H 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 X 10- 13 s. No proton exchange is observed between NH4+ ions and the acidic protons in both phases.