The enhanced stability of endo isomers of C60F60

The structure of C60F60, in which all sixty fluorine atoms lie outside the cluster of sixty carbon atoms, is relatively unstable due to the unfavourable stereochemistry about each carbon atom and to the short F-F distances. An increase in stability is achieved if some of the fluorine atoms are transferred from the exo positions outside the carbon cluster to the endo positions inside the carbon cluster. This transfer results in a decreased crowding in the exo fluorine shell while the puckering of the five and six membered rings in the carbon skeleton leads to more favourable stereochemistries about the carbon atoms. Calculations using the AM1 hamiltonian and the program MOPAC 6.0 indicate that the most stable isomer has eight endo fluorine atoms and 52 exo fluorine atoms. The eight endo atoms are arranged as a dodecahedron. This isomer, designated F-8@C60F52, is 401.6 kcal mol(-1) more stable than the all exo isomer.