Hückel's Rule Diagram of π MOs: Frost Circle

The Frost circle or the inscribed polygon method is a graphical method based on Hückel's rule to determine the number and relative energies of the π molecular orbitals of planar, monocyclic, and conjugated systems.

A Frost circle is drawn by inscribing a polygon, representing the cyclic compound, inside a circle with one of the vertices pointing down.

The intersections where the circle touches the polygon are marked by short horizontal lines representing the number and the relative energies of π molecular orbitals.

Next, a dashed horizontal line is drawn through the circle's center, bisecting the polygon. This line represents the energy of the participating p orbitals.

All the molecular orbitals with energies below this line are bonding, those above the line are antibonding, and those on the line are non-bonding molecular orbitals.

Lastly, the π electrons are added to the energy levels according to Hund's rule and Pauli's exclusion principle.