Molecular dimensions in crystals of methyl-substituted and other derivatives of benz[ a] anthracene

Abstract

The molecular structures of five methylbenz[ a] anthracenes (MBA), 2-MBA, 5-MBA, 8-MBA, 11-MBA and 3,9-DMBA, recently determined by X-ray analyses with bond-length precision σ(CC) − 0.005 Å, are almost planar and exhibit closely similar dimensions. Comparisons among these and with earlier (less precise) analyses of “type I” (nearly planar) BA crystal structures, important in mechanism theories of chemical carcinogenesis, yield reliable estimates of bond lengths in the unsubstituted BA structure, including 1.33 B Å for the short C(5)C(6) K-region bond and 1.46 B Å for the long beach bond C(13)C(18) of the bay region. Mean bond lengths for “type-II” (non-planar) BA structures, such as 1-, 12-, 1,12-D and 7,12-DMBA, differ remarkably little from type-I means, despite mutual inclinations of up to 29° between the extreme aromatic rings caused by steric distortion across the bay region. Endocyclic CCC bond angles at substituent points of type-I structures are about 2° larger than at unsubstituted positions and adjacent bond angles are correspondingly reduced by about 1.3°; the beach bond angles of the bay region, C(12)C(18)C(13) and C(18)C(13)C(1) are about 123° in type-I BA structures and 124° in type-II structures.