Resumo em Inglês:

In this work are presented two modified forms of Kapustinskii equation that could be used to estimate the values of the lattice enthalphies for adducts: DM Hm o=(-n.z+ .z- .10(2)/D).(1-d*/D) .K and DM Hm o=(-n.z+ .z-.10(2)/d).(1-d*/d).K.d. Two new parameters related with steric effects and donor power of the ligands, J anddare introduced. The proposed equations were tested for 49 adducts (mainly from the zinc group halides). The difference between experimental (calorimetric) and calculated values (using the proposed equations) values are less than 5% for 41 of the tested adducts.

Resumo em Inglês:

Studies of the use of a soil from river Pardo basin located at the Ribeirão Preto region, were realized with the aim of preparing catalysts. A clay, high purity kaolin type, was obtained after purification followed by treatment with acid and then calcined. The activity and selectivity of the catalysts were determined using cycloexene as substrate. The majority of the catalysts obtained yield a conversion higher than 70%.

Resumo em Inglês:

Four naphthoquinones, lapachol, a-lapachone, dehydro-a-lapachone and 4-hydroxy-a-lapachone were isolated from the ethanol extract of Zeyheria montana M. roots (Bignoniaceae) and were identified by spectroscopic methods. These naphthoquinones, except 4-hydroxy-a-lapachone, were quantified by HPLC, in the crude ethanol extracts of Z. montana roots. The separation of lapachol, a-lapachone and dehydro-a-lapachone was achieved and these compounds were identified in the chromatograms by their retention times and by their on line UV-spectra. The quantification of the three naphthoquinones, in four different samples showed medium contents of 11,0 mg of lapachol, 6,1 mg of a-lapachone and 4,3 mg of dehydro-a-lapachone, in 100 mg of Z.montana roots.

Resumo em Inglês:

Organotin(IV) derivatives containing bulky R groups have been synthesized and characterized. It is discussed how their syntheses depend on the reaction conditions, such as stoichiometry of starting materials, bulkiness of the organo group attached to the Sn centre as well as temperature in which the reaction is carried out. Finally the molecular structure of Sn2Ph6, determined by X-ray diffraction is reviewed.

Resumo em Inglês:

The flowers of Stiffitia chrysantha Mikam(Asteraceae) contain eriodictiol, quercetin, luteolin and b-D-glycopyranosil-sitosterol. These compounds and its derivatives were identified by their 1H and 13C NMR, infra-red and mass spectra data. The heteronuclear 2D NMR were used to confirm the assignments of the proton and carbon chemical shifts, it was used to eliminate definitively the ambiguous correlation reported in the literature for C-5 and C-9 of quercetin and C-23 and C-25 of b-D-glycopyranosil-sitosterol.

Resumo em Inglês:

The hexane extract of leaves of B. microphylla afforded a mixture of triterpenes esterified with fatty acids. Analyses of spectral data of the mixture and of the derivatives obtained by a transesterification reaction with NaOMe/MeOH permitted to identify the composition of the mixtures as being 24-hydroxy-urs-12-enyl 3b-eicosanate, estearate and palmitate as well as of the 24-hydroxy-olean-12-enyl 3b-eicosanate, estearate and palmitate. From the choroform and ethyl acetate extracts were isolated the oleanolic and 3b,24-dihydroxy-urs-12-en-28-oic acids, quercetin and methyl galic ester, respectively. The compounds were identified through analysis of their spectral data.

Resumo em Inglês:

Selectivity studies for the determination of Cr(VI) using the catalytic oxidation of the o-dianisidine by hydrogen peroxide showed two distincts situations. In the first, when interferents were studied by a univariate procedure, Cr(III) and Cu(II) cause serious interferences even at the 2:1 proportion, relative to Cr(VI), while Fe(III) interfered at the 15:1 ratio and EDTA at the 10:1 ratio. On the other hand, when a multivariate investigation was performed, Cr(III) did not present any significant principal effects and its significant interaction effects were negative, in contrast to EDTA, that presented positive interaction effects although, like Cr(III), did not show significant interaction effects. In view of the interferent's action it become necessary to separate Cr(VI) by extraction with methylisobutylketone in a chloridric acid medium before its determination in vegetals and in wastewater from a cellulose industry samples. Using this procedure, the method precision is ±0,5% at the 10 ng/mL Cr(VI) concentration level. The detection and quantification limits, calculated by means of absorbance measurements of ten replicates of blank reagents were 1,1 and 3,2 ng/mL, respectively. The results obtained with real samples showed a relative standard deviation between 1,2% and 3,0% relative to their reference values.

Resumo em Inglês:

Cr(III) slowly forms a violet complex with EDTA at pH 2.5-5.0 under normal conditions. The complex formation can be accelerated by irradiating the reacting mixture with microwave energy. The complex Cr(III)-EDTA is completely formed within 3 minutes of microwave irradiation and followed by a simple and rapid method for spectrophotometric determination of chromium. The method may be successfully applied to determination of chromium in catalyst.

Resumo em Inglês:

Solid-phase microextraction (SPME) has been applied to direct extraction of 11 organophosphorus pesticides in water using a 100 mm fiber polydimethylsiloxane. The method was evaluated with respect time of exposure, detection limits (LODs), linearity and precision. The detection limits (S/N = 3) depend of each pesticide and varie about ng/L levels. The linearity was satisfactory with coefficients of correlation usually greater than 0.993. The precision of the method was determined by extraction from 4.0 mg/L aqueous standard with coefficients of variation between 5.7 to 17.2%.

Resumo em Inglês:

A simple spectrophotometric method is proposed for the determination of cefaclor. The method involves alkaline hydrolysis of the drug in ammonia buffer solution at pH 10 to yield diketopiperazine-2,5-dione derivative and subsequent measurement at 340 nm. Beer's law is obeyed in the concentration range 1.8 - 55 mg/mL. The proposed method was successfully applied to the determination of cefaclor in pharmaceutical formulations.

Resumo em Inglês:

By using DSC data is shown that there are empirical correlations between lattice enthalpy, melting enthalpy and the temperature of melting for adducts, and that is possible, using only a single DSC curve, estimate the value of DM Hmq.

Resumo em Inglês:

A review of heterocyclic compounds in roasted coffee is presented. The contents, precursors and sensorial properties of furans, pyrroles, oxazoles, thiazoles, thiophenes, pyrazines and pyridines are discussed. The impact heterocyclic compounds of coffee aroma are described.

Resumo em Inglês:

The broad variety of hydrogenation methods of polydienes is presented. Homogeneous and heterogeneous catalysis are reviewed emphasizing also hydrogen transfer from donor compounds.

Resumo em Inglês:

The ability of enzymes to function in aquo-restricted media (commonly reported as non-aqueous media) has greatly enlarged the applications in numerous fields. The development of biosensors was also benefited with this area, expanding their applications toward many previously inaccessible analytes, enlarging significantly the scope of applications. In this review, some basic factors that influence biocatalytical reactions in aquo-restricted media is discussed. Attention is focused on the development, advantages and analytical applications of biosensors in non-aqueous media. Recent examples and relevant applications of their use are presented.

Resumo em Inglês:

The applicability of the silylant agents of the general formula Y3Si-R-X, depends on the reactivity of Y group (halide or alcoxide) attached to silicon and the organic function X (halide, amine, thiol, cyanide, etc) in the extreme position of the chain. Both groups are linked together by an organic chain R, containing usually three methylene groups. A series of these agents can be covalently bonded to an inorganic matrix, since the available OH groups are distributed on the surface, making silica gel the most common support. However, other inorganic oxides, zeolites, lamellar inorganic phosphates and chrysotile can also have these agents anchored. Some illustration are presented for immobilized surface in the use as extractors of cations from dilute aqueous or non-aqueous solutions, catalysts agents, ionic exchanged materials, support for enzyme immobilization, chromatographic applications, use in some industrial features and in many other areas. The evolution of this exciting research field to produce new materials, for many tecnological applications, is strongly dependent on the development of a sensible systematic process for the synthesis of a series of new specific silylant agents.

Resumo em Inglês:

Statistical mechanics Monte Carlo simulation is reviewed as a formalism to study thermodynamic properties of liquids. Considering the importance of free energy changes in chemical processes, the thermodynamic perturbation theory implemented in the Monte Carlo method is discussed. The representation of molecular interaction by the Lennard-Jones and Coulomb potential functions is also discussed. Charges derived from quantum molecular electrostatic potential are also discussed as an useful methodology to generate an adequate set of partial charges to be used in liquid simulation.

Resumo em Inglês:

A selection of common procedures which are used for the identification and analysis of volatile biologically active compounds from insects are described.

Resumo em Inglês:

The mitigation of carbon dioxide is one of the scientific and technological challenges of the 2000s. Among the technologies that are under assessment, the recovery of carbon dioxide from power plants or industrial flue gases plays a strategic role. Recovered carbon dioxide can be either disposed in natural fields or used. The availability of large amounts of carbon dioxide may open new routes to its utilisation in biological, chemical and innovative technological processes. In this paper, the potential of carbon dioxide utilisation in the short-, medium-term is reviewed.

Resumo em Inglês:

Internet publication will radically alter how chemists will publish their research in the next century. In this article, we describe two fundamental changes: enhanced chemical publication which allows chemists to publish materials that cannot be published on paper and end-user customization which allows readers to read articles prepared to meet their specifications. These concepts have been implemented within the Internet Journal of Chemistry, a new journal designed to employ the latest technologies for chemical publications.

Resumo em Inglês:

In this work we report the obtention of a tetrabutylammonium hydroxide (TBAOH) solution in acetonitrile in a one pot process in order to study the interaction ironporphyrinOH- in non-aqueous systems. All the reactions were carried out under dry argon atmosphere to prevent the contamination of the solution with CO2, which leads to the formation of (TBA)2CO3.

Resumo em Inglês:

From an analysis of a learning model based on the theory of information processing four hypothesis were developed for improving the design of laboratory courses. Three of these hypotheses concerned specific procedures to minimise the load on students' working memories (or working spaces) and the fourth hypothesis was concerned with the value of mini-projects in enhancing meaningful learning of the knowledge and skills underpinning the set experiments. A three-year study of a first year undergraduate chemistry laboratory course at a Scottish university has been carried out to test these four hypotheses. This paper reports the results of the study relevant to the three hypotheses about the burden on students' working spaces. It was predicted from the learning model that the load on students working space should be reduced by appropriate changes to the written instructions and the laboratory organisation and by the introduction of prelab-work and prelab-training in laboratory techniques. It was concluded from research conducted over the three years period that all these hypothesised changes were effective both in reducing the load on students' working spaces and in improving their attitudes to the laboratory course.

Resumo em Inglês:

The education of a chemistry teacher is not being seen as a specific preparation for the exercise of a professional activity, which differs from the professional activity of a chemist or a chemistry researcher. This happens because in the academic field, mainly in the exact and natural sciences, the idea that knowing a determined subject is enough to be able to teach it, is tacitly accepted. This is, actually, the first condition to be able to be a university professor. There is another necessary condition for competent teaching: to have specific professional knowledege of teaching. I defend that this knowledge can be constitued in the interaction with other professionals in the form of research. Thus, as a chemist constitutes himself through the research in the interaction with other chemists, the teacher develops himself in this professional field in the interaction with other teachers and the permanent attitude of researching.

Resumo em Inglês:

A résumé of the evolution of quantum chemistry methodologies is presented. The pioneering contributions of John A. Pople and Water Kohn, the 1998 Nobel Prize Laureates in Chemistry, to the development of quantum chemistry computational methods for studying the properties of molecules and their interaction is discussed.