Estimation of a p<i>K</i><sub>a</sub> for Protonated Dimethoxycarbene
Bibliographic record
Abstract
Abstract A pKa is estimated for the protonation of dimethoxycarbene based on the relationship pKa = pKR −pKH2O, where KH2O and KR are equilibrium constants for hydration of the carbene and reaction of the dimethoxymethyl carbocation with water to form dimethoxymethanol, respectively. A value of pKa = 15.5 in aqueous solution is derived based on pKR = −5.7 for the carbocation and an estimate of pKH2O = −21.2 based on a heat of formation of the carbene ΔH°f = −38.8 ± 2 kcal mol−1, calculated at the G3 level in the gas phase. This value for the pKa compares satisfactorily with pKa = 11 inferred from the dependence of experimental rate constants for reaction of the carbene with alcohols and acetic acid upon the pKa of the oxygen acid in acetonitrile, taking account of the difference in solvents for the calculated and measured values. The calculated heat of formation of dimethoxycarbene in the gas phase is compared with an experimental value of −52.6 kcal mol−1 based on photoacoustic measurements of the heat of reaction of the carbene with methanol solvent to form trimethyl orthoformate. Reanalysis of the experimental data using a heat of solution of trimethyl orthoformate in methanol measured in this work suggests that the discrepancy arises in part from a difference in the heat of reaction of the carbene with methanol in solution and in the gas phase and partly (although less certainly) from a relatively high heat of solution of the carbene in methanol. Copyright © 2003 John Wiley & Sons, Ltd.
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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) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
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".