Characterization of Tricoordinate Boron Chemical Shift Tensors: Definitive High-Field Solid-State NMR Evidence for Anisotropic Boron Shielding
Bibliographic record
Abstract
Despite the large known chemical shift (CS) range for boron and the large number of 11 B NMR studies of glasses, no boron CS tensors have been characterized to date. We report the application of solid-state NMR techniques at moderate (9.4 T) and high (17.63 T) applied magnetic field strengths to the characterization of the boron CS tensors in trimesitylborane (BMes 3 ) and triphenyl borate (B(OPh) 3 ). The boron CS tensor of the former compound exhibits a remarkably large span, Ω = 121 ± 1 ppm, which encompasses the known range of isotropic chemical shifts for tricoordinate boron compounds. Conversely, the effect of the boron CS tensor on the 11 B NMR spectra of B(OPh) 3 is difficult to observe and quantify even at field strengths as high as 17.63 T; we find Ω ≤ 10 ppm. This marked difference in the boron nuclear magnetic shielding tensors is reproduced accurately by a series of ab initio and DFT calculations with a range of basis sets. The difference is rationalized in the context of Ramsey's theory of nuclear magnetic shielding by considering contributions to the paramagnetic shielding in the tricoordinate boron plane. Differences in the in-plane shielding tensor components for the molecules considered are a result of variations in the effectiveness of the mixing of occupied σ orbitals with virtual π orbitals under the influence of an applied magnetic field. A similar explanation has been invoked to rationalize 13 C isotropic chemical shifts in classical and nonclassical carbocations. We also report experimental and calculated boron nuclear quadrupolar coupling constants and asymmetry parameters for BMes 3 and B(OPh) 3 . A combination of experimental and theoretical results provides the orientation of the CS and electric field gradient tensors in the molecular framework.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| 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) | 0.000 | 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 teacher head, not a consensus.
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