Electronic State Spectroscopy of the Peroxychloroformyl ClC(O)OO Radical

The peroxychloroformyl (ClC(O)OO) radical is proposed to be a key intermediate involved in the CO₂ cycle, the polysulfur cycle, and the sulfur oxidation cycle in Venus's upper atmosphere. A better understanding of the reactivity of ClC(O)OO is essential to reduce uncertainties in simulations of Venus's photochemical models. However, relevant kinetic studies remain scarce as a result of the lack of suitable spectroscopic methods for gas-phase studies. In this work, the electronic structure of ClC(O)OO was explored through a synergistic combination of experimental and theoretical approaches. Under Cs symmetry, the lowest-excited 1²A' state is bound with an adiabatic excitation energy in the near-infrared (NIR) region. Based on the parameters calculated in this work, the simulated NIR absorption bands suggest the possibility of cis- and trans-isomer selective measurements. In addition, using a multipass transient absorption UV-Vis spectrometer, we recorded the UV spectrum of gas-phase ClC(O)OO in the 210-320 nm region. The absorption band exhibits a peak near 218 nm with an absolute cross-section approximately of σ(218 nm) ≈ 5 × 10⁻¹⁸ cm², along with a shoulder near 255 nm. The spectrum can be fitted to two Gaussian profiles, which were assigned to the 1²A″ ← X²A″ and 2²A″ ← X²A″ transitions. Corresponding orbital transitions were rationalized by a simplified molecular orbital diagram of the -C(O)OO functional group. Our results suggest that the ClC(O)OO radical possesses reactivity similar to those of other acyl peroxy radicals.