The bay-region geometry of some 5-methylchrysenes: Steric effects in 5,6- and 5,12-dimethylchrysenes

The presence of a bay-region methyl group in carcinogenic polycyclic aromatic hydrocarbons leads to considerable distortion in the molecule. This is illustrated In the structures, obtained by X-ray diffraction techniques, of 5,12- and 5,6-dimethylchrysene. The molecular distortions result from steric requirements, such as that the minimum H^{{combining triple dot above}}H distance is 1.8 Å and the minimum C^{{combining triple dot above}}C distance is 2.90 Å; distortions to accommodate these requirements may be both in-plane (by increasing the angles at carbon atoms in the bay-region from 120° to ̃ 124°) and out-of-plane by torsion about certain bonds in the bay-region. It Is shown that more in-plane distortions are found for 5-methylchrysene derivatives than for methylbenz[a]anthracene derivatives and this, it is suggested, results from the nature of the flexibility of the chrysene compared with the benz[a]anthracene fragment at the bay-region.