A theoretical study of the pressure-induced dimerization of C-60 fullerene

We present a theoretical study of the solid-state dimerization of C-60 full erene, which occurs under pressure through [2+2] cycloaddition of double bo nds. The possible crystal packings of (C-60)(2) molecules are calculated by minimization of the lattice energy with a bond charge intermolecular poten tial model proved successful in the previous C-60 studies. The set of dimer lattices that were derived from the fee lattice was used to construct the initial structures for minimization. The final structures found this way, a lthough belonging to various Space-group symmetries, retain approximately f ee arrangement of the constituting C-60 cages. On the other hand, the struc ture obtained from the dimer motif observed in the o-dichlorobenzene solvat e of (C-60)(2) exhibits a:hcp-like C-60 arrangement. The more energetically stable hcp-type dimer seems not to form due to the high potential barrier separating the fee and hcp structures. The relative stability of the predic ted structures changes significantly under pressure. Some of the dimer stru ctures are remarkably similar to the respective theoretical monomeric and p olymeric C-60 structures studied previously. On this basis we propose that the respective structures are connected states in the possible conversion p aths from pristine C-60 to its various polymerization products. One of the dimer structures is geometrically favorable for the formation of the higher C-60 chain oligomers as well as of the infinite polymer.