27Al NMR spectroscopy at multiple magnetic fields and ab initio quantum modeling for kaolinite
Bibliographic record
Abstract
Diffraction methods indicate clearly that there are two crystallographic Al sites in kaolinite with different site symmetry. Many unsuccessful attempts have been made to resolve the two different Al sites in kaolinite by magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy since the early 1980s. This study attempts to resolve these sites, derive accurate quadrupolar coupling parameters at the two sites, and explain the reasons for the lack of separation by combining NMR spectroscopy at various fields with ab initio quantum modeling. At a high field such as 21.06 T, large chemical shift anisotropy and dipolar effects can overwhelm the quadrupolar interaction effects for sites with relatively small Cq values such as the two Al sites in kaolinite. The spectra will then display featureless lineshapes lacking the typical quadrupolar interaction characteristics. At lower fields, such as 4.68 T, the quadrupolar interaction effects become dominant, but due to the similarity of the two Al environments in kaolinite, as well as broadening by residual dipole-dipole and chemical shift anisotropy, the 27 Al MAS NMR peaks still could not be separated. To resolve the two different Al sites spectroscopically, 27 Al MAS NMR experiments at multiple magnetic fields, satellite transition NMR, and multiple-quantum MAS techniques have been utilized. Although these NMR experiments failed to resolve the two Al sites, the spectra have been interpreted with the aid of theoretical ab initio quantum mechanical modeling using full potential linearized augmented plane wave (FP-LAPW) model and the CASTEP software program. Average values of Cq, η, and δ iso for the two Al sites were constrained to be 2.6 MHz, 0.75 and 6.25 ppm with high confidence.
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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 | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 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".