Núcleos exóticos e síntese dos elementos químicos

We have around ninety chemical elements available in nature, which were produced mainly by nuclear reactions inside stars. The fusion reactions are the main synthesis process which generates the light and intermediate masses elements. The synthesis begins with the hydrogen burning reaching the region of iron mass nuclei. Heavier elements are synthesized by neutron capture processes, forming exotic nuclei with large neutron excess. These systems present characteristics very different from nuclei inside of stable atoms; they only occur in particular astrophysical environments or are produced artificially in special laboratory conditions. This work discusses some properties of the exotic nuclei and how they participate in the synthesis of elements.

Energia de superfície para nanossuperfícies de TiO2 na direção (001)

In this work was made an investigation about bulk and surface models (at maximum 20 layers) of the TiO2 material in the (001) direction. TiO2 commercial sample was feature using XDR technique to determine phase and crystallites average size. Bulk and (001) surface models were simulated for TiO2 material using DFT/B3LYP and its results were used for calculating energy surface, electronic levels, superficial atomic displacement and charge maps. Atoms of the first and second layers of the slab model showed electronic densities very well organized in the form of chains or wires.

Synthesis and antifungal activity of halogenated aromatic bis-γ-lactones analogous to avenaciolide

Here we describe the total syntheses and characterization by elemental analyses, infrared and NMR spectroscopy of three new compounds analogous to avenaciolide, a bis-γ-lactone isolated from Aspergillus avenaceus that possesses antifungal activity, where the octyl group of the natural product was replaced by aromatic groups containing chlorine and fluorine atoms. The effects of the avenaciolide, the novel compounds and their synthetic precursors on mycelia development and conidia germination of Colletotrichum gloeosporioides and Fusarium solani were evaluated in vitro. The title compounds were almost as active as avenaciolide. The absolute structures of the chlorinated analogs were determined by X-ray diffraction analysis.

Síntese e controle de qualidade da ureia enriquecida em 13C para diagnóstico da Helicobacterpylori (HP)

The aim of the study was to synthesize the urea (13CO(NH2)2), with 99% 13C atoms, and to perform a quality analysis for the diagnosis (breath test) of Helicobacter pylori. Furthermore, the process was submitted to economic analysis. The reaction was performed in a stainless steel reactor, lined with polytetrafluoroethylene, under low pressure and temperature. The synthesis method was shown to be appropriate (2.35 g; 81.9% yield), evidenced by physico-chemical and microbiological results, according to Brazilian legislation. The production and diagnosis costs were competitive compared with national and international market values, rendering this a valuable tool in clinical medicine.

Synthesis and biological activity of sulfur compounds showing structural analogy with combretastatin A-4

We extended our previous exploration of sulfur bridges as bioisosteric replacements for atoms forming the bridge between the aromatic rings of combretastatin A-4. Employing coupling reactions between 5-iodo-1,2,3-trimethoxybenzene and substituted thiols, followed by oxidation to sulfones with m-CPBA, different locations for attaching the sulfur atom to ring A through the synthesis of nine compounds were examined. Antitubulin activity was performed with electrophoretically homogenous bovine brain tubulin, and activity occurred with the 1,2,3-trimethoxy-4-[(4-methoxyphenyl)thio]benzene (12), while the other compounds were inactive. The compounds were also tested for leishmanicidal activity using promastigote forms of Leishmania braziliensis (MHOM/BR175/M2904), and the greatest activity was observed with 1,2,3-trimethoxy-4-(phenylthio)benzene (10) and 1,2,3-trimethoxy-4-[(4-methoxyphenyl) sulfinyl]benzene (15).

Electron paramagnetic resonance (EPR) spectral components of spin-labeled lipids in saturated phospholipid bilayers: effect of cholesterol

Electron paramagnetic resonance (EPR) spectroscopy was used to study the main structural accommodations of spin labels in bilayers of saturated phosphatidylcholines with acyl chain lengths ranging from 16 to 22 carbon atoms. EPR spectra allowed the identification of two distinct spectral components in thermodynamic equilibrium at temperatures below and above the main phase transition. An accurate analysis of EPR spectra, using two fitting programs, enabled determination of the thermodynamic profile for these major probe accommodations. Focusing the analysis on two-component EPR spectra of a spin-labeled lipid, the influence of 40 mol % cholesterol in DPPC was studied.

O centenário da Teoria de Bohr

The year 2013 marks the centennial of that wondrous year in which Niels Bohr proposed a novel theory about the constitution of atoms and small molecules after which the way we regard atoms and their behaviour began to be drastically altered. Bohr drew on several sources for his original description of the atoms, most importantly on spectroscopy and Balmer's equation thereof, the new quantum hypothesis advanced a few years earlier by Planck, and the planetary atom proposed by Rutherford. Although Bohr's ideas were to be eventually overtaken by the advent of quantum mechanics, his theory was the basis of a new thinking about atoms and molecules which constitutes an invaluable asset in the development of science ever since.

O centenário da molécula de Bohr

A hundred years ago, a twenty-eight year old Danish scientist published a series of three papers in which electron motion was quantized. The Bohr atomic model is surely known by every chemistry student. Nevertheless in this same 1913 trilogy, Bohr studied atoms with several electrons as well as molecules. Chemistry students, in general, are not aware of the Bohr molecule. The present paper aims at rescuing this important classical model. A review of the Bohr atomic model for both one and several electrons is discussed, together with a theoretical presentation of the Bohr molecule.

Os químicos ocultos e sua extraordinária jornada ao mundo dos átomos

At the end of the XIX century, a group of chemists and theosophists called the Occult Chemists suggested it would be possible "to see" atoms by clairvoyance. In a meditative situation, a skilled person in contact with a substance would thus be able to see magnified atoms and molecules, as well as its internal structure. Annie Besant was the leader of this group and, together with Curuppumullage Jinarajadasa, Charles Leadbeater and Bertram Keightley, they devised an extraordinary atomic theory in which atoms consisted of smaller, indivisible units of energy called anu. In this paper, we present the fundamental principles of this unusual theory.

Ressonância: desenvolvimento, utilização e avaliação de um software educacional

Resonance is a useful concept that shows how electrons are shared between two or more atoms and allows a prediction of the chemical reactivity and relative stability of reagents, intermediates, and products. An educational software that enables interactive exploration of the concepts related to resonance and thereby facilitates its understanding was developed. The software was field-tested, and an evaluation questionnaire concerning the software as an educational tool was answered by the students and professors involved in the test. The results led to the conclusion that the developed computer application can be characterized as an auxiliary tool that assists teachers in their lectures and students in their learning process.

Bateria de métricas para avaliação da verdura material de reações de síntese

The use of a battery of three mass metrics (atomic economy - AE, reaction mass efficiency - RME, and mass intensity - MI) for systemic evaluation of the material greenness of synthesis reactions is presented. Material greenness is discussed in terms of materialization/dematerialization of the reaction system and also according to the first two Principles of Green Chemistry, and is shown to involve two components: atomic greenness (incorporation of the atoms provided by reagents into the product, evaluated by AE and RME); and massic greenness (global mass of reagents and non-stoichiometric materials, evaluated by MI, related to the production of residues).

GLOBAL AND LOCAL REACTIVITY DESCRIPTORS FOR PICLORAM HERBICIDE: A THEORETICAL QUANTUM STUDY

In this work, we studied the reactivity of picloram in the aqueous phase at the B3LYP/6-311++G(2d,2p) and MP2/6-311++G(2d,2p) levels of theory through global and local reactivity descriptors. The results obtained at the MP2 level indicate that the cationic form of picloram exhibits the highest hardness while the anionic form is the most nucleophilic. From the Fukui function values, the most reactive site for electrophilic and free radical attacks are on the nitrogen in the pyridine ring. The more reactive sites for nucleophilic attacks are located on the nitrogen atom of the amide group and on the carbon atoms located at positions 2 and 3 in the pyridine ring.

A NEW HOMOGENEOUS ELECTROCATALYST FOR ELECTROCHEMICAL CARBONYLATION OF METHANOL TO DIMETHYL CARBONATE

Electrosynthesis of dimethyl carbonate (DMC) from methanol and carbon monoxide using an Cu(phen)Cl2 catalyst was achieved at room temperature and atmospheric pressure. The catalytic activity of the ligand 1,10-phenanthroline (phen) and the catalytic system were analyzed. The IR characterization results for the complex catalyst showed that copper ions were coordinated by nitrogen atoms of phen. In addition, the effects of the influencing factors, such as reaction time (t), reaction temperature (T) and the surface area of the working electrode (SWE) were studied.

METHANE DRY REFORMING OVER Ni SUPPORTED ON PINE SAWDUST ACTIVATED CARBON: EFFECTS OF SUPPORT SURFACE PROPERTIES AND METAL LOADING

The influence of metal loading and support surface functional groups (SFG) on methane dry reforming (MDR) over Ni catalysts supported on pine-sawdust derived activated carbon were studied. Using pine sawdust as the catalyst support precursor, the smallest variety and lowest concentration of SFG led to best Ni dispersion and highest catalytic activity, which increased with Ni loading up to 3 Ni atoms nm-2. At higher Ni loading, the formation of large metal aggregates was observed, consistent with a lower "apparen" surface area and a decrease in catalytic activity. The H2/CO ratio rose with increasing reaction temperature, indicating that increasingly important side reactions were taking place in addition to MDR.

VIDROS DE OXICARBETO DE SILÍCIO OBTIDOS A PARTIR DE POLISSILOXANOS

AbstractSilicon oxycarbide glasses (SiOC) are a class of amorphous materials with a similar silica glass structure, in which oxygen atoms are partially replaced by tetracoordenated carbon atoms. The presence of carbon atoms covalently bound to the silicon atoms creates a more interconnected structure with better strength, and excellent chemical stability than conventional silica. SiOCs are easily prepared by the pyrolysis of polysiloxanes and can potentially be implemented in several technological applications that require high temperatures. This paper mainly addresses the preparation, structure, and properties of SiOC. Furthermore, potential applications of SiOC are also introduced.