A computational and spectroscopic study of MgCCH (X²Σ⁺): towards characterizing MgCCH⁺

ABSTRACT

New computational and experimental studies have been carried out for the MgCCH radical in its X²Σ⁺ state. Coupled cluster theory [CCSD(T)], was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for MgCCH and its cation. From the quartic force fields, higher-order spectroscopic properties, including rotational constants, were obtained. In tandem, the five lowest energy rotational transitions for MgCCH, N = 1→0 through N = 5→4, were measured for the first time using Fourier transform microwave/millimetre wave methods in the frequency range 9–50 GHz. The radical was created in the Discharge Assisted Laser Ablation Source (DALAS) developed in the Ziurys group. A combined fit of these data with previous millimetre direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within −1.51 to +1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce rovibrational spectroscopic constants for MgCCH⁺, including a rotational constant of B₀ = 5354.5–5359.5 MHz. These new predictions will further the experimental study of MgCCH⁺, and aid in the low-temperature characterisation of MgCCH in the interstellar medium.

GRAPHICAL ABSTRACT

KEYWORDS:

• Magnesium free radicals
• rotational spectroscopy
• quartic force fields
• astrochemistry
• computational chemistry

Acknowledgements

The authors would like to express their heartfelt gratitude to Tim Lee, whose loss is felt deeply. Tim and LMZ were contemporaries during graduate school at University of California-Berkeley. Tim was RCF’s postdoc advisor, and a friend and mentor to NJD. He will be missed, and his insights directly helped to push this research project forward.

RCF acknowledges funding from NASA Grant NNH22ZHA004C, NSF Grant OIA-1757220, and the College of Liberal Arts from the University of Mississippi. LMZ and NJD acknowledge funding from NSF Grant CHE-2154121. JEB was partially supported by a NASA TN Space Grant Fellowship. The authors also thank Prof. José Cernicharo (Instituto de Física Fundamental-CSIC) for providing a table of fitted rotational line values, where previously we had derived the values from visual inspection of figures in reference [5].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by National Aeronautics and Space Administration [grant number NNH22ZHA004C]; National Science Foundation [grant number CHE-2154121]; Office of Integrative Activities [grant number OIA-1757220].