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91.
Griselda Barrera Galland Luciano Forgiarini Da Silva Adriana Nicolini 《Journal of polymer science. Part A, Polymer chemistry》2005,43(20):4744-4753
A systematic study of the influence of the α‐olefin size, the catalyst stereospecificity and the reaction temperature was done on the catalytic activity and tacticity of poly‐α‐olefins from 1‐hexene to 1‐octadecene. The metallocenes used were rac‐Et[Ind2]ZrCl2 ( 1 ) and Me2C[Cp(9‐Flu)]ZrCl2 ( 2 ) to obtain isotactic and syndiotactic polyolefins. Some catalysts giving atactic polymers were also used in order to study all the possible 13C NMR pentades. Catalytic activities increased and isotacticity and syndiotacticity decreased with temperature, but no real trend was found with the α‐olefin size. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4744–4753, 2005 相似文献
92.
Metabolomic profiling of serum in the progression of Alzheimer's disease by capillary electrophoresis–mass spectrometry
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Raúl González‐Domínguez Antonia García Tamara García‐Barrera Coral Barbas José Luis Gómez‐Ariza 《Electrophoresis》2014,35(23):3321-3330
There is high interest in the discovery of early diagnostic biomarkers of Alzheimer's disease, for which metabolomics exhibits a great potential. In this work, a metabolomic approach based on ultrafiltration and analysis by CE‐MS has been used to obtain representative fingerprints of polar metabolites from serum samples in order to distinguish between patients with Alzheimer's disease, mild cognitive impairment, and healthy controls. By the use of partial least squares discriminant analysis it was possible to classify patients according to the disease stage and then identify potential markers. Significant increase was observed with progression of disease in levels of choline, creatinine, asymmetric dimethyl‐arginine, homocysteine‐cysteine disulfide, phenylalanyl‐phenylalanine, and different medium chain acylcarnitines. On the other hand, asparagine, methionine, histidine, carnitine, acetyl‐spermidine, and C5‐carnitine were reduced in these serum samples. In this way, multiple essential pathways were found implicated in the underlying pathology, such as oxidative stress or defects in energy metabolism. However, the most interesting results are related to the association of several vascular risk factors with Alzheimer's disease. 相似文献
93.
Barbra Caballero-Segura Carlos Eduardo Barrera Díaz Jorge Javier Ramírez García Graciela Zarazúa Ricardo Soria 《International journal of environmental analytical chemistry》2013,93(13):1288-1301
The comparison between inductively coupled plasma optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectrometry (TXRF) for simultaneous determination of metal content (Cr, Cu, Fe, Mn, Pb and Zn) in mosses from the Metropolitan Area of the Toluca Valley was performed. Epiphytic mosses (Fabriona ciliaris and Leskea angustata) were collected in two sampling campaigns and were digested with HNO3, HCl and HF for ICP-OES method and HNO3 and HCl for TXRF method. The certified reference material (IAEA-336, Lichen) and the Standard Reference Material (SRM-1573, tomato leaves) were used for the quality control and to evaluate trueness and precision. Linearity, detection and quantification limits were also determined. Results show an ICP-OES and TXRF trueness mean of 101 ± 5% and 97 ± 9%, respectively; the relative standard deviation (RSD percent) was less than 17% in both methods. The moss samples exhibit a satisfactory precision (RSD ≤ 20%), because the RSD percent for ICP-OES, from 2% to 15%, and that for TXRF, from 1% to 17%, were obtained. One factor experimental design and simple regression analysis (α = 0.05) were used to compare the ICP-OES and TXRF metal concentrations. The statistical results do not show significantly different values for Cu, Mn, Pb and Zn in both the sample campaigns. In addition, the average results for Cr in the first sample campaign (30.3 ± 11.4 mg/kg for ICP-OES and 18.6 ± 9.8 mg/kg for TXRF) and Fe values in the second sample campaign (10,810 ± 2980 mg/kg for ICP-OES and 8380 ± 2350 mg/kg for TXRF) were significantly different in both methods. These differences are attributed to an incomplete sample digestion in the TXRF sample preparation. The results of the simple regression analyses show p-value less than 0.05, which indicates an equivalent and significant relation between ICP-OES and TXRF. 相似文献
94.
We propose a fast integer based method for computing square roots of floating point numbers. This implies high accuracy and robustness, since no precision will be lost during the computation. Only integer addition and shifts are necessary to obtain the square root. Comparisons made with the modified Newton method indicate that the suggested method is twice as fast for computing floating point square roots. 相似文献
95.
Fernanda F. Nunes Escher Griselda Barrera Galland Mrcio Ferreira 《Journal of polymer science. Part A, Polymer chemistry》2003,41(16):2531-2541
Many studies have been reported on the 13C NMR characterization of ethylene–α‐olefin copolymers, but only a few have been reported on terpolymers. The incorporation of an α‐olefin into the polyethylene chain changes the structure and, consequently, the properties of the polymer obtained. Looking for new products, we obtained a series of ethylene–propylene–1‐decene terpolymers with the metallocenic system rac‐ethylene bisindenyl zirconium dichloride/methylaluminoxane. We performed a complete 13C NMR characterization of these terpolymers qualitatively and quantitatively. Here we present a detailed study of the 13C NMR chemical shifts, triad sequence distributions, monomer average sequence lengths, and reactivity ratios for these terpolymers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2531–2541, 2003 相似文献
96.
When tested in solar cells, ruthenium polypyridinic dyes with extended π systems show an enhanced light-harvesting capacity that is not necessarily reflected by a high (collected electrons)/(absorbed photons) ratio. Provided that metal-to-ligand charge transfer bands, MLCT, are more effective, due to their directionality, than intraligand (IL) π-π* bands for the electron injection process in the solar cell, it seems important to explore and clarify the nature of the absorption bands present in these types of dyes. This article aims to elucidate if all the absorbed photons of these dyes are potentially useful in the generation of electric current. In other words, their potentiality as dyes must also be analyzed from the point of view of their contribution to the generation of excited states potentially useful for direct injection. Focusing on the assignment of the absorption bands and the nature of the emitting state, a systematic study for a series of ruthenium complexes with 4,4'-distyryl-2,2'-dipyridine (LH) and 4,4'-bis[p-(dimethylamino)-α-styryl]-2,2'-bipyridine (LNMe(2)) "chromophoric" ligands was undertaken. The observed experimental results were complemented with TDDFT calculations to elucidate the nature of the absorption bands, and a theoretical model was proposed to predict the available energy that could be injected from a singlet or a triplet excited state. For the series studied, the results indicate that the percentage of MLCT character to the anchored ligand for the lower energy absorption band follows the order [Ru(deebpy)(2)(LNMe(2))](PF(6))(2) > [Ru(deebpy)(2)(LH)](PF(6))(2) > [Ru(deebpy)(LH)(2)](PF(6))(2), where deebpy is 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine, predicting that, at least from this point of view, their efficiency as dyes should follow the same trend. 相似文献
97.
Mauricio Barrera 《International journal of quantum chemistry》2011,111(12):3097-3111
Within the Kohn‐Sham framework and for a series of single charged monatomic anions, the orbital hardness is calculated as a change in the frontier eigenvalue, which is equivalent to integrate the local hardness function obtained through the derivative of the KS effective potential respect to the occupation number. The local hardness function is composed by the sum of two terms with opposite sign that describe the electrostatic and exchange‐correlation interactions. Moreover, it is found that, at the KS radii, the last term vanishes with the result that the orbital hardness of the anion is a measure of the electrostatic potential exerted by the frontier density at the KS radii. A further derivation leads to establish a direct relationship between hardness and the inverse of the KS radii. The polarizability of the anion is also examined by computing it from the volume of a sphere having the KS radii. These results show that anions from the halide group are hard and little polarizable, whereas anions from the alkali group are soft and highly polarizable. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 相似文献
98.
Carlos Díaz María Luisa Valenzuela Nicolas Yutronic Valeria Villalobos Gonzalo Barrera 《Journal of Cluster Science》2011,22(4):693-704
Pyrolysis of molecular precursors containing vanadium organometallic and cyclic phosphazene affords mixtures of nanostructured
vanadium oxides and pyrophosphates. The products from the molecular precursor [N3P3(OC6H5)5OC5H4N·Cp2VCl][PF6], and of the mixtures Cp2VCl2/N3P3(OC6H4CHO)6 and Cp2VCl2/[NP(O2C12H8)]3 in several relationships 1:1, 1:3, 1:5 and 1:10, pyrolyzed under air and at 400 °C and 600 °C, give mixtures mainly V2O5 and VO(PO3)2. Varied morphologies depending on the molecular or mixture precursors and of the temperature used were observed. Nanowires
with diameters of approximate 40 nm were observed for the 1:5 Cp2VCl2/[NP(O2C12H8)]3 mixture pyrolyzed at 400 °C, while the same mixture pyrolyzed at 600 °C, affords xerogels of V2O5. The products were characterized by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), transmission
electron microscopy (TEM), infra-red (IR) spectroscopy and X-ray diffraction (XRD). The preparation method constitutes a novel
one-pot solid-state way to nanostructured materials with potential applications both in oxidative dehydrogenation of light
hydrocarbons with V2O5, as well as alkenes oxidations with VO(PO3)2. 相似文献
99.
T. Rivera Montalvo L. Olvera Tenorio J. Azorín Nieto M. Barrera Salgado A. M. Soto Estrada C. Furetta 《辐射效应与固体损伤》2013,168(5):181-186
This paper reports the experimental results concerning the thermoluminescent (TL) characteristics of hydrogenated amorphous zirconium oxide (a-Zr:H) powder prepared by the sol–gel method. The advantages of this method are the homogeneity and the purity of the gels associated with a relatively low sintering temperature. Hydrogenated amorphous powder was characterized by thermal analysis and X-ray diffraction. The main TL characteristics investigated were the TL response as a function of the absorbed dose, the reproducibility of the TL readings and the fading. The undoped a-Zr:H powder presents a TL glow curve with two peaks centered at 150 and 260 °C, respectively, after beta irradiation. The TL response a-Zr:H as a function of the absorbed dose showed a linear behavior over a wide range. The results presented open the possibility to use this material as a good TL dosimeter. 相似文献
100.