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

摘要

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.