Photoelectron Spectroscopy Confirms the Gas-Phase Stability of the C₃O₂^- Anion

The carbon suboxide anion with an asymmetric zigzag structure, C₃O₂^-, which was recently characterized in a solid neon matrix, has not been observed in the gas phase. Here, we have experimentally generated C₃O₂^- in a supersonic plasma jet expansion and studied it by negative ion photoelectron spectroscopy. A spectral analysis based on the calculated Franck-Condon factors, in combination with quantum chemical calculations, has for the first time yielded a determination of the electron affinity of the neutral carbon suboxide, EA = +0.48(8) eV. Molecular orbital (MO) analysis indicates that, upon linear-to-zigzag geometric change, additional σ-π MO mixing significantly reduces the energy of the singly occupied MO of C₃O₂^- and thus stabilizes the negative ion in the gas phase. This work provides a benchmark understanding of the gas-phase stability of the C₃O₂^- anion.