Charge Density Study of the Polymorphs of <i>p</i>-Nitrophenol

A careful investigation of the structures and charge densities of the β and α polymorphs of p-nitrophenol has been carried out. Although the two forms crystallize in different monoclinic cells, the crystal densities are similar. There are, however, several differences in the intramolecular structural features of the two forms, including the C−C−O bond angles and the N−O distances. The α form exhibits a large number of intermolecular hydrogen contacts. More importantly, a detailed charge density analysis of the two forms has brought out significant differences in the charge distribution in both the intra- and the intermolecular hydrogen bonding regions. Deformation density maps reveal many differences in the bonding regions of the molecule in the two forms. Charge migration from the benzene ring region of the molecule to the nitro and the hydroxyl groups occurs as the structure changes from the β to the α form. Relief maps of the negative Laplacians in the plane of the intermolecular hydrogen bonds show polarization of the oxygen lone-pair electrons toward hydrogen. The molecular dipole moments in the solid state, derived from the pseudoatomic charges in the β and α structures, are considerably larger (∼20 D) than the value in the free molecule.