Der Einfluss von Wasser und Wasserstoffbrücken auf Reaktionen in Lavaldüsenexpansionen
Bibliographic record
Abstract
There is an active discussion about the influence of weakly bound intermediates in radical-molecule-reactions in the gasphase. Especially hydrogen-bonded complexes are in focus of scientific investigations because of their meaning in atmospheric chemistry. By measuring rate constants over a wide temperature range essential conclusions can be drawn about the molecular mechanism and the meaning for kinetic processes. The present work investigates the temperature dependence of several different reactions of the hydroxyl-radical namely with propanal, nitrogen monoxide, alanine ethyl ester, and the β-diketones 2,4-pentanedione and 3-methyl-2,4-pentanedione in the temperature range between 60 and 300 K. Additionally the influences of small amounts of water on the kinetics of the reactions of propanal and nitrogen monoxide as well as on the dynamics of the β-diketones have been investigated. The cooling process uses an adiabatic expansion through a Laval nozzle which generates a supersonic jet with constant flow velocity, temperature and density. The reactions are initiated by photolysis of an appropriate precursor to generate hydroxyl radicals. Afterwards the temporal evolution of the OH concentration is measured by laser induced fluorescence in the presence of an excess of the other reactant. All reactions show a negative temperature dependence of their rate coefficient. For propanal, alanine ethyl ester and probably also for the β-diketones these observations can be rationalized by the formation of weakly bound complexes along the reaction coordinate. The rate coefficients are in the range of 10^-11 up to 10^-10 cm^3s^-1. Nitrogen monoxide follows a different reaction mechanism. The radical recombination is followed by the stabilization of a primary formed adduct. The termolecular rate coefficient is in the range of 10^-30 and 10^-29 cm^6s^-1. In the presence of water the reaction of propanal as well as the recombination of nitrogen monoxide with hydroxyl radicals the rate constant increases about 50 %. For the aldehyde theoretical investigations showed that this is because of the high stability of water containing prereactive complexes and also the lowering of the effective reaction barrier between the intermediate and the products. However in the case of nitrogen monoxide the explanation is different: the energy transfer of water is highly pronounced in the deactivation of the primary formed adduct. Additionally the complex formation and the deactivation by the radical-complex-mechanism have to be considered. This assumption is supported by MD-calculations for the recombination process in small clusters ((H2O)n, n=1...4). Investigations on the reaction of 2,4-petanedione with OH-radicals show no obvious influence of water. But further measurements indicate that a possible water effect is masked by a competing dynamic of the β-diketone.
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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 | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| 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".