1H NMR studies on the motion of cations in the three solid phases of methylammonium perchlorate including cationic self-diffusion in its highest-temperature solid phase.

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The temperature dependences of the second moment of 1H NMR absorptions, the 1H spin-lattice relaxation time, T1, and the 1H spin–spin relaxation time, T2, of CH3NH3ClO4 (CH3ND3ClO4) were observed in the three solid phases existing over a wide range of temperature. In the phase I stable above 451 K, the onset of the isotropic reorientation as well as the self-diffusion of the cations with the activation energy of 36 kJ mol−1 was observed. In the phase II stable between 451 and 321 K, no remarkable change in the motional state of the cation was observed, but the dynamical orientational disorder of perchlorate anions was expected to occur. In the phase III stable below 321 K, the correlated reorientation defined as the C3 reorientation of the cation having the rigid structure, was observed at lower temperatures for two nonequivalent cations in the crystal with small activation energies of 4.4 and 2.0 kJ mol−1. The uncorrelated reorientation in which both CH3 and NH3+ groups perform C3 reorientation independently could be observed at a higher temperature with the activation energy of 6.4 kJ mol−1. Nonexponential behavior appearing in the observation of T1 below 40 K could be explained by the presence of a contribution to 1H T1 from the rotational tunneling of the cation.
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