Molecular dynamics of amide ions in potassium amide (KNH2) studied with orientation-dependent deuterium spin lattice...

The reorientational molecular dynamics of the amide ions were investigated in three different phases of KND2 by means of 2H NMR line-shape analyses of solid-echo, T1Z as well as T1Q distorted spectra in a temperature range of 80-420 K. The correlation times of the amide dynamics cover roughly eight decades in this temperature range. Due to the nonzero asymmetry parameter (eta approximately 0.2) of the electric field gradient tensor the calculation of the orientation-dependent spectral densities Jm(theta, phi) required for the interpretation of the T1Z and T1Q distorted spectra cannot be simplified as in the case eta = 0 and a numerical approach was used for the calculation of Jm(theta, phi), which allows a maximum flexibility for simulating different models of motion. The amide ion dynamics in the low-temperature phase can be described as a superposition of a thermally activated large angle jump of the amide ions about their two-fold axes in an asymmetric four-well potential and strongly anisotropic molecular librations. The asymmetry of the potential surface of the jump process was found to be a function of temperature. Activation energy EA, attempt frequency tau0(-1) and DND bond angle epsilon were determined to 15.5(2) kJ/mol, 62(6) x 10(12) s(-1) and 104.7(3) degrees. In the middle- and high-temperature phases the amide ions perform 90 degrees jumps about the crystallographic four-fold axes. For the high-temperature modification the correlation times were observed to follow an Arrhenius law with EA = 6.3(2) kJ/mol and tau0(-1) = 32(3) x 10(12) s(-1).