Probing the Location of Hydrogen Atoms in Low-Barrier Hydrogen Bonds in 1,3-Diketone Molecules by H/D Isotope Shifts in Solid-State <sup>17</sup>O Nuclear Magnetic Resonance and <sup>1</sup>H–<sup>17</sup>O Distance Measurements
Bibliographic record
Abstract
1,3-Diketone molecules existing in their cis -keto–enol tautomeric forms in the solid state are classic examples that contain low-barrier hydrogen bonds (LBHBs) of the O–H···O type. We report experimental observations of H/D isotope shifts in 13 C and 17 O solid-state nuclear magnetic resonance (NMR) spectra of three representative 1,3-diketone molecules: dibenzoylmethane, benzoylacetone, and curcumin. While there are several examples of H/D isotope shifts in solid-state 13 C NMR spectra in the literature, this study reports on the first observation of such H/D isotope shifts in solid-state 17 O NMR spectra. We found that the H/D isotope shifts in dibenzoylmethane are considerably larger than those in benzoylacetone and curcumin. To aid interpretation of the observed H/D isotope shifts, we performed direct 1 H– 17 O distance measurement using two-dimensional 17 O/ 1 H quadrupole–dipole correlation NMR spectroscopy. The results of 1 H– 17 O distance measurements provided direct evidence about the “location” of the enol hydrogen atom in the 17 O··· 1 H··· 17 O LBHBs. We demonstrated that H/D isotope shifts in solid-state 17 O NMR and 1 H– 17 O distance measurement are two new ways of probing the nature of LBHBs. Our new solid-state 17 O NMR results are discussed in connection to nuclear quantum effects (i.e., the potential energy curve, zero-point energy, and nuclear wave function) and high-quality structural data available for these compounds in the literature.
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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 | 0.000 |
| 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 | 0.000 |
| 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".