1,721,056 research outputs found
Solid state 1H NMR study, humidity sensitivity and protonic conduction of gel derived phosphosilicate glasses
No full textMultinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)(4)
No full textSolid state H-1, Si-29 and (31)p MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50degreesC. Gels with molar compositions 5, 10, 20 and 30 mol% P2O5 in P2O5-SiO2 were studied. After drying to 400degreesC the gels have very similar structure, formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P2O5 gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix. (C) 2004 Published by Elsevier B.V
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
The N-Terminal Nonapeptide of Cephaibols A and C: A Naturally Occurring Example of Mismatched Helical Screw-Sense Control
No full textThe N-terminal nonapeptide domain of the fungal nonribosomal peptide antibiotics cephaibolA and cephaibolC (AcPheAib(4)LeuIvaGly- Aib) is reported to adopt a right-handed helical conformation in the crystalline state. However, this conformation is at odds with the left-handed helicity observed in solution in related synthetic oligomers capped with Ac-L-PheAib(4) fragments. We report the synthesis of four diastereoisomers of the cephaibol N-terminal nonapeptide, and show by NMR and CD spectroscopy that the peptide containing the chiral amino acids Phe and Leu in the naturally occurring relative configuration exists in solution as an interconverting mixture of helical screw-sense conformers. In contrast, the nonapeptide containing the unnatural relative configuration at Phe and Leu adopts a single, stable helical screw-sense, which is left handed when the N-terminal Phe residue is L and right-handed when the N-terminal Phe residue is D
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Origin of Helical Screw Sense Selectivity Induced by Chiral Constrained Cα-Tetrasubstituted α‑Amino Acids in Aib-based Peptides
No full textThe mechanisms behind the propensity
of chiral constrained Cα-tetrasubstituted
amino acids (cCTAAs) to induce one particular helical screw sense,
when included in the Ac-Aib<sub>2</sub>-cCTAA-Aib<sub>2</sub>-NHMe
peptide model, were studied through replica exchange molecular dynamics,
potential of mean force, and quantum theory of atoms in molecules
calculations. We observed that cCTAAs exert their effect on helical
screw sense selectivity through the positioning of the side chain
to generate steric hindrance in either the (−<i>x</i>, +<i>y</i>, +<i>z</i>) or (+<i>x</i>, +<i>y</i>, −<i>z</i>) sectors of a right-handed
3D Cartesian space, where the <i>z</i> axis corresponds
to the axis of the helix and the Cα lies on the +<i>y</i> semiaxis (0, +<i>y</i>, 0). The different strengthening
of the noncovalent interactions, also comprising C–H···O
interactions, exerted by the cCTAA in the right-handed or left-handed
helix was also found important to define the preference of a cCTAA
for a particular helix screw sense
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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