Regioselectivite des substitutions nucleophiles sur des fluoro olefines fonctionnelles

The fluoroolefines CF₃C$\text{F}\text{β}$=C$\text{F}\text{α}$-Y where Y=C₆H₅, C₆H₄CF₃, CONET₂, COOC₃H₇ have been synthesized. Diethylaminolithium or lithium ethoxide attack preferentially these olefines at the α position when Y=C₆H₅ or exclusively at the β position when YCOOC₃H₇. With Y=CONEt₂ this regioselectivity is not so neat, therefore the polarising effect of CF₃ group should be not very much weaker than the amide's. The increasing of the electronic effect in the order

show that vinylic fluorine atoms have no influence on the transmission of these electronic effects.     

PASSIVE AND ACTIVE ROLE OF FLUORESCERS IN THE CHEMILUMINESCENCE OF TETRAMETHOXYDIOXETANE

Abstract— Tests for the possible catalytic role of fluorescers in the chemiluminescent decomposition of dioxetanes are discussed and applied to the case of tetramethoxydioxetane. This dioxetane, which can undergo direct thermal decomposition yielding excited methyl carbonate with subsequent excitation of fluorescers (for example 9, 10-dibromoanthracene. DBA), is shown to be sensitive also to catalysis by fluorescers such as rubrene. 9, 10-diphenyl-anthracene (DPA) and 9, IOdicyanoanthracene (DCNA). The possibility of a charge-transfer mechanism of chemiluminescent catalysis is suggested.     

A BLUE FLUORESCENT PROTEIN FROM A YELLOW-EMITTING LUMINOUS BACTERIUM

Vibrio fischeri strain Y1 emits yellow light in vivo due to the participation of a yellow fluorescent protein (YFP) in the luciferase reaction. In this study it was found that the organism also produces a protein (referred to as Y1-BFP) emitting strong blue fluorescence. Its molecular weight, about 25 kDa, is the same as or very close to that of YFP. The fluorescence excitation and emission maxima of the purified Y1-BFP are at 416 and 461 nm, respectively, and the fluorescence lifetime is 12.5 ns at 2 degrees C. The molar extinction coefficient of Y1-BFP at 416 nm was estimated to be approx. 9500. With the homologous luciferase, Y1-BFP decreases the intensity and rate of decay in the in vitro reaction but has no effect on its emission spectrum (in contrast to YFP, which has a striking effect on the spectrum). With luciferase isolated from Vibrio harveyi, however, Y1-BFP causes a small blue-shift (approximately 10 nm) in the emission of the enzyme catalyzed reaction, whereas YFP has no effect on the emission spectrum.     

Microscopic approaches to liquid nitromethane detonation properties

In this paper, thermodynamic and chemical properties of nitromethane are investigated using microscopic simulations. The Hugoniot curve of the inert explosive is computed using Monte Carlo simulations with a modified version of the adaptative Erpenbeck equation of state and a recently developed intermolecular potential. Molecular dynamic simulations of nitromethane decomposition have been performed using a reactive potential, allowing the calculation of kinetic rate constants and activation energies. Finally, the Crussard curve of detonation products as well as thermodynamic properties at the Chapman-Jouguet (CJ) point are computed using reactive ensemble Monte Carlo simulations. Results are in good agreement with both thermochemical calculations and experimental measurements.     

Chemical force titration of plasma polymer-modified PDMS substrates by using plasma polymer-modified AFM tips

Plasma polymerization has gained increasing attention in surface functionalization. We use here chemical force titration to characterize PDMS (polydimethylsiloxane) substrates modified by maleic anhydride-pulsed plasma polymerization. The coating is hydrolyzed to promote the formation of dicarboxylic acid groups. To enhance the variation of the adhesion forces as a function of pH, we use AFM tips modified in the same way as the substrates. The pH-dependent adhesion measurements are performed at different KCl concentrations. The dicarboxylic nature of the maleic acid groups clearly emerges from the force titration curves. The surface pK(a) values (pK(a1) = 3.5 +/- 0.5 and pK(a2) = 9.5 +/- 0.5) of the dicarboxylic acids are evaluated from low electrolyte concentration solutions. The values are shifted toward higher pK(a) values when compared to maleic acid in solution. The first pK(a) appears in the titration force curve for low salt concentration as a peak. This peak changes to a sigmoidal shape at higher salt concentrations. The appearance of a peak is attributed to the formation of strong hydrogen bonds between the tip and the substrate as reported in the literature. The effect of the ionic strength on the force curves is explained by the condensation of counterions on the carboxylate groups. At high pH, the adhesion force almost vanishes. On the approach, at high pH, one first observes repulsion between the tip and the substrate, which varies exponentially with the tip/substrate distance. The decay length of this repulsion force is in good agreement with theoretical predictions of the Debye length, attesting to the electrostatic nature of the interactions. We also find that the replacement of monovalent cation K(+) by the divalent cation Ca(2+) leads to significant changes in the force titration curve at high pH where the dicarboxylic groups are fully ionized. We observe that the adhesion force no longer vanishes at high pH but even slightly increases with pH, an effect that is explained by Ca(2+) ions bridging between two carboxylate groups.     

One-loop divergences in chiral perturbation theory and right-invariant metrics onSU(3)

In the framework of chiral perturbation theory, we compute the one-loop divergences of the effective Lagrangian describing strong and non-leptonic weak interactions of pseudoscalar mesons. We use the background field method and the heat-kernel expansion, and underline the geometrical meaning of the different terms, showing how the right-invariance of the metrics onSU(3) allows to clarify and simplify the calculations. Our results are given in terms of a minimal set of independent counterterms, and shorten previous ones of the literature, in the particular case where the electromagnetic fild is the only external source which is considered. We also show that a geometrical construction of the effective Lagrangian at orderO(p ⁴) allows to derive some relations between thefinite parts of the coupling constants. These relations do not depend on the scale used to renormalize.     

Une méthode de comparaison isopérimétrique de fonctionnelles de domaines de la physique mathématique I. Première partie: une démonstration de la conjecture isopérimétrique Pλ2 ≥ πj 0 4 /2 de Pólya et Szegö

Résumé Soient _G la première valeur propre de la membrane à contour fixé sur un domaine simplement connexeG, etP _G la rigidité à la torsion deG. En construisant un cercleK tel queP _K P _G et _{K G}, on démontre la conjecture de Pólya et Szegö [14]; P_{G G} ² j ₀ ² /2. Ce résultat renforce le théorème isopérimétrique classique de Rayleigh [15], Faber [4] et Krahn [10].

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Summary Let _G be the first eigenvalue of the fixed membrane on a simply connected domainG, and letP _G be the torsional rigidity ofG. We prove Pólya-Szegö's conjecture [14]: P_{G G} ² j ₀ ² /2, by constructing a circleK such thatP _K P _G and _{K G}. This result sharpens the classical isoperimetric theorem of Rayleigh [15], Faber [4] and Krahn [10].

     

A new estimate for the topological degree

We establish a new estimate for the topological degree of continuous maps from the sphere ${\mathbb{S}}^N$ into itself, which answers a question raised in Bourgain, Brezis, and Mironescu [Commun. Pure Appl. Math. 58 (2005) 529–551] and extends some of the results proved there, as well as in recent work by these authors (Lifting, degree, and distributional Jacobian revisited, http://ann.jussieu.fr/publications). To cite this article: J. Bourgain et al., C. R. Acad. Sci. Paris, Ser. I 340 (2005).     

Tabu search approaches for solving the two-group classification problem

The two-group classification problem consists in constructing a classifier that can distinguish between the two groups. In this paper, we consider the two-group classification problem which consists in determining a hyperplane that minimizes the number of misclassified points. We assume that the data set is numeric and with no missing data. We develop a tabu search (TS) heuristic for solving this NP-hard problem. The TS approach is based on a more convenient equivalent formulation of the classification problem. We also propose supplementary new intensification phases based on surrogate constraints. The results of the conducted computational experiments show that our TS algorithms produce solutions very close to the optimum and require significantly lower computational effort, so it is a valuable alternative to the MIP approaches. Moreover the tabu search procedures showed in this paper can be extended in a natural way to the general classification problem, which consists of generating more than one separating hyperplanes.     

Infrared Study of the Hydrogen Bond Complexes Between N-Methylsuccinimide and Phenols

Abstract

The hydrogen bonded complexes between N-methylsuccinimide and phenols (pKa = 10.2 → 6) are investigated by infrared spectrometry. The thermodynamic parameters for the 1–1 complexes are determined in carbon tetrachloride-. The formation constants at 298 K range from 15 to 150 dm³ mol^?1, the enthalpies of complex formation from - 20 to - 30 kJ mol^?1, the changes of entropy from - 22 to - 40 J K^?1 mol^?1 and the frequency shifts of the v(OH) stretching vibration from 170 to 340 cm^?1. The complexes are weaker than those involving the monocarbonyl bases. The decrease of the force constant of the bonded carbonyl group ranges from 0.48 to 0.65 N cm^?1. The force constant of the free C=O group slightly increases upon complex formation, in agreement with the cooperatively theory.