By Christine Joblin1 and José Cerni

By Christine Joblin1 and José Cernicharo2 Interstellar clouds are sites of active organic chemistry (1). Many small, gasphase molecules are found in the dark parts of the clouds that are protected from ultraviolet (UV) photons, but these molecules photodissociate in the external layers of the cloud that are exposed to stellar radiation (see the photo). These irradiated regions are populated by large polycyclic aromatic hydrocarbons (PAHs) with characteristic infrared (IR) emission features. These large aromatics are expected to form from benzene (C6H6), which is, however, difficult to detect because it does not have a permanent dipole moment and can only be detected via its IR absorption transitions against a strong background source (2). On page 202 of this issue, McGuire et al. (3) report the detection of benzonitrile (c-C6H5CN) with radio telescopes. Benzonitrile likely forms in the reaction of CN with benzene; from its observation, it is therefore possible to estimate the abundance of benzene itself.Chemical models that include molecular formation and destruction processes, both in the gas phase and at the surface of dust grains, can account reasonably well for the observed abundances of a number of molecular species (4). The situation is different for large aromatics, for which no individual species have been identified, with the exception of the C60 molecule (5). Although PAHs are large molecules, they are considered by astronomers as very small dust particles. They are therefore generally thought to form in the dense and hot environments of the envelopes of evolved stars. Chemical models have been developed that are based on chemical networks in flames (6). More recently, the possibility to form PAHs at the very low temperatures of molecular clouds has been discussed (7) following the demonstration that the reaction CCH  +  C4H