The Structure of Hexabenzotriphenylene and the Problem of Overcrowded “D₃h” Polycyclic Aromatic Compounds

Hexabenzotriphenylene ( 1, dibenzo[ f,j ]phenanthro[9,10- s ]picene) has been prepared in 5% yield by vacuum pyrolysis of phenanthrene-9,10-dicarboxylic anhydride, and its X-ray structure has been determined. Compound 1 is a strongly twisted, D 3 -symmetric molecular propeller, in contrast to other highly substituted triphenylenes (perfluoro- and perchlorotriphenylene) which adopt C 2 -symmetric conformations. Computational studies of these and other overcrowded, nominally D 3 h -symmetric, polycyclic aromatic compounds are reported, and the origins of their conformational preferences and the adequacy of various computational methods for treating these compounds are discussed.