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Crystals of an unknown third crystalline modification of 2,2'-dipyridylamine were obtained from acetone solution and their crystal structure was determined at both 150 K and room temperature. In contrast to the known orthorhombic and triclinic forms, which contain hydrogen-bridged dimers, the molecules of the monoclinic polymorph are arranged in tetramers. The crystallographic results for the monoclinic form are presented here, as well as a detailed comparison of crystal and molecular structures of the three polymorphs. Studies by differential thermal analysis (DTA) and optical microscopy, performed with single crystals, show a transformation of the orthorhombic phase at ~323 K and of the monoclinic form at ~368 K. According to powder diffraction studies, transformation of the low melting orthorhombic polymorph results in a mixture of monoclinic and triclinic phases, whereas the monoclinic modification transforms into the triclinic phase just below its melting point at 368 K. The single crystals of both forms are destroyed during the conversion and, therefore, in both cases a reconstructive transition via nucleation and growth should occur. The conditions for the crystallization of the distinct modifications and their relative thermodynamic stabilities are investigated in different solvents and at different temperatures. Independent of the solvent chosen, the orthorhombic form is the most stable below 263 K. In the range between 263 and 313 K the monoclinic form appears to be thermodynamically advantageous and above that temperature, the stability order is changed in favour of the triclinic polymorph. Based on the experimental results, a qualitative free energy-temperature diagram is provided.
