Aromatic systems have pretty negative NICS values,
antiaromatic systems have strongly positive NICS values, and nonaromatic cyclic systems should have NICS values close to zero [35,42-44].
When the value of the HOMA index is less than zero, the structure is
antiaromatic.
Therefore, according to Huckels Rule, they should be
antiaromatic [1].
Kim, Synthesis, Characterization, and Spectroscopic Analysis of
Antiaromatic Benzofused Metalloporphyrins, Chem.
They're classified as
antiaromatic molecules, which chemical theory predicts will be extremely reactive--in contrast to aromatics, such as benzene, which have unusually stable structures.
The cyclopentadienyl cation is a common textbook example of an
antiaromatic molecule, a molecule so electronically unstable and, therefore, extremely reactive that it should not exist for any length of time.