85 research outputs found
Der elektronische Handel - Potentiale fĂŒr Unternehmen
Die digitale Infrastruktur des Internets wird in zunehmenden MaĂe als Marktplatz genutzt. Auch hier treffen Angebot und Nachfrage zusammen. Der grundlegende Unterschied zu traditionellen MĂ€rkten liegt in der elektronischen Abbildung von Teilen der Transaktion oder der gesamten Transaktion. Diese zunehmende UnterstĂŒtzung bzw. Automatisierung aller Ebenen und Phasen des kommerziellen Handels fĂŒhrt zu elektronischen MarktplĂ€tzen, elektronischem Zahlungsverkehr sowie elektronischen Lieferungen von mediatisierten GĂŒtern und Dienstleistungen. Prof. Dr. Dr. h.c. Arnold Picot, Dominik K. Heger und Dr. Rahild Neuburger UniversitĂ€t MĂŒnchen, analysieren in diesem Beitrag die Einsatzmöglichkeiten das elektronischen Handels und die damit verbundenen Herausforderungen an die Unternehmen.Elektronischer Markt; Unternehmen
Crystal Structure of Zymonic Acid and a Redetermination of Its Precursor, Pyruvic Acid
The structure of zymonic acid (systematic name: 4-hyÂdroxy-2-methyl-5-oxo-2,5-diÂhydroÂfuran-2-carbÂoxyÂlic acid), C6H6O5, which had previously eluded crystallographic determination, is presented here for the first time. It forms by intraÂmolecular condensation of parapyruvic acid, which is the product of aldol condensation of pyruvic acid. A redetermination of the crystal structure of pyruvic acid (systematic name: 2-oxoÂpropanoic acid), C3H4O3, at low temperature (90â
K) and with increased precision, is also presented [for the previous structure, see: Harata et al. (1977). Acta Cryst. B33, 210â212]. In zymonic acid, the hyÂdroxyÂlactone ring is close to planar (r.m.s. deviation = 0.0108â
Ă
) and the dihedral angle between the ring and the plane formed by the bonds of the methyl and carbÂoxyÂlic acid carbon atoms to the ring is 88.68â
(7)°. The torsion angle of the carbÂoxyÂlic acid group relative to the ring is 12.04â
(16)°. The pyruvic acid molÂecule is almost planar, having a dihedral angle between the carbÂoxyÂlic acid and methyl-ketone groups of 3.95â
(6)°. InterÂmolecular interÂactions in both crystal structures are dominated by hydrogen bonding. The common R22(8) hydrogen-bonding motif links carbÂoxyÂlic acid groups on adjacent molÂecules in both structures. In zymonic acid, this results in dimers about a crystallographic twofold of space group C2/c, which forces the carbÂoxyÂlic acid group to be disordered exactly 50:50, which scrambles the carbonyl and hydroxyl groups and gives an apparent equalization of the CâO bond lengths [1.2568â
(16) and 1.2602â
(16)â
Ă
]. The other hydrogen bonds in zymonic acid (OâHO and weak CâHO), link molÂecules across a 21-screw axis, and generate an R22(9) motif. These hydrogen-bonding interÂactions propagate to form extended pleated sheets in the ab plane. Stacking of these zigzag sheets along c involves only van der Waals contacts. In pyruvic acid, inversion-related molÂecules are linked into R22(8) dimers, with van der Waals interÂactions between dimers as the only other interÂmolecular contacts
Evaporating brine from frost flowers with electron microscopy, and implications for atmospheric chemistry and sea-salt aerosol formation
An environmental scanning electron microscope was used for the first time to obtain well-resolved images, in both temporal and spatial dimensions, of lab-prepared frost flowers (FFs) under evaporation within the chamber temperature range from â5â°C to â18â°C and pressures above 500âPa. Our scanning shows temperature-dependent NaCl speciation: the brine covering the ice was observed at all conditions, whereas the NaCl crystals were formed at temperatures below â10â°C as the brine oversaturation was achieved. Finger-like ice structures covered by the brine, with a diameter of several micrometres and length of tens to one hundred micrometres, are exposed to the ambient air. The brine-covered fingers are highly flexible and cohesive. The exposure of the liquid brine on the micrometric fingers indicates a significant increase in the brine surface area compared to that of the flat ice surface at high temperatures, whereas the NaCl crystals can become sites of heterogeneous reactivity at lower temperatures. There is no evidence that, without external forces, salty FFs could automatically fall apart to create a number of sub-particles at the scale of micrometres as the exposed brine fingers seem cohesive and hard to break in the middle. The fingers tend to combine together to form large spheres and then join back to the mother body, eventually forming a large chunk of salt after complete dehydration. A present microscopic observation rationalizes several previously unexplained observations, namely, that FFs are not a direct source of sea salt aerosols and that saline ice crystals under evaporation could accelerate the heterogeneous reactions of bromine liberation
A Photo-Favorskii Ring Contraction Reaction: The Effect of Ring
The effect of ring size on the photo-Favorskii induced ring-contraction reaction of the hydroxybenzocycloalkanonyl acetate and mesylate esters (7aâd, 8aâc) has provided new insight into the mechanism of the rearrangement. By monotonically decreasing the ring size in these cyclic derivatives, the increasing ring strain imposed on the formation of the elusive bicyclic spirocyclopropanone 20 results in a divergence away from rearrangement and toward solvolysis. Cycloalkanones of seven or eight carbons undergo a highly efficient photo-Favorskii rearrangement with ring contraction paralleling the photochemistry of p-hydroxyphenacyl esters. In contrast, the five-carbon ring does not rearrange but is diverted to the photosolvolysis channel avoiding the increased strain energy that would accompany the formation of the spirobicyclic ketone, the âFavorskii intermediate 20â. The six-carbon analogue demonstrates the bifurcation in reaction channels, yielding a solvent-sensitive mixture of both. Employing a combination of time-resolved absorption measurements, quantum yield determinations, isotopic labeling, and solvent variation studies coupled with theoretical treatment, a more comprehensive mechanistic description of the rearrangement has emerged
The genome of Romanomermis culicivorax:revealing fundamental changes in the core developmental genetic toolkit in Nematoda
Background: The genetics of development in the nematode Caenorhabditis elegans has been described in exquisite detail. The phylum Nematoda has two classes: Chromadorea (which includes C. elegans) and the Enoplea. While the development of many chromadorean species resembles closely that of C. elegans, enoplean nematodes show markedly different patterns of early cell division and cell fate assignment. Embryogenesis of the enoplean Romanomermis culicivorax has been studied in detail, but the genetic circuitry underpinning development in this species has not been explored. Results: We generated a draft genome for R. culicivorax and compared its gene content with that of C. elegans, a second enoplean, the vertebrate parasite Trichinella spiralis, and a representative arthropod, Tribolium castaneum. This comparison revealed that R. culicivorax has retained components of the conserved ecdysozoan developmental gene toolkit lost in C. elegans. T. spiralis has independently lost even more of this toolkit than has C. elegans. However, the C. elegans toolkit is not simply depauperate, as many novel genes essential for embryogenesis in C. elegans are not found in, or have only extremely divergent homologues in R. culicivorax and T. spiralis. Our data imply fundamental differences in the genetic programmes not only for early cell specification but also others such as vulva formation and sex determination. Conclusions: Despite the apparent morphological conservatism, major differences in the molecular logic of development have evolved within the phylum Nematoda. R. culicivorax serves as a tractable system to contrast C. elegans and understand how divergent genomic and thus regulatory backgrounds nevertheless generate a conserved phenotype. The R. culicivorax draft genome will promote use of this species as a research model
Photoinitiated polymerisation of monolithic stationary phases in polyimide coated capillaries using visible region LEDs
The spatially controlled synthesis of poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolithic stationary phases in polyimide coated fused silica capillaries by visible light induced radical polymerisation using a three-component initiator and a 660 nm light emitting diode (LED) as a light source is presented here
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Laser flash photolysis study of the photoinduced oxidation of 4-(dimethylamino)benzonitrile (DMABN)
Aromatic amines are aquatic contaminants for which phototransformation in surface waters can be induced by excited triplet states of dissolved organic matter (3DOM*). The first reaction step is assumed to consist of a one-electron oxidation process of the amine to produce its radical cation. In this paper, we present laser flash photolysis investigations aimed at characterizing the photoinduced, aqueous phase one-electron oxidation of 4-(dimethylamino)benzonitrile (DMABN) as a representative of this contaminant class. The production of the radical cation of DMABN (DMABNË+) after direct photoexcitation of DMABN at 266 nm was confirmed in accord with previous experimental results. Moreover, DMABNË+ was shown to be produced from the reactions of several excited triplet photosensitizers (carbonyl compounds) with DMABN. Second-order rate constants for the quenching of the excited triplet states by DMABN were determined to fall in the range of 3 × 107–5 × 109 M−1 s−1, and their variation was interpreted in terms of electron transfer theory using a Rehm–Weller relationship. The decay kinetics of DMABNË+ in the presence of oxygen was dominated by a second-order component attributed to its reaction with the superoxide radical anion (O2Ë−). The first-order rate constant for the transformation of DMABNË+ leading to photodegradation of DMABN was estimated not to exceed ≈5 × 103 s−1.
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