Abstract
Polycyclic aromatic hydrocarbons (PAHs) have been invoked in fundamental molecular mass growth processes in our galaxy. We provide compelling evidence of the formation of the very first ringed aromatic and building block of PAHs-benzene-via the self-recombination of two resonantly stabilized propargyl (C3H3) radicals in dilute environments using isomer-selective synchrotron-based mass spectrometry coupled to theoretical calculations. Along with benzene, three other structural isomers (1,5-hexadiyne, fulvene, and 2-ethynyl-1,3-butadiene) and o-benzyne are detected, and their branching ratios are quantified experimentally and verified with the aid of computational fluid dynamics and kinetic simulations. These results uncover molecular growth pathways not only in interstellar, circumstellar, and solar systems environments but also in combustion systems, which help us gain a better understanding of the hydrocarbon chemistry of our universe.
Highlights
The benzene molecule [C6H6 [1]] isolated 175 years ago by Hofmann [1] has been recognized as the fundamental molecular building block of polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of multiple fused benzenoid rings (Fig. 1) [2]
The recombination of two propargyl radicals leading to benzene formation has been predicted theoretically (Supplementary Materials) [21,22,23,24,25,26,27,28,29,30,31]; there is no experimental verification of the self-recombination of resonantly stabilized free radicals (RSFRs) and the direct in situ detection of benzene along with its isomers limited to the very first aromatic ring (Supplementary Materials; table S1)
We provide compelling evidence of the very first in situ detection of the benzene molecule [C6H6 [1]] along with three structural isomers [1,5-hexadiyne [2], fulvene [3], and 2-ethynyl-1,3-butadiene [4]] (Fig. 1) formed via the barrierless self-recombination of the propargyl radical (C3H3) at conditions mimicking combustion and circumstellar environments using a high-temperature chemical reactor coupled with isomer-specific detection using tunable vacuum ultraviolet (VUV) light
Summary
The benzene molecule [C6H6 [1]] isolated 175 years ago by Hofmann [1] has been recognized as the fundamental molecular building block of polycyclic aromatic hydrocarbons (PAHs)—organic molecules composed of multiple fused benzenoid rings (Fig. 1) [2]. The detailed analysis of the PIE curves resulted in the identification of distinct molecules [benzene [1], 1,5-hexadiyne [2], fulvene [3], 2-ethynyl-1,3-butadiene [4], C6H6; o-benzyne, C6H4 [8]] along with atomic hydrogen abstraction/recombination products of the propargyl radical, i.e., methylacetylene (CH3CCH) and allene (H2CCCH2), Intensity
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