Structures and reaction rates of the gaseous oxidation of SO₂ by an O₋₃ (H₂O)₀₋₅ cluster – a density functional theory investigation

Based on density functional theory calculations we present a study of the gaseous oxidation of SO₂ to SO₃ by an anionic O₃⁻(H₂O)_n cluster, n = 0–5. The configurations of the most relevant reactants, transition states, and products are discussed and compared to previous findings.

Two different classes of transition states have been identified. One class is characterised by strong networks of hydrogen bonds, very similar to the reactant complexes. The other class is characterised by sparser structures of hydration water and is stabilised by high entropy. At temperatures relevant for atmospheric chemistry, the most energetically favourable class of transition states vary with the number of water molecules attached.

A kinetic model is utilised, taking into account the most likely outcomes of the initial SO₂ O₃⁻(H₂O)_n collision complexes. This model shows that the reaction takes place at collision rates regardless of the number of water molecules involved.

A lifetime analysis of the collision complexes supports this conclusion. Hereafter, the thermodynamics of water and O₂ condensation and evaporation from the product SO₃⁻O₂(H₂O)_n cluster is considered and the final products are predicted to be O₂SO₃⁻ and O₂SO₃⁻(H₂O)₁.

The low degree of hydration is rationalised through a charge analysis of the relevant complexes. Finally, the thermodynamics of a few relevant reactions of the O₂SO₃⁻ and O₂SO₃⁻(H₂O)₁ complexes are considered.