Structures of C_(70)X_(38) and C_(70)X_(40), X = H,F

摘要

The structures and stabilities of C_(70)X_(38) and C_(70)X_(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_(70)F_(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_(30)F_(18) in C_(70)F_(36), both C_(30)F_(20) in C_(70)F_(40) and there is one of each type in the intermediate C_(70)F_(38). The unfluorinated carbon atoms are grouped as isolated C_6 rings or as isolated C_2 units. The C_(30)F_(18) hemisphere used was the most stable hemisphere found previously for C_(70)F_(36). All 120 possible C_(30)F_(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_(30)F_(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_(70)F_(40) are grouped as two C_6 rings and nine C=C units, compared with three C_6 and seven C_2 in C_(70)F_(38), and four C_6 and five C_2 in C_(70)F_(36). For each stoichiometry, however, there are many related structures of similar stability. A common feature of the most stable structures of C_(70)F_(36). C_(70)F_(38), and C_(70)F_(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.