1H NMR study of proton dynamics in [(NH4)1 − xRbx]3H(SO4)2 (x = 0.54)

Proton dynamics in [(NH4)1 − xRbx]3H(SO4)2 with x = 0.54 has been studied by means of 1H solid-state NMR. The 1H magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT), and 1H static NMR spectra and spin-lattice relaxation times (T1) were measured in the range of 296–490 K. The 1H chemical shift for the acidic proton (15.4 ppm) indicates hydrogen bonds of intermediate strength between (NH4)3H(SO4)2 and Rb3H(SO4)2. In phase II, a very fast local and anisotropic motion of the acidic protons takes place and NH4+ ions start to diffuse translationally just below the transition temperature. In phase I, both NH4+ ions and the acidic protons diffuse translationally. The acidic protons diffuse with an activation energy of 29 kJ mol− 1 and the inverse of a frequency factor of 6.0 × 10− 13 s. No proton exchange is observed between NH4+ ions and the acidic protons in both phases.