Nucleation of soot: Molecular dynamics simulations of pyrene dimerization

Experimental and numerical studies indicate that particle coagulation plays an important role in soot formation. However, to date, neither method has been able to conclusively determine the stage at which chemical precursors begin to coalesce. In the present study, molecular dynamics (MD) with on-the-fly quantum forces was used to investigate binary collisions between pyrene molecules. The molecular structure and internal motion were treated explicitly. Most of the runs were performed at 1600 K, within the temperature window of soot nucleation inflames. The MD simulations were successful in producing dimers with substantial collisional frequency and lifetimes far exceeding the equilibrium-based predictions, demonstrating that pyrene dimerization is physically realistic in flame environments. The principal finding of the present study is the development of internal rotors by colliding pyrene molecules, the phenomenon responsible for stabilization of the forming dimer. Analysis of the MD results shows that the mechanism of stabilization is rooted in the pattern of energy transfer. The translational energy of the individual colliding molecules is trapped in internal rotors that emerge upon collision and in the vibrational modes of the dimer, including the van der Waals bond established between the pyrene molecules. This model extends the view of stabilization of aromatic species dimerization, with the implication that aromatic dimers of species as small as pyrene can survive long enough to evolve into soot nuclei.