Dimers & oligomers of fullerenes

The discovery of materials where covalently bound C60 molecules exist has attracted great interest both theoretically and experimentally around 1995. The first experimental observation was the photopolymerization of pure C60 film under oxygen-free conditions. The high-pressure high-temperature treatment of solid C60 also led to polymerization. The third example was the cooling of high-temperature alkali-doped fullerenes with a 1:1 stiochometry (A1C60, A=K or Rb). Depending on the cooling rate, linear polymer chains (slow cooling) or dimers (quenching) were formed. It was generally assumed that the covalent bonding between C60 molecules in these materials occurs through a [2+2] cycloaddition, where the (66) short bonds between two hexagons on adjacent molecules are broken and converted to a four-membered ring. The symmetry of such a dimer is D2h. Several theoretical calculations on neutral C60 dimers supported this assumption.

Jenő Kürti and Károly Németh performed calculations on negatively charged C60 dimers and reported the optimized geometry and energetics of such materials for the first time. They found that in contrast to the neutral dimers, the D_D2h (66/66) dimer is not the most stable one among the negatively charged dimers. In fact using the AM1 or the PM3 parametrization, the S_C2h (single bound, trans) dimer turned out to be the most stable one. Meanwhile a group of experimentalists lead by László Forró have drawn similar conclusion by analizing the X-ray diffraction spectra of quenched A1C60 -- the structure of such dimers shows S_C2h symmetry in agreement with Kürti's theoretical prediction.

 D2h
C2h
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C60 dimer geometry. Push the buttons on the right to change the symmetry of the dimer or toggle rotation. No depth

Publications on this topic

[1] J KÜRTI, K NÉMETH: "Structure and energetics of neutral and negatively charged C-60 dimers" In Chemical Physics Letters, 256, 119-125, (1996) DOI.

[2] J KÜRTI, K NÉMETH: "On the differences between neutral and negatively charged C-60 dimers" In Fullerene Science and Technology, 5, 429-442, (1997).

[3] J KÜRTI, P RAJCZY: "Cross-over in the structure of the charged (C-60)(n)(n-) oligomers" In Carbon, 36, 653-656, (1998) DOI.

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fullerene