For two complex aluminium hydrides, the quadrupole coupling parameters of the ²⁷Al nuclei are determined from static NMR spectra and compared with those obtained from MAS NMR spectra and from DFT-GIPAW calculations based on structural data from diffraction methods.

Abstract

The quadrupole coupling constant C _Q and the asymmetry parameter η have been determined for two complex aluminium hydrides from ²⁷Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The thus obtained data for KAlH₄ (C _Q=(1.30±0.02) MHz, η=(0.64±0.02)) and NaAlH₄ (C _Q=(3.11±0.02) MHz, η<0.01) agree very well with data previously determined from MAS NMR spectra. The accuracy with which these parameters can be determined from static spectra turned out to be at least as good as via the MAS approach. The experimentally determined parameters (δ _iso, C _Q and η) are compared with those obtained from DFT-GIPAW (density functional theory – gauge-including projected augmented wave) calculations. Except for the quadrupole coupling constant for KAlH₄, which is overestimated in the GIPAW calculations by about 30 %, the agreement is excellent. Advantages of the application of the Solomon echo sequence for the measurement of less stable materials or for in situ studies are discussed.