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The structures and stabilities of those isomers of C 60 H n and C 60 F n which contain a three-fold axis have been calculated as n increases from 36 to 60. Calculations were carried out using the AM1 Hamiltonian and the program mopac 6.0, and by the density functional technique B3LYP/6-31G ∗ at the AM1 geometry using gaussian 98. The unhydrogenated and unfluorinated carbon atoms are present as CC pairs or delocalised C 6 rings and the most stable isomers are those which maximize the separations between these C 2 and C 6 units. Structures in which there are bare C 6 rings are more favoured for the fluorides than for the hydrides which results in the most stable isomer of C 60 F 42 having one C 6 ring whereas, there are only isolated CC in the most stable isomer of C 60 H 42 . Compared with the AM1 calculations, the density functional calculations also favour structures with bare C 6 rings. Two very stable structures dominate the isomerism in C 60 H 48 and C 60 F 48 , both structures containing six well separated carbon–carbon double bonds. Structures of C 60 F 54 and C 60 F 60 in which all fluorine atoms are added to the outside of the C 60 cage are relatively unstable. More stable structures are obtained, for C 60 F 54 and C 60 F 60 in particular, if some carbon–carbon bonds are broken or if some fluorine atoms are inside the C 60 cage. A mechanism for the formation of these endo compounds is proposed. |
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