1,720,955 research outputs found
Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO<sub>2</sub> Reduction
No full textHerein, we present the reduction chemistry of a [GRAPHIC] dinuclear alpha-diimine rhenium complex, 1, [Re-2(L)(CO)(6)Cl-2], with a proton responsive ligand and its application as a catalyst in the electrochemical CO2, reduction reaction (L = 4-tert-butyl-2,6-bis(6-(1H-imidazol-2-y1)-pyridin-2-yl)phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as alpha-diimine donors. The complex is an active catalyst for the electrochemical CO, reduction reaction. CO is the main product after catalysis, and only small amounts of H-2 were observed, which can be related to the ligand reactivity. The i(c)/i(p) ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)(3)Cl] in MeCN/H2O, albeit 1 requires a slightly larger overpotential (bpy = 2,2-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of 1. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO, The first 2e(-) reduction is ligand based and leads to homolytic N-H bond cleavage reactions at the imidazole units of 1, which is equal to a net double proton removal from 1 forming [Re2(LH-2)(CO)(6)Cl-2](2-). The second 2e(-) reduction process has been identified as an O-H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re-2(LH-3)(CO)(6)Cl](2-). In the presence of CO2, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO, instead of O-H bond cleavage
The generalized invariom database (GID)
No full textInvarioms are aspherical atomic scattering factors that enable structure refinement of more accurate and more precise geometries than refinements with the conventional independent atom model (IAM). The use of single-crystal X-ray diffraction data of a resolution better than sin/ = 0.6 angstrom 1 (or d = 0.83 angstrom) is recommended. The invariom scattering-factor database contains transferable pseudoatom parameters of the HansenCoppens multipole model and associated local atomic coordinate systems. Parameters were derived from geometry optimizations of suitable model compounds, whose IUPAC names are also contained in the database. Correct scattering-factor assignment and orientation reproduces molecular electron density to a good approximation. Molecular properties can hence be derived directly from the electron-density model. Coverage of chemical environments in the invariom database has been extended from the original amino acids, proteins and nucleic acid structures to many other environments encountered in organic chemistry. With over 2750 entries it now covers a wide sample of general organic chemistry involving the elements H, C, N and O, and to a lesser extent F, Si, S, P and Cl. With respect to the earlier version of the database, the main modification concerns scattering-factor notation. Modifications improve ease of use and success rates of automatic geometry-based scattering-factor assignment, especially in condensed hetero-aromatic ring systems, making the approach well suited to replace the IAM for structures of organic molecules
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
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
Theoretical Studies of Ru- and Re-based Catalysts for Artificial Photosynthesis
Full text linkThe conversion of light into energy stored in a sustainable and economic way is one of the main challenges of the 21st century.
Artificial photosynthesis is therefore an active research field in many parts of science.
Molecular catalysts have come a long way in the reduction of carbon dioxide and the oxidation of water to dioxygen.
Although a lot has been learned about the reactivity and stability of these catalysts, many unanswered questions remain.
This thesis hopefully takes a further step towards answering some of these questions.
Since computational studies rely heavily on the investigation of relevant points on the potential hypersurface, the optimisation procedure can become a major bottleneck.
Several approximations have been suggested for the step prediction, second derivatives and coordinates.
The latter have been shown to exhibit a great influence on the efficiency of the minimisation, with approximate decoupling for carefully designed internal coordinates.
Nevertheless, the implementation and thus performance varies widely for different software suites.
A new geometry optimisation software is therefore developed that allows a detailed investigation of each employed algorithm.
The focus is set on the impact of the primitive coordinates, as used in the redundant internals scheme, and tested on the convergence of several drug-like organic molecules.
Some corner-cases of molecular structures and a CO2 reduction catalyst are optimised with several approaches to corroborate the superior performance of the developed procedure.
Investigations into CO2 reduction catalysts have been conducted for some time, with a large number of results reported for rhenium-based complexes.
A recently published dinuclear rhenium catalyst with a proton responsive ligand combines the knowledge gained in those studies and reports a detailed analysis of the occurring reduction intermediates by infrared spectro-electrochemistry.
To achieve a greater insight on the molecular structure of the catalyst in solution, a computational basis for the calculation of infrared frequency shifts is devised.
The calibration of theoretical values to the crystal structure enables further comparison of calculated to the experimental spectra.
An extensive search for possible side-reaction products during reduction is then undertaken to assign each of the experimentally encountered intermediates, which underlines the usefulness of the approach in catalysis investigations.
A novel dinuclear ruthenium catalyst with an activity in the oxidation of water to dioxygen is inspected in the fifth chapter.
It has been proven experimentally to react by a water nucleophilic attack mechanism, contrary to other dinuclear complexes.
The computational examination of this catalyst and its comparison to other molecular systems with a similar active site may thus reveal the subtle structural details that determine the mechanism.
In a first step, the full range of possible reaction pathways is scanned, including all probable multiplicities in two oxidation states.
Fuelled by the failure of some density functionals to reproduce the correct mechanistic preference, a range of different methods and the influence of exact exchange is tested.
With the determined computational protocol the catalyst is compared to two similar water oxidation complexes on the same level of theory, which allows to unearth the difference in their structure and reactivity.
The gained insight suggests further modifications of the ligand structure that could steer the mechanism into one direction or the other
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