Why the CC Stretch in HCC Is So Anharmonic

The marked anharmonicity of the CC stretching mode in the ethynyl radical (C2H) is investigated in terms of a vibronic coupling model. It is demonstrated that the large difference between the harmonic frequency and the fundamental level (about 5–10 times that for triple-bond stretches in the related species HCCH, HCN, HNC and CN) can be attributed to the well-known vibronic interaction between the X̃ 2Σ+ and à 2Π states of CCH. Although the mode has σ symmetry and it is the perturbations of π symmetry that mix the two electronic states, a combination of large intrinsic coupling strength, modest energy gap, and–most importantly–the strong tuning of the gap energy by the CC stretch mode leads to a profound vibronic influence on parts of the anharmonic force field that sample the CC stretch. Finally, calculations of the force field for the X̃ 2Σ+ state with different flavors of coupled-cluster (CC) theory provide insight and underscore an intrinsic advantage of equation-of-motion CC (EOM-CC) methods.