The structure of hexabenzobiphenylene and the problem of overcrowded "D-3h" polycyclic aromatic compounds

Hexabenzotriphenylene (1, dibenzol[fj]phenanthro[9,10-s]picene) has been pr epared in 5% yield by vacuum pyrolysis of phenanthrene-9,10-dicarboxylic an hydride, and its X-ray structure has been determined. Compound 1 is a stron gly twisted, D-3-symmetric molecular propeller, in contrast to other highly substituted triphenylenes (perfluoro- and perchlorotriphenylene) which ado pt C-2-symmetric conformations. Computational studies of these and other ov ercrowded, nominally D-3h-symmetric, polycyclic aromatic compounds are repo rted, and the origins of their conformational preferences and the adequacy of various computational methods for treating these compounds are discussed .