## Abstract

The crystal of 4: 4$^{\prime}$-dinitrodiphenyl [C$_{12}$H$_{8}$(NO$_{2}$)$_{2}$] is an example of one that can for purposes of measurement be referred to orthogonal axes, but which further investigation shows to be structurally monoclinic. The dimensions of the orthogonal unit cell, which contains two molecules, are a = $3\cdot 77$ A, b = $9\cdot 56$ A, c = $15\cdot 39$A, and the space group is Pc. The method of double Fourier series has been applied, and projections of the electron density on two of the axial planes have been made. The most important projection, that on the bc plane, has no centre of symmetry, and the phases of the Fourier components had to be estimated before summing the series. Although so far as the space group is concerned the molecules need have no centre of symmetry, it is almost certain that such centres exist. Within the errors of experiment, the benzene rings are regular hexagons of side $1\cdot 41$ A, the two rings in the molecule being co-planar with a common axis. The C-C link between rings is $1\cdot 42$ A. The C-N links do not lie in the plane of the rings, but make angles of about $\pm $ 2 degrees with it. The two N-O distances in the nitro-group are not quite equal, being $1\cdot 14$ and $1\cdot 21$ A, the O-O distance being $2\cdot 00$ A. The lines joining O to O in the nitro-groups are nearly parallel to the plane of the benzene rings, but lie about $0\cdot 42$ A above and below it. The closest approach of O to CH in adjacent molecules is about $3\cdot 0$ A, and that between CH and CH, $3\cdot 6$ A. A discussion of the packing of the molecules in the structure is given.