Molecular Catalysts that Oxidize Water to Dioxygen

Water into oxygen : A good catalyst for converting water into oxygen is seen as an essential part of any sustainable solar‐energy conversion scheme. Some success has been achieved using molecular complexes as catalysts and the key factors influencing their performance are discussed. The necessity of generating a solid‐state catalytic system is presented and the first attempts to generate supported molecular water‐oxidation catalysts are analyzed.

     

Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

Bridged Ca ²⁺ ???CO???Ca ²⁺ complexes are formed on dual‐cation sites, constituted by a pair of nearby Ca²⁺ cations, when CO is adsorbed on zeolite Ca‐A. Two types of such species can be formed, designated S2–S1 and S1–S1 (see picture). Ca²⁺???CO monocarbonyl species are also identified, and at a relatively high CO equilibrium pressure, dicarbonyl complexes can also form.

     

Hydrogen bonding in the inner-salt zwitterion and in two different charged forms of 5,6-bis(2-pyridyl)pyrazine-2,3-dicarboxylic acid

5,6-Bis(2-pyridyl)­pyrazine-2,3-di­carboxyl­ic acid exists as an inner-salt zwitterion, 3-carboxy-5-(2-pyridinio)-6-(2-pyridyl)­pyrazine-2-carboxyl­ate, (Ia), C₁₆H₁₀N₄O₄. The adjacent pyridine and pyridinium rings are almost coplanar due to the presence of an intramolecular hydrogen bond involving the pyridine N atom and the NH H atom of the pyridinium group. In the crystal of (Ia), symmetry-related mol­ecules are hydrogen bonded via the carboxyl­ic acid OH group and one of the carboxyl­ate O atoms to form a polymer, which exhibits a channel-type structure. In the HCl, HClO₄ and HPF₆ salts, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium chloride 2.25-hydrate, (II), C₁₆H₁₁N₄O₄⁺·Cl⁻·2.25H₂O, 6-­carboxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium perchlor­ate trihydrate, (IIIa), C₁₆H₁₁N₄O₄⁺·ClO₄⁻·3H₂O, and 6-car­boxy-5-carboxyl­ato­pyrazine-2,3-diyldi-2-pyridinium hexa­fluoro­phosphate trihydrate, (IIIb), C₁₆H₁₁N₄O₄⁺·PF₆⁻·3H₂O, both pyridine rings are protonated. In the perchlorate form, and in the isomorphous hexa­fluoro­phosphate form, the mol­ecule possesses C₂ symmetry, with has a symmetrical intramolecular hydrogen bond involving the adjacent carboxyl­ate and carboxyl­ic acid substituents. In the crystals of the chloride and perchlorate (or hexa­fluoro­phosphate) salts, hydrogen-bonded polymers are formed which are three-dimensional and one-dimensional, respectively.     

Quantitative determination of clenbuterol, salbutamol and tulobuterol enantiomers by capillary electrophoresis

Enantiomers of clenbuterol, salbutamol and tulobuterol were directly separated and quantitated from a spiked sample by capillary electrophoresis (CE) using sulfated β-cyclodextrin (SCD) as chiral selector and phosphate as running buffer. The SCD and buffer concentration, pH and field strength were the parameters studied to optimize the separation. Optimal separation was obtained using 50 mM of phosphate monobasic at pH = 2.24, 0.25% (w/w) of sulfated cyclodextrin and a field strength of 10 kV, with 20 min total time analysis. Comparison between two different injection modes (hydrodynamic and electrokinetic) was made. In the hydrodynamic mode, repeatability (expressed as relative standard deviation, RSD) was less than 1.2% for migration times for all the analyte peaks and less than 2% for peak area percentages. With respect to reproducibility, RSD was less than 3.8% for migration time and less than 3% for peak area percentages. Calibration curves were set up for two different sample concentration ranges (1 to 10 μg mL^–1 and 160– 800 ng mL^–1, of each of the racemates studied). Although the electrokinetic injection mode for an aqueous sample appeared to suffer from some enantiodiscrimination, calibration curves were linear in the range between 1 and 10 ng mL^–1 with regression coefficients ranging from 0.9996 to 0.9952. As in the case of hydrodynamic injection, the method was tested with a spiked sample.     

Autoacceleration and inhibition: Free volume. Epoxy-amine kinetics

The epoxy-diamine cure process was studied. We found that the mechanism can be described in three steps: (i) initiation up to 20–25% of conversion. (ii) autoacceleration and (iii) inhibition. It has been observed that after the initiation there is a clear autoacceleration effect which has been explained in terms of free volume, considerations: the volume occupied by the products of reaction diminishes the available volume increasing the “local concentration” of the reactants and therefore the rate of reaction. The reaction was followed by FTIR (near infrared) as the main technique. We used a purified epoxy resin as well as phenyl glycidyl ether cured with m-xylenediamine. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1001–1016, 1998     

Enthalpy relaxation in polymethyl(α-n-alkyl)acrylates: Effect of length of alkyl chain

In this work, we have investigated by DSC the structural relaxation of amorphous polymethyl(α-n-alkyl)acrylates in which it is possible to change the length of the alkyl chain. We have evaluated the Narayanaswamy parameter, x, which controls the relative contribution of temperature and of structure to the relaxation time, the apparent activation energy, Δh*, and the nonexponentiality parameter, β, of the stretched exponential response function. The results suggest that x increases while Δh* decreases and β remains constant as the length of the side chain increases. This allows us to comment on the effect of chemical modification on the relaxation kinetics. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 583–593, 1998     

Tangent Fermions: Dirac or Majorana Fermions on a Lattice Without Fermion Doubling

Methods to discretize the Hamiltonian of a topological insulator or topological superconductor, without giving up on the topological protection of the massless excitations (respectively, Dirac fermions or Majorana fermions) are reviewed. The method of tangent fermions, pioneered by Richard Stacey, is singled out as being uniquely suited for this purpose. Tangent fermions propagate on a $2+1$ dimensional space-time lattice with a tangent dispersion: ${\tan }^2(\mathit{\epsilon }/2)={\tan }^2(k_x/2)+{\tan }^2(k_y/2)$ in dimensionless units. They avoid the fermion doubling lattice artefact that will spoil the topological protection, while preserving the fundamental symmetries of the Dirac Hamiltonian. Although the discretized Hamiltonian is nonlocal, as required by the fermion-doubling no-go theorem, it is possible to transform the wave equation into a generalized eigenproblem that is local in space and time. Applications that are discussed include Klein tunneling of Dirac fermions through a potential barrier, the absence of localization by disorder, the anomalous quantum Hall effect in a magnetic field, and the thermal metal of Majorana fermions.     

Phase separation in a poly(ether sulfone) modified epoxy-amine system studied by temperature modulated differential scanning calorimetry and dielectric relaxation spectroscopy

The phase separation induced by the curing reaction of an epoxy based on diglycidylether of bisphenol A (DGEBA) with methylene dianiline (MDA) modified with poly(ether sulfone) (PES) at a concentration of 20 wt% was studied by temperature modulated differential scanning calorimetry (TMDSC) and dielectric relaxation spectroscopy (DRS). The effect of phase separation on the curing kinetics and vitrification phenomena is analysed. The dependence of the log of the measuring frequency on the degree of conversion allows the correlation between the dipolar relaxation of each phase and the vitrification observed by TMDSC to be established.