Mesophase behaviour of 2,5-dibenzoyloxy-p-benzoquinone derivatives and tetrahydrobenzene tetraesters

The synthesis of 2,5-dibenzoyloxy-p-benzoquinone derivatives, their products after reduction and tetraesters from tetrahydroxybenzene prepared from those products is described. Their phase behaviour was investigated by differential scanning calorimetry and polarizing microscopy and is discussed in terms of their detailed structure.     

Quantitative goldbestimmung in filmmaterialien mittels flammenloser atomabsorption

A graphite rod electrothermal atomizer has been used for the AAS determination of traces of gold in hydrochloric and in hydrobromic acid solutions, and also after extraction into HBr-saturated methyl isobutyl ketone. Photographic film samples were decomposed first by enzyme action then by nitric acid/peroxide oxidation, and the gold was extracted into MIBK. For 10-μl aliquots of solution the 3s limits of detection were 3 × 10^?10g for aqueous solutions, 7 × 10^?10g for MIBK, and 7 × 10^?9 g/cm² for film.     

Preconcentration and determination of Sn- and Pb-organic species in environmental samples by SPME and GC-AED

A new sample preparation and preconcentration technique - solid phase microextraction (SPME) - is reported for the application of several tinorganic compounds and tetrabutyllead in aqueous samples. The solvent-free procedure is rapid in comparison with liquid-liquid extraction or SFE but also sensitive. Analytical variables of the extraction such as adsorption and desorption time, stirring rate and temperature has been investigated. The determination has been performed by GC coupled with atomic emission detection (AED). After optimization of the conditions of SPME a calibration was realized on the basis of a multicomponent standard solution, prepared by ethylation of organotin salts directly in the sample using sodium tetraethylborate (NaBEt(4)) without prior separation of the analytes from the matrix. The method permits preconcentration. Values of about 10 can be reached. A detection limit of 0.09 pg Sn and 0.08 pg Pb can be achieved under optimized conditions. The proposed procedure has been successfully applied to the analysis of organotin compounds in various slurry samples.     

Métallation et alkylation des thioimidoesters: application en synthèse

Alkylation of delocalized anions resulting from metallation of N-phenyl thioimidoesters (precursors of dithioesters and thiolesters) takes place on nitrogen with “saturated” thioimidoesters (alkane thiomidates). On the contrary, unsaturated thiomidoesters (α or β-ethylenic, α-arylated) are regioselectively alkylated on the α carbon atom by alkyl or allylic halides. The possibilities for synthesis offered by successive uses of the reactivities of thioimidoester and dithioester functions (allowing in particular two electrophilic additions on two vicinal carbons) are illustrated by the syntheses of terpenic compounds lavandulal (and lavandulol) and ar-curcumene.     

Chemoenzymatic synthesis of both enantiomers of cispentacin

Based on the lipase-catalysed kinetic resolution of the silyloxyalcohol (1RS,2SR)-5 by transesterification with vinyl acetate in the presence of lipase from Pseudomonas cepacia a synthesis of both enantiomers of the β-amino acid cispentacin (1R,2S)-1 and (1S,2R)-1 using simple functional group interconversions is described.     

Straightforward Synthesis of 2‐Aryl‐1‐alkynylphosphonates and Arylpropiolates

A protocol for the direct cross coupling of trialkylsilyl‐alkynes bearing electron‐withdrawing groups and aryl halides using a Pd/Ag catalytic system is described. The procedure allows the straightforward synthesis of a variety of alkynylphosphonates and aryl propiolates derivatives in good yields.     

Carbon and nitrogen isotope analysis of atrazine and desethylatrazine at sub-microgram per liter concentrations in groundwater

Environmental degradation of organic micropollutants is difficult to monitor due to their diffuse and ubiquitous input. Current approaches—concentration measurements over time, or daughter-to-parent compound ratios—may fall short, because they do not consider dilution, compound-specific sorption characteristics or alternative degradation pathways. Compound-specific isotope analysis (CSIA) offers an alternative approach based on evidence from isotope values. Until now, however, the relatively high limits for precise isotope analysis by gas chromatography—isotope ratio mass spectrometry (GC-IRMS) have impeded CSIA of sub-microgram-per-liter scale micropollutant concentrations in field samples. This study presents the first measurements of C and N isotope ratios of the herbicide atrazine and its metabolite desethylatrazine at concentrations of 100 to 1,000 ng/L in natural groundwater samples. Solid-phase extraction and preparative HPLC were tested and validated for preconcentration and cleanup of groundwater samples of up to 10 L without bias by isotope effects. Matrix interferences after solid-phase extraction could be greatly reduced by a preparative HPLC cleanup step prior to GC-IRMS analysis. Sensitivity was increased by a factor of 6 to 8 by changing the injection method from large-volume to cold-on-column injection on the GC-IRMS system. Carbon and nitrogen isotope values of field samples showed no obvious correlation with concentrations or desethylatrazine-to-atrazine ratios. Contrary to expectations, however, δ ¹³ C values of desethylatrazine were consistently less negative than those of atrazine from the same sites. Potentially, this line of evidence may contain information about further desethylatrazine degradation. In such a case, the common practice of using desethylatrazine-to-atrazine ratios would underestimate natural atrazine degradation.     

Photoleitfähigkeit additiv verfärbtet SrF2-Kristalle

Additively coloured SrF₂ crystals proved to be photoconducting in the region of the coloration bands from 700 to 200 nm, the photocurrents showing some characteristics of photoelectric primary currents. Without applying an external voltage the same photoresponse is measured in the polarizing field of the crystals. Photoelectric excitation (analogous to theF′-centres in crystals of the NaCl-type) could not positively be established. The assignment of the relative spectral photosensitivity to the absorption spectra of the coloured crystals has not yet been possible. In interpreting the photoconduction effects it cannot completely ruled out whether chemical impurities play some part in the photoconduction process.