Large PAHs derived from benzanthrone and naphthanthrone. A semieimpirical study

In a systematic semiempirical study, AM1 and PM3 methods are employed to ca lculate minimum, transition state, and higher order saddle point conformati ons of planar and overcrowded C34H18 4-15 and C38H18 16-21 LPAHs, potential products of peri-peri reductive coupling of benzanthrone (2) and naphthant hrone (3), respectively. The most stable LPAHs in these series are planar C -2v-4 (C34H18) and twisted-folded C-2-21 (C38H18) Among overcrowded regions , two coves are consistently more destabilizing than one fjord, according t o both semiempirical methods. The non-planar members of the C34H18 and C38H 18 series adopt twisted-folded conformations as global minima and folded-tw isted conformations as transition stales. The barriers for enantiomerizatio n 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 .4 kcal/mol (PM3), respectively. A comparison of calculated and experimenta lly determined geometries of LPAHs shows a good correlation between the two types of methods.