Optically implemented broadband blueshift switch in the terahertz regime

We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.     

Glycerol and glycerol carbonate as ultraviscous solvents for mixture analysis by NMR

NMR of weakly polar analytes in an apolar ultraviscous solvent has recently been proposed for mixture analysis as a pertinent alternative to the DOSY experiment. The present article reports the first use of glycerol and glycerol carbonate as polar solvents for the NMR analysis of a model mixture of dipeptides. This work demonstrates the high potentiality of these solvents for the analysis of mixtures made of polar and potentially bioactive compounds. Medium-sized molecules slowly reorient in glycerol and glycerol carbonate under particular temperature conditions, so that solute resonances may show spin diffusion in NOESY spectra, thus opening the way to mixture analysis. Glycerol and glycerol carbonate have turned out to be ultraviscous solvents of choice for the individualization of four structurally close mixed dipeptides: Leu-Val, Leu-Tyr, Gly-Tyr and Ala-Tyr by means of 1D and 2D NOESY experiments. Selective sample excitation and signal detection were implemented to eliminate the intense proton signals of the non-deuterated solvents. Moreover, the recording of a multiplet selective 2D NOESY-TOCSY has shown that the analytical power of NMR in highly viscous solvents is not limited to the extraction of mixture component 1D subspectra but may also yield some supplementary information about atom connectivity within components.     

Exploratory studies toward the synthesis of the peroxylactone unit of plakortolides

Our efforts in construction the 1,2-dioxane ring of plakortolides through two approaches are described. The first one involved as a key step an acid catalyzed 6-endo ring closure of β-hydroperoxy trans-epoxides directed by a vinyl group adjacent to the epoxide function. By this route, an advanced intermediate of plakortolides was obtained in six steps and 35% overall yield. The second approach featured a 1,2-dioxane ring forming by a double opening of bis-epoxides by ethereal hydrogen peroxide. This reaction did not proceed in the expected sense and exclusive formation of hydroperoxy tetrahydropyran derivatives was observed via a tandem oxacyclization–hydroperoxidation sequence.     

Screening of pesticides in blood with liquid chromatography–linear ion trap mass spectrometry

In clinical or forensic toxicology, general unknown screening procedures are used to identify as many xenobiotics as possible, belonging to numerous chemical classes. We present here a general unknown screening procedure based on liquid chromatography coupled with use of a single linear ion trap mass spectrometer, and focus on the identification of pesticides and/or metabolites in whole blood. After solid-phase extraction (SPE), the compounds of interest were separated using a reversed-phase column and identified by the mass spectrometer operated first in the full-scan mass spectrometry (MS) mode, in the positive and negative polarities, followed by MS² and MS³ scanning of ions selected in data-dependent acquisition. The total scan time was 2.45 s. Two mass spectral libraries (MS² and MS³), each of 450 spectra, were created for the 320 pesticides and metabolites detected after injection of pure solutions. Robustness of the spectra and matrix effects were studied and were satisfactory for the present application. Detection limits for the 320 compounds were studied by extracting 1 mL spiked blood at concentrations between 10 μg/L and 10 mg/L. If necessary, it was possible to decrease the detection limits of some compounds by 10–100-fold by scanning MS² in only one polarity, owing to a shorter total scan time. However, at the same time, the detection specificity decreased as no confirmation could be recorded in the following MS³ scan and no information could be registered in the other polarity. So, in these rare cases, confirmation by another method was required.     

Synthesis of 3-Adenyl- and 3-Thyminylcyclobutane-1,1-dimethanols and Their Homo-octameric Phosphodiesters

Starting from 3-(benzyloxy)cyclobutane-1,1-dimethanol, 3-thyminyl- and 3-adenylcyclobutane-1,1-dimethanol were synthesized by direct introduction of the heterocycles (Scheme 1). The mono-O-substituted methoxytrityl derivatives were separately converted to octameric phosphodiesters on an aminomethyl-polystyrene carrier by the phosphotriester method. These oligomers of carba-nucleosides were prepared in order to study their annealing behaviour towards ribo- and deoxyribo-nucleic acids as well as their potential for homologous hybridization.     

First-order distributed feedback dye laser effect in reflection pumping geometry

Tunable distributed feedback lasing output based on reflection grating configuration instead of the traditional transmission one was realized from rhodamine 6G (R6G)-doped ethanol and 4-dicyanomethylene-2-methyl-6- (p-dimethylaminostyryl)-4H-pyran-(DCM)-doped methanol. Pure gain coupling and additional index coupling were obtained in R6G-doped ethanol and DCM-doped methanol, respectively. The tuning, which was found to be independent of the refractive index of the lasing media, went through the entire stimulated emission band for the two cases. Dual-peak lasing emission indicative of the existence of an index grating was observed in DCM-doped methanol. The interval between the two peaks increased with pump energy.     

Nonlinear behaviour of the instability of steady natural convection in a vertical air layer

In this paper we deal with the flow of natural convection between two vertical planes with horizontal temperature gradient. We show that periodic steady flow occur when Rayleigh number increases. Critical values are obtained numerically and a nonlinear analysis is presented according to Center manifold method.     

Convergence to a Single Wave in the Fisher-KPP Equation

The authors study the large time asymptotics of a solution of the Fisher-KPP reaction-diffusion equation,with an initial condition that is a compact perturbation of a step function.A well-known result of Bramson states that,in the reference frame moving as 2t-(3/2) log t+x∞,the solution of the equation converges as t-→ +o∞ to a translate of the traveling wave corresponding to the minimal speed c* =2.The constant x∞ depends on the initial condition u(0,x).The proof is elaborate,and based on probabilistic arguments.The purpose of this paper is to provide a simple proof based on PDE arguments.     

Carbon Dioxide Reforming of Methane Using a Dielectric Barrier Discharge Reactor: Effect of Helium Dilution and Kinetic Model

The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions of CH₄ and CO₂ increase when the amount of helium in the gas mixture increases. This result is attributed to the “penning ionization” phenomenon, which corresponds to an energy transfer from excited He to molecules in ground state (CH₄, CO₂). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation of products resulting from recombination of methyl radicals (such as C₂, C₃ and C₄) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption that the reactant molecules CH₄ or CO₂ are attacked by active species produced by the plasma discharges, and the production of this active species are function of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data we obtained.