HOCCO versus OCCO: Comparative spectroscopy of the radical and diradical reactive intermediates

Formation of a long-lived cyclic isomer of ethylenedione

A century of unsuccessful attempts to identify the neutral ethylenedione molecule combined with the results of quantum-chemical computations resulted in the conclusion on the instability of this molecule.

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.

Electron affinity and excited states of methylglyoxal

Using photoelectron imaging spectroscopy, we characterized the anion of methylglyoxal (X²A″ electronic state) and three lowest electronic states of the neutral methylglyoxal molecule: the closed-shell singlet ground state (X¹A'), the lowest triplet state (a³A″), and the open-shell singlet state (A¹A″). The adiabatic electron affinity (EA) of the ground state, EA(X¹A') = 0.87(1) eV, spectroscopically determined for the first time, compares to 1.10(2) eV for unsubstituted glyoxal. The EAs (adiabatic attachment energies) of two excited states of methylglyoxal were also determined: EA(a³A″) = 3.27(2) eV and EA(A¹A″) = 3.614(9) eV. The photodetachment of the anion to each of these two states produces the neutral species near the respective structural equilibria; hence, the a³A″ ← X²A″ and A¹A″ ← X²A″ photodetachment transitions are dominated by intense peaks at their respective origins. The lowest-energy photodetachment transition, on the other hand, involves significant geometry relaxation in the X¹A' state, which corresponds to a 60° internal rotation of the methyl group, compared to the anion structure. Accordingly, the X¹A' ← X²A″ transition is characterized as a broad, congested band, whose vertical detachment energy, VDE = 1.20(4) eV, significantly exceeds the adiabatic EA. The experimental results are in excellent agreement with the ab initio predictions using several equation-of-motion methodologies, combined with coupled-cluster theory.