Magnetisme dans certains composes organiques : les radicaux libres stables nitroxydes

Nous faisons une brève revue des principaux modèles théoriques proposés pour interpréter les propriétés magnétiques des systèmes organiques présentant des moments magnétiques localisés et appliquons ces modèles aux radicaux libres nitroxydes. Le signe et la grandeur de l'échange ont été mesurées sur une série de ces composés.     

Weak solutions for anisotropic discrete boundary value problems

In this paper, we prove the existence and uniqueness of weak solutions for a family of discrete boundary value problems for data f which belong to a discrete Hilbert space H. Moreover, as an extension, we prove some existence results of weak solutions for more general data f depending on the solution.     

Magnetic behaviour of NpAs2 from Mössbauer spectroscopy

The Mössbauer hyperfine spectra of the 60 keV resonance of ²³⁷Np in powder and single crystal absorbers of NpAs₂ were measured between 4.2 and 60 K. Below 18 K a simple magnetic plus quadrupole pattern is seen in accordance with a ferromagnetic spin structure in tetragonal NpAs₂. The isomer shift favors the 4+ charge state, the hyperfine field of 288 T implies a moment of 1.5μ_B at the Np ion. The large reduction compared to the free ion values points towards a strong mixing of the electronic ground state by crystalline field interactions. Above 18 K the spectrum changes into a complex hyperfine pattern indicating a sinusoidally modulated spin structure. Near 54 K a transition into the paramagnetic state is observed. Both magnetic transitions (18 and 54 K) exhibit a feature typical for a first-order character.     

Rapid screening technique for tetrachlorodibenzo-p-dioxins in complex environmental matrices by gas chromatography/tandem mass spectrometry with an ion-trap detector

A capillary column gas chromatographic/ion-trap tandem mass spectrometric (GC/MS/MS) technique was applied to the determination of tetrachlorodibenzo-p-dioxins (TCDDs) in complex environmental matrices. A detection limit of 500 fg μl^-1 injected, with a signal-to-noise ratio of 6:1, was obtained when MS/MS was combined with broadband ion ejection of matrix ions and selected ion isolation. Calibration graphs were constructed from extracted fragment ion mass chromatograms and were found to be linear over the range 0.5-200 pg. Concentrations of total TCDDs and of 2,3,7,8-TCDD were determined for extracts from fish, air and soil; in most cases, the results found by GC/MS/MS were comparable to those obtained by high-resolution MS and triple quadrupole MS.     

A numerical method for some nonlinear differential equation models in biology

In this paper, the authors present a full discretization of nonlinear generalisations of the Fischer and Burgers equations with the zero flux on the boundary. Efficiency of the method is derived via a numerical comparison between their numerical solution and the exact solution.     

Prenylated xanthone derivatives with antiplasmodial activity from Allanblackia monticola STANER L.C

Further study of the methanol extract of the stem bark of Allanblackia monticola STANER L.C. resulted in the isolation of a new prenylated xanthenedione, designated allanxanthone C, together with the five known xanthones, garciniafuran, tovophyllin A, rubraxanthone, norcowanin and mangostin and one saponin, stigmasterol-3-O-beta-D-glucopyranoside. The structure of the new compound was established by detailed spectroscopic analysis to be 1,2-dihydro-3,6,8-trihydroxy-1,1,7-tri(3-methylbut-2-enyl)xanthen-2,9-dione (3-hydroxyapetalinone C). The methanol extract and pure compounds were tested on two strains of Plasmodium falciparum, F32 (chloroquine sensitive) and FcM29 (chloroquine resistant). The IC50 values obtained ranged from 0.6 to 8.9 microg/ml. Their cytotoxicity was estimated on human melanoma cells (A375) and the cytotoxicity/antiplasmodial ratio was found to be between 15.45 and 30.46. The antimicrobial activities against a range of microorganisms of the crude extract and some of these compounds are also reported.     

Some size-dependent electronic properties in charged MgO clusters

Starting from two cubic pieces of a MgO crystal ((3 x 3 x 3) and (5 x 5 x 5)), both containing a central oxygen atom, two clusters are simulated with the help of a DFT-LDA method. These clusters are charged in order to be equivalent to pieces of a neutral crystal. In each cluster, a neutral vacancy analogous to a F center is created by removing the central oxygen atom. Then, F ⁺ and F ^{+ +} centers are simulated by removing one and two electrons. The main differences and similarities between the two sizes of clusters are studied: geometries, Mulliken charges, electronic distributions, gaps, ionisation potentials. An important result is that in any case, when a F center is simulated, the vacancy does not accept more than about one electron, the second one being spread in the rest of the cluster.Received: 17 March 2003, Published online: 12 August 2003PACS: 31.15.Ar Ab initio calculations - 36.40.Wa Charged clusters - 61.72.Ji Point defects (vacancies, interstitials, color centers, etc.) and defect clusters