Perturbation-graph theory—I: Resonance energies of heteroannulene π systems

Perturbation theory and the graph-theoretical definition of resonance energy are combined and applied to π systems substituted with heteroatoms and external substituents. The systems investigated are monocyclic and contain 3–6 atoms with 2–8 π electrons. In general, heteroatoms are calculated to exert their effects on resonance energy through two separable mechanisms, due either to the electronegativity (h parameter) or the conjugative ability (k parameter) of the substituent atom. There is no first-order effect of electronegativity in species containing 4n+2 (n= integer) π electrons. All electronegative or electropositive substituents are predicted to stabilize 4n π electron systems. Enhanced conjugation stabilizes 4n+2 systems and destabilizes 4n systems. Several examples of ions and molecules are given that qualitatively conform to these rules.