Combined multi-level quantum mechanics theories and molecular mechanics study of water-induced transition state of OH^-+CO₂ reaction in aqueous solution

The presence of a solvent interacting with a system brings about qualitative changes from the corresponding gas-phase 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.