Measurement and calculation of <sup>13</sup> C and <sup>15</sup> N NMR chemical-shift tensors of a push–pull ethylene
Bibliographic record
Abstract
Methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has a remarkably weak carbon–carbon double bond. It has strong electron-withdrawing groups on one end and electron-donating groups on the other: a so-called push–pull ethylene. To investigate this unusual electronic structure, we have determined the crystal structure and measured both the 13 C and 15 N NMR chemical-shift tensors. These measurements are supplemented by shielding-tensor calculations done with density functional methods. The large difference (approximately 100 ppm) between isotropic chemical shifts of the two alkenyl carbons reflects a large charge release from the electron-donating side of C=C double bond to the electron-withdrawing groups. Comparison of the calculated orientations of the principal components of the alkenyl carbons obtained from ab initio calculations shows that the primary changes in charge density occur in the molecular plane. On the other hand, smaller charge density changes above and below the plane of the C=C double bond establish the conjugation of donor and acceptor groups with π* and π molecular orbitals of the central double bond, respectively, which lowers the barrier to rotation about this bond.
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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 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 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".