ABSTRACT
Aromaticity constitutes one of the many useful but loosely dened concepts that conform to the toolbox of modern chemistry concepts. Some of them, aromaticity included, have no precise meaning and do not denote directly measurable quantities, albeit they are based mostly on experimentally “observable” observations. Thus, aside for their “aroma,” not always necessarily nice, it is rmly established that “aromatic” molecules are often more stable and their geometries more regular than expected a priori. Moreover, they are barely reactive, in spite of having several unsaturated bonds. These unsaturated bonds are not localized, but delocalized through the molecule. This imposes that their electronic structure should require delocalized electrons, which are found to be very sensible to their number and less, but still remarkably sensible, to the geometrical deformations of the molecular structure. At the beginning, it was thought that aromatic molecules were annular like with (4n + 2) atomic p-type electrons arranged in spin-coupled pairs into (2n + 1) π-type molecular orbitals, n being an integer number, that is: n ∈ N. However, these earlier thoughts have recently been overcome and, consequently, the aromaticity concept has been extended to s-type electrons, which when (4n + 2), n ∈ N of them are arranged in spin-coupled pairs into (2n + 1) σ-type molecular orbitals give rise to the so-called σ-aromaticity in addition to the former π-aromaticity arising from the above-mentioned π-type molecular orbitals. These two types of aromaticity have been found to occur simultaneously in many molecules. Sometimes they cooperate to increase the aromaticity and sometimes act antagonistically rendering lower aromaticity.
