Scientists study the source of major hydrocarbons on Earth and in space

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Naphthalene—a simple polycyclic aromatic hydrocarbon—is formed from two vibrationally stabilized free radicals, propargyl and benzyl. Credit: Ralph I. Kaiser, University of Hawaii at Manoa

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Naphthalene—a simple polycyclic aromatic hydrocarbon—is formed from two vibrationally stabilized free radicals, propargyl and benzyl. Credit: Ralph I. Kaiser, University of Hawaii at Manoa

Polycyclic aromatic hydrocarbons (PAHs) are a class of organic molecules that contain fused rings made of the chemical benzene. Scientists believe that PAHs are responsible for chemical processes that eventually lead to other carbonaceous nanoparticles on Earth and between stars in deep space. On Earth, PAHs are formed due to incomplete combustion of coal, oil, and other materials and are harmful to human health.

PAHs make up about 30% of all carbon in the universe, whether it’s around stars, interstellar clouds or planets. However, scientists do not fully understand the role of reactions involving two free radicals in how PAHs form in extreme environments. Free radicals are molecules with an unpaired electron that is delocalized over at least three atoms. In a study published in the journal Chemical ScienceThe researchers conducted experiments to determine how the PAH-naphthalene prototype would form from reactions occurring in the gas phase of the material.

The results provide fundamental knowledge of the processes involved in the formation of a simple representative of PAHs, naphthalene, a major constituent of moths. The researchers found that this reaction could occur in the gas phase through the reaction of combustion flames and radicals found in space around carbon-rich stars. This provides new fundamental knowledge about the chemistry and carbon balance of our galaxy.

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Polycyclic aromatic hydrocarbons (PAHs) and their derivative soot particles represent unwanted byproducts of fossil fuel combustion processes, but scientists do not have a complete understanding of the underlying mechanisms of their formation. An isomer-selective product detection reveals the reaction of resonantly stabilized benzyl (C).7H7) and Praburgh (C3H3) radicals synthesizes the simplest representative of PAHs—the 10p Hückel aromatic naphthalene (C10H8) molecule.

The gas-phase preparation of naphthalene provides a radical new concept of the reaction of combustion-related propargyl radicals with aromatic radicals bearing the radical center of the methylene moiety (aromatic-CH).2•), previously overlooked as a source of aromatics in high-temperature environments.

This facile propargyl addition-benzeneannulation (PABA) mechanism of propargyl radicals with other aromatic-CH2• Radicals beyond benzyl lead to higher order PAHs such as anthracene and phenanthrene. This finding is a fundamental shift in the concept that PAHs are mainly formed via hydrogen-acetylene addition (HACA) and phenyl addition dehydrocyclization (PAC) pathways in high-temperature combustion systems.

This PABA mechanism provides versatile and diverse routes to three main classes of aromatic hydrocarbons: acenes (PAHs with linearly fused benzene rings), phenazines (PAHs bearing zig-zag structured benzene rings), and helicenes (ortho-condensed PAHs that form chiral molecules with an angular shape). , thus bringing scientists closer to understanding the aromatic universe in which we live.

More information:
Chao He et al, Unconventional gas-phase preparation of prototypical polycyclic aromatic hydrocarbon naphthalene (C10H8) by hydrogen-atom assisted isomerization via reaction of benzyl (C7H7) and propyrgyl (C3H3) radicals, Chemical Science (2023) DOI: 10.1039/D3SC00911D

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Chemical Science

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