Surface chemistry of Zn3P3 single crystals studied by XPS

X-ray photoemission spectroscopy (XPS) has been used to study the surface reaction of Zn₃P₂ single crystals.The spectra of crystals exposed to H₂, O₂, CO₂, O₂+H₂O or CO₂+H₂O during a four week period were compared to the spectra of as-grown or in UHV scraped samples. For samples contaminated with the wet gases O₂+H₂O and CO₂+H₂O additional phosphorus core levels together with a shift of the zinc core levels were observed. For crystals exposed to atmosphere during several months no phosphorus could be detected on the gasgrown surface, whereas the stochiometry of Zn₃P₂ was maintained within the bulk. Crystals with scraped surfaces showed no moisture sensitivity. No surface contamination was also detected for Zn₃P₂ crystals deposited with up to 1000 L H₂O or exposed to atmosphere during 30 min.     

Work function and barrier heights of transition metal silicides

The work function of 13 polycrystalline transition metal suicides was measured by photoemission in uhv. Their values are discussed in relationship to their Schottky barrier heights on n-Si. While there appears to be a weak correlation for a certain group of transition metal suicides, the values of the 5d-noble metal suicides including some of the lattice matched Ni suicides appear to be completely uncorrelated. Experimental values of work functions are compared to the values proposed previously by Freeouf.     

Menthatrienes and the Oxidation of Limonene

The oxidation of limonene with mercuric acetate gives a mixture containing mentha-1. 3. 8-triene, the hydrocarbon of parsley, and its isomer, mentha-1. 4. 8-triene. Selenium dioxide oxidation of limonene in alcohol gives as the main product mentha-1. 8-dien-4-ol, and pyrolysis of the acetate of the latter, or of mentha-1. 8-dien-10-yl acetate (the main product of the oxidation of limonene with selenium dioxide in acetic anhydride), also gives a mixture of menthatrienes. A discussion of the selenium dioxide oxidation of limonene is presented.     

Localization in small fcc-particles with surface irregularities and disorder

A numerical eigenvector analysis is used to investigate Anderson localization in small fcc-particles ofN=309 andN=147 atoms. Special attention is given to the way size and surface roughness of the particles influence the localization behavior. States begin to localize in a non-exponential regime several lattice spacings from the center of localization and finally converge to a fully exponentially-localized regime for strong disorder. For smooth surface particles, it is found that the states localize first at the band bottom and a mobility edge can clearly be defined for increasing disorder. This doesn't seem to be the case for the rougher particles, where the band middle and the band bottom show similar behavior towards localization. Although particles with surface irregularities show an onset of localization for smaller values of the disorder than smooth particles, the localization length is greater.     

Generation and photoreactions of 2,4,6-trinitreno-1,3,5-triazine, a septet trinitrene

We have studied the matrix photolysis of 2,4,6-triazido-1,3,5-triazine (cyanuric triazide, 1). Stepwise generation of the corresponding mononitrene, dinitrene, and trinitrene was observed by matrix IR and electron paramagnetic resonance (EPR) spectroscopy. The generated species were identified by comparison of their matrix IR spectra with density functional theory (DFT) computational results. The generation of 2,4,6-trinitreno-1,3,5-triazine with a septet ground state was confirmed for the first time by matrix EPR spectroscopy. The trinitrene readily decomposed into three NCN molecules upon further photoirradiation. This process was also confirmed by matrix EPR spectroscopy.     

Synthesis, structure and oxygen nonstoichiometry of La0.4Sr0.6Co1−yFeyO3−δ

The samples of La_0.4Sr_0.6Co_1−yFe_yO_3−δ (y = 0.2 and 0.4) were prepared using both conventional ceramic technique and nitrate–citrate precursors technique. The phase identification was made by X-ray diffraction method. The refinement of structural parameters from the XRD and neutron diffraction measurements was performed by full profile Rietveld analysis. Neutron diffraction showed that both samples possess distorted perovskite-type structure. Oxygen nonstoichiometry was measured by chemical analysis and thermogravimetry (TG) analysis in the range 20 ≤ T/°C ≤ 900 and 2E-5 ≤ pO₂/atm ≤ 4E-1. TG-experiments indicate a relatively fast and reversible oxygen exchange at pO₂ > 1E-2 atm. Mass saturation occurs at T < 300 °C upon cooling. The absolute value of oxygen nonstoichiometry was determined by iodometric titration measurements. It was found that both samples have practically stoichiometric composition at 300 °C in air and δ increases with increasing temperature and decreasing oxygen partial pressure.     

Identification of subnanomole amounts of PTH-amino acids by high performance thin-layer chromatography

Summary High performance thin-layer chromatography (HPTLC) of PTH-amino acids on 5×5 cm silica gel plates precoated with a fluorescence marker gives 10–20 fold increase in sensitivity compared to ordinary silica gel plates. Separation of PTH-Leu from PTH-Ile is easily achieved in contrast to chromatography on polyamide sheets. Only two solvent systems are required and as many as 12 samples can be chromatographed on each plate. However, if the sample is contaminated with N-phenylthiourea a third solvent system is necessary.     

Generation and characterization of the selenocysteinyl radical: direct evidence from time-resolved UV/Vis, electron paramagnetic resonance, and Fourier transform infrared spectroscopy

The selenocysteinyl radical 1 has been generated for the first time by laser flash photolysis (lambda(exc) = 266 nm) of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2 and of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3 in acetonitrile and characterized by time-resolved (TR) UV/Vis, Fourier transform infrared (FTIR), and electron paramagnetic spectroscopy in combination with theoretical methods. A detailed product study was conducted using gas chromatography and one- and two-dimensional NMR spectroscopy. In the case of [(9-fluorenylideneamino)oxycarbonyl]methyl(N-tert-butoxycarbonyl)-l-selenocysteine 3, the (9-fluorenylideneamino)oxycarbonyl moiety serves as a photolabile protection group providing a "caged selenocysteinyl radical" suitable for biophysical applications. Cleavage of the diselenide bridge or the selenium-carbonyl bond by irradiation is possible in high quantum yields. Because of the lack of a good IR chromophore in the mid-IR region, the selenocysteinyl radical 1 cannot be monitored directly by TR FTIR spectroscopy. TR UV/Vis spectroscopy revealed the formation of the selenocysteinyl radical 1 from both precursors. The selenocysteinyl radical 1 has a lifetime tau approximately 63 mus and exhibits a strong band located at lambda(max) = 335 nm. Calculated UV absorptions of the selenocysteinyl radical (UB3LYP/6-311G(d,p)) are in good agreement with the experimental results. The use of TR UV/Vis spectroscopy permitted the determination of the decay rates of the selenocysteinyl radical in the presence of two quenchers. The product studies demonstrated the reversible photoreaction of dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2. Products of the photolysis of the "caged selenocysteinyl radical" precursor 3 are dimethyl bis(N-tert-butoxycarbonyl)-l-selenocystine 2, carbon dioxide, and some further smaller fragments. In addition, the photodecomposition of the (9-fluorenylideneamino)oxycarbonyl moiety produced 9-fluorenone-oxime 4, 9-fluoren-imine 5, and 6 and 7 as products of the dimerization of two 9-fluorenoneiminoxy radicals 8.