Maxima and minima of the displacement components for the Lamb modes

This paper revisits the vanishing of the transverse component of the particle displacement vector in free surfaces of an isotropic homogeneous plate, for both symmetric and antisymmetric Lamb waves. Drawing on well-known analytical expressions from Viktorov's book [(1967) Rayleigh and Lamb Waves: Physical Theory Applications, Chap. II, pp. 67-121], two distinct frequency-thickness product expressions, in cases where this vanishing occurs, are derived: one for the symmetric modes and another for the antisymmetric modes. At these frequency-thickness products, phase and group velocities have appreciable values which are discussed herein. It appears that these velocities depend on the transverse bulk wave velocity only. This is the specific condition of the Lame? modes. Moreover, theoretical and experimental investigations of displacements in the surface of a plate in air have been carried out. The theoretical part shows that the normal and transverse displacements have, respectively, a local maximum and a local minimum in the vicinity of these frequency-thickness products. The experimental part corroborates the presence of the local maximum of the S(0) Lamb mode for various materials.     

Improvement of kinetics,yield, and colloidal stability of biogenic gold nanoparticles using living cells of <Emphasis Type="Italic">Euglena gracilis</Emphasis> microalga

Recent years have witnessed a boom in the biosynthesis of a large variety of nanomaterials using different biological resources among which algae-based entities have been gaining much more attention within the community of material scientists worldwide. In our previously published findings, we explored some factors that governed the biofabrication of gold nanoparticles using living cultures of microalgae, such as the utilized microalgal genera, the phylum they belong to, and the impact of tetrachloroauric acid concentrations on the ability of these strains to perform the biosynthesis of gold nanoparticles once in contact with these cations. As a follow-up, we present in this paper an improvement of the features of bioproduced gold colloids using living cells of Euglena gracilis microalga when this species is grown under either mixotrophic or autotrophic conditions, i.e., exposed to light and grown in an organic carbon-enriched culture medium versus under autotrophic conditions. As an outcome to this alteration, the growth rate of this photosynthetic microorganism is multiplied 7–8 times when grown under mixotrophic conditions compared to autotrophic ones. Therefore, the yield, the kinetics, and the colloidal stability of the biosynthesized gold nanoparticles are dramatically enhanced. Moreover, the shape and the size of the as-produced nano-objects via this biological method are affected. In addition to round-shaped gold nanoparticles, particular shapes, such as triangles and hexagons, appear. These findings add up to the amassed knowledge toward the design of photobioreactors for the scalable and sustainable production of interesting nanomaterials.     

Etude quantitative de la distinction entre une liaison de covalence et une liaison de coordination: les molecules de borazane et d'aminoborane

Résumé On propose un exemple quantitatif de la distinction entre une liaison de covalence et une liaison de coordination; cette différenciation, fondée sur les propriétés de la molécule une fois formée, semble très nette pour les énergies de liaison, les populations de recouvrement et les charges des groupes; en particulier, un critère récent, proposé par deux des auteurs, est remarquablement vérifié. Enfin, on essaie de préciser la signification des symboles chimiques traditionnels.

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Quantitative studies of the difference between a covalence and a coordination bond: The molecules of amine-borane and aminoborane

An illustration of the distinction between these two types of chemical bonds is proposed, which is based on the properties of the molecule once built. Bond energies, overlap populations and group charges vary considerably from one compound to the other; more, a recent criterion given by two of the authors seems to be particularly suitable for describing the character of the bond. Lastly, one tries to give an explicit signification to the usual chemical symbols.

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Zusammenfassung Es wird ein quantitatives Beispiel der Unterscheidung zwischen einer kovalenten und einer koordinativen Bindung angegeben, die auf den Eigenschaften des vorliegenden Moleküls beruht. Sie erscheint sehr günstig für die Bindungsenergien, die Überlagerungspopulationen und die Gruppenladungen. Insbesondere wird ein Kriterium, das kürzlich von zwei der Autoren angegeben wurde, gut verifiziert. Schließlich wird versucht, die üblichen chemischen Symbole zu präzisieren.

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Les auteurs tiennent à remercier très vivement M. M. Berthier et Millié d'avoir mis à leur disposition le programme de localisation indispensable à ce travail, ainsi que Mademoiselle Le Guen et le C.I.R.C.E. pour la réalisation des calculs sur l'ordinateur IBM 360-75.     

Stereoselective Syntheses of Tetrahydroesterastin-β -Lactam Analogues

A total synthesis of the optically active tetrahydroesterastin β -lactam analogue 2 using Miller's hydroxamate approach is described (Scheme 2). Significant modification of published procedures has resulted in a short and facile stereospecific preparation of the N-[(benzyloxycarbonyl)methyl]-β -lactam 17 starting from the readily available D -serine. This material served as intermediate for the preparation of a variety of N-[(benzyloxycarbonyl)methyl]tetrahydroesterastin β-lactam analogues (Scheme 5).     

Synthesen und pharmakologische Eigenschaften von 2,2-Dialkyl-5-aryl-3-pyridylpyrrolidinen

Syntheses and Pharmacological Properties of 2,2-Dialkyl-5-aryl-3-pyridylpyrrolidines Reaction of the photochemically generated benzonitrile methylides 2 with vinylpyridines yields 2-aryl-4-pyridyl-1-pyrrolines 3. Depending on reduction methods, the compounds 3 are selectively transformed to the corresponding cis- or the trans-substituted pyrrolidines 10 or 11 , respectively. Furthermore, a non-photochemical synthesis has been developed: the easily available nitro-ketones 8 provide through reductive cyclization the pyrrolines 3 or directly the pyrrolidines 11. Twenty-seven compounds of types 10 and 11 have been evaluated in the writhing, hot plate and kaolin tests; especially the cis-pyrrolidines 10 exhibit a valuable antinociceptive activity. Some of the pyrrolines and pyrrolidines have been separated into their enantiomers, which are easily interconverted.     

Main chemical engineering parameters affecting radical polymerization processes in heterogeneous media

The main parameters which may affect both reactor performance and polymerization processes in heterogeneous media are described. Viscosity and its influence on heat and mass transfer, residence time distribution in tubular reactor, as well as, influence of agitation on coagulum formation and of monomer feed rate on polymerization rate in semi-batch processes are examined.     

Self-consistent field tight-binding model for neutral and (multi-) charged carbon clusters

A semiempirical model for carbon clusters modeling is presented, along with structural and dynamical applications. The model is a tight-binding scheme with additional one- and two-center distance-dependent electrostatic interactions treated self-consistently. This approach, which explicitly accounts for charge relaxation, allows us to treat neutral and (multi-) charged clusters not only at equilibrium but also in dissociative regions. The equilibrium properties, geometries, harmonic spectra, and relative stabilities of the stable isomers of neutral and singly charged clusters in the range n=1-14, for C(20) and C(60), are found to reproduce the results of ab initio calculations. The model is also shown to be successful in describing the stability and fragmentation energies of dictations in the range n=2-10 and allows the determination of their Coulomb barriers, as examplified for the smallest sizes (C(2) (2+),C(3) (2+),C(4) (2+)). We also present time-dependent mean-field and linear response optical spectra for the C(8) and C(60) clusters and discuss their relevance with respect to existing calculations.     

Acetate- and thiol-capped monodisperse ruthenium nanoparticles: XPS, XAS, and HRTEM studies

Monodisperse ruthenium nanoparticles were prepared by reduction of RuCl3 in 1,2-propanediol. The mean particle size was controlled by appropriate choice of the reduction temperature and the acetate ion concentration. Colloidal solutions in toluene were obtained by coating the metal particles with dodecanethiol. High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XANES and EXAFS for the Ru K-absorption edge) were performed on particles of two different diameters, 2 and 4 nm, and in different environments, polyol/acetate or thiol. For particles stored in polyol/acetate XPS studies revealed superficial oxidation limited to one monolayer and a surface coating containing mostly acetate ions. Analysis of the EXAFS spectra showed both oxygen and ruthenium atoms around the ruthenium atoms with a Ru-Ru coordination number N smaller than the bulk value, as expected for fine particles. In the case of 2 nm acetate-capped particles N is consistent with particles made up of a metallic core and an oxidized monolayer. For 2 nm thiol-coated particles, a Ru-S bond was evidenced by XPS and XAS. For the 4 nm particles XANES and XPS studies showed that most of the ruthenium atoms are in the zerovalent state. Nevertheless, in both cases, when capped with thiol, the Ru-Ru coordination number inferred from EXAFS is much smaller than for particles of the same size stored in polyol. This is attributed to a structural disorganization of the particles by thiol chemisorption. HRTEM studies confirm the marked dependence of the structural properties of the ruthenium particles on their chemical environment; they show the acetate-coated particles to be single crystals, whereas the thiol-coated particles appear to be polycrystalline.     

Exchange monte carlo for molecular simulations with monoelectronic hamiltonians

We introduce a general Monte Carlo scheme for achieving atomistic simulations with monoelectronic Hamiltonians including the thermalization of both nuclear and electronic degrees of freedom. The kinetic Monte Carlo algorithm is used to obtain the exact occupation numbers of the electronic levels at canonical equilibrium, and comparison is made with Fermi-Dirac statistics in infinite and finite systems. The effects of a nonzero electronic temperature on the thermodynamic properties of liquid silver and sodium clusters are presented.