2008A&A...486..493L

An unexpectedly complex polyatomic chemistry exists in diffuse clouds, allowing detection of species such as C₂H, C₃H₂, H₂CO, and NH₃, which have relative abundances that are strikingly similar to those inferred toward the dark cloud TMC-1. We probe the limits of complexity of diffuse cloud polyatomic chemistry. We used the IRAM Plateau de Bure Interferometer to search for galactic absorption from low-lying J=2-1 rotational transitions of A- and E-CH₃OH near 96.740GHz and used the VLA to search for the J=8-7 transition of HC₅N at 21.3GHz. Neither CH₃OH nor HC₅N were detected at column densities well below those of all polyatomics known in diffuse clouds and somewhat below the levels expected from comparison with TMC-1. The HCN/HC₅N ratio is at least 3-10 times higher in diffuse gas than toward TMC-1.