Electron paramagnetic resonance, scanning electron microscopy with energy dispersion X-ray spectrometry, X-ray powder diffraction, and NMR characterization of iron-rich fired clays

The aim of this study is to clarify the structure of an iron-rich clay and the structural changes involved in the firing process as a preliminary step to get information on ancient ceramic technology. To this purpose, illite-rich clay samples fired at different temperatures were characterized using a multitechnique approach, i.e., by electron paramagnetic resonance, scanning electron microscopy with electron dispersion X-ray spectrometry, X-ray powder diffraction, magic angle spinning and multiple quantum magic angle spinning NMR. During firing, four main reaction processes occur: dehydration, dehydroxylation, structural breakdown, and recrystallization. When the results are combined from all characterization methods, the following conclusions could be obtained. Interlayer H2O is located close to aluminum in octahedral sites and is driven off at temperatures lower than 600 degrees C. Between 600 and 700 degrees C dehydroxylation occurs whereas, between 800 and 900 degrees C, the aluminum in octahedral sites disappears, due to the breakdown of the illite structure, and all iron present is oxidized to Fe3+. In samples fired at 1000 and 1100 degrees C iron clustering was observed as well as large single crystals of iron with the occurrence of ferro- or ferrimagnetic effects. Below 900 degrees C the aluminum in octahedral sites presents a continuous distribution of chemical shift, suggesting the presence of slightly distorted sites. Finally, over the whole temperature range, the presence of at least two tetrahedral aluminum sites was revealed, characterized by different values of the quadrupolar coupling constant.     

Structural and infrared spectroscopic characterization of Co6C(CO)12S2: a high-nuclearity carbido carbonyl cluster spontaneously formed from dicobalt octacarbonyl and carbon disulphide

Co₆C(CO)₁₂S₂ (I) has been isolated in crystalline form from the mixture of more than a dozen of carbonyl products formed when Co₂(CO)₈ reacts at room temperature with CS₂. Crystals of I are monoclinic with space group Cc, and lattice constants a  16.250(5), b  9.413(4), c  16.036(5) Å, β  116.77(4)°. Structure refinement gave R  0.034 for 1974 reflections. The CCo₆S₂ core of the molecule possesses idealized D_3h geometry. It is composed of a Co₆ trigonal prism, enclosing a C atom in the centre, and the triangular faces are capped symmetrically by the two S atoms. The core contains two sorts od CoCo distances: short one (2.432 Å) along the triangular edges, and long ones (2.669 Å) along the lateral edges. The average CoC distance is 1.94 Å, and the average CoS distance 2.192 Å.¹³CO-enriched samples were prepared photochemically and their IR spectra used in the assignment of the CO stretching frequencies. The CO stretching force constant was calculated to be 1670(2) Nm^-1.By the use of ¹³CS₂, I has also been obtained in a selectively carbido-¹³C-labelled form. The vibrational frequencies of the carbide atom were observed, and that at 819 cm^-1 (¹³C: 790 cm^-1) assigned to the species

, and that at 548 cm^-1 (¹³C: 535.5 cm^-1) to species E′. For the Co-C(carbide) force constant a value of 155 Nm^-1 was calculated. The cobalt—sulphur stretching frequencies were found at 309 cm^-1 (

) and 239 cm^-1 (E′). The CoS stretching force constant, 78 Nm^-1, is considerably lower than that obtained for SCo₃-(CO)₉, viz. 112 Nm^-1.     

A novel methodology for the synthesis of complexes containing long carbon chains linking metal centres: molecular structures of [Ru(dppe)Cp*]2(mu-C14) and [Co3(mu-dppm)(CO)7]2(mu3:mu3-C16)

Elimination of AuX(PR3)(X = halogen, R = Ph, tol) occurs readily in reactions between compounds containing C(sp)- or C(sp2)-X bonds and alkynyl or polyynyl gold(I) complexes; this reaction has been applied to the syntheses of complexes containing a variety of metal centres linked by C(n) chains (n up to 16).     

Estimation of measurement uncertainty for the determination of aflatoxin M1 in milk using immunoaffinity clean-up procedures

A measurement uncertainty estimated for aflatoxin M₁ determination in milk sample has been calculated using data generated from analytical method validation studies. The protocol adopted is described in detail in document LGC/VAM/1998/088. The uncertainty budget was based on precision, trueness and ruggedness data. The individual contributions are described in detail. The expanded uncertainty for aflatoxin M ₁ at a concentration of 20 ng L⁻¹ was estimated as 2.81 ng L⁻¹. This was calculated using a coverage factor of two which gives a level of confidence of approximately 95%. Presented at AOAC Europe / Eurachem Symposium March 2005, Brussels, Belgium     

Facile synthesis of @-tide beta-strand peptidomimetics: improved assembly in solution and on solid phase

The synthesis of @-tide beta-strand peptidomimetics has been improved such that oligomers now can be obtained from solution- and solid-phase synthesis protocols approaching the efficiency and flexibility of peptide chemistry. These methods enable the synthesis of @-tide oligomers with a variety of amino acids and with lengths up to 13 units. [reaction: see text]     

A novel approach to the synthesis of 1,2-cis-glycopyranosides

Methyl 3,4,6-tri-O-benzyl-α-D-glucopyranoside and methyl 3,4,6-tri-O-benzyl-β-D-mannopyranoside were directly obtained by m-chloroperbenzoic acid treatment of (Z)-(2R,3R,4R)-6-methoxy-1,3,4-tribenzyloxy-5-hexen-2-ol.     

Computational evidence for a variable first shell coordination of the cadmium(II) ion in aqueous solution

In this paper, we present a state-of-the-art 100 ns molecular dynamics simulation of a cadmium(II) aqueous solution that highlights a very flexible ion first coordination shell which transits between hexa- and heptahydrated complexes. From this investigation, a dynamical picture of the water exchange process emerges that takes place through an associative mechanism for the solvent substitution reaction. Our procedure starts from the generation of an effective two-body potential from quantum mechanical ab initio calculations in which the many-body ion-water terms are accounted for by the polarizable continuum method (PCM). This approach is computationally very efficient and has allowed us to carry out extremely long molecular dynamics simulations, indispensable to reproduce the dynamic properties of the cadmium(II) aqueous solution. Quantum mechanical ab initio calculations of the hexa- and heptahydrated complexes extracted from MD configurations have revealed stable minima for both clusters with the water molecules arranged in T(h)() and C(2) symmetries in the hexa- and heptahydrated complexes, respectively, with a slight energetic preference for the heptahydrated one. Finally, a comparison of the calculated hexa- and heptahydrated cluster IR and Raman spectra with the experimental data in the literature, has demonstrated that the IR spectroscopy is not able to distinguish between the two species, whereas the Raman spectrum of the Cd(2+)-(H(2)O)(7) cluster provides a better agreement with the experimental data.     

Fast GC for the analysis of citrus oils

In this investigation, the gas chromatographic (GC) analysis of citrus essential oils is carried out in 3.3 min, with a speed gain of almost 14 times in comparison with traditional GC procedures. The fast method that is developed requires the application of severe experimental conditions (accelerated temperature program rates, high inlet pressures, and split ratios) and, thus, the support of adequate instrumentation. The samples investigated can be considered to be rather complex and, although a slight loss in peak resolution is observed, the overall analytical result is excellent. All data obtained are compared with that of a conventional application on the same matrices. This is done in order to evaluate the effectiveness and advantages of fast GC achieved with narrow bore columns.     

Integral representation of local functionals

We study conditions under which a functional F(u, B) admits an integral representation of the form $$F(u, B) = \int\limits_B {f(x, D^k u(x)) dx.} $$      

Synthesis and structure of heterocyclic compounds. 1-Amino-2,4-Disubstituted imidazoles

A synthesis of 1-aminoimidazoles by reaction between α-halogenoketones and N-acetylamidrazones is described. Structure and position of substituent groups in the synthesized aminoimidazoles were established by spectroscopical methods.