SHS of NbSi2: A Comparison Between Experiments and Simulations

Computer simulation results for the Self-propagating High-temperature Synthesis (SHS) of the intermetallic compound NbSi₂ are discussed in comparison with experiments for SHS of mechanically activated powders. The variation of wave speed of the propagation front is considered while varying Nb grain size and thermal conductivity of the starting mixture. A co-operating effect of these two parameters (due to the change in morphology of reactants after milling) is outlined.     

Volcanic Quartz Growth Zoning Identified by Cathodoluminescence and EPMA Studies

 Quartz is a common phenocryst in acidic volcanic rocks but rarely contributes to the understanding of the magmatic or eruptive processes due to its apparent chemical homogeneity. Cathodoluminescence studies indicate that volcanic quartz is strongly zoned with respect to the trace elements. The determination of this zoning can help to elucidate the crystallisation history of the magmatic rock. Polished thin sections from rhyolitic and dacitic volcanic rocks were examined by cathodoluminescence mounted on an optical microscope and analysed by electron probe microanalysis. X-ray mapping of the trace elements incorporated in the zoned quartz was also performed. The interpretation of the data obtained from these different analytical methods provides useful information on the chemical zoning of single crystals and therefore on the chemico-physical conditions of the melt from which the quartz crystallised. Quartz from dacitic rock shows a complex cathodoluminescence oscillatory zoning, consisting of large red-brown and blue bands concentric to a resorbed core. Quartz from rhyolite shows simpler cathodoluminescence zoning, consisting of dark to bright blue luminescent bands. The EMPA analyses and the X-ray maps combined with the cathodoluminescence images suggest a clear correlation between the concentrations of some trace element (mainly Al and Ti, acting as activators of luminescence) and the cathodoluminescence colours pattern zoning. The incorporation of these trace elements during the growth of the crystals is linked to small variations in the thermo-chemical properties of the magma such as temperature and chemical composition.     

EPMA Major and Trace Element Analysis in Garnet and its Petrological Application

 A comparison between major and trace element concentrations in garnet performed by electron microprobe (EPMA) technique is reported. Quantitative spot analyses and X-ray maps of major elements (Fe, Mg, Mn, Ca) and the trace element yttrium in garnets from metamorphic rocks are presented. The selected garnet samples come from meta-pelitic and meta-basic specimens belonging to the tectonic unit of the Monte Rosa Nappe (Western Alps). In the metapelites, the quantitative Y distribution maps display a prominent increase at the core, the Y abundance varying by over two orders of magnitude, from about 80 ppm (rim) to over 2100 ppm (core). The Y profiles show well defined patterns with sharp features that do not correlate with major element distributions. A roughly comparable pattern can be supposed only with Mn. The Y distribution suggests that the diffusion of Y through the garnet is very slow compared to the major elements, thus the Y results are suitable for geothermometric estimates. In the metabasites, the Y spatial distribution is characterised by an increasing content from the core to the rim, displaying a zoning pattern opposite to the metapelite garnet. Quantitative EPMA analyses range from 1100 ppm at the rim to values lower than the detection limit at the core. Therefore, the Y content in the garnet can be related to several chemical and physical variables such as the bulk rock composition and the phase assemblage. In particular, in the xenotime-bearing metapelitic system the Y distribution seems to be correlated with metamorphic peak temperature.     

Synthesis of 1,3,4-thiadiazolo[3,2-α]pyrimidin-5-one and isomeric 7-one derivatives

Continuing preceeding studies, designed to obtain derivatives of 1,3,4-thiadiazolo[3,2-α]pyrimidin-5-one and of the isomeric 7-one of pharmacological interest, some of the above mentioned compounds were prepared. Their structural identification was obtained by mass spectra.     

An integrated computational tool for the study of the optical properties of nanoscale devices: application to solar cells and molecular wires

We present a combined computational strategy for the study of the optical properties of nanoscale systems, using a combination of codes and techniques based on Density Functional Theory (DFT) and its Time Dependent extension (TDDFT). In particular, we describe the use of Car–Parrinello molecular dynamics simulations for the study of nanoscale devices and show the integration of the obtained results with available quantum chemistry codes for the calculation of TDDFT excitation energies, including solvation effects by continuum solvation models. We review some prototypical applications of this integrated computational strategy, ranging from the interaction of dye sensitizers with TiO₂ nanoparticles, of interest in the field of dye-sensitized solar cells, to transition metal molecular wires exceeding 3 nm length.     

EPR analysis of the interacting properties and the degradation over time and irradiation of mastic resin used for painting protection

Mastic resin used as a covering film for painting protection was analyzed by electron paramagnetic resonance (EPR) spectroscopy, both as received and upon aging in sun-light. The effect of prolonged exposure to sun-light was mimicked by UV and, more so, by xenon lamp irradiation. Solid mastic presented EPR signals due to radicals trapped by PBN in solution. Data in the literature indicated the formation of acyl radicals (RCO·). These radicals preferentially dissolved in medium polarity solvents. The radical concentration in the solid mastic increased over time more than 50 times upon UV irradiation for 96 h and, even more, by xenon irradiation for 800 h. Also the PBN-trapped radicals in solution increased in concentration by irradiation. Small nitroxide radicals (TEMPO) interacted with a polar fraction of mastic dissolved in methanol, but mainly interacted with low polar mastic molecules in hydrophobic solvents. It was suggested, on the basis of both the PBN-spin trapping data and the TEMPO mobility variation in the solvents at different polarities, that terpenoid molecules partially polymerize by a radical mechanism to form low molecular weight products. A polyaromatic-radical (pyrene-TEMPO) and a biomolecule-radical (doxylcholestane) both interact weakly with mastic molecules in cyclohexane solutions. A positively charged surfactant radical (dimethylammonium-TEMPO bromide) was easily adsorbed onto the solid mastic surface suggesting that detergents are responsible for mastic degradation. In conclusion, this study provided information on the degradation mechanism of mastic resin and on its interacting ability towards external molecules and pollutants.     

A simple approach for coupling liquid chromatography and electron ionization mass spectrometry

A miniaturized, aerosol based interface for directly coupling a liquid chromatograph with a mass spectrometer is presented. The interface is entirely within the electron ionization (EI) source of the mass spectrometer and no additional, external devices are needed. This simple and effective approach exploits micro-flow nebulization technology providing a new interface suitable for a variety of applications of environmental and biological interest. The new interface provides necessary linearity, ruggedness, sensitivity, and reproducibility of response for trace level analysis, and readily interpretable mass spectra for unambiguous identification of unknown compounds of small to medium molecular weight.     

Thermal decomposition of n-alkyl[bis(triphenylphosphine)](carbonyl)iridium(I) complex

The decomposition of L₂Ir(CO)R, R = n-alkyl) prepared in situ by the reacton of n-alkyllithium or -magnesium reagents with L₂Ir(CO)Cl(2) produces a mixture of n-alkane and isomerized alkene, the ratio of which is strongly dependent on the concentration [L] of triphenylphosphine as well as certain other additives. When [L] = 0, positional isomerism and isotope scrabling are extensive as is the isomerism of added olefin, suggesting that β-hydride elimination is rapid and reversible and that any olefins participating in an iridium hydride addition-elimination sequence are also capable of exchange with free olefin in solution. When [Ph₃P]/[2] >/ 1, the principal product (>90%) is the 1-alkene. No positional isomerism or isotope scrambling is observed only a minor amount of alkane is produced. A mechanistic scheme consistent these observations is proposed.     

Identification of levoglucosan and related steroisomers in fog water as a biomass combustion tracer by ESI-MS/MS

A conspicuous fraction of the water soluble organic compounds (WSOC) in fog and fine aerosol samples is composed by monosaccharide anhydrides, such as levoglucosan and its stereoisomers, galactosan and mannosan. Levoglucosan is produced exclusively during wood combustion processes, making it a very useful tracer for plant combustion emissions in the atmosphere. This paper describes a new experimental approach, based on electrospray-tandem mass spectrometry (ESI-MS/MS), for the identification of levoglucosan in fog water samples. The analytical method proposed allows to identify the specific sugar anhydrides directly in the liquid phase without the need of any derivatization process.     

An efficient route to tetrahydronaphthols via addition of ortho-lithiated stilbene oxides to alpha,beta-unsaturated fischer carbene Complexes

[reaction: see text] A stereoselective/stereospecific synthesis of polysubstituted tetrahydronaphthols based on the Michael addition of ortho-lithiated stilbene oxides to alpha,beta-unsaturated Fischer carbene complexes followed by an unusual cyclization of the corresponding intermediate in a 6-endo-tet mode is described.