A local perspective on conjugation of double bonds in acyclic polyenes

TL;DR

This paper presents a local, electron-pair based model of conjugation in acyclic polyenes, emphasizing weak interactions and localized delocalization, offering a new perspective on their stability and electronic structure.

Contribution

It introduces a non-canonical molecular orbital approach that models conjugation as local electron pair delocalization, differing from traditional fully delocalized systems.

Findings

Conjugation energy equals the total delocalization energy of electron pairs.

Local environment influences electron delocalization and stability.

Linear and cross-conjugated polyenes exhibit different delocalization patterns.

Abstract

The study is devoted to elaboration of an alternative image of conjugation in acyclic polyenes as a weak and essentially local delocalization of initially-localized pairs of electrons ascribed to individual double bonds (instead of formation of a completely delocalized electron system as usual). To this end, polyenes are modelled as sets of weakly interacting formally-double bonds, where the single bonds represent the interaction between the former and are treated as a perturbation. Mathematically, the above-formulated goal is realized by means of a particular version of the non-canonical method of molecular orbitals (MOs) based on the Brillouin theorem and yielding the expressions both for total energies and for non-canonical (localized) MOs (NCMOs) directly without any reference to usual (canonical) MOs. In addition, total energies and NCMOs are interrelated explicitly in the approach applied, viz. the former are representable via the so-called delocalization coefficients of the latter. Adaptation of these general results to the above-specified model of polyene yields coincidence between the conjugation energy (CE) and the total delocalization energy of all pairs of electrons contained. Moreover, a local relation follows between constitution of the nearest environment of a certain bond, delocalization pattern of the respective pair of electrons and contribution of just this pair to the total CE of the given polyene. As a result, different stabilities of distinct polyenes (e.g. of isomers) prove to be accompanied by variable extents of delocalization of separate pairs of electrons. Linear and cross-conjugated polyene chains are comparatively analyzed.