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Mechanistic insights into the dissociation and decomposition of carbonic acid in water via the hydroxide route: an ab initio metadynamics study.

The dissociation and decomposition of carbonic acid (H2CO3) in water are important reactions in the pH regulation in blood, CO2 transport in biological systems, and the global carbon cycle. H2CO3 is known to have three conformers [cis-cis (CC), cis-trans (CT), and trans-trans (TT)], but their individual reaction dynamics in water has not been probed experimentally. In this paper, we have investigated the energetics and mechanisms of the conformational changes, dissociation (H2CO3 -->/<-- HCO3(-) + H(+)), and decomposition via the hydroxide route (HCO3(-) --> CO2+OH(-)) of all three conformers of H2CO3 in water using Car-Parrinello molecular dynamics (CPMD) in conjunction with metadynamics. It was found that, unlike in the gas phase, the interconversion between the various conformers occurs via two different pathways, one involving a change in one of the two dihedral angles (O=C-O-H) and the other a proton transfer through a hydrogen-bond wire. The free energy barriers/changes for the various conformational changes via the first pathway were calculated and contrasted with the previously calculated values for the gas phase. The CT and TT conformers were found to undergo decomposition in water via a two-step process: first, the dissociation and then the decomposition of HCO3(-) into CO2 and OH(-). The CC conformer does not directly decompose but first undergoes a conformational change to CT or TT prior to decomposition. This is in contrast with the concerted mechanism proposed for the gas phase, which involves a dehydroxylation of one of the OH groups and a simultaneous deprotonation of the other OH group to yield CO2 and H2O. The dissociation in water was seen to involve the repeated formation and breakage of a hydrogen-bond wire with neighboring water molecules, whereas the decomposition is initiated by the diffusion of H(+) away from HCO3(-); this decomposition mechanism differs from that proposed for the water route dehydration (HCO3(-) + H3O(+) --> CO2 + H2O), which involves the participation of a nearbyH3O(+) ion.Our calculated pKa values and decomposition free energy barriers for the CT and TT conformers are consistent with the overall experimental values of 3.45 and 22.28 kcal/mol, respectively, suggesting that the dynamics of the various conformers should be taken into account for a better understanding of aqueous H2CO3 chemistry.