1,720,984 research outputs found
Magnetic criteria for aromaticity
No full textThe coupled Hartree-Fock perturbation theory is used within the framework of the Pariser-Parr-Popleapproximation to compute the magnetic susceptibility, NMR shielding constants, and electric dipole polarizabilityof aromatic molecules. It is shown that the contribution to the magnetic susceptibility Componentperpendicular to the molecular plane serves as a more reliable aromaticity index than that based on magneticanisotropy or NMR data
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
The 1H NMR spectra of quinoline, quinoline N-oxide, the quinolinium ion and their monomethyl derivatives
No full text1H chemical shifts of quinoline, quinoline N-oxide and the quinolinium ion were obtained by complete analysis of their NMR spectra and interpreted critically in an attempt to quantify the possible different effects acting on the shielding constant of protons in these systems. Semi-empirical SCF calculations of electron charge densities and ring current contributions were also performed. It was found that the same effects that act on the shielding of protons directly bonded to the heterocyclic system also act, in an attenuated form, on the proton chemical shift of methyl groups in monomethyl derivatives. Vicinal coupling constants were rationalised in terms of -electron bond order and electro-negativity of neighbouring atoms
Substituent effect on the 1H NMR spectra of trans aryl methyl oxyranes and arylpropenes
No full textThe 1H NMR spectra of 1-aryl-2-methyl oxiranes substituted in the phenyl ring and of the corresponding vinyl derivatives have been analysed. The substituent effect on oxiran protons seems mostly polar in character, as shown by correlations with substituent constants, solvent effects and the fact that perturbation decreases with increasing distance from the substituent. The comparison with the corresponding vinyl derivatives, in which conjugation effects are present, confirms this point. The results also seem to exclude the possibility of substituents causing significant changes on the preferred conformation of the phenyl ring. Ring current contributions on oxirane protons, evaluated by SCF procedure, show that their change with substituents is very small and does not represent a significant part of the change of proton chemical shift with substituents
Variable temperature n.m.r behaviour of 1,5-benzodiazepinones and 1,5-benzodiazepinthiones
No full textThe n.m.r. variable temperature behaviour of the 7-membered ring in 1,3-dihydro-2H-1,5-benzodiazepin-2-one derivatives and in the corresponding 2-thiones has been investigated. The results indicate that the ring inversion barrier is higher in the thiones and dependent on substituents bonded at N-1. The activation parameters are obtained and discussed
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