Hot giant fullerenes eject and capture C₂ molecules: QM/MD simulations with constant density

Quantum chemical molecular dynamics (QM/MD) simulations using periodic boundary conditions show that hot giant fullerene (GF) cages can both eject and capture C₂ molecules dependent on the concentration of noncage carbons in the simulated system, and that the cage size can therefore both increase and decrease under high temperature conditions. The reaction mechanisms for C₂ elimination and incorporation involve sp³ carbon defects and polygonal rings larger than hexagons, and are thus closely related to previously described mechanisms (Murry, R. L.; Strout, D. L.; Odom, G. K.; Scuseria, G. E. Nature1993, 366, 665). The atoms constituting the cage are gradually replaced by the two processes, suggesting that a fullerene cage during high-temperature synthesis is a dissipative structure in the sense of Ilya Prigogines theory of self-organization in nonequilibrium systems. Explicit inclusion of Lennard-Jones-type helium or argon noble gas atoms is found to increase the GF shrinking rate. Large GFs shrink at a greater rate than small GFs. The simulations suggest that in an idealized, closed system the fullerene cage size may grow to a dynamic equilibrium value that depends on initial cage size, temperature, pressure, and overall carbon concentration, whereas in an open system cage shrinking prevails when noncage carbon density decreases as a function of time.