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.
Highlights
Soot emitted into the atmosphere contributes to global warming, and when deposited on ice, soot lowers ice’s albedo, increasing melting.[1]
Recent images of the aromatic molecules in flames have revealed these partially saturated pentagonal edges on aromatic molecules.[20,21]. We demonstrated that these partially saturated edges form highly localized π-radicals,[12,22,24] which were initially hypothesized by Abrahamson in 1977.16 Spectroscopic measurements of these radicals have been undertaken in the astrochemistry community, with their fluorescence suggested to be involved in the unidentified emission from the Red Rectangle proto-planetary nebula.[25]
While these states are resonantly stabilized, with spin density shared among multiple aromatic carbons, they are localized to the edge, maintaining their spin density and high reactivity with enlargement
Summary
Soot emitted into the atmosphere contributes to global warming, and when deposited on ice, soot lowers ice’s albedo, increasing melting.[1]. Quantum molecular dynamics simulations reveal rapid reactions between diradical soot precursors, enabled by internal rotors
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