High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals
Bibliographic record
Abstract
In a previous communication [Hu et al., J. Am. Chem. Soc. 126, 10844 (2004)], an approach was demonstrated that improves the efficiency of the cross-effect polarization mechanism employed in high field dynamic nuclear polarization (DNP) experiments. Specifically, it was shown that tethering two TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) radicals increases the electron-electron dipole coupling from approximately 1 MHz in solutions of monomeric TEMPO to approximately 25 MHz in a tethered biradical. The larger coupling resulted in an increase in the DNP enhancements by a factor of approximately 3-4, from 45-50 to approximately 165. Here, a second approach to improving the efficiency of the polarization process is described that involves approximately satisfying the matching condition |omega(2e)-omega(1e)|=omega(n), where omega(2e) and omega(1e) are two frequencies in the electron paramagnetic resonance (EPR) spectrum and omega(n) is the Larmor frequency of the nuclear spins being polarized. Specifically, in a mixture of TEMPO and trityl [tris (8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) methyl] radicals, the intensity maxima in the EPR spectra of these two species are approximately separated by the (1)H NMR frequency. In this case the frequency difference between the g(yy) value of TEMPO and the narrow pseudo-isotropic g-value of trityl is approximately 224 MHz and the (1)H Larmor frequency is 211 MHz. The optimal magnetic field for DNP using the mixtures was found to coincide with the trityl EPR resonance. At 90 K and 5 T, a mixture of 20 mM TEMPO and 20 mM trityl enhanced the (1)H polarization by a factor of approximately 160, an improvement over the enhancement of approximately 50 with 40 mM TEMPO. The reasons for the improvement are discussed and evidence is presented suggesting that DNP enhancement can be improved further by tethering TEMPO and trityl or two similar radicals.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 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".