An Epilogue on the C_(78)-Fullerene Family: The Discovery and Characterization of an Elusive Isomer

摘要

Among the higher fullerenes—closed carbon clusters with even numbers of atoms greater than 70—C_(78) has been a subject of many experimental, as well as theoretical investigations. According to the isolated pentagon rule (IPR), five isomers with different connectivities are possible for a cage constituted of 78 carbon atoms. The first three representatives of this family—C_(78)(1) (with D3 point group symmetry), C_(78)(2) and C_(78)(3) (both with C_(2v) symmetry), have been found in soot extracts, isolated by means of HPLC techniques, and characterized either crystallographically, or by ~(13)CNMR analyses. The next known stable, but insoluble isomer is C_(78)(5) (D_(3h)), the connectivity pattern of which was recently confirmed through its derivative C_(78)(CF3)_(12). Thus, the only unexplored member of this family, C_(78)(4) (D_(3h)), is needed to complete the first multi membered group of fullerene isomers. Although C_(78)(4) is predicted to be the least stable among all IPR C_(78) isomers across a wide temperature interval, a fact justifying its absence in fullerene extracts, its energy per carbon atom is comparable to that of C_(70). According to a calculation performed at the DFT (desity functional theory) level, C_(78)(4) possesses a large energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 2.47 eV, which is comparable to those of C_(70) (2.69 eV) and C_(60) (2.76 eV). Since the insolubility of fullerenes has been attributed to polymerization due to low or zero HOMO-LUMO gaps, C_(78)(4) is presumably soluble. Kinetic factors were evoked to explain why this isomer has not been experimentally observed so far.