X-ray refinement of the crystal and molecular structure of 1,12-dimethylbenz[a]anthracene.

The molecular and crystal structure of 1,12-dimethylbenz[a]anthracene, one of the least carcinogenic of the benz[a]anthracenes, has been refined, from new X-ray diffractometric data collected at room temperature, to an R index 0.047 over 1217 independent reflections. Improved determination of molecular geometry shows that steric strain arising from the presence of two bay-methyl substituents causes even greater molecular distortion than in the highly carcinogenic 7,12-dimethylbenz[a]anthracene, with the benzo A ring inclined at about 29 degrees to the furthest C and D rings. Methyl carbon atoms are displaced 1.0 and 1.3 A on opposite sides of the mean molecular plane and the beach bond of the bay region is 1.480(4) A long, flanked by C-C-C angles of 124.3 and 125.0 degrees. The shortest carbon-carbon bond is C5-C6 = 1.327(5) A at the K region, with the next shortest C8-C9 = 1.346(6) A. Close intramolecular approaches of methyl hydrogens across the bay and to nearest aromatic hydrogens are 2.3-2.6 A.