Scielo RSS <![CDATA[Química Nova]]> http://www.scielo.br/rss.php?pid=0100-404220010004&lang=pt vol. 24 num. 4 lang. pt <![CDATA[SciELO Logo]]> http://www.scielo.br/img/en/fbpelogp.gif http://www.scielo.br http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400001&lng=pt&nrm=iso&tlng=pt <![CDATA[<B>Constituintes químicos de <I>Gustavia augusta </I>L. (Lecythidaceae)</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400002&lng=pt&nrm=iso&tlng=pt Gustavia augusta is used in the folk medicine against leishmaniosis and showed anti-inflammatory action. The phytochemical studies of the plant stem bark have led to the isolation of (22E)-stigmasta-7,22-dien-3beta-ol, 24alpha(S)-ethyl-5alpha-colesta-7,trans-22-dien-3-one, D-friedoolean-14-en-3beta-ol, D-friedoolean-14-en-3-one and D-friedoolean-14-en-3alpha-ol along with stigmasterol, alpha-amyrin, beta-amyrin, lupeol, 3alpha-hidroxi-lupeol and betulinic acid. The structures of these compounds were identified by IR, GC/MS, ¹H and 13C NMR spectral analysis and comparison with literature data. <![CDATA[<B>Gota suspensa para avaliação de aldeído total no ar interno e externo do ambiente</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400003&lng=pt&nrm=iso&tlng=pt A sensitive and simple system was proposed for the in situ measurement of total aldehyde in outdoor or indoor ambient. The method is based on the use of a reagent drop as an useful interface to preconcentrate the sample prior to determination of total aldehyde as formaldehyde. The drop is formed at the tip of a cylindrical tube that contains two optical fibers placed on opposite sides and in contact with the reagent solution. One optical fiber carries the red light to the drop form a light emitting diode (LED). The transmitted light is measured by a second optical fiber/photodiode system. The analytical signal is read and converted into absorbance. The reagent solution of 3-methyl-2-benzothiazoline hydrazone (MBTH) forms a blue cation during reaction with formaldehyde that can be measured at 660 nm. Some aspects of kinetics reaction formation of dye were reevaluated. The formaldehyde reacts with MBTH and forms the azine in about 12 min. The oxidation of MBTH by Fe (III) and the formation of dye requires 3 min. The absorbance of the reagent drop is proportional to the sampling time and to the analyte concentration. The absorbance signal increases with increased sample gas flow until a maximum is reached then decreases until it forms a plateau. The proposed method was evaluated using both outdoor and indoor samples, and it was shown to viable provide an accurate measure of total aldehyde. <![CDATA[<B>Preparação e caracterização do vinho de laranja</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400004&lng=pt&nrm=iso&tlng=pt A simple method of home made preparation and physical-chemical characterization of orange wine was investigated. Saccharomyces cerevisiae was used as inoculum for wine-making by fermentation. Chemical compositions related to the aroma components seems to be very similar between grape and orange wines. <![CDATA[<B>Coordenação local do Eu(III) em híbridos orgânicos/inorgânicos emissores de luz branca</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400005&lng=pt&nrm=iso&tlng=pt Eu3+ luminescence and EXAFS (Extended X-ray Absorption Fine Structure) results are presented for organic-inorganic hybrid gel hosts composed of a siliceous network to which small chains of oxyethylene units are covalently grafted by means of urea bridges. Coordination numbers for Eu3+ ions range from 12.8 to 9.7 with increasing Eu3+ concentration while the Eu3+-first neighbours mean distance is found to be constant at 2.48-2.49 Å in the same concentration range. Emission spectra display a broad band in the green/blue spectral region superposed to narrow lines appearing in the yellow/red region in such a way that for the eyes emission appears white. The broad band is assigned to intrinsic NH groups emission and also to electron-hole recombination in the nanosised siliceous domains. The narrow lines are assigned to intra-4f6, 5D0->7F0-4 Eu3+ transitions and from the energy position of the 7F0-4 levels a mean distance could be calculated for the Eu3+-first neighbours. The calculated results are in good agreement with the experimental ones obtained from EXAFS analysis. <![CDATA[<B>Estudo termogravimétrico e calorimétrico da algaroba</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400006&lng=pt&nrm=iso&tlng=pt Humidity and ash content of powder and bran of algaroba pods are important quality parameters for biological control during storage. The studies of humidity and ash content were carried out conventional and thermogravimetric methods. Different values were obtained by the various methods. The thermogravimetric method was faster and required less sample for the humidity and ash analyses of the powder and bran of algaroba pods. The differential scanning calorimetry curves of the powder and bran of algaroba pods, dryed at 55 ºC, showed two peaks, one corresponding to the gelatinization of starch and the other to the vaporization of water. The samples dryed at 65, 75, 85, 95 and 105 ºC showed one peak, corresponding to the vaporization of water. <![CDATA[<B>Estudos da deposição em subtensão de cádmio sobre ouro policristalino na presença de diferentes ânions co-adsorvidos</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400007&lng=pt&nrm=iso&tlng=pt Cadmium UPD on Au was studied by voltammetric and microgravimetric measurements. In the oxide formation/reduction potential region, a mass increasing/decreasing of 32 ng cm-2 was associated to incorporation/elimination of one oxygen per active site. The modifications promoted in the voltammetric and mass profiles by 10-5 M Cd(ClO4)2 are restricted to potentials more negative than 0.4 V. After a 120 s potential delay at 0.05 V, the positive sweep reveals an anodic peak with charge of 40 muC cm-2 and mass decrease of 22 ng cm-2, associated to Cd ads dissolution. Sulphate or chloride was added to the solution without significant influence, due to the low coverage with Cd or anions. <![CDATA[<B>Estudo da adsorção de hidrogênio e sulfeto na superfície de paládio: aspectos experimentais (eletroquímica) e teóricos (<I>ab initio </I>e Teoria do Funcional da Densidade)</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400008&lng=pt&nrm=iso&tlng=pt The adsorption of H and S2- species on Pd (100) has been studied with ab initio, density-functional calculations and electrochemical methods. A cluster of five Pd atoms with a frozen geometry described the surface. The computational calculations were performed through the GAUSSIAN94 program, and the basis functions adapted to a pseudo-potential obtained by using the Generator Coordinate Method adapted to the this program. Using the cyclic voltammetry technique through a Model 283 Potentiostat/Galvanostat E.G.&G-PAR obtained the electrochemical results. The calculated chemisorption geometry has a Pd-H distance of 1.55Å, and the potential energy surface was calculated using the Becke3P86//(GCM/DFT/SBK) methodology. The adsorption of S2- ions on Pd surface obtained both through comparison between the experimental and theoretical results, at MP2 level, suggest a S2- absorption into the metallic cluster. The produced Pd-(S2-) system was show to be very stable under the employed experimental conditions. The paper has shows the powerful aid of computational methods to interpret adsorption experimental data. <![CDATA[<B>Análise conformacional de modelos de lignina</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400009&lng=pt&nrm=iso&tlng=pt The conformational equilibrium for two 5,5' biphenyl lignin models have been analyzed using a quantum mechanical semiempirical method. The gas phase and solution structures are discussed based on the NMR and X-ray experimental data. The results obtained showed that the observed conformations are solvent-dependent, being the geometries and the thermodynamic properties correlated with the experimental information. This study shows how a systematic theoretical conformational analysis can help to understand chemical processes at a molecular level. <![CDATA[<B>Triterpenóides pentacíclicos de <I>Mentha villosa</B></I>: <B>identificação estrutural e atribuição dos deslocamentos químicos dos átomos de hidrogênio e carbono</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400010&lng=pt&nrm=iso&tlng=pt The structures of seven oleanene and ursene triterpenoids (1-7) isolated from aerial parts of Mentha villosa were identified. In addition, the complete ¹H and 13C resonance assignments of these triterpenoids were accomplished using 1D and 2D NMR spectroscopic experiments. <![CDATA[<B>Índices de reatividade química a partir da teoria do funcional de densidade</B>: <B>formalismo e perspectivas</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400011&lng=pt&nrm=iso&tlng=pt The fundaments of the modern Density Functional Theory (DFT), its basic theorems, principles and methodology are presented. This review also discuss important and widely used concepts in chemistry but that had not been precisely defined until the development of the DFT. These concepts were proposed and used from an empirical base, but now their precise definition are well established in the DFT formalism. Concepts such as chemical potential (electronegativity), hardness, softness and Fukui function are presented and their consequences to the understanding of chemical reactivity are discussed. <![CDATA[<b>Fluidos supercríticos em química analítica. I. Cromatografia com fluido supercrítico</b>: <b>conceitos termodinâmicos</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400012&lng=pt&nrm=iso&tlng=pt Under the chromatographic point of view, the physico-chemical properties of a supercritical fluid are intermediate to those of the gases and liquids. Many times they approach the best features of each one, as for example, the solubilization power of liquids and low viscosity of gases. The thermodynamic definitions and main physico-chemical features of a supercritical fluid will be presented in this article. The use of supercritical fluids in analytical chemistry has been extremely modest in Brazil, even considering the enormous potential of their applications, and their use in several techniques, such as chromatography (SFC) and supercritical fluid extration (SFE). This article series is intended to discuss the historical evolution, instrumentation features and potential and limitations of the supercritical fluid use in analytical chemistry. A special focus will be centered on chromatography and extration techniques using supercritical fluids. <![CDATA[<B>Glicídios no mel</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400013&lng=pt&nrm=iso&tlng=pt A review about composition, origin and importance of carbohydrates in honey is presented. Fructose and glucose are the major carbohydrates, ranging from 65-85 % of the total soluble solids. Other minor carbohydrates, chiefly di- and trisaccharides, have been also identified. Fructose, glucose and sucrose are mainly originated from nectar. Oligosaccharides are mainly formed by trans-alpha-D-glucosylation reactions catalysed by honeybee alpha-D-glucosidase. The profile of carbohydrates can be useful for the identification of the brazilian region in which honey was produced and may also be useful for testing brazilian honey authenticity. <![CDATA[<B>Derivados organometálicos de estanho(II) - Parte 1. Compostos ciclopentadienílicos</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400014&lng=pt&nrm=iso&tlng=pt This paper provides a brief review of the chemistry of cyclopentadienyl Sn(II) derivatives which includes the preparation, the molecular structure and reactivity associated with such bis-sandwich tin(II) species. It is compared structural and spectroscopic results and it is also discussed how the nature of the cyclopentadienyl ring bonded to the Sn centre plays an important role in the structural and stability features of the derivatives. Bulk rings such as C5HPr i4- , C5Bz5-, C5Me4SiMe2Bu t- and C5Ph5- render air-stable and parallel ring-bonded compounds. <![CDATA[<B>Métodos matemáticos para correção de interferências espectrais e efeitos interelementos na análise quantitativa por fluorescência de raios-X</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400015&lng=pt&nrm=iso&tlng=pt One of the main problems in quantitative analysis of complex samples by x-ray fluorescence is related to interelemental (or matrix) effects. These effects appear as a result of interactions among sample elements, affecting the x-ray emission intensity in a non-linear manner. Basically, two main effects occur; intensity absorption and enhancement. The combination of these effects can lead to serious problems. Many studies have been carried out proposing mathematical methods to correct for these effects. Basic concepts and the main correction methods are discussed here. <![CDATA[<B>Compostos <FONT FACE=Symbol>a</FONT>-diazo carbonílicos</B>: <B>uma estratégia atraente para a síntese orgânica</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400016&lng=pt&nrm=iso&tlng=pt Diazocarbonyl compounds are a very important class of organic substances which have a long history of useful aplications in organic chemistry. The focus of this report deals with the use of diazocarbonyl compounds in a ariety of important reactions and their application in organic synthesis. These reactions are briefly summarized showing their broad scope. <![CDATA[<B>Estudo de compostos orgânicos em lixiviado de aterros sanitários por EFS e CG/EM</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400017&lng=pt&nrm=iso&tlng=pt The main purpose of this work was the qualitative study of organic compounds in landfill leachate. The samples were collected from a sanitary landfill located at Gravataí, a southern Brazilian city, that receive both, industrial and domestic refuse. The samples were submitted to solid phase extraction (SPE) with XAD-4 resin as the stationary phase. The instrumental analysis was performed by Gas Chromatography with a Mass Spectrometry Detector (GC/MSD). The compounds achieved in the SPE extracts were tentatively identified by the GC/MS library. It was found several oxygen and nitrogen compounds like carboxylic acids, ketones, amines and amides. Sulfur compounds and phthalate esters are also identified. <![CDATA[<b>A dinâmica de analisar livros didáticos com os professores de química</b>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400018&lng=pt&nrm=iso&tlng=pt This work evidences a survey conducted during a teacher professional qualification in Rio Grande do Sul, Brazil. This survey analysed the textbooks used by these teachers. The dynamics consisted of choosing the analytic criteria used by teachers, and adding new criteria for examining their difficulties and ways to choose textbooks. This article emphasises the problematics and difficulties teachers have to make choices and their loss of skills and authority to consider these books more profoundly. <![CDATA[<B>Sobre a primeira lei da termodinâmica</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400019&lng=pt&nrm=iso&tlng=pt Entropy is a concept that has long stimulated human curiosity, resulting in an huge intelectual production. The same has not occurred for the first law of thermodynamics, perhaps because of its apparent obviousness. In this article the first law presentation, as displayed in most traditional physical chemistry textbooks, is criticized. An alternative view is suggested, in accordance with temporal thermodynamics. The time derivative local form of the second law is used to stress the entropy concept implications on the notion of internal energy. <![CDATA[<B>Irving langmuir</B>: <B>o milagre da ciência</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400020&lng=pt&nrm=iso&tlng=pt Irving Langmuir received the Nobel Prize in Chemistry in 1932 "...for his outstanding discoveries and investigations within the field of surface chemistry", according to the Swedish Academy. However, few people know that his work comprises other very important contributions, and not only for chemistry, such as the discovery of plasma, the atomic hydrogen, the pure thermoionic phenomenon, the development of the cloud seeding technique for weather modification, among many others. This paper summarizes Langmuir's most important discoveries and theories, with an especial mention for his practical inventions and his work on the atomic theory. <![CDATA[<B>In memoriam</B>]]> http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-40422001000400021&lng=pt&nrm=iso&tlng=pt Irving Langmuir received the Nobel Prize in Chemistry in 1932 "...for his outstanding discoveries and investigations within the field of surface chemistry", according to the Swedish Academy. However, few people know that his work comprises other very important contributions, and not only for chemistry, such as the discovery of plasma, the atomic hydrogen, the pure thermoionic phenomenon, the development of the cloud seeding technique for weather modification, among many others. This paper summarizes Langmuir's most important discoveries and theories, with an especial mention for his practical inventions and his work on the atomic theory.