Molecular dynamics of amide ions in potassium amide (cas 17242-52-3) (KNH2) studied with orientation-dependent deuterium spin lattice relaxation

Add time:08/29/2019    Source:sciencedirect.com

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 80–420K. The correlation times of the amide dynamics cover roughly eight decades in this temperature range. Due to the nonzero asymmetry parameter (η≈0.2) of the electric field gradient tensor the calculation of the orientation-dependent spectral densities Jm(ϑ,φ) required for the interpretation of the T1Z and T1Q distorted spectra cannot be simplified as in the case η=0 and a numerical approach was used for the calculation of Jm(ϑ,φ), 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 τ0−1 and DND bond angle ε were determined to 15.5(2)kJ/mol,62(6)×1012s−1 and 104.7(3)°. In the middle- and high-temperature phases the amide ions perform 90° 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 τ0−1=32(3)×1012s−1.

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