The distortive tendencies of π electronic systems, their relationship to isoelectronic σ bonded analogs, and observables: A description free of the classical paradoxes

Abstract

Ab inito computational experiments are used to decompose the total resistance energies for allylic species and benzene, towards localizing Kekulean distortions, into their σ and π components. While the σ component is always symmetrizing, and responsible for the identical C–C bond lengths of these molecules, the π components are distortive along a Kekulean distortion. As such, the π components must be viewed as unstable electronic species that are forced by the σ frame to adopt a regular rather than bond-alternated geometry. The distortivity of the π components of conjugated molecules is shown to be consistent with a valence bond model for delocalization that is equally valid for isoelectronic species of the σ as well as π varieties. This property unifies the π components of benzene and allylic species with their σ electronic analogs: hydrogen chains, rings and transition states of organic chemical reactions. The π distortivity has some observable consequences. For example, upon excitation of benzene and other aromatic molecules from the ground to the ¹B_2u excited states, such that π resonance is disrupted, the low frequency of the b_2uvibrational mode of the ground states undergoes up-shift (exaltation) in the excited states. Another consequence is that benzene derivatives that possess strong bond localization in the ground states attain almost uniform C–C bond lengths in the ¹B_2u-like excited states. As argued, the traditional view that considers π electronic systems to have intrinsic stability, leads to a number of disturbing paradoxes. By contrast, the distortivity of π electronic component removes all the paradoxes and unifies σ and π electronic systems into a single coherent picture of electronic delocalization and resonance-stabilization.