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.