Theoretical Study on Reactions of HO₂ Radical with Photodissociation Products of Cl₂SO (ClSO and SO)

The possible reactions of HO₂ radical with the intermediates of the Cl₂SO photolysis (ClSO and SO) were studied using G3MP2//B3LYP/cc-pVTZ+d level of theory and Martin’s W1U method. For the reaction between HO₂ and ClSO radicals, the following mechanisms are supposed to be the main reaction pathways HO2 + ClSO → H3OO × ClSO → HOO(Cl)SO → OH + ClSO2 → HO + Cl + SO2HO2 + ClSO → H3OO × ClSO → HOO(Cl)SO → OH + ClSO2 → HO(Cl)SO2 On the basis of G3MP2//B3LYP/cc-pVTZ+d and highly accurate W1U calculations, the reaction of HOO with ³SO species has also been explored, and the following dominant consecutive reactions may describe the fast oxygen transfer HO2 + S3O → H4OO × SO → H2OOSO → OH + SO2 In both reaction mechanisms, the first step is a barrierless formation of relatively stable van der Waals complexes that lead via intersystem crossing to intermediate adducts. Thermodynamically favored decomposition products of ²HOOSO are OH radical and SO₂. In the case of the ClSO and HO₂ reaction, the dissociation of HOO(Cl)SO resulted in OH and ClSO₂. Further decomposition of ClSO₂ to Cl atom and SO₂ competes with formation of HO(Cl)SO₂ via OH addition reaction to ClSO₂. We also report on high-level quantum chemical calculation (W1U) to predict values for the heat of formation of ²HSO₃, ²HOOSO, and ²OOS(H)O radicals using the most reliable thermodynamic data of OH and SO₃: Δ_fH^298.15K(²HSO₃) = −256.2 kJ/mol, Δ_fH^298.15K(²HOOSO) = −152.6 kJ/mol, and Δ_fH^298.15K(²OOS(H)O) = −8.3 kJ/mol. On the basis of W1U standard reaction enthalpy for the reaction ClSO + HOO → HCl + SO₃, the heat of formation for the ClSO radical was also computed to be Δ_fH^298.15K(ClSO) = 102.6 kJ/mol within 4 kJ mol⁻¹ error.