  Much like six-membered rings, five-membered rings are ubiquitous in organic
chemistry, frequently serving as the building blocks for larger molecules,
including many of biochemical importance. From a combination of laboratory
rotational spectroscopy and a sensitive spectral line survey in the radio band
toward the starless cloud core TMC-1, we report the astronomical detection of
1-cyano-1,3-cyclopentadiene, $c$-C$_5$H$_5$CN}, a highly polar, cyano
derivative of cyclopentadiene, $c$-C$_5$H$_6$. The derived abundance of
$c$-C$_5$H$_5$CN} is far greater than predicted from astrochemical models which
well reproduce the abundance of many carbon chains. This finding implies either
an important production mechanism or a large reservoir of aromatic material may
need to be considered. The apparent absence of its closely-related isomer,
2-cyano-1,3-cyclopentadiene, may arise from its lower stability or be
indicative of a more selective pathway for formation of the 1-cyano isomer,
perhaps one starting from acyclic precursors. The absence of N-heterocycles
such as pyrrole and pyridine is discussed in light of the astronomical finding.
