Indirect Nuclear Spin−Spin Coupling Tensors in Diatomic Molecules: A Comparison of Results Obtained by Experiment and First Principles Calculations
Bibliographic record
Abstract
The importance of hyperfine structure observed in molecular beam or high-resolution microwave spectroscopy experiments has been almost completely overlooked by NMR spectroscopists and theoreticians. In the present work, we show for a series of diatomic molecules that the indirect spin−spin coupling tensor, of fundamental importance to magnetic resonance spectroscopy, is completely characterized by the hyperfine measurements. The hyperfine parameter c 4 is known to be equivalent to the isotropic spin−spin coupling constant, J iso; what has not been exploited is the relationship between c 3 and the anisotropic portion of the spin−spin coupling tensor, Δ J . Through comparisons to highly precise experimental data available for LiH, LiF, KF, Na 2, and ClF, multiconfigurational SCF calculations using balanced complete active spaces and large correlation-consistent basis sets have been employed to establish the reliability of such calculations for determining the complete tensor rather than simply J iso . The experimental data are for “isolated” molecules, making them ideal for comparison with ab initio results; agreement is generally within a few percent after accounting for rovibrational effects. These results, combined with further calculations on a larger set of diatomic molecules (HF, BF, AlF, KNa, HCl, NaF), provide new insights into the nature of indirect spin−spin coupling. Calculations indicate the importance of each of the various coupling mechanisms. The influence of the Fermi-contact mechanism, traditionally thought to be the dominant contribution to J iso, is shown to vary considerably even for couplings between first-row elements. General conclusions about the relative importance of all mechanisms to both the isotropic and anisotropic portions of the coupling tensor are discussed, and periodic trends are proposed.
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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) | 0.001 | 0.001 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.002 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 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".