QTAIM Investigation of the Electronic Structure and Large Raman Scattering Intensity of Bicyclo-[1.1.1]-pentane
Bibliographic record
Abstract
Our previous studies of the variation of Raman scattering intensities in saturated hydrocarbons have identified a number of structural descriptors that correlate with calculated polarizability derivatives for particular bond displacements: ring strain, steric hindrance, and alignment and location of a C-H group within the molecular framework (e.g., endo-/exo-, axial/equatorial, in-plane/out-of-plane). The bridgehead C-H bond intensities in bicyclo-[1.1.1]-pentane appear to be extraordinarily large, given its size and structure. Molecular polarizability and derivatives are analyzed here for bicyclo-[1.1.1]-pentane and propane, with HF, MP2, CCSD, B3LYP, M06, and M062X levels of theory and the Dunning AVTZ basis set. Analyses of calculated electronic charge densities were performed with two implementations of QTAIM, including an origin-dependent method and an implementation with origin-independent atomic moments. Numerically accurate atomic partitioning of mean molecular polarizabilities is achievable with either; however, accurate partitioning of polarizability derivatives places stringent requirements on the numerical integration, more so for this highly strained bicyclic structure. QTAIM reveals that most of the polarizability (~90%) can be attributed to charge transfer between atomic basins. Calculated Raman intensities are in accord with our experimental data, notably in the prediction of large trace scattering intensities for stretching of the bridgehead CH in bicyclo-[1.1.1]-pentane and for the methyl in-plane C-H in propane. Density difference plots illustrate the effects of bond displacements on the electron densities and the resultant changes in polarizability. Stretching of the bridgehead C-H bond in bicyclo-[1.1.1]-pentane produces electron density changes that are similar to those encountered upon stretching the methyl in-plane C-H of propane.
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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 | 0.001 | 0.001 |
| 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) | 0.003 | 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".