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

Abstract

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO₂ + nH₂O → 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 ∼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 ∼7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of ∼33 (gas phase) and a free energy barrier of ∼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 ∼15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by ∼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 ∼10 kcal/mol for a free energy barrier of ∼22 kcal/mol for the (CO₂ + 3H₂O)_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 ∼20 (gas phase) and a free energy barrier of ∼19 (in water) kcal/mol were derived for the CO₂ + 4H₂O 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 ∼6−7 kcal/mol) than the corresponding CO₂ complexes and can undergo more facile water-assisted dehydration processes.