Representation of the molecular structure of the polycyclic aromatic hydrocarbon naphthalene

An international team of scientists from Hawaiʻi, Florida and The Netherlands have challenged conventional wisdom by demonstrating that polycyclic aromatic hydrocarbons can be created in the ultra-cold conditions of interstellar space.

Considered key players in astrobiological evolution, polycyclic aromatic hydrocarbons, or PAHs have been found in organic extracts from the Murchison meteorite. Functionalized PAHs can form membrane-like boundary structures—the first indications of a cell type structure, which is requisite to the origin of life, said Ralf Kaiser, University of Hawaiʻi at Mānoa professor of chemistry.

On Earth, polycyclic aromatic hydrocarbons are highly toxic and form readily in high-temperature combustion, such as car engines and burning cigarettes. Depositing easily on leafy vegetables and accumulating in fatty tissues of living organisms, they have been linked to lung cancer, soil contamination, water pollution and food poisoning.

So figuring out how the organic molecules form can advance not only knowledge of how life developed in the first place, but also how to reduce toxic byproducts of hydrocarbon combustion in the future by incorporating these data in combustion models.

Challenging conventional wisdom

Current thinking holds that PAH–producing reactions require high temperatures, such as those present in the outflows of carbon-rich stars and planetary nebulae. However, polycyclic aromatic hydrocarbons in the interstellar medium are rapidly destroyed by photolysis, interstellar shock waves driven by supernova explosions and energetic cosmic rays, more quickly than new material can be injected from carbon-rich sources.

Astrochemist Ralf Kaiser

Thus, the ubiquitous presence of polycyclic aromatic hydrocarbons in the interstellar medium implies a crucial, previously unexplained route to fast chemical growth of the molecules in the cold environment of the interstellar medium, said Kaiser.

To unravel the formation of naphthalene (C10 H8), the simplest polycyclic aromatic hydrocarbon, postdoctoral fellows Dorian Parker, Fangtong Zhang and Seol Kim in Kaiser’s laboratory conducted gas phase crossed molecular beam experiments.

They were able to create naphthalene in a barrier-less and energy-producing reaction between the phenyl radical and vinylacetylene. Mass spectrometer measurements of the reaction products together with isotopic labeling confirmed that the reaction produced naphthalene plus a single hydrogen atom.

Theoretical chemists Alex Landera, Vadim Kislov and Alexander Mebel at Florida International University subsequently merged the experimental results with theoretical work. Mebel’s computations showed that naphthalene is formed from the reaction of a single phenyl radical colliding with vinylacetylene. Moreover, the reaction has no activation energy barrier to prevent it from occurring in the extreme cold of interstellar clouds.

“These findings challenge conventional wisdom that PAH formation only occurs at high temperatures, such as in combustion systems, and implies that low temperature chemistry can initiate the synthesis of the very first polycyclic aromatic hydrocarbons in the interstellar medium,” said co-author Alexander Tielens of Leiden Observatory at the University of Leiden.

Kaiser’s team plans to expand the studies to unravel the formation routes to more complex polycyclic aromatic hydrocarbons, like phenanthrene and anthracene, and to nitrogen-substituted hydrocarbons, such as indole and quinoline.

In cold molecular clouds, barrierless phenyl-type radical reactions could propagate the vinylacetylene-mediated formation of more complex hydrocarbon structures like phenanthrene and anthracene, the team writes.

About the research

Funding for this study was provided by the U.S. Department of Energy, Basic Energy Sciences. Results were published in the January 3 issue of Proceedings of the National Academy of Sciences.

Read the abstract.