Theoretical study on the photochemical reaction mechanism of the HOSO radical with CO

The photo-induced reactions of the hydroxysulfinyl radical (HOSO) play significant roles in atmospheric chemistry and interstellar chemistry. In this work, the multistate complete active space second-order perturbation theory (MS-CASPT2) method has been employed to study the mechanism of the reaction between HOSO and CO to generate SO₂ and HCO under illumination conditions. Our results show that the possibility of the title reaction occurring through direct hydrogen atom transfer is low, as the energy barrier is at least 1.13 eV. We found that the preferred reaction pathway initiates with the photodissociation of HOSO to HO and SO, followed by the respective reactions of HO and SO with CO to form HCO and SO₂, respectively. We revealed that several conical intersections between the D₁ and D₀ states participate in and facilitate these photochemical processes. Our calculations not only clarify the experimental observations (X. Zeng, et al., Nat. Commun., 2022, 13, 7150) but also enrich the understanding of the sulfur and carbon oxide chemistry in the interstellar medium.