ABSTRACT
Some phrases or words tend to be used ubiquitously in chemistry, including for example “the chemical bond,” “aromaticity,” … yet they are notoriously undened quantities or concepts. In some cases, the lack of a proper denition arose because at the time when they were introduced, there was not yet quantum mechanics and the need for a precise denition was not a high priority. The chemical bond, introduced several decades before quantum mechanics [1] is a typical example. Other concepts such as aromaticity are nowadays considered ill dened although this was not entirely the case at the time of introduction. Kekule, in his earlier works on aromaticity [2-4] clearly put that aromatic compounds are composed “atomistically” of C6 units and
13.1 Introduction ..................................................................................................245 13.2 Aromaticity in Al4
2−-Based Compounds .......................................................248 13.3 Theoretical Methods .....................................................................................249
