Electronic structure principles and aromaticity

It is important to note that new chemical reactivity based aromaticity indices, such as Λaromaticity = Λmolecule - Λreference (Λ ≡ E, η, α, ω), can be defined where the reference system may be the corresponding hypothetical localized structure as shown above or a related open-chain system (30). It is obvious that a positive value of Λaromaticity (Λ ≡ η) implies aromaticity and a corresponding negative value implies antiaromaticity. On the other hand a positive value of Λaromaticity (Λ ≡ E, α, ω) implies antiaromaticity and corresponding negative value implies aromaticity. Table 4 presents various Λaromaticity values for benzene, cyclobutadiene, Al42-, and Al44- systems. While benzene and Al42- are aromatic systems according to these criteria, cyclobutadiene is shown to be antiaromatic. Exact nature of Al44- is yet to be ascertained. Aromaticity in other inorganic systems, such as polyacene analogues of Na6 (31), C3N3H3 (32), B3N3H6 (32), B3O3H3 (32) and so forth, is also analyzed. In summary, aromatic molecules are less energetic, ha rder, less polarizable, and less electrophilic whereas antiaromatic molecules are more energetic, softer, more polarizable, and more electrophilic than a suitable reference molecule, as expected from the principles of minimum energy, minimum polarizability, and minimum electrophilicity as well as the principle of maximum hardness.