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991.
R. S. Corrêa S. R. Souza e Silva L. P. Duarte G. D. F. Silva L. C. A. Barbosa J. Ellena A. C. Doriguetto 《Journal of Structural Chemistry》2012,53(1):156-163
The structural study of two (C30H48O2) pentacyclic triterpene (PCTT) isomers is presented. These terpenes, known as 30-hydroxy-lup-20(29)-en-3-one (1) and (11α)-11-hydroxy-lup-20(29)-en-3-one (2), were isolated from Maytenus imbricata Mart. Ex Reissek (Celastraceae). The molecular structure of 1 and 2 differs in the position of the hydroxyl group. Both compounds crystallize in non-centrosymmetric space groups with two molecules
in the asymmetric unit. The crystal structure of 1 shows a triclinic P1 space group (a = 9.5518(1) ?, b = 9.7083(1) ?, c = 14.4696(2) ?, α = 93.832(1)°, β = 102.833(1)°, and γ = 103.307(1)°), while compound 2 crystallizes in a monoclinic P21 one (a = 13.4439(16) ?, b = 14.4463(14) ?, c = 13.5224(9) ? and β = 99.703(8)°). The two molecules independent by symmetry of 1 differ slightly due to the presence of static disorder in oxygen atoms. In addition, the intermolecular geometries of 1 and 2 were analysed, and in each isomer the crystal packing is stabilized by O-H…O intermolecular hydrogen bonds and van der Waals
forces. 相似文献
992.
M. C. D. Silva L. M. da Silva N. A. Santos M. M. Concei??o A. G. Souza A. O. dos Santos 《Journal of Thermal Analysis and Calorimetry》2011,106(2):363-367
Nowadays the growing fuel deficit requires the development of alternative fuel sources. Biodiesel is a good substitute to the conventional diesel because it is quite similar to the fossil fuel in its main characteristics. However, there are some obstacles, as the properties of cold-flow, to the development of a more useful alternative fuel. In this work we use the X-ray diffraction and differential calorimetry scanning to study low temperature properties of ethylic Babassu biodiesel. Our results show that the nucleation of crystals starts below ?8 °C and the crystallization temperature does not change significantly when the sample was submitted to a winterization process. The higher concentrations of ethyl esters from saturated fat acid are probably responsible for this characteristic. The X-ray diffraction, combined with DSC measurements, was efficiently employed in the characterization of cold-flow biodiesel properties, showing to be very helpful techniques. 相似文献
993.
994.
995.
Patricia Gimenez Heleno Bolfarine Enrico A. Colosimo 《Annals of the Institute of Statistical Mathematics》2000,52(4):698-711
In this paper, hypotheses testing based on a corrected score function are considered. Five different testing statistics are proposed and their asymptotic distributions are investigated. It is shown that the statistics are asymptotically distributed according to the chisquare distribution or can be written as a linear combination of chisquare random variables with one degree of freedom. A small scale numerical Monte Carlo study is presented in order to compare the empirical size and power of the proposed tests. A comparative calibration example is used to illustrate the results obtained. 相似文献
996.
A. L. F. Briso M. S. M. Fonseca L. C. A. G. de Almeida S. J. Mauro P. H. dos Santos 《Laser Physics》2010,20(12):2066-2069
This study evaluated the color alteration of teeth subjected to the action of different bleaching agents and the influence
of light sources commonly used in association with these products, In GI, the specimens remained immersed in artificial saliva.
The specimens in GII were bleached with a 10% carbamide peroxide gel 4 hours/day during 3 weeks; the teeth in the other three
groups were subjected to three sessions of three 10-min applications of 35% hydrogen peroxide gel at 7-day intervals. In GIII,
no light was used, while in GIV and GV the gel was associated with a quartz-tungsten-halogen light and a LED/laser source,
respectively. The teeth color was evaluated before and 7 days after the bleaching sessions by reflectance spectrophotometry.
Data were analyzed statistically by ANOVA and Fisher’s test (α = 0.05), and showed that a significant color change was obtained
in all treated groups. After the first week of treatment and at the end of it, the bleaching protocols showed similar results.
The results of the present study indicate that association of a light source is not necessary to obtain the bleaching effect
and that optimal bleaching can be achieved with all techniques tested. 相似文献
997.
998.
The in situ formed Ti(IV) complexes of pyrrolidine-based chiral salen ligands derived from natural l-tartaric acid were evaluated as catalysts in the enantioselective trimethylsilylcyanation of aromatic aldehydes. The different activity and selectivity of the catalysts in the formation of the products were found to be dependent on the N-substituent of the pyrrolidine. 相似文献
999.
We consider a class of convex integral functionals with lagrangeans depending only on the gradient and satisfying a generalized
symmetry assumption, which includes as a particular case the rotational symmetry. Adapting the method by A. Cellina we obtain
a kind of local estimates for minimizers in the respective variational problems, which is applied then to deduce some versions
of the Strong Maximum Principle in the variational setting. 相似文献
1000.
Since the first use of glass electrode to the detection of pH, many efforts have been made to develop new techniques and methods. In this study, the pH sensing properties of an extended gate field effect transistor (EGFET) based on the vanadium pentoxide xerogel thin film is investigated. The vanadium pentoxide was prepared by a sol–gel route. The X-ray diffractogram indicates the presence of a lamellar structure of the vanadium pentoxide xerogel films. The film was investigated as a sensor in the pH range of 2–12 and the corresponding EGFET has a sensitivity of 58.1 mV/pH. This value suggests that the material is a promising candidate for applications as disposable biosensor. 相似文献