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201.
Zinc-copper couple in boiling methanol reduces alkynes to olefins in nearly quantitative yields. Terminal acetylenes are converted to terminal ethylenes, whereas disubstituted acetylenes are transformed to (Z)-olefins. 相似文献
202.
Abstract— A general method for the determination of the extent of the singlet oxygen (1O2) quenching by sensitizer in the dye-sensitized photooxygenation of olefins is used in the case of rose bengal (RB) in methanol and oil-soluble chlorophyll (M) in benzene with 2-methyl-2-pentene and tetramethylethylene as acceptors. Unlike RB, M which contains only a low percentage of pure chlorophyll (Chi), quenches 1O2. It is shown that this very cheap mixture can be used for kinetic studies and that the chemical quenching of 1O2 by M is very weak with respect to the physical quenching. The upper limit for the rate constants of physical and chemical quenching of 1O2 by Chi is estimated. 相似文献
203.
Charles M. Lindsay Keith Smith Charles Allan Brown Kathleen Betterton-Cruz 《Tetrahedron letters》1984,25(9):995-998
Unsymmetrical hexathioorthooxalates of types () and () undergo elimination of dialkyl disulfide on heating in an organic solvent; the reaction, which is catalyzed by acid, proceeds without fission of the central C:C bond and provides the first general, high yield synthesis of unsymmetrical benzotetrathiafulvalenes of types () and (). 相似文献
204.
A. Raith W. T. Perkins N. J. G. Pearce T. E. Jeffries 《Analytical and bioanalytical chemistry》1996,355(7-8):789-792
Laser ablation inductive coupled plasma – mass spectrometry (LA-ICP-MS) was used to analyse minor and trace elements in the hard parts of a shellfish, representing the environmental changes. A large, long-lived bivalve such as Arctica islandica can produce historical records of trace element fluctuations in seawater. Analytical traverses using LA-ICP-MS were performed from the inner to the outer wall of the shell at different locations. The development of the UV laser system now offers spatial resolution of craters of 10 μm diameter, enabling several analyses between the various growthbands. The elemental changes between growthbands of the shell are discussed, showing the potential for precise determination of heavy-metal pollution over the years. 相似文献
205.
Charles W. Jefford Manuel Jimenez Estrada Giacomo Barchietto Tho Berclaz Michel Geoffroy 《Helvetica chimica acta》1990,73(6):1653-1658
The photo-oxygenation of adamantylideneadamantane ( 1 ) on siliceous supports using admixed granules of ion-exchange resin fixed to methylene blue (MB) and rose bengal (RB) gave exclusively the corresponding dioxetane derivative 2 for the former sensitizer, while the latter gave 2 and traces of the epoxide 3. RB and the charge-transfer complex produced from N-ethylcarbazole and 2,4,5,6-tetranitrofluoren-9-one both reacted with chemically generated singlet oxygen to give superoxide radical anion. Trapping of the latter with 5,5-dimethyl-1-pyrroline 1-oxide gave an adduct exhibiting a characteristic ESR spectrum. The treatment of 1 in MeOH with 30% aqueous H2O2 for 22 h at 60° gave 3 in 100% yield. Repetition of this experiment in the presence of 2,6-di(tert-butyl)-p-cresol caused no significant change. These results indicate that singlet oxygen reacts with 1 , in the presence of RB, by two different processes. The first leads to dioxetane formation. The second process involves conversion of singlet oxygen by RB to superoxide radical anion which subsequently gives H2O2 so producing epoxide 3 from 1 . 相似文献
206.
Charles Vassallo 《Optical and Quantum Electronics》1985,17(6):411-426
The effect of curvature on leaky modes with low losses in weakly guiding optical fibres is investigated, within a theoretical model where the cladding (outer cladding in case of multiclad fibres) is extended to infinity. Such modes already radiate rays in the cladding when the fibre is straight and two cases can be considered according to whether this ray pattern is modified near the core area in the bent fibre (strong curvature) or is not modified (weak curvature). A general loss formula is given for the case of weak curvature. The rays which are emitted in the curvature plane, towards the inner side of the bend are captured later on by the core and induce an oscillating modulation of the leakage loss, with an amplitude varying asR
–1/2 but limited to a few per cent in practical cases (for weak curvatures). The case of strong curvatures is analysed in the special case of the LP 01 mode in fibres with a large depressed inner cladding, through a 2-D ray method. It is found that the loss oscillations are rapidly damped while the average loss is considerably increased. 相似文献
207.
208.
Miscible blends through hydrogen bonding have been intensively studied. The effects of a variety of miscible hydrogen bonded polymer blends on properties such as thermal and thermal oxidative stability, moisture sensitivity, modulus and glass transition temperature are discussed. In addition, the preparation of semi-interpenetrating polymer networks (IPNs) and studies of the effect of crosslinking on the miscibility in hydrogen bonded polymer blends are reviewed. 相似文献
209.
210.
An infrared spectral investigation of a number of pyrimidine and pyrimidone hexachloroantimonates is reported. Typical aromatic absorption bands found in pyrimidinium salts were absent in the pyrimidone cations. 2-And 4-Pyrimidone hexachloroantimonates, as well as their N-methyl- and N,N'-dimethyl derivatives, exhibit intense carbonyl absorption bands between 1720–1729 cm?1 (solid state) which corroborates N-protonation of pyrimidones. 相似文献