Some Studies in Molecular Orbital Theory I. Resonance Structures and Molecular Orbitals in Unsaturated Hydrocarbons

The predictions of qualitative resonance theory, as to the electronic structures of unsaturated hydrocarbons, are analyzed in terms of LCAO molecular orbital theory. The following results are proved for ``alternant'' hydrocarbons, that is, hydrocarbons containing no odd-membered unsaturated rings: (1) The number of unpaired electrons present in the ground state is at least as great as the number of carbon atoms having a deficiency of valence bonds in any principal resonance structure. (2) With a few special exceptions, these odd electrons are distributed over just those atoms which have a deficiency of valence bonds in one or more of the principal resonance structures. (3) In singly charged hydrocarbon anions or cations the ionic charge is located on just those atoms which bear charges in the various principal resonance structures. It follows that enumeration of the principal resonance structures of a hydrocarbon molecule or ion gives much valuable information as to its electronic structure; and in particular, an alternant hydrocarbon possessing no normal Kekulé-type structure must have a paramagnetic ground state, and behave as a radical or multiradical. These results do not depend on the neglect of overlap, or the assumption of a constant carbon-carbon resonance integral.