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The [(DABCO)(7)center dot(LiCH2SiMe3)(8)] Octamer: More Aggregated than the Parent Starting Material [LiCH2SiMe3](6) but Also Higher in Reactivity
No full textHerein, we report on the reaggregation of hexameric trimethylsilylmethyllithium [LiCH2SiMe3](6) with the donor base DABCO (1,4-diazabicyclo[2.2.2]octane) to give the unprecedented octamer [(DABCO)(7)center dot(LiCH2SiMe3)(8)] (1). The structure consists of four dimers, forming Li2C2 four-membered rings, connected to two [(DABCO)(3)center dot{(LiCH2SiMe3)(2)}(2)] chain fractions, interconnected by a single DABCO molecule. Interestingly, two different conformers of (LiCH2SiMe3)(2) dimers are present, caused by different steric demand. Higher steric strain in the center of the molecule causes an ecliptic arrangement of the Me3Si group along the Si-C-alpha bond, while at the periphery the more relaxed staggered conformation is enabled. The reactivity of trimethylsilylmethyllithium coordinated by DABCO was tested in the benchmark reaction with toluene. Although the aggregation of 1 is much higher than that of the parent [LiCH2SiMe3](6), the reactivity of the first is higher than that of the starting material, provided the octameric aggregation found in the solid state is maintained in nondonating solvents. While the hexamer would not react with toluene, the octamer gives benzyllithium, coordinated by DABCO. The reaction was monitored by H-1 NMR spectroscopy. Revisiting that known structure with modern technology revealed that [(DABCO)center dot(LiCH2Ph)](infinity) (2) crystallizes in the space group P2(1). 2 still is the only benzyllithium compound featuring the eta(3)-coordination mode to the C-ortho atom of the phenyl ring, presumably triggered by the singly donating DABCO molecule. More donor centers supersede this extra coordination to the carbanion.Danish National Research Foundation (DNRF); Land Niedersachse
Deaggregation of Trimethylsilylmethyllithium
No full textPolyamine donor bases such as N,N,N',N'-tetramethylethylenediamine (TMEDA), (-)-sparteine and N,N,N',N '',N ''-pentamethyldiethylentriamine (PMDETA) have been employed to deaggregate parent trimethylsilylmethyllithium (1). The crystal structures of the dimers [(TMEDA)-LiCH2SiMe3](2) (2) and [{(-)-spareteine)-LiCH2SiMe3](2) (3) were determined and reveal a four-membered ring as the central structural motif. The two lithium atoms are each coordinated by the chelating ligands and the carbanions. The Li-C contacts show alternating bond lengths. The monomer [(PMDETA). LiCH2SiMe3] (4) has the shortest Li-C contact of the aggregates discussed, and the Me3Si group points toward the central nitrogen atom of the PMDETA ligand, even if this alignment seems unfavourable for steric reasons. The bond lengths and angles of compounds 2-4 are discussed in relation to the orientation of the lone pair at the carbanion, ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).Deutsche Forschungsgemeinschaft [1178]; Bruxer axs; INCOATEC, Geesthach
The first asymmetric organolithium tetramers with simple ether donor bases
No full textThe donor-free trimethylsilylmethyllithium [LiCH(2)SiMe(3)](6) hexameric aggregate is for the first time broken up by simple ether donors such as diethyl ether (Et(2)O) and tert-butylmethyl ether ((t)BuOMe) to give the unprecedented asymmetric tetramers while the chelating dimethoxyethane (DME) gives the expected dimer
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
Pure α-metallated benzyllithium from a single-crystal-to-single-crystal transition.
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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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