On the Formation of Proton-Shared and Contact Ion Pair Forms during the Dissociation of Moderately Strong Acids: An Ab Initio Molecular Dynamics Investigation
Bibliographic record
Abstract
Acid ionization and dissociation are phenomena that play a fundamental role in chemistry and biology, but their microscopic details are largely unknown. We use ab initio molecular dynamics to identify and characterize various structures that are formed along the pathway of dissociation of trifluoroacetic acid (p K a = 0.5). The present results demonstrate that solutions of moderately strong (−1 ≤ p K a ≤ 1) carboxylic acids contain significant quantities of at least two forms whose bonding patterns are intermediate between those of covalent and dissociated structures, R−COOH···OH 2 and R−COO − (aq) + H 3 O + (aq), respectively. The first is a proton-shared structure of the form R−COO δ− ·H· δ+ OH 2 which can be distinguished from the covalent form by its characteristic “continuous” absorption below 1700 cm −1 . The second is a contact ion pair whose average structure can be written as R−COO − ·H 3 O + . We demonstrate that the transformation of proton-shared into contact ion pair forms coincides with the disappearance of the carbonyl vibration mode centered around 1720 cm −1 and the appearance of the asymmetric and symmetric carboxylate vibration modes at 1620 and 1380 cm −1, respectively. The aforementioned transformation is accompanied by significant changes in the first solvation shell of the acid molecule which we discuss.
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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 | 0.000 |
| 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 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| 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".