Analyse Conformationnelle en Serie Alcoylidene-5 Dioxanne-1,3

Some 5-alkylidene 2-alkyl 1,3-dioxans have been studied by NMR. Analysis of the spectra leads to the conclusion that these derivatives are predominantly in the chair form, the alkyl substituent on carbon 2 being in the equatorial position. In the case of 5-alkylidene 1,3 dioxans unsubstituted at carbon 2, there is a rapid ring inversion with a very low energy barrier.     

Nuclear magnetic resonance and spin relaxation in biological systems

Proton nuclear spin-lattice relaxation in biological systems is generally distinguished from that in inorganic systems such as rocks by the presence of locally disordered macromolecular environments. Rapid exchange of readily observed labile small molecules among differently oriented macromolecular sites generally nearly averages the spectral anisotropies in the small molecule resonances. The biological tissue is generally distinguished from the inorganic matrix by the presence of a significant population of protons in the solid components that are well connected by dipolar spin couplings. Magnetic coupling between the solid and the liquid components generally dominates the magnetic field dependence of the spin-lattice relaxation rates observed in the small molecule components which is generally described by a power law in the Larmor frequency. Recent theory involving a modification of the spin-phonon class of relaxation mechanism provides a quantitative understanding of these data in terms of the dynamics of the chain molecules generally present in the solid spin systems, folded proteins for example.     

Three-dimensional recomposition of the absorption field inside a nonbuoyant sooting flame

A remote scanning retrieval method was developed to investigate the soot layer produced by a laminar diffusion flame established over a flat plate burner in microgravity. Experiments were conducted during parabolic flights. This original application of an inverse problem leads to the three-dimensional recomposition by layers of the absorption field inside the flame. This technique provides a well-defined flame length that substitutes for other subjective definitions associated with emissions.     

Unprecedented (Cu2Ln)n complexes (Ln = Gd3+, Tb3+): a new "single chain magnet"

The magnetic study of a trinuclear Cu-Gd-Cu complex confirms that such basic units self-assemble to yield a high spin species. A nice fit of the magnetic data is obtained for an infinite chain of tetranuclear Gd(2)Cu(2) motifs linked through the Gd ions located at the opposite vertexes of the tetranuclear motifs according to two Cu-Gd coordination modes, a double bridging through phenoxo and alkoxo oxygen atoms and a single bridging through deprotonated amide functions. The two interaction pathways are ferromagnetic. Alternating current susceptibility measurements confirm that the equivalent copper-terbium entity is a single chain magnet with a barrier height for reversal of the magnetization equal to 28.5 K.     

Unprecedented in situ oxidative ring cleavage of isoxazolidines: diastereoselective transformation of nitronic acids and derivatives into 3-hydroxymethyl 4-nitro tetrahydrofurans and pyrrolidines

[reaction: see text] Nitronic acids undergo an intramolecular 1,3-dipolar cycloaddition to unactivated double bonds, and the resulting isoxazolidines spontaneously evolve by an unprecedented in situ oxidative ring cleavage. The extension of this transformation to silyl nitronates results in a general diastereoselective construction of hydroxymethyl nitro functionalized tetrahydro-furans and -pyrrolidine having up to four consecutive stereogenic centers.     

Rapid determination of mono and dinitrophenols by DPP, in the presence of lead and cadmium and using concentrated CaCl2 electrolyte

The contamination of drinking water and industrial wastewaters is a critical environmental problem. The nitrophenol, dinitrophenol, cadmium, and lead contaminants are classified as hazardous compounds. Their rapid determination may be obtained using differential pulse polarography with concentrated electrolyte. CaCl₂, which is very soluble to levels exceeding 5 mol l^–1, allows separation of coalescent peaks at 0.1 mol l^–1. A systematic study undertaken from 0.1 to 5 mol l^–1 shows good separation of lead and cadmium from the organic compounds, and optimization of the electrolyte concentration according to the objective is described. Preconcentration of real samples is necessary because pollution levels are usually very low.     

Ethics in OR/MS: Past,present and future

The pervasiveness and impact on society and on every day human life of technology has led to a growing awareness that science and technology cannot be considered above or beyond the realm of value judgements and hence of ethics. This is especially true for Operations Research / Management Science (OR/MS), that particular science which is concerned with methodologies for scientifically deciding how to design and operate man-machine systems in an optimal way, usually under conditions requiring the allocation of scarce resources. Here we try to give a historical account of the growing interest for ethics within the OR/MS community from its birth to present days. Starting from attempts to define models and codes of ethical behaviour in our profession, the OR/MS community has arrived at more fundamental questions about the ethical responsibility it faces in a world of growing inequalities and in which the ever greater stress that human activities impose on the environment puts at risk the very survival of human kind.     

Pure surface plasmon resonance enhancement of the first hyperpolarizability of gold core-silver shell nanoparticles

We report the optical second harmonic (SH) response from gold core-silver shell nanoparticles supported at a liquid-liquid interface in the spectral region where the second harmonic generation (SHG) frequency is resonant with the surface plasmon (SP) resonance excitation of the nanoparticles. We compare these results with that obtained by classical linear optical absorption spectroscopy and show that the nonlinear optical response is dominated by the SP resonance enhancement with negligible contributions from the interband transitions. As a result, the SH spectrum exhibits two clear SP resonance bands attributed to the two SP resonances of the composite nanostructure formed by the gold core-silver shell nanoparticles. Absolute values of the hyperpolarizabilities are measured by hyper Rayleigh scattering (HRS) and compared that of pure gold nanoparticles. The hyperpolarizability measured at a harmonic energy of 3.0 eV, enhanced through excitation of the high energy SP resonance of the nanoparticle, increases with the silver content whereas the hyperpolarizability measured at a harmonic energy of 2.4 eV, enhanced through the excitation of the low energy SP resonance of the nanoparticle, decreases because of the shift of this resonance away from the harmonic frequency. The hyperpolarizability determined by HRS and the square root of the SHG intensities, scaling with the nanoparticle hyperpolarizability, have similar trends with respect to the silver content indicative of closely related adsorption properties yielding similar surface concentrations at the liquid-liquid interface.     

Plasma formation dynamics within a water microdroplet on femtosecond time scales

We demonstrate that a highly confined plasma inclusion is formed inside a micrometric water droplet that is excited by a femtosecond laser. An 800-nm laser pulse generates a plasma inclusion by laser-induced breakdown that is subsequently probed by the elastic scattering of a second (400-nm) time-delayed ultrashort pulse. For a 25-microm-radius droplet and an incident intensity of 7 x 10(12) W/cm2 the radius of the highly localized plasma is 1.9 +/- 0.4 microm. We probed the plasma formation dynamics on femtosecond time scales by varying the delay between the pump and the probe pulses. Good agreement with numerical simulations of the process was found.