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991.
992.
Tatiane de A. Maranhão Edmar Martendal Daniel L.G. Borges Eduardo Carasek Bernhard Welz Adilson J. Curtius 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007
Cloud point extraction (CPE) is proposed as a pre-concentration procedure for the determination of Pb and Cd in undigested urine by graphite furnace atomic absorption spectrometry (GF AAS). Aliquots of 0.5 mL urine were acidified with HCl and the chelating agent ammonium O,O-diethyl dithiophosphate (DDTP) was added along with the non-ionic surfactant Triton X-114 at the optimized concentrations. Phase separation was achieved by heating the mixture to 50 °C for 15 min. The surfactant-rich phase was analyzed by GF AAS, employing the optimized pyrolysis temperatures of 900 °C for Pb and 800 °C for Cd, using a graphite tube with a platform treated with 500 μg Ru as permanent modifier. The reagent concentrations for CPE (HCl, DDTP and Triton X-114) were optimized using a Box–Behnken design. The response surfaces and the optimum values were very similar for aqueous solutions and for the urine samples, demonstrating that aqueous standards submitted to CPE could be used for calibration. Detection limits of 40 and 2 ng L− 1 for Pb and Cd, respectively, were obtained along with an enhancement factor of 16 for both analytes. Three control urine samples were analyzed using this approach, and good agreement was obtained at a 95% statistical confidence level between the certified and determined values. Five real samples have also been analyzed before and after spiking with Pb and Cd, resulting in recoveries ranging from 97 to 118%. 相似文献
993.
FTIR methods for measuring optical rotatory dispersion (ORD) and circular dichroism (CD) were developed. The measurements of both phenomena are accomplished by means of various techniques. The study of ORD makes use of linearly polarized IR radiation while that of CD requires a circular polarizer (retarder) able to convert linearly polarized IR radiation into circularly polarized one. Examples of induced cholesteric solutions are shown. 相似文献
994.
Hildebert Wagner Bernhard Kreher Hermann Lotter Matthias O. Hamburger Geoffrey A. Cordell 《Helvetica chimica acta》1989,72(4):659-667
Beside the known naphthoquinones, dehydro-α-lapachone ( 17 ) and lapachol ( 20 ), four new naphtho[2,3-b]-furan-4,9-diones, i.e. the 2-acetyl-5-hydroxy. 2-acetyl-8-hydroxy. (?)-5-hydroxy-2-(1′-hydroxyethyl), and (±)-8-hydroxy-2-(1′-hydroxyethyl) derivatives 16, 15, 12 , and 13 , respectively, and the new compound benzo[b]furan-6-carboxaldehyde ( 8 ) have been isolated from a CHCl3 extract of the inner stem bark of Tabebuia avellanedae LORENTZ ex GRISEB ., together with four known naphthofurandiones, a dihydroisocoumarin derivative, (?)–6-hydroxymellein, and five benzoic-acid and three benzaldehyde derivatives which have not been reported previously from this plant. Structure determination of the isomeric 5- and 8-hydroxynaphtho[2,3?b]furan-4,9-diones was carried out unambiguously by a combination of selective-INEPT experiments and X-ray crystallographic analysis. 相似文献
995.
The bicyclic bisureas 2 react with triflic anhydride to give the dication ether salts 4 . Further transformation of the remaining carbonyl groups of 4 into bis-carbenium ether linkages was not possible. The monocyclic bisurea 8 yields the dicationic heterocycle 9 in an intramolecular fashion. The structure of 9 , which is the first dication ether salt with an endocyclic ether linkage, was established by independent synthesis from 10 as well as by its hydrolysis to give the hemi-protonated bisurea 11 . 相似文献
996.
Faizulla G. Kathawala Bernhard Prager Kapa Prasad Oljan Repi
Michael J. Shapiro Russell S. Stabler Leo Widler 《Helvetica chimica acta》1986,69(4):803-805
The reduction of δ-hydroxy-β-ketoesters 1 was investigated with three different reducing agents. In several instances, high selectivity in favor of syn-1,3-diols was observed. 相似文献
997.
Bernhard Strauß Wolfgang Linert Viktor Gutmann Reginald F. Jameson 《Monatshefte für Chemie / Chemical Monthly》1992,123(6-7):537-546
Summary The iron(II) complexes of the nitrogen donor ligand 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) have been investigated in nonaqueous solvents using various spectrophotometric methods. Measurements in methanol at 20°C revealed thatbzimpy can act as a bi-or tridendate ligand towards iron(II) in this solvent, forming [Fe(bzimpy)]2+ (tridendate, logK=5.54), [Fe(bzimpy)2]2+ (bistridentate, logK=4.12) and [Fe(bzimpy)3]2+ (tris-bidentate, logK=3.85). The pronounced thermochromism of the compound is shown to be the result of both spin-crossover and dissociation equilibria. Furthermore, in solvents of higher donor numbers (30), partial or complete deprotonation of the complex [Fe(bzimpy)3]2+ is observed. Triethylamine added stepwise to methanolic solutions causes successive deprotonation.This paper is dedicated to Professor Dr. Wolfgang Beck on the occasion of his 60th birthday with warmest personal wishes 相似文献
998.
It is shown that 2-allylated anilines (cf. Schemes 2–4, 7, and 8) on irradiation in protic solvents such as H2O. MeOH, and EtOH in the presence of H2SO4 undergo a novel photosolvolysis reaction to yield specifically trans-2-hydroxy- and trans-2-alkoxy-1-methylindanes. Intermediates are presumably tricyclo[4.3.0.01,8]nona-2,4-dienes formed in an intramolecular [2s + 2s] cycloaddition reaction (cf. Scheme 7). On the other hand, N,N,N-trimethyl-2-(1′-methylallyl)anilinium salts 18 (Scheme 6) and 2-(3′-butenyl)-N,N-dimethylaniline ( 17 ) lose on irradiation in MeOH or H2SO4/MeOH the ammonium group reductively to yield (1-methylallyl)benzene ( 19 ) and 1-methylindane ( 20 ), respectively. 相似文献
999.
The kinetics of the reactions of benzhydrylium ions and quinone methides with eight tertiary phosphanes and two phosphites were investigated photometrically. The nucleophilicity parameters N and slope parameters s of these nucleophiles were derived according to the equation log k(20 degrees C) = s(N + E). Correlations of the nucleophilicity parameters N with pK(Ha) and sigma(p) values as well as with the rate constants of reactions with other electrophiles are discussed. In some cases, equilibrium constants for the formation of phosphonium ions were measured, which allow one to determine the Marcus intrinsic barriers of DeltaG(0) (not equal) = 58 kJ mol(-1) for the reactions of triarylphosphanes with benzhydrylium ions. The N parameters [5.51 for P(OPh)3, 10.36 for P(OBu)3, 14.33 for PPh3, 15.49 for PBu3, 18.39 for P(4-Me2NC6H4)3] are compared with the reactivities of other classes of nucleophiles (see, www.cup. uni-muenchen.de/oc/mayr). 相似文献
1000.
Bernhard Heß 《manuscripta mathematica》1977,22(3):209-212
Leptin posed in [1] the problem to determine the class [W] of locally compact groups G characterized by the following property: Every proper closed two-sided idealJ in the Banach-*-algebraL
1(G) is annihilated by some nondegenerate continuous *-representation ofL
1(G) in a Hilbert space. Our main result: A locally compact group G, which is representable as a projective limit of a system of factor groups G/k, k compact normal subgroups, lies in [W] if and only if all the G/k are in [W]. 相似文献