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11.
Sérgio L. C. Ferreira Neyla M. L. Araújo Adriana B. Santos Alailson F. Dantas A. C. Spinola Costa 《Mikrochimica acta》1995,118(1-2):123-129
TAN reacts with zinc(II) forming a red complex with composition 1:2 Zn(II)-TAN and absorption maximum at 582 nm. Zinc can be determined with this reagent in the presence of Triton X-100, in the pH range 6.20-8.00 with a molar absorptivity of 4.5×104 l/mol/cm Beer's Law was obeyed up to least 1.55 g/ml. Copper interference was eliminated with a mixture of thiosulfate and ascorbic acid and nickel separated by precipitation with dimethylglyoxime. The proposed method was used for zinc determination in several copper-base alloys and the results of analysis in comparison with certified values indicated that the procedure was accurate and precise. A derivative procedure is also proposed, allowing zinc determination with high sensitivity (5-400 ng/ml). 相似文献
12.
13.
G. J. T. Fernandes A. S. Araújo V. J. Fernandes Jr. Cs. Novák 《Journal of Thermal Analysis and Calorimetry》2004,75(2):687-692
Thermogravimetry is proposed to study the alumina catalyst regeneration deactivated by coke, after being used in the transformation
of styrene in a fixed bed continuous flow reactor. The model-free kinetic approach has been applied to data for the thermal
oxidation of carbonaceous deposits on the catalyst. The activation energy (E) was calculated as a function of a (conversion) and T (temperature), by using Vyazovkin model-free kinetic method, allowing to estimate time required to remove coke at a given
temperature.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
14.
Stasch A Ferbinteanu M Prust J Zheng W Cimpoesu F Roesky HW Magull J Schmidt HG Noltemeyer M 《Journal of the American Chemical Society》2002,124(19):5441-5448
The reaction of the acetylene RC triple bond CH (R = Ph, CH(2)SiMe(3)) with an excess of AlH(3).NMe(3) in boiling toluene leads to the carbaalane [(AlH)(6)(AlNMe(3))(2)(CCH(2)R)(6)] (R = Ph 1, CH(2)SiMe(3) 2) in good yield. Treatment of 2 with BCl(3) under varying conditions gives the chlorinated products [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(3))(6)] 3 and [(AlCl)(6)(AlNMe(3))(2)(CCH(2)CH(2)SiMe(2)Cl)(6)] 4, respectively. The latter clearly demonstrates that the cluster can be stepwise functionalized within the inner and outer sphere. The X-ray single-crystal structures of 1, 2, and 4 have been determined. All compounds have in common that the central core consists of a cluster having eight aluminum and six carbon atoms. The bonding properties in this cluster are described as a new manifestation of three-dimensional surface aromaticity. Each Al(4)C fragment of the cube is formed by four bonds with three electron pairs, thus leading to a strong delocalization of the electrons. A phenomenological modeling using a three-dimensional Hückel scheme with fitted parameters to reproduce the energies from ab initio calculations revealed that the orbital scheme localized at one Al(4)C fragment possesses an orbital sextet with a large HOMO-LUMO gap. This is in line with the criteria of aromaticity. The idea of aromaticity was sustained also by qualitative valence bond reasons enumerating the different resonance structures by means of graph theoretical methods. 相似文献
15.
Daniela Schlemmer E. R. de Oliveira M. J. Araújo Sales 《Journal of Thermal Analysis and Calorimetry》2007,87(3):635-638
Conventional plastics has a
large impact in increasing the environment’s pollution. That’s
why the interest has turned towards novel partially and completely biodegradable
polymers. In this work, blends of polystyrene and thermoplastic starch with
glycerol and Buriti (Mauritia flexuosa
L.) oil as plasticizers were prepared. Samples were analyzed using TG/DTG
and DSC techniques. The TG results indicated that the blends with Buriti oil
are thermally more stable than those with glycerol. The DSC analysis that
Buriti oil provides a higher degree of plasticization of PS, compared to the
blends plasticized using glycerol under the studied conditions. 相似文献
16.
Araúzo-Bravo MJ Fujii S Kono H Ahmad S Sarai A 《Journal of the American Chemical Society》2005,127(46):16074-16089
Sequence dependence of DNA conformation plays a crucial role in its recognition by proteins and ligands. To clarify the relationship between sequence and conformation, it is necessary to quantify the conformational energy and specificity of DNA. Here, we make a systematic analysis of dodecamer DNA structures including all the 136 unique tetranucleotide sequences at the center by molecular dynamics simulations. Using a simplified conformational model with six parameters to describe the geometry of adjacent base pairs and harmonic potentials along these coordinates, we estimated the equilibrium conformational parameters and the harmonic potentials of mean force for the central base-pair steps from many trajectories of the simulations. This enabled us to estimate the conformational energy and the specificity for any given DNA sequence and structure. We tested our method by using sequence-structure threading to estimate the conformational energy and the Z-score as a measure of specificity for many B-DNA and A-DNA crystal structures. The average Z-scores were negative for both kinds of structures, indicating that the potential of mean force from the simulation is capable of predicting sequence specificity for the crystal structures and that it may be used to study the sequence specificity of both types of DNA. We also estimated the positional distribution of conformational energy and Z-score within DNA and showed that they are strongly position dependent. This analysis enabled us to identify particular conformations responsible for the specificity. The presented results will provide an insight into the mechanisms of DNA sequence recognition by proteins and ligands. 相似文献
17.
Carlo Carlini Marco Di Girolamo Alessandro Macinai Mario Marchionna Marilena Noviello Anna Maria Raspolli Galletti Glauco Sbrana 《Journal of molecular catalysis. A, Chemical》2003,200(1-2):137-146
The synthesis of isobutanol via the Guerbet condensation between methanol and ethanol was studied by using sodium methoxide (MeONa) as soluble basic component and copper-based catalysts as heterogeneous dehydrogenating/hydrogenating metal species. The effect of the nature of the catalyst and the relative amount of its individual components with respect to the reacting alcohols as well as of temperature on productivity and selectivity of the process was investigated. The collected data indicated that the copper chromite/MeONa was more active than Cu-Raney/MeONa system. The reaction was shown to proceed with the formation only of n-propanol and isobutanol. Ethanol conversion up to 61% with selectivity to isobutanol up to 98.4% was obtained. The same catalytic systems were also employed in the reaction of the methanol/ethanol/n-propanol ternary mixture. Again copper chromite/MeONa resulted more active than the Cu-Raney/MeONa system. Finally, experiments were carried out on methanol/n-propanol mixtures in the presence of the copper chromite/MeONa catalytic system by recycling both the recovered solid copper component and the liquid reaction mixture for evidencing eventual copper leaching by MeONa. On the basis of the obtained results it was concluded that in the Guerbet reaction copper chromite works as heterogeneous catalyst. 相似文献
18.
Marialuisa Russo Gabriella Poggi Maria Luisa Navacchia Mila D'Angelantonio Salvatore S Emmi 《Research on Chemical Intermediates》2006,32(2):153-170
Chemical radiolytic oxidation induced by OH addition on 1-(2-furan-2-yl-5-hydroxy-6-hydroxymethyl-[1,3]dioxan-4-yl)-ethan-1,2-diol
(sorbitylfurfural, SF) leads, in the presence of controlled amounts of oxygen, to a permanent functional modification of the
target molecule. The yield of conversion reaches 60% of the starting material. LC-MS analysis allowed the identification,
as final products, of carboxylic acids, butenal and hydroxy-furan derivatives in which the sugar chain remains unbroken, while
the furanic ring is attacked first by OH and then by oxygen, giving in succession an intra-/inter-molecular rearrangement
of the allylperoxyl radicals thus formed. The proposed oxidation of the furanic ring envisages the peroxyl intermediates undergoing
mono- and/or bi-molecular reactions; a reaction path has been outlined and is reported here. The presence of unsaturated bonds
in the final products could provide a further site for radical scavenger activity. Therefore, the fast reaction with O2 and the rearrangement of the produced peroxyl radicals to species, which are likely to be effective OH-capturers, reinforces
the antioxidant ability of SF. 相似文献
19.
Diphenylphosphine oxidatively adds to the ReRe bonds of Re2
X
4(-dppm)2 (X=Cl or Br; dppm=Ph2PCH2PPh2) and Re2Cl4(-dpam)2 (dpam=Ph2AsCH2AsPh2) to afford the dirhenium(III) complexes Re2(-X)(-PPh2)HX
3(-LL)2. The dppm complexes have also been prepared from the reactions of Re2(-O2CCH3)X
4(-dppm)2 with Ph2PH, and a similar strategy has been used to prepare Re2(-Cl)(-PPh2)HCl3(-dmpm)2 (dmpm=Me2PCH2PMe2) from Re2(-O2CCH3)Cl4(dmpm)2. Phenylphosphine likewise reacts with Re2
X
4(-dppm)2 to give Re2(-X)(-PHPh)HX
3(-dppm)2. An X-ray crystal structure determination on Re2(-Cl)(-PPh2)HCl3(-dppm)2 confirms its edge-shared bioctahedral structure. This complex crystallizes in the space group
(No. 148) witha=21.699(3) Å, =84.50(4)°,V=10084(5) Å3, andZ=6. The structure was refined toR=0.049 (R
w
0.069) for 5770 data withI>3.0(I). The Re-Re distance is 2.5918(7) Å. Oxidation of the bromide complex Re2(-Br)(-PPh2)HBr3(-dppm)2 with NOPF6 produces the unusual dirhenium(III, II) cation [Re2(-H)(-Br)[P(O)Ph2]Br2(NO)(-dppm)2]+ which has been structurally characterized as its perrhenate salt, [Re2(-H)(-Br)[P(O)Ph2]Br2(NO)(-dppm)2]ReO4 · 2CH2Cl2. This complex crystallizes in the space group
(No. 2) witha=14.187(7) Å,b=16.419(5) Å,c=16.729(5) Å, =98.76(2)°, =110.11(3)°, =104.66(3)°,V=3414(6) Å3,Z=2. The structure was refined toR=0.040 (R
w
=0.051) for 5736 data withI>3.0(I). The presence of a phosphorus-bound [P(O)Ph2]– ligand, a linear nitrosyl and a bridging hydrido ligand has been confirmed. The Re-Re distance is 2.6273(8) Å. 相似文献
20.