The crystal structure of p-nitroaniline, NO₂.C₆H₄.NH₂

p-Nitroaniline forms monoclinic crystals, space group C_2h⁵-P2₁/n, with four asymmetric molecules in the unit cell. A quantitative X-ray investigation based on visual intensity measurements of about 250 reflections from the axial zones has led to a complete determination of the structure, and the results have been refined by the double Fourier series method, which gives projections of the structure along the three crystal axes. The co-ordinates of the atoms, the molecular dimensions, and the intermolecular distances are given. The nitro-group is symmetrical and lies in the plane of the benzene ring. In addition to rather weak hydrogen bridges connecting the oxygen atoms of the nitro-group to the amino-groups of adjoining molecules, there is a very close approach (2.7-3.0 A.) between one of the nitro-oxygen atoms and three of the aromatic carbon atoms of an adjoining molecule. This appears to be an intermolecular attraction of a new type, and it is suggested that a similar mechanism may be responsible for the formation of a large class of the molecular compounds that are formed between aromatic nitro-compounds and polycyclic hydrocarbons.