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Aromaticity: Molecular‐Orbital Picture of an Intuitive Concept
Author(s) -
Pierrefixe Simon C. A. H.,
Bickelhaupt F. Matthias
Publication year - 2007
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200700206
Subject(s) - delocalized electron , antiaromaticity , aromaticity , cyclobutadiene , chemistry , molecule , chemical physics , ring (chemistry) , computational chemistry , double bond , pi bond , molecular physics , crystallography , bond length , bond order , organic chemistry
Geometry is one of the primary and most direct indicators of aromaticity and antiaromaticity: a regular structure with delocalized double bonds (e.g., benzene) is symptomatic of aromaticity, whereas a distorted geometry with localized double bonds (e.g., 1,3‐cyclobutadiene) is characteristic of antiaromaticity. Here, we present a molecular‐orbital (MO) model of aromaticity that explains, in terms of simple orbital‐overlap arguments, why this is so. Our MO model is based on accurate Kohn–Sham DFT analyses of the bonding in benzene, 1,3‐cyclobutadiene, cyclohexane, and cyclobutane, and how the bonding mechanism is affected if these molecules undergo geometrical deformations between regular, delocalized ring structures, and distorted ones with localized double bonds. We show that the propensity of the π electrons is always, that is, in both the aromatic and antiaromatic molecules, to localize the double bonds, against the delocalizing force of the σ electrons. More importantly, we show that the π electrons nevertheless decide about the localization or delocalization of the double bonds. A key component of our model for uncovering and resolving this seemingly contradictory situation is to analyze the bonding in the various model systems in terms of two interpenetrating fragments that preserve, in good approximation, their geometry along the localization/delocalization modes.