Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH 4 ) 2 .

The thermodynamic properties of key compounds Mg(B 3 H 8 ) 2 , MgB 2 H 6 , MgB 10 H 10 , Mg(B 11 H 14 ) 2 , Mg 3 (B 3 H 6 ) 2 , and MgB 12 H 12 , proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH 4 ) 2 and the uptake of hydrogen by MgB 2 , have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H 2 ) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H 2 ). Although our treatment lowers the calculated formation energy of Mg(B 3 H 8 ) 2 , it is still too high to explain the experimental observation of B 3 H 8 - . We discuss possible reasons for this disparity and propose that the formation of B 3 H 8 - and H - in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B 3 H 8 ) 2 and MgH 2 considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH 4 ) 2 , Mg(B 3 H 8 ) 2 , Mg(B 11 H 14 ) 2 , MgB 10 H 10 , and MgB 12 H 12 provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB 2 H 6 and the proposed trianion species Mg 3 (B 3 H 6 ) 2 that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH 4 ) 2 without the formation of MgH 2 as a coproduct which could phase separate and inhibit rehydrogenation.