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171.
Kim CH  Alexander PW  Smythe LE 《Talanta》1976,23(3):229-233
Molybdenum is extracted as the thiocyanate complex with the quaternary long-chain aliphatic amine Aliquat 336 in chloroform, followed by evaporation of the solvent, dissolution in MIBK, and atomic-absorption spectroscopy. The method is simple, rapid and sensitive, with few interference problems for the determination of the Mo content of soils and sediments in the range 0.1-1.0 ppm with a relative standard deviation better than 5% when 1-g samples are used. Quantitative extraction from large volumes of aqueous solution has also been confirmed, allowing the determination of Mo in natural waters in the ppM range.  相似文献   
172.
A laser-induced fluorescence in graphite furnace (LIF-GF) set-up has been equipped with an intensified CCD detector (ICCD) that enables simultaneous multichannel detection of large wavelength regions. The main advantages of such a system in comparison with conventional photomultiplier detection are: simultaneous detection of several fluorescence wavelengths for easy characterization of excitation and fluorescence spectra and for an increase of the absolute sensitivity and spectral selectivity; simultaneous monitoring of background signals, such as those due to matrix interferences, blackbody radiation and scattered laser light; decrease of the susceptibility to radio-frequency pick-ups emitted from the pump laser due to the delayed read-out procedure; time-resolved studies of fluorescence spectra for improved elemental selectivity or for studies of atomization processes, and a possibility to perform two-dimensional imaging of height distributions of atomization and, in combination with an imaging spectrometer, diffusion processes in the furnace. The first work on LIF-GF with ICCD detection has been performed on Ni. The most sensitive and versatile excitation and detection wavelengths have been identified. Detection limits in water solutions by the LIF-GF technique have been improved by two orders of magnitude and are found to be 0.015 pg with ICCD and 0.01 pg using a photomultiplier at the most sensitive excitation and detection wavelengths. Nickel in concentrations has been detected in aqueous standard reference samples with sodium concentrations ranging from to % (riverine water and estuarine water) with good accuracy and precision. The Ni contents of standard riverine and estuarine water were determined to 1.00 ± 0.11 and 0.75 ± 0.07 ng/ml, respectively. The certified concentrations are 1.03 ± 0.10 and 0.743 ± 0.078 .  相似文献   
173.
A series of structurally related binuclear metallacycles [Cd(NO(3))(2)L](2), where L is an angular exo-bidentate ligand, have been synthesized. Each metallacycle contains two coordinatively unsaturated, chiral metal centers within a single molecule, and the assembly of these metallacycles into polymeric framework structures has been studied systematically for the first time. Stereoselective homochiral association of [Cd(NO(3))(2)L](2) leads to the formation of helical coordination polymers, whereas meso type association results in nonhelical chain structures. The type of stereoselective aggregation depends on the conditions of self-assembly as well as on ligand functionality. Both helical and nonhelical polymeric complexes have been isolated for the metallacycle [Cd(NO(3))(2)(2,4'-pyacph)](2) (2,4'-pyacph = 2,4'-(4-ethynylphenyl)bipyridyl). Homochiral association results in the formation of helical [Cd(NO(3))]( infinity ) chains which link the binuclear [Cd(NO(3))(2)(2,4'-pyacph)](2) metallacycles into racemic two-dimensional sheets which contain both P and M [Cd(NO(3))]( infinity ) helices. In contrast, meso-association leads to the formation of nonhelical one-dimensional chains. It is shown that the product of homochiral association is predominately formed at room temperature and that of meso-association is generated at elevated temperatures. Thus, it may be concluded that the homochiral association appears to be energetically less favorable than the meso-association, a conclusion that has been confirmed by theoretical calculations of the crystal lattice energy. Several high-yield syntheses of bipyridyl-type ligands used for metallacyclic assembly are also reported.  相似文献   
174.
2-Fluoropyrroles were synthesised via 1,3-dipolar cycloaddition of iminiodifluoromethanides derived from corresponding imines and difluorocarbene to dimethyl acetylenedicarboxylate.  相似文献   
175.
Cationic Rh(III) complex [Cp(PMe(3))Rh(SiPh(3))(CH(2)Cl(2))]BAr(4)' (1) activates the carbon-carbon bond of aryl and alkyl cyanides (R-CN, where R = Ph, (4-(CF(3))C(6)H(4)), (4-(OMe)C(6)H(4)), Me, (i)Pr, (t)Bu) to produce complexes of the general formula [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'. With the exception of the (t)BuCN case, every reaction proceeds at room temperature (t(1/2) < 1 h for aryl cyanides, t(1/2) < 14 h for alkyl cyanides). A general mechanism is presented on the basis of (1) an X-ray crystal structure determination of an intermediate isolated from the reaction involving 4-methoxybenzonitrile and (2) kinetic studies performed on the C-C bond cleavage of para-substituted aryl cyanides. Initial formation of an eta(1)-nitrile species is observed, followed by conversion to an eta(2)-iminoacyl intermediate, which was observed to undergo migration of R (aryl or alkyl) to rhodium to form the product [Cp*(PMe(3))Rh(R)(CNSiPh(3))]BAr(4)'.  相似文献   
176.
The formation of a new ternary ion-associate complex of vanadium(V) with 2,3-dihydroxynaphthalene and iodonitrotetrazolium chloride with a composition ratio of 1:2:1 is reported. The complex is quantitatively extracted from water into chloroform. The molar absorptivity (ɛ) of the extract at λ max=340 nm is 2.5 × 104 dm3/mol cm, and Beer’s law is obeyed for concentrations ranging from 0.1 to 0.9 μg/cm3 V(V). The following constants are determined: the extraction constant, the association constant, the distribution constant, and the recovery factor. The effects of foreign ions and reagents are studied. A selective and sensitive method is developed for determination of vanadium in steels.  相似文献   
177.
178.
The [Co2(CO)8]-mediated retro-Diels-Alder reaction of the annelated barrelenes 1 afforded the 1H-indol-2(3H)-one derivatives 3 (Scheme 1), while the hydrobarrelene 4a , under the same conditions, was converted to the anilide 6 (Scheme 2); 4b remained unaffected. The direct irradiation of 1 led to the annelated cyclooctatetraenes 7 (Scheme 3). On irradiation in the presence of excess of [Fe(CO)5], 1a , 1b , and 4a gave the tricarbonyliron complexes 8 , 9 , and 11 , respectively (Schemes 3 and 4); under these conditions, 4b was inert.  相似文献   
179.
The simulation of transport to double microband electrodes in generator–collector mode is reported focusing especially on the ‘titration curve’ approach to electroanalysis in which a titrant is electrogenerated from a redox active precursor on the generator electrode and reacts homogeneously with the target analyte. The current on the detector electrode reflects the amount of titrant ‘surviving’ passage between the two electrodes. The form of the titration curve – plots of detector current as a function of generator current – is shown to be highly sensitive to the electrode kinetics of the redox couple driven at the generator electrode. Accordingly the naïve use of such methodology for analysis without accompanying simulation and kinetic analysis is fraught with danger. Use of the conformal mapping approach in combination with the ADI method for investigation of the ‘titration’ current distributions at the double band system gives fast and precise simulation of this and similar problems. Convergence analysis is described which allows for the automatic selection of the simulation grid size so as to obtain a chosen accuracy (for example 1%) of the current for all experimentally meaningful values of the geometrical and physico-chemical parameters of the system to be investigated.  相似文献   
180.
Structural and mechanistic aspects of orthoplatination of acetophenone and benzaldehyde oximes by the platinum(II) sulfoxide and sulfide complexes [PtCl(2)L(2)] (2, L = SOMe(2) (a), rac-SOMePh (b), R-SOMe(C(6)H(4)Me-4) (c), and SMe(2) (d)) to afford the corresponding platinacycles cis-(C,S)-[Pt(II)(C(6)H(3)-2-CR'=NOH-5-R)Cl(L)] (3, R, R' = H, Me) have been investigated. The reaction of acetophenone oxime with sulfoxide complex 2a in methanol solvent occurs noticeably faster than with sulfide complex 2d due to the fact that the sulfoxide is a much better platinum(II) leaving ligand than the sulfide. Evidence is presented that the orthoplatination is a multistep process. The formation of unreactive dichlorobis(N-oxime)platinum(II) cations accounts for the rate retardation by excess acetophenone oxime and suggests the importance of pseudocoordinatively unsaturated species for the C-H bond activation by Pt(II). A comparative X-ray structural study of dimethyl sulfoxide platinacycle 3b (R = R' = Me) and its sulfide analogue 3e (R = H, R' = Me), as well as of SOMePh complex 3c (R = H, R' = Me), indicated that they are structurally similar and a sulfur ligand is coordinated in the cis position with respect to the sigma-bound phenyl carbon. The differences concern the Pt-S bond distance, which is notably longer in the sulfide complex 3e (2.2677(11) A) as compared to that in sulfoxide complexes 3b (2.201(2)-2.215(2) A) and 3c (2.2196(12) A). Whereas the metal plane is practically a plane of symmetry in 3b due to the H-bonding between the sulfoxide oxygen and the proton at carbon ortho to the Pt-C bond, an S-bonded methyl of SOMePh and SMe(2) is basically in the platinum(II) plane in complexes 3c and 3e, respectively. There are intra- and intermolecular hydrogen bond networks in complex 3b. An interesting structural feature of complex 3c is that the two independent molecules in the asymmetric unit of the crystal reveal an extremely short Pt-Pt contact of 3.337 A.  相似文献   
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