1,721,003 research outputs found
Substrate selectivity and orientation in aromatic Substitution by molecular fluorine
No full textDirect elemental fluorination of representative aromatic substrates, including PhH, PhCH3, PhF, PhCI, PhBr, PhN02, PhCN, and PhOCH3, has been investigated in inert solvents, e.g., CC13F and other fluorocarbons, over the temperature range - 154 to 40 OC. In order to achieve the necessary control of the extremely reactive electrophile, and to minimize unwanted modifications of the reaction environment, the fluorination has been carried out at extremely low rates and correspondingly low conversions, generally below 0.01%, using as a reagent gaseous mixtures of F2 highly diluted ( < I mol %) in N2 or Ar. The partial rate factors, calculated from a consistent set of relative reactivity and orientation data measured at -78 OC in CC13F, correlate smoothly with the u+ constants for all the substituents investigated, giving a p+ value of -2.45 for aromatic substitution by elemental fluorine with a correlation coefficient of 0.993. These results characterize F2 as a highly reactive, and correspondently unselective, reagent, and support a polar electrophilic substitution mechanism that is discussed and compared with other plausible fluorination pathways
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
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
H18f + Sb2o3 - A New Selective Radiofluorinating Agent
No full text18F was selectively introduced into trihalomethyl groups using H18F and Sb2O3. E.g., treatment of PhCClF2 with H18F and Sb2O3 at 220° for 20 min gave 44% PhC18FF2
Labeling of fluorinated aromatics by isotopic exchange with fluoride-18F
No full textFluoroarom. compds. with a suitable activating substituent undergo exchange reactions with Rb18F to give 18F-labeled compds. with sp. activities >30 000 Ci/mol. p-FC6H4NO2 was treated with Rb18F in DMSO 140 min at 145° to give 40% p-18FC6H4NO2 with a sp. activity of 160 Ci/mol
Nucleophilic aromatic substitution; kinetics of fluorine-18 substitution reactions in polyfluorobenzenes. Isotopic exchange between fluorine-18(1-) and polyfluorobenzenes in dimethyl sulfoxide. A kinetic study
No full textNucleophilic arom. substitution reactions of C6F6, C6XF5 (X = H, Cl, Br), C6H2F4, o-C6H4FNO2 and p-C6H4FNO2 by F- were studied in Me2SO utilizing 18F-. Substitution of fluorine by 18F- is the only reaction obsd. with a C6F6 substrate. With other substrates fluorine substitution is predominant. The kinetic studies provide results consistent with an SNAr two-step mechanism and suggest an intermediate analogous to that for electrophilic arom. substitution. Consideration of σ- indicates consistency with MeO- substitution results with similar substrates. The possible utility of these reactions in labeling arom. compds. is noted
Rapid catalytic synthesis of carbon-11-labeled aromatics
No full text11C-labeled C6H6, PhMe, and C6H4Me2 were prepd. by trimerizing HC≡11CH with HC≡CH (I) and MeC≡CH (II) carriers over a SiO2-Al2O3 support activated by K2CrO4 at 24° for 10 min. I was 1.6 times more reactive than II in a competitive trimerization with HC≡11CH. 11C-labeled o-, m-, and p-Me2C6H4 were obtained in the ratio 3.4:10.6:86 in the reaction of HC≡11CH with II
F-18 TO-HALOGEN EXCHANGE-REACTION ON ALPHA,ALPHA,ALPHA-TRIHALOTOLUENES - A USEFUL ROUTE TO NO-CARRIER-ADDED (NCA) F-18 LABELED SUBSTITUTED ALPHA,ALPHA,ALPHA-TRIFLUOROTOLUENES
No full textA general method for the synthesis of substituted α,α,α-[18F]-trifluorotoluenes is described. Reactions of RC6H4CCl3 (R = H, p-Cl, o-F, m-F, m-NO2) and RC6H4CF3 (R = H, m-NO2, p-NO2, o-F, m-F) with two different radiofluorinating agents, [H-18F + Sb2O3] and [H-18F + SbF3], give the corresponding α,α,α-[18F]-trifluoro derivs. in 0-75% radiochem. yield. The radioactive products yields depended upon the nature of the R substituent and the reaction conditions. A discussion on the possible reaction mechanisms involved is reported
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
- …
