Photoionization of HOCO revisited: a new upper limit to the adiabatic ionization energy and lower limit to the enthalpy of formation

A new upper limit to the adiabatic ionization energy of trans-hydroxyoxomethyl, EI(t-HOCO)≤8.195±0.022 eV, is provided, producing a lower limit to the enthalpy of formation, ΔHf 0o(t-HOCO)≥−45.8±0.7 kcal/mol (≥−46.5±0.7 kcal/mol at 298 K). The spectrum shows progressions in CO and C–O stretches of HOCO+ and provides evidence for the excitation of OCO bend. In addition, the data tentatively suggest an ionization onset as low as 8.06±0.03 eV. While it is not clear whether the latter corresponds to cis or trans isomer, it may indicate that ΔHf 0o(HOCO) is even higher.