Theoretical design of stable hydride clusters: isoelectronic transformation in the EnAl4−nH7+n− series

Abstract

New stable hydrogen-rich metallic hydrides are designed by systematic transformations of the stable known Al₄H₇⁻ species, carried out by successive isoelectronic substitutions of one aluminum atom by one E–H unit at a time (where E = Be, Mg, Ca, Sr and Ba atoms). Searches on the potential energy surfaces (PESs) of EAl₃H₈⁻, E₂Al₂H₉⁻, E₃AlH₁₀⁻ and E₄H₁₁⁻ systems indicate that structural analogues of Al₄H₇⁻ become higher energy isomers as the number of E–H units increases. The electronic descriptors: Vertical Electron Affinity (VEA), Vertical Ionization Potential (VIP) and the HOMO–LUMO gap, suggest that the systems composed of EAl₃H₈⁻, E₂Al₂H₉⁻, E₃AlH₁₀⁻, with E = Be and Mg, would be the most stable clusters. Additionally, for a practical application, we found that the Be–H and Mg–H substitutions increase the hydrogen weight percentage (wt%) in the clusters, compared with the isoelectronic analogue Al₄H₇⁻. The good capacity of beryllium and magnesium to stabilize the extra hydrogen atoms is supported by the increment of the bridge-like E–H–Al, 3center–2electron chemical bonds. Finally, explorations on the PESs of the neutral species (using Na⁺ as counterion) indicate that the NaBe₂Al₂H₉, NaBe₃AlH₁₀ and NaMg₃AlH₁₀ minimum-energy structures retain the original geometric shapes of the anionic systems. This analysis supports the potential use of these species as building blocks for cluster-assembled hydrides in the gas phase.