Ab initio molecular dynamics simulations have been performed to study the dissolution of SO
2 in water. It has been obtained that the hydrated SO
2 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 SO
2 + H
2O reaction. The simulations revealed that the hydrated SO
2 forms bisulfite anion and hydronium cation after overcoming a ca. 17 kcal/mol free energy barrier. Direct, one-step H
2SO
3 formation could not been observed, in sharp contrast with earlier cluster calculations. These findings indicate a step-wise H
2SO
3 formation in water. The presence of the sulfur lone pair represents an important constraint on the mechanism: the nucleophilic H
2O attack can occur only from certain angles as shown by the reactive trajectories.
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