Disappearing and Reappearing Polymorphs. The Benzocaine:Picric Acid System.
The low-melting polymorphic modification of the 1:1 complex of benzocaine (BC) and picric acid (PA) had earlier been reported to be an example of a \"disappearing polymorph\". The BC:PA system was reinvestigated by thermomicroscopy, calorimetry, solid-state NMR, and x-ray crystallog. The phase diagram was derived, and robust procedures for the crystn. of the two 1:1 complexes, a hydrate of the 1:1 complex, and a 2:1 complex were devised. The structures of all four phases were detd. and compared using graph set anal. to characterize the hydrogen-bonding patterns. It is shown that the thorough microscopic investigation of the thermal behavior, combined with calorimetric methods, can lead to the development of strategies to crystallize metastable polymorphic forms which may be difficult to obtain once their stable congeners have been obtained.
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