Relation between electron and energy contents of hexagons in pericondensed benzenoid hydrocarbons

Abstract

Relations are established between the π-electron content <i>(EC) </i>and the π-electron energy electron energy content <i>(ec) </i>of hexagons in pericondensed benzenoid hydrocarbons. Whereas in catacondensed benzenoids only four types of differen tly annelated hexagons need to be distinguished [Furtula, Gutman & Turkovic, <i>Indian J Chem </i>43A (2004)], in pericondensed systems there are 12 different annelation types. We show that within each of the 12 classes of equally annelated hexagons there exists a linear correlation between <i>EC </i>and <i>ec. </i>These correlations can be explained by means of the Cruickshank-Sparks equations, which relate the Pauling and Coulson bond orders. In addition, within each annelation class there is a correlation between the <i>ec</i>-value of a hexagon and the effect <i>ef </i>of<i> </i>the same hexagon on the total π-electron energy of the corresponding benzenoid molecule. Most of these latter correlations are curvilinear. Our main conclusions are that (a) <i>ec </i>is proportional to <i>EC </i>and (b) <i>ec </i>is proportional to <i>ef, </i>but (c) the mode of the annelation of the respective hexagon strongly influences the actual form of these interdependencies.