Détermination des forces perturbatrices d'un systeme oscillant à partir de son comportement en fréquence

Résumé La perturbation d'un oscillateur ne peut pas toujours être définie ou mesurée. Il est montré que moyennant une hypothèse plausible pour la perturbation une inversion de la méthode asymptotique en théorie des oscillations non-linéaires peut donner cette information cherchée. Les résultats sont appliqués au défaut d'isochronisme des montres.

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The perturbations of an oscillating system cannot always be defined or measured. It is shown that, given a plausible hypothesis for the perturbation function, this information is obtained by an inversion of the asymptotic methods for the treatment of non-linear oscillations. The results are applied to the isochronism error of watches.

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Zusammenfassung Störkräfte eines Oszillators sind nicht immer definierbar oder messbar. Es wird gezeigt, wie diese Kräfte mittels einer Umkehrung der asymptotischen Methode in der Theorie nicht-linearer Schwingungen erhalten werden, wenn für sie ein plausibler Ansatz gemacht wird. Die Resultate werden auf den Isochronismusfehler von Uhren angewandt.

     

Assessment of endoglucanase activity by analyzing the degree of cellulose polymerization and high-throughput analysis by near-infrared spectroscopy

We developed a novel and practical assessment technique for endoglucanase (EG) activity by measuring the degree of polymerization (DP) of cellulose from Eucalyptus globulus. This evaluation method demonstrated that EG II from Trichoderma reesei had higher endoglucanase activity than EG I, which has not been detected in conventional experiments using carboxymethyl cellulose as a model substrate. In addition, a high-throughput protocol for DP measurement was developed by using near-infrared spectroscopy combined with a multivariate analysis. Interpreting the regression coefficient, a reciprocal correlation was observed between the relative crystallinity of the cellulosic residue after enzymatic hydrolysis and the DP.     

Folded structures of L-leucylglycine oligopeptides and their aggregational behavior in aqueous solution: Raman scattering spectra and proton NMR spin-lattice relaxation studies

The aggregational behavior of three L-leucylglycine oligopeptides (residue numbers of glycine are 3, 4, and 5) in aqueous solution was investigated by the use of Raman scattering and 1H NMR spin-lattice relaxation methods. The results indicate that their oligopeptides take up a folded structure to form dimeric aggregates above their critical aggregation concentration. The application of one-dimensional aggregate theory to these systems provides the following prediction. Elongation up to 6 glycine residues makes it possible to form dimeric aggregates, but further elongation (up to 7 glycine residues) makes the aggregates very unstable, and up to 8 or 9 glycine residues makes the formation of dimeric aggregates very difficult. The one-dimensional aggregate theory may be used to predict the existence of peptide aggregates through intermolecular hydrogen bonding.     

Stereodivergent catalytic doubly diastereoselective nitroaldol reactions using heterobimetallic complexes

Stereodivergent construction of three contiguous stereocenters in catalytic doubly diastereoselective nitroaldol reactions of alpha-chiral aldehydes with nitroacetaldehyde dimethyl acetal using two types of heterobimetallic catalysts is described. A La-Li-BINOL (LLB) catalyst afforded anti,syn-nitroaldol products in >20:1-14:1 selectivity, and a Pd/La/Schiff base catalyst afforded complimentary syn,syn-nitroaldol products in 10:1-5:1 selectivity.     

Self-assembly of polyoxometalate macroanion-capped pd0 nanoparticles in aqueous solution

The self-assembly behavior of polyoxometalate (POM) macroanion-capped 3-nm-radius Pd (0) nanoparticles in aqueous solution is reported. Pd(0) nanoparticles are synthesized from reducing K(2)PdCl(4) by using Dawson-type V-substituted POM K(9)[H(4)PV (IV)W(17)O(62)] (HPV(IV)) clusters as the reductant and stabilizer simultaneously in acidic aqueous solutions. The starting molar ratio of K(2)PdCl(4) to HPV(IV) (R value) in solution is important to the formation of Pd nanoparticles. When R < 0.6, approximately 20-nm-radius Pd(0) colloidal nanocrystals are formed. When R > or = 0.6, HPV-capped (and therefore negatively charged) 3-nm-radius Pd(0) nanoparticles are formed, which can further self-assemble into stable, hollow, spherical, 30-50-nm-radius supramolecular structures in solution without precipitation, as confirmed by light scattering and transmission electron microscopy studies. This structure resembles the unique supramolecular structure formed by hydrophilic POM macroanions in polar solvents, which we refer to as "blackberry" structures. It is the first evidence that the blackberry formation can occur in hydrophobic nanoparticle systems when the surface of nanoparticles is modified to be partially hydrophilic. Counterions play an important role in the self-assembly of Pd nanoparticles, possibly providing an attractive force for blackberry formation, which is the case for blackberry formation in POM macroanionic solutions. Our results suggest that the blackberry formation is not a specific property of POM macroions but most likely a general phenomenon for nanoparticles with relatively hydrophilic surfaces and suitable sizes and charges in a polar solvent.     

Two-dimensional gel permeation chromatography (2D GPC) correlation studies of the aggregate-aggregate interactions in acid-catalyzed triethoxysilyl-terminated polystyrene systems. The effect of specific catalysts on growth process

A set of time resolved gel-permeation chromatography (GPC) profiles, for the reaction mixture of a well-defined polymeric silane coupling agent, triethoxysilyl-terminated polystyrene with molecular weight equal to 8000 (TESi-PS (8000)), catalyzed by HCl (0.1 mol/kg) in tetrahydrofuran (THF), was measured over a long time scale (1–768 h). The GPC profiles were then converted to two-dimensional (2D) correlation spectra. The 2D GPC correlation spectra were compared with those for the CH₃SO₃H (0.1 mol/kg)-catalyzed TESi-PS (8000)-THF system. It has been demonstrated that predominant production of less-reactive oligomers in the HCl-catalyzed system hinders further growth of the oligomer, while formation of reactive oligomers leads to further growth of polymeric precursors in the CH₃SO₃H-catalyzed system.     

Asymmetrically modified silica particles: a simple particulate surfactant for stabilization of oil droplets in water

Spherical silica particles that are able to assemble at a phase boundary of a dual-phase mixture of water and an immiscible organic solvent were prepared by a partial modification of their surface hydroxyl groups with an alkylsilylation agent. Scanning electron microscopic observation of these particles in which their remaining surface hydroxyl groups had been selectively modified with colloidal gold particles revealed that each particle has an asymmetric surface structure: one side of the surface is hydrophilic and the other is hydrophobic. We found that these particles could form a micellar structure in water in the presence of an organic solution of a toluene/polystyrene mixture. The micellar structure was evidenced by formation of golf-ball-like polystyrene particles with dimples imprinting morphologies of the hydrophobic part of modified silica particles.     

Linearity in Optical Absorption Spectra and Gold-Silver Alloy Colloidal Particles in Epoxy Resin

Gold-silver alloy colloids were produced in epoxy resin by thermal decomposition of gold and silver complexes, and their optical absorption spectra were measured. A linear law was found for the maximal absorption wavelengths: the maximal absorption wavelength for gold-silver alloy colloids of arbitrary composition was determined from the wavelengths for pure gold and pure silver colloids and the initial concentration ratio of gold and silver complexes. The change of absorbance and absorption wavelengths with heating periods is presented. Copyright 2001 Academic Press.     

Platinum‐Containing Polyoxometalates: syn‐ and anti‐[PtII2(α‐PW11O39)2]10− and Formation of the Metal–Metal‐Bonded di‐PtIII Derivatives

The first examples of dimeric, di‐Pt^II‐containing heteropolytungstates are reported. The two isomeric di‐platinum(II)‐containing 22‐tungsto‐2‐phosphates [anti‐Pt^II₂(α‐PW₁₁O₃₉)₂]^10? ( 1 a ) and [syn‐Pt^II₂(α‐PW₁₁O₃₉)₂]^10? ( 2 a ) were synthesized in aqueous pH 3.5 medium using one‐pot procedures. Polyanions 1 a and 2 a contain a core comprising two face‐on PtO₄ units, with a Pt???Pt distance of 2.9–3 Å. Both polyanions were investigated by single‐crystal XRD, IR, TGA, UV/Vis, ³¹P NMR, ESI‐MS, CID‐MS/MS, electrochemistry, and DFT. On the basis of DFT and electrochemistry, we demonstrated that the {Pt₂^II} moiety in 1 a and 2 a can undergo fully reversible two‐electron oxidation to {Pt₂^III}, accompanied by formation of a single Pt?Pt bond. Hence we have discovered the novel subclass of Pt^III‐containing heteropolytungstates.