Aromatics and Cyclic Molecules in Molecular Clouds: A New Dimension of
  Interstellar Organic Chemistry
  Astrochemistry lies at the nexus of astronomy, chemistry, and molecular
physics. On the basis of precise laboratory data, a rich collection of more
than 200 familiar and exotic molecules have been identified in the interstellar
medium, the vast majority by their unique rotational fingerprint. Despite this
large body of work, there is scant evidence in the radio band for the basic
building blocks of chemistry on earth -- five and six-membered rings -- despite
long standing and sustained efforts during the past 50 years. In contrast, a
peculiar structural motif, highly unsaturated carbon in a chain-like
arrangement, is instead quite common in space. The recent astronomical
detection of cyanobenzene, the simplest aromatic nitrile, in the dark molecular
cloud TMC-1, and soon afterwards in additional pre-stellar, and possibly
protostellar sources, establishes that aromatic chemistry is likely widespread
in the earliest stages of star formation. The subsequent discovery of
cyanocyclopentadienes and even cyanonapthlenes in TMC-1 provides further
evidence that organic molecules of considerable complexity are readily
synthesized in regions with high visual extinction but where the low
temperature and pressure are remarkably low. This review focuses on laboratory
efforts now underway to understand the rich transition region between linear
and planar carbon structures using microwave spectroscopy. We present key
features, advantages, and disadvantages of current detection methods, a
discussion of the types of molecules found in space and in the laboratory, and
approaches under development to identify entirely new species in complex
mixtures. Studies focusing on the cyanation of hydrocarbons and the formation
of benzene from acyclic precursors are highlighted, as is the role that
isotopic studies might play in elucidating the chemical pathways to ring
formation.
