| Abstract
| - The energy profiles for the reaction OH- + CO2 → HCO3- are analyzed following the results of calculationscarried 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-lessprocess in the gas phase but shows a barrier on the way from the reactants to the product in the dielectriccontinuum medium. In the cluster approach, the reacting species were completely surrounded by 30 watermolecules, each considered as an effective fragment potential (EFP) acting on the quantum system. Positionsof 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 remarkableagreement with the results of the continuum model, showing the barrier in the same region. An analysis ofthe arrangements of the water molecules around the reacting species, as well as changes in geometryconfigurations and electronic distributions of the solute species, allows us to conclude that on the segment ofthe reaction path close to the potential barrier a considerable fraction of the negative charge on OH- transfersto CO2, accompanied by a sharp bending of the O−C−O species. As a result, the hydroxide anion loseswater molecules from its hydration shell. We show that the height of the barrier on the free energy curve forthe reaction OH- + CO2 → HCO3- in water can be estimated within the limits 8−13 kcal/mol, and itsprecise quantity depends on the reference value of experimental free energy of solvation of OH-.
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