Astronomically aromatic: Organics in space

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  • Published: Jan 15, 2018
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Astronomically aromatic: Organics in space

Interstellar

The aromatic molecule benzonitrile was detected by the GBT in the Taurus Molecular Cloud 1 (TMC-1). Credit: B. McGuire, B. Saxton (NRAO/AUI/NSF)

Astronomers using information from the Green Bank Telescope and a lot of chemical nous have made the first definitive interstellar detection of benzonitrile, an aromatic molecule that could provide a missing link between simpler organic compounds and polycyclic aromatic hydrocarbons.

Astronomers were puzzled. They had been for the best part of three decades if not longer. Wherever they looked from the depths of the Milky Way to distant galaxies they could detect a curious infrared glow. This faint cosmic aura is punctuated with a series of spectral lines with no readily identifiable chemical source. The spectra seemed not to coincide with the presence of any known cosmic features, such as giant interstellar clouds, star-forming regions, or even the afterglow of exploded stars, supernovae remnants. It was everywhere and everywhere rather baffling.

Many theories have been posited as to the origins of the spectroscopic cosmos included the presence of immensely long chains of carbon atoms and at one time the famous soccer ball molecule, buckminsterfullerene and its chemical cousins. However, the most likely culprit chemists believed was polycyclic aromatic hydrocarbons (PAHs), which would later be shown to be plentiful in space. Indeed, almost 10 percent of all the carbon in the universe is apparently tied up in PAHs so it was increasingly obvious that PAHs must be the source of the infrared spectral lines. There is a but, of course. Even though, as a collective, PAHs seem to solve this cosmic mystery, astrochemists are yet to detect even one of the hundreds of PAH molecules that can exist, anywhere in interstellar space.

Simply aromatic

Benzonitrile is a simple aromatic compound. But, it is the largest molecule ever recorded using radio astronomy. Indeed, it is the first molecule with a benzene ring detected using radio astronomy. Aromatic rings are commonplace on Earth, present in thousands if not millions of different organic compounds and organometallic species, as well as countless biological molecules.

Now, new information from the US National Science Foundation’s Green Bank Telescope (GBT) in West Virginia have shown, for the first time, not PAHs, but convincing radio frequency fingerprints of their chemical precursor , the molecule benzonitrile. These molecular observations hint at proof of the presence of PAHs in space. If benzonitrile is there, then PAHs will have almost certainly formed. This could be the penultimate piece of the puzzle to explain the astronomical infrared spectra.

Chemist Brett McGuire of the National Radio Astronomy Observatory (NRAO) in Charlottesville, Virginia, and colleagues offered their results recently at the 231st meeting of the American Astronomical Society (AAS) in Washington DC, and have published full details in the journal Science. McGuire and colleagues detected the characteristic radio signals for benzonitrile coming from a nearby star-forming nebula known as the Taurus Molecular Cloud 1 (TCM-1), which is about 430 light-years from Earth. The unique structure of benzonitrile allowed the team to tease out the distinctive radio signature from the background. Moreover, to produce a clear radio fingerprint, molecules must be somewhat asymmetrical. Molecules with more uniform structures, like many PAHs, can have very weak signatures or no signature at all, the team suggests.

Tuning in to chemistry

"These new radio observations have given us more insights than infrared observations can provide," explains McGuire. "Though we haven't yet observed polycyclic aromatic hydrocarbons directly, we understand their chemistry quite well. We can now follow the chemical breadcrumbs from simple molecules like benzonitrile to these larger PAHs."

"The evidence that the GBT allowed us to amass for this detection is incredible," adds McGuire. "As we look for yet larger and more interesting molecules, we will need the sensitivity of the GBT, which has unique capabilities as a cosmic molecule detector." In their Science paper, the team concludes that: "The identification of benzonitrile sheds light on the composition of aromatic material within the interstellar medium—material that will eventually be incorporated into new stars and planets."

Related Links

Science 2018, 359, 202-205: "Detection of the Aromatic Molecule Benzonitrile (c-C6H5CN) in the Interstellar Medium"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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