"Cyclic conjugated polyenes are aromatic if their p electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n p systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in 1H NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes."