Theoretical Study of Diels−Alder Cycloadditions of Butadiene to C70. An Insight into the Chemical Reactivity of C70 as Compared to C60

The eight unique Diels−Alder cycloadditions of butadiene to C70 are analyzed theoretically and compared with the well-established, two possible Diels−Alder cycloadditions of butadiene to C60. Full geometry optimizations of reactants, adducts, and transition states are performed using the AM1 semiempirical method followed by single-point ab initio energy calculations. The results show that the cycloaddition of butadiene to the C70 fullerene in the gas phase is slightly more reactive than that to C60. However, in toluene solution calculations yield that the different solvent effects on C60 and C70 cause a significant decrease of the energy barrier in the C60 cycloaddition, thus predicting a larger reactivity for C60 as compared to the C70 fullerene.