High temperature gas phase syntheses of C20H12 cyclopenta-fused polycyclic aromatic hydrocarbons: Benz[l]acephenanthrylene and benz[j]acephenanthrylene and their selective rearrangement to benzo[j]fluoranthene

The novel C20H12 cyclopenta-fused polycyclic aromatic hydrocarbon benz[l]acephenanthrylene (2) and its isomer benz[j]acephenanthrylene (3) have been obtained by hash vacuum thermolysis of 2-(1-chloroethenyl)benzo[c]phenanthrene (6) and 6-(1-chloroethenyl)chrysene (7), respectively. At T greater than or equal to 900 degrees C 2 and T greater than or equal to 1000 degrees C 3 rearrange selectively to the abundant combustion effluent benzo[j]fluoranthene (1). No evidence for the presence of the related rearrangement products benz[l]aceanthrylene (12) and benz[j]aceanthrylene (13), respectively, is found. Semi-empirical AM1 calculations provide a rationalization for these observations; the conversion of 2 and 3 into 1, instead of 12 and 13, respectively, via consecutive ring-contraction-ring-expansion processes and vice versa is favoured.