A local perspective on the conjugation of double bonds in acyclic polyenes

Abstract

The study is devoted to the elaboration of an alternative image of conjugation in acyclic polyenes as a weak and essentially local delocalization of initially-localized pairs of $$\pi$$ -electrons of individual formally-double (C=C) bonds (instead of formation of a completely delocalized $$\pi$$ -electron system as usual). Polyenes are modelled as sets of weakly-interacting C=C bonds, where the single (C–C) bonds represent the interaction. Expressions for the relevant non-canonical (localized) molecular orbitals (NCMOs) are derived directly without any reference to usual (canonical) MOs in the form of power series with respect to the small resonance parameter of C–C bonds. An explicit interdependence is established between the conjugation energy (CE) of polyene and the shapes of its NCMOs. As a result, a local relation follows between the constitution of the nearest environment of a certain C=C bond, delocalization pattern of the respective single pair of $$\pi$$ -electrons and contribution of just this pair to the total CE. A simple additive scheme is deductively suggested to evaluate the CEs of polyenes in terms of essentially local increments easily found by inspection. The actual order of a C=C bond also is related to the extent of delocalization of the only pair of its $$\pi$$ -electrons. Within a certain rough approximation, the approach yields a simplified image of acyclic conjugation in terms of shifts of separate pairs of $$\pi$$ -electrons from C=C bonds to the nearest C–C bonds in accordance with the picture offered by the curly arrow formalism. The results are illustrated by specific examples.