On the Mechanism of Soot Nucleation. III. The Fate and Facility of the E-Bridge

Abstract

Rotationally excited dimerization of aromatic moieties is a mechanism proposed recently to explain the initial steps of soot particle inception in combustion and pyrolysis of hydrocarbons. The product of such dimerization, termed E-bridge, is an angled molecular structure composed of two aromatic rings sharing a common bond. The present study explores the immediate fate of the E-bridge. The performed theoretical analysis indicates that abstraction of a bridge H atom by a gaseous H leads to a rapid transformation of the angled to planar structure. The implications of this result is that the collisionally activated E-bridge formation followed by its flattening effectively increases the size of "planar" aromatic precursors by combining two aromatic moieties with essentially collisional rates, instead of a slower "atom-by-atom" buildup. The faster growth speeds up PAH reaching a size when physical dimerization takes over. The dimerization can be further assisted by the biradicaloid valence structure of the flattened E-bridge.

Acceleration of a Chemical Reaction due to Nonequilibrium Collisional Dynamics: Dimerization of Polyaromatics

“…A recent critical review of several alternative proposals for the inception mechanism allowed us to center on one of the hypotheses for the formation of chemically bonded dimers that is able to reproduce the rate of nucleationthe E-bridge mechanism, ,, shown in Figure for the case of pyrenyl + acepyrene reaction. , In this mechanism, the PAH-like moieties get linked via two five-membered rings (or via a five- and a six-membered rings) located on the molecular edge, so that two PAH monomers are connected by more than one covalent bond. The E-bridge binding mechanism overcomes the entropic penalty for the mass flow into the chemically bonded dimer, and the dimerized product (P1) survives long enough in the flame to participate in the next step of growth .…”

The Accuracy of Semi-Empirical Quantum Chemistry Methods on Soot Formation Simulation

“…The 3 H-migration and 3 C−C formation reactions were considered in this section because of their important roles in the formation and growth of soot. These reaction pathways were adopted from a previous study [ 47 , 48 , 49 ], where IRC calculations were performed at M06-2x/def2TZVPP level. The reactant, product, and transition state structures were connected by the IRC paths.…”

“…Three isomerization pathways of open-shell E-bridge PAH precursors are illustrated in Figure 4 d–f [ 49 , 50 ]. The relevant energies between the reactant and product of pathway (d) are well predicted by all the seven SE methods.…”

Phenalenyl growth reactions and implications for prenucleation chemistry of aromatics in flames

“…The reaction product, termed as the E-bridge, is formed as an angled PAH dimer but rapidly flattens out through H abstraction. 88 The other proposition pursues the premise of PAH p-dimerization. 12,18,23,[89][90][91][92][93] It has been established, 15,16,22,90,94 though, that p-monoradicals, such as phenalenyl, preferentially form edge s-bonded dimers rather than p-p stacked ones, both bonds being relatively weak.…”