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891.
Kobelnik Marcelo Ferreira Leonardo Miziara Barboza Regasini Luis Octavio Dutra Luiz Antonio da Silva Bolzani Vanderlan Ribeiro Clóvis Augusto 《Journal of Thermal Analysis and Calorimetry》2018,132(1):425-431
Journal of Thermal Analysis and Calorimetry - Chalcones α-β-unsaturated ketones are found in large plant species. Synthesis of chalcones and its three analogues hydroxy group at 2′,... 相似文献
892.
da Silva José Eduardo Estevam Alarcon Rafael Turra Gaglieri Caroline Magdalena Aroldo Geraldo da Silva-Filho Luiz Carlos Bannach Gilbert 《Journal of Thermal Analysis and Calorimetry》2018,133(3):1455-1462
Journal of Thermal Analysis and Calorimetry - This work investigates the thermal polymerization process of a methylene diphenyl diisocyanate (MDI) monomer as well as its thermal degradation... 相似文献
893.
Palladium solutions can be titrated visually and spectrophotometrically with sodium ethylene-bis (thioglycolate). Titrations were done in 2.5 M perchloric acid with nitroso-R salt as indicator. Palladium was determined in the range 5.0–30 mg (visual titration) and 2.0–10 mg (spectrophotometric titration): the tolerance for diverse ions is good, particularly with the spectrophotometric end-point. The probable structure of the isolated complex is given. 相似文献
894.
Alécio Antonio Pimenta Jr. Sílvia Ribeiro de Souza e Silva Grácia Divina de Fátima Silva Luiz Cláudio de Almeida Barbosa Javier Ellena Antonio Carlos Doriguetto 《Structural chemistry》2006,17(1):149-153
A pentacyclic triterpene, 3β,30-dihydroxy-lup-20(29)-ene, was isolated from the powder extract of Maytenus imbricata. The structure and stereochemistry of the compound were established by spectroscopic techniques and unambiguously determined by single crystal X-ray crystallography. The crystal structure shows one molecule in the asymmetric unit. The crystal packing is stabilized by O–H···O intermolecular hydrogen bonds, which give rise to infinite helical chains along the c unit cell axis. The intra-molecular geometry was analyzed using MOGUL, a knowledge base of molecular geometry derived from the Cambridge Structural Database (CSD). 相似文献
895.
We establish in this work sufficient conditions for the existence of periodic solutions for the Liénard equation
. 相似文献
896.
Boscolo M Andrade-Sobrinho LG Lima-Neto BS Franco DW Ferreira MM 《Journal of AOAC International》2002,85(3):744-750
A new methodology was developed for determination of caramel in spirits aged in oak casks. The method is based on differences between the electronic spectra of oak aqueous alcoholic extracts and caramel solutions in the same solvent. The data were treated by 2 different approaches: the simplest one was based on the plot of caramel concentration versus the ratio of absorbance at 210 and 282 nm; the other was based on a partial least squares (PLS) calibration model using the first derivative of the spectral data. Both methodologies were applied to analysis of 159 aged spirit samples. The mean caramel content of several Brazilian sugar cane spirits (cacha?a) and all United States whiskies was smaller than that of Scottish whiskies and other brandies from several countries. Correlation was good between caramel concentrations for the same sample calculated by the 2 methods. The uncertainties following PLS and the absorbance ratio method were 0.01 and 0.03 g/L, respectively, for a sample containing 0.45 g/L caramel. Treatment of UV-VIS spectra by pattern recognition using hierarchical clustering analysis and principal components analysis allowed discrimination of the samples as a function of their caramel content. It was possible to distinguish U.S. whiskies from other whiskies, but a clear differentiation among Brazilian cacha?as as a function of their geographic origin was not feasible. Small caramel quantities as low as 0.08 g/L were clearly detected by these methodologies. 相似文献
897.
Solutions of iodobenzene diacetate in CH3CN, AcOH, MeOH/H2O and MeOH (with or without base) were analyzed by electrospray ionization mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). The major species in CH3CN, AcOH and MeOH/H2O solutions are [PhI(OAc)2Na]+, [PhI(OAc)2K]+, [PhI]+, [PhIOAc]+, [PhIOH]+, [PhIO2Ac]+, [PhIO2H]+ and the dimer [Ph2I2O2Ac]+. On the other hand, MeOH solutions showed [PhIOMe]+ as the most abundant species. A similar result was observed adding 2, 3 or 4 equiv of KOH to MeOH solutions. However, in the presence of 10 equiv of KOH, probably occurs the formation of a polymeric material. Similarly, ESI-MS analysis of the MeOH solution of iodobenzene bis(trifluoroacetate) showed a pattern analogous to that observed for the corresponding solutions of PhI(OAc)2. However, ESI-MS spectral data of PhI(O2CCF3)2 of CH3CN suggests that the main species in solutions is iodosylbenzene, which contrasts with the results obtained for PhI(OAc)2. 相似文献
898.
da Silva Filho LC Lacerda Júnior V Constantino MG da Silva GV Invernize PR 《Beilstein journal of organic chemistry》2005,1(1):14
We have found that some of the usually poor dienophiles (2-cycloenones) can undergo Diels-Alder reaction at -78 degrees C with unusually high stereoselectivity in the presence of niobium pentachloride as a Lewis acid catalyst. A remarkable difference in reaction rates for unsubstituted and alpha- or beta-methyl substituted 2-cycloenones was also observed. 相似文献
899.
Luiz Claudio de Santa Maria Mônica R.M.P Aguiar Marcia C.V Amorim Raphael S.M Almeida Ana J.B Oliveira 《European Polymer Journal》2003,39(2):291-296
A series of porous copolymer beads based on methacrylamide (MA), styrene and divinylbenzene (DVB) was prepared by aqueous suspension polymerization in the presence of diluents to act as precipitants. As MA is totally soluble into aqueous phase, the use of two types of phase transfer agent was investigated, namely: TritonX-100TM [polyoxyethylene-(10)-isooctylphenyl-ether] and tetrabutylammonium perchlorate. The effect of the diluents on the surface appearance and the porous structure of copolymer beads was studied. Suitable particle stabilisation was achieved by using a combination of two suspension agents, namely: gelatin and 2-hydroxy-ethyl-cellulose. The network resins were characterized by optical and scanning electron microscopy, IR spectroscopy, elemental analysis (CHN), apparent density, swelling in different solvents and specific area by BET method. It was observed that the MA incorporation was more effective when TritonTM was employed as phase transfer agent. 相似文献
900.
[structure: see text] The asymmetric total synthesis of pironetin, a compound that shows plant growth regulatory activity, immunosuppressive as well as a remarkable antitumoral activity, is described. The approach involves the use of three very efficient Evans oxazolidinone-mediated syn-aldol condensations, a high-yielding coupling between lithium acetylide ethylenediamine complex and a tosylate followed by methylation, and selective reduction to establish the C12-C13 (E) double bond. 相似文献