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

Abstract

The linear dependence, previously claimed, of the total π-electron energy ( E π ) 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), E π is well reproduced by the expression a + bK − cK − 1 2 , where a, b and c are constant.