Note: This page contains sample records for the topic stable p-heterocyclic carbenes from Science.gov.
While these samples are representative of the content of Science.gov,
they are not comprehensive nor are they the most current set.
We encourage you to perform a real-time search of Science.gov
to obtain the most current and comprehensive results.
Last update: August 15, 2014.
1

A Brief Survey of our Contribution to Stable Carbene Chemistry  

PubMed Central

This personal account summarizes our work, beginning with the discovery of the first stable carbene in 1988 up until the recent isolation of mesoionic carbenes. It explains why we have moved our focus from acyclic to cyclic carbenes, and shows that these stable species are not limited to the role of ligand for transition metals, but that they are also powerful agents for the activation of small molecules, and for the stabilization of highly reactive diamagnetic and paramagnetic species.

Martin, David; Melaimi, Mohand; Soleilhavoup, Michele; Bertrand, Guy

2012-01-01

2

Stable singlet carbenes as mimics for transition metal centers  

PubMed Central

This perspective summarizes recent results, which demonstrate that stable carbenes can activate small molecules (CO, H2, NH3 and P4) and stabilize highly reactive intermediates (main group elements in the zero oxidation state and paramagnetic species). These two tasks were previously exclusive for transition metal complexes.

Martin, David; Soleilhavoup, Michele

2011-01-01

3

Stable Cyclic Carbenes and Related Species beyond Diaminocarbenes  

PubMed Central

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong ?-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger ?-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Melaimi, Mohand; Soleilhavoup, Michele

2011-01-01

4

Comparative reactivity of different types of stable cyclic and acyclic mono- and diamino carbenes with simple organic substrates.  

PubMed

A series of stable carbenes, featuring a broad range of electronic properties, were reacted with simple organic substrates. The N,N-dimesityl imidazolylidene (NHC) does not react with isocyanides, whereas anti-Bredt di(amino)carbene (pyr-NHC), cyclic (alkyl)(amino)carbene (CAAC), acyclic di(amino)carbene (ADAC), and acyclic (alkyl)(amino)carbene (AAAC) give rise to the corresponding ketenimines. NHCs are known to promote the benzoin condensation, and we found that the CAAC, pyr-NHC, and ADAC react with benzaldehyde to give the ketone tautomer of the Breslow intermediate, whereas the AAAC first gives the corresponding epoxide and ultimately the Breslow intermediate, which can be isolated. Addition of excess benzaldehyde to the latter does not lead to benzoin but to a stable 1,3-dioxolane. Depending on the electronic properties of carbenes, different products are also obtained with methyl acrylate as a substrate. The critical role of the carbene electrophilicity on the outcome of reactions is discussed. PMID:24634983

Martin, David; Canac, Yves; Lavallo, Vincent; Bertrand, Guy

2014-04-01

5

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold  

NASA Astrophysics Data System (ADS)

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold–carbon bond relative to that of a gold–sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.

Crudden, Cathleen M.; Horton, J. Hugh; Ebralidze, Iraklii I.; Zenkina, Olena V.; McLean, Alastair B.; Drevniok, Benedict; She, Zhe; Kraatz, Heinz-Bernhard; Mosey, Nicholas J.; Seki, Tomohiro; Keske, Eric C.; Leake, Joanna D.; Rousina-Webb, Alexander; Wu, Gang

2014-05-01

6

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold.  

PubMed

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs. PMID:24755592

Crudden, Cathleen M; Horton, J Hugh; Ebralidze, Iraklii I; Zenkina, Olena V; McLean, Alastair B; Drevniok, Benedict; She, Zhe; Kraatz, Heinz-Bernhard; Mosey, Nicholas J; Seki, Tomohiro; Keske, Eric C; Leake, Joanna D; Rousina-Webb, Alexander; Wu, Gang

2014-05-01

7

An efficient synthetic route to stable bis(carbene)borylenes [(L1)(L2)BH].  

PubMed

Two-electron reduction of bis(carbene) boronium salts allows for the preparation of unsymmetrically substituted nucleophilic boron derivatives of type (L1)(L2)BH, which are characterized by X-ray crystallography. A single electron reduction of the same starting materials leads to the corresponding boron-centered radical cations (L1)(L2)BH?(+), X(-). PMID:24909943

Ruiz, David A; Melaimi, Mohand; Bertrand, Guy

2014-06-24

8

Shape-Controllable Formation of Poly-imidazolium Salts for Stable Palladium N-Heterocyclic Carbene Polymers.  

PubMed

The imidazolium-based main-chain organic polymers are one of promising platforms in heterogeneous catalysis, the size and outer morphology of polymer particles are known to have important effects on their physical properties and catalytic applications, but main-chain ionic polymers usually generate amorphous or spherical particles. Herein, we presented a versatile and facile synthetic route for size- and shape-controllable synthesis of main-chain poly-imidazolium particles. The wire-shaped, spherical and ribbon-shaped morphologies of poly-imidazolium particles were readily synthesized through quaternization of bis-(imidazol-1-yl)methane and 2,4,6-tris(4-(bromomethyl)phenyl)-1,3,5-triazine, and the modification of their size and morphology were realized through adjusting solvent polarity, solubility, concentration and temperatures. The direct complexation of the particles with Pd(OAc)2 produced ionic polymers containing palladium N-heterocyclic carbene units (NHCs) with intactness of original morphologies. The particle morphologies have a significant effect on catalytic performances. Wire-shaped palladium-NHC polymer shows excellent catalytic activity and recyclabilty in heterogeneous Suzuki-Miyaura cross-coupling reaction. PMID:24969738

Zhao, Huaixia; Li, Liuyi; Wang, Yangxin; Wang, Ruihu

2014-01-01

9

Carbenes and Nitrenes  

ERIC Educational Resources Information Center

Summarizes the general methods for carbene and nitrene formation and the reactions in which carbenes and nitrenes are involved such as their reactions with transition metal atoms, alkenes of aromatic compounds, and uncharged oxygen or nitrogen nucleophiles. (CC)

Coyle, J. D.

1974-01-01

10

Air-stable, convenient to handle Pd based PEPPSI (pyridine enhanced precatalyst preparation, stabilization and initiation) themed precatalysts of N\\/O-functionalized N-heterocyclic carbenes and its utility in Suzuki–Miyaura cross-coupling reaction  

Microsoft Academic Search

Several new air-stable, convenient to handle and easily synthesized Pd based PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) type precatalysts supported over N\\/O-functionalized N-heterocyclic carbenes (NHC) namely, trans-(1-(benzyl)-3- (N-t-butylacetamido)imidazol-2-ylidene)Pd(pyridine)Cl2 (2), trans-(1-(2-hydroxy-cyclohexyl)-3- (benzyl)imidazol-2-ylidene)Pd(pyridine)Cl2 (3 )a ndtrans-(1-(o-methoxybenzyl)-3-(t-butyl)imidazol-2- ylidene)Pd(pyridine)Br2 (4), have been designed. Specifically, the Pd-NHC complexes, 2, 3 and 4 ,w ere conveniently synthesized from their respective imidazolium halide salts by

Lipika Ray; Mobin M. Shaikh; Prasenjit Ghosh

2007-01-01

11

Arylation of rhodium(II) azavinyl carbenes with boronic acids.  

PubMed

A highly efficient and stereoselective arylation of in situ-generated azavinyl carbenes affording 2,2-diaryl enamines at ambient temperatures has been developed. These transition-metal carbenes are directly produced from readily available and stable 1-sulfonyl-1,2,3-triazoles in the presence of a rhodium carboxylate catalyst. In several cases, the enamines generated in this reaction can be cyclized into substituted indoles employing copper catalysis. PMID:22913576

Selander, Nicklas; Worrell, Brady T; Chuprakov, Stepan; Velaparthi, Subash; Fokin, Valery V

2012-09-12

12

Phosphinine Derivatives and their Use as Versatile Intermediates in P-Heterocyclic Chemistry  

NASA Astrophysics Data System (ADS)

The simplest preparation of 1,2-dihydrophosphinine oxides is based on ring enlargement of 2,5-dihydro-1H-phosphole oxides involving the addition of dichlorocarbene on the double-bond of the dihydrophosphole oxide that is followed by the opening of the cyclopropane ring so formed. Variation of the substitution pattern and the extent of saturation of the hetero ring made available a variety of six-membered P-heterocycles, such as 1,4-dihydrophosphinine oxides, 1,2,3,6-tetrahydrophosphinine oxides and 1,2,3,4,5,6-hexahydrophosphinine oxides. 3-P(O)Y2-Substituted 1,2,3,6-tetrahydrophosphinine oxides obtained by the Michael reaction of 1,2-dihydrophosphinine oxides form another representative group. The 3-P(O)Y2-tetrahydrophosphinine oxides, along with their saturated derivatives may be useful precursors of bidentate P-ligands. Novel intramolecular interactions were found to determine the conformation of the 3-substituted tetrahydrophosphinine oxides.

Keglevich, György

13

Laser flash photolysis studies of methoxycarbonyl phenyl carbene and its derived carbonyl oxide at room temperature  

SciTech Connect

Laser flash photolysis of methyl ..cap alpha..-diazophenylacetate gives triplet methoxycarbonyl phenyl carbene (/sup 3/MPC) exhibiting a transient absorption spectrum (< 270 nm), which shows a pseudo-first-order decay with lifetime of 461 ns in deaerated Freon 113. The triplet carbene readily reacts with an oxygen molecule to afford a corresponding carbonyl oxide in aerated solvents at room temperature. The formation of carbonyl oxide was demonstrated by the buildup of the transient absorption spectrum (lambda/sub max/ approx. 410 nm) at the expense of the absorption band of the carbene. Quenching rate constants of this triplet carbene by 2-methyl-2-butene and methyl alcohol were obtained to be 7.0 x 10/sup 7/ M/sup -1/ s/sup -1/ and 2.7 x 10/sup 8/ M/sup -1/ s/sup -1/, respectively, by means of monitoring the decay of the triplet carbene. The difference of the reactivity between the triplet carbene and the carbonyl oxide toward methyl alcohol has been investigated. The carbonyl oxide is much more stable with methanol than the parent triplet carbene (/sup 3/MPC). Kinetic data reported in this paper demonstrate that the stability of this carbene (/sup 3/MPC) is regarded as an intermediate between reported diphenylcarbene and fluorenylidene.

Fujiwara, Y.; Tanimoto, Y.; Itoh, M.; Hirai, K.; Tomioka, H.

1987-04-01

14

Synthesis and Properties of Chelating N-Heterocyclic Carbene Rhodium(I) Complexes: Synthetic Experiments in Current Organometallic Chemistry  

ERIC Educational Resources Information Center

The preparation and characterization of two air-stable Rh(I) complexes bearing a chelating N-heterocyclic carbene (NHC) ligand is described. The synthesis involves the preparation of a Ag(I)-NHC complex and its use as carbene transfer agent to a Rh(I) precursor. The so obtained complex can be further reacted with carbon monoxide to give the…

Mata, Jose A.; Poyatos, Macarena; Mas-Marza, Elena

2011-01-01

15

Synthesis, characterization and biological evaluation of some novel P-heterocyclic androst-4-ene derivatives.  

PubMed

The reactions of 21-hydroxyprogesterone with Lawesson's reagent in toluene or [Formula: see text] gave four P-heterocyclic androst-4-ene derivatives (two tautomeric pairs): 4-(3-thioxoandrost-4-en-17[Formula: see text]-yl)-1,3,2-oxathiaphosphole-2- sulfide (2), 4-(3-thioxoandrost-4-en-17[Formula: see text]-ylidene)-1,3,2-oxathiaphospholane-2-sulfide (3), 4-(3-oxoandrost-4-en-17[Formula: see text]-yl)-1,3,2-oxathiaphosphole-2-sulfide (4), and 4-(3-oxoandrost-4-en-17[Formula: see text]-ylidene)-1,3,2- oxathiaphospholane-2-sulfide (5). The structures of all novel 17-substituted steroids were elucidated from their analytic and spectral data (HRMS, IR, 1D NMR and 2D NMR-HSQC, HMBC, NOESY, COSY). The detailed NMR analysis for all compounds revealed the presence of two pairs of signals in approx. 8:2 ratio indicating the existence of two diastereoisomers (a and b) with different configurations at the phosphorus atom. A parallel analysis of heteronuclear 2D [Formula: see text]-[Formula: see text] spectra (HSQC and HMBC) and homonuclear 2D spectra (NOESY and COSY) enabled complete [Formula: see text] and [Formula: see text] assignments of each isomer and provided evidence for the preferred configuration on phosphorus atom. Cytotoxic activity in vitro was tested against four tumor cell lines (human cervix carcinoma HeLa cells, chronic myelogenous leukemia K-562 and two human breast carcinoma MDA-MB-361 and MDA-MB-453 cells). Compounds 3a,b and 4a,b showed a poor activity against HeLa and MDA-MB-453 cell lines, while against MDA-MB-361 cell line, all tested compounds exerted very weak cytotoxic effect. All compounds exerted moderate activity against K562 cells. Antimicrobial activity against Gram-positive, Gram-negative bacteria and fungal cells, and toxicity to brine shrimp Artemia salina were evaluated. All tested compounds showed strong antifungal activity. PMID:23748368

Krsti?, Natalija M; Pavlovi?, Vladimir D; Novakovi?, Irena T; Mati?, Ivana Z; Sladi?, Dušan M

2013-08-01

16

Dynamics of carbene cycloadditions.  

PubMed

Quasiclassical trajectory calculations using quantum mechanical energies and forces generated by the Venus and Gaussian programs provide for the first time a detailed dynamical picture of singlet carbene, CCl(2) and CF(2), cycloadditions to alkenes on the B3LYP/6-31G* surface. For CF(2), B3LYP/6-31G* with exact exchange reduced to 12% HF was also employed to better mimic the high accuracy surface. The range of geometries sampled in reactive trajectories and the timing of bond formation were explored. All trajectories follow the nonlinear approach proposed by Moore and Hoffmann. The reaction of CCl(2) with ethylene is a dynamically concerted reaction, with an average time gap between formation of the two bonds of 50 fs. The reaction of CF(2) with ethylene is dynamically complex with biexponential decay of the diradical species formed from the first bond formation. A general quantitative dynamical classification of cycloaddition mechanisms is proposed, based on the timing of bond formation. PMID:21961828

Xu, Lai; Doubleday, Charles E; Houk, K N

2011-11-01

17

A Three-Step Laboratory Sequence to Prepare a Carbene Complex of Silver(I) Chloride  

ERIC Educational Resources Information Center

We have developed a multistep inorganic synthesis experiment for our second-year undergraduate teaching laboratory that introduces students to modern organometallic chemistry. The ligands are prepared in two simple steps and the preparation of an air-stable silver carbene complex is accomplished in the third step. The students are introduced to…

Canal, John P.; Ramnial, Taramatee; Langlois, Lisa D.; Abernethy, Colin D.; Clyburne, Jason A. C.

2008-01-01

18

Stereoselective [4 + 1] annulation reactions with silyl vinylketenes derived from Fischer carbene complexes.  

PubMed

Stable silyl vinylketenes were prepared via the thermal reaction of Fischer carbene complexes with triisopropylsilyl- or tert-butyldimethylsilyl-substituted alkynes. The ability of these silyl vinylketenes to participate with carbenoid reagents in [4 + 1] annulation reactions was investigated. The best results were obtained with diazomethane and substituted diazomethane reagents, which provided cyclopentenone products in excellent yields and essentially complete stereoselectivity. PMID:16901142

Moser, William H; Feltes, Laura A; Sun, Liangdong; Giese, Matthew W; Farrell, Ryan W

2006-08-18

19

The reactivity game: theoretical predictions for heavy atom tunneling in adamantyl and related carbenes.  

PubMed

The possibility of carbon atom tunneling at cryogenic temperatures for carbene-based ring expansion of adamantane analogues calls for a delicate balance of reactivity to experimentally detect the transpiring reaction. An overly reactive carbene will precipitously decay; an excessively stable carbene will not tunnel. Nevertheless, the factors that affect the quantum-mechanical tunneling (QMT) reactivity - mass, barrier height and width - are strikingly different from the classical "over the barrier" thermal mechanism. Herein, comparisons with experimental values and predictions on measurable rate constants for novel carbene systems are presented by way of small curvature tunneling (SCT) computations. Adamantane, noradamantane and bisnoradamantane have a significantly different C-C bond strain and reactivity, which can be modulated by tinkering with the carbene substituent atom (H, Cl or F) to obtain an observable lifetime of the reactant. The influence of barrier heights and widths, kinetic isotope effects (KIEs), the detection of the tunneling-determining atoms (TDA) and the comparisons with hydrogen-based reactions are discussed with the objective of finding the physical limits for QMT. PMID:24590008

Kozuch, S

2014-05-01

20

Carbenic nitrile imines: properties and reactivity.  

PubMed

Structures and properties of nitrile imines were investigated computationally at B3LYP and CCSD(T) levels. Whereas NBO analysis at the B3LYP DFT level invariably predicts a propargylic electronic structure, CCSD(T) calculations permit a clear distinction between propargylic, allenic, and carbenic structures. Nitrile imines with strong IR absorptions above ca. 2150 cm(–1) have propargylic structures with a CN triple bond (RCNNSiMe3 and R2BCNNBR2), and those with IR absorptions below ca. 2150 cm(–1) are allenic (HCNNH, PhCNNH, and HCNNPh). Nitrile imines lacking significant cumulenic IR absorptions at 1900–2200 cm(–1) are carbenic (R–(C:)–N?N–R?). Electronegative but lone pair-donating groups NR2, OR, and F stabilize the carbenic form of nitrile imines in the same way they stabilize “normal” singlet carbenes, including N-heterocyclic carbenes. NBO analyses at the CCSD(T) level confirm the classification into propargylic, allenic, and carbenic reactivity types. Carbenic nitrile imines are predicted to form azoketenes 21 with CO, to form [2+2] and [2+4] cycloadducts and borane adducts, and to cyclize to 1H-diazirenes of the type 24 in mildly exothermic reactions with activation energies in the range 29–38 kcal/mol. Such reactions will be readily accessible photochemically and thermally, e.g., under the conditions of matrix photolysis and flash vacuum thermolysis. PMID:24400935

Bégué, Didier; Wentrup, Curt

2014-02-01

21

An overview of N-heterocyclic carbenes.  

PubMed

The successful isolation and characterization of an N-heterocyclic carbene in 1991 opened up a new class of organic compounds for investigation. From these beginnings as academic curiosities, N-heterocyclic carbenes today rank among the most powerful tools in organic chemistry, with numerous applications in commercially important processes. Here we provide a concise overview of N-heterocyclic carbenes in modern chemistry, summarizing their general properties and uses and highlighting how these features are being exploited in a selection of pioneering recent studies. PMID:24965649

Hopkinson, Matthew N; Richter, Christian; Schedler, Michael; Glorius, Frank

2014-06-26

22

Carbene and nitrene rearrangements: a theoretical study of cyclic allenes and carbenes, carbodiimides, and azirines.  

PubMed

B3LYP/6-31G(d) calculations of structures, energies, and infrared spectra of several rearrangement products of (hetero)aromatic nitrenes and carbenes are reported. 3-Isoquinolylnitrene 36 ring closes to the azirine 37 prior to ring expansion to the potentially stable but unobserved seven-membered-ring carbodiimide 38 and diazacycloheptatrienylidene C(s)()-39S. A new, stable cycloheptatrienylidene, C(s)()-19S, is located on the naphthylcarbene energy surface. 4-Quinolylnitrene undergoes reaction via the azirine 50 in solution, but ring expansion to the stable seven-membered-ring ketenimine 47 under Ar matrix photolysis conditions. There is excellent agreement between calculated infrared spectra of 1,5-diazacyclohepta-1,2,4,6-tetraene 54 (obtained by photolysis of 4-pyridyl azide), 1-azacyclohepta-1,2,4,6-tetraene 5, 1-azacyclohepta-1,3,5,6-tetraene 55, and 1-azacyclohepta-1,3,4,6-tetraene 56 and the available experimental data. PMID:12467425

Kuhn, Arvid; Vosswinkel, Michael; Wentrup, Curt

2002-12-13

23

Carbene formation upon reactive dissolution of metal oxides in imidazolium ionic liquids.  

PubMed

Metal oxides were found to dissolve in different imidazolium ionic liquids with a hydrogen atom in the C2 position of the imidazolium ring, but not if a methyl substituent was present in the C2 position. The crystal structure of the product that crystallised from an ionic liquid containing dissolved silver(i) oxide showed that this was a silver(i) carbene complex. The presence of carbenes in solution was proven by (13)C NMR spectroscopy and the reactions were also monitored by Raman spectroscopy. The dissolution of other metal oxides, namely copper(ii) oxide, zinc(ii) oxide and nickel(ii) oxide, was also studied in imidazolium ionic liquids and it was found that stable zinc(ii) carbenes were formed in solution, but these did not crystallise under the given experimental conditions. A crystalline nickel(ii) carbene complex could be obtained from a solution of nickel(ii) chloride dissolved in a mixture of 1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium acetate. PMID:24390601

Wellens, Sil; Brooks, Neil R; Thijs, Ben; Meervelt, Luc Van; Binnemans, Koen

2014-03-01

24

Fusing N-heterocyclic carbenes with carborane anions.  

PubMed

Here we describe the fusion of two families of unusual carbon-containing molecules that readily disregard the tendency of carbon to form four chemical bonds, namely N-heterocyclic carbenes (NHCs) and carborane anions. Deprotonation of an anionic imidazolium salt with lithium diisopropylamide at room temperature leads to a mixture of lithium complexes of C-2 and C-5 dianionic NHC constitutional isomers as well as a trianionic (C-2, C-5) adduct. Judicious choice of the base and reaction conditions allows the selective formation of all three stable polyanionic carbenes. In solution, the so-called abnormal C-5 NHC lithium complex slowly isomerizes to the normal C-2 NHC, and the process can be proton-catalyzed by the addition of the anionic imidazolium salt. These results indicate that the combination of two unusual forms of carbon atoms can lead to unexpected chemical behavior, and that this strategy paves the way for the development of a broad new generation of NHC ligands for catalysis. PMID:24664969

El-Hellani, Ahmad; Lavallo, Vincent

2014-04-22

25

A DFT study of the possible role of vinylidene and carbene intermediates in the mechanism of the enzyme acetylene hydratase.  

PubMed

The mode of action of the tungstoprotein, acetylene hydratase, is unresolved at present.We have carried out density functional theory (DFT) calculations to investigate two proposed mechanisms, and also propose a new mechanism based on our calculations. We find that the previously proposed single step mechanisms which involve the attack of a water molecule, catalysed by Asp13, have barriers greater than 40 kcal mol(-1). We propose a new mechanism which involves two tungsten complexes, a vinylidene and a carbene, as stable intermediates. We calculate the potential energy surfaces for their formation, and for the decomposition of the carbene complex to yield acetaldehyde. All the barriers along the pathway are less than 30 kcal mol(-1), except for the barrier to carbene formation which is 34 kcal mol(-1). PMID:20369217

Vincent, Mark A; Hillier, Ian H; Periyasamy, Ganga; Burton, Neil A

2010-04-28

26

Laser-induced carbene-carbene rearrangement in solution: the diphenylcarbene-fluorene rearrangement.  

PubMed

Diphenylcarbene (DPC) generated by high-intensity laser photolysis of diphenyldiazomethane rearranges to fluorene (FL) by two distinct mechanisms as revealed by methyl-group labeling. Thus, excimer laser irradiation of p,p'-dimethyldiphenyldiazomethane generates 3,6-dimethylfluorene (3,6-DMF) and 2,7-dimethylfluorene (2,7-DMF), which were identified by fluorescence measurements as well as GC-MS and comparison with authentic materials. 3,6-DMF corresponds to direct bond formation between ortho positions in DPC, referred to as ortho,ortho' coupling. 2,7-DMF corresponds to a carbene-carbene rearrangement, whereby DPC undergoes ring expansion to phenylcycloheptatetraene (PhCHT) followed by ring contraction to o-biphenylylcarbene (o-BPC), which then cyclizes to FL. The carbene-carbene rearrangement dominates over the ortho,ortho' coupling under all conditions employed. The ortho,ortho' coupling must take place in a higher excited state (most likely S2 or T1) of DPC, because it is not observed at all under thermolysis conditions, where only S1 and T0 are populated. The carbene-carbene rearrangement may take place either in a hot S1 state or more likely in a higher excited state (S2 or T1). PMID:23941300

Régimbald-Krnel, Michèle J; Wentrup, Curt

2013-09-01

27

Isolation of a neutral boron-containing radical stabilized by a cyclic (alkyl)(amino)carbene.  

PubMed

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species. PMID:24917474

Bissinger, Philipp; Braunschweig, Holger; Damme, Alexander; Krummenacher, Ivo; Phukan, Ashwini K; Radacki, Krzysztof; Sugawara, Shun

2014-07-01

28

Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene  

PubMed Central

Two 13C-labeled isomers of the formal Diels?Alder adduct of acetylmethyloxirene to tetramethyl 1,2,4,5-benzenetetracarboxylate have been synthesized. Flash vacuum thermolysis of these adducts leads to various isotopic isomers of acetylmethylketene, the ratios of which have been determined by NMR. The surprising finding that the principal product comes from methylpyruvoyl carbene rather than its more stable isomer diacetylcarbene is explained by MPWB1K density functional calculations, which show that the reactant probably undergoes a unimolecular rearrangement to a norcaradiene derivative prior to its fragmentation. Coupled-cluster calculations on the methylpyruvoyl carbene show that it is capable of undergoing three unimolecular isomerizations. The fastest is 1,2-acetyl migration to give acetylmethylketene directly. The next is rearrangement via acetylmethyloxirene to diacetylcarbene and thence by Wolff rearrangement to acetylmethylketene. The least-favorable reaction is degenerate rearrangement via 1,3-dimethyl-2-oxabicyclo[1.1.0]butan-4-one (the epoxide of dimethylcyclopropenone). The combined experimental and computational results indicate that Wolff rearrangement of the diacetylcarbene occurs with a 2.5:1 ratio of the methyl groups despite the fact that they are related by a twofold axis of symmetry in the carbene. Preliminary molecular dynamics simulations are consistent with this conclusion. Taken together, the results suggest that the Wolff rearrangement is subject to the same kind of nonstatistical dynamical effects detected for other kinds of thermally generated reactive intermediates.

2008-01-01

29

Explicit role of dynamical and nondynamical electron correlation on singlet triplet splitting in carbenes  

NASA Astrophysics Data System (ADS)

Density functional theoretical studies have been performed on carbene systems to determine the singlet-triplet splitting and also to explore the role of electron correlation. Using an approximate method of separation of dynamical and nondynamical correlation, it is found that dynamical and nondynamical electron correlation stabilizes the singlet state relative to the triplet for halo carbenes in both BLYP and B3LYP methods. Calculations performed on higher homologues of methylene suggest that beyond CH(CH 3), both the electron correlations have leveling effect in stabilizing the singlet state relative to the triplet. It has also been observed while dynamical electron correlation fails to provide any substantial degree of stabilization to the singlet states of higher homologues of methylene in B3LYP method, an opposite trend is observed for nondynamical counterpart. Among the larger systems studied, (9-triptycyl)(?-naphthyl)-carbene has the highest stability of the triplet state whereas bis-imidazol-2-ylidenes has the most stable singlet state. Interestingly, the values of the dynamical electron correlation for each state of each system studied are different for the two methods used. The reason behind this apparent discrepancy lies in the fact that the coefficients of the LYP part in B3LYP and BLYP functionals are different.

Seal, Prasenjit; Chakrabarti, Swapan

2007-02-01

30

Protonation of Homoenolate Equivalents Generated by N-Heterocyclic Carbenes  

PubMed Central

Homoenolate equivalents are generated by Lewis basic N-heterocyclic carbene catalysts and then protonated to generate efficiently saturated esters from unsaturated aldehydes. This reactivity is extended to the generation of ?-acylvinyl anions from alkynyl aldehydes. The asymmetric protonation of a homoenolate equivalent generated from a ?,?-disubstituted aldehyde can be accomplished with a chiral N-heterocyclic carbene.

Maki, Brooks E.; Chan, Audrey; Scheidt, Karl A.

2010-01-01

31

1,3-Dimethylimidazolin-2-ylidene carbene donor ligation in lanthanide silylamide complexes  

Microsoft Academic Search

The synthesis and X-ray crystal structures of Y[N(SiHMeâ)â]â(carbene)â (carbene = 1,3-dimethylimidazolin-2-ylidene, x = 1, 2) are described. The donor capability of the strongly nucleophilic carbene ligand is expressed in both the displacement of two THF ligands by one carbene ligand in precursor compound Y[N(SiHMeâ)â]â(THF)â and by the addition of a second carbene ligand to yield the preferred (3 + 2)

Wolfgang A. Herrmann; Florian C. Munck; O. Runte; G. R. J. Artus; Reiner Anwander

1997-01-01

32

N-Heterocyclic Carbene-Catalyzed Conjugate Additions of Alcohols  

PubMed Central

An efficient intermolecular conjugate addition of alcohols to activated alkenes catalyzed by N-heterocyclic carbenes has been developed. With 5 mol % of the free carbene derived from IMes•HCl, unsaturated ketones and esters are competent substrates and a variety of primary and secondary alcohols can be employed as the nucleophile. No oligomerization is observed under these mild conditions for effective hydroalkoxylation. In addition to reactions with activated alkenes, IMes catalyzes the formation of vinyl ethers through the 1,4-addition of alcohols to ynones and promotes tandem conjugate addition/Michael cascade reactions. Preliminary data supports a Brønsted base mechanism with the free carbene.

Phillips, Eric M.; Riedrich, Matthias; Scheidt, Karl A.

2010-01-01

33

Fullerene-carbene Lewis acid-base adducts.  

PubMed

The reaction between a bulky N-heterocylic carbene (NHC) and C(60) leads to the formation of a thermally stable zwitterionic Lewis acid-base adduct that is connected via a C-C single bond. Low-energy absorption bands with weak oscillator strengths similar to those of n-doped fullerenes were observed for the product, consistent with a net transfer of electron density to the C(60) core. Corroborating information was obtained using UV photoelectron spectroscopy, which revealed that the adduct has an ionization potential ?1.5 eV lower than that of C(60). Density functional theory calculations showed that the C-C bond is polarized, with a total charge of +0.84e located on the NHC framework and -0.84e delocalized on the C(60) cage. The combination of reactivity, characterization, and theoretical studies demonstrates that fullerenes can behave as Lewis acids that react with C-based Lewis bases and that the overall process describes n-doping via C-C bond formation. PMID:21761908

Li, Huaping; Risko, Chad; Seo, Jung Hwa; Campbell, Casey; Wu, Guang; Brédas, Jean-Luc; Bazan, Guillermo C

2011-08-17

34

Carbene radicals in cobalt(II)-porphyrin-catalysed carbene carbonylation reactions; a catalytic approach to ketenes.  

PubMed

One-pot radicals: Cobalt(III)-carbene radicals, generated by metallo-radical activation of diazo compounds and N-tosylhydrazone sodium salts with cobalt(II) complexes of porphyrins, readily undergo radical addition to carbon monoxide, allowing the catalytic production of ketenes. These ketenes subsequently react with various amines, alcohols and imines in one-pot tandem transformations to produce differently substituted amides, esters and ?-lactams in good isolated yields. PMID:24038393

Paul, Nanda D; Chirila, Andrei; Lu, Hongjian; Zhang, X Peter; de Bruin, Bas

2013-09-23

35

New homoleptic carbene transfer ligands and related coinage metal complexes  

Microsoft Academic Search

The synthesis, spectroscopic and structural characterization of the carbene ligand precursor bis(4-benzyl-1,2,4-triazol-4-ium-1-yl)dihydroborate has been described. Treatment with Ag2O leads to the silver(I) complex of the new monoanionic bis(4-benzyl-1,2,4-triazol-5-ylidene-1-yl)dihydroborate dicarbene ligand. The resulting dimetallacycle carbene complex has been used to synthesize the homoleptic gold(I) complex by transmetallation reaction.

Grazia Papini; Giuliano Bandoli; Alessandro Dolmella; Giancarlo Gioia Lobbia; Maura Pellei; Carlo Santini

2008-01-01

36

Fundamental spectroscopic studies of carbenes and hydrocarbon radicals  

SciTech Connect

This document details activities during this reporting period topics discussed are: The first spectroscopic identification of the HCCCO and DCCCO radicals; detection of new vibrationally excited states of the carbon chain radicals CCH and CCD and the three-membered carbene ring, cyclopropenylidene; determination of an accurate structure of the cumulene carbene H{sub 2}CCC; analysis the hyperfine structure in the SiC radical; and the undertaking of a systematic search for new sulfur bearing radicals.

Thaddeus, P.; Gottlieb, C.

1992-05-01

37

N Heterocyclic Carbenes: Organocatalysts Displaying Diverse Modes of Action  

Microsoft Academic Search

Within the context of Lewis base catalysis N-heterocyclic carbenes represent an extremely versatile class of organocatalyst that allows for a great variety of different\\u000a transformations. Starting from the early investigations on benzoin, and later Stetter reactions, the mechanistic diversity\\u000a of N-heterocyclic carbenes, depending on their properties, has led to the development of several unprecedented catalytic reactions.\\u000a This article will provide an

K. Zeitler

38

Rhodium-catalyzed carbene transfer to alkynes via 2-furylcarbenes generated from enynones.  

PubMed

A rhodium-catalyzed carbene transfer to alkynes is reported. Easily available enynones served as the carbene source replacing diazocompounds. The overall process involved a cyclization-cyclopropenation-ring enlargement sequence. PMID:24175321

González, María J; López, Enol; Vicente, Rubén

2014-05-25

39

N-Heterocyclic Carbene Complexes in Industrial Processes  

NASA Astrophysics Data System (ADS)

N-Heterocyclic carbene complexes produced on industrial scale are presented in this chapter along with a discussion about their production. Details of processes employing NHC complexes on pilot to industrial scales are discussed. These are frequently oriented towards the synthesis of biologically active molecules, however, examples are given for rubber formation and for 1-octene synthesis, a comonomer for polyethylene synthesis.

Briel, Oliver; Cazin, Catherine S. J.

40

Unexpected ring expansion of an enantiopure imidazoline carbene ligand.  

PubMed

[reaction: see text] We report an unexpected ring expansion reaction of an enantiopure fenchone-derived imidazolinium salt during attempts to form copper complexes of the corresponding imidazoline carbene ligand. A N,N'-difenchyl piperazinone was formed in low yield together with the difenchyl-substituted five-membered urea. PMID:16805549

Pelegrí, Adela Sanchez; Elsegood, Mark R J; McKee, Vickie; Weaver, George W

2006-07-01

41

Reactivities of Metal-Carbenes Toward Alkenes and Alkynes.  

National Technical Information Service (NTIS)

Stabilized metal-carbenes like the P's in eqs 2 and 3 are very much more reactive toward phenylacetylene than toward cyclopentene (by a factor of 119-170) or cyclooctene (by a factor > 10 to the 4th power). The result is strange but accords with 1 (also a...

T. J. Katz E. B. Savage S. J. Lee M. Nair

1980-01-01

42

Characterization of a didehydrobenzoxazine intermediate in a novel carbene-to-carbene transformation.  

PubMed

Irradiation of N(2) matrix-isolated 3-chloro-3-(2-benzoxazolyl)diazirine gives a mixture of syn- and anti-benzoxazolylchlorocarbenes which could be characterized by IR, UV/vis, and B3LYP modeling. Subsequent irradiation of the carbene caused ring opening to the corresponding quinoimine, which was similarly characterized. In turn, the quinoimine photochemically underwent ring-closure to a novel, highly strained cyclic ketenimine. Unrestricted singlet DFT calculations were required to fit the IR spectrum of the ketenimine, suggesting significant diradical character. Photolysis of the ketenimine led to ring cleavage in yet another fashion, to give an isonitrile-phenoxychlorocarbene, whose spectra were similar to those of the previously characterized phenoxychlorocarbene. Finally, at shorter wavelengths, the phenoxycarbene underwent the expected rearrangement to the corresponding benzoyl chloride. PMID:12083916

Nikitina, Asya; Sheridan, Robert S

2002-07-01

43

1,6-Carbene transfer: gold-catalyzed oxidative diyne cyclizations.  

PubMed

In the presence of a gold catalyst an unprecedented oxidative cyclization of diynes takes place. The reaction cascade is initiated by an oxygen transfer from a N-oxide onto a gold-activated alkyne. The formed ?-oxo carbene is transferred across the second alkyne yielding a stabilized vinyl carbene/cation. Alkyl migration or sp(3)-CH insertion then terminates the catalytic cycle by formation of highly substituted functionalized indenones. A 1,6-carbene shift could be supported by the oxidation of the vinyl carbene. This protocol represents an attractive alternative to procedures which are based on the metal-catalyzed decomposition of hazardous, not easily accessible, diazo compounds. PMID:24050384

Nösel, Pascal; dos Santos Comprido, Laura Nunes; Lauterbach, Tobias; Rudolph, Matthias; Rominger, Frank; Hashmi, A Stephen K

2013-10-16

44

Redox behaviour of cymantrene Fischer carbene complexes in designing organometallic multi-tags.  

PubMed

A series of Group?7 Fischer carbene complexes, [Cp(CO)2 Mn(I) =C(OEt)Ar] (Cp=cyclopentadienyl, Ar=Th=thienyl (1?a), Ar=Fu=furyl (2?a), Ar=Fc=ferrocenyl (3?a)) and biscarbene complexes, [Cp(CO)2 Mn?C(OEt)?Ar'?(OEt)C?Mn(CO)2 Cp] (Ar'=Th'=2,5-thienylene (1?b), Ar'=Fu'=2,5-furylene (2?b), Ar'=Fc'=1,1'-ferrocendiyl (3?b)) was synthesized and characterized. Chemical oxidation of [Cp(CO)2 Mn?C(OEt)Fc] (3?a) and isolation of the oxidised species [3?a][PF6 ] possessing a Mn(II) centre proved possible below -30?°C in dichloromethane solution. The ESR spectrum of the transiently stable radical cation, [3?a][PF6 ], confirmed the presence of a low-spin Mn(II) centre characterized by a rhombic g tensor (gx =1.975, gy =2.007 and gz =2.130) in frozen dichloromethane at 77?K with (55) ?Mn hyperfine coupling constants A1 , A2 and A3 of 115, 33 and 43?G, respectively. Electrochemical studies demonstrated the influence of the Ar substituent on the oxidation potential. All complexes showed that the redox potentials of carbene double bond reduction and Mn(I) oxidation were dependent on the type of Ar group, but only 3?b showed resolved oxidations for the two Mn(I) centres. Surprisingly, Mn(I) oxidation occurs at lower potentials than ferrocenyl oxidation. Density functional theory (DFT) calculations were carried out to delineate the nature of the species involved in the oxidation and reduction processes and clearly confirm that oxidation of Mn(I) is favoured over that of ferrocene. PMID:24644017

Bezuidenhout, Daniela I; van der Westhuizen, Belinda; Swarts, Pieter J; Chatturgoon, Teshica; Munro, Orde Q; Fernández, Israel; Swarts, Jannie C

2014-04-22

45

A bonding model for gold(I) carbene complexes  

PubMed Central

The last decade has witnessed dramatic growth in the number of reactions catalyzed by electrophilic gold complexes. While proposed mechanisms often invoke the intermediacy of gold-stabilized cationic species, the nature of bonding in these intermediates remains unclear. Herein, we propose that the carbon-gold bond in these intermediates is comprised of varying degrees of both ? and ?-bonding; however, the overall bond order is generally less than or equal to unity. The bonding in a given gold-stabilized intermediate, and the position of this intermediate on a continuum ranging from gold-stabilized singlet carbene to gold-coordinated carbocation, is dictated by the carbene substituents and the ancillary ligand. Experiments show that the correlation between bonding and reactivity is reflected in the yield of gold-catalyzed cyclopropanation reactions.

Benitez, Diego; Shapiro, Nathan D.; Tkatchouk, Ekaterina; Wang, Yiming; Goddard, William A.; Toste, F. Dean

2009-01-01

46

Deprotonation of a Borohydride: Synthesis of a Carbene-Stabilized Boryl Anion**  

PubMed Central

An acidic hydride! Thanks to the presence of a ?-acceptor cyclic (alkyl)(amino)carbene (CAAC) and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center.

Ruiz, David A.; Ung, Gael; Melaimi, Mohand

2013-01-01

47

Ruthenium catalysts for hydrogenation of aromatic and aliphatic esters: make use of bidentate carbene ligands.  

PubMed

Committed carbenes: The convenient application of bidentate carbene ligands is described for the hydrogenation of carboxylic acid esters. The ligand precursors are easily synthesized through the dimerization of N-substituted imidazoles with diiodomethane. The catalyst is generated in situ and exhibits good activity and functional group tolerance for the hydrogenation of aromatic and aliphatic carboxylic acid esters. PMID:23640857

Westerhaus, Felix A; Wendt, Bianca; Dumrath, Andreas; Wienhöfer, Gerrit; Junge, Kathrin; Beller, Matthias

2013-06-01

48

The Depolymerization of Poly(Ethylene Terephthalate) (PET) Using N-Heterocyclic Carbenes from Ionic Liquids  

ERIC Educational Resources Information Center

The depolymerization of the plastic polyethylene terephthalate (PET or PETE) is described in this laboratory procedure. The transesterification reaction used to depolymerize PET employs a highly efficient N-heterocyclic carbene catalyst derived from a commercially available imidazolium ionic liquid. N-heterocyclic carbenes are potent nucleophilic…

Kamber, Nahrain E.; Tsujii, Yasuhito; Keets, Kate; Waymouth, Robert M.; Pratt, Russell C.; Nyce, Gregory W.; Hedrick, James L.

2010-01-01

49

First Astronomical Detection of the Carbene Chain H2C6  

NASA Technical Reports Server (NTRS)

The cumulene carbenes are important components of hydrocarbon chemistry in low mass star forming cores. Here we report the first astronomical detection of the long chain cumulene carbene H(sub 2)C(sub 6), in the insterstellar cloud TMC1, from observations of two of its rotational transitions:...

Langer, W. D.; Velusamy, T.; Kuiper, T. B. H.; McCarthy, M. C.; Travers, M. J.; Kovacs, A.; Gottlieb, C. A.; Thaddeus, P.

1996-01-01

50

Synthesis, Characterization, and Reactivity of a Uranium(VI) Carbene Imido Oxo Complex.  

PubMed

We report the uranium(VI) carbene imido oxo complex [U(BIPM(TMS) )(NMes)(O)(DMAP)2 ] (5, BIPM(TMS) =C(PPh2 NSiMe3 )2 ; Mes=2,4,6-Me3 C6 H2 ; DMAP=4-(dimethylamino)pyridine) which exhibits the unprecedented arrangement of three formal multiply bonded ligands to one metal center where the coordinated heteroatoms derive from different element groups. This complex was prepared by incorporation of carbene, imido, and then oxo groups at the uranium center by salt elimination, protonolysis, and two-electron oxidation, respectively. The oxo and imido groups adopt axial positions in a T-shaped motif with respect to the carbene, which is consistent with an inverse trans-influence. Complex 5 reacts with tert-butylisocyanate at the imido rather than carbene group to afford the uranyl(VI) carbene complex [U(BIPM(TMS) )(O)2 (DMAP)2 ] (6). PMID:24842784

Lu, Erli; Cooper, Oliver J; McMaster, Jonathan; Tuna, Floriana; McInnes, Eric J L; Lewis, William; Blake, Alexander J; Liddle, Stephen T

2014-06-23

51

Synthesis and reactivity of some tungsten carbene complexes  

SciTech Connect

Dithiocarbene tungsten complexes, W(CO)/sub 5/(C(SR)(SR')), were prepared in good yield. The substituted tetracarbonyl dimethyldithiocarbene tungsten complexes are prepared by an analogous route. Cyclic dithiocarbene tungsten complexes may be prepared in a similar manner. Isocyanide complexes are formed when W(CO)/sub 5/(C(SCH/sub 3/)/sub 2/) reacts with primary amines. Net effect of the reaction of tungsten dithiocarbene complexes with tertiary phosphine is to transfer the carbene group from the tungsten to the phosphorus atom.

Pickering, R.A.

1981-02-01

52

How Stable Is Stable?  

ERIC Educational Resources Information Center

Provides a problem where students are asked to find the point at which a soda can floating in some liquid changes its equilibrium between stable and unstable as the soda is removed from the can. Requires use of Newton's first law, center of mass, Archimedes' principle, stable and unstable equilibrium, and buoyant force position. (MVL)

Baehr, Marie

1994-01-01

53

Actinide metals with multiple bonds to carbon: synthesis, characterization, and reactivity of U(IV) and Th(IV) bis(iminophosphorano)methandiide pincer carbene complexes.  

PubMed

Treatment of ThCl(4)(DME)(2) or UCl(4) with 1 equiv of dilithiumbis(iminophosphorano) methandiide, [Li(2)C(Ph(2)P?NSiMe(3))(2)] (1), afforded the chloro actinide carbene complexes [Cl(2)M(C(Ph(2)P?NSiMe(3))(2))] (2 (M = Th) and 3 (M = U)) in situ. Stable PCP metal-carbene complexes [Cp(2)Th(C(Ph(2)P?NSiMe(3))(2))] (4), [Cp(2)U(C(Ph(2)P?NSiMe(3))(2))] (5), [TpTh(C(Ph(2)P?NSiMe(3))(2))Cl] (6), and [TpU(C(Ph(2)P?NSiMe(3))(2))Cl] (7) were generated from 2 or 3 by further reaction with 2 equiv of thallium(I) cyclopentadienide (CpTl) in THF to yield 4 or 5 or with 1 equiv of potassium hydrotris(pyrazol-1-yl) borate (TpK) also in THF to give 6 or 7, respectively. The derivative complexes were isolated, and their crystal structures were determined by X-ray diffraction. All of these U (or Th)-carbene complexes (4-7) possess a very short M (Th or U)?carbene bond with evidence for multiple bond character. Gaussian 03 DFT calculations indicate that the M?C double bond is constructed by interaction of the 5f and 6d orbitals of the actinide metal with carbene 2p orbitals of both ? and ? character. Complex 3 reacted with acetonitrile or benzonitrile to cyclo-add C?N to the U?carbon double bond, thereby forming a new C-C bond in a new chelated quadridentate ligand in the bridged dimetallic complexes (9 and 10). A single carbon-U bond is retained. The newly coordinated uranium complex dimerizes with one equivalent of unconverted 3 using two chlorides and the newly formed imine derived from the nitrile as three connecting bridges. In addition, a new crystal structure of [CpUCl(3)(THF)(2)] (8) was determined by X-ray diffraction. PMID:21667978

Ma, Guibin; Ferguson, Michael J; McDonald, Robert; Cavell, Ronald G

2011-07-18

54

Activation of water, ammonia, and other small molecules by PC(carbene)P nickel pincer complexes.  

PubMed

Nickel complexes of a PC(carbene)P pincer ligand framework are described. Dehydrobromination of the precursor (PC(sp)(3)P)NiBr in the presence of a donor (PPh3 or NC(t)Bu) leads to the title complexes, which feature a rare nickel-carbene linkage as the pincer ligand anchor point. This strongly donating, nucleophilic carbene center engages in a variety of E-H bond activations (E = H, C, N, O), some of which are reversible. This represents a new mode of bond activation by ligand cooperativity in nickel pincer complexes. PMID:23906261

Gutsulyak, Dmitry V; Piers, Warren E; Borau-Garcia, Javier; Parvez, Masood

2013-08-14

55

Experimental mechanistic studies of the tail-to-tail dimerization of methyl methacrylate catalyzed by N-heterocyclic carbene.  

PubMed

We and others have previously reported the intermolecular umpolung reactions of Michael acceptors catalyzed by an N-heterocyclic carbene (NHC). The representative tail-to-tail dimerization of methyl methacrylate (MMA) has now been intensively investigated, leading to the following conclusions: (1) The catalysis involves the deoxy-Breslow intermediate, which is quite stable and remains active after the catalysis. (2) Addition of the intermediate to MMA and the final catalyst elimination are the rate-limiting steps. Addition of the NHC to MMA and the proton transfers are relatively very rapid. (3) The two alkenyl protons of the first MMA undergo an intermolecular transfer to C3 and C5 of the dimer. (4) The initial proton transfer is intermolecular. (5) Compared with the benzoin condensation, noticeable differences in the kinetics, reversibility, and stability of the intermediates are observed. PMID:23941561

Kato, Terumasa; Ota, Yoshiya; Matsuoka, Shin-ichi; Takagi, Koji; Suzuki, Masato

2013-09-01

56

Magnesium and zinc complexes of functionalised, saturated N-heterocyclic carbene ligands: carbene lability and functionalisation, and lactide polymerisation catalysis.  

PubMed

The synthesis of magnesium and zinc complexes of bidentate anionic alkoxide ligands with saturated-backbone carbene groups is reported. Mono(ligand) and bis(ligand) complexes [M(L(R))N''](2) and [M(L(R))(2)] (M = Mg, Zn, N'' = N(SiMe(3))(2), L(R) = [OCMe(2)CH(2){CNCH(2)CH(2)NR}] R = (i)Pr, Mes, Dipp) have been isolated, and some structurally characterised and compared with the new unsaturated carbene complex [Mg(L)(2)]. Reactions with silyl halides show either addition across the metal carbene bond, or across the metal alkoxide bond, in accordance with the metals' electronegativity difference: the metal alkoxide bonds are stronger for Mg(II) complexes, for which the carbene is silylated to form zwitterionic [MgI(Me(3)SiL(R))N''] (Me(3)SiL(R) = OCMe(2)CH(2){Me(3)SiCNCH(2)CH(2)NR}) while the metal-bound alkoxide group is silylated in the Zn(II) complexes forming [ZnI(Me(3)SiOL(R))N''] (Me(3)SiOL(R) = Me(3)SiOCMe(2)CH(2){CNCH(2)CH(2)NR}). The proligand [HL(R)] is silylated at the alcohol group, forming the iodide salt [Me(3)SiOCMe(2)CH(2){HCNCH(2)CH(2)NR}]I.Preliminary results on the use of these complexes as initiators for the polymerisation of rac-lactide are reported, and suggest different initiation mechanisms are occurring for the two metals, in agreement with the different silylation reactivity observed. The polymerisation reactions are facile at room temperature even without an initiator, and yield polymers of reasonable molecular weight and heterotacticity and with good PDI. These are the first magnesium NHC complexes demonstrated to effect lactide polymerisation.Also, an adduct instead of the anticipated potassium alkoxycarbene is generated from the reaction of the proligand [HL(R)] with potassium amide KN''; this has been structurally characterised. PMID:20449168

Arnold, Polly L; Casely, Ian J; Turner, Zoë R; Bellabarba, Ronan; Tooze, Robert B

2009-09-21

57

Nitrene-carbene-carbene rearrangement. Photolysis and thermolysis of tetrazolo[5,1-a]phthalazine with formation of 1-phthalazinylnitrene, o-cyanophenylcarbene, and phenylcyanocarbene.  

PubMed

1-Azidophthalazine 9A is generated in trace amount by mild FVT of tetrazolo[5,1-a]phthalazine 9T and is observable by its absorption at 2121 cm(-1) in the Ar matrix IR spectrum. Ar matrix photolysis of 9T/9A at 254 nm causes ring opening to generate two conformers of (o-cyanophenyl)diazomethane 11 (2079 and 2075 cm(-1)), followed by (o-cyanophenyl)carbene (3)12, cyanocycloheptatetraene 13, and finally cyano(phenyl)carbene (3)14 as evaluated by IR spectroscopy. The two carbenes (3)12 and (3)14 were observed by ESR spectroscopy (D|hc = 0.5078, E|hc = 0.0236 and D|hc = 0.6488, E|hc = 0.0195 cm(-1), respectively). The rearrangement of 12 ? 13 ? 14 constitutes a carbene-carbene rearrangement. 1-Phthalazinylnitrene (3)10 is observed by means of its UV-vis spectrum in Ar matrix following FVT of 9 above 550 °C. Rearrangement to cyanophenylcarbenes also takes place on FVT of 9 as evidenced by observation of the products of ring contraction, viz., fulvenallenes and ethynylcyclopentadienes 16-18. Thus the overall rearrangement 10 ? 11 ? 12 ? 13 ? 14 can be formulated. PMID:24304417

Høj, Martin; Kvaskoff, David; Wentrup, Curt

2014-01-01

58

N-heterocyclic carbene-catalyzed highly enantioselective synthesis of substituted dihydropyranopyrazolones.  

PubMed

Highly optically pure multisubstituted dihydropyrano[2,3-c]pyrazol-6-(1H)-ones were synthesized via the N-heterocyclic carbene-catalyzed enantioselective [4 + 2] annulation of ?-chloroaldehydes and pyrazolone-derived oxodienes. PMID:23959516

Zhang, Han-Ming; Lv, Hui; Ye, Song

2013-10-01

59

Arylation of ?-substituted acrylates in ionic liquids catalyzed by a Pd–benzothiazole carbene complex  

Microsoft Academic Search

A Pd-catalyst with benzothiazole carbene as ligands allows, in tetrabutylammonium bromide melt as solvent, very fast and efficient reactions of bromoaromatics with 3-hydroxy-2-methylenealkanoates to give ?-arylketones.

Vincenzo Calò; Angelo Nacci; Luigi Lopez; Annalisa Napola

2001-01-01

60

N-heterocyclic carbene catalyzed nucleophilic substitution reaction for construction of benzopyrones and benzofuranones.  

PubMed

N-Heterocyclic carbene as an efficient organic catalyst was employed to catalyze an intramolecular nucleophilic substitution reaction. When R(2) was a phenyl group, the cyclization process underwent isomerization, leading to generation of benzofuranone. PMID:16986969

He, Jinmei; Zheng, Jiyue; Liu, Jian; She, Xuegong; Pan, Xinfu

2006-09-28

61

Electrophilic C(2)-functionalization of nitronyl nitroxides: a reference to N-heterocyclic carbenes  

Microsoft Academic Search

In an attempt to generalize the recently introduced concept of ‘auto-umpolung’, nitronyl nitroxides are identified as precursors of a special class of electron-excess carbenes. The latter can be formally derived from nucleophilic carbenes of the Wanzlick–Arduengo type by replacing redox-inactive N-substituents by lone-pair donor functions. A first systematic access to this class of compounds is achieved by lithiating the nitronyl

Robert Weiss; Norbert Kraut; Frank Hampel

2001-01-01

62

Metal and carbene organocatalytic relay activation of alkynes for stereoselective reactions.  

PubMed

Transition metal and organic catalysts have established their own domains of excellence. It has been expected that merging the two unique domains should provide complimentary or unprecedented opportunities in converting simple raw materials to functional products. N-heterocyclic carbenes alone are excellent organocatalysts. When used with transition metals such as copper, N-heterocyclic carbenes are routinely practiced as strong-coordinating ligands. Combination of an N-heterocyclic carbene and copper therefore typically leads to deactivation of either or both of the two catalysts. Here we disclose the direct merge of copper as a metal catalyst and N-heterocyclic carbenes as an organocatalyst for relay activation of alkynes. The reaction involves copper-catalysed activation of alkynes to generate ketenimine intermediates that are subsequently activated by an N-heterocyclic carbene organocatalyst for stereoselective reactions. Each of the two catalysts (copper metal catalyst and N-heterocyclic carbene organocatalyst) accomplishes its own missions in the activation steps without quenching each other. PMID:24865392

Namitharan, Kayambu; Zhu, Tingshun; Cheng, Jiajia; Zheng, Pengcheng; Li, Xiangyang; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2014-01-01

63

Nitrenes, carbenes, diradicals, and ylides. Interconversions of reactive intermediates.  

PubMed

Rearrangements of aromatic and heteroaromatic nitrenes and carbenes can be initiated with either heat or light. The thermal reaction is typically induced by flash vacuum thermolysis, with isolation of the products at low temperatures. Photochemical experiments are conducted either under matrix isolation conditions or in solution at ambient temperature. These rearrangements are usually initiated by ring expansion of the nitrene or carbene to a seven-membered ring ketenimine, carbodiimide, or allene (that is, a cycloheptatetraene or an azacycloheptatetraene when a nitrogen is involved). Over the last few years, we have found that two types of ring opening take place as well. Type I is an ylidic ring opening that yields nitrile ylides or diazo compounds as transient intermediates. Type II ring opening produces either dienylnitrenes (for example, from 2-pyridylnitrenes) or 1,7-(1,5)-diradicals (such as those formed from 2-quinoxalinylnitrenes), depending on which of these species is better stabilized by resonance. In this Account, we describe our achievements in elucidating the nature of the ring-opened species and unraveling the connections between the various reactive intermediates. Both of these ring-opening reactions are found, at least in some cases, to dominate the subsequent chemistry. Examples include the formation of ring-opened ketenimines and carbodiimides, as well as the ring contraction reactions that form five-membered ring nitriles (such as 2- and 3-cyanopyrroles from pyridylnitrenes, N-cyanoimidazoles from 2-pyrazinyl and 4-pyrimidinylnitrenes, N-cyanopyrazoles from 2-pyrimidinylnitrenes and 3-pyridazinylnitrenes, and so forth). The mechanisms of formation of the open-chain and ring-contraction products were unknown at the onset of this study. In the course of our investigation, several reactions with three or more consecutive reactive intermediates have been unraveled, such as nitrene, seven-membered cyclic carbodiimide, and open-chain nitrile ylide. It has been possible in some cases to observe them all and determine their interrelationships by means of a combination of matrix-isolation spectroscopy, photochemistry, flash vacuum thermolysis, and computational chemistry. These studies have led to a deeper understanding of the nature of reactive intermediates and chemical reactivity. Moreover, the results indicate new directions for further exploration: ring-opening reactions of carbenes, nitrenes, and cyclic cumulenes can be expected in many other systems. PMID:21452850

Wentrup, Curt

2011-06-21

64

Synthesis of the first radiolabeled (188) Re N-heterocyclic carbene complex and initial studies on its potential use in radiopharmaceutical applications.  

PubMed

A novel approach towards the synthesis of radiolabeled organometallic rhenium complexes is presented. We successfully synthesized and analyzed the first (188) Re-labeled N-heterocyclic biscarbene complex, trans-dioxobis(1,1'-methylene-bis(3,3'-diisopropylimidazolium-2-ylidene))(188) rhenium(V) hexafluorophosphate ((188) Re-4) via transmetalation using an air-stable and moisture-stable silver(I) biscarbene complex. In order to assess the viability of this complex as a potential lead structure for in vivo applications, the stability of the (188) Re-NHC complex was tested in physiologically relevant media. Ultimately, our studies illustrate that the complex we synthesized dissociates rapidly and is therefore unsuitable for use in radiopharmaceuticals. However, it is clear that the transmetalation approach we have developed is a rapid, robust, and mild method for the synthesis of new (188) Re-labeled carbene complexes. Copyright © 2014 John Wiley & Sons, Ltd. PMID:24889257

Wagner, Thomas; Zeglis, Brian M; Groveman, Sam; Hille, Claudia; Pöthig, Alexander; Francesconi, Lynn C; Herrmann, Wolfgang A; Kühn, Fritz E; Reiner, Thomas

2014-06-15

65

Probing the nature and extent of stabilization within foiled carbenes: homoallylic participation by a neighboring cyclopropane ring.  

PubMed

Oxadiazoline 6 was synthesized to generate endo-tricyclo[3.2.1.0(2,4)]octan-8-ylidene (3) by either photolysis or thermolysis. Diastereomer 6a thermally decomposed twice as fast as 6b. Carbene 3 was trapped stereoselectively by acrylonitrile and diethylamine in high yields. It behaved as a nucleophilic carbene with electron-poor alkenes, like acrylonitrile, but as an electrophile with very electron-rich species, such as diethylamine. However, when the reactions were performed in cyclohexane and cyclohexene, isomerization of 3 was favored. Replacement of the double bond in 7-norbornenylidene (1) by the single bond in the endo-fused cyclopropane unit of carbene 3 led to similar outcomes. Carbene 3 rightfully belongs to the family of foiled carbenes. PMID:23586674

Apeland, Ingrid Malene; Kählig, Hanspeter; Lorbeer, Eberhard; Brinker, Udo H

2013-05-17

66

Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes.  

PubMed

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:Co(II)(?-Cl)Cl)2 (1) with KC8. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2(•-) and 2(•+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me2-cAAC:)2Co2](•+)OTf(-) (2(•+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(•+) reveals the coupling of the electron spin with 2 equiv (59)Co isotopes, leading to a (Co(0.5))2 state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2(•+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(•+)OTf(-) possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong ?-accepting property of cAAC. 2(•+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(•+) is caused by intermolecular interactions. PMID:24437683

Mondal, Kartik Chandra; Samuel, Prinson P; Roesky, Herbert W; Carl, Elena; Herbst-Irmer, Regine; Stalke, Dietmar; Schwederski, Brigitte; Kaim, Wolfgang; Ungur, Liviu; Chibotaru, Liviu F; Hermann, Markus; Frenking, Gernot

2014-02-01

67

Photochemistry of group 6 Fischer carbene complexes: beyond the photocarbonylation reaction.  

PubMed

The seminal report by Hegedus in 1982, showing that alkoxychromium(0) carbenes reacted with imines under bright Colorado sunlight to yield ?-lactams, marked the beginning of a key reaction in organometallic chemistry. Very little was known about the mechanism of this reaction. In fact, Hegedus proposed the reversible generation of a chromium-coordinated ketene, which would react with nucleophiles. This coordinated species would show all the advantages of ketenes without their shortcomings, namely, dimerization, formation of undesired adducts, and so forth. The quest for the detection of these species and the pursuit of the mechanism of the photocarbonylation (a reaction exclusive to Cr(0) and Mo(0) carbene complexes, not W(0) carbene complexes) remained unabated over the next 15 years. In fact, all attempts to experimentally determine the mechanism of this useful reaction have been fruitless. At the same time, the photocarbonylation of Cr(0) carbenes matured into a valuable synthetic reaction, allowing access to several families of organic compounds. Unfortunately, reactions other than photocarbonylation remained elusive. We used a combination of experimental and computational methodologies to study the photocarbonylation of Cr(0) carbene complexes and the subsequent reaction of the photogenerated ketenes with nucleophiles. In parallel, we discovered new photochemical processes and succeeded in making photoreactive the so-called "unreactive" W(0) carbene complexes. In this Account, we discuss the disentangling of the mechanisms of these transformations, thereby shedding some light onto the photochemistry of group 6 metal (Fischer) carbene complexes. The original designation of the electronic transitions of group 6 carbene complexes was reassigned, and the photocarbonylation step was analyzed again, resulting in the sequence S(0)-T(1)-S(0), which is far removed from conventional organic photochemistry. The T(1) species is a chromacyclopropanone; its unpaired electrons are primarily localized in the metal fragment and in the former carbene carbon atom. The T(1)-S(0) intersystem crossing occurs with the participation of the solvent through an unusual loose-bolt radiationless mechanism. The photogenerated S(0) species reacts with imines to form the final ?-lactams in a mechanism that resembles the organic Staudinger reaction, but here the metal is present during the entire reaction coordinate. The selectivity of these reactions is defined by the nucleophilic attack on the O-bonded metallaketene instead of the subsequent conrotatory ring closure, a distinct departure from the organic reaction. Appropriate modification of the substituents of the carbene ligand or in the coordination sphere of the complex results in new photoprocesses; these include 1,2-metalladyotropic rearrangements as well as ?-fragmentations in which W(0) carbene complexes become photoreactive. Moreover, the inclusion of additional metal centers usually results in new reactions, such as the formation of fulvenes by ?(5)? ?(3) photoslippage, or in the complete inhibition of the photoreactivity. The photochemistry of group 6 metal-carbene complexes thus offers unexplored territory for pursuing new reactions and reaction mechanisms. PMID:21517052

Fernández, Israel; Cossío, Fernando P; Sierra, Miguel A

2011-07-19

68

An unprecedented chemospecific and stereoselective tandem nucleophilic addition/cycloaddition reaction of nucleophilic carbenes with ketenimines.  

PubMed

The first study of the reaction between nucleophilic carbenes and ketenimines is reported. The interaction of thiazole and benzothiazole carbenes with ketenimines proceeded in a chemospecific and stereoselective manner to produce thiazole- and benzothiazole-spiro-pyrrole derivatives generally in good yields. The reaction was proposed to proceed via a tandem nucleophilic addition of carbene to the C=N bond of ketenimine followed by a stepwise [3+2] cycloaddition of the 1,3-dipolar intermediate with the C=C bond of ketenimine. This reaction provides a powerful protocol for the construction of novel polyfunctional thiazole-spiro-pyrrole or benzothiazole-spiro-pyrrole compounds that are not readily accessible by other methods. PMID:19032113

Cheng, Ying; Ma, Yang-Guang; Wang, Xiao-Rong; Mo, Jun-Ming

2009-01-16

69

A non-diazo approach to ?-oxo gold carbenes via gold-catalyzed alkyne oxidation.  

PubMed

For the past dozen years, homogeneous gold catalysis has evolved from a little known topic in organic synthesis to a fully blown research field of significant importance to synthetic practitioners, due to its novel reactivities and reaction modes. Cationic gold(I) complexes are powerful soft Lewis acids that can activate alkynes and allenes toward efficient attack by nucleophiles, leading to the generation of alkenyl gold intermediates. Some of the most versatile aspects of gold catalysis involve the generation of gold carbene intermediates, which occurs through the approach of an electrophile to the distal end of the alkenyl gold moiety, and their diverse transformations thereafter. On the other hand, ?-oxo metal carbene/carbenoids are highly versatile intermediates in organic synthesis and can undergo various synthetically challenging yet highly valuable transformations such as C-H insertion, ylide formation, and cyclopropanation reactions. Metal-catalyzed dediazotizations of diazo carbonyl compounds are the principle and most reliable strategy to access them. Unfortunately, the substrates contain a highly energetic diazo moiety and are potentially explosive. Moreover, chemists need to use energetic reagents to prepare them, putting further constrains on operational safety. In this Account, we show that the unique access to the gold carbene species in homogeneous gold catalysis offers an opportunity to generate ?-oxo gold carbenes if both nucleophile and electrophile are oxygen. Hence, this approach would enable readily available and safer alkynes to replace hazardous ?-diazo carbonyl compounds as precursors in the realm of gold carbene chemistry. For the past several years, we have demonstrated that alkynes can indeed effectively serve as precursors to versatile ?-oxo gold carbenes. In our initial study, we showed that a tethered sulfoxide can be a suitable oxidant, which in some cases leads to the formation of ?-oxo gold carbene intermediates. The intermolecular approach offers excellent synthetic flexibility because no tethering of the oxidant is required, and its reduced form is not tangled with the product. We were the first research group to develop this strategy, through the use of pyridine/quinolone N-oxides as the external oxidants. In this manner, we can effectively make a C-C triple bond a surrogate of an ?-diazo carbonyl moiety in various gold catalyses. With terminal alkynes, we demonstrated that we can efficiently trap exclusively formed terminal carbene centers by internal nucleophiles en route to the formation of cyclic products, including strained oxetan-3-ones and azetidin-3-ones, and by external nucleophiles when a P,N-bidentate ligand is coordinated to gold. With internal alkynes, we generated synthetically useful regioselectivities in the generation of the ?-oxo gold carbene moiety, which enables expedient formation of versatile enone products. Other research groups have also applied this strategy en route to versatile synthetic methods. The ?-oxo gold carbenes appear to be more electrophilic than their Rh counterpart, which many chemists have focused on in a large array of excellent work on metal carbene chemistry. The ease of accessing the reactive gold carbenes opens up a vast area for developing new synthetic methods that would be distinctively different from the known Rh chemistry and promises to generate a new round of "gold rush". PMID:24428596

Zhang, Liming

2014-03-18

70

C4 cumulene and the corresponding air-stable radical cation and dication.  

PubMed

A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(.+) , and dication 1(2+) have been synthesized. The redox property of 1(.+) was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1(.+) is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis. PMID:24623450

Li, Yan; Mondal, Kartik Chandra; Samuel, Prinson P; Zhu, Hongping; Orben, Claudia M; Panneerselvam, Saravanan; Dittrich, Birger; Schwederski, Brigitte; Kaim, Wolfgang; Mondal, Totan; Koley, Debasis; Roesky, Herbert W

2014-04-14

71

A metallacyclic alkyl-amido carbene complex (MCAAC).  

PubMed

The activation of the C?N moiety in the redox-active metalloligand [CpRu{?(3)N(pz)-1}][PF6] (2) (1: ambidentate hybrid ligand, N?C-C(pz)3, with pz = pyrazolyl) was observed in the reaction with [Ir(cod)Cl]2 (cod = 1,5-cyclooctadiene). By performing detailed NMR spectroscopic and X-ray crystallographic investigations the product was found to be a bimetallic Ru(II)-Ir(III) complex of the composition [CpRu{?-1}Ir(cod)Cl2][PF6] (3) consisting of a chemically modified ligand 1'. Most notably, the heterobimetallic complex 3 features an unprecedented metallacyclic alkyl-amido carbene (MCAAC) core structure, which is coordinated to an Ir(III) centre. Density functional theory (DFT) calculations as well as cyclic voltammetry (CV) studies were performed in an effort to establish the formal oxidation states of the metal atoms in 3. Indeed, a quasi-reversible oxidation wave was detected at E(1/2)(0) = 0.36 V, which was attributed to the Ru(II)/Ru(III) redox couple, while two irreversible reduction processes were observed at very negative potentials and have been assigned to the stepwise reduction of Ir(III) to Ir(I). First efforts to elucidate the reaction mechanism have also been performed. PMID:24549226

Trapp, Ina; González-Gallardo, Sandra; Hohnstein, Silvia; Garnier, Delphine; Oña-Burgos, Pascual; Breher, Frank

2014-03-21

72

Rhodium(I) and iridium(I) complexes of pyrazolyl-N-heterocyclic carbene ligands.  

PubMed

Several Rh(I) and Ir(I) complexes containing an N-heterocyclic carbene-pyrazolyl chelate ligand have been synthesised. Determination of the single-crystal X-ray structure of the Ir(I) complex showed a novel binding mode with the iridium centre coordinated to two ligands via two carbene donors in preference to one ligand forming the entropically favoured chelate. The hydrogenation activity of several of these complexes was investigated along with that of previously synthesised Rh(I) and Ir(I) complexes containing an analogous phosphine-pyrazolyl chelate. PMID:16896454

Messerle, Barbara A; Page, Michael J; Turner, Peter

2006-08-28

73

Iron in the Service of Chromium - the ortho-Benzannulation of trans, trans-Dienyl Fischer Carbene Complexes  

PubMed Central

Chromium Fischer carbene complexes with trans, trans-dienyl substituents on the carbene carbon will react with diiron nonacarbonyl to give 2-alkoxycyclohexa-2,4-dienone iron tricarbonyl complexes and/or 2-alkoxyphenols in excellent yields. In the presence of silica gel or base, the cyclohexadienone complex will suffer loss of the iron and aromatization to give 2-alkoxyphenols. The formation of 2-alkoxyphenols from dienyl chromium carbene complexes is a known process (ortho-benzannulation) that only occurs with certain cis, trans-dienyl complexes. Control experiments show that trans, trans-dienyl chromium carbene complexes do not undergo conversion to 2-alkoxyphenols in the absence of an iron source. The process most likely occurs either via coordination of the dienyl unit in the chromium carbene complex to an iron tricarbonyl group and then loss of the chromium, or via direct trans-metallation of the carbene ligand to give an iron carbene complex and then internal coordination to the dienyl unit such that cis to trans isomerization of the ?,?-double bond occurs.

Lian, Yiqian; Wulff, William D.

2008-01-01

74

Iron in the service of chromium: the ortho-benzannulation of trans,trans-dienyl Fischer carbene complexes.  

PubMed

Chromium Fischer carbene complexes with trans,trans-dienyl substituents on the carbene carbon will react with diiron nonacarbonyl to give 2-alkoxycyclohexa-2,4-dienone iron tricarbonyl complexes and/or 2-alkoxyphenols in excellent yields. In the presence of silica gel or base, the cyclohexadienone complex will suffer loss of the iron and aromatization to give 2-alkoxyphenols. The formation of 2-alkoxyphenols from dienyl chromium carbene complexes is a known process (ortho-benzannulation) that only occurs with certain cis,trans-dienyl complexes. Control experiments show that trans,trans-dienyl chromium carbene complexes do not undergo conversion to 2-alkoxyphenols in the absence of an iron source. The process most likely occurs either via coordination of the dienyl unit in the chromium carbene complex to an iron tricarbonyl group and then loss of the chromium or via direct trans-metalation of the carbene ligand to give an iron carbene complex and then internal coordination to the dienyl unit such that cis to trans isomerization of the alpha,beta-double bond occurs. PMID:16332045

Lian, Yiqian; Wulff, William D

2005-12-14

75

Palladium complexes of amido-functionalized N-heterocyclic carbenes as effective precatalysts for the Suzuki–Miyaura C–C cross-coupling reactions of aryl bromides and iodides  

Microsoft Academic Search

A series of air-stable, robust and highly active palladium based precatalysts of amido-functionalized N-heterocyclic carbenes for the Suzuki–Miyaura C–C cross-coupling reaction has been designed. In particular, the [1-R-3-{N-(benzylacetamido)imidazol-2-ylidene]2PdCl2 [R=i-Pr (1c) and CH2Ph (2c)] complexes efficiently carried out the Suzuki–Miyaura coupling of the aryl bromide and iodide substrates with phenyl boronic acid in good to excellent yields in air at 90°C

Sachin Kumar; Mobin M. Shaikh; Prasenjit Ghosh

2009-01-01

76

Pd–benzothiazole carbene catalysed carbonylation of aryl halides in ionic liquids  

Microsoft Academic Search

The carbonylations of aryl halides with the Pd–carbene catalyst 1 were studied both in molecular solvents and in ionic liquids (ILs). Among the ILs, tetrabutylammonium bromide (TBAB) was found to give better results. Under these conditions the catalyst, when recovered and reused, did not show a significant decrease of activity. The role of TBAB on the catalyst stability is discussed.

Vincenzo Calò; Potenzo Giannoccaro; Angelo Nacci; Antonio Monopoli

2002-01-01

77

Bifunctional N-Heterocyclic Carbene-Catalyzed Highly Enantioselective Synthesis of Spirocyclic Oxindolo-?-lactams.  

PubMed

The N-heterocyclic carbene-catalyzed Staudinger reaction of ketenes with isatin-derived ketimines was investigated. The bifunctional NHCs with a free hydroxyl group were demonstrated as efficient catalysts for the reaction, giving the corresponding spirocyclic oxindolo-?-lactams in high yields with excellent diastereo- and enantioselectivities. PMID:24856000

Zhang, Han-Ming; Gao, Zhong-Hua; Ye, Song

2014-06-01

78

Operationally unsaturated pincer/rhenium complexes form metal carbenes from cycloalkenes and metal carbynes from alkanes.  

PubMed

Operationally unsaturated (i.e., 16/18-electron) (PNPR)Re(H)4, where PNPR is N(SiMe2CH2PR2)2, is reactive at 22 degrees C with cyclic olefins. The first observed products are generally (PNPR)Re(H)2(cycloalkylidene), with hydrogenated olefin as the product of hydrogen abstraction from the tetrahydride. The tetrahydride complex with R = tBu generally fails to react (too bulky), that with R = cyclohexyl suffers a (controllable) tendency to abstraction of 3H from one ring, forming an eta3-cyclohexenyl compound, and that with R = iPr generally gives the richest bimolecular reactivity. The cyclic monoolefins studied show distinct reactivity, C6 giving first the carbene and then coordinated cyclohexadiene, C5 giving carbene, then diene, and then eta5-C5H5, C8 giving carbene and then eta2-cyclooctyne, and C12 giving an eta3-allyl. Norbornene gives a pi-complex of the norbornene in thermal equilibrium with its carbene isomer; at 90 degrees C, hydrocarbon ligand Calpha-Cbeta bond cleavage occurs to give, for the first time, a carbyne complex from an internal olefin. Two compounds synthesized here have the formal composition "(PNPR)Re + olefin", and each of these is capable of dehydrogenating the methyl group of a variety of alkanes at 110 degrees C to form (PNP)ReH triple bond (CR). PMID:17429964

Ozerov, Oleg V; Watson, Lori A; Pink, Maren; Caulton, Kenneth G

2007-05-01

79

Formal Diels-Alder reactions of chalcones and formylcyclopropanes catalyzed by chiral N-heterocyclic carbenes.  

PubMed

Highly enantioselective (formal) hetero-Diels-Alder reactions between chalcones and formylcyclopropanes are disclosed. The challenging N-heterocyclic carbene (NHC)-bounded enolate intermediates from formylcyclopropanes were captured for new C-C bond forming reactions. The reaction products were obtained with high diastereo- and enantioselectivities and could be easily transformed to optically pure multisubstituted cyclohexane derivatives. PMID:21877756

Lv, Hui; Mo, Junming; Fang, Xinqiang; Chi, Yonggui Robin

2011-10-01

80

Catalytic dynamic kinetic resolutions with N-heterocyclic carbenes: asymmetric synthesis of highly substituted ?-lactones.  

PubMed

New DKR type: An N-heterocyclic carbene (NHC)-catalyzed dynamic kinetic resolution of racemic ?-substituted ?-keto esters has been developed. This method relies on the epimerization of an NHC-enol intermediate before subsequent aldol/acylation events. Highly substituted ?-lactones are produced in good yield with good to excellent selectivities (see scheme). PMID:22700327

Cohen, Daniel T; Eichman, Chad C; Phillips, Eric M; Zarefsky, Emily R; Scheidt, Karl A

2012-07-16

81

Unexpected thermal decomposition of the "Alder carbene" (iPr2N)2C.  

PubMed

The "Alder carbene" (iPr(2)N)(2)C undergoes a ?-fragmentation in solution already at room temperature, affording propene and N,N,N'-triisopropylformamidine. This stands in sharp contrast to the indefinite stability previously claimed for this iconic compound. PMID:22864196

Schulz, Tim; Leibold, Michael; Färber, Christian; Maurer, Martin; Porsch, Timo; Holthausen, Max C; Siemeling, Ulrich

2012-09-21

82

Enhanced cytotoxicity of silver complexes bearing bidentate N-heterocyclic carbene ligands.  

PubMed

A diverse library of cationic silver complexes bearing bis(N-heterocyclic carbene) ligands have been prepared which exhibit cytotoxicity comparable to cisplatin against the adenocarcinomas MCF7 and DLD1. Bidentate ligands show enhanced cytotoxicity over monodentate and macrocyclic ligands. PMID:22327273

Monteiro, Diana C F; Phillips, Roger M; Crossley, Benjamin D; Fielden, Jake; Willans, Charlotte E

2012-04-01

83

Pd-carbene migratory insertion: application to the synthesis of trifluoromethylated alkenes and dienes.  

PubMed

Pd-catalyzed cross-coupling of halides with CF3-substituted diazo compounds or N-tosylhydrazones has been explored for the synthesis of CF3-substituted alkenes and 1,3-butadienes. Pd-carbene migratory insertion plays the key role in these transformations. PMID:24425676

Wang, Xi; Xu, Yan; Deng, Yifan; Zhou, Yujing; Feng, Jiajie; Ji, Guojing; Zhang, Yan; Wang, Jianbo

2014-01-20

84

N-Heterocyclic Carbene-Catalyzed Enantioselective Annulation of Indolin-3-ones with Bromoenals.  

PubMed

N-Heterocyclic carbene-catalyzed reactions of indolin-3-ones with 2-bromoenals opened an asymmetric access to 3,4-dihydropyrano[3,2-b]indol-2(5?H)-ones in good yields and with good to excellent enantioselectivities. This protocol tolerates a broad substrate scope. In addition, a possible mechanism for the annulation reaction is presented. PMID:24729588

Ni, Qijian; Song, Xiaoxiao; Raabe, Gerhard; Enders, Dieter

2014-06-01

85

Vinyl dihydropyrans and dihydrooxazines: cyclizations of catalytic ruthenium carbenes derived from alkynals and alkynones.  

PubMed

A novel synthesis of 2-vinyldihydropyrans and dihydro-1,4-oxazines (morpholine derivatives) from alkynals and alkynones has been developed. The cyclizations require a mild generation of catalytic ruthenium carbenes from terminal alkynes and (trimethylsilyl)diazomethane followed by trapping with carbonyl nucleophiles. Mechanistic aspects of the new cyclizations are discussed. PMID:24798093

Cambeiro, Fermín; López, Susana; Varela, Jesús A; Saá, Carlos

2014-06-01

86

Synthesis and reactivity of metal complexes with acyclic (amino)(ylide)carbene ligands.  

PubMed

No cycle required: The straightforward synthesis of acyclic (amino)(ylide)carbene gold complexes was achieved by reaction of isocyanide gold complexes with phosphorus and arsenic ylides as well as electron-rich olefins. Their ability to form bimetallic species and to act as ligand-transfer reagents has also been established. PMID:24038894

González-Fernández, Elisa; Rust, Jörg; Alcarazo, Manuel

2013-10-18

87

Singlet-triplet splittings and barriers to Wolff rearrangement for carbonyl carbenes.  

PubMed

High-level ab initio calculations at the G3(MP2)//B3-LYP level have been used to study carbomethoxychlorocarbene and related halogenocarbenes and carbonyl carbenes. Initial calculations at the more accurate W1' level on the subset CH(2), HCCl, HCF, CCl(2), and CF(2) provide support for the reliability of G3(MP2)//B3-LYP for this type of problem. The W1' calculations also suggest that the experimental S-T splitting is slightly underestimated for HCCl and CF(2) and substantially underestimated for CCl(2), in keeping with other recent high-level studies. Whereas the parent carbonyl carbenes, namely formylcarbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to have triplet ground states, their chloro and fluoro derivatives are predicted to have singlet ground states. In particular, carbomethoxychlorocarbene is predicted to have a singlet ground state, with the singlet-triplet splitting estimated as -16.0 kJ mol(-)(1). The barriers to Wolff rearrangement of the singlet carbonyl carbenes generally (but not always) correlate with the exothermicity accompanying the production of ketenes. In the case of the parent carbonyl carbenes, for which the rearrangement reaction is most exothermic, the barriers lie between about 10 and 30 kJ mol(-)(1), whereas for the less exothermic rearrangements of the chloro- and fluoro-substituted carbonyl carbenes, the Wolff rearrangement barriers increase significantly to between 58 and 75 kJ mol(-)(1). The calculated barrier for carbomethoxychlorocarbene is 58.2 kJ mol(-)(1). PMID:11414840

Scott, A P; Platz, M S; Radom, L

2001-06-27

88

Effect of the metal fragment in the thermal cycloaddition between alkynyl metal(0) Fischer carbene complexes and nitrones.  

PubMed

The thermal cycloaddition between alkynyl metal(0) Fischer carbenes and nitrones has been studied computationally within the Density Functional Theory framework. It is found that the [3 + 2] cycloaddition takes place via transition structures that are more asynchronous and less aromatic than their nonorganometallic analogues. These reactions are also found to be completely regioselective in favor of the cycloadduct possessing the Fischer carbene moiety and the oxygen atom of the nitrone in a 1,3-relative disposition. These results are consistent with the role of the Fischer carbene moiety as an electron withdrawing group that enhances the electrophilic character of the alkyne group acting as a Michael acceptor as a dipolarophile. In terms of the isolobal analogy model, it can be concluded that alkynylalkoxy metal(0) carbene complexes act in this reaction as organometallic analogues of organic alkyl-propiolates with enhanced electrophilic character. PMID:16872203

Fernandez, Israel; Sierra, Miguel A; Cossío, Fernando P

2006-08-01

89

An insight into the mechanism of the aerobic oxidation of aldehydes catalyzed by N-heterocyclic carbenes.  

PubMed

N-Heterocyclic carbene catalysis for the aerobic oxidation and esterification of aromatic aldehydes was monitored by ESI-MS (MS/MS) and the key intermediates were intercepted and characterized using the charge-tag strategy. PMID:24413829

Bortolini, O; Chiappe, C; Fogagnolo, M; Giovannini, P P; Massi, A; Pomelli, C S; Ragno, D

2014-02-25

90

Gas-phase reactions of halogenated radical carbene anions with sulfur and oxygen containing species  

Microsoft Academic Search

The reactivities of mono- and dihalocarbene anions (CHCl-, CHBr-, CF2-, CCl2-, and CBrCl-) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS2, COS, CO2, O2, CO, and N2O). These anions were found to demonstrate diverse chemistry as

Stephanie M. Villano; Nicole Eyet; W. Carl Lineberger; Veronica M. Bierbaum

2009-01-01

91

Gas-phase reactions of halogenated radical carbene anions with sulfur and oxygen containing species  

Microsoft Academic Search

The reactivities of mono- and dihalocarbene anions (CHCl?, CHBr?, CF2?, CCl2?, and CBrCl?) were studied using a tandem flowing afterglow-selected ion flow tube instrument. Reaction rate constants and product branching ratios are reported for the reactions of these carbene anions with six neutral reagents (CS2, COS, CO2, O2, CO, and N2O). These anions were found to demonstrate diverse chemistry as

Stephanie M. Villano; Nicole Eyet; W. Carl Lineberger; Veronica M. Bierbaum

2009-01-01

92

Catalytic Dynamic Kinetic Resolutions with N-Heterocyclic Carbenes: Asymmetric Synthesis of Highly Substituted ?-Lactones  

PubMed Central

An N-heterocyclic carbene (NHC)-catalyzed dynamic kinetic resolution of racemic ?-substituted ?-keto esters has been developed. This method relies on the epimerization of an NHC-enol intermediate before subsequent aldol/acylation events. Highly substituted ?-lactones are produced in good yield (50–88%) with good to excellent diastereoselectivity (5:1 to 20:1 dr) and excellent enantioselectivity (up to 99% ee).

Cohen, Daniel T.; Eichman, Chad C.; Phillips, Eric M.; Zarefsky, Emily R.

2012-01-01

93

Cascade olefin isomerization/intramolecular Diels-Alder reaction catalyzed by N-heterocyclic carbenes.  

PubMed

The addition of an N-heterocyclic carbene to the carbonyl group of an ?,?,?,?-unsaturated enol ester affords a hemiacetal azolium intermediate that enables a cascade olefin isomerization/Diels-Alder reaction, for which mechanistic studies implicate Lewis base catalysis. Preliminary studies into the utility of the products have been undertaken with reductive and oxidative cleavage, giving materials for potential use in complex-target synthesis. PMID:24706584

Kowalczyk, Marcin; Lupton, David W

2014-05-19

94

Synthesis, characterization and catalytic activity of saturated and unsaturated N-heterocyclic carbene iridium(i) complexes.  

PubMed

Both saturated and unsaturated N-benzyl substituted heterocyclic carbene (NHC) iridum(i) complexes were synthesized. The unsaturated carbene complex [(un-NHC-Bn)Ir(CO)(2)Cl] in the cis form was prepared via the carbene transfer from the corresponding silver complex to [Ir(COD)(2)Cl](2) followed by ligand substitution with CO, whereas the saturated complex was obtained via the transfer from (sat-NHC-Bn)W(CO)(5). The treatment of phosphines with (NHC)Ir(CO)(2)Cl complexes yielded the products with the phosphine ligand trans to the carbene moiety via substitution. X-Ray structural determination shows that distances of Ir-C((carbene)) in both (un-NHC-Bn)Ir(CO)(PR(3))Cl and (un-NHC-Bn)Ir(CO)(PR(3))Cl are essentially the same. Analyses of spectroscopic and crystal structural data of iridium complexes [(NHC)Ir(CO)(PR(3))Cl] and Vaska's complex show similar corresponding data in both types of complexes, suggesting that the studied NHC ligands and phosphines have similar bonding with Ir(i) metal center. All iridium complexes studied in this work illustrated their catalytically activity on N-alkylation of amine with alcohol via hydrogen transfer reduction. It appears no dramatic difference on the catalytic activity among these iridium carbene complexes; but the saturated carbene complex (sat-NHC-Bn)Ir(CO)(PR(3))Cl appears to be slightly more active. For example, the reaction of benzyl alcohol with aniline in the presence of catalyst (1 mol%) under basic conditions at 100 degrees C provided the secondary amine (N-benzylaniline) in 96% yield. PMID:19156280

Chang, Yung-Hung; Fu, Ching-Feng; Liu, Yi-Hong; Peng, Shei-Ming; Chen, Jwu-Ting; Liu, Shiuh-Tzung

2009-02-01

95

White Phosphorus Degradation with a NacNac Aluminum Carbene Analogue: The Biradical Reaction Mechanism.  

PubMed

The title compound Al-NacNac is isolobal to the imidazol-2-ylidene (NHC); the latter is considered as a nucleophilic carbene. However, the title compound is different from a typical carbene, as aluminum is a heavier group 13 element with a predominant inert s orbital. Its singlet ground state is a poor Lewis donor (acceptor) toward white phosphorus, but its corresponding lowest energy triplet state forms a strong Al-P bond with (opened) white phosphorus. The reaction of Al-NacNac with white phosphorus proceeds in two steps: after the addition of a first carbene analogue, a second one is added, resulting in a transient biradicaloid species. This undergoes facile subsequent rearrangement, and a final ring closure reaction leads to the observed product with a bicyclobutane moiety. It is determined by intramolecular bond formation of two phosphorus centered radicals. Finally, a structure with a large singlet-triplet energy separation is formed. An analogy to the noninnocent ligand character as well as the exciplex view of the monoadduct of white phosphorus with the Al-NacNac system is drawn. PMID:24800980

Schoeller, Wolfgang W; Frey, Guido D

2014-05-19

96

Synthesis, structures, and solution dynamics of palladium complexes of quinoline-functionalized N-heterocyclic carbenes.  

PubMed

A new type of quinoline-functionalized palladium N-heterocyclic carbene (NHC) complexes has been synthesized via silver transmetallation. The quinoline moiety was either directly attached to the imidazole ring or linked to it by a methylene group. NHCs with a methylene linker tend to form trans biscarbene complexes in the reaction of Pd(COD)Cl2, while NHCs without any linker form chelating NHC-quinoline (NHC-N) complexes. These two types of carbenes also react with [Pd(allyl)Cl]2 to give monodentate NHC palladium eta(3)-allyl chlorides [Pd(NHC)(allyl)Cl]. Fluxionality in the NMR time scale was observed for most complexes, and the origin of their dynamic behaviors was discussed for each type of structure. For [Pd(NHC)(allyl)Cl] with a relatively small wing tip group of the NHC, the fluxionality (selective line-broadening of (1)H NMR signals) is caused by selective eta(3)-eta(1)-eta(3) allyl isomerization. For NHC with a bulkier (t)Bu group, a different line-broadening pattern was observed and was ascribed to partially hindered Pd-C(carbene) bond rotation. For cationic chelating complexes [Pd(NHC-N)(allyl)]BF4, the dynamic exchange process likely originates from a dissociative boat-to-boat inversion of 7-membered palladacycles. Activation parameters were measured for this process. Crystal structures were reported for representative complexes in each category. PMID:18707096

Peng, Hong Mei; Song, Guoyong; Li, Yongxin; Li, Xingwei

2008-09-15

97

Role of quantum mechanical tunneling on the ?-effect of silicon on carbenes in 3-trimethylsilylcyclobutylidene.  

PubMed

Quantum mechanical tunneling (QMT) is increasingly being realized as an important phenomenon that can enhance the rate of reactions even at room temperature. Recently, the ability of a trimethylsilane (TMS) group to activate 1,3-H shift to a carbene from a ?-position has been demonstrated. Direct dynamical calculations (using canonical varitational transition state theory) inclusive of small curvature tunneling (CVT-SCT) show that QMT plays a decisive role in such 1,3-hydrogen migration in both the presence and absence of TMS. The presence of a TMS group reduces the activation energy of 1,3-H shift reaction via 1,3-equatorial interaction of the TMS group with the carbene. Tunneling across the smaller barrier enhances the overall forward rate of the reaction. The Arrhenius plot for the reaction shows substantial curvature in comparison to the CVT mechanism at room temperature. Arrhenius plots for the kinetic isotope effects (KIEs) for the ?-deuterated and per deuterated 3-trimethylsilylcyclobutylidene also show strong deviations from the classical over the barrier mechanism. The magnitude of the KIE is suggestive of QMT from the vibrational excited states of the carbenes. PMID:24467461

Karmakar, Sharmistha; Datta, Ayan

2014-03-01

98

Dialkyl bis(bis(trimthylsilyl)amido) Group 4A metal complexes. Preparation of bridging carbene complexes by. gamma. elimination of alkane. Crystal structure of )ZrCHSi(Me)/sub 2/NSiMe/sub 3/(N(SiMe/sub 3/)/sub 2/))/sub 2/  

SciTech Connect

The dialkyls of zirconium of the type R/sub 2/Zr(N(SiMe/sub 3/)/sub 2/)/sub 2/, where is R is Me, ET, or CH/sub 2/SiMe/sub 3/, thermally decompose (60/sup 0/C (10/sup -2/mm)) by elimination of alkane (MeH, EtH, or Me/sub 4/Si, respectively) to yield the bridging carbene compounds. The 1:1 complex with pyridine is described. The related hafnium dialkyls decomposed in a related manner though in this case the decomposition product can only be characterized as its pyridine complex. The NMR spectra of the pyridine complexes of zirconium and hafnium are similar. In contrast, the corresponding titanium compound is thermally stable to 190/sup 0/C. The titanium carbene may be prepared by sodium amalgam reduction of Cl/sub 2/Ti(N(SiMe/sub 3/)/sub 2/)/sub 2/. The crystal structure of the bridging carbene compound of zirconium was determined by single-crystal x-ray crystallography. The crystals were found to be orhtorhombic of space group Pbca, and the cell consists of three, fused planar, four-membered rings giving the molecule a tub configuration. The Zr-C bond distances are 2.16 A, and the Zr-N distances are 2.04 to 2.09 A. 20 references, 4 tables.

Planalp, R.P.; Andersen, R.A.; Zalkin, A.

1983-01-01

99

In vitro antimicrobial studies of silver carbene complexes: activity of free and nanoparticle carbene formulations against clinical isolates of pathogenic bacteria  

PubMed Central

Objectives Silver carbenes may represent novel, broad-spectrum antimicrobial agents that have low toxicity while providing varying chemistry for targeted applications. Here, the bactericidal activity of four silver carbene complexes (SCCs) with different formulations, including nanoparticles (NPs) and micelles, was tested against a panel of clinical strains of bacteria and fungi that are the causative agents of many skin and soft tissue, respiratory, wound, blood, and nosocomial infections. Methods MIC, MBC and multidose experiments were conducted against a broad range of bacteria and fungi. Time-release and cytotoxicity studies of the compounds were also carried out. Free SCCs and SCC NPs were tested against a panel of medically important pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), multidrug-resistant Acinetobacter baumannii (MRAB), Pseudomonas aeruginosa, Burkholderia cepacia and Klebsiella pneumoniae. Results All four SCCs demonstrated strong efficacy in concentration ranges of 0.5–90 mg/L. Clinical bacterial isolates with high inherent resistance to purified compounds were more effectively treated either with an NP formulation of these compounds or by repeated dosing. Overall, the compounds were active against highly resistant bacterial strains, such as MRSA and MRAB, and were active against the biodefence pathogens Bacillus anthracis and Yersinia pestis. All of the medically important bacterial strains tested play a role in many different infectious diseases. Conclusions The four SCCs described here, including their development as NP therapies, show great promise for treating a wide variety of bacterial and fungal pathogens that are not easily killed by routine antimicrobial agents.

Leid, Jeff G.; Ditto, Andrew J.; Knapp, Amanda; Shah, Parth N.; Wright, Brian D.; Blust, Robyn; Christensen, Lanette; Clemons, C. B.; Wilber, J. P.; Young, Gerald W.; Kang, Ae Gyeong; Panzner, Matthew J.; Cannon, Carolyn L.; Yun, Yang H.; Youngs, Wiley J.; Seckinger, Nicole M.; Cope, Emily K.

2012-01-01

100

Synthesis, characterization, and structural studies of multimetallic ferrocenyl carbene complexes of group VII transition metals.  

PubMed

Fischer carbene complexes of the group VII transition metals (Mn and Re) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized. The structural features and their relevance to bonding in the carbene multimetal compounds were investigated, as they represent indicators of possible reactivity sites in polymetallic carbene assemblies. For complexes of the type [ML(x){C(OR)R'}] (ML(x) = MnCp(CO)(2) or Re(2)(CO)(9)), ferrocenyl (Fc) was chosen as the R' substituent, while the OR substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1a (ML(x) = MnCp(CO)(2), R = Et, R' = Fc), 2a (ML(x) = MnCp(CO)(2), R = TiCp(2)Cl, R' = Fc), 3a (ML(x) = Re(2)(CO)(9), R = Et, R' = Fc), and 4a (ML(x) = Re(2)(CO)(9), R = TiCp(2)Cl, R' = Fc). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, resulted in formation of the novel biscarbene complexes with bridging ferrocen-1,1'-diyl (Fc') substituents [{?-Fe(C(5)H(4))(2)-C,C'}{C(OEt)ML(x)}(2)] (1b, ML(x) = MnCp(CO)(2); 3b, ML(x) = Re(2)(CO)(9)) or the unusual bimetallacyclic bridged biscarbene complexes [{?-TiCp(2)O(2)-O,O'}{?-Fe(C(5)H(4))(2)-C,C'}{CML(x)}(2)] (2b, ML(x) = MnCp(CO)(2); 4b, ML(x) = Re(2)(CO)(9)). The target compounds that were isolated displayed a variety of different geometric isomers and conformations. The greater reactivity of the binary dirhenium acylates in solution, compared to that of the cyclopentadienyl manganese acylate, resulted in a complex reaction mixture. Although the stabilization of hydroxycarbene or hydrido-acyl intermediates of dirhenium carbonyls could not be achieved, their existence in solution was confirmed by the isolation of [(?-H)(2)-(Re(CO)(4){C(O)Fc})(2)] (8), the unique dichloro-bridged biscarbene complex fac-[(?-Cl)(2)-(Re(CO)(3){C(OEt)Fc})(2)] (6), the known hydrido complex [Re(3)(CO)(14)H] (5), the acyl complex [Re(CO)(5){C(O)Fc}] (7), and the aldehyde-functionalized eq-[Re(2)(CO)(9){C(OTiCp(2)Cl)(Fc'CHO)}] (9). PMID:21250683

Bezuidenhout, Daniela I; Lotz, Simon; Landman, Marilé; Liles, David C

2011-02-21

101

Synthesis and bonding in carbene complexes of an unsymmetrical dilithio methandiide: a combined experimental and theoretical study.  

PubMed

Herein, we report the preparation of a new unsymmetrical, bis(thiophosphinoyl)-substituted dilithio methandiide and its application for the synthesis of zirconium- and palladium-carbene complexes. These complexes were found to exhibit remarkably shielded (13)C NMR shifts, which are much more highfield-shifted than those of "normal" carbene complexes. DFT calculations were performed to determine the origin of these observations and to distinguish the electronic structure of these and related carbene complexes compared with the classical Fischer and Schrock-type complexes. Various methods show that these systems are best described as highly polarized Schrock-type complexes, in which the metal-carbon bond possesses more electrostatic contributions than in the prototype Schrock systems, or even as "masked" methandiides. As such, geminal dianions represent a kind of "extreme" Schrock-type ligands favoring the ionic resonance structure M(+)-CR2(-) as often used in textbooks to explain the nucleophilic nature of Schrock complexes. PMID:24150833

Gessner, Viktoria H; Meier, Florian; Uhrich, Diana; Kaupp, Martin

2013-12-01

102

The cycloadditions of various substituted carbenes, silylenes, and germylenes onto the diamond (100) surface: a theoretical exploration.  

PubMed

The cycloadditions of 21 singlet substituted carbenes, silylenes, and germylenes onto the diamond (100) surface have been theoretically studied by means of density functional theory coupled with effective cluster models. The calculated reaction energies and reaction pathways have disclosed that the substituents play an important effect on the reaction profiles for the additions of carbenes, silylenes, and germylenes onto the diamond (100) surface. Our theoretical investigations illustrate that, irrespective of carbenes, silylenes, and germylenes, the cycloadditions of those with electropositive substituents (such as H and CH(3)) onto diamond (100) are much more favorable than those with electronegative and pi-donating substituents (such as F and NH(2)) both thermodynamically and kinetically. In broad perspective, we believe that a similar reactivity trend can also be extended to that of Si (100), Ge (100), fullerene, single-walled carbon nanotube, disilenes, digermenes, silenes, and germenes because all of these materials feature an analogous bonding motif. PMID:16836344

Xu, Yi-Jun; Zhang, Yong-Fan; Li, Jun-Qian

2006-07-20

103

A theobromine derived silver N-heterocyclic carbene: synthesis, characterization, and antimicrobial efficacy studies on cystic fibrosis relevant pathogens†  

PubMed Central

The increasing incidence of multidrug-resistant (MDR) pulmonary infections in the cystic fibrosis (CF) population has prompted the investigation of innovative silver based therapeutics. The functionalization of the naturally occurring xanthine theobromine at the N1 nitrogen atom with an ethanol substituent followed by the methylation of the N9 nitrogen atom gives the N-heterocyclic carbene precursor 1-(2-hydroxyethyl)-3,7,9-trimethylxanthinium iodide. The reaction of this xanthinium salt with silver acetate produces the highly hydrophilic silver carbene complex SCC8. The in vitro antimicrobial efficacy of this newly synthesized complex was evaluated with excellent results on a variety of virulent and MDR pathogens isolated from CF patients. A comparative in vivo study between the known caffeine derived silver carbene SCC1 and SCC8 demonstrated the ability of both complexes to improve the survival rates of mice in a pneumonia model utilizing the clinically isolated infectious strain of Pseudomonas aeruginosa PA M57-15.

Panzner, Matthew J.; Hindi, Khadijah M.; Wright, Brian D.; Taylor, Jane B.; Han, Daniel S.

2009-01-01

104

Oxyanion steering and CH-? interactions as key elements in an N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition.  

PubMed

The N-heterocyclic carbene catalyzed [4 + 2] cycloaddition has been shown to give ?,?-unsaturated ?-lactones in excellent enantio- and diastereoselectivity. However, preliminary computational studies of the geometry of the intermediate enolate rendered ambiguous both the origins of selectivity and the reaction pathway. Here, we show that a concerted, but highly asynchronous, Diels-Alder reaction occurs rather than the stepwise Michael-type or Claisen-type pathways. In addition, two crucial interactions are identified that enable high selectivity: an oxyanion-steering mechanism and a CH-? interaction. The calculations accurately predict the enantioselectivity of a number of N-heterocyclic carbene catalysts in the hetero-Diels-Alder reaction. PMID:22765294

Allen, Scott E; Mahatthananchai, Jessada; Bode, Jeffrey W; Kozlowski, Marisa C

2012-07-25

105

Elucidation of carbene ambiphilicity leading to the discovery of reversible ammonia activation.  

PubMed

An N,N'-diamidocarbene (DAC) was found to activate a broad range of primary as well as secondary aliphatic and aromatic amines. The relative rates measured for the insertion of the DAC into the primary amines were consistent with an electrophilic activation mechanism; in contrast, the DAC functioned as a nucleophile upon treatment with secondary aryl amines. Collectively, these results constituted the first ambiphilic process for an isolable carbene. By comparison, an analogous diaminocarbene was found to serve exclusively as a nucleophile under similar conditions and led to the discovery of the first organic reagent to reversibly activate ammonia. PMID:24303890

Moerdyk, Jonathan P; Blake, Garrett A; Chase, Daniel T; Bielawski, Christopher W

2013-12-18

106

Concomitant nitrene and carbene insertion accompanying ring expansion: spectroscopic, X-ray, and computational studies.  

PubMed

Reinvestigation of the thermolysis of azido-meta-hemipinate (I) yielded, in addition to known II, unusual products III and IV. These products are formed via a rare intramolecular nitrene insertion into an adjacent methoxy C-H bond followed by an intermolecular reaction during a ring-expansion and a ring-extrusion reaction followed by a carbene insertion. The structures of the new compounds were confirmed using a battery of techniques, including HRMS (ESI-QTOF) and 2D NMR as well as X-ray crystallography for compound IV. Density functional theory methods were used to support the proposed mechanism of formation of the products. PMID:24410290

Kaur, Divneet; Luk, Hoi L; Coldren, William; Srinivas, Pavan M; Sridhar, Lakshetti; Prabhakar, Sripadi; Raghunathan, Partha; Guru Row, T N; Hadad, Christopher M; Platz, Matthew S; Eswaran, Sambasivan V

2014-02-01

107

Conformation of a MnCp(CO)2-thienothiophene carbene complex: A spectroscopic and computational study  

NASA Astrophysics Data System (ADS)

The carbene complex [MnCp(CO)2C(OEt)(TT)], (TT = 3,6-dimethylthieno[3,2-b]thiophene) 1, was synthesized according to the classical Fischer method. The novel complex was fully characterized using various spectroscopic techniques. The infrared spectrum of the complex, recorded as a solution in dichloromethane, revealed a four-band spectrum in the carbonyl region instead of the expected two-band spectrum. A density functional theory (DFT) study was undertaken to explain the presence of more than one conformer in solution. The crystal structure of 1 is reported.

Landman, Marilé; Barnard, Werner; van Rooyen, Petrus H.; Liles, David C.

2012-08-01

108

CH-? and CF-? interactions lead to structural changes of N-heterocyclic carbene palladium complexes.  

PubMed

The role of CH-? and CF-? interactions in determining the structure of N-heterocyclic carbene (NHC) palladium complexes were studied using (1) H?NMR spectroscopy, X-ray crystallography, and DFT calculations. The CH-? interactions led to the formation of the cis-anti isomers in 1-aryl-3-isopropylimidazol-2-ylidene-based [(NHC)2 PdX2 ] complexes, while CF-? interactions led to the exclusive formation of the cis-syn isomer of diiodobis(3-isopropyl-1-pentafluorophenylimidazol-2-ylidene) palladium(II). PMID:24382838

Xu, Xiangya; Pooi, Benjamin; Hirao, Hajime; Hong, Soon Hyeok

2014-01-27

109

Pd-N-Heterocyclic Carbene (NHC) Organic Silica: Synthesis and Application in Carbon-Carbon Coupling Reactions  

EPA Science Inventory

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica was prepared using sol-gel method and its application in Heck and Suzuki reaction were demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wi...

110

Metal free catalytic hydroboration of multiple bonds in methanol using N-heterocyclic carbenes under open atmosphere.  

PubMed

An easy to operate method of catalytic hydroboration of unsaturated compounds has been developed with wide substrate scope. Reactions of various aldimines, ketimines, ?,?-unsaturated carbonyl compounds, and alkynes were successfully executed with bis(pinacolato)diboron and N-heterocyclic carbenes in methanol without requiring a transition metal or inert atmosphere. PMID:23945776

Wen, Kun; Chen, Jinbo; Gao, Feng; Bhadury, Pinaki S; Fan, Erkang; Sun, Zhihua

2013-10-01

111

Optimized Syntheses of Cyclopentadienyl Nickel Chloride Compounds Containing "N"-Heterocyclic Carbene Ligands for Short Laboratory Periods  

ERIC Educational Resources Information Center

Experiments are described for the preparation of imidazolium chloride precursors to "N"-heterocyclic carbenes and their cyclopentadienyl nickel chloride derivatives. The syntheses have been optimized for second- and third-year undergraduate laboratories that have a maximum programmed length of three hours per week. The experiments are flexible and…

Cooke, Jason; Lightbody, Owen C.

2011-01-01

112

Selective Kumada biaryl cross-coupling reaction enabled by an iron(iii) alkoxide-N-heterocyclic carbene catalyst system.  

PubMed

A catalyst system comprising Fe2(O(t)Bu)6 and an N-heterocyclic carbene ligand enables efficient syntheses of (hetero)biaryls from the reactions of aryl Grignard reagents with a diverse spectrum of (hetero)aryl chlorides. Amongst the alkoxide and amide counterions investigated, tert-butoxide was the most effective in inhibiting the homocoupling of arylmagnesiums. PMID:24947849

Chua, Yi-Yuan; Duong, Hung A

2014-07-01

113

Efficient synthesis of benzofuranones: N-heterocyclic carbene (NHC)/base-catalyzed hydroacylation-Stetter-rearrangement cascade.  

PubMed

A N-heterocyclic carbene/base-catalyzed cascade reaction leading to the formation of functionalized benzofuranones is reported. The reaction proceeds via an intramolecular hydroacylation of unactivated alkynes followed by an intermolecular Stetter reaction and a base-catalyzed chromanone to benzofuranone rearrangement. PMID:21919536

Padmanaban, Mohan; Biju, Akkattu T; Glorius, Frank

2011-10-21

114

A short and modular synthesis of bulky and electron-rich N-phosphinomethyl-functionalised N-heterocyclic carbene complexes.  

PubMed

Bulky and electron-rich N-phosphinomethyl-substituted N-heterocyclic carbene transition metal complexes have been prepared in a short and efficient protocol. The modular synthesis allows one to convert borane-protected phosphino-functionalized imidazolium salts into their corresponding metal chelate complexes in a one-pot procedure. PMID:23903289

Brill, Marcel; Kühnel, Erik; Scriban, Corina; Rominger, Frank; Hofmann, Peter

2013-09-28

115

One-pot synthesis of unsymmetrical N-heterocyclic carbene ligands from N-(2-iodoethyl)arylamine salts.  

PubMed

An approach that provides symmetrical, unsymmetrical, and asymmetric N-heterocyclic carbene (NHC) ligands is reported. Reaction of iodoethanol with aniline provides N-(2-iodoethyl)arylamine salts that are then converted to the corresponding iodide. Reaction with aliphatic or aromatic amines followed by triethyl orthoformate was used to provide 26 different NHC ligands. PMID:19624103

Prasad, B A Bhanu; Gilbertson, Scott R

2009-08-20

116

Nickel N-heterocyclic carbene-pyridine complexes that exhibit selectivity for electrocatalytic reduction of carbon dioxide over water.  

PubMed

We report a homologous series of nickel(II) complexes supported by N-heterocyclic carbene-pyridine ((R)bimpy, R = Me, Et, Pr) ligands that exhibit high selectivity for reducing carbon dioxide over water under electrocatalytic conditions. PMID:21556400

Thoi, V Sara; Chang, Christopher J

2011-06-21

117

Insights into the strength and nature of carbene···halogen bond interactions: a theoretical perspective.  

PubMed

Halogen-bonding, a noncovalent interaction between a halogen atom X in one molecule and a negative site in another, plays critical roles in fields as diverse as molecular biology, drug design and material engineering. In this work, we have examined the strength and origin of halogen bonds between carbene CH? and XCCY molecules, where X?=?Cl, Br, I, and Y?=?H, F, COF, COOH, CF?, NO?, CN, NH?, CH?, OH. These calculations have been carried out using M06-2X, MP2 and CCSD(T) methods, through analyses of surface electrostatic potentials V S(r) and intermolecular interaction energies. Not surprisingly, the strength of the halogen bonds in the CH?···XCCY complexes depend on the polarizability of the halogen X and the electron-withdrawing power of the Y group. It is revealed that for a given carbene···X interaction, the electrostatic term is slightly larger (i.e., more negative) than the dispersion term. Comparing the data for the chlorine, bromine and iodine substituted CH?···XCCY systems, it can be seen that both the polarization and dispersion components of the interaction energy increase with increasing halogen size. One can see that increasing the size and positive nature of a halogen's ?-hole markedly enhances the electrostatic contribution of the halogen-bonding interaction. PMID:23455928

Esrafili, Mehdi D; Mohammadirad, Nafiseh

2013-06-01

118

Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts.  

PubMed

Rh-N-heterocyclic carbene compounds [Rh(?-Cl)(IPr)(?(2)-olefin)](2) and RhCl(IPr)(py)(?(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(?(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step. PMID:22536797

Di Giuseppe, Andrea; Castarlenas, Ricardo; Pérez-Torrente, Jesús J; Crucianelli, Marcello; Polo, Victor; Sancho, Rodrigo; Lahoz, Fernando J; Oro, Luis A

2012-05-16

119

Selective arylation, alkenylation, and cyclization of dibromonaphthols, using visible light, via carbene intermediates.  

PubMed

The photoreactivity of several 3-substituted-1,6-dibromo-2-naphthols has been investigated in neat acetonitrile in the presence of diluted Et3N and in aqueous buffered acetonitrile (pH 8, phosphate buffered), using visible light (450 nm). Hydrobromic acid loss in the presence of the base, for the unsubstituted naphthol, or heterolytic C-Br cleavage directly from the naphtholates, for the more acid 3-substutited naphthols (R = COOCH3, CONH2, CONMe2), generates electrophilic carbene intermediates, which have been successfully trapped by molecular oxygen, pyrrole, acrylonitrile, ethyl vinyl ether, and allyltrimethylsilane. Product distribution analysis reveals three types of products arising from (i) arylation, (ii) alkenylation, and (iii) cyclization reactions. The generation and the reactivity of alpha-ketocarbene intermediates, as electrophilc diradicals, has been supported by laser flash photolysis, with the detection of both the carbene (lambda(max) 510 nm) and 1,2-naphthoquinone-O-oxide (R = CONMe2, lambda(max) 600 nm) in the presence of O2. PMID:19522477

Verga, Daniela; Doria, Filippo; Mella, Mariella; Freccero, Mauro

2009-08-01

120

Asymmetric inter- and intramolecular cyclopropanation of alkenes catalyzed by chiral ruthenium porphyrins. Synthesis and crystal structure of a chiral metalloporphyrin carbene complex.  

PubMed

Extensive investigations of asymmetric intermolecular cyclopropanation of terminal alkenes with diazoacetates catalyzed by ruthenium porphyrin [Ru(P*)(CO)(EtOH)] (1, H2P = 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8-dimethanoanthracene-9-yl]porphyrin) and the application of catalyst 1 to asymmetric intramolecular cyclopropanation of allylic or homoallylic diazoacetates are described. The intermolecular cyclopropanation of styrene and its derivatives with ethyl diazoacetate afforded the corresponding cyclopropyl esters in up to 98% ee with high trans/cis ratios of up to 36 and extremely high catalyst turnovers of up to 1.1 x 10(4). Examination of the effects of temperature, diazoacetate, solvent, and substituent in the intermolecular cyclopropanation reveals that (i) both enantioselectivity and trans selectivity increase with decreasing temperature, (ii) sterically encumbered diazoacetates N2CHCO2R, such as R = Bu(t), and donor solvents, such as diethyl ether and tetrahydrofuran, are beneficial to the trans selectivity, and (iii) electron-donating para substituents on styrene accelerate the cyclopropanations, with the log(k(X)/k(H)) vs sigma(+) plot for para-substituted styrenes p-X-C6H4CH=CH2 (X = MeO, Me, Cl, CF3) exhibiting good linearity with a small negative rho(+) value of -0.44 +/- 0.09. In the case of intramolecular cyclopropanation, complex 1 promoted the decomposition of a series of allylic diazoacetates to form the cyclopropyl lactones in up to 85% ee, contributing the first efficient metalloporphyrin catalyst for an asymmetric intramolecular cyclopropanation. Both the inter- and intramolecular cyclopropanations were proposed to proceed via a reactive chiral ruthenium carbene intermediate. The enantioselectivities in these processes were rationalized on the basis of the X-ray crystal structures of closely related stable chiral carbene complexes [Ru(P*)(CPh2)] (2) and [Ru(P*)(C(Ph)CO2CH2CH=CH2)] (3) obtained from reactions of complex 1 with N2CPh2 and N2C(Ph)CO2CH2CH=CH2, respectively. PMID:11457174

Che, C M; Huang, J S; Lee, F W; Li, Y; Lai, T S; Kwong, H L; Teng, P F; Lee, W S; Lo, W C; Peng, S M; Zhou, Z Y

2001-05-01

121

N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition of ketenes and 3-aroylcoumarins: highly enantioselective synthesis of dihydrocoumarin-fused dihydropyranones.  

PubMed

The N-heterocyclic carbene-catalyzed [4 + 2] cyclization of ketenes and 3-aroylcoumarins has been developed to give the corresponding dihydrocoumarin-fused multisubstituted dihydropyranones in high yield with good diastereoselectivity and high enantioselectivity. PMID:23117299

Jian, Teng-Yue; Chen, Xiang-Yu; Sun, Li-Hui; Ye, Song

2013-01-01

122

Alkynyl Fischer carbene complex as a traceless directing group for the regioselective cycloaddition of dithiolethiones to arylacetylene: synthesis of E-dithiafulvene thione and dithioesters.  

PubMed

A phenylethynyl Fischer carbene complex was used as a traceless directing group for the regioselective cycloaddition of dithiolethiones to arylacetylene, which constitutes the first synthesis of E-dithiafulvene thione or dithioesters. PMID:18188475

Granados, Alejandro M; Fracaroli, Alejandro M; de Rossi, Rita H; Fuertes, Pedro; Torroba, Tomás

2008-01-28

123

Highly regioselective [3 + 2] annulation of azomethine imines with 1-alkynyl Fischer carbene complexes to functionalized N,N-bicyclic pyrazolidin-3-ones.  

PubMed

The highly regioselective [3 + 2] cycloaddition of azomethine imines to 1-alkynyl Fischer carbene complexes has been successfully realized under mild conditions. Oxidative demetalation of the newly formed pyrazolo-pyrazolone carbene complexes with pyridine-N-oxide or ceric ammonium nitrate efficiently afforded pyrazolo-pyrazolone derivatives as well as cycloprop-2-enone and trisubstituted 1H-pyrazoles in some cases, providing a novel route to versatile functionalized N,N-bicyclic pyrazolidin-3-ones. PMID:21634395

Luo, Ning; Zheng, Zhaoyan; Yu, Zhengkun

2011-07-01

124

Synthesis and antimicrobial studies of 1-methyl-2-dimethylaminoethyl-substituted benzimidazolium salts and N-heterocyclic carbene–silver complexes  

Microsoft Academic Search

The synthesis and antimicrobial studies of 1-methyl-2-dimethylaminoethyl-substituted carbene precursors and silver complexes are reported. The carbene precursors (1a–d) have been prepared from 1-methyl-2-dimethylaminoethyl-substituted benzimidazole and various alkyl halides. The silver–NHC complexes (2a–d) were synthesized from the benzimidazolium salts and Ag2O in dichloromethane at room temperature. The new compounds were characterized by H NMR, C NMR, FT-IR, and elemental analyses. The

Beyhan Y???t; Yetk?n Gök; ?lknur Özdem?r; Selam? Günal

2012-01-01

125

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

126

Theoretical study of addition reactions of heavy carbenes to carbon and boron nitride nanotubes.  

PubMed

In an effort to gain insight into the activation energies and reaction enthalpies of the chemical functionalization of carbon and boron nitride nanotubes, calculations using density functional theory have been carried out for the cycloaddition of a heavy carbene to a single-walled carbon (SWCNT; C(130)H(20)) and a boron nitride (SWBNNT; B(65)N(65)H(20)) nanotube. The (CH(3))(2)X + SWCNT and (CH(3))(2)X + SWBNNT (X = C, Si, Ge, Sn, and Pb) reactions are the subject of the present study. All the stationary points were determined at the B3LYP/LANL2DZ level of theory. The major conclusions that can be drawn from this work are as follows: (i) Considering both the activation barrier and reaction enthalpy based on the model calculations presented here, it is found that the order of (CH(3))(2)X reactivity is X = C > Si > Ge > Sn > Pb, irrespective of whether cycloaddition is to a SWCNT or a SWBNNT sidewall. That is to say, (CH(3))(2)C and (CH(3))(2)Si can readily add to the sidewalls of SWCNT and SWBNNT, whereas (CH(3))(2)Ge, (CH(3))(2)Sn, and (CH(3))(2)Pb are unreactive. (ii) Since the chemical reactivities of SWCNT and SWBNNT sidewalls closely resemble those of the small C(16)H(10) and B(8)N(8)H(10) molecules, at least in a qualitative sense, the use of the above small molecules as models is sufficient to provide qualitatively correct results. (iii) Our theoretical observations indicate that all the (5,5) SWCNT and SWBNNT cycloadducts favor opened rather than closed three-membered ring structures. (iv) The theoretical investigations demonstrate that the singlet-triplet splitting of the carbene species (R(2)X) as well as that of the small model molecules can be used as a diagnostic tool to predict the addition reactivities of carbene analogues and sidewalls of various nanotubes, respectively. Moreover, the results obtained in this work allow a number of predictions to be made. PMID:16853811

Su, Ming-Der

2005-11-24

127

Toward molecular rotors: tetra-N-heterocyclic carbene Ag(I)-halide cubane-type clusters.  

PubMed

1,3-Bis(3'-butylimidazol-1'-yl)benzene diiodide (2a), 1,3-bis(3'-but-3''-enyl-imidazolium-3'-yl)benzene diiodide (2b), 1,3-bis(3'-pent-4''-enyl-imidazolium-3'-yl)benzene diiodide (2c), 1,3-bis(4'-butyl-1',2',4'-triazolium-1'-yl)benzene diiodide (2d), or 1,3-bis(4'-butyl-1',2',4'-triazolium-1'-yl)benzene dibromide (2e) was reacted with Ag2O yielding unprecedented tetra-N-heterocyclic carbene-Ag(I)-X cubane-type clusters. These were characterized by (1)H and (13)C NMR spectroscopy, ESI-TOF MS, elemental analysis, and X-ray crystallography. Results from VT (13)C NMR spectroscopy and cross-over experiments are consistent with intramolecular exchange suggesting that the Ag(I) complexes are molecular rotors. PMID:23389555

Clark, Wesley D; Tyson, Ginger E; Hollis, T Keith; Valle, Henry U; Valente, Edward J; Oliver, Allen G; Dukes, Matthew P

2013-05-28

128

Photoswitchable organocatalysis: using light to modulate the catalytic activities of N-heterocyclic carbenes.  

PubMed

A 4,5-dithienylimidazolium salt was found to undergo electrocyclic isomerization upon exposure to UV radiation (?(irr) = 313 nm) under neutral and basic conditions; subsequent exposure to visible light reversed the reaction. Under ambient light and in the presence of base, the imidazolium species catalyzed transesterifications as well as amidations in a manner similar to those of previously reported N-heterocyclic carbene precatalysts. However, upon UV irradiation to effect the aforementioned photocyclization, the rate of the transesterification reaction between vinyl acetate and allyl alcohol was significantly attenuated (k(vis/UV) = 12.5), as was the rate of the condensation of ethyl acetate with aminoethanol (k(vis/UV) = 100). The rates of these reactions were successfully toggled between fast and slow states by alternating exposure to visible and UV light, respectively, thus demonstrating a rare example of a photoswitchable catalyst that operates via photomodulation of its electronic structure. PMID:22809240

Neilson, Bethany M; Bielawski, Christopher W

2012-08-01

129

Exploiting Acyl and Enol Azolium Intermediates via NHeterocyclic Carbene Catalyzed Reactions of Alpha-Reducible Aldehydes  

PubMed Central

N-heterocyclic carbenes are well known for their role in catalyzing benzoin and Stetter reactions: the generation of acyl anion equivalents from simple aldehydes to react with a variety of electrophiles. However, when an aldehyde bearing a leaving group or unsaturation adjacent to the acyl anion equivalent is subjected to an NHC, a new avenue of reactivity is unlocked, leading to a number of novel transformations which can generate highly complex products from simple starting materials, many of which are assembled through unconventional bond disconnections. The field of these new reactions - those utilizing ?-reducible aldehydes to access previously unexplored catalytic intermediates – has expanded rapidly in the past eight years. This review aims to provide the reader with a historical perspective on the underlying discoveries that led to the current state of the art, a mechanistic description of these reactions, and a summary of the recent advances in this area.

Vora, Harit U.; Wheeler, Philip

2013-01-01

130

Tungsten(VI) N-Heterocyclic Carbene Complexes: Synthetic, Structural, and Computational Study  

PubMed Central

The reaction of WOCl4 with 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (Idipp) leads to an orange solid whose spectroscopic data are consistent with the 1:1 adduct [WOCl4(Idipp)]. Computational studies at the DFT level further support this formulation. Exposure of this compound to the atmosphere results in rapid hydrolysis to various imidazolium salts. If air diffuses very slowly into solutions of [WOCl4(Idipp)], it also undergoes slow hydrolysis to form [WO2Cl2(Idipp)]. This has been crystallographically characterized and is the first five-coordinate, 1:1 adduct of WO2Cl2. This complex has also been subject to DFT calculations, and its metal–ligand bonding has been explored. The carbene–metal interaction is primarily ?-donor in nature. The mechanism of the hydrolysis has also been probed by computational methods, revealing a plausible, low-energy reaction pathway.

2011-01-01

131

Dimerisation, rhodium complex formation and rearrangements of N-heterocyclic carbenes of indazoles  

PubMed Central

Summary Deprotonation of indazolium salts at low temperatures gives N-heterocyclic carbenes of indazoles (indazol-3-ylidenes) which can be trapped as rhodium complexes (X-ray analysis). In the absence of Rh, the indazol-3-ylidenes spontaneously dimerize under ring cleavage of one of the N,N-bonds and ring closure to an indazole–indole spiro compound which possesses an exocyclic imine group. The E/Z isomers of the imines can be separated by column chromatography when methanol is used as eluent. We present results of a single crystal X-ray analysis of one of the E-isomers, which equilibrate in solution as well as in the solid state. Heating of the indazole–indole spiro compounds results in the formation of quinazolines by a ring-cleavage/ring-closure sequence (X-ray analysis). Results of DFT calculations are presented.

Guan, Zong; Namyslo, Jan C; Drafz, Martin H H; Nieger, Martin

2014-01-01

132

Photolysis of 4-chlororesorcinol in water: competitive formation of a singlet ketene and a triplet carbene.  

PubMed

Research on photoinduced reactions of halogenated phenols is of interest for environmental photochemistry and for synthetic organic chemistry. Previous studies have uncovered interesting mechanistic features, including ring contraction from benzene to cyclopentadiene from ortho derivatives and two-electron processes forming carbocations and carbenes from para derivatives. In the present work, we studied the aqueous photochemistry of 4-chlororesorcinol (1), which combines the conformational properties of both types of derivatives, using nanosecond transient absorption spectroscopy and photoproduct analysis. The absorption spectra obtained upon pulsed laser excitation of 1 showed the occurrence of both o-Cl and p-Cl elimination, the first observed transients being the ketene 3-hydroxy-6-fulvenone (2, ?(max) = 255 nm) and the carbene 2-hydroxy-4-oxo-2,5-cyclohexadienylidene (3, ?(max) = 405 and 395 nm). The reactivities of 2 and 3, the spectra of the secondary transients and the analysis of the final products showed that the two HCl elimination pathways take place concurrently. Most probably, the bifurcation step is the competition between intersystem crossing on the molecular level and o-Cl elimination on the singlet surface; p-Cl elimination proceeds on the triplet surface. Remarkably, the quantum yield of p-Cl elimination from 1 is lower by one order of magnitude compared to that found in para-halogenated phenols, while that of o-Cl elimination from 1 is comparable to ortho-halogenated phenol. To explain this result, we propose that o-Cl elimination is the major deactivation step, forming an intermediate singlet cation which is able to recombine to ground state 1, thereby limiting the observed photochemical quantum yields. PMID:20859579

Richard, Claire; Krajnik, Paul; Grabner, Gottfried

2010-11-14

133

Electronic structure of the carbyne C-Li and the carbene, Li/sub 2/C  

SciTech Connect

The electronic structure of the carbyne C-Li and the carbene Li/sub 2/C has been investigated using ab initio GVB, SCF, and CI techniques. We find that C-Li has a /sup 4/..sigma../sup -/ ground state with the companion /sup 2/II/sub r/ state approximately 34 kcal/mol higher in energy. This should be contrasted with C-H in which the /sup 2/II/sub r/ is the ground state and the /sup 4/..sigma../sup -/ state is approximately 17 kcal/mol higher. In addition we find a /sup 2/II/sub i/ state some 49 kcal/mol above the ground /sup 4/..sigma../sup -/ state. An analysis of the bonding indicates that these states are highly polar. Li/sub 2/C is an unusual carbene having three triplets below the first singlet: /sup 3/..sigma../sub g//sup -/, R = 3.717 bohrs; /sup 3/II/sub g/ (3), R = 3.507 bohrs; /sup 3/A/sub 2/, THETA = 88.1/sup 0/, R = 3.815 bohrs. At the SCF + 1 + 2 level the /sup 3/..sigma../sub g//sup -/ is the ground state with the /sup 3/A/sub 2/ 6.2 kcal/mol higher. Correcting for unlinked clusters inverts this order and makes the /sup 3/A/sub 2/ the ground state 0.2 kcal/mol below the /sup 3/..sigma../sub g//sup -/ state. An analysis of the bonding in the /sup 3/A/sub 2/ states reveals a dative bond between the doubly occupied Li/sub 2/ /sup 2/sigma/sub g/ orbital and an empty p sigma orbital on C. The implications of this mode of bonding for the dilithiomethanes are discussed. 10 figures, 7 tables.

Mavridis, A. (Michigan State Univ., East Lansing); Harrison, J.F.

1982-07-14

134

Tuning effects for some cyclic aromatic carbenes bearing remote amino groups.  

PubMed

Yamamoto and co-workers synthesized two cyclic aromatic carbenes with remote amino groups. Here we theoretically studied related compounds to explore tuning effects on the singlet-triplet splitting by variations of functional groups. For the Yamamoto compound, the lowest singlet state lies 15.7 kcal/mol below the lowest triplet. The singlet-triplet separation is reduced by ?7 kcal/mol when the dimethylamino groups are replaced by H. In one set of carbenes, when X = O, we substitute S, Se, Te, SO, SeO, and TeO for X; the resulting ?E(S-T) predictions are 9.9, 7.3, 3.9, 4.3, 2.3, and -0.1 kcal/mol, respectively. A different set of X fragments yields triplet electronic ground states with ?E(S-T) values of -8.6 (X = BH), -6.8 (X = AlH), -7.2 (X = GaH), -7.5 (X = InH), and -7.0 kcal/mol (X = TlH). We also predicted ?E(S-T) with N(CH3)2 replaced by PH2, AsH2, SbH2, BiH2, BH2, CH3, OH, and F. Of all the molecules considered, that with N(CH3)2 replaced with BH2 and X = BH most favors the triplet state, lying 13.7 kcal/mol below the singlet. Finally, we have relocated the N(CH3)2 and NH2 groups from the (3, 6) positions to the (4, 5), (2, 7), and (1, 8) terminal ring positions, with very interesting results. PMID:24606014

Zeng, Yi; Feng, Hao; Xie, Yaoming; Schaefer, Henry F

2014-04-01

135

Design, synthesis, and study of main chain poly(n-heterocyclic carbene) complexes: applications in electrochromic devices.  

PubMed

A series of poly(N-heterocyclic carbene) complexes in which the carbene functionalities are orthogonally connected to the main chains of the respective polymers have been synthesized via oxidative electropolymerization of various bis(bithiophene)-substituted monomers with appended transition metal or main group entities (M = Ir, Au, Ag, or S). The polymers were characterized using a range of electrochemical, X-ray photoelectron spectroscopy, UV-vis spectroscopy, profilometry, and four-point probe conductivity measurements. Most of the polymers exhibited an intense absorbance wave at 700 nm under oxidative conditions which was attributable to the formation of polarons along the main chains. The iridium-containing thin film poly(8) was found to possess a significant NIR absorbance at 1100 nm in which the metal moiety effectively functioned as an electron sink. Electrochemical analyses of the polymer thin films revealed that they exhibited highly reversible electrochromic phenomena. PMID:20608677

Powell, Adam B; Bielawski, Christopher W; Cowley, Alan H

2010-07-28

136

Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal N-heterocyclic carbene-gold(i) complexes.  

PubMed

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr(F) 4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X-ray crystal-structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process. PMID:24771293

Manzano, Rubén; Wurm, Thomas; Rominger, Frank; Hashmi, A Stephen K

2014-06-01

137

Enantioselective sulfonation of enones with sulfonyl imines by cooperative N-heterocyclic-carbene/thiourea/tertiary-amine multicatalysis.  

PubMed

Many hands make light work: In an organocatalytic asymmetric sulfonation of enones, the activation of a sulfonyl imine by an N-heterocyclic carbene (NHC) catalyst led to the release of a sulfinic anion, which underwent nucleophilic addition to the enone. The enantioselectivity of the process was controlled by a chiral thiourea/amine co-catalyst through anion recognition and hydrogen-bonding interactions. Tol=p-tolyl. PMID:24115649

Jin, Zhichao; Xu, Jianfeng; Yang, Song; Song, Bao-An; Chi, Yonggui Robin

2013-11-18

138

Reactions of Fischer carbene complexes with Electron-deficient olefins: Scope and limitations of this route to donor-acceptor-substituted cyclopropanes  

SciTech Connect

The Fischer carbene complex ((CO){sub 5}Cr{double bond}C(OMe)Ph) (1) is able to transfer its carbene ligand to a variety of electron-deficient olefins and provides donor-acceptor-substituted cyclopropanes in good yields. Apt activating groups with respect to the alkene are ester, amide, nitrile, sulfone, and dialkyl phosphonate functions. Methyl vinyl ketone (19) affords products in low yield that may arise from an intermediate cyclopropane derivative. Phenyl vinyl sulfoxide (24) mainly acts as an oxidizing agent, transforming 1 into methyl benzoate. for olefin 24 and {alpha}-(N-methylanilino)acrylonitrile the authors found products that should be formed on an olefin metathesis pathway. The methyl-substituted carbene complex 48 also affords the expected donor-acceptor-substituted cyclopropanes; however, acyclic isomers are formed in higher amounts. The molybdenum and tungsten complexes 55 and 56, respectively, also furnish cyclopropane derivatives, but the yields are lower than with the chromium compound 1. Disubstituted olefins and complex 1 still give the cyclopropanes in moderate yields, while all trisubstituted and most of the difunctionalized alkenes do not react with this Fischer carbene complex. The cyclopropanes synthesized can be deprotonated and alkylated or transformed into ring-opened products. These model reactions demonstrate the synthetic potentials of donor-acceptor-substituted cyclopropanes prepared via Fischer carbene complexes.

Wienand, A.; Reissig, H.U. (Inst. fuer Organische Chemie der Technischen Hochschule Darmstadt (West Germany))

1990-12-01

139

Synthesis, structure and reactivity of novel pyridazine-coordinated diiron bridging carbene complexes.  

PubMed

[{mu-(Pyridazine-N(1):N(2))}Fe(2)(mu-CO)(CO)(6)](1) reacts with aryllithium reagents, ArLi (Ar = C(6)H(5), m-CH(3)C(6)H(4)) followed by treatment with Me(3)SiCl to give the novel pyridazine-coordinated diiron bridging siloxycarbene complexes [(C(4)H(4)N(2))Fe(2){mu-C(OSiMe(3))Ar}(CO)(6)](2, Ar = C(6)H(5); 3, Ar =m-CH(3)C(6)H(4)). Complex 2 reacts with HBF(4).Et(2)O at low temperature to yield a cationic bridging carbyne complex [(C(4)H(4)N(2))Fe(2)(mu-CC(6)H(5))(CO)(6)]BF(4)(4). Cationic 4 reacts with NaBH(4) in THF at low temperature to afford the diiron bridging arylcarbene complex [(C(4)H(4)N(2))Fe(2){mu-C(H)C(6)H(5)}(CO)(6)](5). Unexpectedly, the reaction of 4 with NaSCH(3) under similar conditions gave the bridging arylcarbene complex 5 and a carbonyl-coordinated diiron bridging carbene complex [Fe(2){mu-C(SCH(3))C(6)H(5)}(CO)(7)](6), while the reaction of NaSC(6)H(4)CH(3)-p with 4 affords the expected bridging arylthiocarbene complex [(C(4)H(4)N(2))Fe(2){mu-C(SC(6)H(4)CH(3)-p)C(6)H(5)}(CO)(6)](7), which can be converted into a novel diiron bridging carbyne complex with a thiolato-bridged ligand, [Fe(2)(mu-CC(6)H(5))(mu-SC(6)H(4)CH(3)-p)(CO)(6)](8). Cationic can also react with the carbonylmetal anionic compound Na(2)[Fe(CO)(4)] to yield complex 5, while the reactions of 4 with carbonylmetal anionic compounds Na[M(CO)(5)(CN)](M = Cr, Mo, W) produce the diiron bridging aryl(pentacarbonylcyanometal)carbene complexes [(C(4)H(4)N(2))Fe(2)-{mu-C(C(6)H(5))NCM(CO)(5)}(CO)(6)](9, M = Cr; 10, M = Mo; 11, M = W). The structures of complexes 2, 5, 6, 8, and 9 have been established by X-ray diffraction studies. PMID:16172652

Xiao, Nu; Xu, Qiang; Sun, Jie; Chen, Jiabi

2005-10-01

140

Gold (I) N-heterocyclic carbene complex inhibits mouse melanoma growth by p53 upregulation  

PubMed Central

Background Cancer treatment using gold (I) complexes is becoming popular. In this study, a gold (I) N-heterocyclic complex designated as complex 3 was synthesized, its cytotoxicity was examined, and its anti-melanoma activity was evaluated in vitro and in vivo. Methods Viability of cancer cells was determined by MTT assay upon treatment with various concentrations of a gold (I) N-heterocyclic carbene complex (complex 3) in a dose and time dependent manner. Mouse melanoma cells B16F10 were selected for further apoptotic studies, including flowcytometric analysis of annexin binding, cell cycle arrest, intracellular ROS generation and loss in the mitochondrial membrane potential. ELISA based assays were done for caspase activities and western blots for determining the expression of various survival and apoptotic proteins. Immunocytology was performed to visualize the translocation of p53 to the nucleus. B16F10 cells were inoculated into mice and post tumor formation, complex 3 was administered. Immunohistology was performed to determine the expressions of p53, p21, NF-?B (p65 and p50), MMP-9 and VEGF. Student’s t test was used for determining statistical significance. The survival rate data were analyzed by Kaplan-Meier plots. Results Complex 3 markedly inhibited the growth of HCT 116, HepG2, and A549, and induced apoptosis in B16F10 cells with nuclear condensation, DNA fragmentation, externalization of phosphatidylserine, activation of caspase 3 and caspase 9, PARP cleavage, downregulation of Bcl-2, upregulation of Bax, cytosolic cytochrome c elevation, ROS generation, and mitochondrial membrane potential loss indicating the involvement of an intrinsic mitochondrial death pathway. Further, upregulation of p53, p-p53 (ser 15) and p21 indicated the role of p53 in complex 3 mediated apoptosis. The complex reduced tumor size, and caused upregulation of p53 and p21 along with downregulation of NF-?B (p65 and p50), VEGF and MMP-9. These results suggest that it induced anti-melanoma effect in vitro and in vivo by modulating p53 and other apoptotic factors. Conclusions The gold (I) N-heterocyclic carbene complex (C22H26N6AuO2PF6) designated as complex 3 induced ROS and p53 dependent apoptosis in B16F10 cells involving the mitochondrial death pathway along with suppression of melanoma tumor growth by regulating the levels of pro and anti apoptotic factors (p53, p21, NF-?B, VEGF and MMP-9).

2014-01-01

141

Probing Protein Surface with a Solvent Mimetic Carbene Coupled to Detection by Mass Spectrometry  

NASA Astrophysics Data System (ADS)

Much knowledge into protein folding, ligand binding, and complex formation can be derived from the examination of the nature and size of the accessible surface area (SASA) of the polypeptide chain, a key parameter in protein science not directly measurable in an experimental fashion. To this end, an ideal chemical approach should aim at exerting solvent mimicry and achieving minimal selectivity to probe the protein surface regardless of its chemical nature. The choice of the photoreagent diazirine to fulfill these goals arises from its size comparable to water and from being a convenient source of the extremely reactive methylene carbene (:CH2). The ensuing methylation depends primarily on the solvent accessibility of the polypeptide chain, turning it into a valuable signal to address experimentally the measurement of SASA in proteins. The superb sensitivity and high resolution of modern mass spectrometry techniques allows us to derive a quantitative signal proportional to the extent of modification (EM) of the sample. Thus, diazirine labeling coupled to electrospray mass spectrometry (ESI-MS) detection can shed light on conformational features of the native as well as non-native states, not easily addressable by other methods. Enzymatic fragmentation of the polypeptide chain at the level of small peptides allows us to locate the covalent tag along the amino acid sequence, therefore enabling the construction of a map of solvent accessibility. Moreover, by subsequent MS/MS analysis of peptides, we demonstrate here the feasibility of attaining amino acid resolution in defining the target sites.

Gómez, Gabriela E.; Mundo, Mariana R.; Craig, Patricio O.; Delfino, José M.

2012-01-01

142

Conformation analysis of triphenylphosphine in trans and cis triphenylphosphine-substituted Fischer carbene complexes  

NASA Astrophysics Data System (ADS)

The synthesis and selected crystal structures of novel (M = Cr or Mo) and known (M = W) [(CO)4(PPh3)M = C(OEt)R] complexes, M = Cr, R = 2-thienyl (1), 2-furyl (2); M = Mo, R = 2-thienyl (3), 2-furyl (4); M = W, R = 2-thienyl (5), 2-furyl (6), are presented. Experimental crystal structures and DFT calculations of selected trans- and cis-triphenylphosphine-substituted Fischer carbene complexes, illustrate that the minimum energy conformation of triphenylphosphine (PPh3) in octahedral [(CO)4(PPh3)ML]-complexes generally have distinct features that can be described in terms of the "plane of nadir energy", a plane linking all points of minimum steric compression between the ligands. The generally observed orientation of PPh3 involves a correlated feathering of the phenyl groups with the PCipso bond of one phenyl group orientated near parallel to the nadir plane, and a meta carbon (Cm) of the other two phenyl groups orientated as near as possible to the nadir plane, orthogonal to the first. Although the orientation of PPh3 in 6-trans, [(CO)4(PPh3)W = C(OEt)2-furyl], deviates from this, DFT correctly calculated the unexpected and not generally observed PPh3 orientation.

Landman, Marilé; Levell, Tamzyn; van Rooyen, Petrus H.; Conradie, Jeanet

2014-05-01

143

?,?-Unsaturated Acyl Azoliums from N-Heterocyclic Carbene Catalyzed Reactions: Observation and Mechanistic Investigation**  

PubMed Central

Catalytically generated acyl azoliums I and their ?,?-unsaturated counterparts II are thought to be key reactive intermediates in a rapidly growing number of transformations promoted by N-heterocyclic carbene (NHC) catalysts.[1] Acyl azoliums are invoked in the postulated catalytic cycles of nearly all of the new NHC-catalyzed reactions of ?-functionalized aldehydes reported since 2004, in which they are generally assumed to possess the reactivity of an activated carboxylic acid, that is, analogous to an activated ester.[2] In NHC-catalyzed processes, they are most often obtained through internal redox reactions of functionalized aldehydes but have also been prepared by oxidations of the Breslow intermediates[3] or additions to ketenes.[4] Acyl azoliums I are important intermediates in thiamine pyrophosphate (ThPP) dependent enzymatic reactions.[5] Townsend et al. have recently proposed that unsaturated acyl azolium III is the key intermediate in clavulanic acid biosynthesis;[6] despite careful efforts, III or its analogues II have never been characterized or independently synthesized. Here, we document the observation and characterization of ?,?-unsaturated acyl azoliums 1 and 2 (Scheme 1) and demonstrate that their corresponding hemiacetals (1? and 2”) are the kinetically important intermediates in both their acylation and annulation reactions.

Mahatthananchai, Jessada; Zheng, Pinguan; Bode, Jeffrey W.

2011-01-01

144

Mechanism of ultrasound scission of a silver-carbene coordination polymer.  

PubMed

Scission of a supramolecular polymer-metal complex can be carried out using collapsing cavitation bubbles created by ultrasound. Although the most plausible scission mechanism of the coordinative bonds is through mechanical force, the influence of radicals and high hot-spot temperatures on scission has to be considered. A silver(I)-N-heterocyclic carbene complex was exposed to 20 kHz ultrasound in argon, nitrogen, methane, and isobutane saturated toluene. Scission percentages were almost equal under argon, nitrogen, and methane. Radical production differs by a factor of 10 under these gases, indicating that radical production is not a significant contributor to the scission process. A model to describe the displacement of the bubble wall, strain rates, and temperature in the gas shows that critical strain rates for coil-to-stretch transition, needed for scission, are achieved at reactor temperatures of 298 K, an acoustic pressure of 1.2 × 10(5) Pa, and an acoustic frequency of 20 kHz. Lower scission percentages were measured under isobutane, which also shows lower strain rates in model simulations. The activation of the polymer-metal complexes in toluene under the influence of ultrasound occurs through mechanical force. PMID:21827184

Rooze, Joost; Groote, Ramon; Jakobs, Robert T M; Sijbesma, Rint P; van Iersel, Maikel M; Rebrov, Evgeny V; Schouten, Jaap C; Keurentjes, Jos T F

2011-09-29

145

Synthesis and reactivity of new bis(N-heterocyclic carbene) iridium(I) complexes.  

PubMed

New complexes of the type trans-[IrCl(?(2)-COE)(NHC)2] (COE = cis-cyclooctene; NHC = N-heterocyclic carbene) have been prepared in one step from the reaction of ca. 4 equiv of NHC or [AgCl(NHC)] with [IrCl(?(2)-COE)2]2 in benzene. These new complexes have been characterized by techniques including NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. Exposing trans-[IrCl(COE)(I(i)Pr(Me))2] to CO yielded trans-[IrCl(CO)(I(i)Pr(Me))2], which is the only bis(NHC) analogue of Vaska's complex trans-[IrCl(CO)(PPh3)2] known to date. The synthesis of trans-[Ir(CO)(I(i)Pr(Me))2(R)] (R = MeO, PhCC, OSiPh3, O2CPh) complexes has been achieved via deprotonation reactions involving the new hydroxide species trans-[Ir(OH)(CO)(I(i)Pr(Me))2]. PMID:24147789

Nelson, David J; Truscott, Byron J; Slawin, Alexandra M Z; Nolan, Steven P

2013-11-01

146

Canopied trans-chelating bis(N-heterocyclic carbene) ligand: synthesis, structure and catalysis.  

PubMed

The terphspan scaffold was employed to support a bis(N-heterocyclic carbene) ligand () that provides a m-terphenyl canopy over one side of its metal complexes. Single-crystal X-ray diffraction studies on a silver complex of {[Ag()]AgBr(2)}(2) revealed an unusual tetranuclear silver core with a Ag-Ag bond distance of 3.0241(8) A with as a trans-chelating ligand (C-Ag-C = 171 degrees ). A preliminary X-ray structure of pseudo-square planar [PdCl(2)()] showed a similar binding mode of (C-Pd-C = 177 degrees ). High yields were obtained in Suzuki-Miyaura coupling reactions utilizing [PdCl(2)()] as the procatalyst and the results were compared with analogous complexes of trans-spanning diphosphine () and diphosphinite () complexes. The diphosphinite complex, [PdCl(2)()], decomposes to [mu-ClPd(PPh(2)OH)(PPh(2)O)](2) at room temperature. PMID:19259573

Morgan, Brad P; Galdamez, Gabriela A; Gilliard, Robert J; Smith, Rhett C

2009-03-21

147

Aerosolized Antimicrobial Agents Based on Degradable Dextran Nanoparticles Loaded with Silver Carbene Complexes  

PubMed Central

Degradable acetalated dextran (Ac-DEX) nanoparticles were prepared and loaded with a hydrophobic silver carbene complex (SCC) by a single-emulsion process. The resulting particles were characterized for morphology and size distribution using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). The average particle size and particle size distribution were found to be a function of the ratio of the organic phase to the surfactant containing aqueous phase with a 1:5 volume ratio of Ac-DEX CH2Cl2 (organic): PBS (aqueous) being optimal for the formulation of nanoparticles with an average size of 100 ± 40 nm and a low polydispersity. The SCC loading was found to increase with an increase in the SCC quantity in the initial feed used during particle formulation up to 30% (w/w); however, the encapsulation efficiency was observed to be the best at a feed ratio of 20% (w/w). In vitro efficacy testing of the SCC loaded Ac-DEX nanoparticles demonstrated their activity against both Gram-negative and Gram-positive bacteria; the nanoparticles inhibited the growth of every bacterial species tested. As expected, a higher concentration of drug was required to inhibit bacterial growth when the drug was encapsulated within the nanoparticle formulations compared with the free drug illustrating the desired depot release. Compared with free drug, the Ac-DEX nanoparticles were much more readily suspended in an aqueous phase and subsequently aerosolized, thus providing an effective method of pulmonary drug delivery.

Ornelas-Megiatto, Catia; Shah, Parth N.; Wich, Peter R.; Cohen, Jessica L.; Tagaev, Jasur A.; Smolen, Justin A.; Wright, Brian D.; Panzner, Matthew J.; Youngs, Wiley J.; Frechet, Jean M. J.; Cannon, Carolyn L.

2012-01-01

148

Theoretical study on the mechanism of extraction reaction between silylene carbene and its derivatives and thiirane  

NASA Astrophysics Data System (ADS)

The mechanism of the sulfur extraction reaction between singlet silylene carbine and its derivatives and thiirane has been investigated with density functional theory (DFT), including geometry optimization and vibrational analysis for the involved stationary points on the potential energy surface. The energies of the different conformations are calculated by B3LYP/6-311G( d, p) method. From the potential energy profile, it can be predicted that the reaction pathway of this kind consists in two steps: (1) the two reactants firstly form an intermediate through a barrier-free exothermic reaction; (2) the intermediate then isomerizes to a product via a transition state. This kind of reactions has similar mechanism: when the silylene carbene and its derivatives [X2Si=C: (X = H, F, Cl, CH3)] and thiirane approach each other, the shift of 3 p lone electron pair of S in thiirane to the 2 p unoccupied orbital of C in X2Si=C: gives a p ? p donor-acceptor bond, thereby leading to the formation of intermediate (INT). As the p ? p donor-acceptor bond continues to strengthen (that is the C-S bond continues to shorten), the intermediate (INT) generates product (P + C2H4) via transition state (TS). 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, X. H.; Che, X.; Shi, L.; Han, J. F.; Lian, Z. X.; Li, Y. Q.

2010-12-01

149

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis.  

PubMed

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(?-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis. PMID:23288304

Fèvre, Maréva; Pinaud, Julien; Gnanou, Yves; Vignolle, Joan; Taton, Daniel

2013-03-01

150

All-electron relativistic multireference configuration interaction investigation of fluoroiodo carbene.  

PubMed

We present herein the first all-electron relativistic internally contracted multireference configuration interaction with Davidson correction (icMRCI+Q) study on the low-lying states of fluoroiodo carbene, CFI, which contains the most electronegative element (fluorine) and the heavy halogen (iodine). The potential energy surface (PES) of the first excited singlet state (Ã(1)A?) of CFI was carefully examined along the C-I bond distance at the icMRCI+Q/ANO-RCC level, while the other two geometric parameters were optimized at every C-I bond length in contrast to fixing them at the equilibrium values. A reliable barrier height of the Ã(1)A? state was determined to be 625 cm(-1) by our high-level icMRCI+Q calculations with large ANO-RCC basis set and with inclusion of the spin-orbit coupling, core-valence correlation, and zero-point-energy. Finally, the electronic states of CFI with vertical transition energy up to 6 eV were studied. The calculation presented here will provide more comprehensive results about the structure and behavior of electronic states of CFI radical. PMID:24611930

Sun, Erping; Lv, Hang; Shi, Dandan; Wei, Changli; Xu, Haifeng; Yan, Bing

2014-04-01

151

Cytotoxic Gold(I) N-heterocyclic Carbene Complexes with Phosphane Ligands as Potent Enzyme Inhibitors.  

PubMed

Organometallic gold complexes with N-heterocyclic carbene (NHC) ligands have been demonstrating promising properties as novel anticancer agents. Gold(I) NHC complexes containing different phosphanes as secondary ligands were shown to trigger strong cytotoxic effects in cancer cells, and their effective uptake into the cells was quantified by atomic absorption spectroscopy. Moreover, the new compounds strongly inhibited the activity of the seleno-enzyme thioredoxin reductase (TrxR) and of the zinc-finger enzyme poly(ADP-ribose) polymerase?1 (PARP-1). In the case of TrxR inhibition, their activity depended clearly on the size of the alkyl/aryl residues of phosphorus atoms. Density functional theory (DFT) calculations showed that the Au?P bond of the triphenylphosphane complex [Au(I) (NHC)(PPh3 )]I had a lower bond dissociation energy compared to trialkylphosphane complexes [Au(I) (NHC)(PR3 )]I, indicating a higher kinetic reactivity of this particular compound. In fact, [Au(I) (NHC)(PPh3 )]I triggered an enhanced inhibitory activity against PARP-1. PMID:24677779

Rubbiani, Riccardo; Salassa, Luca; de Almeida, Andreia; Casini, Angela; Ott, Ingo

2014-06-01

152

Two Fe3(?3-S)2(CO)8 clusters with terminal N-heterocyclic carbenes.  

PubMed

The title compounds with terminal N-heterocyclic carbenes, namely octacarbonyl(imidazolidinylidene-?C(2))di-?3-sulfido-triiron(II)(2 Fe-Fe), [Fe3(C3H6N2)(?3-S)2(CO)8], (I), and octacarbonyl(1-methylimidazo[1,5-a]pyridin-3-ylidene-?C(3))di-?3-sulfido-triiron(II)(2 Fe-Fe), [Fe3(C8H8N2)(?3-S)2(CO)8], (II), have been synthesized. Each compound contains two Fe-Fe bonds and two S atoms above and below a triiron triangle. One of the eight carbonyl ligands deviates significantly from linearity. In (I), dimers generated by an N-H...S hydrogen bond are linked into [001] double chains by a second N-H...S hydrogen bond. These chains are packed by a C-H...O hydrogen bond to yield [101] sheets. In (II), dimers generated by an N-H...S hydrogen bond are linked by C-H...O hydrogen bonds to form [111] double chains. PMID:24898951

Yang, Wei; Fu, Qiang; Zhao, Jing; Cheng, Huan Ren; Shi, Yao Cheng

2014-06-15

153

Synthesis, structural characterization and properties of new N-heterocyclic carbene Ag(I) complexes  

NASA Astrophysics Data System (ADS)

The syntheses and properties of new N-heterocyclic carbene complexes [{1-(2,3,5,6-tetra methyl benzyl)-3-benzylbimy}AgCl] (2a), [{1-(benzyl)-3-benzylbimy}2Ag][BF4] (2b), [{1-(benzyl)-3-butylbimy}AgBr]2 (2c), [{1-(benzyl)-3-(4-nitro-benzyl)bimy}AgBr] (2d), [{1-(benzyl)-3-(4-bromo-benzyl)bimy}AgBr] (2e), [{1-(benzyl)-3-(4-nitro-benzyl)bimy}2Ag][BF4] (2f), [{1-(benzyl)-3-(4-bromo-benzyl)bimy}2Ag][BF4] (2g) (bimy = benzimidazol-2-ylidene), and [{1-(10-bromo-9-anthracenylmethyl)-3-butylimy}AgBr] (2h) (imy = imidazol-2-ylidene) have been described. Compounds 2a-2h have been characterized by 1H NMR-, UV-vis- LC/MS-spectral studies and elemental analysis. Compounds 2a-2c and 2h are unambiguously characterized by single crystal X-ray crystallography. Compound 2h shows solution emission at room temperature.

Kishore, Ravada; Das, Samar K.

2013-12-01

154

Magnetotelluric Data, Stable Distributions and Stable Regression  

NASA Astrophysics Data System (ADS)

The author has noted for many years that the residuals from robust or bounded influence estimates of the magnetotelluric response function are systematically long tailed compared to a Gaussian or Rayleigh distribution. Consequently, the standard statistical model of a Gaussian core contaminated by a fraction of outlying data is not really valid. However, the typical result is an improvement on ordinary least squares, and has become standard in the electromagnetic induction community. A recent re-evaluation of the statistics of magnetotelluric response function estimation has shown that, in almost all cases, the residuals are alpha stable rather than Gaussian. Alpha stable distributions are characterized by four parameters: a shape parameter lying on (0, 2], a skewness parameter, a scale parameter and a location parameter, and cannot be expressed in closed form except for a few special cases. When the shape parameter is 2, the result is Gaussian, but when it is smaller the resulting distribution has infinite variance. Typical magnetotelluric residuals are alpha stable with a shape parameter lying between 1 and 2. This suggests that robust methods improve response function estimates by eliminating data corresponding to the largest stable residuals while leaving the bulk of the population alone. A better statistical approach is based on stable regression that directly accommodates the actual residual distribution without eliminating the most extreme ones. This paper will introduce such an algorithm, and illustrate its functionality with a variety of magnetotelluric data. Further work remains to produce a robust stable regression algorithm that will eliminate real outliers such as lightning strikes or instrument problems without affecting the bulk stable population. Stable distributions are intimately associated with fractional derivative physical processes. Since the Maxwell equations and the constitutive relations pertaining to the earth do not contain any fractional derivatives, the pervasive occurrence of stable residuals for magnetotelluric response function estimates must have an origin in the ionospheric and magnetospheric sources.

Chave, A. D.

2013-12-01

155

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

156

Reductions of aldehydes and ketones with a readily available N-heterocyclic carbene borane and acetic acid  

PubMed Central

Summary Acetic acid promotes the reduction of aldehydes and ketones by the readily available N-heterocyclic carbene borane, 1,3-dimethylimidazol-2-ylidene borane. Aldehydes are reduced over 1–24 h at room temperature with 1 equiv of acetic acid and 0.5 equiv of the NHC-borane. Ketone reductions are slower but can be accelerated by using 5 equiv of acetic acid. Aldehydes can be selectively reduced in the presence of ketones. On a small scale, products are isolated by evaporation of the reaction mixture and direct chromatography.

Lamm, Vladimir; Pan, Xiangcheng

2013-01-01

157

Stereospecific asymmetric N-heterocyclic carbene (NHC)-catalyzed redox synthesis of trifluoromethyl dihydropyranones and mechanistic insights.  

PubMed

N-heterocyclic carbene (NHC)-catalyzed redox asymmetric hetero-Diels-Alder reactions of ?-aroyloxyaldehydes with ?-trifluoromethyl enones generates synthetically useful dihydropyranones containing a stereogenic trifluoromethyl substituent in good yields (up to 81%) and excellent diastereoselectivity and enantioselectivity (up to >95:5 dr and >99% ee). The process is stereospecific, with use of either (E)- or (Z)-?-trifluoromethyl enones forming syn- or anti-dihydropyranone products, respectively. Mechanistic studies through in situ kinetic analysis of the reaction reveal key differences in reactivity between chiral NHC precursor 1 and an achiral NHC precursor. PMID:23957790

Davies, Alyn T; Taylor, James E; Douglas, James; Collett, Christopher J; Morrill, Louis C; Fallan, Charlene; Slawin, Alexandra M Z; Churchill, Gwydion; Smith, Andrew D

2013-09-20

158

Spiro-fused six-membered N-heterocyclic carbene: a new scaffold toward unique properties and activities.  

PubMed

A six-membered N-heterocyclic carbene fused with a spiro-scaffold is designed. The new NHC shows stronger ?-donation ability than typical 5-membered NHCs. This property leads to interesting reactivities of this spiro-fused six-membered NHC. For example, the NHC-BF3 Lewis pair complex can be readily prepared by using LiBF4 as the BF3 source, or through a direct bond-reconstruction of the tetrafluoroborate salt NHC·HBF4. PMID:24851922

Yang, Bin-Miao; Xiang, Kai; Tu, Yong-Qiang; Zhang, Shi-Heng; Yang, Deng-Tao; Wang, Shao-Hua; Zhang, Fu-Ming

2014-06-10

159

The stable marriage problem  

Microsoft Academic Search

The original work of Gale and Shapley on an assignment method using the stable marriage criterion has been extended to find all the stable marriage assignments. The algorithm derived for finding all the stable marriage assignments is proved to satisfy all the conditions of the problem. Algorithm 411 applies to this paper.

D. G. McVitie; L. B. Wilson

1971-01-01

160

Novel dimetal bridging carbene complexes derived from a terminal carbonyl dimetal compound. Syntheses, structures and reactivities of 7H-indene-coordinated diiron bridging carbene complexes.  

PubMed

Pentacarbonyl-7H-indenediiron, [Fe2(CO)5(eta3,eta5-C9H8)] (1), reacts with aryllithium, ArLi (Ar = C6H5, p-C6H5C6H4), followed by alkylation with Et3OBF4 to give novel 7H-indene-coordinated diiron bridging alkoxycarbene complexes [Fe2{mu-C(OC2H5)Ar}(CO)4(eta4,eta4-C9H8)] (2, Ar = C6H5; 3, Ar = p-C6H5C6H4). Complexes 2 and 3 react with HBF4.Et2O at low temperature to yield cationic bridging carbyne complexes [Fe2(mu-CAr)(CO)4(eta4,eta4-C9H8)]BF4 (4, Ar = C6H5; 5, Ar = p-C6H5C6H4). Cationic 4 and 5 react with NaBH4 in THF at low temperature to afford diiron bridging arylcarbene complexes [Fe2{mu-C(H)Ar}(CO)4(eta4,eta4-C9H8)] (6, Ar = C6H5; 7, Ar = p-C6H5C6H4). The similar reactions of 4 and 5 with NaSC6H4CH3-p produce the bridging arylthiocarbene complexes [Fe2{mu-C(Ar)SC6H4CH3-p}(CO)4(eta4,eta4-C9H8)] (8, Ar = C6H5; 9, Ar = p-C6H5C6H4). Cationic 4 and 5 can also react with anionic carbonylmetal compounds Na[M(CO)5(CN)] (M = Cr, Mo, W) to give the diiron bridging aryl(pentacarbonylcyanometal)carbene complexes [Fe2{mu-C(Ar)NCM(CO)5}(CO)4(eta4,eta4-C9H8)] (10, Ar = C6H5, M = Cr; 11, Ar = p-C6H5C6H4, M = Cr; 12, Ar = C6H5, M = Mo; 13, Ar = p-C6H5C6H4, M = Mo; 14, Ar = C6H5, M = W; 15, Ar = p-C6H5C6H4, M = W). Interestingly, in CH2Cl2 solution at room temperature complexes 10-15 were transformed into the isomerized 7H-indene-coordinated monoiron complexes [Fe(CO)2(eta5-C9H8)C(Ar)NCM(CO)5] (16, Ar = C6H5, M = Cr; 17, Ar = p-C6H5C6H4, M = Cr; 18, Ar = C6H5, M = Mo; 19, Ar = p-C6H5C6H4, M = Mo; 20, Ar = C6H5, M = W; 21, Ar = p-C6H5C6H4, M = W), while complex 3 was converted into a novel ring addition product [Fe2{C(OC2H5)C6H4C6H5-p-(eta2,eta5-C9H8)}(CO)5] (22) under the same conditions. The structures of complexes 2, 6, 8, 14, 18 and 22 have been established by X-ray diffraction studies. PMID:16967119

Zhang, Lei; Sun, Jie; Zhu, Huping; Xu, Qiang; Tsumori, Nobuko; Chen, Jiabi

2006-09-28

161

N-Heterocyclic carbenes and charge separation in heterometallic s-block silylamides.  

PubMed

Addition of the N-heterocyclic carbene (NHC), 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene (IPr), to equimolar quantities of group 1 and group 2 bis(trimethylsilyl)amides results in the isolation of charge separated species, [M(IPr)(2)](+)[M'{N(SiMe(3))(2)}(3)](-) (M = Li, Na, K; M' = Mg, Ca, Sr, Ba). Although these systems were found to be prone to the separation of oily, most likely liquid clathrate, materials, either slow cooling or careful diffusion of the less polar solvent hexane into toluene solutions yielded nine crystalline heterobimetallic complexes in which the coordination sphere of the cationic group 1 center was found by X-ray diffraction analysis to be provided by two IPr ligands. These derivatives are the first examples of any compounds in which coordination at the central alkali metal cation is provided exclusively by NHC ligands and, for the cases where M = Na, are the first instances of any type in which an NHC ligand is bound to sodium. The anionic group 2-containing component of each compound was found to comprise three bis(trimethylsilyl)amido ligands coordinated in an approximately trigonal array about the divalent metal center. The bonding within the unusual cationic components of the compounds has been investigated by density functional theoretical (DFT) methods. Natural Bond Orbital (NBO) analyses have revealed that the coordination is provided by donation of the sp-hydridized IPr lone pair into the valence s-orbital of the alkali metal cation and are consistent with weaker binding, and consequently more labile solution behavior, as group 1 is descended. PMID:21517018

Hill, Michael S; Kociok-Köhn, Gabriele; MacDougall, Dugald J

2011-06-01

162

Synthesis of Pd-N-heterocyclic carbene Pd-catalyst and its application in MW-assisted Heck and Suzuki reaction  

EPA Science Inventory

The first Pd-N-heterocyclic carbene (NHC) complex in the form of organic silica is prepared using sol-gel method and its application in Heck and Suzuki reactions are demonstrated. These C-C coupling reactions proceeded efficiently under the influence of microwave irradiation, wit...

163

An N-Heterocyclic Carbene as a Bidentate Hemilabile Ligand: A Synchrotron X-ray Diffraction and Density Functional Theory Study  

SciTech Connect

The N?heterocyclic carbene ligand IMes was shown by synchrotron crystallography and DFT computations to adopt a hemilabile bidentate coordination mode in CpM(CO)2(IMes)+B(C6F5)4- (M = Mo, W), with a C=C bond of one mesityl weakly coordinated to the metal

Dioumaev, Vladimir K.; Szalda, David J.; Hanson, Jonathan; Franz, James A.; Bullock, Morris

2003-06-11

164

Ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene ligands: synthesis, structure, and catalytic activity.  

PubMed

A series of ruthenium-based olefin metathesis catalysts coordinated with unsymmetrical N-heterocyclic carbene (NHC) ligands has been prepared and fully characterized. These complexes are readily accessible in one or two steps from commercially available [(PCy(3))(2)Cl(2)Ru==CHPh]. All of the complexes reported herein promote the ring-closing of diethyldiallyl and diethylallylmethallyl malonate, the ring-opening metathesis polymerization of 1,5-cyclooctadiene, and the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, in some cases surpassing in efficiency the existing second-generation catalysts. Especially in the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene, all new catalysts demonstrate similar or higher activity than the second-generation ruthenium catalysts and, most importantly, afford improved E/Z ratios of the desired cross-product at conversion above 60 %. The influence of the unsymmetrical NHC ligands on the initiation rate and the activation parameters for the irreversible reaction of these ruthenium complexes with butyl vinyl ether were also studied. Finally, the synthesis of the related chlorodicarbonyl(carbene) rhodium(I) complexes allowed for the study of the electronic properties of the new unsymmetrical NHC ligands that are discussed in detail. PMID:18637651

Vougioukalakis, Georgios C; Grubbs, Robert H

2008-01-01

165

Transition metal-induced activation of alkynes leading to metal carbene species: synthetic application to new ?-conjugated molecules  

NASA Astrophysics Data System (ADS)

The in situ generation of furylcarbene complexes by means of transition metals was applied to catalytic carbene reactions, such as Wittig-type olefination and cycloisomerization reactions. In the presence of [Rh(OAc)2]2 as a catalyst and triphenylphosphine as a carbene accepter, carbonyl-ene-ynes 1a reacted with benzaldehyde to give a 2-styrylfuran derivative 3a. The reaction involving phosphine-ylide formation followed by Wittig-type olefination with aldehydes was applied to synthesis of furfurylidene-containing p-extended conjugated molecules. A Ruthenium catalyst underwent cycloisomerization of 1,2-bis(carbonylenyl)acetylene 10a,b to afford 2,2'-bifuran structures. The crystal structure of 5,5'-diphenyl-2,2'-bifuran 11b was determined by X-ray crystallography. The ORTEP drawings of 11b exhibited transoid and highly planar structure. The emission bands of the bisfuran 11b were observed at 414 and 440 nm, its quantum yield (84%) was higher than that of a similar 5,5'-diphenyl-2,2'-bithiophene.

Abo, T.; Ohe, K.

2008-03-01

166

Organic functionalization of the Si (100) and Ge (100) surfaces by cycloadditions of carbenes and nitrenes: a theoretical prediction.  

PubMed

By means of density functional theory (B3LYP/6-31G*) coupled with effective cluster models, we predict that the well-known cycloaddition reactions of carbenes and nitrenes to alkenes in organic chemistry can be employed as a new type of surface reaction to organically functionalize the Si (100) and Ge (100) surfaces at low temperature. The well-established abundance of carbenes and nitrenes addition chemistry in organic chemistry provides versatile flexibility of functionalizing the surfaces of Si (100) and Ge (100), which can potentially impart new organic functionalities to the semiconductors surface for novel applications in a diversity of fields. Our predictions strongly advance the concept of using organic reactions to modify the solid surface in a controlled manner and quite intriguing chemistry can lie in the material featuring the analogous bonding motif. In further perspective, implications for other theoretical work, regarding disilenes, digermenes, silenes, and germenes that all feature the bonding motif similar to alkenes, are also discussed. PMID:16494329

Xu, Yi-Jun; Zhang, Yong-Fan; Li, Jun-Qian

2006-02-23

167

I-stable ideals  

NASA Astrophysics Data System (ADS)

We define the concept of I-stable ideals in the ring of commutative polynomials over a field, generalizing the so-called stable ideals, which arise as ideals of higher terms under general linear changes of variables. The interest in ideals of this type is motivated by the fact that certain problems concerning homogeneous ideals (for example, the problem of obtaining upper estimates for the graded Betti numbers) can be reduced to the study of stable ideals. I-stable ideals retain many interesting properties of stable ideals. In particular, the minimal resolutions of I-stable ideals constructed in this paper enable us to obtain an explicit formula for the graded Betti numbers, which turn out to be independent of the characteristic of the ground field. Factor rings by I-stable ideals generated by monomials of degree >=2 are Golod rings. We also consider other analogues of stable ideals (strongly and weakly I-stable ideals) and give conditions sufficient for the factor ring by an I-stable ideal to be Cohen-Macaulay or Gorenstein.

Shakin, D. A.

2002-06-01

168

Imidazol(in)ium hydrogen carbonates as a genuine source of N-heterocyclic carbenes (NHCs): applications to the facile preparation of NHC metal complexes and to NHC-organocatalyzed molecular and macromolecular syntheses.  

PubMed

Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations. PMID:22455795

Fèvre, Maréva; Pinaud, Julien; Leteneur, Alexandre; Gnanou, Yves; Vignolle, Joan; Taton, Daniel; Miqueu, Karinne; Sotiropoulos, Jean-Marc

2012-04-18

169

INVERSE STABLE SUBORDINATORS  

PubMed Central

The inverse stable subordinator provides a probability model for time-fractional differential equations, and leads to explicit solution formulae. This paper reviews properties of the inverse stable subordinator, and applications to a variety of problems in mathematics and physics. Several different governing equations for the inverse stable subordinator have been proposed in the literature. This paper also shows how these equations can be reconciled.

MEERSCHAERT, MARK M.; STRAKA, PETER

2013-01-01

170

Photophysical and photochemical trends in tricarbonyl rhenium(I) N-heterocyclic carbene complexes.  

PubMed

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO)3(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the central imidazole ring is substituted at the N3 atom by a butyl, a phenyl, or a mesityl group and substituted at the N1 atom by a pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the ancillary L ligand alternates between chloro and bromo. For the majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus, the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re-C bond cleaves from excited states of metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the pyrimidyl- and quinoxyl-NHC ligands is proposed. PMID:24665819

Vaughan, Jamila G; Reid, Brodie L; Wright, Phillip J; Ramchandani, Sushil; Skelton, Brian W; Raiteri, Paolo; Muzzioli, Sara; Brown, David H; Stagni, Stefano; Massi, Massimiliano

2014-04-01

171

Accessible gold clusters using calix[4]arene N-heterocyclic carbene and phosphine ligands.  

PubMed

We investigate the synthesis of accessible calix[4]arene-bound gold clusters consisting of open "coordinatively unsaturated" active sites, using a comparative approach that relies on calix[4]arene ligands with various upper- and lower-rim substituents. In contrast with a reported Au(I)-tert-butyl-calixarene phosphine complex, which exhibits a single cone conformer in solution, the H upper-rim analog exhibits multiple conformers in solution. This contrasts with observations of the tert-butyl upper-rim analog, which exhibits a single cone conformer in solution under similar conditions. In the solid state, as determined by single-crystal X-ray diffraction, both H and tert-butyl upper-rim analogs exhibit exclusively cone conformer. A detailed structural analysis of these two solid-state structures highlights a CH-? interaction involving a methoxy lower-rim substituent and phenyl substituent on P as the key feature that enforces a tight configuration of Au(I) atoms on the same side of the calix[4]arene lower-rim plane. We hypothesize that such a configuration promotes chelation of the ligand to a gold surface and facilitates the synthesis of small Au11-sized clusters after reduction of both complexes. The new cluster, like the one reported with the tert-butyl analog, has an extraordinary 25% of surface atoms that are open and accessible to a 2-NT (2-naphthalenethiol) probe in solution. We also investigated the effect of calix[4]arene lower-rim substituents that coordinate to the metal, by using N-heterocyclic carbene (NHC) functional groups rather than phosphines. Four small (<1.6 nm diameter) calix[4]arene NHC-bound gold clusters were synthesized, including three using novel calix[4]arene NHC ligands. The smallest calix[4]arene NHC-bound Au cluster consisted of a 1.2 nm gold core, and its number density of accessible and open surface sites was measured. This required development of a new titration method for open sites on gold clusters, using a SAMSA fluorescein dye molecule, which excites and emits at lower energy relative to the previously used 2-NT probe. The number density of open sites on the new calix[4]arene NHC-bound gold cluster measured by the SAMSA fluorescein probe strongly supports the generality of a mechanical model of accessibility, which does not depend on the functional group involved in binding to the gold surface and rather depends on the relative radii of curvature of bound ligands and the gold cluster core. PMID:23917776

Nigra, Michael M; Yeh, Alexander J; Okrut, Alexander; DiPasquale, Antonio G; Yeh, Sheila W; Solovyov, Andrew; Katz, Alexander

2013-09-21

172

Constructing Stable Grasps  

Microsoft Academic Search

This paper shows how all force-closure grasps can be made stable by constructing virtual springs at the contacts, such that the grasped object has a desired stiffness matrix about its stable equilibrium. The paper develops a simple geometric relation between the stiffness of the grasp and the spatial configuration of the virtual springs at the contacts. The stiff ness of

Van-duc Nguyen

1989-01-01

173

Gold(I) N-heterocyclic carbene complexes with naphthalimide ligands as combined thioredoxin reductase inhibitors and DNA intercalators.  

PubMed

Organometallic conjugates consisting of a gold(I) N-heterocyclic carbene (NHC) moiety and a naphthalimide were prepared and investigated as cytotoxic agents that interact with both DNA and the disulfide reductase enzyme thioredoxin reductase (TrxR). The complexes were potent DNA intercalators related to their naphthalimide partial structure, inhibited TrxR as a consequence of the incorporation of the gold(I) moiety, and triggered efficient cytotoxic effects in MCF-7 breast and HT-29 colon adenocarcinoma cells. Strong effects on tumor cell metabolism were noted for the most cytotoxic complex, chlorido[1-(3'-(4''-ethylthio-1'',8''-naphthalimid-N''-yl))-propyl-3-methyl-imidazol-2-ylidene]gold(I) (4?d). In conclusion, the conjugation of naphthalimides with gold(I) NHC moieties provided a useful strategy for the design of bioorganometallic anticancer agents with multiple modes of action. PMID:24803348

Meyer, Andreas; Oehninger, Luciano; Geldmacher, Yvonne; Alborzinia, Hamed; Wölfl, Stefan; Sheldrick, William S; Ott, Ingo

2014-08-01

174

Understanding the mechanism of stereoselective synthesis of cyclopentenes via N-heterocyclic carbene catalyzed reactions of enals with enones.  

PubMed

The N-heterocyclic carbene (NHC) catalyzed addition of enals to enones to yield trans-cyclopentenes has been investigated using DFT methods at B3LYP/6-31G** computational level. This NHC catalyzed reaction comprises several steps. The first one is the formation of a Breslow intermediate, which nucleophilically attacks to the conjugated position of the enone to yield an enol-enolate. This second step is responsible for the trans relationship at the final cyclopentene. An intramolecular aldolic condensation allows for the formation of the alkoxy cyclopentane intermediate, that by intramolecular nucleophilic attack on the carbonyl group yields a bicyclic ether. The extrusion of the NHC catalyst affords a bicyclic lactone, yielding by CO(2) elimination, the final trans-cyclopentene. PMID:20740249

Domingo, Luis R; Zaragozá, Ramón J; Arnó, Manuel

2010-11-01

175

Main-chain organometallic microporous polymers bearing triphenylene-tris(N-heterocyclic carbene)-gold species: catalytic properties.  

PubMed

Two triphenylene-based tris(N-heterocyclic carbene)-gold-acetylide main-chain organometallic microporous polymers (MOMPs) were obtained and fully characterized. Both materials show spherical shapes, and their size is highly dependent on the type of acetylene used in the synthetic protocol. The new solids were tested in the catalytic reduction of nitroarenes with NaBH4 and in the three-component Strecker reaction for the synthesis of ?-aminonitriles, and showed high activity in both processes. Whereas the activity of the solids in the reduction of nitroarenes may be attributed to the formation of Au nanoparticles due to the use of NaBH4 as reducing agent, the activity in the Strecker reaction may originate from the Lewis acidic activation of the ketone or imine on coordination to Au. PMID:24668881

Gonell, Sergio; Poyatos, Macarena; Peris, Eduardo

2014-05-01

176

Carbynes and carbenes in coordination chemistry: A new class of pentaammine and tetraammine complexes of osmium(II)  

SciTech Connect

Since their discovery by Fischer and co-workers in 1973, the study of transition-metal carbyne complexes and their role in alkyne metathesis has rapidly developed into a mature field. Although carbyne complexes are known for a diverse set of early- and mid-transition metals, the vast majority of these complexes contain carbon or phosphine [pi]-acids, or bulky alkoxide ligands, which limit the coordination number. We wish to report the first example of a carbyne species, as well as several heteroatom-carbene derivatives, in which the metal fragment, Os[sup II](NH[sub 3])[sub 5], provides a classical octahedral coordination environment. The carbyne [Os(NH[sub 3])[sub 5]([equivalent to]CPh)](OTf)[sub 3] (2) is synthesized in two steps from Os(NH[sub 3])[sub 5](OTf)[sub 3] and benzaledhyde dimethyl acetal.

Hodges, L.M.; Sabat, M.; Harman, W.D. (Univ. of Virginia, Charlottesville (United States))

1993-02-17

177

Metal-metal interaction in Fischer carbene complexes: a study of ferrocenyl and biferrocenyl tungsten alkylidene complexes.  

PubMed

A series of ferrocenyl (Fc = ferrocenyl; fc = ferrocen-1,1'-diyl) and biferrocenyl (Bfc = 1',1?-biferrocenyl; bfc = 1',1?-biferrocen-1,1?-diyl) mono- and biscarbene tungsten(0) complexes of the type [(CO)5W?C(OMe)R] (1, R = Fc; 3, R = Bfc) and [(CO)5W?C(OMe)-R'-(OMe)C?W(CO)5] (2, R' = fc; 4, R' = bfc) were synthesized according to the classical synthetic methodology by reacting W(CO)6 with LiR (R = Fc, fc, bfc), followed by a subsequent alkylation using methyl trifluoromethanesulfonate. Electrochemical investigations were carried out on these complexes to get a closer insight into the electronic properties of 1-4. The ferrocenyl and biferrocenyl moieties in 1-4 show reversible one-electron redox events. It was further found that the Fischer carbene unit is reducible in an electrochemical one-electron transfer process. For the tungsten carbonyl moieties, irreversible oxidation processes were found. In addition, charge transfer studies were performed on 1-4 using in situ UV-vis-NIR and infrared spectroelectrochemical techniques. During the UV-vis-NIR investigations, typical low energy transitions for the mixed-valent biferrocenyl unit were found. A further observed high energy NIR absorption is attributed to a metal-metal charge transfer transition between the tungsten carbonyl fragment and the ferrocenyl/biferrocenyl group in the corresponding oxidized states, which can be described as class II systems according to Robin and Day. This assignment was verified by infrared spectroelectrochemical studies. The electrochemical investigations are supported by density functional theory calculations. The structural properties of 1-4 in the solid state were investigated by single-crystal X-ray diffraction studies showing no substituent effects on bond lengths and angles. The biferrocenyl derivatives exhibit syn-conformation of the ferrocenyl and carbene building blocks. PMID:24299202

van der Westhuizen, Belinda; Speck, J Matthäus; Korb, Marcus; Friedrich, Joachim; Bezuidenhout, Daniela I; Lang, Heinrich

2013-12-16

178

N-heterocyclic carbene (NHC)-stabilized silanechalcogenones: NHC-->Si(R2)=E (E=O, S, Se, Te).  

PubMed

A series of N-heterocyclic carbene-stabilized silanechalcogenones 2 a,b (Si=O), 3 a,b (Si=S), 4 a,b (Si=Se), and 5 a,b (Si=Te) are described. The silanone complexes 2 a,b were prepared by facile oxygenation of the carbene-silylene adducts 1 a,b with N(2)O, whereas their heavier congeners were synthesized by gentle chalcogenation of 1 a,b with equimolar amounts of elemental sulfur, selenium, and tellurium, respectively. These novel compounds have been isolated in a crystalline form in high yields and have been fully characterized by a variety of techniques including IR spectroscopy, ESIMS, and multinuclear NMR spectroscopy. The structures of 2 b, 3 a, 4 a, 4 b, and 5 b have been confirmed by single-crystal X-ray crystallography. Due to the NHC-->Si donor-acceptor electronic interaction, the Si=E (E=O, S, Se, Te) moieties within these compounds are well stabilized and thus the compounds possess several ylide-like resonance structures. Nevertheless, these species also exhibit considerable Si=E double-bond character, presumably through a nonclassical Si=E pi-bonding interaction between the chalcogen lone-pair electrons and two antibonding Si-N sigma* orbitals, as evidenced by their high stretching vibration modes and the shortening of the Si-E distances (between 5.4 and 6.3%) compared with the corresponding Si-E single-bond lengths. PMID:20013763

Yao, Shenglai; Xiong, Yun; Driess, Matthias

2010-01-25

179

Thermally Stable Organic Polymers.  

National Technical Information Service (NTIS)

Environmentally durable high temperature polymer matrix resins are critical in the design of near-term high performance supersonic aircraft. Currently available 6F based polymers are thermally stable, but are expensive and difficulty processable into void...

R. H. Boschan M. Marrocco J. McGrath

1994-01-01

180

Diastereotopic group selectivity and chemoselectivity of alkylidene carbene reactions on 8-oxabicyclo[3.2.1]- oct-6-ene ring systems.  

PubMed

?-Hydroxyalkylidene carbenes, generated from thermolysis of ?,?-epoxy-N-aziridinylimines, undergo diastereotopic group selective 1,5 C–H insertion reactions on 2,4-dimethyl-8-oxabicyclo[3.2.1]oct-6-ene ring systems. Protection of a tertiary alcohol at C-3 of the bridged oxabicycle as a trimethylsilyl ether reverses the sense of diastereoselectivity. 1,5 C–H insertion into a methine adjacent to an OBn group, 1,5 O–R insertion into a tertiary alcohol (R = H) or silylether (R = TMS) at C-3 to form spirocyclic dihydrofurans, 1,2-rearrangement to an alkyne and fragmentation to a ketone are competing major pathways for 2-benzyloxy-substituted 8-oxabicyclo[3.2.1]oct-6-ene systems. Dihydrofuran formation is shown to be a result of substitution on the oxabicyclic ring system through comparison with other methods of alkylidene carbene formation. PMID:24175334

Munro, Kevin R; Male, Louise; Spencer, Neil; Grainger, Richard S

2013-10-21

181

Lewis base activation of grignard reagents with N-heterocyclic carbenes. Cu-free catalytic enantioselective additions to gamma-chloro-alpha,beta-unsaturated esters.  

PubMed

Direct activation of alkylmagnesium halides with a chiral bidentate N-hetrocyclic carbene (NHC), formed in situ from the imidazolinium chloride precursor, is reported. The Cu-free catalytic asymmetric allylic alkylation (AAA) of alpha-alkyl-gamma-chloro-alpha,beta-unsaturated esters with alkyl-based Grignard reagents is promoted in the presence of 5-10 mol % of the chiral imidazolinium salt to afford synthetically versatile beta,gamma-unsaturated esters. Products bear all-carbon quaternary sterogenic centers generated in 3.5-13.3:1 regioselectivity, 63-98% ee, and in up to 80% isolated yield of the SN2' product. The in-situ-generated chiral NHC promotes enantioselectivity while altering the reactivity of the Grignard reagents: there is only approximately 30% conversion and <2% allylic alkylation in the absence of chiral carbene. PMID:17147366

Lee, Yunmi; Hoveyda, Amir H

2006-12-13

182

Well-defined N-heterocyclic carbenes-palladium(II) precatalysts for cross-coupling reactions.  

PubMed

Metal-catalyzed cross-coupling reactions, notably those permitting C-C bond formation, have witnessed a meteoritic development and are now routinely employed as a powerful synthetic tool both in academia and in industry. In this context, palladium is arguably the most studied transition metal, and tertiary phosphines occupy a preponderant place as ancillary ligands. Seriously challenging this situation, the use of N-heterocyclic carbenes (NHCs) as alternative ligands in palladium-catalyzed cross-coupling reactions is rapidly gaining in popularity. These two-electron donor ligands combine strong sigma-donating properties with a shielding steric pattern that allows for both stabilization of the metal center and enhancement of its catalytic activity. As a result, the number of well-defined NHC-containing palladium(II) complexes is growing, and their use in coupling reactions is witnessing increasing interest. In this Account, we highlight the advantages of this family of palladium complexes and review their synthesis and applications in cross-coupling chemistry. They generally exhibit high stability, allowing for indefinite storage and easy handling. The use of well-defined complexes permits a strict control of the Pd/ligand ratio (optimally 1/1), avoiding the use of excess costly ligand that usually requires end-game removal. Furthermore, it partly removes the "black box" character often associated with cross-coupling chemistry and catalyst formation. In the present Account, four main classes of NHC-containing palladium(II) complexes will be presented: palladium dimers with bridging halogens, palladacycles, palladium acetates and acetylacetonates, and finally pi-allyl complexes. These additional ligands are best described as a protecting shell that will be discarded going from the palladium(II) precatalyst to the palladium(0) true catalyst. The synthesis of all these precatalysts generally requires simple and short synthetic procedures. Their catalytic activity in different cross-coupling reactions is discussed and put into context. Remarkably, some NHC-containing catalytic systems can achieve extremely challenging coupling reactions such as the formation of tetra-ortho-biphenyl compounds and perform reactions at very low loadings of palladium (ppm levels). The chemistry described here, combining fundamental organometallic, catalysis, and pure organic methodology, remains rich in opportunities considering that only a handful of palladium(II) architectures have been studied. Hence, en route to an "ideal catalyst", [(NHC)Pd(II)] compounds exhibit remarkable stability and allow for fine-tuning of the NHC and of surrounding ligands in order to control the activation and the catalytic activity. Finally, unlike [Pd(PPh(3))(4)], [(NHC)Pd(II)] compounds have so far been examined only in palladium-mediated reactions (most often cross-coupling such as the Suzuki-Miyaura and Heck reactions), leaving a treasure trove of exciting discoveries to come. PMID:18774825

Marion, Nicolas; Nolan, Steven P

2008-11-18

183

Asymmetric N-heterocyclic carbene catalyzed addition of enals to nitroalkenes: controlling stereochemistry via the homoenolate reactivity pathway to access ?-lactams.  

PubMed

An asymmetric intermolecular reaction between enals and nitroalkenes to yield ?-nitroesters has been developed, catalyzed by a novel chiral N-heterocyclic carbene. Key to this work was the development of a catalyst that favors the ?-nitroester pathway over the established Stetter pathway. The reaction proceeds in high stereoselectivity and affords the previously unreported syn diastereomer. We also report an operationally facile two-step, one-pot procedure for the synthesis of ?-lactams. PMID:23713683

White, Nicholas A; DiRocco, Daniel A; Rovis, Tomislav

2013-06-12

184

N-heterocyclic carbene catalyzed annulation of benzofuran-2,3-diones and enals: a concise synthesis of spiro-bis-lactone.  

PubMed

The N-heterocyclic carbene catalyzed annulation of benzofuran-2,3-diones and enals via homoenolate intermediates is described. The reaction provided a direct and efficient method for the synthesis of spiro-bis-lactones. The ketone-carbonyl group annulated products and the ester-carbonyl group annulated products can be obtained as major products with good yields by convenient catalyst regulation. Furthermore, commercially available thiazolium salt can also catalyze this reaction with modest yield. PMID:23868545

Wang, Ze-Dong; Wang, Feng; Li, Xin; Cheng, Jin-Pei

2013-09-14

185

Direct in situ synthesis of cationic N-heterocyclic carbene iridium and rhodium complexes from neat ionic liquid: Application in catalytic dehydrogenation of cyclooctadiene  

Microsoft Academic Search

A direct synthetic route to cationic N-heterocyclic carbene (NHC) complexes of rhodium and iridium from neat dialkyl-imidazolium ionic liquids (ILs) has been found. The method uses complexes bearing basic anionic ligands, [M(COD)(PPh3)X], X=OEt, MeCO2, which react with the inactivated imidazolium cation in the absence of external bases yielding one M-NHC moiety and the free protonated base. This new one-pot synthesis

Ulrich Hintermair; Thibaut Gutel; Alexandra M. Z. Slawin; David J. Cole-Hamilton; Catherine C. Santini; Yves Chauvin

2008-01-01

186

How stable are the 'stable ancient shields'?  

NASA Astrophysics Data System (ADS)

"Archean cratons are relatively flat, stable regions of the crust that have remained undeformed since the Precambrian, forming the ancient cores of the continents" (King, EPSL, 2005). While this type of statement is supported by a wealth of constraints in the case of episodes of thoroughgoing ductile deformation affecting shield regions of Archean and also Peleoproterozoic age, a growing amount of research indicates that shields are not nearly as structurally stable within the broad field of environmental conditions leading to brittle deformation. In fact, old crystalline basements usually present compelling evidence of long brittle deformation histories, often very complex and challenging to unfold. Recent structural and geochronological studies point to a significant mechanical instability of the shield areas, wherein large volumes of 'stable' rocks actually can become saturated with fractures and brittle faults soon after regional cooling exhumes them to below c. 300-350° C. How cold, rigid and therefore strong shields respond to applied stresses remains, however, still poorly investigated and understood. This in turn precludes a better definition of the shallow rheological properties of large, old crystalline blocks. In particular, we do not yet have good constraints on the mechanisms of mechanical reactivation that control the partial (if not total) accommodation of new deformational episodes by preexisting structures, which remains a key to untangle brittle histories lasting several hundred Myr. In our analysis, we use the Svecofennian Shield (SS) as an example of a supposedly 'stable' region with Archean nucleii and Paleoproterozoic cratonic areas to show how it is possible to unravel the details of brittle histories spanning more than 1.5 Gyr. New structural and geochronological results from Finland are integrated with a review of existing data from Sweden to explore how the effects of far-field stresses are partitioned within a shield, which was growing progressively saturated with fractures as time passed from its initial consolidation. The comparison of time-constrained paleostress data derived from three different locations of the shield shows a remarkably similar stress evolution through time, despite the different geological frameworks of the investigated areas. This suggests that the southern SS has behaved as a coherent rigid crustal block since the Late Mesoproterozoic. By that time, the SS had already reached structural maturity with respect to the saturation of brittle structural features. Therefore, structural reactivation rather than generation of new faults and fractures is the key mechanism that has controlled the mechanical evolution of the shield and that will steer its future evolution within the brittle regime. Comparable brittle structural histories within parts of the shield that are far apart also imply that far-field stresses can propagate over large distances and can lead to similar deformational histories, with the local geological conditions only playing a second-order role on the final brittle strain pattern recorded by the rock.

Viola, Giulio; Mattila, Jussi

2014-05-01

187

[3,3]-Sigmatropic Rearrangement vs. Carbene Formation in Gold-Catalyzed Transformations of Alkynyl Aryl Sulfoxides: Mechanistic Studies and Expanded Reaction Scope  

PubMed Central

Gold-catalyzed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant is a reaction in gold chemistry of high impact, as it introduced to the field the highly-valued concept of gold carbene generation via alkyne oxidation. The proposed intermediacy of ?-oxo gold carbenes in these reactions, however, has never been substantiated. Detailed experimental studies suggest that the involvement of such reactive intermediates in the formation of dihydrobenzothiepinones is highly unlikely. Instead, a [3,3]-sigmatropic rearrangement of the initial cyclization intermediate offers a reaction path that can readily explain the high reaction efficiency and the lack of sulfonium formation. With internal alkyne substrates, however, the generation of a gold carbene species becomes competitive with the [3,3]-sigmatropic rearrangement. This reactive intermediate, nevertheless, does not proceed to afford the Friedel-Crafts type cyclization product. Extensive Density Functional Theory studies support the mechanistic conclusion that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts type cyclization. With the new mechanistic insight, the product scope of this versatile formation of mid-sized sulfur-containing cycloalkenones has been expanded readily to various dihydrobenzothiocinones, a tetrahydrobenzocyclononenone, and even those without the entanglement of a fused benzene ring. Besides gold, Hg(OTf)2 can be an effective catalyst, thereby offering a cheap alternative for this intramolecular redox reaction.

Lu, Biao; Li, Yuxue; Wang, Youliang; Aue, Donald H.; Luo, Yingdong; Zhang, Liming

2013-01-01

188

[3,3]-Sigmatropic rearrangement versus carbene formation in gold-catalyzed transformations of alkynyl aryl sulfoxides: mechanistic studies and expanded reaction scope.  

PubMed

Gold-catalyzed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant is a reaction of high impact in gold chemistry, as it introduced to the field the highly valued concept of gold carbene generation via alkyne oxidation. The proposed intermediacy of ?-oxo gold carbenes in these reactions, however, has never been substantiated. Detailed experimental studies suggest that the involvement of such reactive intermediates in the formation of dihydrobenzothiepinones is highly unlikely. Instead, a [3,3]-sigmatropic rearrangement of the initial cyclization intermediate offers a reaction path that can readily explain the high reaction efficiency and the lack of sulfonium formation. With internal alkyne substrates, however, the generation of a gold carbene species becomes competitive with the [3,3]-sigmatropic rearrangement. This reactive intermediate, nevertheless, does not proceed to afford the Friedel-Crafts-type cyclization product. Extensive density functional theory studies support the mechanistic conclusion that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts-type cyclization. With the new mechanistic insight, the product scope of this versatile formation of mid-sized sulfur-containing cycloalkenones has been expanded readily to various dihydrobenzothiocinones, a tetrahydrobenzocyclononenone, and even those without the entanglement of a fused benzene ring. Besides gold, Hg(OTf)2 can be an effective catalyst, thereby offering a cheap alternative for this intramolecular redox reaction. PMID:23731178

Lu, Biao; Li, Yuxue; Wang, Youliang; Aue, Donald H; Luo, Yingdong; Zhang, Liming

2013-06-12

189

Intermolecular and regioselective access to polysubstituted benzo- and dihydrobenzo[c]azepine derivatives: modulating the reactivity of group 6 non-heteroatom-stabilized alkynyl carbene complexes.  

PubMed

We highlight the versatility of non-heteroatom-stabilized tungsten-carbene complexes 3 synthesized in situ, which have been used in a modular approach to access 2-benzazepinium isolable intermediates 5. By employing very mild conditions, benzazepinium derivatives 5 have been obtained in high yield from simple compounds, such as acetylides 2, Fischer-type alkoxycarbenes 1, and phenylimines 4. The process, involving a formal [4+3] heterocycloaddition, occurs in a totally regioselective manner, which differs from the approach previously observed in similar procedures for other carbene analogues. This work, which involves three components, reveals a control of the reactivity of non-heteroatom-stabilized carbene complexes 3 ([4+3] vs. [2+2]-heterocycloaddition reactions) depending on the acetylide substitution pattern. The influence of the substitution pattern in the behavior of the complexes has been computationally analyzed and rationalized. Finally, elaboration of the 2-benzazepinium intermediates allows access to 3H-benzo[c]azepines 6 and 3H-1,2-dihydrobenzo[c]azepines 7-9 with high control of the substitution of the nine positions of the heterocycle. PMID:24757080

González, Jairo; Gómez, Aránzazu; Funes-Ardoiz, Ignacio; Santamaría, Javier; Sampedro, Diego

2014-06-01

190

Synthesis, characterization, and antimicrobial activity of silver carbene complexes derived from 4,5,6,7-tetrachlorobenzimidazole against antibiotic resistant bacteria.  

PubMed

Silver N-heterocyclic carbene complexes have been shown to have great potential as antimicrobial agents, affecting a wide spectrum of both Gram-positive and Gram-negative bacteria. A new series of three silver carbene complexes (SCCs) based on 4,5,6,7-tetrachlorobenzimidazole has been synthesized, characterized, and tested against a panel of clinical strains of bacteria. The imidazolium salts and their precursors were characterized by elemental analysis, mass spectrometry, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. The silver carbene complexes, SCC32, SCC33, and SCC34 were characterized by elemental analysis, (1)H and (13)C NMR spectroscopy, and single crystal X-ray diffraction. These complexes proved highly efficacious with minimum inhibitory concentrations (MICs) ranging from 0.25 to 6 ?g mL(-1). Overall, the complexes were effective against highly resistant bacteria strains, such as methicillin-resistant Staphylococcus aureus (MRSA), weaponizable bacteria, such as Yersinia pestis, and pathogens found within the lungs of cystic fibrosis patients, such as Pseudomonas aeruginosa, Alcaligenes xylosoxidans, and Burkholderia gladioli. SCC33 and SCC34 also showed clinically relevant activity against a silver-resistant strain of Escherichia coli based on MIC testing. PMID:22402409

Wright, Brian D; Shah, Parth N; McDonald, Lucas J; Shaeffer, Michael L; Wagers, Patrick O; Panzner, Matthew J; Smolen, Justin; Tagaev, Jasur; Tessier, Claire A; Cannon, Carolyn L; Youngs, Wiley J

2012-06-01

191

Synthesis, characterization, and antimicrobial activity of silver carbene complexes derived from 4,5,6,7-tetrachlorobenzimidazole against antibiotic resistant bacteria†  

PubMed Central

Silver N-heterocyclic carbene complexes have been shown to have great potential as antimicrobial agents, affecting a wide spectrum of both Gram-positive and Gram-negative bacteria. A new series of three silver carbene complexes (SCCs) based on 4,5,6,7-tetrachlorobenzimidazole has been synthesized, characterized, and tested against a panel of clinical strains of bacteria. The imidazolium salts and their precursors were characterized by elemental analysis, mass spectrometry, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction. The silver carbene complexes, SCC32, SCC33, and SCC34 were characterized by elemental analysis, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction. These complexes proved highly efficacious with minimum inhibitory concentrations (MICs) ranging from 0.25 to 6 ?g mL?1. Overall, the complexes were effective against highly resistant bacteria strains, such as methicillin-resistant Staphylococcus aureus (MRSA), weaponizable bacteria, such as Yersinia pestis, and pathogens found within the lungs of cystic fibrosis patients, such as Pseudomonas aeruginosa, Alcaligenes xylosoxidans, and Burkholderia gladioli. SCC33 and SCC34 also showed clinically relevant activity against a silver-resistant strain of Escherichia coli based on MIC testing.

Wright, Brian D.; Shah, Parth N.; McDonald, Lucas J.; Shaeffer, Michael L.; Wagers, Patrick O.; Panzner, Matthew J.; Smolen, Justin; Tagaev, Jasur; Tessier, Claire A.

2013-01-01

192

Cooperativity of anion⋯? and ?⋯? interactions regulates the self-assembly of a series of carbene proligands: Towards quantitative analysis of intermolecular interactions with Hirshfeld surface  

NASA Astrophysics Data System (ADS)

The cooperative effect of weak non-covalent forces between anions and electron deficient aromatics by ?⋯? stacking of a series of carbene proligands (1-3) have been thoroughly explored by crystallographic studies. Structural analysis revealed that the anion⋯? and ?⋯? interactions along with intermolecular hydrogen bonding mutually cooperate to facilitate the assembling of the supramolecular framework. The ?⋯? and corresponding anion⋯? interactions have been investigated in the title carbene proligands despite their association with counter ions. The presence of the anion in the vicinity of the ?-system leads to the formation of anion⋯?/?⋯?/?⋯anion network for an inductive stabilization of the assemblies. To assess the dimensionality of the supramolecular framework consolidated by cooperative anion⋯?/?⋯? interactions and hydrogen bonding, different substituent effects in the carbene backbone have been considered to tune these interactions. These facts show that the supramolecular framework based on these cooperative weak forces may be robust enough for application in molecular recognition. The investigation of close intermolecular interactions between the molecules via Hirshfeld surface analyses is presented in order to reveal subtle differences and similarities in the crystal structures. The decomposition of the fingerprint plot area provides a percentage of each intermolecular interaction, allowing for a quantified analysis of close contacts within each crystal.

Samanta, Tapastaru; Dey, Lingaraj; Dinda, Joydev; Chattopadhyay, Shyamal Kumar; Seth, Saikat Kumar

2014-06-01

193

Electronic structure trends in N-heterocyclic carbenes (NHCs) with varying number of nitrogen atoms and NHC-transition-metal bond properties.  

PubMed

Carbenes derived from five-membered heterocycles with different numbers of nitrogen atoms ranging from two to four lead formally either to normal N-heterocyclic or mesoionic carbenes with, in some cases, the same skeletal structure. The electronic structures of fourteen of these compounds were examined by means of DFT calculations at the B3LYP/aug-cc-pVTZ level. The examined parameters include the energies of the ?-lone pair at Ccarbene and the ?-HOMO of the protonated form, which are correlated to the first and second proton affinities. The singlet-triplet energy gap was used as a measure of the stability of the N-heterocyclic carbene (NHC) towards dimerisation. Natural population analysis provided insight into the variation of the p? population and the natural charge at Ccarbene with NHC structure. Additionally, the transition metal?NHC bond in L-AuCl and L-TiCl4 and the nature of the orbital interactions between the NHC and the transition-metal fragment were analysed in detail by the extended transition state-natural orbitals for chemical valence (ETS-NOCV) approach at the BP86/TZ2P level. Similarities and differences between the NHC?gold and the NHC?titanium bond are discussed, and trends in key bonding properties can be traced back to the variation of the electronic parameters of the NHC. PMID:23955586

Bernhammer, Jan C; Frison, Gilles; Huynh, Han Vinh

2013-09-16

194

Stable Angles of Slopes  

NSDL National Science Digital Library

In this activity, students use a box filled with sand to learn about factors controlling the angle of repose (the stable angle of a slope). They will will see what happens when slopes are oversteepened, and relate the experiment results to general understanding of instability of slopes.

195

Stable Hydride Meisenheimer Adducts.  

National Technical Information Service (NTIS)

The addition of hydride from octahydrotriborate ion to 1-substituted-2,4,6-trinitrobenzenes affords a stable C3-hydride Meisenheimer adduct. Concurrent with this addition reaction, is hydride displacement of the C1-substituent to form 1,3,5-trinitrobenzen...

L. A. Kaplan A. R. Siedle

1970-01-01

196

Stable Local Oscillator Microcircuit.  

National Technical Information Service (NTIS)

This report gives a description of the development of a Stable Local Oscillator (StaLO) Microcircuit. The StaLO accepts a 100MHz input signal and produces output signals at 1.2, 3.3, and 3.6 GHz. The circuit is built as a multi-chip module (MCM), since it...

R. W. Brocato

2006-01-01

197

Stable regions around Pluto  

NASA Astrophysics Data System (ADS)

In a previous work, Giuliatti Winter et al. found several stable regions for test particles in orbit around Pluto associated with families of periodic orbits obtained in the circular, restricted three-body problem. They have shown that a possible eccentricity of the Pluto-Charon binary slightly reduces but does not destroy any of these stable regions. In this work, we extended their results by analysing the cases with the orbital inclination (I) equal to zero and considering the argument of pericentre (?) equal to 90°, 180° and 270°. We explore the influence of the orbital inclination of the particles in these stable regions. In this case, the initial inclination varies from 10° to 170° in steps of 10°. We also present a sample of results for the longitude of the ascending node ? = 90°, considering the cases I = 20°, 50°, 130° and 180°. Our results show that stable regions are present in all of the inclined cases, except when the initial inclination of the particles is equal to 110°. A sample of 3D trajectories of quasi-periodic orbits were found related to the periodic orbits obtained in the planar case by Giuliatti Winter et al.

Giuliatti Winter, S. M.; Winter, O. C.; Vieira Neto, E.; Sfair, R.

2013-04-01

198

Thermally stable laminating resins  

NASA Technical Reports Server (NTRS)

Improved thermally stable laminating resins were developed based on the addition-type pyrolytic polymerization. Detailed monomer and polymer synthesis and characterization studies identified formulations which facilitate press molding processing and autoclave fabrication of glass and graphite fiber reinforced composites. A specific resin formulation, termed P10P was utilized to prepare a Courtaulds HMS reinforced simulated airfoil demonstration part by an autoclave molding process.

Jones, R. J.; Vaughan, R. W.; Burns, E. A.

1972-01-01

199

Comproportionation of Cationic and Anionic Tungsten Complexes Having an N-Heterocyclic Carbene Ligand to Give the Isolable 17-Electron Tungsten Radical, CpW(CO)2(IMes)•  

SciTech Connect

A series consisting of a tungsten anion, radical and cation, supported by the N-heterocyclic carbene IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and spanning formal oxidation states W(0), W(I) and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)2(IMes)H with KH and 18 crown 6 gives the tungsten anion [CpW(CO)2(IMes)]-[K(18 crown 6)]+. The crystal structure of this complex shows that the K+ interacts not only with the oxygen atoms in the crown ether, but also with the carbonyl oxygens. The electrochemical oxidation of [CpW(CO)2(IMes)]- in acetonitrile is fully reversible (E½ = ?1.65 V vs Cp2Fe+•/0) at all scan rates, indicating that CpW(CO)2(IMes)• is a persistent radical. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+PF6 affords [cis-CpW(CO)2(IMes)(MeCN)]+PF6 . Comproportionation of [CpW(CO)2(IMes)]- with [CpW(CO)2(IMes)(MeCN)]+ gives the 17-electron tungsten radical CpW(CO)2(IMes)•. This complex shows paramagnetically shifted resonances in 1H NMR spectra and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)2(IMes)• is very stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)2(IMes)• and [CpW(CO)2(IMes)]- are reported. DFT calculations were carried out on CpW(CO)2(IMes)H, as well as on related complexes bearing NHC ligands with N,N´ substituents Me [CpW(CO)2(IMe)H] or H [CpW(CO)2(IH)H] to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal W H homolytic bond dissociation energies (BDEs) to decrease with increasing steric bulk of the NHC ligand, from 67 for CpW(CO)2(IH)H to 64 for CpW(CO)2(IMe)H to 63 kcal/mol for CpW(CO)2(IMes)H. The calculated spin density at W for CpW(CO)2(IMes)• is 0.63. The W radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)• are calculated to form weak W W bonds. The weakly bonded complexes [CpW(CO)2(IMe)]2 and [CpW(CO)2(IH)]2, are predicted to have W-W BDEs of 6 and 18 kcal/mol, respectively, and to dissociate readily to the W-centered radicals CpW(CO)2(IMe)• and CpW(CO)2(IH)•. This work was supported by the US Department of Energy Basic Energy Sciences' Chemical Sciences, Geosciences & Biosciences Division. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.

Roberts, John A.; Franz, James A.; van der Eide, Edwin F.; Walter, Eric D.; Petersen, Jeffrey L.; DuBois, Daniel L.; Bullock, R. Morris

2011-09-21

200

A chemical-biological evaluation of rhodium(I) N-heterocyclic carbene complexes as prospective anticancer drugs.  

PubMed

Rhodium(I) complexes bearing N-heterocyclic carbene (NHC) ligands have been widely used in catalytic chemistry, but there are very few reports of biological properties of these organometallics. A series of Rh(I)-NHC derivatives with 1,5-cyclooctadiene and CO as secondary ligands were synthesized, characterized, and biologically investigated as prospective antitumor drug candidates. Pronounced antiproliferative effects were noted for all complexes, along with moderate inhibitory activity of thioredoxin reductase (TrxR) and efficient binding to biomolecules (DNA, albumin). Biodistribution studies showed that the presence of albumin lowered the cellular uptake and confirmed the transport of rhodium into the nuclei. Changes in the mitochondrial membrane potential (MMP) were observed as well as DNA fragmentation in wild-type and daunorubicin- or vincristine-resistant Nalm-6 leukemia cells. Overall, these studies indicated that Rh(I)-NHC fragments could be used as partial structures of new antitumor agents, in particular in those drugs designed to address resistant malignant tissues. PMID:24243420

Oehninger, Luciano; Küster, Laura Nadine; Schmidt, Claudia; Muñoz-Castro, Alvaro; Prokop, Aram; Ott, Ingo

2013-12-23

201

A fullerene-carbene adduct as a crystalline molecular rotor: remarkable behavior of a spherically-shaped rotator.  

PubMed

A new fullerene structure was recently obtained from the reaction of a Lewis basic N-heterocyclic carbene (NHC) and the Lewis acidic C60. The molecular features of the zwitterionic adduct can be described as a molecular rotor with the fullerene cage acting as the rotator that spins about one distinct axis given by its C-C single bond linkage with the imidazolium heterocycle stator. A detailed structural analysis of the compound by means of single-crystal X-ray diffraction (XRD) revealed significant differences in the packing motifs of solvent-free and solvent-containing crystals. Variable temperature single-crystal XRD experiments (80 K ? T ? 480 K) carried out to investigate the rotational dynamics of the fullerene group in the higher quality solvent-free structure revealed atomic displacement parameters consistent with fast rotation of the highly symmetric fullerene in the solid state, whereas the imidazolium unit remains in a fixed position and therefore represents the stator. DFT and semiempirical calculations were applied to get insight into the profile of the rotational potential of the fullerene unit, particularly considering interactions with the neighboring molecules in the crystal lattice. The results indicate that the crystal environment leads to the presence of one lowest energy minimum that is connected to seven others that are slightly higher in energy through rotational barriers of approximately 1.5-2.5 kcal mol(-1). PMID:24852314

Lorbach, Andreas; Maverick, Emily; Carreras, Abel; Alemany, Pere; Wu, Guang; Garcia-Garibay, Miguel A; Bazan, Guillermo C

2014-06-01

202

Addition of N-heterocyclic carbenes to a ruthenium(VI) nitrido polyoxometalate: a new route to cyclic guanidines.  

PubMed

The scope of N-atom transfer from the electrophilic ruthenium(VI) nitrido containing polyoxometalate [PW(11)O(39)Ru(VI)N](4-) has been extended to the N-heterocyclic carbene {CH(2)(Mes)N}(2)C and the coupling product {CH(2)(Mes)N}(2)CNH(2)(+) characterized by (1)H NMR and high-resolution mass spectrometry. Because guanidines display many fields of applications ranging from biology to supramolecular chemistry, this could afford an original route to the synthesis of cyclic guanidines. This also enlarges the potential of nitrido complexes in the synthesis of heterocycles, mainly illustrated in the literature through the formation of aziridines through N-atom transfer to alkenes. In the course of the reaction, the ruthenium(III)-containing polyoxometallic intermediate [PW(11)O(39)Ru(III){NC{N(Mes)CH(2)}(2)}](5-) has been thoroughly characterized by continuous-wave and pulsed electron paramagnetic resonance, which nicely confirms the presence of the organic moiety on the polyoxometallic framework, Ru K-edge X-ray absorption near-edge structure, and electrochemistry. PMID:21319776

Besson, Claire; Mirebeau, Jean-Hugues; Renaudineau, Séverine; Roland, Sylvain; Blanchard, Sébastien; Vezin, Hervé; Courillon, Christine; Proust, Anna

2011-03-21

203

Synthesis, photo-, and electrochemistry of ruthenium bis(bipyridine) complexes comprising a N-heterocyclic carbene ligand.  

PubMed

Analogues of [Ru(bpy)3](2+) were prepared in which one pyridine ligand site is substituted by a N-heterocyclic carbene (NHC) ligand, that is, either by an imidazolylidene with a variable wingtip group R (R = Me, 3a; R = Et, 3b; R = iPr, 3c), or by a benzimidazolylidene (Me wingtip group, 3d), or by a 1,2,3-triazolylidene (Me wingtip group, 3e). All complexes were characterized spectroscopically, photophysically, and electrochemically. An increase of the size of the wingtip groups from Me to Et or iPr groups distorts the octahedral geometry (NMR spectroscopy) and curtails the reversibility of the ruthenium oxidation. NHC ligands with methyl wingtip groups display reversible ruthenium oxidation at a potential that reflects the donor properties of the NHC ligand (triazolylidene > imidazolylidene > benzimidazolylidene). The most attractive properties were measured for the triazolylidene ruthenium complex 3e, featuring the smallest gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the series (2.41 eV), a slightly red-shifted absorption profile, and reasonable excited-state lifetime (188 ns) when compared to [Ru(bpy)3](2+). These features demonstrate the potential utility of triazolylidene ruthenium complexes as photosensitizers for solar energy conversion. PMID:23594091

Leigh, Vivienne; Ghattas, Wadih; Lalrempuia, Ralte; Müller-Bunz, Helge; Pryce, Mary T; Albrecht, Martin

2013-05-01

204

Syntheses and structures of ruthenium(II) N,S-heterocyclic carbene diphosphine complexes and their catalytic activity towards transfer hydrogenation.  

PubMed

Phosphine exchange of [Ru(II) Br(MeCOO)(PPh(3))(2)(3-RBzTh)] (3-RBzTh=3-benzylbenzothiazol-2-ylidene) with a series of diphosphines (bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethylene (dppv), 1,1'-bis(diphenylphosphino)ferrocene (dppf), 1,4-bis(diphenylphosphino)butane (dppb), and 1,3-(diphenylphosphino)propane (dppp)) gave mononuclear and neutral octahedral complexes [RuBr(MeCOO)(?(2)-P(2))(3-RBzTh)] (P(2)=dppm (2), dppv (3), dppf (4), dppb (5), or dppp (6)), the coordination spheres of which contained four different ligands, namely, a chelating diphosphine, carboxylate, N,S-heterocyclic carbene (NSHC), and a bromide. Two geometric isomers of 6 (6a and 6b) have been isolated. The structures of these products, which have been elucidated by single-crystal X-ray crystallography, show two structural types, I and II, depending on the relative dispositions of the ligands. Type I structures contain a carbenic carbon atom trans to the oxygen atom, whereas two phosphorus atoms are trans to bromine and oxygen atoms. The type II system comprises a carbene carbon atom trans to one of the phosphorus atoms, whereas the other phosphorus is trans to the oxygen atom, with the bromine trans to the remaining oxygen atom. Complexes 2, 3, 4, and 6a belong to type I, whereas 5 and 6b are of type II. The kinetic product 6b eventually converts into 6a upon standing. These complexes are active towards catalytic reduction of para-methyl acetophenone by 2-propanol at 82 °C under 1% catalyst load giving the corresponding alcohols. The dppm complex 2 shows the good yields (91-97%) towards selected ketones. PMID:21500358

Ding, Nini; Hor, T S Andy

2011-06-01

205

Monomeric bis(anilido)iron(II) Complexes with N-heterocyclic carbene ligation: synthesis, characterization, and redox reactivity toward aryl halides.  

PubMed

Using monodentate N-heterocyclic carbenes as the ancillary ligands, seven monomeric bis(anilido)iron(II) complexes [(IPr(2)Me(2))(2)Fe(NHAr)(2)] (IPr(2)Me(2) = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene; Ar = Ph, C(6)H(4)-2-Pr(i), Mes, C(6)H(3)-2,6-Cl(2), Dipp) and [(IPr)Fe(NHAr)(2)] (IPr = 2,5-di(2,6-diisopropylphenyl)imidazol-1-ylidene; Ar = C(6)H(3)-2,6-Cl(2), Dipp) have been prepared by the one-pot reactions of [Fe(Mes)(2)](2) with the corresponding N-heterocyclic carbenes, and anilines. These high-spin diamido complexes have been fully characterized by (1)H NMR, solution magnetic susceptibility, UV-vis, IR, X-ray diffraction, cyclic voltammetry, as well as elemental analysis. The strong affinity of the N-heterocyclic carbene ligands toward ferrous centers, and the steric protection exerted by the NHC ligands are the key factors to stabilize these bis(anilido)iron complexes in a monomeric manner. Reactivity studies revealed the four-coordinate complex [(IPr(2)Me(2))(2)Fe(NHMes)(2)] can react with 1 equiv of 1-iodo-3,5-dimethylbenzene or 1-bromo-3,5-dimethylbenzene in C(6)D(6) and THF-d(8) to furnish 1-C(6)D(5)-3,5-Me(2)C(6)H(3), and 5-D-1,3-Me(2)C(6)H(3), respectively. Under similar conditions, the three-coordinate compound [(IPr)Fe(NHDipp)(2)] is inert toward these halides. PMID:23249144

Wang, Xiaojie; Mo, Zhenbo; Xiao, Jie; Deng, Liang

2013-01-01

206

The ambivalent chemistry of a free anionic N-heterocyclic carbene decorated with a malonate backbone: the plus of a negative charge.  

PubMed

The anionic heterocycle "[maloNHC](-)", ([1](-)), is the archetype of a growing family of N-heterocyclic carbenes incorporating an anionic backbone; here, a malonate group. A comprehensive experimental exploration of its chemistry as a free entity (in the form of its lithium salt [1]·Li) is presented, and rationalized using DFT calculations at the B3LYP/6-31+G** level of theory. For the sake of comparison, similar computations were performed on other representative carbene types. Reactions of [1]·Li with a broad series of electrophilic reagents were used to ascertain its intrinsic nature as a nucleophilic carbene. Unexpectedly, [1]·Li was also seen to react with the nucleophilic tert-butylisocyanide, to give an anionic ketenimine, which could be subsequently derivatized, either into an imine by protonation of the ketenimine moiety, or into a neutral ketenimine by alkylation of the intracyclic malonate moiety. Further experiments on the electrophilic behavior of [1]·Li revealed its unexpected reactivity toward p-chlorobenzaldehyde, resulting in a formal C-H activation and the first structurally characterized keto-tautomer of the Breslow intermediate. Finally, [1]·Li remarkably activates polar E-H bonds, including N-H bonds from ammonia and amines, Si-H bonds, and B-H bonds. Importantly, DFT calculations indicate the importance of counterion effects. In particular, the key to the observed reactivity appears to be a modulation of energy levels associated with a dynamic variability of the Li-O distance between the remote malonate group and the counterion. PMID:24307368

César, Vincent; Labat, Stéphane; Miqueu, Karinne; Sotiropoulos, Jean-Marc; Brousses, Rémy; Lugan, Noël; Lavigne, Guy

2013-12-01

207

Expanded-ring N-heterocyclic carbenes efficiently stabilize gold(I) cations, leading to high activity in ?-acid-catalyzed cyclizations.  

PubMed

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X(-) = BF4(-), NTf2(-), OTf(-)) were generated in solution. According to their (13)C?NMR spectra, the ionic character of the complexes increases in the order X(-) = Cl(-) < NTf2(-) < OTf(-) < BF4(-). Additional factors for stabilization of the cationic complexes are expansion of the NHC ring and the attachment of bulky substituents at the nitrogen atoms. These er-NHCs are bulkier ligands and stronger electron donors than conventional NHCs as well as phosphines and sulfides and provide more stabilization of [(L)Au(+)] cations. A comparative study has been carried out of the catalytic activities of five-, six-, and seven-membered carbene complexes [(NHC)AuX], [(Ph3 P)AuX], [(Me2S)AuX], and inorganic compounds of gold in model reactions of indole and benzofuran synthesis. It was found that increased ionic character of the complexes was correlated with increased catalytic activity in the cyclization reactions. As a result, we developed an unprecedentedly active monoligand cationic [(THD-Dipp)Au]BF4 (1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydrodiazepin-2-ylidene gold(I) tetrafluoroborate) catalyst bearing seven-membered-ring carbene and bulky Dipp substituents. Quantitative yields of cyclized products were attained in several minutes at room temperature at 1?mol?% catalyst loadings. The experimental observations were rationalized and fully supported by DFT calculations. PMID:24682976

Morozov, Oleg S; Lunchev, Andrey V; Bush, Alexander A; Tukov, Aleksandr A; Asachenko, Andrey F; Khrustalev, Victor N; Zalesskiy, Sergey S; Ananikov, Valentine P; Nechaev, Mikhail S

2014-05-12

208

[CpRu]-Catalyzed Carbene Insertions into Epoxides: 1,4-Dioxene Synthesis through SN 1-Like Chemistry with Retention of Configuration.  

PubMed

Rather than lead to the usual deoxygenation pathway, metal carbenes derived from ?-diazo-?-ketoesters undergo three-atom insertions into epoxides using a combination of 1,10-phenanthroline and [CpRu(CH3 CN)3 ][BArF ] as the catalyst. Original 1,4-dioxene motifs are obtained as single regio- and stereoisomers. A perfect syn?stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an SN 1-like transformation. PMID:24802707

Achard, Thierry; Tortoreto, Cecilia; Poblador-Bahamonde, Amalia I; Guénée, Laure; Bürgi, Thomas; Lacour, Jérôme

2014-06-10

209

Stable Matching of Difference Schemes.  

National Technical Information Service (NTIS)

Approximations that result from the natural matching of two stable dissipative difference schemes across a coordinate line are shown to be stable. The basic ideas is to reformulate the matching scheme consistent to an equivalent initial boundary value pro...

M. Ciment

1972-01-01

210

Stable Isotope Research Pool Inventory.  

National Technical Information Service (NTIS)

This report contains a listing of electromagnetically separated stable isotopes which are available at the Oak Ridge National Laboratory for distribution for non-destructive research use on a loan basis. This inventory includes all samples of stable isoto...

1985-01-01

211

Marginally Stable Nuclear Burning  

NASA Technical Reports Server (NTRS)

Thermonuclear X-ray bursts result from unstable nuclear burning of the material accreted on neutron stars in some low mass X-ray binaries (LMXBs). Theory predicts that close to the boundary of stability oscillatory burning can occur. This marginally stable regime has so far been identified in only a small number of sources. We present Rossi X-ray Timing Explorer (RXTE) observations of the bursting, high- inclination LMXB 4U 1323-619 that reveal for the first time in this source the signature of marginally stable burning. The source was observed during two successive RXTE orbits for approximately 5 ksec beginning at 10:14:01 UTC on March 28, 2011. Significant mHz quasi- periodic oscillations (QPO) at a frequency of 8.1 mHz are detected for approximately 1600 s from the beginning of the observation until the occurrence of a thermonuclear X-ray burst at 10:42:22 UTC. The mHz oscillations are not detected following the X-ray burst. The average fractional rms amplitude of the mHz QPOs is 6.4% (3 - 20 keV), and the amplitude increases to about 8% below 10 keV.This phenomenology is strikingly similar to that seen in the LMXB 4U 1636-53. Indeed, the frequency of the mHz QPOs in 4U 1323-619 prior to the X-ray burst is very similar to the transition frequency between mHz QPO and bursts found in 4U 1636-53 by Altamirano et al. (2008). These results strongly suggest that the observed QPOs in 4U 1323-619 are, like those in 4U 1636-53, due to marginally stable nuclear burning. We also explore the dependence of the energy spectrum on the oscillation phase, and we place the present observations within the context of the spectral evolution of the accretion-powered flux from the source.

Strohmayer, Tod E.; Altamirano, D.

2012-01-01

212

Bi-stable optical actuator  

DOEpatents

The present invention is a bi-stable optical actuator device that is depowered in both stable positions. A bearing is used to transfer motion and smoothly transition from one state to another. The optical actuator device may be maintained in a stable position either by gravity or a restraining device.

Holdener, Fred R. (Tracy, CA); Boyd, Robert D. (Livermore, CA)

2000-01-01

213

Colorado Plateau Stable Isotope Laboratory  

NSDL National Science Digital Library

This website, hosted by Northern Arizona University, describes stable isotopes. The site defines stable isotopes and how to measure them, and lists their research applications. Many of the advances in ecology and environmental sciences in the past decade have relied on stable isotopes.

Colorado Plateau Stable Isotope Laboratory (CPSIL); University, Northern A.

214

Stable angina pectoris.  

PubMed

The stable coronary artery disease (SCAD) population is a heterogeneous group of patients both for clinical presentations and for different underlying mechanisms. The recent European Society of Cardiology guidelines extensively review SCAD from its definition to patients' diagnostic and therapeutic management. In this review, we deal with five topics that, in our opinion, represent the most intriguing, novel and/or clinically relevant aspects of this complex coronary condition. Firstly, we deal with a peculiar SCAD population: patients with angina and 'normal' coronary arteries. Secondly, we reinforce the clinical importance of a diagnostic approach based on the pretest probability of disease. Thirdly, we review and critically discuss the novel pharmacological therapies for SCAD patients. Finally, we analyse the results of the most recent clinical trials comparing revascularization versus optimal medical therapy in SCAD patients and review the currently recommended use of intracoronary functional evaluation of stenosis. PMID:24838374

Valgimigli, Marco; Biscaglia, Simone

2014-07-01

215

Stable string bit models  

NASA Astrophysics Data System (ADS)

In string bit models, the superstring emerges as a very long chain of "bits," in which s fermionic degrees of freedom contribute positively to the ground state energy in a way to exactly cancel the destabilizing negative contributions of d=s bosonic degrees of freedom. We propose that the physics of string formation be studied nonperturbatively in the class of string bit models in which s>d, so that a long chain is stable, in contrast to the marginally stable (s=d=8) superstring chain. We focus on the simplest of these models with s=1 and d=0, in which the string bits live in zero space dimensions. The string bit creation operators are N×N matrices. We choose a Hamiltonian such that the large N limit produces a string moving in one space dimension, with excitations corresponding to one Grassmann light-cone world sheet field (s=1) and no bosonic world sheet field (d=0). We study this model at finite N to assess the role of the large N limit in the emergence of the spatial dimension. Our results suggest that stringlike states with large bit number M may not exist for N?(M-1)/2. If this is correct, one can have finite chains of string bits, but not a continuous string, at finite N. Only for extremely large N can such chains behave approximately like a continuous string, in which case there will also be the (approximate) emergence of a new spatial dimension. In string bit models designed to produce a critical superstring at N=?, we can then expect only approximate Lorentz invariance at finite N, with violations of order 1/N2.

Sun, Songge; Thorn, Charles B.

2014-05-01

216

Construct validity of Stable-2000 and Stable-2007 scores.  

PubMed

We addressed the construct validity of Stable-2000 and Stable-2007 scores by examining correlations between selected items and validated independent measures of relevant constructs in samples of convicted sex offenders. In Study 1, the Child Molester Attitudes item of the Stable-2000 shared 23% of the variance with a self-report measure of beliefs supportive of child molestation, r(19) = .48. The Deviant Sexual Interests items of the Stable-2000 and Stable-2007 shared 7% to 66% of the variance, respectively, with an offense-history-based measure of pedophilic interests, r(18) = .27 for the Stable-2000 and r(11) = .81 for the Stable-2007. In Study 2, the Lovers/Intimate Partners, General Social Rejection/Loneliness, Rapist Attitudes, and Child Molester Attitudes items of the Stable-2000 shared 4% to 19% of the variance with self-report measures of, respectively, intimacy, r(90) = -.44; loneliness, r(88) = .34; beliefs supportive of rape, r(72) = .21; and beliefs supportive of child molestation, r(78) = .36. The results generally suggest that the Stable items examined are associated with measures of similar constructs; however, the degree of convergence was lower than expected. More systematic and comprehensive research is needed to examine convergence of the Stable items with other relevant measures and additional aspects of construct validity. Such efforts will provide a clearer understanding of dynamic risk factors, appropriate areas of focus for treatment efforts, and, more generally, why some sex offenders recidivate. PMID:21543623

Nunes, Kevin L; Babchishin, Kelly M

2012-02-01

217

Synthesis of Well-Defined Copper "N"-Heterocyclic Carbene Complexes and Their Use as Catalysts for a "Click Reaction": A Multistep Experiment that Emphasizes the Role of Catalysis in Green Chemistry  

ERIC Educational Resources Information Center

A multistep experiment for an advanced synthesis lab course that incorporates topics in organic-inorganic synthesis and catalysis and highlights green chemistry principles was developed. Students synthesized two "N"-heterocyclic carbene ligands, used them to prepare two well-defined copper(I) complexes and subsequently utilized the complexes as…

Ison, Elon A.; Ison, Ana

2012-01-01

218

Stoichiometric C?O bond oxidative addition of benzophenone by a discrete radical intermediate to form a cobalt(I) carbene.  

PubMed

Single electron transfer from the Zr(III)Co(0) heterobimetallic complex (THF)Zr(MesNP(i)Pr2)3Co-N2 (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph2CO)Zr(MesNP(i)Pr2)3Co-N2]2 (2) via coupling of two ketyl radicals. In this work, thermolysis of 2 in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C-O bond to generate a Zr/Co ?-oxo species featuring an unusual terminal Co?CPh2 carbene linkage, (?(2)-MesNP(i)Pr2)Zr(?-O)(MesNP(i)Pr2)2Co?CPh2 (3). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex 3 was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to 3 leads to PhSiH2O-Zr(MesNP(i)Pr2)3Co-N2 (5) via a proposed Co-alkyl bond homolysis route. PMID:23574005

Marquard, Seth L; Bezpalko, Mark W; Foxman, Bruce M; Thomas, Christine M

2013-04-24

219

Stable face representations  

PubMed Central

Photographs are often used to establish the identity of an individual or to verify that they are who they claim to be. Yet, recent research shows that it is surprisingly difficult to match a photo to a face. Neither humans nor machines can perform this task reliably. Although human perceivers are good at matching familiar faces, performance with unfamiliar faces is strikingly poor. The situation is no better for automatic face recognition systems. In practical settings, automatic systems have been consistently disappointing. In this review, we suggest that failure to distinguish between familiar and unfamiliar face processing has led to unrealistic expectations about face identification in applied settings. We also argue that a photograph is not necessarily a reliable indicator of facial appearance, and develop our proposal that summary statistics can provide more stable face representations. In particular, we show that image averaging stabilizes facial appearance by diluting aspects of the image that vary between snapshots of the same person. We review evidence that the resulting images can outperform photographs in both behavioural experiments and computer simulations, and outline promising directions for future research.

Jenkins, Rob; Burton, A. Mike

2011-01-01

220

Polypyrrole-functionalized ruthenium carbene catalysts as efficient heterogeneous systems for olefin epoxidation.  

PubMed

New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl2(bpea-pyr)(dmso)] (isomeric complexes 2a and 2b) or [Ru(CN-Me)(bpea-pyr)X)](n+) (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H-imidazol-3-ium-2-ide; X = Cl, 3, or X = H2O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in solution, is dramatically affected by the electropolymerization process leading to the occurrence of two monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the epoxidation of various olefin substrates using PhI(OAc)2 as an oxidant, displaying TON values of several thousands in all cases and good selectivity for the epoxide product. PMID:24848551

Dakkach, Mohamed; Fontrodona, Xavier; Parella, Teodor; Atlamsani, Ahmed; Romero, Isabel; Rodríguez, Montserrat

2014-07-14

221

Spectroscopic characterization of triplet (2- and 3-thienyl)carbenes and development, implementation, and testing of a web-based tool for use in middle and high school classrooms  

NASA Astrophysics Data System (ADS)

Photolysis (lambda> 543 nm) of 3-thienyldiazomethane (1) yields triplet s-Z and s-E 3-thienylcarbene (s-E 5 and s- Z 5), as characterized by IR, UV/vis, and EPR spectroscopy. The EPR spectra of s-E 5 and s-Z 5 are markedly different, and proved to be unusual. NBO calculations and a Mulliken spin density analysis were carried out to explore the complex nature of these spectra. In an effort to obtain a comparison to the unusual EPR spectra of 3-thienylcarbene and to explore how the photochemistry of the system would be perturbed, methyl and deuteriomethyl analogs of the parent compounds were studied. Photolysis (lambda> 534 nm) of 3-thienyl-1-diazoethane (16) yields only one triplet 3-thienylethylidene rotamer (s-E 21), as characterized by UV/vis, and EPR spectroscopy. EPR and UV/visible signals attributed to s-E 21 decreased by approximately 50% after sitting in the dark for 3.5 days, suggesting a "thermal" reaction at 10 K. Photolysis (lambda > 534 nm) of 3-thienyl-1-diazoethane- d3 (16-d3) yields triplet s-E 3-thienylcarbene-d3 (s-E 21-d3), as characterized by IR, UV/vis, and EPR spectroscopy. Analogous to the photochemistry of 16, s-Z 21 -d3 was absent from the matrix. s- E 21-d3 is thermally stable at 10 K. Our attempts at isolating a 2-thienylsubstituted triplet carbene in this family had been unsuccessful up to this point. Spectroscopic detection of a 2-thienylcarbene derivative was realized with the characterization of 2-benzo[b]thienylethylidene (34) and 2-benzo[b]thienylethylidene-d3 (34-d3) by IR, UV/vis, and EPR spectroscopy. The development and implementation of "Today's Science for Tomorrow's Scientist" (TSTS), a Web site that presents current science research in a format suitable for middle school and high school classrooms is discussed. The information presented on TSTS correlates with the National Science Education Standards, allowing for ease of use by teachers. TSTS was beta tested and then fully implemented in middle schools and high schools in the Madison, WI area. The data show that both middle school and high school students improve by a significant amount from content pre survey to content post survey after viewing TSTS.

Pharr, Caroline Rebecca

222

Iridium(III) Hydrido N-Heterocyclic Carbene-Phosphine Complexes as Catalysts in Magnetization Transfer Reactions  

PubMed Central

The hyperpolarization (HP) method signal amplification by reversible exchange (SABRE) uses para-hydrogen to sensitize substrate detection by NMR. The catalyst systems [Ir(H)2(IMes)(MeCN)2(R)]BF4 and [Ir(H)2(IMes)(py)2(R)]BF4 [py = pyridine; R = PCy3 or PPh3; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene], which contain both an electron-donating N-heterocyclic carbene and a phosphine, are used here to catalyze SABRE. They react with acetonitrile and pyridine to produce [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4 and [Ir(H)2(NCMe)(py)(IMes)(PCy3)]BF4, complexes that undergo ligand exchange on a time scale commensurate with observation of the SABRE effect, which is illustrated here by the observation of both pyridine and acetonitrile HP. In this study, the required symmetry breaking that underpins SABRE is provided for by the use of chemical inequivalence rather than the previously reported magnetic inequivalence. As a consequence, we show that the ligand sphere of the polarization transfer catalyst itself becomes hyperpolarized and hence that the high-sensitivity detection of a number of reaction intermediates is possible. These species include [Ir(H)2(NCMe)(py)(IMes)(PPh3)]BF4, [Ir(H)2(MeOH)(py)(IMes)(PPh3)]BF4, and [Ir(H)2(NCMe)(py)2(PPh3)]BF4. Studies are also described that employ the deuterium-labeled substrates CD3CN and C5D5N, and the labeled ligands P(C6D5)3 and IMes-d22, to demonstrate that dramatically improved levels of HP can be achieved as a consequence of reducing proton dilution and hence polarization wastage. By a combination of these studies with experiments in which the magnetic field experienced by the sample at the point of polarization transfer is varied, confirmation of the resonance assignments is achieved. Furthermore, when [Ir(H)2(pyridine-h5)(pyridine-d5)(IMes)(PPh3)]BF4 is examined, its hydride ligand signals are shown to become visible through para-hydrogen-induced polarization rather than SABRE.

2013-01-01

223

STABLE ISOTOPES IN PLANT ECOLOGY  

Microsoft Academic Search

? Abstract The use of stable isotope techniques in plant ecological research has grown steadily during the past two decades. This trend will continue as investigators realize that stable isotopes can serve as valuable nonradioactive tracers and nondestruc- tive integrators of how plants today and in the past have interacted with and responded to their abiotic and biotic environments. At

Todd E. Dawson; Stefania Mambelli; Agneta H. Plamboeck; Pamela H. Templer; Kevin P. Tu

2002-01-01

224

Tampering with a Stable Process.  

ERIC Educational Resources Information Center

Presents a variation of Deming's funnel experiment to illustrate how a stable process operates and how tampering with a stable process leads to increased variability and less desirable results. The goal of the activity is to minimize product variation around the target value. Includes student handout. (Author/MKR)

Krehbiel, Timothy C.

1994-01-01

225

Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands.  

PubMed

This Article reports the synthesis, structural characterization, electrochemistry, and photophysical investigations of five groups of luminescent Pt(II) alkynyl complexes bearing N-heterocyclic carbene (NHC) ligands with varying electronic properties. Complexes of the type [Pt(pmdb)(C?CR)2] 1a-c, [Pt(pm2tz)(C?CR)2] 2a-d, [Pt(pm3tz)(C?CR)2] 3a-c, [Pt(ppim)(C?CR)2] 4(a, b, e), and [Pt(ppbim)(C?CR)2] 5(a, b, e), where pmdb =1,1'-dipentyl-3,3'-methylene-dibenzimidazoline-2,2'-diylidene, pm2tz = 1,1'-dipentyl-3,3'-methylene-di-1,2,4-triazoline-5,5'-diylidene, pm3tz = 1,1'-dipentyl-3,3'-methylene-di-1,3,4-triazoline-5,5'-diylidene, ppim = 3-pentyl-1-picolylimidazoline-2-ylidene, and ppbim = 3-pentyl-1-picolylbenzimidazoline-2-ylidene, and R = 4-C6H4F, C6H5, 4-C6H4OMe, SiMe3, and 4-C6H4N(C6H5)2, were prepared, and the consequences of the electronic properties of the NHC ligands on the phosphorescent emission efficiencies were studied. Moreover, the emission quantum efficiencies of the previously reported complexes [Pt(pmim)(C?CR)2] where pmim = 1,1'-dipentyl-3,3'-methylene-diimidazoline-2,2'-diylidene and R = 4-C6H4F 6a, C6H5 6b, and 4-C6H4OMe 6c were also recorded in neat solid and in 10 wt % PMMA film. The square planar coordination geometry with the alkynyl ligands in cis configuration was corroborated for selected complexes by single crystal X-ray diffraction studies. The observed moderate difference in emission efficiencies of the bis-carbene complexes 6a-c, 1a-c, 2a-c, and 3a-c in conjunction with the decreasing electron-donating nature of the NHC ligands, pmim > pmdb > pm2tz ? pm3tz, can be attributed to the slight modification of the triplet emission parentage among the different complexes. The quantum efficiencies of complexes 4(a, b) and 5(a, b) bearing pyridyl-NHC ligand were significantly low in comparison to the bis-carbene complexes owing to the significant change in the charge transfer character of the triplet manifold. Complexes 4e and 5e bearing diarylamine phenylacetylenes display high ?em of 27% and 33% in 10 wt % PMMA film, respectively. PMID:24392802

Zhang, Yuzhen; Clavadetscher, Jessica; Bachmann, Michael; Blacque, Olivier; Venkatesan, Koushik

2014-01-21

226

Subtle reactivity patterns of non-heteroatom-substituted manganese alkynyl carbene complexes in the presence of phosphorus probes.  

PubMed

The non-heteroatom-substituted manganese alkynyl carbene complexes (eta5-MeC5H4)(CO)2Mn=C(R)C[triple bond]CR'(3; 3a: R = R'= Ph, 3b: R = Ph, R'= Tol, 3c: R = Tol, R'= Ph) have been synthesised in high yields upon treatment of the corresponding carbyne complexes [eta5-MeC5H4)(CO)2Mn[triple bond]CR][BPh4]([2][BPh4]) with the appropriate alkynyllithium reagents LiC[triple bond]CR' (R'= Ph, Tol). The use of tetraphenylborate as counter anion associated with the cationic carbyne complexes has been decisive. The X-ray structures of (eta5-MeC5H4)(CO)2Mn=C(Tol)C[triple bond]CPh (3c), and its precursor [(eta5-MeC5H4)(CO)2Mn=CTol][BPh4]([2b](BPh4]) are reported. The reactivity of complexes toward phosphines has been investigated. In the presence of PPh3, complexes act as a Michael acceptor to afford the zwitterionic sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh3)R' (5) resulting from nucleophilic attack by the phosphine on the remote alkynyl carbon atom. Complexes 5 exhibit a dynamic process in solution, which has been rationalized in terms of a fast [NMR time-scale] rotation of the allene substituents around the allene axis; metrical features within the X-ray structure of (eta5-MeC5H4)(CO)2MnC(Ph)=C=C(PPh3)Tol (5b) support the proposal. In the presence of PMe3, complexes undergo a nucleophilic attack on the carbene carbon atom to give zwitterionic sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PMe3)C[triple bond]CR' (6). Complexes 6 readily isomerise in solution to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PMe3)R (7) through a 1,3 shift of the [(eta5-MeC5H4)(CO)2Mn] fragment. The nucleophilic attack of PPh2Me on 3 is not selective and leads to a mixture of the sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PPh(2)Me)C[triple bond]CR' (9) and the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh(2)Me)R' (10). Like complexes 6, complexes 9 readily isomerize to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PPh2Me)R'). Upon gentle heating, complexes 7, and mixtures of 10 and 10' cyclise to give the sigma-dihydrophospholium complexes (eta5-MeC5H4)(CO)2MnC=C(R')PMe2CH2CH(R)(8), and mixtures of complexes (eta5-MeC5H4)(CO)2MnC=C(Ph)PPh2CH2CH(Tol)(11) and (eta5-MeC5H4)(CO)2MnC=C(Tol)PMe2CH2CH(Ph)(11'), respectively. The reactions of complexes 3 with secondary phosphines HPR(1)(2)(R1= Ph, Cy) give a mixture of the eta2-allene complexes (eta5-MeC5H4)(CO)2Mn[eta2-{R(1)(2)PC(R)=C=C(R')H}](12), and the regioisomeric eta4-vinylketene complexes [eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R)=CHC(R')=C=O}](13) and (eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R')=CHC(R)=C=O}](13'). The solid-state structure of (eta5-MeC5H4)(CO)2Mn[eta2-{Ph2PC(Ph)=C=C(Tol)H}](12b) and (eta5-MeC5H4)(CO)Mn[eta4-{Cy2PC(Ph)=CHC(Ph)=C=O}](13d) are reported. Finally, a mechanism that may account for the formation of the species 12, 13, and 13' is proposed. PMID:15852111

Ortin, Yannick; Lugan, Noêl; Mathieu, René

2005-05-01

227

Stable plastid transformation of petunia.  

PubMed

Petunia hybrida is a commercial ornamental plant and is also an important model species for genetic analysis and transgenic research. Here we describe the steps required to isolate stable plastid transformants in P. hybrida using the commercial Pink Wave cultivar. Wave cultivars are popular spreading Petunias sold as ground cover and potted plants. Transgenes introduced into P. hybrida plastids exhibit stable expression over many generations. The development of plastid transformation in P. hybrida provides an enabling technology to bring the benefits of plastid engineering, including maternal inheritance and stable expression of performance-enhancing trait genes, to the important floriculture and horticulture industries. PMID:24599860

Avila, Elena Martin; Day, Anil

2014-01-01

228

Theoretical study on cooperative effects between X?N and X?Carbene halogen bonds (X?=?F,Cl,Br and I).  

PubMed

Quantum chemical calculations are performed to study the interplay between halogen?nitrogen and halogen?carbene interactions in NCX?NCX?CH2 complexes, where X?=?F, Cl, Br and I. Molecular geometries and interaction energies of dyads and triads are investigated at the MP2/aug-cc-pVTZ level of theory. It is found that the X?N and X?Ccarbene interaction energies in the triads are larger than those in the dyads, indicating that both the halogen bonding interactions are enhanced. The estimated values of cooperative energy E coop are all negative with much larger E coop in absolute value for the systems including iodine. The nature of halogen bond interactions of the complexes is analyzed using parameters derived from the quantum theory atoms in molecules methodology and energy decomposition analysis. PMID:24013450

Esrafili, Mehdi D; Mohammdain-Sabet, Fariba; Esmailpour, Parvin

2013-11-01

229

Correlation between functionality preference of Ru carbenes and exo/endo product selectivity for clarifying the mechanism of ring-closing enyne metathesis.  

PubMed

Functionality preferences of metathesis Ru carbenes to various alkenes and alkynes with electronic and steric diversity were determined by using time-dependent fluorescence quenching. The functionality preferences depend not only on the properties of multiple bonds but also on the ligands on Ru. There was a good correlation between functionality preference and the metathesis reaction outcome. The correlation between functionality preference and exo/endo product ratio offers a solution to resolve the mechanistic issue related with alkene- vs alkyne-initiated pathway in ring-closing enyne metathesis. The correlation indicates the preference is likely to dictate the reaction pathway and eventually the outcome of the reaction. The Ru catalyst favoring alkyne over alkene provides more endo product, indicating that the reaction mainly initiates at the alkyne. By changing the substitution pattern, the preference can be reversed to give an exclusive exo product. PMID:23944769

Lee, Ok Suk; Kim, Kyung Hwan; Kim, Jinwoo; Kwon, Kuktae; Ok, Taedong; Ihee, Hyotcherl; Lee, Hee-Yoon; Sohn, Jeong-Hun

2013-09-01

230

Mechanisms and origins of switchable regioselectivity of palladium- and nickel-catalyzed allene hydrosilylation with N-heterocyclic carbene ligands: a theoretical study.  

PubMed

The mechanisms and origins for the Pd- and Ni-catalyzed regioselective hydrosilylation of allene have been investigated by means of density functional theory (DFT) calculations. The free-energy profiles of Pd- and Ni-catalyzed reactions with small and bulky N-heterocyclic carbene (NHC) ligands are calculated to determine the mechanism for regioselectivities. The calculation results show that different metals (Ni vs Pd) lead to regiochemical reversals for the hydrosilylation of allene. The allylsilane is the major product via palladium catalysis with small NHC ligand, while the vinylsilane is the major product via nickel catalysis with bulky NHC ligand. Both electronic and steric factors play a key role in the regioselectivities for the hydrosilylation of allene via Pd and Ni catalysts. The calculation results are in good agreement with observed regioselectivities and could provide insights into the design of new catalysts for the regioselectivity of hydrosilylation reactions. PMID:24779730

Xie, Hujun; Zhao, Lijiang; Yang, Liu; Lei, Qunfang; Fang, Wenjun; Xiong, Chunhua

2014-05-16

231

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

232

Synthesis of a phosphorano-stabilized U(IV)-carbene via one-electron oxidation of a U(III)-ylide adduct.  

PubMed

Addition of the Wittig reagent Ph(3)P?CH(2) to the U(III) tris(amide) U(NR(2))(3) (R = SiMe(3)) generates a mixture of products from which the U(IV) complex U?CHPPh(3)(NR(2))(3) (2) can be obtained. Complex 2 features a short U?C bond and represents a rare example of a uranium carbene. In solution, 2 exists in equilibrium with the U(IV) metallacycle U(CH(2)SiMe(2)NR)(NR(2))(2) and free Ph(3)P?CH(2). Measurement of this equilibrium as a function of temperature provides ?H(rxn) = 11 kcal/mol and ?S(rxn) = 31 eu. Additionally, the electronic structure of the U?C bond was investigated using DFT analysis. PMID:21488697

Fortier, Skye; Walensky, Justin R; Wu, Guang; Hayton, Trevor W

2011-05-11

233

Influence of pyrazolate vs N-heterocyclic carbene ligands on the slow magnetic relaxation of homoleptic trischelate lanthanide(III) and uranium(III) complexes.  

PubMed

Two isostructural series of trigonal prismatic complexes, M(Bp(Me))3 and M(Bc(Me))3 (M = Y, Tb, Dy, Ho, Er, U; [Bp(Me)](-) = dihydrobis(methypyrazolyl)borate; [Bc(Me)](-) = dihydrobis(methylimidazolyl)borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb(3+), Dy(3+), and U(3+) metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(Bc(Me))3, than for the isomeric pyrazolate complexes, M(Bp(Me))3. Further, investigation of magnetically dilute samples containing 11-14 mol % of Tb(3+), Dy(3+), or U(3+) within the corresponding Y(3+) complex matrix reveals thermally activated relaxation is favored for the M(Bc(Me))3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(Bc(Me))3 exhibits Ueff ? 33 cm(-1), representing the highest barrier yet observed for a U(3+) molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and (1)H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers. PMID:24650296

Meihaus, Katie R; Minasian, Stefan G; Lukens, Wayne W; Kozimor, Stosh A; Shuh, David K; Tyliszczak, Tolek; Long, Jeffrey R

2014-04-23

234

Stable Crystalline Cellulose III Polymorphs.  

National Technical Information Service (NTIS)

The invention relates to production of ammonia mercerized cellulose. More specifically, the invention relates to treating cellulose fiber with vapors of ammonia to produce stable cellulose III polymorphs. The primary objective is to provide a method for p...

L. Y. Yatsu T. A. Calamari R. R. Benerito

1987-01-01

235

Earthquakes in stable continental crust  

SciTech Connect

Earthquakes can strike even in stable crust, well away from the familiar earthquake zones at the edges of tectonic plates, but their mere occurrence is both a source of concern in planning critical facilities such as nuclear power plants. The authors sought answers to two major questions: Just how much seismic activity does take place within the stable parts of continents And are there specific geologic features that make some areas of stable crust particularly susceptible to earthquakes They began by studying North America alone, but it soon became clear that the fairly short record of these rare events on a single continent would not provide enough data for reliable analysis. Hence, they decided to substitute space for time--to survey earthquake frequency and distribution in stable continental areas worldwide. This paper discusses their findings.

Johnson, A.C.; Kanter, L.R. (Memphis State Univ., TN (USA))

1990-03-01

236

Earthquakes in Stable Continental Crust.  

ERIC Educational Resources Information Center

Discussed are some of the reasons for earthquakes which occur in stable crust away from familiar zones at the ends of tectonic plates. Crust stability and the reactivation of old faults are described using examples from India and Australia. (CW)

Johnston, Arch C.; Kanter, Lisa R.

1990-01-01

237

Tris(carbene)borate ligands featuring imidazole-2-ylidene, benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donors. Evaluation of donor properties in four-coordinate {NiNO}10 complexes  

PubMed Central

The synthesis and characterization of new tris(carbene)borate ligand precursors containing substituted benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donor groups, as well as a new tris(imidazol-2-ylidene)borate ligand precursor are reported. The relative donor strength of the tris(carbene)borate ligands have been evaluated by the position of ?(NO) in four-coordinate {NiNO}10 complexes, and follows the order: imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-ylidene. There is a large variation in ?(NO), suggesting these ligands to have a wide range of donor strengths while maintaining a consistent ligand topology. All ligands are stronger donors than Tp* and Cp*.

Munoz, Salvador B.; Foster, Wallace K.; Lin, Hsiu-Jung; Margarit, Charles G.; Dickie, Diane A.

2012-01-01

238

DMF as a Source of Oxygen and Aminomethine: Stereoselective 1,2-Insertion of Rhodium(II) Azavinyl Carbenes into the C?O Bond of Formamides for the Synthesis of cis-Diamino Enones.  

PubMed

A new catalytic reaction in which all the atoms of a formamide are incorporated into the product through a formal stereoselective 1,2-insertion of rhodium(II) azavinyl carbenes, generated in situ from readily available N-sulfonylated 1,2,3-triazoles, into the C?O bond of DMF and other N,N-disubstituted formamides to afford cis-diamino enones is described. PMID:24708029

Jung, Da Jung; Jeon, Hyun Ji; Kim, Ju Hyun; Kim, Youngmee; Lee, Sang-Gi

2014-04-18

239

Phase stable rf transport system.  

National Technical Information Service (NTIS)

This invention is comprised of an RF transport system which delivers a phase-stable RF signal to a load, such as an RF cavity of a charged particle accelerator. A circuit generates a calibration signal at an odd multiple frequency of the RF signal where t...

M. T. Curtin E. F. Natter P. M. Denney

1991-01-01

240

Synthesis of thermally stable polymers  

NASA Technical Reports Server (NTRS)

The reaction of bis triazo linediones with divinyl esters and substituted styrenes was investigated. Twenty new polymers were derived via reaction of two previously synthesized bis triazol linediones and four new bis atriazol linediones with eight styrenes. The structure and polymer properties of these thermally stable polymers was examined. The reaction of triazo linediones with enol esters was also considered.

Butler, G. B.

1978-01-01

241

Globally evolutionarily stable portfolio rules  

Microsoft Academic Search

The paper examines a dynamic model of a financial market with endogenous asset prices determined by short-run equilibrium of supply and demand. Assets pay dividends that are partially consumed and partially reinvested. The traders use fixed-mix investment strategies (portfolio rules), distributing their wealth between assets in fixed proportions. Our main goal is to identify globally evolutionarily stable strategies, allowing an

Igor V. Evstigneev; Thorsten Hens; Klaus Reiner Schenk-Hoppé

2008-01-01

242

Iron stable isotopes: beyond biosignatures  

Microsoft Academic Search

The stable isotope geochemistry of Fe has attracted intense interest in the past five years. This interest was originally motivated by the possible use of Fe isotopes in biosignature applications, particularly in sediments from the ancient Earth or Mars. This application is still being developed, with particular attention to fractionation mechanisms. Understanding such mechanisms should also provide new insights into

A. D. Anbar

2004-01-01

243

Pyridine substituted N-heterocyclic carbene ligands as supports for Au(I)-Ag(I) interactions: formation of a chiral coordination polymer.  

PubMed

Reaction of 1,3-bis(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate, [H(pyCH(2))(2)im]BF(4), with silver oxide in dichloromethane readily yields [Ag((pyCH(2))(2)im)(2)]BF(4), 1.BF(4)(). 1.BF(4) is converted to the analogous Au(I)-containing species, [Au((pyCH(2))(2)im)(2)]BF(4), 3, by a simple carbene transfer reaction in dichloromethane. Further treatment with two equivalents of AgBF(4) produces the trimetallic species [AuAg(2)((pyCH(2))(2)im)(2)(NCCH(3))(2)](BF(4))(3), 4, which contains two silver ions each coordinated to the pyridine moieties on one carbene ligand and to an acetonitrile molecule in a T-shaped fashion. Monometallic [Ag((py)(2)im)(2)]BF(4), 5, and [Au((py)(2)im)(2)]BF(4), 6, are made analogously to 1.BF(4) and 3 starting from 1,3-bis(2-pyridyl)-imidazol-2-ylidene tetrafluoroborate, [H(py)(2)im]BF(4). Addition of excess AgBF(4) to 6 yields the helical mixed-metal polymer, ([AuAg((py)(2)im)(2)(NCCH(3))](BF(4))(2))(n), 7 which contains an extended Au(I)-Ag(I) chain with short metal-metal separations of 2.8359(4) and 2.9042(4) A. Colorless, monometallic [Hg((pyCH(2))(2)im)(2)](BF(4))(2), 8, is easily produced by refluxing [H(pyCH(2))(2)im)]BF(4) with Hg(OAc)(2) in acetonitrile. The related quinolyl-substituted imidazole, [H(quinCH(2))(2)im]PF(6), is produced analogously to [H(pyCH(2))(2)im]BF(4). [Hg((quinCH(2))(2)im)(2)](PF(6))(2), 9, is isolated in good yield as a white solid from the reaction of Hg(OAc)(2) and [H(quinCH(2))(2)im]PF(6). The reaction of [H(quinCH(2))(2)im]PF(6) with excess Ag(2)O produces the triangulo-cluster [Ag(3)((quinCH(2))(2)im)(3)](PF(6))(3), 11. All of these complexes were studied by (1)H NMR spectroscopy, and complexes 3-9 were additionally characterized by X-ray crystallography. These complexes are photoluminescent in the solid state and in solution with spectra that closely resemble those of the ligand precursor. PMID:15332824

Catalano, Vincent J; Malwitz, Mark A; Etogo, Anthony O

2004-09-01

244

Constructing stable grasps in 3D  

Microsoft Academic Search

This paper presents fast and simple algorithms for directly constructing stable grasps in 3D. The synthesis of stable grasps constructs virtual springs at the contacts, such that the grasped object is stable, and has a desired stiffness matrix about its stable equilibrium. The paper develops a simple geometric relation between the stiffness of the grasp and the spatial configuration of

Van-Duc Nguyen

1987-01-01

245

Computing Stable Models by Using the ATMS  

Microsoft Academic Search

An algorithm is described which computes stable mod- els of propositional logic programs with negation as fail- ure using the Assumption Based Truth Maintenance mechanism. Since stable models of logic programs are closely connected to stable expansions of a class of auto- epistemic theories, this algorithm points to a link be- tween stable expansions of a class of autoepistemic the-

Kave Eshghi

1990-01-01

246

Phase stable RF transport system  

DOEpatents

An RF transport system delivers a phase-stable RF signal to a load, such as an RF cavity of a charged particle accelerator. A circuit generates a calibration signal at an odd multiple frequency of the RF signal where the calibration signal is superimposed with the RF signal on a common cable that connects the RF signal with the load. Signal isolating diplexers are located at both the RF signal source end and load end of the common cable to enable the calibration to be inserted and extracted from the cable signals without any affect on the RF signal. Any phase shift in the calibration signal during traverse of the common cable is then functionally related to the phase shift in the RF signal. The calibration phase shift is used to control a phase shifter for the RF signal to maintain a stable RF signal at the load.

Curtin, Michael T. (Los Alamos, NM); Natter, Eckard F. (San Francisco, CA); Denney, Peter M. (Los Alamos, NM)

1992-01-01

247

Stable maps and Quot schemes  

NASA Astrophysics Data System (ADS)

In this paper we study the relationship between two different compactifications of the space of vector bundle quotients of an arbitrary vector bundle on a curve. One is Grothendieck's Quot scheme, while the other is a moduli space of stable maps to the relative Grassmannian. We establish an essentially optimal upper bound on the dimension of the two compactifications. Based on that, we prove that for an arbitrary vector bundle, the Quot schemes of quotients of large degree are irreducible and generically smooth. We precisely describe all the vector bundles for which the same thing holds in the case of the moduli spaces of stable maps. We show that there are in general no natural morphisms between the two compactifications. Finally, as an application, we obtain new cases of a conjecture on effective base point freeness for pluritheta linear series on moduli spaces of vector bundles.

Popa, Mihnea; Roth, Mike

2003-06-01

248

Stable Carbon and Nitrogen Isotopic  

Microsoft Academic Search

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the

Jennifer Lawson; PETER T. DORAN; Fabien Kenig; DAVID J. DES MARAIS; JOHN C. PRISCU

2004-01-01

249

Advanced Thermally Stable Jet Fuels  

SciTech Connect

The Penn State program in advanced thermally stable jet fuels has five components: 1) development of mechanisms of degradation and solids formation; 2) quantitative measurement of growth of sub-micrometer and micrometer-sized particles during thermal stressing; 3) characterization of carbonaceous deposits by various instrumental and microscopic methods; 4) elucidation of the role of additives in retarding the formation of carbonaceous solids; and 5) assessment of the potential of producing high yields of cycloalkanes and hydroaromatics from coal.

A. Boehman; C. Song; H. H. Schobert; M. M. Coleman; P. G. Hatcher; S. Eser

1998-01-01

250

Stable Isotopes in Foraminiferal Carbonate  

Microsoft Academic Search

Analyses of stable oxygen and carbon isotopes from foraminiferal shells have played a pivotal role in palaeoceanography since Emiliani (1955) interpreted the isotopic record from deep-sea cores as a series of Pleistocene climate\\/temperature cycles. Oxygen isotope stratigraphy has become a global correlation tool, and an established dating tool. The primary ice-volume control on (benthic) oxygen isotope records has led to

Steve Cooke; Eelco J. Rohling

251

How Stable Is Stable? Function versus Community Composition  

PubMed Central

The microbial community dynamics of a functionally stable, well-mixed, methanogenic reactor fed with glucose were analyzed over a 605-day period. The reactor maintained constant pH and chemical oxygen demand removal during this period. Thirty-six rrn clones from each of seven sampling events were analyzed by amplified ribosomal DNA restriction analysis (ARDRA) for the Bacteria and Archaea domains and by sequence analysis of dominant members of the community. Operational taxonomic units (OTUs), distinguished as unique ARDRA patterns, showed reproducible distribution for three sample replicates. The highest diversity was observed in the Bacteria domain. The 16S ribosomal DNA Bacteria clone library contained 75 OTUs, with the dominant OTU accounting for 13% of the total clones, but just 21 Archaea OTUs were found, and the most prominent OTU represented 50% of the clones from the respective library. Succession in methanogenic populations was observed, and two periods were distinguished: in the first, Methanobacterium formicicum was dominant, and in the second, Methanosarcina mazei and a Methanobacterium bryantii-related organism were dominant. Higher variability in Bacteria populations was detected, and the temporal OTU distribution suggested a chaotic pattern. Although dominant OTUs were constantly replaced from one sampling point to the next, phylogenetic analysis indicated that inferred physiologic changes in the community were not as dramatic as were genetic changes. Seven of eight dominant OTUs during the first period clustered with the spirochete group, although a cyclic pattern of substitution occurred among members within this order. A more flexible community structure characterized the second period, since a sequential replacement of a Eubacterium-related organism by an unrelated deep-branched organism and finally by a Propionibacterium-like species was observed. Metabolic differences among the dominant fermenters detected suggest that changes in carbon and electron flow occurred during the stable performance and indicate that an extremely dynamic community can maintain a stable ecosystem function.

Fernandez, Ana; Huang, Suiying; Seston, Sherry; Xing, Jian; Hickey, Robert; Criddle, Craig; Tiedje, James

1999-01-01

252

Copper- and copper-N-heterocyclic carbene-catalyzed C-H activating carboxylation of terminal alkynes with CO2 at ambient conditions  

PubMed Central

The use of carbon dioxide as a renewable and environmentally friendly source of carbon in organic synthesis is a highly attractive approach, but its real world applications remain a great challenge. The major obstacles for commercialization of most current protocols are their low catalytic performances, harsh reaction conditions, and limited substrate scope. It is important to develop new reactions and new protocols for CO2 transformations at mild conditions and in cost-efficient ways. Herein, a copper-catalyzed and copper–N-heterocyclic carbene-cocatalyzed transformation of CO2 to carboxylic acids via C?H bond activation of terminal alkynes with or without base additives is reported. Various propiolic acids were synthesized in good to excellent yields under ambient conditions without consumption of any organometallic or organic reagent additives. This system has a wide scope of substrates and functional group tolerances and provides a powerful tool for the synthesis of highly functionalized propiolic acids. This catalytic system is a simple and economically viable protocol with great potential in practical applications.

Yu, Dingyi; Zhang, Yugen

2010-01-01

253

Synthesis, characterization, and photophysical study of luminescent rhenium(I) diimine complexes with various types of N-heterocyclic carbene ligands.  

PubMed

A series of luminescent Re(I) diimine complexes with various types of N-heterocyclic carbene (NHC) ligands has been synthesized through the reaction between isocyano Re(I) diimine complexes with different nucleophiles. These Re(I) NHC complexes were characterized by (1)H and (13)C NMR and IR spectroscopy, mass spectrometry, and elemental analysis. One of the precursor complexes fac-{Re(CO)3[CN(H)C6H4-2-O]2Br} and five of the Re(I) diimine complexes with different types of NHC ligands were also structurally characterized by X-ray crystallography. In the preparation of these Re(I) NHC complexes, it is found that the reactivity of the isocyanide ligands in the synthetic complex precursors is significantly affected by the electronic nature of the trans ligand. All these complexes displayed (3)MLCT [d?(Re) ? ?*(N-N)] phosphorescence in degassed CH2Cl2 and CH3CN solutions at room temperature. Through the study of the photophysical and electrochemical properties of these Re(I) NHC complexes, the electronic properties of different types of NHC ligands were investigated. PMID:24592975

Ng, Chi-On; Yiu, Shek-Man; Ko, Chi-Chiu

2014-03-17

254

Theoretical study on the effect of annelation and carbonylation on the electronic and ligand properties of N-heterocyclic silylenes and germylenes: carbene comparisons begin to break down.  

PubMed

Quantum chemical calculations have been carried out to investigate the effect of annelation and carbonylation on the electronic and ligand properties of N-heterocyclic silylenes and germylenes. The thermodynamic stability of these ligands has been found to increase with annelation, while the reverse is true for carbonylation. This is in sharp contrast to N-heterocyclic carbenes (NHCs) where annelation leads to a decrease in their thermodynamic stabilities. Compared to nonannelated derivatives, annelated and carbonylated ones are found to be weaker ? donors but better ? acceptors. The effect of carbonylation is more pronounced than annelation toward increasing the ? acidity of these ligands. Carbonylation at the ?-position with respect to the N atom attached to the Si/Ge center has been found to be the most effective way of enhancing the ? acidity of these ligands. The computed natural charges reveal that electrophilicity increases upon both annelation and carbonylation. The calculated values of (31)P NMR chemical shifts of corresponding phosphinidene adducts of these ligands have been found to correlate well with the ? acidity of these Si/Ge centers. PMID:24738711

Guha, Ankur Kanti; Phukan, Ashwini K

2014-05-01

255

Carbene- and carbyne-like behavior of the Mo-P multiple bond in a dimolybdenum complex inducing trigonal-pyramidal coordination of a phosphinidene ligand.  

PubMed

The phosphinidene complex [Mo2Cp(micro-kappa1:kappa1,eta5-PC5H4)(CO)2(eta6-R*H)] (2; Cp = eta5-C5H5; R* = 2,4,6-C6H2tBu3) has substantially different Mo-P bonds and displays a high reactivity located at the short Mo-P bond. Sideways cycloaddition or addition processes are observed toward RCCR, HCl, and [Fe2(CO)9], to give respectively metallacyclobutene and arylphosphide-bridged and heterometallic phosphinidene-bridged derivatives, a behavior reminiscent of the nucleophilic mononuclear phosphinidene complexes (carbene-like behavior), which is in good agreement with the ground-state electronic structure of 2 derived from density functional theory calculations. However, the reaction of 2 with [Co2(CO)8] implies the addition of two cobalt fragments to its short Mo-P bond and thus reveals a carbyne-like behavior of compound 2. In most of the new products, the P atom displays an unprecedented trigonal-pyramidal-like environment, instead of the expected tetrahedral distribution of bonds. PMID:17608475

Alvarez, M Angeles; Amor, Inmaculada; García, M Esther; García-Vivó, Daniel; Ruiz, Miguel A

2007-08-01

256

Cross-coupling of diarylborinic acids and anhydrides with arylhalides catalyzed by a phosphite/N-heterocyclic carbene co-supported palladium catalyst system.  

PubMed

A highly efficient cross-coupling of diarylborinic acids and anhydrides with aryl chlorides and bromides has been effected by using a palladium catalyst system co-supported by a strong ?-donor N-heterocyclic carbene (NHC), N,N'-bis(2,6-diisopropylphenyl) imidazol-2-ylidene, and a strong ?-acceptor phosphite, triphenylphosphite, in tert-BuOH in the present of K(3)PO(4)·3H(2)O. Unsymmetrical biaryls with a variety of functional groups could be obtained in good to excellent yields using as low as 0.01, 0.2-0.5, and 1 mol % palladium loadings for aryl bromides and activated and deactivated aryl chlorides, respectively, under mild conditions. A ligand synergy between the ?-donor NHC and the ?-acceptor phosphite in the Pd/NHC/P(OPh)(3) catalytic system has been proposed to be responsible for the high efficacy to arylchlorides in the cross-coupling. A scalable and economical process has therefore been developed for synthesis of Sartan biphenyl from the Pd/NHC/P(OPh)(3) catalyzed cross-coupling of di(4-methylphenyl)borinic acid with 2-chlorobenzonitrile. PMID:22860711

Chen, Xiaofeng; Ke, Haihua; Chen, Yao; Guan, Changwei; Zou, Gang

2012-09-01

257

DFT study on the mechanisms and stereoselectivities of the [4 + 2] cycloadditions of enals and chalcones catalyzed by N-heterocyclic carbene.  

PubMed

The possible reaction mechanisms of stereoselective [4 + 2] cycloaddition of enals and chalcones catalyzed by N-heterocyclic carbene (NHC) have been investigated using density functional theory (DFT). The calculated results indicate that the most favorable reaction channel occurs through five steps. The first step is the nucleophilic attack on the enal by NHC. Then, there are two consecutive acid (AcOH)-assisted proton-transfer steps. Subsequently, the fourth step is the [4 + 2] cycloaddition process associated with the formation of two chiral centers, followed by dissociation of NHC and product. Our computational results demonstrate that the [4 + 2] cycloaddition is the rate-determining and stereoselectivity-determining step. The energy barrier for the SS configurational channel (17.62 kcal/mol) is the lowest one, indicating the SS configurational product should be the main product, which is in agreement with experiment. Moreover, the role of NHC catalyst in the [4 + 2] cycloaddition of enal and chalcone was explored by the analysis of global reactivity indexes. This work should be helpful for realizing the significant roles of catalyst NHC and the additive AcOH and thus provide valuable insights on the rational design of potential catalyst for this kind of reactions. PMID:24628502

Li, Zhenyu; Wei, Donghui; Wang, Yang; Zhu, Yanyan; Tang, Mingsheng

2014-04-01

258

Different behavior of nitrenes and carbenes on photolysis and thermolysis: formation of azirine, ylidic cumulene, and cyclic ketenimine and the rearrangement of 6-phenanthridylcarbene to 9-phenanthrylnitrene.  

PubMed

Flash vacuum thermolysis (FVT) of 9-azidophenanthrene 8, 6-(5-tetrazolyl)phenanthridine 18, and [1,2,3]triazolo[1,5-f]phenanthridine 19 yields 9-cyanofluorene 12 as the principal product and 4-cyanofluorene as a minor product. In all cases, when the product is condensed at or below 77 K, the seven-membered ring ketenimine 24 is detectable by IR spectroscopy (1932 cm(-1)) up to 200 K. Photolysis of Ar matrix isolated 8 at lambda = 308 or 313 nm generates at first the azirine 26, rapidly followed by the ylidic cumulene 27. The latter reverts to azirine 26 at lambda > 405 nm, and the azirine reverts to the ylidic cumulene at 313 nm. Nitrene 9 is observed by ESR spectroscopy following FVT of either azide 8, tetrazole 18, or triazole 19 with Ar matrix isolation of the products. Nitrene 9 and carbene 21 are observed by ESR spectroscopy in the Ar matrix photolyses of azide 8 and triazole 19, respectively. PMID:16277314

Kvaskoff, David; Bednarek, Pawel; George, Lisa; Pankajakshan, Sreekumar; Wentrup, Curt

2005-09-30

259

Z-Stable ASH Algebras  

Microsoft Academic Search

The Jiang--Su algebra Z has come to prominence in the classification program\\u000afor nuclear C*-algebras of late, due primarily to the fact that Elliott's\\u000aclassification conjecture predicts that all simple, separable, and nuclear\\u000aC*-algebras with unperforated K-theory will absorb Z tensorially (i.e., will be\\u000aZ-stable). There exist counterexamples which suggest that the conjecture will\\u000aonly hold for simple, nuclear, separable

Andrew S. Toms; Wilhelm Winter

2008-01-01

260

2. View of stable looking south with garage/stable to the ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

2. View of stable looking south with garage/stable to the right and paddock fence to the left - Richmond Hill Plantation, Stable/Garage, East of Richmond Hill on Ford Neck Road, Richmond Hill, Bryan County, GA

261

Spin Contamination Error in Optimized Geometry of Singlet Carbene (1A1) by Broken-Symmetry Method  

NASA Astrophysics Data System (ADS)

Spin contamination errors of a broken-symmetry (BS) method in optimized structural parameters of the singlet methylene (1A1) molecule are quantitatively estimated for the Hartree-Fock (HF) method, post-HF methods (CID, CCD, MP2, MP3, MP4(SDQ)), and a hybrid DFT (B3LYP) method. For the purpose, the optimized geometry by the BS method is compared with that of an approximate spin projection (AP) method. The difference between the BS and the AP methods is about 10-20° in the HCH angle. In order to examine the basis set dependency of the spin contamination error, calculated results by STO-3G, 6-31G*, and 6-311++G** are compared. The error depends on the basis sets, but the tendencies of each method are classified into two types. Calculated energy splitting values between the triplet and the singlet states (ST gap) indicate that the contamination of the stable triplet state makes the BS singlet solution stable and the ST gap becomes small. The energy order of the spin contamination error in the ST gap is estimated to be 10-1 eV.

Kitagawa, Yasutaka; Saito, Toru; Nakanishi, Yasuyuki; Kataoka, Yusuke; Matsui, Toru; Kawakami, Takashi; Okumura, Mitsutaka; Yamaguchi, Kizashi

2009-10-01

262

Cobalt carbene ion: reactions of Co/sup +/ with C/sub 2/H/sub 4/, cyclo-C/sub 3/H/sub 6/, and cyclo-C/sub 2/H/sub 4/O  

SciTech Connect

An ion beam apparatus is employed to study the formation of the cobalt carbene ion, CoCH/sub 2//sup +/. This ion is produced in the endothermic reaction of cobalt ions with ethene and cyclopropane and in an exothermic reaction with ethylene oxide. A model is proposed to account for the dependence of experimental cross sections on relative kinetic energy for the endothermic reactions. Using this model to interpret the experimental results, a bond dissociation energy D /sup 0/(Co/sup +/-CH/sub 2/) = 3.7 +- 0.3 eV is derived.

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

1981-03-01

263

Stable Dimension Permutations on Boolean Cubes.  

National Technical Information Service (NTIS)

The paper presents lower bounds and algorithms optimal within a small constant factor for stable dimension permutations on Boolean cubes. A stable dimension permutation is a permutation where a global address receives its new content from a global address...

C. T. Ho S. L. Johnsson

1988-01-01

264

Stable Atomic Hydrogen: Polarized Atomic Beam Source.  

National Technical Information Service (NTIS)

We have carried out experiments with stable atomic hydrogen with a view to possible applications in polarized targets or polarized atomic beam sources. Recent results from the stabilization apparatus are described. The first stable atomic hydrogen beam so...

T. O. Niinikoski S. Penttilae J. M. Rieubland A. Rijllart

1985-01-01

265

Uses of stable isotopes in fish ecology  

EPA Science Inventory

Analyses of fish tissues (other than otoliths) for stable isotope ratios can provide substantial information on fish ecology, including physiological ecology. Stable isotopes of nitrogen and carbon frequently are used to determine the mix of diet sources for consumers. Stable i...

266

Temperature and humidity control in livestock stables  

Microsoft Academic Search

The paper describes temperature and humidity control of a livestock stable. It is important to have a correct air flow pattern in the livestock stable in order to achieve proper temperature and humidity control as well as to avoid draught. In the investigated livestock stable the air flow is controlled using wall mounted ventilation flaps. In the paper an algorithm

Michael Hansen; Palle Andersen; Kirsten Moelgaard Nielsen; Tom Soendergaard Pedersen; Martin Riisgaard-Jensen

2010-01-01

267

A Method for Simulating Stable Random Variables  

Microsoft Academic Search

A new algorithm is presented for simulating stable random variables on a digital computer for arbitrary characteristic exponent ?(0 < ? ? 2) and skewness parameter ?(-1 ? ? ? 1). The algorithm involves a nonlinear transformation of two independent uniform random variables into one stable random variable. This stable random variable is a continuous function of each of the

J. M. Chambers; C. L. Mallows; B. W. Stuck

1976-01-01

268

Stable massive particles at colliders  

SciTech Connect

We review the theoretical motivations and experimental status of searches for stable massive particles (SMPs) which could be sufficiently long-lived as to be directly detected at collider experiments. The discovery of such particles would address a number of important questions in modern physics including the origin and composition of dark matter in the universe and the unification of the fundamental forces. This review describes the techniques used in SMP-searches at collider experiments and the limits so far obtained on the production of SMPs which possess various colour, electric and magnetic charge quantum numbers. We also describe theoretical scenarios which predict SMPs, the phenomenology needed to model their production at colliders and interactions with matter. In addition, the interplay between collider searches and open questions in cosmology such as dark matter composition are addressed.

Fairbairn, M.; /Stockholm U.; Kraan, A.C.; /Pennsylvania U.; Milstead, D.A.; /Stockholm U.; Sjostrand, T.; /Lund U.; Skands, P.; /Fermilab; Sloan, T.; /Lancaster U.

2006-11-01

269

Stable States of Biological Organisms  

NASA Astrophysics Data System (ADS)

A novel model of biological organisms is advanced, treating an organism as a self-consistent system subject to a pathogen flux. The principal novelty of the model is that it describes not some parts, but a biological organism as a whole. The organism is modeled by a five-dimensional dynamical system. The organism homeostasis is described by the evolution equations for five interacting components: healthy cells, ill cells, innate immune cells, specific immune cells, and pathogens. The stability analysis demonstrates that, in a wide domain of the parameter space, the system exhibits robust structural stability. There always exist four stable stationary solutions characterizing four qualitatively differing states of the organism: alive state, boundary state, critical state, and dead state.

Yukalov, V. I.; Sornette, D.; Yukalova, E. P.; Henry, J.-Y.; Cobb, J. P.

2009-04-01

270

Dimensionally stable metallic hydride composition  

DOEpatents

A stable, metallic hydride composition and a process for making such a composition. The composition comprises a uniformly blended mixture of a metal hydride, kieselguhr, and a ballast metal, all in the form of particles. The composition is made by subjecting a metal hydride to one or more hydrogen absorption/desorption cycles to disintegrate the hydride particles to less than approximately 100 microns in size. The particles are partly oxidized, then blended with the ballast metal and the kieselguhr to form a uniform mixture. The mixture is compressed into pellets and calcined. Preferably, the mixture includes approximately 10 vol. % or more kieselguhr and approximately 50 vol. % or more ballast. Metal hydrides that can be used in the composition include Zr, Ti, V, Nb, Pd, as well as binary, tertiary, and more complex alloys of La, Al, Cu, Ti, Co, Ni, Fe, Zr, Mg, Ca, Mn, and mixtures and other combinations thereof. Ballast metals include Al, Cu and Ni.

Heung, Leung K. (Aiken, SC)

1994-01-01

271

Bis-N-Heterocyclic Carbene Palladium(IV) Tetrachloride Complexes: Synthesis, Reactivity and Mechanisms of Direct Chlorinations and Oxidations of Organic Substrates  

PubMed Central

This paper describes the preparation and isolation of novel octahedral CH2-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPdIVCl4 [L = (NHC)CH2(NHC)] from LPdIICl2 and Cl2. In intermolecular, non-chelation controlled transformations LPdIVCl4 reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic CH-bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE PdIV-complex. Positive solvent effects as well as kinetic measurements probing the impact of cyclohexene- and chloride concentrations on the rate of alkene chlorination, support a PdIV-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl+-transfer from the pentacoordinated PdIV-intermediate LPdIVCl3+ to olefins. 1-hexene/3-hexene competition experiments rule out both the formation of ?-complexes along the reaction coordinate as well as in situ generated Cl2 from a reductive elimination process. Instead, a ligand-mediated direct Cl+-transfer from LPdIVCl3+ to the ?-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPdIVCl3+. The presence of a large excess of added Cl? slows down cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both PdIV-Cl ionization and Cl+-transfer from LPdIVCl3+. 1H NMR titrations, T1 relaxation time measurements, binding isotherms and Job plot analysis point to the formation of a trifurcated Cl?…H-C bond in the NHC-ligand periphery as a supramolecular cause for the accelerated chemical events involving the metal center.

McCall, A. Scott; Wang, Hongwang; Desper, John M.; Kraft, Stefan

2014-01-01

272

Synthesis and Characterization of Amphiphilic Cyclic Diblock Copolypeptoids from N-Heterocyclic Carbene-Mediated Zwitterionic Polymerization of N-Substituted N-carboxyanhydride  

PubMed Central

N-Heterocyclic carbene (NHC)-mediated ring-opening polymerization of N-decylN-carboxylanhydride monomer (De-NCA) has been shown to occur in a controlled manner, yielding cyclic poly(N-decyl-glycine)s (c-PNDGs) with polymer molecular weights (MW) between 4.8 and 31 kg·mol?1 and narrow molecular weight distributions (PDI < 1.15). The reaction exhibits pseudo-first order kinetics with respect to monomer concentration. The polymer MW increases linearly with conversion, consistent with a living polymerization. ESI MS and SEC analysesconfirm the cyclic architectures of the forming polymers. DSC and WAXS studies reveal that the c-PNDG homopolymers are highly crystalline with two prominent first order transitions at 72–79°C (Tm,1) and 166–177°C (Tm,2), which have been attributed to the side chain and main chain melting respectively. A series of amphiphilic cyclic diblock copolypeptoids [i.e.,poly(N-methyl-glycine)-b-poly(N-decyl-glycine) (c-PNMG-b-PNDG)] with variable molecular weight and composition was synthesized by sequential NHC-mediated polymerization of the corresponding N-methyl N-carboxyanhydride (Me-NCA) and De-NCA monomers. 1H NMR analysis reveals that adjusting the initial monomer to NHC molar ratio can readily control the block copolymer chain length and composition. Time-lapsed light scattering and cryogenic transmission electron microscopy (cryo-TEM) analysis of c-PNDG-b-PNMG samples revealed that the amphiphilic cyclic block copolypeptoids self-assemble into spherical micelles that reorganize into micron-long cylindrical micelles with uniform diameter in room temperature methanol over the course of several days. An identical morphological transition has also been noted for the linear analogs, which occurs more rapidly than for the cyclic copolypeptoids. We tentatively attribute this difference to the different crystallization kinetics of the solvophobic block (i.e., PNDG) in the cyclic and linear block copolypeptoids.

Lee, Chang-Uk; Smart, Thomas P.; Guo, Li; Epps, Thomas H.; Zhang, Donghui

2011-01-01

273

Palladium complexes of abnormal N-heterocyclic carbenes as precatalysts for the much preferred Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium.  

PubMed

A series of new PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed precatalysts of abnormal N-heterocyclic carbenes for the highly desirable Cu-free and amine-free Sonogashira coupling in air in a mixed-aqueous medium is reported. Specifically, the PEPPSI themed (NHC)PdI2(pyridine) type precatalysts, 1b-4b, efficiently carried out the highly convenient Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes in air in a mixed aqueous medium. Complexes, 1b-4b, were synthesized by the direct reaction of the corresponding imidazo[1,2-a]pyridinium iodide salts, 1a-4a, with PdCl2 in pyridine in the presence of K2CO3 as a base while the imidazo[1,2-a]pyridinium iodide salts, 1a-4a, were in turn synthesized by the alkylation reactions of the respective imidazo[1,2-a]pyridine derivatives with alkyl iodides. The density functional theory (DFT) studies revealed that these imidazol-3-ylidene[1,2-a]pyridine derived abnormal carbenes are strongly sigma-donating and consequently significantly weaken the catalytically important labile trans pyridine ligand in 1b-4b. PMID:20023883

John, Alex; Shaikh, Mobin M; Ghosh, Prasenjit

2009-12-21

274

Stable grasp under external influences  

NASA Astrophysics Data System (ADS)

In object manipulation or material handling, the grasped object becomes unstable when external impacts exist. However, previous studies on grasping analysis seldom address such problems as the object is influenced by the external impacts. In this paper, we investigate the grasping stability and optimality issues under the influence of external disturbances. A rotation-displacement geometry model is used in analyzing the changes of equilibrium grasping forces under the disturbances. The changes of the grasping forces can be computed systematically through the invariant grasping configuration base constructed on this geometry model. Based on these results, we introduce the concept of perturbation closure, which plays a central role in our analysis. A method for finding the minimum finger grasping forces required for impact resist grasps is developed using the perturbation closure. A grasp so determined is guaranteed to be stable if the external impacts do not exceed the given threshold. A quantitative measurement is developed that can be used to evaluate the performance of different grasping configurations in resisting any kind of external impacts. Using this performance measurement, we can distinguish a good grasping configuration from a set of feasible grasping configurations of the given object. This actually gives a solution in finding the optimal grasping locations.

Hor, Maw-Kae; Wu, Shiaw-Chian

1997-09-01

275

Stable, intelligible ultrasonic strain imaging  

PubMed Central

Background Freehand quasistatic strain imaging can reveal qualitative information about tissue stiffness with good spatial accuracy. Clinical trials, however, repeatedly cite instability and variable signal-to-noise ratio as significant drawbacks. Methods This study investigates three post-processing strategies for quasistatic strain imaging. Normalisation divides the strain by an estimate of the stress field, the intention being to reduce sensitivity to variable applied stress. Persistence aims to improve the signal-to-noise ratio by time-averaging multiple frames. The persistence scheme presented in this article operates at the pixel level, weighting each frame’s contribution by an estimate of the strain precision. Precision-based display presents the clinician with an image in which regions of indeterminate strain are obscured behind a colour wash. This is achieved using estimates of strain precision that are faithfully propagated through the various stages of signal processing. Results and discussion The post-processing strategy is evaluated qualitatively on scans of a breast biopsy phantom and in vivo head and neck examinations. Strain images processed in this manner are observed to benefit from improved stability and signal-to-noise ratio. There are, however, limitations. In unusual though conceivable circumstances, the normalisation procedure might suppress genuine stiffness variations evident in the unprocessed strain images. In different circumstances, the raw strain images might fail to capture significant stiffness variations, a situation that no amount of post-processing can improve. Conclusion The clinical utility of freehand quasistatic strain imaging can be improved by normalisation, precision-weighted pixel-level persistence and precision-based display. The resulting images are stable and generally exhibit a better signal-to-noise ratio than any of the original, unprocessed strain images.

Gee, Andrew; Lindop, Joel; Treece, Graham; Prager, Richard; Freeman, Susan

2010-01-01

276

Advanced thermally stable jet fuels  

SciTech Connect

The Pennsylvania State University program in advanced thermally stable coal-based jet fuels has five broad objectives: (1) Development of mechanisms of degradation and solids formation; (2) Quantitative measurement of growth of sub-micrometer and micrometer-sized particles suspended in fuels during thermal stressing; (3) Characterization of carbonaceous deposits by various instrumental and microscopic methods; (4) Elucidation of the role of additives in retarding the formation of carbonaceous solids; (5) Assessment of the potential of production of high yields of cycloalkanes by direct liquefaction of coal. Future high-Mach aircraft will place severe thermal demands on jet fuels, requiring the development of novel, hybrid fuel mixtures capable of withstanding temperatures in the range of 400--500 C. In the new aircraft, jet fuel will serve as both an energy source and a heat sink for cooling the airframe, engine, and system components. The ultimate development of such advanced fuels requires a thorough understanding of the thermal decomposition behavior of jet fuels under supercritical conditions. Considering that jet fuels consist of hundreds of compounds, this task must begin with a study of the thermal degradation behavior of select model compounds under supercritical conditions. The research performed by The Pennsylvania State University was focused on five major tasks that reflect the objectives stated above: Task 1: Investigation of the Quantitative Degradation of Fuels; Task 2: Investigation of Incipient Deposition; Task 3: Characterization of Solid Gums, Sediments, and Carbonaceous Deposits; Task 4: Coal-Based Fuel Stabilization Studies; and Task 5: Exploratory Studies on the Direct Conversion of Coal to High Quality Jet Fuels. The major findings of each of these tasks are presented in this executive summary. A description of the sub-tasks performed under each of these tasks and the findings of those studies are provided in the remainder of this volume (Sections 1 through 5).

Schobert, H.H.

1999-01-31

277

Stable Isotopes and M&Ms  

NSDL National Science Digital Library

Students are often confused by the concepts of stable isotope signatures and conventional notation. This activity is designed to introduce students to the del notation (isotopic signature), fractionation, and end member mixing of stable isotopes using familiar objects disguised as isotopes. The activity does not explain the physical manifestation of why isotopic fractionation occurs, but does explain the concepts that one easily observes when interpreting stable isotope data sets.

De Jesus, Roman

278

Parameter Estimates for Symmetric Stable Distributions  

Microsoft Academic Search

Building on results of an earlier article [6], estimators are suggested for the scale parameter and characteristic exponent of symmetric stable distributions, and Monte Carlo studies of these estimators are reported. The powers of various goodness-of-fit tests of a Gaussian null hypothesis against non-Gaussian stable alternatives are also investigated. Finally, a test of the stability property of symmetric stable variables

Eugene F. Fama; Richard Roll

1971-01-01

279

Complexes of an anionic gallium(I) N-heterocyclic carbene analogue with group 14 element(II) fragments: synthetic, structural and theoretical studies.  

PubMed

The reactions of the anionic gallium(I) N-heterocyclic carbene (NHC) analogue, [K(tmeda)][:Ga{[N(Ar)C(H)]2}], Ar = C6H3Pri2-2,6, with the heavier group 14 alkene analogues, R2E=ER2, E = Ge or Sn, R = -CH(SiMe3)2, have been carried out. In 2:1 stoichiometries, these lead to the ionic [K(tmeda)][R2EGa{[N(Ar)C(H)]2}] complexes which exhibit long E-Ga bonds. The nature of these bonds has been probed by DFT calculations, and the complexes have been compared to neutral NHC adducts of group 14 dialkyls. The 4:1 reaction of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with R2Sn=SnR2 leads to the digallyl stannate complex, [K(tmeda)][RSn[Ga{[N(Ar)C(H)]2}]2], presumably via elimination of KR. In contrast, the reaction of the gallium heterocycle with PbR2 affords the digallane4, [Ga{[N(Ar)C(H)]2}]2, via an oxidative coupling reaction. For sake of comparison, the reactions of [K(tmeda)][:Ga{[N(Ar)C(H)]2}] with Ar'2E=EAr'2, E = Ge, Sn or Pb, Ar' = C6H2Pri3-2,4,6, were carried out and led to either no reaction (E = Ge), the formation of [K(tmeda)][Ar'2SnGa{[N(Ar)C(H)]2}] (E = Sn), or the gallium(III) heterocycle, [Ar'Ga{[N(Ar)C(H)]2}] (E = Pb). Salt elimination reactions between [K(tmeda)][:Ga{[N(Ar)C(H)]2}] and the guanidinato group 14 complexes [(Giso)ECl], E = Ge or Sn, Giso = [Pri2NC{N(Ar)}2]-, gave the neutral [(Giso)EGa{[N(Ar)C(H)]2}] complexes. All complexes have been characterized by NMR spectroscopy and X-ray crystallographic studies. PMID:16933925

Green, Shaun P; Jones, Cameron; Lippert, Kai-Alexander; Mills, David P; Stasch, Andreas

2006-09-01

280

Low-spin hexacoordinate Mn(III): synthesis and spectroscopic investigation of homoleptic tris(pyrazolyl)borate and tris(carbene)borate complexes.  

PubMed

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong ?-donor with little or no ?-bonding. PMID:23259486

Forshaw, Adam P; Smith, Jeremy M; Ozarowski, Andrew; Krzystek, J; Smirnov, Dmitry; Zvyagin, S A; Harris, T David; Karunadasa, Hemamala I; Zadrozny, Joseph M; Schnegg, Alexander; Holldack, Karsten; Jackson, Timothy A; Alamiri, Ahmad; Barnes, Diane M; Telser, Joshua

2013-01-01

281

Comparative study of phosphine and N-heterocyclic carbene stabilized Group?13 adducts [L(EH3)] and [L2(E2H(n))].  

PubMed

Quantum chemical calculations using density functional theory at the BP86/TZ2P level have been carried out to determine the geometries and stabilities of Group?13 adducts [(PMe3)(EH3)] and [(PMe3)2(E2H(n))] (E = B-In; n = 4, 2, 0). The optimized geometries exhibit, in most cases, similar features to those of related adducts [(NHC(Me))(EH3)] and [(NHC(Me))2(E2H(n))] with a few exceptions that can be explained by the different donor strengths of the ligands. The calculations show that the carbene ligand L = NHC(Me) (:C(NMeCH)2) is a significantly stronger donor than L = PMe3. The equilibrium geometries of [L(EH3)] possess, in all cases, a pyramidal structure, whereas the complexes [L2(E2H4)] always have an antiperiplanar arrangement of the ligands L. The phosphine ligands in [(PMe3)2(B2H2)], which has Cs symmetry, are in the same plane as the B2H2 moiety, whereas the heavier homologues [(PMe3)2(E2H2)] (E = Al, Ga, In) have Ci symmetry in which the ligands bind side-on to the E2H2 acceptor. This is in contrast to the [(NHC(Me))2(E2H2)] adducts for which the NHC(Me) donor always binds in the same plane as E2H2 except for the indium complex [(NHC(Me))2(In2H2)], which exhibits side-on bonding. The boron complexes [L2(B2)] (L = PMe3 and NHC(Me)) possess a linear arrangement of the LBBL moiety, which has a B?B triple bond. The heavier homologues [L2(E2)] have antiperiplanar arrangements of the LEEL moieties, except for [(PMe3)2(In2)], which has a twisted structure in which the PInInP torsion angle is 123.0°. The structural features of the complexes [L(EH3)] and [L2(E2H(n))] can be explained in terms of donor-acceptor interactions between the donors L and the acceptors EH3 and E2H(n), which have been analyzed quantitatively by using the energy decomposition analysis (EDA) method. The calculations predict that the hydrogenation reaction of the dimeric magnesium(I) compound L'MgMgL' with the complexes [L(EH3)] is energetically more favorable for L = PMe3 than for NHC(Me). PMID:23512819

Holzmann, Nicole; Stasch, Andreas; Jones, Cameron; Frenking, Gernot

2013-05-10

282

Organocatalytic conjugate-addition polymerization of linear and cyclic acrylic monomers by N-heterocyclic carbenes: mechanisms of chain initiation, propagation, and termination.  

PubMed

This contribution presents a full account of experimental and theoretical/computational investigations into the mechanisms of chain initiation, propagation, and termination of the recently discovered N-heterocyclic carbene (NHC)-mediated organocatalytic conjugate-addition polymerization of acrylic monomers. The current study specifically focuses on three commonly used NHCs of vastly different nucleophilicity, 1,3-di-tert-butylimidazolin-2-ylidene (I(t)Bu), 1,3-dimesitylimidazolin-2-ylidene (IMes), and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT), and two representative acrylic monomers, the linear methyl methacrylate (MMA) and its cyclic analog, biomass-derived renewable ?-methyl-?-methylene-?-butyrolactone (MMBL). For MMA, there exhibits an exquisite selectivity of the NHC structure for the three types of reactions it promotes: enamine formation (single-monomer addition) by IMes, dimerization (tail-to-tail) by TPT, and polymerization by I(t)Bu. For MMBL, all three NHCs promote no dimerization but polymerization, with the polymerization activity being highly sensitive to the NHC structure and the solvent polarity. Thus, I(t)Bu is the most active catalyst of the series and converts quantitatively 1000-3000 equiv of MMBL in 1 min or 10,000 equiv in 5 min at room temperature to MMBL-based bioplastics with a narrow range of molecular weights of M(n) = 70-85 kg/mol, regardless of the [MMBL]/[I(t)Bu] ratio employed. The I(t)Bu-catalyzed MMBL polymerization reaches an exceptionally high turnover frequency up to 122 s(-1) and a high initiator efficiency value up to 1600%. Unique chain-termination mechanisms have been revealed, accounting for the production of relative high-molecular-weight linear polymers and the catalytic nature of this NHC-mediated conjugate-addition polymerization. Computational studies have provided mechanistic insights into reactivity and selectivity between two competing pathways for each NHC-monomer zwitterionic adduct, namely enamine formation/dimerization through proton transfer vs polymerization through conjugate addition, and mapped out extensive energy profiles for chain initiation, propagation, and termination steps, thereby satisfactorily explaining the experimental observations. PMID:24245532

Zhang, Yuetao; Schmitt, Meghan; Falivene, Laura; Caporaso, Lucia; Cavallo, Luigi; Chen, Eugene Y-X

2013-11-27

283

Is Solar System Stable? A Remark  

Microsoft Academic Search

It is not yet known (1994) whether the Solar System is stable or not. Common belief is that the Solar System is stable if and only if it is not a resonant system, i.e., whenever its orbital frequencies !i satisfy an inequality j P ni!ij • \\

Vladik Kreinovich; Andrew Bernat

1997-01-01

284

Stable Isotope Signatures for Microbial Forensics  

Microsoft Academic Search

The isotopic distribution of the atoms composing the molecules of microorganisms is a function of the substrates used by the organisms. The stable isotope content of an organism is fixed so long as no further substrate consumption and biosynthesis occurs, while the radioactive isotopic content decays over time. The distribution of stable isotopes of C, N, O and H in

Helen W

2012-01-01

285

Stable terahertz carrier generation with soliton oscillator  

Microsoft Academic Search

This paper focuses the design of an oscillator for stable terahertz carrier generation with soliton pulses. The proposed soliton oscillator generates periodic train of ultra short-width impulses using the background noise from amplifier as an input. The oscillator is made stable with a series combination of NLTL and a unique adaptive bias controlled amplifier. A new NLTL has been designed

Monjur Morshed; Kaiser Habib; Biplob Kumar Daas; M. A. Quaium; M. Akram

2008-01-01

286

A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Toward Catalyst Deactivation During Ketone Hydrogenation  

SciTech Connect

The initial reaction observed between N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO)2(PPh3)H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]+[CpM(CO)2(PPh3)]– . At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO)2(PR3)H (M = Mo, W; R = Me, Ph) produces CpM(CO)2(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph3C+B(C6F5)4- gives CpW(CO)2(IMes)+B(C6F5)4- which was crystallographically characterized using x-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H2, THF, ketones or alcohols. Reaction of CpW(CO)2(IMes)+ with H2 gives the dihydride complex [CpW(CO)2(IMes)(H)2]+. Addition of Et2CH–OH to CpW(CO)2(IMes)+B(C6F5)4- gives the alcohol complex [CpM(CO)2(IMes)(Et2CH–OH)]+[B(C6F5)4]– which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H2 to the ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– produces an equilibrium with the dihydride [CpW(CO)2(IMes)(H)2]+ (Keq = 1.1 ? 103 at 25 °C). The tungsten ketone complex [CpW(CO)2(IMes)(Et2C=O)]+[B(C6F5)4]– serves as a modest catalyst for hydrogenation of Et2C=O to Et2CH–OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]+B(C6F5)4-, indicating that these catalysts with N-heterocyclic carbenes ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Wu, Fan; Dioumaev, Vladimir K.; Szalda, David J.; Hanson, Jonathan; Bullock, R. Morris

2007-09-24

287

Synthesis of B- and P-Heterocycles by Reaction of Cyclic Acetals and Ketals with Borinium and Phosphonium Ions.  

PubMed

Tricoordinated cyclic boron cations result from gas-phase ion/molecule reactions of dicoordinated borinium ions with neutral acetals and ketals and thiazolidine. The reaction, which proceeds via initial cationic binding to a heteroatom followed by a consecutive ring-opening and ring-reclosing process, resembles the Eberlin transacetalization of acylium ions (Eberlin, M. N.; Cooks, R. G. Org. Mass Spectrom. 1993, 28, 679). The cyclic structure of the tricoordinated boron cation is demonstrated by tandem mass spectrometry and further confirmed by comparison with authentic cyclic tricoordinated boron cations. The five-membered cyclic boron cations dissociate by ethylene oxide loss to thus reform the reactant-dicoordinated borinium ion; the six-membered boron cations fragment instead by ethylene loss. Consistent with the proposed mechanism, the ion/molecule reaction efficiency falls in the order CH(3)OB(+)C(2)H(5) > CH(3)OB(+)OCH(3) > CH(3)B(+)CH(3); i.e., the higher the nucleophilicity of the borinium ion, the higher the reaction efficiency. A potential energy surface is calculated for the reaction of CH(3)OB(+)OCH(3) with 2-methyl-1,3-dioxolane, and the reaction is found to be 43.3 kcal/mol exothermic due to initial formation of a strong B-O bond. The analogous reactivity displayed by phosphonium ions is also investigated by both experiment and ab initio calculations. In contrast to the borinium ions, the phosphonium ions exhibit higher regioselectivity for sulfur compared to nitrogen and oxygen. Finally, the present findings indicate that the reaction exothermicity and the regioselectivity are controlled by both the Lewis acidity of the reactant cations and the leaving ability of the released neutrals in the rate-limiting nucleophilic-induced recyclization step. PMID:11674423

Wang, Feng; Tao, W. Andy; Gozzo, Fábio C.; Eberlin, Marcos N.; Cooks, R. Graham

1999-04-30

288

Generating Mammalian Stable Cell Lines by Electroporation  

PubMed Central

Expression of functional, recombinant mammalian proteins often requires expression in mammalian cells (see Single Cell Cloning of a Stable Mammalian Cell Line). If the expressed protein needs to be made frequently, it can be best to generate a stable cell line instead of performing repeated transient transfections into mammalian cells. Here, we describe a method to generate stable cell lines via electroporation followed by selection steps. This protocol will be limited to the CHO dhfr– Urlaub et al. (1983) and LEC1 cell lines, which in our experience perform the best with this method.

Longo, Patti A.; Kavran, Jennifer M.; Kim, Min-Sung; Leahy, Daniel J.

2014-01-01

289

Bi-stable optical element actuator device  

DOEpatents

The present invention is a bistable optical element actuator device utilizing a powered means to move an actuation arm, to which an optical element is attached, between two stable positions. A non-powered means holds the actuation arm in either of the two stable positions. The optical element may be a electromagnetic (EM) radiation or particle source, an instrument, or EM radiation or particle transmissive reflective or absorptive elements. A bearing is used to transfer motion and smoothly transition the actuation arm between the two stable positions.

Holdener, Fred R. (Tracy, CA); Boyd, Robert D. (Livermore, CA)

2002-01-01

290

Magnetic properties of 1 : 4 complexes of CoCl2 and pyridines carrying carbenes (S(0) = 4/2, 6/2, and 8/2) in diluted frozen solution; influence of carbene multiplicity on heterospin single-molecule magnets.  

PubMed

The microcrystalline sample of a parent complex, [CoCl(2)(py)(4)], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, U(eff)/k(B), of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2-4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1 : 4 complexes, CoCl(2)(DYpy)(4); Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, S(total) = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, H(c), at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give U(eff)/k(B) values of 94, 92, 93, and 87 K for CoCl(2)(CYpy)(4); Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl(2)(CYpy)(4); Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5-1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl(2)(CYpy)(4) including [CoCl(2)(py)(4)] and CoCl(2)(C1py)(4) suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, ?(q), due to the spin quantum tunneling magnetization, which became larger with increasing S(total) of the complex. PMID:22898723

Karasawa, Satoru; Nakano, Kimihiro; Tanokashira, Jun-ichi; Yamamoto, Noriko; Yoshizaki, Takahito; Koga, Noboru

2012-11-28

291

Polymeric foams stable at high temperatures  

NASA Technical Reports Server (NTRS)

Crosslinked poly(N-arylenebenzimidazoles) are stable up to 370 C. Polymers are made by mixing appropriate stoichiometric amounts of tetramine and aromatic dicarboxylic acid anhydride with phenol or alkyl-substituted phenol.

Riccitiello, S. R.; Harrison, E. S.; Delano, C. B.

1976-01-01

292

Synthesis of Stable Force-Closure Grasps.  

National Technical Information Service (NTIS)

The thesis addresses the problem of synthesizing grasps that are force-closure and stable. The synthesis of force closure grasps constructs independent regions of contact for the fingertips, such that the motion of the grasped object is totally constraine...

V. D. Nguyen

1986-01-01

293

Synthesis of Stable Grasps in the Plane.  

National Technical Information Service (NTIS)

This paper addresses the problem of synthesizing stable grasps on arbitrary planar polygons. Each finger is a virtual spring whose stiffness and compression can be programmed. The contacts between the finger tips and the object are point contacts without ...

V. D. Nguyen

1985-01-01

294

Computing Stable Outcomes in Hedonic Games  

NASA Astrophysics Data System (ADS)

We study the computational complexity of finding stable outcomes in symmetric additively-separable hedonic games. These coalition formation games are specified by an undirected edge-weighted graph: nodes are players, an outcome of the game is a partition of the nodes into coalitions, and the utility of a node is the sum of incident edge weights in the same coalition. We consider several natural stability requirements defined in the economics literature. For all of them the existence of a stable outcome is guaranteed by a potential function argument, so local improvements will converge to a stable outcome and all these problems are in PLS. The different stability requirements correspond to different local search neighbourhoods. For different neighbourhood structures, our findings comprise positive results in the form of polynomial-time algorithms for finding stable outcomes, and negative (PLS-completeness) results.

Gairing, Martin; Savani, Rahul

295

Stable Isotope Signatures for Microbial Forensics  

SciTech Connect

The isotopic distribution of the atoms composing the molecules of microorganisms is a function of the substrates used by the organisms. The stable isotope content of an organism is fixed so long as no further substrate consumption and biosynthesis occurs, while the radioactive isotopic content decays over time. The distribution of stable isotopes of C, N, O and H in heterotrophic microorganisms is a direct function of the culture medium, and therefore the stable isotope composition can be used to associate samples with potential culture media and also with one another. The 14C content depends upon the 14C content, and therefore the age, of the organic components of the culture medium, as well as on the age of the culture itself. Stable isotope signatures can thus be used for sample matching, to associate cultures with specific growth media, and to predict characteristics of growth media.

Kreuzer, Helen W.

2012-01-03

296

Evolutionary Origin of Asymptotically Stable Consensus  

NASA Astrophysics Data System (ADS)

Consensus is widely observed in nature as well as in society. Up to now, many works have focused on what kind of (and how) isolated single structures lead to consensus, while the dynamics of consensus in interdependent populations remains unclear, although interactive structures are everywhere. For such consensus in interdependent populations, we refer that the fraction of population adopting a specified strategy is the same across different interactive structures. A two-strategy game as a conflict is adopted to explore how natural selection affects the consensus in such interdependent populations. It is shown that when selection is absent, all the consensus states are stable, but none are evolutionarily stable. In other words, the final consensus state can go back and forth from one to another. When selection is present, there is only a small number of stable consensus state which are evolutionarily stable. Our study highlights the importance of evolution on stabilizing consensus in interdependent populations.

Tang, Chang-Bing; Wu, Bin; Wang, Jian-Bo; Li, Xiang

2014-04-01

297

Stable Isotope Inventory Requirements and Enrichment Capabilities.  

National Technical Information Service (NTIS)

The electromagnetic isotope enrichment program established in 1945 has since then continued to provide enriched stable, actinide, and selected radioactive isotopes. These unique materials used in research and medicine and for industrial applications are m...

W. A. Bell J. G. Tracy

1985-01-01

298

Evolutionary Origin of Asymptotically Stable Consensus  

PubMed Central

Consensus is widely observed in nature as well as in society. Up to now, many works have focused on what kind of (and how) isolated single structures lead to consensus, while the dynamics of consensus in interdependent populations remains unclear, although interactive structures are everywhere. For such consensus in interdependent populations, we refer that the fraction of population adopting a specified strategy is the same across different interactive structures. A two-strategy game as a conflict is adopted to explore how natural selection affects the consensus in such interdependent populations. It is shown that when selection is absent, all the consensus states are stable, but none are evolutionarily stable. In other words, the final consensus state can go back and forth from one to another. When selection is present, there is only a small number of stable consensus state which are evolutionarily stable. Our study highlights the importance of evolution on stabilizing consensus in interdependent populations.

Tang, Chang-Bing; Wu, Bin; Wang, Jian-Bo; Li, Xiang

2014-01-01

299

Unconditionally Stable Microwave Si-IMPATT Amplifiers.  

National Technical Information Service (NTIS)

The purpose of this investigation has been the development of an improved understanding of the design and analysis of microwave reflection amplifiers employing the negative resistance property of the IMPATT devices. Unconditionally stable amplifier circui...

M. M. Seddik

1986-01-01

300

Stable Isotope Group 1981 Progress Report.  

National Technical Information Service (NTIS)

Geothermal studies were reported on the Tauhara, Wairakei, Orakeikorako and Mokai areas. We assisted the two N.Z. geothermal consultant companies with isotope analyses and the Geothermal Institute with lectures on stable isotope aspects of the diploma cou...

M. K. Stewart

1982-01-01

301

Stable vector bundles and string theory  

SciTech Connect

In [4], Braun, He, Ovrut and Pantev proposed a model of string theory (based on the Calabi-Yau 3-fold X) whose low energy limit predicts certain properties of the Standard Model of particle Physics. This model depends on two vector bundles that have to be stable. We calculate the ample cone of X, and prove that one of them is stable, and the other one is not.

Gomez, Tomas L.; Sols, Ignacio [ICMAT (CSIC-UAM-UC3M-UCM), Serrano 113bis, 28006 Madrid (Spain); Facultad de Ciencias Matematicas, Universidad Complutense de Madrid, 28040 Madrid (Spain); Lukic, Sergio [Mathematics Department, Imperial College, London SW7 (United Kingdom)

2009-05-06

302

Stieltjes Functions and Hurwitz Stable Entire Functions  

Microsoft Academic Search

The concept of stability, originally introduced for polynomials, will be extended to apply to the class of entire functions.\\u000a This generalization will be called Hurwitz stablility and the class of Hurwitz stable functions will serve as the main focus\\u000a of this paper. A first theorem will show how, given a function of either of the Stieltjes classes, a Hurwitz stable

Victor Katsnelson

2011-01-01

303

Stable isotopes and biomarkers in microbial ecology  

Microsoft Academic Search

The use of biomarkers in combination with stable isotope analysis is a new approach in microbial ecology and a number of papers on a variety of subjects have appeared. We will first discuss the techniques for analysing stable isotopes in biomarkers, primarily gas chromatography-combustion-isotope ratio mass spectrometry, and then describe a number of applications in microbial ecology based on degreesC.

H. T. S. Boschker; J. J. Middelburg

2002-01-01

304

Low work function, stable thin films  

DOEpatents

Generation of low work function, stable compound thin films by laser ablation. Compound thin films with low work function can be synthesized by simultaneously laser ablating silicon, for example, and thermal evaporating an alkali metal into an oxygen environment. For example, the compound thin film may be composed of Si/Cs/O. The work functions of the thin films can be varied by changing the silicon/alkali metal/oxygen ratio. Low work functions of the compound thin films deposited on silicon substrates were confirmed by ultraviolet photoelectron spectroscopy (UPS). The compound thin films are stable up to 500.degree. C. as measured by x-ray photoelectron spectroscopy (XPS). Tests have established that for certain chemical compositions and annealing temperatures of the compound thin films, negative electron affinity (NEA) was detected. The low work function, stable compound thin films can be utilized in solar cells, field emission flat panel displays, electron guns, and cold cathode electron guns.

Dinh, Long N. (Concord, CA); McLean, II, William (Oakland, CA); Balooch, Mehdi (Berkeley, CA); Fehring, Jr., Edward J. (Dublin, CA); Schildbach, Marcus A. (Livermore, CA)

2000-01-01

305

Stable Isotope Enrichment Capabilities at ORNL  

SciTech Connect

The Oak Ridge National Laboratory (ORNL) and the US Department of Energy Nuclear Physics Program have built a high-resolution Electromagnetic Isotope Separator (EMIS) as a prototype for reestablishing a US based enrichment capability for stable isotopes. ORNL has over 60 years of experience providing enriched stable isotopes and related technical services to the international accelerator target community, as well as medical, research, industrial, national security, and other communities. ORNL is investigating the combined use of electromagnetic and gas centrifuge isotope separation technologies to provide research quantities (milligram to several kilograms) of enriched stable isotopes. In preparation for implementing a larger scale production facility, a 10 mA high-resolution EMIS prototype has been built and tested. Initial testing of the device has simultaneously collected greater than 98% enriched samples of all the molybdenum isotopes from natural abundance feedstock.

Egle, Brian [ORNL; Aaron, W Scott [ORNL; Hart, Kevin J [ORNL

2013-01-01

306

High-Order Energy Stable WENO Schemes  

NASA Technical Reports Server (NTRS)

A third-order Energy Stable Weighted Essentially Non-Oscillatory (ESWENO) finite difference scheme developed by Yamaleev and Carpenter was proven to be stable in the energy norm for both continuous and discontinuous solutions of systems of linear hyperbolic equations. Herein, a systematic approach is presented that enables 'energy stable' modifications for existing WENO schemes of any order. The technique is demonstrated by developing a one-parameter family of fifth-order upwind-biased ESWENO schemes; ESWENO schemes up to eighth order are presented in the appendix. New weight functions are also developed that provide (1) formal consistency, (2) much faster convergence for smooth solutions with an arbitrary number of vanishing derivatives, and (3) improved resolution near strong discontinuities.

Yamaleev, Nail K.; Carpenter, Mark H.

2009-01-01

307

High-Order Energy Stable WENO Schemes  

NASA Technical Reports Server (NTRS)

A new third-order Energy Stable Weighted Essentially NonOscillatory (ESWENO) finite difference scheme for scalar and vector linear hyperbolic equations with piecewise continuous initial conditions is developed. The new scheme is proven to be stable in the energy norm for both continuous and discontinuous solutions. In contrast to the existing high-resolution shock-capturing schemes, no assumption that the reconstruction should be total variation bounded (TVB) is explicitly required to prove stability of the new scheme. A rigorous truncation error analysis is presented showing that the accuracy of the 3rd-order ESWENO scheme is drastically improved if the tuning parameters of the weight functions satisfy certain criteria. Numerical results show that the new ESWENO scheme is stable and significantly outperforms the conventional third-order WENO finite difference scheme of Jiang and Shu in terms of accuracy, while providing essentially nonoscillatory solutions near strong discontinuities.

Yamaleev, Nail K.; Carpenter, Mark H.

2008-01-01

308

Stable angina pectoris: current medical treatment.  

PubMed

Stable angina represents the main symptom of established coronary artery disease. In addition atherosclerosis is the common pathological substrate of chronic stable angina as well as acute coronary syndromes. The aim of stable angina management is the symptomatic relief and the secondary prevention. Lifestyle modification and pharmacological therapy are the cornerstones of chronic coronary artery disease management irrespectively of possible surgical or percutaneous revascularization. Optimal medical therapy is a combination of antianginal/antiischemic drugs and disease modifying agents, including nitrates, beta-blockers, calcium channel blockers, antiplatelets, statins and angiotensin converting enzyme inhibitors. Novel classes of treatment with different mechanisms of action have been developed in the last years, including nicorandil, ivabradine, trimetazidine and ranolazine. These drugs, which are currently approved as second-line treatments, have dynamically entered the clinical practice and their long-term effects are still under investigation. PMID:23016717

Siama, Katerina; Tousoulis, Dimitris; Papageorgiou, Nikolaos; Siasos, Gerasimos; Tsiamis, Eleftherios; Bakogiannis, Constantinos; Briasoulis, Alexandros; Androulakis, Emmanuel; Tentolouris, Kostas; Stefanadis, Christodoulos

2013-01-01

309

Stable monolithic interferometer for wavelenghth calibration  

US Patent & Trademark Office Database

Calibration of an arbitrary spectrometer can use a stable monolithic interferometer as a wavelength calibration standard. Light from a polychromatic light source is input to the monolithic interferometer where it undergoes interference based on the optical path difference (OPD) of the interferometer. The resulting wavelength-modulated output beam is analyzed by a reference spectrometer to generate reference data. The output beam from the interferometer can be provided to an arbitrary spectral instrument. Wavelength calibration of the arbitrary spectral instrument may then be performed based on a comparison of the spectral instrument output with the reference data. By appropriate choice of materials for the monolithic interferometer, a highly stable structure can be fabricated that has a wide field and/or is thermally compensated. Because the interferometer is stable, the one-time generated reference data can be used over an extended period of time without re-characterization.

2013-10-29

310

MHD stable regime of the tokamak  

SciTech Connect

A broad family of tokamak current profiles is found to be stable against ideal and resistive MHD kink modes for 1 less than or equal to q(0), with q(a) as low 2. For 0.5 less than or equal to q(0) < and q(a) > 1, current profiles can be found that are unstable only to the m = 1, n = 1 mode. A specific ''optimal'' tokamak profile can be selected from the range of stable solutions, by imposing a common upper limit on dj/dr - corresponding in ohmic equilibrium to a limitation of dT/sub e//dr by anomalous transport.

Cheng, C.Z.; Furth, H.P.; Boozer, A.H.

1986-10-01

311

Linear arrays of stable atmospheric pressure microplasmas  

SciTech Connect

Microdischarges produce cold atmospheric plasma when the discharge current is limited by the quenching of a microwave resonator. A quarter-wavelength microstripline resonator is shown to support stable atmospheric microplasma in pure argon. Electrical characterization of the microplasma shows that its impedance is resistive and capacitive (Z{sub p}=500-j900 {omega}). An array of these linear resonators generates a stable, line-shaped microplasma operating from a single power source due to close-coupling among adjacent resonators. Both simulations and experiments confirm that coupled-mode theory describes the collective behavior of linear microplasma arrays.

Zhang Zhibo; Hopwood, Jeffrey [Tufts University, Medford, Massachusetts 02155 (United States)

2009-10-19

312

Linear arrays of stable atmospheric pressure microplasmas  

NASA Astrophysics Data System (ADS)

Microdischarges produce cold atmospheric plasma when the discharge current is limited by the quenching of a microwave resonator. A quarter-wavelength microstripline resonator is shown to support stable atmospheric microplasma in pure argon. Electrical characterization of the microplasma shows that its impedance is resistive and capacitive (Zp=500-j900 ?). An array of these linear resonators generates a stable, line-shaped microplasma operating from a single power source due to close-coupling among adjacent resonators. Both simulations and experiments confirm that coupled-mode theory describes the collective behavior of linear microplasma arrays.

Zhang, Zhi-Bo; Hopwood, Jeffrey

2009-10-01

313

Transversely stable soliton trains in photonic lattices  

SciTech Connect

We report the existence of transversely stable soliton trains in optics. These stable soliton trains are found in two-dimensional square photonic lattices when they bifurcate from X-symmetry points with saddle-shaped diffraction inside the first Bloch band and their amplitudes are above a certain threshold. We also show that soliton trains with low amplitudes or bifurcated from edges of the first Bloch band ({Gamma} and M points) still suffer transverse instability. These results are obtained in the continuous lattice model and are further corroborated by the discrete model.

Yang Jianke [Department of Mathematics and Statistics, University of Vermont, Burlington, Vermont 05401 (United States)

2011-09-15

314

Stable Spatiotemporal Solitons in Bessel Optical Lattices  

NASA Astrophysics Data System (ADS)

We investigate the existence and stability of three-dimensional solitons supported by cylindrical Bessel lattices in self-focusing media. If the lattice strength exceeds a threshold value, we show numerically, and using the variational approximation, that the solitons are stable within one or two intervals of values of their norm. In the latter case, the Hamiltonian versus norm diagram has a swallowtail shape with three cuspidal points. The model applies to Bose-Einstein condensates and to optical media with saturable nonlinearity, suggesting new ways of making stable three-dimensional solitons and “light bullets” of an arbitrary size.

Mihalache, D.; Mazilu, D.; Lederer, F.; Malomed, B. A.; Kartashov, Y. V.; Crasovan, L.-C.; Torner, L.

2005-07-01

315

Stable spatiotemporal solitons in bessel optical lattices.  

PubMed

We investigate the existence and stability of three-dimensional solitons supported by cylindrical Bessel lattices in self-focusing media. If the lattice strength exceeds a threshold value, we show numerically, and using the variational approximation, that the solitons are stable within one or two intervals of values of their norm. In the latter case, the Hamiltonian versus norm diagram has a swallowtail shape with three cuspidal points. The model applies to Bose-Einstein condensates and to optical media with saturable nonlinearity, suggesting new ways of making stable three-dimensional solitons and "light bullets" of an arbitrary size. PMID:16090683

Mihalache, D; Mazilu, D; Lederer, F; Malomed, B A; Kartashov, Y V; Crasovan, L-C; Torner, L

2005-07-01

316

Nonlinearly stable compact schemes for shock calculations  

NASA Technical Reports Server (NTRS)

The applications of high-order, compact finite difference methods in shock calculations are discussed. The main concern is to define a local mean which will serve as a reference for introducing a local nonlinear limiting to control spurious numerical oscillations while maintaining the formal accuracy of the scheme. For scalar conservation laws, the resulting schemes can be proven total-variation stable in one space dimension and maximum-norm stable in multiple space dimensions. Numerical examples are shown to verify accuracy and stability of such schemes for problems containing shocks. These ideas can also be applied to other implicit schemes such as the continuous Galerkin finite element methods.

Cockburn, Bernardo; Shu, Chi-Wang

1992-01-01

317

Are two modes of thermohaline circulation stable?  

Microsoft Academic Search

The standard version of the coupled ocean atmosphere model developed at the Geophysical Fluid Dynamics Laboratory (GFDL) of NOAA has at least two stable equilibria. One has a realistic and active thermohaline circulation (THC) with sinking regions in the northern North Atlantic Ocean. The other has a reverse THC with extremely weak upwelling in the North Atlantic and sinking in

Syukuro Manabe; Ronald J. Stouffer

1999-01-01

318

STATCOM helps to guarantee a stable system  

Microsoft Academic Search

Transmission system operators are governed by operational security standards that are applied in real time. During system disturbances, the system operators must rely on the installed protection and control equipment, prior to human intervention. New power electronic solutions bring rapid and repeatable responses to disturbances, which will help system operators to guarantee a stable system. Last year, ALSTOM completed the

M. H. Baker; B. D. Gemmell; C. Horwill; D. J. Hanson

2001-01-01

319

Stable Ammonium Bisulfate Clusters in the Atmosphere  

Microsoft Academic Search

Liquid drop model based equilibrium thermodynamics predicts that in the presence of even small ammonia concentrations practically all the atmospheric sulfuric acid molecules are bound to tiny, stable ammonium-bisulfate clusters. Hitherto sulfuric acid has been believed to form hydrates with water. Thermodynamic theory predicts correctly the hydrate formation observed experimentally. Results from ab initio computer simulations contradict the thermodynamic results

Hanna Vehkamäki; Ismo Napari; Markku Kulmala; Madis Noppel

2004-01-01

320

Stable Crack Growth in Aluminum Tensile Specimens.  

National Technical Information Service (NTIS)

Post's white light moire interferometry was used to obtain sequential records of the transient u sub y-displacement fields associated with stable crack growth in 7075-T6 and 2024-O, single edge notched (SEN) specimens with fatigued cracks. The u sub y-dis...

B. S. Kang A. S. Kobayashi D. Post

1986-01-01

321

Hiring Procedures to Implement Stable Allocations  

Microsoft Academic Search

We implement the stable correspondence of a job matching market in Subgame Perfect Equilibrium. We use a simple sequential mechanism in which firms propose a salary to each worker (first stage) and, then, each worker accepts at most one proposal (second stage). Moreover, if agents' preferences are additive, this mechanism implements in Subgame Perfect Equilibrium the firms' optimal corre- spondence

José Alcalde; David Pérez-Castrillo; Antonio Romero-Medina

1998-01-01

322

Regression Estimation from an Individual Stable Sequence  

Microsoft Academic Search

We consider univariate regression estimation from an individual (non-random) sequence , which is stable in the sense that for each interval (i) the limiting relative frequency of A under x1, x2,… is governed by an unknown probability distribution ?, and (ii) the limiting average of those yi with x ? A is governed by an unknown regression function m(·).A computationally

Gusztáv Morvai; Sanjeev R. Kulkarni; Andrew B. Nobel

1999-01-01

323

UNCERTAINTY IN SOURCE PARTITIONING USING STABLE ISOTOPES  

EPA Science Inventory

Stable isotope analyses are often used to quantify the contribution of multiple sources to a mixture, such as proportions of food sources in an animal's diet, C3 vs. C4 plant inputs to soil organic carbon, etc. Linear mixing models can be used to partition two sources with a sin...

324

SRC: stable rate control for streaming media  

Microsoft Academic Search

Rate control, in conjunction with congestion control, is important and necessary to maintain both the stability of the overall network and high quality of individual data transfer flows. We study stable rate control algorithms for streaming data, based on control theory. We introduce various control rules to maintain both sending rate and receiver buffer stability. We also propose an adaptive

Cheng Huang; Lihao Xu

2003-01-01

325

Most stable structure for hard spheres  

Microsoft Academic Search

The hard sphere model is known to show a liquid-solid phase transition, with the solid expected to be either face centered cubic or hexagonal close packed. The differences in free energy of the two structures are very small and various attempts have been made to determine which structure is the more stable. We contrast the different approaches and extend one.

Hans Koch; Charles Radin; Lorenzo Sadun

2005-01-01

326

Substitution of stable isotopes in Chlorella  

NASA Technical Reports Server (NTRS)

Replacement of biologically important isotopes in the alga Chlorella by corresponding heavier stable isotopes produces increasingly greater deviations from the normal cell size and changes the quality and distribution of certain cellular components. The usefulness of isotopically altered organisms increases interest in the study of such permuted organisms.

Flaumenhaft, E.; Katz, J. J.; Uphaus, R. A.

1969-01-01

327

Dynamically stable magnetic suspension/bearing system  

DOEpatents

A magnetic bearing system contains magnetic subsystems which act together to support a rotating element in a state of dynamic equilibrium. However, owing to the limitations imposed by Earnshaw's Theorem, the magnetic bearing systems to be described do not possess a stable equilibrium at zero rotational speed. Therefore, mechanical stabilizers are provided, in each case, to hold the suspended system in equilibrium until its speed has exceeded a low critical speed where dynamic effects take over, permitting the achievement of a stable equilibrium for the rotating object. A state of stable equilibrium is achieved above a critical speed by use of a collection of passive elements using permanent magnets to provide their magnetomotive excitation. The magnetic forces exerted by these elements, when taken together, levitate the rotating object in equilibrium against external forces, such as the force of gravity or forces arising from accelerations. At the same time, this equilibrium is made stable against displacements of the rotating object from its equilibrium position by using combinations of elements that possess force derivatives of such magnitudes and signs that they can satisfy the conditions required for a rotating body to be stably supported by a magnetic bearing system over a finite range of those displacements.

Post, Richard F. (Walnut Creek, CA)

1996-01-01

328

On the complexity group of stable curves  

Microsoft Academic Search

In this paper, we study combinatorial properties of stable curves. To the dual graph of any nodal curve, it is naturally associated a group, which is the group of components of the N\\\\'eron model of the generalized Jacobian of the curve. We study the order of this group, called the complexity. In particular, we provide a partial characterization of the

Simone Busonero; Margarida Melo; Lidia Stoppino

2008-01-01

329

A Tungsten Complex with a Bidentate, Hemilabile N-Heterocyclic Carbene Ligand, Facile Displacement of the Weakly Bound W-(C=C) Bond, and the Vulnerability of the NHC Ligand Towards Catalyst Deactivation During Ketone Hydrogenation  

SciTech Connect

The initial reaction observed between the N-heterocyclic carbene IMes (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) and molybdenum and tungsten hydride complexes CpM(CO){sub 2}(PPh{sub 3})H (M = Mo, W) is deprotonation of the metal hydride by IMes, giving [(IMes)H]{sup +}[CpM(CO){sub 2}(PPh{sub 3})]{sup -}. At longer reaction times and higher temperatures, the reaction of IMes with CpM(CO){sub 2}(PR{sub 3})H (M = Mo, W; R = Me, Ph) produces CpM(CO){sub 2}(IMes)H. Hydride transfer from CpW(CO)2(IMes)H to Ph{sub 3}C{sub +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, which was crystallographically characterized using X-ray radiation from a synchrotron. The IMes is bonded as a bidentate ligand, through the carbon of the carbene as well as forming a weak bond from the metal to a C=C bond of one mesityl ring. The weakly bound C=C ligand is hemilabile, being readily displaced by H{sub 2}, THF, ketones, or alcohols. Reaction of CpW(CO){sub 2}(IMes){sup +} with H{sub 2} gives the dihydride complex [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +}. Addition of Et{sub 2}CH-OH to CpW(CO){sub 2}(IMes){sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -} gives the alcohol complex [CpW(CO){sub 2}(IMes)(Et{sub 2}CH-OH)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}, which was characterized by crystallography and exhibits no evidence for hydrogen bonding of the bound OH group. Addition of H{sub 2} to the ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -} produces an equilibrium with the dihydride [CpW(CO){sub 2}(IMes)(H){sub 2}]{sup +} (K{sub eq} = 1.1 x 10{sup 3} at 25 {sup o}C). The tungsten ketone complex [CpW(CO){sub 2}(IMes)(Et{sub 2}C=O)]{sup +}[B(C{sub 6}F{sub 5}){sub 4}]{sup -}- serves as a modest catalyst for hydrogenation of Et{sub 2}C=O to Et{sub 2}CH-OH in neat ketone solvent. Decomposition of the catalyst produces [H(IMes)]{sup +}B(C{sub 6}F{sub 5}){sub 4}{sup -}, indicating that these catalysts with N-heterocyclic carbene ligands are vulnerable to decomposition by a reaction that produces a protonated imidazolium cation.

Wu,F.; Dioumaev, V.; Szalda, D.; Hanson, J.; Bullock, R.

2007-01-01

330

End-on and side-on ?-acid ligand adducts of gold(I): carbonyl, cyanide, isocyanide, and cyclooctyne gold(I) complexes supported by N-heterocyclic carbenes and phosphines.  

PubMed

N-heterocyclic carbene ligand SIDipp (SIDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and trimesitylphosphine ligand have been used in the synthesis of gold(I) cyanide, t-butylisocyanide, and cyclooctyne complexes (SIDipp)Au(CN) (3), (Mes(3)P)Au(CN) (4), [(Mes(3)P)(2)Au][Au(CN)(2)] (5), [(SIDipp)Au(CN(t)Bu)][SbF(6)] ([6][SbF(6)]), [(SIDipp)Au(cyclooctyne)][SbF(6)] ([8][SbF(6)]), and [(Mes(3)P)Au(cyclooctyne)][SbF(6)] ([9][SbF(6)]). A detailed computational study has been carried out on these and the related gold(I) carbonyl adducts [(SIDipp)Au(CO)][SbF(6)] ([1][SbF(6)]), [(Mes(3)P)Au(CO)][SbF(6)] ([2][SbF(6)]), and [(Mes(3)P)Au(CN(t)Bu)](+) ([7](+)). X-ray crystal structures of 3, 5, [6][SbF(6)], [8][SbF(6)], and [9][SbF(6)] revealed that they feature linear gold sites. Experimental and computational data show that the changes in ?-acid ligand on (SIDipp)Au(+) from CO, CN(-), CN(t)Bu, cyclooctyne as in [1](+), 3, [6](+), and [8](+) did not lead to large changes in the Au-C(carbene) bond distances. A similar phenomenon was also observed in Au-P distance in complexes [2](+), 4, [7](+), and [9](+) bearing trimesitylphosphine. Computational data show that the Au-L bonds of "naked" [Au-L](+) or SIDipp and Mes(3)P supported [Au-L](+) (L = CO, CN(-), CN(t)Bu to cyclooctyne) have higher electrostatic character than covalent character. The Au?L ?-donation and Au?L ?-back-donation contribute to the orbital term with the former being the dominant component, but the latter is not negligible. In the Au-CO adducts [1](+)and [2](+), the cationic gold center causes the polarization of the C-O ? and ? orbitals toward the carbon end making the coefficients at the two atoms more equal which is mainly responsible for the large blue shift in the CO stretching frequency. The SIDipp and Mes(3)P supported gold(I) complexes of cyanide and isocyanide also exhibit a significant blue shift in ?(CN) compared to that of the free ligands. Calculated results for Au(CO)Cl and Au(CF(3))CO suggest that the experimentally observed blue shift in ?(CO) of these compounds may at least partly be caused by intermolecular forces. PMID:23273108

Celik, Mehmet Ali; Dash, Chandrakanta; Adiraju, Venkata A K; Das, Animesh; Yousufuddin, Muhammed; Frenking, Gernot; Dias, H V Rasika

2013-01-18

331

Simulation and Chaotic Behavior of Alpha-stable Stochastic Processes  

Microsoft Academic Search

CONTENTS: Preliminary remarks; Brownian motion, poisson process, alpha-stable Levy motion; Computer simulation of alpha-stable random variables; Stochastic integration; Spectral representations of stationary processes; Computer approximations of continuous time processes; Examples of alpha-stable stochastic modelling; Convergence of approximate methods; Chaotic behaviour of stationary processes; Hierarchy of chaos for stable and ID stationary processes. Appendix - A guide to simulation.

Aleksander Janicki; Aleksander Weron

1994-01-01

332

Stable, selectable, integrative DNA transformation in Physarum.  

PubMed

The Physarum polycephalum actin promoter, PardC, can drive transient expression of heterologous genes in Physarum amoebae. The hph gene, encoding hygromycin (Hy) phosphotransferase, can confer resistance to Hy on a broad spectrum of organisms. When PardC is translationally fused to hph and transformed into yeasts on high-copy-number vectors, the yeasts become Hy resistant (HyR), showing that PardC-hph is a functional, selectable genetic element. To establish a stable transformation system for Physarum, we electroporated plasmids bearing PardC-hph into Physarum amoebae and then selected for HyR transformants. We show that HyR amoebae arise upon the stable integration of PardC-hph into the nuclear genome in single copy. These results establish a transformation system that can be used to add plasmid-borne genetic information to Physarum. PMID:8224865

Burland, T G; Bailey, J; Pallotta, D; Dove, W F

1993-10-15

333

The observation of double strange stable dibaryons  

NASA Astrophysics Data System (ADS)

A few events, detected on the photographs of the JINR 2m propane bubble chamber exposed to a 10 GeV/c proton beam, were interpreted H dibaryons. A reliable identification of the above events needs a multivertex kinematic analysis which is in turn is feasible only using 4?-detectors and high measurement precisions of the sought objects. The 2 m propane bubble chamber is best suited for these purpose. These events are grouped into two following groups. 1. The first group is formed of three neutral, S=-2 stable dibaryons, the masses of which are below the ?? threshold. 2. The second group is formed of two neutral and three positively charged S=-2 heavy stable dibaryons. The masses of all the five dibaryons coincide within the limits of errors are over the ??, ?N, ?? threshold. .

Aslanyan, P. Zh.; Rikhvitskiy, V. S.; Yemelyanenko, V. N.

2001-06-01

334

Thermally Stable, Piezoelectric and Pyroelectric Polymeric Substrates  

NASA Technical Reports Server (NTRS)

A thermally stable, piezoelectric and pyroelectric polymeric substrate was prepared. This thermally stable, piezoelectric and pyroelectric polymeric substrate may be used to prepare electromechanical transducers, thermomechanical transducers, accelerometers. acoustic sensors, infrared sensors, pressure sensors, vibration sensors, impact sensors, in-situ temperature sensors, in-situ stress/strain sensors, micro actuators, switches, adjustable fresnel lenses, speakers, tactile sensors. weather sensors, micro positioners, ultrasonic devices, power generators, tunable reflectors, microphones, and hydrophones. The process for preparing these polymeric substrates includes: providing a polymeric substrate having a softening temperature greater than 1000 C; depositing a metal electrode material onto the polymer film; attaching a plurality of electrical leads to the metal electrode coated polymeric substrate; heating the metal electrode coated polymeric substrate in a low dielectric medium; applying a voltage to the heated metal electrode coated polymeric substrate to induce polarization; and cooling the polarized metal electrode coated polymeric electrode while maintaining a constant voltage.

Simpson, Joycely O. (Inventor); St.Clair, Terry L. (Inventor)

1999-01-01

335

A stable Ti-based quasicrystal  

SciTech Connect

The thermal stability of the icosahedral phase (i phase) in Ti{sub 45}Zr{sub 38}Ni{sub 17} alloys is demonstrated. As-cast alloys containing initially only the C14 hexagonal Laves and {alpha}-solid-solution phases transformed primarily to the icosahedral phase upon annealing in vacuum for 64 h at 570{degree}C. This confirms previous evidence for i-phase stability and firmly establishes this quasicrystal as the first nonaluminum stable icosahedral phase. Diffraction data show that this stable i phase is primitive; energy dispersive x-ray spectroscopy measurements place its composition near Ti{sub 41.5}Zr{sub 41.5}Ni{sub 17}. These and other results suggest that the structure of this i phase is similar to that of i(AlLiCu). {copyright} {ital 1997 American Institute of Physics.}

Kelton, K.F.; Kim, W.J.; Stroud, R.M. [Department of Physics, Washington University, St. Louis, Missouri 63130 (United States)] [Department of Physics, Washington University, St. Louis, Missouri 63130 (United States)

1997-06-01

336

Unexpected Stable Stoichiometries of Sodium Chlorides  

NASA Astrophysics Data System (ADS)

Sodium chloride (NaCl), or rocksalt, is well characterized at ambient pressure. As a result of the large electronegativity difference between Na and Cl atoms, it has highly ionic chemical bonding (with 1:1 stoichiometry dictated by charge balance) and B1-type crystal structure. By combining theoretical predictions and diamond anvil cell experiments, we found that new materials with different stoichiometries emerge at high pressures. Compounds such as Na3Cl, Na2Cl, Na3Cl2, NaCl3, and NaCl7 are theoretically stable and have unusual bonding and electronic properties. To test this prediction, we synthesized cubic and orthorhombic NaCl3 and two-dimensional metallic tetragonal Na3Cl. These experiments establish that compounds violating chemical intuition can be thermodynamically stable even in simple systems at nonambient conditions.

Zhang, Weiwei; Oganov, Artem R.; Goncharov, Alexander F.; Zhu, Qiang; Boulfelfel, Salah Eddine; Lyakhov, Andriy O.; Stavrou, Elissaios; Somayazulu, Maddury; Prakapenka, Vitali B.; Konôpková, Zuzana

2013-12-01

337

Dianionic Tetraborates Do Exist as Stable Entities  

Microsoft Academic Search

To date, B6H6 2- and some of its derivatives are the smallest members of the closo-borates that have been synthesized and analyzed in condensed phases. In contrast, no stable dianionic tetraborate has yet been observed, either in solution or solids or in the gas phase. In this work, the gas-phase stability of dianionic tetraborates B4X4 2- (X ) H, CN,

Andreas Dreuw; Norbert Zint; Lorenz S. Cederbaum

2002-01-01

338

A stable adaptive PI controller for AQM  

Microsoft Academic Search

Active queue management (AQM) mechanisms have been proposed to support end-to-end congestion control mechanisms of transmission control protocol (TCP) in TCP\\/IP networks. This paper introduces a new AQM based on a stable adaptive proportional-integral (SAPI) controller. SAPI adaptively adjusts the PI controller parameters based on estimated network parameters to improve the transient performance of a fixed-gain PI controller. We present

Jinsheng Sun; Moshe Zukerman; Marimuthu Palaniswami

2007-01-01

339

Circular array of stable atmospheric pressure microplasmas  

NASA Astrophysics Data System (ADS)

A circular array composed of six quarter-wavelength microstripline resonators sustains a stable ring-shaped microplasma in atmospheric pressure argon. A single power source (1 GHz, <5 W) drives all six resonators. The operation of the array is modeled by coupled mode theory (CMT) and confirmed by electromagnetic simulations. Non-uniformities in the plasma ring are attributed to parasitic plasma sheath capacitance and confirmed by CMT.

Wu, C.; Zhang, Z.-B.; Hoskinson, A.; Hopwood, J.

2010-12-01

340

Complex symmetric matrices with strongly stable iterates  

NASA Technical Reports Server (NTRS)

Complex-valued symmetric matrices are studied. A simple expression for the spectral norm of such matrices is obtained, by utilizing a unitarily congruent invariant form. A sharp criterion is provided for identifying those symmetric matrices whose spectral norm is not exceeding one: such strongly stable matrices are usually sought in connection with convergent difference approximations to partial differential equations. As an example, the derived criterion is applied to conclude the strong stability of a Lax-Wendroff scheme.

Tadmor, E.

1985-01-01

341

Uncertainty in source partitioning using stable isotopes  

Microsoft Academic Search

Stable isotope analyses are often used to quan- tify the contribution of multiple sources to a mixture, such as proportions of food sources in an animal's diet, or C3 and C4 plant inputs to soil organic carbon. Linear mixing models can be used to partition two sources with a single isotopic signature (e.g., ?13C) or three sources with a second

Donald L. Phillips; Jillian W. Gregg

2001-01-01

342

Stable spatial orbits around collinear libration points  

Microsoft Academic Search

A technique of generation of spatial periodic solutions to the restricted circular three-body problem from periodic orbits\\u000a of the planar problem has been used for the families of orbits around collinear libration points L\\u000a 1 and L\\u000a 2. Developing the families obtained at the 1: 1 resonance, we have obtained stable solutions both in the Earth-Moon system\\u000a and in the

B. B. Kreisman

2010-01-01

343

Constructing simple stable descriptions for image partitioning  

Microsoft Academic Search

A new formulation of the image partitioning problem is presented: construct a complete and stable description of an image-in terms of a specified descriptive language-that is simplest in the sense of being shortest. We show that a descriptive language limited to a low-order polynomial description of the intensity variation within each region and a chain-code-like description of the region boundaries

Yvan G. Leclerc

1989-01-01

344

Stable isotope dynamics in elasmobranch fishes  

Microsoft Academic Search

Carbon and nitrogen stable isotope analyses have improved our understanding of food webs and movement patterns of aquatic\\u000a organisms. These techniques have recently been applied to diet studies of elasmobranch fishes, but isotope turnover rates\\u000a and isotope diet–tissue discrimination are still poorly understood for this group. We performed a diet switch experiment on\\u000a captive sandbar sharks (Carcharhinus plumbeus) as a

John M. LoganMolly; Molly E. Lutcavage

2010-01-01

345

Thermally Stable and Flame Retardant Elastomeric Nanocomposites  

Microsoft Academic Search

\\u000a This chapter is dedicated to thermally stable and flame retardant elastomeric composites. Two approaches are considered: the\\u000a synthesis of elastomeric nanocomposites, where the nanoparticles are dispersed at the nanoscale, and the incorporation of\\u000a nanofillers at high loadings where agglomerate of nanoparticles are observed in the elastomeric matrix. The chapter is mainly\\u000a focused on the key parameter influencing the flame retardancy,

O. Cerin; G. Fontaine; S. Duquesne; S. Bourbigot

346

Mechanisms for stable single bubble sonoluminescence  

SciTech Connect

A gas bubble trapped in water by an oscillating acoustic field is expected to either shrink or grow on a diffusive time scale, depending on the forcing strength and the bubble size. At high ambient gas concentration this has long been observed. However, recent sonoluminescence experiments show that when the ambient gas concentration is low the bubble can be stable for days. This paper discusses mechanisms leading to stability. {copyright} {ital 1996 The American Physical Society.}

Brenner, M.P. [Department of Mathematics, MIT, Cambridge, Massachusetts 02139 (United States)] [Department of Mathematics, MIT, Cambridge, Massachusetts 02139 (United States); Lohse, D. [Fachbereich Physik der Universitaet Marburg, Renthof 6, 35032 Marburg (Germany)] [Fachbereich Physik der Universitaet Marburg, Renthof 6, 35032 Marburg (Germany); [Department of Mathematics, The University of Chicago, Chicago, Illinois 60637 (United States); Oxtoby, D. [Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (United States)] [Department of Chemistry and James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (United States); Dupont, T.F. [Department of Mathematics, The University of Chicago, Chicago, Illinois 60637 (United States)] [Department of Mathematics, The University of Chicago, Chicago, Illinois 60637 (United States)

1996-02-01

347

Stable localized patterns in thin liquid films  

NASA Technical Reports Server (NTRS)

We study a 2-D nonlinear evolution equation which describes the 3-D spatiotemporal behavior of the air-liquid interface of a thin liquid film lying on the underside of a cooled horizontal plate. We show that the Marangoni effect can stabilize the destabilizing effect of gravity (the Rayleigh-Taylor instability) allowing for the existence of stable localized axisymmetric solutions for a wide range of parameter values. Various properties of these structures are discussed.

Deissler, Robert J.; Oron, Alexander

1991-01-01

348

Stable acid phosphatase: I. Demonstration and distribution  

Microsoft Academic Search

After a pH-dependent reactivation a highly stable form of acid phosphatase (SAPhase) could be demonstrated in active cells of the macrophage\\/giant cell\\/osteoclast series and also in epiphyseal chondrocytes, in cells lining bone undergoing resorption and in hamster cosinophils. Because acid phosphatases of epithelial cells in rat, hamster and Macaca sp. tissues did not possess this stability, SAPhase served as a

F. M. Eggert; Jocelyn P. Germain

1980-01-01

349

Overview and first results from project STABLE (STAble Boundary Layer Experiment)  

SciTech Connect

The STABLE project (STAble Boundary Layer Experiment) is a multiyear research effort conceived in 1984 by the Savannah River Laboratory (SRL) and planned by several research groups to study turbulence and diffusion in the stable boundary layer (SBL). The program was jointly planned by Department of Energy (DOE) affiliated laboratories and universities including SRL, Atmospheric Turbulence and Diffusion Division (ATDD) of the National Oceanic and Atmospheric Administration (NOAA), Argonne National Laboratory (ANL), North Carolina State University (NCSU), and The Pennsylvania State University. STABLE's goals are to determine the spatial and temporal distribution of turbulent events during the nighttime, to determine the validity of present models and theories in describing the structure and evolution of the SBL, to determine the role of waves and intermittent turbulence in dispersing chemicals, and to determine better parameterization for describing the mean state and intermittent events in the SBL. By taking advantage of special facility, the program seeks to economize effort and cost. 1 ref., 7 figs.

Weber, A.H.; Kurzeja, R.J.

1988-01-01

350

Calculation of substrate flux using stable isotopes  

SciTech Connect

The use of stable isotope tracers to calculate substrate kinetics in humans is favored over the use of radioactive isotopes because of their greater safety and versatility. However, potential complications not met when dealing with radioactive tracers are caused (1) the natural occurrence of the stable isotope used as a tracer and (2) the necessity to administer the tracer in an amount that cannot be treated as massless. The authors therefore found it desirable to derive a theoretically valid equation for calculating the rate of appearance, R{sub a}, of a substrate under steady-state conditions using a stable isotope tracer. This theoretically valid equation yields results that differ from those of the equations conventionally used to calculate (endogenous) R{sub a} in steady state. Quantitative determination of the error in one of these equations revealed that for tracers commonly used in metabolic studies the error is negligible, whereas the error made in the other equation is likely to be quite high in commonly encountered situations. Finally, to allow for proper use of different definitions of isotopic enrichment that have arisen from practical considerations, they use the results derived above to determine valid equations for R{sub a} appropriate to the two prevalent definitions.

Rosenblatt, J.; Wolfe, R.R. (Univ. of Texas Medical Branch, Galveston (USA))

1988-04-01

351

Stable paclitaxel formulations in oily contrast medium.  

PubMed

Stable paclitaxel/Lipiodol solutions as well as emulsions were developed for the treatment of solid tumors including hepatocellular carcinoma. Paclitaxel could be dissolved in Lipiodol, an oily contrast medium, but precipitated out and formed aggregates with time. Paclitaxel precipitation was due to the inter- and intra-molecular hydrogen bonding of paclitaxel molecules. Time-dependent paclitaxel aggregation was completely prevented by adding small amounts of additional solvents, which are miscible with Lipiodol. It was also notable that paclitaxel helped in stabilizing the water-in-oil (w/o) type emulsion of Lipiodol and Iopamiro. The stability, physical properties and in vitro drug release profiles of the stable paclitaxel solutions and emulsions were characterized. When the stable oily paclitaxel solution was used for the treatment of B16F10 melanoma in C57BL/6 mice, the malignant cells were eradicated completely in 2 weeks, whereas the solid tumor grew rapidly and metastasized to the thigh and to other organs in the control group. Also, the mice survived for more than 1 year after the paclitaxel treatment, whereas all of those in the control group died in 40 days. PMID:15653161

Lee, In-Hyun; Park, Yeong Taek; Roh, Kyungho; Chung, Hesson; Kwon, Ick Chan; Jeong, Seo Young

2005-02-01

352

Metal-Free Catalytic Enantioselective C-B Bond Formation: (Pinacolato)boron Conjugate Additions to ?,?-Unsaturated Ketones, Esters, Weinreb Amides and Aldehydes Promoted by Chiral N-Heterocyclic Carbenes  

PubMed Central

The first broadly applicable metal-free enantioselective method for boron conjugate addition (BCA) to ?,?-unsaturated carbonyls is presented. The C–B bond forming reactions are promoted in the presence of 2.5–7.5 mol % of a readily accessible C1-symmetric chiral imidazolinium salt, which is converted, in situ, to the catalytically active diastereo- and enantiomerically pure N-heterocyclic carbene (NHC) by the common organic base 1,8-diazabicyclo[5.4.0]undec-7-ene (dbu). In addition to the commercially available bis(pinacolato)diboron [B2(pin)2], and in contrast to reactions with the less sterically demanding achiral NHCs, the presence of MeOH is required for high efficiency. Acyclic and cyclic ?,?-unsaturated ketones, as well as acyclic esters, Weinreb amides and aldehydes can serve as suitable substrates; the desired ?-boryl carbonyls are isolated in up to 94% yield and >98:2 enantiomer ratio (er). Transformations are often carried out at ambient temperature. In certain cases, such as when the relatively less reactive unsaturated amides are used, elevated temperatures are required (50–66 °C); nonetheless, reactions remain highly enantioselective. The utility of the NHC-catalyzed method is demonstrated through comparison with the alternative Cu-catalyzed protocols; in cases involving a polyfunctional substrate, unique profiles in chemoselectivity are exhibited by the metal-free approach (e.g., conjugate addition vs reaction with an alkyne, allene or aldehyde).

Wu, Hao; Radomkit, Suttipol; O'Brien, Jeannette M.; Hoveyda, Amir H.

2012-01-01

353

Synthesis and characterization of various ruthenium small molecules and metallopolymers for potential use in photovoltaic devices and luminescence properties of N-heterocyclic carbenes and trifluoro methylated tris(pyrazolyl)borate silver (I) acid-base adducts  

NASA Astrophysics Data System (ADS)

This thesis with its 66 pages, 1 table, 29 figures and 47 references, deals with two major topics involving structure and properties of selected metal complexes and their use in electronic devices. The first chapter discusses the development of ruthenium-based metallopolymer and small molecular complexes toward possible utilization as active materials for solar cells. Such complexes have been found to be good "black absorbers" with solid-state absorption that spans the entire visible region into the near-infrared, representing an impressive overlap with the solar radiation that is more favorable than common molecular solar cell active materials. The second chapter includes photophysical studies of different metallo N-heterocyclic carbenes and acid-base adducts comprising fluorinated tris(pyrazolyl)borate silver(I) complexes coordinated to different solvent and aromatic molecules. These species have been investigated and found to exhibit unusual photophysical properties, including metal-free phosphorescence at room temperature with high quantum yield, multi-color emission from a single compound, and selective sensing of temperature and hazardous vapors.

Chitikuri, Shylaja

354

Directional properties of fluorenylidene moieties in unsymmetrically substituted N-heterocyclic carbenes. Unexpected CH activation of a methylfluorenyl group with palladium. Use in palladium catalysed Suzuki-Miyaura cross coupling of aryl chlorides.  

PubMed

Benzimidazolium salts having their two nitrogen atoms substituted by different 9-alkylfluorenyl groups ( and , alkyl(1)/alkyl(2) = Me/Et, Me/Pr, Me/n-Bu, Me/i-Pr, Me/Bn, Me/CH2SMe have been synthesised in high yields in two or three steps from N,N'-bis(9H-fluoren-9-ylidene)benzene-1,2-diamine (). The imidazolium salts were converted readily into the corresponding PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilisation and initiation), while reaction of the methylthioether-substituted salt with PdCl2/K2CO3/pyridine afforded the palladacycle resulting from metallation of the methyl group attached to the fluorenylidene moiety. NMR and X-ray diffraction studies revealed that the carbene ligands in behave as clamp-like ligands, the resulting metal confinement arising from a combination of the orientational properties of the fluorenylidene moieties that push the alkyl groups towards the metal centre and attractive anagostic interactions involving CH2(fluorenyl) groups. Complexes were assessed in Suzuki-Miyaura cross-coupling reactions. Like their symmetrical analogues they displayed high activity in the coupling of phenyl boronic acid with p-tolylchloride but their performance remained slightly inferior to that of the related, symmetrical Et/Et complex . PMID:24927082

Teci, Matthieu; Brenner, Eric; Matt, Dominique; Gourlaouen, Christophe; Toupet, Loïc

2014-07-22

355

Univalent gallium and indium phosphane complexes: from pyramidal M(PPh3)3(+) to carbene-analogous bent M(PtBu3)2(+) (M=Ga, In) complexes.  

PubMed

In a new oxidative route, Ag(+)[Al(OR(F))(4)](-) (R(F)=C(CF(3))(3)) and metallic indium were sonicated in aromatic solvents, such as fluorobenzene (PhF), to give a precipitate of silver metal and highly soluble [In(PhF)(n)](+) salts (n=2, 3) with the weakly coordinating [Al(OR(F))(4)](-) anion in quantitative yield. The In(+) salt and the known analogous Ga(+)[Al(OR(F))(4)](-) were used to synthesize a series of homoleptic PR(3) phosphane complexes [M(PR(3))(n)](+), that is, the weakly PPh(3)-bridged [(Ph(3)P)(3)In-(PPh(3))-In(PPh(3))(3)](2+) that essentially contains two independent [In(PPh(3))(3)](+) cations or, with increasing bulk of the phosphane, the carbene-analogous [M(PtBu(3))(2)](+) (M=Ga, In) cations. The M(I)-P distances are 27 to 29 pm longer for indium, and thus considerably longer than the difference between their tabulated radii (18 pm). The structure, formation, and frontier orbitals of these complexes were investigated by calculations at the BP86/SV(P), B3LYP/def2-TZVPP, MP2/def2-TZVPP, and SCS-MP2/def2-TZVPP levels. PMID:22623027

Higelin, Alexander; Sachs, Ulf; Keller, Sarah; Krossing, Ingo

2012-08-01

356

A peculiar stable region around Pluto  

NASA Astrophysics Data System (ADS)

Giuliatti Winter et al. found several stable regions for a sample of test particles located between the orbits of Pluto and Charon. One peculiar stable region in the space of the initial orbital elements is located at a = (0.5d, 0.7d) and e = (0.2, 0.9), where a and e are the initial semimajor axis and eccentricity of the particles, respectively, and d is the Pluto-Charon distance. This peculiar region (hereafter called the sailboat region) is associated with a family of periodic orbits derived from the planar, circular, restricted three-body problem (Pluto-Charon-particle). In this work, we study the origin of this stable region by analysing the evolution of such family of periodic orbits. We show that they are not in resonances with Charon. The period of the periodic orbit varies along the family, decreasing with the increase of the Jacobi constant. We also explore the extent of the sailboat region by adopting different initial values of the orbital inclination (I) and argument of the pericentre (?) of the particles. The sailboat region is present for I = [0°, 90°] and for two intervals of ?, ? = [-10°, 10°] and (160°, 200°). A crude estimative of the size of the hypothetical bodies located at the sailboat region can be derived by computing the tidal damping in their eccentricities. If we neglect the orbital evolution of Pluto and Charon, the time-scale for circularization of their orbits is longer than the age of the Solar system for bodies smaller than 500 m in radius.

Giuliatti Winter, S. M.; Winter, O. C.; Vieira Neto, E.; Sfair, R.

2014-04-01

357

Nonperturbative procedure for stable K-brane  

NASA Astrophysics Data System (ADS)

We propose a novel first-order formalism for a type of K-brane system. An example solution is presented and studied. We illustrate how the deviation parameter affects the properties of the model, such as the stability of the solutions and the localization of fermions and gravitons. We argue that our solution is stable against linear perturbations. The tensor zero mode of a graviton can be localized on the brane while the scalar zero mode cannot. The localization condition for fermion is also discussed.

Zhong, Yuan; Liu, Yu-Xiao; Zhao, Zhen-Hua

2014-05-01

358

Convex geometry of max-stable distributions  

Microsoft Academic Search

It is shown that max-stable random vectors in [0,???)\\u000a d\\u000a with unit Fréchet marginals are in one to one correspondence with convex sets K in [0,???)\\u000a d\\u000a called max-zonoids. The max-zonoids can be characterised as sets obtained as limits of Minkowski sums of cross-polytopes\\u000a or, alternatively, as the selection expectation of a random cross-polytope whose distribution is controlled by the

Ilya Molchanov

2008-01-01

359

Stable surface solitons in truncated complex potentials.  

PubMed

We show that surface solitons in the one-dimensional nonlinear Schrödinger equation with truncated complex periodic potential can be stabilized by linear homogeneous losses, which are necessary to balance gain in the near-surface channel arising from the imaginary part of potential. Such solitons become stable attractors when the strength of homogeneous losses acquires values from a limited interval and they exist in focusing and defocusing media. The domains of stability of the surface solitons shrink with an increase in the amplitude of the imaginary part of complex potential. PMID:22743443

He, Yingji; Mihalache, Dumitru; Zhu, Xing; Guo, Lina; Kartashov, Yaroslav V

2012-07-01

360

Stable, Electroinactive Wetting Agent For Fuel Cells  

NASA Technical Reports Server (NTRS)

Straight-chain perfluorooctanesulfonic acid (C8 acid) identified as innocuous and stable wetting agent for use with polytetrafluoroethylene-containing electrodes in liquid-feed direct-oxidation fuel cells suggested for use in vehicles and portable power supplies. C8 acid in small concentrations in aqueous liquid solutions of methanol, trimethoxymethane, dimethoxymethane, and trioxane enables oxidation of these substances by use of commercially available electrodes of type designed originally for use with gases. This function specific to C8 acid molecule and not achieved by other related perfluorolkanesulfonic acids.

Prakash, Surya G.; Olah, George A.; Narayanan, Sekharipuram R.; Surampudi, Subbarao; Halpert, Gerald

1994-01-01

361

Stable metallization for diamond and other materials  

NASA Technical Reports Server (NTRS)

An adherent and metallurgically stable metallization system for diamond is presented. The big improvement in metallurgical stability is attributed to the use of a ternary, amorphous Ti--Si--N diffusion barrier. No diffusion between the layers and no delamination of the metallization was observed after annealing the schemes at 400.degree. C. for 100 hours and at 900.degree. C. for 30 minutes. Thermal cycling experiments in air from -65 to 155.degree. C. and adhesion tests were performed. Various embodiments are disclosed.

Bachli, Andreas (Inventor); Kolawa, Elzbieta (Inventor); Nicolet, Marc-Aurele (Inventor); Vandersande, Jan W. (Inventor)

2000-01-01

362

Stable RNA interference rules for silencing  

PubMed Central

RNA interference has become an indispensable tool for loss-of-function studies across eukaryotes. By enabling stable and reversible gene silencing, shRNAs provide a means to study long-term phenotypes, perform pool-based forward genetic screens and examine the consequences of temporary target inhibition in vivo. However, efficient implementation in vertebrate systems has been hindered by technical difficulties affecting potency and specificity. Focusing on these issues, we analyse current strategies to obtain maximal knockdown with minimal off-target effects.

Fellmann, Christof; Lowe, Scott W.

2014-01-01

363

Stable parallel elimination for boundary valve ODEs  

SciTech Connect

A parallelizable and vectorizable algorithm for solving linear algebraic systems arising from two-point boundary value ODEs is described. The method is equivalent to Gaussian elimination, with row partial pivoting, applied to a certain row- and column-reordered version of the usual almost block-diagonal coefficient matrix. Analytical and numerical evidence is presented to show that the algorithm is stable. Results from implementation on a shared-memory multiprocessor and a vector processor are given. The approach can be extended to handle problems with multipoint and integral conditions or algebraic parameters.

Wright, S.J.

1991-04-01

364

Stable localized patterns in thin liquid films  

NASA Technical Reports Server (NTRS)

A two-dimensional nonlinear evolution equation is studied which describes the three-dimensional spatiotemporal behavior of the air-liquid interface of a thin liquid film lying on the underside of a cooled horizontal plate. It is shown that the equation has a Liapunov functional, and this fact is exploited to demonstrate that the Marangoni effect can stabilize the destabilizing effect of gravity (the Rayleigh-Taylor instability), allowing for the existence of stable localized axisymmetric solutions for a wide range of parameter values. Various properties of these structures are discussed.

Deissler, Robert J.; Oron, Alexander

1992-01-01

365

Is N sub 8 cubane stable  

SciTech Connect

The question of N{sub 8} cubane's stability is addressed via ab initio calculations. An O{sub h} symmetry stationary point is found on the energy hypersurface for all three levels of theory used; these were restricted Hartree-Fock (RHF) self-consistent-field theory using STO-3G, 4-31G, and 4-31G* basis sets. Vibrational frequency calculations, at the same three levels of theory, all indicate that the cubic structure is stable. The effect of correlation on the stable structure energies is examined with Moeller-Plesset perturbation theory up to fourth order; these post-Hartree-Fock calculations were performed at the RHF optimized geometries. The calculations indicate that, if it could be synthesized, N{sub 8} cubane would be a highly energetic material, metastable to dissociation to four N{sub 2} molecules; the energy release for this reaction is predicted to be 530 {plus minus} 50 kcal/mol. The energy barrier to dissociation is estimated via reaction coordinate calculations.

Engelke, R.; Stine, J.R. (Los Alamos National Lab., NM (USA))

1990-07-26

366

Electrochemical Fractionation of Molybdenum Stable Isotopes  

NASA Astrophysics Data System (ADS)

Stable isotope signatures were measured from Molybdenum (Mo) electrodeposited from aqueous solution. As potential varied from -1.35 V to -2.00 V (relative to Ag/AgCl), fractionation decreases from ?97/95Mo = -1.3 ‰ to -0.9 ‰ (?97/95Mo defined as the difference in the 97Mo/95Mo ratio of deposited Mo relative to aqueous Mo). Natural variations of ?97/95Mo span a range of ~ 3 ‰ [Barling, J. and Anbar, A. D., EPSL. 2004, 217: 315], therefore, charge transfer driven fractionation may be responsible for some of the observed variation in Mo stable isotope geochemistry. Following previous approaches with Fe and Zn [Kavner, A. et al. Geochim. Cosmochim. Acta. 2005, 69: 2971; 2008, 72: 1731], Mo was plated in a three-electrode cell from a neutral to slightly alkaline solution (pH ~ 8.7). Voltage was held constant during electrodeposition using an Autolab Potentiostat. In all experiments, less than 0.5 % of the Mo was deposited, which insures that the plating reservoir remains at an approximately constant isotopic composition. Plated Mo was then recovered in acid, and the isotopic composition of samples and stock solutions were measured using a Thermo Scientific Neptune MC-ICP-MS. These experiments show that the redox process induces an isotopic signature with respect to the starting material, with a trend showing that fractionation decreases as a function of applied voltage.

Crawford, J.; Black, J.; Wasylenki, L.; Gordon, G.; Anbar, A.; Kavner, A.

2008-12-01

367

Stable expression recognition abnormalities in unipolar depression.  

PubMed

Although abnormalities in emotion recognition during a depressed episode have frequently been reported in patients with depression, less is known about the stability of these abnormalities. To examine the stability of emotion recognition abnormalities, this longitudinal study assessed patients with unipolar depression on three separate occasions at 3-monthly intervals. Recognition of sad, angry, fearful, disgusted, happy and neutral facial expressions was assessed in a matching task and a labelling task. Patients performed as well as matched healthy controls on the matching task. On the labelling task, patients showed higher accuracy and higher response bias than controls for sad expressions only, which remained stable over a 6-month interval. Over the same period, symptom severity, as measured with the Beck Depression Inventory and the Hamilton Depression Rating Scale, decreased significantly in the patient group. Furthermore, labelling performance for sad expressions was not associated with symptom severity or with changes in severity over time. This stable bias for sad expressions might signal a vulnerability factor for depression, as proposed by cognitive theories of depression. PMID:20478626

Milders, Maarten; Bell, Stephen; Platt, Julie; Serrano, Rosa; Runcie, Olga

2010-08-30

368

New Stable Isotope Tropical Paleoclimate Proxies  

NASA Astrophysics Data System (ADS)

Organized tropical rain systems such as tropical cyclones (TC) and mesoscale convective systems (MCS) produce both water vapor and rainfall with distinctly low isotope ratios. This lowering is caused by recyling of water in organized systems. Therefore, fresh water carbonate organisms have considerable potential to act as proxy recorders of these systems. Ostracoda are ephemeral making them especially attractive candidates. Tropical trees offer another opportunity because the low isotopic spikes produced in both soil waters when heavy rains result and ambient water vapor surronding the trees may be recorded in the tree cellulose. Ostracoda living in the surface waters derived from Tropical Storm Allison (2001) document the passage of the storm in their oxygen isotope ratios. The stable isotopic composition of water vapor along the southwest coast of Mexico shows considerable variation in response to TC and MCS activity offshore even when no rain falls in the region. Potentially a long-term record of this activity may be found in the stable isotopic composition of trees providing low elevation trees of sufficient longevity can be found.

Lawrence, J. R.

2005-05-01

369

Stable-Unstable Transitions in Viscous Fingering  

NASA Astrophysics Data System (ADS)

We have carried out experiences in a Hele-Shaw cell, with a radial injection at constant flow rate. A liquid, oil or glycerol solutions, pushes (or is pushed by) an aqueous polymeric solution of Xanthan. This solution is a shear thinning fluid. An oil and glycerol solution could be considering Newtonian fluids. In this type of displacement, when the mobility control parameter is greater than one, the characteristic Viscous Fingering appears. During the advance of the interface the crossover from a stable to an unstable displacement or vice versa is investigated. On the other hand, we have experimentally obtain the effective mobility of the shear thinning fluid as a function of the depth averaged velocity in order to predict the moment of transition. In the immiscible displacement case, the front becomes unstable and is finally stabilized. In this case, the non-Newtonian fluid displaces the Newtonian one. We observe that the transition occurs approximately at a critical mean velocity where the mobility control parameter reaches one. For miscible fluids, we have studied the displacement where the non-Newtonian fluid is the displacing one. In that case, the front becomes stable and finally instabilities appear. But contrary from we would expect, the critical velocity depends on the injection flow rate Q. As Q increases, the critical velocity increases too. Our experiments seem to show that the critical velocity is fairly constant and not so sensitive to changes in the Newtonian fluid mobility.

Obernauer, S.; Rosen, M.

370

Stable extramarital affairs are breaking the heart.  

PubMed

The relationship between extramarital affairs and cardiovascular risk is still not completely clarified. The aim of this study was to investigate whether extramarital affairs have a protective effect on cardiovascular risk or, conversely, a deleterious one. Among patients studied, 91.8% of the whole sample reported no or occasional extramarital affairs, while 8.2% declared a stable secondary relationship. During a median follow-up of 4 [0-8] years, 95 major adverse cardiovascular events (MACE), eight of which were fatal, were observed. Cox analysis, after adjustment for confounding factors, showed that presence of stable extramarital affair was associated with a higher incidence of MACE (HR = 2.13 [1.12; 4.07], p = 0.023). The introduction in the Cox model of patient perceived partner's hypoactive sexual desire (PPPHSD) attenuates the association (HR 1.86 [0.93; 3.70], p = 0.078). The sample was therefore divided according to PPPHSD. We observed that unadjusted incidence of MACE was significantly associated with presence of extramarital affairs only in men reporting a primal partner without PPPHSD. This association was also confirmed in a Cox regression model, after adjusting for confounders (HR = 2.87 [1.81; 6.98], p = 0.020). We can conclude that to be unfaithful represents an independent risk factor for MACE. Therefore, infidelity induces not only heart trouble in the betrayed partners, but seems to be also able to increase the betrayer's heart-related events. PMID:21631529

Fisher, A D; Bandini, E; Corona, G; Monami, M; Cameron Smith, M; Melani, C; Balzi, D; Forti, G; Mannucci, E; Maggi, M

2012-02-01

371

Color stable manganese-doped phosphors  

SciTech Connect

A process for preparing color stable Mn.sup.+4 doped phosphors includes providing a phosphor of formula I; A.sub.x[MF.sub.y]:Mn.sup.+4 I and contacting the phosphor in particulate form with a saturated solution of a composition of formula II in aqueous hydrofluoric acid; A.sub.x[MF.sub.y]; II wherein A is Li, Na, K, Rb, Cs, NR.sub.4 or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Y, La, Nb, Ta, Bi, Gd, or a combination thereof; R is H, lower alkyl, or a combination thereof; x is the absolute value of the charge of the [MF.sub.y] ion; and y is 5, 6 or 7. In particular embodiments, M is Si, Ge, Sn, Ti, Zr, or a combination thereof. A lighting apparatus capable of emitting white light includes a semiconductor light source; and a phosphor composition radiationally coupled to the light source, and which includes a color stable Mn.sup.+4 doped phosphor.

Lyons, Robert Joseph (Burnt Hills, NY); Setlur, Anant Achyut (Niskayuna, NY); Deshpande, Anirudha Rajendra (Twinsburg, OH); Grigorov, Ljudmil Slavchev (Sofia, BG)

2012-08-28

372

2. Historic American Buildings Survey 'Stable and Smoke House' Photocopy ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

2. Historic American Buildings Survey 'Stable and Smoke House' Photocopy of Plate XXX From: Brown, Glenn, The Octagon, Washington, N.D. - Octagon House, Stable, 1799 New York Avenue Northwest, Washington, District of Columbia, DC

373

Availability of Enriched Stable Isotopes: Present Status and Future Prospects.  

National Technical Information Service (NTIS)

The Electromagnetic Isotope Enrichment Facility (EMIEF) is currently used to produce 225 enriched stable isotopes of 50 elements. Among these are included most of the known elements with stable isotopes except for the noble gases, certain light elements, ...

R. W. Hoff

1986-01-01

374

Direct globally convergent adaptive regulation of stable multivariable plants  

NASA Technical Reports Server (NTRS)

A simple globally convergent direct methodology for adaptive regulation of stable plants, assuming convergence of the parameters, is presented. The development is strictly in terms of the input-output representations and is based on the stable coprime factorization approach.

Ma, C. C. H.; Vidyasagar, M.

1987-01-01

375

Heavy Stable Isotopes: From Exceptional to Expected  

NASA Astrophysics Data System (ADS)

Less than a decade ago, the stable isotope geochemistry of transition metals and other "heavy" elements was a highly specialized niche confined to a few seemingly exceptional elements. This situation was transformed by the development and refinement of MC-ICP-MS techniques, particularly in the last five years. Measurable stable isotope variations turn out to be ubiquitous across the periodic table, from Li to Hg. It is now safe to assume that the isotopic composition of any element with two or more stable isotopes is measurably variable. What was once exceptional is now expected. Among the first of these new systems to be explored were Fe and Mo isotopes. A number of lessons emerging from this work can be applied to the development of other isotope systems. Most important is that initial expectations are often wrong. For example, based on their environmental chemistries it was expected that redox reactions should produce some of the largest isotope effects for both elements. In the case of Fe, theoretical and experimental studies converge to convincingly indicate that a fractionation of ~ 1.5 ‰/amu occurs between Fe(III) and Fe(II) aquo complexes at equilibrium (e.g., Welch et al., 2003; Anbar et al., 2005). Consistent with these findings, most natural variations of are < 1.5 ‰/amu (e.g., Johnson et al., 2004). This redox-related fractionation is at the heart of emerging interpretations of variations in the isotopic composition of Fe and their application to understanding ancient ocean redox (e.g., Dauphas et al., 2004; Rouxel et al., 2005). In contrast, Mo isotope variations turn out to be controlled only indirectly by redox conditions. Instead, one of the most important Mo isotope effects in the environment appears to be a fractionation of ~ 1 ‰/amu during adsorption of Mo to Mn-oxides (Barling et al., 2001; Siebert et al., 2003). This fractionation has been reproduced in the laboratory (Barling and Anbar, 2004) and appears to be an equilibrium isotope effect unrelated to Mo redox chemistry. Because the extent of Mn-oxide deposition on the seafloor is a function of ocean oxygenation, Mo isotopes are nevertheless emerging as a powerful ocean redox proxy (Arnold et al., 2004). Collectively, these and other findings demonstrate that MC-ICP-MS analyses open the door to rich new research areas, particularly when analyses of natural samples are combined with laboratory experiments and computational predictions to gain insight into fractionation mechanisms.

Anbar, A.

2006-12-01

376

Stable CSR in storage rings: A model  

SciTech Connect

A comprehensive historical view of the work done on coherent synchrotron radiation (CSR) in storage rings is given in reference [1]. Here we want just to point out that even if the issue of CSR in storage rings was already discussed over 50 years ago, it is only recently that a considerable number of observations have been reported. In fact, intense bursts of coherent synchrotron radiation with a stochastic character were measured in the terahertz frequency range, at several synchrotron light source storage rings [2-8]. It has been shown [8-11], that this bursting emission of CSR is associated with a single bunch instability, usually referred as microbunching instability (MBI), driven by the fields of the synchrotron radiation emitted by the bunch itself. Of remarkably different characteristics was the CSR emission observed at BESSY II in Berlin, when the storage ring was tuned into a special low momentum compaction mode [12, 13]. In fact, the emitted radiation was not the quasi-random bursting observed in the other machines, but a powerful and stable flux of broadband CSR in the terahertz range. This was an important result, because it experimentally demonstrated the concrete possibility of constructing a stable broadband source with extremely high power in the terahertz region. Since the publication of the first successful experiment using the ring as a CSR source [14], BESSY II has regular scheduled user s shifts dedicated to CSR experiments. At the present time, several other laboratories are investigating the possibility of a CSR mode of operation [15-17] and a design for a new ring optimized for CSR is at an advanced stage [18]. In what follows, we describe a model that first accounts for the BESSY II observations and then indicates that the special case of BESSY II is actually quite general and typical when relativistic electron storage rings are tuned for short bunches. The model provides a scheme for predicting and optimizing the performance of ring-based CSR sources with a stable broadband photon flux in the terahertz region of up to {approx} 9 orders of magnitude larger than in existing ''conventional'' storage rings. Such a scheme is of interest not only for the design of new sources but also for the evaluation and optimization of the CSR performance in existing electron storage rings. The presented results are mainly based on reference [19].

Sannibale, Fernando; Byrd, John M.; Loftsdottir, Agusta; Venturini, Marco; Abo-Bakr, Michael; Feikes, Jorge; Holldack, Karsten; Kuske, Peter; Wustefeld, Godehart; Hubers, Heinz-Willerm; Warnock, Robert

2005-01-03

377

Stable CSR in Storage Rings: A Model  

SciTech Connect

A comprehensive historical view of the work done on coherent synchrotron radiation (CSR) in storage rings is given in reference [1]. Here we want just to point out that even if the issue of CSR in storage rings was already discussed over 50 years ago, it is only recently that a considerable number of observations have been reported. In fact, intense bursts of coherent synchrotron radiation with a stochastic character were measured in the terahertz frequency range, at several synchrotron light source storage rings [2-8]. It has been shown [8-11], that this bursting emission of CSR is associated with a single bunch instability, usually referred as microbunching instability (MBI), driven by the fields of the synchrotron radiation emitted by the bunch itself. Of remarkably different characteristics was the CSR emission observed at BESSY II in Berlin, when the storage ring was tuned into a special low momentum compaction mode [12, 13]. In fact, the emitted radiation was not the quasi-random bursting observed in the other machines, but a powerful and stable flux of broadband CSR in the terahertz range. This was an important result, because it experimentally demonstrated the concrete possibility of constructing a stable broadband source with extremely high power in the terahertz region. Since the publication of the first successful experiment using the ring as a CSR source [14], BESSY II has regular scheduled user's shifts dedicated to CSR experiments. At the present time, several other laboratories are investigating the possibility of a CSR mode of operation [15-17] and a design for a new ring optimized for CSR is at an advanced stage [18]. In what follows, we describe a model that first accounts for the BESSY II observations and then indicates that the special case of BESSY II is actually quite general and typical when relativistic electron storage rings are tuned for short bunches. The model provides a scheme for predicting and optimizing the performance of ring-based CSR sources with a stable broadband photon flux in the terahertz region of up to {approx}9 orders of magnitude larger than in existing ''conventional'' storage rings. Such a scheme is of interest not only for the design of new sources but also for the evaluation and optimization of the CSR performance in existing electron storage rings. The presented results are mainly based on reference [19].

Sannibale, F.

2005-02-02

378

Influence of horse stable environment on human airways  

Microsoft Academic Search

BACKGROUND: Many people spend considerable amount of time each day in equine stable environments either as employees in the care and training of horses or in leisure activity. However, there are few studies available on how the stable environment affects human airways. This study examined in one horse stable qualitative differences in indoor air during winter and late summer conditions

Lena Elfman; Miia Riihimäki; John Pringle; Robert Wålinder

2009-01-01

379

Stable and radioactive tracers in Ru biokinetic studies  

Microsoft Academic Search

The feasibility of ruthenium metabolism studies by stable tracer administration, with a methodology based on proton nuclear activation, is presented. In order to test that the amount of stable tracer administered does not perturb significantly the mechanism investigated, a series of comparative experiments with administration of both radioactive and stable tracers has been performed on animals. As the most critical

M. C. Cantone; D. De Bartolo; A. Giussani; N. Molho; L. Pirola; G. Gambarini; Ch. Hansen; P. Roth; E. Werner

1994-01-01

380

Modelling of the Evolving Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

A single-column model of the evolving stable boundary layer (SBL) is tested for self-similar properties of the flow and effects of ambient forcing. The turbulence closure of the model is diagnostic, based on the K-theory approach, with a semi-empirical form of the mixing length, and empirical stability functions of the Richardson number. The model results, expressed in terms of local similarity scales, are universal functions, satisfied in the entire SBL. Based on similarity expression, a realizability condition is derived for the minimum allowable turbulent heat flux in the SBL. Numerical experiments show that the development of "horse-shoe" shaped, fixed-elevation hodographs in the interior of the SBL around sunrise is controlled by effects imposed by surface thermal forcing.

Sorbjan, Zbigniew

2014-06-01

381

Electromagnetic moments of quasi-stable baryons  

SciTech Connect

We address electromagnetic properties of quasi-stable baryons in the context of chiral extrapolations of lattice QCD results. For particles near their decay threshold we show that an application of a small external magnetic field changes the particle's energy in a non-analytic way. The conventional electromagnetic moments are only well-defined when the background field B satisfies |eB|/2M{sub *}<<|M{sub *}-M-m| where M{sub *} is the mass of the resonance and M, m the masses of the decay products. A direct application of this situation is the chiral extrapolation of the {Delta}(1232)-isobar electromagnetic properties. We discuss such an extrapolation of the {Delta}(1232)-isobar magnetic dipole, electric quadrupole and magnetic octupole moments by a covariant chiral effective field theory.

Ledwig, T.; Pascalutsa, V.; Vanderhaeghen, M. [Institut fuer Kernphysik, Universitaet Mainz, D-55099 Mainz (Germany); Martin-Camalich, J. [Departamento de Fisica Teorica and IFIC, Universidad de Valencia-CSIC (Spain)

2011-10-24

382

Stable spatial orbits around collinear libration points  

NASA Astrophysics Data System (ADS)

A technique of generation of spatial periodic solutions to the restricted circular three-body problem from periodic orbits of the planar problem has been used for the families of orbits around collinear libration points L 1 and L 2. Developing the families obtained at the 1: 1 resonance, we have obtained stable solutions both in the Earth-Moon system and in the Sun-Earth system. Of course, the term “around the libration point” is rather conventional; the obtained orbits become more similar to the orbits around the smaller attracting body. The further development of the family of orbits “around” the libration point L 2 in the Sun-Earth system made it possible to find the orbits satisfying the new, much more rigorous constraints on cooling the spacecraft of the Millimetron project.

Kreisman, B. B.

2010-06-01

383

Stable elliptical vortices in a circular disk.  

PubMed

We show that large elliptical vortices in a finite disk are stable in a two-dimensional (2D) ideal fluid (this also applies to a column of quasi-2D non-neutral plasma in an axial magnetic field). The stability is established by comparison between the energy of elliptical and symmetrical states to satisfy a sufficient condition, without dynamical eigenanalysis. Analytical small ellipticity expansion of system energy and exact numerical values for arbitrary ellipticity are both obtained for uniform vortices. An approximating calculation is presented for general smooth vortices. Numerical simulations of the 2D Euler equation are also performed. The simulations not only confirm the sufficient condition, but also show that the stability persists to smaller vortex sizes. The reason why decaying l=2 modes were obtained by Briggs, Daugherty, and Levy [Phys. Fluids 13, 421 (1970)] using eigenanalysis is also discussed. PMID:11969957

Chen, P

1999-08-01

384

Air-stable ambipolar organic transistors  

NASA Astrophysics Data System (ADS)

The authors report on ambipolar organic transistors based on the soluble dithiolene derivative (diphenylethylenedithiolato)(1,3-dithiol-2-thione-4,5-dithiolato)nickel [Ni(dpedt)(dmit)]. Due to its small band gap, efficient injection of holes and electrons from gold source/drain electrodes is possible. Both carrier mobilities are estimated to be approximately equal with maximum value on the order of 10-4 cm2/V s. The transistors exhibit excellent ambient stability with a shelve lifetime exceeding 3 months. The pronounced stability of Ni(dpedt)(dmit) as well as of several other molecules studied here is correlated to their redox potential. The present findings can be used as a general guide towards design and synthesis of air-stable ambipolar/n-channel molecules.

Anthopoulos, Thomas D.; Anyfantis, G. C.; Papavassiliou, G. C.; de Leeuw, Dago M.

2007-03-01

385

Stable nonlinear Mach-Zehnder fiber switch  

DOEpatents

An all-optical fiber switch is implemented within a short Mach-Zehnder interferometer configuration. The Mach-Zehnder switch is constructed to have a high temperature stability so as to minimize temperature gradients and other thermal effects which result in undesirable instability at the output of the switch. The Mach-Zehnder switch of the preferred embodiment is advantageously less than 2 cm in length between couplers to be sufficiently short to be thermally stable, and full switching is accomplished by heavily doping one or both of the arms between the couplers so as to provide a highly nonlinear region within one or both of the arms. A pump input source is used to affect the propagation characteristics of one of the arms to control the output coupling ratio of the switch. Because of the high nonlinearity of the pump input arm, low pump powers can be used, thereby alleviating difficulties and high cost associated with high pump input powers.

Digonnet, Michel J. F. (Palo Alto, CA); Shaw, H. John (Palo Alto, CA); Pantell, Richard H. (Menlo Park, CA); Sadowski, Robert W. (Camp Doha, KW)

1999-01-01

386

Stable operating regime for traveling wave devices  

DOEpatents

Autophase stability is provided for a traveling wave device (TWD) electron beam for amplifying an RF electromagnetic wave in walls defining a waveguide for said electromagnetic wave. An off-axis electron beam is generated at a selected energy and has an energy noise inherently arising from electron gun. The off-axis electron beam is introduced into the waveguide. The off-axis electron beam is introduced into the waveguide at a second radius. The waveguide structure is designed to obtain a selected detuning of the electron beam. The off-axis electron beam has a velocity and the second radius to place the electron beam at a selected distance from the walls defining the waveguide, wherein changes in a density of the electron beam due to the RF electromagnetic wave are independent of the energy of the electron beam to provide a concomitant stable operating regime relative to the energy noise.

Carlsten, Bruce E. (Los Alamos, NM)

2000-01-01

387

Towards stable silicon nanoarray hybrid solar cells.  

PubMed

Silicon nanoarray hybrid solar cells benefit from the ease of fabrication and the cost-effectiveness of the hybrid structure, and represent a new research focus towards the utilization of solar energy. However, hybrid solar cells composed of both inorganic and organic components suffer from the notorious stability issue, which has to be tackled before the hybrid solar cells could become a viable alternative for harvesting solar energy. Here we show that Si nanoarray/PEDOT:PSS hybrid solar cells with improved stability can be fabricated via eliminating the water inclusion in the initial formation of the heterojunction between Si nanoarray and PEDOT:PSS. The Si nanoarray hybrid solar cells are stable against rapid degradation in the atmosphere environment for several months without encapsulation. This finding paves the way towards the real-world applications of Si nanoarray hybrid solar cells. PMID:24430057

He, W W; Wu, K J; Wang, K; Shi, T F; Wu, L; Li, S X; Teng, D Y; Ye, C H

2014-01-01

388

Measuring In Vivo Ureagenesis With Stable Isotopes  

PubMed Central

Stable isotopes have been an invaluable adjunct to biomedical research for more than 70 years. Indeed, the isotopic approach has revolutionized our understanding of metabolism, revealing it to be an intensely dynamic process characterized by an unending cycle of synthesis and degradation. Isotopic studies have taught us that the urea cycle is intrinsic to such dynamism, since it affords a capacious mechanism by which to eliminate waste nitrogen when rates of protein degradation (or dietary protein intake) are especially high. Isotopes have enabled an appreciation of the degree to which ureagenesis is compromised in patients with urea cycle defects. Indeed, isotopic studies of urea cycle flux correlate well with the severity of cognitive impairment in these patients. Finally, the use of isotopes affords an ideal tool with which to gauge the efficacy of therapeutic interventions to augment residual flux through the cycle.

Yudkoff, Marc; Mew, Nicholas Ah; Daikhin, Yevgeny; Horyn, Oksana; Nissim, Ilana; Nissim, Itzhak; Payan, Irma; Tuchman, Mendel

2010-01-01

389

Highly stable piezoelectrically tunable optical cavities  

NASA Astrophysics Data System (ADS)

We have implemented highly stable and tunable frequency references using optical high finesse cavities which incorporate a piezo actuator. As piezo material we used ceramic PZT, crystalline quartz, or PZN-PT single crystals. Lasers locked to these cavities show a relative frequency stability better than 1× 10^{-14}, which is most likely not limited by the piezo actuators. The piezo cavities can be electrically tuned over more than one free spectral range (>1.5 GHz) with only a minor decrease in frequency stability. Furthermore, we present a novel cavity design, where the piezo actuator is prestressed between the cavity spacer components. This design features a hermetically sealable intra cavity volume suitable for, e.g., cavity enhanced spectroscopy.

Möhle, Katharina; Kovalchuk, Evgeny V.; Döringshoff, Klaus; Nagel, Moritz; Peters, Achim

2013-05-01

390

Stable isotope `vital effects' in coccolith calcite  

NASA Astrophysics Data System (ADS)

Uncertainties about the origin of the many disequilibrium or 'vital effects' in a variety of calcifying organisms, and whether these effects are constant or variable, have hampered paleoceanographic application of carbon and oxygen isotopic ratios. Unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology. Culture of eight species of coccolithophorids, a dominant marine phytoplankton group, reveals a 5‰ array of disequilibrium or 'vital effects' in both the carbon and oxygen isotopic composition of coccolith calcite. In moderate light and nutrient-replete cultures, oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species. However, when growth rates of a single species are increased or decreased by higher or lower light levels, ? 18O is relatively invariant. Likewise, growth rate variations as a function of temperature do not influence coccolith ? 18O; the slope of the ? 18O vs. temperature relation in cultures of both Gephyrocapsa oceanica and Helicosphaera carteri is the same as for abiogenic carbonates. This suggests a constant, species-specific isotopic fractionation which does not vary with cell physiology. The constancy of vital effects suggests that coccolith stable isotopes will provide reliable phase for paleoceanographic reconstruction of temperature and seawater chemistry, as long as monospecific fractions are analyzed or changes in nannofossil assemblages are accounted for with species-specific correction factors. We suspect that the cell size, and its constraints on the rate of CO 2 diffusion relative to C fixation, may be the first order influence on coccolith stable isotope vital effects. A quantitative model of this process may provide important constraints on mechanisms of carbon acquisition of coccolithophorids in both modern and extinct species.

Ziveri, P.; Stoll, H.; Probert, I.; Klaas, C.; Geisen, M.; Ganssen, G.; Young, J.

2003-05-01

391

M?ssbauer, Electron Paramagnetic Resonance, and Theoretical Study of a Carbene-based All-ferrous Fe4S4 Cluster: Electronic Origin and Structural Identification of the Unique Spectroscopic Site  

PubMed Central

It is well established that the cysteinate-coordinated [Fe4S4] cluster of the Fe-protein of nitrogenase from A. vinelandii (Av2) can attain the all-ferrous core oxidation state. Mössbauer and EPR studies have shown that the all-ferrous cluster has a ground state spin S = 4 and an effective 3:1 site symmetry in the spin structure and 57Fe quadrupole interactions. Recently Deng and Holm reported the synthesis of [Fe4S4(Pri2NHCMe2)4],2 1, (Pri2NHCMe2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) and showed that the all-ferrous carbene-coordinated cluster is amenable to physico–chemical studies. Mössbauer and EPR studies of 1, reported here, reveal that the electronic structure of this complex is strikingly similar to that of the protein-bound cluster, suggesting that the ground state spin and the 3:1 site ratio are consequences of spontaneous distortions of the cluster core. To gain insight into the origin of the peculiar ground state of the all-ferrous clusters in 1 and Av2, we have studied a theoretical model that is based on a Heisenberg–Dirac–Van Vleck Hamiltonian whose exchange-coupling constants are a function of the Fe–Fe distances. By combining the exchange energies with the elastic deformation energies in the harmonic approximation, we obtain for a T2 distortion a minimum with spin S = 4 and a C3v core structure in which one iron is unique and three irons are equivalent. This minimum has all the spectroscopic and structural characteristics of the all-ferrous clusters of 1 and Av2. Our analysis maps the unique spectroscopic iron site to a specific site in the X-ray structure of the [4Fe-4S]0 core both in complex 1 and in Av2.

Chakrabarti, Mrinmoy; Deng, Liang; Holm, R. H.; Munck, Eckard; Bominaar, Emile L.

2009-01-01

392

Stable nuclear transformation of Pandorina morum  

PubMed Central

Background Volvocine green algae like Pandorina morum represent one of the most recent inventions of multicellularity diverged from their unicellular relatives. The 8–16 celled P. morum alga and its close multicellular relatives constitute a model lineage for research into cellular differentiation, morphogenesis and epithelial folding, sexual reproduction and evolution of multicellularity. Pandorina is the largest and most complex organism in the volvocine lineage that still exhibits isogamous sexual reproduction. So far, molecular-biological investigations in P. morum were constricted due to the absence of methods for transformation of this species, which is a prerequisite for introduction of reporter genes and (modified) genes of interest. Results Stable nuclear transformation of P. morum was achieved using chimeric constructs with a selectable marker, a reporter gene, promoters and upstream and downstream flanking sequences from heterologous sources. DNA was introduced into the cells by particle bombardment with plasmid-coated gold particles. The aminoglycoside 3?-phosphotransferase VIII (aphVIII) gene of Streptomyces rimosus under control of an artificial, heterologous promoter was used as the selectable marker. The artificial promoter contained a tandem arrangement of the promoter of both the heat shock protein 70A (hsp70A) and the ribulose-1,5-bisphosphat-carboxylase/-oxygenase S3 (rbcS3) gene of Volvox carteri. Due to the expression of aphVIII, transformants gained up to 333-fold higher resistance to paromomycin in comparison to the parent wild-type strain. The heterologous luciferase (gluc) gene of Gaussia princeps, which was previously genetically engineered to match the nuclear codon usage of Chlamydomonas reinhardtii, was used as a co-transformed, unselectable reporter gene. The expression of the co-bombarded gluc gene in transformants and the induction of gluc by heat shock were demonstrated through bioluminescence assays. Conclusion Stable nuclear transformation of P. morum using the particle bombardment technique is now feasible. Functional expression of heterologous genes is achieved using heterologous flanking sequences from Volvox carteri and Chlamydomonas reinhardtii. The aphVIII gene of the actinobacterium S. rimosus can be used as a selectable marker for transformation experiments in the green alga P. morum. The gluc gene of the marine copepod G. princeps, expressed under control of heterologous promoter elements, represents a suitable reporter gene for monitoring gene expression or for other applications in P. morum.

2014-01-01

393

Magnetic Properties of 1:2 Mixed Cobalt(II) Salicylaldehyde Schiff-Base Complexes with Pyridine Ligands Carrying High-Spin Carbenes (Scar = 2/2, 4/2, 6/2, and 8/2) in Dilute Frozen Solutions: Role of Organic Spin in Heterospin Single-Molecule Magnets.  

PubMed

The 1:2 mixtures of Co(p-tolsal)2, p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 3l, 3b, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)2(CXpy)2, with Stotal = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)2(CXpy)2, X = 1, 2, 3l, 3b, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, Ueff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, Hc, of ?0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, ?Q, due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)2(C1py)2, ?2.0 × 10(3) s for Co(p-tolsal)2(C2py)2, and >10(5) s for Co(p-tolsal)2(CXpy)2; X = 3b, 3l, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the ?Q value increased with increasing the Stotal values. PMID:24816331

Karasawa, Satoru; Nakano, Kimihiro; Yoshihara, Daisuke; Yamamoto, Noriko; Tanokashira, Jun-Ichi; Yoshizaki, Takahito; Inagaki, Yuji; Koga, Noboru

2014-06-01

394

Coexisting stable conformations of gaseous protein ions.  

PubMed Central

For further insight into the role of solvent in protein conformer stabilization, the structural and dynamic properties of protein ions in vacuo have been probed by hydrogen-deuterium exchange in a Fourier-transform mass spectrometer. Multiply charged ions generated by electrospray ionization of five proteins show exchange reactions with 2H2O at 10(-7) torr (1 torr = 133.3 Pa) exhibiting pseudo-first-order kinetics. Gas-phase compactness of the S-S cross-linked RNase A relative to denatured S-derivatized RNase A is indicated by exchange of 35 and 135 hydrogen atoms, respectively. For pure cytochrome c ions, the existence of at least three distinct gaseous conformers is indicated by the substantially different values--52, 113, and 74--of reactive H atoms; the observation of these same values for ions of a number--2, 7, and 5, respectively--of different charge states indicates conformational insensitivity to coulombic forces. For each of these conformers, the compactness in vacuo indicated by these values corresponds directly to that of a known conformer structure in the solution from which the conformer ions are produced by electrospray. S-derivatized RNase A ions also exist as at least two gaseous conformers exchanging 50-140 H atoms. Gaseous conformer ions are isometrically stable for hours; removal of solvent greatly increases conformational rigidity. More specific ion-molecule reactions could provide further details of conformer structures. Images

Suckau, D; Shi, Y; Beu, S C; Senko, M W; Quinn, J P; Wampler, F M; McLafferty, F W

1993-01-01

395

Novel risk factors related to stable angina.  

PubMed

Stable angina (SA) pectoris is a common and disabling disorder in patients with coronary artery disease (CAD), with increasing epidemiology and is associated with myocardial infarction and increased mortality. However, within the population of SA patients, an individual's prognosis can vary considerably. Except from conventional risk factors a variety of biomarkers have been evaluated for their prognostic significance in the settings of SA. Novel biomarkers associated with inflammatory status, such as C reactive protein and tumor necrosis factor alpha, with myocardial performance, such as B-type natriuretic peptide, with extracellular matrix remodeling, with vascular calcification such as osteoprotogerin and osteopontin, with myocardial ischemia, such as ischemia modified albumin have been associated with the progression of CAD and with the prognosis of SA patients. Despite the multiplicity of novel biomarkers there is lack of a clinical useful, highly specific for CAD biomarker with the ability to guide treatment decisions. In the context of this evidence in this review article we summarize the so far acquired knowledge of the most promising biomarkers and we discuss the major clinical correlations of novel risk factors with SA physical history, their predictive value for future cardiovascular events and their use in the treatment monitoring of this population. PMID:23016718

Siasos, Gerasimos; Tousoulis, Dimitris; Athanasiou, Dimitrios; Oikonomou, Evangelos; Tourikis, Panagiotis; Gouliopoulos, Nikolaos; Limperi, Maria; Kampoli, Anna-Maria; Toutouzas, Kostas; Papavassiliou, Athanasios G; Stefanadis, Christodoulos

2013-01-01

396

Three new stable L5 Mars Trojans  

NASA Astrophysics Data System (ADS)

Mars was second to Jupiter in being recognized as the host of a population of Trojan minor bodies. Since 1990, five asteroids - 5261 Eureka, (101429) 1998 VF31, (121514) 1999 UJ7, 2001 DH47 and (311999) 2007 NS2 - have been identified as Mars Trojans, one L4 and four L5. Dynamical and spectroscopic evidence suggests that some Mars Trojans may be remnants of the original planetesimal population that formed in the terrestrial planets region. Here, we revisit the long-term dynamical evolution of the previously known Mars Trojans and show that 2011 SC191, 2011 SL25 and 2011 UN63 are also trailing (L5) Mars Trojans. They appear to be as stable as Eureka and may have been Trojans over the age of the Solar system. The fact that five Trojans move in similar orbits and one of them is a binary may point to the disruption of a larger body early in the history of the Solar system. Such a catastrophic event may also explain the apparently strong asymmetry in terms of number of objects (one versus seven) between the L4 and L5 regions. Future spectroscopic observations should be able to reject or confirm a putative common chemical signature that may lend further support to a collisional scenario.

de la Fuente Marcos, C.; de la Fuente Marcos, R.

2013-05-01

397

Stable Isotope Database: present and past archives  

NASA Astrophysics Data System (ADS)

Paleoclimate data provide benchmarks against which the realism of the processes simulated by climate models can be assessed. Within this framework, it is essential to avoid introducing uncertainties associated with transfer functions and therefore to operate with robust proxies. The implementation of stable isotopes of water or carbon inside climate models motivates a synthesis of available data. Supported by the LABEX L-IPSL and involving a team of climate modelers and paleoclimatologists, this project aims to establish a worldwide database of ?18O, ?D ?17O and ?13C from oceanic microfossils, corals, ice cores, cave speleothems, lakes, tree rings, and vegetation leaves wax. The aim is to provide a global vision of the hydrological cycle during the LGM and other selected key periods (last 2000 years, Mid-Holocene, Dansgaard-Oeschger events, and the Eemian). It requires screening through hundreds of published oceanic and continental records, validating the selection of the data based on resolution and chronological information. We extracted ~900 dated ?18O records from 650 marine sediment cores, 65 ?18O records from 50 ice cores, ~200 ?18O speleothems records from 60 caves, and 540 ?13C records from 290 marine sediment cores. An additional aspect of this project consists in the construction of an online portal providing an intuitive and interactive platform allowing selecting, visualizing, and downloading of the records included in this database, thus improving the distribution and comparison of paleoclimatic records from various sites.

Bolliet, Timothé

2014-05-01

398

Arthroscopic medial meniscectomy on stable knees.  

PubMed

We reviewed 74 partial medial meniscectomies in 57 patients with stable knees, to assess the long-term functional and radiological outcome. The International Knee Documentation Committee score and the residual laxity were assessed in both knees. At the time of surgery the mean age of the patients was 36 +/- 11 years and the mean follow-up was 12 +/- 1 years. All had a limited medial meniscectomy. The anterior cruciate ligament was intact in all cases. The meniscal tear was vertical in 95% and complex in 5%. The posterior part of the meniscus was removed in 99%. A peripheral rim was preserved in all cases. After 12 years 95% of the patients were satisfied or very satisfied with their knee(s). Objectively, 57% had grade A function and 43% were grade B. The outcome correlated only with the presence of anterior knee pain at final follow-up. In the 49 cases of arthroscopic meniscectomy for which there was a contralateral normal knee there was narrowing of the 'joint-space' in 16% of the operated knees. There was no correlation between this and other parameters such as age or different meniscal pathologies. PMID:11245533

Hulet, C H; Locker, B G; Schiltz, D; Texier, A; Tallier, E; Vielpeau, C H

2001-01-01

399

Microsatellites grant more stable flanking genes  

PubMed Central

Background Microsatellites, or simple sequence repeats (SSRs), are DNA sequences that include tandem copies of specific sequences no longer than six bases. SSRs are ubiquitous in all genomes and highly mutable. Presentation of the hypothesis Results from previous studies suggest that flanking regions of SSR are exhibit high stability in a wide range of organisms. We hypothesized that the SSRs ability to discard weak DNA polymerases could be responsible for this unusual stability. . When the weak polymerases are being decayed over SSRs, the flanking sequences would have higher opportunity to be replicated by more stable DNA polymerases. We present evidence of the molecular basis of our hypothesis. Testing the hypothesis The hypothesis could be tested by examining the activity of DNA polymerase during and after a number of PCRs. The PCR reactions should be run with the same SSR locus possessing differences in the SSR length. The hypothesis could also be tested by comparing the mutational rate of a transferred gene between two transformations. The first one has a naked T-DNA (transferred DNA), while the second one has the same T-DNA flanked with two SSRs. Implications of the hypothesis In any transformation experiment, flanking the T-DNA fragment with SSR sequences would result in more stably transferred genes. This process would decrease the unpredictable risks that may occur because of the mutational pressure on this foreign segment.

2012-01-01

400

Organized polysaccharide fibers as stable drug carriers  

PubMed Central

Many challenges arise during the development of new drug carrier systems, and paramount among them are safety, solubility and controlled release requirements. Although synthetic polymers are effective, the possibility of side effects imposes restrictions on their acceptable use and dose limits. Thus, a new drug carrier system that is safe to handle and free from side effects is very much in need and food grade polysaccharides stand tall as worthy alternatives. Herein, we demonstrate for the first time the feasibility of sodium iota-carrageenan fibers and their distinctive water pockets to embed and release a wide variety of drug molecules. Structural analysis has revealed the existence of crystalline network in the fibers even after encapsulating the drug molecules, and iota-carrageenan maintains its characteristic and reproducible double helical structure suggesting that the composites thus produced are reminiscent of cocrystals. The melting properties of iota-carrageenan:drug complexes are distinctly different from those of either drug or iota-carrageenan fiber. The encapsulated drugs are released in a sustained manner from the fiber matrix. Overall, our research provides an elegant opportunity for developing effective drug carriers with stable network toward enhancing and/or controlling bioavailability and extending shelf-life of drug molecules using GRAS excipients, food polysaccharides, that are inexpensive and non–toxic.

Janaswamy, Srinivas; Gill, Kristin L.; Campanella, Osvaldo H.; Pinal, Rodolfo

2013-01-01

401

On the Stable Ground State of Mackinawite  

NASA Astrophysics Data System (ADS)

Mackinawite is a layer type iron monosulfide (FeS) with stacked sheets of edge-sharing FeS4 tetrahedra. An important player in iron and sulfur cycles, mackinawite is one of the first-formed metastable iron sulfides in anoxic environments, transforming into greigite (Fe3S4) and pyrite (FeS2) minerals or elemental sulfur (S0) and iron (Fe0) depending on redox conditions. Mackinawite also affects the mobility and oxidation states of toxic metals such as As, Hg, and Se. The mineral, typically found as a nanoparticle, has been characterized experimentally. Its fundamental conducting and magnetic properties, however, are still controversial; e.g., whether mackinawite is metallic and whether it has magnetic order. Mackinawite is believed to be metallic and without magnetic ordering down at 4 K based on Mössbauer spectroscopy studies. We examined these two issues by applying plane-wave density functional theory (DFT) to FeS geometry optimization under different magnetic orderings. We found that antiferromagnetic ordering among the Fe atoms is the stable ground state of mackinawite. In this presentation, we shall discuss this result and how it relates to previous experimental work.

Kwon, K.; Refson, K.; Sposito, G.

2009-12-01

402

Thermoremanence and Stable Memory in Multidomain Hematite  

NASA Astrophysics Data System (ADS)

Thermal demagnetization of thermoremanence (TRM) imparted parallel to the (0001) basal plane of a large natural single crystal of hematite revealed very high unblocking temperatures. Most demagnetization occurs between 670 C and the Neel temperature. In contrast, the decay curve of 1-mT TRM drops exponentially with increasing alternating field, characteristic multidomain (MD) behaviour. Such exponential decay is due to hematite's weak spontaneous magnetization and self-demagnetizating field. Domain walls in MD hematite move almost unhindered to their limiting positions and the TRM intensity approaches to the saturation remanence. This also explains the observed high Mtrm = 1.1 kA/m. The TRM memory recovered after low-temperature cycling (LTC) is parallel to the original TRM and equally resistant to thermal demagnetization. The surviving thermoremanence after LTC is about 30 percent of the original remanence, very similar to single-domain hematites with grain sizes 0.12-0.42 micrometer. Stable TRM and memory with very high unblocking temperatures are due to hard spin-canted ferromagnetism, not to defect ferromagnetism. However, the defect moment seems to couple to the spin-canted moment and renucleate memory in warming through the Morin transition.

Ozdemir, O.

2003-12-01

403

Stable linkage disequilibrium owing to sexual antagonism.  

PubMed

Linkage disequilibrium (LD) is an association between genetic loci that is typically transient. Here, we identify a previously overlooked cause of stable LD that may be pervasive: sexual antagonism. This form of selection produces unequal allele frequencies in males and females each generation, which upon admixture at fertilization give rise to an excess of haplotypes that couple male-beneficial with male-beneficial and female-beneficial with female-beneficial alleles. Under sexual antagonism, LD is obtained for all recombination frequencies in the absence of epistasis. The extent of LD is highest at low recombination and for stronger selection. We provide a partition of the total LD into distinct components and compare our result for sexual antagonism with Li and Nei's model of LD owing to population subdivision. Given the frequent observation of sexually antagonistic selection in natural populations and the number of traits that are often involved, these results suggest a major contribution of sexual antagonism to genomic structure. PMID:20861051

Ubeda, Francisco; Haig, David; Patten, Manus M

2011-03-22

404

Apolipoprotein metabolism: a stable-isotope approach.  

PubMed

Lipids are the major fuel of the body. Efficient functioning as an energy source dictates that lipids must be transportable in the plasma from the point of synthesis and assembly to the storage depots and finally to the tissues to provide energy through oxidative metabolism. Complex lipid forms are transported through the plasma as lipoprotein particles. These particles, secreted from the intestine and liver, have nonpolar outer surface composed of cholesterol, phospholipids, and apolipoproteins. Apolipoproteins are essential for the production, secretion, and continued structural integrity of the various lipoprotein particles and thus play a pivotal role in the control mechanisms of lipid transport. Apolipoproteins regulate specific enzyme activities and modulate plasma lipoprotein clearance through receptor-mediated processes. Quantitative information regarding the rates of synthesis and catabolism of apolipoproteins is vital to an understanding of their metabolism in health and disease. General considerations are followed by a specific use of stable-isotope methodology to quantitative the rate of synthesis of very-low-density-lipoprotein apolipoprotein B-100 (VLDL apo B-100) in control and familial-combined-hyperlipidemic (FCHL) patients. PMID:8475890

Halliday, D; Venkatesan, S; Pacy, P

1993-05-01

405

Structurally stable, thin silicon solar cells  

NASA Technical Reports Server (NTRS)

A fabrication process for structurally stable thin solar cell wafers that produce good power output after irradiation is described. The fabrication process is as follows. A 6 mil, circular wafer is oxidized on both sides. One side is then patterned with a rectangular array of holes in the oxide that are nominally 75 mils square and separated by 2 mil spacings. Wells are then etched into the silicon with KOH to a depth of 4 mils, leaving a 2 mil, unetched thickness. Two areas on the surface are left unetched to provide pads for bonding or testing. All oxide is then removed and the rest of the processing is normal; the unetched face is used as the illuminated face. When all other processing is complete, a 2 X 2 cm cell is sawed from the starting wafer leaving a border that is approximately 10 mils wide. The effective thickness, determined by weighing an unmetallized sample, of such a cell is about 2.4 mil.

Arndt, R. A.; Meulenberg, A.

1984-01-01

406

Magneto-hydrodynamically stable axisymmetric mirrors  

SciTech Connect

Making axisymmetric mirrors magnetohydrodynamically (MHD) stable opens up exciting opportunities for using mirror devices as neutron sources, fusion-fission hybrids, and pure-fusion reactors. This is also of interest from a general physics standpoint (as it seemingly contradicts well-established criteria of curvature-driven instabilities). The axial symmetry allows for much simpler and more reliable designs of mirror-based fusion facilities than the well-known quadrupole mirror configurations. In this tutorial, after a summary of classical results, several techniques for achieving MHD stabilization of the axisymmetric mirrors are considered, in particular: (1) employing the favorable field-line curvature in the end tanks; (2) using the line-tying effect; (3) controlling the radial potential distribution; (4) imposing a divertor configuration on the solenoidal magnetic field; and (5) affecting the plasma dynamics by the ponderomotive force. Some illuminative theoretical approaches for understanding axisymmetric mirror stability are described. The applicability of the various stabilization techniques to axisymmetric mirrors as neutron sources, hybrids, and pure-fusion reactors are discussed; and the constraints on the plasma parameters are formulated.

Ryutov, D. D.; Cohen, B. I.; Molvik, A. W. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); Berk, H. L. [University of Texas, Austin, Texas 78712 (United States); Simonen, T. C. [University of California, Berkeley, California 94720 (United States)

2011-09-15

407

Stable Isotope Spectroscopy for Diagnostic Medicine  

NASA Astrophysics Data System (ADS)

Isotopic tracers have been used in medical research for more than fifty years. Radioactive isotopes have been most used because of the high detection efficiencies possible. With increased awareness of the effects of low level radiation and radioactive waste management problems, the need for safe non radioactive tracers has become apparent. Rare stable isotopes of biologically active elements can be used for metabolic and pharmacokinetic studies provided that both sufficient detection sensitivity can be achieved and reliable cost effective instruments can be developed. High resolution optical spectroscopic methods which can determine isotopic ratios with high precision and accuracy are viable for research and clinical use. The study of 13C/12C ratios in CO2 for breath test diagnostics will be described in detail. Using the laser optogalvonic effect with isotopic lasers a specific medical diagnostic for h-pylori infection, has recently received FDA approval. Opportunities exist to study D/H ratios in water and 18O/16O ratios in CO2 and water for basic metabolism diagnostics and 15N/14N ratios in urine for liver function and related studies.

Murnick, D. E.

2000-06-01

408

Embryotoxicity of stable isotopes and use of stable isotopes in studies of teratogenetic mechanisms  

SciTech Connect

Experiments on teratogenic effects of stable isotopes from our own and other laboratories are evaluated. In the first series of investigations, the enrichment of the stable isotope /sup 13/C derived from U-/sup 13/C-glucose was studied in mouse embryos at various stages of development, including limb buds in organ culture. Preimplantation mouse embryos incubated in vitro in /sup 13/C-enriched medium for 48 hours showed normal development during subsequent differentiation in vitro and also in vivo after embryo transfer to faster mothers. These embryos were 15% to 20% enriched in /sup 13/C. Administration of U-13-C-glucose to pregnant mice during organogenesis led to an increase of the absolute /sup 13/C content of the embryo for several days after the end of isotope administration, whereas the enrichment in maternal tissue decreased. No alterations of embryonic development were detected due to stable isotope enrichment. Development of cultured mouse limb buds was unaffected by incubation with 82 mol% U-/sup 13/C-glucose as judged from morphologic and biochemical criteria. The second part of the article describes the value of deuterium-labeled drugs as probes into the mechanism of activation of teratogenic metabolites. A comparison of the pharmacokinetics as well as the teratogenicity between cyclophosphamide and some specific deuterium-labeled analogues showed that the isotope effect observed can be related to a particular metabolic pathway crucial for teratogenic activation by this drug.

Spielmann, H.; Nau, H.

1986-07-01

409

Stable Vanadium Isotope Fractionation at High Temperatures  

NASA Astrophysics Data System (ADS)

Vanadium is a redox sensitive transition metal existing in multiple valence states at terrestrial conditions. Stable vanadium isotopes (reported as ?51V in % relative to an Alfa Aesar standard [1]) are a potentially powerful tracer of oxidation-reduction processes. However, the determination of ?51V is analytically challenging, primarily due to the extreme abundance ratio between the only two stable isotopes (51V/50V ~ 400) and, also, significant isobaric interferences of 50Ti and 50Cr on the minor 50V isotope. We have developed the first method able to determine ?51V to a precision (2 s.d. ~ 0.15%, [1,2]) that enables application of this isotope system to geological processes. To usefully investigate high temperature processes using vanadium isotopes, knowledge of the isotope composition and range of values present in the ambient mantle is required. Here we discuss the first ?51V measured in igneous materials encompassing peridotites, MORB, and primitive mantle-derived melts such as picrites. This first dataset provides a preliminary reconnaissance of the magnitude of natural fractionation. We find little isotope fractionation in suites of peridotites and MORB (< 0.5 %). However, the small but analytically significant variation appears to be related to secondary processes, with extremely altered peridotites consistently displaying slightly heavier isotope compositions. We find no resolvable ?51V variation between fresh MORB glass and fresh peridotite. Intriguingly, a suite of subduction-related peridotites from the Mariana forearc, previously characterized for fO2 [3], do not display the predicted co-variation between ?51V and fO2, but instead also have compositions identical to MORB glass. This nominally supports recent indications that there is limited difference in the oxygen fugacity of the MORB source and the subarc mantle wedge [e.g., 4, 5]. Finally, we observe large ?51V variations (~ 2 %) in a suite of evolving lavas from Hekla volcano, Iceland, which produces eruptive material with a wide range of SiO2 from a cogenetic source. The ?51V of Hekla lavas are well correlated with indices of differentiation such as MgO and SiO2, suggesting that processes such as fractional crystallization may be a fundamental cause of high temperature ?51V fractionation. This nascent dataset provides a first glimpse into the magnitude of vanadium isotope fractionation that may be expected at high temperatures. The presence of significant isotope variation outside of analytical precision in these materials bodes well for the use of ?51V to address a variety of broad scale questions in high temperature planetary processes. [1] Nielsen, S.G., Prytulak, J., Halliday, A.N. 2011. Geost. Geoanal. Res., in press. [2] Prytulak, J., Nielsen, S.G., Halliday, A.N. 2011. Geost. Geoanal. Res., in press. [3] Parkinson and Pearce, 1998. Journal of Petrology, 39, 1577-1618. [4] Lee et al., 2005. Journal of Petrology, 46, 2313-2336. [5] Cottrell and Kelley, 2011. Earth and Planetary Sciences Letters, 305, 270-282.

Prytulak, J.; Parkinson, I. J.; Savage, P. S.; Nielsen, S. G.; Halliday, A. N.

2011-12-01

410

A? dimers rapidly form stable synaptotoxic protofibrils  

PubMed Central

Non-fibrillar, water-soluble low-molecular weight assemblies of the amyloid ?-protein (A?) are believed to play an important role in Alzheimer’s disease (AD). Aqueous extracts of human brain contain A? assemblies which migrate on SDS-PAGE and elute from size exclusion as dimers (~8 kDa) and can block long term potentiation and impair memory consolidation in the rat. Such species are detected specifically and sensitively in extracts of Alzheimer brain suggesting that SDS-stable dimers may be the basic building blocks of AD-associated synaptotoxic assemblies. Consequently, understanding the structure and properties of A? dimers is of great interest. In the absence of sufficient brain-derived dimer to facilitate biophysical analysis, we generated synthetic dimers designed to mimic the natural species. For this, A?(1-40) containing cysteine in place of serine 26 was used to produce disulphide cross-linked dimer, (A?S26C)2. Such dimers had no detectable secondary structure, produced an analytical ultracentrifugation (AUC) profile consistent for an ~8.6 kDa protein, and had no effect on hippocampal long term potentiation (LTP). However, (A?S26C)2 aggregated more rapidly than either A?S26C or wild type monomers and formed parastable ?-sheet rich, thioflavin T positive, protofibril-like assemblies. Whereas wild type A? aggregated to form typical amyloid fibrils, the protofibril-like structures formed by (A?S26C)2 persisted for prolonged periods and potently inhibited LTP in mouse hippocampus. These data support the idea that A? dimers may stabilize the formation of fibril intermediates by a process distinct from that available to A? monomer and that higher molecular weight pre-fibrillar assemblies are the proximate mediators of A? toxicity.

O'Nuallain, Brian; Freir, Darragh B.; Nicoll, Andrew J.; Risse, Emmanuel; Ferguson, Neil; Herron, Caroline E.; Collinge, John; Walsh, Dominic M.

2010-01-01

411

Reactions of the allenylidenes trans-[IrCl[=C=C=C(ph)R](PiPr3)2] with electrophiles: generation of butatriene-, carbene-, and carbyne- iridium complexes.  

PubMed

The allenylidenciridium(I) complexes trans-[IrX(=C=C-CPh2)(PiPr3)2] (X = Cl: 1; X = I: 2) react with excess methyl iodide by C-C coupling and elimination of HI to give the eta2-butatriene compounds trans-[IrX-(eta2-CH2=C=C=CPh2)(PiPr3)2] (3, 4), of which 3 (X = Cl) was characterized by X-ray crystallography. Treatment of 1 and 5 (containing C=C=C(Ph)tBu as the allenylidene ligand) with HCI leads to the formation of the six-coordinate hydridoiridium(III) complexes [IrHCl2[= C=C=C(Ph)R](PiPr3)2] (6, 7) by oxidative addition at the metal center. In contrast, the reactions of 1 and 5 with both CF3CO2H and CF3SO3H afford the four-coordinate vinylcarbene compounds trans-[IrCl[=C(X)-CH=C(Ph)R[(PiPr3)2] (8-10). For X= CF3CO2, in nitromethane a dissociation of the C-X bond occurs and the cationic iridium carbynes trans-[IrCl[=C-CH=C(Ph)R](PiPr3)2]+ are generated. Upon addition of NaBPh4, the stable carbyne complexes 11b (R= Ph) and 12b (R = tBu) with BPh4 as the counterion were isolated in almost quantitative yields. The X-ray crystal structure analysis of 6 reveals that the chloro ligands are cis and the phosphane ligands trans disposed. PMID:11757655

Ilg, K; Werner, H

2001-11-01

412

Stable grand-unified monopoles with multiple Dirac charge  

NASA Astrophysics Data System (ADS)

Magnetic monopoles with multiple Dirac charge are found to be stable in grand unified theories with symmetry breaking by an adjoint Higgs field under certain conditions. In the SU(5) model, the double, triple, quadruple, and sextuple monopoles are stable for a range of parameters in the Higgs potential. The effects of electroweak symmetry breaking on multiply charged monopoles are discussed. Evidence is also presented for the existence of a stable nonspherically symmetric quadruple monopole.

Gardner, C. L.; Harvey, J. A.

1984-03-01

413

HUNT STABLE COMPLEX LOOKING NORTHWEST TOWARD DOG KENNEL RUINS ACROSS ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

HUNT STABLE COMPLEX LOOKING NORTHWEST TOWARD DOG KENNEL RUINS ACROSS THE GREAT CIRCUS - Overhills, Fort Bragg Military Reservation, Approximately 15 miles NW of Fayetteville, Overhills, Harnett County, NC

414

Efficient Nonlinear Conversion of Frequency Stable Lasers.  

NASA Astrophysics Data System (ADS)

Diode-laser-pumped monolithic Nd:YAG nonplanar ring oscillators produce frequency-stable single-mode output at 1.064 mum. Efficient nonlinear conversion of this output is demonstrated using field enhancement techniques. The use of low loss MgO:LiNbO_3 and novel monolithic cavity designs, where the mirrors are deposited directly on the crystal, has allowed 56% efficient cw second harmonic generation and greater than 60% efficient parametric oscillation. A theory of second harmonic generation using an external cavity resonant at the fundamental is presented that extends previous work to include conversion of the resonated wave to the harmonic field. An impedance-matched 12.5 mm long monolithic MgO:LiNbO_3 ring cavity doubler is used to produce 29.7 mW of cw, single -axial-mode 532-nm radiation from an input of 52.6 mW for a conversion efficiency of 56%. A monolithic MgO:LiNbO_3 doubly resonant optical parametric oscillator (DRO) was operated near degeneracy, pumped by the output of the resonant doubler with the laser producing relaxation oscillation pulses. Pump depletions for the DRO of greater than 60% were observed when pumped with peak powers six times above the calculated 40-mW cw threshold. The DRO output was temperature tuned from 1.01 ?m to 1.13 mum, producing single axial mode output over much of the range. A monolithic MgO:LiNbO_3 singly resonant optical parametric oscillator (SRO) was pumped as both a standing wave and a ring geometry resonator by a long-pulse amplified and frequency-doubled laser. Pump depletions of greater than 60% were observed when pumping four times above the calculated 35-Watt threshold. The SRO output at the resonant signal tuned with temperature from 834 nm to 958 nm while the corresponding idler tuned from 1.47 ?m to 1.2 mu m.

Kozlovsky, William Joseph

415

Frequency distribution of mechanically stable disk packings  

SciTech Connect

Relative frequencies of mechanically stable (MS) packings of frictionless bidisperse disks are studied numerically in small systems. The packings are created by successively compressing or decompressing a system of soft purely repulsive disks, followed by energy minimization, until only infinitesimal particle overlaps remain. For systems of up to 14 particles, most of the MS packings were generated. We find that the packings are not equally probable as has been assumed in recent thermodynamic descriptions of granular systems. Instead, the frequency distribution, averaged over each packing-fraction interval {delta}{phi}, grows exponentially with increasing {phi}. Moreover, within each packing-fraction interval, MS packings occur with frequencies f{sub k} that differ by many orders of magnitude. Also, key features of the frequency distribution do not change when we significantly alter the packing-generation algorithm; for example, frequent packings remain frequent and rare ones remain rare. These results indicate that the frequency distribution of MS packings is strongly influenced by geometrical properties of the multidimensional configuration space. By adding thermal fluctuations to a set of the MS packings, we were able to examine a number of local features of configuration space near each packing. We measured the time required for a given packing to break to a distinct one, which enabled us to estimate the energy barriers that separate one packing from another. We found a gross positive correlation between the packing frequencies and the heights of the lowest energy barriers {epsilon}{sub 0}; however, there is significant scatter in the data. We also examined displacement fluctuations away from the MS packings to assess the size and shape of the local basins near each packing. The displacement modes scale as d{sub i}{approx}{epsilon}{sub 0}{sup {gamma}{sub i}} with {gamma}{sub i} ranging from {approx}0.6 for the largest eigenvalues to 1.0 for the smallest ones. These scalings suggest that the packing frequencies are not determined by the local volume of configuration space near each packing, which would require that the dependence of f{sub k} on {epsilon}{sub 0} is much stronger than the dependence we observe. The scatter in our data implies that in addition to {epsilon}{sub 0} there are also other, as yet undetermined variables that influence the packing probabilities.

Gao Guojie; Blawzdziewicz, Jerzy [Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06520-8284 (United States); O'Hern, Corey S. [Department of Mechanical Engineering, Yale University, New Haven, Connecticut 06520-8284 (United States); Department of Physics, Yale University, New Haven, Connecticut 06520-8120 (United States)

2006-12-15

416

Unsymmetric Ru(II) complexes with N-heterocyclic carbene and/or terpyridine ligands: synthesis, characterization, ground- and excited-state electronic structures and their application for DSSC sensitizers.  

PubMed

Three ruthenium(II) complexes with N-heterocyclic carbene (NHC) or NHC/2,2':6',2''-terpyridine (tpy) hybrid ligands, bis[2,6-bis(3-methylimidazol-3-ium-1-yl)pyridine-4-carboxylic acid]ruthenium(II) (BCN), [2,6-bis(3-methylimidazolium-1-yl)pyridine-4-carboxylic acid](2,2';6'2''-terpyridine)ruthenium(II) (TCN), and [2,6-bis(3-methylimidazol-3-ium-1-yl)pyridine](2,2';6'2''-terpyridine-4'-carboxylic acid)ruthenium(II) (CTN), have been synthesized and characterized by (1)H and (13)C NMR, high-resolution mass spectrometry, and elemental analysis. The molecular geometry of the TCN complex was determined by X-ray crystallography. Electronic absorption spectra of these complexes exhibit typical pi-pi* and metal-to-ligand charge transfer bands in the UV and visible regions, respectively. The lowest energy absorption maxima were 430, 448, and 463 nm with molar extinction coefficients of 28,100, 15,400, and 7400 M(-1)cm(-1) for BCN, TCN, and CTN, respectively. Voltammetric data suggest that energy levels of the highest occupied molecular orbitals (HOMOs) of the three complexes reside within a 10 meV window despite the varying degrees of electronic effect of the constituent ligands. The electronic structures of these complexes calculated via density functional theory (DFT) indicate that the three HOMOs and the three lowest unoccupied MOs (LUMOs) are metal and ligand centered in character, for the former and the latter, respectively. Time-dependent DFT (TD-DFT) calculation predicts that the lowest energy absorption bands of each complex are comprised of multiple one-electron excitations. TD-DFT calculation also suggests that the background of spectral red shift stems most likely from the stabilization of unoccupied MOs rather than the destabilization of occupied MOs. The overall efficiencies of the dye-sensitized solar cell systems of these complexes were found to be 0.48, 0.14, and 0.10% for BCN, TCN, and CTN, respectively, while that of a commercial bis(4,4'-dicarboxylato-2,2'-bipyridine)-bis(isothiocyanoto)ruthenium(II) (N719) system was 6.34%. PMID:20690744

Park, Hee-Jun; Kim, Kyeong Ha; Choi, Soo Young; Kim, Hyeong-Mook; Lee, Wan In; Kang, Youn K; Chung, Young Keun

2010-08-16

417

Convenient synthesis of stable deuterium-labeled alkylpyrazines for use in stable isotope dilution assays.  

PubMed

Stable isotope dilution assays (SIDA) provide for accurate and precise quantitation of aroma components, such as alkylpyrazines, which are often present in low concentrations in complex food matrices. The unavailability of labeled standards is the main limitation to the widespread use of SIDA. This study describes the chlorination of several alkylpyrazines to form the corresponding chloroalkylpyrazine compounds, which are efficient starting materials for the synthesis of deuterium-labeled alkylpyrazines, namely [²H?]-2-methylpyrazine (d-1), [²H?]-2-ethylpyrazine (d-2), [²H?]-2,3(or 6)-dimethylpyrazine (d-3A, d-3B), [²H?]-2,[²H?]-6-dimethylpyrazine (d-3C), [²H?]-2,[²H?]-6-diethylpyrazine (d-4), [²H?]-2-ethyl-3(or 6)-methylpyrazine (d-5A, d-5B), 2,[²H?]-3,5-trimethylpyrazine (d-6), [²H?]-2-ethyl-3,6-dimethylpyrazine (d-7), [²H?]-2-ethyl-3,5-dimethylpyrazine (d-8), and 2,3-diethyl-[²H?]-5-methylpyrazine (d-9), which were obtained in good yields (57-100%) and high purities (86-98%). These stable isotopes were used as internal standards in SIDA to accurately and precisely determine selected alkylpyrazines in commercial peanut butter, cocoa powder, and instant coffee. 2,3-Diethyl-5-methylpyrazine (p-9) and 2-ethyl-3,5-dimethylpyrazine (p-8), despite their low abundance, had the highest odor-active values among the 13 pyrazines quantified in all products due to their very low odor thresholds. PMID:23528050

Fang, Mingchih; Cadwallader, Keith R

2013-04-17

418

Absolute continuity of stable foliations for systems on Banach spaces  

NASA Astrophysics Data System (ADS)

We prove the absolute continuity of stable foliations for C maps of Banach spaces satisfying a globally defined infinitesimal invariant cones condition. Proofs of regularity for center and stable manifolds needed for the main theorem are included. Our results are applicable to dynamical systems generated by ordinary, partial, or functional differential equations, including non-autonomous differential equations that are periodic in time.

Lian, Zeng; Young, Lai-Sang; Zeng, Chongchun

419

The synthesis of stable grasps in the plane  

Microsoft Academic Search

This paper addresses the problem of synthesizing stable grasps on arbitrary planar polygons. Each finger is a virtual spring whose stiffness and compression can be programmed. The contacts between the finger tips and the object are point contacts without friction. We prove that all force-closure grasps can be made stable, and it costs O(n) time to synthesize a set of

Van-Due Nguyen

1986-01-01

420

Stable isotope probing — linking microbial identity to function  

Microsoft Academic Search

Stable isotope probing (SIP) is a technique that is used to identify the microorganisms in environmental samples that use a particular growth substrate. The method relies on the incorporation of a substrate that is highly enriched in a stable isotope, such as 13C, and the identification of active microorganisms by the selective recovery and analysis of isotope-enriched cellular components. DNA

Marc G. Dumont; J. Colin Murrell

2005-01-01

421

Formulation of stable protein powders by supercritical fluid drying  

Microsoft Academic Search

Protein pharmaceuticals are potent drugs for the treatment of several chronic and life-threatening diseases. However, the complex and sensitive nature of protein molecules requires special attention in the development of stable dosage forms. Developing stable aqueous protein formulations is often a problem. Dried formulations have been successfully used to overcome instability of the protein drug during shipping and long-term storage.

N. Jovanovi?

2007-01-01

422

Dynamically-stable Motion Planning for Humanoid Robots  

Microsoft Academic Search

Abstract We present an algorithm for computing stable collision - free motions for humanoid robots given full - body posture goals The motion planner is part of a simulation environment under development for providing high - level software control for humanoid robots Given a robot's internal model of the environment and a statically - stable desired posture, we use a

James J. Kuffner Jr.; K. Nishiwaki; S. Kagami; Masayuki Inaba; Hirochika Inoue

2002-01-01

423

Stable isotopes in fish as indicators of habitat use  

EPA Science Inventory

In our isotopic studies of fish in Oregon Coast Range streams we have found stable isotopes of carbon, oxygen and sulfur to be surprisingly useful in identifying and discriminating specific habitat or tributary use by a variety of fish species. Stable isotopes of carbon can be u...

424

APPROACHES FOR MEASURING STABLE CARBON AND NITROGEN ISOTOPES IN BACTERIA  

EPA Science Inventory

Stable isotopes have been used successfully over the past three decades to trace through aquatic food chairs. his technique, however, has only recently been used to examine aquatic microbial roles in elemental cycling. he major obstacle to measuring stable isotope compositions in...

425

Stable isotopes in daily precipitation at Dome Fuji, East Antarctica  

Microsoft Academic Search

Daily precipitation samples for stable isotope analysis were collected throughout 2003 at Dome Fuji Station, inland East Antarctica. Stable isotopes show significantly depleted values with a large seasonal variability, which have never been obtained. Temporal changes in ?-T relation and d-excess are consistent with those found in geographical distribution. Precipitation was obtained almost everyday, though the amounts were quite small

Koji Fujita; Osamu Abe

2006-01-01

426

Synthesis of Stable Derivatives of Cycloprop-2-ene Carboxylic Acid  

PubMed Central

Large scale syntheses of 3-(cycloprop-2-en-1-oyl)-oxazolidinones from acetylene and ethyl diazoacetate are described. Unlike other cyclopropenes that bear a single substitutent at C-3, these compounds are stable to longterm storage. Although the cyclopropene derivatives are unusually stable, they are reactive toward cyclic and acyclic dienes in stereoselective Diels-Alder reactions.

Yan, Ni; Liu, Xiaozhong; Pallerla, Mahesh K.

2011-01-01

427

An efficient algorithm for the “optimal” stable marriage  

Microsoft Academic Search

In an instance of size n of the stable marriage problem, each of n men and n women ranks the members of the opposite sex in order of preference. A stable matching is a complete matching of men and women such that no man and woman who are not partners both prefer each other to their actual partners under the

Robert W. Irving; Paul Leather; Dan Gusfield

1987-01-01

428

Set of Super-Stable Marriages Forms a Distributive Lattice.  

National Technical Information Service (NTIS)

Replacing the total orders of the preference lists of an instance of the stable marriage problem by arbitrary posets, we show after adjusting the notion of stability to the new problem that the set of stable marriages still forms a distributive lattice.

B. Spieker

1992-01-01

429

A weakly stable algorithm for general Toeplitz systems  

Microsoft Academic Search

We show that a fast algorithm for the QR factorization of a Toeplitz or Hankel matrix A is weakly stable in the sense that R T R is close to A T A. Thus, when the algorithm is used to solve the semi-normal equations R T Rx = A T b, we obtain a weakly stable method for the solution

Adam W. Bojanczyk; Richard P. Brent; Frank R. de Hoog

2010-01-01

430

Stable Isotopes and Mineral Resource Investigations in the United States  

NSDL National Science Digital Library

This USGS handout is a page providing a good, brief summary of stable isotope techniques and applications in the geosciences. A map indicating the locations of current USGS stable isotopic studies accompanies the text. A discussion of mineral weathering effects and case studies from ore deposits in Maine and North Carolina complement the general overview.

431

Role of stable modes in zonal flow regulated ITG turbulence  

NASA Astrophysics Data System (ADS)

Stable modes are studied in zonal flow regulated ITG turbulence using the gyrokinetic code GENE. Proper orthogonal decomposition (POD) modes are employed to investigate the eigenmode space of the distribution function. Both the unstable and stable POD modes show strong nonlinear energy transfer via three wave interactions that include zonal modes. The zonal mode itself absorbs a small fraction of the energy injected by the unstable mode. The remaining energy is deposited in the stable modes of non-zonal wavenumbers that are involved in the three wave coupling. These stable modes lie mostly within the wavenumber range of the instability. This indicates that zonal flows mediate energy transfer from unstable to stable modes, leading to saturation. The amplitude attenuation rate (AAR) of POD modes shows an equipartition across a large range of stable modes. This rate is balanced by three wave correlations of the POD modes and their time dependent amplitudes. These correlations are large if they involve zonal modes and they also show an equipartition for higher mode numbers. A similar analysis using linear eigenmodes also shows rough equipartition among the linear modes. Thus, AAR provides a handle to collectively describe the multitude of stable modes in a gyrokinetic simulation.

Makwana, Kirit; Terry, Paul; Hatch, David; Pueschel, M. J.

2012-10-01

432

Continuous operation of spectroscopy instruments for stable isotope analysis  

NASA Astrophysics Data System (ADS)

Routine Application of Novel Stable Isotope Spectroscopy Instruments; Berkeley, California, 12 December 2010 ; Stable isotopes provide a tracer for biogeochemical processes and for exchange between the biosphere and atmosphere at local to global scales. Recent developments in laser spectroscopy now offer the ability to measure stable isotopes in small molecules such as water vapor and carbon dioxide in unprecedented temporal resolution (up to 10 hertz). In a meeting organized with support from the U.S. National Science Foundation-funded Biogeosphere-Atmosphere Stable Isotope Network (BASIN), an international group consisting of researchers and manufacturers' representatives discussed the challenges of daily use of absorption spectroscopy instruments for stable isotope analysis in continuous field operation. Technological advancement in absorption spectroscopy analysis techniques tied to increased availability of affordable instruments has opened the door to new scientific opportunities and facilitates the use of stable isotope analysis by a larger group of researchers. Despite technological progress, though, stable isotope analysis still requires delicate treatment of samples and setup. Meeting participants tried to identify the key responsibilities for individual labs and the scientific community to help ensure comparability of results and prepare for future regional to global collaborative research efforts. The meeting was concluded with the recommendations below.

Zeeman, Matthias J.; Tu, Kevin P.; Knohl, Alexander

2011-06-01

433

The Dependence Structure for PARMA Models with alpha -Stable Innovations  

NASA Astrophysics Data System (ADS)

In this paper we investigate the dependence structure for PARMA models (i.e. ARMA models with periodic coefficients) with symmetric alpha -stable innovations. In this case the covariance function is not defined and therefore other measures of dependence have to be used. After obtaining the form of the bounded solution of the PARMA system with symmetric alpha -stable innovations, we study the codifference and the covariation -- the most popular measures of dependence defined for symmetric stable time series. We show that both considered measures are periodic. Moreover we determine the cases when the codifference and the covariation are asymptotically proportional with the coefficient of proportionality equal to alpha .

Nowicka-Zagrajek, J.; Wylomanska, A.

2006-11-01

434

Environmental and biomedical applications of natural metal stable isotope variations  

USGS Publications Warehouse

etal stable isotopes are now being used to trace metal contaminants in the environment and as indicators of human systemic function where metals play a role. Stable isotope abundance variations provide information about metal sources and the processes affecting metals in complex natural systems, complementing information gained from surrogate tracers, such as metal abundance ratios or biochemical markers of metal metabolism. The science is still in its infancy, but the results of initial studies confirm that metal stable isotopes can provide a powerful tool for forensic and biomedical investigations.

Bullen, T. D.; Walczyk, T.

2009-01-01

435

A Model Describing Stable Coherent Synchrotron Radiation in Storage Rings  

SciTech Connect

We present a model describing high power stable broadband coherent synchrotron radiation (CSR) in the terahertz frequency region in an electron storage ring. The model includes distortion of bunch shape from the synchrotron radiation (SR), which enhances higher frequency coherent emission, and limits to stable emission due to an instability excited by the SR wakefield. It gives a quantitative explanation of several features of the recent observations of CSR at the BESSY II storage ring. We also use this model to optimize the performance of a source for stable CSR emission.

Sannibale, F.

2004-10-28

436

Differentiable Manifold of Fixed Order Stable Linear Systems.  

National Technical Information Service (NTIS)

Investigations are carried out into problem areas of system theory. Results of Helmke, Ober and Hanzon are generalized by the demonstration that the space of asymptotically stable linear systems of fixed order and the corresponding space of all linear sys...

B. Hanzon

1989-01-01

437

Synthesis and Characterization of the First Stable Cyanocyclophosphazenes.  

National Technical Information Service (NTIS)

Cyclic and high polymeric phosphazenes are known that possess a wide variety of inorganic, organometallic, and organic side groups. However, until now, no stable cyanophosphazenes have been reported, although attempts have been made to isolate such specie...

J. S. Rutt M. Parvez H. R. Allcock

1986-01-01

438

Highly stable high-rate discriminator for nuclear counting  

NASA Technical Reports Server (NTRS)

Pulse amplitude discriminator is specially designed for nuclear counting applications. At very high rates, the threshold is stable. The output-pulse width and the dead time change negligibly. The unit incorporates a provision for automatic dead-time correction.

English, J. J.; Howard, R. H.; Rudnick, S. J.

1969-01-01

439

GENERAL VIEW OF ADOBE STABLES, WITH MISCELLANEOUS OUTBUILDINGS TO LEFT ...  

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

GENERAL VIEW OF ADOBE STABLES, WITH MISCELLANEOUS OUTBUILDINGS TO LEFT (WEST) AND TO RIGHT NEAR SWIMMING POOL (EAST); CAMERA FACING NORTHEAST - Harry Carey Ranch, 28515 San Francisquito Canyon Road, Saugus, Los Angeles County, CA

440

High-Order Entropy Stable Formulations for Computational Fluid Dynamics.  

National Technical Information Service (NTIS)

A systematic approach is presented for developing entropy stable (SS) formulations of any order for the Navier-Stokes equations. These SS formulations discretely conserve mass, momentum, energy and satisfy a mathematical entropy inequality. They are valid...

M. H. Carpenter T. C. Fisher

2013-01-01

441

Distribution of radiocesium and stable elements within a pine tree.  

PubMed

Distributions of (137)Cs and stable elements in different parts of a pine tree collected in Chernobyl-contaminated area in Belarus were determined. Samples include annual tree rings of wood, branches and needles with different ages. The concentrations of (137)Cs and stable Cs in annual tree rings were the highest in cambium and decreased sharply towards inside. The youngest needles and branches contained higher (137)Cs and stable Cs than older ones. The concentration of (137)Cs being highest in growing parts suggests the highest radiation dose to the radiation-sensitive parts of tree. Distribution patterns of stable elements in pine tree differ among the elements. Distributions similar to those of Cs were observed for K and Rb, suggesting that alkaline metals tend to be translocated to young growing parts of pine tree. A similar distribution was also observed for phosphorus. Distributions of alkaline earth metals and several heavy metals were different from those of alkaline metals. PMID:21543348

Yoshida, Satoshi; Watanabe, Masumi; Suzuki, Akira

2011-07-01

</