A Combined Experimental and Theoretical <sup>17</sup>O NMR Study of Crystalline Urea: An Example of Large Hydrogen-bonding Effects
Bibliographic record
Abstract
We report the first experimental determination of the carbonyl 17 O electric-field-gradient (EFG) tensor and chemical-shift (CS) tensor of a urea-type functional group, R 1 NH C(O) NHR 2 . Analysis of magic-angle spinning (MAS) and stationary 17 O NMR spectra of crystalline [ 17 O]urea yields not only the principal components of the carbonyl 17 O EFG and CS tensors, but also their relative orientations. The carbonyl 17 O quadrupole coupling constant (QCC) and the asymmetry parameter (η) in crystalline urea were found to be 7.24 ± 0.01 MHz and 0.92, respectively. The principal components of the 17 O CS tensor were determined: δ 11 = 300 ± 5, δ 22 = 280 ± 5 and δ 33 = 20 ± 5 ppm. The direction with the least shielding, δ 11, is perpendicular to the C O bond and the principal component corresponding to the largest shielding, δ 33, is perpendicular to the N C(O) N plane. The observed 17 O CS tensor suggests that, in crystalline urea, the 17 O paramagnetic shielding contributions from the σ → π* and π → σ* mixing are greater than that from the n → π* mixing. Quantum chemical calculations revealed very large intermolecular H-bonding effects on the 17 O NMR tensors. It is demonstrated that inclusion of a complete intermolecular H-bonding network is necessary in order to obtain reliable 17 O EFG and CS tensors. B3LYP/D95** and B3LYP/6-311++G** calculations with a molecular cluster containing 7 urea molecules yielded 17 O NMR tensors in reasonably good agreement with the experimental data.
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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 | 0.001 |
| 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
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