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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

A carbene-carbene complex equilibrium.  

PubMed

Phenylchlorocarbene, generated by laser flash photolysis of phenylchlorodiazirine, formed highly stable pi-type complexes with 1,3,5-trimethoxybenzene in pentane. The carbene and carbene complexes were in equilibrium. We measured the equilibrium constant (K = 1264 M(-1) at 294 K) and, from its temperature dependence, extracted the associated thermodynamic parameters: DeltaH(o) = -7.1 kcal/mol, DeltaS(o) = -10.2 eu, and DeltaG(o) = -4.1 kcal/mol. The carbene complexes were characterized by UV-vis spectroscopy and computational analysis. PMID:20681697

Moss, Robert A; Wang, Lei; Odorisio, Christina M; Krogh-Jespersen, Karsten

2010-08-11

3

New stable aryl-substituted acyclic imino-N-heterocyclic carbene: synthesis, characterisation and coordination to early transition metals.  

PubMed

The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl)imidazolium salt from the corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons adjacent to the iminic carbon allowed for the first isolation of an imino-N-heterocyclic carbene of this ligand class. The free carbene was isolated, structurally characterised, and coordinated to titanium, zirconium, hafnium and chromium. The resulting metal halide complexes were fully characterised and were tested at room temperature and atmospheric pressure for their activity as ethylene polymerisation catalysts. The Zr(IV) complex was found to be the most active with a productivity of 140 kg PE mol M(-1) h(-1). PMID:22006062

Larocque, Timothy G; Badaj, Anna C; Dastgir, Sarim; Lavoie, Gino G

2011-10-18

4

Chiral N-Heterocyclic Carbene Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates-Claisen Rearrangement  

PubMed Central

A combination of a chiral N-heterocyclic carbene catalyst and ?,?-unsaturated aldehyde leads to a catalytically generated ?,?-unsaturated acyl azolium, which participates in a highly enantioselective annulation to give dihydropyranone products. This full account of our investigations into the scope and mechanism of this reaction reveals the critical role of both the type and substitution pattern of the chiral triazolium precatalyst in inducing and controlling the stereochemistry. In an effort to explain why stable enols such as naphthol, kojic acid, and dicarbonyl are uniquely efficient, we have postulated that this annulation occurs via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal prior to a sigmatropic rearrangement. Detailed kinetic investigations of the catalytic annulation are consistent with this mechanistic postulate.

Mahatthananchai, Jessada; Kaeobamrung, Juthanat; Bode, Jeffrey W.

2012-01-01

5

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

6

Synthesis of a Simplified Version of Stable Bulky and Rigid Cyclic (Alkyl)(Amino)Carbenes (CAACs), and Catalytic Activity of the Ensuing Gold(I) Complex in the Three-Component Preparation of 1,2-Dihydroquinoline Derivatives  

PubMed Central

A 95/5 mixture of cis and trans 2,4-dimethyl-3-cyclohexenecarboxaldehyde (trivertal), a common fragrance and flavor material produced in bulk quantities, serves as the precursor for the synthesis of a stable spirocyclic (alkyl)(amino)carbene, in which the 2-methyl-substituted cyclohexenyl group provides steric protection to an ensuing metal. The efficiency of this carbene as ligand for transition metal based catalysts is first illustrated by the gold(I) catalyzed hydroamination of internal alkynes with secondary dialkyl amines, a process with little precedent. The feasibility of this reaction allows for significantly enlarging the scope of the one-pot three-component synthesis of 1,2-dihydroquinoline derivatives, and related nitrogen-containing heterocycles. Indeed, two different alkynes were used, which include an internal alkyne for the first step.

Zeng, Xiaoming; Frey, Guido D.; Kinjo, Rei; Donnadieu, Bruno; Bertrand, Guy

2009-01-01

7

A persistent (amino)(ferrocenyl)carbene†‡  

PubMed Central

Deprotonation of N,N-diisopropyl-C-ferrocenylaldiminium triflate 2 cleanly leads to the corresponding 1,2-diamino-1,2-diferrocenylethene 3, the dimer of the desired (amino)(ferrocenyl)carbene. Fulvene 6, obtained by addition of the lithium salt of tetramethylcyclopentadiene to methoxyformamidinium methylsulfate 5, reacts with dicarbonylcyclopentadienylbromoiron(II), and with a mixture of FeCl2 and Cp* lithium salt, affording the corresponding tetramethylferrocenylaldiminium salt 7, and nonamethylferrocenylaldiminium salt 8, respectively. Although the deprotonation of 7 gives a complex mixture of products, the treatment of 8 at ?78 °C with sodium hexamethyldisilazide allowed for the isolation of the corresponding (amino)(ferrocenyl)carbene 9 as a yellow powder. However, even in the solid state, it is stable for less than 48 h at ?20 °C. In addition to NMR spectroscopy, evidence for the carbene nature of 9 was found by a trapping experiment with sulfur that leads to the corresponding adduct 10, which was characterized by a single crystal X-ray diffraction study.

DeHope, Alan; Mendoza-Espinosa, Daniel; Donnadieu, Bruno

2012-01-01

8

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

9

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

10

Betaine-carbene interconversions. From N-ylides to zwitterionic N-heterocyclic carbene-borane adducts.  

PubMed

In the presence of NBS 3-methylindole reacted with various imidazoles to give the (indol-2-yl)imidazolium salts 21a-f, which were converted in aqueous solution into the 2-(imidazolium-3-yl)-3-methylindolates 22a-f by base. These conjugated ylides--which represent a subclass of mesomeric betaines--are the exclusively detectable form in the NMR spectra taken in DMSO-d(6). A DFT calculation revealed that the betaine 22a is -9.3 kJ/mol more stable than the tautomeric N-heterocyclic carbene 23a and that the energy for the betaine-carbene interconversion is ?G(‡) = 66.4 kJ/mol. The N-heterocyclic carbenes (3-methyl-indol-2-yl)imidazol-2-ylidenes, however, can be trapped by sulfur, triethylborane, and triphenylborane. Whereas the first trapping reaction yielded the expected imidazolethiones, the borates gave the first representatives of new zwitterionic borane adducts, imidazo[2',1':3,4][1,4,2]diazaborolo[1,5-a]indolium-11-ides 26a-h. We performed DFT calculations on the structures of mesomeric betaine 22a, the carbene 23a, and the mechanisms of the borane adduct formation to 26a-h, NMR spectroscopic investigations including (15)N, (7)Li, and (11)B NMR spectroscopy, and an X-ray single-crystal analysis of one of the borane adducts. PMID:23305205

Pidlypnyi, Nazar; Namyslo, Jan C; Drafz, Martin H H; Nieger, Martin; Schmidt, Andreas

2013-01-18

11

Carbene catalysis: An internal affair  

NASA Astrophysics Data System (ADS)

Preparing powerful reactive intermediates such as enolates and homoenolates for C-C bond formation used to require strong bases and stoichiometric reagents. They can now be catalytically generated from ?-functionalized aldehydes or even from saturated esters under mild conditions using N-heterocyclic carbene catalysts.

Bode, Jeffrey W.

2013-10-01

12

Computational Chemistry Studies on the Carbene Hydroxymethylene  

ERIC Educational Resources Information Center

|A density functional theory computational chemistry exercise on the structure and vibrational spectrum of the carbene hydroxymethylene is presented. The potential energy curve for the decomposition reaction of the carbene to formaldehyde and the geometry of the transition state are explored. The results are in good agreement with recent…

Marzzacco, Charles J.; Baum, J. Clayton

2011-01-01

13

The imidic acids H---N=C(H)---OH and CH3---N=C(H)---OH and their tautomeric carbenes H2N7z.sbnd;C---OH and CH3---N(H)---C---OH: stable species in the gas phase formed by one-electron reduction of their cations  

NASA Astrophysics Data System (ADS)

The prototype imidic acid, formimidic acid, H---N=C(H)---OH, 2, as well as aminohydroxy-carbene, 3, both isomeric with formamide, 1, have been generated in the gas phase by one-electron reduction of the corresponding radical cations. The ionic precursors 2[radical sign]+ and 3[radical sign]+ were produced by appropriate dissociative ionizations, whereas 1[radical sign]+ was formed by direct ionization of formamide. From a detailed analysis of collision induced dissociation (CID) mass spectra, in particular MS/MS/MS data from D- and 15N-labelled isotopomers, it was possible to assign structures to ions 1[radical sign]+, 2[radical sign]+ and 3[radical sign]+ and to ascertain that, for each, ion beams could be obtained which were free from the other two isomers. Reduction of these ions in neutralization-reionization experiments produced intense "survivor" signals. By combining the information contained in CID mass spectra of the "survivor" ions with that from labelling experiments, the existence of 2 and 3 as stable molecules in the gas phase could be established. Among the vibrationally excited molecules we find evidence for the simplest amide-iminol tautomerism, i.e. 1 <--> 2 (a reaction which because of the high barrier is not observed in solution). At higher internal energies decarbonylation occurs. The experimental findings on ions and neutrals 1-3 concur with previously published ab initio MO studies which were supplemented by our own results at the UMP3/6-31G*//4-31G (+ZPVE) level of theory. Using the same experimental methodology, the existence of the imidic acid CH3---N=C(H)---OH and the carbene CH3---N(H)---C---OH, the methyl homologues of 2 and 3, was also established.

McGibbon, Graham A.; Burgers, Peter C.; Terlouw, Johan K.

1994-09-01

14

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

15

Cyclometalated gold(III) complexes with N-heterocyclic carbene ligands as topoisomerase I poisons.  

PubMed

A panel of stable [Au(R-C--N--C)(N-heterocyclic carbene)](+) complexes displays prominent in vitro anticancer properties; [Au(C--N--C)(IMe)]CF(3)SO(3) (1, IMe = 1,3-dimethylimidazol-2-ylidene) significantly poisons topoisomerase I in vitro and suppresses tumor growth in nude mice model. PMID:20401423

Yan, Jessie Jing; Chow, Andy Lok-Fung; Leung, Chung-Hang; Sun, Raymond Wai-Yin; Ma, Dik-Lung; Che, Chi-Ming

2010-04-19

16

Carbene transfer from triazolylidene gold complexes as a potent strategy for inducing high catalytic activity.  

PubMed

A series of gold(I) complexes [AuCl(trz)] were synthesized that contain 1,2,3-triazolylidene (trz) ligands with variable wingtip groups. In the presence of AgBF4, these complexes undergo ligand redistribution to yield cationic complexes [Au(trz)2]BF4 in high yields as a result of efficient carbene transfer. Identical reactivity patterns were detected for carbene gold complexes comprised of Arduengo-type IMes ligands (IMes = N,N'-dimesityl-imidazol-2-ylidene). Reaction of cationic complexes [Au(trz)2](+) with [AuCl(trz')] afforded the heteroleptic complex [Au(trz)(trz')](+) and [AuCl(trz)] (trz, trz' = triazolylidene ligands with different wingtip groups). Carbene transfer occurs spontaneously, yet is markeldy rate-enhanced in the presence of Ag(+). The facile carbene transfer was exploited as a catalyst activation process to form active gold species for the aldol condensation of isocyanides and aldehydes to form oxazolines. The catalytic activity is strongly dependent on the presence of Ag(+) ions to initiate catalyst activation. High turnovers (10(5)) and turnover frequencies (10(4) h(-1)) were accomplished. Structural analysis at early stages of the reaction support the critical role of triazolylidene dissociation to activate the precatalyst and dynamic light scattering revealed the presence of nanoparticles (±100 nm diameter) as potential catalytically active species. Furthermore, the triazolylidene scaffold had no impact on the diastereoselectivity of the oxazoline formation, and chiral triazolylidenes did not induce any asymmetry in the product. The facile dissociation of carbenes from [AuCl(carbene)] in the presence of Ag(+) ions suggests a less stable Au-Ccarbene interaction than often assumed, with potential implications for gold-catalyzed reactions that employ a silver salt as (putative) halide scavenger. PMID:23902160

Canseco-Gonzalez, Daniel; Petronilho, Ana; Mueller-Bunz, Helge; Ohmatsu, Kohsuke; Ooi, Takashi; Albrecht, Martin

2013-08-21

17

Carbene-stabilized phosphenium oxides and sulfides.  

PubMed

Carbene?chalcogenophosphenium adducts, which correspond to an intermolecular stabilization mode of the so far elusive, free oxo- and thiooxophosphenium species [R(2)P(+) = X] (X = O, S) by imidazolylidene (NHC) and diaminocyclopropenylidene (BAC) donors, have been isolated and fully characterized. The dative character of the R(2)C:?P(+)(X)Ph(2) bond was confirmed experimentally by nucleophilic displacement of the carbene donor with a chloride ion and by an exchange reaction of the NHC ligand of the NHC:?P(+)(O)Ph(2) adduct with an independently prepared BAC ligand, thereby giving the BAC:?P(+)(O)Ph(2) adduct. This dative character was further characterized by the DFT-calculated preference of carbene?chalcogenophosphenium systems for a heterolytic dissociation mode over a homolytic one. PMID:23080305

Maaliki, Carine; Lepetit, Christine; Duhayon, Carine; Canac, Yves; Chauvin, Remi

2012-10-18

18

Carbene-stabilized main group diatomic allotropes.  

PubMed

While transition metals are well known for assuming the formal oxidation state of zero in various compounds main group elements have rarely engaged in this practice. Recent reports of N-heterocyclic carbene-stabilized main group diatomic allotropes (i.e., Si(2), Ge(2), P(2), As(2)) denote a breakthrough of zero-oxidation state main group chemistry. This Perspective addresses the synthesis and characterization of these highly reactive main group molecules, with a particular emphasis on the very recent progress in the reactivity study of carbene-stabilized Si(2) and P(2). PMID:21904737

Wang, Yuzhong; Robinson, Gregory H

2011-09-09

19

Synthesis, Activation and Catalytic Activity of N-Heterocyclic Carbene Bearing Palladium Catalysts  

Microsoft Academic Search

The syntheses and characterization of a series of (NHC)Pd(II) complexes (NHC = N-heterocyclic carbene) are described. A variety of architectures and precursors have been employed to lead to numerous air- and moisture-stable complexes. The use of those complexes as pre-catalysts in cross-coupling (Suzuki- Miyaura, Buchwald-Hartwig) and related (catalytic dehalogenation, £-ketone arylation) reactions is also discussed. A comparison of the activity

Oscar Navarro-Fernandez

2006-01-01

20

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

2013-08-13

21

Studies of coal structure using carbene chemistry  

SciTech Connect

The object of this grant was to react coal, derivatized forms of coal, and solvent swelled coal with carbenes (divalent carbon species) under mild conditions. These carbenes were to be prepared by treating the coal with several diazo compounds and then thermally decomposing them at relatively low temperatures (80--130{degree}C). The carbenes were to be chosen to show varying selectively toward aromatic rings containing heteroatom functionalities and toward polynuclear aromatic systems. In some instances, where selectivities toward aromatic and heteroaromatic ring systems were not known, model studies were to be carried out. Because of the generally mild conditions employed and the good selectivity anticipated, and actually observed with one particular system, it was expected that this methodology would provide structural information about the coal, along with data on the extent of occurrence and type of aromatic systems. After carbene reactions, treatment of the coal samples was to include extractions and thermolysis. Physical studies included thermogravimetric analysis, diffuse reflectance FT-IR spectroscopy, NMR ({sup 1}H and {sup 13}C) spectroscopy, gas chromatography, GC/MS and GC/FT-IR. 7 figs., 10 tabs.

Not Available

1990-01-01

22

N-Heterocyclic Carbene-Catalyzed Oxidations  

PubMed Central

N-Heterocyclic carbenes catalyze the oxidation of allylic and benzylic alcohols as well as saturated aldehydes to esters with manganese(IV) oxide in excellent yields. A variety of esters can be synthesized, including protected carboxylates. The oxidation proceeds under mild conditions, with low loadings of a simple triazolium salt pre-catalyst in the presence of base. Substrates containing potentially epimerizable centers are oxidized while preserving stereochemical integrity. The acyl triazolium intermediate generated under catalytic conditions can be employed as a chiral acylating agent in the desymmetrization of meso-diols.

Maki, Brooks E.; Chan, Audrey; Phillips, Eric M.; Scheidt, Karl A.

2009-01-01

23

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

24

New N-Heterocyclic Carbene Ligands in Grubbs and Hoveyda-Grubbs Catalysts  

NASA Astrophysics Data System (ADS)

A series of N-heterocyclic carbene (NHC) ligands bearing aliphatic amino side groups were synthesized and reacted with the Grubbs first generation catalyst. Reactions involving symmetrical, aliphatic NHCs did not allow the isolation of any pure NHC substituted complexes due to their instability. Unsymmetrical NHCs having a planar mesityl group on one amino side reacted with Grubbs catalyst in a favorable manner, and the resulting complexes were stable enough to be isolated. X-ray crystallographic analysis demonstrated that the mesityl group is co-planar with the phenyl ring of the benzylidene, which indicates that a ?-? interaction between the mesityl arm and the benzylidene moiety might constitute an important structural element. Catalysts substituted with an NHC derived from a primary or secondary amino-group were found to surpass the parent-complex for the ROMP of cycloocta-1,5-diene. The catalyst substituted with an NHC derived from tBu-NH2 was considerably less metathesis active. Also new N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbenes were synthesized. These NHC ligands revealed a different reactivity towards Grubbs complexes than the hitherto reported imidazolinylidenes: (i) facile bis(NHC) coordination was found, and (ii) both NHCs on the bis(NHC) complexes can be exchanged with a phosphine, thereupon regenerating the Grubbs first generation complex. Furthermore, a comparison between the classical Hoveyda-Grubbs complexes and complexes substituted with N-alkyl-N'-(aryl) heterocyclic carbenes demonstrates that the introduction of one aliphatic group into the NHC framework does not improve the catalytic activity in any of the tested metathesis reactions. The introduction of two aliphatic amino side groups enhances the reactivity in the ROMP reaction while the increase of steric interactions lowers the CM activity. The lower activity of the N-alkyl-N'-(2,6-diisopropylphenyl) heterocyclic carbene complexes compared with the N-alkyl-N'-mesityl heterocyclic carbene complexes, may analogously be attributed to a more demanding steric environment. While small differences in donor capacities might cause a significantly different catalytic behavior, it is thus plausible that subtle steric differences exert a more determining influence on the activity of the catalysts. In addition, the obtained results confirm that the NHC's amino side groups play a pivotal role in determining the reactivity, selectivity as well as the stability of the corresponding catalysts.

Monsaert, Stijn; Ledoux, Nele; Drozdzak, Renata; van der Voort, Pascal; Verpoort, Francis

25

Carbenes As Catalysts for Transformations of Organometallic Iron Complexes  

PubMed Central

Compared with the enormous arsenal of catalysts used to produce organic compounds, complementary species that are able to mediate sophisticated organometallic transformations are virtually nonexistent. We found that stable N-heterocyclic carbenes (NHCs) can mediate unusual organometallic transformations in solution at room temperature. Depending on the choice of NHC initiator, stoichiometric or catalytic reactions of bis(cyclooctatetraene)iron [Fe(COT)2] ensue. The stoichiometric reaction leads to the isolation of a previously unknown mixed-valent species, featuring distinct and directly bonded Fe(0) and Fe(I) centers. In the catalytic process, three iron atoms are fused to afford the tri-iron cluster Fe3(COT)3, which is a hydrocarbon analog of Dewar’s classic Fe3(CO)12 complex. The key step in both of these processes is proposed to involve the NHC’s ability to induce metal–metal bond formation. These NHC-mediated reactions provide a foundation on which to develop future organometallic transformations that are catalyzed by organic species.

Lavallo, Vincent; Grubbs, Robert H.

2010-01-01

26

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

2013-09-03

27

Synthesis of endohedral metallofullerene glycoconjugates by carbene addition.  

PubMed

Endohedral metallofullerene glycoconjugates were synthesized under mild conditions by carbene addition using appropriate glycosylidene-derived diazirine with La(2)@I(h)-C(80). NMR spectroscopic studies revealed that the glycoconjugate consists of two diastereomers of [6,6]-open mono-adducts. The electronic properties were characterized using Vis/NIR absorption spectroscopy and electrochemical measurements. This study demonstrates that glycosylidene carbene is useful to incorporate carbohydrate moieties onto endohedral metallofullerene surfaces. PMID:22083236

Yamada, Michio; Someya, Chika I; Nakahodo, Tsukasa; Maeda, Yutaka; Tsuchiya, Takahiro; Akasaka, Takeshi

2011-11-14

28

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

29

N-Heterocyclic Carbenes in Organocatalysis  

NASA Astrophysics Data System (ADS)

The use of N-heterocyclic carbenes (NHCs) to promote organocatalytic transformations has rapidly expanded in recent years, building upon the classic use of these compounds to generate acyl anion equivalents from aldehydes. This chapter gives an overview of the recent progress made in this area, describing the use of NHCs to generate synthetic intermediates from a range of readily accessible starting materials and recent developments in their reactivity. The reaction of an NHC can result in the formation of a range of d1, d2 and d3 synthons (acyl anion, azolium enol or enolate, and azolium homoenolate intermediates) or an electrophilic a1 acylazolium species, with typical processes that proceed through each of these intermediates described. The use of NHCs to participate in stereoselective reaction processes within each of these areas is also described, with an emphasis upon a mechanistic understanding of these processes given where appropriate.

Campbell, Craig D.; Ling, Kenneth B.; Smith, Andrew D.

30

N-Heterocyclic carbene adducts of orthopalladated triarylphosphite complexes.  

PubMed

Carbene adducts of orthopalladated triarylphosphite complexes have been synthesised and characterised. The structures of three of these complexes were determined by single-crystal X-ray analysis. The complexes are active in the Suzuki coupling of a range of aryl bromide substrates. PMID:16075119

Bedford, Robin B; Betham, Michael; Blake, Michael E; Frost, Robert M; Horton, Peter N; Hursthouse, Michael B; López-Nicolás, Rosa-María

2005-07-18

31

Direct spectroscopic characterization of a transitory dirhodium donor-acceptor carbene complex.  

PubMed

A multitude of organic transformations catalyzed by dirhodium(II) (Rh2) complexes are thought to proceed via the intermediacy of highly reactive, electrophilic carbenoid intermediates that have eluded direct observation. Herein, we report the generation of a metastable Rh2-carbenoid intermediate supported by a donor-acceptor carbene fragment. This intermediate is stable for a period of ~20 hours in chloroform solution at 0°C, allowing for an exploration of its physical and chemical properties. The Rh=C bond, characterized by vibrational and nuclear magnetic resonance spectroscopy, extended x-ray absorption fine structure analysis, and quantum-chemical calculations, has weak ? and ? components. This intermediate performs stoichiometric cyclopropanation and C-H functionalization reactions to give products that are identical to those obtained from analogous Rh2 catalysis. PMID:24030489

Kornecki, Katherine P; Briones, John F; Boyarskikh, Vyacheslav; Fullilove, Felicia; Autschbach, Jochen; Schrote, Kaitlin E; Lancaster, Kyle M; Davies, Huw M L; Berry, John F

2013-09-12

32

Benzannulation and Cyclopentannulation Reactions of Electron-Poor Amino-Stabilized Alkenyl Fischer Carbene Complexes  

Microsoft Academic Search

The alkenyl(amino)carbene complexes of chromium and tungsten bearing an electron-withdrawing group at the C?-position 5–6 were prepared by condensation of methyl(amino)carbene complexes 1–2 and esters of glyoxylic acid. The complexes 5–6 were readily ?-metallated with n-BuLi affording the deuteriated metal carbene complexes 7–8. The chromium complexes 5, 7 display a remarkable reactivity toward different types of alkynes. These complexes reacted

José Barluenga; Luis A López; Silvia Mart??nez; Miguel Tomás

2000-01-01

33

Synthesis and antimicrobial studies of silver N-heterocyclic carbene complexes bearing a methyl benzoate substituent  

Microsoft Academic Search

Due to the properties of silver as an antimicrobial, our research group has synthesized many different silver carbene complexes. Two new silver N-heterocyclic carbene complexes derived from 4,5-dichloroimidazole and theobromine bearing methyl benzoate substituents were synthesized by in situ carbene formation using silver acetate as the base in the reaction. The new compounds were fully characterized by several methods including

Amanda R. Knapp; Matthew J. Panzner; Doug A. Medvetz; Brian D. Wright; Claire A. Tessier; Wiley J. Youngs

2010-01-01

34

Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study  

Microsoft Academic Search

In order to probe the ligand properties we have examined a series of Cr(CO)5L and Ni(CO)3L complexes using density functional theory (DFT). The ligands included in our study are N-heterocyclic carbenes (NHCs) and Bertrand-type carbenes. Our study shows that the carbene–metal bonds of imidazol-2-ylidenes (1), imidazolin-2-ylidenes (2), thiazo-2-ylidenes (3), and triazo-5-ylidenes (4) are significantly stronger than those of Bertrand-type carbenes

Chun-Liang Lai; Wen-Hsin Guo; Ming-Tsung Lee; Ching-Han Hu

2005-01-01

35

Mechanism of rhodium-catalyzed carbene formation from diazo compounds.  

PubMed

[reaction: see text] A large and normal nitrogen-15 kinetic isotope effect of 1.035 +/- 0.003 provides direct support for the proposed mechanism for the rhodium-catalyzed carbene formation from diazo compounds, which involves the fast formation of a metal-diazo complex followed by rate-limiting extrusion of N2. The large magnitude of the KIE indicates extensive C-N bond fission in the transition state. PMID:17408276

Wong, Freeman M; Wang, Jianbo; Hengge, Alvan C; Wu, Weiming

2007-04-05

36

N-heterocyclic carbene-catalyzed rearrangements of vinyl sulfones†  

PubMed Central

N-heterocyclic carbenes catalyze the rearrangement of 1,1-bis(arylsulfonyl)ethylene to the corresponding trans-1,2-bis(phenylsulfonyl) under mild conditions. Tandem rearrangement/cycloadditions have been developed to capitalize on this new process and generate highly substituted isoxazolines and additional heterocyclic compounds. Preliminary mechanistic studies support a new conjugate addition/Umpolung process involving the ejection and subsequent unusual re-addition of a sulfinate ion.

Atienza, Roxanne L.; Roth, Howard S.

2012-01-01

37

Surface Studies of Enhanced Metal/Carbene Contacts  

NASA Astrophysics Data System (ADS)

Scanning tunneling microscopy and spectroscopy (STM and STS) have proven invaluable in understanding and manipulating the physical and electronic properties of molecular-scale systems. Such systems show great promise in current and future electronic applications. Indeed, self-assembled monolayers (SAMs) composed of alkanethiols have been extensively explored as a possible route to formation of metal-organic semiconductor contacts. While S is effective in this function, it behaves more as a capacitor than a conductor at the metal-adsorbate interface. This presentation will focus on investigations of the formation of metal-carbene complexes. For molecular electronics applications, conjugated metal-carbenes are expected to exhibit significantly greater conductance in transistor applications than corresponding thiols. A particular emphasis of this presentation is the structural and electronic information yielded by ultra-high vacuum (UHV) STM/STS studies of the formation of a Ru-carbene from diazoalkane precursors. Early theoretical predictions and UHV-STS indicate a strong coupling of the HOMO for bromophenyl diazomethane to the Ru d-band.

Lay, Marcus; Flynn, George; Nuckolls, Colin; Tulevski, George; Steigerwald, Mike; Hybertsen, Mark

2006-03-01

38

Structure and energetics of simple carbenes CHâ, CHF, CHCl, CHBr, CFâ, and CClâ  

Microsoft Academic Search

There is very little experimental data concerning the singlet--triplet separations of carbenes. In many cases, it is not even possible to form estimates of this separation from experimental results. CHâ is a distinct exception; here substantial, although contradictory, data are available. The equilibrium geometries of the lowest singlet states of several carbenes have been determined experimentally. However, the geometries of

C. W. Jr. Bauschlicher; H. F. Schaefer; P. S. Bagus

1977-01-01

39

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

40

Nickel-Catalyzed Cycloisomerization of Enynes: Catalyst Generation via C-H Activation of Carbene Ligands  

PubMed Central

The combination of Ni(0) and an N-heterocyclic carbene act as a precatalyst for the cycloisomerization of enynes to afford 1,3-dienes. During the course of the reaction, a nickel hydride is formed from oxidative addition of the ortho C-H on the carbene ligand. Deuteriumn labeling studies are presented.

Tekavec, Thomas N.; Louie, Janis

2011-01-01

41

SBA15 surface functionalization via ferrocenyl Fischer chromium carbene coatings: Technology and textural properties  

Microsoft Academic Search

A ferrocenyl Fischer chromium carbene complex has been covalently grafted on the SBA-15 surface. The method of synthesis of this carbene is a classical one while the surface attachment of such compound on the SBA-15 pores is made via a bridging molecule of 3-aminopropyltriethoxysilane. Important physicochemical properties (XRD, TEM, and N2 sorption) of the preceding substrate, which involves the extra

M. L. Ojeda; A. Campero; C. Velásquez; J. G. López-Cortés; C. Alvarez; J. M. Esparza; F. Rojas

2009-01-01

42

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

2013-05-02

43

Dichlorocarbene addition to C60 from the trichloromethyl anion: carbene mechanism or Bingel mechanism?  

PubMed

The reactions of C(60) and trichloromethyl anion (CCl(3)(-)) via both the Bingel mechanism and the carbene mechanism were comparably studied by means of density functional theory (DFT) computations. The Bingel mechanism is highly competitive as compared with the carbene mechanism that leads to the formation of C(60)(CCl(2)). Unlike the carbene mechanism with a weak regioselectivity and solvent sensitivity, the Bingel mechanism yields the [6,6]-C(60)(CCl(2)) isomer as the exclusive product and favors highly polar solvents. The results receive strong experimental support and simultaneously rationalize these experimental findings. PMID:19317394

Gao, Xingfa; Ishimura, Kazuya; Nagase, Shigeru; Chen, Zhongfang

2009-04-16

44

Chemical functionalization of diamond surfaces by reaction with diaryl carbenes.  

PubMed

A rapid route to the chemical functionalization of hydrogen-terminated diamond surfaces deposited by chemical vapor deposition involving their reaction with substituted diaryl carbenes has been investigated. To avoid difficulties in the handling of highly reactive compounds, the carbene is generated in situ from the thermal decomposition at 400 K of a thin film of the corresponding diaryl diazomethane precursor deposited at the diamond interface. X-ray photoelectron spectroscopy (XPS) has been used to verify that surface functionalization using two starting compounds, bis(4-iodophenyl) diazomethane and bis(4-nitrophenyl) diazomethane, can be achieved using this approach in agreement with recent theoretical studies. The surface grafting density is measured to be around 10(14) cm(-2) in each case. The chemistry observed is found to be insensitive to the detailed properties of the diamond film and to the presence of oxygen contamination at the hydrogen-terminated diamond surface. We further demonstrate the utility of the approach, in the case of the bound nitrophenyl compound, by its reduction to the corresponding primary amine followed by reaction with fluorescein isothiocyanate to achieve fluorescent tagging of the diamond interface. PMID:18177059

Wang, Hao; Griffiths, Jon-Paul; Egdell, Russell G; Moloney, Mark G; Foord, John S

2008-01-05

45

Cross-coupling and dehalogenation reactions catalyzed by (N-heterocyclic carbene)Pd(allyl)Cl complexes.  

PubMed

A series of well-defined, air- and moisture-stable (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes has been used in several catalytic reactions: Suzuki-Miyaura cross-coupling, catalytic dehalogenation of aryl halides, and aryl amination. The scope of the three processes using various substrates was examined. A general system involving the use of (IPr)Pd(allyl)Cl as catalyst and NaO(t)Bu as base has proven to be highly active for the Suzuki-Miyaura cross-coupling of activated and unactivated aryl chlorides and bromides, for the catalytic dehalogenation of aryl chlorides, and for the catalytic aryl amination of aryl triflates. All reactions proceed in short reaction times and at mild temperatures. The system has also proven to be compatible with the microwave-assisted Suzuki-Miyaura cross-coupling and catalytic dehalogenation processes, affording yields similar to those of the conventionally heated analogous reactions. PMID:15104459

Navarro, Oscar; Kaur, Harneet; Mahjoor, Parisa; Nolan, Steven P

2004-04-30

46

GEOMETRY AND ELECTRONIC STRUCTURE OF (CO)3N1CH2. A MODEL TRANSITION METAL CARBENE  

SciTech Connect

The first application of nonempirical molecular electronic structure theory to a realistic transition metal carbene complex is reported. The system chosen was (CO){sub 3}NiCH{sub 2}, methylene (tricarbonyl) nickel(0). All studies were carried out at the self-consistent-field (SCF) level. A large and flexibly contracted basis set was chosen, labeled Ni(15s 11p 6d/11s 8p 3d); C,O(9s 5p/4s 2p); H(5s/3s). In addition, the importance of methylene carbon d functions was investigated. The critical predicted equilibrium geometrical parameters were R [Ni-C (methylene)]=1.83 {Angstrom}, {theta}(HCH)=108°. The sixfold barrier to rotation about the Ni-C (methylene) axis is small, ~o.2 kcal. The electronic structure of (CO){sub 3}NiCH{sub 2} is discussed and compared with those of the "naked" complex NiCH{sub 2} and the stable Ni(CO){sub 4} molecule.

Spangler, Dale; Wendoloski, John J.; Dupuis, Michel; Chen, Maynard M.L.; Schaefer III, Henry F.

1980-04-01

47

Strategies for heterocyclic construction via novel multicomponent reactions based on isocyanides and nucleophilic carbenes.  

PubMed

This Account focuses mainly on our recent endeavors in the area of multicomponent reactions (MCRs) involving zwitterionic species generated by the addition of isocyanides and nucleophilic carbenes such as dimethoxycarbene and N-heterocyclic carbenes to activated alkynes. The strategy employed encompasses the interception of 1:1 zwitterionic species, generated in situ with a wide range of electrophiles. The new MCRs developed offer an efficient and convenient entry into several heterocycles of biological and synthetic importance. PMID:14674781

Nair, Vijay; Rajesh, C; Vinod, A U; Bindu, S; Sreekanth, A R; Mathen, J S; Balagopal, Lakshmi

2003-12-01

48

Design and Synthesis of C2-Symmetric N-Heterocyclic Carbene Precursors and Metal Carbenoids  

PubMed Central

Chiral, C2-symmetric imidazolium and imidazolinium ions, as well as the corresponding copper or silver bound carbenoids, have been prepared. Structural study of these compounds by X-ray crystallography reveals a chiral pocket that surrounds the putative carbene site or the metal-carbene bond, at carbon 2, in three of the four ligands prepared. Preliminary investigation into the application of these complexes has shown one of them to be highly enantioselective in the hydrosilylation of acetophenone.

Albright, Abigail; Eddings, Daniel; Black, Regina; Welch, Christopher J.; Gerasimchuk, Nikolay N.; Gawley, Robert E.

2011-01-01

49

DFT prediction of multitopic N-heterocyclic carbenes using Clar's aromatic sextet theory.  

PubMed

Existence of several multitopic N-heterocyclic carbene (NHC) ligands with up to four carbene centers have been predicted on the basis of Clar's aromatic sextet theory. Assessment on stability and reactivity of NHCs was made by quantifying aromaticity, aromatic stabilization energy (E(aroma)), strength of carbene lone pair, proton affinity, and CuCl binding energy. On NICS(0) and HOMA scales of aromaticity, several NHCs showed high aromaticity, while E(aroma) (17.2-19.4 kcal/mol) indicated substantial stability for the N-heterocycle. Homodesmotic reactions suggested that heat of formation of most of the newly designed carbenes is very close to that of the existing bis-NHCs. Designing a multitopic ligand through branching via C(sp3) linkage was very effective as it improved the stability of the carbene. Electrostatic potential minimum (V(min)) at the carbene lone pair suggested that annelation of heterocycle to a benzenoid ring or branching through C(sp3) linkage can only marginally influence the electron donating power of the ligand. Hence, all multitopic NHCs showed proton affinity (252.3-267.4 kcal/mol) and CuCl binding energy (62.9-66.6 kcal/mol) very close to those of 1,3-dimethylimidazolidine-2-ylidene (1). It has also been demonstrated that branched multitopic 3-dimensional NHCs are attractive for designing metal-organic framework with narrow (1-1.5 nm) cage/pore size. PMID:23461408

Suresh, Cherumuttathu H; Ajitha, Manjaly J

2013-03-15

50

Alkyne hydroarylation with Au N-heterocyclic carbene catalysts  

PubMed Central

Summary Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC) ligands have been employed as catalysts in the intermolecular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III) complexes were able to catalyze the reaction; however, the best results were obtained with the mononuclear gold(I) complex IPrAuCl. This complex, activated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic activity and selectivity compared to more commonly employed palladium(II) catalysts. Moreover, the complex was active, albeit to a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously published complex AuCl(PPh3). Preliminary results on intramolecular hydroarylations using this catalytic system indicate, however, that alkyne hydration by traces of water may become a serious competing reaction.

Tubaro, Cristina; Baron, Marco; Basato, Marino

2013-01-01

51

Alkyne hydroarylation with Au N-heterocyclic carbene catalysts.  

PubMed

Mono- and dinuclear gold complexes with N-heterocyclic carbene (NHC) ligands have been employed as catalysts in the intermolecular hydroarylation of alkynes with simple unfunctionalised arenes. Both mono- and dinuclear gold(III) complexes were able to catalyze the reaction; however, the best results were obtained with the mononuclear gold(I) complex IPrAuCl. This complex, activated with one equivalent of silver tetrafluoroborate, exhibited under acidic conditions at room temperature much higher catalytic activity and selectivity compared to more commonly employed palladium(II) catalysts. Moreover, the complex was active, albeit to a minor extent, even under neutral conditions, and exhibited lower activity but higher selectivity compared to the previously published complex AuCl(PPh(3)). Preliminary results on intramolecular hydroarylations using this catalytic system indicate, however, that alkyne hydration by traces of water may become a serious competing reaction. PMID:23400639

Tubaro, Cristina; Baron, Marco; Biffis, Andrea; Basato, Marino

2013-02-05

52

Nanocrystals: why do silver and gold N-heterocyclic carbene precursors behave differently?  

PubMed

Synthesizing stable Au and Ag nanocrystals of narrow size distribution from metal-N-heterocyclic carbene (NHC) complexes remains a challenge, particularly in the case of Ag and when NHC ligands with no surfactant-like properties are used. The formation of nanocrystals by one-phase reduction of metal-NHCs (metal = Au, Ag) bearing common NHC ligands, namely 1,3-diethylbenzimidazol-2-ylidene (L(1)), 1,3-bis(mesityl)imidazol-2-ylidene (L(2)), and 1,3-bis(2,6-(i)Pr2C6H3)imidazol-2-ylidene (L(3)), is presented herein. We show that both Au and Ag nanocrystals displaying narrow size distribution can be formed by reduction with amine-boranes. The efficiency of the process and the average size and size distribution of the nanocrystals markedly depend on the nature of the metal and NHC ligand, on the sequence in the reactant addition (i.e., presence or absence of thiol during the reduction step), and on the presence or absence of oxygen. Dodecanethiol was introduced to produce stable nanocrystals associated with narrow size distributions. A specific reaction is observed with Ag-NHCs in the presence of thiols whereas Au-NHCs remain unchanged. Therefore, different organometallic species are involved in the reduction step to produce the seeds. This can be correlated to the lack of effect of NHCs on Ag nanocrystal size. In contrast, alteration of Au nanocrystal average size can be achieved with a NHC ligand of great steric bulk (L(3)). This demonstrates that a well-defined route for a given metal cannot be extended to another metal. PMID:24028343

Ling, Xiang; Schaeffer, Nicolas; Roland, Sylvain; Pileni, Marie-Paule

2013-10-01

53

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

54

Stabilization of Fully Reduced Iron-Sulfur Clusters by Carbene Ligation: The [FenSn]0 Oxidation Levels (n = 4, 8)  

PubMed Central

The all-ferrous [Fe4S4]0 state has been demonstrated in the fully reduced Fe protein of the Azotobacter vinelandii nitrogenase complex. We seek synthetic analogues of this state more tractable than the recently prepared but highly unstable cluster [Fe4S4(CN)4]4? (Scott, Berlinguette, Holm, and Zhou, Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The N-heterocyclic carbene 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Pri2NHCMe2) has been found to stabilize the fully reduced clusters [Fe8S8(Pri2NHCMe2)6] (4) and [Fe4S4(Pri2NHCMe2)4] (5), which are prepared by cluster assembly or phosphine substitution of FenSn (n = 8, 16) clusters. Cluster 4 is also obtained by reaction of the carbene with all-ferrous [Fe7S6(PEt3)5Cl2] (3) and cluster 5 by carbene cleavage of 4. Detailed structures of 3 (monocapped prismatic), 4, and 5 are described; the latter two are the first iron–sulfur clusters with Fe–C? bonds. Cluster 4 possesses the [Fe8(?3-S)6(?4-S)2] edge-bridged double cubane structure and 5 the cubane-type [Fe4(?3-S)4] stereochemistry. The all-ferrous formulations of the clusters are confirmed by X-ray structure parameters and 57Fe isomer shifts. Both clusters are stable under conventional aprotic anaerobic conditions, enabling further study of reactivity. The collective properties of 5 indicate that it is a meaningful synthetic analogue of the core of the fully reduced protein-bound cluster.

Deng, Liang

2008-01-01

55

Synthesis and reactivity of triethylborane adduct of N-heterocyclic carbene: versatile synthons for synthesis of N-heterocyclic carbene complexes.  

PubMed

The reaction of an imidazolium salt with LiBEt(3)H afforded triethylborane adduct of imidazol-2-ylidene, which can act as a carbene precursor for the synthesis of a transition metal complex as well as a main group element complex. PMID:15467851

Yamaguchi, Yoshitaka; Kashiwabara, Taigo; Ogata, Kenichi; Miura, Yumiko; Nakamura, Yoshiyuki; Kobayashi, Kimiko; Ito, Takashi

2004-08-20

56

Study of the ESI-mass spectrometry ionization mechanism of Fischer carbene complexes.  

PubMed

By means of deuterium-labeling experiments, we have carried out a systematic ESI-MS study to determine the mechanism of ESI ionization of alkenyl and alkynyl group 6 Fischer carbene complexes. These compounds can be ionized under ESI conditions only in the presence of additives such as hydroquinone (HQ) or tetrathiafulvalene (TTF). Our results demonstrate that in the ESI source an anion-radical is formed after the initial HQ- or TTF-mediated electron transfer to the metallic carbene complex. For alkenyl carbene complexes, this species evolves by extrusion of a hydrogen radical to form an allenylchromium anion that is detected as the [M - H](-) ion in the mass spectrum. The preference for this mechanistic pathway could be rationalized by DFT calculations. In the case of alkynyl carbene complexes, experiments combining deuterated substrate, additive, and solvent demonstrate that the previously proposed allene-anion carbene complex is not formed. Instead, the H transfer from the ethoxy group in the anion radical, followed by extrusion of a hydrogen radical, leads to an allenyl anion that is detected in the ESI-MS as [M - H - CO](-). PMID:15960531

Wulff, William D; Korthals, Keith A; Martínez-Alvarez, Roberto; Gómez-Gallego, Mar; Fernández, Israel; Sierra, Miguel A

2005-06-24

57

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

2013-04-29

58

Olefin cyclopropanation via carbene transfer catalyzed by engineered cytochrome P450 enzymes.  

PubMed

Transition metal-catalyzed transfers of carbenes, nitrenes, and oxenes are powerful methods for functionalizing C=C and C-H bonds. Nature has evolved a diverse toolbox for oxene transfers, as exemplified by the myriad monooxygenation reactions catalyzed by cytochrome P450 enzymes. The isoelectronic carbene transfer to olefins, a widely used C-C bond-forming reaction in organic synthesis, has no biological counterpart. Here we report engineered variants of cytochrome P450(BM3) that catalyze highly diastereo- and enantioselective cyclopropanation of styrenes from diazoester reagents via putative carbene transfer. This work highlights the capacity to adapt existing enzymes for the catalysis of synthetically important reactions not previously observed in nature. PMID:23258409

Coelho, Pedro S; Brustad, Eric M; Kannan, Arvind; Arnold, Frances H

2012-12-20

59

A Continuum of Progress: Applications of N-Hetereocyclic Carbene Catalysis in Total Synthesis  

PubMed Central

N-Heterocyclic carbene (NHC) catalyzed transformations have emerged as powerful tactics for the construction of complex molecules. Since Stetter’s report in 1975 of the total synthesis of cis-jasmon and dihydrojasmon by using carbene catalysis, the use of NHCs in total synthesis has grown rapidly, particularly over the last decade. This renaissance is undoubtedly due to the recent developments in NHC-catalyzed reactions, including new benzoin, Stetter, homoenolate, and aroylation processes. These transformations employ typical as well as Umpolung types of bond disconnections and have served as the key step in several new total syntheses. This Minireview highlights these reports and captures the excitement and emerging synthetic utility of carbene catalysis in total synthesis.

Izquierdo, Javier; Hutson, Gerri E.; Cohen, Daniel T.; Scheidt, Karl A.

2013-01-01

60

Group 7 transition metal complexes with N-heterocyclic carbenes.  

PubMed

This tutorial review summarizes all works and highlights recent advances published in the growing field of group 7 N-heterocyclic carbene (NHC) complexes. It provides a valuable source for all scientists that are interested in conducting research with new compounds bearing these metals. The article provides an overview of all manganese NHC complexes that are known to date. There are a lot of examples where manganese NHC complexes show unpredicted behaviour during synthesis, very different from other transition metal NHC complexes and their higher homologues rhenium and technetium. These differences are depicted and discussed. Furthermore, the chemistry of technetium NHC compounds and their chemical behaviour are discussed. To date published work on technetium NHC chemistry has been restricted to the oxidation state +v. It was found that such compounds are very reactive but show great stability in dry air. Their radioactivity makes such compounds interesting candidates for radiochemical applications. Since most group 7 NHC chemistry was conducted on rhenium NHC complexes, this tutorial review highlights the chemical behaviour of such compounds. Rhenium(i) complexes reveal luminescent properties, making them interesting candidates for applications ranging from biological markers to organic light-emitting diodes (OLEDs). Another interesting feature is the radioactivity of some compounds, which makes them excellent candidates for radiopharmaceutical research; hence their synthesis and reactivity are discussed. PMID:23475206

Hock, Sebastian J; Schaper, Lars-Arne; Herrmann, Wolfgang A; Kühn, Fritz E

2013-06-21

61

Computational insight into the carbenic character of nitrilimines from a reactivity perspective.  

PubMed

Nitrilimines (R-CNN-R) can be described through a carbenic valence bond structure, and although intermolecular carbenic reactions from nitrilimines are unknown, intramolecular reaction products from ortho-vinyl MeCOO-CNN-Ph (1) and Ph-CNN-Ph (2) that seem to have followed two typical carbene reaction mechanisms, [1+2] cycloaddition and C-H insertion, have been reported. This study sheds light on whether such reaction mechanisms are tenable, using the electron density and its Laplacian (QTAIM), natural bond orbital (NBO) descriptions, and reaction profiles. It is shown that the reaction in 1 is distinctly different from the [1+2] cycloadditions of ethene with the typical singlet carbene CF(2) and the nitrilimine F-CNN-F with its large carbenic character, and the formal [1+2] cycloaddition product from 1 is in fact not the primary product. Similarly, it is shown that the reaction in 2 is fundamentally different from the C-H insertions of CF(2), F-CNN-F, and even H-CNN-H with its small carbenic character into methane, and again the formal C-H insertion product from 2 is not the primary product. The small model reactions from CF(2), F-CNN-F, and H-CNN-H were analyzed using B3LYP, MP2, and B2PLYP with the aug-cc-pVTZ basis set, whereas the full study was performed with B3LYP/6-31+G(2d,2p), as it was shown to be sufficient. PMID:21974693

Muchall, Heidi M

2011-10-21

62

Energy barrier by cyclopropylchlorocarbene rearrangement measured by direct observation of the carbene in laser flash spectroscopy  

SciTech Connect

A transient absorption in the 250-265-nm range is assigned to cyclopropylchlorocarbene generated from the laser flash photolysis of 3-chloro-3-cyclopropyldiazirine. Confidence in this assignment was obtained from the following: (a) the decay of this absorption matches the growth of the pyridinium ylide: (b) the values for the rate constant for reaction of the carbene with 1-hexane obtained by monitoring the carbene at 250 nm and the ylide at 370 nm are similar. The kinetic parameters for the rearrangement of cyclopropylchlorocarbene to 1-chlorocyclobutene are log A = 11.1 and E{sub a} = 7.4 kcal/mol.

Liu, M.T.H.; Bonneau, R. (Universite de Bordeaux, Talence (France))

1989-10-19

63

Late-Stage Diversification of Chiral N-Heterocyclic-Carbene Precatalysts for Enantioselective Homoenolate Additions  

PubMed Central

A library of chiral triazolium salts has been prepared by late-state diversification of a triazolium amine salt. By utilizing a primary amine as a functional handle, a single triazolium salt can be transformed into a variety of chiral N-heterocyclic carbene precatalysts. This approach makes the preparation of chiral N-heterocyclic carbenes possible by a single-step modification of a triazolium salt, rather than the usual need for multistep organic synthesis and challenging heterocycle formation for each member of a catalyst library. We have screened these catalysts for control of diastereo- and enantioselectivity in a ?-lactam-forming reaction between ?,?-unsaturated aldehydes and cyclic ketimines.

Zheng, Pinguan; Gondo, Chenaimwoyo A.

2011-01-01

64

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

65

Oxygen-atom transfer from carbon dioxide to a Fischer carbene at (PNP)Ir.  

PubMed

Dehydrogenation of the dihydride (PNP)IrH2 with norbornylene in the presence of t-butyl methyl ether leads to formation of an iridium(I) Fischer carbene complex, (PNP)Ir C(H)OtBu, by double C-H activation and loss of H2. The square planar pincer-type carbene effects quantitative oxygen-atom transfer from CO2 (1 atm) at ambient temperature to generate t-butyl formate and (PNP)Ir-CO. The iridium carbene reacts similarly with carbonyl sulfide and phenyl isocyanate, causing sulfur-atom and nitrene-group transfer, respectively. In the absence of a hydrogen acceptor, thermolysis of (PNP)IrH2 in t-butyl methyl ether under an atmosphere of CO2 also results in the formation of (PNP)Ir-CO and oxidation of t-butyl methyl ether to t-butyl formate via an iridium carbene. Preliminary mechanistic studies indicate that these reactions proceed through an intermediate four-membered metallalactone. PMID:18410098

Whited, Matthew T; Grubbs, Robert H

2008-04-15

66

A Dual Lewis Base Activation Strategy for Enantioselective Carbene-Catalyzed Annulations  

PubMed Central

A dual activation strategy integrating NHC catalysis and a second Lewis base has been developed. NHC-bound homoenolate equivalents derived from ?,?-unsaturated aldehydes combine with transient reactive o-quinione methides in an enatioselective formal [4+3] fashion to access 2-benzoxopinones. The overall approach provides a general blueprint for the integration of carbene catalysis with additional Lewis base activations modes.

Izquierdo, Javier; Orue, Ane; Scheidt, Karl A.

2013-01-01

67

N-Heterocyclic carbene catalyzed C-glycosylation: a concise approach from Stetter reaction.  

PubMed

Described herein is the first example of an organocatalytic approach for acylanion addition to the anomeric carbon of 2-nitroglucal using an N-heterocyclic carbene catalyst. Control over the reaction conditions gives ?-selective and nitro-eliminated C-glycosides, providing opportunities to produce new classes of C-glycoside. PMID:22185609

Vedachalam, Seenuvasan; Tan, Shi Min; Teo, Hui Ping; Cai, Shuting; Liu, Xue-Wei

2011-12-20

68

Synthesis and reactivity of alkyl-palladium N-heterocyclic carbene complexes.  

PubMed

The transamination of alkyl-palladium halide N-heterocyclic carbene complexes has enabled the isolation of products that reveal interesting insights into the factors which might be barriers to the development of a palladium-catalysed alkyl-amination reaction. PMID:17347724

Esposito, Oriana; Lewis, Alexandra K de K; Hitchcock, Peter B; Caddick, Stephen; Cloke, F Geoffrey N

2007-02-14

69

N-heterocyclic carbene-catalyzed stereoselective cascade reaction: synthesis of functionalized tetrahydroquinolines.  

PubMed

The first N-heterocyclic carbene-catalyzed stereoselective aza-Michael-Michael-lactonization cascade reaction of 2'-aminophenylenones and 2-bromoenals for the construction of chiral functionalized tetrahydroquinolines with three consecutive stereogenic centers has been achieved in high yields (up to 98%) with excellent diastereo- (>25:1) and enantioselectivities (up to 98.7% ee). PMID:24020617

Zhang, Hai-Rui; Dong, Zhen-Wen; Yang, Yu-Jie; Wang, Ping-Luan; Hui, Xin-Ping

2013-09-10

70

Pd-catalyzed cyclization and carbene migratory insertion: new approach to 3-vinylindoles and 3-vinylbenzofurans.  

PubMed

A Pd-catalyzed stereoselective synthesis of 3-vinylindoles and 3-vinylbenzofurans has been developed. The reaction merges the alkyne-based Pd-catalyzed cyclization and Pd carbene migratory insertion in a single catalytic cycle, generating a C-C single bond and a C?C double bond in one operation. PMID:24066705

Liu, Zhenxing; Xia, Ying; Zhou, Shiyi; Wang, Long; Zhang, Yan; Wang, Jianbo

2013-09-25

71

Diazirine-modified gold nanoparticle: template for efficient photoinduced interfacial carbene insertion reactions.  

PubMed

Photolysis of a 3-aryl-3-(trifluoromethyl)diazirine-modified monolayer-protected gold nanoparticles (2-C(12)MPNs), with a core size of 1.8 ± 0.3 nm, in the presence of model carbene trapping reagents leads to efficient, essentially quantitative, modification of the interface via carbene insertion reactions. The utility of carbene insertion reactions as a general approach for the modification of Au-MPNs to provide a breadth of new structures available was demonstrated using acetic acid, methanol, benzyl alcohol, phenol, benzylamine, methyl acrylate, and styrene (10a-g, respectively) as electrophilic carbene trapping agents to form the corresponding modified 3a-g-C(12)MPNs. The 1.8 ± 0.3 nm gold nanoparticles bearing a diazirine group (2-C(12)MPNs) were synthesized using the ligand exchange reaction with the requisite 3-aryl-3-(trifluoromethyl)diazirinealkylthiol. The 2-C(12)MPNs and the resulting products of the reaction on the MPN (3a-g-C(12)MPN) were fully characterized by IR, (1)H NMR, and (19)F NMR spectroscopy and, when applicable, transmission electron microscopy (TEM). Verification for the 3a-g-C(12)MPNs was accomplished by comparison of the spectral data to those of obtained for the photoreactions of 3-(3-methoxyphenyl)-3-(trifluoromethyl)-3H-diazirine as a model with 10a-g. PMID:20735050

Ismaili, Hossein; Lee, Soo; Workentin, Mark S

2010-09-21

72

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

2013-09-03

73

Catalytic cross-coupling reactions mediated by palladium\\/nucleophilic carbene systems  

Microsoft Academic Search

In this mini-review, we present a summary of our recent work in the field of palladium-catalyzed cross-coupling reactions, with emphasis on the use of nucleophilic N-heterocyclic carbenes (NHC) as ancillary ligand. The palladium-mediated coupling reactions investigated include the Suzuki–Miyaura, Kumada–Tamao–Corriu, Heck, Sonogashira, Stille, Hiyama and aryl amination reactions.

Anna C Hillier; Gabriela A Grasa; Mihai S Viciu; Hon Man Lee; Chuluo Yang; Steven P Nolan

2002-01-01

74

Abstraction of methyl from neutral Fischer-type carbene complexes: A new site for nucleophilic attack  

SciTech Connect

Reactions of Fischer-type carbene complexes, M(CO){sub 5}(C(OMe)pH) (M = Cr, W), with metal carbonyl anions (M`{sup -} = CpFe(CO){sub 2}{sup -@}, Re(CO){sub 5}{sup -}, Mn(CO){sub 4}PPh{sub 3}{sup -}, Co(CO){sub 3}PPh{sub 3}{sup -}, Cp{sup *}Cr(CO){sub 3}{sup -}, CpMo(CO){sub 3}{sup -}) result in demethylation of the carbene complexes. The products are M(CO){sub 5}C(O)Ph{sup -} and M`-Me, characterized by infrared and NMR spectroscopy. A slower rate for reaction with W(CO){sub 5}(C(OEt)Ph) in comparison to the methyl analogue is consistent with nucleophilic attack of the metal carbonyl anion on the methyl of the methoxy group of the carbene. This is a new type of nucleophilic attack of a Fischer-type carbene. 22 refs., 1 fig., 1 tab.

Toomey, L.M.; Atwood, J.D. [State Univ. of New York, Buffalo, NY (United States)

1997-02-04

75

Regioselective allene hydrosilylation catalyzed by N-heterocyclic carbene complexes of nickel and palladium.  

PubMed

Regioselective methods for allene hydrosilylation have been developed, with regioselectivity being governed primarily by the choice of metal. Alkenylsilanes are produced via nickel catalysis with larger N-heterocyclic carbene (NHC) ligands, and allylsilanes are produced via palladium catalysis with smaller NHC ligands. These complementary methods allow either regioisomeric product to be obtained with exceptional regiocontrol. PMID:24079389

Miller, Zachary D; Li, Wei; Belderrain, Tomás R; Montgomery, John

2013-10-03

76

Cyclopropenylidene carbene ligands in palladium catalysed coupling reactions: carbene ligand rotation and application to the Stille reaction.  

PubMed

Reaction of [Pd(PPh(3))(4)] with 1,1-dichloro-2,3-diarylcyclopropenes gives complexes of the type cis-[PdCl(2)(PPh(3))(C(3)(Ar)(2))] (Ar = Ph 5, Mes 6). Reaction of [Pd(dba)(2)] with 1,1-dichloro-2,3-diarylcyclopropenes in benzene gave the corresponding binuclear palladium complexes trans-[PdCl(2)(C(3)(Ar)(2))](2) (Ar = Ph 7, p-(OMe)C(6)H(4)8, p-(F)C(6)H(4)9). Alternatively, when the reactions were performed in acetonitrile, the complexes trans-[PdCl(2)(NCMe)(C(3)(Ar)(2))] (Ar = Ph 10, p-(OMe)C(6)H(4)11 and p-(F)C(6)H(4)) 12) were isolated. Addition of phosphine ligands to the binuclear palladium complex 7 or acetonitrile adducts 11 and 12 gave complexes of the type cis-[PdCl(2)(PR(3))(C(3)(Ar)(2))] (Ar = Ph, R = Cy 13, Ar = p-(OMe)C(6)H(4), R = Ph 14, Ar = p-(F)C(6)H(4), R = Ph 15). Crystal structures of complexes 6·3.25CHCl(3), 10, 11·H(2)O and 12-15 are reported. DFT calculations of complexes 10-12 indicate the barrier to rotation about the carbene-palladium bond is very low, suggesting limited double bond character in these species. Complexes 5-9 were tested for catalytic activity in C-C coupling (Mizoroki-Heck, Suzuki-Miyaura and, for the first time, Stille reactions) and C-N coupling (Buchwald-Hartwig amination) showing excellent conversion with moderate to high selectivity. PMID:21475742

Chotima, Ratanon; Dale, Tim; Green, Michael; Hey, Thomas W; McMullin, Claire L; Nunns, Adam; Orpen, A Guy; Shishkov, Igor V; Wass, Duncan F; Wingad, Richard L

2011-04-07

77

An N-Heterocyclic Carbene/Lewis Acid Strategy for the Stereoselective Synthesis of Spirooxindole Lactones  

PubMed Central

A cooperative catalysis approach for the enantioselective formal [3+2] addition of ?,?-unsaturated aldehydes to isatins has been developed. The N-heterocyclic carbene (NHC)-catalyzed homoenolate annulations of ?-aryl enals require the addition of lithium chloride for high levels of enantioselectivity. This NHC-catalyzed annulation provides efficient access to the 3-hydroxy indole skeleton and has been applied to the first eantioselective total synthesis of maremycin B.

Dugal-Tessier, Julien; O'Bryan, Elizabeth A.; Schroeder, Thomas B. H.; Cohen, Daniel T.

2012-01-01

78

Cobalt(II)-catalyzed atom\\/group transfer reaction: Stereoselective carbene and nitrene transfer reactions  

Microsoft Academic Search

Metalloporphyrins have been shown to catalyze many fundamental and practically important chemical transformations, some of which represent the first demonstrations of these catalytic processes. The most notable examples include an assortment of atom\\/group transfer reactions, such as oxene, nitrene, and carbene transfers. Atom\\/group transfer reactions allow for the direct conversion of abundant and inexpensive alkenes and alkanes into value-added functional

Joshua V Ruppel

2008-01-01

79

Linking borane with N-heterocyclic carbenes: effective hydrogen-atom donors for radical reactions.  

PubMed

Undue influence: N-heterocyclic carbenes (NHCs) were found to reduce the strength of the B-H bonds of borane by a surprisingly large amount upon the formation of NHC-BH(3) complexes. This property was exploited in the development of a suite of NHC-borane complexes for the reduction of xanthates in radical-mediated Barton-McCombie-type deoxygenation reactions (see scheme). AIBN = azobisisobutyronitrile, Bn = benzyl. PMID:19173269

Walton, John C

2009-01-01

80

Mass spectrometry of laser-initiated carbene reactions for protein topographic analysis.  

PubMed

We report a protein labeling method using nonselective carbene reactions of sufficiently high efficiency to permit detection by mass spectrometric methods. The approach uses a diazirine-modified amino acid (l-2-amino-4,4'-azipentanoic acid, "photoleucine") as a label source, which is converted to a highly reactive carbene by pulsed laser photolysis at 355 nm. Labeling of standard proteins and peptides (CaM, Mb, M13) was achieved with yields up to 390-fold higher than previous studies using methylene. Carbene labeling is sensitive to changes in protein topography brought about by conformational change and ligand binding. The modification of apo-CaM was 45 ± 7% higher than that of holo-CaM. Modification of the CaM-M13 complex reflected a 39 ± 1% reduction in labeling for bound holo-CaM relative to free holo-CaM. Labeling yield is independent of protein concentration over approximately 2 orders of magnitude but is weakly dependent on the presence of other chromophores in a photon-limited apparatus. The current configuration required 2 min of irradiation for full reagent conversion; however, it is shown that comparable yields can be achieved with a single high-energy laser pulse (>100 mJ/pulse, <10 ns), offering a labeling method with high temporal resolution. We suggest a mechanism of labeling governed by limited carbene diffusion and the protein surface activity of the diazirine precursor. This surface activity is speculated to return a measure of selectivity relative to methylene labeling, which ultimately may be tunable. PMID:21425771

Jumper, Chanelle C; Schriemer, David C

2011-03-22

81

Palladium catalyzed asymmetric allylic alkylation using chelating N-heterocyclic carbene–amino ligands  

Microsoft Academic Search

Several silver(I) complexes with chiral amino-N-heterocyclic carbene (NHC) ligands, which are not diastereomerically pure, were prepared and used to generate in situ chelating NHC–amino palladium(II) complexes. The potential of these palladium(II) complexes in asymmetric catalysis was evaluated in the allylic alkylation reaction. The influence of the structure and of the diastereomeric purity of the ligands on enantioselectivity, as well as

Alexandre Flahaut; Sylvain Roland; Pierre Mangeney

2007-01-01

82

N-Heterocyclic carbene–silver complexes: A new class of antibiotics  

Microsoft Academic Search

N-Heterocyclic carbene (NHC)–silver complexes were synthesized from pyridine linked pincer ligands and methylated caffeine. Pincer NHC–silver complexes were found to have more potent antimicrobial activity than the conventionally used silver antimicrobials. Encapsulation of a gem-diol pincer NHC–silver complex in a polymer mat demonstrated a promising method for the sustained delivery of silver ions in wound care applications. An NHC precursor

Aysegul Kascatan-Nebioglu; Matthew J. Panzner; Claire A. Tessier; Carolyn L. Cannon; Wiley J. Youngs

2007-01-01

83

Towards New Generations of Metathesis Metal-Carbene Pre-catalysts  

NASA Astrophysics Data System (ADS)

: A short general introduction combined with some historical milestones in the field of olefin metathesis is presented followed by an overview of recent representatives of metal carbene initiators. This paper attempts to relief the many superb contributions and overwhelming work invested in intelligent design and innovative synthesis in this area. Despites of recent advances there is still a great interest in the generation of new, better performing, and more environment friendly metathesis.

Allaert, Bart; Dieltiens, Nicolai; Stevens, Chris; Drozdzak, Renata; Dragutan, Ileana; Dragutan, Valerian; Verpoort, Francis

84

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

85

Picoline and pyridine functionalised chelate N-heterocyclic carbene complexes of nickel: synthesis and structural studies  

Microsoft Academic Search

The first pyridine and picoline functionalised N-heterocyclic carbene complexes of nickel are described in this paper. The nature of the isolated complexes is critically dependent on the bite angle and the substituents of the ligand used and the synthetic method for the introduction of the ligand on the metal. From NiBr2 (DME), DME=1,2-dimethoxyethane, by transmetallation with one equivalent of Ag(C?N)Cl,

Scott Winston; Neoklis Stylianides; Arran A. D. Tulloch; Joseph A. Wright; Andreas A. Danopoulos

2004-01-01

86

Polyaminoborane main chain scission using N-heterocyclic carbenes; formation of donor-stabilised monomeric aminoboranes.  

PubMed

The reaction of N-heterocyclic carbenes with polyaminoboranes [MeNH-BH2]n or [NH2-BH2]n at 20 °C led to depolymerisation and the formation of labile, monomeric aminoborane-NHC adducts, RNH-BH2-NHC (R = Me or H); a similar NHC adduct of Ph2N[double bond, length as m-dash]BCl2 was characterized by single crystal X-ray diffraction. PMID:23982163

Stubbs, Naomi E; Jurca, Titel; Leitao, Erin M; Woodall, Christopher H; Manners, Ian

2013-10-14

87

Simple and versatile selective synthesis of neutral and cationic copper(I) N-heterocyclic carbene complexes using an electrochemical procedure.  

PubMed

An electrochemical approach for the preparation of copper(I) N-heterocyclic carbene complexes has been developed to include a diverse range of ligand precursors. Importantly, the method is effective for a ligand precursor that contains several acidic protons and for which traditional methods of carbene formation are not suitable. PMID:22498755

Lake, Benjamin R M; Bullough, Emma K; Williams, Thomas J; Whitwood, Adrian C; Little, Marc A; Willans, Charlotte E

2012-04-13

88

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

89

N-Heterocyclic carbene/Br?nsted acid cooperative catalysis as a powerful tool in organic synthesis  

PubMed Central

Summary The interplay between metals and N-heterocyclic carbenes (NHCs) has provided a window of opportunities for the development of novel catalytic strategies within the past few years. The recent successful combination of Brønsted acids with NHCs has added a new dimension to the field of cooperative catalysis, enabling the stereoselective synthesis of functionalized pyrrolidin-2-ones as valuable scaffolds in heterocyclic chemistry. This Commentary will briefly highlight the concept of N-heterocyclic carbene/Brønsted acid cooperative catalysis as a new and powerful methodology in organic chemistry.

De Vreese, Rob

2012-01-01

90

Propagation and termination steps in Rh-mediated carbene polymerisation using diazomethane.  

PubMed

Rh-mediated carbene (co)polymerisation of diazomethane works best in the presence of Rh(III) catalyst precursors, the use of which leads to a significant increase in polymer yield and molecular weight. Chain termination via ?-hydride elimination is severely suppressed for these species, although this process does still occur leading to unsaturated chain ends. Subsequent chain walking leading to the formation of branched polymers seems not to occur. Computational studies describing pathways for both chain propagation and chain termination using a (cycloocta-2,5-dien-1-yl)Rh(III)(alkyl) species as a representative model for the active species revealed that chain propagation is favoured for these species, although ?-hydride elimination is still viable at the applied reaction temperatures. The computational studies are in excellent agreement with all experimental results, and further reveal that chain propagation via carbene insertion (leading to linear poly-methylene) occurs with a much lower energy barrier than insertion of 1-alkenes into either the Rh-H bond after ?-hydride elimination or into the Rh-C bond of the growing polymer chain (leading to branched polymers). These energetic differences conveniently explain why experimentally the formation of branches is not observed in (co)polymerisation reactions employing diazomethane. The formation of substantial amounts of low-M(w) oligomers and dimers in the experimental reactions can be ascribed to the presence of (1,5-cyclooctadiene)Rh(I) species in the reaction mixture, for which chain termination via ?-hydride elimination is clearly favoured over chain propagation. These two species stem from a non-selective catalyst activation process during which the catalyst precursors are in situ activated towards carbene polymerisation, and as such the results in this paper might contribute to further improvements of this reaction. PMID:23283132

Franssen, Nicole M G; Finger, Markus; Reek, Joost N H; de Bruin, Bas

2013-03-28

91

Theoretical studies of mechanisms of cycloaddition reaction between difluoromethylene carbene and acetone  

NASA Astrophysics Data System (ADS)

Mechanisms of the cycloaddition reaction between singlet difluoromethylene carbene and acetone have been investigated with the second-order Møller-Plesset (MP2)/6-31G* method, including geometry optimization and vibrational analysis. Energies for the involved stationary points on the potential energy surface (PES) are corrected by zero-point energy (ZPE) and CCSD(T)/6-31G* single-point calculations. From the PES obtained with the CCSD(T)//MP2/6-31G* method for the cycloaddition reaction between singlet difluoromethylene carbene and acetone, it can be predicted that path B of reactions 2 and 3 should be two competitive leading channels of the cycloaddition reaction between difluoromethylene carbene and acetone. The former consists of two steps: (i) the two reactants first form a four-membered ring intermediate, INT2, which is a barrier-free exothermic reaction of 97.8 kJ/mol; (ii) the intermediate INT2 isomerizes to a four-membered product P2b via a transition state TS2b with an energy barrier of 24.9 kJ/mol, which results from the methyl group transfer. The latter proceeds in three steps: (i) the two reactants first form an intermediate, INT1c, through a barrier-free exothermic reaction of 199.4 kJ/mol; (ii) the intermediate INT1c further reacts with acetone to form a polycyclic intermediate, INT3, which is also a barrier-free exothermic reaction of 27.4 kJ/mol; and (iii) INT3 isomerizes to a polycyclic product P3 via a transition state TS3 with an energy barrier of 25.8 kJ/mol.

Lu, Xiu Hui; Yu, Hai Bin; Wu, Wei Rong; Xu, Yue Hua

92

Pyridinium-derived N-heterocyclic carbene complexes of platinum: synthesis, structure and ligand substitution kinetics.  

PubMed

A series of [(R-iso-BIPY)Pt(CH(3))L ](+)X(-) complexes [R-iso-BIPY = N-(2-pyridyl)-R-pyridine-2-ylidene; (R = 4-H, 1; 4-tert-butyl, 2; 4-dimethylamino, 3; 5-dimethylamino, 4); L = SMe(2), b; dimethyl sulfoxide (DMSO), c; carbon monoxide (CO), d; X = OTf(-) = trifluoromethanesulfonate and/or [BPh(4)](-)] were synthesized by cyclometalation of the [R-iso-BIPY-H](+)[OTF](-) salts 1a-4a ([R-iso-BIPY-H](+) = N-(2-pyridyl)-R-pyridinium) with dimethylplatinum-micro-dimethyl sulfide dimer. X-ray crystal structures for 1b, 2c-4c as well as complexes having bipyridyl and cyclometalated phenylpyridine ligands, [(bipy)Pt(CH(3))(DMSO)](+) (5c) and (C(11)H(8)N)Pt(CH(3))(DMSO) (6c), have been determined. The pyridinium-derived N-heterocyclic carbene complexes display localized C-C and C-N bonds within the pyridinium ligand that are indicative of carbene pi-acidity. The significantly shortened platinum-carbon distance, for "parent" complex 1b, together with NMR parameters and the nu(CO) values for carbonyl cations 1d-4d support a degree of Pt-C10 multiple bonding, increasing in the order 3 < 4 < 2 < 1. Degenerate DMSO exchange kinetics have been determined to establish the nature and magnitude of the trans-labilizing ability of these new N-heterocyclic carbene ligands. Exceptionally large second-order rate constants (k(2) = 6.5 +/- 0.4 M(-1).s(-1) (3c) to 2300 +/- 500 M(-1).s(-1) (1c)) were measured at 25 degrees C using (1)H NMR magnetization transfer kinetics and variable temperature line shape analysis. These rate constants are as much as 4 orders of magnitude greater than those of a series of structurally similar cationic bis(nitrogen)-donor complexes [(N-N)Pt(CH(3))(DMSO)](+) reported earlier, and a factor of 32 to 1800 faster than an analogous charge neutral complex derived from cyclometalated 2-phenylpyridine, (C(11)H(8)N)Pt(CH(3))(DMSO) (k(2) = 0.21 +/- 0.02 M(-1).s(-1) (6c)). The differences in rate constant are discussed in terms of ground state versus transition state energies. Comparison of the platinum-sulfur distances with second order rate constants suggests that differences in the transition-state energy are largely responsible for the range of rate constants measured. The pi-accepting ability and trans-influence of the carbene donor are proposed as the origin of the large acceleration in associative ligand substitution rate. PMID:15225067

Owen, Jonathan S; Labinger, Jay A; Bercaw, John E

2004-07-01

93

Oxygenation of ruthenium carbene complexes containing naphthothiophene or naphthofuran: spectroscopic and DFT studies.  

PubMed

The aryl propargylic alcohol 1-[2-(thiophen-3-yl)phenyl]prop-2-yn-1-ol () is readily prepared from 2-(thiophen-3-yl)benzaldehyde. In the presence of visible light, treatment of with one-half mole equivalent of [Ru]Cl ([Ru]?Cp(dppe)Ru) (dppe=1,2-bis(diphenylphosphino)ethane) and NH4 PF6 in O2 affords the naphtha[2,1-b]thiophene-4-carbaldehyde () in high yields. The cyclization reaction of proceeds through the formation of the carbene complex that contains the naphtha[2,1-b]thiophene ring, which is isolated in a 1:1 stoichiometric reaction. The C?C bond formation between the inner carbon of the terminal triple bond and the heterocyclic ring is confirmed by structure determination of using single-crystal X-ray diffraction analysis. Facile oxygenation of by O2 yields the aldehyde product accompanied by the formation of phosphine oxide of dppe. Oxygen is most likely activated by coordination to the ruthenium center when one PPh2 unit of the dppe ligand dissociates. This dissociated PPh2 unit then reacts with the coordinated oxygen nearby to generate half-oxidized dppe ligand and an unobserved oxo-carbene intermediate. Coupling of the oxo/carbene ligands followed by demetalation then yields . Presumably the resulting complex with the half-oxidized dppe ligand continuously promotes cyclization/oxygenation of to yield the second aldehyde molecule. In alcohol such as MeOH or EtOH, the oxygenation reaction affords a mixture of and the corresponding esters or . Four other aryl propargylic alcohols , which contain thiophen-2-yl, isopropenyl, fur-3-yl, and fur-2-yl, respectively, on the aryl ring are also prepared. Analogous aldehydes are similarly prepared from , respectively. For oxygenations of , , and in alcohol, mixtures of aldehyde , ester , and acetal are obtained. The carbene complex obtained from was also characterized by single-crystal X-ray diffraction analysis. The UV/Vis spectra of and consist of absorption bands with a high extinction coefficient. From DFT calculations on and , the visible light is found to populate the LUMO antibonding orbital of mainly Ru?C bonds, thereby weakening the Ru?C bond and promoting the oxygenation/demetalation reactions of . PMID:23929767

Tsai, Fu-Yuan; Lo, Ji-Xian; Hsu, Hsin-Tzu; Lin, Ying-Chih; Huang, Shou-Ling; Wang, Ju-Chun; Liu, Yi-Hong

2013-08-08

94

Synthesis, characterization and antimicrobial activity of new silver complexes with N-heterocyclic carbene ligands  

Microsoft Academic Search

Synthesis, structure, and antimicrobial studies of silver complexes of N-heterocyclic carbene (NHC) are reported. All the silver–NHC complexes (1a–f) were prepared from the benzimidazolium salts by the reactions with Ag2O in dichloromethane as a solvent at room temperature. The new compounds characterized by 1H NMR, 13 C NMR, IR and elemental analysis techniques which support the proposed structures. Chloro[1-(2,4,6-trimethylbenzyl)-3-(methoxyethyl)benzimidazol-2-ylidene]silver(I) complex

?lknur Özdemir; Serpil Demir; Selami Günal; ?smail Özdemir; Cengiz Ar?c?; Dinçer Ülkü

2010-01-01

95

Furan and thiophene-functionalised bis-carbene ligands: Synthesis, silver(I) complexes, and catalysis  

Microsoft Academic Search

New furan- and thiophene-functionalised nucleophilic heterocyclic carbene (NHC) complexes of Ag(I) were prepared via the reaction of novel furan- and thiophene-functionalised bis-imidazolium salts with Ag2O. Samples of both the N-methyl substituted furan- and thiophene-functionalised Ag(I) complexes suitable for single crystal X-ray studies were obtained following anion metathesis to the tetrafluoroborate salts. The structural characterisations revealed dinuclear [Ag2(MeCEC)2](BF4)2 (E=O, S) formulations

David J. Nielsen; Kingsley J. Cavell; Mihai S. Viciu; Steven P. Nolan; Brian W. Skelton; Allan H. White

2005-01-01

96

Geometry and electronic structure of (CO)âNiCHâ. A model transition-metal carbene  

Microsoft Academic Search

The first application of nonempirical molecular electronic structure theory to a realistic transition-metal carbene complex is reported. The system chosen was (CO)âNiCHâ, methylene(tricarbonyl)nickel(0). All studies were carried out at the self-consistent-field (SCF) level. A large and flexibly contracted basis set was chosen, labeled Ni(15s 11p 6d\\/11s 8p 3d), C,O(9s 5p\\/4s 2p), H(5s\\/3s). The critical predicted equilibrium geometrical parameters were R(Ni-C(methylene))

Dale Spangler; John J. Wendoloski; Michel Dupuis; Maynard M. L. Chen; Henry F. Schaefer

1981-01-01

97

Complexes of borane and N-heterocyclic carbenes: a new class of radical hydrogen atom donor.  

PubMed

Calculations suggest that complexes of borane with N-heterocyclic carbenes (NHC) have B-H bond dissocation energies more then 20 kcal/mol less than free borane, diborane, borane-THF, and related complexes. Values are in the range of popular radical hydrogen atom donors like tin hydrides (70-80 kcal/mol). The resulting prediction that NHC borane complexes could be used as radical hydrogen atom donors was verified by radical deoxygenations of xanthates by using either AIBN or triethylborane as initiator. PMID:18611014

Ueng, Shau-Hua; Brahmi, Malika Makhlouf; Derat, Etienne; Fensterbank, Louis; Lacôte, Emmanuel; Malacria, Max; Curran, Dennis P

2008-07-09

98

Abnormal N-heterocyclic carbene main group organometallic chemistry: a debut to the homogeneous catalysis.  

PubMed

Abnormal N-heterocyclic carbene (aNHC) adducts of zinc(ii) () and aluminum(iii) () were synthesized. The compounds were characterized by NMR spectroscopy and elemental analysis. The solid state structures of these complexes ( and ) were determined by single crystal X-ray study. Furthermore, these organozinc and organoaluminum adducts ( and ) were tested for the ring opening polymerization of cyclic esters. These adducts were found to be quite efficient catalysts for the polymerization of cyclicesters such as rac-lactide (rac-LA), ?-caprolactone (?-CL), and ?-valerolactone (?-VL). Furthermore, aNHC zinc adduct has been used as catalyst for the synthesis of a tri-block copolymer. PMID:23945705

Sen, Tamal K; Sau, Samaresh Chandra; Mukherjee, Arup; Hota, Pradip Kumar; Mandal, Swadhin K; Maity, Bholanath; Koley, Debasis

2013-09-18

99

N-Heterocyclic carbene-catalyzed (4+2) cycloaddition/decarboxylation of silyl dienol ethers with ?,?-unsaturated acid fluorides.  

PubMed

Herein we report the first all-carbon N-heterocyclic carbene-catalyzed (4 + 2) cycloaddition. The reaction proceeds with ?,?-unsaturated acid fluorides and silyl dienol ethers and produces 1,3-cyclohexadienes with complete diastereocontrol (dr >20:1) while demonstrating a new type of reaction cascade exploiting ?,?-unsaturated acyl azoliums. PMID:21391673

Ryan, Sarah J; Candish, Lisa; Lupton, David W

2011-03-10

100

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…

Cooke, Jason; Lightbody, Owen C.

2011-01-01

101

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

2011-09-16

102

Synthesis of phenanthrene derivatives through the net [5+5]-cycloaddition of prenylated carbene complexes with 2-alkynylbenzaldehyde derivatives  

PubMed Central

The reaction of prenylated carbene complexes and 2-alkynylbenzoyl derivatives has been investigated. Phenanthrene derivatives are produced if iodine is added prior to product isolation. Under these conditions alkyl migration reactions occur to form the observed products. The product yields are considerably higher using bis(prenylated) species owing to an increase in the effective molarity of dienophilic entities.

Menon, Suneetha; Sinha-Mahapatra, Dilip

2007-01-01

103

Search for thermal isomerization of olefins to carbenes: Thermal generations of the silicon-nitrogen double bond.  

National Technical Information Service (NTIS)

The first part of this thesis will search for the thermal isomerization of olefins to carbenes which is predicted to be a high energy process by calculations and has only been observed in a few strained olefins. The possibility of thermal isomerization of...

X. Zhang

1990-01-01

104

Synthesis, structure, and electronic properties of extended ?-conjugated Group?6 Fischer alkoxy-bis(carbene) complexes.  

PubMed

The synthesis, structure and electronic properties of novel Group?6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550?nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-?-D-?-A (A = acceptor, D = donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry. PMID:23504973

Chu, Gong M; Fernández, Israel; Sierra, Miguel A

2013-03-15

105

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

2013-03-02

106

Path length determines the tunneling decay of substituted carbenes.  

PubMed

Quantum mechanical tunneling of atoms allows chemical reactions to proceed through barriers too high for thermally activated processes. This causes hydroxycarbenes to decay rapidly and at a temperature-independent rate even at 11 K. In methylhydroxycarbene, tunneling causes decay through a mechanism that reveals a high but thin barrier rather than an alternative with a lower but broader barrier. No accurate estimates of the widths of such barriers and the lengths of tunneling paths were available. Herein, such a measure is provided by calculating the length of the tunneling paths by using instanton theory. Potential energies are provided by density functional theory verified by explicitly correlated coupled cluster CCSD(T) energies. Our results explain the decay efficiency in the known cases and suggest new substitutions to tune the effects of barrier widths and heights. Fluorination and replacement of the hydroxyl group by a thiol group change the qualitative character of the decay. Methylaminocarbene is predicted to be stable for thousands of years. PMID:23609956

Kästner, Johannes

2013-04-22

107

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

PubMed

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

Liu, Zhenxing; Wang, Jianbo

2013-09-19

108

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

109

Bis-N-heterocyclic carbene ruthenium(II) carbonyl complexes: Synthesis, structural characterization and catalytic activities in transfer hydrogenation of ketones  

Microsoft Academic Search

In this contribution, the synthesis and characterization of eight ruthenium(II) carbonyl complexes supported by chelating alkane-bridged bis-N-heterocyclic carbene ligands are reported. The products obtained are analyzed using infrared and NMR spectroscopies. The molecular structures of four metal complexes were determined by X-ray crystallography, which exhibit the six-coordinate octahedral geometry with two carbene carbon atoms from the bidentate Bi-NHCs, two carbonyl

Yong Cheng; Xiang-Yong Lu; Hui-Jun Xu; Yi-Zhi Li; Xue-Tai Chen; Zi-Ling Xue

2010-01-01

110

Chromium-silicon multiple bonds: the chemistry of terminal N-heterocyclic-carbene-stabilized halosilylidyne ligands.  

PubMed

An efficient method for the synthesis of the first N-heterocyclic carbene (NHC)-stabilized halosilylidyne complexes is reported that starts from SiBr(4). In the first step, SiBr(4) was treated with one equivalent of the N-heterocyclic carbene 1,3-bis[2,6-bis(isopropyl)phenyl]imidazolidin-2-ylidene (SIdipp) to give the 4,5-dihydroimidazolium salt [SiBr(3)(SIdipp)]Br (1-Br), which then was reduced with potassium graphite to afford the silicon(II) dibromide-NHC adduct SiBr(2)(SIdipp)(2-Br) in good yields. Heating 2-Br with Li[CpCr(CO)(3)] afforded the complex [Cp(CO)(2)Cr=SiBr(SIdipp)] (3-Br) upon elimination of CO. Complex 3-Br features a trigonal-planar-coordinated silicon center and a very short Cr?Si double bond. Similarly, the reaction of SiCl(2)(SIdipp) (2-Cl) with Li[CpCr(CO)(3)] gave the analogous chloro derivative [Cp(CO)(2)Cr=SiCl(SIdipp)] (3-Cl). Complex 3-Br undergoes an NHC exchange with 1,3-dihydro-4,5-dimethyl-1,3-bis(isopropyl)-2H-imidazol-2-ylidene (IMe(2)iPr(2)) to give the complex [Cp(CO)(2)CrSiBr(IMe(2)iPr(2))(2)] (4-Br). Compound 4-Br features a distorted-tetrahedral four-coordinate silicon center. Bromide abstraction occurs readily from 4-Br with Li[B(C(6)F(5))(4)] to give the putative silylidene complex salt [Cp(CO)(2)Cr=Si(IMe(2)iPr(2))(2)][B(C(6)F(5))(4)], which irreversibly dimerizes by means of an Si-promoted electrophilic activation of one carbonyl oxygen atom to yield the dinuclear siloxycarbyne complex [Cp(CO)Cr{(?-CO)Si(IMe(2)iPr(2))(2)}(2-)Cr(CO)Cp][B(C(6)F(5))(4)](2) (5). All compounds were fully characterized, and the molecular structures of 2-Br-5-Br were determined by single-crystal X-ray diffraction. DFT calculations of 3-Br and 3-Cl and their carbene dissociation products [Cp(CO)(2)Cr=Si-X] (X=Cl, Br) were carried out, and the electronic structures of 3-Br, 3-Cl and [Cp(CO)(2)Cr=Si-X] were analyzed by the natural bond orbital method in combination with natural resonance theory. PMID:22038596

Filippou, Alexander C; Chernov, Oleg; Schnakenburg, Gregor

2011-10-31

111

Carbene-mediated functionalization of the anomeric C-H bond of carbohydrates: scope and limitations.  

PubMed

Herein we investigate the scope and limitations of a new synthetic approach towards ?- and ?-ketopyranosides relying on the functionalization of the anomeric C-H bond of carbohydrates by insertion of a metal carbene. A key bromoacetate grafted at the 2-position is the cornerstone of a stereoselective glycosylation/diazotransfer/quaternarization sequence that makes possible the construction of a quaternary center with complete control of the stereochemistry. This sequence shows a good tolerance toward protecting groups commonly used in carbohydrate chemistry and gives rise to quaternary disaccharides with good efficiency. In the case of a disaccharide with a more restricted conformation, this functionalization process can be hampered by the steric demand next to the targeted anomeric position. In addition, the formation of transient orthoesters during the glycosylation step may also reduce the overall efficiency of the synthetic sequence. PMID:23418074

Boultadakis-Arapinis, Mélissa; Prost, Elise; Gandon, Vincent; Lemoine, Pascale; Turcaud, Serge; Micouin, Laurent; Lecourt, Thomas

2013-02-18

112

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

113

Experimental and theoretical studies of a silver complex of O-functionalized N-heterocyclic carbene  

Microsoft Academic Search

Experimental and theoretical studies of a silver complex namely, [1-i-propyl-3-(2-oxo-2-t-butyl ethyl)imidazol-2-ylidene]AgCl (1b), supported over an O-functionalized N-heterocyclic carbene ligand are reported. Specifically, [1-i-propyl-3-(2-oxo-2-t-butyl ethyl)imidazol-2-ylidene]AgCl (1b) was synthesized by reaction of 1-i-propyl-3-(2-oxo-2-t-butyl ethyl)imidazolium chloride 1a with Ag2O in 42% yield. The 1-i-propyl-3-(2-oxo-2-t-butyl ethyl)imidazolium chloride 1a was synthesized by the alkylation reaction of 1-i-propylimidazole with ?-chloropinacolone in 70% yield. The molecular structures

Manoja K. Samantaray; Dipankar Roy; Ahindra Patra; Raji Stephen; Mobin Saikh; Raghavan B. Sunoj; Prasenjit Ghosh

2006-01-01

114

Silver alkoxide and amino N-heterocyclic carbenes; syntheses and crystal structures  

Microsoft Academic Search

Silver(I) complexes of heterobidentate ligands that incorporate one or two N-heterocyclic carbene moieties coupled with an alcohol or amine group have been made by direct deprotonation of ligands of the form [HOCR1R2CH2(1-HC{NCHCHNR})][X], H2L1X (X=Br, I), [H2NR1CHR2CHR2(1-HC{NCHCHNR})][Br]2 H3L2X2 (X=Cl, Br), and [H2N{CH2CH2(1-HC[NCHCHNMes])}2][X]3 H4L3X3 (X=Cl, Br). Silver(I) oxide is sufficiently basic to deprotonate both the imidazolium and the alcohol functional groups of

Ian S. Edworthy; Mark Rodden; Shaheed A. Mungur; Kate M. Davis; Alexander J. Blake; Claire Wilson; Martin Schröder; Polly L. Arnold

2005-01-01

115

Conjugate reduction of alpha,beta-unsaturated carbonyl compounds catalyzed by a copper carbene complex.  

PubMed

[reaction: see text] An N-heterocyclic carbene copper chloride (NHC-CuCl) complex (2) has been prepared and used to catalyze the conjugate reduction of alpha,beta-unsaturated carbonyl compounds. The combination of catalytic amounts of 2 and NaOt-Bu with poly(methylhydrosiloxane) (PMHS) as the stoichiometric reductant generates an active catalyst for the 1,4-reduction of tri- and tetrasubstituted alpha,beta-unsaturated esters and cyclic enones. The active catalytic species can also be generated in situ from 1,3-bis(2,6-di-isopropylphenyl)-imidazolium chloride (1) CuCl(2).2H(2)O in the presence of NaOt-Bu and PMHS. PMID:12841744

Jurkauskas, Valdas; Sadighi, Joseph P; Buchwald, Stephen L

2003-07-10

116

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

117

?-Carbon activation of saturated carboxylic esters through N-heterocyclic carbene organocatalysis  

NASA Astrophysics Data System (ADS)

The activation of the ?-carbons of carboxylic esters and related carbonyl compounds to generate enolate equivalents as nucleophiles is one of the most powerful strategies in organic synthesis. We reasoned that the horizons of chemical synthesis could be greatly expanded if the typically inert ?-carbons of saturated esters could be used as nucleophiles. However, despite the rather significant fundamental and practical values, direct use of the ?-carbons of saturated carbonyl compounds as nucleophiles remains elusive. Here we report the catalytic activation of simple saturated ester ?-carbons as nucleophiles (?-carbon activation) using N-heterocyclic carbene organocatalysts. The catalytically generated nucleophilic ?-carbons undergo enantioselective reactions with electrophiles such as enones and imines. Given the proven rich chemistry of ester ?-carbons, we expect this catalytic activation mode for saturated ester ?-carbons to open a valuable new arena for new and useful reactions and synthetic strategies.

Fu, Zhenqian; Xu, Jianfeng; Zhu, Tingshun; Leong, Wendy Wen Yi; Chi, Yonggui Robin

2013-10-01

118

Cooperative Catalysis by Carbenes and Lewis Acids in a Highly Stereoselective Route to ?-Lactams  

PubMed Central

Enzymes are a continuing source of inspiration for the design of new chemical reactions that proceed with efficiency, high selectivity and minimal waste. In many biochemical processes, different catalytic species, such as Lewis acids and bases, are involved in precisely orchestrated interactions to activate reactants simultaneously or sequentially. Employing this type of cooperative catalysis, in which two or more catalytic cycles operate concurrently to achieve one overall transformation, has great potential to enhance known reactivity and drive the development of new chemical reactions with high value. In this disclosure, a cooperative N-heterocyclic carbene/Lewis acid catalytic system promotes the addition of homoenolate equivalents to hydrazones generating highly substituted ?-lactams in moderate to good yields and high levels of diastereo- and enantioselectivity.

Raup, Dustin E. A.; Cardinal-David, Benoit; Holte, Dane; Scheidt, Karl A.

2010-01-01

119

?-Carbon activation of saturated carboxylic esters through N-heterocyclic carbene organocatalysis.  

PubMed

The activation of the ?-carbons of carboxylic esters and related carbonyl compounds to generate enolate equivalents as nucleophiles is one of the most powerful strategies in organic synthesis. We reasoned that the horizons of chemical synthesis could be greatly expanded if the typically inert ?-carbons of saturated esters could be used as nucleophiles. However, despite the rather significant fundamental and practical values, direct use of the ?-carbons of saturated carbonyl compounds as nucleophiles remains elusive. Here we report the catalytic activation of simple saturated ester ?-carbons as nucleophiles (?-carbon activation) using N-heterocyclic carbene organocatalysts. The catalytically generated nucleophilic ?-carbons undergo enantioselective reactions with electrophiles such as enones and imines. Given the proven rich chemistry of ester ?-carbons, we expect this catalytic activation mode for saturated ester ?-carbons to open a valuable new arena for new and useful reactions and synthetic strategies. PMID:24056339

Fu, Zhenqian; Xu, Jianfeng; Zhu, Tingshun; Leong, Wendy Wen Yi; Chi, Yonggui Robin

2013-07-21

120

Cleavage of carbon dioxide by an iridium-supported Fischer carbene. A DFT investigation.  

PubMed

The reaction of CO(2), OCS, and PhNCO with an iridium-supported Fischer alkoxycarbene has been investigated with density functional theory. We have confirmed the mechanism for the important CO(2) reaction and successfully rationalized the selective cleavage of the CS and CN bonds in OCS and PhNCO. Armed with this information we have used our model to predict that the same iridium system will preferentially cleave the CS bond in methyl thiocyanate (MeNCS) rather than the CN bond. The formation of the iridium-supported carbene itself has also been investigated and a fascinating autocatalytic mechanism has been discovered which nicely fits the observed experimental behavior. PMID:19338328

Brookes, Nigel J; Ariafard, Alireza; Stranger, Robert; Yates, Brian F

2009-04-29

121

Synthesis and activity of ruthenium alkylidene complexes coordinated with phosphine and N-heterocyclic carbene ligands.  

PubMed

This paper reports the synthesis and characterization of a variety of ruthenium complexes coordinated with phosphine and N-heterocyclic carbene (NHC) ligands. These complexes include several alkylidene derivatives of the general formula (NHC)(PR(3))(Cl)(2)Ru=CHR', which are highly active olefin metathesis catalysts. Although these catalysts can be prepared adequately by the reaction of bis(phosphine) ruthenium alkylidene precursors with free NHCs, we have developed an alternative route that employs NHC-alcohol or -chloroform adducts as "protected" forms of the NHC ligands. This route is advantageous because NHC adducts are easier to handle than their free carbene counterparts. We also demonstrate that sterically bulky bis(NHC) complexes can be made by reaction of the pyridine-coordinated precursor (NHC)(py)(2)(Cl)(2)Ru=CHPh with free NHCs or NHC adducts. Two crystal structures are presented, one of the mixed bis(NHC) derivative (H(2)IMes)(IMes)(Cl)(2)Ru=CHPh, and the other of (PCy(3))(Cl)(CO)Ru[eta(2)-(CH(2)-C(6)H(2)Me(2))(N(2)C(3)H(4))(C(6)H(2)Me(3))], the product of ortho methyl C-H bond activation. Other side reactions encountered during the synthesis of new ruthenium alkylidene complexes include the formation of hydrido-carbonyl-chloride derivatives in the presence of primary alcohols and the deprotonation of ruthenium vinylcarbene ligands by KOBu(t). We also evaluate the olefin metathesis activity of NHC-coordinated complexes in representative RCM and ROMP reactions. PMID:12603143

Trnka, Tina M; Morgan, John P; Sanford, Melanie S; Wilhelm, Thomas E; Scholl, Matthias; Choi, Tae-Lim; Ding, Sheng; Day, Michael W; Grubbs, Robert H

2003-03-01

122

The reaction of amidinato(pyridine) complexes of molybdenum and tungsten with triethylborane adduct of N-heterocyclic carbene (NHC · BEt 3): Investigation on the reactivity of NHC · BEt 3 as carbene precursor toward transition metal complexes  

Microsoft Academic Search

The reaction of triethylborane adduct of N-heterocyclic carbene, NHC·BEt3, (NHC=IiPr=1,3-diisopropylimidazol-2-ylidene (IiPr·BEt3; 1a), NHC=IMes=1,3-dimesitylimidazol-2-ylidene (IMes·BEt3; 1b)), which was prepared by the reaction of the corresponding imidazolium salt with one equivalent of LiBEt3H, with amidinato(pyridine) complex, [M(?3-allyl){?2-(NPh)2CH}(CO)2(NC5H5)] (M=Mo; 2-Mo M=W; 2-W), was investigated. The reaction of compound 1 with complex 2 under toluene-reflux conditions resulted in the formation of carbene complex [M(?3-allyl){?2-(NPh)2CH}(CO)2(NHC)

Kenichi Ogata; Yoshitaka Yamaguchi; Taigo Kashiwabara; Takashi Ito

2005-01-01

123

A search for thermal isomerization of olefins to carbenes: Thermal generations of the silicon-nitrogen double bond  

SciTech Connect

The first part of this thesis will search for the thermal isomerization of olefins to carbenes which is predicted to be a high energy process by calculations and has only been observed in a few strained olefins. The possibility of thermal isomerization of simple olefins to carbenes will be explored. Substitution of a silyl group on the double bond of an olefin allows a potential intermediate which has a {beta}-radical to the silyl group during the cis-trans isomerization. The effects of a trimethylsilyl group on this isomerization are the subject of this study. The second part of this thesis will include the generation and chemistry of intermediates containing a silicon-nitrogen double bond. The isomerization of parent silanimine to the aminosilylene was calculated to be a high energy process. New approaches to the silicon-nitrogen double bond will also be presented. 92 refs., 12 figs., 11 tabs.

Zhang, Xianping.

1990-09-21

124

Silver N-heterocyclic carbene complexes as initiators for bulk ring-opening polymerization (ROP) of l-lactides  

Microsoft Academic Search

Synthetic, structural and catalysis studies of two silver complexes namely, {[1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazol-2-ylidene]2Ag}+Cl?1b, supported over an amido-functionalized N-heterocyclic carbene ligand, and [1-(i-propyl)-3-(benzyl)imidazol-2-ylidene]AgCl 2b, supported over a non-functionalized N-heterocyclic carbene ligand, are reported. Specifically, 1b, a cationic complex bearing 2:1 NHC ligand to metal ratio, was obtained from the reaction of 1-(2,4,6-trimethylphenyl)-3-(N-phenylacetamido)imidazolium chloride 1a with Ag2O in 52% yield. The corresponding 1a was

Manoja K. Samantaray; Vimal Katiyar; Keliang Pang; Hemant Nanavati; Prasenjit Ghosh

2007-01-01

125

Gold(I) N-heterocyclic carbene based initiators for bulk ring-opening polymerization of l-lactide  

Microsoft Academic Search

Synthesis, structures, and catalysis studies of gold(I) complexes of N-heterocyclic carbenes namely, a di-O-functionalized [1-(2-hydroxy-cyclohexyl)-3-(acetophenone)imidazol-2-ylidene], a mono-O-functionalized [1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene] and a non-functionalized [1,3-di-i-propyl-benzimidazol-2-ylidene], are reported. Specifically, the gold complexes, [1-(2-hydroxy-cyclohexyl)-3-(acetophenone)imidazol-2-ylidene]AuCl (1c), [1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]AuCl (2c), and [1,3-di-i-propyl-benzimidazol-2-ylidene]AuCl (3b), were prepared from the respective silver complexes 1b, 2b, and 3a by treatment with (SMe2)AuCl in good yields following the commonly used silver carbene transfer

Lipika Ray; Vimal Katiyar; Samir Barman; Mustafa J. Raihan; Hemant Nanavati; Mobin M. Shaikh; Prasenjit Ghosh

2007-01-01

126

Preparation of a N-Heterocyclic Carbene Nickel(II) Complex: Synthetic Experiments in Current Organic and Organometallic Chemistry  

ERIC Educational Resources Information Center

|A four-part experiment that leads to the synthesis of a cyclopentadienyl chloro-nickel(II) complex bearing a N-heterocyclic carbene (NHC) ligand is presented. In the first part, the preparation of 1,3-bis-(2,4,6-trimethylphenyl)imidazolium chloride (IMes[middle dot]HCl) in a one-pot procedure by reaction of 2,4,6-trimethylaniline with…

Ritleng, Vincent; Brenner, Eric; Chetcuti, Michael J.

2008-01-01

127

O-Glycosidation reactions promoted by in situ generated silver N-heterocyclic carbenes in ionic liquids  

Microsoft Academic Search

We herein report O-glycosidation reactions promoted via silver N-heterocyclic carbene complexes formed in situ in ionic liquids. Seven different room temperature ionic liquids were screened for the glycosidation reaction of 4-nitrophenol with tetra-O-acetyl-?-d-galactopyranosyl bromide. Good to excellent yields were obtained using Ag–NHC complexes derived from imidazolium halide salts to promote the glycosidation reaction, whereas yields considered moderate to low were

Ian Jamie Talisman; Vineet Kumar; Jacqueline Razzaghy; Sanjay V. Malhotra

2011-01-01

128

N-Heterocyclic carbene-palladium(II)-1-methylimidazole complex catalyzed Mizoroki-Heck reaction of aryl chlorides with styrenes  

PubMed Central

Summary A well-defined N-heterocyclic carbene–palladium(II)-1-methylimidazole [NHC-Pd(II)-Im] complex 1 was found to be an effective catalyst for the Mizoroki–Heck reaction of a variety of aryl chlorides with styrenes. Both activated and deactivated aryl chlorides work well to give the corresponding coupling products in good to excellent yields by using tetrabutylammonium bromide (TBAB) as the ionic liquid.

Gao, Ting-Ting; Jin, Ai-Ping

2012-01-01

129

N-Heterocyclic carbene-catalyzed direct cross-aza-benzoin reaction: Efficient synthesis of ?-amino-?-keto esters  

PubMed Central

Summary An efficient catalytic synthesis of ?-amino-?-keto esters has been newly developed. Cross-coupling of various aldehydes with ?-imino ester, catalyzed by N-heterocyclic carbene, leads chemoselectively to ?-amino-?-keto esters in moderate to good yields with high atom efficiency. The reaction mechanism is discussed, and it is proposed that the ?-amino-?-keto esters are formed under thermodynamic control.

Uno, Takuya; Kobayashi, Yusuke

2012-01-01

130

A dinitrosyl iron complex as a platform for metal-bound imidazole to N-heterocyclic carbene conversion.  

PubMed

An N-alkyl imidazole bearing a neutral {Fe(NO)2}(10) dinitrosyliron complex (DNIC) when treated with sodium t-butoxide undergoes base-promoted conversion to the N-heterocyclic carbene (NHC)-DNIC, while maintaining the Fe(NO)2 unit intact. Subsequent alkylation led to the isolation of the NHC-DNIC product; further nitrosylation led to trinitrosyl (NHC)Fe(NO)3(+). Both were isolated and structurally characterized. PMID:23999554

Hsieh, Chung-Hung; Pulukkody, Randara; Darensbourg, Marcetta Y

2013-10-18

131

The total synthesis of (-)-7-deoxyloganin via N-heterocyclic carbene catalyzed rearrangement of ?,?-unsaturated enol esters.  

PubMed

The diastereoselective N-heterocyclic carbene (NHC) catalyzed rearrangement of ?,?-unsaturated enol ester (S)-2b has been used to assemble dihydropyranone (S)-3b, a material embodying the bicyclic core of the iridoid family of natural products. Elaboration of this intermediate, by chemoselective reduction followed by stereoselective ?-glycosylation, has allowed the total synthesis of (-)-7-deoxyloganin (1) to be achieved in four subsequent steps. PMID:20873828

Candish, Lisa; Lupton, David W

2010-11-01

132

N-heterocyclic carbene ligands as mimics of imidazoles/histidine for the stabilization of di- and trinitrosyl iron complexes.  

PubMed

N-heterocyclic carbenes (NHCs) are shown to be reasonable mimics of imidazole ligands in dinitrosyl iron complexes determined through the synthesis and characterization of a series of {Fe(NO)(2)}(10) and {Fe(NO)(2)}(9) (Enemark-Feltham notation) complexes. Monocarbene complexes (NHC-iPr)(CO)Fe(NO)(2) (1) and (NHC-Me)(CO)Fe(NO)(2) (2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene and NHC-Me = 1,3-dimethylimidazol-2-ylidene) are formed from CO/L exchange with Fe(CO)(2)(NO)(2). An additional equivalent of NHC results in the bis-carbene complexes (NHC-iPr)(2)Fe(NO)(2) (3) and (NHC-Me)(2)Fe(NO)(2) (4), which can be oxidized to form the {Fe(NO)(2)}(9) bis-carbene complexes 3(+) and 4(+). Treatment of complexes 1 and 2 with [NO]BF(4) results in the formation of uncommon trinitrosyl iron complexes, (NHC-iPr)Fe(NO)(3)(+) (5(+)) and (NHC-Me)Fe(NO)(3)(+) (6(+)), respectively. Cleavage of the Roussin's Red "ester" (?-SPh)(2)[Fe(NO)(2)](2) with either NHC or imidazole results in the formation of (NHC-iPr)(PhS)Fe(NO)(2) (7) and (Imid-iPr)(PhS)Fe(NO)(2) (10) (Imid-iPr = 2-isopropylimidazole). The solid-state molecular structures of complexes 1, 2, 3, 4, 5(+), and 7 show that they all have pseudotetrahedral geometry. Infrared spectroscopic data suggest that NHCs are slightly better electron donors than imidazoles; electrochemical data are also consistent with what is expected for typical donor/acceptor abilities of the spectator ligands bound to the Fe(NO)(2) unit. Although the monoimidazole complex (Imid-iPr)(CO)Fe(NO)(2) (8) was observed via IR spectroscopy, attempts to isolate this complex resulted in the formation of a tetrameric {Fe(NO)(2)}(9) species, [(Imid-iPr)Fe(NO)(2)](4) (9), a molecular square analogous to the unsubstituted imidazole reported by Li and Wang et al. Preliminary NO-transfer studies demonstrate that the {Fe(NO)(2)}(9) bis-carbene complexes can serve as a source of NO to a target complex, whereas the {Fe(NO)(2)}(10) bis-carbenes are unreactive in the presence of a NO-trapping agent. PMID:21823576

Hess, Jennifer L; Hsieh, Chung-Hung; Reibenspies, Joseph H; Darensbourg, Marcetta Y

2011-08-08

133

Catalytic asymmetric syntheses of quinolizidines by dirhodium-catalyzed dearomatization of isoquinolinium/pyridinium methylides--the role of catalyst and carbene source.  

PubMed

Convenient access to highly enantioenriched substituted quinolizidines has been achieved by chiral dirhodium(II) carboxylate-catalyzed dearomatizing formal [3 + 3]-cycloaddition of isoquinolinium/pyridinium methylides and enol diazoacetates. Coordination of Lewis basic methylides to dirhodium(II) prompts the rearrangement of the enol-carbene that is bound to dirhodium to produce a donor-acceptor cyclopropene. The donor-acceptor cyclopropene is in equilibrium with the dirhodium-bound enol-carbene and undergoes both enantioselective [3 + 3]-cycloaddition from the dirhodium-bound enol-carbene and diastereoselective [3 + 2]-cycloaddition by uncatalyzed reaction of the cyclopropene with isoquinolinium or pyridinium methylides. Increasing the mol % of catalyst loading suppresses the [3 + 2]-cycloaddition pathway. PMID:23889041

Xu, Xichen; Zavalij, Peter Y; Doyle, Michael P

2013-08-09

134

Novel platinum-N-heterocyclic carbene complex is more cardiotoxic than cis-platin in rats.  

PubMed

Cis-platin and other platinum complexes are important chemotherapeutic agents useful in the treatment of several cancers. However, therapeutic usage of cis-platin and other platinum complex are limited by their undesirable side effects including cardiotoxicity. In this context, we aimed to compare the damage caused in heart by cis-platin and novel platinum-N-heterocyclic carbene (Pt-NHC) complex. For this purpose, 35 Sprague-Dawley rats were divided randomly into five equal groups (n = 7 for each group). Cis-platin and novel Pt-NHC complex were intraperitoneally administered at a single dose of 5 mg/kg and 10 mg/kg and then sacrificed 10 days after this treatment. The heart tissues were taken from all rats for determination of oxidative and myocardial damage. Cis-platin and novel Pt-NHC complex caused oxidative and histological damage in the heart tissue in a dose-dependent manner (p < 0.05). On the other hand, at the same dose levels, cis-platin caused lower oxidative and histological damage in heart tissue compared to novel Pt-NHC complex. These results suggest that novel Pt-NHC complex is more cardiotoxic than cis-platin. PMID:21075807

Ciftci, Osman; Ozdemir, Ilknur; Vardi, Nigar; Gurbuz, Nevin

2010-11-12

135

Probing protein surface with a solvent mimetic carbene coupled to detection by mass spectrometry.  

PubMed

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 (:CH(2)). 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. PMID:22006407

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

2011-10-18

136

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

137

Application of silver N-heterocyclic carbene complexes in O-glycosidation reactions  

PubMed Central

We report the efficient O-glycosidation of glycosyl bromides with therapeutically relevant acceptors facilitated by silver N-heterocyclic carbene (Ag-NHC) complexes. A set of four Ag-NHC complexes was synthesized and evaluated as promoters for glycosidation reactions. Two new bis-Ag-NHC complexes derived from ionic liquids 1-benzyl-3-methyl-1H-imidazolium chloride and 1-(2-methoxyethyl)-3-methyl-1H-imidazolium chloride were found to efficiently promote glycosidation, whereas known mono-Ag complexes of 1,3-bis(2,4,6-trimethylphenyl)imidazolium chloride and 1,3-bis(2,6-di-isopropylphenyl)imidazolium chloride failed to facilitate the reaction. The structures of the promoters were established by X-ray crystallography, and these complexes were employed in the glycosidation of different glycosyl bromide donors with biologically valuable acceptors, such as estrone, estradiol, and various flavones. The products were obtained in yields considered good to excellent, and all reactions were highly selective for the ? isomer regardless of neighboring group effects.

Talisman, Ian J.; Kumar, Vineet; Deschamps, Jeffrey R.; Frisch, Mark

2011-01-01

138

Theoretical study of N-heterocyclic carbenes-catalyzed cascade annulation of benzodienones and enals.  

PubMed

Growing attention in developing new N-heterocyclic carbene (NHC)-mediated reactions involving homoenolate intermediates has prompted our interest in exploring the mechanistic details of the related reactions. In this work, we carried out a detailed theoretical study for the NHC-catalyzed annulation reaction of cinnamaldehyde (A) and benzodi(enone) (B) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). By performing density functional theory calculations, we show clearly the detailed reaction mechanism and rationalize the experimental observation. The reaction of A and B falls into two stages: the formation of homoenolate intermediate and the annulation of homoenolate with B. In the homoenolate formation stage, three possible paths are characterized. The pathway involving the DBU-assisted 1,2-proton transfer with a stepwise mechanism is kinetically more favorable, and the DBU-assisted C1 proton departure is the rate-determining step of the total reaction. The annulation of homoenolate with B involves four elementary steps. The conformational difference of homoenolate (cis and trans) leads to two slightly different reaction processes. In the total reaction, the process involving cis-conformation of A is kinetically more feasible. This can be clearly understood through the frontier molecular orbital analysis and the electronic inductive effect. The calculated results are expected to offer valuable information for further design and development of NHC-mediated reactions. PMID:23798330

Sun, Hui; Fang, Xinqiang; Chi, Yonggui Robin; Li, Guohui

2013-06-25

139

Structurally Characterized Cationic Silver(I) and Ruthenium(II)carbene complexes of 1,2,3-Triazol-5-ylidenes.  

PubMed

A novel 1,3,4-substituted 1,2,3-triazolium salt was found to function as an effective precursor for the synthesis of the first structurally characterized cationic silver(I) and ruthenium(II)carbene complexes of overall 1:2 ligand-to-metal stoichiometry. The Ag(I) complex crystallized in the form of an eight silver atom-containing cluster, whereas the Ru(II) complex proved to be a discrete species and was found to be capable of initiating the ring-opening metathesis polymerization of norbornene upon activation with (trimethylsilyl)diazomethane. PMID:22140293

Cai, Jiajia; Yang, Xiaoping; Arumugam, Kuppuswamy; Bielawski, Christopher W; Sessler, Jonathan L

2011-09-26

140

Synthesis and biological studies of silver N-heterocyclic carbene complexes derived from 4,5-diarylimidazole  

Microsoft Academic Search

A novel class of silver N-heterocyclic carbene complexes (5a–f) were synthesized in high yield by reacting silver(I) oxide with 4,5-diarylimidazolium halides (4a–f). The complexes were characterized using NMR and IR spectroscopy. The structure was confirmed on the example of bromo[1,3-diethyl-4,5-bis(4-fluorophenyl)imidazol-2-ylidene]silver(I) (5c) by crystal structure analysis. The X-ray structure indicated a three-dimensional coordination polymer with a repeating unit consisting of a

Wukun Liu; Kerstin Bensdorf; Adelheid Hagenbach; Ulrich Abram; Ben Niu; Aruljothi Mariappan; Ronald Gust

2011-01-01

141

Matrix effects in reactions of formation of triplet carbenes in photodisintegration of diphenyldiazomethane and diazofluorene at 77 K  

SciTech Connect

It was found that the degree of photoconversion of diphenyldiazomethane and diazofluorene into triplet carbenes at 77 K is a function of the nature of the glassy matrix. A correlation between the degree of conversion and the rotational mobility at the time of formation for diphenylmethylene and with the dark rotational mobility in alcohol matrices is observed. This correlation does not exist for fluorenylidene. The observed features are explained with respect to the effect of the microproperties of the matrix on both the possibility of altering the geometry of the molecule in the cage and on its mobility as a whole.

Makarov, B.P.; Babina, O.M.; Gurman, V.S.

1988-11-01

142

Difluoroboroxymolybdenum Fischer carbene complexes as precursors of acyl radicals: dimerization and trapping with electron-deficient alkenes  

PubMed

Pentacarbonyl acyl molybdates 1 react with boron trifluoride to give difluoroboroxy Fischer carbene complexes 2, which undergo loss of the metal fragment at room temperature to form 1,2-diketones 3, 1,2-hydroxy ketones 4, or dimers 5 through a dimerization or decarbonylation-dimerization process of acyl radicals. Decomposition of 2 in the presence of electron-deficient alkenes 11 and 18 furnishes the two-, three-, and four-component coupling products 12, 13, 19, 20, and 21. PMID:10894392

Barluenga; Rodriguez; Fananas

2000-06-01

143

Theoretical insight into electronic spectra of carbon chain carbenes H2Cn (n = 3-10)  

NASA Astrophysics Data System (ADS)

Ground-state geometries of carbenes H2Cn (n = 3-10) have been fully optimized with the C2?-symmetry constraint at the density functional theory and restricted-spin coupled-cluster single-double plus perturbative triple excitation levels of theory, respectively. Comparison of structures corresponding to the X1A1 and B1B1 electronic states has been made by the complete active space self-consistent field calculations. Parity alternation effect on various properties of the ground-state geometries has been discovered in the present study, which generally gives illustration for the relative stabilities of the titled carbon chains. Further calculations on their electronic spectra have been carried out by means of the complete active space second-order perturbation theory method along with the cc-pVTZ basis set. It is found that the vertical excitation energies of the dipole-allowed B1B1 <-- X1A1 transition in the gas phase are 2.28, 4.75, 1.69, 3.66, 1.30, 2.94, 1.12, and 2.49 eV, respectively, which agree very well with the available experimental result for H2C3 (2.27 eV). In addition, the vertical excitation energies for both transitions B1B1 <-- X1A1 and A1A2 <-- X1A1 are found to obey a nonlinear ?E-n relationship as a function of chain size by performing curves fitting.

Zhang, Yanxin; Wang, Li; Li, Yuanyuan; Zhang, Jinglai

2013-05-01

144

Structural and theoretical investigation of 2-iminoimidazolines--carbene analogues of iminophosphoranes.  

PubMed

The preparation of 2-iminoimidazolines - has been accomplished by the Staudinger reaction of the carbenes 1,3-di-tert-butylimidazolin-2-ylidene (), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (), 1,3-diisopropylimidazolin-2-ylidene (), 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (), 1,3-bis(2,6-diisopropylphenylimidazolin-2-ylidene () and 1,3,4,5-tetramethylimidazolin-2-ylidene () with trimethylsilyl azide (Me3SiN3) followed by desilylation of the resulting 2-trimethylsilyliminoimidazolines -. The X-ray crystal structures of and have been established, revealing C1-N1-Si1 angles that are more obtuse than the corresponding P-N-Si angles observed in related trimethylsilyl iminophosphoranes. Together with , the disilylated side product 1,3-diisopropyl-2-(trimethylsilylimino)-4-trimethylsilylimidazoline () has been isolated and structurally characterized. Cleavage of the N-Si bonds in and formation of is easily achieved by stirring in methanol. The molecular structures of the 2-iminoimidazolines are reported, indicating that the structural parameters are best described by non-ylidic resonance structures and that electron delocalization within the imidazole heterocycle does not play a crucial role in these imine systems. Compound forms a head-to-head dimer in the solid state via weak intermolecular N-H...N contacts, which have additionally been characterized by means of compliance constants. To further analyze the electronic structure of these imines in comparison to related guanidine ligands, the proton affinities (PAs) of the model compounds 2-imino-1,3-dimethylimidazoline (), 2-imino-1,3-dimethylimidazolidine () and tetramethylguanidine () have been calculated by means of density functional theory. Finally, the charge distribution in - and the relative contribution of relevant resonance structures have been determined using natural bond orbitals (NBO) and natural resonance theory (NRT). PMID:17252136

Tamm, Matthias; Petrovic, Dejan; Randoll, Sören; Beer, Stephan; Bannenberg, Thomas; Jones, Peter G; Grunenberg, Jörg

2007-01-04

145

Efficient Near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes.  

PubMed

We report on the photophysical studies of two cationic near-UV emitters based on bis-pincer Ir(III) carbene complexes: [Ir(nBu)(CNHC(Me)CCNHC)2]X, where Ir(nBu)(CNHC(Me)CCNHC) is (4,6-dimethyl-1,3-phenylene-?C(2))bis(1-butylimidazol-2-ylidene) and X = I(-) or PF6(-)). The compounds are highly emitting in deaerated CH3CN solution with emission maxima at 384 and 406 nm, and photoluminescence quantum yields of 0.41 and 0.38, for [Ir(nBu)(CNHC(Me)CCNHC)2]I and Ir(nBu)(CNHC(Me)CCNHC)2]PF6, respectively. In order to gain deeper understandings into their structural and electronic features, as well as to ascertain the nature of the excited states involved into the electronic absorption processes, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed on the ground and excited states of the closely related complex [Ir(Me)(CNHC(Me)CCNHC)2](+). In the solid state, an emission at low energy is observed (?max = 500 nm) for both complexes. However, the intensity of the emission at high energy versus the intensity of the new emission at low energy is dependent on the nature of counterions. The origin of this emission is not completely clear, but the experimental data point to the formation of trapping sites induced by aggregation processes involving the interaction between the cationic emitter and the counterion. PMID:24093480

Darmawan, Noviyan; Yang, Cheng-Han; Mauro, Matteo; Raynal, Matthieu; Heun, Susanne; Pan, Junyou; Buchholz, Herwig; Braunstein, Pierre; De Cola, Luisa

2013-09-09

146

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes.  

PubMed

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1-6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C? at formal reduction potentials <-1.7 V vs. FcH/FcH(+). A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent Cr=C species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the Cr=C centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups. PMID:23417243

van der Westhuizen, Belinda; Swarts, Pieter J; Strydom, Ian; Liles, David C; Fernández, Israel; Swarts, Jannie C; Bezuidenhout, Daniela I

2013-02-15

147

Synthesis and crystallographic characterization of multi-donor N-heterocyclic carbene chelating ligands and their silver complexes: Potential use in pharmaceuticals  

Microsoft Academic Search

The potential for N-heterocyclic carbenes (NHCs) to be used as novel chelating ligands for bio-inorganic pharmaceuticals is discussed. In this paper, we design, synthesize and characterize two NHC precursors, 6 and 7, that we believe have potential for use as metal chelators for pharmaceuticals. The NHC precursors are composed of imidazolium and pyridine rings that would form mixed donor NHCs

Jered C. Garrison; Claire A. Tessier; Wiley J. Youngs

2005-01-01

148

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

149

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

150

N-Heterocyclic carbene stabilized adducts of alkyl magnesium amide, bisalkyl magnesium and Grignard reagents: trapping oligomeric organo s-block fragments with NHCs.  

PubMed

Developing N-heterocyclic carbene (NHC) chemistry of simple organomagnesium compounds, this study reports the synthesis, X-ray crystallographic, and NMR spectroscopic characterization of three such new carbene complexes. The 1?:?1 alkyl magnesium amide?:?carbene complexes nBuMg(TMP)·IPr 1 and nBuMg(HMDS)·IPr 2 both exist as mononuclear complexes in the crystal but differ in solution as 2 remains intact whereas 1 undergoes a dynamic exchange involving partial decoordination of IPr [TMP is 2,2,6,6-tetramethylpiperidide; IPr is 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene); HMDS is 1,1,1,3,3,3-hexamethyldisilazide]. Reaction of commercial nBu(2)Mg with IPr surprisingly produced the organoaluminium carbene complex nBu(3)Al·IPr, 3, which also forms a simple mononuclear structure in the crystal. The presence of the Al could be traced to the deliberate addition of a small quantity of Et(3)Al as a stabilizing agent in the commercial nBu(2)Mg reagent. Repeating this reaction with Al-free nBu(2)Mg afforded the hemisolvated carbene complex nBu(8)Mg(4)·2IPr, 4, the stoichiometry of which is dictated by its structure rather than by that used in the initial reaction mixture. The molecular structure of 4 is tetranuclear with a linear chain of 4 Mg centres bridged by nBu ligands and capped at each end by terminal nBu and IPr ligands. Synthesized by treating the Grignard reagent nBuMgCl with IPr, nBuMgCl·IPr, 5, forms a cyclodimer structure with chloro bridges and terminal nBu and IPr ligands. PMID:20733980

Kennedy, Alan R; Mulvey, Robert E; Robertson, Stuart D

2010-08-23

151

BeCH2: The Simplest Metal Carbene. High Levels of Theory.  

PubMed

The simplest metal carbene, BeCH2, is experimentally unknown. Its isomer, HBeCH, lies higher in energy, but has been detected by the infrared matrix isolation [J. Am. Chem. Soc. 1998, 120, 6097]. In the present study the ground and low-lying excited states of the BeCH2 and HBeCH isomers were investigated using state-of-the-art ab initio methods, including coupled-cluster theory with up to full quadruple excitations (CCSDTQ), and complete active space self-consistent field (CASSCF) with multireference configuration interaction with single and double excitations (MRCISD). The relative energies were obtained using the focal point analysis combined with large correlation-consistent cc-pCVXZ basis sets (X = D, T, Q, 5) and were extrapolated to the complete basis set (CBS) limit. The (3)B1 state of BeCH2 (C2v symmetry) is the global minimum on the ground triplet potential energy surface (PES). The (3)?(-) state of the linear isomer HBeCH is located 4.9 kcal mol(-1) above the global minimum, at the CCSDTQ/CBS level of theory. The BeCH2 and HBeCH isomers are connected through the (3)A? transition state lying 46.1 kcal mol(-1) above the global minimum. The higher-lying energy HBeCH structure has much larger Be-C bond dissociation energy (126.6 kcal mol(-1), cf. BDE(BeCH2) = 62.1 kcal mol(-1)). The lowest excited state of BeCH2 is the open-shell (1)B1 state, with a relative energy of only 4.9 kcal mol(-1) above the global minimum, followed by (1)A1 state (16.8 kcal mol(-1)) at the MRCISD/cc-pCVQZ level of theory. For the HBeCH isomer the lowest-energy excited states are (1)? and (1)?(+), lying about 30 kcal mol(-1) above the global minimum. For the ground state of BeCH2 the fundamental vibrational frequencies computed using second-order vibrational perturbation theory (VPT2) at the CCSD(T)/cc-pCVQZ level are reported. We hope that our highly accurate theoretical results will assist in the experimental identification of BeCH2. PMID:23972228

Qiu, Yudong; Sokolov, Alexander Yu; Yamaguchi, Yukio; Schaefer, Henry F

2013-09-12

152

Statistics of stable marriages  

NASA Astrophysics Data System (ADS)

In the stable marriage problem N men and N women have to be matched by pairs under the constraint that the resulting matching is stable. We study the statistical properties of stable matchings in the large N limit using both numerical and analytical methods. Generalizations of the model including singles and unequal numbers of men and women are also investigated.

Dzierzawa, M.; Omero, M.-J.

2000-11-01

153

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

2010-08-26

154

Silver complexes of 1,2,4-triazole derived N-heterocyclic carbenes: Synthesis, structure and reactivity studies  

Microsoft Academic Search

Two silver(I) complexes {[1-R-4-(N-t-butylacetamido)-1,2,4-triazol-5-ylidene]2Ag}?+? Cl??? [R = Et (1b), i-Pr (2b)] of N\\/O-functionalized N-heterocyclic carbenes derived from 1,2,4-triazoles are reported. The silver complexes, 1b and 2b, have been synthesized from the reaction of the N\\/O-functionalized triazolium chloride salts namely, 1-R-4-(N-t-butylacetamido)-1,2,4-triazolium chloride [R = Et (1a), i-Pr (2a)] by treatment with Ag2O in 53–56% yield. The 1,2,4-triazolium chloride salts 1a and

CHANDRAKANTA DASH; MOBIN M SHAIKH; PRASENJIT GHOSH

155

Synthesis, characterization of N -heterocyclic carbene metallacrown palladium complex and catalytic activities in Suzuki and Heck coupling reaction  

Microsoft Academic Search

\\u000a Abstract  A series of new N-heterocyclic carbene Metllacrown palladium complexes with a highly flexible polyether-bridged chain (CH2OCH2)\\u000a n\\u000a (n = 1–3) have been synthesized by the reaction of the corresponding silver complexes and PdCl2(CH3CN)2 in acetonitrile. Complexes 7a,b–9a,b have been structurally characterized. A single-crystal X-ray study of palladium complex 1,1?-(oxy-1,2-ethanediyl)bis[3-(1-naphthalenemethyl)\\u000a imidazol-2-ylidene] palladium hexafluorophosphate 7a adopt a cis configuration. They exhibited excellent catalytic activity

Jun-Wen Wang; Lin-Ying Gao; Fan-Hui Meng; Jing Jiao; Ling-Yan Ding; Li-Fang Zhang

156

Fluorescent silver(I) and gold(I)-N-heterocyclic carbene complexes with cytotoxic properties: mechanistic insights.  

PubMed

Silver(I) and gold(I)-N-heterocyclic carbene (NHC) complexes bearing a fluorescent anthracenyl ligand were examined for cytotoxicity in normal and tumor cells. The silver(I) complex exhibits greater cytotoxicity in tumor cells compared with normal cells. Notably, in cell extracts, this complex determines a more pronounced inhibition of thioredoxin reductase (TrxR), but it is ineffective towards glutathione reductase (GR). Both gold and silver complexes lead to oxidation of the thioredoxin system, the silver(I) derivative being particularly effective. In addition, the dimerization of peroxiredoxin 3 (Prx3) was also observed, demonstrating the ability of these compounds to reach the mitochondrial target. The fluorescence microscopy visualization of the subcellular distribution of the complexes shows a larger diffusion of these molecules in tumor cells with respect to normal cells. PMID:23661165

Citta, Anna; Schuh, Esther; Mohr, Fabian; Folda, Alessandra; Massimino, Maria Lina; Bindoli, Alberto; Casini, Angela; Rigobello, Maria Pia

2013-08-01

157

Synthesis and catalytic activity of neutral salicylaldiminato nickel(II) complexes bearing a single N-heterocyclic carbene ligand  

Microsoft Academic Search

Neutral salicylaldiminato Ni(II) complexes bearing a single N-heterocyclic carbene (NHC) ligand [3,5-tBu2-2-(O)C6H2CHNAr]Ni(C{RNCHCHNiPr})Ph [Ar=2,6-iPr2C6H3, R=Bn (1); Ar=2,6-iPr2C6H3, R=iPr (2)], have been synthesized via a one-pot procedure in high yield. The X-ray structure analysis reveals that both of 1 and 2 adopt distorted square-planar coordination geometry and NHC carbon (Ccarbene) is trans to the ketimine nitrogen. Preliminary study indicates that complex 1

Wan-Fei Li; Hong-Mei Sun; Mu-Zi Chen; Qi Shen; Yong Zhang

2008-01-01

158

Synthesis and structural characterization of aryloxo-functionalized N-heterocyclic carbene complexes of nickel(II)  

Microsoft Academic Search

The reaction of Ni(PPh3)2Br2 with two equivalents of anionic aryloxo-functionalized N-heterocyclic carbene [NaO-4,6-di-tBu-C6H2-2-CH2{C(NCHCHNR)}] (R=iPr, NaL1; R=Bz, NaL2), which are generated in situ by the reaction of the corresponding salt H2LCl with two equivalents of NaN(SiMe3)2, affords bis-ligand complexes of L21Ni (3) and L22Ni (4) in good yield, respectively. It is interesting to note that still the bis-ligand one not the

Wan-Fei Li; Hong-Mei Sun; Zhi-Guo Wang; Mu-Zi Chen; Qi Shen; Yong Zhang

2005-01-01

159

N-benzyl substituted N-heterocyclic carbene complexes of iridium(I): assessment in transfer hydrogenation catalyst.  

PubMed

Iridium(I) complexes of N-heterocyclic carbenes (NHCs) (1a-3a) were obtained by transmetalation reactions from the corresponding Ag(I)-NHC complexes. These complexes have been fully characterized by (1)H, (13)C, heteronuclear multiple-bond correlation NMR spectroscopies, and elemental analysis. X-ray diffraction studies on single crystals of 1a and 2a confirm the square planar geometry at the metal center. [IrCl(CO)2(NHC)] complexes 1b-3b were also synthesized to compare ?-donor/?-acceptor strength of NHC ligands. Transfer hydrogenation (TH) reactions of various ketones and imines have been studied using complexes 1a-3a as precatalysts. N-Benzyl substituted NHC complexes of Ir(I) proved to be highly efficient precatalysts in the reduction of aromatic and aliphatic ketones to afford the corresponding alcohol products with turnover frequencies values up to 24,000 h(-1). PMID:24004417

Gülcemal, Süleyman; Gökçe, Aytaç Gürhan; Cetinkaya, Bekir

2013-09-04

160

Catalytic Asymmetric ?-Acylation of Tertiary Amines Mediated by a Dual Catalysis Mode: N-Heterocyclic Carbene and Photoredox Catalysis  

PubMed Central

Cross-coupling reactions are some of the most widely utilized methods for C-C bond formation; however, the requirement for pre-activated starting materials still presents a major limitation. Methods that take direct advantage of the inherent reactivity of the C-H bond offer an efficient alternative to these methods, negating the requirement for substrate pre-activation. In this process two chemically distinct activation events culminate in the formation of the desired C-C bond with loss of H2 as the only by-product. Herein we report the catalytic asymmetric ?-acylation of tertiary amines with aldehydes facilitated by the combination of chiral N-heterocyclic carbene catalysis and photoredox catalysis.

DiRocco, Daniel A.

2012-01-01

161

Oxyanion-steering and CH-? Interactions as Key Elements in a N-Heterocyclic Carbene-Catalyzed [4+2] Cycloaddition  

PubMed Central

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. In this communication, 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.

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

2012-01-01

162

Copper-free and amine-free Sonogashira coupling in air in a mixed aqueous medium by palladium complexes of N\\/ O-functionalized N-heterocyclic carbenes  

Microsoft Academic Search

Highly convenient copper-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes under amenable conditions in air and in a mixed aqueous medium are reported using several new, user friendly and robust palladium precatalysts (1–5) of N\\/O-functionalized N-heterocyclic carbenes (NHCs). In particular, the precatalysts, 1 and 2, were synthesized from the imidazolium chloride salts by the treatment

Manoja K. Samantaray; Mobin M. Shaikh; Prasenjit Ghosh

2009-01-01

163

Combining oxidative N-heterocyclic carbene catalysis with click chemistry: a facile one-pot approach to 1,2,3-triazole derivatives.  

PubMed

A combination of the oxidative N-heterocyclic carbene catalysis and click chemistry has been explored for the direct, one-pot synthesis of 1,2,3-triazole derivatives from aromatic aldehydes. This procedure was found to be very efficient and a variety of 1,2,3-triazole derivatives could be accessed through their corresponding propargyl esters in moderate-to-good yields under mild conditions. PMID:23606655

Ramanjaneyulu, B T; Reddy, Virsinha; Arde, Panjab; Mahesh, Sriram; Anand, R Vijaya

2013-04-19

164

Dehalogenation of protonated C-halogeno-1,2,4-triazoles: synthesis of new heterocyclic carbenic and ylid radical cations and contrasting behaviour of collision gases  

Microsoft Academic Search

C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization conditions, are found to undergo easy dehalogenation upon 8 keV collisional activation conditions, provided the collision gas is oxygen, not helium. The ions produced under these reactions are demonstrated to be five-membered cyclic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be produced

Robert Flammang; Monique Barbieux-Flammang; Hung Thanh Le; Minh Tho Nguyen; Jacques Berthelot; Jeanine Tortajada

2000-01-01

165

An Electronic Rationale for Observed Initiation Rates in Ruthenium-Mediated Olefin Metathesis: Charge Donation in Phosphine And N-Heterocyclic Carbene Ligands  

SciTech Connect

Ru K-edge XAS data indicate that second generation ruthenium-based olefin metathesis precatalysts (L = N-heterocyclic carbene) possess a more electron-deficient metal center than in the corresponding first generation species (L = tricyclohexylphosphine). This surprising effect is also observed from DFT calculations and provides a simple rationale for the slow phosphine dissociation kinetics previously noted for second-generation metathesis precatalysts.

Getty, K.; Delgado-Jaime, M.U.; Kennepohl, P.

2009-06-01

166

Well-defined N-heterocyclic carbene silver halides of 1-cyclohexyl-3-arylmethylimidazolylidenes: synthesis, structure and catalysis in A3-reaction of aldehydes, amines and alkynes.  

PubMed

Structurally well-defined N-heterocyclic carbene silver chlorides and bromides supported by 1-cyclohexyl-3-benzylimidazolylidene (CyBn-NHC) or 1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene (CyNaph-NHC) were synthesized by reaction of the corresponding imidazolium halides with silver(I) oxide while cationic bis(CyBn-NHC) silver nitrate was isolated under similar conditions using imidazolium iodide in the presence of sodium nitrate. Single-crystal X-ray diffraction revealed a dimeric structure through a nonpolar weak-hydrogen-bond supported Ag-Ag bond for 1-cyclohexyl-3-benzylimidazolylidene silver halides [(CyBn-NHC)AgX](2) (X = Cl, 1; Br, 2) but a monomeric structure for N-heterocyclic carbene silver halides with the more sterically demanding 1-cyclohexyl-3-naphthalen-2-ylmethylimidazolylidene ligand (CyNaph-NHC)AgX (X = Cl, 4; Br, 5). Cationic biscarbene silver nitrate [(CyBn-NHC)(2)Ag](+)NO(3)(-)3 assumed a cis orientation with respect to the two carbene ligands. The monomeric complexes (CyNaph-NHC)AgX 4 and 5 showed higher catalytic activity than the dimeric [(CyBn-NHC)AgX](2)1 and 2 as well as the cationic biscarbene silver nitrate 3 in the model three component reaction of 3-phenylpropionaldehyde, phenylacetylene and piperidine with chloride 4 performing best and giving product in almost quantitative yield within 2 h at 100 °C. An explanation for the structure-activity relationship in N-heterocyclic carbene silver halide catalyzed three component reaction is given based on a slightly modified mechanism from the one in literature. PMID:21264392

Li, Yanbo; Chen, Xiaofeng; Song, Yin; Fang, Ling; Zou, Gang

2011-01-24

167

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

168

Total syntheses of (S)-(-)-zearalenone and lasiodiplodin reveal superior metathesis activity of ruthenium carbene complexes with imidazol-2-ylidene ligands.  

PubMed

Total syntheses of the bioactive orsellinic acid derivatives zearalenone 3 and lasiodiplodin 1 are reported based on a ring-closing metathesis (RCM) reaction of styrene precursors as the key steps. These and closely related macrocyclizations are catalyzed with high efficiency by the "second generation" ruthenium carbene catalyst 5 bearing a N-heterocyclic carbene ligand, whereas the standard Grubbs carbene 4 fails to afford any cyclized product. Only the (E)-isomer of the macrocyclic cycloalkene is formed in all cases. The substrates for RCM can be obtained either via a Stille cross-coupling reaction of tributylvinylstannane or, even more efficiently, by Heck reactions of the aryl triflate precursors with pressurized ethene. Furthermore, the synthesis of 1 via RCM is compared with an alternative approach employing a low-valent titanium-induced McMurry coupling of dialdehyde 47 for the formation of the large ring. This direct comparison clearly ends in favor of metathesis which turned out to be superior in all preparatively relevant respects. PMID:11073608

Fürstner, A; Thiel, O R; Kindler, N; Bartkowska, B

2000-11-17

169

Angina Pectoris | Stable Angina  

MedlinePLUS

Angina Pectoris | Stable Angina Updated:Mar 19,2013 You may have heard the term “angina pectoris” or “stable angina” in your doctor’s office, but ... for you? It’s important to understand the basics. Angina pectoris is the medical term for chest pain or ...

170

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

171

Thermally Stable Laminating Resins.  

National Technical Information Service (NTIS)

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 autoc...

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

1972-01-01

172

Stable superstring relics.  

National Technical Information Service (NTIS)

The authors investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a ''Wilson-line'' breaking of the unifying non-Abelian gau...

S. Chang C. Coriano A. E. Faraggi

1996-01-01

173

Pyridine-Enhanced Head-to-Tail Dimerization of Terminal Alkynes by a Rhodium-N-Heterocyclic-Carbene Catalyst.  

PubMed

A general regioselective rhodium-catalyzed head-to-tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh-N-heterocyclic-carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the ?-alkyne-coordinated Rh(I) species [RhCl(NHC)(?(2) -HC?CCH2 Ph)(py)] (3) and [RhCl(NHC){?(2) -C(tBu)?C(E)CH?CHtBu}(py)] (4) and the Rh(III) -hydride-alkynyl species [RhClH{?C?CSi(Me)3 }(IPr)(py)2 ] (5). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C?H oxidative addition, alkyne insertion, and reductive elimination. A 2,1-hydrometalation of the alkyne is the more favorable pathway in accordance with a head-to-tail selectivity. PMID:24114872

Rubio-Pérez, Laura; Azpíroz, Ramón; Di Giuseppe, Andrea; Polo, Victor; Castarlenas, Ricardo; Pérez-Torrente, Jesús J; Oro, Luis A

2013-09-23

174

Theoretical study of mechanism of cycloaddition reaction between singlet dichlorosilylene carbene (Cl2Si=C:) and formaldehyde  

NASA Astrophysics Data System (ADS)

The mechanism of the cycloaddition reaction between singlet dichlorosilylene carbene (Cl2Si=C:) and formaldehyde has been investigated with MP2/6-31G* method, 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 Zero-point energy and CCSD (T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction pathways. The first dominant reaction pathway consists of two steps: (1) the two reactants (R1, R2) firstly form a four-membered ring intermediate (INT4) through a barrier-free exothermic reaction of 387.9 kJ/mol; (2) intermediate (INT4) then isomerizes to H-transfer product (P4.2) via a transition state (TS4.2) with energy barrier of 4.7 kJ/mol. The second dominant reaction pathway as follows: on the basis of intermediate (INT4) created between R1 and R2, intermediate (INT4) further reacts with formaldehyde (R2) to form the intermediate (INT5) through a barrier-free exothermic reaction of 158.3 kJ/mol. Then, intermediate (INT5) isomerizes to a silicic bis-heterocyclic product (P5) via a transition state (TS5), for which the barrier is 40.1 kJ/mol.

Lu, Xiuhui; Lian, Zhenxia; Xiang, Pingping; Li, Yongqing

2011-01-01

175

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

176

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

2011-04-15

177

On the involvement of NHC carbenes in catalytic reactions by iridium complexes, nanoparticle and bulk metal dispersed in imidazolium ionic liquids.  

PubMed

D/H exchange reactions at C2, C4 and C5 of the imidazolium cation were observed in catalytic hydrogenation reactions promoted by classical Ir(I) colloid precursors and [Ir(0)](n) nanoparticles dispersed in deuterated imidazolium ionic liquids indicating the participation of carbene species in this media. However, no D/H exchange reaction was observed in cyclohexene hydrogenation promoted by iridium bulk metal dispersed in the ionic liquid [BMI]-d(3).NTf(2). The D/H labeling experiments suggest that the ionic liquids interact with the metal centers preferentially as aggregates rather than isolated ions. PMID:18043817

Scholten, Jackson D; Ebeling, Günter; Dupont, Jairton

2007-07-06

178

Copolymer-incarcerated nickel nanoparticles with N-heterocyclic carbene precursors as active cross-linking agents for Corriu-Kumada-Tamao reaction.  

PubMed

We have developed heterogeneous polymer-incarcerated nickel nanoparticles (NPs), which catalyze cross-coupling reactions. The matrix structure of these catalysts incorporates both N-heterocyclic carbenes (NHCs) as ligands and Ni-NPs, thanks to a new design of cross-linking agents in polymer supports. These embedded NHCs were detected by field gradient swollen-resin magic angle spinning NMR analysis. They were successfully applied to Corriu-Kumada-Tamao reactions with a broad substrate scope including functional group tolerance, and the catalyst could be recovered and reused several times without loss of activity. PMID:23815242

Soulé, Jean-François; Miyamura, Hiroyuki; Kobayashi, Sh?

2013-07-09

179

Synthesis, electronic structure, and magnetism of [ni(6-mes)2](+): a two-coordinate nickel(i) complex stabilized by bulky N-heterocyclic carbenes.  

PubMed

The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior. PMID:23971827

Poulten, Rebecca C; Page, Michael J; Algarra, Andrés G; Le Roy, Jennifer J; López, Isidoro; Carter, Emma; Llobet, Antoni; Macgregor, Stuart A; Mahon, Mary F; Murphy, Damien M; Murugesu, Muralee; Whittlesey, Michael K

2013-09-04

180

Stable unstable reliability theory.  

PubMed

Classical reliability theory assumes that individuals have identical true scores on both testing occasions, a condition described as stable. If some individuals' true scores are different on different testing occasions, described as unstable, the estimated reliability can be misleading. A model called stable unstable reliability theory (SURT) frames stability or instability as an empirically testable question. SURT assumes a mixed population of stable and unstable individuals in unknown proportions, with w(i) the probability that individual i is stable. w(i) becomes i's test score weight which is used to form a weighted correlation coefficient r(w) which is reliability under SURT. If all w(i) = 1 then r(w) is the classical reliability coefficient; thus classical theory is a special case of SURT. Typically r(w) is larger than the conventional reliability r, and confidence intervals on true scores are typically shorter than conventional intervals. r(w) is computed with routines in a publicly available R package. PMID:22500569

Thomas, Hoben; Lohaus, Arnold; Domsch, Holger

2011-02-02

181

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

182

Structural characterization and catalytic activity of the rhodium–carbene complex Rh(PPh 3) 2(IMes)Cl (IMes=bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene)  

Microsoft Academic Search

The rhodium–carbene complex Rh(PPh3)2(IMes)Cl (2) is an active catalyst for the hydroboration of simple olefins at room temperature. The reactivity of 2 was also tested in the methylenation of aldehydes. The crystal structure of 2 is also reported.

Gabriela A. Grasa; Zakhia Moore; Kenneth L. Martin; Edwin D. Stevens; Steven P. Nolan; Valérie Paquet; Hélène Lebel

2002-01-01

183

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

184

Stable local oscillator module.  

SciTech Connect

This report gives a description of the development of a Stable Local Oscillator (StaLO) multi-chip module (MCM). It is a follow-on report to SAND2006-6414, Stable Local Oscillator 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 makes use of integrated circuit technologies in silicon and lithium niobate as well as discrete passive components. This report describes the development of an MCM-based version of the complete StaLO, fabricated on an alumina thick film hybrid substrate.

Brocato, Robert Wesley

2007-11-01

185

Handbook of stable strontium  

SciTech Connect

This book presents information on the following topics: chemistry of strontium; biogeochemistry of strontium; uptake of stable strontium by plants and effects on plant growth; divalent cation-dependent deposits in paramecium; effects of strontium ion on the hydrolysis of ATP; stronium ions and membranes - screening versus binding at charged surfaces; mitochondrial granules in the liver of rats kept on stable strontium supplementation; divalent cations and regulation of cyclic nucleotides in nervous systems; strontium as the substitute for calcium in the excitation-contraction coupling of crayfish muscle fibers; hemodynamic effects of strontium in the dog; some mechanical characteristics of strontium-mediated contractions in heart muscle; effects of calcium, magnesium, and strontium on drug-receptor interactions; strontium and histamine secretion; and effects of strontium in human dental enamel.

Skoryna, S.C.

1981-01-01

186

Stable Charged Cosmic Strings  

SciTech Connect

We study the quantum stabilization of a cosmic string by a heavy fermion doublet in a reduced version of the standard model. We show that charged strings, obtained by populating fermionic bound state levels, become stable if the electroweak bosons are coupled to a fermion that is less than twice as heavy as the top quark. This result suggests that extraordinarily large fermion masses or unrealistic couplings are not required to bind a cosmic string in the standard model. Numerically we find the most favorable string profile to be a simple trough in the Higgs vacuum expectation value of radius {approx_equal}10{sup -18} m. The vacuum remains stable in our model, because neutral strings are not energetically favored.

Weigel, H. [Physics Department, Stellenbosch University, Matieland 7602 (South Africa); Quandt, M. [Institute for Theoretical Physics, Tuebingen University, D-72076 Tuebingen (Germany); Graham, N. [Department of Physics, Middlebury College , Middlebury, Vermont 05753 (United States)

2011-03-11

187

Adaptive stable marriage algorithms  

Microsoft Academic Search

Although it takes O(n2) worst-case time to solve a stable marriage problem instance with n men and n women, a trivial O(n) algorithm suffices if all men are known to have identical preference lists and all women also are known to have identical preference lists. Since real-world instances often involve men or women with similar but not necessarily identical preference

John Dabney; Brian C. Dean

2010-01-01

188

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 nondestructive research use on a loan basis. This inventory includes all samples of stable isotop...

1984-01-01

189

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 nondestructive research use on a loan basis. This inventory includes all samples of stable isotop...

1986-01-01

190

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

191

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

192

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.; Boyd, Robert D.

1999-10-15

193

Clear stable gasoline composition  

SciTech Connect

This patent describes a process for making a clear, stable gasoline blend from a hazy, water-saturated gasoline containing a mixture of hydrocarbons in the gasoline boiling range, from about 2.0 to about 12.0 volume percent of methanol, from about 2.0 to about 10.0 volume percent of a cosolvent (C/sub 2/-C/sub 5/) aliphatic alcohol, and 0.1 to 0.5 volume percent of contaminating water. The process consists of adding to the water-saturated gasoline, from about 0.5 to about 1.8 weight percent of a nonanionic surfactant of an adduct of soya sterol and a polyalkylene oxide.

Davis, M.E.; Sung, R.L.

1986-02-04

194

Thermodynamically Stable Pickering Emulsions  

NASA Astrophysics Data System (ADS)

We show that under appropriate conditions, mixtures of oil, water, and nanoparticles form thermodynamically stable oil-in-water emulsions with monodisperse droplet diameters in the range of 30 150 nm. This observation challenges current wisdom that so-called Pickering emulsions are at most metastable and points to a new class of mesoscopic equilibrium structures. Thermodynamic stability is demonstrated by the spontaneous evolution of binary droplet mixtures towards one intermediate size distribution. Equilibrium interfacial curvature due to an asymmetric charge distribution induced by adsorbed colloids explains the growth of emulsion droplets upon salt addition. Moreover, the existence of a minimal radius of curvature with a concomitant expulsion of excess oil is in close analogy with microemulsions.

Sacanna, S.; Kegel, W. K.; Philipse, A. P.

2007-04-01

195

Thermodynamically stable pickering emulsions.  

PubMed

We show that under appropriate conditions, mixtures of oil, water, and nanoparticles form thermodynamically stable oil-in-water emulsions with monodisperse droplet diameters in the range of 30-150 nm. This observation challenges current wisdom that so-called Pickering emulsions are at most metastable and points to a new class of mesoscopic equilibrium structures. Thermodynamic stability is demonstrated by the spontaneous evolution of binary droplet mixtures towards one intermediate size distribution. Equilibrium interfacial curvature due to an asymmetric charge distribution induced by adsorbed colloids explains the growth of emulsion droplets upon salt addition. Moreover, the existence of a minimal radius of curvature with a concomitant expulsion of excess oil is in close analogy with microemulsions. PMID:17501389

Sacanna, S; Kegel, W K; Philipse, A P

2007-04-12

196

Stable Cosmic Vortons  

NASA Astrophysics Data System (ADS)

We present solutions in the gauged U(1)×U(1) model of Witten describing vortons—spinning flux loops stabilized against contraction by the centrifugal force. Vortons were heuristically described many years ago; however, the corresponding field theory solutions were not obtained and so the stability issue remained open. We construct explicitly a family of stationary vortons characterized by their charge and angular momentum. Most of them are unstable and break in pieces when perturbed. However, thick vortons with a small radius preserve their form in the 3+1 nonlinear dynamical evolution. This gives the first ever evidence of stable vortons and impacts several branches of physics where they could potentially exist. These range from cosmology, since vortons could perhaps contribute to dark matter, to QCD and condensed matter physics.

Garaud, Julien; Radu, Eugen; Volkov, Mikhail S.

2013-10-01

197

Stable local oscillator microcircuit.  

SciTech Connect

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 makes use of integrated circuit technologies in silicon and lithium niobate as well as discrete passive components. The StaLO uses a comb generator followed by surface acoustic wave (SAW) filters. The comb generator creates a set of harmonic components of the 100MHz input signal. The SAW filters are narrow bandpass filters that are used to select the desired component and reject all others. The resulting circuit has very low sideband power levels and low phase noise (both less than -40dBc) that is limited primarily by the phase noise level of the input signal.

Brocato, Robert Wesley

2006-10-01

198

Stable superstring relics  

SciTech Connect

The authors investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a ``Wilson-line`` breaking of the unifying non-Abelian gauge symmetry. In the models that they consider the unifying SO(10) gauge symmetry is broken at the string level to SO(6) x SO(4), SU(5) x U(1) or SU(3) x SU(2) x U(1). The exotic matter states are classified according to the patterns of the SO(10) symmetry breaking. In SO(6) x XO(4) and SU(5) x U(1) type models one obtains fractionally charged states with Q{sub e.m.} = {+-}1/2. In SU(3) x SU(2) x U(1) type models one also obtains states with the regular charges under the Standard Model gauge group but with ``fractional`` charges under the U(1){sub z{prime}} symmetry. These states include down-like color triplets and electroweak doublets, as well as states which are Standard Model singlets. By analyzing the renormalizable and nonrenormalizable terms of the superpotential in a specific superstring model, the authors show that these exotic states can be stable. They investigate the cosmological constraints on the masses and relic density of the exotic states. They propose that, while the abundance and the masses of the fractionally charged states are highly constrained, the Standard Model-like states, and in particular the Standard Model singlet, are good dark matter candidates.

Chang, S.; Coriano, C. [Florida Univ., Gainesville, FL (United States). Inst. for Fundamental Theory; Faraggi, A.E. [Florida Univ., Gainesville, FL (United States). Inst. for Fundamental Theory]|[Inst. for Advanced Study, Princeton, NJ (United States). School of Natural Sciences

1996-05-15

199

Are helionitronium trications stable?  

PubMed Central

In a recent article [Olah, G. A., Prakash, G. K. S. & Rasul, G. (1999) Proc. Natl. Acad. Sci. USA 96, 3494–3495] the authors found that the helionitronium trication, HeNO\\documentclass[10pt]{article} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{pmc} \\usepackage[Euler]{upgreek} \\pagestyle{empty} \\oddsidemargin -1.0in \\begin{document} \\begin{equation*}_{2}^{3+}\\end{equation*}\\end{document}, is an unusually stable helium-containing polyatomic ion. This result was based on second-order many-body perturbation (MP2) calculations that showed that strong binding should occur between the oxygen and helium atoms in the assumed singlet ground state. The dissociation energy with respect to NO+ and HeO2+ was predicted to be 7.95 eV. We show here by thorough multireference configuration interaction (MRCI) studies that the ground state for the helionitronium trication is a triplet 3B1 state with He binding to the N atom (C2v). The He—O bound structure of Cs symmetry is not stable. Dissociation of the helionitronium trication occurs toward NO\\documentclass[10pt]{article} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{pmc} \\usepackage[Euler]{upgreek} \\pagestyle{empty} \\oddsidemargin -1.0in \\begin{document} \\begin{equation*}_{2}^{2+}\\end{equation*}\\end{document} and He+, and the trication is bound by at most 0.25 eV. These results indicate that the helionitronium trication is unstable under ambient conditions. The discrepancies between our results and the previous study are explained by the strong multireference character of the wave function of the trication.

Eisfeld, Wolfgang; Francisco, Joseph S.

2002-01-01

200

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

2011-05-04

201

How does the addition of steric hindrance to a typical N-heterocyclic carbene ligand affect catalytic activity in olefin metathesis?  

PubMed

Density functional theory (DFT) calculations were used to predict and rationalize the effect of the modification of the structure of the prototype 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) N-heterocyclic carbene (NHC) ligand. The modification consists in the substitution of the methyl groups of ortho isopropyl substituent with phenyl groups, and here we plan to describe how such significant changes affect the metal environment and therefore the related catalytic behaviour. Bearing in mind that there is a significant structural difference between both ligands in different olefin metathesis reactions, here by means of DFT we characterize where the NHC ligand plays a more active role and where it is a simple spectator, or at least its modification does not significantly change its catalytic role/performance. PMID:23455458

Poater, Albert; Falivene, Laura; Urbina-Blanco, César A; Manzini, Simone; Nolan, Steven P; Cavallo, Luigi

2013-05-28

202

Anionic and zwitterionic copper(I) complexes incorporating an anionic N-heterocyclic carbene decorated with a malonate backbone: synthesis, structure and catalytic applications.  

PubMed

The anionic malonate-derived N-heterocyclic carbenes (maloNHCs) react cleanly and rapidly with copper chloride to generate the anionic complexes of type [(maloNHC)CuCl]·Li, which crystallize in the solid state either in an oligomeric trimer arrangement or in polymeric helixes depending on the substitution pattern and the solvent. Ten zwitterionic heteroleptic Cu(I) complexes combining the anionic maloNHC and a neutral imidazol-2-ylidene are also obtained in a very selective manner and fully characterized. Whereas the anionic complexes are relatively active catalysts for the hydrosilylation of carbonyl compounds, the zwitterionic complexes reveal to be efficient and extremely robust pre-catalysts for the intramolecular cyclopropanation reaction of a diazo ester and outperform the corresponding cationic Cu(i) complexes with classical imidazol-2-ylidenes. PMID:23361332

César, Vincent; Barthes, Cécile; Farré, Yoann C; Cuisiat, Stéphane V; Vacher, Bernard Y; Brousses, Rémy; Lugan, Noël; Lavigne, Guy

2013-05-28

203

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

2013-09-08

204

Visible-light photoredox catalysis: selective reduction of carbon dioxide to carbon monoxide by a nickel N-heterocyclic carbene-isoquinoline complex.  

PubMed

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene-amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni((Pr)bimiq1)](2+) (1c, where (Pr)bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (Ecat = -1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s(-1), respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion. PMID:24033186

Thoi, V Sara; Kornienko, Nikolay; Margarit, Charles G; Yang, Peidong; Chang, Christopher J

2013-09-13

205

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

206

N-heterocyclic carbene gold(I) and copper(I) complexes in C-H bond activation.  

PubMed

Environmental concerns have and will continue to have a significant role in determining how chemistry is carried out. Chemists will be challenged to develop new, efficient synthetic processes that have the fewest possible steps leading to a target molecule, the goal being to decrease the amount of waste generated and reduce energy use. Along this path, chemists will need to develop highly selective reactions with atom-economical pathways producing nontoxic byproduct. In this context, C-H bond activation and functionalization is an extremely attractive method. Indeed, for most organic transformations, the presence of a reactive functionality is required. In Total Synthesis, the "protection and deprotection" approach with such reactive groups limits the overall yield of the synthesis, involves the generation of significant chemical waste, costs energy, and in the end is not as green as one would hope. In turn, if a C-H bond functionalization were possible, instead of the use of a prefunctionalized version of the said C-H bond, the number of steps in a synthesis would obviously be reduced. In this case, the C-H bond can be viewed as a dormant functional group that can be activated when necessary during the synthetic strategy. One issue increasing the challenge of such a desired reaction is selectivity. The cleavage of a C-H bond (bond dissociation requires between 85 and 105 kcal/mol) necessitates a high-energy species, which could quickly become a drawback for the control of chemo-, regio-, and stereoselectivity. Transition metal catalysts are useful reagents for surmounting this problem; they can decrease the kinetic barrier of the reaction yet retain control over selectivity. Transition metal complexes also offer important versatility in having distinct pathways that can lead to activation of the C-H bond. An oxidative addition of the metal in the C-H bond, and a base-assisted metal-carbon bond formation in which the base can be coordinated (or not) to the metal complexes are possible. These different C-H bond activation modes provide chemists with several synthetic options. In this Account, we discuss recent discoveries involving the versatile NHC-gold(I) and NHC-copper(I) hydroxide complexes (where NHC is N-heterocyclic carbene) showing interesting Brønsted basic properties for C-H bond activation or C-H bond functionalization purposes. The simple and easy synthesis of these two complexes involves their halide-bearing relatives reacting with simple alkali metal hydroxides. These complexes can react cleanly with organic compounds bearing protons with compatible pK(a) values, producing only water as byproduct. It is a very simple protocol indeed and may be sold as a C-H bond activation, although the less flashy "metalation reaction" also accurately describes the process. The synthesis of these complexes has led us to develop new organometallic chemistry and catalysis involving C-H bond activation (metalation) and subsequent C-H bond functionalization. We further highlight applications with these reactions, in areas such as photoluminescence and biological activities of NHC-gold(I) and NHC-copper(I) complexes. PMID:22166092

Gaillard, Sylvain; Cazin, Catherine S J; Nolan, Steven P

2011-12-13

207

Stable isotope laser spectroscopy  

NASA Astrophysics Data System (ADS)

Recent advances in semiconductor laser technology have produced a reliable lightweight device ideally suited for a spacecraft high resolution molecular spectrometer. Lead-salt tunable diode lasers (TDL) emit in several spectral modes, each with a very narrow linewidth of -0.0003/cm. This spectral resolution is much narrower than typical Doppler broadened molecular linewidths in the mid-IR range. Thus it is possible to detect individual rotational lines within the vibrational band and measure their intensity, which can be used to determine gas concentration. The narrow spectral lines of any impurity gas tend to lie between the narrow lines of the gas of interest. This represents a major advantage over the accepted gas chromatograph mass spectrometer (GCMS) technique for measuring gas concentrations and isotope ratios. The careful and extensive gas purification procedures required to remove impurities for reliable GCMS measurements will not be required for an IR laser gas analysis. The infrared laser gas analysis technique is being developed to measure stable isotopic ratios of gases such as CO2, CH4, N2O, and NH3. This will eventually lead to development of instruments capable of in situ istopic measurements on planets such as Mars. The carbon (C-12, C-13) isotope ratio is indicative of the type of carbon fixation mechanisms (e.g., photosynthesis, respiration) in operation on a planet, while the nitrogen (N-14, N-15) isotope ratio can probably be used to date nitrogen-bearing Martian samples. The absorbance ratio of two adjacent lines of CO2 in the 2300/cm (4.3 micron) region of the spectrum was measured. The precision of the measurement is presently better than 1 percent and significant improvement is anticipated as rapid sweep-integration techniques and computer controlled data acquistion capabilities are incorporated.

Becker, J. F.; Yaldaei, Ramil; McKay, Christopher P.

1989-03-01

208

Do stable atmospheric layers exist?  

NASA Astrophysics Data System (ADS)

The notion of stable atmospheric layers is a classical idealization used for understanding atmospheric dynamics and thermodynamics. Using state of the art drop sonde data and using conditional, dynamical and convective stability criteria we show that apparently stable layers are typically composed of a hierarchy of unstable layers themselves with embedded stable sublayers, and unstable sub-sub layers etc. i.e. in a Russian Matryoshka doll-like fractal hierarchy. We therefore argue that the notion of stable atmospheric layers is untenable and must be replaced by modern scaling notions.

Lovejoy, S.; Tuck, A. F.; Hovde, S. J.; Schertzer, D.

2008-01-01

209

Tempering the Reactivities of Postulated ?-Oxo Gold Carbenes by Bidentate Ligands: Implication of Tricoordinated Gold Intermediates and the Development of an Expedient Bimolecular Assembly of 2,4-Disubstituted Oxazoles  

PubMed Central

2,4-Oxazole is an important structural motif in various natural products. An efficient modular synthesis of this structure is achieved via a [3+2] annulation between a terminal alkyne and a carboxamide by using a gold-catalyzed oxidation strategy. The postulated reactive intermediate, a terminal ?-oxo gold carbene, previously known to be highly electrophilic and hence impropable to be trapped by stoichiometric external nucleophiles, is coerced to react smoothly with a carboxamide en route to the oxazole ring by a P,N- or P,S-bidentate ligand such as Mor-DalPhos; in stark contrast, often used ligands including monodentate phosphines and NHCs are totally ineffective. The role of these bidentate phosphines in this reaction is attributed to the formation of a tricoordinated gold carbene intermediate, which is less electrophilic and hence more chemoselective when reacting with nucleophiles. The success in using bidentate phosphine ligands to temper the reactivities of in-situ generated gold carbenes would likely open many new opportunities to apply the oxidative gold catalysis to the development of novel methods, and the implication of tricoordinated gold intermediates in homogeneous gold catalysis should stimulate further advance in gold catalysis.

Luo, Yingdong; Ji, Kegong; Li, Yuxue

2012-01-01

210

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

211

Stable Black Families. Final Report.  

ERIC Educational Resources Information Center

|This document is the final report of a study conducted to determine what factors contribute to strong Black family life and how these strong families solve problems, in order to add to the knowledge base on stable families so as to enhance practical intervention with families in need, and to identify models of self-help strategies used by stable

Gary, Lawrence E.; And Others

212

Bayesian Inference for Stable Distributions  

Microsoft Academic Search

Very little work on stable distribution parameter estimation and inference appears in the literature due to the nonexistence of the probability density function. This has led in particular to a dearth of Bayesian work in this area. But Bayesian computation via Markov chain Monte Carlo allows us to sample from the distribution of the parameters of the stable distributions, by

D. J. Buckle

1995-01-01

213

Stable Isotope Research Pool Inventory.  

National Technical Information Service (NTIS)

This report contains a listing of electromagnetically separated stable isotopes which are available for distribution within the United States for nondestructive research use from the Oak Ridge National Laboratory on a loan basis. This inventory includes a...

1982-01-01

214

Stable Isotope Research Pool Inventory.  

National Technical Information Service (NTIS)

This report contains a listing of electromagnetically separated stable isotopes which are available for distribution within the United States for non-destructive research use from the Oak Ridge National Laboratory on a loan basis. This inventory includes ...

1980-01-01

215

Stable isotope research pool inventory  

NASA Astrophysics Data System (ADS)

A listing is presented of electromagnetically separated stable isotopes which are available for distribution within the United States for nondestructive research use from the Oak Ridge National Laboratory on a loan basis. This inventory includes all samples of stable isotopes in the Material Research Collection and does not designate whether a sample is out on loan or in reprocessing. For some of the high abundance naturally occurring isotopes, larger amounts can be made available.

1982-01-01

216

Distributed Weighted Stable Marriage Problem  

NASA Astrophysics Data System (ADS)

The Stable Matching problem was introduced by Gale and Shapley in 1962. The input for the stable matching problem is a complete bipartite K n,n graph together with a ranking for each node. Its output is a matching that does not contain a blocking pair, where a blocking pair is a pair of elements that are not matched together but rank each other higher than they rank their current mates. In this work we study the Distributed Weighted Stable Matching problem. The input to the Weighted Stable Matching problem is a complete bipartite K n,n graph and a weight function W. The ranking of each node is determined by W, i.e. node v prefers node u 1 over node u 2 if W((v,u 1)) > W((v, u 2)). Using this ranking we can solve the original Stable Matching problem. We consider two different communication models: the billboard model and the full distributed model. In the billboard model, we assume that there is a public billboard and each participant can write one message on it in each time step. In the distributed model, we assume that each node can send O(logn) bits on each edge of the K n,n . In the billboard model we prove a somewhat surprising tight bound: any algorithm that solves the Stable Matching problem requires at least n - 1 rounds. We provide an algorithm that meets this bound. In the distributed communication model we provide an algorithm named intermediation agencies algorithm, in short (IAA), that solves the Distributed Weighted Stable Marriage problem in O(sqrt{n}) rounds. This is the first sub-linear distributed algorithm that solves some subcase of the general Stable Marriage problem.

Amira, Nir; Giladi, Ran; Lotker, Zvi

217

Stable isotope research pool inventory  

SciTech Connect

This report contains a listing of electromagnetically separated stable isotopes which are available at the Oak Ridge National Laboratory for distribution for nondestructive research use on a loan basis. This inventory includes all samples of stable isotopes in the Research Materials Collection and does not designate whether a sample is out on loan or is in reprocessing. For some of the high-abundance, naturally occurring isotopes, larger amounts can be made available; for example, Ca-40 and Fe-56.

Not Available

1988-02-01

218

N-Heterocyclic carbene-palladium catalysts for the direct arylation of pyrrole derivatives with aryl chlorides.  

PubMed

New Pd-NHC complexes have been synthesized and employed for palladium-catalyzed direct arylation of pyrrole derivatives by using electron-deficient aryl chlorides as coupling partners. The desired coupling products were obtained in moderate to good yields by using 1 mol % of these air-stable palladium complexes. This is an advantage compared to the procedures employing air-sensitive phosphines, which have been previously shown to promote the coupling of aryl chlorides with heteroarenes. PMID:23504414

Ozdemir, Ismail; Gürbüz, Nevin; Kalo?lu, Nazan; Do?an, Oznur; Kalo?lu, Murat; Bruneau, Christian; Doucet, Henri

2013-02-12

219

N-Heterocyclic carbene-mediated zwitterionic polymerization of N-substituted N-carboxyanhydrides toward poly(?-peptoid)s: kinetic, mechanism, and architectural control.  

PubMed

N-Heterocyclic carbene (NHC)-mediated polymerizations of N-butyl N-carboxyanhydride (Bu-NCA) to produce cyclic poly(N-butyl glycine)s (c-NHC-PNBGs) have been investigated in various solvents with NHCs having differing steric and electronic properties. Control over the polymer molecular weight (MW) and polymerization rate is strongly dependent on the solvent and the NHC structure. Kinetic studies reveal that the propagating intermediates for the polymerization in low dielectric solvents (e.g., THF or toluene) maintain cyclic architectures with two chain ends in close contact through Coulombic interaction. The NHCs not only initiate the polymerization, but also mediate the chain propagation as intramolecular counterions. Side reactions are significantly suppressed in low dielectric solvents due to the reduced basicity and nucleophilicity of the negatively charged chain ends of the zwitterions, resulting in quasi-living polymerization behavior. By contrast, the two charged chain ends of the zwitterionic species are fully dissociated in high dielectric solvents. The chain propagation proceeds as in conventional anionic polymerizations, wherein side reactions (e.g., transamidation) compete with chain propagation, resulting in significantly diminished control over polymer MW. The cyclic zwitterionic propagating species can be converted into their linear polymeric analogues (l-NHC-PNBGs) by end-capping with electrophiles (e.g., acetyl chloride) or the NHC-free cyclic analogues (c-PNBGs) by treatment with NaN(TMS)(2), as evidenced by MALDI-TOF MS, NMR, and SEC analysis. PMID:22568497

Guo, Li; Lahasky, Samuel H; Ghale, Kushal; Zhang, Donghui

2012-05-24

220

Monodentate non-C(2)-symmetric chiral N-heterocyclic carbene complexes for enantioselective synthesis. Cu-catalyzed conjugate additions of aryl- and alkenylsilylfluorides to cyclic enones.  

PubMed

A new class of enantioselective conjugate addition (ECA) reactions that involve aryl- or alkenylsilyl fluoride reagents and are catalyzed by chiral non-C(2)-symmetric Cu-based N-heterocyclic carbene (NHC) complexes are disclosed. Transformations have been designed based on the principle that a catalytically active chiral NHC-Cu-aryl or NHC-Cu-alkenyl complex can be accessed from reaction of a Cu-halide precursor with in situ-generated aryl- or alkenyltetrafluorosilicate. Reactions proceed in the presence of 1.5 equiv of the aryl- or alkenylsilane reagents and 1.5 equiv of tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF). Desired products are isolated in 63-97% yield and 73.5:26.5-98.5:1.5 enantiomeric ratio (47%-97% ee). A major focus of the present studies is the design, evaluation, and development of new chiral imidazolinium salts and their derived NHC-Cu complexes as catalysts that promote reactions of various carbosilanes to a range of electrophilic substrates. Toward this end, nearly 20 new chiral monodentate imidazolinium salts, most of which are non-C(2)-symmetric, have been prepared and fully characterized and their ability to serve as catalysts in the ECA reactions has been investigated. PMID:19445467

Lee, Kang-sang; Hoveyda, Amir H

2009-06-19

221

Metal Stable Isotopes in Paleoceanography  

NASA Astrophysics Data System (ADS)

Considered esoteric only a few years ago, research into the stable isotope geochemistry of transition metals is moving into the geoscience mainstream. Although initial attention focused on the potential use of some of these nontraditional isotope systems as biosignatures, they are now emerging as powerful paleoceanographic proxies. In particular, the Fe and Mo isotope systems are providing information about changes in oxygenation and metal cycling in ancient oceans. Zn, Cu, Tl, and a number of other metals and metalloids also show promise. Here we review the basis of stable isotope fractionation as it applies to these elements, analytical considerations, and the current status and future prospects of this rapidly developing research area.

Anbar, Ariel D.; Rouxel, Olivier

2007-05-01

222

Multi-stable cylindrical lattices  

NASA Astrophysics Data System (ADS)

We present a cylindrical lattice structure that mimics the behaviour of the virus bacteriophage T4 in having two (or more) stable states which differ in their radii and length. While the virus achieves bistability through molecular mechanisms we use composite materials to exploit the interplay between pre-stress, material properties and structural geometry. We demonstrate (computationally) that multi-stability is a robust phenomenon. We also show (analytically) that it is possible to choose the design variables so that the energy is independent of the radius, thus resulting in every state of the structure being stable.

Pirrera, Alberto; Lachenal, Xavier; Daynes, Stephen; Weaver, Paul M.; Chenchiah, Isaac V.

2013-11-01

223

Stable isotopes in tree rings  

Microsoft Academic Search

Stable isotopes in tree rings could provide palaeoclimate reconstructions with perfect annual resolution and statistically defined confidence limits. Recent advances make the approach viable for non-specialist laboratories. The relevant literature is, however, spread across several disciplines, with common problems approached in different ways. Here we provide the first overview of isotope dendroclimatology, explaining the underlying theory and describing the steps

Danny McCarroll; Neil J. Loader

2004-01-01

224

Axiomatic Justification of Stable Equilibria  

Microsoft Academic Search

A solution concept that satisfies the axioms of invariance and strong backward induction selects a stable set of a game's equilibria. The theory of games is inherently incomplete because a typical game has multiple equilibria. Nevertheless, because some equilibria seem more plausible than others, several authors suggest that Nash's (1950) initial definition of equilibrium should be strengthened. Hillas and Kohlberg

Srihari Govindan; Robert Wilson

2005-01-01

225

Periodicity of the stable isotopes  

Microsoft Academic Search

It is demonstrated that all stable (non-radioactive) isotopes are formally interrelated as the products of systematically adding alpha particles to four elementary units. The region of stability against radioactive decay is shown to obey a general trend based on number theory and contains the periodic law of the elements as a special case. This general law restricts the number of

J. C. A. Boeyens

2003-01-01

226

Stable pairs and log flips  

Microsoft Academic Search

This paper has two parts. In the first part, we review stable pairs and triples on curves, leading up to Thaddeus' diagram of flips and contractions starting from the blow-up of projective space along a curve embedded by a complete linear series of the form K + ample. In the second part, we identify log canonical divisors which exhibit Thaddeus'

Aaron Bertram

1997-01-01

227

Possibility of stable multiquark baryons  

Microsoft Academic Search

Some of the heavy baryons (Qqqqq) where Q denotes an heavy quark c or b, and q a light quark u, d or s, are likely to be stable with respect to their dissociation into a meson and a baryon. The binding is achieved by the chromomagnetic interaction between light quarks and lies on a firmer ground than in the

C. Gignoux; B. Silvestre-Brac; J. M. Richard

1987-01-01

228

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

229

Fast linear algebra is stable  

Microsoft Academic Search

In (23) we showed that a large class of fast recursive matrix multiplication algorithms is stable in a normwise sense, and that in fact if multiplication of n-by-n matrices can be done by any algorithm in O(n!+?) operations for any ? > 0, then it can be done stably in O(n!+?) operations for any ? > 0. Here we extend

James Demmel; Ioana Dumitriu; Olga Holtz

2006-01-01

230

Stable Marriages and Search Frictions  

Microsoft Academic Search

We embed a two-sided matching market with non-transferable utility, a marriage market, into a random search model. We study steadystate equilibria and characterize the limit of the corresponding equilibrium matchings as exogenous search frictions become small. The central question is whether the set of such limit matchings coincides with the set of stable matchings for the underlying marriage market. We

Stephan Lauermann; Georg Nöldeke

2012-01-01

231

Nonparaxial eigenmodes of stable resonators  

Microsoft Academic Search

A method to determine the nonparaxial eigenmodes of stable resonators is presented. The method is based on the perturbation theory of Lax et al. For calculating nonparaxial components of the electric field. A matrix formalism which uses a mode expansion into paraxial Hermite-Gaussian modes is applied to describe the nonparaxial propagation and the phase shift at a parabolic and a

Holger Laabs; Ari T. Friberg

1999-01-01

232

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

233

Bis-N-heterocyclic carbene palladium(IV) tetrachloride complexes: synthesis, reactivity, and mechanisms of direct chlorinations and oxidations of organic substrates.  

PubMed

This Article describes the preparation and isolation of novel octahedral CH(2)-bridged bis-(N-heterocyclic carbene)palladium(IV) tetrachlorides of the general formula LPd(IV)Cl(4) [L = (NHC)CH(2)(NHC)] from LPd(II)Cl(2) and Cl(2). In intermolecular, nonchelation-controlled transformations LPd(IV)Cl(4) reacted with alkenes and alkynes to 1,2-dichlorination adducts. Aromatic, benzylic, and aliphatic C-H bonds were converted into C-Cl bonds. Detailed mechanistic investigations in the dichlorinations of alkenes were conducted on the 18VE Pd(IV) 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 Pd(IV)-Cl ionization in the first step. Product stereochemistry and product distributions from various alkenes also support Cl(+)-transfer from the pentacoordinated Pd(IV)-intermediate LPd(IV)Cl(3)(+) 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 Cl(2) from a reductive elimination process. Instead, a ligand-mediated direct Cl(+)-transfer from LPd(IV)Cl(3)(+) to the ?-system is likely to occur. Similarly, C-H bond chlorinations proceed via an electrophilic process with in situ formed LPd(IV)Cl(3)(+). The presence of a large excess of added Cl(-) slows cyclohexene chlorination while the presence of stoichiometric amounts of chloride accelerates both Pd(IV)-Cl ionization and Cl(+)-transfer from LPd(IV)Cl(3)(+). (1)H 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. PMID:21247150

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

2011-01-19

234

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

235

Three-coordinate nickel(I) complexes stabilised by six-, seven- and eight-membered ring n-heterocyclic carbenes: synthesis, EPR/DFT studies and catalytic activity.  

PubMed

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ?3d?z?2? and ?3d?x?2-y?2? character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4?6?5). PMID:23292787

Page, Michael J; Lu, Wei Y; Poulten, Rebecca C; Carter, Emma; Algarra, Andrés G; Kariuki, Benson M; Macgregor, Stuart A; Mahon, Mary F; Cavell, Kingsley J; Murphy, Damien M; Whittlesey, Michael K

2013-01-04

236

A Stable Silicon(0) Compound with a Si=Si Double Bond  

Microsoft Academic Search

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is :C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L,

Yuzhong Wang; Yaoming Xie; Pingrong Wei; R. Bruce King; Henry F. Schaefer; Paul von R. Schleyer; Gregory H. Robinson

2008-01-01

237

Design of stable nanocrystalline alloys.  

PubMed

Nanostructured metals are generally unstable; their grains grow rapidly even at low temperatures, rendering them difficult to process and often unsuitable for usage. Alloying has been found to improve stability, but only in a few empirically discovered systems. We have developed a theoretical framework with which stable nanostructured alloys can be designed. A nanostructure stability map based on a thermodynamic model is applied to design stable nanostructured tungsten alloys. We identify a candidate alloy, W-Ti, and demonstrate substantially enhanced stability for the high-temperature, long-duration conditions amenable to powder-route production of bulk nanostructured tungsten. This nanostructured alloy adopts a heterogeneous chemical distribution that is anticipated by the present theoretical framework but unexpected on the basis of conventional bulk thermodynamics. PMID:22923577

Chookajorn, Tongjai; Murdoch, Heather A; Schuh, Christopher A

2012-08-24

238

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

239

Stable Bose-Einstein correlations  

Microsoft Academic Search

The shape of Bose-Einstein (or HBT) correlation functions is determined for\\u000athe case when particles are emitted from a stable source, obtained after\\u000aconvolutions of large number of elementary random processes. The two-particle\\u000acorrelation function is shown to have a {\\\\it stretched exponential} shape,\\u000acharacterized by the L\\\\'evy index of stability $ 0 < \\\\alpha \\\\le 2$ and the\\u000ascale

S. Hegyi; W. A. Zajc; MTA KFKI RMKI

2004-01-01

240

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

241

Clear stable motor fuel composition  

SciTech Connect

A process is described for making a clear, stable gasoline blend from a hazy, water-saturated gasoline containing a mixture of hydrocarbons in the gasoline boiling range, from about 2.0 to about 12.0 volume percent of methanol, from about 2.0 to about 10.0 volume percent of a cosolvent (C/sub 2/-C/sub 5/) aliphatic alcohol and 0.1 to 0.5 volume percent of contaminating water. The process consists of adding to the water-saturated gasoline, from about 0.05 to about 3.0 weight percent of a nonionic surfactant of an alkanoic acid derivative.

Davis, M.E.; Sung, R.L.

1986-08-26

242

Iron stable isotopes: beyond biosignatures  

NASA Astrophysics Data System (ADS)

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 the environmental biogeochemistry of Fe. At the same time, the Fe isotope system holds promise for other exciting frontiers, including applications in oceanography, solid Earth geochemistry and biomedicine. Such applications will be increasingly attractive as Fe isotope analysis becomes routine.

Anbar, A. D.

2004-01-01

243

Stable polymorph of morphine1  

PubMed Central

In the stable polymorph of the title compound, C17H19NO3 [systematic name: (5?,6?)-7,8-didehydro-4,5-ep­oxy-17-methyl­morphinan-3,6-diol], the mol­ecular conformation is in agreement with the characteristics of previously reported morphine forms. The molecule displays the typical T-shape and its piperidine ring adopts a slightly distorted chair conformation. Inter­molecular O—H?O hydrogen bonds link the mol­ecules into helical chains parallel to the b axis. Intra­molecular O—H?O hydrogen bonds are also observed.

Gelbrich, Thomas; Braun, Doris E.; Griesser, Ulrich J.

2013-01-01

244

Fast linear algebra is stable  

Microsoft Academic Search

In Demmel et al. (Numer. Math. 106(2), 199–224, 2007) we showed that a large class of fast recursive matrix multiplication\\u000a algorithms is stable in a normwise sense, and that in fact if multiplication of n-by-n matrices can be done by any algorithm in O(n\\u000a \\u000a ?+?\\u000a ) operations for any ? >  0, then it can be done stably in O(n\\u000a \\u000a ?+?\\u000a )

James DemmelIoana Dumitriu; Ioana Dumitriu; Olga Holtz

2007-01-01

245

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

246

Persistence Length of Stable Microtubules  

NASA Astrophysics Data System (ADS)

Microtubules are a vital component of the cytoskeleton. As the most rigid of the cytoskeleton filaments, they give shape and support to the cell. They are also essential for intracellular traffic by providing the roadways onto which organelles are transported, and they are required to reorganize during cellular division. To perform its function in the cell, the microtubule must be rigid yet dynamic. We are interested in how the mechanical properties of stable microtubules change over time. Some "stable" microtubules of the cell are recycled after days, such as in the axons of neurons or the cilia and flagella. We measured the persistence length of freely fluctuating taxol-stabilized microtubules over the span of a week and analyzed them via Fourier decomposition. As measured on a daily basis, the persistence length is independent of the contour length. Although measured over the span of the week, the accuracy of the measurement and the persistence length varies. We also studied how fluorescently-labeling the microtubule affects the persistence length and observed that a higher labeling ratio corresponded to greater flexibility.

Hawkins, Taviare; Mirigian, Matthew; Selcuk Yasar, M.; Ross, Jennifer

2011-03-01

247

Methods of Identifying Kinetically Stable Proteins.  

National Technical Information Service (NTIS)

The present invention provides a fast and efficient means for identifying kinetically stable proteins. As used herein the term 'kinetically stable protein' means a protein that is trapped in a specific conformation due to an unusually high unfolding barri...

M. Manning W. Colon

2005-01-01

248

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...

249

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

250

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

251

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

252

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

253

A stable monomeric nickel borohydride.  

PubMed

A stable discrete nickel borohydride complex (Tp*NiBH(4) or Tp*NiBD(4)) was prepared using the nitrogen-donor ligand hydrotris(3,5-dimethylpyrazolyl)borate (Tp*-). This complex represents one of the best characterized nickel(II) borohydrides to date. Tp*NiBH(4) and Tp*NiBD(4) are stable toward air, boiling water, and high temperatures (mp > 230 degrees C dec). X-ray crystallographic measurements for Tp*NiBH(4) showed a six-coordinate geometry for the complex, with the nickel(II) center facially coordinated by three bridging hydrogen atoms from borohydride and a tridentate Tp(-) ligand. For Tp*NiBH(4), the empirical formula is C(15)H(26)B(2)N(6)Ni, a = 13.469(9) A, b = 7.740(1) A, c = 18.851(2) A, beta = 107.605(9) degrees, the space group is monoclinic P2(1)/c, and Z = 4. Infrared measurements confirmed the presence of bridging hydrogen atoms; both nu(B[bond]H)(terminal) and nu(B[bond]H)(bridging) are assignable and shifted relative to nu(B-D) of Tp*NiBD(4) by amounts in agreement with theory. Despite their hydrolytic stability, Tp*NiBH(4) and Tp*NiBD(4) readily reduce halocarbon substrates, leading to the complete series of Tp*NiX complexes (X = Cl, Br, I). These reactions showed a pronounced hydrogen/deuterium rate dependence (k(H)/k(D) approximately 3) and sharp isosbestic points in progressive electronic spectra. Nickel K-edge X-ray absorption spectroscopy (XAS) measurements of a hydride-rich nickel center were obtained for Tp*NiBH(4), Tp*NiBD(4), and Tp*NiCl. X-ray absorption near-edge spectroscopy results confirmed the similar six-coordinate geometries for Tp*NiBH(4) and Tp*NiBD(4). These contrasted with XAS results for the crystallographically characterized pseudotetrahedral Tp*NiCl complex. The stability of Tp*Ni-coordinated borohydride is significant given this ion's accelerated decomposition and hydrolysis in the presence of transition metals and simple metal salts. PMID:14632512

Desrochers, Patrick J; LeLievre, Stacey; Johnson, Rosemary J; Lamb, Brian T; Phelps, Andrea L; Cordes, A W; Gu, Weiwei; Cramer, Stephen P

2003-12-01

254

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

2012-08-17

255

Efficient C-B Bond Formation Promoted by N-Heterocyclic Carbenes. Synthesis of Tertiary and Quaternary B-Substituted Carbons through Metal-Free Catalytic Boron Conjugate Additions to Cyclic and Acyclic ?,?-Unsaturated Carbonyls  

PubMed Central

Metal-free, nucleophilic activation of a B–B bond has been exploited in the development of a highly efficient method for conjugate additions of commercially available bis(pinacolato)diboron to cyclic or acyclic ?,?-unsaturated carbonyls. Reactions are readily catalyzed by 2.5–10 mol % of a simple N-heterocyclic carbene (NHC). A variety of cyclic and acyclic unsaturated ketones and esters can serve as substrates. Transformations deliver ?-boryl carbonyls bearing tertiary as well as quaternary B-substituted carbons in up to >98% yield. Preliminary studies indicate that, although related Cu–NHC-catalyzed reactions are equally efficient, the metal-free variant is more functional group tolerant; in contrast to the Cu-catalyzed reactions, the metal-free processes proceed readily in the presence of a terminal alkyne and do not promote concomitant diboration of an aldehyde. Representative functionalization of the resulting boron enolates demonstrates the strong influence of the Lewis acidic B of the ?-boronate.

Lee, Kang-sang; Zhugralin, Adil R.; Hoveyda, Amir H.

2009-01-01

256

Helical metal cage of silver(I) and mercury(II)-N-heterocyclic carbene complexes of 1,4-phenylene-bis{imidazo[1,5-a]pyridine-4-iumhexaflurophosphate}  

NASA Astrophysics Data System (ADS)

Novel N-heterocyclic carbene (NHC) architecture of 1,4-phenylene-bis{imidazo[1,5-a]pyridine-4 iumhexaflurophosphate}, [1(HPF6)2] has been synthesized in two steps from commercially available starting materials. The p-phylene bridged free annulated imidazolium salt has been synthesized and characterized both in solution and solid-state and has been used to synthesize silver(I) and mercury(II)-NHC complexes using the basic metal precursors. The [Ag2(1)2][PF6]2 (2) was synthesized using [1(HPF6)2] by Ag2O method in acceptable yield. The dinuclear Hg(II) complex [Hg2(1)2][PF6]4 (3) consisting of two biscarbene ligand has been synthesized from Hg(OAc)2 and [1(HPF6)2]. The helical nature of Hg(II)-NHC (3) has been established from solid state X-ray structure.

Rana, Bidyut Kumar; Bertolasi, Valerio; Pal, Satyanarayan; Mitra, Partha; Dinda, Joydev

2013-10-01

257

Benzodifurantrione: a stable phenylogous enol.  

PubMed

The first example of a stable phenylogous enol, resulting from an extended keto-enol tautomerization across a benzene ring, is described. The enol has been isolated, and its structure was proven by X-ray crystallography. The equilibrium between the keto- and enol-tautomers has been extensively studied and quantified in solution by NMR and UV-vis spectroscopy. The position of equilibrium showed a linear correlation to the Kamlet-Taft solvatochromic scale for solvent H-bond acceptor strength (beta(OmicronEta)), and the equilibrium was proven to be fully dynamic, obeying first-order equilibrium kinetics. To attempt to explain why enolization occurs, at what surprisingly appears to be the expense of aromatic resonance stabilization, various structural features have been considered and explored further with the aid of MO calculations. Nucleus independent chemical shift (NICS) index of aromaticity calculations for each of the rings comprising both tautomers showed that while the central benzene ring loses aromaticity on enolization, the alpha-keto-lactone ring showed an unexpected and significant antiaromaticity in the keto-tautomer, which is by no means intuitive. The loss of stabilization energy associated with the central benzene ring is, therefore, to a certain degree compensated by removal of the antiaromatic destabilization of the alpha-keto-lactone ring rendering the two structures much closer in energy than would otherwise be expected. PMID:20055373

Lawrence, Anthony J; Hutchings, Michael G; Kennedy, Alan R; McDouall, Joseph J W

2010-02-01

258

N-heterocyclic carbene-induced zwitterionic ring-opening polymerization of ethylene oxide and direct synthesis of alpha,omega-difunctionalized poly(ethylene oxide)s and poly(ethylene oxide)-b-poly(epsilon-caprolactone) block copolymers.  

PubMed

An N-heterocyclic carbene (NHC), namely, 1,3-bis-(diisopropyl)imidazol-2-ylidene (1), was demonstrated to bring about the metal-free ring-opening polymerization of ethylene oxide at 50 degrees C in dimethyl sulfoxide, in absence of any other reagents. Poly(ethylene oxide) (PEO) of polydispersities <1.2 and molar masses perfectly matching the [monomer]/[(1)] ratio could thus be obtained in quantitative yields, attesting to the controlled/living character of such carbene-initiated polymerizations. It is argued that (1) adds to ethylene oxide to form a zwitterionic species, namely 1,3-bis-(diisopropyl)imidazol-2-ylidinium alkoxide, that further propagates by a zwitterionic ring-opening polymerization (ZROP) mechanism. Through an appropriate choice of terminating agent NuH or NuSiMe(3) at the completion of the polymerization, a variety of end-functionalized PEO chains could be generated. In particular, alpha,omega-bis(hydroxy)-telechelic PEO, alpha-benzyl,omega-hydroxy, and alpha-azido,omega-hydroxy-difunctionalized PEOs were synthesized by NHC (1)-initiated ZROP, using H(2)O, PhCH(2)OH, and N(3)SiMe(3) as terminating agent, respectively. Characterization of these alpha,omega-difunctionalized PEOs by techniques such as (1)H NMR spectroscopy, MALDI-TOF spectrometry, and size exclusion chromatography confirmed the quantitative introduction of functional groups at both alpha and omega positions of the PEO chains and the formation of very narrow molar mass polymers. Finally, the synthesis of a poly(ethylene oxide)-b-poly(epsilon-caprolactone) diblock copolymer by sequential ZROP of the corresponding monomers was successfully achieved using (1) as organic initiator without isolation of the PEO block intermediate. PMID:19209910

Raynaud, Jean; Absalon, Christelle; Gnanou, Yves; Taton, Daniel

2009-03-11

259

Packing and Hausdorff Measures of Stable Trees  

Microsoft Academic Search

\\u000a In this paper we discuss Hausdorff and packing measures of random continuous trees called stable trees. Stable trees form\\u000a a specific class of Lévy trees (introduced by Le Gall and Le Jan in [33]) that contains Aldous’s continuum random tree which\\u000a corresponds to the Brownian case. We provide results for the whole stable trees and for their level sets that

Thomas Duquesne

2010-01-01

260

Prediction of Stationary Max-Stable Processes  

Microsoft Academic Search

We consider prediction of stationary max-stable processes. The usual metric between max-stable variables can be defined in terms of the $L_1$ distance between spectral functions and in terms of this metric a kind of projection can be defined. It is convenient to project onto max-stable spaces; that is, spaces of extreme value distributed random variables that are closed under scalar

Richard A. Davis; Sidney I. Resnick

1993-01-01

261

Stone duality and representation of stable domain  

Microsoft Academic Search

In this paper, the author studies the Stone duality and representation of L-domain w.r.t. stable functions. Two basic notions are introduced, one is D-semilattice and the other is semitopological system. The author first gives the representation of stable D-lattice, then establishes the representation theorem of L-domain and gives the Stone duality of L-domains w.r.t. stable functions in the scheme of

Yixiang Chen

1997-01-01

262

Bayesian Inference for Skewed Stable Distributions  

NASA Astrophysics Data System (ADS)

Stable distributions are a class of distributions which allow skewness and heavy tail. Non-Gaussian stable random variables play the role of normal distribution in the central limit theorem, for normalized sums of random variables with infinite variance. The lack of analytic formula for density and distribution functions of stable random variables has been a major drawback to the use of stable distributions, also in the case of inference in Bayesian framework. Buckle introduced priors for the parameters of stable random variables to obtain an analytic form of posterior distribution. However, many researchers tried to solve the problem, through the Markov chain Monte Carlo methods, e.g. [8] and their references. In this paper a new class of heavy-tailed distribution is introduced, called skewed stable. This class has two main advantages: It has many inferential advantages, since it is a member of exponential family, so the Bayesian inference can be drawn similar to the exponential family of distributions and modelling skew data with stable distributions is dominated by this family. Finally, Bayesian inference for skewed stable arc compared to the stable distributions through a few simulations study.

Shokripour, Mona; Nassiri, Vahid; Mohammadpour, Adel

2011-03-01

263

K Causality Coincides with Stable Causality  

Microsoft Academic Search

It is proven that K-causality coincides with stable causality, and that in a K-causal spacetime the relation K\\u000a + coincides with the Seifert’s relation. As a consequence the causal relation “the spacetime is strongly causal and the closure\\u000a of the causal relation is transitive” stays between stable causality and causal continuity.

E. Minguzzi

2009-01-01

264

Comparison of stable isotope reference samples  

Microsoft Academic Search

Use of light stable isotope ratio measurements has proliferated in the past decade. The need for procuring additional stable isotope reference materials was recognized at an International Atomic Energy Agency (IAEA) consultants' meeting convened in 19761. This group recommended acquisition of two carbonates, two carbon dioxide samples, a biotite, a sulphate, and other reference materials. We report here on the

Tyler B. Coplen; Carol Kendall; Jessica Hopple

1983-01-01

265

Concentration of Stable Elements in Food Products.  

National Technical Information Service (NTIS)

Food samples were taken from commercial markets and analyzed for stable element content. The concentrations of most stable elements (Ag, Al, As, Au, Ba, Br, Ca, Ce, Cl, Co, Cr, Cs, Cu, Fe, Hf, I, K, La, Mg, Mn, Mo, Na, Rb, Sb, Sc, Se, Sr, Ta, Th, Ti, V, Z...

M. A. Montford K. E. Shank C. Hendricks T. W. Oakes

1980-01-01

266

Sexual intercourse and stable angina pectoris  

Microsoft Academic Search

Stable angina pectoris is a common condition associated with chest pain predictable for a given level of exercise. Sexual intercourse does not lead to exaggerated heart rate or blood pressure responses and is interpreted by the heart as one of many forms of activity that may take place in a 24-hour period. Stable angina patients optimally treated are not at

Graham Jackson

2000-01-01

267

Hausdorff Dimension of Operator Stable Sample Paths  

Microsoft Academic Search

The Hausdorff dimension of the sample paths of a stochastic process with stationary independent operator stable increments is computed. With probability one, every sample path has the same dimension, depending on the real parts of the eigenvalues of the operator stable exponent.

Peter Becker-Kern; Mark M. Meerschaert; Hans-Peter Scheffler

2003-01-01

268

Compact Preference Representation in Stable Marriage Problems  

Microsoft Academic Search

The stable marriage problem has many practical applications in two- sided markets like those that assign doctors to hospitals, s tudents to schools, or buyers to vendors. Most algorithms to find stable marriages a ssume that the par- ticipants explicitly expresses a preference ordering. Thi s can be problematic when the number of options is large or has a combinatorial

Enrico Pilotto; Francesca Rossi; Kristen Brent Venable; Toby Walsh

2009-01-01

269

Stable isotope composition of Hellenic bottled waters  

Microsoft Academic Search

Bottled waters are an increasingly significant product in the human diet. In this work, we present a dataset of stable isotope ratios for bottled waters sampled in Greece. A total of 25 domestic brands of bottled still waters, collected on the Greek market in 2009, were analysed for ?18O and ?2H. The measured stable isotope ratios range from ?9.9‰ to

2010-01-01

270

Maximally Stable Local Description for Scale Selection  

Microsoft Academic Search

Scale and ane-in variant local features have shown excellent performance in image matching, object and texture recognition. This pa- per optimizes keypoint detection to achieve stable local descriptors, and therefore, an improved image representation. The technique performs scale selection based on a region descriptor, here SIFT, and chooses re- gions for which this descriptor is maximally stable. Maximal stability is

Gyuri Dorkó; Cordelia Schmid

2006-01-01

271

On Stable Chaos in the Asteroid Belt  

Microsoft Academic Search

Recent numerical integration of Sidlichovsky and Nesvorny showed that 26 out of the first one hundred numbered asteroids have Lyapunov time TL shorter than 50 thousand years. Calculations were performed taking into account both outer and inner planets (without Mercury and Pluto). As the proper elements are reasonably stable for several 10^7 years we have the case of stable chaos.

M. Sidlichovský

1999-01-01

272

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

273

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

274

Portfolio Analysis in a Stable Paretian Market  

Microsoft Academic Search

Recently evidence has come forth which suggests that empirical probability distributions of returns on securities conform better to stable Paretian distributions with infinite variances than to the normal distribution. Using a generalized form of a technique proposed by Sharpe [17] in a recent issue of this journal, this article develops a portfolio analysis model for a stable Paretian market. The

Eugene F. Fama

1965-01-01

275

The effective bandwidth of stable distributions  

Microsoft Academic Search

In this paper the effective bandwidths of stable distributions are studied. Effective bandwidths are being heavily promoted as the most appropriate method for call admission control (CAC) and resource allocation within ATM networks. Previous work in teletraffic modelling has suggested that models based on stable distributions provide an efficient mechanism for capturing the long range dependence and infinite variance associated

Stephen Bates; Steve McLaughlin

1998-01-01

276

The estimation of stable distribution parameters  

Microsoft Academic Search

This paper concerns the estimation of the parameters that describe a stable distribution. Stable distributions are characterised by four parameters which can be estimated using a number of methods and although approximate maximum likelihood estimation (MLE) techniques do exist, they are computationally intensive. There are a number of techniques that are much faster than MLE and these are the focus

Stephen Bates; Steve Mclaughlin

1997-01-01

277

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

278

On stable spherical configurations of artificial protocells  

Microsoft Academic Search

The stability of protocells is discussed in terms of macroscopic energies. It is shown that bulk and surface energy contributions alone cannot lead to stable protocells. If electrostatic energy, due to transmembrane voltage, is taken into account, stable hollow spheres are proved to exist. For several reasons, however, this result should not be regarded as an ultimate explanation.

U. Müller-Herold; G. Nickel

1995-01-01

279

STABLE EQUIVALENCES RELATED WITH SYZYGY FUNCTORS  

Microsoft Academic Search

Let :m od? ? ? mod?? be a stable equivalence betweennite dimensional self-injective algebras over aeld. Then preserves triangles in the triangulated category modif and only if commutes with syzygy functors. As an application, we study some stable equivalence induced by socle equivalence. Throughout this paper K will be axedeld, and all algebras will be basicnite dimensional self-injective K-algebras without

YOSUKE OHNUKI

280

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

281

Stable Isotope Fractionation Studies of Timna Mineralization.  

National Technical Information Service (NTIS)

Stable isotope fractionation studies have been shown to be invaluable geochemical indicators of the processes involved in ore deposition and mineralization. The present work is concerned with the application of carbon and oxygen isotopic studies to an und...

A. Matthews

1983-01-01

282

Stable Isotope Group 1983 Progress Report.  

National Technical Information Service (NTIS)

The work of the Stable Isotope Group of the Institute of Nuclear Sciences in the fields of isotope geology, isotope hydrology, geochronology, isotope biology and related fields, and mass spectrometer instrumentation, during 1983, is described. (Atomindex ...

M. K. Stewart

1984-01-01

283

Stable Particles as Building Blocks of Matter.  

National Technical Information Service (NTIS)

Only absolutely stable particles can be truly elementary. A simple theory of matter based on the three constituents, proton, electron and neutrino (and their antiparticles), bound together by the ordinary magnetic forces is presented, which allows us to g...

A. O. Barut

1979-01-01

284

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

285

Optimal design of thermally stable proteins  

PubMed Central

Motivation: For many biotechnological purposes, it is desirable to redesign proteins to be more structurally and functionally stable at higher temperatures. For example, chemical reactions are intrinsically faster at higher temperatures, so using enzymes that are stable at higher temperatures would lead to more efficient industrial processes. We describe an innovative and computationally efficient method called Improved Configurational Entropy (ICE), which can be used to redesign a protein to be more thermally stable (i.e. stable at high temperatures). This can be accomplished by systematically modifying the amino acid sequence via local structural entropy (LSE) minimization. The minimization problem is modeled as a shortest path problem in an acyclic graph with nonnegative weights and is solved efficiently using Dijkstra's method. Contact: mitchell@biochem.wisc.edu

Bannen, Ryan M.; Suresh, Vanitha; Phillips, George N.; Wright, Stephen J.; Mitchell, Julie C.

2008-01-01

286

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

287

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

288

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

289

Thermally Stable Organometallic Polymers: A Progress Report.  

National Technical Information Service (NTIS)

The need for thermally stable polymers capable of maintaining good mechanical properties in the vicinity of 1000F to be used as coatings, insulators, and adhesives for Army missiles has prompted an attempt to prepare highly metalated polymers containing f...

S. P. Brown

1971-01-01

290

Stable isotope investigations of chlorinated aliphatic hydrocarbons.  

National Technical Information Service (NTIS)

Stable isotope ratio measurements for carbon (C) and chlorine (Cl) can be used to elucidate the processes affecting transformation and transportation of chlorinated aliphatic hydrocarbons (CAHs) in the environment. Methods recently developed in our labora...

T. Abrajano L. J. Heraty B. D. Holt L. Huang N. C. Sturchio

1999-01-01

291

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

292

Aggressive Fibromatosis: Evidence for a Stable Phase  

PubMed Central

Purpose. Aggressive fibromatosis (AF) is an uncommon locally infiltrating benign disease of soft tissue for which treatment comprises complete surgical resection. Radiotherapy can be given postoperatively if the margin is incompletely resected. If the tumour is inoperable radiotherapy provides an alternative treatment. Hormone therapy and cytotoxic chemotherapy have also been used for unresectable or recurrent disease. All treatment modalities carry an associated morbidity. We believe that the natural history of aggressive fibromatosis may include a period of stable disease without progression, during which time, treatment is not always necessary. Patients and methods. We present a retrospective review of 42 patients referred to the Royal Marsden Hospital between 1988 and 1995 with aggressive fibromatosis. Evidence of periods of stable disease and the relationship to delivered treatment was obtained from the case notes, including the natural history prior to referral to our institution. Stable disease was defined as a period of no objective progression for 6 months or longer. Results. Seventeen patients could be assessed for stable disease and all (100%) experienced at least one episode of stable disease, eight of whom whilst receiving hormonal or cytotoxic therapy. Of the 23 patients who could not be assessed for stable disease, as they underwent surgery at presentation or recurrence of disease, only 2 had persisting disease at last follow-up. Both of these patients had had positive surgical resection margins. Discussion. This study demonstrates the variable natural history of AF, which can include a substantial period of stable disease in a significant number of patients. A less aggressive approach to the management of AF may therefore be appropriate, particularly if a subgroup of patients who are likely to experience a period of stable disease can be identified.

Thomas, J. Meirion; Harmer, Clive L.

1998-01-01

293

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

2012-12-28

294

Stable isotope dilution assays in mycotoxin analysis.  

PubMed

The principle and applications of stable isotope dilution assays (SIDAs) in mycotoxin analysis are critically reviewed. The general section includes historical aspects of SIDAs, the prerequisites and limitations of the use of stable isotopically labelled internal standards, and possible calibration procedures. In the application section actual SIDAs for the analysis of trichothecenes, zearalenone, fumonisins, patulin, and ochratoxin A are presented. The syntheses and availability of labelled mycotoxins for use as internal standards is reviewed and specific advances in food analysis and toxicology are demonstrated. The review indicates that LC-MS applications, in particular, require the use of stable isotopically labelled standards to compensate for losses during clean-up and for discrimination due to ion suppression. As the commercial availability of these compounds continues to increase, SIDAs can be expected to find expanding use in mycotoxin analysis. PMID:18060393

Rychlik, Michael; Asam, Stefan

2007-12-01

295

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

296

Searching for the Most Stable Pulsating Stars  

NASA Astrophysics Data System (ADS)

Certain modes in the hot ZZ Ceti stars (HDAVs) exhibit extreme pulsational stability; their periods have been observed to show a slow increase, which is theoretically accounted for by evolutionary cooling of the star. These modes are super-stable, more stable than atomic clocks and most pulsars; they will lose one cycle in a few billion years. We have begun a program to find 100 new HDAVs to do ensemble asteroseismology and to search for planetary companions. HDAVs with an orbiting planet should show a measurable reflex motion around the center of mass of the system, easily distinguishable from their slow evolutionary cooling.

Mukadam, A.; Mullally, F.; Winget, D. E.; Nather, R. E.; Salviander, S.; von Hippel, T.; Reaves, D.; Slaughter, D.; Kepler, S. O.; Sullivan, D. J.; Homeier, D.

297

Moving stable solitons in Galileon theory  

NASA Astrophysics Data System (ADS)

Despite the no-go theorem Endlich et al. (2011) [6] which rules out static stable solitons in Galileon theory, we propose a family of solitons that evade the theorem by traveling at the speed of light. These domain-wall-like solitons are stable under small fluctuations—analysis of perturbation shows neither ghost-like nor tachyon-like instabilities, and perturbative collision of these solitons suggests that they pass through each other asymptotically, which maybe an indication of the integrability of the theory itself.

Masoumi, Ali; Xiao, Xiao

2012-08-01

298

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

299

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

300

Effective stable particle behavior of the ?  

NASA Astrophysics Data System (ADS)

We investigate the reason behind the fact that the ? isobar effectively behaves as a stable particle. We first obtain model-independent conditions for this behavior that depend only on the imaginary part of the delta propagator. We then consider a toy model in which the N? system is decoupled from the NN system and check that the approximate behavior of the ? as a stable particle is not in effect controlled by the coupling to NN channels nor due to the narrow width of the ? resonance. Finally, we describe the N? amplitude in the isospin T=2 channels within a meson-exchange model. Experimental N? T=2 phase shifts would be welcome.

Garcilazo, H.; Pea, M. T.

1991-12-01

301

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

302

Indirect estimation of ®-stable stochastic volatility models  

Microsoft Academic Search

The ®-stable family of distributions constitutes a generalization of the Gaus- sian distribution, allowing for asymmetry and thicker tails. Its many useful p rop- erties, including a central limit theorem, are especially appreciated in the fin ancial field. However, estimation difficulties have up to now hindered its diffusion a mong practitioners. In this paper we propose an indirect estimation approach

Marco J. Lombardi

303

Fractional Calculus and Stable Probability Distributions  

Microsoft Academic Search

Fractional calculus allows one to generalize the linear (one dimensional)diffusion equation by replacing either the first time derivative or the second spacederivative by a derivative of a fractional order. The fundamental solutions of thesegeneralized diffusion equations are shown to provide certain probability density functions,in space or time, which are related to the relevant class of stable distributions. For thespace fractional

Rudolf Gorenflo; Francesco Mainardi

1998-01-01

304

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

305

Method of making stable UOâ fuel pellets  

Microsoft Academic Search

A method is disclosed for producing a dimensionally stable UOâ fuel pellet of large grain mole % and relatively large pore size. A dopant containing an element selected from the group consisting of aluminum, calcium, magnesium, titanium, zirconium, vanadium, niobium, and mixtures thereof is added to a highly sinterable UOâ powder, which is a UOâ powder that is sinterable to

K. C. Radford; J. M. Pope

1984-01-01

306

Asthenospheric shear flow: Thermally stable or unstable?  

Microsoft Academic Search

One-dimensional shear flows may decouple certain lithospheric plates from the deeper mantle, especially beneath continental shields. We investigate the variety of such flows which can occur in a model upper mantle with an olivine rheology and radiogenic heat sources. The thermal stability of these states is examined. The flows are stable if the horizontal velocity at the top of the

David A. Yuen; Gerald Schubert

1977-01-01

307

Dynamically Stable Associative Learning Neural Network System.  

National Technical Information Service (NTIS)

A dynamically stable associative learning neural network system includes a plurality of synapses and a non-linear function circuit and includes an adaptive weight circuit for adjusting the weight of each synapse based upon the present signal and the prior...

D. L. Alkon T. T. Vogl K. L. Blackwell

1989-01-01

308

Stable Fly Project in Campo Grande, Brazil  

Technology Transfer Automated Retrieval System (TEKTRAN)

Andrew Li, Jerry Hogsette, and Adalberto Pérez de León, all USDA-ARS, and Lane Foil, LSU, are collaborating as consultants on an Embrapa-funded research project for the control of large stable fly populations associated with sugar cane production in Brazil. Our Brazilian counterpart, Dr. Paulo Cança...

309

Stable marriage problems with quantitative preferences  

Microsoft Academic Search

The stable marriage problem is a well-known problem of matching men to women so that no man and woman, who are not married to each other, both prefer each other. Such a problem has a wide variety of practical applications, ranging from matching resident doctors to hospitals, to matching students to schools or more generally to any two-sided market. In

Maria Silvia Pini; Francesca Rossi; Brent Venable; Toby Walsh

2010-01-01

310

Roughness of stable, armored gravel beds  

Microsoft Academic Search

The grain roughness of stable armored beds that formed in a laboratory flume under a range of steady flow conditions on rounded, flat, and angular gravel is analyzed. Gravel roughness geometry is determined from bed surface profiles and vertical photographs. These techniques have been employed in field situations. Thus the methodology is potentially applicable to the analysis of grain roughness

Basil Gomez

1993-01-01

311

Stable synchronization of rigid body networks  

Microsoft Academic Search

We address stable synchronization of a network of rotating and translating rigid bodies in three-dimensional space. Motivated by applications that require coordinated spinning spacecraft or diving underwater vehicles, we prove control laws that stably couple and coordinate the dynamics of mul- tiple rigid bodies. We design decentralized, energy shaping control laws for each individual rigid body that depend on the

Sujit Nair; Naomi Ehrich Leonard

2007-01-01

312

High Temperature and Alkaline Stable Catalase.  

National Technical Information Service (NTIS)

The invention relates to thermal and pH stable catalases. One catalase of the invention was purified and characterized from Thermus brockianus. As a part of the characterization, the enzyme was compared to typical catalases from commercial sources and fou...

K. D. Schaller V. Thompson W. A. Apel

2004-01-01

313

Engineering of volume-stable adipose tissues  

Microsoft Academic Search

Autologous adipose tissues have been clinically used for augmentation of soft tissues lost due to mastectomy or lumpectomy in plastic and reconstructive surgery. However, this therapy has problems of absorption and subsequent volume loss of the implanted adipose tissues. In this study, volume-stable adipose tissues were engineered in vivo using mechanical support structures fabricated from biodegradable synthetic polymers. Dome-shaped mechanical

Seung-Woo Cho; Sang-Soo Kim; Jong Won Rhie; Hyun Mi Cho; Cha Yong Choi; Byung-Soo Kim

2005-01-01

314

Panel Discussion on Thermally Stable Polymers  

Microsoft Academic Search

Segal: Three years ago when the Polymer Group of the Southern California Section, ACS, held its last symposium on high-temperature polymers (see High-Temperature Polymers, Marcel Dekker, New York, 1967) a similarly constituted, and equally illustrious, panel was asked: “Where do we go next in regard to the synthesis, characterization, and processing of thermally stable polymers?”

Charles L. Segal; John K. Stille; George F. Pezdirtz; Harold H. Levine; W. E. Gibbs

1969-01-01

315

Formation of a stable canal channel  

Microsoft Academic Search

Conclusions 1.Deformation of the canal cross section occurs where Frov.2.A stable canal section can be obtained by two methods: take cross-sectional shapes and a velocity regime such that the inequality Fro?Frv is fulfilled at all points of the wetted perimeter; partially reinforce the canal side slopes in the region where Froiv.

M. M. Selyametov

1981-01-01

316

Nilpotence and Stable Homotopy Theory II  

Microsoft Academic Search

This paper is a continuation of [7]. Since so much time has lapsedsince its publication a recasting of the context is probably in order.In [15] Ravenel described a series of conjectures getting at thestructure of stable homotopy theory in the large. The theory wasorganized around a family of "higher periodicities" generalizingBott periodicity, and depended on being able to determine thenilpotent

Michael J. Hopkins; H Jeffrey

1992-01-01

317

Stable Isotopes As Tracers In Ocean Sciences  

Microsoft Academic Search

By making use of differences in stable carbon isotope ratios, which have resulted from chemical isotope effects, for carbon reservoirs large scale tracer experiments have been observed. These natural experiments have been used to quantify ocean pollution, sediment sources and duck food-webs.

P. Parker; R. Anderson; J. Winters; R. Scalan

1983-01-01

318

Notes on stable maps and quantum cohomology  

Microsoft Academic Search

These are notes from a jointly taught class at the University of Chicago and lectures by the first author in Santa Cruz. Topics covered include: construction of moduli spaces of stable maps, Gromov-Witten invariants, quantum cohomology, and examples. These notes will appear in the proceedings of the 1995 Santa Cruz conference.

W. Fulton; R. Pandharipande

1996-01-01

319

Generic bi-Lyapunov stable homoclinic classes  

Microsoft Academic Search

We study, for C1 generic diffeomorphisms, homoclinic classes which are Lyapunov stable both for backward and forward iterations. We prove that they must admit a dominated splitting and show that under some hypothesis they must be the whole manifold. As a consequence of our results we also prove that in dimension 2 the class must be the whole manifold and

Rafael Potrie

2010-01-01

320

First order and stable relativistic dissipative hydrodynamics  

NASA Astrophysics Data System (ADS)

Relativistic thermodynamics is derived from kinetic equilibrium in a general frame. Based on a novel interpretation of Lagrange multipliers in the equilibrium state we obtain a generic stable but first order relativistic dissipative hydrodynamics. Although this was believed to be impossible, we circumvent this difficulty by a specific handling of the heat flow.

Ván, P.; Biró, T. S.

2012-03-01

321

On evolutionarily stable behavior in contests  

Microsoft Academic Search

It is argued in this paper that the solution concept of an evolutionary stable strategy (ESS) is an adequate analysis tool for contest theory. Moreover, it is shown that in a contest ESS always differs from Nash equilibrium, the hitherto dominant solution concept in contest theory. Finally, an interpretation of finite population ESS contest behavior in terms of Nash behavior

Wolfgang Leininger

2003-01-01

322

Sugar feeding in adult stable flies.  

PubMed

Adult stable flies (Stomoxys calcitrans L.) are known to feed readily on sugars in the laboratory. However, little is known concerning the extent of stable fly sugar feeding in wild populations. We examined the frequency of sugar feeding in stable flies collected on Alsynite sticky traps in rural and urban environments. In addition, stable flies were visually examined to determine whether blood was present in the gut. In laboratory studies, sugars were detectable with the anthrone technique in stable flies for approximately 3 d after being imbibed, and blood could be visually detected in the gut for 24-48 h after feeding. Twelve percent of the field-collected flies had detectable sugar with a higher percentage of the urban flies having sugar fed than the rural flies, 21 and 8%, respectively. Female flies sugar fed at a slightly higher rate than males, 13 versus 11%, respectively. Less than 1% of the field-collected flies had blood in their guts. The frequency of observable blood was slightly higher in flies collected in an urban environment compared with those collected in a rural environment and did not differ between male and female flies. The number of flies with both blood and sugar was slightly higher than would be expected based on the frequencies of each alone. Seasonal patterns of both sugar feeding and blood feeding were similar in the rural and urban environments; both peaked in the early summer, May to mid-June, and dropped through the summer and fall. Sugar feeding in the urban environment increased again in October. PMID:18559167

Taylor, David B; Berkebile, Dennis R

2008-06-01

323

Forecasting stable long lived Antarctic ozone holes  

NASA Astrophysics Data System (ADS)

Stable Antarctic ozone holes that survive into late November threaten the Antarctic coastal ecology with levels of ultraviolet radiation that exceed tropical levels. Since the breakdown of the lower stratospheric vortex proceeds from the top down it is logical to examine the state of the wintertime upper stratosphere for clues to the ultimate stability of the springtime ozone hole. For instance, the chaotic 2002 Antarctic polar vortex was preceded by a remarkably early (i.e., June) strong sudden stratospheric warming at the 2hPa level, where as, the stable 1987 Antarctic polar vortex did not suffer such a strong upper stratospheric warming until late August. Such wintertime upper stratospheric warmings can now be accurately monitored using NOAA's polar orbiting Advanced Microwave Sounding Units. This paper examines the feasibility of using AMSU wintertime measurements of upper stratospheric temperatures to forecast for the stability of springtime Antarctic ozone holes.

Neuendorffer, A. C.; Long, C.

2005-12-01

324

Stable field emission from nanoporous silicon carbide.  

PubMed

We report on a new type of stable field emitter capable of electron emission at levels comparable to thermal sources. Such an emitter potentially enables significant advances in several important technologies which currently use thermal electron sources. These include communications through microwave electronics, and more notably imaging for medicine and security where new modalities of detection may arise due to variable-geometry x-ray sources. Stable emission of 6 A cm(-2) is demonstrated in a macroscopic array, and lifetime measurements indicate these new emitters are sufficiently robust to be considered for realistic implementation. The emitter is a monolithic structure, and is made in a room-temperature process. It is fabricated from a silicon carbide wafer, which is formed into a highly porous structure resembling an aerogel, and further patterned into an array. The emission properties may be tuned both through control of the nanoscale morphology and the macroscopic shape of the emitter array. PMID:23324430

Kang, Myung-Gyu; Lezec, Henri J; Sharifi, Fred

2013-01-16

325

Stable field emission from nanoporous silicon carbide  

NASA Astrophysics Data System (ADS)

We report on a new type of stable field emitter capable of electron emission at levels comparable to thermal sources. Such an emitter potentially enables significant advances in several important technologies which currently use thermal electron sources. These include communications through microwave electronics, and more notably imaging for medicine and security where new modalities of detection may arise due to variable-geometry x-ray sources. Stable emission of 6 A cm-2 is demonstrated in a macroscopic array, and lifetime measurements indicate these new emitters are sufficiently robust to be considered for realistic implementation. The emitter is a monolithic structure, and is made in a room-temperature process. It is fabricated from a silicon carbide wafer, which is formed into a highly porous structure resembling an aerogel, and further patterned into an array. The emission properties may be tuned both through control of the nanoscale morphology and the macroscopic shape of the emitter array.

Kang, Myung-Gyu; Lezec, Henri J.; Sharifi, Fred

2013-02-01

326

Convection in stable and unstable fronts.  

PubMed

Density gradients across a reaction front can lead to convective fluid motion. Stable fronts require a heavier fluid on top of a lighter one to generate convective fluid motion. On the other hand, unstable fronts can be stabilized with an opposing density gradient, where the lighter fluid is on top. In this case, we can have a stable flat front without convection or a steady convective front of a given wavelength near the onset of convection. The fronts are described with the Kuramoto-Sivashinsky equation coupled to hydrodynamics governed by Darcy's law. We obtain a dispersion relation between growth rates and perturbation wave numbers in the presence of a density discontinuity accross the front. We also analyze the effects of this density change in the transition to chaos. PMID:22400643

Elliott, Drew; Vasquez, Desiderio A

2012-01-18

327

Convection in stable and unstable fronts  

NASA Astrophysics Data System (ADS)

Density gradients across a reaction front can lead to convective fluid motion. Stable fronts require a heavier fluid on top of a lighter one to generate convective fluid motion. On the other hand, unstable fronts can be stabilized with an opposing density gradient, where the lighter fluid is on top. In this case, we can have a stable flat front without convection or a steady convective front of a given wavelength near the onset of convection. The fronts are described with the Kuramoto-Sivashinsky equation coupled to hydrodynamics governed by Darcy's law. We obtain a dispersion relation between growth rates and perturbation wave numbers in the presence of a density discontinuity accross the front. We also analyze the effects of this density change in the transition to chaos.

Elliott, Drew; Vasquez, Desiderio A.

2012-01-01

328

Stable, free-standing Ge nanocrystals  

SciTech Connect

Free-standing Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5 nm are grown in amorphous SiO{sub 2} by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain free-standing nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Free-standing nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For free-standing as opposed to embedded Ge nanocrystals, an additional amorphous-like contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms.

Sharp, I.D.; Xu, Q.; Liao, C.Y.; Yi, D.O.; Beeman, J.W.; Liliental-Weber, Z.; Yu, K.M.; Zakharov, D.N.; Ager III, J.W.; Chrzan,D.C.; Haller, E.E.

2005-01-28

329

Stable magnetic fields in stellar interiors  

NASA Astrophysics Data System (ADS)

We investigate the 50-year old hypothesis that the magnetic fields of the Ap stars are stable equilibria that have survived in these stars since their formation. With numerical simulations we find that stable magnetic field configurations indeed appear to exist under the conditions in the radiative interior of a star. Confirming a hypothesis by Prendergast (1956, ApJ, 123, 498), the configurations have roughly equal poloidal and toroidal field strengths. We find that tori of such twisted fields can form as remnants of the decay of an unstable random initial field. In agreement with observations, the appearance at the surface is an approximate dipole with smaller contributions from higher multipoles, and the surface field strength can increase with the age of the star. The results of this paper were summarised by Braithwaite & Spruit (2004, Nature, 431, 891).

Braithwaite, J.; Nordlund, Å.

2006-05-01

330

Fundamentals of the LISA stable flight formation  

NASA Astrophysics Data System (ADS)

The joint NASA ESA mission, LISA, relies crucially on the stability of the three-spacecraft constellation. Each of the spacecraft is in heliocentric orbit forming a stable triangle. In this paper we explicitly show with the help of the Clohessy Wiltshire equations that any configuration of spacecraft lying in the planes making angles of ±60° with the ecliptic and given suitable initial velocities within the plane, can be made stable in the sense that the inter-spacecraft distances remain constant to first order in the dimensions of the configuration compared with the distance to the Sun. Such analysis would be useful in order to carry out theoretical studies on the optical links, simulators, etc.

Dhurandhar, S. V.; Nayak, K. Rajesh; Koshti, S.; Vinet, J.-Y.

2005-02-01

331

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

332

Stable Spheromaks Sustained by Neutral Beam Injection  

SciTech Connect

It is shown that spheromak equilibria, stable at zero-beta but departing from the Taylor state, could be sustained by non-inductive current drive at acceptable power levels. Stability to both ideal MHD and tearing modes is verified using the NIMROD code for linear stability analysis. Non-linear NIMROD calculations with non-inductive current drive and pressure effects could point the way to improved fusion reactors.

Fowler, T K; Jayakumar, R; McLean, H S

2008-05-14

333

Stable marriage problems with quantitative preferences  

Microsoft Academic Search

The stable marriage problem is a well-known problem of matching men to women\\u000aso that no man and woman, who are not married to each other, both prefer each\\u000aother. Such a problem has a wide variety of practical applications, ranging\\u000afrom matching resident doctors to hospitals, to matching students to schools or\\u000amore generally to any two-sided market. In

Maria Silvia Pini; Francesca Rossi; Kristen Brent Venable; Toby Walsh

2010-01-01

334

Stable non-BPS D-particles  

Microsoft Academic Search

It is shown that the orbifold of type IIB string theory by (?1)FLI4 admits a stable non-BPS Dirichlet particle that is stuck on the orbifold fixed plane. It is charged under the SO(2) gauge group coming from the twisted sector, and transforms as a long multiplet of the D=6 supersymmetry algebra. This suggests that it is the strong coupling dual

Oren Bergman; Matthias R. Gaberdiel

1998-01-01

335

Stable Isotopes in Dendroclimatology: Moving Beyond ‘Potential’  

Microsoft Academic Search

\\u000a When trees grow, they assimilate carbon from atmospheric carbon dioxide, and hydrogen and oxygen from soil water. The stable\\u000a isotope ratios of these three elements carry signals that can be interpreted in terms of past climate because isotope ratios\\u000a are climatically controlled by the tree’s water and gas exchange budgets. The traditional tree-ring proxies form the most\\u000a widespread and arguably

Mary Gagen; Danny McCarroll; Neil J. Loader; Iain Robertson

336

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

337

f-? Power Spectrum and Stable Distribution  

NASA Astrophysics Data System (ADS)

The fractional integral of white noise produces a fractal fluctuation with a f-? power spectrum. By generalizing Holtzmark’s method, we can exactly calculate the distribution function of the fluctuation. The obtained distribution is the Levy’s stable distribution of which the characteristic exponent is a simple function of ?. This result may give a new insight to the basic study of experimentally observed fractal noises, especially those in turbulence.

Takayasu, Hideki

1987-04-01

338

Natural Frequencies of Stable Griffith Cracks  

Microsoft Academic Search

A quasi-statically growing stable crack, if perturbed from its equilibrium position, will accelerate back towards it. Within quasi-static, ideal, Griffith fracture theory, vibrations of the crack and the structure have characteristic natural frequencies. We explore this feature of Griffith fracture theory in two simple geometries: a crack between a bar and a substrate, and a crack in a double-cantilever beam

A. Jagota; P. Rahul-Kumar; S. Saigal

2002-01-01

339

Local search for stable marriage problems  

Microsoft Academic Search

The stable marriage (SM) problem has a wide variety of practical\\u000aapplications, ranging from matching resident doctors to hospitals, to matching\\u000astudents to schools, or more generally to any two-sided market. In the\\u000aclassical formulation, n men and n women express their preferences (via a\\u000astrict total order) over the members of the other sex. Solving a SM problem\\u000ameans

Mirco Gelain; Maria Silvia Pini; Francesca Rossi; Kristen Brent Venable; Toby Walsh

2010-01-01

340

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

341

Unstable to stable transformations during dolomitization  

SciTech Connect

Dolomitization of calcite at 218C in hydrothermal bombs resulted in a series of unstable precursors to the final product, stoichiometric, ordered dolomite. The first phase formed with high-Mg calcite with approximately 35 mol % MgCO{sub 3}. This was replaced by Ca-rich, poorly ordered dolomite that was, in turn, replaced by ordered, stoichiometric dolomite. The rate of dolomitization and the rate at which unstable phases transformed to stoichiometric dolomite was related to the Mg{sup 2+}/Ca{sup 2+} ratio of the solution. There was no simple relationship between the Mg{sup 2+}/Ca{sup 2+} ratio solution and that of the products. SEM analyses of the products are interpreted to indicate that the unstable to stable transformation occurs by three, possibly simultaneous, mechanisms: (1) The small rhombs in the final products indicate that new crystals continue to nucleate during the reaction. (2) The large rhombs indicate that overgrowths of the more stable phase grew on the less stable phase. (3) The very irregular interiors indicate that the initial unstable products underwent a intercrystalline-scale dissolution and reprecipitation replacement.

Sibley, D.F. (Michigan State Univ., East Lansing (USA))

1990-09-01

342

Stable magnetic field configurations in stars  

NASA Astrophysics Data System (ADS)

Long-lived, large-scale magnetic field configurations with similar total fluxes exist in at least three very different, although related kind of stars: upper main sequence stars, white dwarfs, and neutron stars (e.g. Reisenegger 2001). Much or all of the volume of these stars is stably stratified, so there is no convection that could maintain these fields through dynamo processes (except in the cores of upper main sequence stars). Magnetohydrodynamic simulations of stably stratified stars (Braithwaite & Spruit 2004, 2006; Braithwaite & Nordlund 2006) suggest that configurations with linked poloidal and toroidal fields get spontaneously established and might be stable over long times. Physical arguments show that such configurations are in fact natural and that the stable stratification is likely to play a crucial role in their stability. Thus, contrary to assumptions in recent papers, the field is not force-free, and the fluid cannot be taken to be barotropic. Work is in progress to represent these fields analytically and investigate the conditions for their stability. In the case of neutron stars with strong enough fields, the stable stratification can be overcome by long-term, dissipative processes such as beta decays and ambipolar diffusion (Goldreich & Reisenegger 1992; Reisenegger et al. 2005), leading to the release of magnetic energy and potentially explaining the energy source for the "magnetar" phenomenon (Thompson & Duncan 1993, 1996).

Reisenegger, Andreas; Munoz, Francisco; Santos, Raul

343

Isotropic turbulence, stable layers: facts or fictions?  

NASA Astrophysics Data System (ADS)

Using state of the art drop sonde data (from 237 sondes over the Pacific) we examine two classical and fundamental idealizations of atmospheric science showing that they are untenable in the light of the vertical structure. The first is the notion of stable atmospheric layers. This is used for understanding atmospheric dynamics and thermodynamics, including the most advanced dynamical meteorological notions such as potential vorticity. Using the drop sonde data, we show that each apparently stable layer is actually composed of a hierarchy of unstable layers themselves with embedded stable sublayers, each with unstable sub-sub layers etc. i.e. in a Russian doll-like fractal hierarchy whose dimension we estimate. We therefore argue that the notion is untenable and must be replaced by modern scaling notions. The second idealization we examine is the turbulence assumption of isotropy. If we include intermittency, Kolmorogov's landmark proposal that fully developed turbulence has an "inertial subrange" with isotropic energy spectrum E(k)=k**-beta with beta=5/3 has apparently been spectacularly confirmed in both the horizontal direction and in the time domain (k is a wavenumber). For gradients over a horizontal distance deltax this implies deltav=deltaz**Hh (Hh=1/3 corresponds to beta=5/3). Remarkably, Hv for gradients over vertical distances deltaz (deltav=deltaz**Hv) has not been seriously investigated. Using drop sonde data of horizontal wind, we find that from scales of 5 m to 10 km from the surface layer through to the top of the troposphere, Hv is close to (or larger) than the Bolgiano-Obukhov value 3/5. Hv greater than Hh implies that a) the atmosphere becomes progressively less stratified at smaller scales although in a scaling way; b) that at most a single (roughly) isotropic "sphero- scale" exists (often in the range 1-100 cm).

Lovejoy, S.; Tuck, A.; Hovde, S.; Schertzer, D.

2007-05-01

344

A Theory of Multiple Stable Climate Manifolds  

NASA Astrophysics Data System (ADS)

Among the more intriguing enigmas of the climate system is its apparently exquisite sensitivity to quite minor changes of insolation owing to orbital variations, while in a grosser sense, the system is stable, given that the oceans have not boiled away or frozen over. To this enigma may be added the finding from ice core analysis that the climate system is capable of very sudden transitions. An attractive idea that may help explain these features is that the climate system possesses multiple stable manifolds that overlap in some regions of phase space. In the overlap regions, small changes in climate forcing and/or climate noise may lead to transitions from one manifold to another. Here we describe a simple model of the climate system that incorporates three feedback mechanisms that are regarded as essential. The first is the feedback between large scale atmospheric circulation and cloudiness and water vapor, relying on the fact that much of the infrared radiation that escapes to space does so through large patches of dry air in the upper subtropical troposphere. Were the large scale circulation to vanish, these patches would also disappear, leading to a more opaque atmosphere and a warmer climate. The second feedback relies on recent work that strongly suggests that the ocean's thermohaline circulation is largely controlled by global tropical cyclone activity. Tropical cyclones control the strength of the meridional heat flux through their vigorous vertical mixing of the upper tropical ocean. The third major feedback involves the relationship between the partitioning of carbon between the atmosphere and oceans and the temperature and rate of overturning of the oceans. A model based on these three feedback mechanisms produces three stable climate manifolds that we argue correspond to the present climate, glacial episodes, and very hot climates characteristic of the Eocene and part of the Cretaceous.

Emanuel, K.

2001-05-01

345

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

346

A Peculiar Stable Region Around Pluto  

NASA Astrophysics Data System (ADS)

In a previous paper Giuliatti Winter et al. (2010) found several stable regions for a sample of particles located between the orbits of Pluto and Charon. Some of these particles are in orbits around Pluto and some of them are in orbits around Charon. One peculiar stable region (hereafter region 1), located at a=[0.5d, 0.65d], where d is the Pluto-Charon distance, presents large values of the particle's eccentricity e=[0.2, 0.9]. This region is associated to a family of periodic orbits derived from the circular restricted three body problem (Pluto-Charon-particle). The evolution of the periodic and quasi periodic orbits associated to each value of the Jacobi constant is presented in this work. We also analyse the evolution of the region 1 for a set of initial conditions, the nominal values of the argument of pericentre (?) and the inclination (I), of the particles. We concluded that region 1 is present in all values of the inclination of the particles, I= [0, 90°], and for two intervals of ?, ? = [-10°, 10°] and ? = [170°, 190°].

Giuliatti-Winter, Silvia M.; Winter, O.; Vieira Neto, E.; Sfair, R.

2012-10-01

347

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

2011-06-02

348

Engineering of volume-stable adipose tissues.  

PubMed

Autologous adipose tissues have been clinically used for augmentation of soft tissues lost due to mastectomy or lumpectomy in plastic and reconstructive surgery. However, this therapy has problems of absorption and subsequent volume loss of the implanted adipose tissues. In this study, volume-stable adipose tissues were engineered in vivo using mechanical support structures fabricated from biodegradable synthetic polymers. Dome-shaped mechanical support structures were fabricated by reinforcing poly(glycolic acid) fiber-based matrices with poly(L-lactic acid). The support structures were placed into subcutaneous pockets of athymic mice, and human preadipocytes suspended in fibrin matrix were injected into the space under the support structures (group I). Injection of either fibrin matrix without preadipocytes under the support structures (group II) or fibrin matrix containing preadipocytes into subcutaneous spaces with no support structures (group III) served as controls. Six weeks after implantation, the original implant volume was maintained approximately in groups I and II, whereas, group III showed significant implant shrinkage. The compressive modulus of the mechanical support structures did not change significantly over 6-week incubation in phosphate-buffered saline at 37 degrees C. Histological analyses of the implants showed regeneration of adipose tissues in group I. In contrast, groups II and III did not show extensive adipose tissue formation. This study demonstrates that volume-stable adipose tissues can be engineered in vivo using mechanical support structures. This technique offers the potential for augmentation of adipose tissues with volume conservation. PMID:15621248

Cho, Seung-Woo; Kim, Sang-Soo; Rhie, Jong Won; Cho, Hyun Mi; Choi, Cha Yong; Kim, Byung-Soo

2005-06-01

349

Stable counteralignment of a circumbinary disc  

NASA Astrophysics Data System (ADS)

In general, when gas accretes on to a supermassive black hole (SMBH) binary, it is likely to have no prior knowledge of the binary angular momentum. Therefore, a circumbinary disc forms with a random inclination angle ? to the binary. It is known that for ? < 90° the disc will coalign with respect to the binary. If ? > 90°, the disc wholly counteraligns if it satisfies cos ? < -Jd/2Jb, where Jd and Jb are the magnitudes of the disc and binary angular momentum vectors, respectively. If, however, ? > 90° and this criterion is not satisfied, the same disc may counteralign its inner regions and, on longer time-scales, coalign its outer regions. I show that for typical disc parameters, describing an accretion event on to an SMBH binary, a misaligned circumbinary disc is likely to wholly coalign or counteralign with the binary plane. This is because the binary angular momentum dominates the disc angular momentum. However, with extreme parameters (binary mass ratio M2/M1? 1 or binary eccentricity e˜ 1), the same disc may simultaneously coalign and counteralign. It is known that coplanar prograde circumbinary discs are stable. I show that coplanar retrograde circumbinary discs are also stable. A chaotic accretion event on to an SMBH binary will therefore result in a coplanar circumbinary disc that is either prograde or retrograde with respect to the binary plane.

Nixon, Christopher J.

2012-07-01

350

Stable Isotope Constraints of Atmospheric Methane Budgets  

NASA Astrophysics Data System (ADS)

Recent reports of methane concentrations in ice records, including the 800 kyr Antarctic EPICA ice core, confirm that the present day tropospheric methane mixing ratios of over 1.8 ppm are anomalously high for this millennial time period (PIH/LP methane mean is 0.52 ppm). Our understanding and quantification of the changes to and intensity variations of tropospheric methane sources and sinks remains imperfect. Stable carbon and hydrogen isotope ratios of atmospheric methane offer assistance to check our assessments of methane fluxes. However, due to a) the number and shifts of methane sources and sinks, b) substantial overlap in isotope signatures and c) variations on both annual and geologic time scales, the isotope mass balances are under-constrained. Our attempts to reconcile present day and past atmospheric methane mass balances continue to challenge us. Despite this limitation, isotope information is essential to elucidate and characterize the processes and pathways affecting global atmospheric methane systematics. This presentation will outline these mass balancing acts and discuss these issues surrounding the stable isotopes of methane.

Whiticar, M. J.; Melton, J. R.; Kaplan, J. O.; Schaefer, H.

2008-12-01

351

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

352

On feedback and stable price adjustment mechanisms  

NASA Astrophysics Data System (ADS)

Given an excess demand function of an economy, say Z(p), a stable price adjustment mechanism (SPAM) guarantees convergence of solution path p(t,p0) to an equilibrium p solution of Z(p)=0. Besides, all equilibrium points of Z(p) are asymptotically stable. Some SPAMs have been proposed, including Newton and transpose Jacobian methods. Despite this powerful stability property of SPAMs, their acceptation in the economics community has been limited by a lack of interpretation. This paper focuses on this issue. Specifically, feedback control theory is used to link SPAMs and price dynamics models with control inputs, which match the economically intuitive Walrasian Hypothesis (i.e., prices change with excess demand sign). Under mild conditions, it is shown the existence of a feedback function that transforms the price dynamics into a desired SPAM. Hence, a SPAM is interpreted as a fundamental (e.g., Walrasian) price dynamics under the action of a feedback function aimed to stabilize the equilibrium set of the excess demand function.

Fernandez-Anaya, Guillermo; Alvarez-Ramirez, Jose; Ibarra-Valdez, Carlos

2007-04-01

353

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

354

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

355

Ant Colony based Algorithm for Stable Marriage Problem  

Microsoft Academic Search

This paper introduces ant colony system (ACS), a distributed algorithm that is applied to the Stable Marriage Problem (SM).\\u000a The stable marriage problem is an extensively-studied combinatorial problem with many practical applications. It is well known\\u000a that at least one stable matching exists for every stable marriage instance. However, the classical Gale-Shapley [2] algorithm\\u000a produces a marriage that greatly favors

Ngo Anh Vien; Nguyen Hoang Viet; Hyun Kim; SeungGwan Lee; TaeChoong Chung

356

The stable isotope ecology of terrestrial plant succession  

Microsoft Academic Search

We review the relevance and use of stable isotopes for the study of plant community succession. Stable isotope measurements provide information on the origin of resources acquired by plants, the processes governing resource uptake and transformation, and the physiological and environmental conditions of plant growth. When combined with measurements of the stable isotope ratio values of soil microbial biomass, soil

Víctor Resco; Juan P. Ferrio; José A. Carreira; Leonor Calvo; Pere Casals; Ángel Ferrero-Serrano; Elena Marcos; José M. Moreno; David A. Ramírez; M. Teresa Sebastià; Fernando Valladares; David G. Williams

2011-01-01

357

The estimation of stable distribution parameters from teletraffic data  

Microsoft Academic Search

This article is concerned with the estimation of the parameters that describe a stable distribution from teletraffic data. Stable distributions are characterized by four parameters that can be estimated using a number of methods. This article provides an introduction to the stable distribution and a summary of some of the estimation techniques developed for them. It presents a comprehensive performance

Stephen Bates; Steve McLaughlin

2000-01-01

358

Isotropic turbulence, stable layers: atmospheric fictions  

NASA Astrophysics Data System (ADS)

Using state of the art drop sonde data (from 237 sondes over the Pacific) we examine two classical and fundamental idealizations of atmospheric science showing that they are untenable in the light of the vertical structure. The first is the notion of stable atmospheric layers. This is used for understanding atmospheric dynamics and thermodynamics, including notions such as potential vorticity. Using the drop sonde data, we show that each apparently stable layer is actually composed of a hierarchy of unstable layers themselves with embedded stable sublayers, each with unstable sub-sub layers etc. i.e. in a Russian doll-like fractal hierarchy whose dimension we estimate. We therefore argue that the notion is untenable and must be replaced by modern scaling notions. Although the same basic conclusion follows for conditional, dynamical and convective stability, we showed that their correlation exponent (quantifying their sparsenesses) were 0.36±0.056, 0.22±0.037, 0.15±0.016 respectively. The second idealization we examine is the turbulence assumption of isotropy. If we include intermittency, Kolmorogov's landmark proposal that fully developed turbulence has an "inertial subrange" with isotropic energy spectrum E(k) ? k**-? with ??5/3 has apparently been spectacularly confirmed in both the horizontal direction and in the time domain (k is a wavenumber). For gradients over a horizontal distance ?x this implies ?v??z**Hh (Hh=1/3 corresponds to ?=5/3; "<.>" indicates ensemble averaging). Remarkably, Hv for gradients over vertical distances ?z (?v??z**Hv) has not been seriously investigated. Using drop sonde data of horizontal wind, we find that from scales of 5 m to >10 km from the surface layer through to the top of the troposphere, Hv is close to (or larger) than the Bolgiano-Obukhov value 3/5. Hv>Hh implies that a) the atmosphere becomes progressively less stratified at smaller scales although in a scaling way; b) that at most a single (roughly) isotropic "sphero-scale" exists (often in the range 1- 100 cm).

Lovejoy, S.; Tuck, A.; Hovde, S.; Schertzer, D.

2007-12-01

359

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

360

Transport of stable isotopes: I: Development of a kinetic continuum theory for stable isotope transport  

Microsoft Academic Search

Equations are developed describing migration of stable isotopes via a fluid phase infiltrating porous media. The formalism of continuum fluid mechanics is used to deal with the problem of microscopic inhomogeneity. Provision is made explicitly for local equilibrium exchange of isotopes between minerals and fluids as well as for kinetic control of isotopic exchange. Changing characteristic parameters of transport systems

L. P. Baumgartner; D. Rumble

1988-01-01

361

High temperature stable fluid loss control system  

SciTech Connect

The present invention is directed to a high temperature stable composition capable of imparting fluid loss control properties to aqueous systems. The composition is a combination of: a solid, particulate silicate or alumino-silicate material; a chemical compound capable of converting to a higher oxidation state under alkaline conditions; and a reaction product of a polymeric substance selected from polyvinyl alcohol or hydroxyalkyl cellulose reacted with at least about 0.1 percent of stoichiometry of a cross-linking agent selected from a epihalohydrin or an aldehyde containing or generating agents. The invention is further directed to improved drilling fluids containing said composition and to the process of drilling bore holes using said improved fluid.

Green, P. C.; Block, J.

1984-09-25

362

Stable Isotope-Coded Proteomic Mass Spectrometry  

SciTech Connect

The ability to quantify the changes in protein abundance between cells subjected to a variety of extracellular stimuli or the onset of a diseased state remains an extremely active area of proteome research. Although advances in sample preparation, chromatographic separation, mass spectrometry instrumentation and bioinformatics contribute to producing a viable method for comparative proteome-wide analyses, the foundation of quantitation is based in part upon improved methods for chemical and metabolic stable isotope labeling of proteins and peptides. The ability to quantify differences in protein expression and post-translational modifications has been demonstrated, but insights into the biochemical mechanisms that will contribute to the development of new biotechnologies have yet to be realized.

Goshe, Michael B. (BATTELLE (PACIFIC NW LAB)); Smith, Richard D. (BATTELLE (PACIFIC NW LAB))

2003-02-01

363

Stable aqueous solutions of naked titanate nanotubes.  

PubMed

Aqueous solutions of naked nanotubes with Ti concentration up to 10 mM are obtained by hydrothermal synthesis followed by extensive ultrasound treatment. The morphology, surface characteristics, and solution behavior of the solubilized nanotubes are investigated. The time course of the solubilization process driven by ultrasound follows a first-order kinetic law and is mediated by the competition between Na(+) and H(+) for surface sites. The dynamics of interaction with small cations (i.e. the sodium ion) is studied by nuclear magnetic resonance spectroscopy and is demonstrated to be a multifaced process, since Na(+) is in part free to exchange between the binding sites on nanotubes and the bulk and in part is confined to slowly exchanging nanotube sites. The aqueous titanate nanotube solutions are stable for months, thus opening new perspectives for the use of this material in drug delivery and in homogeneous photocatalysis. PMID:23813880

Zennaro, Lucio; Magro, Massimiliano; Vianello, Fabio; Rigo, Adelio; Mariotto, Gino; Giarola, Marco; Froner, Elena; Scarpa, Marina

2013-06-28

364

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

365

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

366

High temperature stable sand control method  

SciTech Connect

This invention relates to a method for treating wells completed in subterranean formations to form a mechanically stable, permeable mass which permits flow of fluids therethrough into or from the well, while restricting the movement of unconsolidated earth formation particles such as sand into the well. This method is especially suitable for treating producing wells in a subterranean formation being subjected to thermal oil recovery stimulation of viscous oil production such as by steam flooding, which causes rapid deterioration of consolidated sand or gravel masses formed with plastic resins. The method comprises contacting sand with an aqueous solution of calcium hydroxide, plus an effective amount of calcium salt having solubility greater than calcium hydroxide, such as calcium chloride, plus an alkalinity agent such as sodium hydroxide.

Park, J.H.

1980-11-11

367

Stable Quantum Resonances in Atom Optics  

NASA Astrophysics Data System (ADS)

Resonances are among the most typical manifestations of quantum mechanical behavior and are very sensitive to values of parameters (for example in standard atomic spectroscopy). Recently resonances, that are stable with respect to variation of parameters, were discovered experimentally for the dynamics of laser cooled Cesium atoms when driven in the presence of gravity. A theoretical explanation of this surprising observation will be presented. Further theoretical predictions that were verified experimentally will be presented as well. In particular it is found that the experimental results depend on the number theoretical properties of the values of parameters. Effects of decoherence that are related to some surprising experimental results will be discussed as well. The theory makes use of invariance properties of the system, that are similar to the ones of solids, as well as of the theory of dynamical systems. The work was done in collaboration with Italo Guarneri, Laura Rebuzzini and Sandro Wimberger.

Fishman, Shmuel

2004-03-01

368

Transient growth in stable collisionless plasma  

SciTech Connect

The first kinetic study of transient growth for a collisionless homogeneous Maxwellian plasma in a uniform magnetic field is presented. A system which is linearly stable may display transient growth if the linear operator describing its evolution is non-normal so that its eigenvectors are nonorthogonal. In order to include plasma kinetic effects, a Landau fluid model is employed. The linear operator of the model is shown to be non-normal and the results suggest that the non-normality of a collisionless plasma is intrinsically related to its kinetic nature, with the transient growth being more accentuated for smaller scales and higher plasma beta. The results based on linear spectral theory have been confirmed with nonlinear simulations.

Camporeale, Enrico; Burgess, David [Queen Mary University of London, Mile End Road, London E1 4NS (United Kingdom); Passot, Thierry [University of Nice Sophia Antipolis, CNRS, Observatoire de la Cote d'Azur, B.P. 4229, 06304 Nice Cedex 4 (France)

2009-03-15

369

Palladium, iridium and ruthenium complexes with acyclic imino-N-heterocyclic carbenes and their application in aqua-phase Suzuki-Miyaura cross-coupling reaction and transfer hydrogenation.  

PubMed

Palladium (4a–4c), iridium (5a–5c) and ruthenium (6a–6c) complexes have been prepared by in situ transmetalation from the corresponding silver complexes of acyclic imino-functionalized imidazolium chlorides [1-(Me)-imidazolium-3-{C(p-CH(3)-Ph)=N(Ar)}]Cl (3) (Ar = 2,4,6-trimethylphenyl (3a), 2,6-diisopropylphenyl (3b) and phenyl (3c)) with [Pd(COD)Cl(2)], [Cp*IrCl(2)](2) or [Ru(p-cymene)Cl(2)](2), respectively. Iridium and ruthenium complexes, 5a[PF(6)]–5c[PF(6)], 6a[PF(6)]–6c[PF(6)], 6c[BF(4)], 6c[BPh(4)] and 6c[NTf(2)], were obtained directly from 5a–5c and 6a–6c through an anion-exchange process with KPF(6), NaBF(4), NaBPh(4) and LiNTf(2) (bis(trifluoromethylsulfonyl)imide lithium), respectively. All complexes were characterized by FT-IR, (1)H and (13)C NMR spectroscopy and elemental analysis. Crystal structures of 4a, 5a and 6c[NTf(2)], and show that five-membered chelate ring is formed in these complexes by the coordination of the carbene carbon and the imino nitrogen atom, and the latter two are cationic compounds with Cl(-) and NTf(2)(-) as counteranion respectively. The catalytic performance of Pd complexes for Suzuki-Miyaura cross-coupling reactions in pure water and Ir and Ru complexes for transfer hydrogenation of ketones and imines was tested in a wide scope of substrates. Pd complex 4b with the largest steric hinder exhibited the best performance to gain moderate to excellent yields on catalyzing Suzuki-Miyaura cross-coupling of aryl chlorides and arylboronic acids in water. While in transfer hydrogenation of various ketones, all the Ir and Ru complexes were effective with good to excellent yields. Among all these complexes, 6c[PF(6)] was found most effective, and moderate yields could be obtained even in the transfer hydrogenation of imines. Moreover, different counteranions of Ru complexes are influential on catalyzing the transfer hydrogenation, with the sequence of PF(6)(-) ? BF(4)(-) > BPh(4)(-) > Cl(-) > NTf(2)(-). PMID:23132042

Guo, Xu-Qing; Wang, Ya-Nong; Wang, Dong; Cai, Li-Hua; Chen, Zhen-Xia; Hou, Xiu-Feng

2012-12-28

370

Stable isotope analysis in primatology: a critical review.  

PubMed

Stable isotope analysis has become an important tool in ecology over the last 25 years. A wealth of ecological information is stored in animal tissues in the relative abundances of the stable isotopes of several elements, particularly carbon and nitrogen, because these isotopes navigate through ecological processes in predictable ways. Stable carbon and nitrogen isotopes have been measured in most primate taxonomic groups and have yielded information about dietary content, dietary variability, and habitat use. Stable isotopes have recently proven useful for addressing more fine-grained questions about niche dynamics and anthropogenic effects on feeding ecology. Here, we discuss stable carbon and nitrogen isotope systematics and critically review the published stable carbon and nitrogen isotope data for modern primates with a focus on the problems and prospects for future stable isotope applications in primatology. PMID:23015270

Sandberg, Paul A; Loudon, James E; Sponheimer, Matt

2012-11-01

371

Characterising the Terrestrial Stable Cr Isotope Cycle  

NASA Astrophysics Data System (ADS)

Cr isotopes were shown to present a sensitive tracer of redox changes in aqueous solutions [1,2]. Here, the potential of stable Cr isotope fractionation in the environment was further investigated by high-precision double- spike MC-ICP-MS measurements. Reduction experiments of Cr(VI) to Cr(III) in aqueous solutions on ion exchange columns confirmed the mass-dependent Cr isotope fractionation of -3.4‰ per atomic mass unit reported by Ellis et al. [1]. Furthermore, these experiments revealed that the fractionation associated with adsorption is minor. This is shown by a very small preferential adsorption of isotopically heavy chromium of most likely Cr(III) species on the anion resin. A similar observation was made for Cr(III) species on cation resin. This is in line with the negligible fractionation effects that have been reported for adsorption of Cr(VI) onto ?-Al2O3 and goethite surfaces [2], respectively. Thus, the large Cr isotope fractionation that accompanies Cr reduction and the small sorption effects of both Cr(VI) and Cr(III) species make stable Cr isotopes a sensitive tracer to detect and quantify redox changes in a variety of geochemical reservoirs. The stable Cr isotope compositions of the principle silicic igneous reservoirs of the Earth do not show measurable variations. ?^{53}Cr/^{52}Cr values (relative to SRM3112a) of six mantle lherzolites (- 0.014±0.083‰; 2 SD), six ultramafic cumulate rocks (-0.034±0.094‰) and five continental and oceanic basalts that represent partial mantle melts (-0.044±0.089‰) are indistinguishable within uncertainties. Thus, unlike as was suggested for Fe isotopes [3], partial mantle melting appears not to fractionate Cr isotopes. Cr(III)-bearing uvarovite and fuchsite minerals from amphibolite facies metamorphic rocks are also equal to those of igneous rock reservoirs. These observations are not unexpected, because there is no apparent redox change of Cr involved during partial mantle melting or metamorphism. Also the fractionation effects caused by potential ligand changes of Cr between solids, and solids and melt are too small to be detected at high-temperatures. Large Cr isotope fractionations with ?^{53}Cr/^{52}Cr values of +0.672 to +1.093‰ were observed for crocoite (PbCr(VI)O4), a mineral that forms in oxidising zones of medium- to high- temperature hydrothermal systems. It has yet to be investigated whether Cr isotopes fractionate during the oxidation of Cr(III) to Cr(VI). It is, however, likely, that Cr behaves like its chemical twin Se that only fractionates during reduction [4]. In this case the observed heavy Cr isotope compositions of crocoites are the result of partial Cr(VI) reduction from the hydrothermal fluid, allowing to quantify changes in redox conditions along the hydrothermal pathway. [1] Ellis, A.S. et al. (2002), Science 295, 2060-2062, [2] Ellis, A.S. et al. (2004), Eniron. Sci. Technol. 38, 3604-3607, [3] Weyer, S. et al. (2005), Earth and Planet. Sci. Letters 240, 251-264, [4] Johnson, T.M. and Bullen, T.D. (2004), Reviews in Mineralogy & Geochemistry 55, 289-317.

Schoenberg, R.; Zink, S.; Staubwasser, M.; von Blanckenburg, F.

2006-12-01

372

Stable nuclear transformation of Gonium pectorale  

PubMed Central

Background Green algae of the family Volvocaceae are a model lineage for studying the molecular evolution of multicellularity and cellular differentiation. The volvocine alga Gonium is intermediate in organizational complexity between its unicellular relative, Chlamydomonas, and its multicellular relatives with differentiated cell types, such as Volvox. Gonium pectorale consists of ~16 biflagellate cells arranged in a flat plate. The detailed molecular analysis of any species necessitates its accessibility to genetic manipulation, but, in volvocine algae, transformation procedures have so far only been established for Chlamydomonas reinhardtii and Volvox carteri. Results Stable nuclear transformation of G. pectorale was achieved using a heterologous dominant antibiotic resistance gene, the aminoglycoside 3'-phosphotransferase VIII gene (aphVIII) of Streptomyces rimosus, as a selectable marker. Heterologous 3'- and 5'-untranslated flanking sequences, including promoters, were from Chlamydomonas reinhardtii or from Volvox carteri. After particle gun bombardment of wild type Gonium cells with plasmid-coated gold particles, transformants were recovered. The transformants were able to grow in the presence of the antibiotic paromomycin and produced a detectable level of the AphVIII protein. The plasmids integrated into the genome, and stable integration was verified after propagation for over 1400 colony generations. Co-transformants were recovered with a frequency of ~30–50% when cells were co-bombarded with aphVIII-based selectable marker plasmids along with unselectable plasmids containing heterologous genes. The transcription of the co-transformed, unselectable genes was confirmed. After heterologous expression of the luciferase gene from the marine copepod Gaussia princeps, which was previously engineered to match the codon usage in C. reinhardtii, Gonium transformants show luciferase activity through light emission in bioluminescence assays. Conclusion Flanking sequences that include promoters from C. reinhardtii and from V. carteri work in G. pectorale and allow the functional expression of heterologous genes, such as the selectable marker gene aphVIII of S. rimosus or the co-transformed, codon-optimized G. princeps luciferase gene, which turned out to be a suitable reporter gene in Gonium. The availability of a method for transformation of Gonium makes genetic engineering of this species possible and allows for detailed studies in molecular evolution using the unicellular Chlamydomonas, the 16-celled Gonium, and the multicellular Volvox.

Lerche, Kai; Hallmann, Armin

2009-01-01

373

Magneto-hydrodynamically stable axisymmetric mirrors  

NASA Astrophysics Data System (ADS)

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.; Berk, H. L.; Cohen, B. I.; Molvik, A. W.; Simonen, T. C.

2011-09-01

374

How stable is the Mississippi Delta?  

NASA Astrophysics Data System (ADS)

Large deltas are commonly believed to exhibit rapid rates of tectonic subsidence, largely due to sediment loading of the lithosphere. As a result, deltaic plains are prone to accelerated relative sea-level rise, coastal erosion, and wetland loss. Hurricane Katrina's devastation testifies to the severe threat that these processes pose to the Mississippi Delta, but the relative role of tectonics versus other mechanisms causing land subsidence remains elusive. Relative sea-level records derived from basal peat have the potential to quantify differential crustal movements over Holocene time scales with exceptionally high accuracy and precision. Here we present new sea-level index points from two study areas in the southwestern Mississippi Delta that essentially coincide with a recently published detailed relative sea-level record from the eastern part of the delta. Our results show that differential vertical movements among the three study areas have been only ˜0.1 mm yr-1. We compare our evidence with a recent sea-level compilation from the Caribbean, to a large extent based on data from areas that are tectonically stable. Our sea-level index points nearly coincide with the Caribbean data, showing surprising tectonic stability for considerable sections of the Mississippi Delta. However, the well-documented high subsidence rates in and near the birdfoot of the Mississippi Delta indicate that different conditions prevail there. The rapid wetland loss in coastal Louisiana is likely due, to a considerable extent, to the compaction of Holocene strata.

Törnqvist, Torbjörn E.; Bick, Scott J.; van der Borg, Klaas; de Jong, Arie F. M.

2006-08-01

375

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

376

Systematic investigations of the stable Cd isotopes  

NASA Astrophysics Data System (ADS)

The Cd nuclei, especially the stable even-even isotopes have been well studied since they were suggested as paradigms of the vibrational, or U(5), limit of the Interacting Boson Model (IBM). In addition to the normal quadrupole phonon states, in many cases suggested up to the three-phonon quintuplet, more deformed 2p4h intruder excitations have been established. Recent investigations with the (n,n^'?) reaction [1,2,3,4] have provided a wealth of information on the low-lying levels, including many lifetimes not previously known. Deviations in the transition B(E2) values for low-spin states from those expected for U(5) nuclei are observed to appear systematically across the Cd isotopes. We have performed detailed calculations using the IBM-2, and find that these deviations cannot be explained through considered mixings with the intruder excitations or mixed-symmetry states, indicating that some physics is missing in the description of these levels. [1] F. Corminboeuf et al., Phys. Rev. C 63, 014305 (2001).[2] P.E. Garrett et al., Phys. Rev. C 75, 014307 (2007).[3] D. Bandyopadhyay et al., to be published. [4] M. Kadi et al., Phys. Rev. 68, 031306 (2003).

Garrett, P. E.; Green, K. L.; Wood, J. L.; Kulp, W. D.

2007-10-01

377

Minute synthesis of extremely stable gold nanoparticles  

NASA Astrophysics Data System (ADS)

We describe a rapid environmentally friendly wet-chemical approach to synthesize extremely stable non-toxic, biocompatible, water-soluble monodispersed gold nanoparticles (AuNPs) in one step at room temperature. The particles have been successfully achieved in just a few minutes by merely adding sodium hydroxide (NaOH) acting as an initiator for the reduction of HAuCl4 in aqueous solution in the presence of polyvinylpyrrolidone (PVP) without the use of any reducing agent. It is also proved to be highly efficient for the preparation of AuNPs with controllable sizes. The AuNPs show remarkable stability in water media with high concentrations of salt, various buffer solutions and physiological conditions in biotechnology and biomedicine. Moreover, the AuNPs are also non-toxic at high concentration (100 µM). Therefore, it provides great opportunities to use these AuNPs for biotechnology and biomedicine. This new approach also involved several green chemistry concepts, such as the selection of environmentally benign reagents and solvents, without energy consumption, and less reaction time.

Zhou, Min; Wang, Baoxiang; Rozynek, Zbigniew; Xie, Zhaohui; Fossum, Jon Otto; Yu, Xiaofeng; Raaen, Steinar

2009-12-01

378

Organized polysaccharide fibers as stable drug carriers.  

PubMed

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. PMID:23544530

Janaswamy, Srinivas; Gill, Kristin L; Campanella, Osvaldo H; Pinal, Rodolfo

2013-01-11

379

Low energy stable plasma calibration facility.  

PubMed

We have designed and fabricated a low energy plasma calibration facility for testing and calibration of rocket-borne charged-particle detectors and for the investigation of plasma sheath formation in an environment with ionospheric plasma energies, densities, and Debye lengths. We describe the vacuum system and associated plasma source, which was modified from a Naval Research Laboratory design [Bowles et al. Rev. Sci. Instrum. 67, 455 (1996)]. Mechanical and electrical modifications to this cylindrical microwave resonant source are outlined together with a different method of operating the magnetron that achieves a stable discharge. This facility produces unmagnetized plasmas with densities from 1x10(3)/cm(3) to 6x10(5)/cm(3), electron temperatures from 0.1 to 1.7 eV, and plasma potentials from 0.5 to 8 V depending on varying input microwave power and neutral gas flow. For the range of input microwave power explored (350-600 W), the energy density of the plasma remains constant because of an inverse relationship between density and temperature. This relationship allows a wide range of Debye lengths (0.3-8.4 cm) to be investigated, which is ideal for simulating the ionospheric plasma sheaths we explore. PMID:17672799

Frederick-Frost, K M; Lynch, K A

2007-07-01

380

The Near-Calm Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

For the near-calm stable boundary layer, nominally 2-m mean wind speed <0.5 ms-1, the time-average turbulent flux is dominated by infrequent mixing events. These events are related to accelerations associated with wave-like motions and other more complex small-scale motions. In this regime, the relationship between the fluxes and the weak mean flow breaks down. Such near-calm conditions are common at some sites. For very weak winds and strong stratification, the characteristics of the fluctuating quantities change slowly with increasing scale and the separation between the turbulence and non-turbulent motions can become ambiguous. Therefore, a new analysis strategy is developed based on the scale dependence of selected flow characteristics, such as the ratio of the fluctuating potential energy to the kinetic energy. In contrast to more developed turbulence, correlations between fluctuating quantities are small, and a significant heat flux is sometimes carried by very weak vertical motions with large temperature fluctuations. The relation of the flux events to small-scale increases of wind speed is examined. Large remaining uncertainties are noted.

Mahrt, Larry

2011-09-01

381

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

382

Management of chronic stable angina pectoris.  

PubMed

Agina pectoris is a discomfort in the chest or adjacent areas caused by myocardial ischemia. It is most commonly caused by the inability of narrowed atherosclerotic coronary arteries to supply adequate oxygen to the heart under conditions of increase demand. This review article will focus in the medical treatment of chronic stable angina, with a focus in new strategies or medications. Treatment by revascularization techniques will not be discussed in this article. The goal of treatment is to improve quality of life, decrease cardiovascular events and mortality. All patients should be evaluated for reversible causes of their angina, such as anemia, hyperthyroidism, sympathomimetic drugs and hypertension. Sublingual nitroglycerin should be used for immediate relief of symptoms. In general, all patients should be on aspirin (ASA) unless they are allergic or other contraindications, if so; clopidogrel should be added to the therapy. In addition to the antiplatelet therapy, which decreases mortality, patients should be started on beta blockers and nitrates. If there is no improvement in symptoms then a calcium channel blockers of the dihydropyridine family should be added. Patients with Diabetes Mellitus and/or left ventricular systolic dysfunction should be also started on angiotensin converting enzyme inhibitors. If the patient continues with limiting angina, ranolazine should be started and finally enhanced external counterpulsation should be considered in those patients who have not responded to maximal drug therapy. PMID:19400528

Rodríguez-Ospina, Luis; Montano-Soto, Luarde

383

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

384

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

385

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

386

Dynamic risk assessment in sexual offenders using STABLE-2000 and the STABLE-2007: an investigation of predictive and incremental validity.  

PubMed

The predictive accuracy of STABLE-2000 and STABLE-2007 was examined within a prospective research design in a German-speaking sample of 263 adult male prison-released sexual offenders followed up for an average of 6.4 years. The STABLE-2007 was significantly related to all outcomes (AUC = 0.67-0.71), whereas the STABLE-2000 demonstrated only weak predictive accuracy for sexual reoffense (AUC = 0.62). Supporting the results of the construction sample, the STABLE-2007 incrementally added to the predictive accuracy of the STATIC-99 for violent and general reoffense (conviction and incarceration). Moreover, the STABLE-2007 total scores and the nominal risk/need categories made significant incremental contributions over the SORAG for predicting sexual reoffense. PMID:21537058

Eher, Reinhard; Matthes, Anna; Schilling, Frank; Haubner-Maclean, Tanja; Rettenberger, Martin

2011-05-02

387

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

388

Stable angina pectoris: antianginal therapies and future directions  

Microsoft Academic Search

Advances in pharmacotherapy for stable angina have produced a wide choice of drugs with various mechanisms of action, potentially enabling individualized, patient-specific treatment strategies to be developed. In this Review, the various treatment options for patients with stable angina are discussed. Data from randomized, clinical trials of established and novel drugs are reviewed, with particular emphasis on the proposed mechanisms

Abhay A. Laddu; Bernard R. Chaitman

2011-01-01

389

A Model Describing Stable Coherent Synchrotron Radiation in Storage Rings  

Microsoft Academic Search

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

Sannibale

2004-01-01

390

STABLE ISOTOPES IN HUMAN NUTRITION LABORATORY METHODS AND RESEARCH APPLICATIONS  

Technology Transfer Automated Retrieval System (TEKTRAN)

The existence of stable, non-radioactive isotopes of the elements and the ability to measure these isotopes by mass spectrometry were first recognized in the early and mid-1920s (Aston, 1927). However, stable isotopes were not used for nutrition-related research until the 1930s, when Schoenheimer a...

391

Lean and Weight Stable: Behavioral Predictors and Psychological Correlates  

Microsoft Academic Search

Objective: To examine behavioral characteristics associated with being lean and weight stable during adulthood.Research Methods and Procedures: Participants included 165 white married couples who were part of a larger longitudinal study. Participants’ height and weight were measured on three occasions at 2-year intervals. Men and women were classified as being lean and weight stable (the target group) if they had

Kirsten Krahnstoever Davison; Leann Lipps Birch

2004-01-01

392

Cerebral Bioenergetics in Stable Chronic Obstructive Pulmonary Disease  

Microsoft Academic Search

Cerebral intracellular energy production (cerebral bioenergetics) via oxidative phosphorylation and the production of adenosine triphosphate (ATP) is critical to cerebral function. To test the hypothesis that patients with chronic stable hypoxia also generate neuronal ATP via an anaerobic metabolism, we studied the changes in cerebral 31 P magnetic resonance spectra ( 31 P MRS) in patients with stable chronic obstructive

RAJAT MATHUR; I. JANE COX; ANGELA OATRIDGE; DANIEL T. SHEPHARD; RORY J. SHAW; SIMON D. TAYLOR-ROBINSON

393

Detection of alternative stable states in marine communities  

Microsoft Academic Search

During the last 20 years there have been several attempts to test the theory of alternative stable states using marine systems. The results have been mixed, and there have been conflicting interpretations of the outcomes. In an attempt to resolve some of the problems, the theory and evidence for alternative stable states are reviewed. There are several different views of

Peter S. Petraitis; Steve R. Dudgeon

394

MINERAL BIOAVAILABILITY AND METABOLISM DETERMINED BY USING STABLE ISOTOPE TRACERS  

Technology Transfer Automated Retrieval System (TEKTRAN)

Definitive data on mineral bioavailability in humans and animals can be obtained by using isotopic tracers. The use of stable isotope tracers to study important issues in mineral nutrition has expanded rapidly in the past two decades, particularly in humans. Stable isotopes have a number of advanta...

395

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...

396

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

397

Gelled mechanically stable high energy fuel composition containing metal platelets  

Microsoft Academic Search

A gelled mechanically stable high energy fuel composition is disclosed comprising a hydrazine selected from the group consisting of hydrazine, unsymmetrical dimethyl hydrazine and mixtures thereof and an amount of metal platelets effective to produce a mechanically stable gel. Said platelets have a diameter of less than about 44 microns and an average diameter of about 1 micron, and said

J. J. Bost; J. A. Cabeal; S. D. Rosenberg

1980-01-01

398

Treatment of stable vitiligo with autologous epidermal grafting and PUVA  

Microsoft Academic Search

Background: Previous reports have shown the benefits of epidermal grafting for vitiligo.Objective: Our purpose was to evaluate the effectiveness and complications of epidermal grafting in combination with PUVA on stable vitiligo refractory to conventional treatments.Methods: In 100 patients with stable refractory vitiligo we performed epidermal grafting with suction blisters followed by PUVA treatment. The grafted sites were examined for repigmentation

Seung Kyung Hann; Sungbin Im; Ha Wook Bong; Yoon-Kee Park

1995-01-01

399

Stable and Metastable Equilibria in the Pb-Cd System  

NASA Astrophysics Data System (ADS)

Thermodynamic and phase diagram data in the Pb-Cd system are reevaluated. A substitutional solution model is used for the liquid and fcc and hcp phases. The stable and metastable equilibria of this system are calculated using the thermodynamic equations derived from equilibrium data. Besides the well-established eutectic reaction at 521 K (248 °C), one stable monotectic reaction at 548 K (275 °C) is found due to the existence of a stable liquid miscibility gap. The stable monotectic reaction has been missed in all previous evaluations. Experimental verifications of the stable and metastable phase equilibria are provided using droplet samples and undercooled liquid alloys. A differential thermal analysis (DTA) method is applied to determine the phase reaction temperatures using both traditional heating and cooling processes and a specially designed cycling process. Additional microstructural evidence is used to elucidate the nature of the phase reactions. The refined thermodynamic descriptions are based upon both the thermochemical and phase diagram stable and metastable data. The agreement between the calculated and experimental data is good. All experimental stable and metastable results are well explained by the new Pb-Cd phase diagram calculations within the experimental accuracy limits. Combined experimental and thermodynamic modeling procedures developed for determining the stable and metastable phase equilibria yield a highly reliable overall phase diagram assessment and a quantitative basis for the interpretation of non-equilibrium solidification processing.

Chuang, Ying-Yu; Paik, J.-S.; Zhang, C.; Perepezko, J. H.; Chang, Y. A.

2013-07-01

400

Stable isotope analysis of the bioelements: an introduction.  

PubMed

The abundances of the stable isotopes of the bioelements are not constant. Subtle, but significant, variations may be induced by physical, physiological and biochemical processes. These variations may be detected and quantified. Often, isotope fingerprints are characteristic of certain processes and may reveal information concerning the sources and origins of compounds of interest. Moreover, natural variabilities of stable isotopes may be exploited in order to perform tracer experiments. The most accurate technology to perform stable isotope analysis is (gas) isotope ratio MS (IRMS). Compound-specific approaches employ hyphenation of GC and LC to IRMS. In these approaches, complete conversion to simple gases prior to MS is required. Analysis by stable isotope ratio spectroscopy currently approaches the accuracy of IRMS. However, for bioanalytical projects, it is still predominantly confined to material synthetically enriched with stable isotopes. PMID:21083079

Flenker, Ulrich

2009-09-01

401

Community assembly: alternative stable states or alternative transient states?  

PubMed Central

The concept of alternative stable states has long been a dominant framework for studying the influence of historical contingency in community assembly. This concept focuses on stable states, yet many real communities are kept in a transient state by disturbance, and the utility of predictions for stable states in explaining transient states remains unclear. Using a simple model of plant community assembly, we show that the conditions under which historical contingency affects community assembly can differ greatly for stable versus transient states. Differences arise because the contribution of such factors as mortality rate, environmental heterogeneity and plant-soil feedback to historical contingency changes as community assembly proceeds. We also show that transient states can last for a long time relative to immigration rate and generation time. These results argue for a conceptual shift of focus from alternative stable states to alternative transient states for understanding historical contingency in community assembly.

Fukami, Tadashi; Nakajima, Mifuyu

2011-01-01

402

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 [(2)H3]-2-methylpyrazine (d-1), [(2)H5]-2-ethylpyrazine (d-2), [(2)H3]-2,3(or 6)-dimethylpyrazine (d-3A, d-3B), [(2)H3]-2,[(2)H3]-6-dimethylpyrazine (d-3C), [(2)H5]-2,[(2)H5]-6-diethylpyrazine (d-4), [(2)H5]-2-ethyl-3(or 6)-methylpyrazine (d-5A, d-5B), 2,[(2)H3]-3,5-trimethylpyrazine (d-6), [(2)H5]-2-ethyl-3,6-dimethylpyrazine (d-7), [(2)H5]-2-ethyl-3,5-dimethylpyrazine (d-8), and 2,3-diethyl-[(2)H3]-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

2013-04-09

403

A Model Describing Stable Coherent Synchrotron Radiation in Storage Rings  

NASA Astrophysics Data System (ADS)

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 BESSYII storage ring. We also use this model to optimize the performance of a source for stable CSR emission.

Sannibale, F.; Byrd, J. M.; Loftsdóttir, Á.; Venturini, M.; Abo-Bakr, M.; Feikes, J.; Holldack, K.; Kuske, P.; Wüstefeld, G.; Hübers, H.-W.; Warnock, R.

2004-08-01

404

On the structure and representations of max-stable processes  

Microsoft Academic Search

We develop classification results for max-stable processes, based on their\\u000aspectral representations. The structure of max-linear isometries and minimal\\u000aspectral representations play important roles. We propose a general\\u000aclassification strategy for measurable max-stable processes based on the notion\\u000aof co-spectral functions. In particular, we discuss the spectrally\\u000acontinuous-discrete, the conservative-dissipative, and the positive-null\\u000adecompositions. For stationary max-stable processes, the latter

Yizao Wang; Stilian A. Stoev

2010-01-01

405

Optical assembly of thermodynamically stable colloidal clusters mediated by depletion  

NASA Astrophysics Data System (ADS)

Colloidal particles with complementary shapes can self-organize into composite structures under the influence of entropic attractions mediated by depletion. What structures can form is governed by the colloidal components' shapes. The structures' stability can be tuned by adjusting the strength of the depletion attraction. Even when a particular colloidal cluster configuration is thermodynamically stable, achieving the stable structure typically involves substantial kinetic barriers. We overcome these kinetic barriers by assembling geometrically organized colloidal clusters using holographic optical tweezers in three dimensions. Once formed, the structures are stable and undergo three-dimensional shape fluctuations that can be measured with video microscopy.

Jyoti Krishnatreya, Bhaskar; Sacanna, Stefano; Edmond, Kazem; Pine, David; Grier, David G.

2013-03-01

406

Stable superhydrophobic surfaces over a wide pH range  

NASA Astrophysics Data System (ADS)

A stable superhydrophobic surface was fabricated by solidifying poly(epoxy-terminated polydimethylsiloxane- co-bisphenol A) [P(ETPDMS- co-BPA)] copolymer on a rough substrate. The low surface energy of the copolymer and the geometric structure at micrometer scale of the surface contribute to the superhydrophobic property. The as-prepared surface shows stable superhydrophobicity over a wide pH range (1-14) and the wettability is excellent stable to heating, water, corrosive solution and organic solvent treatments. The procedure is simple and time-saving as well as utilizing non-fluorine-containing compounds.

Guo, Li; Yuan, Wenfang; Li, Junping; Zhang, Zhijie; Xie, Zemin

2008-01-01

407

A Search for Charged Massive Stable Particles at D0  

SciTech Connect

A search for charged massive stable particles has been performed with the D0 detector at the Fermilab Tevatron. The signature is two particles reconstructed as muons, but with speed and invariant mass inconsistent with beam-produced muons. No excess of events is observed and limits are set on the production cross-section for pair-produced stable stau sleptons based on 390 pb{sup -1} of data. Limits vary from 0.06 pb to 0.62 pb, depending on the stau mass, and are the strictest Tevatron limits to date. Mass limits are also set for stable charginos. The limits are 140 GeV/c{sup 2} for a higgsino-like chargino and 174 GeV/c{sup 2} for a gaugino-like chargino. These are currently the best limits to date for stable charginos.

Eads, Michael T.; /Northern Illinois U.

2005-08-01

408

STABLE ISOTOPES IN ECOLOGICAL STUDIES: NEW DEVELOPMENTS IN MIXING MODELS  

EPA Science Inventory

Stable isotopes are increasingly being used as tracers in ecological studies. One application uses isotopic ratios to quantify the proportional contributions of multiple sources to a mixture. Examples include food sources for animals, water sources for plants, pollution sources...

409

Prediction of Stable Processes: Spectral and Moving Average Representations.  

National Technical Information Service (NTIS)

For stable processes which are Fourier transforms of processes with independent increments, we obtain a wold decomposition, we characterize their regularity and singularity, and, in the discrete-parameter case, we derive their linear predictors. In sharp ...

S. Cambanis A. R. Soltani

1984-01-01

410

2. QUARTERMASTER STOREHOUSE (Bldg. No. 915) IN FOREGROUND WITH STABLES ...  

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

2. QUARTERMASTER STOREHOUSE (Bldg. No. 915) IN FOREGROUND WITH STABLES (Bldg. Nos. 916 and 917) BEHIND AND TO LEFT OF Bldg. No. 915, TAKEN FROM SOUTHEAST. - Fort Lawton, Discovery Park, Seattle, King County, WA

411

Biomedical Research Applications of Electromagnetically Separated Enriched Stable Isotopes.  

National Technical Information Service (NTIS)

The current and projected annual requirements through 1985 for stable isotopes enriched by electromagnetic separation methods were reviewed for applications in various types of biomedical research: (1) medical radioisotope production, labeled compounds, a...

R. M. Lambrecht

1982-01-01

412

Stable isotope views on ecosystem function: challenging or challenged?  

PubMed Central

Stable isotopes and their potential for detecting various and complex ecosystem processes are attracting an increasing number of scientists. Progress is challenging, particularly under global change scenarios, but some established views have been challenged. The IX meeting of the Spanish Association of Terrestrial Ecology (AAET, Úbeda, 18–22 October 2009) hosted a symposium on the ecology of stable isotopes where the linear mixing model approach of partitioning sinks and sources of carbon and water fluxes within an ecosystem was challenged, and new applications of stable isotopes for the study of plant interactions were evaluated. Discussion was also centred on the need for networks that monitor ecological processes using stable isotopes and key ideas for fostering future research with isotopes.

Resco, Victor; Querejeta, Jose I.; Ogle, Kiona; Voltas, Jordi; Sebastia, Maria-Teresa; Serrano-Ortiz, Penelope; Linares, Juan C.; Moreno-Gutierrez, Cristina; Herrero, Asier; Carreira, Jose A.; Torres-Canabate, Patricia; Valladares, Fernando

2010-01-01

413

1. GENERAL VIEW. TRIM, ROOF AND STABLE DOORS ALL ARE ...  

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

1. GENERAL VIEW. TRIM, ROOF AND STABLE DOORS ALL ARE PAINTED RED. HEX SIGNS ARE PAINTED OCHRE, BLACK, RED, WHITE AND BLUE. NOTE PAINTED FLAGS ON SHED - Decorated White Barn, (Maiden Creek Township), Maiden Creek, Berks County, PA

414

Stable isotope views on ecosystem function: challenging or challenged?  

PubMed

Stable isotopes and their potential for detecting various and complex ecosystem processes are attracting an increasing number of scientists. Progress is challenging, particularly under global change scenarios, but some established views have been challenged. The IX meeting of the Spanish Association of Terrestrial Ecology (AAET, Ubeda, 18-22 October 2009) hosted a symposium on the ecology of stable isotopes where the linear mixing model approach of partitioning sinks and sources of carbon and water fluxes within an ecosystem was challenged, and new applications of stable isotopes for the study of plant interactions were evaluated. Discussion was also centred on the need for networks that monitor ecological processes using stable isotopes and key ideas for fostering future research with isotopes. PMID:20015858

Resco, Víctor; Querejeta, José I; Ogle, Kiona; Voltas, Jordi; Sebastià, Maria-Teresa; Serrano-Ortiz, Penélope; Linares, Juan C; Moreno-Gutiérrez, Cristina; Herrero, Asier; Carreira, José A; Torres-Cañabate, Patricia; Valladares, Fernando

2009-12-16

415

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

416

9 CFR 108.10 - Outer premises and stables.  

Code of Federal Regulations, 2013 CFR

...Section 108.10 Animals and Animal Products ANIMAL AND PLANT HEALTH INSPECTION SERVICE...AND ANALOGOUS PRODUCTS; ORGANISMS AND VECTORS FACILITY REQUIREMENTS...premises. (b) Stables or other premises for animals used in the...

2013-01-01

417

How to Control House and Stable Flies Without Using Pesticides.  

National Technical Information Service (NTIS)

Farmers, homeowners, and directors of recreational areas are advised on the control of the house fly and the stable fly with minimum use of insecticides. New management techniques are described, such as surveying barnyard and dumpster areas weekly so that...

E. T. Schmidtmann L. G. Pickens R. W. Miller

1994-01-01

418

Comparative efficiency of six stable fly (Diptera: Muscidae) traps.  

PubMed

Five adhesive traps and the Nzi cloth-target trap were compared to determine their trapping efficiency and biases for stable flies, Stomoxys calcitrans (L.) (Diptera: Muscidae). Two configurations of the BiteFree prototype trap, constructed of polyethylene terephthalate, were most efficient for trapping stable flies, whereas the EZ trap was least efficient. The two Alsynite traps, Broce and Olson, were intermediate to the BiteFree prototype and EZ traps. All adhesive traps collected a ratio of approximately two males for each female. Approximately 50% of the flies collected on the adhesive traps, both male and female, were blood fed, and 20% were vitellogenic. The Nzi trap collected an older component of the stable fly population, 81% blood fed and 62% vitellogenic, but it was much less efficient than the adhesive traps. The effectiveness of the BiteFree prototype trap indicates that materials other than Alsynite are attractive to stable flies. PMID:16937700

Taylor, David B; Berkebile, Dennis

2006-08-01

419

Dynamically-stable motion planning for humanoid robots  

Microsoft Academic Search

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 randomized path planner to search the configuration space

James Kuffner; K. Nishiwaki; Satoshi Kagami; Masayuki Inaba; Hirochika Inoue

2000-01-01

420

Bayesian Segmentation of Magnetic Resonance Images Using the ? Stable Distribution  

Microsoft Academic Search

\\u000a In this work, a segmentation method of Magnetic Resonance images (MRI) is presented. On the one hand, the distribution of\\u000a the grey (GM) and white matter (WM) are modelled using a mixture of ?-stable distributions. A Bayesian ?-stable mixture model for histogram data is used and the unknown parameters are sampled using the Metropolis-Hastings algorithm,\\u000a therefore, voxel intensity information is

Diego Salas-Gonzalez; Matthias Schlögl; Juan Górriz; Javier Ramírez; Elmar Lang

421

Combination treatment with trimetazidine and diltiazem in stable angina pectoris  

Microsoft Academic Search

ObjectiveTo assess antianginal efficacy and possible adverse haemodynamic effects of combination treatment with trimetazidine and diltiazem in patients with stable angina.DesignDouble blind, randomised, placebo controlled trial of four weeks duration.SettingOutpatient department of two Indian hospitals.Subjects64 male patients with stable angina, uncontrolled on diltiazem alone.InterventionsDiltiazem 180 mg and trimetazidine 60 mg, or diltiazem 180 mg and placebo daily.Main outcome measureChange in

S C Manchanda; S Krishnaswami

1997-01-01

422

The use of stable isotopes to study ecosystem gas exchange  

Microsoft Academic Search

Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing.\\u000a In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions\\u000a in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with\\u000a concentration and flux measurements of CO2

D. Yakir; L. da S. L. Sternberg

2000-01-01

423

Tempered stable Levy motion and transient super-difiusion  

Microsoft Academic Search

The space-fractional difiusion equation models anomalous super-difiusion. Its solu- tions are transition densities of a stable Levy motion, representing the accumulation of power-law jumps. Tempered stable Levy motion uses exponential tempering to cool these jumps. A tempered fractional difiusion equation governs the transition densities, which progress from super-difiusive early time to difiusive late-time be- havior. This note provides flnite difierence

Boris Baeumer; Mark M Meerschaert

2009-01-01

424

7. OVERALL VIEW OF ROOM 110 (STABLES), LOOKING EAST. THE ...  

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

7. OVERALL VIEW OF ROOM 110 (STABLES), LOOKING EAST. THE LONG, LOW SPACE IS DIVIDED INTO AISLES BY EXPOSED POSTS AND BEAMS; POSTS ARE PAINTED. CEILING TREATMENT CHANGES FROM GYPSUM BOARD TO WOOD PLANKS IN THE CENTER AISLE. THE CONCRETE SLAB FLOOR IS SLOPED FROM CENTER FOR DRAINAGE PURPOSES. SMALL, HIGH WINDOWS LINE THE NORTHEAST AND SOUTHWEST WALLS. - Presidio of San Francisco, Cavalry Stables, Cowles Street, between Lincoln Boulevard & McDowell Street, San Francisco, San Francisco County, CA

425

Methyl modified MOF-5: a water stable hydrogen storage material.  

PubMed

Water stable methyl modified MOF-5s have been synthesized via a solvothermal route. Methyl- and 2,5-dimethyl-modified MOF-5s show the same topology and hydrogen uptake capability as that of MOF-5. The H(2) uptake capacity of MOF-5, however, drops rapidly when exposed to the ambient air, whereas the H(2) uptake capacities of the methyl modified MOF-5s remain stable for 4 days. PMID:21451855

Yang, Jie; Grzech, Anna; Mulder, Fokko M; Dingemans, Theo J

2011-03-30

426

On Internal Gravity Waves Associated with the Stable Boundary Layer  

NASA Astrophysics Data System (ADS)

The stable planetary boundary layer at the baseof the residual layer supports internalwaves that are unambiguously boundary layer incharacter. Some of these wavesare instabilities and some are neutrally stable modes, but they all have critical levelsin the residual layer. These waves exist for a broad range of conditions and should bea major component of any ducted disturbance that propagates within ninety degreesof the wind direction. The wave properties can be computed without the numericaldifficulties usually associated with critical-level systems.

Chimonas, George

427

Gelled mechanically stable high energy fuel composition containing metal platelets  

SciTech Connect

A gelled mechanically stable high energy fuel composition is disclosed comprising a hydrazine selected from the group consisting of hydrazine, unsymmetrical dimethyl hydrazine and mixtures thereof and an amount of metal platelets effective to produce a mechanically stable gel. Said platelets have a diameter of less than about 44 microns and an average diameter of about 1 micron, and said metal platelets are composed of a metal selected from the group consisting of aluminum, beryllium and aluminumberyllium alloy.

Bost, J.J.; Cabeal, J.A.; Rosenberg, S.D.

1980-02-05

428

Carbon, Nitrogen and Oxygen Stable Isotope Ratios in Chitin  

Microsoft Academic Search

\\u000a Stable isotope ratios in chitin are firmly imprinted during biopolymer biosynthesis and reflect dietary, metabolic, and environmental\\u000a influences. Chitin is a chemically complex amino sugar biopolymer that also includes non-amino-sugar moieties with contrasting\\u000a isotopic compositions. Reproducible N, C, O-stable isotope determinations should rely on a chemically purified chitin substrate\\u000a with limited non-amino-sugar contributions. Insecta, Crustacea and Merostomata are not distinguished

Arndt Schimmelmann

429

Carbon sweating of meta-stable ternary carbides  

NASA Astrophysics Data System (ADS)

Transition metal carbides (TMC) have many attractive and unique physical properties and are nowadays used regularly in many technological applications. In thin films, the TMC can form nanocomposites where nanocrystallites of the carbide are embedded in a carbon matrix. Here we report on a new development where we have investigated meta-stable nanocomposite TMC. By substitutionally dissolving a weak carbide forming metal into a stable monocarbide, the ternary carbide system will become meta-stable and a decomposition into more stable phases will occur. We will show results obtained by density functional theory calculations of meta-stable TMC, where we have alloyed the stable monocarbide TiC with elements from the 3d transition metal series. The stability of the alloyed ternary phase decreases with the amount of alloying metal, which creates a driving force for the release of carbon from the carbide, i.e. the carbide sweats carbon. This mechanism can be controlled by careful tuning of the amount and selection of the alloying metal. These findings are supported by experimental studies on thin films of nanocomposite TMC.

Râsander, Mikael; Lewin, Erik; Sanyal, Biplab; Klintenberg, Mattias; Jansson, Ulf; Eriksson, Olle

2010-03-01

430

Multi-stable perception balances stability and sensitivity  

PubMed Central

We report that multi-stable perception operates in a consistent, dynamical regime, balancing the conflicting goals of stability and sensitivity. When a multi-stable visual display is viewed continuously, its phenomenal appearance reverses spontaneously at irregular intervals. We characterized the perceptual dynamics of individual observers in terms of four statistical measures: the distribution of dominance times (mean and variance) and the novel, subtle dependence on prior history (correlation and time-constant). The dynamics of multi-stable perception is known to reflect several stabilizing and destabilizing factors. Phenomenologically, its main aspects are captured by a simplistic computational model with competition, adaptation, and noise. We identified small parameter volumes (~3% of the possible volume) in which the model reproduced both dominance distribution and history-dependence of each observer. For 21 of 24 data sets, the identified volumes clustered tightly (~15% of the possible volume), revealing a consistent “operating regime” of multi-stable perception. The “operating regime” turned out to be marginally stable or, equivalently, near the brink of an oscillatory instability. The chance probability of the observed clustering was <0.02. To understand the functional significance of this empirical “operating regime,” we compared it to the theoretical “sweet spot” of the model. We computed this “sweet spot” as the intersection of the parameter volumes in which the model produced stable perceptual outcomes and in which it was sensitive to input modulations. Remarkably, the empirical “operating regime” proved to be largely coextensive with the theoretical “sweet spot.” This demonstrated that perceptual dynamics was not merely consistent but also functionally optimized (in that it balances stability with sensitivity). Our results imply that multi-stable perception is not a laboratory curiosity, but reflects a functional optimization of perceptual dynamics for visual inference.

Pastukhov, Alexander; Garcia-Rodriguez, Pedro E.; Haenicke, Joachim; Guillamon, Antoni; Deco, Gustavo; Braun, Jochen

2013-01-01

431

Molecular Structure of Dicarbon Monoxide  

NSDL National Science Digital Library

Dicarbon monoxide is a carbonyl carbene (:C=C=O), a carbon with only two bonds and a pair of non-bonding electrons. These species are building blocks in the synthesis of organic compounds. When the divalent carbon is bonded to a metal, the carbene is more stable and plays an important role in organic synthesis.

2003-06-02

432

Stable Isotope Ratios and the Forensic Analysis of Microorganisms  

SciTech Connect

In the aftermath of the anthrax letters of 2001, researchers have been exploring various analytical signatures for the purpose of characterizing the production environment of microorganisms. One such signature is stable isotope ratios, which in heterotrophs are a function of nutrient and water sources. Here we discuss the use of stable isotope ratios in microbe forensics, using as a database the carbon, nitrogen, oxygen and hydrogen stable isotope ratios of 247 separate cultures of B. subtilis 6051 spores produced on a total of 32 different culture media. In the context of using stable isotope ratios as a signature for sample matching, we present an analysis of variation between individual samples, between cultures produced in tandem, and between cultures produced in the same medium but at different times. Additionally, we correlate the stable isotope ratios of carbon, nitrogen, oxygen, and hydrogen for growth medium nutrients or water with those of spores and show examples of how these relationships can be used to exclude nutrient or water samples as possible growth substrates for specific cultures.

Kreuzer-Martin, Helen W.; Jarman, Kristin H.

2007-06-01

433

Stable Isotope Ratios and Forensic Analysis of Microorganisms?  

PubMed Central

In the aftermath of the anthrax letters of 2001, researchers have been exploring various analytical signatures for the purpose of characterizing the production environment of microorganisms. One such signature is stable isotope ratios, which in heterotrophs, are a function of nutrient and water sources. Here we discuss the use of stable isotope ratios in microbial forensics, using as a database the carbon, nitrogen, oxygen, and hydrogen stable isotope ratios of 247 separate cultures of Bacillus subtilis 6051 spores produced on a total of 32 different culture media. In the context of using stable isotope ratios as a signature for sample matching, we present an analysis of variations between individual samples, between cultures produced in tandem, and between cultures produced in the same medium but at different times. Additionally, we correlate the stable isotope ratios of carbon, nitrogen, oxygen, and hydrogen for growth medium nutrients or water with those of spores and show examples of how these relationships can be used to exclude nutrient or water samples as possible growth substrates for specific cultures.

Kreuzer-Martin, Helen W.; Jarman, Kristin H.

2007-01-01

434

Development of Stable Influenza Vaccine Powder Formulations: Challenges and Possibilities  

PubMed Central

Influenza vaccination represents the cornerstone of influenza prevention. However, today all influenza vaccines are formulated as liquids that are unstable at ambient temperatures and have to be stored and distributed under refrigeration. In order to stabilize influenza vaccines, they can be brought into the dry state using suitable excipients, stabilizers and drying processes. The resulting stable influenza vaccine powder is independent of cold-chain facilities. This can be attractive for the integration of the vaccine logistics with general drug distribution in Western as well as developing countries. In addition, a stockpile of stable vaccine formulations of potential vaccines against pandemic viruses can provide an immediate availability and simple distribution of vaccine in a pandemic outbreak. Finally, in the development of new needle-free dosage forms, dry and stable influenza vaccine powder formulations can facilitate new or improved targeting strategies for the vaccine compound. This review represents the current status of dry stable inactivated influenza vaccine development. Attention is given to the different influenza vaccine types (i.e. whole inactivated virus, split, subunit or virosomal vaccine), the rationale and need for stabilized influenza vaccines, drying methods by which influenza vaccines can be stabilized (i.e. lyophilization, spray drying, spray-freeze drying, vacuum drying or supercritical fluid drying), the current status of dry influenza vaccine development and the challenges for ultimate market introduction of a stable and effective dry-powder influenza vaccine.

Amorij, J-P.; Huckriede, A.; Wilschut, J.; Frijlink, H. W.

2008-01-01

435

STABLE BEHAVIORAL INHIBITION AND GLUCOCORTICOID PRODUCTION AS PREDICTORS OF LONGEVITY  

PubMed Central

Several personality/temperament traits have been linked to health outcomes in humans and animals but underlying physiological mechanisms for these differential outcomes are minimally understood. In this paper, we compared the strength of a behavioral trait (behavioral inhibition) and an associated physiological trait (glucocorticoid production) in predicting life span. In addition, we examined the relative stability of both the behavioral and physiological trait within individuals over a significant portion of adulthood, and tested the hypothesis that a stable behavioral trait is linked with a stable physiological bias. In a sample of 60 Sprague-Dawley male rats, we found that stable inhibition/neophobia was a stronger predictor of life span than stably elevated glucocorticoid production. In addition, these predictors appeared to have an additive influence on life span in that males with both risk factors (stable inhibition and consistently high glucocorticoid production) had the shortest life spans of all, suggesting both traits are important predictors of life span. Across a 4-month period in young adulthood, inhibition and glucocorticoid reactivity were relatively stable traits, however these two traits were not highly correlated with one another. Interestingly, baseline glucocorticoid production was a better predictor of life span than reactivity levels. The results indicate that glucocorticoid production in young adulthood is an important predictor of life span, although not as strong a predictor as inhibition, and that other physiological processes may further explain the shortened life span in behaviorally-inhibited individuals.

Cavigelli, Sonia A.; Ragan, Christina M.; Michael, Kerry C.; Kovacsics, Colleen E.; Bruscke, Alexander P.

2009-01-01

436

Role of stable eignmodes in ETG-driven turbulence  

NASA Astrophysics Data System (ADS)

We investigate the role of the stable eigenmodes in the electron-temperature-gradient driven (ETG) turbulence. The low-wavenumber stable eigenmodes are thought to play a role in the dissipation mechanism leading to the saturation of CTEM [1] and ITG [2] turbulence. Based on the ETG slab fluid model [3], the condition and parameter regime for the saturation by the stable mode are analytically explored through statistical closure theory. In the simulation, the evolution of heat flux and turbulent energy are traced with the correlation of the stable and unstable modes. Possible effect of the stable mode saturation on zonal flow and magnetic fluctuation will be discussed. In the future, we will extend the analysis to the nonlocal ETG fluid model and kinetic model. [1] P. W. Terry, D. A. Baver, and S. Gupta, Phys. Plasmas 13, 022307 (2006). [2] R. Gatto, P. W. Terry, and D. A. Baver, Phys. Plasmas 13, 022306 (2006). [3] W. Horton et. al. Nuclear Fusion 45 (2005).

Kim, Juhyung; Terry, Paul W.

2008-11-01

437

Psychopathology of stable and unstable mixed states: a historical view.  

PubMed

The concept of mixed states was proposed at the end of last century by Kraepelin and Weygandt to confer unity to manic-depressive illness and to better differentiate this disease from dementia praecox. The concept has been further elaborated by the Hamburg and Vienna schools (Mentzos and Berner, respectively), and dichotomized in stable and unstable mixed states. Stable mixed states represent a condition of the synchronic copresence of symptoms of both polarities; unstable mixed states, or "mixed pictures," are characterized by the rapid cycling of these symptoms. The former represent clinical expressions of bipolar disorder, and the latter represent trans-nosological conditions with multiple outcomes. Current North American nosology retains the original Kraepelinian concept of stable mixed states and ignores the concept of unstable mixed states, which is very close to the concept of ultradian cycling proposed by Kramlinger and Post in 1996. PMID:10741882

Maggini, C; Salvatore, P; Gerhard, A; Migone, P

438

Design of partially optically stable reflector systems and prisms  

NASA Astrophysics Data System (ADS)

The characteristics and design method of the total optically stable (TOS) reflector systems/prisms were introduced in an early paper (Tsai and Lin in Appl. Opt. 47:4158-4163, 2008), where only two types of TOS reflector system exist, namely preservation or retroreflection. In this paper, we introduce the partially optically stable (POS) reflector system, which is only optically stable about a specific directional vector; nevertheless, the exiting light ray is not restricted to preservation or retroreflection. The proposed paper also presents an analytic method for the design of POS reflector systems comprised of multiple reflectors. Furthermore, it is shown that a POS prism can be obtained by adding two refracting flat boundary surfaces with specific conditions at the entrance and exit positions of the light ray in an optical system with multiple reflectors.

Tsai, Chuang-Yu

2010-09-01

439

Ranking stability and super-stable nodes in complex networks.  

PubMed

Pagerank, a network-based diffusion algorithm, has emerged as the leading method to rank web content, ecological species and even scientists. Despite its wide use, it remains unknown how the structure of the network on which it operates affects its performance. Here we show that for random networks the ranking provided by pagerank is sensitive to perturbations in the network topology, making it unreliable for incomplete or noisy systems. In contrast, in scale-free networks we predict analytically the emergence of super-stable nodes whose ranking is exceptionally stable to perturbations. We calculate the dependence of the number of super-stable nodes on network characteristics and demonstrate their presence in real networks, in agreement with the analytical predictions. These results not only deepen our understanding of the interplay between network topology and dynamical processes but also have implications in all areas where ranking has a role, from science to marketing. PMID:21772265

Ghoshal, Gourab; Barabási, Albert-László

2011-07-19

440

Stable hydrogenated graphene edge types: Normal and reconstructed Klein edges  

NASA Astrophysics Data System (ADS)

Hydrogenated graphene edges are assumed to be either armchair, zigzag, or a combination of the two. We show that the zigzag is not the most stable fully hydrogenated edge structure along the <21¯1¯0> direction. Instead hydrogenated Klein and reconstructed Klein based edges are found to be energetically more favorable, with stabilities approaching that of armchair edges. These new structures “unify” graphene edge topology, the most stable flat hydrogenated graphene edges always consisting of pairwise bonded C2H4 edge groups, irrespective of the edge orientation. When edge rippling is included, CH3 edge groups are most stable. These new fundamental hydrogen-terminated edges have important implications for graphene edge imaging and spectroscopy, as well as mechanisms for graphene growth, nanotube cutting, and nanoribbon formation and behavior.

Wagner, Philipp; Ivanovskaya, Viktoria V.; Melle-Franco, Manuel; Humbert, Bernard; Adjizian, Jean-Joseph; Briddon, Patrick R.; Ewels, Christopher P.

2013-09-01

441

New fabrication processes for dimensionally stable beryllium mirrors  

NASA Astrophysics Data System (ADS)

Stability of beryllium mirrors is said to be unpredictable. Three recent mirrors demonstrate excellent stability. JPL produced a plano-concave, 0.5 m solid test mirror that was machined from a HIP'ed billet of special I-70 Be powder, polished bare and tested to 4K. It was thermally stable and had no hysteresis. The JPL ITTT 0.85 m primary mirror used similar material and processes, but with more stress relied treatments. Tests of this bare-polished, very lightweight single arch hyperboloidal mirror to 5K showed similar excellent results. The ESO VLT chopping secondary is a 1.12 m, machined lightweight, nickel plated, convex paraboloid. Similar fabrication processes are being used but with higher strength I-220 Be. In-processes testing indicates a stable mirror. The results show beryllium to be a stable and predictable mirror material.

Paquin, Roger A.; Coulter, Daniel R.; Norris, David D.; Augason, Gordon C.; Stier, Mark T.; Cayrel, Marc; Parsonage, Thomas B.

1996-08-01

442

Stable isotopes dissect food webs from top to the bottom  

NASA Astrophysics Data System (ADS)

Stable isotopes have been used extensively to study food web functioning, i.e. the flow of energy and matter among organisms. Traditional food-web studies are based on the natural variability of carbon and nitrogen isotopes and are limited to larger organisms that can be physically separated from their environment. Recent developments allow isotope ratio measurements of microbes and this in turn allows then measurement of entire food webs, i.e. from small producers at the bottom to large consumers at the top. Here, I provide a concise review on the use and potential of stable isotope to reconstruct end-to-end food webs. I will first discuss food web reconstruction based on natural abundances isotope data and will then show that the use of stable isotopes as deliberately added tracers provides complementary information. Finally, challenges and opportunities for end-to-end food web reconstructions in a changing world are discussed.

Middelburg, J. J.

2013-09-01

443

Stable Neutral Atom Trap with a Thin Superconducting Disc  

SciTech Connect

A stable magnetic quadrupole trap for neutral atoms on a superconducting Nb thin-film disc is demonstrated. The quadrupole field is composed of the magnetic field that is generated by vortices on the disc introduced by cooling the disc across the transition temperature with a finite field and an oppositely directed uniform field applied after cooling. The trap is stable when all trapping processes are performed above the dendritic instability temperature T{sub a}. When the field intensity is changed below this temperature, the quadrupole field collapses and the trap disappears. The initial vortex density decreases even when the external field is changed at a temperature T>T{sub a}. However, the vortex density is stabilized at an equilibrium density, whereas at Tstable trap can be formed, even when the initial vortices are introduced through a dendritic avalanche.

Shimizu, Fujio [NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198 (Japan); Institute for Laser Science, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 113-8585 (Japan); Hufnagel, Christoph; Mukai, Tetsuya [NTT Basic Research Laboratories, NTT Corporation, 3-1, Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198 (Japan)

2009-12-18

444

Stable-isotope fingerprints of biological agents as forensic tools.  

PubMed

Naturally occurring stable isotopes of light elements in chemical and biological agents may possess unique "stable-isotope fingerprints" depending on their sources and manufacturing processes. To test this hypothesis, two strains of bacteria (Bacillus globigii and Erwinia agglomerans) were grown under controlled laboratory conditions. We observed that cultured bacteria cells faithfully inherited the isotopic composition (hydrogen, carbon, and nitrogen) of media waters and substrates in predictable manners in terms of bacterial metabolism and that even bacterial cells of the same strain, which grew in media water and substrates of different isotopic compositions, have readily distinguishable isotopic signatures. These "stable-isotopic fingerprints" of chemical and biological agents can be used as forensic tools in the event of biochemical terrorist attacks. PMID:12570211

Horita, Juske; Vass, Arpad A

2003-01-01

445

Free electron lasers using stable-unstable ring resonators  

SciTech Connect

The free electron laser (FEL) simulation code FELEX is used to examine the operation of stable-unstable FEL resonators. These resonators are stable along one transverse axis and unstable along the orthogonal transverse axis. The simulations utilize a ring resonator with an intracavity focus in the unstable plane near the center of the wiggler (close to the same axial position as the waist in the stable plane) thereby enhancing the coupling between the optical and electron beams. Asymmetric output scraping is performed in the back leg of the ring using a reflective mirror inserted from one side of the unstable axis. Resonators with relatively low equivalent Fresnel number (/vert bar/N/sub eq//vert bar/ less than or equal to 10) and magnification (/vert bar/M/sub x//vert bar/ approx. = 1.2) are examined. Optical characteristics including the cavity mode profile at various positions inside the resonator are shown. 8 refs., 8 figs.

Schmitt, M.J.; Paxton, A.H.

1989-01-01

446

Miscible density-stable displacement flows in inclined tube  

NASA Astrophysics Data System (ADS)

We study density-stable laminar miscible displacement flow of two iso-viscous Newtonian fluids in an inclined pipe (diameter \\Dcirc). We present a wide range of novel experimental results. We illustrate the non-monotone relation in displacement efficiency at the density difference moves from positive (density unstable) to negative (density stable), the efficiency being minimal for iso-dense fluids. The density stable configuration has been found to produce highly efficient displacements, with the bulk of the interface moving steadily at the mean velocity. The streamwise length of the stretched interface, or stretch length \\Lcirc, is measured over a wide range of parameters. The stretch length increases with the mean flow velocity, increases with inclination ? from vertical, decreases with density difference, and increases with viscosity. Our data are well represented by the scaled expression L - tan ? = -3680/?, where ? is the ratio of buoyancy and viscous stresses.

Alba, K.; Taghavi, S. M.; Frigaard, I. A.

2012-12-01

447

Intermittent Turbulence in the Very Stable Ekman Layer  

SciTech Connect

INTERMITTENT TURBULENCE IN THE VERY STABLE EKMAN LAYER This study describes a Direct Numerical Simulation (DNS) of a very stable Ekman layer in which a constant downward heat flux is applied at the lower boundary, thus cooling the fluid above. Numerical experiments were performed in which the strength of the imposed heat flux was varied. For downward heat fluxes above a certain critical value the turbulence becomes intermittent and, as the heat flux increases beyond this value, the flow tends to relaminarize because of the very strong ambient stratification. We adopt Mahrt?s (1999) definition of the very stable boundary layer as a boundary layer in which intermittent, rather than continuous turbulence, is observed. Numerical experiments were used to test various hypothesis of where in ?stability parameter space? the very stable boundary layer is found. These experiments support the findings of Howell and Sun (1999) that the boundary layer will exhibit intermittency and therefore be categorized as ?very stable?, when the stability parameter, z/L, exceeds unity. Another marker for the very stable boundary layer, Derbyshire?s (1990) maximum heat flux criterion, was also examined. Using a case study drawn from the simulations where turbulence intermittency was observed, the mechanism that causes the intermittence was investigated. It was found that patchy turbulence originates from a vigorous inflectional, Ekman-like instability -- a roll cell -- that lifts colder air over warmer air. The resulting convective instability causes an intense burst of turbulence. This turbulence is short-lived because the lifting motion of the roll cell, as well as the roll cell itself, is partially destroyed after the patchy turbulence is generated. Examples of intermittent turbulence obtained from the simulations appear to be consistent with observations of intermittency even though the Reynolds number of the DNS is relatively low (400).

Barnard, James C.

2001-01-05

448

Stable and accurate wave-propagation in discontinuous media  

NASA Astrophysics Data System (ADS)

A time stable discretization is derived for the second-order wave equation with discontinuous coefficients. The discontinuity corresponds to inhomogeneity in the underlying medium and is treated by splitting the domain. Each (homogeneous) sub domain is discretized using narrow-diagonal summation by parts operators and, then, patched to its neighbors by using a penalty method, leading to fully explicit time integration. This discretization yields a time stable and efficient scheme. The analysis is verified by numerical simulations in one-dimension using high-order finite difference discretizations, and in three-dimensions using an unstructured finite volume discretization.

Mattsson, Ken; Ham, Frank; Iaccarino, Gianluca

2008-10-01

449

Stable Isotopic Tracing--A Way Forward for Nanotechnology  

PubMed Central

Numerous publications and reports have expressed health and safety concerns about the production and use of nanoparticles, especially in areas of exposure monitoring, personal use, and environmental fate and transport. We suggest that stable isotopic tracers, which have been used widely in the earth sciences and in metabolic and other health-related studies for several decades, could be used to address many of these issues. One such example we are pursuing is the use of stable isotopes to monitor dermal absorption of zinc and titanium oxides in sunscreen preparations and other personal care products. Other potential applications of this tracing approach are discussed.

Gulson, Brian; Wong, Herbert

2006-01-01

450

Metal stable isotopes in low-temperature systems: A primer  

USGS Publications Warehouse

Recent advances in mass spectrometry have allowed isotope scientists to precisely determine stable isotope variations in the metallic elements. Biologically infl uenced and truly inorganic isotope fractionation processes have been demonstrated over the mass range of metals. This Elements issue provides an overview of the application of metal stable isotopes to low-temperature systems, which extend across the borders of several science disciplines: geology, hydrology, biology, environmental science, and biomedicine. Information on instrumentation, fractionation processes, data-reporting terminology, and reference materials presented here will help the reader to better understand this rapidly evolving field.

Bullen, T. D.; Eisenhauer, A.

2009-01-01

451

Stable operation for No. 1 blast furnace at Baosteel  

SciTech Connect

Until Sept. 1994, No. 1 blast furnace in Baoshan Steel (Group) Co. had been operating successfully with high productivity. At present, it still keeps its strong potential for production capacity. The philosophy and methodology of the efficient blast furnace operation will be described. The most important aspect for stable operation and long campaign life of a blast furnace is reasonable gas flow distribution in the furnace. The topics to be discussed include: introduction to ironmaking in Baosteel; operational results; experience of stable operation, philosophy, gas distribution control, stabilization raw material property, furnace maintenance and supervision system.

Cai, X.; Lu, S. [Baoshan Steel Corp., Shanghai (China)

1995-07-01

452

Ice XV: a new thermodynamically stable phase of ice.  

PubMed

A new phase of ice, named ice XV, has been identified and its structure determined by neutron diffraction. Ice XV is the hydrogen-ordered counterpart of ice VI and is thermodynamically stable at temperatures below approximately 130 K in the 0.8 to 1.5 GPa pressure range. The regions of stability in the medium pressure range of the phase diagram have thus been finally mapped, with only hydrogen-ordered phases stable at 0 K. The ordered ice XV structure is antiferroelectric (P1), in clear disagreement with recent theoretical calculations predicting ferroelectric ordering (Cc). PMID:19792330

Salzmann, Christoph G; Radaelli, Paolo G; Mayer, Erwin; Finney, John L

2009-09-02

453

Stable multilayer thin films composed of gold nanoparticles and lysozyme  

NASA Astrophysics Data System (ADS)

It needs appropriately attractive forces to construct multilayer thin films by layer-by-layer (LBL) assembly technique. It is feasible to prepare multilayer thin films on glass slides with negatively charged gold nanoparticles and positively charged lysozyme through the electrostatic LBL assembly technique. The gold nanoparticles/lysozyme multilayer thin films are highly stable; immersion in 0.1 M HCl, NaOH, and surfactant sodium dodecyl sulfate aqueous solutions cannot destroy the films. The highly stable gold nanoparticles/lysozyme multilayer thin films have potential application in long-term antibacterial coating.

Su, Yan-lei; Li, Chao

2008-01-01

454

Thermally stable surfactants and compositions and methods of use thereof  

DOEpatents

There are provided novel thermally stable surfactants for use with fillers in the preparation of polymer composites and nanocomposites. Typically, surfactants of the invention are urethanes, ureas or esters of thiocarbamic acid having a hydrocarbyl group of from 10 to 50 carbons and optionally including an ionizable or charged group (e.g., carboxyl group or quaternary amine). Thus, there are provided surfactants having Formula I: ##STR00001## wherein the variables are as defined herein. Further provided are methods of making thermally stable surfactants and compositions, including composites and nanocomposites, using fillers coated with the surfactants.

Chaiko, David J. (Woodridge, IL)

2008-09-02

455

Stable multilayer structure based on restoring Casimir forces  

NASA Astrophysics Data System (ADS)

The stability of the mesoscopic multilayer structure resulting from the Casimir effect is investigated. For the multilayer structure composed of periodic metal and metamaterial layers, we focus on the formation of the restoring Casimir forces that simultaneously stably equilibrate each layer of the structure with the suitably chosen frequency-dependent electromagnetic properties of the materials. The stable equilibrium distances for the layers are discussed, and the influences of the metamaterial characteristic frequencies and of the layer thickness are shown. Since the system stability is induced only by the Casimir forces, this mesoscopic multilayer can be considered as a stable non-touching "Casimir solid".

Zeng, Ran; Chen, Yanjun; Yang, Yaping

2013-09-01

456

P-stable boundary value methods for second order IVPs  

NASA Astrophysics Data System (ADS)

We introduce a family of Linear Multistep Methods used as Boundary Value Methods for the numerical solution of initial value problems for second order ordinary differential equations of special type. The aim is to obtain P-stable methods with arbitrary order of accuracy. This result allows to overcome the order barrier established by Lambert and Watson which limited to p = 2 the maximum order of a P-stable Linear Multistep Method. In addition, an extension of the methods in the Exponential Fitting framework is also considered.

Aceto, Lidia; Ghelardoni, Paolo; Magherini, Cecilia

2012-09-01

457

Prediction of stable walking for a toy that cannot stand  

NASA Astrophysics Data System (ADS)

Previous experiments [M. J. Coleman and A. Ruina, Phys. Rev. Lett. 80, 3658 (1998)] showed that a gravity-powered toy with no control and that has no statically stable near-standing configurations can walk stably. We show here that a simple rigid-body statically unstable mathematical model based loosely on the physical toy can predict stable limit-cycle walking motions. These calculations add to the repertoire of rigid-body mechanism behaviors as well as further implicating passive dynamics as a possible contributor to stability of animal motions.

Coleman, Michael J.; Garcia, Mariano; Mombaur, Katja; Ruina, Andy

2001-08-01

458

A Stable Compound Containing a Silicon-Silicon Triple Bond  

Microsoft Academic Search

The reaction of 2,2,3,3-tetrabromo-1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasilane with four equivalents of potassium graphite (KC8) in tetrahydrofuran produces 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyl-2-tetrasilyne, a stable compound with a silicon-silicon triple bond, which can be isolated as emerald green crystals stable up to 100°C in the absence of air. The Si?Si triple-bond length (and its estimated standard deviation) is 2.0622(9) angstroms, which shows half the magnitude of the bond

Akira Sekiguchi; Rei Kinjo; Masaaki Ichinohe

2004-01-01

459

Remarks on search methods for stable, massive, elementary particles  

NASA Astrophysics Data System (ADS)

This paper was presented at the 69th birthday celebration of Professor Eugene Commins, honoring his research achievements. These remarks are about the experimental techniques used in the search for new stable, massive particles, particles at least as massive as the electron. A variety of experimental methods such as accelerator experiments, cosmic ray studies, searches for halo particles in the galaxy and searches for exotic particles in bulk matter are described. A summary is presented of the measured limits on the existence of new stable, massive particle. .

Perl, Martin L.

2001-11-01

460

Human-like Running Can Be Open-Loop Stable  

NASA Astrophysics Data System (ADS)

This paper addresses the question if running motions of a human-like robot can be stable without feedback. Exploitation of self-stability is considered to be a crucial factor for biological running and might be the key for success to make bipedal and humanoid robots run in the future, We investigate a two-dimensional simulation model of running with 9 bodies (trunk, thighs, shanks, feet, and arms) powered by torques at all internal joints. Using efficient optimal control techniques and stability optimization, we were able to determine torque inputs and spring-damper parameters that lead to fully open-loop stable running motions.

Mombaur, Katja

461

Indirect estimation of fi-stable stochastic volatility models  

Microsoft Academic Search

The fi-stable family of distributions constitutes a generalization of the Gaus- sian distribution, allowing for asymmetry and thicker tails. Its many useful prop- erties, including a central limit theorem, are especially appreciated in the financial field. However, estimation difficulties have up to now hindered its diffusion among practitioners. In this paper we propose an indirect estimation approach to stochas- tic

Marco J. Lombardi; Giorgio Calzolari

2005-01-01

462

Structurally stable diffeomorphisms with basis sets of codimension one  

NASA Astrophysics Data System (ADS)

We study the topological and homotopical structure of closed n-dimensional manifolds ( n\\geqslant 3) on which there are structurally stable diffeomorphisms with orientable expanding attractors and contracting repellers of codimension one. The results obtained are applied to the topological classification of such diffeomorphisms on the n-dimensional torus T^n, n\\geqslant 3.

Grines, V. Z.; Zhuzhoma, E. V.

2002-04-01

463

Robust strategies for managing rangelands with multiple stable attractors  

Microsoft Academic Search

Savanna rangelands are characterized by dynamic interactions between grass, shrubs, fire and livestock driven by highly variable rainfall. When the livestock are grazers (only or preferentially eating grass) the desirable state of the system is dominated by grass, with scattered trees and shrubs. However, the system can have multiple stable attractors and a perturbation such as a drought can cause

Marco A. Janssen; John M. Anderies; Brian H. Walkerc

2004-01-01

464

Method of producing a dimensionally stable battery separator  

SciTech Connect

A method is disclosed for producing a dimensionally stable battery separator characterized by grafting acrylic acid and/or methacrylic acid onto a polyethylene film, treating the resulting membrane with an aqueous alkaline solution, and drying the treated membrane under application of tension.

Ishigaki, I.; Machi, S.; Murata, K.; Senoo, K.; Sugo, T.; Tanso, S.

1981-08-11

465

France's Administrative Tertiary: Stable Numbers for Occupations in Flux.  

ERIC Educational Resources Information Center

During the past decade, the number of jobs in France's administrative service sector has remained stable. General administrative work has become more infrequent; however, the basic occupations of secretary and accountancy have been maintained. Although the number of typists has declined, the number of "secretarial" jobs has increased dramatically.…

Liaroutzos, Olivier; Meriot, Sylvie-Anne

1995-01-01

466

Standards for stable isotope measurements in natural compounds  

Microsoft Academic Search

RESEARCH based on stable isotope variations in natural compounds is expanding in scientific fields such as geochemistry, hydrology, environmental studies and biochemistry. However, intercomparison of results obtained in different laboratories is often not fully reliable and therefore to improve the intercalibration of deuterium and 18O measurements in natural waters, two water standards have been distributed by the International Atomic Energy

R. Gonfiantini

1978-01-01

467

Apparatus and method for monitoring of gas having stable isotopes  

DOEpatents

Gas having stable isotopes is monitored continuously by using a system that sends a modulated laser beam to the gas and collects and transmits the light not absorbed by the gas to a detector. Gas from geological storage, or from the atmosphere can be monitored continuously without collecting samples and transporting them to a lab.

Clegg, Samuel M; Fessenden-Rahn, Julianna E

2013-03-05

468

Incorporating concentration dependence in stable isotope mixing models  

Microsoft Academic Search

Stable isotopes are often used as natural labels to quantify the contributions of multiple sources to a mixture. For example, C and N isotopic signatures can be used to determine the fraction of three food sources in a consumer's diet. The standard dual isotope, three source linear mixing model assumes that the proportional contribution of a source to a mixture

Donald L. Phillips; Paul L. Koch

2002-01-01

469

Stable isotope composition of precipitation over southeast Asia  

Microsoft Academic Search

Spatial and temporal variability of the stable isotope composition of precipitation in the southeast Asia and western Pacific region is discussed, with emphasis on the China territory, based on the database of the International Atomic Energy Agency\\/World Meteorological Organization Global Network ``Isotopes in Precipitation'' and the available information on the regional climatology and atmospheric circulation patterns. The meteorological and pluviometric

Luis Aragufis-Aragufis; Klaus Froehlich; Kazimierz Rozanski

1998-01-01

470

Palaeoclimate reconstruction from stable isotope variations in speleothems: a review  

Microsoft Academic Search

Speleothems are now regarded as valuable archives of climatic conditions on the continents, offering a number of advantages relative to other continental climate proxy recorders such as lake sediments and peat cores. They are ideal materials for precise U-series dating, yielding ages in calendar years, thereby circumventing the radiocarbon calibration problems associated with most other continental records. Stable isotope studies

Frank McDermott

2004-01-01

471

Source partitioning using stable isotopes: coping with too many sources  

Microsoft Academic Search

Stable isotopes are increasingly being used as tracers in environmental studies. One application is to use isotopic ratios to quantitatively determine the proportional contribution of several sources to a mixture, such as the proportion of various pollution sources in a waste stream. In general, the proportional contributions of n+1 different sources can be uniquely determined by the use of n

Donald L. Phillips; Jillian W. Gregg

2003-01-01

472

Beauty and distance in the stable marriage problem  

Microsoft Academic Search

The stable marriage problem has been introduced in order to describe a complex system where individuals attempt to optimise their own satisfaction, subject to mutually conflicting constraints. Due to the potential large applicability of such model to describe all the situation where different objects has to be matched pairwise, the statistical properties of this model have been extensively studied. In

G. Caldarelli; A. Capocci

2001-01-01

473

Characterizations of the Disjunctive Stable Semantics by Partial Evaluation  

Microsoft Academic Search

There are three most prominent semantics defined for certain subclasses of disjunctive logic programs: GCWA (for positive programs), PERFECT (for stratified programs), and STABLE (defined for the whole class of all disjunctive programs). While there are various competitors based on 3-valued models, notably WFS and its disjunctive counterparts, there are no other semantics consisting of two-valued models. We argue that

Stefan Brass; Jürgen Dix

1997-01-01

474

Stable Isotope Astrobiology at Hispanic Serving Institutions: Si Se Puede!  

NASA Astrophysics Data System (ADS)

A "crawl-walk-run" strategy has been used to create a strong, sustainable program in stable isotope geochemistry with an emphasis on astrobiology. The result was the creation of a vibrant program addressing the record of early life on Earth.

Melchiorre, E. B.; Lopez, A.; Velasquez, C. M.

2010-04-01

475

Insomnia Symptoms and Daytime Function in Stable Heart Failure  

PubMed Central

Objectives: To evaluate insomnia symptoms and the extent to which they are associated with clinical and demographic patient characteristics, daytime symptoms, and functional performance in patients with stable heart failure (HF). Design: Cross-sectional, observational. Setting: Five structured HF disease management programs in the Northeastern U.S. Participants: 173 stable chronic HF patients Interventions: N/A Measurements and Results: Full polysomnography was obtained for one night in participants' homes. Participants completed the six-minute walk test, Medical Outcomes Study SF-36, Epworth Sleepiness Scale, Pittsburgh Sleep Quality Index, Multi-Dimensional Assessment of Fatigue Scale, Centers for the Epidemiological Studies of Depression Scale, and questionnaire items eliciting insomnia symptoms (self-reported difficulty initiating and maintaining sleep and waking too early in the morning). Over half of HF patients reported insomnia symptoms. These were associated with increased daytime symptoms (depression, fatigue), excessive daytime sleepiness, and functional performance in models that statistically controlled for clinical and demographic covariates. These relationships were not explained by sleep disordered breathing. Conclusions: Insomnia symptoms are common in patients with stable heart failure and are associated with daytime symptoms and decrements in functional performance. Citation: Redeker NS; Jeon S; Muench U; Campbell D; Walsleben J; Rapoport DM. Insomnia symptoms and daytime function in stable heart failure. SLEEP 2010;33(9):1210-1216.

Redeker, Nancy S.; Jeon, Sangchoon; Muench, Ulrike; Campbell, Della; Walsleben, Joyce; Rapoport, David M.

2010-01-01

476

Adaptive filtering for non-Gaussian stable processes  

Microsoft Academic Search

A large class of physical phenomena observed in practice exhibit non-Gaussian behavior. In the letter ?-stable distributions, which have heavier tails than Gaussian distributions, are considered to model non-Gaussian signals. Adaptive signal processing in the presence of such a noise is a requirement of many practical problems. Since direct application of commonly used adaptation techniques fail in these applications, new

Orhan Arikan; A. Enis Cetin; Engin Erzin

1994-01-01