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排序方式: 共有5999条查询结果,搜索用时 15 毫秒
22.
Short syntheses of (Z)-7-dodecen-1-yl acetate, (Z)-7-tetradecen-1-yl acetate, (Z)-9-dodecen-1-yl acetate, and (Z)-9-tetradecen-1-yl acetate from 7-hydroxyheptanal and 9-oxononanoic acid precursors obtained by oxidative cleavage of easily available aleuritic acid are reported. The key step in these syntheses is a stereoselective Wittig reaction between aldehyde and alkyl-phosphonium salt. Wittig-Horner type reaction of 7-hydroxyheptanal and diethyl cyanomethylphosphonate gave the ,-unsaturated nitrile derivative which after protection of the hydroxyl group was reduced to the corresponding aldehyde. Wittig reaction of the latter, followed by acetylation, completed the synthesis of (E,Z)-7,9-dodecadien-1-yl acetate, the sex pheromone of the European grapevine mothLobesia botrana Schiff. 相似文献
23.
Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals in sufficient quantity to affect water purification. The theoretical and (practical yield of OH from O3 at high pH, 03/H202, O3/UV and H2O2/UV systems is reviewed. New data is presented which illustrates the importance of direct photolysis in the O3/UV process, the effect of the H202:03 ratio in the O3/H2O2 process, and the impact of the low extinction coefficient of H2O2 in the H202/UV process. 相似文献
24.
The antioxidant vitamin E, -tocopherol, was tested as a candidate elicitor of alterable antiherbivory in soybean plants against cabbage looper larvae. Although a nonspecific antioxidant, vitamin E proved elicitory to the involved sulfhydryl-dependent receptor-energy transducer protein in soybean plasma membrane. Effects of -tocopherol were dependent on dosage, time, and space in the plant. The observed elicited effects were all decreases in herbivory. The best negative phytochemical correlate of looper feeding was the percentage of increased total HPLC peak area of extractables from elicited as compared to nonelicited leaves. Some specific compounds, e.g., glyceollins, were quantitatively major components of the total profile of secondary metabolites. 相似文献
25.
细菌觅食优化算法作为一种新兴的智能优化算法,一般用来解决连续域的问题。为了解决离散域问题,提出了一种改进的细菌觅食优化算法。采用线性递减的思想和随机的游动长度代替固定步长和随机游动方向,改进了趋向性操作方案,并将其应用于解决0-1背包问题。将改进的细菌觅食优化算法与遗传算法、离散粒子群优化算法及基本的离散化细菌觅食优化算法分别在小规模和大规模的0-1背包问题上进行了仿真比较,表明了改进的细菌觅食优化算法能取得较好的效果,寻优能力强。 相似文献
26.
Ya-Ping Sun Robert D. Miller R. Sooriyakumaran Josef Michl 《Journal of Inorganic and Organometallic Polymers》1991,1(1):3-35
A comprehensive study of the electronic absorption and emission spectra and the fluorescence quantum yield and lifetime of seven poly(di-n-alkylsilane)s and of three isotopically labelled poly(di-n-hexylsilane)s in hydrocarbon solution at room temperature is reported. Also reported are fluorescence polarization and carbon tetrachloride quenching of fluorescence of poly(di-n-hexylsilane). The observed fluorescence spectra, quantum yield, and polarization depend on the selected excitation energy in a very characteristic fashion, whereas the fluorescence lifetime does not; however, it depends on the selected emission energy. These characteristic dependencies are qualitatively accounted for by the previously proposed segment distribution model if one assumes that the photophysical behavior at higher excitation energies is strongly affected by the presence of a low-lying weakly allowed state in short-segment chromophores and the behavior at lower excitation energies is dictated by the selective excitation of emitting long-segment chromophores. 相似文献
27.
In-situ EXAFS studies of sulphided Mo/TiO2 catalysts have shown that at low Mo loadings (2–4 wt%), an active species with a short Mo-S distance of 2.25 Å is formed, while on Mo/TiO2 with high Mo loadings as well as on Mo/-Al2O3, bulk MoS2 (Mo-S, 2.42 Å) is formed. The species with the short Mo-S distance has Mo in an oxidation state close to 6 + and is likely to result from the sulphidation of the tetrahedral molybdate species present in the oxidic precursor at low Mo loadings. The calcination temperature of the oxidic precursor appears crucial, a high calcination temperature of 973 K favouring the formation of MoS3 on sulphidation, and a low calcination temperature of 623 K favouring MoS2.Contribution No. 790 from the Solid State and Structural Chemistry Unit. 相似文献
28.
求解0-1背包问题(KP)的最优解的时候,传统遗传算法(GA)的局部求精能力不足而简单局部搜索算法的全局探索能力有限,针对上述问题,将这两个算法整合并提出了混合贪婪遗传算法(HGGA)。在GA全局搜索框架下增加局部搜索模块,并改进传统仅基于物品价值密度的修复算子,增加基于物品价值的贪婪混合选项,从而加速寻优过程。HGGA一方面引导种群在进化的优质解空间中展开精细搜索,另一方面依靠GA的经典操作算子开拓全局搜索空间,从而达到算法求精能力和开拓能力的良好平衡。HGGA分别在三组数据上做了测试,结果表明在第一组15个测试用例中的12个上,HGGA能够百分百找到最优解,成功率达到80%;在第二组小规模数据集上,HGGA的性能明显好于其他同类GA和其他元启发算法;在第三组大规模数据集上,HGGA较其他元启发式算法具有更好的稳定性和高效性。 相似文献
29.
The oxidation behavior of a normalized 2.25Cr-1Mo steel tempered previously for 10 hr at different temperatures between 873 and 1023 K has been studied up to a maximum duration of 1000 hr in air at 773–973 K. The oxidation resistance of the steel was found to decrease significantly with the temperature of tempering. Tempering of this steel is reported to cause microstructural changes involving precipitation of Cr as carbides and a decrease in the effective (free) Cr contents, that could influence the oxidation resistance of the Cr-containing alloys. Relative compositions across the thickness of the oxide scales, as analyzed by SEM/EDX and SIMS, suggest that a less Cr-rich (and less protective) and thicker scale on the steel formed because previous tempering caused extensive depletion of free Cr. 相似文献
30.
D. Banerjee G. K. Lai G. S. Upadhyaya 《Journal of Materials Engineering and Performance》1995,4(5):563-572
For production of fine-grained and corrosion-resistant tungsten carbide (WC) based cemented carbides, addition of chromium
carbide (Cr33C2) in small amounts is standard practice. No systematic study, however, has been made of the effects of large additions (maximum
6 wt % ) of Cr3C2 as a substitute for tungsten carbide. This study focuses on the effect of hard-phase substitution by C3C2 in WC-1OCo cemented carbide. An attempt is also made to modify the binder metal cobalt by partial or complete substitution
of nickel. Specimens were prepared using the standard liquid-phase sintering process and were tested for sintered porosity,
mechanical properties, corrosion resistance, and microstructural parameters. Results confirm the findings of earlier workers
regarding grain refinement and improvement of mechanical properties upon the addition of small amounts (<2 wt%) of Cr3C2. Modification of the binder phase improves indentation fracture toughness and corrosion resistance. Addition of Cr3C2 independent of the binder type improves corrosion resistance. 相似文献