A computational study on the relationship between formation and electrolytic dissociation of carbonic acid

Calculations at the B3LYP/6-311+G**, MPW1K/6-311+G**, and MP2/6-311+G** level theory were carried out for the CO2 + nH2O → H2CO3 + (n − 1)H2O chemical reaction, where n denotes the number of water molecules. For n = 1, 2, 3, and 4 water molecules, a concerted path (without intermediates) for the formation of H2CO3 was obtained. For n = 6(3 + 3) and n = 8(4 + 4) water molecules, the MPW1K and B3LYP/6-311+G** SCRF = PCM methods resulted in an ion-pair intermediate being formed. Here, +3 or +4 stands for the three or four catalytic water molecules. The catalytic water molecules are distinguished from the reaction participating ones (either three or four). For n = 3 + 5 and n = 3 + 10, two ion-pair intermediates were obtained using both the B3LYP and MPW1K DFT methods. In order to check the method and model dependence, furthermore, n = 3 + 17 and n = 3 + 27 reacting systems were examined. Here, a likely mechanism of formation and electrolytic dissociation of carbonic acid was found. First, the O2C–OH2 complex is formed. Second, it is isomerized to the Zundel cation H5O2+. Third, the cation is converted to the carbonic acid. The isomerization, i.e., proton transfer, was computed to occur along the hydrogen-bonded network of the pentagon shape.