Dynamics and kinetics of reversible homo-molecular dimerization of polycyclic aromatic hydrocarbons

Abstract

Reactive MD with a C/H ReaxFF parametrization is used to study reversible physical dimerization of PAHs as a function of temperature, impact parameter, and molecular size, and to extract forward/reverse rate constants compared with a pyrene dimer experiment.

Summary¶

The work reports molecular dynamics with a ReaxFF description of carbon and hydrogen to study reversible homo-molecular dimerization of polycyclic aromatic hydrocarbons (PAHs). Collisions are analyzed over a range of temperatures, impact parameters, and orientations. Coronene dimer energetics from ReaxFF are compared in the supplementary material to semiempirical PM3 and DFT (U)M06-2X/def2SVP references to justify the reactive force field for longer trajectories than feasible with quantum methods. The study quantifies enhancement factors for dimerization, lifetimes within a capture radius as a function of impact parameter, and temperature- and PAH-dependent rate constants used in a reversible kinetic model. The authors conclude that physical dimerization is unlikely to dominate at typical flame temperatures and PAH concentrations, pointing to chemical growth routes instead.

Methods¶

A — Force-field training / fitting: C/H ReaxFF trained against QM data; QEq updates atomic charges each step; bond/angle/torsion, over/under-coordination, vdW, and Coulomb terms as in Sec. II.A of the article.

B — Molecular dynamics / sampling (bimolecular collisions in LAMMPS). PAH monomers (Fig. 1 set including five-membered and six-membered rings) are geometry-minimized (conjugate gradient) then vibrationally equilibrated at the target temperature with 10⁶ iterations at Δt = 0.25 fs using a Nosé–Hoover thermostat. Binary homo-molecular collisions are then integrated in the microcanonical (NVE) ensemble in LAMMPS (reax/c pair style) with 3D periodic boundary conditions, Δt = 0.25 fs, 100 ps total collision time, 10 000 stochastic velocity draws per (PAH, T, impact parameter) case for statistics. System size & composition: two PAH molecules per collision event (~52 atoms for a pyrene–pyrene encounter as a representative lower bound; larger PAHs scale up—see Fig. 1). Impact parameter is scanned to define a capture radius; initial separation is 30 Å (justified against binding-energy decay with distance in the SI). OVITO/VMD-class visualization is used in the paper. N/A — barostat / pressure control: NVE collision dynamics—no thermostat/barostat during the 100 ps collision segment.

C — DFT / static QM: Coronene dimer reference curves vs PM3 and (U)M06-2X/def2SVP (Fig. S1, Table S1 in SI) motivate ReaxFF accuracy for the long reactive trajectories.

D — Review / non-simulation framing. N/A: primary JCP study. Proof PDF note: papers/Mao_Qian_JCPSA6_vol_147_iss_24_244305_1_proof.pdf—confirm final DOI/pagination from the publisher site when citing.

Findings¶

Outcomes and mechanisms. Homo-molecular dimerization enhancement factors grow at lower temperature and for smaller PAHs—the effect is not size-independent. Within the capture radius, dimer lifetimes decrease as impact parameter increases. Forward and reverse rate constants depend on temperature and PAH identity and are extracted from the collision statistics to build a reversible kinetic model.

Comparisons. The reversible model is compared against pyrene dimerization experiments (Sabbah et al., J. Phys. Chem. Lett. 2010 as cited in the article).

Sensitivity and design levers. Temperature, impact parameter, and PAH size/orientation distributions are the primary levers explored in the MD campaign.

Limitations and outlook (as authored). The article argues that under typical flame temperatures and PAH concentrations, physical dimerization is unlikely to dominate relative to chemical growth channels—framing a shift in emphasis for soot inception models.

Corpus / PDF honesty. Detailed kinetics tables live in the SI; this entry is grounded in the proof PDF text—reconcile numbers against the VOR when available.

Limitations¶

Proof PDF; QM benchmarks are limited to smaller systems and short times; ReaxFF binding energies deviate from high-level DFT though trends are matched for parametrization context.

Relevance to group¶

Adri C. T. van Duin is a co-author; the study applies group ReaxFF methodology to combustion-relevant PAH dynamics.

Citations and evidence anchors¶

Prefer the version-of-record J. Chem. Phys. citation when DOI is confirmed from the publisher.

Proof PDF filename: papers/Mao_Qian_JCPSA6_vol_147_iss_24_244305_1_proof.pdf.