Bicarbonate/carbonic acid equilibria in aqueous solutions: Conformational Dynamics, Ion-Pair Speciation, and Free Energy Landscapes from Ab Initio Molecular Dynamics and Metadynamics

Abstract

Carbonic acid (H2CO3) is connected with the dissolved CO2 and bicarbonate (HCO3-) equilibria in aqueous solutions. Its two elementary reactions in water, acid dissociation and dehydration to CO2, are difficult to resolve because both are coupled to proton transfer through hydrogen-bonded networks. In this work, we combine 0.5 ns of unbiased PBE-D3 molecular dynamics of 1 molar solution of H2CO3 with metadynamics on the O - H coordination (H2CO3 = HCO3- + H+) and the C - O distance of the leaving hydroxyl (H2CO3 <=> CO2 + H2O). These unbiased trajectories show frequent cis-cis <=> cis-trans interconversion through long-lived bicarbonate (well separated from solvated proton, H3O+, and also mediated by a short-lived three-water [H2O...H3O+...H2O] ring that acts as a solvent-shared ion pair (SSIP); only 16-23% of deprotonation events reach a solvent-separated ion pair (SShIP). Resolving ion-pairs as SSIP for proper reaction leads to an improved estimate of pKa1 from 1.1 to 2.4, toward the experimental 3.45-3.59 (measured at supposedly infinite dilution). Further, metadynamics simulations provide a dehydration barrier of 17.8 (1D) and 15.3 kcal/mol (2D), suggesting that the final product CO2 + H2O is 2.8 kcal/mol below H2CO3. Our two-dimensional free energy surface also shows that C--O cleavage proceeds through the CO2 + OH- which further goes down by 9.9 kcal/mol to product CO2 + H2O.