Molecular Dynamics Study of sp-Defect Migration in Odd Fullerene: Possible Role in Synthesis of Abundant Isomers of Fullerenes

Abstract

To explain recent experiment showing the role of odd fullerenes in formation of abundant fullerene isomers a reactive molecular dynamics (MD) study has been performed. Three types of bond rearrangement reactions are found by MD simulations at 3000 K in odd fullerenes which contain an extra sp atom among all other sp 2 atoms. The first type is stochastic sp-defect migration analogous to exchange mechanism of adatom migration on a surface. The second type corresponds to changes in the ring configuration of the sp 2-structure assisted by the sp atom which can lead to annealing of seven-membered rings or separation of five-membered rings. The third type is formation of short-living one-coordinated atoms or two additional sp atoms. Annihilation of a pair of sp defects has been also observed in the MD simulations. It is shown that the frequency of sp-defect migration at a lower temperature, as estimated from performed density functional theory calculations of the barriers of sp-defect migration events, is sufficient to deliver the sp atom to defects of sp 2 structure during the fullerene formation time. Based on these results, we propose to supplement the self-organization paradigm of fullerene formation by the following four-stage atomistic mechanism of formation of abundant isomers of fullerenes: 1) attachment of single carbon atoms, 2) sp-defect migration to sp 2-structure defects, 3) sp 2-defect annealing assisted by the sp atom and 4) subsequent annihilation of pairs of sp defects. I.

Multiscale modeling strategy to solve fullerene formation mystery

“…(c and d) SW reactions assisted by the sp atom: (c) the case where the same atom is sp before and after the reaction/ 441 (d) the case where the sp atom becomes the sp 2 atom and vice versa one former sp 2 atom becomes the sp atom. [45] (e) Annihilation of a pair of sp atoms accompanied by pentagon formation/ 21 [2] events by the example of the C61 fullerene with the same sp 2 structure as the CQQ-CJV fullerene but with an sp atom instead of one bond, (b) Generalized scheme of sp-defect migration event. The double-headed arrows point to atoms that form a new bond after breaking the bond indicated by x.…”

“…Other types of reactions that are assisted by an extra sp atom and lead to changes in the numbers of atoms in polygons of the sp 2 structure (analogously to SW reactions) are also possible, as follows from MD simulations. [2] According to DFT calculations, attachment of a single carbon atom to the sp 2 structure of a fullerene and its insertion into this structure in the place of a former bond with formation of an sp atom is a one-stage barrierless reaction^5 5 the sp 2 structure of an odd fullerene can easily move around the fullerene shell at temperature 3000 K (so-called sp-defect migration).^ This bond-rearrangement reaction is analogous to the exchange mechanism for adatom migration on a surface in which is a new atom becomes the sp defect at each migration event. According to the DFT calculations by the example of 10 different sp-defect migration events in the C 69 fullerene, the activation barriers for such reactions lie in the range of 1.3-2.2 eV.…”

“…Up to now, energy changes and activation barriers have been calculated using DFT-based methods for SW reactions in even fullerenes, [44 ' 45 ' 47 ' 51 ' 55 ' 56 ' 65 ' 66] autocatalysis by an extra sp atom of SW reactions in odd fullerenes, [44>45>55>56] C 2 molecule insertion into even fullerenes, [47>50>51>55>56>67] and annihilation of pairs of sp atoms followed by formation of the sp 2 structureJ 55 ^ Energy changes for attachment of single carbon atoms have been also obtained. [55] Comparison of activation barriers calculated within the DFT framework and using empirical reactive potentials for reactions observed in MD simulations allows one to verify the MD results (see, e.g., the study [2] of sp-defect migration events in odd fullerenes).…”

“…[63>64] The parameters of the last version REBO-1990EVC of firstgeneration Brenner potential were fitted to the energies of graphene edges, carbon chains, and monovacancy motion in a graphene layer, which makes this version adequate for simulations of the processes during the fullerene forma-tion^6 41 The adequacy of the REBO-1990EVC potential was confirmed by the good quantitative agreement of energy changes in sp-defect migration events in odd fullerenes with the values obtained by DFT calculations. [2] Nevertheless, only qualitative agreement was achieved for the activation barriers of such events.…”

“…Note also that formation of initial fullerene shells due to self-organization of a chaotic carbon system is the process that is much faster than subsequent selection of abundant fullerene isomers (see, for example, Refs. [2,23]). Therefore, studies of fullerene shell formation and selection of abundant fullerene isomers need different simulation approaches.…”