1,720,990 research outputs found
A combined experimental and computational study of halogen and hydrogen bonding in molecular salts of 5-bromocytosine
Full text linkAlthough natural or artificial modified pyrimidine nucleobases represent important molecules with valuable properties as constituents of DNA and RNA, no systematic analyses of the structural aspects of bromo derivatives of cytosine have appeared so far in the literature. In view of the biochemical and pharmaceutical relevance of these compounds, six different crystals containing proton-transfer derivatives of 5-bromocytosine are prepared and analyzed in the solid-state by single crystal X-ray diffraction. All six compounds are organic salts, with proton transfer occurring to the Nimino atom of the pyridine ring. Experimental results are then complemented with Hirshfeld surface analysis to quantitively evaluate the contribution of different intermolecular interactions in the crystal packing. Furthermore, theoretical calculations, based on different arrangements of molecules extracted from the crystal structure determinations, are carried out to analyze the formation mechanism of halogen bonds (XBs) in these compounds and provide insights into the nature and strength of the observed interactions. The results show that the supramolecular architectures of the six molecular salts involve extensive classical intermolecular hydrogen bonds. However, in all but one proton-transfer adducts, weak to moderate XBs are revealed by C-Br...O short contacts between the bromine atom in the fifth position, which acts as XB donor (electron acceptor). Moreover, the lone pair electrons of the oxygen atom of adjacent pyrimidine nucleobases and/or counterions or water molecules, which acts as XB acceptor (electron donor)
X-ray Single-Crystal Structure and Magnetic Properties of Fe[CH3PO3)]·H2O: A Layered Weak Ferromagnet
No full textThe crystal and molecular structure of the layered weak-ferromagnet Fe[CH3PO3](H2O)-H-. has been solved by X-ray single-crystal diffraction techniques. Crystal data for Fe[CH3PO3](H2O)-H-. are the following: orthorhombic space group Pna2(1); a = 17.538(2), b = 4.814(l), c = 5.719(1) A. The structure is lamellar, and it consists of alternating organic and inorganic layers along the a direction of the unit cell. The inorganic layers are made of Fe(parallel to) ions octahedrally coordinated by five phosphonate oxygen atoms and one from oxygen of the water molecule. Each phosphonate group coordinates four metal ions, through chelation and bridging, making in this way a cross-linked Fe-O network. The resultant layers are then separated by bilayers of the methyl groups, with van der Waals contacts between them. The compound is air stable, and it dehydrates under inert atmosphere at temperatures above 120 degreesC. The oxidation state of the metal ion is +2, and the electronic configuration is d(6) high spin (S = 2), as determined from dc magnetic susceptibility measurements from 150 K to ambient temperature. Below 100 K, the magnetic moment of Fe[CH3PO3](H2O)-H-. rises rapidly to a maximum at T-max congruent to 24 K, and then it decreases again. The onset of peak at T = 25 K is associated with the 3D antiferromagnatic long-range ordering, T-N. The observed critical temperature, TN, is like all the other previously reported Fe(parallel to) phosphonates, and it appears to be nearly independent of the interlayer spacing in this family of hybrid organic-inorganic layered compounds. Below TN, the compound behaves as a "weak ferromagnet', and represents the third kind of magnetic materials with a spontaneous magnetization below a finite critical temperature, ferromagnets and ferrimagnets being the other two types
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
Molecular Structure and Large-Amplitude Motion of p-Diethynylbenzene from Gas-Phase Electron Diffraction and Theoretical Calculations
No full textThe molecular structure of p-diethynylbenzene has been determined by gas-phase electron diffraction and ab initio MO calculations at the HF/6-31G* and MP2/6-31G*(fc) levels. The two ethynyl groups undergo large-amplitude bending motions, making the equilibrium D2h model inadequate to describe the average structure from electron diffraction. Based on spectroscopic information on low-frequency modes, the electron diffraction data were approximated by a model consisting of a mixture of rigid conformers, differing only in the extent of the symmetric out-of-plane bending of the substituents. This gave the following geometrical parameters: Cortho−Cipso−Cortho = 119.2 ± 0.2°, rg(C−C) = 1.402 ± 0.003 Å, rg(Cring−Csp) = 1.431 ± 0.003 Å, and rg(CC) = 1.211 ± 0.003 Å. The computed re values (MP2) are 119.2°, 1.401 Å, 1.430 Å, and 1.223 Å, respectively, with the Cipso−Cortho bond 0.016 Å longer than the central C−C bond. The HF/6-31G* geometries of ethynylbenzene and p-diethynylbenzene indicate that the interaction of the ethynyl group with the ring is not affected by the presence of another ethynyl group in the para position. Comparison with solid-state results shows no appreciable effect of crystal environment on the ring deformation in these molecules
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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