Entendendo estrutura molecular com a molécula de hidrogênio ionizada

In this paper a general view about the modern molecular structure theory is developed discussing the ionized hydrogen molecule. We introduce some necessary approximation methods for the electronic and nuclear spectra study adopting a systematic approach. In addition though, we have performed calculations in order to illustrate these methods.

Sistema automático para determinação seqüencial de cianeto livre e total empregando eletrodo tubular íon-seletivo de membrana homogênea

This study presents an automated system for potentiometric determination of free and total cyanide which employs a homogeneous membrane tubular ion-selective electrode. After the electrode is assembled, it is connected to a system composed of 3 three-way solenoid valves, sample line, carrier line, acid stream, and gas diffusion chamber. A Turbo Pascal® computer program, developed specifically for this task, automatically performs all the steps involved in data acquisition and processing. The proposed analytical procedure offers operational simplicity, since detection is performed by a tubular electrode, whose assembly is fast and easy. The system has shown reproducibility (r.s.d. < 0.5%, n=6) and high speed (30 readings/hour); it is efficient for determination of free and total cyanide in waste waters of starch processing plants. The detection limit was 1.2x10-5 and 1.5x10-5 mol L-1, for determination of free and total cyanide, respectively. The linear response range was between 1.2x10-5 and 1.0x10-2 mol L-1 for free cyanide and between 1.5x10-5 and 1.0x10-2 for total cyanide.

Ligações hidrogênio fortes em ácidos dicarboxílicos e diaminas aromáticas

A very short-strong hydrogen bond (<2 Å, >20kcal/mol) is found in the monoanion of certain dicarboxylic acids derived from maleic and dialkylmalonic acids. Certain aromatic diamines that are known as proton sponge have exceptionally high basicity (pKa) and are only monoprotonated with strong acids like percloric acid. The closed proximity between the two basic centers provokes a strong steric interaction that is relieved upon protonation. Similar effects are found in dicarboxylic acids (hydrogen maleate and hydrogen dialkylmalonates) that present a very short distance between the two oxygens and a short-strong hydrogen bond.

Sistema de injeção em fluxo espectrofotométrico para monitorar peróxido de hidrogênio em processo de fotodegradação por reação foto-Fenton

A flow injection spectrophotometric system was projected for monitoring hydrogen peroxide during photodegradation of organic contaminants in photo-Fenton processes (Fe2+/H2O2/UV). Sample is injected manually in a carrier stream and then receives by confluence a 0.1 mol L-1 NH4VO3 solution in 0.5 mol L-1 H2SO4 medium. The product formed shows absorption at 446 nm which is recorded as a peak with height proportional to H2O2 concentration. The performance of the proposed system was evaluated by monitoring the consumption of H2O2 during the photodegradation of dichloroacetic acid solution by foto-Fenton reaction.

Considerações sobre a eletrogeração de peróxido de hidrogênio

This paper presents some results that may be used as previous considerations to a hydrogen peroxide electrogeneration process design. A kinetic study of oxygen dissolution in aqueous solution is carried out and rate constants for oxygen dissolution are calculated. Voltammetric experiments on vitreous carbon cathode shown that the low saturation concentration drives the oxygen reduction process to a mass transfer controlled process which exhibits low values of limiting currents. Results have shown that the hydrogen peroxide formation and its decomposition to water are separated by 400 mV on the vitreous carbon surface. Diffusion coefficients for oxygen and hydrogen peroxide are calculated using data taken from Levich and Tafel plots. In a series of bulk electrolysis experiments hydrogen peroxide was electrogenerated at several potential values, and concentration profiles as a function of the electrical charged passed were obtained. Data shown that, since limiting current plateaus are poorly defined onto reticulated vitreous carbon, cathodic efficiency may be a good criterion for choosing the potential value in which hydrogen peroxide electrogeneration should be carried out.

Estudo da adsorção de hidrogênio e sulfeto na superfície de paládio: aspectos experimentais (eletroquímica) e teóricos (ab initio e Teoria do Funcional da Densidade)

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.

Triterpenóides pentacíclicos de Mentha villosa: identificação estrutural e atribuição dos deslocamentos químicos dos átomos de hidrogênio e carbono

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.

Correlação entre a estrutura atômica superficial e o processo de adsorção-dessorção reversível de hidrogênio em eletrodos monocristalinos Pt(111), Pt(100) e Pt(110)

Platinum is widely used as electrode in electrocatalytic processes, however the use of polycrystalline electrodes introduces a series of variables in the electrochemical system due to the aleatory contribution of all the crystallographic orientations with different surface packing of atoms. Single crystal platinum electrodes of low Miller index present surface structure of high regularity and serve as model to establish a correlation among the macroscopic and microscopic properties of the electrochemical interface. Therefore, the main aim of this work is the study of the voltammetric profiles of the reversible adsorption-desorption of hydrogen on Pt(100), Pt(110) and Pt(111), in order to correlate the electrochemical properties of each different orientation with the surface atomic structure.

Quatro alternativas para resolver a equação de Schrödinger para o átomo de hidrogênio

Quantum chemistry describes the hydrogen atom as one of the few systems that permits an exact solution of the Schrödinger equation. Students tend to consider that little can be learned from the hydrogen atom and forget that it can be used as a standard to test numerical procedures used to calculate properties of multielectronic systems. In this paper, four different numerical procedures are described in order to solve the Schrödinger equation for the hydrogen atom. The basic motivation is to identify new insights and methods that can be obtained from the application of powerful numerical techniques in a well-known system.

Constituintes químicos isolados de simira glaziovii (K. schum) steyerm. e a atribuição dos deslocamentos químicos dos átomos de carbono e hidrogênio do alcalóide ofiorina e seus derivados

Chromatographic fractionation of bark extracts from Simira glaziovii (Rubiaceae) afforded the steroids beta-sitostenone, stigmastenone, beta-sitosterol and stigmasterol, methyl trans-4-hidroxy-3-methoxycinamate (1), the alkaloids harmane (2) and the new stereoisomer of ophiorine B (3). The structures were established by ¹H and 13C NMR, including 2D techniques and mass spectral analysis, of the natural products and pentaacetyllyalosidic acid (4a) and beta-carboline monoterpene tetraacetylglucoside (5, 1,22-lactamlyaloside) derivatives obtained by chemical transformations.

Peróxido de hidrogênio: importância e determinação

A brief discussion about the hydrogen peroxide importance and its determination is presented. It was emphasized some consideration of the H2O2 as reagent (separated or combined), uses and methods of analysis (techniques, detection limits, linear response intervals, sensor specifications). Moreover, it was presented several applications, such as in environmental, pharmaceutical, medicine and food samples.

Constituintes químicos de Ottonia corcovadensis Miq. da floresta Amazônica: atribuição dos deslocamentos químicos dos átomos de hidrogênio e carbono

In an ethanolic extract of leaves of Ottonia corcovadensis (Piperaceae) were identified sixteen terpenoids of essential oil and the three flavonoids 3',4',5,5',7-pentamethoxyflavone (1), 3',4',5,7-tetramethoxyflavone (2) and 5-hydroxy-3',4',5',7-tetramethoxyflavone (3) and cafeic acid (4). Two amides (5 and 6) were isolated from an ethanolic extract of the roots. The structures were established by spectral analysis, meanly NMR (1D and 2D) and mass spectra. Extensive NMR analysis was also used to complete ¹H and 13C chemical shift assignments of the flavonoids and amides. The components of the essential oil were identified by computer library search, retention indices and visual interpretation of mass spectra.

Microemulsões: estrutura e aplicações como sistema de liberação de fármacos

Depending on formula composition, microemulsions may be used as a vehicle for drug administration. In this work the main applicable parameters used in the development of pharmaceutical microemulsions (ME) are analyzed. The conceptual description of the system, theoretical parameters related to formation of internal phases and some aspects of ME stability are described. The pseudo ternary phase diagram is used to characterize ME boundaries and to describe different structures in several regions of the diagram. Some applications of ME as drug delivery systems for different administration routes are also analyzed. ME offer advantages as drug delivery systems, because they favor drug absorption, being in most cases faster and more efficient than other methods in delivering the same amount of drug.

Hidrogenação de compostos orgânicos utilizando método eletroquímico para geração de hidrogênio in situ: hidrogenação eletrocatalítica

Electrocatalytic hydrogenation (HEC) may be compared to catalytic hydrogenation (HC). The difference between these methods is the hydrogen source: HC needs a hydrogen gas supply; HEC needs a source of protons (solvent) to be reduced at a cathode surface. HEC has presented interesting advances in the last decades due to investigation of the influence of the supporting electrolyte, co-solvent, surfactant, presence of inert gas and the composition of the electrode on the reaction. Several classes of organic compounds have been hydrogenated through HEC: olefins, ketones, aldehydes, aromatics, polyaromatics and nitro-compounds. This paper shows some details about the HEC which may be regarded as a promising technique for the hydrogenation of organic compounds both in industrial processes and in laboratories.

Degradação de corantes reativos pelo sistema ferro metálico/peróxido de hidrogênio

In this work the degradation of aqueous solutions of reactive azo-dyes is reported using a combined reductive/advanced oxidative process based in the H2O2/zero-valent iron system. At optimized experimental conditions (pH 7, H2O2 100 mg L-1, iron 7 g L-1) and using a continuous system containing commercial iron wool, the process afforded almost total discolorization of aqueous solutions of three reactive azo-dyes (reactive orange 16, reactive black 5 and brilliant yellow 3G-P) at a hydraulic retention time of 2.5 min. At these conditions the hydrogen peroxide is almost totally consumed while the released total soluble iron reaches a concentration compatible with the current Brazilian legislation (15 mg L-1).