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1

A C-C bonded phenoxyl radical dimer with a zero bond dissociation free energy.  

PubMed

The 2,6-di-tert-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) Å for the C4-C4a bond. This is significantly longer than typical C-C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed that the Keq for dissociation is 1.3 ± 0.2 M at 20 °C, indicating a C-C bond dissociation free energy of -0.15 ± 0.1 kcal mol(-1). Van't Hoff analysis gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 ± 0.5 kcal mol(-1). To our knowledge, this is the smallest BDE measured for a C-C bond. This very weak bond shows a large deviation from the correlation of C-C bond lengths and strengths, but the computed force constant follows Badger's rule. PMID:23952108

Wittman, Jessica M; Hayoun, Rebecca; Kaminsky, Werner; Coggins, Michael K; Mayer, James M

2013-08-23

2

C=C Bond Formation  

NASA Astrophysics Data System (ADS)

The material presented in this chapter describes the general methodology used for the preparation of unsaturated sugars. The `older' methods (i. e. those being developed since at least the 1950s) which are still very useful and have general application are also presented but they are illustrated by newer examples. The direct formation of the double bond(s) is emphasized, but the methodology based on the rearrangement of unsaturated sugars into other olefinic carbohydrates is also reviewed.

Jarosz, S?awomir; Nowogródzki, Marcin

3

Correlation between shape resonance energies and C-C bond length in carbon-containing molecules: Elastic electron scattering and carbon K-shell excitation by photons.  

National Technical Information Service (NTIS)

We document the correlation of shape resonance energies resulting from (i) elastic electron scattering and (ii) carbon K-shell excitation with ic bond order (C-C bond length) for C(sub 2)H(sub 2), C(sub 2)H(sub 4), C(sub 2)H(sub 6), C(sub 3)H(sub 8), C(su...

M. Kimura

1994-01-01

4

Correlation between shape resonance energies and C-C bond length in carbon-containing molecules: Elastic electron scattering and carbon K-shell excitation by photons  

SciTech Connect

We document the correlation of shape resonance energies resulting from (i) elastic electron scattering and (ii) carbon K-shell excitation with ic bond order (C-C bond length) for C{sub 2}H{sub 2}, C{sub 2}H{sub 4}, C{sub 2}H{sub 6}, C{sub 3}H{sub 8}, C{sub 4}H{sub 10}, and C{sub 6}H{sub 6}. A relationship between K-shell {sigma} resonances and bond length was experimentally pointed out previously. These correlations are qualitatively interpreted to indicate that as molecular size increases (or as bond length increases), the configuration space available for valence electrons increases, reducing energy levels rather uniformly and mowing these correlations to emerge. The similarity of shape resonances in electron scattering and photoexcitation occurs because major events in the resonances take place slightly outside the molecular field and receive little influence from the inner structure of the molecule.

Kimura, Mineo [Argonne National Lab., IL (United States)]|[Rice Univ., Houston, TX (United States). Dept. of Physics

1994-06-01

5

C-C bond formation in diiron complexes.  

PubMed

The growing effort to design new sustainable synthetic methodologies, based on readily available and environmentally friendly transition metals, has boosted research on iron complexes. This review article focuses on C-C-bond-forming reactions occurring at bridging ligands in diiron complexes, aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres. A number of diiron-mediated C-C-bond-forming reactions reported in the literature, including nucleophilic and electrophilic additions and insertion and cycloaddition reactions, have been accumulated over the years, which, together with more recent developments, indicate that diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted C-C bond formation. PMID:22760971

Mazzoni, Rita; Salmi, Mauro; Zanotti, Valerio

2012-07-03

6

a Computational Investigation of cC3H2...HX(X = F, Cl, Br) H-Bonded Complexes  

Microsoft Academic Search

Cyclopropenylidene (c-C3H2) is of significant importance in interstellar chemistry and synthetic chemistry (e.g., transition metal and organic catalysis). Because of its peculiar structure, c-C3H2 can act as a hydrogen-bond donor or acceptor. In order to gain insight into this feature, the ground-state potential energy surfaces of singlet c-C3H2 complexed with hydrogen halides HX (X = F, Cl, Br) have been

Pradeep R. Varadwaj; Arpita Varadwaj; Gilles H. Peslherbe

2011-01-01

7

Determination of effective potentials for the stretching of C? ? C? virtual bonds in polypeptide chains for coarse-grained simulations of proteins from ab initio energy surfaces of N-methylacetamide and N-acetylpyrrolidine  

PubMed Central

The potentials of mean force (PMF’s) for the deformation of the C? ? C? virtual bonds in polypeptide chains were determined from the diabatic energy surfaces of N-methylacetamide (modeling regular peptide groups) and N-acetylpyrrolidine (modeling the peptide groups preceding proline), calculated at the Møller-Plesset (MP2) ab initio level of theory with the 6-31G(d,p) basis set. The energy surfaces were expressed in the C? ? C? virtual-bond length (d) and the H-N-C? ? C? improper dihedral angle (?) that describes the pyramidicity of the amide nitrogen, or in the C?-C?(O)-N-C? dihedral angle (?) and the angle ?. For each grid point, the potential energy was minimized with respect to all remaining degrees of freedom. The PMF’s obtained from the (d, ?) energy surfaces produced realistic free-energy barriers to the trans-cis transition (10 kcal/mol and 13 kcal/mol for the regular and proline peptide groups, respectively, compared to 12.6 – 13.9 kcal/mol and 17.3 – 19.6 kcal/mol determined experimentally for glycylglycine and N-acylprolines, respectively), while those obtained from the (?, ?) energy maps produced either low-quality PMF curves when direct Boltzmann summation was implemented to compute the PMF’s or too-flat curves with too-low free-energy barriers to the trans-cis transition if harmonic extrapolation was used to estimate the contributions to the partition function. An analytical bimodal logarithmic-Gaussian expression was fitted to the PMF’s, and the potentials were implemented in the UNRES force field. Test Langevin-dynamics simulations were carried out for the Gly-Gly and Gly-Pro dipeptides, which showed a 106-fold increase of the simulated rate of the trans-cis isomerization with respect to that measured experimentally; effectively the same result was obtained with the analytical Kramers theory of reaction rate applied to the UNRES representation of the peptide groups. Application of Kramers’ theory to compute the rate constants from the all-atom ab initio energy surfaces of the model compounds studied resulted in isomerization rates close to the experimental values, which demonstrates that the increase of the isomerization rate in UNRES simulations results solely from averaging out the secondary degrees of freedom.

Sieradzan, Adam K.; Scheraga, Harold A.; Liwo, Adam

2012-01-01

8

Determination of effective potentials for the stretching of C(?) ? C(?) virtual bonds in polypeptide chains for coarse-grained simulations of proteins from ab initio energy surfaces of N-methylacetamide and N-acetylpyrrolidine.  

PubMed

The potentials of mean force (PMF's) for the deformation of the C(?) ? C(?) virtual bonds in polypeptide chains were determined from the diabatic energy surfaces of N-methylacetamide (modeling regular peptide groups) and N-acetylpyrrolidine (modeling the peptide groups preceding proline), calculated at the Møller-Plesset (MP2) ab initio level of theory with the 6-31G(d,p) basis set. The energy surfaces were expressed in the C(?) ? C(?) virtual-bond length (d) and the H-N-C(?) ? C' improper dihedral angle (?) that describes the pyramidicity of the amide nitrogen, or in the C(?)-C'(O)-N-C(?) dihedral angle (?) and the angle ?. For each grid point, the potential energy was minimized with respect to all remaining degrees of freedom. The PMF's obtained from the (d, ?) energy surfaces produced realistic free-energy barriers to the trans-cis transition (10 kcal/mol and 13 kcal/mol for the regular and proline peptide groups, respectively, compared to 12.6 - 13.9 kcal/mol and 17.3 - 19.6 kcal/mol determined experimentally for glycylglycine and N-acylprolines, respectively), while those obtained from the (?, ?) energy maps produced either low-quality PMF curves when direct Boltzmann summation was implemented to compute the PMF's or too-flat curves with too-low free-energy barriers to the trans-cis transition if harmonic extrapolation was used to estimate the contributions to the partition function. An analytical bimodal logarithmic-Gaussian expression was fitted to the PMF's, and the potentials were implemented in the UNRES force field. Test Langevin-dynamics simulations were carried out for the Gly-Gly and Gly-Pro dipeptides, which showed a 10(6)-fold increase of the simulated rate of the trans-cis isomerization with respect to that measured experimentally; effectively the same result was obtained with the analytical Kramers theory of reaction rate applied to the UNRES representation of the peptide groups. Application of Kramers' theory to compute the rate constants from the all-atom ab initio energy surfaces of the model compounds studied resulted in isomerization rates close to the experimental values, which demonstrates that the increase of the isomerization rate in UNRES simulations results solely from averaging out the secondary degrees of freedom. PMID:23087598

Sieradzan, Adam K; Scheraga, Harold A; Liwo, Adam

2012-02-24

9

C-C and C-Heteroatom Bond Dissociation Energies in CH3R?C(OH)2: Energetics for Photocatalytic Processes of Organic Diolates on TiO2 Surfaces  

SciTech Connect

The bond energies of a range of gem-diols, CH3R?C(OH)2 (R? = H, F, Cl, Br, CN, NO2, CF3, CH3CH2, CH3CH2CH2, CH3CH2CH2CH2, ((CH3)2)CH, (CH3)3C, ((CH3)2CH)CH2, (CH3CH2)(CH3)CH, C6H5 (CH3CH2)(CH3)CH) which serve as models for binding to a surface have been studied with density functional theory (DFT) and the molecular orbital G3(MP2) methods to provide thermodynamic data for the analysis of the photochemistry of ketones on TiO2. The ultraviolet (UV) photon-induced photodecomposition of adsorbed acetone and 3,3-dimethylbutanone on the rutile TiO2 (110) surface have been investigated with photon stimulated desorption (PSD) and temperature programmed desorption (TPD). The C-CH3 and C-C(R?) bond dissociation energies in CH3R?C(OH)2 were predicted, and our calculated bond dissociation energies are in excellent agreement with the available experimental values. We used a series of isodemic reactions to provide small corrections to the various bond dissociation energies. The calculated bond dissociation energies are in agreement with the observed photodissociation processes except for R? = CF3, suggesting that these processes are under thermodynamic control. For R? = CF3, reaction dynamics also play a role in determining the photodissociation mechanism. The gas phase Brönsted acidities of the gem-diols were calculated. For three molecules, R? = Cl, Br, and NO2, loss of a proton leads to the formation of a complex of acetic acid with the anion Cl-, Br-, and NO2-. The acidities of these three species are very high with the former two having acidities comparable to CF3SO3H. The ketones (R?RC(=O)) are weak Lewis acids except where addition of OH- leads to the dissociation of the complex to form an anion bonded to acetic acid, R' = NO2, Cl, and Br. The X-C bond dissociation energies for a number of X-CO2- species were calculated and these should be useful in correlating with photochemical reactivity studies.

Wang, Tsang-Hsiu; Dixon, David A.; Henderson, Michael A.

2010-08-26

10

The Energy Profile for Rotation about the C-C Bond in Substituted Ethanes: A Multi-Part Experimental Computational Project for the Physical Chemistry Laboratory  

NASA Astrophysics Data System (ADS)

This paper describes a multi-part project for the physical chemistry lab that includes both experimental and computational approaches to the investigation of the energy profile for internal rotation in substituted ethanes. Experimental approaches include dipole moment determinations, high-resolution nmr spectroscopy (including coupling constant determinations from the analysis of C-13 satellites), and infrared spectral analysis, all directed toward the goal of determining the relative populations - and thereby the relative energies - of low energy rotamers of a substituted ethane as a function of substituent, temperature and solvent composition. These analyses are in turn compared with the results of theoretical calculations involving both molecular mechanics and quantum mechanics, including both semi-empirical and ab initio methods. The whole project takes 4-5 lab periods and provides a good example of the close interaction between theory and experiment that is typical of research in physical chemistry.

Erickson, Luther E.; Morris, Kevin F.

1998-07-01

11

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

12

Activation of Ethane C-H and C-C Bonds by Gas Phase Th+ and U+: A Theoretical Study  

NASA Astrophysics Data System (ADS)

Two different approaches of density functional theory were used to analyze the C-H and C-C bond activation mechanisms during the reaction of bare Th+ and U+ ions with ethane. We report a complete exploration of the potential energy surfaces taking into consideration different spin states. According to B3LYP/SDD computations the double dehydrogenation of C2H6 is thermodynamically favorable only in the case of Th+. It is shown that the overall C-H and C-C bond activation processes are exothermic in the case of Th+ and endothermic for U+. In both cases, the C-C insertion transition state barrier exceeds the energy of the ground state reactants, preventing the observation of these species under thermal conditions.

di Santo, E.; Michelini, M. C.; Russo, N.

2009-09-01

13

Activation of Propane C-H and C-C Bonds by Gas-Phase Pt Atom: A Theoretical Study  

PubMed Central

The reaction mechanism of the gas-phase Pt atom with C3H8 has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C3H8 reaction, the major and minor reaction channels lead to PtC3H6 + H2 and PtCH2 + C2H6, respectively, whereas the possibility to form products PtC2H4 + CH4 is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC3H6 + H2, involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ?E?int, which is the actual interaction energy between the deformed reactants in the transition state.

Li, Fang-Ming; Yang, Hua-Qing; Ju, Ting-Yong; Li, Xiang-Yuan; Hu, Chang-Wei

2012-01-01

14

Stereoselective C-C bond formation catalysed by engineered carboxymethylproline synthases  

NASA Astrophysics Data System (ADS)

The reaction of enol(ate)s with electrophiles is used extensively in organic synthesis for stereoselective C-C bond formation. Protein-based catalysts have had comparatively limited application for the stereoselective formation of C-C bonds of choice via enolate chemistry. We describe protein engineering studies on 5-carboxymethylproline synthases, members of the crotonase superfamily, aimed at enabling stereoselective C-C bond formation leading to N-heterocycles via control of trisubstituted enolate intermediates. Active site substitutions, including at the oxyanion binding site, enable the production of substituted N-heterocycles in high diastereomeric excesses via stereocontrolled enolate formation and reaction. The results reveal the potential of the ubiquitous crotonase superfamily as adaptable catalysts for the control of enolate chemistry.

Hamed, Refaat B.; Gomez-Castellanos, J. Ruben; Thalhammer, Armin; Harding, Daniel; Ducho, Christian; Claridge, Timothy D. W.; Schofield, Christopher J.

2011-05-01

15

Vibrational overtone spectroscopy, energy levels, and intensities of (CH3)3C-C?C-H.  

PubMed

The vibrational overtone spectra of the acetylenic (?? = 4, 5) and methyl (?? = 5, 6) C-H stretch transitions of tert-butyl acetylene [(CH(3))(3)C-C?C-H] were obtained using the phase shift cavity ring down (PS-CRD) technique at 295 K. The C-H stretch fundamental and overtone absorptions of the acetylenic (?? = 2 and 3) and methyl (?? = 2-4) C-H bonds have been obtained using a Fourier transform infrared and near-infrared spectrophotometer. Harmonic frequency ?(?(1)) and anharmonicities x(?(1)) and x(?(1), ?(24)) are reported for the acetylenic C-H bond. Molecular orbital calculations of geometry and vibrational frequencies were performed. A harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. Band strength values were obtained experimentally and compared with intensities calculated in terms of the HCAO model where only the C-H modes are considered. No adjustable parameters were used to get order of magnitude agreement with experimental intensities for all pure local mode C-H transitions. PMID:22263573

Perez-Delgado, Yasnahir; Barroso, Jenny Z; Garofalo, Lauren A; Manzanares, Carlos E

2012-02-22

16

Mechanistic studies of C-C bond cleavage of nitriles by dinuclear metal cryptates.  

PubMed

We previously reported that dinuclear copper(II) cryptate [Cu(2)L](4+) cleaves the C-C bond of acetonitrile at room temperature to produce a cyano-bridged dinuclear cryptate and methanol, whereby the reaction mechanism has not yet become clear. We have now systemically investigated this reaction, and four cryptates, [Cu(2)L](ClO(4))(4) (1), [Zn(2)L](ClO(4))(4) (2), [Cu(2)L(H(2)O)(2)](CF(3)SO(3))(4) (5), and [Cu(2)L(OH)(OH(2))](ClO(4))(3) (6) are reported here. Cryptates 1 and 2 can cleave the C--C bonds of acetonitrile, propionitrile, and benzonitrile at room temperature under open atmospheric conditions to give cyano-bridged cryptates [Cu(2)L(CN)](ClO(4))(3) (3) and [Zn(2)L(CN)](ClO(4))(3) (4), respectively, and the corresponding alcohol. In contrast, 5 and 6 do not show any C-C bond activation of nitriles, as the interior axial positions of Cu(II) in 5 and 6 are occupied by water/OH(-). The C-C bond cleavage of (S)-(+)-2-methylbutyronitrile by 2 produced (R)-(-)-2-butanol only; that is, the cleavage reaction proceeds through an S(N)2 pathway (Walden inversion). PMID:19810059

Yang, Li-Zi; Li, Yu; Zhuang, Xiao-Mei; Jiang, Long; Chen, Jia-Mei; Luck, Rudy L; Lu, Tong-Bu

2009-11-16

17

Microbial aldolases as C–C bonding enzymes—unknown treasures and new developments  

Microsoft Academic Search

Aldolases are a specific group of lyases that catalyze the reversible stereoselective addition of a donor compound (nucleophile) onto an acceptor compound (electrophile). Whereas most aldolases are specific for their donor compound in the aldolization reaction, they often tolerate a wide range of aldehydes as acceptor compounds. C–C bonding by aldolases creates stereocenters in the resulting aldol products. This makes

Anne K. Samland; Georg A. Sprenger

2006-01-01

18

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation  

NASA Astrophysics Data System (ADS)

The formation of C-C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C-C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and ?-unsaturated reactants generates electrophile-nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C-H bonds.

Bower, John F.; Krische, Michael J.

19

Formation of C-C Bonds via Iridium-Catalyzed Hydrogenation and Transfer Hydrogenation  

PubMed Central

The formation of C–C bonds via catalytic hydrogenation and transfer hydrogenation enables carbonyl and imine addition in the absence of stoichiometric organometallic reagents. In this review, iridium-catalyzed C–C bond-forming hydrogenations and transfer hydrogenations are surveyed. These processes encompass selective, atom-economic methods for the vinylation and allylation of carbonyl compounds and imines. Notably, under transfer hydrogenation conditions, alcohol dehydrogenation drives reductive generation of organoiridium nucleophiles, enabling carbonyl addition from the aldehyde or alcohol oxidation level. In the latter case, hydrogen exchange between alcohols and ?-unsaturated reactants generates electrophile–nucleophile pairs en route to products of hydro-hydroxyalkylation, representing a direct method for the functionalization of carbinol C–H bonds.

Bower, John F.; Krische, Michael J.

2011-01-01

20

Mechanisms of photoinduced C?-C? bond breakage in protonated aromatic amino acids  

NASA Astrophysics Data System (ADS)

Photoexcitation of protonated aromatic amino acids leads to C?-C? bond breakage among other channels. There are two pathways for the C?-C? bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak C?-C? bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.

Lucas, B.; Barat, M.; Fayeton, J. A.; Perot, M.; Jouvet, C.; Grégoire, G.; Brøndsted Nielsen, S.

2008-04-01

21

Density Functional Theory Study of Selectivity Considerations for C–C Versus C–O Bond Scission in Glycerol Decomposition on Pt(111)  

SciTech Connect

Glycerol decomposition via a combination of dehydrogenation, C–C bond scission, and C–O bond scission reactions is examined on Pt(111) with periodic Density Functional Theory (DFT) calculations. Building upon a previous study focused on C–C bond scission in glycerol, the current work presents a ?rst analysis of the competition between C–O and C–C bond cleavage in this reaction network. The thermochemistry of various species produced from C–O bond breaking in glycerol dehydrogenation intermediates is estimated using an extension of a previously introduced empirical correlation scheme, with parameters ?t to DFT calculations. Brønsted–Evans–Polanyi (BEP) relationships are then used to estimate the kinetics of C–O bond breaking. When combined with the previous results, the thermochemical and kinetic analyses imply that, while C–O bond scission may be competitive with C–C bond scission during the early stages of glycerol dehydrogenation, the overall rates are likely to be very low. Later in the dehydrogenation process, where rates will be much higher, transition states for C–C bond scission involving decarbonylation are much lower in energy than are the corresponding transition states for C–O bond breaking, implying that the selectivity for C–C scission will be high for glycerol decomposition on smooth platinum surfaces. It is anticipated that the correlation schemes described in this work will provide an ef?cient strategy for estimating thermochemical and kinetic energetics for a variety of elementary bond breaking processes on Pt(111) and may ultimately facilitate computational catalyst design for these and related catalytic processes.

Liu, Bin; Greeley, Jeffrey P.

2012-05-01

22

C-C, C-O and C-N bond formation via rhodium(III)-catalyzed oxidative C-H activation.  

PubMed

Rhodium(III)-catalyzed direct functionalization of C-H bonds under oxidative conditions leading to C-C, C-N, and C-O bond formation is reviewed. Various arene substrates bearing nitrogen and oxygen directing groups are covered in their coupling with unsaturated partners such as alkenes and alkynes. The facile construction of C-E (E = C, N, S, or O) bonds makes Rh(III) catalysis an attractive step-economic approach to value-added molecules from readily available starting materials. Comparisons and contrasts between rhodium(III) and palladium(II)-catalyzed oxidative coupling are made. The remarkable diversity of structures accessible is demonstrated with various recent examples, with a proposed mechanism for each transformation being briefly summarized (critical review, 138 references). PMID:22377832

Song, Guoyong; Wang, Fen; Li, Xingwei

2012-02-29

23

Molecular geometry in fullerene C60: A direct determination of the bond-length difference dC-C-dC=C  

NASA Astrophysics Data System (ADS)

The molecular structure factor of fullerene C60 has recently been determined with a high accuracy using neutron scattering over a large momentum-transfer range. The two bond lengths dC-C and dC=C have been measured with a precision of 10-3 Å. As the temperature is lowered from 295 K down to 4 K, the single bond is elongated and the double bond shortened, but the radius of the sphere remains almost unchanged. Unfortunately, the covariance matrix obtained from the fit shows that the two lengths are strongly correlated; this means that any error made on the determination of one bond length is compensated by an opposite error on the other bond length without significantly increasing the ?2 and, thus, the exactness of the reported temperature effect could be argued against. We present in this paper an alternative way of processing the data which allows us to remove almost completely the coupling, using an appropriate change of variables. The temperature effect is confirmed and resolved with a better accuracy.

Damay, P.; Leclercq, F.

1994-03-01

24

Rhodium-Catalyzed C-C Bond Formation via Heteroatom-Directed C-H Bond Activation  

SciTech Connect

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic reagents. In addition to providing an atom economical alternative to standard cross - coupling strategies, C-H bond functionalization also reduces the production of toxic by-products, thereby contributing to the growing field of reactions with decreased environmental impact. In the area of C-C bond forming reactions that proceed via a C-H activation mechanism, rhodium catalysts stand out for their functional group tolerance and wide range of synthetic utility. Over the course of the last decade, many Rh-catalyzed methods for heteroatom-directed C-H bond functionalization have been reported and will be the focus of this review. Material appearing in the literature prior to 2001 has been reviewed previously and will only be introduced as background when necessary. The synthesis of complex molecules from relatively simple precursors has long been a goal for many organic chemists. The ability to selectively functionalize a molecule with minimal pre-activation can streamline syntheses and expand the opportunities to explore the utility of complex molecules in areas ranging from the pharmaceutical industry to materials science. Indeed, the issue of selectivity is paramount in the development of all C-H bond functionalization methods. Several groups have developed elegant approaches towards achieving selectivity in molecules that possess many sterically and electronically similar C-H bonds. Many of these approaches are discussed in detail in the accompanying articles in this special issue of Chemical Reviews. One approach that has seen widespread success involves the use of a proximal heteroatom that serves as a directing group for the selective functionalization of a specific C-H bond. In a survey of examples of heteroatom-directed Rh catalysis, two mechanistically distinct reaction pathways are revealed. In one case, the heteroatom acts as a chelator to bind the Rh catalyst, facilitating reactivity at a proximal site. In this case, the formation of a five-membered metallacycle provides a favorable driving force in inducing reactivity at the desired location. In the other case, the heteroatom initially coordinates the Rh catalyst and then acts to stabilize the formation of a metal-carbon bond at a proximal site. A true test of the utility of a synthetic method is in its application to the synthesis of natural products or complex molecules. Several groups have demonstrated the applicability of C-H bond functionalization reactions towards complex molecule synthesis. Target-oriented synthesis provides a platform to test the effectiveness of a method in unique chemical and steric environments. In this respect, Rh-catalyzed methods for C-H bond functionalization stand out, with several syntheses being described in the literature that utilize C-H bond functionalization in a key step. These syntheses are highlighted following the discussion of the method they employ.

Colby, Denise; Bergman, Robert; Ellman, Jonathan

2010-05-13

25

Palladium(II)-catalyzed dehydrogenative cross-coupling between two C(sp3)-H bonds: unexpected C=C bond formation.  

PubMed

C=C bond construction: A palladium-catalyzed oxidative C(sp3)-H/C(sp3)-H cross-coupling is shown to forge C=C bonds rather than C(sp3)-C(sp3) bonds through reactions of indolin-2-ones or benzofuran-2-ones with O-benzoyl hydroxylamines in the absence of an added oxidant. PMID:23780778

Li, Gaocan; Qian, Shengyou; Wang, Chunxia; You, Jingsong

2013-06-18

26

Recent advances in organocatalytic methods for asymmetric c?c bond formation.  

PubMed

Beyond a doubt organocatalysis belongs to the most exciting and innovative chapters of organic chemistry today. Organocatalysis has emerged not only as a complement to metal-catalyzed reactions and to biocatalysis over the last decade, but also provides new asymmetric organocatalyzed reactions that cannot be accomplished by metal- or biocatalyzed reactions so far. A large number of organocatalytic processes are already well established in organic synthesis. Nevertheless, the number of publications in this field is still on the increase; new important results are produced constantly. This review gives a detailed overview of the latest developments and main streams in organocatalyzed asymmetric C?C bond formation processes of the last three years. It is intended to outline the most important current findings focused on especially new synthetic methodologies. PMID:24115407

Scheffler, Ulf; Mahrwald, Rainer

2013-09-20

27

Electrophile-induced C-C bond activation of vinylcyclopropanes for the synthesis of Z-alkylidenetetrahydrofurans.  

PubMed

We present a detailed study on the behavior of vinylcyclopropanes as masked donor-acceptor system toward the stereoselective synthesis of Z-alkylidenetetrahydrofurans. Results of bromenium catalyzed indirect activation of C-C bond of vinylcyclopropanes and concomitant cyclization to alkylidenetetrahydrofuran and other heterocycles have been discussed. The stereoselective formation of the Z-isomer is strongly controlled by the extent of destabilization of one of the gauche conformers of the vinylcyclopropane. The ring-opening/cyclization step was found to be stereospecific as in the case of DA cyclopropanes. The activation of the C-C bond leads to a tight-carbocation intermediate, which is evident from the complete retention of the stereochemistry. The retention of configuration has been established by a necessary control experiment that rules out the possibility of a double inversion pathway. The present results serve as direct stereochemical evidence in support of a tight ion-pair intermediate versus the controversial S(N)2 pathway. A 2D potential energy scan has been carried out at B3LYP/6-31G(d) level theory to obtain the relative energies of the conformers. The Z-selectivity observed has been explained on the basis of the relative population of the conformers and modeling the intermediate and transition state involved in the reaction at M06-2x/6-31+G(d) level. Energy profile for the cyclization step was modeled considering various possible pathways through which cyclization can happen. The methodology has been successfully demonstrated on vinylcyclobutanes as well. PMID:23176087

Ganesh, Venkataraman; Kundu, Taraknath; Chandrasekaran, Srinivasan

2012-12-18

28

Bond cleavage during isobutanol thermal decomposition and the breaking of C-C bonds in alcohols at high temperatures.  

PubMed

Isobutanol was thermally decomposed in a single pulse shock tube under conditions where chain processes were suppressed. The main reaction is the breaking of C-C bonds. Literature rate expressions, experimentally determined, are found in some cases to be in disagreement. The rate expressions for the decomposition processes at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm are k(isobutanol ? isopropyl + hydroxymethyl) + k(isobutanol ? methyl + 1-hydroxypropyl-2) = 10(16.7±0.3) exp(-41097 ± 750) s(-1), where k(isobutanol ? isopropyl + hydroxymethyl) = 10(16.45 ±0.3) exp(-40910 ± 750/T) s(-1) and k(isobutanol ? methyl + 1-hydroxypropyl-2) = 10(16.38±0.3) exp(-41560 ± 750/T) s(-1). These values permit comparisons with recent estimates including those from ab initio calculations. A new procedure is presented that uses information on the kinetics of bond breaking reactions of alkanes and the effect of OH substitution to derive rate coefficients for similar reactions of alcohols. This leads to the following rate expression for the smaller alcohols, at temperatures of 1090 to 1240 K and pressures of 1.5 and 6 atm, k(ethanol ? methyl + hydroxymethyl) = 10(16.42±0.3) exp(-43496 ± 750 K/T) s(-1), k(isopropanol ? methyl + 1-hydroxyethyl) = 10(16.54±0.3) exp(-42495 ± 750 K/T) s(-1), k(n-propanol ? ethyl + hydroxymethyl) = 10(16.43±0.3) exp(-41696 ± 750 K/T) s(-1), and k(n-propanol ? methyl + 2-hydroxymethyl) = 10(16.53±0.3) exp(-42945 ± 750 K/T) s(-1). Extension of this approach to other alcohols is straightforward. The resulting correlations along with the data on dehydration of alcohols provide novel information of the kinetic stability of alcohols. PMID:23968459

Rosado-Reyes, Claudette; Tsang, Wing

2013-09-19

29

Photo-assisted cyanation of transition metal nitrates coupled with room temperature C-C bond cleavage of acetonitrile.  

PubMed

It is a challenge to use acetonitrile as a cyanating agent because of the difficulty in cleaving its C-CN bond. Herein, we report a mild photo-assisted route to conduct the cyanation of transition metal nitrates using acetonitrile as the cyanating agent coupled with room-temperature C-C bond cleavage. DFT calculations and experimental observations suggest a radical-involved reaction mechanism, which excludes toxicity from free cyanide ions. PMID:23282559

Zou, Shihui; Li, Renhong; Kobayashi, Hisayoshi; Liu, Juanjuan; Fan, Jie

2013-01-03

30

Total Synthesis of Bryostatin 7 via C-C Bond Forming Hydrogenation: Merged Redox-Construction Events for Synthetic Efficiency  

PubMed Central

The marine macrolide bryostatin 7 is prepared in 20 steps (longest linear sequence) and 36 total steps. A total of 5 C-C bonds are formed using hydrogenative methods. The present approach represents the most concise synthesis of any bryostatin reported, to date, setting the stage for practical syntheses of simplified functional analogues.

Lu, Yu; Woo, Sang Kook; Krische, Michael J.

2011-01-01

31

Unprecedented Construction of C?C Double Bonds via Ir-Catalyzed Dehydrogenative and Dehydrative Cross-Couplings.  

PubMed

Unprecedented constructions of C?C double bonds have been achieved by Ir-catalyzed intramolecular dehydrogenative and dehydrative cross-coupling of tertiary amines and ketones. The reactions are proposed to proceed via an Ir-mediated C-H activation mechanism. PMID:23621093

Nie, Shao-Zhen; Sun, Xiang; Wei, Wen-Tao; Zhang, Xue-Jing; Yan, Ming; Xiao, Jian-Liang

2013-04-26

32

Metal-organic cooperative catalysis in C-H and C-C bond activation and its concurrent recovery.  

PubMed

The development of an efficient catalytic activation (cleavage) system for C-H and C-C bonds is an important challenge in organic synthesis, because these bonds comprise a variety of organic molecules such as natural products, petroleum oils, and polymers on the earth. Among many elegant approaches utilizing transition metals to activate C-H and C-C bonds facilely, chelation-assisted protocols based on the coordinating ability of an organic moiety have attracted great attention, though they have often suffered from the need for an intact coordinating group in a substrate. In this Account, we describe our entire efforts to activate C-H or C-C bonds adjacent to carbonyl groups by employing a new concept of metal-organic cooperative catalysis (MOCC), which enables the temporal installation of a 2-aminopyridyl group into common aldehydes or ketones in a catalytic way. Consequently, a series of new catalytic reactions such as alcohol hydroacylation, oxo-ester synthesis, C-C triple bond cleavage, hydrative dimerization of alkynes, and skeletal rearrangements of cyclic ketones was realized through MOCC. In particular, in the quest for an optimized MOCC system composed of a Wilkinson's catalyst (Ph 3P) 3RhCl and an organic catalyst (2-amino-3-picoline), surprising efficiency enhancements could be achieved when benzoic acid and aniline were introduced as promoters for the aldimine formation process. Furthermore, a notable accomplishment of C-C bond activation has been made using 2-amino-3-picoline as a temporary chelating auxiliary in the reactions of unstrained ketones with various terminal olefins and Wilkinson's catalyst. In the case of seven-membered cyclic ketones, an interesting ring contraction to five- or six-membered ones takes place through skeletal rearrangements initiated by the C-C bond activation of MOCC. On the other hand, the fundamental advances of these catalytic systems into recyclable processes could be achieved by immobilizing both metal and organic components using a hydrogen-bonded self-assembled system as a catalyst support. This catalyst-recovery system provides a homogeneous phase at high temperature during the reaction and a heterogeneous phase at room temperature after the reaction. The product could be separated conveniently from the self-assembly support system by decanting the upper layer. The immobilized catalysts of both 2-aminopyridine and rhodium metal species sustained high catalytic activity for up to the eight catalytic reactions. In conclusion, the successful incorporation of an organocatalytic cycle into a transition metal catalyzed reaction led us to find MOCC for C-H and C-C bond activation. In addition, the hydrogen-bonded self-assembled support has been developed for an efficient and effective recovery system of homogeneous catalysts and could be successful in immobilizing both metal and organic catalysts. PMID:18247521

Park, Young Jun; Park, Jung-Woo; Jun, Chul-Ho

2008-02-02

33

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

PubMed

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

Zhang, Chun; Feng, Peng; Jiao, Ning

2013-09-26

34

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

PubMed

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

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

2013-09-17

35

Efficient approach to 1,2-diazepines via formal diazomethylene insertion into the C-C bond of cyclobutenones.  

PubMed

Efficient monocyclic 1,2-diazepine formation via a tandem electrocyclization reaction of cyclobutenones with lithiodiazoacetate is demonstrated. The reaction proceeds through an oxy anion-accelerated 4?-ring opening of cyclobutene followed by an 8?-ring closure of the resultant oxy anion-substituted diazo-diene under mild conditions to furnish a 1,2-diazepine via formal diazomethylene insertion into the C-C bond of cyclobutenone. PMID:22721410

Sugimoto, Kenji; Hayashi, Rie; Nemoto, Hideo; Toyooka, Naoki; Matsuya, Yuji

2012-06-21

36

Raman spectroscopic determination of the length, strength, compressibility, Debye temperature, elasticity, and force constant of the C-C bond in graphene.  

PubMed

From the perspective of bond relaxation and bond vibration, we have formulated the Raman phonon relaxation of graphene, under the stimuli of the number-of-layers, the uni-axial strain, the pressure, and the temperature, in terms of the response of the length and strength of the representative bond of the entire specimen to the applied stimuli. Theoretical unification of the measurements clarifies that: (i) the opposite trends of the Raman shifts, which are due to the number-of-layers reduction, of the G-peak shift and arises from the vibration of a pair of atoms, while the D- and the 2D-peak shifts involve the z-neighbor of a specific atom; (ii) the tensile strain-induced phonon softening and phonon-band splitting arise from the asymmetric response of the C(3v) bond geometry to the C(2v) uni-axial bond elongation; (iii) the thermal softening of the phonons originates from bond expansion and weakening; and (iv) the pressure stiffening of the phonons results from bond compression and work hardening. Reproduction of the measurements has led to quantitative information about the referential frequencies from which the Raman frequencies shift as well as the length, energy, force constant, Debye temperature, compressibility and elastic modulus of the C-C bond in graphene, which is of instrumental importance in the understanding of the unusual behavior of graphene. PMID:22105904

Yang, X X; Li, J W; Zhou, Z F; Wang, Y; Yang, L W; Zheng, W T; Sun, Chang Q

2011-11-21

37

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

PubMed

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

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

2012-12-14

38

Predicting kinetically unstable c?c bonds from the ground-state properties of a molecule.  

PubMed

Determining the weakest link: A simple analysis pinpoints kinetically unstable carbon-carbon bonds. No prior knowledge of reaction pathways is necessary and the approach is particularly helpful for the prediction of novel molecules. krel =relaxed force constant, req =bond length. PMID:23946235

Markopoulos, Georgios; Grunenberg, Jörg

2013-08-14

39

Unlocking Hydrogenation for C-C Bond Formation: A Brief Overview of Enantioselective Methods  

PubMed Central

Hydrogenation of ?-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and ?-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts.

Hassan, Abbas; Krische, Michael J.

2011-01-01

40

Oxidative addition of the ethane C-C bond to Pd. An ab initio benchmark and DFT validation study.  

PubMed

We have computed a state-of-the-art benchmark potential energy surface (PES) for the archetypal oxidative addition of the ethane C-C bond to the palladium atom and have used this to evaluate the performance of 24 popular density functionals, covering LDA, GGA, meta-GGA, and hybrid density functionals, for describing this reaction. The ab initio benchmark is obtained by exploring the PES using a hierarchical series of ab initio methods [HF, MP2, CCSD, CCSD(T)] in combination with a hierarchical series of five Gaussian-type basis sets, up to g polarization. Relativistic effects are taken into account either through a relativistic effective core potential for palladium or through a full four-component all-electron approach. Our best estimate of kinetic and thermodynamic parameters is -10.8 (-11.3) kcal/mol for the formation of the reactant complex, 19.4 (17.1) kcal/mol for the activation energy relative to the separate reactants, and -4.5 (-6.8) kcal/mol for the reaction energy (zero-point vibrational energy-corrected values in parentheses). Our work highlights the importance of sufficient higher angular momentum polarization functions for correctly describing metal-d-electron correlation. Best overall agreement with our ab initio benchmark is obtained by functionals from all three categories, GGA, meta-GGA, and hybrid DFT, with mean absolute errors of 1.5 to 2.5 kcal/mol and errors in activation energies ranging from -0.2 to -3.2 kcal/mol. Interestingly, the well-known BLYP functional compares very reasonably with a slight underestimation of the overall barrier by -0.9 kcal/mol. For comparison, with B3LYP we arrive at an overestimation of the overall barrier by 5.8 kcal/mol. On the other hand, B3LYP performs excellently for the central barrier (i.e., relative to the reactant complex) which it underestimates by only -0.1 kcal/mol. PMID:15880815

De Jong, G Theodoor; Geerke, Daan P; Diefenbach, Axel; Solà, Miquel; Bickelhaupt, F Matthias

2005-07-30

41

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

SciTech Connect

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

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

2013-01-01

42

To protonate or alkylate? Stereoselective Brønsted acid catalysis of C-C bond formation using diazoalkanes.  

PubMed

A new means to activate diazoalkanes has been discovered and applied broadly over the past few years. Brønsted acids, both achiral and chiral, have been used to promote the formation of carbon-carbon and carbon-heteroatom bonds with a growing number of diazoalkane derivatives. Aside from their straightforward ability to build structural and stereochemical complexity in innovative new ways, these transformations are remarkable owing to their ability to skirt competitive diazo protonation--a reaction that has long been used to prepare esters efficiently and cleanly from carboxylic acids. In cases where achiral Brønsted acids are used, high diastereoselection can be achieved. Meanwhile, chiral Brønsted acids can deliver products with both high diastereo- and enantioselectivity. More recently, systems have emerged that combine Brønsted acids and either Lewis acids or transition metals to promote carbon-carbon bond formation from diazoalkanes. PMID:20209537

Johnston, Jeffrey N; Muchalski, Hubert; Troyer, Timothy L

2010-03-22

43

Substituent effect on reactivity of triplet excited state of 2,3-diazabicyclo[2.2.1]hept-2-enes, DBH derivatives: ? C-N bond cleavage versus ? C-C bond cleavage.  

PubMed

The photoreaction of a series of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, in which various substituents (X) were introduced at the methano bridge carbon of C(7), was investigated under direct (>290 nm) and triplet-sensitized (Ph2CO, >370 nm) irradiation conditions of the azo chromophore (?C?–C?–N?N–C?–C??). The azo compounds offered a unique opportunity to see the substituent X effect at the remote position of the azo chromophore on the reactivity of the triplet excited state of bicyclic azoalkanes. The present study led to the first observation of the unusual C?–C? bond-cleavage reaction without the ring stiffness, that is, the structural rigidity, in the triplet state of the cyclic azoalkanes. The stereoelectronic effects were found to play an important role in lowering the activation energy of the C?–C? bond-cleavage reaction. NBO analyses at the M06-2X/cc-PVDZ level of theory confirmed the stereoelectronic effect. PMID:22985271

Abe, Manabu; Watanabe, Shinji; Tamura, Hiroshi; Boinapally, Srikanth; Kanahara, Kousei; Fujiwara, Yoshihisa

2012-10-01

44

Energy transport via coordination bonds  

NASA Astrophysics Data System (ADS)

Vibrational energy transport in transition metal complexes involves stages where energy crosses relatively weak coordination bonds between a coordinated metal atom and the ligands. Understanding the energy transport rules on a molecular level is fundamentally important; it is also essential in relation to a recently proposed structural method, the relaxation-assisted two-dimensional infrared (RA 2DIR) technique, where the vibrational population transport time across the molecule of interest is linked to the transport distance. In this study we report on the energy transport across coordination bonds in tetraethylammonium bis(maleonitriledithiolate)iron(III)nitrosyl complex, studied using dual-frequency RA 2DIR spectroscopy. Three mode pairs, C?N and N=O, N=O and C?N, and N=O and C-C, were interrogated. All three cross-peaks show substantial amplification due to vibrational energy transport from the initially excited mode toward the ``probed'' mode, including a record amplification of 27-fold observed for the C?N/N=O cross-peak. A ninefold amplification measured for the N=O/C?N cross-peak, where the ``probed'' CN mode has higher frequency than the initially excited NO, proves unequivocally that the excitation of the ``probed'' mode via energy transport is not essential for observing stronger cross-peaks and that lower frequency modes serve as the energy accepting modes. A simple modeling of the energy transport is presented highlighting the role of a spatial overlap of the interacting modes. The observed strong cross-peak amplifications and a correlation between the energy transport time and the intermode distance, the distance between atom pairs on which vibrational excitations predominantly reside, demonstrate an applicability of the RA 2DIR method for structural interrogation of transition metal complexes.

Kasyanenko, Valeriy M.; Lin, Zhiwei; Rubtsov, Grigory I.; Donahue, James P.; Rubtsov, Igor V.

2009-10-01

45

Theoretical elucidation of the mechanism of cleavage of the aromatic C-C bond in quinoxaline by a tungsten-based complex [W(PMe3)4(?2-CH2PMe2)H].  

PubMed

The aromatic C-C bond cleavage by a tungsten complex reported recently by Sattler and Parkin offers fresh opportunities for the functionalization of organic molecules. The mechanism of such a process has not yet been determined, which appeals to computational assistance to understand how the unstrained C-C bond is activated at the molecular level. In this work, by performing density functional theory calculations, we studied various possible mechanisms of cleavage of the aromatic C-C bond in quinoxaline (QoxH) by the W-based complex [W(PMe(3))(4)(?(2)-CH(2)PMe(2))H]. The calculated results show that the mechanism proposed by Sattler and Parkin involves an overall barrier of as high as 42.0 kcal mol(-1) and thus does not seem to be consistent with the experimental observation. Alternatively, an improved mechanism has been presented in detail, which involves the removal and recoordination of a second PMe(3) ligand on the tungsten center. In our new mechanism, it is proposed that the C-C cleavage occurs prior to the second C-H bond addition, in contrast to Sattler and Parkin's mechanism in which the C-C bond is broken after the second C-H bond addition. We find that the rate-determining step of the reaction is the ring-opening process of the tungsten complex with an activation barrier of 28.5 kcal mol(-1) after the first PMe(3) ligand dissociation from the metal center. The mono-hydrido species is located as the global minimum on the potential-energy surface, which is in agreement with the experimental observation for this species. The present theoretical results provide new insight into the mechanism of the remarkable C-C bond cleavage. PMID:23055438

Liu, Yuxia; Zhang, Dongju; Gao, Jun; Liu, Chengbu

2012-10-10

46

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

PubMed

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

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

2013-01-18

47

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

PubMed

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

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

2009-02-10

48

C?-C bond cleavage of the peptide backbone in MALDI in-source decay using salicylic acid derivative matrices.  

PubMed

The use of 5-formylsalicylic acid (5-FSA) and 5-nitrosalicylic acid (5-NSA) as novel matrices for in-source decay (ISD) of peptides in matrix-assisted laser desorption/ionization (MALDI) is described. The use of 5-FSA and 5-NSA generated a- and x-series ions accompanied by oxidized peptides [M - 2 H + H](+). The preferential formation of a- and x-series ions was found to be dependent on the hydrogen-accepting ability of matrix. The hydrogen-accepting ability estimated from the ratio of signal intensity of oxidized product [M - 2 H + H](+) to that of non-oxidized protonated molecule [M + H](+) of peptide was of the order 5-NSA > 5-FSA > 5-aminosalicylic acid (5-ASA) ? 2,5-dihydroxyl benzoic acid (2,5-DHB) ? 0. The results suggest that the hydrogen transfer reaction from peptide to 5-FSA and 5-NSA occurs during the MALDI-ISD processes. The hydrogen abstraction from peptides results in the formation of oxidized peptides containing a radical site on the amide nitrogen with subsequent radical-induced cleavage at the C?-C bond, leading to the formation of a- and x-series ions. The most significant feature of MALDI-ISD with 5-FSA and 5-NSA is the specific cleavage of the C?-C bond of the peptide backbone without degradation of side-chain and post-translational modifications (PTM). The matrix provides a useful complementary method to conventional MALDI-ISD for amino acid sequencing and site localization of PTMs in peptides. PMID:21953105

Asakawa, Daiki; Takayama, Mitsuo

2011-04-19

49

Two coordination polymers created via in situ ligand synthesis involving C-N and C-C bond formation.  

PubMed

We report the synthesis and crystal structures of two transition metal-based coordination polymers comprising ligand molecules not included in the original reaction mixtures but instead formed in situ during hydrothermal treatment. Zinc meso-iminodisuccinate hydrate (I), Zn(2)(C(8)H(7)NO(8)).0.57H(2)O, formed from zinc acetate and L-aspartic acid, and tetraaquanickel(II) 5,10-dioxo-5,10-dihydro-4,9-dioxa-pyrene-2,7-dicarboxylate (II), Ni(H(2)O)(4)(C(16)H(4)O(8)), formed from nickel acetate and 5-hydroxyisophthalic acid. We show that the formation of I takes place via a fumaric acid intermediate, while the formation of II requires the formation of a new C-C bond. The structure of I consists of weakly interacting sheets, while the structure of II consists of strongly hydrogen-bonded chains. Crystal data: for I, P2(1)/n (14), a = 10.073 A, b = 9.894 A, c = 12.053 A, beta = 105.605 degrees, V = 1156.87(13) A(3), Z = 4; for II, P1 (2), a = 5.011 A, b = 6.526 A, c = 12.305 A, alpha = 76.868 degrees, beta = 84.988 degrees, gamma = 87.619 degrees, V = 390.3(4) A(3), Z = 1. PMID:17854172

Feller, Russell K; Forster, Paul M; Wudl, Fred; Cheetham, Anthony K

2007-09-14

50

Four-Electron Donor Hemilabile n3-PPh3 Ligand that Binds Through a C = C Bond Rather than an Agostic C-H Interaction, and Displacement of the C = C by Methyl Iodide or Water  

SciTech Connect

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+ BAr'4 [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(?3 PPh3)Mo]+[BAr'4] . Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is four-electron donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)2(?3 PPh3)Mo]+ and [Cp(CO)2(?3 PH2Ph)Mo]+, and DFT/B3LYP on [Cp(CO)2(?3 PHtBuPh)Mo]+ and [Cp(CO)2(?3 PH2Ph)Nb]) provide further information on the bonding, and on the preference for bonding of the metal to the C=C bond rather than an agostic C H interaction found in many related complexes. The hemilabile C=C bond is readily displaced by CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(?3 PPh3)Mo]+ + ICH3 to give [Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq = 5.2 ? 102 M 1 in CD2Cl2 at 22 °C.

Cheng, Tan-Yun; Szalda, David J.; Hanson, Jonathan C.; Muckerman, J. T.; Bullock, R. Morris

2008-08-11

51

Hydrocarbon bond dissociation energies  

SciTech Connect

The best available values for homolytic bond dissociation energies (BDEs) of various classes of neutral compounds are considered in this review. (BDEs in ionic species is a legitimate subject that is touched on briefly and could easily be included in a longer review. The same can be said for heterolytic BDEs, which are not reviewed as such, although some of the ionic thermochemical data discussed yield values for these processes.) The major emphasis is on hydrocarbons and their nitrogen, oxygen, sulfur, halogen, and silicon-containing derivations, but limited data for inorganic molecules are included. The focus is particularly on prototypical radicals whose heats of formation, formerly thought to be well in hand, have recently been called into serious question. The intent is to include all the major types of sigma bonds, if not all specific cases where known or estimatable heats of formation allow bond dissociation energies to be generated. This review attempts to acknowledge all the standard techniques for measuring BDEs in polyatomic molecules and to offer critical analysis of selected portions of the literature. This leaves values that the authors recommend as the most likely to be correct at the time of this writing. 246 references, 9 tables.

McMillen, D.F.; Golden, D.M.

1982-01-01

52

Characterization of adhesive bonded lap joints of C\\/C–SiC composite and Ti–6Al–4V alloy under varying conditions  

Microsoft Academic Search

The effect of glueline thickness, glueline length, etching time, temperature, exposure time and strain rate on C\\/C–SiC composite and Ti–6Al–4V alloy adherences were examined before and after adhesive bonding. The results show that the C\\/C–SiC adherence is more sensitive to strain rate and temperature than the Ti–6Al–4V adherence because of poor oxidizing resistance and high brittleness.

V. K Srivastava

2003-01-01

53

Twinning and multiaxial cyclic plasticity of a low stacking-fault-energy F. C. C. alloy  

Microsoft Academic Search

Out-of-phase cyclic tension and torsion tests followed by metallographic measurements are carried out on Co33Ni specimens. It appears that, even for small strain ranges, nonproportional loadings induce profuse mechanical twinning and a strong kinematic hardening in this low stacking-fault-energy f.c.c. alloy. These results are analyzed and an attempt to incorporate twinning into micromechanical simulations of f.c.c. polycrystals elastoplastic behaviour is

V DOQUET

1993-01-01

54

Bond energy analysis revisited and designed toward a rigorous methodology  

NASA Astrophysics Data System (ADS)

The present study theoretically revisits and numerically assesses two-body energy decomposition schemes including a newly proposed one. The new decomposition scheme is designed to make the equilibrium bond distance equivalent with the minimum point of bond energies. Although the other decomposition schemes generally predict the wrong order of the C-C bond strengths of C2H2, C2H4, and C2H6, the new decomposition scheme is capable of reproducing the C-C bond strengths. Numerical assessment on a training set of molecules demonstrates that the present scheme exhibits a stronger correlation with bond dissociation energies than the other decomposition schemes do, which suggests that the new decomposition scheme is a reliable and powerful analysis methodology.

Nakai, Hiromi; Ohashi, Hideaki; Imamura, Yutaka; Kikuchi, Yasuaki

2011-09-01

55

C-C coupling reactions of superstrong CF3 groups with C(sp2)-H bonds: reactivity and synthetic utility of zero-valent niobium catalyst.  

PubMed

It was found that zero-valent niobium is an efficient catalyst for the intramolecular C-C coupling reactions of o-aryl and o-alkenyl alpha,alpha,alpha-trifluorotoluene derivatives. The superstrong C-F bonds of CF3 groups and neighboring C(sp2)-H bonds were doubly activated, and fluorenes and indenes were obtained in good yields. The niobium fluorocarbenoid species is proposed to be the key intermediate. PMID:18175305

Fuchibe, Kohei; Mitomi, Ken; Suzuki, Ryo; Akiyama, Takahiko

2008-02-01

56

FeCl2-promoted cleavage of the unactivated C-C bond of alkylarenes and polystyrene: direct synthesis of arylamines.  

PubMed

Ironing it out: an efficient and convenient nitrogenation strategy involving C-C bond cleavage for the straightforward synthesis of versatile arylamines is presented. Various alkyl azides and alkylarenes, including the common industrial by-product cumene, react using this protocol. Moreover, this method provides a potential strategy for the degradation of polystyrene. PMID:22689538

Qin, Chong; Shen, Tao; Tang, Conghui; Jiao, Ning

2012-06-11

57

A determination of the stacking-fault energy of some pure F.C.C. metals  

Microsoft Academic Search

A method for determining the stacking-fault energy of f.c.c. materials has been developed using an already-established dependence of preferred orientation on stacking-fault energy. An empirically determined parameter, characteristic of the type of rolling texture exhibited by the metal, is related to the stacking-fault energy value obtained by measurements of dislocation node radius, and from the ? value for the three

I. L. Dillamore; R. E. Smallman; W. T. Roberts

1964-01-01

58

Effect of nitrogen on stacking fault energy of f.c.c. iron-based alloys  

SciTech Connect

A method is proposed to estimate the stacking fault energies of face-centered-cubic (f.c.c.) iron-based alloys. The segregation of alloying elements to stacking faults and the interaction of substitutional and interstitial alloying elements in solid solution and their effect on stacking fault energy have been taken into account. It is shown that at low nitrogen concentrations (e.g., 0.05 wt%), the stacking fault energy is increased mainly due to the effect arising from the bulk of the alloy. At high nitrogen concentration, (e.g., 0.5 wt%), the stacking fault energy is decreased due to the segregation of the alloying elements (mainly nitrogen) on the stacking faults of the alloy. Moreover, it is shown that in nitrogen alloying of f.c.c. iron-based alloys the magnetic contribution of the nitrogen to the stacking fault energy is negligible. The method shows reasonable agreement with existing experimental data.

Yakubtsov, I.A.; Ariapour, A.; Perovic, D.D. [Univ. of Toronto, Ontario (Canada). Dept. of Metallurgy and Materials Science

1999-03-10

59

Effect of nitrogen on stacking fault energy of f.c.c. iron-based alloys  

Microsoft Academic Search

A method is proposed to estimate the stacking fault energies of face-centered-cubic (f.c.c.) iron-based alloys. The segregation of alloying elements to stacking faults and the interaction of substitutional and interstitial alloying elements in solid solution and their effect on stacking fault energy have been taken into account. It is shown that at low nitrogen concentrations (e.g. 0.05wt%), the stacking fault

I. A. Yakubtsov; A. Ariapour; D. D. Perovic

1999-01-01

60

Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament--evidence of both ring C-C and ring Si-C bond cleavages.  

PubMed

The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si-CH(3) bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C-C bond and a ring Si-C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C-C bond is dominant over that of propene via an initial cleavage of a ring Si-C bond. When the collision-free condition is voided, secondary reactions in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene. PMID:20049692

Tong, L; Shi, Y J

2010-02-01

61

Acetate C–C bond formation and decomposition in the anaerobic world: the structure of a central enzyme and its key active-site metal cluster  

Microsoft Academic Search

The structure of carbon monoxide dehydrogenase\\/acetyl-coenzyme A synthase (CODH\\/ACS), a central enzyme in the anaerobic metabolism of acetyl-coenzyme A (acetyl-CoA), has been solved to a resolution of 2.2Å. The active-site metal cluster responsible for catalyzing acetyl C–C bond synthesis and cleavage, designated the A center, was identified as an Fe4S4 iron sulfur cluster with one of its cysteine thiolates acting

David A. Grahame

2003-01-01

62

C-C bond coupling between the organometallic cations CH3Ag2+, CH3Cu2+ and CH3AgCu+ and allyliodide.  

PubMed

Electrospray ionisation on a mixture of AgNO(3) (in MeOH/H(2)O/acetic acid), (CH(3)CO(2))(2)Cu (in MeOH) and acetic acid (in MeOH) yields the metal carboxylate cations CH(3)CO(2)Ag(2)(+), CH(3)CO(2)AgCu(+) and CH(3)CO(2)Cu(2)(+). Collision induced dissociation of these carboxylate cations yields the organometallic cations CH(3)Ag(2)(+), CH(3)AgCu(+) and CH(3)Cu(2)(+). The ion-molecule reactions of these organometallic cations with allyliodide were studied in a quadrupole ion trap mass spectrometer. C-C bond coupling occurred to yield the ionic products Ag(2)I(+), AgCuI(+) and Cu(2)I(+). DFT calculations were carried out on these C-C bond coupling reactions. In all cases, the reactions are highly exothermic and involve initial coordination of the allyliodide to both metal atoms, with the iodine coordinating to one atom and the alkene moiety coordinating to the other. The overall mechanism of C-C bond coupling involves oxidative addition of the allyliodide followed by reductive elimination of 1-butene. PMID:19333507

Khairallah, George N; Waters, Tom; O'Hair, Richard A J

2009-02-20

63

Long live vinylidene! A new view of the H(2)C=C: --> HC triple bond CH rearrangement from ab initio molecular dynamics.  

PubMed

We present complete active space self-consistent field (CASSCF) ab initio molecular dynamics (AIMD) simulations of the preparation of the metastable species vinylidene, and its subsequent, highly exothermic isomerization to acetylene, via electron removal from vinylidene anion (D(2)C=C(-) --> D(2)C=C: --> DC triple bond CD). After equilibrating vinylidene anion-d(2) at either 600 +/- 300 K (slightly below the isomerization barrier) or 1440 K +/- 720 K (just above the isomerization barrier), we remove an electron to form a vibrationally excited singlet vinylidene-d(2) and follow its dynamical evolution for 1.0 ps. Remarkably, we find that none of the vinylidenes equilibrated at 600 K and only 20% of the vinylidenes equilibrated at 1440 K isomerized, suggesting average lifetimes >1 ps for vibrationally excited vinylidene-d(2). Since the anion and neutral vinylidene are structurally similar, and yet extremely different geometrically from the isomerization transition state (TS), neutral vinylidene is not formed near the TS so that it must live until it has sufficient instantaneous kinetic energy in the correct vibrational mode(s). The origin of the delay is explained via both orbital rearrangement and intramolecular vibrational energy redistribution (IVR) effects. Unique signatures of the isomerization dynamics are revealed in the anharmonic vibrational frequencies extracted from the AIMD, which should be observable by ultrafast vibrational spectroscopy and in fact are consistent with currently available experimental spectra. Most interestingly, of those trajectories that did isomerize, every one of them violated conventional transition-state theory by recrossing back to vinylidene multiple times, against conventional notions that expect highly exothermic reactions to be irreversible. The dynamical motion responsible for the multiple barrier recrossings involves strong mode-coupling between the vinylidene CD(2) rock and a local acetylene DCC bend mode that has been recently observed experimentally. The multiple barrier recrossings can be used, via a generalized definition of lifetime, to reconcile extremely disparate experimental estimates of vinylidene's lifetime (differing by at least 6 orders of magnitude). Last, a caveat: These results are constrained by the approximations inherent in the simulation (classical nuclear motion, neglect of rotation-vibration coupling, and restriction to C(s) symmetry); refinement of these predictions may be necessary when more exact simulations someday become feasible. PMID:11456576

Hayes, R L; Fattal, E; Govind, N; Carter, E A

2001-01-31

64

The Significance of Chemical Bond Energies  

Microsoft Academic Search

In view of recently reported departures from bond additivity of chemical energies of formation, a purely empirical study has been made of the significance of chemical bond energies. If one assumes that the energy of formation is made up of bond energies and interaction energies between the bonds as well, it is shown that both of these two types of

C. T. Zahn

1934-01-01

65

Solvent-switched benzylic methylene functionalization: addition, ring-opening, cyclization, and unexpected cleavage of C-O and C-C bonds.  

PubMed

Intermolecular benzylic methylene functionalization of exo-cyclic enol ethers has been achieved using imines as reagents and potassium tert-butoxide as the catalyst. Depending on the solvent used, the reaction proceeds by two pathways. In THF, an addition/elimination reaction of exo-cyclic enol ethers with imines provides dihydroisobenzofuran derivatives in good yield. In DMSO, an addition/ring-opening/cyclization cascade reaction occurs with unexpected cleavage of C-O and C-C bonds, affording isoquinolin-1(2H)-one products in high yield under ambient reaction conditions. PMID:23883188

Li, Deng Yuan; Shang, Xue Song; Chen, Guo Rong; Liu, Pei Nian

2013-07-24

66

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

PubMed Central

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

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

2006-01-01

67

Electron-pair distributions in the C=C double bond. Effects of a push-pull substitution  

SciTech Connect

By application of a recently developed method, the populations of ionic and spin-alternant electron-pair distributions are calculated in ethylene, aminoethylene, nitroethylene, and 2-nitroethenamine, one of the simplest push-pull ethylenes. Occupation numbers, giving a measure of the existence of an ionic electron pair in one carbon atom and of a spin-alternant electron pair in the two carbon atoms of the ethylenic double bond, are investigated from ab initio MO wave functions. The effects of either an electron-donating or an electron-withdrawing group and the cooperative push-pull substitution are discussed.

Marcos, E.S.; Sanz, J.F. (Univ. of Sevilla (Spain)); Karafiloglou, P. (Aristotelian Univ. of Thessaloniki (Greece))

1990-04-05

68

Acetate C-C bond formation and decomposition in the anaerobic world: the structure of a central enzyme and its key active-site metal cluster.  

PubMed

The structure of carbon monoxide dehydrogenase/acetyl-coenzyme A synthase (CODH/ACS), a central enzyme in the anaerobic metabolism of acetyl-coenzyme A (acetyl-CoA), has been solved to a resolution of 2.2A. The active-site metal cluster responsible for catalyzing acetyl C-C bond synthesis and cleavage, designated the A center, was identified as an Fe(4)S(4) iron sulfur cluster with one of its cysteine thiolates acting as a bridge to an adjacent binuclear metal site. Nickel was found at one position in the binuclear site and the other metal was indicated to be copper - a surprising result, implying a previously unrecognized role for copper. Details of the A center provided new insight into the unusual organometallic mechanism of acetyl C-C bond formation and cleavage, with substantial conformational changes indicated for binding of the large methylcorrinoid protein substrate, and a unique intramolecular channel acting to contain carbon monoxide within the protein and transfer it to the site needed for acetyl-CoA synthesis. PMID:12765830

Grahame, David A

2003-05-01

69

Organolanthanide-induced C-C bond formation. Preparation and properties of monomeric lanthanide aldolates and enolates  

SciTech Connect

The reactivity of early-lanthanide carbyls Cp* {sub 2}LnCH(SiMe{sub 3}){sub 2} (1, Ln = La; 2, Ln = Ce) toward ketones has been studied. No reaction was observed with the bulky di-tert-butyl ketone. The corresponding lanthanum hydride [Cp* {sub 2}LaH]{sub 2} (3) is more reactive and afforded the addition product Cp*{sub 2}LaOCH(t-Bu){sub 2}{sup center-dot}O=C(t-Bu){sub 2} (4). Hydrogen transfer and formation of lanthanide aldolates Cp* {sub 2}LnOCMe{sub 2}C(=O)Me (5, Ln = Ce; 6, Ln =La) together with the protonated carbyl CH{sub 2}(SiMe{sub 3}){sub 2} were observed when 1 or 2 was treated with acetone. The molecular structure of the cerium analogue 5 was determined by X-ray diffraction and was found to be a monomeric bent metallocene compound with a {eta}{sup 2}-coordinated aldol fragment. Addition of 3-pentanone to the carbyls 1 and 2 did not result in C-C coupling but provided enolate-ketone adducts Cp*{sub 2}LnOC(Et)=C(H)MeO=CEt{sub 2} (7, Ln = La; 8, Ln = Ce). Reactions of the carbyls 1 and 2 with 4-methyl-5-hydroxy-5ethyl-3-heptanone yielded the enolate-ketone adducts as well and suggest that the differences in reactivity between acetone and 3-pentanone are thermodynamic and not kinetic in origin. 32 refs., 2 figs., 5 tabs.

Heeres, H.J.; Maters, M.; Teuben, J.H. [Univ. of Groningen (Netherlands)] [and others

1992-01-01

70

A substrate-assisted mechanism of nucleophile activation in a ser-his-asp containing C-C bond hydrolase.  

PubMed

The meta-cleavage product (MCP) hydrolases utilize a Ser-His-Asp triad to hydrolyze a carbon-carbon bond. Hydrolysis of the MCP substrate has been proposed to proceed via an enol-to-keto tautomerization followed by a nucleophilic mechanism of catalysis. Ketonization involves an intermediate, ES(red), which possesses a remarkable bathochromically shifted absorption spectrum. We investigated the catalytic mechanism of the MCP hydrolases using DxnB2 from Sphingomonas wittichii RW1. Pre-steady-state kinetic and LC ESI/MS evaluation of the DxnB2-mediated hydrolysis of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid to 2-hydroxy-2,4-pentadienoic acid and benzoate support a nucleophilic mechanism catalysis. In DxnB2, the rate of ES(red) decay and product formation showed a solvent kinetic isotope effect of 2.5, indicating that a proton transfer reaction, assigned here to substrate ketonization, limits the rate of acylation. For a series of substituted MCPs, this rate was linearly dependent on MCP pKa2 (?nuc ? 1). Structural characterization of DxnB2 S105A:MCP complexes revealed that the catalytic histidine is displaced upon substrate-binding. The results provide evidence for enzyme-catalyzed ketonization in which the catalytic His-Asp pair does not play an essential role. The data further suggest that ES(red) represents a dianionic intermediate that acts as a general base to activate the serine nucleophile. This substrate-assisted mechanism of nucleophilic catalysis distinguishes MCP hydrolases from other serine hydrolases. PMID:24067021

Ruzzini, Antonio C; Bhowmik, Shiva; Ghosh, Subhangi; Yam, Katherine C; Bolin, Jeffrey T; Eltis, Lindsay D

2013-10-09

71

Geometry and torsional energies of a C-C-protonated n-alkane  

NASA Astrophysics Data System (ADS)

The geometry and relative energies of torsional conformers of centrally protonated C4H11+ were studied with ab initio methods, to (a) obtain the most accurate geometry of the three-center-two-electron CHC bond to date, (b) evaluate the performance of lower levels of approximation upon this challenging structure, and (c) gain an understanding of the torsional dynamics of C4H11+. Twenty-nine combined levels of theory were used to optimize the geometry of the C2-symmetry minimum for trans-C4H11+, and the most accurate one [CCSD(T)/cc-pVTZ] gave the following CHC bond geometry: ?CHC=122.4°, RCC=2.177 Å, RCH=1.2424 Å. Molecular-orbital-based methods generally perform better than density functional methods for describing the three-center-two-electron bond. A smaller subset of levels of theory was used to optimize other torsional conformers of centrally protonated C4H11+, varying the CCCC dihedral (trans, gauche, cis) and the dihedral for the bridging proton (various eclipsed and staggered positions). The results show that all conformers lie within a 4 kJ mol-1 range, with the lowest-energy conformer being either trans or gauche with a staggered dihedral for the bridging proton. The effect of core-valence correlation was also investigated. Finally, the potential energy surface as a function of the CCCC and bridging-proton dihedral angles was qualitatively estimated and drawn, based on our computed data, to aid in understanding the fluxional character of C4H11+.

Li, Qingbin; Hunter, Ken C.; Seitz, Christa; East, Allan L. L.

2003-10-01

72

Calculation of bond energies in diatomic molecules  

Microsoft Academic Search

An extrapolation method is proposed for an approximative evaluation of covalent bonding powers of some elements from their electronegativity values. Using these values in the bond energy equation, obtained from the principle of electronegativity equalization [3], bond energies can be calculated with an accuracy, comparable with the one, obtained by Evans and Huheey [1], who included an electrostatic attraction energy

G. VAN HOOYDONK

1971-01-01

73

A comparative theoretical investigation into the strength of the trigger-bond in the Na?, Mg²? and HF complexes involving the nitro group of R-NO? (R = -CH?, -NH? and -OCH?) or the C = C bond of (E)-O?N-CH = CH-NO?.  

PubMed

A comparative theoretical investigation into the change in strength of the trigger-bond upon formation of the Na(+), Mg(2+) and HF complexes involving the nitro group of RNO? (R = -CH?, -NH?, -OCH?) or the C = C bond of (E)-O?N-CH = CH-NO? was carried out using the B3LYP and MP2(full) methods with the 6-311++G**, 6-311++G(2df,2p) and aug-cc-pVTZ basis sets. Except for the Mg(2+)?? system with (E)-O2N-CH = CH-NO? (i.e., C?H?N?O??Mg(2+)), the strength of the trigger-bond X-NO? (X = C, N or O) was enhanced upon complex formation. Furthermore, the increment of bond dissociation energy of the X-NO? bond in the Na(+) complex was far greater than that in the corresponding HF system. Thus, the explosive sensitivity in the former might be lower than that in the latter. For C?H?N?O??Mg(2+), the explosive sensitivity might also be reduced. Therefore, it is possible that introducing cations into the structure of explosives might be more efficacious at reducing explosive sensitivity than the formation of an intermolecular hydrogen-bonded complex. AIM, NBO and electron density shifts analyses showed that the electron density shifted toward the X-NO? bond upon complex formation, leading to a strengthened X-NO? bond and possibly reduced explosive sensitivity. PMID:23446440

Zhang, Lin; Ren, Fu-de; Cao, Duan-lin; Wang, Jian-long; Gao, Jian-feng

2013-02-28

74

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

PubMed

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

Murarka, Sandip; Wertz, Sebastian; Studer, Armido

2012-01-01

75

Atomic force microscopy imaging of TiO{sub 2} surfaces active for C-C bond formation reactions in ultrahigh vacuum  

SciTech Connect

TiO{sub 2}(001) single crystal surfaces active for a variety of different chemistries were examined using atomic force microscopy (AFM). C-C bond forming reactions previously identified on these surfaces include carboxylic acid ketonization, aldol condensation, reductive carbonyl coupling, and alkyne cyclotrimerization. The surfaces were prepared in ultrahigh vacuum (UHV) and examined by AFM in air. Surfaces examined included the (011)-faceted surface, (114)-faceted surface, and argon-ion-bombarded surfaces, as well as the mechanically polished single-crystal surface prior to treatment in UHV. The one unifying feature of all the images was their extreme flatness. Root-mean-square roughnesses were routinely less than 10 {Angstrom} in 500 x 500 nm scans. These same scans showed the surfaces to have surface areas exceeding that of an ideal flat surface by no more than 1.2%. Images of the polished surface revealed a variety of surface features, including polishing scratches and particle-like features. The argon-ion-bombarded surface and the faceted surfaces were composed of large flat plateaus ranging in size from 21 to 75 nm. The size of the plateaus was essentially the same for the ion bombarded surface and the (011)-faceted surface. The (114)-faceted surfaced exhibited slightly smaller plateau regions than the other surfaces. The images indicate that argon-ion bombardment, while disordering the surface and causing significant composition changes, does not lead to observable morphological changes on this scale. The relative uniformity of the surfaces examined is consistent with the selectivity of carbon-carbon bond-forming reactions that have been shown to take place on these surfaces. The images also reveal how the surface topography on the scale of the plateau structures observed is only slightly changed during the transformation of the surface unit cell structure from the (011)- to the (114)-faceted surface. 29 refs., 6 figs., 1 tab.

Watson, B.A.; Barteau, M.A. [Univ. of Delaware, Newark, DE (United States)

1994-06-01

76

Four-Electron-Donor Hemilabile N3-PPh3 Ligand that Binds through a C=C Bond Rather than an Agostic C-H Interaction, and Displaceement of the C=C by Methyl Iodide or Water  

SciTech Connect

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr'4- [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3-PPh3)Mo]+[BAr'4]-. Spectroscopic and crystallographic data indicate that one C{double_bond}C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand. Computations (DFT/B3LYP and MP2 on [Cp(CO)2(3-PPh3)Mo]+ and [Cp(CO)2(3-PH2Ph)Mo]+, and DFT/B3LYP on [Cp(CO)2(3-PHtBuPh)Mo]+ and [Cp(CO)2(3-PH2Ph)Nb]) provide further information on the bonding and on the preference for bonding of the metal to the C{double_bond}C bond rather than an agostic C-H interaction found in many related complexes. The hemilabile C{double_bond}C bond is readily displaced by CH3I or H2O, and crystal structures are reported for [Cp(CO)2(PPh3)Mo(ICH3)]+ and [Cp(CO)2(PPh3)Mo(OH2)]+. The equilibrium constant for [Cp(CO)2(3-PPh3)Mo]+ + ICH3 to give [Cp(CO)2(PPh3)Mo(ICH3)]+ is Keq = 5.2 x 102 M-1 in CD2Cl2 at 22 C.

Cheng,T.; Szalda, D.; Hanson, J.; Muckerman, J.; Bullock, R.

2008-01-01

77

Specific cleavage at peptide backbone C?-C and CO-N bonds during matrix-assisted laser desorption/ionization in-source decay mass spectrometry with 5-nitrosalicylic acid as the matrix.  

PubMed

The use of 5-nitrosalicylic acid (5-NSA) as a matrix for in-source decay (ISD) of peptides during matrix-assisted laser desorption/ionization (MALDI) is described herein. Mechanistically, the decay process is initiated by a hydrogen abstraction from a peptide backbone amide nitrogen by 5-NSA. Hydrogen abstraction results in formation of an oxidized peptide containing a radical amide nitrogen. Subsequently, the C(?)-C bond N-terminal to the peptide bond is cleaved to form an a·/x fragment pair. The C(?)-C bonds C-terminal to Gly residues were less susceptible to cleavage than were those of other residues. C(?)-C bonds N-terminal to Pro and Sar residues were not cleaved by the aforementioned mechanism; instead, after hydrogen abstraction from a Pro or Sar C(?)-H bond, the peptide bond N-terminal to the Pro was cleaved yielding b- and y-series ions. We also show that fragments produced by MALDI 5-NSA-induced ISD were formed independently of the ionization process. PMID:21793066

Asakawa, Daiki; Takayama, Mitsuo

2011-09-15

78

Selective activation of C-H and C-C bonds on metal carbides: A comparison of reactions of n-Butane and 1,3-Butadiene on vanadium carbide films on V(110)  

SciTech Connect

The authors have investigated the adsorption and decomposition of n-butane and 1,3-butadiene on clean and carbide-modified vanadium (110) surfaces. By using high-resolution electron energy loss spectroscopy and thermal desorption spectrometry, The authors observe that the formation of carbide significantly modifies the reactivities of vanadium. The 1,3-butadiene molecules interact strongly with clean V(110) via the interaction between the d-band of vanadium and the {pi} orbitals of the adsorbates; the interaction is much weaker on the carbide-modified surfaces. On the other hand, n-butane interacts very weakly and reversibly with the clean surface; the reactivity is enhanced on carbide-modified surfaces. These experimental results are compared to the existing theories on the activation of C-H bonds of alkanes and C-C bonds of unsaturated hydrocarbons on transition metals. Such a comparison indicates that, although the reactivities of clean vanadium agree very well with the theoretical predictions for early transition metals, the properties of carbide-modified surfaces are more similar to those of Group VIIIB metals. 49 refs., 10 figs., 2 tabs.

Chen, J.G. [Exxon Research and Engineering Company, Annandale, NJ (United States)

1995-06-01

79

Four-Electron Donor Hemilabile n3PPh3 Ligand that Binds Through a C = C Bond Rather than an Agostic CH Interaction, and Displacement of the C = C by Methyl Iodide or Water  

Microsoft Academic Search

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+ BAr'4 [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3 PPh3)Mo]+[BAr'4] . Spectroscopic and crystallographic data indicate that one C=C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is four-electron donor ligand. Computations (DFT\\/B3LYP and MP2 on [Cp(CO)2(3 PPh3)Mo]+ and [Cp(CO)2(3 PH2Ph)Mo]+, and DFT\\/B3LYP on [Cp(CO)2(3 PHtBuPh)Mo]+ and [Cp(CO)2(3 PH2Ph)Nb]) provide

Tan-Yun Cheng; David J. Szalda; Jonathan C. Hanson; J. T. Muckerman; R. Morris Bullock

2008-01-01

80

Four-Electron-Donor Hemilabile N3PPh3 Ligand that Binds through a C=C Bond Rather than an Agostic CH Interaction, and Displaceement of the C=C by Methyl Iodide or Water  

Microsoft Academic Search

Hydride transfer from Cp(CO)2(PPh3)MoH to Ph3C+BAr'4- [Ar' = 3,5-bis(trifluoromethyl)phenyl] produces [Cp(CO)2(3-PPh3)Mo]+[BAr'4]-. Spectroscopic and crystallographic data indicate that one C{double_bond}C of a Ph ring is weakly bound to the Mo, so that the PPh3 ligand is a four-electron-donor ligand. Computations (DFT\\/B3LYP and MP2 on [Cp(CO)2(3-PPh3)Mo]+ and [Cp(CO)2(3-PH2Ph)Mo]+, and DFT\\/B3LYP on [Cp(CO)2(3-PHtBuPh)Mo]+ and [Cp(CO)2(3-PH2Ph)Nb]) provide further information on the bonding and on

T. Cheng; D. Szalda; J. Hanson; J. Muckerman; R. Bullock

2008-01-01

81

Bond dissociation energy of astrophysically important molecules.  

NASA Astrophysics Data System (ADS)

The bond dissociation energies for astrophysically important diatomic molecules have been estimated based on the derived relation DAB = D¯AB+32.058??, where D¯AB = (DAA·DBB)1/2, ?? represents Pauling electronegativity difference (?A-?B). Based on the formula suggested by Reddy et al. bond energies are estimated. These values are in reasonably good agreement with the available literature values. The bond energies estimated with the present relation give an error of 12.6%. The corresponding error associated with Pauling's equation is 37.2%.

Reddy, R. R.; Ahammed, Y. N.; Gopal, K. R.; Anjaneyulu, S.

1999-03-01

82

New titanium complexes with symmetric or asymmetric cis-9,10-dihydrophenanthrenediamide ligands formed through sequential intramolecular C-C bond-forming reactions.  

PubMed

A series of new titanium(IV) complexes with symmetric or asymmetric cis-9,10-dihydrophenanthrenediamide ligands, cis-9,10-PhenH(2)(NR)(2)Ti(O(i)Pr)(2) [PhenH(2) = 9,10-dihydrophenanthrene, R = 2,6-(i)Pr(2)C(6)H(3) (2a), 2,6-Et(2)C(6)H(3) (2b), 2,6-Me(2)C(6)H(3) (2c)], cis-9,10-PhenH(2)(NR(1))(NR(2))Ti(O(i)Pr)(2) [R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Et(2)C(6)H(3) (2d); R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Me(2)C(6)H(3) (2e)], and [cis-9,10-PhenH(2)(NR(1))(2)][o-C(6)H(4)(CH=NR(2))]TiO(i)Pr [R(1) = 2,6-(i)Pr(2)C(6)H(3), R(2) = 2,6-Et(2)C(6)H(3) (3a); R(1) = 2,6-(i)Pr(2)C(6)H(3), 2,6-Me(2)C(6)H(3) (3b)], have been synthesized from the reactions of TiCl(2)(O(i)Pr)(2) with o-C(6)H(4)(CH=NR)Li [R = 2,6-(i)Pr(2)C(6)H(3), 2,6-Et(2)C(6)H(3), 2,6-Me(2)C(6)H(3)]. The symmetric complexes 2a-2c were obtained from the reactions of TiCl(2)(O(i)Pr)(2) with 2 equiv of the corresponding o-C(6)H(4)(CH=NR)Li followed by intramolecular C-C bond-forming reductive elimination and oxidative coupling processes, while the asymmetric complexes 2d-2e were formed from the reaction of TiCl(2)(O(i)Pr)(2) with two different types of o-C(6)H(4)(CH=NR)Li sequentially. The complexes 3a and 3b were also isolated from the reactions for complexes 2d and 2e. All complexes were characterized by (1)H and (13)C NMR spectroscopy, and the molecular structures of 2a, 2b, 2e, and 3a were determined by X-ray crystallography. PMID:21114325

Zhao, Dapeng; Gao, Bo; Gao, Wei; Luo, Xuyang; Tang, Duihai; Mu, Ying; Ye, Ling

2010-11-29

83

A theoretical study of c-C 5H 8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding  

NASA Astrophysics Data System (ADS)

In this work we analyzed the geometry and the chemical interactions for c-C 5H 8 adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and dbnd C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average dbnd C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the C dbnd C double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p y and 2p z orbitals corresponding to unsaturated C atoms, 2p z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p z orbitals corresponding to double bond C atoms, 4s and 4p z orbitals of Ge for the adsorption on Ge(0 0 1).

Germán, E.; López-Corral, I.; Juan, A.; Brizuela, G.

2010-08-01

84

?? bond energy from the Nijmegen potentials  

NASA Astrophysics Data System (ADS)

The ?? bond energy ? B?? in ?? hypernuclei is obtained from a G -matrix calculation which includes the coupling between the ?? , ?N , and ?? channels, as well as the effect of Pauli blocking to all orders. The Nijmegen NSC97e model is used as bare baryon-baryon interaction in the strangeness S=-2 sector. The ?? - ?N coupling increases substantially the bond energy with respect to the uncoupled ?? case. However, the additional incorporation of the ?? channel, which couples simultaneously to ?? and ?N states, has a surprisingly drastic effect and reduces the bond energy down to a value closer to that obtained in an uncoupled calculation. We find that a complete treatment of Pauli blocking reduces the repulsive effect on the bond energy to about half of what was claimed before.

Vidaña, I.; Ramos, A.; Polls, A.

2004-08-01

85

The stacking-fault energy of F.C.C. metals  

Microsoft Academic Search

The determination of stacking-fault energy from the dimensions of stacking-fault tetrahedra is critically examined and the results compared with those obtained by other methods. It is concluded that the dimensions of stacking-fault tetrahedra do not provide a reliable measure of stacking-fault energy. New values of the stacking-fault energy of Ag and Al are used as calibration points for the method

I. L. Dillamore; R. E. Smallman

1965-01-01

86

The C-C bond cleavage of a lignin model compound, 1,2-diarylpropane-1,3-diol, with a heme-enzyme model catalyst tetraphenylporphyrinatoiron(III)chloride in the presence of tert-butylhydroperoxide.  

PubMed

The catalytic C-C bond cleavage of a lignin model compound was investigated by use of tetraphenylporphyrinatoiron(III)chloride as a model for enzymic degradation of lignin. The C-C bond of the lignin model compound 1,2-bis(4-ethoxy-3-methoxyphenyl) propane-1,3-diol was oxidatively cleaved by catalysis of iron-porphyrins in the presence of tert-butylhydroperoxide or iodosylbenzene at a room temperature. The products formed after complete oxidation of the substrate were identified as 4-O-ethylvanillin, alpha-hydroxy-4-ethoxy-3-methoxyacetophenone, 4-O-ethylvanillic acid, 4-ethoxy-3-methoxyphenylglycol, 4-ethoxy-3-methoxy-alpha-(4-ethoxy-3-methoxyphenyl)-beta-hydroxypropi ophenone and formaldehyde. PMID:6477560

Shimada, M; Habe, T; Umezawa, T; Higuchi, T; Okamoto, T

1984-08-16

87

C-C (alkynylation) vs C-O (ether) bond formation under Pd/C-Cu catalysis: synthesis and pharmacological evaluation of 4-alkynylthieno[2,3-d]pyrimidines.  

PubMed

The Pd/C-CuI-PPh(3) catalytic system facilitated C-C bond formation between 4-chlorothieno[2,3-d]pyrimidines and terminal alkynes in methanol with high selectivity without generating any significant side products arising from C-O bond formation between the chloro compounds and methanol. A variety of novel 4-alkynylthieno[2,3- d]pyrimidines were prepared via alkynylation of 4-chlorothieno[2,3-d]pyrimidines in good to excellent yields. Some of the compounds synthesized were tested for cytotoxic activity in vitro. PMID:21512602

Gorja, Dhilli Rao; Kumar, K Shiva; Mukkanti, K; Pal, Manojit

2011-03-21

88

Direct, Redox Neutral Prenylation and Geranylation of Secondary Carbinol C-H Bonds: C4 Regioselectivity in Ruthenium Catalyzed C-C Couplings of Dienes to ?-Hydroxy Esters  

PubMed Central

The ruthenium catalyst generated in situ from Ru3(CO)12 and tricyclohexylphosphine, PCy3, promotes the redox-neutral C-C coupling of aryl substituted ?-hydroxy esters to isoprene and myrcene at the diene C4-position, resulting in direct carbinol C-H prenylation and geranylation, respectively. This process enables direct conversion of secondary to tertiary alcohols in the absence of stoichiometric byproducts or premetallated reagents, and is the first example of C4-regioselectivity in catalytic C-C couplings of 2-substituted dienes to carbonyl partners. Mechanistic studies corroborate a catalytic cycle involving diene-carbonyl oxidative coupling.

Leung, Joyce C.; Geary, Laina M.; Chen, Te-Yu; Zbieg, Jason R.

2012-01-01

89

The Central Bond C = C Isotope Effect for Superconductivity in the High-Tc ?*-(ET)2l3 Phase and its Implications Regarding the Superconducting Pairing Mechanism in TTF-Based Organic Superconductors  

Microsoft Academic Search

We report a new study, based on magnetization measurements, of the isotope effect for C substitution in the electronically active central double-bonded carbon atoms (C=C) of the TTF moiety of ET [bis(ethylenedithio)tetrathiafulvalene] in ? -(ET)2I3 [also denoted as ?H-(ET)2[3], which is the crystallographically ordered form of ?-(ET)2I3 produced by application of pressure. A recent report by Merzhanov et al. [C.

K. Douglas Carlson; Jack M. Williams; Urs Geiser; Aravinda M. Kim; H. Hau Wang; Richard A. Klemm; S. Kalyan Kumar; John A. Schlueter; John. R. Ferraro; Keith R. Lykke; Peter Wurz; Deborah Holmes Parker; Jason D. B. Sutin; James E. Schirber; Eugene L. Venturini; Phil Stout

1993-01-01

90

Primary photodissociation pathways of epichlorohydrin and analysis of the C-C bond fission channels from an O((3)P)+allyl radical intermediate.  

PubMed

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H(2)COCH)CH(2)Cl. The three dominant photoproduct channels analyzed are c-(H(2)COCH)CH(2)+Cl, c-(H(2)COCH)+CH(2)Cl, and C(3)H(4)O+HCl. In the second channel, the c-(H(2)COCH) photofission product is a higher energy intermediate on C(2)H(3)O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H(2)CCO. The final primary photodissociation pathway HCl+C(3)H(4)O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H(2)COC)=CH(2); the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C(3)H(5)O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O((3)P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C(2)H(4) products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H(2)CO+C(2)H(3) product channel of the O((3)P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H(3)CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C(3)H(5)O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C(2)H(4), and H(2)CO+C(2)H(3) product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C(2)H(4) and H(2)CO+C(2)H(3) product channels. Using the C(2)H(4) bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C(2)H(4) and HCO yields a value of 11.6+1/-3?Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies. PMID:20831317

Fitzpatrick, Benjamin L; Alligood, Bridget W; Butler, Laurie J; Lee, Shih-Huang; Lin, Jim Jr-Min

2010-09-01

91

The physical origin of large covalent-ionic resonance energies in some two-electron bonds.  

PubMed

This study uses valence bond (VB) theory to analyze in detail the previously established finding that alongside the two classical bond families of covalent and ionic bonds, which describe the electron-pair bond, there exists a distinct class of charge-shift bonds (CS-bonds) in which the fluctuation of the electron pair density plays a dominant role. Such bonds are characterized by weak binding, or even a repulsive, covalent component, and by a large covalent-ionic resonance energy RE(cs) that is responsible for the major part, or even for the totality, of the bonding energy. In the present work, the nature of CS-bonding and its fundamental mechanisms are analyzed in detail by means of a VB study of some typical homonuclear bonds (H-H, H3C-CH3, H2N-NH2, HO-OH, F-F, and Cl-Cl), ranging from classical-covalent to fully charge-shift bonds. It is shown that CS-bonding is characterized by a covalent dissociation curve with a shallow minimum situated at long interatomic distances, or even a fully repulsive covalent curve. As the atoms that are involved in the bond are taken from left to right or from bottom to top of the periodic table, the weakening effect of the adjacent bonds or lone pairs increases, while at the same time the reduced resonance integral, that couples the covalent and ionic forms, increases. As a consequence, the weakening of the covalent interaction is gradually compensated by a strengthening of CS-bonding. The large RE(cs) quantity of CS-bonds is shown to be an outcome of the mechanism necessary to establish equilibrium and optimum bonding during bond formation. It is shown that the shrinkage of the orbitals in the covalent structure lowers the potential energy, V, but excessively raises the kinetic energy, T, thereby tipping the virial ratio off-balance. Subsequent addition of the ionic structures lowers T while having a lesser effect on V, thus restoring the requisite virial ratio (T/-V = 1/2). Generalizing to typically classical covalent bonds, like H-H or C-C bonds, the mechanism by which the virial ratio is obeyed during bond formation is primarily orbital shrinkage, and therefore the charge-shift resonance energy has only a small corrective effect. On the other hand, for bonds bearing adjacent lone pairs and/or involving electronegative atoms, like F-F or Cl-Cl, the formation of the bond corresponds to a large increase of kinetic energy, which must be compensated for by a large participation or covalent-ionic mixing. PMID:17328432

Hiberty, Philippe C; Ramozzi, Romain; Song, Lingchun; Wu, Wei; Shaik, Sason

2007-01-01

92

Adhesive bonding using variable frequency microwave energy  

DOEpatents

Methods of facilitating the adhesive bonding of various components with variable frequency microwave energy are disclosed. The time required to cure a polymeric adhesive is decreased by placing components to be bonded via the adhesive in a microwave heating apparatus having a multimode cavity and irradiated with microwaves of varying frequencies. Methods of uniformly heating various articles having conductive fibers disposed therein are provided. Microwave energy may be selectively oriented to enter an edge portion of an article having conductive fibers therein. An edge portion of an article having conductive fibers therein may be selectively shielded from microwave energy. 26 figs.

Lauf, R.J.; McMillan, A.D.; Paulauskas, F.L.; Fathi, Z.; Wei, J.

1998-08-25

93

Adhesive bonding using variable frequency microwave energy  

DOEpatents

Methods of facilitating the adhesive bonding of various components with variable frequency microwave energy are disclosed. The time required to cure a polymeric adhesive is decreased by placing components to be bonded via the adhesive in a microwave heating apparatus having a multimode cavity and irradiated with microwaves of varying frequencies. Methods of uniformly heating various articles having conductive fibers disposed therein are provided. Microwave energy may be selectively oriented to enter an edge portion of an article having conductive fibers therein. An edge portion of an article having conductive fibers therein may be selectively shielded from microwave energy. 26 figs.

Lauf, R.J.; McMillan, A.D.; Paulauskas, F.L.; Fathi, Z.; Wei, J.

1998-09-08

94

Primary photodissociation pathways of epichlorohydrin and analysis of the C-C bond fission channels from an O({sup 3}P)+allyl radical intermediate  

SciTech Connect

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H{sub 2}COCH)CH{sub 2}Cl. The three dominant photoproduct channels analyzed are c-(H{sub 2}COCH)CH{sub 2}+Cl, c-(H{sub 2}COCH)+CH{sub 2}Cl, and C{sub 3}H{sub 4}O+HCl. In the second channel, the c-(H{sub 2}COCH) photofission product is a higher energy intermediate on C{sub 2}H{sub 3}O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H{sub 2}CCO. The final primary photodissociation pathway HCl+C{sub 3}H{sub 4}O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H{sub 2}COC)=CH{sub 2}; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C{sub 3}H{sub 5}O radical produced from C-Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O({sup 3}P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C{sub 2}H{sub 4} and H{sub 2}CO+C{sub 2}H{sub 3} product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C{sub 2}H{sub 4} products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H{sub 2}CO+C{sub 2}H{sub 3} product channel of the O({sup 3}P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H{sub 3}CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C{sub 3}H{sub 5}O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C{sub 2}H{sub 4}, and H{sub 2}CO+C{sub 2}H{sub 3} product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C{sub 2}H{sub 4} and H{sub 2}CO+C{sub 2}H{sub 3} product channels. Using the C{sub 2}H{sub 4} bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C{sub 2}H{sub 4} and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies.

FitzPatrick, Benjamin L.; Alligood, Bridget W.; Butler, Laurie J. [Department of Chemistry and James Franck Institute, University of Chicago, Chicago, Illinois 60637 (United States); Lee, Shih-Huang [National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan (China); Lin, Jim Jr-Min [Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 10617 (China)

2010-09-07

95

Probing hydrogen bond energies by mass spectrometry.  

PubMed

Mass spectrometry with desorption electrospray ionization (DESI) is demonstrated to be useful for probing the strength of hydrogen bonding, exemplified by various complexes of benzothiazoles and carboxylic acids in the solid state. Efficiencies for fragmentation of the complexes, quantified by collision-induced dissociation (CID) technology, correspond well with energies of the hydrogen bonds of O-H···N and N-H···O bridging each pair of benzothiazole and carboxylic acid. Linear correlations (with correlation factors of 0.8953 and 0.9928) have been established for the calibration curves of normalized collision energy at 100% fragmentation rate vs the length between donor and acceptor (in the hydrogen bond of O-H···N) as well as the slope of the fragmentation efficiency curve vs the average length difference between O-H···N and N-H···O in the complex. The mechanism responsible for determination of the hydrogen bonds is proposed on the basis of the experiments starting from the mixtures of the complexes as well as labeling with deuterium. As a complement of previously available methods (e.g., X-ray diffraction analysis), expectably, the proposed mass spectrometric method seems to be versatile for probing hydrogen bond energies. PMID:23550909

Su, Hai-Feng; Xue, Lan; Li, Yun-Hua; Lin, Shui-Chao; Wen, Yi-Mei; Huang, Rong-Bin; Xie, Su-Yuan; Zheng, Lan-Sun

2013-04-12

96

Bond-Energy and Surface-Energy Calculations in Metals  

ERIC Educational Resources Information Center

|A simple technique appropriate for introductory materials science courses is outlined for the calculation of bond energies in metals from lattice energies. The approach is applied to body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal-closest-packed (hcp) metals. The strength of these bonds is tabulated for a variety metals and is…

Eberhart, James G.; Horner, Steve

2010-01-01

97

Hydrogen migration and C-C bond breaking in 1,3-butadiene in intense laser fields studied by coincidence momentum imaging  

NASA Astrophysics Data System (ADS)

Two-body dissociation processes of H 2C dbnd CH-CH dbnd CH22+ induced by an intense laser field were investigated by the coincidence momentum imaging method. Four dissociation pathways, CH62+ ? CH 2+ + CH4+, CH62+ ? CH 3+ + CH3+, CH62+ ? CH3+ + CH3+ and CH62+ ? CH2+ + CH4+, were identified. The existence of the second and fourth pathways can be regarded as evidences of the chemical bond rearrangement processes associated with hydrogen migration in the intense laser field. It was found that the hydrogen atom bonded originally to one of the two central carbon atoms migrates preferentially to its neighboring terminal carbon atom site.

Xu, Huailiang; Okino, Tomoya; Nakai, Katsunori; Yamanouchi, Kaoru; Roither, Stefan; Xie, Xinhua; Kartashov, Daniil; Schöffler, Markus; Baltuska, Andrius; Kitzler, Markus

2010-01-01

98

Copper(II)-catalyzed aerobic oxidative synthesis of substituted 1,2,3- and 1,2,4-triazoles from bisarylhydrazones via C-H functionalization/C-C/N-N/C-N bonds formation.  

PubMed

An unprecedented copper(II)-catalyzed aerobic oxidative synthesis of 2,4,5-triaryl-1,2,3-triazoles and 1,3,5-triaryl-1,2,4-triazoles from bisarylhydrazones as the common starting precursor has been achieved via cascade C-H functionalization/C-C/N-N/C-N bonds formation under mild reaction conditions. One of the enthralling outcomes of this strategy is the copper(II)-catalyzed room temperature C-H functionalization/C-N bond formation in presence of air, which has been accomplished during the synthesis of substituted 1,2,4-triazoles. This new class of compounds could give prospective luminescence as an iconic component in the area of pharmaceutical and biological sciences. The intermediates for both the processes have been isolated to elucidate the mechanistic scenario. PMID:22571669

Guru, Murali Mohan; Punniyamurthy, Tharmalingam

2012-05-11

99

Methanol conversion to hydrocarbons over zeolite H-ZSM-5: Investigation of the role of CO and ketene in the formation of the initial C-C bond  

SciTech Connect

Mechanistic studies concerning the formation of the initial carbon-carbon bond in the methanol conversion reaction over zeolite H-ZSM-5 are described and discussed. In particular, the possible roles of CO as a reaction intermediate or as a catalyst, via the formation of ketene, are evaluated. Experiments using [sup 13]CH[sub 3]OH/[sup 12]CO reactant mixtures demonstrate that no CO is incorporated into ethene, the primary product of this reaction. In addition, CO is found to have no significant effect on the induction period for this reaction. Model experiments for the methylation of ketene by reaction with Me[sub 2]SO[sub 4] and Me[sub 3]O[sup +]SbCl[sub 6][sup [minus

Hutchings, G.J.; Johnston, P. (Univ. of Liverpool (United Kingdom)); Hunter, R. (Univ. of Cape Town, Rondesbosch (South Africa)); Van Rensburg, L.J. (Univ. of Witwatersrand (South Africa))

1993-08-01

100

Energy Dissipation during Rupture of Adhesive Bonds  

NASA Astrophysics Data System (ADS)

Molecular dynamics simulations were used to study energy-dissipation mechanisms during the rupture of a thin adhesive bond formed by short chain molecules. The degree of dissipation and its velocity dependence varied with the state of the film. When the adhesive was in a liquid phase, dissipation was caused by viscous loss. In glassy films, dissipation occurred during a sequence of rapid structural rearrangements. Roughly equal amounts of energy were dissipated in each of three types of rapid motion: cavitation, plastic yield, and bridge rupture. These mechanisms have similarities to nucleation, plastic flow, and crazing in commercial polymeric adhesives.

Baljon, Arlette R. C.; Robbins, Mark O.

1996-01-01

101

Activation of C-O and C-C bonds and formation of novel HAlOH-ether complexes: an EPR study of the reaction of ground-state Al atoms with methylethyl ether and diethyl ether.  

PubMed

Reaction mixtures, containing Al atoms and methylethyl ether (MEE) or diethyl ether (DEE) in an adamantane matrix, were prepared with the aid of a metal-atom reactor known as a rotating cryostat. The EPR spectra of the resulting products were recorded from 77-260 K, at 10 K intervals. Al atoms were found to insert into methyl-O, ethyl-O, and C-C bonds to form CH(3)AlOCH(2)CH(3), CH(3)OAlCH(2)CH(3), and CH(3)OCH(2)AlCH(3), respectively, in the case of MEE while DEE produced CH(3)CH(2)AlOCH(2)CH(3) and CH(3)AlCH(2)OCH(2)CH(3), respectively. From the intensity of the transition lines attributed to the Al atom C-O insertion products of MEE, insertion into the methyl-O bond is preferred. The Al hyperfine interaction (hfi) extracted from the EPR spectra of the C-O insertion products was greater than that of the C-C insertion products, that is, 5.4% greater for the DEE system and 7% greater for the MEE system. The increase in Al hfi is thought to arise from the increased electron-withdrawing ability of the substituents bonded to Al. Besides HAlOH, resulting from the reaction of Al atoms with adventitious water, novel mixed HAlOH:MEE and HAlOH:DEE complexes were identified with the aid of isotopic studies involving H(2)(17)O and D(2)O. The Al and H hfi of HAlOH were found to decrease upon complex formation. These findings are consistent with the nuclear hfi calculated using a density functional theory (DFT) method with close agreement between theory and experiment occurring at the B3LYP level using a 6-311+G(2df,p) basis set. PMID:22299675

Brunet, François D; Feola, Julie C; Joly, Helen A

2012-03-01

102

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

103

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

Microsoft Academic Search

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

Gertrud Kräuter; Andreas Schumacher; Ulrich Gösele

1998-01-01

104

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

105

Energy-loss near-edge structure changes with bond length in carbon systems  

NASA Astrophysics Data System (ADS)

We show that when the graphene planes of graphite are uniformly expanded, thereby increasing the C?C bond length to 1.7Å , the ?* edge onset of the energy-loss near-edge structure (ELNES) spectrum shifts to lower energies by almost 5eV , meanwhile the ?* edge shifts by less than 0.2eV . The shift of the ?* edge demonstrates that for bond lengths which are typical of some carbon systems such as amorphous carbon, it is possible to find ?* features in the ELNES spectra at energies as low as 286-288eV . Calculations on 64-atom amorphous carbon (a-C) and amorphous carbon nitride model structures characterized by a wide range of bond lengths confirm this. Most of the sp2/sp3 quantification techniques that are available overlook this issue of ?* contamination of the ?* region and assume that all features within this energy range are entirely of ?* origin. We show that the effect of bond length variation on the ?* spectrum of graphite and a-C is minor, thereby supporting the reliability of the former spectrum for sp2/sp3 quantification purposes, as was recently demonstrated [see J. T. Titantah and D. Lamoen, Phys. Rev. B 70, 075115 (2004)].

Titantah, J. T.; Lamoen, D.

2005-11-01

106

Direct-dynamics VTST study of hydrogen or deuterium abstraction and C-C bond formation or dissociation in the reactions of CH3 + CH4, CH3 + CD4, CH3D + CD3, CH3CH3 + H, and CH3CD3 + D.  

PubMed

Direct-dynamics variational transition-state theory calculations are studied at the MPWB1K?6-311++G(d,p) level for the four parts of reactions. The first part is hydrogen or deuterium abstraction in the reactions of CH3 + CH4, CH3 + CD4, and CH3D + CH3. The second part involves C-C bond formation in these reactions. The third one is the reactions of CH3CH3 + H and CH3CD3 + D to form of H2, HD, and D2. The last one is the dissociation of C-C bonds in the last group of reactions. The ground-state vibrational adiabatic potential is plotted for all channels. We have carried out direct-dynamics calculations of the rate constants, including multidimensional tunneling in the temperature range T = 200-2200 K. The results of CVT??OMT rate constants were in good agreement with the experimental data which were available for some reactions. Small-curvature tunneling and Large-curvature tunneling with the LCG4 version were used to include the quantum effects in calculation of the rate constants. To try to find the region of formation and dissociation of bounds we have also reported the variations of harmonic vibrational frequencies along the reaction path. The thermally averaged transmission probability (P(E)exp (-?E?RT)) and representative tunneling energy at 298 K are reported for the reactions in which tunneling is important. We have calculated kinetic isotope effect which shows tunneling and vibrational contributions are noticeable to determine the rate constant. Nonlinear least-squares fitting is used to calculate rate constant expressions in the temperature range 200-2200 K. These expressions revealed that pre-exponential factor includes two parts; the first part is a constant number which is important at low temperatures while the second part is temperature dependent which is significant at high temperatures. PMID:23697416

Ramazani, Shapour

2013-05-21

107

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach  

NASA Astrophysics Data System (ADS)

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the ?/? hyperconjugation in EtFC and the ?-conjugation in VFC and EFC.

Boccia, A.; Lanzilotto, V.; Marrani, A. G.; Stranges, S.; Zanoni, R.; Alagia, M.; Fronzoni, G.; Decleva, P.

2012-04-01

108

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach  

SciTech Connect

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the {sigma}/{pi} hyperconjugation in EtFC and the {pi}-conjugation in VFC and EFC.

Boccia, A.; Lanzilotto, V.; Marrani, A. G.; Zanoni, R. [Dipartimento di Chimica, Universita degli Studi di Roma ''La Sapienza'', piazzale Aldo Moro 5, I-00185 Rome (Italy); Stranges, S. [Dipartimento di Chimica, Universita degli Studi di Roma ''La Sapienza'', piazzale Aldo Moro 5, I-00185 Rome (Italy); IOM-CNR, Laboratorio TASC, I-34149 Basovizza, Trieste (Italy); Alagia, M. [IOM-CNR, Laboratorio TASC, I-34149 Basovizza, Trieste (Italy); Fronzoni, G.; Decleva, P. [Dipartimento di Scienze Chimiche, Universita di Trieste, Via L. Giorgieri 1, I-34127 Trieste, Italy and IOM-CNR Democritos, Trieste (Italy)

2012-04-07

109

Municipal bond financing of solar energy facilities  

SciTech Connect

The application of the laws of municipal bond financing to solar facilities is examined. The type of facilities under consideration are outlined. The general legal principles of municipal securities financing are discussed. The effect of recent decisions applying antitrust liability to municipal corporations is also discussed. Five specific types of municipal bonds are explained. The application of Section 103 of the Internal Revenue Code of 1954, as amended, to the issuance of municipal bonds for solar facility financing is examined also. Five bond laws of five representative states are examined and whether the eight types of solar facilities under consideration could be financed under such law is discussed. The application of the general legal principle is illustrated. Three hypothetical situations are set forth and common legal issues to be confronted by city officials in proposing such financing are discussed. These issues will be raised in most financings, but the purpose is to examine the common context in which they are raised. It is concluded that if it can be shown that the purpose of the solar facilities to be financed is to benefit the public, all legal obstacles to the use of municipal financing of solar facilities can be substantially overcome.

White, S.S.

1979-12-01

110

Measurement of bonding energy in an anhydrous nitrogen atmosphere and its application to silicon direct bonding technology  

NASA Astrophysics Data System (ADS)

Bonding energy represents an important parameter for direct bonding applications as well as for the elaboration of physical mechanisms at bonding interfaces. Measurement of bonding energy using double cantilever beam (DCB) under prescribed displacement is the most used technique thanks to its simplicity. The measurements are typically done in standard atmosphere with relative humidity above 30%. Therefore, the obtained bonding energies are strongly impacted by the water stress corrosion at the bonding interfaces. This paper presents measurements of bonding energies of directly bonded silicon wafers under anhydrous nitrogen conditions in order to prevent the water stress corrosion effect. It is shown that the measurements under anhydrous nitrogen conditions (less than 0.2 ppm of water in nitrogen) lead to high stable debonding lengths under static load and to higher bonding energies compared to the values measured under standard ambient conditions. Moreover, the bonding energies of Si/SiO2 or SiO2/SiO2 bonding interfaces are measured overall the classical post bond annealing temperature range. These new results allow to revisit the reported bonding mechanisms and to highlight physical and chemical phenomena in the absence of stress corrosion effect.

Fournel, F.; Continni, L.; Morales, C.; Da Fonseca, J.; Moriceau, H.; Rieutord, F.; Barthelemy, A.; Radu, I.

2012-05-01

111

The Tautomeric Half-reaction of BphD, a C-C Bond Hydrolase Kinetic and Structural Evidence Supporting a Key Role for Histidine 265 of the Catalytic triad  

SciTech Connect

BphD of Burkholderia xenovorans LB400 catalyzes an unusual C-C bond hydrolysis of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) to afford benzoic acid and 2-hydroxy-2,4-pentadienoic acid (HPD). An enol-keto tautomerization has been proposed to precede hydrolysis via a gem-diol intermediate. The role of the canonical catalytic triad (Ser-112, His-265, Asp-237) in mediating these two half-reactions remains unclear. We previously reported that the BphD-catalyzed hydrolysis of HOPDA ({lambda}{sub max} is 434 nm for the free enolate) proceeds via an unidentified intermediate with a red-shifted absorption spectrum ({lambda}{sub max} is 492 nm) (Horsman, G. P., Ke, J., Dai, S., Seah, S. Y. K., Bolin, J. T., and Eltis, L. D. (2006) Biochemistry 45, 11071-11086). Here we demonstrate that the S112A variant generates and traps a similar intermediate ({lambda}{sub max} is 506 nm) with a similar rate, 1/{tau} {approx} 500 s{sup -1}. The crystal structure of the S112A:HOPDA complex at 1.8-{angstrom} resolution identified this intermediate as the keto tautomer, (E)-2,6-dioxo-6-phenyl-hex-3-enoate. This keto tautomer did not accumulate in either the H265A or the S112A/H265A double variants, indicating that His-265 catalyzes tautomerization. Consistent with this role, the wild type and S112A enzymes catalyzed tautomerization of the product HPD, whereas H265A variants did not. This study thus identifies a keto intermediate, and demonstrates that the catalytic triad histidine catalyzes the tautomerization half-reaction, expanding the role of this residue from its purely hydrolytic function in other serine hydrolases. Finally, the S112A:HOPDA crystal structure is more consistent with hydrolysis occurring via an acyl-enzyme intermediate than a gem-diol intermediate as solvent molecules have poor access to C6, and the closest ordered water is 7{angstrom} away.

Horsman, Geoff P.; Bhowmik, Shiva; Seah, Stephen Y.K.; Kumar, Pravindra; Bolin, Jeffrey T.; Eltis, Lindsay D. (Purdue); (UBC)

2010-01-07

112

CH and NH bond dissociation energies of small aromatic hydrocarbons  

Microsoft Academic Search

A survey of computational methods was undertaken to calculate the homolytic bond dissociation energies (BDEs) of the C-H and N-H bonds in monocyclic aromatic molecules that are representative of the functionalities present in coal. These include six-membered rings (benzene, pyridine, pyridazine, pyrimidine, pyrazine) and five-membered rings (furan, thiophene, pyrrole, oxazole). By comparison of the calculated C-H BDEs with the available

Cynthia Barckholtz; Timothy A. Barckholtz; Christopher M. Hadad

1999-01-01

113

Splitting multiple bonds: A comparison of methodologies on the accuracy of bond dissociation energies.  

PubMed

A benchmarking of different quantum chemical methodologies for the splitting of multiply bonded systems is presented, with an emphasis on quantitative reproduction of experimentally determined dissociation energies. New benchmark full configuration interaction (FCI) calculations are presented for nitrogen and acetylene, and comparisons are made between various methods with both the FCI results and with experiment in an effort to understand qualitatively and quantitatively how well these different methods cope with the bond-breaking process. It is found that the multireference methods fare well, but are sometimes slowly convergent with respect to the size of the basis set, and in particular the angular momenta of the polarization functions. Single-reference (SR) wavefunction-based methods perform poorly against full CI, while CCSD(T) is reasonably accurate for comparison with experiment. Density functional theory with hybrid functionals have very quick basis-set convergence, and offer reliable estimates of bond dissociation energies. Spin-restricted SR wavefunctions are found to be poor for such systems, while unrestricted wavefunctions give reasonably good energies, but with severe spin contamination. © 2013 Wiley Periodicals, Inc. PMID:24037810

Robinson, David

2013-09-13

114

Topology maps of bond descriptors based on the kinetic energy density and the essence of chemical bonding.  

PubMed

Analysis of the kinetic energy density within a molecule identifies patterns in its electronic structure that are intuitively linked to familiar concepts of chemical bonding. The function ?(r), termed localized-orbital locator and based on the positive-definite kinetic energy density ?+, is employed to characterize classes of covalent bonds in terms of its full topology of all critical points of rank three. Not only does ?(r) reveal patterns in chemical bonding, it also discloses features and the influence of extended electronic cores. Gradient paths define the extension of the valence space around an atomic centre; they separate various core level regions from bonding domains, and partition molecules in sensible bonded subunits. Location and ?(r)-values of critical points add a quantitative aspect to the bond characterization; profiles of unconventional chemical linkages such as charge-shift bonds emerge in a natural way. PMID:23450168

Jacobsen, Heiko

2013-04-14

115

Bond dissociation energy of the phenol O sbnd H bond from ab initio calculations  

NASA Astrophysics Data System (ADS)

The phenol O sbnd H bond dissociation energy (BDE) is currently disputed, despite its importance in combustion chemistry and in the reactions of antioxidants such as vitamin E. We have studied this BDE using the computational methods G3, G3B3 and CBS-APNO, with bond-isodesmic work reactions. These calculations yield a BDE of 89.0 ± 1.0 kcal mol -1, which supports other recent calculations [B.J. Costa Cabral, S. Canuto, Chem. Phys. Lett. 406 (2005) 300]. From our BDE we determine the enthalpy of formation of the phenoxy radical to be 13.9 ± 1.0 kcal mol -1. Comparison of the phenol C sbnd H BDE with the vinyl alcohol H sbnd CH dbnd CHOH BDE reveals the bond in phenol to be around 3 kcal mol -1 stronger than that in vinyl alcohol. Replacement of a H atom with the OH group on benzene or ethylene strengthens the C sbnd H bonds on adjacent carbons by 3 to 4 kcal mol -1.

da Silva, Gabriel; Chen, Chiung-Chu; Bozzelli, Joseph W.

2006-06-01

116

Students' interdisciplinary reasoning about "high-energy bonds" and ATP  

NASA Astrophysics Data System (ADS)

Students' sometimes contradictory ideas about ATP (adenosine triphosphate) and the nature of chemical bonds have been studied in the biology and chemistry education literatures, but these topics are rarely part of the introductory physics curriculum. We present qualitative data from an introductory physics course for undergraduate biology majors that seeks to build greater interdisciplinary coherence and therefore includes these topics. In these data, students grapple with the apparent contradiction between the energy released when the phosphate bond in ATP is broken and the idea that an energy input is required to break a bond. We see that students' perceptions of how each scientific discipline bounds the system of interest can influence how they justify their reasoning about a topic that crosses disciplines. This has consequences for a vision of interdisciplinary education that respects disciplinary perspectives while bringing them into interaction in ways that demonstrate consistency amongst the perspectives.

Dreyfus, Benjamin W.; Geller, Benjamin D.; Sawtelle, Vashti; Svoboda, Julia; Turpen, Chandra; Redish, Edward F.

2013-01-01

117

Hydrogen bond energy of the water dimer  

Microsoft Academic Search

Large scale ab initio molecular orbital calculations on the binding energy of the water dimer have been performed. These calculations extend the previous correlation consistent basis set work to include larger basis sets (up to 574 functions), and core\\/valence correlation effects have now been included. The present work confirms the earlier estimate of -4.9 kcal\\/mol as the MP2(FC) basis set

Martin W. Feyereisen; David Feller; David A. Dixon

1996-01-01

118

Bond dissociation energies in second-row compounds.  

PubMed

Heats of formation at 0 and 298 K are predicted for PF3, PF5, PF3O, SF2, SF4, SF6, SF2O, SF2O2, and SF4O as well as a number of radicals derived from these stable compounds on the basis of coupled cluster theory [CCSD(T)] calculations extrapolated to the complete basis set limit. In order to achieve near chemical accuracy (+/-1 kcal/mol), additional corrections were added to the complete basis set binding energies based on frozen core coupled cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, a correction for first-order atomic spin-orbit effects, and vibrational zero-point energies. The calculated values substantially reduce the error limits for these species. A detailed comparison of adiabatic and diabatic bond dissociation energies (BDEs) is made and used to explain trends in the BDEs. Because the adiabatic BDEs of polyatomic molecules represent not only the energy required for breaking a specific bond but also contain any reorganization energies of the bonds in the resulting products, these BDEs can be quite different for each step in the stepwise loss of ligands in binary compounds. For example, the adiabatic BDE for the removal of one fluorine ligand from the very stable closed-shell SF6 molecule to give the unstable SF5 radical is 2.8 times the BDE needed for the removal of one fluorine ligand from the unstable SF5 radical to give the stable closed-shell SF4 molecule. Similarly, the BDE for the removal of one fluorine ligand from the stable closed-shell PF3O molecule to give the unstable PF2O radical is higher than the BDE needed to remove the oxygen atom to give the stable closed-shell PF3 molecule. The same principles govern the BDEs of the phosphorus fluorides and the sulfur oxofluorides. In polyatomic molecules, care must be exercised not to equate BDEs with the bond strengths of given bonds. The measurement of the bond strength or stiffness of a given bond represented by its force constant involves only a small displacement of the atoms near equilibrium and, therefore, does not involve any reorganization energies, i.e., it may be more appropriate to correlate with the diabatic product states. PMID:18351757

Grant, Daniel J; Matus, Myrna H; Switzer, Jackson R; Dixon, David A; Francisco, Joseph S; Christe, Karl O

2008-03-20

119

Structure, energy, vibrational spectrum, and Bader's analysis of ?···H hydrogen bonds and H(-?)···H(+?) dihydrogen bonds.  

PubMed

In this paper, the intermolecular structural study asserted by the vibrational analysis in the stretch frequencies of hydrogen bonds (?···H) and dihydrogen bonds (H(-?)···H(+?)) have definitively been revisited by means of calculations carried out by Density Functional Theory (DFT) and topological parameters derived from the classic treatise of the Quantum Theory of Atoms in Molecules (QTAIM). As a matter of fact the ?···H hydrogen bond is formed between the hydrofluoric acid and the C?C bond of the acetylene, but the QTAIM calculations revealed a distortion in this interaction due to the formation of the ternary complex C(2)H(2)···2(HF). Although the ? bonds of ethylene (C(2)H(4)), propylene (C(2)H(3)(CH(3))), and t-butylene (C(2)H(2)(CH(3))(2)) are considered proton acceptors, two hydrogen-bond types--?···H and C···H--can be observed. Over and above the analysis of the ? hydrogen bonds, theoretical arguments also were used to discuss the red-shifts in the stretch frequencies of the binary dihydrogen complexes formed by BeH(2)···HX with X = F, Cl, CN, and CCH. Although a vibrational blue-shift in the stretch frequency of the H-C bond of HCF(3) due to the formation of the BeH(2)···HCF(3) dihydrogen complex was obtained, unmistakable red-shifts were detected in LiH···HCF(3), MgH(2)···HCF(3), and NaH···HCF(3). Moreover, the alkali-halogen bonds were identified in relation to the formation of the trimolecular systems NaH···2(HCF(3)) and NaH···2(HCCl(3)). At last, theoretical calculations and QTAIM molecular integrations were used to study a novel class of dihydrogen-bonded complexes (mC(2)H(5)(+)···nMgH(2) with m = 1 or 2 and n = 1 or 2) based in the insight that MgH(2) can bind with the non-localized hydrogen H(+?) of the ethyl cation (C(2)H(5)(+)). In an overview, QTAIM calculations were applied to evaluate the molecular topography, charge density, as well as to interpret the shifted frequencies either to red or blue caused by the formation of the hydrogen bonds and dihydrogen bonds. PMID:23138158

de Oliveira, Boaz Galdino

2012-11-09

120

Bonding energies and long-range order in the trialuminides  

SciTech Connect

The degree of long-range order in the trialuminides is determined by X-ray powder diffraction techniques. Long-range order exists to their melting points. For the binary trialuminides Al{sub 3}Ti, Al{sub 73}Ti{sub 27}, and Al{sub 3}Sc, the degree of long-range order is nearly perfect and is a measure of the lack of mixing of the aluminum atoms onto the sublattice occupied by either Ti or Sc. A calculation of the bond energy between neighboring pairs of atoms from the ordering (melting) temperature is made following the Bragg-Williams mean field theory approach. These bond energies compare favorably with more sophisticated calculations. Bond energies are found to be larger than the energy difference between the crystal structure forms DO{sub 22}, Ll{sub 2}, and DO{sub 23}, and therefore, more relevant to understanding the mechanical and chemical behavior of the trialuminides. Ordering or melting temperatures of these intermetallics reflect the strong Al-metal near-neighbor pair potentials and may provide insights to their brittle properties. 11 refs., 2 figs., 2 tabs.

Sparks, C.J.; Specht, E.D.; Ice, G.E.; Zschack, P.; Schneibel, J.

1990-01-01

121

Coulomb energy determination of a single Si dangling bond.  

PubMed

Determination of the Coulomb energy of single point defects is essential because changing their charge state critically affects the properties of materials. Based on a novel approach that allows us to simultaneously identify a point defect and to monitor the occupation probability of its electronic state, we unambiguously measure the charging energy of a single Si dangling bond with tunneling spectroscopy. Comparing the experimental result with tight-binding calculations highlights the importance of the particular surrounding of the localized state on the effective charging energy. PMID:21231404

Nguyen, T H; Mahieu, G; Berthe, M; Grandidier, B; Delerue, C; Stiévenard, D; Ebert, Ph

2010-11-24

122

Bond energies of nitrogen and phosphorous hydrides and fluorides  

SciTech Connect

Recent measurements of bond energies in the N-H/sub n/ and P-H/sub n/ systems by photoionization mass spectrometry are compared with modern ab initio calculations and a semi-empirical theory. Good agreement is noted, providing confirmation for the level of accuracy of the ab initio calculations, and for the essential corrections of the semi-empirical parametrization. However, the N-F/sub n/ and P-F/sub n/ systems, also measured, are currently beyond the capabilities of such high quality ab initio calculations, and the trends observed in the bond energies indicate that other parametrizations are necessary in the semi-empirical approach. 20 refs., 8 figs.

Berkowitz, J.; Gibson, S.T.; Greene, J.P.; Neskovic, O.M.; Ruscic, B.

1985-01-01

123

Atomistic simulation of an f.c.c./b.c.c. interface in Ni-Cr alloys  

SciTech Connect

The embedded atom method is applied to study the atomic structure and energy of an f.c.c./b.c.c. interface in Ni-Cr. The two phases are oriented in a Kurdjumov-Sachs orientation relationship, and the interface considered is the (1 {bar 2} 1){sub f} habit plane adopted by precipitate laths of the b.c.c. phase. The interfacial energy and coherent strain energy at 0 K are calculated for boundaries between an f.c.c. Ni-Cr solid solution and b.c.c. Cr. The calculated interfacial energy varies from 216 mJ/m{sup 2} when the f.c.c. phase is pure Ni to 200 mJ/m{sup 2} when the f.c.c. phase is Ni-50 at.% Cr. Atomic relaxations appear limited to atoms in contact with the interphase boundary. Most of the interfacial energy is attributed to the structural difference across the f.c.c./b.c.c. boundary, and the chemical contribution to the energy is estimated to be less than 20% of the total energy. The values of the calculated energies and the widespread occurrence of the (1{bar 2}1){sub f} habit plane in a variety of alloy systems indicate this boundary orientation has a relatively low interfacial energy.

Chen, J.K. [Exxon Research and Engineering Co., Clinton Township, NJ (United States). Corporate Research Lab.; Reynolds, W.T. Jr. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Materials Science and Engineering Dept.

1997-11-01

124

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

SciTech Connect

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

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

2006-11-09

125

Aliphatic C-H to C-C Conversion: Synthesis of (-)-Cameroonan-7?-ol  

PubMed Central

In the course of a synthesis of the tricyclic sesquiterpene (?)-cameroonan-7?-ol from the acyclic (+)-citronellal, seven aliphatic C-H bonds were converted to C-C bonds, and three rings and four new stereogenic centers were established.

Taber, Douglass F.; Nelson, Christopher G.

2011-01-01

126

Bond Energies in Models of the Schrock Metathesis Catalyst  

SciTech Connect

Heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) of the model Schrock-type metal complexes M(NH)(CRR?)(OH)? (M = Cr, Mo, W; CRR? = CH?, CHF, CF?) and MO?(OH)? compounds, and Brønsted acidities and fluoride affinities for the M(NH)(CH?)(OH) ? transition metal complexes are predicted using high level CCSD(T) calculations. The metallacycle intermediates formed by reaction of C?H4 with M(NH)-(CH?)(OH)2 and MO?(OH)? are investigated at the same level of theory. Additional corrections were added to the complete basis set limit to obtain near chemical accuracy ((1 kcal/mol). A comparison between adiabatic and diabatic BDEs is made and provides an explanation of trends in the BDEs. Electronegative groups bonded on the carbenic carbon lead to less stable Schrock-type complexes as the adiabatic BDEs ofMdCF? andMdCHF bonds are much lower than theMdCH? bonds. The Cr compounds have smaller BDEs than theWorMo complexes and should be less stable. Different M(NH)(OH)?(C?H?) and MO(OH)?(OC?H4) metallacycle intermediates are investigated, and the lowest-energy metallacycles have a square pyramidal geometry. The results show that consideration of the singlet_triplet splitting in the carbene in the initial catalyst as well as in the metal product formed by the retro [2+2] cycloaddition is a critical component in the design of an effective olefin metathesis catalyst in terms of the parent catalyst and the groups being transferred.

Vasiliu, Monica; Li, Shenggang; Arduengo, Anthony J.; Dixon, David A.

2011-06-02

127

Construction of microbial platform for an energy-requiring bioprocess: practical 2?-deoxyribonucleoside production involving a C-C coupling reaction with high energy substrates  

PubMed Central

Background Reproduction and sustainability are important for future society, and bioprocesses are one technology that can be used to realize these concepts. However, there is still limited variation in bioprocesses and there are several challenges, especially in the operation of energy-requiring bioprocesses. As an example of a microbial platform for an energy-requiring bioprocess, we established a process that efficiently and enzymatically synthesizes 2?-deoxyribonucleoside from glucose, acetaldehyde, and a nucleobase. This method consists of the coupling reactions of the reversible nucleoside degradation pathway and energy generation through the yeast glycolytic pathway. Results Using E. coli that co-express deoxyriboaldolase and phosphopentomutase, a high amount of 2?-deoxyribonucleoside was produced with efficient energy transfer under phosphate-limiting reaction conditions. Keeping the nucleobase concentration low and the mixture at a low reaction temperature increased the yield of 2?-deoxyribonucleoside relative to the amount of added nucleobase, indicating that energy was efficiently generated from glucose via the yeast glycolytic pathway under these reaction conditions. Using a one-pot reaction in which small amounts of adenine, adenosine, and acetone-dried yeast were fed into the reaction, 75?mM of 2?-deoxyinosine, the deaminated product of 2?-deoxyadenosine, was produced from glucose (600?mM), acetaldehyde (250?mM), adenine (70?mM), and adenosine (20?mM) with a high yield relative to the total base moiety input (83%). Moreover, a variety of natural dNSs were further synthesized by introducing a base-exchange reaction into the process. Conclusion A critical common issue in energy-requiring bioprocess is fine control of phosphate concentration. We tried to resolve this problem, and provide the convenient recipe for establishment of energy-requiring bioprocesses. It is anticipated that the commercial demand for dNSs, which are primary metabolites that accumulate at very low levels in the metabolic pool, will grow. The development of an efficient production method for these compounds will have a great impact in both fields of applied microbiology and industry and will also serve as a good example of a microbial platform for energy-requiring bioprocesses.

2012-01-01

128

Equilibrium Acidities and Homolytic Bond Dissociation Energies of Acidic C H Bonds in Alpha-Arylacetophenones and Related Compounds  

Microsoft Academic Search

The equilibrium acidities (pKAHs) and the oxidation potentials of the conjugate anions (Eox(A?â)s) were determined in dimethyl sulfoxide (DMSO) for eight ketones of the structure GCOCH3 and twenty of the structure RCOCH2G, (where R= alkyl, phenyl and G= alkyl, aryl). The homolytic bond dissociation energies (BDEs) for the acidic C H bonds of the ketones were estimated using the equation,

Mikhail S. Alnajjar; Xian-Man Zhang; Gerald J. Gleicher; Scott V. Truksa; James A. Franz

2002-01-01

129

The Trouble with Chemical Energy: Why Understanding Bond Energies Requires an Interdisciplinary Systems Approach  

ERIC Educational Resources Information Center

|Helping students understand "chemical energy" is notoriously difficult. Many hold inconsistent ideas about what energy is, how and why it changes during the course of a chemical reaction, and how these changes are related to bond energies and reaction dynamics. There are (at least) three major sources for this problem: 1) the way biologists talk…

Cooper, Melanie M.; Klymkowsky, Michael W.

2013-01-01

130

Bond energy effects on strength, cooperativity and robustness of molecular structures.  

PubMed

A fundamental challenge in engineering biologically inspired materials and systems is the identification of molecular structures that define fundamental building blocks. Here, we report a systematic study of the effect of the energy of chemical bonds on the mechanical properties of molecular structures, specifically, their strength and robustness. By considering a simple model system of an assembly of bonds in a cluster, we demonstrate that weak bonding, as found for example in H-bonds, results in a highly cooperative behaviour where clusters of bonds operate synergistically to form relatively strong molecular clusters. The cooperative effect of bonding results in an enhanced robustness since the drop of strength owing to the loss of a bond in a larger cluster only results in a marginal reduction of the strength. Strong bonding, as found in covalent interactions such as disulphide bonds or in the backbone of proteins, results in a larger mechanical strength. However, the ability for bonds to interact cooperatively is lost, and, as a result, the overall robustness is lower since the mechanical strength hinges on individual bonds rather than a cluster of bonds. The systematic analysis presented here provides general insight into the interplay of bond energy, robustness and other geometric parameters such as bond spacing. We conclude our analysis with a correlation of structural data of natural protein structures, which confirms the conclusions derived from our study. PMID:23050078

Chou, Chia-Ching; Buehler, Markus J

2011-07-27

131

Bond energy effects on strength, cooperativity and robustness of molecular structures  

PubMed Central

A fundamental challenge in engineering biologically inspired materials and systems is the identification of molecular structures that define fundamental building blocks. Here, we report a systematic study of the effect of the energy of chemical bonds on the mechanical properties of molecular structures, specifically, their strength and robustness. By considering a simple model system of an assembly of bonds in a cluster, we demonstrate that weak bonding, as found for example in H-bonds, results in a highly cooperative behaviour where clusters of bonds operate synergistically to form relatively strong molecular clusters. The cooperative effect of bonding results in an enhanced robustness since the drop of strength owing to the loss of a bond in a larger cluster only results in a marginal reduction of the strength. Strong bonding, as found in covalent interactions such as disulphide bonds or in the backbone of proteins, results in a larger mechanical strength. However, the ability for bonds to interact cooperatively is lost, and, as a result, the overall robustness is lower since the mechanical strength hinges on individual bonds rather than a cluster of bonds. The systematic analysis presented here provides general insight into the interplay of bond energy, robustness and other geometric parameters such as bond spacing. We conclude our analysis with a correlation of structural data of natural protein structures, which confirms the conclusions derived from our study.

Chou, Chia-Ching; Buehler, Markus J.

2011-01-01

132

Energy expression of the chemical bond between atoms in metal oxides  

Microsoft Academic Search

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

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

2011-01-01

133

Studies of low-lying triplet states in 1,3-C4F6, c-C4F6 and 2-C4F6 by electron energy-loss spectroscopy and ab initio calculations  

NASA Astrophysics Data System (ADS)

This Letter reports on the first measurements of the lowest lying triplet states as studied by electron energy loss spectroscopy for C4F6 isomers, hexafluoro-1,3-butadiene (1,3-C4F6), hexafluorocyclobutene (c-C4F6) and hexafluoro-2-butyne (2-C4F6). This study has been performed at an incident electron energy of 30 eV, 30°, whilst sweeping the energy loss over the range 2.0-15.0 eV. The electronic state spectroscopy has been investigated and the assignments supported by multi-reference quantum chemical calculations. The transition to the steepest electronic excited potential energy curve, is suggested to be dominant for 2-C4F6, due to the large broadening in its energy profile.

Limão-Vieira, P.; Duflot, D.; Anzai, K.; Kato, H.; Hoshino, M.; Silva, F. Ferreira da; Mogi, D.; Tanioka, T.; Tanaka, H.

2013-06-01

134

Influence of confinement on hydrogen bond energy. The case of the FH···NCH dimer.  

PubMed

The influence of the external pressure on the energy of the intermolecular hydrogen bond is investigated by modeling the pressure effects with helium atoms located at fixed points in space around the hydrogen bonded dimer. Several methods of estimating the energy of the H···N hydrogen bond in the He···FH···NCH···He model system are proposed. They show that the energy of the H···N hydrogen bond in this confined dimer decreases continuously with the reduction of the He···He distance, thus with the increase of the pressure effect exerted on the confined dimer. PMID:20738112

Jab?o?ski, Miros?aw; Solà, Miquel

2010-09-23

135

Enzymatic enantioselective C-C-bond formation in microreactors  

Microsoft Academic Search

We have demonstrated that multiple crude enzyme lysates containing a hydroxynitrile lyase can be used for the enantioselective synthesis of cyanohydrins from aldehydes in microchannels. Using a microreactor setup, two important parameters were efficiently screened consuming only minute amounts of reagents. More impor- tantly, results from the continuous flow reaction were fully consistent with results obtained from larger batchwise processes

K. Koch; R. J. F. van den Berg; P. J. Nieuwland; R. Wijtmans; H. E. Schoemaker; J. C. M. van Hest; F. P. J. T. Rutjes

2008-01-01

136

Hydrogen-bonding contributions to the lattice energy of salts for second harmonic generation  

Microsoft Academic Search

The lattice energies of a series of organic dihydrogenphosphate salts capable of second harmonic generation (SHG) have been calculated. These calculations, coupled with empirical data, indicate that a minimum of 20–25% of the lattice energy arises from hydrogen-bond interactions. Hydrogen bonding is shown to be a strong enough force to have a profound effect on the overall packing and crystal

Christer B. Aakeröy; Kenneth R. Seddon; Maurice Leslie

1992-01-01

137

Structures, energies and bonding in neutral and charged Li microclusters.  

PubMed

Structural and chemical properties of charged and neutral Lithium microclusters are investigated for [Formula: see text]. A total of 18 quantum conformational spaces are randomly walked to produce candidate structures for local minima. Very rich potential energy surfaces are produced, with the largest structural complexity predicted for anionic clusters. Analysis of the electron charge distributions using the quantum theory of atoms in molecules (QTAIM) predicts major stabilizing roles of Non-nuclear attractors (NNAs) via NNA···Li interactions with virtually no direct Li···Li interactions, except in the least stable configurations. A transition in behavior for clusters containing more than seven nuclei is observed by using the recently introduced quantum topology to determine in a quantum mechanically consistent fashion the number of spatial dimensions each cluster has. We experiment with a novel scheme for extracting persistent structural motifs with increase in cluster size. The new structural motifs correlate well with the energetic stability, particularly in highlighting the least stable structures. Quantifying the degree of covalent character in Lithium bonding independently agrees with the observation in the transition in cluster behavior for lithium clusters containing more than seven nuclei. Good correlation with available experimental data is obtained for all properties reported in this work. PMID:22538507

Yepes, Diana; Kirk, Steven Robert; Jenkins, Samantha; Restrepo, Albeiro

2012-04-27

138

Chapter 12 New Expression of the Chemical Bond in Hydrides Using Atomization Energies  

Microsoft Academic Search

Atomization energy diagram is proposed for analyzing the chemical bond in the hydrides including perovskite-type hydrides, metal hydrides and complex hydrides. The atomization energies of hydrogen and metal atoms in them are evaluated theoretically by the energy density analysis (EDA) of the total energy, and used for the construction of the atomization energy diagram. Every hydride can be located on

Yoshifumi Shinzato; Hiroshi Yukawa; Masahiko Morinaga; Takeshi Baba; Hiromi Nakai

2008-01-01

139

An Experimental and Theoretical Study on the Ionization Energies of Polyynes (H-(C = C)n-H; n = 1 - 9)  

SciTech Connect

We present a combined experimental and theoretical work on the ionization energies of polyacetylene -- organic molecules considered as important building blocks to form polycyclic aromatic hydrocarbons (PAHs) in the proto planetary nebulae such as of CRL 618. This set of astrophysical data can be utilized with significant confidence in future astrochemical models of photon-dominated regions and also of the proto planetary nebulae CRL 618. We recommend ionization energies of polyacetylenes from diacetylene up to heptaacetylene with an experimental accuracy of +- 0.05 eV: 10.03 eV (diacetylene), 9.45 eV (triacetylene), 9.08 eV (tetraacetylene), 8.75 eV (pentaacetylene), 8.65 eV (hexaacetylene), and 8.50 eV (heptaacetylene); further, ionization energies and with an accuracy of +- 0.1 eV: 8.32 eV (octaacetylene) and 8.24 eV (nonaacetylene) were computed. Implications of these energies to the redox chemistry involved in the multiply charged metal-ion mediated chemistry of hydrocarbon-rich atmospheres of planets and their moons such as Titan are also discussed.

Kaiser, Ralf I.; Sun, Bian Jian; Lin, Hong Mao; Chang, Agnes H. H.; Mebel, Alexander M.; Kostko, Oleg; Ahmed, Musahid

2010-05-17

140

An Experimental and Theoretical Study on the Ionization Energies of Polyynes (H-(C?C) n -H; n = 1-9)  

NASA Astrophysics Data System (ADS)

We present a combined experimental and theoretical work on the ionization energies of polyacetylene-organic molecules considered as important building blocks to form polycyclic aromatic hydrocarbons in protoplanetary nebulae such as CRL 618. This set of astrophysical data can be utilized with significant confidence in future astrochemical models of photon-dominated regions and also of the protoplanetary nebulae CRL 618. We recommend ionization energies of polyacetylenes from diacetylene up to heptaacetylene with an experimental accuracy of ±0.05 eV: 10.03 eV (diacetylene), 9.45 eV (triacetylene), 9.08 eV (tetraacetylene), 8.75 eV (pentaacetylene), 8.65 eV (hexaacetylene), and 8.50 eV (heptaacetylene). Further, ionization energies with an accuracy of ±0.1 eV: 8.32 eV (octaacetylene) and 8.24 eV (nonaacetylene), were computed. Implications of these energies to the redox chemistry involved in the multiply charged metal-ion mediated chemistry of hydrocarbon-rich atmospheres of planets and their moons such as Titan are also discussed.

Kaiser, Ralf I.; Sun, Bian Jian; Mao Lin, Hong; Chang, Agnes H. H.; Mebel, Alexander M.; Kostko, Oleg; Ahmed, Musahid

2010-08-01

141

The trouble with chemical energy: why understanding bond energies requires an interdisciplinary systems approach.  

PubMed

Helping students understand "chemical energy" is notoriously difficult. Many hold inconsistent ideas about what energy is, how and why it changes during the course of a chemical reaction, and how these changes are related to bond energies and reaction dynamics. There are (at least) three major sources for this problem: 1) the way biologists talk about chemical energy (which is also the way we talk about energy in everyday life); 2) the macroscopic approach to energy concepts that is common in physics and physical sciences; and 3) the failure of chemistry courses to explicitly link molecular with macroscopic energy ideas. From a constructivist perspective, it is unlikely that students can, without a coherent understanding of such a central concept, attain a robust and accurate understanding of new concepts. However, changes are on the horizon, guided by the increasing understanding that difficult concepts require coherent, well-designed learning progressions and the new National Research Council Framework for K-12 Science Education. We provide supporting evidence for our assertions and suggestions for an interdisciplinary learning progression designed to better approach the concept of bond energies, a first step in an understanding chemical energy and behavior of reaction systems that is central to biological systems. PMID:23737636

Cooper, Melanie M; Klymkowsky, Michael W

2013-01-01

142

Broken-Bond Rule for the Surface Energies of FCC Metals  

NASA Astrophysics Data System (ADS)

Using two different full-potential ab initio techniques we introduce a very simple rule based on the number of broken first-neighbor bonds to determine the surface energies of the noble metals and fcc transition metals . When a bond is broken, the rearrangement of the electronic charge for these metals does not lead to a significant change of the remaining bonds. Thus the energy needed to break a bond is independent of the surface orientation, so that the anisotropy of the surface energies is determined by the number of broken nearest-neighbors bonds. While this rule is well obeyed for the fcc noble ans transition metals, significant deviations occur for the fcc sp-metals. This novel finding can lead to the development of simple models to describe the energetics of a surface like step and kink formation, crystal growth, alloy formation, equilibrium shape of mesoscopic crystallites and surface faceting.

Galanakis, Iosif; Bihlmayer, Gustav; Papanikolaou, Nikolaos; Zeller, Rudi; Bluegel, Stefan; Dederichs, Peter H.

2002-03-01

143

Electronic excitation to singlet states of 1,3-C4F6, c-C4F6 and 2-C4F6 by electron impact--electron energy-loss spectroscopy and ab initio calculations.  

PubMed

We report on the first measurements of the electron impact electronic excitation cross sections for C(4)F(6) isomers, hexafluoro-1,3-butadiene (1,3-C(4)F(6)), hexafluorocyclobutene (c-C(4)F(6)), and hexafluoro-2-butyne (2-C(4)F(6)), measured at 100 eV, 3° scattering angle, while sweeping the energy loss over the range 2.0-15.0 eV. Under these experimental conditions, optically allowed transitions are favored. The electronic state spectroscopy has been investigated and the assignments supported by quantum chemical calculations. The n = 3 members of the Rydberg series have been assigned converging to the lowest ionization energy limits of the C(4)F(6) isomers and classified according to the magnitude of the quantum defects (?). PMID:23074974

Limão-Vieira, P; Anzai, K; Kato, H; Hoshino, M; Ferreira da Silva, F; Duflot, D; Mogi, D; Tanioka, T; Tanaka, H

2012-10-17

144

Evaluation of Intrinsic Binding Energy from a Hydrogen Bonding Group in an Enzyme Inhibitor  

Microsoft Academic Search

This and two accompanying reports describe the intrinsic binding energy derived from a single hydrogen bond between an inhibitor and an enzyme. The results were obtained by comparing matched pairs of inhibitors of the zinc endopeptidase thermolysin that bind to the enzyme in an essentially identical manner but differ in the presence or absence of a specific hydrogen bond. This

Paul A. Bartlett; Charles K. Marlowe

1987-01-01

145

Facilitating room-temperature Suzuki coupling reaction with light: Mott-Schottky photocatalyst for C-C-coupling  

NASA Astrophysics Data System (ADS)

The Suzuki coupling reaction is one of the most practiced classes of catalytic C-C bond formation. The development of new means of activating molecules and bonds over old catalysts for C-C bond formation is a fundamental objective for chemists. Here, we report the room-temperature C-C bond formation over heterogeneous Pd catalysts by light-mediated catalyst activation. We employ stimulated electron transfer at the metal-semiconductor interface from optically active mesoporous carbon nitride nanorods to Pd nanoparticles. This photocatalytic pathway is highly efficient for coupling aryl halides with various coupling partners with high activity and selectivity under photo irradiation and very mild conditions.

Li, Xin-Hao; Baar, Moritz; Blechert, Siegfried; Antonietti, Markus

2013-04-01

146

Facilitating room-temperature Suzuki coupling reaction with light: Mott-Schottky photocatalyst for C-C-coupling  

PubMed Central

The Suzuki coupling reaction is one of the most practiced classes of catalytic C-C bond formation. The development of new means of activating molecules and bonds over old catalysts for C-C bond formation is a fundamental objective for chemists. Here, we report the room-temperature C-C bond formation over heterogeneous Pd catalysts by light-mediated catalyst activation. We employ stimulated electron transfer at the metal-semiconductor interface from optically active mesoporous carbon nitride nanorods to Pd nanoparticles. This photocatalytic pathway is highly efficient for coupling aryl halides with various coupling partners with high activity and selectivity under photo irradiation and very mild conditions.

Li, Xin-Hao; Baar, Moritz; Blechert, Siegfried; Antonietti, Markus

2013-01-01

147

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

Microsoft Academic Search

A classification of the hydrogen fluoride H-F-bonded interactions comprising a large number of molecules has been proposed by Espinosa et al. [J. Chem. Phys. 117, 5529 (2002)] based on the ratio |V(rc)|\\/G(rc) where |V(rc)| is the magnitude of the local potential-energy density and G(rc) is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of

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

2006-01-01

148

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

Microsoft Academic Search

A classification of the hydrogen fluoride H–F-bonded interactions comprising a large number of molecules has been proposed by Espinosa &etal; [J. Chem. Phys. 117, 5529 (2002)] based on the ratio |V(rc)|?G(rc) where |V(rc)| is the magnitude of the local potential-energy density and G(rc) is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of the

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

2006-01-01

149

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

Microsoft Academic Search

molecules has been proposed by Espinosa et al. J. Chem. Phys. 117, 5529 2002 based on the ratio Vrc\\/Grc where Vrc is the magnitude of the local potential-energy density and Grc is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of

G. V. Gibbsa; T. D. Crawford; N. L. Ross

150

Hypovalency--a kinetic-energy density description of a 4c-2e bond.  

PubMed

A bond descriptor based on the kinetic energy density, the localized-orbital locator (LOL), is used to characterize the nature of the chemical bond in electron deficient multi-center bonds. The boranes B(2)H(6), B(4)H(4), B(4)H(10), [B(6)H(6)](2-), and [B(6)H(7)](-) serve as prototypical examples of hypovalent 3c-2e and 4c-2e bonding. The kinetic energy density is derived from a set of Kohn-Sham orbitals obtained from pure density functional calculations (PBE/TZVP), and the topology of LOL is analyzed in terms of (3,-3) attractors (Gamma). The B-B-B and B-H-B 3c-2e, and the B-B-H-B 4c-2e bonding situations are defined by their own characteristic LOL profiles. The presence of one attractor in relation to the three or four atoms that are engaged in electron deficient bonding provides sufficient indication of the type of 3c-2e or 4c-2e bond present. For the 4c-2e bond in [B(6)H(7)](-) the LOL analysis is compared to results from an experimental QTAIM study. PMID:19452076

Jacobsen, Heiko

2009-04-22

151

Energy density analysis of the chemical bond between atoms in perovskite-type hydrides  

Microsoft Academic Search

Atomization energy diagram is proposed for analyzing the chemical bond in the perovskite-type hydrides such as M1MgH3 (M1=Na, K, Rb), RbCaH3, CaNiH3 and SrPdH3. The atomization energies of hydrogen and metal atoms in the hydrides are evaluated theoretically by the energy density analysis (EDA) of the total energy, and used for the construction of the atomization energy diagram. Every hydride

Yoshifumi Shinzato; Hiroshi Yukawa; Masahiko Morinaga; Takeshi Baba; Hiromi Nakai

2007-01-01

152

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

153

Pt-catalyzed C-C activation induced by C-H activation.  

PubMed

The catalytic cleavage of two C-C single bonds is achieved by treatment of the hydrocarbon substrate spiro[bicyclo[2.2.1]hept-2-ene-7,1'-cyclopropane] with Pt(II) catalysts such as (Me2bpy)PtPh(NTf2) (Me2bpy = 4,4'-dimethyl-2,2'-bipyridine, NTf2(-) = N(SO2CF3)2(-)). The surprising rearrangement product 1,2,4,7,7a-pentahydroindene is generated in good yield. The mechanism of C-C bond activation is investigated using NMR spectroscopy, electrospray ionization mass spectrometry, and deuterium labeling, along with density functional theory calculations. These studies support an unusual catalytic mechanism in which an initial masked C-H bond activation initiates successive C-C bond cleavage events. PMID:23962182

Bowring, Miriam A; Bergman, Robert G; Tilley, T Don

2013-08-20

154

Hydrogen bond cooperativity in water hexamers: atomic energy perspective of local stabilities.  

PubMed

Atomic energies are used to describe local stability in eight low-lying water hexamers: prism, cage, boat 1, boat 2, bag, chair, book 1, and book 2. The energies are evaluated using the quantum theory of atoms in molecules (QTAIM) at MP2/aug-cc-pVTZ geometries. It is found that the simple, stabilizing cooperativity observed in linear hydrogen-bonded water systems is diminished as clusters move from nearly planar to three-dimensional structures. The prism, cage, and bag clusters can have local water stabilities differing up to 5 kcal mol(-1) as a result of mixed cooperative and anticooperative interactions. At the atomic level, in many cases a water may have a largely stabilized oxygen atom but the net water stability will be diminished due to strong destabilization of the water's hydrogen atoms. Analysis of bond critical point (BCP) electron densities shows that the reduced cooperativity results in a decrease in hydrogen bond strength and an increase in covalent bond strength, most evident in the prism. The chair, with the greatest cooperativity, has the largest average electron density at the BCP per hydrogen bond, whereas the cage has the largest total value for BCP density at all hydrogen bonds. The cage also has the second largest value (after the prism) for covalent bond critical point densities and an oxygen-oxygen BCP which may factor into the experimentally observed stability of the structure. PMID:24067198

Albrecht, Laura; Chowdhury, Saptarshi; Boyd, Russell J

2013-10-08

155

Prediction of RH Homolytic Bond Dissociation Energies from Gas-Phase IR Data on ?CH Values  

Microsoft Academic Search

The main goal of this work is the application of ?CH IR frequencies of gaseous R-H molecules to thermochemistry of carbon-centered Rfree radicals. For the first time the variable ??CH (in R-H molecules) per 1 kcal mol -1 in bond dissociation energies are suggested: 9 cm -1 for alkanes, 11 cm -1 for C-H bond in alkenes, 12 cm -1

Aleksander Golovin; Dmitry Ponomarev; Viatcheslav Takhistov

2004-01-01

156

Rh(I)-catalyzed decarbonylation of diynones via C-C activation: orthogonal synthesis of conjugated diynes.  

PubMed

Utilization of C-C bond activation as a unique mode of reactivity for constructing C-C bonds provides new strategies for preparing important organic molecules. Development of a Rh(I)-catalyzed C-C activation of diynones to synthesize symmetrical and unsymmetrical conjugated diynes through decarbonylation is reported. This C-C cleavage strategy takes advantage of the innate reactivity of conjugated ynones without relying on any ring strain or auxiliary directing group. This alkynation method also has orthogonal properties compared to typical cross-coupling reactions. PMID:23586742

Dermenci, Alpay; Whittaker, Rachel E; Dong, Guangbin

2013-04-15

157

Phase shift cavity ring down and Fourier transform infrared measurements of C-H vibrational transitions, energy levels, and intensities of (CH3)3Si-C?C-H  

NASA Astrophysics Data System (ADS)

Phase shift cavity ring down and Fourier transform IR techniques have been used to observe the C-H stretch fundamental and overtone absorptions of the acetylenic (?? = 1-5) and methyl (?? = 1-6) C-H bonds of trimethyl-silyl-acetylene [(CH3)3CSi?CH] at 295 K. Harmonic frequencies ?(?1), ?a, and ?s and anharmonicities x(?1), ?axa, ?sxs were calculated for the acetylenic, methyl out-of-plane, and methyl in-plane C-H bonds, respectively. The harmonically coupled anharmonic oscillator (HCAO) model was used to determine the overtone energy levels and assign the absorption bands to vibrational transitions of methyl C-H bonds. A hot band, assigned as ??1 + ?24 - ?24 is observed for transitions with ?? = 1-5 in a region near the acetylenic stretch. The intensity of the hot band is reduced considerably at 240 K. The strength of a Fermi resonance between C-Ha transition (??a) and the combination band ((?-1)?a + 2?bend) with (? = 3-6) was calculated using the experimental perturbed energies and relative intensities. The main bands are separated by computer deconvolution and are integrated at each level to get the experimental band strengths. For methyl absorptions, the dipole moment function is expanded as a function of two C-H stretching coordinates and the intensities are calculated in terms of the HCAO model where only the C-H modes are considered. Acetylenic intensities are derived with a one dimensional dipole moment function. The expansion coefficients are obtained from molecular orbital calculations. The intensities are calculated without using adjustable parameters and they are of the same order of magnitude of the experimental intensities for all C-H transitions.

Barroso, Jenny Z.; Perez-Delgado, Yasnahir; Manzanares, Carlos E.

2013-07-01

158

Energy Considerations Show that Low-Barrier Hydrogen Bonds do not Offer a Catalytic Advantage over Ordinary Hydrogen Bonds  

Microsoft Academic Search

Low-barrier hydrogen bonds have recently been proposed as a major factor in enzyme catalysis. Here we evaluate the feasibility of transition state (TS) stabilization by low-barrier hydrogen bonds in enzymes. Our analysis focuses on the facts that (i) a low-barrier hydrogen bond is less stable than a regular hydrogen bond in water, (ii) TSs are more stable in the enzyme

Arieh Warshel; Arno Papazyan

1996-01-01

159

Strong-bonding calcium phosphate coatings on carbon/carbon composites by ultrasound-assisted anodic oxidation treatment and electrochemical deposition  

NASA Astrophysics Data System (ADS)

The current work describes the ultrasound-assisted anodic oxidation treatment of carbon fiber-reinforced carbon (C/C) composites with H3PO4. The aim was to create chemical bonds between the surface-treated C/C composites and subsequently deposited calcium phosphate (CaP) coatings. The surface energy of C/C obviously increased after undergoing ultrasound-assisted anodic oxidation treatment at a current density of 50 mA/cm2 in aqueous 0.5 M H3PO4 solution (USAT-C/C-50). A large number of O-containing groups and some P-containing groups were found on the treated C/C substrate. The formation of O-containing groups may have been promoted by the energy input of the ultrasound and electric field. P-containing species could be bonded to C/C by CPO or COP bonding. Among the species formed, the formation of (H2PO4)- on C/C was promoted by ultrasound. When USAT-C/C-50 was used as the substrate for CaP coatings deposited by ultrasound-assisted electrochemical deposition (USECD), the surface of USAT-C/C-50 acquired strong CaP induction ability, and large amounts of CaP were deposited on it during the initial deposition process. After USECD, very strong-bonding CaP coatings formed on USAT-C/C-50. The average shear bond strength of the coatings obtained on the USAT-C/C-50 substrate [(12.14 ± 1.30) MPa] was about twice that of the coatings obtained on untreated C/C [(6.78 ± 1.06) MPa].

Zhao, Xueni; Li, Hejun; Chen, Mengdi; Li, Kezhi; Wang, Bin; Xu, Zhanwei; Cao, Sheng; Zhang, Leilei; Deng, Hailiang; Lu, Jinhua

2012-04-01

160

Frictional effects of interface bonding energy in blister testing  

Microsoft Academic Search

Blister tests are used to investigate the size effect on the critical energy release rate for the onset of debonding of interfaces between commercial tape and plain steel. The measured data display a size effect if the interfacial friction is not taken into account. A proportional displacement model is thus proposed to simultaneously evaluate the critical energy release rate and

M Zheng; Y. J Su; L. Q Chen; T. Y Zhang

1999-01-01

161

Co-C bond energies in adenosylcobinamide and methylcobinamide in the gas phase and in silico.  

PubMed

Essential to biological activity of adenosylcobalamin (AdoCbl) and methylcobalamin (MeCbl) is the Co-C bond cleavage step. Hence, we report an accurate determination of the homolytic gas-phase Co-C bond dissociation energies in the related adenosyl- and methylcobinamides (41.5 ± 1.2 and 44.6 ± 0.8 kcal/mol, respectively) utilizing an energy-resolved threshold collision-induced dissociation technique. This approach allows for benchmarking of electronic structure methods separate from (often ill-defined) solvent effects. Adequacy of various density functional theory methods has been tested with respect to the experimentally obtained values. PMID:24007238

Kobylianskii, Ilia J; Widner, Florian J; Kräutler, Bernhard; Chen, Peter

2013-09-10

162

XPS study of hydrogen permeation effect on SiC-C films  

NASA Astrophysics Data System (ADS)

70%SiC-C films were deposited with r.f. magnetron sputtering on stainless steel substrates followed by ion beam mixing. These films were permeated by hydrogen gas under the pressure of 3.23×107 Pa for 3 h at 500 K. X-ray photoelectron spectroscopy was used to analyze the chemical bonding states of C and Si in these 70%SiC-C films before and after hydrogen gas permeation. In addition, chemical states of contaminated oxygen were also checked. The effect of hydrogen permeation on those states in the 70%SiC-C films is discussed in this paper.

Huang, N. K.; Wang, D. Z.; Xiong, Q.; Yang, B.

2003-08-01

163

Potential energy surfaces for CH bond cleavage reactions  

SciTech Connect

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

Harding, L.B.

1996-12-31

164

Free energy profiles for H+ conduction along hydrogen-bonded chains of water molecules.  

PubMed Central

The molecular mechanism for proton conduction along hydrogen-bonded chains, or "proton wires," is studied with free energy simulations. The complete transport of a charge along a proton wire requires two complementary processes: 1) translocation of an excess proton (propagation of an ionic defect), and 2) reorientation of the hydrogen-bonded chain (propagation of a bonding defect). The potential of mean force profile for these two steps is computed in model systems comprising a single-file chain of nine dissociable and polarizable water molecules represented by the PM6 model of Stillinger and co-workers. Results of molecular dynamics simulations with umbrella sampling indicate that the unprotonated chain is preferably polarized, and that the inversion of its total dipole moment involves an activation free energy of 8 kcal/mol. In contrast, the rapid translocation of an excess H+ across a chain extending between two spherical solvent droplets is an activationless process. These results suggest that the propagation of a bonding defect constitutes a limiting step for the passage of several protons along single-file chains of water molecules, whereas the ionic translocation may be fast enough to occur within the lifetime of transient hydrogen-bonded water chains in biological membranes.

Pomes, R; Roux, B

1998-01-01

165

Low-energy conformers of pamidronate and their intramolecular hydrogen bonds: a DFT and QTAIM study.  

PubMed

Extensive DFT and ab initio calculations were performed to characterize the conformational space of pamidronate, a typical pharmaceutical for bone diseases. Mono-, di- and tri-protic states of molecule, relevant for physiological pH range, were investigated for both canonical and zwitterionic tautomers. Semiempirical PM6 method were used for prescreening of the single bond rotamers followed by geometry optimizations at the B3LYP/6-31++G(d,p) and B3LYP/6-311++G(d,p) levels. For numerous identified low energy conformers the final electronic energies were determined at the MP2/6-311++G(2df,2p) level and corrected for thermal effects at B3LYP level. Solvation effects were also considered via the COSMO and C-PCM implicit models. Reasonable agreement was found between bond lengths and angle values in comparison with X-ray crystal structures. Relative equilibrium populations of different conformers were determined from molecular partition functions and the role of electronic, vibrational and rotational degrees of freedom on the stability of conformers were analyzed. For no level of theory is a zwitterionic structure stable in the gas-phase while solvation makes them available depending on the protonation state. Geometrically identified intramolecular hydrogen bonds were analyzed by QTAIM approach. All conformers exhibit strong inter-phosphonate hydrogen bonds and in most of them the alkyl-amine side chain is folded on the P-C-P backbone for further hydrogen bond formation. PMID:22940874

Arabieh, Masoud; Karimi-Jafari, Mohammad Hossein; Ghannadi-Maragheh, Mohammad

2012-09-02

166

Orbital Energy Spectra of Electrons in Chemisorption Bonds: O, S, Se on Ni(100)  

Microsoft Academic Search

This work presents the first experimental determinations of orbital energy spectra of electrons in chemisorption bonds. Our method, ion neutralization spectroscopy, determines a transition density function which is essentially the local density of states in the surface region of the solid. For the particular surface formed by chemisorption of the chalcogens in ordered, surface-crystalline arrays on Ni(100), the local density

H. D. Hagstrum; G. E. Becker

1971-01-01

167

Hydrophobic bond energy of non-polar molecules: Application to ? -ionone and 11- cis retinal  

Microsoft Academic Search

A method for calculating the hydrophobic bond energy (HPE) of a nonpolar molecule at room temperature was presented by assuming that the whole HPE is the sum of HPE of each group or atom composing the molecule. This method was applied to ?-ionone and 11-cis retinal and those HPE's were found to be considerable. Some comparison of the present method

T. Kakitani; H. Kakitani; S. Yomosa

1980-01-01

168

Hardware synthesis from C\\/C++ models  

Microsoft Academic Search

Software programming languages, such as C\\/C++, have been used as means for specifying hardware for quite a while. Different design methodologies have exploited the advantages of flexibility and fast simulation of models de- scribed with programming languages. At the same time, the mismatch (of software languages) in expressing power (for hardware systems) has caused several difficulties. In the recent past,

Giovanni De Micheli

1999-01-01

169

Density functional theory with approximate kinetic energy functionals applied to hydrogen bonds  

Microsoft Academic Search

Applicability of the approximate kinetic energy functionals to study hydrogen-bonded systems by means of the formalism of Kohn–Sham equations with constrained electron density (KSCED) [Cortona, Phys. Rev. B 44, 8454 (1991); Wesołowski and Warshel, J. Phys. Chem. 97, 8050 (1993); Wesołowski and Weber, Chem. Phys. Lett. 248, 71 (1996)] is analyzed. In the KSCED formalism, the ground-state energy of a

Tomasz Adam Wesolowski

1997-01-01

170

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy.  

PubMed

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies. PMID:21992301

Blancafort, Lluís; Gatti, Fabien; Meyer, Hans-Dieter

2011-10-01

171

Addition, cycloaddition, and metathesis reactions of the cationic carbyne complexes [Cp(CO)[sub 2]Mn[triple bond]CCH[sub 2]R][sup +] and neutral vinylidene complexes Cp(CO)[sub 2]M=C=C(H)R (M = Mn, Re)  

SciTech Connect

The cationic alkylidyne complexes [Cp(CO)[sub 2]M=VCCH[sub 2]R][sup +] (M = Re, R = H; M = Mn, R = H, Me, Ph) undergo facile deprotonation to give the corresponding neutral vinylidene complexes Cp(CO)[sub 2]M=C=C(H)R. For [Cp(CO)[sub 2]Re=VCCH[sub 3

Terry, M.R.; Mercando, L.A.; Kelley, C.; Geoffroy, G.L. (Pennsylvania State Univ., University Park, PA (United States)); Nombel, P.; Lugan, N.; Mathieu, R. (Laboratoire de Chimie de Coordination du CNRS, Toulouse (France)); Ostrander, R.L.; Owens-Waltermire, B.E.; Rheingold, A.L. (Univ. of Delaware, Newark, DE (United States))

1994-03-01

172

CH Bond Functionalization in Complex Organic Synthesis  

Microsoft Academic Search

Direct and selective replacement of carbon-hydrogen bonds with new bonds (such as C-C, C-O, and C-N) represents an important and long-standing goal in chemistry. These transformations have broad potential in synthesis because C-H bonds are ubiquitous in organic substances. At the same time, achieving selectivity among many different C-H bonds remains a challenge. Here, we focus on the functionalization of

Kamil Godula; Dalibor Sames

2006-01-01

173

Theoretical study on aluminum carbide endohedral fullerene-Al4C@C80.  

PubMed

The possibility of a new endohedral fullerene with a trapped aluminum carbide cluster, Al(4)C@C(80)-I( h ), was theoretical investigated. The geometries and electronic properties of it were investigated using density functional theory methods. The Al(4)C unit formally transfers six electrons to the C(80) cage which induces stabilization of Al(4)C@C(80). A favorable binding energy, relatively large HOMO-LUMO gap, electron affinities and ionization potentials suggested the Al(4)C@C(80) is rather stable. The analysis of vertical ionization potential and vertical electron affinity indicate Al(4)C@C(80) is a good electron acceptor. PMID:23160935

Lu, Qi Liang; Song, Wen Jun; Meng, Jun Wei; Wan, Jian Guo

2012-11-17

174

Hydrogen Induced C-C, C-N, & C-S Bond Activation on Pt & Ni Surfaces  

SciTech Connect

The primary reactions investigated were chosen based on their importance in fuel and chemical production as well as in environmental remediation, and include reactions for hydrodesulfurization (HDS), hydrodenitrogenation (HDN), carbon-carbon hydrogenolysis, and hydrocarbon oxidation.

Gland, J. L.

2004-07-29

175

Fully integrated micro electromagnetic vibration energy harvesters with micro-patterning of bonded magnets  

Microsoft Academic Search

This paper presents the fabrication and characterization of a novel fully integrated micro electromagnetic vibration energy harvester using micro-patterning of bonded magnets. The magnetic material utilized is a polymer composite, consisting of a commercially available NdFeB powder dispersed in epoxy resin to a weight loading percentage of 90%. The prototype is fabricated using UV-LIGA technology and microelectroplating technology. The whole

K. Tao; G. Ding; P. Wang; Z. Yang; Y. Wang

2012-01-01

176

Bond energy and electronic structure in M-bis-terpyridine complexes (M=Os, Co and Ru)  

NASA Astrophysics Data System (ADS)

We investigate the nature of bond energy and electronic structure of M-bis-terpyridine complexes (M = Os, Co and Ru) using Hartree Fock and DFT calculations. The nature of bond energy is analyzed using a Morokuma Kitaura decomposition. DFT results are compared with the Hartree Fock leading to the conclusion that charge transfer and polarization effects are responsible for the bond energy of the complexes. Also we point out the fact that the electronic configuration of the metallic atom plays a dominant role in the charge transfer process.

Morari, C.

2008-03-01

177

Amide-to-E-olefin versus amide-to-ester backbone H-bond perturbations: Evaluating the O-O repulsion for extracting H-bond energies.  

PubMed

Both amide-to-ester and amide-to-E-olefin backbone amide mutation methods were employed to perturb the same H-bond (formed by the NH of F23 and the CO of R14) in the Pin WW domain. Comparison of the thermodynamic folding energies of the ester mutant and the E-olefin mutant, accounting for the transfer free energy differences measured on relevant model compounds, yielded an estimated value of 0.3 kcal/mol for the O-O repulsion term (DeltaGO-Orep) in a beta-sheet context. The value of DeltaGO-Orep enabled us to calculate the intrinsic F23-R14 H-bond free energy to be 1.3 kcal/mol. PMID:17165703

Fu, Yanwen; Gao, Jianmin; Bieschke, Jan; Dendle, Maria A; Kelly, Jeffery W

2006-12-20

178

Benchmark of density functional theory methods on the prediction of bond energies and bond distances of noble-gas containing molecules  

NASA Astrophysics Data System (ADS)

We have tested three pure density functional theory (DFT) functionals, BLYP, MPWPW91, MPWB95, and ten hybrid DFT functionals, B3LYP, B3P86, B98, MPW1B95, MPW1PW91, BMK, M05-2X, M06-2X, B2GP-PLYP, and DSD-BLYP with a series of commonly used basis sets on the performance of predicting the bond energies and bond distances of 31 small neutral noble-gas containing molecules. The reference structures were obtained using the CCSD(T)/aug-cc-pVTZ theory and the reference energies were based on the calculation at the CCSD(T)/CBS level. While in general the hybrid functionals performed significantly better than the pure functionals, our tests showed a range of performance by these hybrid functionals. For the bond energies, the MPW1B95/6-311+G(2df,2pd), BMK/aug-cc-pVTZ, B2GP-PLYP/aug-cc-pVTZ, and DSD-BLYP/aug-cc-pVTZ methods stood out with mean unsigned errors of 2.0-2.3 kcal/mol per molecule. For the bond distances, the MPW1B95/6-311+G(2df,2pd), MPW1PW91/6-311+G(2df,2pd), and B3P86/6-311+G(2df,2pd), DSD-BLYP/6-311+G(2df,2pd), and DSD-BLYP/aug-cc-pVTZ methods stood out with mean unsigned errors of 0.008-0.013 A? per bond. The current study showed that a careful selection of DFT functionals is very important in the study of noble-gas chemistry, and the most recommended methods are MPW1B95/6-311+G(2df,2pd) and DSD-BLYP/aug-cc-pVTZ.

Lai, Tai-Yuan; Yang, Chun-Yu; Lin, Hsiao-Jing; Yang, Chang-Yu; Hu, Wei-Ping

2011-06-01

179

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

SciTech Connect

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

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

2010-06-21

180

Quantum chemical calculations of bond dissociation energies for COOH scission and electronic structure in some acids  

NASA Astrophysics Data System (ADS)

Quantum chemical calculations are performed to investigate the equilibrium C—COOH bond distances and the bond dissociation energies (BDEs) for 15 acids. These compounds are studied by utilizing the hybrid density functional theory (DFT) (B3LYP, B3PW91, B3P86, PBE1PBE) and the complete basis set (CBS—Q) method in conjunction with the 6-311G** basis as DFT methods have been found to have low basis sets sensitivity for small and medium molecules in our previous work. Comparisons between the computational results and the experimental values reveal that CBS—Q method, which can produce reasonable BDEs for some systems in our previous work, seems unable to predict accurate BDEs here. However, the B3P86 calculated results accord very well with the experimental values, within an average absolute error of 2.3 kcal/mol. Thus, B3P86 method is suitable for computing the reliable BDEs of C—COOH bond for carboxylic acid compounds. In addition, the energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of studied compounds are estimated, based on which the relative thermal stabilities of the studied acids are also discussed.

Zeng, Hui; Zhao, Jun; Xiao, Xun

2013-02-01

181

Free energy decomposition analysis of bonding and nonbonding interactions in solution  

NASA Astrophysics Data System (ADS)

A free energy decomposition analysis algorithm for bonding and nonbonding interactions in various solvated environments, named energy decomposition analysis-polarizable continuum model (EDA-PCM), is implemented based on the localized molecular orbital-energy decomposition analysis (LMO-EDA) method, which is recently developed for interaction analysis in gas phase [P. F. Su and H. Li, J. Chem. Phys. 130, 074109 (2009)]. For single determinant wave functions, the EDA-PCM method divides the interaction energy into electrostatic, exchange, repulsion, polarization, desolvation, and dispersion terms. In the EDA-PCM scheme, the homogeneous solvated environment can be treated by the integral equation formulation of PCM (IEFPCM) or conductor-like polarizable continuum model (CPCM) method, while the heterogeneous solvated environment is handled by the Het-CPCM method. The EDA-PCM is able to obtain physically meaningful interaction analysis in different dielectric environments along the whole potential energy surfaces. Test calculations by MP2 and DFT functionals with homogeneous and heterogeneous solvation, involving hydrogen bonding, vdW interaction, metal-ligand binding, cation-?, and ionic interaction, show the robustness and adaptability of the EDA-PCM method. The computational results stress the importance of solvation effects to the intermolecular interactions in solvated environments.

Su, Peifeng; Liu, Hui; Wu, Wei

2012-07-01

182

Free energy decomposition analysis of bonding and nonbonding interactions in solution.  

PubMed

A free energy decomposition analysis algorithm for bonding and nonbonding interactions in various solvated environments, named energy decomposition analysis-polarizable continuum model (EDA-PCM), is implemented based on the localized molecular orbital-energy decomposition analysis (LMO-EDA) method, which is recently developed for interaction analysis in gas phase [P. F. Su and H. Li, J. Chem. Phys. 130, 074109 (2009)]. For single determinant wave functions, the EDA-PCM method divides the interaction energy into electrostatic, exchange, repulsion, polarization, desolvation, and dispersion terms. In the EDA-PCM scheme, the homogeneous solvated environment can be treated by the integral equation formulation of PCM (IEFPCM) or conductor-like polarizable continuum model (CPCM) method, while the heterogeneous solvated environment is handled by the Het-CPCM method. The EDA-PCM is able to obtain physically meaningful interaction analysis in different dielectric environments along the whole potential energy surfaces. Test calculations by MP2 and DFT functionals with homogeneous and heterogeneous solvation, involving hydrogen bonding, vdW interaction, metal-ligand binding, cation-?, and ionic interaction, show the robustness and adaptability of the EDA-PCM method. The computational results stress the importance of solvation effects to the intermolecular interactions in solvated environments. PMID:22830687

Su, Peifeng; Liu, Hui; Wu, Wei

2012-07-21

183

Structure and electronic spectra of the C C6- anion  

NASA Astrophysics Data System (ADS)

Ab initio calculations are employed for determining structures, spectroscopic parameters and transitions to the excited electronic states of the astrophysical relevant C C6- anion, which is a potential intermediate of chemical processes involving carbon chains and polycyclic aromatic hydrocarbons. Calculations confirm the prominent stability of linear carbon chain anions which guarantees their formation. The lowest doublet potential energy surface of C? displays six isomers, whereas in the lowest quartet potential energy surface, a unique minimum energy geometry is found. The most stable form is the linear anion l-C? (X2?u) followed by the c-C?(X2A1')at 9291 cm-1 above in energy, which presents a distorted D3h structure. The other isomers are also located at more than 9000 cm-1 above l-C? (X2?u). For each isomer, a set of spectroscopic parameters including their equilibrium structures, rotational constants at equilibrium, harmonic wavenumbers, dipole moments and electron affinities are predicted. For linear C?, eight electronic states lying below the electron detachment threshold, are computed. Four doublet electronic states, which play important roles in the C? reactivity, lie below 2 eV. Spin-orbit constants for the upper electronic states of l-C? are predicted.

Hammoutene, D.; Hochlaf, M.; Senent, M. L.

2012-08-01

184

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

NASA Astrophysics Data System (ADS)

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

Lundberg, Marcus; Siegbahn, Per E. M.

2005-01-01

185

Designing new free-radical reducing reagents: Theoretical study on Si–H bond dissociation energies of organic silanes  

Microsoft Academic Search

Bond dissociation energies of a series of substituted silanes were studied with the density functional theory methods. The performances of six different density functional methods including B3LYP, B3P86, BH&HLYP, B1LYP, PBE1KCIS, and TPSSLYP1W were examined for the prediction of Si–H bond dissociation energies. The results showed that B3P86 was the most accurate theoretical procedure among these six DFT methods. Using

Hua-Jing Wang; Yao Fu

2009-01-01

186

The chemical bond in external electric fields: Energies, geometries, and vibrational Stark shifts of diatomic molecules  

NASA Astrophysics Data System (ADS)

It is shown that the response of molecular properties of diatomics such as the total energy, the bond length, and the vibrational Stark shift to an external homogenous electric field (EF) can be predicted from field-free observable properties such as the equilibrium bond length, the bond dissociation energy, the polarizability and dipole moment functions, and the vibrational frequency. Delley [J. Mol. Struct.: THEOCHEM 434, 229 (1998)] suggested to approximate the potential energy surface under an EF by a Morse function augmented with a EF term proportional to the internuclear separation. In this work, this term is replaced by the expression of the field-induced energy change which yields a field-perturbed Morse potential that tends to a constant asymptotic limit when the EF term itself become proportional to the sum of the polarizabilities of the separated atoms. The model is validated by comparison with direct calculations on nine diatomics, five homo-nuclear (H2, N2, O2, F2, and Cl2) and four hetero-nuclear (HF, HCl, CO, and NO), covering a range and combinations of dipole moments and polarizabilities. Calculations were conducted at the quadratic configuration interaction with single and double excitations (QCISD) and density functional theory (DFT)-B3LYP levels of theory using the 6-311++G(3df,2pd) basis set. All results agree closely at the two levels of theory except for the Stark effect of NO which is not correctly predicted by QCISD calculations as further calculations, including at the coupled cluster with single and double excitation (CCSD) level of theory, demonstrate.

Sowlati-Hashjin, Shahin; Matta, Chérif F.

2013-10-01

187

Local bond length variations in boron-doped nanocrystalline diamond measured by spatially resolved electron energy-loss spectroscopy  

NASA Astrophysics Data System (ADS)

Variations in local bond length and coordination in boron-doped nanocrystalline diamond (NCD) films have been studied through changes in the fine structure of boron and carbon K-edges in electron energy-loss spectra, acquired in a scanning transmission electron microscope. The presence of high concentrations of B in pristine diamond regions and enrichment of B at defects in single NCD grains is demonstrated. Local bond length variations are evidenced through an energy shift of the carbon 1s --> ?* edge at B-rich defective regions within single diamond grains, indicating an expansion of the diamond bond length at sites with local high B content.

Lu, Ying-Gang; Turner, Stuart; Verbeeck, Johan; Janssens, Stoffel D.; Haenen, Ken; Van Tendeloo, Gustaaf

2013-07-01

188

Evaluation by Rocket Combustor of C/C Composite Cooled Structure Using Metallic Cooling Tubes  

NASA Astrophysics Data System (ADS)

In this study, the cooling performance of a C/C composite material structure with metallic cooling tubes fixed by elastic force without chemical bonding was evaluated experimentally using combustion gas in a rocket combustor. The C/C composite chamber was covered by a stainless steel outer shell to maintain its airtightness. Gaseous hydrogen as a fuel and gaseous oxygen as an oxidizer were used for the heating test. The surface of these C/C composites was maintained below 1500 K when the combustion gas temperature was about 2800 K and the heat flux to the combustion chamber wall was about 9 MW/m2. No thermal damage was observed on the stainless steel tubes that were in contact with the C/C composite materials. The results of the heating test showed that such a metallic tube-cooled C/C composite structure is able to control the surface temperature as a cooling structure (also as a heat exchanger) as well as indicated the possibility of reducing the amount of coolant even if the thermal load to the engine is high. Thus, application of this metallic tube-cooled C/C composite structure to reusable engines such as a rocket-ramjet combined-cycle engine is expected.

Takegoshi, Masao; Ono, Fumiei; Ueda, Shuichi; Saito, Toshihito; Hayasaka, Osamu

189

Study of B¯??c?¯c- and B¯??c+?¯c-K¯ decays at BABAR  

NASA Astrophysics Data System (ADS)

We report measurements of B-meson decays into two- and three-body final states containing two charmed baryons using a sample of 230×106 ?(4S)?BB¯ decays. We find significant signals in two modes, measuring branching fractions B(B-??c+?¯c-K-)=(1.14±0.15±0.17±0.60)×10-3 and B(B-??c0?¯c-)×B(?c0??-?+)=(2.08±0.65±0.29±0.54)×10-5, where the uncertainties are statistical, systematic, and from the branching fraction B(?c+?pK-?+), respectively. We also set upper limits at the 90% confidence level on two other modes: B(B¯0??c+?¯c-)×B(?c+??-?+?+)<5.6×10-5 and B(B¯0??c+?¯c-K¯0)<1.5×10-3. We observe structure centered at an invariant mass of 2.93GeV/c2 in the ?c+K- mass distribution of the decay B-??c+?¯c-K-.

Aubert, B.; Bona, M.; Boutigny, D.; Karyotakis, Y.; Lees, J. P.; Poireau, V.; Prudent, X.; Tisserand, V.; Zghiche, A.; Tico, J. Garra; Grauges, E.; Lopez, L.; Palano, A.; Pappagallo, M.; Eigen, G.; Stugu, B.; Sun, L.; Abrams, G. S.; Battaglia, M.; Brown, D. N.; Button-Shafer, J.; Cahn, R. N.; Groysman, Y.; Jacobsen, R. G.; Kadyk, J. A.; Kerth, L. T.; Kolomensky, Yu. G.; Kukartsev, G.; Pegna, D. Lopes; Lynch, G.; Mir, L. M.; Orimoto, T. J.; Osipenkov, I. L.; Ronan, M. T.; Tackmann, K.; Tanabe, T.; Wenzel, W. A.; Del Amo Sanchez, P.; Hawkes, C. M.; Watson, A. T.; Koch, H.; Schroeder, T.; Walker, D.; Asgeirsson, D. J.; Cuhadar-Donszelmann, T.; Fulsom, B. G.; Hearty, C.; Mattison, T. S.; McKenna, J. A.; Barrett, M.; Khan, A.; Saleem, M.; Teodorescu, L.; Blinov, V. E.; Bukin, A. D.; Druzhinin, V. P.; Golubev, V. B.; Onuchin, A. P.; Serednyakov, S. I.; Skovpen, Yu. I.; Solodov, E. P.; Todyshev, K. Yu.; Bondioli, M.; Curry, S.; Eschrich, I.; Kirkby, D.; Lankford, A. J.; Lund, P.; Mandelkern, M.; Martin, E. C.; Stoker, D. P.; Abachi, S.; Buchanan, C.; Foulkes, S. D.; Gary, J. W.; Liu, F.; Long, O.; Shen, B. C.; Vitug, G. M.; Zhang, L.; Paar, H. P.; Rahatlou, S.; Sharma, V.; Berryhill, J. W.; Campagnari, C.; Cunha, A.; Dahmes, B.; Hong, T. M.; Kovalskyi, D.; Richman, J. D.; Beck, T. W.; Eisner, A. M.; Flacco, C. J.; Heusch, C. A.; Kroseberg, J.; Lockman, W. S.; Schalk, T.; Schumm, B. A.; Seiden, A.; Wilson, M. G.; Winstrom, L. O.; Chen, E.; Cheng, C. H.; Fang, F.; Hitlin, D. G.; Narsky, I.; Piatenko, T.; Porter, F. C.; Andreassen, R.; Mancinelli, G.; Meadows, B. T.; Mishra, K.; Sokoloff, M. D.; Blanc, F.; Bloom, P. C.; Chen, S.; Ford, W. T.; Hirschauer, J. F.; Kreisel, A.; Nagel, M.; Nauenberg, U.; Olivas, A.; Smith, J. G.; Ulmer, K. A.; Wagner, S. R.; Zhang, J.; Gabareen, A. M.; Soffer, A.; Toki, W. H.; Wilson, R. J.; Winklmeier, F.; Altenburg, D. D.; Feltresi, E.; Hauke, A.; Jasper, H.; Merkel, J.; Petzold, A.; Spaan, B.; Wacker, K.; Klose, V.; Kobel, M. J.; Lacker, H. M.; Mader, W. F.; Nogowski, R.; Schubert, J.; Schubert, K. R.; Schwierz, R.; Sundermann, J. E.; Volk, A.; Bernard, D.; Bonneaud, G. R.; Latour, E.; Lombardo, V.; Thiebaux, Ch.; Verderi, M.; Clark, P. J.; Gradl, W.; Muheim, F.; Playfer, S.; Robertson, A. I.; Watson, J. E.; Xie, Y.; Andreotti, M.; Bettoni, D.; Bozzi, C.; Calabrese, R.; Cecchi, A.; Cibinetto, G.; Franchini, P.; Luppi, E.; Negrini, M.; Petrella, A.; Piemontese, L.; Prencipe, E.; Santoro, V.; Anulli, F.; Baldini-Ferroli, R.; Calcaterra, A.; de Sangro, R.; Finocchiaro, G.; Pacetti, S.; Patteri, P.; Peruzzi, I. M.; Piccolo, M.; Rama, M.; Zallo, A.; Buzzo, A.; Contri, R.; Lo Vetere, M.; Macri, M. M.; Monge, M. R.; Passaggio, S.; Patrignani, C.; Robutti, E.; Santroni, A.; Tosi, S.; Chaisanguanthum, K. S.; Morii, M.; Wu, J.; Dubitzky, R. S.; Marks, J.; Schenk, S.; Uwer, U.; Bard, D. J.; Dauncey, P. D.; Flack, R. L.; Nash, J. A.; Vazquez, W. Panduro; Tibbetts, M.; Behera, P. K.; Chai, X.; Charles, M. J.; Mallik, U.; Cochran, J.; Crawley, H. B.; Dong, L.; Eyges, V.; Meyer, W. T.; Prell, S.; Rosenberg, E. I.; Rubin, A. E.; Gao, Y. Y.; Gritsan, A. V.; Guo, Z. J.; Lae, C. K.; Denig, A. G.; Fritsch, M.; Schott, G.; Arnaud, N.; Béquilleux, J.; D'Orazio, A.; Davier, M.; Grosdidier, G.; Höcker, A.; Lepeltier, V.; Le Diberder, F.; Lutz, A. M.; Pruvot, S.; Rodier, S.; Roudeau, P.; Schune, M. H.; Serrano, J.; Sordini, V.; Stocchi, A.; Wang, W. F.; Wormser, G.; Lange, D. J.; Wright, D. M.; Bingham, I.; Burke, J. P.; Chavez, C. A.; Fry, J. R.; Gabathuler, E.; Gamet, R.; Hutchcroft, D. E.; Payne, D. J.; Schofield, K. C.; Touramanis, C.; Bevan, A. J.; George, K. A.; di Lodovico, F.; Sacco, R.; Cowan, G.; Flaecher, H. U.; Hopkins, D. A.; Paramesvaran, S.; Salvatore, F.; Wren, A. C.; Brown, D. N.; Davis, C. L.; Allison, J.; Bailey, D.; Barlow, N. R.; Barlow, R. J.; Chia, Y. M.; Edgar, C. L.; Lafferty, G. D.; West, T. J.; Yi, J. I.; Anderson, J.; Chen, C.; Jawahery, A.; Roberts, D. A.; Simi, G.; Tuggle, J. M.; Blaylock, G.; Dallapiccola, C.; Hertzbach, S. S.; Li, X.; Moore, T. B.; Salvati, E.; Saremi, S.; Cowan, R.; Dujmic, D.; Fisher, P. H.; Koeneke, K.; Sciolla, G.; Spitznagel, M.; Taylor, F.; Yamamoto, R. K.; Zhao, M.; Zheng, Y.; McLachlin, S. E.; Patel, P. M.; Robertson, S. H.; Lazzaro, A.; Palombo, F.; Bauer, J. M.; Cremaldi, L.; Eschenburg, V.; Godang, R.; Kroeger, R.; Sanders, D. A.; Summers, D. J.; Zhao, H. W.; Brunet, S.; Côté, D.; Simard, M.; Taras, P.; Viaud, F. B.; Nicholson, H.; de Nardo, G.; Fabozzi, F.; Lista, L.; Monorchio, D.; Sciacca, C.; Baak, M. A.; Raven, G.; Snoek, H. L.; Jessop, C. P.; Knoepfel, K. J.; Losecco, J. M.; Benelli, G.; Corwin, L. A.; Honscheid, K.; Kagan, H.; Kass, R.; Morris, J. P.; Rahimi, A. M.; Regensburger, J. J.; Sekula, S. J.; Wong, Q. K.

2008-02-01

190

Hydrogen bond energy of CH sub 3 CN-HCl by FTIR photometry  

SciTech Connect

The temperature dependence of an IR absorption band of CH{sub 3}CN-HCl is characterized by FTIR photometry. The heat of dimer formation at 41{degree}C obtained from this data, {Delta}H{sub D} = {minus}13.8 {plus minus} 1.2 kJ/mol, is used to calculate the equilibrium hydrogen bond dissociation energy, D{sub e} = 21.8 {plus minus} 1.7 kJ/mol. These results are compared with recent ab initio theoretical calculations and empirical estimates based on centrifugal distortion parameters.

Ballard, L.; Henderson, G. (Eastern Illinois Univ., Charleston (USA))

1991-01-24

191

Transition metal-catalyzed carbon-carbon bond activation.  

PubMed

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained. PMID:15592626

Jun, Chul-Ho

2004-11-04

192

Synthesis, structure, and reductive elimination in the series Tp'Rh(PR3)(Ar(F))H; determination of rhodium-carbon bond energies of fluoroaryl substituents.  

PubMed

A series of complexes of the type Tp'Rh(PR(3))(Ar(F))H, where PR(3) = PMe(3) (3) and PMe(2)Ph (9), Ar(F) = C(6)F(5) (a), 2,3,4,5-C(6)F(4)H (b), 2,3,5,6-C(6)F(4)H (c), 2,4,6-C(6)F(3)H(2) (d), 2,3-C(6)F(2)H(3) (e), 2,5-C(6)F(2)H(3) (g), and 2-C(6)FH(4) (h) and Tp' = tris(3,5-dimethylpyrazolyl)borate, has been synthesized as stable crystalline compounds by the reactions of the [Tp'Rh(PR(3))] fragment with the corresponding fluorinated aromatic hydrocarbons, and their structures were characterized by NMR spectroscopy and elemental analysis together with X-ray crystallography. The kinetics of the reductive eliminations of fluoroarenes from complexes 3a-h in benzene-d(6) solutions at 140 °C were investigated, but were complicated by the formation of the rhodium(I) bisphosphine complex, Tp'Rh(PMe(3))(2) (4). On the other hand, thermal reactions of (9) in THF-d(8) solutions at 120 °C resulted in the formation of an intramolecular C-H bond activated complex of the phenyl group on the phosphorus atom, Tp'Rh(?(2)-C(6)H(4)-2-PMe(2))H (7), which prevents the formation of the corresponding bisphosphine complex. The experimentally determined rates of the reductive eliminations of fluoroarenes from the complexes 9a-h and their kinetic selectivities for formation in competition with the metallacycle have been used to determine relative Rh-CAr(F) bond energies. The Rh-CAr(F) bond energy is found to be dependent on the number of ortho fluorines. A plot of Rh-CAr(F) vs. C-H bond strengths resulted in a line with a slope R(M-C/C-H) of 2.15 that closely matches the DFT calculated value (slope = 2.05). PMID:20924525

Tanabe, Taro; Brennessel, William W; Clot, Eric; Eisenstein, Odile; Jones, William D

2010-10-05

193

Chemical bonding in 1-(chlorodimethylstannylmethyl)-2-piperidone and its Si and Ge analogues. General trends and O ? M (M = Si, Ge, Sn) coordination bond energy  

NASA Astrophysics Data System (ADS)

Experimental and computational charge density study in the crystal and isolated molecule of 1-(chlorodimethylstannylmethyl)-2-piperidone containing pentacoordinated Sn atom was carried out. Quantum chemical calculations were performed out for Si and Ge analogues of the title compound. Also the calculations were carried out for its Ge and Si analogues. The O ? Sn coordination bond was found to be a weak interaction with mostly ionic character. The integration of electron density over atomic basins with the subsequent computation of atomic charges allowed to estimate the amount of the electron density transferred from the 2-piperidone ligand to the Me2SnCl moiety. The energy of the O ? Sn coordination bond estimated from the strengthening potential was in good agreement with the values obtained from the topological analysis of the charge density.

Korlyukov, Alexander A.; Lyssenko, Konstantin A.; Baukov, Yuri I.; Bylikin, Sergey Yu.

2013-11-01

194

Energy as Money, Chemical Bonding as Business, and Negative ?H and ?G as Investment  

NASA Astrophysics Data System (ADS)

A simple analogy is suggested illustrating the thermodynamic sign convention and the significance of the negative sign of ?H and ?G on spontaneity for general or introductory chemistry students. Enthalpy or energy is associated with money, and atoms or molecules are referred to as people. The suggested metaphor for ?H is net profit or investment into the surroundings; positive ?S is viewed as internal investment into the system. Hence, ?G is similar to gross profit in business; its dependence on temperature is illustrated by the reversal of the market trends for high-tech companies in 2000. The analogy is further expanded to illustrate the significance of ionic lattice enthalpy, the positive sign of atomic ionization energies, and chemical bond enthalpies.

Kozliak, Evguenii I.

2002-12-01

195

Gold for C-C coupling reactions: a Swiss-Army-knife catalyst?  

PubMed

For organic chemists, the construction of C-C bonds is the most essential aspect of the assembly of molecules. Transition-metal-catalyzed coupling reactions have evolved as one of the key tools for this task. Lately, gold has also emerged as a catalyst for this kind of transformation. Gold, with its special properties as a mild carbophilic ? Lewis acid, its ability to insert into C-H bonds, and, as discovered recently, its ability to undergo redox transformations, offers the opportunity to apply all this potent proficiency for the construction of compounds in an efficient and economical way. This Minireview critically presents the C-C coupling reactions enabled by gold catalysts to encourage further research activities in this promising area of oxidation/reduction gold catalysts. PMID:21818831

Wegner, Hermann A; Auzias, Mathieu

2011-08-04

196

Evaluation of N–H bond dissociation energies in some amides using ab initio and density functional methods  

Microsoft Academic Search

The performance of ab initio and DFT methods in the evaluation of N–H bond dissociation energies of formamide, N-methyl formamide, urea, and acetamide molecules has been analyzed. Both restricted and unrestricted HF and MP2 fail to provide reliable results because of unreliable spin localization of the radical. The composite methods G3, G2MP2, and CBS-Q, however, provide fairly accurate bond dissociation

D. Kaur; Rupinder Preet Kaur

2005-01-01

197

Electron energy-loss study of bonding in amorphous silicon-carbon alloy films prepared with hydrogen dilution  

Microsoft Academic Search

Electron energy-loss spectroscopy was used to study the nature of chemical bonding in a-Si1?xCx:H films produced by rf glow discharge decomposition of SiH4\\/CH4 mixtures with hydrogen dilution. The low-loss region, the Si L edge, and the C K edge were studied. It was found that near stoichiometric films have a high degree of Si to C bonding and that substoichiometric

D. R. McKenzie; J. Bruley; G. B. Smith

1988-01-01

198

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

Microsoft Academic Search

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

A. F. Shestakov; E. T. Denisov; N. S. Emel’yanova; T. G. Denisova

2009-01-01

199

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

Microsoft Academic Search

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

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

2009-01-01

200

Potential energy surface and unimolecular dynamics of stretched n-butane  

NASA Astrophysics Data System (ADS)

The potential energy surface (PES) and unimolecular reaction dynamics of stretched n-butane are investigated, as a model for a stretched ``normal'' alkane or straight chain polymer. The nature of the PES for stretched n-butane depends on the extent of stretching. If it is less than that required to reach the inflection points in the C-C stretch potentials and the C-C torsions are considered free rotors, there is only one potential energy minimum, with each bond elongated. However, for stretching past these inflection points, the PES has three minima and each has one bond longer than the other two, i.e., C-C-C-C, C-C-C-C, and C-C-C-C. There are three transition states (TSs) connecting these minima. A linear alkane, consisting of n carbon atoms and stretched past its C-C inflection points, has (n-1) minima and (n-1)(n-2)/2 TSs connecting them. For stretching less than that required to reach the C-C inflection points, the only unimolecular pathways are dissociations to form the C+C-C-C, C-C+C-C, and C-C-C+C products. However, with stretching past the C-C inflection points, isomerizations between the three potential energy minima may also occur. The relative importance of isomerization versus dissociation depends on the relative size of their barriers. For slight stretching past the C-C inflection points, the isomerization barriers are much lower than those for dissociation and relaxation between the minima is much faster than dissociation. Thus, the molecule samples these minima randomly during its dissociation, with a density of states that comprises the complete PES. With extensive stretching past the inflection points, isomerizations between the potential energy minima do not occur and only dissociation for the excited minima occurs, e.g., C-C-C-C-->C+C-C-C. For intermediate stretching past the C-C inflection points, the rates for the isomerization and dissociation pathways are competitive and both must be considered in modeling the dissociation kinetics. Microcanonical chemical dynamics simulations are performed to study the unimolecular kinetics of n-butane in three stretched configurations: Stretched less than the C-C inflection point; stretched slightly beyond the C-C inflection point; and stretched significantly beyond the C-C inflection point. The resulting unimolecular dynamics were found to be in excellent agreement with RRKM theory. Frequency ? factors, determined by fitting the trajectory unimolecular rate constants to the classical harmonic RRKM rate constant expression, depend upon the extent of stretching and vary from 1.0×1012-8.4×1016 s-1. For a molecule with a large number of vibrational degrees of freedom and high excess energy, it is shown that the classical harmonic RRKM and classical harmonic transition state theory rate constants, k(E) and k(T), are equivalent.

Lourderaj, Upakarasamy; McAfee, Jason L.; Hase, William L.

2008-09-01

201

Extra-thermodynamic study on surface diffusion in reversed-phase liquid chromatography using silica gels bonded with alkyl ligands of different chain lengths  

Microsoft Academic Search

Surface diffusion on adsorbents made of silica gels bonded to C, C, C, and C alkyl ligands was studied in reversed-phase liquid chromatography (RPLC) from the viewpoints of two extrathermodynamic relationships: enthalpy-entropy compensation (EEC) and linear free-energy relationship (LFER). First, the values of the surface diffusion coefficient (D{sub s}), normalized by the density of the alkyl ligands, were analyzed with

Kanji Miyabe; Georges A Guiochon

2005-01-01

202

Nano-Bonding of Silicon Oxides-based surfaces at Low Temperature: Bonding Interphase Modeling via Molecular Dynamics and Characterization of Bonding Surfaces Topography, Hydro-affinity and Free Energy  

NASA Astrophysics Data System (ADS)

In this work, a new method, "Nanobonding(TM)" [1,2] is conceived and researched to bond Si-based surfaces, via nucleation and growth of a 2 D silicon oxide SiOxHx interphase connecting the surfaces at the nanoscale across macroscopic domains. Nanobonding cross-bridges two smooth surfaces put into mechanical contact in an O2/H 2O mixed ambient below T ?200 °C via arrays of SiOxH x molecules connecting into a continuous macroscopic bonding interphase. Nano-scale surface planarization via wet chemical processing and new spin technology are compared via Tapping Mode Atomic Force Microscopy (TMAFM) , before and after nano-bonding. Nanobonding uses precursor phases, 2D nano-films of beta-cristobalite (beta-c) SiO2, nucleated on Si(100) via the Herbots-Atluri (H-A) method [1]. beta-c SiO2 on Si(100) is ordered and flat with atomic terraces over 20 nm wide, well above 2 nm found in native oxides. When contacted with SiO2 this ultra-smooth nanophase can nucleate and grow domains with cross-bridging molecular strands of hydroxylated SiOx, instead of point contacts. The high density of molecular bonds across extended terraces forms a strong bond between Si-based substrates, nano- bonding [2] the Si and silica. A new model of beta-cristobalite SiO2 with its <110> axis aligned along Si[100] direction is simulated via ab-initio methods in a nano-bonded stack with beta-c SiO2 in contact with amorphous SiO2 (a-SiO2), modelling cross-bridging molecular bonds between beta-c SiO2 on Si(100) and a-SiO2 as during nanobonding. Computed total energies are compared with those found for Si(100) and a-SiO2 and show that the presence of two lattice cells of !-c SiO2 on Si(100) and a-SiO2 lowers energy when compared to Si(100)/ a-SiO 2 Shadow cone calculations on three models of beta-c SiO2 on Si(100) are compared with Ion Beam Analysis of H-A processed Si(100). Total surface energy measurements via 3 liquid contact angle analysis of Si(100) after H-A method processing are also compared. By combining nanobonding experiments, TMAFM results, surface energy data, and ab-initio calculations, an atomistic model is derived and nanobonding is optimized. [1] US Patent 6,613,677 (9/2/03), 7,851,365 (12/14/10), [2] Patent Filed: 4/30/09, 10/1/2011

Whaley, Shawn D.

203

Rotational spectrum of ethyl cyanoacetylene (C2H5C?C-C?N), a compound of potential astrochemical interest  

NASA Astrophysics Data System (ADS)

Context. New radiotelescopes, such as the very sensitive ALMA, will enable the detection of interstellar molecules in much lower concentrations than previously possible. A successful identification of an interstellar molecule requires that laboratory microwave and millimeter-wave spectra are investigated. Several cyanopolyynes and alkynylcarbonitriles have already been detected in the interstellar medium (ISM). Cyanoacetylene (HC?C-C?N) is abundant in the ISM and its methyl derivative, 2-butynenitrile (CH3C?C-C?N), is also present. The next derivative, ethyl cyanoacetylene, (2-pentynenitrile C2H5C?C-C?N) may also be present in interstellar space. Aims: We report the rotational spectrum of the ethyl cyanoacetylene (C2H5C?C-C?N). This is hoped to facilitate identifying gaseous ethyl cyanoacetylene in the ISM. Methods: We studied the rotational spectrum of C2H5C?C-C?N between 13 and 116 GHz with the microwave spectrometer of the University of Oslo. The spectroscopic study was augmented by high-level quantum-chemical calculations at B3LYP/cc-pVTZ and CCSD/cc-pVTZ levels of theory. Results: We present for the first time the rotational spectrum of the ethyl cyanoacetylene (C2H5C?C-C?N). We assigned 342 transitions of the vibrational ground state, accurate values were obtained for rotational and centrifugal distortion constants, and the dipole moment was determined as well.

Carles, S.; Møllendal, H.; Guillemin, J.-C.

2013-10-01

204

Formation of annealing twins in f.c.c. crystals  

SciTech Connect

A microscopic model for the formation of annealing twins in f.c.c. crystals is proposed. It is argued that Shockley partial loops nucleate on consecutive {l_brace}111{r_brace} planes by growth accidents occurring on migrating {l_brace}111{r_brace} steps associated with a moving grain boundary. The higher the velocity of the boundary, the higher the twin density. The absence of twins in high stacking fault energy materials and the influence of temperature on twin density has been developed by examining the influence of deformation damage on the incidence of twinning in copper and the effect of boron in reducing twin density in annealed nickel.

Mahajan, S. [Carnegie Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering; Pande, C.S.; Imam, M.A.; Rath, B.B. [Naval Research Lab., Washington, DC (United States)

1997-06-01

205

Variable photon energy photoelectron spectroscopic studies of covalent bonding in 3d/sup 10/ transition-metal compounds  

SciTech Connect

Variable photon energy photoelectron spectroscopy (PES) is used to determine the valence band electronic structure and to study the covalent bonding properties of three tetrahedral, 3d/sup 10/ transition-metal complexes with /sigma/-donor, /pi/-donor ligands: ZnO, ZnCl/sub 4//sup 2/minus//, and CuCl. PES data obtained over the photon energy range of 16.7-1486.6 eV show dramatic intensity changes of valence band features that are caused by changes in photoionization cross sections and resonance effects at the metal 3p absorption edge. Intensity changes with photon energy allow detailed assignments of spectra to be made by using molecular orbital theory. Analyses of the peak intensities and binding energies provide a measure of the covalent bonding. Larger ZnO 3d band splitting relative to that for ZnCl/sub 4//sup 2/minus// indicates greater ligand field strength (10Dq) for the oxide ligands, which arises from the stronger /sigma/-bonding interactions of the Zn/sup 2+/ 3d with the O/sup 2/-2p orbitals. The increased effective nuclear charge on Zn/sup 2+/ shifts the d levels below the ligand levels, making them bonding and reversing the sign of the tetrahedral 10Dq value. This greater effective nuclear charge on Zn/sup 2+/ also significantly lowers the energy of the empty metal 4s and 4p levels, resulting in greater covalent stabilization of the ligand bonding levels in ZnCl/sub 4//sup 2/minus// than in CuCl. The relative contributions of ionic and covalent bonding in ZnCl/sub 2/ and CuCl are examined and related to the much greater thermodynamic stability of Zn/sup 2+/ complexes with donor ligands. 74 references, 13 figures, 5 tables.

Didziulis, S.V.; Cohen, S.L.; Butcher, K.D.; Solomon, E.I.

1988-06-29

206

The C-H bond dissociation energy of furan: Photoelectron spectroscopy of the furanide anion  

NASA Astrophysics Data System (ADS)

Using photoelectron spectroscopy, we interrogate the cyclic furanide anion (C4H3O-) to determine the electron affinity and vibrational structure of the neutral furanyl radical and the term energy of its first excited electronic state. We present the 364-nm photoelectron spectrum of the furanide anion and measure the electron affinity of the X~2A' ground state of the ?-furanyl radical to be 1.853(4) eV. A Franck-Condon analysis of the well-resolved spectrum allows determination of the harmonic frequencies of three of the most active vibrational modes upon X~2A' <-- X~1A' photodetachment: 855(25), 1064(25), and 1307(40) cm-1. These modes are ring deformation vibrations, consistent with the intuitive picture of furanide anion photodetachment, where the excess electron is strongly localized on the ?-carbon atom. In addition, the A~2A'' excited state of the ?-furanyl radical is observed 0.68(7) eV higher in energy than the X~2A' ground state. Through a thermochemical cycle involving the known gas-phase acidity of furan, the electron affinity of the furanyl radical yields the first experimental determination of the C-H? bond dissociation energy of furan (DH298(C4H3O-H?)): 119.8(2) kcal mol-1.

Vogelhuber, Kristen M.; Wren, Scott W.; Sheps, Leonid; Lineberger, W. Carl

2011-02-01

207

Determination of bond ionicity using low-energy D[sup +] scattering  

SciTech Connect

We report on the systematic investigation of neutralization of low-energy D[sup +] ions scattered from 17 ionic-compound surfaces with particular emphasis on the relation between the neutralization probability of D[sup +] and the ionicity of the surface atomic bonds. The D[sup +] ions tend to survive neutralization provided that the target atom is completely ionized, whereas almost complete neutralization results if the target has covalent or metallic orbital hybridization with the ligand. The D[sup +] spectra from nitrates, sulfates, and carbonates of Na and K, as well as K[sub 2]SiO[sub 3], exhibit surface peaks of D[sup +] surviving neutralization for both cations and oxygen, indicating large ionicity in the alkali-metal--oxygen bond. Rather exceptional is Na[sub 2]SiO[sub 3], for which a relatively small ionicity is concluded from the absence of surface peaks. For the alkaline-earth compounds, ionicity decreases in going to the light elements as evidenced by the decrease of the surface-peak intensity. The occurrence of charge-exchange relaxation between cations and anions of NiSO[sub 4] and CuSO[sub 4] results in large neutralization probability of D[sup +], while the closed-shell structures of Ag[sup +] (4[ital d][sup 10]) and Pb[sup 2+] (6[ital s][sup 2]) in AgNO[sub 3] and Pb(NO[sub 3])[sub 2], respectively, suppress neutralization to some extent so that the appreciable surface peaks appear.

Souda, R.; Hayami, W.; Aizawa, T.; Ishizawa, Y. (National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305 (Japan))

1994-07-15

208

Theoretical study of the binding energy and bonding of benzene to the Ni (111), (100), and (110) surfaces  

SciTech Connect

The atomic superposition electron delocalization molecular orbital (ASED-MO) method has been applied to a study of the favored bonding sites for benzene adsorbed on the (111), (100), and (110) crystal faces of nickel metal. The different faces were represented in the calculations by clusters, which contained 30 nickel atoms. By using the same size clusters and parameters, it was possible to compare the results among the three different crystal faces. The most favored (highest binding energy) sites for the benzene ring parallel to the surface were found to be the highest coordination site for the (111) and (100) planes and the atop site for the (110) plane. These results are compared with both experimental and other theoretical calculations on these crystal planes. In addition, the bonding of the benzene to the Ni surfaces has been investigated by using [open quotes]bonding plots[close quotes], which are graphical representations of the Mulliken population matrix. By use of these bonding plots, a picture of the bonding of the benzene to the nickel surface is developed and an explanation for the increase of the binding energy with increased cluster size is discussed. 28 refs., 5 figs., 3 tabs.

Grimm, F.A. (Univ. of Tennessee, Knoxville (United States) Oak Ridge National Lab., TN (United States)); Huntley, D.R. (Oak Ridge National Lab., TN (United States))

1993-04-15

209

Tetramethylthiuram monosulfide and tetramethylthiuram disulfide: heats of formation by rotating-bomb calorimetry; the SS thermochemical bond energy  

Microsoft Academic Search

The heats of combustion and formation were determined for tetramethylthiuram monosulfide (bis-(dimethylthiocarbamoyl) sulfide) and tetramethylthiuram disulfide (bis-(dimethylthiocarbamoyl) disulfide). The S-S thermochemical bond energy in tetramethylthiuram disulfide was shown to be about the same as in normal alkane disulfides and in Sâ. Rotating-bomb combustion calorimetry was found satisfactory for compounds that contain both sulfur and nitrogen.

W. D. Good; J. L. Lacina; J. P. McCullough

1961-01-01

210

Effects of hydrogen bonding on vibrational normal modes of pyrimidine.  

PubMed

The effects of weak intermolecular interactions on 10 vibrational normal modes of pyrimidine are investigated by Raman spectroscopy and electronic structure computations. Hydrogen-bonded networks of water induce a shift to higher energy in certain normal modes of pyrimidine with increasing water concentration, while other modes are relatively unaffected. Pyrimidine molecules also exhibit weak C-H...N interactions and shifted normal modes upon crystallization. The selective nature of the shifting of normal modes to higher energy allows for definitive assignments of the nearly degenerate nu(8a) and nu(8b) modes with polarized Raman spectroscopy. Natural bond orbital (NBO) analyses indicate that when water molecules donate hydrogen bonds to the nitrogen atoms of pyrimidine, there is significant charge transfer from pyrimidine to water, much of which can be accounted for by substantial decreases in the populations of the nitrogen lone pair orbitals. Despite the overall decrease of electron density in pyrimidine upon complexation with water, there are concomitant changes in NBO populations that polarize the pi-electron system toward the proton acceptor N atoms, as well as contractions of the bonds associated with the N-C-N and C-C-C regions of the pyrimidine ring. PMID:20527867

Howard, Austin A; Tschumper, Gregory S; Hammer, Nathan I

2010-07-01

211

Solvent dependent competition between fluorescence resonance energy transfer and through bond energy transfer in rhodamine appended hexaphenylbenzene derivatives for sensing of Hg(2+) ions.  

PubMed

Hexaphenylbenzene (HPB) derivatives 5 and 7 having rhodamine B moieties have been designed and synthesized, and have been shown to display solvent dependent. Fluorescence resonance energy transfer (FRET) and through bond energy transfer (TBET) in the presence of Hg(2+) ions among the various cations (Cu(2+), Pb(2+), Zn(2+), Ni(2+), Cd(2+), Ag(+), Ba(2+), Mg(2+), K(+), Na(+), and Li(+)) have been tested. Derivative 5 displays quite high through bond energy transfer efficiency in the presence of Hg(2+) ions in methanol whereas derivative 7 exhibits better FRET efficiency in the presence of Hg(2+) ions in THF and CH(3)CN than derivative 5. PMID:23385981

Bhalla, Vandana; Vij, Varun; Tejpal, Ruchi; Singh, Gopal; Kumar, Manoj

2013-04-01

212

Modelling of c-C2H4O formation on grain surfaces  

NASA Astrophysics Data System (ADS)

Despite its potential reactivity due to ring strain, ethylene oxide (c-C2H4O) is a complex molecule that seems to be stable under the physical conditions of an interstellar dense core; indeed, it has been detected towards several high-mass star-forming regions with a column density of the order of 1013 cm-2. To date, its observational abundances cannot be reproduced by chemical models and this may be due to the significant contribution played by its chemistry on grain surfaces. Recently, Ward & Price have performed experiments in order to investigate the surface formation of ethylene oxide starting with oxygen atoms and ethylene ice as reactants. We present a chemical model which includes the most recent experimental results from Ward & Price on the formation of c-C2H4O. We study the influence of the physical parameters of dense cores on the abundances of c-C2H4O. We verify that ethylene oxide can indeed be formed during the cold phase (when the interstellar medium dense cores are formed), via addition of an oxygen atom across the C=C double bond of the ethylene molecule, and released by thermal desorption during the hot core phase. A qualitative comparison between our theoretical results and those from the observations shows that we are able to reproduce the abundances of ethylene oxide towards high-mass star-forming regions.

Occhiogrosso, A.; Viti, S.; Ward, M. D.; Price, S. D.

2012-12-01

213

Zero kinetic energy photoelectron (ZEKE) spectroscopy of the heterotrimer phenol-water-argon: Interaction between a hydrogen bond and a van der Waals bond  

Microsoft Academic Search

The heterotrimer phenol -water- Ar, a complex containing two different types of intermolecular bonds — a van der Waals bond and a hydrogen bond — has been studied in a supersonic jet with various spectroscopic techniques. The two-photon, two-color (1 + 1?) resonance-enhanced multiphoton ionization (REMPI) spectrum of the S1 state shows striking differences compared to the spectrum of the

Otto Dopfer; Markus Melf; Klaus Müller-Dethlefs

1996-01-01

214

Energy barriers between H-bonded and stacked structures of 9-methyladenine...1-methylthymine and 9-methylguanine...1-methylcytosine complexes.  

PubMed

The transition structures (TS) between H-bonded (H) and stacked (S) structures of 9-methyladenine...1-methylthymine and 9-methylguanine...1-methylcytosine base pairs were localized at the DFT-D/TZVP potential energy surface. The energy barrier between the S and TS structures is considerably higher for the former pair than for the latter, which makes localization of the stacked structure of this pair possible. PMID:20066299

Cerný, Jirí; Hobza, Pavel

2009-11-13

215

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

SciTech Connect

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

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

2004-07-19

216

Exceptional sensitivity of metal-aryl bond energies to ortho-fluorine substituents: influence of the metal, the coordination sphere, and the spectator ligands on M-C/H-C bond energy correlations.  

PubMed

DFT calculations are reported of the energetics of C-H oxidative addition of benzene and fluorinated benzenes, Ar(F)H (Ar(F) = C(6)F(n)H(5-n), n = 0-5) at ZrCp(2) (Cp = eta(5)-C(5)H(5)), TaCp(2)H, TaCp(2)Cl, WCp(2), ReCp(CO)(2), ReCp(CO)(PH(3)), ReCp(PH(3))(2), RhCp(PH(3)), RhCp(CO), IrCp(PH(3)), IrCp(CO), Ni(H(2)PCH(2)CH(2)PH(2)), Pt(H(2)PCH(2)CH(2)PH(2)). The change in M-C bond energy of the products fits a linear function of the number of fluorine substituents, with different coefficients corresponding to ortho-, meta-, and para-fluorine. The values of the ortho-coefficient range from 20 to 32 kJ mol(-1), greatly exceeding the values for the meta- and para-coefficients (2.0-4.5 kJ mol(-1)). Similarly, the H-C bond energies of Ar(F)H yield ortho- and para-coefficients of 10.4 and 3.4 kJ mol(-1), respectively, and a negligible meta-coefficient. These results indicate a large increase in the M-C bond energy with ortho-fluorine substitution on the aryl ring. Plots of D(M-C) vs D(H-C) yield slopes R(M-C/H-C) that vary from 1.93 to 3.05 with metal fragment, all in excess of values of 1.1-1.3 reported with other hydrocarbyl groups. Replacement of PH(3) by CO decreases R(M-C/H-C) significantly. For a given ligand set and metals in the same group of the periodic table, the value of R(M-C/H-C) does not increase with the strength of the M-C bond. Calculations of the charge on the aryl ring show that variations in ionicity of the M-C bonds correlate with variations in M-C bond energy. This strengthening of metal-aryl bonds accounts for numerous experimental results that indicate a preference for ortho-fluorine substituents. PMID:19453181

Clot, Eric; Mégret, Claire; Eisenstein, Odile; Perutz, Robin N

2009-06-10

217

Modeling energy transfer to copper wire for bonding in an inert environment  

Microsoft Academic Search

Copper wire ball bonding has gained popularity in the last few years due to its economic advantage and superior electrical performance. However, because of copper's reactive nature to oxygen, free air ball formation for copper bonding is more crucial than in the gold wire process. To create a free air ball (FAB), the wire bonder uses an electronic flame-off (EFO)

Hong Meng Ho; J. Tan; Yee Chen Tan; Boon Hoe Toh; P. Xavier

2005-01-01

218

Novel carbon-carbon bond formations for biocatalysis  

PubMed Central

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

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

2011-01-01

219

A DFT study on the interaction between adsorbed silver on C?? and disulfide bond.  

PubMed

Adsorption of a silver atom on the surface of Buckyball (C??) was investigated using density functional theory (DFT). The Ag atom tends to occupy the bridge site over C--C bond in pentagon-hexagon ring junction with the binding energy of -38.33 kcal mol?¹. The capability of destroying S--S bond by both a single silver atom and the silver atom adsorbed on C?? was also investigated by DFT calculations using dimethyl disulfide as the molecular model. The results of the natural bond orbital (NBO) and population analysis indicate that the cleavage of the S--S bond effectively occurs by the silver atom adsorbed on C??. Since denaturation of disulfide bonds of envelope glycoprotein (gp) 120 is a key step in the prevention of the spread of HIV-1, the development of the proposed study is promised to HIV-1 research field. PMID:23085174

Azizi, Khaled; Sohrabinia, Ali

2012-06-26

220

Substituent effect on the energy barrier for ?-bond formation from ?-single-bonded species, singlet 2,2-dialkoxycyclopentane-1,3-diyls.  

PubMed

Background: Localized singlet diradicals are in general quite short-lived intermediates in processes involving homolytic bond-cleavage and formation reactions. In the past decade, long-lived singlet diradicals have been reported in cyclic systems such as cyclobutane-1,3-diyls and cyclopentane-1,3-diyls. Experimental investigation of the chemistry of singlet diradicals has become possible. The present study explores the substituents and the effect of their substitution pattern at the C(1)-C(3) positions on the lifetime of singlet octahydropentalene-1,3-diyls to understand the role of the substituents on the reactivity of the localized singlet diradicals. Results: A series of singlet 2,2-dialkoxy-1,3-diaryloctahydropentalene-1,3-diyls DR were generated in the photochemical denitrogenation of the corresponding azoalkanes AZ. The ring-closed products CP, i.e., 3,3-dialkoxy-2,4-diphenyltricyclo[3.3.0.0(2,4)]octanes, were quantitatively obtained in the denitrogenation reaction. The first-order decay process (k = 1/?) was observed for the fate of the singlet diradicals DR (?max ? 580-590 nm). The activation parameters, ?H (‡) and ?S (‡), for the ring-closing reaction (?-bond formation process) were determined by the temperature-dependent change of the lifetime. The energy barrier was found to be largely dependent upon the substituents Ar and Ar'. The singlet diradical DRf (Ar = 3,5-dimethoxyphenyl, OCH2Ar' = OCH2(3,5-dimethoxyphenyl)) was the longest-lived, ?293 = 5394 ± 59 ns, among the diradicals studied here. The lifetime of the parent diradical DR (Ar = Ph, OCH2Ar' = OCH3) was 299 ± 2 ns at 293 K. Conclusion: The lifetimes of the singlet 1,3-diyls are found to be largely dependent on the substituent pattern of Ar and Ar' at the C(1)-C(3) positions. Both the enthalpy and entropy effect were found to play crucial roles in increasing the lifetime. PMID:23766808

Ye, Jianhuai; Fujiwara, Yoshihisa; Abe, Manabu

2013-05-14

221

Financing Public Sector Projects with Clean Renewable Energy Bonds; Fact Sheet Series on Financing Renewable Energy Projects, National Renewable Energy Laboratory (NREL)  

SciTech Connect

Clean renewable energy bonds (CREBs) present a low-cost opportunity for public entities to issue bonds to finance renewable energy projects. The federal government lowers the cost of debt by providing a tax credit to the bondholder in lieu of interest payments from the issuer. Because CREBs are theoretically interest free, they may be more attractive than traditional tax-exempt municipal bonds. In February 2009, Congress appropriated a total of $2.4 billion for the "New CREBs" program. No more than one-third of the budget may be allocated to each of the eligible entities: governmental bodies, electric cooperatives, and public power providers. Applications for this round of "New CREBs" were due to the Internal Revenue Service (IRS) on August 4, 2009. There is no indication Congress will extend the CREBs program; thus going forward, only projects that are approved under the 2009 round will be able to issue CREBs. This factsheet explains the CREBs mechanism and provides guidance on procedures related to issuing CREBs.

Kreycik, C.; Couglin, J.

2009-12-01

222

Prediction of distinct surface segregation effects due to coordination-dependent bond-energy variations in alloy nanoclusters  

NASA Astrophysics Data System (ADS)

The first implementation of a recently introduced method based on the extraction of the coordination dependence of surface bond-energy variations (CBEV) from density-functional theory (DFT) computed pure-metal surface-energy anisotropy is reported. In particular, polynomial functions fitted to DFT data computed previously for Pt, Pd, and Rh are used as input energetics for statistical-mechanical computations of Pt-Pd 923-atom cuboctahedron-cluster compositional structures (and Pt-Rh(111) as a test case) using the free-energy concentration expansion method (FCEM). The major findings concern the roles of preferential strengthening of intrasurface and surface-subsurface interlayer bonds leading to quite unique segregation characteristics: (i) strong Pt segregation at certain (111) surface sites of the Pt-Pd clusters, accompanied, at relatively high overall Pt composition, by weaker Pt segregation forming Pt-Pd ordered (100) structure, whereas Pd segregates mainly at the edge and vertex sites; (ii) dominant Pd subsurface segregation. The high computation efficiency of the CBEV/FCEM approach allows the determination of the complete temperature dependence of atomic-exchange processes among surface sites, as well as between subsurface and deeper sites, reflected in the corresponding configurational heat-capacity curves. Compared to other approaches, the high transparency of this method helps to elucidate the origin of the distinct bond-energy-variation effects on site-specific segregation in alloy nanoclusters.

Rubinovich, Leonid; Polak, Micha

2009-07-01

223

7. Historic American Buildings Survey, C. C. Adams, Photographer August ...  

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

7. Historic American Buildings Survey, C. C. Adams, Photographer August 1931, SEED PACKING ROOM, Gift of New York State Department of Education. - Shaker North Family Washhouse (first), Shaker Road, New Lebanon, Columbia County, NY

224

Double dislocation pole model for deformation twinning in f.c.c. lattices  

Microsoft Academic Search

A double dislocation pole model is proposed for deformation twinning in f.c.c. lattice structures. Operation of the twinning source proposed occurs when the passing stress that poses a high energy barrier to the nucleation and growth of a twin is surmounted by thermal activation. Orowan stress barriers are also involved in the twinning process and reduce the twin nucleation and

S. G. Song; G. T. Gray III

1995-01-01

225

Chemical bonding in view of electron charge density and kinetic energy density descriptors.  

PubMed

Stalke's dilemma, stating that different chemical interpretations are obtained when one and the same density is interpreted either by means of natural bond orbital (NBO) and subsequent natural resonance theory (NRT) application or by the quantum theory of atoms in molecules (QTAIM), is reinvestigated. It is shown that within the framework of QTAIM, the question as to whether for a given molecule two atoms are bonded or not is only meaningful in the context of a well-defined reference geometry. The localized-orbital-locator (LOL) is applied to map out patterns in covalent bonding interaction, and produces results that are consistent for a variety of reference geometries. Furthermore, LOL interpretations are in accord with NBO/NRT, and assist in an interpretation in terms of covalent bonding. PMID:19090572

Jacobsen, Heiko

2009-05-01

226

Electrochemistry of Au(II) and Au(III) pincer complexes: determination of the Au(II)-Au(II) bond energy.  

PubMed

The bond energy of the unsupported Au-Au bond in the Au(ii) dimer [(C(?)N(?)C)Au]2 and the difference between Au(III)-OH and Au(III)-H bond enthalpies have been determined experimentally by electrochemical methods, with Au-OH and Au-H complexes showing unexpected differences in their reduction pathways, supported by DFT modelling. PMID:24051607

Dann, Thomas; Ro?ca, Drago?-Adrian; Wright, Joseph A; Wildgoose, Gregory G; Bochmann, Manfred

2013-10-01

227

Effect of the character of homo- and heteronuclear chemical bond on the intermolecular interaction energy and properties of halogens and hydrogen halides  

Microsoft Academic Search

The effect of the chemical bond character (the degrees of covalence C\\u000a c, metallicity C\\u000a m, and ionicity C\\u000a i) on the bond rigidity and the components of the van der Waals intermolecular interaction have been shown for halogens and\\u000a hydrogen halides as an example. The force constant is determined by the chemical bond character. The intermolecular interaction\\u000a energy of

O. S. Sirotkin; R. O. Sirotkin; P. B. Shibaev

2011-01-01

228

Critical surface energy of composite cement containing MDP (10-methacryloyloxydecyl dihydrogen phosphate) and chemical bonding to hydroxyapatite.  

PubMed

Self-adhesive composite cements are increasingly used for cementing inlays/onlays, intraradicular posts, crowns and laminate veneers. Wider clinical acceptance is driven by simpler and faster handling procedures, much like observed for self-etching adhesives. 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) is a bi-functional monomer incorporated as the reactive ingredient in a contemporary self-adhesive cement. We have examined the surface free energy parameters of this cement and studied the mode of action of the cement on dentine substrate by contact angle measurements to determine the critical surface energy of the cement. Retention of the infrared absorption bands characteristic of the acrylate moieties on the surface of hydroxyapatite particles suggests that MDP contributes to the overall bonding to dentine by forming ionic chemical bonds with surface calcium ions in dentine crystalites. PMID:21310107

Dabsie, Firas; Grégoire, Geneviève; Sharrock, Patrick

2011-02-10

229

Inferring modules of functionally interacting proteins using the Bond Energy Algorithm  

PubMed Central

Background Non-homology based methods such as phylogenetic profiles are effective for predicting functional relationships between proteins with no considerable sequence or structure similarity. Those methods rely heavily on traditional similarity metrics defined on pairs of phylogenetic patterns. Proteins do not exclusively interact in pairs as the final biological function of a protein in the cellular context is often hold by a group of proteins. In order to accurately infer modules of functionally interacting proteins, the consideration of not only direct but also indirect relationships is required. In this paper, we used the Bond Energy Algorithm (BEA) to predict functionally related groups of proteins. With BEA we create clusters of phylogenetic profiles based on the associations of the surrounding elements of the analyzed data using a metric that considers linked relationships among elements in the data set. Results Using phylogenetic profiles obtained from the Cluster of Orthologous Groups of Proteins (COG) database, we conducted a series of clustering experiments using BEA to predict (upper level) relationships between profiles. We evaluated our results by comparing with COG's functional categories, And even more, with the experimentally determined functional relationships between proteins provided by the DIP and ECOCYC databases. Our results demonstrate that BEA is capable of predicting meaningful modules of functionally related proteins. BEA outperforms traditionally used clustering methods, such as k-means and hierarchical clustering by predicting functional relationships between proteins with higher accuracy. Conclusion This study shows that the linked relationships of phylogenetic profiles obtained by BEA is useful for detecting functional associations between profiles and extending functional modules not found by traditional methods. BEA is capable of detecting relationship among phylogenetic patterns by linking them through a common element shared in a group. Additionally, we discuss how the proposed method may become more powerful if other criteria to classify different levels of protein functional interactions, as gene neighborhood or protein fusion information, is provided.

Watanabe, Ryosuke LA; Morett, Enrique; Vallejo, Edgar E

2008-01-01

230

Potential energy surface and unimolecular dynamics of stretched n-butane.  

PubMed

The potential energy surface (PES) and unimolecular reaction dynamics of stretched n-butane are investigated, as a model for a stretched "normal" alkane or straight chain polymer. The nature of the PES for stretched n-butane depends on the extent of stretching. If it is less than that required to reach the inflection points in the C[Single Bond]C stretch potentials and the C[Single Bond]C torsions are considered free rotors, there is only one potential energy minimum, with each bond elongated. However, for stretching past these inflection points, the PES has three minima and each has one bond longer than the other two, i.e., C[Single Bond]C[Single Bond]C[Single Bond]C, C[Single Bond]C[Single Bond]C[Single Bond]C, and C[Single Bond]C[Single Bond]C[Single Bond]C. There are three transition states (TSs) connecting these minima. A linear alkane, consisting of n carbon atoms and stretched past its C[Single Bond]C inflection points, has (n-1) minima and (n-1)(n-2)2 TSs connecting them. For stretching less than that required to reach the C[Single Bond]C inflection points, the only unimolecular pathways are dissociations to form the C+C[Single Bond]C[Single Bond]C, C[Single Bond]C+C[Single Bond]C, and C[Single Bond]C[Single Bond]C+C products. However, with stretching past the C[Single Bond]C inflection points, isomerizations between the three potential energy minima may also occur. The relative importance of isomerization versus dissociation depends on the relative size of their barriers. For slight stretching past the C[Single Bond]C inflection points, the isomerization barriers are much lower than those for dissociation and relaxation between the minima is much faster than dissociation. Thus, the molecule samples these minima randomly during its dissociation, with a density of states that comprises the complete PES. With extensive stretching past the inflection points, isomerizations between the potential energy minima do not occur and only dissociation for the excited minima occurs, e.g., C[Single Bond]C[Single Bond]C[Single Bond]C-->C+C[Single Bond]C[Single Bond]C. For intermediate stretching past the C[Single Bond]C inflection points, the rates for the isomerization and dissociation pathways are competitive and both must be considered in modeling the dissociation kinetics. Microcanonical chemical dynamics simulations are performed to study the unimolecular kinetics of n-butane in three stretched configurations: Stretched less than the C[Single Bond]C inflection point; stretched slightly beyond the C[Single Bond]C inflection point; and stretched significantly beyond the C[Single Bond]C inflection point. The resulting unimolecular dynamics were found to be in excellent agreement with RRKM theory. Frequency nu factors, determined by fitting the trajectory unimolecular rate constants to the classical harmonic RRKM rate constant expression, depend upon the extent of stretching and vary from 1.0 x 10(12)-8.4 x 10(16) s(-1). For a molecule with a large number of vibrational degrees of freedom and high excess energy, it is shown that the classical harmonic RRKM and classical harmonic transition state theory rate constants, k(E) and k(T), are equivalent. PMID:19044880

Lourderaj, Upakarasamy; McAfee, Jason L; Hase, William L

2008-09-01

231

Determination of bond dissociation energies by threshold ion-pair production spectroscopy: An improved D0(HCl)  

Microsoft Academic Search

A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (±0.8 cm?1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric

J. D. D. Martin; J. W. Hepburn

1998-01-01

232

Determination of bond dissociation energies by threshold ion-pair production spectroscopy: An improved D0(HCl)  

Microsoft Academic Search

A recently developed form of threshold ionization spectroscopy has been used to determine the bond energy for HCl to spectroscopic accuracy (+\\/-0.8 cm-1). This method is based on excitation to highly vibrationally excited ion-pair states using single-photon transitions from the ground state of HCl. These metastable Rydberg-like states were selectively detected using electricfield induced dissociation. By systematically varying the electric

J. D. D. Martin; J. W. Hepburn

1998-01-01

233

C,C-diacetylenic phosphaalkenes in palladium-catalyzed cross-coupling reactions.  

PubMed

The reactivity of bis-TMS-substituted C,C-diacetylenic phosphaalkene (A(2)PA) 1 in Sonogashira-Hagihara cross-coupling reactions has been examined. The selective and successive deprotection of the two silyl groups in 1 is enabled by the steric bulk of the Mes* group which renders the acetylene trans to Mes* more reactive and thereby facilitates selective and consecutive couplings with iodoarenes. In situ transformation of the TMS-protected acetylenes into Cu(i)acetylides is the key step in the synthetic sequence and enables the preparation of the first dimeric A(2)PA linked by a phenylene spacer. cis-trans Isomerization across the P[double bond, length as m-dash]C bond is triggered by a tertiary amine and exclusively observed in the case of nitrophenyl-substituted A(2)PAs. The introduced aryl groups are integral parts of the entire ?-system as evidenced by spectroscopic and electrochemical studies. PMID:21792460

Oberg, Elisabet; Geng, Xue-Li; Santoni, Marie-Pierre; Ott, Sascha

2011-07-27

234

Density Functional Study on Relative Energies, Structures, and Bonding of Low-lying Electronic States of Lutetium Dimer  

NASA Astrophysics Data System (ADS)

Low-lying electronic states of the lutetium dimer (Lu2) were studied based on density functional theory (DFT) using ten different density functionals together with three different relativistic effective core pseudopotentials (RECPs). Relative state energies, equilibrium bond lengths, vibrational frequencies, and ground-state dissociation energies were evaluated. It was found that the ground state is a triplet state irrespective of the type of functional and RECP used. This result is in contrast with a previous DFT calculation which gave a singlet ground state for Lu2. By comparing with the high-level ab initio and available experimental results, it is evident that the hybrid-GGA functionals combined with the Stuttgart small-core RECP yield the best overall agreement for the properties under study. The effects of Hartree-Fock exchange in B3LYP functional on the calculated bond length and dissociation energy of the ground state were examined, and rationalized in terms of 5d participation in Lu-Lu covalent bonding.

Xia, Qiong-qiong; Xiao, Wei; Zhang, Yong-fan; Ning, Li-xin; Cui, Zhi-feng

2009-08-01

235

On the influence of the phenyl group of propenylbenzene on the rate of hydrogenation of a double bond conjugated with it  

Microsoft Academic Search

1.On skeletal Ni, the disubstituted ethylenic bond of propenylbenzene is hydrogenated at approximately the same rate as the monosubstituted C=C bond of allylbenzene and allylcyclohexane. Under conditions in which the double bond of allylbenzene is displaced into theß-position, the C=C bond of allylcyclohexane does not migrate. These facts are evidence of activation of the C=C bond of propenylbenzene by the

L. Kh. Freidlin; N. M. Nazarova; M. A. Abduraimova

1967-01-01

236

Experimental determination of the HO-Br bond energy and its relevance to the stratospheric and tropospheric ozone cycles  

SciTech Connect

The HO-Br bond energy (and consequently the heat of formation of HOBr) has been determined experimentally for the first time. HOBr can be photolyzed in the stratosphere to form Br and OH, both of which can react with ozone, and destroy it. Previous calculations modeling stratospheric reactions have assumed a lower stability than that found in the current research. The stability of HOBr was determined by measuring the threshold for formation of Br{sup +} from this molecule, using photoionization mass spectrometry. Their work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences.

Ruscic, B.; Berkowitz, J.

1996-02-01

237

Study of dielectron production in C+C collisions at 1A GeV  

NASA Astrophysics Data System (ADS)

The emission of ee pairs from C+C collisions at an incident energy of 1 GeV per nucleon has been investigated. The measured production probabilities, spanning from the ?-Dalitz to the ?/? invariant-mass region, display a strong excess above the cocktail of standard hadronic sources. The bombarding-energy dependence of this excess is found to scale like pion production, rather than like eta production. The data are in good agreement with results obtained in the former DLS experiment.

Hades Collaboration; Agakishiev, G.; Agodi, C.; Alvarez-Pol, H.; Balanda, A.; Bassini, R.; Bellia, G.; Belver, D.; Belyaev, A.; Blanco, A.; Böhmer, M.; Bortolotti, A.; Boyard, J. L.; Braun-Munzinger, P.; Cabanelas, P.; Castro, E.; Chernenko, S.; Christ, T.; Destefanis, M.; Díaz, J.; Dohrmann, F.; Dybczak, A.; Eberl, T.; Fabbietti, L.; Fateev, O.; Finocchiaro, P.; Fonte, P.; Friese, J.; Fröhlich, I.; Galatyuk, T.; Garzón, J. A.; Gernhäuser, R.; Gil, A.; Gilardi, C.; Golubeva, M.; González-Díaz, D.; Grosse, E.; Guber, F.; Heilmann, M.; Heinz, T.; Hennino, T.; Holzmann, R.; Ierusalimov, A.; Iori, I.; Ivashkin, A.; Jurkovic, M.; Kämpfer, B.; Kajetanowicz, M.; Kanaki, K.; Karavicheva, T.; Kirschner, D.; Koenig, I.; Koenig, W.; Kolb, B. W.; Kotte, R.; Kozuch, A.; Krása, A.; Krizek, F.; Krücken, R.; Kühn, W.; Kugler, A.; Kurepin, A.; Lamas-Valverde, J.; Lang, S.; Lange, J. S.; Lapidus, K.; Lopes, L.; Maier, L.; Mangiarotti, A.; Marín, J.; Markert, J.; Metag, V.; Michalska, B.; Mishra, D.; Morinière, E.; Mousa, J.; Münch, M.; Müntz, C.; Naumann, L.; Novotny, R.; Otwinowski, J.; Pachmayer, Y. C.; Palka, M.; Parpottas, Y.; Pechenov, V.; Pechenova, O.; Pérez Cavalcanti, T.; Pietraszko, J.; Pleskac, R.; Pospísil, V.; Przygoda, W.; Ramstein, B.; Reshetin, A.; Roy-Stephan, M.; Rustamov, A.; Sadovsky, A.; Sailer, B.; Salabura, P.; Schmah, A.; Schroeder, C.; Schwab, E.; Simon, R. S.; Sobolev, Yu. G.; Spataro, S.; Spruck, B.; Ströbele, H.; Stroth, J.; Sturm, C.; Sudol, M.; Tarantola, A.; Teilab, K.; Tlusty, P.; Toia, A.; Traxler, M.; Trebacz, R.; Tsertos, H.; Veretenkin, I.; Wagner, V.; Weber, M.; Wen, H.; Wisniowski, M.; Wojcik, T.; Wüstenfeld, J.; Yurevich, S.; Zanevsky, Y.; Zhou, P.; Zumbruch, P.

2008-05-01

238

Aliphatic C-H bond scission processes in diphenylmethane and 2-benzyl- and 4-benzylpyridine. The heat of formation of the diphenylmethyl and. cap alpha. -phenylethyl radical in the gas phase  

SciTech Connect

Very low-pressure pyrolysis (VLPP) of the title compounds indicates both C-C and C-H bond rupture. Rate constants for all six processes are compatible with A factors of 10/sup 15.3/ s/sup -1/ and activation energies of 81.4 kcal mol/sup -1/ for C-H rupture in diphenylmethane and 82.3 kcal mol/sup -1/ for all the other C-H and C-C bond-breaking processes. The derived heat of formation of diphenylmethyl radical is 69.0 kcal mol/sup -1/. This is in excellent agreement with other studies and indicates that C-H bond rupture in phenylmethanes behaves analogously to C-C rupture. The extra stabilization in diphenylmethyl radical when compared with ..cap alpha..-phenylethyl radical is approx. 4 kcal mol/sup -1/.

Rossi, M.J.; McMillen, D.F.; Golden, D.M.

1984-10-11

239

Metal-Olefin Bond Energies in M(CO)5(C2H4-nCln) M = Cr, Mo, W; n = 0-4: Electron-Withdrawing Olefins Do Not Increase the Bond Strength  

NASA Astrophysics Data System (ADS)

Metal-olefin bond dissociation enthalpies have been calculated for the series of complexes M(CO)5(C2H4-nCln), M = Cr, Mo, W; n = 0-4 using density functional theory. Experimental values of the bond enthalpies have been measured for M(CO)5(C2H4-nCln) M = Cr, Mo, W; n = 2 (vinyl chloride), 3, and 4 using laser photoacoustic calorimetry in n-hexane solution. Experimental and calculated values indicate that the trend in metal-olefin bond energies is opposite to the electron-withdrawing ability of the olefin, which is counter to expectations based on the Dewar-Chatt-Duncanson model for metal-olefin bonding. An in-depth analysis of the metal-olefin interaction using a bond energy decomposition scheme implies that the observed and calculated decreasing trend is influenced by the increase in steric interactions and olefin reorganizational energy which is concomitant to the increase of the number of electron-withdrawing halogen atoms.

Schlappi, Darin N.; Cedeño, David L.

2009-08-01

240

Femtosecond dynamics of cyclopropenylidene, c-C3H2.  

PubMed

The photophysics of the B (1)B(1) state of isolated cyclopropenylidene, c-C(3)H(2), has been studied by femtosecond time-resolved photoionisation and photoelectron spectroscopy. The carbene was produced by flash pyrolysis of 3-chlorocycloprop-1-ene. The bands at 266.9 nm and 264.6 nm have been investigated. The excited state deactivates in a two step process. The first time constant of less than 50 fs corresponds most likely to a nonradiative transition to the A-state, the second one on the order of 200 fs describes the internal conversion to the electronic ground state. The data are compared to those measured for the chlorinated carbene c-C(3)HCl. In the photoelectron spectrum of c-C(3)H(2) resonances were observed which can be assigned to members of a Rydberg d-series. PMID:22173743

Hemberger, Patrick; Köhler, Juliane; Fischer, Ingo; Piani, Giovanni; Poisson, Lionel; Mestdagh, Jean-Michel

2011-12-16

241

Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer  

NASA Astrophysics Data System (ADS)

The highest frequency hydrogen bond fundamental of formic acid dimer, ?24 (Bu), is experimentally located at 264 cm-1. FTIR spectra of this in-plane bending mode of (HCOOH)2 and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three Bu combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D0 = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.

Kollipost, F.; Larsen, R. Wugt; Domanskaya, A. V.; Nörenberg, M.; Suhm, M. A.

2012-04-01

242

Interstellar Detection of c-C3D2  

NASA Astrophysics Data System (ADS)

We report the first interstellar detection of c-C3D2. Doubly deuterated cyclopropenylidene, a carbene, has been detected toward the starless cores TMC-1C and L1544 using the IRAM 30 m telescope. The J_{K_a,K_c} = 3_{0,3} {--} 2_{1,2}, 31, 3-20, 2, and 22, 1-11, 0 transitions of this species have been observed at 3 mm in both sources. The expected 1:2 intensity ratio has been found in the 30, 3-21, 2 and 31, 3-20, 2 lines, belonging to the para and ortho species, respectively. We also observed lines of the main species, c-C3H2, singly deuterated c-C3HD, and the species with one 13C off of the principal axis of the molecule, c-H13CC2H. The lines of c-C3D2 have been observed with high signal-to-noise ratio, better than 7.5? in TMC-1C and 9? in L1544. The abundance of doubly deuterated cyclopropenylidene with respect to the normal species is found to be 0.4%-0.8% in TMC-1C and 1.2%-2.1% in L1544. The deuteration of this small hydrocarbon ring is analyzed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C3D2 can be explained solely by gas-phase processes, supporting the idea that c-C3H2 is a good indicator of gas-phase deuteration. Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).

Spezzano, S.; Brünken, S.; Schilke, P.; Caselli, P.; Menten, K. M.; McCarthy, M. C.; Bizzocchi, L.; Trevinõ-Morales, S. P.; Aikawa, Y.; Schlemmer, S.

2013-06-01

243

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

PubMed Central

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

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

2009-01-01

244

Copper-catalyzed aerobic oxidative C-H and C-C functionalization of 1-[2-(arylamino)aryl]ethanones leading to acridone derivatives.  

PubMed

Efficient copper-catalyzed aerobic oxidative C-H and C-C functionalization of 1-[2-(arylamino)aryl]ethanones leading to acridones has been developed. The procedure involves cleavage of aromatic C-H and acetyl C-C bonds with intramolecular formation of a diarylketone bond. The protocol uses inexpensive Cu(O2CCF3)2 as catalyst, pyridine as additive, and economical and environmentally friendly oxygen as the oxidant, and the corresponding acridones with various functional groups were obtained in moderate to good yields. PMID:23401330

Yu, Jipan; Yang, Haijun; Jiang, Yuyang; Fu, Hua

2013-02-10

245

Phosphino imidazoles and imidazolium salts for Suzuki C-C coupling reactions.  

PubMed

The consecutive syntheses of imidazoles 1-(4-X-C(6)H(4))-4,5-R(2)-(c)C(3)HN(2) (3a, X = Br, R = H; 3b, X = I, R = Me; 3c, X = H, R = Me; 5, X = Fc, R = H; 7, X = C?CFc, R = H; 9, X = C(6)H(5), R = Me; Fc = Fe(?(5)-C(5)H(4))(?(5)-C(5)H(5))), phosphino imidazoles 1-(4-X-C(6)H(4))-2-PR'(2)-4,5-R(2)-(c)C(3)N(2) (11a-k; X = Br, I, Fc, FcC?C, Ph; R = H, Me; R' = Ph, (c)C(6)H(11), (c)C(4)H(3)O), imidazolium salts [1-(4-X-C(6)H(4))-3-R''-4,5-R(2)-(c)C(3)HN(2)]I (16a; X = Br, R = H, R'' = n-Bu; 16b, X = Br, R = H, R'' = n-C(8)H(17); 16c, X = I, R = Me, R'' = n-C(8)H(17), 16d, X = H, R = Me, R'' = n-C(8)H(17)) and phosphino imidazolium salts [1-C(6)H(5)-2-PR'(2)-3-n-C(8)H(17)-4,5-Me(2)-(c)C(3)N(2)]PF(6) (17a, R' = C(6)H(5); 17b, R' = (c)C(6)H(11)) or [1-(4-P(C(6)H(5))(2)-C(6)H(4))-3-n-C(8)H(17)-4,5-Me(2)-(c)C(3)HN(2)]PF(6), (20) and their selenium derivatives 1-(4-X-C(6)H(4))-2-P([double bond, length as m-dash]Se)R'(2)-4,5-R(2)-(c)C(3)N(2) (11a-Se-f-Se; X = Br, I; R = H, Me; R' = C(6)H(5), (c)C(6)H(11), (c)C(4)H(3)O) are reported. The structures of 11a-Se and [(1-(4-Br-C(6)H(4))-(c)C(3)H(2)N(2)-3-n-Bu)(2)PdI(2)] (19) in the solid state were determined. Cyclovoltammetric measurements were performed with the ferrocenyl-containing molecules 5 and 7 showing reversible redox events at E(0) = 0.108 V (?E(p) = 0.114 V) (5) and E(0) = 0.183 V (?E(p) = 0.102 V) (7) indicating that 7 is more difficult to oxidise. Imidazole oxidation does not occur up to 1.3 V in dichloromethane using [(n-Bu)(4)N][B(C(6)F(5))(4)] as supporting electrolyte, whereas an irreversible reduction is observed between -1.2 - -1.5 V. The phosphino imidazoles 11a-k and the imidazolium salts 17a,b and 20, respectively, were applied in the Suzuki C-C cross-coupling of 2-bromo toluene with phenylboronic acid applying [Pd(OAc)(2)] as palladium source. Depending on the electronic character of 11a-k, 17a,b and 20 the catalytic performance of the in situ generated catalytic active species can be predicted. As assumed, more electron-rich phosphines with their higher donor capability show higher activity and productivity. Additionally, 11e was applied in the coupling of 4-chloro toluene with phenylboronic acid showing an excellent catalytic performance when compared to catalysts used by Fu, Beller and Buchwald. Furthermore, 11e is eligible for the synthesis of sterically hindered biaryls under mild reaction conditions. C-C Coupling reactions with the phosphino imidazolium salts 17b and 20 in ionic liquids [BMIM][PF(6)] and [BDMIM][BF(4)] were performed, showing less activity than in common organic solvents. PMID:22450875

Milde, Bianca; Schaarschmidt, Dieter; Rüffer, Tobias; Lang, Heinrich

2012-03-27

246

A Fracture Resisting Molecular Interaction in Trabecular Bone: Sacrificial Bonds and Hidden Length Dissipate Energy as Mineralized Fibrils Separate  

NASA Astrophysics Data System (ADS)

A molecular energy dissipation mechanism in the form of sacrificial bonds and hidden length was previously found in bone constituent molecules of which the efficiency increased with the presence of Ca^2+ ions in the experimental solution. Here we present evidence for how this sacrificial bond-hidden length mechanism contributes to the mechanical properties of the bone composite. From investigations into the nanoscale arrangement of the bone constituents in combination with pico-Newton adhesion force measurements between mineralized collagen fibrils, based on single molecule force spectroscopy, we find evidence that bone consists of mineralized collagen fibrils and a non fibrillar organic matrix which acts as a ``glue'' that holds the mineralized fibrils together. We propose that this ``glue'' resists the separation of mineralized collagen fibrils. Like in the case of the sacrificial bonds in single molecules, the effectiveness of this ``glue'' increases with the presence of Ca^2+ ions. We further investigate how this molecular scale strengthening mechanism increases the fracture toughness of the macroscopic material.

Fantner, Georg E.

2005-03-01

247

First-order correction for bond energy applied to polar molecules: Alkali halides, alkali cyanides, LiCH3, and CH3F  

NASA Astrophysics Data System (ADS)

The ionic bond in molecules containing an electropositive moiety and an electronegative moiety originates from a coupling between the ionic and the covalent contributions. Some representative cases in the example of LiF, LiCl, NaF, NaCl, KF, KCl, LiCH3, CH3F, LiCN, NaCN and KCN are calculated by ab initio and density functional methods. The resulting bond energy can be improved a posteriori by a recently proposed first-order method using the dipole moment or the effective charge. For the ab initio calculations, this method brings about a systematic improvement of the bond energy with respect to the experimental value. While the density functional method gives qualitatively mixed results, application of this first-order method generally improves the bond energy.

Lee, Dong-Ki; Lee, Yoon Sup; Hagebaum-Reignier, D.; Jeung, Gwang-Hi

2006-09-01

248

Low-energy Excitations in the Magnetized State of the Bond-alternating Quantum S=1 Chain System NTENP  

SciTech Connect

High-intensity inelastic neutron scattering experiments on the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C{sub 9}D{sub 24}N{sub 4})(NO{sub 2})ClO{sub 4} (NTENP) are performed in magnetic fields of up to 14.8 T. Excitation in the high field magnetized quantum spin solid (ordered) phase are investigated. In addition to the previously observed coherent long-lived gap excitation [M. Hagiwara et al., Phys. Rev. Lett 94, 177202 (2005)], a broad continuum is detected at lower energies. This observation is consistent with recent numerical studies, and helps explain the suppression of the lowest-energy gap mode in the magnetized state of NTENP. Yet another feature of the excitation spectrum is found at slightly higher energies, and appears to be some kind of multimagnon state.

Regnault, L.-P. [CEA, Grenoble, France; Zheludev, Andrey I [ORNL; Hagiwara, M. [Osaka University; Stunault, A. [Institut Laue-Langevin (ILL)

2006-01-01

249

Chemical Bonds  

NSDL National Science Digital Library

Electrons are key to forming the two broad categories of chemical bonds: covalent and ionic. Atoms, which have a nucleus surrounded by electrons, are represented in several different ways. In the Chemical Bonds activity, students explore the different kinds of chemical bonds that can form, ranging from non-polar covalent to ionic. In the model depicted above students adjust the electronegativity of two atoms and see the effect it has on electron distribution and bond type.

Consortium, The C.

2011-12-11

250

Sticker Bonding.  

ERIC Educational Resources Information Center

|Introduces a science activity on the bonding of chemical compounds. Assigns students the role of either a cation or anion and asks them to write the ions they may bond with. Assesses students' understanding of charge, bonding, and other concepts. (YDS)|

Frazier, Laura Corbin

2000-01-01

251

Sticker Bonding.  

ERIC Educational Resources Information Center

Introduces a science activity on the bonding of chemical compounds. Assigns students the role of either a cation or anion and asks them to write the ions they may bond with. Assesses students' understanding of charge, bonding, and other concepts. (YDS)

Frazier, Laura Corbin

2000-01-01

252

C.C.D. Readout of a Picosecond Streak Camera with an Intensified C.C.D.  

National Technical Information Service (NTIS)

This paper deals with a digital streak camera readout device. The device consists in a low light level television camera made of a solid state C.C.D. array coupled to an image intensifier associated to a video-digitizer coupled to a micro-computer system....

M. Lemonier J. C. Richard C. Cavailler A. Mens G. Raze

1984-01-01

253

Estimation of Impact Damage in C\\/C Composites by Drop Weight Tests  

Microsoft Academic Search

The evaluation of impact damage properties and strength of C\\/C composites is becoming important, due to its low impact strength. In this study, the impact damage is evaluated by using the impact load-deflection diagrams and absorbed energy of specimens on a drop weight impact test. The measured impact load is decomposed in approximation components and detail components by multiple-resolution analysis

Takakazu Yoshioka; Ichiro Takahashi

2008-01-01

254

Rhodium-Catalyzed CC Bond Formation via Heteroatom-Directed CH Bond Activation  

Microsoft Academic Search

Once considered the 'holy grail' of organometallic chemistry, synthetically useful reactions employing C-H bond activation have increasingly been developed and applied to natural product and drug synthesis over the past decade. The ubiquity and relative low cost of hydrocarbons makes C-H bond functionalization an attractive alternative to classical C-C bond forming reactions such as cross-coupling, which require organohalides and organometallic

Denise A. Colby; Robert G. Bergman; Jonathan A. Ellman

2010-01-01

255

On the Construction of Diabatic and Adiabatic Potential Energy Surfaces Based on Ab Initio Valence Bond Theory‡  

PubMed Central

A theoretical model is presented for deriving effective diabatic states based on ab initio self-consistent field valence bond (VBSCF) theory by reducing the multi-configurational VB Hamiltonian into an effective two-state model. We describe two computational approaches for the optimization of the effective diabatic configurations, resulting in two ways of interpreting such effective diabatic states. In the variational diabatic configuration (VDC) method, the energies of the individual diabatic states are variationally minimized. In the consistent diabatic configuration (CDC) method, both the configuration coefficients and orbital coefficients are simultaneously optimized to minimize the adiabatic ground-state energy in VBSCF calculations. In addition, we describe a mixed molecular orbital and valence bond (MOVB) approach to construct the CDC diabatic and adiabatic states for a chemical reaction, whereas the VDC-MOVB method has been described previously. Employing the symmetric SN2 reaction between NH3 and CH3NH3+ as a test system, we found that the results from ab initio VBSCF and from MOVB calculations are in good agreement, suggesting that the computationally efficient MOVB method is a reasonable model for VB simulations of condensed phase reactions. The results indicate that CDC and VDC diabatic states converge, respectively, to covalent and ionic states as the molecular geometries are distorted from the minimum of the respective diabatic state along the reaction coordinate. Furthermore, the resonance energy that stabilizes the energy of crossing between the two diabatic states, resulting in the transition state of the adiabatic ground-state reaction, has a strong dependence on the overlap integral between the two diabatic states and is a function of both the exchange integral and the total diabatic ground-state energy.

Song, Lingchun; Gao, Jiali

2009-01-01

256

High spectral resolution Al2O3/B4C, SiC/Si, SiC/B4C, and SiC/C multilayer structures for the photon energies of 6 keV to 19 keV  

NASA Astrophysics Data System (ADS)

A double multilayer monochromator with each multilayer composed of four stripes with different d-spacing providing spectral resolution of 0.3% to 0.8% in the energy range of 6keV to 19keV has been developed. Test multilayer structures with d-spacing from 2.3nm to 10.6nm have been deposited by magnetron sputtering. X-ray characterization has been performed at OSMIC by using a recently upgraded diffractometer setup and Cu-K? radiation and at the APS. The following material combinations were studied before the final choice of materials for the high energy resolution monochromator has been made: Al2O3/B4C, SiC/Si, SiC/B4C and SiC/C. To minimize the effect of internal stress built in multilayer structure on X-ray characteristics flat and thick 1" diameter silicon substrates supplied by Wave Precision Inc. were used for all calibration coatings. Final coatings were deposited on two 145mm long, 60mm wide and 30mm thick silicon substrates. Resolution of SiC/Si structures with d1=2.3nm, N1=1000 and d2=3nm, N2=700 was measured at Cu-K? with X-ray beam divergence of 14 arcsec to be 0.216% and 0.34% respectively. For plane waves the resolution is expected to be 0.13% and 0.19%, respectively.

Platonov, Yuriy Y.; Martynov, Vladimir V.; Kazimirov, Alexander; Lai, Barry

2004-11-01

257

Surface structure, bonding, and dynamics: The universality of zincblende (110) potential energy surfaces.  

National Technical Information Service (NTIS)

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated, zincblende-structure compound semiconductors exhibit a common ''unive...

T. J. Godin J. P. LaFemina

1991-01-01

258

Amino Acid Mean Excitation Energies and Directional Dependencies from Core and Bond Calculations  

SciTech Connect

We determine the mean excitation energies of several amino acids using a Bragg Rule developed for molecular fragments or functional groups. As the composition of the amino acids is very similar, we find that the amino acids have similar mean excitation energies (approximately 70 eV). Differences arise from variation of the side chains (-R); addition of-CH2-groups decreases the mean excitation energy. We also speculate concerning the directional dependence of the amino acid mean excitation energies.

Sabin, John R.; Oddershede, Jens [Department of Physics, University of Florida, Gainesville, FL 32611 (United States); Institute for Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense M (Denmark); Sauer, Stephan P. A. [Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen (Denmark)

2008-12-08

259

Optimal loading of molecular bonds.  

PubMed

A corollary of the Bell equation of bond rupture is that the bond transfers a maximal impulse during its lifetime when the applied force equals k(B)T/x*. It is proposed that the designs of biological systems converge toward loading bonds with this optimal force to minimize the need for self-healing and to optimize energy transfer. PMID:23025846

Hess, Henry

2012-10-03

260

Theoretical study of the OH addition to the endocyclic and exocyclic double bonds of the d-limonene  

NASA Astrophysics Data System (ADS)

The initial step of the d-limonene + OH gas-phase reaction mechanism was investigated by means of ab initio calculations. We have considered eight different possibilities for the OH addition, corresponding to the two C-C double bonds, the two C atoms of each double bond, and the syn or anti orientation, with respect to the isopropenyl group (endocyclic attack) or the ring cycle (exocyclic attack). Activation energies calculated at the QCISD(T)/6-31G(d)//UMP2/6-31G(d) level, show that there are preferred orientations for the OH addition under atmospheric conditions of temperature and pressure.

Ramirez-Ramirez, Victor M.; Nebot-Gil, Ignacio

2005-06-01

261

A new look at the ylidic bond in phosphorus ylides and related compounds: energy decomposition analysis combined with a domain-averaged fermi hole analysis.  

PubMed

Geometries and bond dissociation energies of the ylide compounds H2CPH3, H2CPMe3, H2CPF3, (BH2)2CPH3, H2CNH3, H2CAsH3, H2SiPH3, and (BH2)2SiPH3 have been calculated using ab initio (MP2, CBS-QB3) and DFT (B3LYP, BP86) methods. The nature of the ylidic bond R2E1-E2X3 was investigated with an energy decomposition analysis and with the domain-averaged Fermi hole (DAFH) analysis. The results of the latter method indicate that the peculiar features of the ylidic bond can be understood in terms of donor-acceptor interactions between closed-shell R2E1 and E2X3 fragments. The DAFH analysis clearly shows that there are two bonding contributions to the ylidic bond. The strength of the donor and acceptor contributions to the attractive orbital interactions can be estimated from the energy decomposition analysis (EDA) calculations, which give also the contributions of the electrostatic attraction and the Pauli repulsion of the chemical bonding. The EDA and DAFH results clearly show that the orbital interactions take place through the singlet ground state of the R2E1 fragment where the donor orbital of E1 yields pi-type back-donation while the E2X3 lone-pair orbital yields sigma-type bonding. Both bonds are polarized toward E2X3 when E2 = P, while the sigma-type bonding remains more polarized at E2X3 when E2 = N, As. This shows that the phosphorus ylides exhibit a particular bonding situation which is clearly different from that of the nitrogen and arsenic homologues. With ylides built around a P-C linkage, the pi-acceptor strength of phosphorus and the sigma-acceptor strength at carbon contribute to a double bond which is enhanced by electrostatic contributions. The strength of the sigma and pi components and the electrostatic attraction are then fine-tuned by the substituents at C and P, which yields a peculiar type of carbon-phosphorus bonding. The EDA data reveal that the relative strength of the ylidic bond may be determined not only by the R2E1 --> E2X3 pi back-donation, but also by the electrostatic contribution to the bonding. The calculations of the R2E1-E2X3 bond dissociation energy using ab initio methods predict that the order of the bond strength is H2C-PMe3 > H2C-PF3 > H2C-PH3 > (BH2)2C-PH3 > H2C-AsH3 > H2C-NH3 approximately H2Si-PH3 approximately (BH2)2Si-PH3. The DFT methods predict a similar trend, but they underestimate the bond strength of (BH2)2CPH3. PMID:17388399

Calhorda, Maria José; Krapp, Andreas; Frenking, Gernot

2007-03-24

262

Comparative study of hydrocarbon, fluorocarbon, and aromatic bonded RP-HPLC stationary phases by linear solvation energy relationships.  

PubMed

The retention properties of eight alkyl, aromatic, and fluorinated reversed-phase high-performance liquid chromatography bonded phases were characterized through the use of linear solvation energy relationships (LSERs). The stationary phases were investigated in a series of methanol/water mobile phases. LSER results show that solute molecular size and hydrogen bond acceptor basicity under all conditions are the two dominant retention controlling factors and that these two factors are linearly correlated when either different stationary phases at a fixed mobile-phase composition or different mobile-phase compositions at a fixed stationary phase are considered. The large variation in the dependence of retention on solute molecular volume as only the stationary phase is changed indicates that the dispersive interactions between nonpolar solutes and the stationary phase are quite significant relative to the energy of the mobile-phase cavity formation process. PCA results indicate that one PCA factor is required to explain the data when stationary phases of the same chemical nature (alkyl, aromatic, and fluoroalkyl phases) are individually considered. However, three PCA factors are not quite sufficient to explain the whole data set for the three classes of stationary phases. Despite this, the average standard deviation obtained by the use of these principal component factors are significantly smaller than the average standard deviation obtained by the LSER approach. In addition, selectivities predicted through the LSER equation are not in complete agreement with experimental results. These results show that the LSER model does not properly account for all molecular interactions involved in RP-HPLC. The failure could reside in the V2 solute parameter used to account for both dispersive and cohesive interactions since "shape selectivity" predictions for a pair of structural isomers are very bad. PMID:10464478

Reta, M; Carr, P W; Sadek, P C; Rutan, S C

1999-08-15

263

Pressure Dependence of Energy Gaps and Refractive Indices of Tetrahedrally Bonded Semiconductors.  

National Technical Information Service (NTIS)

The pressure dependence of energy gaps for a number of elemental and compound semiconductors is investigated employing the empirical pseudopotential method. The compressibility and the empirical pseudopotential form factors appropriate for describing the ...

Y. F. Tsay S. S. Mitra B. Bendow

1974-01-01

264

Surface structure, bonding, and dynamics: Universality of zinc blende (110) potential energy surfaces  

Microsoft Academic Search

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated zinc blende structure compound semiconductors exhibit a common universal'' form if expressed in terms of suitably scaled parameters. TBTE calculations on both III--V and II--VI compounds reveal a linear scaling with bulk lattice constant

T. J. Godin; J. P. LaFemina; C. B. Duke

1992-01-01

265

Calculation of bond energies in compounds of heavy elements by a quasi-relativistic approach  

Microsoft Academic Search

A quasi-relativistic method, in which the valence density is optimized with respect to the first-order relativistic Hamiltonian, has been evaluated by calculations on systems containing heavy elements including third-row transition metals and actinides. The method adopts the statistical energy expression and employs in addition the frozen core approximation. The quasi-relativistic method has been applied in calculations on atomic orbital energies

Tom Ziegler; V. Tschinke; E. J. Baerends; J. G. Snijders; W. Ravenek

1989-01-01

266

A footprint study of bond initiation in gold wire crescent bonding  

Microsoft Academic Search

The morphological features of the crescent bond footprints on the substrate after peeling the wire off were studied to gain an understanding of the effect of process parameters on the crescent bond formation. In the absence of any ultrasonic energy, metallurgical bonding initiated at the peripheral regions of the crescent bond. The bond strength improved at higher substrate temperatures and

Y. n. Zhou; X. Li; N. J. Noolu

2005-01-01

267

A hydroacylation-triggered carbon--carbon triple bond cleavage in alkynes via retro-Mannich type fragmentation.  

PubMed

The carbon-carbon triple bond in alkyne is cleaved via hydroacylation followed by retro-Mannich type fragmentation in the presence of aldehyde, which triggers a successive C-C bond cleavage. PMID:12785769

Lee, Dae-Yon; Hong, Boo-Sun; Cho, Eung-Goo; Lee, Hyuk; Jun, Chul-Ho

2003-05-28

268

Thermodynamics of proton transfer in phenol-acetate hydrogen bonds with large proton polarizability and the conversion of light energy into chemical energy in bacteriorhodopsin  

SciTech Connect

Phenol-acetate solutions in CCl/sub 4/ are studied by IR spectroscopy as a function of the pK/sub a/ of the phenols. The (I) Ar-OH.../sup -/OC in equilibrium Ar-O/sup -/...HOC (II) hydrogen bonds formed show large proton polarizability as indicated by continua in the IR spectra. The percent proton transfer (PT) increases from the p-cresol-acetate to the pentachlorophenol-acetate system from 0% to 56%. The Gibbs free energy, ..delta..G/sup 0//sub PT/, values of the PT equilibria at 295 K are determined as well as the standard enthalpy values, ..delta..H/sup 0//sub PT/, and the standard entropy values, ..delta..S/sup 0//sub PT. The shape of the intensity of the continuum as a function of the ..delta..H/sup 0//sub PT/ value changes. In the classical approximation the average difference between the two minima of the proton double-minimum potential is given by ..delta..H/sup 0//sub PT/. With the decreasing amount of ..delta..H/sup 0//sub PT/, i.e., decreasing degree of asymmetry, the intensity of the continua decreases at higher and increases at lower wavenumbers. This result is in good agreement with the predictions from calculated line spectra. In the photocycle of bacteriorhodopsin a tyrosine-aspartate hydrogen bond is probably of importance for the conversion of light energy into chemical energy. On the basis of the obtained data it is shown that 9.5 kJ/mol can be converted into chemical energy due to a proton transfer induced by a local electrical field in a Tyr-Asp hydrogen bond. Furthermore, if the Ar-O/sup -/...HOC structure is broken afterwards by a conformation change, at least 25 kJ/mol of conformation energy is converted. Thus, altogether 34.5 kJ/mol of Gibbs free energy may be converted into chemical energy and stored by these processes.

Merz, H.; Tangermann, U.; Zundel, G.

1986-11-20

269

Low-energy physical properties of high- Tc superconducting Cu oxides: A comparison between the resonating valence bond and experiments  

NASA Astrophysics Data System (ADS)

In a recent review by Anderson and co-workers, it was pointed out that an early resonating valence bond (RVB) theory is able to explain a number of unusual properties of high-temperature superconducting (SC) Cu oxides. Here we extend previous calculations to study more systematically the low-energy physical properties of the plain vanilla d -wave RVB state, and to compare the results with the available experiments. We use a renormalized mean-field theory combined with variational Monte Carlo and power Lanczos methods to study the RVB state of an extended t-J model in a square lattice with parameters suitable for the hole-doped Cu oxides. The physical observable quantities we study include the specific heat, the linear residual thermal conductivity, the in-plane magnetic penetration depth, the quasiparticle energy at the antinode (?,0) , the superconducting energy gap, the quasiparticle spectra, and the Drude weights. The traits of nodes (including kF , the Fermi velocity vF , and the velocity along Fermi surface v2 ), and the SC order parameter are studied. Comparisons of the theory and the experiments in cuprates show an overall qualitative agreement, especially on their doping dependences.

Yang, Kai-Yu; Shih, C. T.; Chou, C. P.; Huang, S. M.; Lee, T. K.; Xiang, T.; Zhang, F. C.

2006-06-01

270

A Promising Tool to Achieve Chemical Accuracy for Density Functional Theory Calculations on Y-NO Homolysis Bond Dissociation Energies.  

PubMed

A DFT-SOFM-RBFNN method is proposed to improve the accuracy of DFT calculations on Y-NO (Y = C, N, O, S) homolysis bond dissociation energies (BDE) by combining density functional theory (DFT) and artificial intelligence/machine learning methods, which consist of self-organizing feature mapping neural networks (SOFMNN) and radial basis function neural networks (RBFNN). A descriptor refinement step including SOFMNN clustering analysis and correlation analysis is implemented. The SOFMNN clustering analysis is applied to classify descriptors, and the representative descriptors in the groups are selected as neural network inputs according to their closeness to the experimental values through correlation analysis. Redundant descriptors and intuitively biased choices of descriptors can be avoided by this newly introduced step. Using RBFNN calculation with the selected descriptors, chemical accuracy (?1 kcal·mol(-1)) is achieved for all 92 calculated organic Y-NO homolysis BDE calculated by DFT-B3LYP, and the mean absolute deviations (MADs) of the B3LYP/6-31G(d) and B3LYP/STO-3G methods are reduced from 4.45 and 10.53 kcal·mol(-1) to 0.15 and 0.18 kcal·mol(-1), respectively. The improved results for the minimal basis set STO-3G reach the same accuracy as those of 6-31G(d), and thus B3LYP calculation with the minimal basis set is recommended to be used for minimizing the computational cost and to expand the applications to large molecular systems. Further extrapolation tests are performed with six molecules (two containing Si-NO bonds and two containing fluorine), and the accuracy of the tests was within 1 kcal·mol(-1). This study shows that DFT-SOFM-RBFNN is an efficient and highly accurate method for Y-NO homolysis BDE. The method may be used as a tool to design new NO carrier molecules. PMID:22942689

Li, Hong Zhi; Hu, Li Hong; Tao, Wei; Gao, Ting; Li, Hui; Lu, Ying Hua; Su, Zhong Min

2012-06-28

271

A Promising Tool to Achieve Chemical Accuracy for Density Functional Theory Calculations on Y-NO Homolysis Bond Dissociation Energies  

PubMed Central

A DFT-SOFM-RBFNN method is proposed to improve the accuracy of DFT calculations on Y-NO (Y = C, N, O, S) homolysis bond dissociation energies (BDE) by combining density functional theory (DFT) and artificial intelligence/machine learning methods, which consist of self-organizing feature mapping neural networks (SOFMNN) and radial basis function neural networks (RBFNN). A descriptor refinement step including SOFMNN clustering analysis and correlation analysis is implemented. The SOFMNN clustering analysis is applied to classify descriptors, and the representative descriptors in the groups are selected as neural network inputs according to their closeness to the experimental values through correlation analysis. Redundant descriptors and intuitively biased choices of descriptors can be avoided by this newly introduced step. Using RBFNN calculation with the selected descriptors, chemical accuracy (?1 kcal·mol?1) is achieved for all 92 calculated organic Y-NO homolysis BDE calculated by DFT-B3LYP, and the mean absolute deviations (MADs) of the B3LYP/6-31G(d) and B3LYP/STO-3G methods are reduced from 4.45 and 10.53 kcal·mol?1 to 0.15 and 0.18 kcal·mol?1, respectively. The improved results for the minimal basis set STO-3G reach the same accuracy as those of 6-31G(d), and thus B3LYP calculation with the minimal basis set is recommended to be used for minimizing the computational cost and to expand the applications to large molecular systems. Further extrapolation tests are performed with six molecules (two containing Si-NO bonds and two containing fluorine), and the accuracy of the tests was within 1 kcal·mol?1. This study shows that DFT-SOFM-RBFNN is an efficient and highly accurate method for Y-NO homolysis BDE. The method may be used as a tool to design new NO carrier molecules.

Li, Hong Zhi; Hu, Li Hong; Tao, Wei; Gao, Ting; Li, Hui; Lu, Ying Hua; Su, Zhong Min

2012-01-01

272

A coupled cluster approach with a hybrid treatment of connected triple excitations for bond-breaking potential energy surfaces.  

PubMed

An approximate coupled cluster singles, doubles, and triples (CCSDT) method based on the unrestricted Hartree-Fock (UHF) reference, in which the contribution of triple excitations is approximately treated in a hybrid manner [denoted as CCSD(T)-h], is presented. In this approach, canonical UHF molecular orbitals are first transformed into corresponding orbitals so that each alpha-spin orbital is paired with only one beta-spin orbital. Then, active orbitals (occupied or virtual) are automatically selected by setting a threshold for the overlap integrals of corresponding orbitals. With the concept of active orbitals, triple excitations can be divided into two subsets: (1) "active" triples involving at least one occupied active orbital and one virtual active orbital and (2) the remaining triples. The amplitudes of these two classes of triple excitations are obtained via two different approaches. When the present method is employed to study bond-breaking processes, it computationally scales as the seventh power of the system size, because the number of active orbitals involved in such processes is relatively small compared to the total number of the orbitals, and is usually independent on the system size. It has been applied to study the bond-breaking potential energy surfaces in the H(8) model and five small molecules (HF, F(2), CH(4), H(2)O, and N(2)). For all systems under study, the overall performance of CCSD(T)-h is very competitive with that of CCSDT, and much better than that of the UHF-based CCSD(T). PMID:20331289

Shen, Jun; Xu, Enhua; Kou, Zhuangfei; Li, Shuhua

2010-03-21

273

Measurements of the Z partial decay width into c c and multiplicity of charm quarks per b decay  

Microsoft Academic Search

The partial decay width Rc of the Z into c c quark pair and the number of charm quarks nc per b decay are measured with the DELPHI detector at LEP 1. Particle identification provides clear D0, D+, Ds+ and Lambda c+ signatures. The charm hadron production rate is measured in each channel by a fit to the scaled energy,

P. Abreu; P Adzic; I Ajinenko; Z Albrecht; T Alderweireld; G D Alekseev; R Alemany; T Allmendinger; P P Allport; S Almehed; Ugo Amaldi; N Amapane; S Amato; E G Anassontzis; P Andersson; A Andreazza; S Andringa; P Antilogus; W D Apel; Y Arnoud; B Åsman; J E Augustin; A Augustinus; Paul Baillon; P Bambade; F Barão; Guido Barbiellini; R Barbier; Dimitri Yuri Bardin; G Barker; A Baroncelli; Marco Battaglia; M Baubillier; K H Becks; M Begalli; A Behrmann; P Beillière; Yu A Belokopytov; N C Benekos; Alberto C Benvenuti; C Bérat; M Berggren; D Bertini; D Bertrand; M Besançon; M Bigi; S M Bilenky; M A Bizouard; D Bloch; H M Blom; M Bonesini; W Bonivento; M Boonekamp; P S L Booth; A W Borgland; G Borisov; C Bosio; O Botner; E Boudinov; B Bouquet; C Bourdarios; T J V Bowcock; I Boyko; I Bozovic; M Bozzo; P Branchini; T Brenke; R A Brenner; P Brückman; J M Brunet; L Bugge; T Buran; T Burgsmüller; Brigitte Buschbeck; P Buschmann; S Cabrera; M Caccia; M Calvi; T Camporesi; V Canale; F Carena; L Carroll; Carlo Caso; M V Castillo-Gimenez; A Cattai; F R Cavallo; V Chabaud; P Charpentier; L Chaussard; P Checchia; G A Chelkov; R Chierici; P Shlyapnikov; P Chochula; V Chorowicz; J Chudoba; K Cieslik; P Collins; R Contri; E Cortina; G Cosme; F Cossutti; J H Cowell; H B Crawley; D J Crennell; S Crépé; G Crosetti; J Cuevas-Maestro; S Czellar; Martyn Davenport; W Da Silva; A Deghorain; G Della Ricca; P A Delpierre; N Demaria; A De Angelis; Wim de Boer; C De Clercq; B De Lotto; A De Min; L S De Paula; H Dijkstra; Lucia Di Ciaccio; J Dolbeau; K Doroba; M Dracos; J Drees; M Dris; A Duperrin; J D Durand; G Eigen; T J C Ekelöf; Gösta Ekspong; M Ellert; M Elsing; J P Engel; B Erzen; M C Espirito-Santo; E Falk; G K Fanourakis; D Fassouliotis; J Fayot; Michael Feindt; P Ferrari; A Ferrer; E Ferrer-Ribas; F Ferro; S Fichet; A Firestone; U Flagmeyer; H Föth; E Fokitis; F Fontanelli; B J Franek; A G Frodesen; R Frühwirth; F Fulda-Quenzer; J A Fuster; A Galloni; D Gamba; S Gamblin; M Gandelman; C García; C Gaspar; M Gaspar; U Gasparini; P Gavillet; E N Gazis; D Gelé; L N Gerdyukov; N Ghodbane; I Gil; F Glege; R Gokieli; B Golob; G Gómez-Ceballos; P Gonçalves; I González-Caballero; Gian P Gopal; L Gorn; M Górski; Yu Guz; Valerio Gracco; J Grahl; E Graziani; C Green; H J Grimm; P Gris; G Grosdidier; K Grzelak; M Günther; J Guy; F Hahn; S Hahn; S Haider; A Hallgren; K Hamacher; J Hansen; F J Harris; V Hedberg; S Heising; J J Hernández; P Herquet; H Herr; T L Hessing; J M Heuser; E Higón; S O Holmgren; P J Holt; S Hoorelbeke; M A Houlden; Josef Hrubec; K Huet; G J Hughes; K Hultqvist; J N Jackson; R Jacobsson; P Jalocha; R Janik; C Jarlskog; G Jarlskog; P Jarry; B Jean-Marie; E K Johansson; P E Jönsson; C Joram; P Juillot; F Kapusta; K Karafasoulis; S Katsanevas; E C Katsoufis; R Keränen; Borut P Kersevan; B A Khomenko; N N Khovanskii; A P Kiiskinen; B J King; A Kinvig; N J Kjaer; O Klapp; H Klein; P M Kluit; P Kokkinias; M Koratzinos; V Kostyukhin; C Kourkoumelis; O Kuznetsov; E Kriznic; P S Krstic; Z Krumshtein; P Kubinec; J Kurowska; K L Kurvinen; J Lamsa; P Langefeld; V Lapin; J P Laugier; R Lauhakangas; Gerhard Leder; F Ledroit; V Lefébure; L Leinonen; A Leisos; R Leitner; J Lemonne; Georg Lenzen; V Lepeltier; T Lesiak; M Lethuillier; J Libby; D Liko; A Lipniacka; I Lippi; B Lörstad; J G Loken; J H Lopes; J M López; R López-Fernandez; D Loukas; P Lutz; L Lyons; J N MacNaughton; J R Mahon; A Maio; A Malek; T G M Malmgren; S Maltezos; V Malychev; F Mandl; J Marco; R P Marco; B Maréchal; M Margoni; J C Marin; C Mariotti; A Markou; C Martínez-Rivero; F Martínez-Vidal; S Martí i García; J Masik; N Mastroyiannopoulos; F Matorras; C Matteuzzi; Giorgio Matthiae; F Mazzucato; M Mazzucato; M L McCubbin; R McKay; R McNulty; G McPherson; C Meroni; W T Meyer; A Myagkov; E Migliore; L Mirabito; Winfried A Mitaroff; U Mjörnmark; T Moa; M Moch; R Møller; K Mönig; M R Monge; X Moreau; P Morettini; G A Morton; U Müller; K Münich; M Mulders; C Mulet-Marquis; R Muresan; W J Murray; B Muryn; Gerald Myatt; T Myklebust; F Naraghi; M Nassiakou; Francesco Luigi Navarria; S Navas; K Nawrocki; P Negri; S Némécek; N Neufeld; N Neumeister; R Nicolaidou; B S Nielsen; M Nikolenko; V P Nomokonov; Ainsley Normand; A Nygren; V F Obraztsov; A G Olshevskii; A Onofre; Risto Orava; G Orazi; K Österberg; A Ouraou; M Paganoni; S Paiano; R Pain; R Paiva; J Palacios; H Palka; T D Papadopoulou; K Papageorgiou; L Pape; C Parkes; F Parodi; U Parzefall; A Passeri; O Passon; M Pegoraro; L Peralta; Manfred Pernicka; A Perrotta; C Petridou; A Petrolini; H T Phillips; F Pierre; M Pimenta; E Piotto; T Podobnik; M E Pol; G Polok; P Poropat; V Pozdnyakov; P Privitera; N Pukhaeva; Antonio Pullia; D Radojicic; S Ragazzi; H Rahmani; P N Ratoff; A L Read; P Rebecchi; N G Redaelli; Meinhard Regler; D Reid; R Reinhardt; P B Renton; L K Resvanis; F Richard

2000-01-01

274

Surface structure, bonding, and dynamics: Universality of zinc blende (110) potential energy surfaces  

SciTech Connect

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated zinc blende structure compound semiconductors exhibit a common universal'' form if expressed in terms of suitably scaled parameters. TBTE calculations on both III--V and II--VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, InSb, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surfaces of zinc blende structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials.

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

1992-07-01

275

Surface structure, bonding, and dynamics: The universality of zincblende (110) potential energy surfaces  

SciTech Connect

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated, zincblende-structure compound semiconductors exhibit a common universal'' form if expressed in terms of suitably scaled parameters. TBTE calculations on both III-V and II-VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surface of zincblende-structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials. 22 refs., 5 figs., 1 tab.

Godin, T.J.; LaFemina, J.P.

1991-11-01

276

Surface structure, bonding, and dynamics: The universality of zincblende (110) potential energy surfaces  

SciTech Connect

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated, zincblende-structure compound semiconductors exhibit a common ``universal`` form if expressed in terms of suitably scaled parameters. TBTE calculations on both III-V and II-VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surface of zincblende-structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials. 22 refs., 5 figs., 1 tab.

Godin, T.J.; LaFemina, J.P.

1991-11-01

277

Through bond energy transfer: a convenient and universal strategy toward efficient ratiometric fluorescent probe for bioimaging applications.  

PubMed

Fluorescence resonance energy transfer (FRET) strategy has been widely applied in designing ratiometric probes for bioimaging applications. Unfortunately, for FRET systems, sufficiently large spectral overlap is necessary between the donor emission and the acceptor absorption, which would limit the resolution of double-channel images. The through-bond energy transfer (TBET) system does not need spectral overlap between donor and acceptor and could afford large wavelength difference between the two emissions with improved imaging resolution and higher energy transfer efficiency than that of the classical FRET system. It seems to be more favorable for designing ratiometric probes for bioimaging applications. In this paper, we have designed and synthesized a coumarin-rhodamine (CR) TBET system and demonstrated that TBET is a convenient strategy to design an efficient ratiometric fluorescent bioimaging probe for metal ions. Such TBET strategy is also universal, since no spectral overlap between the donor and the acceptor is necessary, and many more dye pairs than that of FRET could be chosen for probe design. As a proof-of-concept, Hg(2+) was chosen as a model metal ion. By combining TBET strategy with dual-switch design, the proposed sensing platform shows two well-separated emission peaks with a wavelength difference of 110 nm, high energy transfer efficiency, and a large signal-to-background ratio, which affords a high sensitivity for the probe with a detection limit of 7 nM for Hg(2+). Moreover, by employing an Hg(2+)-promoted desulfurization reaction as recognition unit, the probe also shows a high selectivity to Hg(2+). All these unique features make it particularly favorable for ratiometric Hg(2+) sensing and bioimaging applications. It has been preliminarily used for a ratiometric image of Hg(2+) in living cells and practical detection of Hg(2+) in river water samples with satisfying results. PMID:23171399

Gong, Yi-Jun; Zhang, Xiao-Bing; Zhang, Cui-Cui; Luo, Ai-Li; Fu, Ting; Tan, Weihong; Shen, Guo-Li; Yu, Ru-Qin

2012-12-04

278

Probing the bonding in nitrogen-doped graphene using electron energy loss spectroscopy.  

PubMed

Precise control of graphene properties is an essential step toward the realization of future graphene devices. Defects, such as individual nitrogen atoms, can strongly influence the electronic structure of graphene. Therefore, state-of-the-art characterization techniques, in conjunction with modern modeling tools, are necessary to identify these defects and fully understand the synthesized material. We have directly visualized individual substitutional nitrogen dopant atoms in graphene using scanning transmission electron microscopy and conducted complementary electron energy loss spectroscopy experiments and modeling which demonstrates the influence of the nitrogen atom on the carbon K-edge. PMID:23869545

Nicholls, Rebecca J; Murdock, Adrian T; Tsang, Joshua; Britton, Jude; Pennycook, Timothy J; Koós, Antal; Nellist, Peter D; Grobert, Nicole; Yates, Jonathan R

2013-07-23

279

Energy localization in HMX-Estane polymer-bonded explosives during impact loading  

NASA Astrophysics Data System (ADS)

We report the results of a mechanistic study of energy localization in aHMX (High Melting point eXplosive octahydro-1,3,5,7-tetranitro-1,2,3,5-tetrazocine)/Estane PBX system during dynamic loading. The focus is on the thermal-mechanical response over the strain rate range of 104 - 105 s-1 under different confinement conditions. A recently developed cohesive finite element method is used to track and analyze the contributions to heating from different constituents, interfaces, deformation and fracture mechanisms, and internal friction. In particular, energy dissipations due to viscoelastic deformation, grain fracture, interfacial debonding, and friction along crack faces are quantified as functions of time and overall deformation. The materials analyzed have HMX volume fractions between 0.69 and 0.82. Calculations show that variation in strain rate can significantly affect the spatial distribution but not the overall number of hot spots. Higher confining stresses lead to more intense heating in the binder and more uniform distribution of hot spots. The evolution of hot spots is quantified as a function of loading condition, deformation and microstructural attributes. The microstructure-response relations obtained can be used to assess the initiation sensitivity of energetic composites.

Barua, A.; Horie, Y.; Zhou, M.

2012-03-01

280

Effect of pyrocarbon content in C\\/C preforms on microstructure and mechanical properties of the C\\/C–SiC composites  

Microsoft Academic Search

Carbon fiber reinforced silicon carbide matrix (C\\/C–SiC) composites were prepared by liquid silicon infiltration (LSI) process, using four kinds of quasi-three-dimensional C\\/C preforms with the same fiber volume fraction (32%) but different pyrocarbon content (from ?20% to ?50%, in volume) filled by chemical vapor infiltration (CVI). The microstructure and mechanical properties of C\\/C–SiC composites have been investigated. X-ray diffractmeter (XRD)

Yonghui Zhang; Zhichao Xiao; Jiping Wang; Jianfeng Yang; Zhihao Jin

2009-01-01

281

Energies and bonding in manganese phosphides. Annual progress report, July 1, 1979-June 30, 1980  

SciTech Connect

Manganese monophosphide and other first row transition metal phosphides have been studied by x-ray photoelectron spectroscopy in collaboration with colleagues at Ames Laboratory. Electron binding energies of both valence-conduction band and core level electrons show an evolution of metallic behavior in the series ScP, TiP, VP, MnP, FeP, and in the series FeP, Fe/sub 2/P, Fe/sub 3/P. Phosphorus dissociation pressure measurements on MnP lead to: 4MnP/sub (s)/ = 2Mn/sub 2/P/sub (s)/ + P/sub 2//sub (g)/, ..delta..H/sup 0//sub 298/ /sub 15//sub R/ = 35.1 +- 1.0 kK when the data are treated by the third law method using estimated entropies. Combination of this result with published data yields: ..delta..H/sup 0//sub f/ /sub 298/ /sub 15/(MnP) = -12.5 +- 0.6 kK which is in satisfactory agreement with the published value: ..delta..H/sup 0//sub f/ /sub 298/ /sub 15/ = -11.6 kK. The apparently low value of the atomization enthalpy of MnP is confirmed and is attributed to a high valence state preparation energy. A study of the congruent vaporization of Mn/sub 2/P has been initiated. Apparatus for solid state galvanic cell measurements on metal phosphide systems has been fabricated and assembled. Studies are proceeding on a high pressure synthesis of MnP/sub 3/.

Myers, C.E.; Jung, E.D.; Simpson, D.J.; Patterson, E.L.; Melzer, J.I.

1980-03-01

282

Lowest-energy site for hydrogen in diamond  

NASA Astrophysics Data System (ADS)

We investigate the locations, electronic energy levels and motion of hydrogen in diamond with tight-binding total energy calculations and Molecular Dynamics simulations. A lowest energy site for hydrogen in diamond is found. It is a sixfold degenerate site (with respect to the C-C bond), giving rise to a dangling bond and to a deep electronic level in the energy gap. The motion of the hydrogen atom in the diamond crystal is jumplike between these stable sites, starting at above ~400 K. It is a coupled-barrier diffusion, with an activation energy of 0.9 eV.

Saada, D.; Adler, Joan; Kalish, R.

2000-04-01

283

Determination of consecutive bond energies by photoionization of SbH[sub [ital n  

SciTech Connect

A photoionization mass spectrometric study of SbH[sub 3] is presented. The adiabatic ionization potential (IP) of SbH[sub 3] is [le]9.40[plus minus]0.02 eV. The lowest energy fragment ion, SbH[sup +] (+H[sub 2]), has an appearance potential (0 K) of 9.73[sub 0][plus minus]0.00[sub 8] eV, while SbH[sub 2][sup +] has an AP of 11.66[plus minus]0.02 eV. The transient species SbH[sub 2] and SbH are generated [ital in] [ital situ] by reacting F atoms with SbH[sub 3]. The IP of SbH[sub 2], forming SbH[sub 2][sup +] ([ital X] [sup 1][ital A][sub 1]), is 8.731[plus minus]0.012 eV. The IP of SbH ([ital X] [sup 3][Sigma][sup [minus

Ruscic, B.; Berkowitz, J. (Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States))

1993-10-15

284

Spectroscopic identification of bond strain and pi interactions in a series of saturated carbon-cage molecules: adamantane, twistane, octahedrane, and cubane  

NASA Astrophysics Data System (ADS)

Novel nanocarbons such as fullerenes, nanotubes, graphene, and nanodiamond reside at the cutting edge of nanoscience and technology. Along with chemical functionalization, geometrical constraints such as extreme curvature or defects in crystallites can modify the electronic properties. This paper presents a fundamental study of how bond strain affects electronic structure in a benchmark series of novel saturated carbon cage compounds. Adamantane, the smallest diamondoid, has carbon atoms commensurate with the diamond lattice. Twistane has the same stoichiometry (C10H16), but introduces some bond strain into the cage. Octahedrane (C12H12) and cubane (C8H8) have increasing strain, culminating in cubane where C-C bonds lie either parallel, or orthogonal to one another. Using gas-phase NEXAFS spectroscopy, we observe the broad C-C ?* splits into two more narrow and intense resonances with increasing strain. Also, LUMO states associated with tertiary C-H ?* broaden and shift to lower energy, and are 3X more intense in cubane than octadedrane. The differences are entirely due to the shape rather than stoichiometry, and indicate, we believe, some ? interaction between parallel C-C bonds in the cubane.

Willey, Trevor M.; Lee, J. R. I.; van Buuren, T.; Landt, L.; Schreiner, P. R.; Fokin, A. A.; Tkachenko, B. A.; Fokina, N. A.; Brehmer, D.

2011-03-01

285

Solar System planetary tests of {dot c/c}  

NASA Astrophysics Data System (ADS)

Analytical and numerical calculations show that a putative temporal variation of the speed of light c, with the meaning of space-time structure constant c ST, assumed to be linear over timescales of about one century, would induce a secular precession of the longitude of the pericenter {\\varpi} of a test particle orbiting a spherically symmetric body. By comparing such a predicted effect to the corrections {?dot\\varpi} to the usual Newtonian/Einsteinian perihelion precessions of the inner planets of the Solar System, recently estimated by E.V. Pitjeva by fitting about one century of modern astronomical observations with the standard classical/relativistic dynamical force models of the EPM epehemerides, we obtained {dot c/c =(0.5± 2)× 10^{-7} yr^{-1}} . Moreover, the possibility that {dot c/cneq 0} over the last century is ruled out at 3-12 ? level by taking the ratios of the perihelia for different pairs of planets. Our results are independent of any measurement of the variations of other fundamental constants which may be explained by a variation of c itself (with the meaning of electromagnetic constant c EM). It will be important to repeat such tests if and when other teams of astronomers will estimate their own corrections to the standard Newtonian/Einsteinian planetary perihelion precessions with different ephemerides.

Iorio, Lorenzo

2010-01-01

286

Chemical Distribution and Bonding of Lithium in Intercalated Graphite: Identification with Optimized Electron Energy Loss Spectroscopy  

SciTech Connect

Direct mapping of the lithium spatial distribution and the chemical state provides critical information on structure-correlated lithium transport in electrode materials for lithium batteries. Nevertheless, probing lithium, the lightest solid element in the periodic table, poses an extreme challenge with traditional X-ray or electron scattering techniques due to its weak scattering power and vulnerability to radiation damage. Here, we report nanoscale maps of the lithium spatial distribution in electrochemically lithiated graphite using electron energy loss spectroscopy in the transmission electron microscope under optimized experimental conditions. The electronic structure of the discharged graphite was obtained from the near-edge fine structure of the Li and C K-edges and ab initio calculations. A 2.7 eV chemical shift of the Li K-edge, along with changes in the density of states, reveals the ionic nature of the intercalated lithium with significant charge transfer to the graphene sheets. Direct mapping of lithium in graphite revealed nanoscale inhomogeneities (nonstoichiometric regions), which are correlated with local phase separation and structural disorder (i.e., lattice distortion and dislocations) as observed by high-resolution transmission electron microscopy. The surface solid-electrolyte interphase (SEI) layer was also imaged and determined to have a thickness of 10-50 nm, covering both edge and basal planes with LiF as its primary inorganic component. The Li K-edge spectroscopy and mapping, combined with electron microscopy-based structural analysis provide a comprehensive view of the structure-correlated lithium intercalation in graphite and of the formation of the SEI layer.

Zhu, Y.; Wang, F.; Graetz, J.; Moreno, M.S.; Ma, C.; Wu, L.; Volkov, V.

2011-02-01

287

3Drefine: consistent protein structure refinement by optimizing hydrogen bonding network and atomic-level energy minimization.  

PubMed

One of the major limitations of computational protein structure prediction is the deviation of predicted models from their experimentally derived true, native structures. The limitations often hinder the possibility of applying computational protein structure prediction methods in biochemical assignment and drug design that are very sensitive to structural details. Refinement of these low-resolution predicted models to high-resolution structures close to the native state, however, has proven to be extremely challenging. Thus, protein structure refinement remains a largely unsolved problem. Critical assessment of techniques for protein structure prediction (CASP) specifically indicated that most predictors participating in the refinement category still did not consistently improve model quality. Here, we propose a two-step refinement protocol, called 3Drefine, to consistently bring the initial model closer to the native structure. The first step is based on optimization of hydrogen bonding (HB) network and the second step applies atomic-level energy minimization on the optimized model using a composite physics and knowledge-based force fields. The approach has been evaluated on the CASP benchmark data and it exhibits consistent improvement over the initial structure in both global and local structural quality measures. 3Drefine method is also computationally inexpensive, consuming only few minutes of CPU time to refine a protein of typical length (300 residues). 3Drefine web server is freely available at http://sysbio.rnet.missouri.edu/3Drefine/. PMID:22927229

Bhattacharya, Debswapna; Cheng, Jianlin

2012-09-26

288

Methodology for hardware\\/software co-verification in C\\/C++  

Microsoft Academic Search

In this paper we present our C\\/C++-based design environment for hardware\\/software co-verification. Our approach is to use C\\/C++ to describe both hardware and software throughout the design flow. Our methodology supports the efficient mapping of C\\/C++ functional descriptions directly into hardware and software. The advantages of a C\\/C++-based flow from the verification point of view are presented. The use of

Luc Séméria; Abhijit Ghosh

2000-01-01

289

Metal cation dependence of interactions with amino acids: bond energies of Rb+ and Cs+ to Met, Phe, Tyr, and Trp.  

PubMed

The interactions of rubidium and cesium cations with four amino acids (AA) including methionine (Met), phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp) are examined in detail. Experimentally, the bond dissociation energies (BDEs) are determined using threshold collision-induced dissociation of the Rb(+)(AA) and Cs(+)(AA) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections include consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion-neutral collisions. 0 K BDEs of 121.0 ± 7.0 (102.8 ± 6.6), 123.8 ± 7.2 (112.9 ± 5.5), 125.8 ± 7.4 (115.6 ± 6.9), and 138.1 ± 7.5 (125.0 ± 6.8) kJ/mol are determined for complexes of Rb(+) (Cs(+)) with Met, Phe, Tyr, and Trp, respectively. Quantum chemical calculations are conducted at the B3LYP, MP2(full), and M06 levels of theory with geometries and zero point energies calculated at the B3LYP level using def2-TZVPPD basis sets. Results obtained using all three levels show good agreement with experiment, with B3LYP values being systematically low and MP2(full) and M06 values being systematically high. At 0 and 298 K, theory predicts the ground-state conformers for M(+)(Met) either have tridentate binding of the metal cation to the carbonyl, amino, and sulfur groups (MP2 and M06) or to both oxygens of a zwitterionic conformation (B3LYP). At 298 K, binding to the carboxylic acid group and the sulfur also becomes competitive. For the aromatic amino acids at 0 K, most levels of theory favor tridentate binding of the metal ions to the backbone carbonyl and amino groups along with the ?-cloud of the ring, whereas for Rb(+)(Trp) and Cs(+)(AA), B3LYP theory favors binding to only the carbonyl and ring groups. At 298 K, B3LYP favors the latter binding mode for all three Rb(+)(aromatic AA) complexes. Comparison of these results to those for the smaller alkali cations provides insight into the trends in binding affinities and structures associated with metal cation variations. PMID:23514190

Armentrout, P B; Yang, Bo; Rodgers, M T

2013-04-01

290

Photochemical tissue bonding  

DOEpatents

Photochemical tissue bonding methods include the application of a photosensitizer to a tissue and/or tissue graft, followed by irradiation with electromagnetic energy to produce a tissue seal. The methods are useful for tissue adhesion, such as in wound closure, tissue grafting, skin grafting, musculoskeletal tissue repair, ligament or tendon repair and corneal repair.

Redmond, Robert W. (Brookline, MA); Kochevar, Irene E. (Charlestown, MA)

2012-01-10

291

Water-stabilized three- and four-atom palladium clusters as highly active catalytic species in ligand-free c?c cross-coupling reactions.  

PubMed

Elite cliques: Palladium clusters with three and four atoms were found to be the catalytically active species for ligand-free palladium-catalyzed C?C bond-forming reactions. These palladium cluster species could be stabilized in water and stored for long periods of time for use on demand with no loss of activity. High yields of products and turnover frequencies (TOFs) of up to 10(5) ?h(-1) were observed. PMID:24038914

Leyva-Pérez, Antonio; Oliver-Meseguer, Judit; Rubio-Marqués, Paula; Corma, Avelino

2013-09-13

292

Diffusion bonding  

DOEpatents

1. A method for joining beryllium to beryllium by diffusion bonding, comprising the steps of coating at least one surface portion of at least two beryllium pieces with nickel, positioning a coated surface portion in a contiguous relationship with an other surface portion, subjecting the contiguously disposed surface portions to an environment having an atmosphere at a pressure lower than ambient pressure, applying a force upon the beryllium pieces for causing the contiguous surface portions to abut against each other, heating the contiguous surface portions to a maximum temperature less than the melting temperature of the beryllium, substantially uniformly decreasing the applied force while increasing the temperature after attaining a temperature substantially above room temperature, and maintaining a portion of the applied force at a temperature corresponding to about maximum temperature for a duration sufficient to effect the diffusion bond between the contiguous surface portions.

Anderson, Robert C. (Oak Ridge, TN)

1976-06-22

293

High-pressure torsion of pure metals: Influence of atomic bond parameters and stacking fault energy on grain size and correlation with hardness  

Microsoft Academic Search

The grain size in pure elements (magnesium, aluminum, silicon, titanium, vanadium, chromium, iron, nickel, copper, zinc, germanium, zirconium, niobium, molybdenum, palladium, silver, indium, tin, hafnium, tantalum, gold and lead) after processing by high-pressure torsion (HPT) reaches steady-state levels where the grain size remains unchanged with straining. The steady-state grain sizes decrease by atomic bond energy and related parameters such as

Kaveh Edalati; Zenji Horita

2011-01-01

294

Proton tunnelling in the hydrogen bonds of halogen-substituted derivatives of benzoic acid studied by NMR relaxometry: the case of large energy asymmetry  

Microsoft Academic Search

The concerted two proton transfer in the hydrogen bonds of para-halogen substituted derivatives of benzoic acid has been investigated using conventional NMR relaxometry combined with field-cycling measurements of the magnetic field dependence of the proton spin–lattice relaxation rate. Thus, the inverse correlation time describing the proton transfer process has been determined over a wide range of temperature. The energy difference

A. J. Horsewill; C. J. McGloin; H. P. Trommsdorff; M. R. Johnson

2003-01-01

295

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

SciTech Connect

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

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

1998-06-01

296

A Study of the Interactions between the Double Bonds in Unsaturated Ketones.  

National Technical Information Service (NTIS)

The interactions between C=C and C-O double bonds in several unsaturated ketones have been studied by comparing MINDO/2 calculations with ionisation potentials determined by photoelectron spectroscopy (PES). With one exception (norbornadienone) the direct...

N. Bodor M. J. S. Dewar Z. B. Maksic

1975-01-01

297

The bond energy of ReO+: Guided ion-beam and theoretical studies of the reaction of Re+ (7S) with O2  

NASA Astrophysics Data System (ADS)

The kinetic-energy dependence of the Re+ + O2 reaction is examined using guided ion-beam mass spectrometry. The cross section for ReO+ formation from ground state Re+ (7S) is unusual, exhibiting two endothermic features. The kinetic energy dependence for ReO+ formation is analyzed to determine D0(Re+-O) = 4.82 +/- 0.05 eV, with the higher energy feature having a threshold 1.35 +/- 0.28 eV higher in energy. This bond energy is consistent with much less precise values determined in the literature. Formation of ReO2+ is also observed with a pressure dependent cross section, establishing that it is formed in an exothermic reaction of ReO+ with O2. The nature of the bonding for ReO+ and ReO2+ is discussed and analyzed primarily using theoretical calculations at the B3LYP/def2-TZVPPD level of theory. The ground state of ReO+ is identified as either 5? or 3?, with the latter favored once estimates of spin-orbit splitting are included. Bond energies for ground state ReO+ are calculated at this level as well as BP86 and CCSD(T,full) levels using several different basis sets. BP86 theoretical bond energies are higher than the experimental value, whereas B3LYP and CCSD(T,full) values are lower, although estimated spin-orbit corrections increase the latter close to experiment. Potential energy surfaces for the reaction of Re+ with O2 are also calculated at the B3LYP/def2-TZVPPD level of theory and reveal that ground state Re+ (7S) inserts into O2 by forming a Re+(O2) (5A'') complex which can then couple with additional surfaces to form ground state ReO2+ (3B1). Several explanations for the unusual dual endothermic features are explored, with no unambiguous explanation being evident. As such, this heavy metal system provides a very interesting experimental phenomenon of both adiabatic and nonadiabatic behavior.

Armentrout, P. B.

2013-08-01

298

The bond energy of ReO+: guided ion-beam and theoretical studies of the reaction of Re+ (7S) with O2.  

PubMed

The kinetic-energy dependence of the Re(+) + O2 reaction is examined using guided ion-beam mass spectrometry. The cross section for ReO(+) formation from ground state Re(+) ((7)S) is unusual, exhibiting two endothermic features. The kinetic energy dependence for ReO(+) formation is analyzed to determine D0(Re(+)-O) = 4.82 ± 0.05 eV, with the higher energy feature having a threshold 1.35 ± 0.28 eV higher in energy. This bond energy is consistent with much less precise values determined in the literature. Formation of ReO2(+) is also observed with a pressure dependent cross section, establishing that it is formed in an exothermic reaction of ReO(+) with O2. The nature of the bonding for ReO(+) and ReO2(+) is discussed and analyzed primarily using theoretical calculations at the B3LYP/def2-TZVPPD level of theory. The ground state of ReO(+) is identified as either (5)? or (3)?, with the latter favored once estimates of spin-orbit splitting are included. Bond energies for ground state ReO(+) are calculated at this level as well as BP86 and CCSD(T,full) levels using several different basis sets. BP86 theoretical bond energies are higher than the experimental value, whereas B3LYP and CCSD(T,full) values are lower, although estimated spin-orbit corrections increase the latter close to experiment. Potential energy surfaces for the reaction of Re(+) with O2 are also calculated at the B3LYP/def2-TZVPPD level of theory and reveal that ground state Re(+) ((7)S) inserts into O2 by forming a Re(+)(O2) ((5)A") complex which can then couple with additional surfaces to form ground state ReO2(+) ((3)B1). Several explanations for the unusual dual endothermic features are explored, with no unambiguous explanation being evident. As such, this heavy metal system provides a very interesting experimental phenomenon of both adiabatic and nonadiabatic behavior. PMID:24006995

Armentrout, P B

2013-08-28

299

Determination of side-chain-rotamer and side-chain and backbone virtual-bond-stretching potentials of mean force from AM1 energy surfaces of terminally-blocked amino-acid residues, for coarse-grained simulations of protein structure and folding. 1. The Method  

PubMed Central

In this and the accompanying paper, we report the development of new physics-based side-chain-rotamer and virtual-bond-deformation potentials which now replace the respective statistical potentials used so far in our physics-based united-reside UNRES force field for large-scale simulations of protein structure and dynamics. In this paper, we describe the methodology for determining the corresponding potentials of mean force (PMF’s) from the energy surfaces of terminally-blocked amino-acid residues calculated with the AM1 quantum-mechanical semiempirical method. The approach is based on minimization of the AM1 energy for fixed values of the angles ? for rotation of the peptide groups about the C? ? C? virtual bonds, and for fixed values of the side-chain dihedral angles ?, which formed a multi-dimensional grid. A harmonic-approximation approach was developed to extrapolate from the energy at a given grid point to other points of the conformational space in order to compute the respective contributions to the PMF. To test the applicability of the harmonic approximation, the rotamer PMF’s of alanine and valine obtained with this approach have been compared with those obtained by using a Metropolis Monte Carlo method. The PMF surfaces computed with the harmonic approximation are more rugged and have more pronounced minima than the MC-calculated surfaces but the harmonic-approximation- and MC-calculated PMF values are linearly correlated. The potentials derived with the harmonic approximation are, therefore, appropriate for UNRES for which the weights (scaling factors) of the energy terms are determined by force-field optimization for foldability.

Kozlowska, Urszula; Liwo, Adam; Scheraga, Harold A.

2009-01-01

300

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. II. Intracluster vibrational energy redistribution of the OH stretching vibration of hydrogen-bonded clusters  

NASA Astrophysics Data System (ADS)

A picosecond time-resolved IR-UV pump-probe spectroscopic study has been carried out for investigating the intracluster vibrational energy redistribution (IVR) and subsequent dissociation of hydrogen-bonded clusters of phenol (C6H5OH) and partially deuterated phenol (C6D5OH, phenol-d5) with various solvent molecules. The H-bonded OH stretching vibration was pumped by a picosecond IR pulse, and the transient S1-S0 UV spectra from the pumped level as well as the redistributed levels were observed with a picosecond UV laser. Two types of hydrogen-bonded clusters were investigated with respect to the effect of the H-bonding strength on the energy flow process: the first is of a strong ``?-type H-bond'' such as phenol-(dimethyl ether)n=1 and phenol dimer, and the second is phenol-(ethylene)n=1 having a weak ``?-type H-bond.'' It was found that the population of the IR-pumped OH level exhibits a single-exponential decay, whose rate increases with the H-bond strength. On the other hand, the transient UV spectrum due to the redistributed levels showed a different time evolutions at different monitoring UV frequency. From an analysis of the time profiles of the transient UV spectra, the following three-step scheme has been proposed for describing the energy flow starting from the IVR of the initially excited H-bonded OH stretching level to the dissociation of the H bond. (1) The intramolecular vibrational energy redistribution takes place within the phenolic site, preparing a hot phenol. (2) The energy flows from the hot phenol to the intermolecular vibrational modes of the cluster. (3) Finally, the hydrogen bond dissociates. Among the three steps, the rate constant of the first step was strongly dependent on the H-bond strength, while the rate constants of the other two steps were almost independent of the H-bond strength. For the dissociation of the hydrogen bond, the observed rate constants were compared with those calculated by the Rice, Ramsperger, Kassel, and Marcus model. The result suggests that dissociation of the hydrogen bond takes place much faster than complete energy randomization within the clusters.

Kayano, Masakazu; Ebata, Takayuki; Yamada, Yuji; Mikami, Naohiko

2004-04-01

301

An analysis of chlorine and bromine oxygen bonding and its implications for stratospheric chemistry  

Microsoft Academic Search

Trends in chlorine\\/oxygen and bromine\\/oxygen single and double bonds are examined for several molecules of interest in stratospheric halogen chemistry. Specifically, the relationships between bond distance and quadratic force constant, and bond distance and ionic bonding character are examined, together with bond energies. Similar to a previous study of FO bonding, it is found that the relationship between bond distance

Timothy J. Lee; Christopher E. Dateo; Julia E. Rice

1999-01-01

302

Chelation-assisted carbon-hydrogen and carbon-carbon bond activation by transition metal catalysts.  

PubMed

Herein we describe the chelation-assisted C-H and C-C bond activation of carbonyl compounds by Rh1 catalysts. Hydroacylation of olefins was accomplished by utilizing 2-amino-3-picoline as a chelation auxiliary. The same strategy was employed for the C-C bond activation of unstrained ketones. Allylamine 24 was devised as a synthon of formaldehyde. Hydroiminoacylation of alkynes with allylamine 24 was applied to the alkyne cleavage by the aid of cyclohexylamine. PMID:12180320

Jun, Chul-Ho; Moon, Choong Woon; Lee, Dae-Yon

2002-06-01

303

More sustainable formation of C-N and C-C bonds for the synthesis of N-heterocycles.  

PubMed

Heterocycles made green: New methodologies for the synthesis of pyrroles were recently developed based on domino Ir- and Ru-catalyzed amination and alkylations of alcohols. The concept provides a greener approach to interesting N-heterocyclic compounds. PMID:23775609

Schranck, Johannes; Tlili, Anis; Beller, Matthias

2013-06-17

304

Fluorescence spectroscopy as a novel method for on-line analysis of biocatalytic C–C bond formations  

Microsoft Academic Search

On-line analysis of bioprocesses is of increasing interest avoiding the time delay for off-line sample preparation and the following analyses via chromatographic methods. Moreover, continuous monitoring of the reaction components during chemo- or biocatalytic transformations provides a direct control of the process. Since productivity of the processes can be controlled simultaneously, on-line monitoring of the processes is attractive for industrial

Selin Kara; Fabienne Anton; Dörte Solle; Markus Neumann; Bernd Hitzmann; Thomas Scheper; Andreas Liese

2010-01-01

305

C-C bond formation via copper-catalyzed conjugate addition reactions to enones in water at room temperature.  

PubMed

Conjugate addition reactions to enones can now be done in water at room temperature with in situ generated organocopper reagents. Mixing an enone, zinc powder, TMEDA, and an alkyl halide in a micellar environemnt containing catalytic amounts of Cu(I), Ag(I), and Au(III) leads to 1,4-adducts in good isolated yields: no organometallic precursor need be formed. PMID:23190029

Lipshutz, Bruce H; Huang, Shenlin; Leong, Wendy Wen Yi; Zhong, Guofu; Isley, Nicholas A

2012-11-28

306

A heterogeneous layered bifunctional catalyst for the integration of aerobic oxidation and asymmetric C-C bond formation.  

PubMed

The design and synthesis of a heterogeneous bifunctional chiral catalyst for the sequential aerobic oxidation-asymmetric Michael reactions between primary allylic alcohols and dibenzyl malonate are described. Interestingly, we found that layering bimetallic nanoparticles over the organocatalyst, within the chiral composite material, is crucial for catalytic activity. PMID:24036576

Miyamura, Hiroyuki; Choo, Gerald C Y; Yasukawa, Tomohiro; Yoo, Woo-Jin; Kobayashi, Shu

2013-10-01

307

Acetylacetone-cleaving enzyme Dke1: a novel C-C-bond-cleaving enzyme from Acinetobacter johnsonii.  

PubMed Central

The toxicity of acetylacetone has been demonstrated in various studies. Little is known, however, about metabolic pathways for its detoxification or mineralization. Data presented here describe for the first time the microbial degradation of acetylacetone and the characterization of a novel enzyme that initiates the metabolic pathway. From an Acinetobacter johnsonii strain that grew with acetylacetone as the sole carbon source, an inducible acetylacetone-cleaving enzyme was purified to homogeneity. The corresponding gene, coding for a 153 amino acid sequence that does not show any significant relationship to other known protein sequences, was cloned and overexpressed in Escherichia coli and gave high yields of active enzyme. The enzyme cleaves acetylacetone to equimolar amounts of methylglyoxal and acetate, consuming one equivalent of molecular oxygen. No exogenous cofactor is required, but Fe(2+) is bound to the active protein and essential for its catalytic activity. The enzyme has a high affinity for acetylacetone with a K (m) of 9.1 microM and a k(cat) of 8.5 s(-1). A metabolic pathway for acetylacetone degradation and the putative relationship of this novel enzyme to previously described dioxygenases are discussed.

Straganz, Grit D; Glieder, Anton; Brecker, Lothar; Ribbons, Douglas W; Steiner, Walter

2003-01-01

308

Correlation of the O-H\\ctdot N bonds energy with the excess compression of binary mixtures of pyridine and its methyl derivatives with methanol and water  

NASA Astrophysics Data System (ADS)

Experimental excess molar isentropic compressions for six binary systems: (pyridine, 2-methylpyridine or 2,6-dimethylpyridine) + (water or methanol) are correlated with the calculated theoretically association energies of the ROH⋯NA complexes that increase in the order: pyridine < 2-methylpyridine < 2,6-dimethylpyridine. In spite of stronger O-H⋯N bonds, the negative excesses for methanolic systems are smaller than for the aqueous ones. That is mainly because of the aggregation of water-amine complexes due to the O-H⋯O bonds. Differences in partial molar compressions of the amines in water and methanol solvents result most probably from the hydrophobic hydration in aqueous solutions.

Czech, Bo?ena; Lodowski, Piotr; Marczak, Wojciech

2013-01-01

309

Dual Bonding between H2O/H2S and AgCl/CuCl: Cu/Ag Bond, Sister Bond to Au Bond.  

PubMed

Recently, Legon et al. reported the first generation and characterization of H2O/H2S···AgCl complexes by rotational spectroscopy and proposed whether there is a silver bond analogous to the more familiar hydrogen and halogen bonds. In this study, a theoretical investigation was performed to answer this question and to deepen the nature of intermolecular interactions for H2O/H2S···M-Cl (M = Cu, Ag, and Au) complexes. NBO analyses reveal that two types of delocalization interactions coexist in these complexes. Apart from the expected ?-donation interaction, the hyperconjugation interaction between H2O/H2S and M-Cl also takes part in the bonding. On the basis of such a dual-bonding mechanism, one class of bond, termed Cu/Ag bond, was defined in this study. In addition, the topological properties at a bond critical point, binding energies, and stretching frequency shifts studied here support that Cu/Ag bond is a sister bond to Au bond put forward previously by Sadlej et al. The Cu/Ag/Au bond is partially covalent and partially electrostatic in nature. Finally, the dual-bonding mechanism of Cu/Ag/Au bond was further discussed. This dual-bonding scheme may be considered a new synergistic bonding model for coordination compounds. PMID:24070212

Zhang, Guiqiu; Zhao, Xingjuan; Chen, Dezhan

2013-10-10

310

Synthesis and reactions of group 6 metal half-sandwich complexes of 2,2-dicyanoethylene-1,1-dichalcogenolates [(Cp*)M[E(2)C=C(CN)(2)](2)]-(M = Mo, W; E = S, Se).  

PubMed

A series of group 6 transition metal half-sandwich complexes with 1,1-dichalcogenide ligands have been prepared by the reactions of Cp*MCl(4)(Cp* = eta(5)-C(5)Me(5); M = Mo, W) with the potassium salt of 2,2-dicyanoethylene-1,1-dithiolate, (KS)(2)C=C(CN)(2) (K(2)-i-mnt), or the analogous seleno compound, (KSe)(2)C=C(CN)(2) (K(2)-i-mns). The reaction of Cp*MCl(4) with (KS)(2)C=C(CN)(2) in a 1:3 molar ratio in CH(3)CN gave rise to K[Cp*M(S(2)C=C(CN)(2))(2)] (M = Mo, 1a, 74%; M = W, 2a, 46%). Under the same conditions, the reaction of Cp*MoCl(4) with 3 equiv of (KSe)(2)C=C(CN)(2) afforded K[Cp*Mo(Se(2)C=C(CN)(2))(2)] (3a) and K[Cp*Mo(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))] (4) in respective yields of 45% and 25%. Cation exchange reactions of 1a, 2a, and 3a with Et(4)NBr resulted in isolation of (Et(4)N)[Cp*Mo(S(2)C=C(CN)(2))(2)] (1b), (Et(4)N)[Cp*W(S(2)C=C(CN)(2))(2)] (2b), and (Et(4)N)[Cp*Mo(Se(2)C=C(CN)(2))(2)] (3b), respectively. Complex 4 crystallized with one THF and one CH(3)CN molecule as a three-dimensional network structure. Inspection of the reaction of Cp*WCl(4) with (KSe)(2)C=C(CN)(2) by ESI-MS revealed the existence of three species in CH(3)CN, [Cp*W(Se(2)C=C(CN)(2))(2)]-, [Cp*W(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))]-, and [Cp*W(Se(Se(2))C=C(CN)(2))(2)]-, of which [Cp*W(Se(2)C=C(CN)(2))(Se(Se(2))C=C(CN)(2))]-(5) was isolated as the main product. Treatment of 2a with 1/4 equiv of S(8) in refluxing THF resulted in sulfur insertion and gave rise to K[Cp*W(S(2)C=C(CN)(2))(S(S(2))C=C(CN)(2))](6), which crystallized with two THF molecules forming a three-dimensional network structure. 6 can also be prepared by refluxing 2a with 1/4 equiv of S(8) in THF. 3a readily added one Se atom upon treatment with 1 mol of Se powder in THF to give 4 in high yield, while the treatment of 3a or 4 with 2 equiv of Na(2)Se in THF led to formation of a dinuclear complex [(Cp*Mo)(2)(mu-Se)(mu-Se(Se(3))C=C(CN)(2))] (7). The structure of 7 consists of two Cp*Mo units bridged by a Se(2-) and a [Se(Se(3))C=C(CN)(2)](2-) ligand in which the triselenido group is arranged in a nearly linear way (163 degrees). The reaction of 2a with 2 equiv of CuBr in CH(3)CN yielded a trinuclear complex [Cp*WCu(2)(mu-Br)(mu(3)-S(2)C=C(CN)(2))(2)] (8), which crystallized with one CH(3)CN and generated a one-dimensional chain polymer through bonding of Cu to the N of the cyano groups. PMID:12206711

Hong, Maochun; Cao, Rong; Kawaguchi, Hiroyuki; Tatsumi, Kazuyuki

2002-09-01

311

Basics of Fidelity Bonding.  

ERIC Educational Resources Information Center

|Fidelity bonds are important for an agency to hold to protect itself against any financial loss that can result from dishonest acts by its employees. Three types of fidelity bonds are available to an agency: (1) public official bonds; (2) dishonesty bonds; and (3) faithful performance bonds. Public official bonds are required by state law to be…

Kahn, Steven P.

312

A Model for the Chemical Bond  

ERIC Educational Resources Information Center

Bond stereochemistry in polyatomic hydrides is explained in terms of the principle of bond energies maximization, which yields X-H straight bonds and suggests the formation of appropriate sp hybrids on the central atom. An introduction to the electron charge distribution in molecules is given, and atomic, overlap, gross and formal charges are…

Magnasco, Valerio

2004-01-01

313

A Model for the Chemical Bond  

ERIC Educational Resources Information Center

|Bond stereochemistry in polyatomic hydrides is explained in terms of the principle of bond energies maximization, which yields X-H straight bonds and suggests the formation of appropriate sp hybrids on the central atom. An introduction to the electron charge distribution in molecules is given, and atomic, overlap, gross and formal charges are…

Magnasco, Valerio

2004-01-01

314

Experimental bond critical point and local energy density properties determined for Mn-O, Fe-O and Co-O bonded interactions for tephroite, Mn2SiO4, fayalite, Fe2SiO4 and Co2SiO4 olivine and selected organic metal complexes: Comparison with properties calculated for non-transition and transition metal M-O bonded interactions for silicates and oxides  

SciTech Connect

Bond critical point, bcp, and local energy density properties for the electron density, ED, distributions, calculated with first principle quantum mechanical methods for divalent transition metal Mn-, Co- and Fe-containing silicates and oxides are compared with experimental model ED properties for tephroite, Mn2SiO4, fayalite, Fe2SiO4 and Co2SiO4 olivine, each determined with high energy synchrotron single crystal X-ray diffraction data. Trends observed between the experimental bond lengths, R(M-O), (M = Mn, Fe, Co), and the calculated bcp properties are comparable with those observed for non-transition M-O bonded interactions. The bcp, local total energy density, H(rc), and bond length trends determined for the Mn-O, Co-O and Fe-O interactions are virtually identical. A comparison is also made with model experimental bcp properties determined for several Mn-O, Fe-O and Co-O bonded interactions for organometallic complexes and several oxides. Despite the complexities of the structures of the organometallic complexes, the agreement between the calculated and the model experimental bcp properties is good in several cases. The G(rc)/p(rc) vs. R(M-O) trends established for non-transition metal M-O bonded interactions hold for the given transition metal M O bonded interactions with the G(rc)/p(rc) ratio increasing in value as H(rc) becomes progressively more negative in value and the shared character of the interaction increases. As observed for the non-transition metal M-O bonded interactions, the Laplacian, ?2p(rc), increases in value as p(rc) increases and as H(rc) decreases. The Mn-O, Fe-O, and Co-O bonded interactions are indicated to be of intermediate character with a substantial component of closed-shell character compared with Fe-S and Ni-S bonded interactions which show greater shared character based on the |V(rc)|/G(rc) bond character indicator. The atomic charges conferred on the transition metal atoms for the three olivines decrease with increasing atomic number from Mn to Fe to Co.

Gibbs, Gerald V.; Downs, R. T.; Cox, David F.; Rosso, Kevin M.; Ross, Nancy L.; Kirfel, Armin; Lippmann, Thomas; Morgenroth, W.; Crawford, T Daniel

2008-09-18

315

Proton tunnelling in the hydrogen bonds of halogen-substituted derivatives of benzoic acid studied by NMR relaxometry: the case of large energy asymmetry  

NASA Astrophysics Data System (ADS)

The concerted two proton transfer in the hydrogen bonds of para-halogen substituted derivatives of benzoic acid has been investigated using conventional NMR relaxometry combined with field-cycling measurements of the magnetic field dependence of the proton spin-lattice relaxation rate. Thus, the inverse correlation time describing the proton transfer process has been determined over a wide range of temperature. The energy difference between the two proton configurations was determined in all four compounds to be significantly larger than in the prototype model system of benzoic acid dimers. This energy difference exceeds the cut-off frequency of the acoustic phonon spectrum of the crystal as well as the frequency of two lowest modes promoting proton tunnelling. The low temperature limit of the tunnelling rate was found to be one order of magnitude higher than in benzoic acid. For the four compounds studied, this rate and its increase at higher temperature exhibit a different behaviour to benzoic acid which is attributed to the details of the level structure and energy gaps between states corresponding predominantly to one or the other proton configuration. A new adaptation of the phonon-assisted theory is proposed which applies to these cases of high energy asymmetry. To assist in the interpretation of the data, a powder neutron diffraction structure determination of 4-bromobenzoic acid is reported together with DFT calculations on the hydrogen bond structures of all members of the 4-halogen substituted derivatives.

Horsewill, A. J.; McGloin, C. J.; Trommsdorff, H. P.; Johnson, M. R.

2003-06-01

316

Tuning the redox potential in molecular monolayers covalently bound to H Si(100) electrodes via distinct C C tethering arms  

NASA Astrophysics Data System (ADS)

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute solvent interactions. Different bond arrangements of the C C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.

Zanoni, R.; Cossi, M.; Iozzi, M. F.; Cattaruzza, F.; Dalchiele, E. A.; Decker, F.; Marrani, A. G.; Valori, M.

2008-10-01

317

Hexacoordinate bonding and aromaticity in silicon phthalocyanine.  

PubMed

Si-E bondings in hexacoordinate silicon phthalocyanine were analyzed using bond order (BO), energy partition, atoms in molecules (AIM), electron localization function (ELF), and localized orbital locator (LOL). Bond models were proposed to explain differences between hexacoordinate and tetracoordinate Si-E bondings. Aromaticity of silicon phthalocyanine was investigated using nucleus-independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), conceptual density functional theory (DFT), ring critical point (RCP) descriptors, and delocalization index (DI). Structure, energy, bonding, and aromaticity of tetracoordinate silicon phthalocyanine were studied and compared with hexacoordinate one. PMID:21105726

Yang, Yang

2010-11-24

318

Hydrogen Bonded Arrays: The Power of Multiple Hydrogen Bonds  

SciTech Connect

Hydrogen bond interactions in small covalent model compounds (i.e. deprotonated polyhydroxy alcohols) were measured by negative ion photoelectron spectroscopy. The experimentally determined vertical and adiabatic electron detachment energies for (HOCH2CH2)2CHO (2a), (HOCH2CH2) 3CO (3a) and (HOCH2CH2CH(OH)CH2)3CO (4a) reveal that hydrogen-bonded networks can provide enormous stabilizations, and that a single charge center not only can be stabilized by up to 3 hydrogen bonds but it can increase the interaction energy between non-charged OH groups by 5.8 kcal mol1 or more per hydrogen bond. This can lead to pKa values that are very different than in water, and provide some of the impetus for catalytic processes.

Shokri, Alireza; Schmidt, Jacob C.; Wang, Xue B.; Kass, Steven R.

2012-02-01

319

Detecting type errors and secure coding in C\\/C++ applications  

Microsoft Academic Search

The programming languages such as C\\/C++ suffer from memory management and security of code especially when their codes are used in critical systems. Therefore, we need an efficient mechanism to detect memory and type errors. Some researches have been done and many tools have been developed to detect these errors and to secure C\\/C++ code. However, theses tools have some

Hamid Mcheick; Heni Dhiab; Mohamad Dbouk; Rakan Mcheik

2010-01-01

320

The enthalpy of formation of fullerene fluoride C60F18 and the C-F bond energy  

NASA Astrophysics Data System (ADS)

The enthalpy of combustion of crystalline fullerene fluoride C60F18 was determined in an isoperibolic calorimeter with a rotating platinized bomb, and the enthalpy of formation of the compound was calculated. The enthalpy of sublimation of C60F18 measured earlier was used to calculate the enthalpy of formation of fullerene fluoride in the gas phase and the mean enthalpy of dissociation of C-F bonds in this compound.

Papina, T. S.; Luk'yanova, V. A.; Goryunkov, A. A.; Ioffe, I. N.; Gol'Dt, I. V.; Buyanovskaya, A. G.; Kabaeva, N. M.; Sidorov, L. N.

2007-10-01

321

High energy X-ray diffraction analysis of strain and residual stress in silicon nitride ceramic diffusion bonds  

Microsoft Academic Search

High resolution X-ray scanning diffractometry is used to study the residual stress in binary metal\\/ceramic (Ni\\/Si3N4) diffusion bonds fabricated by simultaneous high temperature heating and uniaxial pressing. In order to diminish the experimental error on the stress determination, the method consists of three steps: (i) to measure the axial and radial strains following some selected lines at the inner volume

M. Vila; C. Prieto; P. Miranzo; M. I. Osendi; A. E. Terry; G. B. M. Vaughan

2005-01-01

322

Simulation investigations in the binding energy and mechanical properties of HMX-based polymer-bonded explosives  

Microsoft Academic Search

The molecular simulations of the well-known high explosive ?-HMX (cyclotetramethylene tetranitramine) and its fluorine containing\\u000a polymer-bonded explosives (PBXs) were carried out with the combination method of quantum mechanics, molecular mechanics and\\u000a molecular dynamics. The atomic cluster model, containing the ?-HMX molecule and the polymer molecule whose chain dimension\\u000a was about the same as ?-HMX’s, was fully optimized by AM1 and

Jijun Xiao; Guoyong Fang; Guangfu Ji; Heming Xiao

2005-01-01

323

Highly Accurate CCSD(T) and DFT–SAPT Stabilization Energies of H-Bonded and Stacked Structures of the Uracil Dimer  

SciTech Connect

The CCSD(T) interaction energies for the H-bonded and stacked structures of the uracil dimer are determined at the aug-cc-pVDZ and aug-cc-pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug-cc-pVDZ and aug-cc-pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug-cc-pVTZ and aug-cc-pVQZ basis sets) and extrapolated ?CCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H-bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS–MP2 (SCS: spin-component-scaled), SCS(MI)–MP2 (MI: molecular interaction), MP3, dispersion-augmented DFT (DFT–D), M06–2X, and DFT–SAPT (SAPT: symmetry-adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)–MP2 method. Remarkably good binding energies are also obtained with the DFT–SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation.

Pitonak, Michal; Riley, Kevin E.; Neogrady, Pavel; Hobza, Pavel

2008-06-23

324

Highly accurate CCSD(T) and DFT-SAPT stabilization energies of H-bonded and stacked structures of the uracil dimer.  

PubMed

The CCSD(T) interaction energies for the H-bonded and stacked structures of the uracil dimer are determined at the aug-cc-pVDZ and aug-cc-pVTZ levels. On the basis of these calculations we can construct the CCSD(T) interaction energies at the complete basis set (CBS) limit. The most accurate energies, based either on direct extrapolation of the CCSD(T) correlation energies obtained with the aug-cc-pVDZ and aug-cc-pVTZ basis sets or on the sum of extrapolated MP2 interaction energies (from aug-cc-pVTZ and aug-cc-pVQZ basis sets) and extrapolated DeltaCCSD(T) correction terms [difference between CCSD(T) and MP2 interaction energies] differ only slightly, which demonstrates the reliability and robustness of both techniques. The latter values, which represent new standards for the H-bonding and stacking structures of the uracil dimer, differ from the previously published data for the S22 set by a small amount. This suggests that interaction energies of the S22 set are generated with chemical accuracy. The most accurate CCSD(T)/CBS interaction energies are compared with interaction energies obtained from various computational procedures, namely the SCS-MP2 (SCS: spin-component-scaled), SCS(MI)-MP2 (MI: molecular interaction), MP3, dispersion-augmented DFT (DFT-D), M06-2X, and DFT-SAPT (SAPT: symmetry-adapted perturbation theory) methods. Among these techniques, the best results are obtained with the SCS(MI)-MP2 method. Remarkably good binding energies are also obtained with the DFT-SAPT method. Both DFT techniques tested yield similarly good interaction energies. The large magnitude of the stacking energy for the uracil dimer, compared to that of the benzene dimer, is explained by attractive electrostatic interactions present in the stacked uracil dimer. These interactions force both subsystems to approach each other and the dispersion energy benefits from a shorter intersystem separation. PMID:18574830

Pitonák, Michal; Riley, Kevin E; Neogrády, Pavel; Hobza, Pavel

2008-08-01

325

Influence of crystallography upon critical nucleus shapes and kinetics of homogeneous f. c. c. -f. c. c. nucleation-V. The origin of GP zones in Al-Ag and Al-Cu alloys  

SciTech Connect

GP zone formation in aluminum-base alloys has been proposed, by previous investigators, to take place by spinodal decomposition or by homogeneous nucleation and growth. The possibilities of these two mechanisms are examined here for two alloy systems, Al-Ag and Al-Cu, representing two extreme cases, a narrow and a (potentially) very wide miscibility gap, respectively, resulting in low and high interphase boundary energies. The coherent spinodal and the temperature-composition curve corresponding to abundant homogeneous f.c.c.-f.c.c. nucleation were calculated for both systems. In Al-Ag alloys, the nucleation barrier is so low that negligible undercooling is needed to achieve abundant nucleation. Hence cooling into the spinodal region with ordinary quenching rates is virtually impossible. In Al-Cu, however, the inverse situation is present and GP zone formation by (or aided by) spinodal decomposition should be feasible if the diffusivity remains sufficiently high in the spinodal region. Detailed evaluation of the latter proposal is prevented, however, by the incompleteness of thermodynamic information on f.c.c. Al-Cu alloys.

Le Goues, F.K.; Aaronson, H.I.; Lee, Y.W.; Wright, R.N.

1984-10-01

326

Tests of the RPBE, revPBE, ?-HCTHhyb, ?B97X-D, and MOHLYP density functional approximations and 29 others against representative databases for diverse bond energies and barrier heights in catalysis  

NASA Astrophysics Data System (ADS)

Thirty four density functional approximations are tested against two diverse databases, one with 18 bond energies and one with 24 barriers. These two databases are chosen to include bond energies and barrier heights which are relevant to catalysis, and in particular the bond energy database includes metal-metal bonds, metal-ligand bonds, alkyl bond dissociation energies, and atomization energies of small main group molecules. Two revised versions of the Perdew-Burke-Ernzerhof (PBE) functional, namely the RPBE and revPBE functionals, widely used for catalysis, do improve the performance of PBE against the two diverse databases, but give worse results than B3LYP (which denotes the combination of Becke's 3-parameter hybrid treatment with Lee-Yang-Parr correlation functional). Our results show that the Minnesota functionals, M05, M06, and M06-L give the best performance for the two diverse databases, which suggests that they deserve more attention for applications to catalysis. We also obtain notably good performance with the ?-HCTHhyb, ?B97X-D, and MOHLYP functional (where MOHLYP denotes the combination of the OptX exchange functional as modified by Schultz, Zhao, and Truhlar with half of the LYP correlation functional).

Yang, Ke; Zheng, Jingjing; Zhao, Yan; Truhlar, Donald G.

2010-04-01

327

Influence of nitrogen ion implantation energies on surface chemical bonding structure and mechanical properties of nitrogen-implanted silicon carbide ceramics  

Microsoft Academic Search

Nitrogen ions were implanted into silicon carbide ceramics (N+-implanted SiC) at different ions energies. The surface chemical bonding structure of N+-implanted SiC ceramics were investigated by using X-ray photoelectron spectroscopy (XPS). The hardness of N+-implanted SiC ceramics was measured using nano-indenter, and the friction and wear properties of the N+-implanted SiC\\/SiC tribopairs were studied using ball-on-disk type tribo-meter in water

Fei Zhou; Yingguang Yuan; Kangmin Chen; Xiaolei Wang

2009-01-01

328

Tracking the chemistry of unsaturated C3H3 groups adsorbed on a silver surface: propargyl-allenyl-acetylide triple bond migration, self-hydrogenation, and carbon-carbon bond formation.  

PubMed

A diverse array of unsaturated C1 (methylene and methylidyne) and C2 (vinyl, vinylidene, ethylidene, and ethylidyne) bound to metal center(s) and surfaces has received much attention. In sharp contrast to the effort devoted to C1 and C2 ligands, complexes or surfaces bearing C3 fragments have been less explored, especially the M-C3H3 systems, which include propargyl (M-CH2C[triple bond]CH), allenyl (M-CH=C=CH2), and acetylide (M-C[triple bond]CCH3) forms. To understand the bonding and reactivity of these C3 species appended to an extended metal structure, proprargyl bromide (Br-CH2C[triple bond]CH) was utilized as a precursor to generate C3H3 fragments on a Ag(111) surface under ultrahigh vacuum conditions. The molecular transformation process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometries and energies for the various surface intermediates. The computed IR spectra facilitated the vibrational mode assignments. TPD spectra show that C3H3(ad) self-hydrogenates to C3H4 around 300 and 475 K, respectively. In addition to hydrogenation, a C-C coupling product C6H6 (2,4-hexadiyne) is also unveiled as part of the desorption feature at 475 K. Identification of the possible C3H4 isomers (propyne and/or allene) was equivocal, but it was circumvented by using an alpha,alpha-dimethyl-substituted propargylic species--(CH3)2(alpha)C-C[triple bond]CH, which results in hydrogenation products, alkynic (CH3)2CH-C[triple bond]CH and allenic (CH3)2C=C=CH2, distinguishable by the mass spectrometry. The substitution experiments clarify that in the normal case the convoluted TPD feature around 300 K, in fact, consists of both allene at 260 K and propyne at 310 K, while the last hydrogenation product at 475 K is solely propyne. The RAIR spectroscopy demonstrates that at 200 K C3H3(ad) on Ag(111) readily adopts the allenyl formalism involving concerted CBr bond scission and [1,3]-sigmatropic migration (i.e., Br-*CH2C[triple bond]CH --> *CH2=C=CH-Ag), in which the sigma bond moves to a new metal location across the pi-periphery. Single hydrogen incorporation to the alpha-carbon of the surface allenyl rationalizes the allene formation at 260 K. When the surface is heated to the range of 250-300 K, both RAIR and XP spectra reveal drastic changes, indicative of a new species whose spectral characteristics could be duplicated by separate measurements from 1-propyn-1-yl iodide (CH3-C[triple bond]C-I) being a direct source for the surface methylacetylide (CH3-C[triple bond]C-Ag). It is thus suggested that allenyl is further reorganized to render acetylide presumably via [1,3]-hydrogen shift (i.e., *CH2=C=CH-Ag --> *CH3=C[triple bond]C-Ag). The presence of this third Ag-C3H3 isomeric form demonstrates an unprecedented propargyl-allenyl-acetylide multiple rearrangements on a metal surface. Migration of the triple bond from the remote terminal position into the chain, through the stage of allenic structure, is driven by thermodynamic stabilities, supported by the DFT total energy calculations. Consequently, the evolutions of propyne at 310 and 475 K, as well as 2,4-hexadiyne (bismethylacetylide), can all be reasoned out. PMID:18613681

Kung, Hsuan; Wu, Shin-Mou; Wu, Yu-Jui; Yang, Yaw-Wen; Chiang, Chao-Ming

2008-07-10

329

Bonding in Beryllium Clusters  

NASA Astrophysics Data System (ADS)

Beryllium clusters provide an ideal series for exploring the evolution from discrete molecules to the metallic state. The beryllium dimer has a formal bond order of zero, but the molecule is weakly bound. In contrast, bulk-phase beryllium is a hard metal with a high melting point. Theoretical calculations indicate that the bond energies increase dramatically for Ben clusters in the range n=2-6. A triplet ground state is found for n=6, indicating an early emergence of metallic properties. There is an extensive body of theoretical work on smaller Ben clusters, in part because this light element can be treated using high-level methods. However, the apparent simplicity of beryllium is deceptive, and the calculations have proved to be challenging owing to strong electron correlation and configuration interaction effects. Consequently, these clusters have become benchmark systems for the evaluation of a wide spectrum of quantum chemistry methods.

Heaven, Michael C.; Merritt, Jeremy M.; Bondybey, Vladimir E.

2011-05-01

330

Semiconductor wafer bonding  

Microsoft Academic Search

When mirror-polished, flat, and clean wafers are brought into contact, they are locally attracted to each other and adhere or bond. This phenomenon is known as semiconductor wafer bonding. Different adhesion forces (van der Waals forces, hydrogen bonding) are the reason for the bonding effect at room temperature. The different bonding mechanisms acting in dependence on the surface conditions (hydrophilic,

M. Reiche

2006-01-01

331

Detection of Alpha Particles and Low Energy Gamma Rays by Thermo-Bonded Micromegas in Xenon Gas  

NASA Astrophysics Data System (ADS)

Micromegas is a type of micro-pattern gaseous detector currently under R&D for applications in rare event search experiments. Here we report the performance of a Micromegas structure constructed with a micromesh thermo-bonded to a readout plane, motivated by its potential application in two-phase xenon detectors for dark matter and neutrinoless double beta decay experiments. The study is carried out in pure xenon at room temperature. Measurements with alpha particles from the Americium-241 source showed that gas gains larger than 200 can be obtained at xenon pressure up to 3 atm. Gamma rays down to 8 keV were observed with such a device.

Wei, Yuehuan; Guan, Liang; Zhang, Zhiyong; Lin, Qing; Wang, Xiaolian; Ni, Kaixuan; Zhao, Tianchi

2013-08-01

332

Halogen bonding (X-bonding): a biological perspective.  

PubMed

The concept of the halogen bond (or X-bond) has become recognized as contributing significantly to the specificity in recognition of a large class of halogenated compounds. The interaction is most easily understood as primarily an electrostatically driven molecular interaction, where an electropositive crown, or ?-hole, serves as a Lewis acid to attract a variety of electron-rich Lewis bases, in analogous fashion to a classic hydrogen bonding (H-bond) interaction. We present here a broad overview of X-bonds from the perspective of a biologist who may not be familiar with this recently rediscovered class of interactions and, consequently, may be interested in how they can be applied as a highly directional and specific component of the molecular toolbox. This overview includes a discussion for where X-bonds are found in biomolecular structures, and how their structure-energy relationships are studied experimentally and modeled computationally. In total, our understanding of these basic concepts will allow X-bonds to be incorporated into strategies for the rational design of new halogenated inhibitors against biomolecular targets or toward molecular engineering of new biological-based materials. PMID:23225628

Scholfield, Matthew R; Zanden, Crystal M Vander; Carter, Megan; Ho, P Shing

2012-12-29

333

Bond coordinates as an alternative: Low energy reactive collisions of He2+ with He; comparison of TD and TI quantum calculations  

NASA Astrophysics Data System (ADS)

``Reactive'' and ``inelastic'' processes in the ionic He3^+ system[1] have been separated and analyzed through the simulation of the ^3He + ^4He2 collision. The combined use of TD and TI techniques allowed the study for both high and very low kinetic energies; the agreement between the corresponding results in the medium energy range is very good. Influence of the internal excitation of the reagents and implications on the dynamics of evaporation in He clusters will be discussed[2,3]. Emphasis will be made on the TD wavepacket propagation methodology used for the calculation of state-to-state transition probabilies, based bond coordinates: This method[4] was suggested recently by one of the authors, and is applied for the first time to a process with three open channels. Bond coordinates can have several advantages over the use of standard Jacobi ones [1] E. Scifoni, E. Bodo and F. A. Gianturco, Eur. Phys. J. D, 30, 363 (2004) [2] E. Bodo, F. A. Gianturco, A. Dalgarno, J. Phys. B 35 (2002) 2391. [3] E. Bodo and F. A. Gianturco, Eur. Phys. J. D, 31 (2004) 423 [4] M. Lara, A. Aguado, O. Roncero,and M. Paniagua. J. Chem. Phys., 113, 1781 (2000)

Lara, Manuel; Bodo, Enrico; Gianturco, Franco A.

2005-05-01

334

What Determines Bond Costs. Municipal Bonds Series.  

ERIC Educational Resources Information Center

|Public officials in small towns who participate infrequently in the bond market need information about bond financing. This publication, one in a series of booklets published by the Western Rural Development Center using research gathered between 1967-77, discusses factors influencing the marketability and cost of bond financing for towns and…

Young, Douglas; And Others

335

Chemical Bonding Technology.  

National Technical Information Service (NTIS)

Primers employed in bonding together the various material interfaces in a photovoltaic module are being developed. The approach develops interfacial adhesion by generating actual chemical bonds between the various materials bonded together. The current st...

E. Plueddemann

1986-01-01

336

Semiempirical valence bond potential energy surfaces for simple chemical reactions: 1A' states of H2O and H2F  

NASA Astrophysics Data System (ADS)

A semiemprical valence bond method is employed to study the potential energy surfaces (PESs) of the lowest 1A' states of H2O and isoelectronic H2F+. The calculation is based on the search for the electronic configurations which play the most important rôle in the formation of the stable electronic states of the molecules and the relevant diatomic fragments. Several approximations are used to reduce the number of permutations in the calculation of the energy matrix. The Moffit atoms-in-molecule approximation is used to correct for the atomic errors arising from the poor basis set. The computed data for the lowest electronic states of OH, HF, HF+, H2O and H2F+, as well as the general behaviour of the PESs for different molecular geometries, are in good agreement with the available ab initio and experimental data.

Zembekov, A. A.

337

Catalytic asymmetric carbon-carbon bond formation via allylic alkylations with organolithium compounds  

Microsoft Academic Search

Carbon-carbon bond formation is the basis for the biogenesis of nature's essential molecules. Consequently, it lies at the heart of the chemical sciences. Chiral catalysts have been developed for asymmetric C-C bond formation to yield single enantiomers from several organometallic reagents. Remarkably, for extremely reactive organolithium compounds, which are among the most broadly used reagents in chemical synthesis, a general

Manuel Pérez; Martín Fañanás-Mastral; Pieter H. Bos; Alena Rudolph; Syuzanna R. Harutyunyan; Ben L. Feringa

2011-01-01

338

Ab initio study of the excited singlet states of all-trans ?,?-diphenylpolyenes with one to seven polyene double bonds: Simulation of the spectral data within Franck-Condon approximation  

NASA Astrophysics Data System (ADS)

Computational simulations of the electronic spectra with ab initio electronic structure calculations are presented for all-trans ?,?-diphenylpolyenes with the polyene double bond number (N) from 1 to 7. A direct comparison of the fluorescence spectra of diphenylpolyenes was made between the results of highly accurate calculations and the experimental data for the systems with various chain lengths. For the realistic simulation of the emission, the total vibrational wave function was described approximately as a direct product of one-dimensional (1D) vibrational wave functions along the normal coordinates that are determined from the vibrational analysis of the ground state. The observed spectra can be reproduced in a computationally efficient way by selecting effective C-C and C=C stretching modes for the constructions of the 1D vibrational Hamiltonians. The electronic structure calculations were performed using the multireference Møller-Plesset perturbation theory with complete active space configuration interaction reference functions. Based on the vertical excitation energies computed, the lowest singlet excited state of diphenylbutadiene is shown to be the optically forbidden 2 1Ag state. The simulations of fluorescence spectra involving vibronic coupling effects reveal that the observed strong single C=C band consists of two major degenerate vibrational C=C modes for the shorter diphenylpolyenes with N=3 and 5. Further, the relative intensities of the C-C stretching modes in the fluorescence spectra tend to be larger than those of the C=C stretching modes for the systems with N over 5. This indicates that the geometric differences of the energy minima between the ground (1 1Ag) and 2 1Ag states grow larger towards the direction of the C-C stretching mode with increasing N.

Mizukami, Wataru; Kurashige, Yuki; Ehara, Masahiro; Yanai, Takeshi; Itoh, Takao

2009-11-01

339

Ab initio study of the excited singlet states of all-trans alpha,omega-diphenylpolyenes with one to seven polyene double bonds: Simulation of the spectral data within Franck-Condon approximation.  

PubMed

Computational simulations of the electronic spectra with ab initio electronic structure calculations are presented for all-trans alpha,omega-diphenylpolyenes with the polyene double bond number (N) from 1 to 7. A direct comparison of the fluorescence spectra of diphenylpolyenes was made between the results of highly accurate calculations and the experimental data for the systems with various chain lengths. For the realistic simulation of the emission, the total vibrational wave function was described approximately as a direct product of one-dimensional (1D) vibrational wave functions along the normal coordinates that are determined from the vibrational analysis of the ground state. The observed spectra can be reproduced in a computationally efficient way by selecting effective C-C and C=C stretching modes for the constructions of the 1D vibrational Hamiltonians. The electronic structure calculations were performed using the multireference Moller-Plesset perturbation theory with complete active space configuration interaction reference functions. Based on the vertical excitation energies computed, the lowest singlet excited state of diphenylbutadiene is shown to be the optically forbidden 2 (1)A(g) state. The simulations of fluorescence spectra involving vibronic coupling effects reveal that the observed strong single C=C band consists of two major degenerate vibrational C=C modes for the shorter diphenylpolyenes with N=3 and 5. Further, the relative intensities of the C-C stretching modes in the fluorescence spectra tend to be larger than those of the C=C stretching modes for the systems with N over 5. This indicates that the geometric differences of the energy minima between the ground (1 (1)A(g)) and 2 (1)A(g) states grow larger towards the direction of the C-C stretching mode with increasing N. PMID:19895018

Mizukami, Wataru; Kurashige, Yuki; Ehara, Masahiro; Yanai, Takeshi; Itoh, Takao

2009-11-01

340

Linear free energy relationships in C-N bond dissociations in molecular ions of 4-substituted N-(2-furylmethyl)anilines in the gas phase.  

PubMed

The substituent effect on the reactivity of the C-N bond of molecular ions of 4-substituted N-(2-furylmethyl)anilines toward two dissociation pathways was studied. With this aim, six of these compounds were analyzed by mass spectrometry using electron ionization with energies between 7.8 and 69.9 eV. Also, the UB3LYP/6-31G (d,p) and UHF/6-31G (d, p) levels of theory were used to calculate the critical energies (reaction enthalpies at 0 K) of the processes that lead to the complementary ions [C(5)H(5)O](+) and [M - C(5)H(5)O](+), assuming structures that result from the heterolytic and homolytic C-N bond cleavages of the molecular ions, respectively. A kinetic approach proposed in the 1960s was applied to the mass spectral data to obtain the relative rate coefficients for both dissociation channels from ratios of the peak intensities of these ions. Linear relationships were obtained between the logarithms of the relative rate coefficients and the calculated critical energies and other thermochemical properties, whose slopes showed to be conditioned by the energy provided to the compounds within the ion source. Moreover, it was found that the dissociation that leads to [C(5)H(5)O](+) is a process strongly dependent upon the electron withdrawing or donating properties of the substituent, favored by those factors that destabilize the molecular ion. On the contrary, the dissociation that leads to [M - C(5)H(5)O](+) is indifferent to the polar electronic effects of the substituent. The abundance of both products was governed by the rule of Stevenson-Audier, according to which the major ion is the one of less negative electronic affinity. PMID:17687763

Solano Espinoza, Eduardo A; Stashenko, Elena; Martínez, Jairo; Mora, Uriel; Kouznetsov, Vladimir

2007-11-01

341

Molecular dynamics simulations of silicon wafer bonding  

Microsoft Academic Search

Molecular dynamics simulations based on a modified Stillinger-Weber potential are used to investigate the elementary steps of bonding two Si(0 0 1) wafers. The energy dissipation and thus the dynamic bonding behaviour are controlled by the transfer rates for the kinetic energy. The applicability of the method is demonstrated by studying the interaction of perfect wafer surfaces (UHV conditions). First

D. Conrad; K. Scheerschmidt; U. Gösele

1995-01-01

342

Efficient Damage Sensitivity Analysis of advanced Cu Low-k Bond Pad Structures Using Area Release Energy  

Microsoft Academic Search

This paper presents an efficient method to describe the damage sensitivity of three-dimensional multi-layered structures. The index that characterizes this failure sensitivity is an energy measure called the area release energy, which predicts the amount of energy that is released upon crack initiation at an arbitrary position along an interface. The benefits of the method are: (1) the criterion can

O. van der Sluis; R. A. B. Engelen; W. D. van Driel; M. A. J. van Gils; R. B. R. van Silfhout

2006-01-01

343

Strength, ultrasonic and metallurgical evaluation of diffusion bonds  

SciTech Connect

Diffusion bonding allows similar and dissimilar materials to be bonded together in near net shape. However, differentiation of almost perfect diffusion bonds, with little variation in their acoustic response, is of critical importance since relatively significant changes in bond strength may be a consequence. Challenge is to find ultrasonic techniques sensitive enough to detect small imperfections at interface. Diffusion bonds have been produced that show only a slight variation in a single frequency reflection measurement. The total energy, reflected from bond line, can differentiate these diffusion bonds. This evaluation is based on Parseval`s theorem which states that energy in time domain is proportional to energy in the frequency domain. This measurement takes advantage of presence of voids in the diffusion bonds as well as of the interdiffusion zone. Cu and Ni were chosen as the materials to be bonded due to the case of microstructural control. Diffusion bonds fabricated of Ti-6Al-4V are also discussed.

Ojard, G.C.; Buck, O.; Rehbein, D.K.

1993-10-01

344

21. Historic American Buildings Survey Copyright C.C. Pierce Original: ...  

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

21. Historic American Buildings Survey Copyright - C.C. Pierce Original: About 1902 Re-photo: April 1940 QUANDRANGLE (view from north) - Mission Nuestra Senora de la Soledad, Soledad, Monterey County, CA

345

Strength of electron correlations in different types of chemical bonds  

Microsoft Academic Search

A parameter is introduced which characterizes the strength of electron correlations in chemical bonds. It is calculated for a number of bonds. The results show that for a given type of bond (e.g. s, isolated p, conjugated p, etc.) this parameter is almost independent of the types of atoms forming the bond. In contrast, the correlation energies are strongly dependent

A. M. Ole?; F. Pfirsch; P. Fulde; M. C. Böhm

1987-01-01

346

Thermal shock resistance of SiC compositionally graded C\\/C composites  

Microsoft Academic Search

Thermal shock resistance of SiC compositionally graded carbon fiber reinforced carbon composite (CFC) materials that are composed of a CVD SiC layer, a SiC\\/C graded layer and a CFC substrate was studied by thermal shock tests in air. Two types of CFCs were used: 1-D continuous fiber reinforced and 2-D felt reinforced composites. It was found that the SiC\\/C graded

Kimio Fujii; Reiji Yamada

1998-01-01

347

Characterization of chemical bonding in low-k dielectric materialsfor interconnect isolation: a xas and eels study  

SciTech Connect

The use of low dielectric constant materials in the on-chipinterconnect process reduces interconnect delay, power dissipation andcrosstalk noise. To achieve the requirements of the ITRS for 2007-2009minimal sidewall damage from etch, ash or cleans is required. In chemicalvapor deposited (CVD) organo-silicate glass (OSG) which are used asintermetal dielectric (IMD) materials the substitution of oxygen in SiO2by methyl groups (-CH3) reduces the permittivity significantly (from 4.0in SiO2 to 2.6-3.3 in the OSG), since the electronic polarizability islower for Si-C bonds than for Si-O bonds. However, plasma processing forresist stripping, trench etching and post-etch cleaning removes C and Hcontaining molecular groups from the near-surface layer of OSG.Therefore, compositional analysis and chemical bonding characterizationof structured IMD films with nanometer resolution is necessary forprocess optimization. OSG thin films as-deposited and after plasmatreatment are studied using X-ray absorption spectroscopy (XAS) andelectron energy loss spectroscopy (EELS). In both techniques, the finestructure near the C1s absorption or energy loss edge, respectively,allows to identify C-H, C-C, and C-O bonds. This gives the opportunity todifferentiate between individual low-k materials and their modifications.The O1s signal is less selective to individual bonds. XAS spectra havebeen recorded for non-patterned films and EELS spectra for patternedstructures. The chemical bonding is compared for as-deposited andplasma-treated low-k materials. The Fluorescence Yield (FY) and the TotalElectron Yield (TEY) recorded while XAS measurement are compared.Examination of the C 1s near-edge structures reveal a modified bonding ofthe remaining C atoms in the plasma-treated sample regions.

Hoffmann, P.; Schmeisser, D.; Engelmann, H.-J.; Zschech, E.; Stegmann, H.; Himpsel, F.; Denlinger, J.

2006-04-10

348

Detection and Formation of Interstellar c-C_3D_2  

NASA Astrophysics Data System (ADS)

Multiply deuterated molecules are unique observational probes for the earliest stages of star formation. Cyclopropenylidene, c-C_3H_2, is an ideal probe for deuterium chemistry. It is one of the most widespread molecules in our Galaxy and it has the possibility of double deuteration. Furthermore, since c-C_3H_2 is an "early-type" molecule, it is a particular useful tool to investigate early stages of a molecular cloud. This makes observations of its deuterated forms particularly important to test time-dependent chemical codes which include deuteration processes. The centimeter and millimeter wavelength spectra of doubly deuterated cyclopropenylidene have recently been measured in the laboratory, allowing for the first time a search for c-C_3D_2 in space. We report the detection of c-C_3D_2 in two starless cores, L1544 and TMC-1C. The deuteration of this small hydrocarbon ring is analysed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C_3D_2 can be explained solely by gas-phase processes, supporting the idea that c-C_3H_2 is a good indicator of gas-phase deuteration. S. Spezzano, F. Tamassia, S. Thorwirth, P. Thaddeus, C. A. Gottlieb, and M. C. McCarthy Astroph. J. Supp. Series {200}(1), 2012.

Spezzano, Silvia; Brunken, Sandra; Schilke, Peter; Menten, Karl M.; Caselli, Paola; McCarthy, Michael C.; Bizzocchi, Luca; Trevino, Sandra; Aikawa, Yuri; Schlemmer, Stephan

2013-06-01

349

Effect of chemisorption structure on the interfacial bonding characteristics of graphene-polymer composites  

NASA Astrophysics Data System (ADS)

The influence of the chemical functionalization of graphene on the interfacial bonding characteristics between graphene and polymer was investigated using molecular mechanics and molecular dynamics simulations. In this study, three chemical functionalization, (a) phenyl groups, (b) -C6H13 and(c) -C2H4(C2H5)2, which have the same number of carbon atoms, were chosen to investigate the influence of the structure of functionalized groups on the bonding energy and shear stress in the graphene-polyethylene (PE) composites. Our simulations indicated that, the interfacial bonding energy between the graphene modified by -C6H13 groups and PE matrix has the strongest enhancement, but the shear force between the graphene modified by -C2H4(C2H5)2 groups and PE matrix is the strongest in the graphene-polymer composites. Therefore, the suitable structure of chemical groups to the graphene surface may be an effective way to significantly improve the load transfer between the graphene and polymer when graphene is used to produce nanocomposites.

Lv, Cheng; Xue, Qingzhong; Xia, Dan; Ma, Ming

2012-01-01

350

Bond Paths Are Not Chemical Bonds  

NASA Astrophysics Data System (ADS)

This account takes to task papers that criticize the definition of a bond path as a criterion for the bonding between the atoms it links by mistakenly identifying it with a chemical bond. It is argued that the notion of a chemical bond is too restrictive to account for the physics underlying the broad spectrum of interactions between atoms and molecules that determine the properties of matter. A bond path on the other hand, as well as being accessible to experimental verification and subject to the theorems of quantum mechanics, is applicable to any and all of the interactions that account for the properties of matter. It is shown that one may define a bond path operator as a Dirac observable, making the bond path the measurable expectation value of a quantum mechanical operator. Particular attention is given to van der Waals interactions that traditionally are assumed to represent attractive interactions that are distinct from chemical bonding. They are assumed by some to act in concert with Pauli repulsions to account for the existence of condensed states of molecules. It is such dichotomies of interpretation that are resolved by the experimental detection of bond paths and the delineation of their properties in molecular crystals. Specific criticisms of the stabilization afforded by the presence of bond paths derived from spectroscopic measurements performed on dideuteriophenanthrene are shown to be physically unsound. The concept of a bond path as a "bridge of density" linking bonded atoms was introduced by London in 1928 following the definition of the electron density by Schrödinger in 1926. These papers marked the beginning of the theory of atoms in molecules linked by bond paths.

Bader, Richard F. W.

2009-09-01

351

LETTERS TO THE EDITORS: Lightning ball: experiments on creation and hypotheses(comment on "Energy density calculations for ball-lightning-like luminous silicon balls" by G S Paiva, J V Ferreira, C C Bastos, M V P dos Santos, A C Pavão)  

NASA Astrophysics Data System (ADS)

The problems addressed in this paper include estimating: the energy density of luminous silicon balls, the density range of a natural lightning ball, and whether and how the object created and described in the commented paper (Usp. Fiz. Nauk 180 218 (2010) [Phys. Usp. 53 (2) 209 (2010)]) corresponds to the natural phenomenon.

Shabanov, Gennadii D.

2010-05-01

352

Orbital overlap and chemical bonding.  

PubMed

The chemical bonds in the diatomic molecules Li(2)-F(2) and Na(2)-Cl(2) at different bond lengths have been analyzed by the energy decomposition analysis (EDA) method using DFT calculations at the BP86/TZ2P level. The interatomic interactions are discussed in terms of quasiclassical electrostatic interactions DeltaE(elstat), Pauli repulsion DeltaE(Pauli) and attractive orbital interactions DeltaE(orb). The energy terms are compared with the orbital overlaps at different interatomic distances. The quasiclassical electrostatic interactions between two electrons occupying 1s, 2s, 2p(sigma), and 2p(pi) orbitals have been calculated and the results are analyzed and discussed. It is shown that the equilibrium distances of the covalent bonds are not determined by the maximum overlap of the sigma valence orbitals, which nearly always has its largest value at clearly shorter distances than the equilibrium bond length. The crucial interaction that prevents shorter bonds is not the loss of attractive interactions, but a sharp increase in the Pauli repulsion between electrons in valence orbitals. The attractive interactions of DeltaE(orb) and the repulsive interactions of DeltaE(Pauli) are both determined by the orbital overlap. The net effect of the two terms depends on the occupation of the valence orbitals, but the onset of attractive orbital interactions occurs at longer distances than Pauli repulsion, because overlap of occupied orbitals with vacant orbitals starts earlier than overlap between occupied orbitals. The contribution of DeltaE(elstat) in most nonpolar covalent bonds is strongly attractive. This comes from the deviation of quasiclassical electron-electron repulsion and nuclear-electron attraction from Coulomb's law for point charges. The actual strength of DeltaE(elstat) depends on the size and shape of the occupied valence orbitals. The attractive electrostatic contributions in the diatomic molecules Li(2)-F(2) come from the s and p(sigma) electrons, while the p(pi) electrons do not compensate for nuclear-nuclear repulsion. It is the interplay of the three terms DeltaE(orb), DeltaE(Pauli), and DeltaE(elstat) that determines the bond energies and equilibrium distances of covalently bonded molecules. Molecules like N(2) and O(2), which are usually considered as covalently bonded, would not be bonded without the quasiclassical attraction DeltaE(elstat). PMID:17024702

Krapp, Andreas; Bickelhaupt, F Matthias; Frenking, Gernot

2006-12-13

353

Savings Bonds Value Calculator  

NSDL National Science Digital Library

From the Federal Reserve Bank of New York, this site computes the redemption value of users's US savings bonds. Easy-to-use pull-down menus allow visitors to enter information such as the date of issue and face value of their Series E bonds, Series EE bonds, and Series S bonds. After entering the information, the Calculator will then show a chart of issue dates and denominations and actual worth of the bonds, if cashed within a set period of time.

354

Spectroscopic and thermochemical properties of the c-C6H7 radical: A high-level theoretical study  

NASA Astrophysics Data System (ADS)

The electronic ground state (X~2B1) of the cyclohexadienyl radical (c-C6H7) has been studied by explicitly correlated coupled cluster theory at the RCCSD(T)-F12x (x = a, b) level, partly in combination with the double-hybrid density functional method B2PLYP. An accurate equilibrium structure has been established and the ground-state rotational constants are predicted to be A0 = 5347.3 MHz, B0 = 5249.7 MHz, and C0 = 2692.5 MHz. The calculated vibrational wavenumbers agree well with the recent p-H2 matrix IR data [M. Bahou, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 136, 154304 (2012)] and several predictions have been made. A low value of 6.803 +/- 0.005 eV is predicted for the adiabatic ionization energy of c-C6H7. Owing to a moderately large change in the equilibrium structure upon ionization, the first band of the photoelectron spectrum is dominated by the adiabatic peak (100%) and only the peaks corresponding to excitation of the two lowest totally symmetric vibrations (?12 and ?11) by one vibrational quantum have relative intensities of more than 15%. The C6H6-H dissociation energy is calculated to be D0 = 85.7 kJ mol-1, with an estimated error of ~2 kJ mol-1.

Bargholz, Arne; Oswald, Rainer; Botschwina, Peter

2013-01-01

355

Carbon-Hydrogen Bond Strengths in Methane  

Microsoft Academic Search

Earlier formulas for the electron-pair bonding energies of methane, and radicals derivable from methane, are combined with recent data on the energy levels of the carbon atom, the heat of sublimation of carbon, the energy for removal of the first hydrogen, and the dissociation energy of CH to obtain the energies of CH3 and CH2. A consistent set of values

H. H. Voge

1948-01-01

356

Infrared spectroscopy of Si–O bonding in low-dose low-energy separation by implanted oxygen materials  

Microsoft Academic Search

The effects of the processing conditions on the formation of buried oxide (BOX) layers in low-dose low-energy separation by implanted oxygen materials were investigated by using infrared spectroscopy and transmission electron microscopy. In as-implanted samples, the Si–O–Si stretching frequency increases either with increasing the oxygen dose or with decreasing implantation energy because the oxide composition becomes stoichiometric. However, the plateau

Tula Jutarosaga; Jun Sik Jeoung; Supapan Seraphin

2005-01-01

357

Protonic conduction in oxide glasses: Simple relations between electrical conductivity, activation energy, and the OH bonding state  

Microsoft Academic Search

Simple relations between protonic conductivity (sigma) and the peak wave number (nuOH) of O-H infrared absorption band, and between activation energy for the electrical conduction and the wave number (nuOH) were found in oxide glasses containing no alkali- and\\/or transition-metal ions. The present investigation enables one to evaluate the protonic conductivity and the activation energy at a unit proton concentration

Yoshihiro Abe; Hideo Hosono; Yoshio Ohta; L. L. Hench

1988-01-01

358

Ir-Catalyzed Functionalization of C-H Bonds  

NASA Astrophysics Data System (ADS)

The ability to selectively functionalize C-H bonds holds enormous potential value in virtually every sphere of organic chemistry, from fuels to pharmaceuticals. Transition metal complexes have shown great promise in this context. Iridium provided the first examples of oxidative addition of C-H bonds; this addition is key to iridium's leading role in alkane dehydrogenation and related reactions. Catalysts based on iridium have also proven highly effective for valuable borylations of C-H bonds and, to a lesser extent, for C-Si coupling. Compared with other platinum group metals, iridium chemistry has not been developed as extensively for the elaboration of C-C bonds from C-H bonds, but significant promise is indicated, particularly for coupling with simple hydrocarbons which lack functionalities that can act as directing groups.

Choi, Jongwook; Goldman, Alan S.

359

Identification of the strongest bonds in chemistry.  

PubMed

Increasing the effective electronegativity of two atoms forming a triple bond can increase the strength of the latter. The strongest bonds found in chemistry involve protonated species of hydrogen cyanide, carbon monoxide, and dinitrogen. CCSD(T)/CBS (complete basis set) and G4 calculations reveal that bond dissociation energies are misleading strength descriptors. The strength of the bond is assessed via the local stretching force constants, which suggest relative bond strength orders (RBSO) between 2.9 and 3.4 for heavy atom bonding (relative to the CO bond strength in methanol (RBSO = 1) and formaldehyde (RBSO = 2)) in [HCNH](+)((1)?(+)), [HCO](+)((1)?(+)), [HNN](+)((1)?(+)), and [HNNH](2+)((1)?g(+)). The increase in strength is caused by protonation, which increases the electronegativity of the heavy atom and thereby decreases the energy of the bonding AB orbitals (A, B: C, N, O). A similar effect can be achieved by ionization of a nonbonding or antibonding electron in CO or NO. The strongest bond with a RBSO value of 3.38 is found for [HNNH](2+) using scaled CCSD(T)/CBS frequencies determined for CCSD(T)/CBS geometries. Less strong is the NN bond in [FNNH](2+) and [FNNF](2+). PMID:23927609

Kalescky, Robert; Kraka, Elfi; Cremer, Dieter

2013-08-30

360

Sensor/ROIC Integration using Oxide Bonding  

SciTech Connect

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

Ye, Zhenyu; /Fermilab

2009-02-01

361

Investing in Bonds.com  

NSDL National Science Digital Library

Investing in Bonds was created by the Bond Market Association to educate investors about the benefits of bonds investing. The Investor's Guide to Bond Basics educates investors about the types of bonds available, criteria for evaluating a bond, a guide to buying bonds, bond investment strategies and a glossary of bond market terms. The Bond Market section provides an overview of the U.S. bond market while the Investor's Checklist section takes the investor step-by-step through the bond investment decision process. Investors will also find sections with information on municipal bonds, corporate bonds, mortgage securities and U.S. Inflation-Indexed Securities.

362

Direct measurements of chemical bonding at solid surfaces using a unique calorimetric method: Towards understanding surface chemistry in energy technologies  

NASA Astrophysics Data System (ADS)

Measuring the heat released when gas phase species adsorb onto surfaces provides essential information about the energies of surface species and the reactions they undergo. Here, heats of adsorption of technologically-interesting surface species were measured using a unique microcalorimetric technique in ultrahigh vacuum. Specifically, systems were studied which are relevant to understanding and improving transition metal catalysts and organic electronics. Metal adsorption energies were measured which elucidate metal-to-oxide and metal-to-polymer interfacial binding, and molecule adsorption energies were measured to understand how catalyst structure influences the energies of adsorbed reaction intermediates. Oxide-supported metal nanoparticles form the basis for many industrial catalysts. Nanoparticle activity, selectivity and resistance to sintering can depend strongly on particle size, oxide support, and defects on the oxide. To investigate the dependence of catalytic properties on oxide surface defects, defects were introduced on MgO(100) and CeO2(111), and their affect on the adsorption energy of metal atoms and the energy of supported nanoparticles was measured. These measurements help to explain why transition metal catalysts sinter more slowly and maintain smaller particles when supported on CeO 2 compared to other oxides, and how surface defects influence nanoparticle formation and film growth on oxides. The effect of nanoparticle size on the adsorption energy of CO on different-sized Pd nanoparticles on Fe3O 4(111) was measured, providing the first direct evidence that the heat of adsorption of CO decreases with decreasing Pd nanoparticle size. Knowledge of the direction and magnitude of particle size effects is necessary for improving existing catalysts and designing new ones. The metal/polymer interface is important because it impacts charge injection, extraction, and transport in organic electronics. Large-scale energy production using polymer photovoltaics is currently unfeasible due in part to their low efficiency and short lifetimes. Polymer degradation at the interface with the metal electrode is believed to impact device efficiency and lifetime. Calcium adsorption on poly(3-hexylthiophene) was investigated because it is one of the most efficient electrode/polymer combinations. The results were striking: calcium diffused nanometers into the polymer and reacted with the polymer backbone. A method to suppress diffusion was demonstrated, which may lead to improved devices.

Farmer, Jason A.

363

Effects of Anti-Oxidant Migration on Friction and Wear of C/C Aircraft Brakes  

NASA Astrophysics Data System (ADS)

The surfaces of carbon-carbon (C/C) aircraft brakes are usually coated with anti-oxidant to protect them from oxidation. These surfaces do not include the friction surfaces since it is known that when anti-oxidant get onto the friction surface, the friction coefficient decreases. The anti-oxidant migration (AOM), however, happens during processing, heat treatment and application. In this study, phosphorus based anti-oxidants inhibited 3-D C/C aircraft brake system was investigated. The effects of their migration on friction and wear in the 3-D C/C brakes were revealed by sub-scale dynamometer tests and microscopic analysis. Dynamometer results showed that when AOM occurred, both landing and taxi coefficients decreased in humid environment and the wear was slightly lowered. Microscopic study showed that under high humidity conditions there was no formation of the friction film.

Don, Jarlen; Wang, Zhe

2009-04-01

364

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

Microsoft Academic Search

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

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

2009-01-01

365

Direct measurements of chemical bonding at solid surfaces using a unique calorimetric method: Towards understanding surface chemistry in energy technologies  

Microsoft Academic Search

Measuring the heat released when gas phase species adsorb onto surfaces provides essential information about the energies of surface species and the reactions they undergo. Here, heats of adsorption of technologically-interesting surface species were measured using a unique microcalorimetric technique in ultrahigh vacuum. Specifically, systems were studied which are relevant to understanding and improving transition metal catalysts and organic electronics.

Jason A. Farmer

2010-01-01

366

The Bond Market Association  

NSDL National Science Digital Library

The Bond Market Association "represents securities firms and banks that underwrite, trade and sell debt securities." Their web site is divided into several sections. The Research Desk presents the results of research conducted by the association, including forecasts of economic growth and analysis of bond market trends. The Legislative Issues and Regulatory sections provide a summary of legislative and regulatory developments of interest to the bond market community. Market Practices contains guidelines and procedures to be followed in the bond market. The site also provides information to investors on how to invest in corporate bonds, tax-exempt municipal bonds, and mortgage securities.

1998-01-01

367

Investigation of TiC C Eutectic and WC C Peritectic High-Temperature Fixed Points  

NASA Astrophysics Data System (ADS)

TiC C eutectic (2,761°C) and WC C peritectic (2,749°C) fixed points were investigated to compare their potential as high-temperature thermometric reference points. Two TiC C and three WC C fixed-point cells were constructed, and the melting and freezing plateaux were evaluated by means of radiation thermometry. The repeatability of the TiC C eutectic within a day was 60 mK with a melting range roughly 200 mK. The repeatability of the melting temperature of the WC C peritectic within 1 day was 17 mK with a melting range of ˜70 mK. The repeatability of the freezing temperature of the WC C peritectic was 21 mK with a freezing range less than 20 mK. One of the TiC C cells was constructed from a TiC and graphite powder mixture. The filling showed the reaction with the graphite crucible was suppressed and the ingot contained less voids, although the lack of high-purity TiC powder poses a problem. The WC C cells were easily constructed, like metal carbon eutectic cells, without any evident reaction with the crucible. From these results, it is concluded that the WC C peritectic has more potential than the TiC C eutectic as a high-temperature reference point. The investigation of the purification of the TiC C cell during filling and the plateau observation are also reported.

Sasajima, Naohiko; Yamada, Yoshiro

2008-06-01

368

Analysis of the Relationship between Reaction Energies of Electrophilic SWNT Additions and Sidewall Curvature: Chiral Nanotubes  

SciTech Connect

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The relationship of reaction energies for CH2/NH/O exo- and endo-[2 + 1] cycloadditions to chiral singlewalled carbon nanotube (SWNT) sidewalls with the inverse tube diameter (1/d) was investigated using density functional theory (DFT) and density functional tight binding (DFTB) methods. We considered additions to the three nonequivalent C-C bond types t (bond most parallel to tube axis), d (“diagonal” bond, slightly skewed), and p (bond most perpendicular to tube axis), using hydrogen-terminated (2n,n) SWNT model systems with n ) 2-8. Exoadditions are classified into two types, one where the original C-C bond is broken (exo(l)), and one where it remains intact (exo(s)) in the addition complex. Endoadditions are found to always belong to the latter (endo(s)) type. It is found that (a) exoadditions are more exothermic than endo additions, and (b) that exoadditions are more exothermic with larger bond-tube axis angle (p > d > t). A nearly perfect linear relationship between the total reaction energy ?E and 1/d holds only for individual endo, exo(s) and exo(l) addition series to specific t/d/p bonds, while ?E, as well as the SWNT deformation energy (DEF) and the interaction energy (INT) between deformed SWNT and deformed addends, are quadratically dependent on 1/d, when both negative (endo) and positive (exo(s)) bond curvatures are considered in linear regression analysis. Energy decomposition analysis shows that for endo- and exo(s)- series the curvature dependence of ?E is dominated by INT, while for exo(l) series, this quantity is dominated by DEF.

Wang, Zhi; Irle, Stephan; Zheng, Guishan; Morokuma, Keiji

2008-08-21

369

Fracture toughness of f. c. c. nickel and strain aging b. c. c. iron in the temperature range 77-773 K  

SciTech Connect

Ductile initiation fracture toughness J[sub IC] of b.c.c. Armco iron and f.c.c. nickel has been measured in the temperature range 77-773 K. Armco iron exhibits dynamics strain aging (DSA) in the temperature range 383-573 K while nickel of the purity used does not evince DSA. Load vs load line displacement (LLD) plots during fracture toughness testing of Armco iron show serrations in the temperature range 383-573 K similar to those observed in the tensile stress-strain curves. DSA is found to have a beneficial effect on the fracture toughness J[sub IC]. A marked increase in tensile strength and fracture toughness occurs in Armco iron in the DSA regime. The strain hardening exponent, known to have a bearing on the plastic zone size and the void growth rate, seems a clear parameter in terms of which the observed J[sub IC] variation with temperature can be understood. Remarkably, the variation of n with temperature is found to closely follow the observed trend in J[sub IC]. However, the slope of the J-R curve, dJ/da, decreases in the DSA regime with a minimum at 423 K. The decrease has been related to the fracture propagation process which is shown to occur by an alternate fast fracture and the ductile dimpled mode in the DSA regime. In the case of nickel, free from DSA, J[sub IC] or dJ/da are found to be largely unaffected by the test temperature. At room temperature f.c.c. nickel, at closely matching strength levels, possesses higher fracture toughness as compared to b.c.c. Armco iron. The crystal structure effect is more pronounced at temperatures below the ambient. At 77 K, the fracture toughness of iron is drastically reduced due to the onset of cleavage while nickel, not prone to a change in the fracture mode, maintains the same level of J[sub IC], as at the ambient.

Srinivas, M.; Malakondaiah, G.; Rama Rao, P. (Defence Metallurgical Research Lab., Hyderabad (India))

1993-04-01

370

Hot rolling textures of f.c.c. metals. Part 2: Numerical simulations  

SciTech Connect

The development of hot rolling textures in f.c.c. metals has been simulated numerically using a viscoplastic crystal plasticity model assuming different combinations of octahedral and non-octahedral slip systems for several grain-matrix interaction schemes. A combination of quasi-pencil glide and the Lebensohn and Tome self-consistent model gives a satisfactory simulation of typical hot rolling f.c.c. texture components, i.e. well balanced {beta}-fiber components and a small amount of ``Cube.``

Maurice, C.; Driver, J.H. [Ecole des Mines de Saint Etienne (France). Microstructures and Processing Lab.

1997-11-01

371

Long-term tropospheric trend of octafluorocyclobutane (cC4F8 or PFC318)  

Microsoft Academic Search

Air samples collected at Cape Grim, Tasmania\\u000abetween 1978 and 2008 and during a series of more recent\\u000aaircraft sampling programmes have been analysed to determine\\u000athe atmospheric abundance and trend of octafluorocyclobutane\\u000a(c-C4F8 or PFC-318). c-C4F8 has an atmospheric\\u000alifetime in excess of 3000 yr and a global warming potential\\u000a(GWP) of 10 300 (100 yr time horizon), making

D. E. Oram; F. S. Mani; J. C. Laube; M. J. Newland; C. E. Reeves; W. T. Sturges; S. A. Penkett; C. A. M. Brenninkmeijer; T. Röckmann; P. J. Fraser

2012-01-01

372

The Role of Chemical Bonding in Adhesion.  

National Technical Information Service (NTIS)

Strong positive effects have been found when a polybutadiene layer is adhered to a glass substrate by means of interfacial chemical bonds. The bond fracture energy increased by a factor of over 20 when a reactive polysiloxane coating was applied to the gl...

A. N. Gent P. Dreyfuss M. L. Runge

1977-01-01

373

Low temperature bonding techniques for MEMS applications  

Microsoft Academic Search

Direct bonding of two mirror-polished wafers, without any external applied energy and at low temperature, is an attractive technique for the new generation of ICs, due to the flexibility that the technique offers. This technique can be used for advanced CMOS applications and for MEMS ones, that are MOS compatible. The bonded structure can be obtained using wet or dry

C. Dunare; T. Stevenson; A. Gundlach; A. Walton; W. Parkes

2001-01-01

374

Estimation of the thermodynamic parameters of hydrogen bonding in alcohol solutions by the method of infrared spectroscopy  

NASA Astrophysics Data System (ADS)

Hydrogen bonding (H-bonding) is a specific type of intermolecular interaction being formed for favorable mutual orientations of the interacting molecules. One of the authors had developed a model concept relating the H-bonding energy with the change of stretching vibrations ?? = ?OH - ?OH-NC of the alcohol OH-group in acetonitrile and acetone solutions: ? H = 89.24??/?0. The calculated H-bond energy was 10.45 kJ/mole for acetonitrile and ? H = 12.12 kJ/mole for acetone. The results obtained are compared with the data calculated using the equilibrium constant of H-bonding reaction; they can also be used to calculate all other thermodynamic H-bond parameters by measuring the equilibrium constant K c in a certain temperature interval. The equilibrium constant is calculated from the Lambert-Bouguer-Beer law: {K_c} = {{C_{{text{OH}} \\cdots {text{NC}}}}}/{{C_{text{OH}} \\cdot {C_{text{NC}}}}} , ? F = - RT ? ln K c , ? H = RT 2 ? d(ln K c )/ dT, and ? S = {? H - ? F}/T . For the methanol solution in acetonitrile, ?? = 115 cm-1, ? H = 10.87 kJ/mole, and K c = 42 L/mole. For the ethanol solution in acetonitrile, ?? = 118 cm-1, ? H = 10.01 kJ/mole, and K c = 34 L/mole. For the propanol solution in acetonitrile, ?? = 110 cm-1, ? H = 8.36 kJ/mole, and K c = 13 L/mole. All calculations are performed using the developed programs. The spectra are recorded on Perkin-Elmer-180 and Specord-84 IR-spectrometers. The values of the thermodynamic parameters calculated and estimated from K c - f( T) are in good agreement with each other and with the available literature data.

Vedernikova, E. V.; Gafurov, M. M.; Ataev, M. B.

2011-01-01

375

Study of minimum energy conformers of N-substituted derivatives of piperidine and pyrrolidine. Evidence of weak H-bonding by theoretical correlation with experimental NMR data  

NASA Astrophysics Data System (ADS)

Four N-substituted piperidines and four N-substituted pyrrolidines were synthesized and characterized by common spectroscopic techniques. In order to investigate the dynamic behavior of these important heterocyclic building blocks, a conformational study was carried out, using suitable tools of computational chemistry. The comparison of computed and experimental NMR chemical shifts is now accepted as a valuable aid in the elucidation of structures and that was the strategy followed in the present study. After a conformational search by statistical methods, the lowest three minimum-energy conformers of each piperidine/pyrrolidine derivative were analyzed (24 conformers in total). The relative proportion of each conformer was estimated by thermodynamic calculations, involving the Boltzmann weighting factor; Pi. Using DFT methods, the geometry of each conformer was fully optimized and their chemical shifts (1H, 13C) were calculated. A very good coincidence between calculated and experimental data of both, 1H and 13C shifts was found, indicating that the theoretical geometries obtained were reliable enough for the study of structure-properties relationships. For all the conformers, specific weak unusual hydrogen bonding involving H C instead of H X (where X=electronegative atom) was observed. The nitrogen atom and the carbonyl oxygen participate in these interactions and could play an important role in the activity of these compounds and, therefore, they should be considered in the modeling of bioactive compounds. Furthermore, the good quality of theoretical/experimental data validates the theoretical methods used. The solvent effect (CDCl3 in this case) was found of small significance on the observed H-bonding.

Lobato-García, Carlos E.; Guadarrama, Patricia; Lozada, Concepción; Enríquez, Raúl G.; Gnecco, Dino; Reynolds, Wiliam F.

2006-03-01

376

Influence of Radiative Energy Transfer on the Thermal Behavior of Bonded InGaAs\\/GaAs Lasers  

Microsoft Academic Search

Temperature characteristics and thermal resistance of InGaAs\\/GaAs laser diode (LD) is investigated from below to beyond the lasing threshold. Spectrally-resolved emission measurements show that the heat generated in the active region is induced by the radiative energy transfer of free carriers. Below the lasing threshold, nonradiative recombination induces large heat generation in the active region. Beyond the lasing threshold, Joules

J. W. Ronnie Teo; L. S. Kip Goi; L. H. Xiao; Z. F. Wang; G. Y. Li

2009-01-01

377

Laboratory Studies on the Formation of Three C2H4O Isomers-Acetaldehyde (CH3CHO), Ethylene Oxide (c-C2H4O), and Vinyl Alcohol (CH2CHOH)-in Interstellar and Cometary Ices  

NASA Astrophysics Data System (ADS)

Laboratory experiments were conducted to unravel synthetic routes to form three C2H4O isomers-acetaldehyde (CH3CHO), ethylene oxide (c-C2H4O), and vinyl alcohol (CH2CHOH)-in extraterrestrial ices via electronic energy transfer processes initiated by electrons in the track of MeV ion trajectories. Here we present the results of electron irradiation on a 2:1 mixture of carbon dioxide (CO2) and ethylene (C2H4). Our studies suggest that suprathermal oxygen atoms can add to the carbon-carbon ? bond of an ethylene molecule to form initially an oxirene diradical (addition to one carbon atom) and the cyclic ethylene oxide molecule (addition to two carbon atoms) at 10 K. The oxirene diradical can undergo a [1, 2]-H shift to the acetaldehyde molecule. Both the ethylene oxide and the acetaldehyde isomers can be stabilized in the surrounding ice matrix. To a minor amount, suprathermal oxygen atoms can insert into a carbon-hydrogen bond of the ethylene molecule, forming vinyl alcohol. Once these isomers have been synthesized inside the ice layers of the coated grains in cold molecular clouds, the newly formed molecules can sublime as the cloud reaches the hot molecular core stage. These laboratory investigations help to explain astronomical observations by Nummelin et al. and Ikeda et al. toward massive star-forming regions and hot cores, where observed fractional abundances of these isomers are higher than can be accounted for by gas-phase reactions alone. Similar synthetic routes could help explain the formation of acetaldehyde and ethylene oxide in comet C/1995 O1 (Hale-Bopp) and also suggest a presence of both isomers in Titan's atmosphere.

Bennett, Chris J.; Osamura, Yoshihiro; Lebar, Matt D.; Kaiser, Ralf I.

2005-11-01

378

Regulatory properties of araC(c) mutants in the L-arabinose operon of escherichia coliB/r.  

PubMed Central

Merodiploids containing a high-constitutive and a low-constitutive araC(c) allele were assayed for constitutive expression of the ara operon. Low-constitutive araC(c) alleles either were unable to repress the constitutive rate of ara operon expression exhibited by by high-constitutive araC(c) alleles or achieved a partial repression of the high-constitutive rate of operon expression. Either mutation to a low-constitutive araC(c) mutant resulted in a partial or complete loss of repressor function, or subunit mixing between the two araC(c) mutant proteins resulted in a partial or complete dominance of the high-constitutive araC(c) allele. Five of the six araC(c) alleles tested allowed a partial induction of the ara operon in cya crp background. In general, a higher level of ara operon induction was achieved in the cya crp background by high araC(c) alleles than by low araC(c) alleles. Furthermore, several araC(c) mutants exhibited decreased sensitivity to catabolite repression, particularly in the presence of inducer. The results suggest a model in which certain araC(c) gene products can achieve ara operon induction in the presence of either arabinose (inducer) or catabolite activator protein-cyclic adenosine monophosphate, whereas the wild-type araC gene product requires the presence of both of these factors for operon expression.

MacInnes, K R; Sheppard, D E; Falgout, B

1978-01-01

379

Probing the bonding and electronic structure of single atom dopants in graphene with electron energy loss spectroscopy.  

PubMed

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp(3)-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity. PMID:23259533

Ramasse, Quentin M; Seabourne, Che R; Kepaptsoglou, Despoina-Maria; Zan, Recep; Bangert, Ursel; Scott, Andrew J

2013-01-04

380

Density functional theory and ab initio study of bond dissociation energy for peroxonitrous acid and peroxyacetyl nitrate  

Microsoft Academic Search

Geometries and energies of two nitro peroxides, HOONO2 and CH3COOONO2 are calculated by two common ab initio (ROHF and MP2), local (SVWN), four hybrid (BHandH, BHandHLYP, Becke3LYP, and Becke3P86) and two non-local (BLYP and BP86) density functional theory (DFT) methods. In all calculations standard GAUSSIAN-type basis sets [6-31+G(d) and 6-311++G(3df,3pd)] are used. Based on the comparison of computed and experimental

Branko S. Jursic

1996-01-01

381

Societal Bonding and Delinquency  

Microsoft Academic Search

Travis Hirschi's (1969) theory of control has amassed considerable empirical support since its development and has become one of the dominant explanations of delinquent behavior. Control theory's basic premise is that people with strong bonds to society are less likely to deviate from conventional behavior than those with weak bonds. In this article bonding theory is applied to a severely

Karen Witchcoff Knight; Tony Tripodi

1996-01-01

382

Bonded flexible pipe  

Microsoft Academic Search

This paper presents an overview of bonded flexible pipe improvements primarily driven by environmental safety as applied to offshore development using mobile production systems. Bonded pipe is a flexible pipe where the steel reinforcement is integrated and bonded to a vulcanized elastomeric material. Textile material is included in the structure to obtain additional structural reinforcement or to separate elastomeric layers.

Val M. Northcutt

2000-01-01

383

Thin die bonding techniques  

Microsoft Academic Search

Picking and bonding 50-micron thick chips presents new challenges to the die bonder industry. Extensive development efforts have identified a number of process challenges and solutions. This paper summarizes Esec's most recent learning's pertaining to the thin die bonding process. Problems with standard die bonding chip pick up and epoxy-dispensing techniques are discussed and solutions identified. Two novel pick solutions

J. Medding; R. Stalder; M. Niederhauser; P. Stoessel

2004-01-01

384

Hydrophobic silicon wafer bonding  

Microsoft Academic Search

Silicon wafers with hydrophilic surfaces can be bonded at room temperature (RT). This has been attributed to the presence of OH groups on the mating surfaces that form hydrogen bonds between the two wafers.19 Hydrophobic Si wafers prepared by a dip in diluted HF without subsequent water rinse have shown a similar RT bonding performance.3 Dispersion van der Waals forces

Q.-Y. Tong; E. Schmidt; U. Gösele; M. Reiche

1994-01-01

385

Using Linear Genetic Programming to Develop a C\\/C++ Simulation Model of a Waste Incinerator  

Microsoft Academic Search

We explore whether Linear Genetic Programming (LGP) can evolve a C\\/C++ computer simulation model that accurately models the performance of a waste incinerator. Human expert written simulation models are used worldwide in a variety of industrial and business applications. They are expensive to develop, may or may not be valid for the specific process that is being modeled, and may

Larry M. Deschaine; Janardan J. Patel; Ronald D. Guthrie; Joseph T. Grumski

2001-01-01

386

Characterization of C–C chemokine receptor subfamily in teleost fish  

Microsoft Academic Search

Chemokines and their receptors play important roles in nervous and immune systems. Little information, however, exists concerning this gene family in teleost fish. In the present study, 17 C–C chemokine receptors genes were identified from Danio rerio, 9 from Gasterosteus aculeatus, 10 from Oryzias latipes, 8 from Takifugu rubripes and 5 from Tetraodon nigroviridis. Phylogenetic analysis showed that the orthologs

Y. Liu; M. X. Chang; S. G. Wu; P. Nie

2009-01-01

387

Dynamics of the Antarctic Circumpolar Current (A.C.C.) Part I.  

National Technical Information Service (NTIS)

Equations of motion and continuity of the volume transport of the Antarctic Circumpolar Current (A.C.C.) are derived for the barotropic mode, including the advective terms, bottom friction and horizontal stress terms. In the zonal component of equations o...

T. Ichiye

1967-01-01

388

The Structure and Decomposition of Titanium and Zirconium B.C.C. Solid Solutions.  

National Technical Information Service (NTIS)

The research was concerned with studying the structure, the mechanical properties and the decomposition of Ti and Zr b.c.c. (e.g. Zr-Nb, Ti-V) solid solutions. The experimental techniques used in this study were electron microscopy, electron diffraction, ...

S. L. Sass

1974-01-01

389

Imidazole-nitrile or imidazole-isonitrile c?c coupling on rhenium tricarbonyl complexes.  

PubMed

Ligand activation: Deprotonation of the nitrile or isonitrile complexes [Re(CO)3 (N-RIm)2 (L)](+) (N-RIm=N-alkylimidazole; L=N?CtBu, C?NtBu) selectively afforded alkylidenamido or iminoacyl derivatives, respectively, in which C?C coupling has occurred. Protonation of the latter complex leads to aminocarbene products. PMID:24038657

Viguri, Maialen Espinal; Huertos, Miguel A; Pérez, Julio; Riera, Lucía

2013-09-03

390

Cooperativity between the dihydrogen bond and the NHC hydrogen bond in LiH-(HCN)n Complexes.  

PubMed

The cooperativity between the dihydrogen bond and the NHC hydrogen bond in LiH-(HCN)(n) (n=2 and 3) complexes is investigated at the MP2 level of theory. The bond lengths, dipole moments, and energies are analyzed. It is demonstrated that synergetic effects are present in the complexes. The cooperativity contribution of the dihydrogen bond is smaller than that of the NHC hydrogen bond. The three-body energy in systems involving different types of hydrogen bonds is larger than that in the same hydrogen-bonded systems. NBO analyses indicate that orbital interaction, charge transfer, and bond polarization are mainly responsible for the cooperativity between the two types of hydrogen bonds. PMID:18712732

Li, Qing-Zhong; Hu, Ting; An, Xiu-Lin; Gong, Bao-An; Cheng, Jian-Bo

2008-09-15

391

Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations.  

PubMed

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products. PMID:15549803

Baciocchi, Enrico; Giacco, Tiziana Del; Elisei, Fausto; Gerini, Maria Francesca; Lapi, Andrea; Liberali, Prisca; Uzzoli, Barbara

2004-11-26

392

25 CFR 215.13 - Bond.  

Code of Federal Regulations, 2013 CFR

...215.13 Section 215.13 Indians BUREAU OF INDIAN AFFAIRS, DEPARTMENT OF THE INTERIOR ENERGY AND MINERALS LEAD AND ZINC MINING OPERATIONS AND LEASES, QUAPAW AGENCY § 215.13 Bond. Every mineral lease made and entered into...

2013-04-01

393

25 CFR 215.13 - Bond.  

Code of Federal Regulations, 2011 CFR

...215.13 Section 215.13 Indians BUREAU OF INDIAN AFFAIRS, DEPARTMENT OF THE INTERIOR ENERGY AND MINERALS LEAD AND ZINC MINING OPERATIONS AND LEASES, QUAPAW AGENCY § 215.13 Bond. Every mineral lease made and entered into...

2011-04-01

394

25 CFR 225.30 - Bonds.  

Code of Federal Regulations, 2013 CFR

...DEPARTMENT OF THE INTERIOR ENERGY AND MINERALS OIL AND GAS, GEOTHERMAL, AND SOLID MINERALS AGREEMENTS...a $75,000 bond for all geothermal, mining, or oil and...nationwide coverage to cover all geothermal or oil and gas...

2013-04-01

395

Hydrogen multicentre bonds  

NASA Astrophysics Data System (ADS)

The concept of a chemical bond stands out as a major development in the process of understanding how atoms are held together in molecules and solids. Lewis' classical picture of chemical bonds as shared-electron pairs evolved to the quantum-mechanical valence-bond and molecular-orbital theories, and the classification of molecules and solids in terms of their bonding type: covalent, ionic, van der Waals and metallic. Along with the more complex hydrogen bonds and three-centre bonds, they form a paradigm within which the structure of almost all molecules and solids can be understood. Here, we present evidence for hydrogen multicentre bonds-a generalization of three-centre bonds-in which a hydrogen atom equally bonds to four or more other atoms. When substituting for oxygen in metal oxides, hydrogen bonds equally to all the surrounding metal atoms, becoming fourfold coordinated in ZnO, and sixfold coordinated in MgO. These multicentre bonds are remarkably strong despite their large hydrogen-metal distances. The calculated local vibration mode frequency in MgO agrees with infrared spectroscopy measurements. Multicoordinated hydrogen also explains the dependence of electrical conductivity on oxygen partial pressure, resolving a long-standing controversy on the role of point defects in unintentional n-type conductivity of ZnO (refs 8-10).

Janotti, Anderson; van de Walle, Chris G.

2007-01-01

396

Temperature dependence of electron attachment and detachment in SF[sub 6] and [ital c]-C[sub 4]F[sub 6  

SciTech Connect

In the temperature, [ital T], range of 300--600 K and the mean electron energy range [l angle][epsilon][r angle] of 0.19--1.0 eV, the total electron attachment rate constant for SF[sub 6] and [ital c]-C[sub 4]F[sub 6] measured in dilute mixtures with N[sub 2], is virtually independent of [ital T]. Under the same experimental conditions the stabilized SF[sup [minus

Datskos, P.G.; Christophorou, L.G.; Carter, J.G. (Atomic, Molecular, and High Voltage Physics Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6122 (United States))

1993-12-01

397

Small Carbonaceous Molecules, Ethylene Oxide (c-C2H4O) and Cyclopropenylidene (c-C3H2): Sources of the Unidentified Infrared Bands?  

NASA Astrophysics Data System (ADS)

We suggest that small carbonaceous molecules (SCMs) may be the sources of the unidentified infrared bands (UIRs) and the underlying continuum. We show that the IR spectroscopy of ethylene oxide (EO, c-C2H4O) and cyclopropenylidene (CP, c-C3H2) closely correlates with the major UIR bands at 3.3, 6.2, 7.7, 8.6, and 11.2 ?m, the often seen strong bands at 12.7 and 16.4 ?m, as well as many minor features. The differences in band locations and shapes between laboratory EO absorption spectra and astrophysical UIR emission spectra are attributed to vibrational anharmonicity, Fermi resonance splitting of nearly degenerate vibration levels, and rotational envelope narrowing due to the low temperatures in space. The excitation mechanism is absorption of UV radiation, primarily Ly?, by SCMs. Photon trapping for this very optically thick transition enhances the absorption by several orders of magnitude. Our abundance analysis for NGC 7027 reveals that the SCM abundance, relative to H2, is ~3 × 10-9 which compares well to radio measurements of the CP abundance range of ~10-9-10-7. The origin of the UIR continuum is discussed in terms of emission from vibrationally and rotationally hot SCM UV photodissociation products and UV excitation of rotationally hot SCM species. Radio lines of CP have been seen in numerous astronomical objects, most displaying the UIR bands. EO is also seen, but in fewer objects, none displaying the UIR bands. We theorize that in UIR objects, EO is formed on, and primarily resides on, carbonaceous grains, precluding radio detection of rotational lines. We suggest laboratory experiments, astronomical observations, and theoretical investigations to further evaluate the SCM mechanism for the UIR bands and continuum.

Bernstein, Lawrence S.; Lynch, David K.

2009-10-01

398

Mechanistic Investigation of Phosphate Ester Bond Cleavages of Glycylphosphoserinyltryptophan Radical Cations under Low-Energy Collision-Induced Dissociation  

NASA Astrophysics Data System (ADS)

Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G p SW]•+) fragments differently from its tautomer with the radical initially generated on the ?-carbon atom of the glycine residue ([G• p SW]+). The dissociation of [G• p SW]+ is dominated by the neutral loss of H3PO4 (98 Da), with backbone cleavage forming the [b2 - H]•+/y1 + pair as the minor products. In contrast, for [G p SW]•+, competitive cleavages along the peptide backbone, such as the formation of [G p SW - CO2]•+ and the [c2 + 2H]+/[z1 - H]•+ pair, significantly suppress the loss of neutral H3PO4. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G• p SW]+ and [G p SW]•+ and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H3PO4 from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G• p SW]+ results in its charge being more mobile, thereby favoring charge-driven loss of H3PO4; in contrast, radical-driven pathways are more competitive during the CID of [G p SW]•+.

Quan, Quan; Hao, Qiang; Song, Tao; Siu, Chi-Kit; Chu, Ivan K.

2013-04-01

399

Direct observation of catch bonds involving cell-adhesion molecules  

Microsoft Academic Search

Bonds between adhesion molecules are often mechanically stressed. A striking example is the tensile force applied to selectin-ligand bonds, which mediate the tethering and rolling of flowing leukocytes on vascular surfaces. It has been suggested that force could either shorten bond lifetimes, because work done by the force could lower the energy barrier between the bound and free states (`slip'),

Bryan T. Marshall; Mian Long; James W. Piper; Tadayuki Yago; Rodger P. McEver; Cheng Zhu

2003-01-01

400

Integrated and Automated Abstract Interpretation, Verification and Testing of C\\/C++ Modules  

Microsoft Academic Search

\\u000a Starting from the perspective of safety-critical systems development in avionics, railways and the automotive domain, we advocate\\u000a an integrated verification approach for C\\/C++ modules combining abstract interpretation, formal verification and conventional\\u000a testing. It is illustrated how testing and formal verification can benefit from abstract interpretation results and, vice\\u000a versa, how test automation techniques may help to reduce the well known

Jan Peleska

2010-01-01

401

A Unified Approach to Abstract Interpretation, Formal Verification and Testing of C\\/C++ Modules  

Microsoft Academic Search

In this paper, a unified approach to abstract interpretation, formal verification and testing is described. The approach is\\u000a applicable for verifying and testing C\\/C++ functions and methods and complies with the requirements of today’s applicable\\u000a standards for the development of safety-critical systems in the avionics and railway domains. We give an overview over the\\u000a techniques required and motivate why an

Jan Peleska

2008-01-01

402

Long-term tropospheric trend of octafluorocyclobutane (c-C4F8 or PFC-318)  

NASA Astrophysics Data System (ADS)

Air samples collected at Cape Grim, Tasmania between 1978 and 2008 and during a series of more recent aircraft sampling programmes have been analysed to determine the atmospheric abundance and trend of octafluorocyclobutane (c-C4F8 or PFC-318). c-C4F8 has an atmospheric lifetime in excess of 3000 yr and a global warming potential (GWP) of 10 300 (100 yr time horizon), making it one of the most potent greenhouse gases detected in the atmosphere to date. The abundance of c-C4F8 in the Southern Hemisphere has risen from 0.35 ppt in 1978 to 1.2 ppt in 2010, and is currently increasing at a rate of around 0.03 ppt yr-1. It is the third most abundant perfluorocarbon (PFC) in the present day atmosphere, behind CF4 (~75 ppt) and C2F6 (~4 ppt). Although a number of potential sources of c-C4F8 have been reported, including the electronics and semi-conductor industries, there remains a large discrepancy in the atmospheric budget. Using a 2-D global model to derive top-down global emissions based on the Cape Grim measurements yields a recent (2007) emission rate of around 1.1 Gg yr-1 and a cumulative emission up to and including 2007 of 38.1 Gg. Emissions reported on the EDGAR emissions database for the period 1986-2005 represent less than 1% of the top-down emissions for the same period, which suggests there is a large unaccounted for source of this compound. It is also apparent that the magnitude of this source has varied considerably over the past 30 yr, declining sharply in the late 1980s before increasing again in the mid-1990s.

Oram, D. E.; Mani, F. S.; Laube, J. C.; Newland, M. J.; Reeves, C. E.; Sturges, W. T.; Penkett, S. A.; Brenninkmeijer, C. A. M.; Röckmann, T.; Fraser, P. J.

2012-01-01

403

A Package for the Automatic Differentiation of Algorithms Written in C\\/C  

Microsoft Academic Search

The C++ package ADOL-C described here facilitates the evaluation of first andhigher derivatives of vector functions that are defined by computer programs written inC or C++. The resulting derivative evaluation routines may be called from C\\/C++,Fortran, or any other language that can be linked with C.The numerical values of derivative vectors are obtained free of truncation errors ata small multiple

Andreas Griewank; David Juedes; Jean Utke

1995-01-01

404

Long-term tropospheric trend of octafluorocyclobutane (cC4F8 or PFC318)  

Microsoft Academic Search

Air samples collected at Cape Grim, Tasmania between 1978 and 2008 and during a series of more recent aircraft sampling programmes have been analysed to determine the atmospheric abundance and trend of octafluorocyclobutane (-C4F8 or PFC-318). c-C4F8 has an atmospheric lifetime in excess of 3000 yr and a global warming potential (GWP) of 10 300 (100 yr time horizon), making

D. E. Oram; F. S. Mani; J. C. Laube; M. J. Newland; C. E. Reeves; W. T. Sturges; S. A. Penkett; C. A. M. Brenninkmeijer; T. Röckmann; P. J. Fraser

2011-01-01

405

Transversely isotropic elastic properties of single-walled carbon nanotubes by a rectangular beam model for the CC bonds  

Microsoft Academic Search

Continuum mechanics modeling of carbon nanotubes has long been an attractive issue, but how to reflect exactly the physics essential of the atomic bonds still remains to be a challenging problem. To capture the distinguishing in-plane sigma-sigma and out-of-plane sigma-pi bond angle bending rigidities of C-C bonds in carbon nanotubes, an equivalent beam element with rectangular section is proposed and

Haijun Li; Wanlin Guo

2008-01-01

406

The Role of Multiple, Reformable Parallel Bonds on the Self-healing Behavior of Dual Crosslinked Nanogel Materials  

NASA Astrophysics Data System (ADS)

Using computational modeling, we design novel self-healing materials composed of nanoscopic polymer gel particles, or nanogels. The particles are interconnected via both labile bonds (e.g., disulfide bonds) and stronger, less reactive bonds (e.g, C-C bonds) and therefore the nanogels form a "dual crosslinked" network. The stable bonds provide a rigid backbone while the labile bonds allow the material to undergo a dynamic reconfiguration in response to stress. We adapt the Hierarchical Bell Model (HBM) to describe the labile bonding interactions. The HBM effectively allows us to model cases where the ligands on neighboring nanogels interact through multiple sites. We show that the introduction of a small number of labile bonds that lie in parallel significantly increases the strength of the material relative to samples crosslinked solely by the stable bonds. We also isolate an optimal range of labile interconnections that provide high-strength, tough materials that are capable of self-repair.

Salib, Isaac G.; Kolmakov, German V.; Gnegy, Chet N.; Matyjaszewski, Krzysztof; Balazs, Anna C.

2011-03-01

407

Dichotomous effects of C-C chemokines in HIV-1 pathogenesis.  

PubMed

Chemokines play a critical role in shaping innate and adaptive immunity. These molecules also participate in maintaining the immune balance in the body. Apart from their regulatory role, these mediators are involved in several inflammatory and autoimmune diseases including viral infection such as HIV-1/AIDS. Chemokine co-receptor CCR5 and CXCR4 and their ligands significantly contribute to HIV-1 disease progression. C-C chemokines such CCL3, CCL4 and CCL5 have been shown to possess antiviral effects by binding to HIV-1 co-receptors. CCL2, a member of the C-C chemokine family, displays a different feature instead. It is a potential enhancer rather than inhibitor of viral replication, a property exhibited by most of the C-C chemokine members. In addition, the role of CCL2 is well established in forming a Th2 type of response by directing differentiation of Th0 cells towards Th2 type, a unique feature of HIV-1 disease. We propose a hypothesis in which the chemotactic nature of CCL2 drives recruitment of target cells to the site of infection as one of the mechanisms operating in vivo that favours viral replication and eventually a high viral load in infected individuals. PMID:17434211

Ansari, A Wahid; Heiken, Hans; Moenkemeyer, Maren; Schmidt, Reinhold E

2007-03-26

408

Exploring for 3D photonic bandgap structures in the 11 f.c.c. space groups  

NASA Astrophysics Data System (ADS)

The promise of photonic crystals and their potential applications has attracted considerable attention towards the establishment of periodic dielectric structures that in addition to possessing robust complete bandgaps, can be easily fabricated with current techniques. A number of theoretical structures have been proposed. To date, the best complete photonic bandgap structure is that of diamond networks having Fd3m symmetry (2-3 gap). The only other known complete bandgap in a face-centred-cubic (f.c.c.) lattice structure is that of air spheres in a dielectric matrix (8-9 gap; the so called 'inverse-opal' structure). Importantly, there is no systematic approach to discovering champion photonic crystal structures. Here we propose a level-set approach based on crystallography to systematically examine for photonic bandgap structures and illustrate this approach by applying it to the 11 f.c.c. groups. This approach gives us an insight into the effects of symmetry and connectivity. We classify the F-space groups into four fundamental geometries on the basis of the connectivity of high-symmetry Wyckoff sites. Three of the fundamental geometries studied display complete bandgaps-including two: the F-RD structure with Fm'3 m symmetry and a group 216 structure with F 4'3m symmetry that have not been reported previously. By using this systematic approach we were able to open gaps between the 2-3, 5-6 and 8-9 bands in the f.c.c. systems.

Maldovan, Martin; Ullal, Chaitanya K.; Carter, W. Craig; Thomas, Edwin L.

2003-10-01

409

An unusal case of facile non-degenerate P-C bond making and breaking.  

PubMed

Oxidation of Li/X phosphinidenoid complex 2, obtained via selective deprotonation from the P-H precursor 1, with [Ph(3)C]BF(4) led to the formation of two P-F substituted diorganophosphane complexes 6,7; the latter tautomer 7 formed via H-shift from 6. In contrast, oxidation of 2 with [(p-Tol)(3)C]BF(4) led to three major and one minor intermediates at low temperature, which we tentatively assign to two pairs of P-C atropisomers 10?a,a' and 10?c,c' and which differ by the relative orientations of their CH(SiMe(3))(2) and W(CO)(5) groups. Conversion of all isomers led finally to complex 11 having a ligand with a long P-C bond to the central trityl* carbon atom, firmly established by single-crystal X-ray analysis. DFT calculations at the B3LYP/def2-TZVPP//BP86/def2-TZVP level of theory on real molecular entities revealed the structures of the in situ formed combined singlet diradicals (4+5 and 5+9) and the nature of intermediates on the way to the final product, complex 11. Remarkable is that all isomers of 11 possess relative energies in the narrow energy regime of about 20?kcal ?mol(-1). A preliminary study revealed that complex 11 undergoes selective P-C bond cleavage at 75?°C in toluene solution. PMID:22488865

Nesterov, Vitaly; Özbolat-Schön, Aysel; Schnakenburg, Gregor; Shi, Lili; Cangönül, Asli; van Gastel, Maurice; Neese, Frank; Streubel, Rainer

2012-04-05

410