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| Main Authors: | , |
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| Format: | Preprint |
| Published: |
2024
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| Subjects: | |
| Online Access: | https://arxiv.org/abs/2410.07731 |
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Table of Contents:
- The quadrupole coupling constant $C_{\text{Q}}$ and the asymmetry parameter $η$ of the aluminium nuclei in two polymorphs of the complex aluminium hydride CsAlH4 are determined from both $^{27}$Al MAS NMR spectra and $^{27}$Al NMR spectra recorded for stationary samples by using the Solomon echo sequence. The accuracy with which these parameters can be determined from the static spectra (CsAlH4(o): $C_{\text{Q}}=(1.42\pm0.01)$ MHz, $η=(0.62\pm0.01)$ and CsAlH4(t): $C_{\text{Q}}=(1.43\pm0.02)$ MHz, $η<0.03$) seems to be slightly higher than via the MAS approach. The experimentally determined parameters ($δ_{\text{iso}}$, $C_{\text{Q}}$ and $η$) are compared with those obtained from DFT-GIPAW (density functional theory - gauge-including projected augmented wave) calculations. When using DFT-optimized structures, the magnitude of the quadrupole coupling constant is overestimated by about 45% for both polymorphs. Further calculations in which the geometry of the AlH4 tetrahedra was varied show a high sensitivity of $C_{\text{Q}}$ on the H--Al--H angles in particular. Modest changes in the angles on the order of one to three degrees are sufficient to achieve near-perfect agreement between GIPAW calculations and experiment. The deviations found for the DFT-optimized structures are explained with the neglect of thermal motion, which typically leads to a reduction of distortions of the AlH4 tetrahedra. From a broader perspective, the uncertainty in the positions of the hydrogen atoms renders the accurate reproduction or prediction of quadrupole coupling constants for aluminium hydrides challenging.