SO₂ Hydrolysis: Ab initio MD Study of the Formation of Bisulfite Ion

Abstract

Ab initio molecular dynamics simulations have been performed to study the dissolution of SO₂ in water. It has been obtained that the hydrated SO₂ is surrounded by the water molecules without any S-H hydrogen bond, restraining the sulfonate anion formation but allowing the bisulfite isomer formation. The metadynamics method has been employed to explore the free energy surface of the SO₂ + H₂O reaction. The simulations revealed that the hydrated SO₂ forms bisulfite anion and hydronium cation after overcoming a ca. 17 kcal/mol free energy barrier. Direct, one-step H₂SO₃ formation could not been observed, in sharp contrast with earlier cluster calculations. These findings indicate a step-wise H₂SO₃ formation in water. The presence of the sulfur lone pair represents an important constraint on the mechanism: the nucleophilic H₂O attack can occur only from certain angles as shown by the reactive trajectories.