1,720,977 research outputs found
Isomeric alkyl cation/arene complexes in the gas phase
No full textThe kinetics and the stereochemistry of the protonation-induced unimolecular isomerization of (S)-(+)-1-D-1-3-(p-tolyl)butane have been investigated in the, gas phase in the 100-160degreesC range. The process leads to the almost exclusive formation of the relevant meta isomer with complete racemization and partial 1,2-H shift in the migrating sec-butyl group. These results, together with the relevant activation parameters, point to the occurrence of low-energy, tightly bound isomeric sec-butyl cation/toluene complexes of defined structure and stability along the isomerization coordinate. The existence and the eta(1)-type structure of these low-energy intermediate species are confirmed by ab initio calculations on closely related systems at the MP2(full)/6-3l1++G**//HF/6-31 + G** level of theory. Their role in the relevant energy surface clearly emerges from the comparison of the present results with those concerning sec-butylation of toluene carried out under comparable experimental conditions
Theoretical aspects of physical-chemical parameters for the correct conservation of mummies on display in museums and preserved in storage rooms
No full textThis study is aimed at evaluating physical and chemical parameters which are considered as the most appropriate for the long-term preservation of mummies, distinguishing between those which are on display in museums and those which are preserved in storage rooms. The objective of this study is also to compare and discuss differences among the most widely known guidelines and norms of the field and to highlight the single phenomena which affect the decay of this kind of cultural asset
Crucial Role of Elusive Isomeric h-Complexes in Gas-Phase Electrophilic Aromatic Alkylations
No full textThe kinetics and stereochem. of the protonation-induced unimol. isomerization of (R)-1-D1-3-(p-fluorophenyl)butane have been investigated in the gas phase at 40-100 C and 70-760 Torr. This process leads to the formation of the relevant meta and ortho isomers with partial racemization of the migrating sec-Bu moiety. Complete racemization is obsd., instead, when the isomerization reaction involves a 1,2-H shift in the moving alkyl group. These results, together with the relevant activation parameters, fully confirm the previous evidence of the occurrence in the alkyl cation/arene PES of noncovalent .eta.-type intermediates of defined structure and stability, lying well below the classical .pi.-complexes, as confirmed by ab initio calcns. Their crucial role in detg. the positional selectivity of gas-phase electrophilic arom. substitutions clearly emerges from the comparison of the present results with the site selectivity measured in the corresponding bimol. arene alkylation carried out at the same temps. and pressures
Substituent effects on the stereochemistry of gas-phase intracomplex nucleophilic substitutions
No full textThe mechanism and stereochemistry of the intracomplex solvolysis of proton-bound complexes [Y center dot center dot center dot H center dot center dot center dot M](+) between M = (CH3OH)-O-18 and Y = 1-arylethanol [(S)-1-(para-tolyl)ethanol (1(S)), (S)-1-(para-chlorophenyl)ethanol (2(S)), (S)-1-(meta-alpha,alpha,alpha-trifluoromethylphenyl)ethanol (3(S)), (S)-1-(Para-alpha,alpha,alpha-trifluoromethylphenyl)-ethanol (4(S)), (R)-1-(pentafluorophenyl)ethanol (5(R)), (R)-alpha-(trifluoromethyl)benzyl alcohol (6(R)), and (R)-1-phenylethanol (7(R))] have been investigated in the gas phase (CH3F; 720 Torr) in the 25-140 degrees C temperature range. Gas-phase solvolysis of [Y center dot center dot center dot H center dot center dot center dot M](+) (Y = 2(S), 3(S), 4(S), and 7(R)) leads to extensive racemization above a characteristic temperature t(#) (e.g. at t(#) > 60 degrees C for 7(R)), whereas below that temperature the reaction displays a preferential retention of configuration. Predominant retention of configuration is instead observed in the intracomplex solvolysis of [Y center dot center dot center dot H center dot center dot center dot M](+) (Y = 1(S), 4(S), 5(R), and 6(R)) with the temperature range investigated (25 <= T <= 120 degrees C). These results indicate that the intracomplex solvolysis proceeds through the intermediacy of the relevant benzylic cations (Bz(+)), which is electrostatically coordinated to a H2O and a (CH3OH)-O-18 molecule (a pure S(N)1 mechanism). The obtained gas-phase mechanism is discussed in the light of related solution data. It is concluded that the stereochemistry of unimolecular solvolytic reactions is determined by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature and the dynamics of the solvent cage as well
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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