Structures of C60Hn and C60Fn, n=36-60

The structures and stabilities of those isomers of C60Hn and C60Fn 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 C=C 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 C60F42 having one C-6 ring whereas, there are only isolated C=C in the most stable isomer of C60H42. 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 C60H48 and C60F48, both structures containing six well separated carbon-carbon double bonds. Structures of C60F54 and C60F60 in which all fluorine atoms are added to the outside of the C-60 cage are relatively unstable. More stable structures are obtained, for C60F54 and C60F60 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. (C) 2002 Elsevier Science B.V. All rights reserved.