Theoretical Study of the Structure and Stability of Stepwise Hydrogenated Aluminum Clusters Al₄₄H _n (n = 1−24)

Abstract

The energies and structural and spectroscopic characteristics of a series of model stepwise hydrogenated aluminum clusters Al₄₄H _n (n = 1−24) obtained by successive introduction of hydrogen atoms into various surface positions of the Al₄₄ cluster have been calculated by the density functional theory method (B3LYP). According to these calculations, the [Al₃₉] surface layer of the cage retains a closed “nested doll” shape with a pentaatomic inner core [Al₅]. With increasing n, both the surface layer and the core tend to experience increasing asymmetric distortions. The surface is corrugated and undergoes significant axial and equatorial extensions and contractions, some of the Al−H two-center terminal bonds are transformed into threecenter hydrogen bridges, and some Al atoms are displaced from the surface layer to the outer sphere and are bound to the surface through hydrogen bridges. The inner core [Al₅] at n = 24 loses its bipyramidal shape and shifts to the surface layer so that one or two of its atoms are “built-in” into the concave regions of the surface layer. The calculated average energies of Al−H bonds are within the range ~55.5 ± 2.5 kcal/mol. The averaged energies of the Al₄₄H _n → Al₄₄H_n–2 + Н₂ dissociation reactions with elimination of a hydrogen molecule are on the order of a few kilocalories per mole and are evidence of small exothermicity (or isothermicity) of these reactions. For the Al₄₄H, Al₄₄H₂, and Al₄₄H₆ clusters as an example, the relative stabilities of isomers with terminal Al−H bonds in various nonequivalent positions of the [Al₃₉] surface layer are compared.