Polymorphs of p-nitrophenol as studied by variable-temperature X-ray diffraction and calorimetry: comparison with m-nitrophenol.

Crystal structures of two polymorphic forms of p-nitrophenol have been determined at several temperatures between 120 and 375 K. The thermal expansion tensor has been determined for both polymorphs. The rigid-body mean-square amplitudes of molecular translations and librations and the amplitudes of the internal torsions of the nitro group have been calculated at different temperatures. Differential scanning calorimetry was used to find the temperature and enthalpy of the polymorphic transformation. The results were compared with those recently obtained for m-nitrophenol polymorphs. Some conclusions concerning the polymorphism of p- and m-nitrophenols are presented.