Rifampicin is a first-line drug widely used in the treatment of tuberculosis both in the intensive and in the treatment phase. In this work we characterized the crystal structure of form I of anhydrous rifampicin mainly by using X-ray powder diffraction data combined with the Rietveld method. Other complementary techniques such as Fourier transform infrared spectroscopy and thermal analysis were also employed. It crystallized in a monoclinic crystal system with space group C2 and the following unit cell parameters: a = 25.8846(2) Å, b = 14.2965(2) Å, c = 14.2796(2) Å, β = 122.98(1)°, V = 4432.81(7) Å3, Z = 4, Z′ = 1 and ρcalc = 1.23310(2) g cm−3. A BFDH model was used to inspect the crystal morphology prediction, and great similarity with crystals observed on an optical microscope was found. The FTIR spectrum confirmed the results obtained by X-ray powder diffraction, which indicates that all the functional groups involved in H bonding are intramolecularly connected. This polymorphic form presented a thermal stability up to approximately 230 °C.

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