Crystal structure determination from two-dimensional powders: A combined experimental and theoretical approach

Abstract

A combined experimental and theoretical approach is used to solve the unknown crystal structure of para-cyano-quaterphenylene. Thin films are prepared by physical vapor deposition on isotropic surfaces, whereby a two-dimensional powder is formed. Using specular and grazing incidence X-ray diffraction, the crystallographic unit cell of the dominant polymorph phase is obtained. A monoclinic lattice with unit cell parameters ${\rm a}=5.56\,{\rm \AA}$, ${\rm b}=7.67\,{\rm \AA}$, ${\rm c}=20.85\,{\rm \AA}$, and β=97.4^○ is determined. These experimental parameters are used as input for ab initio calculations within the framework of density-functional theory to determine the molecular packing within the crystal structure. Three types of molecular arrangements are found, exhibiting very close energies of formation. From the comparison of calculated structure factors with the experimentally observed intensities two solutions could be excluded. The molecule is found to pack in a herringbone pattern with layers of upright standing molecules.