Bonding in Polycyclic Aromatic Hydrocarbons in Terms of the Electron Density and of Electron Delocalization
Bibliographic record
Abstract
We show that the delocalization indices calculated within the framework of the quantum theory of atoms in molecules provide an excellent basis for the definition of a bond order in polycyclic aromatic hydrocarbons. We show that the two-electron information contained within the delocalization index can be estimated from the electron density at the bond critical point, a mapping that has no a priori reason to exist. The mapping between the bond order and the electron density at the bond critical point can be exploited to provide a fast estimate of bond orders from theoretical or experimental electron densities. Bond orders in these molecules are shown to be associated to other one-electron properties. We provide evidence to the strong coupling of the σ and π contributions to the electron density at the bond critical point, a nonseparability by virtue of which the electron density at the bond critical point reflects both contributions. Another remarkable finding is that the delocalization indices between bonded carbon atoms are also strongly negatively correlated to the electronic energy density at the bond critical point, the latter being another example of a two electron property. In this manner, bond order is associated to a stabilizing effect quantified by the electronic energy density at the bond critical point. Because aromaticity is rooted in electron delocalization within a ring system, the delocalization index is used to define an aromaticity index that measures alternation in the delocalization of electrons within a ring of a polycyclic aromatic hydrocarbon. The proposed aromaticity index represents a “local aromaticity measure”. We have analyzed in detail the effects of ring annealation on bond orders and on the aromatic character of a ring and explored its correlation with independent measures of aromaticity such as NICS and HOMA.
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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 | 0.001 |
| 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 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| 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".