Theovetical Study of Silicon Clusters and Metal-Containing Carbon Clusters

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1. Silicon clusters and silicon-containing carbon clustersFor the Si_n (n=1-20) and SiC_n (n=2-5) clusters, it has been investigated how the structures and electronic states change as the size (n) increases.2. Spherical carbon clusters containing transition metal atoms(1) Encapsulation of metal atoms insides hollow carbon clusters (fullerenes) has been investigated in order to disclose the interesting properties. Through the theoretical calculations of M@C_<82>, Sc_2 @C_<84>, and M_2 @C_<80> (M=Sc, Y,La, Ce, Pr, Eu, Gd, Yb, and Lu), the following has been disclosed ; (a) the electronic sates depend strongly on the kind of encapsulated metal atoms, (b) two or three electrons are transferred from metals to carbon cages, (c) the net charge on metals are little changed even when an electron is added or removed, endohedral metallofullerene being regarded as a sort of "superatom" , (d) endohedralmetallofullerene can act as stronger electron acceptors and donors, and (e) endohedral metaldoping leads to interesting reactivities which differ significantly from those of the empty cases. The first structural determination of Sc_2 @C_<84> and La_2 @C_<80> is especially noteworthy since it provides new insight into the growth mechanism. It has been suggested that the structures are determined during the final annealing process and energetically stable isomers with a large HOMO-LUMO gap are most abundantly produced and extracted.(2) To disclose the dynamic behavior of entrapped metals, Sc_2 @C_<84> and La_2 @C_<80> have been investigated in detail. It has been found that the two Sc atoms are oscillating with considerable amplitude while the two La atoms are circulating at high speed even at room temperature. The circular motion of two La^<3+> atoms has also been experimentally by observing the ^<13>C and ^<139>La NMR spectra and is expected to provide an interesting molecular device with new electronic and magnetic properties.