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Resonance-stabilized hydrocarbon-radical chain reactions may explain soot inception and growth

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Science  07 Sep 2018:
Vol. 361, Issue 6406, pp. 997-1000
DOI: 10.1126/science.aat3417
  • Fig. 1 Schematic overview of the clustering of hydrocarbons by radical-chain reactions (CHRCR) mechanism.

    (A) The CHRCR mechanism is initiated and propagated by resonantly stabilized radicals sequentially generated through radical-chain reactions involving acetylene or vinyl. (B) To form an incipient particle, these RSRs can cluster a wide range of hydrocarbons, including radicals, stable PAHs, and unsaturated aliphatic species, through radical-chain reactions fueled by loss and gain of extended conjugation. (C) Cyclopentadienyl-type moieties on cluster surfaces are posited to further propagate growth via the CHRCR mechanism.

  • Fig. 2 VUV-AMS spectrum demonstrating a sequence of radicals.

    This VUV-AMS spectrum was recorded on a sample extracted from an atmospheric-pressure laminar premixed ethylene-oxygen flame using a photon energy of 9.4 eV. Masses are indicated for species posited to drive the CHRCR mechanism. Our best estimates of the predominant structures are shown. Four hypothesized isomers are shown for mass 165 u. There are many more potential isomers for higher radical masses not shown.

  • Fig. 3 Representative potential energy surface for hydrocarbon clustering.

    The potential energy diagram shows formation of an indenyl-phenyl σ-dimer and subsequent H abstraction, which stabilizes the bond and regenerates an RSR whose extended conjugation requires an unpaired electron (see supplementary materials for details on the calculations). These reactions are examples of possible steps in the hydrocarbon-clustering scheme proposed. Phenyl represents an arbitrary PAH radical; the cluster is shown for illustrative purposes. The y axis displays differences in the zero-point–corrected electronic energies. Abstraction steps, energy changes, and barriers are evaluated with R=H. The total number of H atoms was identical for all of the calculated energies, but, in the figure, species involved during the H-abstraction reactions are only shown when active.

Supplementary Materials

  • Resonance-stabilized hydrocarbon-radical chain reactions may explain soot inception and growth

    K. O. Johansson, M. P. Head-Gordon, P. E. Schrader, K. R. Wilson, H. A. Michelsen

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S11
    • References
    • Caption for Data File S1
    Data File S1
    Tabulated data for VUV-AMS spectra in Fig. 2 and Figs. S1-S3.

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