High-Level Ab Initio Quartic Force Fields and Spectroscopic Characterization of C2N−
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− . 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 , suggesting that other alternative pathways might dominate their synthesis . Recent laboratory studies by Chacko et al.  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.
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