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21.
Abstract— Oxygen effects on the photocyclization of N-methyldiphenylamine to N-methylcarbazole were investigated in n-hexane, water, and aqueous surfactant solutions by steady state irradiations and flash photolysis measurements. The reaction sequence in micelles was found to involve the same intermediate steps as in homogeneous solutions. In aerated micellar solutions, the quantum yield of N-methylcarbazole is significantly higher than in n-hexane, while the rate constants of the unimolecular reaction steps show no solvent dependence. The bimolecular dehydrogenation of the intermediate 4a, 4b-dihydro-N-methylcarbazole by oxygen is enhanced in aqueous and micellar solutions, whereas the quenching rate of triplet intermediates by oxygen was not affected. The lesuhs are interpreted using a dispersed phase model of micellar solutions. Special 'micellar effects' need not be invoked since the dependence of the quantum yield on the solvent is shown to be due to the difference in the overall oxygen concentration. 相似文献
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Christophe Curty Norbert Engel Jos Iturraspe Albert Gossauer 《Photochemistry and photobiology》1995,61(6):552-556
Photooxygenation of (pyropheophorbidato a methyl ester)cadmium (II) was studied using 18,18O2 labeling of the molecular oxygen required for cleavage of the macrocycle. After reductive demetallation of the primary oxidation product (4,5-dioxo-4,5-secopyropheophorbidato a methyl ester)cadmium (II), the isotope content of formylbilinone 4a was analyzed by repeated-scan fast atom bombardment mass spectrometry. Comparison of the spectroscopic data of the labeled pigment 4a with the statistical probabilities of18 O isotope incorporation calculated for four possible reaction mechanisms clearly proves that photooxidative ring cleavage occurred by the one-molecule mechanism, i.e. the terminal oxygen atoms of 4a were derived from one oxygen molecule. Furthermore, a study of the exchange of the18 O-labeled atoms revealed that no exchange occurs within the pH 4.5–9.5 range. In stronger alkaline or acidic solutions, only the oxygen atom of the formyl group is exchanged. Hydrolysis of the methyl ester group of 4a was achieved, without loss of the18 O label on the formyl group, at pH 7.2 in the presence of pig liver esterase. 相似文献
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26.
P. Hoffmann V. K. Karandashev T. Sinner H. M. Ortner 《Analytical and bioanalytical chemistry》1997,357(8):1142-1148
Data are reported for a first measurement campaign for a great number of cations (the range of concentrations for main components
is given in ng/mL: Na:<70–880; Mg: 12–160; Al:<7–120; K: 200–1600; NH4
+: 2500–9000; Ca: 300–2500; Fe:<140–1200; Zn: 20–320) and anions (HCOO-: 100–1800; CH3COO-: 400–14500; C2O2-
4: <100–460; NO-
2: <20–55; NO-
3: 660–9900; SO2-
3:<100–1150; SO4
2-: 450–19700; Cl-: 170–3200) in rain and snow samples of the Chernogolovka region 80 km northeast of the centre of Moscow. These data serve
as a first assessment of the situation of the atmosphere in this region. For the determination of fluoride, formate, acetate,
nitrite, bromide, nitrate, sulfite, and sulfate by IC, a relevant development of the method was carried out. For some metallic
cations, a critical quality control was achieved by parallel measurements with ICP-MS, TXRF, and IC. Furthermore, time resolved
measurements were performed for a rain event. Filtration experiments were undertaken to differentiate between the presence
of a series of elements and species in liquid or solid state in the collected aqueous samples.
Received: 25 July 1996 / Revised: 26 September 1996 / Accepted: 13 October 1996 相似文献
27.
A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst. 相似文献
28.
Nowadays, about 150 natural products comprising an allenic or cumulenic structure are known. The chemistry of these compounds has turned out to be a very attractive and prolific area of interest: advances in the isolation and characterization of new allenic natural products have led to the establishment of efficient synthetic procedures which in many cases also open up an access to enantiomerically pure target molecules. Inspired by the intriguing biological activities of many allenic natural products, allene moieties are now systematically introduced in pharmacologically active classes of compounds (steroids, prostaglandins, amino acids, nucleosides). The functionalized allenes thus obtained often exhibit impressive activities as mechanism-based enzyme inhibitors, cytotoxic, or antiviral agents. A prerequisite for further developments in this field is the efficient stereoselective synthesis of allene derivatives. 相似文献
29.
Jin R Jureller JE Kim HY Scherer NF 《Journal of the American Chemical Society》2005,127(36):12482-12483
Femtosecond laser excited second harmonic (SH) activity from single Ag nanoparticles is reported. A correlation of SH single-particle measurements with high-resolution imaging of particle morphology by TEM was achieved by creating position markers on an optical and electron transparent substrate (Si3N4 thin film, approximately 100 nm). We compared the SH activity of single Ag nanoparticles (nanospheres versus nanorods) and cluster structures (composed of two or multiple particles, e.g., dimers and trimers). The direct correlation of single-particle structures and SH activity, spectral and power dependence, strongly suggests one-photon resonant driven nonlinear oscillator response mechanism. 相似文献
30.
Heinz Falk Gabriele Streßler Norbert Müller 《Monatshefte für Chemie / Chemical Monthly》1988,119(4):505-508
Using a specific force field model, relative stabilities of 2,2-bipyrrole and 2,2-furylpyrrole conformers as well as of prodigiosene tautomers, configurations, and conformations, are studied. 2,2-Bipyrrole adopts anantiperiplanar conformation in accord with other theoretical calculations and experimental findings. Its rotational barrier calculated by this method compares favourably to the one derived by other means. For the parent prodigiosene the (Z)sp,sp arrangement is found to be the most stable one. The two possible tautomers within its dipyrrin fragment do not show a significant energy difference. 相似文献