Dipolar-Chemical Shift and Rotational Resonance<sup>13</sup>C NMR Studies of the Carboxyl−Methylene Carbon Spin Pair in Solid Phenylacetic Acid and Potassium Hydrogen Bisphenylacetate
Bibliographic record
Abstract
The dipolar-chemical shift (CS) method has been applied to analyze the carboxyl−methylene carbon isolated spin pair in phenylacetic- 13 C 2 acid and potassium hydrogen bisphenylacetate- 13 C 2 . The span, Ω, of the CS tensor is decreased significantly for both carbon atoms in the potassium acid salt compared to the acid. The orientations of the carboxyl CS tensor and, more notably, the methylene CS tensor, have a marked dependence on the protonation state of the carboxyl group. Ab initio calculations [RHF/6-311G*, RHF/6-311++G(2d,2p)] support the experimental findings. In addition to these studies, we demonstrate how rotational resonance (RR) NMR spectroscopy complements the dipolar-CS method in a study of the isolated 13 C spin pairs in phenylacetic- 13 C 2 acid. In particular, the higher-order RR experiments provide a stringent check on the CS parameters and the dipolar coupling constant, R, derived from the dipolar-CS analysis. The dipolar-CS method, in combination with a two-dimensional spin−echo experiment, yields R = 2150 ± 30 Hz for phenylacetic acid, whereas RR indicates that R = 2100 ± 15 Hz. Although n = 1 RR can be applied reliably to determine internuclear distances in the absence of CS tensor data, these data are critical for simulations of the n = 2 RR effects. Specifically, longitudinal magnetization exchange curves are shown to be sensitive to slight rotations of the carboxyl carbon CS tensor about an axis perpendicular to the carboxyl plane, a phenomenon observed upon moving from the acid to the acid salt. Simulations indicate that altering the MAS rate by only a few tens of hertz can drastically alter the higher-order RR line shapes. The ab initio calculations of chemical shielding tensors provide data that are useful in the simulation of rotational resonance effects. We propose phenylacetic- 13 C 2 acid as a setup sample for rotational resonance and other homonuclear dipolar recoupling experiments.
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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 | 0.000 | 0.000 |
| 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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".