Spectroscopy of Ethylenedione and Ethynediolide: A Reinvestigation

In an effort to characterize the electronic states of ethylenedione, OCCO, photoelectron-photofragment coincidence (PPC) spectroscopy was applied to measure anions at m/z 56 and 57 using a pulsed discharge of glyoxal vapor and N₂ O. PPC measurements at a photon energy of 3.20 eV yield photoelectron spectra in coincidence with either neutral photofragments or stable neutral products. The measurements showed that primarily stable neutral products were formed, with photoelectron spectra consistent with the oxyallyl diradical, C₃ H₄ O, and acetone enolate radical, C₃ H₅ O. The spectra were also found to have features nearly identical to those reported for OCCO and HOCCO by Sanov and co-workers. The stability of the neutral products, as well as an examination of spectra reported for the oxyallyl anion and acetone enolate show that the previous assignments of OCCO and HOCCO are in error, and are instead attributed here to the oxyallyl diradical, C₃ H₄ O, and the acetone enolate radical, C₃ H₅ O.