1,721,127 research outputs found
Enantiopure furo[3,4-c]pyrazole derivatives by intramolecular nitrilimine cycloaddition: a stereoselectivity rationale based upon MP2 calculations
No full textSilver carbonate treatment of hydrazonoyl chloride 4 promoted the in situ generation of the corresponding nitrilimine bearing a stereocentre at the alpha-position of the ethylenic dipolarophile. Intramolecular cycloaddition of the latter intermediate involves the formation of 4-(S)-methyl-6-oxo-3,3a,4,5-tetra hydro-furo[3,4-c]pyrazole derivatives with very good yield and diastereoselectivity. Full rationalization of the experimentally observed stereoselectivity has been pursued by means of MP2 calculations
Uncommon aqueous media for nitrilimine cycloadditions. II. Computational study of the effect of water on reaction rate
No full textThe effect of water on the 1,3-dipolar cycloaddition reaction of nitrilimines to alkenes has been studied by HF and DFT ab initio calculations. As solvation models we used the polarizable continuum model and a model comprising small clusters consisting of reactants hydrogen-bonded to water molecules. In addition, a combination of these two models was considered. Reactivity has been qualitatively estimated by the familiar frontier molecular orbital approach and quantitatively computed by a recent extension to the hard soft acid base principle involving molecular reactivity indices. The main conclusions drawn from the extensive computational results are that (i) the effect of water is in general rather small and (ii) therefore, water is not directly responsible for the large acceleration of the 1,3-dipolar cycloaddition reaction of nitrilimines to alkenes. Its role is surely minor with respect to that played by the ionic catalysts present in the reaction mixture, in agreement with the conclusions drawn in the experimental companion paper
Assessment of mechanistic hypotheses of 1,3-dipolar cycloaddition of (arylsulfonyl)allene to nitrilimines by DFT reactivity indices
No full textThe mechanism of 1,3-dipolar cycloaddition between 1-[2-(acetylamino)phenylsulfonyl]-1,2-propadiene 1 and a series of N-(4-substituted)phenyl-C-methoxycarbonylnitrilimines 2 has been reinvestigated on the basis of DFT calculations and a quantitative formulation of the HSAB principle. Comparison of the new computational results with experimental findings led to the formulation of a mechanistic scheme involving both 1,3-prototropic and 1,3-arylsulfonyl shifts. The role of silver cation is also pointed out. (C) 2003 Elsevier Science Ltd. All rights reserved
DFT-based quantitative prediction of regioselectivity: Cycloaddition of nitrilimines to methyl propiolate
No full textDFT-HSAB prediction of regioselectivity in 1,3-dipolar cycloadditions: behavior of (4-substituted)benzonitrile oxides towards methyl propiolate
No full textThe regioselectivity of 1,3-dipolar cycloadditions between (4-substituted)benzonitrile oxides and methyl propiolate cannot be rationalized on the basis of the electron demand of the reactants or frontier molecular-orbital theory. To this problem, we have applied a quantitative formulation of the hard-soft acid-base principle developed within the density functional theory. Global and local reactivity indices were computed at B3LYP/6311+G(d,p) level. The details of charge transfer upon the reactive encounter have been elucidated, and the computed regioselectivity has been shown to be in good agreement with experimental data
Arylazide cycloaddition to methyl propiolate: DFT-based quantitative prediction of regioselectivity
No full textSeveral 1-(4-substituted)-phenyl-4- or 5-methoxycarbonyl-1,2,3-triazoles have been synthesized by 1,3-dipolar cycloaddition of the corresponding arylazides to methyl propiolate in carbon tetrachloride. The regioselectivity of these reactions cannot be rationalized on the basis of the electronic demands of the reactants or frontier molecular-orbital theory. Therefore, we applied to this problem a quantitative formulation of the HSAB principle to this problem developed within density functional theory. Global and local reactivity indices were computed at B3LYP/6-311 + G(d,p) level both in vacuo and in carbon tetrachloride (by the COSMO approach). The direction of charge transfer upon reactive encounter has been determined and the computed regioselectivity has been shown to be in good agreement with the experimental results. The relationship between computed and experimental data and how it related to the transition state energy. On these grounds, a generalization of the local HSAB principle has been recently introduced, ([21]) which enables one to compute, from mu and s of the reactants only, the grand potential variation DeltaOmega([22]) due to the charge transfer occurring in the very first step of the bond-forming interaction between specific atoms of the reactants. For the above reasons, DeltaOmega is expected to be proportional to the transition state energy and to provide a quantitative prediction of regioselectivity without the need of locating the transition state. This method has been successfully applied by us to the 1,3-DC between nitrilimines and alkynyl- or alkenyl dipolarophiles.[23] Continuing our investigations, we present here the first quantitative prediction of the regioselectivity involved in the cycloaddition between 1-(4-substituted)phenylazides 2 and methyl propiolate 3, which is based upon the DFT theory and the HSAB principle
Angular momentum and the two-dimensional free particle
No full textThe model system consisting of a free particle is revisited in order to fully exploit it as a teaching resource. Because of this pedagogical aim, we chose the two-dimensional free particle system as the best choice between the opposite requests of algebraic simplicity and physical interest. Indeed, this is the simplest system possessing angular momentum which is thus introduced without undue reference to rotational motion. After briefly reviewing the classical mechanics, the quantum mechanics of a free particle is described in some detail. It is shown how two different sets of eigenfunctions naturally arise from requiring the particle to have a well-defined linear momentum or a well-defined angular momentum. The reasons for the different constant of motions and quantization schemes appearing in the two sets are discussed. The complex eigenfunctions are plotted and the significance of such plots is critically assessed. Finally, it is shown how the two alternative descriptions of the free particle are related to each other
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
Surface properties of dispersed Fe oxide catalytic phases supported over mesoporous silica
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