The crystal and molecular structure of coronene

Crystals of coronene, C24H12, are monoclinic, a — 16·119, b = 4·702, c = 10·102, β = 110·9°, Z = 2, space group P21/a . The intensities of 936 reflexions were measured with a scintillation counter and CuKα radiation, the gross features of the structure previously determined from projections were confirmed with the new three-dimensional data, and the positional and thermal parameters of the carbon and hydrogen atoms were refined by least squares. The anisotropic thermal parameters of the carbon atoms were interpreted in terms of rigid-body vibrations of the molecule, and the measured carbon-carbon bond lengths were then corrected for rotational oscillation effects. The bonds in the central ring measure 1·425 ± 0·004 Å, the ‘spokes’ 1·433 ± 0·004 Å, and the two types of outer bond 1·346 ± 0·005 Å and 1·415 ± 0·003 Å. These distances agree closely with the previous two-dimensional results, and with the values predicted by valence-bond and by molecular orbital calculations, except that the measured distance of the shortest bonds (1·346 Å) is significantly less than the theoretical values. This discrepancy is observed also for all other polynuclear aromatic hydro- ­ carbons for which accurate bond distances are available, and an amended order/length correlation curve improves the agreement between theory and experiment for these shorter bonds. There are small, but significant, deviations from complete planarity (maximum displacement for carbon atoms 0·029 Å), which reduce the molecular symmetry from 6mmm to 3. This distortion may be a result of intra- or intermolecular forces. All the intermolecular separations correspond to van der Waals interactions; the perpendicular distance between the molecular planes is 3·46 Å.