How many water molecules are actively involved in the neutral hydration of carbon dioxide?

The detailed reaction pathways for the hydration of carbon dioxide by water and water clusters containing two, three, and four water molecules (CO2+nH(2)O-->H2CO3+(n-1)H2O, n=1-4) have been investigated in both gas phase and aqueous solution using ab initio molecular orbital (MO) theory up to the quadratic configuration interaction QCISD(T)/6-31G(d,p)//MP2/6-31G(d,p) level, both SCRF and PCM models of continuum theory, and a mixed approach based on MO calculations in conjunction with Monte Carlo and reaction field simulations. It is confirmed that the CO2 hydration constitutes a case of active solvent catalysis where solvent molecules actively participate as a catalyst in the chemical process. In aqueous solution the hydration mechanism is multimolecular, where geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an attack of a water oxygen to the CO2 carbon in concert with a proton transfer to a CO2 oxygen. The proton transfer is assisted by a chain of water molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the transition structures, nonspecific electrostatic interactions between solute and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest that although a water tetramer (n=4) seems to be necessary for CO2 hydration in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n=3) is likely to be actively involved in the neutral hydration of CO2 in aqueous solution.