The presence of a solvent interacting with a system brings about qualitative changes from the corresponding gasphase reactions.A solvent can not only change the energetics along the reaction pathway,but also radically alter the reaction mechanism.Here,we investigated the water-induced transition state of the OH-+ CO2 → HCO3-reaction using a multi-level quantum mechanics and molecular mechanics method with an explicit water model.The solvent energy contribution along the reaction pathway has a maximum value which induces the highest energy point on the potential of mean force.The charge transfer from OH-to CO2 results in the breaking of the OH-solvation shell and the forming of the CO2 solvation shell.The loss of hydrogen bonds in the OH-solvation shell without being compensated by the formation of hydrogen bonds in the CO2 solvation shell induces the transition state in the aqueous solution.The calculated free energy reaction barrier at the CCSD(T)/MM level of theory,11.8 kcal/mol,agrees very well with the experimental value,12.1 kcal/mol.