1,720,963 research outputs found
Quantitative Ligand Affinity Scales for Metal Triflate Salts: Application to Isomer Differentiation
No full textOwing to the importance of metal triflates in catalysis, the affinity of the cationic center for a selection of organic ligands was explored for InIII and ZnII triflates. The organic Lewis bases include a variety of carbonyls (amides, unsaturated ketones, a lactone) and cyclic 1,2-diols. The relative affinity of the ligands for the cationic center in triflates was quantitatively determined on the basis of relative ion concentrations determined by electrospray-ionization mass spectrometry. The affinity scales were discussed with reference to gas-phase proton basicity and Lewis basicity scales. Structural isomers and stereoisomers display significant affinity differences in several cases. In the case of isomer mixtures, a model describing the relative peak intensities in the mass spectra was developed. On this basis, an isomer titration method was set up. Remarkably, this MS-based method overcame the blindness of mass spectrometry to isomers without the need for isotope labeling or MS/MS experiments. This model may prove to have applications in analytical chemistry and catalysis
A quantitative approach of the interaction between metal triflates and organic ligands using electrospray mass spectrometry
No full textThe interaction between two Lewis "superacid" catalysts Zn(OTf)2 and In(OTf)3 and series of amide and phosphate ligands is quantitatively characterized by electrospray ionization mass spectrometry (ESI-MS). A specific feature of the ESI-MS spectra of the mixture of metal triflates and Lewis bases is the formation of ionic adducts resulting from the displacement of one triflate anion by two neutral ligands. A ligand competition model is developed, which describes the relative intensities of the ionic adducts as a function of relative ligand concentrations. The relative affinities deduced from the ligand competition method are combined in an affinity scale for the metal triflate. © 2012 American Society for Mass Spectrometry
Energetics and Structures of Adducts of JohnPhos(Au+), PPh3(Au+), and IPr(Au+) with Organic Substrates: A Mass Spectrometry and DFT Study
No full textWe report a mass spectrometry study of the interaction between three representative Au(I) catalysts containing the ligands L = JohnPhos (2-biphenylyl[bis(2-methyl-2-propanyl)]phosphine), TPP (PPh3, triphenylphosphine), the N-heterocyclic carbene IPr (carbene form of 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-imidazole), and 27 organic molecules S (substrates) bearing organic functionalities often present in reactants (alkenes, alkynes, allenes, enol ethers, aldehydes, ketones, and epoxides). An experimental scale of gas-phase relative binding energy between three L(Au+) ions and the organic substrates was established by energy-resolved experiments of the 81 cationic two-coordinate gold adducts L(Au+)S using a quadrupole ion trap mass spectrometer. The experimental scale is expressed in units of normalized collision energy for 50% dissociation (NCE50) of the precursor ion. In parallel, the gas-phase bond dissociation energetics and the structure of adducts were probed by DFT calculations. The experimental affinity order of each substrate for the three cationic gold complexes L(Au+), IPr(Au+) > TPP(Au+) > JohnPhos(Au+) is well reproduced by the calculated bond dissociation energies ΔE. At the computational level used in this study, the agreement between the calculated ΔE and the experimental NCE50 values is limited to series of substrates with the same functionality, and reasonable correlations NCE50 vs ΔE are observed within series. The DFT-optimized structures are discussed and compared with available X-ray crystal structures. Although no general trend can be observed between bond lengths or their changes upon coordination with the L(Au+) cations and dissociation energies, a significant correlation between the Au-O distance (S = O-bases) and ΔE is observed
Catalytic intramolecular carbonyl-ene reaction with ketones: Evidence for a retro-ene process
No full textThe ene-process with unsaturated ketones was catalyzed by Lewis acids such as bismuth or indium triflates. Unlike aldehydes, the reverse ene-process occurs with ketones, resulting in incomplete conversions, as shown by control experiments and analysis by ESI-MS
Metal triflates and triflimides as Lewis "superacids": Preparation, synthetic application and affinity tests by mass spectrometry
No full textMetal salts of triflic acid, CF3SO2OH and of triflimidic acid, [CF3SO2]2NH, often called "Lewis superacids", are powerful catalysts for several classes of reactions. Typical applications developed at the Institut de Chimie de Nice during the last decade, mostly in the domain of inter- and intramolecular carbon-carbon, carbon-oxygen and carbon-sulfur bond formation, are illustrated. During the last three years, the characterization of metal triflates and triflimides by electrospray mass spectrometry was also accomplished in our Institute. Displacement of one anion of the salt by strong neutral ligands was developed as a mean for generating characteristic positive ions. In an effort toward the quantitative description of the ligand/metal bonding and the catalytic role of the Lewis acid, a ligand competition method was devised. An exploratory study attests that subtle structural effects on the donor/acceptor interaction can be evidenced using this simple mass spectrometric method. Copyright © 2012 John Wiley & Sons, Ltd
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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