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51.
Karla A. Barrera‐Rivera Ángel Marcos‐Fernández Ricardo Vera‐Graziano Antonio Martínez‐Richa 《Journal of polymer science. Part A, Polymer chemistry》2009,47(21):5792-5805
Yarrowia lipolytica (YLL), Candida rugosa (CRL), and porcine pancreatic lipase (PPL) were employed successfully as catalysts in the enzymatic ring‐opening polymerization (ROP) of ε‐caprolactone in the presence of 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM][BF4]), 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([BMIM][BF4]), 1‐butylpyridinium tetrafluoroborate ([BuPy][BF4]), 1‐butylpyridinium trifluoroacetate ([BuPy][CF3COO]), 1‐ethyl‐3‐methylimidazolium nitrate ([EMIM][NO3]) ionic liquids. Poly(ε‐caprolactone)s (PCLs) with molecular weights (Mn) in the range of 300–9000 Da were obtained. 1H‐ and 13C‐NMR analyses on PCLs formed by YLL, CRL, and PPL showed asymmetric telechelic α‐hydroxy‐ω‐carboxylic acid end groups. Differences between CP‐MAS and MAS spectra are observed and discussed in terms of morphology. MALDI‐TOF spectra show the formation of at least seven species. Differential scanning calorimetry (DSC) and Wide Angle X‐Ray Scattering (WAXS) results demonstrate the high degree of crystallinity present in all the polyesters. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5792–5805, 2009 相似文献
52.
It is shown that the introduction of a cut-off wavenumber corresponding to electron-hole excitations leads to a 10% reduction of the Lifshitz van der Waals attraction of low density metallic half-spaces at small separations. 相似文献
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Coy Barrera CA Coy Barrera ED Granados Falla DS Delgado Murcia G Cuca Suarez LE 《Chemical & pharmaceutical bulletin》2011,59(7):855-859
A novel seco-limonoid, rel-(1S,5R,9S,7R,8S,9R,10S,11R,13S,14R,15R,17R)-11,19-dihydroxy-7-acetoxy-7-deoxoichangin (raputiolide) (1), and two novel quinolone alkaloids N-methyl-2-phenoxyquinolin-4(1H)-one (heptaphyllone A) (2) and 6-methylbenzofuro[2,3-b]quinolin-4(1H)-one (heptaphyllone B) (3), along with the known seco-limonoid ichangin (4), were isolated from Raputia heptaphylla PITTIER (Rutaceae) stem bark. Five known alkaloids, N-methyl-8-methoxyflindersine (5), skimmianine (6), kokusaginine (7), dictamnine (8) and flindersiamine (9), were also isolated from R. heptaphylla leaves. Their structures were established on the basis of full spectroscopic data interpretation supported by data from the pertinent literature. seco-Limonoid 1 configuration was determined by enhanced nuclear Overhauser effect spectroscopy (NOESY) experiments and density functional theory (DFT) molecular modeling. The antileishmanial effect of the isolated compounds was evaluated on Leishmania Viannia panamensis (promastigotes and amastigotes). Whereas alkaloids 2-3, 6-8 and limonoid 4 exhibited no significant parasitocide activity against internalized L. (V.) panamensis amastigotes, limonoid 1 and alkaloid 5 had leishmanicidal activity on intracellular amastigotes (EC??: 8.7 μg/ml) and promastigotes (EC(50): 14.3 μg/ml), respectively. 相似文献
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Marvin Cummings Sebastian Gliga Boris Lukanov Eric I. Altman Matthias Bode Enrique V. Barrera 《Surface science》2011,605(1-2):72-80
Here, the interactions of C60 at the surface of pseudomorphic Ni/Cu(100) and Co/Ru(0001) thin films and its effect on film growth and morphology were determined using in-situ scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). The novel development of C60-metallic based nanosystems, such as C60 molecular junction transistors, hinges on our ability to understand the factors governing structural stability in these nanosystems and the nature of the bond interactions at the C60–metal interface. In this study, C60 deposited onto the Ni(100) film surface is observed to be fairly immobile and uniformly distributed across the Ni surface. On the Co(0001) film surface however, C60 mobility is observed to be severely limited in some regions and highly mobile in others dependent upon Co film surface reconstruction, resulting in a non-uniform distribution of C60 across the Co film surface. Despite the presence of C60 on the Ni surface, there is no obvious influence of the C60 on further Ni film growth. In contrast, during Co film growth, islands only nucleate and grow from step edges or locally around C60 molecules. The strength of the Co–C60 bond interaction appears stronger than the Co–Co bond on Co film terrace. Generally, the Ni and Co films both continue epitaxial film growth in the presence of molecular C60. AES results indicate the C60 molecules maintain their chemical integrity during growth. 相似文献
58.
Fernanda F. Nunes Escher Griselda Barrera Galland 《Journal of polymer science. Part A, Polymer chemistry》2004,42(10):2474-2482
We report the complete 13C NMR characterization of a series of ethylene–propylene–1‐hexene terpolymers obtained with the metallocenic system rac‐ethylene bis‐indenyl zirconium dichloride, with different comonomer ratios. A detailed study of 13C NMR chemical shifts, triad sequence distributions, monomer‐average sequence lengths, and reactivity ratios for these terpolymers is presented. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2474–2482, 2004 相似文献
59.
Daniela Bianchini Griselda Barrera Galland Joo Henrique Z. dos Santos Roberto J. J. Williams Diana P. Fasce Ignacio E. Dell'Erba Raul Quijada Mnica Perez 《Journal of polymer science. Part A, Polymer chemistry》2005,43(22):5465-5476
A polyhedral oligomeric silsesquioxane (POSS) consisting of a mixture of perfect and imperfect polyhedra with 8–10 Si atoms bonded to bulky and branched organic groups functionalized with (β‐hydroxy)‐tertiary amines, was adsorbed on an activated silica from a toluene solution. POSS fixation was confirmed by measuring its concentration in the solutions resulting from filtering and repeatedly washing with toluene. Diffuse reflectance infrared Fourier‐transformed (DRIFTS) spectra of the modified silica showed a decrease in the absorption of isolated SiOH groups. A POSS previously marked by reaction with an isocyanate was also used, and the presence of the carbonyl in the urethane group was recorded in the DRIFTS spectra of the modified silica, confirming the presence of adsorbed POSS. X‐ray photoelectron spectroscopy (XPS) was used to measure the amount of Si (2p) of POSS adsorbed on the SiO2 surface as a function of the POSS concentration in the toluene solution. A linear increase of adsorbed POSS as a function of its concentration in toluene was found, even after successive washings with toluene. A metallocene, (nBuCp)2ZrCl2, was then adsorbed on the POSS‐modified silica from a toluene solution, and the fixed Zr amount was measured by Rutherford backscattering spectrometry (RBS). The activity of the resulting catalyst for ethylene polymerization in toluene, using methylaluminoxane (MAO) as a cocatalyst, was determined in two different devices operating in the range of 1.6–7.0 bar. Under every experimental condition, POSS‐modification of the silica support led to an increase of about 50% in the activity of the resulting catalyst when compared to the use of the unmodified support. Reasons for the observed increase in activity are discussed. The molar mass distribution and crystallinity of the resulting polyethylenes was not affected by the POSS modification of the silica support. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5465–5476, 2005 相似文献
60.
A general analytical strategy for mercury speciation in seafood samples has been proposed to increase sample throughput. This consists of the initial determination of total mercury content, and then mercury speciation using gas chromatography coupled to atomic fluorescence spectroscopy. The appropriate sample treatment for mercury speciation is selected between a method based on aqueous ethylation with sodium tetraethylborate (Approach A: a rapid methodology for samples with methylmercury concentrations between 150 and 2000 ng g?1) and another one based on the determination of organomercury chlorides (Approach B: a much more time‐consuming methodology, applicable to samples with methylmercury at 1.2–200 ng g?1). Both procedures have been used together for the analysis of bivalves and fish samples. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献