Multiple valence superatoms

Abstract

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al(13) cluster behaves like a halogen atom, whereas an Al(14) cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al(13)(-) is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called superatoms. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this article, we provide evidence of an additional class of superatoms, namely Al(7)(-), that exhibit multiple valences, like some of the elements in the periodic table, and hence have the potential to form stable compounds when combined with other atoms. These findings support the contention that there should be no limitation in finding clusters, which mimic virtually all members of the periodic table.

Fig. 1.

Benzene is introduced in the source aluminum plasma to produce Al_n⁻ (●) and Al_nC⁻ (). The clusters are subsequently reacted with molecular oxygen at thermal energy.

Fig. 2.

Structure and energetics of aluminum compound clusters. (a) Al₇C⁻-optimized geometry. (b) Energy gained by adding an Al atom to Al_n-1C⁻ species and HOMO–LUMO gap for the Al_nC⁻ clusters. (c) Electron charge density of the HOMO in Al₇C⁻ clusters. (d) Al₇O⁻-optimized geometry. (e) Energy gained by adding an Al atom to Al_n-1O⁻ species and HOMO–LUMO gap for the Al_nO⁻ clusters. (f) Electron charge density of the HOMO in Al₇O⁻.

Fig. 3.

One-electron levels in Al₇⁻, C, Al₇C⁻, O, and Al₇O⁻. The continuous thin lines represent single occupied levels, the continuous thicker lines represent double occupied levels, the dotted thin lines correspond to single unfilled states, the dotted thicker lines correspond to double unfilled states, and the arrows indicate the majority (up) and minority (down) spin states.

Fig. 4.

Energetics of the Al₇M clusters. (a) BE of Al₇M⁻ clusters, M is an atom of the second, third, and fourth row of the periodic table. (b) HOMO–LUMO gap for the Al₇M⁻ clusters. (c) BE and HOMO–LUMO gap of the Al₇I_m⁻ clusters.

Fig. 5.

Mass spectra of Al_n⁻ clusters (A) reacted with I₂⁻ (B).