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Modern valence‐bond description of the electronic structure of benzocyclobutadiene
Author(s) -
Karadakov Peter B.,
Gerratt Joseph,
Cooper David L.,
Raimondi Mario,
Sironi Maurizio
Publication year - 1996
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/(sici)1097-461x(1996)60:1<545::aid-qua53>3.0.co;2-6
Subject(s) - cyclobutadiene , antiaromaticity , wave function , chemistry , valence bond theory , complete active space , ring (chemistry) , fulvene , atomic orbital , computational chemistry , molecule , ground state , generalized valence bond , electron , valence (chemistry) , atomic physics , physics , quantum mechanics , aromaticity , stereochemistry , organic chemistry
The ground state of benzocyclobutadiene, a bicyclic molecule with 8π electrons containing a benzene and a cyclobutadiene ring, is studied by means of modern valence bond (VB) theory in its spin‐coupled (SC) form and the complete‐active‐space self‐consistent field (CAS SCF) approach. The CAS SCF wave function is used to optimize the geometry, and SC theory—to obtain a well‐correlated and easy to visualize and understand model of the active space hosting the π electrons. It is shown that the π system of the ground state of benzocyclobutadiene can be described with sufficient accuracy as a combination of the π systems of a distorted benzene ring and an isolated double bond. Each of the eight SC orbitals is found to be well‐localized about one carbon atom only, with small distortions toward its nearest neighbors. The analysis of the optimal active‐space spin‐coupling pattern within the SC wave function for benzocyclobutadiene shows unambiguously that this molecule inherits neither the aromatic nor the antiaromatic character of its cyclic components, and it is most appropriate to regard it as nonaromatic. © 1996 John Wiley & Sons, Inc.