Spin diffusion and ¹H spin-lattice relaxation in Cs₂(HSO₄)(H₂PO₄) containing a small amount of ammonium ions

Inorganic solid acid salts with hydrogen bond networks frequently show very long spin-lattice relaxation times even for ¹H because the hydrogen bonds suppress motions. In the present work, the ¹H spin-lattice relaxation in Cs₂(HSO₄)(H₂PO₄) containing a small amount of ammonium ions were studied in detail by use of the effect of magic angle spinning (MAS) on the relaxation. The ¹H spin-lattice relaxation times of the acid protons decrease with increase in the content of ammonium ions. Reorientation of the NH₄ group fluctuates the dipole-dipole interaction and relaxes the ammonium protons as well as the acid protons. The ¹H relaxation times of the acid protons are a little bit longer than those of the ammonium protons at the MAS rate of 8 kHz. The spinning at 50 kHz makes the relaxation times of the acid protons longer and those of the ammonium protons shorter. Spin diffusion between the acid and the ammonium protons averages partially the ¹H relaxation of the acid and the ammonium protons at the MAS rate of 8 kHz. The spin diffusion is suppressed completely at the MAS rate of 50 kHz. Spin diffusion between the acid protons is not suppressed at the MAS rate of 50 kHz. The acid protons always show the same relaxation times. The intrinsic relaxation times not affected by spin diffusion are evaluated quantitatively for both the acid and the ammonium protons. Those values are independent of the ammonium content. Contribution of the spin diffusion between the acid and the ammonium protons to the relaxation is estimated quantitatively. Using those parameters, the effect of ammonium ions on the ¹H spin-lattice relaxation can be predicted. The ¹H spin-lattice relaxation is a sensitive tool to study the distribution of ammonium ions in solids.