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991.
Navarro I Basset JF Hebbe S Major SM Werner T Howsham C Bräckow J Barrett AG 《Journal of the American Chemical Society》2008,130(31):10293-10298
Diketo-1,3-dioxin-2-ones underwent retro-Diels-Alder reaction on heating in toluene at 110 degrees C to generate alpha,gamma,-triketo-ketenes. These were trapped with alcohols to provide 2,4,6-triketocarboxylates, which were smoothly aromatized by sequential reaction with potassium carbonate and methanolic hydrogen chloride to give resorcylate esters. The reaction was applied in the total synthesis of the marine antifungal agents 15G256beta (1), 15G256iota (2), and 15G256pi (3) and the mycotoxin S-(-)-zearalenone (4). 相似文献
992.
Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt(111) electrode 总被引:1,自引:0,他引:1
Skúlason E Karlberg GS Rossmeisl J Bligaard T Greeley J Jónsson H Nørskov JK 《Physical chemistry chemical physics : PCCP》2007,9(25):3241-3250
We present results of density functional theory calculations on a Pt(111) slab with a bilayer of water, solvated protons in the water layer, and excess electrons in the metal surface. In this way we model the electrochemical double layer at a platinum electrode. By varying the number of protons/electrons in the double layer we investigate the system as a function of the electrode potential. We study the elementary processes involved in the hydrogen evolution reaction, 2(H(+) + e(-)) --> H(2), and determine the activation energy and predominant reaction mechanism as a function of electrode potential. We confirm by explicit calculations the notion that the variation of the activation barrier with potential can be viewed as a manifestation of the Br?nsted-Evans-Polanyi-type relationship between activation energy and reaction energy found throughout surface chemistry. 相似文献
993.
Bohinc K Gimsa J Kralj-Iglic V Slivnik T Iglic A 《Bioelectrochemistry (Amsterdam, Netherlands)》2005,67(1):91-99
The physical properties of organic nanotubes attract increasing attention due to their potential benefit in technology, biology and medicine. We study the effect of ion size on the electrical properties of cylindrical nanotubes filled with electrolyte solution within a modified Poisson-Boltzmann (PB) approach. For comparison purposes, small hollow nanospheres filled with electrolyte solution are considered. The finite size of the particles in the inner electrolyte solution is described by the excluded volume effect within a lattice statistics approach. We found that an increased ion size reduces the number of counterions near the charged inner surface of the nanotube, leading to an enlarged electrostatic surface potential. The concentration of counterions close to the inner surface saturates for higher surface charge densities and larger ions. In the case of saturation, the closest counterion packing is achieved, all lattice sites near the surface are occupied and an actual counterion condensation is observed. By contrast, the counterion concentration at the axis of the nanotube steadily increases with increasing surface charge density. This growth is more pronounced for smaller nanotube radii and larger ions. At larger nanotube radii for small ion size counterion condensation may also be observed according to the Tsao criterion, i.e. the counterion concentration at the centre is independent of the number of counterions in the system. With decreasing radius the Tsao condensation effect is shifted towards physiologically unrealistic surface charge densities. 相似文献
994.
Henno Brandsma Jan van Mill 《Proceedings of the American Mathematical Society》1998,126(11):3407-3411
We show that Kunen's example of a compact L-space, constructed under CH, can be modified so that it has a non-monolithic hyperspace. This answers a question of Bell's. This example is also relevant to a question of Arhangel'skii's.
995.
996.
Jan Henk Marsman Berthold B. Breman Antonie A. C. M. Beenackers 《Journal of separation science》1993,16(3):141-147
The main problems with complete analysis of the components of fuel methanol, or in Fischer-Tropsch studies, are the several classes of compound present in the sample (permanent gases, water, alcohols, hydrocarbons), its wide range of components, its boiling point range, and the wide range of component concentrations. A flexible on-line GC method has been developed for kinetic study of catalyzed chemical reactions of hydrogen and oxides of carbon. The single capillary column, temperature programmed method was designed for complete analysis of reactants and products (hydrogen, carbon monoxide, carbon dioxide, water, C1-C10 hydrocarbons, and C1-C6 alcohols): a sample selection valve is used to switch between either the heated line used for input of the synthesis gases or the heated line used to transport reaction products from the reactor. Sample is introduced to the capillary column by means of a 10-port heated gas sampling valve with two external injection loops (0.07 and 1.95 cm3); this results in the determination of components over a wide range of concentrations in the sample (ppm to percentage levels). Helium from a pressure-controlled supply is used as carrier gas and detection of the components is performed by serial connection of thermal conductivity and flame ionization detectors. Peak identification is performed by mass spectrometry and by comparison of component retention times. The automated analytical equipment is integrated with a process control computer and delivers repeatable analytical results for the individual components (RSDs varying between 0.3 and 10% depending strongly on the concentration of the component and the accuracy of the determination of its peak area). 相似文献
997.
Simplicial complexes X provide commutative rings A(X) via the Stanley- Reisner construction. We calculated the cotangent cohomology, i.e., T1 and T2 of A(X) in terms of X. These modules provide information about the deformation theory of the algebro geometric objects assigned to X. 相似文献
998.
Czechowski T Krzyminiewski R Jurga J Chlewicki W 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2008,195(1):52-59
New EPR resonators were developed by using a ceramic material with a high dielectric constant, epsilon=160. The resonators have a high quality factor, Q=10(3), and enhance the sensitivity of an EPR spectrometer up to 170 times. Some advantages of the new ceramic resonators are: (1) cheaper synthesis and simplified fabricating technology; (2) wider temperature range; and (3) ease of use. The ceramic material is produced with a titanate of complex oxides of rare-earth and alkaline metals, and has a perovskite type structure. The resonators were tested with X-band EPR spectrometers with cylindrical (TE(011)) and rectangular (TE(102)) cavities at 300 and 77K. We discovered that EPR signal strength enhancement depends on the dielectric constant of the material, resonator geometry and the size of the sample. Also, an unusual resonant mode was found in the dielectric resonator-metallic cavity structure. In this mode, the directions of microwave magnetic fields of the coupled resonators are opposite and the resonant frequency of the structure is higher than the frequency of empty metallic cavity. 相似文献
999.
Pimková K Bocková M Hegnerová K Suttnar J Cermák J Homola J Dyr JE 《Analytical and bioanalytical chemistry》2012,402(1):381-387
The surface plasmon resonance (SPR) biosensor system with dispersionless microfluidics for the direct and label-free detection
of a soluble vascular endothelial growth factor receptor (sVEGFR-1) is described. The detection approach takes advantage of
an affinity interaction between sVEGFR-1 and its ligand, vascular endothelial growth factor (VEGF-A), which is covalently
immobilized on the surface of the SPR sensor. The ability of the immobilized VEGF-A to specifically bind the sVEGFR-1 receptor
is demonstrated in a buffer. The detection of sVEGFR-1 in 2% human blood plasma is carried out by using the sequential injection
approach. The detection limit of 25 ng/mL is achieved. In addition, we demonstrate that the functional surface of the sensor
can be regenerated for repeated use. 相似文献
1000.
Nga H. Nguyen Xuefei Leng Hao‐Jan Sun Virgil Percec 《Journal of polymer science. Part A, Polymer chemistry》2013,51(15):3110-3122
The efficient Cu(0) wire‐catalyzed single‐electron transfer‐living radical polymerization (SET‐LRP) of oligo(ethylene oxide) methyl ether methacrylate (OEOMA) in DMSO and binary mixtures of DMSO with H2O is reported. Addition of 10–80% H2O to DMSO resulted in an increase in the apparent rate constant of propagation ( ), corresponding to an increase in the polarity and extent of disproportionation. At higher H2O content, decreases, and in H2O is slightly lower than that in DMSO. This unexpected behavior was attributed to the physical inaccessibility of Cu(0) wire catalyst to the hydrophobic reactive centers of OEOMA and initiator which self‐assemble in H2O into micellar aggregates and vesicles. This hypothesis was confirmed by the faster polymerization in H2O than in DMSO during catalysis with Cu(0) nanoparticles generated by disproportionation of CuBr. SET‐LRP of OEOMA can be performed in protic and dipolar aprotic solvents in air by the addition of hydrazine hydrate. The polymerization exhibited no induction period and identical as in the degassed experiment, and led to polymers with narrow molecular weigh distribution. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3110–3122 相似文献