The concept of spatial and electronic structure of cyclic molecular systems displaying
the effects of cyclic electron
delocalization 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
resonance energy. 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
diamagnetic ring current induced in a conjugated cyclic molecule by an external magnetic field
and manifested by an exaltation and
anisotropy of
magnetic susceptibility). Although originally introduced for characterization of peculiar properties of cyclic
conjugated
hydrocarbons and their ions, the concept of aromaticity has been extended to their homoderivatives
(see
homoaromaticity), conjugated
heterocyclic compounds (heteroaromaticity), saturated cyclic compounds (σ-aromaticity) as well as to three-dimensional
organic and
organometallic compounds (three-dimensional aromaticity). A common feature of the electronic structure inherent
in all aromatic molecules is the close nature of their
valence 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.
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