Quantum chemical HF/4-31G calculations on buckminsterfullerene intermediates

Abstract

Quantum chemical ab initio (U)HF/4-31G investigation on buckminsterfullerene and some proposed intermediates in its formation is carried out in this study with a view to better understanding how small carbon species carry out self-assembly to form fullerenes. The calculations on 19 carefully designed fullerene intermediates reveal that the core of an intermediate, rather than the number of its dangling bonds or abutting pentagon rings, has an intrinsic effect on its energy. The computational results show that hexagonal-core structures have lower energies than pentagonal-core structures. In addition, the pentagonal core enclosed completely by hexagonal rings has the highest energy. The UHF/4-31G results also suggest that some intermediates such as C₁₈, C₂₁ and C₃₀ with hexagonal cores have unusually low energies in comparison with their isomers or neighbours. Based on these calculated results, we outline the possible pathways from precursor to intermediates to fullerenes, subject to synthesis conditions and raw materials. These pathways support some existing proposals, such as medium monocyclic ring stacking and small ring polymerization mechanisms. However, our results do not suggest that the numbers of dangling bonds or abutting pentagonal rings have the highest impact on fullerene formation. The calculated thermodynamic parameters of the dimerization and addition reactions between two bowl-shaped intermediates suggest that these reactions are favorable to fullerene formation, and that the concentration of bowl-shaped fullerene intermediates should be very low in all detectable carbon species.