IONIZATION AND ATTACHMENT IN VALENCE-BOND THEORY

The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), introduced and used extens ively by the Pullmans in the fifties, continue to enjoy great populari ty as indicators of the ease with which a molecule may lose or gain an electron: they are particularly valuable in the discusson of oxidatio n-reduction reactions. Valence bond theory, on the other hand, has bee n little used in the context of ionization and attachment processes. T he idea of 'resonance' among alternative VB structures had a unique pi ctorial appeal for chemists, especially in the the interpretation of d e-localized pi-bonding in conjugated systems, but was usually applied only to discussions of the stability of molecules in their neutral gro und states. The further development of VB theory was also seriously im peded by mathematical difficulties that precluded any kind of ab initi o calculation - whereas MO theory provided a simple and attractive bas is for modem 'computational chemistry'. The intention of this paper is to demonstrate that ab initio VB theory is perfectly capable of deali ng with electron removal and attachment processes and can provide valu able insight into the resultant distribution of charges, bonds and unp aired electrons. Numerical applications to the rr-electron systems of benzene and pyridine show that 30 covalent VB structures are sufficien t to give accurate energies (2-3 eV better than MO-SCF values) and a l arge amount of useful information on the weights of the various types of VB structure.