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

The sequential bonding of N-ethyl-2-pyrrolidone (EP), a monomer unit of poly(vinylpyrrolidone) (PVP), to an open-Shell superatom Al-13 was studied by density functional theory calculations. The first three EP ligands prefer to be chemisorbed on the atop sites of Al-13 via the carbonyl O atom, mainly due to bonding interaction between molecular orbitals of EP and the IS or ID superatomic orbital of Al-13. The fourth EP ligand, however, prefers to be bound electrostatically to one of the chemisorbed EP ligands rather than to be chemisorbed on Al-13. 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 Al-13 moiety increases with the number of EP ligands chemisorbed and reaches nearly -1 e in Al-13(EP)(3), suggesting the closure of the electronic shell of Al-13 by ligation of three EP ligands. However, the spin density analysis revealed that the superatomic orbital IF of Al-13 remains singly occupied even after chemisorption of three EP ligands. In conclusion, the Al-13 moiety stabilized by PVP remains to be an open-shell superatom although it accepts electronic charge through polarized Al-O bonding.