Probing the Low-Lying Electronic States of Cyclobutanetetraone (C4O4) and Its Radical Anion: A Low-Temperature Anion Photoelectron Spectroscopic Approach.

Despite a seemingly simple appearance, cyclobutanetetraone (C4O4) has four low-lying electronic states. Determining the energetic ordering of these states and the ground state of C4O4(-) theoretically has been proven to be considerably challenging and remains largely unresolved to date. Here, we report a low-temperature negative ion photoelectron spectroscopic approach. Well-resolved spectra were obtained at both 193 and 266 nm. Combined with recent theoretical studies and our own Franck-Condon factors simulations, the ground state of C4O4(-) and the ground and two low-lying excited states of C4O4 are determined to be (2)A2u, (3)B2u, (1)A1g (8π), and (1)B2u, respectively. The frequency of the ring breathing mode (1810 ± 20 cm(-1)), the electron affinity (3.475 ± 0.005 eV), and the term values of (1)A1g (8π) (6.27 ± 0.5 kJ/mol) and (1)B2u (13.50 ± 0.5 kJ/mol) are also directly obtained from the experiments.