A computational and spectroscopic study of MgCCH (X²⁺): towards characterizing MgCCH⁺

New computational and experimental studies have been carried out for the MgCCH radical in its X-2 Sigma(+) 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 B-0 = 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.