Thermal interconversions of the C16H10 cyclopenta-fused polycyclic aromatic hydrocarbons fluoranthene, acephenanthrylene and aceanthrylene revisited

The thermal interconversion of the C16H10 cydopenta-fused polycyclic aromatic hydrocarbons (CP-PAH) fluoranthene (1), acephenanthrylene (2) and aceanthrylene (3) has been studied using Flash Vacuum Thermolysis (FVT). In contrast to previously reported flow-pyrolysis results, which gave the unexpected thermodynamic stability order 1 > 3 > 2, FVT temperature-canversion data unequivocally show the extended order 1 > 2 > 3. These results are supported by independent FVT experiments with 9-ethynylphenanthrene (4), 9-ethynylanthracene (5) and 5-(anthryl-9'-methylene)-2, 2-dimethyl-1,3-dioxan-4,6-dione (6), which are precursors for acephenanthrylene (2) and aceanthrylene (3), respectively. The interpretation of the FVT results is corroborated by semiempirical AM1 calculations of the C16H10 potential energy surface.