The vacuum UV photolysis of various C4 and C5 olefins: The energy content of the α- and β-methallyl fragments

In recent publications from this laboratory, we have shown that the fragmentation of photoexcited olefinic molecules in the vacuum UV region leads mainly to the cleavage of a C—C bond located in the ß position relative to the double bond. The allyl fragment bears away part of the excess energy of the photon. At low pressure, this excited radical is capable of undergoing further decomposition. From the pressure effect, we were able to measure the first order rate constant for this secondary fragmentation. In this paper we shall use RRKM calculations in order to get a better idea on how the energy is distributed among the primary fragments. In cases where α- and β;-methallyl radicals were involved, the results show that an important part of the excess energy is located in the methallyl fragment in the 7.1 and 7.6 eV photolysis of 3-methyl-1-butene, 2-methyl-1-butene, and cis-2-pentene.     

Algorithme matriciel et connexions d'ordre supérieur dans des fibrés non vectoriels

Résumé Le calcul des jets devenant rapidement presqu'inextricable, un calcul matriciel partiel est proposé qui pallie en partie à cet inconvénient et permet de mieux saisir la structure algebrique des jets holonomes (chap1). Cet algorithme est ensuite utilisé dans le calcul des jets et permet de définir des connexions d'ordre supériéur ainsi que les différentielles absolues correspondantes de manière très simple (chap2). Ceci généralise naturellement l'algorithme classique mis sous forme adéquate (voir introduction). Entrata in Redazione il 13 dicembre 1969.     

Synthesis, reactivity, and ligand properties of a stable alkyl carbene

Stable tert-butyl diisopropylamino carbene is more nucleophilic but also more electrophilic than diamino carbenes; it undergoes cyclopropanation reactions and easily binds to metal fragments.     

Reduction of β-ketosulfoxides : a highly efficient asymmetric synthesis of both enantiomers of methyl carbinols from the corresponding esters

It is shown that reductions of β-ketosulfoxides of identical chirality (R) at sulfur, lead to reduction products of opposite stereochemistry according to the reducing agent used. The high enantiomeric excesses obtained (80 to 100%) provide a general route to both enantiomers of methylcarbinols from the corresponding esters.     

Études calorimétriques en milieu solvant organique: II Enthalpies de dissolution et de dilution du boranate de lithium (LiBH4) et de l'alanate de lithium (LiAlH4) dans le tétrahydrofuranne

The enthalpies of dissolution and dilution of LiBH₄ and LiAlH₄ in THF have been determined.Dissolutions are exothermic. The endothermic dilution of LiAlH₄ suggests for this compound, either a dissociation or an association equilibrium.     

High Probe Intensity Photobleaching Measurement of Lateral Diffusion in Cell Membranes

Lateral diffusion measurements, most commonly accomplished through Fluorescence Photobleaching Recovery (FPR or FRAP), provide important information on cell membrane molecules' size, environment and participation in intermolecular interactions. However, serious difficulties arise when these techniques are applied to weakly expressed proteins of either of two types: fusions of membrane receptors with visible fluorescent proteins or membrane molecules on autofluorescent cells. To achieve adequate sensitivity in these cases, techniques such as interference fringe FPR are needed. However, in such measurements, cytoplasmic species contribute to the fluorescence recovery signal and thus yield diffusion parameters not properly representing the small number of surface molecules. A new method helps eliminate these difficulties. High Probe Intensity (HPI)-FPR measurements retain the intrinsic confocality of spot measurements to eliminate interference from fluorescent cytoplasmic species. However, HPI-FPR methods lift the previous requirement that FPR procedures be performed at probe beam intensities low enough to not induce bleaching in samples during measurements. The high probe intensities now employed provide much larger fluorescence signals and thus more information on molecular diffusion from each measurement. We report successful measurement of membrane dynamics by this technique.     

Practical aspects of shimming a high resolution magic angle spinning probe

High resolution magic angle spinning (HRMAS) has become an extremely versatile tool to study heterogeneous systems. HRMAS relies on magic angle spinning of the sample to average out to zero magnetic susceptibility differences in the sample and to obtain resonance linewidths approaching those of liquid state NMR. Shimming such samples therefore becomes an important issue. By analyzing the different sources of magnetic field perturbations present in a sample under MAS conditions, we propose a simple protocol to obtain optimum shim settings in HRMAS. In the case of aqueous samples, we show that the lock level cannot be used as a reliable indicator of the quality of the shims at high spinning speeds. This effect is explained by the presence of temperature gradients imparted by the sample rotation.     

Factors affecting d-block metal-ligand bond lengths: toward an automated library of molecular geometry for metal complexes

Metal-ligand (M-L) bond lengths for a range of ligands (carboxylates, chlorides, pyridines, water, tertiary phosphines, and alkenes) and a variety of metals have been retrieved from the Cambridge Structural Database, CSD. Analysis of the factors which affect M-L bond lengths (for example, ligand coordination mode, oxidation state, metal coordination number and geometry, spin and Jahn-Teller effects, and ligand trans to M-L bond) shows that it is generally possible to subdivide the M-L data sets systematically to obtain better defined, unimodal, bond length distributions with means and sample standard deviations (SSDs) which reflect the nature of the bond in question. Typically, the SSDs for the M-L data sets can be reduced to 0.04-0.05 A by these methods. This work is an extension to tables of bond lengths in organometallic compounds and coordination complexes published in 1989. The importance of the factors which affect M-L bond lengths for particular metal-ligand groups are discussed. From the case studies reported, an algorithm is proposed by which compilation of a library of molecular geometry for metal complexes may be automated. The points that need to be considered to produce such a molecular library from the data stored in the CSD are discussed. The development of such a library would allow users to retrieve chemically well-defined geometric data rapidly and accurately. This should be of use, for example, to crystallographers and molecular modelers.     

Theory of mesoscopic magnetism in photonic crystals

We provide a rigorous theoretical basis for the artificial magnetic activity of metamaterials near resonances. Our approach is a renormalization-based scheme that authorizes a completely general theory. The major result is an explicit expression of the effective permeability, in terms of resonant frequencies. The theoretical results are checked numerically, and we give applications of our theory to left-handed media and to the solution of the Pokrovski-Efros paradox.