A Theoretical Study of the Iminoxyl/Oxime Self-Exchange Reaction. A Five-Center, Cyclic Proton-Coupled Electron Transfer
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Bibliographic record
Abstract
In solution, the self-exchange reactions for oxygen-centered pi-radicals, e.g., PhO. + PhOH <==>PhOH + PhO., are known to occur with low activation enthalpies (E(a) approximately equal to 2 kcal/mol). For the PhO./PhOH couple and, we conclude, for other O-centered pi-radicals, exchange occurs by proton-coupled electron transfer (PCET) with the proton transferred between oxygen electron pairs while the electron migrates between oxygen orbitals orthogonal to the -O- - -H- - -O- transition state plane (Mayer et al. J. Am. Chem. Soc. 2002, 123, 11142). Iminoxyls, R(2)C=NO., are sigma-radicals with substantial spin density on the nitrogen. The R(2)C=NO./R(2)C=NOH self-exchange has a significant E(a) (Mendenhall et al. J. Am. Chem. Soc. 1973, 95, 627). For this exchange, DFT calculations have revealed a counterintuitive cisoid transition state in which the seven atoms, >C=NO- - -H- - -ON=C<, lie in a plane (R = H, Me) or, for steric reasons, two planes twisted at 45.2 degrees (R = Me(3)C). The planar transition state has the two N-O dipoles close to each other and pointing in the same direction and an O- - -H- - -O angle of 165.4 degrees . A transoid transition state for R = H lies 3.4 kcal/mol higher in energy than the cisoid despite a more favorable arrangement of the dipoles and a near linear O- - -H- - -O. It is concluded that iminoxyl/oxime self-exchange reactions occur by a five-center, cyclic PCET mechanism with the proton being transferred between electron pairs on the oxygens and the electron migrating between in-plane orbitals on the two nitrogens (R(N-N) = 2.65 A). The calculated E(a) values (8.8-9.9 kcal/mol) are in satisfactory agreement with the limited experimental data.
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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 | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.001 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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