Synthesis and structural, spectroscopic and magnetic studies of two new polymorphs of Mn(SeO3)·H2O

Two new manganese(II) selenite polymorphs with formula Mn(SeO₃)·H₂O have been synthesized by slow evaporation from an aqueous solution. The crystal structure of both compounds (1) and (2) have been solved from X-ray diffraction data. The structure of (1) was determined from single-crystal X-ray diffraction techniques. The compound crystallizes in the Ama2 space group, with a=5.817(1), b=13.449(3), and Z=4. The structure of (2) has been solved from X-ray powder diffraction data. This phase crystallizes in the P2₁/n space group with unit-cell parameters of a=4.921(3), b=13.121(7), , β=90.03(2)° and Z=4. Both polymorphs exhibit a layered structure formed by isolated sheets of MnO₆ octahedra and (SeO₃)²⁻ trigonal pyramids in the (010) plane. These layers, which contain one manganese and selenium atom crystallographically independent, are formed by octahedra linked between them through the selenite oxoanions. The difference of both compounds consists in the stacking of the layers along the b-axis. The IR spectra show the characteristic bands of the selenite anion. Studies of luminescence performed at 6 K and diffuse reflectance spectroscopy have been carried out for both phases. The Dq and Racah (B and C) parameters, from luminescence and diffuse reflectance spectroscopy, are Dq=705, B=750, for (1) and Dq=720, B=745, for (2). The ESR spectra of both compounds are isotropic with g-values of 1.99(1). Magnetic measurements indicate the presence of antiferromagnetic couplings in both phases. The J-exchange parameters have been estimated by fitting the experimental magnetic data to a model for square-planar lattice. The values obtained are J/k=-0.83, −0.91 K and J^′/k=-0.97, −1.20 K, for polymorphs (1) and (2), respectively.     

Copper-catalyzed dimerization of chromium Fischer carbene complexes: synthesis of dialkoxytrienes and their Nazarov-type cyclization to 2-alkoxy-2-cyclopentenones

Fischer carbene complexes 1 underwent a clean ligand dimerization reaction yielding functionalized olefins and trienes 4 in the presence of copper (I) catalysts. If treated with trifluoroacetic acid (TFA), trienes 4c, d, f undergo a cyclization process (Nazarov reaction) which furnishes cyclopentenone derivatives 6c, d, 7c, d and 8 in good yields. Finally, the Fischer aminocarbene 9 efficiently cyclodimerizes to the substituted arene 10 in the presence of CuBr.     

Plastic crystals and their potential use in new technologies

The paper gives some comments about the potential use of plastic crystals in energy storage. The main point deals with the possibilities of binary solid solution formation. Three binary phase diagrams in the pentaerythritol series are commented on.     

Convergent stereocontrolled synthesis of substituted exo-glycals by Stille cross-coupling of halo-exo-glycals and stannanes

(Z)-exo-Glycals can be conveniently prepared in a convergent manner by Stille cross-coupling of (Z)-halo(Br,I)-exo-glycals and aryl or alkenyl stannanes, the latter are readily obtained by addition of tributylstannyl radicals to terminal alkynes.     

Theoretical prediction of the damping of a railway wheel with sandwich-type dampers

This paper presents a procedure for predicting the damping added to a railway wheel when sandwich-type dampers are installed. Although there are different ways to reduce the noise generated by a railway wheel, most devices are based on the mechanism of increasing wheel damping. This is why modal damping ratios are a clear indicator of the efficiency of the damping device and essential when a vibro-acoustic study of a railway wheel is carried out. Based on a number of output variables extracted from the wheel and damper models, the strategy explained herein provides the final damping ratios of the damped wheel. Several different configurations are designed and experimentally tested. Theoretical and experimental results agree adequately, and it is demonstrated that this procedure is a good tool for qualitative comparison between different solutions in the design stages.     

Emulsion Copolymerization of Vinyl Acetate and Vinyl Silanes: Kinetics and Development of Microstructure

The batch emulsion copolymerization of vinyl acetate with different vinyl silane functional monomers (vinyl trimethoxysilane [VTMS], vinyl triethoxysilane [VTES], and vinyl silanetriol [VSTO]) is studied. The nature of the silane strongly affects the development of the microstructure and crosslinking ability of the latexes. A combination of techniques (Soxhlet extraction, centrifugation, assymetric‐flow field flow fractionation AF4/MALS/RI) shows that the factor controlling the molar mass and crosslinking density is the degree of hydrolysis of the alkoxysilane, producing higher molar masses and degrees of crosslinking when the degree of hydrolysis is high. Thus, the copolymer containing VSTO produced a very crosslinked latex, the one with VTMS produced a latex with a low degree of crosslinking in the wet state that can yield high degrees of crosslinking upon drying, and the latex with VTES do not produce significant amounts of crosslinking neither before nor after drying.     

Site-specific C content by quantitative isotopic C Nuclear Magnetic Resonance spectrometry: A pilot inter-laboratory study

Isotopic ¹³C NMR spectrometry, which is able to measure intra-molecular ¹³C composition, is of emerging demand because of the new information provided by the ¹³C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic ¹³C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular ¹³C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic ¹³C NMR was then assessed on vanillin from three different origins associated with specific δ¹³C_i profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ¹³C_i in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.     

Study of medieval enamelling on gilded objects combining SEM-EDAX and PIXE

A set of fragments of metallic artefacts from the medieval period excavated from Ciudad Real in Spain has been studied. The objects are gilded copper buckles with a champlevé enamelling decoration. The composition of predominantly blue-coloured enamels has been analysed using three non-destructive techniques, SEM-EDAX (scanning electron microscopy coupled with energy-dispersive X-ray analysis), X-ray fluorescence (XRF) and PIXE (particle-induced X-ray emission). Analyses show that Co is responsible for the blue colour. The results of the two techniques are compared, as well as the main components which constitute the enamel. Analyses suggest that Cu is responsible for red colour.     

Optimization and validation of a method of analysis for fenitrothion and its main metabolites in forestry air samples using sorbent tubes with thermal desorption cold trap injection and gas chromatography-mass spectrometry

An analytical methodology using thermal-desorption cold trap (TCT) and GC-MS was developed for the determination of the insecticide fenitrothion and its main metabolites, 3-methyl-4-nitrophenol and fenitrooxon, in forestry atmospheres. The sampled atmosphere was pumped through a glass tube containing 100 mg of Tenax adsorbent at a flow rate of 50 ml min(-1). Adsorption/thermal desorption and breakthrough experiments were performed to test the ability to quantitatively trap the compounds. The detection limits of method for these compounds ranged between 1.6 and 2.1 ng m(-3). This methodology was developed to evaluate the persistence of fenitrothion in forest atmospheres after treatment. Spray application at 21.5 mg active ingredient m(-2) resulted in atmosphere levels of the insecticide of 78.3 ng m(-3) (after 2 h of application). Within 2-4 days following treatment, the presence of fenitrooxon fell to 50-55%. During this period residues of metabolites began to appear, disappearing 19 days later.