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991.
992.
We introduce and study vertex cover algebras of weighted simplicial complexes. These algebras are special classes of symbolic Rees algebras. We show that symbolic Rees algebras of monomial ideals are finitely generated and that such an algebra is normal and Cohen-Macaulay if the monomial ideal is squarefree. For a simple graph, the vertex cover algebra is generated by elements of degree 2, and it is standard graded if and only if the graph is bipartite. We also give a general upper bound for the maximal degree of the generators of vertex cover algebras. 相似文献
993.
The field dependence of the nuclear spin-lattice relaxation (SLR) of cold implanted 82Br (T ≤ 25 mK) in α-Fe single crystals was investigated with nuclear magnetic resonance of oriented nuclei (NMR/ON) at low temperatures
as experimental technique. The SLR at the lattice sites with the hyperfine fields found by earlier NMR/ON experiments was
measured as a function of the applied external magnetic field B
ext parallel to the three principle axes [100], [110] and [111] of the iron single crystal. The data were evaluated with the
full relaxation formalism in the single impurity limit and for comparison also with the often employed model of a single exponential function with an effective relaxation time
T
1′. With a phenomenological model the high field values of the relaxation rates r
∞, [100]′ = 6.6(2) · 10−15 T2sK−1, r
∞, [110] = 5.4(2) · 10−15 T2sK−1 and r
∞, [111] = 5.2(1) · 10−15 T2sK−1 were obtained. 相似文献
994.
Emanuela De Negri Jü rgen Herzog 《Proceedings of the American Mathematical Society》1998,126(12):3467-3473
In this paper we study ideals which are generated by lexsegments of monomials. In contrast to initial lexsegments, the shadow of an arbitrary lexsegment is in general not again a lexsegment. An ideal generated by a lexsegment is called completely lexsegment, if all iterated shadows of the set of generators are lexsegments. We characterize all completely lexsegment ideals and describe cases in which they have a linear resolution. We also prove a persistence theorem which states that all iterated shadows of a lexsegment are again lexsegments if the first shadow has this property.
995.
Tsvetomir Tsonev Michael Herzog Sanchi Nenkova 《Central European Journal of Chemistry》2013,11(12):2058-2065
Shape memory polyurethanes (SMPUs) were synthesized by 4,4′-diphenylmethane diisocyanate (MDI), hexane-1,6-diol (HD), polypropylene glycol (PPG), and recycled polyvinyl butyral (PVB). Dynamic mechanical analysis, differential scanning calorimetry and Fourier transformation infrared attenuated total reflection spectroscopy was used to characterize the poly (vinylbutyral-urethanes). Micro-phase domain separation of hard and soft segments and phase inversion were investigated. Increasing the hard segment content, i.e., average hard segment molecular weight, leads to an increase in the degree of micro-phase separation, hard domain order and crystallinity. The crystalline hard segment structures combined with the elastic nature of soft segment matrix provide enough physical and chemical crosslinks to have shape memory effect. 相似文献
996.
Talita T. Silva Yan F. Silva Antonio E. H. Machado Pedro Ivo S. Maia Carlos R. B. Tasso Benedito S. Lima-Neto 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(12):1132-1140
AbstractNickel(II) complexes of Schiff base derived from cycloalkylamines (cycloalkyl?=?cyclopentyl (cpen), cyclohexyl (chex), and cycloheptyl (chep)) were synthesized: [NiII(Lcpen)2] (1), [NiII(Lchex)2] (2), and [NiII(Lchep)2] (3). The Schiff base-NiII complexes 1-3 were characterized by FTIR, UV–Vis, elemental analysis, and computational methods. Electrodeposited films of complexes 1–3 were obtained by potential cycling CH2Cl2 on platinum electrode, and their electrochemical behavior were characterized by cyclic voltammetry. The polymerization of vinyl acetate (VAc) initiated by azobisisobutyronitrile (AIBN) at 55?°C were conducted according to an organometallic-mediated radical polymerization (OMRP) mechanism. The best conversions were obtained using a ratio of [VAc]/[AIBN]/[Ni]?=?542/3.25/1, reaching 50, 69 and 85% in 12?h for 1, 2 and 3, respectively. The kinetic of polymerization mediated by complex 1 exhibited a linear dependence of ln([VAc]0/[VAc]) versus time, supporting a constant radical concentration; while for the complexes 2 and 3, the radical concentration was constant for a short period of time. The increase of molecular weights with the conversion coupled with low polydispersities indicate a certain level of control of the polymerization when using the complexes [NiII(LR)2] as controlling agents. 相似文献
997.
Dr. Julia Merz Maximilian Dietz Yvonne Vonhausen Frederik Wöber Dr. Alexandra Friedrich Dr. Daniel Sieh Dr. Ivo Krummenacher Prof. Dr. Holger Braunschweig Michael Moos Dr. Marco Holzapfel Prof. Dr. Christoph Lambert Prof. Dr. Todd B. Marder 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(2):438-453
We synthesized new pyrene derivatives with strong bis(para-methoxyphenyl)amine donors at the 2,7-positions and n-azaacene acceptors at the K-region of pyrene. The compounds possess a strong intramolecular charge transfer, leading to unusual properties such as emission in the red to NIR region (700 nm), which has not been reported before for monomeric pyrenes. Detailed photophysical studies reveal very long intrinsic lifetimes of >100 ns for the new compounds, which is typical for 2,7-substituted pyrenes but not for K-region substituted pyrenes. The incorporation of strong donors and acceptors leads to very low reduction and oxidation potentials, and spectroelectrochemical studies show that the compounds are on the borderline between localized Robin-Day class-II and delocalized Robin-Day class-III species. 相似文献
998.
Marina Lučić Dr. Dimitri A. Svistunenko Prof. Michael T. Wilson Dr. Amanda K. Chaplin Bradley Davy Dr. Ali Ebrahim Dr. Danny Axford Dr. Takehiko Tosha Dr. Hiroshi Sugimoto Dr. Shigeki Owada Dr. Florian S. N. Dworkowski Dr. Ivo Tews Dr. Robin L. Owen Dr. Michael A. Hough Dr. Jonathan A. R. Worrall 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(48):21840-21846
Obtaining structures of intact redox states of metal centers derived from zero dose X-ray crystallography can advance our mechanistic understanding of metalloenzymes. In dye-decolorising heme peroxidases (DyPs), controversy exists regarding the mechanistic role of the distal heme residues aspartate and arginine in the heterolysis of peroxide to form the catalytic intermediate compound I (FeIV=O and a porphyrin cation radical). Using serial femtosecond X-ray crystallography (SFX), we have determined the pristine structures of the FeIII and FeIV=O redox states of a B-type DyP. These structures reveal a water-free distal heme site that, together with the presence of an asparagine, imply the use of the distal arginine as a catalytic base. A combination of mutagenesis and kinetic studies corroborate such a role. Our SFX approach thus provides unique insight into how the distal heme site of DyPs can be tuned to select aspartate or arginine for the rate enhancement of peroxide heterolysis. 相似文献
999.
Dr. Jana Vacek Chocholoušová Dr. Jaroslav Vacek Dr. Angelina Andronova Dr. Jiří Míšek Dr. Olivier Songis Michal Šámal Dr. Irena G. Stará Dr. Michel Meyer Mélanie Bourdillon Assoc. Prof. Dr. Lubomír Pospíšil Dr. Ivo Starý 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(3):877-893
A comprehensive study on the physicochemical properties of a series of mono‐ and diaza[5]helicenes as well as mono‐ and diaza[6]helicenes is reported. Through the use of both computational and experimental methods, these helically chiral pyridohelicenes with the nitrogen atom(s) in various positions are characterised according to their inversion barriers, protonation constants and redox potentials. By using DFT calculations, kinetic measurements, UV/Vis titrations, cyclic voltammetry and EPR spectroscopy, a self‐contained picture of their behaviour under conventional treatment by heat, acids and oxidising/reducing agents is provided. 相似文献
1000.
In this article, a comprehensive overview of the most recent developments in the field of stripping voltammetry at regular micro-interfaces (both solid–liquid and liquid–liquid interfaces) is presented. This review will report on the most conventional arrays of metallic micro-electrodes but also on the rapidly growing field of electrochemistry at arrays of micro-interfaces between two immiscible electrolyte solutions (μITIES). The main fabrication methods, together with some design considerations and diffusion phenomena at such interfaces are discussed. Main applications of micro-interface arrays are presented including heavy metals detection at micro-electrode arrays and detection of organic molecules (amino acids, vitamins, peptides and drugs) at the μITIES. Stripping analysis at micro-interface arrays is suitable for the detection of analytes in several real media including water, soil extracts and biological fluids (blood and saliva) with high specificity, sensitivity (detection limits of nM, ppb level) and reliability. Stripping analysis at μITIES and micro-electrode arrays are two complementary approaches that have the advantages of being cost effective, simple to use and easily adaptable to field measurement. 相似文献