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 (X) over tilde (2)Sigma(+) and (A) over tilde (2)Pi states of CCH. Although the mode has sigma symmetry and it is the perturbations of pi 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 (X) over tilde (2)Sigma(+) state with different flavors of coupled-cluster (CC) theory provide insight and underscore an intrinsic advantage of equation-of-motion CC (EOM-CC) methods.

Automated summary

The study investigates the marked anharmonicity of the CC stretching mode in the ethynyl radical using a vibronic coupling model, attributing the large difference between the harmonic frequency and the fundamental level to vibronic interaction between electronic states. The profound vibronic influence due to strong tuning of gap energy by the CC stretch mode is highlighted, emphasizing the advantage of equation-of-motion CC methods for calculations.