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Unusual egg-shaped fullerene molecules are rulebreakers because they do what no other fullerenes seem to do - fuse three pentagons of carbon atoms, according to chemists in China. The discovery of these molecules could lead to new insights into fullerene chemistry as well as offering new opportunities for synthesising novel materials. Yuan-Zhi Tan, Jia Li, Feng Zhu, Xiao Han, Wen-Sheng Jiang, Rong-Bin Huang, Zhiping Zheng, Zhuo-Zhen Qian, Rui-Ting Chen, Zhao-Jiang Liao, Su-Yuan Xie, Xin Lu, and Lan-Sun Zheng of the State Key Laboratory for Physical Chemistry of Solid Surfaces, and Department of Chemistry, College of Chemistry and Chemical Engineering, at Xiamen University discuss the research in Nature Chemistry. Fullerenes are a family of cage-like carbon clusters, made up of hexagons and pentagons of carbon atoms. There are two general classes of fullerenes: the relatively stable form in which pentagons are isolated and the highly reactive form that has adjacent pentagons. Three-dimensional geometry would suggest that it should be possible to build fullerenes containing the triple sequentially fused pentagons (TSFP) motif - three neighbouring pentagons fused one after the other as opposed to three fused into a triangular structure. However, from the early days of research into this novel form of carbon, chemists recognized that such a subunit would seriously violate the isolated pentagon rule (IPR), which says that no two carbon pentagons can be direct neighbours in a stable fullerene and instead must be separated by a six-membered carbon ring, a hexagon. Three fused pentagons might be considered a ring too far. The underlying physical phenomenon from which the rule is derived is the inherent strain across carbon bonds that would results from the existence of fused pentagons that would have to be extremely contorted to form a closed carbon cage. The fused pentagon motif had not previously been observed in carbon cages lending support to the IPR. As such, most carbon chemists have not pursued fullerenes that contain TSFPs either experimentally or theoretically. Now, Tan and colleagues have discovered four chlorinated derivatives of three different fullerene cages all of which contain the TSFP motif. Their X-ray crystallographic analyses of the structures indicate that the molecular strain inherent in pentagons that are adjacent can be relieved when the fullerene structure is chlorinated in a particular way, on the outside of the cluster, exohedrally. The effect renders one of the four pentagon fusion sites unsaturated and also gives the molecule chirality, handedness. The four novel fullerene chloro-derivatives have the formulae C54Cl8, C56Cl12, C66Cl6, and C66Cl10, each made by the chlorination of the corresponding non-IPR fullerenes C54, C56, and C66. "They are the first experimentally established fullerene derivatives featuring TSFP," the team says, "More importantly, the sharing of this motif suggests its prevalence in the making of various fullerenes, and further research on this particular aspect of fullerene chemistry is expected to shed new light on the mechanism(s) responsible for their gas-phase formation." The crystallographic studies also reveal a unique, incomplete chlorination pattern of the otherwise highly strained and thus reactive TSFP sites," the team explains. The team has rationalized their findings using density functional theory calculations, something that was not possible for previous fullerene work with fused motifs. They now anticipate that other chemists will carry out follow-up studies of these new members of the fullerene family, both theoretically and experimentally. The mechanistic computational studies of the chlorination process offer insights into the thermodynamics of the stepwise chlorination of these egg-shaped fullerenes. "This work is significant because of the prevalence of this newly found structural motif, as suggested by the examples presented herein, which suggests that there are many non-IPR fullerenes yet to be discovered," the team concludes.
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