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951.
Nilo Zanatta Fabio M. da Silva Luciana S. da Rosa Louíse Jank Helio G. Bonacorso Marcos A.P. Martins 《Tetrahedron letters》2007,48(37):6531-6534
A simple one-pot procedure to obtain a series of new 3-alkoxy-3-cyanocarboxylic acids from the reaction of 4-alkoxy-1,1,1-trichloro-but-3-en-2-ones with sodium cyanide is described. 相似文献
952.
A procedure for the determination of trace level of copper(II) and cadmium(II) by FAAS using an on-line preconcentration system has been proposed. In this system, copper and cadmium ions were adsorbed onto a minicolumn packed with silica gel modified with niobium(V) oxide (Nb2O5-SiO2), followed by nitric acid elution in reverse mode and determination on-line by flame atomic absorption spectrometry (AAS) without interference of the matrix. Chemical and flow variables as well as concomitant ions were studied in the developed procedure. The enrichment factor for copper(II) and cadmium(II) was 34.2 and 33.0, respectively, using a preconcentration time of 2 min. The limit of detection for copper(II) and cadmium(II) was 0.4, and 0.1 μg l−1, respectively. The precision of the method, evaluated as the relative standard deviation in solutions containing 15 μg l−1 of copper and 10 μg l−1 of cadmium, by analyzing a series of seven replicates, was 1.8 and 1.6%, respectively. The accuracy was assessed through recovery experiments of certified material and water samples. 相似文献
953.
Organocyanamides, Ntbd1;CNR(2) (R = Me or Et), react with trans-[Mo(N(2))(2)(dppe)(2)] (1, dppe = Ph(2)PCH(2)CH(2)PPh(2)), in THF, to give the first mixed molybdenum dinitrogen-cyanamide complexes trans-[Mo(N(2))(NCNR(2))(dppe)(2)] (R = Me 2a or Et 2b) which are selectively protonated at N(2) by HBF(4) to yield the hydrazide(2-) complexes trans-[Mo(NNH(2))(NCNR(2))(dppe)(2)][BF(4)](2) (R = Me, 3a, or Et, 3b). On treatment with Ag[BF(4)], oxidation and metal fluorination occur, and the ligating cyanamide undergoes an unprecedented beta-protonation at the unsaturated C atom to form trans-[MoF(NCHNR(2))(dppe)(2)][BF(4)](2) (R = Me, 4a, or Et, 4b) compounds which present the novel amidoazavinylidene (or amidomethyleneamide) ligands. Complexes 4 are also formed from the corresponding compounds 3, with liberation of ammonia and hydrazine. The crystal structure of 2b was determined by single-crystal X-ray diffraction analysis which indicates that the N atom of the amide group has a trigonal planar geometry. 相似文献
954.
Berenice da Silva Junkes 《Analytica chimica acta》2003,477(1):29-39
The novel semi-empirical topological index (IET), previously developed by Heinzen and Yunes, was extended to predict the chromatographic retention of saturated alcohols on low polarity stationary phases (OV-1). The predictive ability of IET was also verified on stationary phases of different polarity (SE-30, OV-3, OV-7, OV-11, OV-17 and OV-25). Simple linear regressions between the retention indices and the semi-empirical topological index (RI=a+bIET) were established for each stationary phase separately, showing good statistical parameters. Statistical analysis showed that the QSRR model used on stationary phases of low polarity (OV-1) has high internal stability and good predictive ability for external groups. The polarity of the SE-30, OV-3, OV-7, OV-11, OV-17 and OV-25 stationary phases, indicated by retention polarity (PR) given by Tarján et al., is reflected in the ‘a’ (intercept) and ‘b’ (slope) coefficients of the equations obtained for each of these phases. The linear relationship between the ‘a’ coefficient and PR showed satisfactory statistical quality. Thus, it was possible to generate a single combined model of QSRR, including a parameter that represents the property of the stationary phase PR. The combined model also has a satisfactory predictive quality, as shown by the plot of calculated versus experimental retention indices for saturated alcohols on six stationary phases of different polarity (r2=0.9956; S.D.=9.54). 相似文献
955.
Pierre Juanda Jean Louis Sebedio William W. Christie 《Journal of separation science》1994,17(5):321-324
A method has been developed for separation of linolenic acid and its seven isomers by HPLC on a silver-ion-loaded column. The standard 18:3 isomers, isolated from a heated linseed oil or prepared by isomerization of linolenic acid, were converted into phenacyl esters and detected by UV at 238 nm. The use of low temperature (10 °C) combined with a gradient of dichloromethane and methanol enabled separation of all the cis/trans isomers. The peaks were identified by comparison of ECL values with those of a standard mixture, by chromatographing collected HPLC fractions on a polar GC column. HPLC quantification was compared with GC analysis. There was satisfactory agreement between the tow methods. This method could be used for seperation, collection and quantification of 18:3 fatty acids with trans double bonds in different oils and foods. 相似文献
956.
da Silva VM da Cunha Veloso MC de Oliveira AS Santos GV de P Pereira PA de Andrade JB 《Talanta》2005,68(2):323-328
Brominated phenols 2- and 4-bromophenol (2-BP and 4-BP); 2,4- and 2,6-dibromophenol (2,4-DBP and 2,6-DBP) and 2,4,6-tribromophenol (2,4,6-TBP) have been identified as key flavor compounds found in seafoods. Depending on their concentrations, they were responsible for marine or ocean flavor (shrimp/crab/fish/sea salt-like) or for phenolic/iodine/iodoform-like off-flavor. In this work a new analytical methodology was developed to determine, simultaneously, such bromophenols in fish meats, based on reversed-phased high-performance liquid chromatographic separation (RP-HPLC). The separation of bromophenols was made onto a Lichrospher 100 RP-18 column using water:acetonitrile gradient at a flow rate of 1.0 mL min−1, using absorbance detection at 286 nm, were the 2-BP, 4-BP, 2,4- and 2,6-DBP show significant absorbtivity values and at 297 nm for 2,4,6-TBP. They were separated in 20 min with a good chromatographic resolution (Rs) for the isomeric compounds: 2- and 4-BP, Rs = 1.23; 2,4- and 2,6-DBP, Rs = 1.63. The calibration curves were linear in the bromophenols concentration range of 200.0-1000 ng mL−1. Under optimized conditions, the detection limit of the HPLC method was 127 ng mL−1 for 2-BP; 179 ng mL−1 for 4-BP; 89.0 ng mL−1 for 2,4-DBP; 269 ng mL−1 for 2,6-DBP and 232 ng mL−1 for 2,4,6-TBP. This method has been applied in determination of bromophenols, isolated by combined steam distillation-solvent extraction with 2 mL of pentane/diethyl ether (6:4), from Brazilian fishes samples, collected on the Atlantic coast of Bahia (13°01′S and 38°31′W), Brazil. The concentration range determined were 0.20 ng g−1 (2-BP) to 299 ng g−1 (2,4,6-TBP). The method proposed here is rapid and suitable for simultaneous quantification of simple bromophenols in fish meat. As long as we know, it is the first analytical methodology, using RP-HPLC/UV, which was developed to determine simple bromophenols in fish meat. 相似文献
957.
Analysis of application of Langmuir isotherm to heterogeneous systems: high-pressure conditions 总被引:2,自引:0,他引:2
Cordeiro CN da Rocha MS Faleiros AC Iha K 《Journal of colloid and interface science》2005,286(2):459-461
In this paper, an adsorption model that is based on the statistical mechanics approach was applied to study the saturation phenomena in adsorption to calculate the minimum pressure needed to attain the complete surface coverage on a physical adsorption. The fundamental integral equation, Theta(T)(Q)=integral N(Q)Theta(Q)dQ, for the calculation of the coverage degree of the surface was developed for an exponential distribution function, N(Q)=(m/RT)exp(-mQ/RT), and the representation of local adsorption sites is given by the Langmuir expression, Theta(Q)=b(0)C exp(Q/RT)/(1+b(0)C exp(Q/RT)). At high values of the pressure C, a solution of the fundamental integral equation was obtained by imposing the condition b(0)C>1. The expression for the saturation condition, b(0)C>m/(m+1), was obtained; that is, the saturation phenomenon is dependent on two parameters, correlated with the energetic heterogeneity and adsorption energy of the system. The pressure in the analysis of the cited expression shows that, for low m values (more heterogeneous systems), saturation is attained for b(0)C>m, while for m congruent with 1 (more homogeneous systems), the saturation is attained for b(0)C>0.5. 相似文献
958.
Moro AV Nogueira CW Barbosa NB Menezes PH da Rocha JB Zeni G 《The Journal of organic chemistry》2005,70(13):5257-5268
We present here the reaction of diorganoyl dichalcogenides with terminal alkynes under catalyst-free conditions, by a one-pot procedure, to prepare bis- and tris-chalcogenide alkenes selectively, avoiding the previous preparation of chalcogen alkynes. The reaction proceeded cleanly under mild reaction conditions, and the addition of dichalcogenides to alkynes occurred stereoselectively to give exclusively the corresponding Z isomers. We observed that the selectivity control was governed by the effective participation of the hydroxyl group from propargyl alcohols. In addition, the bis-chalcogenide alkenes were exclusively obtained with propargyl alcohol having the acidic hydroxyl group proton. Conversely, the alkynes with no potentially acidic hydroxyl group proton, at propargyl positions, gave exclusively the tris-chalcogenide alkenes. 相似文献
959.
A series of cis nitrosyl complexes containing polypyridyl ligands were prepared and characterized as cis-[RuL(bpy)2(NO)](PF6)3 (L = pyridine, 4-picoline, or 4-acetylpyridine), by elemental analysis, u.v.–vis. and i.r. spectroscopy, and by electrochemical techniques such as cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and coulometry. The complexes exhibit stretching frequencies (NO) at ca. 1950 cm–1 indicating that nitrosyl group has a sufficiently high degree of nitrosonium ion (NO+) character. In non-aqueous solution, the reduction of these complexes induce nitrosyl to nitro conversion. In aqueous solution the reduction product is cis-[RuL(bpy)2(NH3)]2+ formed by a six electron mechanism. The nitrosyl compounds are susceptible to nucleophilic attack by hydroxide ion. The equilibrium constants were determined. 相似文献
960.
A module for continuous high pressure-temperature leaching is proposed. The overall approach involves the coupling of an extractor with a continuous-flow manifold where the analyte forms a complex with SCN(-), which is driven to a flow-cell packed with a suitable material for retention and continuous monitoring of this process which in turn is a way for indirect monitoring of the leaching kinetics. The approach has been used for the determination of iron in soil after leaching and derivatization. The sensitivity of the method (ng level) and the short time required (only 15 min versus 24 h for leaching and 30 min for the measurement step required by the conventional method), together with its precision (RSD less than 9%) makes this method an excellent alternative to the conventional procedure. 相似文献