1,721,100 research outputs found
Redox Chemistry of Selenenic Acids and the Insight It Brings on Transition State Geometry in the Reactions of Peroxyl Radicals
No full textThe redox chemistry of selenenic acids has been explored for the first time using a persistent selenenic acid, 9-triptyceneselenenic acid (RSeOH), and the results have been compared with those we recently obtained with its lighter chalcogen analogue, 9-triptycenesulfenic acid (RSOH). Specifically, the selenenyl radical was characterized by EPR spectroscopy and equilibrated with a phenoxyl radical of known stability in order to determine the O–H bond dissociation enthalpy of RSeOH (80.9 ± 0.8 kcal/mol): ca. 9 kcal/mol stronger than in RSOH. Kinetic measurements of the reactions of RSeOH with peroxyl radicals demonstrate that it readily undergoes H-atom transfer reactions (e.g., k = 1.7 × 105 M–1 s–1 in PhCl), which are subject to kinetic solvent effects and kinetic isotope effects similar to RSOH and other good H-atom donors. Interestingly, the rate constants for these reactions are only 18- and 5-fold smaller than those measured for RSOH in PhCl and CH3CN, respectively, despite being 9 kcal/mol less exothermic for RSeOH. IR spectroscopic studies demonstrate that RSeOH is less H-bond acidic than RSOH, accounting for these solvent effects and enabling estimates of the pKas in RSeOH and RSOH of ca. 15 and 10, respectively. Calculations suggest that the TS structures for these reactions have significant charge transfer between the chalcogen atom and the internal oxygen atom of the peroxyl radical, which is nominally better for the more polarizable selenenic acid. The higher than expected reactivity of RSeOH toward peroxyl radicals is the strongest experimental evidence to date for charge transfer/secondary orbital interactions in the reactions of peroxyl radicals with good H-atom donors
6,835,216 - Chain-breaking antioxidants
No full textCompounds, preferably 5-pyrimidinol and 3-pyridinol derivatives, that act as effective chain breaking antioxidants of both the lipid and water-soluble variety (analogous to the natural Vitamins E and C), many of which are more reactive toward peroxyl radicals than the most potent form of Vitamin E. These compounds may exhibit many chemopreventive effects associated with conditions in which free radical-mediated cellular damage or disruption is implicated and Vitamins E and C are shown to have protective effects. Additionally, these compounds should be excellent oxidation inhibitors as additives to fuels, lubricants, rubber, polymers, chemicals, solvents and foodstuffs
Inhibitor combination, resin mixture containing same and use of same
No full textThe use of a mixture of 5-pyrimidinol derivatives with sterically hindered phenol derivatives for adjusting the reactivity and the gel time of resin mixtures and reactive resin mortars based on radically polymerizable compounds is described. Furthermore, a resin mixture containing the inhibitor combination, a reactive resin mortar containing this resin mixture and two-component mortar systems with the reactive resin mortar according to the invention and a hardener with improved stability in storage and good low-temperature hardening properties are also described. The resin mixture according to the invention is suitable in particular for chemical fastening of construction elements in boreholes in various substrates
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
3-Pyridinols and 5-pyrimidinols: Tailor-made for use in synergistic radical-trapping co-antioxidant systems
No full textThe incorporation of nitrogen atoms into the aromatic ring of phenolic compounds has enabled the development of some of the most potent radical-trapping antioxidants ever reported. These compounds, 3-pyridinols and 5-pyrimidinols, have stronger O–H bonds than equivalently substituted phenols, but possess similar reactivities toward autoxidation chain-carrying peroxyl radicals. These attributes suggest that 3-pyridinols and 5-pyrimidinols will be particularly effectiveco-antioxidants when used in combination with more common, but less reactive, phenolic antioxidants such as 2,6-di-tert-butyl-4-methylphenol (BHT), which we demonstrate herein. The antioxidants function in a synergistic manner to inhibit autoxidation; taking advantage of the higher reactivity of the 3-pyridinols/5-pyrimidinols to trap peroxyl radicals and using the less reactive phenols to regenerate them from their corresponding aryloxyl radicals. The present investigations were carried out in chlorobenzene and acetonitrile in order to provide some insight into the medium dependence of the synergism and the results, considered with some from our earlier work, prompt a revision of the H-bonding basicity value of acetonitrile to β2H of 0.39. Overall, the thermodynamic and kinetic data presented here enable the design of co-antioxidant systems comprising lower loadings of the more expensive 3-pyridinol/5-pyrimidinol antioxidants and higher loadings of the less expensive phenolic antioxidants, but which are equally efficacious as the 3-pyridinol/5-pyrimidinol antioxidants alone at higher loadings
Variations on the Author
Get PDF“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
Get PDFWe 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
Get PDFWe 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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