Metodologia AGOA: a modelagem de clusters de hidratação no complexo aziridina···ácido fluorídrico

We present a theoretical study of solvent effect on C2H5N···HF hydrogen-bonded complex through the application of the AGOA methodology. By using the TIP4P model to orientate the configuration of water molecules, the hydration clusters generated by AGOA were obtained through the analysis of the molecular electrostatic potential (MEP) of solute (C2H5N···HF). Thereby, it was calculated the hydration energies on positive and negative MEP fields, which are maxima (PEMmax) and minima (PEMmin) when represent the -CH2- methylene groups and hydrofluoric acid, respectively. By taking into account the higher and lower hydration energy values of -370.6 kJ mol-1 and -74.3 kJ mol-1 for PEMmax and PEMmin of the C2H5N···HF, our analysis shows that these results corroborate the open ring reaction of aziridine, in which the preferential attack of water molecules occurs at the methylene groups of this heterocyclic.

Termocromismo em soluções de alcóxidos de Vanádio(IV): uma abordagem pela modelagem molecular

The thermochromic behavior exhibited by vanadium(IV) alkoxides, [V2(μ-OPr i)2(OPr i) 6] and [V2(μ-ONep)2(ONep)6 ], OPr i = isopropoxide and ONep = neopentoxide, was studied by molecular modeling using DFT, TDDFT and INDO/S methods. The vibrational and electronic spectra calculated for [V2(μ-OPr i)2(OPr i) 6] were very similar to the experimental data registered for crystalline samples of the complex and for its solutions at low temperature (< 210 K), while spectra recorded at high temperature (> 315 K) were compatible with those calculated for the monomeric form, [V(OPr i)4]. These results consistently point to a monomer/dimer equilibrium as an explanation for the solution thermochromism of {V(OPr i)4}n. In spite of the structural similarity between [V2(μ-ONep)2(ONep)6 ] and [V2(μ-OPr i)2(OPr i) 6] in the solid state, the thermochromic behavior of the former could not be explained by the same model, and the possibility of tetranuclear aggregation at low temperatures was also investigated.

Simulação Monte Carlo no ensino de luminescência e cinética de decaimento de estados excitados

A software based in the Monte Carlo method have been developed aiming the teaching of important cases of mechanisms found in luminescence and in excited states decay kinetics, including: multiple decays, consecutive decays and coupled systems decays. The Monte Carlo Method allows the student to easily simulate and visualize the luminescence mechanisms, focusing on the probabilities of the related steps. The software CINESTEX was written for FreeBASIC compiler; it assumes first-order kinetics and any number of excited states, where the pathways are allowed with probabilities assigned by the user.

Propriedades dinâmicas de fluidos por simulação computacional: métodos híbridos atomístico-contínuo

Computational methods for the calculation of dynamical properties of fluids might consider the system as a continuum or as an assembly of molecules. Molecular dynamics (MD) simulation includes molecular resolution, whereas computational fluid dynamics (CFD) considers the fluid as a continuum. This work provides a review of hybrid methods MD/CFD recently proposed in the literature. Theoretical foundations, basic approaches of computational methods, and dynamical properties typically calculated by MD and CFD are first presented in order to appreciate the similarities and differences between these two methods. Then, methods for coupling MD and CFD, and applications of hybrid simulations MD/CFD, are presented.

Simulação Monte Carlo de mecanismo de transferência de energia de excitação eletrônica: modelo de Perrin para a supressão estática da luminescência

A software based in the Monte Carlo method has been developed aiming the teaching of the Perrin´s model for static luminescence quenching. This software allows the student to easily simulate the luminescence decays of emissive molecules in the presence of quenching ones. The software named PERRIN was written for FreeBASIC compiler and it can be applied for systems where the molecules remain static during its excited state lifetime. The good agreement found between the simulations and the expected theoretical results shows that it can be used for the luminescence and excited states decay kinetic teaching.

Uso de modelagem molecular no estudo dos conceitos de nucleofilicidade e basicidade

Molecular modeling enables the students to visualize the abstract relationships underlying theoretical concepts that explain experimental data on the molecular and atomic levels. With this aim we used the free software "Arguslab 4.0.1" (semi-empirical method) to study the reaction of 1-chloropropane with ethoxide in solution, known to lead to methyl propyl ether, through the S N2 mechanism, and propene, through the E2 mechanism. This tool allows users to calculate some properties (i. e. heat formation or electric charges) and to produce 3D images (molecular geometry, electrostatic potential surface, etc.) that render the comprehension of the factors underlying the reaction's progress, which are related to the structure of the reagents, and the process kinetic clearer and easier to understand by the students

Modelagem da cinética de biodegradação de paclobutrazol em dois solos do semiárido do nordeste brasileiro

Mathematical models can help to prevent high levels of toxic substances in soil or fruits of plants treated with pesticides and indicate that such substances should be systematically monitored. The aim of this research was to study the kinetics of paclobutrazol biodegradation by soil native bacteria using mathematical models. Three models were used to assess the kinetics of paclobutrazol biodegradation obtained experimentally. Excellent fits were obtained using dual kinetic and logistic models. The use of glycerol as additional carbon source increased the biodegradation of PBZ and consequently decreased the time required for a given PBZ initial concentration be halved.

Simulação do intemperismo natural de pilhas zinco-carbono e alcalinas

Spent alkaline and Zn-C batteries were placed in seawater, rainwater or landfill leachate at room temperature for up 30 days in order to simulate natural weathering. After the experiments pH and electrical conductivity of the liquid were measured. The precipitate formed and the filtrate were submitted to metal analysis by ICP-OES. Seawater is the most corrosive medium, followed by landfill leachate. Pb, Cd and Hg were mainly in the filtrate. Fe, Mn and Zn were generally dominant in the precipitate. Na and K account for the electrical conductivity and are good indicators of the corrosion stage of the batteries.

Modelagem termodinâmica por extração por solvente de metais divalentes em meio sulfato usando D2EHPA

The extraction of divalent metals (Mn2+, Ni2+, Co2+ and Cu2+) in the system MSO4 - H2SO4 - H2O - D2EHPA in isoparaffin (17/21) was studied by a thermodynamic model based on chemical equilibria with mass and charge balance equations. The activity coefficients of all solutes in the aqueous phase were calculated by Davies equation. By applying this model, the equilibrium concentrations of solutes were calculated from de concentration of divalent metals and pH. The predicted distribution coefficients for the divalents metals were in good agreement with experimental results.

Software para simulação de mecanismo de supressão da luminescência: modelo cinético de Stern-Volmer

A software that includes both Stochastic and Molecular Dynamics procedures has been developed with the aim of visualizing the Stern-Volmer kinetic mechanism of dynamic luminescence quenching. The software allows the student to easily simulate and graphically visualize the molecular collisions, the molecular speed distributions, the luminescence decay curves, and the Stern-Volmer graphs. The software named "SternVolmer" is written for the FreeBASIC compiler and can be applied to dynamic systems where luminescent molecules, during their excited state lifetimes, are able to collide with quenching molecules (collisional quenching). The good agreement found between the simulations and the expected results shows that this software can be used as an effective teaching aid for the study of luminescence and kinetic decay of excited states.

Simulação de sinais de RMN através das equações de Bloch

The aim of this paper was to present a simple and fast way of simulating Nuclear Magnetic Resonance signals using the Bloch equations. These phenomenological equations describe the classical behavior of macroscopic magnetization and are easily simulated using rotation matrices. Many NMR pulse sequences can be simulated with this formalism, allowing a quantitative description of the influence of many experimental parameters. Finally, the paper presents simulations of conventional sequences such as Single Pulse, Inversion Recovery, Spin Echo and CPMG.

TRIHALOMETANOS EM ÁGUA POTÁVEL E RISCOS DE CÂNCER: SIMULAÇÃO USANDO POTENCIAL DE INTERAÇÃO E TRANSFORMAÇÕES DE BÄCKLUND

We briefly investigated the well-known correlation of trihalomethanes, present in fresh water, with cancer hazard in humans. A transient alternative method of chemical simulation using Bäcklund Transformations and Quantum Mechanics is presented. Finally, the method was applied to simulate the interaction between Trichloridemethane and Alanine - as well as its amino and carboxyl groups.

REDUÇÃO DE VARIÁVEIS DE ENTRADA DE REDES NEURAIS ARTIFICIAIS A PARTIR DE DADOS DE ANÁLISE DE COMPONENTES PRINCIPAIS NA MODELAGEM DE OXIGÊNIO DISSOLVIDO

The objective of this work is to demonstrate the efficient utilization of the Principal Components Analysis (PCA) as a method to pre-process the original multivariate data, that is rewrite in a new matrix with principal components sorted by it's accumulated variance. The Artificial Neural Network (ANN) with backpropagation algorithm is trained, using this pre-processed data set derived from the PCA method, representing 90.02% of accumulated variance of the original data, as input. The training goal is modeling Dissolved Oxygen using information of other physical and chemical parameters. The water samples used in the experiments are gathered from the Paraíba do Sul River in São Paulo State, Brazil. The smallest Mean Square Errors (MSE) is used to compare the results of the different architectures and choose the best. The utilization of this method allowed the reduction of more than 20% of the input data, which contributed directly for the shorting time and computational effort in the ANN training.

Efeitos da simulação da solarização do solo com materiais vegetais sobre o crescimento micelial de fungos fitopatogênicos habitantes do solo

A incorporação de materiais vegetais específicos associados à solarização do solo tem sido um avanço promissor no controle de fungos fitopatogênicos habitantes do solo. O objetivo do trabalho foi avaliar determinados efeitos da incorporação e decomposição de brócolis, mamona, mandioca brava e mansa, no solo, em condições de microcosmo mantido em BOD (37±2ºC), sobre o micélio de Fusarium oxysporum f. sp. lycopersici Raça 2, Macrophomina phaseolina, Rhizoctonia solani AG-4 HGI e de Sclerotium rolfsii. Assim, quatro ensaios idênticos foram instalados em conjunto de microcosmos, com cinco tratamentos e quatro períodos de tempo diferentes e independentes (7, 14, 21 e 28 dias). O parâmetro avaliado foi os efeitos inócuo, fungistático e fungicida dos tratamentos sobre o micélio dos fungos. Verificou-se efeito fungistático e fungicida no crescimento micelial de F. oxysporum f. sp. lycopersici Raça 2, R. solani AG-4 HGI e de S. rolfsii. Os fungos que apresentaram efeito fungistático apresentaram uma velocidade média de crescimento micelial inferior ao controle geral, que consistiu na incubação dos fungos em temperatura de 25±2ºC. O efeito fungicida ocorreu aos 21 dias de incubação para F. oxysporum e R. solani e aos 28 dias para S. rolfsii. Para M. phaseolina, observou-se apenas efeito inócuo. Associação da temperatura de 37±2ºC mais o período de tempo dos tratamentos foi o fator responsável pelos efeitos fungistático e fungicida no micélio dos fitopatógenos estudados. Essa associação também interferiu na velocidade do crescimento micelial dos fungos que apresentaram efeito fungistático.