A nanometric window on fullerene formation in the interstellar medium: Insights from molecular dynamics studies

Abstract

Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules such as from graphite, which is known to occur in the ISM, in particular circumstellar environments . Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions, and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures such as pristine and metastable molecules. The pathways leading to molecular formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone-Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie formation. Moreover, they clearly demonstrate that top-down synthesis of from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter.

Carbon Condensation via [4 + 2] Cycloaddition of Highly Unsaturated Carbon Chains

“…A myriad of chemical models for the formation of fullerene have been proposed and can be categorized into two groups: pathways that start from large carbon aggregates that degrade into fullerene (“size-down”) and pathways that start from small carbon aggregates and condense into fullerene (“size-up”). The “size-down” pathways that have been proposed include the degradations of graphene, , graphene nanoflakes, carbon nanotubes, and giant fullerenes. , The “size-up” pathways proposed include the fullerene road, pentagon road, closed network growth, − and ring coalescence and annealing. ,, All of the “size-up” pathways except the ring coalescence and annealing model progress from smaller carbon aggregates to larger fullerenes through the steady incorporation of C 2 fragments. In contrast, the ring coalescence and annealing model proposes the addition of medium-sized (C 12 –C 20 ) cyclic polyynes, and subsequent annealing through a cascading radical mechanism, resulting in fullerenes .…”

“…7 A myriad of chemical models for the formation of fullerene have been proposed and can be categorized into two groups: pathways that start from large carbon aggregates that degrade into fullerene ("size-down") and pathways that start from small carbon aggregates and condense into fullerene ("size-up"). The "size-down" pathways that have been proposed include the degradations of graphene, 8,9 graphene nanoflakes, 10 carbon nanotubes, 11 and giant fullerenes. 12,13 The "size-up" pathways proposed include the fullerene road, 14 pentagon road, 15 closed network growth, 15−17 and ring coalescence and annealing.…”