1,720,965 research outputs found
Complex with Tetracarbene Ligation and a Linear FeNO Unit
No full textThe iron(II) complex 1 of a macrocyclic tetracarbene binds NO to form a low-spin (S = (1/2)) {FeNO}(7) complex (2) with a linear FeNO unit and a short Fe-NO bond. IR, electron paramagnetic resonance, and Mossbauer spectroscopies as well as density functional theory calculations suggest some (FeNO+)-N-I character and reveal that the singly occupied molecular orbital of 2, resulting from the s-antibonding interaction of Fe dz(2) and the NO lone pair, is largely iron-based. Reduction yields a quite stable {FeNO}(8) species (3); both 2 and 3 feature very low Mossbauer isomer shifts (similar to 0.0 mm s(-1)).Fonds der Chemischen Industrie (Kekule-Scholarship); DFG [IRTG 1422
Disproportionation Equilibrium of a μ ‐Oxodiiron(III) Complex Giving Rise to C−H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct
No full textModel of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer
No full textMitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S-2} ligation which could be structurally characterized in its diferric form, 5(2-). In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 5(2-) and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 4(2-) are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H(2-) (230 +/- 4 kJ mol(-1)) and the free energy Delta G degrees(PCET) for the reaction with TEMPO (-48.4 kJ mol(-1)) are very similar to values for the homoleptic cluster 4H(2-) (232 +/- 4 kJ mol(-1),-46.3 kJ mol(-1)) the latter is found to react significantly faster than the mitoNEET model (data for 5H(2-): k = 135 +/- 27 M-1 s(-1), Delta H double dagger = 17.6 +/- 3.0 kJ mol(-1), Delta S double dagger = -143 +/- 11 J mol(-1) K-1, and Delta G double dagger = 59.8 kJ mol(-1) at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.DFG [1422, ME 1313/13-1]; U.S. NIH [5R01GM050422
Organometallic μ-Nitridodiiron Complexes in Oxidation States Ranging from (III/III) to (IV/IV)
No full textComplete Series of {FeNO}(8), {FeNO}(7), and {FeNO}(6) Complexes Stabilized by a Tetracarbene Macrocycle
No full textUse of a macrocyclic tetracarbene ligand, which is topologically reminiscent of tetrapyrrole macrocycles though electronically distinct, has allowed for the isolation, X-ray crystallographic characterization and comprehensive spectroscopic investigation of a complete set of {FeNO}(x) complexes (x = 6, 7, 8). Electrochemical reduction, or chemical reduction with CoCp2, of the {FeNO}(7) complex 1 leads to the organometallic {FeNO}(8) species 2. Its crystallographic structure determination is the first for a nonheme iron nitroxyl {FeNO}(8) and has allowed to identify structural trends among the series of {FeNO}(x) complexes. Combined experimental data including Fe-57 Mossbauer, IR, UV-vis NIR, NMR and K beta X-ray emission spectroscopies in concert with DFT calculations suggest a largely metal centered reduction of 1 to form the low spin (S = 0) {FeNO}(8) species 2. The very strong sigma-donor character of the tetracarbene ligand imparts unusual properties and spectroscopic signatures such as low Fe-57 Mossbauer isomer shifts and linear Fe N-O units with high IR stretching frequencies for the NO ligand. The observed metal centered reduction leads to distinct reactivity patterns of the {FeNO}(8) species. In contrast to literature reported {FeNO}(8) complexes, 2 does not undergo NO protonation under strictly anaerobic conditions. Only in the presence of both dioxygen and protons is rapid and clean oxidation to the {FeNO}(7) complex 1 observed. While 1 is stable toward dioxygen, its reaction with dioxygen under NO atmosphere forms the {FeNO}(6)(ONO) complex 3 that features an unusual O-nitrito ligand trans to the NO. 3 is a rare example of a nonheme octahedral {FeNO}(6) complex. Its electrochemical or chemical reduction triggers dissociation of the O-nitrito ligand and sequential formation of the {FeNO}(7) and {FeNO}(8) compounds 1 and 2. A consistent electronic structure picture has been derived for these unique organometallic variants of the key bioinorganic {FeNO}(x) functional units
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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