1,721,129 research outputs found
Un voyage en Suède et Norvège
No full textLenoir D. Un voyage en Suède et Norvège. In: Le Globe. Revue genevoise de géographie, tome 26, 1887. pp. 51-54
Dechlorination of perchlorinated aromatic compounds by graphite-potassium-intercalate
Full text linkLissel M, Kottmann J, Lenoir D. Dechlorination of perchlorinated aromatic compounds by graphite-potassium-intercalate. Chemosphere. 1989;19(10-11):1499-1502
What is the nature of the first-formed intermediates in the electrophilic halogenation of alkenes, alkynes, and allenes?
No full textThe pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K-f. Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K-i value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of, the bromonium ion. The effect of various polar groups situated in equatorial homo-allyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series
Synthesis and characterization of the syn-bromonium ion of 4-equ chloroadamantylidenadamantane, towards a chiral bromination reagent
No full textSynthesis and characterization of the stable bromonium ion of 4-equ-chloroadamantylideneadamantane is described. C-13 and H-1 NMR spectroscopy reveal preferred formation of the syn-isomer, which might be used as chiral bromination reagent using resolved starting material. (C) 2004 Elsevier Ltd. All rights reserved
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
Concentration dependence of the steric course of bromine addition to acenaphthylene. A product and kinetic study
No full textThe ratios of Z- to E-1,2-dibromo-1,2-dihydroacenaphthylene obtained in the bromination of acenaphthylene in chlorinated solvents have been determined by NMR as a function of the reagents concentrations. Kinetic measurements have shown that always the reaction occured through the same rate-determining step. A rationalization involving tight or solvent-separated ion pairs intermediates, is proposed. Copyright (C) 1996 Elsevier Science Lt
NATURE OF THE INTERACTION OF OLEFIN-BROMINE COMPLEXES - INFERENCE FROM (E)-2,2,5,5-TETRAMETHYL-3,4-DIPHENYLHEX-3-ENE, THE FIRST EXAMPLE OF AN OLEFIN WHOSE REACTION WITH BROMINE STOPS AT THE STAGE OF PI-COMPLEX-FORMATION
No full textA novel synthesis of tetramesityldisilene
No full textDimesityldiclorosilane undergoes reductive coupling with potassium-graphite to afford a mixture of cyclic polysilanes in a high yield and purity; this mixture is quantitatively converted to the title compound by photochemical irradiation. (c) 2006 Elsevier Ltd. All rights reserved
The first intermediates in the bromination of bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane, combination of experiments and theoretical results
No full textBicyclo[3.3.1] nonylidenebicyclo[ 3.3.1] nonane ( 1) and adamantylideneadamantane (Ad = Ad) are two caged olefins with closely related structures at the double bond. Both compounds react instantaneously with Br(2) in chlorinated hydrocarbon solvents to give mixtures of olefin - Br(2) aggregates identified as the 1: 1 pi-complex and bromonium tribromide, bromonium pentabromide ion pairs. The stoichiometry, formation constants and the electronic spectra of all the species present at equilibrium (pi-complex and bromonium ions), obtained by addition of bromine to alkene 1, have been determined in 1,2-dichloroethane at 25 degrees C and compared with the values that characterize the corresponding aggregates arising from Ad = Ad. The absence of the two bridging CH(2) groups in 1 significantly affects all the formation constants. Moreover, at variance with Ad=Ad, olefin 1 reacts with bromine to give, depending on reagent concentration, a substitution product. DFT (B3LYP) and ONIOM computations of 1: 1 Br(2) - olefin complexes for 1 and Ad = Ad confirm that the association energy is larger for the complex 1 - Br(2). The higher stability of this species seems to be correlated to the greater IP of 1 with respect to Ad = Ad which is able to compensate the reduced polarizability. The experimental value of the formation constant found for the complex 1 - Br(2), 643 vs 289 M(-1) further supports the primary role exerted by dispersion interactions in alkene-Br(2) pi-complexes
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