Theoretical study of polyoxide clusters Sc20O30, P20O50, Ti20O30F20, and V20O30F20

The structural, electronic, and vibrational characteristics and energies of the isolated polyoxide clusters Sc₂₀O₃₀, P₂₀O₅₀, Ti₂₀O₃₀F₂₀, and V₂₀O₃₀F₂₀ and ammonia complexes Sc₂₀O₃₀ · nNH₃ were calculated by the density functional theory B3LYP method with several basis sets. The computation results show that a fullerene-like closo structure I _h with oxygen bridges located above the midpoints of the edges of an empty M₂₀] dodecahedron is preferable for the Ti₂₀O₃₀F₂₀ and V₂₀O₃₀F₂₀ clusters with four-coordinate metal atoms protected by the outer M-F bonds. This structure with a cage diameter of ∼1 nm and the diameter of nearly planar decagonal faces (windows) of ∼0.5 nm is stable to dissociation into fragments and to strong geometric distortions and retains its closo shape when molecules like NH₃ and anions like H⁻ are attached to the cage. An analogous closo structure is favorable for the P₂₀O₅₀ cluster; however, in this structure, the P₂₀] cage is severely distorted and all 12 windows are strongly corrugated. For Sc₂₀O₃₀, the I _h dodecahedron with bare three-coordinate Sc atoms corresponds to a local minimum of the potential energy surface, which is 170–200 kcal/mol less favorable than compact puck-shaped isomers in which four- and five-coordinate metal atoms and three- and four-coordinate oxygen atom prevail. “Solvation” of the dodecahedral and puck-shaped Sc₂₀O₃₀ isomers by ammonia molecules strongly decreases the energy gap between the isomers; however, the dodecahedron I _h in all cases remains a high-lying intermediate. According to calculations, most polyoxides under consideration have a high electron affinity (comparable with or higher than that of fullerenes) and is able to add three to five or more alkali-metal atoms to form radical salts in which clusters are in the state of polyanions. Because of large sizes of the M₂₀] cages and their windows, the interior of the cage (as distinct from fullerenes) can accommodate a considerable number of atoms and several small molecules. The V₂₀O₃₀F₂₀ cluster has 20 unpaired electrons and can be treated as a molecular magnet. The properties of the M₂₀] cages depend only slightly on the outer substituents. It is suggested that the pattern will be retained upon the substitution of OH groups for the F atoms and that the hydroxo-substituted clusters can bind to each other through hydrogen bridges and serve as building blocks for self-assembly into ordered nanometer and crystalline structures of various dimensions. Original Russian Text ? O.P. Charkin, N.M. Klimenko, D.O. Charkin, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 5, pp. 775–785.