High-level ab initio quartic force fields and spectroscopic
  characterization of C$_{2}$N$^{-}$
  While it is now well established that large carbon chain species and
radiative electron attachment (REA) are key ingredients triggering interstellar
anion chemistry, the role played by smaller molecular anions, for which REA
appears to be an unlikely formation pathway, is as yet elusive. Advancing this
research undoubtedly requires the knowledge of their astronomical abundances
which, for the case of C$_{2}$N$^{-}$, is largely hindered by a lack of
accurate spectroscopic signatures. In this work, we provide such data for both
ground $\ell$-CCN$^{-}$($^{3}\Sigma^{-}$) and low-lying
$c$-CNC$^{-}$($^{1}A_{1}$) isomers by means of state-of-the-art rovibrational
quantum chemical techniques. Their quartic force fields are herein calibrated
using a high-level composite energy scheme that accounts for extrapolations to
both one-particle and (approximate) ${N}$-particle basis set limits, in
addition to relativistic effects, with the final forms being subsequently
subject to nuclear motion calculations. Besides standard spectroscopic
attributes, the full set of computed properties includes fine and hyperfine
interaction constants and can be readily introduced as guesses in conventional
experimental data reduction analyses through effective Hamiltonians. On the
basis of benchmark calculations, the target accuracies are determined to be
better than 0.1% of experiment for rotational constants and 0.3% for
vibrational fundamentals. Apart from laboratory investigations, the results
here presented are expected to also prompt future astronomical surveys on
C$_{2}$N$^{-}$. Using the theoretically-predicted spectroscopic constants, the
rotational spectra of both $\ell$-CCN$^{-}$($^{3}\Sigma^{-}$) and
$c$-CNC$^{-}$($^{1}A_{1}$) are also derived and their likely detectability in
the interstellar medium is further explored in connection with working
frequency ranges of powerful astronomical facilities.
