| Afiliação do(s) autor(es): | [1] Univ Sao Paulo, Fac Filosofia Ciencias & Letras Ribeirao Preto, Dept Quim, BR-14040901 Ribeirao Preto, SP - Brazil
Número total de Afiliações: 1
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We have studied prop-2-en-1-imine (1), prop-2-enal (2), ethane-1,2-diimine (3), ethanedial (4), and 2-iminoacetaldehyde (5) to investigate the influence of the negative hyperconjugation in pi-pi{*} interaction with the substitution of =CH2 by =NH and/or =O in trans-buta-1,3-diene (6). The analyzes of the pi-pi{*} interaction performed from evaluation of the pi molecular orbital diagrams and electron localization function method demonstrated, that compared to 6, the substituted compounds 1-5 presented lower electron conjugation, especially in the structures bearing =O. The geometric parameters, natural bond orders, and topological analysis realized by quantum theory of atoms in molecules method indicated a predominant C-C and C=C character for the simple and double C-C bonds in the substituted compounds, 1-5, as compared to 6. Compound 4 had the highest enthalpy of formation, which reflected the lowest pi-pi{*} interaction, maintained by the two =O conjugated groups. The natural bond orbital (NBO) and natural resonance theory (NRT) methods revealed that the pi-pi{*} electron delocalization in substituted compounds, 1-5, is lower than in 6 from, firstly, of the less favorable interactions: pi(X=C) -> pi{*}(C=C) and pi(X=C) -> pi{*}(C=X), despite of the larger pi(C=C) -> pi{*}(C=X) conjugation, with X = N and/or O, of 1-5 than pi(C=C) -> pi{*}(C=C) of 6. But, most importantly, the weight of the interaction: n(pi)(O) -> sigma{*}(C-C), was determined from NBO and NRT methods as proportional to the pi-pi{*} conjugation and thus demonstrating be decisive to establish the level of pi electronic delocalization. (AU) |