The Role of Hydrogen Bonding in the Decomposition of H<sub>2</sub>CO<sub>3</sub> in Water: Mechanistic Insights from Ab Initio Metadynamics Studies of Aqueous Clusters
Bibliographic record
Abstract
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.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".