Trivalent Cation Substitution Effect into Layered Double Hydroxides Co2Fey Al1−y(OH)6Cl·nH2O: Study of the Local Order: Ionic Conductivity and Magnetic Properties

A series of layered double hydroxide materials of composition Co₂Fe_yAl_1−y(OH)₆Cl·nH₂O (0≤y≤1) was prepared via chimie douce. The crystalline parameter related to the cation to cation distance obeys the expected variation, showing that the substitution is effective over the entire range. Local order around Co and Fe cations is studied by X-ray absorption spectroscopy. Moduli of the Fourier transform at the Fe K-edge are superimposable, in agreement with an ordered model, although present in small domains since no superlattice is depicted. The ionic resistivity of the samples is highly dependent on the water molecule content. The conductivity is found to be thermally assisted, and the variation of the slope in the Arrhenius diagram is explained by a Vogel-Tamman-Fulcher-type behavior. Magnetic susceptibility measurements support the proposed cation composition and indicate the onset of local magnetic order at low temperature (below 10 K). The inter-sheet distance influences the magnetic response at low temperature, showing the presence of weak interactions between lamellae.     

Quasi-solitonic behavior of self-written waveguides created by photopolymerization

We investigated the condition of unique self-written channel and multichannel propagation inside bulk photopolymerizable materials. Light was introduced in the medium by a single-mode optical fiber. At a very low beam power of 5 muW , a unique uniform-channel waveguide without any broadening was obtained by polymerization. When the input power is increased to 100 muW , the guide becomes chaotic and multichannel. We connected two fibers separated by a 1-cm distance. The results open the door to studies of the optical and electro-optical properties of photopolymerized guides doped by nonlinear optical chromophores and to possible applications in integrated optical devices.     

Richness of isomerism in labile octahedral Werner-type cobalt(II) complexes demonstrated by 19F NMR spectroscopy: structure and stability

The cobalt(II) complexes [CoL2(R2-Py)2] (1-4) where HLA = 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione, R2-Py = 4-methylpyridine (1), HLB = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione, R2-Py = 4-methylpyridine (2), 4-phenylpyridine (3) and S-(-)-1-(4-pyridyl)ethanol (4) were prepared by two-step reactions. X-ray structure analysis of [CoLA2(CH3-Py)2] revealed the {trans(N)-trans(CF3)-trans} configuration for the complex obtained by crystallization from ethanol. A dynamic equilibrium between the five possible stereoisomers was observed for each complex 1-4 in solution by 19F NMR spectroscopy. The criteria used for full NMR assignment (180-265 K) include comparison of integral ratios, cis(N) and trans(N) differentiation in presence of the chiral amine [S-(-)-1-(4-pyridyl)ethanol], effect of solvent polarity on the relative stabilities of the five isomers and observation of trans influences in a mixture of complexes. Thermodynamic parameters for the equilibria between the isomers of 2 in CD2Cl2 (DeltaHi,j, DeltaSi,j and Ki,j) were obtained from signal integrals. The two trans(N) isomers are slightly more stable than the three cis(N) isomers at low temperature [DeltaGdegreesi,j (max) = 2.8 kJ mol(-1) at 179.8 K], but this stability difference almost vanishes with increasing temperature [DeltaGdegreesi,j (max) = 1.0 kJ mol(-1) at 265.0 K]. The values found for DeltaHdegreesi,j are relatively small and largely entropy compensated.     

Redox-triggered molecular movement in a multicomponent metal complex in solution and in the solid state

The Cu(I) and Cu(II) complexes of the new 1,8-diferro-cenylmethyl-4,11-dimethyl-1,4,8,11-tetraazacyclotetra-decane ligand (denoted L) have been isolated and characterized by X-ray structure determination and electrochemical studies. The Cu(I) complex presents an unprecedented stability toward dioxygen. The two complexes adopt two energetically distinct and stable geometries, which differ mainly by the relative positioning of the substituents above or below the cyclam plane. Triggered by a copper-centered electron transfer, a fast and reversible motion of the noncoordinating subunits is obtained in homogeneous solution and in the solid state.     

Heterogeneity of the surface energy on unused C18-chromolith adsorbents in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis (FA) for phenol and caffeine on a new C18-Chromolith column (Merck, Darmstadt, Germany), using a water-rich mobile phase (methanol/water, 15/85, v/v). These data were modeled for best agreement between the experimental data points and the adsorption isotherm model. The adsorption-energy distributions, based on the expectation-maximization (EM) procedure, were also derived and used for the selection of the best isotherm model. The adsorption energy distributions (AEDs) for phenol and caffeine converged toward a trimodal and a quadrimodal distribution, respectively. Energy distributions with more than two modes had not been reported before for the adsorption of these compounds on packed columns. The third high energy mode observed for both phenol and caffeine seems to be specific of the surface of the monolithic column while the first and second low energy modes have the same physical origin as the two modes detected on packed columns. These results suggest significant differences between the structures of the porous silica in these different materials.     

Effect of pressure and temperature upon tin alkoxide-promoted ring-opening polymerisation of epsilon-caprolactone in supercritical carbon dioxide

Ring-opening polymerisation of epsilon-caprolactone in supercritical carbon dioxide is slowed down by a carbonation reaction, resulting in a positive volume of activation and a higher energy of activation as compared to polymerisation in a regular hydrocarbon solvent.     

Scattering on Reissner-Nordstrøm Metric for Massive Charged Spin 1/2 Fields

We consider massive charged Dirac fields on the Reissner-Nordstrøm metric. We prove the existence and asymptotic completeness of wave operators, classical at horizon and modified at infinity. Communicated by Piotr Chrusciel submitted 24/05/02, accepted: 14/02/03     

Sensitivity indices for multivariate outputs

We define and study a generalization of Sobol sensitivity indices for the case of a vector output.