Theoretical Study of OH and H2O Addition to SO2

The G2 computational method is applied to the study of the hydroxyl radical oxidation of SO 2 to SO 3 as well as the hydrolysis of SO 2 to H 2 SO 3 . A key intermediate in the oxidation process is the HOSO 2 radical, which is predicted to have a S−OH bond enthalpy (Δ H 298 ) of 26.2 kcal/mol, 4.3 kcal/mol lower than the currently accepted value of 30.5 kcal/mol. The radical is characterized by a 2c-2e S−OH bond with an unpaired electron delocalized into the π* orbital of the SO 2 moiety. The hydrolysis of SO 2 to H 2 SO 3 was computed with and without a catalytic water. The SO 2 ·H 2 O and SO 2 ·2H 2 O complexes and transition states are very similar to those computed for SO 3 plus water. The uncatalyzed reaction has an activation barrier of 33.9 kcal/mol, which is reduced to 20.0 kcal/mol with one catalytic water. Since the reaction of SO 2 with two waters is nearly thermoneutral (4.5 kcal/mol endothermic), the reaction may be more amenable to thermodynamic study compared to the SO 3 + 2H 2 O reaction, which is much more exothermic.