Aromaticity in molecules and transition structures: from atomic and molecular orbitals to simple ring current models

Abstract

Abstract In this chapter, the main aspects that govern the relationship between aromaticity (as well as antiaromaticity) and molecular orbitals (MOs) are presented and discussed. The Huckel and Mobius theories are discussed first. The main concepts derived from this theoretical level are transferred to the self-consistent field-MO (SCF-MO, canonical orbitals) and density functional theory (DFT, Kohn–Sham orbitals) frameworks. These ideas are also extended to transition structures, showing selected examples associated with Huckel and Mobius aromaticity. Valence bond theory is also discussed emphasizing its complementary character with respect to SCF-MO and DFT theories. Finally, delocalization models involving atomic orbitals are discussed, with special emphasis on the link between aromaticity and cyclic electron delocalization.