Study of the polymer latex coalescence by dielectric measurements at microwave frequency—feasibility of the method

The measurement of the complex dielectric constant ( ^*=–j) in the dipolar absorption domain of the free water molecule (microwave region) permits us to follow quasi-specifically and precisely the water circulation during the coalescence process of a latex. Weight losses and dielectric constants variations have been simultaneously recorded upon latex drying, in a resonant cavity at 5 and 9 GHz and under controlled atmosphere and temperature. Two different latexes were studied (polybutylacrylate (PBuA) and polystyrene (PS)) from which the glass transition temperature effect was investigated. It is found that the harder the polymer particles, the more clearly evidenced the flocculation and packing points are. This method appears to be quite reliable for discriminating the various steps in the film formation process of latexes.     

Chemoselective and stereoselective synthesis of gem-difluoro-β-aminoesters or gem-difluoro-β-lactams from ethylbromodifluoroacetate and imines during Reformatsky reaction

The chemoselective and stereoselective synthesis of gem-difluoro-β-aminoesters or gem-difluoro-β-lactams was investigated from ethylbromodifluoroacetate and imines during Reformatsky reaction. Influence of various reaction parameters, such as nature of the amine part, nature of the chiral auxiliary, was evaluated. High levels of stereoselectivity (up to 98%) were obtained for gem-difluoro-β-aminoesters and gem-difluoro-β-lactams using either (R)-phenylglycinol or (R)-methoxyphenylglycinol.     

Exploring the self-assembly of dumbbell-shaped polyoxometalate hybrids,from molecular building units to nanostructured soft materials

The formation of hierarchical nanostructures using preformed dumbbell-like species made of covalent organic–inorganic polyoxometalate (POM)-based hybrids is herein described. In this system, the presence of charged subunits (POM, metal linkers, and counter ions) in the complex molecular architecture can drive their aggregation, which results from a competition between the solvation energy of the discrete species and intermolecular electrostatic interactions. We show that the nature of the POM and the charge of the metal linker are key parameters for the hierarchical nanoorganization. The experimental findings were corroborated with a computational investigation combining DFT and molecular dynamics simulation methods, which outlines the importance of solvation of the counter ion and POM/counter ion association in the aggregation process. The dumbbell-like species can also form gels, in the presence of a poorer solvent, displaying similar nanoorganization of the aggregates. We show that starting from the designed molecular building units whose internal charges can be controlled by redox trigger we can achieve their implementation into soft nanostructured materials through the control of their supramolecular organization.

The formation of hierarchical nanostructures using supramolecular dumbbell-like species made of organic–inorganic polyoxometalate-based hybrids is investigated by combination of SAXS and computational methods.     

A novel approach to study the inclusion mechanism of imidazole derivatives in micellar chromatography

A chromatographic approach was proposed to describe the existence of surfactant micelles in a surfactant/hydroorganic phosphate buffer mobile phase. Using this mixture as a mobile phase, a novel mathematical theory is presented to describe the inclusion mechanism of imidazole derivatives in surfactant micelles. Using this model, enthalpy, entropy and the Gibbs free energy were determined for two chromatographic chemical processes: (i) the transfer of the imidazole derivative from the mobile phase to the stationary phase; and (ii) the imidazole derivative inclusion in surfactant micelles. These thermodynamic data indicate that the main parameter determining chromatographic retention is distribution of the imidazole derivatives to micelles of surfactant while the interaction with the stationary phase play a minor role.     

Analyse destructive de traces de rubidium dans des echantillons mineralogiques par radioactivation au moyen de neutrons de 14 MeV

Increasing interest is devoted in geochemistry to the determination of minor elements in rocks and minerals with the best analytical precision. The use of different methods is quite useful to point out and overcome the systematic and specific interferences of the geological matrix. 14 MeV neutron activation analysis can be used for such a purpose as an additional check method. A successful attempt has been made in the determination of rubidium in different geochemical standards. A sensitivity of 100 ppm can be reached with a 5% precision without any interference problem.     

Molecular dynamics study of hydration in ethanol-water mixtures using a polarizable force field

The abnormal physicochemical characteristics of ethanol solvation in water are commonly attributed to the phenomenon of hydrophobic hydration. To investigate the structural organization of hydrophobic hydration in water-ethanol mixtures, we use molecular dynamics simulations based on detailed atomic models. Induced polarization is incorporated into the potential function on the basis of the classical Drude oscillator model. Water-ethanol mixtures are simulated at 11 ethanol molar fractions, from 0.05 to 0.9. Although the water and ethanol models are parametrized separately to reproduce the vaporization enthalpy, static dielectric constant, and self-diffusion constant of neat liquids at ambient conditions, they also reproduce the energetic and dynamical properties of the mixtures accurately. Furthermore, the calculated dielectric constant for the various water-alcohol mixtures is in excellent agreement with experimental data. The simulations provide a detailed structural characterization of the mixtures. A depletion of water-water hydrogen bonding in the first hydration shell of ethanol is compensated by an enhancement in the second hydration shell. The structuring effect from the second solvation shell gives rise to a net positive hydrogen-bonding excess for ethanol molar fractions up to approximately 0.5. For larger molar fractions, the second hydration shell is not sufficiently populated to overcome the net H-bond depletion from the first shell.     

Direct access to CF3-propargyl amines and conversion to difluoromethyl imines

Trifluoromethyl aldimines reacted with acetylides in toluene at −78 °C to provide propargyl amines in good yields. From a chiral trifluoromethyl aldimine, the propargyl amines were obtained with excellent diastereoselectivities (de >98%). Trifluoromethyl propargyl amines could be further converted into difluoromethyl imines under basic conditions.     

Electrospray ionization mass spectrometry studies of noncovalent myosin VI complexes reveal a new specific calmodulin binding site

Among the myosin superfamily, myosin VI differs from all others by a reverse directionality and a particular motility. Little structural information is available for myosin VI. It is known that it binds one calmodulin (CaM) by means of a single "IQ motif" and that myosin VI contains a specific insert located at the junction between the motor domain (MD) and the lever arm, likely to play a critical role for the unusual motility previously observed. Electrospray ionization mass spectrometry (MS) was used to determine the CaM and Ca2+ stoichiometries in several myosin VI constructs. In particular, the experimental conditions required for the observation of multiprotein/Ca2+ noncovalent assemblies are detailed for two truncated MD constructs (less than 20 kDa) and for three full MD constructs (more than 90 KDa). The specificity of the detected stoichiometries is discussed for each construct and the resolving power of Time of Flight mass spectrometry is stressed, in particular for the detection of metal ions binding to high molecular weight complexes. MS reveals a new CaM binding site for myosin VI and highlights a different behavior for the five myosin VI constructs versus Ca2+ binding. In addition to these stoichiometry based experiments, gas-phase dissociation analyses on intact complexes are described. They reveal that Ca2+ transfer between protein partners occurs during the dissociation process for one construct with a full MD. Charge-transfer and dissociation behavior has allowed to draw structural assumptions for the interaction of the MD with the CaM N-terminal lobe.     

Preequilibrium versus thermalized light-charged particle emissions in the40Ar (1100MeV)+24Mg reaction. Dynamical calculations

In an attempt to separate preequilibrium and thermalised emissions of light particles in low impact parameter heavy-ion collisions, the⁴⁰Ar+²⁴Mg reaction has been studied at 27.5 MeV/nucleon. Exclusive measurements have permitted us to examine, in some detail, heavy fragments and charged particles (p, d, t,-particle). The fragments recognized as evaporation residues have been selected and, due to inverse kinematic conditions, the related preequilibrium and statistical emissions of light particles resulting from incomplete fusion reaction appear to be distinguishable to a fair extent. This separation is fully supported by Monte Carlo calculations. Some experimental characteristics of the light particles have been examined and compared to the predictions of dynamical calculations. These calculations, associating a preequilibrium (interpreted as prompt emitted particles) model with a statistical-decay model, follow the evolution of the collision from the point of contact between the projectile and the target to the final evaporation-residue formation on an event by event basis. The predictions of these calculations have been compared to experimental data and satisfactory agreement is achieved for fragment-mass distribution, proton-energy spectra, and proton-angular distribution.     

Synthesis of chiral α-substituted N-[((2S)-2-hydroxy-2-phenyl)-ethyl]-2-phenylglycine derivatives by diastereocontrolled alkylation of (6 R)-2,3,5,6-tetrahydro-3,6-diaryl-N-[(2′R)-(2′-methyl)phenyl-methyl]-4H-1,4-oxazin-2-ones

The synthesis of α-substituted N-[((2S)-2-hydroxy-2-phenyl)-ethyl]-2-phenylglycine derivatives is reported. The key step of the sequence is the highly diastereoselective alkylation of (6R)-2,3,5,6-tetrahydro-3,6-diaryl-N-[(2′R)-(2′-methyl)phenylmethyl]-4H-1,4-oxazin-2-ones after deprotonation with t-BuOK. Opening of the resulting oxazinone with ethanolic KOH, followed by hydrogenolysis of the corresponding N-[(2R)-(2-methyl)phenylmethyl] compound to furnish the expected 2-phenylglycine derivative, is also described.