The spin-vibronic energy levels of the cyanoacetylene cation have been measured using the one-photon zero-kinetic energy (ZEKE) photoelectron spectroscopic method. All three degenerate vibrational modes showing vibronic coupling, i.e., Renner-Teller (RT) effect, have been observed. All the splitting spin-vibronic energy levels of the fundamental H—C≡C bending vibration (v5) have been determined. The spin-vibronic energy levels of the degenerate vibrational modes have also been calculated using a diabatic model in which the harmonic terms as well as all the second-order vibronic coupling terms are used. The theoretical predictions are in good agreement with the experimental data and are used to assign the ZEKE spectrum. It is found that the RT effects for the H—(CC)—CN bending (v7) and the C—C≡N bending (v6) vibrations are weak, whereas they are strong for the H—C≡C bending (v5) vibration. The cross-mode RT couplings between any of the two degenerate vibrations are strong. The spin-orbit resolved fundamental vibrational energy levels of the C≡N stretching (v2) and C—H stretching (v1) vibrations have also been observed. The spin-orbit energy splitting of the ground state has been determined for the first time as 43 ± 2 cm−1, and the ionization energy of HCCCN is found to be 93 903.5 ± 2 cm−1.

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