Shapes of carcinogenic benz[a]anthracenes: the crystal and molecular structure of 1-methylbenz[a]anthracene.

The crystal structure of 1-methylbenz[a]lanthracene, which is weakly carcinogenic, has been determined by application of direct methods to single-crystal X-ray diffractometric data and refined by least squares to R = 0.09 over 845 independent reflections. Crystals are monoclinic, space group P2(1), with a = 8.491(2), b = 7.138(2), c = 10.500(2)ABEta = 95.06(01), Z = 2. As in other benz[a]anthracenes, the K-region bond C(5)-C(6) is short [1.34(1)A]. The distinctive bay geometry, with a methyl group opposite to a hydrogen, H(12), peri to another hydrogen, H(11), has a long bond C(13)--C(18) = 1.47(1)A in the bay, and the angular benz-ring is inclined at 16.5 degrees to the mean plane of the anthracene fragment. The methyl carbon atom is 0.79 A out of the mean molecular plane (or 0.19 A out of the plane of the benz-ring) and the 1.50 A long C(1)-methyl bond makes angles of 117 degrees and 125 degrees at C(1).