7,12-Dimethylbenz[a]anthracene: refined structure, electron density distribution and endo-peroxide structure.

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The crystal structure of 7,12-dimethylbenz[a]anthracene (DMBA) has been refined from new X-ray diffraction data collected at low temperature (180 K). This has allowed the location of the hydrogen atom positions not previously reported in earlier structure determinations and refinements; a more precise molecular geometry is therefore now presented. In addition, an analysis of the electron density in this carcinogenic molecule has been made by multipole refinement. These two types of studies give information on the amount of strain in the bay region and the distribution of electron density in the molecule. The molecule is highly distorted in the bay region as a result of steric overcrowding between hydrogen atoms (minimum H ... H 2.06 A) so that torsion angles of 18 degrees and 22 degrees occur in this area. The bonds in the bay region and to the two methyl groups appear to be electron-rich; however, while the K-region of DMBA has a high pi-bond density computed from interatomic distances, the multipole analysis does not indicate that it is highly electron-rich. The 7- and 12-positions (equivalent to the 9- and 10-positions of anthracene) are highly reactive and appear to show a deficiency of electron density. Molecular dioxygen can add across these positions to give a peroxy compound. The crystal structure of such an endo-peroxide of DMBA has also been studied at 180 K although not to the high precision obtained for the parent compound. Some distortions are apparent in this molecule; in particular small CH3-C-O angles (101-104 degrees) are observed, indicative of some strain in the molecule. A computer graphics analysis of the diol epoxides of DMBA, generated from X-ray coordinates of DMBA and reported values for a diol epoxide of benzo[a]pyrene, show that steric overcrowding may affect the conformation of certain isomers of the diol epoxides.