The structures of C70Xn, X = H, F, Br, C6H5 and n=2-12

The structures and stabilities of isomers of C70Hn, where n = 2-12 have been investigated using the AM1 Hamiltonian and the program MOPAC 6.0. Two possibilities an addition of pairs of hydrogen atoms to the hex-hex carbon-carbon bonds, or addition of six hydrogen atoms to form a skew pentagonal pyramid. Both types of addition occur at the hemispherical ends of C-70 and are similar to the addition found for C-60. A third possibility is consecutive para addition to C-6 rings around the central part of the molecule, between the hemispherical ends, and has no counterpart in additions to C-60. A number of isomers of similar stability are found for C70H2 and C70H4, but at higher levels of addition the string structures found by consecutive para-addition become of dominant stability. At C70H10 this pam string forms a continuous belt by the insertion of an ortho C6H2 unit into the string. Similar behaviour is found for C70Fn, C70Brn and C70Phn. These results are in agreement with structures which have been experimentally determined. (C) 1999 Elsevier Science B.V. All rights reserved.