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, P2(1)/n, Z = 4, a = 10.7029 (3), b = 18.8262 (1), c = 6.8266 (2) angstrom, beta = 111.306 (2)-degrees. 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.