Continuation of travelling‐wave solutions of the Navier–Stokes equations

An efficient way of obtaining travelling waves in a periodic fluid system is described and tested. We search for steady states in a reference frame travelling at the wave phase velocity using a first‐order pseudospectral semi‐implicit time scheme adapted to carry out the Newton's iterations. The method is compared to a standard Newton–Raphson solver and is shown to be highly efficient in performing this task, even when high‐resolution grids are used. This method is well suited to three‐dimensional calculations in cylindrical or spherical geometries. Copyright © 2006 John Wiley & Sons, Ltd.     

Polymer chain dynamics in a random environment: heterogeneous mobilities

We present a neutron scattering investigation on a miscible blend of two polymers with greatly different glass-transition temperatures Tg. Under such conditions, the nearly frozen high-Tg component imposes a random environment on the mobile chain. The results demand the consideration of a distribution of heterogeneous mobilities in the material and demonstrate that the larger scale dynamics of the fast component is not determined by the average local environment alone. This distribution of mobilities can be mapped quantitatively on the spectrum of local relaxation rates measured at high momentum transfers.     

Non-Gaussian nature of the alpha relaxation of glass-forming polyisoprene

We report quasielastic neutron scattering experiments exploring the alpha relaxation in polyisoprene over an unprecedented range in momentum transfer. Corroborating and validating earlier molecular dynamics simulations, the measurements reveal a crossover from a Gaussian regime of sublinear diffusion to a strongly non-Gaussian regime at short distances. We show that a consistent interpretation in terms of a distribution of finite jumps underlying the alpha process is possible. This model leads to a time-dependent non-Gaussian parameter exhibiting all features revealed so far from various simulations.     

Palladium-catalysed direct alkenylation of sydnones

Sydnones have been directly functionalised with alkenyl halides and an alkynyl bromide under palladium catalysis.     

Quasielastic neutron scattering in soft matter

The general trend in soft matter is to study systems of increasing complexity which are more technologically and biologically relevant. This is facilitated by the capability of quasielastic neutron scattering (QENS) to selectively probe spatially resolved dynamical modes at a molecular level. The large number of recent publications using QENS for investigating complex and multi-component soft matter systems, serves as recognition of the suitability of this technique by the scientific community. Exploiting its complementarity with molecular dynamics (MD) simulations and other experimental techniques is the basis of a successful methodology for this scientific challenge. We illustrate the potential of QENS with three kinds of soft materials whose structural units increase in size/complexity: lipids, polymers and biomolecules.     

Atomic motions in the alphabeta-merging region of 1,4-polybutadiene: a molecular dynamics simulation study

We present fully atomistic molecular dynamics simulations on 1,4-polybutadiene in a wide temperature range from 200 to 280 K, i.e., in the region where the alpha- and beta-relaxations merge and above. A big computational effort has been performed-especially for the lowest temperatures investigated-to extend the simulation runs to very long times (up to 1 mus for 200 K). The simulated sample has been carefully validated by using previous neutron scattering data on the real sample with similar microstructure. Inspecting the trajectories of the different hydrogens in real space, we have observed a heterogeneous dynamical behavior (each kind of hydrogen moves in a different way) with signatures of combined hopping and diffusive motions in the whole range investigated. The application of a previously proposed model [Colmenero et al., Europhys. Lett. 71, 262 (2005)] is successful and a characterization of the local motions and diffusion is possible. The comparison of our results to those reported in the literature provides a consistent scenario for polybutadiene dynamics and puts into a context the different experimental observations. We also discuss the impact of the hopping processes on the observation and interpretation of experimentally accessible magnitudes and the origin of the deviations from Gaussian behavior in this system.     

A Hexameric Cationic Copper(II) Metallacrown as a Pertechnetate and Perrhenate Scavenger

Materials based on the cationic copper(II) hexanuclear 18‐membered metallacrown [18‐MC ‐6]⁶⁺ (2phH=2‐piconyl hydrazide) and tetrafluoroborate, perchlorate, nitrate, sulfate, and perrhenate anions were prepared by an easy method in aqueous medium. Single‐crystal X‐ray characterization of six members of this new family of complexes showed that the anions are attached to the metallacrown by direct coordination to a copper cation or by hydrogen‐bonding interaction with the center of the hexamer. The stable cationic nature of the complexes and their ability to bind different anions allows them to adsorb and immobilize environmentally relevant anions such as MO₄^? (M=Tc, Re). The MO₄^? trapping capacities suggest that these materials would be useful in the treatment of oxoanionic contaminants in water.     

Metamorphosis of a Commodity Plastic like PVC to Efficient Catalytic Single-Chain Nanoparticles

We perform the conversion of a commodity plastic of common use in pipes, window frames, medical devices, flexible hoses, etc. like polyvinyl chloride (PVC) to single-chain nanoparticles (SCNPs). SCNPs are versatile, protein-mimetic soft nano-objects of growing interest for catalysis, sensing, and nanomedicine, among other uses. We demonstrate that the metamorphosis process -as induced through metal-free click chemistry- leads to well-defined, uniform SCNPs that are stable during storage in the solid state for months. All the conversion process (from PVC isolation to PVC-SCNPs synthesis) can be run in a green, dipolar aprotic solvent and involving, when required, a simple mixture of ethanol and water (1/1 vol.) as non-solvent. The resulting PVC-SCNPs are investigated as recyclable, metalloenzyme-mimetic catalysts for several representative Cu(II)-catalyzed organic reactions. The method could be valid for the metamorphosis and valorization of other commodity plastics in which it is feasible to install azide functional groups in their linear polymer chains.     

Improving real-time measurement of H/D exchange using a FTIR biospectroscopic probe

We describe the improvement of a novel approach to investigating hydrogen/deuterium (H/D) exchange kinetics in biomolecules using transmission infrared spectroscopy. The method makes use of a Fourier transform infrared spectrometer coupled with a microdialysis flow cell to determine exchange rates of labile hydrogens. With this cell system, the monitoring of exchange reactions has been studied here as a function of some cell characteristics such as: (a) dialysis membrane surface contacting both the H₂O and D₂O compartments; (b) molecular cutoff of dialysis membrane; and (c) distance between the cell-filling holes. The best improvement has been obtained by increasing the dialysis membrane surface followed by increase of molecular cutoff. However, not significant differences were found using various distances between filling holes. The fastest exchange rate which can be measured with the cell system used here is found to be k = 0.41 ± 0.02 min⁻¹, that is, about threefold greater than the one got in a previous work. This microdialysis flow cell has been used here for the study of H/D exchange in nucleic acids with subsequent structural analysis by 2D correlation spectroscopy.