THEORETICAL-STUDY OF IONIZATION-POTENTIALS IN MONOSUBSTITUTED BENZENES

Monosubstituted benzenes, in which the substituents participate in the pi-electron system, are studied following a classification in two cla sses according to the pi-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 pi-character and belong to the a2 and b1 species if a C2v point group is assumed. Simpl e symmetry arguments lead to the conclusion that the a2 Orbitals have, essentially, an intraring character, whereas the pi-orbitals of the s ubstituents do give an important contribution to the b1 orbitals. Ther efore, an a2 electron must have a larger interaction with the benzene ring and a smaller kinetic energy, whereas a b1 electron must have a l arger interaction with the substituent and a larger kinetic energy. It is also expected that the changes in the pi-electronic structure of t he substituent must much more influence the variations on the b1 energ ies and on the components of orbital energies associated with the subs tituent than the variations on the a2 energies and on the intraring co mponents of the orbital energies. A modified version of the MOPAC prog ram was prepared to perform the decomposition of the orbital energies in their kinetic and potential energy components and these in their mo nocentric and bicentric terms. MNDO calculations on nine monosubstitut ed benzenes, using the modified MOPAC program, give good confirmation of the symmetry predictions and prove the consistency of the classific ation of the substituents that is introduced.