Theoretical Study of Polyoxide Clusters Sc_(20)O_(30), P_(20)O_(50), Ti_(20)O_(30)F_(20), and V_(20)O_(30)F_(20)

摘要

The structural, electronic, and vibrational characteristics and energies of the isolated polyoxide clusters Sc_(20)O_(30), P_(20)O_(50), Ti_(20)O_(30)F_(20), and V_(20)O_(30)F_(20) and ammonia complexes Sc_(20)O_(30) centre dot nNH_3 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 [M_(20)] dodecahedron is preferable for the Ti_(20)O_(30)F_(20) and V_(20)O_(30)F_(20) 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 approx 0.5 nm is stable to dissociation into fragments and to strong geometric distortions and retains its closo shape when molecules like NH_3 and anions like H~ are attached to the cage. An analogous closo structure is favorable for the P_(20)O_(50) cluster; however, in this structure, the [P_(20)] cage is severely distorted and all 12 windows are strongly corrugated. For Sc_(20)O_(30), 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 Sc_(20)O_(30) 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 [M_(20)] 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 V_(20)O_(30)F_(20) cluster has 20 unpaired electrons and can be treated as a molecular magnet. The properties of the [M_(20)] 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.