14N Nuclear Quadrupole Resonance Study of Piroxicam: Confirmation of New Polymorphic Form V

A new polymorphic crystal form of piroxicam was discovered while preparing crystalline samples of piroxicam for 14N nuclear quadrupole resonance (NQR) analysis. The new crystal form, designated as V, was prepared by evaporative recrystallization from dichloromethane. Three known polymorphic forms (I, II, and III) were also prepared. Our aim was to apply 14N NQR to characterize the new polymorphic form of piroxicam and compare the results with those of the other known polymorphic forms. Additional analytical methods used for characterization were X-ray powder diffraction (XRPD), thermal analysis, and vibrational spectroscopy. For the first time, a complete set of nine characteristic 14N NQR frequencies was found for each prepared polymorph of piroxicam. The consistent set of measured frequencies and calculated characteristic quadrupole parameters found for the new polymorphic form V is a convincing evidence that we are dealing with a new form. The already known piroxicam polymorphic forms were characterized similarly. The XRPD results were in accordance with the conclusions of 14N NQR analysis. The performed study clearly demonstrates a strong potential of 14N NQR method to be applied as a highly discriminative spectroscopic analytical tool to characterize polymorphic forms.