Annulenes

Cyclic polyenes C_nH_n generally have nonplanar geometry with no conjugation. Planar conjugated cyclic polyenes are called [n]-annulenes. To be planar they must obey some rules, namely: 1) all angles must be 120°, 2) there must be no severe steric interaction. This sets the minimum length to 18 (with exceptional length 6). Annulenes obeying n=4k+2 rule may have aromaticity (completely delocalized π-orbitals). For aromatic annulenes there is a critical n=14-30 above which there is bond length alteration. Some polyenes allow Moebius cycling, see e.g. C18H18.

Some particular cases:

• [4] cyclobutadiene [pdb] has planar rectangular geometry and is unstable.
• [5] cyclopentadienyl (anion) [xyz] is aromatic.
• [6] benzene is exceptional.
• [8] cyclooctatetraene [pdb] has nonplanar geometry as expected (its dianion is aromatic).
• [10] cyclodecapentaene has highly strained nonplanar structure of D2 symmetry.
• [14] cyclotetradecaheptaene has unstable planar structure of D2h symmetry.
• [18] cyclooctadecanonaene [xyz] is aromatic with presumably no bond length alteration.