Structures of C 70X 38 and C 70X 40, X=H,F

Abstract

The structures and stabilities of C 70X 38 and C 70X 40 have been investigated using the AM1 Hamiltonian and the program MOPAC 6.0. Very similar results are obtained for X=H and F. As in C 70F 36, structures are based on an equatorial plane of 10 bare carbon atoms separating two C 30 hemispheres, which do not interact strongly with each other. Both hemispheres are C 30F 18 in C 70F 36, both C 30F 20 in C 70F 40 and there is one of each type in the intermediate C 70F 38. The unfluorinated carbon atoms are grouped as isolated C 6 rings or as isolated C 2 units. The C 30F 18 hemisphere used was the most stable hemisphere found previously for C 70F 36. All 120 possible C 30F 20 hemispheres were considered, 101 of which exist as a pair of optical isomers. Each hemisphere can be rotated relative to the other by 0, 72, 144, 216 or 288° relative to the other. Four C 30F 20 hemispheres were found to be more stable than the others, all of which contained one unfluorinated C 6 ring sharing an edge with the equatorial plane. For the most stable isomers, the 30 unfluorinated carbon atoms in C 70F 40 are grouped as two C 6 rings and nine CC units, compared with three C 6 and seven C 2 in C 70F 38, and four C 6 and five C 2 in C 70F 36. For each stoichiometry, however, there are many related structures of similar stability. A common feature of the most stable structures of C 70F 36, C 70F 38, and C 70F 40 is the presence of an unfluorinated C 6 ring attached to each side of the unfluorinated equatorial plane, the fluorine atoms being concentrated on the most curved parts of the molecules, which are most suited to tetrahedral carbon atoms.