Mechanism of the Hydration of Carbon Dioxide: Direct Participation of H2O versus Microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO2 + nH(2)O -> H2CO3 + (n - 1)H2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of similar to 50 kcal/mol was predicted for the addition of one water molecule to CO2 (n = 1). This barrier is lowered in cyclic H-bonded systems of CO2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of similar to 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of similar to 33 (gas phase) and a free energy barrier of similar to 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of similar to 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by similar to 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO3-/H3OH2O+) ion pair. Bulk solvation reduces the free energy barrier of the first path by similar to 10 kcal/mol for a free energy barrier of similar to 22 kcal/mol for the (CO2 + 3H(2)O)(aq) reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of similar to 20 (gas phase) and a free energy barrier of similar to 19 (in water) kcal/mol were derived for the CO2 + 4H(2)O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules (n = 3) in the eight-member cyclic TS for the CO2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by similar to 6-7 kcal/mol) than the corresponding CO2 complexes and can undergo more facile water-assisted dehydration processes.