Pluralité des densités moyennes de moments multipolaires dans un milieu matériel. Application à un milieu diélectrique

Demonstration are often easier when more than one simplified model are taken into consideration separately. Since only one model is used at a time, all the results tend to be considered as absolute ; this applies specially to the values of the mean densities of multipolar moments. The relativity of mean densities of moments compared to the models considered may hardly be expected a priori, since the mean density of a multipolar moment consists of the total moment within a macroscopic volume-unit, and furthermore moments are additive when particles are gathered. Nevertheless, the present study shows that the dependence of the mean densities on the model, is evident in a dielectric médium ; at the same time their stability is observed, when spatial averaging is varied. The existence of many mean densities for a given moment, reconciliates some contradictory points of views, about the charge density in dielectrics. Moreover, the comparison of several well formalized models shows the danger of using on the same term macroscopic densities of moments that are issued from different models ; this also induces one to define intrinsic mean distributions in an impartial manner.Le fait de considérer, pour un milieu matériel, tantôt une représentation microscopique simplifiée, tantôt une autre, apporte une certaine souplesse dans les démonstrations. Comme on utilise généralement un seul modèle à la fois, tous les résultats tendent à être considérés comme absolus ; il en est en particulier ainsi de la valeur des densités moyennes de moments multipolaires. Le caractère relatif des densités moyennes de moments par rapport aux modèles considérés, peut d'ailleurs difficilement être soupçonné a priori, puisque la densité moyenne d'un moment multipolaire est le moment total dans un volume unité macroscopique, et que les moments sont additifs quand on réunit les particules. Pourtant, l'étude présente met en évidence dans un milieu diélectrique la dépendance des densités moyennes vis-à-vis du modèle, en même temps qu'elle montre leur relative stabilité lorsqu'on change la définition précise du passage à la moyenne. La pluralité des densités moyennes de moments concilie certaines appréciations contradictoires concernant la densité de charge dans un diélectrique ; surtout, la comparaison de plusieurs modèles bien formalisés montre le danger de manipuler sur un même plan des densités macroscopiques de moments qui sont issues de modèles différents, et incite à définir impartialement des distributions moyennes intrinsèques

Moment multipolaire électrique. Quelques propriétés des moments cartésien, irréductible, sphérique

Pour calculer l'énergie d'interaction électrostatique de deux systèmes de charges rapprochés, leur développement en multipôles électriques doit être poussé jusqu'à un ordre n élevé. Pour cette raison, on s'applique ici à généraliser à tout ordre plusieurs propriétés des moments multipolaires connues pour les premiers ordres. Le moment cartésien X(n) n'a pas d’autres propriétés que sa symétrie. Le moment irréductible Y(n) s’exprime simplement en fonction de X(n) , et, pour un corps à symétrie axiale en fonction du moment multipolaire scalaire Qn-. Sa norme ׀׀Y(n)׀׀ s'exprime simplement en fonction du moment sphérique ([math])

Modeling of substituent effects: self-parametrization, a case of interdependence of the coefficients, illustrated on the linear plus cross-term models

International audienceThe effect of one or several substituents X-i on a molecular property f (reaction rate, spectroscopic property, etc.) is often described in terms of substituent parameters lambda(i) (X) depending on the nature X of the substituent, and the position i of substitution. We call self-parametrization the case when the substituents are parameterized by the value lambda(i) (X) = f(i) (X) of f itself in the molecule monosubstituted by X in position i. On the example of the linear plus cross-term models, we show that self-parametrization implies that the coefficients of the model are interdependent. The theoretical relations between them are given, for equivalent or nonequivalent positions of substitution, and tested on measured acid-base equilibrium constants, and NMR coupling constants

Quantum microscopic vs. classical macroscopic calculations on the phenomenon of electrostatic influence

In order to compare microscopic and macroscopic approaches to the phenomenon of electrostatic influence, we have studied the atomic charges of an electric conductor, obtained either from macroscopic classical electrostatics, or microscopic quantum ab initio calculations. A torus was chosen as conducting material, built from valence monoelectronic atoms and influenced by an external point charge. The classical electric charges are obtained by integrating the macroscopic density over “atomic" sectors. This density is determined from a numerical integration of linearized electrostatic equations. The quantum charges are defined from Natural Orbitals in MP2/6-31G* calculations on clusters of different sizes. The overall agreement is good, with reasonable discrepancies due (i) to the continuity of the macroscopic model, which ignores the oscillations on atomic distances; and (ii) to the linearity constraint in the macroscopic equations

Altered sialylation of alveolar macrophages in HIV-1-infected individuals

In previous studies, we have demonstrated that O-glycans at the surface of HIV-1-infected cell lines were hyposialylated. Moreover, we and others have shown that HIV+ individuals produced autoantibodies that react with hyposialylated CD43, on T cell lines. Since the autoantigen responsible for this abnormal immune response was not easily found in the peripheral blood cells of corresponding patients, we searched for its possible presence in other sites. Using fluorescence staining of alveolar macrophages with various lectins, we show that the binding of the PNA lectin specific for asialo O-glycans is much more efficient on cells from HIV-1-infected individuals. Moreover, the degree of reactivity of PNA is correlated with the clinical stage of the illness

The hydrogen atom as an antenna. Retarded radiation field and spontaneous emission

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Anisotropic Organization and Microscopic Manipulation of Self-Assembling Synthetic Porphyrin Microrods That Mimic Chlorosomes: Bacterial Light-Harvesting Systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems. (1, 2) They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces