Insights of the peroxychloroformyl radical ClC(O)OO via microwave spectrum†

The exploration of Venus has received much attention in the past and will keep growing due to the starting of the NASA DAVINCI project. To explain the extremely low O₂ : CO ratio observed in Venus’ atmosphere, a chlorine-initiated CO oxidation catalytic cycle has been proposed. However, relevant studies on the key intermediates, such as the peroxychloroformyl radical (ClC(O)OO), are rare. In this study, the ClC(O)OO radical was observed using Fourier-Transform Microwave (FTMW) spectroscopy under the supersonic expansion condition. Two conformers, trans-ClC(O)OO and cis-ClC(O)OO, and their chlorine isotopologues were detected. The molecular constants including the fine and hyperfine constants, were determined. Based on the experimental and the ab initio calculations, the unpaired electron is mostly located at the terminal oxygen atom, supported by the small magnetic hyperfine constants of chlorine. In addition, the angles between the Cl–C bond and the a-axis of the trans-³⁵ClC(O)OO and trans-³⁷ClC(O)OO are similar, but these angles are different for cis-ClC(O)OO, making the quadrupole coupling tensors in the inertial axes disagree with the ratio of the quadrupole moments of ³⁵Cl and ³⁷Cl. Finally, we concluded that the ClC(O)OO radicals should behave similarly to other peroxyl radicals, as assumed in the current photochemical model of Venus.