The average Wiener index of hexagonal chains

The average value of the Wiener index of hexagonal chains (= unbranched cat acondensed benzenoid systems) with a fixed number of hexagons, is calculate d. This average value differs somewhat from what earlier was reported (Gutm an et al., 1990. Chem. Phys. Lett. 173, 403). The origin of this difference is clarified. A remarkable collective property is established for the chai ns possessing no linearly annelated hexagons: the sum of their Wiener indic es is divisible by the number of chains. (C) 1999 Elsevier Science Ltd. All rights reserved.