The Structure of Two Anhydrous Polymorphs of Caffeine from Single-Crystal Diffraction and Ultrahigh-Field Solid-State <sup>13</sup>C NMR Spectroscopy
Bibliographic record
Abstract
In this work, we examine two anhydrous polymorphs of caffeine, as well as the monohydrate, by high-field (21 T) NMR spectroscopy, and since suitable single crystals of the anhydrous forms could be obtained, the crystal structures were also determined. At high field, the 13 C NMR spectra are simplified considerably over those obtained at low field as the effect of the 14 N nuclear quadrupoles on the 13 C resonances becomes minimal. The spectra of the two anhydrous polymorphs provide information about the number of distinct caffeine sites and indicate structural disorder. The NMR observations are consistent with single-crystal X-ray diffraction, which shows that the structures are indeed complicated by disorder. Furthermore, the space groups obtained previously from powder diffraction were, in fact, incorrect, R 3̄ c rather than R 3 c for the low-temperature polymorph and C 2/ c rather than Cc for the high-temperature polymorph. As a result, the structures are different from those proposed on the basis of modeling calculations.
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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 | 0.000 |
| 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".