Ionization and attachment in valence bond theory

Abstract

Abstract The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), introduced and used extensively by the Pullmans in the fifties, continue to enjoy great popularity as indicators of the ease with which a molecule may lose or gain an electron: they are particularly valuable in the discusson of oxidation-reduction reactions. Valence bond theory, on the other hand, has been little used in the context of ionization and attachment processes. The idea of ‘resonance’ among alternative VB structures had a unique pictorial appeal for chemists, especially in the the interpretation of de-localized π-bonding in conjugated systems, but was usually applied only to discussions of the stability of molecules in their neutral ground states. The further development of VB theory was also seriously impeded by mathematical difficulties that precluded any kind of ab initio calculation - whereas MO theory provides a simple and attractive basis for modern ‘computational chemistry’. The intention of this paper is to demonstrate that ab initio VB theory is perfectly capable of dealing with electron removal and attachment processes and can provide valuable insight into the resultant distribution of charges, bonds and unpaired electrons. Numerical applications to the π-electron systems of benzene and pyridine show that 30 covalent VB structures are sufficient to give accurate energies (2–3 eV better than MO-SCF values) and a large amount of useful information on the weights of the various types of VB structure.