The role of molecular properties on the dimerization of aromatic compounds

Recent results have shown the presence and importance of oxygen chemistry during the growth of aromatic compounds, leading to the formation of oxygenated structures that have been identified in various environments. Since the formation of polycyclic aromatic compounds (PAC) bridge the formation of gas-phase species with particle inception, in this work we report a detailed analysis of the effects of molecular characteristics on physical growth of PAC via dimerization. We have included oxygen content, mass, type of bonds (rigid versus rotatable), and shape as main properties of the molecules and studied their effect on the propensity of these structures to form homo-molecular and hetero-molecular dimers. Using enhanced sampling molecular dynamics techniques, we have computed the free energy of dimerization in the temperature range 500-1680 K. Initial structures used in this study were obtained from experimental data. The results show that the effects of shape, presence of oxygen, mass, and internal bonds are tightly intertwined, and that their relative importance changes with temperature. In general, mass and the presence of rotatable bonds are the most influential factor to predict dimerization propensity among the one considered. The results provide knowledge on the inception step and the role that particle characteristics play during inception. In addition, our study highlights the fact that current models that use stabilomers as monomers for physical aggregation are overestimating the importance of their dimerization during particle nucleation. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.