A monocyclic array of orbitals in which there is a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hückel systems; with 4n electrons it is stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). In the excited state 4n + 2 Möbius pi-electron systems are stabilized, and 4n systems are destabilized. No examples of ground-state Möbius pi systems are known, but the concept has been applied to transition states of pericyclic reactions [see aromatic (3)].
The name is derived from the topological analogy of such an arrangement of orbitals to a Möbius strip. HEILBRONNER (1964); ZIMMERMAN (1971). See also Hückel (4n + 2) rule.
