Szeged indices of benzenoid hydrocarbons

The Szeged index (Sz) is a recently proposed structure descriptor based on distances between vertices of the molecular graph. Numerous properties of Sz were previously shown to parallel that of the long-known Wiener index (W). We report explicit combinatorial expressions for Sz of a variety of homologous series of benzenoid hydrocarbons. Comparing Sz with W it is found that in extruded benzenoid systems, Sz is linearly proportional to W. In compact benzenoids, however, Sz increases much faster than W, indicating that the two indices reflect different features of the molecular structure.