Stereochemical patterns in bromofullerenes, C60Br12 to C60Br24

The stabilities of different isomers of C60Brn have been calculated for n = 14-24. Two types of structure are found to be important. The first type involves the linking of alternating 1,4-C6Br2 rings and 1,3-C6Br2 rings, forming -Br-BrBr-Br- strings over the molecule. Additional stability is achieved if the string ends are eliminated by the formation of cyclic strings, and particularly stable structures incorporate the Br-12 cycle of S-6 symmetry. The strings may branch at 1,3,5-C6Br3, rings with the formation of polycyclic string structures, culminating in the rhombicuboctahedral bromine pattern in C60Br24 These patterns are related to the conformation of the Cg rings, which form shallow boats in 1,4-C6Br2 and chairs in 1,3,5-C6Br3. A second important structural feature is the presence of discrete Br-6 skew pentagonal pyramids, one in C60BT6, two in C60Br12 and three in C60Br18 These results indicate the possibility of forming C60Br12 and C60Br18 under the appropriate conditions.