Crystal structure and physical properties of two polymorphs of ropivacaine HCl.

The crystal structure of two polymorphs of ropivacaine HCl have been determined, as well as their relative stability up to 100 degrees C. A geometric restriction for a solid-state transition between the two polymorphs has been identified. The packing density along the H-bonded chains form the basis for a model explaining the kinetic crystallization of the metastable form. The difference in stability and physicochemical properties between the two polymorphs can be attributed to the difference in crystal structure.