Molecular motional and structural study of ammoniated titanium disulfide by proton NMR

Proton spin-lattice relaxation times and linewidths have been measured as a function of temperature at 25 MHz for (NH₄⁺)_0.21Ti₂^0.21- and at 25 and 50 MHZ for (NH₄⁺)_0.22(NH₃)_0.37TiS₂^0.22-. In the former compound, the NH₄⁺ cations reorient randomly and rapidly on trigonal prismatic sites in the van der Waals gap of TiS₂, and there is no evidence of other thermally activated motion between 3 K and the deintercalation temperature. In the latter compound, two distinct types of motions can be discerned from their characteristic NMR signals. At lower temperatures the NH₃ molecules rotate about their three-fold axes and above 270 K translational diffusion occurs. An average proton-proton distance of 1.79±0.03 A ̊ in NH₃ has been evaluated from the temperature dependence of the relaxation time associated with the C₃ axis rotation. This large distance relative to that in normal pyramidal NH₃ (1.65 Å) is consistent with a planar NH₃ geometry and perhaps some lengthening of the normal NH bond length (1.008 Å). The activation energy for NH₃ rotation (2.41 ± 0.02 kcal/mol NH₃) is nearly equal to that measured in solid NH₃ (2.3 kcal/mol NH₃). A high-temperature minimum in the relaxation time has been attributed to NH₃ diffusion, with an activation energy (13.3±3kcal/mol NH₃) that is in good agreement with the NH₃ deintercalation enthalpy.