# Journal of Applied Crystallography

### Volume 46, Part 1 (February 2013)

## research papers

### Cimetidine, C_{10}H_{16}N_{6}S, form C: crystal structure and modelling of polytypes using the superspace approach

**Abstract:** 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 *C*2/*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* = 6*a*' originates from a sixfold *LLLL*'*L*'*L*' sequence with *L* and *L*' differing by 0.5*b*. 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 *X*2/*c*(0)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 *n*M with lattice parameter *a*_{n}_{M} = *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 *n*M polytypes suggests that the powder samples published previously probably contain a mixture of various polytypes.

**Keywords: cimetidine; crystal structure; polymorphism; polytypism; superspace modelling; synchrotron radiation.**

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