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Atomistic insights into the dynamics of binary collisions between gaseous molecules and polycyclic aromatic hydrocarbon dimers

Summary

Polycyclic aromatic hydrocarbon (PAH) dimers are frequently invoked as soot nuclei, yet most kinetic models either treat dimerization as irreversible or neglect collisions with hot bath gases present in engines and flames. The PCCP study therefore occupies a middle ground between supersonic jet experiments that emphasize low-temperature physical dimers and flame simulations that historically lumped bath gas effects into effective sticking coefficients without explicit collision dynamics. Mao, Zhou, Luo, and van Duin perform ReaxFF molecular dynamics of binary collisions between N\(_2\), CO, CO\(_2\), and related projectiles with stacked PAH dimers sampled from prior dimerization studies. The work classifies scattering outcomes by N\(_2\) residence time inside the dimer scaffold, links energy transfer to dimer lifetime, and quantifies how projectile mass biases decomposition back to monomers.

Methods

2 — Force-field training (as used, not re-fit here). The C/H/O/N ReaxFF set combines C-2013 carbon parameters with the broader hydrocarbon/gas-phase training of the C/H/O/N library documented in the paper; ESI tabulates the ReaxFF bond-order parameters used in this work.

1 — MD application (binary collision trajectories). ReaxFF MD is implemented in LAMMPS. Stacked PAH dimers are taken from prior dimerization work (same program). Binary-collision cells place a PAH dimer in a cubic (70 Å)³ PBC box with 50 Å initial separation to the projectile gas molecule; impact parameters and collision angles are sampled as specified in §2.2 (including 1000 independent trajectories per (temperature, dimer) case in the PCCP text). Collisions are integrated in the microcanonical (NVE) ensemble with a 0.1 fs timestep for 10⁶ steps (~100 ps total per trajectory in the article’s equation-of-motion block). Thermostat: N/A for these NVE segments—no Nosé–Hoover/Berendsen thermal bath is applied during the quoted collision integration. Initial translational and rotational energies of the dimer and projectile are assigned from Maxwell / equipartition-style distributions at the target temperature; N₂, CO, and CO₂ are treated with ReaxFF-consistent internal bond models (harmonic oscillators for diatomic/polyatomic projectiles, per §2.2). Thermostat/barostat during the collision segment: N/A — NVE binary-collision dynamics (not constant-T NVT production). Mean hydrostatic pressure as an NPT target: N/A — isolated collision studies without bulk pressure coupling. External electric field: N/A. Replica / enhanced sampling: N/A. System sizes / compositions: as defined by the pyrene- and larger-PAH dimer models in Figs. 1–2; exact atom counts follow from those dimer constructions plus one gaseous projectile.

Findings

When N\(_2\) remains trapped only briefly, specular-like scattering dominates, whereas long residence enables inelastic energy accommodation; temperature and dimer size shift the balance, with specular channels favored at high T on small dimers and inelastic channels more visible at low T on large dimers. Collisional energy transfer excites internal vibrational modes that accelerate decomposition to monomers relative to isolated dimers. Heavier projectiles are more effective at promoting decomposition at comparable collision conditions. Larger dimers and lower temperatures reduce decomposition rates, but any gas-phase collisions net destabilize dimers compared with vacuum baselines discussed in the article. The PCCP paper explicitly motivates engine-relevant collision frequencies by citing Frenklach-style arguments that N\(_2\) collision times can be comparable to dimer lifetimes under high-pressure combustion, justifying gas-mediated kinetic models beyond vacuum dimerization alone. Projectiles beyond N\(_2\) (CO, CO\(_2\)) extend the same framework to oxidizing versus inert bath compositions.

Limitations

ReaxFF accuracy for PAH–gas collisions should be benchmarked against higher-level QM for selected configurations.

Relevance to group

Extends ReaxFF soot-precursor work by explicitly coupling PAH dimers to realistic gaseous collision partners relevant to combustion environments.

Citations and evidence anchors