화학공학소재연구정보센터
Journal of Physical Chemistry B, Vol.103, No.21, 4497-4507, 1999
Reorientational dynamics of amide ions in isotypic phases of strontium and calcium amide II. Solid state NMR
H-2 and H-1 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 degrees reorientational jumps about the C-2 axis of the ND2- ions were detected. The jump rates tau(-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 tau(0)(-1) equal to 6(2) x 10(12) Hz (Sr(ND2)(2)), 2.3(7) x 10(13) Hz (Ca(ND2)(2)), respectively. The DND bond angle was determined to be 106.4(5)degrees and 105.7(6)degrees 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 (eta approximate to 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.(12).