Molecular Conformations of the Polymorphic Forms of Cimetidine from¹³C Solid-State NMR Distance and Angle Measurements

The polymorphic drug compound cimetidine, a histamine H-2 receptor antagonist, was synthesized containing sites of C-13 enrichment at the imidazolium methine carbon C2 and at the guanidinium methyl carbon C16. The structures of four crystalline forms of double C-13-labeled cimetidine, three anhydrates (A, B, and C) and a monohydrate (M1), were examined using C-13 cross polarization magic angle spinning (CPMAS) NMR methods. Rotational resonance magnetization exchange curves obtained for forms A and M1 were consistent with C2-C16 interatomic distances of 3.78 and 3.82 Angstrom as measured from their crystal structures. Exchange curves for forms B and C, for which crystal structures have not been obtained, indicated that in both cases the C2-C16 interatomic distance lies between 5.2 and 5.8 Angstrom, suggesting that cimetidine adopts a partially extended conformation in these forms. In addition, double quantum heteronuclear local field (2Q-HLF) NMR was used to determine the relative orientations of the C-13-H bonds at the two C-13-labeled sites. The experimental data were consistent with the known geometry of forms A and M1 and, in the case of form C, with a limited number of possible structures. Energetically favorable molecular conformations of form C, which were in agreement with the distance and angle measurements, fell into just six distinct clusters. These results demonstrate the feasibility of determining the complete solid-state structures of pharmaceutical compounds, and other materials not amenable to crystallography, using CP-MAS NMR combined with a minimal isotope labeling strategy.