No abstract

No abstract

No abstract

No abstract

No abstract

The crystal and molecular structure of the monoclinic modification of 1 : 2-5 : 6-dibenzanthracene has been determined, and the co-ordinates of all the carbon atoms are given. The space group is P2,, and the molecule does not display any crystallographic symmetry. The final results, however, indicate a planar structure with the molecular symmetry required by the chemical formula. All the bond lengths have been evaluated and agree well with those obtained from a study of the orthorhombic modification, already described. When the most reliable bond-length measurements are selected from these two independent crystal-structure determinations, excellent agreement is obtained with the theoretical calculations of Vroelant and Daudel.Two distinct crystalline modifications of 1 : 2-5 : 6-dibenzanthracene are known. The orthorhombic form, first described by Krishnan and Banerjee and by Iball,2 has been analysed in detail by the present authorsS3 The unit cell contains four centro-symmetric molecules in the space group Pcab (DiE), and in the best projection of the structure 18 of the 22 carbon atoms in the molecule were separately resolved, yielding measurements of 18 of the 26 carbon-carbon bond lengths.Only preliminary data have so far been recorded4 for the second, monoclinic, form. We have now completed the analysis of this modification, and the results are of interest in completing the bond-length data for this hydrocarbon and in providing one of the few examples of polymorphous organic crystal structures where reasonably accurate values of the atomic co-ordinates are known for both modifications. The monoclinic form is of low symmetry, with two crystallographically asymmetric molecules in the space group P2, (C,Z). With 22 carbon atoms in the asymmetric crystal unit the analysis is thus considerably more complex than for the orthorhombic form. But the final results yield a clear projection of the molecule, with 18 of the 22 carbon atoms resolved (Fig. 1). By assuming a centre of symmetry in the molecule (as demanded by the chemical structure but not required crystallographically) co-ordinates can be assigned to the unresolved atoms

No abstract

No abstract