Revisiting a simple regularity for benzenoid hydrocarbons. Total π-electron energy versus the number of Kekulé structures

The linear dependence, previously claimed, of the total π-electron energy (Eπ) of isomeric benzenoid hydrocarbons on the number of Kekulé structures (K) is found to be a satisfactory approximation only for benzenoids having eight or fewer six-membered rings (h ⩽ 8), and is violated if h ⩾ 9. For sets of isomeric ubranched catocondensed benzenoids (having fixed values of h, h ⩾ 9), Eπ is well reproduced by the expression a + bK − cK−12, where a, b and c are constant.