Theoretical study of excited state proton transfer in 3,6-bis(benzoxazolyl)pyrocatechol (BBPC)
Bibliographic record
Abstract
Ab initio density functional theory and approximate instanton methods are used to study proton transfer processes in the first excited electronic state of 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). Geometries of di-enol, keto-enol, and di-ketone tautomers as well as transition states for single and double proton transfer processes and the corresponding force fields are obtained with the CIS/6-31G* method and verified with CISD/6-31G* single point calculations. It is shown that keto-enol tautomer is the most stable in the S1 state while the least stable is di-ketone. The single proton transfer in the 2A1 state of di-enol leads to a somewhat more stable keto-enol tautomer. This result nicely reproduces the experimental assignment stating that BBPC, a symmetric molecule with two equivalent proton transfer reaction sites, undergoes a single proton transfer in the S1 state. The excited system has to overcome the barrier of about 9 kcal/mol and proton transfer is therefore dominated by tunneling. Dynamics calculations with the instanton method yield the rate of transfer of 9.8×1010 s−1, again in a very good agreement with the experimental value of kPT=(5.1±0.4)×1010 s−1 [Chem. Phys. Lett. 169, 450 (1990)]. Theory predicts a large kinetic isotope effect on this process. It is also shown that the reverse proton transfer leading back to di-enol has the rate strongly dependent on the stabilization energy of keto-enol. It effectively competes with the radiative decay of the latter, resulting in the observed weak di-enol fluorescence of BBPC. Finally, the calculations demonstrate why BBPC is not a photochrome unlike many typical Schiff bases.
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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 | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.001 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 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".