The structure of cimetidine (C₁₀H₁₆N₆S) solved from synchrotron-radiation X-ray powder diffraction data

The crystal structure of cimetidine (C₁₀H₁₆N₆S) has been solved and refined from high-resolution synchrotron X-ray powder data collected on station 8.3 at Daresbury Laboratory. The structure is monoclinic, P2₁/n, Z = 4, a = 10.7029 (3), b = 18.8262 (1), c = 6.8266 (2) Å, β = 111.306 (2)°. The data were auto-indexed and the integrated intensities were obtained by pattern decomposition. The structure was solved by direct methods followed by iterative cycles of least-squares refinement and Fourier syntheses. All 17 non-H atoms were located. The structure was then refined using the method of Rietveld [J. Appl. Cryst. (1969), 2, 65–71]. A difference Fourier synthesis after the convergence of the non-H-atom model showed positive electron density at the expected positions of all 16 H atoms, ten of which were significantly above the background. The H atoms were allowed to refine unconstrained and the final agreement factors were R_I = 1.9, R_wp = 8.5 and R_ex = 6.9%; the equivalent R-factors without the H atoms were R_I = 10.3 and R_wp = 16.2%, demonstrating the significant contribution of the H atoms to the profile.