Stability of XSO2 (X=F, Cl, and Br) radical: impact of the basis set on X–S bonding energy in ab initio and DFT calculations

XSO2 is a labile intermediate, which can be formed upon pyrolysis of SO2X2 or upon interaction between halogen atom and sulphur dioxide in the troposphere. Detailed theoretical investigation demonstrated that even high correlated ab initio methods reproduce incorrectly BDE(X–SO2), unless very extended basis set is used. Thus, the bond energies derived previously using small basis sets may not be correct. At the same time, B3LYP with medium size basis sets produced results close to those obtained from Gaussian and CBS model chemistries. The source of discrepancies between ab initio and DFT methods is analysed and the implication of our results in the oxidation of tropospheric SO2 is discussed.