Water-induced Transition State Of The OH^-+CO₂ Reaction In Aqueous Solution With Multi-Level QM/MM Method

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 origin of water-induced transition state of the OH-+ CO₂ → HCO₃-reaction with a multi-level quantum mechanics and molecular mechanics method.The solvent energy contribution has a maximum which induces the highest energy point on the potential of mean force.The charge transfer from OH-to CO₂ originates the breaking of the OH-solvation shell and the forming of the CO₂ solvation shell.The loss of hydrogen bonds in OH-solvation shell without being compensated by the formation of hydrogen bonds in CO₂ solvation shell induces the transition state in aqueous solution.The calculated free energy reaction barrier,11.8 kcal/mol,agrees very well with the experimental value,12.1 kcal/mol.