Structural characterization of two polymorphic forms of piroxicam pivalate

The crystal and molecular structures of two polymorphs of piroxicam pivalate are presented and discussed. A peculiarity of the high melting (154 degrees C) polymorph is the association of piroxicam pivalate molecules as centrosymmetric dimers by hydrogen bonding. Two centrosymmetrically related N-H...N hydrogen bonds maintain the dimer structure involving the amido nitrogen atom as donor and the pyridine nitrogen atom as acceptor. Molecular association of this type does not occur in the crystal structures of drugs belonging to the oxicam class of nonsteroidal antiinflammatory drugs. Two distinct conformations coexist in the crystal of the low melting polymorph (136 degrees C) with differing hydrogen bonding arrangements within domains of the crystallographically independent molecules. The occurrence of different molecular conformations (conformational polymorphism) associated with different hydrogen bonding schemes in discrete domains is an unusual structural feature. Structural data for the two polymorphs are also correlated with the relevant infrared spectra. Computer-generated X-ray powder diffraction patterns for the two polymorphs of piroxicam pivalate are in very good agreement with the experimental ones, thus confirming the validity of the single-crystal X-ray models.