Parallel Session: Theory, Contributed Talk (15min)

High-Level Ab Initio Quartic Force Fields and Spectroscopic Characterization of C2N

C. M. Rocha1, H. Linnartz1*
1Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden, The Netherlands

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.