1,721,070 research outputs found
Solid state 13C nuclear magnetic resonance spectrum of syndiotactic poly(4-methyl-1-pentene)
No full textThe chain conformation and the crystal packing of syndiotactic poly(4-methyl-1-pentene) have been studied by solid state 13C NMR cross polarization/magic angle spinning (CPMAS) spectroscopy. The presence in the CPMAS spectrum of two distinct resonances separated by ≈5 ppm (one γ-gauche effect) for the methyl carbons indicates that the two methyl carbons belonging to the monomeric unit are nonequivalent. This has been explained on the basis of the conformation of the lateral groups proposed in the literature. A single resonance of the backbone methylene carbons, although broad and with various shoulders, is present. The absence of the splitting of ≈10 ppm for this resonance, typical of other syndiotactic polymers having TTGG helical conformation, like polypropylene and poly(1-butene), is still fully consistent with the helical TTGG conformation of the chains of syndiotactic poly(4-methyl-1-pentene), since the conformational assignment of the lateral groups makes the two methylene carbons in conformational environments TG.GT and GT.GT, nearly equivalent. The narrow splitting of the resonance of the backbone methine carbons indicates that the helical TTGG conformation of the chains, approximately described with a s(12/7)2 symmetry, should be better described with a complex nonuniform helix with slightly different values of the backbone torsion angles of different structural units. In this nonuniform helix, backbone methine carbons of different structural units are nonequivalent since they experience different amounts of the γ-gauche shielding effect resulting in the asymmetric splitting of the corresponding resonance. Both resonances of the methyl carbons present narrow splittings, which can be traced back to packing effect. These splittings are easily interpreted on the basis of the model of packing proposed in the literature
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
A 29Si-27Al magic-angle spinning NMR study of natural silica glass from the Libyan Desert (Egypt)
No full textSamples of impactite glass, known as Libyan Desert Glass (LDG), were analyzed by NMR MAS in order to study the coordination of silicon and aluminum. In all samples, silicon was observed in a fourfold coordination. A very small amount of aluminum in fourfold, fivefold and sixfold coordinations was also observed in the three studied samples. The occurrence of five and six coordinated aluminum may be due to high pressure effects or due to the presence of Al-enriched micro-domains
Unsaturated alkoxy-substituted poly(p-phenylene 1,3,4-oxadiazole)s: Synthesis and chemical-physical characterization
No full textAppropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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