Dynamics of Pyrene-Dimer Association and Ensuing Pyrene-Dimer Dissociation.

To address the possible role of pyrene dimers in soot, chemical dynamics simulations are reported to provide atomistic details for the process of collisional association of pyrene dimers and ensuing decomposition of pyrene dimers. The simulations are performed at 600, 900, 1200, 1600, and 2000 K temperatures (T) with different collisional impact parameters (b; 0-18 Å) using the all-atom optimized potentials for liquid simulations intermolecular force field. Corresponding to each b, ensembles of 1000 trajectories are computed up to a maximum time of 110 ps at each T. Microcanonical association rate constants for the pyrene-dimerization processes decrease with an increase in T. The ensuing dissociation of the pyrene dimers is statistical and could be well represented by the Rice-Ramsperger-Kassel-Marcus theory of unimolecular dissociation. Fits of the dissociation rate constants versus the harmonic Rice-Ramsperger-Kassel equation revealed that partial energy randomization occurs among the inter- and intramolecular vibrational modes during the dissociation of pyrene dimers, whereas rotational and translational modes play a significant role. Based on the low probability of association and short lifetime at 1600 (∼13.3 ps) and 2000 (∼12.8 ps) K, it is concluded that pyrene dimers are unlikely to play any major role in soot nucleation processes.