The structure of hexabenzotriphenylene and the problem of overcrowded "D3h" polycyclic aromatic compounds

Hexabenzotriphenylene (1, dibenzol[fj]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-3h-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.