Calcul théorique des énergies de divers arrangements géométriques de la molécule (η5-C5H5)2TiCl2

The molecular energy of the complex (η⁵-C₅H₅)₂TiCl₂ has been calculated for experimental geometry and for several hypothetical forms by a semi-empirical CNDO/2 approach. The energy difference between experimental (quasi-tetrahedral) geometry and a planar geometry is ca. 20 kcal mol^?1. This difference is sufficiently high to explain the difficulty of the inversion process.     

Fluorescence from the second excited singlet state of [18] annulenes

The emission spectra of the second excited singlet state of [18] annulene and of monofluoro [18] annulene (in a 3-methylpentane glass at 4 K) are reported. The large energy gap between the first and second excited singlet states inhibits fast internal conversion and favours the appearance of S₂ → S₀ emission. In addition, fluorescence from the S₁ state can be observed in monofluoro [13] annulene by exciting into the S₂ or directly into the S₁ absorption.     

Comportement de LiAlH4 en fonction de la dilution dans la racemisation d'un silane optiquement actif en milieu thf

The optically active methylphenyl-α-naphthylsilane racemises quickly in tetrahydrofuran in the presence of LiAlH₄.Kinetic results show a different behaviour of the hydride depending on its concentration. Activation parameters have been obtained and mechanisms proposed.     

Determination de la constante d'acidite intrinseque d'un echangeur d'ions carboxylique

To obtain the intrinsic acidity constant of a carboxylic ion-exchanger, we propose a method of graphical double extrapolation. The results for an ion-exchanger made of polyvinyl alcohol grafted with acrylic acid are close to those published for the corresponding linear polyacid.     

Sodium-23 Nuclear Magnetic Resonance Spectroscopy

With the availability of Fourier transform spectrometers, ²³Na-NMR spectroscopy has become an important tool. It affords direct insight into solvation and ion pairing phenomena, both in organic and in bio-inorganic systems. Monitoring the chemical shifts and linewidths of ²³Na signals gives access to binding constants, reorientational correlation times and the microdynamics of the sodium coordination shell. The binding of other cations can also be studied by ²³Na-NMR spectroscopy, in competition experiments.     

CHEMILUMINESCENCE AND ENERGY TRANSFER FROM SINGLET OXYGEN PAIRS

Abstract— Chemiluminescence and photochemiluminescence of systems generating excited (singlet) molecular oxygen has been studied at low temperature in fluid media. A possible interpretation is proposed: that dimers of singlet oxygen are directly or indirectly responsible for the observed emission, the species [¹δg +¹σg⁺] predominating. Energy transfer to a fluorescent molecule is possible only when conditions of spectral overlap, required for transfer by inductive resonance, are fulfilled. Evidence is also presented for the formation of a transient complex between oxygen and a sensitiser or acceptor molecule [S…O₂], which is the precursor of singlet oxygen in these systems. However, other possible mechanisms cannot be excluded.     

19F NMR chemical shifts of n-F-alkyl compounds

The examination of ¹⁹F chemical shifts for ca. 650 F-alkylated compounds of general formula CF₃(CF₂)_nCF₂X led to the following conclusions: the CF₂ groups α to X are very sensitive to the nature of X, and are spread over a range of 85 ppm. The effect of the length of the F-alkyl chain decreases rapidly, so that δCF₂(α) can already be considered as characteristic of X for n = 1 or 2 for most practical purposes. Solvent effects (in 9 different solvents having ε = 1.8 to 52.1) were found to be rather small except for the F-alkyl iodides. A chart which indicates the domain in which the CF₂(α) resonance signal is to be expected is given for 42 different series of F-alkylated compounds; it is expected to provide the synthetic chemist with a useful tool for the identification and characterization of such compounds.     

Dilute solution properties of a polyurethane. I. Linear polymers

A linear polyurethane of high molecular weight was prepared in solution by the polyaddition of equimolar amounts of ethylene glycol and methylene bis(4-phenyl isocyanate). The polymer was fractionated by using a direct sequential extraction procedure, with a solvent–nonsolvent system consisting of N,N′-dimethylformamide (DMF) and acetone (A). The resulting fractions were characterized by viscosity and lightscattering measurements. The relationship between the intrinsic viscosity and molecular weight was found in DMF at 25°C. to be [η] = 3.64 × 10^?4M^0.71. The unperturbed polymer chain dimensions were determined from intrinsic viscosity measurements carried out under experimentally determined theta conditions.     

Study of the ternary system FeCrTe around the composition FeCr2Te4

The system FeCrTe was investigated around the composition FeCr₂Te₄. FeCr₂Te₄ is a metastable compound. Single crystals with Fe_0·93Cr_1·76Te₄ composition were grown by vapor transport or the Bridgman method. These crystals are metallic and anisotropic ferromagnets.     

k-spaces and Borel filters on the set of integers

We say that a countable, Hausdorff, topological space with one and only one accumulation point is a point-space. For such a space, we give several properties which are equivalent to the property of being a k-space. We study some free filters on the set of integers and we determine if the associated point-spaces are k-spaces or not. We show that the filters of Lutzer-van Mill-Pol are k-filters. We deduce that, for each countable ordinal , there exists a free filter of true additive class (Baire's classification) and a free filter of true multiplicative class for which the associated point-spaces are k-spaces but not , the existence being true in the additive case for . In particular, we answer negatively a question raised in J. Calbrix, C. R. Acad. Sci. Paris 305 (1987), 109--111.