Carbon nanotubes/carbon xerogel-nafion electrodes: a comparative study of preparation methods

We report the preparation and electrochemical characterization of carbon nanotubes (CNT)/carbon xerogel-nafion (CXN) electrodes obtained by casting carbon nanotube inks on carbon xerogel-nafion matrixes under terrestrial (g) and enhanced (13?g) gravity. The impregnated electrodes were compared with composites prepared by mixing CXN dispersions with CNT inks. For casted CNT, alternate current scanning electrochemical microscopy studies along the film-electrode area showed differences that can be correlated with the position of the electronic conducting CNT phase on the more resistive matrix. It revealed the transition from a conductive to dielectric surface when impregnation takes place at terrestrial and enhanced gravity, respectively. Although the addition of CNT enhances the capacitance and mechanical properties of CXN in all preparation methods, the largest specific capacitance was observed in electrodes impregnated at 1?g. Electrodes prepared by mixing and those casted at 13?g show similar capacitance values regardless of contrasting conductivity. A mechanism explaining the microstructural, electrical, and adsorptive differences brought out by the various preparation methods is proposed.     

Does cation dehydration drive the binding of metal ions to polyelectrolytes in water? What we can learn from the behaviour of aluminium(III) and chromium(III)

Much stronger binding is seen in aqueous solutions between the anionic polyelectrolyte potassium poly(vinyl sulfate) and the substitution labile aluminium(III) than with the kinetically inert chromium(III). This strongly supports the idea that entropy driven water loss from the hydration sphere of the metal ion plays a major role in driving binding of the trivalent metal ion to the polyelectrolyte.     

Dynamics of H2 dissociation on the 1/2 ML c(2 × 2)-Ti/Al(100) surface

The dissociation of H(2) on Ti-covered Al surfaces is relevant to the rehydrogenation and dehydrogenation of the NaAlH(4) hydrogen storage material. The energetically most stable structure for a 1/2 monolayer of Ti deposited on the Al(100) surface has the Ti atoms in the second layer with a c(2 × 2) structure, as has been confirmed by both low-energy electron diffraction and low-energy ion scattering experiments and density functional theory studies. In this work, we investigate the dynamics of H(2) dissociation on a slab model of this Ti/Al(100) surface. Two six-dimensional potential energy surfaces (PESs) have been built for this H(2) + Ti/Al(100) system, based on the density functional theory PW91 and RPBE exchange-correlation functionals. In the PW91 (RPBE) PES, the lowest H(2) dissociation barrier is found to be 0.65 (0.84) eV, with the minimum energy path occurring for H(2) dissociating above the bridge to top sites. Using both PESs, H(2) dissociation probabilities are calculated using the classical trajectory (CT), the quasi-classical trajectory (QCT), and the time-dependent wave-packet methods. We find that the QCT H(2) dissociation probabilities are in good agreement with the quantum dynamics results in the collision energy range studied up to 1.0 eV. We have also performed molecular beam simulations and present predictions for molecular beam experiments. Our molecular beam simulations show that H(2) dissociation on the 1/2 ML Ti/Al(100) surface is an activated process, and the reaction probability is found to be 6.9% for the PW91 functional and 1.8% for the RPBE at a nozzle temperature of 1700 K. Finally, we have also calculated H(2) dissociation rate constants by applying transition state theory and the QCT method, which could be relevant to modeling Ti-catalyzed rehydrogenation and dehydrogenation of NaAlH(4).     

Synthesis of CDE molecular fragments related to sendanin mediated by titanocene(III)

A practical, brief, and diastereoselective synthesis of limonoid CDE fragments from a readily available starting material is described. The key step was the titanocene(III)-promoted cyclization of unsaturated epoxylactones, readily prepared from α-cyclocitral. In this way, we confirm the viability of our procedure for the synthesis of a limonoid model with different functionalization patterns. We also report the antifeedant activity of epoxylactones 18 and 19, which show significant antifeedant activity against Spodoptera littoralis and Spodoptera frugiperda, two insect species with different feeding ecologies.     

Disulfide linkage Raman markers: a reconsideration attempt

During the last decades, Raman spectroscopy has been routinely used for probing the conformational features of disulfide linkages in peptides and proteins. However, the interpretation of disulfide Raman markers is currently performed by a simple rule derived from the earliest observations on dialkyl disulfides. More precisely, this rule consists of the following: (1) in analyzing the Raman bands in the 550–500 cm⁻¹ region ascribed to disulfide bond stretch motion, namely, ν(S‐S), and (2) assigning the three types of Raman markers observed at ~500, ~520, and ~540 cm⁻¹ to three families of rotamers defined along the three successive bonds of the ‐C‐S‐S‐C‐ moiety, referred to as ggg, ggt, and tgt. In this report, we attempt to show that an accurate analysis of disulfide vibrational features needs the use of the five torsion angles (χ₁, χ₂, χ₃, χ₂', and χ₁') along the five successive bonds joining the two α‐carbon atoms in the cystine (Cys‐Cys) unit. The present work is inspired by the disulfide conformational investigations performed by a statistical scan of numerous protein crystal and nuclear magnetic resonance data, taking into account the handedness (right and left) of a disulfide bridge, its spatial shape (Staple, Hook, and Spiral), as well as the signs of the two extreme torsion angles χ₁ and χ₁'. It appears that the combined use of the old and recent conformational notations allows a more accurate structural and vibrational analysis of disulfide linkage. Copyright © 2014 John Wiley & Sons, Ltd.     

A new computational approach for electrical analysis of biological tissues

A new computational approach for electrical analysis especially designed for application in biological tissues is presented. It is based on the modelling of the electrical properties of the medium by means of lumped circuit elements, such as capacitance, conductance and current sources. The cell scale model is suitable for modelling the local anisotropy around cell membranes. It permits to obtain the electric potential, ionic concentrations and current densities around cells in time steps in an iterative process. The tissue scale model utilises volume-averaged values of conductivity and permittivity and models suitably the dispersive characteristic of biological tissues. It permits to obtain potential and current distributions in large volumes of tissue in the time or frequency domain. An example of analysis of skeletal muscle is presented aiming to demonstrate the features of the method.     

An Infrared Spectroscopic Study of Crystalline Copper (II) Propionate and Butyrate

Abstract

Abstract: The vibrational spectra of crystalline anhydrous copper (II) propionate and butyrate were studied by FT-IR spectroscopy. A detailed assignment of the spectra of both compounds is presented and discussed. It is shown that the presence of two non-equivalent sets of carboxylate ligands within the crystals gives rise to several band splittings, which are particularly evident in those bands ascribable to normal modes involving predominantly the -C_βH₃ or -C_βH₂-fragments. In addition, the vibrational assignments made for the studied molecules are shown to be very useful to help understanding the much more complex spectra of long chain copper carboxylates.     

Influence of the experimental conditions on the specific heat critical behaviour of TGS

Using a conduction calorimeter the specific heat critical behaviour of a TGS crystal has been measured. The ranges of the deviations from the classical behaviour below and above the transition point are 1 and 20 K respectively. These deviations seem to be well related to LK theory predictions. In a range of 0.1 K below the transition point thermal hysteresis appears which does not depend on the measuring temperature rate. The same behaviour is obtained with the sample at atmospheric pressure and under a high vacuum.     

Some exact values of the Harborth constant and its plus-minus weighted analogue

The Harborth constant of a finite abelian group is the smallest integer ${\ell}$ such that each subset of G of cardinality ${\ell}$ has a subset of cardinality equal to the exponent of the group whose elements sum to the neutral element of the group. The plus-minus weighted analogue of this constant is defined in the same way except that instead of considering the sum of all elements of the subset, one can choose to add either the element or its inverse. We determine these constants for certain groups, mainly groups that are the direct sum of a cyclic group and a group of order 2. Moreover, we contrast these results with existing results and conjectures on these problems.