Valores elevados da primeira(β) e segunda(γ) hiperpolarizabilidades foram obtidos para as moléculas apresentando as pontes mistas que contêm os anéis mesoiônicos inseridos entre unidades poliênicas. As magnitudes de β e γ foram calculadas pela metodologia semi-empírica AM1/TDHF. Pares doador(D)/receptor(A) com diferentes forças foram usados para testar o potencial do novo modelo de ponte. O melhor desempenho foi obtido para odicianometileno(A) e a fenilamina(D) inseridos nas extremidades opostas das pontes. A fenilamina é ligada ao heteroátomo nitrogênio em contraste com o relatado na literatura em que o sítio de ligação ocorre no anel aromático. O efeito da natureza e tamanho da ponte na magnitude das hiperpolarizabilidades foi investigado. Os sistemas apresentando as pontes estendidas, contendo dois anéis mesoiônicos, apresentam as maiores magnitudes das hiperpolarizabilidades.Os resultados demonstram que a combinação da ponte selecionadae dos pares D/Apodem contribuir para magnitudes muito elevadas das hiperpolarizabilidades β e γ.

Large values of the static first(β) and second(γ) hyperpolarizabilities were obtained for donor-acceptor molecules having mixed bridges with mesoionic rings inserted between polyenic moieties. The magnitudes of β and γ were calculated at the AM1/TDHF semiempirical level. Donor(D)/acceptor(A) pairs of different strengths were used to test the potential of the new model bridges. The largest value was obtained by linking thedicyanomethylene(A) and phenylamine(D) at the opposite ends of mixed bridges.Unlike D/A molecules traditionally reported in the literature, the phenylamine was linked to the bridge by the nitrogen heteroatom. The effect of the nature and size of the bridge in the magnitude of the hyperpolarizabilities was investigated. The push-pull systems with the extended bridges, containing two mesoionic rings and polyenic moieties, present the larger magnitudes values of β and γ.The results show that the combination of the selectedbridges and D/Apairs can be responsible for very large hyperpolarizabilities.

Cálculos de orbitais moleculares ab initio RHF e MP2 usando os conjuntos de base 4-31G**, 6-311G** e cc-pVTZ têm revelado que os valores de G D,A mostram uma boa correlação com transferências de cargas atômicas intermoleculares obtidas a partir de diferentes esquemas de partição para os complexos de hidrogênio CNH…CNH, NCH…CNH, CNH…NCH e NCH…NCH. Isto é especialmente evidente quando a distância da ligação de hidrogênio é progressivamente aumentada até 4,5 Å. Entretanto, valores de G D,A mostram uma melhor correlação linear com valores de deltaQ usando as cargas Mülliken corrigidas, que são obtidas do modelo carga-fluxo de carga-recobrimento (CCFO) para intensidades no infravermelho. Neste caso, ambos G D,A e deltaQ formam duas curvas exponenciais praticamente superpostas. Por outro lado, valores de G D,A mostram uma menor concordância com valores de deltaQ obtidos das cargas atômicas derivadas dos orbitais de ligação naturais. Isso é claramente verificado quando é considerada a taxa de decaimento exponencial de primeira ordem de G D,A versus deltaQ obtida de diferentes esquemas de partição de carga.

RHF and MP2 ab initio molecular orbital calculations using the 4-31G**, 6-311G** and cc-pVTZ basis sets have revealed that the Green's function matrix element (G D,A) values show a good correlation with the amount of intermolecular transferred charges obtained from different charge partitioning schemes for the CNH…CNH, NCH…CNH, CNH…NCH and NCH…NCH hydrogen bonded complexes. This is evident specially when the hydrogen bond distance is progressively increased from the equilibrium position until 4.5 Å. However, G D,A values show a better linear correlation with deltaQ values using corrected Mülliken charges, which are obtained from the charge-charge flux-overlap (CCFO) model for infrared intensities. In this case, both G D,A and deltaQcorr form two practically superposed exponential curves. On the other hand, G D,A values show a smaller agreement with deltaQ values obtained from atomic charges derived from natural bonding orbitals. This is clearly verified when considering the first order exponential decay rate of G D,A versus deltaQ obtained from different charge partitioning schemes.

The graduate programs in Chemistry were evaluated and classified from the data collected for the period 1998-2000 by CAPES, the Minister of Education agency that is responsible for the regulation and support of this level of human resources formation. The next classification will be made in 2004, from the data of the period 2001-2003. The years in between, like the present year, are used for an evaluation without classification, from what comments are emitted to the programs to know how their data are improving or not, before the new ranking reporting. The progresses in the area of chemistry in Brazil are analysed from the confrontation of the present data with that of all previous evaluations, since 1983. The results confirm that the research and graduate formation in the area of Chemistry is continuously improving in Brazil, particularly during the last seven years.

The concept of molecular conductance is discussed in terms of the propagation of an electronic interaction, between electron donor and acceptor groups, through the bonds of a molecular structure where these groups are embedded. The electronic interaction propagation is described by a Green's function matrix element, in a donor-bridge-acceptor molecular system reduced to a two-level representation.

A broad picture of the graduate programs in chemistry in Brazil is analyzed from the data collected during the period 1996/1997 for evaluation by CAPES. The results are compared with those from previous evaluations, indicating significant quantitative and qualitative progresses.

In the opportunity of the 20th year of foundation of the Brazilian Chemical Society, the role of this organization in behalf of the development of Science, in particular of Chemistry, is analysed by the way it is observed by someone that has been working in the Northeast of this Country.