Interstellar Detection of 2-cyanocyclopentadiene, C₅H₅CN, a Second Five-membered Ring toward TMC-1

Abstract Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used combination stacking, Markov Chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, low-lying isomer 1-cyanocyclopentadiene, which was recently discovered there by same methods. The new observational data also yield considerably improved detection significance more stable several individual transitions between 23–32 GHz. Through our MCMC analysis, derive cospatial, total column densities 8.3 × 10 11 1.9 cm −2 1- respectively, corresponding ratio ∼4.4 favoring former. derived abundance ratios point common formation pathway—most likely being cyanation cyclopentadiene analogy benzonitrile.