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, giving rise to rovibronic transition bands in the near-infrared (NIR) region. This NIR absorption band potentially allows measurements with selectivity of cis- and trans-isomers. In addition, using a multipass transient absorption UV-Vis spectrometer, we recorded the gas-phase spectrum of 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 (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 that of other acyl peroxy radicals.