$^{27}$Al NMR spectroscopic and DFT computational study of the quadrupole coupling of aluminium in two polymorphs of the complex aluminium hydride CsAlH4

TL;DR

This study combines NMR spectroscopy and DFT calculations to analyze the quadrupole coupling of aluminium in two polymorphs of CsAlH4, revealing high sensitivity to structural details and the impact of thermal motion.

Contribution

It provides a detailed comparison of experimental NMR parameters with DFT calculations, highlighting the importance of structural accuracy and thermal effects in predicting quadrupole couplings.

Findings

Experimental parameters are more accurately determined from static spectra.

DFT calculations overestimate quadrupole coupling constants by about 45%.

Small geometric variations significantly improve agreement between theory and experiment.

Abstract

The quadrupole coupling constant $C_{\text{Q}}$ and the asymmetry parameter $\eta$ 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, $\eta=(0.62\pm0.01)$ and CsAlH4(t): $C_{\text{Q}}=(1.43\pm0.02)$ MHz, $\eta<0.03$) seems to be slightly higher than via the MAS approach. The experimentally determined parameters ($\delta_{\text{iso}}$, $C_{\text{Q}}$ and $\eta$) 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.