Rate Coefficients and Kinetic Isotope Effect for the C<sub>2</sub>H Reactions with NH<sub>3</sub> and ND<sub>3</sub> in the 104−294 K Temperature Range
Bibliographic record
Abstract
Reactions of C 2 H with NH 3 and ND 3 are studied at low temperature using a pulsed Laval nozzle apparatus. The C 2 H radical is prepared by 193 nm photolysis of acetylene, and the C 2 H concentration is monitored using CH(A 2 Δ) chemiluminescence from the C 2 H + O 2 reaction. The rate constants for the C 2 H + NH 3 and C 2 H + ND 3 reactions are measured at three temperatures, 104 ± 5 K, 165 ± 15 K, and 296 ± 2 K. Measured rate constants are fit to power law expressions, k ( T ) = A ( T /298) n, for ease of comparison with the results for the related CN + NH 3 reaction and to emphasize the importance of the attractive part of intermolecular interaction potential in the reaction mechanism. The rate constants are (2.9 ± 0.7) × 10 -11 × ( T /298 K) (−0.90 ± 0.15) cm 3 molecule -1 s -1 and (1.1 ± 0.2) × 10 -11 × ( T /298 K) (−0.82 ± 0.026) cm 3 molecule -1 s -1 for NH 3 and ND 3, respectively. A large kinetic isotope effect is observed, k (C 2 H + NH 3 )/ k (C 2 H + ND 3 ) = 2.0 ± 0.2, which within experimental uncertainty does not depend on the temperature in the 104−296 K range. Previous theoretical work shows that a hydrogen abstraction channel, C 2 H + NH 3 → C 2 H 2 + NH 2, is a possible mechanism for the C 2 H + NH 3 reaction since the minimum energy path for this channel does not have an activation barrier. This theoretical prediction is consistent with the strong negative temperature dependence of the rate coefficients for the C 2 H + NH 3 reaction observed in this work, which clearly shows that the C 2 H + NH 3 reaction does not have a barrier.
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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 | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.001 | 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 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.004 | 0.001 |
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".