In silico calculation of acidity constants of carbonic acid conformers

In order to explore the aqueous acid chemistry of carbonic acid, we employ a constrained ab initio molecular dynamics (AIMD) technique to study acid dissociations of its three conformers including CC (cis-cis), CT (cis-trans), and TT (trans-trans). The simulations of reagent states reveal similar hydration characteristics for them: the hydroxyls donate H-bonds to solvating waters but no obvious H-bonding exists between hydroxyl oxygen atoms and waters. It is found that the CC conformer dissociates spontaneously to bicarbonate within picoseconds whereas the other two can stay for relatively long simulation times. This suggests that CC has the strongest acidity among the three conformers and it is not stable in water. The simulations indicate that the symmetrical hydroxyls of TT conformer have a pKa value of 3.11 and the two asymmetrical hydroxyls of CT show different pKa values: 2.60 and 3.75, respectively. Overall, these results confirm the recent experimental measurement: about 4.0 for deuterated carbonic acid. By analyzing the dissociation processes, it is revealed that the differences of the acid constants stem from the initial steps of hydroxyls stretches. This simulation study provides a quantitative and microscopic basis for better understanding the reactivity of aqueous carbonate species.