On <img src='/image/spc_char/pi.gif' border=0>-electron configuration of cyclopenta-derivatives of benzenoid hydrocarbons

853-860Within the Hückel molecular orbital model, Kekulèan benzenoid hydrocarbons have equal number of bonding and antibonding molecular orbitals, and no non-bonding molecular orbitals. In cyclopenta-derivatives of benzenoid hydrocarbons this regularity may be either preserved or violated, depending on the number and position of the five-membered rings. Some general regularities along these lines are established. In particular, the problem is completely solved in the case of prolate-rectangle-shaped benzenoids; the number of bonding molecular orbitals always exceeds the number of antibonding molecular orbitals, except if all five-membered rings lie on the same side of the benzenoid system