Aromaticity: Types, Rules, and Quantification

Aromaticity is not a well-defined concept because of its non-observable nature. Despite the imprecise character of aromaticity, it is a concept of utmost importance in Chemistry. In this Chapter, we discuss first the different types of aromaticity that have been described in the literature. We show that despite the lack of precise definition, there are a number of very simple electron counting rules that are followed by a large number of organic and inorganic aromatic compounds. Finally, we analyze the most usual ways to quantify aromaticity and we emphasize the fact that different descriptors do not speak always with the same voice.