¹H NMR Studies on the Dynamics of Planar [C(NH₂)₃]⁺ Cations in the Crystal of Various Guanidinium Salts

The temperature dependences of 1 H NMR spin‐lattice relaxation times and 1 H NMR second moments were observed for guanidinium chloride, bromide, iodide, thiocyanate, nitrate, and perchlorate. The results of the NMR experiments carried out with the crystals of these compounds except the last one can be interpreted in terms of the C 3 reorientation of the cation about its symmetry axis. The solid‐solid phase transition at 454 K reported for guanidinium perchlorate could be resolved into two phase transitions taking place successively at 450 and 452 K. In the low temperature phase of this salt, the C 3 reorientation of the cation is activated. The occurrence of cationic self‐diffusion in the high temperature phase was confirmed by the measurements of 1 H NMR relaxation times. The NMR motional parameters are evaluated for these cationic motions in the present salts. The phase transition temperature of guanidinium iodide was strongly depended on the grain size of the sample employed. Guanidinium bromide yielded a single 81 Br NQR line at room temperature indicating that all bromide ions in the crystal are crystallographically equivalent in agreement with the results of X‐ray analysis.