Quantum Chemical Molecular Dynamics Simulations of Dynamic Fullerene Self-Assembly in Benzene Combustion

Abstract

Using density-functional tight-binding (DFTB)-based quantum chemical molecular dynamics at 2500 and 3000 K, we have performed simulations of benzene combustion by gradually reducing the hydrogen to carbon (H/C) ratio. The accuracy of DFTB for these simulations was found to be on the order of 7-9 kcal/mol when compared to higher-level B3LYP and G3-like quantum chemical methods in extensive benchmark calculations. Ninety direct-dynamics trajectories were run for up to 225 ps simulation time, during which hydrocarbon cluster size, curvature, and C(x)H(y) composition, carbon hybridization type, and ring count statistics were recorded. Giant fullerene cage formation was observed only after hydrogen was completely eliminated from the reaction mixture, with yields of around 50% at 2500 K and 42% at 3000 K. Cage sizes are mostly in the range from 152 to 202 carbon atoms, with the distribution shifting toward larger cages at lower temperature. In contrast to previous simulations of dynamics fullerene assembly from ensembles of C(2) molecules, we find that the resulting cages show smaller number of attached carbon chains (antenna) surviving until cage closure. Again, no direct formation pathway for C(60) from smaller fragments was observed. Our results challenge the idealized picture of "ordered" growth of PAHs along a route involving only maximally condensed and fully hydrogenated graphene platelets, and favor instead fleeting open-chains with ring structures attached, featuring a large number of hydrogen defects, pentagons, and other nonhexagon ring species.

Atomistic Mechanism of Carbon Nanostructure Self-Assembly as Predicted by Nonequilibrium QM/MD Simulations

“…We named the combination of dynamic GF self-assembly and cage shrinking the "shrinking hot giant" (SHG) road of fullerene formation. Our more recent QM/MD simulations [136,137] of combustion fullerene synthesis [114] confirmed that SHG applies also in the case where hydrocarbon fuels are burnt in an oxygen-lean environment. The presence of hydrogen was found to effectively delay ring condensation, which is stage (ii) described in Sect.…”

Magic and hot giant fullerenes formed inside ion irradiated weakly bound C60 clusters

“…It is envisaged that C 60 and C 70 then will form with high probabilities. 9,10 This combination of size-up and long-range, multistep, size-down fullerene formation is clearly different from earlier models in which C 60 and C 70 are assumed to be formed through size-up processes followed by fragmentation in few steps only. Here, we will discuss such formation scenarios-and various types of coalescence reactions in view of the intensities and kinetic energies of even-n C 70 + -C 180 + ions which we measure following 400 keV head-on Xe 20+ collisions with weakly bound ͓C 60 ͔ m clusters.…”

“…8 Recently Irle et al presented quantum chemical molecular dynamics simulations suggesting that hot giant fullerenes, C n with n Ͼ ϳ 90, self-assemble in hot carbon and hydrocarbon vapors. 9,10 In these calculations, the hot giant fullerenes ͑vibrationally highly excited and with distorted structures͒ shrink spontaneously to smaller fullerenes-mostly by C 2 -emissions far from thermal equilibrium. It is envisaged that C 60 and C 70 then will form with high probabilities.…”

Silicon Carbide Derived Carbons: Experiments and Modeling

“…Fig. 3 displays the ´calculated angle distribution for V 30 . As an illustration of the graphitization tendencies, note the emerging peak near 120° in the angle distribution function of the most stable vacancy cluster for n=30, chosen for illustrative purposes here.…”