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New computational and experimental studies have been carried out for the MgCCH radical in its X2Σ+ state. Coupled cluster theory with single, double, and perturbative triples, CCSD(T), was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for this molecule. 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/millimeter 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 millimeter direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within 1.51-1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce accurate 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 characterization of MgCCH, detected towards the circumstellar shell of IRC+10216, a carbon-rich star. 

In this computational study, we describe a self-consistent trajectory simulation approach to capture the effect of neutral gas pressure on ion–ion mutual neutralization (MN) reactions. The electron transfer probability estimated using Landau–Zener (LZ) transition state theory is incorporated into classical trajectory simulations to elicit predictions of MN cross sections in vacuum and rate constants at finite neutral gas pressures. Electronic structure calculations with multireference configuration interaction and large correlation consistent basis sets are used to derive inputs to the LZ theory. The key advance of our trajectory simulation approach is the inclusion of the effect of ion-neutral interactions on MN using a Langevin representation of the effect of background gas on ion transport. For H+ − H− and Li+ − H(D)−, our approach quantitatively agrees with measured speed-dependent cross sections for up to ∼105 m/s. For the ion pair Ne+ − Cl−, our predictions of the MN rate constant at ∼1 Torr are a factor of ∼2 to 3 higher than the experimentally measured value. Similarly, for Xe+ − F− in the pressure range of ∼20 000–80 000 Pa, our predictions of the MN rate constant are ∼20% lower but are in excellent qualitative agreement with experimental data. The paradigm of using trajectory simulations to self-consistently capture the effect of gas pressure on MN reactions advanced here provides avenues for the inclusion of additional nonclassical effects in future work.