THEORETICAL-STUDIES OF SELECTED C60H2 AND C70H2 ISOMERS

Semi-empirical MNDO methods predict that 2 of 23 structurally distinct C60H2 isomers and 4 of 143 C70H2 isomers have particularly low heats of formation. These isomers represent either 1,2-addition across a 6:6 -ring fusion or 1,4-addition across a 6-ring, with both hydrogens exte rnally bound. Fully optimized ab initio structures are computed for th ese low-lying isomers at the Hartree-Fock level using 3-2 1 G and 6-3 1 G basis sets. For C60H2, all three methods agree on the isomer orde ring, and the lowest energy structure is also the only one that has be en observed experimentally. The energy separations among the lowest fo ur C70H2 isomers are found to be quite small, but only the ab initio o rdering is consistent with experimental results.