SOOT INCEPTION: A DFT STUDY OF ? AND ? DIMERIZATION OF RESONANTELY STABILIZED AROMATIC RADICALS

Recent advances in the literature have shown experimental evidences of a consistent amount of ?-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. The inclusion of such experimental observations in the mechanism of soot particle formation will improve the comprehension of the physics and chemistry involved during the evolution of soot particles in flames. In this regards ?-radical could have a fundamental role, particularly at the onset of soot formation, when gas-phase molecules convert to a condensed-phase material. According to the literature of highly delocalized ?-radicals, two type of dimerization process can be taken into account. These are: a ?-stacking mechanism that occurs by forming multi-electron/multi-center delocalized bonds, thus covalent-like, with shorter intermolecular distances and stronger interactions with respect to van der Waal interactions; a conventional ?-bond formation, with longer distance and weaker strength than regular C-C bond. In this work we use density functional theory (DFT) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify two radicals having different topological structures in term of localization/delocalization of the unpaired electron, and in turn binding energy and optical band gap characteristics of the clustering of two different molecular radicals. The ambition of this study can be represented by the investigation of the two polycyclic aromatic, labeled as radical 1 (R1), and radical 2 (R2), experimentally determined by Atomic Force Microscopy (AFM) studies and calculated by ab-initio quantum investigations. As results the different clustering of the two polycyclic aromatic hydrocarbons with a couple of unpaired electron delocalized, in different spin multiplicity (Sz=1, 3), leads to a different reductions of the optical gap.