On the pairing of molecular orbital energy levels of heteroconjugated molecules

The pairing theorem (discovered by Coulson and Rushbrooke in 1940) predicts pairing of the molecular orbital energy levels of alternant hydrocarbons, as calculated within the Hückel molecular orbital (HMO) model. In the general case, the pairing theorem does not apply to nonalternant hydrocarbons or alternant conjugated systems containing heteroatoms. It was recently observed that, in certain heteroconjugated molecules (both alternant and nonalternant), some HMO energy levels remain paired irrespective of the nature of the heteroatoms, i.e., irrespective of the values of the HMO parameters h and k. We now provide an explanation of this regularity.