ChemInform Abstract: A Different Story of π‐Delocalization — The Distortivity of π‐Electrons and Its Chemical Manifestations

Abstract

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Structures of Annulenes and Model Annulene Systems in the Ground and Lowest Excited States

“…Due to second order Jahn-Teller effect [42], electronic states close to each other may interact and for n sufficiently large a structure with bond alternation may be energetically advantageous even for the higher annulenes satisfying the Hückel rule [43,44]. Following early recognitions of the propensity of π electrons to distortivity [18,45], the rather surprising statement was advanced in a series of important papers [17,[46][47][48][49][50][51][52][53]. The delocalized structure of benzene was seen as intermediate configuration between the localized K A and K B structures in the bond exchange process K A → K B along the distortion mode where CC stretchings alternate with CC shrinkings.…”

“…On one hand, aromaticity has been recovered lowering as much as possible the molecular flexibility by the introduction of chemical bridges between opposite ends of the annulene ring [16] or by replacement of double with triple bonds along the cycle [1]. On the other, the theoretical foundations responsible for the delocalized arrangement of the π electrons in aromatic systems have been critically reexamined leading to the view of π propensity to distortion in benzene and cyclobutadiene [17]. In addition, it has been found that the transition from bond equalized to bond alternating structures in (4n + 2) π systems, theoretically predicted long ago [18], may occur for rings of 30 C atoms [19] or lower [20].…”

“…Further, it has been thought to be useful to associate to our results an updated outline of the aromaticity/antiaromaticity theory [17] and a short description of isomeric structures up to [18]annulene [10,15,32,33]. The paper is organized in two Sections, one dedicated to basic aromatic/antiaromatic systems and the second to higher annulenes and model annulene systems to which the research activity of our group has mostly contributed.…”

On the Importance of Clar Structures of Polybenzenoid Hydrocarbons as Revealed by the π-Contribution to the Electron Localization Function

“…Shaik, Hiberty and co-workers, have revealed that forces within the σ-framework of benzene lead to the observed D 6h symmetric structure, whereas the six -electrons exert a -distortive effect that strives towards a bond alternant D 3h symmetry [58][59][60]. Furthermore, Maksić and co-workers deduced that σ-type interactions are also the major contributors to the Clar structures of PBHs, and they showed that these structures are the only minima on the potential energy surfaces [57].…”

“…They concluded that the driving force leading to Clar structures are favorable interactions in the -electron framework [57]. With regard to the -electron framework in annulenes and polyenes it has been concluded based on thorough valence bond theoretical studies by Shaik, Hiberty and co-workers that it is distortive and prefers a CC bond alternant structure (in benzene D 3h symmetry) [58][59][60]. Yet, -systems are simultaneously resonance stabilized, and for benzene this resonance stabilization is largest at the symmetric hexagon, although it has been concluded that for the -network the distortive "Kekulé term always wins over the resonance term [60].…”