Experimental and Theoretical Investigation of the Reaction of C<sub>2</sub>H with Formaldehyde (CH<sub>2</sub>O) at Very Low Temperatures and Application to Astrochemical Models
Bibliographic record
Abstract
High Resolution Image Download MS PowerPoint Slide Rate coefficients for the reaction of C 2 H with CH 2 O were measured for the first time over the temperature range of 37–603 K, with the C 2 H radicals produced by pulsed laser photolysis and detected by CH radical chemiluminescence following their reaction with O 2 . The low temperature measurements (≤93 K) relevant to the interstellar medium were made within a Laval nozzle gas expansion, while higher temperature measurements (≥308 K) were made within a temperature controlled reaction cell. The rate coefficients display a negative temperature dependence below 300 K, reaching (1.3 ± 0.2) × 10 –10 cm 3 molecule –1 s –1 at 37 K, while only a slight positive temperature dependence is observed at higher temperatures above 300 K. Ab initio calculations of the potential energy surface (PES) were combined with rate theory calculations using the MESMER master-equation program in order to predict rate coefficients and branching ratios. The three lowest energy entrance channels on the PES all proceed via the initial formation of a weakly bound prereaction complex, bound by ∼5 kJ mol –1, followed by either a submerged barrier on the route to the H-abstraction products (C 2 H 2 + CHO), or emerged barriers on the routes to the C- or O-addition species. MESMER calculations indicated that over the temperature range investigated (10–600 K) the two addition channels were uncompetitive, accounting for less 0.3% of the total product yield even at 600 K. The PES containing only the H-abstraction product channel was fit to the experimentally determined rate coefficients, with only a minor adjustment to the height of the submerged barrier (from −2.6 to −5.9 kJ mol –1 ) required. Using this new submerged barrier height, and including the subsequent dissociation of the CHO product into CO + H in the PES, rate coefficients and branching ratios were calculated over a wide range of temperatures and pressures and these used to recommend best-fit modified Arrhenius expressions for use in astrochemical modeling. Inclusion of the new rate coefficients and branching ratios in a UMIST chemical model of an outflow from an asymptotic giant branch (AGB) star yielded no significant changes in the abundances of the reactants or the products of the reaction, however, removal of the C-addition channel currently in the UMIST Rate22 database did result in a significant reduction in the abundance of propynal (HCCCHO).
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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) | 0.000 | 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) | 0.000 | 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".