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991.
[reaction: see text] A highly versatile approach to the enantioselective synthesis of securinega alkaloids is presented. Crucial steps are a palladium-catalyzed enantioselective imide alkylation, a vinylogous Mannich reaction, and a ring-closing metathesis process. Through this strategy, the synthesis of (-)-norsecurinine has been accomplished in nine steps and 11% overall yield.  相似文献   
992.
Summary Radiochemical neutron activation procedures using liquid-liquid extraction with tetraphenylarsonium chloride in chloroform from 1M HCl and solid extraction with ALIQUAT 336 incorporated in a polyacrylonitrile binding matrix from 0.1M HCl were developed for accurate determination of rhenium in biological and environmental samples at the sub-ng . g-1 level. Concentrations of Re in the range of 0.1 to 2.4 ng . g-1 were determined in several botanical reference materials (RM), while in a RM of road dust a value of ~10 ng . g-1 was found. Significantly elevated values of Re, up to 90 ng . g-1 were found in seaweed (brown algae). Results for Re in the brown algae Fucus vesiculosus in which elevated 99Tc values had previously been determined suggested possible competition between Re and Tc in the accumulation process.  相似文献   
993.
Infant formula and powdered milk samples were suspended in a medium containing 20% ethanol, 0.65% nitric acid, 6% hydrogen peroxide and 0.1% ammonium dihydrogenphosphate and directly introduced in the electrothermal atomizer. Liquid milk samples were diluted with this suspension solution. The build-up of carbonaceous residues inside the atomizer was avoided by using a combination of hydrogen peroxide and nitric acid. Fast programs were used to simplify the heating cycles. Calibration was carried out using aqueous standards and the detection limits for copper and zinc were found to be 70 and 15 pg, respectively. The metal contents of several milk samples obtained by direct introduction of the samples agree with those obtained by means of a conventional procedure based on the total dissolution of the samples. The reliability of the methods was also confirmed by analyzing three certified reference materials. The free and protein binding fractions were measured by ultrafiltration membrane permeability of the milk samples.  相似文献   
994.
We have developed a simple, rapid, inexpensive method for the determination of benzo[a]pyrene (BP, a known carcinogen) in smoking-flavour agents (water-soluble liquid smoke; WSLS). After purification of the WSLS by a single passage through a Sep Pak C18 Plus cartridge, BP in the hexane eluate was determined by second derivative constant-wavelength synchronous spectrofluorimetry. Method precision (RSD < 6%) and recovery ( approximately 92%) were satisfactory, and the detection and quantification limits (1.05 and 2.28 mug kg(-1) respectively) indicated that the current maximum permissible concentration of BP in smoke flavourings (10 mug kg(-1)) can be monitored by this method.  相似文献   
995.
Zusammenfassung Zur Messung geringer226Radium- und222Radongehalte werden auch auf dem Gebiet der Strahlenhygiene verschiedene Verfahren verwendet. Eine zylindrische Szintillationskammer günstigster Abmessungen bietet die Möglichkeit, Personen, die226Ra inkorporiert haben, ohne vorhergehende Anreicherung des Atem-Radons zu kontrollieren sowie Lebensmittel und andere Proben von niedrigem226Ra- und222Rn-Gehalt zu analysieren.
Measurement of small amounts of radium and radon
Summary Various procedures employed for the measurement of slight226radium and222radon contents have also been used for the radiation hygiene. A cylindrical scintillation chamber of the most favorable dimensions offers the possibility of checking persons who have incorporated226Ra, without prior concentration of the breath-radon, and also in the analysis of foods and other samples that contain minute amounts of226Ra and222Ra.
  相似文献   
996.
Résumé On montre à partir de considérations théoriques qu'une molécule conjugée dont un substituant donneur d'électrons est le centre d'acidité doit devenir plus acide quand eile passe de l'état fondamental au premier état électronique excité. Le contraire doit se produire lorsque le substituant est accepteur d'électrons. Enfin une molécule conjugée contenant un hétéroatome centre de basicité doit devenir plus basique au cours de la mÊme transition. On suggère, par ailleurs, une interprétation du fait que dans cette série de molécules le premier état excité triplet possède une force acido-basique très differente de celle du premier état excité singlet.
Theoretical study on the pK of excited states
From theoretical considerations it is shown, that a conjugated molecule, in which an electron-donator is the center of acidity, the first excited state is more acid than the ground state. If the substituent is an electron-acceptor, then the contrary will happen. If a conjugated molecule has a heteroatom as center of basicity, then the first excited state is more basic than the ground state. Further an interpretation is proposed for the fact, that in this series of molecules the first singlet has an acido-basic force, which is quite different from that of the first triplet.

Zusammenfassung Mit Hilfe theoretischer überlegungen wird gezeigt, da\ ein konjugiertes Molekül, bei dem ein Elektronendonator als Substituent das AciditÄtszentrum ist, im ersten angeregten Zustand saurer ist, als im Grundzustand. Ist der Substituent ein Elektronenacceptor so tritt der entgegengesetzte Effekt ein. Besitzt ein konjugiertes Molekül ein Heteroatom als BasizitÄtszentrum so wird es bei demselben übergang basischer. Darüber hinaus wird eine Interpretation der Tatsache vorgeschlagen, da\ in dieser Reihe von Molekülen der erste Singulett-Zustand eine vom ersten Triplett-Zustand verschiedene SÄure-BasenstÄrke besitzt.
  相似文献   
997.
Extensive molecular dynamics simulations have been done to study the evaporation of a 13-atom Lennard-Jones cluster. The survival probability and the evaporative lifetime are calculated as a function of the cluster total energy from a classical trajectory analysis. The results are interpreted in terms of the RRK theory of unimolecular dissociation. The calculation of the binding energy of the evaporated species from the evaporation rate and the average kinetic energy release is discussed.  相似文献   
998.
Cyclic voltammetry and electron spin resonance (ESR) techniques were used in the investigation of several potential antiprotozoal thiosemicarbazones nitrofurane derivatives. A self-protonation process involving the protonation of the nitro group due to the presence of an acidic proton in the thiosemicarbazone moiety was observed in the first step of a CEE(rev) reduction mechanism of these derivatives. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. AM1 methodology was used to obtain the optimized geometries and UB3LYP calculations were performed to obtain the theoretical hyperfine coupling constants. The theoretical study exhibited an unusual assignment of the spin densities showing a free radical centered in the thiosemicarbazone moiety rather than the nitro which are in agreement with the experimental hyperfine pattern.  相似文献   
999.
The coupled-column (LC-LC) system, consisting of a first column packed with internal surface reversed phase (ISRP) (50 x 4.6mm ID) and a Chrompack C18 (100 x 4.6 mm ID) as second column, allowed the simultaneous determination of five benzoylurea insecticides in dichloromethane (CH2Cl2) extracts of vegetable samples without any clean-up step. This system was combined with a photochemically induced fluorescence (PIF) post-column derivatization in order to provide strongly fluorescent photoproducts from the non-fluorescent benzoylureas. Limits of detection ranged from 0.21 to 0.98 microg L(-1) of pesticide (equivalent to 0.14-0.65 microg kg(-1) in vegetable samples) and limits of determination ranged from 4.0 to 10.0 microg L(-1) (equivalent to 2.7-6.7 microg kg(-1)). Linearity of the method was established between 2 and 1800 microg L(-1), depending upon the compound. Validation of the total method was performed by randomly analyzing recoveries of four vegetable samples (aubergine, cucumber, green bean, and tomato) spiked at two levels of concentration (10.0 and 33.3 microg kg(-1)). The combination of the LC-LC system with PIF detection provides a sensitive, selective, and rapid method for the determination of pesticides in vegetable samples at levels lower than the maximum residue levels (MRLs) established for these compounds by Spanish legislation.  相似文献   
1000.
In this work, we report the synthesis and a study on the degree of electronic delocalization in the asymmetric mixed valence complexes [CpFe(C(8)H(6))Fe(C(8)H(7))](+), 3a(+), and [CpCo(C(8)H(6))Fe(C(8)H(7))](+), 3b(+), (Cp = C(5)Me(5), C(8)H(6) = pentalenyde, C(8)H(7) = hydropentalenyde, and = ((3,5(CF(3))(2)C(6)H(3))(4)B(-))). Electrochemical methods, (57)Fe M?ssbauer spectroscopy, electronic spectroscopy, and electron paramagnetic resonance were used for this purpose. Although the anti conformation of the complexes precludes any metal-metal interaction, all the techniques employed show that 3a(+) is a electronic delocalized system, while 3b(+) behaves as two individual metallic centers with localized electron density.  相似文献   
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