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 molecul es (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 theor y, and a mixed approach based on MO calculations in conjunction with M onte Carlo and reaction field simulations. It is confirmed that the CO 2 hydration constitutes a case of active solvent catalysis where solve nt molecules actively participate as a catalyst in the chemical proces s. In aqueous solution the hydration mechanism is multimolecular, wher e geometric parameters of the solvent fully intervene in the reaction coordinate. The hydration reaction was found to proceed through an att ack of a water oxygen to the CO2 carbon in concert with a proton trans fer to a CO2 oxygen. The proton transfer is assisted by a chain of wat er molecules, which is necessary for a proton relay between different oxygens. Owing to a significantly larger charge separation in the tran sition structures, nonspecific electrostatic interactions between solu te and solvent continuum also play a more important stabilizing role. Regarding the answer to the title question, our calculations suggest t hat although a water tetramer (n=4) seems to be necessary for CO2 hydr ation in the gaseous phase, a reaction channel involving formation of a bridge containing three water molecules (n=3) is likely to be active ly involved in the neutral hydration of CO2 in aqueous solution.