| Abstract
| - Structural and solid-state changes of piroxicam in its crystalline form under mechanical stresswere investigated using cryogenic grinding, powder X-ray diffractometry, diffuse-reflectance solid-stateultraviolet−visible spectroscopy, variable-temperature solid-state 13C nuclear magnetic resonance spectroscopy, and solid-state diffuse-reflectance infrared Fourier transform spectroscopy. Crystalline piroxicamanhydrate exists as colorless single crystals irrespective of the polymorphic form and contains neutralpiroxicam molecules. Under mechanical stress, these crystals become yellow amorphous piroxicam, whichhas a strong propensity to recrystallize to a colorless crystalline phase. The yellow color of amorphouspiroxicam is attributed to charged piroxicam molecules. Variable-temperature solid-state 13C NMRspectroscopy indicates that most of the amorphous piroxicam consists of neutral piroxicam molecules; thecharged species comprise only about 8% of the amorphous phase. This ability to quantify the fractions ofcharged and neutral molecules of piroxicam in the amorphous phase highlights the unique capability ofsolid-state NMR to quantify mixtures in the absence of standards. Other compounds of piroxicam, whichare yellow, are known to contain zwitterionic piroxicam molecules. The present work describes a systemin which proton transfer accompanies both solid-state disorder and a change in color induced by mechanicalstress, a phenomenon which may be termed mechanochromism of piroxicam.
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