Physicochemical Properties and X-ray Structural Studies of the Trigonal Polymorph of Carbamazepine

The trigonal polymorph of carbamazepine (α-carbamazepine) was obtained by crystallization from a number of solvents. It was characterized by means of differential scanning calorimetry, thermogravimetric analysis, infrared spectroscopy, X-ray powder diffraction, thermal microscopy, and powder and intrinsic dissolution rates. The crystal and molecular structures were determined by single-crystal, three dimensional X-ray analysis. A comparison is drawn between the physicochemical properties of the monoclinic (β) and trigonal (α) forms. Structural features of carbamazepine occurring in these forms and in the acetone solvate are compared. Substantial differences were detected between the two polymorphic forms with regard to infrared and differential scanning calorimetry data, thermomicroscopical behavior, morphology, and molecular conformation.