RESEARCH HIGHLIGHTS Nature Reviews Chemistry | Published online 31 Jan 2018; doi:10.1038/s41570-018-0113 SPECTROSCOPY Aromatic astrochemistry We know that polycyclic aromatic vibrational spectra of hundreds of hydrocarbons (PAH) are out in space, potential interstellar molecules, either but we do not know their exact struc- measured or predicted, have been tures or origins. A big step towards collated. “Sergei Kalenskii — a co-­ structure elucidation comes with the author of the present study — started first detection of rotational features analysing archival data from TMC-1 of an aromatic compound bearing a using a clever trick to see if there was six-membered ring in the interstellar signal from large, undetected mole- medium. Indeed, a group led by Brett cules hiding under the noise signal,” McGuire have reported in Science an explains McGuire. “He chose to look Rachael Tremlett/Macmillan Publishers Limited assignment of spectroscopic data to for aromatic molecules because they signatures of benzonitrile (C6H5CN) have long fascinated astronomers and — a possible precursor of the PAHs. astrochemists, but have eluded nearly for these molecules. For C6H5CN Vibrational and rotational spec- all our attempts at detection so far”. in particular, the neutral-neutral troscopies are common techniques The group selected 12 small aromatic reaction between CN and C6H6 was Detection of used in most labs to study molecular molecules — including cyclopenta­ included as a possible path. These benzonitrile structure and dynamics. The same diene, furan, pyrrole, phenol, fluorene models appear to be in good agree- in a molecular techniques are used to study the and C6H5CN — and in each case ment with experimental data. Also, properties of molecules present in added all the spectra that contained certain unassigned infrared bands cloud means the interstellar medium, although positive intensities corresponding are attributable to C6H5CN and as a that we the complexity of these spectra to known signals of that particular result the group hypothesize that it is can now makes their a priori interpretation molecule. The signal-to-noise ratio a plausible precursor to PAHs in the difficult. Most of the emission bands should increase if a particular spike interstellar medium. investigate in the mid-infrared region arise from corresponds to a specific vibrational “We have detected C6H5CN, aromatic molecular vibrations, but many have or rotational transition. Among the the largest molecule ever seen by chemistry yet to be attributed to specific mole- 12 molecules analysed, only C H CN radioastronomy in terms of number 6 5 at the cules. However, emissions have been exhibited an enhancement of the of atoms, and the first ever seen observed at energies comparable to signal-to-noise ratio in the summed with a six-membered aromatic furthest point the vibrational transitions of aromatic spectra. After seeing the signal-to- ring. Detection of this molecule in a back in its C–C and C–H bonds — an indica- noise enhancement in the archival molecular cloud representing one of evolutionary tion that some of these unassigned data, the group of scientists obtained the earliest stages of star and planet infraredbands might belong to the a completely new and vastly superior formation means that we can now history family of PAHs. Yet, the structures set of high resolution measurements investigate aromatic chemistry at the of these PAHs and how they came to of specific rotational transitions of furthest point back in its evolutionary be in space remain open questions C6H5CN. Eight of the nine rotational history,” points out McGuire. “This to astrochemists. Searching for an transitions observed were in agree- detection, in combination with theo- answer, McGuire and co-workers­ ment with predictions, thus confirm- retical work and modelling, indicates explored an alternative approach ing the presence of C6H5CN in the that we can use C6H5CN as a reliable by looking for possible precursors interstellar medium. tag for C6H6, which is invisible to of large hydrocabons in the Taurus The obvious next question is: how radioastronomy,” he concludes. This Molecular Cloud 1 (TMC-1), a dense does C6H5CN form in space? The capability opens up opportunities to area in space in which stars will group led by McGuire modelled the investigate the historical evolution of eventually form. TMC-1 is of par- temporal evolution of the column aromatic molecules. ticular interest because it is known density — the amount of material per Gabriella Graziano to contain unsaturated hydrocarbons unit area in a cloud in space — for such as cyanopolyynes. C6H5CN and other organics such as ORIGINAL ARTICLE McGuire, B. A. et al. The characterization of the inter- benzene and cyanopolyynes. This Detection of the aromatic molecule benzonitrile (c-C H CN) in the interstellar medium. Science stellar medium is not a new scientific model takes into account most of the 6 5 359, 202–205 (2018) endeavour and both rotational and known possible formation pathways NATURE REVIEWS | CHEMISTRY www.nature.com/natrevchem ©2018 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. .