Relative validity of the valence bond theory and of the molecular orbital theory for description of conjugated hydrocarbons

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Abstract
It is shown that MO theory predicts the butadiene molecule to be flat, and that VB theory predicts a skewed configuration of this molecule. The calculations of the resonance energy of conjugated systems, using either VB or MO theory, are discussed and compared. Benzede is taken as an example. Emphasis is laid on the requirement of similarity between the theoretical calculation and the empirical determination of the resonance energy. Using simple Hückel MO theory, the resonance energy of benzene is then predicted with the correct sign and in the right order of magnitude. However, with valence bond theory one finds the wrong sign for the resonance energy of benzene. The cause of this failure of valence bond theory is discussed. It is concluded that description of conjugated systems by the MO theory is far superior.

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