The influence of the negative hyperconjugation is relevant for the analysis of the π-π* conjugation with the mono-substitution and di-substitution of H2C= by O= and/or HN= in <italic>trans</italic>-buta-1,3-diene?

We have studied prop-2-en-1-imine (<bold>1</bold>), prop-2-enal (<bold>2</bold>), ethane-1,2-diimine (<bold>3</bold>), ethanedial (<bold>4</bold>), and 2-iminoacetaldehyde (<bold>5</bold>) to investigate the influence of the negative hyperconjugation in π-π* interaction with the substitution of =CH2 by =NH and/or =O in <italic>trans</italic>-buta-1,3-diene (<bold>6</bold>). The analyzes of the π-π* interaction performed from evaluation of the π molecular orbital diagrams and electron localization function method demonstrated, that compared to <bold>6</bold>, the substituted compounds <bold>1</bold>-<bold>5</bold> 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, <bold>1</bold>-<bold>5</bold>, as compared to <bold>6</bold>. Compound <bold>4</bold> had the highest enthalpy of formation, which reflected the lowest π-π* interaction, maintained by the two =O conjugated groups. The natural bond orbital (NBO) and natural resonance theory (NRT) methods revealed that the π-π* electron delocalization in substituted compounds, <bold>1</bold>-<bold>5</bold>, is lower than in <bold>6</bold> from, firstly, of the less favorable interactions: π(X=C) ➔ π*(C=C) and π(X=C) ➔ π*(C=X), despite of the larger π(C=C) ➔ π*(C=X) conjugation, with X = N and/or O, of <bold>1</bold>-<bold>5</bold> than π(C=C) ➔ π*(C=C) of <bold>6</bold>. But, most importantly, the weight of the interaction: nπ(O) ➔ σ*(C-C), was determined from NBO and NRT methods as proportional to the π-π* conjugation and thus demonstrating be decisive to establish the level of π electronic delocalization. [ABSTRACT FROM AUTHOR]