Cláudia Flores Braga

Conformational analysis of protonated cyclo-[(S)-phenylalanyl-(S)-histidyl] and its complex with benzaldehyde within metal-organic frameworks (MOFs)

A analise conformacional do dipeptideo protonado ciclo-[(S)-fenilalanil-(S)-histidil], denominado ciclo-[(S)-Phe-(S)-His-H+], dentro das cavidades de novos materiais porosos do tipo IRMOFs-phen (redes metal-orgânicas isoreticulares com pontes de 2,7-dicarboxilato fenantreno substituido) foi realizada com o metodo de quimica quântica AM1. Foram consideradas duas formas do ciclo-[(S)-Phe-(S)-His-H+], a saber, a forma ligada, em que o peptideo esta quimicamente ligado ao anel fenantreno, e a forma nao-ligada, em que o peptideo esta livre dentro da cavidade. Os conformeros mais estaveis dentro da cavidade foram comparados aqueles encontrados em fase gas ou em agua (modelados pelo metodo de solvente implicito SM5.4). As conformacoes preferenciais dentro da cavidade da IRMOF-phen sao bem distintas das encontradas em fase gas ou em agua. O ambiente quimico da cavidade e tambem relevante, pois a substituicao de um atomo de hidrogenio por CH3 (IRMOF-phen-CH3) ou Br (IRMOF-phen-Br) forneceu novas IRMOFs-phen que alteram significativamente as populacoes dos conformeros, assim como o sitio de adsorcao do dipeptideo. Estes resultados tem implicacoes importantes no controle da estereo-seletividade de reacoes na presenca de indutores quirais. De fato, esta e a primeira vez que uma conformacao fechada para o ciclo-[(S)-Phe-(S)-His-H+] e determinada, assim como a obtencao de uma estrutura estavel para o complexo ciclo-[(S)-Phe-(S)-His-H+]-benzaldeido usando o metodo ONIOM(PBE1:AM1). Este complexo e compativel com a estrutura proposta para o estado de transicao da hidrocianacao do benzaldeido que explica a enantio-seletividade observada.

Matrix Isolation FTIR Spectroscopic and Theoretical Study of 3,3-Dichloro-1,1,1-Trifluoropropane (HCFC-243)

The molecular structure and infrared spectrum of the atmospheric pollutant 3,3-dichloro-1,1,1-trifluoropropane (HCFC-243) were characterized experimentally and theoretically. The theoretical calculations show the existence of two conformers, with the gauche (G) and trans (T) orientation around the HCCC dihedral angle. Conformer G was calculated to be more stable than form T by more than 10 kJ mol (-1). In consonance with the large predicted relative energy of conformer T, only the G form was identified spectroscopically in cryogenic argon (10 K) and xenon (20 K) matrices prepared from room-temperature equilibrium vapor of the compound. The observed infrared spectra of the matrix-isolated HCFC-243 were interpreted with the aid of high-level density functional theory calculations and normal coordinate analysis. For experimental identification of the weakest IR absorption bands, the spectrum of HCFC-243 in the neat solid state at 145 K was obtained. This spectrum also confirmed the sole presence of the G conformer in the sample. Natural bond orbital and atomic charge analyses were carried out for the two conformers to shed light on the most important intramolecular interactions in the two conformers, in particular those responsible for their relative stability.

Estudo de aerossóis atmosféricos e aplicação de modelos numéricos

The main purpose of the present work is to study the concentration of atmospheric particles (PM10 and PM2.5) in the Candiota (RS) region using HV PM10 and dichotomous samplers. Four sampling sites at a distance of 50 km from the emission source were selected: Acegua, Aeroporto, 8 de Agosto and Pedras Altas. Samples were collected from December 2000 to December 2001. The values obtained with the ISCST (Industrial Source Complex Term) model and with the samplers were compared on January 21st, April 5th, July 14th, August 1st, and October 13th 2001, and are representative of frontal systems occurring in the study area.

Análises seqüenciais de agrupamentos aplicadas ao estudo de partículas atmosféricas

The present paper has as objective to apply a sequential Cluster Analysis to the atmospheric particles: Hierarchical Cluster Analysis followed by Nonhierarchical Cluster Analysis. The hierarchical cluster analysis results were used as start point for the nonhierarchical cluster analysis as an agglomerative technique. These particles were taken from two areas of the metropolitan region of Porto Alegre, Charqueadas and Sapucaia do Sul., from may /97 to may/98, using a High Volume Sampler (Hi-Vol). Around 10,000 particles were analysed by Scanning Electron Microscope with Energy-Dispersive X-Ray microanalysis (SEM-EDS). The Hierarchical Cluster Analysis allowed the identification of five groups of particles, whose amounts were differentiated according to the summer and the winter campaigns. The abundance of each type of particles inside each group according to the different sections was verified by the Nonhierarchical Cluster Analysis, resulting in information about the emissions sources. The groups of particles of Si/Al and Si and of Fe/Zn and Fe for Charqueadas were more significant in section 2 and 3 (NW and W wind directions) and in section 1 (SE wind direction), evidencing the influence of the coal power plant and steel industry, respectively located in these quadrants. In Sapucaia do Sul the data were more heterogeneous, causing a certain difficulty to identify the source as anthropogenic. Nevertheless the group of particles containing Fe was found in sectors of NW/W wind directions which shows the influence of the steel plant.

O ensino de reações orgânicas usando química computacional: I. reações de adição eletrofílica a alquenos

Basic concepts that play an important role in some organic reactions are revisited in this paper, which reports a pedagogical experience involving undergraduate and graduate students. A systematic procedure has been applied in order to use widespread available computational tools. This paper aims to discuss the use of computers in teaching electrophilic addition reactions to alkenes. Two classical examples have been investigated: addition to non-conjugated alkenes and addition to conjugated dienes. The results were compared with those normally discussed in organic textbooks. Several important concepts, such as conformational analysis and energy control (kinetic and thermodynamic) involved in reaction mechanisms can be taught more efficiently if one connects theoretical and practical tools.