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.
