Alkali Metal NMR Chemical Shielding as a Probe of Local Structure: An Experimental and Theoretical Study of Rb<sup>+</sup> in Halide Lattices
Bibliographic record
Abstract
When Rb + is incorporated into different cubic halide lattices at very low concentrations, the 87 Rb NMR chemical shift becomes more shielded by as much as 282 ppm when the cation is changed from Na + to Cs + . Similarly, in mixed crystals of KCl and RbCl, the average resonance frequency shifts with the degree of incorporation. The 87 Rb chemical shift data for each halide show a near-linear correlation with the average alkali metal−halide interionic distance in the different crystals studied, in good agreement with ab initio calculations which show that indeed the chemical shielding has a strong dependence on the nearest neighbor shell of halide anions and their distance from the Rb + . When the Rb concentration is increased as in KCl/RbCl solid solutions, the 87 Rb NMR spectra are influenced by a large distribution of K/Rb configurations in the next-nearest-neighbor shell of 12 cations. While the total shift is still largely dependent on the Rb−Cl distance, shells containing both K and Rb atoms give rise to second-order quadrupolar effects and an offset from the isotropic chemical shift. This work is a remarkable demonstration of the sensitivity of the 87 Rb nucleus to its physical environment.
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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 | 0.001 | 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".