Structure of a metastable phase of piracetam from X-ray powder diffraction using the atom–atom potential method

Abstract

The crystal structure of the room-temperature metastable phase of 2-oxo-1-pyrrolidineacetamide (piracetam), C 6 H 10 N 2 O 2 , has been solved using the atom-atom potential method and refined by Rietveld refinement from powder diffraction data collected with a curved position-sensitive detector (INEL CPS 120) using Debye-Scherrer diffraction geometry with monochromatic X-rays. In the first stage, the indexing of the powder pattern was performed by the successive dichotomy method from data collected with a high-resolution diffractometer using the Bragg-Brentano geometry. The lattice parameters are a = 6.747 (2), b = 13.418 (3), c = 8.090 (2) A, β = 99.01 (3)°, and the space group is P2 1 /n. The structure model was obtained from minimization of the crystal-lattice potential energy calculated with semi-empirical atom-atom potentials using the PMC [Packing of Molecules in Crystals; Dzyabchenko, Belsky & Zorkii (1979). Kristallografiya, 24, 221-226] program. The Rietveld refinement converged to the final crystal-structure model indicator R B = 0.04 and profile factors Rp = 0.03 and R wp = 0.04. The molecular packing is characterized by intermolecular hydrogen bonds (N-H...O). The structure consists of two types of perpendicular infinite chains of piracetam molecules, which build parallel layers containing rings. No cyclic dimers were found. For a description of the hydrogen-bond patterns, the method based on graph theory was employed