Détermination de la structure cristalline du ferrite de baryum Ba2Fe6O11

The crystal structure of dibarium triferrite Ba₂Fe₆O₁₁ has been solved by direct methods, using intensity data collected by means of an automated diffractometer (MoKα radiation) and corrected for absorption. It crystallizes in the orthorhombic space group Pnnm: a = 23.024(10)Å, b = 5.181(3) Å, c = 8.900(4) Å, Z = 4. Program MULTAN was successfully used for locating Ba²⁺ and most of the Fe³⁺ ions. The structure was further refined by conventional Fourier and least-squares methods (full-matrix program) to a final R value of 0.045 for 1448 observed reflections. Fe³⁺ ions occur in both octahedral (FeO mean distance: 2.02 Å) and tetrahedral (FeO mean distance: 1.865 Å) coordination. Two types of Ba²⁺ ions are found, with six and seven neighboring oxygen atoms. The structure consists of sheets of edge-shared FeO₆ octahedra which are connected by means of corner-shared tetrahedra.     

A General Method for the Synthesis of N,N-Dialkylaminobutylamines

A general method for the synthesis of N,N-dialkylaminobutylamines 4 from readily available chloroacetamides 6 is described.     

H and Mu diffusion in ice

Detailed studies of muonium in ice carried out at TRIUMF in the mid eighties directly inspired EPR investigation of H and D atoms in ice at the Argonne National Laboratory, using a pulsed EPR spectrometer in conjunction with a Van de Graaff electron accelerator. T₂ measurements were made by the spin-echo method, Subsequent work resulted in a measurement of the longitudinal spin relaxation time T₁ at a single temperature. In combination with T₂ this permits an unambiguous determination of the strength of the magnetic interaction giving rise to relaxation, and hence permits conversion of all the existing T₂ data to diffusional correlation times, . We have recently initiated a further study of muonium in ice, this time concentrating on T₁ measurements. Preliminary work shows the feasibility of determining from the field dependence of T₁ at a given temperature. The result is in good agreement with our earlier, less direct determinations.     

Synthesis of chromophores combining second harmonic generation and two photon induced fluorescence properties

The design of new chromophores presenting simultaneous SHG and TPEF properties is reported.     

Monodisperse fluorene oligomers exhibiting strong dipolar coupling interactions

Well-defined fluorene oligomers (n = 1 to 6) were prepared step by step using Suzuki and Yamamoto couplings, while absorption and photoluminescence properties evidenced very large dipolar coupling interactions between fluorene moieties.     

Ionic interactions in solutions. III. Derivation of the ″associated″ electrolyte formulation from the Onsager treatment of conductance

The system of equations of the Onsager treatment of conductance is rederived in a systematic way by the use of several sets of hierarchy equations generalizing to other quantities the classical Born-Bogoliubov-Green-Kirkwood-Yvon (BBGKY) hierarchy of internal force. The monitoring term of the Onsager continuity equation obtained is compared to former theoretical studies. It is inferred in particular that the perturbation on the anion-cation pair distribution for a symmetrical binary electrolyte is proportional not only to the external fieldX but also to the conductance coefficientf _A=1+X/X+ _el/ ₀. This result, called the echo effect, leads to a new formulation of molar conductance which shows a great similarity to the empirical conductance function obtained by grafting the chemical model of Bjerrum onto the conductance equations derived on the basis of the Debye approximations as proposed formerly by the author from experimental observations. The result allows a simple generalization of the conductance equation to any direct short-range anion-cation energy-potential model.     

Lp resolvent estimates for magnetic Schrödinger operators with unbounded background fields

We prove L^p and smoothing estimates for the resolvent of magnetic Schrödinger operators. We allow electromagnetic potentials that are small perturbations of a smooth, but possibly unbounded background potential. As an application, we prove an estimate on the location of eigenvalues of magnetic Schrödinger and Pauli operators with complex electromagnetic potentials.     

Synthese ambidoselective d'enamines et d'imines siliciees

Trimethylchlorosilane reacts with organotin imines or enamines leading mainly (or only) to the corresponding silicon enamine. In contrast, the silylation of lithium or magnesium iminates gives only the isomeric imine when the nitrogen is hindered by bulky groups (i-Pr, t-Bu).The study of the NSi ? CSi equilibration shows clearly that in both cases the isomer obtained is the kinetic product of each reaction.These results show that a proper choice of both the metal associated with the iminate and of the iminate itself allows the formation of each isomer in a pure state.