Resumo em Inglês:

Open chain hydroxamic acid (Hx) can exist as Z and E diastereomers of two tautomers, hydroxamic acid and hydroximic acid. The conformational stability of the formohydroxamic acid isomers evaluated by PM3 compared better to ab initio results from the literature than AM1 results. Structural data of the cyclic Hx 2,4-dihydroxy-7-metoxy-2H-1,4-benzoxazin-3(4 H)-one (DIMBOA) obtained by both semiempirical methods compared well to ab initio results. pKa data from the literature for derivatives of the aldolic isomer of DIMBOA were compared to the stability of the anions resulting from the loss of protons of their phenol and hydroxamic acid groups, determined as the difference in heat of formation between anionic and neutral forms, calculated by AM1 and PM3 methods. Good correlations between theoretical and experimental data were obtained for both semiempirical methods.

Resumo em Inglês:

This paper describes a sequential injection analysis (SIA) set-up coupled to a flame atomic absorption spectrometer (FAAS) to accomplish the determination of low concentrations of copper in drinking waters. Copper is first retained under neutral media in an on-line 29x1.6 mm column filled with poly(ethylenimine) immobilised on silica gel. The retained analyte is then eluted by flowing through the column 250 <FONT FACE=Symbol>m</font>L of a nitric acid solution. The selection of 3.85 ml of sample enabled to obtain a detection limit of 0.27 mug/L and a sampling rate of about 24 samples/h. There was a good agrement between the results of 12 samples furnished by the proposed procedure and by electrothermal atomic absorption spectrometry. Repeatability assessment gave a relative standard deviation of 1.3 % after ten replicate analysis of a sample containing about 70 mug/L in copper..

Resumo em Inglês:

The reactions of four new unsymmetrical N,O-donor ligands, {H2BBPETEN= [N-(2-hydroxybenzyl) - N,N' - bis(2 methylpyridyl) -N'-(hydroxyethyl) ethylenodiamine], H3BPETEN=[N,N'- bis(2-hydroxybenzyl) -N- (2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine], HTPETEN=[N,N,N'- tris(2-methylpyridyl) -N'- (hydroxyethyl) ethylenodiamine] and H3BIMETEN=[N,N'-(2-hydroxybenzyl)-N-(1-methylimidazol-2-il-methyl)-N'- (hydroxyethyl)ethylenodiamine]}, with Cu(II) salts afforded the following mononuclear compounds: [CuII(HBBPETEN)]ClO4, [CuII(H2BPETEN)]ClO4 , [CuII(HTPETEN)](PF6)2 and [CuII(H2BIMETEN)]ClO4 . All were characterized by EPR, electronic spectroscopy and electrochemistry. The four copper (II) compounds showed interesting electrochemistry properties. All presented an anodic wave that can be attributed to the Cu (I) oxide formation at the electrode surface, or to a Cu0 sediment at the same surface or yet, to Cu(I) -> Cu(II) oxidation process with coupled chemistry reaction, due to their irreversibility. Two of the complexes are described as interesting synthetic models for the active site of the metalloenzyme galactose oxidase.

Resumo em Inglês:

The compounds 5-nitro-8-quinolinol and 5,7-dinitro-8-quinolinol were obtained by nitration of the chelant 8-quinolinol. The compounds were characterized through EA, MNR, XRD, IR, TG, DTA and DSC. It was verified through thermal analysis that the compounds show consecutive processes of sublimation, fusion and vaporization. During the vaporization process, partial thermal decomposition was observed, with formation of carbonaceous residues. Considering a slower heating rate, the sublimation is the prevalent process to the nitro-derivatives while the vaporization is the main process to 8-quinolinol. The thermal stability follows the decreasing order from 5,7-dinitro-8-quinolinol to 5-nitro-8-quinolinol to 8-quinolinol.

Resumo em Inglês:

Chalcone (1) and its fluorinated derivatives 2-4, as well as their cyclic analogues 5-10, were synthesized through an aldol condensation reaction between the corresponding ketone and aldehyde. These compounds were characterized by IR, EIMS and ¹H and 13C NMR spectral data. Modern NMR techniques allowed us to conclude that the compounds obtained show E configuration. These techniques were also employed to investigate the equilibrium involving the s-cis and s-trans conformations of 1-4, with this equilibrium being dependent on the fluorine substitution on both aromatic rings, A or B. IR studies indicated that the yield of the s-cis conformation in the fluorinated derivatives is 57.4±1.4; 88.1±0.4 and 66.4±0.7%, for 2, 3 and 4, respectively, based on previous ¹H NMR calculations for chalcone. Theoretical calculations, using the MMX method, were employed to justify the variation of chemical shifts for the fluorinated derivatives and cyclic analogues. These chemical shifts are consequence of the anisotropic effect showed by the carbonyl group on these compounds.

Resumo em Inglês:

This paper presents a study of adsorption of xanthate with alkyl chain of two (C2XK), four (C4XK) and eight (C8XK) atoms of carbon, on pyrite from Santa Catarina, Brazil. The results showed that pyrite surface changes from hydrophilic to hydrophobic when xanthate is adsorbed increasing the contact angle to 35º for C2XK, and to 90º for C4XK and C8XK. The rate of flotation of pyrite particles after adsorption increases with the increase of the number of carbon atoms in the alkyl chain in agreement with the results of contact angle measurements.

Resumo em Inglês:

A monoterpene, 3beta,6beta-dihydroxy-p-menth-1-ene has been isolated from the seeds of Xylopia sericea along with four kaurane, one beyerene, one atisene and four trachylobane diterpenoids, including the trachyloban-18- and 19-methyl esters. The X-ray crystal structure of methyl ent-trachyloban-18-oate was determined in order to make an unambiguous distinction between the 18- and 19-esters. The 13C NMR data for ent-15alpha-hydroxy-trachyloban-19-oic acid has been revised.

Resumo em Inglês:

From the hexane extract of leaves, stem and roots, of Vellozia aff. carunculares, fourteen cleistanthane diterpenoids were detected by HRGC-MS, through the analysis of their fragmentation patterns. Comparison of their MS spectra with those of authentic standards previously isolated from other species of the family Velloziaceae allowed the identification, with a high degree of confidence, of eigth known cleistanthane diterpenoids. Of these, four were isolated and identified from their MS, IR and ¹H and 13C NMR spectra, as the known cleistantha-8,11,13-trien-7-one, cleistantha-8,11,13-trien-3,7-dione, cleistantha-1,8,11,13-tetraen-3,7-dione and the first reported 6-alpha-hydroxy-cleistantha-8,11,13-trien-7-one. The triterpenoid beta-amyrin was also isolated.

Resumo em Inglês:

In this review article, we give a general introduction on the mechanisms involved in organic chemiluminescence, where three basic models for excited state formation are presented. The chemiluminescence properties of 1,2-dioxetanes - four membered ring peroxides - are briefly outlined in the second part. In the main part, the mechanisms involved in the decomposition of 1,2-dioxetanes and analogous peroxides are discussed: (i) the unimolecular decomposition of 1,2-dioxetanes; (ii) the electron transfer catalyzed decomposition of peroxides by an intermolecular CIEEL (Chemically Initiated Electron Exchange Luminescence) mechanism; (iii) 1,2-dioxetane decomposition catalyzed by an intramolecular electron transfer mechanism (intramolecular CIEEL). Special emphasis is given to the latter subject, where recent examples with potential analytical applications are presented.

Resumo em Inglês:

A review on the electrophilic addition of iodine to alkenes in the presence of oxygen containing nucleophiles (cohalogenation reaction) is presented. The intermolecular reactions are discussed with emphasis in methods of reaction and synthetic applications of the resulting vicinal iodo-functionalized products (iodohydrins, beta-iodoethers and beta-iodocarboxylates).

Resumo em Inglês:

The use of water as solvent in organic reactions has been uncommon for several reasons, among them the low solubility of the reactants, the incompatibility of the intermediates with water, and the competition between the desired reaction and hydrolysis. Breslow in 1980, demonstrated that the hydrophobic effect accelerates Diels-Alder reactions and gives a high endo/exo selectivity. Since then, many other reactions were studied in this medium, and below we show the principal results. Besides the academic interest, human and economic aspects are included in this study. Water as a solvent minimizes environmental impact, costs and increases operational safety.

Resumo em Inglês:

The present paper aims to bring under discussion some theoretical and practical aspects about the proposition, validation and analysis of QSAR models based on multiple linear regression. A comprehensive approach for the derivation of extrathermodynamic equations is reviewed. Some examples of QSAR models published in the literature are analyzed and criticized.

Resumo em Inglês:

The work here presented was based on 4th International Symposium on Biocatalysis and Biotransformations "BIOTRANS '99" held in Giardini - Naxos, Italy, from september 26 to October 1, 1999. Some of the lectures presented during the congress will be comented which exemplifies the technological inovations and the actual tendencies in the area of biocatalysis.

Resumo em Inglês:

The purpose of this work is to demonstrate the usefulness of low cost high performance computers. It is presented technics and software packages used by computational chemists. Access to high-performance computing power remains crucial for many computational quantum chemistry. So, this work introduces the concept of PC cluster, an economical computing plataform.

Resumo em Inglês:

Thalidomide, first synthesized in 1953, was widely prescribed for morning sickness of pregnant women from 1957 to 1961, when it was found to be seriously teratogenic, having caused serious birth defect. Nowadays, a quarter of a century later, it appears that it may be a miracle drug for such diseases as leprosy, AIDS, cancer and tuberculosis.

Resumo em Inglês:

An important component for the automation of flow injection analysis (FIA) systems is the sample injection valve. A simple and inexpensive commutator with 16 pinch valves (8 normally open and 8 closed) was developed and configured as a multichannel injection valve. It is activated by a single solenoid of 3 Kgf, powered by a pulsed driver circuit, controlled by a microcomputer or a switch. FIA with spectrophometric detection of potassium dichromate solution was used for the evaluation of the new injection valve and its comparison with other valves, for sample loops of 50, 100, 200, 300 and 500 muL. The repeatability was favorable (RSD 1.0% for 15 injections at each loop volume) compared to a manual injector, an electropneumatic injector and an injector configured with three mini solenoid valves (RSD 1.1, 1.3 and 1.0%, respectively, for15 injections at each loop volume).

Resumo em Inglês:

This paper discuss the consequences of the equipartition principle when used to calculate the heat capacity of atoms and molecules, a discussion that appeared at the end of XIX century and beginning of the XX century. Classical molecular thermodynamics prediction of the heat capacity is introduced, followed by a presentation of the degrees of freedom of a system. The historical discussion that appeared at the time, by Dulong, Petit, Maxwell, Boltzmann, Rayleigh and Kelvin is discussed afterwards. The necessity of a new theory is also presented as a direct consequence of the equipartition principle collapse.

Resumo em Inglês:

A didactic experiment is proposed in order to identify inorganic phosphates in home laundry detergents by ascending paper chromatography. Chemical principles related with hard water are also investigated. The role of polyphosphates in detergent formulations is discussed and the softening of hard water by sodium tripolyphosphate is illustrated. Several chemical concepts, related to the daily experience of the students, can be explored in post-lab discussions.

Resumo em Inglês:

The historiography of science has undergone a profound change from the traditional perspective of a few decades ago, according to which only mainstream science was worth pursuing as a research field. Such a restricted outlook was not conducive to a thorough understanding of the whole process of the construction of science. New approaches have breathed new life into the discipline, taking into account not only scientific endeavours from so-called peripheral societies, but also from different quarters within or close to the central groups. From this a new and far more interesting view has emerged, showing a complex and richer texture.