1,720,960 research outputs found
On the occurrence of an electron-transfer step in aromatic nitration
No full textThe possibility that aromatic nitration proceeds via the formation of an electron donor-acceptor complex and its possible evolution in a ''contact'' radical pair is discussed on the basis of ab-initio configuration interaction computations on benzene/toluene NO2+ systems. The analysis of the region of the potential energy hypersurfaces corresponding to the two reactants kept at van der Waals distances shows the existence of a conical intersection between the ground state and the first excited charge transfer singlet, leading to electron transfer from the aromatic substrate to the nitronium ion, The activated state for electron transfer (ET) might then be identified with the ''early'' transition state invoked by Olah to explain the high positional selectivity of substitution in spite of a low substrate selectivity. The retention of positional selectivity at encounter-limited rates may then be ascribed to the fact that the formation of a Wheland intermediate corresponds to a radical pair recombination and as such is spin density driven. The objection that ET is a kinetically difficult step is met, the computed upper limit of the barrier to ET being 13 kcal/mol for the toluene substrate
Chemical Effects and Surface-properties - the Nature of An Adsorbed Complex
No full textThe general Born-Oppenheimer Hamiltonian of an adsorbate is expressed with reference to an AO basis by means of the second quantization technique, and the part representing the in situ adcomplex (admolecule possibly involving adsorption site atoms) is decomposed so as to show how contributions of different effects can be separated. A basic reference Hamiltonian of the adcomplex is defined, and the suggestion is made that it essentially represents a molecular system susceptible of treatment in terms of ordinary localized bonds. This suggestion is shown to be valid for one thing in the highly unsaturated surface complex FeCO (compared with Fe5CO), provided maximum-localization hybrids are used as elements of the AO basis. The example treated has also been used to show how two kinds of effects can be introduced and discussed: inductive effects, which appear to result from hybridization and from feedback compensations to charge shifts, and charge transfer between the adcomplex and the surface. The proposed local (''chemical'') description of an adcomplex appears to be an extremely useful tool for describing such complex adsorbates as those associated with Fischer-Tropsch catalysts. (C) 1994 John Wiley and Sons, Inc
On the protonation of the guanidine arm of cimetidine: A theoretical study
No full textThe hypothesis that the activity of cimetidine, a member of a family of anti-ulcer drugs, may depend on the protonation ability of the unsaturated nitrogens of its guanidine arm is studied quantitatively on the basis of energies and charge distributions determined by the PM3 method. At variance of histamine, whose two sites susceptible of proton activity are more or less energetically equivalent, in cimetidine protonation at the imidazole cycle is favoured. However, the ability of the cyano-guanidine arm to fur a proton depends on the conformation of the whole molecule and, in certain cases, it becomes comparable with that of the imidazole nitrogen. These points may provide a key to a better understanding of the mechanism of action of drugs of the cimetidine group
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
Hydrogen Bridges and Electron-transfer In Biomolecules - Study of A Possible Mechanism On A Model Charge Recombination System
No full textt is suggested that internal electron transfer in biomolecules such as multiheme proteins can take place along H-bridge/peptide-link chains by a proton-relay process followed by radicalic return to the initial condition. The plausibility of such a process is assessed by (a) reference to published suggestions and evidence; (b) estimates of vibrational characteristics and of the free energies of the various states of a model system consisting of three H-bridges and one peptide group. It appears that the proposed mechanism is highly efficient and very fast, at least if the H-bonds formed by a peptide group evolve in a concerted manner
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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