Comparison of High Pressure-Induced Phases of Mg(AlH4)2 as Hydrogen Storage Using Ab Initio Calculated NQCC Parameters

ABSTRACT

Nuclear quadrupole resonance (NQR) spectroscopy is proven to be a very sensitive technique for measuring distribution of electric charge around quadrupolar nuclei. Quadrupolar parameters of nuclei can be used as a tool to understand the electronic structure of compounds. The electronic structure of magnesium alanate, Mg(AlH4)2, as promising hydrogen storage materials for hydrogen fuel cell-powered automobile applications, has been studied in detail by ab initio calculated NQR parameters. Furthermore, using calculated nuclear quadrupole coupling constants (NQCCs) of hydrogen atoms (2H-NQCC), the electronic structure of α-Mg(AlH4)2 with its high-pressure forms, β- and γ- Mg(AlH4)2, was compared. The electric field gradient (EFG) at the site of 2H atoms was calculated to obtain NQCC parameters. The results show that in the γ-Mg(AlH4)2, 2H-NQCCs are smaller than that of other considered phases. In other words, Al–H bonds in γ-Mg(AlH4)2 nanocrystal is weaker than others and the charge transfer from Al to hydrogen atom is less than the others and therefore these hydrogens have weaker bonds with Al and easier condition for dehydrogenation is expected in γ-Mg(AlH4)2. Comparison of calculated dehydrogenation enthalpies of various Mg(AlH4)2 phases verifies this prediction. All calculations performed using Gaussian 03 at the HF/3-21G level of theory. The selected level and basis set give the rather acceptable qualitative NQCCs of hydrogen atoms.

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