CROATICA CHEMICA ACTA, vol.78, no.4, pp.485-488, 2005 (Peer-Reviewed Journal)
The McClelland number of a conjugated hydrocarbon is the integer k, satisfying the condition 2(-(1/2)k) root 2nm <= E < 2(-(1/2)k+1) root 2nm, where E is the HMO total pi-electron energy, n the number of carbon atoms, and m the number of carbon-carbon bonds. If k = 3, then the respective conjugated system is said to be energy-regular. If k ! 2 and k ! 4, then one speaks of energy-poor and energy-rich pi-electron systems, respectively. We found that all polycyclic Kekulean hydrocarbons, possessing condensed rings, are energy-regular, with only three exceptions: naphthalene, phenanthrene, and triphenylene (which are energy-rich). Energy-poor pi-electron systems are some (but not all) non-Kekuleans, whereas many of the polycyclic Kekulean hydrocarbons with non-condensed rings (polyphenyls, phenyl-substituted polyenes and similar) are energy-rich.