Stereochemical patterns in [76]fullerenes, C76 to C76H28

The structures and stabilities of C-76 and various isomers of C76Hn as n increases from 2 to 28 have been calculated using the AM1 Hamiltonian and the program MOPAC 6.0. C-76 forms an elongated structure with relatively hat areas in which some carbon atoms are at the junction of three six-membered rings. The bonding in some of these graphite-type rings is delocalised whereas in the remainder of the structure the double bonds are localised on edges linking two hexagonal faces. Hydrogenation occurs at these double bonds on the more curved part of the structure where each carbon atom is at the junction of one five-membered and two six-membered rings. The first stage of successive hydrogenations of this type leads to C76H20 in which there are two C10H10 zig-zag strings, each string formed by the edge sharing of four C6H4 rings. The second stage of hydrogenation leads to C76H28 in which these strings are extended by the hydrogenation of Dent-hex edges. The stereochemical pattern is determined by the instability of CH groups on the triple hexagonal junctions, the localisation of double bonds on pent-hex edges, and the formation of C6H6 rings. (C) 2001 Elsevier Science B.V. All rights reserved.