1,720,968 research outputs found
Di-mu-chloro-bis[bis(cumylcyclopentadienyl)yttrium(III)]
No full textThe title yttrocene, [Y-2(C14H15)(4)Cl-2] or [Y-2(cCp)(4)Cl-2] (cCp = cumylcyclopentadienyl), belongs to a class of complexes bearing pendant phenyl substituents which are potentially active as catalytic precursors in the stereospecific polymerization of olefins. In the crystal structure, the molecules possess a crystallographically imposed inversion centre and may be described as centrosymmetric mu-Cl-bridged dimers. No pi-aryl coordination to the metal is observed. A distorted tetrahedral coordination geometry is found around the two metal atoms
Highly stereoregular polymerization of 1,3-cyclohexadiene in the presence of Cp2Ni-MAO catalyst
No full textHighly stereoregular polymerization of 1,3-cyclohexadiene in the presence of Cp2Ni-MAO catalyst
No full textNeutral and cationic heteroscorpionate aluminium complexes: syntesis, structure, and ring-opening polymerization of e-caprolactone
No full textTreatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate
ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)-
AlR2 (R ) Me (1); R ) Et (2); R ) iBu (3)) in high yield. In the solid state, complexes 1-3 were
isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp
ligand 2-coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl
rings. The investigation of the solution structure of 1-3 by means of VT 1H NMR spectroscopy revealed
fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion
between the two enantiomers. The activation enthalpy (¢H#) for racemization processes (10.1 ( 0.2
kcalâmol-1 (1); 11.6 (0.3 kcalâmol-1 (2); 14.1 ( 0.2 kcalâmol-1 (3)) was dependent on steric hindrance
at the aluminum center. Reaction of 1 or 2 with B(C6F5)3 proceeds through net alkyl abstraction, forming
the expected cationic aluminum complexes [(bpzmp)AlR]+[RB(C6F5)3]- (R ) Me (4); R ) Et (5)), in
which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic
compounds (B(C6F5)3 or [Ph3C][B(C6F5)4]) proceeds by net â-H abstraction from the isobutyl group,
forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Bro ̈nsted
acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ringopening
polymerization of -caprolactone (-CL), producing high-molecular-weight polymers. The 1H
NMR monitoring of the reaction between 4 and -CL in 1:1 molar ratio showed that the initiation involves
monomer insertion into the Al-O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe]+-
[MeB(C6F5)4]- (bpzphe ) 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate
(8). The growth of the polymer chain occurs through continuous insertions of the monomer in
the Al-alkoxide bond
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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