Structures and energetics of regioisomers of C60 dimer and trimers

Ten different regioisomers of C-60 trimers with intact cages were studied using the empirical tight-binding (TB) total energy calculation and the semiempirical PM3, AM1, and MNDO methods. We determined the stable atomic structure and energies of 10 (C-60)(3) regioisomers with ETBTEC, PM3, AM1, and MNDO calculations. The dimerization of C-60 is exothermic according to the density-functional-based nonorthogonal tight-binding (DF-TB) method; the semiempirical PM3, AM1, and MNDO; and the ab initio 3-21G LDA and HF calculations, but it is endothermic in both the TB and the ab initio B3LYP6-31G*//4-21G B3LYP calculations. Therefore, according to the TB calculations, the most stable regioisomer of C-60 trimers is equatorial. The energies of the trans-3(C-2), trans-1(D-2h), trans-2(C-2), trans-4(C-s), cis-1 (C-3v), Cis-2(C-s), cis-3(C-2), cis-1(C-s), and cis-2(C-3v) regioisomers relative to the equatorial regioisomer are 13,19, 20, 30, 66, 99, 169, 215, and 673 meV, respectively.