Water-accelerated OH addition to sulfur dioxide SO₂: direct ab initio molecular dynamics (AIMD) study

Ionization dynamics of water microsolvated sulfur dioxide SO2(H2O)n (n = 1-3 and 6) have been investigated by means of direct ab initio molecular dynamics (AIMD) method to elucidate the hydration effects of OH addition reaction to SO2 following the ionization. The calculations showed that the neutral 1:1 complex SO2-H2O has a C(s) symmetry and the sulfur of SO2 interacts with the oxygen of H2O with an eclipsed form. In the case of ionization of SO2-H2O 1:1 complex (n = 1), the cation complex composed of [H2O-SO2](+) with a face-to-face form was obtained as the product. The OH addition reactions to SO2 were found in larger systems (n = 2, 3, and 6) following the ionization. The reaction was expressed as SO2(+)(H2O)n → SO2(OH)···H(+)(H2O)(n-1) (n = 2, 3, and 6). The proton generated as (SO2-H2O)(+) → (HSO3) + H(+) was stabilized by the second water molecule as the reaction: H(+) + H2O → H3O(+). These processes occurred and were completed within the cluster. The OH addition mechanism in SO2(+)(H2O)n cluster was discussed on the basis of the present results.