## Abstract

The method of double Fourier series has been applied to determine the crystal structure of the complex between 4: 4$^{\prime}$-dinitrodiphenyl and 4-hydroxydiphenyl, which may be represented as (O$_{2}$NC$_{6}$H$_{4}$C$_{6}$H$_{4}$NO$_{2}$)$_{3}$ (C$_{6}$H$_{5}$C$_{6}$H$_{4}$OH). Projections of the electron density on two axial planes have been made, and the parameters of the structure measured. The dimensions of the unit cell are a = 20$\cdot $06A, b = $9\cdot 46$A, c = $11\cdot 13$A, $\beta $ = 99 degrees 39$^{\prime}$. The space-group is C$_{m}$, and the unit cell contains only two of the complex groups. The hydroxydiphenyl molecules lie completely in the mirror planes (020), while the dinitrodiphenyl molecules lie across the mirror planes, and are approximately parallel to one another in the planes $(20\overline{6})$. In this structure, therefore, the benzene rings of the hydroxydiphenyl molecules have a plane of symmetry in the plane of the rings, and the benzene rings of the dinitrodiphenyl molecules have a mirror plane passing through the terminal carbon atoms and perpendicular to the plane of the rings. The nitro groups possess a plane of symmetry through the nitrogen atom perpendicular to the plane of the group. The thermal vibrations in these crystals produce very pronounced diffuse spectra of two types, which are consistent with the presence of long, flat molecules lying in the planes $(20\overline{6})$. All the molecules are approximately equally spaced from one another, and the intermolecular distances are no shorter than are normally found in aromatic nitro compounds. The molecular dimensions determined with greatest accuracy are those between atoms which are reflected across the mirror planes, namely, the distance of 1$\cdot $90 A between the oxygen atoms of the nitro groups, and the C to C distance of 2$\cdot $42 A across the benzene rings.