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 Å. The thickness of each layer L is equal to a′ = a/6 = 13.82 Å, 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(α0γ)00, where X = 00; (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 anM = 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.