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While it is now well established that large carbon chain species andradiative electron attachment (REA) are key ingredients triggering interstellaranion chemistry, the role played by smaller molecular anions, for which REAappears to be an unlikely formation pathway, is as yet elusive. Advancing thisresearch undoubtedly requires the knowledge of their astronomical abundanceswhich, for the case of C$_{2}$N$^{-}$, is largely hindered by a lack ofaccurate spectroscopic signatures. In this work, we provide such data for bothground $\ell$-CCN$^{-}$($^{3}\Sigma^{-}$) and low-lying$c$-CNC$^{-}$($^{1}A_{1}$) isomers by means of state-of-the-art rovibrationalquantum chemical techniques. Their quartic force fields are herein calibratedusing a high-level composite energy scheme that accounts for extrapolations toboth one-particle and (approximate) ${N}$-particle basis set limits, inaddition to relativistic effects, with the final forms being subsequentlysubject to nuclear motion calculations. Besides standard spectroscopicattributes, the full set of computed properties includes fine and hyperfineinteraction constants and can be readily introduced as guesses in conventionalexperimental data reduction analyses through effective Hamiltonians. On thebasis of benchmark calculations, the target accuracies are determined to bebetter than 0.1% of experiment for rotational constants and 0.3% forvibrational fundamentals. Apart from laboratory investigations, the resultshere presented are expected to also prompt future astronomical surveys onC$_{2}$N$^{-}$. Using the theoretically-predicted spectroscopic constants, therotational spectra of both $\ell$-CCN$^{-}$($^{3}\Sigma^{-}$) and$c$-CNC$^{-}$($^{1}A_{1}$) are also derived and their likely detectability inthe interstellar medium is further explored in connection with workingfrequency ranges of powerful astronomical facilities.

High-level ab initio quartic force fields and spectroscopic characterization of C$_{2}$N$^{-}$