Steady-state free precession NMR in the presence of heteronuclear couplings and decoupling: More than meets the eye
Bibliographic record
Abstract
Fourier Transform (FT) has been a mainstay of analytical 13C/15N NMR. On the other hand, it has been shown that Steady-State Free Precession (SSFP) experiments, which depart from this scheme, can, under certain conditions, endow 13C/15N small-molecule NMR with comparable sensitivity and resolution. SSFP is one of the earliest and most widely used NMR pulse sequences, yet its analyses have focused on isolated spin-1/2 ensembles such as water. The present study demonstrates that significant deviations from such isolated spin-1/2 behavior may occur when SSFP is applied in the presence of spin-spin couplings. Even in the simplest case supporting such couplings, a single 13C J-coupled to a 1H, departures from the isolated spin-1/2 behavior arise in the 13C SSFP response-both in the absence and in the presence of 1H spin decoupling. In the former case, deviations are produced by the differential relaxation of antiphase two-spin terms generated by the pulse train; in the latter case, magnified interferences may arise between the SSFP pulses and the coherent perturbation arising upon 1H decoupling. Although both phenomena are also known in FT NMR, the spectral distortions that they will originate may be much larger in the SSFP case-particularly if interpulse delays in large flip-angle SSFP pulse trains "resonate" with the coupling perturbations. The origins of these effects are here analyzed for heteronuclear spin-1/2 systems and corroborated with 13C NMR SSFP experiments recorded under different conditions. Additional considerations aimed at magnifying or suppressing these effects, as well as extensions to more complex scenarios, are also 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 | 0.003 | 0.003 |
| Meta-epidemiology (narrow) | 0.001 | 0.001 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.001 | 0.003 |
| Scholarly communication | 0.003 | 0.010 |
| Open science | 0.002 | 0.002 |
| Research integrity | 0.004 | 0.006 |
| Insufficient payload (model declined to judge) | 0.004 | 0.002 |
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".