Theoretical study of C₂₀ fullerene dimerization: A facile [2+2] cycloaddition

Dimerization of the C₂₀ cage was studied with HF and DFT methods. Eight dimers were found. Of these, open-[2+2] isomer turned out to be the most stable dimer. [2+2] isomer is the second most stable due to the high ring strain of the C₂₀ cage, in contrast to the case of C₆₀ dimers where [2+2] isomer is more stable. The number of inter-fullerene bonds varies from 1 to 4 in the eight isomers. Asymmetric stepwise reaction channel leading to the open-[2+2] isomer with no net activation barrier was found, showing that it is both thermodynamically and kinetically favorable.