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991.
Bracher F Eisenreich WJ Mühlbacher J Dreyer M Bringmann G 《The Journal of organic chemistry》2004,69(25):8602-8608
The first biarylic bis-morphinanedienone alkaloids, saludimerines A (3a) and B (3b), isolated from a tree of Croton flavens (Euphorbiaceae) are described. These naturally occurring dimers of the known alkaloid salutaridine are joined together via a rotationally hindered biaryl axis, giving rise to atropo-diastereomers that are configurationally stable at room temperature but slowly interconvert in methanolic solution within several days. Their structures were established by spectroscopic methods and by partial synthesis, which was achieved by a highly atropo-diastereoselective biomimetic oxidative coupling of the monomeric precursor, salutaridine. Their axial configurations were elucidated by circular dichroism (CD) investigations, which succeeded despite the fact that the two atropo-diastereomers exhibit near-identical CD spectra. This remarkable phenomenon was rationalized by quantum chemical CD calculations. The configurational assignment of saludimerines A (3a) as P-axial and B (3b) as M was corroborated by atropisomer-specific NOE interactions between protons of the one molecular half with nuclei in the other. 相似文献
992.
G. Pokol F. Hevesi Tóth I. Péter J. Madarász T. Kocsis S. Gál 《Journal of Thermal Analysis and Calorimetry》1990,36(5):1867-1888
Empirical parameters were chosen for the characterization of peaks (or steps) of thermoanalytical curves. The parameters were applied in studies on the repeatability, the relationship between kinetic constants and peak shape, the effect of sample thermal resistance. Kinetic constants can be estimated on the basis of peak shape parameters. Besides, approximate criteria were formulated for experiments allowing kinetic evaluation with the neglect of the heat transport within the sample. The empirical parameters were also used in checking the suitability of DSC data for purity analysis and in detecting changes of the thermal decomposition of papers caused by ageing.Zur Beschreibung von Peaks bzw. Stufen thermoanalytischer Kurven wurden emperische Parameter ermittelt, die bei der Untersuchung der Reproduziertbarkeit, der Beziehung zwischen kinetischen Konstanten und Peakform so wie des Einflusses des thermischen Wiederstandes der Probe angewendet wurden. Kinetische Konstanten können auf der Basis von Peakformparametern geschÄtzt werden. Weiterhin wurden Kriterien für Experimente ermittelt, die eine kinetische NÄherungslösung unter VernachlÄssigung des WÄrmetransportes innerhalb der Probe ermöglichen. Die empirischen Parameter wurden au\erdem bei der Untersuchung der Eignung von DSC-daten zur Reinheitsanalyse angewendet, weiterhin zum Nachweis von alterungsbedingten Änderungen der thermischen Zerzetzung von Papier.
The authors express their thanks to Mrs E. Tóth for her valuable technical help. The permission to reproduce Figs 2–6 and 8–10 from Analytica Chimica Acta and Thermochimica Acta (Elsevier, Amsterdam) is also acknowledged. 相似文献
The authors express their thanks to Mrs E. Tóth for her valuable technical help. The permission to reproduce Figs 2–6 and 8–10 from Analytica Chimica Acta and Thermochimica Acta (Elsevier, Amsterdam) is also acknowledged. 相似文献
993.
Renáta Oriňáková Hans-Dieter Wiemhöfer Jürgen Paulsdorf Veronika Barinková Anna Bednáriková Roger M. Smith 《Journal of Solid State Electrochemistry》2006,10(7):458-464
The electrochemical impedance method was applied during the electrochemical deposition of a binary Ni–Co coating on iron powder in a fluidised bed electrode system. The influence of the suspension density on the charge transfer in the course of the electro-deposition process was studied. At a potential of −900 mV (vs. Ag/AgCl/3 M KCl), when the binary Ni–Co layer was formed, the impedance data were characterised by two semicircles with the semicircle at high frequencies being larger in magnitude. A contribution of the diffusion process to the overall current was observed. The optimal suspension density for the charge transfer in the bed was 10×10−3–15×10−3 (i.e., 4–6 g of iron powder in 50 ml of electrolyte). The most probable mechanism of the charge transfer for the studied concentrations of powder particles is the convective mechanism. The iron particles dispersed in the electrolyte were considered to act as either a depolariser or an additional working electrode depending on the applied electrode potential and on the suspension density. 相似文献
994.
Remedios González-Luque Manuela Merchán Piotr Borowski Björn O. Roos 《Theoretical chemistry accounts》1993,86(6):467-476
Summary Multiconfigurational electron correlation methods have been analyzed in order to theoretically compute the electron affinity (EA) of ozone. The near-degeneracy correlation effects, which are so important in O3 and O
3
–
, have been described using complete active space (CAS) SCF wave functions. Remaining dynamic correlation effects are computed using second-order perturbation theory (the CASPT2 method). The best calculated adiabatic value (including zero-point energy corrections), 2.19 eV, is about 0.09 eV larger than the experimental value. Comparative studies using size-consistent coupled pair functional approaches (CPF and ACPF) have also been performed. The harmonic frequencies in O
3
–
have been determined to be: 1=992, 2=572, and 3=879 cm–1, which gives a zero-point energy of 0.151 eV. 相似文献
995.
Mid infrared spectroscopy is a non-destructive technique that can provide detailed information on important, molecule-specific features such as the conformation and functional groups of a large range of compounds. Infrared spectroscopy is now an established and frequently used technique for qualitative analysis, i.e. the identification of chemical constituents in a sample. In addition, its use for quantitative purposes has grown dramatically in recent years. It is important to realise that the analytical problem defines the mode of operation and implementation of the FTIR technique. This Highlight article focuses on the advantages and scope of on-line FTIR detection strategies. However, in common with all techniques, on-line FTIR detection has a number of potential shortcomings, which are also discussed. 相似文献
996.
J. Krägel J. B. Li R. Miller M. Bree G. Kretzschmar H. Möhwald 《Colloid and polymer science》1996,274(12):1183-1187
The surface viscoelastic properties of monolayers of two phospholipids DPPC (L--dipalmitoylphosphatidylcholine) and DMPE (L--dimyristoylphosphatidyl-ethanolamin), at the air-water interface have been investigated. Two techniques were used for the investigation. One involved use of an interfacial shear rheometer (torsion pendulum apparatus ISR1), to provide measurements of the shear viscosity s as a function of surface pressure, and the second, a modified LB trough with an oscillating barrier to generate periodie dilation and compression so as to measure the dilational elastic modulusE as a function of surface area.Results indicate a strong dependence of s andE upon monolayer phases. This suggests that the viscoelastic relaxation of monolayers can be understood as molecular rearrangements, domain exchange and molecular reorientations between different monolayer states. 相似文献
997.
The enzyme catalyzed conversion of R/S-(±)-2-octanol with hexanoic acid to R/S-(±)-2-octyl hexanoate has been studied in
different microenvironments and in the presence of the competing substrate ethanol. The reactions were performed in various
gels made from aqueous gelatin solutions and liposome dispersions or isotropic liquid solutions, with or without oil and ethanol.
The lipase Candida sp. (SP 525) was dissolved in the dispersions or solutions stabilized by the naturally occurring zwitterionic surfactant
soybean lecithin. The sectioned porous gel was immersed in hexane containing 0.33 mol dm-3 of racemic 2-octanol and hexanoic acid. Since ethanol acts both as a substrate and as a part of the gel it is of fundamental
interest to know the phase behaviour of the used systems. Partial phase diagrams for the systems ethanol–water–soybean lecithin
and ethanol/water (7:3)–oil–soybean lecithin were determined at 298.2 K. The oil was either castor oil or hexadecane. The
conversion of R-2-octyl hexanoate was about 0.45 when no or small amounts of ethanol was present, but decreased considerably
with high amounts of ethanol present and ethyl hexanoate became the main product. Hydrolysis of R-2-octyl hexanoate was favoured
in the latter systems and hexanoic acid formed was immediately esterified to ethyl hexanoate. The conversion of R-2-octyl
hexanoate and ethyl hexanoate depends only on the ethanol content present in the systems and is thus independent of the environment
of the enzyme. However, the chiral esters synthesized from racemic 2-octanol and hexanoic acid showed high optical purities
regardless of the ethanol content.
Received: 1 July 1996 Accepted: 30 August 1996 相似文献
998.
The measurement is carried out of aqueous solutions. The burner is supplied with a mixture of acetylene and nitrous oxide. Interfering influences by acid and iron concentrations of the sample solution as well as by organic solvents are shown. The standard deviation is ± 0.001%. The agreement with conventional analysis methods is very good. By this procedure a rapid and safe determination of aluminium will be possible. 相似文献
999.
Magda Mrton-Mersz Erzsbet Zra-Kaczin Sndor Boros Pter Mtyus 《Journal of heterocyclic chemistry》1997,34(3):1033-1036
Reaction of 2-acetonyl-4,5-dimethoxy-3′-chlorobenzophenone (1) with ethylenediamine afforded the imidazo[2,1-a]isoquinoline 2, whereas 6-(3-chlorophenyl)-8,9-dimethoxybenzo[b]phenazine (3) and naphthol 4 were obtained with ortho-phenylenediamine. 相似文献
1000.
Eberling J Braun P Kowalczyk D Schultz M Kunz H 《The Journal of organic chemistry》1996,61(8):2638-2646
The selective C-terminal deprotection of O-glycopeptide (methoxyethoxy)ethyl esters is achieved under mild conditions (pH 6.6, 37 degrees C) by enzymatic hydrolysis using papain or lipase M from Mucor javanicus to give building blocks useful for chain-extending glycopeptide synthesis. On the other hand, the selective removal of acetyl protecting groups from the saccharide portion of glycopeptides is accomplished by alternative enzymatic hydrolysis with lipase WG from wheat germ to furnish model substrates for enzymatic glycosyl transfer reactions in order to extend the carbohydrate side chain of these conjugates. 相似文献