The crystal and molecular structure of perylene

Abstract

Perylene, C 20 H 12 , crystallizes in the monoclinic system, space group P 2 1 / a , with four molecules in the unit cell. The structure resembles that of pyrene and 1:12-benzperylene, and has been determined by trial and error and Fourier series methods. In the principal electron-density projection eleven of the twenty carbon atoms in the molecule can be resolved, and the positions of the others can be estimated. The accuracy is not very high, but after averaging in accordance with the expected molecular symmetry the bond lengths can be determined to within about ± 0·04 Å. The results agree with molecular orbital calculations, and in particular it is found that a pair of unusually long bonds ( ~ 1·50 Å) connect the two naphthalene nuclei. The structure of the molecule is discussed in relation to other physical and chemical evidence. The carbon skeleton is at least approximately planar, but the 1,12 and 6,7 hydrogen atoms may deviate from the mean molecular plane.