1,720,965 research outputs found
THEORETICAL SECONDARY KINETIC ISOTOPE EFFECTS AND THE INTERPRETATION OF TRANSITION-STATE GEOMETRIES .2. THE DIELS-ALDER REACTION TRANSITION-STATE GEOMETRY
No full textTheoretical secondary deuterium and primary C-14 kinetic isotope effects have been calculated for concerted and stepwise Diels-Alder reactions of butadiene with ethylene and acrolein. Comparisons between theory and experiment on substituted cases give definitive evidence for the mechanism and transition state geometries of the Diels-Alder reactions. Five levels of ab initio theory up to MCSCF/6-31G* are used for isotope effect calculations. The isotope effects for the retro-Diels-Alder reaction of ethanoanthracene are calculated and compared directly to experiment
EMPIRICAL FORCE-FIELD MODELS FOR THE TRANSITION-STATES OF INTRAMOLECULAR DIELS-ALDER REACTIONS BASED UPON ABINITIO TRANSITION STRUCTURES
No full textA quantitative model based upon Allinger's MM2 force field has been devised to calculate the diastereoselectivity of intramolecular Diels-Alder (IDA) reactions. The parameters for the modified MM2 force field were derived whenever possible from ab initio calculations on the intermolecular transition structures for the Diels-Alder reactions of butadiene plus ethylene, acrolein, and acrolein coordinated to BH3. The force field reproduces the ab initio 3-21G transition structures for the intramolecular Diels-Alder reactions of 1,3,8-nonatriene and 1,3,9-decatriene. The force field was developed for both thermal and acid-catalyzed reactions and provides insight into the origins of the diastereoselectivity in the IDA cycloaddition for a wide variety of nonatrienes and decatrienes. The flexibility of the transition structure and the conformational effects due to the chain connecting the two reacting moieties were shown tote of the greatest importance in determining the stereochemical outcome of these reactions. The use of the parameters in the new MM3 force field was tested
A FORCE-FIELD MODEL FOR THE TRANSITION STRUCTURES OF INTRAMOLECULAR NITRILE OXIDE CYCLOADDITIONS
No full textA force field model for nitrile oxide cycloadditions accounts for observed diastereoselectivities in inter- and intramolecular reactions controlled by steric factors
CONCERTED AND STEPWISE MECHANISMS IN CYCLOADDITION REACTIONS - POTENTIAL SURFACES AND ISOTOPE EFFECTS
No full textCASSCF/6-31G* calculations have been performed on concerted and stepwise Diels-Alder reactions of butadiene with ethene, the dimerization of butadiene, and the dimerization of cyclobutadiene. The relative energies of concerted and stepwise mechanisms are compared, and the factors influencing these 'energies of concert' are discussed. The comparison of calculated isotope effects to experimental data provides support for theoretical results
THE INSIDE ALKOXY EFFECT IN INTRAMOLECULAR NITRILE OXIDE CYCLOADDITIONS - ABINITIO AND FORCE-FIELD MODELING
No full textMM2 parameters based on ab initio transition structures for nitrile oxide cycloaddition on allyl ethers were developed for INOC reactions
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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