1,720,991 research outputs found
o-Quinone Methide as Alkylating Agent of Nitrogen, Oxygen and Sulfur Nucleophiles. The Role of H-bonding and Solvent Effects on the Reactivity through a DFT Computational Study
No full textThe reactivity of the alkylating agent o-quinone methide (o-QM) toward NH3, H2O, and H2S, prototypes of nitrogen-, oxygen-, and sulfur-centered nucleophiles, has been studied by quantum chemical methods in the frame of DF theory (B3LYP) in reactions modeling its reactivity in water with biological nucleophiles
Concerted vs Stepwise Mechanism in 1,3-dipolar Cycloaddition of Nitrone to Ethene, Cyclobutadiene and Benzocyclobutadiene. A Computational Study.
No full textThe problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT (R(U)B3LYP/6-31G*) and post HF methods. According to calculations this reaction should take place via the concerted cycloaddition pat
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
Local environment of electrons trapped at the MgO surface: Spin density on the oxygen ions from O- 17 hyperfine coupling constants
No full textAl- and Ga-Doped TiO2, ZrO2, and HfO2: The Nature of O 2p Trapped Holes from a Combined Electron Paramagnetic Resonance (EPR) and Density Functional Theory (DFT) Study
Full text linkThe nature of hole centers in a series of MeO2 (TiO2, ZrO2, HfO2) metal oxides doped with trivalent Al or Ga ions has been investigated coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) technique with advanced density functional theory (DFT) calculations. The insertion of an aliovalent ion in the structure of the tetravalent oxides is compensated by the creation of oxygen vacancies leading to diamagnetic defective systems. The hole centers are observed by EPR after irradiation using ultraviolet (UV) frequencies (with consequent formation of an electronhole pair) and trapping of the photogenerated electron. The distortion imparted by the presence of the dopant stabilizes these centers. This generates a rich superhyperfine structure, since the dopants employed in this investigation (Al and Ga) have a nonzero nuclear spin. The DFT calculations performed on a wide set of possible hole-trapping sites occurring in the solid, allow us to identify (comparing the calculated EPR parameters of various models with the experimental ones) the nature of the observed hole centers in all cases. These are always three-coordinated oxygen ions with one Al (or Ga) ion in the first coordinative sphere. As it has been observed in other cases of holes centers, the spin density associated with the unpaired electron is concentrated in an O p-orbital with a modest delocalization toward the first neighboring ions
Cerium-doped zirconium dioxide, a visible-light-sensitive photoactive material of third generation
Full text linkThe dispersion of small amounts of Ce4+ ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (lambda > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material
Reactivity and Diastereoselectivity in the Diels-Alder Reaction of o-Quinodimethanes. An Experimental and DFT Computational Study
No full textEndo–exo Selectivity in Diels–Alder cycloadditions of several o-quinodimethanes (1–4) with acrylonitrile, 2-(5H)-furanone and N-methylmaleimide has been investigated in acetonitrile solution. Theoretical data reproduce fairly well both experimental absolute reaction rates and diastereoisomer ratios. The high endo selectivity has been rationalized mainly as a result of solvation effects (acetonitrile, PCM model) and reactant deformations. The latter is due to steric interactions
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