Density Functional Theory Study on Stabilization of the Al₁₃ Superatom by Poly(vinylpyrrolidone)

Abstract

The sequential bonding of <i>N</i>-ethyl-2-pyrrolidone (EP), a monomer unit of poly­(vinylpyrrolidone) (PVP), to an open-shell superatom Al₁₃ was studied by density functional theory calculations. The first three EP ligands prefer to be chemisorbed on the atop sites of Al₁₃ via the carbonyl O atom mainly due to bonding interaction between molecular orbitals of EP and the 1S or 1D superatomic orbital of Al₁₃. The fourth EP ligand, however, prefers to be bound electrostatically to one of the chemisorbed EP ligands rather than to be chemisorbed on Al₁₃. This behavior suggests that the maximum number of PVP that can be chemisorbed on an Al cluster is determined not only by the steric repulsion between adjacent PVP but also by the electronic charge accumulated on the Al cluster. The gross Mulliken charge accumulated on the Al₁₃ moiety increases with the number of EP ligands chemisorbed and reaches nearly −1 e in Al₁₃(EP)₃, suggesting the closure of the electronic shell of Al₁₃ by ligation of three EP ligands. However, the spin density analysis revealed that the superatomic orbital 1F of Al₁₃ remains singly occupied even after chemisorption of three EP ligands. In conclusion, the Al₁₃ moiety stabilized by PVP remains to be an open-shell superatom although it accepts electronic charge through polarized Al–O bonding