Large‐Scale Synthesis of Enantiopure Molecular Cages: Chiroptical and Recognition Properties

A convenient and efficient gram‐scale synthesis for enantiopure hemicryptophane–tren (tren=tris(2‐aminoethyl)amine) derivatives has been developed. The four‐step synthesis is based on the optical resolution of a key intermediate, cyclotriveratrylene, for which the energy barrier for racemization has been measured to ensure that no racemization occurs during the two last steps of the synthetic pathway. The assignments of the absolute configurations have been performed by electronic circular dichroism and the enantiopurity was determined by NMR spectroscopy in the presence of enantiopure camphor sulfonic acid. To highlight the interest of such compounds, the recognition of norephedrine neurotransmitter was investigated and showed a remarkable enantioselectivity towards the C₃ symmetrical hosts. Finally, this highly modular synthetic pathway was used to provide eight enantiopure hemicryptophanes with different sizes, shapes, and functionalities. These results underline the high potential of this approach, which could lead to many applications in chiral recognition or asymmetric supramolecular catalysis.     

Electron-ion recombination in a helium plasma produced by laser

Experimental investigation of the recombination coefficients in a helium plasma produced by laser is reported. The relative contribution of electronic collision processes and of atomic collision processes on the values of the recombination coefficients are pointed out according to the value of the neutral pressure of helium.     

80 Ev Imaging of Laser Produced Plasmas for Different Z Materials at 1.06, 0.53 and 0.26 μM

The VUV emissions (155 Å) of laser produced plasma on massive plane targets with different Z numbers are recorded according three laser wavelengths with a pinhole camera associated with a multilayered mirror. The 2D dimensions of these VUV images are compared with the sizes of the focal spots through the 1 Kev pinhole image sizes recorded at the same laser intensity (510¹⁴ W/cm², 600 ps). The lateral and axial dimensions of the VUV emissions of Al, Cu, Au plasmas are analyzed and give informations on the radiations-hydrodynamic behavior of Laser heated plane targets with respect to the wavelength of the incident laser.     

Spin fluctuation effects in substituted CeRu2Si2 and YbPd2Si2 alloys

Effects of chemical substitution in CeRu₂Si₂, a well-studied heavy fermion system and YbPd₂Si₂ have been investigated through magnetic susceptibility and x-ray diffraction in the systems CeRu _x Si₂, CeRu_2−x Os _x Si₂, CeRu₂Si_2−x Ge _x and YbPd₂Si_2−x Ge _x . Replacing silicon by germanium generates normal chemical pressure effect, namely, Ce and Yb atoms in CeRu₂Si₂ and YbPd₂Si₂ became more and less magnetic respectively. With increasing Ge concentration, CeRu₂Si_2−x Ge _x exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd₂Si_2−x Ge _x , increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu_2−x Os _x Si₂ though CeRu₂Si₂ and CeOs₂Si₂ have very nearly the same lattice parameters. It is conjectured that CeRu₂Si_2−x Ge _x may be the first Ce-based heavy fermion having a magnetic ground state. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

151Eu-Mössbauer investigation in some new ternary Eu2M3Si5 systems (M=Ni,Pd, Cu,Rh)

The series Eu₂M₃Si₅ (M=Ni, Cu, Pd, Rh) have been synthesised for the first time. The Ni, Cu, Pd systems form in U₂Co₃Si₅ type structure.¹⁵¹Eu Mössbauer spectroscopy shows that Eu is in valence fluctuating (VF) state in Ni system, while in Cu and Pd systems it is in stable divalent state. These observations are further confirmed by magnetic susceptibility studies. The material Eu₂Rh₃Si₅ shows two lines in the Mössbauer resonance, one of which shows VF behaviour.     

Recombination of doubly ionized atoms in the afterglow of a helium plasma produced by laser

Experimental investigation of the collisional-radiative decay coefficient of doubly ionized helium atoms in a helium plasma produced by laser is reported. Comparison of our experimental values with the theoretical values determined by Drawin-Emard is made. A reasonable agreement is obtained if we take into account the optical thickness of the plasma.     

New promising bulk thermoelectrics: intermetallics,pnictides and chalcogenides

The need of alternative “green” energy sources has recently renewed the interest in thermoelectric (TE) materials, which can directly convert heat to electricity or, conversely, electric current to cooling. The thermoelectric performance of a material can be estimated by the so-called figure of merit, zT = σ α ² T/λ (α the Seebeck coefficient, σ α ² the power factor, σ and λ the electrical and thermal conductivity, respectively), that depends only on the material. In the middle 1990s the “phonon glass and electron crystal” concept was developed, which, together with a better understanding of the parameters that affect zT and the use of new synthesis methods and characterization techniques, has led to the discovery of improved bulk thermoelectric materials that start being implemented in applications. During last decades, special focus has been made on skutterudites, clathrates, half-Heusler alloys, Si_1?x Ge _x-, Bi₂Te_3- and PbTe-based materials. However, many other materials, in particular based on intermetallics, pnictides, chalcogenides, oxides, etc. are now emerging as potential advanced bulk thermoelectrics. Herein we discuss the current understanding in this field, with special emphasis on the strategies to reduce the lattice part of the thermal conductivity and maximize the power factor, and review those new potential thermoelectric bulk materials, in particular based on intermetallics, pnictides and chalcogenides. A final chapter, discussing different shaping techniques leading to bulk materials (eventually from nanostructured TE materials), is also included.