Sulfur dioxide is one of the most well-known common chemicals that have a direct bearing on human life. Both of natural and anthropogenic origin, SO 2 is largely responsible for acidic rain and formation of particulates, with damaging consequences on climate, forest ecology, fish and aquatic organisms. [1] A number of thermochemical properties are available in current databases, [2] including those concerning proton-transfer reactions. These also govern other important chemical transformations such as hydrogen-atom abstraction reactions, a class of which are the proton-coupled electron transfer reactions (PCET). [4] This mechanism, relevant to biological and metalmediated oxidations, [5] has been suggested as a viable route to OΓH bond activation by oxygen-centered radicals. [4d] Recent gas-phase experiments have reported PCET in the double CΓH activation of ethane by the oxygen-centered SO 2 C + radical cation, that also efficiently activates the strong OΓH bond of water. [6] Despite the relevance of these properties, the proton affinity (PA) and gas-phase basicity (GB) of SO 2 at 298 K, currently available in the NIST database, prove to be overestimated by at least 8 kcal mol Γ1 .
The NIST PA and GB of SO 2 , taken from the Hunter and Lias compilation, amount to 160.7 and 153.8 kcal mol Γ1 , respectively. Briefly, the PA and GB are the negative changes of the enthalpy and Gibbs free energy, ΓDH T and ΓDG T , respectively, of Equation (1):
According to the procedure generally followed in the compilation, PA 298 and GB 298 (henceforth denoted as PA and GB) of any species (M) were obtained by referencing M to primary standards S with well-established gas basicities. The final values were attained following critical evaluation of DG 298 of