Cationic Motions Involving Self-Diffusion and Structural Phase Transitions in Solid Dimethyl- and Trimethylammonium Nitrates as Studied by Differential Thermal Analysis and 1H NMR Techniques

Differential thermal analysis and the temperature dependence of broad-line 1H NMR absorptions and of 1H spin-lattice relaxation times in both laboratory and rotating frames were observed for (CH3)2NH2NO3, (CH3)ND2NO3, and (CH3)3NHNO3. A solid-solid phase transition was located at 297 K for (CH3)2NH2NO3. while two phase transitions at 359 and 407 K were found for (CH3)3NHNO3. The 1H spin-lattice relaxation times in the laboratory frame at different Larmor frequencies and those in the rotating frame at the rf field of 8 G observed for the above salts were analyzed by referring to the 1H NMR second moments determined. The motional parameters of the cations in each solid phase of the salts were evaluated by a fitting calculation to the relaxation times observed. In the highest-temperature solid phase of (CH3)3NHNO3, the cation was found to perform self-diffusion as well as overall rotation with the relatively low activation energy of 29 kJ mol−1. The motions of the cations in each newly found solid phase are characterized.