Aspirin Polymorph Found

In the late 1960s, there were indications that aspirin might have a second crystalline form, but it continued to escape detection. Now, Michael J. Zaworotko, chemistry professor at the University of South Florida, Tampa, and coworkers at South Florida and TransForm Pharmaceuticals, Lexington, Mass., have found this elusive polymorph (J. Am. Chem. Soc. 2005, 127, 16802).

This second form was obtained during cocrystallization experiments with aspirin and other compounds. Form II is kinetically stable at 100 K, but it converts back to form I at ambient conditions. Both forms contain a hydrogen-bonded carboxylic acid dimer (shown). But they differ in the way the dimers adjoin one another through their acetyl groups: The familiar form I assembles into dimers of dimers, whereas the new polymorph forms chains of dimers.