1,720,963 research outputs found
pH Basics
No full textThe exposition of the pervasive concept of pH, of its foundations and implementation as a meaningful quantitative measurement, in nonspecialist university texts is often not easy to follow because too many of its theoretical and operative underpinnings are neglected. To help the inquiring student we provide a concise introduction to the depth just necessary to allow a consistent coverage of this topic
Effect of Methoxy Substituents on the Structural and Electronic Properties of Fluorinated Cyclobutenes : A Study of Hexafluorocyclobutene and its Vinyl Methoxy Derivatives by XRD and Periodic DFT Calculations
No full textThe effect of the methoxy substituent on the structure, crystal packing, and electrostatic properties of
hexafluorocyclobutene (C4F6) was investigated in the solid-state with DFT-B3LYP calculations. Full geometry
optimizations were done for the parent compound and its two vinyl methoxy derivatives C4F5OCH3 and
C4F4(OCH3)2, starting from the structures obtained by single-crystal X-ray diffraction at low temperature. A full topological analysis, followed by the calculation of several electrostatic properties, was performed on the periodic electron density using the quantum theory of atoms in molecules. Eventually, the cohesive energies
of the three crystals were estimated. In the cyclobutene plane, the methoxy substitution yields a significant
electronic rearrangement involving the π-electrons. The solid-state (periodic) results agree with those obtained
by gas-phase calculations on C4F6 and its derivatives at a comparable level of theory. It was found that the substitution of one or two vinylic fluorine atoms with the OCH3 group considerably influences the molecular
dipole moment, which undergoes an enhancement in both the solid and the gas phase as large as 200% and
235% for C4F5OCH3 and C4F4(OCH3)2, respectively, with respect to that calculated for C4F6. The charge
rearrangement due to the substituents provides a significant electrostatic contribution to the lattice energy,
and in turn it can be related to the change in the observed crystal packing on going from C4F6 (space group P21/c) to both of its derivatives (space group P1bar). It is also shown that the dispersion energy significantly contributes to the lattice stability in all three compounds. Since the DFT calculations, in the limit of large
separations, entirely miss the dispersion term, this was estimated by applying a recently proposed dampening function to the semiempirical atom-atom C6 R-6 potentials in the mainframe of Spackman’s energy decomposition scheme for Mulliken multipoles
A study on the electrochemical behaviour of the 1,2 carbodicyano¬cyclobuten 3,4 dione structure
No full text
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
Appropriate 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
Determination of low levels of free fibres of chrysotile in contaminated soils by X-ray diffraction and FTIR spectroscopy
No full textA new analytical method for the determination of low levels (0.01-1 wt%) of free fibres of chrysotile in contaminated clayey, sandy and sandy-organic soils is described. The detection limit of 0.01 wt% is reached with an enrichment of free fibres of chrysotile in the sample using a standard laboratory elutriator for sedimentation analysis. The chrysotile quantitative determination is performed both by X-ray powder diffraction, using the internal standard and reference intensity ratio methods, and by Fourier-transform infrared absorption spectroscopy. The procedure can be successfully applied to different soils after removal, by a thermal treatment, of the matrix components which can interfere. This straightforward method fulfils the request of public institutions and private companies for an appropriate quantitative determination of chrysotile-free fibres in contaminated soils
- …
