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Calixarene Hosts and Ammonium Guests: from endo-cavity Complexes to Interpenetrated Systems with Self-sorting Abilities
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Absolute Configuration Assignment of Chiral Resorcin[4]arenes from ECD Spectra
No full textRacemates of five chiral resorcin[4]arenes, four tetra-O-substituted and one hepta-O-substituted, have been resolved by enantioselective HPLC, and their ECD spectra have been recorded online by stopped-flow measurements. The absolute configuration has been assigned by comparison of the experimental ECD spectra with DFT and semiempirical calculations. For the four tetra-O-substituted resorcin[4]arenes, the ECD exciton couplet at longer wavelength depends on the chirality induced in the arene scaffold by the substituents rather than on the precise nature of the substituents themselves. Accordingly, the exciton chirality model with excitons localized on the arene scaffold, here generalized to Cn symmetry, accurately describes the relationship between stereochemistry and chiroptical properties for this couplet, while its application at shorter wavelengths is unsafe. For the significantly larger hepta-O-substituted system the assignment particularly benefits from the use of the semiempirical ZINDO method
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
Cavity Filling and Chirality Effects in Calixarene Threading
Full text link2014 - 2015In 2010, the Prof. Neri’s group showed that the calix[6]arene hosts were able to recognize dialkylammonium axles when they were coupled with the weakly coordinating Tetrakis[3,5 bis(triFluoromethyl)Phenyl]Borate (TFPB−) “superweak anion” that gives very loose ion-pairs with dialkylammonium cations in solution.1 These interpenetrated structures have been defined in supramolecular chemistry as pseudorotaxane and can be considered as synthetic precursors of catenane and rotaxane architectures2 which have showed appealing properties as molecular machines3. Interestingly, the threading of directional alkylbenzylammonium guests with calix[6]arene wheels, led to the stereopreference for the endo-alkyl complexation over the endo-benzyl one and this observed stereo-selectivity brought to the definition of the so called “endo-alkyl rule”4. On these basis we have studied the recognition abilities of calix[6]arene derivatives toward alkylbenzylammonium guests bearing aliphatic chains with different shape and length. In particular, we studied the validity of the endo-alkyl rule with alkylbenzylammonium axles bearing alkyl chains with different lengths and using guests bearing branched alkyl chains.
Then the thesis work has been extended to the threading of chiral systems. Special attention was paid to the possibility of obtaining, after threading, the chiral recognition of chiral alkylbenzylammonium ions using chiral calix[6]arene derivatives. The investigation techniques chosen for this purpose have been NMR spectroscopy and Mass Spectrometry by means of the "Enantiomer Labelled Method".
Finally the search for new and attractive potential chiral hosts resulted in the application of the "p-bromodienone route" for the first time on calix[6]arene derivatives5 and the study of the optical and electronic properties of inherently chiral resorcin[4]arene derivatives. [edited by Author]XIV n.s
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