Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

LIGHT-INDUCIBLE ABSORBANCE CHANGES AND VANADATE-SENSITIVE ATPase ACTIVITY ASSOCIATED WITH THE PRESUMPTIVE PLASMA MEMBRANE FRACTION FROM CAULIFLOWER INFLORESCENCES

Sucrose density gradient centrifugation of a microsomal membrane fraction of cauliflower inflorescences showed a strong correlation between a blue light mediated cytochrome b reduction (LIAC) and an ion stimulated nitrate-insensitive but a vanadate-sensitive ATPase activity at 38-40% sucrose. LIAC activity and vanadate-sensitive ATPase might be assigned to the same type of membrane different from ER, Golgi, tonoplast and mitochondria. The Mg²⁺-dependent ATP-hydrolytic activity obtained after purification of the microsomal fraction on an aqueous polymer two phase system was partially characterized. Temperature optimum (40°C), pH optimum (pH 7.0), vanadate inhibition (I₅₀ at 20 μ M ), substrate kinetics ( K _m= 1.37 m M Mg.ATP) and inhibitor studies all point to the presence of the frequently described plasma membrane ATPase. Potassium and Na⁺ stimulated the enzyme activity (20-40%). In general our data arc strongly in favour of the hypothesis that LIAC activity is localized on the plant plasma membrane. The cytochrome b involved in the light reaction has a midpoint potential near +150 mV. This cytochrome which has been previously shown in a cauliflower microsomal fraction is a constituent of the plasma membrane.     

A Titration Microcalorimetric Study of the Effects of Halide Counterions on Vesicle-Forming Aggregation in Aqueous Solution of Branched-Chain Alkylpyridinium Surfactants

Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press.     

Molecular structure of 1,1-difluoroethylene as determined by gas phase electron diffraction and microwave data

The structure of 1,1-difluoroethylene was determined, from gas phase electron diffraction data obtained independently in Leiden and Tokyo and the rotational constants of F₂CCH₂, F₂CCHD and F₂CCD₂ derived from the microwave study by Chauffoureaux. The two electron diffraction data agreed without significant discrepancy. From a joint least squares analysis of the diffraction and microwave data, the following r_g bond distances and r_z bond angles were derived: CC = 1.340 ± 0.006 Å, C-F = 1.315 ± 0.003 Å, C-H = 1.091 ± 0.010 Å, ∠C-C-F = 124.7 ± 0.3°, ∠C-C-H = 119.0 ± 0.4°, where the uncertainties represent estimated limits of error.     

Thionation of the monoacetal of pentacyclo[5.4.0.0.0.0]undecane-8,11-dione

As part of a programme to synthesize thione derivatives with pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane moieties it was decided to sulfurize the monoacetal 6 of pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane-8,11-dione 2. Unexpectedly the diol 9 was isolated as the product.     

Effects of Adsorbed Polyaniline on Redox Process on MoO3 Surface

The effects exhibited by adsorbed conducting polyaniline on the redox process on a molybdenum oxide surface were studied. Thermogravimetric results indicate a 4% polyaniline deposition. Cyclic voltammograms of the adsorbed polymer on MoO3 show that polyaniline exerts remarkable effects on the molybdenum blue oxidation-reduction process, with oxidation and reduction potentials of 0.33 and 0.18 V, respectively. This effect strongly enhances the electrode response, and can be used as an important tool in qualitative and/or quantitative determinations of molybdenum in solution as well as in any substrate. Copyright 1999 Academic Press.     

Single molecule spectroscopy as a probe for dye-polymer interactions

Experimental (Single Molecule Spectroscopy) and theoretical (quantum-chemical calculations and Monte Carlo and molecular dynamics simulations) techniques are combined to investigate the behavior and dynamics of a polymer-dye molecule system. It is shown that the dye molecule of interest (1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-dicarbocyanine) adopts two classes of conformations, namely planar and nonplanar ones, when embedded in a poly(styrene) matrix. From an in-depth analysis of the fluorescence lifetime trajectories, the planar conformers can be further classified according to the way their alkyl side chains interact with the surrounding poly(styrene) chains.     

Study of the carbonyl products of terpene/OH radical reactions: detection of the 2,4-DNPH derivatives by HPLC-MS

The oxidation of the terpenes - and -pinene, limonene and ³-carene by hydroxyl radicals has been investigated in a fast-flow reactor coupled to a liquid nitrogen trap for collecting the carbonyl compounds. Identification of the products was performed via 2,4-dinitrophenylhydrazone (DNPH) derivatization of the carbonyls to form the mono- and di-DNPH derivatives, which were analysed by high-performance liquid chromatographic (HPLC)-DAD (diode array detector) and HPLC-mass spectrometry (HPLC-MS). Both electrospray ionization [ESI(–)] and atmospheric pressure chemical ionization [APCI(–)] were suitable for the detection of the DNPH derivatives of formaldehyde, acetaldehyde, myrtanal, campholene aldehyde, perillaldehyde, acetone, nopinone, trans-4-hydroxynopinone and 4-acetyl-1-methylcyclohexene. Also the mono-DNPH derivatives of the dicarbonyl compounds pinonaldehyde, endolim and caronaldehyde could be identified. The MS² spectra generated in the ion trap of the mass spectrometer allowed us to distinguish between aldehydes and ketones on the basis of the characteristic fragment ion m/z 163 for the aldehydes. For the quantitative analysis of the mono-DNPH derivatives, ESI(–) in combination with single ion monitoring (SIM) detection showed the lowest detection limits. For the quantification of the dicarbonyl compounds, the acid-sensitive di-DNPH derivatives had to be formed by keeping the acidity in the acid-catalysed derivatization reaction at about 1.7 mM H₂SO₄. Detection of these dicarbonyl compounds can only be performed by APCI(–) with somewhat lesser sensitivity than by HPLC-DAD.     

Diels-Alder reactions of pyridinone o-quinodimethanes generated from substituted sulfolene[3,4-c]pyridin-4(1H)-ones

1-Benzyl-3-(bromomethyl)-2(1H)-pyrazinone was converted to [3,4-c] sulfolene pyridinone 8a and further (1- or 3-) substituted derivatives having a dienophilic side chain on the sulfolene ring. Thermolytic extrusion of sulfur dioxide from o-QDM precursor 8 led to generation of 3,4-dimethylene-2(1H)-pyrazinone 9, which was reacted in situ with various dienophiles. Thermolysis of the substituted precursors resulted in intramolecular cycloaddition of the corresponding o-QDM intermediates