A Solid-State Multinuclear Magnetic Resonance Investigation of Hexamethylborazine
Bibliographic record
Abstract
Analyses of 11 B, 13 C, and 2 H NMR spectra of solid hexamethylborazine, I, provide conclusive evidence for rapid in-plane jumps of the borazine ring at room temperature. Boron-11 NMR spectra of magic-angle spinning (MAS) samples, acquired at low (4.7 T), moderate (9.4 T), and high (18.8 T) external applied magnetic field strengths, have been simulated to yield the 11 B nuclear quadrupolar coupling constant ( C Q ), asymmetry parameter, and isotropic chemical shift; their values at 298 K are 2.98 ± 0.03 MHz, 0.01 ± 0.01, and 36.0 ± 0.4 ppm, respectively. Simulations of 13 C CP/MAS NMR spectra provide the carbon−boron isotropic indirect spin−spin coupling constant, J iso, the sign of C Q ( 11 B), the relative orientations of the boron electric field gradient (EFG) and the 13 C− 11 B dipolar coupling tensors, and the motionally averaged 13 C− 11 B dipolar coupling constant. Variable-temperature 2 H NMR spectra of a partially deuterated sample of I indicate that the in-plane jumps of the borazine ring are slow with respect to C Q ( 2 H) -1 (i.e., τ jump ≥ 10 -4 s) at temperatures less than 130 K. Over the temperature range 180 to 128 K, 2 H NMR line shape analysis yields an activation energy of 30.1 ± 1.5 kJ mol -1 for the in-plane jumps of the borazine ring. Although a precise experimental determination of boron chemical shift anisotropy was impeded by intramolecular and intermolecular boron−boron dipolar interactions and heteronuclear nitrogen−boron dipolar interactions, simulations of high-field 11 B NMR spectra of a stationary sample of I suggest a value of 55 ± 15 ppm for the motionally averaged span of the chemical shift tensor. Lastly, high-level ab initio and density functional theory calculations provide values of the boron EFG tensor and the boron and nitrogen magnetic shielding tensors for a rigid molecule of I .
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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".