Structure determination of forms I and II of phenobarbital from X-ray powder diffraction

From pure powders of forms I and II of phenobarbital, X‐ray diffraction patterns were recorded at room temperature. The starting crystal structural models were found by a Monte‐Carlo simulated annealing method. The structures of the two forms were obtained through Rietveld refinements. Soft restraints were applied on bond lengths and bond angles, all H‐atom positions were calculated. The cell of form I is monoclinic with the space group P21/n, Z = 12, Z′ = 3. Form II has a triclinic cell, with the space group , Z = 6, Z′ = 3. For both forms, the crystal cohesion is achieved by networks of N—H⋯O hydrogen bonds along [101]. The broadening of the Bragg peak profiles is interpreted in terms of isotropic strain effects and anisotropic size effects.