Dimerization of C60:: Symmetry considerations

Orbital-symmetry analysis (OCAMS) of the dimerization of C-60 via [2 + 2] cycloaddition indicates that the reactant monomers should approach one another along a pathway in which C-2h symmetry is conserved. Point-by-point computations (AMI/UHF) confirm this prediction: a low-energy pathway leads to a single-bonded dimer 2 with C-2h symmetry. Closure to the stable D-2h dimer 1 is effected by relatively facile rotation about the single bond. A similar symmetry analysis was performed in a second isomer 3 with D-2h symmetry, the moieties of which are linked by two two-atom chains. It raises the possibility that 3, the so-called 'window' isomer, may be interconvertible with I along a pathway that retains C, (S-2) symmetry, Although the computational results indicate that C-60 is in thermal equilibrium with its stable dimer I at moderate temperatures, the latter is not observed in the gas phase for thermodynamic reasons. According to THERMO computations (AMI/RHF), the equilibrium is shifted strongly towards the monomer pair at temperatures where vaporization of the solid C-60 is observed (>400).