On the origin of potential barrier for the reaction OH-+CO2 → HCO3- in water:: Studies by using continuum and cluster solvation methods

The energy profiles for the reaction OH- + CO2 --> HCO3- are analyzed following the results of calculations carried out using both a continuum solvation model and a cluster approach. The minimum energy path, computed with the quantum chemistry LMP2 and B3LYP approximations, corresponds to the activation-less process in the gas phase but shows a barrier on the way from the reactants to the product in the dielectric continuum medium. In the cluster approach, the reacting species were completely surrounded by 30 water molecules, each considered as an effective fragment potential (EFP) acting on the quantum system. Positions of all particles were optimized along the reaction coordinate in this quantum mechanical-molecular mechanical (QM/MM) approximation. The energy profile obtained with the QM/MM(EFP) approach is in remarkable agreement with the results of the continuum model, showing the barrier in the same region. An analysis of the arrangements of the water molecules around the reacting species, as well as changes in geometry configurations and electronic distributions of the solute species, allows us to conclude that on the segment of the reaction path close to the potential barrier a considerable fraction of the negative charge on OH- transfers to CO2, accompanied by a sharp bending of the O-C-O species. As a result, the hydroxide anion loses water molecules from its hydration shell. We show that the height of the barrier on the free energy curve for the reaction OH- + CO2 --> HCO3- in water can be estimated within the limits 8-13 kcal/mol, and its precise quantity depends on the reference value of experimental free energy of solvation of OH-.