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

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.