Fullerenes and PAHs: A Novel Model Explains their Formation via Sequential Cycloaddition Reactions Involving C2 Dimers

Based on a new understanding of the nature of the bonding in the C₂ dimer, a novel bottom-up model is offered that can explain the growth processes of fullerenes and polycyclic aromatic hydrocarbons (PAHs). It is shown how growth sequences involving C₂ dimers, that take place in carbon vapor, combustion systems, and the ISM, could give rise to a variety of bare carbon clusters. Among the bare carbon clusters thus formed, some could lead to fullerenes, while others, after hydrogenation, could lead to PAHs. We propose that the formation of hexagonal rings present in these bare carbon clusters are the result of [2+2+2] and [4+2] cycloaddition reactions that involve C₂ dimers. In the C₆₀ and C₇₀ fullerenes, each of the twelve pentagons is surrounded by five hexagons and thereby “isolated” from each other. To explain why, we suggest that in bowl-shaped clusters the pentagons can form as the consequence of closures of “cove regions” that exist between hexagonal rings, and that they can also form during cage-closure, which results in the creation of six “isolated” pentagonal rings, and five hexagonal rings. The proposed growth mechanisms can account for the formation of fullerene isomers without the need to invoke the widely used Stone-Wales rearrangement. Considering processes that take place in combustion systems, it is proposed that soot particle inception can result from oligomerization of the bare carbon clusters that originated from cycloadditions involving C₂ dimers. In future work, the insights offered in this article could provide a better understanding of nanotube growth, with or without defects.