The crystal and molecular structure of perylene

Crystals of perylene, C20H12, are monoclinic with four molecules in a unit cell of dimensions a = 11.277, b = 10.826, c = 10.263Å, B = 100.55°, space group P21/a The intensities of 1135 reflexions were measured with a scintillation counter and MoKa radiation, and the 436 strongest reflexions were used in the structure refinement. The gross features of the structure previously determined from two projections, have been confirmed by the new threedimensional data, the positional and thermal parameters of the carbon atoms have been refined by least-squares and differential syntheses, and the hydrogen atoms have been approximately located. There are small, but significant deviations from a completely planar arrangement ; this minor distortion of the molecule is the result of slight intermolecular steric effects, and does not appear to be associated with any possible overcrowding in the 1, 12 and 6, 7 positions. The mean bond distances agree fairly closely with the values predicted by valence-bond and molecular-orbital calculations; the peri-bond lengths are 1.471 + 0.006 Å. All the intermolecular separations correspond to normal van der Waals interactions; the perpendicular distance between mean molecular planes is 3.46 Å.