Electronic Structure of the Ground and Excited States of β‐Carboline

Coupled‐cluster calculations are used to compute the energy of conversion between the neutral and the zwitterionic forms of β‐carboline. The stability of the different species is discussed in terms of charge separation and aromatic character, which is related to magnetic criteria. By means of a linear response formalism the vertical excitation energies and oscillator strengths of the lowest singlet states of both structures as well as of the cationic species are determined. General agreement of the relative position and intensity of the different peaks with experimental data is achieved, but the overall spectra are slightly displaced because of solvent effects.     

Affability of Euclidean tilings

We prove that every minimal equivalence relation on a Cantor set arising from the continuous hull of an aperiodic and repetitive Euclidean tiling is affable. To cite this article: F. Alcalde Cuesta et al., C. R. Acad. Sci. Paris, Ser. I 347 (2009).     

Feuilletages en surfaces, cycles évanouissants et variétés de Poisson

Surface foliations, vanishing cycles and Poisson manifolds. Afoliated cylinder of a foliated manifold (M,F) is a path of integral loopsc _t forF. Such a cylinder defines anon-trivial vanishing cycle c ₀ ifc _t is null-homotopic in its supportF _t for eacht>0, butc ₀ is not null-homotopic in its supportF ₀. Vanishing cycles were introduced by S. P. Novikov to study qualitative aspects of codimension one foliations. In this paper we apply this notion to the study of higher codimensional foliations.The first aim is to show the influence that the triviality of vanishing cycles exerts on the topology of thehomotopy groupoid ofF. It is natural to try to reduce the study of triviality to more regular vanishing cycles, as well as to obtain a nice criterion of triviality. In this way, we introduce the notion ofregular vanishing cycle as the orthogonal version (for a riemannian metric onM) of the classical notion of immersed vanishing cycle and the notion ofcoherent vanishing cycle, i.e. an integral discD ₁ with boundaryc ₁ extends to a global foliated homotopyD _t such thatc _t is the boundary ofD _t for eacht>0. We also prove that the triviality of these vanishing cycles implies the triviality of all vanishing cycles. For compact foliated manifolds, we obtain the following criterion: a regular coherent vanishing cycle is non-trivial if and only if the area of the discsD _t converges to infinity.Finally, we give two applications of these results to surface foliations: we generalize the Reeb stability theorem to higher codimensions and we resolve the problem of the symplectic realization of Poisson structures supported by surface foliations.

     

New ultrasound assisted chemical oxygen demand determination

A new method for the chemical oxygen demand (COD) determination assisted by ultrasonic radiation has been evaluated for the first time. The suggested method uses an instrumentation simpler and cheaper than the previous ones used for the same purpose. With this preliminary version of the method the optimized experimental conditions are: high ultrasonic power (143 W, 95% of maximum nominal power; 126.5 W/cm2 of power density); high sulphuric acid concentration (> 60%); and a sonication time of 2 min. Under these conditions this first prototype shows the same limitations as the official COD method as regards the type of organic compounds. It works successfully with easily oxidative organic matter (sodium oxalate, glucose and salicylic acid) but the COD values obtained with more difficult organic matter are poor. When the ultrasonic assisted method is applied to real waste waters the precision is statistically not different to that of the conventional semi-micro method but the COD values obtained lie between 50 and 60% of the values obtained with the conventional semi-micro method. Hence, the use of the ultrasonic radiation for COD determination seems to be an interesting and promising alternative to the conventional ones but still much more research efforts must be done in order to improve the instrumental set-up. The suggested direction of these developments must be a more efficient interaction between the ultrasonic radiation and the sample.     

Synthesis of (-)-nakamurol A and assignment of absolute configuration of diterpenoid (+)-nakamurol A

The total synthesis of the enantiomer of the marine sponge diterpenoid nakamurol A and determination of the absolute configuration of this natural product are reported. This first synthetic entry to thelepogane-type diterpenoids involves the use of the bicyclic enone (-)-3, which after a tandem difunctionalization process and elongation of the side chain leads to the formation of the ketone (-)-13. From (-)-13, two approaches to ent-nakamurol A (1) are reported: the straightforward but nonselective way, by reaction with vinylmagnesium bromide, and a longer but stereocontrolled route, through the primary allylic alcohol 20, which is submitted to a Sharpless epoxidation followed by a tellurium-promoted reductive-epoxide ring-opening cascade reaction.     

Effect of ZnO and organo-modified montmorillonite on thermal degradation of poly(methyl methacrylate) nanocomposites

Since a few years ago, a topic of interest consists in developing composites filled with nanofillers to improve thermal degradation and flammability property of poly(methyl methacrylate) (PMMA). In the present work, the effects of ZnO nanoparticles and organo-modified montmorillonite (OMMT) on the thermal degradation of PMMA were investigated by thermogravimetric analysis (TGA). PMMA-ZnO and PMMA-OMMT nanocomposites were prepared by melt blending with different (2, 5, and 10 wt%) loadings. SEM and TEM analyses of nanocomposites were performed in order to investigate the dispersion of nanofillers in the matrix. According to TGA results, the addition of ZnO nanoparticles does not affect the thermal degradation of PMMA under an inert atmosphere. However, in an oxidative atmosphere, two contrary effects were observed, a catalytic effect at lower concentration of ZnO in the PMMA matrix and a stabilizing effect when the ZnO concentration is higher (10 wt%). In contrast, the presence of OMMT stabilizes the thermal degradation of PMMA whatever be the atmosphere. Differential thermal analysis (DTA) curves showed surprising results, because a dramatic change of exothermic reaction of the PMMA degradation process to an endothermic reaction was observed only in the case of OMMT. During the degradation of PMMA-ZnO nanocomposites, pyrolysis-gas chromatography coupled to mass spectrometer (Py-GC/MS) showed an increase in the formation of methanol and methacrylic acid while a decrease in the formation of propanoic acid methyl ester occurred. In the case of PMMA-OMMT systems, a very significant reduction in the quantity of all these degradation products of PMMA was observed with increasing OMMT concentration. It is also noted that during PMMA-OMMT degradation less energy was released as the decomposition is an endothermic reaction and the material was cooled.