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Quantum chemical study of atomic structure evolution of the Co$$_{x}$$/C$$_{60}$$(x$$leq$$2) composites

Avramov, P.; Naramoto, Hiroshi; Sakai, Seiji; Narumi, Kazumasa; Lavrentiev, V.

Atomic and electronic structure of a new class of Co$$_{x}$$/C$$_{60}$$ (x$$leq$$2) composites synthesized using energy beams has been studied using B3LYP/6-31G$$^{*}$$ approach. For the x$$leq$$1 the $$eta$$$$^{2}$$ type of coordination is the lowest energy structure. Other isomers like $$eta$$$$^{2'}$$ or mixed structures $$eta$$$$^{2}$$/$$eta$$$$^{5}$$, $$eta$$$$^{2'}$$/$$eta$$$$^{5}$$ and $$eta$$$$^{2'}$$/$$eta$$$$^{2}$$ have energies between the $$eta$$$$^{2}$$ and $$eta$$$$^{5}$$ clusters ($$sim$$30kcal/mol). The theoretical data confirm the indirect qualitative structural data. A comparison of the theoretical Co-C bond lengths with XAFS analysis shows that B3LYP/6-31G$$^{*}$$ data (2.06 ${AA}$ ) match well the experimental value (2.01 ${AA}$). For the clusters with two cobalt ions a formation of metallic dimmers is preferable even in case of formation of additional C-C bonds between the fullerene molecules. To study the atomic structure evolution of Co$$_{x}$$/C$$_{60}$$ composites we have calculated the structure of all possible linear and bent (C$$_{60}$$-Co-C$$_{60}$$-Co-C$$_{60}$$) isomers of Co$$_{2}$$(C$$_{60}$$)$$_{3}$$ clusters. Like in the case of Co(C$$_{60}$$)$$_{2}$$ clusters, the $$eta$$$$^{2}$$:$$eta$$$$^{2}$$/$$eta$$$$^{2}$$:$$eta$$$$^{2}$$ and $$eta$$$$^{2}$$:$$eta$$$$^{2}$$/$$eta$$$$^{2}$$:$$eta$$$$^{2'}$$ types of coordination have lowest energies whereas the $$eta$$$$^{5}$$:$$eta$$$$^{5}$$/$$eta$$$$^{5}$$:$$eta$$$$^{2'}$$ and $$eta$$$$^{5}$$:$$eta$$$$^{5}$$/$$eta$$$$^{5}$$:$$eta$$$$^{5}$$ are the high energy isomers. It was shown that the closer the cobalt ions are, the lower the relative energy of the structures is. The formation of the cobalt dimmers on the final stage of the transition metal migration around the C$$_{60}$$ cage sufficiently ($$sim$$20-50kcal/mol) lowers the relative energies of the structures.

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