Carbonaceous Nanoparticle Molecular Inception from Radical Addition and van der Waals Coagulation of Polycyclic Aromatic Hydrocarbon-Based Systems. A Theoretical Study

Carbon nanopartides, generated during combustion at relatively low [O-2] or under pyrolysis conditions, can be seen both as soot precursors and as primary pollutants in themselves. Soot particle inception, with transition of relatively low-mass molecular systems from the gaseous phase to a solid nature through coagulation/condensation, is believed to take place via chemical reactions as well as van der Waals (vdW) interactions involving polycyclic aromatic hydrocarbons (PAHs) or derivatives. In this Article, radical addition between open and closed shell molecular PAH or PAR-like systems (R-center dot + P --> R-P-center dot) is examined by density functional theory, and different sigma bond formations are compared to the stacking of the aromatic components (R-center dot:P, previously discussed in J. Phys. Chem. C 2011, 115, 1732-1739). Energetic and entropic effects are examined. At higher T, formation of aliphatic bridges (hence reticulation) appears to be of the utmost importance to link PAR-like moieties, with a preference for more extended arrangements (due to entropic effects). More packed structures, promoted by vdW interactions (an energetic effect), may be favored by lower T. Thus, when the gas in the flame cools, reticulation could be followed by inter- or intramolecular stacking. These distinct processes can take place within different T ranges, but are not mutually exclusive: in particular, sigma bond formation helps subsequent stacking, because crystallites would be more easily produced at lower T by stacking of already bound elements. Therefore, the mechanistic picture offered by the calculations bears out a structural model for carbonaceous particle growth in which an initial more amorphous core is generated at higher T through successive radical attacks and sigma bond formations, hence reticulation involving growing adducts of R-P-center dot type. The core can subsequently become enclosed in an external shell, which grows at a lower-T regime and presents more ordered zones. It cannot be excluded that limited transitions from amorphous zones to more ordered zones could take place within internal regions of the particle, provided the local textures of these regions were sufficiently sparse to allow rotations in the R-P-center dot adducts from extended (anti) to packed (syn) arrangements.