Although the existence and importance of negative molecular ions had been conjectured in the early days of astrochemistry, it was not until 2006 that the first interstellar anion, C₆H⁻, was finally detected. This led to a resurgence of interest of chemists, physicists, and astrophysicists in anions, motivating new surveys as well as theoretical and laboratory studies. As a result, several other negatively charged species were soon identified like C₄H⁻, C₈H⁻, C₃N⁻, C₅N⁻ and CN⁻ [1]. Assuming electron radiative attachment (REA) as their major formation route, previous anion astrochemical models have been successful in reproducing the observed abundances of the larger, highly-dipolar carbon-chain anions like C₈H⁻, C₆H⁻, and C₅N⁻. However, for the smallest anionic species (e.g., CN⁻ and C₃N⁻) for which REA to their parent neutrals are theorized to very slow, notable discrepancies have soon appeared between the modeled and observed anion-to-neutral ratios [1], suggesting that other alternative pathways might dominate their synthesis [2]. Recent laboratory studies by Chacko et al. [3] pointed out the dominance of the (as yet unobserved) C₂N^- species as fragmentation product of larger carbonitrile anions in UV-abundant circumstellar media, thereby offering new prospects into its omnipresence in the external layers of the carbon-rich star IRC+10216. Motivated by these most recent findings and the general lack of spectral signatures of this anion, in this talk, I will discuss our most recent efforts to obtain accurate rovibrational spectroscopic constants and anharmonic vibrational frequencies for l-C₂N^-(³Σ^-) and c-CNC(¹A₁) by means of a high-level theoretical approach. Special attention will be paid into the characterization and computation of their quartic force fields (QFFs) [4,5] using state-of-the-art electronic structure composite methods followed by nuclear motion calculations. It is then expected that the new spectroscopic data here reported prompt future high-resolution laboratory and observational studies on this target molecular anion.

[1] Tomas J. Millar, Catherine Walsh, Thomas A. Field, Chemical Reviews, 2017, 117, 1765-1795.

[2] Simon Petrie, Monthly Notices of the Royal Astronomical Society, 1996, 281, 137-144.

[3] Roby Chacko, Shreyak Banhatti, M. Nrisimhamurty, J. K. Yadav, A. K. Gupta, G. Aravind, The Astrophysical Journal, 2020, 905, 90-95.

[4] Attila G. Császár, WIREs Computational Molecular Science, 2012, 2, 273-289.

[5] Ryan C. Fortenberry, Timothy J. Lee, Xinchuan Huang, Physical Chemistry Chemical Physics,

2017, 19, 22860-22869.