27Al NMR spectroscopic and DFT computational study of the quadrupole coupling of aluminum in two polymorphs of the complex aluminum hydride CsAlH4

Abstract

The quadrupole coupling constant CQ and the asymmetry parameter η of the aluminum nuclei in two polymorphs of the complex aluminum hydride CsAlH4 are determined from both 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra and 27Al 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): CQ = (1.42 ± 0.01) MHz, η = (0.62 ± 0.01) and CsAlH4(t): CQ = (1.43 ± 0.02) MHz, η < 0.03] seems to be slightly higher than via the MAS approach. The experimentally determined parameters (δiso, CQ, 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 is varied show a high sensitivity of CQ to the H–Al–H angles. Modest changes in the angles on the order of one to three degrees are sufficient to achieve a near-perfect agreement between GIPAW calculations and experiments. The deviations found for the DFT-optimized structures are explained with the neglect of thermal motion, which typically leads to a reduction in the distortion 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 aluminum hydrides challenging.