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871.
ABSTRACTWe present theoretical investigations on the nature and persistence of the strong central hydrogen bond of a tetraamido/diamino quaternized macrocycle. Our theoretical study of the NMR properties of the central hydrogen bond proved difficult to reconcile with the available experimental results, suggesting the possibility of the non-persistence of the central hydrogen bond of the macrocycle in solution. We demonstrate alternative scenarios, in which a tautomerization of the macrocycle in solution gives rise to the experimentally observed NMR shifts. 相似文献
872.
AbstractA systematic investigation of the brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using the active braze alloy (ABA) Ag–35.25Cu–1.75Ti wt.% has been undertaken to study the chemical reactions at the interfaces of the joints. The extent to which silica-based secondary phases in the Al2O3 participate in the reactions at the ABA/Al2O3 interface has been clarified. Another aspect of this work has been to determine the influence of various brazing parameters, such as the peak temperature, Tp, and time at Tp, τ, on the resultant microstructure. As a consequence, the microstructural evolution of the joints as a function of Tp and τ is discussed in some detail. The formation of a Fe2Ti layer on the Kovar® and its growth, along with adjacent Ni3Ti particles in the ABA, dominate the microstructural developments at the ABA/Kovar® interface. The presence of Kovar® next to the ABA does not change the intrinsic chemical reactions occurring at the ABA/Al2O3 interface. However, the extent of these reactions is limited if the purity of the Al2O3 is high, and so it is necessary to have some silica-rich secondary phase in the Al2O3 to facilitate the formation of a Ti3Cu3O layer on the Al2O3. Breakdown of the Ti3Cu3O layer, together with fracture of the Fe2Ti layer and separation of this layer from the Kovar®, has been avoided by brazing at temperatures close to the liquidus temperature of the ABA for short periods of time, e.g., for Tp between 820 and 830 °C and τ between 2 and 8 min. 相似文献
873.
Mohammednoor Altarawneh Phillip V. Smith Eric M. Kennedy 《Applied Surface Science》2008,254(14):4218-4224
The interaction between a 2-chlorophenol (C6H4OHCl) molecule and the Cu(1 1 1) surface has been investigated using density functional theory as an initial step in gaining a better understanding of the catalyzed formation of dioxin compounds on a clean copper surface. The 2-chlorophenol molecule is found to form several weakly bonded, horizontally and vertically oriented configurations. Dissociative modes have also been investigated. For the latter, the formation of phenyl and benzyne fragments is found to be more energetically favourable than the formation of 2-chlorophenoxy radicals. 相似文献
874.
Fox PA Griffin ST Reichert WM Salter EA Smith AB Tickell MD Wicker BF Cioffi EA Davis JH Rogers RD Wierzbicki A 《Chemical communications (Cambridge, England)》2005,(29):3679-3681
Readily prepared imidazole-based boronium ions form stable, hydrophobic, room-temperature ionic liquids (RTIL) with unique electronic and spectroscopic characteristics. 相似文献
875.
In this paper we develop a first-order system of conservation laws for finite deformation in solids, describe its characteristic structure, and use this analysis to develop a second-order numerical method for problems involving finite deformation and plasticity. The equations of mass, momentum, and energy conservation in Lagrangian and Eulerian frames of reference are combined with kinetic equations of state for the stress and with caloric equations of state for the internal energy, as well as with auxiliary equations representing equality of mixed partial derivatives of the deformation gradient. Particular attention is paid to the influence of a curl constraint on the deformation gradient, so that the characteristic speeds transform properly between the two frames of reference. Next, we consider models in rate-form for isotropic elastic-plastic materials with work-hardening, and examine the circumstances under which these models lead to hyperbolic systems for the equations of motion. In spite of the fact that these models violate thermodynamic principles in such a way that the acoustic tensor becomes nonsymmetric, we still find that the characteristic speeds are always real for elastic behavior, and essentially always real for plastic response. These results allow us to construct a second-order Godunov method for the computation of three-dimensional displacement in a one-dimensional material viewed in the Lagrangian frame of reference. We also describe a technique for the approximate solution of Riemann problems in order to determine numerical fluxes in this algorithm. Finally, we present numerical examples of the results of the algorithm. 相似文献
876.
877.
Zheng X Aniol K Armstrong DS Averett TD Bertozzi W Binet S Burtin E Busato E Butuceanu C Calarco J Camsonne A Cates GD Chai Z Chen JP Choi S Chudakov E Cusanno F Leo RD Deur A Dieterich S Dutta D Finn JM Frullani S Gao H Gao J Garibaldi F Gilad S Gilman R Gomez J Hansen JO Higinbotham DW Hinton W Horn T De Jager CW Jiang X Kaufman L Kelly J Korsch W Kramer K LeRose J Lhuillier D Liyanage N Margaziotis DJ Marie F Markowitz P McCormick K Meziani ZE Michaels R Moffit B Nanda S Neyret D Phillips SK 《Physical review letters》2004,92(1):012004
We have measured the neutron spin asymmetry A(n)(1) with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q(2)=2.7, 3.5, and 4.8 (GeV/c)(2), respectively. Our results unambiguously show, for the first time, that A(n)(1) crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation. 相似文献
878.
Schaldach CM Bourcier WL Paul PH Wilson WD 《Langmuir : the ACS journal of surfaces and colloids》2004,20(24):10744-10750
We have developed a method of calculation of the dielectrophoretic force on a nanoparticle in a fluid environment where variations in the electric field and electric field gradients are on the same nanoscale as the particle. The Boundary Element Dielectrophoretic Force (BEDF) method involves constructing a solvent-accessible or molecular surface surrounding the particle, calculating the normal component of the electric field at the surface boundary elements, and then solving a system of linear equations for the induced surface polarization charge on each element. Different surface elements of the molecule may experience quite different polarizing electric fields, unlike the situation in the point dipole approximation. A single 100-A-radius ring test configuration is employed to facilitate comparison with the well-known point dipole approximation (PDA). We find remarkable agreement between the forces calculated by the BEDF and PDA methods for a 1 A polarizable sphere. However, for larger particles, the differences between the methods become qualitative as well as quantitative; the character of the force changes from attractive at the origin of the ring for a 50-A sphere, to repulsive for a 75-A sphere. Equally dramatic differences are found in a more complex electrical environment involving two sets of 10 rings. 相似文献
879.
Katritzky AR Hayden AE Kirichenko K Pelphrey P Ji Y 《The Journal of organic chemistry》2004,69(15):5108-5111
Reactions of secondary amides 2a-i with 1-chloro-1H-benzotriazole and triphenylphosphine give imidoylbenzotriazoles 3a-i. The treatment of 3a,b,e,g with silyl enol ethers 5a,b in the presence of potassium tert-butoxide provides a new general approach to enaminoketones 6a-h. 相似文献
880.
Rendle PM Seger A Rodrigues J Oldham NJ Bott RR Jones JB Cowan MM Davis BG 《Journal of the American Chemical Society》2004,126(15):4750-4751
The continuing ability of bacteria to resist current antibiotic treatments highlights the need for alternative strategies for inhibiting their pathogenicity. Bacterial attachment is a major factor in infectivity and virulence. This key binding phase of bacteria to any potential host is mediated by adhesin proteins and so these present an attractive therapeutic target for antiinfective blocking strategies. However, the natural ligands to adhesins are large, typically complex molecules that are difficult to mimic with small molecules. We describe here a method that creates precise synthetic mimics of glycoproteins that are designed to bind adhesins. By using protein-degrading enzymes as the basis for these mimics we have created large-molecule protein ligands that inhibit aggregation of pathogenic bacteria at levels greater than a million-fold higher than small-molecule inhibitors of adhesins. 相似文献