The concept of spatial and electronic structure of cyclic molecular systems displaying the effects of cyclic electron @D01583@ which provide for their enhanced thermodynamic stability (relative to acyclic structural analogues) and tendency to retain the structural type in the course of chemical transformations. A quantitative assessment of the degree of aromaticity is given by the value of the @R05333@. It may also be evaluated by the energies of relevant isodesmic and homodesmotic reactions. Along with energetic criteria of aromaticity, important and complementary are also a structural criterion (the lesser the alternation of bond lengths in the rings, the greater is the aromaticity of the molecule) and a magnetic criterion (existence of the @D01668@ ring current induced in a conjugated cyclic molecule by an external magnetic field and manifested by an exaltation and @AT06776@ of @M03689@). Although originally introduced for characterization of peculiar properties of cyclic conjugated @H02889@ and their ions, the concept of aromaticity has been extended to their homoderivatives (see @H02839@), conjugated @H02798@ (heteroaromaticity), saturated cyclic compounds (σ-aromaticity) as well as to three-dimensional organic and @O04328@ (three-dimensional aromaticity). A common feature of the electronic structure inherent in all aromatic molecules is the close nature of their @V06588@ electron shells, i.e., double electron occupation of all bonding MOs with all antibonding and delocalized nonbonding MOs unfilled. The notion of aromaticity is applied also to transition states.
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1923 [Terms] [Paper]