1,720,979 research outputs found
Chenoalbicin, a novel cinnamic acid amide alkaloid from chenopodium album
No full textThe roots of Chenopodium album were infused in MeOH, and the extract was partitioned between AcOEt and H2O. AcOEt-Soluble material was subjected to different silica-gel column chromatographies and then purified by reverse-phase HPLC to afford a new cinnamic acid amide alkaloid as a racemic mixture. The new compound, named chenoalbicin (1), was characterized by extensive spectroscopic investigation, especially 1D and 2D NMR spectroscopy. Its effects on the germination and growth of Lactuca sativa L. has been studied. The results are reported as percentage differences of germination, root elongation, and shoot elongation from the control at concentrations ranging from 10-4 to 10-7 M. © 2004 Verlag Helvetica Chimica Acta AG, Zürich
CS-SYMMETRIC CONSTRAINED-GEOMETRY-TYPE CATALYSTS: WHAT IS THE TRUE ORIGIN OF THE STEREOCONTROL?
No full textSYNDIOTACTIC POLYPROPYLENE FROM [ME2SI(3,6-DI-T-BUTYL-9-FLUORENYL)(N-T-BUTYL)]TICL2BASED CATALYSTS: CHAIN-END OR ENANTIOTOPIC-SITES STEREOCONTROL?
No full textFROM POLYPROPYLENE MICROSTRUCTURE TO PROPENE POLYMERIZATION KINETICS: CS-SYMMETRIC ANSA-ZIRCONOCENE CATALYSTS
No full textMETALLOCENE-CATALYZED PROPENE POLYMERIZATION: FROM MICROSTRUCTURE TO KINETICS, 1 -
No full textA microstructural method for the determination of the reaction order with respect to the monomer in coordinated 1-alkene polymerizations is proposed. The method is applicable whenever enantioselective monomer polyinsertion competes with an intramolecular process introducing stereodefects of known and recognizable nature in the growing polymer chains at a rate independent of monomer concentration, [M] (e.g., chain or site epimerization for C-2-symmetric and C-s-symmetric ansa-metallocene catalysts, respectively). Indeed, in such cases, the stereoregularity of the polymer obtained at a given temperature results from the balance between the rates of the two competing processes, and is obviously a function of [M] because so is the rate of polyinsertion, . Therefore, from the way polymer stereoregularity changes with [M], the corresponding change of can be inferred. Compared with the conventional macroscopic approach based on measurements of monomer consumption, the microstructural route is more reproducible, and allows one to discriminate between "authentic" and apparent higher-order polyinsertion rate laws. In this paper, in particular, we show that the "trigger" hypothesis invoked to explain the observed pseudo-second-order kinetics of propene polymerization promoted by C-2-symmetric ansa-metallocene catalysts is inconsistent with the experimental results and that an alternative explanation assuming first-order kinetics at catalytic species which interconvert between a propagating and a "resting" state is more plausible
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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