Theoretical spectroscopic characterization of small interstellar anions: C2N- at the focal point

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. In the forthcoming years, several other anionic species have been soon identified in space such as C₄H⁻, C₈H⁻, C₃N⁻, C₅N⁻ and CN⁻. Assuming electron radiative attachment (REA) as their major formation route, chemical models have been successful in reproducing the observed abundances of the larger carbon-chain anions like C₈H⁻, C₆H⁻, and C₅N⁻. However, for the smallest 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. Recent laboratory studies 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 spectroscopic constants for l-C₂N^-(³Σ^-) and c-CNC(¹A₁) by means of a high-level theoretical approach. Special attention will be paid into the computation of their quartic force fields (QFFs) 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.