Modelos contínuos do solvente: fundamentos

Continuum solvation models are nowadays widely used in the modeling of solvent effects and the range of applications goes from the calculation of partition coefficients to chemical reactions in solution. The present work presents a detailed explanation of the physical foundations of continuum models. We discuss the polarization of a dielectric and its representation through the volume and surface polarization charges. The Poisson equation for a dielectric was obtained and we have also derived and discuss the apparent surface charge method and its application for free energy of solvation calculations.

The role of aromaticity in determining the molecular structure and reactivity of (endohedral metallo)fullerenes

The encapsulation of metal clusters in endohedral metallofullerenes (EMFs) takes place in cages that in most cases are far from being the most stable isomer in the corresponding hollow fullerenes. There exist several possible explanations for the choice of the hosting cages in EMFs, although the final reasons are actually not totally well understood. Moreover, the reactivity and regioselectivity of (endohedral metallo)fullerenes have in the past decade been shown to be generally dependent on a number of factors, such as the size of the fullerene cage, the type of cluster that is being encapsulated, and the number of electrons that are transferred formally from the cluster to the fullerene cage. Different rationalizations of the observed trends had been proposed, based on bond lengths, pyramidalization angles, shape and energies of (un)occupied orbitals, deformation energies of the cages, or separation distances between the pentagon rings. Recently, in our group we proposed that the quest for the maximum aromaticity (maximum aromaticity criterion) determines the most suitable hosting carbon cage for a given metallic cluster (i.e. EMF stabilization), including those cases where the IPR rule is not fulfilled. Moreover, we suggested that local aromaticity plays a determining role in the reactivity of EMFs, which can be used as a criterion for understanding and predicting the regioselectivity of different reactions such as Diels-Alder cycloadditions or Bingel-Hirsch reactions. This review highlights different aspects of the aromaticity of fullerenes and EMFs, starting from how this can be measured and ending by how it can be used to rationalize and predict their molecular structure and reactivity

Building complexity in O2-binding copper complexes : site-selective metalation and intermolecular O2-binding at dicopper and heterometallic complexes derived from an unsymmetric ligand

A novel unsymmetric dinucleating ligand (LN3N4) combining a tridentate and a tetradentate binding sites linked through a m-xylyl spacer was synthesized as ligand scaffold for preparing homo- and dimetallic complexes, where the two metal ions are bound in two different coordination environments. Site-selective binding of different metal ions is demonstrated. LN3N4 is able to discriminate between CuI and a complementary metal (M′ = CuI, ZnII, FeII, CuII, or GaIII) so that pure heterodimetallic complexes with a general formula [CuIM′(LN3N4)]n+ are synthesized. Reaction of the dicopper(I) complex [CuI 2(LN3N4)]2+ with O2 leads to the formation of two different copper-dioxygen (Cu2O2) intermolecular species (O and TP) between two copper atoms located in the same site from different complex molecules. Taking advantage of this feature, reaction of the heterodimetallic complexes [CuM′(LN3N4)]n+ with O2 at low temperature is used as a tool to determine the final position of the CuI center in the system because only one of the two Cu2O2 species is formed

Triggering the generation of an iron(IV)-oxo compound and its reactivity toward sulfides by RuII photocatalysis

The preparation of [FeIV(O)(MePy2tacn)]2+ (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [FeII(MePy2tacn)(solvent)]2+ (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru II(bpy)3]2+ as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru II(bpy)3]2+ are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [RuIII(bpy)3] 3+ oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide

Antioxidantes na manutenção do equilíbrio redox cutâneo: uso e avaliação de sua eficácia

The skin is a very complex organ, continuously exposed to physical, chemical and microbiological agents. Enzymes as well as low-molecular weight antioxidants are present in the cutaneous tissue to counterbalance the deleterious effect caused by an oxidative stress and thus maintain homeostasis. Antioxidants such as vitamins C and E, carotenoids, and extracts with these properties have been extensively used for treatment of pathologies and skin aging prevention. We review here different mechanisms that can interfere in the redox equilibrium of the skin, as well as the chemical reactions involved in these processes. Moreover, we discuss the importance of endogenous or exogenous antioxidants that can be acquired from the diet or from oral or topical administration, and methodologies that have been developed to evaluate their efficacy.

Uma estratégia adaptada para síntese de magnetita

Chemical reactions in the solid state are often not included in undergraduate chemistry curricula, due to requirements for special laboratory facilities such as ovens and precision weighing balances. This work aims to describe novel and relatively simple magnetite chemical syntheses in dry medium, which could also be used as an alternative for freshmen chemistry experiments. The proposed reaction was carried out in a muffle furnace by heating (i) a sol-gel preparation and (ii) natural hematite, under nitrogen atmosphere at 400 ºC. The synthesized magnetite was characterized with powder X-ray diffraction and Mössbauer spectroscopy. Results show that magnetite samples have chemical properties as well as crystalline structure quite similar to those of standard natural magnetite.

Uso de scanner em espectrofotometria de absorção molecular: aplicação em experimento didático enfocando a determinação de ácido ascórbico

Quantitative analysis of colored substances is generally performed using a spectrophotometer for detection of radiation. However, the combination of digital images and mathematical conversion of colors is an alternative for quantitative procedures based on colored chemical reactions. This work proposes a didactic experiment using a scanner for determination of ascorbic acid in medicines based on the formation of the Prussian blue complex. Results were in good agreement with a conventional spectrophotometer method and the procedure is attractive for instrumental analysis undergraduate courses.

Conceitos de química dos ingressantes nos cursos de graduação do Instituto de Química da Universidade de São Paulo

A diagnostic instrument was developed to evaluate the basic chemistry concepts held by freshmen students of the three Chemistry undergraduate courses offered by the University of São Paulo. The instrument minimizes the use of algorithms or memorization by students and values high-order cognitive skills. Analysis of the students' performances reveals systematic use of "displacement reaction" as an algorithm and a mechanical use of Le Chatelier's Principle. Failure in comprehending the chemical equation and chemical language drives students to alternative models for chemical reactions in aqueous solution. For instance, reaction would occur between "ionic pairs" and/or between species situated in separate compartments.

Uma métrica gráfica para avaliação holística da verdura de reacções laboratoriais - "Estrela Verde"

A new semi-quantitative metrics, Green Star (GS), for evaluation of the global greenishness of chemical reactions used in teaching laboratories has been developed. Its aim is to help choosing the more acceptable reactions for implementing Green Chemistry (GC) and to identify suitable modifications of reaction protocols to improve the greenishness of chemistry. GS considers globally all the Twelve Principles of GC. To illustrate its construction, the tetraamminecopper(II) sulfate monohydrate laboratory synthesis, performed under several sets of conditions to pursue greenishness, is presented. A comparative study with other GC metrics showed the advantages of GS and that it accomplishes its purpose.

A Equação-Mestra: atingindo o equilíbrio

In the last years, a great interest in nonequilibrium systems has been witnessed. Although the Master Equations are one of the most common methods used to describe these systems, the literature about these equations is not straightforward due to the mathematical framework used in their derivations. The goals of this work are to present the physical concepts behind the Master Equations development and to discuss their basic proprieties via a matrix approach. It is also shown how the Master Equations can be used to model typical nonequilibrium processes like multi-wells chemical reactions and radiation absorption processes.

Luz: um raro produto de reação

The production of visible light by chemical reactions constitutes interesting and fascinating phenomena and several reaction mechanisms are discussed to rationalize excited state formation. Most efficient chemiluminescence reactions are thought to involve one or more electron transfer steps and chemiexcitation is believed to occur by radical annihilation. A brief introduction to the general principles of light production and the main known chemiexcitation mechanisms will be given here. Subsequently, recent results on the mechanistic elucidation of efficient chemiluminescence systems, as the peroxyoxalate reaction, the induced decomposition of phenoxy-substituted 1,2-dioxetanes and the catalyzed decomposition of new a-peroxylactones will be discussed.

Avaliação dos efeitos do ultrassom terapêutico sobre a cafeína e verificação da liberação em sistema de difusão vertical

Currently, the physiotherapists use the phonoforesis, which consists in the therapeutical ultrasound (US) used to increase the drug molecules migration through the skin, however, the US can shows oxidative effects, and is used, for example, in chemical reactions acceleration. The present study aimed to perform the electrochemical evaluation and the diffusion investigation of gel/caffeine 5% solutions submitted to therapeutical US (continuous mode, 1.0 W cm-2 and 1 MHz). It this study, it has been verified diffusion increase and a possible oxidation of the caffeine molecules, when subjected to therapeutical US.

Furfural - da biomassa ao laboratório de química orgânica

The goal of this manuscript is provide to students of Chemistry and related areas an alternative experiment in which they can obtain a compound and learn to observe and interpret properties and predict organic structure by obtaining furfural from biomass. Furfural is an organic compound, obtained through acid hydrolysis of pentosans, commonly used in the chemical and pharmaceutical industries. Students are guided to get furfural through extractive procedures and chemical reactions adapted to semi-micro laboratory scale. Characterization of furfural was done by chemical tests and physical properties. Identification was accomplished by a series of spectroscopic and spectrometric techniques.

Geração de mesoporos em zeólitas ZSM-5 e seus efeitos na conversão do etanol em olefinas

Developing mesoporosity in HZSM-5 zeolites is an alternative for improving their catalytic performance on chemical reactions. In this work, alkaline and thermal treatments were used to produce mesoporosity. These treatments increased mesoporous area and volume. They also influenced the acid properties of the samples. Concerning catalytic performance, treatments modified reaction product distribution. Whereas alkaline treatment favored formation of olefins and increased propene ratio in the beginning of the reaction, thermal treatment resulted in formation of only ethylene due to the low acidity of the sample.

Miniprojeto para ensino de química geral experimental baseado na fermentação do caldo de cana-de-açúcar

This paper describes a five-week mini-project for a general chemistry laboratory course. Activities are included preparations of ethanol and sucrose solutions, calculation of concentrations, determination of densities with densimeters, sugarcane juice fermentation with CO2 capture in alkaline solution, distillation, and determination of amounts of ethanol and CO2 formed. Abilities and concepts normally present in practical general chemistry courses are covered: use of balances, volumetric glassware and densimeters, preparation of solutions, performing of dilutions, determination of solution densities, observation of chemical reactions, stoichiometric calculations, separation of mixtures, and titration.