1,720,963 research outputs found
The application of the Hammett equation to 13C N.M.R. spectroscopy: correlations in the β-nitrostyrenes
No full textA series of 33 β-nitrostyrenes
has been prepared and their 13C N.M.R. spectra have been determined
in dimethyl sulphoxide and in deuterochloroform (where soluble). Hammett
correlations between substituent and the chemical shift of both the para ring
carbons and the α-carbon of the ethenyl group have been shown to be only
approximate. In contrast, an excellent correlation has been observed between
the chemical shift of the β-carbon of the ethenyl group and Hammett σ+
constants in all cases except those involving para substituents capable of
exerting a - R effect. Possible reasons for this are considered. The system
appears to be the first reported one where 13C measurements for both
meta- and para-substituted compounds give rise to correlations in which agreement
lies well inside the limits of experimental error, and shows promise as a
convenient method for calculating substituent constants and studying their
variation with solvent.
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The application of the Hammett equation to 13C N.M.R. spectrometry. IV. Solvent effects
No full textThe chemical shifts for Cβ of the vinyl group of styrene and 16 of
its meta-or para-substituted derivatives have been measured in seven solvents:
75% aqueous ethanol, ethanol, dimethyl sulfoxide, acetone, deuterochloroform,
carbon tetrachloride and benzene. Attempts have been made to correlate the data
by means of the Hammett equation, discrepancies being discussed in terms of
solvent dependence of σ and p.</jats:p
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
The Baker-Nathan Order: Hyperconjugation or a Solvent Effect?
No full textThe electronic effect of alkyl groups attached to an aromatic ring has been determined under varying conditions of electron demand by measuring their influence on the 13C n.rn.r. chemical shifts of the β-carbon of β-substituted styrenes. When the group, is para to the side chain their ability to donate electrons follows the order Me > Et > Pri > But, and, when meta, But > Pri > Et > Me. This order is the same in CCl4, CDCl3, Me2SO and EtOH solvent, the results suggesting that the Baker-Nathan effect is a hyperconjugative rather than solvation phenomenon. </jats:p
The Determination of Substituent Effects by 13C Nuclear Magnetic Resonance Spectrometry. The Effect of Solvent on CH2X Groups
No full textA previous study of the effect of meta- and para-CH2X substituents on the 13C n.m.r. chemical shifts of the α and β side-chain carbons of β- methoxycarbonylstyrenes (methyl cinnamates ) in ethanol has been extended to cover five additional solvents (Me2SO, Me2CO, CDCl3, CCl4 and C6H6). The results support the earlier claim that, for most substituents , the magnitudes of the substituent -induced shifts are proportional to the inductive effect of X. The major contributor to the latter appears to be the field effect generated by the C-X dipole, with the lines of force passing mainly through the molecule. In non-polar solvents, however, there is evidence that lines of force passing directly through the solvent can also influence the shifts in both the meta and para series. </jats:p
The application of the Hammett equation to 13C N.M.R. spectrometry. V. The transmission of electronic effects through azophenylene and ethenylphenylene groups
No full textThe 13C n.m.r. spectra of a number of ethyl (3-aryl)azo-, (4-aryl)azo-,
2-(4-arylethenyl)-cinnamates have been measured in
CDCl3. Comparison of the data with those for the corresponding ethyl
3-arylpropenoates has been used to investigate the attenuation of electronic
effects by the 3- and 4-azophenylene and the 4-ethenylphenylene linkages.</jats:p
The reductive halogenation of aromatic azo compounds
No full textThe conversion of compounds
of the type Ar-N=N-Y into p-X-ArNHNHY
by the reaction with hydrogen halides has been re-examined and has been found
to be not as general a reaction as expected. The most likely mechanism is one
involving nucleophilic addition of halide ion to the aromatic nucleus at the
position para to the azo group, this being facilitated by the strong -R
character of the -N=N-YH+ substituent. This addition is made
essentially irreversible by a series of prototropic shifts resulting in the
formation of a stable hydrazo compound.
</jats:p
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