Theoretical study of ionization potentials in monosubstituted benzenes

Monosubstituted benzenes, in which the substituents participate in the π-electron system, are studied following a classification in two classes according to the π-electronic structure of the substituent. For this type of molecule, a relation is established between the nature of the substituent and, on the one hand, the energies of the two highest occupied molecular orbitals and, on the other hand, their respective differences. The two orbitals referred to above have π-character and belong to the a₂ and b₁ species if a C_2v point group is assumed. Simple symmetry arguments lead to the conclusion that the a₂ orbitals have, essentially, an intraring character, whereas the π-orbitals of the substituents do give an important contribution to the b₁ orbitals. Therefore, an a₂ electron must have a larger interaction with the benzene ring and a smaller kinetic energy, whereas a b₁ electron must have a larger interaction with the substituent and a larger kinetic energy. It is also expected that the changes in the π-electronic structure of the substituent must much more influence the variations on the b₁ energies and on the components of orbital energies associated with the substituent than the variations on the a₂ energies and on the intraring components of the orbital energies. A modified version of the MOPAC program was prepared to perform the decomposition of the orbital energies in their kinetic and potential energy components and these in their monocentric and bicentric terms. MNDO calculations on nine monosubstituted benzenes, using the modified MOPAC program, give good confirmation of the symmetry predictions and prove the consistency of the classification of the substituents that is introduced. © 1993 John Wiley & Sons, Inc.