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 distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".