Cimetidine, C10H16N6S, form C: crystal structure and modelling of polytypes using the superspace approach

The H2 antagonist cimetidine forms many polymorphs, several of which have resisted structural analysis thus far. Using single-crystal X-ray measurements obtained from synchrotron radiation, the crystal structure of cimetidine form C has been solved. This layered structure crystallizes in space group C2/c with an unusually large lattice parameter, a = 82.904 angstrom. The thickness of each layer L is equal to a' = a/6 = 13.82 angstrom, and a = 6a' originates from a sixfold LLLL'L'L' sequence with L and L' differing by 0.5b. This packing is reminiscent of polytypic stacking in metals. A (3 + 1)-dimensional superspace model is derived and used to explain and predict many polytypic modifications. This model is characterized by (i) the (3 + 1)-dimensional symmetry group X2/c(alpha 0 gamma)00, where X = 0 1/2 0 1/2; (ii) the lattice parameter a' and modulation vector q = 1/n(a'*); (iii) the atomic positions of a single molecule of cimetidine form C; (iv) the primary variable, 1/n. The model reproduces the previously solved structure, the 6M polytype, and generates the related polytypes nM with lattice parameter a(nM) = na' for n = 1, 2, 3, 4 and 5. A comparison of powder X-ray diffraction patterns available for cimetidine form C with those simulated for the nM polytypes suggests that the powder samples published previously probably contain a mixture of various polytypes.