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A cyclically conjugated ion with a stability (due to delocalization)significantly greater than that of a hypothetical localized structure (e.g. Kekul???????????? structure). If the structure is of higher energy (less stable) than such a hypothetical classical structure, the ion is ??????????????????????antiaromatic?????????????????????????. Ions that obey the H?????????????ckel (4n + 2) rule will exhibit aromatic characters: monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain (4n + 2) p-electrons (where n is a non-negative integer). The rule is generally limited to n = 0??????????????????????????5. Examples of aromatic ions are the cyclopropenyl cation (n = 0) and the cyclopentadienyl anion (n = 1) whereas the cyclopentadienyl cation is ??????????????????????antiaromatic?????????????????????????.
