Orientations of Molecules in the p-Benzoquinone Crystal

Abstract

THE crystal structure of p-benzoquinone has recently been studied by W. A. Caspari¹ by X-ray methods. The crystal belongs to the monoclinic system and is assigned by Caspari to the space-group C⁵_2h; there are two molecules in the unit cell, the molecules possessing a centre of symmetry. His X-ray measurements are not sufficient to determine uniquely the orientations of the molecules in the unit cell. However, by combining his X-ray data with certain considerations concerning the crystal habit and the dimensions of the molecules, he concludes that the molecular planes are parallel to (201) and that the lines joining the two oxygen atoms of the molecule lie in the (010) plane. This orientation of the molecules secures a large concentration of the oxygen atoms in the (201) plane, which concentration, according to Caspari, will account for the exceptional development in the crystal of so peculiar a face as (201).