Kinetik der Reaktionen des Hydroxylradikals mit ungesättigten Kohlenwasserstoffen in einer Lavaldüsenexpansion
Bibliographic record
Abstract
The investigation of hydroxyl radical reactions with unsaturated hydrocarbons like ethyne, ethylene, propene and isoprene at low temperatures plays a significant role for the understanding of tropospheric chemistry. The temperature dependence of rate constants provides information about mechanistic details regarding the observed reaction. Bimolecular rate constant are then very sensitive to details of the potential energy surface at chemical significant energies.This thesis presents experimental investigations of the hydroxyl radical reactions with several unsaturated hydrocarbons at low temperatures. The carrier gas (nitrogen) is cooled down to 58 K using the expansion through a Laval nozzle. The Laval nozzle produces a uniform flow (temperature, density and Mach number are constant) of the carrier gas. Hydroxyl radicals are generated by means of laser photolysis of hydrogen peroxide at 193 nm and 248 nm. The time dependent hydroxyl radical concentration is observed via laser induced fluorescence (LIF). To estimate the rate constants of the observed reactions calculations according to the statistical adiabatic channel model SACM) are performed, the required educt and product properties are obtained by means of DFT methods.The determined rate constants between 60 and 300 K range from 10-12 to 10-10 cm3s-1. A systematic increase of the rate constants from ethyne to ethylene, propene and isoprene is observed. Moreover there is (at least up to 100 K) a negative temperature dependence of the rate constants. This feature appears due to long range electrostatic forces directly connected to transition states located at great reactant distances. The vibrational relaxation of the hydroxyl radical is used for the determination of high pressure limiting rate constants of the hydroxyl radical reaction with ethyne and ethylene. The OH recombination reaction with propene and particularly isoprene distinguish themselves by a decrease of the rate constants below 100 K. This effect may be due to the existence of intermediate isomers. Analysis of the rate constants reveals a significant barrier for the OH ethyne reaction, which dominates the negligible temperature dependence between 300 and 60 K, whereas the respective ethylene reaction shows a negative temperature dependence within the measured range.
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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 | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".