A proposed mechanism for soot nucleation, CHRCR (clustering of hydrocarbons by radical chain reactions), suggests that aromatic hydrocarbons can grow quickly by radical-radical chain reactions. In these reactions, two radicals associate but lose an H atom in a well-skipping reaction to form a larger radical. We have observed three such reactions in a controlled pyrolysis environment using time-of-flight mass spectrometry: phenyl + propargyl, phenyl + cyclopentadienyl, and phenyl + benzyl. In each case, we observe the closed-shell adduct as well as the radical product of the well-skipping H loss. In the first reaction, our results are consistent with calculations that predict mostly indenyl, although we cannot distinguish phenylpropargyl radicals from indenyl radicals. For the second reaction, we record the observation of 5-phenylcyclopentadiene and 5-phenylcyclopentadienyl radical, and we investigate the ring-opening reactions of the radical. In the final reaction, a second phenyl addition to the diphenylmethyl radical yields triphenylmethyl radical, propagating the chain reaction. These radical well-skipping reactions are most likely to occur at high temperatures in radical rich environments, such as fuel-rich flames, or in very low-pressure environments such as interstellar molecular clouds. In both environments, radical-radical chain reactions could potentially help close the gap between observations and chemical models.

Gas phase radical-radical chain reactions: Hydrocarbon growth without requiring reactivation

ACS Spring 2022

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