Overcrowded polycyclic aromatic enes (PAEs) are starting materials for the formation of fullerene fragments, including buckybowls. Semiempirical (PM3) and ab initio DFT (B3LYP/STO-3G and B3LYP/6-31G*) calculations of C_(26)H_n, C_(27)H_nO and C_(28)H_nO_2 species 1 - 7 and 11 -14 including the PAEs bifluorenylidene (1), bianthrone (6) and fluorenylidene-anthrone (11) have been carried out, with special emphasis given to the energetics of dehydrocyclization and decarbonylation reactions, leading to the buckybowl C_(26)H_(12) 2. The majority of the dehydrocyclizations are exothermic, while all the decarbonylations are endothermic. The overall reaction 6 -> 2 is highly endothermic, 67.3 (B3LYP/6-31G*) and 53.7 (PM3) kcal/mol. In almost all cases, the PM3 method overestimates both the exothermicity and the endothermicity of the reactions, as compared with the ab initio methods. Noting anti-folded, twisted, planar, and bowl conformations as a, t, p, and b, respectively, the steric course of the double-dehydrocyclization - double-decarbonylation pathway is a-6 -> t-12 -> p-7 -> p-13 -> b-2. The alternative doube-decarbonylation - double-dehydrocyclization pathway is a-6 -> a-11 -> t-1 -> p-4 -> b-2. In both pathways, the motif is a -> t -> p -> b. The endothermicity is most severe in the final planar to bowl steps 4 -> 2 and 13 -> 2. The fullerene fragment 4, formed by dehydrocyclization of 1, is planar. The decarbonylation of planar mesonaphthobianthrone (7) to the planar 7H-indeno[1,2,3,4-defg]naphthaleno[3,2,1,8-mnop]chrysene-7-one (13) is highly endothermic, 36.2 kcal/mole (B3LYP/6-31G*).
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