Term: alternancy symmetry
https://doi.org/10.1351/goldbook.AT06985

Definition:
A topological property of the molecular graphs of alternant hydrocarbons 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).

Related Terms:
1) alternant (http://doi.org/10.1351/goldbook.A00249).
2) hydrocarbons (http://doi.org/10.1351/goldbook.H02889).

Source: PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1922 (https://doi.org/10.1351/pac199971101919)

Citation: 'alternancy symmetry' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.AT06985

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