Growth of silicene layers on Ag(111): unexpected effect of the substrate temperature

The deposition of one silicon monolayer on the silver (111) substrate in the temperature range 150-300 °C gives rise to a mix of (4 × 4), (2√3 × 2√3)R30° and (√13 × √13)R13.9° superstructures which strongly depend on the substrate temperature. We deduced from a detailed analysis of the LEED patterns and the STM images that all these superstructures are given by a quasi-identical silicon single layer with a honeycomb structure (i.e. a silicene-like layer) with different rotations relative to the silver substrate. The morphologies of the STM images are explained from the position of the silicon atoms relative to the silver atoms. A complete analysis of all possible rotations of the silicene layer predicts also a (√7 × √7)R19.1° superstructure which has not been observed so far.     

Amine-Directed Palladium-Catalyzed C−H Halogenation of Phenylalanine Derivatives

An efficient primary-amine-directed, palladium-catalyzed C−H halogenation (X=I, Br, Cl) of phenylalanine derivatives is reported on a range of quaternary amino acid (AA) derivatives thanks to suitable conditions employing trifluoroacetic acid as additive. The extension of this original native functionality-directed ortho-selective halogenation was even demonstrated with the more challenging native phenylalanine as tertiary AA.     

Application des orbitales atomiques perturbées au calcul de la constante de couplage entre les spins nucléaires de la molécule HD

Nuclear spin-spin coupling constant J _HD of the deuterated hydrogen molecule is determined by minimizing the self-coupling energy evaluated with the Gregson, Hall and Rees operator. The family of variational functions used is suggested by the form of the hydrogen-like atomic orbitals perturbed by this operator.The comparison of the results with those previously obtained with a different family of functions shows that it is necessary for those calculations to use a perturbated function which is as perfectly adapted to the perturbation in the immediate neighbourhood of the nucleus, as in the remainder of the molecular domain.

     

General synthesis for chiral 4-alkyl-4-hydroxycyclohexenones

[reaction: see text] Some selective transformations of resorcinol-derived cyclohexadienone are reported. Efforts led to a structure reported to display anticancer properties. On the basis of the results, the structures for natural products reported to contain a 4,6-dihydroxy-4-alkyl-cyclohexenone nucleus are corrected.     

Near-Wake Turbulence Properties in the High Reynolds Number Incompressible Flow Around a Circular Cylinder Measured by Two- and Three-Component PIV

The main objective of the present experimental study is to analyse the turbulence properties in unsteady flows around bluff body wakes and to provide a database for improvement and validation of turbulence models, concerning the present class of non-equilibrium flows. The flow around a circular cylinder with a low aspect ratio () and a high blockage coefficient () is investigated. This confined environment is used in order to allow direct comparisons with realisable 3D Navier–Stokes computations avoiding ‘infinite’ conditions. The flow is investigated in the critical regime at Reynolds number 140,000. A cartography of the velocity fields in the near wake of the cylinder is obtained by PIV and Stereoscopic PIV techniques. Statistical means and phase-averaged quantities are determined. Furthermore, POD analysis is performed on the data set in order to extract coherent structures of the flow and to compare the results with those obtained by the conditional sampling technique. The Reynolds stresses, the strain-rate and vorticity fields as well as the turbulence production terms are determined.     

Stereoselective synthesis of the lituarine tricyclic spiroacetal

[reaction: see text] Oxidative cyclizations of 2-(4-hydroxybutyl)furan derivatives provide spirobutenolide acetals directly; on the basis of this methodology, we describe an asymmetric synthesis of a tricyclic spirobutenolide precursor to the C(7-18) fragment common to lituarines A-C.     

Enlarging the library of poly‐(L‐lysine citramide) polyelectrolytic drug carriers

Poly‐(L ‐lysine citramide) is a degradable drug carrier of the polyelectrolyte type that is composed of citric acid and L ‐lysine building blocks. In a previous work, poly‐(L ‐lysine citramide) was synthesized by the interfacial polycondensation of α‐hydroxy acid protected citryl dichloride with COOH‐protected lysine diamine. Because of head‐to‐head and head‐to‐tail and tail‐to‐tail linkages in the chains as well as various side reactions such as deprotection of the α‐hydroxy acid moieties and intramolecular imide ring formation, a very large family of degradable polyelectrolyte copolymers was obtained. All the members of this family hydrolytically degrade to the same end products. In this study, another route was explored based on the polycondensation of α‐hydroxy acid protected citric acid pentafluorophenyl diesters, namely, citrobenzal dipentafluorophenyl and citrochloral dipentafluorophenyl with N‐N′‐trimethylsilylated COOH‐protected L ‐lysine. The resulting polymers were characterized by IR, NMR, and size exclusion chromatographic analyses. The resulting chain structures and repeat units were identified from these characterizations and are discussed as compared with characteristics exhibited by analogous polymers resulting from interfacial polycondensation. Differences observed at the intermediate stage involving protected polymers were largely erased during the final deprotection stage because of imide formation during final hydrolysis under the selected conditions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3475–3484, 2001     

Nanocomposites composed of P3HT:PCBM and nanoparticles synthesized by laser ablation of a bulk PbS target in liquid

Nanoparticles synthesized by laser ablation of bulk target materials in liquids have ligand-free surfaces since no chemical precursors are used for their synthesis, and thus, they are ideally suited for applications in the fabrication of organic solar cells in which the properties of the interface between the nanoparticles and the polymer blend matrix largerly determine the exciton splitting and transport of carriers to the external electrodes, properties crucial for the device operation and performance. Narrow band gap semiconducting quantum dots can act as sensitizers, increasing the absorption of the device active layer in the infrared part of the solar spectrum. In this work, a bulk PbS target was laser ablated (450 fs, 1,064 nm, 1 kHz) in ethanol for the synthesis of nanoparticle colloidal solutions. The solutions exhibit a broad absorption which extends at the longest wavelength measured of ~1,700 nm and beyond. The nanoparticles were directly mixed with the blend P3HT:PCBM for the formation of nanocomposites. The nanocomposites with the nanoparticles exhibit lower transmission in the whole spectral range as compared to the blend without the nanoparticles.