Investigation of Slow Molecular Motions and Chemical Exchange in the High Loaded Form of <i>p</i>-Xylene in ZSM-5 by Two-Dimensional <sup>13</sup>C Solid-State NMR Exchange Experiments
Bibliographic record
Abstract
The slow molecular motions and exchange processes in the high loaded form of p -xylene in zeolite ZSM-5 have been investigated by a variety of solid-state NMR techniques. The 13 C CP MAS NMR spectra of [ 13 C, CH 3 ] p -xylene show four resonances in the slow exchange limit, reflecting the four inequivalent methyl positions in the two organic molecules in the asymmetric unit. From the known assignment of the silicon resonances in the MAS NMR spectrum, the four carbon signals were assigned from the cross-correlations in a 1 H/ 13 C/ 29 Si 2D CP TEDOR experiment. Using this information, the chemical exchange processes between the methyls was invesitgated by a series of 2D CP NOESY experiments with different mixing times at different temperatures. Two types of processes were identified: intramolecular exchanges with activation energies of 65 ± 2 kJ/m for the molecule in the zigzag channel and 56 ± 13 kJ/m for the molecule in the straight channel (the substantial error in the latter value reflecting the very limited temperature range over which measurements can be made). There is also intermolecular exchange between the two molecules with a much higher energy barrier. Possible mechanisms for the exchanges and the implications of slow motional processes for the use of solid-state NMR for the determination of sorbate/framework structures are 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 | 0.000 | 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 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 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".