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31.
32.
Jose A. López‐Barrera Alejandro Avila‐Ortega Juan Morales Jorge Cervantes Roberto Olayo 《应用有机金属化学》2007,21(10):858-861
The surface of quarry stone was modified by continuous plasma polymerization of hexamethyldisiloxane. The hydrophilic surface of the quarry stone was made hydrophobic and impermeable to water. Three different reaction times were analyzed. All of them resulted in the formation of a homogenous layer on the quarry stone surface. Contact angle and FT‐IR analyses show that the hydrophobic character of the surface is due to methyl groups on the surface. The change in the contact angle with temperature and the wetting temperature (Tw) are also discussed. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
33.
Elena E. Stashenko Martha Cervantes Yajaira Combariza Hernn Fuentes Jairo Ren Martínez 《Journal of separation science》1999,22(6):343-349
Steam distillation (SD), simultaneous distillation-solvent extraction (SDE), microwave-assisted solvent extraction (MWE), and supercritical (CO2) extraction (SFE) were used to isolate secondary metabolites from Lepechinia schiedeana. The various extracts were analyzed by capillary gas-chromatography, on poly (dimethylsiloxane) (DB-1) and poly(ethyleneglycol) (INNOWAX), 60 m columns, using FID or MSD (EI, 70 eV). Kováts indexes, mass spectra, or standard compounds were employed for compound identification. 43, 61, 67, and 79 compounds at concentrations above 0.01% were detected in the SD, SDE, MWE, and SFE extracts, respectively. Ledol, C15H26O, was the major constituent (20.04–36.87%) in all extracts. Oxygenated sesquiterpenes (24.36–43.14%), C10H16, monoterpenes (27.70–39.87%), and C15H24, sesquiterpenes (10.04–22.22%) were the main groups of compounds present in SD, SDE, MWE, and SFE extracts. Heavy hydrocarbons (Cn > 15), diterpenoids, and phytosterols were found only in MWE and SFE extracts. The antioxidant activity of Lepechinia schiedeana was measured by the HRGC quantification of the volatile carbonyl compounds, final products of lipoxidation, released in a model lipid system (sunflower oil) by the effect of the Fenton reagent. The concentration of volatile carbonyl compounds decreased by 65% when lipid oxidation was induced in the presence of macerated Lepechinia plant. The protection of polyunsaturated acids in sunflower oil was also studied by measuring their concentrations after heating of the oil (180°C, 2 h) with and without macerated Lepechinia plant. 相似文献
34.
A. C. D. de Medeiros N. A. B. de Cervantes A. P. B. Gomes R. O. Macêdo 《Journal of Thermal Analysis and Calorimetry》2001,64(2):745-750
TG and DSC data were used to determine the thermal parameters of prednisone drug and tablets. Two formulations of prednisone
20 mg were analysed in the form of tablets. The TG curves of prednisone drug and tablets A and B displayed six, eight and
seven thermal decomposition processes, respectively. Analysis of the DSC data pointed to chemical interactions between prednisone
drug and the excipients of tablets A and B, suggested by alterations in the melting temperature of prednisone. The analysis
revealed that prednisone drug is more stable than tablets A and B.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
35.
Jorge Cervantes Guillermo Mendoza-Díaz Dolores Elena Alvarez-Gasca Antonio Martinez-Richa 《Solid state nuclear magnetic resonance》1999,13(4):481-269
We report the application of 29Si and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) studies on building stones from historical monuments to obtain direct information about the degree of degradation and to observe the changes in the consolidated material after treatment with tetraethoxysilane (TEOS). Using the data obtained from deteriorated materials, a diagnostic laboratory, a suitable treatment and recommendations for building conservation may infer. A case study is presented using stones from Guanajuato City Main Church (Central Mexico). X-ray diffraction patterns to characterize the species present in the stones agree with the solid state NMR results. 相似文献
36.
Synthesis and Characterization of m‐Terphenyl (1,3‐Diphenylbenzene) Compounds Containing Trifluoromethyl Groups
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Alma T. Corona‐Armenta Mario Ordoñez Jorge A. López Jorge Cervantes Oracio Serrano 《Helvetica chimica acta》2015,98(3):359-367
The bis(silyl)triazene compound 2,6‐(Me3Si)2‐4‐Me‐1‐(N?N? NC4H8)C6H2 ( 4 ) was synthesized by double lithiation/silylation of 2,6‐Br2‐4‐Me‐1‐(N?N? NC4H8)C6H2 ( 1 ). Furthermore, 2,6‐bis[3,5‐(CF3)2‐C6H3]‐4‐Me‐C6H2‐1‐(N?N? NC4H8)C6H2 derivative 6 can be easily synthesized by a C,C‐bond formation reaction of 1 with the corresponding aryl‐Grignard reagent, i.e., 3,5‐bis[(trifluoromethyl)phenyl]magnesium bromide. Reactions of compound 4 with KI and 6 with I2 afforded in good yields novel phenyl derivatives, 2,6‐(Me3Si)2‐4‐MeC6H2? I and 2,6‐bis[3,5‐(CF3)2? C6H3]‐4‐MeC6H2? I ( 5 and 7 , resp.). On the other hand, the analogous m‐terphenyl 1,3‐diphenylbenzene compound 2,6‐bis[3,5‐(CF3)2? C6H3]C6H3? I ( 8 ) could be obtained in moderate yield from the reaction of (2,6‐dichlorophenyl)lithium and 2 equiv. of aryl‐Grignard reagent, followed by the reaction with I2. Different attempts to introduce the tBu (Me3C) or neophyl (PhC(Me)2CH2) substituents in the central ring were unsuccessful. All the compounds were fully characterized by elemental analysis, melting point, IR and NMR spectroscopy. The structure of compound 6 was corroborated by single‐crystal X‐ray diffraction measurements. 相似文献
37.
Dividing chains have been used as conditions to isolate adequate subclasses of simple theories. In the first part of this
paper we present an introduction to the area. We give an overview on fundamental notions and present proofs of some of the
basic and well-known facts related to dividing chains in simple theories. In the second part we discuss various characterizations
of the subclass of low theories. Our main theorem generalizes and slightly extends a well-known fact about the connection
between dividing chains and Morley sequences (in our case: independent sequences). Moreover, we are able to give a proof that
is shorter than the original one. This result motivates us to introduce a special property of formulas concerning independent
dividing chains: For any dividing chain there exists an independent dividing chain of the same length. We study this property
in the context of low, short and ω -categorical simple theories, outline some examples and define subclasses of low and short theories, which imply this property.
The results give rise to further studies of the relationships between some subclasses of simple theories.
Research supported by CNPq grant 150309/2003-1.
Research supported by CNPq grant 304365/2003-3 (Modelos, Provas e Algoritmos) 相似文献
38.
Jorge L. Arocha Imre Bárány Javier Bracho Ruy Fabila Luis Montejano 《Discrete and Computational Geometry》2009,42(2):142-154
We prove several colorful generalizations of classical theorems in discrete geometry. Moreover, the colorful generalization
of Kirchberger’s theorem gives a generalization of the theorem of Tverberg on non-separated partitions. 相似文献
39.
F.A. Hopf A. Tomita G. Al-Jumaily M. Cervantes T. Liepmann 《Optics Communications》1981,36(6):487-490
Experiments on interferometers based on second harmonic generation of light are described. These make use of the distortions of phase and amplitude produced by the phase mismatch of angle-matched crystals to provide information. The interferometers are directly sensitive to small wavefront tilts and do not require additional reference wavefronts. 相似文献
40.
P Cervantes Z SlanicF Bridges E KnittleQ Williams 《Journal of Physics and Chemistry of Solids》2002,63(10):1927-1933
The indirect energy gap and electrical resistivity of FeS2-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS2 is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10−2 eV/GPa. If this linear trend continues, FeS2 is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects. 相似文献