Structure et mecanisme de cristallisation des produits obtenus par hyper-trempe dans les systemes BaO·Fe2O3 et SrO·Fe2O3

Splat cooling applications to the BaO·Fe₂O₃ and SrO·Fe₂O₃ systems are described. The complete study of structural and crystallographic properties is given for several compositions in these systems. The mechanism of the new cristallization after vitrification is explained for these phases. New compounds are formed, so the interest of the method is proved for the study of binary diagrams. Application of these compounds may be possible on the basis of their magnetic properties.     

Measurement of the susceptibility of solid 3He along the melting curve from 20 mK down to the nuclear ordering temperature

The susceptibility of solid ³He is deduced from multiple echoes observed in pulsed NMR experiments. It follows a Curie Weiss law at high temperature, increases faster than this law below 5 mK and drops abruptly around 1 mK.     

Elaboration,caracterisation et proprietes magnetiques de cristaux d'oxyde Sm2O3

Single crystals of samarium oxide Sm₂O₃ with monoclinic B structure were prepared by the Verneuil process adapted to a plasma torch. Some properties of these crystals have been studied (cleavage, hardness …) especially the magnetic properties. Paramagnetic susceptibilities measurements have been performed for various orientations of the crystal with respect to the magnetic field at constant temperatures (5, 77, 300 K) as well as at increasing temperatures (5–1000 K) and indicate an anisotropy. An interpretation of this phenomena is proposed.     

N-acylation of 1,3-dimethyl-6-aminouracils. A reversal in the regiospecificity of an electrophilic substitution induced by an intramolecular proton-transfer

Electrophilic reactions of 1,3-dimethyl-6-aminouracil lead to 5-substituted derivatives. The introduction of a dialkylaminoalkylamino chain in the 6-position of 1,3-dimethyluracil modifies the regiospecificity of the acylation reaction to give N-6 acylated compounds. This reversal in acylation is induced by an intramolecular proton-transfer which introduces a change in the electron density of the enamine system. The cyclic transition state and the spatial conformation of the final products substantiate the proposed mechanism, on the basis of X-ray and ¹H nmr data.     

Kinetic enolate formation by lithium arylamide: effects of basicity on selectivity

Five ketones R1COCH2R2 (1a-e) were enolized in tetrahydrofuran solvent employing lithium arylamides with different electron-withdrawing and -donating substituents on the phenyl ring (4a-e). Enolate selectivity is unaffected by a moderate electron-releasing or -withdrawing group, but significantly enhanced by strong electron-withdrawing substituents to yield predominantly Z-enolate. Outstanding selectivity was achieved with lithium trichloroanilide (5) and lithium diphenylamide (6). The results are rationalized in terms of electronic effects on the tightness of the transition states.     

Synthesis of [Sar1, Val5, (4′-Azido-3′,5′-ditritio)Phe8] Angiotensin II,a Photoaffinity Label for the Isolation of Angiotensin II Receptors Communication 1

The synthesis of [Sar¹, Val⁵, (4′-azido-3′,5′-ditritio)Phe⁸] angiotensin II from a iodinated precursor peptide is described. The principal problems of this synthesis and their resolution are discussed: (i) The β-induced autophotolysis of the highly tritiated (73 Ci/mmol) and photosensitive label, and (ii) the absorption problems encountered during the purification of microgramm quantities. Such photoaffinity labels are being used for specific labeling and isolation of peptide hormone receptors.     

Suppression of high-p{T} neutral pion production in central Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV relative to p+C and p+Pb collisions

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[S{NN}]=17.4 GeV at midrapidity (2.3 less than or approximately equal eta{lab} less than or approximately equal 3.0) over the range 0.7 less than or approximately equal p{T} less than or approximately equal 3.5 GeV/c. The spectra are compared to pi{0} spectra measured in Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV in the same experiment. For a wide range of Pb+Pb centralities (N{part} less than or approximately equal 300), the yield of pi{0}'s with p{T} greater than or approximately equal 2 GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N{coll}), while for central Pb+Pb collisions with N{part}greater than or approximately equal 350, the pi{0} yield is suppressed.     

Interferometry of direct photons in central 208Pb + 208Pb collisions at 158A GeV

Two-particle correlations of direct photons were measured in central 208Pb+208Pb collisions at 158A GeV. The invariant interferometric radii were extracted for 100相似文献