Trimorphism of N-(3-pyridyl)-benzamide

Three polymorphic modifications of N-(3-pyridyl)-benzamide (form I, monoclinic, space group P2(1)/c, Z = 4, form II, monoclinic, space group P2(1), Z = 4 and form III, monoclinic, space group C2/c, Z = 8) were obtained and characterized by single crystal X-ray diffraction. In all forms significant differences in the torsion of the phenyl rings and the arrangement of the molecules in the crystal are observed. Quantum chemical calculations of the conformational barriers reveal only small differences and indicate that the conformation of form III is energetically favoured but this form does not represent the minimum energy conformation. In the crystal structures of all forms the molecules are connected via intermolecular N-H center dot center dot center dot N (form I) and N-H center dot center dot center dot O (form II and III) hydrogen bonds into chains. Solvent mediated conversion and DSC experiments indicate that form I represents the thermodynamically stable form between -20 degrees C and its melting point. Solidification of the melt or sublimation leads directly to form I or to an amorphous material that transforms into form I on crystallization. Form II and form III cannot be obtained as phase-pure materials. Mixtures of form I and II with form II as the minor component can be obtained sometimes by crystallization under kinetic control. Upon heating these mixtures, form II transforms into form I in an endothermic reaction, which indicates that both modifications are enantiotropically related with form II being stable at lower temperatures. Thermomicroscopic investigations on selected single crystals of form III show a polymorphic transition presumably to form I because melting is observed at exactly the same temperature as measured for I.