Coloration mechanism in silicate gels and gel-glasses containing iron

The present investigation was carried out in order to study the incorporation of iron oxide in several samples with a general composition xFeO-(100-x)SiO₂ (x between 0.1 and 9.5) and 5R_nO-xFeO-(95-x)SiO₂ (R=Li, Na or Ca) prepared by the sol-gel method. Samples were gelated and dried at 60°C and thermally treated between 200 and 1000°C in oxidizing and reducing atmospheres. The color change and structural evolution were studied by VIS and NIR spectrophotometry, TEM and X-ray diffraction.When the temperature increases, the color changes from yellow to brown-reddishness, going through a bright ruby-red, at the same time the absorption edge shifts to higher wavelengths and at elevated temperatures to smaller wavelengths newly. A part of iron oxide is incorporated into the glass network and the remaining as colloidal particles, producing a double mechanism of coloration by optical absorption and dispersion.In the samples heat-treated in oxidizing conditions all iron is present in the ferric state. When the heat-treatment is carried out in reducing conditions the presence of iron (II) was detected at temperatures of nearly 1000°C.     

Determination of cadmium and lead in table salt by sequential multi-element flame atomic absorption spectrometry

In the present paper, a simultaneous pre-concentration procedure for the sequential determination of cadmium and lead in table salt samples using flame atomic absorption spectrometry is proposed. This method is based on the liquid-liquid extraction of cadmium(II) and lead(II) ions as dithizone complexes and direct aspiration of the organic phase for the spectrometer. The sequential determination of cadmium and lead is possible using a computer program. The optimization step was performed by a two-level fractional factorial design involving the variables: pH, dithizone mass, shaking time after addition of dithizone and shaking time after addition of solvent. In the studied levels these variables are not significant. The experimental conditions established propose a sample volume of 250 mL and the extraction process using 4.0 mL of methyl isobutyl ketone. This way, the procedure allows determination of cadmium and lead in table salt samples with a pre-concentration factor higher than 80, and detection limits of 0.3 ng g⁻¹ for cadmium and 4.2 ng g⁻¹ for lead. The precision expressed as relative standard deviation (n = 10) were 5.6 and 2.6% for cadmium concentration of 2 and 20 ng g⁻¹, respectively, and of 3.2 and 1.1% for lead concentration of 20 and 200 ng g⁻¹, respectively. Recoveries of cadmium and lead in several samples, measured by standard addition technique, proved also that this procedure is not affected by the matrix and can be applied satisfactorily for the determination of cadmium and lead in saline samples. The method was applied for the evaluation of the concentration of cadmium and lead in table salt samples consumed in Salvador City, Bahia, Brazil.     

4,4'-dithiodipyridine as a bridging ligand in osmium and ruthenium complexes: the electron conductor ability of the -S-S- bridge

The compounds [Ru(NH(3))(5)(dtdp)](TFMS)(3), [Os(NH(3))(5)(dtdp)](TFMS)(3), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](TFMS)(6), [(NH(3))(5)Os(dtdp)Ru(NH(3))(5)](TFMS)(3)(PF(6))(2), and [(NH(3))(5)Os(dtdp)Fe(CN)(5)] (dtdp = 4,4'-dithiodipyridine, TFMS = trifluoromethanesulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and (1)H NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os(NH(3))(5)(dtdp)](3+) as a function of the solution pH enable us to calculate the pK(a) for the [Os(NH(3))(5)(dtdpH)](4+) and [Os(NH(3))(5)(dtdpH)](3+) acids as 3.5 and 5.5, respectively. The comparison of the above pK(a) data with that for the free ligand (pK(1) = 4.8) provides evidence for the -S-S- bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](6+), is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the -S-S- bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin-spin interaction parameter of 150-200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the pi orbitals of the dttp ligand relative to that found in [Os(NH(3))(5)(dtdp)](3+). The comproportionation constant, K(c) = 2.0 x 10(5), for the equilibrium equation [Os(II)Os(II)] + [Os(III)Os(III)] right harpoon over left harpoon 2[Os(II)Os(III)] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](5+) (lambda(MMCT) = 1665 nm, epsilon = 3.5 x 10(3) M(-)(1) cm(-)(1), deltanu(1/2) = 3.7 x 10(3) cm(-)(1), alpha = 0.13, and H(AB) = 7.8 x 10(2) cm(-)(1)), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH(3))(5)Os(III)(dtdp)Ru(II)(NH(3))(5)](5+) (lambda(MMCT) = 965 nm, epsilon = 2.2 x 10(2) M(-)(1) cm(-)(1), deltanu(1/2) = 3.0 x 10(3) cm(-)(1), and H(AB) = 2.2 x 10(2) cm(-)(1)) and [(NH(3))(5)Os(III)(dtdp)Fe(II)(CN)(5)] (lambda(MMCT) = 790 nm, epsilon = 7.5 x 10 M(-)(1) cm(-)(1), deltanu(1/2) = 5.4 x 10(3) cm(-)(1), and H(AB) = 2.0 x 10(2) cm(-)(1)) also suggest a considerable electron delocalization through the S-S bridge. As indicated by a comparison of K(c) and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order: Fe < Ru < Os.     

The synthesis of tetrahydropyridopyrimidones as a new scaffold for HIV-1 integrase inhibitors

An efficient synthesis of methyl 9-amino-3-hydroxy-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidine-2-carboxylate as a late stage intermediate for a new class of HIV-1 integrase inhibitors is described. After construction of the bicyclic core scaffold, a stereoselective introduction of the chiral amino group in the 9-position is achieved.     

A Pre-Concentration Procedure Using Cloud Point Extraction for the Determination of Manganese in Saline Effluents of a Petroleum Refinery by Flame Atomic Absorption Spectrometry

This paper describes the development of a cloud point extraction procedure for the determination of manganese in saline effluents by flame atomic absorption spectrometry (FAAS). The optimization step was performed using the Doehlert matrix involving the following variables: buffer concentration, pH and centrifugation time. The validation process was assessed as: parameters of the analytical curve, precision, effect of other ions in the proposed procedure, robustness test and accuracy. The proposed procedure allows the determination of manganese with a detection limit (3δ/S) of 0.60 μg L⁻¹, and a precision expressed as relative standard deviation (RSD) of 2.2 (n = 8) and 1.5% (n = 8) for a manganese concentration of 1 and 5 μg L⁻¹, respectively. The pre-concentration factor obtained was 84. The recovery achieved for the determination of manganese in the presence of several other metal ions demonstrated that this procedure could be satisfactorily applied to the analysis of environmental samples. The accuracy was confirmed by the analysis of CRM trace elements in water (NIST 1643d). This procedure was applied to the determination of manganese in saline effluents of a petroleum refinery. For three analyzed samples the manganese content varied between 44.9 and 67.9 μg L⁻¹.     

Specific detection of Clostridium botulinum types A,B, E,and F using the polymerase chain reaction

Clostridium botulinum organisms generally produce 1 of 4 neurotoxin types (A, B, E, and F) associated with human illness. Neurotoxin type determination is important in identification of the bacterium. A polymerase chain reaction (PCR) method was developed to identify 24 h botulinal cultures as potential types A, B, E, and F neurotoxin producers as well as other clostridial species which also produce neurotoxins. Components of the PCR and amplification conditions were adjusted for optimal amplification of toxin gene target regions to enable simultaneous testing for types A, B, E, and F in separate tubes using a single thermal cycler. Each primer set was specific for its corresponding toxin type. A DNA extraction procedure was also included to remove inhibitory substances that may affect amplification. This procedure is rapid, sensitive, and specific for identification of toxigenic C. botulinum.     

Sequential determination of iron and titanium by flow-injection analysis

A flow-injection method has been developed for the sequential spectrophotometric determination of iron and titanium using 3,4 dihydroxybenzoic acid as chromogenic reagent. The system involves the sequential measurement of the absorbances of the complexes at 380 and 570 nm. The system is designed using a simultaneous injection of sample and reagent into separate carrier streams. The proposed method is characterized by a precision of about 2%, a sampling rate of about 50 samples per hour, and a reagent consumption of 200 mul (0.50% solution) per sample. It is relatively free of interferences and was used for the sequential determination of titanium and iron in rocks.     

Partition and location of nimesulide in EPC liposomes: a spectrophotometric and fluorescence study

Study of the mechanism of action of anti-inflammatory drugs (NSAIDs) and their side-effects may fall in the domain of membranology. In this work the extent of the interaction between an NSAID (nimesulide) and membrane phospholipids was quantified by the partition coefficient, K_p , using egg phosphatidylcholine (EPC) liposomes as cell membrane models. The liposome/aqueous phase partition coefficients were determined under physiological conditions, by derivative spectrophotometry and fluorescence quenching. Derivative spectrophotometry allows elimination of background signal effects (light scattering) due to the presence of liposomes. Theoretical models, accounting for simple partition of the NSAID between two different media, were used to fit the experimental data, allowing the determination of K_p in multilamellar vesicles (MLVs) and large unilamellar vesicles (LUVs). The location of nimesulide in MLVs and LUVs was evaluated by fluorescence quenching using spectroscopic probes located at different sites on the membrane. All n-AS probes were quenched and the relative quenching efficiencies were ordered as 2-AS<6-AS9-AS<12-AS; this suggests the drug is deeply buried in the membrane. Fluorescence quenching using the 12-AS probe was also used to determine the partition coefficient of the drug in MLVs and LUVs. The two techniques yield similar results. Finally, measurement of zeta-potential in the presence of different concentrations of nimesulide was performed to investigate possible changes in the zwitterionic phosphatidylcholine membranes. The membrane surface potential was not altered, which seems to be an indication that nimesulide binds to lipid bilayer mostly by hydrophobic interactions.     

π‐Stacked dimers in 6‐methoxy‐3,3‐dimethyl‐3H‐benzo[f]chromene,and centrosymmetric dimers containing C—H...π(arene) hydrogen bonds in racemic 3‐bromo‐2,2,6,6‐tetramethyl‐3,4‐dihydro‐2H,6H‐1,5‐dioxatriphenylene

The title compounds, namely 6‐methoxy‐3,3‐dimethyl‐3H‐benzo[f]chromene, C₁₆H₁₆O₂, (III), and racemic 3‐bromo‐2,2,6,6‐tetramethyl‐3,4‐dihydro‐2H,6H‐1,5‐dioxatriphenylene, C₂₀H₂₁BrO₂, (IV), were both synthesized in one‐step regioselective Wittig reactions from substituted 1,2‐naphthoquinones. The new ring in both compounds adopts a screw‐boat conformation. A single π–π stacking interaction links the molecules of (III) into centrosymmetric dimeric aggregates, and a single C—H...π(arene) hydrogen bond links the molecules of (IV) into centrosymmetric dimers.     

High kinetic resolution in the addition of a racemic allenylzinc onto enantiopure N-tert-butanesulfinimines: concise synthesis of enantiopure trans-2-ethynylaziridines

Enantiopure trans-ethynyl N-tert-butanesulfinylaziridines (R(S))-6 were prepared in good to excellent yields by the condensation of the racemic allenylzinc species 1 derived from 3-chloro-1-trimethylsilylpropyne onto the corresponding enantiopure N-tert-butanesulfinimines (R(S))-5. The absolute stereochemistry of enantiopure N-tert-butanesulfinylaziridines (R(S))-6 was shown to be (R(S),2R,3R) and results from a chelate-type transition state in which the zinc atom of allenylzinc 1 is coordinated by both the nitogen and the oxygen atoms of the imine. Further removal of the N-tert-butanesulfinyl auxiliary of alkyl 3-substituted and 3,3-disubstituted ethynyl N-tert-butanesulfinylaziridines (R(S))-6 could be achieved by treatment with HCl in MeOH affording the corresponding deprotected aziridines (2R,3R)-9 and (2R)-9 respectively as enantiomerically pure compounds.