Overcrowding motifs in large PAHs. An ab initio study

Planar and overcrowded LPAHs C₃₄H₁₈ anthra[9,1,2-cde]benzo[rst]penaphene (1), benzo[rst]phenanthro[1O,1,2-cde]pentaphene (2), tetrabenzo[a,cd,j,lm]perylene (3), tetrabenzo[a,cd,lm,o]perylene (4), and LPAHs C₃₈H₁₈ anthra[2,1,9,8-klmno]naphtho[3,2,1,8,7-vwxyz]hexaphene (5), dianthra[2,1,9,8-stuva;2′,1′,9′,8′-hijkl]pentacene (6), dibenzo[jk,uv]dinaphtho[2,1,8,7-defg;2′,1′,8′, 7′-opqr]perylene (7), diphenanthro[5,4,3-abcd;3′,4′,5′-dmno]perylene (8), potential products of periperi reductive couplings of benzanthrone and of naphthanthrone, respectively, were subjected to an ab initio study with emphasis on overcrowding motifs. The HF and DFT B3LYP methods were employed to calculate energies and geometries of the minima conformations of these LPAHs. The most stable LPAHs in these series were found to be planar C_2v-1 and C_2v-5, respectively. Among overcrowded LPAHs, twisted-folded C₂-3 and C₂-7 with two cove regions were found to be more stable than their respective isomers twisted-folded C₂-4 and C₂-8 with one fjord region each, in contrast to the semiempirical predictions. The energy differences between the most stable planar isomer and the overcrowded isomers were significantly smaller in the C₃₈H₁₈ series, than in the C₃₄H₁₈ series. Overcrowded twisted-folded C₂-7 with two coves was found to be more stable than planar C_2h-6 by 2.0 kJ/mol (at B3LYP/6-311G**), indicating enhanced role of aromatic stabilization and decreased destabilization due to overcrowding, with increasing the number of aromatic rings. Heats of formation of LPAHs 1 - 8 were derived from the ab initio total energies (at B3LYP/6-31G*). A search of the conformational spaces of 3 and 4 revealed an anti-folded local minimum C_i-3 and a syn-folded transition state C_s-4, 23.7 and 120.3 kJ/mol higher in energy than the twisted-folded C₂-3 and C₂-4, respectively (at B3LYP/6-31G*). The cove and fjord torsion angles in the C₃₈H₁₈ series were found to be smaller than in the C₃₄H₁₈ series. The nonbonding distances between carbon atoms at cove and fjord regions of the overcrowded LPAHs were found to be smaller than the sum of the van der Waals radii of two carbon atoms.