A Quantum Chemical and Classical Transition State Theory Explanation of Negative Activation Energies in OH Addition To Substituted Ethenes
Bibliographic record
Abstract
The OH addition to ethene has been modeled using ab initio quantum chemical calculations and classical transition state theory (CTST). The results agree with the hypothesis of Singleton and Cvetanovic (Singleton, D. L.; Cvetanovic, R. J. J. Am. Chem. Soc. 1976, 98, 6812) that the reaction is not elemental, and that it consists of a reversible first step involving the formation of a prereactive complex, followed by the irreversible formation of an addition adduct. The overall rate depends on the rates of two competitive reactions, i.e., the reverse of the first step and the second step, the former being more favored by an increase in temperature than the latter. Applying CTST to the proposed mechanism, we obtain an overall rate constant of 11.7 × 10 - 12 cm 3 molecule - 1 s - 1, which agrees very well with the experimental results. New results for the activation energies of the OH addition to a series of substituted ethenes have also been obtained assuming that the above mechanism holds. The activation energies were calculated from projected second-order Moller−Plesset total energies obtained with the 6-311G** basis set. We find that a plot of these data vs the logarithm of the experimental rate constants has a correlation coefficient of 0.996. This seems to imply that the preexponential factor for the series should be approximately constant, in contradiction with the reported values for A . Moreover, a plot of our effective activation energies for OH addition vs the activation energies of a similar reaction, the addition of atomic oxygen to the same series of alkenes, also yields a good correlation. Indeed, for this reaction, the reported A factors are similar to each other. We suggest that the experimental Arrhenius parameters of the OH addition reaction need to be revised.
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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 | 0.001 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 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".