Theoretical studies on the pK~a~ values of perfluoroalkyl carboxylic acids: Non-helical conformation acidity constants for n-perfluorooctanoic acid (n-PFOA)
Bibliographic record
Abstract
Abstract A molecular mechanics force field method (MMFF94) systematic rotor conformational search was conducted on the well known environmental contaminant n-perfluorooctanoic acid (n-PFOA). The MMFF94 conformational search yielded 2915 separate low- through high-energy conformers of n-PFOA. Ranking of these structures gave the 94 lowest MMFF94 energy conformations that were subjected to subsequent density functional theory (DFT) computational investigations at the B3LYP/6-311++G(d,p) level of theory. Application of a thermodynamic cycle approach, coupled with aqueous and gas phase DFT calculations on the molecular and anionic forms for each of the 94 lowest energy n-PFOA conformers, gave estimated acidity constants (pK~a~ values) for a final converged set of twelve n-PFOA conformers, including the global minimum helical perfluoroalkyl chain geometry and eleven non-helical higher energy conformations. Collectively, the twelve lowest energy conformers appear to encompass 85% of the total conformational space for n-PFOA, indicating no higher energy conformations likely remain that could appreciably influence the observed composite aqueous pK~a~ of this compound. The estimated pK~a~ values of all eleven non-helical n-PFOA conformers were within 0.2 pK~a~ units of the helical conformer pK~a~. The findings support previous predictions and recent experimental evidence for an aqueous phase n-PFOA monomeric pK~a~ of about zero.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.002 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.002 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.001 | 0.003 |
| 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; both teacher heads agree on what is shown here.
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".