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991.
992.
993.
Fink K 《Physical chemistry chemical physics : PCCP》2006,8(13):1482-1489
Oxygen vacancies at the polar O terminated (0001) surface of ZnO are of particular interest, because they are discussed as active sites in the methanol synthesis. In general, the polar ZnO surfaces are stabilized by OH groups, therefore O vacancies can be generated by removing either O atoms or OH or H2O groups from the surface. These defects differ in the number of electrons in the vacancy and the number of OH groups in the neighborhood. In the present study, the electronic structure and the adsorption properties of four different types of oxygen vacancies have been investigated by means of embedded cluster calculations. We performed ab initio calculations on F+ like surface excitations for the different defect types and found that the transition energies are above the optical band-gap, while F+ centers in bulk ZnO show a characteristic optical excitation at 3.19 eV. Furthermore, we studied the adsorption of CO2 and CO at the different defect sites by DFT calculations. We found that CO2 dissociates at electron rich vacancies into CO and an O atom which remains in the vacancy. At the OH vacancy which contains an unpaired electron CO2 adsorbed in the form of CO2-, while it adsorbed as a linear neutral molecule at the H2O defect. CO adsorbed preferentially at the H2O defect and the OH defect, both with a binding energy of 0.3 eV. 相似文献
994.
Bergström SK Goiny M Danielsson R Ungerstedt U Andersson M Markides KE 《Journal of chromatography. A》2006,1120(1-2):21-26
A novel method is described to follow known and unknown compounds in biological processes using microdialysis sampling and mass spectrometric detection. By implementation of internal standard, desalting/enrichment for the sample work-up, and multivariate data analysis, this methodology is a basis for future applications in early diagnosis of diseases and organ damage, as a complement to the routinely used clinical methods for biological samples. The present study includes screening without specific target analytes, of samples collected by microdialysis from liver of anaesthetized rats before and after local damage to this organ. Sample series were classified by principal component analysis, and the stimulation was identified in the chemical patterns produced by the presented analytical tool. 相似文献
995.
Ljungdahl T Pettersson K Albinsson B Mårtensson J 《The Journal of organic chemistry》2006,71(4):1677-1687
In this paper, our attempts to optimize the Heck alkynylation (copper-free Sonogashira) reaction are presented. An efficient copper-free coupling protocol was needed for the synthesis of gold/zinc porphyrin dimers because previous methods had failed. Previous studies have usually focused on ligands, whereas this work focuses on the choice of solvent and base. The catalytic system throughout the investigation was formed from the stable precursor [Pd2(dba)3.CHCl3] together with the ligand triphenylarsine, an easy-to-handle, air-stable ligand. A model study was conducted to examine the dependence of the Heck alkynylation on the solvent and base. The most successful modification proved to be the addition of methanol, as a cosolvent, in combination with a nucleophilic tertiary base. The success of the methanol additive is hypothesized to be caused by the presence of a rate-determining deprotonation step featuring a charge-separated transition state. Finally, the very high yielding and successful synthesis of a series of porphyrin systems using these new conditions is presented. For the first time, gold porphyrin substrates could efficiently be coupled in Heck alkynylation reactions. 相似文献
996.
In insulators, the method of Marzari and Vanderbilt [Phys. Rev. B 56, 12 847 (1997)] can be used to generate maximally localized Wannier functions whose centers are related to the electronic polarization. In the case of layered insulators, this approach can be adapted to provide a natural definition of the local polarization associated with each layer, based on the locations of the nuclear charges and one-dimensional Wannier centers comprising each layer. Here, we use this approach to compute and analyze layer polarizations of ferroelectric perovskite superlattices, including changes in layer polarizations induced by sublattice displacements (i.e., layer-decomposed Born effective charges) and local symmetry breaking at the interfaces. The method provides a powerful tool for analyzing the polarization-related properties of complex layered oxide systems. 相似文献
997.
The preparation and application of dynamically coated ligand-exchange chromatography phases for enantioseparation is described. The phases were prepared by pumping a solution of N-decyl-L-4-hydroxyproline, N-hexadecyl-L-4-hydroxyproline, or N-2-hydroxydodecyl-L-4-hydroxyproline through a commercially available monolithic RP-18 column. These coatings are stable against desorption for months at ambient temperature when aqueous mobile phases are used. The columns were applied to the chiral separation of amino acids, glycyl dipeptides and diastereomeric dipeptides, and tripeptides. The chiral selector can be removed or changed easily by washing the column with ACN or methanol. Ultrafast separations in the range of seconds were achieved using high flow rates. 相似文献
998.
Karin Halupczok 《Journal of Number Theory》2006,117(2):292-300
For
999.
Karin Cvetko-Vah 《Semigroup Forum》2006,73(2):267-272
Skew lattices form a class of non-commutative lattices. Spinks' Theorem [Matthew Spinks, On middle distributivity for skew
lattices, ] states that for symmetric skew lattices the two distributive identities
and
are equivalent. Up to now only computer proofs of this theorem have been known. In the present paper the author presents
a direct proof of Spinks' Theorem. In addition, a new result is proved showing that the assumption of symmetry can be omitted
for cancellative skew lattices. 相似文献
1000.
Gary Chinga-Carrasco Per Olav Johnsen Karin ?yaas 《Micron (Oxford, England : 1993)》2010,41(6):648-659
Wood fibres have been utilized by our society as an important component of paper products and are presently gaining more interest as reinforcement in composite materials. During the last decades biochemical treatments have also found applications in the processing of wood fibres. The chemical, mechanical and biochemical treatments affect the morphology of the fibre wall structure at the micro- and nano-level. In this study, we present a modern approach where field-emission SEM (FE-SEM) and relevant computerized image analysis are applied to quantify the fibre wall characteristics. Details such as surface roughness and texture of the fibre walls are quantified objectively. Global polar plots are generated, which are considered to represent the fingerprint of a given pulp. The approach offers a novel perspective in the characterisation of surface structures, moving forward from performing subjective evaluations to performing objective quantifications of wood pulp fibre surfaces. 相似文献