The Site of Protonation Affects the Dissociation of Protonated α‐ and β‐Pinene Ions
Bibliographic record
Abstract
RATIONALE: In electrospray ionization and atmospheric pressure chemical ionization, the protonation site directly guides the ion's dissociation. But what if the site of protonation is ambiguous? In this study, we explored the unimolecular reactions of protonated α- and β-pinene ions with a combination of tandem mass spectrometry and theory. Each has multiple potential protonation sites that influence their chemistry. METHODS: Atmospheric pressure chemical ionization was employed to form the protonated pinene isomers. The unimolecular chemistry of these ions was explored with a Waters Ultima triple-quadrupole mass spectrometer using energy-resolved collision-induced dissociation with argon collision gas. Reaction mechanisms were calculated with CBS-QB3 single-point energy calculations on B3LYP/6-311+G(d,p) optimized structures. RESULTS: The two main dissociation reactions in each ion lead to the loss of neutral propene and isobutene. Both ions were found to dissociate over the same minimum energy reaction pathway, the only difference being the site of initial protonation. α-Pinene preferentially protonates at the bridging carbon, while β-pinene can only significantly protonate at the exocyclic double bond. This leads to a lower appearance energy for loss of isobutene, and thus relatively greater m/z 81 fragment ion abundance for β-pinene. CONCLUSIONS: The distinct sites of initial protonation result in the subtle differences observed in the CID of α- and β-pinene. The work highlights that it is not necessarily the "lowest energy" ion that will be formed in the ion source, and any distribution of initial structures must be accounted for when examining CID mass spectra.
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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 | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 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".