High-level ab initio quartic force fields and spectroscopic characterization of C₂N

Area of Science:

• Astrochemistry
• Computational Quantum Chemistry
• Molecular Spectroscopy

Background:

• Interstellar anion chemistry is primarily driven by large carbon chain species and radiative electron attachment (REA).
• The role of smaller molecular anions, where REA is unlikely, remains poorly understood due to limited spectroscopic data.
• Accurate spectroscopic signatures for C2N- are essential for determining its astronomical abundance and role in interstellar chemistry.

Purpose of the Study:

• To provide accurate spectroscopic data for the ground ( l-CCN-) and low-lying (c-CNC-) isomers of C2N- and their isotopologues.
• To enable the modeling and determination of astronomical abundances for C2N-.
• To facilitate the search for C2N- in interstellar medium through astronomical surveys.

Main Methods:

• State-of-the-art rovibrational quantum chemical techniques were employed.
• Quartic force fields were calibrated using a high-level composite energy scheme, including extrapolation to basis set limits and relativistic effects.
• Nuclear motion calculations were performed to derive spectroscopic attributes, including fine and hyperfine interaction constants.

Main Results:

• Accurate spectroscopic constants for both l-CCN- (3Σ-) and c-CNC- (1A1) isomers and their isotopologues were computed.
• Target accuracies of better than 0.1% for rotational constants and 0.3% for vibrational fundamentals relative to experiment were achieved.
• The rotational spectra of both isomers were derived, and their interstellar detectability was explored.

Conclusions:

• The computed spectroscopic data are suitable for direct use in experimental data reduction and astronomical observations.
• The study provides essential data to prompt future astronomical surveys targeting C2N-.
• The c-CNC- (1A1) isomer is theoretically estimated to be approximately 15.3 kcal mol-1 higher in energy than the ground-state l-CCN- (3Σ-) species.