3,4:9,10-Perylenetetracarboxylic dianhydride (PTCDA) by electron crystallography.

The crystal structures of the alpha and beta modifications of PTCDA were analyzed as projected structures along the a axes by electron crystallography using an imaging plate. The results for the alpha modification agree well with the sheet-and-stack structure obtained by X-ray diffraction by M. L. Kaplan et al. (private communication, full set of crystal structure data). Projected onto the (102) plane, which is parallel to the molecular sheets, the long molecular axis makes an angle of 42 degrees with the b axis and the hexagonal benzene rings appear slightly elongated, indicating a slight inclination of the molecular plane from the (102) lattice plane. For the beta modification, it was concluded that the molecules are aligned in a herringbone packing scheme on the (102) plane similar to that of the alpha modification, but with a slightly different angle of the long molecular axis with the b axis (38 degrees ).