of Mo to the dinitrosyl enhances N-N coupling while simultaneously weakening the N-O bond, possibly explaining the activity of MoO3 for NO reduction. STM images were simulated for 0.5 ML sulfur overlayer on Mo(110) in order to resolve descrepancies between LEED and STM. The bright spots in the experimental STM images do not correspond to the atomic sulfur positions, but rather to orbitals associated with the sulfur atom, explaining the differences between the experimental measurements. STM can also be used as a tool to identify reaction products on the surface. For example, the [4+2] and the [2+2] products of butadiene reaction on Si(100)-2x1 give distinct STM images. We show using a slab model for the Si(100) surface that the Diels-Alder [4+2] product is favored over the [2+2] product, in agreement with cluster calculations. The ability of density functional theory to accurately predict structures and bonding demonstrates its usefulness as another tool in elucidating surface phenomenon.

Chen, Melvin

1999-11-01

122

Oxidation bonding of porous silicon nitride ceramics with high strength and low dielectric constant  

Microsoft Academic Search

Silica (SiO2) bonded porous silicon nitride (Si3N4) ceramics were fabricated from ?-Si3N4 powder in air at 12001500C by the oxidation bonding process. Si3N4 particles are bonded by the oxidation-derive SiO2 and the pores derived from the stack of Si3N4 particles and the release of N2 and SiO gas during sintering. The influence of the sintering temperature and holding time on

Shuqiang Ding; Yu-Ping Zeng; Dongliang Jiang

2007-01-01

123

Breakdown of the spectator model for the OH bonds in studying the H+H2O reaction.  

PubMed

The time-dependent wave packet method is used to study the exchange and abstraction processes for the H+H2O reaction with both OH bonds in the H2O reactant treated as reactive bonds in full dimension. The calculation clearly shows that it is necessary to treat both OH bonds in this way in order to accurately investigate the exchange process. However, for the abstraction process, the spectator model works very well. Nonreactive treatment of one OH bond by using a few vibrational basis functions or even freezing the bond can yield very accurate abstraction reaction probability. PMID:12225190

Zhang, Dong H; Yang, Minghui; Lee, Soo-Y

2002-08-14

124

Theoretical Bond Dissociation Energies of Halo-Heterocycles: Trends and Relationships to Regioselectivity in Palladium-Catalyzed Cross-Coupling Reactions  

PubMed Central

Selectivity of the palladium-catalyzed cross-coupling reactions of heterocycles bearing multiple identical halogens is mainly determined by the relative ease of oxidative addition. This is related to both the energy to distort the carbon halogen bond to the transition-state geometry (related to the CX bond-dissociation energy) and to the interaction between the heterocycle ?* (LUMO) and PdL2 HOMO (J. Am. Chem. Soc. 2007, 129, 12664). The computed bond dissociation energies of a larger series of halo-heterocycles have been explored with B3LYP and higher accuracy G3B3 calculations. Quantitative trends in bond dissociation energies have been identified for five- and six-membered chloro and bromo substituted heterocycles with N, O, and S heteroatoms.

Garcia, Yeimy; Schoenebeck, Franziska; Legault, Claude Y.; Merlic, Craig A.; Houk, K. N.

2009-01-01

125

Studying Chemical Reactions, One Bond at a Time, with Single Molecule AFM Techniques  

NASA Astrophysics Data System (ADS)

The mechanisms by which mechanical forces regulate the kinetics of a chemical reaction are unknown. In my lecture I will demonstrate how we use single molecule force-clamp spectroscopy and protein engineering to study the effect of force on the kinetics of thiol/disulfide exchange. Reduction of disulfide bond via the thiol/disulfide exchange chemical reaction is crucial in regulating protein function and is of common occurrence in mechanically stressed proteins. While reduction is thought to proceed through a substitution nucleophilic bimolecular (SN2) reaction, the role of a mechanical force in modulating this chemical reaction is unknown. We apply a constant stretching force to single engineered disulfide bonds and measure their rate of reduction by dithiothreitol (DTT). We find that while the reduction rate is linearly dependent on the concentration of DTT, it is exponentially dependent on the applied force, increasing 10-fold over a 300 pN range. This result predicts that the disulfide bond lengthens by 0.34 at the transition state of the thiol/disulfide exchange reaction. In addition to DTT, we also study the reduction of the engineered disulfide bond by the E. coli enzyme thioredoxin (Trx). Thioredoxins are enzymes that catalyze disulfide bond reduction in all organisms. As before, we apply a mechanical force in the range of 25-450 pN to the engineered disulfide bond substrate and monitor the reduction of these bonds by individual enzymes. In sharp contrast with the data obtained with DTT, we now observe two alternative forms of the catalytic reaction, the first requiring a reorientation of the substrate disulfide bond, causing a shortening of the substrate polypeptide by 0.760.07 , and the second elongating the substrate disulfide bond by 0.210.01 . These results support the view that the Trx active site regulates the geometry of the participating sulfur atoms, with sub-ngstrm precision, in order to achieve efficient catalysis. Single molecule atomic force microscopy (AFM) techniques, as shown here, can probe dynamic rearrangements within an enzyme's active site which cannot be resolved with any other current structural biological technique. Furthermore, our work at the single bond level directly demonstrates that thiol/disulfide exchange in proteins is a force-dependent chemical reaction. Our findings suggest that mechanical force plays a role in disulfide reduction in vivo, a property which has never been explored by traditional biochemistry. 1.-Wiita, A.P., Ainavarapu, S.R.K., Huang, H.H. and Julio M. Fernandez (2006) Force-dependent chemical kinetics of disulfide bond reduction observed with single molecule techniques. Proc Natl Acad Sci U S A. 103(19):7222-7 2.-Wiita, A.P., Perez-Jimenez, R., Walther, K.A., Grter, F. Berne, B.J., Holmgren, A., Sanchez-Ruiz, J.M., and Fernandez, J.M. (2007) Probing the chemistry of thioredoxin catalysis with force. Nature, 450:124-7.

Fernandez, Julio M.

2008-03-01

126

Heterogeneous ozone oxidation reactions of 1-pentene, cyclopentene, cyclohexene, and a menthenol derivative studied by sum frequency generation.  

PubMed

We report vibrational sum frequency generation (SFG) spectra of glass surfaces functionalized with 1-pentene, 2-hexene, cyclopentene, cyclohexene, and a menthenol derivative. The heterogeneous reactions of ozone with hydrocarbons covalently linked to oxide surfaces serve as models for studying heterogeneous oxidation of biogenic terpenes adsorbed to mineral aerosol surfaces commonly found in the troposphere. Vibrational SFG is also used to track the C=C double bond oxidation reactions initiated by ozone in real time and to characterize the surface-bound product species. Combined with contact angle measurements carried out before and after ozonolysis, the kinetic and spectroscopic studies presented here suggest reaction pathways involving vibrationally hot Criegee intermediates that compete with pathways that involve thermalized surface species. Kinetic measurements suggest that the rate limiting step in the heterogeneous C=C double bond oxidation reactions is likely to be the formation of the primary ozonide. From the determination of the reactive uptake coefficients, we find that ozone molecules undergo between 100 and 10000 unsuccessful collisions with C=C double bonds before the reaction occurs. The magnitude of the reactive uptake coefficients for the cyclic and linear olefins studied here does not follow the corresponding gas-phase reactivities but rather correlates with the accessibility of the C=C double bonds at the surface. PMID:18942815

Stokes, Grace Y; Buchbinder, Avram M; Gibbs-Davis, Julianne M; Scheidt, Karl A; Geiger, Franz M

2008-10-23

127

A comparative evaluation of cobalt chromium oxide, cobalt manganese oxide, cobalt manganese oxide, and copper manganese oxide as catalysts for the water-gas shift reaction  

Microsoft Academic Search

Cobalt chromium oxide, cobalt manganese oxide, and copper manganese oxide have been compared as catalysts for the water-gas shift reaction. Cobalt chromium oxide and cobalt manganese oxide catalysts can give both high activity and long lifetimes for this reaction. Cobalt chromium oxide catalysts display higher activity compared to the other catalyst systems and this is shown that both the cobalt

G. J. Hutchings; R. G. Copperthwaite; F. M. Gottschalk; R. Hunter; J. Mellor; S. W. Orchard; T. Sangiorgio

1992-01-01

128

Electronic structure analysis of multistate reactivity in transition metal catalyzed reactions: the case of C-H bond activation by non-heme iron(IV)-oxo cores.  

PubMed

This perspective discusses the principles of the multistate scenario often encountered in transition metal catalyzed reactions, and is organized as follows. First, several important theoretical concepts (physical versus formal oxidation states, orbital interactions, use of (spin) natural and corresponding orbitals, exchange enhanced reactivity and the connection between valence bond and molecular orbital based electronic structure analysis) are presented. These concepts are then used to analyze the electronic structure changes occurring in the reaction of C-H bond oxidation by Fe(IV)oxo species. The analysis reveals that the energy separation and the overlap between the electron donating orbitals and electron accepting orbitals of the Fe(IV)oxo complexes dictate the reaction stereochemistry, and that the manner in which the exchange interaction changes depends on the identity of these orbitals. The electronic reorganization of the Fe(IV)oxo species during the reaction is thoroughly analyzed and it is shown that the Fe(IV)oxo reactant develops oxyl radical character, which interacts effectively with the ?CH orbital of the alkane. The factors that determine the energy barrier for the reaction are discussed in terms of molecular orbital and valence bond concepts. PMID:23632340

Ye, Shengfa; Geng, Cai-Yun; Shaik, Sason; Neese, Frank

2013-05-01

129

Energy expression of the chemical bond between atoms in metal oxides  

Microsoft Academic Search

The chemical bond between atoms in metal oxides is expressed in an energy scale. Total energy is partitioned into the atomic energy densities of constituent elements in the metal oxide, using energy density analysis. The atomization energies, ?EM for metal atom and ?EO for O atom, are then evaluated by subtracting the atomic energy densities from the energy of the

Yoshifumi Shinzato; Yuki Saito; Masahito Yoshino; Hiroshi Yukawa; Masahiko Morinaga; Takeshi Baba; Hiromi Nakai

2011-01-01

130

Application of chemical structure and bonding of actinide oxide materials for forensic science  

Microsoft Academic Search

We are interested in applying our understanding of actinide chemical structure and bonding to broaden the suite of analytical tools available for nuclear forensic analyses. Uranium- and plutonium-oxide systems form under a variety of conditions, and these chemical species exhibit some of the most complex behavior of metal oxide systems known. No less intriguing is the ability of AnO (An:

Wilkerson; Marianne Perry

2010-01-01

131

Reactions of a uranium (IV) tertiary alkyl bond. Facile ligand-assisted reduction and insertion of ethylene and carbon monoxide  

SciTech Connect

Reaction of (MeC{sub 5}H{sub 4}){sub 3}UX (X = Cl, MeC{sub 5}H{sub 4}) with t-BuLi affords the tertiary alkyl complex (MeC{sub 5}H{sub 4}){sub 3}U(t-Bu). Despite uranium(IV) generally being the preferred oxidation state in organometallic systems, (MeC{sub 5}H{sub 4}){sub 3}U(t-Bu) reacts with Lewis bases (L = PMe{sub 3}, THF, RCN, RNC) to yield the reduced uranium(III) base adducts (MeC{sub 5}H{sub 4}){sub 3}U(L). Carbon monoxide undergoes migratory insertion into the metal tertiary alkyl bond. The resulting acyl derivative decomposes at 90{degree}C to yield insoluble uranium-containing products and a mixture of tert-butyltoluenes by ring expansion of a methylcyclopentadienyl ligand. Ethylene also undergoes migratory insertion into the metal tertiary alkyl bond. No subsequent insertion of ethylene into the metal carbon bond takes place after the first equivalent has inserted. In marked contrast, reaction of various (MeC{sub 5}H{sub 4}){sub 3}ThX (X = Cl, I, MeC{sub 5}H{sub 4}, O-2,6-Me{sub 2}C{sub 6}H{sub 3}, OTs) compounds with t-BuLi gave intractable materials. However, reaction of the cationic species [(RC{sub 5}H{sub 4}){sub 3}Th](BPh{sub 4}) (R = Me{sub 3}Si, t-Bu) with t-BuLi yields the new thorium hydrides (RC{sub 5}H{sub 4}){sub 3}ThH. 40 refs., 2 figs.

Weydert, M.; Brennan, J.G.; Andersen, R.A.; Bergman, R.G. [Lawrence Berkeley Lab., CA (United States)

1995-08-01

132

Deamination, oxidation, and C-C bond cleavage reactivity of 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxycytosine.  

PubMed

Three new cytosine derived DNA modifications, 5-hydroxymethyl-2'-deoxycytidine (hmdC), 5-formyl-2'-deoxycytidine (fdC) and 5-carboxy-2'-deoxycytidine (cadC) were recently discovered in mammalian DNA, particularly in stem cell DNA. Their function is currently not clear, but it is assumed that in stem cells they might be intermediates of an active demethylation process. This process may involve base excision repair, C-C bond cleaving reactions or deamination of hmdC to 5-hydroxymethyl-2'-deoxyuridine (hmdU). Here we report chemical studies that enlighten the chemical reactivity of the new cytosine nucleobases. We investigated their sensitivity toward oxidation and deamination and we studied the C-C bond cleaving reactivity of hmdC, fdC, and cadC in the absence and presence of thiols as biologically relevant (organo)catalysts. We show that hmdC is in comparison to mdC rapidly oxidized to fdC already in the presence of air. In contrast, deamination reactions were found to occur only to a minor extent. The C-C bond cleavage reactions require the presence of high concentration of thiols and are acid catalyzed. While hmdC dehydroxymethylates very slowly, fdC and especially cadC react considerably faster to dC. Thiols are active site residues in many DNA modifiying enzymes indicating that such enzymes could play a role in an alternative active DNA demethylation mechanism via deformylation of fdC or decarboxylation of cadC. Quantum-chemical calculations support the catalytic influence of a thiol on the C-C bond cleavage. PMID:23980549

Schiesser, Stefan; Pfaffeneder, Toni; Sadeghian, Keyarash; Hackner, Benjamin; Steigenberger, Barbara; Schrder, Arne S; Steinbacher, Jessica; Kashiwazaki, Gengo; Hfner, Georg; Wanner, Klaus T; Ochsenfeld, Christian; Carell, Thomas

2013-09-17

133

Correlation among electronegativity, cation polarizability, optical basicity and single bond strength of simple oxides  

NASA Astrophysics Data System (ADS)

A suitable relationship between free-cation polarizability and electronegativity of elements in different valence states and with the most common coordination numbers has been searched on the basis of the similarity in physical nature of both quantities. In general, the cation polarizability increases with decreasing element electronegativity. A systematic periodic change in the polarizability against the electronegativity has been observed in the isoelectronic series. It has been found that generally the optical basicity increases and the single bond strength of simple oxides decreases with decreasing the electronegativity. The observed trends have been discussed on the basis of electron donation ability of the oxide ions and type of chemical bonding in simple oxides.

Dimitrov, Vesselin; Komatsu, Takayuki

2012-12-01

134

Low Temperature Bonding with Oxides for Electronic Device Applications.  

National Technical Information Service (NTIS)

We have found that silicon wafers can be bonded together at temperatures as low as 450 deg C, using a layer of boron glass between the wafers. With further development, this discovery could be used to advantage for silicon on insulator work, for sensor fa...

L. A. Field R. S. Muller

1988-01-01

135

Concerted O Atom-Proton Transfer in the O---O Bond Forming Step of Water Oxidation  

SciTech Connect

As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H{sub 2}O ? O{sub 2 + 4e{sup -} + 4H{sup +}) is key, but it imposes a significant mechanistic challenge with requirements for both 4e{sup -}/4H{sup +} loss and OO bond formation. Significant progress in water oxidation catalysis has been achieved recently by use of single-site Ru metal complex catalysts such as [Ru(Mebimpy)(bpy)(OH{sub 2})]{sup 2+} [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2?-bipyridine]. When oxidized from Ru{sup II}-OH{sub 2}{sup 2+} to Ru{sup V} = O{sup 3+}, these complexes undergo OO bond formation by O-atom attack on a H{sub 2}O molecule, which is often the rate-limiting step. Microscopic details of OO bond formation have been explored by quantum mechanical/molecular mechanical (QM/MM) simulations the results of which provide detailed insight into mechanism and a strategy for enhancing catalytic rates. It utilizes added bases as proton acceptors and concerted atomproton transfer (APT) with O-atom transfer to the O atom of a water molecule in concert with proton transfer to the base (B). Base catalyzed APT reactivity in water oxidation is observed both in solution and on the surfaces of oxide electrodes derivatized by attached phosphonated metal complex catalysts. These results have important implications for catalytic, electrocatalytic, and photoelectrocatalytic water oxidation.

Chen, Zuofeng; Concepcion, Javier C.; Hu, Xiangqian; Yang, Weitao; Hoertz, Paul G.; Meyer, Thomas J

2010-01-01

136

Thermal Oxidative Degradation Reactions of Perfluoroalkylethers.  

National Technical Information Service (NTIS)

The mechanisms operative in thermal oxidative degradation of Fomblin Z and hexafluoropropene oxide derived fluids and the effect of alloys and additives upon these processes are investigated. The nature of arrangements responsible for the inherent thermal...

K. L. Paciorek T. I. Ito R. H. Kratzer

1981-01-01

137

Room temperature reactions involving silicon dangling bond centers and molecular hydrogen in amorphous SiO2 thin films on silicon  

Microsoft Academic Search

Exposing thin films of amorphous SiO2 to molecular hydrogen at room temperature converts some silicon dangling bond defects, E' centers, into two hydrogen coupled complexes. It is argued that these reactions may play important roles in radiation and hot carrier instabilities in metal\\/oxide\\/silicon devices. The results suggest that an extension of the recent model of M.R. Shaneyfelt et al. (1990)

J. F. Conley; P. M. Lenahan

1992-01-01

138

A new approach to carbon-carbon bond formation: Development of aerobic Pd-catalyzed reductive coupling reactions of organometallic reagents and styrenes  

PubMed Central

Alkenes are attractive starting materials for organic synthesis and the development of new selective functionalization reactions are desired. Previously, our laboratory discovered a unique Pd-catalyzed hydroalkoxylation reaction of styrenes containing a phenol. Based upon deuterium labeling experiments, a mechanism involving an aerobic alcohol oxidation coupled to alkene functionalization was proposed. These results inspired the development of a new Pd-catalyzed reductive coupling reaction of alkenes and organometallic reagents that generates a new carbon-carbon bond. Optimization of the conditions for the coupling of both organostannanes and organoboronic esters is described and the initial scope of the transformation is presented. Additionally, several mechanistic experiments are outlined and support the rationale for the development of the reaction based upon coupling alcohol oxidation to alkene functionalization.

Gligorich, Keith M.; Iwai, Yasumasa; Cummings, Sarah A.; Sigman, Matthew S.

2009-01-01

139

Dependence of oxide surface structure on surface topology and local chemical bonding  

Microsoft Academic Search

The atomic geometries of the charge neutral surfaces of several oxides exhibiting different crystal structures and varying participation of O(2{ital p}) electrons in the chemical bonding have been calculated using tight-binding total energy models. Surface structures have been computed for exemplary cubic (MgO), wurtzite (ZnO), β-tridymite, and ideal β-cristobalite (SiO) oxides. The cubic oxide exhibits a minimum energy structure involving

John P. LaFemina; C. B. Duke

1991-01-01

140

Evidence of Trigonal Dangling Bonds at the Ge(111)/Oxide Interface by Electrically Detected Magnetic Resonance  

NASA Astrophysics Data System (ADS)

Despite a renewed interest in Ge as a competitor with Si for a broad range of electronic applications, the microstructure and the electronic properties of the dangling bonds that, in analogy with Si, are expected at the Ge/oxide interface have escaped a firm spectroscopy observation and characterization. Clear evidence based on contactless electrically detected magnetic resonance spectroscopy of a dangling bond at the Ge(111)/GeO2 interface is reported in this Letter. This result supports the similarity between dangling bonds at the Si(111)/oxide and Ge(111)/oxide interfaces, both showing C3v trigonal point symmetry with the main axis oriented along the ?111? direction. In contrast, at the Ge(001)/oxide interface the absence of the trigonal center in favor of a lower symmetry dangling bond marks the difference with the Si(001)/oxide interface, where both centers are present and the one having higher point symmetry prevails. This fact is rationalized in terms of suboxide interface rearrangement and oxide viscoelasticity, which promote the generation of the nonaxial centers at distorted dimers. The unambiguous identification of the centers at the Ge/oxide interfaces yields a deeper insight into the physical properties of the suboxide interface structure and offers a valid indicator for the evaluation of different surface capping and passivation techniques, with the potential to boost the Ge-related technology.

Paleari, S.; Baldovino, S.; Molle, A.; Fanciulli, M.

2013-05-01

141

Carbon-oxygen bond cleavage reactions by electron transfer. 1. Electrochemical studies on the formation and subsequent reaction pathways of cyanoanisole radical anions  

SciTech Connect

The radical anions of three isomers of cyanoanisole have been electrochemically generated and subsequently shown to react by at least three different reaction pathways in dry N,N-dimethylformamide (DMF). The o-cyanoanisole radical anion (E/sub pc/ = -2.3 V vs. SCE) dimerizes (k/sub 2/ = 3.2 X 10/sup 2/ M/sup -1/s/sup -1/) to form an intermediate dimeric dianion. The dianion, which can be oxidized (E/sub pa = 1.1 V vs. SCE), undergoes a slow intramolecular disproportionation reaction to form o-cyanophenoxide ion, methide ion, and unreduced substrate (k/sub 3/ = 1.9 X 10/sup -2/s/sup -1/). Subsequent protonation results in the formation of methane and p-cyanophenol in an overall two-electron process. The m-cyanoanisole radical anion (E/sub pc/ = -2.3 V vs. SCE) is very stable (t/sub 1/2/ > 10/sup 3/ s) under anhydrous DMF conditions. Overall, slow carbon-carbon bond cleavage with loss of cyanide occurs competitively with ..beta.. carbon-oxygen bond cleavage to produce anisole and m-cyanophenol, respectively. The final products of the reduction of p-cyanoanisole are p-cyanophenol and methane; however, the radical anion of p-cyanoanisole (E/sub pc/ = -2.5 V vs. SCE) undergoes a relatively rapid unimolecular fragmentation reaction (k/sub 1/ = 7 s/sup -1/). The initial products of the fragmentation are p-cyanophenoxide ion and the methyl radical, which is reduced further to methide ion. Hydrogen atom abstraction reactions by the methyl radical can also occur in the bulk solution to produce methane.

Koppang, M.D.; Woolsey, N.F.; Bartak, D.E.

1984-05-16

142

A bond-topological approach to theoretical mineralogy: crystal structure, chemical composition and chemical reactions  

NASA Astrophysics Data System (ADS)

Here, I describe a theoretical approach to the structure and chemical composition of minerals based on their bond topology. This approach allows consideration of many aspects of minerals and mineral behaviour that cannot be addressed by current theoretical methods. It consists of combining the bond topology of the structure with aspects of graph theory and bond-valence theory (both long range and short range), and using the moments approach to the electronic energy density-of-states to interpret topological aspects of crystal structures. The structure hierarchy hypothesis states that higher bond-valence polyhedra polymerize to form the (usually anionic) structural unit, the excess charge of which is balanced by the interstitial complex (usually consisting of large low-valence cations and (H2O) groups). This hypothesis may be justified within the framework of bond topology and bond-valence theory, and may be used to hierarchically classify oxysalt minerals. It is the weak interaction between the structural unit and the interstitial complex that controls the stability of the structural arrangement. The principle of correspondence of Lewis acidity-basicity states that stable structures will form when the Lewis-acid strength of the interstitial complex closely matches the Lewis-base strength of the structural unit, and allows us to examine the factors that control the chemical composition and aspects of the structural arrangements of minerals. It also provides a connection between a structure, the speciation of its constituents in aqueous solution and its mechanism of crystallization. The moments approach to the electronic energy density-of-states provides a link between the bond topology of a structure and its thermodynamic properties, as indicated by correlations between average anion coordination number and reduced enthalpy of formation from the oxides for [6]Mg{/m [4]}Si n O( m+2 n) and MgSO4(H2O) n .

Hawthorne, Frank C.

2012-11-01

143

Minimum energy spin crossings for an O-O bond formation reaction  

NASA Astrophysics Data System (ADS)

The O-O bond formation reaction in the synthetic O 2 formation catalyst, [(terpy)(H 2O)Mn IV(?-O) 2Mn III(H 2O)(terpy)] 3+ (terpy = 2,2':6,2?-terpyridine), goes through a transition between two spin surfaces. Using DFT with the B3LYP functional, three different minimum energy crossing points have been located for this redox reaction. The system is predicted to perform the spin crossing close to (+0.1 kcal/mol) the low-spin Mn III-OOH product and this transition does not significantly affect the kinetics. The location of spin crossings in this system is relevant for discussions of O-O bond formation in the oxygen evolving complex of photosystem II.

Lundberg, Marcus; Siegbahn, Per E. M.

2005-01-01

144

The role of intramolecular hydrogen bonds in nucleophilic addition reactions of ketenaminals  

NASA Astrophysics Data System (ADS)

Quantum-chemical calculations of the geometries and electronic structures of molecules of ketenaminals 3-(diaminomethylene)-2,4-pentanedione and dimethyl-2-(diaminomethylene)-malonate and calculations of the structures of intermediates in the reaction of the nucleophilic addition of the ketenaminals to the acetonitrile molecule are performed by B3LYP/6-31+G** method. Two possible scenarios of the process are shown, depending on the mutual orientation of reacting molecules. The nucleophilic addition proceeds in two stages. It is found that the rate-limiting stage of the process is the transfer of the proton of the intramolecular hydrogen bond in a ketenaminal molecule. The experimentally observed faster reaction of pyrimidine formation for the 3-(diaminomethylene)-2,4-pentanedione molecule relative to that for dimethyl-2-(diaminomethylene)-malonate is explained by the hydrogen bond being stronger and the barrier of proton transfer from the aminogroup to the ketogroup oxygen falling upon nucleophilic attack in the former molecule.

Isaev, A. N.

2012-08-01

145

A novel solution for hydroxylated PAHs removal by oxidative coupling reaction using Mn oxide  

Microsoft Academic Search

In this study, removals of 1-naphthol by oxidative-coupling reaction using birnessite, one of the natural Mn oxides present in soil, was investigated in various experimental conditions (reaction time, Mn oxide loadings, pH). The removal efficiency of 1-naphthol by birnessite was high in all the experimental conditions, and UV-visible and mass spectrometric analyses on the supernatant after reaction confirmed that the

Ki-Hoon Kang; Dong-Min Lim; Hyun-Sang Shin

2008-01-01

146

Direct functionalization of M-C (M = Pt(II), Pd(II)) bonds using environmentally benign oxidants, O2 and H2O2.  

PubMed

Atom economy and the use of "green" reagents in organic oxidation, including oxidation of hydrocarbons, remain challenges for organic synthesis. Solutions to this problem would lead to a more sustainable economy because of improved access to energy resources such as natural gas. Although natural gas is still abundant, about a third of methane extracted in distant oil fields currently cannot be used as a chemical feedstock because of a dearth of economically and ecologically viable methodologies for partial methane oxidation. Two readily available "atom-economical" "green" oxidants are dioxygen and hydrogen peroxide, but few methodologies have utilized these oxidants effectively in selective organic transformations. Hydrocarbon oxidation and C-H functionalization reactions rely on Pd(II) and Pt(II) complexes. These reagents have practical advantages because they can tolerate moisture and atmospheric oxygen. But this tolerance for atmospheric oxygen also makes it challenging to develop novel organometallic palladium and platinum-catalyzed C-H oxidation reactions utilizing O(2) or H(2)O(2). This Account focuses on these challenges: the development of M-C bond (M = Pt(II), Pd(II)) functionalization and related selective hydrocarbon C-H oxidations with O(2) or H(2)O(2). Reactions discussed in this Account do not involve mediators, since the latter can impart low reaction selectivity and catalyst instability. As an efficient solution to the problem of direct M-C oxidation and functionalization with O(2) and H(2)O(2), this Account introduces the use of facially chelating semilabile ligands such as di(2-pyridyl)methanesulfonate and the hydrated form of di(2-pyridyl)ketone that enable selective and facile M(II)-C(sp(n)) bond functionalization with O(2) (M = Pt, n = 3; M = Pd, n = 3 (benzylic)) or H(2)O(2) (M = Pd, n = 2). The reactions proceed efficiently in protic solvents such as water, methanol, or acetic acid. With the exception of benzylic Pd(II) complexes, the organometallic substrates studied form isolable high-valent Pt(IV) or Pd(IV) intermediates as a result of an oxidant attack at the M(II) atom. The resulting high-valent M(IV) intermediates undergo C-O reductive elimination, leading to products in high yields. Guidelines for the synthesis of products containing other C-X bonds (X = OAc, Cl, Br) while using O(2) or H(2)O(2) as oxidants are also discussed. Although the M(II)-C bond functionalization reactions including high-valent intermediates are well understood, the mechanism for the aerobic functionalization of benzylic Pd(II) complexes will require a more detailed exploration. Importantly, further optimization of the systems suitable for stoichiometric M(II)-C bond functionalization led to the development of catalytic reactions, including selective acetoxylation of benzylic C-H bonds with O(2) as the oxidant and hydroxylation of aromatic C-H bonds with H(2)O(2) in acetic acid solutions. Both reactions proceed efficiently with substrates that contain a directing heteroatom. This Account also describes catalytic methods for ethylene dioxygenation with H(2)O(2) using M(II) complexes supported by facially chelating ligands. Mechanistic studies of these new oxidation reactions point to important ways to improve their substrate scope and to develop "green" CH functionalization chemistry. PMID:22087633

Vedernikov, Andrei N

2011-11-16

147

The energy and geometric characteristics of the transition state in reactions of RO 2 with carbonyl compound CH bonds  

Microsoft Academic Search

The energy and geometry of the transition state in reactions of the ethyl peroxyl radical with ethane, ethanol (its ? and\\u000a ? C-H bonds), acetone, butanone-2, and acetaldehyde were calculated by the density functional theory method. In all these\\u000a reactions (except EtO2\\/ + ethanol ? C-H bond), the CHO reaction center has an almost linear configuration (? = 176

A. F. Shestakov; E. T. Denisov; N. S. Emelyanova; T. G. Denisova

2009-01-01

148

The energy and geometric characteristics of the transition state in reactions of RO{2\\/} with carbonyl compound CH bonds  

Microsoft Academic Search

The energy and geometry of the transition state in reactions of the ethyl peroxyl radical with ethane, ethanol (its alpha and beta C-H bonds), acetone, butanone-2, and acetaldehyde were calculated by the density functional theory method. In all these reactions (except EtO2\\/ + ethanol alpha C-H bond), the C...H...O reaction center has an almost linear configuration (phi = 176

A. F. Shestakov; E. T. Denisov; N. S. Emel'Yanova; T. G. Denisova

2009-01-01

149

Study on tool wear characteristics in diamond turning of reaction-bonded silicon carbide  

Microsoft Academic Search

Tool wear is one of the most critical problems in machining hard, brittle materials. In the present work, diamond turning\\u000a experiments were performed on reaction-bonded silicon carbide, and the tool wear characteristics were investigated. A special\\u000a kind of wear pattern, namely periodical groove wear, was identified on the flank face of the tool, where the periodicity of\\u000a the microgrooves was

Zhiyu Zhang; Jiwang Yan; Tsunemoto Kuriyagawa

150

Mechanism for material removal in diamond turning of reaction-bonded silicon carbide  

Microsoft Academic Search

Reaction-bonded silicon carbide (RB-SiC) is a new ceramic material that has extremely high strength and hardness. Diamond turning experiments were performed on RB-SiC to investigate the microscopic material removal mechanism. Diamond tools with large nose radii of 10mm were used for machining. It was found that the surface roughness was not significantly affected by the tool feed rate, but was

Jiwang Yan; Zhiyu Zhang; Tsunemoto Kuriyagawa

2009-01-01

151

The dependence of the elastic moduli of reaction bonded alumina on porosity  

Microsoft Academic Search

The resonant beam technique (RBT) allows the simultaneous determination of the Young's and the shear modulus. In the present paper this method is extended to materials covering an extremely wide range of porosities from nearly dense to the limit of stability, which is shown for ZrO2-toughened reaction-bonded Al2O3 (RBAO) ceramics as an example. Additional characterisation of the pore structure and

Stephan Puchegger; Frank Dose; Dieter Loidl; Karl Kromp; Rolf Janssen; D. Brandhuber; N. Hsing; Herwig Peterlik

2007-01-01

152

Recent advances in reaction bonded silicon carbide optics and optical systems  

Microsoft Academic Search

SSG Precision Optronics, Inc. (SSG) has recently developed a number of Reaction Bonded (RB) Silicon Carbide (SiC) optical systems for space-based remote sensing and astronomical observing applications. RB SiC's superior material properties make it uniquely well suited to meet the image quality and long term dimensional stability requirements associated with these applications. An overview of the RB SiC manufacturing process

Joseph Robichaud; Jay Schwartz; David Landry; William Glenn; Brian Rider; Michael Chung

2005-01-01

153

Manufacture of Phi1.2m reaction bonded silicon carbide mirror blank CFID  

Microsoft Academic Search

Silicon carbide (SiC) is a new type candidate material for large-scale lightweight space mirror. Its low thermal distortion, high stiffness, fine optical quality and dimensional stability, make SiC an ideal material for large space born telescope. Since ten years Changchun institute optics, fine mechanics and physics (CIOMP) has developed reaction bonded SiC (RB-SiC) technology for space application, and can fabricate

Ge Zhang; Rucheng Zhao; Wenxing Zhao; Jianxun Bao

2010-01-01

154

Asymmetric vinylogous aldol reaction via H-bond-directing dienamine catalysis.  

PubMed

The enantioselective direct vinylogous aldol reaction of 3-methyl 2-cyclohexen-1-one with ?-keto esters has been developed. The key to success was the design of a bifunctional primary amine-thiourea catalyst that can combine H-bond-directing activation and dienamine catalysis. The simultaneous dual activation of the two reacting partners results in high reactivity while securing high levels of stereocontrol. PMID:23259650

Bastida, David; Liu, Yankai; Tian, Xu; Escudero-Adn, Eduardo; Melchiorre, Paolo

2012-12-21

155

Kinetics of Chlorine Oxide Reactions. Ii. The Reaction of Nitrogen Atoms with Chlorine Monoxide.  

National Technical Information Service (NTIS)

The reaction was studied mass spectrometrically in a fast-flow system at pressure near 0.3 torr. Final reaction products were N2, O2 and Cl2. The ratio of Cl2O to N consumed at long reaction times was 0.93 with a fivefold excess of Cl2O. No nitric oxide w...

C. G. Freeman L. F. Phillips

1968-01-01

156

E1 reaction-induced synthesis of hydrophilic oxide nanoparticles in a non-hydrophilic solvent.  

PubMed

In this paper, tert-amyl alcohol was employed to directly react with metal chlorides for the preparation of oxide nanoparticles. Some typical metal oxide or hydroxides with different morphologies, such as TiO(2) nanoparticles, TiO(2) nanorods, FeOOH nanowires, Fe(2)O(3) nanoparticles, and SnO(2) nanoparticles, can be easily fabricated through such simple chemical reactions. E1 reaction was found to play the leading role in the synthesis of metal oxides attributed to better stability of tertiary carbocations in tert-amyl alcohol and the strong interaction of metal chlorides with hydroxyl groups that results in the easy dissociation of carbon-oxygen bonds in tert-amyl alcohol. S(N)1 reaction can also occur in certain reactions due to nucleophilic substitution of chloride ions for hydroxyl groups. As-prepared metal oxides show good compatibility with an aqueous system while they were synthesized in a non-hydrophilic solvent probably attributed to the specific E1 reaction mechanism involving the generation of water, and can be directly incorporated into an aqueous soluble polymer, such as PVA, to exhibit many promising applications. PMID:22990362

Hu, Ming-Jun; Gao, Jiefeng; Yang, Shiliu; Dong, Yucheng; Wong, Julia Shuk Ping; Xu, Jiaju; Shan, Guangcun; Li, Robert K Y

2012-10-21

157

Evaluation of reaction mechanism of coal-metal oxide interactions in chemical-looping combustion  

SciTech Connect

The knowledge of reaction mechanism is very important in designing reactors for chemical-looping combustion (CLC) of coal. Recent CLC studies have considered the more technically difficult problem of reactions between abundant solid fuels (i.e. coal and waste streams) and solid metal oxides. A definitive reaction mechanism has not been reported for CLC reaction of solid fuels. It has often been assumed that the solid/solid reaction is slow and therefore requires that reactions be conducted at temperatures high enough to gasify the solid fuel, or decompose the metal oxide. In contrast, data presented in this paper demonstrates that solid/solid reactions can be completed at much lower temperatures, with rates that are technically useful as long as adequate fuel/metal oxide contact is achieved. Density functional theory (DFT) simulations as well as experimental techniques such as thermo-gravimetric analysis (TGA), flow reactor studies, in situ X-ray photo electron spectroscopy (XPS), in situ X-ray diffraction (XRD) and scanning electron microscopy (SEM) are used to evaluate how the proximal interaction between solid phases proceeds. The data indicate that carbon induces the Cu-O bond breaking process to initiate the combustion of carbon at temperatures significantly lower than the spontaneous decomposition temperature of CuO, and the type of reducing medium in the vicinity of the metal oxide influences the temperature at which the oxygen release from the metal oxide takes place. Surface melting of Cu and wetting of carbon may contribute to the solid-solid contacts necessary for the reaction. (author)

Siriwardane, Ranjani; Richards, George; Poston, James [US Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, WV 26507-0880 (United States); Tian, Hanjing; Miller, Duane; Simonyi, Thomas [US Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, P.O. Box 880, Morgantown, WV 26507-0880 (United States); URS, 3610 Collins Ferry Road, Morgantown, WV 26505 (United States)

2010-11-15

158

Cofactor-apoprotein hydrogen bonding in oxidized and fully reduced flavodoxin monitored by trans-hydrogen-bond scalar couplings.  

PubMed

Hydrogen bonding plays a key role in the tight binding of the FMN cofactor and the regulation of its redox properties in flavodoxins. Hydrogen bonding interactions can be directly observed in solution by multidimensional heteronuclear NMR spectroscopy through the scalar couplings between donor and acceptor nuclei. Here we report on the detection of intermolecular trans-hydrogen-bond couplings ((h)J) between the flavin ring system and the backbone of Desulfovibrio vulgaris flavodoxin in the oxidized and the two-electron reduced states. For this purpose, experiments are adapted from pulse sequences previously applied to determining (h)J coupling constants in nucleic acid-base pairs and proteins. The resulting (h2)J(N,N), (h4)J(N,N), (h3)J(C,N), and (h1)J(H,N) couplings involve the (15)N(1), (13)C(2), and (15)N(3) nuclei of the pyrimidine moiety of FMN, whereas no such interactions are detectable for (13)C(4) and (15)N(5). Several long-range (15)N-(15)N, (13)C-(15)N, and (1)H-(15)N J-coupling constants within the flavin are obtained as "by-products". The magnitudes of both (h)J and regular J couplings are found to be dependent on the redox state. In general, good correlations between (h)J coupling constants and donor-group (1)H chemical shifts and also crystallographic donor-acceptor distances are observed. PMID:15515086

Lhr, Frank; Yalloway, Gary N; Mayhew, Stephen G; Rterjans, Heinz

2004-11-01

159

Pd(II)-catalyzed cross-coupling of sp3 C-H Bonds with sp2 and sp3 boronic acids using air as the oxidant.  

PubMed

O-Methyl hydroxamic acids, readily available from carboxylic acids, are found to be extremely reactive for beta-C-H activation by Pd(OAc)2. This reactivity is exploited to develop the first example of cross-coupling sp3 C-H bonds with sp3 boronic acids. Air was shown to be a suitable stoichiometric oxidant for the catalytic oxidative coupling reaction. A biologically active natural product is readily converted to its novel analogues through this coupling reaction. PMID:18479089

Wang, Dong-Hui; Wasa, Masayuki; Giri, Ramesh; Yu, Jin-Quan

2008-05-14

160

Isomerization of the double bonds of a conjugated fatty acid during ?-oxidation  

Microsoft Academic Search

The ?-oxidation of an unsaturated fatty acid containing conjugated double bonds at odd-numbered carbon atoms has not previously\\u000a been studied. It is, therefore, not clear whether, during the ?-oxidation of such an acid, the double bonds will be isomerized\\u000a by enoyl-CoA isomerase (?3-?2-enoyl-CoA isomerase) with the loss or retention of its conjugated nature. To investigate the problem, (E,E)-3,5-octadienoyl-CoA\\u000a was synthesized

L. M. Du Plessis; N. Grobbelaar

1979-01-01

161

Iodine Oxide Thermite Reactions: Physical and Biological Effects  

Microsoft Academic Search

We investigated the potential for some thermite-like material reactions to kill bacteria spores. Iodine oxides and silver oxides react vigorously with metals like aluminum, tantalum, and neodymium. These reactions theoretically produce temperatures as high as 8000K, leading to vaporization of the reactants, producing very hot iodine and\\/or silver gases. We performed a series of computations and experiments to characterize these

Rod Russell; Michelle Pantoya; Stephan Bless; William Clark

2009-01-01

162

Kinetics of Metal Atom Oxidation Reactions.  

National Technical Information Service (NTIS)

The tubular fast-flow reactor, previously used for reaction kinetic measurements at temperatures up to only 1000K, has been adapted to reach temperatures up to 2000K and has been applied to studies of gas phase reactions of metal atoms with O2. The most e...

A. Fontijn S. C. Kurzius J. J. Houghton

1972-01-01

163

Ternary electrocatalysts for oxidizing ethanol to carbon dioxide: making ir capable of splitting C-C bond.  

PubMed

Splitting the C-C bond is the main obstacle to electrooxidation of ethanol (EOR) to CO(2). We recently demonstrated that the ternary PtRhSnO(2) electrocatalyst can accomplish that reaction at room temperature with Rh having a unique capability to split the C-C bond. In this article, we report the finding that Ir can be induced to split the C-C bond as a component of the ternary catalyst. We characterized and compared the properties of several carbon-supported nanoparticle (NP) electrocatalysts comprising a SnO(2) NP core decorated with multimetallic nanoislands (MM' = PtIr, PtRh, IrRh, PtIrRh) prepared using a seeded growth approach. An array of characterization techniques were employed to establish the composition and architecture of the synthesized MM'/SnO(2) NPs, while electrochemical and in situ infrared reflection absorption spectroscopy studies elucidated trends in activity and the nature of the reaction intermediates and products. Both EOR reactivity and selectivity toward CO(2) formation of several of these MM'/SnO(2)/C electrocatalysts are significantly higher compared to conventional Pt/C and Pt/SnO(2)/C catalysts. We demonstrate that the PtIr/SnO(2)/C catalyst with high Ir content shows outstanding catalytic properties with the most negative EOR onset potential and reasonably good selectivity toward ethanol complete oxidation to CO(2). PMID:23210450

Li, Meng; Cullen, David A; Sasaki, Kotaro; Marinkovic, Nebojsa S; More, Karren; Adzic, Radoslav R

2012-12-14

164

Investigating N-methoxy-N'-aryl ureas in oxidative C-H olefination reactions: an unexpected oxidation behaviour.  

PubMed

Herein, we report a urea derived directing group for mild and highly selective oxidative C-H bond olefination. Subsequent intramolecular Michael addition affords dihydroquinazolinones in good yields. The N-O bond of the urea substrate exhibits superior oxidative behaviour compared to a variety of other external oxidants. PMID:21625722

Willwacher, Jens; Rakshit, Souvik; Glorius, Frank

2011-05-31

165

Bond length and radii variations in fluoride and oxide molecules and crystals  

NASA Astrophysics Data System (ADS)

Molecular orbital calculations completed on fluoride molecules containing first and second row cations have generated bond lengths, R, that match those observed for coordinated polyhedra in crystals to within 0.04 , on average. The calculated bond lengths and those observed for fluoride crystals can be ranked with the expression R=Kp -0.22, where p=s/r, s is the Pauling strength of the bond, r is the row number of the cation and K=1.34. The exponent -0.22 (? -2/9) is the same as that observed for oxide, nitride and sulfide molecules and crystals. Bonded radii for the fluoride anion, obtained from theoretical electron density maps, increase linearly with bond length. Those calculated for the cations as well as for the fluoride anion match calculated promolecule radii to within 0.03 , on average, suggesting that the electron density distributions in the vicinity of the minima along the bond paths possess a significant atomic component despite bond type. Bonded radii for Si and O ions provided by experimental electron density maps measured for the oxides coesite, danburite and stishovite match those calculated for a series of monosilicic acid molecules. The resulting radii increase with bond length and coordination number with the radius of the oxide ion increasing at a faster rate than that of the Si cation. The oxide ion within danburite exhibits several distinct radii, ranging between 0.9 and 1.2 , rather than a single radius with each exhibiting a different radius along each of the nonequivalent bonds with B, Si and Ca. Promolecule radii calculated for the coordinated polyhedra in danburite match procrystal radii obtained in a structure analysis to within 0.002 . The close agreement between these two sets of radii and experimentally determined bonded radii lends credence to Slater's statement that the difference between the electron density distribution observed for a crystal and that calculated for a procrystal (IAM) model of the crystal would be small and subtle, and very hard to determine by examination of the total charge density.

Nicoll, J. S.; Gibbs, G. V.; Boisen, M. B.; Downs, R. T.; Bartelmehs, K. L.

1994-05-01

166

Guided ion beam studies of the reactions of V{sub n}{sup +} (n=2{endash}17) with O{sub 2}: Bond energies and dissociation pathways  

SciTech Connect

The kinetic energy dependence of the reactions of V{sub n}{sup +} (n=2{endash}17) with oxygen is studied using a guided ion beam mass spectrometer. In all but the smallest clusters, the primary reaction process at low energies is the formation of a vanadium cluster dioxide ion which then loses one or two vanadium atoms or a vanadium oxide diatom (VO). Vanadium atom loss is the preferred reaction pathway for large clusters (n{ge}5), whereas loss of VO is more favorable for the smallest reactant clusters (n{le}4). As the collision energy is increased, these primary products dissociate further by loss of additional vanadium atoms. Bond dissociation energies of the vanadium cluster oxides are determined by analysis of the kinetic energy dependence of several different products. The effect of oxygen atoms on the stabilities of vanadium cluster ions is discussed and compared with bulk phase thermochemistry. {copyright} {ital 1998 American Institute of Physics.}

Xu, J.; Rodgers, M.T.; Griffin, J.B.; Armentrout, P.B. [Department of Chemistry, University of Utah, Salt Lake City, Utah84112 (United States)

1998-06-01

167

Kinetic Modeling of Nitric-Oxide-Associated Reaction Network  

Microsoft Academic Search

PurposeNitric oxide and superoxide are the two important free radicals in the biological system. The coexistence of both free radicals in the physiological milieu gives rise to intricate oxidative and nitrosative reactions, which have been implicated in many physiological and\\/or pathophysiological conditions, such as vasodilatation and inflammation. It is difficult, if not impossible, to study the complexity of the nitric

Teh-Min Hu; William L. Hayton; Susan R. Mallery

2006-01-01

168

Mixed peroxides from the chloroperoxidase-catalyzed oxidation of conjugated dienoic esters with a trisubstituted terminal double bond  

Microsoft Academic Search

The chloroperoxidase (CPO)-catalyzed oxidations of conjugated dienoic esters with a trisubstituted terminal double bond were studied by using tert-butyl hydroperoxide as the terminal oxidant. Most of the substrates gave disubstituted mixed peroxides as the major products.

Despina J. Bougioukou; Ioulia Smonou

2002-01-01

169

CHEMICAL REACTIONS OF AQUATIC HUMIC MATERIALS WITH SELECTED OXIDANTS  

EPA Science Inventory

A study was conducted to identify the specific organic reaction products of natural aquatic humic materials with selected oxidants (KMnO4, HOCl, Cl02, O3 and monochloramine). Reaction products were identified by GC/MS after solvent extraction and derivatization. The two most reac...

170

HETEROGENEOUS REACTIONS OF NITROGEN OXIDES IN SIMULATED ATMOSPHERES  

EPA Science Inventory

A laboratory study has been conducted on heterogeneous reactions of nitrogen dioxide and nitric oxide to evaluate their potential role in reaction in polluted urban atmosphere. The results of this study suggest that nitrogen dioxide decomposes on a wide variety of solids likely t...

171

Thermal expansion and thermal strength of oxide refractories bonded with silicon nitrides  

Microsoft Academic Search

In this article the results are reported of an experimental investigation of the thermal expansion and certain aspects of the thermal strength of granular oxide refractories based on corundum and periclase with various binders. The oxide bonds were produced by firing specimens containing 70~ corundum or periclase grains 0.25-0.5 mm in diameter and 30~ fine-ground powders of the same chemical

I. Ya. Guzman; E. V. Kosokina; E. I. Tumakova

1974-01-01

172

O-demethylations catalyzed by rieske nonheme iron monooxygenases involve the difficult oxidation of a saturated C-h bond.  

PubMed

Dicamba monooxygenase (DMO) catalyzes the O-demethylation of dicamba (3,6-dichloro-2-methoxybenzoate) to produce 3,6-dichlorosalicylate and formaldehyde. Recent crystallographic studies suggest that DMO catalyzes the challenging oxidation of a saturated C-H bond within the methyl group of dicamba to form a hemiacetal intermediate. Testing of this hypothesis was made possible by our development of two new independent techniques. As a novel method to allow use of (18)O2 to follow reaction products, bisulfite was used to trap newly formed (18)O-formaldehyde in the stable adduct, hydroxymethanesulfonate (HMS(-)), and thereby prevent the rapid exchange of (18)O in formaldehyde with (16)O in water. The second technique utilized unique properties of Pseudomonas putida formaldehyde dehydrogenase that allow rapid conversion of (18)O-formaldehyde into stable and easily detectable (18)O-formic acid. Experiments using these two new techniques provided compelling evidence for DMO-catalyzed oxidation of the methyl group of dicamba, thus validating a mechanism for DMO [and for vanillate monooxygenase, a related Rieske nonheme iron monooxygenase] that involves the difficult oxidation of a saturated C-H bond. PMID:23719540

Jiang, Wenzhi; Wilson, Mark A; Weeks, Donald P

2013-06-10

173

Trends in CO and CN bond formations over transition metal surfaces: An insight into kinetic sensitivity in catalytic reactions  

Microsoft Academic Search

Transition metal catalyzed bond formation is a fundamental process in catalysis and is of general interest throughout chemistry. To date, however, the knowledge of association reactions is rather limited, relative to what is known about dissociative processes. For example, surprisingly little is known about how the bond-forming ability of a metal, in general, varies across the Periodic Table. In particular,

Paul Crawford; P. Hu

2007-01-01

174

Engaging zwitterions in carbon-carbon and carbon-nitrogen bond-forming reactions: A promising synthetic strategy.  

PubMed

An Account of carbon-carbon and carbon-nitrogen bond-forming reactions mediated by zwitterions generated by the addition of organic nucleophiles to activated unsaturated systems highlighting their synthetic potential is presented. PMID:16906748

Nair, Vijay; Menon, Rajeev S; Sreekanth, A R; Abhilash, N; Biju, A T

2006-08-01

175

Method for producing bio-fuel that integrates heat from carbon-carbon bond-forming reactions to drive biomass gasification reactions  

DOEpatents

A low-temperature catalytic process for converting biomass (preferably glycerol recovered from the fabrication of bio-diesel) to synthesis gas (i.e., H.sub.2/CO gas mixture) in an endothermic gasification reaction is described. The synthesis gas is used in exothermic carbon-carbon bond-forming reactions, such as Fischer-Tropsch, methanol, or dimethylether syntheses. The heat from the exothermic carbon-carbon bond-forming reaction is integrated with the endothermic gasification reaction, thus providing an energy-efficient route for producing fuels and chemicals from renewable biomass resources.

Cortright, Randy D. (Madison, WI); Dumesic, James A. (Verona, WI)

2012-04-10

176

Method for producing bio-fuel that integrates heat from carbon-carbon bond-forming reactions to drive biomass gasification reactions  

DOEpatents

A low-temperature catalytic process for converting biomass (preferably glycerol recovered from the fabrication of bio-diesel) to synthesis gas (i.e., H.sub.2/CO gas mixture) in an endothermic gasification reaction is described. The synthesis gas is used in exothermic carbon-carbon bond-forming reactions, such as Fischer-Tropsch, methanol, or dimethylether syntheses. The heat from the exothermic carbon-carbon bond-forming reaction is integrated with the endothermic gasification reaction, thus providing an energy-efficient route for producing fuels and chemicals from renewable biomass resources.

Cortright, Randy D.; Dumesic, James A.

2013-04-02

177

Method for producing bio-fuel that integrates heat from carbon-carbon bond-forming reactions to drive biomass gasification reactions  

DOEpatents

A low-temperature catalytic process for converting biomass (preferably glycerol recovered from the fabrication of bio-diesel) to synthesis gas (i.e., H.sub.2/CO gas mixture) in an endothermic gasification reaction is described. The synthesis gas is used in exothermic carbon-carbon bond-forming reactions, such as Fischer-Tropsch, methanol, or dimethylether syntheses. The heat from the exothermic carbon-carbon bond-forming reaction is integrated with the endothermic gasification reaction, thus providing an energy-efficient route for producing fuels and chemicals from renewable biomass resources.

Cortright, Randy D. (Madison, WI); Dumesic, James A. (Verona, WI)

2011-01-18

178

Disulfide Bond Formation and Activation of Escherichia coli ?-Galactosidase under Oxidizing Conditions  

PubMed Central

Escherichia coli ?-galactosidase is probably the most widely used reporter enzyme in molecular biology, cell biology, and biotechnology because of the easy detection of its activity. Its large size and tetrameric structure make this bacterial protein an interesting model for crystallographic studies and atomic mapping. In the present study, we investigate a version of Escherichia coli ?-galactosidase produced under oxidizing conditions, in the cytoplasm of an Origami strain. Our data prove the activation of this microbial enzyme under oxidizing conditions and clearly show the occurrence of a disulfide bond in the ?-galactosidase structure. Additionally, the formation of this disulfide bond is supported by the analysis of a homology model of the protein that indicates that two cysteines located in the vicinity of the catalytic center are sufficiently close for disulfide bond formation.

Seras-Franzoso, Joaquin; Affentranger, Roman; Ferrer-Navarro, Mario; Daura, Xavier; Villaverde, Antonio

2012-01-01

179

Evaluation of the surface bonding energy of an InP membrane bonded oxide-free to Si using instrumented nanoindentation  

NASA Astrophysics Data System (ADS)

Instrumented nanoindentation is used in conjunction with scanning transmission electron microscopy to evaluate the mechanical resistance at the bonding interface of a 450 nm thick InP membrane bonded oxide-free to Si. Indentation using a Berkovich tip is shown to cause the planes in InP to rotate by as much as 16. The shear stress resulting from this rotation causes the InP membrane to buckle, forming a debonded blister around the indented zone. The geometry of this blister is used to compute the surface bond energy of InP bonded oxide-free to Si. An average surface bonding energy of 585 mJ m-2 is reported.

Pantzas, Konstantinos; Patriarche, Gilles; Le Bourhis, Eric; Troadec, David; Itawi, Ahmad; Beaudoin, Grgoire; Sagnes, Isabelle; Talneau, Anne

2013-08-01

180

Diffusion bonding of aluminium oxide to stainless steel using stress relief interlayers  

Microsoft Academic Search

Aluminium oxide was diffusion bonded to AISI 304 steel using Ti, Cu or Mo as interlayer materials. It was observed that Ti joins easily to both ceramic and steel parts, giving an average shear strength equal to 20 MPa. However, within the experimental conditions applied, the adhesion between Al2O3 and Cu or Mo was unsuccessful. A finite element modelling (FEM)

Dilermando Travessa; Maurizio Ferrante; Gert den Ouden

2002-01-01

181

Applicability of Oxidative Systems to Initiate Grafting on and Bonding of Wood  

Microsoft Academic Search

Graft polymerization techniques were applied to induce bonding in wood composites. Wood surfaces were activated with an oxidant such as hydrogen peroxide, nitric acid, peracetic acid, potassium ferricyanide or sodium dichromate. The activated wood surfaces were then chemically crosslinked with polymerizing materials such as furfuryl alcohol, ammonium lignosulfonate, mixtures of lignosulfonate with furfuryl alcohol, with formaldehyde or with maleic acid.

John L. Phllippou

1981-01-01

182

Synthesis and reactions of the oxides of hexafluoropropylene trimers  

SciTech Connect

By the oxidation of the hexafluoropropylene trimers with an aqueous solution of sodium hypochlorite in the presence of acetonitrile the following ..cap alpha..-oxides were obtained: 2,3-Epoxyperfluoro-3-isopropyl-4-methylpentane and 2,3-epoxyperfluoro-3-ethyl-2,4-dimethylpentane. According to the /sup 19/F NMR data, the epoxidation takes place stereoselectively with the formation of only one conformer of the ..cap alpha..-oxide in each case. The determining effect of the steric factors on the reactivity of the oxides of hexafluoropropylene trimers in reaction with nucleophiles was demonstrated.

Zapevalov, A.Ya.; Filyakova, T.I.; Peschanskii, N.V.; Kodess, M.I.; Kolenko, I.P.

1986-03-10

183

Mode- and bond-selected reactions of vibrationally excited methane and monodeuterated methane with chlorine atoms  

NASA Astrophysics Data System (ADS)

Direct infrared absorption prepares CH4 in two nearly isoenergetic vibrationally excited states, the symmetric stretch-bend combination (nu 1 + nu4) and the antisymmetric stretch-bend combination (nu3 + nu4), for a study of the effect of stretching vibrations of CH4 on the reaction, CH4 + Cl( 2P3/2) ? CH3 + HCl. Comparison of intensities in the action spectra with those in the simulated absorption spectra shows that vibrational excitation of methane to the nu1 + nu4 state promotes the reaction more efficiently than excitation to the nu3 + nu4 state by a factor of 1.9 +/- 0.5. The reduced symmetry of CH3D allows us to explore the relative reactivity of the fundamental symmetric and the antisymmetric C-H stretches. We excite three vibrational eigenstates of CH3D near 3000 cm -1 that contain different amounts of symmetric C-H stretch (nu 1), antisymmetric C-H stretch (nu4), and two quanta of bend (2nu5). Analyzing the action spectra with the simulation and the composition of the eigenstates reveals that the nu1 vibration is 6 +/- 1 times more reactive than the nu4 vibration. Ab initio calculations of the vibrational eigenfunctions along the reaction coordinate show that as the Cl atom approaches, the nu1 vibration of CH3D is transformed into localized vibrational excitation in the C-H bond pointing toward the Cl atom, promoting the reaction, and the nu 4 vibrational energy flows into the distal C-H bonds that remain unaffected during the reaction, consistent with our experimental results. Selective vibrational excitation permits control of the outcome of a reaction with two competing channels. Vibrational excitation of the first overtone of C-D stretch (2nu2) of CH3D at 4300 cm-1 exclusively increases the probability of breaking the C-D bond, yielding CH3 but no CH2D. By contrast, excitation of the nu1 vibration, the nu4 vibration, or the combination vibration of C-H stretch and CH3 umbrella bend (nu4 + nu3) preferentially promotes the H-atom abstraction to produce CH2D over CH3. The vibrational action spectra for the two products permit the separation of the two sets of interleaved transitions to give band origins and rotational constants of the 2nu 2 state and the nu4 + nu3 state of CH 3D.

Yoon, Sangwoon

184

Study of a major intermediate in the oxidative folding of leech carboxypeptidase inhibitor: contribution of the fourth disulfide bond.  

PubMed

The oxidative folding pathway of leech carboxypeptidase inhibitor (LCI; four disulfide bonds) proceeds through the formation of two major intermediates (III-A and III-B) that contain three native disulfide bonds and act as strong kinetic traps in the folding process. The III-B intermediate lacks the Cys19-Cys43 disulfide bond that links the beta-sheet core with the alpha-helix in wild-type LCI. Here, an analog of this intermediate was constructed by replacing Cys19 and Cys43 with alanine residues. Its oxidative folding follows a rapid sequential flow through one, two, and three disulfide species to reach the native form; the low accumulation of two disulfide intermediates and three disulfide (scrambled) isomers accounts for a highly efficient reaction. The three-dimensional structure of this analog, alone and in complex with carboxypeptidase A (CPA), was determined by X-ray crystallography at 2.2A resolution. Its overall structure is very similar to that of wild-type LCI, although the residues in the region adjacent to the mutation sites show an increased flexibility, which is strongly reduced upon binding to CPA. The structure of the complex also demonstrates that the analog and the wild-type LCI bind to the enzyme in the same manner, as expected by their inhibitory capabilities, which were similar for all enzymes tested. Equilibrium unfolding experiments showed that this mutant is destabilized by approximately 1.5 kcal mol(-1) (40%) relative to the wild-type protein. Together, the data indicate that the fourth disulfide bond provides LCI with both high stability and structural specificity. PMID:16126224

Arolas, Joan L; Popowicz, Grzegorz M; Bronsoms, Slvia; Aviles, Francesc X; Huber, Robert; Holak, Tad A; Ventura, Salvador

2005-09-30

185

Lanthanide Triflate-Catalyzed Carbon-Carbon Bond-Forming Reactions in Organic Synthesis  

Microsoft Academic Search

Versatile carbon-carbon bond-forming reactions using lanthanide triflates (Ln(OTf)3) as catalysts are discussed. Lanthanide triflates are new types of Lewis acids different from typical Lewis acids such as\\u000a AlCl3, BF3, SnCl4, etc. While most Lewis acids are decomposed or deactivated in the presence of water, lanthanide triflates are stable and\\u000a works as Lewis acids in water solutions. Many nitrogen-containing compounds such

Sh? Kobayashi

186

Cyclic Oxidation Behavior of HVOF Bond Coatings Deposited on La- and Y-doped Superalloys  

SciTech Connect

One suggested strategy for improving the performance of thermal barrier coating (TBC) systems used to protect hot section components in gas turbines is the addition of low levels of dopants to the Ni-base superalloy substrate. To quantify the benefit of these dopants, the oxidation behavior of three commercial superalloys with different Y and La contents was evaluated with and without a NiCoCrAlYHfSi bond coating deposited by high velocity oxygen fuel (HVOF) spraying. Cyclic oxidation experiments were conducted in dry O{sub 2} at 1050, 1100 and 1150 C. At the highest temperature, the bare superalloy without La showed more attack due to its lower Al content but no difference in oxidation rate or scale adhesion was noted at lower temperatures. With a bond coating, the alumina scale was non-uniform in thickness and spalled at each temperature. Among the three coated superalloys, no clear difference in oxide growth rate or scale adhesion was observed. Evaluations with a YSZ top coat and a bond coating without Hf are needed to better determine the effect of superalloy dopants on high temperature oxidation performance.

Pint, Bruce A [ORNL; Bestor, Michael A [ORNL; Haynes, James A [ORNL

2011-01-01

187

Does hydrogen-bonding donation to manganese(IV)-oxo and iron(IV)-oxo oxidants affect the oxygen-atom transfer ability? A computational study.  

PubMed

Iron(IV)-oxo intermediates are involved in oxidations catalyzed by heme and nonheme iron enzymes, including the cytochromes P450. At the distal site of the heme in P450 Compound?I (Fe(IV) -oxo bound to porphyrin radical), the oxo group is involved in several hydrogen-bonding interactions with the protein, but their role in catalysis is currently unknown. In this work, we investigate the effects of hydrogen bonding on the reactivity of high-valent metal-oxo moiety in a nonheme iron biomimetic model complex with trigonal bipyramidal symmetry that has three hydrogen-bond donors directed toward a metal(IV)-oxo group. We show these interactions lower the oxidative power of the oxidant in reactions with dehydroanthracene and cyclohexadiene dramatically as they decrease the strength of the O?H bond (BDEOH ) in the resulting metal(III)-hydroxo complex. Furthermore, the distal hydrogen-bonding effects cause stereochemical repulsions with the approaching substrate and force a sideways attack rather than a more favorable attack from the top. The calculations, therefore, give important new insights into distal hydrogen bonding, and show that in biomimetic, and, by extension, enzymatic systems, the hydrogen bond may be important for proton-relay mechanisms involved in the formation of the metal-oxo intermediates, but the enzyme pays the price for this by reduced hydrogen atom abstraction ability of the intermediate. Indeed, in nonheme iron enzymes, where no proton relay takes place, there generally is no donating hydrogen bond to the iron(IV)-oxo moiety. PMID:23362213

Latifi, Reza; Sainna, Mala A; Rybak-Akimova, Elena V; de Visser, Sam P

2013-01-30

188

Experimental and Molecular Dynamics Simulations of Tribochemical Reactions with ZDDP: Zinc PhosphateIron Oxide Reaction  

Microsoft Academic Search

Zinc phosphate glass is considered to be the main constituent of tribofilms generated under boundary lubrication with zinc dialkyldithiophosphate (ZDDP), a well-known antiwear additive. The reaction occurring during friction between zinc phosphate glasses and steel native iron oxide layer is investigated by both an experimental approach and by Molecular Dynamics simulations (MD). The importance of this tribochemical reaction in the

Clotilde Minfray; Thierry Le Mogne; Jean-Michel Martin; Tasuku Onodera; Sayaka Nara; Shuko Takahashi; Hideyuki Tsuboi; Michihisa Koyama; Akira Endou; Hiromitsu Takaba; Momoji Kubo; Carlos A. Del Carpio; Akira Miyamoto

2008-01-01

189

Reactions of Propylene Oxide on Supported Silver Catalysts: Insights into Pathways Limiting Epoxidation Selectivity  

SciTech Connect

The reactions of propylene oxide (PO) on silver catalysts were studied to understand the network of parallel and sequential reactions that may limit the selectivity of propylene epoxidation by these catalysts. The products of the anaerobic reaction of PO on Ag/a-Al2O3 were propanal, acetone and allyl alcohol for PO conversions below 23%. As the conversion of PO was increased either by increasing the temperature or the contact time, acrolein was formed at the expense of propanal, indicating that acrolein is a secondary reaction product in PO decomposition. With addition of oxygen to the feedstream the conversion of PO increased moderately. In contrast to the experiments in absence of oxygen, CO2 was a signi?cant product while the selectivity to propanal decreased as soon as oxygen was introduced in the system. Allyl alcohol disappeared completely from the product stream in the presence of oxygen, reacting to form acrolein and CO2. The product distribution may be explained by a network of reactions involving two types of oxametallacycles formed by ring opening of PO: one with the oxygen bonded to C1 (OMC1, linear) and the other with oxygen bonded to C2 (OMC2, branched). OMC1 reacts to form PO, propanal, and allyl alcohol.

Kulkarni, Apoorva; Bedolla-Pantoja, Marco; Singh, Suyash; Lobo, Raul F.; Mavrikakis, Manos; Barteau, Mark A.

2012-02-04

190

Specific Bonds between an Iron Oxide Surface and Outer Membrane Cytochromes MtrC and OmcA from Shewanella oneidensis MR1  

Microsoft Academic Search

Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration.  A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface.  Atomic force microscopy (AFM) was used to detect whether a specific

Brian H. Lower; Liang Shi; Ruchirej Yongsunthon; Timothy C. Droubay; David E. Mccready; Steven K. Lower

2007-01-01

191

Effect of varying oxidation parameters on the generation of C-dangling bond centers in oxidized SiC  

SciTech Connect

SiC is perhaps the most appropriate material to replace Si in power-metal-oxide semiconductor-field-effect-transistors (MOSFETs), because, unlike the other wide band-gap semiconductors, SiC can be thermally oxidized similarly to Si to form a SiO{sub 2} insulating layer. In studies of oxidized SiC, the authors have used electron paramagnetic resonance (EPR) to identify C-dangling bonds generated by hydrogen release from C-H bonds. While hydrogen's effect on SiC-based MOSFETs is uncertain, studies of Si-based MOSFETs indicate that it is important to minimize hydrogen in MOS structures. To examine the role of hydrogen, they have studied the effects of SiC/SiO{sub 2} fabrication on the density of C-related centers, which are made EPR active by a dry heat-treatment. Here, the authors examine the starting and ending procedures of their oxidation routine. The parameter that appears to have the greatest effect on center density is the ending step of the oxidation procedure. For example, samples that were removed from the furnace in flowing O{sub 2} produced the smallest concentration of centers after dry heat-treatment. They report on the details of these experiments and use their results to suggest an oxidation procedure that limits center production.

Macfarlane, P.J.; Zvanut, M.E.

1999-07-01

192

New synthesis of 2-heteroarylperfluoropropionic acids derivatives by reaction of azine N-oxides with hexafluoropropene.  

PubMed

Hexafluoropropene reacts with aromatic azine N-oxides under mild conditions to produce fluorides of 2-heteroarylperfluoropropionic acids. The reaction proceeds as 1,3-dipolar cycloaddition followed by spontaneous scission of the N--O bond in the isoxazolidine ring and elimination of HF. When the reaction is carried out in the presence of alcohols or N-alkyl anilines, the in situ formed acyl fluorides give the corresponding esters and amides. They can be also treated separately with nucleophiles to produce the respective acylation products, whereas their hydrolysis leads to unstable carboxylic acids that undergo spontaneous decarboxylation to 1-aryl-1,2,2,2-tetrafluoroethanes. This new reaction provides a simple and general method of synthesizing 2-heteroarylperfluoropropionic acid derivatives that were previously unknown and unavailable. PMID:18189255

Loska, Rafa?; Makosza, Mieczys?aw

2008-01-01

193

Bimetallic bonding and mixed oxide formation in the Ga-Pd-CeO2 system  

NASA Astrophysics Data System (ADS)

The interaction of gallium and palladium with 2 nm CeO2(111) layers grown on Cu(111) was studied by core level photoelectron spectroscopy and resonant valence band spectroscopy. Palladium alone interacted weakly with ceria layers. Gallium deposited on cerium dioxide formed a mixed Ga2O3-Ce2O3 oxide of 1:1 stoichiometry (cerium gallate CeGaO3), with both metals in the M3+ oxidation state. Increasing Ga coverages led to the formation of lower oxidation states, i.e., Ga1+ in Ga2O oxide and metallic Ga0. Palladium deposited onto this complex system interacted with gallium leading to a breakage of Ga-ceria bonds, a decrease of the oxidation state of gallium, and formation of a Ga-Pd intermetallic alloy in which all components (CeO2, CeGaO3, Ga2O, Ga-Pd, and Pd) are in equilibrium.

Skla, Tom; Tsud, Nataliya; Prince, Kevin C.; Matoln, Vladimr

2011-08-01

194

Fabrication and characterization of reaction bonded silicon carbide/carbon nanotube composites  

NASA Astrophysics Data System (ADS)

Carbon nanotubes have generated considerable excitement in the scientific and engineering communities because of their exceptional mechanical and physical properties observed at the nanoscale. Carbon nanotubes possess exceptionally high stiffness and strength combined with high electrical and thermal conductivities. These novel material properties have stimulated considerable research in the development of nanotube-reinforced composites (Thostenson et al 2001 Compos. Sci. Technol. 61 1899, Thostenson et al 2005 Compos. Sci. Technol. 65 491). In this research, novel reaction bonded silicon carbide nanocomposites were fabricated using melt infiltration of silicon. A series of multi-walled carbon nanotube-reinforced ceramic matrix composites (NT-CMCs) were fabricated and the structure and properties were characterized. Here we show that carbon nanotubes are present in the as-fabricated NT-CMCs after reaction bonding at temperatures above 1400 C. Characterization results reveal that a very small volume content of carbon nanotubes, as low as 0.3 volume %, results in a 75% reduction in electrical resistivity of the ceramic composites. A 96% decrease in electrical resistivity was observed for the ceramics with the highest nanotube volume fraction of 2.1%.

Thostenson, Erik T.; Karandikar, Prashant G.; Chou, Tsu-Wei

2005-11-01

195

Computational studies of polysiloxanes : oxidation potentials and decomposition reactions.  

SciTech Connect

Silicon-containing solvents have tremendous potential for application as electrolytes for electrical energy storage devices such as lithium-ion (air) batteries and supercapacitors. Quantum chemical methods were employed to investigate trends in oxidation potentials and decomposition reactions of a series of polysiloxanes. Various electron-donating and -withdrawing substituents can be used to tune the oxidation potential in shorter chain siloxanes but not in longer ones. Decomposition reactions of siloxanes in their oxidized states were investigated and compared against their carbon analogues. These studies suggest that the Si-O group provides added stability for siloxanes over their carbon analogues. Computational studies have also been performed for various disiloxanes and siloxanes with spacer groups to understand their thermochemical stability and oxidation potentials.

Assary, R. S.; Curtiss, L. A.; Redfern, P. C.; Zhang, Z.; Amine, K. (Center for Nanoscale Materials); ( CSE); ( MSD); (Northwestern Univ.)

2011-06-23

196

Oxidation reactions of steryl esters in a saturated lipid matrix.  

PubMed

In the present study, the formation and further reactions of intact steryl ester hydroperoxides were followed in a tripalmitin matrix maintained at 100C. The effects of the acyl moiety and its unsaturation degree, and of the sterol structure were investigated. Intact steryl ester hydroperoxides were isolated from the lipid matrix by a developed solid-phase extraction (SPE) method and were determined by HPLC-ELSD. Further reactions of hydroperoxides were followed by determining secondary oxidation products of sterol by GC-FID/MS and oligomers by HPSEC-RI. The oxidation of sterol decreased when its solubility in the lipid medium was increased by introduction of acyl moiety. Increased unsaturation of the acyl or steryl moiety of steryl ester shortened the induction period and increased the oxidation of both steryl and acyl moieties. Thus, by changing the chemical and physical properties of sterols, their oxidation may be greatly affected. PMID:23442653

Lehtonen, Mari; Lampi, Anna-Maija; Riuttamki, Mari-Anna; Piironen, Vieno

2012-04-09

197

Oxidation of a polycrystalline silver foil by reaction with ozone  

Microsoft Academic Search

The surface oxidation of a polycrystalline silver foil by reaction with ozone (5mol% O3 in O2) at 300K and ambient pressure was investigated. The morphology, microstructure and chemical composition of the oxide scale which developed on the foil surface during exposure to ozone was characterised by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy

G. I. N. Waterhouse; G. A. Bowmaker; J. B. Metson

2001-01-01

198

Hydrolysis of Surfactants Containing Ester Bonds: Modulation of Reaction Kinetics and Important Aspects of Surfactant Self-Assembly  

ERIC Educational Resources Information Center

|The effects of self-assembly on the hydrolysis kinetics of surfactants that contain ester bonds are discussed. A number of examples on how reaction rates and apparent reaction orders can be modulated by changes in the conditions, including an instance of apparent zero-order kinetics, are presented. Furthermore, it is shown that the examples on

Lundberg, Dan; Stjerndahl, Maria

2011-01-01

199

Final Report: Catalytic Hydrocarbon Reactions over Supported Metal Oxides, August 1, 1995 - July 31, 1999  

SciTech Connect

The research program focused on the catalysis of hydrodesulfurization (HDS) over molybdenum-based catalysts and how catalyst composition, redox ability, structure and neighboring sites control the catalytic properties of metal oxides. We sought to understand the catalytic features/sites that control hydrogenation, hydrogenolysis, and isomerization during HDS. Unprompted silica-supported molybdenum oxides and molybdenum sulfides were studied. Model catalyst systems were prepared from organometallic precursors or cluster compounds to generate supported structures that feature Mo(II) and Mo(IV) cations that are isolated or in ensembles and that have either Mo-O or Mo-S bonds. Conventional MOS{sub 2} catalysts, which contain both edge and rim sites, were be studied. Finally, single-layer MOS{sub 2} structures were also prepared from 2H-MoS{sub 2} powder so that the model systems could be compared against a disulfide catalyst that only involves rim sites. Catalytic reactions for thiophene and tetrahydrothione were studied over the various catalysts. Oxidation states were determined using X-ray photoelectron spectroscopy. X-ray crystallography was used to characterize and follow changes in the MOS{sub 2} structures. The program on metal oxides prepared supported oxides that have a specific structure and oxidation state to serve as model templates for the more complex commercial catalysts and then employed these structures in reaction studies. This focus area examined the relationships between structure and cation redox characteristics in oxidation catalysis. Infrared and Raman spectroscopy were used to characterize the cations and reaction intermediates.

Ekerdt, John G.

1999-07-31

200

a Global Reaction Rate for Nitric Oxide Reburning  

Microsoft Academic Search

An investigation of a global reburning-NO reaction, which is the reduction pathway of nitric oxide (NO) by reaction with hydrocarbons, was conducted. The global reburning-NO rate expression and its rate constants were determined. This global reburning-NO rate constant can be expressed as 2.72 times 10 ^6exp(-18,800\\/RT) (gmole\\/cm^3 s). This expression is applicable to atmospheric pressure, an equivalence ratio range of

Wei Chen

1994-01-01

201

Manufacture of ?1.2m reaction bonded silicon carbide mirror blank CFID  

NASA Astrophysics Data System (ADS)

Silicon carbide (SiC) is a new type candidate material for large-scale lightweight space mirror. Its low thermal distortion, high stiffness, fine optical quality and dimensional stability, make SiC an ideal material for large space born telescope. Since ten years Changchun institute optics, fine mechanics and physics (CIOMP) has developed reaction bonded SiC (RB-SiC) technology for space application, and can fabricate RB-SiC mirror with scale less than 1.0 meter for telescope. The green body is prepared with gel-casting method which is an attractive new ceramic forming process for making high-quality, complex-shaped ceramic parts. And then unmolding, drying, binder burning out, reacting bonded, the RB-SiC can be obtained. But with the development of space-born or ground telescope, the scale of primary mirror has exceeded 1.0 meter. So CIOMP has developed an assembly technique which called novel reaction-formed joint technology for larger RB-SiC mirror blank. The steps include joining of green bodies with mixture comprised of SiC particles and phenolic resin etc, firing, machining and sintering. Joining the ?1.2 meter RB-SiC mirror blank by the novel reaction-formed joint technology. And testing the welding layer's performance, the results show that the thickness of 54-77?m, the microstructure and thermal property can be comparable to the substrate and the mechanical property are excellent in bending strength of 307MPa.

Zhang, Ge; Zhao, Rucheng; Zhao, Wenxing; Bao, Jianxun

2010-05-01

202

Reactions of Hydrogen Atoms with Hydrazine, Ammonia, and Nitrous Oxide  

Microsoft Academic Search

Reactions of hydrogen atoms with hydrazine, ammonia, and nitrous oxide have been investigated in a flow system employing a mass spectrometer for the measurements of the steady-state concentrations of the gases. It is shown that reaction with hydrazine proceeds through the mechanism: N2H4+H=H2+N2H3,k1=3.51011e?2000?RTmole?1cc sec?1;2N2H3=2NH3+N2,k2⩾31012 mole?1cc sec?1 (at 150∘C). The mechanism of the reactions of hydrogen atoms with ammonia or nitrous

M. Schiavello; G. G. Volpi

1962-01-01

203

Extended protein/water H-bond networks in photosynthetic water oxidation.  

PubMed

Oxidation of water molecules in the photosystem II (PSII) protein complex proceeds at the manganese-calcium complex, which is buried deeply in the lumenal part of PSII. Understanding the PSII function requires knowledge of the intricate coupling between the water-oxidation chemistry and the dynamic proton management by the PSII protein matrix. Here we assess the structural basis for long-distance proton transfer in the interior of PSII and for proton management at its surface. Using the recent high-resolution crystal structure of PSII, we investigate prominent hydrogen-bonded networks of the lumenal side of PSII. This analysis leads to the identification of clusters of polar groups and hydrogen-bonded networks consisting of amino acid residues and water molecules. We suggest that long-distance proton transfer and conformational coupling is facilitated by hydrogen-bonded networks that often involve more than one protein subunit. Proton-storing Asp/Glu dyads, such as the D1-E65/D2-E312 dyad connected to a complex water-wire network, may be particularly important for coupling protonation states to the protein conformation. Clusters of carboxylic amino acids could participate in proton management at the lumenal surface of PSII. We propose that rather than having a classical hydrophobic protein interior, the lumenal side of PSII resembles a complex polyelectrolyte with evolutionary optimized hydrogen-bonding networks. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial. PMID:22503827

Bondar, Ana-Nicoleta; Dau, Holger

2012-04-04

204

Quantitative determination of double bond positions in unsaturated fatty acids after oxidative cleavage  

Microsoft Academic Search

The position and amt of unsaturation in fatty acids have been determined, especially in pure fractions of partially hydrogenated\\u000a fats. In developing a quantitative method for determination of ethylenic bonds in monounsaturated and polyunsaturated fatty\\u000a acids several procedures were combined. Key features include oxidative cleavage; recovery of cleaved acids as salts; and their\\u000a conversion to methyl, ethyl or butyl esters

E. P. Jones; V. L. Davison

1965-01-01

205

Double bond isomerization of 5-vinylbicyclo[2.2.1]hept-2-ene to 5-ethylidenebicyclo[2.2.1]hept-2-ene over alkaline earth oxides  

Microsoft Academic Search

All the alkaline earth oxides exhibit activities for double, bond isomerization of 5-vinylbicyclo[2.2.1]hept-2-ene (VBH) to 5-ethylidenebicyclo[2.2.1]hept-2-ene (EBH) at a reaction temperature of 273 K. The order of the activity on the basis of unit weight of catalyst was CaO>MgO>SrO>BaO when compared under optimum pretreatment conditions. The E\\/Z ratio in the products is determined by the reaction temperature regardless of the

Hajime Kabashima; Hideto Tsuji; Hideshi Hattori

1996-01-01

206

Catalyzed oxidation reactions. IV. Picolinic acid catalysis of chromic acid oxidations  

Microsoft Academic Search

Picolinic acid and several closely related acids are effective catalysts in the chromic acid oxidation of primary and secondary alcohols; the oxidation of other substrates is accelerated only moderately. The reaction is first order in chromium-(VI), alcohol, and picolinic acid; it is second order in hydrogen ions at low acidity and approaches acidity independence at high perchloric acid concentrations. A

Jan Rocek; T. Y. Peng

1977-01-01

207

Ternary Electrocatalysts for Oxidizing Ethanol to Carbon Dioxide: Making Ir Capable of Splitting C-C bond  

SciTech Connect

Splitting the C-C bond is the main obstacle to electroxidation of ethanol (EOR) to CO2. We recently demonstrated that the ternary PtRhSnO2 electrocatalyst can accomplish that reaction at room temperature with Rh having a unique capability to split the C-C bond. In this article we report the finding that Ir can be induced to split the C-C bond as a component of the ternary catalyst. We synthesized, characterized and compared the properties of several ternary electrocatalysts. Carbon-supported nanoparticle (NP) electrocatalysts comprising a SnO2 NP core decorated with multi-metallic nanoislands (MM = PtIr, PtRh, IrRh, PtIrRh) were prepared using a seeded growth approach. An array of characterization techniques were employed to establish the composition and architecture of the synthesized MM /SnO2 NPs, while electrochemical and in situ infrared reflection absorption spectroscopy studies elucidated trends in activity and the nature of the reaction intermediates and products. Both EOR reactivity and selectivity towards CO2 formation of several of these MM /SnO2/C electrocatalysts are significantly higher compared to conventional Pt/C and Pt/SnO2/C catalysts. We demonstrate that the PtIr/SnO2/C catalyst with high Ir content shows outstanding catalytic property with the most negative EOR onset potential and reasonably good selectivity towards ethanol complete oxidation to CO2. PtRh/SnO2/C catalysts with a moderate Rh content exhibit the highest EOR selectivity, as deduced from infrared studies.

Li, Meng [Brookhaven National Laboratory (BNL); Cullen, David A [ORNL; Sasaki, Kotaro [Brookhaven National Laboratory (BNL); Marinkovic, N. [University of Delaware; More, Karren Leslie [ORNL; Adzic, Radoslav R. [Brookhaven National Laboratory (BNL)

2013-01-01

208

Uric Acid Oxidation by Peroxynitrite: Multiple Reactions, Free Radical Formation, and Amplification of Lipid Oxidation  

Microsoft Academic Search

Uric acid has been considered to be an efficient scavenger of peroxynitrite but the reaction between urate and peroxynitrite has been only partially characterized. Also, previous studies have indicated that urate may increase peroxynitrite-mediated oxidation of low density lipoprotein (LDL). Here, we examined the reaction between urate and peroxynitrite by combining kinetic, oxygen consumption, spin trapping, and product identification studies;

Clio X. C. Santos; Edson I. Anjos; Ohara Augusto

1999-01-01

209

The utilization of microwave heating for the fabrication of sintered reaction-bonded silicon nitride  

SciTech Connect

The results of studies in which microwave heating was used to fabricate sintered reaction-bonded silicon nitride (SRBSN) are reviewed. These results are compared to parallel studies where conventional heating was used for the fabrication of these materials. Microwave fabrication of SRBSN involves a single heating cycle, whereas conventional processing requires two separate furnace runs and sample packaging steps. SRBSN containing high levels of sintering aids which were fabricated by microwave heating showed improved strength and toughness, as compared to those materials fabricated using a conventional resistance-heated furnace. An analysis of the microstructures of the microwave fabricated materials showed enhanced acicular grain growth as compared to conventionally heated material. Results are presented on studies involving the scale-up of the microwave fabrication process.

Kiggans, J.O.; Tiegs, T.N.; Lin, H.T.; Holcombe, C.E.

1995-12-31

210

Gelcasting of silicon preforms for the production of sintered reaction-bonded silicon nitride  

SciTech Connect

Gelcasting of silicon metal for the production of sintered reaction-bonded silicon nitride (SRBSN) was investigated in order to identify associated advantages over conventional forming techniques, i.e., die and isostatic pressing. Compacts were formed from identical powder mixtures by both gelcasting and pressing, and were nitrided and sintered to produce SRBSN ceramics using both conventional and microwave heating. Characterization of the samples included measurement of green density, green and nitrided pore structure, weight gain during nitridation, final density, microstructure, toughness, and flexural strength. It was found that a more uniform pore structure existed in the green gelcast samples. It is believed that this pore configuration aided in nitridation, and manifested itself in a more uniform final microstructure. In addition, improved mechanical properties were achieved in the gelcast samples. This improvement can be attributed to green microstructure homogeneity. An additional finding of this study was that microwave hearing combined with gelcast forming resulted in SRBSN materials with improved mechanical properties.

Kiggans, J.O. Jr.; Nunn, S.D.; Tiegs, T.N.; Davisson, C.C.; Coffey, D.W. [Oak Ridge National Lab., TN (United States); Maria, J.P. [Pennsylvania State Univ., University Park, PA (United States)

1995-12-31

211

Flower-like porous hematite nanoarchitectures achieved by complexation-mediated oxidation-hydrolysis reaction.  

PubMed

Flower-like porous hematite (?-Fe(2)O(3)) nanoarchitectures composed of ultra-thin nanoflakes were prepared by annealing the iron oxide precursor formed via the oxidation-hydrolysis reaction between Fe(II) ions and Tris(hydroxymethyl)aminomethane (abbreviated as Tris). The microstructure of the prepared FeOOH and hematite samples were fully characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction analysis, Fourier-transforming infrared spectra, thermogravimetric analysis, and nitrogen adsorption-desorption isotherm. Based on the influences of reactant concentrations, reaction time and reaction temperature on the morphologies of the resultant samples, a formation mechanism of etching was proposed, Fe(II)-Tris complexes were self-assembled via hydrogen bonds into brick-like building blocks, which then aggregated into rudimentary nanoparticles, and the synergistic effect between the crystallization of FeOOH and dissociation of Fe(II)-Tris complexes make the rudimentary nanoparticles evolve into the flower-like products. The as-prepared flower-like ?-Fe(2)O(3) nanostructures possessed a Brunauer-Emmett-Teller specific surface area of 191.63 m(2)g(-1), hierarchical pore distribution ranging from micropores to macropores, and good crystallinity, and excellent visible photocatalysis in terms of removing chemical oxygen demand of dimethyl sulfoxide industrial wastewater. The current work provides a reliable approach for building functional hierarchical nanoarchitectures and the prepared iron oxide nanomaterials demonstrate an excellent ability to remove toxic pollutants in industrial wastewater. PMID:21353233

Huang, Xing; Guan, Jianguo; Xiao, Zhidong; Tong, Guoxiu; Mou, Fangzhi; Fan, Xi'an

2011-02-24

212

Selective oxidation of carbolide C-H bonds by an engineered macrolide P450 mono-oxygenase  

PubMed Central

Regio- and stereoselective oxidation of an unactivated CH bond remains a central challenge in organic chemistry. Considerable effort has been devoted to identifying transition metal complexes, biological catalysts, or simplified mimics, but limited success has been achieved. Cytochrome P450 mono-oxygenases are involved in diverse types of regio- and stereoselective oxidations, and represent a promising biocatalyst to address this challenge. The application of this class of enzymes is particularly significant if their substrate spectra can be broadened, selectivity controlled, and reactions catalyzed in the absence of expensive heterologous redox partners. In this study, we engineered a macrolide biosynthetic P450 mono-oxygenase PikC (PikCD50N-RhFRED) with remarkable substrate flexibility, significantly increased activity compared to wild-type enzyme, and self-sufficiency. By harnessing its unique desosamine-anchoring functionality via a heretofore under-explored substrate engineering strategy, we demonstrated the ability of PikC to hydroxylate a series of carbocyclic rings linked to the desosamine glycoside via an acetal linkage (referred to as carbolides) in a regioselective manner. Complementary analysis of a number of high-resolution enzyme-substrate cocrystal structures provided significant insights into the function of the aminosugar-derived anchoring group for control of reaction site selectivity. Moreover, unexpected biological activity of a select number of these carbolide systems revealed their potential as a previously unrecorded class of antibiotics.

Li, Shengying; Chaulagain, Mani Raj; Knauff, Allison R.; Podust, Larissa M.; Montgomery, John; Sherman, David H.

2009-01-01

213

General aspect of catalytic hydrogenation and isomerization reactions on oxide catalyst. Characteristics of the reactions via ionic and nonionic intermediates  

SciTech Connect

An analysis of new and available data, mostly published by Tanaka and Okuhara, on the hydrogenation of alkenes and conjugated dienes and the hydrogen-deuterium exchange on the oxides of cadmium, magnesium, zinc, cobalt, chromium, thorium, zirconium, and lanthanum, and on molybdenum disulfide suggested that the partial hydrogenation of conjugated dienes catalyzed by cadmium and magnesium oxide proceeds by a mechanism which is similar to that of diene hydrogenation on supported alkali metals or on alkali metal hydride and involves allyl carbanion intermediates. The same ionic mechanism apparently also occurs in the double-bond migration of n-alkenes on cadmium oxide, and the isomerization catalyzed by molybdenum disulfide. The nonionic reaction mechanism via o-alkyl and o- or m-allyl intermediates apparently proceeds on active sites of a definite structure, i.e., a specific degree of coordination, but the ionic mechanism seems to depend only on the acidity or basicity of the sites. The nonspecific nature of the site for the ionic mechanism may result in high resistance to poisoning.

Tanaka, K.; Okuhara, T.

1980-09-01

214

Oxidation of Annelated Diarylamines: Analysis of Reaction Pathways to Nitroxide Diradical and Spirocyclic Products  

SciTech Connect

Oxidation of diaryldiamine 2, a tetrahydrodiazapentacene derivative, provides diarylnitroxide diradical 1 accompanied by an intermediate nitroxide monoradical and a multitude of isolable diamagnetic products. DFT-computed tensors for EPR spectra and paramagnetic {sup 1}H NMR isotropic shifts for nitroxide diradical 1 show good agreement with the experimental EPR spectra in rigid matrices and paramagnetic {sup 1}H NMR spectra in solution, respectively. Examination of the diamagnetic products elucidates their formation via distinct pathways involving C-O bond-forming reactions, including Baeyer-Villiger-type oxidations. An unusual diiminoketone structure and two spirocyclic structures of the predominant diamagnetic products are confirmed by either X-ray crystallography or correlations between DFT-computed and experimental spectroscopic data such as {sup 1}H, {sup 13}C, and {sup 15}N NMR chemical shifts and electronic absorption spectra.

Rajca, Andrzej; Shiraishi, Kouichi; Boraty; #324; ski, Przemyslaw J.; Pink, Maren; Miyasaka, Makoto; Rajca, Suchada (UNL); (Indiana)

2012-02-06

215

Rhodium-catalyzed cascade oxidative annulation leading to substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp2)-H/C(sp3)-H and C(sp2)-H/O-H bonds.  

PubMed

The cascade oxidative annulation reactions of benzoylacetonitrile with internal alkynes proceed efficiently in the presence of a rhodium catalyst and a copper oxidant to give substituted naphtho[1,8-bc]pyrans by sequential cleavage of C(sp(2))-H/C(sp(3))-H and C(sp(2))-H/O-H bonds. These cascade reactions are highly regioselective with unsymmetrical alkynes. Experiments reveal that the first-step reaction proceeds by sequential cleavage of C(sp(2))-H/C(sp(3))-H bonds and annulation with alkynes, leading to 1-naphthols as the intermediate products. Subsequently, 1-naphthols react with alkynes by cleavage of C(sp(2))-H/O-H bonds, affording the 1:2 coupling products. Moreover, some of the naphtho[1,8-bc]pyran products exhibit intense fluorescence in the solid state. PMID:22989331

Tan, Xing; Liu, Bingxian; Li, Xiangyu; Li, Bin; Xu, Shansheng; Song, Haibin; Wang, Baiquan

2012-09-25

216

Oxidative stress drives disulfide bond formation between basic helix-loop-helix transcription factors.  

PubMed

Basic helix-loop-helix (bHLH) transcription factors including Twist1 and E2a proteins regulate essential processes. These factors bind DNA as homo- or heterodimers and the choice of binding partners determines their functional output. To investigate potential regulators of bHLH dimerization, cells were exposed to the oxidative agent hydrogen peroxide (H(2)O(2)). Western blot analysis in the presence or absence of reducing agents, revealed that H(2)O(2) induces the rapid formation of an intermolecular disulfide bond between Twist1 homodimers and Twist/E2a proteins heterodimers. The disulfide bond is first observed between Twist1 homodimers at 25 mM H(2)O(2) and between Twist1 heterodimers at 75 mM H(2)O(2). This response is dependent upon cell density as H(2)O(2) did not induce disulfide bridge formation between bHLH proteins in cells seeded at high density. In the presence of E proteins, the formation of Twist1/E2a proteins heterodimers is favored over Twist1 homodimers, identifying an oxidative stimulus as an important factor in modulating binding partner specificity. We further demonstrated that a cysteine residue located at the C-terminus of Twist1 and E2a proteins is involved in this response. Disulfide bond formation between Twist1 homodimers significantly reduced its ability to interact with two of its binding partners, Runx2 and HDAC4, indicating that disulfide dimerization in response to H(2)O(2) has functional significance. These data support the conclusion that disulfide bond formation in response to an oxidative stimulus contributes to Twist1 homo- and heterodimerization and raises the possibility that the redox status of a cell may represent an important step in bHLH transcriptional regulation. PMID:19950203

Danciu, Theodora E; Whitman, Malcolm

2010-02-01

217

Development of Adhesive Bonds to Epichlorohydrin-Ethylene Oxide (ECO) Coated Fabric for Use in Collapsible Fuel Storage Tanks.  

National Technical Information Service (NTIS)

The study is directed specifically toward development of a (polyethylene oxide-polyepichlorahydrin copolymer single ply coated fabric and the adhesive bonds that are suitable for manufacture of self-supporting collapsible fuel storage tanks. The report di...

H. R. Bork

1974-01-01

218

Metal-catalyzed nitrogen-atom transfer methods for the oxidation of aliphatic C-H bonds.  

PubMed

For more than a century, chemists have endeavored to discover and develop reaction processes that enable the selective oxidation of hydrocarbons. In the 1970s, Abramovitch and Yamada described the synthesis and electrophilic reactivity of sulfonyliminoiodinanes (RSO(2)N?IPh), demonstrating the utility of this new class of reagents to function as nitrene equivalents. Subsequent investigations by Breslow, Mansuy, and Mller would show such oxidants to be competent for alkene and saturated hydrocarbon functionalization when combined with transition metal salts or metal complexes, namely those of Mn, Fe, and Rh. Here, we trace our own studies to develop N-atom transfer technologies for C-H and ?-bond oxidation. This Account discusses advances in both intra- and intermolecular amination processes mediated by dirhodium and diruthenium complexes, as well as the mechanistic foundations of catalyst reactivity and arrest. Explicit reference is given to questions that remain unanswered and to problem areas that are rich for discovery. A fundamental advance in amination technology has been the recognition that iminoiodinane oxidants can be generated in situ in the presence of a metal catalyst that elicits subsequent N-atom transfer. Under these conditions, both dirhodium and diruthenium lantern complexes function as competent catalysts for C-H bond oxidation with a range of nitrogen sources (e.g., carbamates, sulfamates, sulfamides, etc.), many of which will not form isolable iminoiodinane equivalents. Practical synthetic methods and applications thereof have evolved in parallel with inquiries into the operative reaction mechanism(s). For the intramolecular dirhodium-catalyzed process, the body of experimental and computational data is consistent with a concerted asynchronous C-H insertion pathway, analogous to the consensus mechanism for Rh-carbene transfer. Other studies reveal that the bridging tetracarboxylate ligand groups, which shroud the dirhodium core, are labile to exchange under standard reaction conditions. This information has led to the generation of chelating dicarboxylate dinuclear rhodium complexes, exemplified by Rh(2)(esp)(2). The performance of this catalyst system is unmatched by other dirhodium complexes in both intra- and intermolecular C-H amination reactions. Tetra-bridged, mixed-valent diruthenium complexes function as effective promoters of sulfamate ester oxidative cyclization. These catalysts can be crafted with ligand sets other than carboxylates and are more resistant to oxidation than their dirhodium counterparts. A range of experimental and computational mechanistic data amassed with the tetra-2-oxypyridinate diruthenium chloride complex, [Ru(2)(hp)(4)Cl], has established the insertion event as a stepwise pathway involving a discrete radical intermediate. These data contrast dirhodium-catalyzed C-H amination and offer a cogent model for understanding the divergent chemoselectivity trends observed between the two catalyst types. This work constitutes an important step toward the ultimate goal of achieving predictable, reagent-level control over product selectivity. PMID:22546004

Roizen, Jennifer L; Harvey, Mark Edwin; Du Bois, J

2012-04-30

219

Redox control and hydrogen bonding networks: proton-coupled electron transfer reactions and tyrosine Z in the photosynthetic oxygen-evolving complex.  

PubMed

In photosynthetic oxygen evolution, redox active tyrosine Z (YZ) plays an essential role in proton-coupled electron transfer (PCET) reactions. Four sequential photooxidation reactions are necessary to produce oxygen at a Mn(4)CaO(5) cluster. The sequentially oxidized states of this oxygen-evolving cluster (OEC) are called the S(n) states, where n refers to the number of oxidizing equivalents stored. The neutral radical, YZ, is generated and then acts as an electron transfer intermediate during each S state transition. In the X-ray structure, YZ, Tyr161 of the D1 subunit, is involved in an extensive hydrogen bonding network, which includes calcium-bound water. In electron paramagnetic resonance experiments, we measured the YZ recombination rate, in the presence of an intact Mn(4)CaO(5) cluster. We compared the S(0) and S(2) states, which differ in Mn oxidation state, and found a significant difference in the YZ decay rate (t(1/2) = 3.3 0.3 s in S(0); t(1/2) = 2.1 0.3 s in S(2)) and in the solvent isotope effect (SIE) on the reaction (1.3 0.3 in S(0); 2.1 0.3 in S(2)). Although the YZ site is known to be solvent accessible, the recombination rate and SIE were pH independent in both S states. To define the origin of these effects, we measured the YZ recombination rate in the presence of ammonia, which inhibits oxygen evolution and disrupts the hydrogen bond network. We report that ammonia dramatically slowed the YZ recombination rate in the S(2) state but had a smaller effect in the S(0) state. In contrast, ammonia had no significant effect on YD, the stable tyrosyl radical. Therefore, the alterations in YZ decay, observed with S state advancement, are attributed to alterations in OEC hydrogen bonding and consequent differences in the YZ midpoint potential/pK(a). These changes may be caused by activation of metal-bound water molecules, which hydrogen bond to YZ. These observations document the importance of redox control in proton-coupled electron transfer reactions. PMID:23346921

Keough, James M; Zuniga, Ashley N; Jenson, David L; Barry, Bridgette A

2013-01-24

220

Versatile synthesis of isocoumarins and ?-pyrones by ruthenium-catalyzed oxidative C-H/O-H bond cleavages.  

PubMed

An inexpensive cationic ruthenium(II) catalyst enabled the expedient synthesis of isocoumarins through oxidative annulations of alkynes by benzoic acids. This C-H/O-H bond functionalization process also proved applicable to the preparation of ?-pyrones and was shown to proceed by rate-limiting C-H bond ruthenation. PMID:22273364

Ackermann, Lutz; Pospech, Jola; Graczyk, Karolina; Rauch, Karsten

2012-01-24

221

Bond forming reactions of carbyne and nitrene complexes. Final technical report for DE-FG02-96ER14608  

SciTech Connect

An isolobal relationship among terminal carbyne, nitrene and oxo ligands provided the basis for our efforts to explore new synthetic routes to such complexes and to probe bond forming reactions of these ligands. The specific goals of this project were to explore reactions of carbyne and nitrene ligands, and a summary of our results follows. Manipulation of metal-ligand pi bonds to control reactivity patterns provided the conceptual basis for this work. New transformations and coupling reactions of the CR and NR moieties bound to metal centers have been explored and transformations of carbyne or nitrene ligands have been achieved. Perhaps the most exciting results have come in the area of the simplest ligand: the CH carbyne unit. Treatment of [Tp'(CO){sub 2}W{triple_bond}C-PPh{sub 3}] [PF{sub 6}] Tp' = hydridotris(3,5-dimethylpyrazolylborate) with Na[HBEt{sub 3}] forms the methylidyne complex Tp' (CO){sub 2}W{triple_bond}C-H via formyl and carbene intermediates. Protonation of the Tp'(CO){sub 2}W{triple_bond}C-H methylidyne complex yields the cationic agostic methylidene complex, [Tp'(CO){sub 2}W=CH{sub 2}][BF{sub 4}]. The methylidyne complex with a pK{sub a} of 28.7 can be deprotonated to provide the anionic terminal carbide Tp' (CO){sub 2}W{triple_bond}C-Li; a resonance at 556 ppm in the {sup 13}C NMR spectrum has been assigned to the carbide carbon. Addition of excess Na[HBEt{sub 3}] to Tp'(CO){sub 2}W{triple_bond}C-H generates the anionic methylidene complex [Na] [Tp'(CO){sub 2}W{triple_bond}CH{sub 2}].

Templeton, J.L.

2002-09-01

222

Reactions of metal ions at surfaces of hydrous iron oxide  

USGS Publications Warehouse

Cu, Ag and Cr concentrations in natural water may be lowered by mild chemical reduction involving ferric hydroxide-ferrous ion redox processes. V and Mo solubilities may be controlled by precipitation of ferrous vanadate or molybdate. Concentrations as low as 10-8.00 or 10-9.00 M are readily attainable for all these metals in oxygen-depleted systems that are relatively rich in Fe. Deposition of manganese oxides such as Mn3O4 can be catalyzed in oxygenated water by coupling to ferrous-ferric redox reactions. Once formed, these oxides may disproportionate, giving Mn4+ oxides. This reaction produces strongly oxidizing conditions at manganese oxide surfaces. The solubility of As is significantly influenced by ferric iron only at low pH. Spinel structures such as chromite or ferrites of Cu, Ni, and Zn, are very stable and if locally developed on ferric hydroxide surfaces could bring about solubilities much below 10-9.00 M for divalent metals near neutral pH. Solubilities calculated from thermodynamic data are shown graphically and compared with observed concentrations in some natural systems. ?? 1977.

Hem, J. D.

1977-01-01

223

Microscopic effects of the bonding configuration of nitrogen-doped graphene on its reactivity toward hydrogen peroxide reduction reaction.  

PubMed

We report a density functional theory (DFT) study of microscopic detailed effects of the bonding configuration of nitrogen-doped graphene (N-graphene) within the carbon lattice (including pyridinic, pyrrolic, and graphitic N) on the reactivity and mechanistic processes of H2O2 reduction reaction. We simulated the adsorption process of H2O2, analyzed the mechanistic processes, and calculated the reversible potential of each reaction step of the H2O2 reduction reaction on N-graphene. The results indicate that the adsorption of H2O2 on the pristine and N-doped graphene surfaces occurs via physisorption without the formation of a chemical bond. When H(+) is introduced into the system, a series of reactions can occur, including the breakage of the O-O bond, the formation of an O-C chemical bond between oxygen and graphene, and the creation of water molecules. The results also indicate a decrease in the energy of the system and a positive reversible potential for each reaction step. The calculations of the relative energy of each reaction step and the value of the onset potential for H2O2 reduction reaction suggest that the reactivity of pristine and N-doped graphene has the following order: pyridinic N-graphene > pyrrolic N-graphene > graphitic N-graphene > pristine graphene. We also proposed an explanation based on electrostatic potential calculations for this dependence of the reactivity order on the bond configuration of the doping in N-graphene. The results of this study should help in the atomic-scale understanding of the dependence of the reactivity of N-graphene on its microstructure, inspire the study of various types of heteroatom-doped graphenes to improve their catalytic efficiency, and provide a theoretical framework to analyze their reactivities. PMID:23549636

Wu, Ping; Du, Pan; Zhang, Hui; Cai, Chenxin

2013-04-03

224

Formation, structure and bond dissociation thresholds of gas-phase vanadium oxide cluster ions  

NASA Astrophysics Data System (ADS)

The formation and structure of gas-phase vanadium oxide cluster anions are examined using a guided ion beam mass spectrometer coupled with a laser vaporization source. The dominant peaks in the anion total mass distribution correspond to clusters having stoichiometries of the form (VO2)n(VO3)m(O2)q-. Collision-induced dissociation studies of the vanadium oxide species V2O4-6-, V3O6-9-, V4O8-10-, V5O11-13-, V6O13-15-, and V7O16-18- indicate that VO2, VO3, and V2O5 units are the main building blocks of these clusters. There are many similarities between the anion mass distribution and that of the cation distribution studied previously. The principal difference is a shift to higher oxygen content by one additional oxygen atom for the stoichiometric anions (VxOy-) as compared to the cations with the same number of vanadium atoms, which is attributed to the extra pair of electrons of the anionic species. The oxygen-rich clusters, VxOy(O2)-, are shown to more tightly adsorb molecular oxygen than those of the corresponding cationic clusters. In addition, the bond dissociation thresholds for the vanadium oxide clusters ?E(V+-O)=6.09+/-0.28 eV, ?E(OV+-O)=3.51+/-0.36 eV, and ?E(O2V--O)=5.43+/-0.31 eV are determined from the energy-dependent collision-induced dissociation cross sections with Xe as the collision partner. To the best of our knowledge, this is the first bond dissociation energy reported for the breaking of the V-O bond of a vanadium oxide anion.

Bell, R. C.; Zemski, K. A.; Justes, D. R.; Castleman, A. W.

2001-01-01

225

The effect of vitamin E-bonded polysulfone membrane dialyzer on a new oxidative lipid marker.  

PubMed

The use of vitamin E-bonded cellulose membrane dialyzers has been reported to cause a decrease in oxidative lipid marker levels (Nakai et al., Ther Apher Dial 14:505-540, 1; Nakai et al., J Jpn Soc Dial Ther 45:1-47, 2; Mashiba et al., Arterioscler Thromb Vasc Biol 21:1801-1808, 3). However, few studies have identified this effect with vitamin E-bonded polysulfone membranes, and no studies report the same effect on alpha (1) antitrypsin-LDL complex, a new oxidative lipid marker. This prompted us to examine the influence of use of VPS-HA vitamin E-bonded polysulfone high-flux membrane dialyzers on this new oxidative lipid marker. The subjects were 17 patients who had been dialyzed with VPS-HA for 12 months. The subjects' baseline characteristics were as follows. Their average age was 65.6 13.1 years, comprising 8 males and 9 females; hemodialysis vintage was 83.8 85.4 months. Eight had chronic glomerular nephropathy and five had diabetic nephropathy. The primary outcome was defined as alpha (1) antitrypsin-LDL complex level after 12 months, as a post-study using VPS-HA. Secondary outcomes included triglycerides, total cholesterol, HDL cholesterol and LDL cholesterol levels. The data were analyzed pre-study and after 3, 6, 9 and 12 months for alpha (1) antitrypsin-LDL complex, and pre-study and post-study for the other indicators. Twelve months after switching to VPS-HA, alpha (1) antitrypsin-LDL complex, total cholesterol and LDL cholesterol had significantly decreased. Triglycerides and HDL cholesterol had not significantly changed. Hemodialysis therapy with VPS-HA was shown to decrease alpha (1) antitrypsin-LDL complex, an index of oxidative stress, and also to decrease some lipid markers. PMID:23397123

Kitamura, Yuki; Kamimura, Kumi; Yoshioka, Noriko; Hosotani, Yoko; Tsuchida, Kenji; Koremoto, Masahide; Minakuchi, Jun

2013-02-10

226

Bond and mode selectivity in the OH + NH2D reaction: a quasi-classical trajectory calculation.  

PubMed

A state-to-state dynamics study was performed to analyze the effects of vibrational excitation on the dynamics of the OH + NH2D gas-phase reaction, which are connected to issues such as bond and mode selectivity. This reaction can evolve along two channels: H-abstraction, H2O(?) + NHD(?); and D-abstraction, HOD(?) + NH2(?). Based on an analytical potential energy surface previously developed by our group, quasi-classical trajectory calculations and subsequent normal mode analysis were performed. While vibrational excitation of the NH-sym mode of NH2D slightly favours H-abstraction over the D-abstraction, vibrational excitation of the ND mode shows that there is no clear preference for the H- or D-abstraction. These results show that this reaction does not exhibit bond selectivity, suggesting a breakdown of the spectator model. For H-abstraction, vibrational excitation of the non-reactive ND mode is partially retained in the NHD product; and for D-abstraction, excitation of the non-reactive NH mode is also partially retained in the products, indicating that this reaction exhibits mode selectivity only partially. In sum, we rule out bond and mode selectivity for this reaction. All these results were interpreted on the basis of strong coupling between modes along the reaction path, a behaviour which seems to be more the general tendency than the exception in polyatomic reactions. PMID:24105098

Monge-Palacios, M; Espinosa-Garcia, J

2013-10-23

227

Metastable structures and isotope exchange reactions in polyoxometalate ions provide a molecular view of oxide dissolution  

NASA Astrophysics Data System (ADS)

Reactions involving minerals and glasses in water are slow and difficult to probe spectroscopically but are fundamental to the performance of oxide materials in green technologies such as automotive thermoelectric power generation, CO2 capture and storage and water-oxidation catalysis; these must be made from geochemically common elements and operate in hydrous environments. Polyoxometalate ions (POMs) have structures similar to condensed oxide phases and can be used as molecular models of the oxide/water interface. Oxygen atoms in POM exchange isotopes at different rates, but, at present, there is no basis for predicting how the coordination environment and metal substitution influences rates and mechanisms. Here we identify low-energy metastable configurations that form from the breaking of weak bonds between metals and underlying highly coordinated oxygen atoms, followed by facile hydroxide, hydronium or water addition. The mediation of oxygen exchange by these stuffed structures suggests a new view of the relationship between structure and reactivity at the oxide/solution interface.

Rustad, James R.; Casey, William H.

2012-03-01

228

Chemical bonds and vibrational properties of ordered (U, Np, Pu) mixed oxides  

NASA Astrophysics Data System (ADS)

We use density functional theory +U to investigate the chemical bonding characters and vibrational properties of the ordered (U, Np, Pu) mixed oxides (MOXs), UNpO4, NpPuO4, and UPuO4. It is found that the 5f electronic states of different actinide elements keep their localized characters in all three MOXs. The occupied 5f electronic states of different actinide elements do not overlap with each other and tend to distribute over the energy band gap of the other actinide element's 5f states. As a result, the three ordered MOXs all show smaller band gaps than those of the component dioxides, with values of 0.91, 1.47, and 0.19 eV for UNpO4, NpPuO4, and UPuO4, respectively. Through careful charge density analysis, we further show that the U-O and Pu-O bonds in MOXs show more ionic character than in UO2 and PuO2, while the Np-O bonds show more covalent character than in NpO2. The change in covalencies in the chemical bonds leads to vibrational frequencies of oxygen atoms that are different in MOXs.

Yang, Yu; Zhang, Ping

2013-01-01

229

Origin of the synchronicity in bond formation in polar Diels-Alder reactions: an ELF analysis of the reaction between cyclopentadiene and tetracyanoethylene.  

PubMed

The origin of the synchronicity in C-C bond formation in polar Diels-Alder (P-DA) reactions involving symmetrically substituted electrophilic ethylenes has been studied by an ELF analysis of the electron reorganization along the P-DA reaction of cyclopentadiene (Cp) with tetracyanoethylene (TCE) at the B3LYP/6-31G* level. The present study makes it possible to establish that the synchronicity in C-C bond formation in P-DA reactions is controlled by the symmetric distribution of the electron-density excess reached in the electrophile through the charge transfer process, which can be anticipated by an analysis of the spin electron-density at the corresponding radical anion. The ELF comparative analysis of bonding along the DA reactions of Cp with ethylene and with TCE asserts that these DA reactions, which have a symmetric electron reorganization, do not have a cyclic electron reorganization as the pericyclic mechanism states. Due to the very limited number of cases of symmetrically substituted ethylenes, we can conclude that the synchronous mechanism is an exception of DA reactions. PMID:22527420

Domingo, Luis R; Prez, Patricia; Sez, Jose A

2012-05-21

230

Basic ancillary ligands promote O-O bond formation in iridium-catalyzed water oxidation: a DFT study.  

PubMed

The cationic iridium complex [Ir(OH(2))(2)(phpy)(2)](+) (phpy = o-phenylpyridine) is among the most efficient mononuclear catalysts for water oxidation. The postulated active species is the oxo complex [Ir(O)(X)(phpy)(2)](n), with X = OH(2) (n = +1), OH(-) (n = 0) or O(2-) (n = -1), depending on the pH. The reactivity of these species has been studied computationally at the DFT(B3LYP) level. The three [Ir(O)(X)(phpy)(2)](n) complexes have an electrophilic Ir(v)-oxo moiety, which yields an O-O bond by undergoing a nucleophilic attack of water in the critical step of the mechanism. In this step, water transfers one proton to either the Ir(V)-oxo moiety or the ancillary X ligand. Five different reaction pathways associated with this acid/base mechanism have been characterized. The calculations show that the proton is preferably accepted by the X ligand, which plays a key role in the reaction. The higher the basicity of X, the lower the energy barrier associated with O-O bond formation. The anionic species, [Ir(O)(2)(phpy)(2)](-), which has the less electrophilic Ir(V)-oxo moiety but the most basic X ligand, promotes O-O bond formation through the lowest energy barrier, 14.5 kcal mol(-1). The other two active species, [Ir(O)(OH)(phpy)(2)] and [Ir(O)(OH(2))(phpy)(2)](+), which have more electrophilic Ir(V)-oxo moieties but less basic X ligands, involve higher energy barriers, 20.2 kcal mol(-1) and 25.9 kcal mol(-1), respectively. These results are in good agreement with experiments showing important pH effects in similar catalytic systems. The theoretical insight given by the present study can be useful in the design of more efficient water oxidation catalysts. The catalytic activity may increase by using ligand scaffolds bearing internal bases. PMID:21918767

Vilella, Laia; Vidossich, Pietro; Balcells, David; Lleds, Agust

2011-09-14

231

Effect of maillard reaction volatile products on lipid oxidation  

Microsoft Academic Search

Maillard reaction volatile compounds (MRV), prepared by heating a glucose-glycine solution, were tested as antioxidants in\\u000a soybean oil (SBO) thermoxidation. The volatiles were transferred into the oil by stripping with a stream of Nitrogen and substituting\\u000a the atmosphere above the oil with air containing MRV. Standard accelerated oxidation was performed by heating the SBO. Peroxide\\u000a value measurement and headspace gas

B. E. Elizalde; M. Dalla Rosa; C. R. Lerici

1991-01-01

232

The Influence of Hydrogen Bonding on Hydrogen-Atom Abstraction Reactions of Dehydropyridinium Cations in the Gas Phase  

PubMed Central

The reactions of several substituted, positively-charged dehydropyridinium cations with cyclohexane, methanol and tetrahydrofuran have been examined in a Fourier-transform ion cyclotron resonance mass spectrometer. All of the charged monoradicals react with the neutral reagents exclusively via hydrogen atom abstraction. For cyclohexane, there is a good correlation between the reaction efficiencies and the calculated electron affinities at the radical sites; that is, the greater the electron affinity of the charged monoradical at the radical site, the faster the reaction. The reaction efficiencies with methanol and tetrahydrofuran, however, do not correlate with the calculated electron affinities. Density functional theory (DFT) calculations indicate that for these reagents a stabilizing hydrogen bonding interaction exists in the hydrogen atom abstraction transition states for some of the charged monoradicals but not for others. At both the MPW1K and G3MP2B3 levels of theory, there is a good correlation between the calculated activation enthalpies and the observed reaction efficiencies although the G3MP2B3 method provides a slightly better correlation than the MPW1K method. The extent of enhancement in the reaction efficiencies caused by the hydrogen bonding interactions parallels the calculated hydrogen bond lengths in the transition states.

Adeuya, Anthony; Nash, John J.; Kenttamaa, Hilkka I.

2010-01-01

233

Model reaction studies on vanadium oxide nanostructures on Pd(111)  

NASA Astrophysics Data System (ADS)

Deuterium desorption and reaction between deuterium and oxygen to water has been studied on ultrathin vanadium oxide structures prepared on Pd(111). The palladium sample was part of a permeation source, thus enabling the supply of atomic deuterium to the sample surface via the bulk. Different vanadium oxide films have been prepared by e-beam evaporation in UHV under oxygen atmosphere. The structure of these films was determined using low energy electron diffraction and scanning tunneling microscopy. The mean translational energy of the desorption and reaction products has been measured with a time-of-flight spectrometer. The most stable phases for monolayer and submonolayer VOx are particular surface-V2O3 and VO phases at 523 and 700 K, respectively. Thicker films grow in the form of bulk V2O3. The mean translational energy of the desorbing deuterium species corresponds in all cases to the thermalized value. Apparent deviations from this energy distribution could be attributed to different adsorption/desorption and/or accommodation behaviors of molecular deuterium from the gas phase on the individual vanadium oxide films. The water reaction product shows a slightly hyperthermal mean translational energy, suggesting that higher energetic permeating deuterium contributes with higher probability to the water formation.

Kratzer, M.; Surnev, S.; Netzer, F. P.; Winkler, A.

2006-08-01

234

Bond Length and Local Energy Density Property Connections for Non-transition- Metal-Oxide-Bonded Interactions  

SciTech Connect

For a variety of molecules and Earth materials, the theoretical local kinetic energy density, G(rc), increases and the local potential energy density, V(rc), decreases as the MO bond lengths (M = first and second row metal atoms) decrease and electron density, ?(rc), is localized at the bond critical points, rc. Despite claims that the ratio, G(rc)/?(rc), classifies bonded interactions as shared covalent when less than unity and closed shell ionic when greater than unity, the ratio was found to increase from 0.5 to 2.5 a.u. as the local electronic energy density H(rc) = G(rc) + V(rc) decreases and becomes progressively more negative. In any event, the ratio is indicated to be a measure of the character for a given M-O bond, the greater the ratio, the larger the value of ?(rc), the smaller the coordination number of the M atom and the more covalent the bond. H(rc)/?(rc) vs. G(rc)/?(rc) scatter diagrams categorize the M-O bond data into domains with the H(rc)/?(rc) ratio tending to increase as the electronegativity of the M atoms increase. Estimated values of G(rc) and V(rc), using an expression based on gradient corrected electron gas theory, are in good agreement with theoretical values, particularly for bonded interactions involving second row M atoms. The agreement is poorer for the more covalent C-O and N-O bonds.

Gibbs, Gerald V.; Spackman, M. A.; Jayatilaka, Dylan; Rosso, Kevin M.; Cox, David F.

2006-11-09

235

Chemical analysis and bonding reaction of RelyX Unicem and Bifix compositesA comparative study  

Microsoft Academic Search

ObjectivesThe chemical and physical properties of the dual curing luting composites RelyX Unicem (3M ESPE) and Bifix (VOCO) were analyzed with regard to their elemental composition, surface morphology and polymerization reaction. The bonding of both materials to hydroxyapatite (HAp) was studied.

Hans U. V. Gerth; Till Dammaschke; Harald Zchner; Edgar Schfer

2006-01-01

236

Transformation of acetaminophen using manganese dioxide-mediated oxidative processes: reaction rates and pathways.  

PubMed

This study investigates the oxidative transformation kinetics of acetaminophen (APAP) by ?-MnO2 under different conditions. APAP was rapidly oxidized by ?-MnO2 with the generation of Mn(2+). The measured APAP reaction rate considerably increased with an increase in initial ?-MnO2 and APAP concentration, but decreased as pH increased. The APAP reaction rate also increased with an increase in temperature. The addition of inorganic ions (Mn(2+), Ca(2+), and Fe(3+)) and substituted phenols (guaiacol, caffeic acid, and p-coumaric acid) as co-solutes remarkably decreased the transformation rate of APAP. The UV-Vis absorption spectra exhibited the ? ? ?* transition, typical for aromatic rings. In addition, the intensity of the absorption peak gradually improved with increasing reaction time, suggesting that APAP can polymerize to form oligomers. Moreover, the secondary mass spectra of the dimers elucidated that the dimers were formed by the covalent bonding of phenol aromatic rings. Moreover, the higher-degree oligomers were formed by the coupling polymerization of phenolic and anilidic groups of dimers. These results are useful in understanding the fate of APAP in natural systems. PMID:23434488

Xiao, Hong; Song, Haiyan; Xie, Hongqin; Huang, Wei; Tan, Jun; Wu, Jianzhong

2013-02-04

237

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex  

PubMed Central

The mechanism of formation of [FeIV(O)(N4Py)]2+ (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [FeII(N4Py)(CH3CN)]2+ (1) with m-chloroperbenzoic acid (mCPBA) in CH2Cl2 at ?30 C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv peracid and takes place in two successive one-electron steps via an [FeIII(N4Py)OH]2+(3) intermediate. The first oxidation step uses 0.5 equiv peracid and produces 0.5 equiv 3-chlorobenzoic acid, while the second step uses 1 equiv peracid and affords byproducts derived from chlorophenyl radical. We conclude that the FeII(N4Py) center promotes O-O bond heterolysis, while the FeIII(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state.

Ray, Kallol; Lee, Sang Mok; Que, Lawrence

2008-01-01

238

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex.  

PubMed

The mechanism of formation of [Fe(IV)(O)(N4Py)](2+) (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [Fe(II)(N4Py)(CH(3)CN)](2+) (1) with m-chloroperbenzoic acid (mCPBA) in CH(2)Cl(2) at -30 C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv peracid and takes place in two successive one-electron steps via an [Fe(III)(N4Py)OH](2+)(3) intermediate. The first oxidation step uses 0.5 equiv peracid and produces 0.5 equiv 3-chlorobenzoic acid, while the second step uses 1 equiv peracid and affords byproducts derived from chlorophenyl radical. We conclude that the Fe(II)(N4Py) center promotes O-O bond heterolysis, while the Fe(III)(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state. PMID:18443654

Ray, Kallol; Lee, Sang Mok; Que, Lawrence

2008-03-01

239

Phase transformation and bond coat oxidation behavior of plasma-sprayed zirconia thermal barrier coating  

Microsoft Academic Search

ZrO2CeO2Y2O3 and ZrO2Y2O3 thermal barrier coatings were prepared using the air plasma spray process. Phase transformation in the ceramic top coating, bond coat oxidation and thermal barrier properties were investigated to compare ZrO2CeO2Y2O3 with ZrO2Y2O3 at 1300C under high temperature thermal cycles. In the as-sprayed condition, both coatings showed a 7?11% porosity fraction and typical lamellar structures formed by continuous

C. H. Lee; H. K. Kim; H. S. Choi; H. S. Ahn

2000-01-01

240

Palladium-catalyzed oxidative acetoxylation of benzylic C-h bond using bidentate auxiliary.  

PubMed

Pd(OAc)2-catalyzed oxidative acetoxylation of benzylic C-H bonds utilizing a bidentate system has been explored. A variety of picolinoyl- or quinoline-2-carbonyl-protected toluidine derivatives react with PhI(OAc)2 in the presence of Pd(OAc)2 to afford the acetoxylated products in synthetically useful yields. A broad of functionalities, such as CH3, F, Cl, Br, I, COCH3, CO2Et, SO2CH3, and NO2, were tolerated. This transformation provides easy access to 2-hydroxymethylaniline derivatives. PMID:24106909

Ju, Long; Yao, Jinzhong; Wu, Zaihong; Liu, Zhanxiang; Zhang, Yuhong

2013-10-24

241

Auger parameter determination of bonding states on thinly oxidized silicon nitride  

SciTech Connect

Silicon nitride powders have been thermally oxidized between 700 and 1,200 C in a high purity N{sub 2}--20% O{sub 2} gas environment. The powders were subsequently analyzed by x-ray photoelectron and Auger electron spectroscopy for evidence of oxynitride surface states. Measurements were made on the Si 2p, O 1s, N 1s, C 1s, F 1s, and Si KLL transitions, the latter being obtained using bremsstrahlung radiation from the Mg x-ray source. As a function of increasing temperature the data show a clear progression of spectral binding energies and peak shapes that are indicative of more advanced surface oxidation. However, definitive analysis of these data rests on the combined use of both Auger and photoelectron data to define the oxidized surface states for a system that involves two electrically insulating end states, silicon nitride and silicon dioxide. Curve fitting the Si 2p and Si KLL transitions as a function of oxidation, coupled with use of Auger parameters for the starting silicon nitride and final silicon dioxide, reveals no measurable evidence for an interphase oxynitride in the thin oxide scales of this study, where the silicon nitride substrate is detectable. Possible incorrect assignment of oxynitride bonding, from shifted Si 2p states in the carbon referenced spectra, is attributable to band bending as the transition is made from incipient to fully formed silicon dioxide.

Taylor, T.N.; Butt, D.P. [Los Alamos National Lab., NM (United States); Pantano, C.G. [Pennsylvania State Univ., University Park, PA (United States). Dept. of Materials Science and Engineering

1997-10-01

242

Double proton transfer behavior and one-electron oxidation effect in double H-bonded glycinamide-formic acid complex  

NASA Astrophysics Data System (ADS)

The behavior of double proton transfer occurring in a representative glycinamide-formic acid complex has been investigated at the B3LYP/6-311++G** level of theory. Thermodynamic and, especially, kinetic parameters, such as tautomeric energy, equilibrium constant, and barrier heights have been discussed, respectively. The relevant quantities involved in the double proton transfer process, such as geometrical changes, interaction energies, and intrinsic reaction coordinate calculations have also been studied. Computational results show that the participation of a formic acid molecule favors the proceeding of the proton transfer for glycinamide compared with that without mediate-assisted case. The double proton transfer process proceeds with a concerted mechanism rather than a stepwise one since no ion-pair complexes have been located during the proton transfer process. The calculated barrier heights are 11.48 and 0.85 kcal/mol for the forward and reverse directions, respectively. However, both of them have been reduced by 2.95 and 2.61 kcal/mol to 8.53 and -1.76 kcal/mol if further inclusion of zero-point vibrational energy corrections, where the negative barrier height implies that the reverse reaction should proceed with barrierless spontaneously, analogous to that occurring between glycinamide and formamide. Furthermore, solvent effects on the thermodynamic and kinetic processes have also been predicted qualitatively employing the isodensity surface polarized continuum model within the framework of the self-consistent reaction field theory. Additionally, the oxidation process for the double H-bonded glycinamide-formic acid complex has also been investigated. Contrary to that neutral form possessing a pair of two parallel intermolecular H bonds, only a single H bond with a comparable strength has been found in its ionized form. The vertical and adiabatic ionization potentials for the neutral complex have been determined to be about 9.40 and 8.69 eV, respectively, where ionization is mainly localized on the glycinamide fragment. Like that ionized glycinamide-formamide complex, the proton transfer in the ionized complex is characterized by a single-well potential, implying that the proton initially attached to amide N4 in the glycinamide fragment cannot be transferred to carbonyl O13 in the formic acid fragment at the geometry of the optimized complex.

Li, Ping; Bu, Yuxiang

2004-11-01

243

Specific Bonds between an Iron Oxide Surface and Outer Membrane Cytochromes MtrC and OmcA from Shewanella oneidensis MR-1  

SciTech Connect

Shewanella oneidensis MR-1 is purported to express outer membrane cytochromes (e.g., MtrC and OmcA) that transfer electrons directly to Fe(III) in a mineral during anaerobic respiration. A prerequisite for this type of reaction would be the formation of a stable bond between a cytochrome and an iron oxide surface. Atomic force microscopy (AFM) was used to detect whether a specific bond forms between a hematite (Fe2O3) thin film, created with oxygen plasma assisted molecular beam epitaxy (MBE), and recombinant MtrC or OmcA molecules coupled to gold substrates. Force spectra displayed a unique force signature indicative of a specific bond between each cytochrome and the hematite surface. The strength of the OmcA-hematite bond was approximately twice as strong as the MtrC-hematite bond, but direct binding to hematite was twice as favorable for MtrC. Reversible folding/unfolding reactions were observed for mechanically denatured MtrC molecules bound to hematite. The force measurements for the hematite-cytochrome pairs were compared to spectra collected between an iron oxide and S. oneidensis under anaerobic conditions. There is a strong correlation between the whole cell and pure protein force spectra suggesting that the unique binding attributes of each cytochrome complement one another and allow both MtrC and OmcA to play a prominent role in the transfer of electrons to Fe(III) in minerals. Finally, by comparing the magnitude of binding force for the whole cell vs. pure protein data, we were able to estimate that a single bacterium of S. oneidensis (2 x 0.5 ?m) expresses ~104 cytochromes on its outer surface.

Lower, Brian H.; Shi, Liang; Yongsunthon, Ruchirej; Droubay, Timothy; Mccready, David E.; Lower, Steven

2007-07-31

244

Palladium- and copper-mediated N-aryl bond formation reactions for the synthesis of biological active compounds  

PubMed Central

Summary N-Arylated aliphatic and aromatic amines are important substituents in many biologically active compounds. In the last few years, transition-metal-mediated N-aryl bond formation has become a standard procedure for the introduction of amines into aromatic systems. While N-arylation of simple aromatic halides by simple amines works with many of the described methods in high yield, the reactions may require detailed optimization if applied to the synthesis of complex molecules with additional functional groups, such as natural products or drugs. We discuss and compare in this review the three main N-arylation methods in their application to the synthesis of biologically active compounds: Palladium-catalysed BuchwaldHartwig-type reactions, copper-mediated Ullmann-type and ChanLam-type N-arylation reactions. The discussed examples show that palladium-catalysed reactions are favoured for large-scale applications and tolerate sterically demanding substituents on the coupling partners better than ChanLam reactions. ChanLam N-arylations are particularly mild and do not require additional ligands, which facilitates the work-up. However, reaction times can be very long. Ullmann- and BuchwaldHartwig-type methods have been used in intramolecular reactions, giving access to complex ring structures. All three N-arylation methods have specific advantages and disadvantages that should be considered when selecting the reaction conditions for a desired CN bond formation in the course of a total synthesis or drug synthesis.

Fischer, Carolin

2011-01-01

245

Palladium- and copper-mediated N-aryl bond formation reactions for the synthesis of biological active compounds.  

PubMed

N-Arylated aliphatic and aromatic amines are important substituents in many biologically active compounds. In the last few years, transition-metal-mediated N-aryl bond formation has become a standard procedure for the introduction of amines into aromatic systems. While N-arylation of simple aromatic halides by simple amines works with many of the described methods in high yield, the reactions may require detailed optimization if applied to the synthesis of complex molecules with additional functional groups, such as natural products or drugs. We discuss and compare in this review the three main N-arylation methods in their application to the synthesis of biologically active compounds: Palladium-catalysed Buchwald-Hartwig-type reactions, copper-mediated Ullmann-type and Chan-Lam-type N-arylation reactions. The discussed examples show that palladium-catalysed reactions are favoured for large-scale applications and tolerate sterically demanding substituents on the coupling partners better than Chan-Lam reactions. Chan-Lam N-arylations are particularly mild and do not require additional ligands, which facilitates the work-up. However, reaction times can be very long. Ullmann- and Buchwald-Hartwig-type methods have been used in intramolecular reactions, giving access to complex ring structures. All three N-arylation methods have specific advantages and disadvantages that should be considered when selecting the reaction conditions for a desired C-N bond formation in the course of a total synthesis or drug synthesis. PMID:21286396

Fischer, Carolin; Koenig, Burkhard

2011-01-14

246

Oxygen switch in visible-light photoredox catalysis: radical additions and cyclizations and unexpected C-C-bond cleavage reactions.  

PubMed

Visible light photoredox catalyzed inter- and intramolecular C-H functionalization reactions of tertiary amines have been developed. Oxygen was found to act as chemical switch to trigger two different reaction pathways and to obtain two different types of products from the same starting material. In the absence of oxygen, the intermolecular addition of N,N-dimethyl-anilines to electron-deficient alkenes provided ?-amino nitriles in good to high yields. In the presence of oxygen, a radical addition/cyclization reaction occurred which resulted in the formation of tetrahydroquinoline derivatives in good yields under mild reaction conditions. The intramolecular version of the radical addition led to the unexpected formation of indole-3-carboxaldehyde derivatives. Mechanistic investigations of this reaction cascade uncovered a new photoredox catalyzed C-C bond cleavage reaction. PMID:23330701

Zhu, Shaoqun; Das, Arindam; Bui, Lan; Zhou, Hanjun; Curran, Dennis P; Rueping, Magnus

2013-01-18

247

Role of bond strength on the lattice thermal expansion and oxide ion conductivity in quaternary pyrochlore solid solutions.  

PubMed

Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 ). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides. PMID:22280449

Radhakrishnan, A N; Prabhakar Rao, P; Mahesh, S K; Thampi, D S Vaisakhan; Koshy, Peter

2012-01-26

248

Formation of phenol under conditions of the reaction of oxidative carbonylation of benzene to benzoic acid  

SciTech Connect

This paper describes conditions for the oxidation of benzene to phenol. It is shown that a reaction mixture of water, carbon monoxide, and oxygen are essential to the oxidation. The oxidation is a side reaction found to occur during the oxidative carbonylation of benzene to benzoic acid in a medium of trifluoroacetic acid.

Kalinovsky, I.O.; Leshcheva, A.N.; Pogorelov, V.V.; Gelbshtein, A.I.

1993-12-31

249

Effect of composition on the processing and properties of sintered reaction-bonded silicon nitride  

SciTech Connect

The type of silicon powder and sintering additive were found to influence the processing and final mechanical properties of sintered reaction bonded silicon nitride. High purity silicon powders produced low {alpha}-Si{sub 3}N{sub 4} content during nitridation. The Si powder type had no apparent effect on densification. More complete nitridation and higher room temperature mechanical properties were observed for the Si powders with higher Fe contents. However, the higher Fe contents resulted in greater high temperature strength degradation and so there was better high temperature strength retention with the higher purity Si. High {alpha}-Si{sub 3}N{sub 4} contents were found after nitridation with {alpha}-Si{sub 3}N{sub 4} seeded materials and with MgO-Y{sub 2}O{sub 3} as the sintering additive. Densification was inhibited by refractory additives, such as Y{sub 2}O{sub 3}-SiO{sub 2}. The highest room temperature strength and fracture toughness values correlated to high nitrided {alpha}-Si{sub 3}N{sub 4} contents. The high temperature strength behavior was similar for all additive types.

Tiegs, T.N.; Kiggans, J.O.; Montgomery, F.C.; Lin, H.T.; Barker, D.L.; Snodgrass, J.D.; Sabolsky, E.M.; Coffey, D.W.

1996-04-01

250

Recent advances in reaction bonded silicon carbide optics and optical systems  

NASA Astrophysics Data System (ADS)

SSG Precision Optronics, Inc. (SSG) has recently developed a number of Reaction Bonded (RB) Silicon Carbide (SiC) optical systems for space-based remote sensing and astronomical observing applications. RB SiC's superior material properties make it uniquely well suited to meet the image quality and long term dimensional stability requirements associated with these applications. An overview of the RB SiC manufacturing process is presented, along with a summary description of recently delivered RB SiC flight hardware. This hardware includes an RB SiC telescope and Pointing Mirror Assembly (PMA) for the Geostationary Imaging Fourier Transform Spectrometer (GIFTS) mission and an imaging telescope for the Long-Range Reconnaissance Imager (LORRI) mission. SSG continues to advance the state-of-the-technology with SiC materials and systems. A summary of development activities related to a low-cost, fracture tough, fiber reinforced RB SiC material formulation, novel tooling to produce monolithic, partially closed back mirror geometries, and extension of the technology to large aspheric mirrors is also provided.

Robichaud, Joseph; Schwartz, Jay; Landry, David; Glenn, William; Rider, Brian; Chung, Michael

2005-08-01

251

Oxidative Addition of Carbon-Carbon Bonds with a Redox-Active Bis(imino)pyridine Iron Complex  

PubMed Central

Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(?2-N2) (RPDI = 2,6-(2,6-R2C6H3 N=CMe)2C5H3N; R = Me, iPr), resulted in oxidative addition of a CC bond at ambient temperature to yield the corresponding iron biphenyl compounds, (RPDI)Fe-(biphenyl). The molecular structures of the resulting bis-(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mssbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.

Darmon, Jonathan M.; Stieber, S. Chantal E.; Sylvester, Kevin T.; Fernandez, Ignacio; Lobkovsky, Emil; Semproni, Scott P.; Bill, Eckhard; Wieghardt, Karl; DeBeer, Serena; Chirik, Paul J.

2013-01-01

252

Temperature-dependent size exclusion chromatography for the in situ investigation of dynamic bonding/debonding reactions.  

PubMed

Polymers capable of dynamic bonding/debonding reactions are of great interest in modern day research. Potential applications can be found in the fields of self-healing materials or printable networks. Since temperature is often used as a stimulus for triggering reversible bonding reactions, an analysis operating at elevated temperatures is very useful for the in situ investigation of the reaction mechanism, as unwanted side effects can be minimized when performing the analyses at the same temperature at which the reactions occur. A temperature-dependent size exclusion chromatographic system (TD SEC) has been optimized for investigating the kinetics of retro Diels-Alder-based depolymerization of Diels-Alder polymers. The changing molecular weight distribution of the analyzed polymers during depolymerization gives valuable quantitative information on the kinetics of the reactions. Adequate data interpretation methods were developed for the correct evaluation of the chromatograms. The results are confirmed by high-temperature dynamic light scattering, thermogravimetric analysis, and time-resolved nuclear magnetic resonance spectroscopy at high temperatures. In addition, the SEC system and column material stability under application conditions were assessed using thermoanalysis methods, infrared spectroscopy, nitrogen physisorption, and scanning electron microscopy. The findings demonstrate that the system is stable and, thus, we can reliably characterize such dynamically bonding/debonding systems with TD SEC. PMID:23877179

Brandt, Josef; Guimard, Nathalie K; Barner-Kowollik, Christopher; Schmidt, Friedrich G; Lederer, Albena

2013-07-23

253

One-electron oxidation of ruthenocene: reactions of the ruthenocenium ion in gentle electrolyte media.  

PubMed

The electrochemical oxidation of ruthenocene, RuCp(2) (Cp = eta(5)-C(5)H(5)), 1, has been studied in dichloromethane using a supporting electrolyte containing either the [B(C(6)F(5))(4)](-) (TFAB) or the [B(C(6)H(3)(CF(3))(2))(4)](-) (BArF(24)) counteranion. A quasi-Nernstian process was observed in both cases, with E(1/2) values of 0.41 and 0.57 V vs FeCp(2) in the respective electrolyte media. The ruthenocenium ion 1(+) equilibrates with a metal-metal bonded dimer [Ru(2)Cp(4)](2+), 2(2+), that is increasingly preferred at low temperatures. Dimerization equilibrium constants determined by digital simulation of cyclic voltammetry (CV) curves were in the range of 10(2)-10(4) M(-1) at temperatures of 256 to 298 K. Near room temperature, and particularly when BArF(24) is the counteranion, the dinuclear species [Ru(2)Cp(2)(sigma:eta(5)-C(5)H(4))(2)] (2+), 3(2+), in which each metal is sigma-bonded to a cyclopentadienyl ring, was the preferred electrolytic oxidation product. Cathodic reduction of 3(2+) regenerated ruthenocene. The two dinuclear products, 2(2+) and 3(2+), were characterized by (1)H NMR spectroscopy on anodically electrolyzed solutions of 1 at low temperatures in CD(2)Cl(2)/[NBu(4)][BArF(24)]. The variable temperature NMR behavior of these solutions showed that 3(2+) and 2(2+) take part in a thermal equilibrium, the latter being dominant at the lowest temperatures. Ruthenocene hydride, [1-H](+), was also identified as being present in the electrolysis solutions. The oxidation of ruthenocene is shown to be an inherent one-electron process, giving a ruthenocenium ion which is highly susceptible to reactions that allow it to regain an 18-electron configuration. In a dry non-donor solvent, and in the absence of nucleophiles, this electronic configuration is attained by self-reactions involving formation of Ru-Ru or Ru-C bonds. The present data offer a mechanistic explanation for the previously described results on the chemical oxidation of osmocene (Droege, M.W.; Harman, W.D.; Taube, H. Inorg. Chem. 1987, 26, 1309) and are relevant to the manner in which sigma:eta(5)-C(5)H(4)-complexes of other second and third-row metals are formed. PMID:19235975

Swarts, Jannie C; Nafady, Ayman; Roudebush, John H; Trupia, Sabrina; Geiger, William E

2009-03-01

254

Influence of Sulfur, Platinum, and Hafnium on the Oxidation Behavior of CVD NiAl Bond Coatings  

Microsoft Academic Search

The influences of S, Pt, and Hf on the oxidation behavior of chemical vapor deposition (CVD) NiAl bond coatings on high-S and low-S superalloys were investigated. Cyclic and isothermal-oxidation testing at 1150C revealed that alumina-scale adherence to NiAl coatings was very sensitive to substrate S impurities. Scale spallation, as well as the growth of voids at the oxidemetal interface, increased

J. A. Haynes; B. A. Pint; K. L. More; Y. Zhang; I. G. Wright

2002-01-01

255

Destruction of hydrogen bonds of poly(N-isopropylacrylamide) aqueous solution by trimethylamine N-oxide  

NASA Astrophysics Data System (ADS)

Trimethylamine N-oxide (TMAO) is a compatible or protective osmolyte that stabilizes the protein native structure through non-bonding mechanism between TMAO and hydration surface of protein. However, we have shown here first time the direct binding mechanism for naturally occurring osmolyte TMAO with hydration structure of poly(N-isopropylacrylamide) (PNIPAM), an isomer of polyleucine, and subsequent aggregation of PNIPAM. The influence of TMAO on lower critical solution temperature (LCST) of PNIPAM was investigated as a function of TMAO concentration at different temperatures by fluorescence spectroscopy, viscosity (?), multi angle dynamic light scattering, zeta potential, and Fourier transform infrared (FTIR) spectroscopy measurements. To address some of the basis for further analysis of FTIR spectra of PNIPAM, we have also measured FTIR spectra for the monomer of N-isopropylacrylamide (NIPAM) in deuterium oxide (D2O) as a function of TMAO concentration. Our experimental results purportedly elucidate that the LCST values decrease with increasing TMAO concentration, which is mainly contributing to the direct hydrogen bonding of TMAO with the water molecules that are bound to the amide (-CONH) functional groups of the PNIPAM. We believed that the present work may act as a ladder to reach the heights of understanding of molecular mechanism between TMAO and macromolecule.

Reddy, P. Madhusudhana; Taha, Mohamed; Venkatesu, Pannuru; Kumar, Awanish; Lee, Ming-Jer

2012-06-01

256

Temporary zinc oxide-eugenol cement: eugenol quantity in dentin and bond strength of resin composite.  

PubMed

Uptake of eugenol from eugenol-containing temporary materials may reduce the adhesion of subsequent resin-based restorations. This study investigated the effect of duration of exposure to zinc oxide-eugenol (ZOE) cement on the quantity of eugenol retained in dentin and on the microtensile bond strength (?TBS) of the resin composite. The ZOE cement (IRM Caps) was applied onto the dentin of human molars (21 per group) for 1, 7, or 28 d. One half of each molar was used to determine the quantity of eugenol (by spectrofluorimetry) and the other half was used for ?TBS testing. The ZOE-exposed dentin was treated with either OptiBond FL using phosphoric acid (H?PO?) or with Gluma Classic using ethylenediaminetetraacetic acid (EDTA) conditioning. One group without conditioning (for eugenol quantity) and two groups not exposed to ZOE (for eugenol quantity and ?TBS testing) served as controls. The quantity of eugenol ranged between 0.33 and 2.9 nmol mg? of dentin (median values). No effect of the duration of exposure to ZOE was found. Conditioning with H?PO? or EDTA significantly reduced the quantity of eugenol in dentin. Nevertheless, for OptiBond FL, exposure to ZOE significantly decreased the ?TBS, regardless of the duration of exposure. For Gluma Classic, the ?TBS decreased after exposure to ZOE for 7 and 28 d. OptiBond FL yielded a significantly higher ?TBS than did Gluma Classic. Thus, ZOE should be avoided in cavities later to be restored with resin-based materials. PMID:23841789

Koch, Tamara; Peutzfeldt, Anne; Malinovskii, Vladimir; Flury, Simon; Hner, Robert; Lussi, Adrian

2013-05-09

257

Residues in Human Arsenic (+3 Oxidation State) Methyltransferase Forming Potential Hydrogen Bond Network around S-adenosylmethionine  

PubMed Central

Residues Tyr59, Gly78, Ser79, Met103, Gln107, Ile136 and Glu137 in human arsenic (+3 oxidation state) methyltransferase (hAS3MT) were deduced to form a potential hydrogen bond network around S-adenosylmethionine (SAM) from the sequence alignment between Cyanidioschyzon merolae arsenite S-adenosylmethyltransferase (CmArsM) and hAS3MT. Herein, seven mutants Y59A, G78A, S79A, M103A, Q107A, I136A and E137A were obtained. Their catalytic activities and conformations were characterized and models were built. Y59A and G78A were completely inactive. Only 7.0%, 10.6% and 13.8% inorganic arsenic (iAs) was transformed to monomethylated arsenicals (MMA) when M103A, Q107A and I136A were used as the enzyme. The Vmax (the maximal velocity of the reaction) values of M103A, Q107A, I136A and E137A were decreased to 8%, 22%, 15% and 50% of that of WT-hAS3MT, respectively. The KM(SAM) (the Michaelis constant for SAM) values of mutants M103A, I136A and E137A were 15.7, 8.9 and 5.1 fold higher than that of WT-hAS3MT, respectively, indicating that their affinities for SAM were weakened. The altered microenvironment of SAM and the reduced capacity of binding arsenic deduced from KM(As) (the Michaelis constant for iAs) value probably synergetically reduced the catalytic activity of Q107A. The catalytic activity of S79A was higher than that of WT despite of the higher KM(SAM), suggesting that Ser79 did not impact the catalytic activity of hAS3MT. In short, residues Tyr59 and Gly78 significantly influenced the catalytic activity of hAS3MT as well as Met103, Ile136 and Glu137 because they were closely associated with SAM-binding, while residue Gln107 did not affect SAM-binding regardless of affecting the catalytic activity of hAS3MT. Modeling and our experimental results suggest that the adenine ring of SAM is sandwiched between Ile136 and Met103, the amide group of SAM is hydrogen bonded to Gly78 in hAS3MT and SAM is bonded to Tyr59 with van der Waals, cation-? and hydrogen bonding contacts.

Li, Xiangli; Cao, Jing; Wang, Shuping; Geng, Zhirong; Song, Xiaoli; Hu, Xin; Wang, Zhilin

2013-01-01

258

Residues in Human Arsenic (+3 Oxidation State) Methyltransferase Forming Potential Hydrogen Bond Network around S-adenosylmethionine.  

PubMed

Residues Tyr59, Gly78, Ser79, Met103, Gln107, Ile136 and Glu137 in human arsenic (+3 oxidation state) methyltransferase (hAS3MT) were deduced to form a potential hydrogen bond network around S-adenosylmethionine (SAM) from the sequence alignment between Cyanidioschyzon merolae arsenite S-adenosylmethyltransferase (CmArsM) and hAS3MT. Herein, seven mutants Y59A, G78A, S79A, M103A, Q107A, I136A and E137A were obtained. Their catalytic activities and conformations were characterized and models were built. Y59A and G78A were completely inactive. Only 7.0%, 10.6% and 13.8% inorganic arsenic (iAs) was transformed to monomethylated arsenicals (MMA) when M103A, Q107A and I136A were used as the enzyme. The Vmax (the maximal velocity of the reaction) values of M103A, Q107A, I136A and E137A were decreased to 8%, 22%, 15% and 50% of that of WT-hAS3MT, respectively. The KM(SAM) (the Michaelis constant for SAM) values of mutants M103A, I136A and E137A were 15.7, 8.9 and 5.1 fold higher than that of WT-hAS3MT, respectively, indicating that their affinities for SAM were weakened. The altered microenvironment of SAM and the reduced capacity of binding arsenic deduced from KM(As) (the Michaelis constant for iAs) value probably synergetically reduced the catalytic activity of Q107A. The catalytic activity of S79A was higher than that of WT despite of the higher KM(SAM) , suggesting that Ser79 did not impact the catalytic activity of hAS3MT. In short, residues Tyr59 and Gly78 significantly influenced the catalytic activity of hAS3MT as well as Met103, Ile136 and Glu137 because they were closely associated with SAM-binding, while residue Gln107 did not affect SAM-binding regardless of affecting the catalytic activity of hAS3MT. Modeling and our experimental results suggest that the adenine ring of SAM is sandwiched between Ile136 and Met103, the amide group of SAM is hydrogen bonded to Gly78 in hAS3MT and SAM is bonded to Tyr59 with van der Waals, cation-? and hydrogen bonding contacts. PMID:24124590

Li, Xiangli; Cao, Jing; Wang, Shuping; Geng, Zhirong; Song, Xiaoli; Hu, Xin; Wang, Zhilin

2013-10-04

259

Effect of deposition temperature on the bonding configurations and properties of fluorine doped silicon oxide film  

Microsoft Academic Search

In our study, fluorine-doped silicon oxide (SiOF) films were prepared using a mixture of SiH4, N2O, and CF4 in a conventional plasma enhanced chemical vapor deposition system at various deposition temperatures. Deposition behaviors are determined by the deposition temperature. Our results show that for temperatures below 300C the process is surface-reaction-limited controlled, but becomes diffusion-limited when the deposition temperature exceeds

Wei-Lun Lu; Ting-Wei Kuo; Chun-Hsien Huang; Na-Fu Wang; Yu-Zen Tsai; Ming-Wei Wang; Chen-I. Hung; Mau-Phon Houng

2011-01-01

260

Unlocking the binding and reaction mechanism of hydroxyurea substrates as biological nitric oxide donors  

PubMed Central

Hydroxyurea is the only FDA approved treatment of sickle cell disease. It is believed the primary mechanism of action is associated with the pharmacological elevation of nitric oxide in the blood; however, the exact details of this are still unclear. In the current work, we investigate the atomic level details of this process using a combination of flexible-ligand / flexible-receptor virtual screening coupled with energetic analysis that decomposes interaction energies. Utilizing these methods we were able to elucidate the previously unknown substrate binding modes of a series of hydroxyurea analogs to hemoglobin and the concomitant structural changes of the enzyme. We identify a backbone carbonyl that forms a hydrogen bond with bound substrates. Our results are consistent with kinetic and EPR measurements of hydroxyurea-hemoglobin reactions and a full mechanism is proposed that offers new insights into possibly improving substrate binding and/or reactivity.

Vankayala, Sai Lakshmana; Hargis, Jacqueline C.; Woodcock, H. Lee

2012-01-01

261

Initial reaction of hafnium oxide deposited by remote plasma atomic layer deposition method  

SciTech Connect

A remote plasma atomic layer deposition (RPALD) method has been applied to grow a hafnium oxide thin film on the Si substrate. The deposition process was monitored by in situ XPS and the as-deposited structure and chemical bonding were examined by TEM and XPS. The in situ XPS measurement showed the presence of a hafnium silicate phase at the initial stage of the RPALD process up to the 20th cycle and indicated that no hafnium silicide was formed. The initial hafnium silicate was amorphous and grew to a thickness of approximately 2 nm. Based on these results and model reactions for silicate formation, we proposed an initial growth mechanism that includes adatom migration at nascent step edges. Density functional theory calculations on model compounds indicate that the hafnium silicate is thermodynamically favored over the hafnium silicide by as much as 250 kJ/mol.

Won, Youngdo; Park, Sangwook; Koo, Jaehyoung; Kim, Seokhoon; Kim, Jinwoo; Jeon, Hyeongtag [Department of Chemistry, Hanyang University, Seoul 133-791 (Korea, Republic of); Division of Materials Science and Engineering, Hanyang University, Seoul 133-791 (Korea, Republic of)

2005-12-26

262

Fly Ash and Mercury Oxidation/Chlorination Reactions  

SciTech Connect

Mercury is a known pollutant that has detrimental effect on human health and environment. The anthropogenic emissions of mercury account for 10 to 30% of worldwide mercury emissions. There is a need to control/reduce anthropogenic mercury emissions. Many mercury control technologies are available but their effectiveness is dependent on the chemical form of mercury, because different chemical forms of mercury have different physical and chemical properties. Mercury leaves the boiler in its elemental form but goes through various transformations in the post-combustion zone. There is a need to understand how fly ash and flue gas composition affect speciation, partitioning, and reactions of mercury under the full range of post-combustion zone conditions. This knowledge can then be used to predict the chemical transformation of mercury (elemental, oxidized or particulate) in the post combustion zone and thus help with the control of mercury emissions from coal-burning power plants. To accomplish this goal present study was conducted using five coal fly ashes. These ashes were characterized and their catalytic activity was compared under selected reaction conditions in a fixed bed reactor. Based on the results from these fly ash experiments, three key components (carbon, iron oxide and calcium oxide) were chosen. These three components were then used to prepare model fly ashes. Silica/alumina was used as a base for these model fly ashes. One, two or three component model fly ashes were then prepared to investigate mercury transformation reactions. The third set of experiments was performed with CuO and CuCl2 catalysts to further understand the mercury oxidation process. Based on the results of these three studies the key components were predicted for different fly ash compositions under variety of flue gas conditions. A fixed bed reactor system was used to conduct this study. In all the experiments, the inlet concentration of Hg0(g) was maintained at 35 {micro}g/m3 using a diffusion tube as the source of Hg0(g). All experiments were conducted using 4% O2 in nitrogen mix as a reaction gas, and other reactants (HCl, H2O and SO2, NO2, Br2) were added as required. The fixed bed reactor was operated over a temperature range of 200 to 400 C. In each experiment, the reactor effluent was analyzed using the modified Ontario-Hydro method. After each experiment, fly ash particles were also analyzed for mercury. The results show that the ability of fly ash to adsorb and/or oxidize mercury is primarily dependent on its carbon, iron and calcium content. There can be either one or more than one key component at a particular temperature and flue gas condition. Surface area played a secondary role in effecting the mercury transformations when compared to the concentration of the key component in the fly ash. Amount of carbon and surface area played a key important role in the adsorption of mercury. Increased concentration of gases in the flue gas other than oxygen and nitrogen caused decreased the amount of mercury adsorbed on carbon surface. Mercury adsorption by iron oxide primarily depended on the crystalline structure of iron oxide. {alpha}-Iron oxide had no effect on mercury adsorption or oxidation under most of the flue gas conditions, but ?-iron oxide adsorbed mercury under most of the flue gas conditions. Bromine is a very good oxidizing agent for mercury. But in the presence of calcium oxide containing fly ashes, all the oxidized mercury would be reduced to elemental form. Among the catalysts, it was observed that presence of free lattice chlorine in the catalyst was very important for the oxidation of mercury. But instead of using the catalyst alone, using it along with carbon may better serve the purpose by providing the adsorption surface for mercury and also some extra surface area for the reaction to occur (especially for fly ashes with low surface area).

Sukh Sidhu; Patanjali Varanasi

2008-12-31

263

Catalytic oxidation reactions of aromatic diamines by transition metal complexes  

Microsoft Academic Search

The catalytic reactions of M(TPP)Cl (M = Fe (1), Mn (2)), Fe(TMP)Cl (3), and Fe(TDCPP)Cl (4) (H2TPP = 5,10,15,20-tetraphenylporphyrin; H2TMP = 5,10,15,20-tetramesitylporphyrin; H2TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) with 2,4-diaminotoluene (5) and p-phenylenediamine (6) were carried out using t-BuOOH (3 M isooctane solution) as oxidant. With the former substrate, the nitro derivatives 2-amino-4-nitrotoluene (5a) and 2,4-dinitrotoluene (5b) (total yield of 5a + 5b

Francesca Porta; Fabio Colonna; Francesca Arciprete; Stefano Banfi; Fausta Coppa

1996-01-01

264

Porous platinum mesoflowers with enhanced activity for methanol oxidation reaction  

NASA Astrophysics Data System (ADS)

Porous Pt and Pt-Ag alloy mesoflowers (MFs) with about 2 ?m in diameter and high porosity were synthesized using Ag mesoflowers as sacrificial template by galvanic reaction. The silver content in Pt-Ag alloys can be facilely controlled by nitric acid treatment. And the pure Pt MFs can be obtained by selective removal of silver element from Pt72Ag28 MFs electrochemically. Both Pt45Ag55, Pt72Ag28 and pure Pt show a high catalytic performance in methanol oxidation reaction (MOR). Especially, pure Pt MFs exhibited a 2 to 3 times current density enhancement in MOR compared with the commercial used Pt black, which can be attributed to their porous nanostructure with 3-dimentional nature and small crystal sizes.

Zhuang, Lina; Wang, Wenjin; Hong, Feng; Yang, Shengchun; You, Hongjun; Fang, Jixiang; Ding, Bingjun

2012-07-01

265

Reactions of oxidatively activated arylamines with thiols: reaction mechanisms and biologic implications. An overview.  

PubMed Central

Aromatic amines belong to a group of compounds that exert their toxic effects usually after oxidative biotransformation, primarily in the liver. In addition, aromatic amines also undergo extrahepatic activation to yield free arylaminyl radicals. The reactive intermediates are potential promutagens and procarcinogens, and responsible for target tissue toxicity. Since thiols react with these intermediates at high rates, it is of interest to know the underlying reaction mechanisms and the toxicologic implications. Phenoxyl radicals from aminophenols and aminyl radicals from phenylenediamines quickly disproportionate to quinone imines and quinone diimines. Depending on the structure, Michael addition or reduction reactions with thiols may prevail. Products of sequential oxidation/addition reactions (e.g., S-conjugates of aminophenols) are occasionally more toxic than the parent compounds because of their higher autoxidizability and their accumulation in the kidney. Even after covalent binding of quinone imines to protein SH groups, the resulting thioethers are able to autoxidize. The quinoid thioethers can then cross-link the protein by addition to neighboring nucleophiles. The reactions of nitrosoarenes with thiols yield a so-called "semimercaptal" from which various branching reactions detach, depending on substituents. Compounds with strong pi-donors, like 4-nitrosophenetol, give a resonance-stabilized N-(thiol-S-yl)-arylamine cation that may lead to bicyclic products, thioethers, and DNA adducts. Examples of toxicologic implications of the interactions of nitroso compounds with thiols are given for nitrosoimidazoles, heterocyclic nitroso compounds from protein pyrolysates, and nitrosoarenes. These data indicate that interactions of activated arylamines with thiols may not be regarded exclusively as detoxication reactions.

Eyer, P

1994-01-01

266

Recent advancements and challenges of palladiumII-catalyzed oxidation reactions with molecular oxygen as the sole oxidant  

PubMed Central

During the past 10 years there have been significant advances in PdII-catalyzed oxidation reactions where the use of ligands has led to the development of catalytic systems capable of achieving high turnover numbers, which employ molecular oxygen as the sole stoichiometric oxidant. This Feature article will highlight some of the recent developments in direct molecular oxygen-coupled PdII-catalyzed oxidation reactions with an emphasis on enhanced catalytic systems and new reactions. Additionally, limitations of current catalytic systems, such as ligand oxidation, are presented and their implications for the development of new reactions are discussed.

Gligorich, Keith M.; Sigman, Matthew S.

2010-01-01

267

Analysis of diatomic bond dissociation and formation in terms of the reaction force and the position-dependent reaction force constant.  

PubMed

Bond dissociation and formation in diatomic molecules are analyzed in terms of the reaction force F(R) and the reaction force constant kappa(R). These were determined for a group of 13 molecules from their extended-Rydberg potential energy functions V(R), which are of near-experimental quality. From F(R) and kappa(R) comes a two-stage description of dissociation/formation. In dissociation, the first stage involves stretching of the bond, which is opposed by an increasingly negative retarding force F(R). This reaches a minimum and then begins to weaken in the second stage, which is the transition from stretched molecule to free atoms. Bond formation begins with the reverse transition, driven by a positive F(R) which reaches a maximum for the stretched molecule and then becomes a decreasing restoring force. In the stages in which the system is a stretched molecule, kappa(R) is positive with its maximum at the equilibrium bond length; it is zero at the minimum or maximum of F(R), and negative throughout the transition stages, going through a minimum. kappa(R) <0 has been found to characterize the transition portion of a reaction. This description of dissociation/formation is reinforced by computed B3LYP and Hartree-Fock force constants at different atom separations for the singlet molecules. Hartree-Fock wave function stability assessments suggest that, for the single-bonded singlet molecules, the onset of electron unpairing in dissociation comes in the neighborhood of the F(R) minimum. PMID:19052782

Murray, Jane S; Toro-Labb, Alejandro; Clark, Tim; Politzer, Peter

2008-12-04

268

Electronic properties and chemical bonding in quaternary arsenide oxides LaZnAsO and YZnAsO  

Microsoft Academic Search

First principles FLAPW-GGA band structure calculations are employed to obtain the structural, electronic properties and chemical bonding picture for two related phases, namely, quaternary arsenide oxides LaZnAsO and YZnAsO. These compounds are found to be direct-transition type semiconductors with the GGA gaps of about 0.651.30eV. The peculiarities of chemical bonding in these phases are investigated and discussed in comparison with

V. V. Bannikov; I. R. Shein; A. L. Ivanovskii

2009-01-01

269

Low-barrier hydrogen bond plays key role in active photosystem II A new model for photosynthetic water oxidation  

Microsoft Academic Search

The function and mechanism of TyrZ in active photosystem II (PSII) is one of the long-standing issues in the study of photosynthetic water oxidation. Based on recent investigations on active PSII and theoretical studies, a new model is proposed, in which D1-His190 acts as a bridge, to form a low-barrier hydrogen bond (LBHB) with TyrZ, and a coordination bond to

Chunxi Zhang

2007-01-01

270

Respiratory chain strongly oxidizes the CXXC motif of DsbB in the Escherichia coli disulfide bond formation pathway.  

PubMed Central

Escherichia coli DsbB has four essential cysteine residues, among which Cys41 and Cys44 form a CXXC redox active site motif and the Cys104-Cys130 disulfide bond oxidizes the active site cysteines of DsbA, the disulfide bond formation factor in the periplasm. Functional respiratory chain is required for the cell to keep DsbA oxidized. In this study, we characterized the roles of essential cysteines of DsbB in the coupling with the respiratory chain. Cys104 was found to form the inactive complex with DsbA under respiration-defective conditions. While DsbB, under normal aerobic conditions, is in the oxidized state, having two intramolecular disulfide bonds, oxidation of Cys104 and Cys130 requires the presence of Cys41-Cys44. Remarkably, the Cys41-Cys44 disulfide bond is refractory to reduction by a high concentration of dithiothreitol, unless the membrane is solubilized with a detergent. This reductant resistance requires both the respiratory function and oxygen, since Cys41-Cys44 became sensitive to the reducing agent when membrane was prepared from quinone- or heme-depleted cells or when a membrane sample was deaerated. Thus, the Cys41-Val-Leu-Cys44 motif of DsbB is kept both strongly oxidized and strongly oxidizing when DsbB is integrated into the membrane with the normal set of respiratory components.

Kobayashi, T; Ito, K

1999-01-01

271

Stabilization of sulfide radical cations through complexation with the peptide bond: mechanisms relevant to oxidation of proteins containing multiple methionine residues.  

PubMed

The recent study on the *OH-induced oxidation of calmodulin, a regulatory "calcium sensor" protein containing nine methionine (Met) residues, has supported the first experimental evidence in a protein for the formation of S therefore N three-electron bonded radical complexes involving the sulfur atom of a methionine residue and the amide groups in adjacent peptide bonds. To characterize reactions of oxidized methionine residues in proteins containing multiple methionine residues in more detail, in the current study, a small model cyclic dipeptide, c-(L-Met-L-Met), was oxidized by *OH radicals generated via pulse radiolysis and the ensuing reactive intermediates were monitored by time-resolved UV-vis spectroscopic and conductometric techniques. The picture that emerges from this investigation shows there is an efficient formation of the Met (S therefore N) radicals, in spite of the close proximity of two sulfur atoms, located in the side chains of methionine residues, and in spite of the close proximity of sulfur atoms and oxygen atoms, located in the peptide bonds. Moreover, it is shown, for the first time, that the formation of Met(S therefore N) radicals can proceed directly, via H+-transfer, with the involvement of hydrogen from the peptide bond to an intermediary hydroxysulfuranyl radical. Ultimately, the Met(S therefore N) radicals decayed via two different pH-dependent reaction pathways, (i) conversion into sulfur-sulfur, intramolecular, three-electron-bonded radical cations and (ii) a proposed hydrolytic cleavage of the protonated form of the intramolecular, three-electron-bonded radicals [Met(S therefore N)/Met(S therefore NH)+] followed by electron transfer and decarboxylation. Surprisingly, also alpha-(alkylthio)alkyl radicals enter the latter mechanism in a pH-dependent manner. Density functional theory computations were performed on the model c-(L-Met-Gly) and its radicals in order to obtain optimizations and energies to aid in the interpretation of the experiments on c-(L-Met-L-Met). PMID:17658786

Bobrowski, Krzysztof; Hug, Gordon L; Pogocki, Dariusz; Marciniak, Bronislaw; Schneich, Christian

2007-07-21

272

Hydrogen bonding to the cysteine ligand of superoxide reductase: acid-base control of the reaction intermediates.  

PubMed

Superoxide reductase (SOR) is a non-heme iron metalloenzyme that detoxifies superoxide radical in microorganisms. Its active site consists of an unusual non-heme Fe(2+) center in a [His4Cys1] square pyramidal pentacoordination, with the axial cysteine ligand proposed to be an essential feature in catalysis. Two NH peptide groups from isoleucine 118 and histidine 119 establish hydrogen bonds involving the sulfur ligand (Desulfoarculus baarsii SOR numbering). To investigate the catalytic role of these hydrogen bonds, the isoleucine 118 residue of the SOR from Desulfoarculus baarsii was mutated into alanine, aspartate, or serine residues. Resonance Raman spectroscopy showed that the mutations specifically induced an increase of the strength of the Fe(3+)-S(Cys) and S-C?(Cys) bonds as well as a change in conformation of the cysteinyl side chain, which was associated with the alteration of the NH hydrogen bonding involving the sulfur ligand. The effects of the isoleucine mutations on the reactivity of SOR with O2 (-) were investigated by pulse radiolysis. These studies showed that the mutations induced a specific increase of the pK a of the first reaction intermediate, recently proposed to be an Fe(2+)-O2 (-) species. These data were supported by density functional theory calculations conducted on three models of the Fe(2+)-O2 (-) intermediate, with one, two, or no hydrogen bonds involving the sulfur ligand. Our results demonstrated that the hydrogen bonds between the NH (peptide) and the cysteine ligand tightly control the rate of protonation of the Fe(2+)-O2 (-) reaction intermediate to form an Fe(3+)-OOH species. PMID:23917995

Tremey, Emilie; Bonnot, Florence; Moreau, Yohann; Berthomieu, Catherine; Desbois, Alain; Favaudon, Vincent; Blondin, Genevive; Houe-Levin, Chantal; Nivire, Vincent

2013-08-06

273

The Effect of Substrate Surface Oxides on the Bonding of NiCr Alloy Particles HVAF Thermally Sprayed onto Aluminum Substrates  

NASA Astrophysics Data System (ADS)

The effect of substrate surface oxides on splat-substrate bonding was investigated by thermally spraying NiCr particles onto aluminum substrates with surface oxide layers grown hydrothermally and electrochemically. Cross sections of bonded solid and molten splats revealed substantial deformation of both the substrate and the surface oxide. In spite of the substantial substrate deformation, there was no significant loss of the surface oxide material and there was no observed diffusion of the substrate oxide into the NiCr particle or vice versa. For solid splats, the substrate oxide was still present over the entire splat-substrate interface, however for molten splats, the oxide had been penetrated in several locations allowing close proximity of the splat metal to the substrate metal. These results strengthen the theory that oxide layers impede bonding and that successful bonding occurs only when the surface oxide is substantially deformed or disrupted to produce mechanically interlocking features at the interface.

Trompetter, W.; Hyland, M.; McGrouther, D.; Munroe, P.; Markwitz, A.

2010-09-01

274

Infrared laser single photon absorption reaction chemistry in the solid state. Reactions of nitrogen oxides with sulfur hexafluoride  

Microsoft Academic Search

The infrared laser induced reactions between SF as a guest reactant matrix isolated within nitrogen oxides as host reactants at low temperatures are described. These reactions proceed via single-photon excitation of the ..nu.. band of SF from the upsilon = 0 ..-->.. 1 vibrational states. They are further examples of a process known as single-photon absorption reaction chemistry in the

Edward Catalano; Robert E. Barletta

1980-01-01

275

Mechanism of heterogeneous reaction of carbonyl sulfide on magnesium oxide.  

PubMed

Heterogeneous reaction of carbonyl sulfide (OCS) on magnesium oxide (MgO) under ambient conditions was investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), quadrupole mass spectrometer (QMS), and density functional theory (DFT) calculations. It reveals that OCS can be catalytically hydrolyzed by surface hydroxyl on MgO to produce carbon dioxide (CO2) and hydrogen sulfide (H2S), and then H2S can be further catalytically oxidized by surface oxygen or gaseous oxygen on MgO to form sulfite (SO3(2-)) and sulfate (SO4(2-)). Hydrogen thiocarbonate (HSCO2-) was found to be the crucial intermediate. Surface hydrogen sulfide (HS), sulfur dioxide (SO2), and surface sulfite (SO3(2-)) were also found to be intermediates for the formation of sulfate. Furthermore, the surface hydroxyl contributes not only to the formation of HSCO2- but also to HSCO2- decomposition. On the basis of experimental results, the heterogeneous reaction mechanism of OCS on MgO was discussed. PMID:17455919

Liu, Yongchun; He, Hong; Xu, Wenqing; Yu, Yunbo

2007-04-25

276

Bond Formation By Wood Surface Reactions Part Iv Analysis of Furfuryl Alcohol, Tannin and Maleic Acid Bridging Agents  

Microsoft Academic Search

Solid wood panels (Acer saccharum Marsh.) were bonded with various bridging materials following nitric acid activation. These chemical bridging materials included tannin, furfuryl alcohol and mixtures of the two with and without maleic acid. High shear strengths were achieved with a tannin-furfuryl alcohol-maleic acid mixture (T-F-M) with or without the nitric acid activation.The curing reactions of the bridging material were

S. S. Kelley; R. A. Young; R. M. Rammon; R. H. Gillespie

1982-01-01

277

[Selective carbon oxygen bond scission during reactions of oxygenates on single crystal catalysts]. Progress report  

SciTech Connect

We have discovered that the carbon-oxygen bond in methanol can be selectively broken if the surface structure of the platinum catalyst is appropriately tailored. The objective of this project is to determine if variations in surface structure allow one to selectively break C-O and C-H bonds. The decomposition of a wide range of oxygenates on several carefully chosen faces of group VIII metals will be examined to see when C-O bond scission occurs and what new chemistry we can find on stepped surfaces.

Not Available

1992-08-01

278

Nitric oxide induces acrosome reaction in cryopreserved bovine spermatozoa.  

PubMed

The aim of this work was to study the effect of nitric oxide on acrosome reaction (AR) and the participation of protein kinases and reactive oxygen species in the AR of cryopreserved bovine spermatozoa. Spermatozoa were capacitated in Tyrode's albumin lactate pyruvate medium with heparin (10 IU ml(-1)) and then incubated with different concentrations of sodium nitroprusside (SNP) (1-200 micromol l(-1)). Methylene blue and haemoglobin were used to confirm the role of nitric oxide as an inducer of the AR. The participation of protein kinase A (PKA) , protein kinase C (PKC) and protein tyrosine kinase was evaluated using specific inhibitors of these enzymes (H-89, 50 micromol l(-1); bisindolylmaleimide I, 0.1 micromol l(-1) and genistein, 3 micromol l(-1)). The role of hydrogen peroxide or superoxide anion was evaluated by incubation with catalase or superoxide dismutase respectively. AR percentages were determined by the fluorescence technique with chlortetracycline. The highest levels of AR were obtained in capacitated spermatozoa treated with 5-200 micromol l(-1) SNP (24.8 +/- 1.8%). The presence of PKA, PKC and protein tyrosine kinase inhibitors likewise decreased AR percentages. The addition of superoxide dismutase had no effect on the AR level but catalase completely blocked it. These results indicate that nitric oxide induces AR in capacitated spermatozoa involving hydrogen peroxide and the participation of PKA, PKC and protein tyrosine kinase as part of the signal transduction mechanism which lead to the AR in cryopreserved bovine spermatozoa. PMID:16266394

Rodriguez, P C; O'Flaherty, C M; Beconi, M T; Beorlegui, N B

2005-10-01

279

Porous platinum mesoflowers with enhanced activity for methanol oxidation reaction  

SciTech Connect

Porous Pt and Pt-Ag alloy mesoflowers (MFs) with about 2 {mu}m in diameter and high porosity were synthesized using Ag mesoflowers as sacrificial template by galvanic reaction. The silver content in Pt-Ag alloys can be facilely controlled by nitric acid treatment. And the pure Pt MFs can be obtained by selective removal of silver element from Pt{sub 72}Ag{sub 28} MFs electrochemically. Both Pt{sub 45}Ag{sub 55}, Pt{sub 72}Ag{sub 28} and pure Pt show a high catalytic performance in methanol oxidation reaction (MOR). Especially, pure Pt MFs exhibited a 2 to 3 times current density enhancement in MOR compared with the commercial used Pt black, which can be attributed to their porous nanostructure with 3-dimentional nature and small crystal sizes. - Graphical Abstract: The CVs of MOR on Pt (red) and Pt black (green) catalysts in 0.1 M HClO{sub 4} and 0.5 M CH{sub 3}OH for specific mass current. The insert shows the SEM images of two porous Pt MFs. Platinum mesoflowers (MFs) with about 2 {mu}m in diameter and high porosity were synthesised with Ag mesoflowers as sacrificial template by galvanic replacement. The porous Pt MFs exhibited a more than 3 times enhancement in electrocatalytic performance for methanol oxidation reaction compared the commercial used Pt black. Highlights: Black-Right-Pointing-Pointer Porous Pt and Pt-Ag mesoflowers (MFs) were synthesized using Ag MFs sacrifical template. Black-Right-Pointing-Pointer Pt MFs presents an improved catalytic activity in MOR compared with Pt black. Black-Right-Pointing-Pointer We provided a facile approach for the development of high performance Pt electrocatalysts for fuel cells.

Zhuang Lina; Wang Wenjin; Hong Feng [School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an 710049 (China); Yang Shengchun, E-mail: ysch1209@mail.xjtu.edu.cn [School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an 710049 (China); You Hongjun, E-mail: hjyou@mail.xjtu.edu.cn [School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an 710049 (China); Fang Jixiang; Ding Bingjun [School of Science, MOE Key Laboratory for Non-equilibrium Synthesis and Modulation of Condensed Matter, Xi'an Jiaotong University, Xi'an 710049 (China)

2012-07-15

280

Long hydrogen-bonded rod of molecular oxide: a hexatantalate tetramer.  

PubMed

A tetra-n-butylammonium (TBA) salt of [H(4.5)(Ta(6)O(19))](3.5-) was synthesized by reacting hydrous tantalum oxide with TBAOH. X-ray structural analysis of TBA(3.5)[H(4.5)(Ta(6)O(19))]2THF5.5H(2)O (THF = tetrahydrofuran) revealed that this compound consists of a hydrogen-bonded, rod-shaped tetramer of hexatantalate that is almost 30 long together with TBA cations and solvent molecules of crystallization [a = 20.6354(5) , b = 25.5951(7) , c = 37.2058(8) , ? = 77.092(1), ? = 86.177(1), ? = 88.683(1), V = 19110.9(8) (3), Z = 8, and space group P ?1]. (1)H NMR spectra showed that this tetrameric structure is maintained in solution. PMID:22587659

Matsumoto, Miki; Ozawa, Yoshiki; Yagasaki, Atsushi

2012-05-15

281

Examining the structure and bonding in complex oxides using aberration-corrected imaging and spectroscopy  

SciTech Connect

Our ability to directly characterize the atomic and electronic structures is crucial to developing a fundamental understanding of structure-property relationships in complex-oxide materials. Here, we examine one specific example, the misfit-layered thermoelectric material Ca3Co4O9, which exhibits a high Seebeck coefficient governed by spin-entropy transport as well as hopping-mediated electron transport. However, the role of oxygen and its bonding with cobalt in thermoelectric transport remains unclear. We use atomic-resolution annular bright-field imaging to directly image the oxygen sublattice and to combine our experimental data with multislice image calculations to find that the oxygen atoms in the CoO2 subsystem are highly ordered, while the oxygen-atomic columns are displaced in the Ca2CoO3 subsystem. Atomic-column-resolved electron energy-loss spectroscopy and spectrum image calculations are used to quantify the bonding in the different subsystems of incommensurate Ca3Co4O9. We find that the holes in the CoO2 subsystem are delocalized, which could be responsible for the p-type conductivity found in the CoO2 subsystem.

Klie, Robert F [University of Illinois, Chicago; Qiao, Q [University of Illinois at Chicago, Chicago, Illinois 60607, USA; Paulauskas, T [University of Illinois at Chicago, Chicago, Illinois 60607, USA; Ramassee, Q [SuperSTEM, Daresbury, United Kingdom; Oxley, Mark P [ORNL; Idrobo Tapia, Juan C [ORNL

2012-01-01

282

High-Temperature Thermoelectric Characterization of III-V Semiconductor Thin Films by Oxide Bonding  

NASA Astrophysics Data System (ADS)

A device fabrication and measurement method utilizing a SiO2-SiO2 covalent bonding technique is presented for high-temperature thermoelectric characterization of thin-film III-V semiconductor materials that suffer from the side-effect of substrate conduction at high temperatures. The proposed method includes complete substrate removal, high-temperature surface passivation, and metallization with a Ti-W-N diffusion barrier. A thermoelectric material, thin-film ErAs:InGaAlAs metal/semiconductor nanocomposite grown on a lattice-matched InP substrate by molecular beam epitaxy, was transferred onto a sapphire substrate using the oxide bonding technique at 300C, and its original InP substrate, which is conductive at high temperatures, was removed. Electrical conductivities and Seebeck coefficients were measured from room temperature to 840 K for this material on both the InP and sapphire substrates, and the measurement results clearly show that the InP substrate effect was eliminated for the sample on the sapphire substrate. A strain experiment has been conducted to investigate the effect of strain on electrical conductivity.

Bahk, Je-Hyeong; Zeng, Gehong; Zide, Joshua M. O.; Lu, Hong; Singh, Rajeev; Liang, Di; Ramu, Ashok T.; Burke, Peter; Bian, Zhixi; Gossard, Arthur C.; Shakouri, Ali; Bowers, John E.

2010-08-01

283

Mass transfer model for two-layer TBP oxidation reactions  

SciTech Connect

To prove that two-layer, TBP-nitric acid mixtures can be safely stored in the canyon evaporators, it must be demonstrated that a runaway reaction between TBP and nitric acid will not occur. Previous bench-scale experiments showed that, at typical evaporator temperatures, this reaction is endothermic and therefore cannot run away, due to the loss of heat from evaporation of water in the organic layer. However, the reaction would be exothermic and could run away if the small amount of water in the organic layer evaporates before the nitric acid in this layer is consumed by the reaction. Provided that there is enough water in the aqueous layer, this would occur if the organic layer is sufficiently thick so that the rate of loss of water by evaporation exceeds the rate of replenishment due to mixing with the aqueous layer. This report presents measurements of mass transfer rates for the mixing of water and butanol in two-layer, TBP-aqueous mixtures, where the top layer is primarily TBP and the bottom layer is comprised of water or aqueous salt solution. Mass transfer coefficients are derived for use in the modeling of two-layer TBP-nitric acid oxidation experiments. Three cases were investigated: (1) transfer of water into the TBP layer with sparging of both the aqueous and TBP layers, (2) transfer of water into the TBP layer with sparging of just the TBP layer, and (3) transfer of butanol into the aqueous layer with sparging of both layers. The TBP layer was comprised of 99% pure TBP (spiked with butanol for the butanol transfer experiments), and the aqueous layer was comprised of either water or an aluminum nitrate solution. The liquid layers were air sparged to simulate the mixing due to the evolution of gases generated by oxidation reactions. A plastic tube and a glass frit sparger were used to provide different size bubbles. Rates of mass transfer were measured using infrared spectrophotometers provided by SRTC/Analytical Development.

Laurinat, J.E.

1994-09-28

284

Oxidation Numbers, Oxidants, and Redox Reactions: Variants of the Electrophilic Bromination of Alkenes and Variants of the Application of Oxone  

ERIC Educational Resources Information Center

|Oxidation-state and donor-acceptor concepts are important areas in the chemical education. Student worksheets containing problems that emphasize oxidation numbers, redox reactions of organic compounds, and stoichiometric reaction equations are presented. All of the examples are incorporated under one unifying topic: the production of vicinal

Eissen, Marco; Strudthoff, Merle; Backhaus, Solveig; Eismann, Carolin; Oetken, Gesa; Kaling, Soren; Lenoir, Dieter

2011-01-01

285

Iron and hydroxyl radicals in lipid oxidation: Fenton reactions in lipid and nucleic acids co-oxidized with lipid  

Microsoft Academic Search

Hydroxyl radicals can initiate lipid peroxidation in liquids, but their high reactivity affords reaction paths so short that they are unlikely to reach lipids in membrane bilayers when formed exteriorly. EPR studies of Fenton-like reactions inducing oxidation in bulk lipids indicate that iron-dependent initiation of lipid oxidation in organelles and vesicles may result from hydroxyl radicals formed within the hydrophobic

D. C. Borg; K. M. Schaich

1987-01-01

286

Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti[triple bond]C linkage.  

PubMed

The neopentylidene-neopentyl complex (PNP)Ti=CH(t)Bu(CH2(t)Bu) (2; PNP(-) = N[2-P(CHMe2)(2-)4-methylphenyl]2), prepared from the precursor (PNP)Ti[triple bond]CH(t)Bu(OTf) (1) and LiCH2(t)Bu, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)Ti[triple bond]C(t)Bu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti[triple bond]C linkage to generate (PNP)Ti=CH(t)Bu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters DeltaH = 24(7) kcal/mol and DeltaS = -2(3) cal/mol.K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti[triple bond]C(t)Bu(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 degrees C over 48 h generates 3-d(6), thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) x 10-5 s(-1), and with activation parameters DeltaH = 31(16) kcal/mol and DeltaS = 3(9) cal/mol x K. At 95 degrees C for one week, the EIE for the 2 --> 3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)Ti=CHtBu(CH2SiMe3) (4) and (PNP)Ti=CHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)Ti=CHSiMe3(CH2tBu) (5) and (PNP)Ti=CHPh(CH2tBu) (7). By means of similar alkylations of (PNP)Ti=CHSiMe3(OTf) (8), the degenerate complex (PNP)Ti=CHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)Ti=CHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)Ti=CHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)Ti=CHtBu(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)Ti=CHR(R') (R = 3,5-Me2C6H2, R' = CH2-3,5-Me2C6H2; R = SiMe3, R' = CH2SiMe3; R = SiMe2CCSiMe3, R' = CH2SiMe2CCSiMe3; R = SiMe2OSiMe3, R' = CH2SiMe2OSiMe3). PMID:17592842

Bailey, Brad C; Fan, Hongjun; Huffman, John C; Baik, Mu-Hyun; Mindiola, Daniel J

2007-06-26

287

Oxidation of vacuolar and apoplastic phenolic substrates by peroxidase: Physiological significance of the oxidation reactions  

Microsoft Academic Search

Phenolic components and peroxidases are localized in vacuoles. Vacuolar peroxidase can oxidize phenolics when H2O2 is formed in vacuoles or tonoplasts, or when H2O2 formed outside of vacuoles is diffused into the organelles. In a mixture of phenolics containing a good and a poor substrate for peroxidase, a radical transfer reaction is possible from the radicals of the good substrate

Umeo Takahama

2004-01-01

288

Application of recyclable, polymer-immobilized iodine(III) oxidants in catalytic CH bond functionalization  

Microsoft Academic Search

This paper describes the palladium-catalyzed oxidative functionalization of arene and alkane CH bonds using the polymer-immobilized iodine(III) oxidant poly-4-(diacetoxyiodo)styrene (PSI(OAc)2) and its derivatives. These transformations have been applied to a wide range of organic substrates, and typically proceed with excellent levels of regio- and chemoselectivity as well as with high functional group tolerance. In addition, the polymer-immobilized iodine(I) byproducts of

Eric W. Kalberer; Salena R. Whitfield; Melanie S. Sanford

2006-01-01

289

Instability of Si?F bonds in fluorinated silicon oxide (SiOF) films formed by various techniques  

Microsoft Academic Search

Instability of Si?F bonds in fluorinated silicon oxide (SiOF) films is studied. Al wiring corrosion and underlayer SiO2 etching problems are the major issues for the use of SiOF interlayer dielectric films. To clarify the mechanism, three kinds of SiOF films have been used for this study. They are: (i) a fluorinated silicon oxide (SiOF) film prepared by room-temperature chemical

Tetsuya Homma

1996-01-01

290

Oxidation of drying oils containing non-conjugated and conjugated double bonds catalyzed by a cobalt catalyst  

Microsoft Academic Search

The oxidation of drying oil containing non-conjugated (linseed oil) and conjugated (tung oil) double bonds catalyzed by Co(II)-2-ethylhexanoate (Co-EH) is investigated. There is distinctive difference in the oxidation mechanism between the two model oils. For the non-conjugated linseed oil, H-abstraction occurs via the most susceptible double allylic H atom. A large amount of oxygen is taken up, leading to the

Z. O. Oyman; W. Ming; R. van der Linde

2005-01-01

291

Oxidation of humic substances supports denitrification reactions in agricultural soils.  

NASA Astrophysics Data System (ADS)

Humic substances (HS) are a ubiquitous, recalcitrant, and diverse class of compounds arising from degradation and condensation of plant and microbial biopolymers. Many bacteria oxidize hydroquinones within humic substances to their quinone analogs, providing electrons for respiratory processes such as nitrate reduction. Microbial hydroquinone oxidation contributes to the redox state of HS and supports denitrification, which may be of import to agricultural soils where nitrate retention is critical and HS are prevalent. Most probable number counts were performed on soils collected from a Nebraska farm, with the model humic hydroquinone 2,6- anthrahydroquinone disulfonate (AHDS) serving as an electron donor and nitrate as the electron acceptor. Results indicated that AHDS oxidizing, nitrate reducing bacteria were present in soils from bluegrass fields (104 cells/g) and aspen groves (106 cells/g), as well as in plots of corn (106 cells/g), and soybean treated (106 cells/g) and un-treated (105 cells/g) with pig slurry. These results demonstrate that microorganisms participating in the proposed metabolism are prevalent within agricultural soils. Upflow glass columns were constructed, containing a support matrix of glass beads amended with 10% w/w soil from the corn plot previously mentioned. All columns were subjected to a continual flow of phosphate-buffered water amended with sodium nitrate. Above the point source for nitrate injection, phosphate-buffered water containing electron donor treatments were continually injected. The impacts of electron donor treatments (no donor, oxidized HS, reduced HS, and acetate) on denitrification and other geochemical parameters were observed. Column studies were able to resolve effects of electron donor treatment both spatially as a function of distance from the injection point source, and temporally, as a function of time of donor treatment. Four sample ports in each column were routinely analyzed for concentrations of nitrate, nitrite, Fe(II), and humic-born hydroquinones. All data were analyzed with respect to dilution factors obtained through analysis of a conservative bromide tracer present in electron donor medium. Addition of oxidized HS, reduced HS, and acetate all resulted in significant loss of nitrate from the columns. Significant nitrite accumulation was not observed. Of all the electron donor treatments, reduced HS, enriched for hydroquinone-containing functional moieties, supported the greatest degree of denitrification. The participation of excess hydroquinones in denitrification accounted for approximately 104% of the difference in nitrate reduction between reduced and oxidized HS treatments. This electron balance allowed for assignment of respiratory activity due to hydroquinone oxidation, rather than degradation of humic substances or associated electron-donating compounds. These results suggest that denitrification reactions catalyzed by microbial oxidation of reduced HS may be prevalent in agricultural soils. Likewise, these results demonstrate for the first time that respiratory behavior due to hydroquinone oxidation, as well as impact upon local geochemistry, can be analyzed in complex flow-through model systems.

van Trump, J. I.; Coates, J. D.

2007-12-01

292

Energetics of the N-O bonds in 2-hydroxyphenazine-di-N-oxide.  

PubMed

The standard enthalpy of formation and the enthalpy of sublimation of crystalline 2-hydroxyphenazine-di-N-oxide, at T = 298.15 K, were determined from isoperibol static bomb combustion calorimetry and from Knudsen effusion experiments, as -76.7 +/- 4.2 kJ.mol(-1) and 197 +/- 5 kJ.mol(-1), respectively. The sum of these two quantities gives the standard enthalpy of formation in the gas-phase for this compound, delta(f)H(m)degrees(g) = 120 +/- 6 kJ.mol(-1). This value was combined with the gas-phase standard enthalpy of formation for 2-hydroxyphenazine retrieved from a group estimative method yielding the mean (N-O) bond dissociation enthalpy, in the gas-phase, for 2-hydroxyphenazine-di-N-oxide. The result obtained with this strategy is (DH(m)degrees (N - O)) = 263 +/- 4 kJ.mol(-1), which is in excellent agreement with the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G(d) computed value, 265 kJ.mol(-1). PMID:16853057

Gomes, Jos R B; Sousa, Emanuel A; Gonalves, Jorge M; Monte, Manuel J S; Gomes, Paula; Pandey, Siddharth; Acree, William E; Ribeiro da Silva, Maria D M C

2005-08-25

293

Exploring hydrogen bond in the excited state leading toward intramolecular proton transfer: detailed analysis of the structure and charge density topology along the reaction path using QTAIM.  

PubMed

Excited state intramolecular proton transfer (ESIPT) reaction along the O-H[Symbol: see text][Symbol: see text][Symbol: see text][Symbol: see text]O hydrogen bond of o-hydroxy benzaldehyde (OHBA), methyl salicylate (MS) and salicylic acid (SA) was investigated by ab-initio quantum chemical calculation and theory of atoms and molecules (QTAIM) for the first time. Variation in several geometric as well as QTAIM parameters along the reaction coordinate was monitored in the fully relaxed excited state potential energy curve (PEC) obtained from intrinsic reaction coordinate (IRC) analysis. Although, the excited state barrier height for the forward reaction (?E (0) (#) ) reduces substantially in all the systems, MS and SA do not show any obvious asymmetry for proton transfer. For MS and SA, the crossover of the bond index as well as the lengths of the participating bonds at the saddle point is assigned due to this symmetry in accordance with bond energy - bond order (BEBO) model, which does not hold true in OHBA both in the ground and excited states. Bond ellipticity, covalent and metallic character were examined for different structures along the reaction path within the QTAIM framework. The QTAIM analysis was found to be able to uniquely distinguish between the ground and excited states of the OHBA molecule as well as both determining the effects on the bonding character of adding different substituent groups and differentiating between the ESIPT reactions in the SA and MS molecules. PMID:22555772

Mitra, Sivaprasad; Chandra, Asit K; Gashnga, Pynsakhiat Miki; Jenkins, Samantha; Kirk, Steven R

2012-05-04

294

Mapping chemical bonding of reaction intermediates with femtosecond X-ray laser spectroscopy  

NASA Astrophysics Data System (ADS)

We determine the pathways in the photo-dissociation reactions of Fe(CO)5 both in the gas phase and in solution by mapping the valence electronic structure of the reaction intermediates with femtosecond X-ray laser spectroscopy.

Wernet, Ph.; Beye, M.; de Groot, F.; Dsterer, S.; Gaffney, K.; Grbel, S.; Hartsock, R.; Hennies, F.; Josefsson, I.; Kennedy, B.; Kunnus, K.; Leitner, T.; Mazza, T.; Meyer, M.; Nordlund, D.; Odelius, M.; Quevedo, W.; Radcliffe, P.; Rajkovic, I.; Schlotter, B.; Scholz, M.; Schreck, S.; Suljoti, E.; Techert, S.; Turner, J.; Weniger, C.; Zhang, W.; Fhlisch, A.

2013-03-01

295

Infrared spectra of XC[triple bond]IrX(3) and CX(2)=IrX(2) prepared by reactions of laser-ablated iridium atoms with halomethanes.  

PubMed

Small iridium high oxidation-state complexes with carbon-iridium multiple bonds are identified in the product matrix infrared spectra from reactions of laser-ablated Ir atoms with tetra-, tri- and dihalomethanes. In contrast to the previously studied Rh case, Ir carbyne complexes (XC[triple bond]IrX(3)) are generated in reactions of tetrahalomethanes, and their short Ir-C bond lengths of 1.725-1.736 A are appropriate for the carbon-metal triple bonds. DFT calculations also show that the Ir carbynes with an Ir-F bond have unusual square planar structures, similar to the recently discovered Ru planar complexes. Diminishing preference for the carbyne complexes leads to methylidene product absorptions in the tri- and dihalomethane spectra, marking a limit for generation of small metal carbynes. The insertion complexes, on the other hand, are not observed in this study, suggesting that X migration from C to Ir following initial C-X insertion is swift. PMID:20442951

Cho, Han-Gook; Andrews, Lester

2010-05-05

296

Phosphine oxide-catalysed chlorination reactions of alcohols under Appel conditions.  

PubMed

A phosphine oxide-catalysed chlorination reaction of primary and secondary alcohols has been developed. This process represents the first triphenylphosphine oxide-catalysed alcohol chlorination under Appel conditions. PMID:20386856

Denton, Ross M; An, Jie; Adeniran, Beatrice

2010-03-26

297

Preparation of superacids by metal oxides for reactions of butanes and pentanes  

Microsoft Academic Search

Recent works on preparation of solid superacid catalysts which are active for reactions of butanes and pentanes are reviewed. Sulfated metal oxides are obtained by adsorbing sulfate ion onto amorphous oxides of Fe, Ti, Zr, Hf, Sn, and Si followed by calcination in air; a superacid of Al2O3 is prepared from the crystallized oxide. Superacids by metal oxides are synthesized

Kazushi Arata

1996-01-01

298

Bite angle effects of diphosphines in C-C and C-X bond forming cross coupling reactions.  

PubMed

Catalytic reactions of C-C and C-X bond formation are discussed in this critical review with particular emphasis on cross coupling reactions catalyzed by palladium and wide bite angle bidentate diphosphine ligands. Especially those studies have been collected that allow comparison of the ligand bite angles for the selected ligands: dppp, BINAP, dppf, DPEphos and Xantphos. Similarities with hydrocyanation and CO/ethene/MeOH reactions have been highlighted, while rhodium hydroformylation has been mentioned as a contrasting example, in which predictability is high and steric and electronic effects follow smooth trends. In palladium catalysis wide bite angles and bulkiness of the ligands facilitate generally the reductive elimination thus giving more efficient cross coupling catalysis (174 references). PMID:19421583

Birkholz, Mandy-Nicole; Freixa, Zoraida; van Leeuwen, Piet W N M

2009-02-10

299

Novel carbon-carbon bond formations for biocatalysis  

PubMed Central

Carboncarbon bond formation is the key transformation in organic synthesis to set up the carbon backbone of organic molecules. However, only a limited number of enzymatic CC bond forming reactions have been applied in biocatalytic organic synthesis. Recently, further name reactions have been accomplished for the first time employing enzymes on a preparative scale, for instance the Stetter and PictetSpengler reaction or oxidative CC bond formation. Furthermore, novel enzymatic CC bond forming reactions have been identified like benzylation of aromatics, intermolecular Diels-Alder or reductive coupling of carbon monoxide.

Resch, Verena; Schrittwieser, Joerg H; Siirola, Elina; Kroutil, Wolfgang

2011-01-01

300

Reaction of superoxide with nitric oxide to form peroxonitrite in alkaline aqueous solution  

Microsoft Academic Search

Although aqueous superoxide often acts as a one-electron reductant or less frequently as an oxidant, it rarely undergoes covalent bond formation with simple organic or inorganic compounds in water, perhaps owing to its poor nucleophilicity in this solvent. In this communication it is shown, however, that superoxide can react with nitric oxide to form the peroxionitrite anion in deaerated aqueous

Neil V. Blough; Oliver C. Zafiriou

1985-01-01

301

Preparation of dicarboxylic acid-type gemini surfactant via Diels-Alder reaction & ozone oxidation.  

PubMed

We wish to report a novel preparation method for Gemsurf analogs as well as dicarboxylic acid-type Gemini surfactant from Diels-Alder adducts of 2-trimethylsilyloxy-1,3-butadiene, in which ozone oxidation is adopted to convert C=C double bond to dicarboxylic acid without any additional oxidant. PMID:23728332

Chau, Huynh N; Kawase, Tokuzo; Oida, Tatsuo

2013-01-01

302

Kinetic Mechanism of Reaction between TT-Phase Niobium Oxide and Carbon Tetrachloride (CCl 4)  

Microsoft Academic Search

The kinetic mechanism of reaction between TT-phase niobium oxide and carbon tetrachloride (CCl4) at low (453 K) and high (above 573 K) temperatures was investigated. The reaction of the TT-phase niobium oxide with CCl4 at 453 K was controlled by the diffusion of CCl4 through the outer layer of the reaction product, NbOCl3, formed around the surface. For the reaction

Kohki Ebitani; Yoshiaki Hirano; Akira Morikawa

1995-01-01

303

Influence of prebonding cleaning on the electrical properties of the buried oxide of bond-and-etchback silicon-on-insulator materials  

Microsoft Academic Search

Three different groups of metal-oxide-semiconductor devices were manufactured of bond-and-etchback silicon-on-insulator wafers where the buried oxide functioned as the gate dielectric. The groups differed in the procedure used to clean the surfaces prior to bonding and in the location of the bonded interface. The surfaces were cleaned using either the standard RCA cleaning procedure without HF dip or by rinsing

Per Ericsson; Stefan Bengtsson; Ulf Sdervall

1995-01-01

304

Kinetics and Mechanism of Iodide Oxidation by Iron(III): A Clock Reaction Approach  

ERIC Educational Resources Information Center

|A simple method for studying the kinetics of a chemical reaction is described and the significance of reaction orders in deducing reaction mechanisms is demonstrated. In this student laboratory experiment, oxidation of iodide by iron(III) ions in an acidic medium is transformed into a clock reaction. By means of the initial rates method, it is

Bauer, Jurica; Tomisic, Vladislav; Vrkljan, Petar B. A.

2008-01-01

305

Pentavalent Uranium Oxide via Reduction of [UO2]2+ Under Hydrothermal Reaction Conditions  

SciTech Connect

The synthesis, crystal structure and spectroscopic characterization of [UV(H2O)2(UVIO2)2O4(OH)](H2O)4 (1), a mixed-valent UV/UVI oxide material, are reported. The hydrothermal reaction of UO22+ with Zn and hydrazine at 120 C for three days yields 1 in the form of a dark red crystalline solid. Compound 1 has been characterized by a combination of single-crystal and powder X-ray diffraction (XRD), elemental analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The structure consists of an extended sheet of edge and point shared UVI pentagonal bipyramids that are further connected by edge sharing to square bipyramidal UV units. The overall topology is similar to the mineral ianthinite. The uranium L|||-edge XAS revealed features consistent with those observed by single-crystal X-ray diffraction. High resolution XPS data analysis of the U4f region confirmed the oxidation states of U as originally assigned from XRD analysis and bond valence summations.

Belai, Nebebech; Frisch, Mark; Ilton, Eugene S.; Ravel, Bruce; Cahill, Christopher L.

2008-11-03

306

Hydrogen-bond mediated regio- and enantioselectivity in a C-H amination reaction catalysed by a supramolecular Rh(ii) complex.  

PubMed

Enantioselective (up to 74% ee) and regioselective C-H amination reactions of 3-benzylquinolones were achieved with a chiral C2-symmetric Rh(ii) complex exhibiting two lactam binding sites for substrate coordination via hydrogen bonds. PMID:23917404

Hke, Thorsten; Herdtweck, Eberhardt; Bach, Thorsten

2013-08-05

307

Bond selection in the photoisomerization reaction of anionic green fluorescent protein and kindling fluorescent protein chromophore models.  

PubMed

The chromophores of the most widely known fluorescent proteins (FPs) are derivatives of a core p-hydroxybenzylidene-imidazolinon-5-one (HBI) motif, which usually occurs as a phenolate anion. Double bond photoisomerization of the exocyclic bridge of HBI is widely held to be an important internal conversion mechanism for FP chromophores. Herein we describe the ground and excited-state electronic structures and potential energy surfaces of two model chromophores: 4- p-hydroxybenzylidiene-1,2-dimethyl-imidazolin-5-one anion (HBDI), representing green FPs (GFPs), and 2-acetyl-4-hydroxybenylidene-1-methyl-imidazolin-5-one anion (AHBMI), representing kindling FPs (KFPs). These chromophores differ by a single substitution, but we observe qualitative differences in the potential energy surfaces which indicate inversion of bond selection in the photoisomerization reaction. Bond selection is also modulated by whether the reaction proceeds from a Z or an E conformation. These configurations correspond to fluorescent and nonfluorescent states of structurally characterized FPs, including some which can be reversibly switched by specific illumination regimes. We explain the difference in bond selectivity via substituent stabilization effects on a common set of charge-localized chemical structures. Different combinations of these structures give rise to both optically active (planar) and twisted intramolecular charge-transfer (TICT) states of the molecules. We offer a prediction of the gas-phase absorption of AHBMI, which has not yet been measured. We offer a hypothesis to explain the unusual fluorescence of AHBMI in DMF solution, as well as an experimental proposal to test our hypothesis. PMID:18597428

Olsen, Seth; Smith, Sean C

2008-07-01

308

Tuning the mechanical properties of graphene oxide paper and its associated polymer nanocomposites by controlling cooperative intersheet hydrogen bonding.  

PubMed

The mechanical properties of pristine graphene oxide paper and paper-like films of polyvinyl alcohol (PVA)-graphene oxide nanocomposite are investigated in a joint experimental-theoretical and computational study. In combination, these studies reveal a delicate relationship between the stiffness of these papers and the water content in their lamellar structures. ReaxFF-based molecular dynamics (MD) simulations elucidate the role of water molecules in modifying the mechanical properties of both pristine and nanocomposite graphene oxide papers, as bridge-forming water molecules between adjacent layers in the paper structure enhance stress transfer by means of a cooperative hydrogen-bonding network. For graphene oxide paper at an optimal concentration of ~5 wt % water, the degree of cooperative hydrogen bonding within the network comprising adjacent nanosheets and water molecules was found to optimally enhance the modulus of the paper without saturating the gallery space. Introducing PVA chains into the gallery space further enhances the cooperativity of this hydrogen-bonding network, in a manner similar to that found in natural biomaterials, resulting in increased stiffness of the composite. No optimal water concentration could be found for the PVA-graphene oxide nanocomposite papers, as dehydration of these structures continually enhances stiffness until a final water content of ~7 wt % (additional water cannot be removed from the system even after 12 h of annealing). PMID:22188595

Compton, Owen C; Cranford, Steven W; Putz, Karl W; An, Zhi; Brinson, L Catherine; Buehler, Markus J; Nguyen, SonBinh T

2012-02-22

309

Hydrogen Oxidation Reaction at the Ni/YSZ Anode of Solid Oxide Fuel Cells from First Principles  

NASA Astrophysics Data System (ADS)

By means of ab initio simulations we here provide a comprehensive scenario for hydrogen oxidation reactions at the Ni/zirconia anode of solid oxide fuel cells. The simulations have also revealed that in the presence of water chemisorbed at the oxide surface, the active region for H oxidation actually extends beyond the metal/zirconia interface unraveling the role of water partial pressure in the decrease of the polarization resistance observed experimentally.

Cucinotta, Clotilde S.; Bernasconi, Marco; Parrinello, Michele

2011-11-01

310

Bonding principles of the Passivation Mechanism at III-V -- oxide Interfaces  

NASA Astrophysics Data System (ADS)

It has always been much more difficult to make FETs from GaAs than Si, because of `Fermi level pinning' and the difficulty of passivating its surfaces and interfaces. These issues have been discussed from the early days of PCSI by Spicer et al [1] with their ``unified defect model.'' Hasegawa [2] introduced the idea of ``Disorder Induced Gap states'' (DIGS). Since 1997 it has been possible to make inverted MOSFETs on GaAs using the epitaxial Gadolinium gallium oxide [3], but the main impetuous has been since 2003 to use atomic layer dpeosition to make scalable FETs, as recently acheived by Intel [4]. The obvious question is why GaAs is so much more difficult to passivate than Si. The early answer was that the native oxide was poor. But since the advent of good deposited ALD oxides on Si such as HfO2 or Al2O3, this answer is no good, as they should also work on GaAs. They do to an extent, but the interfacial density of states is still too large and the CV p lots are distorted. The cause of the defects is cannot be due to stress. The reason must be some underlying chemical reason. I show that the reason is the polar nature of bonding in GaAa and other III-Vs, and the driving force to keep the Fermi level in a gap. The electron counting rule of Pashley [5] that describes surface reconstruction is shown to be a variant of auto-compensation, and it is proposed to work more generally, at each layer deposition or growth on GaAs [6]. This leads to a continuous generation of defects if it is not satisfied. So the answer is to deposit oxide layers that meet this rule, and also break up any surface reconstructions that would lead to As-As dimers [6]. [4pt] [1] W E Spicer, Phys Rev Lett 44 420 (1980)[0pt] [2] H Hasegawa, J Vac Sci Technol B 5 1097 (1987)[0pt] [3] M Hong et al, Science 283 1897 (1997)[0pt] [4] M Radosavljevic, et al, IEDM (2009) p13.1[0pt] [5] M D Pashley, Phys Rev B 40 10481 (1989)[0pt] [6] J Robertson, L Lin, App Phys Letts (submitted), App Phys Lett 98 082903 (2011)

Robertson, John

2012-02-01

311

A comparative evaluation of cobalt chromium oxide, cobalt manganese oxide, cobalt manganese oxide, and copper manganese oxide as catalysts for the water-gas shift reaction  

SciTech Connect

Cobalt chromium oxide, cobalt manganese oxide, and copper manganese oxide have been compared as catalysts for the water-gas shift reaction. Cobalt chromium oxide and cobalt manganese oxide catalysts can give both high activity and long lifetimes for this reaction. Cobalt chromium oxide catalysts display higher activity compared to the other catalyst systems and this is shown that both the cobalt chromium oxide and the cobalt manganese oxide catalysts are not sensitive to the presence of up to 240 ppm feedstock sulphur impurities (H{sub 2}S and COS) and high catalyst activity can be maintained either in the presence or in the absence of feedstock sulphur. The copper manganese oxide catalysts are demonstrated to be particularly sensitive to feedstock sulphur, being significantly poisoned at levels as low as 1 ppm H{sub 2}S. An investigation of the mechanism of the water-gas shift mechanism over the three catalyst systems is also described using both kinetic and model reagent studies. Based on these studies, a mechanism is proposed for the cobalt- and copper-containing catalyst systems involving the formation of a surface formate intermediate.

Hutchings, G.J. (Univ. of Liverpool (United Kingdom)); Copperthwaite, R.G.; Gottschalk, F.M.; Hunter, R.; Mellor, J.; Orchard, S.W.; Sangiorgio, T. (Univ. of the Witwatersrand, Johannesburg (South Africa))

1992-10-01

312

Comparison of properties of sintered and sintered reaction-bonded silicon nitride fabricated by microwave and conventional heating  

SciTech Connect

A comparison of microwave and conventional processing of silicon nitride-based ceramics was performed to identify any differences between the two, such as improved fabrication parameters or increased mechanical properties. Two areas of thermal processing were examined: sintered silicon nitride (SSN) and sintered reaction-bonded silicon nitride (SRBSN). The SSN powder compacts showed improved densification and enhanced grain growth. SRBSN materials were fabricated in the microwave with a one-step process using cost-effective raw materials. The SRBSN materials had properties appropriate for structural applications. Observed increases in fracture toughness for the microwave processed SRBSN materials were attributable to enhanced elongated grain growth.

Tiegs, T.N.; Kiggans, J.O. Jr.; Lin, H.T. [and others

1995-10-01

313

Retro-reaction of singly bonded La@C82 derivatives.  

PubMed

Retro-reaction of radical monoadducts of a paramagnetic endohedral metallofullerene, La@C(2v)-C(82), is shown using thermal reaction in the presence of a radical trapping reagent, affording pristine La@C(2v)-C(82) in high yield (96%). PMID:20862446

Takano, Yuta; Ishitsuka, Midori O; Tsuchiya, Takahiro; Akasaka, Takeshi; Kato, Tatsuhisa; Nagase, Shigeru

2010-09-23

314

Catalytic asymmetric carbon-carbon bond-forming reaction utilizing rare earth metal complexes  

Microsoft Academic Search

Novel optically active rare earth complexes have made possible a catalytic asymmetric nitroaldol reaction for the first time. Structural elucidation reveals that the complexes consist of one rare earth metal, three lithium atoms, and three BINOL units. Applications of the catalytic asymmetric nitroaldol reaction to syntheses of several p-blockers and erythro-AYA have been also achieved. Although the lithium containing rare

Masakatsu Shibasaki; Hiroaki Sasai

1996-01-01

315

Hydrogen bonding in picolinic acid N-oxide. Part II: A proposal for dissipative laser driven proton transfer dynamics  

NASA Astrophysics Data System (ADS)

Laser control of proton dynamics in the medium-strong intramolecular hydrogen bond of picolinic acid N-oxide (PANO) is investigated. This work is an extension of our recent article dealing with the "statical" effects of hydrogen bonding in PANO [J. Mol. Struct. (Theochem) 500 (2000) 429-440]. A two-dimensional model potential is extracted from DFT calculations that include the proton transfer motion and the heavy atom mode. The effects of the environmental degrees of freedom were treated by means of their spectral density within the density matrix formalism. The proton dynamics is monitored over time by calculating the nonlinear optical response nonperturbatively in the driving field.

Doli?, Nadja; Stare, Jernej; Mavri, Janez

2001-07-01

316

Haptoglobin alters oxygenation and oxidation of hemoglobin and decreases propagation of peroxide-induced oxidative reactions.  

PubMed

We compared oxygenation and anaerobic oxidation reactions of a purified complex of human hemoglobin (Hb) and haptoglobin (Hb-Hp) to those of uncomplexed Hb. Under equilibrium conditions, Hb-Hp exhibited active-site heterogeneity and noncooperative, high-affinity O(2) binding (n(1/2)=0.88, P(1/2)=0.33 mm Hg in inorganic phosphate buffer at pH 7 and 25 C). Rapid-reaction kinetics also exhibited active-site heterogeneity, with a slower process of O(2) dissociation and a faster process of CO binding relative to uncomplexed Hb. Deoxygenated Hb-Hp had significantly reduced absorption at the ?(max) of 430 nm relative to uncomplexed Hb, as occurs for isolated Hb subunits that lack T-state stabilization. Under comparable experimental conditions, the redox potential (E(1/2)) of Hb-Hp was found to be +54 mV, showing that it is much more easily oxidized than uncomplexed Hb (E(1/2)=+125 mV). The Nernst plots for Hb-Hp oxidation showed no cooperativity and slopes less than unity indicated active-site heterogeneity. The redox potential of Hb-Hp was unchanged by pH over the range of 6.4-8.3. Exposure of Hb-Hp to excess hydrogen peroxide (H(2)O(2)) produced ferryl heme, which was found to be more kinetically inert in the Hb-Hp complex than in uncomplexed Hb. The negative shift in the redox potential of Hb-Hp and its stabilized ferryl state may be central elements in the protection against Hb-induced oxidative damage afforded by formation of the Hb-Hp complex. PMID:22841869

Banerjee, Sambuddha; Jia, Yiping; Siburt, Claire J Parker; Abraham, Bindu; Wood, Francine; Bonaventura, Celia; Henkens, Robert; Crumbliss, Alvin L; Alayash, Abdu I

2012-07-27

317

Basic character of rare earth metal alkoxides. Utilization in catalytic CC bond-forming reactions and catalytic asymmetric nitroaldol reactions  

Microsoft Academic Search

In a recent paper, the authors reported that Zr(O-t-Bu) was an efficient and convenient basic reagent in organic synthesis. However, all reactions examined were performed with stoichiometric quantities of the reagent. The authors envisioned that rare earth metal alkoxides would be stronger bases than group 4 metal alkoxides due to the lower ionization potential (ca. 5.4-6.4 eV) and the lower

Hiroaki Sasai; Takeyuki Suzuki; Shigeru Arai; Takayoshi Arai; Masakatsu Shibasaki

1992-01-01

318

Niobium oxide based materials as catalysts for acidic and partial oxidation type reactions  

Microsoft Academic Search

Niobic acid, H8Nb6O19xH2O, was synthesized and studied for its acidic features as a function of its dehydroxylation extent. It was observed to be strongly acidic, using NH3 adsorption calorimetry and isopropanol conversion reaction as probe techniques, and to be weakly acidic on its dehydrated form, Nb2O5. The mixed oxide Al2o3:Nb2o5in 1:1 molar ratio prepared from aluminum and niobium oxalates was

J. C. Vdrine; G. Coudurier; A. Ouqour; P. G. Pries de Oliveira; J. C. Volta

1996-01-01

319

Reactions of copper cluster cations with nitrous oxide: Oxidation and sequential N2O adsorption  

NASA Astrophysics Data System (ADS)

Reactions of size-selected copper cluster cations, Cun+ (n = 3-7) and OO(N2O)l, involving both the oxidation and the N2O adsorption. Preadsorption of oxygen atoms promotes the sequential N2O-adsorption significantly, and in particular, CuO(N2O)3, CuO2+(N2O)3, and CuO2+(N2O)4 are produced dominantly from CuO, CuO2+, and CuO2+, which shows the saturation of the adsorption with N2O.

Hirabayashi, Shinichi; Ichihashi, Masahiko; Kondow, Tamotsu

2012-04-01

320

The reaction of skin to primers used in the "single-step" bonding systems.  

PubMed

A large number of direct bonding systems have been introduced for use by dentists and dental hygienists. Although these agents are used for direct bonding of brackets to enamel, contact with skin, oral mucosa, and gingiva is not uncommon. These products have wide clinical use, but their biocompatibility characteristics have not been extensively investigated. The present study was undertaken to determine the effects of the primer component of Mono-Lok (Rocky Mountain) and the primer component of Control (Lancer Pacific) on skin. Three adult Macaca mulatta monkeys were used in the study. The skin on their backs was shaved and the primer component of Control, primer component of Mono-Lok, and saline solution were applied at different sites every 3 days for up to four applications. The skin was examined clinically and, 5 days after the last application, a biopsy was performed for histologic evaluation. The skin in contact with the primer component of Mono-Lok exhibited pronounced inflammatory changes and was characterized by swelling, vesiculation, and ulceration. Histologic observations confirmed these findings by showing a marked inflammatory cellular response characterized by eosinophils. In contrast to these findings, the skin in contact with the primer component of Control or saline solution exhibited normal architecture. Histologic observations supported this appearance and showed minimal inflammatory cell infiltration. These results show that there are differences in the biocompatibility of direct bonding systems and that further studies are needed to clarify their long-term effects on patients and dental personnel. PMID:2949598

Altuna, G; Freeman, E

1987-02-01

321

Anti-oxidant enzyme activities and expression and oxidative damage in patients with non-immediate reactions to drugs  

PubMed Central

Adverse drug reactions with an immunological basis (ADRIB) may involve activation of other concomitant, non-specific mechanisms, amplifying the specific response and contributing to the severity and duration. One concomitant mechanism could be the generation of reactive oxygen species (ROS) and/or their detoxification by anti-oxidants, including anti-oxidant enzymes. We analysed the activity of the anti-oxidant enzymes Cu/Zn-superoxide dismutase (SOD), catalase (CAT) and cellular glutathione peroxidase (GPX), as well as certain markers of oxidative damage (thiobarbituric acid reactive substances (TBARS) and carbonyl content) in peripheral blood mononuclear cells from patients with non-immediate ADRIB using spectrophotometric methods and the anti-oxidant enzymes expression by quantitative real-time reverse transcriptionpolymerase chain reaction. SOD activity and expression were increased in all types of non-immediate reactions (urticaria, maculopapular exanthema and toxic epidermal necrolysis). Regarding oxidative damage, TBARS were increased in urticaria and maculopapular exanthema, and carbonyl groups in all types of reactions. Our observations indicate that oxidative damage occurs in non-immediate reactions. Carbonyl stress and the inadequacy of the anti-oxidant defences are probable causes.

Cornejo-Garcia, J A; Mayorga, C; Torres, M J; Fernandez, T D; R-Pena, R; Bravo, I; Mates, J M; Blanca, M

2006-01-01

322

Effect of Oxidation and SiO2 Coating on the Bonding Strength of Ti-Porcelain  

NASA Astrophysics Data System (ADS)

Investigations on the effect of oxidation on titanium-ceramic adhesion were performed. Cast pure titanium was subjected to surface modification by preoxidation and introduction of an intermediate layer of SiO2 by sol-gel process. Specimens were characterized by TG-DSC, XRD, and SEM/EDS. The adhesion between the titanium and porcelain was evaluated by three-point flexure bond test. Failure of the titanium-porcelain with preoxidation treatment predominantly occurred at the titanium-oxide interface. Preoxidation treatment did not affect the fracture mode of the titanium-ceramic system and did not increase the bonding strength of Ti-porcelain. The SEM results revealed the existence of microcracks on the SiO2 coating surface oxidized at 800 C in an air furnace. During the porcelain fusion, minute amounts of oxygen were able to penetrate the cracks and caused localized oxidation of the Ti-substrate. Failure of the titanium-porcelain with SiO2 coating predominantly occurred at the SiO2 layer. The SiO2 coating served as an effective oxygen diffusion barrier and improved the mechanical and chemical bonding between porcelain and titanium.

Guo, Litong; Liu, Xiaochen; Zhu, Yabo; Xu, Cheng; Gao, Jiqiang; Guo, Tianwen

2010-11-01

323

Novel Ti-O-Ti bonding species constructed in a metal-oxide cluster.  

PubMed

The preparation and structural characterization of a novel Ti-O-Ti bonding complex constructed in the mono-lacunary alpha-Keggin polyoxometalate (POM), are described. The water-soluble, crystalline complex with a formula of K5H2[[{Ti(OH)(ox)}2(micro-O)](alpha-PW11O39)] x 13H2O 1 was prepared in 30.2% (0.60 g scale) yield in a 1 : 3 molar-ratio reaction of the tri-lacunary species of alpha-Keggin POM, Na9[A-PW9O34] x 19H2O, with the titanium(IV) source, K2TiO(ox)2 x 2H2O (H2ox = oxalic acid), in HCl-acidic solution (pH 0.08), and characterized by complete elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solution (31P, 183W, 1H and 13C) NMR spectroscopy and X-ray crystallography. The complex was also obtained in 47.6% (0.81 g scale) yield in a 1 : 2 molar-ratio reaction of the mono-lacunary Keggin POM, K7[PW11O39] x 10H2O, with the anionic titanium(IV) complex under acidic conditions. The molecular structure of [[{Ti(OH)(ox)}2(micro-O)](alpha-PW11O39)]7- 1a, was successfully determined. This POM in the solid state is composed of one host (mono-lacunary site) and two guests (two octahedral Ti groups), in contrast to most titanium (IV)-substituted POMs consisting of one host and one guest. On the other hand, the 31P NMR measurements revealed that in aqueous solution this POM was present under a dissociation equilibrium which depends upon both temperature and pH. PMID:16471056

Hayashi, Kunihiko; Takahashi, Mizuto; Nomiya, Kenji

2005-12-01

324

An Intrinsically Disordered Photosystem II Subunit, PsbO, Provides a Structural Template and a Sensor of the Hydrogen-bonding Network in Photosynthetic Water Oxidation.  

PubMed

Photosystem II (PSII) is a membrane-bound enzyme that utilizes solar energy to catalyze the photooxidation of water. Molecular oxygen is evolved after four sequential light-driven oxidation reactions at the Mn4CaO5 oxygen-evolving complex, producing five sequentially oxidized states, Sn. PSII is composed of 17 membrane-spanning subunits and three extrinsic subunits, PsbP, PsbQ, and PsbO. PsbO is intrinsically disordered and plays a role in facilitation of the water oxidizing cycle. Native PsbO can be removed and substituted with recombinant PsbO, thereby restoring steady-state activity. In this report, we used reaction-induced Fourier transform infrared spectroscopy to obtain information concerning the role of PsbP, PsbQ, and PsbO during the S state cycle. Light-minus-dark difference spectra were acquired, monitoring structural changes associated with each accessible flash-induced S state transition in a highly purified plant PSII preparation (Triton X-100, octylthioglucoside). A comparison of S2 minus S1 spectra revealed that removal of PsbP and PsbQ had no significant effect on the data, whereas amide frequency and intensity changes were associated with PsbO removal. These data suggest that PsbO acts as an organizational template for the PSII reaction center. To identify any coupled conformational changes arising directly from PsbO, global (13)C-PsbO isotope editing was employed. The reaction-induced Fourier transform infrared spectra of accessible S states provide evidence that PsbO spectral contributions are temperature (263 and 277 K) and S state dependent. These experiments show that PsbO undergoes catalytically relevant structural dynamics, which are coupled over long distance to hydrogen-bonding changes at the Mn4CaO5 cluster. PMID:23940038

Offenbacher, Adam R; Polander, Brandon C; Barry, Bridgette A

2013-08-12

325

Influence of interfacial shear strength on the mechanical properties of SiC fiber-reinforced reaction-bonded silicon nitride matrix composites. Technical memo  

SciTech Connect

The influence of fiber/matrix interface microstructure and interfacial shear strength on the mechanical properties of a fiber-reinforced ceramic composite has been evaluated. The composite consisted of about 30 vol % uniaxially aligned 142 micron diameter SiC fibers (Textron SCS-6) in a reaction-bonded Si3N4 matrix (SiC/RBSN). The interface microstructure was varied by controlling the composite fabrication conditions and by heat treating the composite in an oxidizing environment. Interfacial shear strength was determined by the matrix crack spacing method. The results of microstructural examination indicate that the carbon-rich coating provided with the as-produced SiC fibers was stable in composites fabricated at 1200 C in a nitrogen or in a nitrogen plus 4 percent hydrogen mixture for 40 hr. However this coating degraded in composites fabricated at 1350 C in N2 + 4%H2 for 40 and 72 hr and also in composites heat treated in an oxidizing environment at 600 C for 100 hr after fabrication at 1200 C in a nitrogen. It was determined that degradation occurred by carbon removal which in turn had a strong influence on interfacial shear strength and other mechanical properties. Specifically, as the carbon coating was removed, the composite interfacial shear strength, primary elastic modulus, first matrix cracking stress, and ultimate tensile strength decreased, but the first matrix cracking strain remained nearly the same.

Bhatt, R.T.

1990-01-01

326

Oxo iron(IV) as an oxidative active intermediate of p-chlorophenol in the Fenton reaction: a DFT study.  

PubMed

Debate continues over which active species plays the role of oxidative agent during the Fenton reaction-the HO? radical or oxo iron [Fe(IV)O](2+). In this context, the present study investigates the oxidation of p-chlorophenol by [Fe(IV)O(H(2)O)(5)](2+) using DFT calculations, within gas-phase and micro-solvated models, in order to explore the possible role of oxo iron as a reactant. The results show that the chlorine atom substitution of p-chlorophenol by oxo iron is a highly stabilising step (?H = -83 kcal mol(-1)) with a free energy barrier of 5.8 kcal mol(-1) in the micro-solvated model. This illustrates the high oxidising power of the [Fe(IV)O(H(2)O)(5)](2+) complex. On the other hand, the breaking of the Fe-O bond, leading to the formation of hydroquinone, is observed to be the rate-determining step of the reaction. The rather large free energy barrier corresponding to this bond cleavage amounts to 10.2 and 9.3 kcal mol(-1) in the gas-phase and micro-solvated models, respectively. Elsewhere, the lifetime of the HO? radical has previously been shown to be extremely small. These facts, combined with observations of oxo iron under certain experimental conditions, suggest that oxo iron is a highly plausible oxidative species of the reaction. In addition, a trigonal bipyramidal iron complex, coordinated either by hydroxyl groups and/or by water molecules, has been found in all described mechanisms. This structure appears to be a stable intermediate; and to our knowledge, it has not been characterised by previous studies. PMID:22322483

Mignon, Pierre; Pera-Titus, Marc; Chermette, Henry

2012-02-09

327

Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts  

Microsoft Academic Search

Density functional theory (DFT) calculations were carried out on a vanadium oxide cluster containing four vanadium atoms to probe the mechanism of the selective catalytic reduction (SCR) of NO with ammonia. The interaction of ammonia with Brnsted acid sites on this V4-cluster leads to the formation of NH4 species bonded to two vanadyl (V?O) groups, with a bonding energy of

Mark Anstrom; Nan-Yu Topse; J. A. Dumesic

2003-01-01

328

Activation Energy of Tantalum-Tungsten Oxide Thermite Reaction  

SciTech Connect

The activation energy of a high melting temperature sol-gel (SG) derived tantalum-tungsten oxide thermite composite was determined using the Kissinger isoconversion method. The SG derived powder was consolidated using the High Pressure Spark Plasma Sintering (HPSPS) technique to 300 and 400 C to produce pellets with dimensions of 5 mm diameter by 1.5 mm height. A custom built ignition setup was developed to measure ignition temperatures at high heating rates (500-2000 C {center_dot} min{sup -1}). Such heating rates were required in order to ignite the thermite composite. Unlike the 400 C samples, results show that the samples consolidated to 300 C undergo an abrupt change in temperature response prior to ignition. This change in temperature response has been attributed to the crystallization of the amorphous WO{sub 3} in the SG derived Ta-WO{sub 3} thermite composite and not to a pre-ignition reaction between the constituents. Ignition temperatures for the Ta-WO{sub 3} thermite ranged from approximately 465-670 C. The activation energy of the SG derived Ta-WO{sup 3} thermite composite consolidated to 300 and 400 C were determined to be 37.787 {+-} 1.58 kJ {center_dot} mol{sup -1} and 57.381 {+-} 2.26 kJ {center_dot} mol{sup -1}, respectively.

Cervantes, O; Kuntz, J; Gash, A; Munir, Z

2010-02-25

329

Control of bimolecular reactions: Bond-selected reaction of vibrationally excited CH3D with Cl (2P3/2)  

NASA Astrophysics Data System (ADS)

Selective vibrational excitation permits control of the outcome of a reaction with two competing channels. The thermal reaction of CH3D with Cl (2P3/2) yields two reaction products: CH3 from the D-atom abstraction and CH2D from the H-atom abstraction. We prepare the first overtone of the C-D stretching vibration (2?2) at ~4300 cm-1 and react the vibrationally excited molecule with photolytic Cl atoms. The 2+1 resonance enhanced multiphoton ionization spectra for the products show that the 2?2 vibrational excitation of CH3D exclusively increases the probability of breaking the C-D bond, yielding CH3 but no CH2D. By contrast, vibrational excitation of the combination of the antisymmetric C-H stretch and CH3 umbrella (?4+?3) vibrations, which has total energy similar to that of 2?2, preferentially promotes the H-atom abstraction reaction to produce CH2D over CH3. The vibrational action spectra for the two products permit the separation of the two sets of interleaved transitions to give band origins and rotational constants of the 2?2 state and the ?4+?3 state of CH3D.

Yoon, Sangwoon; Holiday, Robert J.; Crim, F. Fleming

2003-09-01

330

Development of Nitric Oxide Oxidation Catalysts for the Fast SCR Reaction  

SciTech Connect

This study was undertaken in order to assess the potential for oxidizing NO to NO{sub 2} in flue gas environments, with the aim of promoting the so-called fast SCR reaction. In principle this can result in improved SCR kinetics and reduced SCR catalyst volumes. Prior to commencing experimental work, a literature study was undertaken to identify candidate catalysts for screening. Selection criteria comprised (1) proven (or likely) activity for NO oxidation, (2) low activity for SO2 oxidation (where data were available), and (3) inexpensive component materials. Catalysts identified included supported base metal oxides, supported and unsupported mixed metal oxides, and metal ion exchanged ZSM-5 (Fe, Co, Cu). For comparison purposes, several low loaded Pt catalysts (0.5 wt% Pt) were also included in the study. Screening experiments were conducted using a synthetic feed gas representative of flue gas from coal-fired utility boilers: [NO] = 250 ppm, [SO{sub 2}] = 0 or 2800 ppm, [H{sub 2}O] = 7%, [CO{sub 2}] = 12%, [O{sub 2}] = 3.5%, balance = N{sub 2}; T = 275-375 C. Studies conducted in the absence of SO{sub 2} revealed a number of supported and unsupported metal oxides to be extremely active for NO oxidation to NO{sub 2}. These included known catalysts (Co{sub 3}O{sub 4}/SiO{sub 2}, FeMnO{sub 3}, Cr{sub 2}O{sub 3}/TiO{sub 2}), as well as a new one identified in this work, CrFeO{sub x}/SiO{sub 2}. However, in the presence of SO{sub 2}, all the catalysts tested were found to be severely deactivated with respect to NO oxidation. Of these, Co{sub 3}O{sub 4}/SiO{sub 2}, Pt/ZSM-5 and Pt/CeO{sub 2} showed the highest activity for NO oxidation in the presence of SO{sub 2} (based on peak NO conversions to NO{sub 2}), although in no cases did the NO conversion exceed 7%. Reactor studies indicate there are two components to SO{sub 2}-induced deactivation of Co{sub 3}O{sub 4}/SiO{sub 2}, corresponding to an irreversible deactivation due to sulfation of the surface of the Co{sub 3}O{sub 4} phase, together with a reversible inhibition due to competitive adsorption of SO{sub 2} with NO on the catalyst. In an effort to minimize the deactivating effect of SO{sub 2} on Co{sub 3}O{sub 4}/SiO{sub 2}, two synthetic approaches were briefly examined. These consisted of (1) the incorporation of highly dispersed Co(II) ions in silica, as a non-sulfating matrix, via the sol-gel preparation of CoO-SiO{sub 2}; and (2) the sol-gel preparation of a mixed metal oxide, CoO-Nb{sub 2}O{sub 5}-SiO{sub 2}, with the aim of exploiting the acidity of the niobium oxide to minimize SO2 adsorption. While both catalysts showed almost no activity for NO oxidation in the absence of SO{sub 2}, when SO{sub 2} was present low activity was observed, indicating that SO{sub 2} acts as a promoter for NO oxidation over these materials. The kinetics of NO oxidation over Co{sub 3}O{sub 4}/SiO{sub 2}, Pt/SiO{sub 2} and Pt/CeO{sub 2} were also examined. Co{sub 3}O{sub 4}/SiO{sub 2} was found to exhibit a higher apparent activation energy for NO oxidation than the Pt catalysts, while the combined reaction order in NO and O{sub 2} for the three catalysts was very close to one. CO{sub 2} was found to have no effect on the kinetics of NO oxidation over these catalysts. The presence of H{sub 2}O caused a decrease in NO conversion for both Co{sub 3}O{sub 4}/SiO{sub 2} and Pt/CeO{sub 2} catalysts, while no effect was observed for Pt/SiO{sub 2}. The inhibiting effect of water was reversible and is attributed to competitive adsorption with the reactants. In sum, this study has shown that a variety of base metal catalysts are very active for NO oxidation. However, all of the catalysts studied are strongly deactivated in the presence of 2800 ppm SO{sub 2} at typical flue gas temperatures; consequently improving catalyst resistance to SO{sub x} will be a pre-requisite if the fast SCR concept is to be applied to coal-fired flue gas conditions.

Mark Crocker

2005-09-30

331

Construction materials for reaction unit in the liquid-phase synthesis of propylene oxide  

Microsoft Academic Search

The main components of the reaction medium in equipment for the synthesis of propylene oxide by liquid-phase oxidation of gaseous propylene with peracetic acid are propylene, peracetic acid, ethyl acetate, acetic acid, propylene oxide, carbon dioxide, oxygen, methane, and propylene glycol acetates. The operating conditions of the equipment and content of the main components of the medium are shown. Results

V.-I. D. Zaritskii; V. I. D

1987-01-01

332

Fatty acid oxidation in anoxic marine sediments: the importance of hydrogen sensitive reactions  

Microsoft Academic Search

In anoxic marine sediments fatty acids may be oxidized directly by sulfate reducing bacteria, or may be oxidized by pathways which result in hydrogen production. Some of these latter reactions are quite sensitive to hydrogen concentrations ... in other words if hydrogen concentrations become elevated, fatty acid oxidation will cease. Thus sulfate reducers may actually play two important roles in

Matthew A. Monetti; Mary I. Scranton

1992-01-01

333

Reaction mechanisms of the thermal conversion of Pu(IV) oxalate into plutonium oxide  

Microsoft Academic Search

During the oxalic conversion of plutonium into oxide at the end of the PUREX process, the thermal transition of Pu(IV) oxalate into oxide plays a significant role leading to the main properties of the oxide. The structure of the solid intermediates is often speculative and the reaction mechanisms are quite misunderstood and divergent from one study to another. This work

N. Vigier; S. Grandjean; B. Arab-Chapelet; F. Abraham

2007-01-01

334

Biochemical Reaction Products of Nitric Oxide as Quantitative Markers of Primary Pulmonary Hypertension  

Microsoft Academic Search

Primary pulmonary hypertension (PPH) is a rare and fatal disease of unknown etiology. Inflammatory oxidant mechanisms and deficiency in nitric oxide (NO) have been implicated in the pathogenesis of pulmonary hypertension. In order to investigate abnormalities in oxidants and antioxidants in PPH, we studied intrapulmonary NO levels, biochemical reaction products of NO, and antioxidants (glu- tathione (GSH), glutathione peroxidase (GPx),

F. TAKAO KANEKO; ALEJANDRO C. ARROLIGA; RAED A. DWEIK; SUZY A. COMHAIR; DANIEL LASKOWSKI; RITA OPPEDISANO; MARY JANE THOMASSEN; SERPIL C. ERZURUM

1998-01-01

335

High-temperature reaction of aluminum oxide with boron in vacuum  

Microsoft Academic Search

The reaction of aluminum oxide with boron in vacuum was studied over a wide range of temperatures. Analysis of the reaction products was carried out by chemical, x-ray diffraction, optical, and metallographic methods. The temperature dependence of the reaction was determined. A high boron phase of the composition AlB1831 is produced by the borothermic reduction of aluminum oxide. The crystalline

A. I. Kharlamov; V. V. Fomenko; N. V. Kirillova

1996-01-01

336

Oxidation and reduction characteristics of oxygen carrier particles and reaction kinetics by unreacted core model  

Microsoft Academic Search

The reaction kinetics of the oxygen carrier particles, which are used as bed material for a fluidized bed chemical looping\\u000a combustor (CLC), has been studied experimentally by a conventional thermal gravimetrical analysis technique. The weight percent of nickel\\u000a and nickel oxide in oxygen carrier particles and reaction temperature were considered as experimental variables. After oxidation\\u000a reaction, the pure nickel particle

Ho-Jung Ryu; Dal-Hee Bae; Keun-Hee Han; Seung-Yong Lee; Gyoung-Tae Jin; Jeong-Hoo Choi

2001-01-01

337

Kinetic mechanism of reactions of carbon tetrachloride with TT-niobium oxide and niobium phosphate  

Microsoft Academic Search

The rate of reaction of CCl4 with lattice oxygen atoms of the TT-phase niobium oxide (TT-Nb2O5) at 453 K was limited by the diffusion of CCl4 through the layer of a solid reaction product, NbOCl3, formed around the outer surface of niobium oxide. This contrasts to the reaction occurring on the amorphous niobium phosphate in which an inner niobium phosphate

Akira Morikawa; Kohki Ebitani; Yoshiaki Hirano

1996-01-01

338

The kinetics of reactions of hexacarbonyls of chromium, molybdenum, and tungsten with hydroxylamine and trimethylamine oxide  

SciTech Connect

The mechanism of the reactions of M(CO){sub 6}(M=Cr, Mo, and W) with hydroxylamine was studied. As follows from kinetic data, the reaction results in the oxidation of CO to CO{sub 2} and an intramolecular transfer of the formed amine to a central metal atom. The mechanisms of reactions of M(CO){sub 6} with hydroxylamine and trimethylamine oxide are compared.

Maksakov, V.A.; Ershova, V.A. [Institute of Inorganic Chemistry, Novosibirsk (Russian Federation)

1994-04-01

339

Desulfurization of fuel oil by oxidation and extraction. Part 2; Kinetic modeling of oxidation reaction  

SciTech Connect

The oxidation of Arabian atmospheric gas oil (AGO) with nitric acid to remove sulfur from the oil is similar to the gradual processes involved in the storage instability of petroleum distillates and synfuels, except that in this process the instability process is accelerated by adding a strong oxidizing agent, nitric acid. The elemental composition of these sediments is compared. A mathematical kinetic model is presented to describe the kinetics of sulfur removal in the oxidation of AGO using a CSTR. This model employs lumping of the sulfur compounds in the oil into four groups according to their retention times (hence, boiling points) in the gas chromatograph, and residue (R) containing some of the other three sulfur groups. The first group had a very fast reaction rate. The second group reacts with second-order kinetics with a rate constant of 17 g of oil/(g of S2 {center dot} min) at 25{degrees}C. The third group was not present in the unoxidized oil, and as it was formed, it equilibrated between the oil and the residue phases.

Tan, P.S.; Eldridge, J.W. (Massachusetts Univ., Amherst, MA (USA). Dept. of Chemical Engineering); Kittrell, J.R. (KSE, Inc., Amherst, MA (US))

1990-03-01

340

Multilayered thermal insulation formed of zirconia bonded layers of zirconia fibers and metal oxide fibers and method for making same  

DOEpatents

A multilayered thermal insulating composite is formed of a first layer of zirconia-bonded zirconia fibers for utilization near the hot phase or surface of a furnace or the like. A second layer of zirconia-bonded metal oxide fibers is attached to the zirconia fiber layer by a transition layer formed of intermingled zirconia fibers and metal oxide fibers. The thermal insulation is fabricated by vacuum molding with the layers being sequentially applied from aqueous solutions containing the fibers to a configured mandrel. A portion of the solution containing the fibers forming the first layer is intermixed with the solution containing the fibers of the second layer for forming the layer of mixed fibers. The two layers of fibers joined together by the transition layer are saturated with a solution of zirconium oxynitrate which provides a zirconia matrix for the composite when the fibers are sintered together at their nexi.

Wrenn, Jr., George E. (Clinton, TN); Holcombe, Jr., Cressie E. (Farragut, TN)

1988-01-01

341

Multilayered thermal insulation formed of zirconia bonded layers of zirconia fibers and metal oxide fibers and method for making same  

DOEpatents

A multilayered thermal insulating composite is formed of a first layer of zirconia-bonded zirconia fibers for utilization near the hot phase or surface of a furnace or the like. A second layer of zirconia-bonded metal oxide fibers is attached to the zirconia fiber layer by a transition layer formed of intermingled zirconia fibers and metal oxide fibers. The thermal insulation is fabricated by vacuum molding with the layers being sequentially applied from aqueous solutions containing the fibers to a configured mandrel. A portion of the solution containing the fibers forming the first layer is intermixed with the solution containing the fibers of the second layer for forming the layer of mixed fibers. The two layers of fibers joined together by the transition layer are saturated with a solution of zirconium oxynitrate which provides a zirconia matrix for the composite when the fibers are sintered together at their nexi.

Wrenn, G.E. Jr.; Holcombe, C.E. Jr.

1988-09-13

342

Identification of nitriding mechanisms in high purity reaction bonded silicon nitride  

SciTech Connect

The rapid, low-temperature nitriding results from surface effects on the Si particles beginning with loss of chemisorbed H and sequential formation of thin amorphous Si nitride layers. Rapid complete conversion to Si[sub 3]N[sub 4] during the fast reaction can be inhibited when either too few or too many nuclei form on Si particels. Optimally, [approximately] 10 Si[sub 3]N[sub 4] nuclei form per Si particles under rapid, complete nitridation conditions. Nitridation during the slow reaction period appears to progress by both continued reaction of nonpreferred Si[sub 3]N[sub 4] growth interfaces and direct nitridation of the remaining Si/vapor interfaces.

Haggerty, J.S.

1993-03-01

343

Visible Light Mediated Oxidative C-N Bond Formation/Aromatization Cascade: A New Photocatalytic Entry to N-Arylindoles  

PubMed Central

Indoles: A joint effort of light and air We have developed a mild aerobic oxidation protocol using visible light photocatalysis to synthesize structurally diverse N-arylindoles. The procedure employs 4 mol% [Ru(bpz)3](PF6)2, 18W LED light, and is performed open to the atmosphere. Readily prepared o-stryryl anilines are converted to a variety of indoles via a cascade sequence composed of oxidation of anilines, C-N bond formation, and aromatization. A 1,2-carbon shift can be also incorporated into this cascade event to further extend the substrate scope of the method. bpz = 2, 2?-Bipyrazine

Maity, Soumitra

2012-01-01

344

Simplified Reaction Mechanisms for the Oxidation of Hydrocarbon Fuels in Flames  

Microsoft Academic Search

Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and

CHARLES K. WESTBROOK; FREDERICK L DRYER

1981-01-01

345

Reaction pattern and mechanism of light induced oxidative water splitting in photosynthesis  

Microsoft Academic Search

This mini review is an attempt to briefly summarize our current knowledge on light driven oxidative water splitting in photosynthesis. The reaction leading to molecular oxygen and four protons via photosynthesis comprises thermodynamic and kinetic constraints that require a balanced fine tuning of the reaction coordinates. The mode of coupling between electron (ET) and proton transfer (PT) reactions is shown

Gernot Renger; Philipp Khn

2007-01-01

346

Redox-neutral ?-C-H bond functionalization of secondary amines with concurrent C-P bond formation/N-alkylation.  

PubMed

Redox-neutral formation of C-P bonds in the ?-position of amines was achieved via a process that features a combination of an oxidative ?-C-H bond functionalization and a reductive N-alkylation. Benzoic acid functions as an efficient catalyst in this three-component reaction of cyclic secondary amines, aldehydes and phosphine oxides to provide rapid access to ?-amino phosphine oxides not easily accessible by classic Kabachnik-Fields reactions. PMID:23957378

Das, Deepankar; Seidel, Daniel

2013-08-19

347

Modeling of the reaction-based processing of aluminum oxide (RBAO) and alumina-aluminide alloys (3A)  

NASA Astrophysics Data System (ADS)

Transient material and energy balances have been utilized to model the reaction-bonded aluminum oxide (RBAO) and alumina-aluminide alloys (3A) processes. The model for the RBAO process considers the diffusion of a gas-phase reactant into a porous solid followed by a solid-gas reaction, while the 3A model considers a solid-solid reaction taking place within a porous solid. The modeling work on the RBAO process reveals that the process may proceed via an ignition/extinguishment phenomenon with thermal runaway. It is believed that this type of behavior can lead to stress development, and subsequent sample cracking. Thus, the model is used to determine conditions under which RBAO bodies may be fired in a controlled manner (i.e., avoiding the runaway reaction). A complimentary experimental study, utilizing simultaneous thermogravimetry (TG) and differential thermal analysis (DTA), in-situ temperature measurements, and analysis of samples fired in a box furnace, verifies the predicted reaction behavior and shows that by controlling the reaction, high Al content powders can be used to produce crack-free RBAO samples. The modeling work on the 3A process demonstrates the effects of various processing parameters on the general reaction behavior. After considering the general behavior, the model is used to predict the reaction behavior of the TiOsb2/Al system. A reaction sequence for the TiOsb2/Al system (based on XRD data) is proposed and used to model the system. The effects of the heating rate, the convective heat transfer coefficient, and sample size are investigated.

Gaus, Shaun Patrick

348

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study.  

PubMed

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol. PMID:15880815

De Jong, G Theodoor; Geerke, Daan P; Diefenbach, Axel; Sol, Miquel; Bickelhaupt, F Matthias

2005-07-30

349

Cobalt(II) and cobalt(III) complexes of thioether-containing hexadentate pyrazine amide ligands: C-S bond cleavage and cyclometallation reaction.  

PubMed

Anaerobic reaction of Co(O2CMe)2.4H2O with the thioether-containing acyclic pyrazine amide hexadentate ligand 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,4-dithiobutane (H2L1) (-CH2CH2- spacer between the two pyrazine amide tridentate coordination units) furnishes [CoII(L1)].MeOH (1a) having CoN2(pyrazine)N'2(amide)S2(thioether) coordination. It exhibits an eight-line EPR spectrum, attesting to a low-spin (S = 1/2) state of CoII. A similar reaction in air, however, furnishes [CoIII(L3a)(L3b)].2MeOH (2a) (S = 0), resulting from a C-S bond cleavage reaction triggered by an acetate ion as a base, having CoN2(pyrazine)N'2(amide)S(thioether)S'(thiolate) coordination. On the other hand, the reaction of Co(O2CMe)2.4H2O with 1,4-bis[o-(pyrazine-2-carboxamidophenyl)]-1,5-dithiopentane (H2) (-CH2CH2CH2- spacer between the two pyrazine amide tridentate coordination units) in air affords a cobalt(II) complex [CoII(L2)].MeOH (1b.MeOH) (S = 1/2); its structurally characterized variety has the composition 1b.C6H6. Interestingly, 1b.MeOH undergoes facile metal-centred oxidation by aerial O2-H2O2-[Fe(eta5-C5H5)2][PF6], which led to the isolation of the corresponding cobalt(iii) complex [CoIII(L2)][ClO4] (2b). When treated with methanolic KOH, 2b affords a low-spin (S = 0) organocobalt(III) complex [Co(III)((L2')] (3). Structures of all complexes, except 1a, have been authenticated by X-ray crystallography. A five-membered chelate-ring forming ligand L1(2-) effects C-S bond cleavage and a six-membered chelate-ring forming ligand L2(2-) gives rise to Co-C bond formation, in cobalt(III)-coordinated thioether functions due to alpha C-H bond activation by the base. A rationale has been provided for the observed difference in the reactivity properties. The spectroscopic properties of the complexes have also been investigated. Cyclic voltammetry experiments in MeCN-CH2Cl2 reveal facile metal-centred reversible-to-quasireversible CoIV-CoIII (or a ligand-centred redox process; 2a), CoIII-CoII (1a, 1b.MeOH, 2a, 2b and 3), CoII-CoI (1a, 1b.MeOH, 2aand 2b), and CoI-Co0 (1a, 1b.MeOH and 2b) redox processes. PMID:18097493

Singh, Akhilesh Kumar; Mukherjee, Rabindranath

2007-10-22

350

Liquid-phase reaction-bonding of silicon carbide using alloyed silicon-molybdenum melts  

Microsoft Academic Search

The authors have investigated reaction-forming of silicon carbide by the infiltration of carbonaceous preforms using alloyed silicon melts, in order to synthesize composite materials free of the residual silicon phase that has previously limited mechanical properties and upper use temperatures. In this approach, rejection of the alloying component(s) from the primary silicon carbide phase into the remaining melt results in

Robert P. Messner; Yet-Ming Chiang

1990-01-01

351

Coupling of anodic and cathodic reactions for phenol electro-oxidation using three-dimensional electrodes  

Microsoft Academic Search

We studied the electrochemical oxidation of phenol by the coupling of anodic and cathodic reactions. The experiments were done in an electrochemical filter press cell equipped with an Sb-doped SnO2-coated titanium foam and a RVC cathode. The oxidation occurs by a direct oxidation on the anodic side, while on the cathodic side oxidation occurs via an electro-Fenton mechanism. We studied

E Fockedey; A Van Lierde

2002-01-01

352

Molecular recognition in Mn-catalyzed C-H oxidation. Reaction mechanism and origin of selectivity from a DFT perspective.  

PubMed

Experimental studies have shown that the C-H oxidation of Ibuprofen and methylcyclohexane acetic acid can be carried out with high selectivities using [(terpy')Mn(OH(2))(mu-O)(2)Mn(OH(2))(terpy')](3+) as catalyst, where terpy' is a terpyridine ligand functionalized with a phenylene linker and a Kemp's triacid serving to recognize the reactant via H-bonding. Experiments, described here, suggest that the sulfate counter anion, present in stoichiometric amounts, coordinates to manganese in place of water. DFT calculations have been carried out using [(terpy')Mn(O)(mu-O)(2)Mn(SO(4))(terpy')](+) as a model catalyst, to analyze the origin of selectivity and its relation to molecular recognition, as well as the mechanism of catalyst inhibition by tert-butyl benzoic acid. The calculations show that a number of spin states, all having radical oxygen character, are energetically accessible. All these spin states promote C-H oxidation via a rebound mechanism. The catalyst recognizes the substrate by a double H bond. This interaction orients the substrate inducing highly selective C-H oxidation. The double hydrogen bond stabilizes the reactant, the transition state and the product to the same extent. Consequently, the reaction occurs at lower energy than without molecular recognition. The association of the catalyst with tert-butyl benzoic acid is shown to shield the access of unbound substrate to the reactive oxo site, hence preventing non-selective hydroxylation. It is shown that the two recognition sites of the catalyst can be used in a cooperative manner to control the access to the reactive centre. PMID:19623399

Balcells, David; Moles, Pamela; Blakemore, James D; Raynaud, Christophe; Brudvig, Gary W; Crabtree, Robert H; Eisenstein, Odile

2009-06-17

353

Molecular Recognition in Mn-Catalyzed C-H Oxidation. Reaction Mechanism and Origin of Selectivity from a DFT Perspective  

PubMed Central

Experimental studies have shown that the C-H oxidation of ibuprofen and methylcyclohexane acetic acid can be carried out with high selectivies using [(terpy)Mn(OH2)(?-O)2Mn(OH2)(terpy)]3+ as catalyst, where terpy is a terpyridine ligand functionalized with a phenylene linker and a Kemps triacid serving to recognize the reactant via H-bonding. Experiments, described here, suggest that the sulfate counter anion, present in stochiometric amounts, coordinates to manganese in place of water. DFT calculations have been carried out using [(terpy)Mn(O)(?-O)2Mn(SO4)(terpy)]+ as model catalyst, to analyze the origin of selectivity and its relation to molecular recognition, as well as the mechanism of catalyst inhibition by tert-butyl benzoic acid. The calculations show that a number of spin states, all having radical oxygen character, are energetically accessible. All these spin states promote C-H oxidation via a rebound mechanism. The catalyst recognizes the substrate by a double H bond. This interaction orients the substrate inducing highly selective C-H oxidation. The double hydrogen bond stabilizes the reactant, the transition state and the product to the same extent. Consequently, the reaction occurs at lower energy than without molecular recognition. The association of the catalyst with tert-butyl benzoic acid is shown to shield the access of unbound substrate to the reactive oxo site, hence preventing non-selective hydroxylation. It is shown that the two recognition sites of the catalyst can be used in a cooperative manner to control the access to the reactive centre.

Balcells, David; Moles, Pamela; Blakemore, James; Raynaud, Christophe; Brudvig, Gary W.; Crabtree, Robert H.

2010-01-01

354

Identification and cleavage of breakable single bonds by selective oxidation, reduction, and hydrolysis. Quarterly report No. 9, October 1-December 31, 1980  

SciTech Connect

Objective is to determine the structure of bituminous coal with emphasis on the crosslinks and breakable single bonds. Some of the highlights of this quarter are: (1) large-scale extraction of benzylamine-extracted coal with ethylene diamine-dimethyl sulfoxide (EDA-DMSO) led to 50.4% recovery of soluble material and 40.5% recovery of undissolved coal; (2) EDA-DMSO extraction appears to have removed essentially all of the ether and ester links in both fractions, leaving only hydrocarbon links; (3) extraction of whole coal by a mixture of N-methylpyrrolidinone (NMP) and EDA is no better than EDA-DMSO, even though NMP alone is a better solvent than DMSO alone; (4) investigators have no evidence that NaOCl will oxidize sulfide links to sulfuric acid and NaOCl appears to be unsuitable for determining sulfide links in coal; (5) black acid fraction from several NaOCl oxidations precipitated between pH 4.98 and 4.66 was characterized; (6) measurements of hydroxyl and carboxyl contents on pyridine and benzylamine extracts and extracted coal show increases in carboxyl content on reactions of these fractions with alcoholic KOH, and an increase in hydroxyl content on reactions of the pyridine-extracted coal with benzylamine, consistent with ester cleavage. 6 tables.

Pavelka, L.A.; Mayo, F.R.; Zevely, J.

1981-03-24

355

Selective Covalent Bond Formation in Polypeptide Ions via Gas-Phase Ion/Ion Reaction Chemistry  

PubMed Central

Primary amines present in protonated polypeptides can be covalently modified via gas-phase ion/ion reactions using bifunctional reagent ions. The use of reagent anions with a charge bearing site that leads to strong interactions with the polypeptide, such as sulfonic acid, gives rise to the formation of a long-lived adduct. A distinct reactive functional group, an aldehyde in the present case, can then undergo a reaction with the peptide. Collisional activation of the adduct ion formed from a reagent with an aldehyde group and a peptide ion with a primary amine gives rise to water loss in conjunction with imine (Schiff base) formation. The covalently-bound modification is retained upon subsequent collisional activation. This work demonstrates the ability to selectively modify polypeptide ions in the gas-phase within the context of a multi-stage mass spectrometry experiment.

Han, Hongling; McLuckey, Scott A.

2009-01-01

356

The role of Si 3 N 4 additions in the reaction bonding of silicon compacts  

Microsoft Academic Search

The effects of Si3N4 additions to silicon prior to slip casting and nitriding have recently been reported by Williams and co-workers [1, 2] who concluded that the primary effect is to prevent the closing of the pore structure thereby maintaining space for nitridation to occur. These additions resulted in a greater extent of reaction in a given thickness sample, and

O. J. Gregory; M. H. Richman

1984-01-01

357

The Crystal Structure of Arabidopsis thaliana Allene Oxide Cyclase: Insights into the Oxylipin Cyclization Reaction[W  

PubMed Central

We describe the crystallization and structure elucidation of Arabidopsis thaliana allene oxide cyclase 2 (AOC2), a key enzyme in the biosynthesis of jasmonates. In a coupled reaction with allene oxide synthase, AOC2 releases the first cyclic and biologically active metabolite, 12-oxo-phytodienoic acid (OPDA). AOC2 (AT3G25770) folds into an eight-stranded antiparallel ?-barrel with a C-terminal partial helical extension. The protein forms a hydrophobic binding cavity with two distinct polar patches. AOC2 is trimeric in crystals, in vitro and in planta. Based on the observed folding pattern, we assigned AOC2 as a low molecular weight member of the lipocalin family with enzymatic activity in plants. We determined the binding position of the competitive inhibitor vernolic acid (a substrate analog) in the binding pocket. Based on models for bound substrate 12,13-epoxy-9,11,15-octadecatrienoic acid and product OPDA, we propose a reaction scheme that explains the influence of the C15 double bond on reactivity. Reaction is promoted by anchimeric assistance through a conserved Glu residue. The transition state with a pentadienyl carbocation and an oxyanion is stabilized by a strongly bound water molecule and favorable ?? interactions with aromatic residues in the cavity. Stereoselectivity results from steric restrictions to the necessary substrate isomerizations imposed by the protein.

Hofmann, Eckhard; Zerbe, Philipp; Schaller, Florian

2006-01-01

358

Nano-Bonding of Silicon Oxides-based surfaces at Low Temperature: Bonding Interphase Modeling via Molecular Dynamics and Characterization of Bonding Surfaces Topography, Hydro-affinity and Free Energy  

NASA Astrophysics Data System (ADS)

In this work, a new method, "Nanobonding(TM)" [1,2] is conceived and researched to bond Si-based surfaces, via nucleation and growth of a 2 D silicon oxide SiOxHx interphase connecting the surfaces at the nanoscale across macroscopic domains. Nanobonding cross-bridges two smooth surfaces put into mechanical contact in an O2/H 2O mixed ambient below T ?200 C via arrays of SiOxH x molecules connecting into a continuous macroscopic bonding interphase. Nano-scale surface planarization via wet chemical processing and new spin technology are compared via Tapping Mode Atomic Force Microscopy (TMAFM) , before and after nano-bonding. Nanobonding uses precursor phases, 2D nano-films of beta-cristobalite (beta-c) SiO2, nucleated on Si(100) via the Herbots-Atluri (H-A) method [1]. beta-c SiO2 on Si(100) is ordered and flat with atomic terraces over 20 nm wide, well above 2 nm found in native oxides. When contacted with SiO2 this ultra-smooth nanophase can nucleate and grow domains with cross-bridging molecular strands of hydroxylated SiOx, instead of point contacts. The high density of molecular bonds across extended terraces forms a strong bond between Si-based substrates, nano- bonding [2] the Si and silica. A new model of beta-cristobalite SiO2 with its <110> axis aligned along Si[100] direction is simulated via ab-initio methods in a nano-bonded stack with beta-c SiO2 in contact with amorphous SiO2 (a-SiO2), modelling cross-bridging molecular bonds between beta-c SiO2 on Si(100) and a-SiO2 as during nanobonding. Computed total energies are compared with those found for Si(100) and a-SiO2 and show that the presence of two lattice cells of !-c SiO2 on Si(100) and a-SiO2 lowers energy when compared to Si(100)/ a-SiO 2 Shadow cone calculations on three models of beta-c SiO2 on Si(100) are compared with Ion Beam Analysis of H-A processed Si(100). Total surface energy measurements via 3 liquid contact angle analysis of Si(100) after H-A method processing are also compared. By combining nanobonding experiments, TMAFM results, surface energy data, and ab-initio calculations, an atomistic model is derived and nanobonding is optimized. [1] US Patent 6,613,677 (9/2/03), 7,851,365 (12/14/10), [2] Patent Filed: 4/30/09, 10/1/2011

Whaley, Shawn D.

359

Kinetics of fatty acid oxidation in low density lipoproteins evaluated by registration of the oxidizer consumption and reaction product yield.  

PubMed

Oxidation of arachidonic acid by ROS in vitro can be evaluated by the formation of reaction products (conjugated dienes); this is preceded by a lag period caused by the action of antioxidants (alpha-tocopherol, beta-carotene, and ascorbic acid). In case of ozone titration the oxidizer is consumed even during the lag period, when conjugated dienes are not yet forming. Comparison of the oxidation rate constants for antioxidants, arachidonic and oleic monoenic fatty acids suggests that during the lag period Cu(2+)-initiated forms of O(2) oxidize primarily oleic acid, whose reaction rate constant is much higher than those of antioxidants. Presumably, the duration of lag period during oxidation of arachidonic acid and formation of conjugated dienes is determined also by the content of triglycerides and oleic fatty acid in low density lipoproteins. PMID:16254615

Titov, V N; Konovalova, G G; Lisitsyn, D M; Razumovskii, S D; Nezhdanova, I B; Kukharchuk, V V

2005-07-01

360

C-C coupling reactions of superstrong CF3 groups with C(sp2)-H bonds: reactivity and synthetic utility of zero-valent niobium catalyst.  

PubMed

It was found that zero-valent niobium is an efficient catalyst for the intramolecular C-C coupling reactions of o-aryl and o-alkenyl alpha,alpha,alpha-trifluorotoluene derivatives. The superstrong C-F bonds of CF3 groups and neighboring C(sp2)-H bonds were doubly activated, and fluorenes and indenes were obtained in good yields. The niobium fluorocarbenoid species is proposed to be the key intermediate. PMID:18175305

Fuchibe, Kohei; Mitomi, Ken; Suzuki, Ryo; Akiyama, Takahiko

2008-02-01

361

Aerosol synthesis and electrochemical analysis of niobium mixed-metal oxides for the ethanol oxidation reaction in acid and alkaline electrolyte  

NASA Astrophysics Data System (ADS)

Direct ethanol fuel cells are especially important among emerging electrochemical power systems with the potential to offset a great deal of the energy demand currently met through the use of fossil fuels. Ethanol can be refined from petroleum sources or attained from renewable biomass, and is more easily and safely stored and transported than hydrogen, methanol or gasoline. The full energy potential of ethanol in fuel cells can only be realized if the reaction follows a total oxidation pathway to produce CO2. This must be achieved by the development of advanced catalysts that are electrically conductive, stable in corrosive environments, contain a high surface area on which the reaction can occur, and exhibit a bi-functional effect for the ethanol oxidation reaction (EOR). The latter criterion is achievable in mixed-metal systems. Platinum is an effective metal for catalyzing surface reactions of many adsorbates and is usually implemented in the form of Pt nanoparticles supported on inexpensive carbon. This carbon is believed to be neutral in the catalysis of Pt. Instead, carbon can be replaced with carefully designed metals and metal oxides as co-catalysis or support structures that favorably alter the electronic structure of Pt slightly through a strong metal support interaction, while also acting as an oxygen source near adsorbates to facilitate the total oxidation pathway. Niobium mixed-metal-oxides were explored in this study as bi-functional catalyst supports to Pt nanoparticles. We developed a thermal aerosol synthesis process by which mesoporous powders of mixed-metal-oxides decorated with Pt nanoparticles could be obtained from liquid precursors within 5 seconds or less, followed by carefully refined chemical and thermal post-treatments. Exceptionally high surface areas of 170--180m2/g were achieved via a surfactant-templated 3D wormhole-type porosity, comparable on a per volume basis to commercial carbon blacks and high surface area silica supports. For the first time, in situ FTIR measurements in acid electrolyte showed that highly dispersed Pt nanoparticles (2--5nm) on NbRuyO z (at% 8Nb:1Ru) catalyze the formation of CO2 from ethanol in greater yield, and 0.35--0.4V lower, than Pt(111). Compared to conventional Pt/carbon, this indicates that, (1) Pt supported on NbRuyO z can be more effective at splitting the C---C bond in ethanol and, (2) the scission occurs at potentials more ideal for a higher efficiency fuel cell anode. Ex situ-microscopy revealed the polarization-induced two- and three-dimensional formation of Pt-NbOx interfacial adsorption sites responsible for the facilitation of the total oxidation pathway of ethanol. The results show that synthesis and post-treatment of niobia supports can bias the utility of Pt/niobia systems towards the ethanol oxidation reaction at the anode or the oxygen reduction reaction at the cathode. Experimental and computational-theoretical analyses indicate that the mechanism of interfacial site formation is dependent upon the local oxygen concentration, as well as the availability of multiple, energetically accessible oxidation states like those inherent to niobia. Future directions for the development of highly active, niobium-based materials tailored for efficient catalysis of the total oxidation pathway of ethanol are discussed.

Konopka, Daniel A.

362

Studies on the scale-up of the microwave-assisted nitridation and sintering of reaction-bonded silicon nitride  

SciTech Connect

Studies using laboratory test samples have shown that microwave heating produces sintered reaction-bonded silicon nitride materials with improved properties. The final challenge for processing this material by microwave heating is the development of a technology for processing larger batch-size quantities of these materials. Initial microwave scale-up experiments were performed using powder compacts of a bucket tappet geometry. In experiments using microwave-transparent boron nitride sample crucibles, temperature gradients within some crucibles led to larger variations in the sample densities than were obtained with the conventionally processed samples. The use of a microwave-suscepter type crucible made of silicon carbide and boron nitride resulted in an improved temperature uniformity and in density variations comparable to those obtained for the control groups.

Kiggans, J.O.: Tiegs, T.N.; Kimrey, H.D.

1996-05-01

363

Processing and properties of SiC whisker- and particulate-reinforced reaction bonded Si3N4  

SciTech Connect

The microstructure and mechanical properties of reaction bonded Si3N4 (RBSN) reinforced with SiC whiskers of particles were investigated using RBSN composites made from colloidally pressed octanol dispersions of high-purity Si powders mixed with either SiC whiskers or alpha-SiC particles. Results of investigations, revealing high conversions of Si to Si3N4, specific surface areas, and constant relative densities and strengths, showed that the uniform microstructure and small flaw size of the matrix were maintained in the composites and that no degradation of the reinforcements was taking place. Neither the monolithic nor the composite materials exhibited R-curve behavior. A modest increase in fracture toughness was observed only in the RBSN containing 33 vol pct SiC(p). 37 refs.

Lightfoot, A.; Ewart, L.; Haggerty, J.; Cai, Z.Q.; Ritter, J.; Nair, S.

1991-08-01

364

Effect of grain size on microstructure, properties, and surface roughness of reaction bonded SiC ceramics  

NASA Astrophysics Data System (ADS)

Silicon carbide (SiC) based ceramics have received significant study for optical applications due to high specific stiffness, high thermal conductivity, and low coefficient of thermal expansion (CTE). Reaction bonded SiC ceramics, which are composites of SiC and Si, are of particular interest due to large size and complex shape capability. The behavior of these ceramics is very much affected by the grain size of the SiC phase. The present work examines SiC grain sizes ranging from 6 to 50 ?m, with the goal of optimizing properties and finishing capability for optical uses. Microstructures are reviewed; physical, mechanical and thermal properties are presented; and post-polishing surface roughness data are provided. In particular, results demonstrate that properties can be tailored by SiC particle size selection, and that measureable enhancement in surface roughness can be achieved by moving to smaller SiC grain size.

Aghajanian, Michael; Emmons, Craig; Rummel, Steve; Barber, Paul; Robb, Chris; Hibbard, Doug

2013-09-01

365

In vitro evaluation of shear bond strengths of resin to densely-sintered high-purity zirconium-oxide ceramic after long-term storage and thermal cycling  

Microsoft Academic Search

Statement of problemThe few available studies on the resin bond to zirconium-oxide ceramic recommend airborne-particle abrasion and modified resin luting agents containing adhesive monomers for superior and long-term durable bond strengths. It is unknown whether this regimen can also be successfully applied to the intaglio surface of a commercial zirconia-based all-ceramic system.

Markus B Blatz; Avishai Sadan; Javier Martin; Brien Lang

2004-01-01

366

Trinuclear organooxotin assemblies from solvothermal synthesis reaction: Crystal structure, hydrogen bonding and ? ? stacking interaction  

NASA Astrophysics Data System (ADS)

Two new trinuclear mono-organooxotin(IV) complexes with 2,3,4,5-tetrafluorobenzoic acid and sodium perchlorate of the types: [(SnR)3(OH)(2,3,4,5-F4C6HCO2)4 ClO4] [O2CC6HF4](R = PhCH2, 1; o-F-PhCH2 for 2), have been solvothermally synthesized and structurally characterized by elemental, IR, 1H, 13C and 119Sn NMR and X-ray crystallography diffraction analyses. Complex 2 is also characterized by X-ray crystallography diffraction analyses. In complex 2, four carboxyl groups and a perchlorate bridged three tin atoms in a cyclohexane chair arrangement and form the basic framework. A hydroxyl group comprises the oxygen components of the stannoxane ring system. In these complexes, weak but significant intramolecular hydrogen bonding and ? ? stacking interaction are also shown. These contacts lead to aggregation and supramolecular assembly of complexes 1 and 2 into 1D or 2D framework.

Ma, Chunlin; Sun, Junshan; Zhang, Rufen

2007-05-01

367

Kinetic study of the hydrogen oxidation reaction on sub-stoichiometric titanium oxide-supported platinum electrocatalyst in acid solution  

Microsoft Academic Search

The kinetics and mechanism of the hydrogen oxidation reaction were studied in 0.5moldm?3 HClO4 solution on an electrode based on titanium oxide with Magneli phase structure-supported platinum electrocatalyst applied on rotation Au disk electrode. Pt catalyst was prepared by impregnation method from 2-propanol solution of Pt(NH3)2(NO2)2 and sub-stoichiometric titanium oxide powder. Sub-stiochiometric titanium oxide support was characterized by X-ray diffraction

B. Babi?; J. Gulicovski; Lj. Gaji?-Krstaji?; N. Elezovi?; V. R. Radmilovi?; N. V. Krstaji?; Lj. M. Vra?ar

2009-01-01

368

THE USE OF ISOTOPE CROSSOVER EXPERIMENTS IN INVESTIGATING CARBON-CARBON BOND FORMING REACTIONS OF BINUCLEAR DIALKYL COBALT COMPLEXES  

SciTech Connect

Our present understanding of the mechanisms of organometallic reactions stems almost completely from investigation of complexes containing only one metal. Recently interest has been increasing in the synthesis, structure elucidation and reaction mechanisms of polynuclear clusters, complexes containing more than one metal. This attention derives partially from the possibility that polynuclear catalysts and reagents might be designed in such a way that the metals could interact, generating cooperative systems which might be much more selective than their mononuclear analogs. Another stimulant to this work has been the relationship of cluster complexes to larger multi-metal systems, such as heterogeneous catalysts. Many polynuclear clusters have been prepared and characterized, and some of these have been found to function as unique catalysts or catalyst precursors. However, very little is yet known about how chemical transformations take place at multinuclear reaction centers. Given this paucity of information, they decided a few years ago to initiate mechanistic study of simple cluster systems containing two metal centers, in which each of the metals has a {sigma}-bound organic ligand attached to it. They also choose to focus on reactions of these complexes in which new carbon-carbon or carbon-hydrogen bonds are formed. This Account describes the work on such a system: a binuclear alkyl cobalt complex capable of transferring both alkyl groups to a molecule of carbon monoxide. In this work they have adopted as one of our highest priorities the determination of whether the cluster 'holds together' during its reactions, a question that is in our opinion too often ignored in such studies. They have found that isotope crossover experiments provide a powerful tool for investigating this structural integrity questions, and in this Account they outline a number of examples in which such crossover experiments have provided important, and occasionally surprising, informationa bout the mechanisms involved in the reactions of binuclear cluster complexes. Also summarized are studies of the reactions of related mononuclear complexes which have provided information critical to understanding the chemistry of these binuclear system.

Bergman, Robert G.

1980-01-01

369

Ion conductivity of poly(ethylene oxide)\\/ligand complexes with the hydrogen-bond interaction  

Microsoft Academic Search

New polymer\\/ligand complexes could be easily obtained due to the molecular self-assembly of carboxylic acid and hydrogen acceptor fragments through the intermolecular hydrogen-bonding. The polymer\\/ligand complexes consisting of poly(ethyleneglycol)bis(carboxymethyl)ether (PEO?CA), operating as a hydrogen-bonding donor, and pyrazine, 4,4?-bipyridine, or 1,3,5-triazine, serving as a hydrogen-bonding acceptor, exhibited a gel-like structure at room temperature. The high ion conductivity of the order of

Koji Tsuruhara; Kazuhiro Hara; Mitsuyasu Kawahara; Masahiro Rikukawa; Kohei Sanui; Naoya Ogata

2000-01-01

370

Molybdenum 17- and 18-electron bis- and tris(butadiene) complexes: electronic structures, spectroscopic properties, and oxidative ligand substitution reactions.  

PubMed

New results on the electronic structures, spectroscopic properties, and reactivities of the molybdenum tris(butadiene) and tris(2,3-dimethylbutadiene) complexes [Mo(bd)3] (1(bd)) and [Mo(dmbd)3] (1(dmbd)), respectively, are reported. Importantly, the metal ligand bonding interaction can be weakened by oxidizing the metal center with ferrocenium salts. The addition of the bidentate phosphine ligand 1,2-bis(diphenylphosphino)ethane then leads to a new type of stable 17-electron complex, [Mo(dmbd)2(dppe)](X) (2; X = BF4(-), PF6(-), BPh4(-)), where one of the butadiene ligands is exchanged by a chelating phosphine. Reduction of the cationic complexes 2 generates the corresponding 18-electron complex [Mo(dmbd)2(dppe)] (3), thus establishing a new strategy for ligand substitution reactions in [Mo(bd)3] complexes via one-electron oxidized intermediates. The new heteroleptic molybdenum complexes are characterized by X-ray structure analysis; vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT calculations are performed to explain the structural and specroscopic trends observed experimentally. For compound 1(bd), a normal coordinate analysis is presented, providing additional information on the bonding situation in this type of complex. PMID:23627292

Stephan, Gerald C; Nther, Christian; Peters, Gerhard; Tuczek, Felix

2013-04-29

371

Methionine oxidation by reactive oxygen species: reaction mechanisms and relevance to Alzheimer's disease.  

PubMed

The oxidation of methionine plays an important role in vivo, during biological conditions of oxidative stress, as well as for protein stability in vitro. Depending on the nature of the oxidizing species, methionine may undergo a two-electron oxidation to methionine sulfoxide or one-electron oxidation to methionine radical cations. Both reaction mechanisms derive catalytic support from neighboring groups, which stabilize electron-deficient reaction centers. In vivo, methionine sulfoxide is subject to reduction by the methionine sulfoxide reductase (Msr) system, suggesting that some methionine sulfoxide residues may only be transiently involved in the deactivation of proteins through reactive oxygen species (ROS). Other methionine sulfoxide residues may accumulate, depending on the accessibility to Msr. Moreover, methionine sulfoxide levels may increase as a result of a lower abundance of active Msr and/or the required cofactors as a consequence of pathologies and biological aging. On the other hand, methionine radical cations will enter predominantly irreversible reaction channels, which ultimately yield carbon-centered and/or peroxyl radicals. These may become starting points for chain reactions of protein oxidation. This review will provide detailed mechanistic schemes for the reactions of various prominent, biologically relevant ROS with methionine and organic model sulfides. Emphasis will be given on the one-electron oxidation pathway, characterizing the physico-chemical parameters, which control this mechanism, and its physiological relevance, specifically for the oxidation and neurotoxicity of the Alzheimer's disease beta-amyloid peptide (betaAP). PMID:15680219

Schneich, Christian

2004-10-27

372

Photochemistry and proton transfer reaction chemistry of selected cinnamic acid derivatives in hydrogen bonded environments  

NASA Astrophysics Data System (ADS)

Proton transfer reactions between cinnamic acid derivatives (MH) and ammonia are studied using a time-of-flight mass spectrometer equipped with a supersonic nozzle to entrain neutral species formed by 337 nm laser desorption. The supersonic nozzle is used to form clusters of the type MH(NH3)n where n ranges to numbers greater than 20. Multimeric clusters of MH, e.g. MH2(NH3)n are not detected in this experiment or are of low abundance. Photoexcitation of MH(NH3)n clusters by using 355 nm photons yields ionic species that correspond to direct multiphoton ionization, e.g. MH+[middle dot](NH3)n, and proton transfer reactions, e.g. H+(NH3)n. Analogous product ions are formed by photoexcitation of the methylamine, MH(CH3NH2)n, and ammonia/methanol, MH(NH3)(CH3OH)n, clusters. Detailed analysis of energetics data suggests that proton transfer occurs through neutral excited stare species, and a mechanism analogous to one proposed previously is used to rationalize the data. The energetics of proton transfer via a radical cation form of the cinnarnic acid dimer is also consistent with the data. The relevance of this work to fundamental studies of matrix-assisted laser desorption ionization (MALDI) is discussed. In particular, the role of excited state proton transfer (ESPT) in MALDI is discussed.

Huang, Yong; Russell, David H.

1998-05-01

373

Oxidation reactions catalyzed by vanadium chloroperoxidase from Curvularia inaequalis  

Microsoft Academic Search

Vanadium haloperoxidases have been reported to mediate the oxidation of halides to hypohalous acid and the sulfoxidation of organic sulfides to the corresponding sulfoxides in the presence of hydrogen peroxide. However, traditional heme peroxidase substrates were reported not to be oxidized by vanadium haloperoxidases. Surprisingly, we have now found that the recombinant vanadium chloroperoxidase from the fungus Curvularia inaequalis catalyzes

Hilda B ten Brink; Henk L Dekker; Hans E Schoemaker; Ron Wever

2000-01-01

374

Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels  

SciTech Connect

Oscillating reaction rates have been observed in the steam oxidation of 2 1/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500-550C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.

Hurdus, M.H.; Tomlinson, L.; Titchmarsh, J.M. (AEA Technology, Oxfordshire (England))

1990-12-01

375

Sporicidal effects of iodine-oxide thermite reaction products  

NASA Astrophysics Data System (ADS)

Iodine pentoxide-aluminum thermite reactions have been driven by impacts at 1000 m/s on steel plates 3 mm or thicker. This reaction releases iodine gas that is known to be a sporicide. To test the impact reactions for sporicidal effects, reactions took place in closed chambers containing dried Bacillus subtilis spores. The reduction in colony-forming units was dependent on the exposure time; long exposure times resulted in a 105 decrease in germination rate. This was shown to be due to the gas exposure rather than the heat or turbulence. Sporicidal effectiveness was increased by adding neodymium and saran resin.

Russell, Rod; Bless, Stephan; Blinkova, Alexandra; Chen, Tiffany

2012-03-01

376

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities  

Microsoft Academic Search

A classification of the hydrogen fluoride H-F-bonded interactions comprising a large number of molecules has been proposed by Espinosa et al. [J. Chem. Phys. 117, 5529 (2002)] based on the ratio |V(rc)|\\/G(rc) where |V(rc)| is the magnitude of the local potential-energy density and G(rc) is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of

Gerald V. Gibbs; David Cox; T Daniel Crawford; Kevin M. Rosso; Nancy Ross; R. T. Downs

2006-01-01

377

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities  

Microsoft Academic Search

A classification of the hydrogen fluoride HF-bonded interactions comprising a large number of molecules has been proposed by Espinosa &etal; [J. Chem. Phys. 117, 5529 (2002)] based on the ratio |V(rc)|?G(rc) where |V(rc)| is the magnitude of the local potential-energy density and G(rc) is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of the

G. V. Gibbs; D. F. Cox; T. D. Crawford; K. M. Rosso; N. L. Ross; R. T. Downs

2006-01-01

378

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities  

Microsoft Academic Search

molecules has been proposed by Espinosa et al. J. Chem. Phys. 117, 5529 2002 based on the ratio Vrc\\/Grc where Vrc is the magnitude of the local potential-energy density and Grc is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of

G. V. Gibbsa; T. D. Crawford; N. L. Ross

379

Bulk Gold-Catalyzed Reactions of Isocyanides, Amines, and Amine N-Oxides  

SciTech Connect

Bulk gold powder (550 ?m particles) catalyzes the reactions of isocyanides with amines and amine N-oxides to produce ureas. The reaction of n-butyl isocyanide (nBuN?C) with di-n-propylamine and N-methylmorpholine N-oxide in acetonitrile, which was studied in the greatest detail, produced 3-butyl-1,1-dipropylurea (O?C(NHnBu)(NnPr2)) in 99% yield at 60 C within 2 h. Sterically and electronically different isocyanides, amines, and amine N-oxides react successfully under these conditions. Detailed studies support a two-step mechanism that involves a gold-catalyzed reaction of adsorbed isocyanide with the amine N-oxide to form an isocyanate (RN?C?O), which rapidly reacts with the amine to give the urea product. These investigations show that bulk gold, despite its reputation for poor catalytic activity, is capable of catalyzing these reactions.

Klobukowski, Erik; Angelici, Robert; Woo, Keith L.

2012-01-26

380

Reactions of sulfur dioxide with ammonia: Dependence on oxygen and nitric oxide  

SciTech Connect

The influence of oxygen and nitric oxide on the reactions of sulfur dioxide with ammonia were studied in a simulated flue gas in the range of 0--20% oxygen and 0--300 ppm nitric oxide at temperatures in the range of 40--60 C. A Fourier transform infrared spectrometer (FT-IR) analyzed the reaction products deposited on the reactor surface and revealed that ammonium sulfate was the main product of the reactions, with sulfamic acid and ammonium sulfamate as the minor products. The results showed that oxygen and nitric oxide enhanced the oxidation reactions of sulfur dioxide to form ammonium sulfate. The yield of the minor products markedly increased in the presence of nitrogen dioxide. The size and number concentration of product aerosols increased at lower temperature. The fraction of sulfur dioxide which formed aerosols increased with sulfur dioxide removal.

Hirota, Koichi; Maekelae, J.; Tokunaga, Okihiro [Japan Atomic Energy Research Inst., Takasaki, Gunma (Japan)

1996-10-01

381

Oxidative tailoring reactions catalyzed by nonheme iron-dependent enzymes: streptorubin B biosynthesis as an example.  

PubMed

Tailoring enzymes catalyze reactions that modify natural product backbone structures before, during, or after their biosynthesis to create a final product with specific biological activities. Such reactions can be catalyzed by a myriad of different enzyme families and are responsible for a wide variety of transformations including regio- and/or stereospecific acylation, alkylation, glycosylation, halogenation, and oxidation. Within a broad group of oxidative tailoring enzymes, there is a rapidly growing family of nonheme iron- and oxygen-dependent enzymes that catalyze a variety of remarkable hydroxylation, desaturation, halogenation, and oxidative cyclization reaction in the biosynthesis of several important metabolites, including carbapenems, penicillins, cephalosporins, clavams, prodiginines, fosfomycin, syringomycin, and coronatine. In this chapter, we report an expedient method for analyzing tailoring enzymes that catalyze oxidative cyclization reactions in prodiginine biosynthesis via expression of the corresponding genes in a heterologous host, feeding of putative biosynthetic intermediates to the resulting strains, and liquid chromatography-mass spectrometry analyses of the metabolites produced. PMID:23034230

Sydor, Paulina K; Challis, Gregory L

2012-01-01

382

Hybrid Quantum/Classical Molecular Dynamics Simulations of the Proton Transfer Reactions Catalyzed by Ketosteroid Isomerase: Analysis of Hydrogen Bonding, Conformational Motions, and Electrostatics  

PubMed Central

Hybrid quantum/classical molecular dynamics simulations of the two proton transfer reactions catalyzed by ketosteroid isomerase are presented. The potential energy surfaces for the proton transfer reactions are described with the empirical valence bond method. Nuclear quantum effects of the transferring hydrogen increase the rates by a factor of ~8, and dynamical barrier recrossings decrease the rates by a factor of 34. For both proton transfer reactions, the donor-acceptor distance decreases substantially at the transition state. The carboxylate group of the Asp38 side chain, which serves as the proton acceptor and donor in the first and second steps, respectively, rotates significantly between the two proton transfer reactions. The hydrogen bonding interactions within the active site are consistent with the hydrogen bonding of both Asp99 and Tyr14 to the substrate. The simulations suggest that a hydrogen bond between Asp99 and the substrate is present from the beginning of the first proton transfer step, whereas the hydrogen bond between Tyr14 and the substrate is virtually absent in the first part of this step but forms nearly concurrently with the formation of the transition state. Both hydrogen bonds are present throughout the second proton transfer step until partial dissociation of the product. The hydrogen bond between Tyr14 and Tyr55 is present throughout both proton transfer steps. The active site residues are more mobile during the first step than during the second step. The van der Waals interaction energy between the substrate and the enzyme remains virtually constant along the reaction pathway, but the electrostatic interaction energy is significantly stronger for the dienolate intermediate than for the reactant and product. Mobile loop regions distal to the active site exhibit significant structural rearrangements and, in some cases, qualitative changes in the electrostatic potential during the catalytic reaction. These results suggest that relatively small conformational changes of the enzyme active site and substrate strengthen the hydrogen bonds that stabilize the intermediate, thereby facilitating the proton transfer reactions. Moreover, the conformational and electrostatic changes associated with these reactions are not limited to the active site but rather extend throughout the entire enzyme.

Chakravorty, Dhruva K.; Soudackov, Alexander V.; Hammes-Schiffer, Sharon

2009-01-01

383

Mechanism of the Quenching of the Tris(bipyridine)ruthenium(II) Emission by Persulfate: Implications for Photoinduced Oxidation Reactions.  

PubMed

A revised mechanism for the oxidation of the excited state of Ru(bpy)3(2+) with the persulfate anion is described in this work. The formation of the precursor complex in the electron transfer reaction involves ion pairing between the metal complex in ground and excited states and S2O8(2-). The equilibrium constant for the ion-pair formation (KIP = 2.7 M(-1)) was determined from electrochemical measurements and analysis of thermal reaction between Ru(bpy)3(2+) and persulfate. It was found to be consistent with the calculated value estimated from the Debye-Hckel model. The analysis of rate constants for reactions between persulfate and various metal complexes indicates that thermal and photochemical reactions most likely proceed through a common pathway. Extremely high reorganization energy (ca. 3.54 eV) for the electron transfer obtained from fitting experimental data with the Marcus equation is indicative of significant nuclear reorganization during the electron transfer step. In view of these results the electron transfer can be described as dissociative probably involving substantial elongation or complete scission of the O-O bond. The proposed model accurately describes experimental results for the quenching of *Ru(bpy)3(2+) over a wide range of persulfate concentrations and resolves some discrepancies between the values of KIP and ket previously reported. The implications of various factors such as the ionic strength and dielectric constant of the medium are discussed in relation to measurements of the quantum yields in photodriven oxidation reactions employing the Ru(bpy)3(2+)/persulfate couple. PMID:24040757

Lewandowska-Andralojc, A; Polyansky, D E

2013-09-26

384

Reaction Mechanism of Oxidative Carbonylation of Methanol to Dimethyl Carbonate in CuY Zeolite  

Microsoft Academic Search

The Cu(I)Y catalyst, as prepared by solid-state ion exchange at 650C, shows higher productivity and less deactivation than the known carbon-supported CuCl2catalyst for gas-phase oxidative carbonylation of methanol to make dimethyl carbonate. The ion-exchanged Cu(II)Y showed very little activity for the same reaction. Anin situFTIR technique was applied to elucidate the reaction mechanism. The first step is the oxidation of

S. T King

1996-01-01

385

Antioxidation reaction mechanism studies of phenolic lignans, identification of antioxidation products of secoisolariciresinol from lipid oxidation  

Microsoft Academic Search

The chain-breaking antioxidation reaction mechanism of a phenolic lignan in a lipid oxidation system was investigated. The 2,2?-azobis(isobutyronitrile) (AIBN)-induced radical oxidation reaction in a large amount of ethyl linoleate in the presence of secoisolariciresinol, one of the potent antioxidative lignans widely distributed in edible plants, produced two types of peroxides as radical termination products, as well as a cyclic derivative.

Toshiya Masuda; Jun Akiyama; Aya Fujimoto; Satoshi Yamauchi; Tomomi Maekawa; Yoshiaki Sone

2010-01-01

386

The effect of inherent and added inorganic matter on low-temperature oxidation reaction of coal  

Microsoft Academic Search

The influence of inherent and added inorganic matter on low-temperature oxidation reactions of coal and the effectiveness of the additives to affect the oxidation reactions are examined in this paper. A Victorian brown coal was selected for this study. Samples of the raw coal, water-washed coal, and acid-washed coal were prepared. The acid-washed coal was also doped with seven additives,

Wiwik S Watanabe; Dong-ke Zhang

2001-01-01

387

A heterogeneous layered bifunctional catalyst for the integration of aerobic oxidation and asymmetric C-C bond formation.  

PubMed

The design and synthesis of a heterogeneous bifunctional chiral catalyst for the sequential aerobic oxidation-asymmetric Michael reactions between primary allylic alcohols and dibenzyl malonate are described. Interestingly, we found that layering bimetallic nanoparticles over the organocatalyst, within the chiral composite material, is crucial for catalytic activity. PMID:24036576

Miyamura, Hiroyuki; Choo, Gerald C Y; Yasukawa, Tomohiro; Yoo, Woo-Jin; Kobayashi, Shu

2013-10-01

388

The interaction of reaction-bonded silicon carbide and inconel 600 with a nickel-based brazing alloy  

NASA Astrophysics Data System (ADS)

The objective of the present research was to join reaction-bonded silicon carbide (RBSC) to INCONEL 600 (a nickel-based superalloy) for use in advanced heat engine applications using either direct brazing or composite interlayer joining. Direct brazing experiments employed American Welding Society (AWS) BNi-5, a commercial nickel-based brazing alloy, as a filler material; composite interlayers consisted of intimate mixtures of ?-SiC and BNi-5 powders. Both methods resulted in the liquid filler metal forming a Ni-Si liquid with the free Si in the RBSC, which, in turn, reacted vigorously with the SiC component of the RBSC to form low melting point constituents in both starting materials and Cr carbides at the metal-ceramic interface. Using solution thermodynamics, it was shown that a Ni-Si liquid of greater than 60 at. pct Ni will decompose a-SiC at the experimental brazing temperature of 1200 C; these calculations are consistent with the experimentally observed composition profiles and reaction morphology within the ceramic. It was concluded that the joining of RBSC to INCONEL 600 using a nickel-based brazing alloy is not feasible due to the inevitability of the filler metal reacting with the ceramic, degrading the high-temperature properties of the base materials.

McDermid, J. R.; Pugh, M. D.; Drew, R. A. L.

1989-09-01

389

Revealing the involvement of extended hydrogen-bond networks in the cooperative function between distant sites in bacterial reaction centres.  

SciTech Connect

In reaction center proteins of photosynthetic bacteria, the amplitude of proton uptake induced by the one-electron reduction of either of the two quinone electron acceptors (Q{sub A} and Q{sub B}) is an intrinsic observable of the electrostatic interactions associated with the redox function of the complex. We report here that, in Rhodobacter capsulatus, complete restoration of proton uptake (upon formation of Q{sub A}{sup -} and Q{sub B}{sup -}) to the level found in the wild type is observed in a mutant reaction center in which a tyrosine substitution in the Q{sub A} environment (AlaM247 {yields} Tyr) is coupled with mutations of acidic residues near QB (GluL212 right-arrow Ala/AspL213 right-arrow Ala) that initially cancel the proton uptake above pH 8. This result demonstrates that proton uptake occurs by strong cooperation between structural motifs, such as hydrogen-bonded networks, that span the 18 Angstroms distance between the two quinone acceptors.

Tandori, J.; Baciou, L.; Alexov, E.; Maroti, P.; Schiffer, M.; Hanson, D. K.; Sebban, P.; Biosciences Division; CNRS; Univ. of Szeged; Columbia Univ.

2001-12-07

390

Inhibition of C-H and C-O bond activation by surface oxygen: Stabilization of surface phenoxide in the reaction of phenol on oxygen-precovered Mo(110)  

SciTech Connect

The presence of surface oxygen on Mo(110) is shown to increase the kinetic stability of surface phenoxide formed after saturation phenol exposure with respect to decomposition. Temperature-programmed reaction and x-ray photoelectron spectroscopies have been used to study the reactions of phenol (C{sub 6}H{sub 5}OH) on oxygen-precovered Mo(110) ({theta}{sub 0} = 0.33). At saturation coverage, surface phenoxide is formed from cleavage of the phenol O-H bond at temperatures below 400 K and is stable on the surface to 650 K, where it undergoes disproportionation to gaseous phenol (E{sub d} = 42 kcal/mol) and the decomposition products gaseous water, gaseous dihydrogen, and surface carbon. A trace amount of benzene is also produced from decomposition of surface phenoxide. The hydrogen released by O-H bond cleavage in phenol at low temperature reacts with oxygen in the oxide overlayer to form gaseous water at temperatures below 400 K. Intact molecular phenol is also desorbed at temperatures below 350 K. The atomic oxygen overlayer dramatically stabilizes C-O and C-H bonds in the surface phenoxide species, which are cleaved at {approx} 370 K on the clean Mo(110) surface. The stability of surface phenoxide on the oxygen-precovered surface is dependent on the coverage of phenol-derived surface species, with decomposition occurring at lower temperature for lower phenol exposures. No gaseous phenol is reformed from phenoxide for low phenol exposures.

Serafin, J.G.; Fried, C.M. (Harvard Univ., Cambridge, MA (USA))

1989-06-07

391

Oxidation kinetics of reaction products formed in uranium metal corrosion.  

SciTech Connect

The oxidation behavior of uranium metal ZPPR fuel corrosion products in environments of Ar-4%O{sub 2} and Ar-20%O{sub 2} were studied using thermo-gravimetric analysis (TGA). These tests were performed to extend earlier work in this area specifically, to assess plate-to-plate variations in corrosion product properties and the effect of oxygen concentration on oxidation behavior. The corrosion products from two relatively severely corroded plates were similar, while the products from a relatively intact plate were not reactive. Oxygen concentration strongly affected the burning rate of reactive products, but had little effect on low-temperature oxidation rates.

Totemeier, T. C.

1998-04-22

392

Oxides reactions with a high-chrome sesquioxide refractory  

SciTech Connect

In slagging coal-gasifier systems, the combination of oxides present as impurities in coal and combustion temperatures that can exceed 1650 degrees C restrict the use of liner materials in the coal combustion chambers to refractories. In this study, the slag-refractory interactions of a new high chrome sesquioxide refractory was characterized. High-temperature cup tests showed that the molten oxides infused into the refractory and that the sesquioxide refractory reacts with the oxides in a manner similar to spinel phase refractories. Studies of the coal slags individual oxide components showed CaO reacts with the chrome refractory to form a low melting Ca(CrO2)2. FeO reacts with the sesquioxide to form a interface layer of (Cr,Fe)3O4 spinel phase. Results of this study now make it possible to design studies for improving corrosion resistance to increase refractory life.

Rawers, James C.; Collins, W. Keith; Peck, M.

2001-10-01

393

Highly active catalysts of bisphosphine oxides for asymmetric Heck reaction.  

PubMed

Bisphosphine oxides formed highly active asymmetric Heck catalysts, which were applied in asymmetric synthesis of pharmacologically active azacycles. Olefin insertion proceeded via cis pathways, different from P,N-ligands. PMID:24005808

Hu, Jian; Lu, Yunpeng; Li, Yongxin; Zhou, Jianrong Steve

2013-10-21

394

Pi-nucleophilicity in carbon-carbon bond-forming reactions.  

PubMed

Which electrophiles react with which nucleophiles? The correlation log k(20 degrees Celsius) = s(E + N), in which electrophiles (carbocations, metal-pi-complexes, diazonium ions) are characterized by one (E) and nucleophiles are characterized by two parameters (N, s), proved to be applicable for a wide variety of electrophile-nucleophile combinations. Since the introduction of this correlation in 1994 (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), numerous new reagents have been characterized, and in 2001 (J. Am. Chem. Soc. 2001, 123, 9500-9512), a new method of parametrization was proposed that facilitates a continuous extension of the data sets without the need for reparametrization of existing data. This Account adjusts the N and s parameters of all presently characterized pi-nucleophiles (arenes, alkenes, organometallics) to the new parametrization and illustrates how to employ the resulting reactivity scales for analyzing synthetic and mechanistic problems in organic and macromolecular chemistry. Predictions of absolute rate constants, inter- and intramolecular selectivities, and analyses of reaction mechanisms are discussed. We outline how new compounds can be added to the scales and present our view on the scope and limitations of this approach to polar organic reactivity. PMID:12534306

Mayr, Herbert; Kempf, Bernhard; Ofial, Armin R

2003-01-01

395

Bond cleavage reactions in oxygen and nitrogen heterocycles by a rhodium phosphine complex  

SciTech Connect

The reactions of (C[sub 5]Me[sub 5])Rh(PMe[sub 3])PhH with furan, 2,5-dimethylfuran, 2,3-dihydrofuran, dibenzofuran, pyrrole, 1-methylpyrrole, 2,5-dimethylpyrrole, 1,2,5-trimethylpyrrole, carbazole, 9-methylcarbazole, pyrrolidine, pyridine, 3,5-lutidine, 2,4,6-collidine, pyrazole, 3-methylpyrazole, and piperidine have been investigated. While the oxygen heterocycles give only C-H activation, the nitrogen heterocycles yield C-H and N-H insertion products. The chloro derivative (C[sub 5]Me[sub 5])Rh(PMe[sub 3])[2-(1-methylpyrrole)]Cl was found to crystallize in the monoclinic space group C2/c with a = 13.753 (6) A, b = 9.665 (5) A, c = 30.14 (2) A, [beta] = 99.77 (5)-[degree], Z = 8, and V = 3949 (4.1) A[sup 3] while (C[sub 5]Me[sub 5])Rh(PMe[sub 5])[2-(3,5-lutidine)]Cl was found to crystallize in the monoclinic space group P2[sub 1]/c with a = 14.976 (8) A, b = 8.613 (5) A, c = 17.12 (2) A, [beta] = 101.90 (6)[degree], Z = 4, and V = 2160 (5.2) A[sup 3]. 30 refs., 2 figs., 3 tabs.

Jones, W.D.; Dong, L.; Myers, A.W. (Univ. of Rochester, NY (United States))

1995-02-01

396

AQUEOUS-PHASE OXIDATION OF SLUDGE USING THE VERTICAL REACTION VESSEL SYSTEM  

EPA Science Inventory

The overall objective of the study was to provide plant-scale operating data on the wet-oxidation of municipal wastewater sludge utilizing the Vertical Reaction Vessel System and the effect of the return flow from the wet-oxidation process on the operation of the wastewater treat...

397

Heterogeneous oxidation reactions relevant to tropospheric aerosol chemistry studied by sum frequency generation  

Microsoft Academic Search

Unsaturated organic molecules (terpenes) that commonly form molecular films on tropospheric aerosols can be oxidized by ozone, influencing the microphysics of cloud formation and thus the earth's climate. Using a laboratory approach that combines organic synthesis with surface spectroscopy, we track the ozone oxidation reactions of tropospherically relevant terpenes bound to glass surfaces that serve as mimics for mineral dust.

Grace Stokes; Avram Buchbinder; Julianne Gibbs-Davis; Karl Scheidt; Franz Geiger

2008-01-01

398

EFFECTS OF SOLAR RADIATION ON MANGANESE OXIDE REACTIONS WITH SELECTED ORGANIC COMPOUNDS  

EPA Science Inventory

The effects of sunlight on aqueous redox reactions between manganese oxides (MnOx) and selected organic substances are reported. o sunlight-induced rate enhancement was observed for the MnOx oxidation of substituted phenols, anisole, o-dichlorobenzene, or p-chloroaniline. n the o...

399

Reaction of ethylene oxide or propylene oxide with long-chain fatty acids. Mono and diester formation  

Microsoft Academic Search

SummaryThe alkali-catalyzed reaction of ethylene oxide or propylene oxide with fatty acids was shown to be complex. Mono- and diesters\\u000a were formed in comparable amounts even at an early stage when only about one equivalent of the cyclic ether had been condensed.\\u000a \\u000a The following compounds were isolated in a pure state: ethylene glycol monolaurate, ethylene glycol dilaurate, diethylene\\u000a glycol dilaurate,

A. N. Wrigley; F. D. Smith; A. J. Stirton

1959-01-01

400

Rapid and effective oxidative pretreatment of woody biomass at mild reaction conditions and low oxidant loadings  

PubMed Central

Background One route for producing cellulosic biofuels is by the fermentation of lignocellulose-derived sugars generated from a pretreatment that can be effectively coupled with an enzymatic hydrolysis of the plant cell wall. While woody biomass exhibits a number of positive agronomic and logistical attributes, these feedstocks are significantly more recalcitrant to chemical pretreatments than herbaceous feedstocks, requiring higher chemical and energy inputs to achieve high sugar yields from enzymatic hydrolysis. We previously discovered that alkaline hydrogen peroxide (AHP) pretreatment catalyzed by copper(II) 2,2?-bipyridine complexes significantly improves subsequent enzymatic glucose and xylose release from hybrid poplar heartwood and sapwood relative to uncatalyzed AHP pretreatment at modest reaction conditions (room temperature and atmospheric pressure). In the present work, the reaction conditions for this catalyzed AHP pretreatment were investigated in more detail with the aim of better characterizing the relationship between pretreatment conditions and subsequent enzymatic sugar release. Results We found that for a wide range of pretreatment conditions, the catalyzed pretreatment resulted in significantly higher glucose and xylose enzymatic hydrolysis yields (as high as 80% for both glucose and xylose) relative to uncatalyzed pretreatment (up to 40% for glucose and 50% for xylose). We identified that the extent of improvement in glucan and xylan yield using this catalyzed pretreatment approach was a function of pretreatment conditions that included H2O2 loading on biomass, catalyst concentration, solids concentration, and pretreatment duration. Based on these results, several important improvements in pretreatment and hydrolysis conditions were identified that may have a positive economic impact for a process employing a catalyzed oxidative pretreatment. These improvements include identifying that: (1) substantially lower H2O2 loadings can be used that may result in up to a 50-65% decrease in H2O2 application (from 100 mg H2O2/g biomass to 3550 mg/g) with only minor losses in glucose and xylose yield, (2) a 60% decrease in the catalyst concentration from 5.0 mM to 2.0 mM (corresponding to a catalyst loading of 25 ?mol/g biomass to 10 ?mol/g biomass) can be achieved without a subsequent loss in glucose yield, (3) an order of magnitude improvement in the time required for pretreatment (minutes versus hours or days) can be realized using the catalyzed pretreatment approach, and (4) enzyme dosage can be reduced to less than 30 mg protein/g glucan and potentially further with only minor losses in glucose and xylose yields. In addition, we established that the reaction rate is improved in both catalyzed and uncatalyzed AHP pretreatment by increased solids concentrations. Conclusions This work explored the relationship between reaction conditions impacting a catalyzed oxidative pretreatment of woody biomass and identified that significant decreases in the H2O2, catalyst, and enzyme loading on the biomass as well as decreases in the pretreatment time could be realized with only minor losses in the subsequent sugar released enzymatically. Together these changes would have positive implications for the economics of a process based on this pretreatment approach.

2013-01-01

401

Studies on high temperature corrosion reactions involving metal oxides and sodium sulfate  

Microsoft Academic Search

Purpose The purpose of this paper is to present the results of studies on the reaction of metal oxides such as Cr2O3 and Al2O3 with Na2SO4 in flowing SO2 (g) at 1,100 and 1,200 K. Design\\/methodology\\/approach The oxides chosen for the studies were initial scaling products during the oxidation of industrial alloys and invariably are involved in hot-corrosion

M. Mobin; S. K. Hasan

2008-01-01

402

Electrocatalytic activity of nanosized Pt alloys in the methanol oxidation reaction  

Microsoft Academic Search

The electrocatalysis of the methanol oxidation reaction on carbon supported nanosized Pt and PtM (M: Ru, Pd, Rh and Sn) electrocatalysts was investigated in 0.5M H2SO4+0.1M CH3OH at room temperature by cyclic voltammetry and electrochemical impedance spectroscopy. The onset potential for methanol oxidation at GC\\/PtRu\\/V was shifted to lower potentials and showed improved specific activity towards the methanol oxidation compared

I. vila-Garca; C. Ramrez; J. M. Hallen Lpez; E. M. Arce Estrada

2010-01-01

403

Iron and hydroxyl radicals in lipid oxidation: Fenton reactions in lipid and nucleic acids co-oxidized with lipid  

SciTech Connect

Hydroxyl radicals can initiate lipid peroxidation in liquids, but their high reactivity affords reaction paths so short that they are unlikely to reach lipids in membrane bilayers when formed exteriorly. EPR studies of Fenton-like reactions inducing oxidation in bulk lipids indicate that iron-dependent initiation of lipid oxidation in organelles and vesicles may result from hydroxyl radicals formed within the hydrophobic membrane interiors, where they would be inaccessible to typical hydrophilic radical scavengers. The cytotoxic or cytogenetic results of lipid peroxidation, especially in nuclear membranes, may include radiominetic chemical damage to adjacent DNA or nucleoprotein. Preliminary product analyses of nucleic acid basis cooxidized with lipids in vitro support this view.

Borg, D.C.; Schaich, K.M.

1987-01-01

404

Preparation of recoverable Ru catalysts for liquid-phase oxidation and hydrogenation reactions  

Microsoft Academic Search

We here report the synthesis, characterization and catalytic performance of new supported Ru(III) and Ru(0) catalysts. In contrast to most supported catalysts, these new developed catalysts for oxidation and hydrogenation reactions were prepared using nearly the same synthetic strategy, and are easily recovered by magnetic separation from liquid phase reactions. The catalysts were found to be active in both forms,

Marcos J. Jacinto; Osvaldo H. C. F. Santos; Renato F. Jardim; Richard Landers; Liane M. Rossi

2009-01-01

405

Microwave induced reactions of sulfur dioxide and nitrogen oxides in char and anthracite bed  

Microsoft Academic Search

Microwaves applied to a pyrolytic carbon matrix enhance the chemical reactions of nitric oxide (NO) and sulfur dioxide (SO2) with carbon to produce nitrogen, sulfur, and carbon dioxide. These microwave-induced reactions were investigated to find the feasibility of applying microwaves to directly destroy NO and SO2 in the combustion product gases or to minimize the formation of these pollutants during

Chang Yul Cha; Dong Sik Kim

2001-01-01

406

Kinetics of the reaction of 4,4'-methylenedianiline with propylene oxide in ethylene glycol  

Microsoft Academic Search

The kinetics for the reaction of 4,4'- methylenedianiline, MDA, with propylene oxide in ethylene glycol as a solvent to generate secondary and tertiary aromatic alkanolamines were investigated. This reaction is important in converting glycolyzed, liquefied, recycled polyurethane foam, containing high levels of MDA and primary aromatic amines, into a high-performance, recycled polyol substitute that can be used in the manufacture

R. M. Machado; J. W. Mitchell; J. P. Bullock; B. E. Farrell

1996-01-01

407

Ionic Polymerization of 1,2Butylene Oxide: NMR Study of the Reaction Products  

Microsoft Academic Search

An NMR study on the reaction products of the ionic polymerization of 1,2-butylene oxide has been carried out. Polymers prepared via a cationic mechanism by using a trityl salt as the initiator are built up of repeat monomer units, and the propagation reaction follows Bernoullian statistics. Polymers prepared via an anionic mechanism with the use of sodium metal as initiator,

S. L. Malhotra; L. P. Blanchard

1977-01-01

408

Mutagenicity screening of reaction products from the enzyme-catalyzed oxidation of phenolic pollutants  

SciTech Connect

Phenol-oxidizing enzymes such as peroxidases, laccases, and mushroom polyphenol oxidase are capable of catalyzing the oxidation of a wide range of phenolic pollutants. Although the use of these enzymes in waste-treatment applications has been proposed by a number of investigators, little information exists on the toxicological characteristics of the oxidation products. The enzymes chloroperoxidase, horseradish peroxidase, lignin peroxidase, and mushroom polyphenol oxidase were used in this study to catalyze the oxidation of phenol, several mono-substituted phenols, and pentachlorophenol. Seventeen reaction mixtures representing selected combinations of enzyme and parent phenol were subjected to mutagenicity screening using the Ames Salmonella typhimurium plate incorporation assay; five selected mixtures were also incubated with the S9 microsomal preparation to detect the possible presence of promutagens. The majority of reaction mixtures tested were not directly mutagenic, and none of those tested with S9 gave a positive response. Such lack of mutagenicity of enzymatic oxidation products provides encouragement for establishing the feasibility of enzyme-catalyzed oxidation as a waste-treatment process. The only positive responses were obtained with reaction products from the lignin peroxidase-catalyzed oxidation of 2-nitrophenol and 4-nitrophenol. Clear positive responses were observed when strain TA100 was incubated with 2-nitrophenol reaction-product mixtures, and when strain TA98 was incubated with the 4-nitrophenol reaction mixture. Additionally, 2,4-dinitrophenol was identified as a reaction product from 4-nitrophenol, and preliminary evidence indicates that both 2,4- and 2,6-dinitrophenol are produced from the oxidation of 2-nitrophenol. Possible mechanism by which these nitration reactions occur are discussed.

Massey, I.J.; Aitken, M.D.; Ball, L.M.; Heck, P.E. (Univ. of North Carolina, Chapel Hill, NC (United States). Dept. of Environmental Sciences and Engineering)

1994-11-01

409