Large PAHs derived from benzanthrone and naphthanthrone. A semiempirical study

In a systematic semiempirical study, AM1 and PM3 methods are employed to calculate minimum, transition state, and higher order saddle point conformations of planar and overcrowded C₃₄H₁₈ 4-15 and C₃₈H₁₈ 16-21 LPAHs, potential products of peri-peri reductive coupling of benzanthrone (2) and naphthanthrone (3), respectively. The most stable LPAHs in these series are planar C_2v-4 (C₃₄H₁₈) and twisted-folded C₂-21 (C₃₈H₁₈). Among overcrowded regions, two coves are consistently more destabilizing than one fjord, according to both semiempirical methods. The non-planar members of the C₃₄H₁₈ and C₃₈H₁₈ series adopt twisted-folded conformations as global minima and folded-twisted conformations as transition states. The barriers for enantiomerization of the LPAHs with a fjord, cove, and two coves are 18.5-24.0 (AM1) and 16.0-19.3 (PM3), 11.1-11.8 (AM1) and 7.2-7.5 (PM3), 12.0-12.4 (AM1) and 8.1-8.4kcal/mol (PM3), respectively. A comparison of calculated and experimentally determined geometries of LPAHs shows a good correlation between the two types of methods.