Effect of non-bonding molecular orbitals on total π-electron energy

The presence of non-bonding molecular orbitals diminishes the HMO total π-electron energy. A method is elaborated to quantitatively assess this effect. The effect is found to be much smaller than earlier anticipated. Contrary to earlier claims, it depends on the size of the conjugated system and, as a good approximation, decreases as the pth power of the number of vertices of the molecular graph; p≈0.95. The effect is almost insensitive to other structural features present in the conjugated system.