STRUCTURES AND VIBRATIONAL FREQUENCIES OF C60, C70, AND C84

The structures and vibrational frequencies of C60, C70, and C84 have been determined with the semiempirical MNDO method. Two highly stable minimum-energy structures have been identified for C84. Of the two, a D6h form, which contains five distinct carbon atoms, is found to be slightly more stable than a T(d) form with four distinct carbons. Both isomers are found to be significantly more stable than either C60 or C70, consistent with the presence of a greater proportion of six-membered rings in C84. An additional structure containing seven-membered rings has also been considered for C84, but is significantly less stable than structures containly only five- and six-membered rings. Finally, the geometries of C60, C70, and the two stable forms of C84 have been completely optimized by using ab initio Hartree-Fock theory with the double-zeta-3-21G basis set.