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Differential scanning calorimetry, the temperature dependence of proton and deuteron lineshapes and spin-lattice relaxation times are reported for the isotopic species CH 3 NH 3 ClO 4 , CD 3 NH 3 ClO 4  and CH 3 ND 3 ClO 4  of monomethylammonium Perchlorate. The data confirmed the existence of three different phase modifications in monomethylammonium Perchlorate and its selectively deuterated analogues. In addition they were used to identify the molecular motions occurring in the respective phases and to determine their activation parameters. In the low temperature phase III, stable below 320 K. the CH 3  and NH 3  groups reorient about their threefold symmetry axes C 3  with different frequencies. In the low-temperature range of this phase the deuteron quadrupole coupling constant indicates N - H ... O hydrogen bonds between the monomethylammonium and the Perchlorate ions. In the intermediate phase II, between 320 K and 451 K for CH 3 NH 3 ClO 4  and CD 3 NH 3 ClO 4  and 320 K and 437 K for CH 3 ND 3 ClO 4  , the methylammonium ions reorient about an axis inclined at an angle of 18 degrees to the C 3  axis. The analysis of the entropy changes, associated with the III- II transitions indicates that the ClO 4  −  ions have a large motional freedom in phase II, presumably because of breaking or weakening of N - H . . . O hydrogen bonds. In phase I the monomethylammonium ions undergo isotropic motion along with translational diffusion between different sites of the primitive cubic unit cell.

Interrelation between Molecular Motions and Phase Transitions in Monomethylammonium Perchlorate. A Study by DSC, Proton, and Deuteron NMR