Numerical analysis of conservative unstructured discretisations for low Mach flows

Unstructured meshes allow easily representing complex geometries and to refine in regions of interest without adding control volumes in unnecessary regions. However, numerical schemes used on unstructured grids have to be properly defined in order to minimise numerical errors. An assessment of a low Mach algorithm for laminar and turbulent flows on unstructured meshes using collocated and staggered formulations is presented. For staggered formulations using cell‐centred velocity reconstructions, the standard first‐order method is shown to be inaccurate in low Mach flows on unstructured grids. A recently proposed least squares procedure for incompressible flows is extended to the low Mach regime and shown to significantly improve the behaviour of the algorithm. Regarding collocated discretisations, the odd–even pressure decoupling is handled through a kinetic energy conserving flux interpolation scheme. This approach is shown to efficiently handle variable‐density flows. Besides, different face interpolations schemes for unstructured meshes are analysed. A kinetic energy‐preserving scheme is applied to the momentum equations, namely, the symmetry‐preserving scheme. Furthermore, a new approach to define the far‐neighbouring nodes of the quadratic upstream interpolation for convective kinematics scheme is presented and analysed. The method is suitable for both structured and unstructured grids, either uniform or not. The proposed algorithm and the spatial schemes are assessed against a function reconstruction, a differentially heated cavity and a turbulent self‐igniting diffusion flame. It is shown that the proposed algorithm accurately represents unsteady variable‐density flows. Furthermore, the quadratic upstream interpolation for convective kinematics scheme shows close to second‐order behaviour on unstructured meshes, and the symmetry‐preserving is reliably used in all computations. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis of Nature‐Inspired Medium‐Sized Fused Heterocycles from Amino Acids

Herein, we describe the synthesis of molecular scaffolds consisting of medium‐sized fused heterocycles using amino acids, which are some of the most useful building blocks used by nature as well as chemists to create structural diversity. The acyclic precursors were assembled by using traditional Merrifield solid‐phase peptide synthesis, and cyclization was carried out through acid‐mediated tandem endocyclic N‐acyliminium ion formation, followed by nucleophilic addition with internal nucleophiles. The synthesis of molecular scaffolds consisting of seven‐, eight‐, and nine‐membered rings proceeded with full stereocontrol of the newly generated stereogenic center in most cases.     

Determination of calibration function in thermal field flow fractionation under thermal field programming

A new procedure for determining the calibration function able to relate retention and operative parameters to molecular weight of the species in thermal field flow (ThFFF) under thermal field programming (TFP) conditions is presented. The procedure involves determining the average values of retention parameters under TFP and determining a numerical function related to the temperature variations that occur during TFP. The calibration parameters are obtained by a procedure fitting the retention and operative parameters that hold true at the beginning of the TFP. The procedure is closely related to the one previously developed to calibrate the retention time axis under TFP ThFFF and, together, they constitute a full calibration procedure. Experimental validation was performed with reference to polystyrene (PS)-decalin and PS-THF systems. The calibration functions here obtained were compared to those derived by the classical procedure at constant thermal field ThFFF to obtain the calibration function at variable cold wall temperatures. Excellent agreement was found in all cases proving "universality" of the ThFFF calibration concept, i.e. it is independent of the particular system on which it was determined and can thus be extended to ThFFF operating under TFP. The new procedure is simpler than the classical one since it requires less precision in setting the instrumentation and can be obtained with fewer experiments. The potential applications for the method are discussed.     

Stereoisomerism of molecular multipropellers. 1. Static stereochemistry of bis- and tris-triaryl systems

The static stereoisomerism of bis- and tris-triaryl systems has been analyzed by a systematic stereochemical analysis, and the resulting theoretical predictions have been experimentally confirmed by using reversed-phase HPLC and ESR and (1)H NMR spectroscopies with a family of seven distinct polychlorinated aromatic multipropellers. To analyze the static stereochemistry of these molecules, we have developed a specific procedure that uses a symmetry-adapted symbolic notation, allowing the theoretical prediction of both the number and symmetry of the isomers of the investigated molecules. Due to the steric hindrance introduced by the presence of bulky chlorine substituents, (all) conformational isomers can be characterized experimentally by several independent techniques confirming the theoretical stereochemical predictions. The different propeller moieties that constitute the molecule appear to be nearly independent of each other. Consequently, most of the observed isomers show comparable populations in solution at room temperature.     

Stereoisomerism of molecular multipropellers. 2. Dynamic stereochemistry of bis- and tris-triaryl systems

The dynamic stereochemistry of bis- and tris-triaryl systems, the most simple "molecular multipropellers", is discussed on the basis of an extension of a systematic stereochemical analysis based on a symmetry-adapted symbolic notation developed specifically for these molecules. A suitable theoretical basis for our study is provided by the classical hypotheses concerning the dynamics of simple triaryl systems as formulated by Mislow and co-workers (J. Am. Chem. Soc. 1973, 95, 1535-1547), which, once applied to molecular multipropellers, show the existence of two modes of rearrangement for each propeller. Interconversion graphs for all molecules under study, covering a wide span of structural complexity, are presented. A complete NMR study of a two- and a three-propeller molecule indicates that all experimentally observable exchange pathways are indeed predicted by theoretical analysis. Moreover, quantitative analysis of 2D-EXSY experiments affords the activation energy of the subset of pathways that give rise to observable interconversions on the NMR time scale. Assuming that two-ring flips are the threshold mechanism for individual propeller interconversion, the experimental evidence indicates a preference for the flip of the central ring and one of the outer rings over the flip of two outer rings.     

Hollow and Yolk‐Shell Iron Oxide Nanostructures on Few‐Layer Graphene in Li‐Ion Batteries

We report a simple and template‐free strategy for the synthesis of hollow and yolk‐shell iron oxide (FeO_x) nanostructures sandwiched between few‐layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X‐ray diffraction, X‐ray absorption near‐edge structure, X‐ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li‐ion batteries and compared with those of solid FeO_x/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18 wt. %, the presence of metallic iron in the hollow and yolk‐shell FeO_x/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97 % after 120 cycles at 1000 mA g^?1 in the voltage range of 0.1–3.0 V.     

Preparation of uniform rich cholesterol unilamellar nanovesicles using CO2-expanded solvents

Nanoscopic and uniform unilamellar vesicles, rich in cholesterol, have been prepared by a new procedure, named "DELOS-SUSP", which is based on the depressurization of a cholesterol solution in CO2-expanded acetone into an aqueous solution containing a surfactant. The CO2 is used here as a cosolvent medium, allowing the straightforward preparation of vesicular systems with controlled size distribution, uniform shape, and stability unachievable by conventional mixing technologies. The resulting nanoscopic vesicular systems dispersed in water were characterized using dynamic light scattering, zeta potential, turbidity, and cryogenic transmission electron microscopy. The influence of operational parameters of this new methodology on the physicochemical characteristics of the vesicular systems is also reported.     

Solvent tuning from normal to inverted marcus region of intramolecular electron transfer in ferrocene-based organic radicals

The solvent dependence of spectroscopic data of two neutral paramagnetic donor-acceptor dyads, based on a polychlorinated triphenylmethyl radical acceptor unit linked through a vinylene pi-bridge to a ferrocene (compound 1) or a nonamethylferrocene donor (compound 2) unit, is described. Both compounds exhibit broad absorptions in the near-IR region, with band maxima appearing around 1000 and 1500 nm for 1 and 2, respectively. These bands correspond to the excitation of a neutral DA ground state to the charge-separated D+A- state, indicative of an intramolecular electron-transfer process. Compounds 1 and 2 show two reversible one-electron redox processes associated with the oxidation of the ferrocene and the reduction of the polychlorotriphenylmethyl radical subunits. The solvent dependence of the redox potentials was also investigated, allowing the determination of the redox asymmetries DeltaG degrees of both dyads. The latter values, along with the experimental Eopt spectroscopic data, allow us to estimate, using the total energy balance Eopt = lambda + DeltaG degrees , the reorganization energy values, lambda, and their solvent polarity dependence. Since DeltaG degrees and lambda are of the same order of magnitude but exhibit opposite trends in their solvent polarity dependence, a unique shift from the normal to the inverted Marcus region with the change in solvent polarity is found. The kinetics of the charge recombination step of the excited charge-separated D+A- state was studied by picosecond transient absorption spectroscopy, which allows us to observe and monitor for the first time the charge-separated D+A- state, thereby confirming unambiguously the photoinduced electron-transfer phenomena.     

Influence of ultrathin poly-(3,4-ethylenedioxythiophene) (PEDOT) film supports on the electrodeposition and electrocatalytic activity of discrete platinum nanoparticles

Coating a carbon electrode surface, specifically highly oriented pyrolytic graphite (HOPG) with an ultrathin film of poly-(3,4-ethylenedioxythiophene), PEDOT, provides a support on which a high density of uniformly dispersed Pt nanoparticles (NPs) can readily be formed by electrodeposition. The NPs tend to be much smaller, have a higher surface coverage, better dispersion and show a much lower tendency to aggregate, than Pt NPs produced under identical electrochemical conditions on HOPG alone. The electrocatalytic activity of the NPs was investigated for methanol (MeOH) and formic acid (HCOOH) oxidation. Significantly, for similarly prepared particles, Pt NP-PEDOT arrays exhibited higher catalytic activity (in terms of current density, based on the Pt area), towards MeOH oxidation, by an order of magnitude, and towards HCOOH oxidation at high potentials, than Pt NPs supported on native HOPG. These findings can be rationalised in terms of the enhanced oxidation of adsorbed CO, a key reaction intermediate and a catalyst poison. This research provides strong evidence that employing conducting polymers, such as PEDOT, as a support substrate, can greatly improve particular catalytic reactions, allowing for better catalyst utilisation in fuel cell technology.