Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic (III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic (V) to arsenic (III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800 degrees C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2 microg/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3sigma) of 0.01 microg/L using a 0.5 mL sample. The reagent blank was 0.001+/-0.0003 absorbance units and the standard deviation of 10 measurements of the 2 microg/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS     

Synthesis of [(MeCyt)2H]I-structure and stability of a dimeric threefold hydrogen-bonded 1-methylcytosinium 1-methylcytosine cation

Reaction of trimethylsilyl-protected cytosine with methyl iodide afforded N1-methylated product. Subsequent treatment with ethanol resulted in cleavage of the protection group forming [(MeCyt)2H]I (4). Identity of was confirmed by microanalysis, mass spectrometry, 1H and 13C NMR spectroscopy and by single-crystal X-ray diffraction analysis. Crystals of consist of dimeric [(MeCyt)2H]+ cations and I- anions. These ions are arranged in the crystal such that there is a strong base stacking (mean stacking distance 3,467 angstroms) and, furthermore, pi interactions between I- and cytosine rings (mean distance 3,737 angstroms). The dimeric [(MeCyt)2H]+ cations are centrosymmetric having three strong hydrogen bonds, namely two terminal N4-H...O' ones (N4...O' 2.815(4) angstroms) and a central N3-H...N3' (N3...N3' 2.813(4) angstroms) one. Quantum chemical calculations on the DFT level of theory show that the gas phase structure of the dimeric cation exhibits two different terminal N-HO hydrogen bonds, a stronger (N4...O' 2.722 angstroms) and a weaker one (N4'...O 2.960 angstroms). The central N3-HN3[prime or minute] hydrogen bond (N3...N3' 2.852 angstroms) was characterized to have an unsymmetrically located proton and a typical double minimum potential with a very low activation barrier. The interaction energy between [(MeCyt)H]+ and MeCyt yielding [(MeCyt)2H]+ was calculated to be -42.4 kcal mol(-1)(ZPE and BSSE corrected). Comparison with the interaction energy (calculated on the same level of the theory) between cytosine and guanine yielding the triply hydrogen-bonded Watson-Crick dimer (-24.2 kcal mol(-1)) revealed a much higher stability of the hydrogen bonds in [(MeCyt)2H]+.     

Sekundäre D-isotopieeffekte bei der hydrolyse von p-nitroacetanilid

The OH^--catalysed hydrolysis of p-nitroacetanilide and p-nitroacetanilide-1-d₃ has been studied between p_H11·5 and 13·5 at 30°. The value of the secundary isotope effect is changed with respect to the OH^--concentration. The inverse istope effects at high OH^--concentrations (^Hk_korr/^Dk_korr = 0·87 ±0·05) and the opposite effects in the lower OH^?-concentration ranges (^Hk_korr/^Dk_korr = 1·08 ± 0·04)are discussed on the basis of change in the rate limiting step.     

Separation of organoselenium compounds and their electrochemical detection

A simplified procedure based on ion-exchange separation of selenourea (Se-U) and selenocystamine (Se-CM), which have very close half-wave potential when they are simultaneously analyzed by voltammetric techniques, has been developed and optimized. Thus, selenocystamine remains in the cation exchanger Purolite C 100 H, whereas selenourea is found in the effluent and is determined by square wave cathodic stripping voltammetry using Na₂CO₃ as electrolyte. Selenocystamine is then eluted from the cation exchanger using 4 M HCl and analyzed by square wave cathodic stripping voltammetry in the HCl solution. For each voltammetric determination the corresponding parameters were investigated and optimized; the obtained detection limits were 0.3 ng Se mL^–1 for Se-CM and 2 ng Se mL^–1 for Se-U. A flow sheet for the separation of inorganic (Se(IV) and Se(VI)) and organoselenium compounds (Se-U, Se-CM, (CH₃)₂Se₂, and (CH₃)₂Se) developed for their electrochemical detection is presented and it was successfully applied to a certified reference material, an environmental soil sample, and a urine sample.     

Olefin Metathesis and Beyond A list of abbreviations can be found at the end of this article

The advent of well-defined catalysts for olefin metathesis which combine high activity, durability, and excellent tolerance towards polar functional groups has revolutionized the field. The past decade has seen the rapid embrace of these reagents as tools for advanced organic and polymer chemistry and the success of this development is witnessed by a plethora of elegant applications to the synthesis of natural and nonnatural products. This review article provides an overview of these developments and intends to familiarize the reader with some very recent advances which hold the promise to expand the scope of the reaction even further. Moreover, the positive impact of metathesis on the fundamental logic of retrosynthetic planning is demonstrated by means of typical examples. Finally, it will be shown that metathesis is by no means restricted to alkenes as substrates, and some comments on metathesis reactions following unconventional mechanistic pathways will also be presented.     

Beitrag zur Bestimmung der Spurenverunreinigungen in Rheniummetall

Zusammenfassung Spuren von Zn, Cu, Ni, Co, Fe, Mn und Mo in Rheniummetall werden aus einer Einwaage ermittelt. Mo wird photometrisch als ternäres Mo(VI)Hydroxylamin-PAR-Chelat bestimmt. Die übrigen Elemente werden als PAN-Chelate gefällt und im abgetrennten Niederschlag mittels Röntgenfluoreszenzanalyse bestimmt.

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A contribution to the determination of trace contaminants in rhenium metal

Summary Traces of Zn, Cu, Ni, Co, Fe, Mn, and Mo in rhenium metal were determined in a single weighed sample. Molydenum was determined photometrically as ternary Mo(VI)-hydroxylamine-PAR-chelate. The other elements were precipitated as PAN-chelates and then determined in the isolated precipitate by means of roentgen fluorescence analysis.

     

Zur synthese von siloxanen: I. Die hydrolyse von chlorsiloxanen

The kinetics of hydrolysis reactions of some chlorodisiloxanes and of the chloropentasiloxanes (Me₃) SiO)₂Si(Me)OSiAr(Cl)OSiMeAr′ (with Ar = Ph and Ar′ = p-ClC₆H₄, C₆H₅ or p-MeC₆H₄; or Ar′ = Ph and Ar = p-ClC₆H₄, C₆H₅ or p-MeC₆H₄ in dioxane were studied by ¹H NMR spectroscopy.Because of their high hydrolysis rate the chlorodisiloxanes can be reacted only with equimolar amounts of H₂O. The formal second order rate constants and also the ? value evaluated can be used only in a tentative way.From hydrolysis reactions of the chloropentasiloxanes with an excess of H₂O and different amounts of HCl or HClO₄ the following results were obtained: hydrolysis is catalysed by HCl and HClO₄. The reaction is of first order with respect to chlorosiloxane. With respect to HCl at a tenfold excess of H₂O the order is in the range 0.5–0.7. Our results indicate, that there is a “non-catalysed” hydrolysis besides the acid catalysed reaction, ?-Values for the variation of Ar (0.7) and Ar′ (0.3) can be derived from the rate constants of the acid catalysed reactions of chloropentasiloxanes with a tenfold amount of water. The results are interpreted in mechanistic terms.     

Activation analytical determination of129I in bovine thyroid glands

The continued erection of nuclear installations which is expected in the future involves a greater production of long-lived fission products, which result in a concentration increase in the biosphere, the hydrosphere and the atmosphere. In the frame of a broad survey program, a routine method has been developed with the help of the European Communities to investigate the¹²⁹I content of thyroid glands of cows. It is the purpose of this project to determine the present concentration situation of this nuclide, which in the Federal Republic of Germany mainly concerns the North German low plains. Between autumn '76 and summer '77 18 cows have been investigated with respect to¹²⁹I, and thyroid glands have been taken at different times in November '76 and August '77. Without exception, the animals were 13-month old bulls, part of which has pastured, while the other part has been fed. The detection limit of the method is 20fCi of¹²⁹I.     

MO-LCAO-calculations on sulfur containing π-electron systems

Zusammenfassung Halbempirische SCF-CI-Berechnungen wurden auf Thiokohlensäurederivate ausgedehnt, um spezifische Eigenschaften intensiver Absorptionen im nahen UV zu erklären. Auch ohne besondere Anpassung der Parameter wurde gute Übereinstimmung mit experimentellen Daten erzielt. Die Änderungen der Elektronenverteilung bei der optischen Anregung sind komplizierter, als für die Thiokohlensäurederivate als gekreuzt konjugierte Systeme bisher angenommen wurde.

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MO-LCAO-calculations on sulfur containing -electron systemsXVI. PPP calculations on thiocarbonic acid derivatives as crossed conjugated systems

Semiempirical SCF-CI-calculations were extended to derivatives of thiocarbonic acid to explain specific properties of intense near UV-absorptions. Good agreement with experimental data was obtained although parameters were not adjusted. The change of electron distribution by optical excitation is more complicated as has been assumed for thiocarbonic acid as crossed conjugated systems.

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Résumé Des calculs SCF-CI semiempiriques sont étendus aux dérivés d'acide thiocarbonique en vue d'expliquer des propriétés spécifiques des absorptions intensives dans l'UV proche. Même sans ajustement des paramètres les résultats obtenus étaient en bon accord avec les dates experimentelles. Les changements de la distribution électronique par l'excitation optique sont plus compliques qu'on ne les estimait.

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XV. Mitteilung: Fabian, J.: Z. Chem. 8, 274 (1968).

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Diese Arbeit wurde in der Schwefelarbeitsgruppe unter Leitung von Herrn Prof. Dr. R. Mayer durchgeführt.     

Structure and Jahn-Teller effect in mixed crystals Rb2Cr1−xMnxCl4: A single-crystal neutron diffraction study and spectroscopic results

The structures of single crystals Rb₂Cr_1?xMn_xCl₄ (0 ? x ? 1) have been studied by neutron diffraction. A crystal of composition x = 0.01 shows a superstructure of the K₂NiF₄ type [space group Bbcm; a = b = 7.262Å, c = 15.733Å]. The structural refinement [R = 0.043] yields an antiferrodistortive order of tetragonally elongated octahedra (superimposed by a small orthorhombic component) with CrCl bond lengths of 2.43 Å (∥[001]) and 2.40 Å, 2.74 Å (⊥[001]). Structural results for x = 0.01/0.08/0.53/0.63/0.83/0.91/0.97 in space group $\text{I4}\text{mmm}$ [K₂NiF₄ type]—in particular the anomalous ms displacements of Cl(1) in the (001) plane—give evidence that the distortion of the (Mn, Cr)Cl₆ octahedra decreases with increasing x. AOM calculations based on experimental ligand field energies indicate that the individual CrCl₆ polyhedra are more strongly distorted than the (Jahn-Teller stable) MnCl₆ octahedra in mixed crystals with larger x values.