alternancy symmetry
A topological property of the molecular graphs of @A00249@ @H02889@ which allows the carbon atoms to be divided into two subsets in such a way that no two atoms of the same subset are adjacent. A consequence of this property is the symmetrical arrangement of the energy levels of bonding and antibonding Hückel MOs relative to the level of a nonbonding orbital (energy level of the p AO of a carbon atom).
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1922 [Terms] [Paper]