Antonio C. Pavão

MODELING THE ADSORPTION AND DISSOCIATION OF CO ON TRANSITION METAL SURFACES

Abstract The interaction between transition metal surfaces and simple diatomic molecules such as CO may lead to breaking and making chemical bonds and trigger important surface catalyzed reactions. We discuss the most common surface interaction and orientation models taking into consideration the behavior of each transition metal, the influence of structure, bonding, and coordination of the molecules. We emphasize the importance of the precursor tilted state in the dissociation of diatomic molecules on transition metal surfaces.

Vulnerabilidades e situações de riscos relacionados ao uso de agrotóxicos na fruticultura irrigada

A determinacao de situacoes de riscos na populacao que faz uso de agrotoxico e complexa. O objetivo desse estudo foi conhecer o contexto social, economico e cultural e algumas das vulnerabilidades para a saude relacionadas com o processo produtivo quimico dependente da fruticultura do Vale do Sao Francisco. Questionario semiestruturado foi aplicado a uma amostra aleatoria de trabalhadores rurais, para obtencao de informacoes socioeconomicas e culturais, assim como a morbidade relacionada com intoxicacao por agrotoxicos. Os trabalhadores rurais da regiao tem em sua maioria baixo grau de escolaridade, sendo esta uma importante vulnerabilidade para compreensao da rotulagem dos agrotoxicos e sua implicacao toxicologica e ambiental. Foi observado uso indiscriminado de agrotoxicos em condicoes inseguras de trabalho que comprometem a saude dos expostos. A sintomatologia sugestiva de intoxicacao por agrotoxicos aponta para quadros relacionados com exposicao a organofosforados, carbamatos e piretroides, em congruencia com os biocidas utilizados na fruticultura. 7% da populacao estudada referiram ter sofrido pelo menos um caso de intoxicacao no decorrer da vida. A assistencia tecnica relacionada com o manejo de agrotoxicos e precaria e nao se observaram acoes de protecao no âmbito da saude, do trabalho, da previdencia ou do ambiente. Estas vulnerabilidades institucionais corroboram aquelas relacionadas com o modelo tecnologico que objetiva meramente a produtividade e o rendimento financeiro.

Structure and activity of aflatoxins B and G

Abstract AM1 heats of formation, HOMO/LUMO energies, bond orders, and atomic charges are used to analyse the observed activity of the aflatoxins B and G. The low LUMO energy indicates correctly that the aflatoxins are electrophiles. The present thermodynamic analysis shows that the epoxide and the hydroxylated product of the aflatoxin B1 may be considered to be intermediate species in the metabolism of the aflatoxins, in agreement with the experiments. Optimized bond lengths of aflatoxin B1 are in accord with X-ray crystallography determinations.

Synthesis, characterization and interaction mechanism of new oxadiazolo-phthalimides as peripheral analgesics. IV

Abstract The synthesis, characterization and spectroscopic studies of compounds 6a–g with analgesic activity is described. A new model of interaction between the drug and the enzyme is suggested. Application of the Resonance Valence Bond theory led us to propose, for the first time, an entirely new mechanism involving an electron transfer from the amino acid residue of the enzyme to the drug. Theoretical studies of various transition states involved in the interaction mechanism employing the semi-empirical molecular orbital calculations (AM1 method) have been carried out. This article also deals with an extensive study of the structure–activity relationships of seven oxadiazolo-phthalimides 6a–g.

Carcinogenic potential of endosulfan and its metabolites based on a quantum chemical model.

The aim of the present study was to investigate the carcinogenic potential of endosulfan and its metabolites through electronic parameters that characterize the action of carcinogens, i.e. descriptors such as electron affinity, Δ (HOMO-LUMO), dipole moments, electrostatic attraction, formation heat (H(f)) and permeability of the cell membrane (c Log P). The results reveal that both endosulfan and its metabolites are electrophilic and have carcinogenic potential. Although there are few data on its carcinogenicity in the literature, the findings of the present study indicate that the use of this pesticide represents a risk to the health of the general population, especially rural workers.

A molecular orbital analysis of the DNA bases

Abstract The carcinogen–DNA interaction can be described as an electron transfer from the DNA (HOMO) to the carcinogen (LUMO). To get a description of the DNAs HOMO, the 35 tautomers of the DNA bases are analyzed. Koopmans’ (ROHF and OVGF) and ΔSCF vertical/adiabatic AM1 ionization potentials combined with other calculated electronic properties provide additional insight on the stability and relative population of the tautomers. It is correctly found that the one-electron transfer and the electrophilic attack occurs preferentially at C6 and N7 of guanine.

Theoretical study of the O4 molecule

Abstract MCSCF molecular orbital calculations show that the D2h structure of the O4 molecule is more stable than the D4th structure. The O 4 (D 2h , 1 A 1 ) → 2O 2 ( 3 Σ g − ) dissociation energy is in agreement with the dissociation energy predicted by Lewis. The unsynchronized resonating valence bond theory also predicts the formation of the O4 molecule with D2h geometry.

Theoretical assessment of the nonlinear optical properties of substituted oligoacenes

The β and γ nonlinear optical coefficients of substituted oligoacenes (1–10 rings) with the donors (D) groups –CH3, –CH2CH3, –CH2CH2CH3, –CH2CH2CH2CH3, –C(CH3)3, –CH3CHN(CH3)2, –N(CH3)2, –OCH3, –OCH2CH3, –OCH2CH2CH3, –OCH2CH2CH2CH3, –OC(CH3)3, and –OCHCH3N(CH3)2 and the acceptor (R) group –CHC(CN)2 is calculated by the AM1/TDHF method. A partial least squares regression analysis of electronic and structural parameters is performed to investigate their correlation with β and γ hyperpolarizabilities. It is found that the dipole moment, the HOMO–LUMO energy gap, the ionization potential, the number of π-electrons, and the number of rings in the bridge are parameters that significantly affect β and γ magnitudes. The dipole moment and the number of π-electrons are positively correlated with β values, while the HOMO–LUMO energy gap and the ionization potential are negatively correlated. Calculations indicate that the hyperpolarizabilities increase with the number of rings in the bridge and that the dimethylamine/dicyanoethenyl pair leads to the highest hyperpolarizabilities values. Optimization of β and γ is obtained through several substitution patterns of the oligoacenes. The present methodology shows that more extended systems containing two D/R pairs have high values of β and γ coefficients, which indicates that these systems can be employed in manufacturing nonlinear optics devices.

An electron transfer mechanism of O4 formation

A resonating valence bond electron transfer mechanism of combining two O2 molecules to form an O4 molecule is presented. The predicted molecular states of the reaction path D1h! C2v! D2h are supported by the present ab initio molecular orbital calculations. The CASPT2BSSE calculations yield a stable diamagnetic D 2h O4 molecule with a very weak chemical bond between the monomers, in good agreement with experiments. A low activation barrier energy of � 26 cal/mol for the O4 formation is found. 2003 Elsevier Science B.V. All rights reserved.

Anomeric effect enhancement in C-5-substituted 2-methoxytetrahydropyrans

Abstract cis - and trans -2,5-Dimethoxytetrahydropyrans, cis -2,5-dimethoxy-6-methyltetrahydropyran and 2-methoxy-5-methyltetrahydropyran have been examined to see the effect of an OCH 3 group at position 5 on the degree of anomeric effect in substituted 2-methoxytetrahydropyrans. The present study shows that this group stabilises the C-2 electronegative substituent in the axial position. Semi-empirical and ab initio molecular orbital calculations support this view. AM1 calculation gives lower enthalpies as well as lower dipole moments for the compounds having an OCH 3 group in the axial position at C-2 over the equatorial form in 2-methoxytetrahydropyrans. This enhanced stabilisation is attributed to the electrostatic interaction between the partial positive charge at C-5 and the partial negative charge of the aglycone oxygen atom.