Conformers of Vanillic Acid and Its Monohydrate: A Rotational Spectroscopic and Theoretical Study
Bibliographic record
Abstract
Organic acids are released during wildfire combustion and can influence aerosol formation and growth. Conformational flexibility is thought to be advantageous in stabilizing the precritical nucleus in the process of aerosol particle formation and allowing for further complexation with other atmospheric constituents. We describe here a study of the conformational flexibility of vanillic acid and its monohydrate using electronic structure calculations and Fourier transform microwave spectroscopy. Computationally, 12 and 28 conformers were found for the monomer and monohydrate, respectively. The two lowest energy conformers of both the vanillic acid monomer and the vanillic acid–water complex could be experimentally identified. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TZVP levels of theory is less than 1%. No tunneling splittings were 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 for the vanillic acid monomer, in agreement with the computed zero c -dipole moment component of the two lowest energy structures. For the monohydrate the absence of c -type transitions is rationalized by averaging over a large amplitude motion involving the free H atom of the water unit. From the theoretical structures, it is apparent that intramolecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further characterize the intramolecular interactions in the monomer and intra- and intermolecular interactions in the monohydrate, quantum theory of atoms-in-molecules (QTAIM), noncovalent interactions (NCI), and intrinsic bond strength index (IBSI) analyses were performed. The atmospheric abundance of the vanillic acid monohydrate relative to the monomer was evaluated to assess its atmospheric significance.
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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 | 0.000 | 0.000 |
| 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) | 0.002 | 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".