Interstellar Detection of 2-Cyanocyclopentadiene, C$_5$H$_5$CN, a Second Five-Membered Ring Toward TMC-1

Using radio observations with the Green Bank Telescope, evidence has now been found for a second five-membered ring in the dense cloud Taurus Molecular Cloud-1 (TMC-1). Based on additional observations of an ongoing, large-scale, high-sensitivity spectral line survey (GOTHAM) at centimeter wavelengths toward this source, we have used a combination of spectral stacking, Markov chain Monte Carlo (MCMC), and matched filtering techniques to detect 2-cyanocyclopentadiene, a low-lying isomer of 1-cyanocyclopentadiene, which was recently discovered there by the same methods. The new observational data also yields a considerably improved detection significance for the more stable isomer and evidence for several individual transitions between 23 - 32 GHz. Through our MCMC analysis, we derive total column densities of $8.3\times10^{11}$ and $1.9\times10^{11}$ cm$^{-2}$ for 1- and 2-cyanocyclopentadiene respectively, corresponding to a ratio of 4.4(6) favoring the former. The derived abundance ratios point towards a common formation pathway - most likely being cyanation of cyclopentadiene by analogy to benzonitrile.