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排序方式: 共有426条查询结果,搜索用时 281 毫秒
381.
Nik Norulaini Nik Abd. Rahman Salman Zhari Md. Zaidul Islam Sarker Sahena Ferdosh Mohd Azizi Che Yunus Mohd Omar Abd Kadir 《中国化学会会志》2012,59(4):507-514
A rapid identification, classification and discrimination tool, using Fourier Transform Infrared (FTIR) spectroscopy combined with Principal Component Analysis (PCA), was developed and applied to determine the profile of the Supercritical Fluid Extraction (SFE) of Parkia speciosa seeds under various temperature and pressure conditions (313, 323, 333, 343, 353 and 363 K and 20.68, 27.58, 34.47, 41.37, 48.26, and 55.16 MPa). The separation and identification of the compounds was carried out by Gas Chromatography coupled with Time of Flight Mass Spectrometry (GC/TOF‐MS). This technique has made it possible to detect the variability obtained under different SFE conditions and the separation of different chemical compounds in P. speciosa seeds. The FTIR‐PCA results were verified by GC/TOF‐MS, and the FTIR‐PCA method successfully identified the unsaturated carboxylic acids with the highest percentage area under the different conditions. 相似文献
382.
Meryem Fistikci Ozlem Gundogdu Derya Aktas Hasan Secen M. Fethi Sahin Ramazan Altundas Yunus Kara 《Tetrahedron》2012,68(12):2607-2610
A new synthetic strategy for (R)- and (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butanoic acid, a building block in the preparation of sitagliptin and its derivatives, was developed. Pd(OAc)2 catalyzed coupling of 2,4,5-trifluoro-1-iodobenzene with allyl alcohol gave 3-(2,4,5-trifluorophenyl)propanal in a yield of 95%. l-Proline catalyzed reaction of the 3-phenylpropanal (in only 1.2 molar equiv) with nitrosobenzene followed by reduction with NaBH4 and Pd/C catalyzed hydrogenation gave (R)-3-(2,4,5-trifluorophenyl)propane-1,2-diol with >99% ee and 65% yield. Selective tosylation of primary hydroxyl group of the 1,2-propandiol unit followed by cyanide displacement afforded (R)-3-hydroxy-4-(2,4,5-trifluorophenyl)butanenitrile (80%). The nitrile was converted to the title β-hydroxy acid under basic hydrolysis in a yield of 90%. Thus, (R)-3-hydroxy-4-(2,4,5-trifluorophenyl)butanoic acid was prepared enantioselectively from the starting material in four steps and 45% overall yield. The reaction sequence was repeated with d-proline as the catalyst to give (S)-3-hydroxy-4-(2,4,5-trifluorophenyl)butanoic acid in 45% overall yield and >99% enantiomeric excess. 相似文献
383.
Türkmen YE Montavon TJ Kozmin SA Rawal VH 《Journal of the American Chemical Society》2012,134(22):9062-9065
A highly effective silver-catalyzed formal inverse electron-demand Diels-Alder reaction of 1,2-diazines and siloxy alkynes has been developed. The reactions provide ready access to a wide range of siloxy naphthalenes and anthracenes, which are formed in good to high yields, under mild reaction conditions, using low catalyst loadings. 相似文献
384.
The growth of Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylate), also known as CuBTC and HKUST-1, Metal-Organic Framework (MOF) nanostructures on silk fibers were achieved by layer-by-layer technique in alternating bath of Cu(OAc)2·2H2O and H3BTC solutions under ultrasound irradiation. The effect of pH, reaction time, ultrasound irradiation and sequential dipping steps in growth of the CuBTC Metal-Organic Framework nanostructures has been studied. These systems depicted a decrease in the size accompanying a decrease in the sequential dipping steps. In addition, dense coating of silk fibers with CuBTC MOF results in decrease the emission intensity of silk fibers. The silk fibers containing CuBTC Metal-Organic Framework exhibited high antibacterial activity against Escherichia coli and Staphylococcus aureus. The samples were characterized with powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) spectra and scanning electron microscopy (SEM). XRD analyses indicated that the prepared CuBTC MOF nanostructures on silk fibers were crystalline. 相似文献
385.
Saeideh Ebrahimiasl Wan Md. Zin Wan Yunus Zulkarnain Zainal Anuar Kassim 《Optical and Quantum Electronics》2012,43(11-15):129-136
Hybrid photovoltaic structures based on transparent conductive SnO2 and electrically conductive polypyrrole (PPy) were prepared. Nanocrystalline SnO2 is considered an n-type barrier and window layer on p-type PPy layer in cell structures. The surface morphology and thickness of the layers were studied using scanning electron microscopy. The optical absorbance data showed an increase of absorbance in contrast with PPy and SnO2. There was a red shift in absorbance wavelengths and a decrease in band gaps for the prepared PV structures. To investigate the electrical properties of the obtained structures, current-voltage characteristic was measured. The best structure showed an open-circuit voltage of 0.170?V, a short-circuit current density of 0.017?mA/cm2, a fill factor of 0.36 and power conversion efficiency of 0.076. 相似文献
386.
In a globalised world where risks spread through contagion, the decision of an entity to invest in securing its premises from stochastic risks no longer depends solely on its own actions but also on the actions of other interacting entities in the system. This phenomenon is commonly seen in many domains including airline, logistics and computer security and is referred to as Interdependent Security (IDS). An IDS game models this decision problem from a game-theoretic perspective and deals with the behavioural dynamics of risk-reduction investments in such settings. This paper enhances this model and investigates the spatio-temporal aspects of the IDS games. The spatio-temporal dynamics are studied using simple replicator dynamics on a variety of network structures and for various security cost tradeoffs that lead to different Nash equilibria in an IDS game. The simulation results show that the neighbourhood configuration has a greater effect on the IDS game dynamics than network structure. An in-depth empirical analysis of game dynamics is carried out on regular graphs, which leads to the articulation of necessary and sufficient conditions for dominance in IDS games under spatial constraints. 相似文献
387.
Hai-Ning Liang Kamran Sedig 《International Journal of Computers for Mathematical Learning》2010,15(3):191-224
Many epistemic activities, such as spatial reasoning, sense-making, problem solving, and learning, are information-based.
In the context of epistemic activities involving mathematical information, learners often use interactive 3D mathematical
visualizations (MVs). However, performing such activities is not always easy. Although it is generally accepted that making
these visualizations interactive can improve their utility, it is still not clear what role interaction plays in such activities.
Interacting with MVs can be viewed as performing low-level epistemic actions on them. In this paper, an epistemic action signifies
an external action that modifies a given MV in a way that renders learners’ mental processing of the visualization easier,
faster, and more reliable. Several, combined epistemic actions then, when performed together, support broader, higher-level
epistemic activities. The purpose of this paper is to examine the role that interaction plays in supporting learners to perform
epistemic activities, specifically spatial reasoning involving 3D MVs. In particular, this research investigates how the provision
of multiple interactions affects the utility of 3D MVs and what the usage patterns of these interactions are. To this end,
an empirical study requiring learners to perform spatial reasoning tasks with 3D lattice structures was conducted. The study
compared one experimental group with two control groups. The experimental group worked with a visualization tool which provided
participants with multiple ways of interacting with the 3D lattices. One control group worked with a second version of the
visualization tool which only provided one interaction. Another control group worked with 3D physical models of the visualized
lattices. The results of the study indicate that providing learners with multiple interactions can significantly affect and
improve performance of spatial reasoning with 3D MVs. Among other findings and conclusions, this research suggests that one
of the central roles of interaction is allowing learners to perform low-level epistemic actions on MVs in order to carry out
higher-level cognitive and epistemic activities. The results of this study have implications for how other 3D mathematical
visualization tools should be designed. 相似文献
388.
A series of asymmetrically substituted phthalocyanines conjugated to four different carbohydrate units has been designed to be used as photosensitisers for potential selective recognition by targeted cells. 相似文献
389.
Kamran M. Dadkhah Fatemeh Zahedi 《The Journal of the Operational Research Society》1986,37(7):695-704
All existing tests and criteria for model evaluation are based on the sum of squared residuals. These measures do not evaluate the performance of a model in predicting turning points in the variable of interest. Here, three tests are proposed to evaluate models on the basis of their ability to predict turning points and to give an unbiased prediction of the relative order of magnitude of changes. Five models are evaluated using the proposed tests. Results indicate that the tests expose facets of models that R2 and sum of squared residuals are unable to highlight. Hence, the tests are suggested to complement existing model evaluation techniques. 相似文献
390.
Yunus KaraMetin Balci 《Tetrahedron》2003,59(12):2063-2066
The photooxygenation of trans-8-(acetyloxy)bicyclo[4.2.0]octa-2,4-dien-7-yl acetate afforded the bicyclic endoperoxide. Reduction of the endoperoxide with thiourea followed by acetylation gave the corresponding tetraacetate. The KMnO4 oxidation of the tetraacetate followed by acetylation gave dihydroxytetraacetate Ammonolysis of tetraacetate afforded the bis-homoinositol, bicyclo[4.2.0]octane-2,3,4,5,7,8-hexol. 相似文献