1,721,012 research outputs found
Magnetic response of dithiin molecules: Is there anti-aromaticity in nature?
No full textAb initio current density formalism is used to investigate the response to external magnetic fields of the only known naturally occurring moieties which are formally anti-aromatic, i.e., dithiines. Magnetic susceptibility, nuclear shielding constants, and the topology of induced current densities indicate that although these molecules satisfy Hückel's rule for being anti-aromatic, they are not. In chiral dithiines, the multipolar expansion of the response contains non-vanishing anapole terms associated with 'spinning cuff' current lines. © 2003 Elsevier Science B.V. All rights reserved
Molecular response to a time-independent non-uniform magnetic-field
No full textThe response of a molecule to a static inhomogeneous magnetic-field is rationalized via multipole magnetic susceptibilities and induced magnetic multipole and anapole moments. The energy of the molecule interacting with the external field is expressed as a Taylor series in the powers of the field and its gradient at the origin of the coordinate system. It involves magnetic multipole tensors of increasing rank, which can be evaluated via quantum mechanical approaches. An electronic energy shift is caused by the feed-back interaction between the induced magnetic dipole moment and the external magnetic field, and between the induced magnetic quadrupole moment and the gradient of the magnetic field. It is shown that, for a static magnetic field with uniform gradient, the magnetic quadrupole moment is origin-dependent, but the total interaction energy and the induced magnetic dipole are invariant to a translation of the coordinate system. The formal advantages of a Geertsen approach to third- and fourth-rank mixed-multipole susceptibilities are discussed. © 2004 Elsevier B.V. All rights reserved
Topology of magnetic-field-induced current-density field in diatropic monocyclic molecules
No full textConcise information on the general features of the quantum-mechanical current density induced in the
electrons of a molecule by a spatially uniform, time-independent magnetic field is obtained via a stagnation
graph that shows the isolated singularities and the lines at which the current-density vector field vanishes.
Stagnation graphs provide compact description of current-density vector fields and help the interpretation of
molecular magnetic response, e.g., magnetic susceptibility and nuclear magnetic shielding. The stagnation
graph of six cyclic, planar aromatic molecules has been obtained at the Hartree-Fock level via a procedure
based on continuous transformation of the origin of the current density formally annihilating the diamagnetic
contribution. Some common distinctive elements observed for cyclic aromatic rings CnHn, with n
=3,4, . . . ,8, in the presence of a magnetic field normal to the molecular plane, are discussed. The results can
be used for a general discussion of diatropism in aromatic systems
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
- …
