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991.
Efficient stereospecific 4-methoxylation of both 2,3-trans- and 2,3-cis-flavan-3-ol methyl ethers with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in CHCl3-MeOH solution is of both synthetic and degradative significance in oligomeric flavanoid chemistry. 相似文献
992.
Monteiro B Roitershtein D Ferreira H Ascenso JR Martins AM Domingos A Marques N 《Inorganic chemistry》2003,42(13):4223-4231
Reaction of yttrium and lanthanide trichlorides (Ln = La, Eu, Yb) with 1 equiv of the trisodium salt of 1,4,7-tris(dimethylsilylaniline)-1,4,7-triazacyclononane (Na(3)[(SiMe(2)NPh)(3)-tacn](THF)(2)) gives good yields of the compounds [M[(SiMe(2)NPh)(3)-tacn]] (M = Y (1), Eu (3), Yb (4)) and [La[(SiMe(2)NPh)(3)-tacn](THF)] (2). Reduction of 3 with Na/Hg followed by recrystallization in the presence of diglyme yielded crystals of [Eu[(SiMe(2)NPh)(3)-tacn]][Na(diglyme)(2)] (5). Synthesis of the uranium(III) complex [U[(SiMe(2)NPh)(3)-tacn]] (6) is achieved by reaction of 1 equiv of Na(3)[(SiMe(2)NPh)(3)-tacn](THF)(2) with uranium triiodide. The U(IV) complexes, [U[(SiMe(2)NPh)(3)-tacn]X] (X = Cl (7); I (8)), were prepared via oxidation of 6 with benzyl chloride or I(2), but salt metathesis from UCl(4) provided a higher yield route for 7. The solid-state structures of 1-7 were determined by single-crystal X-ray diffraction. The ligand [(SiMe(2)NPh)(3)-tacn] generates a trigonal prismatic coordination environment for the metal center in the neutral complexes 1, 3, 4, and 6 and the ionic 5. In 2 the six nitrogen atoms of the ligand are in a trigonal prismatic configuration with the oxygen atom of the THF capping one of the triangular faces of the trigonal prism. In 7 the coordination geometry around the uranium atom is best described as bicapped trigonal bipyramidal. 相似文献
993.
Fabio de S. Dias Walter N. L. dos Santos Antônio C. S. Costa Bernhard Welz Maria Goreti R. Vale Sérgio L. C. Ferreira 《Mikrochimica acta》2007,158(3-4):321-326
A direct method is proposed for the determination of lead in naphtha and petroleum condensate by electrothermal atomic absorption
spectrometry (ET AAS) using palladium as a permanent modifier. The procedure includes the dilution of 3 mL of sample (naphtha
or petroleum condensate) to a final volume of 10 mL with xylene, and direct injection of 30 μL of this solution into the graphite
furnace. The optimization of the instrumental conditions was performed using multivariate techniques. Firstly, a 23 full factorial design was performed for preliminary evaluation of the factors: pyrolysis time, pyrolysis temperature and
atomization temperature. This experiment showed that in the studied levels only the factors pyrolysis time and atomization
temperature were significant. Then, a 32 full factorial design was performed for the determination of the critical conditions of these variables. The method allows
the determination of lead using the standard calibration technique with a calibration curve from 2.6 to 30 μg L−1 (correlation coefficient higher than 0.998). A limit of detection (3σ) of 0.8 μg L−1 and a characteristic mass of 35 pg were obtained in the presence of palladium as modifier. The precision expressed as relative
standard deviation (RSD) was 1.5 and 0.8% for lead concentrations of 3.0 and 30 μg L−1 (n = 10). Recovery studies demonstrate that lead can be determined in naphtha and petroleum condensate using calibration with
organic standard solutions. This method was applied for the determination of lead in three petroleum condensate and two naphtha
samples. The concentrations found for the petroleum condensate was between 2.7 and 5.7 μg L−1, while the naphtha samples did not contain any detectable lead. 相似文献
994.
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997.
Lucas C. F. Ferreira Marcelo Montenegro Matheus C. Santos 《Journal d'Analyse Mathématique》2016,128(1):1-49
We define the Polish space R of non-degenerate rank-1 systems. Each non-degenerate rank-1 system can be viewed as a measure-preserving transformation of an atomless, σ-finite measure space and as a homeomorphism of a Cantor space. We completely characterize when two non-degenerate rank-1 systems are topologically isomorphic. We also analyze the complexity of the topological isomorphism relation on R, showing that it is \({F_\sigma }\) as a subset of R× R and bi-reducible to E0. We also explicitly describe when a non-degenerate rank-1 system is topologically isomorphic to its inverse. 相似文献
998.
L. N. Jorge L. S. Ferreira S. A. Leão A. A. Caparica 《Brazilian Journal of Physics》2016,46(5):556-564
In this work, we use a refined entropic sampling technique based on the Wang-Landau method to study the spin- 1/2 Baxter-Wu model. We adopt the total magnetization as the order parameter and, as a result, do not divide the system into three sub-lattices. The static critical exponents were determined as α = 0.6697(54), β = 0.0813(67), γ = 1.1772(33), and ν = 0.6574(61). The estimate for the critical temperature was Tc = 2.26924(2). We compare the present results with those obtained from other well-established approaches, and we find a very good closeness with the exact values, besides the high precision reached for the critical temperature. 相似文献
999.
Reggiani Vilela Gonçalves Rômulo Dias Novaes Mariáurea Matias Sarandy João Paulo Viana Leite Emerson Ferreira Vilela Marli do Carmo Cupertino 《Natural product research》2016,30(23):2738-2742
The aim of this study was to investigate the effect of leaves extract of Schizocalyx cuspidatus (A. St.-Hil.) Kainul. & B. Bremer on hepatic morphofunctional dysfunction induced by carbon tetrachloride (CCl4). Liver lesions were induced via intraperitoneal administration of CCl4 every 48 h for 12 days. Forty-nine rats were randomised into seven groups: G1: CCl4; G2: CCl4 (animals euthanised 24 h after last CCl4 application); G3: CCl4 + DMSO; G4: SCE 400 mg/kg; G5: DMSO (700 μl); G6: CCl4 + SCE 200 mg/kg and G7: CCl4 + SCE 400 mg/kg. SCE administration resulted in reduction in hydroperoxide levels, lipidic droplets and necrosis compared to G1, G2 and G3. There was an increase in the amount of collagen fibres in G1, G2 and G3 compared to the groups. These results show that the extract of SCE leaves has great potential for the recovery of liver damage after the application of CCl4. 相似文献
1000.
In this paper we study eigenfunctions and fundamental solutions for the three parameter fractional Laplace operator \(\Delta _+^{(\alpha , \beta , \gamma )}:= D_{x_0^+}^{1+\alpha } +D_{y_0^+}^{1+\beta } +D_{z_0^+}^{1+\gamma },\) where \((\alpha , \beta , \gamma ) \in \,]0,1]^3\), and the fractional derivatives \(D_{x_0^+}^{1+\alpha }, D_{y_0^+}^{1+\beta }, D_{z_0^+}^{1+\gamma }\) are in the Riemann–Liouville sense. Applying operational techniques via two-dimensional Laplace transform we describe a complete family of eigenfunctions and fundamental solutions of the operator \(\Delta _+^{(\alpha ,\beta ,\gamma )}\) in classes of functions admitting a summable fractional derivative. Making use of the Mittag–Leffler function, a symbolic operational form of the solutions is presented. From the obtained family of fundamental solutions we deduce a family of fundamental solutions of the fractional Dirac operator, which factorizes the fractional Laplace operator. We apply also the method of separation of variables to obtain eigenfunctions and fundamental solutions. 相似文献