Cyclodehydrogenation reactions to cyclopentafused polycyclic aromatic hydrocarbons

B3LYP/6-31G(d,p) electronic structure calculations are employed to elucidate the reaction mechanisms for the conversion of the alternant C(18)H(12) polycyclic aromatic hydrocarbon benzoc]phenanthrene into the nonalternant C(18)H(10) PAHs cyclopentacd]pyrene and benzoghi]fluoranthene. Isomerization reactions such as 5/6-ring switching and hydrogen atom scrambling are analyzed. Bay region chemistry, involving the rupture of one benzene ring followed by the formation of a new five-membered ring, is also studied, together with the mechanism for the formation of an aryne. The rearrangement of the latter yields annelated cyclopentadienylidenecarbene, which is then trapped intramolecularly.