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1

Mechanisms and kinetics of reaction-bonded aluminium oxide ceramics  

Microsoft Academic Search

Reaction-bonded Al[sub 2]O[sub 3] (RBAO) ceramics were fabricated starting from mechanically alloyed Al[sub 2]O[sub 3]\\/Al, Al[sub 2]O[sub 3]\\/Al\\/ZrO[sub 2], and Al[sub 2]O[sub 3]\\/Al\\/ZrO[sub 2]\\/Zr mixtures. Isopressed compacts were heat-treated in air up to 1,550 C. Reaction-bonding mechanisms, kinetics, and the influence of ZrO[sub 2] and Zr additions are investigated. Independent of additive, oxidation of Al proceeds both as solid\\/gas and

Suxing Wu; Dietmar Holz; Nils Claussen

1993-01-01

2

Ignition phenomena and controlled firing of reaction-bonded aluminum oxide  

Microsoft Academic Search

The ignition phenomena that can occur during the reaction-bonding of aluminum oxide is investigated. It is experimentally characterized through measurements of the combustion temperature and the velocity of the ignition front. One- and two-dimensional, transient, simultaneous material and energy balances are used to model the phenomena in a cylindrical geometry. The models are used to predict the aluminum concentration, oxygen

Matthew James Watson

1999-01-01

3

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

4

C-H bond oxidation initiated pummerer- and Knoevenagel-type reactions of benzyl sulfide and 1,3-dicarbonyl compounds.  

PubMed

A novel Pummerer-type reaction was developed via o-chloranil-mediated C-H bond oxidation. The reaction provides a simple and efficient method to construct sulfide derivatives. A Knoevenagel-type reaction was also achieved via multiple C-H bonds activation under neutral reaction conditions. PMID:18237178

Li, Zhiping; Li, Haijun; Guo, Xingwei; Cao, Lin; Yu, Rong; Li, Huanrong; Pan, Shiguang

2008-03-01

5

Reactions of Co sup + , Ni sup + , and Cu sup + with cyclopropane and ethylene oxide. Metal-methylidene ion bond energies  

SciTech Connect

The reactions of atomic cobalt, nickel, and copper ions with cyclopropane and ethylene oxide have been studied by using guided ion beam mass spectrometry. A predominant process in all these systems is formation of MCH{sub 2}{sup +}. Analyses of these endothermic reactions yield the bond energies D{sup 0} (Co{sup +}-CH{sub 2}) = 77.5 {plus minus} 2.3 kcal/mol, D{sup 0}(Ni{sup +}-CH{sub 2}) = 75.2 {plus minus} 1.8 kcal/mol, and D{sup 0}(Cu{sup +}-CH{sub 2}) = 63.9 {plus minus} 1.6 kcal/mol. Differences between these values and those derived from earlier studies for Co{sup +} and Ni{sup +} are discussed. In addition to D{sup 0} (M{sup +}-CH{sub 2}), bond energies for Co{sup +}-H, M-H, (M = Co, Ni, Cu) and M{sup +}-O and M-O (M = Co, Ni) are evaluated and lower limits are placed on D{sup 0} (M{sup +}-C{sub 2}H{sub 4}) and D{sup 0} (M{sup +}-C{sub 2}H{sub 2}) (M = Co, Ni, Cu). The reaction mechanism for these reactions is also discussed in detail.

Fisher, E.R.; Armentrout, P.B. (Univ. of Utah, Salt Lake City (USA))

1990-02-22

6

Oxidation effects on the mechanical properties of SiC fiber-reinforced reaction-bonded silicon nitride matrix composites  

NASA Technical Reports Server (NTRS)

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 percent uniaxially aligned 142 micron diameter SiC fibers in a reaction bonded Si3N4 matrix. The results indicate that composites heat treated in a nitrogen environment at temperatures to 1400 C showed deformation and fracture behavior equivalent to that of the as-fabricated composites. Also, the composites heat treated in an oxidizing environment beyond 400 C yielded significantly lower tensile strength values. Specifically in the temperature range from 600 to 1000 C, composites retained approx. 40 percent of their as-fabricated strength, and those heat treated in the temperatures from 1200 to 1400 C retained 70 percent. Nonetheless, for all oxygen heat treatment conditions, composite specimens displayed strain capability beyond the matrix fracture stress; a typical behavior of a tough composite.

Bhatt, Ramakrishna T.

1989-01-01

7

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

8

Reactions of fourth-period metal ions (Ca sup + endash Zn sup + ) with O sub 2 : Metal-oxide ion bond energies  

SciTech Connect

Reactions of Ca{sup +}, Zn{sup +} and all first-row atomic transition metal ions with O{sub 2} are studied using guided ion beam techniques. While reactions of the ground states of Sc{sup +}, Ti{sup +}, and V{sup +} are exothermic, the remaining metal ions react with O{sub 2} in endothermic processes. Analyses of these endothermic reactions provide new determinations of the M{sup +}--O bond energies for these eight elements. Source conditions are varied such that the contributions of excited states of the metal ions can be explicitly considered for Mn{sup +}, Co{sup +}, Ni{sup +}, and Cu{sup +}. Results (in eV) at 0 K are {ital D}{sup 0}(Ca{sup +}--O)=3.57{plus minus}0.05, {ital D}{sup 0}(Cr{sup +}--O)=3.72{plus minus}0.12, {ital D}{sup 0}(Mn{sup +}--O)=2.95{plus minus}0.13, {ital D}{sup 0}(Fe{sup +}--O)=3.53{plus minus}0.06 (reported previously), {ital D}{sup 0}(Co{sup +}--O)=3.32{plus minus}0.06, {ital D}{sup 0}(Ni{sup +}--O)=2.74{plus minus}0.07, {ital D}{sup 0}(Cu{sup +}--O)=1.62{plus minus}0.15, and {ital D}{sup 0}(Zn{sup +}--O)=1.65{plus minus}0.12. These values along with literature data for neutral metal oxide bond energies and ionization energies are critically evaluated. Periodic trends in the ionic metal oxide bond energies are compared with those of the neutral metal oxides and those of other related molecules.

Fisher, E.R.; Elkind, J.L.; Clemmer, D.E.; Georgiadis, R.; Loh, S.K.; Aristov, N.; Sunderlin, L.S.; Armentrout, P.B. (Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (USA) Department of Chemistry, University of California, Berkeley, CA (USA))

1990-08-15

9

Stereoelectronic effects in C-h bond oxidation reactions of ni(i) N-heterocyclic carbene complexes.  

PubMed

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(?-OH)(?-O-6-Mes')NiBr (3) and Ni(7-Mes)(Br)(?-OH)(?-O-7-Mes')NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-?-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV-visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. -80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O?PPh3)2Br2 and (6-MesDAC)?O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2. PMID:24972307

Poulten, Rebecca C; López, Isidoro; Llobet, Antoni; Mahon, Mary F; Whittlesey, Michael K

2014-07-21

10

Side reaction of S-to-N acetamidomethyl shift during disulfide bond formation by iodine oxidation of S-acetamidomethyl-cysteine in a glutamine-containing peptide.  

PubMed

During the time course of disulfide bond formation by iodine oxidation (in a methanolic and hydrochloric acid solution) of a cysteinyl(S-acetamidomethyl)-glutaminyl tridecapeptide, we observed by ESI, FAB mass spectrometry (pseudo-molecular ion and ion-fragments) and 1H-NMR a side reaction due to a shift of the Acm leaving group from cysteine to the carboxamide side chain of glutamine. This type of Acm-shift at low level was described previously by L.W. Mendelson et al. (Int. J. Pept. Protein Res. 35:249-257) for an aspariginyl-cysteinyl(S-acetamidomethyl) peptide in an anhydrous hydrochloric solution. We report here the efficiency of glutamine as a scavenger to suppress the S-->N shift of the acetamidomethyl group during S-acetamidomethyl cleavage and sulfhydryl oxidation with iodine, as the folded tridecapeptide was obtained with the expected molecular weight. PMID:8838414

Lamthanh, H; Virelizier, H; Frayssinhes, D

1995-01-01

11

Guided ion-beam studies of the reactions of Co(n)+ (n=2-20) with O2: cobalt cluster-oxide and -dioxide bond energies.  

PubMed

The kinetic-energy dependence for the reactions of Co(n)+ (n=2-20) with O2 is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co(m)+, Co(m)O+, and Co(m)O2+ (m < or = n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co(n)+ cations with O2 are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co(n)+-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O2 chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters. PMID:16122305

Liu, Fuyi; Li, Feng-Xia; Armentrout, P B

2005-08-01

12

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

13

Oxidation effects on the mechanical properties of a SiC-fiber-reinforced reaction-bonded Si3N4 matrix composite  

NASA Technical Reports Server (NTRS)

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 percent uniaxially aligned 142 micron diameter SiC fibers in a reaction bonded Si3N4 matrix. The results indicate that composites heat treated in a nitrogen environment at temperatures to 1400 C showed deformation and fracture behavior equivalent to that of the as-fabricated composites. Also, the composites heat treated in an oxidizing environment beyond 400 C yielded significantly lower tensile strength values. Specifically in the temperature range from 600 to 1000 C, composites retained approx. 40 percent of their as-fabricated strength, and those heat treated in the temperatures from 1200 to 1400 C retained 70 percent. Nonetheless, for all oxygen heat treatment conditions, composite specimens displayed strain capability beyond the matrix fracture stress; a typical behavior of a tough composite.

Bhatt, Ramakrishna T.

1992-01-01

14

Bulk gold catalyzed oxidation reactions of amines and isocyanides and iron porphyrin catalyzed N-H and O-H bond insertion/cyclization reactions of diamines and aminoalcohols  

SciTech Connect

This work involves two projects. The first project entails the study of bulk gold as a catalyst in oxidation reactions of isocyanides and amines. The main goal of this project was to study the activation and reactions of molecules at metal surfaces in order to assess how organometallic principles for homogeneous processes apply to heterogeneous catalysis. Since previous work had used oxygen as an oxidant in bulk gold catalyzed reactions, the generality of gold catalysis with other oxidants was examined. Amine N-oxides were chosen for study, due to their properties and use in the oxidation of carbonyl ligands in organometallic complexes. When amine N-oxides were used as an oxidant in the reaction of isocyanides with amines, the system was able to produce ureas from a variety of isocyanides, amines, and amine N-oxides. In addition, the rate was found to generally increase as the amine N-oxide concentration increased, and decrease with increased concentrations of the amine. Mechanistic studies revealed that the reaction likely involves transfer of an oxygen atom from the amine N-oxide to the adsorbed isocyanide to generate an isocyanate intermediate. Subsequent nucleophilic attack by the amine yields the urea. This is in contrast to the bulk gold-catalyzed reaction mechanism of isocyanides with amines and oxygen. Formation of urea in this case was proposed to proceed through a diaminocarbene intermediate. Moreover, formation of the proposed isocyanate intermediate is consistent with the reactions of metal carbonyl ligands, which are isoelectronic to isocyanides. Nucleophilic attack at coordinated CO by amine N-oxides produces CO{sub 2} and is analogous to the production of an isocyanate in this gold system. When the bulk gold-catalyzed oxidative dehydrogenation of amines was examined with amine N-oxides, the same products were afforded as when O{sub 2} was used as the oxidant. When the two types of oxidants were directly compared using the same reaction system and conditions, it was found that the oxidative dehydrogenation of dibenzylamine to Nbenzylidenebenzylamine, with N-methylmorpholine N-oxide (NMMO), was nearly quantitative (96%) within 24 h. However, the reaction with oxygen was much slower, with only a 52% yield of imine product over the same time period. Moreover, the rate of reaction was found to be influenced by the nature of the amine N-oxide. For example, the use of the weakly basic pyridine N-oxide (PyNO) led to an imine yield of only 6% after 24 h. A comparison of amine N-oxide and O2 was also examined in the oxidation of PhCH{sub 2}OH to PhCHO catalyzed by bulk gold. In this reaction, a 52% yield of the aldehyde was achieved when NMMO was used, while only a 7% product yield was afforded when O{sub 2} was the oxidant after 48 h. The bulk gold-catalyzed oxidative dehydrogenation of cyclic amines generates amidines, which upon treatment with Aerosil and water were found to undergo hydrolysis to produce lactams. Moreover, 5-, 6-, and 7-membered lactams could be prepared through a one-pot reaction of cyclic amines by treatment with oxygen, water, bulk gold, and Aerosil. This method is much more atom economical than industrial processes, does not require corrosive acids, and does not generate undesired byproducts. Additionally, the gold and Aerosil catalysts can be readily separated from the reaction mixture. The second project involved studying iron(III) tetraphenylporphyrin chloride, Fe(TPP)Cl, as a homogeneous catalyst for the generation of carbenes from diazo reagents and their reaction with heteroatom compounds. Fe(TPP)Cl, efficiently catalyzed the insertion of carbenes derived from methyl 2-phenyldiazoacetates into O-H bonds of aliphatic and aromatic alcohols. Fe(TPP)Cl was also found to be an effective catalyst for tandem N-H and O-H insertion/cyclization reactions when 1,2-diamines and 1,2-alcoholamines were treated with diazo reagents. This approach provides a one-pot process for synthesizing piperazinones and morpholinones and related analogues such as quinoxalinones and benzoxazin-2-ones.

Klobukowski, Erik

2011-12-29

15

Guided ion beam studies of the reactions of Cr{sub n}{sup +} (n=1{endash}18) with CO{sub 2}: Chromium cluster oxide bond energies  

SciTech Connect

The kinetic energy dependence of the reactions of Cr{sub n}{sup +} (n=1{endash}18) with CO{sub 2} are studied in a guided ion beam mass spectrometer. The primary product ions are Cr{sub n}O{sup +}, which then decompose by sequential loss of chromium atoms as the kinetic energy is increased. Simple collision-induced dissociation to form the Cr{sub n{minus}1}{sup +} product ions is also observed. Large cluster ions, n{ge}9, form the Cr{sub n}CO{sub 2}{sup +} adduct at low kinetic energies. For many cluster sizes, the cross section for the primary reaction, Cr{sub n}{sup +}+CO{sub 2}{r_arrow}Cr{sub n}O{sup +}+CO, exhibits an interesting bimodal energy behavior that is discussed in some detail. Cr{sub n}{sup +}{endash}O bond energies are measured and found to compare well with measurements obtained from guided ion beam studies of the Cr{sub n}{sup +}+O{sub 2} systems. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values. {copyright} {ital 1998 American Institute of Physics.}

Griffin, J.B.; Armentrout, P.B. [Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (United States)] [Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (United States)

1998-05-01

16

Activation of the C-H bond by electrophilic attack: theoretical study of the reaction mechanism of the aerobic oxidation of alcohols to aldehydes by the Cu(bipy)(2+)/2,2,6,6-tetramethylpiperidinyl-1-oxy cocatalyst system.  

PubMed

We have investigated the reaction mechanism of the selective aerobic oxidation of primary alcohols into aldehydes using a bipy-copper complex and the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical as cocatalysts (Gamez et al. Chem. Commun. 2003, 2412-2415) and compared it to the well-known oxidation by the TEMPO(+) ion. Our theoretical investigation shows that (a) the oxidation of alcohols to aldehydes by uncoordinated TEMPO(+) takes place by electrophilic attack on the C-H(alpha) bond of the alcohol; (b) the Cu(bipy)(2+) complex has two functions, namely, (1) it acts as a template that brings TEMPO and the (deprotonated) alcohol in proximity by coordinating these moieties in adjacent coordination sites, and (2) it oxidizes the TEMPO radical to (coordinated) TEMPO(+) ion. The H abstraction from alcohol by TEMPO(+) then proceeds as an intramolecular reaction, very much analogous to one of the reaction pathways with free TEMPO(+) and with a remarkably low barrier. We stress that compared to other A-H bonds (A = C, N, O, F), the relatively high-lying C-H bonds are particularly susceptible to electrophilic attack, and notably the C-H(alpha) bond next to the O in an alcohol is so because it is pushed up by an O lone pair. Electrophilic attack, being common to the particular catalytic system studied in this paper and the well-known biotic and abiotic oxidation catalysis by heme and non-heme complexes of the ferryl (Fe(IV)O(2+)) ion, appears to be a unifying electronic structure principle of C-H(alpha) hydroxylation and oxidation reactions. PMID:19938864

Michel, Carine; Belanzoni, Paola; Gamez, Patrick; Reedijk, Jan; Baerends, Evert Jan

2009-12-21

17

A fluxless oxidation-free bonding technology  

Microsoft Academic Search

A fluxless oxidation-free bonding technology is reported. The technology uses the direct deposition of multilayer composite in high vacuum to prevent oxidation. The outer layer of the composite is either gold or copper which protects the inner layers from oxidation when the composite is later exposed to atmosphere. As a result of oxidation prevention neither flux nor scrubbing action is

Chin C. Lee; Yi-Chia Chen; Goran Matijasevic; Richard Metzler

1994-01-01

18

Effect of processing parameters on reaction bonding of silicon nitride  

NASA Technical Reports Server (NTRS)

Reaction bonded silicon nitride was developed. The relationship between the various processing parameters and the resulting microstructures was to design and synthesize reaction bonded materials with improved room temperature mechanical properties.

Richman, M. H.; Gregory, O. J.; Magida, M. B.

1980-01-01

19

Reactions of Metal-Metal Triple Bonds. 7. Addition of the Halogens (Cl2, Br2 and I2) and Diisopropylperoxide to Hexaisopropoxydimolybdenum (M-M). Dinuclear Oxidative-Addition Reactions Accompanied by Metal-Metal Bond Order Changes from Three to Two to One.  

National Technical Information Service (NTIS)

In mononuclear transition metal chemistry, the term oxidative-addition is often used to describe reactions in which both the oxidation number and coordination number of the metal are increased by two. The microscopic reverse reaction is called reductive e...

M. H. Chisholm C. C. Kirkpatrick J. C. Huffman

1980-01-01

20

Reaction-bonded Si3N4 and SiC matrix composites  

NASA Technical Reports Server (NTRS)

A development status evaluation is presented for the reaction-bonded SiC- and Si3N4-matrix types of fiber-reinforced ceramic-matrix composite (FRCMC). A variety of reaction-bonding methods are being pursued for FRCMC fabrication: CVI, CVD, directed metal oxidation, and self-propagating high-temperature synthesis. Due to their high specific modulus and strength, toughness, and fabricability, reaction-bonded FRCMC are important candidate materials for such heat-engine components as combustor liners, nozzles, and turbine and stator blading. The improvement of long-term oxidative stability in these composites is a major goal of current research.

Bhatt, Ramakrishna T.; Behrendt, Donald R.

1992-01-01

21

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

2013-09-13

22

Guided ion-beam studies of the reactions of Co{sub n}{sup +} (n=2-20) with O{sub 2}: Cobalt cluster-oxide and -dioxide bond energies  

SciTech Connect

The kinetic-energy dependence for the reactions of Co{sub n}{sup +} (n=2-20) with O{sub 2} is measured as a function of kinetic energy over a range of 0 to 10 eV in a guided ion-beam tandem mass spectrometer. A variety of Co{sub m}{sup +}, Co{sub m}O{sup +}, and Co{sub m}O{sub 2}{sup +} (m{<=}n) product ions is observed, with the dioxide cluster ions dominating the products for all larger clusters. Reaction efficiencies of Co{sub n}{sup +} cations with O{sub 2} are near unity for all but the dimer. Bond dissociation energies for both cobalt cluster oxides and dioxides are derived from threshold analysis of the energy dependence of the endothermic reactions using several different methods. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry and the stabilities of oxygenated cobalt clusters are discussed. The bond energies of Co{sub n}{sup +}-O for larger clusters are found to be very close to the value for desorption of atomic oxygen from bulk-phase cobalt. Rate constants for O{sub 2} chemisorption on the cationic clusters are compared with results from previous work on cationic, anionic, and neutral cobalt clusters.

Liu Fuyi; Li Fengxia; Armentrout, P.B. [Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (United States)

2005-08-08

23

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

Microsoft Academic Search

Reaction sinter bonding is a process that aims to bond two materials for improvement in properties through reactive sintering\\u000a technique. The process has been effectively used to sinter hard materials like borides in situ which not only possess excellent\\u000a oxidation resistance, good corrosion resistance but also resistant to abrasive wear. Sinter bonding is a unique surface modification\\u000a process achieved through

B. Palanisamy; A. Upadhyaya

2011-01-01

24

ChemTeacher Resource: Oxidation Reduction Reactions  

NSDL National Science Digital Library

This computer resource goes over oxidation reduction reactions and how they are broken down into half reactions. It explains which half reaction is the oxidation reaction and which one is the reduction, then has some summary questions at the end.

Dr. Martin McClinton, Debbie McClinton, Dr. Miriam Douglass

2011-01-01

25

Microstructural evolution in reaction bonded alumina/mullite ceramics  

SciTech Connect

Alumina, Al, and SiC powders were milled, compacted and heat-treated during which both the Al and SiC are oxidized. The Al2O3 and SiO2 that form then react to produce mullite, 3Al2O3*2SiO2. The goal of this process was to compensate for shrinkage that occurs during densification by expansion that occurs during oxidation and mullite formation to produce a body with little or no shape change during firing. The microstructure that developed during reaction bonding was in most cases not uniform and exhibited a complicated ring structure on polished cross-sections observed using polarized light optical microscopy. The outer surface was composed of a relatively dense ceramic bounded by a ring of porosity located just inside the dense layer. Inside the porous ring were many rings of alternating contrast and a black oval in the center of the body. The ring structure was more pronounced as the green density, SiC content and component size was increased. The phase distribution across the ring structure was investigated using XRD and XPS analyses. The results of these studies showed the presence of unoxidized SiC and ZrO in the center of fully heat-treated bodies. The ZrO comes from reduction of ZrO2 present in the milled powders due to wear of the ZrO2 milling media. These results indicate that the body was not completely reacted during the heat treatment and that a reducing environment existed inside of the ceramic during reaction bonding. The reducing environment is generated by densification of the outer surface before the SiC has been completely oxidized. The accumulation of CO gas that evolves during SiC oxidation and the reduction in oxygen diffusion into the body are believed to cause the observed reduction of zirconia. The microstructure of fully and partially reacted bodies was characterized by TEM.

Fletcher, T.D.

1992-01-01

26

Photochemistry of intermolecular C-H bond activation reactions.  

National Technical Information Service (NTIS)

Goals at the onset were to obtain photophysical and photochemical data on transition-metal organometallic complexes which undergo light-induced intermolecular C-H bond activation reactions with hydrocarbon substrates. The complexes included these of rhodi...

A. J. Lees

1994-01-01

27

Carbon-carbon bond-forming reactions using alkyl fluorides  

Microsoft Academic Search

This account summarizes our recent results on C-C bond formation reactions using alkyl fluorides mostly focusing on the transition-metal-catalyzed reactions. These reactions proceed efficiently under mild conditions by the combined use of Grignard reagents and tran- sition-metal catalysts, such as Ni, Cu, and Zr. It is proposed that ate complex intermediates formed by the reaction of these transition metals with

Jun Terao; Hirohisa Todo; Hiroyasu Watabe; Aki Ikumi; Yoshiaki Shinohara; Nobuaki Kambe

2008-01-01

28

Pressure bonding molybdenum alloy (TZM) to reaction-bonded silicon nitride  

Microsoft Academic Search

Topping cycles could boost the energy efficiencies of a variety of systems by using what is now waste heat. One such topping cycle uses a ceramic helical expander and would require that a reaction-bonded silicon nitride (RBSN) rotor be bonded to a shaft of TZM (Mo-0.5 wt % Ti-0.08 wt % Zr). Coupon studies show that TZM can be bonded

S. A. Huffsmith; R. L. Landingham

1978-01-01

29

Microstructural evolution in reaction-bonded silicon carbide  

Microsoft Academic Search

A detailed microstructural investigation of reaction-bonded silicon carbide has been performed using both fully-bonded and quenched samples and other specially prepared specimens containing large original single crystals of known crystallographic habit. The development of the epitaxial SiC overgrowth on the original SiC particles has been followed and found to proceed by the progressive growth and coalescence of identically-oriented nuclei. This

J. N. Ness; T. F. Page

1986-01-01

30

Interdiffusion reactions and degradation in gold-aluminum ball bonds  

NASA Astrophysics Data System (ADS)

The present study addresses the interdiffusion reactions and the degradation in Au-Al ball bonds as a function of time and the Cu content of the Al metallization during constant elevated-temperature exposure. Sample preparation methodology was developed to characterize the phase transformations and the degradation in the thermally-exposed Au-Al ball bonds by scanning electron microscopy. The as-bonded microstructure of a Au ball bond consisted of three regions; a columnar grain zone, an equiaxed grain zone and an interdiffusion zone. A line of discontinuous voids (void line) was observed in the interdiffusion zone. Thermal exposure of Au-Al ball bonds at 175°C and 250°C resulted in the nucleation and the growth of Au4Al, Au8Al 3 and Au2Al across the ball bond. Once the metallization below the bond is completely consumed, the Al-rich phases reverse transformed eventually resulting in the formation of Au4Al, in the entire reaction zone. With prolonged thermal exposure, lateral interdiffusion resulted in the nucleation and the growth of AuAl2, AuAl, Au2Al and Au8Al3 between Al around the ball bond and Au 4Al. Theoretical analysis of the partial reactions at each interphase boundary showed that the phase transformations across the ball bond result in volumetric shrinkage, which promote tensile stresses at the void line. It is these shrinkage stresses that aid in the nucleation and the growth of cavities and degrade Au-Al ball bonds during constant elevated-temperature exposure. The line of discontinuous voids acts as a site for the nucleation and the growth of cavities. Aided by thermal exposure, the cavity growth is higher at the crack front due to stress concentration at the crack front, which is initially at the edge of the void line. The crack propagation into the bond occurred by the linking of sufficiently grown cavities at the void line. Crack propagation completely into the ball bond results in the separation of the ball bond from the Si chip. Scanning electron microscopy observations of the thermally-exposed ball bond cross sections revealed lower cavity growth and higher rate of crack propagation in ball bonds on Al-0.75Cu metallization than in bonds on Al metallization. The interdiffusion coefficients of the phases Au4Al, Au 8Al3 and Au2Al formed across ball bonds on Al and Al-Cu alloy metallizations were very comparable. From these interdiffusion coefficients and the similar nature of phase transformations in Au ball bonds on Al and Al-Cu alloy metallizations, it is inferred that Cu additions to the Al metallization do not influence the interdiffusion reactions between the Au ball bond and the metallization.

Noolu, Narendra Janakiram

31

Chemistry of chloroethylenes on Cu(100): bonding and reactions  

NASA Astrophysics Data System (ADS)

The bonding and reactions of chloroethylenes (vinyl chloride, trans- and cis-dichloroethylene, trichloroethylene) on a Cu(100) surface have been investigated by temperature-programmed desorption and reaction, near-edge X-ray absorption fine structure (NEXAFS) studies and measurements of changes of the surface work function. The monolayer molecules adsorb molecularly intact at 95 K via ? coordination. The molecules orient with their molecular planes parallel to the surface with little increase ( <0.02 Å) in the C?C bond distance compared with the gas phase values. The degree of thermal dissociation increases with the extent of halogenation, and no chlorine is present in the hydrocarbon reaction products liberated from the surface. Both cis- and trans-1.2-chloroethylene produce acetylene and a small amount of benzene, while dissociation of trichloroethylene generates acetylene and deposits a carbon residue on the surface. Vinyl chloride desorbs from Cu(100) without reaction.

Yang, Michael X.; Kash, Phillip W.; Sun, Dong-Hong; Flynn, George W.; Bent, Brian E.; Holbrook, Michael T.; Bare, Simon R.; Fischer, Daniel A.; Gland, John L.

1997-05-01

32

Wet milling of Al-containing powder mixtures as precursor materials for reaction bonding of alumina (RBAO) and reaction sintering of alumina–aluminide alloys (3A)  

Microsoft Academic Search

Wet milling of powders containing metallic and ceramic particles in different organic milling media has been investigated. These powders are used as precursors for the reaction bonding of aluminum oxide (RBAO) and for the reaction sintering of alumina–aluminide alloys (3A). An attempt is made to explain effects on the milling efficiency in terms of interactions between the organic medium and

Frank Essl; Rolf Janssen; Nils Claussen

1999-01-01

33

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

Microsoft Academic Search

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,

J. Yu; R. C. Flagan; J. H. Seinfeld

1998-01-01

34

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 type…

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

2010-01-01

35

Reactions of a quintuply bonded chromium dimer with alkynes.  

PubMed

Quintuply bonded [(H)L(iPr)Cr](2) reacts with alkynes RC?CR (R = Me, Et, Ph, CF(3)) to form exclusively 1?:?1 adducts [(H)L(iPr)Cr](2)(RCCR). All products feature relatively short Cr-Cr distances (1.919-1.962 Å) and elongated C-C bonds (1.315-1.436 Å), consistent with [2+2] cycloaddition reactions. The hydrocarbon adducts are 4-membered metallacycles, the bridging alkynes of which are progressively skewed with respect to the Cr-Cr axis. In contrast, perfluoroalkyne adds across the metal ligand moiety. PMID:21986955

Shen, Jingmei; Yap, Glenn P A; Werner, Jan-Philipp; Theopold, Klaus H

2011-11-28

36

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. García-Godoy; F. R. Tay; D. H. Pashley; S. Imazato; N. M. King; S. C. N. Lai

2002-01-01

37

Some factors influencing the formation of reaction-bonded silicon nitride  

Microsoft Academic Search

Several salient factors influencing the formation of reaction-bonded silicon nitride (RBSN) compacts have been studied. These include the effects of mullite and alumina furnace tubes typically employed during “high-purity” nitridation studies, pre-sintering of green silicon compacts, free powder versus compact nitridation, and the influence of metal\\/metal oxide additions. The latter studies have provided experimental evidence for enhancement due to dissociated

R. G. Pigeon; A. Varma; A. E. Miller

1993-01-01

38

Tetrel bond-?-hole bond as a preliminary stage of the SN2 reaction.  

PubMed

MP2/aug-cc-pVTZ calculations were carried out on complexes of ZH4, ZFH3 and ZF4 (Z = C, Si and Ge) molecules with HCN, LiCN and Cl(-) species acting as Lewis bases through nitrogen centre or chlorine ion. Z-Atoms in these complexes usually act as Lewis acid centres forming ?-hole bonds with Lewis bases. Such noncovalent interactions may adopt a name of tetrel bonds since they concern the elements of the group IV. There are exceptions for complexes of CH4 and CF4, as well as for the F4SiNCH complex where the tetrel bond is not formed. The energetic and geometrical parameters of the complexes were analyzed and numerous correlations between them were found. The Quantum Theory of 'Atoms in Molecules' and Natural Bonds Orbital (NBO) method used here should deepen the understanding of the nature of the tetrel bond. An analysis of the electrostatic potential surfaces of the interacting species is performed. The electron charge redistribution, being the result of the tetrel bond formation, is the same as that of the SN2 reaction. The energetic and geometrical parameters of the complexes analyzed here correspond to different stages of the SN2 process. PMID:24296531

Grabowski, S?awomir J

2014-02-01

39

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

40

Engineered Pathways for Correct Disulfide Bond Oxidation  

PubMed Central

Abstract Correct formation of disulfide bonds is critical for protein folding. We find that cells lacking protein disulfide isomerases (PDIs) can use alternative mechanisms for correct disulfide bond formation. By linking correct disulfide bond formation to antibiotic resistance, we selected mutants that catalyze correct disulfide formation in the absence of DsbC, Escherichia coli's PDI. Most of our mutants massively overproduce the disulfide oxidase DsbA and change its redox status. They enhance DsbA's ability to directly form the correct disulfides by increasing the level of mixed disulfides between DsbA and substrate proteins. One mutant operates via a different mechanism; it contains mutations in DsbB and CpxR that alter the redox environment of the periplasm and increases the level of the chaperone/protease DegP, allowing DsbA to gain disulfide isomerase ability in vivo. Thus, given the proper expression level, redox status, and chaperone assistance, the oxidase DsbA can readily function in vivo to catalyze the folding of proteins with complex disulfide bond connectivities. Our selection reveals versatile strategies for correct disulfide formation in vivo. Remarkably, our evolution of new pathways for correct disulfide bond formation in E. coli mimics eukaryotic PDI, a highly abundant partially reduced protein with chaperone activity. Antioxid. Redox Signal. 14, 2399–2412.

Ren, Guoping

2011-01-01

41

C-H Bond Functionalizations with Palladium(II): Intramolecular Oxidative Annulations of Arenes  

PubMed Central

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 C-H bond functionalization event, followed by olefin insertion and ?-hydride elimination.

Ferreira, Eric M.; Zhang, Haiming

2008-01-01

42

Copper-catalyzed dehydrogenative cross-coupling reaction between allylic C-H bonds and ?-C-H bonds of ketones or aldehydes.  

PubMed

A dehydrogenative cross-coupling reaction between allylic C-H bonds and the ?-C-H bond of ketones or aldehydes was developed using Cu(OTf)2 as a catalyst and DDQ as an oxidant. This synthetic approach to ?,?-unsaturated ketones and aldehydes has the advantages of broad scope for both ketones and aldehydes as reactants, mild reaction conditions, good yields and atom economy. A plausible mechanism using Cu(OTf)2 as a Lewis acid catalyst was also proposed (DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone; Tf=trifluoromethanesulfonate). PMID:24757128

Huang, Xing-Fen; Salman, Muhammad; Huang, Zhi-Zhen

2014-05-26

43

Diffusion bonding of the oxide dispersion strengthened steel PM2000  

NASA Astrophysics Data System (ADS)

Ferritic oxide dispersion strengthened (ODS) steels are well suited as structural materials, e.g. for claddings in fission reactors and for plasma facing components in fusion power plants due to their high mechanical and oxidation stability at high temperatures and their high irradiation resistance. PM2000 is an iron based ODS ferritic steel with homogeneously distributed nanometric yttria particles. Melting joining techniques are not suitable for such ODS materials because of the precipitation and agglomeration of the oxide particles and hence the loss of their strengthening effect. Solid state diffusion bonding is thus chosen to join PM2000 and is investigated in this work with a focus on oxide particles. The diffusion bonding process is aided by the computational modeling, including the influence of the ODS particles. For modeling the microstructure stability and the creep behavior of PM2000 at various, diffusion bonding relevant temperatures (50-80% Tm) are investigated. Particle distribution (TEM), strength (tensile test) and toughness (Charpy impact test) obtained at temperatures relevant for bonding serve as input for the prediction of optimal diffusion bonding parameters. The optimally bonded specimens show comparable strength and toughness relative to the base material.

Sittel, Wiebke; Basuki, Widodo W.; Aktaa, Jarir

2013-11-01

44

Nitrosobenzene-Mediated C-C Bond Cleavage Reactions and Spectral Observation of an oxazetidin-4-one Ring System  

PubMed Central

While bond formation processes have traditionally garnered the attention of the chemical community, methods facilitating bond-breaking remain relatively undeveloped. We report a novel, transition metal-free oxidative C-C bond cleavage process for a broad range of ester and dicarbonyl compounds involving carbanion addition to nitrosobenzene. ReactIR experiments on the nitrosobenzene-mediated oxidative decarboxylation of esters indicate the reaction proceeds via fragmentation of a previously unobserved oxazetidin-4-one heterocycle, characterized by an intense IR stretching frequency at 1846 cm-1. These mechanistic studies have allowed further expansion of this protocol to ketone cleavage reactions of a diverse array of ?-ketoester and 1,3-diketone substrates. The conceptual and mechanistic insights offered by this study are likely to provide a platform for further development of bond-breaking methodologies.

Payette, Joshua N.; Yamamoto, Hisashi

2009-01-01

45

Nitrosobenzene-mediated C-C bond cleavage reactions and spectral observation of an oxazetidin-4-one ring system.  

PubMed

While bond formation processes have traditionally garnered the attention of the chemical community, methods facilitating bond breaking remain relatively undeveloped. We report a novel, transition-metal-free oxidative C-C bond cleavage process for a broad range of ester and dicarbonyl compounds involving carbanion addition to nitrosobenzene. ReactIR experiments on the nitrosobenzene-mediated oxidative decarboxylation of esters indicate the reaction proceeds via fragmentation of a previously unobserved oxazetidin-4-one heterocycle, characterized by an intense IR stretching frequency at 1846 cm-1. These mechanistic studies have allowed further expansion of this protocol to ketone cleavage reactions of a diverse array of beta-ketoester and 1,3-diketone substrates. The conceptual and mechanistic insights offered by this study are likely to provide a platform for further development of bond-breaking methodologies. PMID:18722431

Payette, Joshua N; Yamamoto, Hisashi

2008-09-17

46

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 C–H 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

47

Copper-catalyzed aerobic oxidative n?s bond functionalization for c?s bond formation: regio- and stereoselective synthesis of sulfones and thioethers.  

PubMed

A regio- and stereoselective synthesis of sulfones and thioethers by means of Cu(I) -catalyzed aerobic oxidative N?S bond cleavage of sulfonyl hydrazides, followed by cross-coupling reactions with alkenes and aromatic compounds to form the C?sp?2?S bond, is described herein. N2 and H2 O are the byproducts of this transformation, thus offering an environmentally benign process with a wide range of potential applications in organic synthesis and medicinal chemistry. PMID:24860978

Li, Xianwei; Xu, Yanli; Wu, Wanqing; Jiang, Chang; Qi, Chaorong; Jiang, Huanfeng

2014-06-23

48

Uranium oxidation: characterization of oxides formed by reaction with water  

SciTech Connect

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 at the oxide/metal interface. Single laminae are released if liquid water is present due to the prizing penetration at the reaction zone. The rate of reaction of water with uranium is directly proportional to the amount of adsorbed water on the oxide product. Rapid transport is effected through the open hydrous oxide product. Dehydration of the hydrous oxide irreversibly forms a more inert oxide which cannot be rehydrated to the degree that prevails in the original hydrous product of uranium oxidation with water. 27 figures.

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

1983-04-27

49

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

50

Allenes and computational chemistry: from bonding situations to reaction mechanisms.  

PubMed

The present review is focused on the application of computational/theoretical methods to the wide and rich chemistry of allenes. Special emphasis is made on the interplay and synergy between experimental and computational methodologies, rather than on recent developments in methods and algorithms. Therefore, this review covers the state-of-the-art applications of computational chemistry to understand and rationalize the bonding situation and vast reactivity of allenes. Thus, the contents of this review span from the most fundamental studies on the equilibrium structure and chirality of allenes to recent advances in the study of complex reaction mechanisms involving allene derivatives in organic and organometallic chemistry. PMID:24553788

Soriano, Elena; Fernández, Israel

2014-05-01

51

Palladium-catalyzed oxidative C-N bond coupling involving a solvent-controlled regioselective bromination process.  

PubMed

Stereoselective palladium-catalyzed oxidative C-N bond coupling reactions between aromatic amines and alkenes involving a solvent-controlled regioselective bromination process under 1 atm of oxygen atmosphere are disclosed, providing easy access to two different brominated enamines. The addition of hydrogen peroxide (30% aq) as a co-oxidant in the system is crucial for the dehydrogenative aminohalogenation under molecular oxygen (1 atm), and in such a case, the C-N bond coupling/electrophilic bromination reaction cascade is proposed. Furthermore, the different reaction media leads to a switched regioselectivity of the process. PMID:25010511

Ji, Xiaochen; Huang, Huawen; Xiong, Wenfang; Huang, Kunbo; Wu, Wanqing; Jiang, Huanfeng

2014-08-01

52

Palladium-catalyzed oxidative carbonylation reactions.  

PubMed

Palladium-catalyzed coupling reactions have become a powerful tool for advanced organic synthesis. This type of reaction is of significant value for the preparation of pharmaceuticals, agrochemicals, as well as advanced materials. Both, academic as well as industrial laboratories continuously investigate new applications of the different methodologies. Clearly, this area constitutes one of the major topics in homogeneous catalysis and organic synthesis. Among the different palladium-catalyzed coupling reactions, several carbonylations have been developed and widely used in organic syntheses and are even applied in the pharmaceutical industry on ton-scale. Furthermore, methodologies such as the carbonylative Suzuki and Sonogashira reactions allow for the preparation of interesting building blocks, which can be easily refined further on. Although carbonylative coupling reactions of aryl halides have been well established, palladium-catalyzed oxidative carbonylation reactions are also interesting. Compared with the reactions of aryl halides, oxidative carbonylation reactions offer an interesting pathway. The oxidative addition step could be potentially avoided in oxidative reactions, but only few reviews exist in this area. In this Minireview, we summarize the recent development in the oxidative carbonylation reactions. PMID:23307763

Wu, Xiao-Feng; Neumann, Helfried; Beller, Matthias

2013-02-01

53

Template syntheses of copper(II) complexes from arylhydrazones of malononitrile and their catalytic activity towards alcohol oxidations and the nitroaldol reaction: hydrogen bond-assisted ligand liberation and E/Z isomerisation.  

PubMed

A one-pot template condensation of 2-(2-(dicyanomethylene)hydrazinyl)benzenesulfonic acid (H(2)L(1), 1) or 2-(2-(dicyanomethylene)hydrazinyl)benzoic acid (H(2)L(2), 2) with methanol (a), ethylenediamine (b), ethanol (c) or water (d) on copper(II), led to a variety of metal complexes, that is, mononuclear [Cu(H(2)O)(2)(?O(1),?N(2)L(1a)] (3) and [Cu(H(2)O)(?O(1),?N(3)L(1b))] (4), tetranuclear [Cu(4)(1?O(1),?N(2):2?O(1)L(2a))(3)-(1?O(1), ?N(2):2?O(2)L(2a))] (5), [Cu(2)(H(2)O)(1?O(1), ?N(2):2?O(1)L(2c))-(1?O(1),1?N(2):2?O(1),2??N(1)- L(2c))](2) (6) and [Cu(2)(H(2)O)(2)(?O(1),?N(2)- L(1dd))-(1?O(1),?N(2):2?O(1)L(1dd))(?-H(2)O)](2·) 2H(2)O (7·2H(2)O), as well as polymer- ic [Cu(H(2)O)(?O(1),1?N(2):2?N(1)L(1c))](n) (8) and [Cu(NH(2)C(2)H(5))(?O(1),1?N(2):2?N(1)L(2a))](n) (9). The ligands 2-SO(3)H-C(6)H(4)-(NH)N=C{(CN)[C(NH(2))-(=NCH(2)CH(2)NH(2))]} (H(2)L(1b), 10), 2-CO(2)H-C(6)H(4)-(NH)N={C(CN)[C(OCH(3))-(=NH)]} (H(2)L(2a), 11) and 2-SO(3)H-C(6)H(4)-(NH)N=C{C(=O)-(NH(2))}(2) (H(2)L(1dd), 12) were easily liberated upon respective treatment of 4, 5 and 7 with HCl, whereas the formation of cyclic zwitterionic amidine 2-(SO(3)(?))-C(6)H(4)-N=NC(-C=(NH(+))CH(2)CH(2)NH)(=CNHCH(2)CH(2)NH) (13) was observed when 1 was treated with ethylenediamine. The hydrogen bond-induced E/Z isomerization of the (HL(1d))(?) ligand occurs upon conversion of [{Na(H(2)O)(2)(?-H(2)O)(2)}(HL(1d))](n) (14) to [Cu(H(2)O)(6)][HL(1d)](2)·2H(2)O (15) and [{CuNa(H(2)O)-(?N(1),1?O(2):2?O(1)L(1d))(2)}K(0.5)(?-O)(2)]n·H(2)O (16). The synthesized complexes 3–9 are catalyst precursors for both the selective oxidation of primary and secondary alcohols (to the corresponding carbonyl compounds) and the following diastereoselective nitroaldol (Henry) reaction, with typical yields of 80–99%. PMID:23180736

Kopylovich, Maximilian N; Mizar, Archana; Guedes da Silva, M Fátima C; Mac Leod, Tatiana C O; Mahmudov, Kamran T; Pombeiro, Armando J L

2013-01-01

54

Oxidative stretching of metal-metal bonds to their limits.  

PubMed

Oxidation of quadruply bonded Cr2(dpa)4, Mo2(dpa)4, MoW(dpa)4, and W2(dpa)4 (dpa = 2,2'-dipyridylamido) with 2 equiv of silver(I) triflate or ferrocenium triflate results in the formation of the two-electron-oxidized products [Cr2(dpa)4](2+) (1), [Mo2(dpa)4](2+) (2), [MoW(dpa)4](2+) (3), and [W2(dpa)4](2+) (4). Additional two-electron oxidation and oxygen atom transfer by m-chloroperoxybenzoic acid results in the formation of the corresponding metal-oxo compounds [Mo2O(dpa)4](2+) (5), [WMoO(dpa)4](2+) (6), and [W2O(dpa)4](2+) (7), which feature an unusual linear M···M?O structure. Crystallographic studies of the two-electron-oxidized products 2, 3, and 4, which have the appropriate number of orbitals and electrons to form metal-metal triple bonds, show bond distances much longer (by >0.5 Å) than those in established triply bonded compounds, but these compounds are nonetheless diamagnetic. In contrast, the Cr-Cr bond is completely severed in 1, and the resulting two isolated Cr(3+) magnetic centers couple antiferromagnetically with J/kB= -108(3) K [-75(2) cm(-1)], as determined by modeling of the temperature dependence of the magnetic susceptibility. Density functional theory (DFT) and multiconfigurational methods (CASSCF/CASPT2) provide support for "stretched" and weak metal-metal triple bonds in 2, 3, and 4. The metal-metal distances in the metal-oxo compounds 5, 6, and 7 are elongated beyond the single-bond covalent radii of the metal atoms. DFT and CASSCF/CASPT2 calculations suggest that the metal atoms have minimal interaction; the electronic structure of these complexes is used to rationalize their multielectron redox reactivity. PMID:24746142

Brogden, David W; Turov, Yevgeniya; Nippe, Michael; Li Manni, Giovanni; Hillard, Elizabeth A; Clérac, Rodolphe; Gagliardi, Laura; Berry, John F

2014-05-01

55

Regioselectivity of the oxidative C-S bond formation in ergothioneine and ovothiol biosyntheses.  

PubMed

Ergothioneine (5) and ovothiol (8) are two novel thiol-containing natural products. Their C-S bonds are formed by oxidative coupling reactions catalyzed by EgtB and OvoA enzymes, respectively. In this work, it was discovered that in addition to catalyzing the oxidative coupling between histidine and cysteine (1 ? 6 conversion), OvoA can also catalyze a direct oxidative coupling between hercynine (2) and cysteine (2 ? 4 conversion), which can shorten the ergothioneine biosynthetic pathway by two steps. PMID:24016264

Song, Heng; Leninger, Maureen; Lee, Norman; Liu, Pinghua

2013-09-20

56

Method of densifying an article formed of reaction bonded silicon nitride  

NASA Technical Reports Server (NTRS)

A method of densifying an article formed of reaction bonded silicon nitride is disclosed. The reaction bonded silicon nitride article is packed in a packing mixture consisting of silicon nitride powder and a densification aid. The reaction bonded silicon nitride article and packing powder are sujected to a positive, low pressure nitrogen gas treatment while being heated to a treatment temperature and for a treatment time to cause any open porosity originally found in the reaction bonded silicon nitride article to be substantially closed. Thereafter, the reaction bonded silicon nitride article and packing powder are subjected to a positive high pressure nitrogen gas treatment while being heated to a treatment temperature and for a treatment time to cause a sintering of the reaction bonded silicon nitride article whereby the strength of the reaction bonded silicon nitride article is increased.

Mangels, John A. (Inventor)

1982-01-01

57

The development of Cu bonding wire with oxidation-resistant metal coating  

Microsoft Academic Search

Although Cu bonding wire excels over Au bonding wire in some respects such as production costs, it has not been widely used because of its poor bondability at second bonds due to surface oxidation. We conceived an idea of electroplating oxidation-resistant metal on the Cu bonding wire to prevent the surface oxidation. The electroplating of Au, Ag, Pd, and Ni

Shingo Kaimori; Tsuyoshi Nonaka; Akira Mizoguchi

2006-01-01

58

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

59

Guided ion beam studies of the reactions of Cr{sub n}{sup +} (n=2{endash}18) with O{sub 2}: Chromium cluster oxide and dioxide bond energies  

SciTech Connect

The kinetic energy dependence of the reactions of Cr{sub n}{sup +} (n=2{endash}18) with O{sub 2} are studied in a guided ion beam mass spectrometer. A variety of Cr{sub m}O{sub 2}{sup +}, Cr{sub m}O{sup +}, and Cr{sub m}{sup +} product ions, where m{le}n, are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the product cross sections is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to chromium cluster ions. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values. {copyright} {ital 1998 American Institute of Physics.}

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

1998-05-01

60

Hydrogen multicenter bond in oxide and nitride semiconductors  

NASA Astrophysics Data System (ADS)

Hydrogen is a very reactive atom, occurring in virtually all organic and in many inorganic compounds. It can form a purely covalent bond, in which two hydrogen atoms share a pair of electrons in a two-electron two-center bond, as well as polar covalent bonds, such as in an H2O molecule. In solids, hydrogen is usually considered as an interstitial impurity. In elemental semiconductors, such as silicon, hydrogen forms a three-center bond when located at the bond center. In compound semiconductors, hydrogen bonds to the anionic species in p-type material, and to the cationic species in n-type. Thus far, hydrogen in solids has been found to form chemical bonds with one, two, or at most three other atoms. Higher coordination numbers are exceedingly rare and have been reported only for clusters. In this talk we will show that hydrogen is capable of forming multicenter bonds in solids, occupying substitutional sites. As examples, we discuss substitutional hydrogen impurities in oxides (ZnO, MgO, SnO2, TiO2) [1,2] and nitrides (InN, AlN, GaN) [3]. Based on first-principles calculations we show that hydrogen replaces oxygen (nitrogen) and forms genuine chemical bonds with multiple metal atoms, in truly multicoordinated configurations. These multicenter bonds are surprisingly strong despite the large hydrogen-metal distances when compared to typical values in hydrogen two-center bonds. Hydrogen in the multicenter bond configuration is a shallow donor in a number of materials. In conducting oxides, it provides a consistent explanation for the observed dependence of electrical conductivity on oxygen partial pressure, thus resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity [1,2]. [4pt] [1] A. Janotti and C. G Van de Walle, Nature Materials 6, 44 (2007). [0pt] [2] A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. 101, 055502 (2008). [0pt] [3] A. Janotti and C. G. Van de Walle, Appl. Phys. Lett. 92, 032104 (2008).

Janotti, Anderson

2009-03-01

61

Bonding of sapphire to sapphire by eutectic mixture of aluminum oxide and zirconium oxide  

NASA Technical Reports Server (NTRS)

Bonding of an element comprising sapphire, ruby or blue sapphire to another element of such material with a eutectic mixture of aluminum oxide and zirconium oxide is discussed. The bonding mixture may be applied in the form of a distilled water slurry or by electron beam vapor deposition. In one embodiment the eutectic is formed in situ by applying a layer of zirconium oxide and then heating the assembly to a temperature above the eutectic temperature and below the melting point of the material from which the elements are formed. The formation of a sapphire rubidium maser cell utilizing eutectic bonding is shown.

Deluca, J. J. (inventor)

1975-01-01

62

Bonding of sapphire to sapphire by eutectic mixture of aluminum oxide and zirconium oxide  

NASA Technical Reports Server (NTRS)

An element comprising sapphire, ruby or blue sapphire can be bonded to another element of such material with a eutectic mixture of aluminum oxide and zirconium oxide. The bonding mixture may be applied in the form of a distilled water slurry or by electron beam vapor deposition. In one embodiment the eutectic is formed in situ by applying a layer of zirconium oxide and then heating the assembly to a temperature above the eutectic temperature and below the melting point of the material from which the elements are formed. The formation of a sapphire rubidium maser cell utilizing eutectic bonding is shown.

Deluca, J. J. (inventor)

1979-01-01

63

?-Bond Activation of Small Molecules and Reactions Catalyzed by Transition-Metal Complexes: Theoretical Understanding of Electronic Processes.  

PubMed

?-Bond activations of R1-R2 and R1-X1 (R1, R2 = H, alkyl, aromatics, etc.; X1 = electronegative group) by transition-metal complexes are classified into two main categories: ?-bond activation by a metal (M) center and that by a metal-ligand bond. The former is classified into two subcategories: concerted oxidative addition to M and stepwise oxidative addition via nucleophilic attack of M. The latter is also classified into two subcategories: heterolytic activaton by M-X2 (X2 = anion ligand) and oxidative addition to M-L (L = neutral ligand). In the concerted oxidative addition, charge transfer (CT) occurs from the M d orbital to the ?* antibonding orbital of R1-R2, the clear evidence of which is presented here. The concerted oxidative additions of Ph-CN, Me-CN, and Ph-Cl to a nickel(0) complex are discussed as examples. The stepwise oxidative addition occurs through nucleophilic attack of M to R1-X1 to form an ion-pair intermediate. In the nucleophilic attack, CT occurs from the M d? to either the ?* orbital or empty p? orbital of R1-X1. Solvation plays a crucial role in stabilizing the transition state and ion-pair intermediate. The oxidative addition reactions of Ph-I, CH3-Br, and Br2B(OSiH3) to platinum(0), platinum(II), and palladium(0) complexes are discussed. In the heterolytic activation of R1-R2 by an M-X2 bond, R1 and R2 are bound with M and X2, respectively, indicating that R1 becomes anion-like and R2 becomes cation-like. CT mainly occurs from the X2 ligand to the ?* antibonding orbital of R1-R2 and also from R1 to the M empty d orbital. In the oxidative addition to an M-L moiety, R1 is bound with M, R2 is bound with L, and thus-formed L-R2 is bound with M. The oxidative addition reaction of the Si-H bond of silane to Cp2Zr(C2H4) and that of the H-H bond of H2 to Ni[MesB(o-Ph2PC6H4)2] are discussed as examples. The importance of the ?-bond activation in such catalytic reactions as nickel(0)-catalyzed phenylcyanation of alkyne, nickel(0)-catalyzed carboxylation of phenyl chloride, ruthenium(II)-catalyzed hydrogenation of carbon dioxide, and the Hiyama cross-coupling reaction is discussed based on theoretical studies. PMID:24784994

Guan, Wei; Sayyed, Fareed Bhasha; Zeng, Guixiang; Sakaki, Shigeyoshi

2014-07-01

64

Partial Oxidation Reactions of Methane and Oxygen  

Microsoft Academic Search

In considering the typical experimental conditions used for studies of methane oxidation to oxygenates or hydrocarbons in the presence of catalysts, it is clear that a significant potential exists for the appearance of non-catalyzed, thermally activated gas-phase reactions of methane and oxygen. In order to identify and describe the contribution of the gas phase processes during catalyzed reactions, a chemical

M. W. Droege; L. M. Hair; W. J. Pitz; C. K. Westbrook

1989-01-01

65

Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability  

NASA Astrophysics Data System (ADS)

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires—Au-1 wt.% Cu wire and Au-1 wt.% Pd wire—were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.

Gam, Sang-Ah; Kim, Hyoung-Joon; Cho, Jong-Soo; Park, Yong-Jin; Moon, Jeong-Tak; Paik, Kyung-Wook

2006-11-01

66

Cu-catalyzed C(sp³)-H bond activation reaction for direct preparation of cycloallyl esters from cycloalkanes and aromatic aldehydes.  

PubMed

Cu-catalyzed dehydrogenation-olefination and esterification of C(sp(3))-H bonds of cycloalkanes with TBHP as an oxidant has been developed. The reaction involves four C-H bond activations and gives cycloallyl ester products directly from cycloalkanes and aromatic aldehydes. PMID:24758330

Zhao, Jincan; Fang, Hong; Han, Jianlin; Pan, Yi

2014-05-01

67

Efficient oxidative cycloreversion reaction of photochromic dithiazolythiazole.  

PubMed

Electrochemical response of photochromic tearylenes was surveyed by means of cyclic voltammetry, DFT calculations, and spectroelectrochemistry. 4,5-Bis(2-phenyl-5-methylthiazolyl)-2-phenylthiazole was found to show electrochemical oxidative ring-cycloreversion reaction. The net current efficiency of the cycloreversion reaction under constant potential electrolysis was as high as 900%, which is ascribed to an electrochemical local-cell mechanism and a chain reaction mechanism. Electron transfer stopped-flow study using a chemical oxidant successfully identified radical cation intermediates of both closed- and open-ring isomers, involved in the oxidative cycloreversion process. The significantly long-lived radical cation of open-ring isomer with the lifetime of 33 s takes part in the indirect electron transfer process from the neutral closed-ring isomer to the radical cation of open-ring isomer in the chain reaction manner. PMID:23163734

Nakashima, Takuya; Kajiki, Yoshiyuki; Fukumoto, Sayo; Taguchi, Maki; Nagao, Satoshi; Hirota, Shun; Kawai, Tsuyoshi

2012-12-01

68

Graphite Oxidation Thermodynamics/Reactions.  

National Technical Information Service (NTIS)

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 c...

W. A. Propp

1998-01-01

69

Carbon-chalcogen bond cleavage reactions characterized for dinuclear sulfur-bridged cyclopentadienyl molybdenum complexes  

Microsoft Academic Search

The types of carbon-chalcogen bond cleavage reactions which have been characterized for dinuclear cyclopentadienylmolybdenum complexes are reviewed. The reactions include both the homolytic cleavage and the protonolysis of carbon-chalcogen bonds in neutral ?-thiolate derivatives of MoIII, nucleophilic cleavage of carbon-chalcogen bonds in certain cationic derivatives of MoIII or MoIV, and the reactions of certain dinuclear complexes with molecular hydrogen. Unique

1997-01-01

70

Theoretical model for oxidative activation of the O-H bond to platinum(0) complexes  

NASA Astrophysics Data System (ADS)

A density functional study has been used to investigate potential energy surfaces of the oxidative addition reaction PtL 2+H-OCH 3?PtL 2(H)(OCH 3) (L=CO, PH 3, L 2=H 2PCH 2PH 2, H 2PCH 2CH 2PH 2). The B3LYP/LANL2DZ results suggest that a smaller L-M-L angle and a better electron-donating ligand should promote oxidative addition of the O-H bond of CH 3OH. In contrast, a linear structure and a better electron-withdrawing ligand would be a good candidate for methanol elimination.

Su, Ming-Der; Chu, San-Yan

1998-01-01

71

Oxidation of copper pads and its influence on the quality of Au\\/Cu bonds during thermosonic wire bonding process  

Microsoft Academic Search

To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200°C under an air atmosphere. The bondability and bonding strength of the Au\\/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated

Cheng-li Chuang; Jong-ning Aoh; Rong-fong Din

2006-01-01

72

A Study of Protein Secondary Structure Hydrogen Bonds under Oxidizing Conditions  

Microsoft Academic Search

Molecular simulations show that hydrogen bond energies are stable between oxidized and unoxidized variants of the same protein, and that, consequentially, the secondary-structure elements are stable. Whilst there is no change in the energy of hydrogen bonds, it is shown that oxidation increases the number of hydrogen bonds between the oxidized histone octamer and its solvation layer. This analysis shows

Christopher M. Wood; H. J. Kadim

2007-01-01

73

Chemical Reactions and Mechanical Properties of the Directly Bonded Ge-Si Interface  

NASA Astrophysics Data System (ADS)

In this study, chemical reactions and mechanical properties of directly bonded Ge-Si interfaces are investigated. The Ge-Si bonded interface has been systematically characterized as a function of the thermal budget (200 °C and 300 °C), which demonstrated that the formation of a thin GeO2 cap layer by radical pre-treatment can reduce the generation rate of voids at the bonded interface significantly. Patterning of one of the wafers prior to bonding can help to achieve high bonding quality thanks to enhanced out-diffusion of reaction by-products and stress reduction at the bonded interface. Both numerical modeling and structural analysis show that the presence of diffusion path (channels) at the bonded interfaces result in a maximum bond strength and minimum stress at the bonded interface.

Byun, Ki Yeol; Ferain, Isabelle; Yu, Ran; Colinge, Cindy

2011-12-01

74

If C-H Bonds Could Talk - Selective C-H Bond Oxidation  

PubMed Central

C–H oxidation has a long history and an ongoing presence in research at the forefront of chemistry and interrelated fields. As such, numerous highly useful texts and reviews have been written on this subject. Logically, these are generally written from the perspective of the scope and limitations of the reagents employed. This minireview instead attempts to emphasize chemoselectivity imposed by the nature of the substrate. Consequently many landmark discoveries in the field of C–H oxidation are not discussed, but hopefully the perspective taken herein will allow for the more ready incorporation of C–H oxidation reactions into synthetic planning.

Newhouse, Timothy; Baran, Phil S.

2014-01-01

75

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

76

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

77

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

2012-06-19

78

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

79

Mullite fiber reinforced reaction bonded Si3N4 composites  

NASA Technical Reports Server (NTRS)

Fracture toughnesses of brittle ceramic materials have been improved by introducing reinforcements and carefully tailored interface layers. Silicon carbide and Si3N4 have been emphasized as matrices of structural composites intended for high temperature service because they combine excellent mechanical, chemical, thermal and physical properties. Both matrices have been successfully toughened with SiC fibers, whiskers and particles for ceramic matrix composite (CMC) parts made by sintering, hot pressing or reaction forming processes. These SiC reinforced CMCs have exhibited significantly improved toughnesses at low and intermediate temperature levels, as well as retention of properties at high temperatures for selected exposures; however, they are vulnerable to attack from elevated temperature dry and wet oxidizing atmospheres after the matrix has cracked. Property degradation results from oxidation of interface layers and/or reinforcements. The problem is particularly acute for small diameter (-20 tim) polymer derived SiC fibers used for weavable toes. This research explored opportunities for reinforcing Si3N4 matrices with fibers having improved environmental stability; the findings should also be applicable to SiC matrix CMCs.

Saleh, T.; Sayir, A.; Lightfoot, A.; Haggerty, J.

1996-01-01

80

SURFACE REACTIONS OF OXIDES OF SULFUR  

EPA Science Inventory

Surface reactions of several sulfur-containing molecules have been studied in order to understand the mechanism by which sulfate ions are formed on atmospheric aerosols. At 25C the heterogeneous oxidation of SO2 by NO2 to sulfuric acid and sulfate ions occurred on hydrated silica...

81

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

82

An Unusual Carbon-Carbon Bond Cleavage Reaction During Phosphinothricin Biosynthesis  

PubMed Central

Natural products containing phosphorus-carbon bonds have found widespread use in medicine and agriculture1. One such compound, phosphinothricin tripeptide (PTT), contains the unusual amino acid phosphinothricin (PT) attached to two alanine residues (Fig. 1). Synthetic PT (glufosinate) is a component of two top-selling herbicides (Basta® and Liberty®), and is widely used with resistant transgenic crops including corn, cotton and canola. Recent genetic and biochemical studies showed that during PTT biosynthesis 2-hydroxyethylphosphonate (HEP) is converted to hydroxymethylphosphonate (HMP) (Fig. 1)2. Reported here are the in vitro reconstitution of this unprecedented C(sp3)-C(sp3) bond cleavage reaction and X-ray crystal structures of the enzyme. The protein is a mononuclear non-heme iron(II)-dependent dioxygenase that converts HEP to HMP and formate. In contrast to most other members of this family, the oxidative consumption of HEP does not require additional cofactors or the input of exogenous electrons. The current study expands the scope of reactions catalyzed by the 2-His-1-carboxylate mononuclear non-heme iron family of enzymes.

Cicchillo, Robert M.; Zhang, Houjin; Blodgett, Joshua A.V.; Whitteck, John T.; Li, Gongyong; Nair, Satish K.; van der Donk, Wilfred A.; Metcalf, William W.

2010-01-01

83

Formation of porous surface layers in reaction bonded silicon nitride during processing  

NASA Technical Reports Server (NTRS)

Microstructural examination of reaction bonded silicon nitride (RBSN) has shown that there is often a region adjacent to the as-nitrided surfaces that is even more porous than the interior of this already quite porous material. Because this layer of large porosity is considered detrimental to both the strength and oxidation resistance of RBSN, a study was undertaken to determine if its formation could be prevented during processing. All test bars studied were made from a single batch of Si powder which was milled for 4 hours in heptane in a vibratory mill using high density alumina cylinders as the grinding media. After air drying the powder, bars were compacted in a single acting die and hydropressed.

Shaw, N. J.; Glasgow, T. K.

1979-01-01

84

Interfacial bonding stabilizes rhodium and rhodium oxide nanoparticles on layered Nb oxide and Ta oxide supports.  

PubMed

Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and ?-Zr(HPO4)2·H2O. High surface area SiO2 and ?-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ?H values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and ?-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and ?-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports. PMID:24654835

Strayer, Megan E; Binz, Jason M; Tanase, Mihaela; Shahri, Seyed Mehdi Kamali; Sharma, Renu; Rioux, Robert M; Mallouk, Thomas E

2014-04-16

85

Concerted Proton-Electron Transfer in the Oxidation of Hydrogen-Bonded Phenols  

PubMed Central

Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (–CPh2NH2), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, •OAr-BH+, a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and ??G‡/??G°. The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [•+HOAr-B] or initial proton transfer to give the zwitterions [?OAr-BH+]. The rate constant for heterogeneous electron transfer from HOAr-NH2 to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH2, the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus Theory. The CPET reorganization energies, ? = 23 – 56 kcal mol?1, are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly nonadiabatic, based on minimum values of Hrp derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.

Rhile, Ian J.; Markle, Todd F.; Nagao, Hirotaka; DiPasquale, Antonio G.; Lam, Oahn P.; Lockwood, Mark A.; Rotter, Katrina; Mayer, James M.

2008-01-01

86

Chemical bonding and stability of multilayer graphene oxide layers  

NASA Astrophysics Data System (ADS)

The chemistry of graphene oxide (GO) and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is however crucial to enable future applications of the material that typically are subject to environmental conditions. The nature of the initial GO is also highly dependent on the preparation and the form of the initial carbon material. Here, we consider both standard GO made from oxidizing graphite and layered GO made from oxidizing epitaxial graphene on SiC, and examine their evolution under different stimuli. The effect of the solvent on the thermal evolution of standard GO in vacuum is first investigated. In situ infrared absorption measurements clearly show that the nature of the last solvent in contact with GO prior to deposition on a substrate for vacuum annealing studies substantially affect the chemical evolution of the material as GO is reduced. Second, the stability of GO derived from epitaxial graphene (on SiC) is examined as a function of time. We show that hydrogen, in the form of CH, is present after the Hummers process, and that hydrogen favors the reduction of epoxide groups and the formation of water molecules. Importantly, this transformation can take place at room temperature, albeit slowly (~ one month). Finally, the chemical interaction (e.g. bonding) between GO layers in multilayer samples is examined with diffraction (XRD) methods, spectroscopic (IR, XPS, Raman) techniques, imaging (APF) and first principles modeling.

Gong, Cheng; Kim, Suenne; Zhou, Si; Hu, Yike; Acik, Muge; de Heer, Walt; Berger, Claire; Bongiorno, Angelo; Riedo, Eliso; Chabal, Yves

2014-03-01

87

Ion beam studies of the reactions of atomic cobalt ions with alkanes: determination of metal-hydrogen and metal-carbon bond energies and an examination of the mechanism by which transition metals cleave carbon-carbon bonds  

SciTech Connect

An ion beam apparatus is employed to study the reactions of singly charged cobalt positive ions with hydrogen and 17 alkanes. Reaction cross sections and product distributions as a function of kinetic energy are determined. Exothermic carbon-carbon bond cleavage reactions are observed for all alkanes but methane and ethane. A mechanism involving oxidative addition of C-C and C-H bonds to cobalt as a first step is demonstrated to account for all major reactions at all energies. Interpretation of several endothermic processes allows the extraction of thermochemical data. The bond dissociation energies obtained are D/sup 0/(Co/sup +/-H) = 52 +- 4 kcal/mol, D/sup 0/(Co-H) = 39 +- 6 kcal/mol, D/sup 0/(Co/sup +/-CH/sub 3/) = 61 +- 4 kcal/mol, and D/sup 0/(Co-CH/sub 3/) = 41 +- 10 kcal/mol.

Armentrout, P.B.; Beauchamp, J.L.

1981-02-25

88

Strength of hot isostatically pressed and sintered reaction bonded silicon nitrides containing Y2O3  

NASA Technical Reports Server (NTRS)

The hot isostatic pressing of reaction bonded Si3N4 containing Y2O3 produced specimens with greater room temperature strengths than those by high pressure nitrogen sintering of the same material. Average room temperature bend strengths for hot isostatically pressed reaction bonded silicon nitride and high pressure nitrogen sintered reaction bonded silicon nitride were 767 and 670 MPa, respectively. Values of 472 and 495 MPa were observed at 1370 C. For specimens of similar but lower Y2O3 content produced from Si3N4 powder using the same high pressure nitrogen sintering conditions, the room temperature strength was 664 MPa and the 1370 C strength was 402 MPa. The greater strengths of the reaction bonded silicon nitride materials in comparison to the sintered silicon nitride powder material are attributed to the combined effect of processing method and higher Y2O3 content.

Sanders, William A.; Mieskowski, Diane M.

1989-01-01

89

Halogen bond-assisted electron transfer reactions of aliphatic bromosubstituted electrophiles.  

PubMed

Isolation of pre-reactive complexes and analysis of kinetics of redox reactions of tetrabromomethane and tribromonitromethane demonstrated that halogen bonding results in significant lowering of the activation barriers for electron transfer, and that the rate constants of such (inner-sphere) processes can be evaluated from spectral, structural, and thermodynamic characteristics of the halogen-bonded associates. PMID:24336817

Rosokha, Sergiy V; Vinakos, Michael K

2014-02-01

90

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

91

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

PubMed Central

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

2013-01-01

92

Proton transfer reactions and hydrogen-bond networks in protein environments.  

PubMed

In protein environments, proton transfer reactions occur along polar or charged residues and isolated water molecules. These species consist of H-bond networks that serve as proton transfer pathways; therefore, thorough understanding of H-bond energetics is essential when investigating proton transfer reactions in protein environments. When the pKa values (or proton affinity) of the H-bond donor and acceptor moieties are equal, significantly short, symmetric H-bonds can be formed between the two, and proton transfer reactions can occur in an efficient manner. However, such short, symmetric H-bonds are not necessarily stable when they are situated near the protein bulk surface, because the condition of matching pKa values is opposite to that required for the formation of strong salt bridges, which play a key role in protein-protein interactions. To satisfy the pKa matching condition and allow for proton transfer reactions, proteins often adjust the pKa via electron transfer reactions or H-bond pattern changes. In particular, when a symmetric H-bond is formed near the protein bulk surface as a result of one of these phenomena, its instability often results in breakage, leading to large changes in protein conformation. PMID:24284891

Ishikita, Hiroshi; Saito, Keisuke

2014-02-01

93

Oxide treatments of Al 2024 for adhesive bonding- surface characterization  

NASA Astrophysics Data System (ADS)

A study was made to characterize the surface of oxides formed on Al 2024 in comparison to those formed on Al 1100, using Auger, ESCA, SIMS, FTIR and SEM/EDAX techniques. The pretreatments studied were chromate conversion coating, chromic acid anodizing and sulfuric acid anodizing. The specimens were evaluated as treated and after exposure to laboratory conditions ( T=25?3°C and relative humidity of 60?10%) for 6 months. In addition, a new surface pretreatment was studied, which was based on McDonnell Douglas P.S. 13201 specification. Adhesive bond strength of silicone rubbers and epoxy adhesives to pretreated Al 2024 was determined, using single lap joint specimens. It was shown that the main difference between the anodic coatings on Al 1100 and Al 2024 was the presence of copper on the surface in the latter. The presence of copper led us to postulate the mechanism of polymerization inhibition of 2 parts polysiloxane adhesive. The most suitable treatments for adhesive joining were found to be chromic acid sealed or unsealed anodizing for both Al 2024 and Al 1100. Our standard chromic acid anodizing gave higher bond strength for aluminum-adhesive joints than the "Douglas" process. as determined with several epoxy adhesives. Correlation between microscopic and macroscopic phenomena was established.

Fin, N.; Dodiuk, H.; Yaniv, A. E.; Drori, L.

1987-03-01

94

A general nonaqueous route to binary metal oxide nanocrystals involving a C-C bond cleavage.  

PubMed

A widely applicable solvothermal route to nanocrystalline iron, indium, gallium, and zinc oxide based on the reaction between the corresponding metal acetylacetonate as metal oxide precursor and benzylamine as solvent and reactant is presented. Detailed XRD, TEM, and Raman studies prove that, with the exception of the iron oxide system, where a mixture of the two phases magnetite and maghemite is formed, only phase pure materials are obtained, gamma-Ga(2)O(3), zincite ZnO, and cubic In(2)O(3). The particle sizes lie in the range of 15-20 nm for the iron, 10-15 nm for the indium, 2.5-3.5 nm for gallium, and around 20 nm for zinc oxide. GC-MS analysis of the final reaction solution after removal of the nanoparticles showed that the composition is rather complex consisting of more than eight different organic compounds. Based on the fact that N-isopropylidenebenzylamine, 4-benzylamino-3-penten-2-one, and N-benzylacetamide were the main species found, we propose a detailed formation mechanism encompassing solvolysis of the acetylacetonate ligand, involving C-C bond cleavage, as well as ketimine and aldol-like condensation steps. PMID:15826200

Pinna, Nicola; Garnweitner, Georg; Antonietti, Markus; Niederberger, Markus

2005-04-20

95

The role of reactant and product bond energies in determining limitations to selective catalytic oxidations  

Microsoft Academic Search

A wide range of selective oxidation reactions occurring in the gas phase over oxide catalysts have been surveyed. The reactions include oxidative dehydrogenation of alkanes, oxidation of alkenes and alcohols to aldehydes and the oxidation of alkanes to acids. The literature data was gathered into a series of selectivity-conversion plots and each plot was constructed with data from a variety

C. Batiot; B. K. Hodnett

1996-01-01

96

Metal Clusters and Extended Metal-Metal Bonding in Metal Oxide Systems.  

National Technical Information Service (NTIS)

The results of recent investigations of the synthesis and structures of ternary and quaternary molybdenum oxide compounds having metal-metal bonded cluster units or infinite chains are presented. In all cases the average oxidation state of molybdenum is l...

R. E. McCarley

1982-01-01

97

Microstructural evolution in reaction bonded alumina\\/mullite ceramics  

Microsoft Academic Search

Alumina, Al, and SiC powders were milled, compacted and heat-treated during which both the Al and SiC are oxidized. The Al2O3 and SiO2 that form then react to produce mullite, 3Al2O3*2SiO2. The goal of this process was to compensate for shrinkage that occurs during densification by expansion that occurs during oxidation and mullite formation to produce a body with little

1992-01-01

98

Oxidation chemistry of axially protected Mo(2) and W(2) quadruply bonded compounds.  

PubMed

Reported is a facile, high-yielding one-pot synthesis of the quadruply bonded ditungsten (II,II) compound W(2)(dpa)(4) (1) (dpa = 2,2'-dipyridylamide), which was obtained from W(CO)(6) at high temperature in naphthalene. A similar reaction in 1,2-dichlorobenzene furnished a ditungsten (III, III) species as the major product that was crystallized as [W(2)(dpa)(3)Cl(2)][BPh(4)] (3). The [W(2)(dpa)(3)Cl(2)](+) cation is better prepared by oxidation of 1 with SO(2)Cl(2). Compound 1 was characterized by X-ray crystallography and cyclic-voltammetry, and is compared with its earlier reported molybdenum analogue, Mo(2)(dpa)(4) (2). One-electron oxidation products of 1 and 2, [W(2)(dpa)(4)][BPh(4)] (1BPh(4)) and [Mo(2)(dpa)(4)][BPh(4)] (2BPh(4)), respectively, have also been synthesized. The crystallographically determined metal-metal distances of 2.23 A and 2.14 A in 1BPh(4) and 2BPh(4), respectively, are in agreement with metal-metal bond orders of 3.5. Unlike most previously reported Mo(2)(5+) and W(2)(5+) compounds, the primary coordination spheres around the M(2)-units in 1/1BPh(4) and 2/2BPh(4) remain unchanged upon one-electron oxidation, because the tridentate dpa ligand hinders axial coordination of exogenous ligands. PMID:19921777

Nippe, Michael; Victor, Eric; Berry, John F

2009-12-21

99

Reaction bonding and mechanical properties of mullite/silicon carbide composites  

SciTech Connect

Based on the RBAO technology, low-shrinkage mullite/SiC/Al[sub 2]O[sub 3]/ZrO[sub 2] composite were fabricated. A powder mixture of 40 vol% Al, 30 vol% Al[sub 2]O[sub 3], and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO[sub 2] wear debris into the mixture. The precursor powder was isopressed at 300--900 MPa and heat-treated in air by two different cycles resulting in various phase ratios in the final products. During heating, Al oxidizes to Al[sub 2]O[sub 3] completely, while SiC oxidizes to SiO[sub 2] only on its surface. Fast densification (at > 1300 C) and mullite formation (at 1400 C) prevent further oxidation of the SiC particles. Because of the volume expansion associated with the oxidation of Al (28%), SiC (108%), and the mullitization (4.2%), sintering shrinkage is effectively compensated. The reaction-bonded composites exhibit low linear shrinkages and high strengths: shrinkages of 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 490 MPa, corresponding to compaction pressure of 300, 600, and 900 MPa, respectively, were achieved in samples containing 49--55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 MPa and a toughness of 4.1 MPa[center dot]m[sup 1/2] increased to 890 MPa and 6 MPa[center dot]m[sup 1/2], respectively.

Wu, S. (Lehigh Univ., Bethlehem, PA (United States). Materials Research Center); Claussen, N. (Technische Univ. Hamburg-Harburg, Hamburg (Germany). Advanced Ceramics Group)

1994-11-01

100

REACTIONS OF 1,2-OXAPHOSPHOLENE-2- OXIDES 5. CONSEQUENCES OF REDUCTION  

Microsoft Academic Search

The reactions of 5,5-dimethyl-2-phenyl-1,2-oxaphosphol-3-ene 2-oxide (1a) with a variety of reducing agents have been studied. Catalytic hydrogenation of 1a (Pd\\/C) resulted in reduction of the C=C bond, giving 5,5-dimethyl-2-phenyl-1,2-oxaphospholane 2-oxide (6). By contrast, hydroboration with excess BH3-THF left the double bond intact, and reduced the P=O group. The product, 5,5-dimethyl-2-phenyl-l,2-oxaphosphol-3-ene (7), is itself unstable, but it can be isolated as

Maher Mualla; Roger S. Macomber

1990-01-01

101

Hypersensitivity reactions to ethylene oxide: clinical experience.  

PubMed

A hypersensitivity reaction occurring in the first minute of the dialysis procedure was observed in seven haemodialysis patients in one day. Hollow-fibre dialysers were used, five made of saponified cellulose ester (SCE) and two of cuprammonium cellulose (CC). All were sterilised with ethylene oxide (ETO) and used for the first time. The severity of the reactions was grade 2. The whole series of dialysers was examined for the presence of ETO concentration. A significantly higher concentration of ETO was found in the polyurethane potting than in the capillaries. The ETO concentrations were 122, 185, 440, 274, 342, and 280 p.p.m. in the following dialysers: Cordis Dow (cellulose acetate CA), Cordis Dow-Plivadial (SCE), Fresenius C-1.3 (CC), Fresenius E-2 (CC), Fresenius E-3 (CC), and Travenol-Medial S 11 flate plate (CC) respectively. According to the clinical signs, ETO concentrations in the dialysers and the lack of reaction when extensive rinsing was used, it can be presumed that these reactions are related to ETO although other mechanisms cannot be excluded. PMID:1775268

Masin, G; Polenakovi?, M; Ivanovski, N; Atanasov, N; Olivera, S; Cakalaroski, K

1991-01-01

102

Reaction of nitrosonium cation with resorc[4]arenes activated by supramolecular control: covalent bond formation.  

PubMed

Resorc[4]arenes 1 and 2, which previously proved to entrap NO(+) cation within their cavities under conditions of host-to-guest excess, were treated with a 10-fold excess of NOBF4 salt in chloroform. Kinetic and spectral UV-visible analyses revealed the formation of isomeric 1:2 complexes as a direct evolution of the previously observed event. Accordingly, three-body 1-(NO(+))2 and 2-(NO(+))2 adducts were built by MM and fully optimized by DFT calculations at the B3LYP/6-31G(d) level of theory. Notably, covalent nitration products 4, 5 and 6, 7 were obtained by reaction of NOBF4 salt with host 1 and 2, respectively, involving macrocycle ring-opening and insertion of a nitro group in one of the four aromatic rings. In particular, compounds 4 and 6, both containing a trans-double bond in the place of the methine bridge, were oxidized to aldehydes 5 and 7, respectively, after addition of water to the reaction mixture. Calculation of the charge and frontier orbitals of the aromatic donor (HOMO) and the NO(+) acceptor (LUMO) clearly suggests an ipso electrophilic attack by a first NO(+) unit on the resorcinol ring, mediated by the second NO(+) unit. PMID:23786237

Ghirga, Francesca; D'Acquarica, Ilaria; Delle Monache, Giuliano; Mannina, Luisa; Molinaro, Carmela; Nevola, Laura; Sobolev, Anatoly P; Pierini, Marco; Botta, Bruno

2013-07-19

103

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

2013-10-18

104

Guided ion beam studies of the reactions of Fe{sub n}{sup +} (n=2{endash}18) with O{sub 2}: Iron cluster oxide and dioxide bond energies  

SciTech Connect

The kinetic energy dependence of the reactions of Fe{sub n}{sup +} (n=2{endash}18) with O{sub 2} are studied in a guided ion beam mass spectrometer. A variety of Fe{sub m}O{sub 2}{sup +}, Fe{sub m}O{sup +}, and Fe{sub m}{sup +}, where m{le}n, product ions are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependence of the products is analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to iron cluster ions. The trends in this thermochemistry are discussed and compared to bulk phase values. {copyright} {ital 1997 American Institute of Physics.}

Griffin, J.B.; Armentrout, P.B. [Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (United States)] [Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (United States)

1997-03-01

105

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

NASA Technical Reports Server (NTRS)

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.

1994-01-01

106

Cascade radical reaction of substrates with a carbon-carbon triple bond as a radical acceptor  

PubMed Central

Summary The limitation of hydroxamate ester as a chiral Lewis acid coordination moiety was first shown in an intermolecular reaction involving a radical addition and sequential allylation processes. Next, the effect of hydroxamate ester was studied in the cascade addition–cyclization–trapping reaction of substrates with a carbon–carbon triple bond as a radical acceptor. When substrates with a methacryloyl moiety and a carbon–carbon triple bond as two polarity-different radical acceptors were employed, the cascade reaction proceeded effectively. A high level of enantioselectivity was also obtained by a proper combination of chiral Lewis acid and these substrates.

Asada, Ryuta

2013-01-01

107

Constraint Theory and Roken Bond Bending Constraints in Oxide Glasses  

NASA Astrophysics Data System (ADS)

The molecular structure of sodium tellurate glasses was established using ^{125}Te absorption and ^{129}I emission Mossbauer spectroscopies, differential scanning calorimetry (DSC), molar volume measurements and powder x-ray diffraction (XRD). The local atomic arrangement in these glasses is found to be different from that in corresponding crystals. This picture does not follow the usual thinking (Ioffe-Regel rule) about glass structure. The experimental evidence for this conclusion derives not only from Mossbauer spectroscopy but also from time-temperature -transformation curve and powder XRD measurements used to examine the crystallization of the bulk glasses. The TTT -curve exhibits both nucleation and growth branches, while XRD scans reveal growth of metastable phases before forming the stable crystalline phases. These results are in harmony with ^{23}Na solid state NMR results that reveal that sodium local environment in a x = 0.20 glass differs qualitatively from that of the crystalline counterpart. Results from DCS and XRD measurements reveal that at x = 0.18 several observables such as, dT_{g}/dx, activation energy for enthalpy relaxation, molar volume and Lamb-Mossbauer f factor, each display a threshold behavior. We believe that the physical origin of this threshold behavior comes from the rigidity percolation threshold. The constraint theory has recently been extended to include one-fold coordinated species and broken bond bending (beta) constraints. The latter was developed and has been applied successfully to many glass systems including the oxides, as we did for the first time in our Science paper, but also to chalcogenides and chalcohalides, etc.. In the experiments, the observed threshold apparently shifts to the over-constrained regime, i.e. > 2.4 in many glass systems. This shift is largely due to broken beta -constraint at some two-fold coordinated atoms, e.g. Se/S in chain segments and oxygen atoms. An example is g-Ge _{x}Se_{1-x } where one can understand the rigidity percolation threshold shift from x = 0.20 to x = 0.23, if one assumes a fraction of 20% chalcogen atoms have their bond angle constraints broken. A simple interpretation is that these chalcogen atoms (with broken bond bending constraints) represent short floppy chain-segments connecting the more rigid tetrahedral Ge(Se_{1/2} )_4 units. Thus the concept of broken bond bending constraints plays an important role in promoting glass forming tendency of materials. The extended constraint theory has also found application in aspect of mechanical property of hydrogenated diamond like carbon, silicon carbide and silicon thin films. We have established for the first time a linear relationship between measured hardness and hardness index, a geometric parameter derived from constraint theory. The slopes of such linear functions for different type materials are determined by chemical effect that reflect bonding type and interaction strength among atoms.

Zhang, Min

108

Kinetics and Mechanism of the Reaction of Ozone with Double Bonds  

NASA Astrophysics Data System (ADS)

Analysis of the results of recent studies has shown that the reaction of ozone with the double bonds of organic compounds apparently takes place not by a synchronous addition mechanism but through a reversible stage involving the formation of an intermediate complex of ozone with the double bond. The subsequent stages of the reaction involve the formation of a primary ozonide, its decomposition into two fragments, their combination, and a number of other transformations. The properties of the intermediate products, the relationship between the structure of the original alkenes and their reactivity towards ozone, the influence of the nature of the solvent on the direction and rate of the reaction, and the practical application of the reaction of ozone with double bonds in research and chemical technology have been examined. The bibliography contains 205 references.

Razumovskii, S. D.; Zaikov, Gennadii E.

1980-12-01

109

Novel dehydrogenase catalyzes oxidative hydrolysis of carbon-nitrogen double bonds for hydrazone degradation.  

PubMed

Hydrazines and their derivatives are versatile artificial and natural compounds that are metabolized by elusive biological systems. Here we identified microorganisms that assimilate hydrazones and isolated the yeast, Candida palmioleophila MK883. When cultured with adipic acid bis(ethylidene hydrazide) as the sole source of carbon, C. palmioleophila MK883 degraded hydrazones and accumulated adipic acid dihydrazide. Cytosolic NAD+- or NADP+-dependent hydrazone dehydrogenase (Hdh) activity was detectable under these conditions. The production of Hdh was inducible by adipic acid bis(ethylidene hydrazide) and the hydrazone, varelic acid ethylidene hydrazide, under the control of carbon catabolite repression. Purified Hdh oxidized and hydrated the C=N double bond of acetaldehyde hydrazones by reducing NAD+ or NADP+ to produce relevant hydrazides and acetate, the latter of which the yeast assimilated. The deduced amino acid sequence revealed that Hdh belongs to the aldehyde dehydrogenase (Aldh) superfamily. Kinetic and mutagenesis studies showed that Hdh formed a ternary complex with the substrates and that conserved Cys is essential for the activity. The mechanism of Hdh is similar to that of Aldh, except that it catalyzed oxidative hydrolysis of hydrazones that requires adding a water molecule to the reaction catalyzed by conventional Aldh. Surprisingly, both Hdh and Aldh from baker's yeast (Ald4p) catalyzed the Hdh reaction as well as aldehyde oxidation. Our findings are unique in that we discovered a biological mechanism for hydrazone utilization and a novel function of proteins in the Aldh family that act on C=N compounds. PMID:18096698

Itoh, Hideomi; Suzuta, Tetsuya; Hoshino, Takayuki; Takaya, Naoki

2008-02-29

110

Tandem application of C-C bond-forming reactions with reductive ozonolysis  

PubMed Central

Several variants of reductive ozonolysis, defined here as the in situ generation of aldehydes or ketones during ozonolytic cleavage of alkenes, are demonstrated to work effectively in tandem with a number of C-C bond-forming reactions. For reactions involving basic nucleophiles (1,2-addition of Grignard reagents, Wittig or Horner-Emmons olefinations, and directed Aldol reactions of lithium enolates) the one-pot process offers a rapid and high-yielding alternative to traditional two-step protocols.

Willand-Charnley, Rachel; Dussault, Patrick H.

2012-01-01

111

Concerted O atom-proton transfer in the O--O bond forming step in water oxidation  

PubMed Central

As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H2O ? O2 + 4e- + 4H+) is key, but it imposes a significant mechanistic challenge with requirements for both 4e-/4H+ loss and O—O 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)(OH2)]2+ [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2?-bipyridine]. When oxidized from to RuV = O3+, these complexes undergo O—O bond formation by O-atom attack on a H2O molecule, which is often the rate-limiting step. Microscopic details of O—O 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 atom–proton 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 J.; Hu, Xiangqian; Yang, Weitao; Hoertz, Paul G.; Meyer, Thomas J.

2010-01-01

112

Bonding  

NSDL National Science Digital Library

Bonding Purpose: To review the 3 Types of Bonds and the 4 Intermolecular Forces. Assignment: Answer the following questions on a separate piece of paper: 1) List the 3 types of bonding, give descriptions and one example of each 2) List the 4 intermolecular forces and draw examples of each. Be ...

Cutting, Mrs.

2008-03-08

113

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

114

On the electronic nature of low-barrier hydrogen bonds in enzymatic reactions.  

PubMed

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 an "atoms in molecules" analysis of the electron density, it is found that the hydrogen atom is stabilized by covalent bonds to both oxygens. Large atomic partial charges on the hydrogen-bonded atoms are found experimentally and theoretically. Therefore, the hydrogen bond gains stabilization from both covalency and from the normal electrostatic interactions found for long, weak hydrogen bonds. Based on comparisons with other systems having short-strong hydrogen bonds or LBHBs, it is proposed that all short-strong and LBHB systems possess similar electronic features of the hydrogen-bonded region, namely polar covalent bonds between the hydrogen atom and both heteroatoms in question. PMID:9788994

Schiøtt, B; Iversen, B B; Madsen, G K; Larsen, F K; Bruice, T C

1998-10-27

115

Promotion effect of metal oxides on CO bond dissociation in CO hydrogenation over Rh\\/SiO 2 Catalyst — possible role of partially reduced state cations  

Microsoft Academic Search

In CO hydrogenation over Rh\\/SiO2 catalysts, the effect of additive metal oxides on C-O bond dissociation was studied by using pulse surface reaction rate\\u000a analysis (PSRA). The addition of oxides of Al, Ti, Cr, V, and Mn resulted in an increase in the rate constant for the dissociation\\u000a according to this sequence, while the oxides of Cu, Zn, and Ag

Hajime Kato; Masaki Nakashima; Yoshihiko Mori; Toshiaki Mori; Tadashi Hattori; Yuichi Murakami

1995-01-01

116

Isothermal oxidation of air plasma spray NiCrAlY bond coatings  

Microsoft Academic Search

The isothermal oxidation behaviors of plasma-sprayed NiCrAlY bond coatings (Ni22Cr10Al1Y and Ni31Cr11Al 0.6Y) have been evaluated. Two unique microstructures, oxide stringers and improperly flattened zones, can be observed in the air plasma spray (APS) bond coatings. The structures and chemical compositions of the oxide stringers were examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Improperly flattened zones

Hanshin Choi; Byounghyun Yoon; Hyungjun Kim; Changhee Lee

2002-01-01

117

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

2013-06-21

118

Electrochromic reactions in manganese oxides I. Raman analysis  

Microsoft Academic Search

Like nickel oxide, manganese oxide is a widely studied material in the primary batteries field. The reactions taking place during voltametric cycling of manganese oxides can be determined using in situ Raman spectroscopy. The main difficulty for the oxide identification is to obtain relevant Raman reference spectra because of the many possible compounds and, for some of these compounds, of

M. C. Bernard; A. Hugot-Le Goff; B. V. Thi; S. Cordoba de Torresi

1993-01-01

119

Overcoming the "oxidant problem": strategies to use O2 as the oxidant in organometallic C-H oxidation reactions catalyzed by Pd (and Cu).  

PubMed

Oxidation reactions are key transformations in organic chemistry because they can increase chemical complexity and incorporate heteroatom substituents into carbon-based molecules. This principle is manifested in the conversion of petrochemical feedstocks into commodity chemicals and in the synthesis of fine chemicals, pharmaceuticals, and other complex organic molecules. The utility and function of these molecules correlate directly with the presence and specific placement of oxygen and nitrogen heteroatoms and other functional groups within the molecules. Methods for selective oxidation of C-H bonds have expanded significantly over the past decade, and their role in the synthesis of organic chemicals will continue to increase. Our group's contributions to this field are linked to our broader interest in the development and mechanistic understanding of aerobic oxidation reactions. Molecular oxygen (O(2)) is the ideal oxidant. Its low cost and lack of toxic byproducts make it a highly appealing reagent that can address key "green chemistry" priorities in industry. With strong economic and environmental incentives to use O(2), the commmodity chemicals industry often uses aerobic oxidation reactions. In contrast, O(2) is seldom used to prepare more-complex smaller-volume chemicals, a limitation that reflects, in part, the limited synthetic scope and utility of existing aerobic reactions. Pd-catalyzed reactions represent some of the most versatile methods for selective C-H oxidation, but they often require stoichiometric transition-metal or organic oxidants, such as Cu(II), Ag(I), or benzoquinone. This Account describes recent strategies that we have identified to use O(2) as the oxidant in these reactions. In Pd-catalyzed C-H oxidation reactions that form carbon-heteroatom bonds, the stoichiometric oxidant is often needed to promote difficult reductive elimination steps in the catalytic mechanism. To address this challenge, we have identified new ancillary ligands for Pd that promote reductive elimination, or replaced Pd with a Cu catalyst that undergoes facile reductive elimination from a Cu(III) intermediate. Both strategies have enabled O(2) to be used as the sole stoichiometric oxidant in the catalytic reactions. C-H oxidation reactions that form the product via ?-hydride or C-C reductive elimination steps tend to be more amenable to the use of O(2). The use of new ancillary ligands has also overcome some of the limitations in these methods. Mechanistic studies are providing insights into some (but not yet all) of these advances in catalytic reactivity. PMID:22263575

Campbell, Alison N; Stahl, Shannon S

2012-06-19

120

Facile one-pot synthesis of [1,?2,?3]triazolo[1,?5-a]pyridines from 2-acylpyridines by copper(II)-catalyzed oxidative N-N bond formation.  

PubMed

An efficient and simple method for the synthesis of various [1,?2,?3]triazolo[1,?5-a]pyridines has been established. The method involves a copper(II)-catalyzed oxidative N-N bond formation that uses atmospheric oxygen as the terminal oxidant following hydrazonation in one pot. The use of ethyl acetate as the solvent dramatically promotes the oxidative N-N bond-formation reaction and enables the application of oxidative cyclization in the efficient one-pot reaction. A mechanism for the reaction was proposed on the basis of the results of a spectroscopic study. PMID:24615898

Hirayama, Tasuku; Ueda, Satoshi; Okada, Takahiro; Tsurue, Norihiko; Okuda, Kensuke; Nagasawa, Hideko

2014-04-01

121

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

122

Reaction-diffusion analysis for one-step plasma etching and bonding of microfluidic devices  

NASA Astrophysics Data System (ADS)

A self-similar reaction front develops in reactive ion etching when the ions penetrate channels of shallow height h. This relates to the patterning of microchannels using a single-step etching and bonding, as described by Rhee et al. [Lab Chip 5, 102 (2005)]. Experimentally, we report that the front location scales as xf~ht1/2 and the width is time-invariant and scales as ?~h. Mean-field reaction-diffusion theory and Knudsen diffusion give a semiquantitative understanding of these observations and allow optimization of etching times in relation to bonding requirements.

Rosso, Michel; van Steijn, Volkert; de Smet, Louis C. P. M.; Sudhölter, Ernst J. R.; Kleijn, Chris R.; Kreutzer, Michiel T.

2011-04-01

123

Preparation and properties of UV curable acrylic PSA by vinyl bonded graphene oxide  

NASA Astrophysics Data System (ADS)

Acrylic pressure sensitive adhesives (PSAs) with higher thermal stability for thin wafer handling were successfully prepared by forming composite with the graphene oxide (GO) nanoparticles modified to have vinyl groups via subsequent reaction with isophorone diisocyanate and 2-hydroxyethyl methacrylate. The acrylic copolymer was synthesized as a base resin for PSAs by solution radical polymerization of ethyl acrylate, 2-ethylhexyl acrylate, and acrylic acid followed by further modification with GMA to have the vinyl groups available for UV curing. The peel strength of PSA decreased with the increase of gel content which was dependent on both modified GO content and UV dose. Thermal stability of UV-cured PSA was improved noticeably with increasing the modified GO content mainly due to the strong and extensive interfacial bonding formed between the acrylic copolymer matrix and GO fillers

Pang, Beili; Ryu, Chong-Min; Jin, Xin; Kim, Hyung-Il

2013-11-01

124

Intramolecular Pd(II)-catalyzed oxidative biaryl synthesis under air: reaction development and scope.  

PubMed

New reaction conditions for intramolecular palladium(II)-catalyzed oxidative carbon-carbon bond formation under air are described. The use of pivalic acid as the reaction solvent, instead of acetic acid, results in greater reproducibility, higher yields, and broader scope. This includes the use of electron-rich diarylamines as illustrated in the synthesis of three naturally occurring carbazole products: Murrayafoline A, Mukonine, and Clausenine. A variety of side products have also been isolated, casting light on competing reaction pathways and revealing new reactivity with palladium(II) catalysis. PMID:18543991

Liégault, Benoît; Lee, Doris; Huestis, Malcolm P; Stuart, David R; Fagnou, Keith

2008-07-01

125

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

Fernández, Israel; Solà, Miquel; Bickelhaupt, F Matthias

2013-06-01

126

Studies of reductive elimination reactions to form carbon-oxygen bonds from Pt(IV) complexes.  

PubMed

The platinum(IV) complexes fac-L(2)PtMe(3)(OR) (L(2) = bis(diphenylphosphino)ethane, o-bis(diphenylphosphino)benzene, R = carboxyl, aryl; L = PMe(3), R = aryl) undergo reductive elimination reactions to form carbon-oxygen bonds and/or carbon-carbon bonds. The carbon-oxygen reductive elimination reaction produces either methyl esters or methyl aryl ethers (anisoles) and L(2)PtMe(2), while the carbon-carbon reductive elimination reaction affords ethane and L(2)PtMe(OR). Choice of reaction conditions allows the selection of either type of coupling over the other. A detailed mechanistic study of the reductive elimination reactions supports dissociation of the OR(-) ligand as the initial step for the C-O bond formation reaction. This is followed by a nucleophilic attack of OR(-) upon a methyl group bound to the Pt(IV) cation to produce the products MeOR and L(2)PtMe(2). C-C reductive elimination proceeds from L(2)PtMe(3)(OR) by initial L (L = PMe(3)) or OR(-) (L(2) = dppe, dppbz) dissociation, followed by C-C coupling from the resulting five-coordinate intermediate. Our studies demonstrate that both C-C and C-O reductive elimination reactions from Pt(IV) are more facile in polar solvents, in the presence of Lewis acids, and for OR(-) groups that contain electron withdrawing substituents. PMID:11456927

Williams, B S; Goldberg, K I

2001-03-21

127

Arylpalladium Phosphonate Complexes as Reactive Intermediates in Phosphorus-Carbon Bond Forming Reactions  

SciTech Connect

Phosphorus-carbon bond formation from discrete transition metal complexes have been investigated through a combination of synthetic, spectroscopic, crystallographic, and computational methods. Reactive intermediates of the type (diphosphine)Pd(aryl)(P(O)(OEt)(2)) have been prepared, characterized, and studied as possible intermediates in metal-mediated coupling reactions. Several of the reactive intermediates were characterized crystallographicaliy, and a discussion of the solid state structures is presented. In contrast to other carbon-heteroelement bond forming reactions, palladium complexes containing electron-donating substituents on the aromatic fragment exhibited faster rates of reductive elimination. Large bite angle diphosphine ligands induced rapid rates of elimination, while bipyridine and small bite angle diphosphine ligands resulted in much slower rates of elimination. An investigation of the effect of typical impurities on the elimination reaction was carried out. While excess diphosphine, pyridine, and acetonitrile had little effect on the observed rate, the addition of water slowed the phosphorus-carbon bond forming reaction. Coordination of water to the complex was observed spectroscopically and crystallographically. Computational studies were utilized to probe the reaction pathways for P-C bond formation via Pd catalysis.

Kohler, Mark C. [Bucknell University; Grimes, Thomas V. [University of North Texas; Wang, Xiaoping [ORNL; Cundari, Thomas R. [University of North Texas; Stockland, Robert A. Jr. [Bucknell University

2009-01-01

128

Modeling of metal–oxide semiconductor: Analytical bond-order potential for cupric oxide  

NASA Astrophysics Data System (ADS)

Atomistic potentials for cupric element and cupric oxide are derived based on the analytical bond-order scheme that was presented by Brenner [Brenner D W, “Erratum: Empirical potential for hydrocarbons for use in simulating the chemical vapor deposition of diamond films”, Phys. Rev. B 1992, 46 1948]. In this paper, for the pure cupric element, the energy and structural parameters for several bulk phases as well as dimmer structure are well reproduced. The reference data are taken from our density functional theory calculations and the available experiments. The model potential also provides a good description of the bulk properties of various solid structures of cupric oxide compound structures, including cohesive energies, lattice parameters, and elastic constants.

Li, Kun; Yang, Wen; Wei, Ji-Lin; Du, Shi-Wen; Li, Yong-Tang

2014-04-01

129

Reaction of Metal Sulfates with Molybdenum Oxide, a High Temperature Reaction Step for Thermochemical Cycles.  

National Technical Information Service (NTIS)

The reaction of sulfates with molybdenum oxide at high temperature was studied to determine the feasibility of using these reactions in a thermochemical process for making hydrogen. The experiments were performed by heating the blended powders to temperat...

C. M. Hollabaugh T. C. Wallace M. G. Bowman W. M. Jones

1980-01-01

130

The Reaction of HppE with Substrate Analogues: Evidence for Carbon-Phosphorus Bond Cleavage by a Carbocation Rearrangement  

PubMed Central

(S)-2-Hydroxypropylphosphonic acid ((S)-2-HPP) epoxidase (HppE) is an unusual mononuclear non-heme iron enzyme that catalyzes the oxidative epoxidation of (S)-2-HPP in the biosynthesis of the antibiotic fosfomycin. Recently, HppE has been shown to accept (R)-1-hydroxypropylphosphonic acid ((R)-1-HPP) as a substrate and convert it to an aldehyde product in a reaction involving a biologically unprecedented 1,2-phosphono migration. In this study, a series of substrate analogues were designed, synthesized, and used as mechanistic probes to study this novel enzymatic transformation. The resulting data, together with insights obtained from density functional theory calculations, are consistent with a mechanism of HppE-catalyzed phosphono group migration that involves the formation of a carbocation intermediate. As such, this reaction represents a new paradigm for biological C-P bond cleavage.

Chang, Wei-chen; Mansoorabadi, Steven O.; Liu, Hung-wen

2013-01-01

131

Modeling of the reaction-based processing of aluminum oxide (RBAO) and alumina-aluminide alloys (3A)  

Microsoft Academic Search

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

Shaun Patrick Gaus

1998-01-01

132

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

Microsoft Academic Search

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

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

1995-01-01

133

Thermochemical benchmarking of hydrocarbon bond separation reaction energies: Jacob's ladder is not reversed!  

Microsoft Academic Search

We reinvestigate the performance of Kohn–Sham density functional (DF) methods for a thermochemical test set of bond separation reactions of alkanes (BSR36) published recently by Steinmann et al. [J. Chem. Theory Comput. 5, 2950 (2009)]. According to our results, the tested approximations perform for this rather special benchmark as usual. We show that the choice of reference enthalpies plays a

Helge Krieg; Stefan Grimme

2010-01-01

134

Effect of silicon oxide, silicon nitride and polysilicon layers on the electrostatic pressure during anodic bonding  

Microsoft Academic Search

The quality of the anodic bonding between glass and silicon wafers coated with silicon oxide, silicon nitride and polysilicon layers has been investigated. We have used an electrostatic test to study the effect of these layers on the quality of the bond. The electrostatic test shows how the electrostatic pressure decreases with the increase of the thickness of the silicon

J. A. Plaza; J. Esteve; E. Lora-Tamayo

1998-01-01

135

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

NASA Astrophysics Data System (ADS)

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 TiO2 nanoparticles, TiO2 nanorods, FeOOH nanowires, Fe2O3 nanoparticles, and SnO2 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. SN1 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.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 TiO2 nanoparticles, TiO2 nanorods, FeOOH nanowires, Fe2O3 nanoparticles, and SnO2 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. SN1 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. Electronic supplementary information (ESI) available. See DOI: 10.1039/c2nr32255b

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

2012-09-01

136

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities  

SciTech Connect

A classification of the HF 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 density evaluated at the bond critical points, rc. A calculation of the ratio for the MO bonded interactions comprising a relatively large number of molecules and earth materials, together with the constraints imposed by the values of Ñ2?(rc) and the local electronic energy density H(rc) = G(rc) + V(rc) in the HF study, yielded the same classification for the oxides as found for the fluorides. This is true despite the different trends of the bond critical point and local energy properties with the bond length displayed by the HF and MO bonded interactions. LiO, NaO and MgO bonded interactions classify as closed shell ionic bonds, BeO, AlO, SiO, BO and PO bonded interactions classify as bonds of intermediate character and NO bonded interactions classify as shared covalent bonds. CO and SO bonded interactions classify as both intermediate and covalent bonded interactions. The CO triple bonded interaction classifies as a bond of intermediate character and the CO single bonded interaction classifies as a covalent bond whereas their H(rc) value indicates that they are both covalent bonds. The |V(rc)|/G(rc) ratios for the BeO, AlO and SiO bonded interactions indicate that they have a substantial component of ionic character despite their classification as bonds of intermediate character. The trend between |V(rc)|/G(rc) and the character of the bonded interaction is consistent with trends expected from electronegativity considerations. The connection between the net charges and the experimental SiO bond length evaluated for the Si and O atoms comprising two orthosilicates are examined in terms of the |V(rc)|/G(rc) values.

Gibbs, Gerald V.; Cox, David; Crawford, T Daniel; Rosso, Kevin M.; Ross, Nancy; Downs, R. T.

2006-02-28

137

Mimicking the photosynthetic system with strong hydrogen bonds to promote proton electron concerted reactions  

Microsoft Academic Search

A Copper(2+) complex with a CuII–C bond containing sp3 configuration was used to investigate the role of strong hydrogen bonds in proton coupled electron transfer (PCET) reactions.\\u000a The only example of a CuII–C system realized so far is that using tris-(pyridylthio)methyl (tptm) as a tetradentate tripodal ligand. Using this ligand,\\u000a [CuF(tptm)] and [Cu(tptm)(OH)] have been prepared. The former complex forms

Isamu Kinoshita; Hideki Hashimoto; Takanori Nishioka; Riichi Miyamoto; Naoto Kuwamura; Yuki Yoshida

2008-01-01

138

Reaction of thiols with N-bonded sulfenamide complexes of cobalt(III): steric effect and reaction pathway.  

PubMed

The products and kinetics of the reaction of several thiols (RSH = 2-aminoethanethiol, cysteine, penicillamine, cysteine ethyl ester) with N-bonded sulfenamide complexes ([Co(en)(2)(NH(2)S(CH(2))(2)NH(2)](3+) (IA), [Co(en)(2)(NH(2)SCH(2)CH(CO(2)H)NH(2)](3+) (IC), [Co(en)(2)(NH(2)SC(CH(3))(2)CH(CO(2)H)NH(2)](3+) (IP)) have been studied. The reaction proceeds by nucleophilic attack at sulfur with cleavage of the N-S bond to form a disulfide and leave a coordinated NH(3) ligand. The kinetics (pH 4-10) reveal that the deprotonated thiol, RS(-), is the reactive nucleophile and that the N-deprotonated sulfenamide complex is unreactive. The reactions of IP are approximately 10(4) times slower than those of IA or IC, and the reasons and consequences of this large steric effect are discussed. It is concluded, on the basis of these and other observations from the literature, that there will be substantial steric retardation to nucleophilic attack at two-coordinate sulfur in a R-C(CH(3))(2)-S-X-R' unit because of the regiospecificity of the reaction. The acid dissociation constants of IP and the X-ray structure of its bromide salt also are reported. PMID:15651875

Sisley, Margaret J; Ferguson, Michael J; Jordan, Robert B

2005-01-24

139

Anodic Oxide Formation on Ti-6Al-4V in Chromic Acid for Adhesive Bonding.  

National Technical Information Service (NTIS)

Chromic acid anodization (CAA) of Ti-6Al-4V alloy has been shown to produce desirable oxide for adhesive bonding. The highly porous oxide layer provides mechanical interlocking with the adhesive or the primer forming a much stronger interface. This leads ...

A. M. T. Cheng D. W. Dwight

1984-01-01

140

Green oxidations: Titanium dioxide induced tandem oxidation coupling reactions  

PubMed Central

Summary The application of titanium dioxide as an oxidant in tandem oxidation type processes is described. Under microwave irradiation, quinoxalines have been synthesized in good yields from the corresponding ?-hydroxyketones.

Jeena, Vineet

2009-01-01

141

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

142

Cleavage of ether, ester, and tosylate C(sp3)-O bonds by an iridium complex, initiated by oxidative addition of C-H bonds. Experimental and computational studies.  

PubMed

A pincer-ligated iridium complex, (PCP)Ir (PCP = ?(3)-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp(3))-O bonds of methyl esters (CH3-O2CR'), methyl tosylate (CH3-OTs), and certain electron-poor methyl aryl ethers (CH3-OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C-H bonds followed by oxygenate migration, rather than by direct C-O addition. Thus, methyl aryl ethers react via addition of the methoxy C-H bond, followed by ?-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C-H bond addition at the carbomethoxy group to give (PCP)Ir(H)[?(2)-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(?(2)-O2CMe) (6-Me) in which the methoxy C-O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(?(2)-O2CR), the net product of methoxy group C-O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses ?-C-H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C-O bond oxidative additions, these reactions also proceed via initial activation of a C-H bond; in this case, C-H addition at the ?-position is followed by ?-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the ?-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C-O bond is cleaved with no direct participation by the iridium center. PMID:23469859

Kundu, Sabuj; Choi, Jongwook; Wang, David Y; Choliy, Yuriy; Emge, Thomas J; Krogh-Jespersen, Karsten; Goldman, Alan S

2013-04-01

143

Topochemical reactions of layered transition-metal oxides  

NASA Astrophysics Data System (ADS)

Low-temperature topochemical reactions performed on complex transition metal oxides allow the synthesis of novel, metastable phases often with unusual transition metal oxidation states and/or coordination geometries. Layered materials offer many opportunities to control the structural and chemical selectivity of these types of reactions to allow the preparation of desired phases in a rational manner. Using complex oxides adopting Ruddlesden–Popper type structures as examples, recent progress in the low-temperature topochemical reduction by anion de-intercalation, and oxidation by anion insertion, is reviewed.

Hayward, M. A.

2014-06-01

144

Theoretical study of reaction pathways for the rhodium phosphine-catalysed borylation of C-H bonds with pinacolborane.  

PubMed

The reaction mechanism of the rhodium-phosphine catalysed borylation of methyl-substituted arenes using pinacolborane (HBpin) has been investigated theoretically using DFT calculations at the B3PW91 level. Factors affecting selectivity for benzylic vs. aromatic C-H bond activation have been examined. It was found that [Rh(PR3)2(H)] is the active species which oxidatively adds the C-H bond leading to an eta3-benzyl complex which is the key to determining the unusual benzylic regioselectivity observed experimentally for this catalyst system. Subsequent reaction with HBpin leads to a [Rh(PR3)2(eta3-benzyl)(H)(Bpin)] complex from which B-C reductive elimination provides product and regenerates the catalyst. The electrophilic nature of the boryl ligand assists in the reductive elimination process. In contrast to Ir(L)2(boryl)3-based catalysts, for which Ir(III)-Ir(V) cycles have been proposed, the Rh(I)-Rh(III) cycle is operating with the system addressed herein. PMID:15252604

Lam, Wai Han; Lam, King Chung; Lin, Zhenyang; Shimada, Shigeru; Perutz, Robin N; Marder, Todd B

2004-05-21

145

Reaction of arene oxides with nucleosides.  

PubMed

Inosine was reacted with phenanthrene-9,10-oxide and 7,12-dimethylbenz(a)anthracene-5,6-oxide (DMBA-5,6-oxide) to yield the corresponding N-1-alkylinosines. They were shown to undergo hydrolysis to 5-amino-4-imidazole-N-alkylcarboxamide ribosides when heated in the presence of sodium carbonate. Guanosine was reacted with DMBA-5,6-oxide to yield a mixture of N-7-alkylguanines along with a smaller amount of an orcine positive product exhibiting an ultraviolet spectrum resembling that of DMBA-5,6-dihydrodiol. PMID:409483

Friesel, H; Hecker, E

1977-09-01

146

Effects of sputtered metal oxide films on the ceramic-to-metal bond.  

PubMed

The application of a thin oxide film is seen as a method of improving the ceramic-to-metal bond. Using sputter coating to form oxide films allows control of its thickness. The thickness of the oxide layer can also be controlled by sputter coating. Films produced by sputtering, in themselves, have superior bond strengths in that the high-energy levels that are used in the technique drive the coating material into the atomic lattice of the substrate material (dental gold). Remaining to be determined are optimum film thickness and the oxide to be used. However, the present study opens a field of study showing promise of improving porcelain-to-metal bonding in dental restoration. PMID:3908655

Bullard, J T; Dill, R E; Marker, V A; Payne, E V

1985-12-01

147

Nondestructive Impedance Spectroscopy Evaluation of the Bond Coat Oxidation in Thermal Barrier Coatings  

NASA Astrophysics Data System (ADS)

In this paper, the impedance spectroscopy technique was employed to examine nondestructively the isothermal oxidation of air plasma sprayed (APS) thermal barrier coatings (TBCs) in air at 800°C. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were also used to characterize the microstructure evolution of TBCs. After oxidation, the thermally grown oxide (TGO), which was mainly composed of alumina as confirmed by EDX, formed at the upper ceramic coat/bond coat interface, the lower bond coat/substrate interface, and the bond coat. Impedance diagrams obtained from impedance measurements at room temperature were analyzed according to the equivalent circuit model proposed for the TBCs. Various observed electrical responses relating to the growth of oxides and the sintering of YSZ were explained by simulating the impedance spectra of the TBCs.

Yang, L.; Zhou, Y. C.; Mao, W. G.; Liu, Q. X.

148

Electrochromic reactions in manganese oxides I. Raman analysis  

SciTech Connect

Like nickel oxide, manganese oxide is a widely studied material in the primary batteries field. The reactions taking place during voltametric cycling of manganese oxides can be determined using in situ Raman spectroscopy. The main difficulty for the oxide identification is to obtain relevant Raman reference spectra because of the many possible compounds and, for some of these compounds, of their instability in the laser beam. As a consequence, several modifications of different tetra-, tri- and divalent manganese oxides and oxyhydroxides were carefully studied. The electrochromic behavior of three types of manganese oxides, two prepared by thermal oxidations and the other by electrochemical deposition, were then compared. The presence of nonstoichiometry in the pristine material was necessary to obtain a reversible electrochromic effect. The reaction during electrochromic cycling is more complicated than a simple passage from MnO[sub 2] to MnOOH.

Bernard, M.C.; Hugot-Le Goff, A.; Thi, B.V. (Univ. Pierre et Marie Curie, Paris (France). UPR 15 du CNRS Physique des Liquides et Electrochimie); Cordoba de Torresi, S. (Univ. Estadual de Campinas (Brazil). Dept. de Fisica Aplicada)

1993-11-01

149

Reaction of metal sulfates with molybdenum oxide, a high temperature reaction step for thermochemical cycles  

Microsoft Academic Search

The reaction of sulfates with molybdenum oxide at high temperature was studied to determine the feasibility of using these reactions in a thermochemical process for making hydrogen. The experiments were performed by heating the blended powders to temperatures of 1075 K and higher and analyzing the evolved gas for acid content. It was found that the reaction rate over the

C. M. Hollabaugh; T. C. Wallace; M. G. Bowman; W. M. Jones

1980-01-01

150

Studies of reactions of small titanium oxide cluster cations toward oxygen at thermal energies  

NASA Astrophysics Data System (ADS)

Reactions of small titanium oxide cluster cations with oxygen are studied using a selected-ion drift tube operated at thermal energies, in conjunction with a laser vaporization source. Clusters of most compositions are observed to react rapidly with oxygen in the drift tube reactor, but a few of them display very inert behavior. The studies of size-selected oxide clusters indicate that the inert clusters belong to two series, namely, (TiO2)+n (n [greater-or-equal, slanted] 3) and TiO3(TiO2)+m (m [greater-or-equal, slanted] 1). Interestingly, reactions of the oxygen-poor clusters with the oxygen molecule are seen to yield products containing fewer titanium atoms, while the clusters with higher oxygen-to-titanium ratios undergo attachment of one or two oxygen atoms. The reaction patterns are interpreted in terms of the difference in the bonding structure of the clusters.

Guo, B. C.; Kerns, K. P.; Castleman, A. W.

1992-09-01

151

Unusual pathways for metal-assisted C[bond]C and C[bond]P coupling reactions using allenylidenerhodium complexes as precursors.  

PubMed

The rhodium allenylidenes trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = Ph (1), p-Tol (2)] react with NaC(5)H(5) to give the half-sandwich type complexes [(eta(5)-C(5)H(5))Rh[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))] (3, 4). The reaction of 1 with the Grignard reagent CH(2)[double bond]CHMgBr affords the eta(3)-pentatrienyl compound [Rh(eta(3)-CH(2)CHC[double bond]C[double bond]CPh(2))(PiPr(3))(2)] (6), which in the presence of CO rearranges to the eta(1)-pentatrienyl derivative trans-[Rh[eta(1)-C(CH[double bond]CH(2))[double bond]C[double bond]CPh(2)](CO)(PiPr(3))(2)] (7). Treatment of 7 with acetic acid generates the vinylallene CH(2)[double bond]CH[bond]CH[double bond]=C=CPh(2) (8). Compounds 1 and 2 react with HCl to give the five-coordinate allenylrhodium(III) complexes [RhCl(2)[CH[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (10, 11). An unusual [C(3) + C(2) + P] coupling process takes place upon treatment of 1 with terminal alkynes HC[triple bond]CR', leading to the formation of the eta(3)-allylic compounds [RhCl[eta(3)-anti-CH(PiPr(3))C(R')C[double bond]C[double bond]CPh(2)](PiPr(3))] [R' = Ph (12), p-Tol (13), SiMe(3) (14)]. From 12 and RMgBr the corresponding phenyl and vinyl rhodium(I) derivatives 15 and 16 have been obtained. The previously unknown unsaturated ylide iPr(3)PCHC(Ph)[double bond]C[double bond]C[double bond]CPh(2) (17) was generated from 12 and CO. A [C(3) + P] coupling process occurs on treatment of the rhodium allenylidenes 1, 2, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(p-Anis)(2)](PiPr(3))(2)] (20) with either Cl(2) or PhICl(2), affording the ylide-rhodium(III) complexes [RhCl(3)[C(PiPr(3))C[double bond]C(R)R'](PiPr(3))] (21-23). The butatrienerhodium(I) compounds trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (28-31) were prepared from 1, 20, and trans-[RhCl[[double bond]C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] [R = CF(3) (26), tBu (27)] and diazomethane; with the exception of 30 (R = CF(3), R' = Ph), they thermally rearrange to the isomers trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C[double bond]C(R)R'](PiPr(3))(2)] (32, 33, and syn/anti-34). The new 1,1-disubstituted butatriene H(2)C[double bond]C[double bond]C[double bond]C(tBu)Ph (35) was generated either from 31 or 34 and CO. The iodo derivatives trans-[RhI(eta(2)-H(2)C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] [R = Ph (38), p-Anis (39)] were obtained by an unusual route from 1 or 20 and CH(3)I in the presence of KI. While the hydrogenation of 1 and 26 leads to the allenerhodium(I) complexes trans-[RhCl[eta(2)-H(2)C[double bond]C[double bond]C(Ph)R](PiPr(3))(2)] (40, 41), the thermolysis of 1 and 20 produces the rhodium(I) hexapentaenes trans-[RhCl(eta(2)-R(2)C[double bond]C[double bond]C[double bond]C[double bond]C[double bond]CR(2))(PiPr(3))(2)] (44, 45) via C-C coupling. The molecular structures of 3, 7, 12, 21, and 28 have been determined by X-ray crystallography. PMID:12059220

Werner, Helmut; Wiedemann, Ralf; Laubender, Matthias; Windmüller, Bettina; Steinert, Paul; Gevert, Olaf; Wolf, Justin

2002-06-19

152

Reaction Pathways and Energetics of Etheric C?O Bond Cleavage Catalyzed by Lanthanide Triflates  

SciTech Connect

Efficient and selective cleavage of etheric C?O bonds is crucial for converting biomass into platform chemicals and liquid transportation fuels. In this contribution, computational methods at the DFT B3LYP level of theory are employed to understand the efficacy of lanthanide triflate catalysts (Ln(OTf)3, Ln = La, Ce, Sm, Gd, Yb, and Lu) in cleaving etheric C?O bonds. In agreement with experiment, the calculations indicate that the reaction pathway for C?O cleavage occurs via a C?H ? O?H proton transfer in concert with weakening of the C?O bond of the coordinated ether substrate to ultimately yield a coordinated alkenol. The activation energy for this process falls as the lanthanide ionic radius decreases, reflecting enhanced metal ion electrophilicity. Details of the reaction mechanism for Yb(OTf)3-catalyzed ring opening are explored in depth, and for 1-methyl-d3-butyl phenyl ether, the computed primary kinetic isotope effect of 2.4 is in excellent agreement with experiment (2.7), confirming that etheric ring-opening pathway involves proton transfer from the methyl group alpha to the etheric oxygen atom, which is activated by the electrophilic lanthanide ion. Calculations of the catalytic pathway using eight different ether substrates indicate that the more rapid cleavage of acyclic versus cyclic ethers is largely due to entropic effects, with the former C?O bond scission processes increasing the degrees of freedom/particles as the transition state is approached.

Assary, Rajeev S.; Atesin, Abdurrahman C.; Li, Zhi; Curtiss, Larry A.; Marks, Tobin J.

2013-07-15

153

Oxidation Reaction of Pyrolytic Carbon Coating.  

National Technical Information Service (NTIS)

The behaviour of pyrolytic carbon coatings on commercial grade graphite substrate in oxidizing environment is described. Specimens were examined under sputtering in plasma of oxygen and argon, or in an oxidizing solution of K sub 2 CrO sub 7 +H sub 3 PO s...

A. Raveh A. Inspektor U. Carmi E. Rabinovitz R. Avni

1985-01-01

154

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

155

Reaction-bonded silicon carbide tube fabricated by continuous sintering of double-walled preform  

Microsoft Academic Search

A reaction-bonded silicon carbide radiant tube has been successfully fabricated by the continuous sintering of a double-walled preform which was composed of inner Si\\/C and outer SiC\\/C layers. Sequential centrifugal casting was employed to develop double-walled preform and process shrinkages were minimized by adjusting the particle packing characteristics with multimodal particle size distribution. Debinding behavior was optimized in a two-step

Hyung-Woo Jun; Hae-Weon Lee; Huesup Song; Byung Ho Kim; Jowoong Ha

2004-01-01

156

Effect of hot isostatic pressing on reaction-bonded silicon nitride  

NASA Technical Reports Server (NTRS)

Specimens of nearly theoretical density have been obtained through the isostatic hot pressing of reaction-bonded silicon nitride under 138 MPa of pressure for two hours at 1850, 1950, and 2050 C. An amorphous phase that is introduced by the hot isostatic pressing partly accounts for the fact that while room temperature flexural strength more than doubles, the 1200 C flexural strength increases significantly only after pressing at 2050 C.

Watson, G. K.; Moore, T. J.; Millard, M. L.

1984-01-01

157

Evaluation of catalyst acidity and substrate electronic effects in a hydrogen bond-catalyzed enantioselective reaction.  

PubMed

A modular catalyst structure was applied to evaluate the effects of catalyst acidity in a hydrogen bond-catalyzed hetero Diels-Alder reaction. Linear free energy relationships between catalyst acidity and both rate and enantioselectivity were observed, where greater catalyst acidity leads to increased activity and enantioselectivity. A relationship between reactant electronic nature and rate was also observed, although there is no such correlation to enantioselectivity, indicating the system is under catalyst control. PMID:20919686

Jensen, Katrina H; Sigman, Matthew S

2010-11-01

158

Optical surfacing of one-meter-class reaction bonded silicon carbide  

Microsoft Academic Search

Optical fabrication methods for a one meter reaction bonded optical grade silicon carbide (RBO SiC) substrate have been successfully developed and demonstrated by Itek Optical Systems in Lexington, Massachusetts. A 1.125 m multiplied by .825 m RBO SiC panel, the largest SiC ever fabricated, has been ground and polished to a surface figure of lambda\\/20 rms at lambda equals .6328

Robert S. Breidenthal; Regina Galat-Skey; John J. Geany

1995-01-01

159

Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement  

Microsoft Academic Search

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration–to–detonation characteristics and explosion\\/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and

W. Lee Perry; Peter M. Dickson; Gary R. Parker; Blaine W Asay

2005-01-01

160

Quantification of reaction violence and combustion enthalpy of plastic bonded explosive 9501 under strong confinement  

Microsoft Academic Search

The confinement experienced by an explosive during thermal self-initiation can substantially affect performance in terms of deflagration-to-detonation characteristics and explosion\\/detonation violence. To this end, we have developed an experiment to quantitatively observe enthalpy change and reaction violence in thermally initiated plastic bonded explosive (PBX) 9501. Traditionally, researchers attempt to quantify violence using terminal observations of fragment size, fragment velocity, and

W. Lee Perry; Peter M. Dickson; Gary R. Parker; B. W. Asay

2005-01-01

161

B-H bond activation using an electrophilic metal complex: insights into the reaction pathway.  

PubMed

A highly electrophilic ruthenium center in the [RuCl(dppe)(2)][OTf] complex brings about the activation of the B-H bond in ammonia borane (H(3)N·BH(3), AB) and dimethylamine borane (Me(2)HN·BH(3), DMAB). At room temperature, the reaction between [RuCl(dppe)(2)][OTf] and AB or DMAB results in trans-[RuH(?(2)-H(2))(dppe)(2)][OTf], trans-[RuCl(?(2)-H(2))(dppe)(2)][OTf], and trans-[RuH(Cl)(dppe)(2)], as noted in the NMR spectra. Mixing the ruthenium complex and AB or DMAB at low temperature (198/193 K) followed by NMR spectral measurements as the reaction mixture was warmed up to room temperature allowed the observation of various species formed enroute to the final products that were obtained at room temperature. On the basis of the variable-temperature multinuclear NMR spectroscopic studies of these two reactions, the mechanistic insights for B-H bond activation were obtained. In both cases, the reaction proceeds via an ?(1)-B-H moiety bound to the metal center. The detailed mechanistic pathways of these two reactions as studied by NMR spectroscopy are described. PMID:23244598

Kumar, Rahul; Jagirdar, Balaji R

2013-01-01

162

The Menshutkin reaction in the gas phase and in aqueous solution: a valence bond study.  

PubMed

The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J. Phys. Chem. A 2004, 108, 6017-6024) valence bond method coupled to a polarized continuum model (VBPCM) is applied to the Menshutkin reaction, NH3+CH3Cl-->CH3NH3(+)+Cl-, in the gas phase and in aqueous solution. The computed barriers and reaction energies at the level of the breathing orbital VB method (P. C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimental values in solution. The gas-phase reaction is endothermic and leads to an ion-pair complex via a late transition state. By contrast, the reaction in the aqueous phase is exothermic and leads to separate solvated ions as reaction products, via an early transition state. The VB calculations provide also the reactivity parameters needed to apply the valence bond state correlation diagram method, VBSCD (S. Shaik, A. Shurki, Angew. Chem. Int. Ed. 1999, 38, 586). It is shown that the reactivity parameters along with their semiempirical derivations provide together a satisfactory qualitative and quantitative account of the barriers. PMID:18061916

Su, Peifeng; Ying, Fuming; Wu, Wei; Hiberty, Philippe C; Shaik, Sason

2007-12-21

163

Molecular dynamics simulation of oxides with ionic–covalent bonds  

Microsoft Academic Search

A “semi-classical” method was developed for molecular dynamics simulation of a system with ionic–covalent bonds like silica. The ionic charges were calculated by minimization of the potential energy on each step of molecular dynamics simulation. Ionic–covalent potential was used in modeling of SiO2 molecule, non-crystalline silica, and calcium metasilicate. The internal energy of a system includes energies of silicon ionization,

D. K Belashchenko; O. I Ostrovski

2001-01-01

164

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

2012-06-19

165

High temperature heterogeneous reaction kinetics and mechanisms of tungsten oxidation  

NASA Astrophysics Data System (ADS)

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 oxides that have higher melting or boiling points than the pure metal (e.g., aluminum, iron). Understanding heterogeneous oxidation and vaporization processes may allow for the expansion and improvement of high temperature tungsten applications. In order to increase understanding of the oxidation processes of tungsten, there is a need to develop reaction mechanisms and kinetics for oxidation processes involving oxidizers and environmental conditions of interest. Tungsten oxidation was thoroughly studied in the past, and today there is a good phenomenological understanding of these processes. However, as the design of large scale systems increasingly relies on computer modeling there becomes a need for improved descriptions of chemical reactions. With the increase in computing power over the last several decades, and the development of quantum chemistry and physics theories, heterogeneous systems can be modeled in detail at the molecular level. Thermochemical parameters that may not be measured experimentally may now be determined theoretically, a tool that was previously unavailable to scientists and engineers. Additionally, chemical kinetic modeling software is now available for both homogeneous and heterogeneous reactions. This study takes advantage of these new theoretical tools, as well as a thermogravimetric (TG) flow reactor developed as part of this study to learn about mechanisms and kinetics of tungsten oxidation. Oxidizers of interest are oxygen (O2), carbon dioxide (CO 2), water (H2O), and other oxidizers present in combustion and energy systems. The primary application for this research topic is the migration of erosion processes in solid rocket motor nozzles. Since oxidation is the primary erosion mechanism of tungsten based nozzles, mitigation of this process through improved comprehension of the chemical mechanisms will increase performance of future rocket systems. In this dissertation, results of the high temperature reaction rates of bulk tungsten are studied using TG analysis in oxidizing atmospheres of O2, CO2, and H2O using helium (He) as an inert carrier gas. Isothermal reaction rates were determined at temperatures up to 1970 K, and oxidizing species partial pressures up to 64.6 torr. Kinetic parameters such as activation energies, frequency factors, and pressure exponents were determined for each reactive system. An important contribution of this work was quantifying the effects of carbon monoxide (CO) on the CO2 reaction, and hydrogen (H2) on the H2O reaction. In both cases the non-oxidizing species significantly reduced oxidation rates. Results have led to new interpretations and thought processes for limiting nozzle erosion in rocket motors. Combined with the TG analysis, as well as recent theoretical interpretations of reaction thermodynamics and kinetics, a new mechanism for tungsten and O2 oxidation has been developed using a one-dimensional numerical model of the TG flow reactor. Important chemical processes and species are also identified for reaction systems involving H2O and CO2. In the future, additional studies are needed to improve our understanding of these chemical species and processes so that more advanced kinetic mechanisms may be developed. In addition to a detailed analysis of high temperature tungsten corrosion processes, synthetic graphite corrosion processes are studied in detail as well. Details of these studies are presented in an attached appendix of this dissertation. These studies considered not only oxidation processes, but decomposition of synthetic graphite in the presence of reducing and inert gas environments.

Sabourin, Justin L.

166

CONTAMINANT ADSORPTION AND OXIDATION VIA FENTON REACTION  

EPA Science Inventory

A ground water treatment process is proposed involving two cgemical processes: adsorption and oxidation. Adsorption of an organic compound onto granulated activated carbon (GAC) containing iron conveniently results in immobilizing and concentrating contaminants from the ground w...

167

Preparation on oligostilbenes of isorhapontigenin by oxidative coupling reaction.  

PubMed

Four new compounds 1-4 were obtained from an oxidative coupling reaction of (E)-isorhapontigenin using FeCl(3) as oxidant. Their structures and stereochemistry were determined on the basis of spectroscopic evidence [UV, IR, MS, (1)H-, (13)C-NMR, NOE and 2D NMR], and their possible formation mechanisms were also discussed, respectively. PMID:14758010

Yao, Chun-Suo; Zhou, Li-Xin; Lin, Mao

2004-02-01

168

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...

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

Chemical Reactions of Aquatic Humic Materials with Selected Oxidants.  

National Technical Information Service (NTIS)

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 derivati...

A. A. Stevens D. S. Millington J. D. Johnson R. F. Christman

1983-01-01

171

Gold-Catalyzed Oxidative Coupling Reactions with Aryltrimethylsilanes  

PubMed Central

During continuing studies with a novel oxidative gold oxyarylation reaction, arylsilanes were found to be competent coupling partners, providing further evidence for an intramolecular electrophilic aromatic substitution mechanism. While providing complementary yields to the previously described boronic acid methods, the use of trimethylsilanes reduces the observation of homocoupling byproducts and allows for facile intramolecular coupling reactions.

Brenzovich, William E.; Brazeau, Jean-Francois; Toste, F. Dean

2010-01-01

172

A critical study of the role of the surface oxide layer in titanium bonding  

NASA Technical Reports Server (NTRS)

The molecular understanding of the role which the surface oxide layer of the adherend plays in titanium bonding is studied. The effects of Ti6-4 adherends pretreatment, bonding conditions, and thermal aging of the lap shear specimens were studied. The use of the SEM/EDAX and ESCA techniques to study surface morphology and surface composition was emphasized. In addition, contact angles and both infrared and visible reflection spectroscopy were used in ancillary studies.

Dias, S.; Wightman, J. P.

1982-01-01

173

The Electrochemical Displacement Reaction of Lithium with Metal Oxides  

Microsoft Academic Search

The electrochemical reaction of lithium with a-LiFeO2, b-Li5FeO4 , and CoO is studied by in situ X-ray diffraction and in situ Mossbauer measurements. The results of the measurements show that these metal oxides are immediately decomposed during discharge to form lithia and the reduced metal. This reaction proceeds through a single intermediate or surface phase. The reaction products are nanometer-sized,

M. N. Obrovac; R. A. Dunlap; R. J. Sanderson; J. R. Dahn

2001-01-01

174

A critical study of the role of the surface oxide layer in titanium bonding  

NASA Technical Reports Server (NTRS)

Scanning electron microscope/X-ray photoelectron spectroscopy (SEM/XPS) analysis of fractured adhesively bonded Ti 6-4 samples is discussed. The text adhesives incuded NR 056X polyimide, polypheylquinoxaline (PPQ), and LARC-13 polyimide. Differentiation between cohesive and interfacial failure was based on the absence of presence of a Ti 2p XPS photopeak. In addition, the surface oxide layer on Ti-(6A1-4V) adherends is characterized and bond strength and durability are addressed. Bond durability in various environmental conditions is discussed.

Dias, S.; Wightman, J. P.

1983-01-01

175

Kinetics of the reactions of hydrogen fluoride with calcium oxide  

SciTech Connect

This paper studies the kinetics of interaction of gaseous hydrogen fluoride with calcium oxide at temperatures 300-700 degrees. The experiments were conducted in a laboratory adsorption apparatus modified and adapted for work with corrosive hydrogen fluoride. Calcium oxide samples in granulated form and deposited on gamma-alumina were used in the experiments. Kinetic curves representing variations of the degree of conversion of the solid samples with time are shown. The influence of retardation dure to diffusion was observed in the experiments. The influence of diffusion control on the reaction rate was also observed in a study of the reaction kinetics on supported layers of calcium oxide.

Kossaya, A.M.; Belyakov, B.P.; Kuchma, Z.V.; Sandrozd, M.K.; Vasil'eva, V.G.

1986-08-01

176

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)] [Department of Chemistry, University of Utah, Salt Lake City, Utah84112 (United States)

1998-06-01

177

Oxidation characteristics of a platinized MCrAlY bond coat for TBC systems during cyclic oxidation at 1000 °C  

Microsoft Academic Search

The scale growth mechanisms of a platinized NiCoCrAlY bond coat (BC) beneath a ceramic thermal barrier coating (TBC) were studied during cyclic oxidation in air at 1000 °C. The thermally grown oxide scales (TGO) after exposure times ranging from 100 to 9100 h were characterized by scanning electron microscopy and laser fluorescence spectroscopy. The TGO appeared to consist of alumina

W. J. Quadakkers; V. Shemet; D. Sebold; R. Anton; E. Wessel; L. Singheiser

2005-01-01

178

Efficient and directed peptide bond formation in the gas phase via ion/ion reactions.  

PubMed

Amide linkages are among the most important chemical bonds in living systems, constituting the connections between amino acids in peptides and proteins. We demonstrate the controlled formation of amide bonds between amino acids or peptides in the gas phase using ion/ion reactions in a mass spectrometer. Individual amino acids or peptides can be prepared as reagents by (i) incorporating gas phase-labile protecting groups to silence otherwise reactive functional groups, such as the N terminus; (ii) converting the carboxyl groups to the active ester of N-hydroxysuccinimide; and (iii) incorporating a charge site. Protonation renders basic sites (nucleophiles) unreactive toward the N-hydroxysuccinimide ester reagents, resulting in sites with the greatest gas phase basicities being, in large part, unreactive. The N-terminal amines of most naturally occurring amino acids have lower gas phase basicities than the side chains of the basic amino acids (i.e., those of histidine, lysine, or arginine). Therefore, reagents may be directed to the N terminus of an existing "anchor" peptide to form an amide bond by protonating the anchor peptide's basic residues, while leaving the N-terminal amine unprotonated and therefore reactive. Reaction efficiencies of greater than 30% have been observed. We propose this method as a step toward the controlled synthesis of peptides in the gas phase. PMID:24474750

McGee, William M; McLuckey, Scott A

2014-01-28

179

Acid-Catalyzed Oxidative Addition of a C-H Bond to a Square Planar d(8) Iridium Complex.  

PubMed

While the addition of C-H bonds to three-coordinate Ir(I) fragments is a central theme in the field of C-H bond activation, addition to square planar four-coordinate complexes is far less precedented. The dearth of such reactions may be attributed, at least in part, to kinetic factors elucidated in seminal work by Hoffmann. C-H additions to square planar carbonyl complexes in particular are unprecedented, in contrast to the extensive chemistry of oxidative addition of other substrates (e.g., H2, HX) to Vaska's Complex and related species. We report that Bronsted acids will catalyze the addition of the alkynyl C-H bond of phenylacetylene to the pincer complex (PCP)Ir(CO). The reaction occurs to give exclusively the trans-C-H addition product. Our proposed mechanism, based on kinetics and DFT calculations, involves initial protonation of (PCP)Ir(CO) to generate a highly active five-coordinate cationic intermediate, which forms a phenylacetylene adduct that is then deprotonated to give product. PMID:24896665

Hackenberg, Jason D; Kundu, Sabuj; Emge, Thomas J; Krogh-Jespersen, Karsten; Goldman, Alan S

2014-06-25

180

DFT Study of Mechanism of Extraction Reaction Between Germylene Carbene (H2Ge=C:) and Its Derivatives and Ethylene Oxide  

NASA Astrophysics Data System (ADS)

The mechanism of the oxide extraction reaction between singlet germylene carbene and its derivatives X2Ge=C: (X=H, F, Cl, CH3) and ethylene oxide has been investigated with B3LYP/6-311G(d,p) method. The results show that this kind of reaction has similar mechanism, the shift of 2p lone electron pair of O in ethylene oxide to the 2p unoccupied orbital of C in X2Ge=C: gives a p?p donor-acceptor bond, thereby leading to the formation of intermediate. As the p?p donor-acceptor bond continues to strengthen, that is the C—O bond continues to shorten, the intermediate generates product (P+C2H4) via transition state. It is the substituent electronegativity that mainly affect the extraction reactions. When the substituent electronegativity is greater, the energy barrier is lower, and the reaction rate is greater.

Lu, Xiu-hui; Che, Xin; Li, Yong-qing; Wang, Zhi-na

2011-06-01

181

Reaction kinetics and transformation products of 1-naphthol by Mn oxide-mediated oxidative-coupling reaction  

Microsoft Academic Search

In this study, the transformation of 1-naphthol via oxidative-coupling reaction was investigated using Mn oxides. 1-Naphthol was transformed completely by birnessite, which is one of the natural Mn oxides present in soil. The surface area-normalized specific rate constant, ksurf, for 1-naphthol was determined to be 9.66×10?4L\\/m2min using observed pseudo-first-order rate constants with respect to birnessite loading. The transformation of 1-naphthol

Hyun-Sang Shin; Dong-Min Lim; Doo-Hee Lee; Ki-Hoon Kang

2009-01-01

182

Discovery and synthetic applications of novel silicon-carbon bond cleavage reactions based on the coordination number change of organosilicon compounds  

PubMed Central

Some synthetically useful transformations of organosilicon compounds have been developed since the mid 1970s, based on the new concept that the silicon-carbon bonds are activated toward electrophilic cleavage via the formation of penta- and hexa-coordinate species. This review mainly consists of the following aspects: (1) a general concept for the activation of the silicon-carbon bond via penta- and hexa-coordinate species, (2) synthetic application of hexa-coordinate organopentafluorosilicates, and (3) development of the H2O2 oxidation of the silicon-carbon bond and its synthetic applications via the intramolecular hydrosilylation, silicon-tethered intramolecular radical cyclization and Diels-Alder reaction, and some silicon-containing organometallic reagents for nucleophilic hydroxymethylation and hydroxyallylation synthons.

TAMAO, Kohei

2008-01-01

183

Thermal oxidative degradation reactions of perfluoroalkylethers  

NASA Technical Reports Server (NTRS)

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 oxidative instability of the Fomblin Z fluids is not established. It was determined that this behavior is not associated with hydrogen end groups or peroxy linkages. The degradation rate of these fluids at elevated temperatures in oxidizing atmospheres is dependent on the surface/volume ratio. Once a limiting ratio is reached, a steady rate appears to be attained. Based on elemental analysis and oxygen consumption data, CF2OCF2CF2O2, no. CF2CF2O, is one of the major arrangements present. The action of the M-50 and Ti(4 Al, 4 Mn) alloys is much more drastic in the case of Fomblin Z fluids than that observed for the hexafluoropropene derived materials. The effectiveness of antioxidation anticorrosion additives, P-3 and phospha-s-triazine, in the presence of metal alloys is very limited at 316 C; at 288 C the additives arrested almost completely the fluid degradation. The phospha-s-triazine appears to be at least twice as effective as the P-3 compound; it also protected the coupon better. The Ti(4 Al, 4 Mn) alloy degraded the fluid mainly by chain scission processes this takes place to a much lesser degree with M-50.

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

1981-01-01

184

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities.  

PubMed

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 /Vr(c)/ / Gr(c) where /Vr(c)/ is the magnitude of the local potential-energy density and Gr(c) is the local kinetic-energy density, each evaluated at a bond critical point r(c). A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of oxide molecules and earth materials, together with the constraints imposed by the values of inverted Delta2rho r(c) and the local electronic energy density, Hr(c) = Gr(c) + Vr(c), in the H-F study, yielded practically the same classification for the oxides. This is true despite the different trends that hold between the bond critical point and local energy density properties with the bond lengths displayed by the H-F and M-O bonded interactions. On the basis of the ratio, Li-O, Na-O, and Mg-O bonded interactions classify as closed-shell ionic bonds, Be-O, Al-O, Si-O, B-O, and P-O interactions classify as bonds of intermediate character with the covalent character increasing from Be-O to P-O. N-O interactions classify as shared covalent bonds. C-O and S-O bonded interactions classify as both intermediate and covalent bonded interactions. The C-O double- and triple-bonded interactions classify as intermediate-bonded interactions, each with a substantial component of covalent character and the C-O single-bonded interaction classifies as a covalent bond whereas their local electronic energy density values indicate that they are each covalent bonded interactions. The ratios for the Be-O, Al-O, and Si-O bonded interactions indicate that they have a substantial component of ionic character despite their classification as bonds of intermediate character. The trend between the ratio and the character of the bonded interactions is consistent with trends expected from electronegativity considerations. The ratio increases as the net charges and the coordination numbers for the atoms for several Ni-sulfides decrease. On the contrary, the ratio for the Si-O bonded interactions for the orthosilicate, forsterite, Mg2SiO4, and the high-pressure silica polymorph, stishovite, decreases as the observed net atomic charges and the coordination numbers of Si and O increase in value. The ratio for the Ni-Ni bonded interactions for the Ni-sulfides and bulk Ni metal indicate that the interactions are intermediate in character with a substantial component of ionic character. PMID:16512733

Gibbs, G V; Cox, D F; Crawford, T D; Rosso, K M; Ross, N L; Downs, R T

2006-02-28

185

Redox reactions in liquid plasma during iron oxide and oxide-hydroxide nanoparticles synthesis  

Microsoft Academic Search

A new synthesis method of iron oxide and iron oxide-hydroxide nanoparticles by pulsed liquid plasma process is presented. The redox reaction rates in liquid phase depend on the relative number densities of oxygen and hydrogen atoms produced in plasma gas phase and on the injected energy per pulse. The oxidation rate was higher than the reduction one in both regimes

M. A. Bratescu; N. Saito; O. Takai

186

Thermal oxidative degradation reactions of perfluoroalklethers  

NASA Technical Reports Server (NTRS)

The objective of this contract was to investigate the mechanisms operative in thermal and thermal oxidative degradation of Fomblin Z and hexafluoropropene oxide derived fluids and the effect of alloys and additives upon these processes. The nature of arrangements responsible for the inherent thermal oxidative instability of the Fomblin Z fluids has not been established. It was determined that this behavior was not associated with hydrogen end-groups or peroxy linkages. The degradation rate of these fluids at elevated temperatures in oxidizing atmospheres was found to be dependent on the surface/volume ratio. Once a limiting ratio was reached, a steady rate appeared to be attained. Based on elemental analysis and oxygen consumption data, -CF2OCF2CF2O-, not -CF2CF2O-, is one of the major arrangements present. The action of the M-50 and Ti(4 Al, 4 Mn) alloys was found to be much more drastic in the case of Fomblin Z fluids than that observed for the hexalfuoropropane oxide derived materials. The effectiveness of antioxidation/anticorrosion additives, P-3 and phospha-s-triazine, in the presence of metal alloys was very limited at 316 C; at 288 C the additives arrested almost completely the fluid degradation. The phospha-s-triazine appeared to be at least twice as effective as the P-3 compound; it also protected the coupon better. The Ti(4 Al, 4 Mn) alloy degraded the fluid mainly by chain scission processes; this took place to a much lesser degree with M-50.

Paciorek, K. L.; Harris, D. H.; Smythe, M. E.; Kratzer, R. H.

1983-01-01

187

Reactions of silicon carbide with oxidizing atmospheres  

Microsoft Academic Search

During the operation of silicon carbide materials in thermally stressed elements of electrothermal, metallurgical, and power plants (various types of lining, heat exchangers, electric heating elements, etc.) heterogeneous reactions take place between the solid silicon carbide and chemically active gaseous media (air, organic fuel combustion products, and the like). Air and combustion products consist mainly of nitrogen, oxygen, water vapor,

I. A. Yavorskii; V. I. Elchin; G. G. Gnesin

1978-01-01

188

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

189

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

190

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

191

Hydrogen bonding between histidine and lignin model compounds or redox mediators as calculated with the DFT method. Effects on the ease of oxidation.  

PubMed

Using the Density Functional Theory method, the effect of hydrogen bonding between imidazole (IM) and ten benzyl alcohol derivatives (BA) on the ionization potentials of the latter is calculated. IM is used as a model for histidine, which is found in the reaction sites of laccases and lignin peroxidases, and the BA-derivatives serve as lignin model compounds. A marked decrease ([similar]15 kcal mol(-1)) is found for the IP's of the BA-derivatives when paired with IM. This should facilitate the one-electron oxidation of BA in the reaction site of the enzyme. The same effect is found for the known redox mediators violuric acid, 1-hydroxybenzotriazole and N-hydroxyacetanilide which are assumed to enter the reaction site of the enzymes. Furthermore, upon one-electron oxidation the strength of the H-bond from BA to IM is considerably increased and in the case of the mediators this effect is so pronounced that the relevant proton shifts from them to IM. If this occurs in the active site of the enzyme then the oxidized redox mediators are released into the aqueous phase in their neutral form rather than as radical cations (deprotonation of the radical cations). The oxidation power of the neutral radical mediators, however, is too low to initialize oxidation of lignin. A more likely reaction pathway is oxidation of the substrates via hydrogen abstraction. The pertinent bond dissociation energies are similar for the BA-derivatives and the redox mediators, which in principle allows the reaction to occur. PMID:14770237

Reynisson, Jóhannes; Steenken, Steen

2004-02-21

192

A copper-mediated tandem reaction through isocyanide insertion into N-H bonds: efficient access to unsymmetrical tetrasubstituted ureas.  

PubMed

A copper-mediated multi-component reaction was developed through isocyanide insertion into N-H bonds of less active secondary arylamines. This approach leads to an efficient synthesis of unsymmetrical tetrasubstituted ureas in one pot. PMID:24358461

Huang, Xiaomei; Xu, Shuguang; Tan, Qitao; Gao, Mingchun; Li, Minjie; Xu, Bin

2014-02-11

193

Ceramic Tube Material and Processing Development: Development of Large Reaction Bonded Silicon Carbide Components for Gas-Fired Furnaces.  

National Technical Information Service (NTIS)

Ceramic component manufacturing technology for reaction bonded silicon carbide (SCRB-210) has been developed for production of large thin-walled components up to 8 inch diameter and 8 feet in length. Available property data, component performance informat...

D. W. Roy K. E. Green C. J. Dobos

1987-01-01

194

Effects of Cu and Pd addition on Au bonding wire\\/Al pad interfacial reactions and bond reliability  

Microsoft Academic Search

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I\\/Os. However, finer Au wires are\\u000a more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions\\u000a for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and

Sang-Ah Gam; Hyoung-Joon Kim; Jong-Soo Cho; Yong-Jin Park; Jeong-Tak Moon; Kyung-Wook Paik

2006-01-01

195

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

196

Reactions of calcium orthosilicate and barium zirconate with oxides and sulfates of various elements  

NASA Technical Reports Server (NTRS)

Calcium orthosilicate and barium zirconate were evaluated as the insulation layer of thermal barrier coatings for air cooled gas turbine components. Their reactions with various oxides and sulfates were studied at 1100 C and 1300 C for times ranging up to 400 and 200 hours, respectively. These oxides and sulfates represent potential impurities or additives in gas turbine fuels and in turbine combustion air, as well as elements of potential bond coat alloys. The phase compositions of the reaction products were determined by X-ray diffraction analysis. BaZrO3 and 2CaO-SiO2 both reacted with P2O5, V2O5, Cr2O3, Al2O3, and SiO2. In addition, 2CaO-SiO2 reacted with Na2O, BaO, MgO, and CoO and BaZrO3 reacted with Fe2O3.

Zaplatynsky, I.

1979-01-01

197

Oxidative reaction of oxindole-3-acetic acids.  

PubMed

The oxindole-3-acetic acids, oxidative metabolites of indole-3-acetic acid, were isolated from a byproduct of a corn starch manufacturing plant, and were further converted to the 3-hydroxyl derivatives in the presence of metal ion. The mechanical study was followed by a chemical analysis including other byproducts, and suggested the presence of an intermediate that had a radical at the C-3 position of oxindole-3-acetic acids. PMID:14519969

Niwa, Toshio; Ishii, Sayuri; Hiramatsu, Atsushi; Osawa, Toshihiko

2003-09-01

198

Cluster reactivity experiments: Employing mass spectrometry to investigate the molecular level details of catalytic oxidation reactions  

PubMed Central

Mass spectrometry is the most widely used tool in the study of the properties and reactivity of clusters in the gas phase. In this article, we demonstrate its use in investigating the molecular-level details of oxidation reactions occurring on the surfaces of heterogeneous catalysts via cluster reactivity experiments. Guided ion beam mass spectrometry (GIB-MS) employing a quadrupole–octopole–quadrupole (Q–O–Q) configuration enables mass-selected cluster ions to be reacted with various chemicals, providing insight into the effect of size, stoichiometry, and ionic charge state on the reactivity of catalyst materials. For positively charged tungsten oxide clusters, it is shown that species having the same stoichiometry as the bulk, WO3+, W2O6+, and W3O9+, exhibit enhanced activity and selectivity for the transfer of a single oxygen atom to propylene (C3H6), suggesting the formation of propylene oxide (C3H6O), an important monomer used, for example, in the industrial production of plastics. Furthermore, the same stoichiometric clusters are demonstrated to be active for the oxidation of CO to CO2, a reaction of significance to environmental pollution abatement. The findings reported herein suggest that the enhanced oxidation reactivity of these stoichiometric clusters may be due to the presence of radical oxygen centers (W–O?) with elongated metal–oxygen bonds. The unique insights gained into bulk-phase oxidation catalysis through the application of mass spectrometry to cluster reactivity experiments are discussed.

Johnson, Grant E.; Tyo, Eric C.; Castleman, A. W.

2008-01-01

199

Effect of thermally grown oxide (TGO) microstructure on the durability of TBCs with PtNiAl diffusion bond coats  

Microsoft Academic Search

The role of pre-oxidation surface treatments on the oxide microstructure and the failure mechanism of multi-layer thermal barrier systems based on Pt-modified NiAl bond coats and electron beam deposited thermal barrier coatings (TBCs) have been studied. The primary pre-oxidation experimental variable was the partial pressure of oxygen in the pre-oxidizing atmosphere at constant temperature and bond coat composition. The durability

Irene Spitsberg; Karren More

2006-01-01

200

High-temperature tensile properties of fiber reinforced reaction bonded silicon nitride  

NASA Technical Reports Server (NTRS)

Measurements of tensile properties of unidirectional silicon carbide fiber-reinforced reaction-bonded silicon nitride (SiC/RBSN) composite specimens were carried out in air at 25, 1300, and 1500 C, using a new testing technique and a specially designed gripping system that minimizes bending moment and assures that failure always occurred in the gage section. The material was found to display metallike stress-strain behavior at all temperatures tested, and a noncatastrophic failure beyond the matrix fracture. The tensile properties were found to be temperature dependent, with the values of the ultimate tensile strength decreasing with temperature, from 543 MPa at 25 C to 169 at 1500 C.

Jablonski, David A.; Bhatt, Ramakrishna T.

1990-01-01

201

Bend strengths of reaction bonded silicon nitride prepared from dry attrition milled silicon powder  

NASA Technical Reports Server (NTRS)

Dry attrition milled silicon powder was compacted, sintered in helium, and reaction bonded in nitrogen-4 volume percent hydrogen. Bend strengths of bars with as-nitrided surfaces averaged as high as 210 MPa at room temperature and 220 MPa at 1400 C. Bars prepared from the milled powder were stronger than those prepared from as-received powder at both room temperature and at 1400 C. Room temperature strength decreased with increased milling time and 1400 C strength increased with increased milling time.

Herbell, T. P.; Glasgow, T. K.

1979-01-01

202

Relationships between toughness and microstructure of reaction bonded Si3N4  

NASA Technical Reports Server (NTRS)

Fracture toughnesses of nominally identical batches of reaction bonded silicon nitride (RBSN) differed significantly (about 2.0 and about 2.7 MPa sq rt m). Detailed fractographic and microstructural characterizations investigated underlying factors. Subtile differences between high and low toughness RBSN and between constituent Si powders have been revealed through SEM/FEG, TEM, BET, Hg-porosimetry, and XRD. The results illustrate the need for behavioral models to guide microstructural design and to interpret properties of brittle materials with intermediate levels of porosity.

Lightfoot, Annamarie; Sigalovsky, Julia; Haggerty, John S.

1992-01-01

203

Selective molecular recognition, C-H bond activation, and catalysis in nanoscale reaction vessels  

SciTech Connect

Supramolecular chemistry represents a way to mimic enzyme reactivity by using specially designed container molecules. We have shown that a chiral self-assembled M{sub 4}L{sub 6} supramolecular tetrahedron can encapsulate a variety of cationic guests, with varying degrees of stereoselectivity. Reactive iridium guests can be encapsulated and the C-H bond activation of aldehydes occurs, with the host cavity controlling the ability of substrates to interact with the metal center based upon size and shape. In addition, the host container can act as a catalyst by itself. By restricting reaction space and preorganizing the substrates into reactive conformations, it accelerates the sigmatropic rearrangement of enammonium cations.

Fiedler, Dorothea; Leung, Dennis H.; Raymond, Kenneth N.; Bergman, Robert G.

2004-11-27

204

The Oxidation Behavior of TBC with Cold Spray CoNiCrAlY Bond Coat  

NASA Astrophysics Data System (ADS)

Cold gas dynamic spray (CGDS) has been considered a potential technique to produce the metallic bond coat for TBC applications, because of its fast deposition rate and low deposition temperature. This article presents the influence of spray processes for bond coat, including air plasma spray, high velocity oxy-fuel, and in particular CGDS, on the oxidation performance of TBCs with a Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat and an air plasma sprayed topcoat. Oxidation behavior of the TBCs was evaluated by examining the coating microstructural evolution, TGO growth behavior, and crack propagation during thermal exposure at 1050 °C. The relationship between the TGO growth and crack propagation will be discussed.

Chen, W. R.; Irissou, E.; Wu, X.; Legoux, J.-G.; Marple, B. R.

2011-01-01

205

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

206

Disulfide bond formation and activation of Escherichia coli ?-galactosidase under oxidizing conditions.  

PubMed

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. PMID:22286993

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

2012-04-01

207

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

208

Surface rumpling of a (Ni, Pt)Al bond coat induced by cyclic oxidation  

Microsoft Academic Search

The surface of an initially flat, platinum-modified nickel aluminide bond coat formed on a single crystal superalloy is shown to progressively roughen (“rumple”) with thermal cycling in air. Far less surface roughening occurs after isothermal oxidation or after the same number of thermal cycles but with a shorter high-temperature exposure in each cycle. Mechanisms of the observed rumpling and the

V. K Tolpygo; D. R Clarke

2000-01-01

209

Theoretical study of the reaction formalhydrazone with singlet oxygen. Fragmentation of the C=N bond, ene reaction and other processes.  

PubMed

Photobiologic and synthetic versatility of hydrazones has not yet been established with (1)O2 as a route to commonly encountered nitrosamines. Thus, to determine whether the "parent" reaction of formalhydrazone and (1)O2 leads to facile C=N bond cleavage and resulting nitrosamine formation, we have carried out CCSD(T)//DFT calculations and analyzed the energetics of the oxidation pathways. A [2 + 2] pathway occurs via diradicals and formation of 3-amino-1,2,3-dioxazetidine in a 16 kcal/mol(-1) process. Reversible addition or physical quenching of (1)O2 occurs either on the formalhydrazone carbon for triplet diradicals at 2-3 kcal mol(-1), or on the nitrogen (N(3)) atom forming zwitterions at ~15 kcal/mol(-1), although the quenching channel by charge-transfer interaction was not computed. The computations also predict a facile conversion of formalhydrazone and (1)O2 to hydroperoxymethyl diazene in a low-barrier 'ene' process, but no 2-amino-oxaziridine-O-oxide (perepoxide-like) intermediate was found. A Benson-like analysis (group increment calculations) on the closed-shell species are in accord with the quantum chemical results. PMID:24354600

Rudshteyn, Benjamin; Castillo, Alvaro; Ghogare, Ashwini A; Liebman, Joel F; Greer, Alexander

2014-01-01

210

Intramolecular Hydrogen Bonding: A Potential Strategy for More Bioavailable Inhibitors of Neuronal Nitric Oxide Synthase  

PubMed Central

Selective neuronal nitric oxide synthase (nNOS) inhibitors have therapeutic applications in the treatment of numerous neurodegenerative diseases. Here we report the synthesis and evaluation of a series of inhibitors designed to have increased cell membrane permeability via intramolecular hydrogen bonding. Their potencies were examined in both purified enzyme and cell-based assays; a comparison of these results demonstrates that two of the new inhibitors display significantly increased membrane permeability over previous analogs. NMR spectroscopy provides evidence of intramolecular hydrogen bonding under physiological conditions in two of the inhibitors. Crystal structures of the inhibitors in the nNOS active site confirm the predicted non-intramolecular hydrogen bonded binding mode. Intramolecular hydrogen bonding may be an effective approach for increasing cell membrane permeability without affecting target protein binding.

Labby, Kristin Jansen; Xue, Fengtian; Kraus, James M.; Ji, Haitao; Mataka, Jan; Li, Huiying; Martasek, Pavel; Roman, Linda J.; Poulos, Thomas L.; Silverman, Richard B.

2012-01-01

211

Thermochemical properties and bond dissociation enthalpies of 3- to 5-member ring cyclic ether hydroperoxides, alcohols, and peroxy radicals: cyclic ether radical + (3)O(2) reaction thermochemistry.  

PubMed

The formation of cyclic ethers is a major product in the oxidation of hydrocarbons, and the oxidation of biomass derived alcohols. Cyclic ethers are formed in the initial reactions of alkyl radicals with dioxygen in combustion and precombustion processes that occur at moderate temperatures. They represent a significant part of the oxygenated pollutants found in the exhaust gases of engines. Cyclic ethers can also be formed from atmospheric reactions of olefins. Additionally, cyclic ethers have been linked to the formation of the secondary organic aerosol (SOA) in the atmosphere. In combustion and thermal oxidation processes these cyclic ethers will form radicals that react with (3)O2 to form peroxy radicals. Density functional theory and higher level ab initio calculations are used to calculate thermochemical properties and bond dissociation enthalpies of 3 to 5 member ring cyclic ethers (oxirane, yC2O, oxetane, yC3O, and oxolane, yC4O), corresponding hydroperoxides, alcohols, hydroperoxy alkyl, and alkyl radicals which are formed in these oxidation reaction systems. Trends in carbon-hydrogen bond dissociation energies for the ring and hydroperoxide group relative to ring size and to distance from the ether group are determined. Bond dissociation energies are calculated for use in understanding effects of the ether oxygen in the cyclic ethers, their stability, and kinetic properties. Geometries, vibration frequencies, and enthalpies of formation, ?H°f,298, are calculated at the B3LYP/6-31G(d,p), B3LYP/6-31G(2d,2p), the composite CBS-QB3, and G3MP2B3 methods. Entropy and heat capacities, S°(T) and Cp°(T) (5 K ? T ? 5000), are determined using geometric parameters and frequencies from the B3LYP/6-31G(d,p) calculations. The strong effects of ring strain on the bond dissociation energies in these peroxy systems are also of fundamental interest. Oxetane and oxolane exhibit a significant stabilization, 10 kcal mol(-1), lower ?fH°298 when an oxygen group is on the ether carbon relative to the isomer with the oxygen group on a secondary carbon. Relative to alkane systems the ether oxygen decreases bond dissociation energies (BDEs) on carbon sites adjacent to the ether by ?5 kcal mol(-1), and increases BDEs on nonether carbons ?1 kcal mol(-1). The cyclic structures have significant effects on the C-H, CO-OH, COO-H, and CO-H bond dissociation enthalpies. These values can be used to help calibrate calculations of larger more complex bicyclic and tricyclic hydrocarbon and ether species. PMID:24660891

Auzmendi-Murua, Itsaso; Bozzelli, Joseph W

2014-05-01

212

Saturation Kinetics in Phenolic O-H Bond Oxidation by a Mononuclear Mn(III)-OH Complex Derived from Dioxygen.  

PubMed

The mononuclear hydroxomanganese(III) complex, [Mn(III)(OH)(dpaq)](+), which is supported by the amide-containing N5 ligand dpaq (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) was generated by treatment of the manganese(II) species, [Mn(II)(dpaq)](OTf), with dioxygen in acetonitrile solution at 25 °C. This oxygenation reaction proceeds with essentially quantitative yield (greater than 98% isolated yield) and represents a rare example of an O2-mediated oxidation of a manganese(II) complex to generate a single product. The X-ray diffraction structure of [Mn(III)(OH)(dpaq)](+) reveals a short Mn-OH distance of 1.806(13) Å, with the hydroxo moiety trans to the amide function of the dpaq ligand. No shielding of the hydroxo group is observed in the solid-state structure. Nonetheless, [Mn(III)(OH)(dpaq)](+) is remarkably stable, decreasing in concentration by only 10% when stored in MeCN at 25 °C for 1 week. The [Mn(III)(OH)(dpaq)](+) complex participates in proton-coupled electron transfer reactions with substrates with relatively weak O-H and C-H bonds. For example, [Mn(III)(OH)(dpaq)](+) oxidizes TEMPOH (TEMPOH = 2,2'-6,6'-tetramethylpiperidine-1-ol), which has a bond dissociation free energy (BDFE) of 66.5 kcal/mol, in MeCN at 25 °C. The hydrogen/deuterium kinetic isotope effect of 1.8 observed for this reaction implies a concerted proton-electron transfer pathway. The [Mn(III)(OH)(dpaq)](+) complex also oxidizes xanthene (C-H BDFE of 73.3 kcal/mol in dimethylsulfoxide) and phenols, such as 2,4,6-tri-t-butylphenol, with BDFEs of less than 79 kcal/mol. Saturation kinetics were observed for phenol oxidation, implying an initial equilibrium prior to the rate-determining step. On the basis of a collective body of evidence, the equilibrium step is attributed to the formation of a hydrogen-bonding complex between [Mn(III)(OH)(dpaq)](+) and the phenol substrates. PMID:25010596

Wijeratne, Gayan B; Corzine, Briana; Day, Victor W; Jackson, Timothy A

2014-07-21

213

Reactions of the oxides of vanadium with liquid potassium  

Microsoft Academic Search

The reactions of the oxides VâOâ, VâOâ, V0â; , and VO with liquid potassium were studied at temperatures up to 400 deg C. The ; oxides VâOâ and VOâ react at 63 deg C (the mp of potassium); V\\/; s ub 2\\/Oâ at 180 deg C; and VO was found not to react at up to 400 deg C. ;

Marten G. Barker; Alan J. Hooper; Roger M. Lintonbon

1973-01-01

214

Involvement of incipient metal oxidation products in organic oxidation reactions at noble metal anodes  

SciTech Connect

The object of the present work is to demonstrate that in many organic electro-oxidation reactions at noble metal anodes in aqueous media, interfacial cyclic redox behavior involving incipient metal oxidation products (as intermediates) may be more important that the widely accepted adsorption-based view of this type of reaction. The activated chemisorption model is still accepted as being valid for many reactions involving hydrogen gas, e.g., hydrogen evolution or ionization, and catalytic hydrogenation reactions of unsaturated organic materials at platinized platinum electrodes. The alternative view is outlined here for gold followed by a brief discussion of some other noble metals.

Burke, L.D.; Cunnane, V.J.

1986-08-01

215

An approach to benzophosphole oxides through silver- or manganese-mediated dehydrogenative annulation involving C-C and C-P bond formation.  

PubMed

Benzophosphole construction was achieved through the Ag(I) -mediated dehydrogenative annulation of phenylphosphine oxides with internal alkynes in a process involving C?C and C?P bond formation. A wide range of asymmetrical phenylacetylenes could be employed and the reactions proceeded with perfect regioselectivity. Moreover, the annulation could be conducted even at room temperature when a Mn(III) promoter was used in place of Ag(I) . PMID:24127410

Unoh, Yuto; Hirano, Koji; Satoh, Tetsuya; Miura, Masahiro

2013-12-01

216

Indium(III) chloride-catalyzed oxidative cleavage of carbon–carbon multiple bonds by tert-butyl hydroperoxide in water—a safer alternative to ozonolysis  

Microsoft Academic Search

An efficient and general method for the oxidative cleavage of alkenes and alkynes using tert-butyl hydroperoxide and indium(III) chloride as catalyst in water to give the corresponding carboxylic acids or ketones has been achieved. The reaction conditions are compatible with sensitive moieties such as peptide bonds, tert-butyl carboxylic esters and N-Boc-protected tryptophan. The catalyst could be recycled.

Brindaban C. Ranu; Sukalyan Bhadra; Laksmikanta Adak

2008-01-01

217

Chemical Characterization and Reactivity of Fuel-Oxidizer Reaction Product  

NASA Technical Reports Server (NTRS)

Fuel-oxidizer reaction product (FORP), the product of incomplete reaction of monomethylhydrazine and nitrogen tetroxide propellants prepared under laboratory conditions and from firings of Shuttle Reaction Control System thrusters, has been characterized by chemical and thermal analysis. The composition of FORP is variable but falls within a limited range of compositions that depend on three factors: the fuel-oxidizer ratio at the time of formation; whether the composition of the post-formation atmosphere is reducing or oxidizing; and the reaction or post-reaction temperature. A typical composition contains methylhydrazinium nitrate, ammonium nitrate, methylammonium nitrate, and trace amounts of hydrazinium nitrate and 1,1-dimethylhydrazinium nitrate. Thermal decomposition reactions of the FORP compositions used in this study were unremarkable. Neither the various compositions of FORP, the pure major components of FORP, nor mixtures of FORP with propellant system corrosion products showed any unusual thermal activity when decomposed under laboratory conditions. Off-limit thruster operations were simulated by rapid mixing of liquid monomethylhydrazine and liquid nitrogen tetroxide in a confined space. These tests demonstrated that monomethylhydrazine, methylhydrazinium nitrate, ammonium nitrate, or Inconel corrosion products can induce a mixture of monomethylhydrazine and nitrogen tetroxide to produce component-damaging energies. Damaging events required FORP or metal salts to be present at the initial mixing of monomethylhydrazine and nitrogen tetroxide.

David, Dennis D.; Dee, Louis A.; Beeson, Harold D.

1997-01-01

218

Does a higher metal oxidation state necessarily imply higher reactivity toward H-atom transfer? A computational study of C-H bond oxidation by high-valent iron-oxo and -nitrido complexes.  

PubMed

In this work, the reactions of C-H bond activation by two series of iron-oxo ( (Fe(IV)), (Fe(V)), (Fe(VI))) and -nitrido model complexes ( (Fe(IV)), (Fe(V)), (Fe(VI))) with a nearly identical coordination geometry but varying iron oxidation states ranging from iv to vi were comprehensively investigated using density functional theory. We found that in a distorted octahedral coordination environment, the iron-oxo species and their isoelectronic nitrido analogues feature totally different intrinsic reactivities toward C-H bond cleavage. In the case of the iron-oxo complexes, the reaction barrier monotonically decreases as the iron oxidation state increases, consistent with the gradually enhanced electrophilicity across the series. The iron-nitrido complex is less reactive than its isoelectronic iron-oxo species, and more interestingly, a counterintuitive reactivity pattern was observed, i.e. the activation barriers essentially remain constant independent of the iron oxidation states. The detailed analysis using the Polanyi principle demonstrates that the different reactivities between these two series originate from the distinct thermodynamic driving forces, more specifically, the bond dissociation energies (BDEE-Hs, E = O, N) of the nascent E-H bonds in the FeE-H products. Further decomposition of the BDEE-Hs into the electron and proton affinity components shed light on how the oxidation states modulate the BDEE-Hs of the two series. PMID:24492533

Geng, Caiyun; Ye, Shengfa; Neese, Frank

2014-04-28

219

Favorite Demonstration: Demonstrating Indigo Carmine Oxidation-Reduction Reactions--A Choreography for Chemical Reactions  

NSDL National Science Digital Library

The indigo carmine demonstration (Ferguson et al. 1973), also referred to as a traffic-light demonstration (Flinn Scientific 2007a), is an example of a set of oxidation-reduction reactions that occurs within one solution. This type of demonstration can be used to introduce the concept of chemical reaction to undergraduate nonscience majors. Through their observations guided by the instructor, students begin to develop and construct the following concepts: color changes, reaction rates, reversible reactions, energy requirements (endothermic/exothermic), and equilibrium.

Majerich, David M.; Schmuckler, Joseph S.

2008-03-01

220

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

221

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

222

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.

Skála, Tomáš; Tsud, Nataliya; Prince, Kevin C.; Matolín, Vladimír

2011-08-01

223

Oxidative Half-reaction of Arabidopsis thaliana Sulfite Oxidase  

PubMed Central

Vertebrate forms of the molybdenum-containing enzyme sulfite oxidase possess a b-type cytochrome prosthetic group that accepts reducing equivalents from the molybdenum center and passes them on to cytochrome c. The plant form of the enzyme, on the other hand, lacks a prosthetic group other than its molybdenum center and utilizes molecular oxygen as the physiological oxidant. Hydrogen peroxide is the ultimate product of the reaction. Here, we present data demonstrating that superoxide is produced essentially quantitatively both in the course of the reaction of reduced enzyme with O2 and during steady-state turnover and only subsequently decays (presumably noncatalytically) to form hydrogen peroxide. Rapid-reaction kinetic studies directly following the reoxidation of reduced enzyme demonstrate a linear dependence of the rate constant for the reaction on [O2] with a second-order rate constant of kox = 8.7 × 104 ± 0.5 × 104 m?1s?1. When the reaction is carried out in the presence of cytochrome c to follow superoxide generation, biphasic time courses are observed, indicating that a first equivalent of superoxide is generated in the oxidation of the fully reduced Mo(IV) state of the enzyme to Mo(V), followed by a slower oxidation of the Mo(V) state to Mo(VI). The physiological implications of plant sulfite oxidase as a copious generator of superoxide are discussed.

Byrne, Robert S.; Hansch, Robert; Mendel, Ralf R.; Hille, Russ

2009-01-01

224

THE CATALYSIS OF THE HYDROGEN-OXYGEN REACTION BY AQUEOUS SLURRIES OF THORIUM OXIDE AND THORIUM-URANIUM OXIDE  

Microsoft Academic Search

Aqueous slurries of thorium oxide and thorium oxide containing urarium ; were investigated for their catalytic activity for the reaction of hydrogen and ; oxygen to form water. Pure thorium oxide. thorium-uranium oxide mixed crystals ; prepared by calcining coprecipitated oxalates, and thorium oxide with uranium ; oxide sorbed on the surface were used after calcining at 650, 800, and

Krohn

1960-01-01

225

Oxidation of triclosan by ferrate: reaction kinetics, products identification and toxicity evaluation.  

PubMed

The oxidation of triclosan by commercial grade aqueous ferrate (Fe(VI)) was investigated and the reaction kinetics as a function of pH (7.0-10.0) were experimentally determined. Intermediate products of the oxidation process were characterized using both GC-MS and RRLC-MS/MS techniques. Changes in toxicity during the oxidation process of triclosan using Fe(VI) were investigated using Pseudokirchneriella subcapitata growth inhibition tests. The results show that triclosan reacted rapidly with Fe(VI), with the apparent second-order rate constant, k(app), being 754.7 M(-1) s(-1) at pH 7. At a stoichiometric ratio of 10:1 (Fe(VI):triclosan), complete removal of triclosan was achieved. Species-specific rate constants, k, were determined for reaction of Fe(VI) with both the protonated and deprotonated triclosan species. The value of k determined for neutral triclosan was 6.7(±1.9)×10(2) M(-1) s(-1), while that measured for anionic triclosan was 7.6(±0.6)×10(3) M(-1) s(-1). The proposed mechanism for the oxidation of triclosan by the Fe(VI) involves the scission of ether bond and phenoxy radical addition reaction. Coupling reaction may also occur during Fe(VI) degradation of triclosan. Overall, the degradation processes of triclosan resulted in a significant decrease in algal toxicity. The toxicity tests showed that Fe(VI) itself dosed in the reaction did not inhibit green algae growth. PMID:21093982

Yang, Bin; Ying, Guang-Guo; Zhao, Jian-Liang; Zhang, Li-Juan; Fang, Yi-Xiang; Nghiem, Long Duc

2011-02-15

226

Appearance of the. nu. (Fe sup IV double bond O) vibration from a ferryl-oxo intermediate in the cytochrome oxidase/dioxygen reaction  

SciTech Connect

Time-resolved resonance Raman spectra have been recorded during the reaction of fully reduced (a{sup 2+}a{sub 3}{sup 2+}) cytochrome oxidase with dioxygen at room temperature. In the spectrum recorded at 800 {mu}s subsequent to carbon monoxide photolysis, a mode is observed at 790 cm{sup {minus}1} that shifts to 755 cm{sup {minus}1} when the experiment is repeated with {sup 18}O{sub 2}. The frequency of this vibration and the magnitude of the {sup 18}O{sub 2} isotopic frequency shift lead the authors to assign the 790-cm{sup {minus}1} mode to the Fe{sup IV}{double bond}O stretching vibration of a ferryl-oxo cytochrome a{sub 3} intermediate that occurs in the reaction of fully reduced cytochrome oxidase with dioxygen. The appearance and vibrational frequency of this mode were not affected when D{sub 2}O was used as a solvent. This result suggests that the ferryl-oxo intermediate is not hydrogen bonded. They have also recorded Raman spectra in the high-frequency region during the oxidase/O{sub 2} reaction that show that the oxidation of cytochrome a{sup 2+} is biphasic. These results on the kinetics of the redox activity of cytochrome a are consistent with the branched pathway discussed by Hill et al. for the oxidation of reduced cytochrome oxidase by O{sub 2} at room temperature.

Varotsis, C.; Babcock, G.T. (Michigan State Univ., East Lansing (USA))

1990-08-14

227

Heme-protein covalent bonds in peroxidases and resistance to heme modification during halide oxidation.  

PubMed

Plant peroxidases, as typified by horseradish peroxidase (HRP), primarily catalyze the one-electron oxidation of phenols and other low oxidation potential substrates. In contrast, the mammalian homologues such as lactoperoxidase (LPO) and myeloperoxidase primarily oxidize halides and pseudohalides to the corresponding hypohalides (e.g., Br(-) to HOBr, Cl(-) to HOCl). A further feature that distinguishes the mammalian from the plant and fungal enzymes is the presence of two or more covalent bonds between the heme and the protein only in the mammalian enzymes. The functional roles of these covalent links in mammalian peroxidases remain uncertain. We have previously reported that HRP can oxidize chloride and bromide ions, but during oxidation of these ions undergoes autocatalytic modification of its heme vinyl groups that virtually inactivates the enzyme. We report here that autocatalytic heme modification during halide oxidation is not unique to HRP but is a general feature of the oxidation of halide ions by fungal and plant peroxidases, as illustrated by studies with Arthromyces ramosus and soybean peroxidases. In contrast, LPO, a prototypical mammalian peroxidase, is protected from heme modification and its heme remains intact during the oxidation of halide ions. These results support the hypothesis that the covalent heme-protein links in the mammalian peroxidases protect the heme from modification during the oxidation of halide ions. PMID:16375846

Huang, Liusheng; Ortiz de Montellano, Paul R

2006-02-01

228

Reaction mechanism, bonding, and thermal stability of 1-alkanethiols self-assembled on halogenated Ge surfaces.  

PubMed

We have employed synchrotron radiation photoemission spectroscopy to study the reaction mechanism, surface bonding, and thermal stability of 1-octadecanethiolate (ODT) self-assembled monolayers (SAMs) at Cl- and Br-terminated Ge(100) surfaces. Density functional theory (DFT) calculations were also carried out for the same reactions. From DFT calculations, we have found that adsorption of 1-octadecanethiol on the halide-terminated surface via hydrohalogenic acid elimination is kinetically favorable on both Cl- and Br-terminated Ge surfaces at room temperature, but the reactions are more thermodynamically favorable at Cl-terminated Ge surfaces. After ODT SAM formation at room temperature, photoemission spectroscopy experiments show that Ge(100) and (111) surfaces contain monothiolates and possibly dithiolates together with unbound thiol and atomic sulfur. Small coverages of residual halide are also observed, consistent with predictions by DFT. Annealing studies in ultrahigh vacuum show that the Ge thiolates are thermally stable up to 150 degrees C. The majority of the surface thiolates are converted to sulfide and carbide upon annealing to 350 degrees C. By 430 degrees C, no sulfur remains on the surface, whereas Ge carbide is stable to above 470 degrees C. PMID:20433151

Ardalan, Pendar; Sun, Yun; Pianetta, Piero; Musgrave, Charles B; Bent, Stacey F

2010-06-01

229

Reversibility of heme-nitric oxide reactions for use in an inhaled nitric oxide sensor  

NASA Astrophysics Data System (ADS)

Nitric Oxide is a simple gaseous compound which serves as a regulatory molecule in a number of physiological processes. Due to its biological role as a potent local vasodilator,there has been widespread interest in the therapeutic use of gaseous nitric oxide a selective pulmonary vasodilator. Our goal is the development of a sensor for the direct and continuous measurement of inhaled nitric oxide concentrations. This study evaluated the reversibility of potential sensing compounds upon reaction with nitric oxide. Previously, absorption spectroscopy was used to study the sensitivity of the Fe II, Fe III and oxygenated forms of three biologically active hemes known to rapidly react with NO: hemoglobin, myoglobin, and cytochrome-c. This study focused on the photo-reversibility of the hem's reaction with nitric oxide. Hemoglobin, myoglobin and cytochrome-c in the Fe III state reversibly reacted with nitric oxide. Hemoglobin and myoglobin in the Fe II state non-reversibly reacted with nitric oxide to form an unstable product. Cytochrome-c (FeII) does not react with nitric oxide. The oxy forms of hemoglobin and myoglobin react with nitric oxide to form their respective met forms, unreversible via photolysis. For all reversible reactions, photolysis was gradual and complete within five minutes.

Parikh, Bhairavi R.; Soller, Babs R.; Rencus, Tal

1997-06-01

230

Biotransformations Utilizing ?-Oxidation Cycle Reactions in the Synthesis of Natural Compounds and Medicines.  

PubMed

?-Oxidation cycle reactions, which are key stages in the metabolism of fatty acids in eucaryotic cells and in processes with a significant role in the degradation of acids used by microbes as a carbon source, have also found application in biotransformations. One of the major advantages of biotransformations based on the ?-oxidation cycle is the possibility to transform a substrate in a series of reactions catalyzed by a number of enzymes. It allows the use of sterols as a substrate base in the production of natural steroid compounds and their analogues. This route also leads to biologically active compounds of therapeutic significance. Transformations of natural substrates via ?-oxidation are the core part of the synthetic routes of natural flavors used as food additives. Stereoselectivity of the enzymes catalyzing the stages of dehydrogenation and addition of a water molecule to the double bond also finds application in the synthesis of chiral biologically active compounds, including medicines. Recent advances in genetic, metabolic engineering, methods for the enhancement of bioprocess productivity and the selectivity of target reactions are also described. PMID:23443116

Swizdor, Alina; Panek, Anna; Milecka-Tronina, Natalia; Ko?ek, Teresa

2012-01-01

231

Fabrication of Direct Silicon Bonded Hybrid Orientation Substrate by Separation by Implanted Oxygen Layer Transfer and Oxide Dissolution Annealing  

NASA Astrophysics Data System (ADS)

The quasi direct Si bonded (DSB) hybrid orientation substrate with a 3 nm interfacial oxide layer between the (100) superficial Si and the (110) handle wafer is fabricated by the separation by implanted oxygen layer transfer (SLT) process. The quasi DSB hybrid orientation substrates are annealed in oxygen-containing and oxygen-free ambient. The cross-sectional transmission electron microscopy (XTEM) results show the oxide-free (100) Si/(110) Si bonding interface, indicating that the direct Si--Si bonded structure is realized by these two processes. The anisotropic bonding interface morphology of the DSB hybrid orientation substrates is observed, and the formation mechanism is discussed in detail.

Wei, Xing; Xue, Zhongying; Wu, Aimin; Cao, Gongbai; Zhang, Bo; Lin, Chenglu; Zhang, Miao; Wang, Xi

2011-03-01

232

Method for facilitating catalyzed oxidation reactions, device for facilitating catalyzed oxidation reactions  

DOEpatents

A catalytic process for the oxidation of organic. Oxygen is loaded into a metal foil by heating the foil while in contact with an oxygen-containing fluid. After cooling the oxygen-activated foil to room temperature, oxygen diffuses through the foil and oxidizes reactants exposed to the other side of the foil.

Beuhler, Robert J. (East Moriches, NY) [East Moriches, NY; White, Michael G. (Blue Point, NY) [Blue Point, NY; Hrbek, Jan (Rocky Point, NY) [Rocky Point, NY

2006-08-15

233

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

234

Surface/subsurface observation and removal mechanisms of ground reaction bonded silicon carbide  

NASA Astrophysics Data System (ADS)

Reaction Bonded Silicon Carbide (RBSiC) has long been recognized as a promising material for optical applications because of its unique combination of favorable properties and low-cost fabrication. Grinding of silicon carbide is difficult because of its high hardness and brittleness. Grinding often induces surface and subsurface damage, residual stress and other types of damage, which have great influence on the ceramic components for optical application. In this paper, surface integrity, subsurface damage and material removal mechanisms of RBSiC ground using diamond grinding wheel on creep-feed surface grinding machine are investigated. The surface and subsurface are studied with scanning electron microscopy (SEM) and optical microscopy. The effects of grinding conditions on surface and subsurface damage are discussed. This research links the surface roughness, surface and subsurface cracks to grinding parameters and provides valuable insights into the material removal mechanism and the dependence of grind induced damage on grinding conditions.

Yao, Wang; Zhang, Yu-Min; Han, Jie-cai; Zhang, Yun-long; Zhang, Jian-han; Zhou, Yu-feng; Han, Yuan-yuan

2006-01-01

235

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

236

Experimental investigation on material migration phenomena in micro-EDM of reaction-bonded silicon carbide  

NASA Astrophysics Data System (ADS)

Material migration between tool electrode and workpiece material in micro electrical discharge machining of reaction-bonded silicon carbide was experimentally investigated. The microstructural changes of workpiece and tungsten tool electrode were examined using scanning electron microscopy, cross sectional transmission electron microscopy and energy dispersive X-ray under various voltage, capacitance and carbon nanofibre concentration in the dielectric fluid. Results show that tungsten is deposited intensively inside the discharge-induced craters on the RB-SiC surface as amorphous structure forming micro particles, and on flat surface region as a thin interdiffusion layer of poly-crystalline structure. Deposition of carbon element on tool electrode was detected, indicating possible material migration to the tool electrode from workpiece material, carbon nanofibres and dielectric oil. Material deposition rate was found to be strongly affected by workpiece surface roughness, voltage and capacitance of the electrical discharge circuit. Carbon nanofibre addition in the dielectric at a suitable concentration significantly reduced the material deposition rate.

Liew, Pay Jun; Yan, Jiwang; Kuriyagawa, Tsunemoto

2013-07-01

237

Identification and cleavage of breakable single bonds by selective oxidation, reduction, and hydrolysis. Annual report, October 1, 1980-September 30, 1981  

SciTech Connect

The objective of this project is to determine the structure of bituminous coal by determining the proportions of the various kinds of connecting bonds and how they can best be broken. Results obtained during the past quarter are presented for the following tasks: (1) extractions and fractionations of coal products which covers pyridine extraction, fractionation of TIPS fractions, EDA extraction of Illinois No. 6 coal and swelling ratios of coal samples; (2) experiments on breakable single bonds which cover reactions of ethylenediamine and model ethers, reaction of pyridine-extracted coal with Me/sub 3/SiI, Baeyer-Villiger oxidations, reaction to diphenylmethane with 15% HNO/sub 3/, cleavage of TIPS with ZnI/sub 2/, and cleavage of black acids; and (3) oxygen oxidation No. 18. Some of the highlights of these studies are: (1) some model ethers are not cleaved by EDA under extraction conditions; (2) oxidation of diaryl ketones with m-chloroperbenzoic acid and saponification of the resulting esters in promising for identifying ketones, (3) treatment of a black acid with pyridine hydroiodide reduced the acid's molecular weight and increased its solubility in pyridine, but treatment with ZnI/sub 2/ was ineffective; (4) in comparison with 0.1 M K/sub 2/S/sub 2/O/sub 8/, 0.01 M persulfate is relatively ineffective in accelerating oxidation of BnNH/sub 2/-extracted coal in water suspension. 2 figures, 3 tables.

Hirschon, A.S.; Zevely, J.; Mayo, F.R.

1981-11-12

238

Forming mechanism of nitrogen doped graphene prepared by thermal solid-state reaction of graphite oxide and urea  

Microsoft Academic Search

Nitrogen doped graphene was synthesized from graphite oxide and urea by thermal solid-state reaction. The samples were characterized by transmission electron microscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectra, element analysis, and electrical conductivity measurement. The results reveal that there is a gradual thermal transformation of nitrogen bonding configurations from amide form nitrogen to

Zhigang Mou; Xiaoye Chen; Yukou Du; Xiaomei Wang; Ping Yang; Suidong Wang

239

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

240

Dark reaction of oxidation of iodine by hydrogen peroxide  

SciTech Connect

The oxidation of iodine in darkness was studied in the system H/sub 2/O/sub 2/-I/sub 2/-HNO/sub 3/ by a potentiometric method using an ion-selective electrode and a spectrophotometric method. The concentration limits of the reaction was established. The reaction rate is satisfactorily described by the equation of a first-order reaction. It was established experimentally that the maximum of the reaction rate constant lies in the region of 0.2-0.6 M HNO/sub 3/ and 0.06 M H/sub 2/O/sub 2/ and reaches a value of 0.1 min/sup /minus/1/.

Moskalev, P.N.; Sedov, V.P.; Isupov, V.K.

1989-01-01

241

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

242

The oxidative burst reaction in mammalian cells depends on gravity  

PubMed Central

Gravity has been a constant force throughout the Earth’s evolutionary history. Thus, one of the fundamental biological questions is if and how complex cellular and molecular functions of life on Earth require gravity. In this study, we investigated the influence of gravity on the oxidative burst reaction in macrophages, one of the key elements in innate immune response and cellular signaling. An important step is the production of superoxide by the NADPH oxidase, which is rapidly converted to H2O2 by spontaneous and enzymatic dismutation. The phagozytosis-mediated oxidative burst under altered gravity conditions was studied in NR8383 rat alveolar macrophages by means of a luminol assay. Ground-based experiments in “functional weightlessness” were performed using a 2 D clinostat combined with a photomultiplier (PMT clinostat). The same technical set-up was used during the 13th DLR and 51st ESA parabolic flight campaign. Furthermore, hypergravity conditions were provided by using the Multi-Sample Incubation Centrifuge (MuSIC) and the Short Arm Human Centrifuge (SAHC). The results demonstrate that release of reactive oxygen species (ROS) during the oxidative burst reaction depends greatly on gravity conditions. ROS release is 1.) reduced in microgravity, 2.) enhanced in hypergravity and 3.) responds rapidly and reversible to altered gravity within seconds. We substantiated the effect of altered gravity on oxidative burst reaction in two independent experimental systems, parabolic flights and 2D clinostat / centrifuge experiments. Furthermore, the results obtained in simulated microgravity (2D clinorotation experiments) were proven by experiments in real microgravity as in both cases a pronounced reduction in ROS was observed. Our experiments indicate that gravity-sensitive steps are located both in the initial activation pathways and in the final oxidative burst reaction itself, which could be explained by the role of cytoskeletal dynamics in the assembly and function of the NADPH oxidase complex.

2013-01-01

243

Chromato-mass spectral study of the reactions of organylsiloxanes with metal oxides  

Microsoft Academic Search

A study was carried out on the reaction of 1,1,3,3-tetramethyldisiloxane with metal oxides such as CuO, HgO, and Sb2O5 in the presence of compounds containing a siloxane bond such as hexamethyldisiloxane, octamethylcyclotetrasiloxane, and trimethylacetoxysilane, which, at 100–110°C over 0.5–10 h, leads to the formation of linear organyl siloxanes (CH3-[-Si(CH3)2O-]n-Si(CH3)3, CH3-[-Si(CH3)2O-]n-Si (CH3)2H, and H-[-Si(CH3)2O-]n-Si(CH3)2H, where n=2–6, and cyclic organylsiloxanes. The reaction

M. G. Voronkov; S. V. Basenko; S. M. Nozdrya; V. Yu. Vitkovskii; R. G. Mirskov

1990-01-01

244

Palladium-catalyzed coupling reactions of bromothiophenes at the C-H bond adjacent to the sulfur atom with a new activator system, AgNO3/KF.  

PubMed

[reaction: see text] Bromothiophene derivatives react with aryl iodides catalyzed by a palladium complex in the presence of a silver(I) nitrate/potassium fluoride system to induce coupling at the C-H bond, while the carbon-bromine bond is intact. The produced coupling product bearing the C-Br bond allows further palladium-catalyzed C-C bond-forming reactions in reasonable yields. PMID:16235963

Kobayashi, Kei; Sugie, Atsushi; Takahashi, Masabumi; Masui, Kentaro; Mori, Atsunori

2005-10-27

245

Time-resolved spectroscopic characterization of a novel photodecarboxylation reaction mediated by homolysis of a carbon ?-bond in flurbiprofen.  

PubMed

Flurbiprofen (Fp), a nonsteroidal anti-inflammatory drug (NSAID) currently in use for arthritis pain relief and in clinical trials for metastatic prostate cancer, can induce photosensitization and phototoxicity upon exposure to sunlight. The mechanisms responsible for Fp phototoxicity are poorly understood and deserve investigation. In this study, the photodecarboxylation reaction of Fp, which has been assumed to underpin its photoinduced side effects, was explored by femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopic techniques in pure acetonitrile (MeCN) solvent. Density functional theory (DFT) calculations were also performed to facilitate the assignments of transient species. The resonance Raman and DFT calculation results reveal that the neutral form of Fp was the predominant species present in MeCN. Analysis of the ultraviolet/visible absorption spectrum and results from TD-DFT calculations indicate that the second excited singlet (S2) can be excited by 266 nm light. Due to its intrinsic instability, S2 rapidly underwent internal conversion (IC) to decay to the lowest lying excited singlet (S1), which was observed in the fs-TA spectra at very early delay times. Intriguingly, three distinct pathways for S1 decay seem to coexist. Specifically, other than fluorescence emission back to the ground state and transformation to the lowest triplet state T1 through intersystem crossing (ISC), the homolysis of the carbon ?-bond decarboxylation reaction proceeded simultaneously to give rise to two radical species, one being carboxyl and another being the residual, denoted as FpR. The coexistence of the triplet Fp (T1) and FpR species was verified by means of TR(3) spectra along with ns-TA spectra. As a consequence of its apparent high reactivity, the FpR intermediate was observed to undergo oxidation under oxygen-saturated conditions to yield another radical species, denoted as FOR, which subsequently underwent intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to form a final product, which could react with the carboxyl from the decarboxylation reaction to generate a minor final product. TD-DFT and transient state (TS) calculations for predicting the absorption bands and activation energies of the transient species produced in the photodecarboxylation reaction have provided valuable mechanistic insights for the assignment of the intermediate species observed in the time-resolved spectroscopy experiments reported here. The results of the time-resolved spectroscopy experiments and DFT calculations were used to elucidate the reaction mechanisms and intermediates involved in the photochemistry of Fp. PMID:23750456

Su, Tao; Ma, Jiani; Wong, Naikei; Phillips, David Lee

2013-07-18

246

An in vitro study to evaluate the effects of addition of zinc oxide to an orthodontic bonding agent  

PubMed Central

Objective: The objective of this study is to test the antimicrobial effect of zinc oxide when incorporated into an orthodontic bonding material and to check the effect of addition of zinc oxide on the shear bond strength of the bonding material. Materials and Methods: Zinc oxide was added to a resin modified light cure glass ionomer cement (GIC) (Fuji Ortho LC GC America, Alsip, Ill) to make modified bonding agent containing 13% and 23.1% ZnO and the antimicrobial assay was done using agar disc diffusion method. Discs of the modified bonding agent were prepared and a culture of Streptococcus mutans mixed with soft agar was poured over it and incubated at 38°C for 48 h and zones of inhibition were measured. The test was repeated after a month to check the antimicrobial effect. In addition shear bond strength of the brackets bonded with the modified bonding agent was tested. Results: The agar disc showed zones of inhibition around the modified bonding agent and the antimicrobial activity was more when the concentration of ZnO was increased. The antimicrobial effect was present even after a month. The shear bond strength decreased as the concentration of ZnO increased. Conclusion: The incorporation of ZnO into a resin modified light cure GIC (Fuji Ortho LC GC America, Alsip, Ill) added antimicrobial property to the original compound.

Jatania, Archana; Shivalinga, B. M.

2014-01-01

247

Determination of nitrogen oxides by reaction gas chromatography  

SciTech Connect

The determination of nitrogen oxides as well as of nitrites, nitrates, and nitric acid is one of the difficult tasks related to analytical control in industry to the analysis of pollutants of the environment. The conversion of oxygenated nitrogen compounds into nitrobenzene has been used for the gas chromatographic determination of these compounds in exhaust gases from diesel engines and gunpowder gases. The nitrogen oxides were trapped and preconcentrated from air by reaction and sorption, a procedure based on the preliminary removal of interfering impurities from the analyzed air (gas) by means of a reactor-precolumn packed with a sorbent. This approach has significantly improved the selectivity and accuracy of the analysis.

Vsemirnova, E.A.; Drugov, Y.S.; Tul'chinskii, V.M.

1986-05-01

248

A highly efficient oxidative condensation reaction for selective protein conjugation.  

PubMed

We hereby report a mild and efficient coupling reaction between alkyl aldehydes and aryl diamines. In the presence of a Cu(2+) or a Zn(2+) ion, oxygen (O2) in air is able to promote the oxidative condensation of the two readily preparable functional groups, forming stable benzimidazole linkages in neutral aqueous solution at room temperature (RT). We demonstrated that the reaction could be utilized to label a T4 lysozyme protein containing a chemically installed aryl diamine group with a fluorescent aldehyde dye molecule at 37 °C. PMID:24875017

Ji, Ao; Ren, Wei; Ai, Hui-Wang

2014-06-17

249

Further characterization of the aluminum peroxide oxide, AlO 2, formed by interfacial reaction between Pt and ?-Al 2O 3  

Microsoft Academic Search

A new metastable aluminum peroxide oxide, AlO2, was recently shown to form by an interfacial reaction in the presence of a kinetic constraint during diffusion-bonding of Pt and ?-Al2O3. Further studies of the properties, bonding, formation kinetics and chemistry of AlO2 are described and the current state of knowledge of the kinetics of its formation is summarized. Raman spectroscopy on

Y.-C. Lu; S. Agnew; R. Dieckmann; S. L. Sass

1995-01-01

250

Rhodium-catalyzed direct oxidative carbonylation of aromatic C-H bond with CO and alcohols.  

PubMed

A general protocol for the rhodium-catalyzed oxidative carbonylation of arenes to form esters has been developed. A broad substrate scope has been demonstrated allowing carbonylation of electron-rich, electron-poor, and heterocyclic arenes, and the reaction shows wide functional group tolerance and excellent regioselectivities. Up to 96% yield of ortho-substituted aryl or heteroaryl carboxylic esters were obtained with this methodology. The possible mechanism for the rhodium-catalyzed oxidative carbonylation reaction was proposed in this article. Studies show that Oxone play an important role in the transformation. PMID:19099479

Guan, Zheng-Hui; Ren, Zhi-Hui; Spinella, Stephen M; Yu, Shichao; Liang, Yong-Min; Zhang, Xumu

2009-01-21

251

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

2013-02-01

252

Reaction of scandium ions with ethane. First and second hydride-scandium ion bond energies  

SciTech Connect

Reactions of atomic scandium ions with ethane (d/sub 0/, 1,1,1-d/sub 3/, and d/sub 6/) are examined by using guided ion beam mass spectrometry. The present results for ethane-d/sub 0/ are in qualitative agreement with the previous work of Tolbert and Beauchamp. Although the dominant reaction at low energies is exothermic dehydrogenation, this is inefficient. Strong inter- and intramolecular isotope effects suggest that this inefficiency results from the need for a triplet-singlet surface crossing. At low energies dehydrogenation forms a Sc/sup +/-ethene complex, while at higher energies, the product is Sc/sup +/-ethylidene. Double dehydrogenation of ethane has two components: an exothermic one and an apparent endothermic one. Several possible explanations are discussed for this unusual result. The results are analyzed to yield both the first and second metal hydride bond energies, D/sup 0/ /sub 298/(Sc/sup +/-H) = 56.2 +/- 2 and D/sup 0/ /sub 298/(HSc/sup +/H) = 59.3 +/- 3.7 as well as D/sup 0/ /sub 298/(Sc/sup +/-CH/sub 3/) = 59.0 +/- 3 and D/sup 0/ /sub 298/(Sc/sup +/-CH/sub 2/) greater than or equal to 93.4 +/- 2.5, all in kcal/mol.

Sunderlin, L.; Aristov, N.; Armentrout, P.B.

1987-01-07

253

Palladium-catalyzed oxidative arylalkylation of activated alkenes: dual C-H bond cleavage of an arene and acetonitrile.  

PubMed

Not one but two: The title reaction proceeds through the dual C-H bond cleavage of both aniline and acetonitrile. The reaction affords a variety of cyano-bearing indolinones in excellent yield. Mechanistic studies demonstrate that this reaction involves a fast arylation of the olefin and a rate-determining C-H activation of the acetonitrile. PMID:22076660

Wu, Tao; Mu, Xin; Liu, Guosheng

2011-12-23

254

Oxidative C(sp(3))-H bond cleavage, C-C and C[double bond, length as m-dash]C coupling at a boron center with O2 as the oxidant mediated by platinum(ii).  

PubMed

Dimethyl- and diphenylplatinum(ii) fragments Pt(II)R2 (R = Me, Ph) enable facile and efficient oxidative C(sp(3))-H bond cleavage and stepwise C-C and C[double bond, length as m-dash]C coupling at the boron atom of a coordinated 1,5-cyclooctanediyldi(2-pyridyl)borato ligand with O2 as the sole oxidant. PMID:24287586

Pal, S; Zavalij, P Y; Vedernikov, A N

2014-05-25

255

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

2013-11-28

256

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

257

Synthesis and oxidation behavior of nanocrystalline MCrAlY bond coatings  

NASA Astrophysics Data System (ADS)

Thermal barrier coating systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work, the oxidation behavior of conventional and nanostructured high-velocity oxyfuel (HVOF) NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Freestanding bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at 1000 °C for different time periods to form the thermally grown oxide layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared with that of the conventional one. The observed behavior is a result of the formation of a continuous Al2O3 layer on the surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.

Ajdelsztajn, L.; Tang, F.; Schoenung, J. M.; Kim, G. E.; Provenzano, V.

2005-03-01

258

Reaction Mechanism and Kinetics of Enargite Oxidation at Roasting Temperatures  

NASA Astrophysics Data System (ADS)

Roasting of enargite (Cu3AsS4) in the temperature range of 648 K to 898 K (375 °C to 625 °C) in atmospheres containing variable amounts of oxygen has been studied by thermogravimetric methods. From the experimental results of weight loss/gain data and X-ray diffraction (XRD) analysis of partially reacted samples, the reaction mechanism of the enargite oxidation was determined, which occurred in three sequential stages:

Padilla, Rafael; Aracena, Alvaro; Ruiz, Maria C.

2012-10-01

259

Bond strength of selected composite resin-cements to zirconium-oxide ceramic  

PubMed Central

Objectives: The aim of this study was to evaluate bond strengths of zirconium-oxide (zirconia) ceramic and a selection of different composite resin cements. Study Design: 130 Lava TM cylinders were fabricated. The cylinders were sandblasted with 80 µm aluminium oxide or silica coated with CoJet Sand. Silane, and bonding agent and/or Clearfil Ceramic Primer were applied. One hundred thirty composite cement cylinders, comprising two dual-polymerizing (Variolink II and Panavia F) and two autopolymerizing (Rely X and Multilink) resins were bonded to the ceramic samples. A shear test was conducted, followed by an optical microscopy study to identify the location and type of failure, an electron microscopy study (SEM and TEM) and statistical analysis using the Kruskal-Wallis test for more than two independent samples and Mann-Whitney for two independent samples. Given the large number of combinations, Bonferroni correction was applied (?=0.001). Results: Dual-polymerizing cements provided better adhesion values (11.7 MPa) than the autopolymerizing (7.47 MPa) (p-value M-W<0.001). The worst techniques were Lava TM + sandblasting + Silane + Rely X; Lava TM + sandblasting + Silane + Multilink and Lava TM + CoJet + silane + Multilink. Adhesive failure (separation of cement and ceramic) was produced at a lesser force than cohesive failure (fracture of cement) (p-value M-W<0.001). Electron microscopy confirmed that the surface treatments modified the zirconium-oxide ceramic, creating a more rough and retentive surface, thus providing an improved micromechanical interlocking between the cement and the ceramic. Key words:Shear bond strength, silica coating, surface treatment, zirconia ceramics, phosphate monomer.

Fons-Font, Antonio; Amigo-Borras, Vicente; Granell-Ruiz, Maria; Busquets-Mataix, David; Panadero, Ruben A.; Sola-Ruiz, Maria F.

2013-01-01

260

The reaction of vapor-deposited Al with Cu oxides  

SciTech Connect

Interfaces formed by controlled deposition of Al on Cu oxides at 300K have been characterized using Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). When Al is deposited onto a thin oxide grown on Cu(110) by atmospheric exposure, it completely scavenges the oxygen from the substrate material, increasing the O(1s) binding energy by 2.0 eV to give the value found for atmospheric oxidation of a thin Al film. Similar oxygen behavior is seen for Al deposition on sputter-deposited CuO with an enriched oxygen surface region, where multilayers of Al erase the shakeup satellites in the Cu(2p) region of the XPS spectrum to give features like those exhibited by Cu{sub 2}O or metallic Cu. Having calibrated the fluence of the Al source with Rutherford backscattering spectrometry, the attenuation of the Cu 2p{sub 1/2} satellite after approximately one monolayer of Al deposition is associated with the removal of oxygen from the top 20 {angstrom} of the CuO. Approximately 7--8 equivalent monolayers of Al are converted to an oxide in the initial rapid reaction process. Further deposition leads to progressive development of the metallic Al signature in both the XPS and AES spectra. These measurements clearly demonstrate the dominant role played by Al, a strong oxide former, when it is placed in intimate contact with the distinctively weaker Cu oxide. 9 refs., 5 figs.

Taylor, T.N.; Martin, J.A.

1990-01-01

261

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

262

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; Pérez, Patricia; Sáez, Jose A

2012-05-21

263

Fabrication of near-net-shaped reaction-bonded silicon carbide tubes by thermoset molding and reaction infiltration of silicon melt  

Microsoft Academic Search

A reaction-bonded silicon carbide tube has been fabricated by a thermoset molding (transfer molding) technique using a mixture of thermoplastic and thermosetting polymer resins. Shrinkage and stress development during the centrifugal thermoset molding have been minimized by tailoring the particle packing characteristics with a multimodal particle size distribution. Debinding behavior was optimized in a two-step process consisting of the capillary

Hyung-Woo Juna; Hae-Weon Lee; Huesup Song; Hyunick Shin; Joosun Kim; Jowoong Hab

264

Reaction rate oscillations during catalytic CO oxidation: A brief overview  

NASA Technical Reports Server (NTRS)

It is not the intent here to present a comprehensive review of the dynamic behavior of the catalytic oxidation of CO. This reaction is one of the most widely studied in the field of catalysis. A review paper by Engel and Ertl has examined the basic kinetic and mechanistic aspects, and a comprehensive paper by Razon and Schmitz was recently devoted to its dynamic behavior. Those interested in further study of the subject should consult these reviews and a number of general review papers on catalytic reaction dynamics. The goal is to present a brief overview of certain interesting aspects of the dynamic behavior of this reaction and to discuss a few questions and issues, which are still the subject of study and debate.

Tsotsis, T. T.; Sane, R. C.

1987-01-01

265

Laminate behavior for SiC fiber-reinforced reaction-bonded silicon nitride matrix composites  

NASA Technical Reports Server (NTRS)

The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained approx 30 volume fraction of aligned 142-micron diameter SiC fiber in a porous RBSN matrix. Three types of laminate studied were unidirectional: (1) (0) sub 8, (2) (10) sub 8, and (3) (45) sub 8, and (90) sub 8; cross plied laminates (0 sub 2/90 sub 2); and angle plied laminates: (+45 sub 2/-45 sub 2). Each laminate contained eight fiber plies. Results of the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane properties. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Splitting parallel to the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. In-plane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for (0 sub 2/90 sub 2) sub s and (+45/-45 sub 2) sub s laminates showed that their matrix failure strains were similar to that for (0) sub 8 laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle-ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.

Bhatt, Ramakrishna T.; Phillips, Ronald E.

1990-01-01

266

Laminate behavior for SiC fiber-reinforced reaction-bonded silicon nitride matrix composites  

NASA Technical Reports Server (NTRS)

The room temperature mechanical properties of SiC fiber reinforced reaction-bonded silicon nitride matrix composite laminates (SiC/RBSN) have been measured. The laminates contained approx 30 volume fraction of aligned 142-micron diameter SiC fiber in a porous RBSN matrix. Three types of laminate studied were unidirectional: (1) (0) sub 8, (2) (10) sub 8, and (3) (45) sub 8, and (90) sub 8; cross plied laminates (0 sub 2/90 sub 2); and angle plied laminates: (+45 sub 2/-45 sub 2). Each laminate contained eight fiber plies. Results of the unidirectionally reinforced composites tested at various angles to the reinforcement direction indicate large anisotropy in in-plane properties. In addition, strength properties of these composites along the fiber direction were independent of specimen gage length and were unaffected by notches normal to the fiber direction. Splitting parallel to the fiber at the notch tip appears to be the dominant crack blunting mechanism responsible for notch insensitive behavior of these composites. In-plane properties of the composites can be improved by 2-D laminate construction. Mechanical property results for (0 sub 2/90 sub 2)sub s and (+45/-45 sub 2) sub s laminates showed that their matrix failure strains were similar to that for (0) sub 8 laminates, but their primary elastic moduli, matrix cracking strengths, and ultimate composite strengths were lower. The elastic properties of unidirectional, cross-ply, and angle-ply composites can be predicted from modified constitutive equations and laminate theory. Further improvements in laminate properties may be achieved by reducing the matrix porosity and by optimizing the bond strength between the SiC fiber and RBSN matrix.

Rhatt, R. T.; Phillips, R. E.

1988-01-01

267

Structural and mechanistic studies of Pd-catalyzed C C bond formation: The case of carbonylation and Heck reaction  

Microsoft Academic Search

The extraordinary catalytic activity of palladium in CC bond forming processes is reviewed in respect to Heck and carbonylation reactions. Examples of catalytic systems based on palladium complexes with phosphorus ligands and phosphine-free systems and based on the Pd(0) colloid are presented. The two key steps of both reaction mechanisms: first, reduction of Pd(II) to Pd(0) complexes or Pd(0) colloids

Anna M. Trzeciak; Józef J. Zió?kowski

2005-01-01

268

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

2013-06-01

269

Pd nanoparticles supported on MIL-101: an efficient recyclable catalyst in oxidation and hydrogenation reactions.  

PubMed

Pd nanoparticles supported on the chromium terephthalate metal organic framework MIL-101 (Pd/MIL-101) in different loadings (0.9 and 4.5 wt%) have been successfully prepared through a simple Pd-acetate adsorption and reduction in acetone, and tested as catalyst for selected liquid phase oxidation and hydrogenation reactions. The materials were characterized by XRD, N2 adsorption--desorption isotherm, TEM, SEM-EDX and ICP analysis. The parent MIL-101 structure was found well preserved after formation of Pd nanoparticles and after catalytic reaction runs. The present catalyst afforded good activity and selectivity for the oxidation of benzyl alcohol to benzaldehyde with 85% conversion and 97% selectivity using air (1 atm) at 85 degrees C after 14 h. The catalyst also showed good activity in the hydrogenation of the C=C bond in alkenes to corresponding alkanes and also benzaldehyde to benzyl alcohol at room temperature using H2 (1 atm). Rigorous test results confirmed that Pd-nanoparticles supported on MIL-101 are responsible for the catalytic reactions occurred. Pd/MIL-101 was reusable several times without losing the structural integrity and initial activity, and demonstrated significantly higher catalytic activities than those by a commercial Pd catalyst supported on activated carbon. PMID:24745261

Bhattacharjee, Samiran; Kim, Jun; Ahn, Wha-Seung

2014-03-01

270

Bis(trifluoromethyl)methylene Addition to Vinyl-Terminated SAMs: A Gas-Phase C-C Bond-Forming Reaction on a Surface  

PubMed Central

Vinyl-terminated self-assembled monolayers (SAMs) on silicon oxide substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene group in a rare gas-phase C–C bond-forming reaction to directly generate films carrying terminal CF3 groups. The vinyl-terminated films were treated with hexafluoroacetone azine (HFAA) for modification. The films were characterized with ellipsometry, contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In this study, we find that for optimized conditions clean reactions occur on a surface between SAMs with terminal olefins and HFAA, and the product is consistent with bis(trifluoromethyl)cyclopropanation formation after nitrogen extrusion.

2014-01-01

271

Bis(trifluoromethyl)methylene Addition to Vinyl-Terminated SAMs: A Gas-Phase C-C Bond-Forming Reaction on a Surface.  

PubMed

Vinyl-terminated self-assembled monolayers (SAMs) on silicon oxide substrates were chemically modified by the addition of a bis(trifluoromethyl)methylene group in a rare gas-phase C-C bond-forming reaction to directly generate films carrying terminal CF3 groups. The vinyl-terminated films were treated with hexafluoroacetone azine (HFAA) for modification. The films were characterized with ellipsometry, contact angle measurements, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). In this study, we find that for optimized conditions clean reactions occur on a surface between SAMs with terminal olefins and HFAA, and the product is consistent with bis(trifluoromethyl)cyclopropanation formation after nitrogen extrusion. PMID:24806554

Adamkiewicz, Malgorzata; O'Hagan, David; Hähner, Georg

2014-05-20

272

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

SciTech Connect

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{sub 2} (An: U, Pu) to form non-stoichiometric species described as AnO{sub 2+x}. Environmental studies have shown the value of utilizing the chemical signatures of these actinide oxide materials to understand transport following release into the environment. Chemical speciation of actinide-oxide samples may also provide clues as to the age, source, or process history of the material. The scientific challenge is to identify, measure and understand those aspects of speciation of actinide analytes that carry information about material origin and history most relevant to forensics. Here, we will describe our efforts in material synthesis and analytical methods development that we will use to provide the fundamental science to characterize actinide oxide molecular structures for forensic science. Structural properties and initial results to measure structural variability of uranium oxide samples using synchrotron-based X-ray Absorption Fine Structure will be discussed.

Wilkerson, Marianne Perry [Los Alamos National Laboratory

2010-01-01

273

Forming mechanism of nitrogen doped graphene prepared by thermal solid-state reaction of graphite oxide and urea  

NASA Astrophysics Data System (ADS)

Nitrogen doped graphene was synthesized from graphite oxide and urea by thermal solid-state reaction. The samples were characterized by transmission electron microscopy, atomic force microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectra, element analysis, and electrical conductivity measurement. The results reveal that there is a gradual thermal transformation of nitrogen bonding configurations from amide form nitrogen to pyrrolic, then to pyridinic, and finally to "graphitic" nitrogen in graphene sheets with increasing annealing temperature from 200 to 700 °C. The products prepared at 600 °C and 700 °C show that the quantity of nitrogen incorporated into graphene lattice is ˜10 at.% with simultaneous reduction of graphite oxide. Oxygen-containing functional groups in graphite oxide are responsible for the doping reaction to produce nitrogen doped graphene.

Mou, Zhigang; Chen, Xiaoye; Du, Yukou; Wang, Xiaomei; Yang, Ping; Wang, Suidong

2011-12-01

274

Insertion of arynes into the carbon–oxygen double bond of amides and its application into the sequential reactions  

Microsoft Academic Search

The reaction of arynes, generated from ortho-(trimethylsilyl)aryl triflates, with the CO bond of formamides gave salicylaldehyde derivatives via the formation of formal [2+2] adducts. The sequential transformation of arynes into ortho-disubstituted arenes, o-aminoalkylphenols or o-hydroxyalkylphenols, was achieved by one-pot procedure using dialkylzincs.

Eito Yoshioka; Hideto Miyabe

275

Organic reaction pathways in the nonaqueous synthesis of metal oxide nanoparticles.  

PubMed

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods. PMID:16927442

Niederberger, Markus; Garnweitner, Georg

2006-09-25

276

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

277

Composition Effects on Aluminide Oxidation Performance: Objectives for Improved Bond Coats  

SciTech Connect

Formerly, the role of metallic coatings on Ni-base superalloys was simply to limit environmental attack of the underlying substrate. However, a new paradigm has been established for metallic coatings adapted as bond coats for thermal barrier coatings. It is no longer sufficient for the coating to just minimize the corrosion rate. The metallic coating must also form a slow-growing external Al{sub 2}O{sub 3} layer beneath the overlying low thermal conductivity ceramic top coat. This thermally grown oxide or scale must have near-perfect adhesion in order to limit spallation of the top coat, thereby achieving a long coating lifetime. While oxidation is not the only concern in complex thermal barrier coating systems, it is, however, a primary factor in developing the next generation of bond coats. Therefore, a set of compositional guidelines for coatings is proposed in order to maximize oxidation performance. These criteria are based on test results of cast alloy compositions to quantify an d understand possible improvements as a basis for further investigations using coatings made by chemical vapor deposited (CVD). Experimental work includes furnace cycle testing and in-depth characterization of the alumina scale, including transmission electron microscopy (TEM).

Pint, BA

2001-09-06

278

Joining of silicon carbide ceramic for optical application by reaction bonded technology  

NASA Astrophysics Data System (ADS)

Silicon carbide ceramic is a prospective candidate for the next generation space telescope with a large-scale reflector. In order to reduce the cost of the fabrication of complex shaped component, the joining technology of SiC ceramic is investigated. In this work, SiC ceramic was joined by reaction bonded technology using SiC green preforms successfully. SiC green preforms were fastened together by epoxy resin and joined during the process of one-step Si infiltration. The microstructure and composition of SiC substrate ceramic and the joint is homogeneous and there have no micro-cracks or defects in the joint. The interface reaction layer become smaller and even disappears with carefully surface grinding of the SiC green preforms. The surface roughness of the joined SiC in the area of the joint is 0.95nm rms and there has no significant difference in the surface roughness of the joint area and SiC substrate ceramic. The bending strength of the joined SiC is about 300MPa and the crack occurred inside the SiC substrate but not in the joint. The most important advantage of the method is the joining of SiC ceramics was achieved via one-step Si infiltration process and the joints are homogeneous with the substrate, which is economical and effective. The mechanical and optical properties of the joined SiC can satisfy the requirement of the large-scale telescope in many astronomical instruments.

Zhang, Aifang; Chen, Yichao; Chen, Zhiqiang; Liu, Hong; Fang, Jingzhong

2010-05-01

279

Synthesis, structure, and properties of compounds with a chalcogen-nitrogen bond. X. Reaction of tellurimides with thiols and thioamides  

Microsoft Academic Search

Like their isoelectronic analogs (the sulfoxides), N-sulfonylsulfimides oxidize thiophenols to diaryl disulfides in 24 h at room temperature, but the oxidation of alkanethiols by these compounds takes place under more rigorous conditions. In order to investigate the relationships governing the effect of the nature of the chalcogen atom on the reactivity of chalcogen imides the authors studied the reactions of

V. I. Naddaka; K. V. Avanesyan; M. L. Cherkinskaya; V. I. Minkin

1989-01-01

280

Ligand-based carbon-nitrogen bond forming reactions of metal dinitrosyl complexes with alkenes and their application to C-H bond functionalization.  

PubMed

Over the past few decades, researchers have made substantial progress in the development of transition metal complexes that activate and functionalize C-H bonds. For the most part, chemists have focused on aliphatic and aromatic C-H bonds and have put less effort into complexes that activate and functionalize vinylic C-H bonds. Our groups have recently developed a novel method to functionalize vinylic C-H bonds that takes advantage of the unique ligand-based reactivity of a rare class of metal dinitrosyl complexes. In this Account, we compare and discuss the chemistry of cobalt and ruthenium dinitrosyl complexes, emphasizing alkene binding, C-H functionalization, and catalysis. Initially discovered in the early 1970s by Brunner and studied more extensively in the 1980s by the Bergman group, the cyclopentadienylcobalt dinitrosyl complex CpCo(NO)2 reacts reversibly with alkenes to give, in many cases, stable and isolable cobalt dinitrosoalkane complexes. More recently, we found that treatment with strong bases, such as lithium hexamethyldisilazide, Verkade's base, and phosphazene bases, deprotonates these complexes and renders them nucleophilic at the carbon ? to the nitroso group. This conjugate anion of metal dinitrosoalkanes can participate in conjugate addition to Michael acceptors to form new carbon-carbon bonds. These functionalized cobalt complexes can further react through alkene exchange to furnish the overall vinylic C-H functionalized organic product. This stepwise sequence of alkene binding, functionalization, and retrocycloaddition represents an overall vinylic C-H functionalization reaction of simple alkenes and does not require directing groups. We have also developed an asymmetric variant of this reaction sequence and have used this method to synthesize C1- and C2-symmetric diene ligands with high enantioinduction. Building upon these stepwise reactions, we eventually developed a simple one-pot procedure that uses stoichiometric amounts of a cobalt dinitrosoalkane complex for both inter- and intramolecular C-H functionalization. We can achieve catalysis in one-pot intramolecular reactions with a limited range of substrates. Our groups have also reported an analogous ruthenium dinitrosyl complex. In analogy to the cobalt complex, this ruthenium complex reacts with alkenes in the presence of neutral bidentate ligands, such as TMEDA, to give octahedral dinitrosoalkane complexes. Intramolecular functionalization or cyclization of numerous ruthenium dinitrosoalkane complexes proceeds under mild reaction conditions to give the functionalized organic products in excellent yields. However, despite extensive efforts, so far we have not been able to carry out intermolecular reactions of these complexes with a variety of electrophiles or C-H functionalization reactions. Although additional work is necessary to further boost the catalytic capabilities of both cobalt and ruthenium dinitrosyl complexes for vinylic C-H functionalization of simple alkenes, we believe this ligand-based vinylic C-H functionalization reaction has provided chemists with a useful set of tools for organic synthesis. PMID:24359109

Zhao, Chen; Crimmin, Mark R; Toste, F Dean; Bergman, Robert G

2014-02-18

281

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

282

The Influence of Interfaces on Reactions in Oxide Ceramics  

NASA Astrophysics Data System (ADS)

Many technologically important properties of crystalline solids are either determined by or strongly influenced by the presence and behavior of defects. Interfaces between two crystals (grain or phase boundaries) or between a crystal and a gas (surfaces) are perhaps the most technologically important class of defects because of the influence they exert on other defects (point defects, dislocations, pores and second-phase particles). Obtaining a thorough and fundamental description of interfaces including their influence on macroscale properties of solids is a daunting scientific challenge; in many important polycrystalline solids there will be a large number of interfaces with diverse, non-periodic atomic structures. In oxide ceramics, the situation is further complicated by, for example, segregated impurity atoms and amorphous interfacial films. This dissertation will describe a number of experiments involving reactions at interfaces in quite different oxide ceramic systems. By comparing observations in these different systems and with the observations of previous researchers, some general principles applicable to different types of interfaces in oxides will be described. However it will also be seen that in many cases reaction behavior will depend in a complicated way on the details of the interface and the external variables such as temperature. For this research, "reactions" refers primarily to charged point-defect reactions including segregation phenomena and phase transformations in which point-defect movement is a necessary feature. Dislocations will also make several brief appearances. These reactions are of particular practical interest because of their importance in the performance of solid oxide fuel cells and catalysts, both of which are important for alternative-energy technology. The primary experimental tool has been the transmission electron microscope (TEM) which combines high spatial resolution (0.1 nm or better) with the capability to detect many different signals resulting from electron scattering phenomena in solids, most importantly for this research, ionization events and constructive interference of electron waves. Analysis of these scattering phenomena enables local crystallographic, microstructural and compositional information to be obtained. These capabilities (particularly at high spatial resolution) are unique to the TEM and essential for understanding defects in solids; the technique also has several limitations that will be discussed.

Winterstein, Jonathan Paul

283

Formation of porous surface layers in reaction bonded silicon nitride during processing  

NASA Technical Reports Server (NTRS)

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 prepared from wet vibratory milled Si powder. Sintering and nitriding were each done under three different conditions:(1) bars directly exposed to the furnance atmosphere; (2) bars packed in Si powder; (3) bars packed in Si3N4 powder. Packing the bars in either Si of Si3N4 powder during sintering retarded formation of the layer of large porosity. Only packing the bars in Si prevented formation of the layer during nitridation. The strongest bars (316 MPa) were those sintered in Si and nitrided in Si3N4 despite their having a layer of large surface porosity; failure initiated at very large pores and inclusions. The alpha/beta ratio was found to be directly proportional to the oxygen content; a possible explanation for this relationship is discussed.

Shaw, N. J.; Glasgow, T. K.

1979-01-01

284

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

285

Optical surfacing of one-meter-class reaction bonded silicon carbide  

NASA Astrophysics Data System (ADS)

Optical fabrication methods for a one meter reaction bonded optical grade silicon carbide (RBO SiC) substrate have been successfully developed and demonstrated by Itek Optical Systems in Lexington, Massachusetts. A 1.125 m multiplied by .825 m RBO SiC panel, the largest SiC ever fabricated, has been ground and polished to a surface figure of lambda/20 rms at lambda equals .6328 nm. The single most critical technology area for RBO SiC optics has been attaining a high quality optical surface finish; this bimodal material requires particular processing techniques due to the hardness differences between the silicon and silicon carbide components. The measured surface finish quality for this one meter mirror is 15.5 angstrom rms, nearly twice as good as the specified 25 angstrom rms. Furthermore, the variation of surface finish over the mirror area is very low, sigma equals 2 angstrom. The mirror represents the state-of-the-art for lightweight, high quality SiC optical mirrors.

Breidenthal, Robert S.; Galat-Skey, Regina; Geany, John J.

1995-10-01

286

Bond length and local energy density property connections for non-transition-metal oxide-bonded interactions.  

PubMed

For a variety of molecules and earth materials, the theoretical local kinetic energy density, G(r(c)), increases and the local potential energy density, V(r(c)), decreases as the M-O bond lengths (M = first- and second-row metal atoms bonded to O) decrease and the electron density, rho(r(c)), accumulates at the bond critical points, r(c). Despite the claim that the local kinetic energy density per electronic charge, G(r(c))/rho(r(c)), classifies bonded interactions as shared interactions when less than unity and closed-shell when greater, the ratio was found to increase from 0.5 to 2.5 au as the local electronic energy density, H(r(c)) = G(r(c)) + V(r(c)), decreases and becomes progressively more negative. The ratio appears to be a measure of the character of a given M-O bonded interaction, the greater the ratio, the larger the value of rho(r(c)), the smaller the coordination number of the M atom and the more shared the bonded interaction. H(r(c))/rho(r(c)) versus G(r(c))/rho(r(c)) scatter diagrams categorize the M-O bonded interactions into domains with the local electronic energy density per electron charge, H(r(c))/rho(r(c)), tending to decrease as the electronegativity differences for the bonded pairs of atoms decrease. The values of G(r(c)) and V(r(c)), estimated with a gradient-corrected electron gas theory expression and the local virial theorem, are in good agreement with theoretical values, particularly for the bonded interactions involving second-row M atoms. The agreement is poorer for shared C-O and N-O bonded interactions. PMID:17078623

Gibbs, G V; Spackman, M A; Jayatilaka, D; Rosso, K M; Cox, D F

2006-11-01

287

Mechanism and regioselectivity for the reactions of propylene oxide with X(100)-2x1 surfaces (X = C, Si, Ge): a density functional cluster model investigation.  

PubMed

We have performed density functional cluster model calculations to explore the mechanism and regioselectivity for the reactions of propylene oxide with X(100)-2x1 surfaces (X = C, Si, and Ge). The computations reveal the following: (i) the reactions on Si(100) and Ge(100) are barrierless and highly exothermic; (ii) the reactions on X(100) (X = Si and Ge) are initiated by the formation of a dative-bonded precursor state followed by regioselective cleavage of the C2-O bond (C2 directly connected to the methyl-substituent) in propylene oxide, giving rise to a five-membered ring surface species; and (iii) the reaction on C(100), although highly exothermic, requires a large activation energy and would be kinetically forbidden at room temperature. PMID:16722754

Guo, Zheng; Lu, Xin

2006-06-01

288

Calmodulin Methionine Residues are Targets For One-Electron Oxidation by Hydroxyl Radicals: Formation of S therefore N three-electron bonded Radical Complexes  

SciTech Connect

The one-electron (1e) oxidation of organic sulfides and methionine (Met) constitutes an important reaction mechanism in vivo.1,2 Evidence for a Cu(II)-catalyzed oxidation of Met35 in the Alzheimer's disease -amyloid peptide was obtained,3 and, based on theoretical studies, Met radical cations were proposed as intermediates.4 In the structure of -amyloid peptide, the formation of Met radical cations appears to be facilitated by a preexisting close sulfur-oxygen (S-O) interaction between the Met35 sulfur and the carbonyl oxygen of the peptide bond C-terminal to Ile31.5 Substitution of Ile31 with Pro31 abolishes this S-O interaction,5 significantly reducing the ability of -amyloid to reduce Cu(II), and converts the neurotoxic wild-type -amyloid into a non-toxic peptide.6 The preexisting S-O bond characterized for wild-type -amyloid suggests that electron transfer from Met35 to Cu(II) is supported through stabilization of the Met radical cation by the electron-rich carbonyl oxygen, generating an SO-bonded7 sulfide radical cation (Scheme 1, reaction 1).5

Nauser, Thomas; Jacoby, Michael E.; Koppenol, Willem H.; Squier, Thomas C.; Schoneich, Christian

2005-02-01

289

Competition between Covalent Bonding and Charge Transfer at Complex-Oxide Interfaces  

NASA Astrophysics Data System (ADS)

Here we study the electronic properties of cuprate-manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high Tc superconductor YBa2Cu3O7-? and the colossal magnetoresistance compound (La,Ca)MnO3. A net transfer of electrons from manganite to cuprate with a peculiar nonmonotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales.

Salafranca, Juan; Rincón, Julián; Tornos, Javier; León, Carlos; Santamaria, Jacobo; Dagotto, Elbio; Pennycook, Stephen J.; Varela, Maria

2014-05-01

290

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

Microsoft Academic Search

ZrO2–CeO2–Y2O3 and ZrO2–Y2O3 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 ZrO2–CeO2–Y2O3 with ZrO2–Y2O3 at 1300°C 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

291

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

292

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 Å). 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

2012-02-20

293

Thermochemical properties and bond dissociation energies of C3-C5 cycloalkyl hydroperoxides and peroxy radicals: cycloalkyl radical + (3)O2 reaction thermochemistry.  

PubMed

Cyclic aliphatic hydrocarbons are major components in modern fuels; they can be present in the reactants, and they can be formed during the gas-phase oxidation processes. In combustion and thermal oxidation processes, these cyclics will form radicals that react with (3)O(2) to form peroxy radicals. In this study, density functional theory and higher level ab initio calculations are used to calculate thermochemical properties and bond dissociation energies of 3-5-membered cycloalkanes, corresponding hydroperoxides, hydroperoxycycloalkyl radicals, and cycloalkyl radicals that occur in these reaction systems. Geometries, vibration frequencies, and thermochemical properties, ?H(f 298)°, are calculated with the B3LYP/6-31 g(d,p), B3LYP/6-31 g(2d,2p), composite CBS-QB3, and G3MP2B3 methods. Standard enthalpies of formation at 298 K are evaluated using isodesmic reaction schemes with several work reactions for each species. Group additivity contributions are developed, and application of group additivity with comparison to calculated values is illustrated. Entropy and heat capacities, S°(T) and C(p)°(T) (5 K ? T ? 5000), are determined using geometric parameters and frequencies from the B3LYP/6-31 g(d,p) calculations. PMID:22779400

Auzmendi-Murua, Itsaso; Bozzelli, Joseph W

2012-07-19

294

Multiple-Site Concerted Proton-Electron Transfer Reactions of Hydrogen-Bonded Phenols are Non-adiabatic and Well Described by Semi-Classical Marcus Theory  

PubMed Central

Photo-oxidations of hydrogen-bonded phenols using excited state polyarenes are described, to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol-bases having ?CPh2NH2, ?Py, and ?CH2Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH2, HOAr-Py, and HOAr-CH2Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-tBu3C6H2OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical kCPET values with those from prior thermal stopped-flow kinetic studies gives datasets for the oxidations of HOAr-NH2 and of HOAr-CH2Py that span over 107 in kCPET and nearly 0.9 eV in driving force (?Go?). Plots of log(kCPET) vs. ?Go? define a single Marcus parabola in each case, each including both excited state anthracenes and ground state aminium radical cations. These two datasets are thus well described by semi-classical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give ?CPET ? 1.15–1.2 eV and Hab ? 20–30 cm?1. These experiments represent the most direct measurements of Hab for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 1012 s?1. Thus, this is a very clear demonstration that the reactions are non-adiabatic. The non-adiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-tBu3C6H2OH, which lacks a base, show that both have substantially lower ? and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.

Schrauben, Joel N.; Cattaneo, Mauricio; Day, Thomas C.; Tenderholt, Adam L.; Mayer, James M.

2012-01-01

295

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

296

The Identification of Stable Reaction Intermediates on Aluminum Oxide Surfaces with Inelastic Electron Tunneling Spectroscopy  

NASA Astrophysics Data System (ADS)

The stable surface intermediates that were formed in several heterogeneous reactions on aluminum oxide films were identified with inelastic electron tunneling spectroscopy. The alumina films were synthesized by the plasma oxidation of aluminum metal films. The temperature and exposure dependent interaction of cyclopropane carboxylic acid with alumina films was studied. Cyclopropane carboxylate and n-butane carboxylate were the only adsorbed species formed. The n-butane carboxylate results from hydrogenolysis of the cyclopropyl ring of the adsorbed cyclopropane carboxylate with hydrogen supplied by surface hydroxyl groups. The relative populations of the two surface species are strongly dependent upon coverage and temperature. The adsorption of gaseous dimethyl methyl phosphonate (DMMP) on alumina films was investigated. Surface temperatures ranged between 200 K and 673 K, and exposures ranged between 3 x 10('-4) and 10 Torr-s. Tunneling spectra of deuterium labeled DMMP, perdeutero methyl alcohol, methyl methyl phosphonic acid, methyl phosphonic acid and trimethyl phosphine oxide, all adsorbed on aluminum oxide surfaces, were used to clarify the structures of the species resulting from the adsorption and decomposition of DMMP. At 200 K, DMMP is adsorbed molecularly with high surface coverages. At surface temperatures above 295 K, DMMP is adsorbed dissociatively in low coverages. Surface temperatures above 473 K lead to the dealkylation of the dissociatively adsorbed adspecies, which results in the formation of adsorbed methyl phosphonate. The adsorption and reaction of three phosphonate esters on alumina films was comparatively examined. The phosphonate esters were diisopropyl methyl phosphonate (DIMP), dimethyl methyl phosphonate (DMMP) and diphenyl methyl phosphonate (DPMP). The adsorption temperatures ranged from 200 to 673 K. At 373 K, DIMP was found to adsorb dissociatively in low coverages as isopropyl methyl phosphonate. Above 373 K the isopropyl methyl phosphonate decomposed to the metastable hydroxy methyl phosphonate, which, in turn, decomposed completely to the methyl phosphonate above 573 K. Adsorbed DMMP decomposed similarly through the hydroxy methyl phosphonate to form the methyl phosphonate. However, in this case the hydroxy methyl phosphonate was a short -lived intermediate. DPMP was found to adsorb dissociatively at 295 K, and the adsorbed species formed is stable through 673 K. Consistent with these observations, mechanisms for the adsorption and decomposition of phosphonate esters on aluminum oxide are proposed that involve P-O bond cleavage upon adsorption but O-C bond cleavage upon decomposition of the adspecies.

Templeton, Michael Karpovich

297

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

298

Study of oxidation behavior of nanostructured NiCrAlY bond coatings deposited by cold spraying  

Microsoft Academic Search

Nanostructured NiCrAlY bond coating was deposited using a milled powder by cold spraying. A shot-peening treatment was then applied to the as-sprayed coating to modify the coating surface morphology. The oxidation behavior of the coating with the as-sprayed surface and shot-peened surface was investigated under isothermal oxidation at 900 °C and 1000 °C for different times. The oxidation behavior of the coating

Qiang Zhang; Chang-Jiu Li; Cheng-Xin Li; Guan-Jun Yang; Siu-Ching Lui

2008-01-01

299

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-Labbé, Alejandro; Clark, Tim; Politzer, Peter

2009-06-01

300

Initial Reactions in Anaerobic Oxidation of m-Xylene by the Denitrifying Bacterium Azoarcus sp. Strain T  

PubMed Central

The initial enzymatic steps in anaerobic m-xylene oxidation were studied in Azoarcus sp. strain T, a denitrifying bacterium capable of mineralizing m-xylene via 3-methylbenzoate. Permeabilized cells of m-xylene-grown Azoarcus sp. strain T catalyzed the addition of m-xylene to fumarate to form (3-methylbenzyl)succinate. In the presence of succinyl coenzyme A (CoA) and nitrate, (3-methylbenzyl)succinate was oxidized to E-(3-methylphenyl)itaconate (or a closely related isomer) and 3-methylbenzoate. Kinetic studies conducted with permeabilized cells and whole-cell suspensions of m-xylene-grown Azoarcus sp. strain T demonstrated that the specific rate of in vitro (3-methylbenzyl)succinate formation accounts for at least 15% of the specific rate of in vivo m-xylene consumption. Based on these findings, we propose that Azoarcus sp. strain T anaerobically oxidizes m-xylene to 3-methylbenzoate (or its CoA thioester) via (3-methylbenzyl)succinate and E-(3-methylphenyl)itaconate (or its CoA thioester) in a series of reactions that are analogous to those recently proposed for anaerobic toluene oxidation to benzoyl-CoA. A deuterium kinetic isotope effect was observed in the (3-methylbenzyl)succinate synthase reaction (and the benzylsuccinate synthase reaction), suggesting that a rate-determining step in this novel fumarate addition reaction involves breaking a C-H bond.

Krieger, Cynthia J.; Beller, Harry R.; Reinhard, Martin; Spormann, Alfred M.

1999-01-01

301

Lubricating Properties of Some Bonded Fluoride and Oxide Coatings for Temperature to 1500 F  

NASA Technical Reports Server (NTRS)

The lubricating properties of some experimental ceramic coatings, diffusion-bonded fluoride coatings, and ceramic-bonded fluoride coatings were determined. The experiments were conducted in an air atmosphere at a sliding velocity of 430 feet per minute and at temperatures from 75 to 1500 F. Several ceramic coatings provided substantial reductions in friction coefficient and rider wear (compared with the unlubricated metals). For example, a cobaltous oxide (CoO) base coating gave friction coefficients of 0.24 to 0.36 within the temperature range of 75 to 1400 F; serious galling and welding of the metal surfaces were prevented. The friction coefficients were higher than the arbitrary maximum (0.2) usually considered for effective boundary lubrication. However, when a moderately high friction coefficient can be tolerated, this type of coating may be a useful antiwear composition. Diffusion-bonded calcium fluoride (CaF2) on Haynes Stellite 21 and on Inconel X gave friction coefficients of 0.1 to 0.2 at 1500 F. Endurance life was dependent on the thermal history of the coating; life improved with increased exposure time at elevated temperatures prior to running. Promising results were obtained with ceramic-bonded CaF2 on Inconel X. Effective lubrication and good adherence were obtained with a 3 to 1 ratio of CaF2 to ceramic. A very thin sintered and burnished film of CaF2 applied to the surface of this coating further improved lubrication, particularly above 1350 F. The friction coefficient was 0.2 at 500 F and decreased with increasing temperature to 0.06-at 1500 F. It was 0.25 at 75 F and 0.22 at 250 F.

Sliney, Harold E.

1960-01-01

302

Gold-Aluminum Intermetallics: Ball Bond Shear Testing and Thin Film Reaction Couples  

Microsoft Academic Search

The gold-aluminum intermetallic phases are known to contribute to the long term degradation of gold wire ball bonds to aluminum metallization on semiconductor devices. The properties of these intermetallics were investigated through two techniques: 1) ball bond shear testing and 2) preparation of known intermetallic compositions by using thin film Au-Al diffusion couples. The influence of thermal aging on gold

GUY V. CLATTERBAUGH; JOEL A. WEINER; HARRY K. CHARLES

1984-01-01

303

Thermal barrier coating life and isothermal oxidation of low-pressure plasma-sprayed bond coat alloys  

NASA Technical Reports Server (NTRS)

The paper investigates the isothermal oxidation kinetics of Ni-35Cr-6Al-0.95Y, Ni-18Cr-12Al-0.3Y, and Ni-16Cr-6Al-0.3Y low-pressure plasma-sprayed bond coat alloys and examines the effect of these alloys on the thermal barrier coating (TBC) cyclic life. TBC life was examined by cycling substrates coated with the different bond coats and a ZrO2-7 wt pct Y2O3 TBC in an air-rich burner rig flame between 1150 C and room temperature. The oxidation kinetics of the three bond coat alloys was examined by isothermal oxidation of monolithic NJiCrAlY coupons at 1083 C. The Ni-35Cr-6Al-0.95Y alloy exhibits comparatively high isothermal oxidation weight gains and provides the longest TBC life, whereas the Ni-16Cr-6Al-0.3Y alloy had the lowest weight gains and provided the shortest TBC life. The results show that, although bond coat oxidation is known to have a strong detrimental effect on TBC life, it is not the only bond coat factor that determines TBC life.

Brindley, W. J.; Miller, R. A.

1990-01-01

304

Effects of alkoxy groups on arene rings of lignin ?-O-4 model compounds on the efficiencies of single electron transfer-promoted photochemical and enzymatic C-C Bond Cleavage Reactions.  

PubMed

To gain information about how alkoxy substitution in arene rings of ?-O-4 structural units within lignin governs the efficiencies/rates of radical cation C1-C2 bond cleavage reactions, single electron transfer (SET) photochemical and lignin peroxidase-catalyzed oxidation reactions of dimeric/tetrameric model compounds have been explored. The results show that the radical cations derived from less alkoxy-substituted dimeric ?-O-4 models undergo more rapid C1-C2 bond cleavage than those of more alkoxy-substituted analogues. These findings gained support from the results of DFT calculations, which demonstrate that C1-C2 bond dissociation energies of ?-O-4 radical cations decrease as the degree of alkoxy substitution decreases. In SET reactions of tetrameric compounds consisting of two ?-O-4 units, containing different degrees of alkoxy substitution, regioselective radical cation C-C bond cleavage was observed to occur in one case at the C1-C2 bond in the less alkoxy-substituted ?-O-4 moiety. However, regioselective C1-C2 cleavage in the more alkoxy-substituted ?-O-4 moiety was observed in another case, suggesting that other factors might participate in controlling this process. These observations show that lignins containing greater proportions of less rather than more alkoxylated rings as part of ?-O-4 units would be more efficiently cleaved by SET mechanisms. PMID:23992466

Lim, Suk Hyun; Nahm, Keepyung; Ra, Choon Sup; Cho, Dae Won; Yoon, Ung Chan; Latham, John A; Dunaway-Mariano, Debra; Mariano, Patrick S

2013-09-20

305

Diffusion and reaction of nitric oxide in suspension cell cultures.  

PubMed Central

A reaction-diffusion model was developed to predict the fate of nitric oxide (NO) released by cells of the immune system. The model was used to analyze data obtained previously using macrophages attached to microcarrier beads suspended in a stirred vessel. Activated macrophages synthesize NO, which is oxidized in the culture medium by molecular oxygen and superoxide (O2-, also released by the cells), yielding mainly nitrite (NO2-) and nitrate (NO3-) as the respective end products. In the analysis the reactor was divided into a "stagnant film" with position-dependent concentrations adjacent to a representative carrier bead and a well-mixed bulk solution. It was found that the concentration of NO was relatively uniform in the film. In contrast, essentially all of the O2- was calculated to be consumed within approximately 2 microm of the cell surfaces, due to its reaction with NO to yield peroxynitrite. The decomposition of peroxynitrite caused its concentration to fall to nearly zero over a distance of approximately 30 microm from the cells. Although the film regions (which had an effective thickness of 63 microm) comprised just 2% of the reactor volume and were predicted to account for only 6% of the NO2- formation under control conditions, they were calculated to be responsible for 99% of the NO3- formation. Superoxide dismutase in the medium (at 3.2 microM) was predicted to lower the ratio of NO3- to NO2- formation rates from near unity to <0.5, in reasonable agreement with the data. The NO3-/NO2- ratio was predicted to vary exponentially with the ratio of O2- to NO release rates from the cells. Recently reported reactions involving CO2 and bicarbonate were found to have important effects on the concentrations of peroxynitrite and nitrous anhydride, two of the compounds that have been implicated in NO cytotoxicity and mutagenesis.

Chen, B; Keshive, M; Deen, W M

1998-01-01

306

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

307

Selectivity of Chemisorbed Oxygen in C–H Bond Activation and CO Oxidation and Kinetic Consequences for CH?–O? Catalysis on Pt and Rh Clusters  

SciTech Connect

Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O* reactions with CO and CH?, during CH?–O? catalysis on Pt and Rh clusters. CO? and H?O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O*) are depleted from cluster surfaces. Oxygen selectivities, measured using ¹²CO–¹³CH?–O? reactants, increase with O?/ CO ratio and O* coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O* reacts much faster with CO than with CH?, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O* to produce CO? at any residence time required for detectable extents of CH? conversion. O* selectivities were also calculated by averaging DFTderived activation barriers for CO and CH? oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8 nm diameter, 201 Pt atoms) at low O* coverages, which are prevalent at low O? pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O* atoms (on *–O* site pairs). CH? oxidation occurs via kinetically relevant C–H bond activation on *–* site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH?, forming a three-centered HC?–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH? and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH? * binding energies. Ensemble-averaged O* selectivities increase linearly with O?/CO ratios, which define the O* coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O? dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33 nm) at all temperatures (573–1273 K) relevant for CH?–O? reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O* selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in ¹²CO–¹³CH?–O? mixtures, consistent with lower CO/O? ratios measured by varying the residence time and O?/CH? ratio independently in CH?–O? reactions. These mechanistic assessments and theoretical treatments for O* selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH? to CO (and H?) does not occur at the molecular scale on Pt and Rh clusters. CO (and H?) are predominantly formed upon complete O? depletion from the sequential reforming steps.

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

2011-10-06

308

[Preparation of covalently bonded cellulose tris (4-methylbenzoate) derivative chiral stationary phases through a polymerization reaction].  

PubMed

Cellulose tris (4-methylbenzoate) derivatives (CTMB) having methacryloyl groups were synthesized with regio-selective or nonselective procedures, and were immobilized on gamma-methacrylatepropylated silica (gamma-MAPS) through a polymerization reaction. The obtained chiral stationary phases ( CSPs) were evaluated by high performance liquid chromatographic (HPLC) resolution of 9 racemates (3-butyl-phthalide, ketoprofen, trans-stilbene oxide, benzoin, benzoin analogue, Tröger' s base, warfarin, 4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2,2'-dicarboxylate and 4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2-methylcarboxylate-2'-ethylcarboxylate). For regio-nonselective procedure, increasing the content of vinyl group on CTMB would lead to higher immobilization efficiency and enantio-selectivity. CSPs prepared by regio-selective procedure possessed slightly higher enantioselectivities than those prepared by regio-nonselective procedure, while the latter showed some advantages for rapid and facile preparation. CSPs prepared with silica gel II (Fuji, 5 microm, 30 nm, ca. 150 m2/g) showed higher enantioselectivities than those prepared with silica gel I (Kromasil, 5 microm, 20 nm, ca. 240 m2/g). The prepared CSPs could keep stable under different eluents, even with 40% of tetrahydrofuran (THF) in mobile phases. PMID:15807104

Qin, Feng; Chen, Xiaoming; Liu, Yueqi; Kong, Liang; Zou, Hanfa

2004-11-01

309

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

310

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

311

Redox-controlled hydrogen bonding: turning a superbase into a strong hydrogen-bond donor.  

PubMed

Herein the synthesis, structures and properties of hydrogen-bonded aggregates involving redox-active guanidine superbases are reported. Reversible hydrogen bonding is switched on by oxidation of the hydrogen-donor unit, and leads to formation of aggregates in which the hydrogen-bond donor unit is sandwiched by two hydrogen-bond acceptor units. Further oxidation (of the acceptor units) leads again to deaggregation. Aggregate formation is associated with a distinct color change, and the electronic situation could be described as a frozen stage on the way to hydrogen transfer. A further increase in the basicity of the hydrogen-bond acceptor leads to deprotonation reactions. PMID:24757064

Wild, Ute; Neuhäuser, Christiane; Wiesner, Sven; Kaifer, Elisabeth; Wadepohl, Hubert; Himmel, Hans-Jörg

2014-05-12

312

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

313

Pre-treatments applied to oxidized aluminum surfaces to modify the interfacial bonding with bis-1,2-(triethoxysilyl)ethane (BTSE). Part I. High-purity Al with native oxide  

NASA Astrophysics Data System (ADS)

A remote microwave-generated H 2 plasma and heating to 250 °C were separately used to modify high-purity oxidized aluminum surfaces and to assess whether these treatments can help enhance adhesion with bis-1,2-(triethoxysilyl)ethane (BTSE) coatings. Different initial oxide surfaces were considered, corresponding to the native oxide and to surfaces formed by the Forest Products Laboratory (FPL) treatment applied for either 15 or 60 min. BTSE is applied from solution at pH 4, and competing processes of etching, protonation (to form OH groups) and coupling (to form Al sbnd O sbnd Si interfacial bonds) occur at the solid-liquid interface. Scanning electron microscopy (SEM) was used to determine how the topographies of the modified Al surfaces changed with the different pre-treatments and with exposure to a buffer solution of pH 4. Secondary-ion mass spectrometry (SIMS) was used to determine the direct amount of Al sbnd O sbnd Si interfacial bonds by measuring the ratio of peak intensities 71-70 amu, while X-ray photoelectron spectroscopy (XPS) was used to determine the overall strength of the silane coating adhesion by measuring the Si 2p signals before and after application of an ultrasonic rinse to the coated sample. Measured Al 2p and O 1s spectra helped assess how the different pre-treatments modified the various Al oxidized surfaces prior to BTSE coating. Pre-treated samples that showed increased Al sbnd O sbnd Si bonding after BTSE coating corresponded to surfaces, which did not show evidence of significant etching after exposure to a pH 4 environment. This suggests that such surfaces are more receptive to the coupling reaction during exposure to the BTSE coating solution. These surfaces include all H 2 plasma-treated samples, the heated native oxide and the sample that only received the 15 min FPL treatment. In contrast, other surfaces that show evidence of etching in pH 4 environments are samples that received lower amounts of Al sbnd O sbnd Si interfacial bonding. Overall, heating improved the BTSE adhesive bonding for the native Al oxide, while H 2 plasma treatment improved the BTSE bonding for surfaces that had initially been FPL-treated for 15 and 60 min.

Teo, M.; Kim, J.; Wong, P. C.; Wong, K. C.; Mitchell, K. A. R.

2005-12-01

314

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

315

Oxidative addition of the C?-C? bond in ?-O-4 linkage of lignin to transition metals using a relativistic pseudopotential-based ccCA-ONIOM method.  

PubMed

A multi-level multi-layer QM/QM method, the relativistic pseudopotential correlation-consistent composite approach within an ONIOM framework (rp-ccCA-ONIOM), was applied to study the oxidative addition of the C(?)-C(?) bond in an archetypal arylglycerol ?-aryl ether (?-O-4 linkage) substructure of lignin to Ni, Cu, Pd and Pt transition metal atoms. The chemically active high-level layer is treated using the relativistic pseudopotential correlation-consistent composite approach (rp-ccCA), an efficient methodology designed to reproduce an accuracy that would be obtained using the more computationally demanding CCSD(T)/aug-cc-pCV?Z-PP, albeit at a significantly reduced computational cost, while the low-level layer is computed using B3LYP/cc-pVTZ. The thermodynamic and kinetic feasibilities of the model reactions are reported in terms of enthalpies of reactions at 298 K (?H°(298)) and activation energies (?H-act). The results obtained from the rp-ccCA:B3LYP hybrid method are compared to the corresponding values using CCSD(T) and several density functionals including B3LYP, M06, M06 L, B2PLYP, mPWPLYP and B2GP-PLYP. The energetics of the oxidative addition of C?C bond in ethane to Ni, Cu, Pd and Pt atoms are also reported to demonstrate that the rp-ccCA method effectively reproduces the accuracy of the CCSD(T)/aug-cc-pCV?Z method. Our results show that in the catalytic activation of the C(?)-C(?) bond of ?-O-4, the use of platinum metal catalysts will lead to the most thermodynamically favored reaction with the lowest activation barrier. PMID:22144374

Oyedepo, Gbenga A; Wilson, Angela K

2011-12-01

316

Synthesis, structure, and properties of compounds with a chalcogen-nitrogen bond. X. Reaction of tellurimides with thiols and thioamides  

SciTech Connect

Like their isoelectronic analogs (the sulfoxides), N-sulfonylsulfimides oxidize thiophenols to diaryl disulfides in 24 h at room temperature, but the oxidation of alkanethiols by these compounds takes place under more rigorous conditions. In order to investigate the relationships governing the effect of the nature of the chalcogen atom on the reactivity of chalcogen imides the authors studied the reactions of tellurimides with thiols and thioamides. They established that N-sulfonyldiarytellurimides oxidize thiophenol and benzyl hydrosulfide to the corresponding disulfides, thiourea to cyanamide, thiobenzamide to benzonitrile, and 1,3-diphenylthiourea to diphenylcarbodiimide.

Naddaka, V.I.; Avanesyan, K.V.; Cherkinskaya, M.L.; Minkin, V.I.

1989-01-10

317

Wollastonite based-Chemically Bonded Phosphate Ceramics with lead oxide contents under gamma irradiation  

NASA Astrophysics Data System (ADS)

The shielding properties to gamma rays as well as the effect of lead concentration incorporated into Chemically Bonded Phosphate Ceramics (CBPCs) composites are presented. The Wollastonite-based CBPC was fabricated by mixing a patented aqueous phosphoric acid formulation with Wollastonite powder. CBPC has been proved to be good structural material, with excellent thermal resistant properties, and research already showed their potential for radiation shielding applications. Wollastonite-based CBPC is a composite material itself with several crystalline and amorphous phases. Irradiation experiments were conducted on different Wollastonite-based CBPCs with lead oxide. Radiation shielding potential, attenuation coefficients in a broad range of energies pertinent to engineering applications and density experiments showing the effect of the PbO additions (to improve gamma shielding capabilities) are also presented. Microstructure was identified by using scanning electron microscopy and X-ray diffraction.

Colorado, H. A.; Pleitt, J.; Hiel, C.; Yang, J. M.; Hahn, H. T.; Castano, C. H.

2012-06-01

318

Competition between Covalent Bonding and Charge Transfer at Complex-Oxide Interfaces.  

PubMed

Here we study the electronic properties of cuprate-manganite interfaces. By means of atomic resolution electron microscopy and spectroscopy, we produce a subnanometer scale map of the transition metal oxidation state profile across the interface between the high Tc superconductor YBa_{2}Cu_{3}O_{7-?} and the colossal magnetoresistance compound (La,Ca)MnO_{3}. A net transfer of electrons from manganite to cuprate with a peculiar nonmonotonic charge profile is observed. Model calculations rationalize the profile in terms of the competition between standard charge transfer tendencies (due to band mismatch), strong chemical bonding effects across the interface, and Cu substitution into the Mn lattice, with different characteristic length scales. PMID:24877959

Salafranca, Juan; Rincón, Julián; Tornos, Javier; León, Carlos; Santamaria, Jacobo; Dagotto, Elbio; Pennycook, Stephen J; Varela, Maria

2014-05-16

319

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 300°C, 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

320

Chemical Bonding, Interfaces and Defects in Hafnium Oxide/Germanium Oxynitride Gate Stacks on Ge (100)  

SciTech Connect

Correlations among interface properties and chemical bonding characteristics in HfO{sub 2}/GeO{sub x}N{sub y}/Ge MIS stacks were investigated using in-situ remote nitridation of the Ge (100) surface prior to HfO{sub 2} atomic layer deposition (ALD). Ultra thin ({approx}1.1 nm), thermally stable and aqueous etch-resistant GeO{sub x}N{sub y} interfaces layers that exhibited Ge core level photoelectron spectra (PES) similar to stoichiometric Ge{sub 3}N{sub 4} were synthesized. To evaluate GeO{sub x}N{sub y}/Ge interface defects, the density of interface states (D{sub it}) was extracted by the conductance method across the band gap. Forming gas annealed (FGA) samples exhibited substantially lower D{sub it} ({approx} 1 x 10{sup 12} cm{sup -2} eV{sup -1}) than did high vacuum annealed (HVA) and inert gas anneal (IGA) samples ({approx} 1x 10{sup 13} cm{sup -2} eV{sup -1}). Germanium core level photoelectron spectra from similar FGA-treated samples detected out-diffusion of germanium oxide to the HfO{sub 2} film surface and apparent modification of chemical bonding at the GeO{sub x}N{sub y}/Ge interface, which is related to the reduced D{sub it}.

Oshima, Yasuhiro; /Stanford U., Materials Sci. Dept.; Sun, Yun; /SLAC, SSRL; Kuzum, Duygu; /Stanford U.; Sugawara, Takuya; Saraswat, Krishna C.; Pianetta, Piero; /SLAC, SSRL; McIntyre, Paul C.; /Stanford U., Materials Sci. Dept.

2008-10-31

321

Chemical bonding, optical constants, and electrical resistivity of sputter-deposited gallium oxide thin films  

NASA Astrophysics Data System (ADS)

Gallium oxide (Ga2O3) thin films were made by sputter deposition employing a Ga2O3 ceramic target for sputtering. The depositions were made over a wide range of substrate temperatures (Ts), from 25 to 600 °C. The effect of Ts on the chemical bonding, surface morphological characteristics, optical constants, and electrical properties of the grown films was evaluated using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), spectroscopic ellipsometry (SE), and four-point probe measurements. XPS analyses indicate the binding energies (BE) of the Ga 2p doublet, i.e., the Ga 2p3/2 and Ga 2p1/2 peaks, are located at 1118.0 and 1145.0 eV, respectively, characterizing gallium in its highest chemical oxidation state (Ga3+) in the grown films. The core level XPS spectra of O 1s indicate that the peak is centered at a BE ˜ 531 eV, which is also characteristic of Ga-O bonds in the Ga2O3 phase. The granular morphology of the nanocrystalline Ga2O3 films was evident from AFM measurements, which also indicate that the surface roughness of the films increases from 0.5 nm to 3.0 nm with increasing Ts. The SE analyses indicate that the index of refraction (n) of Ga2O3 films increases with increasing Ts due to improved structural quality and packing density of the films. The n(?) of all the Ga2O3 films follows the Cauchy's dispersion relation. The room temperature electrical resistivity was high (˜200 ?-cm) for amorphous Ga2O3 films grown at Ts = RT-300 °C and decreased to ˜1 ?-cm for nanocrystalline Ga2O3 films grown at Ts ? 500-600 °C. A correlation between growth conditions, microstructure, optical constants, and electrical properties of Ga2O3 films is derived.

Ramana, C. V.; Rubio, E. J.; Barraza, C. D.; Miranda Gallardo, A.; McPeak, Samantha; Kotru, Sushma; Grant, J. T.

2014-01-01

322

TOPICAL REVIEW: Bond-length fluctuations in the copper oxide superconductors  

NASA Astrophysics Data System (ADS)

Superconductivity in the copper oxides occurs at a crossover from localized to itinerant electronic behaviour, a transition that is first order. A spinodal phase segregation is normally accomplished by atomic diffusion; but where it occurs at too low a temperature for atomic diffusion, it may be realized by cooperative atomic displacements. Locally cooperative, fluctuating atomic displacements may stabilize a distinguishable phase lying between a localized-electron phase and a Fermi-liquid phase; this intermediate phase exhibits quantum-critical-point behaviour with strong electron-lattice interactions making charge transport vibronic. Ordering of the bond-length fluctuations at lower temperatures would normally stabilize a charge-density wave (CDW), which suppresses superconductivity. It is argued that in the copper oxide superconductors, crossover occurs at an optimal doping concentration for the formation of ordered two-electron/two-hole bosonic bags of spin S = 0 in a matrix of localized spins; the correlation bags contain two holes in a linear cluster of four copper centres ordered within alternate Cu-O-Cu rows of a CuO2 sheet. This ordering is optimal at a hole concentration per Cu atom of p approx 1/6, but it is not static. Hybridization of the vibronic electrons with the phonons that define long-range order of the fluctuating (Cu-O) bond lengths creates barely itinerant, vibronic quasiparticles of heavy mass. The heavy itinerant vibrons form Cooper pairs having a coherence length of the dimension of the bosonic bags. It is the hybridization of electrons and phonons that, it is suggested, stabilizes the superconductive state relative to a CDW state.

Goodenough, John B.

2003-02-01

323

Do amines react with protonated peptides in the gas phase via transacylation reactions to induce peptide bond cleavage?  

PubMed

The proposal that protonated peptides react with NH(3) in the gas phase via transacylation reactions (Tabet et al., Spectros. Int. J. 5: 253 1987) has been investigated by studying the reactions of the fixed charge derivatives [RC(O)NMe(2)CH(2)CO(2)H](+) (R=Me and Ph) with pyridine and triethylamine and the reactions of protonated glycine oligomers and leucine enkenphalin with butylamine. Under the near thermal conditions of the quadrupole ion trap, both the fixed charge derivatives as well as the protonated peptides react with the amines via either proton transfer or proton bound dimer formation. Collision induced dissociation of protonated peptides in the presence of butylamine yields b(n) and y(n) sequence ions as well as [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions. MS(3) experiments reveal that a major route to these [b(n) + BuNH(2)](+) and [y(n) + BuNH(2)](+) ions involves ion-molecule reactions between the b(n) and y(n) sequence ions and butylamine. MS(4) experiments, carried out to determine the nature of the [b(n) + BuNH(2)](+) ions, reveal that they correspond to a mixture of hydrogen bonded (i.e. proton bound dimer) and covalent amide bond structures. PMID:10962495

O'Hair, R A; Androutsopoulos, N K; Reid, G E

2000-01-01

324

Hydrogen bonds in 1:2 complexes of substituted pyridine N-oxides with pentachlorophenol  

NASA Astrophysics Data System (ADS)

Pyridine N-oxides form two types of crystalline complexes with phentachlorophenol, with 1:1 and 1:2 base-to-acid ratios. The 1:2 complex of 2,6-dimethylpyridine N-oxide with pentachlorophenol crystallizes in space group P1¯ with a = 7.335(1) Å, b = 11.324(2) Å, c = 15.824(2) Å, ? = 100.38(1)°, ? = 94.63(1)°, ? = 106.60(1)°, V = 1226.7(6) Å3 and Z = 2. The structure has been refined to R = 0.046 for 3408 observed Mo K? reflections. The oxygen atom of the N-oxide group accepts hydrogen bonds from two molecules of pentachlorophenol, with Otctdot;O distances of 2.639(5) and 2.642(5) Å and OHO angles of 141.2° and 157.6°, respectively. Both N?Otctdot;H?O bridges are formed in, or near, the directions of the electron lone-pairs of the N-oxygen atom. The two pentachlorophenol rings (A and B) are nearly parallel to each other and they are almost perpendicular to the pyridine ring. FTIR spectra of eleven 1:2 complexes in the solid state are similar and independent of the proton acceptor properties of the N-oxides. Five lines in the 35Cl NQR spectra of the 1:2 complexes provide evidence that both molecules of pentachlorophenol are equivalent. In CHCl 3 solution, all the 1:2 complexes exist as a mixture of the 1:1 complex and pentachlorophenol.

Dega-Szafran, Zofia; Kosturkiewicz, Zofia; Tykarska, Ewa; Szafran, Miroslaw; Lema?ski, Dariusz; Nogaj, Boles?aw

1997-02-01

325

Atomic-Scale Chemical Imaging of Composition and Bonding at Perovskite Oxide Interfaces  

NASA Astrophysics Data System (ADS)

Scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) has proven to be a powerful technique to study buried perovskite oxide heterointerfaces. With the recent addition of 3^rd order and now 5^th order aberration correction, which provides a factor of 100x increase in signal over an uncorrected system, we are now able to record 2D maps of composition and bonding of oxide interfaces at atomic resolution [1]. Here, we present studies of the microscopic structure of oxide/oxide multilayers and heterostructures by STEM in combination with EELS and its effect on the properties of the film. Using atomic-resolution spectroscopic imaging we show that the degradation of the magnetic and transport properties of La0.7Sr0.3MnO3/SrTiO3 multilayers correlates with atomic intermixing at the interfaces and the presence of extended defects in the La0.7Sr0.3MnO3 layers. When these defects are eliminated, metallic ferromagnetism at room temperature can be stabilized in 5 unit cell thick manganite layers, almost 40% thinner than the previously reported critical thickness of 3-5 nm for sustaining metallic ferromagnetism below Tc in La0.7Sr0.3MnO3 thin films grown on SrTiO3.[4pt] [1] D.A. Muller, L. Fitting Kourkoutis, M. Murfitt, J.H. Song, H.Y. Hwang, J. Silcox, N. Dellby, O.L. Krivanek, Science 319, 1073-1076 (2008).

Fitting Kourkoutis, L.

2010-03-01

326

Ruthenium-catalyzed alkylation of indoles with tertiary amines by oxidation of a sp3 C-H bond and Lewis acid catalysis.  

PubMed

Ruthenium porphyrins (particularly [Ru(2,6-Cl(2)tpp)CO]; tpp=tetraphenylporphinato) and RuCl(3) can act as oxidation and/or Lewis acid catalysts for direct C-3 alkylation of indoles, giving the desired products in high yields (up to 82% based on 60-95% substrate conversions). These ruthenium compounds catalyze oxidative coupling reactions of a wide variety of anilines and indoles bearing electron-withdrawing or electron-donating substituents with high regioselectivity when using tBuOOH as an oxidant, resulting in the alkylation of N-arylindoles to 3-{[(N-aryl-N-alkyl)amino]methyl}indoles (yield: up to 82%, conversion: up to 95%) and the alkylation of N-alkyl or N-H indoles to 3-[p-(dialkylamino)benzyl]indoles (yield: up to 73%, conversion: up to 92%). A tentative reaction mechanism involving two pathways is proposed: an iminium ion intermediate may be generated by oxidation of an sp(3) C-H bond of the alkylated aniline by an oxoruthenium species; this iminium ion could then either be trapped by an N-arylindole (pathway A) or converted to formaldehyde, allowing a subsequent three-component coupling reaction of the in situ generated formaldehyde with an N-alkylindole and an aniline in the presence of a Lewis acid catalyst (pathway B). The results of deuterium-labeling experiments are consistent with the alkylation of N-alkylindoles via pathway B. The relative reaction rates of [Ru(2,6-Cl(2)tpp)CO]-catalyzed oxidative coupling reactions of 4-X-substituted N,N-dimethylanilines with N-phenylindole (using tBuOOH as oxidant), determined through competition experiments, correlate linearly with the substituent constants sigma (R(2)=0.989), giving a rho value of -1.09. This rho value and the magnitudes of the intra- and intermolecular deuterium isotope effects (k(H)/k(D)) suggest that electron transfer most likely occurs during the initial stage of the oxidation of 4-X-substituted N,N-dimethylanilines. Ruthenium-catalyzed three-component reaction of N-alkyl/N-H indoles, paraformaldehyde, and anilines gave 3-[p-(dialkylamino)benzyl]indoles in up to 82% yield (conversion: up to 95%). PMID:20391566

Wang, Ming-Zhong; Zhou, Cong-Ying; Wong, Man-Kin; Che, Chi-Ming

2010-05-17

327

Unified view of oxidative C-H bond cleavage and sulfoxidation by a nonheme iron(IV)-oxo complex via Lewis acid-promoted electron transfer.  

PubMed

Oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by a nonheme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), were remarkably enhanced by the presence of triflic acid (HOTf) and Sc(OTf)3 in acetonitrile at 298 K. All the logarithms of the observed second-order rate constants of both the oxidative C-H bond cleavage and sulfoxidation reactions exhibit remarkably unified correlations with the driving forces of proton-coupled electron transfer (PCET) and metal ion-coupled electron transfer (MCET) in light of the Marcus theory of electron transfer when the differences in the formation constants of precursor complexes between PCET and MCET were taken into account, respectively. Thus, the mechanisms of both the oxidative C-H bond cleavage of toluene derivatives and sulfoxidation of thioanisole derivatives by [(N4Py)Fe(IV)(O)](2+) in the presence of HOTf and Sc(OTf)3 have been unified as the rate-determining electron transfer, which is coupled with binding of [(N4Py)Fe(IV)(O)](2+) by proton (PCET) and Sc(OTf)3 (MCET). There was no deuterium kinetic isotope effect (KIE) on the oxidative C-H bond cleavage of toluene via the PCET pathway, whereas a large KIE value was observed with Sc(OTf)3, which exhibited no acceleration of the oxidative C-H bond cleavage of toluene. When HOTf was replaced by DOTf, an inverse KIE (0.4) was observed for PCET from both toluene and [Ru(II)(bpy)3](2+) (bpy =2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+). The PCET and MCET reactivities of [(N4Py)Fe(IV)(O)](2+) with Brønsted acids and various metal triflates have also been unified as a single correlation with a quantitative measure of the Lewis acidity. PMID:24605985

Park, Jiyun; Morimoto, Yuma; Lee, Yong-Min; Nam, Wonwoo; Fukuzumi, Shunichi

2014-04-01

328

Kinetic and Photochemical Data for Atmospheric Chemistry Reactions of the Nitrogen Oxides.  

National Technical Information Service (NTIS)

This report contains 24 individual data sheets for thermal and photochemical reactions of importance in the atmospheric chemistry of the nitrogen oxides. For each reaction the available experimental data are summarized and critically evaluated, and a pref...

R. F. Hampson

1980-01-01

329

Chemical Characterization and Reactivity Testing of Fuel-Oxidizer Reaction Product (Test Report).  

National Technical Information Service (NTIS)

The product of incomplete reaction of monomethylhydrazine (MMH) and nitrogen tetroxide (NTO) propellants, or fuel-oxidizer reaction product (FORP), has been hypothesized as a contributory cause of an anomaly which occurred in the chamber pressure (PC) tra...

1996-01-01

330

Extending Pummerer reaction chemistry. Synthesis of (+/-)-dibromophakellstatin by oxidative cyclization of an imidazole derivative.  

PubMed

The diastereoselective oxidative cyclization of a dihydrooroidin derivative is reported. The thioimidate product formed by application of a new variant of the Pummerer reaction serves as a precursor to dibromophakellstatin. [reaction: see text] PMID:15727477

Feldman, Ken S; Skoumbourdis, Amanda P

2005-03-01

331

Simultaneous carbon coating and lithiation of oxides by contact reaction.  

PubMed

Chemical lithiation and carbon coating of cathode materials can lead to strongly improved electrochemical properties, especially if the active materials have low electronic conductivity. This behavior is quite often the case for new high-capacity materials. A novel synthesis method is presented in which the two processes are performed simultaneously by employing Li2 C2 as both the carbon and the lithium source. In this contact reaction, the acetylide anion C2 (2-) is oxidized to carbon and deposited directly on the surface of the active material, while lithium is reductively inserted into the oxidant. Two different synthesis routes are demonstrated: a tribochemical approach at room temperature and heat treatments between 150 and 600?°C. The applicability of these new carbon-coating methods are demonstrated on various crystalline and amorphous Lix V2 O5 phases. The composites obtained were characterized by powder X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. In addition, electrochemical data confirm the chemical lithiation and show that lithiated Lix V2 O5 with specific phases can be prepared selectively. PMID:24692318

Wächter, Florian; Krumeich, Frank; Nesper, Reinhard

2014-04-25

332

Porphyrin-kaolinite as efficient catalyst for oxidation reactions.  

PubMed

The preparation, characterization, and application in oxidation reactions of new biomimetic catalysts are reported. Brazilian Sao Simao kaolinite clay has been functionalized with [meso-tetrakis(pentafluorophenyl)porphinato]iron(III), Fe(TPFPP). To obtain the functionalized clay, the natural clay was purified by dispersion-sedimentation, expanded by insertion of dimethyl sulfoxide (DMSO), and functionalized with amino groups by substitution of DMSO with ethanolamine. These previous steps allowed clay functionalization with Fe(TPFPP), leading to a layered material with a basal spacing of 10.73 A. Clay functionalization with the porphyrin was confirmed by formation of the secondary amine, as demonstrated by FTIR bands at 3500-3700 cm(-1). UV-vis spectroscopy revealed a red shift in the Soret band of the iron porphyrin in the functionalized material as compared to the parent iron porphyrin catalyst in solution, indicating Fe(III)P --> Fe(II)P reduction. The catalytic performance of the functionalized clay was evaluated in the epoxidation of cyclooctene, with complete selectivity for the epoxide (100% epoxide yield), and ketonization of cyclohexane, cyclohexanone being the major product. The novel catalyst was also evaluated in the Baeyer-Villiger (BV) oxidation of cyclohexanone, with 85% conversion of cyclohexanone in epsilon-caprolactone, with total selectivity to epsilon-caprolactone. PMID:20356141

Bizaia, Natalia; de Faria, Emerson H; Ricci, Gustavo P; Calefi, Paulo S; Nassar, Eduardo J; Castro, Kelly A D F; Nakagaki, Shirley; Ciuffi, Katia J; Trujillano, Raquel; Vicente, Miguel A; Gil, Antonio; Korili, Sophia A

2009-11-01

333

Kinetics modeling and reaction mechanism of ferrate(VI) oxidation of benzotriazoles.  

PubMed

Benzotriazoles (BTs) are high production volume chemicals with broad application in various industrial processes and in households, and have been found to be omnipresent in aquatic environments. We investigated oxidation of five benzotriazoles (BT: 1H-benzotriazole; 5MBT: 5-methyl-1H-benzotriazole; DMBT: 5,6-dimethyl-1H-benzotriazole hydrate; 5CBT: 5-chloro-1H-benzotriazole; HBT: 1-hydroxybenzotriazole) by aqueous ferrate (Fe(VI)) to determine reaction kinetics as a function of pH (6.0-10.0), and interpreted the reaction mechanism of Fe(VI) with BTs by using a linear free-energy relationship. The pK(a) values of BT and DMBT were also determined using UV-Visible spectroscopic method in order to calculate the species-specific rate constants, and they were 8.37 ± 0.0 and 8.98 ± 0.08 respectively. Each of BTs reacted moderately with Fe(VI) with the k(app) ranged from 7.2 to 103.8 M(-1)s(-1) at pH 7.0 and 24 ± 1 °C. When the molar ratio of Fe(VI) and BTs increased up to 30:1, the removal rate of BTs reached about >95% in buffered milli-Q water or secondary wastewater effluent. The electrophilic oxidation mechanism of the above reaction was illustrated by using a linear free-energy relationship between pH-dependence of species-specific rate constants and substituent effects (?(p)). Fe(VI) reacts initially with BTs by electrophilic attack at the 1,2,3-triazole moiety of BT, 5MBT, DMBT and 5CBT, and at the N-OH bond of HBT. Moreover, for BT, 5MBT, DMBT and 5CBT, the reactions with the species HFeO(4)(-) predominantly controled the reaction rates. For HBT, the species H(2)FeO(4) with dissociated HBT played a major role in the reaction. The results showed that Fe(VI) has the ability to degrade benzotriazoles in water. PMID:21334710

Yang, Bin; Ying, Guang-Guo; Zhang, Li-Juan; Zhou, Li-Jun; Liu, Shan; Fang, Yi-Xiang

2011-03-01

334

Catalysis of Reduction and Oxidation Reactions for Application in Gas Particle Filters  

SciTech Connect

The present study is a first part of an investigation addressing the simultaneous occurrence of oxidation and reduction reactions in catalytic filters. It has the objectives (a) to assess the state of knowledge regarding suitable (types of) catalysts for reduction and oxidation, (b) to collect and analyze published information about reaction rates of both NOx reduction and VOC oxidation, and (c) to adjust a lab-scale screening method to the requirements of an activity test with various oxidation/reduction catalysts.

Udron, L.; Turek, T.

2002-09-19

335

Reaction mechanism of the CCN radical with nitric oxide.  

PubMed

To investigate the possibility of the carbyne radical CCN in removal of nitric oxide, a detailed computational study is performed at the Gaussian-3//B3LYP/6-31G(d) level on the CCN + NO reaction by constructing the singlet and triplet electronic state [C(2)N(2)O] potential energy surfaces (PESs). The barrierless formation of the chain-like isomers NCCNO (singlet at -106.5, triplet cis at -48.2 and triplet trans at -47.6 kcal/mol) is the most favorable entrance attack on both singlet and triplet PESs. Subsequently, the singlet NCCNO takes an O-transfer to form the branched intermediate singlet NCC(O)N (-85.6), which can lead to the fragments CN + NCO (-51.2) via the intermediate singlet NCOCN (-120.3). The simpler evolution of the triplet NCCNO is the direct N-O rupture to form the weakly bound complex triplet NCCN...O (-56.2) before the final fragmentation to NCCN + (3)O (-53.5). However, the lower lying products (3)NCN + CO (-105.6) and (3)CNN + CO (-74.6) are kinetically much less competitive. All the involved transition states for generation of CN + NCO and NCCN + (3)O lie much lower than the reactants. Thus, the novel reaction CCN + NO can proceed effectively even at low temperatures and is expected to play a role in both combustion and interstellar processes. Significant differences are found on the singlet PES between the CCN + NO and CH + NO reaction mechanisms. PMID:16544348

Jin, Lin; Ding, Yi-Hong; Wang, Jian; Sun, Chia-Chung

2006-05-01

336

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

337

Factors limiting selectivity in C3 and C4 amm(oxidation) reactions  

Microsoft Academic Search

An extensive data-mining study of C3 and C4 selective oxidation and ammoxidation reactions occurring in the gas phase over solid catalysts has been conducted. The reactions include the amm(oxidation) of propane, propene, isobutane and isobutene to produce ?,?-unsaturated mononitriles and unsaturated carboxylic acids. Selectivity–conversion plots were generated for each reaction from the patent and scientific literature. For each reaction there

A. Costine; B. K. Hodnett

2005-01-01

338

Nitric oxide in star-forming regions - Further evidence for interstellar N-O bonds  

NASA Technical Reports Server (NTRS)

Nitric oxide has been newly detected toward several star-forming clouds, including Orion-KL, Sgr B2(N), W33A, W51M, and DR21(OH) via its J = 3/2-1/2 transitions near 150 GHz, using the FCRAO 14 m telescope. Both lambda-doubling components of NO were observed toward all sources. Column densities derived for nitric oxide in these clouds are 10 to the 15th-10 to the 16th/sq cm, corresponding to fractional abundances of 0.5-1.0 x 10 to the -8th, relative to H2. Toward Orion-KL, the NO line profile suggests that the species arises primarily from hot, dense gas. Nitric oxide may arise from warm material toward the other clouds as well. Nitric oxide in star-forming regions could be synthesized by high-temperature reactions, although the observed abundances do not disagree with values predicted from low-temperature, ion-molecule chemistry by more than one order of magnitude.

Ziurys, L. M.; Mcgonagle, D.; Minh, Y.; Irvine, W. M.

1991-01-01

339

Nitric oxide in star-forming regions - Further evidence for interstellar N-O bonds  

SciTech Connect

Nitric oxide has been newly detected toward several star-forming clouds, including Orion-KL, Sgr B2(N), W33A, W51M, and DR21(OH) via its J = 3/2{minus}1/2 transitions near 150 GHz, using the FCRAO 14 m telescope. Both lambda-doubling components of NO were observed toward all sources. Column densities derived for nitric oxide in these clouds are 10 to the 15th{minus}10 to the 16th/sq cm, corresponding to fractional abundances of 0.5{minus}1.0 {times} 10 to the {minus}8th, relative to H2. Toward Orion-KL, the NO line profile suggests that the species arises primarily from hot, dense gas. Nitric oxide may arise from warm material toward the other clouds as well. Nitric oxide in star-forming regions could be synthesized by high-temperature reactions, although the observed abundances do not disagree with values predicted from low-temperature, ion-molecule chemistry by more than one order of magnitude. 40 refs.

Ziurys, L.M.; Mcgonagle, D.; Minh, Y.; Irvine, W.M. (Arizona State University, Tempe (USA) Steward Observatory, Tucson (USA) Five College Radio Astronomy Observatory, Amherst, MA (USA))

1991-06-01

340

A quantum chemical topological analysis of the C-C bond formation in organic reactions involving cationic species.  

PubMed

ELF topological analysis of the ionic Diels-Alder (I-DA) reaction between the N,N-dimethyliminium cation and cyclopentadiene (Cp) has been performed in order to characterise the C-C single bond formation. The C-C bond formation begins in the short range of 2.00-1.96 Åvia a C-to-C pseudoradical coupling between the most electrophilic center of the iminium cation and one of the two most nucleophilic centers of Cp. The electron density of the pseudoradical center generated at the most electrophilic carbon of the iminium cation comes mainly from the global charge transfer which takes place along the reaction. Analysis of the global reactivity indices indicates that the very high electrophilic character of the iminium cation is responsible for the negative activation energy found in the gas phase. On the other hand, the analysis of the radical P Parr functions of the iminium cation, and the nucleophilic Pk(-) Parr functions of Cp makes the characterisation of the most favourable two-center interaction along the formation of the C-C single bond possible. PMID:24901220

Domingo, Luis R; Pérez, Patricia

2014-07-21

341

Long term oxidation of a SiC fiber-bonded composite in air at 1500 °C  

Microsoft Academic Search

The oxidation behavior of the SiC fiber-bonded composite (Tyrannohex™) was investigated at 1500 °C up to 4000 h in air. During heating of the samples, it was observed that an amorphous SiO2 scale was formed and that bubbles of SiO2 were generated due to the gaseous products upon oxidation. A large mass loss was also observed after 4000 h heating in

Akihiko Otsuka; Yoshikazu Matsumura; Kazuya Hosono; Ryohei Tanaka

2003-01-01

342

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

343

Uraninite oxidation and dissolution induced by manganese oxide: A redox reaction between two insoluble minerals  

NASA Astrophysics Data System (ADS)

The longevity of subsurface U(IV) produced by reduction of U(VI) during in situ bioremediation can be limited by reoxidation to more mobile U(VI) species. Coupling of the biogeochemical cycles of U and Mn may affect the fate and transport of uranium. Manganese oxides can act as a powerful oxidant that accelerates the oxidative dissolution of UO2. This study investigated the physical and chemical factors controlling the interaction between UO2 and MnO2, which are both poorly soluble minerals. A multi-chamber reactor with a permeable membrane was used to eliminate direct contact of the two minerals while still allowing transport of aqueous species. The oxidation of UO2 was not significantly enhanced by MnO2 if the two solids were physically separated. Complete mixing of MnO2 with UO2 led to a much greater extent and rate of U oxidation. When direct contact is not possible, the reaction slowly progresses through release of soluble U(IV) with its adsorption and oxidation on MnO2. Continuously-stirred tank reactors (CSTRs) were used to quantify the steady-state rates of UO2 dissolution induced by MnO2. MnO2 dramatically promoted UO2 dissolution, but the degree of promotion leveled off once the MnO2:UO2 ratio exceeded a critical value. Substantial amounts of U(VI) and Mn(II) were retained on MnO2 surfaces. The total production of Mn(II) was less than that of U(VI), indicating that the fate of Mn products and their impact on UO2-MnO2 reaction kinetics were complicated and may involve formation of Mn(III) phases. At higher dissolved inorganic carbon concentrations, UO2 oxidation by MnO2 was faster and less U(VI) was adsorbed to MnO2. Such an inverse relationship suggested that U(VI) may passivate MnO2 surfaces. A conceptual model was developed to describe the oxidation rate of UO2 by MnO2. This model is potentially applicable to a broad range of water chemistry conditions and is relevant to other environmental redox processes involving two poorly soluble minerals.

Wang, Zimeng; Lee, Sung-Woo; Kapoor, Pratyul; Tebo, Bradley M.; Giammar, Daniel E.

2013-01-01

344

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

2010-06-21

345

Oxidative cyclization reactions: controlling the course of a radical cation-derived reaction with the use of a second nucleophile.  

PubMed

Construction of new ring systems: Oxidative cyclizations (see picture; RVC=reticulated vitreous carbon) have been conducted that use two separate intramolecular nucleophiles to trap an enol ether-derived radical cation intermediate. The reactions provide a means for rapidly trapping the radical cation intermediate in a manner that avoids competitive decomposition reactions. PMID:24254900

Redden, Alison; Perkins, Robert J; Moeller, Kevin D

2013-12-01

346

Competition between covalent bonding and charge transfer tendencies at complex-oxides interfaces  

NASA Astrophysics Data System (ADS)

Interfaces alter the subtle balance among different degrees of freedom responsible for exotic phenomena in complex oxides, such as cuprate-manganite interfaces. We study these interfaces by means of scanning transmission electron microscopy and theoretical calculations. Microscopy and EEL spectroscopy indicate that the interfaces are sharp, and the chemical profile is symmetric with two equivalent interfaces. Spectroscopy also allows us to establish an oxidation state profile with sub-nanometer resolution. We find an anomalous charge redistribution: a non-monotonic behavior of the occupancy of d orbitals in the manganite layers as a function of distance to the interface. Relying on model calculations, we establish that this profile is a result of the competition between standard charge transfer tendencies involving materials with different chemical potentials and strong bonding effects across the interface. The competition can be tuned by different factors (temperature, doping, magnetic fields...). As examples, we report different charge distributions as a function of doping of the manganite layers. ACKNOWLEDGEMENTS ORNL:U.S. DOE-BES, Material Sciences and Engineering Division & ORNL's ShaRE. UCM:Juan de la Cierva, Ramon y Cajal, & ERC Starting Investigator Award programs.

Salafranca, J.; Tornos, J.; García-Barriocanal, J.; León, C.; Santamaria, J.; Rincón, J.; Álvarez, G.; Pennycook, S. J.; Dagotto, E.; Varela, M.

2013-03-01

347

Influence of a reaction medium on the oxidation of aromatic nitrogen-containing compounds by peroxyacids  

NASA Astrophysics Data System (ADS)

The influence of different solvents on the oxidation reaction rate of pyridine (Py), quinoline (QN), acridine (AN), ?-oxyquinoline (OQN) and ?-picolinic acid (APA) by peroxydecanoic acid (PDA) was studied. It was found that the oxidation rate grows in the series Py < QN < AN, and the rate of the oxidation reaction of compounds containing a substituent in the ? position from a reactive center is significantly lower than for unsubstituted analogues. The effective energies of activation of the oxidation reaction were found. It was shown that in the first stage, the reaction mechanism includes the rapid formation of an intermediate complex nitrogen-containing compound, peroxyacid, which forms products upon decomposing in the second stage. A kinetic equation that describes the studied process is offered. The constants of equilibrium of the intermediate complex formation ( K eq) and its decomposition constant ( k 2) in acetone and benzene were calculated. It was shown that the nature of the solvent influences the numerical values of both K p and k 2. It was established that introduction of acetic acid (which is able to form compounds with Py) into the reaction medium slows the rate of the oxidation process drastically. Correlation equations linking the polarity, polarizability, electrophilicity, and basicity of solvents with the constant of the PDA oxidation reaction rate for Py were found. It was concluded that the basicity and polarity of the solvent have a decisive influence on the oxidation reaction rate, while the polarizability and electrophilicity of the reaction medium do not influence the oxidation reaction rate.

Dutka, V. S.; Matsyuk, N. V.; Dutka, Yu. V.

2011-01-01

348

Nickel-catalyzed carbon-carbon bond-forming reactions of unactivated tertiary alkyl halides: Suzuki arylations.  

PubMed

The first Suzuki cross-couplings of unactivated tertiary alkyl electrophiles are described. The method employs a readily accessible catalyst (NiBr(2)·diglyme/4,4'-di-tert-butyl-2,2'-bipyridine, both commercially available) and represents the initial example of the use of a group 10 catalyst to cross-couple unactivated tertiary electrophiles to form C-C bonds. This approach to the synthesis of all-carbon quaternary carbon centers does not suffer from isomerization of the alkyl group, in contrast with the umpolung strategy for this bond construction (cross-coupling of a tertiary alkylmetal with an aryl electrophile). Preliminary mechanistic studies are consistent with the generation of a radical intermediate along the reaction pathway. PMID:23281960

Zultanski, Susan L; Fu, Gregory C

2013-01-16

349

Theoretical study of bond distances and dissociation energies of actinide oxides AnO and AnO2.  

PubMed

In the present study we evaluated trends in the bond distances and dissociation enthalpies of actinide oxides AnO and AnO(2) (An = Th-Lr) on the basis of consistent computed data obtained by using density functional theory in conjunction with relativistic small-core pseudopotentials. Computations were carried out on AnO (An = Th-Lr) and AnO(2) (An = Np, Pu, Bk-Lr) species, while for the remaining AnO(2) species recent literature data (Theor. Chem. Acc. 2011, 129, 657) were utilized. The most important computed properties include the geometries, vibrational frequencies, dissociation enthalpies, and several excited electronic states. These molecular properties of the late actinide oxides (An = Bk-No) are reported here for the first time. We present detailed analyses of the bond distances, covalent bonding properties, and dissociation enthalpies. PMID:22471700

Kovács, Attila; Pogány, Peter; Konings, Rudy J M

2012-04-16

350

Dienamine-mediated inverse-electron-demand hetero-diels-alder reaction by using an enantioselective H-bond-directing strategy.  

PubMed

Giving directions: optically active dihydropyrans bearing three contiguous stereogenic centers can be efficiently prepared by the title reaction. High stereo- and regiocontrol can be achieved by employing a bifunctional H-bond-directing aminocatalyst. PMID:23148030

Albrecht, Lukasz; Dickmeiss, Gustav; Weise, Christian F; Rodríguez-Escrich, Carles; Jørgensen, Karl Anker

2012-12-21

351

Microstructure, bonding strength and thermal shock resistance of ceramic coatings on steels prepared by plasma electrolytic oxidation  

Microsoft Academic Search

Ceramic coatings were successfully prepared on steel by plasma electrolytic oxidation (PEO) in aluminate electrolyte and silicate electrolyte, respectively. The microstructure of the coatings including surface morphology, phase and element composition were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The bonding strength between the ceramic coating and the substrate was tested using different methods including tensile

Yunlong Wang; Zhaohua Jiang; Zhongping Yao

2009-01-01

352

Novel carbon-carbon bond formations for biocatalysis  

PubMed Central

Carbon–carbon 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 C–C 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 Pictet–Spengler reaction or oxidative C–C bond formation. Furthermore, novel enzymatic C–C 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

353

C-F bond activation with an apparently benign ethynyl dithiocarbamate, and subsequent fluoride transfer reactions.  

PubMed

F-act finding mission: Selective activation of the para C-F bonds of perfluorinated aromatic derivatives was achieved with a simple ethynyl dithiocarbamate. The resulting adducts can be used as non-ionic, anhydrous fluoride transfer reagents towards various electrophiles (see scheme). PMID:22961643

Ung, Gaël; Bertrand, Guy

2012-10-01

354

Sequence-Specific Cu(II)-Dependent Peptide Bond Hydrolysis: Similarities and Differences with the Ni(II)-Dependent Reaction.  

PubMed

Potentiometry and UV-vis and circular dichroism spectroscopies were applied to characterize Cu(II) coordination to the Ac-GASRHWKFL-NH2 peptide. Using HPLC and ESI-MS, we demonstrated that Cu(II) ions cause selective hydrolysis of the Ala-Ser peptide bond in this peptide and characterized the pH and temperature dependence of the reaction. We found that Cu(II)-dependent hydrolysis occurs solely in 4N complexes, in which the equatorial coordination positions of the Cu(II) ion are saturated by peptide donor atoms, namely, the pyridine-like nitrogen of the His imidazole ring and three preceding peptide bond nitrogens. Analysis of the reaction products led to the conclusion that Cu(II)-dependent hydrolysis proceeds according to the mechanism demonstrated previously for Ni(II) ions ( Kopera , E. ; Kr??el , A. ; Protas , A. M. ; Belczyk , A. ; Bonna , A. ; Wys?ouch-Cieszy?ska , A. ; Pozna?ski , J. ; Bal , W. Inorg. Chem. 2010 , 49 , 6636 - 6645 ). However, the pseudo-first-order reaction rate found for Cu(II) is, on average, 100 times lower than that for Ni(II) ions. The greater ability of Cu(II) ions to form 4N complexes at lower pH partially compensates for this difference in rates, resulting in similar hydrolytic activities for the two ions around pH 7. PMID:24735221

Belczyk-Ciesielska, Agnieszka; Zawisza, Izabela A; Mital, Mariusz; Bonna, Arkadiusz; Bal, Wojciech

2014-05-01

355

Experimental and theoretical study of the reactions between neutral vanadium oxide clusters and ethane, ethylene, and acetylene.  

PubMed

Reactions of neutral vanadium oxide clusters with small hydrocarbons, namely C2H6, C2H4, and C2H2, are investigated by experiment and density functional theory (DFT) calculations. Single photon ionization through extreme ultraviolet (EUV, 46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is used to detect neutral cluster distributions and reaction products. The most stable vanadium oxide clusters VO2, V2O5, V3O7, V4O10, etc. tend to associate with C2H4 generating products V(m)O(n)C2H4. Oxygen-rich clusters VO3(V2O5)(n=0,1,2...), (e.g., VO3, V3O8, and V5O13) react with C2H4 molecules to cause a cleavage of the C=C bond of C2H4 to produce (V2O5)(n)VO2CH2 clusters. For the reactions of vanadium oxide clusters (V(m)O(n)) with C2H2 molecules, V(m)O(n)C2H2 are assigned as the major products of the association reactions. Additionally, a dehydration reaction for VO3 + C2H2 to produce VO2C2 is also identified. C2H6 molecules are quite stable toward reaction with neutral vanadium oxide clusters. Density functional theory calculations are employed to investigate association reactions for V2O5 + C2H(x). The observed relative reactivity of C2 hydrocarbons toward neutral vanadium oxide clusters is well interpreted by using the DFT calculated binding energies. DFT calculations of the pathways for VO3+C2H4 and VO3+C2H2 reaction systems indicate that the reactions VO3+C2H4 --> VO2CH2 + H2CO and VO3+C2H2 --> VO2C2 + H2O are thermodynamically favorable and overall barrierless at room temperature, in good agreement with the experimental observations. PMID:18198869

Dong, Feng; Heinbuch, Scott; Xie, Yan; Rocca, Jorge J; Bernstein, Elliot R; Wang, Zhe-Chen; Deng, Ke; He, Sheng-Gui

2008-02-13

356

Effect of surface treatment on the initial bond strength of different luting cements to zirconium oxide ceramic.  

PubMed

The objective of this study was to compare the shear bond strength to zirconium oxide ceramic of adhesive-phosphate-monomer-containing (APM) and non-APM-containing (nAPM) luting cements after different surface treatments. nAPM cements: Bifix QM, Dual Cement, Duo Cement Plus, Multilink Automix, ParaCem Universal DC, PermaCem Smartmix, RelyX ARC, Variolink Ultra, and Variolink II; APM cements: Panavia EX, Panavia F2.0, and RelyX UniCem. Groups of ten test specimens were each prepared by layering luting cement, using cylindrical Teflon molds, onto differently treated zirconium dioxide discs. The surface treatments were airborne-particle abrasion with 110 mum alumina particles, silica coating (SC) using 30 mum alumina particles modified by silica (Rocatec System) or SC and silanization. Bifix QM and Multilink Automix were used in combination with an additional bonding/priming agent recommended by the manufacturers. After 48 h of water storage, each specimen was subjected to a shear test. Combinations involving APM-containing cements (14.41-23.88 MPa) generally exhibited higher shear bond strength than those without APM (4.29-17.34 MPa). Exceptions were Bifix QM (14.20-25.11 MPa) and Multilink Automix (19.14-23.09 MPa) in combination with system-specific silane or priming agent, which were on the upper end of shear bond strength values. With the use of the Rocatec system, a partially significant increase in shear bond strength could be achieved in nAPM cement. Modified surface treatment modalities increased the bond strength to zirconium oxide, although the most important factor in achieving a strong bond was the selection of a suitable cement. System-specific priming or bonding agents lead to further improvement. PMID:18758827

Nothdurft, F P; Motter, P J; Pospiech, P R

2009-06-01

357

Role of metal-oxide interfacial reactions on the interactions between oxidation and deformation  

SciTech Connect

The influence of chemical environment on mechanical properties and behavior of materials can be illustrated by many examples. At high temperature, the strong effect of oxygen and oxidation on the creep and fatigue properties of Ni-base superalloys is well documented and has been previously discussed and reviewed. The different types of damage induced by oxidation for long-term exposures at high temperatures have been clearly determined and analyzed. At lower temperatures, i.e., at temperatures which do not exceed 0.6 T{sub m}(K) or about 650 C for Ni-base superalloys, such an analysis is more difficult. The diffusion of reacting and/or detrimental species through the oxide scale or within the underlying alloy is not the only operative process. The interfacial reaction steps associated with scale growth are among the processes which can play an important role at intermediate temperatures. As illustrated by the recent analysis by Hirth, Pieraggi and Rapp, the scale-alloy interface must be able to annihilate or to provide the flux of defects involved in the diffusion processes acting in both the oxide scale and the underlying alloy. Therefore, the goal of the present article is to present some possible mechanisms based on the role of the scale-alloy interface in scale growth, which permit an analysis and explanation of the observed effect of chemical environment on creep and crack growth rate for some industrial Ni-base alloys.

Andrieu, E.; Pieraggi, B. [ENSCT, Toulouse (France)] [ENSCT, Toulouse (France); Gourgues, A.F. [ENSMP, Corbeil (France). Centre des Materiaux] [ENSMP, Corbeil (France). Centre des Materiaux

1998-08-04

358

Temperature evolution of structure and bonding of formic acid and formate on fully oxidized and highly reduced CeO2(111)  

SciTech Connect

Adsorption of formate on oxide surfaces plays a role in water-gas shift (WGS) and other reactions related to H2 production and CO2 utilization. CeO2 is of particular interest because its reducibility affects the redox of organic molecules. In this work, the adsorption and thermal evolution of formic acid and formate on highly ordered films of fully oxidized CeO2(111) and highly reduced CeOx(111) surfaces have been studied using reflection absorption infrared spectroscopy (RAIRS) under ultra-high vacuum conditions, and the experimental results are combined with density functional theory (DFT) calculations to probe the identity, symmetry, and bonding of the surface intermediates. Disordered ice, ordered a-polymorph and molecular formic acid bonded through the carbonyl are observed at low temperatures. By 250 K, desorption and deprotonation lead to formate coexisting with hydroxyl on CeO2(111), identified to be a bridging bidentate formate species that is coordinated to Ce cations in nearly C2v symmetry and interacting strongly with neighboring H. Changes in the spectra at higher temperatures are consistent with additional tilting of the formate, resulting in Cs(2) or lower symmetry. This change in bonding is caused primarily by interaction with oxygen vacancies introduced by water desorption at 300 K. On reduced CeOx, multiple low-symmetry formate states exist likewise due to interactions with oxygen vacancies. Isotopic studies demonstrate that the formyl hydrogen does not contribute to H incorporated in hydroxyl on the surface, and that both formate oxygen atoms may exchange with lattice oxygen at 400 K. The combined experimental and theoretical results thus provide important insights on the surface reaction pathways of formic acid on ceria.

Gordon, Wesley O [ORNL; Xu, Ye [ORNL; Mullins, David R [ORNL; Overbury, Steven {Steve} H [ORNL

2009-01-01

359

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

360

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)â 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

361

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: (1) sintered silicon nitride (SSN) and (2) 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. [Oak Ridge National Lab., TN (United States)] [Oak Ridge National Lab., TN (United States); Willkens, C.A. [St. Gobain/Norton Industrial Ceramics, Northboro, MA (United States)] [St. Gobain/Norton Industrial Ceramics, Northboro, MA (United States)

1994-10-01

362

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] and others

1995-10-01

363

Some observations of the reactions between molten oxides and solid carbon  

Microsoft Academic Search

Observations of the reactions between molten metal oxides and various forms of carbon have been made using X-ray transmission and optical techniques. Particular attention has been paid to the wetting characteristics and the reaction rates between the melts and carbon. The critical slag surface tension for complete wetting of metallurgical coke is predicted. The results also indicate that non-wetting oxide

P. C. Hayes; D. A. Okongwu; J. M. Toguri

1995-01-01

364

Rh-Catalyzed Intermolecular Reactions of Cyclic ?-Diazocarbonyl Compounds with Selectivity Over Tertiary C-H Bond Migration  

PubMed Central

Intermolecular Rh-catalyzed reactions of cyclic ?-diazocarbonyl compounds with chemoselectivity over ?-hydride elimination are described. These methods represent the first general intermolecular reactions of Rh-carbenoids that are selective over tertiary ?-C?H bond migration. Successful transformations include cyclopropanation, cyclopropenation, and various X–H insertion reactions with a broad scope of substrates. We propose that the intermolecular approach of substrates to carbenes from acyclic diazo precursors may be relatively slow due to a steric interaction with the ester function, which is perpendicular to the ?-system of the carbene. For carbenes derived from 5- and 6-membered cyclic ?-diazocarbonyls, it is proposed that the carbene is constrained to be more conjugated with the carbonyl, thereby relieving the steric interaction for intermolecular reactions, and accelerating the rate of intermolecular reactivity relative to intramolecular ?-hydride migration. However, attempts to use ?-diazo-?-ethylcaprolactone in intermolecular cyclopropanation with styrene were unsuccessful. It is proposed that the conformational flexibility of the 7-membered ring allows the carbonyl to be oriented perpendicular to Rh-carbene. The significant intermolecular interaction between the carbonyl and approaching substrate is in agreement with the poor ability of ?-diazo-?-ethylcaprolactone to participate in intermolecular cyclopropanation reactions. DFT calculations provide support for the mechanistic proposals that are described.

DeAngelis, Andrew; Dmitrenko, Olga; Fox, Joseph M.

2012-01-01

365

Reaction pathways of glucose oxidation by ozone under acidic conditions.  

PubMed

The ozonation of d-glucose-1-(13)C, 2-(13)C, and 6-(13)C was carried out at pH 2.5 in a semi-batch reactor at room temperature. The products present in the liquid phase were analyzed by GC-MS, HPAEC-PAD, and (13)C NMR spectroscopy. Common oxidation products of glucose have also been submitted to identical ozonation conditions. For the first time, a pentaric acid was identified and its formation quantitatively correlated to the loss of C-6 of glucose in the form of carbon dioxide. Potential mechanisms for the formation of this pentaric acid are discussed. The well-accepted pathway involving the anomeric position in glucose, gluconic acid, arabinose, and carbon dioxide is reinvestigated. The origin of small molecules such as tartaric, erythronic, and oxalic acids is clarified. Finally, new reaction pathways and tentative mechanisms consistent with the formation of ketoaldonic acids and smaller acids are proposed. PMID:19524217

Marcq, Olivier; Barbe, Jean-Michel; Trichet, Alain; Guilard, Roger

2009-07-27

366

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

367

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

368

C-H and C-O oxidative addition in reactions of aryl carboxylates with a PNP pincer-ligated Rh(I) fragment.  

PubMed

Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO(2)CPh only underwent C-H oxidative addition (OA). PhO(2)CCF(3) chiefly underwent acyl-oxygen OA. PhO(2)CBu(t) and PhO(2)CNEt(2) initially underwent OA of an ortho-C-H bond of the phenyl group but continued thermolysis led to the phenyl-oxygen OA products. PMID:22089427

Zhu, Yanjun; Smith, Dan A; Herbert, David E; Gatard, Sylvain; Ozerov, Oleg V

2012-01-01

369

Effect of ascorbic acid in dough: reaction of oxidized glutathione with reactive thiol groups of wheat glutelin.  

PubMed

The reactions of oxidized glutathione generated from endogenous glutathione by the addition of ascorbic acid (AA) prior to dough mixing on free thiol groups of gluten proteins have been investigated. A small amount of (35)S-labeled glutathione was added as a tracer to identify the reaction products of GSSG and free protein thiols by radioactivity measurement. First, gluten was isolated from the dough, then the gliadins were extracted, and residual glutenin was partially hydrolyzed with thermolysin. After preseparation by gel permeation chromatography, the fractions with the highest radioactivity were separated by high-performance liquid chromatography. Radioactive peptides were identified, isolated, sequenced, and assigned to amino acid sequences of gluten protein components. The isolated peptides contained exclusively the cysteine residues C(b) and C(x) of low molecular weight subunits of glutenin, which are supposed to be highly reactive in forming intermolecular disulfide bonds. From these results it can be assumed that the cysteine residues C(b) and C(x) of the low molecular weight subunits of glutenin are at least partly present in the thiol form in flour. During dough mixing they are converted to protein-protein disulfides or glutathione-protein mixed disulfides by thiol/disulfide interchange reactions. Oxidized glutathione necessary for this reaction is generated from glutathione by the action of AA. These results are in accordance with the major hypothesis about the mechanism of action of AA. PMID:12903952

Koehler, Peter

2003-08-13

370

Photo-Fenton reaction of graphene oxide: a new strategy to prepare graphene quantum dots for DNA cleavage.  

PubMed

Graphene quantum dots (GQDs) are great promising in various applications owing to the quantum confinement and edge effects in addition to their intrinsic properties of graphene, but the preparation of the GQDs in bulk scale is challenging. We demonstrated in this work that the micrometer sized graphene oxide (GO) sheets could react with Fenton reagent (Fe(2+)/Fe(3+)/H(2)O(2)) efficiently under an UV irradiation, and, as a result, the GQDs with periphery carboxylic groups could be generated with mass scale production. Through a variety of techniques including atomic force microscopy, X-ray photoelectron spectroscopy, gas chromatography, ultraperformance liquid chromatography-mass spectrometry, and total organic carbon measurement, the mechanism of the photo-Fenton reaction of GO was elucidated. The photo-Fenton reaction of GO was initiated at the carbon atoms connected with the oxygen containing groups, and C-C bonds were broken subsequently, therefore, the reaction rate depends strongly on the oxidization extent of the GO. Given the simple and efficient nature of the photo-Fenton reaction of GO, this method should provide a new strategy to prepare GQDs in mass scale. As a proof-of-concept experiment, the novel DNA cleavage system using as-generated GQDs was constructed. PMID:22813062

Zhou, Xuejiao; Zhang, Yan; Wang, Chong; Wu, Xiaochen; Yang, Yongqiang; Zheng, Bin; Wu, Haixia; Guo, Shouwu; Zhang, Jingyan

2012-08-28

371

Structural and functional investigation of graphene oxide-Fe3O4 nanocomposites for the heterogeneous Fenton-like reaction  

PubMed Central

Graphene oxide–iron oxide (GO–Fe3O4) nanocomposites were synthesised by co-precipitating iron salts onto GO sheets in basic solution. The results showed that formation of two distinct structures was dependent upon the GO loading. The first structure corresponds to a low GO loading up to 10?wt%, associated with the beneficial intercalation of GO within Fe3O4 nanoparticles and resulting in higher surface area up to 409?m2 g?1. High GO loading beyond 10?wt% led to the aggregation of Fe3O4 nanoparticles and the undesirable stacking of GO sheets. The presence of strong interfacial interactions (Fe-O-C bonds) between both components at low GO loading lead to 20% higher degradation of Acid Orange 7 than the Fe3O4 nanoparticles in heterogeneous Fenton-like reaction. This behaviour was attributed to synergistic structural and functional effect of the combined GO and Fe3O4 nanoparticles.

Zubir, Nor Aida; Yacou, Christelle; Motuzas, Julius; Zhang, Xiwang; Diniz da Costa, Joao C.

2014-01-01

372

Structural and functional investigation of graphene oxide–Fe3O4 nanocomposites for the heterogeneous Fenton-like reaction  

NASA Astrophysics Data System (ADS)

Graphene oxide-iron oxide (GO-Fe3O4) nanocomposites were synthesised by co-precipitating iron salts onto GO sheets in basic solution. The results showed that formation of two distinct structures was dependent upon the GO loading. The first structure corresponds to a low GO loading up to 10 wt%, associated with the beneficial intercalation of GO within Fe3O4 nanoparticles and resulting in higher surface area up to 409 m2 g-1. High GO loading beyond 10 wt% led to the aggregation of Fe3O4 nanoparticles and the undesirable stacking of GO sheets. The presence of strong interfacial interactions (Fe-O-C bonds) between both components at low GO loading lead to 20% higher degradation of Acid Orange 7 than the Fe3O4 nanoparticles in heterogeneous Fenton-like reaction. This behaviour was attributed to synergistic structural and functional effect of the combined GO and Fe3O4 nanoparticles.

Zubir, Nor Aida; Yacou, Christelle; Motuzas, Julius; Zhang, Xiwang; Diniz da Costa, João C.

2014-04-01

373

Putative Hydrogen Bond to Tyrosine M208 in Photosynthetic Reaction Centers from Rhodobacter capsulatus Significantly Slows Primary Charge Separation.  

PubMed

Slow, ?50 ps, P* ? P(+)HA(-) electron transfer is observed in Rhodobacter capsulatus reaction centers (RCs) bearing the native Tyr residue at M208 and the single amino acid change of isoleucine at M204 to glutamic acid. The P* decay kinetics are unusually homogeneous (single exponential) at room temperature. Comparative solid-state NMR of [4'-(13)C]Tyr labeled wild-type and M204E RCs show that the chemical shift of Tyr M208 is significantly altered in the M204E mutant and in a manner consistent with formation of a hydrogen bond to the Tyr M208 hydroxyl group. Models based on RC crystal structure coordinates indicate that if such a hydrogen bond is formed between the Glu at M204 and the M208 Tyr hydroxyl group, the -OH would be oriented in a fashion expected (based on the calculations by Alden et al., J. Phys. Chem. 1996, 100, 16761-16770) to destabilize P(+)BA(-) in free energy. Alteration of the environment of Tyr M208 and BA by Glu M204 via this putative hydrogen bond has a powerful influence on primary charge separation. PMID:24902471

Saggu, Miguel; Carter, Brett; Zhou, Xiaoxue; Faries, Kaitlyn; Cegelski, Lynette; Holten, Dewey; Boxer, Steven G; Kirmaier, Christine

2014-06-19

374

Hydrogen-bond-mediated cascade reaction involving chalcones: facile synthesis of enantioenriched trisubstituted tetrahydrothiophenes.  

PubMed

A bifunctional squaramide catalyzed sulfa-Michael/aldol cascade reaction between 1,4-dithiane-2,5-diol and chalcones with a low catalyst loading has been developed. Trisubstituted tetrahydrothiophenes with three contiguous stereogenic centers are obtained in a highly stereocontrolled manner. Additionally, a remarkable temperature effect on reaction efficiency was observed and a synthetically potential gram-synthesis was also conducted. PMID:22313116

Ling, Jun-Bing; Su, Yu; Zhu, Hai-Liang; Wang, Guan-Yu; Xu, Peng-Fei

2012-02-17

375

Bonding Constraints,Elasticity and Electronic Heterogeneity in Transition Metal Oxides*  

NASA Astrophysics Data System (ADS)

We describe a multiscale "systems" scenario for doped transition metal oxides and related "strongly correlated" electronic materials, in which local polarizable "hotspots" (pairing centers, polarons, charge-transfer centers, etc.) induce elastic strains, which self-consistently drive self-assembly and coherent responses - macroscopically but heterogeneously. The elastic fields are a result of the highly-directional, local bonding "compatibility" constraints, and provide structural templates which couple strongly to electronic wave-functions because of (oxygen) polarizabilities and (metal-oxygen) charge-transfers. This leads to intrinsic "landscapes" of heterogeneous ground and metastable states (and associated multiscale dynamics), which are globally sensitive to local perturbations. Heterogeneity at atomic scales (e.g. filamentary/clump charge/spin localization and perovskite unit cell distortions) are intimately coupled, in multiscale systems, to mesoscale structural textures (twinning, tweed, etc). We describe selected signatures of heterogeneous textures in spin, charge and lattice degrees-of-freedom. We emphasize the importance of soft filamentary phases as the typical consequences of coexisiting anistropic short- and long-range fields and suggest generalizations to other hard, soft and biological matter. We speculate on the role of strain fields in controlling pseudogroups, correlated percolation, soft mesoscopic dynamics, filamentary dielectric breakdown and proximity effects, and effective pairing symmetry. 1)Intrinsic multiscale structure and dynamics in complex electronic oxides, eds. A. Bishop, S. Shenoy, S. Sridhar (World Scientific 2003); 2) A. Bishop et al, Euro. Phys. Lett. 63, 289 (2003); 3) J-X Zhu et al, Phys. Rev. Lett. 91, 057004; 4) J. C. Phillips et al, Rep. Prog. Phys. 66, 2111 (2003). *This work was performed in collaboration with K. Ahn, T. Lookman, A. Saxena, S. Shenoy, J-X Zhu, and work at Los Alamos was supported by the USDOE Office of Basic Energy Sciences-Division of Materials

Bishop, Alan

2004-03-01

376

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

377

Sr Diffusion and Reaction within Fe Oxides: Evaluation of the Rate-Limiting Mechanism for Sorption  

Microsoft Academic Search

Sorption studies were conducted to study intraparticle diffusion of strontium in amorphous iron oxide. Oxides of manganese and especially iron often control contaminant sorption in the subsurface environment. In the two-step sorption of inorganics by oxide minerals, the first step is a fast, reversible reaction between the bulk aqueous phase and the adsorbent external surface sites. The second step is

Lisa Axe; Paul R. Anderson

1995-01-01

378

Aryl nitrile oxide cycloaddition reactions in the presence of pinacol boronic acid ester  

PubMed Central

Summary An aryl substrate with dual functionality consisting of a nitrile oxide and a pinacolyl boronate ester was prepared by mild hypervalent iodine oxidation (diacetoxyiodobenzene) of the corresponding aldoxime, without decomposition of the boronate functionality. The nitrile oxide was trapped in situ with a variety of dipolarophiles to yield aryl isoxazolines with the boronate ester function intact and available for subsequent reaction.

Harding, Sarah L; Marcuccio, Sebastian M

2012-01-01

379

Comparative analysis of oxidation methods of reaction-sintered silicon carbide for optimization of oxidation-assisted polishing.  

PubMed

Combination of the oxidation of reaction-sintered silicon carbide (RS-SiC) and the polishing of the oxide is an effective way of machining RS-SiC. In this study, anodic oxidation, thermal oxidation, and plasma oxidation were respectively conducted to obtain oxides on RS-SiC surfaces. By performing scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDX) analysis and scanning white light interferometry (SWLI) measurement, the oxidation behavior of these oxidation methods was compared. Through ceria slurry polishing, the polishing properties of the oxides were evaluated. Analysis of the oxygen element on polished surfaces by SEM-EDX was conducted to evaluate the remaining oxide. By analyzing the three oxidation methods with corresponding polishing process on the basis of schematic diagrams, suitable application conditions for these methods were clarified. Anodic oxidation with simultaneous polishing is suitable for the rapid figuring of RS-SiC with a high material removal rate; polishing of a thermally oxidized surface is suitable for machining RS-SiC mirrors with complex shapes; combination of plasma oxidation and polishing is suitable for the fine finishing of RS-SiC with excellent surface roughness. These oxidation methods are expected to improve the machining of RS-SiC substrates and promote the application of RS-SiC products in the fields of optics, molds, and ceramics. PMID:24216836

Shen, Xinmin; Dai, Yifan; Deng, Hui; Guan, Chaoliang; Yamamura, Kazuya

2013-11-01

380

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

381

[10] Disulfide bond formation in peptides  

Microsoft Academic Search

The goal of this review has been to present different chemical approaches for the formation of disulfide bonds in synthetic peptides and small proteins. Three general types of approaches have been described: (1) oxidation starting from the unprotected thiols; (2) oxidation starting from protected thiols; and (3) directed methods for formation of unsymmetrical disulfides. Individual or sequential disulfide-forming reactions can

Ioana Annis; Balazs Hargittai; George Barany

1997-01-01

382

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

383

A Comprehensive Chemical Kinetic Reaction Mechanism for Oxidation and Pyrolysis of Propane and Propene  

Microsoft Academic Search

Abstract—A detailed chemical kinetic reaction mechanism is developed to describe the oxidation and pyrolysis of propane and propene. The mechanism consists of 163 elementary reactions among 4l chemical species. New rate expressions are developed for a number of reactions of propane, propene, and intermediate hydrocarbon species with radicals including H, 0, and OH. The mechanism is tested by comparisons between

CHARLES K. WESTBROOK; WILLIAM J. PITZ

1984-01-01

384

Coordinate Contribution of Lipid Oxidation and Maillard Reaction to the Nonenzymatic Food Browning  

Microsoft Academic Search

Lipid oxidation and the Maillard reaction are probably the two most important reactions in Food Science. Both include a whole network of different reactions in which an extraordinary complex mixture of compounds are obtained in very different amounts and produce important changes in food flavor, color, texture, and nutritional value, with positive and negative consequences. This article analyzes the interactions

ROSARIO ZAMORA; FRANCISCO J. HIDALGO

2005-01-01

385

A highly oxidizing and isolable oxoruthenium(V) complex [Ru(V)(N4O)(O)]2+: electronic structure, redox properties, and oxidation reactions investigated by DFT calculations.  

PubMed

The electronic structure and redox properties of the highly oxidizing, isolable Ru(V)=O complex [Ru(V)(N4O)(O)](2+), its oxidation reactions with saturated alkanes (cyclohexane and methane) and inorganic substrates (hydrochloric acid and water), and its intermolecular coupling reaction have been examined by DFT calculations. The oxidation reactions with cyclohexane and methane proceed through hydrogen atom transfer in a transition state with a calculated free energy barrier of 10.8 and 23.8?kcal?mol(-1), respectively. The overall free energy activation barrier (?G(?)=25.5?kcal?mol(-1)) of oxidation of hydrochloric acid can be decomposed into two parts: the formation of [Ru(III)(N4O)(HOCl)](2+) (?G=15.0?kcal?mol(-1)) and the substitution of HOCl by a water molecule (?G(?)=10.5?kcal?mol(-1)). For water oxidation, nucleophilic attack on Ru(V)=O by water, leading to O-O bond formation, has a free energy barrier of 24.0?kcal?mol(-1), the major component of which comes from the cleavage of the H-OH bond of water. Intermolecular self-coupling of two molecules of [Ru(V)(N4O)(O)](2+) leads to the [(N4O)Ru(IV)-O2-Ru(III)(N4O)](4+) complex with a calculated free energy barrier of 12.0?kcal?mol(-1). PMID:23788366

Guan, Xiangguo; Chan, Sharon Lai-Fung; Che, Chi-Ming

2013-09-01

386

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

387

The binding and reactions of nucleotides and polynucleotides on iron oxide hydroxide polymorphs.  

PubMed

The iron oxide hydroxide minerals goethite and akaganéite were likely constituents of the sediments present in, for instance, geothermal regions of the primitive earth. They may have adsorbed organics and catalyzed the condensation processes which led to the origins of life. The binding to and reactions of nucleotides and oligonucleotides with these minerals was investigated. The adsorption of adenosine, 5'-AMP, 3'-AMP, 5'-UMP, and 5'-CMP to these minerals was studied. Adenosine did not bind to goethite and akaganéite. The adsorption isotherms for the binding of the nucleotides revealed that they all had close to the same affinity for the mineral. Binding to goethite was about four times stronger than to akaganéite. There was little difference in the adsorption of each nucleotide suggesting the binding was between the negative charge on the phosphate group and the positive charges on the mineral surface. The absence of binding of adenosine is consistent with this explanation. Binding decreases as the pH increases due to the titration of the positive (acidic) centers on the minerals. Two times as many moles of polynucleotides were bound to these minerals as compared to the mononucleotides. Watson-Crick hydrogen bonding of adenosine and 5'-AMP to poly(U) complexes with goethite and akaganéite was observed. There was no interaction of uridine with the poly(U)-goethite complex as expected if Watson-Crick hydrogen bonding is taking place. Neither goethite nor akaganéite catalyzed the oligomerization of the phosphorimidazolide of adenosine (ImpA). The template directed synthesis of oligomers of 5'-GMP on the poly(C) bound to goethite was observed. Higher molecular weight oligomers were observed when the poly(C) was bound to goethite than was found in the absence of the mineral. PMID:8316350

Holm, N G; Ertem, G; Ferris, J P

1993-06-01

388

Hydrogen bonding effects on the reorganization energy for photoinduced charge separation reaction between porphyrin and quinone studied by nanosecond laser flash photolysis.  

PubMed

Alcohol concentration dependences of photoinduced charge separation (CS) reaction of zinc tetraphenyl-porphyrin (ZnTPP) and duroquinone (DQ) were investigated in benzonitrile by a nanosecond laser flash photolysis technique. The photoinduced CS reaction was accelerated by the addition of alcohols, whereas the addition of acetonitrile caused little effect on the CS reactions. The simple theory was developed to calculate an increase in reorganization energies induced by the hydrogen bonding interactions between DQ and alcohols using the chemical equilibrium constants for the hydrogen bonding complexes through the concerted pathway and the stepwise one. The experimental results were analyzed by using the Marcus equation where we took into account the hydrogen bonding effects on the reorganization energy and the reaction free energy for the CS reaction. The observed alcohol concentration dependence of the CS reaction rates was well explained by the formation of the hydrogen bonding complexes through the concerted pathway, demonstrating the increase in the reorganization energy by the hydrogen bonding interactions. PMID:20121118

Yago, Tomoaki; Gohdo, Masao; Wakasa, Masanobu

2010-02-25

389

Reactions of transition-metal ions with methylsilanes in the gas phase. The formation and characteristics of strong transition metal-silylene bonds  

SciTech Connect

Reactions of transition-metal ions (Ti/sup +/, V/sup +/, Cr/sup +/, Fe/sup +/, Co/sup +/, and Ni/sup +/) with organosilanes are investigated in the gas phase with an ion beam apparatus. Co/sup +/ and Ni/sup +/ react with silane to yield metal silylenes as exothermic products. Collision-induced dissociation studies of the product CoSiH/sub 2//sup +/ and nascent CoSiH/sub 4//sup +/ adducts provide additional information concerning the product structure and reaction mechanisms. Reactions with methylsilanes lead to formation and metal silylenes as major reaction channels, along with several other processes including hydride abstraction, dehydrogenation, and methane loss. Reactions with hexamethyldisilane are also investigated, with major products indicating Si-Si bond cleavage. An examination of the reaction enthalpies for the observed metal silylene products provides estimates for metal ion-silylene bond energies, which include D/sup 0/(M/sup +/ - SiH/sub 2/) = 67 +/- 6 kcal mol/sup -1/ (M = Co, Ni). Correlation between the metal ion-silylene bond energies and the electronic structure of the metal ions supports a bonding scheme in which silylene donates its nonbonding lone pair electrons to an empty 4s orbital of the metal center. For Co/sup +/ and Ni/sup +/, back-donation of paired 3d electrons from the metal into the empty 3p orbital on silicon is suggested to account for the stronger bond deduced for these metals.

Kang, H.; Jacobson, D.B.; Shin, S.K.; Beauchamp, J.L.; Bowers, M.T.

1986-09-17

390

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

391

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

392

Evolution of chemical bonding and electron density rearrangements during D3h ? D3d reaction in monolayered TiS2 : A QTAIM and ELF study.  

PubMed

Monolayered titanium disulfide TiS2 , a prospective nanoelectronic material, was previously shown to be subject to an exothermic solid-state D3h -D3d reaction that proceeds via a newly discovered transition state. Here, we study the reaction in detail using topological methods of quantum chemistry (quantum theory of atoms in molecules and electron localization function analysis) and show how electron density and chemical bonding between the atoms change in the course of the reaction. The reaction is shown to undergo a series of topological catastrophes, associated with elementary chemical events such as break and formation of bonds (including the unexpected formation of S?S bonding between sulfur layers), and rearrangement of electron density of outer valence and core shells. © 2014 Wiley Periodicals, Inc. PMID:24943852

Ryzhikov, Maxim R; Slepkov, Vladimir A; Kozlova, Svetlana G; Gabuda, Svyatoslav P

2014-08-15

393

[Investigation of the oxidation reaction of O3 with bromide ion in aqueous solution].  

PubMed

The reaction mechanism of O3 with bromide ion in aqueous solution was studied by ion chromatography and UV-Vis spectrometry instruments. Ion chromatography analysis showed that only 10% of Br- which was oxidized by ozone was formed into bromate ion. The results demonstrated that the final products of the oxidation reaction were identified as Br2 and Br3- except for BrO3-. The formation of Br3- which was yielded from the reaction of Br2 with Br- was the major process in the reaction of Br attacked by O3. The characteristic absorption spectrum of Br3- with an absorption peak at 260 nm was also investigated. The results may provide helpful information about the mechanism of the oxidation reaction of Br- with O3 and fate of Br- or its derivatives in the environment by the oxidation processes. PMID:23243871

Yu, Xiao-Ting; Zhang, Jia-Hui; Pan, Xun-Xi; Zhang, Ren-Xi; Hou, Hui-Qi

2012-09-01

394

Infrared spectra of methylidynes formed in reactions of re atoms with methane, methyl halides, methylene halides, and ethane: methylidyne C-H stretching absorptions, bond lengths, and s character.  

PubMed

Rhenium carbyne complexes (HC identical with ReH 3, HC identical with ReH 2X, HC identical with ReHX 2, [X = F, Cl, and Br] and CH 3C identical with ReH 3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and alpha-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC identical with ReH 3 and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C s structures, and as a result the HC identical with ReH 3 and CH 3C identical with ReH 3 complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC identical with WH 3 with three equivalent W-H bonds. PMID:18257553

Cho, Han-Gook; Andrews, Lester

2008-03-01

395

Structural and medium effects on the reactions of the cumyloxyl radical with intramolecular hydrogen bonded phenols. The interplay between hydrogen-bonding and Acid-base interactions on the hydrogen atom transfer reactivity and selectivity.  

PubMed

A time-resolved kinetic study on the reactions of the cumyloxyl radical (CumO(•)) with intramolecularly hydrogen bonded 2-(1-piperidinylmethyl)phenol (1) and 4-methoxy-2-(1-piperidinylmethyl)phenol (2) and with 4-methoxy-3-(1-piperidinylmethyl)phenol (3) has been carried out. In acetonitrile, intramolecular hydrogen bonding protects the phenolic O-H of 1 and 2 from attack by CumO(•) and hydrogen atom transfer (HAT) exclusively occurs from the C-H bonds that are ? to the piperidine nitrogen (?-C-H bonds). With 3 HAT from both the phenolic O-H and the ?-C-H bonds is observed. In the presence of TFA or Mg(ClO4)2, protonation or Mg(2+) complexation of the piperidine nitrogen removes the intramolecular hydrogen bond in 1 and 2 and strongly deactivates the ?-C-H bonds of the three substrates. Under these conditions, HAT to CumO(•) exclusively occurs from the phenolic O-H group of 1-3. These results clearly show that in these systems the interplay between intramolecular hydrogen bonding and Brønsted and Lewis acid-base interactions can drastically influence both the HAT reactivity and selectivity. The possible implications of these findings are discussed in the framework of the important role played by tyrosyl radicals in biological systems. PMID:24892501

Salamone, Michela; Amorati, Riccardo; Menichetti, Stefano; Viglianisi, Caterina; Bietti, Massimo

2014-07-01

396

In Pursuit of an Ideal C-C Bond-Forming Reaction  

PubMed Central

Attempts to introduce the highly versatile vinyl group into other organic molecules in a chemo-, regio- and stereoselective fashion via catalytic activation of ethylene provided challenging opportunities to explore new ligand and salt effects in homogeneous catalysis. This review provides a personal account of the development of enantioselective reactions involving ethylene.

RajanBabu, T. V.

2009-01-01

397

Controlling selectivity in photochemical reactions through confinement and non bonding interactions  

Microsoft Academic Search

Work presented in this thesis is the consolidation of our experiments aimed at achieving selectivity in photochemical reactions by making use of confinement provided by the host and the weak intermolecular interactions existing between the host and the guest(s) molecules. Two supramolecules namely Pd-nanocage (a water soluble self assembled coordination cage) and commercially available Y zeolites were used as hosts

Karthikeyan Sivasubramanian

2007-01-01

398

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

399

Palladium and gold nanotubes as oxygen reduction reaction and alcohol oxidation reaction catalysts in base.  

PubMed

Palladium (PdNTs) and gold nanotubes (AuNTs) were synthesized by the galvanic displacement of silver nanowires. PdNTs and AuNTs have wall thicknesses of 6?nm, outer diameters of 60?nm, and lengths of 5-10 and 5-20??m, respectively. Rotating disk electrode experiments showed that the PdNTs and AuNTs have higher area normalized activities for the oxygen reduction reaction (ORR) than conventional nanoparticle catalysts. The PdNTs produced an ORR area activity that was 3.4, 2.2, and 3.7?times greater than that on carbon-supported palladium nanoparticles (Pd/C), bulk polycrystalline palladium, and carbon-supported platinum nanoparticles (Pt/C), respectively. The AuNTs produced an ORR area activity that was 2.3, 9.0, and 2.0?times greater than that on carbon-supported gold nanoparticles (Au/C), bulk polycrystalline gold, and Pt/C, respectively. The PdNTs also had lower onset potentials than Pd/C and Pt/C for the oxidation of methanol (0.236?V), ethanol (0.215?V), and ethylene glycol (0.251?V). In comparison to Pt/C, the PdNTs and AuNTs further demonstrated improved alcohol tolerance during the ORR. PMID:24757078

Alia, Shaun M; Duong, Kathlynne; Liu, Toby; Jensen, Kurt; Yan, Yushan

2014-06-01

400

High-temperature oxidation behavior of reaction-formed silicon carbide ceramics  

NASA Technical Reports Server (NTRS)

The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, k(sub p)), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced k(sub p) values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi, in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.

Ogbuji, Linus U. J. T.; Singh, M.

1995-01-01

401

[Spectral characterization for capsanthin molecule with in vitro oxidative defense reaction].  

PubMed

The changes of UV-Vis and FTIR spectroscopic properties for capsanthin before and after reaction with exogenous superoxide anion (*O2(-)), hydrogen peroxide (H2O2) and hydroxyl radical (*OH), catalase (CAT), peroxidase (POD) and lipoxygenase (LOX) were explored. The results showed that, the UV-Vis spectral absorption of capsanthin treated with reactive oxygen species had a blue-shift. At the same time, the FTIR spectra changed significantly. The number of FTIR spectral peaks reduced and theFTIR strength weakened for capsanthin molecule treated with *O2(-) and *OH. The characteristic and strong peaks moved to shorter wavelengths when treated with H2O2. And LOX caused breakage of capsanthin molecule and reduction of peak number or groups without carbonyl. Exogenous H2O2 + CAT or H2O2 + POD treatment could not affect the UV-Vis and FTIR spectra significantly. So ROS could cause oxidative degradation of capsanthin and destroy chromophoric groups such as carbon-carbon double bond and carbonyl, then grow colorless alcohols. Hence ROS and LOX should transforms the conjugate system of capsanthin molecules, while CAT and POD could protect the capsanthin. PMID:24822401

Zheng, Wen-Yu; Ding, Zhu-Hong; Liu, Hai

2014-02-01

402

Reactivity of Biomimetic Iron(II)-2-aminophenolate Complexes toward Dioxygen: Mechanistic Investigations on the Oxidative C-C Bond Cleavage of Substituted 2-Aminophenols.  

PubMed

The isolation and characterization of a series of iron(II)-2-aminophenolate complexes [(6-Me3-TPA)Fe(II)(X)](+) (X = 2-amino-4-nitrophenolate (4-NO2-HAP), 1; X = 2-aminophenolate (2-HAP), 2; X = 2-amino-3-methylphenolate (3-Me-HAP), 3; X = 2-amino-4-methylphenolate (4-Me-HAP), 4; X = 2-amino-5-methylphenolate (5-Me-HAP), 5; X = 2-amino-4-tert-butylphenolate (4-(t)Bu-HAP), 6 and X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-(t)Bu-HAP), 7) and an iron(III)-2-amidophenolate complex [(6-Me3-TPA)Fe(III)(4,6-di-(t)Bu-AP)](+) (7(Ox)) supported by a tripodal nitrogen ligand (6-Me3-TPA = tris(6-methyl-2-pyridylmethyl)amine) are reported. Substituted 2-aminophenols were used to prepare the biomimetic iron(II) complexes to understand the effect of electronic and structural properties of aminophenolate rings on the dioxygen reactivity and on the selectivity of C-C bond cleavage reactions. Crystal structures of the cationic parts of 5·ClO4 and 7·BPh4 show six-coordinate iron(II) centers ligated by a neutral tetradentate ligand and a monoanionic 2-aminophenolate in a bidentate fashion. While 1·BPh4 does not react with oxygen, other complexes undergo oxidative transformation in the presence of dioxygen. The reaction of 2·ClO4 with dioxygen affords 2-amino-3H-phenoxazin-3-one, an auto-oxidation product of 2-aminophenol, whereas complexes 3·BPh4, 4·BPh4, 5·ClO4 and 6·ClO4 react with O2 to exhibit C-C bond cleavage of the bound aminophenolates. Complexes 7·ClO4 and 7(Ox)·BPh4 produce a mixture of 4,6-di-tert-butyl-2H-pyran-2-imine and 4,6-di-tert-butyl-2-picolinic acid. Labeling experiments with (18)O2 show the incorporation of one oxygen atom from dioxygen into the cleavage products. The reactivity (and stability) of the intermediate, which directs the course of aromatic ring cleavage reaction, is found to be dependent on the nature of ring substituent. The presence of two tert-butyl groups on the aminophenolate ring in 7·ClO4 makes the complex slow to cleave the C-C bond of 4,6-di-(t)Bu-HAP, whereas 4·BPh4 containing 4-Me-HAP displays fastest reactivity. Density functional theory calculations were conducted on [(6-Me3-TPA)Fe(III)(4-(t)Bu-AP)](+) (6(Ox)) to gain a mechanistic insight into the regioselective C-C bond cleavage reaction. On the basis of the experimental and computational studies, an iron(II)-2-iminobenzosemiquinonate intermediate is proposed to react with dioxygen resulting in the oxidative C-C bond cleavage of the coordinated 2-aminophenolates. PMID:24787025

Chakraborty, Biswarup; Bhunya, Sourav; Paul, Ankan; Paine, Tapan Kanti

2014-05-19

403

Radical O-O coupling reaction in diferrate-mediated water oxidation studied using multireference wave function theory.  

PubMed

The O-O (oxygen-oxygen) bond formation is widely recognized as a key step of the catalytic reaction of dioxygen evolution from water. Recently, the water oxidation catalyzed by potassium ferrate (K2FeO4) was investigated on the basis of experimental kinetic isotope effect analysis assisted by density functional calculations, revealing the intramolecular oxo-coupling mechanism within a di-iron(vi) intermediate, or diferrate [Sarma et al., J. Am. Chem. Soc., 2012, 134, 15371]. Here, we report a detailed examination of this diferrate-mediated O-O bond formation using scalable multireference electronic structure theory. High-dimensional correlated many-electron wave functions beyond the one-electron picture were computed using the ab initio density matrix renormalization group (DMRG) method along the O-O bond formation pathway. The necessity of using large active space arises from the description of complex electronic interactions and varying redox states both associated with two-center antiferromagnetic multivalent iron-oxo coupling. Dynamic correlation effects on top of the active space DMRG wave functions were additively accounted for by complete active space second-order perturbation (CASPT2) and multireference configuration interaction (MRCI) based methods, which were recently introduced by our group. These multireference methods were capable of handling the double shell effects in the extended active space treatment. The calculations with an active space of 36 electrons in 32 orbitals, which is far over conventional limitation, provide a quantitatively reliable prediction of potential energy profiles and confirmed the viability of the direct oxo coupling. The bonding nature of Fe-O and dual bonding character of O-O are discussed using natural orbitals. PMID:24686771

Kurashige, Yuki; Saitow, Masaaki; Chalupský, Jakub; Yanai, Takeshi

2014-06-28