1,720,962 research outputs found
Sviluppo di metodolgie sotenibili per la catalisi omogenea di oro
No full textNegli ultimi due decenni, la catalisi omogenea d’oro(I) è diventata uno strumento utile per la trasformazione chimica di composti organici. Sono stati fatti grandi sforzi per la comprensione del ciclo catalitico e di altre variabili come la struttura del catalizzatore, la natura del legante, l’effetto degli additivi, ecc. Anche se in letteratura sono presenti molte informazioni sulla catalisi omogenea d’oro riguardo gli effetti dei leganti, l'ingegnerizzazione di nuovi catalizzatori tipo L-Au-X e lo studio sulle reazioni, è ancora basata su “trial and error” con un risultati a volte imprevedibili. Recentemente, è stato scoperto che l'anione abbia una partecipazione attiva in ogni singolo passo della catalisi e questo dipende sia dalla sua natura (capacità di coordinazione e basicità) sia dalla sua posizione. Sicuramente, l'effetto dell'anione dipende anche dal tipo di reazione e dall'RDS del ciclo catalitico. In questa tesi, grazie a una completa razionalizzazione dell'effetto dell’anione, derivante da studi sperimentali e teorici, sono state sviluppate metodologie sostenibili altamente efficienti. Pertanto, le reazioni possono essere eseguite convenientemente a temperatura ambiente, usando solventi neoterici o lavorando in condizioni “neat”, in assenza si sali d’argento e di acidi. Le principali reazioni studiate sono state l'idratazione e l'alcossilazione degli alchini, che generalmente richiedono grandi quantità di additivi acidi e/o alte temperature. Qui viene riportato per la prima volta un metodo innovativo e “green” per eseguire queste reazioni. Infine, questo lavoro dimostra che l'interazione tra il legante, l’anione, l'additivo e il solvente è cruciale per ottenere buoni risultati in termini di efficienza della reazione.In the last two decades, homogeneous gold(I) catalysis has become a useful tool for chemical transformation of organic compounds, and great efforts have been made in the understandings of the catalytic cycle and other variables like structure of the catalyst, nature of the ligand, effect of the additives, etc. Even if a large number of information on the ligand effects is present in the literature on homogeneous gold catalysis, the engineering of new L-Au-X catalysts and reactions is still based on trial and error, sometimes with an unpredictable outcome. Recently, it appears that the anion has an active participation in each single step of the catalysis and this depends both on its nature (co-ordination ability and basicity) and position. Surely, the effect of the anion also depends on the type of reaction as well as the RDS of the catalytic cycle. In this thesis, thanks to a complete rationalization of the counterion effect, resulting from experimental and theoretical studies, highly efficient sustainable methodologies have been developed. Thus, reactions can be conveniently run at room or mild temperature, using neoteric solvents or working in solvent-, silver-, and acid-free conditions. The main reactions studied were the hydration and alkoxylation of alkynes, that generally require large amounts of acidic additives and/or high temperature. Here is reported for the first time an innovative and green method for carrying out these reactions. Lastly, this work demonstrates that the interplay between the ligand, the counterion, the additive and the solvent is crucial in order to obtain good results in terms of efficiency of the reaction
New heteroscorpionate lanthanide complexes for ring-opening polymerisation of ε-caprolactone and rac-lactide
No full textYttrium and lanthanum amido-complexes with bis(pyrazol-1-yl) acetates in their coordination spheres were studied as the catalysts in epsilon-caprolactone and lactide ring-opening polymerisation. A high molecular mass poly(epsilon-caprolactone) (PCL) was obtained in almost quantitative yield under mild conditions. rac-Lactide polymerisations were less efficient and required quite harsh experimental conditions to obtain atactic PLA samples with moderate yields. The average chain-length of PCL was dependent upon the choice of the metal centre and the presence of substituents on the pyrazole rings of the ancillary ligand. The ground-state geometries of the complexes and the first stages of epsilon-caprolactone polymerisation were computationally modelled by means of DFT calculations
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
Hydration and alkoxylation of alkynes catalyzed by NHC-Au-OTf
No full textThe NHC-Au-OTf [NHC =
1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] catalyst was tested
in the alkoxylation and hydration of alkynes in a wide set of neoteric
solvents for the first time. We found that most of these solvents (ethyl
lactate, glycerol, propylene carbonate, d-limonene, -valerolactone, and
p-cymene) are comparable or better solvents with respect to traditional
VOSs. An important beneficial effect of -valerolactone was found in the
hydration of inactive diphenylacetylene and one order of magnitude
higher TOF was obtained with respect to the literature values. Kinetic
experiments and DFT calculations seem to indicate that the polarity of
the solvent and the presence of suitable proton acceptor/donor
functionalities on the solvent itself could be taken into account
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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