Scielo RSS <![CDATA[Journal of the Chilean Chemical Society]]> vol. 50 num. 4 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>SHORT NANOTUBES OR BIG MOLECULES?</b>: <b>THE CONCEPT OF MINIMAL LENGTH</b>]]> We have studied the problem of what must be the minimal length of a nanotube with a given diameter to reasonably assert that its electronic properties can be extrapolated to longer structures. For this purpose the variation of the electronic chemical potential (ECP) was plotted against the number of units composing the nanotube for three systems: (5,5) armchair, (9,0) zigzag and (10,0) zigzag nanotubes. We found that the ECP curve defines a different minimal length for each system. This minimal length corresponds to the boundary between large molecules (short nanotubes) and nanotubes properly speaking. We suggest that this minimal length exists for at least all armchair and zigzag nanotubes <![CDATA[<b>ASSESSMENT OF THE HYDROLYTIC DEGRADATION OF LOVASTATIN BY HPLC</b>]]> In this work an HPLC stability-indicating method was developed and applied to study the hydrolytic behaviour of lovastatin in different simulated fluids. The selected chromatographic conditions were a C-18 column, acetonitrile/methanol/phosphate buffer solution pH 4 (32/33/35) as mobile phase, 45ºC temperature column, flux of 1.5 mL/min and UV detection at 238 nm. The developed method exhibited an adequate repeatability and reproducibility (CV 0.057% and 0.73%, respectively) and a recovery higher than 98%. Furthermore, the detection and quantitation limits were 9.1×10-7 M and 2.8×10-6 M. Lovastastin exhibited a pH-dependent degradation with an instantaneous hydrolysis in alkaline media at room temperature. One or two degradation products could be observed when lovastatin is hydrolyzed in alkaline or acid medium, respectively. The degradation products from lovastatin retain the UV-spectra of the parent drug, evidencing that the chromophore structure remains unaltered. Also, lovastatin hydrolysis in different media follows a pseudo first-order kinetic. The rank-order for lovastatin stability in different media was: simulated gastric medium without pepsin > 0.06 M HCl > 0.1 M HCl > phosphate buffer pH 7.4 + sodium laurylsulphate > phosphate buffer pH 7.4 <![CDATA[<b>REPPE REACTION CATALYZED BY SOLUBLE CARBONYLRHODIUM COMPLEXES</b>]]> The catalysis of the hydroesterification and hydroformylation-acetalization of 1-hexene by rhodium(I), cis-[Rh(CO)2(amine)2](PF6) complexes (amine = pyridine, 2-picoline, 3-picoline, 4-picoline, 3,5-lutidine or 2,6-lutidine) dissolved in 10 mL of ethanol or 10 mL of 80% aqueous ethanol, 0.4 mL (3 x 10-3 mol) of 1-hexene, [Rh] = 5 x 10-5 mol, 1-hexene/Rh = 64 under P(CO) = 0.9 atm at 100 °C for 4 h, is described in this work. The mayor products are ethyl-heptanoate, heptanal and 1,1-diethoxyheptane coming from the hydroesterification, hydroformylation reactions and the nucleophilic addition reaction of the ethanol over the formed heptanal, respectively. The reaction product distribution depends on the nature of the coordinate amine to the rhodium center <![CDATA[<b>CaO-MgO CATALYSTS FOR SOOT COMBUSTION</b>: <b>KNO<sub>3</sub> AS SOURCE FOR DOPING WITH POTASSIUM</b>]]> MgO, CaO, an equal ratio mixture of the oxides (50-CaO) and a sample with 10% potassium added to the previous mixture, showed increasing activity for the catalytic combustion of carbon black, a model compound for soot. These catalysts were characterized by XPS, BET, DRIFTS and AAS. Activity was measured by TGA and by TPO; CO and CO2 concentration in the outlet gas flow were measured by NDIR. Pure MgO was barely active. The 50-CaO catalyst was as active as the pure CaO in spite of a lower content of calcium. This was attributed to greater contact area and better dispersion of calcium, lesser bulk and surface carbonation, cooperative effect of magnesium that diminishes the stability of adsorbed surface compounds, and a greater accessibility of O2(g) to calcium sites. Potassium introduction over the 50-CaO produced high activity, related to a increase of the surface concentration of oxygen on the catalyst and larger CO2/CO ratio in the exit gases. Moreover, potassium addition resulted in a lower content of calcium on the surface, hence less carbonation of the catalyst <![CDATA[<b>SECONDARY PRODUCTS FROM <i>Pannaria tavaresii</i> (LICHENS)</b>]]> Pannarin, chloroatranorin, leprolomin, ursolic acid and ergosterol peroxide were isolated from Pannaria tavaresii P.M. Jørg. The results confirm that the chlorinated depsidone pannarin is the most representative compound in this lichen genus <![CDATA[<b>PARA-Ni-TETRAAMINOPHENYLPORPHYRIN/Co-COBALTITE/SnO<sub>2</sub>:F MODIFIED ELECTRODES</b>: <b>ELECTROCATALYTIC BEHAVIOR TOWARD THE OXIDATION OF HIDRAZINE</b>]]> The aim of this work was the study of three different modified electrodes where the substrate and the porphyrin used to modify the electrodic surface was the same. The surface electrode was a glass covered by a layer of SnO2 doped with fluor. On this layer was formed a cubic spinel of Co-cobaltite. On this modified electrode was deposited or polymerized para-tetraaminophenylporphyrin of Ni (II). In each case, the treatment of the deposition of the porphyrin was different and the electrocatalytic behavior of the modified electrodes toward the oxidation of hydrazine drastically changes. In the case of a simple physically adsorbed layer of porphyrin, practically no electrocatalytic activity was found. In the case of a porphyrin deposited by reflux and cycled in basic solution, there is an important electrocatalytic activity giving N2 as product. This electrode was proved as an amperometric sensor of hydrazine and a very good linear relationship between current and concentration was determinate. Also, the polymeric electrode showed an important catalytic behavior toward the oxidation of hydrazine <![CDATA[<b>CHROMIUM ADSORPTION FROM TANNERY EFFLUENTS BY ACTIVATED CARBONS PREPARED FROM COCONUT SHELLS BY CHEMICAL ACTIVATION WITH KOH AND ZNCL<sub>2</sub></b>]]> Activated carbons of high adsorbents properties for adsorption of chromium species from aqueous solution and Tannery effluents were prepared from coconut shells by means of chemical activation using as activating agents the KOH and ZnCL2, in atmosphere of N2(g) and CO2(g), respectively. It was found that the adsorption capacity of the produced activated carbons are significantly higher than that of the commercial one that was tested. Based on the results obtained from a variety of characterization methods, it has been determined the presence of functional organic groups, phenols and carbonyls, which are responsible for the activated carbons surface charge and it favored the chromium species adsorption, zeta potential measurements helped to explain the metal adsorption in terms of electrostatic attraction-repulsion behavior of the activated carbon surface and cations of Cr(III) and oxyanions of Cr(VI), respectively. It was observed that the activated carbons CK12 and CZ32 displayed the biggest adsorbents properties, it was adsorbed approximately 721.38 mg of chromium with the CK12, 534.43 mg with the CZ32, and only 274.02 mg with the commercial carbon (CC), by gram of activated carbon, respectively, at pH 4.45 from Tannery effluents<hr/>Se prepararon carbones activados de elevadas propiedades adsorbentes desde cáscaras de coco mediante activación química usando como agentes activantes el KOH y ZnCL2, en atmósfera de N2(g) y CO2(g), respectivamente, para adsorber especies de cromo desde soluciones acuosas y efluentes de Curtiembres. Se halló que la capacidad de adsorción de los carbones activados obtenidos es significativamente mayor que el carbón comercial usado. En base a resultados obtenidos de los diversos métodos de caracterización, se determinó la presencia de grupos funcionales orgánicos, fenoles y carbonilos, responsables de la carga superficial de los carbones activados que favoreció la adsorción de las especies de cromo, medidas de potencial zeta ayudaron a explicar la adsorción del metal en términos de la acción electrostática atracción-repulsión de la superficie del carbón activado y los cationes Cr(III) y oxianiones Cr(VI), respectivamente. Se observó que los carbones activados CK12 y CZ32 muestran las propiedades adsorbentes más altas, se adsorbió aproximadamente 721,38 mg de cromo con el CK12, 534,43 mg con el CZ32 y solamente 274,02 mg con el carbón comercial (CC), por gramo de carbón activado, respectivamente, a pH 4,45 desde los efluentes de Curtiembres <![CDATA[<b>ADSORPTION OF METHIONINE ON MILD STEEL</b>]]> The adsorption of methionine at the mild steel surface from acidic solutions is studied using gravimetric technique. The adsorbability of methionine (values of surface coverage) depends on the nature of the acid and the concentration of methionine. The surface coverage with the adsorbed methionine is used to calculate the free energy of adsorption, DG0ad of methionine using Bockris - Swinkel isotherm. The dependence of free energy of adsorption, DG0ad with surface coverage, q is ascribed to surface heterogeneity of the adsorbent. The effect of methionine is discussed from the viewpoint of adsorption model. The adsorption of methionine molecules on the surface occurs without modifying the kinetic of corrosion process <![CDATA[<b><i>RETENTION OF PB (II) ION FROM AQUEOUS SOLUTION BY NIPAH PALM (NYPA FRUTICANS WURMB) PETIOLE BIOMASS</i></b>]]> The retention of Pb (II) from aqueous solution by pure (PB) biomass and thioglycolic acid (TGA) modified biomass (MB) of Nipah palm (Nypa fruticans Wurmb) petiole was studied using batch sorption technique. The equilibrium retention capacity of Pb (II) was determined from the Langmuir equation and found to be 15.06 mg g-1 and 17.60 mg g-1 for pure and modified biomass, indicating a small difference in Pb(II) retention after treatment with TGA. The data showed that chemisorption process could be the rate-limiting step in the retention mechanism. Studies on the retention of Pb (II) by Nipah palm petiole biomass are important because it may contribute in aiding the innovative removal and recovery of metal ions from contaminated industrial effluents <![CDATA[<b>CHEMICAL PROFILES IN LAKE SEDIMENTS IN LAGUNA CHICA DE SAN PEDRO (BIO-BIO REGION, CHILE)</b>]]> Metal profiles in lake sediments could provide historical environmental information on impacts caused by human activities and natural events, with view to the formulation of effective environmental policies. This paper presents data on sedimentary metal profiles in Laguna Chica de San Pedro (LCSP) lake, located in the Bio Bio Region in Southern Chile, where important industrial activities are concentrated. Sediment properties (organic and inorganic matter, grain size, particle distribution, biogenic silica, Mn/Fe ratio), major (Al, Ca, K, Mg, Ti, Na), trace (V, As, Co, Cu, Zn, Ni, Cr, Pb, Sr), and mobile (Fe, Mn, P, S) elements, as well as mineral profiles (plagioclase, quartz, clays, amorphous material) are presented, up to 65 cm depth. In general, relatively constant concentration profiles are observed in the sedimentary core mid section, for most elements. Most changes in composition are seen at the top (recent industrial period) and bottom (before human intervention) sections. Variable redox conditions, generated by biological activity at the sediment-water interface are likely to account for composition profiles at the sediment-water interface. On the other hand, physical processes seem to be mostly responsible for concentration changes in Pre-industrial sediments. Mineral content profiles, such as plagioclase, clay and quartz, as well as total clay content remain fairly constant in most of the core, showing significant changes at its bottom part. High excess V, As, and S values, especially at the upper sediment, arise as a consequence of redox conditions in the lake. Certainly, such metal enrichment is mainly associated with natural sedimentary matter supply from the watershed <![CDATA[<b>Zn(II) AND Cu(II) UPTAKE FROM ACID SOLUTIONS WITH MICROCAPSULES CONTAINING A NON-SPECIFIC ALKYL PHOSPHONIC EXTRACTANT</b>]]> It is studied the uptake of Zn(II) and Cu(II) from acid aqueous solutions using microcapsules containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester, a non-specific extractant, as an alternative method for metals separation in place of current techniques as ion-exchange solid resins or solvent extraction. The microcapsules employed in this study were prepared by an in situ radical polimerization method. The amount of extractant held within each microcapsule was determined by potentiometric titration. Many experimental tests were carried out to measure the hydrometallurgical capacity of the microcapsules to remove these metallic ions from the aqueous solutions <![CDATA[<b>REGIO AND STEREOSELECTIVE HYDROXYLATION OF A-AGAROFURAN BY BIOTRANSFORMATION OF RHIZOPUS NIGRICANS</b>]]> A new synthesis of 9a-hydroxy-a-agarofuran (6a) is described, using a microbiological hydroxylation a-agarofuran (5) as the key reaction. The stereochemistry of the biohydroxylation was determined on the basis of a NOESY-experiment and GIAO calculations at the B3LYP/cc-pVDZ level. A strong g-effect was observed at C15 of the agarofuran ring which was correctly predicted by the GIAO-B3LYP calculations <![CDATA[<b>SYNTHESIS AND CHARACTERIZATION OF NEW CHITOSAN-O-ETHYL PHOSPHONATE</b>]]> The chitin is a natural polymer that is extracted commercially from the shells of crustaceans generated as raw material from the fishing industry. Their chemical structure is constituted by residual units of N-acetyl glucosamine linked by b (1 ® 4) and its derivatives can be obtained from alkaline chitosan. The chitin, chitosan and their derivatives present very good perspectives to be used in agriculture. These derivatives are biodegradable and they exhibit fungicides, germicides, nemacides properties and natural defensive mechanisms of the plants. The chlorophosphonic-2-acid (Ethephon®) is a commercial product employed for the early maturation of fruits. The objective of the present work is the synthesis and characterization of chitosan-O-ethyl phosphonate chitosan to obtain a controlled released system with potential plant growth regulation properties. Alkaline chitosan was obtained; and then reacted with the 2-chlorophosphonic acid. The synthesis of new chitosan derivatives and its complete characterization by FT-IR, 13C, ¹H and 31P NMR is described and gas chromatographic, the effects on field blueberries are also tested. A chitosan 99 % deacetylated and Mv = 89,000 g/mol is prepared. The O-(ethyl phosphonic) chitosan (QOEP) with a degree of substitution of 58% is obtained <![CDATA[<b>SURFACE ENHANCED RAMAN SPECTRUM OF NANOMETRIC MOLECULAR SYSTEMS</b>]]> Surface-enhanced Raman spectroscopy (SERS) was demonstrated to be a useful and sensitive structural spectroscopic tool for the characterization of different types of supramolecular architectures, ideally suited for trace analysis. SERS spectra allowed to distinguish cell membrane components of the biomining bacterium Acidithiobacillus ferrooxidans and their physical and chemical variations caused by different growth media. Several SERS spectral modifications were arguments to propose different structures for two isolated model blue cooper proteins, i.e., hemocyanins belonging to mollusc Concholepas concholepas (CCH) and Megathura crenulata (KLH). The complex structure of a humic acid was demonstrated to be highly sensitive to the pH of the medium; a globular conformation predominated at lower pH, while a decurled structure was observed at higher pH. Finally, the SERS technique using different excitation laser lines allowed to characterizing a carbon nanotube as a single-walled species, with metallic electronic characteristics; these species coexisted with different diameters <![CDATA[<b><i>THEORETICAL AND CHEMOMETRIC STUDY OF SUBSTITUTED OXAZINES</i></b>]]> The application of the Principal Component of Analysis (PCA) over the theoretical data of a series of substituted 1,3-oxazines helps to determine the factors that rule the conformational equilibrium. Repulsive and intertribal syn-1,3-diaxial interactions have been used as determiners for the axial structures, whereas dipolar interactions are preferred in the equatorial conformation. Theoretical variables such as hardness, charges, nuclear repulsion energy and bond order in the different nuclear sectors have been used. PCA method has been successfully used in predicting the hyperconjugative interactions that play an important role in the conformational equilibrium between a given pair of conformers of 1,3-oxazines. The studied 1,3-oxazines belong to three different groups of structures having the heterocyclic ring as a common feature. The axial structures are stabilized by two-electron/two-orbital interactions, whereas the equatorial structures are believed to be stabilized by syn-1,3-diaxial and dipole-dipole interactions <![CDATA[<b>OPTIMIZATION OF PHYSICO-CHEMICAL MODELS USING THE GAUSS-NEWTON METHOD</b>]]> This study proposes the use of a numerical calculation method for optimizing non-linear physico-chemical models based on the Gauss-Newton algorithm. This method can be applicable to the Wagner-Traud model used to determine the parameters of corrosion and to adsorption isotherm models, like those of Langmuir and Langmuir-Freundlich, to determine the free energy of adsorption. This method in present work was applicated to experimental data of polarization of the iron electrode in medium of sulfuric acid 0.5 M and experimental values of adsorption of 3-Mercaptopropyltrimethoxisilane on copper surface. For the first case (polarization) the results showed with the proposed method was obtained smaller relative errors than the relative errors obtained by the Polynomial method and the computational method of Betacrunch. In the second case, the results showed that the adsorption parameters agree with the model of isotherm of Langmuir-Freundlich, obtaining a free energy of adsorption of 40 kJ/mol <![CDATA[<b>THE INTERACTION OF ATOMIC OXYGEN WITH SMALL COPPER CLUSTERS</b>: <b>A THEORETICAL APPROACH</b>]]> A theoretical study of atomic oxygen adsorption on small Cu n (n ­ 8) clusters was carried out using density functional methods. The copper-oxygen system is important to understand the mechanism of the oxygen oxidation catalyzed by copper. It is found that the oxygen reactivity is strongly dependent of the size of copper clusters and Cu n with n odd exhibiting the highest reactivity. The adsorption energy has values from 37 to 93 kcal/mol. In general, in clusters with even number of atoms the oxygen adsorption is carried out on a top type site, on the other hand, in clusters with odd number of atoms the adsorption is carried out on highly coordinated sites. In comparison with bare Cu n clusters, the relative stability of copper clusters under oxygen adsorption is not largely affected, exhibiting only small modifications in the structure of the cluster. Electronic and symmetrical effects seen to govern the oxygen adsorption process