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 Kekulé structures in them. Depending on the position of the oxo groups, the pattern of cyclic conjugation varies significantly, as does the Kekulé structure count. Some general regularities along these lines are established.