1,721,014 research outputs found
A proposed modification of CBS-4M model chemistry for application to molecules of increasing molecular size
No full textIn order to calculate more accurately the enthalpies of formation, DeltaH(t)(o)(298 K), for large molecules using the CBS-4M method, a new formulation of the empirical higher-level correction to the energy is proposed: posed: DeltaE = a\S\I-2(ii)ii + b(n(alpha) + n(beta)) + c Delta [S-2] + Sigman(i)d(i). The new methodology (CBS-4MB) applied to a set of 114 molecules of different size significantly decreases the mean absolute deviation from 3.78 to 2.06 kcal/mol
Ab initio study of the oxidation of CH3SH to CH3SSCH3
No full textThe thermodynamic and kinetic aspects of the oxidation of methanethiol to dimethyl disulfide by oxygen were studied using ab-initio molecular techniques. The reaction was considered as a multi-step process. Geometrical parameters and molecular energies for critical points were determined and characterized at the MP2(Full)/6-31G* level. Afterwards, a study was carried out employing different methods of theoretical model chemistry (G2, G2(MP2), G3 and CBS-4 MB), for which the equilibrium geometry at MP2(Full)/6-31G* was assumed as starting point. The steps governing the rate of the whole reaction were determined and prove to be the same for any level of theory employed. Although the MP2(Full)/6-31G* level is able to afford a good qualitative description of the reaction pathway, the quantitative aspect is less satisfactory. Results from the methods of theoretical model chemistry, on the other hand, accord well with experimental data, if available, and are very useful for studying the thermodynamic and kinetic aspects of chemical reactions
Solvent effect on keto-enol tautomerism in a new B-diketone: a comparison between experimental data and different theoretical approaches.
No full textThe novel -diketo compound (3-acetyl-4-oxopentanoic acid) OPAA is here synthesized, completely characterized in the solid state by means of X-ray crystallography and in solution by potentiometry and 1H and 13C NMR spectroscopy. In the solid state, OPAA exhibits the di-keto (DK) structure, while in solution a strongly solvent dependent tautomeric equilibrium is observed. Theoretical ab-initio calculations employing DFT at B3LYP/6-311G** level and different methods of theoretical model chemistry (CBS-4M, G3MP2, CBS-QB3) are used to extensively investigate the tautomeric equilibrium in comparison with experimental data. Solvent effects are evaluated using CPCM continuum solvation method; among all implied methods, CBS-4M is the one that better predicts experimental data and is able to qualitatively describe tautomeric equilibrium in solution, allowing thermodynamic calculation of pKa. Furthermore a supermolecular solvent approach is used to better analyze solvent-solute interactions in order to forecast chemical properties
A Theoretical Investigation on the Oligomerization of 4-Alkylsulfanyl bithiophene
No full textDesnity functional B3PW91 with Midix basis set was performed in order to investigate various mechanisms of growth of a symmetric octithiophene
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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