Acemetacin polymorphs: a rare case of carboxylic acid catemer and dimer synthons

Acemetacin is the first example of an API polymorph with a carboxylic acid catemer and dimer O–H⋯O synthons. The auxiliary stabilization to the catemer motif from stronger C–H⋯O and C–Cl⋯O interactions compared to those in the dimer structure is discussed for a polymorph pair. The crystal structure of the stable dimer polymorph was solved by structure determination from powder X-ray diffraction data (SDPD).