| Abstract
| - Planar and overcrowded LPAHs C34H18 anthra[9,1,2-cde]benzo[rst]penaphene (1), benzo[rst]phenanthro[10,1,2-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‘-lmno]perylene (8), potential products of peri-peri reductive couplings of benzanthrone and of naphthanthrone, respectively, were subjected toan ab initio study with emphasis on overcrowding motifs. The HF and DFT B3LYP methods wereemployed to calculate energies and geometries of the minima conformations of these LPAHs. Themost stable LPAHs in these series were found to be planar C2v-1 and C2v-5, respectively. Amongovercrowded LPAHs, twisted-folded C2-3 and C2-7 with two cove regions were found to be morestable than their respective isomers twisted-folded C2-4 and C2-8 with one fjord region each, incontrast to the semiempirical predictions. The energy differences between the most stable planarisomer and the overcrowded isomers were significantly smaller in the C38H18 series, than in theC34H18 series. Overcrowded twisted-folded C2-7 with two coves was found to be more stable thanplanar C2h-6 by 2.0 kJ/mol (at B3LYP/6-311G**), indicating enhanced role of aromatic stabilizationand 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 minimumCi-3 and a syn-folded transition state Cs-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 theC38H18 series were found to be smaller than in the C34H18 series. The nonbonding distances betweencarbon atoms at cove and fjord regions of the overcrowded LPAHs were found to be smaller thanthe sum of the van der Waals radii of two carbon atoms
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