[R,S]-Ethambutol dihydrochloride: Variable-temperature studies of a dimorphic system with very similar packing

The two polymorphs (Forms I and II) of [R,S]-ethambutol dihydrochloride transform enantiotropically and reversibly in a single-crystal-to-single-crystal phase transformation mode. These structurally very similar forms have been characterized and their thermodynamic relationship has been investigated by variable-temperature solid-state carbon-13 nuclear magnetic resonance, variable-temperature powder X-ray diffraction, differential scanning calorimetry, and optical microscopy. The nuclear magnetic resonance results are compared with those for the two polymorphs of the [S,S] diastereomer with known structures. (C) 2004 Wiley-Liss, Inc. and the American Pharmacists Association.