Reorientational Dynamics of Amide Ions in Isotypic Phases of Strontium and Calcium Amide II. Solid State NMR

2H and 1H wide-line NMR spectroscopy were used to study molecular dynamics of amide ions in a large temperature range for strontium and calcium amides. Line shape analyses resulted in two reorientational processes on very different time scales. Above 340 K (Sr(ND2)2) and 390 K (Ca(ND2)2), respectively, slow 180° reorientational jumps about the C 2 axis of the ND2 - ions were detected. The jump rates τ-1 follow an Arrhenius law with activation energies of 56.8(6) kJ/mol (Sr(ND2)2), 69.2(8) kJ/mol (Ca(ND2)2), and τ0 -1 equal to 6(2) × 1012 Hz (Sr(ND2)2), 2.3(7) × 1013 Hz (Ca(ND2)2), respectively. The DND bond angle was determined to be 106.4(5)° and 105.7(6)° for strontium and calcium amides. Furthermore, fast rotational dynamics was observed which could be identified as strongly anisotropic, librational motion of the amide ions. This can be clearly seen from the large asymmetry parameter of the electric field gradient tensor even at low temperatures (η ≈ 0.2). A simple model allows to obtain amplitudes of libration. Magnitude and temperature dependence fit well to results from recent high-resolution neutron powder diffraction investigations.