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The application of the Valence Bond method to the theoretical description of concerted reactions furnishes an example of the importance of higher-order permutations.

On the basis of quantum chemical calculations and general theoretical arguments a discussion has been going on regarding the stability towards distortion of benzene and the monocyclic polyene C2nH2n in general. Also the polyene radicals C2n+1H2n+3, of which the allyl- radical is the first representative, are of interest. These molecules contain a {sigma}-skeleton and a {pi}-electron system with two main contributing and, in a symmetric geometry, equivalent resonance structures. The question asked is whether (in)stability towards distortion away from a symmetric geometry is a {pi}-electron property or, on the contrary, primarily connected to the {sigma}-skeleton. In this communication the question is answered firstly by means of simple molecular orbital and valence bond arguments in the case of the {pi}-electron system, and secondly with the characteristic difference between 2p{pi} - and 2p{sigma} overlap integrals for the discussion of the {sigma}-skeleton. The outcome is inescapable: notwithstanding the symmetrizing influence of resonance, the {pi}-electron system as a whole is unstable towards distortion.

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