π-Diradical Aromatic Soot Precursors in Flames

Abstract

Soot emitted from incomplete combustion of hydrocarbon fuels contributes to global warming and causes human disease. The mechanism by which soot nanoparticles form within hydrocarbon flames is still an unsolved problem in combustion science. Mechanisms proposed to date involving purely chemical growth are limited by slow reaction rates, whereas mechanisms relying on solely physical interactions between molecules are limited by weak intermolecular interactions that are unstable at flame temperatures. Here, we show evidence for a reactive π-diradical aromatic soot precursor imaged using non-contact atomic force microscopy. Localization of π-electrons on non-hexagonal rings was found to allow for Kekulé aromatic soot precursors to possess a triplet diradical ground state. Barrierless chain reactions are shown between these reactive sites, which provide thermally stable aromatic rim-linked hydrocarbons under flame conditions. Quantum molecular dynamics simulations demonstrate physical condensation of aromatics that survive for tens of picoseconds. Bound internal rotors then enable the reactive sites to find each other and become chemically cross-linked before dissociation. These species provide a rapid, thermally stable chain reaction toward soot nanoparticle formation and could provide molecular targets for limiting the emission of these toxic combustion products.

Figure 1

Aromatic soot precursor collection: Burner configuration, collection location, and schematic of soot formation.

Figure 2

Imaging, electronic structure, and reactions of aromatic radicals. (a, d) HR-AFM images of aromatic soot precursor species (Laplace-filtered sharing the same scale shown in (a)). (b, e) Spin density surfaces (iso = 0.025 au) for the electronic ground state. (c, f, g) The dominant Kekulé structures are also shown. (h, k) The bond energy E_b in kJ/mol is shown for each cross-link. (i, l) The geometries of one such cross-link. (j, m) Reaction mechanisms for the doublet monoradical and the triplet diradical with the later allowing subsequent polymerization with no loss in reactivity.

Figure 3

Radical recombination of aromatic radicals. (a, b) Formation of diradicals from hydrogen abstraction of species imaged with nc-AFM from Commodo et al. (c) Fraction of effective reactive collisions determined using QM/MM simulations for (d–f) σ-monoradicals and σ-diradicals with filled and open triangle symbols, respectively. (g–j) π-Monoradicals filled square symbols. (k–m) π-Diradicals open square symbols derived from HR-AFM structures. (n) A single reactive trajectory is shown for the diradical in (m). (o) Distances between the centers of mass (d_C.O.M.) and the reactive sites that bond, d₁₁, are plotted. (o–q) Insets show the geometries of the (o) species approaching, (p) stacking unbonded, and (q) stacking and bonding. (r) Angle, θ₁₁, between the vectors, r₁₁, from the center of mass to the reactive sites (see o) shows the rotation preceding the bond formation. (s) Inset shows a different orientation of the (p) geometry with the direction of rotation in the plane of the aromatic highlighted.