σ‐Conjugation and σ‐Aromaticity

Resonance integrals between different hybrid AOs do not vanish, contrary to statements in current texts. Bonds in saturated molecules are therefore not localized. Indeed β(sp 3 , sp 3 ) is much greater than the CC resonance integral in conjugated hydrocarbons. Paraffins are therefore σ‐conjugated analogs of π‐conjugated polyenes. Additivity of collective properties applies equally to both, for reasons explained by PMO theory. This is why the localized bond model successfully interprets collective properties. Cyclopropanes should then be isoconjugate with benzene and hence σ‐aromatic. Various anomalies concerning the chemistry of small ring compounds can be explained in this way. Delocalization in alkyl anions and radicals, due to σ‐conjugation, is discussed.