CONFORMERS OF VANILLIC ACID: A ROTATIONAL SPECTROSCOPIC AND THEORETICAL STUDY
Bibliographic record
Abstract
Vanillic acid is produced in significant quantities in forest fires and is used as a biomass burning tracer for softwood and hardwood combustion events. Vanillic acid has also been found in secondary organic aerosol and can be oxidized in aqueous atmospheric processes to humic-like substances. Hydrates of vanillic acid play undoubtedly important roles in the processes involved, and we present here a study of conformers of the vanillic acid monomer to lay the groundwork for subsequent studies of the mono- and di-hydrates. \n\nComputationally, 12 conformers were found within an energy range of 51 kJ/mol using the Conformer–Rotamer Ensemble Sampling Tool (CREST).$^1$ The two lowest energy conformers could then be identified in the spectrum recorded with a chirped pulse Fourier transform microwave spectrometer in the 2-6 GHz range. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TVPZ levels of theory is less than 1\\%. No tunnelling splitting was observed which suggests a relatively high barrier to methyl internal rotation, in agreement with other, previously studied vanillin derivatives. Furthermore, no c-type transitions could be observed in agreement with computed zero c-dipole moment component of the lowest energy structures. From the theoretical structures, it is apparent that intra-molecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further explore the intra-molecular interactions in vanillic acid, QTAIM and NCI analyses were performed. \n\n\n\n$^1$P. Pracht, F. Bohle, S. Grimme, Phys. Chem. Chem. Phys. 22, 7169-7192 (2020).
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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 | 0.000 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 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".