The role of hydrogen bonding in the decomposition of H₂CO₃ in water: mechanistic insights from ab initio metadynamics studies of aqueous clusters.

Both concerted and stepwise mechanisms have been proposed for the decomposition of H₂CO₃ in bulk water based on electronic structure and ab initio molecular dynamics calculations. To consistently determine which, if any, mechanism predominates in bulk water, we performed ab initio metadynamics simulations of the decomposition of H₂CO₃ in water clusters of increasing size. We found that, in the small clusters (containing six and nine water molecules), the decomposition occurs according to a concerted proton shuttle mechanism via a cyclic transition state, whereas, in the larger clusters (containing 20 and 45 water molecules), the decomposition occurs according to a two-step mechanism via a solvent-separated HCO₃⁻/H₃O⁺ ion pair intermediate. Due to the additional water molecules in the larger clusters, the dissociation of H₂CO₃ into the metastable solvent-separated ion pair was found to be energetically favorable, thereby preventing the formation of the cyclic transition state and committing the decomposition to the sequential route. An analysis of the solvation environment around the H₂CO₃ molecule in the various clusters revealed that the transition from the concerted mechanism to the stepwise mechanism precisely hinges upon the number of water molecules hydrogen bonded to the H₃O⁺ intermediate, which changes as the size of the cluster increases. The larger clusters contain a sufficient number of water molecules to fully solvate the H₃O⁺ intermediate, indicating that they can provide a bulk-like environment for this reaction. Therefore, these results strongly demonstrate that the decomposition of H₂CO₃ in bulk water occurs via the stepwise mechanism.