X-ray structural characterization of anhydrous metronidazole benzoate and metronidazole benzoate monohydrate

Single crystals of anhydrous metronidazole benzoate and its monohydrate were isolated from the same aqueous solution and studied by X-ray diffraction. Anhydrous metronidazole benzoate gives crystals belonging to the triclinic space group P1 and, at ambient temperature, a = 6.649 (2), b = 8.666(1), c = 11.940(3) A, alpha = 76.70(2)degrees, beta = 76.72(2)degrees, gamma = 87.56(2)degrees, V = 651.6(3) A3, Z = 2, and Rw(F) = 0.053. Metronidazole benzoate monohydrate gives crystals belonging to the triclinic space group P1 and, at ambient temperature, a = 7.544(1), b = 7.990(1), c = 12.329(4) A, alpha = 94.33(2)degrees, beta = 97.40(2)degrees, gamma = 101.36(1)degrees, V = 718.6(3) A3, Z = 2, and Rw(F) = 0.041. Thermomicroscopy, differential scanning calorimetry, and thermogravimetry were used for initial characterization of the title species and to investigate possible phase changes on heating. The crystal structure analyses revealed that the metronidazole benzoate molecule adopts different conformations in the two crystal forms. Crystal cohesion in the anhydrous form is due to van der Waals interactions only, whereas in the monohydrate, there is strong intermolecular hydrogen bonding mediated by water molecules. Computer-generated X-ray powder patterns for the two species are distinctly different and serve as reference for their identification.