Improved method for isotopic and quantitative analysis of dissolved inorganic carbon in natural water samples

We present here an improved and reliable method for measuring the concentration of dissolved inorganic carbon (DIC) and its isotope composition (delta(13)C(DIC)) in natural water samples. Our apparatus, a gas chromatograph coupled to an isotope ratio mass spectrometer (GCIRMS), runs in a quasi-automated mode and is able to analyze about 50 water samples per day. The whole procedure (sample preparation, CO(2(g))-CO(2(aq)) equilibration time and GCIRMS analysis) requires 2 days. It consists of injecting an aliquot of water into a H(3)PO(4)-loaded and He-flushed 12 mL glass tube. The H(3)PO(4) reacts with the water and converts the DIC into aqueous and gaseous CO(2). After a CO(2(g))-CO(2(aq)) equilibration time of between 15 and 24 h, a portion of the headspace gas (mainly CO(2)+He) is introduced into the GCIRMS, to measure the carbon isotope ratio of the released CO(2(g)), from which the delta(13)C(DIC) is determined via a calibration procedure. For standard solutions with DIC concentrations ranging from 1 to 25 mmol . L(-1) and solution volume of 1 mL (high DIC concentration samples) or 5 mL (low DIC concentration samples), delta(13)C(DIC) values are determined with a precision (1sigma) better than 0.1 per thousand. Compared with previously published headspace equilibration methods, the major improvement presented here is the development of a calibration procedure which takes the carbon isotope fractionation associated with the CO(2(g))-CO(2(aq)) partition into account: the set of standard solutions and samples has to be prepared and analyzed with the same 'gas/liquid' and 'H(3)PO(4)/water' volume ratios. A set of natural water samples (lake, river and hydrothermal springs) was analyzed to demonstrate the utility of this new method.     

Copper-catalyzed asymmetric conjugate addition of Grignard reagents to trisubstituted enones. Construction of all-carbon quaternary chiral centers

The copper-catalyzed asymmetric conjugate addition of Grignard reagents to trisubstituted cyclic enones affords enantioenriched all-carbon quaternary centers with up to 96% ee. The chiral ligand is a diaminocarbene, directly generated in situ. The combination of Grignard reagent and diaminocarbene is unprecedented in conjugate addition, and the additon of the phenyl group, on such enones, cannot be done by other conjugate addition methods.     

The structure of pyrazoles in the solid state: a combined CPMAS, NMR, and crystallographic study

The structures of six N-unsubstituted pyrazoles, three already known and three newly synthesized, have been studied by a combination of X-ray crystallography, multinuclear NMR (solution and solid state), and density functional theory (DFT) calculations. In those cases where crystal structure and CPMAS NMR were available, the agreement was almost perfect, allowing a prediction of the tautomer (with certitude) and the tetrameric structure (with high probability) in the case of 5-isopropyl-3-phenyl-1H-pyrazole without knowing the X-ray structure. In the case of the 5-(2-benzylphenyl)-3-trifluoromethyl-1H-pyrazole above represented, the DFT calculations at the B3LYP/6-31G level justify the great stability of this tautomer by the presence of an intramolecular N-H...pi interaction, present in solution.     

Sucrose analogs: an attractive (bio)source for glycodiversification

Covering: up to April 2012Sucrose is a widespread carbohydrate in nature and is involved in many biological processes. Its natural abundance makes it a very appealing renewable raw material for the synthetic production of high-valued molecules. To further diversify the structure and the inherent properties of these molecules, the access to sucrose analogs is of utmost interest and has historically been widely explored through chemical means. Nature also offers a large panel of sucrose-scaffold derivatives, including phosphorylated or highly substituted phenylpropanoid esters amenable to transformation. Additionally, the use of microorganisms or enzymes could provide an alternative ecologically-compatible manner to diversify sucrose-scaffold derivatives to enable the synthesis of oligo- or polysaccharides, glycoconjugates or polymers that could exhibit original properties for biotechnological applications. This review covers the main biological routes to sucrose derivatives or analogs that are prevalent in nature, that can be obtained via enzymatic processes and the potential applications of such sucrose derivatives in sugar bioconversion, in particular through the engineering of substrates, enzymes or microorganisms.     

Structural modulation of internal charge transfer in small molecular donors for organic solar cells

Donor-acceptor molecules with small chain extension have been synthesized and used as active material in organic solar cells. The effect of fusion of a phenyl group on the end dicyanovinylene acceptor is discussed.     

Validation of a method for the analysis of biogenic amines: Histamine instability during wine sample storage

This paper reports on the development of an optimized method for the simultaneous analysis of eight biogenic amines (histamine, methylamine, ethylamine, tyramine, putrescine, cadaverine, phenethylamine, and isoamylamine). The analytical method thus proposed has the following advantages: the easy derivatization of wine, the quantification of biogenic amines and a complete degradation of excess derivatization reagent during sample preparation in order to preserve the column. It consists in reversed phase separation by HPLC and UV–vis detection of the aminoenones formed by the reaction of amino compounds with the derivatization reagent diethyl ethoxymethylenemalonate (DEEMM). The usefulness of this technique was confirmed by an alternative oenological analytical method for the validation, quality control and uncertainty assessment (OIV Oeno 10/2005). The method was validated and proposed as a reference method to the International Organization of Vine and Wine (OIV). As a specific application of the proposed method, the biogenic amine content of Rhône valley wines was investigated.     

Banach Spaces of Solutions of the Helmholtz Equation in the Plane

The purpose of this article is to study the Hilbert space W²\mathcal{ W}^2 consisting of all solutions of the Helmholtz equation Du+u=0\Delta u+u=0 in \BbbR²\Bbb{R}^2 that are the image under the Fourier transform of L²L^2 densities in the unit circle. We characterize this space as a close subspace of the Hilbert space H²\mathcal{ H}^2 of all functions belonging to L²( | x | ^-3dx) L^2( | x | ^{-3}dx) jointly with their angular and radial derivatives, in the complement of the unit disk in \BbbR²\Bbb{R}^2. We calculate the reproducing kernel of W²\mathcal{ W}^2 and study its reproducing properties in the corresponding spaces H^p\mathcal{H}^p, for $p>1$p>1.     

Using E-Exercise Bases in Mathematics: Case Studies at University

E-Exercise Bases (EEB) are now used in the teaching of mathematics, especially at university. We discuss here the consequences of their use on the students’ activity during computer lab sessions. Results stem from observations of several teaching designs organised in different French universities with three e-exercise bases. The analysis focuses on new tasks and on specific solving strategies, which appear using these resources. Moreover, specific didactic contract clauses are studied.     

Semiclassical determination of exponentially small intermode transitions for 1 + 1 spacetime scattering systems

We consider the semiclassical limit of systems of autonomous PDEs in 1 + 1 spacetime dimensions in a scattering regime. We assume the matrix‐valued coefficients are analytic in the space variable, and we further suppose that the corresponding dispersion relation admits real‐valued modes only with one‐dimensional polarization subspaces. Hence a BKW‐type analysis of the solutions is possible. We typically consider time‐dependent solutions to the PDE that are carried asymptotically in the past and as x → ?∞ along one mode only and determine the piece of the solution that is carried for x → +∞ along some other mode in the future. Because of the assumed nondegeneracy of the modes, such transitions between modes are exponentially small in the semiclassical parameter; this is an expression of the Landau‐Zener mechanism. We completely elucidate the spacetime properties of the leading term of this exponentially small wave, when the semiclassical parameter is small, for large values of x and t, when some avoided crossing of finite width takes place between the involved modes. © 2006 Wiley Periodicals, Inc.     

Spray-drying as a tool to disperse conductive carbon inside Na<Subscript>2</Subscript>FePO<Subscript>4</Subscript>F particles by addition of carbon black or carbon nanotubes to the precursor solution

In this work, Na₂FePO₄F-carbon composite powders were prepared by spray-drying a solution of inorganic precursors with 10 and 20 wt% added carbon black (CB) or carbon nanotubes (CNTs). In order to compare the effect of CB and CNT when added to the precursor solutions, the structural, electrochemical, and morphological properties of the synthesized Na₂FePO₄F-xCB and Na₂FePO₄F-xCNT samples were systematically investigated. In both cases, X-ray diffraction shows that calcination at 600 °C in argon leads to the formation of Na₂FePO₄F as the major inorganic phase. ⁵⁷Fe Mössbauer spectroscopy was used as complementary technique to probe the oxidation states, local environment, and identify the composition of the iron-containing phases. The electrochemical performance is markedly better in the case of Na₂FePO₄F-CNT (20 wt%), with specific capacities of about 100 mAh/g (Na₂FePO₄F-CNT) at C/4 rate vs. 50 mAh/g for Na₂FePO₄F-CB (20 wt%). SEM characterization of Na₂FePO₄F-CB particles revealed different particle morphologies for the Na₂FePO₄F-CNT and Na₂FePO₄F-CB powders. The carbon-poor surface observed for Na₂FePO₄F-CB could be due to a slow diffusion of carbon in the droplets during drying. On the contrary, Na₂FePO₄F-CNT shows a better CNT dispersion inside and at the surface of the NFPF particles that improves the electrochemical performance.