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1.
Wenzhan Yang Rahul Manek William M. Kolling Marius Brits Wilna Liebenberg 《Supramolecular chemistry》2013,25(6):485-496
In this study, the thermal behavior of three hydrated water-soluble 4-sulphonato calix[n]arenes was investigated. The melting points, heats of fusion, and heats of solution of the calix[4]arene, calix[6]arene and calix[8]arene were 277, 262, and 270°C; 192, 242 and 351 kJ/mol; and 30, 58 and 63 kJ/mol, respectively. Lower heat of fusion, smaller increase in entropy and smaller heat of solution of the calix[4]arene compared to the calix[6]arene and calix[8]arene showed that less heat was required to break up the crystal lattice of the smaller macromolecule. This apparent anomaly is rationalized in terms of smaller cooperativity of interaction between the molecules of calix[4]arene in the crystal lattice, although the strength of the individual interactions is stronger as evidenced by the higher melting point. TGA analysis indicated that about 17–20% of water was associated with the calix[n]arenes. Both TGA and hot stage microscopy results indicated that upon heating these molecules underwent stepwise water loss. TGA kinetics showed that the 4-sulphonato-calix[8]arene lost water easier than the other two calixarenes. The moisture adsorption behavior of all calixarenes followed type II isotherms. For the same amount of material, the calix[6]arene adsorbed more moisture than the calix[4]arene and the calix[8]arene. Moreover, dehydrated less crystalline 4-sulphonic-calix[n]arenes powders are hydroscopic. 相似文献
2.
Hebert Vinicius Pereira Victória Silva Amador Marcelo Martins Sena Rodinei Augusti Evandro Piccin 《Analytica chimica acta》2016
Paper spray mass spectrometry (PS-MS) combined with partial least squares discriminant analysis (PLS-DA) was applied for the first time in a forensic context to a fast and effective differentiation of beers. Eight different brands of American standard lager beers produced by four different breweries (141 samples from 55 batches) were studied with the aim at performing a differentiation according to their market prices. The three leader brands in the Brazilian beer market, which have been subject to fraud, were modeled as the higher-price class, while the five brands most used for counterfeiting were modeled as the lower-price class. Parameters affecting the paper spray ionization were examined and optimized. The best MS signal stability and intensity was obtained while using the positive ion mode, with PS(+) mass spectra characterized by intense pairs of signals corresponding to sodium and potassium adducts of malto-oligosaccharides. Discrimination was not apparent neither by using visual inspection nor principal component analysis (PCA). However, supervised classification models provided high rates of sensitivity and specificity. A PLS-DA model using full scan mass spectra were improved by variable selection with ordered predictors selection (OPS), providing 100% of reliability rate and reducing the number of variables from 1701 to 60. This model was interpreted by detecting fifteen variables as the most significant VIP (variable importance in projection) scores, which were therefore considered diagnostic ions for this type of beer counterfeit. 相似文献
3.
M. Keil T. Kolling K. Bergmann W. Meyer 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,7(1):55-64
The process of dissociative attachment (DA) of low-energy electrons ) to vibrationally excited sodium dimer molecules is studied with high electron energy resolution () in a supersonic molecular beam. A novel photoelectron source, based on two-step photoionization of the sodium atoms in the
beam, may deliver a current of up to 1 nA and has been used with a current of typically 0.2 nA in this experiment. The energy
dependence of the rate of sodium anion formation is determined by ion detection based on a time-of-flight analysis. The molecules
are selectively excited to levels using the technique of coherent population transfer by delayed pulses (STIRAP). The comparison of the experimental data with
recent resonance model calculations based on improved potential curves reveals generally good agreement for levels v
”>12. For some distinct differences between theoretical and experimental results persist.
Received: 21 November 1998 / Received in final form: 7 April 1999 相似文献
4.
Thayana da C. Alves Mariele R. S. Gonçalves Francyne C. S. Correia Virgínia C. da Silva Paulo T. de Sousa Jr. Mário G. de Carvalho Raimundo Braz‐Filho Evandro L. Dall'Oglio 《Helvetica chimica acta》2014,97(11):1469-1474
From the hexane and MeOH extracts of Annona coriacea Mart . (Annonaceae) seeds, two novel acetogenins, coriapentocins A and B ( 1 and 2 , resp.) were isolated. The known acetogenin bullacin ( 3 ) was also isolated from the hexane extract. The structures of compounds 1 – 3 were elucidated by NMR and MS analysis, and relative configurations were established by comparison with literature data. 相似文献
5.
Valdecir Antoninho Dalpasquale Dcio Sperandio Luiz Henry Monken e Silva Evandro Kolling 《Applied mathematics and computation》2008,200(2):590
This work is concerned with the numerical simulation of fixed-bed corn drying using MSU (Michigan State University) drying model. The classical numerical procedure for MSU model relies on an explicit method of finite differences which requires certain stability conditions between the step sizes of the time and space variables. The objective of the present paper is to establish a stable implicit method based on backward finite differences, in both time and space variables, which takes into account some specific empirical aspects of the problem. Computational results illustrate the efficiency and the flexibility of method. 相似文献
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Castaldelli E Triboni ER Demets GJ 《Chemical communications (Cambridge, England)》2011,47(19):5581-5583
This naphthalene diimide derivative, DC18, forms highly conjugated semiconducting stacked assemblies over electrodes after electrochemical conditioning. These molecular materials are very efficient towards electrochemical photoreduction of oxygen under visible light. 相似文献
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Since the microstructure of short fiber reinforced composites is inhomogeneous, the application of micromechanical models is useful, that take into account their characteristics like the fiber orientation and the aspect ratio of fibers. Two different methods are considered in this work: A two-step approach is utilized to get approximately the upper and lower bounds of the elastic properties. Furthermore, an approximation for the elastic properties is calculated by the self-consistence method. Both methods use discretely microstructural information including the length, the diameter and the orientation of each single fiber. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
Evandro A. de Morais Luis V.A. Scalvi Alberto A. Cavalheiro Américo Tabata José Brás B. Oliveira 《Journal of Non》2008,354(42-44):4840-4845
Some very relevant optical, electrical, and structural properties of SnO2 doped with rare-earth ions Er3+ and Eu3+ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er3+ and Eu3+ ions are expected to act as acceptors in SnO2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO2 lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO2 and SnO2:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO2 than in Er-doped SnO2, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu3+ centers. 相似文献