Loading-dependent structures of CO2 in the flexible molecular van der Waals host p-tert-butylcalix[4]arene with 1 : 1 and 2 : 1 guest–host stoichiometries
Bibliographic record
Abstract
The adsorption of CO(2) into the low density form of p-tert-butylcalix[4]arene (tBC) has been studied by (13)C solid state NMR, single crystal X-ray diffraction and volumetric adsorption measurements. The experimental results indicate that tBC and carbon dioxide can form two distinct inclusion compounds. At low loadings the structure of the empty low-density form of the tBC framework (space group P2(1)/n) is preserved with the included CO(2) molecules located within the conical cavities of the tBC molecules. The ideal composition of this form is therefore 1 : 1 (CO(2) : tBC). With higher applied CO(2) pressures the guest loading increases and the structure of the tBC framework transforms to a well studied tetragonal (space group P4/n) form. In this form an additional CO(2) molecule is located on an interstitial site resulting in an ideal composition 2 : 1 (CO(2) : tBC). In agreement with SCXRD and the gas adsorption measurements, (13)C NMR measurements show the change in structure that takes place as a function of sample loading. Inclusion of CO(2) is a rather slow activated process that can be accelerated by increasing the temperature and the transition between crystal forms is inhomogeneous over a bulk sample. After gas release, the empty (or near empty) P4/n structure survives, thus providing another low density phase of tBC. The magnitude and temperature variation of the (13)C chemical shift anisotropy of CO(2) in both low and high occupancy complexes with tBC indicates restricted motion of the CO(2) molecules. The location and dynamics of CO(2) molecules inside the tBC structure are discussed and a motional model for CO(2) is proposed. The CO(2) molecules in the highly loaded compound are shown to exchange rapidly as a single resonance is observed for the two distinct CO(2) molecules.
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How this classification was reachedexpand
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) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| 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.
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".