1,720,957 research outputs found
CATALYSIS IN APROTIC-SOLVENTS - INTERMOLECULAR AND INTRAMOLECULAR HYDROGEN-BONDING COMPLEXATION
No full textA mechanistic investigation is reported of aminolysis reactions of 2-hydroxy-5-nitro-alpha-toluenesulfonic acid
sultone (1) in aprotic solvents. The n-butylaminolysis of 1 in acetonitrile and in toluene requires two and three
molecules of amine, respectively. In the latter solvent, general bases strongly catalyze the reaction, and their
catalytic constants are well correlated by the hydrogen bonding parameter pKm. These results are interpreted
by a multistep mechanism where each intermediate can be stabilized via hydrogen bonding by general bases.
The mechanistic features depend on the stability of the intermediates and on the solvent characteristics. When
diamines such as polyoxyethylenediamines H 2NCH2(CH2OCH2)nCH2NH2 (2, n=2; 3, n = 4; 4, n = 6) are used as nucleophiles for the reaction with sultone 1 in toluene, much higher reactivities are observed when compared
to reactions of monoamines and alkylenediamines. This represents a novel type of intramolecular catalysis due
to intramolecular hydrogen bonding complexation between oxygen atoms and the ammonium group of the reaction
intermediates (Scheme 111). In toluene 2-4 also display a large basicity
Neutral Cobalt Carbonyl Catalysts for the Equilibration between CO and P(OPh)3 Ligands in Carbonylcobaltates
No full textThe substitution reaction of [Co(CO)(4)](-) by P(OPh)(3) was investigated in THF tinder a CO atmosphere, the reaction consists of an equilibrium, which, in order to be established, needs [Co-2(CO)(6)(P(OPh)(3))(2)]. The catalysis by this neutral carbonyl is formally related to a previously reported case, where either [Co-2(CO)(8)] or [Co-4(CO)(12)] was found to be necessary for the (CO)-C-13 scrambling in [Co(CO)(4)](-). The kinetic analysis reported in this paper indicates, however, that the two CO labilizations differ in their mechanism-the substitution by P(OPh)(3) is ascribed to a disproportionation-synpro-portionation of [Co-2(CO)(6)(P(OPh)(3))(2)]
Degenerate lithium-hydrogen exchange reactions: an alternative mechanism for metalation of CH4 in gas phase and in THF solution
No full textWe report here the results of an ab initio study of the lithium-hydrogen exchange reaction of CH4 + (CH3Li)(2) both in gas phase and in tetrahydrofuran (THF) solution. All the species involved in the reaction have been characterized at the Hartree-Fock, second-order Moller-Plesset (MP2)(full), and density functional theory (B3LYP) levels using the 6-31G(d,p) basis set. The effect of the solvent (THF) has been modeled using the Polarizable Continuum Model developed by the group in Pisa that includes both electrostatic and nonelectrostatic (cavitation and dispersion repulsion terms) contributions to the solvation energy. A main result of this study is the finding of a nonplanar transition state structure that leads to a barrier similar to 2 kcal/mol lower at the MP2 level than the one calculated based on a C-s six-membered ring transition state previously reported by Schleyer et al. for the same reaction (J. Comput. Chem. 10, 437 (1989)). We include here a detailed discussion of the differences between these two mechanistic alternatives and the effect of the solvent on both of them. The performance of the B3LYP hybrid functional is examined against our MP2 results to assess whether this methodology is reliable for the study of more complex metalation reactions in which the size of the reactant system prevents the use of MP2 methods as a way for including electron correlation
Synthesis of 1,3-Benzodioxoles, 1,3-Benzoxathioles and 1,3-Benzodithioles from Allenic Derivatives
No full text
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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