Temperature-dependent interconversion of phosphoramidite-Cu complexes detected by combined diffusion studies, 31P NMR, and low-temperature NMR spectroscopy

For copper-catalyzed enantioselective conjugate additions, knowledge about the precatalytic and catalytic complexes has not yet been sufficiently developed to understand the strong influence of different temperatures on these famous reactions. Therefore, NMR experiments with four Cu(I) salts and two phosphoramidite ligands have been performed to elucidate the temperature dependence and the low-temperature structures of these copper complexes. The existence of the precatalytic binuclear complex with a mixed trigonal/tetrahedral coordination on copper is for the first time proven with direct NMR spectroscopic methods. Below 200 K, intermolecular interactions between free ligands and [Cu2X2L3] complexes induce binuclear [Cu2X2L4] complexes similar to the crystal structures. By combining diffusion experiments and (31)P integrals at different temperatures, it is for the first time possible to follow the formation of stoichiometrically different complexes, even under experimental conditions in which the (31)P signals of the complexes are spectroscopically not resolved due to exchange processes. This allows a first correlation between the complex species observed and the synthetic conditions reported. Furthermore, different preferences to build homo- or heterochiral complexes are detected for binaphthol and biphenol phosphoramidite complexes.     

Anharmonicity in a fragile glass-former probed by inelastic neutron scattering

Within the overall understanding of the glass transition, the relationship between microscopic dynamics and fragility is still to be clarified. Decalin is an organic glass former, for which a cis/trans mixture exhibits the highest known degree of fragility in a molecular system. It is therefore an ideal system for the investigation of microscopic dynamics in fragile systems. In the present study, the microscopic dynamics of pure cis-decalin has been measured by inelastic and quasi-elastic incoherent neutron scattering, giving the single particle self-correlation function. The fast relaxation dynamics and low-frequency vibrational modes are reported here. Both in the glass and in the crystal the vibrations show strong anharmonic behavior. In the glass phase, the short time microscopic dynamics evolve rapidly with temperature, however do not exhibit any significant change around the glass transition temperature T(g). The elastic intensity provides a measure of the mean square displacements which are comparable to those measured in other fragile glass formers, in particular, the archetypical fragile glass former orthoterphenyl. It appears that the microscopic relaxation gets unfrozen, relative to T(g), at much lower temperature than in other fragile systems.     

A MEMS skin-friction sensor for time resolved measurements in separated flows

A MEMS surface fence sensor for skin-friction measurements in separated flows has been developed and tested successfully in low-speed flows. The new sensor is able to distinguish between forward and reverse flow and features a temporal resolution of up to 1 kHz. Calibrations of the sensor have been obtained in the range of –0.7 N m^-2_w0.7 N m^-2 with a resolution of 0.02 N m^-2. Comparative measurements with the wall-pulsed wire technique in a reverse-flow region show excellent agreement with respect to the mean skin-friction coefficient c_f but also reveal some discrepancies for the fluctuating part c_f.     

The effects of viscosity on the linear stability of damped Stokes waves,downshifting, and rogue wave generation

We investigate a higher order nonlinear Schrödinger equation with linear damping and weak viscosity, recently proposed as a model for deep water waves exhibiting frequency downshifting. Through analysis and numerical simulations, we discuss how the viscosity affects the linear stability of the Stokes wave solution, enhances rogue wave formation, and leads to permanent downshift in the spectral peak. The novel results in this work include the analysis of the transition from the initial Benjamin–Feir instability to a predominantly oscillatory behavior, which takes place in a time interval when most rogue wave activity occurs. In addition, we propose new criteria for downshifting in the spectral peak and determine the relation between the time of permanent downshift and the location of the global minimum of the momentum and the magnitude of its second derivative.     

Facile and efficient synthesis of C3-symmetric benzoxazine: a novel tri-arm molecular scaffold

Six novel C₃-symmetric benzoxazines (1a–f) were synthesized via a one-pot condensation, in which twelve covalent bonds were formed up to 96% yield. Two X-ray crystal structures confirmed their proposed conformations and showed their potential as a novel tri-arm molecular scaffold.     

Regioselective enzymatic carboxylation of phenols and hydroxystyrene derivatives

The enzymatic carboxylation of phenol and styrene derivatives using (de)carboxylases in carbonate buffer proceeded in a highly regioselective fashion: Benzoic acid (de)carboxylases selectively formed o-hydroxybenzoic acid derivatives, phenolic acid (de)carboxylases selectively acted at the β-carbon atom of styrenes forming (E)-cinnamic acids.     

Analytical model for the calculation of the optical signal-to- noise ratio (SNR) of WDM EDFA chains

A new, simplified analytical model for the calculation of the signal-to-noise ratio (SNR) in multiwavelength erbium-doped fibre amplifier (EDFA) cascades is presented. Comparisons with numerical results give excellent agreement, confirming the validity of this model and its significance for optimising system parameters.     

Decay of mesoscopically localized vibrations in porous materials

The low-temperature decay of a vibrational eigenstate of a crystalline mesoscopic particle surrounded by other such particles of approximately the same size is considered. The decay time is determined by the anharmonicity and the coupling between adjacent mesoscopic particles. Under some limiting conditions for particles of a typical size of 50 Å the decay time at low temperatures can be several milliseconds. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 51–56 (10 January 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

On the structure of support points in the class Σ

Journal d'Analyse Mathématique -