1H–2H cross-polarization NMR in fast spinning solids by adiabatic sweeps
Bibliographic record
Abstract
Cross-polarization (CP) experiments employing frequency-swept radiofrequency (rf) pulses have been successfully used in static spin systems for obtaining broadband signal enhancements. These experiments have been recently extended to heteronuclear I, S = spin-1/2 nuclides under magic-angle spinning (MAS), by applying adiabatic inversion pulses along the S (low-γ) channel while simultaneously applying a conventional spin-locking pulse on the I-channel (1H). This study explores an extension of this adiabatic frequency sweep concept to quadrupolar nuclei, focusing on CP from 1H (I = 1/2) to 2H spins (S = 1) undergoing fast MAS (νr = 60 kHz). A number of new features emerge, including zero- and double-quantum polarization transfer phenomena that depend on the frequency offsets of the swept pulses, the rf pulse powers, and the MAS spinning rate. An additional mechanism found operational in the 1H–2H CP case that was absent in the spin-1/2 counterpart, concerns the onset of a pseudo-static zero-quantum CP mode, driven by a quadrupole-modulated rf/dipolar recoupling term arising under the action of MAS. The best CP conditions found at these fast spinning rates correspond to double-quantum transfers, involving weak 2H rf field strengths. At these easily attainable (ca. 10 kHz) rf field conditions, adiabatic level-crossings among the {|1⟩,|0⟩,|−1⟩} mS energy levels, which are known to complicate the CP MAS of quadrupolar nuclei, are avoided. Moreover, the CP line shapes generated in this manner are very close to the ideal 2H MAS spectral line shapes, facilitating the extraction of quadrupolar coupling parameters. All these features were corroborated with experiments on model compounds and justified using numerical simulations and average Hamiltonian theory models. Potential applications of these new phenomena, as well as extensions to higher spins S, are briefly discussed.
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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 | 0.001 | 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 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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".