Theoretical Study of the Structure and Stability of Layerwise Hydrogenated Aluminum Clusters Al₄₄H_n and Al₈₉H_m

Abstract

Structural parameters, energies, and spectroscopic characteristics of two series of layerwise hydrogenated aluminum clusters Al₄₄H_n (n = 27–44) and Al₈₉H_m (m = 15, 24, 39, and 63) have been calculated by the density functional theory method. It has been shown that increasing number of H atoms in both series entails rapid enhancement of structural distortions up to cooperative rearrangements accompanied by a change in the shape and composition of the surface layer and internal core of the cluster. At the end of the first series Al₄₄H_n, several surface atoms migrate to the outer sphere of the cage to form valence-unsaturated “outer-surface” AlH_n and Al₂H_n moieties, which can be active sites at the stages of deeper hydrogenation. Simultaneously, the inner core [Al]₅ disintegrates, and its atoms are introduced into the surface layer. A family of “inverted” Al₄₂H₄₂ isomers with the hollow [Al₄₂] cage has been localized; the isomers contain the endohedral AlH₄ group and “inner” Al-H bonds with their hydrogen end directed to the center of the inner cavity. At the end of the second series, five alanate groups AlH₄ and two Al₃H₂ fragments bonded to the surface through hydrogen bridges are formed in the outer sphere of the $\rm{Al}_{89}H_{63}^-$ cluster. The results are of interest for DFT modeling of hydrogenation of nanosized aluminum clusters at the molecular level.