Aromaticity: what does it mean?

Aromaticity/aromatic belongs to one of the most useful and popular terms in organic chemistry and related fields. However, aromaticity is not an unambiguous term; therefore, its definition is enumerative. The criteria are based on energy (increased stability), molecular geometry (very low bond lengths alternation), magnetism (induction of the diatropic ring current by external magnetic field) and reactivity (tendency to maintain π-electron structure in chemical reactions). The energetic criterion is based on resonance energy and aromatic stabilization energy, whereas harmonic oscillator model of aromaticity—on molecular geometry. Magnetism-based criteria are illustrated by local indicators (for individual rings): nucleus independent chemical shifts and proton nuclear magnetic resonance chemical shifts as well as the global aromaticity index—exaltation of the magnetic susceptibility. For selected homo- and hetero-cyclic compounds, illustrative data are presented in tables, which allow the comparison of the above-mentioned indices. Finally, examples of agreements or disagreements between these various aromaticity indices are presented for few representative cases. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s40828-015-0012-2) contains supplementary material, which is available to authorized users.