Monomer size effect in inelastic collisional dynamics of non-equilibrium soot nucleation.

Molecular dynamics (MD) simulations of the collisional dynamics of the coronene-acepyrene and coronene radical-acepyrene pairs have been carried out to investigate the size effect of monomers of polycyclic aromatic hydrocarbons (PAH) on their non-equilibrium dimerization. The results compared to the previous MD simulations of the smaller pyrene-acepyrene and pyrenyl-acepyrene systems corroborate the non-equilibrium hypothesis of crosslinking PAH dimerization enhanced by physical interaction between the monomers. The phenomenon of inelastic collisional dynamics responsible for non-equilibrium van der Waals dimerization, which fosters a covalent bond formation between the monomers, amplifies with increasing PAH size. The increase in the size of the colliding monomers enhances the non-equilibrium effects as the growing pool of low-frequency modes provides a larger sink for the energy of the colliding PAH monomers. Based on the direct count of the crosslinking reaction events observed in the MD simulations, the forward rate constant for the coronene radical-acepyrene association is estimated at ∼10-11 cm3 molecule-1 s-1, showing a 15-fold increase with respect to the value from the statistical Rice-Ramsperger-Kassel-Marcus calculations. A comparison with the eightfold increase reported previously for the pyrenyl-acepyrene system shows that the statistical (equilibrium-based) calculations increasingly underestimate the reaction rate with the increasing size of the interacting PAHs from pyrene to coronene. The total increase of the MD-assessed rate constant for the coronene radical-acepyrene dimerization reaction as compared to pyrenyl-acepyrene is a factor of 2.4, with the overall collision efficiency to produce dimerized products growing by 30%.