188,239 research outputs found
Activation of the C-H bond by electrophilic attack: theoretical study of the reaction mechanism of the aerobic oxidation of alcohols to aldehydes by the Cu(bipy) 2+ /2,2,6,6-tetramethylpiperidinyl-1-oxy cocatalyst system
No full textWe have investigated the reaction mechanism of the selective aerobic oxidation of primary alcohols into aldehydes using a bipy-copper complex and the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical as cocatalysts (Gamez et al. Chem. Commun. 2003, 2412-2415) and compared it to the well-known oxidation by the TEMPO+ ion. Our
theoretical investigation shows that (a) the oxidation of alcohols to aldehydes by uncoordinated TEMPO+ takes place by electrophilic attack on the C-HR bond of the alcohol; (b) the Cu(bipy)2+ complex has two functions, namely, (1) it acts as a template that brings TEMPO and the (deprotonated) alcohol in proximity by coordinating these moieties in adjacent coordination sites, and (2) it oxidizes the TEMPO radical to (coordinated) TEMPO+ ion. The H abstraction from alcohol by TEMPO+ then proceeds as an intramolecular reaction, very much analogous to one of the reaction pathways with free TEMPO+ and with a remarkably low barrier. We stress that compared to other A-H bonds (A=C, N, O, F), the relatively high-lying C-H bonds are particularly susceptible to electrophilic attack, and notably the C-HR bond next to the O in an alcohol is so because it is pushed up by an O lone pair. Electrophilic attack, being common to the particular catalytic system studied in this paper and the well-known biotic and abiotic oxidation catalysis by heme
and non-heme complexes of the ferryl (FeIVO)2+ ion, appears to be a unifying electronic structure principle of C-HR hydroxylation and oxidation reactions
A construção da coerência em cenários pedagógicos on-line: uma metodologia para apoiar a transformação de cursos presenciais que migram para a modalidade de educação a distância
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia de Produção.A presente pesquisa enquadra-se em um cenário atual de transição, do qual fazem parte diversas universidades que passam a oferecer cursos a distância, utilizando-se das inúmeras vantagens que as novas tecnologias de informação e comunicação proporcionam para este fim. Particularmente, focou-se a discussão em torno do processo de redesign de disciplinas que migram do ensino presencial para a modalidade on-line de educação a distância, entendendo que esse processo implica considerar um conjunto complexo de fatores, sobretudo em se tratando de reutilizar materiais pedagógicos, adaptando-os e transformando-os para a nova modalidade. O termo Reengenharia Pedagógica (RP) foi utilizado para definir esse processo de transformação. A carência de metodologias que orientem a transição entre modalidades de ensino motivou este trabalho de tese. Propõe uma metodologia que apóie o design de cursos a distância, focando o alcance da coerência na construção de cenários pedagógicos. Por coerência entende-se a adequada combinação entre parâmetros de RP durante a composição de um cenário de ensino e aprendizagem a distância. Apoiando-se em abordagens ergonômicas empíricas, três pesquisas qualitativas foram realizadas. Na primeira foram identificados, a partir da análise de situações de ensino presencial, quais elementos e estratégias pedagógicas devem ser modificados (ajustados, suprimidos ou alterados) para transformar uma disciplina para a modalidade on-line. Na segunda pesquisa procurou-se compreender quais são as etapas e as estratégias utilizadas por professores em um processo de RP, observando como eles procedem para realizar a tarefa quando esta é feita sem o apoio de ajudas conceituais. Na terceira pesquisa foi verificado como os professores procedem diante da possibilidade de conduzir o processo examinando um conjunto de dez parâmetros que auxiliam a composição da coerência dos cenários pedagógicos. Estes estudos concluem propondo uma metodologia para RP de cursos a distância, na modalidade on-line
Crystallographic Evidence of Nitrate-p Interactions Involving the Electron-Deficient 1,3,5-Triazine Ring.
No full textThe reaction of Zn(NO3)2·6H2O or Cu(NO3)2·3H2O with the star-shaped ligand 2,4,6-tris(di-2-picolylamino)[1,3,5]triazine (dipicatriz) in acetonitrile results in the formation of the mono- or trinuclear coordination compounds [Zn(dipicatriz)(NO3)2] (1), [Zn3(dipicatriz)(NO3)6](CH3CN)3 (2), and [Cu3(dipicatriz)(NO3)2(H2O)6](NO3)4 (3), depending on the metal-to-ligand ratios used during the crystallization process. Their crystal structures exhibit unique supramolecular interactions. Compounds 1 and 2 show anion−π interactions between coordinated nitrate ions and the s-triazine ring. Compound 3 exhibits remarkable interactions between two noncoordinated nitrate anions and the two faces of the electron-deficient heteroaromatic ring, corroborating earlier theoretical investigations in this area. New theoretical investigations have been carried out on nitrate−π interactions, taking into account the particular position of the anion toward the aromatic ring observed in the crystal structures
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Solvent control in the synthesis of Zn(II) and Cd(II) supramolecular compounds with N,N'-{2,4-di-[(di-pyridin-2-yl)amine]-1,3,5-triazine}ethylenediamine.
No full textAppropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
Crystallographic and Theoretical Evidence of Acetonitrile-p Interactions with the Electron-Deficient 1,3,5-Triazine Ring
No full textReaction of zinc(II) chloride with the ligand N,N‘-{2,4-(di-2-pyridylamino)-1,3,5-triazin-6-yl}-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (oxodendtriz) in acetonitrile/methanol results in the formation the tetranuclear complex [Zn4(oxodendtriz)Cl8](CH3CN)2(H2O)1.5. Notably, its crystal structure reveals unique π-interactions with the first experimental evidence of electron-rich/electron-poor molecule pairing, which is corroborated by computational studies. In addition, infinite helical water chains with both right- and left-handed configurations are present in the crystal lattice
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