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排序方式: 共有289条查询结果,搜索用时 15 毫秒
1.
S. Moss de Oliveira P. M. C. de Oliveira F. C. de Sá Barreto 《Journal of statistical physics》1995,78(5-6):1619-1627
Using the finite-size scaling renormalization group, we obtain the two-dimensional flow diagram of the Blume-Capel model forS=1 andS=3/2. In the first case our results are similar to those of mean-field theory, which predicts the existence of first- and second-order transitions with a tricritical point. In the second case, however, our results are different. While we obtain in theS=1 case a phase diagram presenting a multicritical point, the mean-field approach predicts only a second-order transition and a critical endpoint. 相似文献
2.
We study the evolution of radiating and viscous fluid spheres assuming an additional homothetic symmetry on the spherically symmetric space-time. We match a very simple solution to the symmetry equations with the exterior one (Vaidya). We then obtain a system of two ordinary differential equations which rule the dynamics, and find a self-similar collapse which is shear-free and with a barotropic equation of state. Considering a huge set of initial self-similar dynamics states, we work out a model with an acceptable physical behavior. 相似文献
3.
The collapse of a charged and radiating ball in the diffusion limit is studied using a method reported by Herrera and collaborators. The interior solution is matched with the exterior Reissner-Nordström-Vaidya metric to obtain a system of differential equations at the surface of the distribution, which can be integrated numerically for some set of initial data. For one model, the profiles of the physical variables are obtained at any piece of the material in terms of Schwarzschild-like coordinates. 相似文献
4.
3-Methyl-2-butenenitrile (1) cyclodimerised on treatment with lithium diisopropylamide in dimethoxyethane at temperatures between ?78°C and 0°C to 3-amino-4-cyano-1,5,5-trimethyl-1,3-cyclohexadiene (2) the structure of which was established by acid hydrolysis to the known 4-cyano-1,5,5-trimethyl-1-cyclohexene-3-one (3). 相似文献
5.
R. Centoducatte E. P. Muniz F. E. Jorge M. T. Barreto 《Journal of Molecular Structure》2004,680(1-3):143-147
The molecular improved generator coordinate Hartree–Fock (MIGCHF) method is used to generate accurate basis sets of primitive Gaussian-type functions for the H2O molecule. Sequences of increasing size atom centered basis sets are employed to explore the accuracy that can be achieved with this method. Using the O(24s14p8d5f2g1h);H(22s9p5d2f1g) basis set, the HF and second-order electron correlation energies of the H2O ground state at the experimental geometry are computed as −76.0674680 and −0.3491935 hartree, respectively. The HF energy is in error by 20 μhartree and the second-order correlation energy corresponds to 96.5% of an estimate of the limiting value. The relevance of the present calculations is to show the accuracy that can be achieved in studies of small polyatomic molecules with the MIGCHF method. 相似文献
6.
7.
L. A. Schuch D. J. R. Nordemann W. O. Barreto A. Cardoso A. Zago 《Journal of Radioanalytical and Nuclear Chemistry》1994,177(1):39-49
Soil samples from Paraná State, Brazil, were collected in 1991 and compared with others collected at the same location in March 1977 and at the end of 1983. Pedological analyses were practiced on these samples and137Cs and232Th,226Ra and40K activities were determined by gammaray spectrometry. A latitude dependence of137Cs was found as well. It was impossible to determine the137Cs contribution from Chernobyl nuclear accident because of low fallout and intense leaching, erosion and re-suspension in soils of regions with high annual precipitation. Natural radionuclides did not show such effects. 相似文献
8.
Liquid and solid cyclic peroxides derived from aliphatic ketones are explosive materials so their enthalpies of vaporization
and other thermodynamic or condensed-phase properties cannot be measured directly. In this work the enthalpies of vaporization
of peroxides at 298.15 K were estimated simply from gas chromatographic retention times measured at different temperatures.
The technique correlates changes in the retention times of compounds whose enthalpies of vaporization are known (called the
reference series), with those of the compounds of interest. If t
R′ is the adjusted retention time (retention time of each compound minus the retention time of unretained diethyl ether, used
as solvent) a plot of ln t
R′ against 1/T for each compound (reference compounds and cyclic peroxides) results in a straight line (r
2 > 0.99 for all compounds). The enthalpy of transfer from solution to the vapor state (Δsolg
H
m) can be obtained by multiplying the slope by the gas constant (R). A second plot correlates the enthalpies of transfer from
solution to the vapor state (Δsolg
H
m), as measured by gas–liquid chromatography (GLC), with enthalpies of vaporization of reference materials (Δvap
H
m at 298.15 K) available in the literature. C9–C15 fatty acid methyl esters and hydrocarbons were used as reference compounds. The enthalpies of vaporization of the cyclic
organic peroxides were calculated from the equation of the line obtained in this second correlation, the slope of which was
Δvap
H
m (at 298.15 K)/Δg
sol
H
m. The experiments were performed under isothermal conditions with a DB-5 capillary column, flame-ionization detection (FID),
and nitrogen as carrier gas. The column temperature was varied over a range of at least 30–70 K between 403 and 473 K, with
chromatograms being acquired at 10 K intervals. Enthalpies of vaporization of cyclic organic peroxides are not available in
the literature, and the values given in this paper, obtained by gas chromatography, are the first to be reported. 相似文献
9.
Wagner J. Barreto Henrique de Santana Flaveli A. S. Almeida Dilson N. Ishikawa Yoshio Kawano 《Journal of Analytical and Applied Pyrolysis》2003,70(2):719-210
The synthesis, characterization, and thermal decomposition of the [Ni(SCN)2(H+SCN)2(4-mepy)2] compound with an octahedral structure in polymeric chain were reported, in which SCN groups form bridges among Ni(II) ions. The compound decomposes in water resulting in a pH<4 solution. The FT-IR spectrum presented doublet bands at 2117; 2128 cm−1, 788; 773 cm−1 assigned to ν(C---N) and ν(C---S) stretching modes, respectively, and δ(SCN) deformation modes at 468; 476 cm−1. The Raman spectrum of the compound presented the ν(C---N) stretching as a strong doublet at 2122; 2128 cm−1, ν(C---S) at 783; 770 cm−1, and δ(SCN) at 468; 477 cm−1. No significant changes were observed in the 4-mepy ligand bands compared with the vibrational frequencies of the pure compound or the compound in aqueous solution 0.2 mol l−1. The crystal UV–vis reflectance spectrum presented two bands centered in 626 and 424 nm tentatively assigned to the d→d type transitions, 3A2g→3T1g and 3A2g→3T1g, for a symmetry close to Oh. The TG curve showed a mass loss between 120 and 200 °C assigned to the loss of the two 4-mepy molecules; from 200 to 265 °C, the loss of the two H+SCN groups; and from 265 to 450 °C, the loss of the two SCN groups that formed the bridges among the nickel atoms. Based on these mass loss data, a mechanism of thermal decomposition for the compound was proposed. 相似文献
10.
Ariana Borges Souza Monique Rodrigues Moreira Carly Henrique Gambeta Borges Marília Rodrigues Simão Jairo Kenupp Bastos João Paulo Barreto de Sousa Sérgio Ricardo Ambrosio Rodrigo Cassio Sola Veneziani 《Biomedical chromatography : BMC》2013,27(3):280-283
The Copaifera species (Leguminoseae) are popularly known as ‘copaíba’ or ‘copaíva’ and are grown in the states of Amazonas, Pará and Ceará in northern Brazil. The oleoresins obtained from these species have been extensively used owing to their pharmacological potential and their application in cosmetic and pharmaceutical preparations. In the present study, the development and validation of a novel, rapid and efficient RP‐HPLC methodology for the analysis of the diterpene (?)‐copalic acid (CA), pointed out as the only chemical marker of the Copaifera genus, are described. The regression equation (Y = 26,707x ? 29,498) was obtained with good linearity (r2 = 0.9993) and the limits of quantification and detection were 9.182 and 3.032 µg/mL, respectively. The precision and the accuracy of the method were adequate (lower than 4%). Finally, the validation parameters evaluated were satisfactorily met, so the developed method represents a suitable tool for application in the quality control of such natural products. Further studies aiming to develop analytical methodologies for each Copaifera species using a more representative number of chemical markers should be performed. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献