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

We present a photoelectron imaging study of three glyoxal derivatives: the ethylenedione anion (OCCO⁻), ethynediolide (HOCCO⁻), and glyoxalide (OHCCO⁻). These anions provide access to the corresponding neutral reactive intermediates: the OCCO diradical and the HOCCO and OHCCO radicals. Contrasting the straightforward deprotonation pathway in the reaction of O⁻ with glyoxal (OHCCHO), which is expected to yield glyoxalide (OHCCO⁻), OHCCO⁻ is shown to be a minor product, with HOCCO⁻ being the dominant observed isomer of the m/z = 57 anion. In the HOCCO/OHCCO anion photoelectron spectrum, we identify several electronic states of this radical system and determine the adiabatic electron affinity of HOCCO as 1.763(6) eV. This result is compared to the corresponding 1.936(8) eV value for ethylenedione (OCCO), reported in our recent study of this transient diradical [A. R. Dixon, T. Xue, and A. Sanov, Angew. Chem., Int. Ed. 54, 8764–8767 (2015)]. Based on the comparison of the HOCCO⁻/OHCCO⁻ and OCCO⁻ photoelectron spectra, we discuss the contrasting effects of the hydrogen connected to the carbon framework or the terminal oxygen in OCCO.