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It has been shown by experimental and theoretical studies that exchange interactions in non planar π systems are strongly dependent on the geometrical parameters characterizing the non planarity. A study of the exchange interaction in isolated nitroxide biradicals is presented for non planar paraphenylene bis nitroxide 3 [1,4-di-t-butyl-2,5-bis(t-butylaminoxyl) benzene (anti)], non planar metaphenylene bis nitroxides 7 and planar metaphenylene bis nitroxide 9. Determination of positive or weak negative exchange interactions in isolated molecules in dilute solution is difficult and can be inaccurate. 3 is a ground state singlet 2J/k = −35K. Metaphenylene bis nitroxide 7g [1-t-butyl,3,5-bis(t-butylaminoxyl) benzene] and 9 [1,2,9-tetrahydro 2,2,4,4,6,6,8,8-octamethyl-4H,6H-benzo[1,2d; 4,5[dcirc]]bis(1,3 oxazine)-1,9 dioxyl] are probably ground state singlets, the tripletsinglet separation being smaller than 10K, contrary to theoretical predictions. X-Ray structure of 7a A [1,3,5-trimethyl-2,6-bis(t-...

A fullerene-like ball is found in a second century mosaic from Porta Marina Roman Baths near Ostia. Although it could be an hexadecahedral ball (28 vertices, 4 hexagonal and 12 pentagonal faces), it is probably an erroneous representation of a dodecahedral ball.

High amounts of micronized C60 have been injected intraperitoneally into Swiss mice. Until the fourteenth day, they were still alive without any behaviour trouble. C60 was well absorbed, and found localized in spleen and liver. Inside the liver, C60 was

In order to check its possible acute toxicity, C60 was incorporated into living human phagocytes. It was observed that C60 has no influence on the survival of human leukocytes.

New experimental data on the vibrational spectrum of C60 have been recorded with neutron inelastic scattering (NIS). These data modify the positions of the lattice modes, which previously were reported at too high frequencies. New information on the frequencies of the intramolecular modes has also been obtained using a better instrumental resolution.

Fluorination of pure C60 and a C60/C70 mixture has been studied under different experimental conditions: reaction time, temperature, fluorine pressure. The products obtained have been characterized by IR and NMR spectroscopy, X-ray diffraction, and mass spectrometry analysis by positive and negative ions.

A single crystal of C60 grown from a toluene solution has been studied by X-ray diffraction at room temperature. The lattice is face-centered cubic, with space group Fm3m and Z = 4, which agrees with previous powder diffraction measurements. It is shown that, contrary to what is obtained in other plastic crystals, the Pauling-Fowler model (the so-called free rotation one), which implies an isotropic molecular disorder, gives a better description of the molecular packing than the site model does. It is concluded that the molecules undergo a rotational diffusion as previous molecular dynamics simulations have described.

La chaleur specifique du tetramethyl-2,2,6,6 piperidinol-4 oxyle-1 a ete mesuree de 1,3°k a 74°k. La courbe Cp = f(T) montre une anomalie dont le maximum se situe a 4°k, que l'on interprete par l'existence d'une interaction d'echange antiferromagnetique entre proches voisins. Les resultats numeriques concordent bien avec les previsions theoriques attendues pour un modele lineaire d'Heisenberg.

The electron resonance spectra of two ground state triplets have been measured in the isotropic and nematic mesophase of p-azoxyanisole. The extra transitions observed when the biradicals are in the nematic mesophase are interpreted in terms of an unaveraged zero-field splitting. The nitrogen hyperfine splitting is also found to decrease in the nematic mesophase. The observation of these extra lines in the spectra provides excellent quantitative confirmation for the triplet character of these radicals. The changes in the spectra are satisfactorily accounted for using the swarm model of nematic liquid crystals.

The synthesis of three nitroxide radicals labelled with 13C in the α position makes possible the unambiguous determination of the corresponding 13C nuclear-electron contact coupling constant: di-t-butyl nitroxide (1) (quaternary 13C), N-methyl 13C,N-t-octyl nitroxide (2), and N-methyl 13C,N-phenyl (d 5) nitroxide (3). The order of magnitude of this interaction is consistent with a polarization of the N-Cα bond by the nitrogen spin density ρn π. Variations of nitrogen and carbon hyperfine splittings a n, a c α and a cβ have been studied through the solvent effect on radical (1). For radicals (2) and (3), a c α has the same value. In order to interpret these results (and the similar variation of a h α in H-NO-R nitroxides) we propose to take into account the influence of oxygen spin density ρo π and of the non-planarity of the CNOC system, at least in non-aromatic nitroxides. Study of selective broadening of particular lines in the hyperfine spectra shows that α-carbon interactions are probably negative.

La comparaison des intensites de signaux de resonance paramagnetique electronique de solutions de biradicaux a echange interelectronique faible, fort ou intermediaire, montre que le terephthalate est un biradical a echange faible.

L'influence de la non-planeite du groupement nitroxyde sur les differents ecarts hyperfins observes par resonance paramagnetique electronique est etudiee dans les trois premiers termes de la serie des radicaux nitroxydes (aminoxyle, methyl et dimethyl nitroxydes). La methode de calcul utilisee derive des theories du champ self-consistant et tient compte de tous les electrons de valence. Il est montre que l'ecart hyperfin du a l'azote croit au fur et a mesure que l'angle de torsion du nitroxyde augmente; par contre, les densites de spin sur les atomes de carbone voisins ne varient presque pas. L'augmentation de l'angle de torsion hors du plan de la liaison N-O conduit a une diminution des ecarts hyperfins dus aux hydrogenes en libre rotation. On montre que la dependance stereochimique des interactions hyperfines peut etre traduite par un systeme d'equations de la forme a Hβ = B 0 + B 2 cos2 θ pour un nitroxyde a structure pyramidale en inversion rapide au niveau de l'azote. Ces equations se reduisent a a H...