1,721,013 research outputs found
Can electron delocalization be revealed through the Source Function?
No full textThe Source Function (SF) [1,2] enables one to view chemical bonding and other chemical paradigms under a new perspective and using only information from the electron density observable, rho, and its derivatives. Being completely independent from the tools used to get rho, the SF represents a very useful descriptor, able in many cases to bridge the gap between the rich information ine gains from an ab-initio wavefunction of an ideal system and that, quite often more limited, but referred to a real system, obtained from an experimental rho derived from X-ray diffraction data.
The potential uses of SF are, however, not fully explored. In this lecture we discuss our recent work where the question of wheter the SF is or not capable to reveal electron delocalization has been addressed [3,4].
[1] R. F. W. Bader, C. Gatti, Chem. Phys. Lett. 1998, 287, 233-238. [2] C. Gatti, C. Cargnoni et al., J. Comput. Chem. 2003, 24, 422-436. [3] C. Gatti, Struct. Bond. 2011, 1, DOI: 10.1007/430_2010_31 [4] E. Monza, C. Gatti, L. Lo Presti, E. Ortoleva, J. Phys. Chem. Article ASAP DOI: 10.1021/jp204000d, Richard Bader's Festchrifte issu
Revealing electron conjugation through an observable
No full textThe Source Function (SF) [1,2] enables one to view chemical
bonding and other chemical paradigms under a new perspective and
using only information from the electron density observable, ρ, and
its derivatives. Being completely independent from the tools used to
get ρ, the SF represents a very useful descriptor, able in some cases to
bridge the gap between the rich information one gains from an ab-initio wavefunction of an ideal system and that, quite often more limited, but
referred to a real system, obtained from an experimental ρ derived from
X-ray diffraction data.
The potential uses of the SF are, however, yet not fully explored. In
a preliminary work, we addressed the question of whether the SF is or is
not capable to reveal electron conjugation [3]. Question arose because
of a recent claim [4] according to which “π-electron delocalization in
the benzene ring is not manifest in the SF when the reference point (rp)
- the point at which the atomic sources for its density are calculated
- is taken at the C-C bond critical point (bcp)”. Reasoning behind this
statement is the null contribution from π molecular orbitals (MOs)
to ρ in their nodal plane. However, since σ- and π-distributions are
not independent, but self-consistently interrelated, we conjectured
that some, albeit small, effect of electron conjugation could also be
manifest when the rp lies in the π-nodal plane, even though π-orbitals
do not obviously yield direct contributions to ρ in that plane. Results
on a series of increasingly π-conjugated systems demonstrate that this
is actually the case. By looking at the C-C bcp electron density for
the shortest bond(s) in cyclohexene, cyclohexadiene, benzene, i.e.
those bonds with largest double-bond character, one observes that
both the SF and the SF% contributions of the C atoms other than those
directly involved in such a bond increase with decreasing double bond
character and electron localization of the bond. The enhanced S%
value then becomes largely more evident when analysed using rps for
which the effect of π-electron conjugation takes place directly through
π-electron distribution rather than, indirectly, through σ-π electron
interdependency. In this work, the analysis is extended to more complex systems,
formed by more than one ring, with fully conjugated or partially
interrupted sequence of formal double-bonds and with planar or non
planar geometry. In the case of benzene, the analysis is also performed
on a ρ derived through multipole refinement of a set of X-ray diffraction
data taken on a benzene molecular crystal. In the inspected cases and
regardless of the theoretical or experimental origin of ρ, the SF reveals
capable to detect electron conjugation. Such an ability is independent
from a σ and π separation of ρ, since the SF tool was applied to the total
ρ. This observation is important in view of the possibility to recover
electron conjugation effects using both ρ’s derived experimentally
(hence without σ and π separation being allowed) and ρ’s where
the departure from symmetry inhibits a proper separation of σ and π
contributions. Using a MO approach, the σ and π contributions to the
SF values can also be revealed and quantified. [1] R.F.W. Bader, C. Gatti, Chem Phys Lett 1998, 287, 233-238. [2] C. Gatti,
F. Cargnoni et al., J Comput Chem 2003 24, 422-436. [3] C. Gatti, Struct Bond
2011 [4] L.J. Farrugia, P. Macchi, J. Phys. Chem. 2009, A113, 10058-10067
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
Dimensione educativa
No full textLa ricerca promossa dall’AMISM è finalizzata a monitorare la qualità del servizio offerto dalle singole scuole dell’infanzia che la compongono e dalla realtà associativa nel suo insieme. Il volume documenta gli esiti del percorso di ricerca, restituendo un profilo globale sulla salute dell’associazione e sulle sue prospettive di sviluppo, così come emerge dai dati forniti dalle oltre 200 scuole delle province di Milano e Monza-Brianza che hanno partecipato alla ricerca
Indagine di percezione: dati associativi medi
No full textLa ricerca promossa dall’AMISM è finalizzata a monitorare la qualità del servizio offerto dalle singole scuole dell’infanzia che la compongono e dalla realtà associativa nel suo insieme. Il volume documenta gli esiti del percorso di ricerca, restituendo un profilo globale sulla salute dell’associazione e sulle sue prospettive di sviluppo, così come emerge dai dati forniti dalle oltre 200 scuole delle province di Milano e Monza-Brianza che hanno partecipato alla ricerca
Indagine di percezione: dati medi per componenti
No full textLa ricerca promossa dall’AMISM è finalizzata a monitorare la qualità del servizio offerto dalle singole scuole dell’infanzia che la compongono e dalla realtà associativa nel suo insieme. Il volume documenta gli esiti del percorso di ricerca, restituendo un profilo globale sulla salute dell’associazione e sulle sue prospettive di sviluppo, così come emerge dai dati forniti dalle oltre 200 scuole delle province di Milano e Monza-Brianza che hanno partecipato alla ricerca
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
Revealing Electron Delocalization through the Source Function
No full textThe source function (SF) introduced in late 90s by Bader and Gatti quantifies the influence of each atom in a system in determining the amount of electron density at a given point, regardless of the atom’s remote or close location with respect to the point. The SF may thus be attractive for
studying directly in the real space somewhat elusive molecular properties, such as “electron conjugation” and “aromaticity”,
that lack rigorous definitions as they are not directly associated to quantum-mechanical observables. In this work, the results of a preliminary test aimed at understanding whether the SF descriptor is capable to reveal electron delocalization effects are corroborated by further examination of the previously investigated benzene, 1,3-cyclohexadiene, and cyclohexene series and by extending the analysis to some benchmark organic systems with different unsaturated bond patterns. The SF can actually reveal, order, and quantify π-electron delocalization effects
for formal double, single conjugated, and allylic bonds, in terms of the influence of distant atoms on the electron density at given bond critical points. In polycyclic aromatic hydrocarbons, the SF neatly reveals themutual influence of the benzenoid subunits. In naphthalene it provides a rationale for the changes observed in the local aromatic character of one ring when the other is partially hydrogenated. The SF analysis describes instead biphenyl as made up by two weakly interacting benzene rings, only slightly perturbed by the combination of mutual steric and electronic effects. Eventually, a new SF-based indicator of local aromaticity is introduced, which shows excellent correlation with the aromatic index developed byMatta and Hernandez-Trujillo, based on the delocalization indices. At variance with this latter and other commonly employed quantum-mechanical (local) aromaticity descriptors, the SF-based indicator does not require the knowledge of the pair density, nor the system wave function, being therefore promising for applications to experimentally derived charge density distributions
- …
