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971.
972.
In this work, a powerful analytical method, called Liao’s homotopy analysis method is used to study the limit cycle of a two-dimensional nonlinear dynamical system, namely the van der Pol oscillator with delayed amplitude limiting. It is shown that the solutions are valid for a wide range of variation of the system parameters. Comparison of the obtained solutions with those achieved by numerical solutions and by other perturbation techniques shows that the utilized method is effective and convenient to solve this type of problems with the desired order of approximation. 相似文献
973.
Mohammad Rahimizadeh Tahmineh Bazazan Ali Shiri Mehdi Bakavoli Hassan Hassani 《中国化学快报》2011,22(4):435-438
Preyssler-type heteropoly acid is introduced as a new,mild and efficient catalyst for protection of a variety of carbonyl compounds with 1,3-propane dithiol. 相似文献
974.
Asad Ali Munawar Hussain Imran Malik Alexander Villinger Christine Fischer Peter Langer 《Helvetica chimica acta》2010,93(9):1764-1772
Double Heck cross‐coupling reactions of 2,3‐ and 3,5‐dibromopyridine with various alkenes afforded the corresponding novel di(alkenyl)pyridines. The Heck reaction of 2,5‐dibromopyridine unexpectedly afforded 5,5′‐di(alkenyl)‐2,2′‐bipyridines by palladium‐catalyzed dimerization to give 5,5′‐dibromo‐2,2′‐bipyridine and subsequent twofold Heck reaction. 相似文献
975.
Influences of Different Enzymatic Treatment on Denim Garment 总被引:1,自引:0,他引:1
In current study, the effects of different enzymatic treatment including acid cellulases, neutral cellulases, and combination
of laccases with cellulases on denim garment were investigated. The color changes of different samples were compared by colorimetric
indices for the garment surface and back and also white pocket. Abrasion resistance, tensile strength, and crease recovery
angle of the samples were measured within the standard methods. Also, surfaces of the treated samples were observed by SEM.
The results showed that the combination of laccases with cellulases help to improve the lightness and decrease staining on
both back of garment and on white pocket. 相似文献
976.
The ordinary generalized geometric–arithmetic index of graphs is introduced and some properties especially lower and upper bounds in terms of other graph invariants and topological indices are obtained. 相似文献
977.
978.
The aminocarbonylation and alkoxycarbonylation reactions of terminal alkynes took place smoothly and efficiently using a catalyst system Pd(OAc)2–dppb–p‐TsOH? CH3CN? CO under relatively mild experimental conditions. The catalytic system was tested and optimized using two different nucleophiles: alcohols and amines. Phenylacetylene (1a) was considered as an alkyne along with diisobutylamine (2b1) and methanol (2c1) as nucleophiles. The results showed significant differences in the conversion of 1a and in the selectivity towards the gem or trans unsaturated esters or amides with these nucleophiles. The effects of the type of palladium catalysts, the type of ligands, the amount of dppb and the solvents were carefully studied. With diisobutylamine (2b1), excellent regioselectivity towards the 2‐acrylamides (gem isomer, 3ab1) was almost always observed, while trans‐α,β‐unsaturated esters 4ac1 was the predominant product with methanol (2c1) as a nucleophile. This remarkable sensitivity in the selectivity of the reaction indicates two different possible mechanistic pathways for these carbonylation reactions. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
979.
<正>A new flavonoidic glycoside,tricin-4'-O-β-L-arabinoside(1) was isolated from the leaves of Alstonia macrophylla along with two known flavonoids,vitexin and myricetin-3′-rhamnoside-3-O-galactoside.Their structures were established by chemical and spectral evidences.The known compounds were reported for the first time from this plant.Moreover compound 1 was tested for antifungal and antibacterial activities. 相似文献
980.
Rabea Parveen Sayeed Ahmad Sanjula Baboota Javed Ali Ahuja Alka 《Biomedical chromatography : BMC》2010,24(6):639-647
In the present study a novel stability‐indicating high‐performance thin‐layer chromatography (HPTLC) method for quantitative determination of silybin in bulk drug and nanoemulsion formulation has been developed and validated on silica using solvent chloroform–acetone–formic acid (9 : 2 : 1 v/v/v) (Rf of silybin 0.46 ± 0.05) in the absorbance mode at 296 nm. The method showed a good linear relationship (r2 ± 0.999) in the concentration range 25–1500 ng per spot. It was found to be linear, accurate, precise, specific, robust and stability‐indicating and can be applied for quality control and standardization of several multi‐component hepatoprotective formulations as well as for stability testing of different dosage forms. The method proposed was also used to investigate the kinetics of acidic and alkaline degradation processes by quantification of drug at different temperature to calculate the activation energy and half‐life for silymarin degradation. Copyright © 2009 John Wiley & Sons, Ltd 相似文献