On dioxo derivatives of benzenoid hydrocarbons

Extending a problem occurring in connection with the stability order of the tautomers of hypericin, we examine the topological factors influencing the stability of the isomeric dioxo derivatives of benzenoid hydrocarbons, in particular the rules determining the number of Kekule structures in them. D epending on the position of the oxo groups, the pattern of cyclic conjugati on varies significantly, as does the Kekule structure count. Some general r egularities along these lines are established (C) 1999 Elsevier Science B.V . All rights reserved.